Lactosamine modulates the rate of migration of GnRH neurons during mouse development

Nasal Placode Development, GnRH Neuronal Migration and Kallmann Syndrome

The development of Gonadotropin releasing hormone-1 (GnRH) neurons is important for a functional reproduction system in vertebrates. Disruption of GnRH results in hypogonadism and if accompanied by anosmia is termed Kallmann Syndrome (KS). From their origin in the nasal placode, GnRH neurons migrate along the olfactory-derived vomeronasal axons to the nasal forebrain junction and then turn caudally into the developing forebrain. Although research on the origin of GnRH neurons, their migration and genes associated with KS has identified multiple factors that influence development of this system, several aspects still remain unclear. This review discusses development of the olfactory system, factors that regulate GnRH neuron formation and development of the olfactory system, migration of the GnRH neurons from the nose into the brain, and mutations in humans with KS that result from disruption of normal GnRH/olfactory systems development.

Gonadoliberin – Synthesis, Secretion, Molecular Mechanisms and Targets of Action

Decapeptide gonadoliberin (GnRH) is the most important regulator of the hypothalamic-pituitary-gonadal (HPG) axis that controls the synthesis and secretion of the luteinizing and follicle-stimulating hormones by gonadotrophs in the adenohypophysis. GnRH is produced by the specialized hypothalamic neurons using the site-specific proteolysis of the precursor protein and is secreted into the portal pituitary system, where it binds to the specific receptors. These receptors belong to the family of G protein-coupled receptors, and they are located on the surface of gonadotrophs and mediate the regulatory effects of GnRH on the gonadotropins production. The result of GnRH binding to them is the activation of phospholipase C and the calcium-dependent pathways, the stimulation of different forms of mitogen-activated protein kinases, as well as the activation of the enzyme adenylyl cyclase and the triggering of cAMP-dependent signaling pathways in the gonadotrophs. The gonadotropins, kisspeptin, sex steroid hormones, insulin, melatonin and a number of transcription factors have an important role in the regulation of GnRH1 gene expression, which encodes the GnRH precursor, as well as the synthesis and secretion of GnRH. The functional activity of GnRH-producing neurons depends on their migration to the hypothalamic region at the early stages of ontogenesis, which is controlled by anosmin, ephrins, and lactosamine-rich surface glycoconjugate. Dysregulation of the migration of GnRH-producing neurons and the impaired production and secretion of GnRH, lead to hypogonadotropic hypogonadism and other dysfunctions of the reproductive system. This review is devoted to the current state of the problem of regulating the synthesis and secretion of GnRH, the mechanisms of migration of hypothalamic GnRH-producing neurons at the early stages of brain development, the functional activity of the GnRH-producing neurons in the adult hypothalamus and the molecular mechanisms of GnRH action on the pituitary gonadotrophs. New experimental data are analyzed, which significantly change the current understanding of the functioning of GnRH-producing neurons and the secretion of GnRH, which is very important for the development of effective approaches for correcting the functions of the HPG axis.

Haploinsufficiency of SIX3 Abolishes Male Reproductive Behavior Through Disrupted Olfactory Development, and Impairs Female Fertility Through Disrupted GnRH Neuron Migration

Mating behavior in males and females is dependent on olfactory cues processed through both the main olfactory epithelium (MOE) and the vomeronasal organ (VNO). Signaling through the MOE is critical for the initiation of male mating behavior, and the loss of MOE signaling severely compromises this comportment. Here, we demonstrate that dosage of the homeodomain gene Six3 affects the degree of development of MOE but not the VNO. Anomalous MOE development in Six3 heterozygote mice leads to hyposmia, specifically disrupting male mounting behavior by impairing detection of volatile female estrus pheromones. Six3 is highly expressed in the MOE, main olfactory bulb (MOB), and hypothalamus; all regions essential in the proper migration of the gonadotropin-releasing hormone (GnRH) neurons, a key reproductive neuronal population that migrates along olfactory axons from the developing nose into the brain. Interestingly, we find that the reduction in Six3 expression in Six3 heterozygote mice compromises development of the MOE and MOB, resulting in mis-migration of GnRH neurons due to improper olfactory axon targeting. This reduction in the hypothalamic GnRH neuron population, by 45% in adulthood, leads to female subfertility, but does not impact male hormone levels, suggesting that male infertility is not related to GnRH neuron numbers, but exclusively linked to abnormal olfaction. We here determine that Six3 is haploinsufficient for MOE development, GnRH neuron migration, and fertility, and represents a novel candidate gene for Kallmann syndrome, a form of inherited infertility.

