Targeted disruption of the cation-dependent or cation-independent mannose 6-phosphate receptor does not decrease the content of acid glycosidases in the acrosome

The acrosome is a unique organelle containing acid hydrolases common to lys osomes as well as unique enzymes. Its ultimate exocytosis, as well as the a bsence of several lysosomal markers, has led to the speculation that it sho uld be considered a secretory or zymogen vesicle rather than a specialized lysosome. The basic targeting machinery for eukaryotic lysosomal acid glyco sidases are the two mannose B-phosphate receptors. Mouse testicular germ ce lls are known to express both the cation-independent (CI-MPR) and cation-de pendent (CD-MPR) forms of the mannose 6-phosphate receptors, but the CD-MPR is predominant. In this report, we utilized the recent targeted disruption of the CD-MPR and CI-MPR genes to determine whether these mutations affect targeting of acid glycosidases to the acrosome. Antibody to luminal fluid beta -D- galactosidase was used to examine the targeting of immunoreactive product within the acrosome of permeabilized spermatozoa and testicular spe rmatids. No obvious changes in acrosomal immunoreactivity in either MPR hom ozygous mutant were observed when compared with the case of wild-type litte rmates. In addition, targeted disruption of either MPR did not result in de creased levels of beta -D-galactosidase, alpha -D-mannosidase, or N-acetylg lucosaminidase activities in spermatozoa from either MPR-homozygous mutant. These results suggest that the targeted disruption of either MPR does not result in decreased acrosomal targeting efficiency.