Gene expression in cortical interneuron precursors is prescient of their mature function

GABA system as the cause and effect in early development

GABA is the primary inhibitory neurotransmitter in the adult brain and through its actions on GABAARs, it protects against excitotoxicity and seizure activity, ensures temporal fidelity of neurotransmission, and regulates concerted rhythmic activity of neuronal populations. In the developing brain, the development of GABAergic neurons precedes that of glutamatergic neurons and the GABA system serves as a guide and framework for the development of other brain systems. Despite this early start, the maturation of the GABA system also continues well into the early postnatal period. In this review, we organize evidence around two scenarios based on the essential and protracted nature of GABA system development: 1) disruptions in the development of the GABA system can lead to large scale disruptions in other developmental processes (i.e., GABA as the cause), 2) protracted maturation of this system makes it vulnerable to the effects of developmental insults (i.e., GABA as the effect). While ample evidence supports the importance of GABA/GABAAR system in both scenarios, large gaps in existing knowledge prevent strong mechanistic conclusions.

Spatial enhancer activation influences inhibitory neuron identity during mouse embryonic development

The mammalian telencephalon contains distinct GABAergic projection neuron and interneuron types, originating in the germinal zone of the embryonic basal ganglia. How genetic information in the germinal zone determines cell types is unclear. Here we use a combination of in vivo CRISPR perturbation, lineage tracing and ChIP-sequencing analyses and show that the transcription factor MEIS2 favors the development of projection neurons by binding enhancer regions in projection-neuron-specific genes during mouse embryonic development. MEIS2 requires the presence of the homeodomain transcription factor DLX5 to direct its functional activity toward the appropriate binding sites. In interneuron precursors, the transcription factor LHX6 represses the MEIS2-DLX5-dependent activation of projection-neuron-specific enhancers. Mutations of Meis2 result in decreased activation of regulatory enhancers, affecting GABAergic differentiation. We propose a differential binding model where the binding of transcription factors at cis-regulatory elements determines differential gene expression programs regulating cell fate specification in the mouse ganglionic eminence.

Connectivity and molecular profiles of Foxp2- and Dbx1-lineage neurons in the accessory olfactory bulb and medial amygdala

In terrestrial vertebrates, the olfactory system is divided into main (MOS) and accessory (AOS) components that process both volatile and nonvolatile cues to generate appropriate behavioral responses. While much is known regarding the molecular diversity of neurons that comprise the MOS, less is known about the AOS. Here, focusing on the vomeronasal organ (VNO), the accessory olfactory bulb (AOB), and the medial amygdala (MeA), we reveal that populations of neurons in the AOS can be molecularly subdivided based on their ongoing or prior expression of the transcription factors Foxp2 or Dbx1, which delineate separate populations of GABAergic output neurons in the MeA. We show that a majority of AOB neurons that project directly to the MeA are of the Foxp2 lineage. Using single-neuron patch-clamp electrophysiology, we further reveal that in addition to sex-specific differences across lineage, the frequency of excitatory input to MeA Dbx1- and Foxp2-lineage neurons differs between sexes. Together, this work uncovers a novel molecular diversity of AOS neurons, and lineage and sex differences in patterns of connectivity.

Transcriptional maintenance of cortical somatostatin interneuron subtype identity during migration

Although cardinal cortical interneuron identity is established upon cell-cycle exit, it remains unclear whether specific interneuron subtypes are pre-established, and if so, how their identity is maintained prior to circuit integration. We conditionally removed Sox6 (Sox6-cKO) in migrating somatostatin (Sst+) interneurons and assessed the effects on their mature identity. In adolescent mice, five of eight molecular Sst+ subtypes were nearly absent in the Sox6-cKO cortex without a reduction in cell number. Sox6-cKO cells displayed electrophysiological maturity and expressed genes enriched within the broad class of Sst+ interneurons. Furthermore, we could infer subtype identity prior to cortical integration (embryonic day 18.5), suggesting that the loss in subtype was due to disrupted subtype maintenance. Conversely, Sox6 removal at postnatal day 7 did not disrupt marker expression in the mature cortex. Therefore, Sox6 is necessary during migration for maintenance of Sst+ subtype identity, indicating that subtype maintenance requires active transcriptional programs.

Human pallial MGE-type GABAergic interneuron cell therapy for chronic focal epilepsy

Mesial temporal lobe epilepsy (MTLE) is the most common focal epilepsy. One-third of patients have drug-refractory seizures and are left with suboptimal therapeutic options such as brain tissue-destructive surgery. Here, we report the development and characterization of a cell therapy alternative for drug-resistant MTLE, which is derived from a human embryonic stem cell line and comprises cryopreserved, post-mitotic, medial ganglionic eminence (MGE) pallial-type GABAergic interneurons. Single-dose intrahippocampal delivery of the interneurons in a mouse model of chronic MTLE resulted in consistent mesiotemporal seizure suppression, with most animals becoming seizure-free and surviving longer. The grafted interneurons dispersed locally, functionally integrated, persisted long term, and significantly reduced dentate granule cell dispersion, a pathological hallmark of MTLE. These disease-modifying effects were dose-dependent, with a broad therapeutic range. No adverse effects were observed. These findings support an ongoing phase 1/2 clinical trial (NCT05135091) for drug-resistant MTLE.

Chloride transporters controlling neuronal excitability

Synaptic inhibition plays a crucial role in regulating neuronal excitability, which is the foundation of nervous system function. This inhibition is largely mediated by the neurotransmitters GABA and glycine that activate Cl^--permeable ion channels, which means that the strength of inhibition depends on the Cl^- gradient across the membrane. In neurons, the Cl^- gradient is primarily mediated by two secondarily active cation-chloride cotransporters (CCCs), NKCC1 and KCC2. CCC-mediated regulation of the neuronal Cl^- gradient is critical for healthy brain function, as dysregulation of CCCs has emerged as a key mechanism underlying neurological disorders including epilepsy, neuropathic pain, and autism spectrum disorder. This review begins with an overview of neuronal chloride transporters before explaining the dependent relationship between these CCCs, Cl^- regulation, and inhibitory synaptic transmission. We then discuss the evidence for how CCCs can be regulated, including by activity and their protein interactions, which underlie inhibitory synaptic plasticity. For readers who may be interested in conducting experiments on CCCs and neuronal excitability, we have included a section on techniques for estimating and recording intracellular Cl^-, including their advantages and limitations. Although the focus of this review is on neurons, we also examine how Cl^- is regulated in glial cells, which in turn regulate neuronal excitability through the tight relationship between this nonneuronal cell type and synapses. Finally, we discuss the relatively extensive and growing literature on how CCC-mediated neuronal excitability contributes to neurological disorders.

