COUP-TFII is preferentially expressed in the caudal ganglionic eminence and is involved in the caudal migratory stream

Fate determination of cerebral cortical GABAergic interneurons and their derivation from stem cells

Cortical GABAergic interneurons modulate cortical excitation, and their dysfunction is implicated in a multitude of neuropsychiatric disorders including autism, schizophrenia and epilepsy. Consequently, the study of cortical interneuron development, and their derivation from stem cells for transplantation therapy, has garnered intense scientific interest. In this review, we discuss some of the molecular signals involved in cortical interneuron fate determination, and describe how this has informed the use of mouse and human embryonic stem cell biology in generating cortical interneurons in vitro. We highlight the tremendous progress that has been made recently using stem cells to derive cortical interneurons, as well as challenges that have arisen. This article is part of a Special Issue entitled SI:StemsCellsinPsychiatry.

The COUP-TFII/Neuropilin-2 is a molecular switch steering diencephalon-derived GABAergic neurons in the developing mouse brain

The preoptic area (POa) of the rostral diencephalon supplies the neocortex and the amygdala with GABAergic neurons in the developing mouse brain. However, the molecular mechanisms that determine the pathway and destinations of POa-derived neurons have not yet been identified. Here we show that Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII)-induced expression of Neuropilin-2 (Nrp2) and its down-regulation control the destination of POa-derived GABAergic neurons. Initially, a majority of the POa-derived migrating neurons express COUP-TFII and form a caudal migratory stream toward the caudal subpallium. When a subpopulation of cells steers toward the neocortex, they exhibit decreased expression of COUP-TFII and Nrp2. The present findings show that suppression of COUP-TFII/Nrp2 changed the destination of the cells into the neocortex, whereas overexpression of COUP-TFII/Nrp2 caused cells to end up in the medial part of the amygdala. Taken together, these results reveal that COUP-TFII/Nrp2 is a molecular switch determining the pathway and destination of migrating GABAergic neurons born in the POa.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.

Transcription factors COUP-TFI and COUP-TFII are required for the production of granule cells in the mouse olfactory bulb

Neural stem cells (NSCs) persist in the adult mammalian subventricular zone (SVZ) of the lateral ventricle. Primary NSCs generate rapidly dividing intermediate progenitor cells, which in turn generate neuroblasts that migrate along the rostral migratory stream (RMS) to the olfactory bulb (OB). Here, we have examined the role of the COUP-TFI and COUP-TFII orphan nuclear receptor transcription factors in mouse OB interneuron development. We observed that COUP-TFI is expressed in a gradient of low rostral to high caudal within the postnatal SVZ neural stem/progenitor cells. COUP-TFI is also expressed in a large number of migrating neuroblasts in the SVZ and RMS, and in mature interneurons in the OB. By contrast, very few COUP-TFII-expressing (+) cells exist in the SVZ-RMS-OB pathway. Conditional inactivation of COUP-TFI resulted in downregulation of tyrosine hydroxylase expression in the OB periglomerular cells and upregulation of COUP-TFII expression in the SVZ, RMS and OB deep granule cell layer. In COUP-TFI/COUP-TFII double conditional mutant SVZ, cell proliferation was increased through the upregulation of the proneural gene Ascl1. Furthermore, COUP-TFI/II-deficient neuroblasts had impaired migration, resulting in ectopic accumulation of calretinin (CR)+ and NeuN+ cells, and an increase in apoptotic cell death in the SVZ. Finally, we found that most Pax6+ and a subset of CR+ granular cells were lost in the OB. Taken together, these results suggest that COUP-TFI/II coordinately regulate the proliferation, migration and survival of a subpopulation of Pax6+ and CR+ granule cells in the OB.

Extended Production of Cortical Interneurons into the Third Trimester of Human Gestation

In humans, the developmental origins of interneurons in the third trimester of pregnancy and the timing of completion of interneuron neurogenesis have remained unknown. Here, we show that the total and cycling Nkx2.1(+)and Dlx2(+)interneuron progenitors as well as Sox2(+)precursor cells were higher in density in the medial ganglionic eminence (MGE) compared with the lateral ganglionic eminence and cortical ventricular/subventricular zone (VZ/SVZ) of 16-35 gw subjects. The proliferation of these progenitors reduced as a function of gestational age, almost terminating by 35 gw. Proliferating Dlx2(+)cells were higher in density in the caudal ganglionic eminence (CGE) compared with the MGE, and persisted beyond 35 gw. Consistent with these findings, Sox2, Nkx2.1, Dlx2, and Mash1 protein levels were higher in the ganglionic eminences relative to the cortical VZ/SVZ. The density of gamma-aminobutyric acid-positive (GABA(+)) interneurons was higher in the cortical VZ/SVZ relative to MGE, but Nkx2.1 or Dlx2-expressing GABA(+)cells were more dense in the MGE compared with the cortical VZ/SVZ. The data suggest that the MGE and CGE are the primary source of cortical interneurons. Moreover, their generation continues nearly to the end of pregnancy, which may predispose premature infants to neurobehavioral disorders.

Molecular and Electrophysiological Characterization of GABAergic Interneurons Expressing the Transcription Factor COUP-TFII in the Adult Human Temporal Cortex

Transcription factors contribute to the differentiation of cortical neurons, orchestrate specific interneuronal circuits, and define synaptic relationships. We have investigated neurons expressing chicken ovalbumin upstream promoter transcription factor II (COUP-TFII), which plays a role in the migration of GABAergic neurons. Whole-cell, patch-clamp recording in vitro combined with colocalization of molecular cell markers in the adult cortex differentiates distinct interneurons. The majority of strongly COUP-TFII-expressing neurons were in layers I–III. Most calretinin (CR) and/or cholecystokinin- (CCK) and/or reelin-positive interneurons were also COUP-TFII-positive. CR-, CCK-, or reelin-positive neurons formed 80%, 20%, or 17% of COUP-TFII-positive interneurons, respectively. About half of COUP-TFII-/CCK-positive interneurons were CR-positive, a quarter of them reelin-positive, but none expressed both. Interneurons positive for COUP-TFII fired irregular, accommodating and adapting trains of action potentials (APs) and innervated mostly small dendritic shafts and rarely spines or somata. Paired recording showed that a calretinin-/COUP-TFII-positive interneuron elicited inhibitory postsynaptic potentials (IPSPs) in a reciprocally connected pyramidal cell. Calbindin, somatostatin, or parvalbumin-immunoreactive interneurons and most pyramidal cells express no immunohistochemically detectable COUP-TFII. In layers V and VI, some pyramidal cells expressed a low level of COUP-TFII in the nucleus. In conclusion, COUP-TFII is expressed in a diverse subset of GABAergic interneurons predominantly innervating small dendritic shafts originating from both interneurons and pyramidal cells.

