Conserved Genoarchitecture of the Basal Hypothalamus in Zebrafish Embryos

A role for Retinoblastoma 1 in hindbrain morphogenesis by regulating GBX family

The hindbrain, which develops from the anterior end of the neural tube expansion, can differentiate into the metencephalon and myelencephalon, with varying sizes and functions. The midbrain-hindbrain boundary (MHB) and hindbrain myelencephalon/ventral midline (HMVM) are known to be the source of the progenitors for the anterior hindbrain and myelencephalon, respectively. However, the molecular networks regulating hindbrain morphogenesis in these structures remain unclear. In this study, we show that retinoblastoma 1 (rb1) is highly expressed at the MHB and HMVM in zebrafish. Knocking out rb1 in mice and zebrafish results in an enlarged hindbrain due to hindbrain neuronal hyperproliferation. Further study reveals that Rb1 controls the hindbrain morphogenesis by suppressing the expression of Gbx1/Gbx2, essential transcription factors for hindbrain development, through its binding to E2f3/Hdac1, respectively. Interestingly, we find that Gbx1 and Gbx2 are expressed in different types of hindbrain neurons, suggesting distinct roles in hindbrain morphogenesis. In summary, our study clarifies the specific role of RB1 in hindbrain neural cell proliferation and morphogenesis by regulating the E2f3-Gbx1 axis and the Hdac1-Gbx2 axis. These findings provide a research paradigm for exploring the differential proliferation of neurons in various brain regions.

Genoarchitectonics of the larval zebrafish diencephalon

The brain is spatially organized into subdivisions, nuclei and areas, which often correspond to functional and developmental units. A segmentation of brain regions in the form of a consensus atlas facilitates mechanistic studies and is a prerequisite for sharing information among neuroanatomists. Gene expression patterns objectively delineate boundaries between brain regions and provide information about their developmental and evolutionary histories. To generate a detailed molecular map of the larval zebrafish diencephalon, we took advantage of the Max Planck Zebrafish Brain (mapzebrain) atlas, which aligns hundreds of transcript and transgene expression patterns in a shared coordinate system. Inspection and co-visualization of close to 50 marker genes have allowed us to resolve the tripartite prosomeric scaffold of the diencephalon at unprecedented resolution. This approach clarified the genoarchitectonic partitioning of the alar diencephalon into pretectum (alar part of prosomere P1), thalamus (alar part of prosomere P2, with habenula and pineal complex), and prethalamus (alar part of prosomere P3). We further identified the region of the nucleus of the medial longitudinal fasciculus, as well as the posterior and anterior parts of the posterior tuberculum, as molecularly distinct basal parts of prosomeres 1, 2, and 3, respectively. Some of the markers examined allowed us to locate glutamatergic, GABAergic, dopaminergic, serotoninergic, and various neuropeptidergic domains in the larval zebrafish diencephalon. Our molecular neuroanatomical approach has thus (1) yielded an objective and internally consistent interpretation of the prosomere boundaries within the zebrafish forebrain; has (2) produced a list of markers, which in sparse combinations label the subdivisions of the diencephalon; and is (3) setting the stage for further functional and developmental studies in this vertebrate brain.

The proneural factors Ascl1a and Ascl1b contribute to the terminal differentiation of dopaminergic GABAergic dual transmitter neurons in zebrafish

The proneural factor Ascl1 is involved in several steps of neurogenesis, from neural progenitor maintenance to initiation of terminal differentiation and neuronal subtype specification. In neural progenitor cells, Ascl1 initiates the cell-cycle exit of progenitors, and contributes to their differentiation into mainly GABAergic neurons. Several catecholaminergic neuron groups in the forebrain of zebrafish use GABA as co-transmitter, but a potential role of the two paralogues Ascl1a and Ascl1b in their neurogenesis is not understood. Here, we show that ascl1a, ascl1b double mutant embryos develop a significantly reduced number of neurons in all GABAergic and catecholaminergic dual transmitter neuron anatomical clusters in the fore- and hindbrain, while glutamatergic catecholaminergic clusters develop normally. However, none of the affected catecholaminergic cell clusters are lost completely, suggesting an impairment in progenitor pools, or a requirement of Ascl1a/b for differentiation of a subset of neurons in each cluster. Early progenitors which are dlx2a+, fezf2 + or emx2 + are not reduced whereas late progenitors and differentiating neurons marked by the expression of dlx5a, isl1 and arxa are severely reduced in ascl1a, ascl1b double mutant embryos. This suggests that Ascl1a and Ascl1b play only a minor or no role in the maintenance of their progenitor pools, but rather contribute to the initiation of terminal differentiation of GABAergic catecholaminergic neurons.

