Cortical interneuron specification: the juncture of genes, time and geometry

Development of Auditory Cortex Circuits

The ability to process and perceive sensory stimuli is an essential function for animals. Among the sensory modalities, audition is crucial for communication, pleasure, care for the young, and perceiving threats. The auditory cortex (ACtx) is a key sound processing region that combines ascending signals from the auditory periphery and inputs from other sensory and non-sensory regions. The development of ACtx is a protracted process starting prenatally and requires the complex interplay of molecular programs, spontaneous activity, and sensory experience. Here, we review the development of thalamic and cortical auditory circuits during pre- and early post-natal periods.

Interneuron development and dysfunction

Understanding excitation and inhibition balance in the brain begins with the tale of two basic types of neurons, glutamatergic projection neurons and GABAergic interneurons. The diversity of cortical interneurons is contributed by multiple origins in the ventral forebrain, various tangential migration routes, and complicated regulations of intrinsic factors, extrinsic signals, and activities. Abnormalities of interneuron development lead to dysfunction of interneurons and inhibitory circuits, which are highly associated with neurodevelopmental disorders including schizophrenia, autism spectrum disorders, and intellectual disability. In this review, we mainly discuss recent findings on the development of cortical interneuron and on neurodevelopmental disorders related to interneuron dysfunction.

Cellular transcriptomics reveals evolutionary identities of songbird vocal circuits

Birds display advanced behaviors, including vocal learning and problem-solving, yet lack a layered neocortex, a structure associated with complex behavior in mammals. To determine whether these behavioral similarities result from shared or distinct neural circuits, we used single-cell RNA sequencing to characterize the neuronal repertoire of the songbird song motor pathway. Glutamatergic vocal neurons had considerable transcriptional similarity to neocortical projection neurons; however, they displayed regulatory gene expression patterns more closely related to neurons in the ventral pallium. Moreover, while γ-aminobutyric acid-releasing neurons in this pathway appeared homologous to those in mammals and other amniotes, the most abundant avian class is largely absent in the neocortex. These data suggest that songbird vocal circuits and the mammalian neocortex have distinct developmental origins yet contain transcriptionally similar neurons.

Early development of human ganglionic eminences assessed in vitro by using 7.04 Tesla micro-MRI - a pilot study

Background and aims

Ganglionic eminences are temporary structures which appear during the 5^th week post-fertilization on the floor of telencephalic vesicles and disappear until the 35^th week of gestation. The aim of this descriptive study of morphological research is to depict the ganglionic eminences within the embryonic and early fetal brains by using micro-MRI.

Methods

Six human embryos and fetuses ranging from 21 mm crown-rump length CRL (9 gestational week GW) to 85 mm CRL (14 GW) were examined in vitro by micro-MRI. The investigation was performed with a Bruker BioSpec 70/16USR scanner (Bruker BioSpin MRI GmbH, Ettlingen, Germany) operating at 7.04 Tesla.

Results

We describe the morphological characteristics of the ganglionic eminences at different gestational ages. The acquisition parameters were modified for each subject in order to obtain an increased spatial resolution. The remarkable spatial resolution of 27 μm/voxel allows visualization of millimetric structures of the developing brain on high quality micro-MR images.

Conclusion

In our study we give the description of the ganglionic eminences within the embryonic and early fetal brains by using micro-MRI, which, to the best of our knowledge, have not been previously documented in literature. Micro-MRI provides accurate images, which are comparable with the histological slices.

Parvalbumin interneuron vulnerability and brain disorders

Parvalbumin-expressing interneurons (PV-INs) are highly vulnerable to stressors and have been implicated in many neuro-psychiatric diseases such as schizophrenia, Alzheimer's disease, autism spectrum disorder, and bipolar disorder. We examined the literature about the current knowledge of the physiological properties of PV-INs and gathered results from diverse research areas to provide insight into their vulnerability to stressors. Among the factors that confer heightened vulnerability are the substantial energy requirements, a strong excitatory drive, and a unique developmental trajectory. Understanding these stressors and elaborating on their impact on PV-IN health is a step toward developing therapies to protect these neurons in various disease states and to retain critical brain functions.

Cellular birthdate predicts laminar and regional cholinergic projection topography in the forebrain

The basal forebrain cholinergic system projects broadly throughout the cortex and constitutes a critical source of neuromodulation for arousal and attention. Traditionally, this system was thought to function diffusely. However, recent studies have revealed a high degree of spatiotemporal specificity in cholinergic signaling. How the organization of cholinergic afferents confers this level of precision remains unknown. Here, using intersectional genetic fate mapping, we demonstrate that cholinergic fibers within the mouse cortex exhibit remarkable laminar and regional specificity and that this is organized in accordance with cellular birthdate. Strikingly, birthdated cholinergic projections within the cortex follow an inside-out pattern of innervation. While early born cholinergic populations target deep layers, late born ones innervate superficial laminae. We also find that birthdate predicts cholinergic innervation patterns within the amygdala, hippocampus, and prefrontal cortex. Our work reveals previously unappreciated specificity within the cholinergic system and the developmental logic by which these circuits are assembled.

