GUANIDINOACETATE METHYLTRANSFERASE IN THE MOUSE - EXTENSIVE EXPRESSION IN SERTOLI CELLS OF TESTIS AND IN MICROVILLI OF CAPUT EPIDIDYMIS

Guanidinoacetate methyltransferase (GAMT) catalyzes the last step of t he biosynthetic pathway to creatine (Cr), the transfer of a methyl gro up from S-adenosylmethionine to guanidinoacetate. Despite the importan ce of this methylation reaction in cellular energy metabolism and in m ethyl group metabolism, a systematic study of the expression and cellu lar localization of this enzyme is lacking. As a means for determining why the testis and seminal vesicles contain high levels of Cr, we ana lyzed GAMT protein and mRNA levels in mouse tissues by Western and Nor thern blot analyses. The results show that GAMT was regulated at tissu e-specific levels; the enzyme was most highly expressed in testis, cap ut epididymis, ovary, and liver. Differences were also noted between s exes; the levels of GAMT mRNA and protein were higher in female liver than in male liver. Furthermore, as the male mouse developed from neon atal stages through sexual maturity, the GAMT protein level increased in testis but decreased in liver. Immunohistochemical labeling showed that GAMT was localized primarily in Sertoli cells of the testis and i n microvilli of the epithelial cells lining the caput epididymis. GAMT expression in Sertoli cells was confirmed by biochemical and molecula r analyses of purified testicular cells, testes from prepubertal mice of different ages, and germ cell-deficient mutant mouse testes. These results indicate that Cr is synthesized extensively in the epithelia o f seminiferous tubules and caput epididymis, suggesting that GAMT or m etabolic pathways related to Cr biosynthesis may be important for germ cell development or function.