Neurog1 and Olig2 integrate patterning and neurogenesis signals in development of zebrafish dopaminergic and glutamatergic dual transmitter neurons

Dopaminergic neurons develop in distinct neural domains by integrating local patterning and neurogenesis signals. While the proneural proteins Neurog1 and Olig2 have been previously linked to development of dopaminergic neurons, their dependence on local prepatterning and specific contributions to dopaminergic neurogenesis are not well understood. Here, we show that both transcription factors are differentially required for the development of defined dopaminergic glutamatergic subpopulations in the zebrafish posterior tuberculum, which are homologous to A11 dopaminergic neurons in mammals. Both Olig2 and Neurog1 are expressed in otpa expressing progenitor cells and appear to act upstream of Otpa during dopaminergic neurogenesis. Our epistasis analysis confirmed that Neurog1 acts downstream of Notch signaling, while Olig2 acts downstream of Shh, but upstream and/or in parallel to Notch signaling during neurogenesis of A11-type dopaminergic clusters. Furthermore, we identified Olig2 to be an upstream regulator of neurog1 in dopaminergic neurogenesis. This regulation occurs through Olig2-dependent repression of the proneural repressor and Notch target gene her2. Our study reveals how Neurog1 and Olig2 integrate local patterning signals, including Shh, with Notch neurogenic selection signaling, to specify the progenitor population and initiate neurogenesis and differentiation of A11-type dopaminergic neurons.

Brain-wide mapping of efferent projections of glutamatergic (Onecut3+ ) neurons in the lateral mouse hypothalamus

AIM

This study mapped the spatiotemporal positions and connectivity of Onecut3+ neuronal populations in the developing and adult mouse brain.

METHODS

We generated fluorescent reporter mice to chart Onecut3+ neurons for brain-wide analysis. Moreover, we crossed Onecut3-iCre and Mapt-mGFP (Tau-mGFP) mice to visualize axonal projections. A dual Cre/Flp-dependent AAV construct in Onecut3-iCre cross-bred with Slc17a6-FLPo mice was used in an intersectional strategy to map the connectivity of glutamatergic lateral hypothalamic neurons in the adult mouse.

RESULTS

We first found that Onecut3 marks a hitherto undescribed Slc17a6+ /Vglut2+ neuronal cohort in the lateral hypothalamus, with the majority expressing thyrotropin-releasing hormone. In the adult, Onecut3+ /Vglut2+ neurons of the lateral hypothalamus had both intra- and extrahypothalamic efferents, particularly to the septal complex and habenula, where they targeted other cohorts of Onecut3+ neurons and additionally to the neocortex and hippocampus. This arrangement suggests that intrinsic reinforcement loops could exist for Onecut3+ neurons to coordinate their activity along the brain's midline axis.

CONCLUSION

We present both a toolbox to manipulate novel subtypes of hypothalamic neurons and an anatomical arrangement by which extrahypothalamic targets can be simultaneously entrained.

The importance of thyroid hormone signaling during early development: Lessons from the zebrafish model

The zebrafish is an optimal experimental model to study thyroid hormone (TH) involvement in vertebrate development. The use of state-of-the-art zebrafish genetic tools available for the study of the effect of gene silencing, cell fate decisions and cell lineage differentiation have contributed to a more insightful comprehension of molecular, cellular, and tissue-specific TH actions. In contrast to intrauterine development, extrauterine embryogenesis observed in zebrafish has facilitated a more detailed study of the development of the hypothalamic-pituitary-thyroid axis. This model has also enabled a more insightful analysis of TH molecular actions upon the organization and function of the brain, the retina, the heart, and the immune system. Consequently, zebrafish has become a trendy model to address paradigms of TH-related functional and biomedical importance. We here compilate the available knowledge regarding zebrafish developmental events for which specific components of TH signaling are essential.

