bioRxiv Subject Collection: Neuroscience's Journal

Neuronal or vascular receptive fields? On the relation between BOLD-fMRI population receptive field (pRF) estimates and cortical vascularization.
This study investigates the contribution of different vascular compartments on population receptive field (pRF) size estimates within the early visual cortex (V1, V2, and V3) using BOLD-fMRI. We employed T2*-weighted gradient-echo (GE) and T2-weighted spin-echo (SE) sequences at 7 Tesla (7T) and a multi-band GE sequence at 3T to explore how different vessel sensitivities across these sequences influence pRF modeling. Our results confirm the expected pRF size increase across eccentricity and visual areas but found no significant differences in pRF size estimates across MR sequences, voxel sizes, or field strengths. BOLD signal amplitudes were influenced by MR sequence, with the largest signal changes observed for 7TGE, and amplitude increases were noted across cortical depth for GE sequences but not for SE sequences. Contrary to our hypotheses, pRF size estimates were not noticeably affected by the local vascularization, suggesting that pRFs primarily reflect neuronal activity rather than vascular compartment characteristics. Our study highlights the robust nature of pRF size estimates across various fMRI conditions and points toward the decoupling of pRF properties from vascular factors.

Novel Body-Selective Regions Responsive to Bodies Away from the Center of Gaze
We report the existence of two previously undescribed body-selective visual regions in the human brain, which we term the Ventromedial Body Area (VMBA) and the Medial Body Area (MBA). We localize these regions based on high-signal localizer contrasts and characterize selectivity in them with encoding model analysis of BOLD fMRI responses to thousands of naturalistic images. These regions respond to images of bodies away from the center of gaze.

Effects of binge-like ethanol drinking on nest building behavior in mice
Nest building is a natural behavior that can readily be analyzed in mice in the home cage environment. Nest building is involved in thermoregulation, positive motivational states, and motor function, and has been proposed as an index for ethanol withdrawal severity in mice. However, nest building outcomes after voluntary ethanol consumption have not been examined. Here, we tested male and female C57BL/6J mice on a 4-day drinking in the dark (DID) paradigm of binge-like drinking with either ethanol or a water control and analyzed nest scores at two time points (48 hours and 7 days) after the last DID session. At 48 hours after the last DID session, there were no differences between the two groups in nest quality. At 7 days after DID, ethanol-drinking animals showed significantly lower nest scores than the water group. No differences were found between the ethanol- and water-drinking groups in locomotor activity or anxiety-like behavior at this timepoint, indicating that nest building deficits in the ethanol group were likely not due to underlying differences in these behaviors. Together, these results validate the use of nest building as a naturalistic assessment of withdrawal-associated behavioral changes following voluntary binge-like ethanol consumption.

The representation of emotion knowledge in hippocampal-prefrontal systems
Emotional experiences involve more than bodily reactions and momentary feelings - they depend on knowledge about the world that spans contexts and time. Although it is well established that individuals conceptualize emotions using a low-dimensional space organized by valence and arousal, the neural mechanisms giving rise to this configuration remain unclear. Here, we examine whether hippocampal-prefrontal circuits - regions implicated in forming cognitive maps - also support the abstraction of emotional experiences. Using functional MRI data collected as participants viewed emotionally evocative film clips, we found that activity in hippocampal and prefrontal cortex predicted self-reported emotion across schematically distinct videos, consistent with a role in structural learning. Computational modeling of emotion transitions revealed that hippocampal responses to films and emotion self-reports could be predicted based on the statistical regularities of emotion transitions across different temporal scales. These findings demonstrate that hippocampal-prefrontal systems represent emotion concepts at multiple levels of abstraction, offering new insight into how the brain organizes emotion knowledge.

