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

Control of tuberal hypothalamic development and its implications in metabolic disorders

The tuberal hypothalamus regulates a range of crucial physiological processes, including energy homeostasis and metabolism. In this Review, we explore the intricate molecular mechanisms and signalling pathways that control the development of the tuberal hypothalamus, focusing on aspects that shape metabolic outcomes. Major developmental events are discussed in the context of their effect on the establishment of both functional hypothalamic neuronal circuits and brain-body interfaces that are pivotal to the control of metabolism. Emerging evidence indicates that aberrations in molecular pathways during tuberal hypothalamic development contribute to metabolic dysregulation. Understanding the molecular underpinnings of tuberal hypothalamic development provides a comprehensive view of neurodevelopmental processes and offers a promising avenue for future targeted interventions to prevent and treat metabolic disorders.

The embryonic patterning gene Dbx1 governs the survival of the auditory midbrain via Tcf7l2-Ap2δ transcriptional cascade

At the top of the midbrain is the inferior colliculus (IC), which functions as the major hub for processing auditory information. Despite the functional significance of neurons in the IC, our understanding of their formation is limited. In this study, we identify the embryonic patterning gene Dbx1 as a key molecular player that governs genetic programs for IC survival. We find that Dbx1 plays a critical role in preventing apoptotic cell death in postnatal IC by transcriptionally repressing c-Jun and pro-apoptotic BH3 only factors. Furthermore, by employing combined approaches, we uncover that Tcf7l2 functions downstream of Dbx1. Loss of Tcf7l2 function causes IC phenotypes with striking similarity to those of Dbx1 mutant mice, which include defective embryonic maturation and postnatal deletion of the IC. Finally, we demonstrate that the Dbx1-Tcf7l2 cascade functions upstream of Ap-2δ, which is essential for IC development and survival. Together, these results unravel a novel molecular mechanism for IC maintenance, which is indispensable for normal brain development.

Diversity within olfactory sensory derivatives revealed by the contribution of Dbx1 lineages

In vertebrates, the embryonic olfactory epithelium contains progenitors that will give rise to distinct classes of neurons, including olfactory sensory neurons (OSNs; involved in odor detection), vomeronasal sensory neurons (VSNs; responsible for pheromone sensing), and gonadotropin-releasing hormone (GnRH) neurons that control the hypothalamic-pituitary-gonadal axis. Currently, these three neuronal lineages are usually believed to emerge from uniform pools of progenitors. Here, we found that the homeodomain transcription factor Dbx1 is expressed by neurogenic progenitors in the developing and adult mouse olfactory epithelium. We demonstrate that Dbx1 itself is dispensable for neuronal fate specification and global organization of the olfactory sensory system. Using lineage tracing, we characterize the contribution of Dbx1 lineages to OSN, VSN, and GnRH neuron populations and reveal an unexpected degree of diversity. Furthermore, we demonstrate that Dbx1-expressing progenitors remain neurogenic in the absence of the proneural gene Ascl1. Our work therefore points to the existence of distinct neurogenic programs in Dbx1-derived and other olfactory lineages.

Dorsoventral Arrangement of Lateral Hypothalamus Populations in the Mouse Hypothalamus: a Prosomeric Genoarchitectonic Analysis

The lateral hypothalamus (LH) has a heterogeneous cytoarchitectonic organization that has not been elucidated in detail. In this work, we analyzed within the framework of the prosomeric model the differential expression pattern of 59 molecular markers along the ventrodorsal dimension of the medial forebrain bundle in the mouse, considering basal and alar plate subregions of the LH. We found five basal (LH1-LH5) and four alar (LH6-LH9) molecularly distinct sectors of the LH with neuronal cell groups that correlate in topography with previously postulated alar and basal hypothalamic progenitor domains. Most peptidergic populations were restricted to one of these LH sectors though some may have dispersed into a neighboring sector. For instance, histaminergic Hdc-positive neurons were mostly contained within the basal LH3, Nts (neurotensin)- and Tac2 (tachykinin 2)-expressing cells lie strictly within LH4, Hcrt (hypocretin/orexin)-positive and Pmch (pro-melanin-concentrating hormone)-positive neurons appeared within separate LH5 subdivisions, Pnoc (prepronociceptin)-expressing cells were mainly restricted to LH6, and Sst (somatostatin)-positive cells were identified within the LH7 sector. The alar LH9 sector, a component of the Foxg1-positive telencephalo-opto-hypothalamic border region, selectively contained Satb2-expressing cells. Published studies of rodent LH subdivisions have not described the observed pattern. Our genoarchitectonic map should aid in systematic approaches to elucidate LH connectivity and function.

