Epididymosomes: transfer of fertility‑modulating proteins to the sperm surface
Date
2015-06-26
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Shanghai Materia Medica, Shanghai Jiao Tong University.
Abstract
A variety of glycosylphosphatidylinositol (GPI)‑linked proteins are acquired on spermatozoa from epididymal luminal fluids (ELF) during
sperm maturation. These proteins serve roles in immunoprotection and in key steps of fertilization such as capacitation, acrosomal
exocytosis and sperm‑egg interactions. Their acquisition on sperm cells is mediated both by membrane vesicles (epididymosomes,
EP) which were first reported to dock on the sperm surface, and by lipid carriers which facilitate the transfer of proteins associated
with the membrane‑free fraction of ELF. While the nonvesicular fraction is more efficient, both pathways are dependent on
hydrophobic interactions between the GPI‑anchor and the external lipid layer of the sperm surface. More recently proteomic
and hypothesis‑driven studies have shown that EP from several mammals carry transmembrane (TM) proteins, including plasma
membrane Ca2+‑ATPase 4 (PMCA4). Synthesized in the testis, PMCA4 is an essential protein and the major Ca2+ efflux pump in
murine spermatozoa. Delivery of PMCA4 to spermatozoa from bovine and mouse EP during epididymal maturation and in vitro
suggests that the docking of EP on the sperm surface precedes fusion, and experimental evidence supports a fusogenic mechanism
for TM proteins. Fusion is facilitated by CD9, which generates fusion–competent sites on membranes. On the basis of knowledge
of PMCA4’s interacting partners a number of TM and membrane‑associated proteins have been identified or are predicted to be
present, in the epididymosomal cargo deliverable to spermatozoa. These Ca2+‑dependent proteins, undetected in proteomic studies,
play essential roles in sperm motility and fertility, and their detection highlights the usefulness of the hypothesis‑driven approach.
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Citation
Asian Journal of Andrology (2015) 17, 720–725; doi: 10.4103/1008-682X.155538