bioRxiv Subject Collection: Neuroscience's Journal

Integration of Transplanted Interneurons Over a New Period of Ocular Dominance Plasticity in Adult Mouse Visual Cortex
Cortical interneurons play an important role in mediating the juvenile critical period for ocular dominance plasticity in the mouse primary visual cortex. Previously, we showed that transplantation of cortical interneurons derived from the medial ganglionic eminence (MGE) opens a robust period of ocular dominance plasticity 33-35 days after transplantation into neonatal host visual cortex. The plasticity can be induced by transplanting either PV or SST MGE-derived cortical interneurons; it requires transplanted interneurons to express the vesicular GABAergic transporter; and it is manifested by changes to the host visual circuit. Here, we show that transplantation of MGE-derived cortical interneurons into the adult host visual cortex also opens a period of ocular dominance plasticity. The transplanted interneurons must be active to induce plasticity, and the neuronal activity and tuning of visually evoked responses in transplanted and host PV and SST interneurons are modulated by the locomotor state of the host. We also show that changes in activity over the period of plasticity induction are different between PV and SST interneurons but similar between host and transplanted interneurons of each type. The present findings demonstrate that the transplant-induced plasticity generated in adult visual cortex has many features in common with the role of these interneurons during the normal, juvenile critical period.

Latrophilin-3 conditional knockout in tyrosine hydroxylase neurons (Lphn3-Th-Cre) compared with Lphn3 global KO rats: Role of Lphn3 in tyrosine hydroxylase neurons on cognitive and behavioral effects of this ADHD susceptibility gene
Latrophilin-3 (LPHN3) is a brain specific adhesion G-protein coupled receptor associated with elevated risk of attention deficit hyperactivity disorder (ADHD). We developed a global Lphn3 knock-out (gKO) rat using CRISPR/Cas9 to delete exon-3. Here we report the development of a floxed Lphn3 rat crossed with tyrosine hydroxylase (Th-Cre) rats to create a conditional Lphn3 KO rat specific for catecholaminergic- positive cells. The gKO rats are hyperactive and have egocentric and allocentric navigation deficits but showed sparing of conditioned contextual and novel object recognition memory. Here we compared gKO and cKO rats controlling for litter effects. Both gKO and cKO rats were hyperactive and were impaired in egocentric navigation in the Cincinnati water maze (CWM) with deficits greater in gKO rats. The gKO rats were impaired in allocentric navigation in the Morris water maze (MWM) whereas cKO rats were only slightly affected compared with WT, cre, and floxed rats. Striatal tyrosine hydroxylase and dopamine D1 receptors were not significantly different in either model, nor were NMDA-NR1 or NMDA-NR2 in the hippocampus. We previously showed, however, that dopamine is released more rapidly in the striatum of gKO rats by fast- scan cyclic voltammetry. The cKO model shows an important role of catecholamines in the phenotype of LPHN3 disruption and add evidence that this synaptic protein plays a role in neuroplasticity that are consistent with ADHD.

Morphometric Identification of Parvalbumin-Positive Interneurons: A Data-Driven Approach
Traditionally, anatomical studies of parvalbumin-positive (PV+) labelled interneurons describe them as a homogeneous population of neurons. In contrast, recent single-cell RNAseq studies have identified multiple transcriptomically distinct categories of PV+ cells. That difference between a single anatomical category of PV+ neurons and multiple transcriptomic categories presents a problem in understanding the role of these neurons in cortical function. One gap that might contribute to this discrepancy is that PV+ morphology is typically addressed using qualitative descriptions and simple quantifications, while single-cell RNAseq studies use big data and high dimensional analyses. PV+ neurons play critical roles in the experience-dependent development of the cortex and are often involved in disease-related changes associated with neurodegenerative and neuropsychiatric disorders. Here, we developed a modern data-driven analysis pipeline to quantify PV+ morphology. We quantified 97 morphometric features from 14274 PV+ neurons and applied unsupervised clustering that identified 13 different PV+ morphologies. We extended the analysis to compare PV+ dendritic arbour patterns and cell body morphologies. Finally, we compared the morphologies of PV+ neurons with the cell body morphologies of neurons expressing various genes associated with PV+ transcriptomic cell types. This approach identified a range of PV+ morphologies similar to the number of transcriptomic categories. It also found that the PV+ morphologies have cortical area, laminar, and transcriptomic biases that might contribute to cortical function.

