The germinal zones of the basal ganglia but not the septum generate GABAergic interneurons for the cortex

Developmental origin and local signals cooperate to determine septal astrocyte identity

Astrocyte specification during development is influenced by both intrinsic and extrinsic factors, but the precise contribution of each remains poorly understood. Here we show that septal astrocytes from Nkx2.1 and Zic4 expressing progenitor zones are allocated into non-overlapping domains of the medial (MS) and lateral septal nuclei (LS) respectively. Astrocytes in these areas exhibit distinctive molecular and morphological features tailored to the unique cellular and synaptic circuit environment of each nucleus. Using single-nucleus (sn) RNA sequencing, we trace the developmental trajectories of cells in the septum and find that neurons and astrocytes undergo region and developmental stage-specific local cell-cell interactions. We show that expression of the classic morphogens Sonic hedgehog (Shh) and Fibroblast growth factors (Fgfs) by MS and LS neurons respectively, functions to promote the molecular specification of local astrocytes in each region. Finally, using heterotopic cell transplantation, we show that both morphological and molecular specifications of septal astrocytes are highly dependent on the local microenvironment, regardless of developmental origins. Our data highlights the complex interplay between intrinsic and extrinsic factors shaping astrocyte identities and illustrates the importance of the local environment in determining astrocyte functional specialization.

A developmentally defined population of neurons in the lateral septum controls responses to aversive stimuli

When interacting with their environment, animals must balance exploratory and defensive behavior to evaluate and respond to potential threats. The lateral septum (LS) is a structure in the ventral forebrain that calibrates the magnitude of behavioral responses to stress-related external stimuli, including the regulation of threat avoidance. The complex connectivity between the LS and other parts of the brain, together with its largely unexplored neuronal diversity, makes it difficult to understand how defined LS circuits control specific behaviors. Here, we describe a mouse model in which a population of neurons with a common developmental origin (Nkx2.1-lineage neurons) are absent from the LS. Using a combination of circuit tracing and behavioral analyses, we found that these neurons receive inputs from the perifornical area of the anterior hypothalamus (PeFAH) and are specifically activated in stressful contexts. Mice lacking Nkx2.1-lineage LS neurons display increased exploratory behavior even under stressful conditions. Our study extends the current knowledge about how defined neuronal populations within the LS can evaluate contextual information to select appropriate behavioral responses. This is a necessary step towards understanding the crucial role that the LS plays in neuropsychiatric conditions where defensive behavior is dysregulated, such as anxiety and aggression disorders.

Interneuron odyssey: molecular mechanisms of tangential migration

Cortical GABAergic interneurons are critical components of neural networks. They provide local and long-range inhibition and help coordinate network activities involved in various brain functions, including signal processing, learning, memory and adaptative responses. Disruption of cortical GABAergic interneuron migration thus induces profound deficits in neural network organization and function, and results in a variety of neurodevelopmental and neuropsychiatric disorders including epilepsy, intellectual disability, autism spectrum disorders and schizophrenia. It is thus of paramount importance to elucidate the specific mechanisms that govern the migration of interneurons to clarify some of the underlying disease mechanisms. GABAergic interneurons destined to populate the cortex arise from multipotent ventral progenitor cells located in the ganglionic eminences and pre-optic area. Post-mitotic interneurons exit their place of origin in the ventral forebrain and migrate dorsally using defined migratory streams to reach the cortical plate, which they enter through radial migration before dispersing to settle in their final laminar allocation. While migrating, cortical interneurons constantly change their morphology through the dynamic remodeling of actomyosin and microtubule cytoskeleton as they detect and integrate extracellular guidance cues generated by neuronal and non-neuronal sources distributed along their migratory routes. These processes ensure proper distribution of GABAergic interneurons across cortical areas and lamina, supporting the development of adequate network connectivity and brain function. This short review summarizes current knowledge on the cellular and molecular mechanisms controlling cortical GABAergic interneuron migration, with a focus on tangential migration, and addresses potential avenues for cell-based interneuron progenitor transplants in the treatment of neurodevelopmental disorders and epilepsy.

Cortical interneuron specification and diversification in the era of big data

Inhibition in the mammalian cerebral cortex is mediated by a small population of highly diverse GABAergic interneurons. These largely local neurons are interspersed among excitatory projection neurons and exert pivotal regulation on the formation and function of cortical circuits. We are beginning to understand the extent of GABAergic neuron diversity and how this is generated and shaped during brain development in mice and humans. In this review, we summarise recent findings and discuss how new technologies are being used to further advance our knowledge. Understanding how inhibitory neurons are generated in the embryo is an essential pre-requisite of stem cell therapy, an evolving area of research, aimed at correcting human disorders that result in inhibitory dysfunction.

Fate mapping reveals mixed embryonic origin and unique developmental codes of mouse forebrain septal neurons

The septum is a key structure at the core of the forebrain that integrates inputs and relays information to other brain areas to support cognition and behaviours such as feeding and locomotion. Underlying these functions is a rich diversity of neuronal types and an intricate complexity of wiring across and within the septal region. We currently have very little understanding of how septal neuronal diversity emerges during development. Using transgenic mice expressing Cre in different subsets of telencephalic precursors we explored the origins of the three main neuronal types of the septal complex: GABAergic, cholinergic and glutamatergic neurons. We find that septal neurons originate from distinct neuroepithelial domains of the developing septum and are born at different embryonic time points. An exception to this is the GABAergic medial septal Parvalbumin-expressing population which is generated outside the septum from surrounding germinal zones. We identify the transcription factor BSX as being expressed in the developing glutamatergic neuron population. Embryonic elimination of BSX in the septum results in a reduction of septal glutamatergic cell numbers and a consequent deficit in locomotion. Further refinement of septal neuron diversity is needed to understand the multiple roles of septal neurons and their contribution to distinct behaviours.

MTG8 interacts with LHX6 to specify cortical interneuron subtype identity

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

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

Origin, Development, and Synaptogenesis of Cortical Interneurons

The mammalian cerebral cortex represents one of the most recent and astonishing inventions of nature, responsible of a large diversity of functions that range from sensory processing to high-order cognitive abilities, such as logical reasoning or language. Decades of dedicated study have contributed to our current understanding of this structure, both at structural and functional levels. A key feature of the neocortex is its outstanding richness in cell diversity, composed by multiple types of long-range projecting neurons and locally connecting interneurons. In this review, we will describe the great diversity of interneurons that constitute local neocortical circuits and summarize the mechanisms underlying their development and their assembly into functional networks.

Single cell enhancer activity distinguishes GABAergic and cholinergic lineages in embryonic mouse basal ganglia

During brain development, neurons are generated by spatially and temporally distinct processes that remain to be fully characterized. The ganglionic eminences (GEs) in the embryonic subpallium give rise to GABAergic and cholinergic neuron lineages that form the basal ganglia or migrate to the cerebral cortex. Beyond a limited set of canonical RNA markers, the transcriptional states of GE progenitors and immature neurons cells remain poorly defined. We combine enhancer labeling, single-cell transcriptomics using transcription factor-anchored clustering, and integration with in situ hybridization data to distinguish emerging neuronal populations in embryonic mouse basal ganglia. Our results demonstrate the specificity of enhancer-based labeling at single-cell resolution and reveal developmental origins and specification processes of critical neuronal lineages.