RNA-seq reveals transcriptomic differences in circadian-related genes of the choroid plexus in a preclinical chronic migraine model.
Background: Migraine patients show choroid plexus (CP) changes, impairing the blood-CSF barrier. The CP regulates circadian rhythms, but links between CP circadian genes and migraine are unexplored. Objective: This study examined CP circadian gene transcriptome changes in a chronic migraine rat model versus controls to identify migraine-related pathways. Design: Chronic migraine model: Sprague Dawley rats (3 females, 3 males) received nitroglycerine (NTG) every other day for 9 days; controls (3 females, 3 males) got saline. CP from the 4th ventricle was collected 2 hours post-final injection for RNAseq. Methods: Migraine Behavior: Von Frey hair tests on days 1, 5, and 9, pre- and 2 hours post-NTG/saline injection, assessed basal and NTG-induced pain thresholds. RNAseq & Analysis: Differentially expressed genes (p < 0.05, fold change > 1) were identified. GO, KEGG, and Reactome enrichment analyses evaluated circadian gene expression changes. Results: NTG group showed reduced basal and NTG-induced pain thresholds on days 1, 5, and 9. Females had more upregulated genes (MT2A, SLC7A11), males upregulated ZBTB16, S100A8. SLC7A11, SCG2, GRIA1 showed inverse regulation (up in females, down in males). Circadian gene expression altered: 10 genes upregulated (e.g., SERPINE1, MAPK9, ATF4), 13 downregulated (e.g., PER2, DBP, EZH2). Sex-specific differences: females (FBXL12, GPR157), males (NKX2-1, ATF4, CLOCK). GO/KEGG analyses revealed significant enrichment of circadian rhythm-related pathways, insulin resistance, and inflammatory response processes, with sex-specific differences: females showed HIF-1 signaling and hemoglobin-related pathways, while males exhibited arachidonic acid and leukotriene production. Conclusion: CP transcriptomics in the rat migraine model revealed sex-specific gene regulation, with females upregulating antioxidant genes (MT2A, SLC7A11) and males upregulating inflammatory factors (ZBTB16, S100A8), alongside circadian disruption (e.g., SERPINE1 upregulated, PER2 downregulated). Pathway analyses indicate enriched circadian rhythms, HIF-1 signaling (females), inflammatory processes (males), lipid metabolism (PPAR), and heme signaling, highlighting sex-specific and circadian targets for migraine therapy.

PP2A METHYLESTERASE, PME-1, AND PP2A METHYLTRANSFERASE, LCMT-1, CONTROL SENSITIVITY TO IMPAIRMENTS CAUSED BY INJURY-RELATED OLIGOMERIC TAU
Oligomeric species of tau are a hallmark of multiple neurodegenerative diseases such as Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). Given the evidence implicating protein phosphatase 2A (PP2A) in the molecular pathogenesis of tau-related neurodegenerative disorders, we sought to determine whether manipulating the expression of enzymes that regulate PP2A activity, such as leucine carboxyl methyltransferase 1 (LCMT-1) and protein methyl esterase 1 (PME-1), might impact pathological responses to oligomeric tau. Here, we tested the effect of transgenic overexpression of LCMT-1 or PME-1 on cognitive and electrophysiological impairments caused by exposure to either recombinant oligomeric human tau or oligomeric tau prepared from mice subjected to blast-induced traumatic brain injury. We found that overexpression of LCMT-1 reduced sensitivity to tau-induced impairments, while overexpression of PME-1 increased sensitivity to these impairments. Moreover, we found that shockwave exposure increased the propensity of endogenous tau to form toxic oligomers. These results suggest that manipulating LCMT-1 or PME-1 activity may represent novel therapeutic approaches for disorders involving exposure to pathogenic forms of oligomeric tau.

Implantable CMOS Deep-Brain Fluorescence Imager with Single-Neuron Resolution
Despite the advantages of optical imaging over electrophysiology, such as cell-type specificity, its application has been limited to the investigation of shallow brain regions (< 2 mm) because of the light scattering property of brain tissue. Passive optical conduits such as graded-index lenses and waveguides have permitted access to deeper locales but with restricted resolution and field of view, while creating massive lesions along the inserted path, with little pathway to improvement in the technology. As an alternative, we present the Acus device, an active implantable complementary metal-oxide-semiconductor (CMOS) neural imager with a 512-pixel silicon image sensor post-processed into a 4.1-mm-long, 120-m-wide shank with a collinear fiber for illumination, which is able to record transient fluorescent signals in deep brain regions at 400 frames/sec. Acus can achieve single-neuron resolution in functional imaging of GCaMP6s-expressing neurons at a frame rate of 400 frames/sec.

