Caudal Ganglionic Eminence Research Articles

Impaired GABAergic regulation and developmental immaturity in interneurons derived from the medial ganglionic eminence in the tuberous sclerosis complex

GABAergic interneurons play a critical role in maintaining neural circuit balance, excitation–inhibition regulation, and cognitive function modulation. In tuberous sclerosis complex (TSC), GABAergic neuron dysfunction contributes to disrupted network activity and associated neurological symptoms, assumingly in a cell type-specific manner. This GABAergic centric study focuses on identifying specific interneuron subpopulations within TSC, emphasizing the unique characteristics of medial ganglionic eminence (MGE)- and caudal ganglionic eminence (CGE)-derived interneurons. Using single-nuclei RNA sequencing in TSC patient material, we identify somatostatin-expressing (SST+) interneurons as a unique and immature subpopulation in TSC. The disrupted maturation of SST+ interneurons may undergo an incomplete switch from excitatory to inhibitory GABAergic signaling during development, resulting in reduced inhibitory properties. Notably, this study reveals markers of immaturity specifically in SST+ interneurons, including an abnormal NKCC1/KCC2 ratio, indicating an imbalance in chloride homeostasis crucial for the postsynaptic consequences of GABAergic signaling as well as the downregulation of GABAA receptor subunits, GABRA1, and upregulation of GABRA2. Further exploration of SST+ interneurons revealed altered localization patterns of SST+ interneurons in TSC brain tissue, concentrated in deeper cortical layers, possibly linked to cortical dyslamination. In the epilepsy context, our research underscores the diverse cell type-specific roles of GABAergic interneurons in shaping seizures, advocating for precise therapeutic considerations. Moreover, this study illuminates the potential contribution of SST+ interneurons to TSC pathophysiology, offering insights for targeted therapeutic interventions.

Protracted neuronal recruitment in the temporal lobes of young children

The temporal lobe of the human brain contains the entorhinal cortex (EC). This region of the brain is a highly interconnected integrative hub for sensory and spatial information; it also has a key role in episodic memory formation and is the main source of cortical hippocampal inputs1–4. The human EC continues to develop during childhood5, but neurogenesis and neuronal migration to the EC are widely considered to be complete by birth. Here we show that the human temporal lobe contains many young neurons migrating into the postnatal EC and adjacent regions, with a large tangential stream persisting until the age of around one year and radial dispersal continuing until around two to three years of age. By contrast, we found no equivalent postnatal migration in rhesus macaques (Macaca mulatta). Immunostaining and single-nucleus RNA sequencing of ganglionic eminence germinal zones, the EC stream and the postnatal EC revealed that most migrating cells in the EC stream are derived from the caudal ganglionic eminence and become LAMP5+RELN+ inhibitory interneurons. These late-arriving interneurons could continue to shape the processing of sensory and spatial information well into postnatal life, when children are actively interacting with their environment. The EC is one of the first regions of the brain to be affected in Alzheimer’s disease, and previous work has linked cognitive decline to the loss of LAMP5+RELN+ cells6,7. Our investigation reveals that many of these cells arrive in the EC through a major postnatal migratory stream in early childhood.

LIF signaling regulates outer radial glial to interneuron fate during human cortical development.

Radial glial (RG) development is essential for cerebral cortex growth and organization. In humans, the outer radial glia (oRG) subtype is expanded and gives rise to diverse neurons and glia. However, the mechanisms regulating oRG differentiation are unclear. oRG cells express leukemia-inhibitory factor (LIF) receptors during neurogenesis, and consistent with a role in stem cell self-renewal, LIF perturbation impacts oRG proliferation in cortical tissue and organoids. Surprisingly, LIF treatment also increases the production of inhibitory interneurons (INs) in cortical cultures. Comparative transcriptomic analysis identifies that the enhanced IN population resembles INs produced in the caudal ganglionic eminence. To evaluate whether INs could arise from oRGs, we isolated primary oRG cells and cultured them with LIF. We observed the production of INs from oRG cells and an increase in IN abundance following LIF treatment. Our observations suggest that LIF signaling regulates the capacity of oRG cells to generate INs.

Understanding and modeling regional specification of the human ganglionic eminence.