Transcriptional Profile of the Developing Subthalamic Nucleus

The subthalamic nucleus (STN) is a small, excitatory nucleus that regulates the output of basal ganglia motor circuits. The functions of the STN and its role in the pathophysiology of Parkinson's disease are now well established. However, some basic characteristics like the developmental origin and molecular phenotype of neuronal subpopulations are still being debated. The classical model of forebrain development attributed the origin of STN within the diencephalon. Recent studies of gene expression patterns exposed shortcomings of the classical model. To accommodate these findings, the prosomeric model was developed. In this concept, STN develops within the hypothalamic primordium, which is no longer a part of the diencephalic primordium. This concept is further supported by the expression patterns of many transcription factors. It is interesting to note that many transcription factors involved in the development of the STN are also involved in the pathogenesis of neurodevelopmental disorders. Thus, the study of neurodevelopmental disorders could provide us with valuable information on the roles of these transcription factors in the development and maintenance of STN phenotype. In this review, we summarize historical theories about the developmental origin of the STN and interpret the gene expression data within the prosomeric conceptual framework. Finally, we discuss the importance of neurodevelopmental disorders for the development of the STN and its potential role in the pathophysiology of neurodevelopmental disorders.

Genetic Parameter Estimation and Genome-Wide Association Analysis of Social Genetic Effects on Average Daily Gain in Purebreds and Crossbreds

Simple Summary

Average daily gain (ADG) is influenced by both an individual’s direct genetic effect (DGE) and by a social genetic effect (SGE) derived from pen mates. Therefore, identifying the DGE and SGE on ADG is essential for a better understanding of pig breeding systems. We conducted this study to elucidate the genetic characteristics and relationships of DGE and SGE on ADG using purebred and crossbred pigs. We found that the DGE and SGE both contributed to ADG in both populations. In addition, the SGE of purebred pigs was highly correlated with the DGE of crossbred pigs. Furthermore, we identified several genomic regions that may be associated with the DGE and SGE on ADG. Our findings will contribute to future genomic evaluation studies of socially affected traits.

Abstract

Average daily gain (ADG) is an important growth trait in the pig industry. The direct genetic effect (DGE) has been studied mainly to assess the association between genetic information and economic traits. The social genetic effect (SGE) has been shown to affect ADG simultaneously with the DGE because of group housing systems. We conducted this study to elucidate the genetic characteristics and relationships of the DGE and SGE of purebred Korean Duroc and crossbred pigs by single-step genomic best linear unbiased prediction and a genome-wide association study. We used the genotype, phenotype, and pedigree data of 1779, 6022, and 7904 animals, respectively. Total heritabilities on ADG were 0.19 ± 0.04 and 0.39 ± 0.08 for purebred and crossbred pigs, respectively. The genetic correlation was the greatest (0.77 ± 0.12) between the SGE of purebred and DGE of crossbred pigs. We found candidate genes located in the quantitative trait loci (QTLs) for the SGE that were associated with behavior and neurodegenerative diseases, and candidate genes in the QTLs for DGE that were related to body mass, size of muscle fiber, and muscle hypertrophy. These results suggest that the genomic selection of purebred animals could be applied for crossbred performance.

Ontogenetic rules for the molecular diversification of hypothalamic neurons

The hypothalamus is an evolutionarily conserved endocrine interface that, among other roles, links central homeostatic control to adaptive bodily responses by releasing hormones and neuropeptides from its many neuronal subtypes. In its preoptic, anterior, tuberal and mammillary subdivisions, a kaleidoscope of magnocellular and parvocellular neuroendocrine command neurons, local-circuit neurons, and neurons that project to extrahypothalamic areas are intermingled in partially overlapping patches of nuclei. Molecular fingerprinting has produced data of unprecedented mass and depth to distinguish and even to predict the synaptic and endocrine competences, connectivity and stimulus selectivity of many neuronal modalities. These new insights support eminent studies from the past century but challenge others on the molecular rules that shape the developmental segregation of hypothalamic neuronal subtypes and their use of morphogenic cues for terminal differentiation. Here, we integrate single-cell RNA sequencing studies with those of mouse genetics and endocrinology to describe key stages of hypothalamus development, including local neurogenesis, the direct terminal differentiation of glutamatergic neurons, transition cascades for GABAergic and GABAergic cell-derived dopamine cells, waves of local neuronal migration, and sequential enrichment in neuropeptides and hormones. We particularly emphasize how transcription factors determine neuronal identity and, consequently, circuit architecture, and whether their deviations triggered by environmental factors and hormones provoke neuroendocrine illnesses.

The enteric nervous system of the C. elegans pharynx is specified by the Sine oculis-like homeobox gene ceh-34

Overarching themes in the terminal differentiation of the enteric nervous system, an autonomously acting unit of animal nervous systems, have so far eluded discovery. We describe here the overall regulatory logic of enteric nervous system differentiation of the nematode Caenorhabditis elegans that resides within the foregut (pharynx) of the worm. A C. elegans homolog of the Drosophila Sine oculis homeobox gene, ceh-34, is expressed in all 14 classes of interconnected pharyngeal neurons from their birth throughout their life time, but in no other neuron type of the entire animal. Constitutive and temporally controlled ceh-34 removal shows that ceh-34 is required to initiate and maintain the neuron type-specific terminal differentiation program of all pharyngeal neuron classes, including their circuit assembly. Through additional genetic loss of function analysis, we show that within each pharyngeal neuron class, ceh-34 cooperates with different homeodomain transcription factors to individuate distinct pharyngeal neuron classes. Our analysis underscores the critical role of homeobox genes in neuronal identity specification and links them to the control of neuronal circuit assembly of the enteric nervous system. Together with the pharyngeal nervous system simplicity as well as its specification by a Sine oculis homolog, our findings invite speculations about the early evolution of nervous systems.

