Migration of neurons containing gonadotropin releasing hormone (GnRH) in slices from embryonic nasal compartment and forebrain

The great migration: how glial cells could regulate GnRH neuron development and shape adult reproductive life

In mammals, reproductive function is under the control of hypothalamic neurons named Gonadotropin-Releasing Hormone (GnRH) neurons. These neurons migrate from the olfactory placode to the brain, during embryonic development. For the past 40 years, these neurons have been considered an example of tangential migration, i.e., dependent on the olfactory/vomeronasal/terminal nerves. Numerous studies have highlighted the factors involved in the migration of these neurons but thus far overlooked the cellular microenvironment that produces them. Many of these factors are dysregulated in hypogonadotropic hypogonadism, resulting in subfertility/infertility. Nevertheless, over the past ten years, several papers have reported the influence of glial cells (named olfactory ensheathing cells [OECs]) in the migration and differentiation of GnRH neurons. This review will describe the atypical origins, migration, and differentiation of these neurons, focusing on the latest discoveries. There will be a more specific discussion on the involvement of OECs in the development of GnRH neurons, during embryonic and perinatal life; as well as on their potential implication in the development of congenital or idiopathic hypogonadotropic hypogonadism (such as Kallmann syndrome).

Mammalian D-cysteine: A novel regulator of neural progenitor cell proliferation: Endogenous D-cysteine, the stereoisomer with rapid spontaneous in vitro racemization rate, has major neural roles: Endogenous D-cysteine, the stereoisomer with rapid spontaneous in vitro racemization rate, has major neural roles

D-amino acids are being recognized as functionally important molecules in mammals. We recently identified endogenous D-cysteine in mammalian brain. D-cysteine is present in neonatal brain in substantial amounts (mM) and decreases with postnatal development. D-cysteine binds to MARCKS and a host of proteins implicated in cell division and neurodevelopmental disorders. D-cysteine decreases phosphorylation of MARCKS in neural progenitor cells (NPCs) affecting its translocation. D-cysteine controls NPC proliferation by inhibiting AKT signaling. Exogenous D-cysteine inhibits AKT phosphorylation at Thr 308 and Ser 473 in NPCs. D-cysteine treatment of NPCs led to 50% reduction in phosphorylation of Foxo1 at Ser 256 and Foxo3a at Ser 253. We hypothesize that in the developing brain endogenous D-cysteine is as a physiologic regulator of NPC proliferation by inhibiting AKT signaling mediated by Foxo1 and Foxo3a. Endogenous D-cysteine may regulate mammalian neurodevelopment with roles in schizophrenia and Alzheimer's disease (AD).

Deletion of the Homeodomain Protein Six6 From GnRH Neurons Decreases GnRH Gene Expression, Resulting in Infertility

Hypothalamic GnRH (luteinizing hormone-releasing hormone) neurons are crucial for the hypothalamic-pituitary-gonadal (HPG) axis, which regulates mammalian fertility. Insufficient GnRH disrupts the HPG axis and is often associated with the genetic condition idiopathic hypogonadotropic hypogonadism (IHH). The homeodomain protein sine oculis-related homeobox 6 (Six6) is required for the development of GnRH neurons. Although it is known that Six6 is specifically expressed within a more mature GnRH neuronal cell line and that overexpression of Six6 induces GnRH transcription in these cells, the direct role of Six6 within the GnRH neuron in vivo is unknown. Here we find that global Six6 knockout (KO) embryos show apoptosis of GnRH neurons beginning at embryonic day 14.5 with 90% loss of GnRH neurons by postnatal day 1. We sought to determine whether the hypogonadism and infertility reported in the Six6KO mice are generated via actions within the GnRH neuron in vivo by creating a Six6-flox mouse and crossing it with the LHRHcre mouse. Loss of Six6 specifically within the GnRH neuron abolished GnRH expression in ∼0% of GnRH neurons. We further demonstrated that deletion of Six6 only within the GnRH neuron leads to infertility, hypogonadism, hypogonadotropism, and delayed puberty. We conclude that Six6 plays distinct roles in maintaining fertility in the GnRH neuron vs in the migratory environment of the GnRH neuron by maintaining expression of GnRH and survival of GnRH neurons, respectively. These results increase knowledge of the role of Six6 in the brain and may offer insight into the mechanism of IHH.

From organotypic culture to body-on-a-chip: A neuroendocrine perspective

The methods used to study neuroendocrinology have been as diverse as the discoveries to come out of the field. Maintaining live neurones outside of a body in vitro was important from the beginning, building on methods that dated back to at least the first decade of the 20th Century. Neurosecretion defines an essential foundation of neuroendocrinology based on work that began in the 1920s and 1930s. Throughout the first half of the 20th Century, many paradigms arose for studying everything from single neurones to whole organs in vitro. Two of these survived as preeminent systems for use throughout the second half of the century: cell cultures and explant systems. Slice cultures and explants that emerged as organotypic technologies included such neuroendocrine organs such as the brain, pituitary, adrenals and intestine. The vast majority of these studies were carried out in static cultures for which media were changed over a time scale of days. Tissues were used for experimental techniques such as electrical recording of neuronal physiology in single cells and observation by live microscopy. When maintained in vitro, many of these systems only partially capture the in vivo physiology of the organ system of interest, often because of a lack of cellular diversity (eg, neuronal cultures lacking glia). Modern microfluidic methodologies show promise for organ systems, ranging from the reproductive to the gastrointestinal to the brain. Moving forward and striving to understand the mechanisms that drive neuroendocrine signalling centrally and peripherally, there will always be a need to consider the heterogeneous cellular compositions of organs in vivo.

