SP8 and SP9 coordinately promote D2-type medium spiny neuron production by activating Six3 expression

CD200-based cell sorting results in homogeneous transplantable striatal neuroblasts for human cell therapy for Huntington's disease

Neurodegenerative diseases are characterized by selective loss of neurons. Cell replacement therapies are the most promising therapeutic strategies to restore the neuronal functions lost during these neurodegenerative processes. However, cell replacement-based clinical trials for Huntington's (HD) and Parkinson's diseases (PD) failed due to the large heterogeneity of the samples. Here, we identify CD200 as a cell surface marker for human striatal neuroblasts (NBs) using massively parallel single-cell RNA sequencing. Next, we set up a CD200-based immunomagnetic sorting pipeline that allows high-yield enrichment of human striatal NBs from in vitro differentiation of human pluripotent stem cells (hPSCs). We also show that sorted CD200-positive cells are striatal projection neuron (SPN)-committed NBs which survive upon intra-striatal transplantation in adult mice with no evidence of graft overgrowth in vivo. In conclusion, we implemented a new CD200 cell selection strategy that reduces the heterogeneity and batch-to-batch variation and potentially decreases the teratogenic risk of hPSC-based cell therapy for neurodegenerative diseases.

Reevaluating the role of Pou3f1 in striatal development: Evidence from transgenic mouse models

The striatum, a critical component of the basal ganglia, is essential for motor control, cognitive processing, and emotional regulation. Medium spiny neurons (MSNs) are the primary neuronal population in the striatum, classified into D1 and D2 subtypes. The transcription factor Pou3f1 has been hypothesized to play a crucial role in the development of pyramidal neurons. Recently, a comprehensive analysis of the human embryonic scRNA-seq dataset predicted and emphasized the bridging function of POU3F1 between striatal progenitor cells and immature neurons, though this finding lacked genetic validation. In this study, we found that Pou3f1 expression was significantly reduced after Six3 deletion. However, Pou3f1 deletion does not significantly affect the number or subtype composition of MSNs, nor the proliferation and differentiation of progenitor cells, in our Pou3f1 conditional knockout (cko) mice, challenging the in silico predictions based on human data. These results suggest that Pou3f1 is not required for the specification, generation, or differentiation of MSNs, though its potential involvement in other aspects of striatal development cannot be entirely ruled out.

Cyclosporine A Accelerates Neurorecovery Transcriptional Trajectory in a Swine Model of Diffuse Traumatic Brain Injury

Mild traumatic brain injury (mTBI) is a leading cause of morbidity in children with both short- and long-term neurological, cognitive, cerebrovascular, and emotional deficits. These deficits have been attributed to ongoing pathophysiological cascades that occur acutely and persist post-injury. Given our limited understanding of the transcriptional changes associated with these pathophysiological cascades, we studied formalin-fixed paraffin-embedded (FFPE) tissues from the frontal cortex (FC) and the hippocampus + amygdala (H&A) regions of swine (N = 40) after a sagittal rapid non-impact head rotation (RNR). We then sequenced RNA to define transcriptional changes at 1 day and 1 week after injury and investigated the protective influence of cyclosporine A (CsA) treatment. Differentially expressed genes (DEGs) were classified into five temporal patterns (Early, Transient, Persistent, Intensified, Delayed, or Late). DEGs were more abundant at 1 week than 1 day. Shared significant gene ontology annotations in both regions included terms associated with neuronal distress at 1 day and neurorecovery at 1 week. CsA (20 mg/kg/day) infused for 1 day (beginning at 6 h after injury) accelerated 466 DEGs in the FC and 2794 DEGs in the H&A, such that the CsA-treated transcriptional profile was associated with neurorecovery. Overall, our data reveal the effects of anatomic region and elapsed time on gene expression post-mTBI and motivate future studies of CsA treatment.

Mammalian TatD DNase domain containing 1 (TATDN1) is a proteostasis-responsive gene with roles in ventricular structure and neuromuscular function

The characterization of highly conserved but poorly understood genes often reveals unexpected biological roles, advancing our understanding of disease mechanisms. One such gene is Mammalian TatD DNase domain containing 1 (Tatdn1), the mammalian homolog of bacterial Twin-arginine translocation D (TatD), a protein proposed to have roles either in DNA degradation or protein quality control in unicellular organisms. Despite its association with different pathologies, including several cancer types and cardiovascular diseases, the role of TATDN1 in mammals remains unexplored. Here, we demonstrate that Tatdn1 encodes a cytoplasmic protein that does not participate in DNA degradation but is upregulated in cells under proteostasis stress. Tatdn1-deficient mice exhibit dysregulated expression of genes involved in membrane and extracellular protein biology, along with mild dilated cardiomyopathy and impaired motor coordination. These findings identify TATDN1 as a key player in cytosolic processes linked to protein homeostasis, with significant physiological implications for cardiac and neurological function.

The Principle of Cortical Development and Evolution

Human's robust cognitive abilities, including creativity and language, are made possible, at least in large part, by evolutionary changes made to the cerebral cortex. This paper reviews the biology and evolution of mammalian cortical radial glial cells (primary neural stem cells) and introduces the concept that a genetically step wise process, based on a core molecular pathway already in use, is the evolutionary process that has molded cortical neurogenesis. The core mechanism, which has been identified in our recent studies, is the extracellular signal-regulated kinase (ERK)-bone morphogenic protein 7 (BMP7)-GLI3 repressor form (GLI3R)-sonic hedgehog (SHH) positive feedback loop. Additionally, I propose that the molecular basis for cortical evolutionary dwarfism, exemplified by the lissencephalic mouse which originated from a larger gyrencephalic ancestor, is an increase in SHH signaling in radial glia, that antagonizes ERK-BMP7 signaling. Finally, I propose that: (1) SHH signaling is not a key regulator of primate cortical expansion and folding; (2) human cortical radial glial cells do not generate neocortical interneurons; (3) human-specific genes may not be essential for most cortical expansion. I hope this review assists colleagues in the field, guiding research to address gaps in our understanding of cortical development and evolution.

Modelling a pathological GSX2 variant that selectively alters DNA binding reveals hypomorphic mouse brain defects

Gsx2 is a homeodomain transcription factor critical for development of the ventral telencephalon and hindbrain in mouse. Loss of Gsx2 function results in severe basal ganglia dysgenesis and defects in the nucleus tractus solitarius (nTS) of the hindbrain, together with respiratory failure at birth. De Mori et al. (2019) reported two patients with severe dystonia and basal ganglia dysgenesis that encode distinct recessive GSX2 variants, including a missense variant within the homeodomain (GSX2Q251R). Hence, we modelled the homologous Gsx2 mutation (i.e. Gsx2Q252R) in mouse, and our biochemical analysis revealed that this variant selectively altered DNA binding. Moreover, mice carrying the Gsx2Q252R allele exhibited basal ganglia dysgenesis, albeit to a lesser extent than did Gsx2 null mice. A notable difference between Gsx2Q252R and Gsx2 null mice was that Gsx2Q252R mice survived, and hindbrain analysis revealed relative sparing of the glutamatergic nTS neurons and catecholaminergic A1/C1 and A2/C2 groups. Thus, the Gsx2Q252R variant is a hypomorph that compromises a subset of Gsx2-dependent neuronal subtypes and highlights a critical role for distinct thresholds of catecholaminergic and/or glutamatergic nTS neurons for viability.