Inhibitory neurons originating from the ventral forebrain are associated with several neurological conditions. Distinct ventral forebrain subpopulations are generated from topographically defined zones; lateral-, medial- and caudal ganglionic eminences (LGE, MGE and CGE), yet key specification factors often span across developing zones contributing to difficulty in defining unique LGE, MGE or CGE profiles. Here we use human pluripotent stem cell (hPSC) reporter lines (NKX2.1-GFP and MEIS2-mCherry) and manipulation of morphogen gradients to gain greater insight into regional specification of these distinct zones. We identified Sonic hedgehog (SHH)-WNT crosstalk in regulating LGE and MGE fate and uncovered a role for retinoic acid signaling in CGE development. Unraveling the influence of these signaling pathways permitted development of fully defined protocols that favored generation of the three GE domains. These findings provide insight into the context-dependent role of morphogens in human GE specification and are of value for invitro disease modeling and advancement of new therapies.

Genetic Insights of Schizophrenia via Single Cell RNA-Sequencing Analyses.

Schizophrenia is a complex and heterogeneous disorder involving multiple regions and types of cells in the brain. Despite rapid progress made by genome-wide association studies (GWAS) of schizophrenia, the mechanisms of the illness underlying the GWAS significant loci remain less clear. We investigated schizophrenia risk genes using summary-data-based Mendelian randomization based on single-cell sequencing data, and explored the types of brain cells involved in schizophrenia through the expression weighted cell-type enrichment analysis. We identified 54 schizophrenia risk genes (two-thirds of these genes were not identified using sequencing data of bulk tissues) using single-cell RNA-sequencing data. Further cell type enrichment analysis showed that schizophrenia risk genes were highly expressed in excitatory neurons and caudal ganglionic eminence interneurons, suggesting putative roles of these cells in the pathogenesis of schizophrenia. We also found that these risk genes identified using single-cell sequencing results could form a large protein-protein interaction network with genes affected by disease-causing rare variants. Through integrative analyses using expression data at single-cell levels, we identified 54 risk genes associated with schizophrenia. Notably, many of these genes were only identified using single-cell RNA-sequencing data, and their altered expression levels in particular types of cells, rather than in the bulk tissues, were related to the increased risk of schizophrenia. Our results provide novel insight into the biological mechanisms of schizophrenia, and future single-cell studies are necessary to further facilitate the understanding of the disorder.

Histone Methyltransferase SETDB1 Regulates the Development of Cortical Htr3a-Positive Interneurons and Mood Behaviors

BackgroundGABAergic (gamma-aminobutyric acidergic) interneurons (INs) are highly heterogeneous, and Htr3a labels a subpopulation of cortical INs originating from the embryonic caudal ganglionic eminence. SETDB1 is one of the histone H3K9 methyltransferases and plays an essential role in the excitatory neurons, but its role in regulating cortical inhibitory INs remains largely unknown. MethodsIn this study, we generated transgenic mice with conditional knockout of Setdb1 in neural progenitor cells (Setdb1-NS-cKO) and GABAergic neurons (Setdb1-Gad2-cKO). In addition, we performed RNA sequencing, ATAC-seq (assay for transposase-accessible chromatin with sequencing), chromatin immunoprecipitation sequencing, luciferase assay, chromatin conformation capture, and CRISPR (clustered regularly interspaced short palindromic repeats)/dCas9 to study the epigenetic mechanism underlying SETDB1-mediated transcriptional regulation of Htr3a. We also performed in situ hybridization and whole-cell recording to evaluate the functional properties of cortical Htr3a+ INs and behavioral tests for mood. ResultsWe detected significant upregulation of Htr3a expression in the embryonic ganglionic eminence of Setdb1-NS-cKO and identified the endogenous retroviral sequence RMER21B as a new target of SETDB1. RMER21B showed enhancer activity and formed distal chromatin interaction with the promoter of Htr3a. In addition, we observed an increased number and enhanced excitability of Htr3a+ INs in the knockout cortex. Moreover, Setdb1-Gad2-cKO mice exhibited anxiety- and depressive-like behaviors, which were partially reversed by a 5-HT3 receptor antagonist. ConclusionsThese findings suggest that SETDB1 represses Htr3a transcription via RMER21B-mediated distal chromatin interaction in the embryonic ganglionic eminence and regulates the development of cortical Htr3a+ INs and mood behaviors.

