Cell signaling in sperm midpiece ensures quiescence and survival in cauda epididymis

Abstract

Sperm in most mammalian species including rat, mice and human are kept completely quiescent (motionless) and viable for up to a few weeks in the cauda epididymis before ejaculation. Vigorous motility is initiated almost instantly upon sperm release from cauda during ejaculation. The molecular mechanisms that suppress sperm motility but increase cell-survival during storage in cauda epididymis are not known. Intracellular signalling via phosphorylation cascades are quick events that may regulate motility and survival of transcriptionally inactive sperm. Pathscan® intracellular signalling array provided the preliminary picture of cell-signaling in quiescent and motile rat sperm, indicating upregulation of cell-survival pathways in quiescent sperm, which were downregulated during motility activation. Interactome of signalling-proteins involved in motility activation was constructed by STRING-software, which identified MAPK-p38, AKT, mTOR and their downstream target p70S6K as the key kinases regulating sperm function. Further validation was achieved by western-blotting and pathway activators/inhibitors. Immunofluorescence localized the kinase proteins in the sperm mid-piece region (mitochondria), a known extra-nuclear target for these signalling pathways. Activators of these kinases inhibited sperm motility but increased viability, and vice-versa was true for inhibitors, in most of the cases. Activators and inhibitors also affected sperm mitochondrial membrane potential, ATP content and ROS levels. Data suggest that sperm motility and survival are inversely complementary and critically regulated by intracellular cell signalling. Aberrant cell signalling in caudal sperm may affect cell survival (sperm concentration) and motility of ejaculated sperm.

MAPKs signaling is obligatory for male reproductive function in a development-specific manner

“…Building on these findings, Devi et al found phosphorylated levels of p38 MAPK, Akt, mTOR, and p70S6K were greater in quiescent rat sperm compared to motile spermatozoa from the caudal epididymis (63). Activating each of these kinases in motile sperm decreased the percentage of motile cells, while inhibiting these same kinases increased the percentage of motile sperm (63). The study furthermore found that p70S6K was downstream of p38, Akt, and mTOR, suggesting that p38 acts as one of the master kinases regulating the transition from quiescent to motile sperm.…”

“…An early study focused on defining the mechanisms by which spermatozoa transition from the quiescent to motile state demonstrated active p38 MAPK was higher in quiescent rat spermatozoa than motile cells, where in contrast, pERK1/2 MAPK was higher in motile cells (62). Building on these findings, Devi et al found phosphorylated levels of p38 MAPK, Akt, mTOR, and p70S6K were greater in quiescent rat sperm compared to motile spermatozoa from the caudal epididymis (63). Activating each of these kinases in motile sperm decreased the percentage of motile cells, while inhibiting these same kinases increased the percentage of motile sperm (63).…”