Enhancers integrate transcription factor signaling pathways that drive cell fate specification in the developing brain. We paired enhancer labeling and single-cell RNA-sequencing (scRNA-seq) to delineate and distinguish specification of neuronal lineages in mouse medial, lateral, and caudal ganglionic eminences (MGE, LGE, and CGE) at embryonic day (E)11.5. We show that scRNA-seq clustering using transcription factors improves resolution of regional and developmental populations, and that enhancer activities identify specific and overlapping GE-derived neuronal populations. First, we mapped the activities of seven evolutionarily conserved brain enhancers at single-cell resolution in vivo, finding that the selected enhancers had diverse activities in specific progenitor and neuronal populations across the GEs. We then applied enhancer-based labeling, scRNA-seq, and analysis of in situ hybridization data to distinguish transcriptionally distinct and spatially defined subtypes of MGE-derived GABAergic and cholinergic projection neurons and interneurons. Our results map developmental origins and specification paths underlying neurogenesis in the embryonic basal ganglia and showcase the power of scRNA-seq combined with enhancer-based labeling to resolve the complex paths of neuronal specification underlying mouse brain development.

Interneuron origin and molecular diversity in the human fetal brain

Precise generation of excitatory neurons and inhibitory interneurons is crucial for proper formation and function of neural circuits in the mammalian brain. Because of the size and complexity of the human brain, it is a challenge to reveal the rich diversity of interneurons. To decipher origin and diversity of interneurons in the human fetal subpallium, here we show molecular features of diverse subtypes of interneuron progenitors and precursors by conducting single-cell RNA sequencing and in situ sequencing. Interneuron precursors in the medial and lateral ganglionic eminence simultaneously procure temporal and spatial identity through expressing a combination of specific sets of RNA transcripts. Acquisition of various interneuron subtypes in adult human brains occurs even at fetal stages. Our study uncovers complex molecular signatures of interneuron progenitors and precursors in the human fetal subpallium and highlights the logic and programs in the origin and lineage specification of various interneurons.

Analysis of Islet-1, Nkx2.1, Pax6, and Orthopedia in the forebrain of the sturgeon Acipenser ruthenus identifies conserved prosomeric characteristics

The distribution patterns of a set of conserved brain developmental regulatory transcription factors were analyzed in the forebrain of the basal actinopterygian fish Acipenser ruthenus, consistent with the prosomeric model. In the telencephalon, the pallium was characterized by ventricular expression of Pax6. In the subpallium, the combined expression of Nkx2.1/Islet-1 (Isl1) allowed to propose ventral and dorsal areas, as the septo-pallidal (Nkx2.1/Isl1+) and striatal derivatives (Isl1+), respectively, and a dorsal portion of the striatal derivatives, ventricularly rich in Pax6 and devoid of Isl1 expression. Dispersed Orthopedia (Otp) cells were found in the supracommissural and posterior nuclei of the ventral telencephalon, related to the medial portion of the amygdaloid complex. The preoptic area was identified by the Nkx2.1/Isl1 expression. In the alar hypothalamus, an Otp-expressing territory, lacking Nkx2.1/Isl1, was identified as the paraventricular domain. The adjacent subparaventricular domain (Spa) was subdivided in a rostral territory expressing Nkx2.1 and an Isl1+ caudal one. In the basal hypothalamus, the tuberal region was defined by the Nkx2.1/Isl1 expression and a rostral Otp-expressing domain was identified. Moreover, the Otp/Nkx2.1 combination showed an additional zone lacking Isl1, tentatively identified as the mamillary area. In the diencephalon, both Pax6 and Isl1 defined the prethalamic domain, and within the basal prosomere 3, scattered Pax6- and Isl1-expressing cells were observed in the posterior tubercle. Finally, a small group of Pax6 cells was observed in the pretectal area. These results improve the understanding of the forebrain evolution and demonstrate that its basic bauplan is present very early in the vertebrate lineage.

Cellular resolution anatomical and molecular atlases for prenatal human brains

Increasing interest in studies of prenatal human brain development, particularly using new single‐cell genomics and anatomical technologies to create cell atlases, creates a strong need for accurate and detailed anatomical reference atlases. In this study, we present two cellular‐resolution digital anatomical atlases for prenatal human brain at postconceptional weeks (PCW) 15 and 21. Both atlases were annotated on sequential Nissl‐stained sections covering brain‐wide structures on the basis of combined analysis of cytoarchitecture, acetylcholinesterase staining, and an extensive marker gene expression dataset. This high information content dataset allowed reliable and accurate demarcation of developing cortical and subcortical structures and their subdivisions. Furthermore, using the anatomical atlases as a guide, spatial expression of 37 and 5 genes from the brains, respectively, at PCW 15 and 21 was annotated, illustrating reliable marker genes for many developing brain structures. Finally, the present study uncovered several novel developmental features, such as the lack of an outer subventricular zone in the hippocampal formation and entorhinal cortex, and the apparent extension of both cortical (excitatory) and subcortical (inhibitory) progenitors into the prenatal olfactory bulb. These comprehensive atlases provide useful tools for visualization, segmentation, targeting, imaging, and interpretation of brain structures of prenatal human brain, and for guiding and interpreting the next generation of cell census and connectome studies.

Human Neural Stem Cells for Cell-Based Medicinal Products

Neural stem cells represent an attractive tool for the development of regenerative therapies and are being tested in clinical trials for several neurological disorders. Human neural stem cells can be isolated from the central nervous system or can be derived in vitro from pluripotent stem cells. Embryonic sources are ethically controversial and other sources are less well characterized and/or inefficient. Recently, isolation of NSC from the cerebrospinal fluid of patients with spina bifida and with intracerebroventricular hemorrhage has been reported. Direct reprogramming may become another alternative if genetic and phenotypic stability of the reprogrammed cells is ensured. Here, we discuss the advantages and disadvantages of available sources of neural stem cells for the production of cell-based therapies for clinical applications. We review available safety and efficacy clinical data and discuss scalability and quality control considerations for manufacturing clinical grade cell products for successful clinical application.

Transcriptional profiling of sequentially generated septal neuron fates

The septum is a ventral forebrain structure known to regulate innate behaviors. During embryonic development, septal neurons are produced in multiple proliferative areas from neural progenitors following transcriptional programs that are still largely unknown. Here, we use a combination of single-cell RNA sequencing, histology, and genetic models to address how septal neuron diversity is established during neurogenesis. We find that the transcriptional profiles of septal progenitors change along neurogenesis, coinciding with the generation of distinct neuron types. We characterize the septal eminence, an anatomically distinct and transient proliferative zone composed of progenitors with distinctive molecular profiles, proliferative capacity, and fate potential compared to the rostral septal progenitor zone. We show that Nkx2.1-expressing septal eminence progenitors give rise to neurons belonging to at least three morphological classes, born in temporal cohorts that are distributed across different septal nuclei in a sequential fountain-like pattern. Our study provides insight into the molecular programs that control the sequential production of different neuronal types in the septum, a structure with important roles in regulating mood and motivation.