Orderly compartmental mapping of premotor inhibition in the developing zebrafish spinal cord

In vertebrates, faster movements involve the orderly recruitment of different types of spinal motor neurons. However, it is not known how premotor inhibitory circuits are organized to ensure alternating motor output at different movement speeds. We found that different types of commissural inhibitory interneurons in zebrafish form compartmental microcircuits during development that align inhibitory strength and recruitment order. Axonal microcircuits develop first and provide the most potent premotor inhibition during the fastest movements, followed by perisomatic microcircuits, and then dendritic microcircuits that provide the weakest inhibition during the slowest movements. The conversion of a temporal sequence of neuronal development into a spatial pattern of inhibitory connections provides an "ontogenotopic" solution to the problem of shaping spinal motor output at different speeds of movement.

A 'Marginal' tale: the development of the neocortical layer 1

The development of neocortical layer 1 is a very dynamic process and the scene of multiple transient events, with Cajal-Retzius cell death being one of the most characteristic ones. Layer 1 is also the route of migration for a substantial number of GABAergic interneurons during embryogenesis and where some of which will ultimately remain in the adult. The two cell types, together with a diverse set of incoming axons and dendrites, create an early circuit that will dramatically change in structure and function in the adult cortex to give prominence to inhibition. Through the engagement of a diverse set of GABAergic inhibitory cells by bottom-up and top-down inputs, adult layer 1 becomes a powerful computational platform for the neocortex.

Analysis of pallial/cortical interneurons in key vertebrate models of Testudines, Anurans and Polypteriform fishes

The organization of the pallial derivatives across vertebrates follows a comparable elementary arrangement, although not all of them possess a layered cortical structure as sophisticated as the cerebral cortex of mammals. However, its expansion along evolution has only been possible by the development and coevolution of the cellular networks formed by excitatory neurons and inhibitory interneurons. Thus, the comparative analysis of interneuron types in vertebrate models of key evolutionary significance will provide important information, due to the extraordinary anatomical sophistication of their interneuron systems with simpler behavioral implications. Particularly in mammals, the main consensus for classifying interneuron types is based on non-overlapping markers, which do not form a single population, but consist of several distinct classes of inhibitory cells showing co-expression of other markers. In our study, we analyzed immunohistochemically the expression of the main markers like somatostatin (SOM), parvalbumin (PV), calretinin (CR), calbindin (CB), neuropeptide Y (NPY) and/or nitric oxide synthase (NOS) at the pallial regions of three different models of Osteichthyes. First, we selected two tetrapods, one amniote from the genus Pseudemys belonging to the order Testudine, at the base of the amniote diversification and with a three-layered simple cortex, and the Anuran Xenopus laevis, an anamniote tetrapod with a non-layered evaginated pallium, and finally the order Polypteriform, a small fish group at the base of the actinopterygian diversification with an everted telencephalon. SOM was the most conserved interneuron type in terms of its distribution and co-expression with other markers such as CR, in contrast to PV, which showed a different pattern between the models analyzed. In addition, the SOM expression supports a homological relationship between the medial pallial derivatives in all the models. CR and CB expressions in the tetrapods were observed, particularly, CR expressing cells were detected in the medial and the dorsal pallial derivatives, in contrast to CB, which appeared only in discrete scattered populations. However, the pallium of Polypteriforms fishes was almost devoid of CR cells, in contrast to the important number of CB cells observed in all the pallial regions. The NPY immunoreactivity was detected in all the pallial domains of all the models, as well as cells coexpressing CR. Finally, the pallial nitrergic expression was also conserved, which allows to postulate the homological relationships between the ventropallial and the amygdaloid derivatives. In summary, even in basal pallial models the neurochemically characterized interneurons indicate that their first appearance took place before the common ancestor of amniotes. Thus, our results suggest a shared pattern of interneuron types in the pallium of all Osteichthyes.

Interneuron Types as Attractors and Controllers

Cortical interneurons display striking differences in shape, physiology, and other attributes, challenging us to appropriately classify them. We previously suggested that interneuron types should be defined by their role in cortical processing. Here, we revisit the question of how to codify their diversity based upon their division of labor and function as controllers of cortical information flow. We suggest that developmental trajectories provide a guide for appreciating interneuron diversity and argue that subtype identity is generated using a configurational (rather than combinatorial) code of transcription factors that produce attractor states in the underlying gene regulatory network. We present our updated three-stage model for interneuron specification: an initial cardinal step, allocating interneurons into a few major classes, followed by definitive refinement, creating subclasses upon settling within the cortex, and lastly, state determination, reflecting the incorporation of interneurons into functional circuit ensembles. We close by discussing findings indicating that major interneuron classes are both evolutionarily ancient and conserved. We propose that the complexity of cortical circuits is generated by phylogenetically old interneuron types, complemented by an evolutionary increase in principal neuron diversity. This suggests that a natural neurobiological definition of interneuron types might be derived from a match between their developmental origin and computational function.

