Nicotinic Acetylcholine Receptor Subunits and Associated Proteins in Human Sperm

We demonstrated previously the involvement of a nicotinic acetylcholine receptor containing an α7 subunit in the human sperm acrosome reaction (a modified exocytotic event essential to fertilization). Here we report the presence in human sperm of α7, α9, α3, α5, and β4 nicotinic acetylcholine receptor subunits and the following proteins known to be associated with the receptor in the somatic cell: rapsyn and the tyrosine kinases c-SRC and FYN. The α7 subunit appears to exist as a homomer in the posterior post-acrosomal and neck regions of sperm and is probably linked to the cytoskeleton via rapsyn. The α3, α5, and β4 subunits are present in the sperm flagellar mid-piece of sperm and possibly exist as α3α5β4 and/or α3β4 channels. The α9 subunit is present in the sperm mid-piece. We detected the FYN and c-SRC tyrosine kinases in the flagellar mid-piece region. Both co-precipitated only with the nicotinic acetylcholine receptor β4 subunit. Immunolocalization with a C-terminal SRC kinase antibody, which recognizes several members of SRC kinase family, detected a SRC kinase co-localized with the α7 subunit in the neck region of sperm. Immunoprecipitation studies with that antibody demonstrated that the α7 subunit is associated with a SRC kinase. Antagonists of tyrosine phosphorylation inhibited the acetylcholine-initiated acrosome reaction, suggesting the involvement of a SRC kinase in the acrosome reaction.