Sp9 Regulates Medial Ganglionic Eminence-Derived Cortical Interneuron Development

Immature neurons generated by the subpallial MGE tangentially migrate to the cortex where they become parvalbumin-expressing (PV+) and somatostatin (SST+) interneurons. Here, we show that the Sp9 transcription factor controls the development of MGE-derived cortical interneurons. SP9 is expressed in the MGE subventricular zone and in MGE-derived migrating interneurons. Sp9 null and conditional mutant mice have approximately 50% reduction of MGE-derived cortical interneurons, an ectopic aggregation of MGE-derived neurons in the embryonic ventral telencephalon, and an increased ratio of SST+/PV+ cortical interneurons. RNA-Seq and SP9 ChIP-Seq reveal that SP9 regulates MGE-derived cortical interneuron development through controlling the expression of key transcription factors Arx, Lhx6, Lhx8, Nkx2-1, and Zeb2 involved in interneuron development, as well as genes implicated in regulating interneuron migration Ackr3, Epha3, and St18. Thus, Sp9 has a central transcriptional role in MGE-derived cortical interneuron development.

Cortical distribution of GABAergic interneurons is determined by migration time and brain size

Cortical interneurons (CINs) originate in the ganglionic eminences (GEs) and migrate tangentially to the cortex guided by different attractive and repulsive cues. Once inside the cortex, the cellular and molecular mechanisms determining the migration of CINs along the rostrocaudal axis are less well understood. Here, we investigated the cortical distribution of CINs originating in the medial and caudal GEs at different time points. Using molecular and genetic labeling, we showed that, in the mouse, early- and late-born CINs (E12 versus E15) are differentially distributed along the rostrocaudal axis. Specifically, late-born CINs are preferentially enriched in cortical areas closer to their respective sites of origin in the medial or caudal GE. Surprisingly, our in vitro experiments failed to show a preferential migration pattern along the rostrocaudal axis for medial- or caudal-born CINs. Moreover, in utero transplantation experiments suggested that the rostrocaudal dispersion of CINs depends on the developmental stage of the host brain and is limited by the migration time and the increasing size of the developing brain. These data suggest that the embryonic expansion of the cortex contributes to the rostrocaudal distribution of CINs.

The role of the diencephalon in the guidance of thalamocortical axons in mice

Thalamocortical axons (TCAs) cross several tissues on their journey to the cortex. Mechanisms must be in place along the route to ensure they connect with their targets in an orderly fashion. The ventral telencephalon acts as an instructive tissue, but the importance of the diencephalon in TCA mapping is unknown. We report that disruption of diencephalic development by Pax6 deletion results in a thalamocortical projection containing mapping errors. We used conditional mutagenesis to test whether these errors are due to the disruption of pioneer projections from prethalamus to thalamus and found that, although this correlates with abnormal TCA fasciculation, it does not induce topographical errors. To test whether the thalamus contains navigational cues for TCAs, we used slice culture transplants and gene expression studies. We found the thalamic environment is instructive for TCA navigation and that the molecular cues netrin 1 and semaphorin 3a are likely to be involved. Our findings indicate that the correct topographic mapping of TCAs onto the cortex requires the order to be established from the earliest stages of their growth by molecular cues in the thalamus itself.

Summary: The diencephalon plays a role in the correct organization of thalamocortical axons. The thalamic environment is instructive for their correct medial-lateral position, while prethalamic pioneer axons help to avoid premature fasciculation.

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.

Expression of ventral telencephalon transcription factors ASCL1 and DLX2 in the early fetal human cerebral cortex

In rodent ventral telencephalon, diffusible morphogens induce expression of the proneural transcription factor ASCL1, which in turn induces expression of the transcription factor DLX2 that controls differentiation of cortical interneuron precursors and their tangential migration to the cerebral cortex. RNAseq analysis of human fetal samples of dorsal telencephalon revealed consistently high cortical expression of ASCL1 and increasing expression of DLX2 between 7.5 and 17 post-conceptional weeks (PCW). We explored whether cortical expression of these genes represented a population of intracortically derived interneuron precursors. Immunohistochemistry revealed an ASCL1+ /DLX2+ population of progenitor cells in the human ganglionic eminences between 6.5 and 12 PCW, but in the cortex there also existed a population of ASCL1+ /DLX2- progenitors in the subventricular zone (SVZ) that largely co-expressed cortical markers PAX6 or TBR2, although a few ASCL1+ /PAX6- progenitors were observed in the ventricular zone (VZ) and ASCL1+ cells expressing the interneuron marker GAD67 were present in the SVZ. Although rare in the VZ, DLX2+ cells progressively increased in number between 8 and 12 PCW across the cortical wall and the majority co-expressed LHX6 and originated either in the MGE, migrating to the lateral cortex, or from the septum, populating the medial wall. A minority co-expressed COUP-TFII, which identifies cells from the caudal ganglionic eminence (CGE). By 19 PCW, a significant increase in expression of DLX2 and ASCL1 was observed in the cortical VZ with a small proportion expressing both proteins. The DLX2+ cells did not co-express a cell division marker, so were not progenitors. The majority of DLX2+ cells throughout the cortical plate expressed COUP-TFII rather than LHX6+ . As the VZ declined as a proliferative zone it appeared to be re-defined as a migration pathway for COUP-TFII+ /DLX2+ interneurons from CGE to cortex. Therefore, in developing human cortex, ASCL1 expression predominantly marks a population of intermediate progenitors giving rise to glutamatergic neurons. DLX2 expression predominantly defines post-mitotic interneuron precursors.

New insights into the development of the human cerebral cortex

The cerebral cortex constitutes more than half the volume of the human brain and is presumed to be responsible for the neuronal computations underlying complex phenomena, such as perception, thought, language, attention, episodic memory and voluntary movement. Rodent models are extremely valuable for the investigation of brain development, but cannot provide insight into aspects that are unique or highly derived in humans. Many human psychiatric and neurological conditions have developmental origins but cannot be studied adequately in animal models. The human cerebral cortex has some unique genetic, molecular, cellular and anatomical features, which need to be further explored. The Anatomical Society devoted its summer meeting to the topic of Human Brain Development in June 2018 to tackle these important issues. The meeting was organized by Gavin Clowry (Newcastle University) and Zoltán Molnár (University of Oxford), and held at St John's College, Oxford. The participants provided a broad overview of the structure of the human brain in the context of scaling relationships across the brains of mammals, conserved principles and recent changes in the human lineage. Speakers considered how neuronal progenitors diversified in human to generate an increasing variety of cortical neurons. The formation of the earliest cortical circuits of the earliest generated neurons in the subplate was discussed together with their involvement in neurodevelopmental pathologies. Gene expression networks and susceptibility genes associated to neurodevelopmental diseases were discussed and compared with the networks that can be identified in organoids developed from induced pluripotent stem cells that recapitulate some aspects of in vivo development. New views were discussed on the specification of glutamatergic pyramidal and γ-aminobutyric acid (GABA)ergic interneurons. With the advancement of various in vivo imaging methods, the histopathological observations can be now linked to in vivo normal conditions and to various diseases. Our review gives a general evaluation of the exciting new developments in these areas. The human cortex has a much enlarged association cortex with greater interconnectivity of cortical areas with each other and with an expanded thalamus. The human cortex has relative enlargement of the upper layers, enhanced diversity and function of inhibitory interneurons and a highly expanded transient subplate layer during development. Here we highlight recent studies that address how these differences emerge during development focusing on diverse facets of our evolution.