DANCR promotes glioma cell autophagy and proliferation via the miR‑33b/DLX6/ATG7 axis

Long non‑coding RNAs (lncRNAs) are common in the human body. Misregulated lncRNA expression can cause a variety of diseases in the human body. The present study aimed to investigate the effect of lncRNA differentiation antagonizing non‑protein‑coding RNA (DANCR) on glioma proliferation and autophagy through the microRNA (miR)‑33b/distal‑less homeobox 6 (DLX6)/autophagy‑related 7 (ATG7) axis. Reverse transcription‑quantitative PCR was used to detect DANCR and miR‑33b expression. Cell Counting Kit‑8 assay and flow cytometry were used to detect cell proliferation and apoptosis, respectively. Transmission electron microscopy was used to determine the autophagy level by observing intracellular autophagosomes. A western blot assay was used to detect protein expression levels and determine the level of autophagy in different cells. The binding sites of miR‑33b and DANCR or DLX6 were detected using a dual‑luciferase reporter assay. A chromatin immunoprecipitation assay confirmed DLX6 as a transcript of ATG7. In vivo tumorigenesis of glioma cells was validated in nude mice. DANCR and DLX6 were highly expressed in glioma cells, while miR‑33b showed low expression in glioma cells. DANCR reduced the targeted binding of miR‑33b to DLX6 by sponging miR‑33b. The result verified that DANCR could promote ATG7 protein expression through miR‑33b/DLX6, promote intracellular autophagy and proliferation and reduce apoptosis. The present study identified the role of the DANCR/miR‑33b/DLX6/ATG7 axis in regulating autophagy, proliferation, and apoptosis in glioma cells, providing new ideas for glioma treatment.

Epigenetic regulation of GABAergic differentiation in the developing brain

In the vertebrate brain, GABAergic cell development and neurotransmission are important for the establishment of neural circuits. Various intrinsic and extrinsic factors have been identified to affect GABAergic neurogenesis. However, little is known about the epigenetic control of GABAergic differentiation in the developing brain. Here, we report that the number of GABAergic neurons dynamically changes during the early tectal development in the Xenopus brain. The percentage of GABAergic neurons is relatively unchanged during the early stages from stage 40 to 46 but significantly decreased from stage 46 to 48 tadpoles. Interestingly, the histone acetylation of H3K9 is developmentally decreased from stage 42 to 48 (about 3.5 days). Chronic application of valproate acid (VPA), a broad-spectrum histone deacetylase (HDAC) inhibitor, at stage 46 for 48 h increases the acetylation of H3K9 and the number of GABAergic cells in the optic tectum. VPA treatment also reduces apoptotic cells. Electrophysiological recordings show that a VPA induces an increase in the frequency of mIPSCs and no changes in the amplitude. Behavioral studies reveal that VPA decreases swimming activity and visually guided avoidance behavior. These findings extend our understanding of histone modification in the GABAergic differentiation and neurotransmission during early brain development.

MTG8 interacts with LHX6 to specify cortical interneuron subtype identity

Cortical interneurons originating in the embryonic medial ganglionic eminence (MGE) diverge into a range of different subtypes found in the adult mouse cerebral cortex. The mechanisms underlying this divergence and the timing when subtype identity is set up remain unclear. We identify the highly conserved transcriptional co-factor MTG8 as being pivotal in the development of a large subset of MGE cortical interneurons that co-expresses Somatostatin (SST) and Neuropeptide Y (NPY). MTG8 interacts with the pan-MGE transcription factor LHX6 and together the two factors are sufficient to promote expression of critical cortical interneuron subtype identity genes. The SST-NPY cortical interneuron fate is initiated early, well before interneurons migrate into the cortex, demonstrating an early onset specification program. Our findings suggest that transcriptional co-factors and modifiers of generic lineage specification programs may hold the key to the emergence of cortical interneuron heterogeneity from the embryonic telencephalic germinal zones.

There is a large diversity of inhibitory interneurons in the mammalian cerebral cortex. How this emerges during embryogenesis remains unclear. Here, the authors identify MTG8 as a co-factor of LHX6 and a new regulator of cortical interneuron development.

Transcriptional heterogeneity of ventricular zone cells in the ganglionic eminences of the mouse forebrain

The ventricular zone (VZ) of the nervous system contains radial glia cells that were originally considered relatively homogenous in their gene expression, but a detailed characterization of transcriptional diversity in these VZ cells has not been reported. Here, we performed single-cell RNA sequencing to characterize transcriptional heterogeneity of neural progenitors within the VZ and subventricular zone (SVZ) of the ganglionic eminences (GEs), the source of all forebrain GABAergic neurons. By using a transgenic mouse line to enrich for VZ cells, we characterize significant transcriptional heterogeneity, both between GEs and within spatial subdomains of specific GEs. Additionally, we observe differential gene expression between E12.5 and E14.5 VZ cells, which could provide insights into temporal changes in cell fate. Together, our results reveal a previously unknown spatial and temporal genetic diversity of VZ cells in the ventral forebrain that will aid our understanding of initial fate decisions in the forebrain.

Gabaergic Interneurons in Early Brain Development: Conducting and Orchestrated by Cortical Network Activity

Throughout early phases of brain development, the two main neural signaling mechanisms—excitation and inhibition—are dynamically sculpted in the neocortex to establish primary functions. Despite its relatively late formation and persistent developmental changes, the GABAergic system promotes the ordered shaping of neuronal circuits at the structural and functional levels. Within this frame, interneurons participate first in spontaneous and later in sensory-evoked activity patterns that precede cortical functions of the mature brain. Upon their subcortical generation, interneurons in the embryonic brain must first orderly migrate to and settle in respective target layers before they can actively engage in cortical network activity. During this process, changes at the molecular and synaptic level of interneurons allow not only their coordinated formation but also the pruning of connections as well as excitatory and inhibitory synapses. At the postsynaptic site, the shift of GABAergic signaling from an excitatory towards an inhibitory response is required to enable synchronization within cortical networks. Concomitantly, the progressive specification of different interneuron subtypes endows the neocortex with distinct local cortical circuits and region-specific modulation of neuronal firing. Finally, the apoptotic process further refines neuronal populations by constantly maintaining a controlled ratio of inhibitory and excitatory neurons. Interestingly, many of these fundamental and complex processes are influenced—if not directly controlled—by electrical activity. Interneurons on the subcellular, cellular, and network level are affected by high frequency patterns, such as spindle burst and gamma oscillations in rodents and delta brushes in humans. Conversely, the maturation of interneuron structure and function on each of these scales feeds back and contributes to the generation of cortical activity patterns that are essential for the proper peri- and postnatal development. Overall, a more precise description of the conducting role of interneurons in terms of how they contribute to specific activity patterns—as well as how specific activity patterns impinge on their maturation as orchestra members—will lead to a better understanding of the physiological and pathophysiological development and function of the nervous system.

Neurotransmitter signaling regulates distinct phases of multimodal human interneuron migration

Inhibitory GABAergic interneurons migrate over long distances from their extracortical origin into the developing cortex. In humans, this process is uniquely slow and prolonged, and it is unclear whether guidance cues unique to humans govern the various phases of this complex developmental process. Here, we use fused cerebral organoids to identify key roles of neurotransmitter signaling pathways in guiding the migratory behavior of human cortical interneurons. We use scRNAseq to reveal expression of GABA, glutamate, glycine, and serotonin receptors along distinct maturation trajectories across interneuron migration. We develop an image analysis software package, TrackPal, to simultaneously assess 48 parameters for entire migration tracks of individual cells. By chemical screening, we show that different modes of interneuron migration depend on distinct neurotransmitter signaling pathways, linking transcriptional maturation of interneurons with their migratory behavior. Altogether, our study provides a comprehensive quantitative analysis of human interneuron migration and its functional modulation by neurotransmitter signaling.