Phylogenetic variation of immature neurons in mammalian amygdala: high prevalence in primate expanded nuclei projecting to neocortex
Structural changes involving new neurons can occur through stem cell-driven neurogenesis and late-maturing "immature" neurons, namely undifferentiated neuronal precursors frozen in a state of arrested maturation. The latter exist in the cerebral cortex, being particularly abundant in large-brained mammals. Similar cells have been described in the amygdala of some species, although their interspecies variation remain poorly understood. Here, their occurrence, number, molecular expression, and morphology were systematically analyzed in eight diverse mammalian species widely differing in neuroanatomy, brain size, lifespan, and socioecology. We show remarkable phylogenetic variation of the immature neurons in the amygdala, with a significantly greater prevalence in primates. The cells are associated with the amygdalas basolateral complex that in evolution has expanded in primates in conjunction with cortical projections, thus mimicking the general trend of the neocortex. These results support the emerging view that large brains performing complex socio-cognitive functions rely on wide reservoirs of immature neurons.

An Independent Coding Scheme for Distance versus Position in the Hippocampus
Animals navigate using cognitive maps of their environment, integrating external landmarks and distances between them. Hippocampal place cells are the neuronal substrate of these cognitive maps. However, while hippocampal allocentric position coding in reference to external landmarks is well characterized, the determinants of idiothetic hippocampal distance coding remain poorly understood. Using virtual reality, electrophysiological recordings in mice, and local cue manipulations we could dissociate distance from position coding. In the cue-poor condition, we found pervasive distance coding with high distance indices in all bidirectional place cells including both superficial and deep CA1 pyramidal cells. In this condition, the mapping of distance onto a low-dimensional manifold and rigid distance relationships between place fields suggested strong attractor dynamics similar to those observed for grid cells. Inactivation of the medial septum (MS), which disrupts grid cells, significantly reduced both distance coding and rigid distance dynamics, suggesting an alteration (but not complete abolition) of the underlying attractor. In contrast, allocentric position coding could be observed in cue-rich environments, predominantly engaged deep CA1 pyramidal cells, and persisted during MS inactivation. These results are consistent with a selective contribution of grid cells and associated rigid attractor dynamics to hippocampal idiothetic distance coding but not allocentric position coding.

Best Cochlear Locations for Delivering Interaural Timing Cues in Electric Hearing
Growing numbers of children and adults who are deaf are eligible to receive cochlear implants (CI), which provide access to everyday sound. CIs in both ears (bilateral CIs or BiCIs) are becoming standard of care in many countries. However, their effectiveness is limited because they do not adequately restore the acoustic cues essential for sound localization, particularly interaural time differences (ITDs) at low frequencies. The cochlea, the auditory sensory organ, typically transmits ITDs more effectively at the apical region, which is specifically "tuned" to low frequencies. We hypothesized that effective restoration of robust ITD perception through electrical stimulation with BiCIs depends on targeting cochlear locations that transmit information most effectively. Importantly, we show that these locations can occur anywhere along the cochlea, even on the opposite end of the frequency map from where ITD cues are most dominantly encoded in an acoustic hearing system.