White matter neuron biology and neuropathology in schizophrenia

Schizophrenia is considered a neurodevelopmental disorder as it often manifests before full brain maturation and is also a cerebral cortical disorder where deficits in GABAergic interneurons are prominent. Whilst most neurons are located in cortical and subcortical grey matter regions, a smaller population of neurons reside in white matter tracts of the primate and to a lesser extent, the rodent brain, subjacent to the cortex. These interstitial white matter neurons (IWMNs) have been identified with general markers for neurons [e.g., neuronal nuclear antigen (NeuN)] and with specific markers for neuronal subtypes such as GABAergic neurons. Studies of IWMNs in schizophrenia have primarily focused on their density underneath cortical areas known to be affected in schizophrenia such as the dorsolateral prefrontal cortex. Most of these studies of postmortem brains have identified increased NeuN+ and GABAergic IWMN density in people with schizophrenia compared to healthy controls. Whether IWMNs are involved in the pathogenesis of schizophrenia or if they are increased because of the cortical pathology in schizophrenia is unknown. We also do not understand how increased IWMN might contribute to brain dysfunction in the disorder. Here we review the literature on IWMN pathology in schizophrenia. We provide insight into the postulated functional significance of these neurons including how they may contribute to the pathophysiology of schizophrenia.

Long-range projections from sparse populations of GABAergic neurons in murine subplate

The murine subplate contains some of the earliest generated populations of neurons in the cerebral cortex, which play an important role in the maturation of cortical inhibition. Here we present multiple lines of evidence, that the subplate itself is only very sparsely populated with GABAergic neurons at postnatal day (P)8. We used three different transgenic mouse lines, each of which labels a subset of GABAergic, ganglionic eminence derived neurons. Dlx5/6-eGFP labels the most neurons in cortex (on average 11% of NEUN+ cells across all layers at P8) whereas CGE-derived Lhx6-Cre::Dlx1-Venusfl cells are the sparsest (2% of NEUN+ cells across all layers at P8). There is significant variability in the layer distribution of labeled interneurons, with Dlx5/6-eGFP and Lhx6-Cre::R26R-YFP being expressed most abundantly in Layer 5, whereas CGE-derived Lhx6-Cre::Dlx1-Venusfl cells are least abundant in that layer. All three lines label at most 3% of NEUN+ neurons in the subplate, in contrast to L5, in which up to 30% of neurons are GFP+ in Dlx5/6-eGFP. We assessed all three GABAergic populations for expression of the subplate neuron marker connective tissue growth factor (CTGF). CTGF labels up to two-thirds of NEUN+ cells in the subplate, but was never found to colocalize with labeled GABAergic neurons in any of the three transgenic strains. Despite the GABAergic neuronal population in the subplate being sparse, long-distance axonal connection tracing with carbocyanine dyes revealed that some Gad65-GFP+ subplate cells form long-range axonal projections to the internal capsule or callosum.

Alzheimer's disease phospholipase C-gamma-2 (PLCG2) protective variant is a functional hypermorph

BACKGROUND

Recent Genome Wide Association Studies (GWAS) have identified novel rare coding variants in immune genes associated with late onset Alzheimer's disease (LOAD). Amongst these, a polymorphism in phospholipase C-gamma 2 (PLCG2) P522R has been reported to be protective against LOAD. PLC enzymes are key elements in signal transmission networks and are potentially druggable targets. PLCG2 is highly expressed in the hematopoietic system. Hypermorphic mutations in PLCG2 in humans have been reported to cause autoinflammation and immune disorders, suggesting a key role for this enzyme in the regulation of immune cell function.

METHODS

We assessed PLCG2 distribution in human and mouse brain tissue via immunohistochemistry and in situ hybridization. We transfected heterologous cell systems (COS7 and HEK293T cells) to determine the effect of the P522R AD-associated variant on enzymatic function using various orthogonal assays, including a radioactive assay, IP-One ELISA, and calcium assays.

RESULTS

PLCG2 expression is restricted primarily to microglia and granule cells of the dentate gyrus. Plcg2 mRNA is maintained in plaque-associated microglia in the cerebral tissue of an AD mouse model. Functional analysis of the p.P522R variant demonstrated a small hypermorphic effect of the mutation on enzyme function.

CONCLUSIONS

The PLCG2 P522R variant is protective against AD. We show that PLCG2 is expressed in brain microglia, and the p.P522R polymorphism weakly increases enzyme function. These data suggest that activation of PLCγ2 and not inhibition could be therapeutically beneficial in AD. PLCγ2 is therefore a potential target for modulating microglia function in AD, and a small molecule drug that weakly activates PLCγ2 may be one potential therapeutic approach.

Heparan Sulfate Sulfation by Hs2st Restricts Astroglial Precursor Somal Translocation in Developing Mouse Forebrain by a Non-Cell-Autonomous Mechanism

Heparan sulfate (HS) is a cell surface and extracellular matrix carbohydrate extensively modified by differential sulfation. HS interacts physically with canonical fibroblast growth factor (FGF) proteins that signal through the extracellular signal regulated kinase (ERK)/mitogen activated protein kinase (MAPK) pathway. At the embryonic mouse telencephalic midline, FGF/ERK signaling drives astroglial precursor somal translocation from the ventricular zone of the corticoseptal boundary (CSB) to the induseum griseum (IG), producing a focus of Slit2-expressing astroglial guidepost cells essential for interhemispheric corpus callosum (CC) axon navigation. Here, we investigated the cell and molecular function of a specific form of HS sulfation, 2-O HS sulfation catalyzed by the enzyme Hs2st, in midline astroglial development and in regulating FGF protein levels and interaction with HS. Hs2st^−/− embryos of either sex exhibit a grossly enlarged IG due to precocious astroglial translocation and conditional Hs2st mutagenesis and ex vivo culture experiments show that Hs2st is not required cell autonomously by CC axons or by the IG astroglial cell lineage, but rather acts non-cell autonomously to suppress the transmission of translocation signals to astroglial precursors. Rescue of the Hs2st^−/− astroglial translocation phenotype by pharmacologically inhibiting FGF signaling shows that the normal role of Hs2st is to suppress FGF-mediated astroglial translocation. We demonstrate a selective action of Hs2st on FGF protein by showing that Hs2st (but not Hs6st1) normally suppresses the levels of Fgf17 protein in the CSB region in vivo and use a biochemical assay to show that Hs2st (but not Hs6st1) facilitates a physical interaction between the Fgf17 protein and HS.