Zebrafish - The Neurobehavioural Model in Trend

Zebrafish (Danio rerio) is currently in vogue as a prevalently used experimental model for studies concerning neurobehavioural disorders and associated fields. Since the 1960s, this model has succeeded in breaking most barriers faced in the hunt for an experimental model. From its appearance to its high parity with human beings genetically, this model renders itself as an advantageous experimental lab animal. Neurobehavioural disorders have always posed an arduous task in terms of their detection as well as in determining their exact etiology. They are still, in most cases, diseases of interest for inventing or discovering novel pharmacological interventions. Thus, the need for a harbinger experimental model for studying neurobehaviours is escalating. Ensuring the same model is used for studying several neuro-studies conserves the results from inter-species variations. For this, we need a model that satisfies all the pre-requisite conditions to be made the final choice of model for neurobehavioural studies. This review recapitulates the progress of zebrafish as an experimental model with its most up-to-the-minute advances in the area. Various tests, assays, and responses employed using zebrafish in screening neuroactive drugs have been tabulated effectively. The tools, techniques, protocols, and apparatuses that bolster zebrafish studies are discussed. The probable research that can be done using zebrafish has also been briefly outlined. The various breeding and maintenance methods employed, along with the information on various strains available and most commonly used, are also elaborated upon, supplementing Zebrafish's use in neuroscience.

The Neuromeric/Prosomeric Model in Teleost Fish Neurobiology

The neuromeric/prosomeric model has been rejuvenated by Puelles and Rubenstein [Trends Neurosci. 1993;16(11):472-9]. Here, its application to the (teleostean) fish brain is detailed, beginning with a historical account. The second part addresses three main issues with particular interest for fish neuroanatomy and looks at the impact of the neuromeric model on their understanding. The first one is the occurrence of four early migrating forebrain areas (M1 through M4) in teleosts and their comparative interpretation. The second issue addresses the complex development and neuroanatomy of the teleostean alar and basal hypothalamus. The third topic is the vertebrate dopaminergic system, with the focus on some teleostean peculiarities. Most of the information will be coming from zebrafish studies, although the general ductus is a comparative one. Throughout the manuscript, comparative developmental and organizational aspects of the teleostean amygdala are discussed. One particular focus is cellular migration streams into the medial amygdala.

Gut-derived metabolites influence neurodevelopmental gene expression and Wnt signaling events in a germ-free zebrafish model

BACKGROUND

Small molecule metabolites produced by the microbiome are known to be neuroactive and are capable of directly impacting the brain and central nervous system, yet there is little data on the contribution of these metabolites to the earliest stages of neural development and neural gene expression. Here, we explore the impact of deriving zebrafish embryos in the absence of microbes on early neural development as well as investigate whether any potential changes can be rescued with treatment of metabolites derived from the zebrafish gut microbiota.

RESULTS

Overall, we did not observe any gross morphological changes between treatments but did observe a significant decrease in neural gene expression in embryos raised germ-free, which was rescued with the addition of zebrafish metabolites. Specifically, we identified 354 genes significantly downregulated in germ-free embryos compared to conventionally raised embryos via RNA-Seq analysis. Of these, 42 were rescued with a single treatment of zebrafish gut-derived metabolites to germ-free embryos. Gene ontology analysis revealed that these genes are involved in prominent neurodevelopmental pathways including transcriptional regulation and Wnt signaling. Consistent with the ontology analysis, we found alterations in the development of Wnt dependent events which was rescued in the germ-free embryos treated with metabolites.

CONCLUSIONS

These findings demonstrate that gut-derived metabolites are in part responsible for regulating critical signaling pathways in the brain, especially during neural development. Video abstract.

