The development of MGE-derived cortical interneurons: An Lhx6 tale

The cerebral cortex contains two main neuronal cell populations, the excitatory pyramidal neurons and the inhibitory interneurons, which constitute 20-30% of all cortical neurons. Cortical interneurons are characterized by a remarkable morphological, molecular and functional diversity. A swathe of research activity in the last 20 years aimed to determine how cortical interneurons acquire their mature cellular and functional features, identified a number of transcription factors that function at different stages of interneuron development. Here, we review all current knowledge concerning the multiple functions of the “master regulator” LIMHomeodomain transcription factor Lhx6; a gene expressed in the medial ganglionic eminence of the basal telencephalon that controls the development of somatostatin and parvalbumin expressing interneurons.

Interneuron origins in the embryonic porcine medial ganglionic eminence

Interneurons contribute to the complexity of neural circuits and maintenance of normal brain function. Rodent interneurons originate in embryonic ganglionic eminences, but developmental origins in other species are less understood. Here, we show that transcription factor expression patterns in porcine embryonic subpallium are similar to rodents, delineating a distinct medial ganglionic eminence (MGE) progenitor domain. On the basis of Nkx2.1, Lhx6 and Dlx2 expression, in vitro differentiation into neurons expressing GABA and robust migratory capacity in explant assays, we propose that cortical and hippocampal interneurons originate from a porcine MGE region. Following xenotransplantation into adult male and female rat hippocampus, we further demonstrate that porcine MGE progenitors, like those from rodents, migrate and differentiate into morphologically distinct interneurons expressing GABA. Our findings reveal that basic rules for interneuron development are conserved across species, and that porcine embryonic MGE progenitors could serve as a valuable source for interneuron-based xenotransplantation therapies.Significance StatementHere we demonstrate that porcine MGE, like rodents, exhibit a distinct transcriptional and pallial interneuron-specific antibody profile, in vitro migratory capacity and are amenable to xenotransplantation. This is the first comprehensive examination of embryonic pallial interneuron origins in the pig, and because a rich neurodevelopmental literature on embryonic mouse MGE exists (with some additional characterizations in other species like monkey and human) our work allows direct neurodevelopmental comparisons with this literature.

Transcriptional regulation of MGE progenitor proliferation by PRDM16 controls cortical GABAergic interneuron production

ABSTRACTThe mammalian cortex is populated by neurons derived from neural progenitors located throughout the embryonic telencephalon. Excitatory neurons are derived from the dorsal telencephalon, whereas inhibitory interneurons are generated in its ventral portion. The transcriptional regulator PRDM16 is expressed by radial glia, neural progenitors present in both regions; however, its mechanisms of action are still not fully understood. It is unclear whether PRDM16 plays a similar role in neurogenesis in both dorsal and ventral progenitor lineages and, if so, whether it regulates common or unique networks of genes. Here, we show that Prdm16 expression in mouse medial ganglionic eminence (MGE) progenitors is required for maintaining their proliferative capacity and for the production of proper numbers of forebrain GABAergic interneurons. PRDM16 binds to cis-regulatory elements and represses the expression of region-specific neuronal differentiation genes, thereby controlling the timing of neuronal maturation. PRDM16 regulates convergent developmental gene expression programs in the cortex and MGE, which utilize both common and region-specific sets of genes to control the proliferative capacity of neural progenitors, ensuring the generation of correct numbers of cortical neurons.

NMDAR-mediated transcriptional control of gene expression in the specification of interneuron subtype identity

Medial ganglionic eminence (MGE)-derived parvalbumin (PV)+, somatostatin (SST)+ and Neurogliaform (NGFC)-type cortical and hippocampal interneurons, have distinct molecular, anatomical and physiological properties. However, the molecular mechanisms regulating their diversity remain poorly understood. Here, via single-cell transcriptomics, we show that the obligate NMDA-type glutamate receptor (NMDAR) subunit gene Grin1 mediates subtype-specific transcriptional regulation of gene expression in MGE-derived interneurons, leading to altered subtype identities. Notably, MGE-specific conditional Grin1 loss results in a systemic downregulation of diverse transcriptional, synaptogenic and membrane excitability regulatory programs. These widespread gene expression abnormalities mirror aberrations that are typically associated with neurodevelopmental disorders, particularly schizophrenia. Our study hence provides a road map for the systematic examination of NMDAR signaling in interneuron subtypes, revealing potential MGE-specific genetic targets that could instruct future therapies of psychiatric disorders.

