Seasonal Changes In Immunoreactivities Of Activin Signaling Components In The Wild Ground Squirrel(Citellus Dauricus Brandt) Testis

Wildlife possesses many fascinating physiological features, such as seasonal reproduction and hibernation. As the environmental condition (i.e. photoperiod and temperature) alters, there are breeding and nonbreeding seasons in wild animals of the temperate regions. The wild ground squirrel, which is a typical seasonal breeder, usually inhabits in semi-desert grasslands in north China. Their breeding season lasts from early April to the middle of May. Both inhibin and activin belong to the transforming growth factor β (TGF-β) superfamily. They are mainly secreted by testes in males and are postulated to have paracrine and autocrine roles in the regulation of steroidogenesis and spermatogenesis in addition to their endocrine regulatory effect on FSH secretion. Previously, we have investigated the seasonal changes in testicular weight, size and seminiferous diameters by histological observations and have detected the distributions of inhibin/activin a, βA and βb subunits in different reproductive seasons. Our results showed significant differences between breeding and nonbreeding seasons in terms of testicular morphology and histology. To test the hypothesis that inhibin/activin may play an important autocrine and/or paracrine role(s) in the spermatogenesis and seasonal reproduction of the wild ground squirrels, we have observed the immunolocalization and protein expression levels of activin signaling components (ActRIIB, phospho-SMAD2/3, SMAd4and SMAD7) in testes of both breeding and nonbreeding seasons, by immunohistochemistry and western blotting. Our data indicated that the activin signaling components are widely expressed in the Sertoli cells, Leydig cells and spermatogenic cells, particularly the round spermatids in the breeding season, while in the nonbreeding season they are localized mainly in the spermatogonia cytoplasm. The expression levels of all signaling components are generally higher in testis of the breeding season than in the nonbreeding season. In conclusion, our data suggest that activin signal, mediated by its receptor and Smads, might play an important paracrine role in the seasonal spermatogenesis of the wild ground squirrel. Moreover, the localization of these proteins might suggest that Activin/Smad signaling might play distinct roles in the seasonal spermatogenesis of the wild ground squirrel.