Maternal P7C3-A20 Treatment Protects Offspring from Neuropsychiatric Sequelae of Prenatal Stress

Aims: Impaired embryonic cortical interneuron development from prenatal stress is linked to adult neuropsychiatric impairment, stemming in part from excessive generation of reactive oxygen species in the developing embryo. Unfortunately, there are no preventive medicines that mitigate the risk of prenatal stress to the embryo, as the underlying pathophysiologic mechanisms are poorly understood. Our goal was to interrogate the molecular basis of prenatal stress-mediated damage to the embryonic brain to identify a neuroprotective strategy. Results: Chronic prenatal stress in mice dysregulated nicotinamide adenine dinucleotide (NAD⁺) synthesis enzymes and cortical interneuron development in the embryonic brain, leading to axonal degeneration in the hippocampus, cognitive deficits, and depression-like behavior in adulthood. Offspring were protected from these deleterious effects by concurrent maternal administration of the NAD⁺-modulating agent P7C3-A20, which crossed the placenta to access the embryonic brain. Prenatal stress also produced axonal degeneration in the adult corpus callosum, which was not prevented by maternal P7C3-A20. Innovation: Prenatal stress dysregulates gene expression of NAD⁺-synthesis machinery and GABAergic interneuron development in the embryonic brain, which is associated with adult cognitive impairment and depression-like behavior. We establish a maternally directed treatment that protects offspring from these effects of prenatal stress. Conclusion: NAD⁺-synthesis machinery and GABAergic interneuron development are critical to proper embryonic brain development underlying postnatal neuropsychiatric functioning, and these systems are highly susceptible to prenatal stress. Pharmacologic stabilization of NAD⁺ in the stressed embryonic brain may provide a neuroprotective strategy that preserves normal embryonic development and protects offspring from neuropsychiatric impairment. Antioxid. Redox Signal. 35, 511-530.

Single-cell analysis of the ventricular-subventricular zone reveals signatures of dorsal & ventral adult neurogenesis

The ventricular-subventricular zone (V-SVZ), on the walls of the lateral ventricles, harbors the largest neurogenic niche in the adult mouse brain. Previous work has shown that neural stem/progenitor cells (NSPCs) in different locations within the V-SVZ produce different subtypes of new neurons for the olfactory bulb. The molecular signatures that underlie this regional heterogeneity remain largely unknown. Here, we present a single-cell RNA-sequencing dataset of the adult mouse V-SVZ revealing two populations of NSPCs that reside in largely non-overlapping domains in either the dorsal or ventral V-SVZ. These regional differences in gene expression were further validated using a single-nucleus RNA-sequencing reference dataset of regionally microdissected domains of the V-SVZ and by immunocytochemistry and RNAscope localization. We also identify two subpopulations of young neurons that have gene expression profiles consistent with a dorsal or ventral origin. Interestingly, a subset of genes are dynamically expressed, but maintained, in the ventral or dorsal lineages. The study provides novel markers and territories to understand the region-specific regulation of adult neurogenesis.

Nerve cells, or neurons, are the central building blocks of brain circuits. Their damage, death or loss of function leads to cognitive decline. Neural stem/progenitor cells (NSPCs) first appear during embryo development, generating most of the neurons found in the nervous system. However, the adult brain retains a small subpopulation of NSPCs, which in some species are an important source of new neurons throughout life.

In the adult mouse brain the largest population of NSPCs, known as B cells, is found in an area called the ventricular-subventricular zone (V-SVZ). These V-SVZ B cells have properties of specialized support cells known as astrocytes, but they can also divide and generate intermediate ‘progenitor cells’ called C cells. These, in turn, divide to generate large numbers of young ‘A cells’ neurons that undertake a long and complex migration from V-SVZ to the olfactory bulb, the first relay in the central nervous system for the processing of smells.

Depending on their location in the V-SVZ, B cells can generate different kinds of neurons, leading to at least ten subtypes of neurons. Why this is the case is still poorly understood.

To examine this question, Cebrián-Silla, Nascimento, Redmond, Mansky et al. determined which genes were expressed in B, C and A cells from different parts of the V-SVZ. While cells within each of these populations had different expression patterns, those that originated in the same V-SVZ locations shared a set of genes, many of which associated with regional specification in the developing brain. Some, however, were intriguingly linked to hormonal regulation.

Salient differences between B cells depended on whether the cells originated closer to the top (‘dorsal’ position) or to the bottom of the brain (‘ventral’ position). This information was used to stain slices of mouse brains for the RNA and proteins produced by these genes in different regions. These experiments revealed dorsal and ventral territories containing B cells with distinct ‘gene expression’.

This study highlights the heterogeneity of NSPCs, revealing key molecular differences among B cells in dorsal and ventral areas of the V-SVZ and reinforcing the concept that the location of NSPCs determines the types of neuron they generate. Furthermore, the birth of specific types of neurons from B cells that are so strictly localized highlights the importance of neuronal migration to ensure that young neurons with specific properties reach their appropriate destination in the olfactory bulb.

The work by Cebrián-Silla, Nascimento, Redmond, Mansky et al. has identified sets of genes that are differentially expressed in dorsal and ventral regions which may contribute to regional regulation. Furthering the understanding of how adult NSPCs differ according to their location will help determine how various neuron types emerge in the adult brain.

Cell-Type-Specific Gene Expression in Developing Mouse Neocortex: Intermediate Progenitors Implicated in Axon Development

Cerebral cortex projection neurons (PNs) are generated from intermediate progenitors (IPs), which are in turn derived from radial glial progenitors (RGPs). To investigate developmental processes in IPs, we profiled IP transcriptomes in embryonic mouse neocortex, using transgenic Tbr2-GFP mice, cell sorting, and microarrays. These data were used in combination with in situ hybridization to ascertain gene sets specific for IPs, RGPs, PNs, interneurons, and other neural and non-neural cell types. RGP-selective transcripts (n = 419) included molecules for Notch receptor signaling, proliferation, neural stem cell identity, apical junctions, necroptosis, hippo pathway, and NF-κB pathway. RGPs also expressed specific genes for critical interactions with meningeal and vascular cells. In contrast, IP-selective genes (n = 136) encoded molecules for activated Delta ligand presentation, epithelial-mesenchymal transition, core planar cell polarity (PCP), axon genesis, and intrinsic excitability. Interestingly, IPs expressed several “dependence receptors” (Unc5d, Dcc, Ntrk3, and Epha4) that induce apoptosis in the absence of ligand, suggesting a competitive mechanism for IPs and new PNs to detect key environmental cues or die. Overall, our results imply a novel role for IPs in the patterning of neuronal polarization, axon differentiation, and intrinsic excitability prior to mitosis. Significantly, IPs highly express Wnt-PCP, netrin, and semaphorin pathway molecules known to regulate axon polarization in other systems. In sum, IPs not only amplify neurogenesis quantitatively, but also molecularly “prime” new PNs for axogenesis, guidance, and excitability.