Diet-Induced Obesity Induces Transcriptomic Changes in Neuroimmunometabolic-Related Genes in the Striatum and Olfactory Bulb

The incidence of obesity has markedly increased globally over the last several decades and is believed to be associated with the easier availability of energy-dense foods, including high-fat foods. The reinforcing hedonic properties of high-fat foods, including olfactory cues, activate reward centers in the brain, motivating eating behavior. Thus, there is a growing interest in the understanding of the genetic changes that occur in the brain that are associated with obesity and eating behavior. This growing interest has paralleled advances in genomic methods that enable transcriptomic-wide analyses. Here, we examined the transcriptomic-level differences in the olfactory bulb and striatum, regions of the brain associated with olfaction and hedonic food-seeking, respectively, in high-fat-diet (HFD)-fed obese mice. To isolate the dietary effects from obesity, we also examined transcriptomic changes in normal-chow-fed and limited-HFD-fed groups, with the latter being pair-fed with an HFD isocaloric to the consumption of the normal-chow-fed mice. Using RNA sequencing, we identified 274 differentially expressed genes (DEGs) in the striatum and 11 in the olfactory bulb of ad libitum HFD-fed mice compared to the chow-fed group, and thirty-eight DEGs in the striatum between the ad libitum HFD and limited-HFD-fed groups. The DEGs in both tissues were associated with inflammation and immune-related pathways, including oxidative stress and immune function, and with mitochondrial dysfunction and reward pathways in the striatum. These results shed light on potential obesity-associated genes in these regions of the brain.

The transcriptomic and spatial organization of telencephalic GABAergic neuronal types

The telencephalon of the mammalian brain comprises multiple regions and circuit pathways that play adaptive and integrative roles in a variety of brain functions. There is a wide array of GABAergic neurons in the telencephalon; they play a multitude of circuit functions, and dysfunction of these neurons has been implicated in diverse brain disorders. In this study, we conducted a systematic and in-depth analysis of the transcriptomic and spatial organization of GABAergic neuronal types in all regions of the mouse telencephalon and their developmental origins. This was accomplished by utilizing 611,423 single-cell transcriptomes from the comprehensive and high-resolution transcriptomic and spatial cell type atlas for the adult whole mouse brain we have generated, supplemented with an additional single-cell RNA-sequencing dataset containing 99,438 high-quality single-cell transcriptomes collected from the pre- and postnatal developing mouse brain. We present a hierarchically organized adult telencephalic GABAergic neuronal cell type taxonomy of 7 classes, 52 subclasses, 284 supertypes, and 1,051 clusters, as well as a corresponding developmental taxonomy of 450 clusters across different ages. Detailed charting efforts reveal extraordinary complexity where relationships among cell types reflect both spatial locations and developmental origins. Transcriptomically and developmentally related cell types can often be found in distant and diverse brain regions indicating that long-distance migration and dispersion is a common characteristic of nearly all classes of telencephalic GABAergic neurons. Additionally, we find various spatial dimensions of both discrete and continuous variations among related cell types that are correlated with gene expression gradients. Lastly, we find that cortical, striatal and some pallidal GABAergic neurons undergo extensive postnatal diversification, whereas septal and most pallidal GABAergic neuronal types emerge simultaneously during the embryonic stage with limited postnatal diversification. Overall, the telencephalic GABAergic cell type taxonomy can serve as a foundational reference for molecular, structural and functional studies of cell types and circuits by the entire community.

DLX genes and proteins in mammalian forebrain development

The vertebrate Dlx gene family encode homeobox transcription factors that are related to the Drosophila Distal-less (Dll) gene and are crucial for development. Over the last ∼35 years detailed information has accrued about the redundant and unique expression and function of the six mammalian Dlx family genes. DLX proteins interact with general transcriptional regulators, and co-bind with other transcription factors to enhancer elements with highly specific activity in the developing forebrain. Integration of the genetic and biochemical data has yielded a foundation for a gene regulatory network governing the differentiation of forebrain GABAergic neurons. In this Primer, we describe the discovery of vertebrate Dlx genes and their crucial roles in embryonic development. We largely focus on the role of Dlx family genes in mammalian forebrain development revealed through studies in mice. Finally, we highlight questions that remain unanswered regarding vertebrate Dlx genes despite over 30 years of research.

Spatially resolved cell atlas of the teleost telencephalon and deep homology of the vertebrate forebrain

The telencephalon has undergone remarkable diversification and expansion throughout vertebrate evolution, exhibiting striking variations in structural and functional complexity. Nevertheless, fundamental features are shared across vertebrate taxa, such as the presence of distinct regions including the pallium, subpallium, and olfactory structures. Teleost fishes have a uniquely "everted" telencephalon, which has confounded comparisons of their brain regions to other vertebrates. Here we combine spatial transcriptomics and single nucleus RNA-sequencing to generate a spatially-resolved transcriptional atlas of the Mchenga conophorus cichlid fish telencephalon. We then compare cell-types and anatomical regions in the cichlid telencephalon with those in amphibians, reptiles, birds, and mammals. We uncover striking transcriptional similarities between cell-types in the fish telencephalon and subpallial, hippocampal, and cortical cell-types in tetrapods, and find support for partial eversion of the teleost telencephalon. Ultimately, our work lends new insights into the organization and evolution of conserved cell-types and regions in the vertebrate forebrain.

De novo variants in SP9 cause a novel form of interneuronopathy characterized by intellectual disability, autism spectrum disorder, and epilepsy with variable expressivity

PURPOSE

Interneuronopathies are a group of neurodevelopmental disorders characterized by deficient migration and differentiation of gamma-aminobutyric acidergic interneurons resulting in a broad clinical spectrum, including autism spectrum disorders, early-onset epileptic encephalopathy, intellectual disability, and schizophrenic disorders. SP9 is a transcription factor belonging to the Krüppel-like factor and specificity protein family, the members of which harbor highly conserved DNA-binding domains. SP9 plays a central role in interneuron development and tangential migration, but it has not yet been implicated in a human neurodevelopmental disorder.

METHODS

Cases with SP9 variants were collected through international data-sharing networks. To address the specific impact of SP9 variants, in silico and in vitro assays were carried out.

RESULTS

De novo heterozygous variants in SP9 cause a novel form of interneuronopathy. SP9 missense variants affecting the glutamate 378 amino acid result in severe epileptic encephalopathy because of hypomorphic and neomorphic DNA-binding effects, whereas SP9 loss-of-function variants result in a milder phenotype with epilepsy, developmental delay, and autism spectrum disorder.