High-dimensional neuronal activity from low-dimensional latent dynamics: a solvable model
Computation in recurrent networks of neurons has been hypothesized to occur at the level of low-dimensional latent dynamics, both in artificial systems and in the brain. This hypothesis seems at odds with evidence from large-scale neuronal recordings in mice showing that neuronal population activity is high-dimensional. To demonstrate that low-dimensional latent dynamics and high-dimensional activity can be two sides of the same coin, we present an analytically solvable recurrent neural network (RNN) model whose dynamics can be exactly reduced to a low-dimensional dynamical system, but generates an activity manifold that has a high linear embedding dimension. This raises the question: Do low-dimensional latents explain the high-dimensional activity observed in mouse visual cortex? Spectral theory tells us that the covariance eigenspectrum alone does not allow us to recover the dimensionality of the latents, which can be low or high, when neurons are nonlinear. To address this indeterminacy, we develop Neural Cross-Encoder (NCE), an interpretable, nonlinear latent variable modeling method for neuronal recordings, and find that high-dimensional neuronal responses to drifting gratings and spontaneous activity in visual cortex can be reduced to low-dimensional latents, while the responses to natural images cannot. We conclude that the high-dimensional activity measured in certain conditions, such as in the absence of a stimulus, is explained by low-dimensional latents that are nonlinearly processed by individual neurons.

Intermingled representation of oral cavity in mouse trigeminal ganglion
Somatotopy serves as a fundamental principle underlying sensory information processing, traditionally emphasized in the study of the cerebral cortex. However, little effort has been directed towards unraveling the spatial organization characterizing the earlier stages of somatosensory pathways. In this study, we developed a novel methodology to visualize individual neurons within the trigeminal ganglion, a crucial cluster of cell bodies of sensory neurons innervating the face. Our investigations revealed a reliable sensory response to stimulation of the lower incisor or lip within this ganglion. The responsive neurons were confined to a specific portion of the trigeminal ganglion, consistent with innervation of the lower oral cavity by the mandibular nerve (V3). Contrary to our expectations, we did not observe a discernible map differentiating the lower tooth and the lip. Instead, the spatial representation of the tooth and lip within this portion of trigeminal ganglion exhibited intermingling with no clear border between tooth-responding and lip-responding neurons. These findings contrast with earlier studies that identified tooth and lip responding regions in the somatosensory cortex. Our study sheds light on the complex spatial organization of sensory processing in the trigeminal system, highlighting the need for further research to elucidate the underlying mechanisms and implications for sensory perception and clinical interventions.

Gene therapies alleviate absence epilepsy associated with Scn2a deficiency in DBA/2J mice
Mutations in the voltage-gated sodium channel gene SCN2A, which encodes the NaV1.2 channel, cause severe epileptic seizures. Patients with SCN2A loss-of-function (LoF) mutations, such as protein-truncating mutations, often experience later-onset and drug-resistant epilepsy, highlighting an urgent unmet clinical need for new therapies. We previously developed a gene-trap Scn2a (Scn2agt/gt) mouse model with a global NaV1.2 reduction in the widely used C57BL/6N (B6) strain. Although these mice display multiple behavioral abnormalities, EEG recordings indicated only mild epileptiform discharges, possibly attributable to the seizure-resistant characteristics associated with the B6 strain. To enhance the epileptic phenotype, we derived congenic Scn2agt/gt mice in the seizure-susceptible DBA/2J (D2J) strain. Notably, we found that these mice exhibit prominent spontaneous absence seizures, marked by both short and long spike-wave discharges (SWDs). Restoring NaV1.2 expression in adult mice substantially reduced their SWDs, suggesting the possibility of SCN2A gene replacement therapy during adulthood. RNA sequencing revealed significant alterations in gene expression in the Scn2agt/gt mice, in particular a broad downregulation of voltage-gated potassium channel (KV) genes, including KV1.1. The reduction of KV1.1 expression was further validated in human cerebral organoids with SCN2A deficiency, highlighting KV1.1 as a promising therapeutic target for refractory seizures associated with SCN2A dysfunction. Importantly, delivery of exogenous human KV1.1 expression via adeno-associated virus (AAV) in D2J Scn2agt/gt mice substantially reduced absence seizures. Together, these findings underscore the influence of mouse strain on seizure severity and highlight the potential of targeted gene therapies for treating SCN2A deficiency-related epilepsies.

