Laminar organization of the early developing anterior hypothalamus

Molecular Diversity of Neuron Types in the Salamander Amygdala and Implications for Amygdalar Evolution

The amygdala is a complex brain structure in the vertebrate telencephalon, essential for regulating social behaviors, emotions, and (social) cognition. In contrast to the vast majority of neuron types described in the many nuclei of the mammalian amygdala, little is known about the neuronal diversity in non-mammals, making reconstruction of its evolution particularly difficult. Here, we characterize glutamatergic neuron types in the amygdala of the urodele amphibian Pleurodeles waltl. Our single-cell RNA sequencing data indicate the existence of at least ten distinct types and subtypes of glutamatergic neurons in the salamander amygdala. These neuron types are molecularly distinct from neurons in the ventral pallium (VP), suggesting that the pallial amygdala and the VP are two separate areas in the telencephalon. In situ hybridization for marker genes indicates that amygdalar glutamatergic neuron types are located in three major subdivisions: the lateral amygdala, the medial amygdala, and a newly defined area demarcated by high expression of the transcription factor Sim1. The gene expression profiles of these neuron types suggest similarities with specific neurons in the sauropsid and mammalian amygdala. In particular, we identify Sim1+ and Sim1+ Otp+ expressing neuron types, potentially homologous to the mammalian nucleus of the lateral olfactory tract (NLOT) and to hypothalamic-derived neurons of the medial amygdala, respectively. Taken together, our results reveal a surprising diversity of glutamatergic neuron types in the amygdala of salamanders, despite the anatomical simplicity of their brain. These results offer new insights on the cellular and anatomical complexity of the amygdala in tetrapod ancestors.

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

Ectopic expression of Irx3 and Irx5 in the paraventricular nucleus of the hypothalamus contributes to defects in Sim1 haploinsufficiency

The paraventricular nucleus of the hypothalamus (PVH) contains a heterogeneous cluster of Sim1-expressing neurons critical for feeding regulation. Sim1 haploinsufficiency results in hyperphagic obesity with disruption of PVH neurons, yet the molecular profiles of PVH neurons and the mechanism underlying the defects of Sim1 haploinsufficiency are not well understood. By single-cell RNA sequencing, we identified two major populations of Sim1⁺ PVH neurons, which are differentially affected by Sim1 haploinsufficiency. The Iroquois homeobox genes Irx3 and Irx5 have been implicated in the hypothalamic control of energy homeostasis. We found that Irx3 and Irx5 are ectopically expressed in the Sim1⁺ PVH cells of Sim1^+/− mice. By reducing their dosage and PVH-specific deletion of Irx3, we demonstrate that misexpression of Irx3 and Irx5 contributes to the defects of Sim1^+/− mice. Our results illustrate abnormal hypothalamic activities of Irx3 and Irx5 as a central mechanism disrupting PVH development and feeding regulation in Sim1 haploinsufficiency.

Specification of oxytocinergic and vasopressinergic circuits in the developing mouse brain

Oxytocin (OXT) and arginine vasopressin (AVP) support a broad range of behaviors and homeostatic functions including sex-specific and context-appropriate social behaviors. Although the alterations of these systems have been linked with social-related disorders such as autism spectrum disorder, their formation and developmental dynamics remain largely unknown. Using novel brain clearing techniques and 3D imaging, we have reconstructed the specification of oxytocinergic and vasopressinergic circuits in the developing mouse brain with unprecedented cellular resolution. A systematic quantification indicates that OXT and AVP neurons in the hypothalamus display distinctive developmental dynamics and high cellular plasticity from embryonic to early postnatal stages. Our findings reveal new insights into the specification and consolidation of neuropeptidergic systems in the developing CNS.

Cellular and molecular properties of neural progenitors in the developing mammalian hypothalamus

The neuroendocrine hypothalamus is the central regulator of vital physiological homeostasis and behavior. However, the cellular and molecular properties of hypothalamic neural progenitors remain unexplored. Here, hypothalamic radial glial (hRG) and hypothalamic mantle zone radial glial (hmRG) cells are found to be neural progenitors in the developing mammalian hypothalamus. The hmRG cells originate from hRG cells and produce neurons. During the early development of hypothalamus, neurogenesis occurs in radial columns and is initiated from hRG cells. The radial glial fibers are oriented toward the locations of hypothalamic subregions which act as a scaffold for neuronal migration. Furthermore, we use single-cell RNA sequencing to reveal progenitor subtypes in human developing hypothalamus and characterize specific progenitor genes, such as TTYH1, HMGA2, and FAM107A. We also demonstrate that HMGA2 is involved in E2F1 pathway, regulating the proliferation of progenitor cells by targeting on the downstream MYBL2. Different neuronal subtypes start to differentiate and express specific genes of hypothalamic nucleus at gestational week 10. Finally, we reveal the developmental conservation of nuclear structures and marker genes in mouse and human hypothalamus. Our identification of cellular and molecular properties of neural progenitors provides a basic understanding of neurogenesis and regional formation of the non-laminated hypothalamus.

Hypothalamic IRX3: A New Player in the Development of Obesity

Genome-wide association studies (GWASs) have identified SNPs of the fat mass and obesity (FTO) gene as the most important risk alleles for obesity. However, how the presence of risk alleles affect phenotype is still a matter of intense investigation. In 2014, a study revealed that long-range enhancers from the intronic regions of the FTO gene regulate iroquois-class homeobox protein (IRX)3 expression. IRX3 is expressed in hypothalamic pro-opiomelanocortin (POMC) neurons and changes in its expression levels affect body adiposity by modifying food intake and energy expenditure. These findings have placed IRX3 as a potential target for the treatment of obesity. Here, we review studies that evaluated the roles of IRX3 in development, neurogenesis, and body energy homeostasis.