Haploinsufficiency of Homeodomain Proteins Six3, Vax1, and Otx2 Causes Subfertility in Mice via Distinct Mechanisms

Haploinsufficiency occurs when loss of one copy of a diploid gene (hemizygosity) causes a phenotype. It is relatively rare, in that most genes can produce sufficient mRNA and protein from a single copy to prevent any loss of normal activity and function. Reproduction is a complex process relying on migration of GnRH neurons from the olfactory placode to the hypothalamus during development. We have studied 3 different homeodomain genes Otx2, Vax1, and Six3 and found that the deletion of one allele for any of these genes in mice produces subfertility or infertility in one or both sexes, despite the presence of one intact allele. All 3 heterozygous mice have reduced numbers of GnRH neurons, but the mechanisms of subfertility differ significantly. This review compares the subfertility phenotypes and their mechanisms.

The hypothalamus-pituitary-gonad axis: Tales of mice and men

Reproduction is controlled by the hypothalamic-pituitary-gonadal (HPG) axis. Gonadotropin-releasing hormone (GnRH) neurons play a central role in this axis through production of GnRH, which binds to a membrane receptor on pituitary gonadotrophs and stimulates the biosynthesis and secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Multiple factors affect GnRH neuron migration, GnRH gene expression, GnRH pulse generator, GnRH secretion, GnRH receptor expression, and gonadotropin synthesis and release. Among them anosmin is involved in the guidance of the GnRH neuron migration, and a loss-of-function mutation in its gene leads to a failure of their migration from the olfactory placode to the hypothalamus, with consequent anosmic hypogonadotropic hypogonadism (Kallmann syndrome). There are also cases of hypogonadotropic hypogonadim with normal sense of smell, due to mutations of other genes. Another protein, kisspeptin plays a crucial role in the regulation of GnRH pulse generator and the pubertal development. GnRH is the main hypothalamic regulator of the release of gonadotropins. Finally, FSH and LH are the essential hormonal regulators of testicular functions, acting through their receptors in Sertoli and Leydig cells, respectively. The main features of the male HPG axis will be described in this review.

Accelerating Gene Discovery by Phenotyping Whole-Genome Sequenced Multi-mutation Strains and Using the Sequence Kernel Association Test (SKAT)

Forward genetic screens represent powerful, unbiased approaches to uncover novel components in any biological process. Such screens suffer from a major bottleneck, however, namely the cloning of corresponding genes causing the phenotypic variation. Reverse genetic screens have been employed as a way to circumvent this issue, but can often be limited in scope. Here we demonstrate an innovative approach to gene discovery. Using C. elegans as a model system, we used a whole-genome sequenced multi-mutation library, from the Million Mutation Project, together with the Sequence Kernel Association Test (SKAT), to rapidly screen for and identify genes associated with a phenotype of interest, namely defects in dye-filling of ciliated sensory neurons. Such anomalies in dye-filling are often associated with the disruption of cilia, organelles which in humans are implicated in sensory physiology (including vision, smell and hearing), development and disease. Beyond identifying several well characterised dye-filling genes, our approach uncovered three genes not previously linked to ciliated sensory neuron development or function. From these putative novel dye-filling genes, we confirmed the involvement of BGNT-1.1 in ciliated sensory neuron function and morphogenesis. BGNT-1.1 functions at the trans-Golgi network of sheath cells (glia) to influence dye-filling and cilium length, in a cell non-autonomous manner. Notably, BGNT-1.1 is the orthologue of human B3GNT1/B4GAT1, a glycosyltransferase associated with Walker-Warburg syndrome (WWS). WWS is a multigenic disorder characterised by muscular dystrophy as well as brain and eye anomalies. Together, our work unveils an effective and innovative approach to gene discovery, and provides the first evidence that B3GNT1-associated Walker-Warburg syndrome may be considered a ciliopathy.

Model organisms are useful tools for uncovering new genes involved in a biological process via genetic screens. Such an approach is powerful, but suffers from drawbacks that can slow down gene discovery. In forward genetics screens, difficult-to-map phenotypes present daunting challenges, and whole-genome coverage can be equally challenging for reverse genetic screens where typically only a single gene’s function is assayed per strain. Here, we show a different approach which includes positive aspects of forward (high-coverage, randomly-induced mutations) and reverse genetics (prior knowledge of gene disruption) to accelerate gene discovery. We paired a whole-genome sequenced multi-mutation C. elegans library with a rare-variant associated test to rapidly identify genes associated with a phenotype of interest: defects in sensory neurons bearing sensory organelles called cilia, via a simple dye-filling assay to probe the form and function of these cells. We found two well characterised dye-filling genes and three genes, not previously linked to ciliated sensory neuron development or function, that were associated with dye-filling defects. We reveal that disruption of one of these (BGNT-1.1), whose human orthologue is associated with Walker-Warburg syndrome, results in abrogated uptake of dye and cilia length defects. We believe that our novel approach is useful for any organism with a small genome that can be quickly sequenced and where many mutant strains can be easily isolated and phenotyped, such as Drosophila and Arabidopsis.