CONCLUSION

De novo heterozygous SP9 variants are responsible for a neurodevelopmental disease. Interestingly, variants located in conserved DNA-binding domains of KLF/SP family transcription factors may lead to neomorphic DNA-binding functions resulting in a combination of loss- and gain-of-function effects.

E3 Ubiquitin Ligase Uhrf2 Knockout Reveals a Critical Role in Social Behavior and Synaptic Plasticity in the Hippocampus

The hippocampal formation, particularly the CA2 subregion, is critical for social memory formation and memory processing, relying on synaptic plasticity-a fundamental mechanism by which synapses strengthen. Given the role of the ubiquitin-proteasome system (UPS) in various nervous system processes, including learning and memory, we were particularly interested in exploring the involvement of RING-type ubiquitin E3 ligases, such as UHRF2 (NIRF), in social behavior and synaptic plasticity. Our results revealed altered social behavior in mice with systemic Uhrf2 knockout, including changes in nest building, tube dominance, and the three-chamber social novelty test. In Uhrf2 knockout mice, the entorhinal cortex-CA2 circuit showed significant reductions in synaptic plasticity during paired-pulse facilitation and long-term potentiation, while the inability to evoke synaptic plasticity in the Schaffer-collateral CA2 synapses remained unaffected. These changes in synaptic plasticity correlated with significant changes in gene expression including genes related to vesicle trafficking and transcriptional regulation. The effects of Uhrf2 knockout on synaptic plasticity and the observed gene expression changes highlight UHRF2 as a regulator of learning and memory processes at both the cellular and systemic levels. Targeting E3 ubiquitin ligases, such as UHRF2, may hold therapeutic potential for memory-related disorders, warranting further investigation.

Context-dependent regulation of Notch signaling in glial development and tumorigenesis

In the mammalian brain, Notch signaling maintains the cortical stem cell pool and regulates the glial cell fate choice and differentiation. However, the function of Notch in regulating glial development and its involvement in tumorigenesis have not been well understood. Here, we show that Notch inactivation by genetic deletion of Rbpj in stem cells decreases astrocytes but increases oligodendrocytes with altered internal states. Inhibiting Notch in glial progenitors does not affect cell generation but instead accelerates the growth of Notch-deprived oligodendrocyte progenitor cells (OPCs) and OPC-related glioma. We also identified a cross-talk between oligodendrocytes and astrocytes, with premyelinating oligodendrocytes secreting BMP4, which is repressed by Notch, to up-regulate GFAP expression in adjacent astrocytes. Moreover, Notch inactivation in stem cells causes a glioma subtype shift from astroglia-associated to OPC-correlated patterns and vice versa. Our study reveals Notch's context-dependent function, promoting astrocytes and astroglia-associated glioma in stem cells and repressing OPCs and related glioma in glial progenitors.

Early-life stress and ovarian hormones alter transcriptional regulation in the nucleus accumbens resulting in sex-specific responses to cocaine

Early-life stress and ovarian hormones contribute to increased female vulnerability to cocaine addiction. Here, we reveal molecular substrates in the reward area, the nucleus accumbens, through which these female-specific factors affect immediate and conditioning responses to cocaine. We find shared involvement of X chromosome inactivation-related and estrogen signaling-related gene regulation in enhanced conditioning responses following early-life stress and during the low-estrogenic state in females. Low-estrogenic females respond to acute cocaine by opening neuronal chromatin enriched for the sites of ΔFosB, a transcription factor implicated in chronic cocaine response and addiction. Conversely, high-estrogenic females respond to cocaine by preferential chromatin closing, providing a mechanism for limiting cocaine-driven chromatin and synaptic plasticity. We find that physiological estrogen withdrawal, early-life stress, and absence of one X chromosome all nullify the protective effect of a high-estrogenic state on cocaine conditioning in females. Our findings offer a molecular framework to enable understanding of sex-specific neuronal mechanisms underlying cocaine use disorder.

OnTarget: in silico design of MiniPromoters for targeted delivery of expression

MiniPromoters, or compact promoters, are short DNA sequences that can drive expression in specific cells and tissues. While broadly useful, they are of high relevance to gene therapy due to their role in enabling precise control of where a therapeutic gene will be expressed. Here, we present OnTarget (http://ontarget.cmmt.ubc.ca), a webserver that streamlines the MiniPromoter design process. Users only need to specify a gene of interest or custom genomic coordinates on which to focus the identification of promoters and enhancers, and can also provide relevant cell-type-specific genomic evidence (e.g. accessible chromatin regions, histone modifications, etc.). OnTarget combines the provided data with internal data to identify candidate promoters and enhancers and design MiniPromoters. To illustrate the utility of OnTarget, we designed and characterized two MiniPromoters targeting different cell populations relevant to Parkinson Disease.

Endogenous neural stem cells characterization using omics approaches: Current knowledge in health and disease

Neural stem cells (NSCs), an invaluable source of neuronal and glial progeny, have been widely interrogated in the last twenty years, mainly to understand their therapeutic potential. Most of the studies were performed with cells derived from pluripotent stem cells of either rodents or humans, and have mainly focused on their potential in regenerative medicine. High-throughput omics technologies, such as transcriptomics, epigenetics, proteomics, and metabolomics, which exploded in the past decade, represent a powerful tool to investigate the molecular mechanisms characterizing the heterogeneity of endogenous NSCs. The transition from bulk studies to single cell approaches brought significant insights by revealing complex system phenotypes, from the molecular to the organism level. Here, we will discuss the current literature that has been greatly enriched in the “omics era”, successfully exploring the nature and function of endogenous NSCs and the process of neurogenesis. Overall, the information obtained from omics studies of endogenous NSCs provides a sharper picture of NSCs function during neurodevelopment in healthy and in perturbed environments.

DNA methylation and hydroxymethylation characterize the identity of D1 and D2 striatal projection neurons

Neuronal DNA modifications differ from those in other cells, including methylation outside CpG context and abundant 5-hydroxymethylation whose relevance for neuronal identities are unclear. Striatal projection neurons expressing D1 or D2 dopamine receptors allow addressing this question, as they share many characteristics but differ in their gene expression profiles, connections, and functional roles. We compare translating mRNAs and DNA modifications in these two populations. DNA methylation differences occur predominantly in large genomic clusters including differentially expressed genes, potentially important for D1 and D2 neurons. Decreased gene body methylation is associated with higher gene expression. Hydroxymethylation differences are more scattered and affect transcription factor binding sites, which can influence gene expression. We also find a strong genome-wide hydroxymethylation asymmetry between the two DNA strands, particularly pronounced at expressed genes and retrotransposons. These results identify novel properties of neuronal DNA modifications and unveil epigenetic characteristics of striatal projection neurons heterogeneity.

