Plasma membrane calcium ATPase 4 (PMCA4) co-ordinates calcium and nitric oxide signaling in regulating murine sperm functional activity.

Reduced sperm motility (asthenospermia) and resulting infertility arise from deletion of the Ca²⁺ -ATPase 4 (Pmca4) gene which encodes the highly conserved Ca²⁺ efflux pump, PMCA4. This is the major Ca²⁺ clearance protein in murine sperm. Since the mechanism underlying asthenospermia in PMCA4's absence or reduced activity is unknown, we investigated if sperm PMCA4 negatively regulates nitric oxide synthases (NOSs) and when absent NO, peroxynitrite, and oxidative stress levels are increased. Using co-immunoprecipitation (Co-IP) and Fluorescence Resonance Energy Transfer (FRET), we show an association of PMCA4 with the NOSs in elevated cytosolic [Ca²⁺ ] in capacitated and Ca²⁺ ionophore-treated sperm and with neuronal (nNOS) at basal [Ca²⁺ ] (ucapacitated sperm). FRET efficiencies for PMCA4-eNOS were 35% and 23% in capacitated and uncapacitated sperm, significantly (p < 0.01) different, with the molecules being <10 nm apart. For PMCA4-nNOS, this interaction was seen only for capacitated sperm where FRET efficiency was 24%, significantly (p < 0.05) higher than in uncapacitated sperm (6%). PMCA4 and the NOSs were identified as interacting partners in a quaternary complex that includes Caveolin1, which co-immunoprecipitated with eNOS in a Ca²⁺ -dependent manner. In Pmca4^-/- sperm NOS activity was elevated twofold in capacitated/uncapacitated sperm (vs. wild-type), accompanied by a twofold increase in peroxynitrite levels and significantly (p < 0.001) increased numbers of apoptotic germ cells. The data support a quaternary complex model in which PMCA4 co-ordinates Ca²⁺ and NO signaling to maintain motility, with increased NO levels resulting in asthenospermia in Pmca4^-/- males. They suggest the involvement of PMCA4 mutations in human asthenospermia, with diagnostic relevance.

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