Mammalian sex determination and testicular function
Our long-standing interest lies in the elucidation of the molecular mechanisms regulating gonadal differentiation and testicular function. We use molecular, cellular and mouse functional genomics to investigate the complex gene networks that regulate primary sex determination, testis development and functions.
More precisely, we are investigating the following topics:
Congenital defects such as disorders of sexual differentiation (DSD) are rare diseases that present considerable challenges for physicians, parents and affected individuals. Unfortunately a large fraction of human cases of DSD are unexplained, clearly indicating that a significant number of genes (or regulatory regions of known genes) involved in sex determination remain to be identified. There are two classical and complementary ways to identify and characterize the function of genes involved in the process of sex determination: the first relies on in vivo gain- and loss-of-function experiments in mice. In fact, mouse functional genomics has remained over the years the gold standard for candidate gene validation/characterization. In parallel, the identification of novel pathogenic mutations in affected patients with unresolved cases of DSD has also been a successful approach to identify new genes involved in testicular or ovarian differentiation processes.
We are using cutting edge techniques in human and mouse genetics, to investigate the complex mechanisms of gonadal differentiation. Our first axis of research relies on human genetics: we apply genetic tools such as CGH arrays and exome sequencing to a cohort of unresolved cases of patient suffering from 46,XY DSD, 46 XY complete gonadal dysgenesis (CGD) and 46,XX DSD to reveal new genes and pathways involved in gonadal development. As a second step, we use mouse functional genetics to validate and characterize the function of candidate genes involved in the process of sex determination
Understanding how distinct cell populations diverge from multi-potent progenitors is a major goal in developmental biology. The adrenal gland and the gonads represent the two major steroidogenic organs in mammals, and are absolutely central to the regulation of body homeostasis, metabolism, sexual development and reproduction. Both organs share a common developmental origin, the adrenogonadal primordium (AGP), composed of a population of uncharacterized SF1+ progenitor cells. AGP specification and differentiation into testis or ovaries, and adrenal glands represents a unique model system for studying cell fate decisions during mammalian organ development. While major outstanding questions regarding the identity of these progenitors and the transcriptional events that drive cell lineage specification remain, traditional experimental approaches, such as genome-wide transcription analysis, are rendered completely ineffective by the very small number of progenitor cells.
Here, we build on our previous achievements and combine single-cell RNA sequencing and mouse lineage tracing experiments to investigate how cell-fate decisions are made during testicular, ovarian and adrenal development. We plan to identify progenitor cells, map lineage progression and cell type diversification, and reconstruct the genetic programs controlling their differentiation into testicular, ovarian and adrenocortical cells.
Figure:Model of Nr5a1-GFP+ Cell-Lineage Specification during Male Sex Determination (Stévant et al Cell Rep. 2018)
This project should provide a comprehensive transcriptional blueprint outlining steroidogenic organ specification and differentiation. These results should have also broad physiological and clinical applications, contributing to a detailed understanding of both normal physiology and development, and impairment and disturbances in sexual development, fertility and endocrinology.
Male reproductive health has been declining since World War II in many European countries. With the current trend for couples to have children later in life, lower sperm quality combined with the declining fertility of older women are impairing spontaneous conceptions and reducing couple’s fertility, resulting in significant social and financial challenges for modern societies. The drop of semen quality occurs over a short timescale which suggests that the causes must be lifestyle and environmental rather than genetics.
In Switzerland, no information is available on male reproductive health including semen parameters and exposure to EDCs. A large-scale epidemiological study on the male reproductive health in Switzerland was therefore launched with the aim to collect data on 3000 conscripts originating from all regions of the country during their visit to the national army’s recruitment centers. Our goals are first to complete data collection. We plan then to analyze all available data in details - to describe the distribution of seminal and hormonal characteristics and the prevalence of male genital disorders; - to identify their variability according to the places of residence and socio-demographic characteristics of the volunteers and their parents; to compare the distribution of fertility indicators to those reported in other countries of the world.Finally, we will assess the associations between environmental exposure variables and male reproductive health outcomes (seminal quality, hormones). The following exposure variables will be considered: a) lifestyle (tobacco, alcohol, recreational drugs,…) and occupational factors in volunteers and in parents at the time of conception, b) chemical exposures determined by the measurement of environmental chemicals in biological fluids as above mentioned.
Today, infertility rates are exceptionally high and in many countries infertility affects 1 in 7 couples of reproductive age. Involuntary childlessness due to suboptimal reproductive capacities or infertility of the male partner occurs in more than 40% of infertile couples. Spermatogenesis is a highly coordinated developmental process by which diploid spermatogonial stem cells develop into mature haploid spermatozoa. This complex process is governed by an intricate system of endocrine and paracrine factors. Advances in the management of male infertility will only occur when our understanding of the regulation of spermatogenesis has progressed further. They are considerable data indicating that the complex regulation of spermatogenesis is finely controlled by autocrine and paracrine factors at the testicular level. It includes the insulin-like family of growth factor composed of insulin and the insulin-like growth factor I (IGF1) and II (IGF2).
We initiated few years ago a project aimed at dissecting in each relevant cell-type of the testis the role played by the IGF system. For this purpose, we are using mouse functional genetics and the Cre/Lox system of conditional gene targeting to delete in a cell-specific manner either the ligands , their receptors or some key downstream signaling effectors to decipher in vivo the roles played by IGF system in mediating testis development, spermatogenesis and steroidogenesis.
The ultimate goal is to provide a better understanding of intricate cell–cell interactions mediated by the IGF system within the whole testis and their physiological roles and functions. These results may have broad physiological and clinical applications, contributing to detailed understanding of both normal testis physiology and development, and impairment and disturbances in fertility and endocrinology.