Dysregulation of Hypothalamic Gene Expression and the Oxytocinergic System by Soybean Oil Diets in Male Mice

Soybean oil consumption has increased greatly in the past half-century and is linked to obesity and diabetes. To test the hypothesis that soybean oil diet alters hypothalamic gene expression in conjunction with metabolic phenotype, we performed RNA sequencing analysis using male mice fed isocaloric, high-fat diets based on conventional soybean oil (high in linoleic acid, LA), a genetically modified, low-LA soybean oil (Plenish), and coconut oil (high in saturated fat, containing no LA). The 2 soybean oil diets had similar but nonidentical effects on the hypothalamic transcriptome, whereas the coconut oil diet had a negligible effect compared to a low-fat control diet. Dysregulated genes were associated with inflammation, neuroendocrine, neurochemical, and insulin signaling. Oxt was the only gene with metabolic, inflammation, and neurological relevance upregulated by both soybean oil diets compared to both control diets. Oxytocin immunoreactivity in the supraoptic and paraventricular nuclei of the hypothalamus was reduced, whereas plasma oxytocin and hypothalamic Oxt were increased. These central and peripheral effects of soybean oil diets were correlated with glucose intolerance but not body weight. Alterations in hypothalamic Oxt and plasma oxytocin were not observed in the coconut oil diet enriched in stigmasterol, a phytosterol found in soybean oil. We postulate that neither stigmasterol nor LA is responsible for effects of soybean oil diets on oxytocin and that Oxt messenger RNA levels could be associated with the diabetic state. Given the ubiquitous presence of soybean oil in the American diet, its observed effects on hypothalamic gene expression could have important public health ramifications.

The partial inhibition of hypothalamic IRX3 exacerbates obesity

Background

The Iroquois homeobox 3 (Irx3) gene has been identified as a functional long-range target of obesity-associated variants within the fat mass and obesity-associated protein (FTO) gene. It is highly expressed in the hypothalamus, and both whole-body knockout and hypothalamic restricted abrogation of its expression results in a lean phenotype, which is mostly explained by the resulting increased energy expenditure in the brown adipose tissue. Because of its potential implication in the pathogenesis of obesity, we evaluated the hypothalamic cell distribution of Irx3 and the outcomes of inhibiting its expression in a rodent model of diet-induced obesity.

Methods

Bioinformatics tools were used to evaluate the correlations between hypothalamic Irx3 and neurotransmitters, markers of thermogenesis and obesity related phenotypes. Droplet-sequencing analysis in >20,000 hypothalamic cells was used to explore the types of hypothalamic cells expressing Irx3. Lentivirus was used to inhibit hypothalamic Irx3 and the resulting phenotype was studied.

Findings

IRX3 is expressed predominantly in POMC neurons. Its expression is inhibited during prolonged fasting, as well as when mice are fed a high-fat diet. The partial inhibition of hypothalamic Irx3 using a lentivirus resulted in increased diet-induced body mass gain and adiposity due to increased caloric intake and reduced energy expenditure.

Interpretation

Contrary to the results obtained when lean mice are submitted to complete inhibition of Irx3, partial inhibition of hypothalamic Irx3 in obese mice causes an exacerbation of the obese phenotype. These data suggest that at least some of the Irx3 functions in the hypothalamus are regulated according to a hormetic pattern, and modulation of its expression can be a novel approach to modifying the body's energy-handling regulation.

Fund

Sao Paulo Research Foundation grants 2013/07607-8 (LAV) and 2017/02983-2 (JDJ); NIH grants R01DK083567 (YBK).

The Paraventricular Nucleus of the Hypothalamus: Development, Function, and Human Diseases

The paraventricular nucleus of the hypothalamus (PVH), located in the ventral diencephalon adjacent to the third ventricle, is a highly conserved brain region present in species from zebrafish to humans. The PVH is composed of three main types of neurons, magnocellular, parvocellular, and long-projecting neurons, which play imperative roles in the regulation of energy balance and various endocrinological activities. In this review, we focus mainly on recent findings about the early development of the hypothalamus and the PVH, the functions of the PVH in the modulation of energy homeostasis and in the hypothalamus-pituitary system, and human diseases associated with the PVH, such as obesity, short stature, hypertension, and diabetes insipidus. Thus, the investigations of the PVH will benefit not only understanding of the development of the central nervous system but also the etiology of and therapy for human diseases.

High-resolution transcriptional dissection of in vivo Atoh1-mediated hair cell conversion in mature cochleae identifies Isl1 as a co-reprogramming factor

In vivo direct conversion of differentiated cells holds promise for regenerative medicine; however, improving the conversion efficiency and producing functional target cells remain challenging. Ectopic Atoh1 expression in non-sensory supporting cells (SCs) in mouse cochleae induces their partial conversion to hair cells (HCs) at low efficiency. Here, we performed single-cell RNA sequencing of whole mouse sensory epithelia harvested at multiple time points after conditional overexpression of Atoh1. Pseudotemporal ordering revealed that converted HCs (cHCs) are present along a conversion continuum that correlates with both endogenous and exogenous Atoh1 expression. Bulk sequencing of isolated cell populations and single-cell qPCR confirmed 51 transcription factors, including Isl1, are differentially expressed among cHCs, SCs and HCs. In transgenic mice, co-overexpression of Atoh1 and Isl1 enhanced the HC conversion efficiency. Together, our study shows how high-resolution transcriptional profiling of direct cell conversion can identify co-reprogramming factors required for efficient conversion.

The ongoing ATOH1 gene therapy clinical trial offers promise for hearing restoration in humans. However, in animal models, Atoh1-mediated sensory regeneration is inefficient and incomplete. Here we performed high-resolution gene expression profiling of single cochlear cells at multiple time points in a mouse model whereby we discovered a continuous regeneration process that leads to the formation of immature sensory cells. We identified 51 key reprogramming transcription factors that may increase the efficiency and completion of the regeneration process and confirmed that Isl1 in transgenic mice promotes Atoh1-mediated sensory regeneration as a co-reprogramming factor. Our studies identify molecular mechanisms and novel co-reprogramming factors for sensory restoration in humans with irreversible hearing loss.

Fezf1 is a novel regulator of female sex behavior in mice

Female sexual behavior is a complex process regulated by multiple brain circuits and influenced by sex steroid hormones acting in the brain. Several regions in the hypothalamus have been implicated in the regulation of female sexual behavior although a complete circuitry involved in female sexual behavior is not understood. Fez family zinc finger 1 (Fezf1) gene is a brain specific gene that has been mostly studied in the context of olfactory development, although in a recent study, FEZF1 has been identified as one of the genes responsible for the development of Kallman syndrome. In the present study, we utilized shRNA approach to downregulate Fezf1 in the ventromedial nucleus of the hypothalamus (VMN) with the aim to explore the role of this gene. Adult female mice were stereotaxically injected with lentiviral vectors encoding shRNA against Fezf1 gene. Mice injected with shRNA against Fezf1 had significantly reduced female sexual behavior, presumably due to the downregulation of estrogen receptor alpha (ERα), as the number of ERα-immunoreactive cells in the VMN of Fezf1 mice was significantly lower in comparison to controls. However, no effect on body weight or physical activity was observed in mice with downregulated Fezf1, suggesting that the role of Fezf1 in the VMN is limited to the regulation of sexual behavior.

