bioRxiv Subject Collection: Neuroscience's Journal

APP beta-CTF triggers cell-autonomous synaptic toxicity independent of Abeta
Abeta is believed to play a significant role in synaptic degeneration observed in Alzheimer's disease (AD) and is primarily investigated as a secreted peptide. However, the contribution of intracellular Abeta or other cleavage products of its precursor protein (APP) to synaptic loss remains uncertain. In this study, we conducted a systematic examination of their cell-autonomous impact using a sparse expression system. Here, these proteins/peptides were overexpressed in a single neuron, surrounded by thousands of untransfected neurons. Surprisingly, we found that APP induced dendritic spine loss only when co-expressed with BACE1. This effect was mediated by beta-CTF, a beta-cleavage product of APP, through an endosome-related pathway independent of Abeta. Neuronal expression of beta-CTF in mouse brains resulted in defective synaptic transmission and cognitive impairments, even in the absence of amyloid plaques. These findings unveil a beta-CTF-initiated mechanism driving synaptic toxicity irrespective of amyloid plaque formation and suggest a potential intervention by inhibiting the endosomal GTPase Rab5.

Beyond Static Models: The Dynamic Interplay of Facial Emotions and Attentional Scope
The interplay between emotion and attention has long been intensely scrutinized, with competing theories proposing divergent mechanisms. Building on our previous work, here we present evidence that refines these perspectives, revealing a nuanced, temporally dynamic relationship between emotional stimuli and attentional focus. Using a modified Flanker task with facial emotion cues, we demonstrate that the effects of emotional stimuli on attention evolve over time, contrary to traditional fixed-effect assumptions. Our results show distinct temporal patterns: Neutral faces elicited typical flanker effects initially, but only interference persisted later. Early-stage happy faces amplified flanker facilitation but not interference, while threat faces augmented flanker interference but not facilitation. In the late stage, flanker facilitation disappeared across all emotion conditions, and interference patterns converged, mirroring the neutral face condition. These findings indicate emotion's influence on attention is more complex and dynamic than previously recognized, potentially reflecting learning or habituation processes. We propose a new framework for understanding emotion-attention interactions that transcends traditional dichotomies of attention focus and approach-avoidance, offering a more nuanced perspective on this critical cognitive interface.

H105A peptide eye drops promote photoreceptor survival in murine and human models of retinal degeneration
Photoreceptor death causes blinding inheritable retinal diseases, such as retinitis pigmentosa (RP). As disease progression often outpaces therapeutic advances, finding effective treatments is urgent. This study focuses on developing a targeted approach by evaluating the efficacy of small peptides derived from pigment epithelium-derived factor (PEDF), known to restrict common cell death pathways associated with retinal diseases. Peptides with affinity for the PEDF receptor, PEDF-R, (17-mer and H105A) delivered via eye drops reached the retina, efficiently promoted photoreceptor survival, and improved retinal function in RP mouse models based on both the rd10 mutation and the rhodopsin P23H mutation. Additionally, intravitreal delivery of AAV-H105A vectors delayed photoreceptor degeneration in the latter RP mouse model. Furthermore, peptide H105A specifically prevented photoreceptor death induced by oxidative stress, a contributing factor to RP progression, in human retinal organoids. This promising approach for peptide eye drop delivery holds significant potential as a therapeutic for preventing photoreceptor death in retinal disorders, offering a high safety profile, low invasiveness and multiple delivery options.

Spatial Transcriptomics and Single-Nucleus Multi-omics Analysis Revealing the Impact of High Maternal Folic Acid Supplementation on Offspring Brain Development
Folate, an essential vitamin B9, is crucial for diverse biological processes including neurogenesis. Folic acid (FA) supplementation during pregnancy is a standard practice for preventing neural tube defects (NTDs). However, concerns are growing over the potential risks of excessive maternal FA intake. Here, we employed mouse model and spatial transcriptomics and single-nucleus multi-omics approaches to investigate the impact of high maternal FA supplementation during the periconceptional period on offspring brain development. Maternal high FA supplementation affected gene pathways linked to neurogenesis and neuronal axon myelination across multiple brain regions, as well as gene expression alterations related to learning and memory in thalamic and ventricular regions. Single-nucleus multi-omics analysis revealed that maturing excitatory neurons in the dentate gyrus (DG) are particularly vulnerable to high maternal FA intake, leading to aberrant gene expressions and chromatin accessibility in pathways governing ribosomal biogenesis critical for synaptic formation. Our findings provide new insights into specific brain regions, cell types, gene expressions and pathways that can be affected by maternal high FA supplementation.

Visual system structural and functional connections during face viewing in body dysmorphic disorder
Individuals with body dysmorphic disorder (BDD) perceive distortions in their appearance, which could be due to imbalances in global and local visual processing. The vertical occipital fasciculus connects dorsal and ventral visual stream regions, integrating global and local information, yet the role of this structural connection in BDD has not been explored. Here, we investigated the vertical occipital fasciculus's white matter microstructure in those with BDD and healthy controls and tested associations with psychometric measures and effective connectivity while viewing their face during fMRI. We analyzed diffusion MRI and fMRI data in 17 unmedicated adults with BDD and 21 healthy controls. For diffusion MRI, bundle-specific analysis was performed, enabling quantitative estimation of neurite density and orientation dispersion of the vertical occipital fasciculus. For task fMRI, participants naturalistically viewed photos of their own face, from which we computed effective connectivity from dorsal to ventral visual regions. In BDD, neurite density was negatively correlated with appearance dissatisfaction and negatively correlated with effective connectivity. Further, those with weaker effective connectivity while viewing their face had worse BDD symptoms and worse insight. In controls, no significant relationships were found between any of the measures. There were no significant group differences in neurite density or orientation dispersion. Those with BDD with worse appearance dissatisfaction have a lower fraction of tissue having axons or dendrites along the vertical occipital fasciculus bundle, possibly reflecting impacting the degree of integration of global and local visual information between the dorsal and ventral visual streams. These results provide early insights into how the vertical occipital fasciculus's microstructure relates to the subjective experience of one's appearance, as well as the possibility of distinct functional-structural relationships in BDD.

Local keratinocyte-nociceptor interactions enhance obesity-mediated small fiber neuropathy via NGF-TrkA-PI3K signaling axis
The pathology of diabetic small fiber neuropathy, characterized by neuropathic pain and axon degeneration, develops locally within the skin during the stages of obesity and pre-diabetes. However, the initiation and progression of morphological and functional abnormalities in skin sensory nerves remains elusive. To address this, we utilized ear skin from mice with diet-induced obesity (DIO), the mouse models for obesity and pre-type 2 diabetes. We evaluated pain-associated wiping behavior and conducted ex vivo live Ca2+ imaging of the DIO ear skin to detect sensory hypersensitivity. Our findings reveal sensory hypersensitivity in skin nociceptive axons followed by axon degeneration. Further mechanistic analysis identified keratinocytes as a major source of nerve growth factor (NGF) in DIO skin, which locally sensitizes nociceptors through NGF-mediated signaling. Indeed, the local inactivation of NGF and its receptor TrkA-mediated downstream signaling, including the phosphoinositide 3-kinases (PI3K) pathway, suppresses sensory hypersensitivity in DIO skin. Thus, targeting these local interactions between keratinocytes and nociceptors offers a therapeutic strategy for managing neuropathic pain, avoiding the adverse effects associated with systemic interventions.