SIGNIFICANCE STATEMENT We report a novel non-cell-autonomous mechanism regulating cell signaling in developing brain. Using the developing mouse telencephalic midline as an exemplar, we show that the specific sulfation modification of the cell surface and extracellular carbohydrate heparan sulfate (HS) performed by Hs2st suppresses the supply of translocation signals to astroglial precursors by a non-cell-autonomous mechanism. We further show that Hs2st modification selectively facilitates a physical interaction between Fgf17 and HS and suppresses Fgf17 protein levels in vivo, strongly suggesting that Hs2st acts selectively on Fgf17 signaling. HS interacts with many signaling proteins potentially encoding numerous selective interactions important in development and disease, so this class of mechanism may apply more broadly to other biological systems.

Zic4-Lineage Cells Increase Their Contribution to Visual Thalamic Nuclei during Murine Embryogenesis If They Are Homozygous or Heterozygous for Loss of Pax6 Function

Our aim was to study the mechanisms that contribute to the development of discrete thalamic nuclei during mouse embryogenesis (both sexes included). We characterized the expression of the transcription factor coding gene Zic4 and the distribution of cells that expressed Zic4 in their lineage. We used genetic fate mapping to show that Zic4-lineage cells mainly contribute to a subset of thalamic nuclei, in particular the lateral geniculate nuclei (LGNs), which are crucial components of the visual pathway. We observed that almost all Zic4-lineage diencephalic progenitors express the transcription factor Pax6 at variable location-dependent levels. We used conditional mutagenesis to delete either one or both copies of Pax6 from Zic4-lineage cells. We found that Zic4-lineage cells carrying either homozygous or heterozygous loss of Pax6 contributed in abnormally high numbers to one or both of the main lateral geniculate nuclei (LGNs). This could not be attributed to a change in cell production and was likely due to altered sorting of thalamic cells. Our results indicate that positional information encoded by the levels of Pax6 in diencephalic progenitors is an important determinant of the eventual locations of their daughter cells.

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.

Transcriptome and in Vitro Differentiation Profile of Human Embryonic Stem Cell Derived NKX2.1-Positive Neural Progenitors

The generation of inhibitory interneuron progenitors from human embryonic stem cells (ESCs) is of great interest due to their potential use in transplantation therapies designed to treat central nervous system disorders. The medial ganglionic eminence (MGE) is a transient embryonic structure in the ventral telencephalon that is a major source of cortical GABAergic inhibitory interneuron progenitors. These progenitors migrate tangentially to sites in the cortex and differentiate into a variety of interneuron subtypes, forming local synaptic connections with excitatory projection neurons to modulate activity of the cortical circuitry. The homeobox domain-containing transcription factor NKX2.1 is highly expressed in the MGE and pre-optic area of the ventral subpallium and is essential for specifying cortical interneuron fate. Using a combination of growth factor agonists and antagonists to specify ventral telencephalic fates, we previously optimized a protocol for the efficient generation of NKX2.1-positive MGE-like neural progenitors from human ESCs. To establish their identity, we now characterize the transcriptome of these MGE-like neural progenitors using RNA sequencing and demonstrate the capacity of these cells to differentiate into inhibitory interneurons in vitro using a neuron-astrocyte co-culture system. These data provide information on the potential origin of interneurons in the human brain.

Midazolam prevents motor neuronal death from oxidative stress attack mediated by JNK-ERK pathway

Midazolam is a sedative used by patients with mechanical ventilation. However, the potential clinical value is not fully explored. In this report, we made use of a neuroblastoma-spinal cord hybrid motor neuron-like cell line NSC34, and elucidated the potential role of Midazolam on these cells under the insult of oxidative stress. We found the protective effect of Midazolam on motor neurons against cytotoxicity induced by the combination of oligomycin A and rotenone (O/R) or phenylarsine oxide. The characteristics of apoptosis, such as the ratio of TUNEL+ cells or the expression level of cleaved Caspase-3, was decreased by 22 or 45% in the presence of Midazolam. Furthermore, this effect was correlated with the JNK-ERK signaling pathway. Either phosphorylation of ERK or JNK was positively or negatively modulated with the treatment of Midazolam in NSC34 cells attacked by reactive oxygen species. Meanwhile, inhibition or activation of the JNK-ERK pathway regulated the protective effect of Midazolam on NSC34 cells with oxidative stress insult. Collectively, this study elucidated a previously unidentified clinical effect of Midazolam, and put forward the great promise that Midazolam may be considered as a potential candidate to the treatment of motor neuron disease.

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.

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.

Conserved rules in embryonic development of cortical interneurons

This review will focus on early aspects of cortical interneurons (cIN) development from specification to migration and final positioning in the human cerebral cortex. These mechanisms have been largely studied in the mouse model, which provides unique possibilities of genetic analysis, essential to dissect the molecular and cellular events involved in cortical development. An important goal here is to discuss the conservation and the potential divergence of these mechanisms, with a particular interest for the situation in the human embryo. We will thus cover recent works, but also revisit older studies in the light of recent data to better understand the developmental mechanisms underlying cIN differentiation in human. Because cIN are implicated in severe developmental disorders, understanding the molecular and cellular mechanisms controlling their differentiation might clarify some causes and potential therapeutic approaches to these important clinical conditions.

Charting the protomap of the human telencephalon

The cerebral cortex is divided stereotypically into a number of functionally distinct areas. According to the protomap hypothesis formulated by Rakic neural progenitors in the ventricular zone form a mosaic of proliferative units that provide a primordial species-specific cortical map. Positional information of newborn neurons is maintained during their migration to the overlying cortical plate. Much evidence has been found to support this hypothesis from studies of primary cortical areas in mouse models in particular. Differential expansion of cortical areas and the introduction of new functional modules during evolution might be the result of changes in the progenitor cells. The human cerebral cortex shows a wide divergence from the mouse containing a much higher proportion of association cortex and a more complicated regionalised repertoire of neuron sub-types. To what extent does the protomap hypothesis hold true for the primate brain? This review summarises a growing number of studies exploring arealised gene expression in the early developing human telencephalon. The evidence so far is that the human and mouse brain do share fundamental mechanisms of areal specification, however there are subtle differences which could lead us to a better understanding of cortical evolution and the origins of neurodevelopmental diseases.

Septal contributions to olfactory bulb interneuron diversity in the embryonic mouse telencephalon: role of the homeobox gene Gsx2

Background

Olfactory bulb (OB) interneurons are known to represent diverse neuronal subtypes, which are thought to originate from a number of telencephalic regions including the embryonic dorsal lateral ganglionic eminence (dLGE) and septum. These cells migrate rostrally toward the OB, where they then radially migrate to populate different OB layers including the granule cell layer (GCL) and the outer glomerular layer (GL). Although previous studies have attempted to investigate regional contributions to OB interneuron diversity, few genetic tools have been used to address this question at embryonic time points when the earliest populations are specified.