SHH and Notch regulate SOX9+ progenitors to govern arcuate POMC neurogenesis

Pro-opiomelanocortin (POMC)-expressing neurons in the hypothalamic arcuate nucleus (ARC) play key roles in feeding and energy homoeostasis, hence their development is of great research interest. As the process of neurogenesis is accompanied by changes in adhesion, polarity, and migration that resemble aspects of epithelial-to-mesenchymal transitions (EMTs), we have characterised the expression and regulation within the prospective ARC of transcription factors with context-dependent abilities to regulate aspects of EMT. Informed by pseudotime meta-analysis of recent scRNA-seq data, we use immunohistochemistry and multiplex in situ hybridisation to show that SOX2, SRY-Box transcription factor 9 (SOX9), PROX1, Islet1 (ISL1), and SOX11 are sequentially expressed over the course of POMC neurogenesis in the embryonic chick. Through pharmacological studies ex vivo, we demonstrate that while inhibiting either sonic hedgehog (SHH) or Notch signalling reduces the number of SOX9+ neural progenitor cells, these treatments lead, respectively, to lesser and greater numbers of differentiating ISL1+/POMC+ neurons. These results are consistent with a model in which SHH promotes the formation of SOX9+ progenitors, and Notch acts to limit their differentiation. Both pathways are also required to maintain normal levels of proliferation and to suppress apoptosis. Together our findings demonstrate that hypothalamic neurogenesis is accompanied by dynamic expression of transcription factors (TFs) that mediate EMTs, and that SHH and Notch signalling converge to regulate hypothalamic cellular homoeostasis.

Developmental genoarchitectonics as a key tool to interpret the mature anatomy of the chondrichthyan hypothalamus according to the prosomeric model

The hypothalamus is a key vertebrate brain region involved in survival and physiological functions. Understanding hypothalamic organization and evolution is important to deciphering many aspects of vertebrate biology. Recent comparative studies based on gene expression patterns have proposed the existence of hypothalamic histogenetic domains (paraventricular, TPa/PPa; subparaventricular, TSPa/PSPa; tuberal, Tu/RTu; perimamillary, PM/PRM; and mamillary, MM/RM), revealing conserved evolutionary trends. To shed light on the functional relevance of these histogenetic domains, this work aims to interpret the location of developed cell groups according to the prosomeric model in the hypothalamus of the catshark Scyliorhinus canicula, a representative of Chondrichthyans (the sister group of Osteichthyes, at the base of the gnathostome lineage). To this end, we review in detail the expression patterns of ScOtp, ScDlx2, and ScPitx2, as well as Pax6-immunoreactivity in embryos at stage 32, when the morphology of the adult catshark hypothalamus is already organized. We also propose homologies with mammals when possible. This study provides a comprehensive tool to better understand previous and novel data on hypothalamic development and evolution.

A versatile transcription factor: Multiple roles of orthopedia a (otpa) beyond its restricted localization in dopaminergic systems of developing and adult zebrafish (Danio rerio) brains

Many transcription factors boost neural development and differentiation in specific directions and serve for identifying similar or homologous structures across species. The expression of Orthopedia (Otp) is critical for the development of certain cell groups along the vertebrate neuraxis, for example, the medial amygdala or hypothalamic neurosecretory neurons. Therefore, the primary focus of the present study is the distribution of Orthopedia a (Otpa) in the larval and adult zebrafish (Danio rerio) brain. Since Otpa is also critical for the development of zebrafish basal diencephalic dopaminergic cells, colocalization of Otpa with the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) is studied. Cellular colocalization of Otpa and dopamine is only seen in magnocellular neurons of the periventricular posterior tubercular nucleus and in the posterior tuberal nucleus. Otpa-positive cells occur in many additional structures along the zebrafish neuraxis, from the secondary prosencephalon down to the hindbrain. Furthermore, Otpa expression is studied in shh-GFP and islet1-GFP transgenic zebrafish. Otpa-positive cells only express shh in dopaminergic magnocellular periventricular posterior tubercular cells, and only colocalize with islet1-GFP in the ventral zone and prerecess caudal periventricular hypothalamic zone and the perilemniscal nucleus. The scarcity of cellular colocalization of Otpa in islet1-GFP cells indicates that the Shh-islet1 neurogenetic pathway is not active in most Otpa-expressing domains. Our analysis reveals detailed correspondences between mouse and zebrafish forebrain territories including the zebrafish intermediate nucleus of the ventral telencephalon and the mouse medial amygdala. The zebrafish preoptic Otpa-positive domain represents the neuropeptidergic supraopto-paraventricular region of all tetrapods. Otpa domains in the zebrafish basal plate hypothalamus suggest that the ventral periventricular hypothalamic zone corresponds to the otp-expressing basal hypothalamic tuberal field in the mouse. Furthermore, the mouse otp domain in the mammillary hypothalamus compares partly to our Otpa-positive domain in the prerecess caudal periventricular hypothalamic zone (Hc-a).