Ex Utero Electroporation and Organotypic Slice Cultures of Embryonic Mouse Brains for Live-Imaging of Migrating GABAergic Interneurons

GABAergic interneurons (INs) are critical components of neuronal networks that drive cognition and behavior. INs destined to populate the cortex migrate tangentially from their place of origin in the ventral telencephalon (including from the medial and caudal ganglionic eminences (MGE, CGE)) to the dorsal cortical plate in response to a variety of intrinsic and extrinsic cues. Different methodologies have been developed over the years to genetically manipulate specific pathways and investigate how they regulate the dynamic cytoskeletal changes required for proper IN migration. In utero electroporation has been extensively used to study the effect of gene repression or overexpression in specific IN subtypes while assessing the impact on morphology and final position. However, while this approach is readily used to modify radially migrating pyramidal cells, it is more technically challenging when targeting INs. In utero electroporation generates a low yield given the decreased survival rates of pups when electroporation is conducted before e14.5, as is customary when studying MGE-derived INs. In an alternative approach, MGE explants provide easy access to the MGE and facilitate the imaging of genetically modified INs. However, in these explants, INs migrate into an artificial matrix, devoid of endogenous guidance cues and thalamic inputs. This prompted us to optimize a method where INs can migrate in a more naturalistic environment, while circumventing the technical challenges of in utero approaches. In this paper, we describe the combination of ex utero electroporation of embryonic mouse brains followed by organotypic slice cultures to readily track, image and reconstruct genetically modified INs migrating along their natural paths in response to endogenous cues. This approach allows for both the quantification of the dynamic aspects of IN migration with time-lapse confocal imaging, as well as the detailed analysis of various morphological parameters using neuronal reconstructions on fixed immunolabeled tissue.

In Vitro, Ex Vivo and In Vivo Techniques to Study Neuronal Migration in the Developing Cerebral Cortex

Neuronal migration is a fundamental biological process that underlies proper brain development and neuronal circuit formation. In the developing cerebral cortex, distinct neuronal populations, producing excitatory, inhibitory and modulatory neurotransmitters, are generated in different germinative areas and migrate along various routes to reach their final positions within the cortex. Different technical approaches and experimental models have been adopted to study the mechanisms regulating neuronal migration in the cortex. In this review, we will discuss the most common in vitro, ex vivo and in vivo techniques to visualize and study cortical neuronal migration.

Viewing cell movements in the developing neuroendocrine brain

Many studies suggest that migratory guidance cues within the developing brain are diverse across many regions. To better understand the early development and differentiation of select brain regions, an in vitro method was developed using selected inbred and transgenic strains of embryonic mice. In particular, organotypic slices are used to test factors that influence the movements of neurons during brain development. Thick 250 μm slices cut on a vibrating microtome are prepared and maintained in vitro for 0-3 days. Nissl stain analyses often show a uniform distribution of cells in the regions of interest on the day of plating (embryonic days 12-15). After 3 days in vitro, cellular aggregation suggesting nuclear formation or the changing position of cells with a defined phenotype show that reasonably normal cell movements occur in several regions. Movements in vitro that mimic changes in vivo suggest that key factors reside locally within the plane of the slices. Video microscopy studies are used to follow the migration of fluorescently labeled cells in brain slices from mice maintained in serum-free media for 1 to 3 days. Transgenic mice with selective promoter driven expression of fluorescent proteins allow us to view specific cell types (e.g., neurons expressing gonadotropin-releasing hormone). The accessibility of an in vitro system that provides for relatively normal brain development over key brief windows of time allows for the testing of important mechanisms.

Hormonal programming across the lifespan

Hormones influence countless biological processes across an animal's lifespan. Many hormone-mediated events occur within developmental sensitive periods, during which hormones have the potential to cause permanent tissue-specific alterations in anatomy and physiology. There are numerous selective critical periods in development with different targets being affected during different periods. This review outlines the proceedings of the Hormonal Programming in Development session at the US-South American Workshop in Neuroendocrinology in August 2011. Here we discuss how gonadal steroid hormones impact various biological processes within the brain and gonads during early development and describe the changes that take place in the aging female ovary. At the cellular level, hormonal targets in the brain include neurons, glia, or vasculature. On a genomic/epigenomic level, transcription factor signaling and epigenetic changes alter the expression of critical hormone receptor genes across development and following ischemic brain insult. In addition, organizational hormone exposure alters epigenetic processes in specific brain nuclei and may be an important mediator of sexual differentiation of the neonatal brain. Brain targets of hormonal programming, such as the paraventricular nucleus of the hypothalamus, may be critical in influencing the development of peripheral targets, such as the ovary. Exposure to excess hormones can cause abnormalities in the ovary during development leading to polycystic ovarian syndrome (PCOS). Exposure to excess androgens during fetal development also has a profound effect on the development of the male reproductive system. In addition, increased activity of the sympathetic nerve and stress during early life have been linked to PCOS symptomology in adulthood. Finally, we describe how age-related decreases in fertility are linked to high levels of nerve growth factor (NGF), which enhances sympathetic nerve activity and alters ovarian function.