Convergent and divergent brain-cognition relationships during development revealed by cross-sectional and longitudinal analyses in the ABCD Study
How brain networks and cognition co-evolve during development remains poorly understood. Using longitudinal data collected at baseline and Year 2 from 2,949 individuals (ages 8.9-13.5) in the Adolescent Brain Cognitive Development (ABCD) study, we show that baseline resting-state functional connectivity (FC) more strongly predicts future cognitive ability than concurrent cognitive ability. Models trained on baseline FC to predict baseline cognition generalize better to Year 2 data, suggesting that brain-cognition relationships strengthen over time. Intriguingly, baseline FC outperforms longitudinal FC change in predicting future cognitive ability. Differences in measurement reliability do not fully explain this discrepancy: although FC change is less reliable (intraclass correlation, ICC = 0.24) than baseline FC (ICC = 0.56), matching baseline FC reliability by shortening scan time only partially narrows the predictive gap. Furthermore, neither baseline FC nor FC change meaningfully predicts longitudinal change in cognitive ability. We also identify converging and diverging predictive network features across cross-sectional and longitudinal models of brain-cognition relationships, revealing a multivariate twist on Simpsons paradox. Together, these findings suggest that during early adolescence, stable individual differences in brain functional network organization exert a stronger influence on future cognitive outcomes than short-term changes.

Central nervous system atlas of larval zebrafish constructed using the morphology of single excitatory and inhibitory neurons
Comprehensive single-neuron mapping is essential for understanding the brain connectome. However, focusing only on mapping the projections of neurons from a single brain region gives limited insight into the global organizational principles of brain connectivity. Here, we present a whole body-wide Common Physical Space (CPSv1.0) for larval zebrafish, integrating neuroanatomical parcellations, cytoarchitecture atlases of excitatory (E) and inhibitory (I) neurons, and a dendrite-axon-annotated projectome comprising 12,219 E and 7,792 I neuronal morphologies (~25% of total E and I neurons). These neurons span all regions of the brain, spinal cord, and peripheral ganglia, forming 531 hierarchical morphotypes and generating weighted and directed inter-cell-type and inter-region connectomes. Projection and network analyses reveal structural divergence between E and I neurons, combinatory and modular projection rules, behavior-linked pathways, structured E and I connectivities, and multimodal sensorimotor hubs. Hosted on an open interactive platform, these digital atlases establish a scalable framework for multimodal neural data integration, offering insights into the global neural architecture across the whole central nervous system.

Multidimensional components of impulsivity during early adolescence: Relationships with brain networks and future substance-use in the Adolescent Brian and Cognitive Development (ABCD) study
Impulsivity is a multifaceted construct that typically increases during adolescence and is implicated in risk for substance use disorders that develop later in life. Here, we take a multivariate approach to identify latent dimensions of impulsivity, broadly defined, among youth enrolled in the Adolescent Brain and Cognitive Development (ABCD) study and explore associations with individual differences in demographics, substance-use initiation and canonical resting state networks (N=11,872, ages ~9-10). Using principal component analysis, we identified eight latent impulsivity dimensions, the top three of which together accounted for the majority of the variance across all impulsivity assessments. The first principal component (PC1) was a general impulsivity factor that mapped onto all impulsivity-related assessments. PC2 mapped onto a 'mixed' impulsivity style related to both poorer, less attentive performance on the SST and decreased delay discounting. PC3 linked externalizing behaviors across multiple measures with indices of delay discounting, making delay discounting the only impulsivity-related assessment to load on all three of the top PCs. Multiple impulsivity PCs were significantly associated with subsequent initiation of alcohol and cannabis use. Finally, we found both cross-sectional and longitudinal associations between the PCs and functional connectivity between and within frontoparietal, cingulo-opercular, and default mode networks. These data provide a critical empirical baseline for how facets of impulsivity covary in early adolescence which may be tracked through future waves of ABCD data, enabling longitudinal elucidation of how dimensions of impulsivity interact with other individual and environmental factors to influence risk for substance use later in life.