Generation of human appetite-regulating neurons and tanycytes from stem cells
The balance between energy intake and expenditure is controlled by the hypothalamus, a small brain region characterised by high neuronal diversity. Specifically, the arcuate nucleus (ARC) and ventromedial hypothalamus (VMH) are key hypothalamic nuclei controlling appetite through behavioural response to circulating humoral signals. Yet, despite their physiological importance, the cellular and functional characteristics of this highly specialised neural region has been studied mainly in animals due to a lack of human models. Here, we fine-tuned the differentiation of human pluripotent stem cells toward the ARC and VMH hypothalamic nuclei and identified key subtype-specific progenitor markers of these subregions. We demonstrate that the timing for initiation and termination of bone morphogenetic protein (BMP) signalling is essential for controlling subregional specification of tuberal hypothalamic progenitors along the anterior-posterior axis, balancing VMH versus ARC fates. A particular population of SHH-/NKX2.1+/FGF10high/RAXhigh/TBX3high posterior tuberal progenitors was identified as the source for generation of ARC-associated agouti-related peptide (AGRP) neurons and tanycytes whilst anterior tuberal SHH+/NKX2.1+/FGF10low/RAXlow/TBX3low progenitors generated VMH phenotypes including NR5A1 neurons. Upon maturation in vitro and in xenografts, ARC-patterned progenitors gave rise to key appetite-regulating cell types including those producing AGRP, prepronociceptin (PNOC), growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH) and pro-opiomelanocortin (POMC), as well as tanycyte glial cells. Differentiated ARC cultures showed high transcriptomic similarity to the human ARC and displayed evidence of functionality by AGRP secretion and responsiveness to leptin and fibroblast growth factor 1 (FGF1). In summary, our work provides insights into the developmental lineages underlying hypothalamic subregional specification and enables access to highly characterised human ARC and VMH cultures, which will provide novel opportunities for investigating the cellular and molecular pathways triggered by obesity-associated genetic variants and weight-regulating stimuli.

Cell-type specific binocular interactions in mouse visual thalamus
Projections from each eye are segregated in separate domains within the dorsal lateral geniculate nucleus (dLGN). Yet, in vivo studies indicate that the activity of single dLGN neurons can be influenced by visual stimuli presented to either eye. In this study we explored whether intrinsic circuits mediate binocular interactions in the mouse dLGN. We employed dual color optogenetics in vitro to selectively activate input from each eye and recorded synaptic responses in thalamocortical (relay) cells as well as inhibitory interneurons, which have extensive dendritic arbors that are not confined to eye specific domains. While most relay cells received monocular retinal input, most interneurons received binocular retinal input; consequently, the majority of dLGN relay cells received binocular retinogeniculate-evoked inhibition. Moreover, in recordings from adjacent pairs of relay cells and interneurons, the most common relationship observed was binocular excitation of interneurons paired with binocular inhibition of adjacent relay cells. Finally, we found that dLGN interneurons are interconnected, displaying both monocular and binocular inhibition in response to retinal activation. In sum, our results indicate that geniculate interneurons provide one of the first locations where signals from the two eyes can be compared, integrated, and adjusted before being transmitted to cortex, shedding new light on the role of the thalamus in binocular vision.

REM sleep quality is associated with balanced tonic activity of the locus coeruleus during wakefulness
Objective: Animal studies established that the locus coeruleus (LC) plays important roles in sleep and wakefulness regulation. Whether it contributes to sleep variability in humans is not yet established. Here, we investigated if the in vivo activity of the LC is related to the variability in the quality of Rapid Eye Movement (REM) sleep. Methods: We assessed the LC activity of 34 healthy younger (~22y) and 18 older (~61y) individuals engaged in bottom-up and top-down cognitive tasks using 7-Tesla functional Magnetic Resonance Imaging (fMRI). We further recorded their sleep electroencephalogram (EEG) to evaluate associations between LC fMRI measures and REM sleep EEG metrics. Results: Theta oscillation energy during REM sleep was positively associated with LC response in the top-down task. In contrast, REM sleep theta energy was negatively associated with LC activity in older individuals during the bottom-up task. Importantly, sigma oscillations power immediately preceding a REM sleep episode was positively associated with LC activity in the top-down task. Interpretation: LC activity during wakefulness was related to REM sleep intensity and to a transient EEG change preceding REM sleep, a feature causally related to LC activity in animal studies. The associations depend on the cognitive task, suggesting that a balanced level of LC tonic activity during wakefulness is required for optimal expression of REM sleep. The findings may have implications for the high prevalence of sleep complaints reported in aging and for disorders such as insomnia, Alzheimers, and Parkinsons disease, for which the LC may play pivotal roles through sleep.

Tracking changes in functionality and morphology of repopulated microglia in young and old mice
Microglia (MG) are myeloid cells of the central nervous system supporting its homeostasis and instigating neuroinflammation in pathologies. Single-cell RNA sequencing (scRNA-seq) revealed the functional heterogeneity of MG in mice brains. Inhibition of colony-stimulating factor 1 receptor (CSF1R) signaling with inhibitors deplete microglia which rapidly repopulate. Functionalities of repopulated microglia are poorly known. We combined scRNA-seq, bulk RNA-seq, immunofluorescence and confocal imaging to study functionalities and morphology of repopulated microglia. CSRF1R inhibitor (BLZ-945) depleted MG in 21 days and their numbers were restored 7 days later as evidenced by TMEM119 staining and flow cytometry. ScRNA-seq and computational analyses demonstrate that repopulated MG originate from preexisting MG progenitors and reconstitute functional clusters but upregulate inflammatory genes. Percentages of proliferating, immature MG displaying inflammatory gene expression increase in aging mice. Morphometric analysis of MG cell body and branching shows distinct morphology of repopulated MG, particularly in old mouse brains. We demonstrate that with aging some repopulated MG fail to reach the homeostatic phenotype. These differences microglia may contribute to the deterioration of microglia protective functions with age.

Persistent activity during working memory maintenance predicts long-term memory formation in the human hippocampus
Working Memory (WM) and Long-Term Memory (LTM) are often viewed as separate cognitive systems. Little is known about how these systems interact when forming memories. We recorded single neurons in the human medial temporal lobe while patients maintained novel items in WM and a subsequent recognition memory test for the same items. In the hippocampus but not the amygdala, the level of WM content-selective persist activity during WM maintenance was predictive of whether the item was later recognized with high confidence or forgotten. In contrast, visually evoked activity in the same cells was not predictive of LTM formation. During LTM retrieval, memory-selective neurons responded more strongly to familiar stimuli for which persistent activity was high while they were maintained in WM. Our study suggests that hippocampal persistent activity of the same cell supports both WM maintenance and LTM encoding, thereby revealing a common single-neuron component of these two memory systems.

Big Data in Myoelectric Control: LargeMulti-User Models Enable Robust Zero-ShotEMG-based Discrete Gesture Recognition
Myoelectric control, the use of electromyogram (EMG) signals generated during muscle contractions to control a system or device, is a promising modality for enabling always-available control of emerging ubiquitous computing applications. However, its widespread use has historically been limited by the need for user-specific machine learning models because of behavioural and physiological differences between users. Leveraging the publicly available 612-user EMG-EPN612 dataset, this work dispels this notion, showing that true zero-shot cross-user myoelectric control is achievable without user-specific training. By taking a discrete approach to classification (i.e., recognizing the entire dynamic gesture as a single event), a classification accuracy of 93.0% for six gestures was achieved on a set of 306 unseen users (who provided no training data), showing that big data approaches (compared to most EMG studies, which typically employ only 10-20 users) can enable robust cross-user myoelectric control. By organizing the results into a series of mini-studies, this work provides an in-depth analysis of discrete cross-user models to answer unknown questions and uncover new research directions. In particular, this work explores the number of participants required to build cross-user models, the impact of transfer learning for fine-tuning these models, and the effects of under-represented end-user demographics in the training data, among other issues. Additionally, in order to further evaluate the performance of the created cross-user models, a completely new data set was created (using the same recording device) that includes known covariate factors such as cross-day use and limb-position variability. The results show that the large data models can effectively generalize to new datasets and mitigate the impact of common confounding factors that have historically limited the adoption of EMG-based inputs.

