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Rat Caltrin Protein Modulates the Acrosomal Exocytosis During Sperm Capacitation¹

Laboratorio de Bioquímica y Biología Reproductiva, Instituto de Ciencia y Tecnología de Alimentos, Facultad de Ciencias Exactas, Físicas y Naturales,³ and Cátedra de Bioquímica y Biología Molecular,⁴ and Centro de Microscopia Electronica,⁵ Facultad de Ciencias Médicas, Universidad Nacional de Córdoba, 5016 Córdoba, Argentina Instituto de Biología y Medicina Experimental,⁶ 1428 Buenos Aires, Argentina

ABSTRACT

Caltrin is a small and basic protein of the seminal vesicle secretion that inhibits sperm calcium uptake. The influence of rat caltrin on sperm physiological processes related to fertilizing competence was studied by examining its effect on 1) spontaneous acrosomal exocytosis, 2) protein tyrosine phosphorylation, and 3) sperm-egg interaction. Results show that the presence of caltrin during in vitro capacitation both reduced the rate of spontaneous acrosomal exocytosis without altering the pattern of protein tyrosine phosphorylation, and enhanced the sperm ability to bind to the zona pellucida (ZP). The significantly higher proportion of sperm with intact acrosome observed in the presence of caltrin was accompanied by a strong inhibition in the acrosomal hyaluronidase release. Enhancement of sperm-ZP binding was evident by the increase in the percentage of eggs with bound spermatozoa as well as in the number of bound sperm per egg. Similar results were obtained when the assays were performed using spermatozoa preincubated with caltrin and then washed to remove the unbound protein, indicating that the sperm-bound caltrin was the one involved in both acrosomal exocytosis inhibition and sperm-ZP binding enhancement. Caltrin bound to the sperm head was partially released during the acrosomal exocytosis induced by Ca-ionophore A23187. Indirect immunofluorescence and immunoelectron microscopy studies revealed that caltrin molecules distributed on the dorsal sperm surface disappeared after ionophore exposure, whereas those on the ventral region remained in this localization after the treatment. The present data suggest that rat caltrin molecules bound to the sperm head during ejaculation prevent the occurrence of the spontaneous acrosomal exocytosis along the female reproductive tract. Consequently, more competent spermatozoa with intact and functional acrosome would be available in the oviduct to participate in fertilization.

acrosome reaction, calcium, male reproductive tract, seminal vesicles, sperm capacitation

¹ Supported by grants from Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Secretaria de Ciencia y Tecnología de la Universidad Nacional de Córdoba. C.M., P.S.C., and C.E.C. are the recipients of a Research Career Award from CONICET. S.D.M. and J.A.M. are Fellows of CONICET.

Correspondence: ²Carlos E. Coronel, Edificio de Investigaciones Biológicas y Tecnológicas, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Vélez Sarsfield 1611, 5016 Córdoba, Argentina. FAX: 54 351 433 3072; e-mail: ccoronel{at}efn.uncor.edu