ჰიპოკამპის გამა სიხშირის ოსცილაციების უჯრედული მექანიზმები

Gamma oscillations are known as cognitive rhythms due to their occurrence during the cognitive functions such as working memory, episodic memory, memory encoding and retrieval, sensory binding and attention. Gamma oscillations can be easily transferred into epileptic activity. Altered gamma rhythms are seen during the brain disorders such as schizophrenia, dementia and autism. Hence, studying the mechanisms of gamma rhythms is of great importance. Recent discoveries revealed new details of gamma oscillations. The classical view about the parvalbumin containing perisomatic basket cells that drives the gamma oscillation is valid for CA3 region of hippocampus but it may not be so for CA1 region. However, medial ganglionic eminence (MGE) derived axo-axonic cells action potential discharge follows the gamma rhythm in CA1 together with caudal ganglionic eminence (CGE) derived trilaminar and back-projecting interneurons. Oriense-Lacunosum-Moleculare (OLM) cells appear to have dual origine and can be modulated by gamma frequency. Using the modern technologies and relying on the current knowledge and new insights about neuronal elements in gamma frequency oscillation will help the scientists to study the mechanisms of cognitive rhythms in more details.

Identification of cortical interneuron cell markers in mouse embryos based on machine learning analysis of single-cell transcriptomics.

Mammalian cortical interneurons (CINs) could be classified into more than two dozen cell types that possess diverse electrophysiological and molecular characteristics, and participate in various essential biological processes in the human neural system. However, the mechanism to generate diversity in CINs remains controversial. This study aims to predict CIN diversity in mouse embryo by using single-cell transcriptomics and the machine learning methods. Data of 2,669 single-cell transcriptome sequencing results are employed. The 2,669 cells are classified into three categories, caudal ganglionic eminence (CGE) cells, dorsal medial ganglionic eminence (dMGE) cells, and ventral medial ganglionic eminence (vMGE) cells, corresponding to the three regions in the mouse subpallium where the cells are collected. Such transcriptomic profiles were first analyzed by the minimum redundancy and maximum relevance method. A feature list was obtained, which was further fed into the incremental feature selection, incorporating two classification algorithms (random forest and repeated incremental pruning to produce error reduction), to extract key genes and construct powerful classifiers and classification rules. The optimal classifier could achieve an MCC of 0.725, and category-specified prediction accuracies of 0.958, 0.760, and 0.737 for the CGE, dMGE, and vMGE cells, respectively. The related genes and rules may provide helpful information for deepening the understanding of CIN diversity.

Serotonergic regulation of bipolar cell survival in the developing cerebral cortex.

One key factor underlying the functional balance of cortical networks is the ratio of excitatory and inhibitory neurons. The mechanisms controlling the ultimate number of interneurons are beginning to be elucidated, but to what extent similar principles govern the survival of the large diversity of cortical inhibitory cells remains to be investigated. Here, we investigate the mechanisms regulating developmental cell death in neurogliaform cells, bipolar cells, and basket cells, the three main populations of interneurons originating from the caudal ganglionic eminence and the preoptic region. We found that all three subclasses of interneurons undergo activity-dependent programmed cell death. However, while neurogliaform cells and basket cells require glutamatergic transmission to survive, the final number of bipolar cells is instead modulated by serotonergic signaling. Together, our results demonstrate that input-specific modulation of neuronal activity controls the survival of cortical interneurons during the critical period of programmed cell death.

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

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.

Individual human cortical progenitors can produce excitatory and inhibitory neurons.

