Calcium influx is a major step in the cascade of events regulating sperm physiological processes such as acrosome reaction and motility. Whereas the existence and the role(s) of calcium-binding proteins have been largely characterized in many cell types, these proteins have been little studied in sperm cells.

The aim of the present study was to evaluate the presence and possible role(s) of calcium-binding proteins in mouse spermatozoa.

Descriptive investigations and functional studies compared Wild-type (WT), Calretinin knock-out (CR-/-), Calbindin D-28k knock-out (CB-/-) and Calretinin/Calbindin D-28k/Parvalbumin knock-out (CR-/-CB-/-PV-/-) mice.

Indirect immunofluorescence and Western blotting indicated the presence of CB and CR in mouse spermatozoa, as well as in cerebellum used as control tissue. While a signal was still observed in knock-out sperm by indirect immunofluorescence, the Western blotting methods confirmed the absence of CR and/or CB in knock-out extracts but the presence of these proteins in WT spermatozoa.

CASA analysis of motility parameters from WT spermatozoa showed an increase in the percentage of hyperactivated sperm following incubation with NH₄Cl (25mM). This increase was present although being more pronounced in CR-/- sperm. Preliminary results indicated that the percentage of spontaneous acrosome reaction appeared higher in CR-/- sperm than in WT sperm.

In conclusion, this study documents, for the first time, the presence of CB and CR in mouse spermatozoa. Our preliminary results indicate that calcium-binding proteins might play a role in the calcium signaling cascade involved in sperm physiological processes. Measurements of intracellular calcium concentrations will be performed to complete the functional studies.