Methods

In this study, we utilized Zic3-lacZ and Gsx2e-CIE transgenic mice as genetic fate-mapping tools to study OB interneuron contributions derived from septum and LGE, respectively. Moreover, to address the regional (i.e. septal) requirements of the homeobox gene Gsx2 for OB interneuron diversity, we conditionally inactivated Gsx2 in the septum, leaving it largely intact in the dLGE, by recombining the Gsx2 floxed allele using Olig2^Cre/+ mice.

Results

Our fate mapping studies demonstrated that the dLGE and septum gave rise to OB interneuron subtypes differently. Notably, the embryonic septum was found to give rise largely to the calretinin⁺ (CR⁺) GL subtype, while the dLGE was more diverse, generating all major GL subpopulations as well as many GCL interneurons. Moreover, Gsx2 conditional mutants (cKOs), with septum but not dLGE recombination, showed impaired generation of CR⁺ interneurons within the OB GL. These Gsx2 cKOs exhibited reduced proliferation within the septal subventricular zone (SVZ), which correlated well with the reduced number of CR⁺ interneurons observed.

Conclusions

Our findings indicate that the septum and LGE contribute differently to OB interneuron diversity. While the dLGE provides a wide range of OB interneuron subtypes, the septum is more restricted in its contribution to the CR⁺ subtype. Gsx2 is required in septal progenitors for the correct expansion of SVZ progenitors specified toward the CR⁺ subtype. Finally, the septum has been suggested to be the exclusive source of CR⁺ interneurons in postnatal studies. Our results here demonstrate that dLGE progenitors in the embryo also contribute to this OB neuronal subtype.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0090-5) contains supplementary material, which is available to authorized users.

NKX2-1 Is Required in the Embryonic Septum for Cholinergic System Development, Learning, and Memory

The transcription factor NKX2-1 is best known for its role in the specification of subsets of cortical, striatal, and pallidal neurons. We demonstrate through genetic fate mapping and intersectional focal septal deletion that NKX2-1 is selectively required in the embryonic septal neuroepithelium for the development of cholinergic septohippocampal projection neurons and large subsets of basal forebrain cholinergic neurons. In the absence of NKX2-1, these neurons fail to develop, causing alterations in hippocampal theta rhythms and severe deficiencies in learning and memory. Our results demonstrate that learning and memory are dependent on NKX2-1 function in the embryonic septum and suggest that cognitive deficiencies that are sometimes associated with pathogenic mutations in NKX2-1 in humans may be a direct consequence of loss of NKX2-1 function.

In Vitro, Ex Vivo and In Vivo Techniques to Study Neuronal Migration in the Developing Cerebral Cortex

Neuronal migration is a fundamental biological process that underlies proper brain development and neuronal circuit formation. In the developing cerebral cortex, distinct neuronal populations, producing excitatory, inhibitory and modulatory neurotransmitters, are generated in different germinative areas and migrate along various routes to reach their final positions within the cortex. Different technical approaches and experimental models have been adopted to study the mechanisms regulating neuronal migration in the cortex. In this review, we will discuss the most common in vitro, ex vivo and in vivo techniques to visualize and study cortical neuronal migration.

Transplantation of GABAergic interneurons for cell-based therapy

Many neurological disorders stem from defects in or the loss of specific neurons. Neuron transplantation has tremendous clinical potential for central nervous system therapy as it may allow for the targeted replacement of those cells that are lost in diseases. Normally, most neurons are added during restricted periods of embryonic and fetal development. The permissive milieu of the developing brain promotes neuronal migration, neuronal differentiation, and synaptogenesis. Once this active period of neurogenesis ends, the chemical and physical environment of the brain changes dramatically. The brain parenchyma becomes highly packed with neuronal and glial processes, extracellular matrix, myelin, and synapses. The migration of grafted cells to allow them to home into target regions and become functionally integrated is a key challenge to neuronal transplantation. Interestingly, transplanted young telencephalic inhibitory interneurons are able to migrate, differentiate, and integrate widely throughout the postnatal brain. These grafted interneurons can also functionally modify local circuit activity. These features have facilitated the use of interneuron transplantation to study fundamental neurodevelopmental processes including cell migration, cell specification, and programmed neuronal cell death. Additionally, these cells provide a unique opportunity to develop interneuron-based strategies for the treatment of diseases linked to interneuron dysfunction and neurological disorders associated to circuit hyperexcitability.

An Overview of the Mechanisms of Abnormal GABAergic Interneuronal Cortical Migration Associated with Prenatal Ethanol Exposure

GABAergic Interneuronal migration constitutes an essential process during corticogenesis. Derived from progenitor cells located in the proliferative zones of the ventral telencephalon, newly generated GABAergic Interneuron migrate to their cortical destinations. Cortical dysfunction associated with defects in neuronal migration results in severe developmental consequences. There is growing evidence linking prenatal ethanol exposure to abnormal GABAergic interneuronal migration and subsequent cortical dysfunction. Investigating the pathophysiological mechanisms behind disrupted GABAergic interneuronal migration encountered with prenatal alcohol exposure is crucial for understanding and managing fetal alcohol spectrum disorders. This review explores the molecular pathways regulating GABAergic interneuronal cortical migration that might be altered by prenatal ethanol exposure thus opening new avenues for further research in this topic.

Distinct cortical and sub-cortical neurogenic domains for GABAergic interneuron precursor transcription factors NKX2.1, OLIG2 and COUP-TFII in early fetal human telencephalon

The extent of similarities and differences between cortical GABAergic interneuron generation in rodent and primate telencephalon remains contentious. We examined expression of three interneuron precursor transcription factors, alongside other markers, using immunohistochemistry on 8–12 post-conceptional weeks (PCW) human telencephalon sections. NKX2.1, OLIG2, and COUP-TFII expression occupied distinct (although overlapping) neurogenic domains which extended into the cortex and revealed three CGE compartments: lateral, medial, and ventral. NKX2.1 expression was very largely confined to the MGE, medial CGE, and ventral septum confirming that, at this developmental stage, interneuron generation from NKX2.1+ precursors closely resembles the process observed in rodents. OLIG2 immunoreactivity was observed in GABAergic cells of the proliferative zones of the MGE and septum, but not necessarily co-expressed with NKX2.1, and OLIG2 expression was also extensively seen in the LGE, CGE, and cortex. At 8 PCW, OLIG2+ cells were only present in the medial and anterior cortical wall suggesting a migratory pathway for interneuron precursors via the septum into the medial cortex. By 12 PCW, OLIG2+ cells were present throughout the cortex and many were actively dividing but without co-expressing cortical progenitor markers. Dividing COUP-TFII+ progenitor cells were localized to ventral CGE as previously described but were also numerous in adjacent ventral cortex; in both the cases, COUP-TFII was co-expressed with PAX6 in proliferative zones and TBR1 or calretinin in post-mitotic cortical neurons. Thus COUP-TFII+ progenitors gave rise to pyramidal cells, but also interneurons which not only migrated posteriorly into the cortex from ventral CGE but also anteriorly via the LGE.

