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Localization and Characterization of an Orphan Receptor, Guanylyl Cyclase-G, in Mouse Testis and Sperm

Department of Biochemistry and Graduate Institute of Medical Sciences (Y.-H.H., C.-C.W., Y.-Y.C.) School of Medicine, Taipei Medical University, Taipei 110, Taiwan; Institute of Reproductive Medicine, University of Muenster, Muenster D-48149, Germany (T.G.C., C.-H.Y.); Institute of Biological Chemistry, Academia Sinica and Institute of Biochemical Sciences (S.-T.C.), College of Science, National Taiwan University, Taipei 106, Taiwan; Institute of Pharmacology, School of Medicine (R.-B.Y.), National Yang-Ming University, Taipei 112, Taiwan; and Institute of Biomedical Sciences (B.-T.W., Y.-H.S., C.-F.T., M.-T.T., R.-B.Y.), Academia Sinica, Taipei 115, Taiwan

Address all correspondence and requests for reprints to: Dr. Ruey-Bing Yang, Institute of Biomedical Sciences, Academia Sinica, 128, Academia Road, Section 2, Taipei 11529, Taiwan. E-mail: rbyang{at}ibms sinica.edu.tw.

We recently identified a novel testis-enriched receptor guanylyl cyclase (GC) in the mouse, designated mGC-G. To further investigate its protein expression and function, we generated a neutralizing antibody specifically against the extracellular domain of this receptor. RT-PCR and immunohistochemical analyses show that mGC-G is predominantly expressed from round spermatids to spermatozoa in mouse testis at both the mRNA and protein levels. Flow cytometry and confocal immunofluorescence reveal that mGC-G is a cell surface protein restricted to the plasma membrane overlying the acrosome and midpiece of the flagellum in mature sperm. Interestingly, Western blot analysis demonstrates that testicular mGC-G is approximately 180 kDa but is subject to limited proteolysis during epididymal sperm transport, resulting in a smaller fragment tethered on the mature sperm surface. On Fluo-3 cytometrical analysis and computer-assisted sperm assay, we found that serum albumin-induced elevation of sperm intracellular Ca²⁺ concentration, protein tyrosine phosphorylation, and progressive motility associated with capacitation are markedly reduced by preincubation of the anti-mGC-G neutralizing antibody. Together, these results indicate that mGC-G is proteolytically modified in mature sperm membrane and suggest that mGC-G-mediated signaling may play a critical role in gamete/reproductive biology.

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