The early fetal development of human neocortical GABAergic interneurons

GABAergic interneurons are crucial to controlling the excitability and responsiveness of cortical circuitry. Their developmental origin may differ between rodents and human. We have demonstrated the expression of 12 GABAergic interneuron-associated genes in samples from human neocortex by quantitative rtPCR from 8 to 12 postconceptional weeks (PCW) and shown a significant anterior to posterior expression gradient, confirmed by in situ hybridization or immunohistochemistry for GAD1 and 2, DLX1, 2, and 5, ASCL1, OLIG2, and CALB2. Following cortical plate (CP) formation from 8 to 9 PCW, a proportion of cells were strongly stained for all these markers in the CP and presubplate. ASCL1 and DLX2 maintained high expression in the proliferative zones and showed extensive immunofluorescent double-labeling with the cell division marker Ki-67. CALB2-positive cells increased steadily in the SVZ/VZ from 10 PCW but were not double-labeled with Ki-67. Expression of GABAergic genes was generally higher in the dorsal pallium than in the ganglionic eminences, with lower expression in the intervening ventral pallium. It is widely accepted that the cortical proliferative zones may generate CALB2-positive interneurons from mid-gestation; we now show that the anterior neocortical proliferative layers especially may be a rich source of interneurons in the early neocortex.

Prospero-related homeobox 1 (Prox1) at the crossroads of diverse pathways during adult neural fate specification

Over the last decades, adult neurogenesis in the central nervous system (CNS) has emerged as a fundamental process underlying physiology and disease. Recent evidence indicates that the homeobox transcription factor Prox1 is a critical intrinsic regulator of neurogenesis in the embryonic CNS and adult dentate gyrus (DG) of the hippocampus, acting in multiple ways and instructed by extrinsic cues and intrinsic factors. In the embryonic CNS, Prox1 is mechanistically involved in the regulation of proliferation vs. differentiation decisions of neural stem cells (NSCs), promoting cell cycle exit and neuronal differentiation, while inhibiting astrogliogenesis. During the complex differentiation events in adult hippocampal neurogenesis, Prox1 is required for maintenance of intermediate progenitors (IPs), differentiation and maturation of glutamatergic interneurons, as well as specification of DG cell identity over CA3 pyramidal fate. The mechanism by which Prox1 exerts multiple functions involves distinct signaling pathways currently not fully highlighted. In this mini-review, we thoroughly discuss the Prox1-dependent phenotypes and molecular pathways in adult neurogenesis in relation to different upstream signaling cues and cell fate determinants. In addition, we discuss the possibility that Prox1 may act as a cross-talk point between diverse signaling cascades to achieve specific outcomes during adult neurogenesis.

Emergence of neuronal diversity from patterning of telencephalic progenitors

During central nervous system (CNS) development, hundreds of distinct neuronal subtypes are generated from a single layer of multipotent neuroepithelial progenitor cells. Within the rostral CNS, initial regionalization of the telencephalon marks the territories where the cerebral cortex and the basal ganglia originate. Subsequent refinement of the primary structures determines the formation of domains of differential gene expression, where distinct fate-restricted progenitors are located. To understand how diversification of neural progenitors and neurons is achieved in the telencephalon, it is important to address early and late patterning events in this context. In particular, important questions include: How does the telencephalon become specified and regionalized along the major spatial axes? Within each region, are the differences in neuronal subtypes established at the progenitor level or at the postmitotic stage? If distinct progenitors exist that are committed to subtype-specific neuronal lineages, how does the diversification emerge? What is the contribution of positional and temporal cues and how is this information integrated into the intrinsic programs of cell identity? WIREs For further resources related to this article, please visit the WIREs website.

Serotonin receptor 3A controls interneuron migration into the neocortex

Neuronal excitability has been shown to control the migration and cortical integration of reelin-expressing cortical interneurons (INs) arising from the caudal ganglionic eminence (CGE), supporting the possibility that neurotransmitters could regulate this process. Here we show that the ionotropic serotonin receptor 3A (5-HT_3AR) is specifically expressed in CGE-derived migrating interneurons and upregulated while they invade the developing cortex. Functional investigations using calcium imaging, electrophysiological recordings and migration assays indicate that CGE-derived INs increase their response to 5-HT_3AR activation during the late phase of cortical plate invasion. Using genetic loss-of-function approaches and in vivo grafts, we further demonstrate that the 5-HT_3AR is cell autonomously required for the migration and proper positioning of reelin-expressing CGE-derived INs in the neocortex. Our findings reveal a requirement for a serotonin receptor in controlling the migration and laminar positioning of a specific subtype of cortical IN.

During brain development, neuronal excitability controls the laminar migration of cortical interneurons from the caudal ganglionic eminences (CGEs). Here the authors identify the 5-HT_3A receptor as a specific marker of CGE-derived cortical interneurons (cINs), and as a stimulator of cIN migration.

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.

The complexity of the calretinin-expressing progenitors in the human cerebral cortex

The complex structure and function of the cerebral cortex critically depend on the balance of excitation and inhibition provided by the pyramidal projection neurons and GABAergic interneurons, respectively. The calretinin-expressing (CalR⁺) cell is a subtype of GABAergic cortical interneurons that is more prevalent in humans than in rodents. In rodents, CalR⁺ interneurons originate in the caudal ganglionic eminence (CGE) from Gsx2⁺ progenitors, but in humans it has been suggested that a subpopulation of CalR⁺ cells can also be generated in the cortical ventricular/subventricular zone (VZ/SVZ). The progenitors for cortically generated CalR⁺ subpopulation in primates are not yet characterized. Hence, the aim of this study was to identify patterns of expression of the transcription factors (TFs) that commit cortical stem cells to the CalR fate, with a focus on Gsx2. First, we studied the expression of Gsx2 and its downstream effectors, Ascl1 and Sp8 in the cortical regions of the fetal human forebrain at midgestation. Next, we established that a subpopulation of cells expressing these TFs are proliferating in the cortical SVZ, and can be co-labeled with CalR. The presence and proliferation of Gsx2⁺ cells, not only in the ventral telencephalon (GE) as previously reported, but also in the cerebral cortex suggests cortical origin of a subpopulation of CalR⁺ neurons in humans. In vitro treatment of human cortical progenitors with Sonic hedgehog (Shh), an important morphogen in the specification of interneurons, decreased levels of Ascl1 and Sp8 proteins, but did not affect Gsx2 levels. Taken together, our ex-vivo and in vitro results on human fetal brain suggest complex endogenous and exogenous regulation of TFs implied in the specification of different subtypes of CalR⁺ cortical interneurons.

