Hypothalamic neuroglial plasticity is regulated by anti-Müllerian hormone and disrupted in polycystic ovary syndrome.

BACKGROUND: Polycystic ovary syndrome (PCOS) is the most common reproductive-endocrine disorder affecting between 5 and 18% of women worldwide. An elevated frequency of pulsatile luteinizing hormone (LH) secretion and higher serum levels of anti-Müllerian hormone (AMH) are frequently observed in women with PCOS. The origin of these abnormalities is, however, not well understood. METHODS: We studied brain structure and function in women with and without PCOS using proton magnetic resonance spectroscopy (MRS) and diffusion tensor imaging combined with fiber tractography. Then, using a mouse model of PCOS, we investigated by electron microscopy whether AMH played a role on the regulation of hypothalamic structural plasticity. FINDINGS: Increased AMH serum levels are associated with increased hypothalamic activity/axonal-glial signalling in PCOS patients. Furthermore, we demonstrate that AMH promotes profound micro-structural changes in the murine hypothalamic median eminence (ME), creating a permissive environment for GnRH secretion. These include the retraction of the processes of specialized AMH-sensitive ependymo-glial cells called tanycytes, allowing more GnRH neuron terminals to approach ME blood capillaries both during the run-up to ovulation and in a mouse model of PCOS. INTERPRETATION: We uncovered a central function for AMH in the regulation of fertility by remodeling GnRH terminals and their tanycytic sheaths, and provided insights into the pivotal role of the brain in the establishment and maintenance of neuroendocrine dysfunction in PCOS. FUNDING: INSERM (U1172), European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement n° 725149), CHU de Lille, France (Bonus H).

GnRH neurons recruit astrocytes in infancy to facilitate network integration and sexual maturation.

Neurons that produce gonadotropin-releasing hormone (GnRH), which control fertility, complete their nose-to-brain migration by birth. However, their function depends on integration within a complex neuroglial network during postnatal development. Here, we show that rodent GnRH neurons use a prostaglandin D2 receptor DP1 signaling mechanism during infancy to recruit newborn astrocytes that 'escort' them into adulthood, and that the impairment of postnatal hypothalamic gliogenesis markedly alters sexual maturation by preventing this recruitment, a process mimicked by the endocrine disruptor bisphenol A. Inhibition of DP1 signaling in the infantile preoptic region, where GnRH cell bodies reside, disrupts the correct wiring and firing of GnRH neurons, alters minipuberty or the first activation of the hypothalamic-pituitary-gonadal axis during infancy, and delays the timely acquisition of reproductive capacity. These findings uncover a previously unknown neuron-to-neural-progenitor communication pathway and demonstrate that postnatal astrogenesis is a basic component of a complex set of mechanisms used by the neuroendocrine brain to control sexual maturation.

Preclinical Assessment of Leptin Transport into the Cerebrospinal Fluid in Diet-Induced Obese Minipigs.

OBJECTIVE: A minipig model was employed to explore the changes in endogenous leptin transport into the central nervous system and in hypothalamic sensitivity to exogenous leptin when individuals are placed on high-fat diet (HFD) compared with standard diet. METHODS: Serum and cerebrospinal fluid (CSF) leptin concentrations during 10 weeks of HFD versus standard diet and exogenous leptin-induced STAT3 phosphorylation in the hypothalamus of minipigs were assessed, and the hypothalamic leptin-sensitive cells were characterized by immunofluorescence. RESULTS: The efficiency of the passage of endogenous blood-borne leptin into the CSF (measured as the log [CSF:serum leptin ratio]) decreased over time in minipigs fed a HFD (ß = -0.04 ± 0.005 per kilogram of weight gain in HFD; P < 0.0001), while it remained stable in minipigs fed a standard diet. However, the ability of peripherally administered leptin to activate its receptor in hypothalamic neurons was preserved in obese minipigs at 10 weeks of HFD. CONCLUSIONS: Together, these data are consistent with the existence of an early-onset tranport deficiency for endogenous circulating leptin into the brain in individuals developing obesity, preceding the acquisition of hypothalamic leptin resistance. Although additional studies are required to identify the underlying mechanisms, our study paves the way for the development of new preclinical pharmacological models targeting the restoration of the shuttling of peripheral leptin into the central nervous system to manage obesity.

Seasonal reorganization of hypothalamic neurogenic niche in adult sheep.

Neurogenesis is the process by which new neurons are generated. This process, well established during development, persists in adulthood owing to the presence of neural stem cells (NSCs) localized in specific brain areas called neurogenic niches. Adult neurogenesis has recently been shown to occur in the hypothalamus, a structure involved in the neuroendocrine regulation of reproduction and metabolism, among others. In the adult sheep-a long-lived mammalian model-we have previously reported the existence of such a neurogenic niche located in the hypothalamic arcuate nucleus and the median eminence. In addition, in this seasonal species, the proliferation as well as neuroblasts production varies depending on the time of the year. In the present study, we provide a better characterization of the hypothalamic neurogenic niche by identifying the main components (NSCs, migrating cells, glial cells and blood vessels) using immunohistochemistry for validated markers. Then, we demonstrate the strong sensitivity of these various neurogenic niche components to the season, particularly in the arcuate nucleus. Further, using an electron microscopic approach, we reveal the cellular and cytoarchitectural reorganization of the arcuate nucleus niche following exposure to contrasting seasons. This study provides evidence that the arcuate nucleus and the median eminence contain two independent niches that react differently to the season. In addition, our results support the view that the cytoarchitectural organization of the sheep arcuate nucleus share comparable features with the structure of the subventricular zone in humans and non-human primates.