Selective requirement for polycomb repressor complex 2 in the generation of specific hypothalamic neuronal subtypes

The hypothalamus displays staggering cellular diversity, chiefly established during embryogenesis by the interplay of several signalling pathways and a battery of transcription factors. However, the contribution of epigenetic cues to hypothalamus development remains unclear. We mutated the polycomb repressor complex 2 gene Eed in the developing mouse hypothalamus, which resulted in the loss of H3K27me3, a fundamental epigenetic repressor mark. This triggered ectopic expression of posteriorly expressed regulators (e.g. Hox homeotic genes), upregulation of cell cycle inhibitors and reduced proliferation. Surprisingly, despite these effects, single cell transcriptomic analysis revealed that most neuronal subtypes were still generated in Eed mutants. However, we observed an increase in glutamatergic/GABAergic double-positive cells, as well as loss/reduction of dopamine, hypocretin and Tac2-Pax6 neurons. These findings indicate that many aspects of the hypothalamic gene regulatory flow can proceed without the key H3K27me3 epigenetic repressor mark, but points to a unique sensitivity of particular neuronal subtypes to a disrupted epigenomic landscape.

Summary: Polycomb repressor complex 2 inactivation results in selective effects on mouse hypothalamic development, increasing glutamatergic/GABA cells, while reducing dopamine, Hcrt and Tac2-Pax6 cells.

Characterization of Hypothalamic MCH Neuron Development in a 3D Differentiation System of Mouse Embryonic Stem Cells

Hypothalamic melanin-concentrating hormone (MCH) neurons are important regulators of multiple physiological processes, such as sleep, feeding, and memory. Despite the increasing interest in their neuronal functions, the molecular mechanism underlying MCH neuron development remains poorly understood. We report that a three-dimensional culture of mouse embryonic stem cells (mESCs) can generate hypothalamic-like tissues containing MCH-positive neurons, which reproduce morphologic maturation, neuronal connectivity, and neuropeptide/neurotransmitter phenotype of native MCH neurons. Using this in vitro system, we demonstrate that Hedgehog (Hh) signaling serves to produce major neurochemical subtypes of MCH neurons characterized by the presence or absence of cocaine- and amphetamine-regulated transcript (CART). Without exogenous Hh signals, mESCs initially differentiated into dorsal hypothalamic/prethalamic progenitors and finally into MCH⁺CART⁺ neurons through a specific intermediate progenitor state. Conversely, activation of the Hh pathway specified ventral hypothalamic progenitors that generate both MCH⁺CART⁻ and MCH⁺CART⁺ neurons. These results suggest that in vivo MCH neurons may originate from multiple cell lineages that arise through early dorsoventral patterning of the hypothalamus. Additionally, we found that Hh signaling supports the differentiation of mESCs into orexin/hypocretin neurons, a well-defined cell group intermingled with MCH neurons in the lateral hypothalamic area (LHA). The present study highlights and improves the utility of mESC culture in the analysis of the developmental programs of specific hypothalamic cell types.

Sex-Specific Social Behavior and Amygdala Proteomic Deficits in Foxp2 +/- Mutant Mice

In humans, mutations in the transcription factor encoding gene, FOXP2, are associated with language and Autism Spectrum Disorders (ASD), the latter characterized by deficits in social interactions. However, little is known regarding the function of Foxp2 in male or female social behavior. Our previous studies in mice revealed high expression of Foxp2 within the medial subnucleus of the amygdala (MeA), a limbic brain region highly implicated in innate social behaviors such as mating, aggression, and parental care. Here, using a comprehensive panel of behavioral tests in male and female Foxp2 +/- heterozygous mice, we investigated the role Foxp2 plays in MeA-linked innate social behaviors. We reveal significant deficits in olfactory processing, social interaction, mating, aggressive, and parental behaviors. Interestingly, some of these deficits are displayed in a sex-specific manner. To examine the consequences of Foxp2 loss of function specifically in the MeA, we conducted a proteomic analysis of microdissected MeA tissue. This analyses revealed putative sex differences expression of a host of proteins implicated in neuronal communication, connectivity, and dopamine signaling. Consistent with this, we discovered that MeA Foxp2-lineage cells were responsive to dopamine with differences between males and females. Thus, our findings reveal a central and sex-specific role for Foxp2 in social behavior and MeA function.