New Perspectives on Genomic Imprinting, an Essential and Multifaceted Mode of Epigenetic Control in the Developing and Adult Brain

Mammalian evolution entailed multiple innovations in gene regulation, including the emergence of genomic imprinting, an epigenetic regulation leading to the preferential expression of a gene from its maternal or paternal allele. Genomic imprinting is highly prevalent in the brain, yet, until recently, its central roles in neural processes have not been fully appreciated. Here, we provide a comprehensive survey of adult and developmental brain functions influenced by imprinted genes, from neural development and wiring to synaptic function and plasticity, energy balance, social behaviors, emotions, and cognition. We further review the widespread identification of parental biases alongside monoallelic expression in brain tissues, discuss their potential roles in dosage regulation of key neural pathways, and suggest possible mechanisms underlying the dynamic regulation of imprinting in the brain. This review should help provide a better understanding of the significance of genomic imprinting in the normal and pathological brain of mammals including humans.

Loss of Growth arrest specific gene 6 (Gas6) results in altered GnRH neuron migration, delayed vaginal opening and sexual maturation in mice

Prior work has shown the importance of TAM (Tyro3, Axl, Mer) receptor tyrosine kinases in GnRH neuronal development and reproductive function. It is unclear if TAM receptor actions are dependent on ligand activation for their functional effects; thus, we characterized reproductive phenotype of ligand Growth arrest specific gene (Gas6) null mice. Gas6 null mice showed delayed vaginal opening and delayed first estrus. Animals eventually attained normal estrous cycles as adults. The GnRH neuronal population was significantly decreased in Gas6 null adults and embryos, but the final positioning of cell bodies in the hypothalamus was normal. Vaginal tissue showed up-regulation of TAM receptor mRNAs in the absence of the ligand. These data confirm that Gas6 plays a role in early GnRH neuronal development and during vaginal opening. The phenotype of Gas6 KO mice suggests that TAMs function in a ligand-dependent and independent manner to control GnRH neuron development to modulate normal reproductive function.

Netrin-1 stimulates developing GnRH neurons to extend neurites to the median eminence in a calcium- dependent manner

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons are required for fertility in all mammalian species studied to date. In rodents, GnRH neuron cell bodies reside in the rostral hypothalamus, and most extend a single long neuronal process in the caudal direction to terminate at the median eminence (ME), the site of hormone secretion. The molecular cues that GnRH neurites use to grow and navigate to the ME during development, however, remain poorly described. Reverse transcription-PCR (RT-PCR) identified mRNAs encoding Netrin-1, and its receptor, DCC, in the fetal preoptic area (POA) and mediobasal hypothalamus (MBH), respectively, from gestational day 12.5 (GD12.5), a time when the first GnRH neurites extend toward the MBH. Moreover, a subpopulation of GnRH neurons from GD14.5 through GD18.5 express the Netrin-1 receptor, DCC, suggesting a role for Netrin-1/DCC signaling in GnRH neurite growth and/or guidance. In support of this notion, when GD15.5 POA explants, containing GnRH neurons actively extending neurites, were grown in three-dimensional collagen gels and challenged with exogenous Netrin-1 (100 ng/ml or 400 ng/ml) GnRH neurite growth was stimulated. In addition, Netrin-1 provided from a fixed source was able to stimulate outgrowth, although it did not appear to chemoattract GnRH neurites. Finally, the effects of Netrin-1 on the outgrowth of GnRH neurites could be inhibited by blocking either L-type voltage-gated calcium channels (VGCCs) with nifedipine (10 µM), or ryanodine receptors with ryanodine (10 µM). This is consistent with the role of Ca2+ from extra- and intracellular sources in Netrin-1/DCC-dependent growth cone motility in other neurons. These results indicate that Netrin-1 directly stimulates the growth of a subpopulation of GnRH neurites that express DCC, provide further understanding of the mechanisms by which GnRH nerve terminals arrive at their site of hormone secretion, and identify an additional neuronal population whose neurites utilize Netrin-1/DCC signaling for their development.

Gonadotropin-releasing hormone (GnRH) neuron migration: initiation, maintenance and cessation as critical steps to ensure normal reproductive function

GnRH neurons follow a carefully orchestrated journey from their birth in the olfactory placode area. Initially, they migrate along with the vomeronasal nerve into the brain at the cribriform plate, then progress caudally to sites within the hypothalamus where they halt and send projections to the median eminence to activate pituitary gonadotropes. Many factors controlling this precise journey have been elucidated by the silencing or over-expression of candidate genes in mouse models. Importantly, a number of these factors may not only play a role in normal physiology of the hypothalamic-pituitary-gonadal axis but also be mis-expressed to cause human disorders of GnRH deficiency, presenting as a failure to undergo normal pubertal development. This review outlines the current cadre of candidates thought to modulate GnRH neuronal migration. The further elucidation and characterization of these factors that impact GnRH neuron development may shed new light on human reproductive disorders and provide potential targets to develop new pro-fertility or contraceptive agents.

