Overproduction of upper-layer neurons in the neocortex leads to autism-like features in mice

Stabilizing axin leads to optic nerve hypoplasia in a mouse model of autism

Autism spectrum disorder (ASD) is a group of neurodevelopment disorders characterized by deficits in social interaction and communication, and repetitive or stereotyped behavior. Autistic children are more likely to have vision problems, and ASD is unusually common among blind people. However, the mechanisms behind the vision disorders in autism are unclear. Stabilizing WNT-targeted scaffold protein Axin2 by XAV939 during embryonic development causes overproduction of cortical neurons and leads to autistic-like behaviors in mice. In this study, we investigated the relationship between vision abnormality and autism using an XAV939-induced mouse model of autism. We found that the mice receiving XAV939 had decreased amplitude of bright light-adaptive ERG. The amplitudes and latency of flash visual evoked potential recorded from XAV939-treated mice were lower and longer, respectively than in the control mice, suggesting that XAV939 inhibits visual signal processing and conductance. Anatomically, the diameters of RGC axons were reduced when Axin2 was stabilized during the development, and the optic fibers had defective myelin sheaths and reduced oligodendrocytes. The results suggest that the WNT signaling pathway is crucial for optic nerve development. This study provides experimental evidence that conditions interfering with brain development may also lead to visual problems, which in turn might exaggerate the autistic features in humans.

Auts2 enhances neurogenesis and promotes expansion of the cerebral cortex

INTRODUCTION

The AUTS2 gene is associated with various neurodevelopmental and psychiatric disorders and has been suggested to play a role in acquiring human-specific traits. Functional analyses of Auts2 knockout mice have focused on postmitotic neurons, and the reported phenotypes do not faithfully recapitulate the whole spectrum of AUTS2-related human diseases.

OBJECTIVE

The objective of the study is to assess the role of AUTS2 in the biology of neural progenitor cells, cortical neurogenesis and expansion; and understand how its deregulation leads to neurological disorders.

METHODS

We screened the literature and conducted a time point analysis of AUTS2 expression during cortical development. We used in utero electroporation to acutely modulate the expression level of AUTS2 in the developing cerebral cortex in vivo, and thoroughly characterized cortical neurogenesis and morphogenesis using immunofluorescence, cell tracing and sorting, transcriptomic profiling, and gene ontology enrichment analyses.

RESULTS

In addition to its expression in postmitotic neurons, we showed that AUTS2 is also expressed in neural progenitor cells at the peak of neurogenesis. Upregulation of AUTS2 dramatically altered the differentiation program and fate determination of cortical progenitors. Notably, it increased the number of basal progenitors and neurons and changed the expression of hundreds of genes, among which 444 have not been implicated in mouse brain development or function.

CONCLUSION

The study provides evidence that AUTS2 is expressed in germinal zones and plays a key role in fate decision of neural progenitor cells with impact on corticogenesis. It also presents comprehensive lists of AUTS2 target genes thus advancing the molecular mechanisms underlying AUTS2-associated diseases and the evolutionary expansion of the cerebral cortex.

Loss of protein tyrosine phosphatase receptor delta PTPRD increases the number of cortical neurons, impairs synaptic function and induces autistic-like behaviors in adult mice

BACKGROUND

The brain cortex is responsible for many higher-level cognitive functions. Disruptions during cortical development have long-lasting consequences on brain function and are associated with the etiology of brain disorders. We previously found that the protein tyrosine phosphatase receptor delta Ptprd, which is genetically associated with several human neurodevelopmental disorders, is essential to cortical brain development. Loss of Ptprd expression induced an aberrant increase of excitatory neurons in embryonic and neonatal mice by hyper-activating the pro-neurogenic receptors TrkB and PDGFRβ in neural precursor cells. However, whether these alterations have long-lasting consequences in adulthood remains unknown.

RESULTS

Here, we found that in Ptprd+/- or Ptprd-/- mice, the developmental increase of excitatory neurons persists through adulthood, affecting excitatory synaptic function in the medial prefrontal cortex. Likewise, heterozygosity or homozygosity for Ptprd also induced an increase of inhibitory cortical GABAergic neurons and impaired inhibitory synaptic transmission. Lastly, Ptprd+/- or Ptprd-/- mice displayed autistic-like behaviors and no learning and memory impairments or anxiety.

CONCLUSIONS

These results indicate that loss of Ptprd has long-lasting effects on cortical neuron number and synaptic function that may aberrantly impact ASD-like behaviors.

Decoding Molecular Bases of Rodent Social Hetero-Grooming Behavior Using in Silico Analyses and Bioinformatics Tools

Highly prevalent in laboratory rodents, 'social' hetero-grooming behavior is translationally relevant to modeling a wide range of neuropsychiatric disorders. Here, we comprehensively evaluated all known to date mouse genes linked to aberrant hetero-grooming phenotype, and applied bioinformatics tools to construct a network of their established protein-protein interactions (PPI). We next identified several distinct molecular clusters within this complex network, including neuronal differentiation, cytoskeletal, WNT-signaling and synapsins-associated pathways. Using additional bioinformatics analyses, we further identified 'central' (hub) proteins within these molecular clusters, likely key for mouse hetero-grooming behavior. Overall, a more comprehensive characterization of intricate molecular pathways linked to aberrant rodent grooming may markedly advance our understanding of underlying cellular mechanisms and related neurological disorders, eventually helping discover novel targets for their pharmacological or gene therapy interventions.

Manipulation of the nuclear envelope-associated protein SLAP during mammalian brain development affects cortical lamination and exploratory behavior

Here, we report the first characterization of the effects resulting from the manipulation of Soluble-Lamin Associated Protein (SLAP) expression during mammalian brain development. We found that SLAP localizes to the nuclear envelope and when overexpressed causes changes in nuclear morphology and lengthening of mitosis. SLAP overexpression in apical progenitors of the developing mouse brain altered asymmetric cell division, neurogenic commitment and neuronal migration ultimately resulting in unbalance in the proportion of upper, relative to deeper, neuronal layers. Several of these effects were also recapitulated upon Cas9-mediated knockdown. Ultimately, SLAP overexpression during development resulted in a reduction in subcortical projections of young mice and, notably, reduced their exploratory behavior. Our study shows the potential relevance of the previously uncharacterized nuclear envelope protein SLAP in neurodevelopmental disorders.

Neural conditional ablation of the protein tyrosine phosphatase receptor Delta PTPRD impairs gliogenesis in the developing mouse brain cortex

Neurodevelopmental disorders are characterized by alterations in the development of the cerebral cortex, including aberrant changes in the number and function of neural cells. Although neurogenesis is one of the most studied cellular processes in these pathologies, little evidence is known about glial development. Genetic association studies have identified several genes associated with neurodevelopmental disorders. Indeed, variations in the PTPRD gene have been associated with numerous brain disorders, including autism spectrum disorder, restless leg syndrome, and schizophrenia. We previously demonstrated that constitutive loss of PTPRD expression induces significant alterations in cortical neurogenesis, promoting an increase in intermediate progenitors and neurons in mice. However, its role in gliogenesis has not been evaluated. To assess this, we developed a conditional knockout mouse model lacking PTPRD expression in telencephalon cells. Here, we found that the lack of PTPRD in the mouse cortex reduces glial precursors, astrocytes, and oligodendrocytes. According to our results, this decrease in gliogenesis resulted from a reduced number of radial glia cells at gliogenesis onset and a lower gliogenic potential in cortical neural precursors due to less activation of the JAK/STAT pathway and reduced expression of gliogenic genes. Our study shows PTPRD as a regulator of the glial/neuronal balance during cortical neurodevelopment and highlights the importance of studying glial development to understand the etiology of neurodevelopmental diseases.