7 Tesla MRS estimates of GABA concentration relate to physiological measures of tonic inhibition in the human motor cortex
GABAergic neurotransmission within the cortex plays a key role in learning and is altered in several brain diseases. Quantification of bulk GABA in the human brain is typically obtained by Magnetic Resonance Spectroscopy (MRS). However, the interpretation of MRS-GABA is still debated. A recent mathematical simulation contends that MRS detects extrasynaptic GABA, mediating tonic inhibition. Nevertheless, no empirical data have yet confirmed this hypothesis. Here we collected ultra-high field 7 Tesla MRS and Transcranial Magnetic Stimulation coupled with high-density Electroencephalography (TMS-hdEEG) from the motor cortex of twenty healthy participants (age 23.95+-6.4), while they were at rest. We first applied a Neural Mass Model to TMS-evoked potentials to disentangle the contribution of different GABAergic pools. We then assessed to which of these different pools MRS-GABA was related to by means of Parametric Empirical Bayesian (PEB) analysis. We found that MRS-GABA was mostly positively related to the NMM-derived measures of tonic inhibition and overall functionality of the GABAergic synapse. This relationship was reliable enough to predict MRS-GABA from NMM-GABA. These findings clarify the mesoscopic underpinnings of GABA levels measured by MRS and will contribute to the concretization of MRS-GABA promises to improve our understanding of human behaviour, brain physiology and pathophysiology.

The flow state is not accompanied by frontal-midline theta activity: An EEG investigation of more than 700 video gameplay sessions
People sometimes experience a "flow state"- characterized by hyperfocus, time distortion, and loss of self-awareness - during sports or video gameplay. Previous neuropsychological studies using simple laboratory tasks have reported that the flow state is associated with activation in the frontal lobe, reflected in theta (4-7 Hz) band rhythmic neural activity in medial prefrontal regions (frontal-midline theta [FMT] activity). However, the findings of previous studies might be problematic because they did not appropriately capture the neural activity associated with the flow state for the following reasons: 1) they used unfamiliar and unmotivating tasks; 2) they defined the neural basis of the flow state as neural activity occurring during tasks of optimal difficulty, disregarding trial-to-trial variations in subjective experience of the flow state; 3) the duration of the experiment or the number of trials was not sufficient to capture the rare experience of flow; or 4) they ignored individual differences in neural activities related to flow experiences. Thus, we examined the relationship between the flow state and FMT activity, recorded via scalp electroencephalography, in an experimental paradigm that addressed these four issues. First, participants played their favorite competitive video games, which they had been routinely playing. Second, task difficulty was kept as uniform as possible across trials by employing rank matching to directly examine the correlation between subjective flow level and FMT activity across trials. Third, to address the concern regarding the low frequency of the flow experience, more than 100 trials were completed over 10 days by each participant. Lastly, we adopted a within-participant statistical approach to examine individual differences in the nature of the flow experience. The results showed no correlation between FMT activity and the degree of subjective flow in six out of seven participants, contrary to previous reports. Our results challenge the conventional view that frontal lobe activity, as reflected in FMT activity, is instrumental in entering into the flow state.

The Neural efficiency score: Validation and application
We propose an indirect measure of the efficiency of neural processing: the neural efficiency score (NES). The basis for this measure is the hazard function on the reaction time distribution from a task, h(t), which can be interpreted as an instantaneous measure of work being accomplished, and which has been foundational in characterizations of perceptual and cognitive workload capacity (e.g., Townsend & Ashby, 1978; Townsend & Nozawa, 1995; Townsend & Wenger, 2004). We suggest that the global field power on electroencephalographic (EEG) data (Skrandies, 1989, 1990) can function as a proxy for actual energy expended, and then place h(t) and GFP in a ratio to give a measure that can be interpreted as work accomplished relative to energy expended. To make this proposal plausible, we first need to show that the GFP can be interpreted in terms of energy expended, and we do this using previously unpublished data from an earlier study (Wenger, DellaValle, Murray-Kolb, & Haas, 2017) in which we simultaneously collected EEG and metabolic data during the performance of a cognitive task. Having shown that the GFP can be used as a proxy for energy expended, we then demonstrate the interpretability of the NES by applying it to previously unpublished data from a more recent study (Newbolds & Wenger, 2024). These outcomes suggest the potential for broad applicability of the NES and its potential for characterizing the efficiency of neural energy expenditure in the performance of perceptual and cognitive work.

Hearing and cognitive decline in aging differentially impact neural tracking of context-supported versus random speech across linguistic timescales
Cognitive decline and hearing loss are common in older adults and often co-occur while investigated separately, affecting the neural processing of speech. This study investigated the interaction between cognitive decline, hearing loss, and contextual cues in speech processing. Participants aged 60 years and older were assessed for cognitive decline using the Montreal Cognitive Assessment and for hearing ability using a four-frequency pure tone average. They listened to in-house-designed matrix-style sentences that either provided supportive context or were random. Neurophysiological responses were analyzed through auditory evoked potentials and speech tracking at different linguistic timescales (i.e., phrase, word, syllable and phoneme rate) using phase-locking values. The results showed that cognitive decline was associated with a lower chance of correct responses in a speech recognition task. Cognitive decline significantly impacted the P2 component of auditory evoked potentials, while hearing loss influenced speech tracking at the word and phoneme rates, but not at the phrase or syllable rates. Contextual cues enhanced speech tracking at the syllable rate. These findings suggest that cognitive decline and hearing loss differentially affect the neural mechanisms underlying speech processing, with contextual cues playing a significant role in enhancing syllable rate tracking. This study emphasises the importance of considering both cognitive and auditory factors when studying speech processing in older people and highlights the need for further research to investigate the interplay between cognitive decline, hearing loss and contextual cues in speech processing.

Structural comparisons of human and mouse fungiform taste buds
Taste buds are commonly studied in rodent models, but some differences exist between mice and humans in terms of gustatory mechanisms and sensitivities. Whether these functional differences are reflected in structural differences between species is unclear. Using immunofluorescent image stacks, we compared morphological and molecular characteristics of mouse and human fungiform taste buds. The results suggest that while the general features of fungiform taste buds are similar between mice and humans, several characteristics differ significantly. Human taste buds are larger and taller than those of mice, yet they contain similar numbers of taste cells. Taste buds in humans are more heavily innervated by gustatory nerve fibers expressing the purinergic receptor P2X3 showing a 40% higher innervation density than in mice. Like Type II cells of mice, a subset (about 30%) of cells in human taste buds is immunoreactive for PLC{beta}2. These PLC{beta}2-immunoreactive cells display CALHM1-immunoreactive puncta closely apposed to gustatory nerve fibers suggestive of channel-type synapses described in mice. These puncta, used as a measure of synaptic contact, are however significantly larger in humans compared to mice. Altogether these findings suggest that while many similarities exist in the structural organization of murine and human fungiform taste buds, significant differences do exist in taste bud size, innervation density, and size of synaptic contacts that may impact gustatory signal transmission.

Rapid eye and hand responses in an interception task are differentially modulated by context-dependent predictability
Humans can quickly generate eye and hand responses to unpredictable changes in the environment. Here, we investigated eye-hand coordination in a rapid interception task where human participants used a virtual paddle to intercept a moving target. The target moved vertically down a computer screen and could suddenly jump to the left or right. In high-certainty blocks, the target always jumped, and in low-certainty blocks, the target only jumped in a portion of trials. Further, we manipulated response urgency by varying the time of target jumps, with early jumps requiring less urgent and late jumps requiring more urgent responses. Our results highlighted differential effects of certainty and urgency on eye-hand coordination. Participants initiated both eye and hand responses earlier for high-certainty compared to low-certainty blocks. Hand reaction times decreased, and response vigor increased with increasing urgency levels. However, eye reaction times were lowest for medium-urgency levels, and eye vigor was unaffected by urgency. Across all trials, we found a weak positive correlation between eye and hand responses. Taken together, these results suggest that the limb and oculomotor systems use similar early sensorimotor processing; however, rapid responses are modulated differentially to attain system-specific sensorimotor goals.