Truncated TrkB: The predominant TrkB Isoform in Nociceptors
Truncated TrkB (TrkBT1), traditionally considered a dominant-negative regulator of full-length TrkB (TrkBTK+), remains poorly understood in peripheral sensory neurons, particularly nociceptors. Furthermore, sensory neuronal TrkB expression and function has been traditionally associated with non-nociceptive neurons, particularly A{delta} low-threshold mechanoreceptors. This study challenges prevailing assumptions by demonstrating that TrkBT1 is the predominant TrkB isoform expressed in sensory neurons and plays a functional role in modulating neuronal activity. We demonstrate that TrkBT1 is the predominant isoform expressed in nociceptors, identified by markers such as TRPV1, TRPA1, TRPM8 and 5HT3A, as well as non-nociceptors, while the full-length isoform (TrkBTK+) is restricted to non-nociceptive subpopulation. Functionally, we show that acute application of BDNF induces modest calcium influx in nociceptors and prolonged BDNF exposure significantly potentiates capsaicin-induced calcium influx, an effect blocked by the TrkB-specific antagonist ANA12. Additionally, BDNF also promotes the survival of both nociceptive and non-nociceptive neurons in culture, an effect dependent on TrkBT1 activity. Our data also reveal that ANA12 inhibits BDNF-mediated neuronal sensitization and survival in a concentration-dependent manner, implicating distinct TrkBT1 signaling pathways in these processes. Collectively, our findings redefine TrkBT1 as a functional modulator of nociceptor activity rather than a passive regulator of full-length TrkB. By uncovering its dual roles in nociceptor sensitization and survival, this study provides new insights into the molecular mechanisms of BDNF/TrkB signaling in pain. Future work evaluating the role of TrkBT1 in sensory biology could offer new perspectives on how this receptor contributes to neuronal function and plasticity during chronic pain conditions.

Subregional Biomarkers in FDG PET for Alzheimer's Diagnosis and Staging: An Interpretable and Explainable model
ObjectiveTo investigate the radiomics features of the hippocampus and the amygdala subregions in FDG-PET images that can best differentiate Mild Cognitive Impairment (MCI), Alzheimers Disease (AD), and healthy patients.

MethodsBaseline FDG-PET data from 555 participants in the ADNI dataset were analyzed, comprising 189 cognitively normal (CN) individuals, 201 with MCI, and 165 with AD. The hippocampus and amygdala were segmented based on the DKT-Atlas, with additional subdivisions guided by probabilistic atlases from Freesurfer. Then radiomic features (n=120) were extracted from 38 hippocampal subregions and 18 amygdala nuclei using PyRadiomics. Various feature selection techniques, including ANOVA, PCA, Chi-square, and LASSO, were applied alongside nine machine learning classifiers.

ResultsThe Multi-Layer Perceptron (MLP) model combined with LASSO demonstrated excellent classification performance: ROC AUC of 0.957 for CN vs. AD, ROC AUC of 0.867 for MCI vs. AD, and ROC AUC of 0.782 for CN vs. MCI. Key regions, including the accessory basal nucleus, presubiculum head, and CA4 head, were identified as critical biomarkers. Features including GLRLM (Long Run Emphasis) and Small Dependence Emphasis (GLDM) showed strong diagnostic potential, reflecting subtle metabolic and microstructural changes often preceding anatomical alterations.

ConclusionSpecific hippocampal and amygdala subregions and their four radiomic features were found to have a significant role in the early diagnosis of AD, its staging, and its severity assessment by capturing subtle shifts in metabolic patterns. Furthermore, these features offer potential insights into the diseases underlying mechanisms and model interpretability.

Huntingtin interactome reveals huntingtin role in regulation of double strand break DNA damage response (DSB/DDR), chromatin remodeling and RNA processing pathways
Huntingtons Disease (HD), a progressive neurodegenerative disorder with no disease-modifying therapies, is caused by a CAG repeat expansion in the HD gene encoding polyglutamine-expanded huntingtin (HTT) protein. Mechanisms of HD cellular pathogenesis and cellular functions of the normal and mutant HTT proteins are still not completely understood. HTT protein has numerous interaction partners, and it likely provides a scaffold for assembly of multiprotein complexes many of which may be altered in HD. Previous studies have implicated DNA damage response in HD pathogenesis. Gene transcription and RNA processing has also emerged as molecular mechanisms associated with HD. Here we used multiple approaches to identify HTT interactors in the context of DNA damage stress. Our results indicate that HTT interacts with many proteins involved in the regulation of interconnected DNA repair/remodeling and RNA processing pathways. We present evidence for a role for HTT in double strand break repair mechanism. We demonstrate HTT functional interaction with a major DNA damage response kinase DNA-PKcs and association of both proteins with nuclear speckles. We show that S1181 phosphorylation of HTT is regulated by DSB, and can be carried out (at least in vitro) by DNA-PK. Furthermore, we show HTT interactions with RNA binding proteins associated with nuclear speckles, including two proteins encoded by genes at HD modifier loci, TCERG1 and MED15, and with chromatin remodeling complex BAF. These interactions of HTT may position it as an important scaffolding intermediary providing integrated regulation of gene expression and RNA processing in the context of DNA repair mechanisms.