Effect of Oral Chondroitin Sulfate Supplementation on Acute Brain Injury in a Murine Necrotizing Enterocolitis Model

BACKGROUND

Necrotizing enterocolitis (NEC) is a devastating condition where inflammatory changes and necrosis in the gut results in activation of brain microglia and subsequent neurodevelopmental impairment. Chondroitin sulfate (CS) is a glycosaminoglycan in human breast milk that is absent in conventional formulas. We hypothesized that oral formula supplementation with CS during a murine model of experimental NEC would not only attenuate intestinal injury, but also brain injury.

STUDY DESIGN

NEC was induced in mouse pups on postnatal days (PNDs) 5 to 8. Three conditions were studied: (1) breastfed controls, (2) NEC, and (3) NEC+enteral CS (formula+200 mg/kg/d of CS). Pups were euthanized on PND 9 or reunited with dams by the evening of PND 8. Intestinal segments were H&E stained, and immunohistochemistry was performed on brain tissue for Iba-1 to assess for microglial morphology and cortical changes. Neurodevelopmental assays were performed on mice reunited with foster dams on PND 9. Single-cell RNA-sequencing analysis was performed on human intestinal epithelial cells exposed to (1) nothing, (2) hydrogen peroxide (H 2 O 2 ) alone, or (3) H 2 O 2 + CS to look at the differential gene expression between groups. Groups were compared with ANOVA or Kruskal-Wallis tests as appropriate with p < 0.05 considered significant.

RESULTS

Compared with NEC, mice treated with oral CS showed improved clinical outcomes, decreased intestinal injury, and attenuated microglial activation and deleterious cortical change. Mice with CS performed better on early neurodevelopmental assays when compared with NEC alone. Single-cell analysis of HIEC-6 cells demonstrated that CS treatment down regulated several inflammatory pathways including nuclear factor κB-suggesting an explanation for the improved Th17 intestinal cytokine profile.

CONCLUSIONS

Oral CS supplementation improved both physiological, clinical, and developmental outcomes. These data suggest that CS is a safe compound for formula supplementation for the prevention of NEC.

Srsf1 and Elavl1 act antagonistically on neuronal fate choice in the developing neocortex by controlling TrkC receptor isoform expression

The seat of higher-order cognitive abilities in mammals, the neocortex, is a complex structure, organized in several layers. The different subtypes of principal neurons are distributed in precise ratios and at specific positions in these layers and are generated by the same neural progenitor cells (NPCs), steered by a spatially and temporally specified combination of molecular cues that are incompletely understood. Recently, we discovered that an alternatively spliced isoform of the TrkC receptor lacking the kinase domain, TrkC-T1, is a determinant of the corticofugal projection neuron (CFuPN) fate. Here, we show that the finely tuned balance between TrkC-T1 and the better known, kinase domain-containing isoform, TrkC-TK+, is cell type-specific in the developing cortex and established through the antagonistic actions of two RNA-binding proteins, Srsf1 and Elavl1. Moreover, our data show that Srsf1 promotes the CFuPN fate and Elavl1 promotes the callosal projection neuron (CPN) fate in vivo via regulating the distinct ratios of TrkC-T1 to TrkC-TK+. Taken together, we connect spatio-temporal expression of Srsf1 and Elavl1 in the developing neocortex with the regulation of TrkC alternative splicing and transcript stability and neuronal fate choice, thus adding to the mechanistic and functional understanding of alternative splicing in vivo.

Layer-Specific Changes in the Prefrontal Glia/Neuron Ratio Characterizes Patches of Gene Expression Disorganization in Children with Autism

Autism spectrum disorder (ASD) is manifested by abnormal cell numbers and patches of gene expression disruption in higher-order brain regions. Here, we investigated whether layer-specific changes in glia/neuron ratios (GNR) characterize patches in the dorsolateral prefrontal cortex (DL-PFC) of children with ASD. We analyzed high-resolution digital images of postmortem human brains from 11 ASD and 11 non-ASD children obtained from the Autism Study of the Allen Human Brain Atlas. We found the GNR is overall reduced in the ASD DL-PFC. Moreover, layers II-III belonging to patches presented a lower GNR in comparison with layers V-VI. We here provide a new insight into how brain cells are arranged within patches that contributes to elucidate how neurodevelopmental programs are altered in ASD.

Neurovascular crosstalk and cerebrovascular alterations: an underestimated therapeutic target in autism spectrum disorders

Normal brain development, function, and aging critically depend on unique characteristics of the cerebrovascular system. Growing evidence indicated that cerebrovascular defects can have irreversible effects on the brain, and these defects have been implicated in various neurological disorders, including autism spectrum disorder (ASD). ASD is a neurodevelopmental disorder with heterogeneous clinical manifestations and anatomical changes. While extensive research has focused on the neural abnormalities underlying ASD, the role of brain vasculature in this disorder remains poorly understood. Indeed, the significance of cerebrovascular contributions to ASD has been consistently underestimated. In this work, we discuss the neurovascular crosstalk during embryonic development and highlight recent findings on cerebrovascular alterations in individuals with ASD. We also discuss the potential of vascular-based therapy for ASD. Collectively, these investigations demonstrate that ASD can be considered a neurovascular disease.

Dysregulation of the Wnt/β-catenin signaling pathway via Rnf146 upregulation in a VPA-induced mouse model of autism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder associated with impaired social behavior and communication, repetitive behaviors, and restricted interests. In addition to genetic factors, environmental factors such as prenatal drug exposure contribute to the development of ASD. However, how those prenatal factors induce behavioral deficits in the adult stage is not clear. To elucidate ASD pathogenesis at the molecular level, we performed a high-resolution mass spectrometry-based quantitative proteomic analysis on the prefrontal cortex (PFC) of mice exposed to valproic acid (VPA) in utero, a widely used animal model of ASD. Differentially expressed proteins (DEPs) in VPA-exposed mice showed significant overlap with ASD risk genes, including differentially expressed genes from the postmortem cortex of ASD patients. Functional annotations of the DEPs revealed significant enrichment in the Wnt/β-catenin signaling pathway, which is dysregulated by the upregulation of Rnf146 in VPA-exposed mice. Consistently, overexpressing Rnf146 in the PFC impaired social behaviors and altered the Wnt signaling pathway in adult mice. Furthermore, Rnf146-overexpressing PFC neurons showed increased excitatory synaptic transmission, which may underlie impaired social behavior. These results demonstrate that Rnf146 is critical for social behavior and that dysregulation of Rnf146 underlies social deficits in VPA-exposed mice.

In utero Exposure to Valproic Acid throughout Pregnancy Causes Phenotypes of Autism in Offspring Mice

Valproic acid (VPA) is an antiepileptic drug that inhibits the epileptic activity of neurons mainly by inhibiting sodium channels and GABA transaminase. VPA is also known to inhibit histone deacetylases, which epigenetically modify the cell proliferation/differentiation characteristics of stem/progenitor cells within developing tissues. Recent clinical studies in humans have indicated that VPA exposure in utero increases the risk of autistic features and intellectual disabilities in offspring; we have previously reported that low-dose VPA exposure in utero throughout pregnancy increases the production of projection neurons from neuronal stem/progenitor cells that are distributed in the superficial neocortical layers of the fetal brain. In the present study, we found that in utero VPA-exposed mice exhibited abnormal social interaction, changes in cognitive function, hypersensitivity to pain/heat, and impaired locomotor activity, all of which are characteristic symptoms of autism spectrum disorder in humans. Taken together, our findings indicate that VPA exposure in utero throughout pregnancy alters higher brain function and predisposes individuals to phenotypes that resemble autism and intellectual disability. Furthermore, these symptoms are likely to be due to neocortical dysgenesis that was caused by an increased number of projection neurons in specific layers of the neocortex.