The Prop1-like homeobox gene unc-42 specifies the identity of synaptically connected neurons

Many neuronal identity regulators are expressed in distinct populations of cells in the nervous system, but their function is often analyzed only in specific isolated cellular contexts, thereby potentially leaving overarching themes in gene function undiscovered. We show here that the Caenorhabditis elegans Prop1-like homeobox gene unc-42 is expressed in 15 distinct sensory, inter- and motor neuron classes throughout the entire C. elegans nervous system. Strikingly, all 15 neuron classes expressing unc-42 are synaptically interconnected, prompting us to investigate whether unc-42 controls the functional properties of this circuit and perhaps also the assembly of these neurons into functional circuitry. We found that unc-42 defines the routes of communication between these interconnected neurons by controlling the expression of neurotransmitter pathway genes, neurotransmitter receptors, neuropeptides, and neuropeptide receptors. Anatomical analysis of unc-42 mutant animals reveals defects in axon pathfinding and synaptic connectivity, paralleled by expression defects of molecules involved in axon pathfinding, cell-cell recognition, and synaptic connectivity. We conclude that unc-42 establishes functional circuitry by acting as a terminal selector of functionally connected neuron types. We identify a number of additional transcription factors that are also expressed in synaptically connected neurons and propose that terminal selectors may also function as 'circuit organizer transcription factors' to control the assembly of functional circuitry throughout the nervous system. We hypothesize that such organizational properties of transcription factors may be reflective of not only ontogenetic, but perhaps also phylogenetic trajectories of neuronal circuit establishment.

Specific contribution of neurons from the Dbx1 lineage to the piriform cortex

The piriform cortex (PC) is a major cortical processing center for the sense of smell that receives direct inputs from the olfactory bulb. In mice, the PC consists of three neuronal layers, which are populated by cells with distinct developmental origins. One origin of PC neurons is the pool of Dbx1-expressing neural progenitors located in the ventral pallium at the pallial-subpallial boundary. Since the precise mechanisms of PC neuron development are largely unknown, we sought to define the distribution, timing of neurogenesis, morphology and projection patterns of PC neurons from the Dbx1 lineage. We found that Dbx1-lineage neurons are preferentially distributed in layer 2 and enriched in the ventral portion of the PC. Further, Dbx1 neurons are early-born neurons and contribute to most neuronal subtypes in the PC. Our data also revealed an enrichment of Dbx1-lineage neurons in the ventral anterior PC that project to the orbitofrontal cortex. These findings suggest a specific association between the developmental origin of PC neurons and their neuronal properties.

The cellular and molecular landscape of hypothalamic patterning and differentiation from embryonic to late postnatal development

The hypothalamus is a central regulator of many innate behaviors essential for survival, but the molecular mechanisms controlling hypothalamic patterning and cell fate specification are poorly understood. To identify genes that control hypothalamic development, we have used single-cell RNA sequencing (scRNA-Seq) to profile mouse hypothalamic gene expression across 12 developmental time points between embryonic day 10 and postnatal day 45. This identified genes that delineated clear developmental trajectories for all major hypothalamic cell types, and readily distinguished major regional subdivisions of the developing hypothalamus. By using our developmental dataset, we were able to rapidly annotate previously unidentified clusters from existing scRNA-Seq datasets collected during development and to identify the developmental origins of major neuronal populations of the ventromedial hypothalamus. We further show that our approach can rapidly and comprehensively characterize mutants that have altered hypothalamic patterning, identifying Nkx2.1 as a negative regulator of prethalamic identity. These data serve as a resource for further studies of hypothalamic development, physiology, and dysfunction.

Loss of the centrosomal protein Cenpj leads to dysfunction of the hypothalamus and obesity in mice

Cenpj is a centrosomal protein located at the centrosomes and the base of cilia, it plays essential roles in regulating neurogenesis and cerebral cortex development. Although centrosomal and cilium dysfunction are one of the causes of obesity, insulin resistance, and type 2 diabetes, the role that Cenpj plays in the regulation of body weight remains unclear. Here, we deleted Cenpj by crossing Cenpj^flox/flox mice with Nkx2.1-Cre mice. Loss of the centrosomal protein Cenpj in Nkx2.1-expressing cells causes morbid obesity in mice at approximately 4 months of age with expended brain ventricles but no change of brain size. We found that hypothalamic cells exhibited reduced proliferation and increased apoptosis upon Cenpj depletion at the embryonic stages, resulting in a dramatic decrease in the number of Proopiomelanocortin (POMC) neurons and electrophysiological dysfunction of NPY neurons in the arcuate nucleus (ARC) in adults. Furthermore, depletion of Cenpj also reduced the neuronal projection from the ARC to the paraventricular nucleus (PVN), with decreased melanocortin-4 receptors (MC4R) expression in PVN neurons. The study defines the roles that Cenpj plays in regulating hypothalamus development and body weight, providing a foundation for further understanding of the pathological mechanisms of related diseases.

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.

Conditional knockout of leptin receptor in neural stem cells leads to obesity in mice and affects neuronal differentiation in the hypothalamus early after birth