NEUROLINGUA: A Neuroimaging Database Tailored to Unravel the Complexity of Multilingual Comprehension
The neural mechanisms underlying language processing involve a well-defined brain network, including mainly left perisylvian areas. Yet, the extent of its individual variability remains largely unexplored, particularly in bilingual and multilingual contexts. Differences in linguistic profiles (e.g., age of acquisition, exposure, proficiency) provide an opportunity to assess how network topology adapts to sociolinguistic factors. To address this, we developed NEUROLINGUA, a comprehensive database of functional and structural MRI data, enriched with sociodemographic, sociolinguistic, and behavioral information. It includes 725 healthy individuals aged 18-82 immersed in a Basque-Spanish multilingual environment, ranging from near-monolinguals to highly proficient multilinguals. Participants completed a functional MRI language localizer with both auditory and visual comprehension tasks, enabling cross-modal comparisons. Additionally, this localizer included sentences involving arithmetic problem-solving. Exploratory analyses confirmed associations between structural MRI, sociodemographic, and cognitive measures. Functional MRI validated NEUROLINGUA's capacity to localize the language comprehension network and capture linguistic profile effects. This integrative dataset offers an unparalleled resource to investigate the factors influencing language network adaptability and variability in diverse sociolinguistic contexts.

Causal Lesion Evidence for Two Motor Speech Coordination Networks in the Brain
Speech production is supported by sensory-to-motor transformations to coordinate activity of the larynx and orofacial muscles. Here, we show that lesions to left temporal lobe areas involved in pitch processing cause reduced neural responses when repeating sentences and when humming piano melodies in a dorsal portion of the left precentral gyrus linked to laryngeal motor control. In contrast, lesions to left inferior parietal areas involved in somatosensory processing of speech cause reduced neural responses when repeating sentences but not when humming piano melodies in a ventral portion of the left precentral gyrus linked to orofacial motor control. Analyses in neurotypical participants converge in showing that the dorsal and ventral portions of the left precentral gyrus exhibit strong functional connectivity to left temporal and inferior parietal regions, respectively. These results provide causal lesion evidence that dissociable networks underlie distinct sensory-to-motor transformations supporting laryngeal and orofacial motor control for speech production.

Stress induces distinct social behavior states encoded by the ventral hippocampus
A single, acute traumatic experience can result in a host of negative impacts on behavior, such as increased violence, reduced sociability, and exaggerated fear responses. Despite the large body of research on the neurobiology of stress, we have a poor understanding of how trauma rewires social circuits in the brain. To identify how social circuits are re-organized by stress, we interrogated the role of the ventral hippocampus (VH), a key node for both the orchestration of emotionally-relevant behavior and the integration of sensory information. Using a footshock-based model of traumatic stress, we established that a single exposure to a series of unpredictable, inescapable footshocks was sufficient to negatively alter social behavior, resulting in increased violence and social hesitancy. Critically, we found that stress-induced changes to social behavior engaged neural ensembles in the VH, a critical node in the regulation of emotion states. Using a virally-mediated, activity-dependent cellular tagging approach to label two neural ensembles activated by temporally distinct experiences, we were surprised to find that stress-induced violence and stress-induced social hesitancy recruited largely non-overlapping populations of cells in the VH, in contrast to higher degrees of ensemble overlap in unstressed control mice, suggesting that stress biases the brain towards stronger differentiation of distinct social states. Additionally, we found that activity of VH neurons was required for stress-induced aggression. Finally, we found that stress-induced changes to social behaviors and VH activity profiles could be reversed by the introduction of social buffering post-stress. Collectively, our findings suggest that stress drives the VH to dissociably encode specific behavioral states, rather than a single state of negative valence, consistent with a role for multiple structures and distributed ensembles in the modulation of traumatic stress.

Frequency tagging evidence supports perceptual separation of rapid stimuli in human fetuses
In early human development, perceptual processes grow faster with maturation, as inferred using the duration of the attentional blink and multisensory integration window. The consequences of this developmental trend for sensory-cognition in fetuses are unclear: does the fetus perceive rapid stimuli as discrete events or, rather, one fused stimulus? We addressed this question using frequency tagging in two experiments with rapid auditory stimuli while neural responses were recorded in the third trimester with fetal magnetoencephalography (MEG). Our results are the first successful demonstration of frequency tagging in the fetal MEG amplitude spectrum and show that the fetal cortex generates separate neural responses to discrete auditory stimuli in both experiments, and similar results were also obtained when one experiment was repeated in newborns. While we cannot rule out perceptual fusion of rapid stimuli in higher-order association cortices, our results weaken the hypothesis that fetuses fuse rapid auditory stimuli into a single prolonged percept. Finally, our work points to frequency tagging analysis as a solution which avoids the uncertainties surrounding immature neural response latencies in time-domain analysis of fetal MEG.

