Cellular and molecular analysis of GnRH-1 neuronal development: insights into hypogonadotropic hypogonadisms. – MUSeS (Migration Under Semaphorins Stimu

The goal of my research is to gain a comprehensive view on the molecular events governing GnRH cell migration from nose to brain and its role in the establishment and maintenance of neural circuits leading to reproductive functions. During these last few years, combining mouse genetics, ex vivo manipulations, molecular biology and imaging techniques, I have investigated: i) the molecular mechanisms controlling the specification and migratory routes of GnRH cells in vivo and in vitro; ii) the genes involved in the correct pathfinding of GnRH migratory process and in the maturation of these neurons; iii) and finally, the genetic pathways leading to KS. The first goal was accomplished by functional experiments in vitro using primary and immortalized GnRH cell lines. The second goal was accomplished isolating GnRH neurons at different embryonic and postnatal stages using fluorescent cell sorting techniques (FACS) coupled with quantitative-RT-PCR experiments and by the pathohistological analysis of mutant mice for the candidate semaphorins and plexins. Finally, the third aim was tackled in collaboration with a group of genetists (Prof. Hardelin and Dr. Dode, Cochin Institute, Paris).

The relevant studies that have emerged from this project allowed showing that semaphorins are involved in the cell migration and axonal growth of GnRH neurons.
We showe that mutant mice for semaphorins or their receptors have a Kallmann syndrome-like phenotype and/or reproductive dysfunctions and we provide genetic evidence that insufficient semaphorin signaling can contribute to the KS phenotype in man. Moreover, we provide new insights into how semaphorins’ signaling contribute to morphogenesis, differentiation and plasticity of fundamental hormonal systems regulating key physiological processes such as reproduction. This regulation could act at different levels of GnRH-1 development and or maturation. For instance, some semaphorins modulate the proper migration of GnRH neurons from nose to brain during embryonic development. The same molecules, in combination with others, in the adult brain regulate the correct progression of the estrous cycle in mammals, acting on the GnRH-1 axonal plasticity and therefore on the secretion of the neurohormone. We generated several transgenic animals which carry mutations in different semaphorins genes and provided evidences that these mutations are responsible for infertility or subfertility of these animals. Strikingly, the same mutations have been found in humans affected by the genetic disease known as Kallmann’s syndrome.

Kallmann syndrome is an inherited neurodevelopmental disorder defined as the association of hypogonadotropic hypogonadism, due to gonadotropin-releasing hormone (GnRH-1) deficiency, and the inability to smell (anosmia or hyposmia), related to abnormal development of the peripheral olfactory system (olfactory nerves and olfactory bulbs). Pathohistological studies of fetuses with olfactory bulb agenesis have shown that the reproductive phenotype of KS results from a premature interruption of the olfactory, vomeronasal and terminal nerve fibers in the frontonasal region which disrupts the migration of neuroendocrine GnRH-1 cells. Barely 30% of the KS patients have mutations in any of the eight genes known so far, and current efforts thus concentrate on the identification of other genes that contribute to this disorder.

The scientific production since the beginning of this project has been quite relevant and led to important advances regarding the control of the hypothalamic-pituitary-gonadal axis by semaphorins. Thanks to the ANR financial support, we published 6 papers, most of them published in high-impact factor journals, thus supporting the high quality of the research that we’ve been able to pursue. These publications strongly enhanced my international visibility as demonstrated by the number of lectures/seminars that I was invited to give in France and abroad (see list below).

Reproduction in mammals is under the control of the hypothalamic neuropeptide GnRH-1. GnRH-1-secreting neurons, like olfactory neurons, originate in the olfactory placode, which will later develop into the olfactory epithelium. During normal development, olfactory neurons send their axons to the olfactory bulb, while GnRH-1 neurons migrate from nose to brain along these axons. Developmental alterations in this migratory process result in reproductive failure as demonstrated by patients affected by the human genetic disease known as Kallmann’s syndrome (KS) which results in anosmia (reduced or absent odor perception) and hypogonadism (lack of gonadal development). Only approximately 30% of all KS cases are attributable to mutations of known genes, suggesting that other genetic pathways might be relevant for this pathogenesis. Even though, many progresses have been made, especially during the last 15 years, in the field of developmental neuroendocrinology, the elaboration of new therapeutic strategies for reproductive disorders requires the identification of the molecules orchestrating migration and differentiation of GnRH-1 neurons.
The goal of this project is to gain a comprehensive view on the molecular events governing GnRH-1 cell migration from nose to brain and its role in the establishment of neural circuits leading to reproductive functions. Combining mouse genetics, ex vivo manipulations and imaging techniques, I propose to investigate: i) the molecular mechanisms controlling the specification and migratory routes of GnRH-1 in vivo; ii) the nature of GnRH-1 cells interactions with olfactory axons as well as their role in the correct targeting to their final destination areas. iii) In parallel, we will investigate how adult GnRH-1 axons terminals rearrange in the median eminence under the influence of different chemotropic molecules. Our combination of approaches will generate a novel framework to understand how the hypothalamic-pituitary-gonadal axis connectivity is established during normal and pathological development.