Human fetal brain-derived neural stem/progenitor cells grafted into the adult epileptic brain restrain seizures in rat models of temporal lobe epilepsy

Cell transplantation has been suggested as an alternative therapy for temporal lobe epilepsy (TLE) because this can suppress spontaneous recurrent seizures in animal models. To evaluate the therapeutic potential of human neural stem/progenitor cells (huNSPCs) for treating TLE, we transplanted huNSPCs, derived from an aborted fetal telencephalon at 13 weeks of gestation and expanded in culture as neurospheres over a long time period, into the epileptic hippocampus of fully kindled and pilocarpine-treated adult rats exhibiting TLE. In vitro, huNSPCs not only produced all three central nervous system neural cell types, but also differentiated into ganglionic eminences-derived γ-aminobutyric acid (GABA)-ergic interneurons and released GABA in response to the depolarization induced by a high K⁺ medium. NSPC grafting reduced behavioral seizure duration, afterdischarge duration on electroencephalograms, and seizure stage in the kindling model, as well as the frequency and the duration of spontaneous recurrent motor seizures in pilocarpine-induced animals. However, NSPC grafting neither improved spatial learning or memory function in pilocarpine-treated animals. Following transplantation, grafted cells showed extensive migration around the injection site, robust engraftment, and long-term survival, along with differentiation into β-tubulin III⁺ neurons (∼34%), APC-CC1⁺ oligodendrocytes (∼28%), and GFAP⁺ astrocytes (∼8%). Furthermore, among donor-derived cells, ∼24% produced GABA. Additionally, to explain the effect of seizure suppression after NSPC grafting, we examined the anticonvulsant glial cell-derived neurotrophic factor (GDNF) levels in host hippocampal astrocytes and mossy fiber sprouting into the supragranular layer of the dentate gyrus in the epileptic brain. Grafted cells restored the expression of GDNF in host astrocytes but did not reverse the mossy fiber sprouting, eliminating the latter as potential mechanism. These results suggest that human fetal brain-derived NSPCs possess some therapeutic effect for TLE treatments although further studies to both increase the yield of NSPC grafts-derived functionally integrated GABAergic neurons and improve cognitive deficits are still needed.

Spatio-temporal extension in site of origin for cortical calretinin neurons in primates

The vast majority of cortical GABAergic neurons can be defined by parvalbumin, somatostatin or calretinin expression. In most mammalians, parvalbumin and somatostatin interneurons have constant proportions, each representing 5-7% of the total neuron number. In contrast, there is a threefold increase in the proportion of calretinin interneurons, which do not exceed 4% in rodents and reach 12% in higher order areas of primate cerebral cortex. In rodents, almost all parvalbumin and somatostatin interneurons originate from the medial part of the subpallial proliferative structure, the ganglionic eminence (GE), while almost all calretinin interneurons originate from its caudal part. The spatial pattern of cortical GABAergic neurons origin from the GE is preserved in the monkey and human brain. However, it could be expected that the evolution is changing developmental rules to enable considerable expansion of calretinin interneuron population. During the early fetal period in primates, cortical GABAergic neurons are almost entirely generated in the subpallium, as in rodents. Already at that time, the primate caudal ganglionic eminence (CGE) shows a relative increase in size and production of calretinin interneurons. During the second trimester of gestation, that is the main neurogenetic stage in primates without clear correlates found in rodents, the pallial production of cortical GABAergic neurons together with the extended persistence of the GE is observed. We propose that the CGE could be the main source of calretinin interneurons for the posterior and lateral cortical regions, but not for the frontal cortex. The associative granular frontal cortex represents around one third of the cortical surface and contains almost half of cortical calretinin interneurons. The majority of calretinin interneurons destined for the frontal cortex could be generated in the pallium, especially in the newly evolved outer subventricular zone that becomes the main pool of cortical progenitors.

Revisiting enigmatic cortical calretinin-expressing interneurons

Cortical calretinin (CR)-expressing interneurons represent a heterogeneous subpopulation of about 10-30% of GABAergic interneurons, which altogether total ca. 12-20% of all cortical neurons. In the rodent neocortex, CR cells display different somatodendritic morphologies ranging from bipolar to multipolar but the bipolar cells and their variations dominate. They are also diverse at the molecular level as they were shown to express numerous neuropeptides in different combinations including vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), neurokinin B (NKB) corticotrophin releasing factor (CRF), enkephalin (Enk) but also neuropeptide Y (NPY) and somatostatin (SOM) to a lesser extent. CR-expressing interneurons exhibit different firing behaviors such as adapting, bursting or irregular. They mainly originate from the caudal ganglionic eminence (CGE) but a subpopulation also derives from the dorsal part of the medial ganglionic eminence (MGE). Cortical GABAergic CR-expressing interneurons can be divided in two main populations: VIP-bipolar interneurons deriving from the CGE and SOM-Martinotti-like interneurons originating in the dorsal MGE. Although bipolar cells account for the majority of CR-expressing interneurons, the roles they play in cortical neuronal circuits and in the more general metabolic physiology of the brain remained elusive and enigmatic. The aim of this review is, firstly, to provide a comprehensive view of the morphological, molecular and electrophysiological features defining this cell type. We will, secondly, also summarize what is known about their place in the cortical circuit, their modulation by subcortical afferents and the functional roles they might play in neuronal processing and energy metabolism.