Distribution of corticotropin-releasing factor neurons in the mouse brain: a study using corticotropin-releasing factor-modified yellow fluorescent protein knock-in mouse

We examined the morphological features of corticotropin-releasing factor (CRF) neurons in a mouse line in which modified yellow fluorescent protein (Venus) was expressed under the CRF promoter. We previously generated the CRF-Venus knock-in mouse, in which Venus is inserted into the CRF gene locus by homologous recombination. In the present study, the neomycin phosphotransferase gene (Neo), driven by the pgk-1 promoter, was deleted from the CRF-Venus mouse genome, and a CRF-Venus∆Neo mouse was generated. Venus expression is much more prominent in the CRF-Venus∆Neo mouse when compared to the CRF-Venus mouse. In addition, most Venus-expressing neurons co-express CRF mRNA. Venus-expressing neurons constitute a discrete population of neuroendocrine neurons in the paraventricular nucleus of the hypothalamus (PVH) that project to the median eminence. Venus-expressing neurons were also found in brain regions outside the neuroendocrine PVH, including the olfactory bulb, the piriform cortex (Pir), the extended amygdala, the hippocampus, the neocortices, Barrington's nucleus, the midbrain/pontine dorsal tegmentum, the periaqueductal gray, and the inferior olivary nucleus (IO). Venus-expressing perikarya co-expressing CRF mRNA could be observed clearly even in regions where CRF-immunoreactive perikarya could hardly be identified. We demonstrated that the CRF neurons contain glutamate in the Pir and IO, while they contain gamma-aminobutyric acid in the neocortex, the bed nucleus of the stria terminalis, the hippocampus, and the amygdala. A population of CRF neurons was demonstrated to be cholinergic in the midbrain tegmentum. The CRF-Venus∆Neo mouse may be useful for studying the structural and functional properties of CRF neurons in the mouse brain.

Evidence That the Laminar Fate of LGE/CGE-Derived Neocortical Interneurons Is Dependent on Their Progenitor Domains

Neocortical interneurons show tremendous diversity in terms of their neurochemical marker expressions, morphology, electrophysiological properties, and laminar fate. Allocation of interneurons to their appropriate regions and layers in the neocortex is thought to play important roles for the emergence of higher functions of the neocortex. Neocortical interneurons mainly originate from the medial ganglionic eminence (MGE) and the caudal ganglionic eminence (CGE). The diversity and the laminar fate of MGE-derived interneurons depend on the location of their birth and birthdate, respectively. However, this relationship does not hold for CGE-derived interneurons. Here, using the method of in utero electroporation, which causes arbitrary occurrence of labeled progenitor domains, we tracked all descendants of the lateral ganglionic eminence (LGE)/CGE progenitors in mice. We provide evidence that neocortical interneurons with distinct laminar fate originate from distinct progenitor domains within the LGE/CGE. We find layer I interneurons are predominantly labeled in a set of animals, whereas other upper layer neurons are predominantly labeled in another set. We also find distinct subcortical structures labeled between the two sets. Further, interneurons labeled in layer I show distinct neurochemical properties from those in other layers. Together, these results suggest that the laminar fate of LGE/CGE-derived interneurons depends on their spatial origin.

SIGNIFICANCE STATEMENT

Diverse types of neocortical interneurons have distinct laminar fate, neurochemical marker expression, morphology, and electrophysiological properties. Although the specifications and laminar fate of medial ganglionic eminence-derived neocortical interneurons depend on their location of embryonic origin and birthdate, no similar causality of lateral/caudal ganglionic eminence (LGE/CGE)-derived neocortical interneurons is known. Here, we performed in utero electroporation on mouse LGE/CGE and found two groups of animals, one with preferential labeling of layer I and the other with preferential labeling of other layers. Interneurons labeled in these two groups show distinct neurochemical properties and morphologies and are associated with labeling of distinct subcortical structures. These findings suggest that the laminar fate of LGE/CGE-derived neocortical interneurons depends on their spatial origin.

Prox1 Regulates the Subtype-Specific Development of Caudal Ganglionic Eminence-Derived GABAergic Cortical Interneurons

Neurogliaform (RELN+) and bipolar (VIP+) GABAergic interneurons of the mammalian cerebral cortex provide critical inhibition locally within the superficial layers. While these subtypes are known to originate from the embryonic caudal ganglionic eminence (CGE), the specific genetic programs that direct their positioning, maturation, and integration into the cortical network have not been elucidated. Here, we report that in mice expression of the transcription factor Prox1 is selectively maintained in postmitotic CGE-derived cortical interneuron precursors and that loss of Prox1 impairs the integration of these cells into superficial layers. Moreover, Prox1 differentially regulates the postnatal maturation of each specific subtype originating from the CGE (RELN, Calb2/VIP, and VIP). Interestingly, Prox1 promotes the maturation of CGE-derived interneuron subtypes through intrinsic differentiation programs that operate in tandem with extrinsically driven neuronal activity-dependent pathways. Thus Prox1 represents the first identified transcription factor specifically required for the embryonic and postnatal acquisition of CGE-derived cortical interneuron properties.

SIGNIFICANCE STATEMENT

Despite the recognition that 30% of GABAergic cortical interneurons originate from the caudal ganglionic eminence (CGE), to date, a specific transcriptional program that selectively regulates the development of these populations has not yet been identified. Moreover, while CGE-derived interneurons display unique patterns of tangential and radial migration and preferentially populate the superficial layers of the cortex, identification of a molecular program that controls these events is lacking.Here, we demonstrate that the homeodomain transcription factor Prox1 is expressed in postmitotic CGE-derived cortical interneuron precursors and is maintained into adulthood. We found that Prox1 function is differentially required during both embryonic and postnatal stages of development to direct the migration, differentiation, circuit integration, and maintenance programs within distinct subtypes of CGE-derived interneurons.

Crosstalk between intracellular and extracellular signals regulating interneuron production, migration and integration into the cortex

During embryogenesis, cortical interneurons are generated by ventral progenitors located in the ganglionic eminences of the telencephalon. They travel along multiple tangential paths to populate the cortical wall. As they reach this structure they undergo intracortical dispersion to settle in their final destination. At the cellular level, migrating interneurons are highly polarized cells that extend and retract processes using dynamic remodeling of microtubule and actin cytoskeleton. Different levels of molecular regulation contribute to interneuron migration. These include: (1) Extrinsic guidance cues distributed along migratory streams that are sensed and integrated by migrating interneurons; (2) Intrinsic genetic programs driven by specific transcription factors that grant specification and set the timing of migration for different subtypes of interneurons; (3) Adhesion molecules and cytoskeletal elements/regulators that transduce molecular signalings into coherent movement. These levels of molecular regulation must be properly integrated by interneurons to allow their migration in the cortex. The aim of this review is to summarize our current knowledge of the interplay between microenvironmental signals and cell autonomous programs that drive cortical interneuron porduction, tangential migration, and intergration in the developing cerebral cortex.

