Sperm Capacitation and Acrosome Reaction Mechanisms
Summary
Spermatozoa produced in the testes are initially incapable of fertilising an oocyte. To acquire this capacity, they undergo capacitation within the female reproductive tract, a complex process involving cholesterol efflux, changes in membrane fluidity and surface charge, influx of bicarbonate and calcium ions, and activation of cyclic adenosine monophosphate (cAMP)–dependent pathways. These events trigger a cascade of protein kinase A (PKA) and tyrosine phosphorylation reactions that remodel the actin cytoskeleton, stabilise acrosomal integrity and enable hyperactivated motility. Upon encountering the zona pellucida, capacitated sperm experience a further rise in intracellular calcium, engagement of ion channels such as CatSper, and activation of membrane fusion machinery. The ensuing acrosome reaction—an exocytotic event releasing hydrolytic enzymes—allows penetration of the zona matrix and facilitates gamete fusion. Together, capacitation and the acrosome reaction underpin male fertility and represent targets for assisted reproduction and non-hormonal contraception.
Research from Nature Portfolio
Recent studies have elucidated the role of multidrug resistance-associated protein 4 (MRP4) as a key regulator of cAMP efflux during capacitation. Inhibition of MRP4 impairs capacitation-associated protein phosphorylation, F-actin polymerisation and motility parameters in bovine sperm, while restoration of extracellular cAMP rescues these defects. This work highlights cAMP efflux not merely as waste removal but as a driver of membrane and cytoskeletal remodelling. Another approach has exploited graphene oxide to selectively extract cholesterol from the sperm plasma membrane. This treatment enhances membrane fluidity and capacitation markers without compromising viability, leading to improved fertilisation rates in vitro. Such findings open avenues for engineering sperm membrane composition to boost assisted-reproduction outcomes.
Sperm Capacitation and Acrosome Reaction Mechanisms publication trend
The graph below shows the total number of articles in sperm capacitation and acrosome reaction mechanisms across all publications each year (not limited to Nature Index journals).
Technical terms
Capacitation: The physiological and biochemical maturation process that sperm undergo in the female tract to gain fertilising competence.
Acrosome Reaction: A calcium-triggered exocytotic event in which enzymes are released from the acrosomal vesicle to enable zona pellucida penetration.
cAMP: A second messenger synthesised by soluble adenylate cyclase in sperm, activating PKA and downstream phosphorylation cascades.
MRP4: A membrane transporter that exports cAMP from sperm, thereby modulating extracellular signalling and capacitation-linked processes.
F-actin: Filamentous actin formed during capacitation that reinforces acrosomal stability and is disassembled prior to exocytosis.
Adenosine A2A Receptor: A G-protein-coupled receptor on sperm that influences motility, calcium influx and fertilisation outcomes.
References
- Mechanisms That Protect Mammalian Sperm from the Spontaneous Acrosome Reaction. International Journal of Molecular Sciences (2023).
- The adenosine A2A receptor in human sperm: its role in sperm motility and association with in vitro fertilization outcomes. Frontiers in Endocrinology (2024).
- Cyclic AMP efflux through MRP4 regulates actin dynamics signalling pathway and sperm motility in bovines. Scientific Reports (2020).
- Graphene Oxide increases mammalian spermatozoa fertilizing ability by extracting cholesterol from their membranes and promoting capacitation. Scientific Reports (2019).
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