The Transcription Factor LHX1 Regulates the Survival and Directed Migration of POA-derived Cortical Interneurons

The delicate balance of excitation and inhibition is crucial for proper function of the cerebral cortex, relying on the accurate number and subtype composition of inhibitory gamma-aminobutyric (GABA)-expressing interneurons. Various intrinsic and extrinsic factors precisely orchestrate their multifaceted development including the long-range migration from the basal telencephalon to cortical targets as well as interneuron survival throughout the developmental period. Particularly expressed guidance receptors were described to channel the migration of cortical interneurons deriving from the medial ganglionic eminence (MGE) and the preoptic area (POA) along distinct routes. Hence, unveiling the regulatory genetic networks controlling subtype-specific gene expression profiles is key to understand interneuron-specific developmental programs and to reveal causes for associated disorders. In contrast to MGE-derived interneurons, little is known about the transcriptional networks in interneurons born in the POA. Here, we provide first evidence for the LIM-homeobox transcription factor LHX1 as a crucial key player in the post-mitotic development of POA-derived cortical interneurons. By transcriptional regulation of related genes, LHX1 modulates their survival as well as the subtype-specific expression of guidance receptors of the Eph/ephrin family, thereby affecting directional migration and layer distribution in the adult cortex.

PDK1 regulates the survival of the developing cortical interneurons

Inhibitory interneurons are critical for maintaining the excitatory/inhibitory balance. During the development cortical interneurons originate from the ganglionic eminence and arrive at the dorsal cortex through two tangential migration routes. However, the mechanisms underlying the development of cortical interneurons remain unclear. 3-Phosphoinositide-dependent protein kinase-1 (PDK1) has been shown to be involved in a variety of biological processes, including cell proliferation and migration, and plays an important role in the neurogenesis of cortical excitatory neurons. However, the function of PDK1 in interneurons is still unclear. Here, we reported that the disruption of Pdk1 in the subpallium achieved by crossing the Dlx5/6-Cre-IRES-EGFP line with Pdk1^fl/fl mice led to the severely increased apoptosis of immature interneurons, subsequently resulting in a remarkable reduction in cortical interneurons. However, the tangential migration, progenitor pools and cell proliferation were not affected by the disruption of Pdk1. We further found the activity of AKT-GSK3β signaling pathway was decreased after Pdk1 deletion, suggesting it might be involved in the regulation of the survival of cortical interneurons. These results provide new insights into the function of PDK1 in the development of the telencephalon.

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.

Developmental characterization of Zswim5 expression in the progenitor domains and tangential migration pathways of cortical interneurons in the mouse forebrain

GABAergic interneurons play an essential role in modulating cortical networks. The progenitor domains of cortical interneurons are localized in developing ventral forebrain, including the medial ganglionic eminence (MGE), caudal ganglionic eminence (CGE), preoptic area (POA), and preoptic hypothalamic border domain (POH). Here, we characterized the expression pattern of Zswim5, an MGE-enriched gene in the mouse forebrain. At E11.5-E13.5, prominent Zswim5 expression was detected in the subventricular zone (SVZ) of MGE, POA, and POH, but not CGE of ventral telencephalon where progenitors of cortical interneurons resided. At E15.5 and E17.5, Zswim5 expression remained in the MGE/pallidum primordium and ventral germinal zone. Zswim5 mRNA was markedly decreased after birth and was absent in the adult forebrain. Interestingly, the Zswim5 expression pattern resembled the tangential migration pathways of cortical interneurons. Zswim5-positive cells in the MGE appeared to migrate from the MGE through the SVZ of LGE to overlying neocortex. Indeed, Zswim5 was co-localized with Nkx2.1 and Lhx6, markers of progenitors and migratory cortical interneurons. Double labeling showed that Ascl1/Mash1-positive cells co-expressed Zswim5. Zswim5 expressing cells contained none or at most low levels of Ki67 but co-expressed Tuj1 in the SVZ of MGE. These results suggest that Zswim5 is immediately upregulated as progenitors exiting cell cycle become postmitotic. Given that recent studies have elucidated that the cell fate of cortical interneurons is determined shortly after becoming postmitotic, the timing of Zswim5 expression in early postmitotic interneurons suggests a potential role of Zswim5 in regulation of neurogenesis and tangential migration of cortical interneurons.

DNA Methylation-Mediated Modulation of Endocytosis as Potential Mechanism for Synaptic Function Regulation in Murine Inhibitory Cortical Interneurons

The balance of excitation and inhibition is essential for cortical information processing, relying on the tight orchestration of the underlying subcellular processes. Dynamic transcriptional control by DNA methylation, catalyzed by DNA methyltransferases (DNMTs), and DNA demethylation, achieved by ten–eleven translocation (TET)-dependent mechanisms, is proposed to regulate synaptic function in the adult brain with implications for learning and memory. However, focus so far is laid on excitatory neurons. Given the crucial role of inhibitory cortical interneurons in cortical information processing and in disease, deciphering the cellular and molecular mechanisms of GABAergic transmission is fundamental. The emerging relevance of DNMT and TET-mediated functions for synaptic regulation irrevocably raises the question for the targeted subcellular processes and mechanisms. In this study, we analyzed the role dynamic DNA methylation has in regulating cortical interneuron function. We found that DNMT1 and TET1/TET3 contrarily modulate clathrin-mediated endocytosis. Moreover, we provide evidence that DNMT1 influences synaptic vesicle replenishment and GABAergic transmission, presumably through the DNA methylation-dependent transcriptional control over endocytosis-related genes. The relevance of our findings is supported by human brain sample analysis, pointing to a potential implication of DNA methylation-dependent endocytosis regulation in the pathophysiology of temporal lobe epilepsy, a disease characterized by disturbed synaptic transmission.