The cerebral cortex is a cellularly complex structure comprising a rich diversity of neuronal and glial cell types. Cortical neurons can be broadly categorized into two classes-excitatory neurons that use the neurotransmitter glutamate, and inhibitory interneurons that use γ-aminobutyric acid (GABA). Previous developmental studies in rodents have led to a prevailing model in which excitatory neurons are born from progenitors located in the cortex, whereascortical interneurons are born from a separate population of progenitors located outside the developing cortex in the ganglionic eminences1-5. However, the developmental potential of human cortical progenitors has not been thoroughly explored. Here we show that, in addition to excitatory neurons and glia, human cortical progenitors are also capable of producing GABAergic neurons with the transcriptional characteristics and morphologies of cortical interneurons. By developing a cellular barcoding tool called 'single-cell-RNA-sequencing-compatible tracer for identifying clonal relationships' (STICR), we were able to carry out clonal lineage tracing of 1,912 primary human cortical progenitors from six specimens, and to capture both the transcriptional identities and the clonal relationships of their progeny. A subpopulation of corticallyborn GABAergic neurons was transcriptionally similar to cortical interneurons born from the caudal ganglionic eminence, and these cells were frequently related to excitatory neurons and glia. Our results show that individual human cortical progenitors can generate both excitatory neurons and cortical interneurons, providing a new framework for understanding the origins of neuronal diversity in the human cortex.

Identification of Vulnerable Interneuron Subtypes in 15q13.3 Microdeletion Syndrome Using Single-Cell Transcriptomics

BackgroundA number of rare copy number variants (CNVs) have been linked to neurodevelopmental disorders. However, because CNVs encompass many genes, it is often difficult to identify the mechanisms that lead to developmental perturbations. MethodsWe used 15q13.3 microdeletion to propose and validate a novel strategy to predict the impact of CNV genes on brain development that could further guide functional studies. We analyzed single-cell transcriptomics datasets containing cortical interneurons to identify their developmental vulnerability to 15q13.3 microdeletion, which was validated in mouse models. ResultsWe found that Klf13—but not other 15q13.3 genes—is expressed by precursors and neuroblasts in the medial and caudal ganglionic eminences during development, with a peak of expression at embryonic day (E)13.5 and E18.5, respectively. In contrast, in the adult mouse brain, Klf13 expression is negligible. Using Df(h15q13.3)/+ and Klf13+/− embryos, we observed a precursor subtype-specific impairment in proliferation in the medial ganglionic eminence and caudal ganglionic eminence at E13.5 and E17.5, respectively, corresponding to vulnerability predicted by Klf13 expression patterns. Finally, Klf13+/− mice showed a layer-specific decrease in parvalbumin and somatostatin cortical interneurons accompanied by changes in locomotor and anxiety-related behavior. ConclusionsWe show that the impact of 15q13.3 microdeletion on precursor proliferation is grounded in a reduction in Klf13 expression. The lack of Klf13 in Df(h15q13.3)/+ cortex might be the major reason for perturbed density of cortical interneurons. Thus, the behavioral defects seen in 15q13.3 microdeletion could stem from a developmental perturbation owing to selective vulnerability of cortical interneurons during sensitive stages of their development.

Characterization and Stage-Dependent Lineage Analysis of Intermediate Progenitors of Cortical GABAergic Interneurons.

Intermediate progenitors of both excitatory and inhibitory neurons, which can replenish neurons in the adult brain, were recently identified. However, the generation of intermediate progenitors of GABAergic inhibitory neurons (IPGNs) has not been studied in detail. Here, we characterized the spatiotemporal distribution of IPGNs in mouse cerebral cortex. IPGNs generated neurons during both embryonic and postnatal stages, but the embryonic IPGNs were more proliferative. Our lineage tracing analyses showed that the embryonically proliferating IPGNs tended to localize to the superficial layers rather than the deep cortical layers at 3 weeks after birth. We also found that embryonic IPGNs derived from the medial and caudal ganglionic eminence (CGE) but more than half of the embryonic IPGNs were derived from the CGE and broadly distributed in the cerebral cortex. Taken together, our data indicate that the broadly located IPGNs during embryonic and postnatal stages exhibit a different proliferative property and layer distribution.

Islet1 Precursors Contribute to Mature Interneuron Subtypes in Mouse Neocortex.

Cortical interneurons (GABAergic cells) arise during embryogenesis primarily from the medial and caudal ganglionic eminences (MGE and CGE, respectively) with a small population generated from the preoptic area (POA). Progenitors from the lateral ganglionic eminence (LGE) are thought to only generate GABAergic medium spiny neurons that populate the striatum and project to the globus pallidus. Here, we report evidence that neuronal precursors that express the LGE-specific transcription factor Islet1 (Isl1) can give rise to a small population of cortical interneurons. Lineage tracing and homozygous deletion of Nkx2.1 in Isl1 fate-mapped mice showed that neighboring MGE/POA-specific Nkx2.1 cells and LGE-specific Isl1 cells make both common and distinct lineal contributions towards cortical interneuron fate. Although the majority of cells had overlapping transcriptional domains between Nkx2.1 and Isl1, a population of Isl1-only derived cells also contributed to the adult cerebral cortex. The data indicate that Isl1-derived cells may originate from both the LGE and the adjacent LGE/MGE boundary regions to generate diverse neuronal progeny. Thus, a small population of neocortical interneurons appear to originate from Isl-1-positive precursors.