Genetically Labeled Premyelinating Oligodendrocytes: Bridging Oligodendrogenesis and Neuronal Activity
To myelinate axons, oligodendrocyte precursor cells (OPCs) must stop dividing and differentiate into premyelinating oligodendrocytes (preOLs). PreOLs are thought to survey and begin ensheathing nearby axons, and their maturation is often stalled at human demyelinating lesions. Lack of genetic tools to visualize and manipulate preOLs has left this critical differentiation stage woefully understudied. Here, we generated a knock-in mouse line that specifically labels preOLs across the central nervous system. Genetically labeled preOLs exhibit distinct morphology, unique transcriptomic and electrophysiological features, and do not overlap with OPCs. PreOL lineage tracing revealed that subsets of them undergo prolonged maturation and that different brain regions initiate oligodendrogenesis with the spatiotemporal specificity. Lastly, by fate mapping preOLs under sensory deprivation, we find that neuronal activity functions within a narrow time window of preOL maturation to promote their survival and successful integration. Our work provides a new tool to probe this critical cell stage during axon ensheathment, allowing for fine dissection of axon-oligodendrocyte interactions.

Personalized electric field simulations of deformable large TMS coils based on automatic position and shape optimization
BackgroundTranscranial Magnetic Stimulation (TMS) therapies use both focal and unfocal coil designs. Unfocal designs often employ bendable windings and moveable parts, making realistic simulations of their electric fields in inter-individually varying head sizes and shapes challenging. This hampers comparisons of the various coil designs and prevents systematic evaluations of their dose-response relationships.

ObjectiveIntroduce and validate a novel method for optimizing the position and shape of flexible coils taking individual head anatomies into account. Evaluate the impact of realistic modeling of flexible coils on the electric field simulated in the brain.

MethodsAccurate models of four coils (Brainsway H1, H4, H7; MagVenture MST-Twin) were derived from computed tomography data and mechanical measurements. A generic representation of coil deformations by concatenated linear transformations was introduced and validated. This served as basis for a principled approach to optimize the coil positions and shapes, and to optionally maximize the electric field strength in a region of interest (ROI).

ResultsFor all four coil models, the new method achieved configurations that followed the scalp anatomy while robustly preventing coil-scalp intersections on N=1100 head models. In contrast, setting only the coil center positions without shape deformation regularly led to physically impossible configurations. This also affected the electric field calculated in the cortex, with a median peak difference of [~]16%. In addition, the new method outperformed grid search-based optimization for maximizing the electric field of a standard figure 8 coil in a ROI with a comparable computational complexity.

ConclusionOur approach alleviates practical hurdles that so far hampered accurate simulations of bendable coils. This enables systematic comparison of dose-response relationships across the various coil designs employed in therapy.

HighlightsO_LIautomatic positioning and shape optimization of large deformable TMS coils
C_LIO_LIensures adherence to the head anatomy and prevents coil-head intersections
C_LIO_LIenable automatic electric field maximization in target brain regions
C_LIO_LIoutperforms grid search for standard flat coils
C_LIO_LIprovides accurate computational models of four coils used in clinical practice
C_LI

A Blueprint of Sex-Specific Neuronal Regulation in the C. elegans Nervous System at Single-Cell Resolution
Sex-specific behaviors within a species are often attributed to variances in neuronal wiring and molecular signatures. However, how the genetic sex shapes the molecular architecture of the nervous system at a single neuron level is still unknown. To address this gap, we used single-cell RNA-sequencing (scRNA-seq) to profile the transcriptome of the sex-shared nervous system of adult male and hermaphrodite Caenorhabditis elegans. By ranking neurons based on the degree of molecular dimorphism, we uncover novel sexually dimorphic neurons such as PLM neurons, where functional dimorphism was corroborated by diminished posterior touch responses in hermaphrodites. We identify sex-specific regulators of mechanosensory behavior and neuronal function by combining our dataset with reverse genetic screens. While most sex-shared neurons retain their neurotransmitter identity, the male neuropeptide connectome undergoes substantial remodeling, with most of the neuropeptides showing a male-bias expression. This reinforces the notion that neuropeptides are crucial for diversifying connectome outputs. Furthermore, by correlating gene expression with outgoing synaptic connectivity, we identified regulatory candidates for synaptic wiring, including both shared and sex-specific genes. This comprehensive resource provides foundational insights into the molecular drivers of sexual dimorphism, facilitating future exploration of regulatory mechanisms and their impact on sex-specific behaviors in higher organisms.