Distinct developmental origins manifest in the specialized encoding of movement by adult neurons of the external globus pallidus

Transcriptional codes initiated during brain development are ultimately realized in adulthood as distinct cell types performing specialized roles in behavior. Focusing on the mouse external globus pallidus (GPe), we demonstrate that the potential contributions of two GABAergic GPe cell types to voluntary action are fated from early life to be distinct. Prototypic GPe neurons derive from the medial ganglionic eminence of the embryonic subpallium and express the transcription factor Nkx2-1. These neurons fire at high rates during alert rest, and encode movements through heterogeneous firing rate changes, with many neurons decreasing their activity. In contrast, arkypallidal GPe neurons originate from lateral/caudal ganglionic eminences, express the transcription factor FoxP2, fire at low rates during rest, and encode movements with robust increases in firing. We conclude that developmental diversity positions prototypic and arkypallidal neurons to fulfil distinct roles in behavior via their disparate regulation of GABA release onto different basal ganglia targets.

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Arkypallidal and prototypic GPe cells have distinct origins and transcriptional codes

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Arkypallidal neurons rapidly and robustly increase firing rate at movement onset

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Movement is accurately encoded by single arkypallidal or prototypic neurons

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Two GPe cell types are fated to affect different targets in distinct ways in behavior

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.

Visualization of corticotropin-releasing factor neurons by fluorescent proteins in the mouse brain and characterization of labeled neurons in the paraventricular nucleus of the hypothalamus

Corticotropin-releasing factor (CRF) is the key regulator of the hypothalamic-pituitary-adrenal axis. CRF neurons cannot be distinguished morphologically from other neuroendocrine neurons in the paraventricular nucleus of the hypothalamus (PVH) without immunostaining. Thus, we generated a knock-in mouse that expresses modified yellow fluorescent protein (Venus) in CRF neurons (CRF-Venus), and yet its expression is driven by the CRF promoter and responds to changes in the interior milieu. In CRF-Venus, Venus-expressing neurons were distributed in brain regions harboring CRF neurons, including the PVH. The majority of Venus-expressing neurons overlapped with CRF-expressing neurons in the PVH, but many neurons expressed only Venus or CRF in a physiological glucocorticoid condition. After glucocorticoid deprivation, however, Venus expression intensified, and most Venus neurons coexpressed CRF. Conversely, Venus expression was suppressed by excess glucocorticoids. Expression of copeptin, a peptide encoded within the vasopressin gene, was induced in PVH-Venus neurons by glucocorticoid deprivation and suppressed by glucocorticoid administration. Thus, Venus neurons recapitulated glucocorticoid-dependent vasopressin expression in PVH-CRF neurons. Noradrenaline increased the frequency of glutamate-dependent excitatory postsynaptic currents recorded from Venus-expressing neurons in the voltage clamp mode. In addition, the CRF-iCre knock-in mouse was crossed with a CAG-CAT-EGFP reporter mouse to yield the Tg(CAG-CAT-EGFP/wt);CRF(iCre/wt) (EGFP/CRF-iCre) mouse, in which enhanced green fluorescent protein (EGFP) is driven by the CAG promoter. EGFP was expressed more constitutively in the PVH of EGFP/CRF-iCre mice. Thus, CRF-Venus may have an advantage for monitoring dynamic changes in CRF neurons and CRF networks in different glucocorticoid states.

Novel Striatal GABAergic Interneuron Populations Labeled in the 5HT3a(EGFP) Mouse

Histological and morphological studies indicate that approximately 5% of striatal neurons are cholinergic or γ-aminobutyric acidergic (GABAergic) interneurons (gINs). However, the number of striatal neurons expressing known interneuron markers is too small to account for the entire interneuron population. We therefore studied the serotonin (5HT) receptor 3a-enhanced green fluorescent protein (5HT3a(EGFP)) mouse, in which we found that a large number of striatal gINs are labeled. Roughly 20% of 5HT3a(EGFP)-positive cells co-express parvalbumin and exhibit fast-spiking (FS) electrophysiological properties. However, the majority of labeled neurons do not overlap with known molecular interneuron markers. Intrinsic electrical properties reveal at least 2 distinct novel subtypes: a late-spiking (LS) neuropeptide-Y (NPY)-negative neurogliaform (NGF) interneuron, and a large heterogeneous population with several features resembling low-threshold-spiking (LTS) interneurons that do not express somatostatin, NPY, or neuronal nitric oxide synthase. Although the 5HT3a(EGFP) NGF and LTS-like interneurons have electrophysiological properties similar to previously described populations, they are pharmacologically distinct. In direct contrast to previously described NPY(+) LTS and NGF cells, LTS-like 5HT3a(EGFP) cells show robust responses to nicotine administration, while the 5HT3a(EGFP) NGF cell type shows little or no response. By constructing a molecular map of the overlap between these novel populations and existing interneuron populations, we are able to reconcile the morphological and molecular estimates of striatal interneuron numbers.

Cilia: traffic directors along the road of cortical development

While the presence of a primary cilium on neural progenitors and on post-mitotic neurons was noted long ago, a primary cilium has been observed on migrating cortical interneurons only recently. As in fibroblasts, the cilium of interneurons controls the directionality of migration. It plays an important role in the reorientation of cortical interneurons toward the cortical plate. The morphogen Shh, which is expressed in the migratory pathway of interneurons, is one of the signals that control this reorientation. After a short description of the migratory pathways of cortical interneurons, we focus on cellular mechanisms that allow interneurons to reorient their trajectory during their long-distance migration. Then we examine the role of the primary cilium in cell migration and how ciliogenesis might be related to the migration cycle in interneurons. Finally, we review the molecular mechanisms at the basis of the sensory function of the primary cilium and examine how Shh signals could influence the migratory behavior of cortical interneurons. These novel data provide a cellular basis to further understanding cognitive deficits associated with human ciliopathies.

Olig1 function is required to repress dlx1/2 and interneuron production in Mammalian brain

Abnormal GABAergic interneuron density, and imbalance of excitatory versus inhibitory tone, is thought to result in epilepsy, neurodevelopmental disorders, and psychiatric disease. Recent studies indicate that interneuron cortical density is determined primarily by the size of the precursor pool in the embryonic telencephalon. However, factors essential for regulating interneuron allocation from telencephalic multipotent precursors are poorly understood. Here we report that Olig1 represses production of GABAergic interneurons throughout the mouse brain. Olig1 deletion in mutant mice results in ectopic expression and upregulation of Dlx1/2 genes in the ventral medial ganglionic eminences and adjacent regions of the septum, resulting in an ∼30% increase in adult cortical interneuron numbers. We show that Olig1 directly represses the Dlx1/2 I12b intergenic enhancer and that Dlx1/2 functions genetically downstream of Olig1. These findings establish Olig1 as an essential repressor of Dlx1/2 and interneuron production in developing mammalian brain.