An In Vivo Platform for Rebuilding Functional Neocortical Tissue

Recent progress in cortical stem cell transplantation has demonstrated its potential to repair the brain. However, current transplant models have yet to demonstrate that the circuitry of transplant-derived neurons can encode useful function to the host. This is likely due to missing cell types within the grafts, abnormal proportions of cell types, abnormal cytoarchitecture, and inefficient vascularization. Here, we devised a transplant platform for testing neocortical tissue prototypes. Dissociated mouse embryonic telencephalic cells in a liquid scaffold were transplanted into aspiration-lesioned adult mouse cortices. The donor neuronal precursors differentiated into upper and deep layer neurons that exhibited synaptic puncta, projected outside of the graft to appropriate brain areas, became electrophysiologically active within one month post-transplant, and responded to visual stimuli. Interneurons and oligodendrocytes were present at normal densities in grafts. Grafts became fully vascularized by one week post-transplant and vessels in grafts were perfused with blood. With this paradigm, we could also organize cells into layers. Overall, we have provided proof of a concept for an in vivo platform that can be used for developing and testing neocortical-like tissue prototypes.

Prenatal low-dose methylmercury exposure causes premature neuronal differentiation and autism-like behaviors in a rodent model

Aberrant neurodevelopment is a core deficit of autism spectrum disorder (ASD). Here we ask whether a non-genetic factor, prenatal exposure to the environmental pollutant methylmercury (MeHg), is a contributing factor in ASD onset. We showed that adult mice prenatally exposed to non-apoptotic MeHg exhibited key ASD characteristics, including impaired communication, reduced sociability, and increased restrictive repetitive behaviors, whereas in the embryonic cortex, prenatal MeHg exposure caused premature neuronal differentiation. Further single-cell RNA sequencing (scRNA-seq) analysis disclosed that prenatal exposure to MeHg resulted in cortical radial glial precursors (RGPs) favoring asymmetric differentiation to directly generate cortical neurons, omitting the intermediate progenitor stage. In addition, MeHg exposure in cultured RGPs increased CREB phosphorylation and enhanced the interaction between CREB and CREB binding protein (CBP). Intriguingly, metformin, an FDA-approved drug, can reverse MeHg-induced premature neuronal differentiation via CREB/CBP repulsion. These findings provide insights into ASD etiology, its underlying mechanism, and a potential therapeutic strategy.

Spontaneous Activity Predicts Survival of Developing Cortical Neurons

Spontaneous activity plays a crucial role in brain development by coordinating the integration of immature neurons into emerging cortical networks. High levels and complex patterns of spontaneous activity are generally associated with low rates of apoptosis in the cortex. However, whether spontaneous activity patterns directly encode for survival of individual cortical neurons during development remains an open question. Here, we longitudinally investigated spontaneous activity and apoptosis in developing cortical cultures, combining extracellular electrophysiology with calcium imaging. These experiments demonstrated that the early occurrence of calcium transients was strongly linked to neuronal survival. Silent neurons exhibited a higher probability of cell death, whereas high frequency spiking and burst behavior were almost exclusively detected in surviving neurons. In local neuronal clusters, activity of neighboring neurons exerted a pro-survival effect, whereas on the functional level, networks with a high modular topology were associated with lower cell death rates. Using machine learning algorithms, cell fate of individual neurons was predictable through the integration of spontaneous activity features. Our results indicate that high frequency spiking activity constrains apoptosis in single neurons through sustained calcium rises and thereby consolidates networks in which a high modular topology is reached during early development.

Genetics of glutamate and its receptors in autism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental impairment characterized by deficits in social interaction skills, impaired communication, and repetitive and restricted behaviors that are thought to be due to altered neurotransmission processes. The amino acid glutamate is an essential excitatory neurotransmitter in the human brain that regulates cognitive functions such as learning and memory, which are usually impaired in ASD. Over the last several years, increasing evidence from genetics, neuroimaging, protein expression, and animal model studies supporting the notion of altered glutamate metabolism has heightened the interest in evaluating glutamatergic dysfunction in ASD. Numerous pharmacological, behavioral, and imaging studies have demonstrated the imbalance in excitatory and inhibitory neurotransmitters, thus revealing the involvement of the glutamatergic system in ASD pathology. Here, we review the effects of genetic alterations on glutamate and its receptors in ASD and the role of non-invasive imaging modalities in detecting these changes. We also highlight the potential therapeutic targets associated with impaired glutamatergic pathways.

Coordinating cerebral cortical construction and connectivity: Unifying influence of radial progenitors

Radial progenitor development and function lay the foundation for the construction of the cerebral cortex. Radial glial scaffold, through its functions as a source of neurogenic progenitors and neuronal migration guide, is thought to provide a template for the formation of the cerebral cortex. Emerging evidence is challenging this limited view. Intriguingly, radial glial scaffold may also play a role in axonal growth, guidance, and neuronal connectivity. Radial glial cells not only facilitate the generation, placement, and allocation of neurons in the cortex but also regulate how they wire up. The organization and function of radial glial cells may thus be a unifying feature of the developing cortex that helps to precisely coordinate the right patterns of neurogenesis, neuronal placement, and connectivity necessary for the emergence of a functional cerebral cortex. This perspective critically explores this emerging view and its impact in the context of human brain development and disorders.

Integration of Spatial and Temporal Patterning in the Invertebrate and Vertebrate Nervous System

The nervous system is one of the most sophisticated animal tissues, consisting of thousands of interconnected cell types. How the nervous system develops its diversity from a few neural stem cells remains a challenging question. Spatial and temporal patterning mechanisms provide an efficient model through which diversity can be generated. The molecular mechanism of spatiotemporal patterning has been studied extensively in Drosophila melanogaster, where distinct sets of transcription factors define the spatial domains and temporal windows that give rise to different cell types. Similarly, in vertebrates, spatial domains defined by transcription factors produce different types of neurons in the brain and neural tube. At the same time, different cortical neuronal types are generated within the same cell lineage with a specific birth order. However, we still do not understand how the orthogonal information of spatial and temporal patterning is integrated into the progenitor and post-mitotic cells to combinatorially give rise to different neurons. In this review, after introducing spatial and temporal patterning in Drosophila and mice, we discuss possible mechanisms that neural progenitors may use to integrate spatial and temporal information. We finally review the functional implications of spatial and temporal patterning and conclude envisaging how small alterations of these mechanisms can lead to the evolution of new neuronal cell types.

Impaired bidirectional communication between interneurons and oligodendrocyte precursor cells affects social cognitive behavior

Cortical neural circuits are complex but very precise networks of balanced excitation and inhibition. Yet, the molecular and cellular mechanisms that form the balance are just beginning to emerge. Here, using conditional γ-aminobutyric acid receptor B1- deficient mice we identify a γ-aminobutyric acid/tumor necrosis factor superfamily member 12-mediated bidirectional communication pathway between parvalbumin-positive fast spiking interneurons and oligodendrocyte precursor cells that determines the density and function of interneurons in the developing medial prefrontal cortex. Interruption of the GABAergic signaling to oligodendrocyte precursor cells results in reduced myelination and hypoactivity of interneurons, strong changes of cortical network activities and impaired social cognitive behavior. In conclusion, glial transmitter receptors are pivotal elements in finetuning distinct brain functions.