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Effect of flicker-induced retinal stimulation of mice revealed by full-field electroretinography
PurposeTo investigate the effects of brief flickering light stimulation (FLS) on retinal electrophysiology and its blood flow in normal C57BL6J mice.

MethodsRetinal blood flow (RBF) and full-field electroretinography (ffERG) were measured before and after a 60-second long FLS (12 Hz, 0.1 cd{middle dot}s/m2) in a cohort of 8-12 weeks old C57BL6J mice (n=10) under anaesthetic and light-adapted conditions. A separate set of age-matched mice (n=9) underwent RBF and ffERG measurements before and after steady light stimulation (SLS) at 1 cd/m2 under similar conditions. The changes in RBF (arterial and venous flow) and ffERG responses (amplitudes and implicit times of a- and b-wave) were analyzed.

ResultsFLS significantly increased both arterial (p=0.003) and venous (p=0.018) blood flow as well as b-wave amplitudes (p=0.017) compared to SLS, which did not have any significant changes in both RBF and ERG. However, no significant differences were found in other ffERG responses (amplitude and implicit time of a-wave and b-wave implicit time) between the two groups after light stimulation. An increase in b-wave amplitude was positively associated with increase in both arterial (r=0.655, p=0.040) and venous blood flow (r=0.638, p=0.047) in the FLS group.

ConclusionsTransient FLS induced a significant increase in both RBF and electro-retinal activity, but such increase was not observed after SLS. Our results suggest the role of FLS, which exerts metabolic stress on the retina, in triggering retinal neurovascular coupling.

Long-term, cell type-specific effects of prenatal stress on dorsal striatum and relevant behaviors in mice
Maternal stress during pregnancy, or prenatal stress, is a risk factor for neurodevelopmental disorders in offspring, including autism spectrum disorder (ASD). In ASD, dorsal striatum displays abnormalities correlating with symptom severity, but there is a gap in knowledge about dorsal striatal cellular and molecular mechanisms that may contribute. Using a mouse model, we investigated how prenatal stress impacted striatal-dependent behavior in adult offspring. We observed enhanced motor learning and earlier response times on an interval timing task, with accompanying changes in time-related medium spiny neuron (MSN) activity. We performed adult dorsal striatal single-cell RNA sequencing following prenatal stress which revealed differentially expressed genes (DEGs) in multiple cell types; downregulated DEGs were enriched for ribosome and translational pathways consistently in MSN subtypes, microglia, and somatostatin neurons. DEGs in MSN subtypes over-represented ASD risk genes and were enriched for synapse-related processes. These results provide insights into striatal alterations relevant to neurodevelopmental disorders.

SLC35A2 loss of function variants affect glycomic signatures, neuronal fate, and network dynamics
SLC35A2 encodes a UDP-galactose transporter essential for glycosylation of proteins and galactosylation of lipids and glycosaminoglycans. Germline genetic SLC35A2 variants have been identified in congenital disorders of glycosylation and somatic SLC35A2 variants have been linked to intractable epilepsy associated with malformations of cortical development. However, the functional consequences of these pathogenic variants on brain development and network integrity remain elusive.

In this study, we use an isogenic human induced pluripotent stem cell-derived neuron model to comprehensively interrogate the functional impact of loss of function variants in SLC35A2 through the integration of cellular and molecular biology, protein glycosylation analysis, neural network dynamics, and single cell electrophysiology.