Disruption of Transient SERT Expression in Thalamic Glutamatergic Neurons Alters Trajectory of Postnatal Interneuron Development in the Mouse Cortex

In mice, terminal differentiation of subpopulations of interneurons occurs in late postnatal stages, paralleling the emergence of the adult cortical architecture. Here, we investigated the effects of altered initial cortical architecture on later interneuron development. We identified that a class of somatostatin (SOM)-expressing GABAergic interneurons undergoes terminal differentiation between 2nd and 3rd postnatal week in the mouse somatosensory barrel cortex and upregulates Reelin expression during neurite outgrowth. Our previous work demonstrated that transient expression (E15-P10) of serotonin uptake transporter (SERT) in thalamocortical projection neurons regulates barrel elaboration during cortical map establishment. We show here that in thalamic neuron SERT knockout mice, these SOM-expressing interneurons develop at the right time, reach correct positions and express correct neurochemical markers, but only 70% of the neurons remain in the adult barrel cortex. Moreover, those neurons that remain display altered dendritic patterning. Our data indicate that a precise architecture at the cortical destination is not essential for specifying late-developing interneuron identities, their cortical deposition, and spatial organization, but dictates their number and dendritic structure ultimately integrated into the cortex. Our study illuminates how disruption of temporal-specific SERT function and related key regulators during cortical map establishment can alter interneuron development trajectory that persists to adult central nervous system.

Genetic Mechanisms Underlying Cortical Evolution in Mammals

The remarkable sensory, motor, and cognitive abilities of mammals mainly depend on the neocortex. Thus, the emergence of the six-layered neocortex in reptilian ancestors of mammals constitutes a fundamental evolutionary landmark. The mammalian cortex is a columnar epithelium of densely packed cells organized in layers where neurons are generated mainly in the subventricular zone in successive waves throughout development. Newborn cells move away from their site of neurogenesis through radial or tangential migration to reach their specific destination closer to the pial surface of the same or different cortical area. Interestingly, the genetic programs underlying neocortical development diversified in different mammalian lineages. In this work, I will review several recent studies that characterized how distinct transcriptional programs relate to the development and functional organization of the neocortex across diverse mammalian lineages. In some primates such as the anthropoids, the neocortex became extremely large, especially in humans where it comprises around 80% of the brain. It has been hypothesized that the massive expansion of the cortical surface and elaboration of its connections in the human lineage, has enabled our unique cognitive capacities including abstract thinking, long-term planning, verbal language and elaborated tool making capabilities. I will also analyze the lineage-specific genetic changes that could have led to the modification of key neurodevelopmental events, including regulation of cell number, neuronal migration, and differentiation into specific phenotypes, in order to shed light on the evolutionary mechanisms underlying the diversity of mammalian brains including the human brain.

N-type calcium channels control GABAergic transmission in brain areas related to fear and anxiety

N-type (CaV2.2) calcium channels are key for action potential-evoked transmitter release in the peripheral and central nervous system. Previous studies have highlighted the functional relevance of N-type calcium channels at both the peripheral and central level. In the periphery, the N-type calcium channels regulate nociceptive and sympathetic responses. At the central level, N-type calcium channels have been linked to aggression, hyperlocomotion, and anxiety. Among the areas of the brain that are involved in anxiety are the basolateral amygdala, medial prefrontal cortex, and ventral hippocampus. These three areas share similar characteristics in their neuronal circuitry, where pyramidal projection neurons are under the inhibitory control of a wide array of interneurons including those that express the peptide cholecystokinin. This type of interneuron is well-known to rely on N-type calcium channels to release GABA in the hippocampus, however, whether these channels control GABA release from cholecystokinin-expressing interneurons in the basolateral amygdala and medial prefrontal cortex is not known. Here, using mouse models to genetically label cholecystokinin-expressing interneurons and electrophysiology, we found that in the basolateral amygdala, N-type calcium channels control ~50% of GABA release from these neurons onto pyramidal cells. By contrast, in the medial prefrontal cortex N-type calcium channels are functionally absent in synapses of cholecystokinin-expressing interneurons, but control ~40% of GABA release from other types of interneurons. Our findings provide insights into the precise localization of N-type calcium channels in interneurons of brain areas related to anxiety.

Generation of cerebral cortical GABAergic interneurons from pluripotent stem cells

The cerebral cortex functions by the complex interactions of intrinsic and extrinsic neuronal activities, glial actions, and the effects of humoral factors. The intrinsic neuronal influences are mediated by two major subclasses: excitatory glutamatergic neurons that generally have axonal projections extending beyond the neuron's locality and inhibitory GABAergic neurons that generally project locally. These interneurons can be grouped based on morphological, neurochemical, electrophysiological, axonal targeting, and circuit influence characteristics. Cortical interneurons (CIns) can also be grouped based on their origins within the subcortical telencephalon. Interneuron subtypes, of which a dozen or more are thought to exist, are characterized by combinations of these subgrouping features. Due to their well-documented relevance to the causes of and treatments for neuropsychiatric disorders, and to their remarkable capacity to migrate extensively following transplantation, there has been tremendous interest in generating cortical GABAergic interneurons from human pluripotent stem cells. In this concise review, we discuss recent progress in understanding how interneuron subtypes are generated in vivo, and how that progress is being applied to the generation of rodent and human CIns in vitro. In addition, we will discuss approaches for the rigorous designation of interneuron subgroups or subtypes in transplantation studies, and challenges to this field, including the protracted maturation of human interneurons.

Transcriptomic and epigenomic dynamics associated with development of human iPSC-derived GABAergic interneurons

GABAergic interneurons (GINs) are a heterogeneous population of inhibitory neurons that collectively contribute to the maintenance of normal neuronal excitability and network activity. Identification of the genetic regulatory elements and transcription factors that contribute toward GIN function may provide new insight into the pathways underlying proper GIN activity while also indicating potential therapeutic targets for GIN-associated disorders, such as schizophrenia and epilepsy. In this study, we examined the temporal changes in gene expression and chromatin accessibility during GIN development by performing transcriptomic and epigenomic analyses on human induced pluripotent stem cell-derived neurons at 22, 50 and 78 days (D) post-differentiation. We observed 13 221 differentially accessible regions (DARs) of chromatin that associate with temporal changes in gene expression at D78 and D50, relative to D22. We also classified families of transcription factors that are increasingly enriched at DARs during differentiation, indicating regulatory networks that likely drive GIN development. Collectively, these data provide a resource for examining the molecular networks regulating GIN functionality.

ALK4 coordinates extracellular and intrinsic signals to regulate development of cortical somatostatin interneurons

Göngrich et al. show that the activin receptor ALK4 is a key regulator of the specification of somatostatin interneurons. They find that intrinsic transcriptional programs interact with extracellular signals present in the environment of MGE cells to regulate cortical interneuron specification.

Although the role of transcription factors in fate specification of cortical interneurons is well established, how these interact with extracellular signals to regulate interneuron development is poorly understood. Here we show that the activin receptor ALK4 is a key regulator of the specification of somatostatin interneurons. Mice lacking ALK4 in GABAergic neurons of the medial ganglionic eminence (MGE) showed marked deficits in distinct subpopulations of somatostatin interneurons from early postnatal stages of cortical development. Specific losses were observed among distinct subtypes of somatostatin⁺/Reelin⁺ double-positive cells, including Hpse⁺ layer IV cells targeting parvalbumin⁺ interneurons, leading to quantitative alterations in the inhibitory circuitry of this layer. Activin-mediated ALK4 signaling in MGE cells induced interaction of Smad2 with SATB1, a transcription factor critical for somatostatin interneuron development, and promoted SATB1 nuclear translocation and repositioning within the somatostatin gene promoter. These results indicate that intrinsic transcriptional programs interact with extracellular signals present in the environment of MGE cells to regulate cortical interneuron specification.