Regulation of the Human IL-10RB Gene Expression by Sp8 and Sp9

BACKGROUND

Interleukin-10 (IL-10) is a classic anti-inflammatory cytokine that exerts its effects via the receptor complexes IL-10RA and IL-10RB. Loss of IL-10RB results in many diseases. Moreover, IL-10RB is closely associated with neuronal survival and synaptic formation. However, the regulation of IL-10RB gene expression remains elusive.

OBJECTIVE

To investigate whether the expression of IL-10RB gene is increased in brain of Alzheimer's disease (AD) and its transcriptional regulation.

METHODS

We examined the gene expression of AD patient brain from public database and detected the protein expression of AD model mouse brain by western blot. We constructed a variety of reporter gene plasmids with different lengths or mutation sites, tested the promoter activity and defined the functional region of the promoter with the luciferase reporter assay. The protein-DNA binding between transcription factors and the promoter was analyzed using chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assay (EMSA).

RESULTS

We found that the IL-10RB is elevated in the brain of AD patient and AD model mice. The minimal promoter of the IL-10RB gene is located in the -90 to +51 bp region (relative to the transcriptional start site) and is sufficient for high-level expression of the IL-10RB gene. Transcription factors Sp8 and Sp9 bind to the IL-10RB promoter in vitro. The overexpression or knockdown of Sp8 and Sp9 affected the IL-10RB promoter activity and its gene expression.

CONCLUSION

Our study functionally characterized the promoter of the IL-10RB gene and demonstrated that Sp8 and Sp9 regulated its expression.

Transcription factor Sp9 is a negative regulator of D1-type MSN development

The striatum is the main input structure of the basal ganglia, receiving information from the cortex and the thalamus and consisting of D1- and D2- medium spiny neurons (MSNs). D1-MSNs and D2-MSNs are essential for motor control and cognitive behaviors and have implications in Parkinson’s Disease. In the present study, we demonstrated that Sp9-positive progenitors produced both D1-MSNs and D2-MSNs and that Sp9 expression was rapidly downregulated in postmitotic D1-MSNs. Furthermore, we found that sustained Sp9 expression in lateral ganglionic eminence (LGE) progenitor cells and their descendants led to promoting D2-MSN identity and repressing D1-MSN identity during striatal development. As a result, sustained Sp9 expression resulted in an imbalance between D1-MSNs and D2-MSNs in the mouse striatum. In addition, the fate-changed D2-like MSNs survived normally in adulthood. Taken together, our findings supported that Sp9 was sufficient to promote D2-MSN identity and repress D1-MSN identity, and Sp9 was a negative regulator of D1-MSN fate.

Evolutionarily conservative and non-conservative regulatory networks during primate interneuron development revealed by single-cell RNA and ATAC sequencing

The differences in size and function between primate and rodent brains, and the association of disturbed excitatory/inhibitory balance with many neurodevelopmental disorders highlight the importance to study primate ganglionic eminences (GEs) development. Here we used single-cell RNA and ATAC sequencing to characterize the emergence of cell diversity in monkey and human GEs where most striatal and cortical interneurons are generated. We identified regional and temporal diversity among progenitor cells which give rise to a variety of interneurons. These cells are specified within the primate GEs by well conserved gene regulatory networks, similar to those identified in mice. However, we detected, in human, several novel regulatory pathways or factors involved in the specification and migration of interneurons. Importantly, comparison of progenitors between our human and published mouse GE datasets led to the discovery and confirmation of outer radial glial cells in GEs in human cortex. Our findings reveal both evolutionarily conservative and nonconservative regulatory networks in primate GEs, which may contribute to their larger brain sizes and more complex neural networks compared with mouse.

Dlx1/2-dependent expression of Meis2 promotes neuronal fate determination in the mammalian striatum

The striatum is a central regulator of behavior and motor function through the actions of D1 and D2 medium-sized spiny neurons (MSNs), which arise from a common lateral ganglionic eminence (LGE) progenitor. The molecular mechanisms of cell fate specification of these two neuronal subtypes are incompletely understood. Here, we found that deletion of murine Meis2, which is highly expressed in the LGE and derivatives, led to a large reduction in striatal MSNs due to a block in their differentiation. Meis2 directly binds to the Zfp503 and Six3 promoters and is required for their expression and specification of D1 and D2 MSNs, respectively. Finally, Meis2 expression is regulated by Dlx1/2 at least partially through the enhancer hs599 in the LGE subventricular zone. Overall, our findings define a pathway in the LGE whereby Dlx1/2 drives expression of Meis2, which subsequently promotes the fate determination of striatal D1 and D2 MSNs via Zfp503 and Six3.

Deletion of Six3 in post-proliferative neurons produces weakened SCN circadian output, improved metabolic function, and dwarfism in male mice

OBJECTIVE

The increasing prevalence of obesity makes it important to increase the understanding of the maturation and function of the neuronal integrators and regulators of metabolic function.

METHODS

Behavioral, molecular, and physiological analyses of transgenic mice with Sine oculis 3 (Six3) deleted in mature neurons using the Synapsin^creallele.

RESULTS

Conditional deletion of the homeodomain transcription factor Six3 in mature neurons causes dwarfism and weakens circadian wheel-running activity rhythms but increases general activity at night, and improves metabolic function, without impacting pubertal onset or fertility in males. The reduced growth in 6-week-old Six3^fl/fl:Synapsin^cre (Six3^syn) males correlates with increased somatostatin (SS) expression in the hypothalamus and reduced growth hormone (GH) in the pituitary. In contrast, 12-week-old Six3^syn males have increased GH release, despite an increased number of the inhibitory SS neurons in the periventricular nucleus. GH is important in glucose metabolism, muscle function, and bone health. Interestingly, Six3^syn males have improved glucose tolerance at 7, 12, and 18 weeks of age, which, in adulthood, is associated with increased % lean mass and increased metabolic rates. Further, 12-week-old Six3^syn males have reduced bone mineralization and a lower bone mineral density, indicating that reduced GH levels during early life cause a long-term reduction in bone mineralization.

CONCLUSION

Our study points to the novel role of Six3 in post-proliferative neurons to regulate metabolic function through SS neuron control of GH release.

Mouse and human share conserved transcriptional programs for interneuron development

[Figure: see text].