We show that loss of function variants in SLC35A2 result in disrupted glycomic signatures and precocious neurodevelopment, yielding hypoactive, asynchronous neural networks. This aberrant network activity is attributed to an inhibitory/excitatory imbalance as characterization of neural composition revealed preferential differentiation of SLC35A2 loss of function variants towards the GABAergic fate. Additionally, electrophysiological recordings of synaptic activity reveal a shift in excitatory/inhibitory balance towards increased inhibitory drive, indicating changes occurring specifically at the pre-synaptic terminal.

Our study is the first to provide mechanistic insight regarding the early development and functional connectivity of SLC35A2 loss of function variant harboring human neurons, providing important groundwork for future exploration of potential therapeutic interventions.

TMEM63A, associated with hypomyelinating leukodystrophies, is an evolutionarily conserved regulator of myelination
Infantile hypomyelinating leukodystrophy 19 (HLD19) is a rare genetic disorder where patients exhibit reduced myelin in central nervous system (CNS) white matter tracts and present with varied neurological symptoms. The causative gene TMEM63A encodes a mechanosensitive ion channel whose role in myelination has not been explored. Our study shows that TMEM63A is a major regulator of OL-driven myelination in the CNS. In mouse and zebrafish, Tmem63a inactivation led to early deficits in myelination, recapitulating the HLD19 phenotype. OL-specific conditional mouse knockouts of Tmem63a exhibited transient reductions in myelin, indicating that TMEM63A regulates myelination cell-autonomously. We show that TMEM63A is present at plasma membrane and on lysosomes and modulates myelin/myelin-associated protein production. Intriguingly, HLD19-associated TMEM63A variants from patients blocked trafficking to cell membrane. Together, our results reveal an ancient role for TMEM63A in fundamental aspects of myelination in vivo and highlight two exciting new models for the development of treatments for devastating hypomyelinating leukodystrophies.

The effects of object category training on the responses of macaque inferior temporal cortex are consistent with performance-optimizing updates within a visual hierarchy
How does the primate brain coordinate plasticity to support its remarkable ability to learn object categories? To address this question, we measured the consequences of category learning on the macaque inferior temporal (IT) cortex, a key waypoint along the ventral visual stream that is known to support object recognition. First, we observed that neural activity across task-trained monkeys IT showed increased object category selectivity, enhanced linear separability (of categories), and overall more categorical representations compared to those from task-naive monkeys. To model how these differences could arise, we next developed a computational hypothesis-generation framework of the monkeys learning process using anatomically-mapped artificial neural network (ANN) models of the primate ventral stream that we combined with various choices of learning algorithms. Our simulations revealed that specific gradient-based, performance-optimizing updates of the ANNs internal representations substantially approximated the observed changes in the IT cortex. Notably, we found that such models predict novel training-induced phenomena in the IT cortex, including changes in category-orthogonal representations and ITs alignment with behavior. This convergence between experimental and modeling results suggests that plasticity in the visual ventral stream follows principles of task optimization that are well approximated by gradient descent. We propose that models like the ones developed here could be used to make accurate predictions about visual plasticity in the ventral stream and its transference - or lack thereof - to any future test image.

Exact linear theory of perturbation response in a space- and feature-dependent cortical circuitmodel
What are the principles that govern the responses of cortical networks to their inputs and the emergence of these responses from recurrent connectivity? Recent experiments have probed these questions by measuring cortical responses to two-photon optogenetic perturbations of single cells in the mouse primary visual cortex. A robust theoretical framework is needed to determine the implications of these responses for cortical recurrence. Here we propose a novel analytical approach: a formulation of the dependence of cell-type-specific connectivity on spatial distance that yields an exact solution for the linear perturbation response of a model with multiple cell types and space- and feature-dependent connectivity. Importantly and unlike previous approaches, the solution is valid in regimes of strong as well as weak intra-cortical coupling. Analysis reveals the structure of connectivity implied by various features of single-cell perturbation responses, such as the surprisingly narrow spatial radius of nearby excitation beyond which inhibition dominates, the number of transitions between mean excitation and inhibition thereafter, and the dependence of these responses on feature preferences. Comparison of these results to existing optogenetic perturbation data yields constraints on cell-type-specific connection strengths and their tuning dependence. Finally, we provide experimental predictions regarding the response of inhibitory neurons to single-cell perturbations and the modulation of perturbation response by neuronal gain; the latter can explain observed differences in the feature-tuning of perturbation responses in the presence vs. absence of visual stimuli.

