Amazingly, it is estimated that greater than 4% of protein encoding genes in mammals are expressed exclusively during the processes of spermatogenesis or spermiogenesis. Our lab is currently directing efforts to use cultures of sperm cells as powerful vectors to unravel molecular mechanisms that govern sperm maturation. The unique expression profile of such “sperm-specific” genes makes them ideal candidate targets for new male contraceptives. To first determine if such candidate genes are required for fertilization, we have constructed a repertoire of “DNA reporter vectors” to allow these candidate genes to be stably repressed within spermatogonial lines. The generated “reporter” spermatogonial lines provide our lab with the advantage of a unique genetic system to rapidly screen for essential spermatogenic factors during culture in vitro, and during culture within recipient rat testes, thus eliminating a primary requirement to generate transgenic rodent colonies to study candidate gene function. Additionally, we have unique transgenic rat models available with spermatogenic cell-specific reporters that can be used to complement our genetic screens (Figure 6).
In total, >5% of the male population is infertile, with male factors being attributed to nearly 50% of all infertile couples (7-8). On a global scale, >1% of all males are inflicted with a severe defect in sperm production termed azoospermia (9-11). Fundamentally, because azoospermia results in an inability to reproduce by natural mating, it seems enigmatic as to why this disease remains so prevalent in the human population. Such an epidemiological trend clearly points to the existence of potent environmental factors that disrupt the process of sperm production (i.e. spermatogenesis) (10, 12), or a substantial repertoire of gene mutations that could render one sterile, but otherwise healthy (13). This is in fact true in mammalian species where mutations in >100 distinct genes have already been shown to disrupt sperm development or function in mice (13-14).
In addition to direct genetic effectors of germ cell function, another prominent concern resides in the fact that each year an increasing number of young women and men are left infertile due to the potent, toxic side-effects of life-saving cancer therapies on gametogenesis (12, 15-16). It is estimated that ~30% of male childhood cancer survivors are inflicted with azoospermia due to degenerative side effects of chemotherapy on spermatogenesis (15). Bilateral cryptorchidism, which is the lack of testicular descent after birth, is another major cause of azoospermia in about 1/500 males born (11). Thus, the best hope for these boys to someday father their own children currently depends on the cryopreservation of their spermatogonial stem cells. Still, this is under anticipation that cellular therapies for curing spermatogenic arrest are established as a clinical option early enough in their future.
Ostensibly, the ability to propagate human spermatogonial lines in culture, prior to using them to produce functional spermatozoa by transplanting them back into the testes of their own donor, presents a clear strategy to cure many existing types of male infertility (Figure 7). Establishing these culture methods would overcome the foreseeable barrier of obtaining enough pure, donor spermatogonia from a minimally invasive testis biopsy to effectively restore a patient’s fertility. To date, a majority of studies in this area have been performed in mice (3, 17-19). However, due in large part to the multipotent nature of isolated germline stem cells in culture (20-23), and the potential for introducing defective cells back into patients, it will be informative to evaluate therapeutic efficacies of spermatogonia in additional pre-clinical mammalian models, as we recently reported, using azoospermic DAZL-deficient rats (24).