From Progenitors to Progeny: Shaping Striatal Circuit Development and Function

Understanding how neurons of the striatum are formed and integrate into complex synaptic circuits is essential to provide insight into striatal function in health and disease. In this review, we summarize our current understanding of the development of striatal neurons and associated circuits with a focus on their embryonic origin. Specifically, we address the role of distinct types of embryonic progenitors, found in the proliferative zones of the ganglionic eminences in the ventral telencephalon, in the generation of diverse striatal interneurons and projection neurons. Indeed, recent evidence would suggest that embryonic progenitor origin dictates key characteristics of postnatal cells, including their neurochemical content, their location within striatum, and their long-range synaptic inputs. We also integrate recent observations regarding embryonic progenitors in cortical and other regions and discuss how this might inform future research on the ganglionic eminences. Last, we examine how embryonic progenitor dysfunction can alter striatal formation, as exemplified in Huntington's disease and autism spectrum disorder, and how increased understanding of embryonic progenitors can have significant implications for future research directions and the development of improved therapeutic options.SIGNIFICANCE STATEMENT This review highlights recently defined novel roles for embryonic progenitor cells in shaping the functional properties of both projection neurons and interneurons of the striatum. It outlines the developmental mechanisms that guide neuronal development from progenitors in the embryonic ganglionic eminences to progeny in the striatum. Where questions remain open, we integrate observations from cortex and other regions to present possible avenues for future research. Last, we provide a progenitor-centric perspective onto both Huntington's disease and autism spectrum disorder. We suggest that future investigations and manipulations of embryonic progenitor cells in both research and clinical settings will likely require careful consideration of their great intrinsic diversity and neurogenic potential.

Addiction-Associated Genetic Variants Implicate Brain Cell Type- and Region-Specific Cis-Regulatory Elements in Addiction Neurobiology

Recent large genome-wide association studies have identified multiple confident risk loci linked to addiction-associated behavioral traits. Most genetic variants linked to addiction-associated traits lie in noncoding regions of the genome, likely disrupting cis-regulatory element (CRE) function. CREs tend to be highly cell type-specific and may contribute to the functional development of the neural circuits underlying addiction. Yet, a systematic approach for predicting the impact of risk variants on the CREs of specific cell populations is lacking. To dissect the cell types and brain regions underlying addiction-associated traits, we applied stratified linkage disequilibrium score regression to compare genome-wide association studies to genomic regions collected from human and mouse assays for open chromatin, which is associated with CRE activity. We found enrichment of addiction-associated variants in putative CREs marked by open chromatin in neuronal (NeuN+) nuclei collected from multiple prefrontal cortical areas and striatal regions known to play major roles in reward and addiction. To further dissect the cell type-specific basis of addiction-associated traits, we also identified enrichments in human orthologs of open chromatin regions of female and male mouse neuronal subtypes: cortical excitatory, D1, D2, and PV. Last, we developed machine learning models to predict mouse cell type-specific open chromatin, enabling us to further categorize human NeuN+ open chromatin regions into cortical excitatory or striatal D1 and D2 neurons and predict the functional impact of addiction-associated genetic variants. Our results suggest that different neuronal subtypes within the reward system play distinct roles in the variety of traits that contribute to addiction.SIGNIFICANCE STATEMENT We combine statistical genetic and machine learning techniques to find that the predisposition to for nicotine, alcohol, and cannabis use behaviors can be partially explained by genetic variants in conserved regulatory elements within specific brain regions and neuronal subtypes of the reward system. Our computational framework can flexibly integrate open chromatin data across species to screen for putative causal variants in a cell type- and tissue-specific manner for numerous complex traits.

Unbiased identification of novel transcription factors in striatal compartmentation and striosome maturation

Many diseases are linked to dysregulation of the striatum. Striatal function depends on neuronal compartmentation into striosomes and matrix. Striatal projection neurons are GABAergic medium spiny neurons (MSNs), subtyped by selective expression of receptors, neuropeptides, and other gene families. Neurogenesis of the striosome and matrix occurs in separate waves, but the factors regulating compartmentation and neuronal differentiation are largely unidentified. We performed RNA- and ATAC-seq on sorted striosome and matrix cells at postnatal day 3, using the Nr4a1-EGFP striosome reporter mouse. Focusing on the striosome, we validated the localization and/or role of Irx1, Foxf2, Olig2, and Stat1/2 in the developing striosome and the in vivo enhancer function of a striosome-specific open chromatin region 4.4 Kb downstream of Olig2. These data provide novel tools to dissect and manipulate the networks regulating MSN compartmentation and differentiation, including in human iPSC-derived striatal neurons for disease modeling and drug discovery.

The transcription factors SIX3 and VAX1 are required for suprachiasmatic nucleus circadian output and fertility in female mice

The homeodomain transcription factors sine oculis homeobox 3 (Six3) and ventral anterior homeobox 1 (Vax1) are required for brain development. Their expression in specific brain areas is maintained in adulthood, where their functions are poorly understood. To identify the roles of Six3 and Vax1 in neurons, we conditionally deleted each gene using Synapsincre , a promoter targeting maturing neurons, and generated Six3syn and Vax1syn mice. Six3syn and Vax1syn females, but not males, had reduced fertility, due to impairment of the luteinizing hormone (LH) surge driving ovulation. In nocturnal rodents, the LH surge requires a precise timing signal from the brain's circadian pacemaker, the suprachiasmatic nucleus (SCN), near the time of activity onset. Indeed, both Six3syn and Vax1syn females had impaired rhythmic SCN output, which was associated with weakened Period 2 molecular clock function in both Six3syn and Vax1syn mice. These impairments were associated with a reduction of the SCN neuropeptide vasoactive intestinal peptide in Vax1syn mice and a modest weakening of SCN timekeeping function in both Six3syn and Vax1syn mice. Changes in SCN function were associated with mistimed peak PER2::LUC expression in the SCN and pituitary in both Six3syn and Vax1syn females. Interestingly, Six3syn ovaries presented reduced sensitivity to LH, causing reduced ovulation during superovulation. In conclusion, we have identified novel roles of the homeodomain transcription factors SIX3 and VAX1 in neurons, where they are required for proper molecular circadian clock function, SCN rhythmic output, and female fertility.

LISA2: Learning Complex Single-Cell Trajectory and Expression Trends

Single-cell transcriptional and epigenomics profiles have been applied in a variety of tissues and diseases for discovering new cell types, differentiation trajectories, and gene regulatory networks. Many methods such as Monocle 2/3, URD, and STREAM have been developed for tree-based trajectory building. Here, we propose a fast and flexible trajectory learning method, LISA2, for single-cell data analysis. This new method has two distinctive features: (1) LISA2 utilizes specified leaves and root to reduce the complexity for building the developmental trajectory, especially for some special cases such as rare cell populations and adjacent terminal cell states; and (2) LISA2 is applicable for both transcriptomics and epigenomics data. LISA2 visualizes complex trajectories using 3D Landmark ISOmetric feature MAPping (L-ISOMAP). We apply LISA2 to simulation and real datasets in cerebellum, diencephalon, and hematopoietic stem cells including both single-cell transcriptomics data and single-cell assay for transposase-accessible chromatin data. LISA2 is efficient in estimating single-cell trajectory and expression trends for different kinds of molecular state of cells.