Lhx6 directly regulates Arx and CXCR7 to determine cortical interneuron fate and laminar position

Cortical GABAergic interneurons have essential roles for information processing and their dysfunction is implicated in neuropsychiatric disorders. Transcriptional codes are elucidating mechanisms of interneuron specification in the MGE (a subcortical progenitor zone), which regulate their migration, integration, and function within cortical circuitry. Lhx6, a LIM-homeodomain transcription factor, is essential for specification of MGE-derived somatostatin and parvalbumin interneurons. Here, we demonstrate that some Lhx6⁻/⁻ MGE cells acquire a CGE-like fate. Using an in vivo MGE complementation/transplantation assay, we show that Lhx6-regulated genes Arx and CXCR7 rescue divergent aspects of Lhx6⁻/⁻ cell-fate and laminar mutant phenotypes and provide insight into a neonatal role for CXCR7 in MGE-derived interneuron lamination. Finally, Lhx6 directly binds in vivo to an Arx enhancer and to an intronic CXCR7 enhancer that remains active in mature interneurons. These data define the molecular identity of Lhx6 mutants and introduce technologies to test mechanisms in GABAergic interneuron differentiation.

COUP-TFII in pancreatic adenocarcinoma: clinical implication for patient survival and tumor progression

Despite the accumulating knowledge of alterations in pancreatic cancer molecular pathways, no substantial improvements in the clinical prognosis have been made and this malignancy continues to be a leading cause of cancer death in the Western World. The orphan nuclear receptor COUP-TFII is a regulator of a wide range of biological processes and it may exert a pro-oncogenic role in cancer cells; interestingly, indirect evidences suggest that the receptor could be involved in pancreatic cancer. The aim of this study was to evaluate the expression of COUP-TFII in human pancreatic tumors and to unveil its role in the regulation of pancreatic tumor growth. We evaluated COUP-TFII expression by immunohistochemistry on primary samples. We analyzed the effect of the nuclear receptor silencing in human pancreatic cancer cells by means of shRNA expressing cell lines. We finally confirmed the in vitro results by in vivo experiments on nude mice. COUP-TFII is expressed in 69% of tested primary samples and correlates with the N1 and M1 status and clinical stage; Kaplan-Meier and Cox regression analysis show that it may be an independent prognostic factor of worst outcome. In vitro silencing of COUP-TFII reduces the cell growth and invasiveness and it strongly inhibits angiogenesis, an effect mediated by the regulation of VEGF-C. In nude mice, COUP-TFII silencing reduces tumor growth by 40%. Our results suggest that COUP-TFII might be an important regulator of the behavior of pancreatic adenocarcinoma, thus representing a possible new target for pancreatic cancer therapy.

Mechanisms regulating GABAergic neuron development

Neurons using gamma-aminobutyric acid (GABA) as their neurotransmitter are the main inhibitory neurons in the mature central nervous system (CNS) and show great variation in their form and function. GABAergic neurons are produced in all of the main domains of the CNS, where they develop from discrete regions of the neuroepithelium. Here, we review the gene expression and regulatory mechanisms controlling the main steps of GABAergic neuron development: early patterning of the proliferative neuroepithelium, production of postmitotic neural precursors, establishment of their identity and migration. By comparing the molecular regulation of these events across CNS, we broadly identify three regions utilizing distinct molecular toolkits for GABAergic fate determination: telencephalon-anterior diencephalon (DLX2 type), posterior diencephalon-midbrain (GATA2 type) and hindbrain-spinal cord (PTF1A and TAL1 types). Similarities and differences in the molecular regulatory mechanisms reveal the core determinants of a GABAergic neuron as well as provide insights into generation of the vast diversity of these neurons.

Genetic dissection of GABAergic neural circuits in mouse neocortex

Diverse and flexible cortical functions rely on the ability of neural circuits to perform multiple types of neuronal computations. GABAergic inhibitory interneurons significantly contribute to this task by regulating the balance of activity, synaptic integration, spiking, synchrony, and oscillation in a neural ensemble. GABAergic interneurons display a high degree of cellular diversity in morphology, physiology, connectivity, and gene expression. A considerable number of subtypes of GABAergic interneurons diversify modes of cortical inhibition, enabling various types of information processing in the cortex. Thus, comprehensively understanding fate specification, circuit assembly, and physiological function of GABAergic interneurons is a key to elucidate the principles of cortical wiring and function. Recent advances in genetically encoded molecular tools have made a breakthrough to systematically study cortical circuitry at the molecular, cellular, circuit, and whole animal levels. However, the biggest obstacle to fully applying the power of these to analysis of GABAergic circuits was that there were no efficient and reliable methods to express them in subtypes of GABAergic interneurons. Here, I first summarize cortical interneuron diversity and current understanding of mechanisms, by which distinct classes of GABAergic interneurons are generated. I then review recent development in genetically encoded molecular tools for neural circuit research, and genetic targeting of GABAergic interneuron subtypes, particularly focusing on our recent effort to develop and characterize Cre/CreER knockin lines. Finally, I highlight recent success in genetic targeting of chandelier cells, the most unique and distinct GABAergic interneuron subtype, and discuss what kind of questions need to be addressed to understand development and function of cortical inhibitory circuits.

Genetic programs controlling cortical interneuron fate

•

Cortical interneurons originate in the embryonic subcortical telencephalon.

•

Spatial and temporal control of progenitor differentiation generates diversity.

•

Genetic pathways of interneuron cell fate specification.

•

Intrinsic pathways and extrinsic cues interplay in interneuron specification.

The origins of cortical interneurons in rodents have been localized to the embryonic subcortical telencephalon where distinct neuroepithelial precursors generate defined interneuron subsets. A swathe of research activity aimed at identifying molecular determinants of subtype identity has uncovered a number of transcription factors that function at different stages of interneuron development. Pathways that lead to the acquisition of mature interneuron traits are therefore beginning to emerge. As genetic programs are influenced by external factors the search continues not only into genetic determinants but also extrinsic influences and the interplay between the two in cell fate specification.

Decision making during interneuron migration in the developing cerebral cortex

Appropriate interneuron migration and distribution is essential for the construction of functional neuronal circuitry and the maintenance of excitatory/inhibitory balance in the brain. Gamma-aminobutyric acid (GABA)ergic interneurons originating from the ventral telencephalon choreograph a complex pattern of migration to reach their target destinations within the developing brain. This review examines the cellular and molecular underpinnings of the major decision-making steps involved in this process of oriental navigation of cortical interneurons.