On the Geometry of Somatosensory Representations in the Cortex
It is well-known that cortical areas specializing in the processing of somatosensory information from different parts of the body are arranged in an orderly manner along the cortex. It is also generally accepted that in the cortex, somatosensory information is initially processed in the primary somatosensory cortex and from there, it is hierarchically processed in other cortical regions. Previous studies have focused on the organization of representation at a level of a single or few cortical regions, identifying multiple body maps. However, the question of the large-scale organization of these different maps, and their relation to the hierarchical organization has received little attention. This is primarily because the highly convoluted shape of the cortical surface makes it difficult to characterize the relationship between cortical areas that are centimeters apart. Here, we used functional MRI to characterize cortical responses to full-body light touch stimulation. Our results indicate that the organization of both body representation and hierarchy is radial, with a small number of extrema that reign over a large number of cortical regions. Quantitatively computing the local relationship between the gradients of body and hierarchy maps, we show that the interaction between these two radial geometries, body representation and hierarchy in S1 are approximately orthogonal. However, this orthogonality is restricted to S1. Similar organizational patterns in the visual and auditory systems suggest that radial topography may be a common feature across sensory systems.

Mild neonatal hypoxia disrupts adult hippocampal learning and memory and is associated with CK2-mediated dysregulation of synaptic calcium-activated potassium channel KCNN2
ObjectiveAlthough nearly half of preterm survivors display persistent neurobehavioral dysfunction including memory impairment without overt gray matter injury, the underlying mechanisms of neuronal or glial dysfunction, and their relationship to commonly observed cerebral white matter injury are unclear. We developed a mouse model to test the hypothesis that mild hypoxia during preterm equivalence is sufficient to persistently disrupt hippocampal neuronal maturation related to adult cellular mechanisms of learning and memory.

Methods: Neonatal (P2) mice were exposed to mild hypoxia (8%O2) for 30 min and evaluated for acute injury responses or survived until adulthood for assessment of learning and memory and hippocampal neurodevelopment.

ResultsNeonatal mild hypoxia resulted in clinically relevant oxygen desaturation and tachycardia without bradycardia and was not accompanied by cerebral gray or white matter injury. Neonatal hypoxia exposure was sufficient to cause hippocampal learning and memory deficits and abnormal maturation of CA1 neurons that persisted into adulthood. This was accompanied by reduced hippocampal CA3-CA1 synaptic strength and LTP and reduced synaptic activity of calcium-sensitive SK2 channels, key regulators of spike timing dependent neuroplasticity, including LTP. Structural illumination microscopy revealed reduced synaptic density, but intact SK2 localization at the synapse. Persistent loss of SK2 activity was mediated by altered casein kinase 2 (CK2) signaling.

InterpretationClinically relevant mild hypoxic exposure in the neonatal mouse is sufficient to produce morphometric and functional disturbances in hippocampal neuronal maturation independently of white matter injury. Additionally, we describe a novel persistent mechanism of potassium channel dysregulation after neonatal hypoxia. Collectively our findings suggest an unexplored explanation for the broad spectrum of neurobehavioral, cognitive and learning disabilities that paradoxically persist into adulthood without overt gray matter injury after preterm birth.

Direct piriform-to-auditory cortical projections shape auditory-olfactory integration
In a real-world environment, the brain must integrate information from multiple sensory modalities, including the auditory and olfactory systems. However, little is known about the neuronal circuits governing how odors influence and modulate sound processing. Here, we investigated the mechanisms underlying auditory-olfactory integration using anatomical, electrophysiological, and optogenetic approaches, focusing on the auditory cortex as a key locus for cross-modal integration. First, retrograde and anterograde viral tracing strategies revealed a direct projection from the piriform cortex to the auditory cortex. Next, using in vivo electrophysiological recordings of neuronal activity in the auditory cortex of awake mice, we found that odor stimuli modulate auditory cortical responses to sound. Finally, we used in vivo optogenetic manipulations during electrophysiology to demonstrate that olfactory modulation in auditory cortex, specifically, odor-driven enhancement of sound responses, depends on direct input from the piriform cortex. Together, our results identify a novel cortical circuit shaping olfactory modulation in the auditory cortex, shedding new light on the neuronal mechanisms underlying auditory-olfactory integration.

Chaotic synchronization in adaptive networks of pulse-coupled oscillators
Ensembles of phase-oscillators are known to exhibit a variety of collective regimes. Here, we show that a simple mean-field model involving two heterogenous populations of pulse-coupled oscillators, exhibits, in the strong-coupling limit, a robust irregular macroscopic dynamics. The resulting, strongly synchronized, regime is sustained by a homeostatic mechanism induced by the shape of the phase-response curve combined with adaptive coupling strength, included to account for energy dissipated by the pulse emission. The proposed setup mimicks a neural network composed of excitatory and inhibitory neurons.

Tempo-dependent selective enhancement of neural responses at the beat frequency can be explained by both an oscillator and an evoked model
The synchronization of neural oscillations with an external regularity such as a musical beat has been regarded as an important mechanism for the brain to make sense of our auditory environment. Such synchronization is often quantified as phase locking of neural oscillations to a stimulus, but this method has been criticized for not differentiating between entrainment - the rate-dependent adjustment of an ongoing endogenous oscillation to an external regularity - and evoked neural responses to the rhythmic stimulus. Here, we aimed to differentiate between these two accounts by measuring EEG responses to non-isochronous rhythmic sequences played at five different rates. Behaviorally, participants shifted the perceived level of regularity depending on the tempo, towards the preferred beat rate (~2 Hz). We found a similar shift in the EEG data, with strongest neural phase locking at the level of the note rate for slow tempi, and at the level of a hierarchical beat for faster tempi, independent of active attention to the sounds. While this pattern of results is in line with entrainment accounts of beat perception and could indeed be mimicked by an oscillator model, it was explained equally well using a model simulating evoked responses. An additional phase concentration metric of the EEG data fell in between the predictions of these two models. In conclusion, we show that neural responses to rhythm are selectively enhanced at the beat rate in a tempo-dependent manner, but that this selective neural enhancement can be explained by successive evoked responses as well as by assuming the presence of oscillatory entrainment.

Neural specialisation for concrete and abstract concepts revealed through meta-analysis
Identifying the brain systems that process concrete and abstract concepts is key to understanding the neural architecture of thought, memory and language. We review current theories of concreteness effects and test their neural predictions in a meta-analysis of 72 neuroimaging studies. Concrete concepts preferentially activated visual and action processing regions, particularly when presented in sentences, while abstract concepts preferentially activated networks for language, social cognition and semantic control. Specialisation for both concept types was present in the default mode network (DMN), with effects dissociating along a social-spatial axis. Concrete concepts generated greater activation in a medial temporal DMN component, implicated in constructing mental models of spatial contexts and scenes. Abstract concepts showed greater activation in frontotemporal DMN regions involved in theory-of-mind and language. These results support claims that generating models of situations and events is a core DMN function and indicate specialisation within DMN for different aspects of these models.

Spatially organized striatum-wide acetylcholine dynamics for the learning and extinction of Pavlovian cues and actions
Striatal acetylcholine (ACh) has been linked to behavioral flexibility. A key component of flexibility is down-regulating responding as valued cues and actions become decoupled from positive outcomes. We used array fiber photometry in mice to investigate how ACh release across the striatum evolves during learning and extinction of Pavlovian associations. Changes in multi-phasic release to cues and consummatory actions were bi-directional and region-specific. Following extinction, increases in cue-evoked ACh release emerged in the anterior dorsal striatum (aDS) which preceded a down-regulation of anticipatory behavior. Silencing ACh release from cholinergic interneurons in the aDS blocked behavioral extinction. Dopamine release dipped below baseline for down-shifted cues, but glutamate input onto cholinergic interneurons did not change, suggesting an intrastriatal mechanism for the emergence of ACh increases. Our large- scale mapping of striatal ACh dynamics during learning pinpoints region-specific elevations in ACh release positioned to down-regulate behavior during extinction, a central feature of flexible behavior.