Early postnatal interruption of the bidirectional GABA/TNFSF12 signaling between parvalbumin-positive interneurons and oligodendrocyte precursor cells impairs correct prefrontal cortical network activity and social cognitive behavior later in life.

TRIM32 Deficiency Impairs the Generation of Pyramidal Neurons in Developing Cerebral Cortex

Excitatory-inhibitory imbalance (E/I) is a fundamental mechanism underlying autism spectrum disorders (ASD). TRIM32 is a risk gene genetically associated with ASD. The absence of TRIM32 causes impaired generation of inhibitory GABAergic interneurons, neural network hyperexcitability, and autism-like behavior in mice, emphasizing the role of TRIM32 in maintaining E/I balance, but despite the description of TRIM32 in regulating proliferation and differentiation of cultured mouse neural progenitor cells (NPCs), the role of TRIM32 in cerebral cortical development, particularly in the production of excitatory pyramidal neurons, remains unknown. The present study observed that TRIM32 deficiency resulted in decreased numbers of distinct layer-specific cortical neurons and decreased radial glial cell (RGC) and intermediate progenitor cell (IPC) pool size. We further demonstrated that TRIM32 deficiency impairs self-renewal of RGCs and IPCs as indicated by decreased proliferation and mitosis. A TRIM32 deficiency also affects or influences the formation of cortical neurons. As a result, TRIM32-deficient mice showed smaller brain size. At the molecular level, RNAseq analysis indicated reduced Notch signalling in TRIM32-deficient mice. Therefore, the present study indicates a role for TRIM32 in pyramidal neuron generation. Impaired generation of excitatory pyramidal neurons may explain the hyperexcitability observed in TRIM32-deficient mice.

Integration of structural MRI and epigenetic analyses hint at linked cellular defects of the subventricular zone and insular cortex in autism: Findings from a case study

Introduction

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social interaction, communication and repetitive, restrictive behaviors, features supported by cortical activity. Given the importance of the subventricular zone (SVZ) of the lateral ventrical to cortical development, we compared molecular, cellular, and structural differences in the SVZ and linked cortical regions in specimens of ASD cases and sex and age-matched unaffected brain.

Methods

We used magnetic resonance imaging (MRI) and diffusion tractography on ex vivo postmortem brain samples, which we further analyzed by Whole Genome Bisulfite Sequencing (WGBS), Flow Cytometry, and RT qPCR.

Results

Through MRI, we observed decreased tractography pathways from the dorsal SVZ, increased pathways from the posterior ventral SVZ to the insular cortex, and variable cortical thickness within the insular cortex in ASD diagnosed case relative to unaffected controls. Long-range tractography pathways from and to the insula were also reduced in the ASD case. FACS-based cell sorting revealed an increased population of proliferating cells in the SVZ of ASD case relative to the unaffected control. Targeted qPCR assays of SVZ tissue demonstrated significantly reduced expression levels of genes involved in differentiation and migration of neurons in ASD relative to the control counterpart. Finally, using genome-wide DNA methylation analyses, we identified 19 genes relevant to neurological development, function, and disease, 7 of which have not previously been described in ASD, that were significantly differentially methylated in autistic SVZ and insula specimens.

Conclusion

These findings suggest a hypothesis that epigenetic changes during neurodevelopment alter the trajectory of proliferation, migration, and differentiation in the SVZ, impacting cortical structure and function and resulting in ASD phenotypes.

Intrauterine Viral Infections: Impact of Inflammation on Fetal Neurodevelopment

Intrauterine viral infections during pregnancy by pathogens such as Zika virus, Cytomegalovirus, Rubella and Herpes Simplex virus can lead to prenatal as well as postnatal neurodevelopmental disorders. Although maternal viral infections are common during pregnancy, viruses rarely penetrate the trophoblast. When they do cross, viruses can cause adverse congenital health conditions for the fetus. In this context, maternal inflammatory responses to these neurotropic pathogens play a significant role in negatively affecting neurodevelopment. For instance, intrauterine inflammation poses an increased risk of neurodevelopmental disorders such as microcephaly, schizophrenia, autism spectrum disorder, cerebral palsy and epilepsy. Severe inflammatory responses have been linked to stillbirths, preterm births, abortions and microcephaly. In this review, we discuss the mechanistic basis of how immune system shapes the landscape of the brain and how different neurotropic viral pathogens evoke inflammatory responses. Finally, we list the consequences of neuroinflammation on fetal brain development and discuss directions for future research and intervention strategies.

Endocannabinoid markers in autism spectrum disorder: A scoping review of human studies

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by social communication deficits and patterns of restrictive and repetitive behavior. Although the neurological underpinnings of ASD remain elusive, the endocannabinoid system (ECS) may play a role in modulating social behavior in ASD. Preclinical studies have suggested that alterations in the ECS result in ASD-like phenotypes, but currently no reviews have examined ECS abnormalities in human studies. This scoping review investigated any evidence of ECS alterations in humans with ASD. A comprehensive literature search was conducted and five studies were eligible for review. Three studies reported a significant reduction of anandamide in ASD compared to controls. Other alterations included decreased 2-arachidonoylglycerol, oleoylethanolamide, and palmitoylethanolamide and elevated diacylglycerol lipase and monoacylglycerol lipase. Some discrepant findings were also noted, which included elevated or reduced CB2 receptor in three studies and elevated or reduced N-acyl phosphatidylethanolamine phospholipase D and fatty acid amide hydrolase in two studies. We conclude from this preliminary investigation that the ECS may be altered in humans with ASD. Potential limitations of the reviewed studies include medication use and psychiatric comorbidities. Further research, such as positron emission tomography studies, are necessary to fully understand the relationship between ECS markers and ASD.

Dysregulation of Neuronal Nicotinic Acetylcholine Receptor-Cholesterol Crosstalk in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a set of complex neurodevelopmental diseases that include impaired social interaction, delayed and disordered language, repetitive or stereotypic behavior, restricted range of interests, and altered sensory processing. The underlying causes of the core symptoms remain unclear, as are the factors that trigger their onset. Given the complexity and heterogeneity of the clinical phenotypes, a constellation of genetic, epigenetic, environmental, and immunological factors may be involved. The lack of appropriate biomarkers for the evaluation of neurodevelopmental disorders makes it difficult to assess the contribution of early alterations in neurochemical processes and neuroanatomical and neurodevelopmental factors to ASD. Abnormalities in the cholinergic system in various regions of the brain and cerebellum are observed in ASD, and recently altered cholesterol metabolism has been implicated at the initial stages of the disease. Given the multiple effects of the neutral lipid cholesterol on the paradigm rapid ligand-gated ion channel, the nicotinic acetylcholine receptor, we explore in this review the possibility that the dysregulation of nicotinic receptor-cholesterol crosstalk plays a role in some of the neurological alterations observed in ASD.

