A potential molecular mechanism for regulating pre-mRNA splicing of implantation-related genes through unique uterine expression of splicing factor SC35 in women and rhesus monkeys

Splicing factor SC35 is an essential component of the spliceosome, the cellular apparatus that removes introns from pre-mRNA to provide alternatively spliced isoforms. Many proteins associated with development of uterine receptivity and embryo implantation are present as isoforms, the tissue-specific expression of which may be regulated through alternative splicing. SC35 was identified as being increased at implantation sites during early pregnancy in mice. However, the present study has demonstrated that SC35 is present in human and rhesus monkey endometrium, that the protein is increased during the secretory phase of the oestrous cycle compared with the proliferative phase in both these primates and that it is present in a distinct pattern within the nucleus of both epithelial and stromal cells, as well as in cells of the vasculature. Both the intensity of immunoreactive protein and the proportion of cells that stain for SC35 alter with the phase of the oestrous cycle. A very precise expression pattern of SC35 (both protein and mRNA) was seen during early placentation in rhesus monkeys. At implantation sites between day 24 and day 35 of early pregnancy, SC35 was expressed strongly in cytotrophoblasts within the trophoblastic shell, in syncytiotrophoblast at the periphery of the cell column and in both cytotrophoblast and syncytiotrophoblast in the floating villi. In the adjacent maternal decidua, expression of SC35 was weak. These results indicate a role for SC35 in preparation of a receptive uterus, in the provision of secreted proteins to support blastocyst development and in trophoblast invasion.