New insights into transduction pathways that regulate boar sperm function

Detailed molecular mechanisms mediating signal transduction cascades that regulate boar sperm function involving Ser/Thr and tyrosine phosphorylation of proteins have been reviewed previously. Therefore, this review will focus in those kinase pathways identified recently (<10 years) in boar spermatozoa that regulate different functional spermatozoa processes. AMP-activated protein kinase (AMPK) is a cell energy sensor kinase that was first identified in mammalian spermatozoa in 2012, and since then it has emerged as an essential regulator of boar sperm function. Signaling pathways leading to AMPK activation in boar sperm are highlighted in this review (PKA, CaMKKα/β, and PKC as well as Ca(2+) and cAMP messengers as upstream regulators). Interestingly, stimuli considered as cell stress (hyperosmotic stress, inhibition of mitochondrial activity, absence of intracellular Ca(2+)) markedly activate AMPK in boar spermatozoa. Moreover, AMPK plays a remarkable and necessary regulatory role in mammalian sperm function, controlling essential boar sperm functional processes such as motility, viability, mitochondrial membrane potential, organization and fluidity of plasma membrane, and outer acrosome membrane integrity. These mentioned processes are all required under fluctuating environment of spermatozoa when transiting through the female reproductive tract to achieve fertilization. An applied role of AMPK in artificial insemination techniques is also suggested as during boar seminal doses preservation at 17 °C, physiological levels of AMPK activity markedly increase (maximum on Day 7) and result essential to maintain the aforementioned fundamental sperm processes. Moreover, regulation of sperm function exerted by the glycogen synthase kinase 3 and Src family kinase pathways is summarized.