Role and Regulation of Sperm Gelsolin Prior to Fertilization

Abstract

To acquire fertilization competence, spermatozoa should undergo several biochemical changes in the female reproductive tract, known as capacitation. The capacitated spermatozoon can interact with the egg zona pellucida resulting in the occurrence of the acrosome reaction, a process that allowed its penetration into the egg and fertilization. Sperm capacitation requires actin polymerization, whereas F-actin must disperse prior to the acrosome reaction. Here, we suggest that the actin-severing protein, gelsolin, is inactive during capacitation and is activated prior to the acrosome reaction. The release of bound gelsolin from phosphatidylinositol 4,5-bisphosphate (PIP 2 ) by PBP10, a peptide containing the PIP 2 -binding domain of gelsolin, or by activation of phospholipase C, which hydrolyzes PIP 2 , caused rapid Ca 2؉ -dependent F-actin depolymerization as well as enhanced acrosome reaction. Using immunoprecipitation assays, we showed that the tyrosine kinase SRC and gelsolin coimmunoprecipitate, and activating SRC by adding 8-bromo-cAMP (8-Br-cAMP) enhanced the amount of gelsolin in this precipitate. Moreover, 8-Br-cAMP enhanced tyrosine phosphorylation of gelsolin and its binding to PIP 2(4,5) , both of which inactivated gelsolin, allowing actin polymerization during capacitation. This actin polymerization was blocked by inhibiting the Src family kinases, suggesting that gelsolin is activated under these conditions. These results are further supported by our finding that PBP10 was unable to cause complete F-actin breakdown in the presence of 8-BrcAMP or vanadate. In conclusion, inactivation of gelsolin during capacitation occurs by its binding to PIP 2 and tyrosine phosphorylation by SRC. The release of gelsolin from PIP 2 together with its dephosphorylation enables gelsolin activation, resulting in the acrosome reaction.

Mechanism of sperm capacitation and the acrosome reaction: role of protein kinases

“…The free gelsolin becomes activated as a result of elevated levels of Ca 21 and tyrosine dephosphorylation by tyrosine phosphatases, leading to F-actin dispersion and the AR. 70 This interpretation is consistent with the role of PIP 2(4,5) as a cofactor for PLD activation which mediates actin polymerization in capacitation. 71,101 It has been shown in several cell types that PLD, the enzyme that hydrolyzes phosphatidylcholine to phosphatidic acid and choline, 102 is involved in the regulation of the actin cytoskeleton.…”

“…35 Src and gelsolin coimmunoprecipitate, and Src phosphorylates gelsolin on tyrosine-438 and Sperm capacitation and acrosome reaction D Ickowicz et al 818 inhibits its activity. 70 Actin polymerization is blocked by inhibiting the SFKs, suggesting that gelsolin is activated under these conditions.…”

“…[102][103][104] We showed elsewhere that PLD-dependent actin polymerization is a necessary step in the cascade leading to bull sperm capacitation. 70,71 PLD as well as actin polymerization is also involved in regulating cell motility. 103 In our recent study, we showed that the development of HAM in mouse sperm during capacitation depends upon actin polymerization.…”

“…93,94 We recently showed that gelsolin is inactive during capacitation, and is activated prior to the AR. 70 The release of bound gelsolin from phosphatidylinositol 4,5-bisphosphate (PIP 2 (4,5) ) by PBP10, a peptide containing the PIP 2 -binding domain of gelsolin, or by activation of PLC, which hydrolyzes PIP 2 , causes rapid Ca 21 -dependent F-actin depolymerization as well as an enhanced AR. 70 The activation of PLC, which hydrolyzes PIP 2 (4,5) , releases the bound gelsolin and enables its activity of breaking down F-actin, resulting in the AR.…”

“…The AR requires free extracellular calcium; 67,68 however, activation of PKC induces the AR in a calcium-independent fashion. 69 A calcium-dependent isotype of PLCc is activated during sperm capacitation, 70 leading to activation of a broad range of PKC isotypes at the Inhibitors of PKC block motility, while application of a crude PKC agonist increases motility. Studies on human sperm 69 demonstrated that the crude PKC agonist could also induce the AR in the absence of an elevation of intracellular-free calcium as long as the sperm had previously undergone capacitation.…”