The nuclear receptors COUP-TF: a long-lasting experience in forebrain assembly

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are nuclear receptors belonging to the superfamily of the steroid/thyroid hormone receptors. Members of this family are internalized to the nucleus both in a ligand-dependent or -independent manner and act as strong transcriptional regulators by binding to the DNA of their target genes. COUP-TFs are defined as orphan receptors, since ligands regulating their activity have not so far been identified. From the very beginning of metazoan evolution, these molecules have been involved in various key events during embryonic development and organogenesis. In this review, we will mainly focus on their function during development and maturation of the central nervous system, which has been well characterized in various animal classes ranging from ctenophores to mammals. We will start by introducing the current knowledge on COUP-TF mechanisms of action and then focus our discussion on the crucial processes underlying forebrain ontogenesis, with special emphasis on mammalian development. Finally, the conserved roles of COUP-TFs along phylogenesis will be highlighted, and some hypotheses, worth exploring in future years to gain more insight into the mechanisms controlled by these factors, will be proposed.

Production and organization of neocortical interneurons

Inhibitory GABA (γ-aminobutyric acid)-ergic interneurons are a vital component of the neocortex responsible for shaping its output through a variety of inhibitions. Consisting of many flavors, interneuron subtypes are predominantly defined by their morphological, physiological, and neurochemical properties that help to determine their functional role within the neocortex. During development, these cells are born in the subpallium where they then tangentially migrate over long distances before being radially positioned to their final location in the cortical laminae. As development progresses into adolescence, these cells mature and form chemical and electrical connections with both glutamatergic excitatory neurons and other interneurons ultimately establishing the cortical network. The production, migration, and organization of these cells are determined by vast array of extrinsic and intrinsic factors that work in concert in order to assemble a proper functioning cortical inhibitory network. Failure of these cells to undergo these processes results in abnormal positioning and cortical function. In humans, this can bring about several neurological disorders including schizophrenia, epilepsy, and autism spectrum disorders. In this article, we will review previous literature that has revealed the framework for interneuron neurogenesis and migratory behavior as well as discuss recent findings that aim to elucidate the spatial and functional organization of interneurons within the neocortex.

The development of hippocampal cellular assemblies

The proper assembly of a cohort of distinct cell types is a prerequisite for building a functional hippocampus. In this review, we describe the major molecular events of the developmental program leading to the cellular construction of the hippocampus. Data from rodent studies are used here to elaborate on our understanding of these processes.

Cdk5 phosphorylation of ErbB4 is required for tangential migration of cortical interneurons

Interneuron dysfunction in humans is often associated with neurological and psychiatric disorders, such as epilepsy, schizophrenia, and autism. Some of these disorders are believed to emerge during brain formation, at the time of interneuron specification, migration, and synapse formation. Here, using a mouse model and a host of histological and molecular biological techniques, we report that the signaling molecule cyclin-dependent kinase 5 (Cdk5), and its activator p35, control the tangential migration of interneurons toward and within the cerebral cortex by modulating the critical neurodevelopmental signaling pathway, ErbB4/phosphatidylinositol 3-kinase, that has been repeatedly linked to schizophrenia. This finding identifies Cdk5 as a crucial signaling factor in cortical interneuron development in mammals.

Non-coding-regulatory regions of human brain genes delineated by bacterial artificial chromosome knock-in mice

BACKGROUND

The next big challenge in human genetics is understanding the 98% of the genome that comprises non-coding DNA. Hidden in this DNA are sequences critical for gene regulation, and new experimental strategies are needed to understand the functional role of gene-regulation sequences in health and disease. In this study, we build upon our HuGX ('high-throughput human genes on the X chromosome') strategy to expand our understanding of human gene regulation in vivo.

RESULTS

In all, ten human genes known to express in therapeutically important brain regions were chosen for study. For eight of these genes, human bacterial artificial chromosome clones were identified, retrofitted with a reporter, knocked single-copy into the Hprt locus in mouse embryonic stem cells, and mouse strains derived. Five of these human genes expressed in mouse, and all expressed in the adult brain region for which they were chosen. This defined the boundaries of the genomic DNA sufficient for brain expression, and refined our knowledge regarding the complexity of gene regulation. We also characterized for the first time the expression of human MAOA and NR2F2, two genes for which the mouse homologs have been extensively studied in the central nervous system (CNS), and AMOTL1 and NOV, for which roles in CNS have been unclear.

CONCLUSIONS

We have demonstrated the use of the HuGX strategy to functionally delineate non-coding-regulatory regions of therapeutically important human brain genes. Our results also show that a careful investigation, using publicly available resources and bioinformatics, can lead to accurate predictions of gene expression.

PROX1: a lineage tracer for cortical interneurons originating in the lateral/caudal ganglionic eminence and preoptic area

The homeobox-encoding gene Prox1 and its Drosophila homologue prospero are key regulators of cell fate-specification. In the developing rodent cortex a sparse population of cells thought to correspond to late-generated cortical pyramidal neuron precursors expresses PROX1. Using a series of transgenic mice that mark cell lineages in the subcortical telencephalon and, more specifically, different populations of cortical interneurons, we demonstrate that neurons expressing PROX1 do not represent pyramidal neurons or their precursors but are instead subsets of cortical interneurons. These correspond to interneurons originating in the lateral/caudal ganglionic eminence (LGE/CGE) and a small number of preoptic area (POA)-derived neurons. Expression within the cortex can be detected from late embryonic stages onwards when cortical interneurons are still migrating. There is persistent expression in postmitotic cells in the mature brain mainly in the outer cortical layers. PROX1(+ve) interneurons express neurochemical markers such as calretinin, neuropeptide Y, reelin and vasoactive intestinal peptide, all of which are enriched in LGE/CGE- and some POA-derived cells. Unlike in the cortex, in the striatum PROX1 marks nearly all interneurons regardless of their origin. Weak expression of PROX1 can also be detected in oligodendrocyte lineage cells throughout the forebrain. Our data show that PROX1 can be used as a genetic lineage tracer of nearly all LGE/CGE- and subsets POA-derived cortical interneurons at all developmental and postnatal stages in vivo.

