How SF May Activate the PI Pathway
The mechanism by which the sperm/SF stimulates the PI pathway remains unresolved. Of the 10 mammalian PLC isozymes identified to date, mammalian sperm express PLCpl, PLC7I, and PLC72. In addition, mouse testis expresses high levels of PLC81 mRNA, and this mRNA appears confined to spermatogonia cells. Further, two splicing variants of PLC84, ALTI and ALTII, are abundantly expressed in rat testis. Based on these reports and on additional functional studies, it has been proposed that a PLC is the active component of SF. Subsequent studies, however, showed that addition/injection of several recombinant PLCs and tissue extracts containing native PLCs into sea urchin egg extracts and mouse eggs failed to trigger Ca2+ release despite the fact that the specific PLC activity of recombinant PLCs in in vitro assays was significantly higher than SF PLC activity.
Our fractionation results confirm and extend these findings by demonstrating that although pig testis extracts and SF contain PLC71, PLC72, and PLC84 and/or its splice variants, these enzymes are present in inactive fractions. The splice variants of PLC84 are the most testis-specific of all the known isoforms of PLC, and the finding that they do not represent the active component of testis extracts suggests that the sperm/SF may trigger IP3 production by delivering a different type of molecule. However, we cannot exclude the possibility that one of the untested PLC isoforms, or a novel undiscovered testicular isoform, may be the active component of SF.