Possible role of transcription factors (BSX, NKX2.1, IRX3 and SIRT1) in the regulation of appetite in goldfish (Carassius auratus)

The homeobox genes play important roles in the embryonic development of animals. Recent evidence suggests they might also regulate feeding and act as transcription factors of appetite regulators. Examples of these genes are a brain-specific homeobox transcription factor (BSX), NK2 homeobox 1 (NKX2.1) and the Iroquois homeobox 3 (IRX3). Sirtuin1 (SIRT1) acts as a transcription factor for nutrient (e.g. lipid, glucose) homeostasis and responds to stress and nutrient availability, and has been shown to interact with appetite regulators. Very little is known about the role of these genes in the regulation of feeding and nutrient homeostasis in fish. In this study, we assessed the roles of BSX, NKX2.1, IRX3 and SIRT1 in the central regulation of feeding in goldfish by examining their mRNA brain distribution, assessing the effects of fasting on their brain expression and assessing the effects of peripheral injections of cholecystokinin (CCK, a brain-gut peptide), on their brain expression. All genes showed a widespread distribution in the brain, with high levels in the hypothalamus. In both hypothalamus and telencephalon, fasting induced increases in BSX, IRX3 and NKX2.1 expressions but had no effect on SIRT1 expression levels. CCK injections increased hypothalamic expression levels of IRX3 and SIRT1, and telencephalic expression levels of NKX2.1 and SIRT1, with no effect on either hypothalamic BSX or NKX2.1 expression levels or telencephalon BSX or IRX3 expression levels. Our results suggest that, in goldfish as in mammals, central BSX, NKX2.1, IRX3 and SIRT1 are present in regions of the brain regulating feeding, are sensitive to nutrient status and interact with appetite-regulating peptides.

Expression Pattern of nos1 in the Developing Nervous System of Ray-Finned Fish

Fish have colonized nearly all aquatic niches, making them an invaluable resource to understand vertebrate adaptation and gene family evolution, including the evolution of complex neural networks and modulatory neurotransmitter pathways. Among ancient regulatory molecules, the gaseous messenger nitric oxide (NO) is involved in a wide range of biological processes. Because of its short half-life, the modulatory capability of NO is strictly related to the local activity of nitric oxide synthases (Nos), enzymes that synthesize NO from L-arginine, making the localization of Nos mRNAs a reliable indirect proxy for the location of NO action domains, targets, and effectors. Within the diversified actinopterygian nos paralogs, nos1 (alias nnos) is ubiquitously present as a single copy gene across the gnathostome lineage, making it an ideal candidate for comparative studies. To investigate variations in the NO system across ray-finned fish phylogeny, we compared nos1 expression patterns during the development of two well-established experimental teleosts (zebrafish and medaka) with an early branching holostean (spotted gar), an important evolutionary bridge between teleosts and tetrapods. Data reported here highlight both conserved expression domains and species-specific nos1 territories, confirming the ancestry of this signaling system and expanding the number of biological processes implicated in NO activities.

Prosomeric Hypothalamic Distribution of Tyrosine Hydroxylase Positive Cells in Adolescent Rats

Most of the studies on neurochemical mapping, connectivity, and physiology in the hypothalamic region were carried out in rats and under the columnar morphologic paradigm. According to the columnar model, the entire hypothalamic region lies ventrally within the diencephalon, which includes preoptic, anterior, tuberal, and mamillary anteroposterior regions, and sometimes identifying dorsal, intermediate, and ventral hypothalamic partitions. This model is weak in providing little or no experimentally corroborated causal explanation of such subdivisions. In contrast, the modern prosomeric model uses different axial assumptions based on the parallel courses of the brain floor, alar-basal boundary, and brain roof (all causally explained). This model also postulates that the hypothalamus and telencephalon jointly form the secondary prosencephalon, separately from and rostral to the diencephalon proper. The hypothalamus is divided into two neuromeric (transverse) parts called peduncular and terminal hypothalamus (PHy and THy). The classic anteroposterior (AP) divisions of the columnar hypothalamus are rather seen as dorsoventral subdivisions of the hypothalamic alar and basal plates. In this study, we offered a prosomeric immunohistochemical mapping in the rat of hypothalamic cells expressing tyrosine hydroxylase (TH), which is the enzyme that catalyzes the conversion of L-tyrosine to levodopa (L-DOPA) and a precursor of dopamine. This mapping was also combined with markers for diverse hypothalamic nuclei [agouti-related peptide (Agrp), arginine vasopressin (Avp), cocaine and amphetamine-regulated transcript (Cart), corticotropin releasing Hormone (Crh), melanin concentrating hormone (Mch), neuropeptide Y (Npy), oxytocin/neurophysin I (Oxt), proopiomelanocortin (Pomc), somatostatin (Sst), tyrosine hidroxilase (Th), and thyrotropin releasing hormone (Trh)]. TH-positive cells are particularly abundant within the periventricular stratum of the paraventricular and subparaventricular alar domains. In the tuberal region, most labeled cells are found in the acroterminal arcuate nucleus and in the terminal periventricular stratum. The dorsal retrotuberal region (PHy) contains the A13 cell group of TH-positive cells. In addition, some TH cells appear in the perimamillary and retromamillary regions. The prosomeric model proved useful for determining the precise location of TH-positive cells relative to possible origins of morphogenetic signals, thus aiding potential causal explanation of position-related specification of this hypothalamic cell type.