Subcortical origins of human and monkey neocortical interneurons

Cortical GABAergic inhibitory interneurons have crucial roles in the development and function of the cerebral cortex. In rodents, nearly all neocortical interneurons are generated from the subcortical ganglionic eminences. In humans and nonhuman primates, however, the developmental origin of neocortical GABAergic interneurons remains unclear. Here we show that the expression patterns of several key transcription factors in the developing primate telencephalon are very similar to those in rodents, delineating the three main subcortical progenitor domains (the medial, lateral and caudal ganglionic eminences) and the interneurons tangentially migrating from them. On the basis of the continuity of Sox6, COUP-TFII and Sp8 transcription factor expression and evidence from cell migration and cell fate analyses, we propose that the majority of primate neocortical GABAergic interneurons originate from ganglionic eminences of the ventral telencephalon. Our findings reveal that the mammalian neocortex shares basic rules for interneuron development, substantially reshaping our understanding of the origin and classification of primate neocortical interneurons.

Non-epithelial stem cells and cortical interneuron production in the human ganglionic eminences

GABAergic cortical interneurons underlie the complexity of neural circuits and are particularly numerous and diverse in humans. In rodents, cortical interneurons originate in the subpallial ganglionic eminences, but their developmental origins in humans are controversial. We characterized the developing human ganglionic eminences and found that the subventricular zone (SVZ) expanded massively during the early second trimester, becoming densely populated with neural stem cells and intermediate progenitor cells. In contrast with the cortex, most stem cells in the ganglionic eminence SVZ did not maintain radial fibers or orientation. The medial ganglionic eminence exhibited unique patterns of progenitor cell organization and clustering, and markers revealed that the caudal ganglionic eminence generated a greater proportion of cortical interneurons in humans than in rodents. On the basis of labeling of newborn neurons in slice culture and mapping of proliferating interneuron progenitors, we conclude that the vast majority of human cortical interneurons are produced in the ganglionic eminences, including an enormous contribution from non-epithelial SVZ stem cells.

Prenatal ontogeny as a susceptibility period for cortical GABA neuron disturbances in schizophrenia

Cognitive deficits in schizophrenia have been linked to disturbances in GABA neurons in the prefrontal cortex (PFC). Furthermore, cognitive deficits in schizophrenia appear well before the onset of psychosis and have been reported to be present during early childhood and even during the first year of life. Taken together, these data raise the following question: Does the disease process that produces abnormalities in prefrontal GABA neurons in schizophrenia begin prenatally and disrupt the ontogeny of cortical GABA neurons? Here, we address this question through a consideration of evidence that genetic and/or environmental insults that occur during gestation initiate a pathogenetic process that alters cortical GABA neuron ontogeny and produces the pattern of GABA neuron abnormalities, and consequently cognitive difficulties, seen in schizophrenia. First, we review available evidence from postmortem human brain tissue studies characterizing alterations in certain subpopulations of prefrontal GABA neuron that provide clues to a prenatal origin in schizophrenia. Second, we review recent discoveries of transcription factors, cytokine receptors, and other developmental regulators that govern the birth, migration, specification, maturation, and survival of different subpopulations of prefrontal GABA neurons. Third, we discuss recent studies demonstrating altered expression of these ontogenetic factors in the PFC in schizophrenia. Fourth, we discuss the potential role of disturbances in the maternal-fetal environment such as maternal immune activation in the development of GABA neuron dysfunction. Finally, we propose critical questions that need to be answered in future research to further investigate the role of altered GABA neuron ontogeny in the pathogenesis of schizophrenia.

Nuclear receptor COUP-TFII-expressing neocortical interneurons are derived from the medial and lateral/caudal ganglionic eminence and define specific subsets of mature interneurons

Neocortical GABAergic interneurons in rodents originate from subpallial progenitor zones. The majority of mouse neocortical interneurons are derived from the medial and caudal ganglionic eminences (MGE and CGE, respectively) and the preoptic area (POA). It is controversial whether the lateral ganglionic eminence (LGE) also generates neocortical interneurons. Previously it was shown that the transcription factor COUP-TFII is expressed in the CGE; here we show that COUP-TFII is also expressed in the dorsal MGE, dorsal LGE (dMGE and dLGE, respectively), and POA. In the adult neocortex, COUP-TFII+/somatostatin (SOM)+ interneurons are mainly located in layer V. Using a genetic fate-mapping approach (Shh-Cre and Nkx2.1-Cre), we demonstrate that the POA and ventral MGE do not give rise to COUP-TFII+ neocortical interneurons, suggesting that the dMGE is the source of COUP-TFII+/SOM+ neocortical interneurons. We also observed a migratory stream of COUP-TFII+/Sp8+ cells emanating from the dLGE and CGE to the neocortex mainly through the subventricular zone at later embryonic stages. Slice culture experiments in which dLGE progenitors were labeled with BrdU provided additional evidence that the dLGE generates neocortical interneurons. While earlier-born dMGE-derived COUP-TFII+ interneurons occupy cortical layer V, later-born dLGE- and CGE-derived COUP-TFII+ ones preferentially occupy superficial cortical layers. A similar laminar distribution was observed following neonatal transplantation of embryonic day (E)14.5 dMGE and E15.5 dLGE. These results provide novel information about interneuron fate and position from spatially and temporally distinct origins in the ganglionic eminences.

Development and specification of GABAergic cortical interneurons

GABAergic interneurons are inhibitory neurons of the nervous system that play a vital role in neural circuitry and activity. They are so named due to their release of the neurotransmitter gamma-aminobutyric acid (GABA), and occupy different areas of the brain. This review will focus primarily on GABAergic interneurons of the mammalian cerebral cortex from a developmental standpoint. There is a diverse amount of cortical interneuronal subtypes that may be categorized by a number of characteristics; this review will classify them largely by the protein markers they express. The developmental origins of GABAergic interneurons will be discussed, as well as factors that influence the complex migration routes that these interneurons must take in order to ultimately localize in the cerebral cortex where they will integrate with the neural circuitry set in place. This review will also place an emphasis on the transcriptional network of genes that play a role in the specification and maintenance of GABAergic interneuron fate. Gaining an understanding of the different aspects of cortical interneuron development and specification, especially in humans, has many useful clinical applications that may serve to treat various neurological disorders linked to alterations in interneuron populations.

DISC1: a key lead in studying cortical development and associated brain disorders

For the past decade, DISC1 has been studied as a promising lead to understand the biology underlying major mental illnesses, such as schizophrenia. Consequently, many review articles on DISC1 have been published. In this article, rather than repeating comprehensive overviews of research articles, we will introduce the utility of DISC1 in the study of cortical development in association with a wide range of developmental brain disorders. Cortical development involves cell autonomous and cell nonautonomous mechanisms as well as host responses to environmental factors, all of which involve DISC1 function. Thus, we will discuss the significance of DISC1 in forming an overall understanding of multiple mechanisms that orchestrate corticogenesis and can serve as therapeutic targets in diseases caused by abnormal cortical development.