Wnt/β-Catenin-Dependent Transcription in Autism Spectrum Disorders

Autism spectrum disorders (ASD) is a heterogeneous group of neurodevelopmental disorders characterized by synaptic dysfunction and defects in dendritic spine morphology. In the past decade, an extensive list of genes associated with ASD has been identified by genome-wide sequencing initiatives. Several of these genes functionally converge in the regulation of the Wnt/β-catenin signaling pathway, a conserved cascade essential for stem cell pluripotency and cell fate decisions during development. Here, we review current information regarding the transcriptional program of Wnt/β-catenin signaling in ASD. First, we discuss that Wnt/β-catenin gain and loss of function studies recapitulate brain developmental abnormalities associated with ASD. Second, transcriptomic approaches using patient-derived induced pluripotent stem cells (iPSC) cells, featuring mutations in high confidence ASD genes, reveal a significant dysregulation in the expression of Wnt signaling components. Finally, we focus on the activity of chromatin-remodeling proteins and transcription factors considered high confidence ASD genes, including CHD8, ARID1B, ADNP, and TBR1, that regulate Wnt/β-catenin-dependent transcriptional activity in multiple cell types, including pyramidal neurons, interneurons and oligodendrocytes, cells which are becoming increasingly relevant in the study of ASD. We conclude that the level of Wnt/β-catenin signaling activation could explain the high phenotypical heterogeneity of ASD and be instrumental in the development of new diagnostics tools and therapies.

Unraveling the Role of Dopaminergic and Calretinin Interneurons in the Olfactory Bulb

The perception and discriminating of odors are sensory activities that are an integral part of our daily life. The first brain region where odors are processed is the olfactory bulb (OB). Among the different cell populations that make up this brain area, interneurons play an essential role in this sensory activity. Moreover, probably because of their activity, they represent an exception compared to other parts of the brain, since OB interneurons are continuously generated in the postnatal and adult period. In this review, we will focus on periglomerular (PG) cells which are a class of interneurons found in the glomerular layer of the OB. These interneurons can be classified into distinct subtypes based on their neurochemical nature, based on the neurotransmitter and calcium-binding proteins expressed by these cells. Dopaminergic (DA) periglomerular cells and calretinin (CR) cells are among the newly generated interneurons and play an important role in the physiology of OB. In the OB, DA cells are involved in the processing of odors and the adaptation of the bulbar network to external conditions. The main role of DA cells in OB appears to be the inhibition of glutamate release from olfactory sensory fibers. Calretinin cells are probably the best morphologically characterized interneurons among PG cells in OB, but little is known about their function except for their inhibitory effect on noisy random excitatory signals arriving at the main neurons. In this review, we will mainly describe the electrophysiological properties related to the excitability profiles of DA and CR cells, with a particular view on the differences that characterize DA mature interneurons from cells in different stages of adult neurogenesis.

Biopsychological correlates of repetitive and restricted behaviors in autism spectrum disorders

BACKGROUND

Autism Spectrum Disorder (ASD) is considered a neurodevelopmental condition that is characterized by alterations in social interaction and communication, as well as patterns of restrictive and repetitive behaviors (RRBs). RRBs are defined as broad behaviors that comprise stereotypies, insistence on sameness, and attachment to objects or routines. RRBs can be divided into lower-level behaviors (motor, sensory, and object-manipulation behaviors) and higher-level behaviors (restrictive interests, insistence on sameness, and repetitive language). According to the DSM-5, the grade of severity in ASD partially depends on the frequency of RRBs and their consequences for disrupting the life of patients, affecting their adaptive skills, and increasing the need for parental support.

METHODS

We conducted a systematic review to examine the biopsychological correlates of the symptomatic domains of RRBs according to the type of RRBs (lower- or higher-level). We searched for articles from the National Library of Medicine (PubMed) using the terms: autism spectrum disorders, ASD, and autism-related to executive functions, inhibitory control, inflexibility, cognitive flexibility, hyper or hypo connectivity, and behavioral approaches. For describing the pathophysiological mechanism of ASD, we also included animal models and followed PRISMA guidelines.

RESULTS

One hundred and thirty-one articles were analyzed to explain the etiology, continuance, and clinical evolution of these behaviors observed in ASD patients throughout life.

CONCLUSIONS

Biopsychological correlates involved in the origin of RRBs include alterations in a) neurotransmission system, b) brain volume, c) inadequate levels of growth factors, d) hypo- or hyper-neural connectivity, e) impairments in behavioral inhibition, cognitive flexibility, and monitoring and f) non-stimulating environments. Understanding these lower- and higher-level of RRBs can help professionals to improve or design novel therapeutic strategies.

Innate immunity at the crossroads of healthy brain maturation and neurodevelopmental disorders

The immune and nervous systems have unique developmental trajectories that individually build intricate networks of cells with highly specialized functions. These two systems have extensive mechanistic overlap and frequently coordinate to accomplish the proper growth and maturation of an organism. Brain resident innate immune cells - microglia - have the capacity to sculpt neural circuitry and coordinate copious and diverse neurodevelopmental processes. Moreover, many immune cells and immune-related signalling molecules are found in the developing nervous system and contribute to healthy neurodevelopment. In particular, many components of the innate immune system, including Toll-like receptors, cytokines, inflammasomes and phagocytic signals, are critical contributors to healthy brain development. Accordingly, dysfunction in innate immune signalling pathways has been functionally linked to many neurodevelopmental disorders, including autism and schizophrenia. This review discusses the essential roles of microglia and innate immune signalling in the assembly and maintenance of a properly functioning nervous system.

Neuronal and glial cell number is altered in a cortical layer-specific manner in autism

LAY ABSTRACT

The cerebral cortex affected with autism spectrum disorder presents changes in the number of neurons and glia cells, possibly leading to a dysregulation of brain circuits and affecting behavior. However, little is known about cell number alteration in specific layers of the cortex in autism spectrum disorder. We found an increase in the number of neurons and a decrease in the number of astrocytes in specific layers of the prefrontal cortex in postmortem human brains from autism spectrum disorder cases. We hypothesize that this may be due to a failure in neural stem cells to shift differentiation from neurons to glial cells during prenatal brain development. These data provide key anatomical findings that contribute to the bases of autism spectrum disorder pathogenesis.

Exposure to sevoflurane results in changes of transcription factor occupancy in sperm and inheritance of autism

One in 54 children in the U.S. is diagnosed with Autism Spectrum Disorder (ASD). De novo germline and somatic mutations cannot account for all cases of ASD, suggesting that epigenetic alterations triggered by environmental exposures may be responsible for a subset of ASD cases. Human and animal studies have shown that exposure of the developing brain to general anesthetic (GA) agents can trigger neurodegeneration and neurobehavioral abnormalities but the effects of general anesthetics on the germ line have not been explored in detail. We exposed pregnant mice to sevoflurane during the time of embryonic development when the germ cells undergo epigenetic reprogramming and found that more than 38% of the directly exposed F1 animals exhibit impairments in anxiety and social interactions. Strikingly, 44-47% of the F2 and F3 animals, which were not directly exposed to sevoflurane, show the same behavioral problems. We performed ATAC-seq and identified more than 1200 differentially accessible sites in the sperm of F1 animals, 69 of which are also present in the sperm of F2 animals. These sites are located in regulatory regions of genes strongly associated with ASD, including Arid1b, Ntrk2, and Stmn2. These findings suggest that epimutations caused by exposing germ cells to sevoflurane can lead to ASD in the offspring, and this effect can be transmitted through the male germline inter and trans-generationally.