Hyperpolarization-activated cation channels confer tonotopic specialization for temporal encoding of sound frequency in the cochlear nucleus
Sensory neurons are equipped with physiological properties vital for accurate signal processing. The functional importance of such properties is exemplified in auditory circuits where intrinsic excitability is optimized to detect frequency-specific features. In birds, the neurons of nucleus magnocellularis (NM) receive primary auditory input (Rubel and Parks, 1975a; Parks and Rubel, 1978; Jackson et al., 1982) and are arranged tonotopically. NM comprises a superficially homogenous neural population, but several physiological properties vary systematically along its tonotopic frequency axis. In particular, expression of voltage-gated conductances plays a pivotal role in creating selectivity that enables temporal precision. Here, we identify a previously undescribed gradient of hyperpolarization-activated cation channels (IH). Whole cell patch clamp techniques and immunostaining for HCN1, an IH channel subunit, demonstrated an expression gradient corresponding to NM's tonotopic axis. To investigate the function of tonotopic IH expression in NM, we applied a depolarizing ramp injection protocol to measure the impact of pharmacologically blocking IH on neural active properties (Ferragamo and Oertel, 2002; McGinley and Oertel, 2006; Oline et al. 2016). Next, we investigated whether this tonotopic patterning of HCN facilitates encoding of temporally patterned inputs. We injected depolarizing current pulse trains before and during HCN channel block. During pharmacological block, there was a reduction of NM spike entrainment to input pulses suggesting a key contribution of HCN channels to NM's ability to encode its synaptic drive. Results show that there is tonotopic distribution of HCN channels in NM which provides a novel mechanism that enables NM neurons to encode temporally patterned excitatory input.

Impairment in axonal translation and cytoplasmic viscosity during aging in sensory neurons
Mitochondria are trafficked along axons and provide the energy required for several intracellular mechanisms including molecular transport and local translation, which is believed to contribute to the homeostasis of the axonal compartment. Decline in mitochondria activity is one of the hallmarks of aging. It is still unclear, though, whether this decline corresponds to a concomitant reduction in the extent of axonal translation during aging. Using live cell imaging of sensory neurons, we found a significant decrease in the number of active mitochondria and the percentage of mitochondria localized to axons in aged mice compared to young mice. This decrease was mirrored by a loss of intracellular ATP as well as an ATP-dependent decrease in axoplasmic viscosity. In addition, the size of G3BP1 positive axonal granules and the number of FMRP axonal granules increased. Cumulatively, we found a functional decrease in the overall level of axonal translation in aged neurons. We were able to rescue this effect by increasing ATP synthesis, which induced a global decrease in axoplasmic viscosity, while promoting RNA granule solubilization and boosting axonal translation. Proteomic analysis of newly synthesized proteins in axons of aged vs young neurons revealed a dysregulation of pathways related to axonal biology and growth. We identified MAP1B and STAT3 as proteins whose axonal local synthesis was impaired in aged axons, and more notably show that this impairment could be rescued by increasing ATP synthesis. We believe that this research sheds light on axonal translation in aged neurons and its relationship with energy sources inside the axonal compartment, possibly presenting an opportunity for future therapeutics.

Respiratory coordination of excitability states across the human wake-sleep cycle
While the respiratory rhythm is increasingly recognized as a key modulator of oscillatory brain activity across the wake-sleep cycle in humans, very little is known about its influence on aperiodic brain activity during sleep. This broadband activity indicates spontaneous fluctuations in excitation-inhibition (E:I) balance across vigilance states and has recently been shown to systematically covary across the respiratory cycle during waking resting state. We used simultaneous EEG and respiratory recordings over a full night of sleep collected from N = 23 healthy participants to unravel the nested dynamics of respiration phase-locked excitability states across the wake-sleep cycle. We demonstrate a prominent phase shift in the coupling of aperiodic brain activity to respiratory rhythms as participants were transitioning from wakefulness to sleep. Moreover, respiration-brain coupling became more consistent both across and within participants, as interindividual as well as intraindividual variability systematically lessened from wakefulness and the transition to sleep towards deeper sleep stages. Our results suggest that respiration phase-locked changes in E:I balance conceivably add to sleep stage-specific neural signatures of REM and NREM sleep, highlighting the complexity of brain-body coupling during sleep.

