Thyroid hormone -mediated control of Sertoli cell proliferation – TimeOfLife2

In agronomy, research in male reproduction is intensive in the aim to optimize sperm production in males with desirable genetic traits. In all mammals, the number of Sertoli cells (SC), the unique somatic cells of the seminiferous tubules, directly conditions the spermatogenesis yield. This number is determined before puberty, during the prepubertal life, and depends on endocrine regulation involving both Folliculo-Stimulating-Hormone FSH and Thyroid hormone T3. T3 possesses several nuclear receptors that repress transcription when unliganded (because of binding with co-repressors). The effect of T3 on testis is likely mediated by its ubiquitous receptor TRa1, as shown by TRa1KO that leads to a characteristic increase in testis weight and sperm production (like observed in hypothyroidism). Whether this High Testicular Size (HTS) phenotype observed in TRa1KO is the result of either direct perturbations at the testicular level, involving Sertoli cells (SC) and/or Leydig cells (LC), or indirect perturbations at the central level, involving pituitary dysfunction (FSH/LH production) is still unkonw. To answer this question, the two partners propose to create transgenic mice that express a dominant-negative mutant of TRa1 named TRAmi. This mutant receptor has been produced by partner 2 (F Flamant); its avoids activation of the transcription by inhibiting interaction with co-activators, and can be induced in specific cells (using Cre-lox system). To decipher T3 regulation of SC proliferation, two lines have already been created in partner 1’s lab:
1) TRAmi-SC line that expresses TRAmi in SC (by crossing with AMH-cre); the phenotype has already been achieved in the adult;
2) TRAmi-SC/LC line (in production) that expresses TRAmi both in SC and LC (by crossing with AROMATASE-cre line). Interestingly, TRAmi-SC line presents a HTS phenotype, substantiating the hypothesis that SC TRa1 participates, at least in part, in T3 action for controlling SC proliferation. However, the putative contribution of LC remains to be explored and will be evaluated using TRAmi-SC/LC line (in production); besides, the hypothesis of a pituitary contribution will be deciphered by creating a new line expressing TRAmi specifically in gonadotrope cells (TRAmi-GC line), by crossing TRAmi line with ßLH-cre line. In all these lines, we will explore testicular and pituitary functions (post-natal development, endocrine status, SC proliferation index). Then, the second (and independent) aim of the project is to decipher the genomic T3-dependent regulation of SC proliferation: which genes are activated/repressed by TRa1 in SC? As few T3-regulated genes are known in SC, we plan to identify a large set of these genes using transcriptomic investigations (microarrays) to compare gene expression profiles of SC in TRAmi-SC males versus controls. Some of these genes are expected to be FSH-mediated signaling intermediates and/or cell cycle regulators. The present project, which mainly relies on in vivo investigations by exploiting specific and unique transgenic mice obtained with the CRE-Lox system, takes advantage of the partners’ expertise on the murine genetic manipulation, combined with high knowledge on testis function and its endocrine regulation. This project is expected to help decipher cellular mechanisms and genes involved in SC proliferation/differentiation, and to understand how the number of SC is established in a developing male in the mouse. As perspectives of the project, we will propose 1) to explore FSH/T3 cross-talking (the two hormones having opposite effects on SC proliferation during the prebupertal period); 2) to initiate phylogenic and comparative genomic analyses to extend to other species (farm species) the data concerning T3-regulated genes in mouse SC. Importantly, the tasks of the project are independent and will be conducted in parallel.