Autism Spectrum Disorder: Signaling Pathways and Prospective Therapeutic Targets

The Autism Spectrum Disorder (ASD) consists of a prevalent and heterogeneous group of neurodevelopmental diseases representing a severe burden to affected individuals and their caretakers. Despite substantial improvement towards understanding of ASD etiology and pathogenesis, as well as increased social awareness and more intensive research, no effective drugs have been successfully developed to resolve the main and most cumbersome ASD symptoms. Hence, finding better treatments, which may act as "disease-modifying" agents, and novel biomarkers for earlier ASD diagnosis and disease stage determination are needed. Diverse mutations of core components and consequent malfunctions of several cell signaling pathways have already been found in ASD by a series of experimental platforms, including genetic associations analyses and studies utilizing pre-clinical animal models and patient samples. These signaling cascades govern a broad range of neurological features such as neuronal development, neurotransmission, metabolism, and homeostasis, as well as immune regulation and inflammation. Here, we review the current knowledge on signaling pathways which are commonly disrupted in ASD and autism-related conditions. As such, we further propose ways to translate these findings into the development of genetic and biochemical clinical tests for early autism detection. Moreover, we highlight some putative druggable targets along these pathways, which, upon further research efforts, may evolve into novel therapeutic interventions for certain ASD conditions. Lastly, we also refer to the crosstalk among these major signaling cascades as well as their putative implications in therapeutics. Based on this collective information, we believe that a timely and accurate modulation of these prominent pathways may shape the neurodevelopment and neuro-immune regulation of homeostatic patterns and, hopefully, rescue some (if not all) ASD phenotypes.

Dysregulation of Multiple Signaling Neurodevelopmental Pathways during Embryogenesis: A Possible Cause of Autism Spectrum Disorder

Understanding the autistic brain and the involvement of genetic, non-genetic, and numerous signaling pathways in the etiology and pathophysiology of autism spectrum disorder (ASD) is complex, as is evident from various studies. Apart from multiple developmental disorders of the brain, autistic subjects show a few characteristics like impairment in social communications related to repetitive, restricted, or stereotypical behavior, which suggests alterations in neuronal circuits caused by defects in various signaling pathways during embryogenesis. Most of the research studies on ASD subjects and genetic models revealed the involvement of mutated genes with alterations of numerous signaling pathways like Wnt, hedgehog, and Retinoic Acid (RA). Despite significant improvement in understanding the pathogenesis and etiology of ASD, there is an increasing awareness related to it as well as a need for more in-depth research because no effective therapy has been developed to address ASD symptoms. Therefore, identifying better therapeutic interventions like "novel drugs for ASD" and biomarkers for early detection and disease condition determination are required. This review article investigated various etiological factors as well as the signaling mechanisms and their alterations to understand ASD pathophysiology. It summarizes the mechanism of signaling pathways, their significance, and implications for ASD.

Is Cannabidiol During Neurodevelopment a Promising Therapy for Schizophrenia and Autism Spectrum Disorders?

Schizophrenia and autism spectrum disorders (ASD) are psychiatric neurodevelopmental disorders that cause high levels of functional disabilities. Also, the currently available therapies for these disorders are limited. Therefore, the search for treatments that could be beneficial for the altered course of the neurodevelopment associated with these disorders is paramount. Preclinical and clinical evidence points to cannabidiol (CBD) as a promising strategy. In this review, we discuss clinical and preclinical studies on schizophrenia and ASD investigating the behavioral, molecular, and functional effects of chronic treatment with CBD (and with cannabidivarin for ASD) during neurodevelopment. In summary, the results point to CBD's beneficial potential for the progression of these disorders supporting further investigations to strengthen its use.

Stick around: Cell-Cell Adhesion Molecules during Neocortical Development

The neocortex is an exquisitely organized structure achieved through complex cellular processes from the generation of neural cells to their integration into cortical circuits after complex migration processes. During this long journey, neural cells need to establish and release adhesive interactions through cell surface receptors known as cell adhesion molecules (CAMs). Several types of CAMs have been described regulating different aspects of neurodevelopment. Whereas some of them mediate interactions with the extracellular matrix, others allow contact with additional cells. In this review, we will focus on the role of two important families of cell-cell adhesion molecules (C-CAMs), classical cadherins and nectins, as well as in their effectors, in the control of fundamental processes related with corticogenesis, with special attention in the cooperative actions among the two families of C-CAMs.

Endocannabinoid signalling in stem cells and cerebral organoids drives differentiation to deep layer projection neurons via CB1 receptors

The endocannabinoid (eCB) system, via the cannabinoid CB₁ receptor, regulates neurodevelopment by controlling neural progenitor proliferation and neurogenesis. CB₁ receptor signalling in vivo drives corticofugal deep layer projection neuron development through the regulation of BCL11B and SATB2 transcription factors. Here, we investigated the role of eCB signalling in mouse pluripotent embryonic stem cell-derived neuronal differentiation. Characterization of the eCB system revealed increased expression of eCB-metabolizing enzymes, eCB ligands and CB₁ receptors during neuronal differentiation. CB₁ receptor knockdown inhibited neuronal differentiation of deep layer neurons and increased upper layer neuron generation, and this phenotype was rescued by CB₁ re-expression. Pharmacological regulation with CB₁ receptor agonists or elevation of eCB tone with a monoacylglycerol lipase inhibitor promoted neuronal differentiation of deep layer neurons at the expense of upper layer neurons. Patch-clamp analyses revealed that enhancing cannabinoid signalling facilitated neuronal differentiation and functionality. Noteworthy, incubation with CB₁ receptor agonists during human iPSC-derived cerebral organoid formation also promoted the expansion of BCL11B⁺ neurons. These findings unveil a cell-autonomous role of eCB signalling that, via the CB₁ receptor, promotes mouse and human deep layer cortical neuron development.

Examining volumetric gradients based on the frustum surface ratio in the brain in autism spectrum disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder that is accompanied by neurodevelopmental differences in regional cortical volume (CV), and a potential layer‐specific pathology. Conventional measures of CV, however, do not indicate how volume is distributed across cortical layers. In a sample of 92 typically developing (TD) controls and 92 adult individuals with ASD (aged 18–52 years), we examined volumetric gradients by quantifying the degree to which CV is weighted from the pial to the white surface of the brain. Overall, the spatial distribution of Frustum Surface Ratio (FSR) followed the gyral and sulcal pattern of the cortex and approximated a bimodal Gaussian distribution caused by a linear mixture of vertices on gyri and sulci. Measures of FSR were highly correlated with vertex‐wise estimates of mean curvature, sulcal depth, and pial surface area, although none of these features explained more than 76% variability in FSR on their own. Moreover, in ASD, we observed a pattern of predominant increases in the degree of FSR relative to TD controls, with an atypical neurodevelopmental trajectory. Our findings suggest a more outward‐weighted gradient of CV in ASD, which may indicate a larger contribution of supragranular layers to regional differences in CV.

Shifting Developmental Trajectories During Critical Periods of Brain Formation

Critical periods of brain development are epochs of heightened plasticity driven by environmental influence necessary for normal brain function. Recent studies are beginning to shed light on the possibility that timely interventions during critical periods hold potential to reorient abnormal developmental trajectories in animal models of neurological and neuropsychiatric disorders. In this review, we re-examine the criteria defining critical periods, highlighting the recently discovered mechanisms of developmental plasticity in health and disease. In addition, we touch upon technological improvements for modeling critical periods in human-derived neural networks in vitro. These scientific advances associated with the use of developmental manipulations in the immature brain of animal models are the basic preclinical systems that will allow the future translatability of timely interventions into clinical applications for neurodevelopmental disorders such as intellectual disability, autism spectrum disorders (ASD) and schizophrenia.