Decoding the development of the human hippocampus

The hippocampus is an important part of the limbic system in the human brain that has essential roles in spatial navigation and the consolidation of information from short-term memory to long-term memory^1,2. Here we use single-cell RNA sequencing and assay for transposase-accessible chromatin using sequencing (ATAC-seq) analysis to illustrate the cell types, cell linage, molecular features and transcriptional regulation of the developing human hippocampus. Using the transcriptomes of 30,416 cells from the human hippocampus at gestational weeks 16-27, we identify 47 cell subtypes and their developmental trajectories. We also identify the migrating paths and cell lineages of PAX6⁺ and HOPX⁺ hippocampal progenitors, and regional markers of CA1, CA3 and dentate gyrus neurons. Multiomic data have uncovered transcriptional regulatory networks of the dentate gyrus marker PROX1. We also illustrate spatially specific gene expression in the developing human prefrontal cortex and hippocampus. The molecular features of the human hippocampus at gestational weeks 16-20 are similar to those of the mouse at postnatal days 0-5 and reveal gene expression differences between the two species. Transient expression of the primate-specific gene NBPF1 leads to a marked increase in PROX1⁺ cells in the mouse hippocampus. These data provides a blueprint for understanding human hippocampal development and a tool for investigating related diseases.

Dosage-dependent requirements of Magoh for cortical interneuron generation and survival

Embryonic interneuron development underlies cortical function and its disruption contributes to neurological disease. Yet the mechanisms by which viable interneurons are produced from progenitors remain poorly understood. Here, we demonstrate dosage-dependent requirements of the exon junction complex component Magoh for interneuron genesis in mouse. Conditional Magoh ablation from interneuron progenitors, but not post-mitotic neurons, depletes cortical interneuron number through adulthood, with increased severity in homozygotes. Using live imaging, we discover that Magoh deficiency delays progenitor mitotic progression in a dosage-sensitive fashion, with 40% of homozygous progenitors failing to divide. This shows that Magoh is required in progenitors for both generation and survival of newborn progeny. Transcriptome analysis implicates p53 signaling; moreover, p53 ablation in Magoh haploinsufficient progenitors rescues apoptosis, completely recovering interneuron number. In striking contrast, in Magoh homozygotes, p53 loss fails to rescue interneuron number and mitotic delay, further implicating mitotic defects in interneuron loss. Our results demonstrate that interneuron development is intimately dependent upon progenitor mitosis duration and uncover a crucial post-transcriptional regulator of interneuron fate relevant for neurodevelopmental pathologies.This article has an associated 'The people behind the papers' interview.

Hippocampal CA1 Somatostatin Interneurons Originate in the Embryonic MGE/POA

Oriens lacunosum-moleculare (O-LM) interneurons constitute 40% of hippocampal interneurons expressing Somatostatin (SST). Recent evidence has indicated a dual origin for these cells in the medial and caudal ganglionic eminences (MGE and CGE), with expression of Htr3a as a distinguishing factor. This is strikingly different from cortical SST interneurons that have a single origin within the MGE/preoptic area (POA). We reassessed the origin of hippocampal SST interneurons using a range of genetic lineage-tracing mice combined with single-cell transcriptomic analysis. We find a common origin for all hippocampal SST interneurons in NKX2-1-expressing progenitors of the telencephalic neuroepithelium and an MGE/POA-like transcriptomic signature for all SST clusters. This suggests that functional heterogeneity within the SST CA1 population cannot be attributed to a differential MGE/CGE genetic origin.

Large-Scale Generation and Characterization of Homogeneous Populations of Migratory Cortical Interneurons from Human Pluripotent Stem Cells

During development, cortical interneurons (cINs) are generated from the ventral telencephalon, robustly migrate to the dorsal telencephalon, make local synaptic connections, and critically regulate brain circuitry by inhibiting other neurons. Thus, their abnormality is associated with various brain disorders. Human pluripotent stem cell (hPSC)-derived cINs can provide unlimited sources with which to study the pathogenesis mechanism of these disorders as well as provide a platform to develop novel therapeutics. By employing spinner culture, we could obtain a >10-fold higher yield of cIN progenitors compared to conventional culture without affecting their phenotype. Generated cIN spheres can be maintained feeder-free up to 10 months and are optimized for passaging and cryopreservation. In addition, we identified a combination of chemicals that synchronously matures generated progenitors into SOX6⁺KI67⁻ migratory cINs and extensively characterized their maturation in terms of metabolism, migration, arborization, and electrophysiology. When transplanted into mouse brains, chemically matured migratory cINs generated grafts that efficiently disperse and integrate into the host circuitry without uncontrolled growth, making them an optimal cell population for cell therapy. Efficient large-scale generation of homogeneous migratory cINs without the need of feeder cells will play a critical role in the full realization of hPSC-derived cINs for development of novel therapeutics.