Defective gonadotropin-releasing hormone neuron migration in mice lacking SEMA3A signalling through NRP1 and NRP2: implications for the aetiology of hypogonadotropic hypogonadism

Kallmann syndrome (KS) is a genetic disease characterized by hypogonadotropic hypogonadism and impaired sense of smell. The genetic causes underlying this syndrome are still largely unknown, but are thought to be due to a developmental defect in the migration of gonadotropin-releasing hormone (GnRH) neurons. Understanding the causes of the disease is hampered by lack of appropriate mouse models. GnRH neurons are hypothalamic cells that centrally control reproduction in mammals by secreting the GnRH decapeptide into the portal blood vessels of the pituitary to stimulate the production of gonadotropins. During development, these cells are born in the nasal placode outside the brain and migrate in association with olfactory/vomeronasal axons to reach the forebrain and position themselves in the hypothalamus. By combining the analysis of genetically altered mice with in vitro models, we demonstrate here that a secreted guidance cue of the class 3 semaphorin family, SEMA3A, is essential for the development of the GnRH neuron system: loss of SEMA3A signalling alters the targeting of vomeronasal nerves and the migration of GnRH neurons into the brain, resulting in reduced gonadal size. We found that SEMA3A signals redundantly through both its classical receptors neuropilin (NRP) 1 and, unconventionally, NRP2, while the usual NRP2 ligand SEMA3F is dispensable for this process. Strikingly, mice lacking SEMA3A or semaphorin signalling through both NRP1 and NRP2 recapitulate the anatomical features of a single case of KS analysed so far, and may therefore be used as genetic models to elucidate the pathogenesis of KS.

A novel developmental role for kisspeptin in the growth of gonadotrophin-releasing hormone neurites to the median eminence in the mouse

The puberty- and fertility-regulating neuropeptide kisspeptin (KISS1) exerts dramatic effects on the physiology of adult gonadotrophin-releasing hormone (GnRH) neurones as a master regulator of mammalian reproduction. Given the action of KISS1 directly on adult GnRH neurones, and that KISS1 activates a signal transduction cascade involved in neurite growth in other neurones, we investigated whether KISS1 may play a role in the normal growth of GnRH neurites to the median eminence. A reverse transcription-polymerase chain reaction demonstrated the expression of Kiss1 mRNA in the embryonic mediobasal hypothalamus, the target region for GnRH neurite termination, as early as embryonic day 13.5 (E13.5), a time when the first GnRH neurites are arriving. Complementary expression of the mRNA encoding the KISS1 receptor, Kiss1r, in the preoptic area (POA) at E13.5 was also observed, suggesting that POA-resident GnRH neurones can respond to KISS1 from an early age. To examine the effects of KISS1 on GnRH neurite growth in isolation, E15.5 POA explants, containing GnRH neurones actively extending neurites, were grown in three-dimensional collagen gels. In the presence of KISS1 (1 μm), both the number and length of GnRH neurites were increased significantly compared to controls without KISS1. The effects of KISS1 on GnRH neurite growth could be inhibited by pretreatment with the phospholipase C inhibitor U73122 (50 μm), indicating that embryonic and adult GnRH neurones respond to KISS1 with the same intracellular signalling pathway. KISS1 provided in a concentration gradient from a fixed source had no effect on GnRH neurite growth, indicating that KISS1 does not function as a long-range chemoattractant. Taken together, these results identify KISS1 as a stimulator of GnRH neurite growth, and suggest that it influences GnRH neurites at close-range to innervate the median eminence. These data add a novel developmental role to the repertoire of the functions of KISS1 in mammalian reproduction.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

From nose to fertility: the long migratory journey of gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons, a small number of cells dispersed in the hypothalamic region of the basal forebrain, play an important role in reproductive function. These neurons originate in the nasal placode and migrate, first in the nasal compartment, then through the cribriform plate and finally through the basal forebrain, before they attain their positions in the hypothalamus. Their movement through changing molecular environments suggests that numerous factors are involved in different phases of their migration. In humans, failure of GnRH neurons to migrate normally results in delayed or absent pubertal maturation and infertility. Advances in genetic and molecular biologic techniques in this decade have allowed us to gain insights into several molecules that affect the migration of GnRH neurons and, consequently, play a role in the establishment and maintenance of reproductive function.