Calcium dynamics in gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neurons represent the key output cells of the neuronal network controlling fertility. Intracellular calcium ion concentration ([Ca(2+)](i)) is likely to be a key signaling tool used by GnRH neurons to regulate and co-ordinate multiple cell processes. This review examines the dynamics and control of [Ca(2+)](i) in GT1 cells, embryonic GnRH neurons in the nasal placode culture, and adult GnRH neurons in the acute brain slice preparation. GnRH neurons at all stages of development display spontaneous [Ca(2+)](i) transients driven, primarily, by their burst firing. However, the intracellular mechanisms generating [Ca(2+)](i) transients, and the control of [Ca(2+)](i) by neurotransmitters, varies markedly across the different developmental stages. The functional roles of [Ca(2+)](i) transients are beginning to be unraveled with one key action being that of regulating the dynamics of GnRH neuron burst firing.

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.

Hepatocyte growth factor acts as a motogen and guidance signal for gonadotropin hormone-releasing hormone-1 neuronal migration

Reproduction in mammals is under the control of the hypothalamic neuropeptide gonadotropin hormone-releasing hormone-1 (GnRH-1). GnRH-1-secreting neurons originate during embryonic development in the nasal placode and migrate into the forebrain along olfactory nerves. Gradients of secreted molecules may play a role in this migratory process. In this context, hepatocyte growth factor (HGF) is a potential candidate, because it promotes cell motility in developing brain and has been shown previously to act as a motogen on immortalized GnRH-1 neurons (GN11). In this study, the role of HGF and its receptor Met during development of the GnRH-1 system was examined. GnRH-1 cells express Met during their migration and downregulate its expression once they complete this process. Tissue-type plasminogen activator (tPA), a known HGF activator, is also detected in migratory GnRH-1 neurons. Consistent with in vivo expression, HGF is present in nasal explants, and GnRH-1 neurons express Met. HGF-neutralizing antibody was applied to explants to examine the role of the endogenous growth factor. Migration of GnRH-1 cells and olfactory axon outgrowth were significantly reduced, in line with disruption of a guidance gradient. Exogenous application of HGF to explants increased the distance that GnRH-1 cells migrated, suggesting that HGF also acts as a motogen to GnRH-1 neurons. Functional experiments, performed on organotypic slice cultures, show that creation of an opposing HGF gradient inhibits GnRH-1 neuronal migration. Finally, tPA(-/-):uPA(-/-) (urokinase-type plasminogen activator(-/-)) knock-out mice exhibit strong reduction of the GnRH-1 cell population. Together, these data indicate that HGF signaling via Met receptor influences the development of GnRH-1.

Minireview: recent progress in gonadotropin-releasing hormone neuronal migration

Neurons that synthesize GnRH are critical brain regulators of the reproductive axis, yet they originate outside the brain and must migrate over long distances and varied environments to get to their appropriate positions during development. Many studies, past and present, are providing clues for the types of molecules encountered and movements expected along the migratory route. Recent studies provide real-time views of the behavior of GnRH neurons in the context of in vitro preparations that model those in vivo. Live images provide direct evidence of the changing behavior of GnRH neurons in their different environments, showing that GnRH neurons move with greater frequency and with more alterations in direction after they enter the brain. The heterogeneity of molecular phenotypes for GnRH neurons likely ensures that multiple external factors will be found that regulate the migration of different portions of the GnRH neuronal population at different steps along the route. Molecules distributed in gradients both in the peripheral olfactory system and basal forebrain may be particularly influential in directing the appropriate movement of GnRH neurons along their arduous migration. Molecules that mediate the adhesion of GnRH neurons to changing surfaces may also play critical roles. It is likely that the multiple external factors converge on selective signal transduction pathways to engage the mechanical mechanisms needed to modulate GnRH neuronal movement and ultimately migration.

Reelin provides an inhibitory signal in the migration of gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons, a small number of cells scattered in the hypothalamic region of the basal forebrain, play an important role in reproductive function. These cells originate in the olfactory placode and migrate into the basal forebrain in late embryonic life. Here, we show that reelin, which is expressed along the route of the migrating cells, has an inhibitory role in guiding GnRH neurons to the basal forebrain. Only a small(approximately 5%) subpopulation of these neurons expresses one of the reelin receptors (ApoER2/Lrp8), and all GnRH neurons appear to lack the intracellular adaptor protein Dab1, suggesting that the function of reelin is not mediated by the conventional signal transduction pathway. The importance of reelin in the establishment of GnRH neurons in the hypothalamus was confirmed by our finding that the brains of developing and adult reeler mice of both sexes contained a markedly reduced number of these neuroendocrine neurons. Furthermore, the testes of adult males showed dilation of seminiferous tubules and reduction in their density when compared with controls. Mutants lacking the reelin receptors ApoER2 and Vldlr, and scrambler mice lacking Dab1, showed a normal complement of GnRH neurons in the hypothalamus,confirming that the effect of reelin in their migration is independent of Dab1.