SIGNIFICANCE STATEMENT

Fezf1 gene has been identified in the present study as a regulator of female sexual behavior in mice. Regulation of the female sexual behavior could be through the regulation of estrogen receptor alpha expression in the ventromedial nucleus of the hypothalamus, as the expression of this receptor was reduced in mice with downregulated Fezf1. As expression of Fezf1 is very specific in the brain, this gene could present a potential target for the development of novel drugs regulating hypoactive sexual desire disorder in women, if similar function of FEZF1 will be confirmed in humans.

Oxytocin Signaling in the Early Life of Mammals: Link to Neurodevelopmental Disorders Associated with ASD

Oxytocin plays a role in various functions including endocrine and immune functions but also parent-infant bonding and social interactions. It might be considered as a main neuropeptide involved in mediating the regulation of adaptive interactions between an individual and his/her environment. Recently, a critical role of oxytocin in early life has been revealed in sensory processing and multi-modal integration that are essential for normal postnatal neurodevelopment. An early alteration in the oxytocin-system may disturb its maturation and may have short-term and long-term pathological consequences such as autism spectrum disorders. Here, we will synthesize the existing literature on the development of the oxytocin system and its role in the early postnatal life of mammals (from birth to weaning) in a normal or pathological context. Oxytocin is required in critical windows of time that play a pivotal role and that should be considered for therapeutical interventions.

Genetic programs of the developing tuberal hypothalamus and potential mechanisms of their disruption by environmental factors

The hypothalamus is a critical regulator of body homeostasis, influencing the autonomic nervous system and releasing trophic hormones to modulate the endocrine system. The developmental mechanisms that govern formation of the mature hypothalamus are becoming increasingly understood as research in this area grows, leading us to gain appreciation for how these developmental programs are susceptible to disruption by maternal exposure to endocrine disrupting chemicals or other environmental factors in utero. These vulnerabilities, combined with the prominent roles of the various hypothalamic nuclei in regulating appetite, reproductive behaviour, mood, and other physiologies, create a window whereby early developmental disruption can have potent long-term effects. Here we broadly outline our current understanding of hypothalamic development, with a particular focus on the tuberal hypothalamus, including what is know about nuclear coalescing and maturation. We finish by discussing how exposure to environmental or maternally-derived factors can perhaps disrupt these hypothalamic developmental programs, and potentially lead to neuroendocrine disease states.

The stage-dependent roles of Ldb1 and functional redundancy with Ldb2 in mammalian retinogenesis

The Lim domain-binding proteins are key co-factor proteins that assemble with LIM domains of the LMO/LIM-HD family to form functional complexes that regulate cell proliferation and differentiation. Using conditional mutagenesis and comparative phenotypic analysis, we analyze the function of Ldb1 and Ldb2 in mouse retinal development, and demonstrate overlapping and specific functions of both proteins. Ldb1 interacts with Lhx2 in the embryonic retina and both Ldb1 and Ldb2 play a key role in maintaining the pool of retinal progenitor cells. This is accomplished by controlling the expression of the Vsx2 and Rax, and components of the Notch and Hedgehog signaling pathways. Furthermore, the Ldb1/Ldb2-mediated complex is essential for generation of early-born photoreceptors through the regulation of Rax and Crx. Finally, we demonstrate functional redundancy between Ldb1 and Ldb2. Ldb1 can fully compensate the loss of Ldb2 during all phases of retinal development, whereas Ldb2 alone is sufficient to sustain activity of Lhx2 in both early- and late-stage RPCs and in Müller glia. By contrast, loss of Ldb1 disrupts activity of the LIM domain factors in neuronal precursors. An intricate regulatory network exists that is mediated by Ldb1 and Ldb2, and promotes RPC proliferation and multipotency; it also controls specification of mammalian retina cells.

Research Resource: The Dexamethasone Transcriptome in Hypothalamic Embryonic Neural Stem Cells

Exposure to excess glucocorticoids during fetal development has long-lasting physiological and behavioral consequences, although the mechanisms are poorly understood. The impact of prenatal glucocorticoids exposure on stress responses in juvenile and adult offspring implicates the developing hypothalamus as a target of adverse prenatal glucocorticoid action. Therefore, primary cultures of hypothalamic neural-progenitor/stem cells (NPSCs) derived from mouse embryos (embryonic day 14.5) were used to identify the glucocorticoid transcriptome in both males and females. NPSCs were treated with vehicle or the synthetic glucocorticoid dexamethasone (dex; 100nM) for 4 hours and total RNA analyzed using RNA-Sequencing. Bioinformatic analysis demonstrated that primary hypothalamic NPSC cultures expressed relatively high levels of a number of genes regulating stem cell proliferation and hypothalamic progenitor function. Interesting, although these cells express glucocorticoid receptors (GRs), only low levels of sex-steroid receptors are expressed, which suggested that sex-specific differentially regulated genes identified are mediated by genetic and not hormonal influences. We also identified known or novel GR-target coding and noncoding genes that are either regulated equivalently in male and female NPSCs or differential responsiveness in one sex. Using gene ontology analysis, the top functional network identified was cell proliferation and using bromodeoxyuridine (BrdU) incorporation observed a reduction in proliferation of hypothalamic NPSCs after dexamethasone treatment. Our studies provide the first characterization and description of glucocorticoid-regulated pathways in male and female embryonically derived hypothalamic NPSCs and identified GR-target genes during hypothalamic development. These findings may provide insight into potential mechanisms responsible for the long-term consequences of fetal glucocorticoid exposure in adulthood.