Multifaceted actions of Zeb2 in postnatal neurogenesis from the ventricular-subventricular zone to the olfactory bulb

The transcription factor Zeb2 controls fate specification and subsequent differentiation and maturation of multiple cell types in various embryonic tissues. It binds many protein partners, including activated Smad proteins and the NuRD co-repressor complex. How Zeb2 subdomains support cell differentiation in various contexts has remained elusive. Here, we studied the role of Zeb2 and its domains in neurogenesis and neural differentiation in the young postnatal ventricular-subventricular zone (V-SVZ), in which neural stem cells generate olfactory bulb-destined interneurons. Conditional Zeb2 knockouts and separate acute loss- and gain-of-function approaches indicated that Zeb2 is essential for controlling apoptosis and neuronal differentiation of V-SVZ progenitors before and after birth, and we identified Sox6 as a potential downstream target gene of Zeb2. Zeb2 genetic inactivation impaired the differentiation potential of the V-SVZ niche in a cell-autonomous fashion. We also provide evidence that its normal function in the V-SVZ also involves non-autonomous mechanisms. Additionally, we demonstrate distinct roles for Zeb2 protein-binding domains, suggesting that Zeb2 partners co-determine neuronal output from the mouse V-SVZ in both quantitative and qualitative ways in early postnatal life.

Single-cell transcriptomic evidence for dense intracortical neuropeptide networks

Seeking new insights into the homeostasis, modulation and plasticity of cortical synaptic networks, we have analyzed results from a single-cell RNA-seq study of 22,439 mouse neocortical neurons. Our analysis exposes transcriptomic evidence for dozens of molecularly distinct neuropeptidergic modulatory networks that directly interconnect all cortical neurons. This evidence begins with a discovery that transcripts of one or more neuropeptide precursor (NPP) and one or more neuropeptide-selective G-protein-coupled receptor (NP-GPCR) genes are highly abundant in all, or very nearly all, cortical neurons. Individual neurons express diverse subsets of NP signaling genes from palettes encoding 18 NPPs and 29 NP-GPCRs. These 47 genes comprise 37 cognate NPP/NP-GPCR pairs, implying the likelihood of local neuropeptide signaling. Here, we use neuron-type-specific patterns of NP gene expression to offer specific, testable predictions regarding 37 peptidergic neuromodulatory networks that may play prominent roles in cortical homeostasis and plasticity.

Dlx5 and Dlx6 expression in GABAergic neurons controls behavior, metabolism, healthy aging and lifespan

Dlx5 and Dlx6 encode two homeobox transcription factors expressed by developing and mature GABAergic interneurons. During development, Dlx5/6 play a role in the differentiation of certain GABAergic subclasses. Here we address the question of the functional role of Dlx5/6 in the mature central nervous system. First, we demonstrate that Dlx5 and Dlx6 are expressed by all subclasses of adult cortical GABAergic neurons. Then we analyze VgatΔDlx5-6 mice in which Dlx5 and Dlx6 are simultaneously inactivated in all GABAergic interneurons. VgatΔDlx5-6 mice present a behavioral pattern suggesting reduction of anxiety-like behavior and obsessive-compulsive activities, and a lower interest in nest building. Twenty-month-old VgatΔDlx5-6 animals have the same size as their normal littermates, but present a 25% body weight reduction associated with a marked decline in white and brown adipose tissue. Remarkably, both VgatΔDlx5-6/+ and VgatΔDlx5-6 mice present a 33% longer median survival. Hallmarks of biological aging such as motility, adiposity and coat conditions are improved in mutant animals. Our data imply that GABAergic interneurons can regulate healthspan and lifespan through Dlx5/6-dependent mechanisms. Understanding these regulations can be an entry point to unravel the processes through which the brain affects body homeostasis and, ultimately, longevity and healthy aging.

The Chromatin Environment Around Interneuron Genes in Oligodendrocyte Precursor Cells and Their Potential for Interneuron Reprograming

Oligodendrocyte precursor cells (OPCs), also known as NG2 glia, arise from neural progenitor cells in the embryonic ganglionic eminences that also generate inhibitory neurons. They are ubiquitously distributed in the central nervous system, remain proliferative through life, and generate oligodendrocytes in both gray and white matter. OPCs exhibit some lineage plasticity, and attempts have been made to reprogram them into neurons, with varying degrees of success. However, little is known about how epigenetic mechanisms affect the ability of OPCs to undergo fate switch and whether OPCs have a unique chromatin environment around neuronal genes that might contribute to their lineage plasticity. Our bioinformatic analysis of histone posttranslational modifications at interneuron genes in OPCs revealed that OPCs had significantly fewer bivalent and repressive histone marks at interneuron genes compared to astrocytes or fibroblasts. Conversely, OPCs had a greater degree of deposition of active histone modifications at bivalently marked interneuron genes than other cell types, and this was correlated with higher expression levels of these genes in OPCs. Furthermore, a significantly higher proportion of interneuron genes in OPCs than in other cell types lacked the histone posttranslational modifications examined. These genes had a moderately high level of expression, suggesting that the "no mark" interneuron genes could be in a transcriptionally "poised" or "transitional" state. Thus, our findings suggest that OPCs have a unique histone code at their interneuron genes that may obviate the need for erasure of repressive marks during their fate switch to inhibitory neurons.

Activity Regulates Cell Death within Cortical Interneurons through a Calcineurin-Dependent Mechanism

We demonstrate that cortical interneurons derived from ventral eminences, including the caudal ganglionic eminence, undergo programmed cell death. Moreover, with the exception of VIP interneurons, this occurs in a manner that is activity-dependent. In addition, we demonstrate that, within interneurons, Calcineurin, a calcium-dependent protein phosphatase, plays a critical role in sequentially linking activity to maturation (E15-P5) and survival (P5-P20). Specifically, embryonic inactivation of Calcineurin results in a failure of interneurons to morphologically mature and prevents them from undergoing apoptosis. By contrast, early postnatal inactivation of Calcineurin increases apoptosis. We conclude that Calcineurin serves a dual role of promoting first the differentiation of interneurons and, subsequently, their survival.

A new mouse model of ARX dup24 recapitulates the patients' behavioral and fine motor alterations

The aristaless-related homeobox (ARX) transcription factor is involved in the development of GABAergic and cholinergic neurons in the forebrain. ARX mutations have been associated with a wide spectrum of neurodevelopmental disorders in humans, among which the most frequent, a 24 bp duplication in the polyalanine tract 2 (c.428_451dup24), gives rise to intellectual disability, fine motor defects with or without epilepsy. To understand the functional consequences of this mutation, we generated a partially humanized mouse model carrying the c.428_451dup24 duplication (Arx^dup24/0) that we characterized at the behavior, neurological and molecular level. Arx^dup24/0 males presented with hyperactivity, enhanced stereotypies and altered contextual fear memory. In addition, Arx^dup24/0 males had fine motor defects with alteration of reaching and grasping abilities. Transcriptome analysis of Arx^dup24/0 forebrains at E15.5 showed a down-regulation of genes specific to interneurons and an up-regulation of genes normally not expressed in this cell type, suggesting abnormal interneuron development. Accordingly, interneuron migration was altered in the cortex and striatum between E15.5 and P0 with consequences in adults, illustrated by the defect in the inhibitory/excitatory balance in Arx^dup24/0 basolateral amygdala. Altogether, we showed that the c.428_451dup24 mutation disrupts Arx function with a direct consequence on interneuron development, leading to hyperactivity and defects in precise motor movement control and associative memory. Interestingly, we highlighted striking similarities between the mouse phenotype and a cohort of 33 male patients with ARX c.428_451dup24, suggesting that this new mutant mouse line is a good model for understanding the pathophysiology and evaluation of treatment.