Distinct Expression of SLM2 Underlies Splicing-Dependent Trans-Synaptic Signaling of Neurexin Across GABAergic Neuron Subtypes.

The mammalian brain contains multiple types of neuronal cells with complex assemblies and distinct structural and functional properties encoded by divergent gene programs. There is increasing evidence that alternative splicing (AS) plays fundamental roles in transcriptomic diversity and specifying synaptic properties of each neuronal cell type. However, the mechanisms underlying AS regulation and whether it controls synapse formation across GABAergic interneurons have not been fully elucidated. Here we show the differential expression levels of Sam68-like molecule 2 (SLM2), a major splicing regulator of neurexin (NRX), in GABAergic neuronal subtypes and its contribution to GABAergic synapse specification. Cortical SLM2 is strongly expressed not only in excitatory neurons but also in a subpopulation of GABAergic interneurons, especially in VIP-positive neurons that are originated from late-born caudal ganglionic eminence (GE)- derived cells. Using artificial synapse formation assay, we found that GE containing cortices form a strong synapse with LRRTM2, a trans-synaptic receptor of the alternatively spliced segment 4 (AS4)(-) of NRX. SLM2 knock-down reduced the NRX AS4(-) isoform expression and hence weaken LRRTM2-induced synapse formation. The addition of NRX AS4(-) was sufficient to rescue the synaptic formation by LRRTM2 in SLM2 knock-down neurons. Thus, our findings suggest a novel function of SLM2 in modifying network formation of a specific population of GABAergic interneurons and contribute to a better understanding of the roles AS plays in regulating synapse specificity and neuronal molecular diversity.

Single-Cell Transcriptome Profiling of Thyroid Hormone Effectors in the Human Fetal Neocortex: Expression of SLCO1C1, DIO2, and THRB in Specific Cell Types.

Background: Thyroid hormones are crucial for brain development, acting through the thyroid hormone nuclear receptors (TR)α1 and β to control gene expression. Triiodothyronine (T3), the receptor-ligand, is transported into the brain from the blood by the monocarboxylate transporter 8 (MCT8). Another source of brain T3 is from the local deiodination of thyroxine (T4) by type 2 deiodinase (DIO2). While these mechanisms are very similar in mice and humans, important species-specific differences confound our understanding of disease using mouse models. To fill this knowledge gap on thyroid hormone action in the human fetal brain, we analyzed the expression of transporters, DIO2, and TRs, which we call thyroid hormone effectors, at single-cell resolution. Methods: We analyzed publicly available single-cell transcriptome data sets of isolated cerebral cortex neural cells from three different studies, with expression data from 393 to almost 40,000 cells. We generated Uniform Manifold Approximation and Projection scatterplots and cell clusters to identify differentially expressed genes between clusters, and correlated their gene signatures with the expression of thyroid effectors. Results: The radial glia, mainly the outer radial glia, and astrocytes coexpress SLCO1C1 and DIO2, indicating close cooperation between the T4 transporter OATP1C1 and DIO2 in local T3 formation. Strikingly, THRB was mainly present in two classes of interneurons: a majority expressing CALB2/calretinin, from the caudal ganglionic eminence, and in somatostatin-expressing interneurons from the medial ganglionic eminence. By contrast, many cell types express SLC16A2 and THRA. Conclusions:SLCO1C1 and DIO2 coexpression in the outer radial glia, the universal stem cell of the cerebral cortex, highlights the likely importance of brain-generated T3 in neurogenesis. The unique expression of THRB in discrete subsets of interneurons is a novel finding whose pathophysiological meaning deserves further investigation.