Sex differences in the location of immunochemically defined cell populations in the mouse preoptic area/anterior hypothalamus

The preoptic area/anterior hypothalamus (POA/AH) is sexually dimorphic in many vertebrates. We have defined specific cell populations within the POA/AH using immunocytochemical markers for estrogen receptor beta (ERbeta) and the R1 subunit of the GABA(B) receptor (GABA(B)R1). Our previous finding of sex differences in cell migration in this region in embryonic day 15 mice led us to examine sex differences in the location or size of chemically identified cell groups. At embryonic day 17 (E17), cells containing immunoreactive (ir) ERbeta in females were located more dorsal and lateral than those in males. In contrast to this positional sex difference seen at E17, ERbeta expression at P0 and adulthood showed a sex difference in cell number and area of immunoreactivity with a higher expression of ERbeta in males than females. Furthermore, in animals that were genetically deprived of gonadal and adrenal hormones by virtue of a disrupted gene coding for steroidogenic factor 1, cells containing ir ERbeta followed a female phenotype for location at E17 and a female phenotype for number of ir cells at P0 regardless of genetic sex, suggesting that circulating hormones may be influencing cell position in the POA/AH. A second phenotypically identified cell group containing ir GABA(B)R1 also had a sex difference in cell positions at E17. Females expressed GABA(B)R1 in cells with a more dorsal position than in males. These results provide support for the suggestion that sex differences in cellular organization in the developing hypothalamus arise from sex differences in cell migration.

Live view of gonadotropin-releasing hormone containing neuron migration

Neurons that synthesize GnRH control the reproductive axis and migrate over long distances and through different environments during development. Prior studies provided strong clues for the types of molecules encountered and movements expected along the migratory route. However, our studies provide the first real-time views of the behavior of GnRH neurons in the context of an in vitro preparation that maintains conditions comparable to those in vivo. The live views provide direct evidence of the changing behavior of GnRH neurons in their different environments, showing that GnRH neurons move with greater frequency and with more changes in direction after they enter the brain. Perturbations of guiding fibers distal to moving GnRH neurons in the nasal compartment influenced movement without detectable changes in the fibers in the immediate vicinity of moving GnRH neurons. This suggests that the use of fibers by GnRH neurons for guidance may entail selective signaling in addition to mechanical guidance. These studies establish a model to evaluate the influences of specific molecules that are important for their migration.

Overexpression of glutamic acid decarboxylase-67 (GAD-67) in gonadotropin-releasing hormone neurons disrupts migratory fate and female reproductive function in mice

gamma-Aminobutyric acid (GABA) inhibits the embryonic migration of GnRH neurons and regulates hypothalamic GnRH release. A subset of GnRH neurons expresses GABA along their migratory route in the nasal compartment before entering the brain, suggesting that GABA produced by GnRH neurons may help regulate the migratory process. To examine this hypothesis and the possibility that persistence of GABA production by GnRH neurons may affect subsequent reproductive function, we generated transgenic mice in which the expression of glutamic acid decarboxylase-67 (GAD-67), a key enzyme in GABA synthesis, is targeted to GnRH neurons under the control of the GnRH gene promoter. On embryonic d 15, when GnRH neurons are still migrating, the transgenic animals had more GnRH neurons in aberrant locations in the cerebral cortex and fewer neurons reaching the hypothalamic-preoptic region, whereas migration into the brain was not affected. Hypothalamic GnRH content in mutant mice was low during the first week of postnatal life, increasing to normal values during infantile development (second week after birth) in the presence of increased pulsatile GnRH release. Consistent with these changes, serum LH and FSH levels were also elevated. Gonadotropin release returned to normal values by the time steroid negative feedback became established (fourth week of life). Ovariectomy at this time demonstrated an enhanced gonadotropin response in transgenic animals. Although the onset of puberty, as assessed by the age at vaginal opening and first ovulation, was not affected in the mutant mice, estrous cyclicity and adult reproductive capacity were disrupted. Mutant mice had reduced litter sizes, increased time intervals between deliveries of litters, and a shorter reproductive life span. Thus, GABA produced within GnRH neurons does not delay GnRH neuronal migration, but instead serves as a developmental cue that increases the positional diversity of these neurons within the basal forebrain. In addition, the results suggest that the timely termination of GABA production within the GnRH neuronal network is a prerequisite for normal reproductive function. The possibility arises that similar abnormalities in GABA homeostasis may contribute to syndromes of hypothalamic amenorrhea/oligomenorrhea in humans.

DLX5 regulates development of peripheral and central components of the olfactory system

Induction, neurogenesis, and synaptogenesis of the olfactory bulb are thought to require interactions with the olfactory epithelium. The Dlx family of homeobox genes is expressed in both the olfactory bulb and olfactory epithelium. In particular, Dlx5 is expressed in the olfactory placode, olfactory epithelium, and local circuit neurons of the olfactory bulb. Here we analyzed mice lacking DLX5 function. The Dlx5-/- mutation reduces the size of the olfactory epithelium. Although some olfactory neurons are formed, they fail to generate olfactory axons that innervate the olfactory bulb. Despite the lack of innervation, the olfactory bulb forms, and neurogenesis of projection and local circuit neurons proceeds. However, the mutation has a cell-autonomous effect on the ability of neural progenitors to produce olfactory bulb local circuit neurons, with granule cells more severely affected than periglomerular cells. In addition, the mutation has a noncell-autonomous effect on the morphogenesis of mitral cells.