Precision in the development of neocortical architecture: From progenitors to cortical networks

Of all brain regions, the 6-layered neocortex has undergone the most dramatic changes in size and complexity during mammalian brain evolution. These changes, occurring in the context of a conserved set of organizational features that emerge through stereotypical developmental processes, are considered responsible for the cognitive capacities and sensory specializations represented within the mammalian clade. The modern experimental era of developmental neurobiology, spanning 6 decades, has deciphered a number of mechanisms responsible for producing the diversity of cortical neuron types, their precise connectivity and the role of gene by environment interactions. Here, experiments providing insight into the development of cortical projection neuron differentiation and connectivity are reviewed. This current perspective integrates discussion of classic studies and new findings, based on recent technical advances, to highlight an improved understanding of the neuronal complexity and precise connectivity of cortical circuitry. These descriptive advances bring new opportunities for studies related to the developmental origins of cortical circuits that will, in turn, improve the prospects of identifying pathogenic targets of neurodevelopmental disorders.

Cortical Dysmaturation in Congenital Heart Disease

Congenital heart disease (CHD) is among the most common birth defects. Children with CHD frequently display long-term intellectual and behavioral disability. Emerging evidence indicates that cardiac anomalies lead to a reduction in cerebral oxygenation, which appears to profoundly impact on the maturation of cerebral regions responsible for higher-order cognitive functions. In this review we focus on the potential mechanisms by which dysregulation of cortical neuronal development during early life may lead to the significant cognitive impairments that commonly occur in children with CHD. Further understanding of the mechanisms underlying cortical dysmaturation due to CHD will be necessary to identify strategies for neonatal neuroprotection and for mitigating developmental delays in this patient population.

Rbfox1 Mediates Cell-type-Specific Splicing in Cortical Interneurons

Cortical interneurons display a remarkable diversity in their morphology, physiological properties, and connectivity. Elucidating the molecular determinants underlying this heterogeneity is essential for understanding interneuron development and function. We discovered that alternative splicing differentially regulates the integration of somatostatin- and parvalbumin-expressing interneurons into nascent cortical circuits through the cell-type-specific tailoring of mRNAs. Specifically, we identified a role for the activity-dependent splicing regulator Rbfox1 in the development of cortical interneuron-subtype-specific efferent connectivity. Our work demonstrates that Rbfox1 mediates largely non-overlapping alternative splicing programs within two distinct but related classes of interneurons.

Polypyrimidine tract-binding protein blocks miRNA-124 biogenesis to enforce its neuronal-specific expression in the mouse

MicroRNA (miRNA)-124 is expressed in neurons, where it represses genes inhibitory for neuronal differentiation, including the RNA binding protein PTBP1. PTBP1 maintains nonneuronal splicing patterns of mRNAs that switch to neuronal isoforms upon neuronal differentiation. We find that primary (pri)-miR-124-1 is expressed in mouse embryonic stem cells where mature miR-124 is absent. PTBP1 binds to this precursor RNA upstream of the miRNA stem-loop to inhibit mature miR-124 expression in vivo and DROSHA cleavage of pri-miR-124-1 in vitro. This function for PTBP1 in repressing miR-124 biogenesis defines an additional regulatory loop in the already intricate interplay between these two molecules. Applying mathematical modeling to examine the dynamics of this regulation, we find that the pool of pri-miR-124 whose maturation is blocked by PTBP1 creates a robust and self-reinforcing transition in gene expression as PTBP1 is depleted during early neuronal differentiation. While interlocking regulatory loops are often found between miRNAs and transcriptional regulators, our results indicate that miRNA targeting of posttranscriptional regulators also reinforces developmental decisions. Notably, induction of neuronal differentiation observed upon PTBP1 knockdown likely results from direct derepression of miR-124, in addition to indirect effects previously described.

Timing matters: A strategy for neurons to make diverse connections

Neurogenesis proceeds like a continuous wave, in which each type of neurons is produced over a few days to several days. During this protracted time window, early-born and late-born neurons are sequentially produced with a considerable time lag. Even if they are identical in their genetic and molecular specifications, they could develop different characteristics under the influences of the timing of their birth. In this review, we discuss the potential influences of "timing" as a generic parameter affecting neuronal differentiation, particularly on axon guidance and connections. These ideas have rarely been tested experimentally, but may provide a new strategy by which phenotypic diversity is increased in neurons.

Elucidating the developmental trajectories of GABAergic cortical interneuron subtypes

GABAergic interneurons in the neocortex play pivotal roles in the feedforward and feedback inhibition that control higher order information processing and thus, malfunction in the inhibitory circuits often leads to neurodevelopmental disorders. Very interestingly, a large diversity of morphology, synaptic targeting specificity, electrophysiological properties and molecular expression profiles are found in cortical interneurons, which originate within the distantly located embryonic ganglionic eminences. Here, I will review the still ongoing effort to understand the developmental trajectories of GABAergic cortical interneuron subtypes.

