Epididymal SPAM1 and its impact on sperm function

Abstract

The most widely conserved mammalian sperm antigen is sperm adhesion molecule 1, SPAM1/PH-20, which is also the major testicular hyaluronidase. This multifunctional glycosyl phosphatidylinositol (GPI)-linked protein plays several roles in fertilization and is encoded by a gene that resides among hyaluronidase family members in a cluster on human 7q31/mouse 6A2. In the human cluster, SPAM1 is the only functional hyaluronidase and of all six hyaluronidases in the genome it is the best characterized, both structurally and functionally. While SPAM1 transcripts are abundantly expressed only in the testis, specifically in spermatids, the RNA and protein are present in the male reproductive tract and accessory organs and in the female tract of mice. Our investigation of the post-testicular expression of SPAM1 shows that the protein is widely expressed in the epididymis. Like testicular SPAM1, epididymal SPAM1 (ES) has hyaluronidase activity and is conserved in at least five species (mouse, rat, bull, macaque, and human) all of which have putative androgen response elements in the gene promoters, consistent with androgen regulation. Testicular lumicrine factors have also been implicated in ES regulation. Based on regional expression, the protein is likely to play a role in both sperm maturation and storage. A minor secretory glycoprotein, ES is present in the epididymal luminal fluid in both a soluble and insoluble form (epididymosomes), with the latter having an intact lipid anchor. Genetic approaches have provided evidence for sperm uptake of ES in vivo, and in vitro uptake has been demonstrated with the use of Spam1 null mice. In vitro acquisition of ES on the sperm surface results in a pattern that mimics the wild-type distribution. More importantly it significantly increases the ability of null sperm to penetrate the cumulus of oocytes via hyaluronidase activity, directly relating ES uptake with fertilizing ability and indicating that ES is a marker of sperm maturation.