Neuronal nitric oxide synthase expressing neurons: a journey from birth to neuronal circuits

Nitric oxide (NO) is an important signaling molecule crucial for many physiological processes such as synaptic plasticity, vasomotricity, and inflammation. Neuronal nitric oxide synthase (nNOS) is the enzyme responsible for the synthesis of NO by neurons. In the juvenile and mature hippocampus and neocortex nNOS is primarily expressed by subpopulations of GABAergic interneurons. Over the past two decades, many advances have been achieved in the characterization of neocortical and hippocampal nNOS expressing neurons. In this review, we summarize past and present studies that have characterized the electrophysiological, morphological, molecular, and synaptic properties of these neurons. We also discuss recent studies that have shed light on the developmental origins and specification of GABAergic neurons with specific attention to neocortical and hippocampal nNOS expressing GABAergic neurons. Finally, we summarize the roles of NO and nNOS-expressing inhibitory neurons.

COUP-TFII controls amygdala patterning by regulating neuropilin expression

The development of the progenitor zones in the pallium, lateral ganglionic eminence (LGE) and medial ganglionic eminence (MGE) in the subpallium has been well studied; however, so far the role of the caudal ganglionic eminence (CGE), a posterior subpallial domain, in telencephalon patterning remains poorly understood. COUP-TFII, an orphan nuclear receptor, is preferentially expressed in the CGE. We generated COUP-TFII mouse mutants, using Rx-Cre (RxCre;COUP-TFII(F/F)), to study its function in telencephalon development. In these mutants, we found severe defects in the formation of the amygdala complex, including the lateral (LA), basolateral (BLA) and basomedial (BMA) amygdala nuclei. Molecular analysis provided evidence that the migration of CGE-derived Pax6(+) cells failed to settle into the BMA nucleus, owing to reduced expression of neuropilin 1 (Nrp1) and Nrp2, two semaphorin receptors that regulate neuronal cell migration and axon guidance. Our ChIP assays revealed that Nrp1 and Nrp2 genes are the direct targets of COUP-TFII in the telencephalon in vivo. Furthermore, our results showed that the coordinated development between the CGE originated subpallial population (Pax6(+) cells) and pallial populations (Tbr1(+) and Lhx2(+) cells) was essential for patterning the amygdala assembly. Our study presented novel genetic evidence that the caudal ganglionic eminence, a distinct subpallial progenitor zone, contributes cells to the basal telencephalon, such as the BMA nucleus.

Migratory pathways of GABAergic interneurons when they enter the neocortex

Inhibitory gamma-aminobutyric-acid-containing interneurons play important roles in the functions of the neocortex. During rodent development, most neocortical interneurons are generated in the subpallium and migrate tangentially toward the neocortex. They migrate through multiple pathways to enter the neocortex. Failure of interneuron migration through these pathways during development leads to an abnormal distribution and abnormal functions of interneurons in the postnatal brain. Because of recent discoveries regarding the novel origins and migratory pathways of neocortical interneurons, in this article we review the literature on the migratory pathways of interneurons when they enter the neocortex.

Activin induces cortical interneuron identity and differentiation in embryonic stem cell-derived telencephalic neural precursors

Understanding the mechanisms underlying neural progenitor differentiation and neuronal fate specification is critical for the use of embryonic stem cells (ESCs) for regenerative medicine. Cortical interneurons are of particular interest for cell transplantation; however, only a limited subset of these neurons can be generated from ESCs. Here we uncover a pivotal role for Activin in regulating the differentiation and identity of telencephalic neural precursors derived from mouse and human ESCs. We show that Activin directly inhibits the mitogenic sonic hedgehog pathway in a Gli3-dependent manner while enhancing retinoic acid signalling, the pro-neurogenic pathway. In addition, we demonstrate that Activin provides telencephalic neural precursors with positional cues that specifically promote the acquisition of a calretinin interneuron fate by controlling the expression of genes that regulate cortical interneuron identity. This work demonstrates a novel means for regulating neuronal differentiation and specification of subtype identity.

Neurons on the move: migration and lamination of cortical interneurons

The modulation of cortical activity by GABAergic interneurons is required for normal brain function and is achieved through the immense level of heterogeneity within this neuronal population. Cortical interneurons share a common origin in the ventral telencephalon, yet during the maturation process diverse subtypes are generated that form the characteristic laminar arrangement observed in the adult brain. The long distance tangential and short-range radial migration into the cortical plate is regulated by a combination of intrinsic and extrinsic signalling mechanisms, and a great deal of progress has been made to understand these developmental events. In this review, we will summarize current findings regarding the molecular control of subtype specification and provide a detailed account of the migratory cues influencing interneuron migration and lamination. Furthermore, a dysfunctional GABAergic system is associated with a number of neurological and psychiatric conditions, and some of these may have a developmental aetiology with alterations in interneuron generation and migration. We will discuss the notion of additional sources of interneuron progenitors found in human and non-human primates and illustrate how the disruption of early developmental events can instigate a loss in GABAergic function.

Intrinsic and extrinsic mechanisms control the termination of cortical interneuron migration

During development, neurons migrate from their site of origin to their final destinations. Upon reaching this destination, the termination of their migration is crucial for building functional architectures such as laminated structures and nuclei. How this termination is regulated, however, is not clear. Here, we investigated the contribution of cell-intrinsic mechanisms and extrinsic factors. Using GAD67-GFP knock-in mice and in utero electroporation cell labeling, we visualized GABAergic neurons and analyzed their motility in vitro. We find that the motility of GABAergic neurons in cortical slices gradually decreases as development proceeds and is almost abolished by the end of the first postnatal week. Consistent with this, a reduction of embryonic interneuron motility occurred in dissociated cultures. This is in part due to cell-intrinsic mechanisms, as a reduction in motility is observed during long-term culturing on glial feeder cells. Cell-intrinsic regulation is further supported by observations that interneurons labeled in early stages migrated more actively than those labeled in late stages in the same cortical explant. We found evidence suggesting that upregulation of the potassium-chloride cotransporter KCC2 underlies this intrinsic regulation. Reduced motility is also observed when embryonic interneurons are plated on postnatal cortical feeder cells, suggesting extrinsic factors derived from the postnatal cortex too contribute to termination. These factors should include secreted molecules, as cultured postnatal cortical cells could exercise this effect without directly contacting the interneuron. These findings suggest that intrinsic mechanisms and extrinsic factors coordinate to reduce the motility of migrating neurons, thereby leading to the termination of migration.

