Sperm from the calmegin-deficient mouse have normal abilities for binding and fusion to the egg plasma membrane

Calmegin is a putative testis-specific molecular chaperone required for the heterodimerization of fertilin alpha/beta and the appearance of fertilin beta on the sperm surface. Calmegin-deficient mice are almost completely sterile. The cause of the sterility initially was considered to be impaired abilities in sperm/zona pellucida (ZP) and sperm/egg plasma membrane (EPM) binding, and in the ascension of sperm to the oviduct, phenotypes similar to those seen in sperm from fertilin beta-deficient animals. We have developed a new method in which eggs were prepared without any detectable ZP3 on their surfaces by using a piezo-driven micromanipulator. Using these eggs and sperm containing the green fluorescent protein in their acrosomes, which can distinguish acrosome-intact from acrosome-reacted sperm, the binding and fusing abilities of calmegin-deficient sperm were reexamined. Under these conditions, acrosome-reacted sperm retained their ability to bind to and fuse with the EPM. The reduction in EPM binding of sperm from the calmegin(-/-) animals was apparently due to the artifactual binding of large numbers of acrosome-intact sperm from calmegin(+/-) mice to ZP remnants remaining on the EPM prepared with acidic Tyrode's solution. Thus, the sperm defect in calmegin-null animals is not at the level of sperm-EPM binding but rather may involve either sperm-ZP binding and/or sperm transit to the oviduct. Because fertilin beta is absent from calmegin-deficient mice, these results also suggest that the role of fertilin beta in sperm-EPM interaction needs to be reevaluated.