Deletion of Semaphorin 3F in Interneurons Is Associated with Decreased GABAergic Neurons, Autism-like Behavior, and Increased Oxidative Stress Cascades

Autism and epilepsy are diseases which have complex genetic inheritance. Genome-wide association and other genetic studies have implicated at least 500+ genes associated with the occurrence of autism spectrum disorders (ASD) including the human semaphorin 3F (Sema 3F) and neuropilin 2 (NRP2) genes. However, the genetic basis of the comorbid occurrence of autism and epilepsy is unknown. The aberrant development of GABAergic circuitry is a possible risk factor in autism and epilepsy. Molecular biological approaches were used to test the hypothesis that cell-specific genetic variation in mouse homologs affects the formation and function of GABAergic circuitry. The empirical analysis with mice homozygous null for one of these genes, Sema 3F, in GABAergic neurons substantiated these predictions. Notably, deletion of Sema 3F in interneurons but not excitatory neurons during early development decreased the number of interneurons/neurites and mRNAs for cell-specific GABAergic markers and increased epileptogenesis and autistic behaviors. Studies of interneuron cell-specific knockout of Sema 3F signaling suggest that deficient Sema 3F signaling may lead to neuroinflammation and oxidative stress. Further studies of mouse KO models of ASD genes such as Sema 3F or NRP2 may be informative to clinical phenotypes contributing to the pathogenesis in autism and epilepsy patients.

Homochronic Transplantation of Interneuron Precursors into Early Postnatal Mouse Brains

Neuronal fate determination and maturation requires an intricate interplay between genetic programs and environmental signals. However, disentangling the roles of intrinsic vs. extrinsic mechanisms that regulate this differentiation process is a conundrum for all developmental neurobiologists. This issue is magnified for GABAergic interneurons, an incredibly heterogeneous cell population that is born from transient embryonic structures and undergo a protracted migratory phase to disperse throughout the telencephalon. To explore how different brain environments affect interneuron fate and maturation, we developed a protocol for harvesting fluorescently labeled immature interneuron precursors from specific brain regions in newborn mice (P0-P2). At this age, interneuron migration is nearly complete and these cells are residing in their final resting environments with relatively little synaptic integration. Following collection of single cell solutions via flow cytometry, these interneuron precursors are transplanted into P0-P2 wildtype postnatal pups. By performing both homotopic (e.g., cortex-to-cortex) or heterotopic (e.g., cortex-to-hippocampus) transplantations, one can assess how challenging immature interneurons in new brain environments affects their fate, maturation, and circuit integration. Brains can be harvested in adult mice and assayed with a wide variety of posthoc analysis on grafted cells, including immunohistochemical, electrophysiological and transcriptional profiling. This general approach provides investigators with a strategy to assay how distinct brain environments can influence numerous aspects of neuron development and identify if specific neuronal characteristics are primarily driven by hardwired genetic programs or environmental cues.

Identification of genes regulating GABAergic interneuron maturation

During embryonic development, GABAergic interneurons, a main inhibitory component in the cerebral cortex, migrate tangentially from the ganglionic eminence (GE) to cerebral cortex. After reaching the cerebral cortex, they start to extend their neurites for constructing local neuronal circuits around the neonatal stage. Aberrations in migration or neurite outgrowth are implicated in neurological and psychiatric disorders such as epilepsy, schizophrenia and autism. Previous studies revealed that in the early phase of cortical development the neural population migrates tangentially from the GE in the telencephalon and several genes have been characterized as regulators of migration and specification of GABAergic interneurons. However, much less is known about the molecular mechanisms of GABAergic interneurons-specific maturation at later stages of development. Here, we performed genome-wide screening to identify genes related to the later stage by flow cytometry based-microarray (FACS-array) and identified 247 genes expressed in cortical GABAergic interneurons. Among them, Dgkg, a member of diacylglycerol kinase family, was further analyzed. Correlational analysis revealed that Dgkg is dominantly expressed in somatostatin (SST)-expressing GABAergic interneurons. The functional study of Dgkg using GE neurons indicated alteration in neurite outgrowth of GABAergic neurons. This study shows a new functional role for Dgkg in GABAergic interneurons as well as the identification of other candidate genes for their maturation.

Default Patterning Produces Pan-cortical Glutamatergic and CGE/LGE-like GABAergic Neurons from Human Pluripotent Stem Cells

Default differentiation of human pluripotent stem cells has been promoted as a model of cortical development. In this study, a developmental transcriptome analysis of default-differentiated hPSNs revealed a gene expression program resembling in vivo CGE/LGE subpallial domains and GABAergic signaling. A combination of bioinformatic, functional, and immunocytochemical analysis further revealed that hPSNs consist of both cortical glutamatergic and CGE-like GABAergic neurons. This study provides a comprehensive characterization of the heterogeneous group of neurons produced by default differentiation and insight into future directed differentiation strategies.

Transcriptional Architecture of Synaptic Communication Delineates GABAergic Neuron Identity

Understanding the organizational logic of neural circuits requires deciphering the biological basis of neuronal diversity and identity, but there is no consensus on how neuron types should be defined. We analyzed single-cell transcriptomes of a set of anatomically and physiologically characterized cortical GABAergic neurons and conducted a computational genomic screen for transcriptional profiles that distinguish them from one another. We discovered that cardinal GABAergic neuron types are delineated by a transcriptional architecture that encodes their synaptic communication patterns. This architecture comprises 6 categories of ∼40 gene families, including cell-adhesion molecules, transmitter-modulator receptors, ion channels, signaling proteins, neuropeptides and vesicular release components, and transcription factors. Combinatorial expression of select members across families shapes a multi-layered molecular scaffold along the cell membrane that may customize synaptic connectivity patterns and input-output signaling properties. This molecular genetic framework of neuronal identity integrates cell phenotypes along multiple axes and provides a foundation for discovering and classifying neuron types.

INTERNEURONOPATHIES AND THEIR ROLE IN EARLY LIFE EPILEPSIES AND NEURODEVELOPMENTAL DISORDERS

GABAergic interneurons control the neural circuitry and network activity in the brain. The advances in genetics have identified genes that control the development, maturation and integration of GABAergic interneurons and implicated them in the pathogenesis of epileptic encephalopathies or neurodevelopmental disorders. For example, mutations of the Aristaless-Related homeobox X-linked gene (ARX) may result in defective GABAergic interneuronal migration in infants with epileptic encephalopathies like West syndrome (WS), Ohtahara syndrome or X-linked lissencephaly with abnormal genitalia (XLAG). The concept of "interneuronopathy", i.e. impaired development, migration or function of interneurons, has emerged as a possible etiopathogenic mechanism for epileptic encephalopathies. Treatments that enhance GABA levels, may help seizure control but do not necessarily show disease modifying effect. On the other hand, interneuronopathies can be seen in other conditions in which epilepsy may not be the primary manifestation, such as autism. In this review, we plan to outline briefly the current state of knowledge on the origin, development, and migration and integration of GABAergic interneurons, present neurodevelopmental conditions, with or without epilepsy, that have been associated with interneuronopathies and discuss the evidence linking certain types of interneuronal dysfunction with epilepsy and/or cognitive or behavioral deficits.