HBL-1 patterns synaptic remodeling in C. elegans

During development, circuits are refined by the dynamic addition and removal of synapses; however, little is known about the molecular mechanisms that dictate where and when synaptic refinement occurs. Here we describe transcriptional mechanisms that pattern remodeling of C. elegans neuromuscular junctions (NMJs). The embryonic GABAergic DD motor neurons remodel their synapses, whereas the later born VD neurons do not. This specificity is mediated by differential expression of a transcription factor (HBL-1), which is expressed in DD neurons but is repressed in VDs by UNC-55/COUP-TF. DD remodeling is delayed in hbl-1 mutants whereas precocious remodeling is observed in mutants lacking the microRNA mir-84, which inhibits hbl-1 expression. Mutations increasing and decreasing circuit activity cause corresponding changes in hbl-1 expression, and corresponding shifts in the timing of DD plasticity. Thus, convergent regulation of hbl-1 expression defines a genetic mechanism that patterns activity-dependent synaptic remodeling across cell types and across developmental time.

Sticking out of the crowd: the molecular identity and development of cholecystokinin-containing basket cells

Certain essential cognitive processes require the precise temporal interplay between glutamatergic (excitatory) pyramidal cells and γ-aminobutyric acid (GABA)-releasing inhibitory interneurons in the hippocampus. Basket cells, the main class of interneurons, target pyramidal cell somata and proximal dendrites and thus are poised to modify network oscillations. Though only present in limited numbers, the impaired development of basket cells can result in changes in the hippocampal circuitry leading to neurological disorders, such as schizophrenia. The diversity of the spatial origins, neurochemical make-up, cytoarchitecture and network contributions amongst basket cells is a provocative example of interneuron heterogeneity in the hippocampus. This review discusses recent data concerned with the developmental trajectories of one subclass, the cholecystokinin-containing basket cell, and emphasizes the significance of the short-range intercellular guidance cues that have recently emerged to impact the formation and function of their inhibitory synapses.

COUP-TFII expressing interneurons in human fetal forebrain

Transcription factor COUP-TFII in rodents is important for migration of cortical interneurons from caudal ganglionic eminence (CGE) to the neocortex. Since in human, unlike in rodents, cortical interneurons have both ganglionic eminence (GE) and dorsal cortical origin, we studied the distribution of COUP-TFII in the human developing neocortex from 9 to 22 gestational weeks. COUP-TFII is expressed at all stages studied in the GE and in various cortical zones, from the proliferative ventricular/subventricular zone (VZ/SVZ) to layer I. Gradients of COUP-TFII expression are present in the GE, with peak expression in the CGE, and in the neocortex, from high expression in the temporal and occipital cortex to moderate in the frontal and dorsal cortex. Double immunofluorescence with γ-aminobutyric acid (GABA), calretinin, or calbindin, established that subpopulations of interneurons express COUP-TFII. A small fraction of COUP-TFII(+) cells are progenitor cells that proliferate in the CGE (3.4 ± 0.3%) and in the cortical VZ/SVZ (1.7 ± 0.1%). In summary, COUP-TFII is expressed in the human fetal forebrain in GABAergic cells, according to its possible role in migration of cortical interneurons. The source of these cells seems to be the CGE and, to a smaller extent, the cortical VZ/SVZ.

Origins of neocortical interneurons in mice

GABAergic interneurons influence the development and function of the cerebral cortex through the actions of a variety of subtypes. Despite the relevance to cortical function and dysfunction, including seizure disorders and neuropsychiatric illnesses, the molecular determinants of interneuron fate remain largely unidentified. Challenges to this endeavor include the difficulty of studying fate determination of cells that even in rodents do not fully mature until weeks after their embryonic birth. However, in recent years a strong literature has grown on the temporal and spatial origins of distinct interneuron groups and types. Here we seek to highlight these findings, particularly in mice. Our goal is to lay the groundwork for future studies that use mouse genetics to study cortical interneuron fate determination and function.

Interneurons in the developing human neocortex

Cortical interneurons play a crucial role in the functioning of cortical microcircuitry as they provide inhibitory input to projection (pyramidal) neurons. Despite their involvement in various neurological and psychiatric disorders, our knowledge about their development in human cerebral cortex is still incomplete. Here we demonstrate that at the beginning of corticogenesis, at embryonic 5 gestation weeks (gw, Carnegie stage 16) in human, early neurons could be labeled with calretinin, calbindin, and GABA antibodies. These immunolabeled cells show a gradient from the ganglionic eminences (GE) toward the neocortex, suggesting that GE is a well conserved source of early born cortical interneurons from rodents to human. At mid-term (20 gw), however, a subset of calretinin(+) cells proliferates in the cortical subventricular zone (SVZ), suggesting a second set of interneuron progenitors that have neocortical origin. Neuropeptide Y, somatostatin, or parvalbumin cells are sparse in mid-term cerebral cortex. In addition to the early source of cortical interneurons in the GE and later in the neocortical SVZ, other regions, such as the subpial granular layer, may also contribute to the population of human cortical interneurons. In conclusion, our findings from cryosections and previous in vitro results suggest that cortical interneuron progenitor population is more complex in humans relative to rodents. The increased complexity of progenitors is probably evolutionary adaptation necessary for development of the higher brain functions characteristic to humans.

New insights into cortical interneurons development and classification: contribution of developmental studies

The concerted development of GABAergic interneurons and glutamatergic neurons is a key feature in the construction of the cerebral cortex. In contrast with glutamatergic neurons, GABAergic interneurons are heterogeneous differing by their axonal and dendritic morphologies, biochemical markers, connectivity, and physiology. Furthermore, interneurons have recently been shown to be generated in a variety of telencephalic structures (the ganglionic eminences, the entopeduncular and preoptic areas and the cortex). This review describes the origin, specification and differentiation of interneurons. These recent developmental studies may help to clarify the classification of mature interneurons. In particular recent studies, including our own, provide compelling evidences that most interneurons are specify after their last division in their region of origin before migration. The roles of target tissues in determining the final physiological properties of interneurons are also discussed.