Female sex hormones exacerbate retinal neurodegeneration
Neurodegenerative disorders such as Alzheimer's disease and macular degeneration represent major sources of human suffering, yet the factors influencing disease severity remain poorly understood. Sex has been implicated as one potential modifying factor. Here, we show that female sex is a risk factor for worsened outcomes in a model of retinal degeneration. Further, we show that this susceptibility is caused by the presence of female-specific circulating sex hormones. The adverse effect of female sex hormones was specific to diseased retinal neurons, and depletion of these hormones ameliorated this phenotypic effect. These findings provide novel insights into the pathogenesis of neurogenerative diseases and how sex hormones can impact the severity of disease. These findings have far-reaching implications for clinical trial design and the use of hormonal therapy in females with certain neurogenerative disorders.

Functional analysis of conserved C. elegans bHLH family members uncovers lifespan control by a peptidergic hub neuron
Throughout the animal kingdom, several members of the basic helix-loop-helix (bHLH) family act as proneural genes during early steps of nervous system development. Roles of bHLH genes in specifying terminal differentiation of postmitotic neurons have been less extensively studied. We analyze here the function of five C. elegans bHLH genes, falling into three phylogenetically conserved subfamilies, which are continuously expressed in a very small number of postmitotic neurons in the central nervous system. We show (a) that two orthologs of the vertebrate bHLHb4/b5 genes, called hlh-17 and hlh-32, function redundantly to specify the identity of a single head interneuron (AUA), as well as an individual motor neuron (VB2), (b) that the PTF1a ortholog hlh-13 acts as a terminal selector to control terminal differentiation and function of the sole octopaminergic neuron class in C. elegans, RIC, and (c) that the NHLH1/2 ortholog hlh-15 controls terminal differentiation and function of the peptidergic AVK head interneuron class, a known neuropeptidergic signaling hub in the animal. Strikingly, through null mutant analysis and cell-specific rescue experiments, we find that loss of hlh-15/NHLH in the peptidergic AVK neurons and the resulting abrogation of neuropeptide secretion causes a substantially expanded lifespan of the animal, revealing an unanticipated impact of a central, peptidergic hub neuron in regulating lifespan, which we propose to be akin to hypothalamic control of lifespan in vertebrates. Taken together, our functional analysis reveals themes of bHLH gene function during terminal differentiation that are complementary to the earlier lineage specification roles of other bHLH family members. However, such late functions are much more sparsely employed by members of the bHLH transcription factor family, compared to the function of the much more broadly employed homeodomain transcription factor family.

Appropriate data segmentation improves speech encoding models
In recent decades, research on the neural processing of speech and language increasingly investigated ongoing responses to continuously presented naturalistic speech, allowing researchers to ask interesting questions about different representations of speech and their relationships. This requires statistical models that can dissect different sources of variance occurring in the processing of naturalistic speech. One commonly used family of models are temporal response functions (TRFs) which can predict neural responses to speech as a weighted combination of different features and points in time. TRFs model the brain as a linear time-invariant (LTI) system whose responses can be characterized by constant transfer functions. This implicitly assumes that the underlying signals are stationary, varying to a fixed degree around a constant mean. However, continuous neural recordings commonly violate this assumption. Here, we use simulations and EEG recordings to investigate how non-stationarities affect TRF models for continuous speech processing. Our results suggest that non-stationarities may impair the performance of TRF models, but that this can be partially remedied by dividing the data into shorter segments that approximate stationarity.

Residual Microglia Following Short-term PLX5622 Treatment in 5xFAD Mice Exhibit Diminished NLRP3 Inflammasome and mTOR Signaling, and Enhanced Autophagy
Chronic neuroinflammation represents a prominent hallmark of Alzheimer's disease (AD). While moderately activated microglia are pivotal in clearing amyloid beta (Abeta), hyperactivated microglia perpetuate neuroinflammation. Prior investigations have indicated that the elimination of ~80% of microglia through a month-long inhibition of the colony-stimulating factor 1 receptor (CSF1R) during the advanced stage of neuroinflammation in 5xFamilial AD (5xFAD) mice mitigates synapse loss and neurodegeneration without impacting Abeta levels. Furthermore, prolonged CSF1R inhibition diminished the development of parenchymal plaques. Nonetheless, the immediate effects of short-term CSF1R inhibition during the early stages of neuroinflammation on residual microglial phenotype or metabolic fitness are unknown. Therefore, we investigated the effects of 10-day CSF1R inhibition in three-month-old female 5xFAD mice, a stage characterized by the onset of neuroinflammation and minimal Abeta plaques. We observed ~65% microglia depletion in the hippocampus and cerebral cortex. The leftover microglia demonstrated a noninflammatory phenotype, with highly branched and ramified processes and reduced NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome complexes. Moreover, plaque-associated microglia were reduced in number with diminished Clec7a (dectin-1) expression. Additionally, both microglia and neurons displayed reduced mechanistic target of rapamycin (mTOR) signaling and autophagy. Biochemical assays validated the inhibition of NLRP3 inflammasome activation, decreased mTOR signaling, and enhanced autophagy. However, short-term CSF1R inhibition did not influence Abeta plaques, soluble Abeta-42 levels, or hippocampal neurogenesis. Thus, short-term CSF1R inhibition during the early stages of neuroinflammation in 5xFAD mice promotes the retention of homeostatic microglia with diminished inflammasome activation and mTOR signaling, alongside increased autophagy.

Cerebellar output shapes cortical preparatory activity during motor adaptation
The cerebellum plays a key role in motor adaptation by driving trial-to-trial recalibration of movements based on previous errors. In primates, this adaptive response is achieved by cerebellar modulation of motor cortical signals, but the nature and timing of this process are unknown. Specifically, cortical correlates of adaptation are encoded already in the pre-movement motor plan, but these early cortical signals could be driven by a cerebellar-to-cortical information flow or evolve independently through intracortical mechanisms. To address this question, we trained monkeys to reach against a viscous force field while blocking cerebellar outflow. During the force field trials, the cerebellar block led to impaired adaptation and a compensatory, re-aiming-like shift in motor cortical preparatory activity. In the null-field conditions, the cerebellar block altered neural preparatory activity by increasing task-representation dimensionality and impeding generalization. A computational model indicated that low-dimensional (cerebellar-like) feedback is sufficient to replicate these findings. We conclude that cerebellar signals carry task structure information that constrains the dimensionality of the cortical preparatory manifold and promotes generalization. In the absence of these signals, cortical mechanisms are harnessed to partially restore adaptation.

Muller glia cell cycle re-activation by simultaneous cyclin D1 overexpression and p27kip1 knockdown promotes retinal regeneration in mice
Harnessing the regenerative potential of endogenous stem cells to restore lost neurons is a promising strategy for treating neurodegenerative disorders. Muller glia (MG), the primary glial cell type in the retina, exhibit remarkable regenerative abilities in lower vertebrate species, such as zebrafish and amphibians, where injury induces MG to proliferate and differentiate into various retinal neuron types. The regenerative potential of mammalian MG is constrained by their inherent inability to re-enter the cell cycle, likely due to high levels of the cell cycle inhibitor p27Kip1 and low levels of cyclin D1 observed in adult mouse MG. In this study, we found that adeno-associated virus (AAV)-mediated cyclin D1 overexpression and p27Kip1 knockdown exerts a strong synergistic effect on MG proliferation. MG proliferation induced by this treatment was potent but self-limiting, as MG did not undergo uncontrolled proliferation or lead to retinal neoplasia. Single-cell RNA sequencing (scRNA-seq) revealed that cell cycle reactivation leads to immunosuppression and dedifferentiation of MG. Notably, scRNA-seq analysis identified a new cluster of rod-like MG cells expressing both rod and MG genes, which was further validated by RNA in situ hybridization. Cell cycle reactivation also led to de novo genesis of bipolar- and amacrine-like cells from MG. Overall, our findings suggest that AAV-mediated cyclin D1 overexpression and p27Kip1 knockdown stimulate MG proliferation and promote MG reprogramming. This approach may be a promising strategy, especially when combined with other regeneration-promoting factors, to enhance MG-mediated retinal repair.