Semaphorin7A regulates neuroglial plasticity in the adult hypothalamic median eminence.

Reproductive competence in mammals depends on the projection of gonadotropin-releasing hormone (GnRH) neurons to the hypothalamic median eminence (ME) and the timely release of GnRH into the hypothalamic-pituitary-gonadal axis. In adult rodents, GnRH neurons and the specialized glial cells named tanycytes periodically undergo cytoskeletal plasticity. However, the mechanisms that regulate this plasticity are still largely unknown. We demonstrate that Semaphorin7A, expressed by tanycytes, plays a dual role, inducing the retraction of GnRH terminals and promoting their ensheathment by tanycytic end feet via the receptors PlexinC1 and Itgb1, respectively. Moreover, Semaphorin7A expression is regulated during the oestrous cycle by the fluctuating levels of gonadal steroids. Genetic invalidation of Semaphorin7A receptors in mice induces neuronal and glial rearrangements in the ME and abolishes normal oestrous cyclicity and fertility. These results show a role for Semaphorin7A signalling in mediating periodic neuroglial remodelling in the adult ME during the ovarian cycle.

Brain endothelial cells control fertility through ovarian-steroid-dependent release of semaphorin 3A.

Neuropilin-1 (Nrp1) guides the development of the nervous and vascular systems, but its role in the mature brain remains to be explored. Here we report that the expression of the 65 kDa isoform of Sema3A, the ligand of Nrp1, by adult vascular endothelial cells, is regulated during the ovarian cycle and promotes axonal sprouting in hypothalamic neurons secreting gonadotropin-releasing hormone (GnRH), the neuropeptide controlling reproduction. Both the inhibition of Sema3A/Nrp1 signaling and the conditional deletion of Nrp1 in GnRH neurons counteract Sema3A-induced axonal sprouting. Furthermore, the localized intracerebral infusion of Nrp1- or Sema3A-neutralizing antibodies in vivo disrupts the ovarian cycle. Finally, the selective neutralization of endothelial-cell Sema3A signaling in adult Sema3aloxP/loxP mice by the intravenous injection of the recombinant TAT-Cre protein alters the amplitude of the preovulatory luteinizing hormone surge, likely by perturbing GnRH release into the hypothalamo-hypophyseal portal system. Our results identify a previously unknown function for 65 kDa Sema3A-Nrp1 signaling in the induction of axonal growth, and raise the possibility that endothelial cells actively participate in synaptic plasticity in specific functional domains of the adult central nervous system, thus controlling key physiological functions such as reproduction.

Tanycytic VEGF-A boosts blood-hypothalamus barrier plasticity and access of metabolic signals to the arcuate nucleus in response to fasting.

The delivery of blood-borne molecules conveying metabolic information to neural networks that regulate energy homeostasis is restricted by brain barriers. The fenestrated endothelium of median eminence microvessels and tight junctions between tanycytes together compose one of these. Here, we show that the decrease in blood glucose levels during fasting alters the structural organization of this blood-hypothalamus barrier, resulting in the improved access of metabolic substrates to the arcuate nucleus. These changes are mimicked by 2-deoxyglucose-induced glucoprivation and reversed by raising blood glucose levels after fasting. Furthermore, we show that VEGF-A expression in tanycytes modulates these barrier properties. The neutralization of VEGF signaling blocks fasting-induced barrier remodeling and significantly impairs the physiological response to refeeding. These results implicate glucose in the control of blood-hypothalamus exchanges through a VEGF-dependent mechanism and demonstrate a hitherto unappreciated role for tanycytes and the permeable microvessels associated with them in the adaptive metabolic response to fasting.

Kisspeptin-GPR54 signaling in mouse NO-synthesizing neurons participates in the hypothalamic control of ovulation.

Reproduction is controlled in the brain by a neural network that drives the secretion of gonadotropin-releasing hormone (GnRH). Various permissive homeostatic signals must be integrated to achieve ovulation in mammals. However, the neural events controlling the timely activation of GnRH neurons are not completely understood. Here we show that kisspeptin, a potent activator of GnRH neuronal activity, directly communicates with neurons that synthesize the gaseous transmitter nitric oxide (NO) in the preoptic region to coordinate the progression of the ovarian cycle. Using a transgenic Gpr54-null IRES-LacZ knock-in mouse model, we demonstrate that neurons containing neuronal NO synthase (nNOS), which are morphologically associated with kisspeptin fibers, express the kisspeptin receptor GPR54 in the preoptic region, but not in the tuberal region of the hypothalamus. The activation of kisspeptin signaling in preoptic neurons promotes the activation of nNOS through its phosphorylation on serine 1412 via the AKT pathway and mimics the positive feedback effects of estrogens. Finally, we show that while NO release restrains the reproductive axis at stages of the ovarian cycle during which estrogens exert their inhibitory feedback, it is required for the kisspeptin-dependent preovulatory activation of GnRH neurons. Thus, interactions between kisspeptin and nNOS neurons may play a central role in regulating the hypothalamic-pituitary-gonadal axis in vivo.