Region and Cell Type Distribution of TCF4 in the Postnatal Mouse Brain

Transcription factor 4 is a class I basic helix-loop-helix transcription factor regulating gene expression. Altered TCF4 gene expression has been linked to non-syndromic intellectual disability, schizophrenia, and a severe neurodevelopmental disorder known as Pitt-Hopkins syndrome. An understanding of the cell types expressing TCF4 protein in the mouse brain is needed to help identify potential pathophysiological mechanisms and targets for therapeutic delivery in TCF4-linked disorders. Here we developed a novel green fluorescent protein reporter mouse to visualize TCF4-expressing cells throughout the brain. Using this TCF4 reporter mouse, we observed prominent expression of TCF4 in the pallial region and cerebellum of the postnatal brain. At the cellular level, both glutamatergic and GABAergic neurons express TCF4 in the cortex and hippocampus, while only a subset of GABAergic interneurons express TCF4 in the striatum. Among glial cell groups, TCF4 is present in astrocytes and immature and mature oligodendrocytes. In the cerebellum, cells in the granule and molecular layer express TCF4. Our findings greatly extend our knowledge of the spatiotemporal and cell type-specific expression patterns of TCF4 in the brain, and hence, lay the groundwork to better understand TCF4-linked neurological disorders. Any effort to restore TCF4 functions through small molecule or genetic therapies should target these brain regions and cell groups to best recapitulate TCF4 expression patterns.

Genetics of structural and functional brain changes in autism spectrum disorder

Autism spectrum disorder (ASD) is a neurological and developmental disorder characterized by social impairment and restricted interactive and communicative behaviors. It may occur as an isolated disorder or in the context of other neurological, psychiatric, developmental, and genetic disorders. Due to rapid developments in genomics and imaging technologies, imaging genetics studies of ASD have evolved in the last few years. Increased risk for ASD diagnosis is found to be related to many specific single-nucleotide polymorphisms, and the study of genetic mechanisms and noninvasive imaging has opened various approaches that can help diagnose ASD at the nascent level. Identifying risk genes related to structural and functional changes in the brain of ASD patients provide a better understanding of the disease's neuropsychiatry and can help identify targets for therapeutic intervention that could be useful for the clinical management of ASD patients.

STAT3 Regulates Mouse Neural Progenitor Proliferation and Differentiation by Promoting Mitochondrial Metabolism

The proliferation and differentiation of neural progenitor lay the foundation for brain development. In neural progenitors, activation of Signal Transducer and Activator of Transcription 3 (STAT3) has been found to promote proliferation and astrocytogenesis while suppressing neurogenesis. However, our study found that Stat3 conditional knockout in neural progenitors (Stat3 cKO) also results in increased proliferation and suppressed neurogenesis. To investigate how STAT3 regulates these processes, we attempted to identify potential STAT3 target genes by RNA-seq profiling of the control (CTL) and Stat3 cKO neural progenitors. We found that STAT3 promotes the expression of genes involved in the mitochondrial oxidative phosphorylation (OXPHOS), and thereby promotes mitochondrial respiration and negatively regulates reactive oxygen species (ROS) production. In addition, we demonstrated that Stat3 loss-of-function promotes proliferation via regulation of mitochondrial metabolism and downstream signaling pathways. Our study provides novel insights into the relation between STAT3, mitochondrial metabolism and the process of embryonic neurogenesis.

Prenatal Origins of ASD: The When, What, and How of ASD Development

Autism spectrum disorder (ASD) is a largely heritable, multistage prenatal disorder that impacts a child's ability to perceive and react to social information. Most ASD risk genes are expressed prenatally in many ASD-relevant brain regions and fall into two categories: broadly expressed regulatory genes that are expressed in the brain and other organs, and brain-specific genes. In trimesters one to three (Epoch-1), one set of broadly expressed (the majority) and brain-specific risk genes disrupts cell proliferation, neurogenesis, migration, and cell fate, while in trimester three and early postnatally (Epoch-2) another set (the majority being brain specific) disrupts neurite outgrowth, synaptogenesis, and the 'wiring' of the cortex. A proposed model is that upstream, highly interconnected regulatory ASD gene mutations disrupt transcriptional programs or signaling pathways resulting in dysregulation of downstream processes such as proliferation, neurogenesis, synaptogenesis, and neural activity. Dysregulation of signaling pathways is correlated with ASD social symptom severity. Since the majority of ASD risk genes are broadly expressed, many ASD individuals may benefit by being treated as having a broader medical disorder. An important future direction is the noninvasive study of ASD cell biology.

Distinct Developmental Features of Olfactory Bulb Interneurons

The olfactory bulb (OB) has an extremely higher proportionof interneurons innervating excitatory neurons than otherbrain regions, which is evolutionally conserved across species.Despite the abundance of OB interneurons, little is knownabout the diversification and physiological functions ofOB interneurons compared to cortical interneurons. In thisreview, an overview of the general developmental processof interneurons from the angles of the spatial and temporalspecifications was presented. Then, the distinct featuresshown exclusively in OB interneurons development andmolecular machinery recently identified were discussed.Finally, we proposed an evolutionary meaning for thediversity of OB interneurons.

The Neurodevelopment of Autism from Infancy Through Toddlerhood

Autism spectrum disorder (ASD) emerges during early childhood and is marked by a relatively narrow window in which infants transition from exhibiting normative behavioral profiles to displaying the defining features of the ASD phenotype in toddlerhood. Prospective brain imaging studies in infants at high familial risk for autism have revealed important insights into the neurobiology and developmental unfolding of ASD. In this article, we review neuroimaging studies of brain development in ASD from birth through toddlerhood, relate these findings to candidate neurobiological mechanisms, and discuss implications for future research and translation to clinical practice.

Constructing multilayered neural networks with sparse, data-driven connectivity using biologically-inspired, complementary, homeostatic mechanisms

The immense complexity of the brain requires that it be built and controlled by intrinsic, self-regulating mechanisms. One such mechanism, the formation of new connections via synaptogenesis, plays a central role in neuronal connectivity and, ultimately, performance. Adaptive synaptogenesis networks combine synaptogenesis, associative synaptic modification, and synaptic shedding to construct sparse networks. Here, inspired by neuroscientific observations, novel aspects of brain development are incorporated into adaptive synaptogenesis. The extensions include: (i) multiple layers, (ii) neuron survival and death based on information transmission, and (iii) bigrade growth factor signaling to control the onset of synaptogenesis in succeeding layers and to control neuron survival and death in preceding layers. Also guiding this research is the assumption that brains must achieve a compromise between good performance and low energy expenditures. Simulations of the network model demonstrate the parametric and functional control of both performance and energy expenditures, where performance is measured in terms of information loss and classification errors, and energy expenditures are assumed to be a monotonically increasing function of the number of neurons. Major insights from this study include (a) the key role a neural layer between two other layers has in controlling synaptogenesis and neuron elimination, (b) the performance and energy-savings benefits of delaying the onset of synaptogenesis in a succeeding layer, and (c) how the elimination of neurons in a preceding layer provides energy savings, code compression, and can be accomplished without significantly degrading information transfer or classification performance.

Genetic Suppression of mTOR Rescues Synaptic and Social Behavioral Abnormalities in a Mouse Model of Pten Haploinsufficiency

Heterozygous mutations in PTEN, which encodes a negative regulator of the mTOR and β-catenin signaling pathways, cause macrocephaly/autism syndrome. However, the neurobiological substrates of the core symptoms of this syndrome are poorly understood. Here, we investigate the relationship between cerebral cortical overgrowth and social behavior deficits in conditional Pten heterozygous female mice (Pten cHet) using Emx1-Cre, which is expressed in cortical pyramidal neurons and a subset of glia. We found that conditional heterozygous mutation of Ctnnb1 (encoding β-catenin) suppresses Pten cHet cortical overgrowth, but not social behavioral deficits, whereas conditional heterozygous mutation of Mtor suppresses social behavioral deficits, but not cortical overgrowth. Neuronal activity in response to social cues and excitatory synapse markers are elevated in the medial prefrontal cortex (mPFC) of Pten cHet mice, and heterozygous mutation in Mtor, but not Ctnnb1, rescues these phenotypes. These findings indicate that macroscale cerebral cortical overgrowth and social behavioral phenotypes caused by Pten haploinsufficiency can be dissociated based on responsiveness to genetic suppression of Ctnnb1 or Mtor. Furthermore, neuronal connectivity appears to be one potential substrate for mTOR-mediated suppression of social behavioral deficits in Pten haploinsufficient mice. Autism Res 2019, 12: 1463-1471. © 2019 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: A subgroup of individuals with autism display overgrowth of the head and the brain during development. Using a mouse model of an autism risk gene, Pten, that displays both brain overgrowth and social behavioral deficits, we show here that that these two symptoms can be dissociated. Reversal of social behavioral deficits in this model is associated with rescue of abnormal synaptic markers and neuronal activity.