Developmental Characterization of Schizophrenia-Associated Gene Zswim6 in Mouse Forebrain

Schizophrenia is a devastating neuropsychiatric disease with a globally 1% life-long prevalence. Clinical studies have linked Zswim6 mutations to developmental and neurological diseases, including schizophrenia. Zswim6’s function remains largely unknown. Given the involvement of Zswim6 in schizophrenia and schizophrenia as a neurodevelopmental disease, it is important to understand the spatiotemporal expression pattern of Zswim6 in the developing brain. Here, we performed a comprehensive analysis of the spatiotemporal expression pattern of Zswim6 in the mouse forebrain by in situ hybridization with radioactive and non-radioactive-labeled riboprobes. Zswim6 mRNA was detected as early as E11.5 in the ventral forebrain. At E11.5–E13.5, Zswim6 was highly expressed in the lateral ganglionic eminence (LGE). The LGE consisted of two progenitor populations. Dlx+;Er81+ cells in dorsal LGE comprised progenitors of olfactory bulb interneurons, whereas Dlx+;Isl1+ progenitors in ventral LGE gave rise to striatal projection neurons. Zswim6 was not colocalized with Er81 in the dorsal LGE. In the ventral LGE, Zswim6 was colocalized with striatal progenitor marker Nolz-1. Zswim6 was highly expressed in the subventricular zone (SVZ) of LGE in which progenitors undergo the transition from proliferation to differentiation. Double labeling showed that Zswim6 was not colocalized with proliferation marker Ki67 but was colocalized with differentiation marker Tuj1 in the SVZ, suggesting Zswim6 expression in early differentiating neurons. Zswim6 was also expressed in the adjacent structures of medial and caudal ganglionic eminences (MGE, CGE) that contained progenitors of cortical interneurons. At E15.5 and E17.5, Zswim6 was expressed in several key brain regions that were involved in the pathogenesis of schizophrenia, including the striatum, cerebral cortex, hippocampus, and medial habenular nucleus. Zswim6 was persistently expressed in the postnatal brain. Cell type analysis indicated that Zswim6 mRNA was colocalized with D1R-expressing striatonigral and D2R-expressing striatopallidal neurons of the adult striatum with a higher colocalization in striatopallidal neurons. These findings are of particular interest as striatal dopamine D2 receptors are known to be involved in the pathophysiology of schizophrenia. In summary, the comprehensive analysis provides an anatomical framework for the study of Zswim6 function and Zswim6-associated neurological disorders.

Multiple Origins of Secretagogin Expressing Cortical GABAergic Neuron Precursors in the Early Human Fetal Telencephalon.

Secretagogin (SCGN) which acts as a calcium signaling sensor, has previously been shown to be expressed by a substantial population of cortical GABAergic neurons at mid-gestation in humans but not in mice. The present study traced SCGN expression in cortical GABAergic neurons in human fetal forebrain from earlier stages than previously studied. Multiple potential origins of SCGN-expressing neurons were identified in the caudal ganglionic eminence (CGE) lateral ganglionic eminence (LGE) septum and preoptic area; these cells largely co-expressed SP8 but not the medial ganglionic eminence marker LHX6. They followed various migration routes to reach their target regions in the neocortex, insular and olfactory cortex (OC) and olfactory bulbs. A robust increase in the number of SCGN-expressing GABAergic cortical neurons was observed in the midgestational period; 58% of DLX2+ neurons expressed SCGN in the cortical wall at 19 post-conceptional weeks (PCW), a higher proportion than expressed calretinin, a marker for GABAergic neurons of LGE/CGE origin. Furthermore, although most SCGN+ neurons co-expressed calretinin in the cortical plate (CP) and deeper layers, in the marginal zone (MZ) SCGN+ and calretinin+ cells formed separate populations. In the adult mouse, it has previously been shown that in the rostral migratory stream (RMS), SCGN, annexin V (ANXA5), and matrix metalloprotease 2 (MMP2) are co-expressed forming a functioning complex that exocytoses MMP2 in response to calcium. In the present study, ANXA5 showed widespread expression throughout the cortical wall, although MMP2 expression was very largely limited to the CP. We found co-expression of these proteins in some SCGN+ neurons in the subventricular zones (SVZ) suggesting a limited role for these cells in remodeling the extracellular matrix, perhaps during cell migration.

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.

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

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