Fc-engineered large molecules targeting blood-brain barrier transferrin receptor and CD98hc have distinct central nervous system and peripheral biodistribution compared to standard antibodies
The blood-brain barrier (BBB) poses a significant challenge drug delivery to the brain. BBB-crossing molecules are emerging as a new class of therapeutics with significant potential for central nervous system (CNS) indications. In particular, transferrin receptor (TfR)- and CD98 heavy chain (CD98hc)-targeting molecules have been demonstrated to cross the BBB for enhanced brain delivery. Previously, we reported TfR and CD98hc antibody transport vehicles (ATVTfR and ATVCD98hc) that utilize these BBB receptors to improve CNS drug delivery1,2. Here, we provide a comprehensive and unbiased biodistribution characterization of ATVTfR and ATVCD98hc compared to a standard IgG at a multiscale level, ranging from whole-body to brain region- and cell type-targeting specificity. Mouse whole-body tissue clearing revealed distinct organ localization for each molecule. In the CNS, ATVTfR and ATVCD98hc not only achieves enhanced brain delivery but importantly, much broader parenchymal distribution in contrast to the severely limited distribution observed with a standard antibody that was not able to be improved even at very high dose levels. Using cell sorting and single-cell RNA sequencing of mouse brain, we revealed that standard IgG predominantly localizes to perivascular and leptomeningeal cells and reaches the CNS by entering the CSF, rather than crossing the BBB. In contrast, ATVTfR and ATVCD98hc enables broad parenchymal cell-specific distribution via transcytosis through brain endothelial cells (BECs) along the neurovasculature. Finally, we extended the translational relevance of our findings by revealing enhanced and broad brain and spinal cord biodistribution of ATVTfR compared to standard IgG in cynomolgus monkey. Taken together, this multiscale analysis reveals in-depth biodistribution differences between ATVTfR, ATVCD98hc, and standard IgG. These results may better inform platform selection for specific therapeutic targets of interest, optimally matching platforms to desired CNS target engagement, peripheral organ exposures, and predict or potentially reduce off-target effects.

How the window of visibility varies around polar angle
Contrast sensitivity, the amount of contrast required to detect or discriminate an object, depends on spatial frequency (SF): The Contrast Sensitivity Function (CSF) peaks at intermediate SFs and drops at lower and higher SFs and is the basis of computational models of visual object recognition. The CSF varies from foveal to peripheral vision, but only a couple studies have assessed changes around polar angle of the visual field. Sensitivity is generally better along the horizontal than the vertical meridian, and better at the lower vertical than the upper vertical meridian, yielding polar angle asymmetries. Here, we investigate CSF attributes at polar angle locations at both group and individual levels, using Hierarchical Bayesian Modeling. This method enables precise estimation of CSF parameters by decomposing the variability of the dataset into multiple levels and analyzing covariance across observers. At the group level, peak contrast sensitivity and corresponding spatial frequency with the highest sensitivity are higher at the horizontal than vertical meridian, and at the lower than upper vertical meridian. At an individual level, CSF attributes (e.g., maximum sensitivity, the most preferred SF) across locations are highly correlated, indicating that although the CSFs differ across locations, the CSF at one location is predictive of the CSF at another location. Within each location, the CSF attributes co-vary, indicating that CSFs across individuals vary in a consistent manner (e.g., as maximum sensitivity increases, so does the SF at which sensitivity peaks), but more so at the horizontal than the vertical meridian locations. These results show similarities and uncover some critical polar angle differences across locations and individuals, suggesting that the CSF should not be generalized across iso-eccentric locations around the visual field. Our window of visibility varies with polar angle: It is enhanced and more consistent at the horizontal meridian.

A SMARTR workflow for multi-ensemble atlas mapping and brain-wide network analysis
In the last decade, activity-dependent strategies for labelling multiple immediate early gene (IEG) ensembles in mice have generated unprecedented insight into the mechanisms of memory encoding, storage, and retrieval. However, few strategies exist for brain-wide mapping of multiple ensembles, including their overlapping population, and none incorporate capabilities for downstream network analysis. Here, we introduce a scalable workflow to analyze traditionally coronally-sectioned datasets produced by activity-dependent tagging systems. Intrinsic to this pipeline is simple multi-ensemble atlas registration and statistical testing in R (SMARTR), an R package which wraps mapping capabilities with functions for statistical analysis and network visualization. We demonstrate the versatility of SMARTR by mapping the ensembles underlying the acquisition and expression of learned helplessness (LH), a robust stress model. Applying network analysis, we find that exposure to inescapable shock (IS), compared to context training (CT), results in decreased centrality of regions engaged in spatial and contextual processing and higher influence of regions involved in somatosensory and affective processing. During LH expression, the substantia nigra emerges as a highly influential region which shows a functional reversal following IS, indicating a possible regulatory function of motor activity during helplessness. We also report that IS results in a robust decrease in reactivation activity across a number of cortical, hippocampal, and amygdalar regions, indicating suppression of ensemble reactivation may be a neurobiological signature of LH. These results highlight the emergent insights uniquely garnered by applying our analysis approach to multiple ensemble datasets and demonstrate the strength of our workflow as a hypothesis-generating toolkit.

Extracellular Vesicles from hiPSC-derived NSCs Protect Human Neurons against Abeta-42 Oligomers Induced Neurodegeneration, Mitochondrial Dysfunction and Tau Phosphorylation
Background: One of the hallmarks of Alzheimer's disease (AD) is the buildup of amyloid-beta 42 (A-beta 42) in the brain, which leads to various adverse effects. Therefore, therapeutic interventions proficient in reducing A-beta 42-induced toxicity in AD are of great interest. One promising approach is to use extracellular vesicles from human induced pluripotent stem cell-derived neural stem cells (hiPSC-NSC-EVs) because they carry multiple therapeutic miRNAs and proteins capable of protecting neurons against A-beta 42-induced pathological changes. Therefore, this in vitro study investigated the proficiency of hiPSC-NSC-EVs to protect human neurons derived from two distinct hiPSC lines from A-beta 42 oligomers (A-beta 42o)-induced neurodegeneration. Methods: We isolated hiPSC-NSC-EVs using chromatographic methods and characterized their size, ultrastructure, expression of EV-specific markers and proficiency in getting incorporated into mature human neurons. Next, mature human neurons differentiated from two different hiPSC lines were exposed to 1 micromolar A-beta 42o alone or with varying concentrations of hiPSC-NSC-EVs. The protective effects of hiPSC-NSC-EVs against A-beta 42o-induced neurodegeneration, increased oxidative stress, mitochondrial dysfunction, impaired autophagy, and tau phosphorylation were ascertained using multiple measures and one-way ANOVA with Newman-Keuls multiple comparisons post hoc tests. Results: Significant neurodegeneration was observed when human neurons were exposed to A-beta 42o alone. Notably, neurodegeneration was associated with elevated levels of oxidative stress markers malondialdehyde (MDA) and protein carbonyls (PCs), increased expression of proapoptotic Bax and Bad genes and proteins, reduced expression of the antiapoptotic gene and protein Bcl-2, increased expression of genes encoding mitochondrial complex proteins, decreased expression of autophagy-related proteins Beclin-1 and microtubule-associated protein 1 light chain 3B, and increased phosphorylation of tau. However, the addition of an optimal dose of hiPSC-NSC-EVs (6 billion EVs) to human neuronal cultures exposed to A-beta 42o significantly reduced the extent of neurodegeneration, along with diminished levels of MDA and PCs, normalized expressions of Bax, Bad, and Bcl-2, and genes linked to mitochondrial complex proteins, and reduced tau phosphorylation. Conclusions: The findings demonstrate that an optimal dose of hiPSC-NSC-EVs could significantly decrease the degeneration of human neurons induced by A-beta 42o. The results also support further research into the effectiveness of hiPSC-NSC-EVs in AD, particularly their proficiency in preserving neurons and slowing disease progression.