The terminal nerve in turbot, Psetta maxima:

The ontogeny and organization of the terminal nerve (TN) during turbot development was studied using an antiserum to neuropeptide Y. First immunoreactive cells were detected in the olfactory placode at hatching time. At 1 day after hatching, a loose group of labeled neurons form an extracranial primordial ganglion of the TN. During the subsequent larval development, more perikarya displaying increased immunoreactivity were found along the course of the olfactory nerve. Moreover, labeled cells cross the meninx of the forebrain gathering in the olfactory bulb of larval turbot. Projections from these cells, directed both to the caudal brain and to the retina, develop when the cells become established in the olfactory bulb. The generation of immunoreactive cells in the olfactory organ extends into the metamorphic period, when a pronounced asymmetry affects the turbot morphology. At this time, the topological location of the immunoreactive cells in the TN becomes distorted. This developmental pattern was compared with those found in other teleosts and in other vertebrates. Preabsorption experiments of anti-neuropeptide Y serum with neuropeptide Y and FMRF-amide suggests that immunoreactive material observed in TN cells was not neuropeptide Y, and raises the possibility that other peptides, e.g. FMRF-amide-like peptides, exist in this neural system.

Gamma-aminobutyric acidB receptors and the development of the ventromedial nucleus of the hypothalamus

Gamma-aminobutyric acid (GABA) is a highly abundant neurotransmitter in the brain and the ligand for GABA(A), GABA(B), and GABA(C) receptors. Unlike GABA(A) and GABA(C) receptors, which are chloride channels, GABA(B) receptors are G-protein linked and alter cell-signaling pathways. Electrophysiological studies have found GABA(B) receptors in cultured embryonic hypothalamus, but the distribution of these receptors remains unknown. In the present study, we examined the expression of GABA(B) receptors in the ventromedial nucleus of the hypothalamus (VMH) during embryonic mouse development. GABA(B) receptors were present in the VMH at all ages examined, from embryonic day 13 to postnatal day 6. Using a brain slice preparation, we examined the effect of GABA(B) receptor activation on cell movement in the embryonic VMH as the nucleus forms in vitro. The GABA(B) receptor agonist baclofen decreased the rate of cell movement in a dose-dependent manner. Baclofen reduced cell movement by up to 56% compared with vehicle-treated controls. The percentage of cells moving per field and the angles of cell movement were not affected. With our previous findings of GABA(A) receptor activation, it is likely that GABA influences VMH development via multiple mechanisms.

Developmental aspect of the gonadotropin-releasing hormone system

Gonadotropin-releasing hormone (GnRH) regulates the hypothalamo-pituitary-gonadal (HPG) axis in all vertebrates studied. GnRH neurons that regulate the HPG axis are primarily derived from progenitor cells in the nasal compartment (NC) and migrate along olfactory system derived fibers across the cribriform plate to destinations in the forebrain. Across their long and uncommon migratory route many factors are likely important for their successful development. Several classes of molecules are being studied for their potential influences on migration, including those related to cell surface interactions (membrane receptors, adhesion molecules, extracellular matrix (ECM) molecules, etc.) and those related to communication across distances (neurotransmitters, peptides, chemoattractant or repellent molecules). Of the classes of molecules associated with cell surface interactions, glycoconjugates with terminal galactose, are temporally and spatially expressed on olfactory fibers that guide GnRH neurons and may play role(s) in migration. Of the molecules associated with communication across distances, the neurotransmitter gamma-aminobutyric acid (GABA) is associated with the GnRH migration pathway and influences the position and organization of GnRH neurons in vitro and in vivo. Furthermore, galactose-containing glycoconjugates and GABA are associated with GnRH neurons in species ranging from humans to lamprey. In mice and rats, GABA is found transiently within a subpopulation of GnRH neurons as they migrate through the NC. One of the key elements in considering regulators of GnRH neuron migration is the diversity of GnRH synthesizing cells. For example, only subpopulations of GnRH neurons also contain GABA, specific GABA receptors, or select glycoconjugates. Similarly, treatments that influence GnRH neuronal migration may only affect specific subsets and not the entire population. It is likely that we will not be able to characterize the migration of all GnRH neurons by a single factor. By combining molecular inquiries with genetic models, single cell analyses, and an in vitro migration model, we are beginning to decipher one of the most critical events in the establishment of the reproductive axis.

GABA influences the development of the ventromedial nucleus of the hypothalamus

The region that becomes the ventromedial nucleus of the hypothalamus (VMH) is surrounded by cells and fibers containing immunoreactive gamma-aminobutyric acid (GABA) by embryonic day 13 (E13), several days before the nucleus emerges in Nissl stains. As GABA plays many roles during neural development, we hypothesized that it influences VMH development, perhaps by providing boundary information for migrating neurons. To test this hypothesis we examined the VMH in embryonic mice in which the beta3 subunit of the GABA(A)-receptor, a receptor subunit that is normally highly expressed in this nucleus, was disrupted by gene targeting. In beta3 -/- embryos the VMH was significantly larger, and the distribution of cells containing immunoreactive estrogen receptor-alpha was expanded compared to controls. Using in vitro brain slices from wild-type C57BL/6J mice killed at E15 we found that treatment with the GABA(A) antagonist bicuculline increased the number of cells migrating per video field analyzed in the VMH. In addition, treatment with either bicuculline or the GABA(A) agonist muscimol altered the orientation of cell migration in particular regions of this nucleus. These data suggest that GABA is important for the organization of cells during VMH formation.

