Ventolin Inhalers

- Sperm Factor Induces Intracellular Free Calcium: DISCUSSION(5)


Another possibility is that the sperm/SF may carry an activator of the egg PLCs. Mammalian eggs express PLC|31, PLC(33, PLC71, and PLC72, and because of their much greater volume, they are likely to contain in excess of 1000-fold greater amounts of the isoforms present in sperm. In addition, the egg PLCs are easily activated to produce [Ca2+]i oscillations. For example, [Ca2+]i rises are initiated in mammalian eggs after injection of GTP7S or when eggs expressing foreign cell-surface receptors coupled to G-proteins or tyrosine kinase pathways are stimulated with appropriate agonists. Thus, it appears unlikely that SF would induce [Ca2+]i oscillations without involving the egg PLCs.

In echinoderms and ascidians, fertilization and SF (as-cidians) appear to activate the egg PLCs; expression of Src homology 2 (SH2) domains of PLC71 inhibited the sperm-induced Ca2+ responses. In these eggs, a Src-like tyrosine kinase appears to be required to induce activation of PLC7 and initiation of Ca2+ release. In vertebrates (Xenopus and mouse), the role of eggs PLCs is not as obvious because fertilization-induced Ca2+ responses were not inhibited by competing PLC71 SH2 domains or injection of antibodies against Gaq proteins, which are supposed to prevent the activation of PLCp [8, 34, 74]. PLO7 can be activated by mechanisms other than tyrosine phosphorylation at SH2 domains, and in most studies, the anti-Gaq antibody was raised against the C-terminal portion of this protein, a domain that does not appear to be important for interaction with PLCs. In Xenopus and mouse eggs, fertilization-induced Ca2+ responses appear sensitive to the addition of tyrosine kinase inhibitors, and in Xenopus eggs, Src family kinases seem to activate PLC7 at fertilization. Whether and how SF may activate a Src-like kinase in mammalian eggs is not known, although in somatic cells an Src kinase was shown to be activated by Ga-type proteins, and it has been reported that mammalian sperm express a Gaq protein. Thus, the active component(s) of SF may stimulate IP3 production by acting upstream of the egg PLCs, although the precise mechanism remains to be elucidated.

October 15, 2013 Sperm
Tags: calcium developmental biology fertilization