Cellular ontogeny of RBMY during human spermatogenesis and its role in sperm motility

The Y-chromosome-encoded gene RBMY (RNA-binding motif on Y) is a male germline RNA-binding protein and is postulated to be a RNA-splicing regulator. In order to understand the roles of RBMY in different stages of male gamete maturation, the present study aimed at determining its cellular expression during spermatogenesis, spermeogenesis and in mature spermatozoa. In the spermatogonia (cKIT-positive cells), RBMY immunolocalized as two distinct foci, one in the nucleolus and the other in the subnuclear region; in the spermatocytes (cKIT-negative cells), the nucleus had punctuate staining with a subnuclear foci; in the pachytene cells, the protein was localized as a punctuate pattern in the nucleus spread along the elongating chromosomes. In the round and the elongating spermatids, the protein expression was polarized and restricted to the cytoplasm and in the developing mid-piece. In testicular and ejaculated sperm, RBMY was localized to the mid-piece region and weakly in the tail. Incubation of spermatozoa with the RBMY antibody reduced its motility. The spatial differences in expression of RBMY in the germ cells and the presences of this protein in post-meiotic cells and in transcriptionally inert spermatozoa suggest its involvement in multiple functions beyond RNA splicing. One such possible function of RBMY could be its involvement in sperm motility.