Homeobox Gene Six3 is Required for the Differentiation of D2-Type Medium Spiny Neurons

Medium spiny neurons (MSNs) in the striatum, which can be divided into D1 and D2 MSNs, originate from the lateral ganglionic eminence (LGE). Previously, we reported that Six3 is a downstream target of Sp8/Sp9 in the transcriptional regulatory cascade of D2 MSN development and that conditionally knocking out Six3 leads to a severe loss of D2 MSNs. Here, we showed that Six3 mainly functions in D2 MSN precursor cells and gradually loses its function as D2 MSNs mature. Conditional deletion of Six3 had little effect on cell proliferation but blocked the differentiation of D2 MSN precursor cells. In addition, conditional overexpression of Six3 promoted the differentiation of precursor cells in the LGE. We measured an increase of apoptosis in the postnatal striatum of conditional Six3-knockout mice. This suggests that, in the absence of Six3, abnormally differentiated D2 MSNs are eliminated by programmed cell death. These results further identify Six3 as an important regulatory element during D2 MSN differentiation.

Diversity in striatal synaptic circuits arises from distinct embryonic progenitor pools in the ventral telencephalon

Synaptic circuits in the brain are precisely organized, but the processes that govern this precision are poorly understood. Here, we explore how distinct embryonic neural progenitor pools in the lateral ganglionic eminence contribute to neuronal diversity and synaptic circuit connectivity in the mouse striatum. In utero labeling of Tα1-expressing apical intermediate progenitors (aIP), as well as other progenitors (OP), reveals that both progenitors generate direct and indirect pathway spiny projection neurons (SPNs) with similar electrophysiological and anatomical properties and are intermingled in medial striatum. Subsequent optogenetic circuit-mapping experiments demonstrate that progenitor origin significantly impacts long-range excitatory input strength, with medial prefrontal cortex preferentially driving aIP-derived SPNs and visual cortex preferentially driving OP-derived SPNs. In contrast, the strength of local inhibitory inputs among SPNs is controlled by birthdate rather than progenitor origin. Combined, these results demonstrate distinct roles for embryonic progenitor origin in shaping neuronal and circuit properties of the postnatal striatum.

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The Tα1 promoter distinguishes two embryonic progenitor pools in the LGE

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Both pools generate intermixed spiny projection neurons in dorsomedial striatum

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Excitatory cortical inputs are biased toward SPNs of different embryonic origin

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Neurogenic stage rather impacts local inhibitory connections among SPNs

Single-Cell Analysis of Foxp1-Driven Mechanisms Essential for Striatal Development

The striatum is a critical forebrain structure integrating cognitive, sensory, and motor information from diverse brain regions into meaningful behavioral output. However, the transcriptional mechanisms underlying striatal development at single-cell resolution remain unknown. Using single-cell RNA sequencing (RNA-seq), we examine the cellular diversity of the early postnatal striatum and show that Foxp1, a transcription factor strongly linked to autism and intellectual disability, regulates the cellular composition, neurochemical architecture, and connectivity of the striatum in a cell-type-dependent fashion. We also identify Foxp1-regulated target genes within distinct cell types and connect these molecular changes to functional and behavioral deficits relevant to phenotypes described in patients with FOXP1 loss-of-function mutations. Using this approach, we could also examine the non-cell-autonomous effects produced by disrupting one cell type and the molecular compensation that occurs in other populations. These data reveal the cell-type-specific transcriptional mechanisms regulated by Foxp1 that underlie distinct features of striatal circuitry.

Transcriptional profiling reveals the transcription factor networks regulating the survival of striatal neurons

The striatum is structurally highly diverse, and its organ functionality critically depends on normal embryonic development. Although several studies have been conducted on the gene functional changes that occur during striatal development, a system-wide analysis of the underlying molecular changes is lacking. Here, we present a comprehensive transcriptome profile that allows us to explore the trajectory of striatal development and identify the correlation between the striatal development and Huntington's disease (HD). Furthermore, we applied an integrative transcriptomic profiling approach based on machine learning to systematically map a global landscape of 277 transcription factor (TF) networks. Most of these TF networks are linked to biological processes, and some unannotated genes provide information about the corresponding mechanisms. For example, we found that the Meis2 and Six3 were crucial for the survival of striatal neurons, which were verified using conditional knockout (CKO) mice. Finally, we used RNA-Seq to speculate their downstream targets.

Neurogenesis of medium spiny neurons in the nucleus accumbens continues into adulthood and is enhanced by pathological pain

In mammals, most adult neural stem cells (NSCs) are located in the ventricular-subventricular zone (V-SVZ) along the wall of the lateral ventricles and they are the source of olfactory bulb interneurons. Adult NSCs exhibit an apico-basal polarity; they harbor a short apical process and a long basal process, reminiscent of radial glia morphology. In the adult mouse brain, we detected extremely long radial glia-like fibers that originate from the anterior-ventral V-SVZ and that are directed to the ventral striatum. Interestingly, a fraction of adult V-SVZ-derived neuroblasts dispersed in close association with the radial glia-like fibers in the nucleus accumbens (NAc). Using several in vivo mouse models, we show that newborn neurons integrate into preexisting circuits in the NAc where they mature as medium spiny neurons (MSNs), i.e., a type of projection neurons formerly believed to be generated only during embryonic development. Moreover, we found that the number of newborn neurons in the NAc is dynamically regulated by persistent pain, suggesting that adult neurogenesis of MSNs is an experience-modulated process.

Gene Expression Changes in the Ventral Tegmental Area of Male Mice with Alternative Social Behavior Experience in Chronic Agonistic Interactions

Daily agonistic interactions of mice are an effective experimental approach to elucidate the molecular mechanisms underlying the excitation of the brain neurons and the formation of alternative social behavior patterns. An RNA-Seq analysis was used to compare the ventral tegmental area (VTA) transcriptome profiles for three groups of male C57BL/6J mice: winners, a group of chronically winning mice, losers, a group of chronically defeated mice, and controls. The data obtained show that both winners and defeated mice experience stress, which however, has a more drastic effect on defeated animals causing more significant changes in the levels of gene transcription. Four genes (Nrgn, Ercc2, Otx2, and Six3) changed their VTA expression profiles in opposite directions in winners and defeated mice. It was first shown that Nrgn (neurogranin) expression was highly correlated with the expression of the genes involved in dopamine synthesis and transport (Th, Ddc, Slc6a3, and Drd2) in the VTA of defeated mice but not in winners. The obtained network of 31 coregulated genes, encoding proteins associated with nervous system development (including 24 genes associated with the generation of neurons), may be potentially useful for studying their role in the VTA dopaminergic neurons maturation under the influence of social stress.

Striatal circuit development and its alterations in Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder that usually starts during midlife with progressive alterations of motor and cognitive functions. The disease is caused by a CAG repeat expansion within the huntingtin gene leading to severe striatal neurodegeneration. Recent studies conducted on pre-HD children highlight early striatal developmental alterations starting as soon as 6 years old, the earliest age assessed. These findings, in line with data from mouse models of HD, raise the questions of when during development do the first disease-related striatal alterations emerge and whether they contribute to the later appearance of the neurodegenerative features of the disease. In this review we will describe the different stages of striatal network development and then discuss recent evidence for its alterations in rodent models of the disease. We argue that a better understanding of the striatum's development should help in assessing aberrant neurodevelopmental processes linked to the HD mutation.