Significance StatementThe cerebral cortex is strongly re-currently connected with complex wiring rules. This circuitry can now be probed by studying responses to optogenetic perturbations of one or small numbers of cells. However, we currently lack a general theory connecting these responses to underlying circuitry. Here we develop a novel, exactly solvable theory to determine responses to small perturbations from the underlying connectivity. Analysis of these equations reveals simple rules that govern perturbation response patterns. Comparison with experimental data yields new constraints on the connectivity parameters. The theory yields predictions for the responses of unmeasured cell types and in new experimental conditions.

Altered neuronal network activity and connectivity in human Down Syndrome excitatory cortical neurons
Down syndrome (DS) is the most common genetic cause of intellectual disability, affecting one in 600 live births worldwide, and is caused by trisomy of the human chromosome 21 (Hsa21). Here, we investigated whether trisomy 21 results in changes in excitatory neuron network development, that could contribute to the neurodevelopmental phenotypes of DS. Replaying cerebral cortex development in vitro with Trisomy 21 and control isogenic and non-isogenic human induced pluripotent stem cells (hiPSC) enabled the analysis of the effect of Hsa21 triplication on neural network activity and connectivity specifically in developing excitatory cortical neurons. Network activity analysis revealed a significant decrease in neuronal activity in TS21 neurons early in development. TS21 neurons showed a marked reduction of synchronised bursting activity up to 80 days in vitro and over 5 months in vivo following transplantation into the mouse forebrain. Viral transynaptic tracing identified significant reduction of neuronal connectivity in TS21 neuronal networks in vitro, suggesting that reduced network connectivity contributes to the absence of synchronised bursting. Expression of voltage-gated potassium channels was significantly reduced in TS21 neurons, and single neuron recordings confirmed the lack of hyperpolarization-activated currents, indicating a functional loss of the potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1) in DS TS21 neurons. We conclude that the trisomy of Hsa21 leads to changes in ion channel composition, network activity and connectivity in cortical excitatory neuron networks, all of which collectively are likely to contribute to some of the neurodevelopmental features of DS.

Pathological α-synuclein elicits granulovacuolar degeneration independent of tau
BackgroundPathologic heterogeneity is a hallmark of Lewy body dementia (LBD), yet the impact of Lewy pathology on co-pathologies is poorly understood. Lewy pathology, containing -synuclein, is often associated with regional tau pathology burden in LBD. Similarly, granulovacuolar degeneration bodies (GVBs) have been associated with tau pathology in Alzheimers disease. Interestingly, GVBs have been detected in a broad range of neurodegenerative conditions including both -synucleinopathies and tauopathies. Despite the frequent co-occurrence, little is known about the relationship between -synuclein, tau, and granulovacuolar degeneration.

MethodsWe developed a mouse model of limbic-predominant -synucleinopathy by stereotactic injection of -synuclein pre-formed fibrils (PFFs) into the basal forebrain. This model was used to investigate the relationship of -synuclein pathology with tau and GVB formation.

ResultsOur model displayed widespread -synuclein pathology with a limbic predominant distribution. Aberrantly phosphorylated tau accumulated in a subset of -synuclein inclusion-bearing neurons, often colocalized with lysosomes. Many of these same neurons also contained CHMP2b- and CK1{delta}-positive granules, established markers of GVBs, which suggests a link between tau accumulation and GVB formation. Despite this observation, GVBs were also detected in tau-deficient mice following PFF-injection, suggesting that pathological -synuclein alone is sufficient to elicit GVB formation.

ConclusionsOur findings support that -synuclein pathology can independently elicit granulovacuolar degeneration. The frequent co-accumulation of tau and GVBs suggests a parallel mechanism of cellular dysfunction. The ability of -synuclein pathology to drive GVB formation in the absence of tau highlights the broader relevance of this process to neurodegeneration with relevance to the pathobiology of LBD.