Sonic Hedgehog Signaling Rises to the Surface: Emerging Roles in Neocortical Development

The mammalian neocortex is composed of a diverse population of neuronal and glial cells that are crucial for cognition and consciousness. Orchestration of molecular events that lead to the production of distinct cell lineages is now a major research focus. Recent studies in mammalian animal models reveal that Sonic Hedgehog (Shh) signaling plays crucial roles in this process. In this review, we will evaluate these studies and provide insights on how Shh signaling specifically influence cortical development, beyond its established roles in telencephalic patterning, by specifically focusing on its impact on cells derived from the cortical radial glial (RG) cells. We will also assess how these findings further advance our knowledge of neurological diseases and discuss potential roles of targeting Shh signaling in therapies.

Cortical progenitor biology: key features mediating proliferation versus differentiation

The cerebral cortex is a highly organized structure whose development depends on diverse progenitor cell types, namely apical radial glia, intermediate progenitors, and basal radial glia cells, which are responsible for the production of the correct neuronal output. In recent years, these progenitor cell types have been deeply studied, particularly basal radial glia and their role in cortical expansion and gyrification. We review here a broad series of factors that regulate progenitor behavior and daughter cell fate. We first describe the different neuronal progenitor types, emphasizing the differences between lissencephalic and gyrencephalic species. We then review key factors shown to influence progenitor proliferation versus differentiation, discussing their roles in progenitor dynamics, neuronal production, and potentially brain size and complexity. Although spindle orientation has been considered a critical factor for mode of division and daughter cell output, we discuss other features that are emerging as crucial for these processes such as organelle and cell cycle dynamics. Additionally, we highlight the importance of adhesion molecules and the polarity complex for correct cortical development. Finally, we briefly discuss studies assessing progenitor multipotency and its possible contribution to the production of specific neuronal populations. This review hence summarizes recent aspects of cortical progenitor cell biology, and pinpoints emerging features critical for their behavior.

Heterotopic Transplantations Reveal Environmental Influences on Interneuron Diversity and Maturation

During embryogenesis, neural progenitors in the ganglionic eminences give rise to diverse GABAergic interneuron subtypes that populate all forebrain regions. The extent to which these cells are genetically predefined or determined by postmigratory environmental cues remains unknown. To address this question, we performed homo- and heterotopic transplantation of early postnatal MGE-derived cortical and hippocampal interneurons. Grafted cells migrated, and displayed neurochemical, electrophysiological, morphological, and neurochemical profiles similar to endogenous interneurons. Our results indicate that the host environment regulates the proportion of interneuron classes in the brain region. However, some specific interneuron subtypes retain characteristics representative of their donor brain regions.

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.

From Human Pluripotent Stem Cells to Cortical Circuits

Understanding the development of the human brain in relation with evolution is an important frontier field in developmental biology. In particular, investigating the mechanisms underlying the greatly increased relative size and complexity of the cerebral cortex, the seat of our enhanced cognitive abilities, remains a fascinating yet largely unsolved question. Though many advances in our understanding have been gained from the study of animal models, as well as human genetics and embryology, large gaps remain in our knowledge of the molecular mechanisms that control human cortical development. Interestingly, many aspects of corticogenesis can be recapitulated in vitro from mouse and human embryonic or induced pluripotent stem cells (PSCs), using a variety of experimental systems from 2D models to organoids to xenotransplantation. This has provided the opportunity to study these processes in an accessible and physiologically relevant setting. In this chapter, we will discuss how conserved and divergent features of primate/human corticogenesis can be modeled and studied mechanistically using PSC-based models of corticogenesis. We will also review what has been learned through these approaches about pathological defects of human corticogenesis, from early neurogenesis to late neuronal maturation and connectivity.

Linking neuronal lineage and wiring specificity

Brain function requires precise neural circuit assembly during development. Establishing a functional circuit involves multiple coordinated steps ranging from neural cell fate specification to proper matching between pre- and post-synaptic partners. How neuronal lineage and birth timing influence wiring specificity remains an open question. Recent findings suggest that the relationships between lineage, birth timing, and wiring specificity vary in different neuronal circuits. In this review, we summarize our current understanding of the cellular, molecular, and developmental mechanisms linking neuronal lineage and birth timing to wiring specificity in a few specific systems in Drosophila and mice, and review different methods employed to explore these mechanisms.

A single-cell RNA-seq survey of the developmental landscape of the human prefrontal cortex

The mammalian prefrontal cortex comprises a set of highly specialized brain areas containing billions of cells and serves as the centre of the highest-order cognitive functions, such as memory, cognitive ability, decision-making and social behaviour. Although neural circuits are formed in the late stages of human embryonic development and even after birth, diverse classes of functional cells are generated and migrate to the appropriate locations earlier in development. Dysfunction of the prefrontal cortex contributes to cognitive deficits and the majority of neurodevelopmental disorders; there is therefore a need for detailed knowledge of the development of the prefrontal cortex. However, it is still difficult to identify cell types in the developing human prefrontal cortex and to distinguish their developmental features. Here we analyse more than 2,300 single cells in the developing human prefrontal cortex from gestational weeks 8 to 26 using RNA sequencing. We identify 35 subtypes of cells in six main classes and trace the developmental trajectories of these cells. Detailed analysis of neural progenitor cells highlights new marker genes and unique developmental features of intermediate progenitor cells. We also map the timeline of neurogenesis of excitatory neurons in the prefrontal cortex and detect the presence of interneuron progenitors in early developing prefrontal cortex. Moreover, we reveal the intrinsic development-dependent signals that regulate neuron generation and circuit formation using single-cell transcriptomic data analysis. Our screening and characterization approach provides a blueprint for understanding the development of the human prefrontal cortex in the early and mid-gestational stages in order to systematically dissect the cellular basis and molecular regulation of prefrontal cortex function in humans.