Assembly of functionally integrated human forebrain spheroids

The development of the nervous system involves a coordinated succession of events including the migration of GABAergic neurons from ventral to dorsal forebrain and their integration into cortical circuits. However, these interregional interactions have not yet been modelled with human cells. Here, we generate from human pluripotent cells three-dimensional spheroids resembling either the dorsal or ventral forebrain and containing cortical glutamatergic or GABAergic neurons. These subdomain-specific forebrain spheroids can be assembled to recapitulate the saltatory migration of interneurons similar to migration in fetal forebrain. Using this system, we find that in Timothy syndrome– a neurodevelopmental disorder that is caused by mutations in the Ca_V1.2 calcium channel, interneurons display abnormal migratory saltations. We also show that after migration, interneurons functionally integrate with glutamatergic neurons to form a microphysiological system. We anticipate that this approach will be useful for studying development and disease, and for deriving spheroids that resemble other brain regions to assemble circuits in vitro.

Single-cell RNA sequencing identifies distinct mouse medial ganglionic eminence cell types

Many subtypes of cortical interneurons (CINs) are found in adult mouse cortices, but the mechanism generating their diversity remains elusive. We performed single-cell RNA sequencing on the mouse embryonic medial ganglionic eminence (MGE), the major birthplace for CINs, and on MGE-like cells differentiated from embryonic stem cells. Two distinct cell types were identified as proliferating neural progenitors and immature neurons, both of which comprised sub-populations. Although lineage development of MGE progenitors was reconstructed and immature neurons were characterized as GABAergic, cells that might correspond to precursors of different CINs were not identified. A few non-neuronal cell types were detected, including microglia. In vitro MGE-like cells resembled bona fide MGE cells but expressed lower levels of Foxg1 and Epha4. Together, our data provide detailed understanding of the embryonic MGE developmental program and suggest how CINs are specified.

Neuronal activity controls the development of interneurons in the somatosensory cortex

BACKGROUND

Neuronal activity in cortical areas regulates neurodevelopment by interacting with defined genetic programs to shape the mature central nervous system. Electrical activity is conveyed to sensory cortical areas via intracortical and thalamocortical neurons, and includes oscillatory patterns that have been measured across cortical regions.

OBJECTIVE

In this work, we review the most recent findings about how electrical activity shapes the developmental assembly of functional circuitry in the somatosensory cortex, with an emphasis on interneuron maturation and integration. We include studies on the effect of various neurotransmitters and on the influence of thalamocortical afferent activity on circuit development. We additionally reviewed studies describing network activity patterns.

METHODS

We conducted an extensive literature search using both the PubMed and Google Scholar search engines. The following keywords were used in various iterations: "interneuron", "somatosensory", "development", "activity", "network patterns", "thalamocortical", "NMDA receptor", "plasticity". We additionally selected papers known to us from past reading, and those recommended to us by reviewers and members of our lab.

RESULTS

We reviewed a total of 132 articles that focused on the role of activity in interneuronal migration, maturation, and circuit development, as well as the source of electrical inputs and patterns of cortical activity in the somatosensory cortex. 79 of these papers included in this timely review were written between 2007 and 2016.

CONCLUSIONS

Neuronal activity shapes the developmental assembly of functional circuitry in the somatosensory cortical interneurons. This activity impacts nearly every aspect of development and acquisition of mature neuronal characteristics, and may contribute to changing phenotypes, altered transmitter expression, and plasticity in the adult. Progressively changing oscillatory network patterns contribute to this activity in the early postnatal period, although a direct requirement for specific patterns and origins of activity remains to be demonstrated.

De novo synaptogenesis induced by GABA in the developing mouse cortex

Dendrites of cortical pyramidal neurons contain intermingled excitatory and inhibitory synapses. We studied the local mechanisms that regulate the formation and distribution of synapses. We found that local γ-aminobutyric acid (GABA) release on dendrites of mouse cortical layer 2/3 pyramidal neurons could induce gephyrin puncta and dendritic spine formation via GABA type A receptor activation and voltage-gated calcium channels during early postnatal development. Furthermore, the newly formed inhibitory and excitatory synaptic structures rapidly gained functions. Bidirectional manipulation of GABA release from somatostatin-positive interneurons increased and decreased the number of gephyrin puncta and dendritic spines, respectively. These results highlight a noncanonical function of GABA as a local synaptogenic element shaping the early establishment of neuronal circuitry in mouse cortex.

Experience-Dependent Bimodal Plasticity of Inhibitory Neurons in Early Development

Inhibitory neurons are heterogeneous in the mature brain. It is unclear when and how inhibitory neurons express distinct structural and functional profiles. Using in vivo time-lapse imaging of tectal neuron structure and visually evoked Ca(2+) responses in tadpoles, we found that inhibitory neurons cluster into two groups with opposite valence of plasticity after 4 hr of dark and visual stimulation. Half decreased dendritic arbor size and Ca(2+) responses after dark and increased them after visual stimulation, matching plasticity in excitatory neurons. Half increased dendrite arbor size and Ca(2+) responses following dark and decreased them after stimulation. At the circuit level, visually evoked excitatory and inhibitory synaptic inputs were potentiated by visual experience and E/I remained constant. Our results indicate that developing inhibitory neurons fall into distinct functional groups with opposite experience-dependent plasticity and as such, are well positioned to foster experience-dependent synaptic plasticity and maintain circuit stability during labile periods of circuit development.

A Robust Single Primate Neuroepithelial Cell Clonal Expansion System for Neural Tube Development and Disease Studies

Developing a model of primate neural tube (NT) development is important to promote many NT disorder studies in model organisms. Here, we report a robust and stable system to allow for clonal expansion of single monkey neuroepithelial stem cells (NESCs) to develop into miniature NT-like structures. Single NESCs can produce functional neurons in vitro, survive, and extensively regenerate neuron axons in monkey brain. NT formation and NESC maintenance depend on high metabolism activity and Wnt signaling. NESCs are regionally restricted to a telencephalic fate. Moreover, single NESCs can turn into radial glial progenitors (RGPCs). The transition is accurately regulated by Wnt signaling through regulation of Notch signaling and adhesion molecules. Finally, using the “NESC-TO-NTs” system, we model the functions of folic acid (FA) on NT closure and demonstrate that FA can regulate multiple mechanisms to prevent NT defects. Our system is ideal for studying NT development and diseases.