Maf and Mafb control mouse pallial interneuron fate and maturation through neuropsychiatric disease gene regulation

​Maf (c-Maf) and Mafb transcription factors (TFs) have compensatory roles in repressing somatostatin (SST+) interneuron (IN) production in medial ganglionic eminence (MGE) secondary progenitors in mice. Maf and Mafb conditional deletion (cDKO) decreases the survival of MGE-derived cortical interneurons (CINs) and changes their physiological properties. Herein, we show that (1) Mef2c and Snap25 are positively regulated by Maf and Mafb to drive IN morphological maturation; (2) Maf and Mafb promote Mef2c expression which specifies parvalbumin (PV+) INs; (3) Elmo1, Igfbp4 and Mef2c are candidate markers of immature PV+ hippocampal INs (HIN). Furthermore, Maf/Mafb neonatal cDKOs have decreased CINs and increased HINs, that express Pnoc, an HIN specific marker. Our findings not only elucidate key gene targets of Maf and Mafb that control IN development, but also identify for the first time TFs that differentially regulate CIN vs. HIN production.

Nf1deletion results in depletion of theLhx6transcription factor and a specific loss of parvalbumin+cortical interneurons

Neurofibromatosis 1 (NF1) is caused by mutations in theNF1gene, which encodes the protein, neurofibromin, an inhibitor of Ras activity. Cortical GABAergic interneurons (CINs) are implicated in NF1 pathology, but the cellular and molecular changes to CINs are unknown. We deleted mouseNf1from the medial ganglionic eminence, which gives rise to both oligodendrocytes and CINs that express somatostatin and parvalbumin.Nf1loss led to a persistence of immature oligodendrocytes that prevented later-generated oligodendrocytes from occupying the cortex. Moreover, molecular and cellular properties of parvalbumin (PV)-positive CINs were altered by the loss ofNf1, without changes in somatostatin (SST)-positive CINs. We discovered that loss ofNf1results in a dose-dependent decrease inLhx6expression, the transcription factor necessary to establish SST+and PV+CINs, which was rescued by the MEK inhibitor SL327, revealing a mechanism whereby a neurofibromin/Ras/MEK pathway regulates a critical CIN developmental milestone.

Tsc1 represses parvalbumin expression and fast-spiking properties in somatostatin lineage cortical interneurons

Medial ganglionic eminence (MGE)-derived somatostatin (SST)+ and parvalbumin (PV)+ cortical interneurons (CINs), have characteristic molecular, anatomical and physiological properties. However, mechanisms regulating their diversity remain poorly understood. Here, we show that conditional loss of the Tuberous Sclerosis Complex (TSC) gene, Tsc1, which inhibits the mammalian target of rapamycin (MTOR), causes a subset of SST+ CINs, to express PV and adopt fast-spiking (FS) properties, characteristic of PV+ CINs. Milder intermediate phenotypes also occur when only one allele of Tsc1 is deleted. Notably, treatment of adult mice with rapamycin, which inhibits MTOR, reverses the phenotypes. These data reveal novel functions of MTOR signaling in regulating PV expression and FS properties, which may contribute to TSC neuropsychiatric symptoms. Moreover, they suggest that CINs can exhibit properties intermediate between those classically associated with PV+ or SST+ CINs, which may be dynamically regulated by the MTOR signaling.

Nf1 deletion results in depletion of the Lhx6 transcription factor and a specific loss of parvalbumin+ cortical interneurons

SummaryNeurofibromatosis-1 (NF-1) is a monogenic disorder caused by mutations in the NF1 gene, which encodes the protein, Neurofibromin, an inhibitor of Ras GTPase activity. While NF-1 is often characterized by café-au-lait skin spots and benign tumors, the mechanisms underlying cognitive changes in NF-1 are poorly understood. Cortical GABAergic interneurons (CINs) are implicated in NF-1 pathology but cellular and molecular changes to CINs are poorly understood. We deleted Nf1 from the medial ganglionic eminence (MGE), which gives rise to both oligodendrocytes and CINs that express somatostatin and parvalbumin. Loss of Nf1 led to a persistence of immature oligodendrocytes that prevented later born oligodendrocytes from occupying the cortex. Moreover, PV+ CINs were uniquely lost, without changes in SST+ CINs. We discovered that loss of Nf1 results in a graded decrease in Lhx6 expression, the transcription factor necessary to establish SST+ and PV+ CINs, revealing a mechanism whereby Nf1 regulates a critical CIN developmental milestone.