Generation of diverse cortical inhibitory interneurons

First described by Ramon y Cajal as 'short-axon' cells over a century ago, inhibitory interneurons in the cerebral cortex make up ~20-30% of the neuronal milieu. A key feature of these interneurons is the striking structural and functional diversity, which allows them to modulate neural activity in diverse ways and ultimately endow neural circuits with remarkable computational power. Here, we review our current understanding of the generation of cortical interneurons, with a focus on recent efforts to bridge the gap between progenitor behavior and interneuron production, and how these aspects influence interneuron diversity and organization. WIREs Dev Biol 2018, 7:e306. doi: 10.1002/wdev.306 This article is categorized under: Nervous System Development > Vertebrates: General Principles.

Stranger in a Strange Land: Using Heterotopic Transplantations to Study Nature vs Nurture in Brain Development

The mammalian brain develops from a simple sheet of neuroepithelial cells into an incredibly complex structure containing billions of neurons with trillions of synapses. Understanding how intrinsic genetic programs interact with environmental cues to generate neuronal diversity and proper connectivity is one of the most daunting challenges in developmental biology. We recently explored this issue in forebrain GABAergic inhibitory interneurons, an extremely diverse population of neurons that are classified into distinct subtypes based on morphology, neurochemical markers, and electrophysiological properties. Immature interneurons were harvested from one brain region and transplanted into a different region, allowing us to assess how challenging cells in a new environment affected their fate. Do these grafted cells adopt characteristics of the host environment or retain features from the donor environment? We found that the proportion of interneuron subgroups is determined by the host region, but some interneuron subtypes maintain features attributable to the donor environment. In this commentary, I expound on potential mechanisms that could underlie these observations and explore the implications of these findings in a greater context of developmental neuroscience.

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.

Mechanisms of radial glia progenitor cell lineage progression

The mammalian cerebral cortex is responsible for higher cognitive functions such as perception, consciousness, and acquiring and processing information. The neocortex is organized into six distinct laminae, each composed of a rich diversity of cell types which assemble into highly complex cortical circuits. Radial glia progenitors (RGPs) are responsible for producing all neocortical neurons and certain glia lineages. Here, we discuss recent discoveries emerging from clonal lineage analysis at the single RGP cell level that provide us with an inaugural quantitative framework of RGP lineage progression. We further discuss the importance of the relative contribution of intrinsic gene functions and non‐cell‐autonomous or community effects in regulating RGP proliferation behavior and lineage progression.

The BRAIN Initiative Cell Census Consortium: Lessons Learned toward Generating a Comprehensive Brain Cell Atlas

A comprehensive characterization of neuronal cell types, their distributions, and patterns of connectivity is critical for understanding the properties of neural circuits and how they generate behaviors. Here we review the experiences of the BRAIN Initiative Cell Census Consortium, ten pilot projects funded by the U.S. BRAIN Initiative, in developing, validating, and scaling up emerging genomic and anatomical mapping technologies for creating a complete inventory of neuronal cell types and their connections in multiple species and during development. These projects lay the foundation for a larger and longer-term effort to generate whole-brain cell atlases in species including mice and humans.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Inter-progenitor pool wiring: An evolutionarily conserved strategy that expands neural circuit diversity

Diversification of neuronal types is key to establishing functional variations in neural circuits. The first critical step to generate neuronal diversity is to organize the compartmental domains of developing brains into spatially distinct neural progenitor pools. Neural progenitors in each pool then generate a unique set of diverse neurons through specific spatiotemporal specification processes. In this review article, we focus on an additional mechanism, 'inter-progenitor pool wiring', that further expands the diversity of neural circuits. After diverse types of neurons are generated in one progenitor pool, a fraction of these neurons start migrating toward a remote brain region containing neurons that originate from another progenitor pool. Finally, neurons of different origins are intermingled and eventually form complex but precise neural circuits. The developing cerebral cortex of mammalian brains is one of the best examples of inter-progenitor pool wiring. However, Drosophila visual system development has revealed similar mechanisms in invertebrate brains, suggesting that inter-progenitor pool wiring is an evolutionarily conserved strategy that expands neural circuit diversity. Here, we will discuss how inter-progenitor pool wiring is accomplished in mammalian and fly brain systems.

