Developmental expression of SRC-related tyrosine kinases in the mouse testis

An increase in protein tyrosine phosphorylation occurs during sperm capacitation in numerous species. The involvement of Src-related tyrosine kinases in this phenomenon has been demonstrated using different inhibitors specifically targeting this family of enzymes. In mammals, this group of nonreceptor tyrosine kinases is made up of 8 members with similar SRC homology domain 3 (SH3) and SH2 domains. Although some members of this group of enzymes can compensate for one another, showing some redundancy, each is unique and may perform specific functions during male germ cell development. To further characterize the importance of Src-related tyrosine kinases in the events leading to proper sperm formation, and because no inhibitor affecting a single gene product exists, expression of Src, Yes1, Fyn, Lyn, Lck, Hck, Blk, and Fgr was assessed by real-time polymerase chain reaction in developing mouse testes and in enriched populations of mouse spermatogenic cells, revealing distinct expression profiles for each kinase during testis development and in isolated male germ cells. Immunolocalization of SRC, LYN, and HCK in adult mouse testes as well as in mature spermatozoa further confirmed differential localization of these kinases during spermatogenesis. Although mRNA levels of these latter kinases were higher in spermatogonia and spermatocytes than in spermatids, protein levels were highest in spermatids, suggesting delayed transcript translation. Taken together, these results clearly show an uneven expression of each kinase in different spermatogenic cells, indicating that each member may play a different role during spermatogenesis, in addition to highlighting the complexity of Src-related kinase expression regulation in male germ cells. Furthermore, differential localization of these tyrosine kinases in mature spermatozoa also suggests a specific role for each member in sperm function and integrity.