PDZRN4 acts as a suppressor of cell proliferation in human liver cancer cell lines

Recently, some reports show that Ligand of Numb Protein-X 1 (LNX1) could be a suppressor gene in gliomas, while our current research has firstly shown that PDZ domain containing ring finger 4 (PDZRN4), another member of LNX family, could also be a potential suppressor in hepatocellular carcinoma (HCC). PDZRN4, also named LNX4 (Ligand of Numb Protein-X 4), is a member of the LNX family. We recently found that PDZRN4, but not LNX1, was down-regulated in HCC samples, and the role of PDZRN4 in the progression of HCC had not been studied before. To address this question, firstly, we evaluated the expression of PDZRN4 in HCC samples and adjacent non-cancerous tissues. Semi-quantitative polymerase chain reaction showed that PDZRN4 was down-regulated in 24/36 (66.7%) HCC samples separately. In addition, our research shows that PDZRN4 is silenced in all of the 12 HCC cell lines tested. Subsequently, cell-based functional assay exhibited that ectopic expression of PDZRN4 inhibits the proliferation, plate colony formation and anchorage-independent colony formation of HCC cells. Collectively, our results showed that PDZRN4 might be a potential tumour suppressor gene and had anti-proliferative effect on HCC cell proliferation, which would be of great significance to the researches on HCC.

Developmental specification of metabolic circuitry

The hypothalamus contains a core circuitry that communicates with the brainstem and spinal cord to regulate energy balance. Because metabolic phenotype is influenced by environmental variables during perinatal development, it is important to understand how these neural pathways form in order to identify key signaling pathways that are responsible for metabolic programming. Recent progress in defining gene expression events that direct early patterning and cellular specification of the hypothalamus, as well as advances in our understanding of hormonal control of central neuroendocrine pathways, suggest several key regulatory nodes that may represent targets for metabolic programming of brain structure and function. This review focuses on components of central circuitry known to regulate various aspects of energy balance and summarizes what is known about their developmental neurobiology within the context of metabolic programming.

Protocadherins and hypothalamic development: do they play an unappreciated role?

Normal brain development requires coordinated cell movements at precise times. It has long been established that cell-cell adhesion proteins of the cadherin superfamily are involved in the adhesion and sorting of cells during tissue morphogenesis. In the present review, we focus on protocadherins, which form the largest subfamily of the cadherin superfamily and mediate homophilic cell-cell adhesion in the developing brain. These molecules are highly expressed during neural development and the exact roles that they play are still emerging. Although, historically, protocadherins were considered to provide mechanical and chemical connections between adjacent cells, recent research suggests that they may also serve as molecular identity markers of neurones to help guide cell recognition and sorting, cell migration, outgrowth of neuronal processes, and synapse formation. This phenomenon of single cell diversity stems, in part, from the vast variation in protein structure, genomic organisation and molecular function of the protocadherins. Although expression profiles and genetic manipulations have provided evidence for the role of protocadherins in the developing brain, we have only begun to construct a complete understanding of protocadherin function. We examine our current understanding of how protocadherins influence brain development and discuss the possible roles for this large superfamily within the hypothalamus. We conclude that further research into these underappreciated but vitally important genes will shed insight into hypothalamic development and perhaps the underlying aetiology of neuroendocrine disorders.

Specification of select hypothalamic circuits and innate behaviors by the embryonic patterning gene dbx1

The hypothalamus integrates information required for the production of a variety of innate behaviors such as feeding, mating, aggression, and predator avoidance. Despite an extensive knowledge of hypothalamic function, how embryonic genetic programs specify circuits that regulate these behaviors remains unknown. Here, we find that in the hypothalamus the developmentally regulated homeodomain-containing transcription factor Dbx1 is required for the generation of specific subclasses of neurons within the lateral hypothalamic area/zona incerta (LH) and the arcuate (Arc) nucleus. Consistent with this specific developmental role, Dbx1 hypothalamic-specific conditional-knockout mice display attenuated responses to predator odor and feeding stressors but do not display deficits in other innate behaviors such as mating or conspecific aggression. Thus, activity of a single developmentally regulated gene, Dbx1, is a shared requirement for the specification of hypothalamic nuclei governing a subset of innate behaviors. VIDEO ABSTRACT.

Patterning, specification, and differentiation in the developing hypothalamus

Owing to its complex structure and highly diverse cell populations, the study of hypothalamic development has historically lagged behind that of other brain regions. However, in recent years, a greatly expanded understanding of hypothalamic gene expression during development has opened up new avenues of investigation. In this review, we synthesize existing work to present a holistic picture of hypothalamic development from early induction and patterning through nuclear specification and differentiation, with a particular emphasis on determination of cell fate. We will also touch on special topics in the field including the prosomere model, adult neurogenesis, and integration of migratory cells originating outside the hypothalamic neuroepithelium, and how these topics relate to our broader theme.

Ontogenesis of oxytocin pathways in the mammalian brain: late maturation and psychosocial disorders

Oxytocin (OT), the main neuropeptide of sociality, is expressed in neurons exclusively localized in the hypothalamus. During the last decade, a plethora of neuroendocrine, metabolic, autonomic and behavioral effects of OT has been reported. In the urgency to find treatments to syndromes as invalidating as autism, many clinical trials have been launched in which OT is administered to patients, including adolescents and children. However, the impact of OT on the developing brain and in particular on the embryonic and early postnatal maturation of OT neurons, has been only poorly investigated. In the present review we summarize available (although limited) literature on general features of ontogenetic transformation of the OT system, including determination, migration and differentiation of OT neurons. Next, we discuss trajectories of OT receptors (OTR) in the perinatal period. Furthermore, we provide evidence that early alterations, from birth, in the central OT system lead to severe neurodevelopmental diseases such as feeding deficit in infancy and severe defects in social behavior in adulthood, as described in Prader-Willi syndrome (PWS). Our review intends to propose a hypothesis about developmental dynamics of central OT pathways, which are essential for survival right after birth and for the acquisition of social skills later on. A better understanding of the embryonic and early postnatal maturation of the OT system may lead to better OT-based treatments in PWS or autism.

Ontogenesis of peptidergic neurons within the genoarchitectonic map of the mouse hypothalamus

During early development, the hypothalamic primordium undergoes anteroposterior and dorsoventral regionalization into diverse progenitor domains, each characterized by a differential gene expression code. The types of neurons produced selectively in each of these distinct progenitor domains are still poorly understood. Recent analysis of the ontogeny of peptidergic neuronal populations expressing Sst, Ghrh, Crh and Trh mRNAs in the mouse hypothalamus showed that these cell types originate from particular dorsoventral domains, characterized by specific combinations of gene markers. Such analysis implies that the differentiation of diverse peptidergic cell populations depends on the molecular environment where they are born. Moreover, a number of these peptidergic neurons were observed to migrate radially and/or tangentially, invading different adult locations, often intermingled with other cell types. This suggests that a developmental approach is absolutely necessary for the understanding of their adult distribution. In this essay, we examine comparatively the ontogenetic hypothalamic topography of twelve additional peptidergic populations documented in the Allen Developmental Mouse Brain Atlas, and discuss shared vs. variant aspects in their apparent origins, migrations and final distribution, in the context of the respective genoarchitectonic backgrounds. This analysis should aid ulterior attempts to explain causally the development of neuronal diversity in the hypothalamus, and contribute to our understanding of its topographic complexity in the adult.