Leptin, secreted by peripheral adipocytes, binds the leptin receptor (Lepr) in the hypothalamus, thereby contributing to the regulation of satiety and body weight. Lepr is expressed in the embryonic brain as early as embryonic day 12.5. However, the function of Lepr in neural precursor cells in the brain has not been resolved. To address this issue, we crossed the Lepr^flox/flox mice with each of Shh-Cre mice (Shh, sonic hedgehog) and Nestin (Nes)-Cre mice. We found that deletion of Lepr specifically in nestin-expressing cells led to extreme obesity, but the conditional null of Lepr in Shh-expressing cells had no obvious phenotype. Moreover, the level of leptin-activated pSTAT3 decreased in the anterior and central subregions of the arcuate hypothalamus of Shh-Cre; Lepr^flox/flox mice compared with the controls. By contrast, in Nes-Cre; Lepr^flox/flox mice, the level of leptin-activated pSTAT3 decreased in all subregions including the anterior, central, and posterior arcuate hypothalamus as well as the dorsomedial, ventromedial, and median eminence of the hypothalamus, revealing that the extensive lack of Lepr in the differentiated neurons of the hypothalamus in the conditional null mice. Notably, conditional deletion of Lepr in nestin-expressing cells enhanced the differentiation of neural precursor cells into neurons and oligodendroglia but inhibited differentiation into astrocytes early in postnatal development of hypothalamus. Our results suggest that Lepr expression in neural precursor cells is essential for maintaining normal body weight as well as the differentiation of neural precursor cells to the neural/glial fate in the hypothalamus shortly after birth.

Molecular, spatial, and functional single-cell profiling of the hypothalamic preoptic region

The hypothalamus controls essential social behaviors and homeostatic functions. However, the cellular architecture of hypothalamic nuclei-including the molecular identity, spatial organization, and function of distinct cell types-is poorly understood. Here, we developed an imaging-based in situ cell-type identification and mapping method and combined it with single-cell RNA-sequencing to create a molecularly annotated and spatially resolved cell atlas of the mouse hypothalamic preoptic region. We profiled ~1 million cells, identified ~70 neuronal populations characterized by distinct neuromodulatory signatures and spatial organizations, and defined specific neuronal populations activated during social behaviors in male and female mice, providing a high-resolution framework for mechanistic investigation of behavior circuits. The approach described opens a new avenue for the construction of cell atlases in diverse tissues and organisms.

Development of neuroendocrine neurons in the mammalian hypothalamus

The neuroendocrine system consists of a heterogeneous collection of (mostly) neuropeptidergic neurons found in four hypothalamic nuclei and sharing the ability to secrete neurohormones (all of them neuropeptides except dopamine) into the bloodstream. There are, however, abundant hypothalamic non-neuroendocrine neuropeptidergic neurons developing in parallel with the neuroendocrine system, so that both cannot be entirely disentangled. This heterogeneity results from the workings of a network of transcription factors many of which are already known. Olig2 and Fezf2 expressed in the progenitors, acting through mantle-expressed Otp and Sim1, Sim2 and Pou3f2 (Brn2), regulate production of magnocellular and anterior parvocellular neurons. Nkx2-1, Rax, Ascl1, Neurog3 and Dbx1 expressed in the progenitors, acting through mantle-expressed Isl1, Dlx1, Gsx1, Bsx, Hmx2/3, Ikzf1, Nr5a2 (LH-1) and Nr5a1 (SF-1) are responsible for tuberal parvocellular (arcuate nucleus) and other neuropeptidergic neurons. The existence of multiple progenitor domains whose progeny undergoes intricate tangential migrations as one source of complexity in the neuropeptidergic hypothalamus is the focus of much attention. How neurosecretory cells target axons to the medial eminence and posterior hypophysis is gradually becoming clear and exciting progress has been made on the mechanisms underlying neurovascular interface formation. While rat neuroanatomy and targeted mutations in mice have yielded fundamental knowledge about the neuroendocrine system in mammals, experiments on chick and zebrafish are providing key information about cellular and molecular mechanisms. Looking forward, data from every source will be necessary to unravel the ways in which the environment affects neuroendocrine development with consequences for adult health and disease.

Neurochemical Heterogeneity Among Lateral Hypothalamic Hypocretin/Orexin and Melanin-Concentrating Hormone Neurons Identified Through Single-Cell Gene Expression Analysis

The lateral hypothalamic area (LHA) lies at the intersection of multiple neural and humoral systems and orchestrates fundamental aspects of behavior. Two neuronal cell types found in the LHA are defined by their expression of hypocretin/orexin (Hcrt/Ox) and melanin-concentrating hormone (MCH) and are both important regulators of arousal, feeding, and metabolism. Conflicting evidence suggests that these cell populations have a more complex signaling repertoire than previously appreciated, particularly in regard to their coexpression of other neuropeptides and the machinery for the synthesis and release of GABA and glutamate. Here, we undertook a single-cell expression profiling approach to decipher the neurochemical phenotype, and heterogeneity therein, of Hcrt/Ox and MCH neurons. In transgenic mouse lines, we used single-cell quantitative polymerase chain reaction (qPCR) to quantify the expression of 48 key genes, which include neuropeptides, fast neurotransmitter components, and other key markers, which revealed unexpected neurochemical diversity. We found that single MCH and Hcrt/Ox neurons express transcripts for multiple neuropeptides and markers of both excitatory and inhibitory fast neurotransmission. Virtually all MCH and approximately half of the Hcrt/Ox neurons sampled express both the machinery for glutamate release and GABA synthesis in the absence of a vesicular GABA release pathway. Furthermore, we found that this profile is characteristic of a subpopulation of LHA glutamatergic neurons but contrasts with a broad population of LHA GABAergic neurons. Identifying the neurochemical diversity of Hcrt/Ox and MCH neurons will further our understanding of how these populations modulate postsynaptic excitability through multiple signaling mechanisms and coordinate diverse behavioral outputs.

