A microRNA switch regulates the rise in hypothalamic GnRH production before puberty

Abstract

A sparse population of a few hundred primarily hypothalamic neurons forms the hub of a complex neuroglial network that controls reproduction in mammals by secreting the 'master molecule' gonadotropin-releasing hormone (GnRH). Timely postnatal changes in GnRH expression are essential for puberty and adult fertility. Here we report that a multilayered microRNA-operated switch with built-in feedback governs increased GnRH expression during the infantile-to-juvenile transition and that impairing microRNA synthesis in GnRH neurons leads to hypogonadotropic hypogonadism and infertility in mice. Two essential components of this switch, miR-200 and miR-155, respectively regulate Zeb1, a repressor of Gnrh transcriptional activators and Gnrh itself, and Cebpb, a nitric oxide-mediated repressor of Gnrh that acts both directly and through Zeb1, in GnRH neurons. This alteration in the delicate balance between inductive and repressive signals induces the normal GnRH-fuelled run-up to correct puberty initiation, and interfering with this process disrupts the neuroendocrine control of reproduction.

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“…Our results show that, during an infantile "critical period" lasting just a few days, a dramatic switch in the microRNA expression pattern of GnRH neurons inverts the balance between inductive and repressive signals, triggering increased hypothalamic GnRH expression and controlling the crucial transition from the early infantile phase, when its levels are low, to the GnRH-fuelled run-up to puberty (Figure 1) [2]. This infantile "critical period" corresponds to a centrally-driven gonad-independent activation of the HPG axis and the resulting surge in gonadotropin levels (on postnatal days 10-16 in mice).…”

“…However, despite the identification of several factors known to influence GnRH production and release, the molecular mechanisms controlling the postnatal increase in GnRH expression and its timing have remained a mystery until now. We have recently made a significant advance in understanding these processes by uncovering a specific set of microRNAs, embedded in the genetic network controlling GnRH transcription, that drive the infantile switch from the repression to the induction of GnRH expression [2].…”

“…Specifically, we have shown that during this critical period, a switch in the expression of several microRNA species in the hypothalamus in turn flips a switch in a multilayered array of GnRH gene activators and repressors, permitting the sustained increase of the neurohormone required for subsequent sexual maturation [2]. Two microRNA species act as the linchpins of this process: the miR-200/429 family, which is not only upregulated during the critical period but selectively enhanced in GnRH neurons, and miR-155, which appears to act on other hypothalamic cell types as well, and mediates, for example, the effects of a release of nitric oxide known to occur concomitantly upstream of GnRH neurons .…”

Loss of microRNA-7a2 induces hypogonadotropic hypogonadism and infertility

“…Our results show that the hypogonadotropic hypogonadism in mir-7a2 KO mice is highly unlikely to be causally linked to miR-7 function in hypothalamic neurons because (a) absolute quantification of miR-7 levels demonstrates that miR-7a expression is 10-fold lower in hypothalamus as compared with pituitary and therefore, based on miR-7b levels, in a range where no phenotype and target gene regulation is expected; (b) hypothalamic GnRH expression levels are indistinguishable between mir-7a2 KO and control littermates; (c) mir-7a2 KO mice show unaltered olfaction and normal expression of hypothalamic genes that are known to be involved in hypothalamus-dependent hypopituitarism; and (d) expression of the GnRH receptor in the pituitary, which is downregulated in animal models of GnRH deficiency, is expressed at similar levels in mir-7a2 KO and WT control mice (48). In addition, miR-7 is not among the 53 miRNAs that are enriched in GnRH neurons compared with the surrounding nonGnRH cells (19), further suggesting that miR-7 is unlikely to have a specific function in these neurons.…”

“…Moreover, the importance of individual miRNAs in normal physiology and disease has been established by the discovery of mutations in miRNAs and their targets (16)(17)(18). Recently, the collective role of miRNAs in the HPG axis was demonstrated by genetic ablation of Dicer1 in GnRH neurons (19) and gonadotrophs (20), resulting in hypogonadism and infertility. Furthermore, miR-200b and miR-429, both members of the miR-200 family, as well as their target, ZEB1, are required for female ovulation and reproduction in mice (21).…”