Ovulation is triggered by a cyclical modulation of gonadotropes into a hyperexcitable state.

Gonadotropes in the anterior pituitary gland are essential for fertility and provide a functional link between the brain and the gonads. To trigger ovulation, gonadotrope cells release massive amounts of luteinizing hormone (LH). The mechanism underlying this remains unclear. Here, we utilize a mouse model expressing a genetically encoded Ca2+ indicator exclusively in gonadotropes to dissect this mechanism in intact pituitaries. We demonstrate that female gonadotropes exclusively exhibit a state of hyperexcitability during the LH surge, resulting in spontaneous [Ca2+]i transients in these cells, which persist in the absence of any in vivo hormonal signals. L-type Ca2+ channels and transient receptor potential channel A1 (TRPA1) together with intracellular reactive oxygen species (ROS) levels ensure this state of hyperexcitability. Consistent with this, virus-assisted triple knockout of Trpa1 and L-type Ca2+ subunits in gonadotropes leads to vaginal closure in cycling females. Our data provide insight into molecular mechanisms required for ovulation and reproductive success in mammals.

Bitter taste cells in the ventricular walls of the murine brain regulate glucose homeostasis.

The median eminence (ME) is a circumventricular organ at the base of the brain that controls body homeostasis. Tanycytes are its specialized glial cells that constitute the ventricular walls and regulate different physiological states, however individual signaling pathways in these cells are incompletely understood. Here, we identify a functional tanycyte subpopulation that expresses key taste transduction genes including bitter taste receptors, the G protein gustducin and the gustatory ion channel TRPM5 (M5). M5 tanycytes have access to blood-borne cues via processes extended towards diaphragmed endothelial fenestrations in the ME and mediate bidirectional communication between the cerebrospinal fluid and blood. This subpopulation responds to metabolic signals including leptin and other hormonal cues and is transcriptionally reprogrammed upon fasting. Acute M5 tanycyte activation induces insulin secretion and acute diphtheria toxin-mediated M5 tanycyte depletion results in impaired glucose tolerance in diet-induced obese mice. We provide a cellular and molecular framework that defines how bitter taste cells in the ME integrate chemosensation with metabolism.

Intra-pituitary follicle-stimulating hormone signaling regulates hepatic lipid metabolism in mice.

Inter-organ communication is a major hallmark of health and is often orchestrated by hormones released by the anterior pituitary gland. Pituitary gonadotropes secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH) to regulate gonadal function and control fertility. Whether FSH and LH also act on organs other than the gonads is debated. Here, we find that gonadotrope depletion in adult female mice triggers profound hypogonadism, obesity, glucose intolerance, fatty liver, and bone loss. The absence of sex steroids precipitates these phenotypes, with the notable exception of fatty liver, which results from ovary-independent actions of FSH. We uncover paracrine FSH action on pituitary corticotropes as a mechanism to restrain the production of corticosterone and prevent hepatic steatosis. Our data demonstrate that functional communication of two distinct hormone-secreting cell populations in the pituitary regulates hepatic lipid metabolism.

Transient receptor potential (TRP) channel function in the reproductive axis.

Transient receptor potential (TRP) channels play important functional roles in the signal transduction machinery of hormone-secreting cells and have recently been implicated in reproductive physiology. While expression studies have demonstrated TRP channel expression at all levels of the hypothalamic-pituitary-gonadal (hpg) axis, functional details about TRP channel action at the level of the individual cells controlling reproduction are just beginning to emerge. Canonical TRP (TRPC) channels are prominently expressed in the reproductive center of the neuroendocrine brain, i.e. in kisspeptin and gonadotropin-releasing hormone (GnRH) neurons. Kisspeptin neurons are depolarized by leptin via activation of TRPC channels and kisspeptin depolarizes GnRH neurons through TRPC4 activation. Recent studies have functionally identified TRPC channels also in gonadotrope cells in the anterior pituitary gland, which secrete gonadotropins in response to GnRH and thus regulate gonadal function. TRP channel expression in these cells exhibits remarkable plasticity and depends on the hormonal status of the animal. Subsequent functional analyses have demonstrated that TRPC5 in gonadotropes contributes to depolarization of the plasma membrane upon GnRH stimulation and increases the intracellular Ca2+ concentration via its own Ca2+ permeability and via the activation of voltage-gated Ca2+ channels. However, conditional gene targeting experiments will be needed to unambiguously dissect the physiological role of TRPC channels in the different cell types of the reproductive axis in vivo.

Functional Characterization of Transient Receptor Potential (TRP) Channel C5 in Female Murine Gonadotropes.

Gonadotrope cells in the anterior pituitary gland secrete gonadotropins regulating gonadal function in mammals. Recent results have implicated transient receptor potential (TRP) cation channels in pituitary physiology; however, if and how TRP channels contribute to gonadotrope function is not known. Here, we report that 14 out of 28 TRP channels encoded in the mouse genome are expressed in murine gonadotropes with highest expression levels found for canonical TRP (TRPC) channel 5 in juvenile females. We show that TRP channel expression in these cells exhibits considerable plasticity and that it depends on the sex and the developmental and hormonal status of the animal. We then combine different genetic strategies including genetic confocal Ca2+ imaging in whole-mount pituitary gland preparations to characterize TRPC5 channel function in gonadotropes from juvenile females. We show that the TRPC5 agonist Englerin A activates a cytosolic Ca2+ signal and a whole-cell current in these cells, which is absent in TRPC5-deficient mice, and demonstrate that TRPC5 forms functional heteromultimers with TRPC1 in gonadotropes. We further show that the Englerin A-activated TRPC5-dependent Ca2+ signal is mediated by Ca2+ influx both via TRPC5 and via l-type voltage-gated Ca2+ channels, activated by the depolarization through TRPC5-mediated cation influx. Finally, we demonstrate that the gonadotropin-releasing hormone (GnRH)-mediated net depolarization is significantly reduced in gonadotropes isolated from TRPC5-deficient mice. In conclusion, our data suggest that TRPC5 contributes to depolarization of the plasma membrane in gonadotropes upon GnRH stimulation and increases the intracellular Ca2+ concentration via its own Ca2+ permeability and via the activation of voltage-gated Ca2+ channels.

Gene Targeting in Neuroendocrinology.

Research in neuroendocrinology faces particular challenges due to the complex interactions between cells in the hypothalamus, in the pituitary gland and in peripheral tissues. Within the hypothalamus alone, attempting to target a specific neuronal cell type can be problematic due to the heterogeneous nature and level of cellular diversity of hypothalamic nuclei. Because of the inherent complexity of the reproductive axis, the use of animal models and in vivo experiments are often a prerequisite in reproductive neuroendocrinology. The advent of targeted genetic modifications, particularly in mice, has opened new avenues of neuroendocrine research. Within this review, we evaluate various mouse models used in reproductive neuroendocrinology and discuss the different approaches to generate genetically modified mice, along with their inherent advantages and disadvantages. We also discuss a variety of versatile genetic tools with a focus on their potential use in reproductive neuroendocrinology.