Characterization of the hypothalamus of Xenopus laevis during development. I. The alar regions

The patterns of expression of a set of conserved developmental regulatory transcription factors and neuronal markers were analyzed in the alar hypothalamus of Xenopus laevis throughout development. Combined immunohistochemical and in situ hybridization techniques were used for the identification of subdivisions and their boundaries. The alar hypothalamus was located rostral to the diencephalon in the secondary prosencephalon and represents the rostral continuation of the alar territories of the diencephalon and brainstem, according to the prosomeric model. It is composed of the supraoptoparaventricular (dorsal) and the suprachiasmatic (ventral) regions, and limits dorsally with the preoptic region, caudally with the prethalamic eminence and the prethalamus, and ventrally with the basal hypothalamus. The supraoptoparaventricular area is defined by the orthopedia (Otp) expression and is subdivided into rostral and caudal portions, on the basis of the Nkx2.2 expression only in the rostral portion. This region is the source of many neuroendocrine cells, primarily located in the rostral subdivision. The suprachiasmatic region is characterized by Dll4/Isl1 expression, and was also subdivided into rostral and caudal portions, based on the expression of Nkx2.1/Nkx2.2 and Lhx1/7 exclusively in the rostral portion. Both alar regions are mainly connected with subpallial areas strongly implicated in the limbic system and show robust intrahypothalamic connections. Caudally, both regions project to brainstem centers and spinal cord. All these data support that in terms of topology, molecular specification, and connectivity the subdivisions of the anuran alar hypothalamus possess many features shared with their counterparts in amniotes, likely controlling similar reflexes, responses, and behaviors.

Rax is a selector gene for mediobasal hypothalamic cell types

The brain plays a central role in controlling energy, glucose, and lipid homeostasis, with specialized neurons within nuclei of the mediobasal hypothalamus, namely the arcuate (ARC) and ventromedial (VMH), tasked with proper signal integration. Exactly how the exquisite cytoarchitecture and underlying circuitry becomes established within these nuclei remains largely unknown, in part because hypothalamic developmental programs are just beginning to be elucidated. Here, we demonstrate that the Retina and anterior neural fold homeobox (Rax) gene plays a key role in establishing ARC and VMH nuclei in mice. First, we show that Rax is expressed in ARC and VMH progenitors throughout development, consistent with genetic fate mapping studies demonstrating that Rax+ lineages give rise to VMH neurons. Second, the conditional ablation of Rax in a subset of VMH progenitors using a Shh::Cre driver leads to a fate switch from a VMH neuronal phenotype to a hypothalamic but non-VMH identity, suggesting that Rax is a selector gene for VMH cellular fates. Finally, the broader elimination of Rax throughout ARC/VMH progenitors using Six3::Cre leads to a severe loss of both VMH and ARC cellular phenotypes, demonstrating a role for Rax in both VMH and ARC fate specification. Combined, our study illustrates that Rax is required in ARC/VMH progenitors to specify neuronal phenotypes within this hypothalamic brain region. Rax thus provides a molecular entry point for further study of the ontology and establishment of hypothalamic feeding circuits.

HDAC4 governs a transcriptional program essential for synaptic plasticity and memory

Neuronal activity influences genes involved in circuit development and information processing. However, the molecular basis of this process remains poorly understood. We found that HDAC4, a histone deacetylase that shuttles between the nucleus and cytoplasm, controls a transcriptional program essential for synaptic plasticity and memory. The nuclear import of HDAC4 and its association with chromatin is negatively regulated by NMDA receptors. In the nucleus, HDAC4 represses genes encoding constituents of central synapses, thereby affecting synaptic architecture and strength. Furthermore, we show that a truncated form of HDAC4 encoded by an allele associated with mental retardation is a gain-of-function nuclear repressor that abolishes transcription and synaptic transmission despite the loss of the deacetylase domain. Accordingly, mice carrying a mutant that mimics this allele exhibit deficits in neurotransmission, spatial learning, and memory. These studies elucidate a mechanism of experience-dependent plasticity and define the biological role of HDAC4 in the brain.

Wnt signaling: role in Alzheimer disease and schizophrenia

Wnt signaling function starts during the development of the nervous system and is crucial for synaptic plasticity in the adult brain. Clearly Wnt effects in synaptic and plastic processes are relevant, however the implication of this pathway in the prevention of neurodegenerative diseases that produce synaptic impairment, is even more interesting. Several years ago our laboratory found a relationship between the loss of Wnt signaling and the neurotoxicity of the amyloid-β-peptide (Aβ), one of the main players in Alzheimer's disease (AD). Moreover, the activation of the Wnt signaling cascade prevents Aβ-dependent cytotoxic effects. In fact, disrupted Wnt signaling may be a direct link between Aβ-toxicity and tau hyperphosphorylation, ultimately leading to impaired synaptic plasticity and/or neuronal degeneration, indicating that a single pathway can account for both neuro-pathological lesions and altered synaptic function. These observations, suggest that a sustained loss of Wnt signaling function may be a key relevant factor in the pathology of AD. On the other hand, Schizophrenia remains one of the most debilitating and intractable illness in psychiatry. Since Wnt signaling is important in organizing the developing brain, it is reasonable to propose that defects in Wnt signaling could contribute to Schizophrenia, particularly since the neuro-developmental hypothesis of the disease implies subtle dys-regulation of brain development, including some core components of the Wnt signaling pathways such as GSK-3β or Disrupted in Schizophrenia-1 (DISC-1). This review focuses on the relationship between Wnt signaling and its potential relevance for the treatment of neurodegenerative and neuropsychiatric diseases including AD and Schizophrenia.