Genomic Resolution of DLX-Orchestrated Transcriptional Circuits Driving Development of Forebrain GABAergic Neurons

DLX transcription factors (TFs) are master regulators of the developing vertebrate brain, driving forebrain GABAergic neuronal differentiation. Ablation of Dlx1&2 alters expression of genes that are critical for forebrain GABAergic development. We integrated epigenomic and transcriptomic analyses, complemented with in situ hybridization (ISH), and in vivo and in vitro studies of regulatory element (RE) function. This revealed the DLX-organized gene regulatory network at genomic, cellular, and spatial levels in mouse embryonic basal ganglia. DLX TFs perform dual activating and repressing functions; the consequences of their binding were determined by the sequence and genomic context of target loci. Our results reveal and, in part, explain the paradox of widespread DLX binding contrasted with a limited subset of target loci that are sensitive at the epigenomic and transcriptomic level to Dlx1&2 ablation. The regulatory properties identified here for DLX TFs suggest general mechanisms by which TFs orchestrate dynamic expression programs underlying neurodevelopment.

Lindtner et al. reveal the regulatory wiring organized by DLX transcription factors in forebrain GABAergic neuronal specification, by integrating functional genomic, epigenomic, and genetic data on a transgenic mouse model. This network determines key sequence-encoded regulatory elements and implicates a combination of histone modifications and biophysical interactions.

Nolz1 expression is required in dopaminergic axon guidance and striatal innervation

Midbrain dopaminergic (DA) axons make long longitudinal projections towards the striatum. Despite the importance of DA striatal innervation, processes involved in establishment of DA axonal connectivity remain largely unknown. Here we demonstrate a striatal-specific requirement of transcriptional regulator Nolz1 in establishing DA circuitry formation. DA projections are misguided and fail to innervate the striatum in both constitutive and striatal-specific Nolz1 mutant embryos. The lack of striatal Nolz1 expression results in nigral to pallidal lineage conversion of striatal projection neuron subtypes. This lineage switch alters the composition of secreted factors influencing DA axonal tract formation and renders the striatum non-permissive for dopaminergic and other forebrain tracts. Furthermore, transcriptomic analysis of wild-type and Nolz1-/- mutant striatal tissue led to the identification of several secreted factors that underlie the observed guidance defects and proteins that promote DA axonal outgrowth. Together, our data demonstrate the involvement of the striatum in orchestrating dopaminergic circuitry formation.

Caudate nuclei volume alterations and cognition and mood dysfunctions in adolescents with single ventricle heart disease

Adolescents with single ventricle heart disease (SVHD) exhibit mood and cognitive deficits, which may result from injury to the basal ganglia structures, including the caudate nuclei. However, the integrity of the caudate in SVHD adolescents is unclear. Our aim was to examine the global and regional caudate volumes, and evaluate the relationships between caudate volumes and cognitive and mood scores in SVHD and healthy adolescents. We acquired two high-resolution T1-weighted images from 23 SVHD and 37 controls using a 3.0-Tesla MRI scanner, as well as assessed mood (Patient Health Questionnaire-9 [PHQ-9]; Beck Anxiety Inventory [BAI]) and cognition (Montreal Cognitive Assessment [MoCA]; Wide Range Assessment of Memory and Learning-2; General Memory Index [GMI]) functions. Both left and right caudate nuclei were outlined, which were then used to calculate and compare volumes between groups using ANCOVA (covariates: age, gender, and head-size), as well as perform 3D surface morphometry. Partial correlations (covariates: age, gender, and head-size) were used to examine associations between caudate volumes, cognition, and mood scores in SVHD and controls. SVHD subjects showed significantly higher PHQ-9 and BAI scores, indicating more depressive and anxiety symptoms, as well as reduced GMI scores, suggesting impaired cognition, compared to controls. SVHD patients showed significantly reduced caudate volumes (left, 3,198.8 ± 490.1 vs. 3,605.0 ± 480.4 mm³ , p < 0.004; right, 3,162.1 ± 475.4 vs. 3,504.8 ± 465.9 mm³ , p < 0.011) over controls, and changes were localized in the rostral, mid-dorsolateral, and caudal areas. Significant negative correlations emerged between caudate volumes with PHQ-9 and BAI scores and positive correlations with GMI and MoCA scores in SVHD and controls. SVHD adolescents show significantly reduced caudate volumes, especially in sites that have projections to regulate mood and cognition, which may result from developmental and/or hypoxia-/ischemia-induced processes.

Parcellation of the striatal complex into dorsal and ventral districts

The striatal complex of basal ganglia comprises two functionally distinct districts. The dorsal district controls motor and cognitive functions. The ventral district regulates the limbic function of motivation, reward, and emotion. The dorsoventral parcellation of the striatum also is of clinical importance as differential striatal pathophysiologies occur in Huntington's disease, Parkinson's disease, and drug addiction disorders. Despite these striking neurobiologic contrasts, it is largely unknown how the dorsal and ventral divisions of the striatum are set up. Here, we demonstrate that interactions between the two key transcription factors Nolz-1 and Dlx1/2 control the migratory paths of striatal neurons to the dorsal or ventral striatum. Moreover, these same transcription factors control the cell identity of striatal projection neurons in both the dorsal and the ventral striata including the D1-direct and D2-indirect pathways. We show that Nolz-1, through the I12b enhancer, represses Dlx1/2, allowing normal migration of striatal neurons to dorsal and ventral locations. We demonstrate that deletion, up-regulation, and down-regulation of Nolz-1 and Dlx1/2 can produce a striatal phenotype characterized by a withered dorsal striatum and an enlarged ventral striatum and that we can rescue this phenotype by manipulating the interactions between Nolz-1 and Dlx1/2 transcription factors. Our study indicates that the two-tier system of striatal complex is built by coupling of cell-type identity and migration and suggests that the fundamental basis for divisions of the striatum known to be differentially vulnerable at maturity is already encoded by the time embryonic striatal neurons begin their migrations into developing striata.

