Rat sperm surface mannosidase is first expressed on the plasma membrane of testicular germ cells

In previous publications (Tulsiani et al., Biochem J 1993; 290:427-436 and Tulsiani et al., Dev Biol 1995; 167:584-595), we reported that sperm surface mannosidase is present in rat testis and is modified during spermatogenesis and sperm maturation. The present studies were directed towards examining the origin of alpha-D-mannosidase activity present on fertile spermatozoa. Mixed germ cells prepared after sequential enzymatic digestions of rat testis were separated by unit gravity sedimentation using 2-4% linear bovine serum albumin gradient. Fractions enriched in spermatocytes, round spermatids, and condensed/elongated spermatids (> 95% pure cells) were separately pooled and assayed for [3H]Man9-mannosidase activity before (intact) and after lysis with Triton X-100. Interestingly, the cells contained a significant level of alpha-D-mannosidase activity. Approximately 70% of the total [3H]Man9-mannosidase activity present in the detergent-solubilized germ cell extract cross-reacted with anti-rat sperm mannosidase, and 25% of the activity cross-reacted with anti-Golgi mannosidase I. This result indicates that most of the mannosidase activity present in the germ cell extract is antigenically similar to the enzyme present on the cauda spermatozoa. Using cell fractionation techniques, we obtained evidence suggesting that the germ cell-associated mannosidase activity is an integral component of the plasma membranes. Taken together, these results indicate that sperm surface mannosidase is first expressed on the testicular germ cells.