ROLE OF FORELIMB MORPHOLOGY IN MUSCLE SENSORIMOTOR FUNCTIONS DURING LOCOMOTION IN THE CAT
Previous studies established strong links between morphological characteristics of mammalian hindlimb muscles and their sensorimotor functions during locomotion. Less is known about the role of forelimb morphology in motor outputs and generation of sensory signals. Here, we measured morphological characteristics of 46 forelimb muscles from 6 cats. These characteristics included muscle attachments, physiological cross-sectional area (PCSA), fascicle length, etc. We also recorded full-body mechanics and EMG activity of forelimb muscles during level overground and treadmill locomotion in 7 and 16 adult cats of either sex, respectively. We computed forelimb muscle forces along with force- and length-dependent sensory signals mapped onto corresponding cervical spinal segments. We found that patterns of computed muscle forces and afferent activities were strongly affected by the muscles moment arm, PCSA, and fascicle length. Morphology of the shoulder muscles suggests distinct roles of the forelimbs in lateral force production and movements. Patterns of length-dependent sensory activity of muscles with long fibers (brachioradialis, extensor carpi radialis) closely matched patterns of overall forelimb length, whereas the activity pattern of biceps brachii matched forelimb orientation. We conclude that cat forelimb muscle morphology contributes substantially to locomotor function, particularly to control lateral stability and turning, rather than propulsion.

High fat diet induces differential age- and gender-dependent changes in neuronal function linked to redox stress.
The prevalence of neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, is steadily increasing, posing significant challenges to global healthcare systems. Emerging evidence suggests that dietary habits, particularly consumption of high-fat diets specify which fats, may play a pivotal role in the development and progression of neurodegenerative disorders. Moreover, several studies have shed light on the intricate communication between the gut and the brain, known as the gut-brain axis and its involvement in neurodegenerative processes. This study aims to assess the effects of a high-fat dietary intake on various aspects of neuronal function during aging and following gender separation to help understand the potential contributions of diet to neuronal function. To investigate the effects of a high-fat diet, Drosophila melanogaster was used and exposed to standard normal food diet (NF) and high-fat diet (HF). Adults were grouped at 10 and 45 days of age in male and female flies reared under the same conditions. Multiple assays were conducted, showing differential gender- and HF diet-induced oxidative stress levels as determined by malondialdehyde (MDA) measurements, enhanced caspase-3 expression and reduced climbing activity. Adult lifespan under both dietary conditions was unchanged but odour-associated learning ability was reduced in larvae reared in a HF diet. This is the first study to characterise effects of diet on neuronal phenotypes in an age- and gender-specific manner in a Drosophila model. Our findings suggest a HF diet induces differential forms of neuronal dysfunction with age and sex-specific outcomes, characterised by enhanced oxidative stress and cell death which impacts on neuronal and behavioural functions.

Redundancy protects processing speed in healthy individuals with accelerated brain aging
Recent advancements in computational learning techniques have enabled the estimation of brain age (BA) from neuroimaging data. The difference between chronological age (CA) and BA, known as the BA gap, can potentially serve as a biomarker of brain health. Studies, however, have documented low correlations between BA gap and cognition in healthy aging. This suggests that protective mechanisms in the brain may help counter the effect of accelerated brain aging. Here, we investigated whether redundancy in brain networks may protect cognitive function in individuals with accelerated brain aging. First, we employed deep learning to estimate individual brain ages from structural magnetic resonance imaging (MRI). Next, we associated CA, BA, and BA gap, with cognitive measures and network topology derived from diffusion MRI and tractography. We found that CA and BA were both similarly related to cognitive measures and network topology, while BA gap did not show strong relationships in either domain. Despite observing no strong relationships between brain-age gap (BA gap) and demographic variables, cognitive measures, or topological features in healthy aging, individuals with accelerated aging (BA gap+) exhibited lower average degree and redundancy within the dorsal attention network compared to those with delayed aging (BA gap-). Furthermore, redundancy in the dorsal attention network was positively associated with processing speed in BA gap+ individuals. These results indicate a potential neuroprotective role of redundancy in structural brain networks for mitigating the impact of accelerated brain atrophy on cognitive performance in healthy aging.

Cortical layer-specific and cell type-specific dendritic expression of Arc (Arg 3.1) in an in vitro model of slow-wave sleep
An abundance of evidence shows sleep homeostatically regulates synaptic plasticity and memory consolidation but the underlying molecular regulators of this important function of sleep have been difficult to directly elucidate. Arc is an immediate early gene that has a fundamental role in several aspects of synaptic plasticity including regulation and maintenance. Using a physiological brain slice model of slow-wave sleep, here we show that Arc protein has a characteristic spatial and cell-type specific distribution during persistent sleep-related delta oscillations in neocortical slices, which may correlate with aspects of sleep-dependent regulation of synaptic plasticity in cortical regions. In delta oscillating slices, Arc is highly expressed in layer 2/3 dendrites of the primary and secondary association cortex and this dendritic localisation is specific to intrinsically bursting cells (IB) whose cell bodies are in layer 5. Moreover, Arc immunopositive dendrites are clustered together arranged in a quasi hexagonal arrangement with a spacing of ~ 50 m between clusters. The cytoarchitectural distribution of Arc across the association cortex has implications for the mechanisms of sleep and for the synaptic homeostasis hypothesis regarding the function of sleep.

Induction of long-term hyperexcitability by memory-related cAMP signaling in isolated nociceptor cell bodies
Persistent hyperactivity of nociceptors is known to contribute significantly to long-lasting sensitization and ongoing pain in many clinical conditions. It is often assumed that nociceptor hyperactivity is mainly driven by continuing stimulation from inflammatory mediators. We have tested an additional possibility: that persistent increases in excitability promoting hyperactivity can be induced by a prototypical cellular signaling pathway long known to induce late-phase long-term potentiation (LTP) of synapses in brain regions involved in memory formation. This cAMP-PKA-CREB-gene transcription-protein synthesis pathway was tested using whole-cell current clamp methods on small dissociated sensory neurons (primarily nociceptors) from dorsal root ganglia (DRGs) excised from previously uninjured (naive) rats. Six-hour treatment with the specific Galphas-coupled 5-HT4 receptor agonist, prucalopride, or with the adenylyl cyclase activator, forskolin, induced long-term hyperexcitability (LTH) in DRG neurons that manifested 12-24 hours later as action potential (AP) discharge (ongoing activity, OA) during artificial depolarization to -45 mV, a membrane potential that is normally subthreshold for AP generation. Prucalopride treatment also induced significant long-lasting depolarization of resting membrane potential (from -69 to -66 mV), enhanced depolarizing spontaneous fluctuations (DSFs) of membrane potential, and indications of reduced AP threshold and rheobase. LTH was prevented by co-treatment of prucalopride with inhibitors of PKA, CREB, gene transcription, and protein synthesis. As in the induction of synaptic memory, many other cellular signals are likely to be involved. However, the discovery that this prototypical memory induction pathway can induce nociceptor LTH, along with reports that cAMP signaling and CREB activity in DRGs can induce hyperalgesic priming, suggest that early, temporary, cAMP-induced transcriptional and translational mechanisms can induce nociceptor LTH that might last for long periods. An interesting possibility is that these mechanisms can also be reactivated by re-exposure to inflammatory mediators such as serotonin during subsequent challenges to bodily integrity, reconsolidating the cellular memory and thereby extending the duration of persistent nociceptor hyperexcitability.