Subdivisions of the turtle Pseudemys scripta hypothalamus based on the expression of regulatory genes and neuronal markers

The patterns of distribution of a set of conserved brain developmental regulatory transcription factors and neuronal markers were analyzed in the hypothalamus of the juvenile turtle, Pseudemys scripta. Combined immunohistochemical techniques were used for the identification of the main boundaries and subdivisions in the optic, paraventricular, tuberal, and mammillary hypothalamic regions. The combination of Tbr1 and Pax6 with Nkx2.1 allowed identification of the boundary between the telencephalic preoptic area, rich in Nkx2.1 expression, and the prethalamic eminence, rich in Tbr1 expression. In addition, at this level Nkx2.2 expression defined the boundary between the telencephalon and the hypothalamus. The dorsalmost hypothalamic domain was the supraoptoparaventricular region that was defined by the expression of Otp/Pax6 and the lack of Nkx2.1/Isl1. It is subdivided into rostral, rich in Otp and Nkx2.2, and caudal, only Otp-positive, portions. Ventrally, the suprachiasmatic area was identified by its catecholaminergic groups and the lack of Otp, and could be further divided into a rostral portion, rich in Nkx2.1 and Nkx2.2, and a caudal portion, rich in Isl1 and devoid of Nkx2.1 expression. The expressions of Nkx2.1 and Isl1 defined the tuberal hypothalamus, whereas only the rostral portion expressed Otp. Its caudal boundary was evident by the lack of Isl1 in the adjacent mammillary area, which expressed Nkx2.1 and Otp. All these results provide an important set of data on the interpretation of the hypothalamic organization in a reptile, and hence make a useful contribution to the understanding of hypothalamic evolution.

Wired for behaviors: from development to function of innate limbic system circuitry

The limbic system of the brain regulates a number of behaviors that are essential for the survival of all vertebrate species including humans. The limbic system predominantly controls appropriate responses to stimuli with social, emotional, or motivational salience, which includes innate behaviors such as mating, aggression, and defense. Activation of circuits regulating these innate behaviors begins in the periphery with sensory stimulation (primarily via the olfactory system in rodents), and is then processed in the brain by a set of delineated structures that primarily includes the amygdala and hypothalamus. While the basic neuroanatomy of these connections is well-established, much remains unknown about how information is processed within innate circuits and how genetic hierarchies regulate development and function of these circuits. Utilizing innovative technologies including channel rhodopsin-based circuit manipulation and genetic manipulation in rodents, recent studies have begun to answer these central questions. In this article we review the current understanding of how limbic circuits regulate sexually dimorphic behaviors and how these circuits are established and shaped during pre- and post-natal development. We also discuss how understanding developmental processes of innate circuit formation may inform behavioral alterations observed in neurodevelopmental disorders, such as autism spectrum disorders, which are characterized by limbic system dysfunction.

Molecular evolution of the LNX gene family

Background

LNX (Ligand of Numb Protein-X) proteins typically contain an amino-terminal RING domain adjacent to either two or four PDZ domains - a domain architecture that is unique to the LNX family. LNX proteins function as E3 ubiquitin ligases and their domain organisation suggests that their ubiquitin ligase activity may be targeted to specific substrates or subcellular locations by PDZ domain-mediated interactions. Indeed, numerous interaction partners for LNX proteins have been identified, but the in vivo functions of most family members remain largely unclear.

Results

To gain insights into their function we examined the phylogenetic origins and evolution of the LNX gene family. We find that a LNX1/LNX2-like gene arose in an early metazoan lineage by gene duplication and fusion events that combined a RING domain with four PDZ domains. These PDZ domains are closely related to the four carboxy-terminal domains from multiple PDZ domain containing protein-1 (MUPP1). Duplication of the LNX1/LNX2-like gene and subsequent loss of PDZ domains appears to have generated a gene encoding a LNX3/LNX4-like protein, with just two PDZ domains. This protein has novel carboxy-terminal sequences that include a potential modular LNX3 homology domain. The two ancestral LNX genes are present in some, but not all, invertebrate lineages. They were, however, maintained in the vertebrate lineage, with further duplication events giving rise to five LNX family members in most mammals. In addition, we identify novel interactions of LNX1 and LNX2 with three known MUPP1 ligands using yeast two-hybrid asssays. This demonstrates conservation of binding specificity between LNX and MUPP1 PDZ domains.

Conclusions

The LNX gene family has an early metazoan origin with a LNX1/LNX2-like protein likely giving rise to a LNX3/LNX4-like protein through the loss of PDZ domains. The absence of LNX orthologs in some lineages indicates that LNX proteins are not essential in invertebrates. In contrast, the maintenance of both ancestral LNX genes in the vertebrate lineage suggests the acquisition of essential vertebrate specific functions. The revelation that the LNX PDZ domains are phylogenetically related to domains in MUPP1, and have common binding specificities, suggests that LNX and MUPP1 may have similarities in their cellular functions.

Sim1 is a novel regulator in the differentiation of mouse dorsal raphe serotonergic neurons

BACKGROUND

Mesencephalic dopaminergic neurons (mDA) and serotonergic (5-HT) neurons are clinically important ventral neuronal populations. Degeneration of mDA is associated with Parkinson's disease; defects in the serotonergic system are related to depression, obsessive-compulsive disorder, and schizophrenia. Although these neuronal subpopulations reveal positional and developmental relationships, the developmental cascades that govern specification and differentiation of mDA or 5-HT neurons reveal missing determinants and are not yet understood.

METHODOLOGY

We investigated the impact of the transcription factor Sim1 in the differentiation of mDA and rostral 5-HT neurons in vivo using Sim1-/- mouse embryos and newborn pups, and in vitro by gain- and loss-of-function approaches.

PRINCIPAL FINDINGS

We show a selective significant reduction in the number of dorsal raphe nucleus (DRN) 5-HT neurons in Sim1-/- newborn mice. In contrast, 5-HT neurons of other raphe nuclei as well as dopaminergic neurons were not affected. Analysis of the underlying molecular mechanism revealed that tryptophan hydroxylase 2 (Tph2) and the transcription factor Pet1 are regulated by Sim1. Moreover, the transcription factor Lhx8 and the modulator of 5-HT(1A)-mediated neurotransmitter release, Rgs4, exhibit significant higher expression in ventral hindbrain, compared to midbrain and are target genes of Sim1.

