The effect of oviductal deleted in malignant brain tumor 1 over porcine sperm is mediated by a signal transduction pathway that involves pro-AKAP4 phosphorylation

Abstract

The interaction between sperm and oviduct results in the selection of sperm with certain qualities. Porcine oviductal deleted in malignant brain tumor 1, DMBT1 (previously called sperm-binding glycoprotein, SBG), has been proposed to be implicated in sperm selection through acrosome alteration and suppression of motility of a subpopulation of sperm that have begun capacitation prematurely. It produces in vitro acrosome alteration and decrease of motility of boar sperm, concomitant with tyrosine phosphorylation of a 97 kDa sperm protein (p97). We hypothesized that the phosphorylation of p97 may be a link between DMBT1 sensing by a subpopulation of boar sperm and its biological effect. In this work, p97 was identified by mass spectrometry and immunoprecipitation as a porcine homologue of AKAP4. Pro-AKAP4 was localized by immunofluorescence and subcellular fractionation to the periacrosomal membranes and was shown to be tyrosine phosphorylated by DMBT1 regardless of the presence of calcium or bicarbonate, and of cAMP analogs, protein kinase A inhibitors, or a protein kinase C inductor. A processed ∼80 kDa form of AKAP4 was also detected at the tail of boar sperm, which was not tyrosine phosphorylated by DMBT1 under the conditions tested. Immunohistochemistry of testis showed presence of AKAP4 in boar sperm precursor cells. The evidence presented here supports the involvement of AKAP4 in the formation of the fibrous sheath on boar precursor sperm cells and implicates the phosphorylation of pro-AKAP4 as an early step in the signal transduction pathway gated by DMBT1 that leads to sperm selection through acrosome alteration.