Rac1 is necessary for capacitation and acrosome reaction in guinea pig spermatozoa

Abstract

Actin cytoskeleton remodeling is a critical process for the acquisition of fertilizing capacity by spermatozoa during capacitation. However, the molecular mechanism that regulates this process has not been fully elucidated. In somatic cells, Ras‐related C3 botulinum toxin substrate 1 protein (Rac1) promotes the polymerization of actin by participating in the modeling of two structures: lamellipodia and adhesion complexes linked with the plasma membrane. Rac1 is expressed in mammalian spermatozoa; however, the role of Rac1 in sperm physiology is unknown. This study aimed to elucidate the participation of Rac1 in capacitation and acrosome reaction (AR). Rac1 was found to be dispersed throughout the acrosome and without changes in the middle piece. After 60 minutes of capacitation, Rac1 was found in the apical region of the acrosome only, which concurred with an increase in Rac1‐GTP. Rac1 inhibition prevented such changes. In the middle piece, Rac1 localization remained unchanged. Besides, Rac1 inhibition blocked capacitation and AR. The present study demonstrates that Rac1 participates only in the actin cytoskeleton remodeling that occurs in the acrosomal apical region during capacitation, a region where a large amount of actin is polymerized and shaped in a diadem‐like structure. Our data also show that this actin cytoskeleton organized by Rac1 interacts with filamin‐1, and such interaction was blocked by the inhibition of Rac1, which led to a different organization of the actin cytoskeleton. All these outcomes imply that the formation of an F‐actin cytoskeleton in the acrosomal apical region is a necessary event for capacitation and AR, and which is Rac1 driven.

CDC42 drives RHOA activity and actin polymerization during capacitation

“…Another possibility is that other RHO protein, not inhibited by Y-27632, would be related to the actin polymerization between the external plasma and acrosomal membranes, an event required to avoid early AR, and that this sperm process occurs in the usual way (Spungin et al 1995, Hernandez-Gonzalez et al 2000, Breitbart et al 2005. We have recently reported that RAC1 could be related to the formation of this cytoskeleton (Ramirez-Ramirez et al 2020).…”

Role of cytoskeleton-related proteins in the acrosome reaction of Eriocheir sinensis spermatozoa

“…RAS-related C3 botulinum toxin substrate 1 (RAC1) has a small GTPase-binding protein structural domain that binds to small GTPases and is involved in actin cytoskeleton remodeling in the apical region of the acrosome during guinea pig spermatozoa capacitation. The formation of the actin cytoskeleton in the apical region of the acrosome is considered a necessary event for spermatozoa capacitation and acrosome reaction in guinea pigs and is driven by Rac1 [ 36 ]. In this study, we also found that RAC1 is differentially expressed in the acrosome-reacting spermatozoa of E. sinensis , suggesting that RAC1 may also regulate actin cytoskeleton remodeling in E. sinensis spermatozoa through its own specific structural domain and participate in the process of acrosome reaction.…”

Role of Integrins in Sperm Activation and Fertilization

“…The cytoplasmic tail of β4 integrin is significantly longer than other β integrin subunits and has the ability to be involved in cytoskeletal reorganization. It affects, for example, the Rac1 protein (Ras related C3 botulinum toxin substrate 1 protein) and its activation [67], which plays an important role in capacitation and the acrosome reaction, especially in the remodeling of actin in the apical part of the acrosome [68]. Unlike other integrins, β4 binds not only to actin and tubulin but also to intermediate filaments, such as keratin 5 via its counterpart plectin, which surrounds the sperm nucleus [68].…”

“…It affects, for example, the Rac1 protein (Ras related C3 botulinum toxin substrate 1 protein) and its activation [67], which plays an important role in capacitation and the acrosome reaction, especially in the remodeling of actin in the apical part of the acrosome [68]. Unlike other integrins, β4 binds not only to actin and tubulin but also to intermediate filaments, such as keratin 5 via its counterpart plectin, which surrounds the sperm nucleus [68]. This binding ability is due to the structural differences of the cytoplasmic domain of β4 (Figure 3) and may contribute to the mechanical and structural stability of a cell.…”