CONCLUSIONS

The results demonstrate for the first time a selective transcription factor dependence of the 5-HT cell groups, and introduce Sim1 as a regulator of DRN specification acting upstream of Pet1 and Tph2. Moreover, Sim1 may act to modulate serotonin release via regulating RGS4. Our study underscores that subpopulations of a common neurotransmitter phenotype use distinct combinations of transcription factors to control the expression of shared properties.

Integrated genetic and epigenetic analysis identifies haplotype-specific methylation in the FTO type 2 diabetes and obesity susceptibility locus

Recent multi-dimensional approaches to the study of complex disease have revealed powerful insights into how genetic and epigenetic factors may underlie their aetiopathogenesis. We examined genotype-epigenotype interactions in the context of Type 2 Diabetes (T2D), focussing on known regions of genomic susceptibility. We assayed DNA methylation in 60 females, stratified according to disease susceptibility haplotype using previously identified association loci. CpG methylation was assessed using methylated DNA immunoprecipitation on a targeted array (MeDIP-chip) and absolute methylation values were estimated using a Bayesian algorithm (BATMAN). Absolute methylation levels were quantified across LD blocks, and we identified increased DNA methylation on the FTO obesity susceptibility haplotype, tagged by the rs8050136 risk allele A (p = 9.40×10⁻⁴, permutation p = 1.0×10⁻³). Further analysis across the 46 kb LD block using sliding windows localised the most significant difference to be within a 7.7 kb region (p = 1.13×10⁻⁷). Sequence level analysis, followed by pyrosequencing validation, revealed that the methylation difference was driven by the co-ordinated phase of CpG-creating SNPs across the risk haplotype. This 7.7 kb region of haplotype-specific methylation (HSM), encapsulates a Highly Conserved Non-Coding Element (HCNE) that has previously been validated as a long-range enhancer, supported by the histone H3K4me1 enhancer signature. This study demonstrates that integration of Genome-Wide Association (GWA) SNP and epigenomic DNA methylation data can identify potential novel genotype-epigenotype interactions within disease-associated loci, thus providing a novel route to aid unravelling common complex diseases.

Roles for gamma-aminobutyric acid in the development of the paraventricular nucleus of the hypothalamus

The development of the hypothalamic paraventricular nucleus (PVN) involves several factors that work together to establish a cell group that regulates neuroendocrine functions and behaviors. Several molecular markers were noted within the developing PVN, including estrogen receptors (ER), neuronal nitric oxide synthase (nNOS), and brain-derived neurotrophic factor (BDNF). By contrast, immunoreactive gamma-aminobutyric acid (GABA) was found in cells and fibers surrounding the PVN. Two animal models were used to test the hypothesis that GABA works through GABA(A) and GABA(B) receptors to influence the development of the PVN. Treatment with bicuculline to decrease GABA(A) receptor signaling from embryonic day (E) 10 to E17 resulted in fewer cells containing immunoreactive (ir) ERalpha in the region of the PVN vs. control. GABA(B)R1 receptor subunit knockout mice were used to examine the PVN at P0 without GABA(B) signaling. In female but not male GABA(B)R1 subunit knockout mice, the positions of cells containing ir ERalpha shifted from medial to lateral compared with wild-type controls, whereas the total number of ir ERalpha-containing cells was unchanged. In E17 knockout mice, ir nNOS cells and fibers were spread over a greater area. There was also a significant decrease in ir BDNF in the knockout mice in a region-dependent manner. Changes in cell position and protein expression subsequent to disruption of GABA signaling may be due, in part, to changes in nNOS and BDNF signaling. Based on the current study, the PVN can be added as another site where GABA exerts morphogenetic actions in development.

Molecular genetics of the developing neuroendocrine hypothalamus

Formation of the mammalian endocrine system and neuroendocrine organs involves complex regulatory networks resulting in a highly specialized cell system able to secrete a diverse array of peptide hormones. The hypothalamus is located in the mediobasal region of the brain and acts as a gateway between the endocrine and nervous systems. From an endocrinology perspective, the parvicellular neurons of the hypothalamus are of particular interest as they function as a control centre for several critical physiological processes including growth, metabolism and reproduction by regulating hormonal signaling from target cognate cell types in the anterior pituitary. Delineating the genetic program that controls hypothalamic development is essential for complete understanding of parvicellular neuronal function and the etiology of congenital disorders that result from hypothalamic-pituitary axis dysfunction. In recent years, studies have shed light on the interactions between signaling molecules and activation of transcription factors that regulate hypothalamic cell fate commitment and terminal differentiation. The aim of this review is to summarize the recent molecular and genetic findings that have advanced our understanding of the emergence of the known important hypophysiotropic signaling molecules in the hypothalamus. We have focused on reviewing the literature that provides evidence of the dependence on expression of specific genes for the normal development and function of the cells that secrete these neuroendocrine factors, as well as studies of the elaboration of the spatial or temporal patterns of changes in gene expression that drive this development.

A neuronal migratory pathway crossing from diencephalon to telencephalon populates amygdala nuclei

Neurons usually migrate and differentiate in one particular encephalic vesicle. We identified a murine population of diencephalic neurons that colonized the telencephalic amygdaloid complex, migrating along a tangential route that crosses a boundary between developing brain vesicles. The diencephalic transcription factor OTP was necessary for this migratory behavior.

Genetic mapping of Foxb1-cell lineage shows migration from caudal diencephalon to telencephalon and lateral hypothalamus

The hypothalamus is a brain region with vital functions, and alterations in its development can cause human disease. However, we still do not have a complete description of how this complex structure is put together during embryonic and early postnatal stages. Radially oriented, outside-in migration of cells is prevalent in the developing hypothalamus. In spite of this, cell contingents from outside the hypothalamus as well as tangential hypothalamic migrations also have an important role. Here we study migrations in the hypothalamic primordium by genetically labeling the Foxb1 diencephalic lineage. Foxb1 is a transcription factor gene expressed in the neuroepithelium of the developing neural tube with a rostral expression boundary between caudal and rostral diencephalon, and therefore appropriate for marking migrations from caudal levels into the hypothalamus. We have found a large, longitudinally oriented migration stream apparently originating in the thalamic region and following an axonal bundle to end in the anterior portion of the lateral hypothalamic area. Additionally, we have mapped a specific expansion of the neuroepithelium into the rostral diencephalon. The expanded neuroepithelium generates abundant neurons for the medial hypothalamus at the tuberal level. Finally, we have uncovered novel diencephalon-to-telencephalon migrations into septum, piriform cortex and amygdala.