Radial glia in the ventral telencephalon

The ventral telencephalon is the developmental origin of the basal ganglia and the source of neuronal and glial cells that integrate into developing circuits in other areas of the brain. Radial glia in the embryonic subpallium give rise to an enormous diversity of mature cell types, either directly or through other transit-amplifying progenitors. Here, we review current knowledge about these subpallial neural stem cells and their progeny, focusing on the period of neurogenesis. We describe their cell biological features and the extrinsic and intrinsic molecular codes that guide their fate specification in defined temporal and spatial sequences. We also discuss the role of clonal lineage in the organization and specification of mature neurons.

Playing Well with Others: Extrinsic Cues Regulate Neural Progenitor Temporal Identity to Generate Neuronal Diversity

During neurogenesis, vertebrate and Drosophila progenitors change over time as they generate a diverse population of neurons and glia. Vertebrate neural progenitors have long been known to use both progenitor-intrinsic and progenitor-extrinsic cues to regulate temporal patterning. In contrast, virtually all temporal patterning mechanisms discovered in Drosophila neural progenitors (neuroblasts) involve progenitor-intrinsic temporal transcription factor cascades. Recent results, however, have revealed several extrinsic pathways that regulate Drosophila neuroblast temporal patterning: nutritional cues regulate the timing of neuroblast proliferation/quiescence and a steroid hormone cue that is required for temporal transcription factor expression. Here, we discuss newly discovered extrinsic cues regulating neural progenitor temporal identity in Drosophila, highlight conserved mechanisms, and raise open questions for the future.

Testing hypotheses of developmental constraints on mammalian brain partition evolution, using marsupials

There is considerable debate about whether the partition volumes of the mammalian brain (e.g. cerebrum, cerebellum) evolve according to functional selection, or whether developmental constraints of conserved neurogenetic scheduling cause predictable partition scaling with brain size. Here we provide the first investigation of developmental constraints on partition volume growth, derived from contrast-enhanced micro-computed tomography of hydrogel-stabilized brains from three marsupial species. ANCOVAs of partition vs. brain volume scaling, as well as growth curve comparisons, do not support several hypotheses consistent with developmental constraints: brain partition growth significantly differs between species, or between developing vs. adult marsupials. Partition growth appears independent of adult brain volume, with no discernable growth spurts/lags relatable to internal structural change. Rather, adult proportion differences appear to arise through growth rate/duration heterochrony. Substantial phylogenetic signal in adult brain partitions scaling with brain volume also counters expectations of development-mediated partition scaling conservatism. However, the scaling of olfactory bulb growth is markedly irregular, consistent with suggestions that it is less constrained. The very regular partition growth curves suggest intraspecific developmental rigidity. We speculate that a rigid, possibly neuromer-model-like early molecular program might be responsible both for regular growth curves within species and impressions of a link between neurogenesis and partition evolution.

Genetic and activity-dependent mechanisms underlying interneuron diversity

The proper construction of neural circuits requires the generation of diverse cell types, their distribution to defined regions, and their specific and appropriate wiring. A major objective in neurobiology has been to understand the molecular determinants that link neural birth to terminal specification and functional connectivity, a task that is especially daunting in the case of cortical interneurons. Considerable evidence supports the idea that an interplay of intrinsic and environmental signalling is crucial to the sequential steps of interneuron specification, including migration, selection of a settling position, morphogenesis and synaptogenesis. However, when and how these influences merge to support the appropriate terminal differentiation of different classes of interneurons remains uncertain. In this Review, we discuss a wealth of recent findings that have advanced our understanding of the developmental mechanisms that contribute to the diversification of interneurons and suggest areas of particular promise for further investigation.

Single-Cell Profiling of an In Vitro Model of Human Interneuron Development Reveals Temporal Dynamics of Cell Type Production and Maturation

GABAergic interneurons are essential for neural circuit function, and their loss or dysfunction is implicated in human neuropsychiatric disease. In vitro methods for interneuron generation hold promise for studying human cellular and functional properties and, ultimately, for therapeutic cell replacement. Here we describe a protocol for generating cortical interneurons from hESCs and analyze the properties and maturation time course of cell types using single-cell RNA-seq. We find that the cell types produced mimic in vivo temporal patterns of neuron and glial production, with immature progenitors and neurons observed early and mature cortical neurons and glial cell types produced late. By comparing the transcriptomes of immature interneurons to those of more mature neurons, we identified genes important for human interneuron differentiation. Many of these genes were previously implicated in neurodevelopmental and neuropsychiatric disorders.

Regulation of neuronal survival by DNA methyltransferases

The limited regenerative capacity of neuronal cells requires tight orchestration of cell death and survival regulation in the context of longevity, age-associated diseases as well as during the development of the nervous system. Subordinate to genetic networks epigenetic mechanisms like DNA methylation and histone modifications are involved in the regulation of neuronal development, function and aging. DNA methylation by DNA methyltransferases (DNMTs), mostly correlated with gene silencing, is a dynamic and reversible process. In addition to their canonical actions performing cytosine methylation, DNMTs influence gene expression by interactions with histone modifying enzymes or complexes increasing the complexity of epigenetic transcriptional networks. DNMTs are expressed in neuronal progenitors, post-mitotic as well as adult neurons. In this review, we discuss the role and mode of actions of DNMTs including downstream networks in the regulation of neuronal survival in the developing and aging nervous system and its relevance for associated disorders.