Effects of muscle-specific UCP1 overexpression on spermatogenesis and mitochondrial sirtuin expression in the testis

Background: In a society where obesity is a growing problem, science looks for possibilities to balance energy intake and energy expenditure. Increasing energy expenditure by exercising would be one way to achieve this. However, our western society has evolved to restrain from exercising, which is actually part of the underlying obesity pandemic. An alternative option would be to uncouple oxidative phosphorylation and shunt excess energy into heat production. Since obesity is possibly linked with reduced fertility, we wanted to study whether increasing expenditure in skeletal muscle would impact the male reproductive system. For this, we used a mouse model where UCP1 (uncoupling protein 1) was overexpressed specifically in skeletal muscle. It was previously shown that whole-body energy expenditure was increased, but at the same time energy availability to other organs than skeletal muscle was reduced. The mitochondrial sirtuins, SIRT3 and SIRT4, are located in mitochondria and control metabolic pathways via their NAD+-dependent deacylation activity and could therefore play a role in controlling energy balance in the testis.
Methods: We used testicular tissue from multiple mouse transgenic lines that had UCP1 overexpression in skeletal muscle in combination with whole-body FGF21 knock-out (KO). FGF21 functions as a hormone that can signal energy deprivation to induce a coping mechanism and was previously shown to be highly expressed in UCP1 transgenic mice. Spermatogenesis was analysed by histology, whereas cell metabolism was studied through immunohistochemical staining for SIRT3 and SIRT4.
Results and Discussion: Histological analysis showed that UCP1 overexpression did not negatively affect reproduction, although some small changes in germ cell apoptosis and seminiferous tubule epithelial cell layer thickness were present, specifically in stage IX of spermatogenesis. SIRT3 staining was increased in both UCP1 transgenic mice and UCP1 transgenic/FGF21 KO mice. SIRT4 staining showed opposite results with high staining in the testis of wild type mice and less intense SIRT4 staining in both UCP1 transgene groups. Staining was most prominently present in Leydig cells, interstitial cells that contain large amounts of mitochondria, necessary for steroid production. Since the opposing expression of SIRT3 and SIRT4 has also been observed in liver and adipose tissue to regulate energy balance and lipid metabolism, it could be possible that SIRT3/SIRT4 regulate similar molecular pathways in the testis. This study thus shows that overexpression of UCP1 in skeletal muscle as well as a combination of UCP1 transgene/FGF21 KO mice induced an adaptive response in testis cells through upregulation of SIRT3 and downregulation of SIRT4, without having severe effects on spermatogenesis.