Genetic ablation of the Bsx homeodomain transcription factor in zebrafish: Impact on mature pineal gland morphology and circadian behavior

The pineal gland is a neuroendocrine structure in the brain, which produces and secretes the hormone melatonin at nighttime and is considered a key element in the circadian clock system. Early morphogenesis of the gland is controlled by a number of transcription factors, some of which remain active in adult life. One of these is the brain-specific homeobox (Bsx), a highly conserved homeodomain transcription factor with a developmental role in the pineal gland of several species, including zebrafish, and regulatory roles in mature pinealocytes of the rat. To determine the role of Bsx in circadian biology, we here examined the effects of a bsx loss-of-function mutation on the pineal gland in adult zebrafish and on behavioral circadian rhythms in larvae. In pineal cell type-specific Gfp/Egfp reporter zebrafish lines, we did not detect fluorescence signals in the pineal area of homozygous (bsx^-/- ) mutants. Interestingly, a nonpigmented area on the dorsal surface of the head above the gland, known as the pineal window, was pigmented in the homozygous mutants. Furthermore, a structure corresponding to the pineal gland was not detectable in the midline of the adult brain in histological sections analyzed by Nissl staining and S-antigen immunohistochemistry. Moreover, the levels of pineal transcripts were greatly reduced in bsx^-/- mutants, as revealed by quantitative real-time polymerase chain reaction analysis. Notably, analysis of locomotor activity at the larval stage revealed altered circadian rhythmicity in the bsx mutants with periods and phases similar to wildtype, but severely reduced amplitudes in locomotor activity patterns. Thus, Bsx is essential for full development of the pineal gland, with its absence resulting in a phenotype of morphological pineal gland ablation and disrupted circadian behavior.

Developmental neuroanatomy of the rosy bitterling Rhodeus ocellatus (Teleostei: Cypriniformes)-A microCT study

Bitterlings are carp-like teleost fish (Cypriniformes: Acheilanathidae) known for their specialized brood parasitic lifestyle. Bitterling embryos, in fact, develop inside the gill chamber of their freshwater mussel hosts. However, little is known about how their parasitic lifestyle affects brain development in comparison to nonparasitic species. Here, we document the development of the brain of the rosy bitterling, Rhodeus ocellatus, at four embryonic stages of 165, 185, 210, 235 hours postfertilization (hpf) using micro-computed tomography (microCT). Focusing on developmental regionalization and brain ventricular organization, we relate the development of the brain divisions to those described for zebrafish using the prosomeric model as a reference paradigm. Segmentation and three-dimensional visualization of the ventricular system allowed us to identify changes in the longitudinal brain axis as a result of cephalic flexure during development. The results show that during early embryonic and larval development, histological differentiation, tissue boundaries, periventricular proliferation zones, and ventricular spaces are all detectable by microCT. The results of this study visualized with differential CT profiles are broadly consistent with comparable histological studies, and with the genoarchitecture of teleosts like the zebrafish. Compared to the zebrafish, our study identifies distinct developmental heterochronies in the rosy bitterling, such as a precocious development of the inferior lobe.

