In guinea pig spermatozoa, the procaine-promoted synchronous acrosome reaction results in highly fertile cells exhibiting normal F-actin distribution

Acquisition of fertilization competence in guinea pig spermatozoa under different capacitation protocols

Guinea pig in vitro fertilization (IVF) are poorly developed due to the limited accessibility to oocytes and the lack of an efficient method of sperm capacitation. Thus, we aimed to evaluate different capacitation protocols that we validated through sperm analysis and using heterologous (He) IVF with zona-intact bovine oocytes. Spermatozoa of guinea pigs were collected and processed separately by 4 different protocols: A) Spermatozoa were obtained by flushing the lumen of one cauda epididymis and incubated in a minimal culture medium (MCM); B) One epididymis was placed in a prewarmed of M2 medium and gently minced with fine scissors. Spermatozoa were incubated in a modified human tubal fluid medium (HTF). In both protocols, the spermatozoa were capacitated at 37 °C under an atmosphere of 5% CO2 for 2 h. In the protocols C and D, the spermatozoa were collected by flushing the lumen of the cauda epididymis and selected by commercial density gradient Bovipure® (Nidacon Laboratories AB, Göthenborg, Sweden), according to the manufacturer's instructions. Then for Protocol C) spermatozoa were incubated in MCM medium supplemented with 10 mg/mL heparin (MCM-Hep); while for Protocol D) spermatozoa were incubated in FERT medium supplemented 10 mg/mL heparin (FERT-Hep). Incubation of C and D protocols were performed at 38.5 °C under an atmosphere of 5% CO₂ for 2 h. Capacitation protocols C and D showed a higher percentage of viability, total and hyperactive-like motility, and acrosome reaction compared to protocols A and B. For this reason, protocols C and D were used for further He-IVF analysis. Guinea pig sperm and matured zona-intact bovine oocytes were co-incubated at 5% CO₂ and 38.5 °C. Sperm-oocyte interaction was assessed at 2.5 h post-insemination (hpi) and pronuclear formation (PrF) were evaluated at 18, 20, 22, 24 and 26 hpi, while the cleavage rate was evaluated at 48 hpi. In protocol D, PrF was significantly higher than in protocol C (P ≤ 0.05) at every time point evaluated. Also, the cleavage rate at 48 hpi was higher (P ≤ 0.05) in He-IVF protocol D (69.8 ± 1.7%) compared to He-IVF protocol C (49.1 ± 1.1%). In conclusion, we determined the most adequate sperm capacitation conditions for guinea pig that allow zona-intact bovine oocyte penetration and lead to hybrid embryo formation, suggesting that these conditions could be optimal to develop IVF in guinea pigs.

Focal adhesion kinase is required for actin polymerization and remodeling of the cytoskeleton during sperm capacitation

Several focal adhesion proteins are known to cooperate with integrins to link the extracellular matrix to the actin cytoskeleton; as a result, many intracellular signaling pathways are activated and several focal adhesion complexes are formed. However, how these proteins function in mammalian spermatozoa remains unknown. We confirm the presence of focal adhesion proteins in guinea pig spermatozoa, and we explore their role during capacitation and the acrosome reaction, and their relationship with the actin cytoskeleton. Our results suggest the presence of a focal adhesion complex formed by β1-integrin, focal adhesion kinase (FAK), paxillin, vinculin, talin, and α-actinin in the acrosomal region. Inhibition of FAK during capacitation affected the protein tyrosine phosphorylation associated with capacitation that occurs within the first few minutes of capacitation, which caused the acrosome reaction to become increasingly Ca²⁺ dependent and inhibited the polymerization of actin. The integration of vinculin and talin into the complex, and the activation of FAK and paxillin during capacitation, suggests that the complex assembles at this time. We identify that vinculin and α-actinin increase their interaction with F-actin while it remodels during capacitation, and that during capacitation focal adhesion complexes are structured. FAK contributes to acrosome integrity, likely by regulating the polymerization and the remodeling of the actin cytoskeleton.

Summary: We describe the role of FAK and focal adhesion proteins in capacitation, acrosome reaction, polymerization and remodeling of actin cytoskeleton, and how inhibition of FAK affects sperm physiology.

The Macaque Sperm Actin Cytoskeleton Reorganizes in Response to Osmotic Stress and Contributes to Morphological Defects and Decreased Motility1

Sperm undergo extreme variations in temperature and osmolality during cryopreservation, resulting in cell damage that includes plasma membrane defects, changes in cell volume, decreased motility, and flagellar defects. However, the fundamental biologic mechanisms underlying these events are poorly understood. We investigated the effects of osmotic stress and cytochalasins b (CB) and d (CD), naturally occurring toxins that disrupt actin organization, on the actin cytoskeleton and motility of Rhesus macaque sperm (Macaca mulatta). Sperm were diluted in media of low, medium, or high osmolality, or medium-osmolality media containing CB or CD, were stained with phalloidin-fluorescein isothiocyanate, and were processed for microscopy. The majority of sperm incubated in medium-osmolality media exhibited postacrosomal stain, whereas the minority displayed banding patterns of F-actin stain in the head. High-osmolality media, as well as CB and CD incubation, resulted in reorganization of F-actin into bands of stain in the majority of sperm heads. Cytochalasin b treatment also resulted in curled and looped tails, a phenomenon of hyposmotic stress, and CB and CD caused significant, dose-dependent decreases in motility determined by computer-assisted sperm assessment. Rho A cell populations were determined using flow cytometry, and immunocytochemistry analysis demonstrated that Rho A localization was altered after osmotic stress. Together, our results support a mechanism in which reorganization of the actin cytoskeleton induced by osmotic stress and potentially mediated by a Rho A signaling pathway contributes to sublethal sperm flagellar and motility defects.

F-actin involvement in guinea pig sperm motility

Sperm motility is a must for natural fertilization to occur. During their travel through the epididymis, mammalian spermatozoa gradually acquire the ability to move. This is accomplished through a sliding movement of the outer doublet microtubules of the axoneme which is energized by the dynein ATPase. Within its complex structure, the mammalian sperm flagellum contains F-actin and thus, we decided to test in the guinea pig sperm flagellum the role of F-actin in motility. During maturation, capacitation, and the acrosome reaction, a gradual decrease of the relative concentration of F-actin was observed. Motility increased as spermatozoa became able to fertilize. Gelsolin, phalloidin, and KI inhibited sperm motility. Gelsolin canceled sperm motility within 20 min of treatment while 0.6 M KI had immediate effects. Phalloidin diminished hyperactive sperm motility slightly. All three compounds significantly increased the relative concentration of F-actin. Latrunculins are conventional drugs that destabilize the F-actin cytoskeleton. Latrunculin A (LAT A) did not affect sperm motility; but significantly increased F-actin relative concentration. The results suggested that in guinea pig spermatozoa, randomly severing F-actin filaments inhibits flagellar motility; while end filament alteration does not. Thus, specific filament regions seem to be important for sperm motility.