Dlx1/2 are Central and Essential Components in the Transcriptional Code for Generating Olfactory Bulb Interneurons

Generation of olfactory bulb (OB) interneurons requires neural stem/progenitor cell specification, proliferation, differentiation, and young interneuron migration and maturation. Here, we show that the homeobox transcription factors Dlx1/2 are central and essential components in the transcriptional code for generating OB interneurons. In Dlx1/2 constitutive null mutants, the differentiation of GSX2+ and ASCL1+ neural stem/progenitor cells in the dorsal lateral ganglionic eminence is blocked, resulting in a failure of OB interneuron generation. In Dlx1/2 conditional mutants (hGFAP-Cre; Dlx1/2F/- mice), GSX2+ and ASCL1+ neural stem/progenitor cells in the postnatal subventricular zone also fail to differentiate into OB interneurons. In contrast, overexpression of Dlx1&2 in embryonic mouse cortex led to ectopic production of OB-like interneurons that expressed Gad1, Sp8, Sp9, Arx, Pbx3, Etv1, Tshz1, and Prokr2. Pax6 mutants generate cortical ectopia with OB-like interneurons, but do not do so in compound Pax6; Dlx1/2 mutants. We propose that DLX1/2 promote OB interneuron development mainly through activating the expression of Sp8/9, which further promote Tshz1 and Prokr2 expression. Based on this study, in combination with earlier ones, we propose a transcriptional network for the process of OB interneuron development.

Dynamic postnatal development of the cellular and circuit properties of striatal D1 and D2 spiny projection neurons

KEY POINTS

Imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs) are thought to contribute to many basal ganglia disorders, including early-onset neurodevelopmental disorders such as obsessive-compulsive disorder, attention deficit hyperactivity disorder and Tourette's syndrome. This study provides the first detailed quantitative investigation of development of D1 and D2 SPNs, including their cellular properties and connectivity within neural circuits, during the first postnatal weeks. This period is highly dynamic with many properties changing, but it is possible to make three main observations: many aspects of D1 and D2 SPNs progressively mature in parallel; there are notable exceptions when they diverge; and many of the defining properties of mature striatal SPNs and circuits are already established by the first and second postnatal weeks, suggesting guidance through intrinsic developmental programmes. These findings provide an experimental framework for future studies of striatal development in both health and disease.

ABSTRACT

Many basal ganglia neurodevelopmental disorders are thought to result from imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs). Insight into these disorders is reliant on our understanding of normal D1 and D2 SPN development. Here we provide the first detailed study and quantification of the striatal cellular and circuit changes occurring for both D1 and D2 SPNs in the first postnatal weeks using in vitro whole-cell patch-clamp electrophysiology. Characterization of their intrinsic electrophysiological and morphological properties, the excitatory long-range inputs coming from cortex and thalamus, as well their local gap junction and inhibitory synaptic connections reveals this period to be highly dynamic with numerous properties changing. However it is possible to make three main observations. Firstly, many aspects of SPNs mature in parallel, including intrinsic membrane properties, increases in dendritic arbours and spine densities, general synaptic inputs and expression of specific glutamate receptors. Secondly, there are notable exceptions, including a transient stronger thalamic innervation of D2 SPNs and stronger cortical NMDA receptor-mediated inputs to D1 SPNs, both in the second postnatal week. Thirdly, many of the defining properties of mature D1 and D2 SPNs and striatal circuits are already established by the first and second postnatal weeks, including different electrophysiological properties as well as biased local inhibitory connections between SPNs, suggesting this is guided through intrinsic developmental programmes. Together these findings provide an experimental framework for future studies of D1 and D2 SPN development in health and disease.

Embryonic and postnatal development of mouse olfactory tubercle

The olfactory tubercle (OT) is located in the ventral-medial region of the brain where it receives primary input from olfactory bulb (OB) projection neurons and processes olfactory behaviors related to motivation, hedonics of smell and sexual encounters. The OT is part of the dopamine reward system that shares characteristics with the striatum. Together with the nucleus accumbens, the OT has been referred to as the "ventral striatum". However, despite its functional importance little is known about the embryonic development of the OT and the phenotypic properties of the OT cells. Here, using thymidine analogs, we establish that mouse OT neurogenesis occurs predominantly between E11-E15 in a lateral-to-medial gradient. Then, using a piggyBac multicolor technique we characterized the migratory route of OT neuroblasts from their embryonic point of origin. Following neurogenesis in the ventral lateral ganglionic eminence (vLGE), neuroblasts destined for the OT followed a dorsal-ventral pathway we named "ventral migratory course" (VMC). Upon reaching the nascent OT, neurons established a prototypical laminar distribution that was determined, in part, by the progenitor cell of origin. A phenotypic analysis of OT neuroblasts using a single-color piggyBac technique, showed that OT shared the molecular specification of striatal neurons. In addition to primary afferent input from the OB, the OT also receives a robust dopaminergic input from ventral tegmentum (Ikemoto, 2007). We used tyrosine hydroxylase (TH) expression as a proxy for dopaminergic innervation and showed that TH onset occurs at E13 and progressively increased until postnatal stages following an 'inside-out' pattern. Postnatally, we established the myelination in the OT occurring between P7 and P14, as shown with CNPase staining, and we characterized the cellular phenotypes populating the OT by immunohistochemistry. Collectively, this work provides the first detailed analysis of the developmental and maturation processes occurring in mouse OT, and demonstrates the striatal nature of the OT as part of the ventral striatum (vST).

Transcription Factors Sp8 and Sp9 Regulate Medial Ganglionic Eminence-Derived Cortical Interneuron Migration

Cortical interneurons are derived from the subpallium and reach the developing cortex through long tangential migration. Mature cortical interneurons are characterized by remarkable morphological, molecular, and functional diversity. The calcium-binding protein parvalbumin (PV) and neuropeptide somatostatin (SST) identify most medial ganglionic eminence (MGE)-derived cortical interneurons. Previously, we demonstrated that Sp9 plays a curial transcriptional role in regulating MGE-derived cortical interneuron development. Here, we show that SP8 protein is weekly expressed in the MGE mantle zone of wild type mice but upregulated in Sp9 null mutants. PV⁺ cortical interneurons were severely lost in Sp8/Sp9 double conditional knockouts due to defects in tangential migration compared with Sp9 single mutants, suggesting that Sp8/9 coordinately regulate PV⁺ cortical interneuron development. We provide evidence that Sp8/Sp9 activity is required for normal MGE-derived cortical interneuron migration, at least in part, through regulating the expression of EphA3, Ppp2r2c, and Rasgef1b.

Active intermixing of indirect and direct neurons builds the striatal mosaic

The striatum controls behaviors via the activity of direct and indirect pathway projection neurons (dSPN and iSPN) that are intermingled in all compartments. While such cellular mosaic ensures the balanced activity of the two pathways, its developmental origin and pattern remains largely unknown. Here, we show that both SPN populations are specified embryonically and intermix progressively through multidirectional iSPN migration. Using conditional mutant mice, we found that inactivation of the dSPN-specific transcription factor Ebf1 impairs selective dSPN properties, including axon pathfinding, while molecular and functional features of iSPN were preserved. Ebf1 mutation disrupted iSPN/dSPN intermixing, resulting in an uneven distribution. Such architectural defect was selective of the matrix compartment, highlighting that intermixing is a parallel process to compartment formation. Our study reveals while iSPN/dSPN specification is largely independent, their intermingling emerges from an active migration of iSPN, thereby providing a novel framework for the building of striatal architecture.