Development of the hypothalamus: conservation, modification and innovation

The hypothalamus, which regulates fundamental aspects of physiological homeostasis and behavior, is a brain region that exhibits highly conserved anatomy across vertebrate species. Its development involves conserved basic mechanisms of induction and patterning, combined with a more plastic process of neuronal fate specification, to produce brain circuits that mediate physiology and behavior according to the needs of each species. Here, we review the factors involved in the induction, patterning and neuronal differentiation of the hypothalamus, highlighting recent evidence that illustrates how changes in Wnt/β-catenin signaling during development may lead to species-specific form and function of this important brain structure.

The molecular mechanisms controlling morphogenesis and wiring of the habenula

The habenula is an evolutionarily conserved brain region comprising bilaterally paired nuclei that plays a key role in processing reward information and mediating aversive responses to negative stimuli. An important aspect underlying habenula function is relaying information between forebrain and mid- and hindbrain areas. This is mediated by its complex organization into multiple subdomains and corresponding complexity in circuit organization. Additionally, in many species habenular nuclei display left-right differences at the anatomical and functional level. In order to ensure proper functional organization of habenular circuitry, sophisticated molecular programs control the morphogenesis and wiring of the habenula during development. Knowledge of how these mechanisms shape the habenula is crucial for obtaining a complete understanding of this brain region and can provide invaluable tools to study habenula evolution and function. In this review we will discuss how these molecular mechanisms pattern the early embryonic nervous system and control the formation of the habenula, how they shape its asymmetric organization, and how these mechanisms ensure proper wiring of the habenular circuit. Finally, we will address unexplored aspects of habenula development and how these may direct future research.

Lef1-dependent hypothalamic neurogenesis inhibits anxiety

While innate behaviors are conserved throughout the animal kingdom, it is unknown whether common signaling pathways regulate the development of neuronal populations mediating these behaviors in diverse organisms. Here, we demonstrate that the Wnt/ß-catenin effector Lef1 is required for the differentiation of anxiolytic hypothalamic neurons in zebrafish and mice, although the identity of Lef1-dependent genes and neurons differ between these 2 species. We further show that zebrafish and Drosophila have common Lef1-dependent gene expression in their respective neuroendocrine organs, consistent with a conserved pathway that has diverged in the mouse. Finally, orthologs of Lef1-dependent genes from both zebrafish and mouse show highly correlated hypothalamic expression in marmosets and humans, suggesting co-regulation of 2 parallel anxiolytic pathways in primates. These findings demonstrate that during evolution, a transcription factor can act through multiple mechanisms to generate a common behavioral output, and that Lef1 regulates circuit development that is fundamentally important for mediating anxiety in a wide variety of animal species.

Embryonic transcription factor expression in mice predicts medial amygdala neuronal identity and sex-specific responses to innate behavioral cues

The medial subnucleus of the amygdala (MeA) plays a central role in processing sensory cues required for innate behaviors. However, whether there is a link between developmental programs and the emergence of inborn behaviors remains unknown. Our previous studies revealed that the telencephalic preoptic area (POA) embryonic niche is a novel source of MeA destined progenitors. Here, we show that the POA is comprised of distinct progenitor pools complementarily marked by the transcription factors Dbx1 and Foxp2. As determined by molecular and electrophysiological criteria this embryonic parcellation predicts postnatal MeA inhibitory neuronal subtype identity. We further find that Dbx1-derived and Foxp2+ cells in the MeA are differentially activated in response to innate behavioral cues in a sex-specific manner. Thus, developmental transcription factor expression is predictive of MeA neuronal identity and sex-specific neuronal responses, providing a potential developmental logic for how innate behaviors could be processed by different MeA neuronal subtypes.

DOI:http://dx.doi.org/10.7554/eLife.21012.001

Within the brain, a set of interconnected structures called the limbic system is involved in emotion, motivation and memory. This system – and in particular a structure called the medial amygdala – also contributes to behavioral drives that help an animal to survive and reproduce. These include the drive to avoid predators, to defend territory, and to find a mate. Such behaviors are thought to be inborn or innate. This means that animals display them instinctively whenever specific triggers are present, without the need to learn them beforehand.

However, just as a computer must be programmed to perform specific tasks, these innate behavioral responses must also be programmed into the brain. Given that animals do not learn these behaviors, Lischinsky et al. reasoned that specific events during the development of the brain must provide the animal’s brain with the necessary instructions. To test this idea, they studied how the development of the medial amygdala in mouse embryos may give rise to differences in innate mating behavior seen between male and female mice.

The medial amygdala contains many subtypes of neurons, which show different responses to sex hormones such as estrogen and androgen. Lischinsky et al. show that two sets of cells give rise to some of the different neurons of the adult medial amygdala. One set of these precursor cells makes a protein called Dbx1 and the other makes a protein called Foxp2. These two sets of precursors generate medial amygdala neurons with different arrays of sex hormone receptors in male and female mice. Moreover, while the two sets of medial amygdala neurons are activated during aggressive encounters, they show different patterns of activation in male and female animals during mating.