Immunohistochemical localization of calbindin D-28k in the migratory pathway from the rat olfactory placode

The spatiotemporal localization of calbindin D-28k (Calb), a calcium-binding protein, was examined immunohistochemically in the developing rat olfactory system with special reference to cell migration from the olfactory placode. Calb immunoreactivity was first detected at embryonic day 12 (E12) in a few cells just outside the olfactory epithelium, and at E13, Calb-immunoreactive cells were found scattered in the laminin-rich mesenchyme. By E14, Calb-immunoreactive cells had increased in number and were seen along the entire migratory route between the vomeronasal organ, a derivative of the medial olfactory pit, and the ventromedial surface of the telencephalic vesicle. Calb neurones were not seen in the olfactory epithelium, a derivative of the lateral olfactory pit. Although the distribution pattern of Calb-immunoreactive cells was similar to that of luteinizing hormone releasing hormone (LHRH)-producing neurones, which are known to originate in the vomeronasal organ and migrate into the forebrain, Calb and LHRH immunoreactivities were contained in separate neuronal populations. Calb-immunoreactive cells were localized along the vomeronasal nerves, identified by labelling the vomeronasal organ with the lipophilic dye, DiI, and strongly immunoreactive for neural cell adhesion molecule (NCAM). These data strongly suggest that, in addition to LHRH neurones, the rat vomeronasal organ generates Calb-immunoreactive neurones which migrate along the vomeronasal nerves to enter the forebrain. The final fate and functional importance of these cells remains to be determined.

Myocyte enhancer factors-2B and -2C are required for adhesion related kinase repression of neuronal gonadotropin releasing hormone gene expression

Synthesis of the hypothalamic peptide, gonadotropin releasing hormone (GnRH), is paramount for reproductive function. GnRH neurons originate in the olfactory region and migrate into the forebrain during development. We recently implicated adhesion related kinase (Ark) in GnRH neuron development based on its differential expression in two GnRH producing cell lines, GT1-7 and Gn10. The Ark membrane receptor encodes an extracellular domain resembling cell adhesion molecules and an intracellular tyrosine kinase. Ark is expressed in Gn10 cells derived from migrating GnRH neurons but not GT1-7 cells of the post-migratory phenotype. Here, we show that Ark and GnRH transcripts are colocalized in the cribriform plate at midgestation, suggesting that Ark is expressed in migrating GnRH neurons in vivo. Furthermore, we have identified the GnRH gene as a downstream target of Ark signaling. Ark inhibits GnRH gene expression in GnRH neuronal cells via the coordinated binding of myocyte enhancer factor-2B and -2C (MEF-2B and -2C) and a putative homeoprotein within the proximal rat GnRH promoter. Given that MEF-2 proteins are widely expressed in the brain, these studies provide further evidence for MEF-2 action during neuronal development. Moreover, our studies elucidate a potential role for Ark in regulating GnRH gene expression during GnRH neuronal migration.

Immortalized hypothalamic luteinizing hormone-releasing hormone (LHRH) neurons induce a functional switch in the growth factor responsiveness of astroglia: involvement of basic fibroblast growth factor

Recent evidence indicates that astroglial-derived growth factors (GFs) participate in the development of luteinizing hormone-releasing hormone (LHRH) neurons, but it is still unknown whether LHRH neurons may exert a reciprocal modulation of glial cell function. Using immortalized hypothalamic LHRH (GT1-1) neurons in co-culture with glial cells, we have recently shown that basic fibroblast growth factor (bFGF) plays a prominent role in the glial-induced acquisition of the mature LHRH phenotype by GT1-1 cells. We have resorted to this model and combined biochemical and morphological approaches to study whether the response of glial cells to a number of GFs (including bFGF, insulin-like growth factor I, IGF-I, epidermal growth factor, EGF and insulin) expressed during LHRH neuron differentiation, is modulated by co-culture with pure LHRH neurons. Pre-treatment of hypothalamic astrocytes with an inactive ('priming') dose of bFGF for 12 h powerfully increased astroglia proliferative response to IGF-I (10 ng/ml), EGF (10 g/ml) and insulin (10 microg/ml), inducing a 65-100% increase in the [3H]thymidine incorporation compared to untreated cultures. When astroglial cells and developing GT1-1 neurons were co-cultured for 5 days in vitro (DIV), the [3H]thymidine incorporation was significantly higher than in astroglial cells cultured without neurons. Application of the different GFs to the co-culture for either 12 or 24 h further stimulated DNA synthesis to various extent according to the GF applied and the time of application. Localization of the proliferating cells by dual immunohistochemical staining, followed by cell counting and bromodeoxiuridine (BrdU) labeling index calculation, revealed that the incorporation of BrdU was restricted to the nuclei of LHRH-immunopositive neurons. Such changes were accompanied by extensive morphological alterations of astroglial and LHRH fiber networks, whereas neutralization of bFGF activity in GT1-1 neuron-glial co-cultures by a bFGF-antibody, dramatically counteracted the observed effects. The functional switch of astroglia proliferative response to GFs coupled to the potent morphological and functional modifications of developing glia and pure LHRH neurons observed in vitro, support a bidirectional interaction between immortalized LHRH neurons and astroglial cells and identify bFGF as a key player in this crosstalk.