Lack of mRNA Methylation in Schwann Cells Results in Demyelination and Regenerative Failure
Schwann cells are essential for peripheral nerve myelination and regeneration. N6-methyladenosine (m6A) RNA methylation, regulated by methyltransferase-like 14 (Mettl14), is a critical post-transcriptional modification, but its role in Schwann cell biology remains unclear. Using a conditional knockout (cKO) mouse model, we investigated the impact of Mettl14-mediated m6A methylation on Schwann cells. Mice born with Schwann cell-specific genetic deletion of Mettl14 developed normally but starting in young adulthood exhibited progressive motor deficits, severe demyelination, and axonal degeneration, confirmed by behavioral assessments and histological analyses. Mettl14-deficient Schwann cells displayed impaired proliferation and mitochondrial dysfunction in vitro. Following sciatic nerve injury, Mettl14 cKO mice showed defective macrophage recruitment, slowed axonal degeneration, and impaired regeneration. These findings suggest that Mettl14-mediated m6A methylation is critical for Schwann cell maintenance but not development. Given that Mettl14 cKO mice developed a demyelinating polyneuropathy, it is possible that manipulation of m6A methylation in Schwann cells is a promising therapeutic strategy targeting peripheral nerve repair and myelination.

A conserved Arf-GEF modulates axonal integrity through RAB-35 by altering neuron-epidermal attachment
Neurites of sensory neurons densely innervate the skin and are embedded within it. These delicate structures are exposed to acute and chronic mechanical strain and yet their integrity is maintained throughout life. Although evidence suggests a neuroprotective role for the skin, the molecular pathways involved are still poorly understood. In C. elegans, the cytoskeletal molecule UNC-70/{beta}-spectrin functions in synergy with the small GTPase RAB-35 within the skin to stabilize neuron-epidermal attachment structures against mechanical strain and prevent movement-induced damage to mechanosensitive axons. However, the full suite of molecules regulating these specialized attachments remains elusive. Here, through an unbiased forward genetic screen we have identified a guanine nucleotide exchange factor (GEF) previously associated with the endocytic-recycling machinery, AGEF-1a, that impacts axonal maintenance. We show that AGEF-1a functions selectively within the skin to regulate the integrity of the embedded axons. Mechanistically, we reveal that this effect is achieved through an interaction between AGEF-1a and epidermal RAB-35 to facilitate its activation, which in turn modulates the neuron-epidermal attachments. Finally, we demonstrate that the function of this GEF is highly conserved, with the expression of its human ortholog, BIG2 capable of replacing AGEF-1a. Together, we reveal the specific molecular machinery responsible for fine-tuning neuron-epidermal attachments and maintaining axonal integrity during life.

LINE-1 replication in a mouse TDP-43 model of neurodegeneration marks motor cortex neurons for cell-intrinsic and non-cell autonomous programmed cell death.
A key pathological feature of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) is the loss of nuclear localization and the accumulation of cytoplasmic inclusions of hyperphosphorylated TAR-DNA binding protein 43 (TDP-43). TDP-43 is a nucleic acid-binding protein involved in transcriptional repression, mRNA splicing, and the regulation of retrotransposable elements (RTEs) and endogenous retroviruses (ERVs). RTEs and ERVs are mobile genetic elements that constitute about 45% of our genome. These virus-like elements encode the capacity to replicate through an RNA intermediate and insert cDNA copies at de novo chromosomal locations. Their expression is a proven source of DNA damage and inflammatory signaling. Research in Drosophila has demonstrated a causal role of RTEs/ERVs in mediating both intracellular toxicity of TDP-43 and the intercellular spread of these toxic effects from glia to neurons. RTEs and ERVs are inappropriately expressed in postmortem tissues from ALS, FTD, and Alzheimers Disease (AD) patients, as well as in cell culture in response to TDP-43 disruption, suggesting that the findings in Drosophila may be conserved across species. But the role of RTEs and ERVs has not yet been examined in a vertebrate model of TDP-43 pathology. To investigate the functional contributions of RTEs in vertebrates, we utilized an established transgenic mouse model that overexpresses moderate levels of either human wild-type TDP-43 (hTDP-43-WT) or a mutant version with a specific causal amino acid substitution (hTDP-43-Q331K) associated with some inherited forms of the disease. Through RNA-sequencing of the motor cortex, and imaging of a LINE-1-EGFP retrotransposon indicator cassette, we found that the TDP-43 transgenic animals exhibit broad expression of RTEs and ERVs, along with replication of LINE-1 in glia and neurons in the motor cortex. This expression begins at the age of onset of neurological phenotypes, earlier in the hTDP-43-Q331K animals and much later in hTDP-43-WT. Although the motor defects progressively worsen over time, the LINE-1-EGFP replication reporter transiently labels spatially clustered groups of neurons and glia at the time of onset of motor symptoms. These EGFP-labeled neurons undergo cell death and are therefore lost over time. Unlabeled cells also die as a function of distance from the clusters of LINE-1-EGFP labeled neurons and glial cells. Together, these findings support the hypothesis that TDP-43 pathology triggers RTE/ERV expression in the motor cortex, that such expression marks cells for programmed cell death, with non-cell autonomous effects on nearby neurons and glial cells.

