Update on mammalian sperm capacitation: how much does the horse differ from other species?

Abstract

In contrast to various other mammalian species, conventional in vitro fertilization (IVF) with horse gametes is not reliably successful. In particular, stallion spermatozoa fails to penetrate the zona pellucida, most likely due to incomplete activation of stallion spermatozoa (capacitation) under in vitro conditions. In other mammalian species, specific capacitation triggers have been described; unfortunately, none of these is able to induce full capacitation in stallion spermatozoa. Nevertheless, knowledge of capacitation pathways and their molecular triggers might improve our understanding of capacitation-related events observed in stallion sperm. When sperm cells are exposed to appropriate capacitation triggers, several molecular and biochemical changes should be induced in the sperm plasma membrane and cytoplasm. At the level of the sperm plasma membrane, (1) an increase in membrane fluidity, (2) cholesterol depletion and (3) lipid raft aggregation should occur consecutively; the cytoplasmic changes consist of protein tyrosine phosphorylation and elevated pH, cAMP and Ca 2+ concentrations. These capacitation-related events enable the switch from progressive to hyperactivated motility of the sperm cells, and the induction of the acrosome reaction. These final capacitation triggers are indispensable for sperm cells to migrate through the viscous oviductal environment, penetrate the cumulus cells and zona pellucida and, finally, fuse with the oolemma. This review will focus on molecular aspects of sperm capacitation and known triggers in various mammalian species. Similarities and differences with the horse will be highlighted to improve our understanding of equine sperm capacitation/fertilizing events.Reproduction (2019) 157 R181-R197

Effect of Seminal Plasma Protein Fractions on Stallion Sperm Cryopreservation

“…Based on the evaluation of phosphorylated proteins as one of the crucial known parameters for sperm capacitation, our study showed no significant effect of SP protein fraction on decreased cryocapacitation during the freezing–thawing of stallion sperm. Although there is no protocol which is able to fully capacitate spermatozoa [ 69 ] stallion spermatozoa also displayed an increase in protein tyrosine phosphorylation when capacitated in vitro [ 68 ]. Pommer et al, 2003 [ 70 ] reported an increase of phosphorylation in the mid and principal portions of sperm flagella when freshly ejaculated stallion sperm were capacitated.…”

“…The individual predisposition of a stallion’s semen significantly influences the effectiveness of capacitation [ 79 ]. Reliable success has not yet been achieved in inducing the full capacitation state of stallion spermatozoa by specific capacitating triggers [ 69 ].…”

Signaling Enzymes Required for Sperm Maturation and Fertilization in Mammals

“…These signaling enzymes, protein phosphatase 1 (PP1), glycogen synthase kinase 3 (GSK3) and the calcium activated phosphatase (PP2B), affect sperm function in the epididymis and in the female reproductive tract. Several reviews detail the roles of sperm cAMP-PKA and calcium metabolism (Visconti et al, 2002;Burton and McKnight, 2007;Buffone et al, 2014;Stival et al, 2016;Freitas et al, 2017;Balbach et al, 2018;Leemans et al, 2019;Stewart and Davis, 2019). Following a brief summary of the actions of cAMP-PKA, calcium and pH, the remainder of the review will be focused largely devoted to examination of the key roles of PP1, GSK3, and calcium activated protein phosphatase, PP2B (PPP3R2/CC) in mediating the actions of the second messengers during sperm maturation and fertilization.…”

Male Infertility: Shining a Light on Lipids and Lipid-Modulating Enzymes in the Male Germline

“…Chief among these changes are the efflux of cholesterol and resultant increase in membrane fluidity, permeability, and fusibility characteristics, which signal the onset of capacitation [118][119][120]. Cholesterol removal is also permissive of membrane remodeling, including the repositioning of receptors and fusion machinery needed to prime the sperm cell for acrosomal exocytosis and downstream oocyte interactions [121][122][123][124]. Although the mechanisms by which sterols are depleted during capacitation is not established for all species, in porcine and mouse spermatozoa, bicarbonate-induced ROS formation appears to promote the oxidation of sterols at the sperm surface.…”