Wnt/β-catenin signaling promotes neurogenesis in the diencephalospinal dopaminergic system of embryonic zebrafish

Wnt/β-catenin signaling contributes to patterning, proliferation, and differentiation throughout vertebrate neural development. Wnt/β-catenin signaling is important for mammalian midbrain dopaminergic neurogenesis, while little is known about its role in ventral forebrain dopaminergic development. Here, we focus on the A11-like, Otp-dependent diencephalospinal dopaminergic system in zebrafish. We show that Wnt ligands, receptors and extracellular antagonist genes are expressed in the vicinity of developing Otp-dependent dopaminergic neurons. Using transgenic Wnt/β-catenin-reporters, we found that Wnt/β-catenin signaling activity is absent from these dopaminergic neurons, but detected Wnt/β-catenin activity in cells adjacent to the caudal DC5/6 clusters of Otp-dependent dopaminergic neurons. Pharmacological manipulations of Wnt/β-catenin signaling activity, as well as heat-shock driven overexpression of Wnt agonists and antagonists, interfere with the development of DC5/6 dopaminergic neurons, such that Wnt/β-catenin activity positively correlates with their number. Wnt/β-catenin activity promoted dopaminergic development specifically at stages when DC5/6 dopaminergic progenitors are in a proliferative state. Our data suggest that Wnt/β-catenin signaling acts in a spatially and temporally restricted manner on proliferative dopaminergic progenitors in the hypothalamus to positively regulate the size of the dopaminergic neuron groups DC5 and DC6.

nr0b1 (DAX1) loss of function in zebrafish causes hypothalamic defects via abnormal progenitor proliferation and differentiation

The nuclear receptor DAX-1 (encoded by the NR0B1 gene) is presented in the hypothalamic tissues in humans and other vertebrates. Human patients with NR0B1 mutations often have hypothalamic-pituitary defects, but the involvement of NR0B1 in hypothalamic development and function is not well understood. Here, we report the disruption of the nr0b1 gene in zebrafish causes abnormal expression of gonadotropins, a reduction in fertilization rate, and an increase in post-fasting food intake, which is indicative of abnormal hypothalamic functions. We find that loss of nr0b1 increases the number of prodynorphin (pdyn)-expressing neurons but decreases the number of pro-opiomelanocortin (pomcb)-expressing neurons in the zebrafish hypothalamic arcuate region (ARC). Further examination reveals that the proliferation of progenitor cells is reduced in the hypothalamus of nr0b1 mutant embryos accompanying with the decreased expression of genes in the Notch signaling pathway. Additionally, the inhibition of Notch signaling in wild-type embryos increases the number of pdyn neurons, mimicking the nr0b1 mutant phenotype. In contrast, ectopic activation of Notch signaling in nr0b1 mutant embryos decreases the number of pdyn neurons. Taken together, our results suggest that nr0b1 regulates neural progenitor proliferation and maintenance to ensure normal hypothalamic neuron development.

Gonadotropin-inhibitory hormone (GnIH) distribution in the brain of the ancient fish Atractosteus tropicus (Holostei, Lepisosteiformes)

The Holostei group occupies a critical phylogenetic position as the sister group of the Teleostei. However, little is known about holostean pituitary anatomy or brain distribution of important reproductive neuropeptides, such as the gonadotropin-inhibitory hormone (GnIH). Thus, the present study set out to characterize the structure of the pituitary and to localize GnIH-immunoreactive cells in the brain of Atractosteus tropicus from the viewpoint of comparative neuroanatomy. Juveniles of both sexes were processed for general histology and immunohistochemistry. Based on the differences in cell organization, morphology, and staining properties, the neurohypophysis and three regions in the adenohypophysis were identified: the rostral and proximal pars distalis (PPD) and the pars intermedia. This last region was found to be innervated by the neurohypophysis. This organization, together with the presence of a saccus vasculosus, resembles the general teleost pituitary organization. A vast number of blood vessels were also recognized between the infundibulum floor of the hypothalamus and the PPD, evidencing the characteristic presence of a median eminence and a portal system. However, this well-developed pituitary portal system resembles that of tetrapods. As regards the immunohistochemical localization of GnIH, we found four GnIH-immunoreactive (GnIH-ir) populations in three hypothalamic nuclei (suprachiasmatic, retrotuberal, and tuberal nuclei) and one in the diencephalon (prethalamic nucleus), as well as a few scattered neurons throughout the olfactory bulbs, the telencephalon, and the intersection between them. GnIH-ir fibers showed a widespread distribution over almost all brain regions, suggesting that GnIH function is not restricted to reproduction only. In conclusion, the present study describes, for the first time, the pituitary of A. tropicus and the neuroanatomical localization of GnIH in a holostean fish that exhibits a similar distribution pattern to that of teleosts and other vertebrates, suggesting a high degree of phylogenetic conservation of this system.