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Long-term cultured neuroepithelial stem cells (NESCs) can be induced from monkey ESCs

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Single NESCs can self-organize into miniature neural tube (NT) structures

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NESCs have high metabolism activity and are restricted to a telencephalic fate

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The “NESC-TO-NTs” system can model and study RPGC transition and NT defect disease

Functional synergy between cholecystokinin receptors CCKAR and CCKBR in mammalian brain development

Cholecystokinin (CCK), a peptide hormone and one of the most abundant neuropeptides in vertebrate brain, mediates its actions via two G-protein coupled receptors, CCKAR and CCKBR, respectively active in peripheral organs and the central nervous system. Here, we demonstrate that the CCK receptors have a dynamic and largely reciprocal expression in embryonic and postnatal brain. Using compound homozygous mutant mice lacking the activity of both CCK receptors, we uncover their additive, functionally synergistic effects in brain development and demonstrate that CCK receptor loss leads to abnormalities of cortical development, including defects in the formation of the midline and corpus callosum, and cortical interneuron migration. Using comparative transcriptome analysis of embryonic neocortex, we define the molecular mechanisms underlying these defects. Thus we demonstrate a developmental, hitherto unappreciated, role of the two CCK receptors in mammalian neocortical development.

NPAS1 represses the generation of specific subtypes of cortical interneurons

Little is known about genetic mechanisms that regulate the ratio of cortical excitatory and inhibitory neurons. We show that NPAS1 and NPAS3 transcription factors (TFs) are expressed in progenitor domains of the mouse basal ganglia (subpallium, MGE, and CGE). NPAS1(-/-) mutants had increased proliferation, ERK signaling, and expression of Arx in the MGE and CGE. NPAS1(-/-) mutants also had increased neocortical inhibition (sIPSC and mIPSC) and generated an excess of somatostatin(+) (SST) (MGE-derived) and vasoactive intestinal polypeptide(+) (VIP) (CGE-derived) neocortical interneurons, but had a normal density of parvalbumin(+) (PV) (MGE-derived) interneurons. In contrast, NPAS3(-/-) mutants showed decreased proliferation and ERK signaling in progenitors of the ganglionic eminences and had fewer SST(+) and VIP(+) interneurons. NPAS1 repressed activity of an Arx enhancer, and Arx overexpression resulted in increased proliferation of CGE progenitors. These results provide insights into genetic regulation of cortical interneuron numbers and cortical inhibitory tone.

Characterization of a subpopulation of developing cortical interneurons from human iPSCs within serum-free embryoid bodies

Production and isolation of forebrain interneuron progenitors are essential for understanding cortical development and developing cell-based therapies for developmental and neurodegenerative disorders. We demonstrate production of a population of putative calretinin-positive bipolar interneurons that express markers consistent with caudal ganglionic eminence identities. Using serum-free embryoid bodies (SFEBs) generated from human inducible pluripotent stem cells (iPSCs), we demonstrate that these interneuron progenitors exhibit morphological, immunocytochemical, and electrophysiological hallmarks of developing cortical interneurons. Finally, we develop a fluorescence-activated cell-sorting strategy to isolate interneuron progenitors from SFEBs to allow development of a purified population of these cells. Identification of this critical neuronal cell type within iPSC-derived SFEBs is an important and novel step in describing cortical development in this iPSC preparation.

Dystroglycan binding to α-neurexin competes with neurexophilin-1 and neuroligin in the brain

α-Neurexins (α-Nrxn) are mostly presynaptic cell surface molecules essential for neurotransmission that are linked to neuro-developmental disorders as autism or schizophrenia. Several interaction partners of α-Nrxn are identified that depend on alternative splicing, including neuroligins (Nlgn) and dystroglycan (αDAG). The trans-synaptic complex with Nlgn1 was extensively characterized and shown to partially mediate α-Nrxn function. However, the interactions of α-Nrxn with αDAG, neurexophilins (Nxph1) and Nlgn2, ligands that occur specifically at inhibitory synapses, are incompletely understood. Using site-directed mutagenesis, we demonstrate the exact binding epitopes of αDAG and Nxph1 on Nrxn1α and show that their binding is mutually exclusive. Identification of an unusual cysteine bridge pattern and complex type glycans in Nxph1 ensure binding to the second laminin/neurexin/sex hormone binding (LNS2) domain of Nrxn1α, but this association does not interfere with Nlgn binding at LNS6. αDAG, in contrast, interacts with both LNS2 and LNS6 domains without inserts in splice sites SS#2 or SS#4 mostly via LARGE (like-acetylglucosaminyltransferase)-dependent glycans attached to the mucin region. Unexpectedly, binding of αDAG at LNS2 prevents interaction of Nlgn at LNS6 with or without splice insert in SS#4, presumably by sterically hindering each other in the u-form conformation of α-Nrxn. Thus, expression of αDAG and Nxph1 together with alternative splicing in Nrxn1α may prevent or facilitate formation of distinct trans-synaptic Nrxn·Nlgn complexes, revealing an unanticipated way to contribute to the identity of synaptic subpopulations.

Npas4 regulates excitatory-inhibitory balance within neural circuits through cell-type-specific gene programs

The nervous system adapts to experience by inducing a transcriptional program that controls important aspects of synaptic plasticity. Although the molecular mechanisms of experience-dependent plasticity are well characterized in excitatory neurons, the mechanisms that regulate this process in inhibitory neurons are only poorly understood. Here, we describe a transcriptional program that is induced by neuronal activity in inhibitory neurons. We find that, while neuronal activity induces expression of early-response transcription factors such as Npas4 in both excitatory and inhibitory neurons, Npas4 activates distinct programs of late-response genes in inhibitory and excitatory neurons. These late-response genes differentially regulate synaptic input to these two types of neurons, promoting inhibition onto excitatory neurons while inducing excitation onto inhibitory neurons. These findings suggest that the functional outcomes of activity-induced transcriptional responses are adapted in a cell-type-specific manner to achieve a circuit-wide homeostatic response.

An epigenetic signature of developmental potential in neural stem cells and early neurons

A cardinal property of neural stem cells (NSCs) is their ability to adopt multiple fates upon differentiation. The epigenome is widely seen as a read-out of cellular potential and a manifestation of this can be seen in embryonic stem cells (ESCs), where promoters of many lineage-specific regulators are marked by a bivalent epigenetic signature comprising trimethylation of both lysine 4 and lysine 27 of histone H3 (H3K4me3 and H3K27me3, respectively). Bivalency has subsequently emerged as a powerful epigenetic indicator of stem cell potential. Here, we have interrogated the epigenome during differentiation of ESC-derived NSCs to immature GABAergic interneurons. We show that developmental transitions are accompanied by loss of bivalency at many promoters in line with their increasing developmental restriction from pluripotent ESC through multipotent NSC to committed GABAergic interneuron. At the NSC stage, the promoters of genes encoding many transcriptional regulators required for differentiation of multiple neuronal subtypes and neural crest appear to be bivalent, consistent with the broad developmental potential of NSCs. Upon differentiation to GABAergic neurons, all non-GABAergic promoters resolve to H3K27me3 monovalency, whereas GABAergic promoters resolve to H3K4me3 monovalency or retain bivalency. Importantly, many of these epigenetic changes occur before any corresponding changes in gene expression. Intriguingly, another group of gene promoters gain bivalency as NSCs differentiate toward neurons, the majority of which are associated with functions connected with maturation and establishment and maintenance of connectivity. These data show that bivalency provides a dynamic epigenetic signature of developmental potential in both NSCs and in early neurons.