These findings suggest that the development of Dbx1-derived and Foxp2+ neurons in the medial amygdala helps program innate reproductive and aggressive behaviors into the brain. The new findings also provide insights into why these behaviors differ in male and female mice. The next challenge is to identify the inputs and outputs of these two distinct subpopulations of medial amygdala neurons. This should make it possible to work out exactly how these populations of cells control innate behaviors in male and female animals.

DOI:http://dx.doi.org/10.7554/eLife.21012.002

Hubs and spokes of the lateral hypothalamus: cell types, circuits and behaviour

The hypothalamus is among the most phylogenetically conserved regions in the vertebrate brain, reflecting its critical role in maintaining physiological and behavioural homeostasis. By integrating signals arising from both the brain and periphery, it governs a litany of behaviourally important functions essential for survival. In particular, the lateral hypothalamic area (LHA) is central to the orchestration of sleep-wake states, feeding, energy balance and motivated behaviour. Underlying these diverse functions is a heterogeneous assembly of cell populations typically defined by neurochemical markers, such as the well-described neuropeptides hypocretin/orexin and melanin-concentrating hormone. However, anatomical and functional evidence suggests a rich diversity of other cell populations with complex neurochemical profiles that include neuropeptides, receptors and components of fast neurotransmission. Collectively, the LHA acts as a hub for the integration of diverse central and peripheral signals and, through complex local and long-range output circuits, coordinates adaptive behavioural responses to the environment. Despite tremendous progress in our understanding of the LHA, defining the identity of functionally discrete LHA cell types, and their roles in driving complex behaviour, remain significant challenges in the field. In this review, we discuss advances in our understanding of the neurochemical and cellular heterogeneity of LHA neurons and the recent application of powerful new techniques, such as opto- and chemogenetics, in defining the role of LHA circuits in feeding, reward, arousal and stress. From pioneering work to recent developments, we review how the interrogation of LHA cells and circuits is contributing to a mechanistic understanding of how the LHA coordinates complex behaviour.

Neuronal fate specification by the Dbx1 transcription factor is linked to the evolutionary acquisition of a novel functional domain

Background

Dbx1 is a homeodomain transcription factor involved in neuronal fate specification belonging to a widely conserved family among bilaterians. In mammals, Dbx1 was proposed to act as a transcriptional repressor by interacting with the Groucho corepressors to allow the specification of neurons involved in essential biological functions such as locomotion or breathing.

Results

Sequence alignments of Dbx1 proteins from different species allowed us to identify two conserved domains related to the Groucho-dependent Engrailed repressor domain (RD), as well as a newly described domain composed of clusterized acidic residues at the C-terminus (Cter) which is present in tetrapods but also several invertebrates. Using a heterologous luciferase assay, we showed that the two putative repressor domains behave as such in a Groucho-dependent manner, whereas the Cter does not bear any intrinsic transcriptional activity. Consistently with in vitro data, we found that both RDs are involved in cell fate specification using in vivo electroporation experiments in the chick spinal cord. Surprisingly, we show that the Cter domain is required for Dbx1 function in vivo, acting as a modulator of its repressive activity and/or imparting specificity.

Conclusion

Our results strongly suggest that the presence of a Cter domain among tetrapods is essential for Dbx1 to regulate neuronal diversity and, in turn, nervous system complexity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0055-5) contains supplementary material, which is available to authorized users.

Hubs and spokes of the lateral hypothalamus: cell types, circuits and behaviour

The hypothalamus is among the most phylogenetically conserved regions in the vertebrate brain, reflecting its critical role in maintaining physiological and behavioural homeostasis. By integrating signals arising from both the brain and periphery, it governs a litany of behaviourally important functions essential for survival. In particular, the lateral hypothalamic area (LHA) is central to the orchestration of sleep-wake states, feeding, energy balance and motivated behaviour. Underlying these diverse functions is a heterogeneous assembly of cell populations typically defined by neurochemical markers, such as the well-described neuropeptides hypocretin/orexin and melanin-concentrating hormone. However, anatomical and functional evidence suggests a rich diversity of other cell populations with complex neurochemical profiles that include neuropeptides, receptors and components of fast neurotransmission. Collectively, the LHA acts as a hub for the integration of diverse central and peripheral signals and, through complex local and long-range output circuits, coordinates adaptive behavioural responses to the environment. Despite tremendous progress in our understanding of the LHA, defining the identity of functionally discrete LHA cell types, and their roles in driving complex behaviour, remain significant challenges in the field. In this review, we discuss advances in our understanding of the neurochemical and cellular heterogeneity of LHA neurons and the recent application of powerful new techniques, such as opto- and chemogenetics, in defining the role of LHA circuits in feeding, reward, arousal and stress. From pioneering work to recent developments, we review how the interrogation of LHA cells and circuits is contributing to a mechanistic understanding of how the LHA coordinates complex behaviour.