Emergence and long-term maintenance of modularity in plastic networks of spiking neurons
In the last three decades it has become clear that cortical regions, interconnected via white-matter fibers, form a modular and hierarchical network. This type of organization, which has also been recognized at the microscopic level in the form of interconnected neural assemblies, is typically believed to support the coexistence of segregation (specialization) and integration (binding) of information. A fundamental open question is to understand how this complex structure can emerge in the brain. Here, we made a first step to address this question and propose that adaptation to various inputs could be the key driving mechanism for the formation of structural assemblies. To test this idea, we develop a model of quadratic integrate-and-fire spiking neurons, trained to stimuli targetting distinct sub-populations. The model is designed to satisfy several biologically plausible constraints: (i) the network contains excitatory and inhibitory neurons with Hebbian and anti-Hebbian spike-timing-dependent plasticity (STDP); and (ii) neither the neuronal activity nor the synaptic weights are frozen after the learning phase. Instead, the network is allowed to continue firing spontaneously while synaptic plasticity remains active. We find that only the combination of the two inhibitory STDP sub-populations allows for the formation of stable modular organization in the network, with each sub-population playing a distinct role. The Hebbian sub-population controls for the firing rate, while the anti-Hebbian mediates pattern selectivity. After the learning phase, the network activity settles into an asynchronous irregular resting-state-resembling the behaviour typically observed in-vivo in the cortex. This post-learning activity also displays spontaneous memory recalls, which are fundamental for the long-term consolidation of the learned memory items. The model here introduced can represent a starting point for the joint investigation of neural dynamics, connectivity and plasticity.

Peripheral CaV2.2 channels in skin regulate prolonged heat hypersensitivity during neuroinflammation
Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuro-immune cell signaling but their roles in specific behavioral responses are not fully elucidated. Voltage-gated CaV2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal capsaicin via IL-1 cytokine signaling. CaV2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the same animal model. Here, we show that CaV2.2 channels are also critical for heat hypersensitivity induced by the intradermal (id) Complete Freunds Adjuvant (CFA) model of chronic neuroinflammation that involves ongoing cytokine signaling for days. Ongoing CFA-induced cytokine signaling cascades in skin lead to pronounced edema, and hypersensitivity to sensory stimuli. Peripheral CaV2.2 channel activity in skin is required for the full development and week-long time course of heat hypersensitivity induced by id CFA. CaV2.2 channels, by contrast, are not involved in paw edema and mechanical hypersensitivity. CFA induced increases in cytokines in hind paws including IL-6 which was dependent on CaV2.2 channel activity. Using IL-6 specific neutralizing antibodies, we show that IL-6 contributes to heat hypersensitivity and, neutralizing both IL-1 and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between CaV2.2 channel activity and the release of IL-6 in skin and show that CaV2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in skin.

Significance StatementNeuroinflammation can lead to chronic maladaptive pain. Neurotransmitters, ion channels, cytokines, and cytokine receptors are implicated in neuron-immune signaling, but their importance in mediating specific behavioral responses are not fully elucidated. We show that the activity of peripheral CaV2.2 calcium ion channels in skin play a unique role in the induction and maintenance of heat hypersensitivity in the CFA model of prolonged neuroinflammation, without accompanying effects on edema and mechanical hypersensitivity. Blocking peripheral CaV2.2 channel activity reduces local cytokine levels in hind paws injected with CFA including IL-6 and neutralizing IL-6 reduces CFA- induced heat hypersensitivity. Our studies define key signaling molecules that act locally in skin to trigger and maintain heat hypersensitivity during chronic neuroinflammation.

Geometrical determinant of nonlinear synaptic integration in human cortical pyramidal neurons
Neurons integrate synaptic inputs and convert them to action potential output at electrically distant locations. The computational power of a neuron is hence enhanced by subcellular compartmentalization and nonlinear synaptic integration, but the biophysical determinants of these features in human neurons are not completely understood. By examining the synaptic input-output function of human neocortical pyramidal neurons, we found that the nonlinearity threshold at the soma was linearly determined by the shortest path distance from the synapse to the apical trunk, and the slope of this relationship was consistent throughout the dendritic arbor. Analogous rules were found from both supragranular and infragranular layers of the rodent cortex, suggesting that these represent a fundamental property of pyramidal neurons. Additionally, we found that neurons associated with tumor or epilepsy had distinct membrane properties, but the nonlinearity threshold was shifted in amplitude such that the slope of its relationship with synaptic distance remained consistent.

Taming the chaos gently: a Predictive Alignment learning rule in recurrent neural networks
Recurrent neural circuits often face inherent complexities in learning and generating their desired outputs, especially when they initially exhibit chaotic spontaneous activity. While the celebrated FORCE learning rule can train chaotic recurrent networks to produce coherent patterns by suppressing chaos, it requires non-local plasticity rules and extremely quick plasticity, raising the question of how synapses adapt on local, biologically plausible timescales to handle potential chaotic dynamics. We propose a novel framework called "Predictive Alignment", which tames the chaotic recurrent dynamics to generate a variety of patterned activities via a biologically plausible plasticity rule. Unlike most recurrent learning rules, predictive alignment does not aim to directly minimize output error to train recurrent connections, but rather it tries to efficiently suppress chaos by aligning recurrent prediction with chaotic activity. We show that the proposed learning rule can perform supervised learning of multiple target signals, including complex low-dimensional attractors, delay matching tasks that require short-term temporal memory, and finally even dynamic movie clips with high-dimensional pixels. Our findings shed light on how predictions in recurrent circuits can support learning.

Dendritically localized RNAs are packaged as diversely composed Ribonucleoprotein particles with heterogeneous copy number states
Localization of mRNAs to dendrites is a fundamental mechanism by which neurons achieve spatiotemporal control of gene expression. Translationally repressed neuronal mRNA transport granules, also referred to as ribonuclear proteins (RNPs), have been shown to be trafficked as single or low copy number RNPs and as larger complexes with multiple copies and/or species of mRNAs. However, there is little evidence of either population in intact neuronal circuits. Using single molecule fluorescence in situ hybridization studies in the dendrites of adult rat and mouse hippocampus, we provide evidence that supports the existence of multi-transcript RNPs with the constituents varying in amounts for each RNA species. By competing-off fluorescently labeled probe with serial increases of unlabeled probe, we detected stepwise decreases in Arc RNP number and fluorescence intensity, suggesting Arc RNAs localize to dendrites in both low- and multiple-copy number RNPs. When probing for multiple mRNAs, we find that localized RNPs are heterogeneous in size and colocalization patterns that vary per RNA. Further, localized RNAs that are targeted by the same trans-acting element (FMRP) display greater levels of colocalization compared to an RNA not targeted by FMRP. Simultaneous visualization and assessment of colocalization using highly multiplexed imaging of a dozen mRNA species targeted by FMRP demonstrates that dendritic RNAs are mostly trafficked as heteromeric cargoes of multiple types of RNAs (at least one or more RNAs). Moreover, the composition of these RNA cargoes correlates with the abundance of the transcripts even after accounting for expression. Collectively, these results suggest that dendritic RNPs are packaged as heterogeneous co-assemblies of different mRNAs and that RNP contents may be driven, at least partially, by highly abundant dendritic RNAs; a model that favors efficiency over fine-tuned control for sustaining long-distance trafficking of thousands of messenger molecules.