Expression of Robo/Slit and Semaphorin/Plexin/Neuropilin family members in the developing hypothalamic paraventricular and supraoptic nuclei

The hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON) contain neuroendocrine cells that modulate pituitary secretion to maintain homeostasis. These two nuclei have a common developmental origin but they eventually form at locations distant from each other. Little is known about the molecular cues that direct the segregation of PVN and SON. As a means to identify potential factors, we have documented expression patterns of genes with known guidance roles in neural migration. Here, we focus on two groups of ligand/receptor families classified to mediate chemo-repulsion of neurons and their axons: the Slit/Robo and the Semaphorin/Plexin/Neuropilin families. Their dynamic expression patterns within and around the common PVN/SON progenitor as well as the mature PVN and SON may provide a framework for understanding the formation of these two important nuclei.

Dynamic expression of thyrotropin-releasing hormone in the mouse definitive endoderm

Thyrotropin-releasing hormone (TRH) is a well-characterized regulator of the hypothalamic-pituitary-thyroid endocrine axis. Here, we describe the expression of Trh during early embryonic development in the mouse. We find Trh to be highly expressed during postimplantation stages in the mouse embryo, with expression first observed in the epiblast at embryonic day (E) 6.5. During gastrulation, Trh is expressed in the newly formed definitive endoderm cells, and at embryonic day (E) 7.75, marks the entire definitive endoderm. Subsequently, Trh mRNA levels rapidly decrease such that, by E9.0, expression in the definitive endoderm is no longer detected, after which neural expression predominates. Thus, Trh is expressed dynamically and specifically in the developing mouse definitive endoderm from E7.0 to E8.5. Trh is unique among definitive endoderm markers as it transiently marks the entire definitive endoderm population and is not expressed in the extraembryonic endoderm. Trh will be a valuable tool to study definitive endoderm formation in the mouse embryo.

The neonatal ventromedial hypothalamus transcriptome reveals novel markers with spatially distinct patterning

The ventromedial hypothalamus (VMH) is a distinct morphological nucleus involved in feeding, fear, thermoregulation, and sexual activity. It is essentially unknown how VMH circuits underlying these innate responses develop, in part because the VMH remains poorly defined at a cellular and molecular level. Specifically, there is a paucity of cell-type-specific genetic markers with which to identify neuronal subgroups and manipulate development and signaling in vivo. Using gene profiling, we now identify approximately 200 genes highly enriched in neonatal (postnatal day 0) mouse VMH tissue. Analyses of these VMH markers by real or virtual (Allen Brain Atlas; http://www.brain-map.org) experiments revealed distinct regional patterning within the newly formed VMH. Top neonatal markers include transcriptional regulators such as Vgll2, SF-1, Sox14, Satb2, Fezf1, Dax1, Nkx2-2, and COUP-TFII, but interestingly, the highest expressed VMH transcript, the transcriptional coregulator Vgll2, is completely absent in older animals. Collective results from zebrafish knockdown experiments and from cellular studies suggest that a subset of these VMH markers will be important for hypothalamic development and will be downstream of SF-1, a critical factor for normal VMH differentiation. We show that at least one VMH marker, the AT-rich binding protein Satb2, was responsive to the loss of leptin signaling (Lep(ob/ob)) at postnatal day 0 but not in the adult, suggesting that some VMH transcriptional programs might be influenced by fetal or early postnatal environments. Our study describing this comprehensive "VMH transcriptome" provides a novel molecular toolkit to probe further the genetic basis of innate neuroendocrine behavioral responses.

GABAB receptors role in cell migration and positioning within the ventromedial nucleus of the hypothalamus

The ventromedial (VMN) and arcuate (ARC) nuclei of the hypothalamus are bilateral nuclear groups at the base of the hypothalamus that are organized through the aggregation of neurons born along the third ventricle that migrate laterally. During development, GABAergic neurons and fibers surround the forming (or primordial) VMN while neurons containing GABA receptors are found within the boundaries of the emerging nucleus. To investigate the role that GABAB receptors play in establishing the VMN, Thy-1 yellow fluorescent protein (YFP) mice were utilized for live video microscopy studies. The Thy-1 promoter drives YFP expression in regions of the hypothalamus during development. Administration of the GABAB receptor antagonist saclofen and the GABAA receptor antagonist bicuculline selectively increased the rate of VMN cell movement in slices placed in vitro at embryonic day 14, when cells that form both the ARC and VMN are migrating away from the proliferative zone surrounding the third ventricle. To further test the role of GABAB receptors in VMN development, GABAB receptor knockout mice were used to examine changes in the positions of phenotypically identified cells within the VMN. Cells containing immunoreactive estrogen receptors (ER) alpha were located in the ventrolateral quadrant of the wild type VMN. In GABABR1 knockout mice, these ERalpha positive neurons were located in more dorsal positions at postnatal day (P) 0 and P4. We conclude that GABA alters cell migration and its effect on final cell positioning may lead to changes in the circuitry and connections within specific nuclei of the developing hypothalamus.

Allocation of paraventricular and supraoptic neurons requires Sim1 function: a role for a Sim1 downstream gene PlexinC1

SIM1 is a transcription factor essential for the developmental expression of the endocrine hormone genes, e.g. vasopressin (Vp) and oxytocin (Ot), in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus. Mice mutant for Sim1 lack structural PVN and SON, attributed in previous studies to the death of the PVN/SON progenitor cells. Here, we use a tau-LacZ knock-in allele at the Sim1 locus to trace Sim1 mutant cells and show that they are generated normally and survive to birth, contrasting to the previous proposal. Mutant cells adopt neuronal characteristics and maintain their PVN/SON identity as they continue to express PVN/SON progenitor markers. However, they occupy an ectopic position between the normal PVN and SON, indicating a defect in neuronal migration. To explore candidate molecular cues that contribute to PVN/SON neuronal migration, we focused on the Plexin family of genes. We found that PlexinA1 is expressed in regions surrounding the PVN and SON, whereas PlexinC1 is expressed within the PVN and SON. PlexinA1 expression becomes up-regulated in Sim1 mutant cells, whereas PlexinC1 expression is down-regulated. Finally, the PlexinC1 mutant has a selective defect in partitioning the VP and OT neurons coherently into the PVN and SON. Together, our results uncover a transcriptional regulation of neuronal migration cues initiated by Sim1 that contribute to the organization of the PVN and SON.