Developmental Genes and Malformations in the Hypothalamus

The hypothalamus is a heterogeneous rostral forebrain region that regulates physiological processes essential for survival, energy metabolism, and reproduction, mainly mediated by the pituitary gland. In the updated prosomeric model, the hypothalamus represents the rostralmost forebrain, composed of two segmental regions (terminal and peduncular hypothalamus), which extend respectively into the non-evaginated preoptic telencephalon and the evaginated pallio-subpallial telencephalon. Complex genetic cascades of transcription factors and signaling molecules rule their development. Alterations of some of these molecular mechanisms acting during forebrain development are associated with more or less severe hypothalamic and pituitary dysfunctions, which may be associated with brain malformations such as holoprosencephaly or septo-optic dysplasia. Studies on transgenic mice with mutated genes encoding critical transcription factors implicated in hypothalamic-pituitary development are contributing to understanding the high clinical complexity of these pathologies. In this review article, we will analyze first the complex molecular genoarchitecture of the hypothalamus resulting from the activity of previous morphogenetic signaling centers and secondly some malformations related to alterations in genes implicated in the development of the hypothalamus.

Topological atlas of the hypothalamus in adult rhesus monkey

The prosomeric model explains the embryological development of the central nervous system (CNS) shared by all vertebrates as a Bauplan. As a primary event, the early neural plate is patterned by intersecting longitudinal plates and transverse segments, forming a mosaic of progenitor units. The hypothalamus is specified by three prosomeres (hp1, hp2, and the acroterminal domain) of the secondary prosencephalon with corresponding alar and basal plate parts, which develop apart from the diencephalon. Mounting evidence suggests that progenitor units within alar and basal plate parts of hp1 and hp2 give rise to distinct hypothalamic nuclei, which preserve their relative invariant positioning (topology) in the adult brain. Nonetheless, the principles of the prosomeric model have not been applied so far to the hypothalamus of adult primates. We parcellated hypothalamic nuclei in adult rhesus monkeys (Macaca mulatta) using various stains to view architectonic boundaries. We then analyzed the topological relations of hypothalamic nuclei and adjacent hypothalamic landmarks with homology across rodent and primate species to trace the origin of adult hypothalamic nuclei to the alar or basal plate components of hp1 and hp2. We generated a novel atlas of the hypothalamus of the adult rhesus monkey with developmental ontologies for each hypothalamic nucleus. The result is a systematic reinterpretation of the adult hypothalamus whose prosomeric ontology can be used to study relationships between the hypothalamus and other regions of the CNS. Further, our atlas may serve as a tool to predict causal patterns in physiological and pathological pathways involving the hypothalamus.

Bsx Is Essential for Differentiation of Multiple Neuromodulatory Cell Populations in the Secondary Prosencephalon

The hypothalamus is characterized by great neuronal diversity, with many neuropeptides and other neuromodulators being expressed within its multiple anatomical domains. The regulatory networks directing hypothalamic development have been studied in detail, but, for many neuron types, control of differentiation is still not understood. The highly conserved Brain-specific homeobox (Bsx) transcription factor has previously been described in regulating Agrp and Npy expression in the hypothalamic arcuate nucleus (ARC) in mice. While Bsx is expressed in many more subregions of both tuberal and mamillary hypothalamus, the functions therein are not known. Using genetic analyses in zebrafish, we show that most bsx expression domains are dependent on Nkx2.1 and Nkx2.4 homeodomain transcription factors, while a subset depends on Otp. We show that the anatomical pattern of the ventral forebrain appears normal in bsx mutants, but that Bsx is necessary for the expression of many neuropeptide encoding genes, including agrp, penka, vip, trh, npb, and nts, in distinct hypothalamic anatomical domains. We also found Bsx to be critical for normal expression of two Crh family members, crhb and uts1, as well as crhbp, in the hypothalamus and the telencephalic septal region. Furthermore, we demonstrate a crucial role for Bsx in serotonergic, histaminergic and nitrergic neuron development in the hypothalamus. We conclude that Bsx is critical for the terminal differentiation of multiple neuromodulatory cell types in the forebrain.