The neurobiology of reproductive development

This brief review has highlighted some of the major advances in the last decade or so in understanding the central control of puberty. These include the discovery that GnRH-I neurons develop in the olfactory placode and migrate into the forebrain, the recognition that puberty is a reactivation of GnRH secretion, the identification of leptin as a metabolic signal which may permit puberty to occur, unraveling the molecular basis of the circadian clock which underlies photoperiodic control of puberty in seasonal species, the identification of the structure of pheromones in urine, and the discovery of other populations of GnRH neurons in mammals expressing the GnRH-II gene. Such advances generate further questions: what regulates the migratory pathways of GnRH neurons, and what controls axon outgrowth and targeting to the median eminence? What is the mechanism which causes GnRH secretion to decline between the neonatal and pubertal phase of development? How do leptin and other sensory inputs finally communicate to the GnRH neuron? How do GnRH neurons communicate with each other such that co-ordinated pulsatile release of GnRH occurs? What is the function of GnRH-II? Some of these issues may be better addressed using the transgenic technologies which allow the identification and thus the recording, sampling and observation of GnRH neurons in living tissue, but in order to understand how internal and external cues influence puberty it will also be important to study a variety of other mammalian models in which the relative importance of such inputs differs.

Basic fibroblast growth factor (bFGF) acts on both neurons and glia to mediate the neurotrophic effects of astrocytes on LHRH neurons in culture

Luteinizing hormone-releasing hormone (LHRH) neurons play a pivotal role in the neuroendocrine control of mammalian reproduction. Astrocytes were shown to be involved in the regulation of LHRH neuronal function, but little is known about the contribution of astroglial-derived factors in the regulation of LHRH neuron development. In order to gain insight into the mechanisms regulating the development of these cells, at morphological and biochemical levels we characterized the neurotrophic effects exerted by young astrocytes (maintained in culture for 8 days in vitro) and old astrocytes (maintained 26 days) on the differentiation, proliferation, and phenotypic expression of immortalized hypothalamic LHRH (GT(1-1)) neurons in vitro. Culturing GT(1-1) cells in the presence of young glia for different time intervals caused a marked acceleration in the acquisition of their neuronal phenotype. At all times examined, GT(1-1) cells cocultured with young glia exhibited a significantly greater extension of processes/cell, larger number of processes/cell and greater surface area of growth cones than GT(1-1) cells grown over nonglial adhesive substrates (polylysine). By contrast, when GT(1-1) neurons were cocultured with old glia, the length of neuronal processes and the growth cone surface area were significantly lower than in control GT(1-1) neurons cultured in the absence of glia. At 3 days in vitro (DIV), GT(1-1) neurons cocultured with young glia exhibited a 50% lower incorporation of [(3)H]thymidine than GT(1-1) neurons cultured without glia. By contrast, in the presence of old glia [(3)H]thymidine incorporation was significantly higher in cells cocultured with glia than in GT(1-1) neurons cultured alone. Localization of the proliferating cells by dual immunohistochemical staining revealed that the incorporation of bromodeoxiuridine (BrdU) was restricted to nuclei of GT(1-1) neurons when these were cocultured with young glia, but associated with both neurons and astrocytes in the presence of old glia. At the functional level, coculture of GT(1-1) neurons with young glia increased the spontaneous release of LHRH as compared to GT(1-1) neurons grown in the absence of glia. By contrast, in the presence of old glia LHRH release in the medium was significantly lower than in controls. Conditioned medium of young glia (ACM-Y) induced significant neurotrophic and functional effects on GT(1-1) cells, but these effects were 50% less potent than the coculture itself. Heat denaturation of ACM-Y totally abolished its neurotrophic and functional properties, indicating that they involved a peptide factor. Suppression of bFGF activity in ACM-Y reduced its neurotrophic activity by approximately 40%, but did not affect its LHRH release-promoting effects. By contrast, neutralization of endogenous bFGF activity in GT(1-1) neurons cocultured with young glia counteracted both neurotrophic and functional effects of young glia. Treatment of old glia with bFGF rescued its neurotrophic and functional effects on GT(1-1) cells. Moreover, the ACM of aged bFGF-treated old glia was the most powerful neurotrophic stimulus for GT(1-1) neurons. These results suggest that: 1) soluble peptidic factors, including bFGF, and mechanism(s) requiring coculture are responsible for the highly potent neurotrophic and functional effects of young glia; 2) the inhibitory effects of old glia on neurite outgrowth and LHRH release are mediated in part by soluble inhibitory molecules and in part by factors requiring coculture with old glia; 3) old glia may revert to a growth-supporting state when treated with bFGF and this functional shift involves a diffusible molecule with potent neurotrophic and functional effects on immortalized LHRH neurons. (c) 2000 Wiley-Liss, Inc.