PDK1 Regulates Transition Period of Apical Progenitors to Basal Progenitors by Controlling Asymmetric Cell Division

The six-layered neocortex consists of diverse neuron subtypes. Deeper-layer neurons originate from apical progenitors (APs), while upper-layer neurons are mainly produced by basal progenitors (BPs), which are derivatives of APs. As development proceeds, an AP generates two daughter cells that comprise an AP and a deeper-layer neuron or a BP. How the transition of APs to BPs is spatiotemporally regulated is a fundamental question. Here, we report that conditional deletion of phoshpoinositide-dependent protein kinase 1 (PDK1) in mouse developing cortex achieved by crossing Emx1Cre line with Pdk1fl/fl leads to a delayed transition of APs to BPs and subsequently causes an increased output of deeper-layer neurons. We demonstrate that PDK1 is involved in the modulation of the aPKC-Par3 complex and further regulates the asymmetric cell division (ACD). We also find Hes1, a downstream effecter of Notch signal pathway is obviously upregulated. Knockdown of Hes1 or treatment with Notch signal inhibitor DAPT recovers the ACD defect in the Pdk1 cKO. Thus, we have identified a novel function of PDK1 in controlling the transition of APs to BPs.

Disruption of TCF4 regulatory networks leads to abnormal cortical development and mental disabilities

The TCF4 gene is the subject of numerous and varied investigations of it's role in the genesis of neuropsychiatric disease. The gene has been identified as the cause of Pitt-Hopkins syndrome (PTHS) and it has been implicated in various other neuropsychiatric diseases, including schizophrenia, depression, and autism. However, the precise molecular mechanisms of the gene's involvement in neurogenesis, particularly, corticogenesis, are not well understood. Here, we present data showing that TCF4 is expressed in a region-specific manner in the radial glia and stem cells of transient embryonic zones at early gestational ages in both humans and mice. TCF4 haploinsufficiency mice exhibit a delay in neuronal migration, and a significant increase in the number of upper-layer cortical neurons, as well as abnormal dendrite and synapse formation. Our research also reveals that TCF3 up-regulates Tcf4 by binding to the specific "E-box" and its flank sequence in intron 2 of the Tcf4 gene. Additionally, our transcriptome study substantiates that Tcf4 transcriptional function is essential for locomotion, cognition, and learning. By activating expression of TCF4 in the regulation of neuronal proliferation and migration to the overlaying neocortex and subsequent differentiation leading to laminar formation TCF4 fulfills its normal function, but if not, abnormalities such as those reported here result. These findings provide new insight into the specific roles of Tcf4 molecular pathway in neocortical development and their relevance in the pathogenesis of neuropsychiatric diseases.

Baker S. Non-invasive assessment of superficial and deep layer circuits in human motor cortex

KEY POINTS

The first indirect (I) corticospinal volley from stimulation of the motor cortex consists of two parts: one that originates from infragranular layer 5 and a subsequent part with a delay of 0.6 ms to which supragranular layers contribute. Non-invasive probing of these two parts was performed in humans using a refined electrophysiological method involving transcranial magnetic stimulation and peripheral nerve stimulation. Activity modulation of these two parts during a sensorimotor discrimination task was consistent with previous results in monkeys obtained with laminar recordings.

ABSTRACT

Circuits in superficial and deep layers play distinct roles in cortical computation, but current methods to study them in humans are limited. Here, we developed a novel approach for non-invasive assessment of layer-specific activity in the human motor cortex. We first conducted brain slice and in vivo experiments on monkey motor cortex to investigate the output timing from layer 5 (including corticospinal neurons) following extracellular stimulation. Neuron responses contained cyclical waves. The first wave was composed of two parts: the earliest part originated only from stimulation of layer 5; after 0.6 ms, stimuli to superficial layers 2/3 could also contribute. In healthy humans we then assessed different parts of the first corticospinal volley elicited by transcranial magnetic stimulation (TMS), by interacting TMS with stimulation of the median nerve generating an H-reflex. By adjusting the delay between stimuli, we could assess the earliest volley evoked by TMS, and the part 0.6 ms later. Measurements were made while subjects performed a visuo-motor discrimination task, which has been previously shown in monkey to modulate superficial motor cortical cells selectively depending on task difficulty. We showed a similar selective modulation of the later part of the TMS volley, as expected if this part of the volley is sensitive to superficial cortical excitability. We conclude that it is possible to segregate different cortical circuits which may refer to different motor cortex layers in humans, by exploiting small time differences in the corticospinal volleys evoked by non-invasive stimulation.

Identifying Windows of Susceptibility by Temporal Gene Analysis

Increased understanding of developmental disorders of the brain has shown that genetic mutations, environmental toxins and biological insults typically act during developmental windows of susceptibility. Identifying these vulnerable periods is a necessary and vital step for safeguarding women and their fetuses against disease causing agents during pregnancy and for developing timely interventions and treatments for neurodevelopmental disorders. We analyzed developmental time-course gene expression data derived from human pluripotent stem cells, with disease association, pathway, and protein interaction databases to identify windows of disease susceptibility during development and the time periods for productive interventions. The results are displayed as interactive Susceptibility Windows Ontological Transcriptome (SWOT) Clocks illustrating disease susceptibility over developmental time. Using this method, we determine the likely windows of susceptibility for multiple neurological disorders using known disease associated genes and genes derived from RNA-sequencing studies including autism spectrum disorder, schizophrenia, and Zika virus induced microcephaly. SWOT clocks provide a valuable tool for integrating data from multiple databases in a developmental context with data generated from next-generation sequencing to help identify windows of susceptibility.

Loss of Satb2 in the Cortex and Hippocampus Leads to Abnormal Behaviors in Mice

Satb2-associated syndrome (SAS) is a genetic disorder that results from the deletion or mutation of one allele within the Satb2 locus. Patients with SAS show behavioral abnormalities, including developmental delay/intellectual disability, hyperactivity, and symptoms of autism. To address the role of Satb2 in SAS-related behaviors and generate an SAS mouse model, Satb2 was deleted in the cortex and hippocampus of Emx1-Cre; Satb2^flox/flox [Satb2 conditional knockout (CKO)] mice. Satb2 CKO mice showed hyperactivity, increased impulsivity, abnormal social novelty, and impaired spatial learning and memory. Furthermore, we also found that the development of neurons in cortical layer IV was defective in Satb2 CKO mice, as shown by the loss of layer-specific gene expression and abnormal thalamocortical projections. In summary, the abnormal behaviors revealed in Satb2 CKO mice may reflect the SAS symptoms associated with Satb2 mutation in human patients, possibly due to defective development of cortical neurons in multiple layers including alterations of their inputs/outputs.