Mice Deficient in CHRNA7, a Subunit of the Nicotinic Acetylcholine Receptor, Produce Sperm with Impaired Motility1

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Abstract
In this study we investigate the role of the CHRNA7 subunit (also known as the alpha7 subunit) of the nicotinic acetylcholine receptor in mouse sperm function. We confirm by reverse-transcription-polymerase chain reaction the expression in adult mouse testis of Chrna7 mRNA and demonstrate the subunit’s presence in mouse sperm by immunoblot. Alpha-bungarotoxin binds a range of nicotinic acetylcholine receptor subunits, including the CHRNA7 subunit. Localization studies using a fluorescent alpha-bungarotoxin-tetramethyl-rhodamine conjugate revealed specific binding sites on the midpiece of mouse sperm with fainter alpha-bungarotoxin binding on the remainder of the flagellum. Mice engineered with a double-null disruption of the Chrna7 gene displayed only faint fluorescence on the midpiece, suggesting that the CHRNA7 contributed the majority of the observed alpha-bungarotoxin binding sites. The location of alpha-bungarotoxin binding suggested that nicotinic acetylcholine receptors may play an ionotropic role in sperm motility. Sperm from Chrna7−/− mice display no difference in number, morphology, viability or spontaneous acrosome reaction rate compared with Chrna7+/+ sperm. Studies using computer-assisted sperm analysis indicate the motility of Chrna7−/− sperm is significantly impaired. This impairment is characterized by significantly reduced swimming velocities, failure to maintain vigorous swimming, and lower levels of hyperactivated swimming patterns in Chrna7−/− sperm compared with Chrna7+/+ sperm. This is the first genetic evidence that sperm nicotinic acetylcholine receptors are important for maintenance of normal sperm motility.