PP1, PP2A and PP2B Interplay in the Regulation of Sperm Motility: Lessons from Protein Phosphatase Inhibitors

Abstract

Male fertility relies on the ability of spermatozoa to fertilize the egg in the female reproductive tract (FRT). Spermatozoa acquire activated motility during epididymal maturation; however, to be capable of fertilization, they must achieve hyperactivated motility in the FRT. Extensive research found that three protein phosphatases (PPs) are crucial to sperm motility regulation, the sperm-specific protein phosphatase type 1 (PP1) isoform gamma 2 (PP1γ2), protein phosphatase type 2A (PP2A) and protein phosphatase type 2B (PP2B). Studies have reported that PP activity decreases during epididymal maturation, whereas protein kinase activity increases, which appears to be a requirement for motility acquisition. An interplay between these PPs has been extensively investigated; however, many specific interactions and some inconsistencies remain to be elucidated. The study of PPs significantly advanced following the identification of naturally occurring toxins, including calyculin A, okadaic acid, cyclosporin, endothall and deltamethrin, which are powerful and specific PP inhibitors. This review aims to overview the protein phosphorylation-dependent biochemical pathways underlying sperm motility acquisition and hyperactivation, followed by a discussion of the PP inhibitors that allowed advances in the current knowledge of these pathways. Since male infertility cases still attain alarming numbers, additional research on the topic is required, particularly using other PP inhibitors.

Keywords: capacitation; protein phosphatase inhibitors; protein phosphatase type 1; protein phosphatase type 2A; protein phosphatase type 2B; sperm motility.

Figure 1

Representation of the PPs and PKs involved in activated motility acquisition in the epididymis, as well as in hyperactivation in the female reproductive tract. The colour green represents catalytic activity, whereas red stands for enzymatic inactivity. (a) Epididymis representation including its three epididymal subdivisions: caput, corpus, and cauda, as well as deferent and efferent ducts. In caput epididymis spermatozoa are immotile, the PPs PP1γ2, PP2A, PP2B and the PK GSK3 present catalytic activity, while PKA is inactive. In cauda, mature and progressively motile sperm are characterized by inactive PP1γ2, PP2A, PP2B and GSK3 and active PKA. (b) Feminine reproductive tract representation where hyperactivated spermatozoa presents inactive PP1γ2 and PP2A, whereas PP2B, GSK3 and PKA present catalytic activity.

Figure 2

Interplay between the PPs, PP1γ2, PP2A and PP2B, and PKs, GSK3 and PKA, regarding sperm motility regulation. The color green represents catalytic activity, whereas red stands for enzymatic inactivity. Blue arrows define phosphorylation reactions, while orange represents dephosphorylation processes. (a) In caput immotile sperm, Ca²⁺ increase promotes PP2B interaction with Ca²⁺-CAM complex that activates it. PP2B dephosphorylates GSK3α increasing its activity. Phosphorylated PP2A also contributes to the phosphorylation of both GSK3 isoforms. GSK3 phosphorylates the I2 that disassociates from PP1γ2, rendering it active, solely in a complex with I3. The inhibitor SDS22 is bound to p17. PP1γ2 dephosphorylates GSK3 and Ser/Thr residues, resulting in immotile spermatozoa. PKA presents no significant catalytic activity. (b) In cauda epididymis, Ca²⁺ concentration is lower, causing CAM dissociation from PP2B and its subsequent inactivity. PPME1 decreases allowing PP2A methylation and inactivity. Simultaneously, sAC is activated and produces cAMP, which in turn activates PKA. cAMP degradation is due to PDE activity. PKA phosphorylates PP1γ2 and GSK3 that decreases its activity and is no longer able to phosphorylate I2, which forms a complex with PP1γ2 and SDS22 that dissociates from p17. Lastly, PKA’s increased activity causes phosphorylation of Ser/Thr residues which are a requirement for activated motility acquisition. (c) The increase in Ca²⁺ once again activates PP2B that dephosphorylates GSK3α rendering it active and able to phosphorylate I2. Due to sAC activation by HCO₃⁻ and Ca²⁺, cAMP further increases and stimulates PKA activity. PKA phosphorylates GSK3. PP1y2 is phosphorylated and in a complex with SDS22 and I3. SFK also contributes to decrease PP1y2 activity along with PP2A, that remains methylated. The increase in phosphorylation of Tyr residues contributes to achieve hyperactivated motility.

Figure 2

Interplay between the PPs, PP1γ2, PP2A and PP2B, and PKs, GSK3 and PKA, regarding sperm motility regulation. The color green represents catalytic activity, whereas red stands for enzymatic inactivity. Blue arrows define phosphorylation reactions, while orange represents dephosphorylation processes. (a) In caput immotile sperm, Ca²⁺ increase promotes PP2B interaction with Ca²⁺-CAM complex that activates it. PP2B dephosphorylates GSK3α increasing its activity. Phosphorylated PP2A also contributes to the phosphorylation of both GSK3 isoforms. GSK3 phosphorylates the I2 that disassociates from PP1γ2, rendering it active, solely in a complex with I3. The inhibitor SDS22 is bound to p17. PP1γ2 dephosphorylates GSK3 and Ser/Thr residues, resulting in immotile spermatozoa. PKA presents no significant catalytic activity. (b) In cauda epididymis, Ca²⁺ concentration is lower, causing CAM dissociation from PP2B and its subsequent inactivity. PPME1 decreases allowing PP2A methylation and inactivity. Simultaneously, sAC is activated and produces cAMP, which in turn activates PKA. cAMP degradation is due to PDE activity. PKA phosphorylates PP1γ2 and GSK3 that decreases its activity and is no longer able to phosphorylate I2, which forms a complex with PP1γ2 and SDS22 that dissociates from p17. Lastly, PKA’s increased activity causes phosphorylation of Ser/Thr residues which are a requirement for activated motility acquisition. (c) The increase in Ca²⁺ once again activates PP2B that dephosphorylates GSK3α rendering it active and able to phosphorylate I2. Due to sAC activation by HCO₃⁻ and Ca²⁺, cAMP further increases and stimulates PKA activity. PKA phosphorylates GSK3. PP1y2 is phosphorylated and in a complex with SDS22 and I3. SFK also contributes to decrease PP1y2 activity along with PP2A, that remains methylated. The increase in phosphorylation of Tyr residues contributes to achieve hyperactivated motility.