Lipopolysacharide Rapidly and Completely Suppresses AgRP Neuron-Mediated Food Intake in Male Mice

Although Agouti-related peptide (AgRP) neurons play a key role in the regulation of food intake, their contribution to the anorexia caused by proinflammatory insults has yet to be identified. Using a combination of neuroanatomical and pharmacogenetics experiments, this study sought to investigate the importance of AgRP neurons and downstream targets in the anorexia caused by the peripheral administration of a moderate dose of lipopolysaccharide (LPS) (100 μg/kg, ip). First, in the C57/Bl6 mouse, we demonstrated that LPS induced c-fos in select AgRP-innervated brain sites involved in feeding but not in any arcuate proopiomelanocortin neurons. Double immunohistochemistry further showed that LPS selectively induced c-Fos in a large subset of melanocortin 4 receptor-expressing neurons in the lateral parabrachial nucleus. Secondly, we used pharmacogenetics to stimulate the activity of AgRP neurons during the course of LPS-induced anorexia. In AgRP-Cre mice expressing the designer receptor hM3Dq-Gq only in AgRP neurons, the administration of the designer drug clozapine-N-oxide (CNO) induced robust food intake. Strikingly, CNO-mediated food intake was rapidly and completely blunted by the coadministration of LPS. Neuroanatomical experiments further indicated that LPS did not interfere with the ability of CNO to stimulate c-Fos in AgRP neurons. In summary, our findings combined together support the view that the stimulation of select AgRP-innervated brain sites and target neurons, rather than the inhibition of AgRP neurons themselves, is likely to contribute to the rapid suppression of food intake observed during acute bacterial endotoxemia.

Molecular and behavioral profiling of Dbx1-derived neurons in the arcuate, lateral and ventromedial hypothalamic nuclei

BACKGROUND

Neurons in the hypothalamus function to regulate the state of the animal during both learned and innate behaviors, and alterations in hypothalamic development may contribute to pathological conditions such as anxiety, depression or obesity. Despite many studies of hypothalamic development and function, the link between embryonic development and innate behaviors remains unexplored. Here, focusing on the embryonically expressed homeodomain-containing gene Developing Brain Homeobox 1 (Dbx1), we explored the relationship between embryonic lineage, post-natal neuronal identity and lineage-specific responses to innate cues. We found that Dbx1 is widely expressed across multiple developing hypothalamic subdomains. Using standard and inducible fate-mapping to trace the Dbx1-derived neurons, we identified their contribution to specific neuronal subtypes across hypothalamic nuclei and further mapped their activation patterns in response to a series of well-defined innate behaviors.

RESULTS

Dbx1-derived neurons occupy multiple postnatal hypothalamic nuclei including the lateral hypothalamus (LH), arcuate nucleus (Arc) and the ventral medial hypothalamus (VMH). Within these nuclei, Dbx1 (+) progenitors generate a large proportion of the Pmch-, Nesfatin-, Cart-, Hcrt-, Agrp- and ERα-expressing neuronal populations, and to a lesser extent the Pomc-, TH- and Aromatase-expressing populations. Inducible fate-mapping reveals distinct temporal windows for development of the Dbx1-derived LH and Arc populations, with Agrp(+) and Cart(+) populations in the Arc arising early (E7.5-E9.5), while Pmch(+) and Hcrt(+) populations in the LH derived from progenitors expressing Dbx1 later (E9.5-E11.5). Moreover, as revealed by c-Fos labeling, Dbx1-derived cells in male and female LH, Arc and VMH are responsive during mating and aggression. In contrast, Dbx1-lineage cells in the Arc and LH have a broader behavioral tuning, which includes responding to fasting and predator odor cues.

CONCLUSION

We define a novel fate map of the hypothalamus with respect to Dbx1 expression in hypothalamic progenitor zones. We demonstrate that in a temporally regulated manner, Dbx1-derived neurons contribute to molecularly distinct neuronal populations in the LH, Arc and VMH that have been implicated in a variety of hypothalamic-driven behaviors. Consistent with this, Dbx1-derived neurons in the LH, Arc and VMH are activated during stress and other innate behavioral responses, implicating their involvement in these diverse behaviors.

Laterodorsal tegmentum interneuron subtypes oppositely regulate olfactory cue-induced innate fear

Innate fear has a critical role in survival of animals. Unlike conditioned fear, the neuronal circuitry underlying innate fear is largely unknown. We found that the laterodorsal tegmentum (LDT) and lateral habenula (LHb) are specifically activated by the mouse predator odorant trimethylthiazoline (TMT). Using optogenetics to selectively stimulate GABAergic neurons in the LDT immediately produced fear-like responses (freezing, accelerated heart rate and increased serum corticosterone), whereas prolonged stimulation caused anxiety-like behaviors. Notably, although selective stimulation of parvalbumin (PV)-positive interneurons similarly induced fear-like responses, stimulation of somatostatin-positive interneurons or inhibition of PV neurons in the LDT suppressed TMT-induced fear-like responses without affecting conditioned fear. Finally, activation of LHb glutamatergic inputs to LDT interneurons was sufficient to generate fear-like responses. Thus, the LHb-LDT pathway is important for regulating olfactory cue-induced innate fear. Our results provide a potential target for therapeutic intervention for anxiety disorder.