Sex differences in cell migration in the preoptic area/anterior hypothalamus of mice

The preoptic area/anterior hypothalamus (POA/AH) sits as a boundary region rostral to the classical diencephalic hypothalamus and ventral to the telencephalic septal region. Numerous studies have pointed to the region's importance for sex-dependent functions. Previous studies suggested that migratory guidance cues within this region might be particularly unique in their diversity. To better understand the early development and differentiation of the POA/AH, cytoarchitectural, birthdate, immunocytochemical, and cell migration studies were conducted in vivo and in vitro using embryonic C57BL/6J mice. A medial preoptic nucleus became discernible using Nissl stain in males and females between embryonic days (E) E15 and E17. Cells containing immunoreactive estrogen receptor-alpha were detected in the POA/AH by E13, and increased in number with age in both sexes. From E15 to E17, examination of the radial glial fiber pattern by immunocytochemistry confirmed the presence of dual orientations for migratory guidance ventral to the anterior commissure (medial-lateral and dorsal-ventral) and uniform orientation more caudally (medial-lateral). Video microscopy studies followed the migration of DiI-labeled cells in coronal 250-microm brain slices from E15 mice maintained in serum-free media for 1-3 days. Analyses showed significant migration along a dorsal-ventral orientation in addition to medial-lateral. The video analyses showed significantly more medial-lateral migration in males than females in the caudal POA/AH. In vivo, changes in the distribution of cells labeled by the mitotic indicator bromodeoxyuridine (BrdU) suggested their progressive migration through the POA/AH. BrdU analyses also indicated significant movement from dorsal to ventral regions ventral to the anterior commissure. The significant dorsal-ventral migration of cells in the POA/AH provides additional support for the notion that the region integrates developmental information from both telencephalic and diencephalic compartments. The sex difference in the orientation of migration of cells in the caudal POA/AH suggests one locus for the influence of gonadal steroids in the embryonic mouse forebrain.

Special relationship of gamma-aminobutyric acid to the ventromedial nucleus of the hypothalamus during embryonic development

The ventromedial nucleus of the hypothalamus (VMH) is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. We conducted immunocytochemical analyses by using antisera directed against gamma-aminobutyric acid (GABA), its synthetic enzyme glutamic acid decarboxylase (GAD67), GABA-A receptor subunits (alpha2, beta3, epsilon), estrogen receptor-alpha, and Neuropeptide Y (NPY) in the region of the VMH in embryonic mice to identify potential patterning elements for VMH formation. Cells and fibers containing GABA and GAD67 encircled the primordial VMH as early as embryonic day 13 (E13) when the cytoarchitecture of the VMH was not recognizable by Nissl stain. At E16-17 the cytoarchitecture of the VMH became recognizable by Nissl stain as GABAergic fibers invaded the nucleus, continued postnatally, and by adulthood the density of GABAergic fibers was greater inside than outside the VMH. GABA-A receptor subunit expression (beta3 by E13 and alpha2 by E15) within the primordial VMH suggested potential sensitivity to the surrounding GABA signal. Brain slices were used to test whether fibers from distal or proximal sites influenced VMH development. Coronal Vibratome slices were prepared and maintained in vitro for 0-3 days. Nissl stain analyses showed a uniform distribution of cells in the region of the VMH on the day of plating (E15). After 3 days in vitro, cellular aggregation suggesting VMH formation was seen. Nuclear formation in vitro suggests that key factors resided locally within the coronal plane of the slices. It is suggested that either GABA intrinsic to the region nearby the VMH directly influences the development and organization of the VMH, or along with other markers provides an early indicator of pattern determination that precedes the cellular organization of the VMH.

Gonadotropin-releasing hormone containing neurons and olfactory fibers during development: from lamprey to mammals

Gonadotropin releasing-hormone (GnRH) regulates the hypothalamo-pituitary-gonadal axis in all vertebrates. The vast majority of GnRH neurons are thought to be derived from progenitor cells in medial olfactory placodes. Several antibodies and lectins that recognize cell surface carbohydrates have been useful for delineating the migratory pathway from the olfactory placodes and vomeronasal organ, through the nasal compartment, and across the cribriform plate into the brain. In rats, alpha-galactosyl-linked glycoconjugates (immunoreactive with the CC2 monoclonal antibody) are expressed on fibers along the GnRH migration pathway and approximately 10% of the GnRH neuronal population. In lamprey, the alpha-galactosyl binding lectin, Grifonia simplicifolia-I (GS-1), identifies cells and fibers of the developing olfactory system. In contrast to the CC2 immunoreactive GnRH neurons in rats, the GS-1 does not label a subpopulation of presumptive GnRH neurons in lamprey. Results from these and other experiments suggest that GnRH neurons in developing lamprey do not originate within the olfactory placode, but rather within proliferative zones of the diencephalon. However, the overlap of olfactory- and GnRH-containing fibers from prolarval stages to metamorphosis, suggest that olfactory stimuli may play a major role in the regulation of GnRH secretion in lamprey throughout life. By contrast, olfactory fibers are directly relevant to the migration of GnRH neurons from the olfactory placodes in mammalian species. Primary interactions between olfactory fibers and GnRH neurons are likely transient in mammals, and so in later life olfactory modulation of GnRH secretion is likely to be indirect.