Parkinson's disease-vulnerable and -resilient dopamine neurons display opposite responses to excitatory input
Dopamine (DA) neurons of the substantia nigra (SN) are essential for motor control and selectively degenerate in Parkinson's disease (PD). However, DA neurons are molecularly heterogeneous, with some showing greater vulnerability and others resilience. Here, we show that the DA subtype marker Anxa1, identified in mice, labels PD-vulnerable DA neurons in human SN. Using mice, we found that excitatory inputs from subthalamic (STN) and pedunculopontine (PPN) nuclei evoked frequency dependent excitation in SN GABA neurons, but complex multiphasic DA neuron responses, suggesting heterogeneous DA subtype responses. Indeed, excitatory inputs evoked differential DA responses in striatal subregions, an increase in caudal striatum, but inhibition followed by rebound in dorsolateral striatum. Additionally, PD resilient Vglut2+ DA neurons were excited by STN/PPN input, while vulnerable Anxa1+ DA neurons were inhibited. These findings demonstrate that DA subtypes are embedded in distinct functional networks, suggesting that some therapeutic interventions may differentially impact vulnerable and resilient DA subtypes.

Stochastic Misfolding Drives the Emergence of Distinct α-Synuclein Strains
The existence of -synuclein conformational strains provides a potential explanation for the clinical and pathological differences among synucleinopathies such as Parkinson's disease and multiple system atrophy. However, how distinct -synuclein strains are formed in vivo remains unknown. Here, we examined whether unique strains of self-propagating -synuclein aggregates can arise within a consistent molecular environment. Unexpectedly, we observed conformational heterogeneity between individual preparations of -synuclein pre-formed fibrils (PFFs) generated by polymerizing recombinant wild-type or A53T-mutant human -synuclein under identical conditions. Moreover, we found that -synuclein aggregates formed spontaneously in the brains of a transgenic synucleinopathy mouse model were conformationally diverse, leading to the identification of three distinct disease subtypes. Propagation of putative PFF- and brain-derived -synuclein strains in mice initiated several distinct synucleinopathies, characterized by differences in disease onset times, cerebral -synuclein deposition patterns, and the conformational attributes of -synuclein aggregates. The conformational diversity of -synuclein aggregates across PFF preparations and between the brains of individual transgenic mice demonstrates that -synuclein can spontaneously form multiple self-propagating strains within an identical environment both in vitro and in vivo. This suggests that stochastic misfolding into distinct aggregate structures drives the emergence of -synuclein strains and implies that the intrinsic variability of common synucleinopathy research tools must be considered when designing and interpreting experiments.

A New Pathway for the Decussation of Corticospinal Tracts
Each brain hemisphere controls the movements of the opposite side of the body because the motor axons cross the midline, known as CST decussation. The current theory on CST decussation suggests CSTs decussate as a single tract at the junction between medulla and spinal cord. Although this theory is widely accepted, this theory is based on selective analyses and is therefore incomplete. Here, we employed new approaches, including the horizontal analyses and a non-invasive anterograde tracing method to examine CST decussation thoroughly. We analyzed all CS axons in 3 planes. These approaches led to the discovery of a new pathway for CST decussation. We found CSTs turned back to medulla, moving anteriorly and decussated in an oval structure. In this structure, each CST split into 4 fascicle groups and interdigitated with the corresponding groups from the opposite CST to cross the midline. The significance of this pathway was apparent after decussation where these 4 groups reversed direction, moving posteriorly toward spinal cord. While moving, the motor axons gradually separated at different locations and subsequently turned and occupied the correct positions in the dorsal funiculus for proper limb control. In addition to CSTs, we also characterized several components in the oval structure.