| Testicular homeostasis requires tightly regulated germ cell proliferation to be balanced by germ cell apoptosis. This homeostatic regulation is the sum of intracellular, autocrine, paracrine, and endocrine signaling systems working in concert to optimize sperm output. Due to a finite amount of structural and nutritional support from Sertoli cells, germ cell overpopulation derails testicular homeostasis and leads to atrophic seminiferous tubules. The apoptotic machinery that controls testicular homeostasis and the testicular response to injury is the focus of this work.; Ubiquitin and NF-kappaB are two important modulators of cellular response and the two factors investigated here. Small ubiquitin polypeptides can form enzyme-dependent chains covalently bound to proteins to signal their degradation by the 26S proteasome. The protein degradation attributes of ubiquitin can be blocked through mutation of lysine 48 to arginine (K48R). The transcription factor NF-kappaB is a key regulator of apoptotic gene expression and has an overall anti-apoptotic function in most cell types. To assess the roles of ubiquitin and NF-kappaB in the testis, these systems were inhibited in two mouse models. Transgenic mice expression a K48R mutant dominant negative ubiquitin transgene (K48R Ub) and IkappaB-Kinase activating complex ( IkkbetaF/Delta) disrupted NF-kappaB knockdown mice did not have altered baseline spermatogenesis compared to controls.; To unmask the role of ubiquitin and NF-kappaB in the testis, we exposed K48R Ub and IkkbetaF/Delta NF-kappaB knockdown mice to testicular injury. Expression of the K48R ubiquitin transgene conferred resistance to germ cell apoptosis and testicular atrophy from experimental cryptorchid and aging, suggesting a proapoptotic role for K48-linked ubiquitin. Similarly, IkkbetaF/Delta mice were resistant to gamma-radiation-induced germ cell apoptosis, also indicating a pro-apoptotic role for NF-kappaB in testis. Interestingly, neither of these gene alterations was deleterious to normal testicular homeostasis, and only through cellular stress could their biological functions be unmasked. |
