Sperm total motility and hyperactivated motility are

Sperm total motility and hyperactivated motility are mediated by PLD-dependent ras pathway polymerization [20]. Reduction of PIP2 synthesis inhibited actin polymerization and motility, and increasing PIP2 synthesis enhanced these activities. Furthermore, sperm demonstrating low motility contained low levels of PIP2 and F-actin. During capacitation, there was an increase in PIP2 and F-actin levels in the sperm head and a decrease in the tail [18]. Moreover, the localization of gelsolin in the sperm cell influenced the sperm motility. In sperm with high progressive motility, gelsolin was mainly localized to the sperm head before capacitation, whereas in low motility sperm, most of the gelsolin was localized to the tail before capacitation and translocated to the head during capacitation [18]. F-actin also increases in the sperm tail during capacitation, which is important for the development of hyper-activated motility [20]. Thus, it is likely that there are two reasons for the translocation of gelsolin from the tail to the head. First gelsolin is required in the head to depolymerize F-actin to allow the occurrence of the acrosomal exocytosis and second the exclusion of gelsolin from the tail prevents F-actin depolymerization in the tail allowing the development of hyper-activated motility. F-actin formation during sperm capacitation is also controlled by Rho GTPases [12], [38]. The small GTPases Rho A, Rho B, Rac1 and Cdc42 are present in the head and tail of several mammalian spermatozoa [39]. These small GTPases can interact with Wasp, which involves in actin nucleation together with Arp2/3 and profilins [40]. Wasp, profilin, Arp2/3 and cdc42 are localized to the head and tail of sperm cells [10]. It was shown that Wasp, RhoA, RhoB and Cdc42 are involved in actin remodeling in sperm capacitation and the acrosomal exocytosis processes [38].
Gelsolin and cofilin activities are inhibited during capacitation The presence of actin severing proteins such as gelsolin and cofilin in mammalian sperm suggests that the assembly and disassembly of F-actin are well-controlled events. Gelsolin severs assembled actin filaments, and caps the fast growing plus end of free or newly severed filaments in response to Ca2+, and is inhibited by binding to PIP2 and by phosphorylation on tyr-438. We showed that gelsolin must be inhibited during capacitation for actin polymerization to occur [41]. Gelsolin can be inhibited by its Src-dependent phosphorylation on tyr-438 and/or its binding to PIP2, two processes that occur during sperm capacitation. In human sperm, Src is involved in regulating capacitation, Ca2+ fluxes, protein tyrosine phosphorylation and the acrosomal exocytosis [42], [43], [44]. The binding of gelsolin to PIP2 promotes its tyrosine phosphorylation by Src, which keeps gelsolin in an inactive form, allowing F-actin formation during capacitation [41]. Increase in PIP2 in the sperm head during capacitation occurs simultaneously to the increase in F-actin and gelsolin in the sperm head. Moreover, gelsolin phosphorylation on Tyr438 is enhanced during sperm capacitation mainly towards the mid and end of the capacitation process [18]. Inhibition of PIP2 synthesis prevented the translocation of gelsolin to the head while enhancing PIP2 synthesis significantly increased it. Furthermore, tyr-438 phosphorylation/inhibition of gelsolin is reduced when PIP2 levels are decreased and vice versa [18]. Previous studies suggested that Src is not directly involved in protein tyrosine phosphorylation during sperm capacitation, but rather, inhibits protein phosphatase resulting in an increase in tyrosine phosphorylation of proteins [45]. It was shown elsewhere that capacitation is regulated by activation of PKA, which activates Src leading to inactivation of Ser/Thr phosphatase [43], [46] (Fig. 1). We recently showed that activation of Src by PKA, inhibits the Ser/Thr phosphatase PP1 resulting in CaMKII activation leading to activation of Pyk2 which phosphorylates PI3K on tyrosine-845 [47] (See Fig. 1). Src was found in the sperm tail and head and is localized to the membrane fraction [46], similar to gelsolin localization and consistent with our assumption about its inactivating function. These assumptions were verified when Src-dependent phosphorylation of gelsolin on tyr-438 during sperm capacitation was found [18]. In addition, we found that activation of PKA/Src cause F-actin formation which was inhibited by inhibiting Src activity [41], suggesting that activation of Src causes gelsolin inhibition and an increase in F-actin. In addition, PBP10-induced F-actin depolymerization is inhibited by activating Src or by inhibition of tyrosine-phosphatase, suggesting that although gelsolin is released from its binding to PIP2, it is still highly phosphorylated and inactive. PBP10 is a peptide which contains the gelsolin binding domain to PIP2, competes with gelsolin to bind to PIP2 resulting in the release of gelsolin from PIP2. The levels of Tyr-p-gelsolin are enhanced by elevating the cellular levels of PIP2 and vice versa [18], suggesting that binding of gelsolin to PIP2 increases its phosphorylation. These findings is further supported by showing a decrease in Tyr-p-gelsolin by releasing gelsolin from PIP2 by activation of phospholipase C (PLC) which hydrolyze PIP2 or by using PBP10. These findings also suggest that free Tyr-p-gelsolin is more sensitive to Tyr-phosphatase activity compared to the PIP2-bound p-gelsolin. Similarly, activation of sperm EGFR caused PLC-dependent dephosphorylation of p-gelsolin, after 1 h of incubation under capacitation conditions [18]. These results confirm our hypothesis that gelsolin is inhibited during capacitation due to its binding to PIP2 and its Tyr-phosphorylation by Src.