Possible Function of the ADAM1a/ADAM2 Fertilin Complex in the Appearance of ADAM3 on the Sperm Surface

Abstract

In mouse, two different isoforms of ADAM1 (fertilin α), ADAM1a and ADAM1b, are produced in the testis. ADAM1a is localized within the endoplasmic reticulum of testicular germ cells, whereas epididymal sperm contain only ADAM1b on the plasma membrane. In this study, we show that the loss of ADAM1a results in the male infertility because of the severely impaired ability of sperm to migrate from the uterus into the oviduct through the uterotubal junction. However, epididymal sperm of ADAM1a-deficient mice were capable of fertilizing cumulus-intact, zona pellucida-intact eggs in vitro despite the delayed dispersal of cumulus cells and the reduced adhesion/binding to the zona pellucida. Among testis (sperm)-specific proteins examined, only the level of ADAM3 (cyritestin) was strongly reduced in ADAM1a-deficient mouse sperm. Moreover, the appearance of ADAM3 on the sperm surface was dependent on the formation of a fertilin protein complex between ADAM1a and ADAM2 (fertilin β) in testicular germ cells, although no direct interaction between the fertilin complex and ADAM3 was found. These results suggest that ADAM1a/ADAM2 fertilin may be implicated in the selective transport of specific sperm proteins including ADAM3 from the endoplasmic reticulum of testicular germ cells onto the cell surface. These proteins then can participate in sperm migration into the oviduct, the dispersal of cumulus cells, and sperm binding to the zona pellucida. In mouse, two different isoforms of ADAM1 (fertilin α), ADAM1a and ADAM1b, are produced in the testis. ADAM1a is localized within the endoplasmic reticulum of testicular germ cells, whereas epididymal sperm contain only ADAM1b on the plasma membrane. In this study, we show that the loss of ADAM1a results in the male infertility because of the severely impaired ability of sperm to migrate from the uterus into the oviduct through the uterotubal junction. However, epididymal sperm of ADAM1a-deficient mice were capable of fertilizing cumulus-intact, zona pellucida-intact eggs in vitro despite the delayed dispersal of cumulus cells and the reduced adhesion/binding to the zona pellucida. Among testis (sperm)-specific proteins examined, only the level of ADAM3 (cyritestin) was strongly reduced in ADAM1a-deficient mouse sperm. Moreover, the appearance of ADAM3 on the sperm surface was dependent on the formation of a fertilin protein complex between ADAM1a and ADAM2 (fertilin β) in testicular germ cells, although no direct interaction between the fertilin complex and ADAM3 was found. These results suggest that ADAM1a/ADAM2 fertilin may be implicated in the selective transport of specific sperm proteins including ADAM3 from the endoplasmic reticulum of testicular germ cells onto the cell surface. These proteins then can participate in sperm migration into the oviduct, the dispersal of cumulus cells, and sperm binding to the zona pellucida. A family of ADAM (adisintegrin and metalloprotease) transmembranous proteins belonging to the zinc protease superfamily is implicated in various biological processes such as fertilization, neurogenesis, myogenesis, cancer, and inflammation (1Schlöndorff J. Blobel C.P. J. Cell Sci. 1999; 112: 3603-3617Crossref PubMed Google Scholar, 2Primakoff P. Myles D.G. Trends Genet. 2000; 16: 83-87Abstract Full Text Full Text PDF PubMed Scopus (518) Google Scholar, 3Seals D.F. Courtneidge S.A. Genes Dev. 2003; 17: 7-30Crossref PubMed Scopus (904) Google Scholar). ADAMs have a unique organization containing an N-terminal signal peptide domain followed by pro-, metalloprotease, disintegrin, cysteine-rich, epidermal growth factor-like, transmembrane, and cytoplasmic tail domains, each of which has been believed to fulfill distinct function(s). The metalloprotease domain exhibits a “sheddase” activity toward the ectodomain of membranous precursor proteins (4Hooper N.M. Karran E.H. Turner A.J. Biochem. J. 1997; 321: 265-279Crossref PubMed Scopus (562) Google Scholar), whereas cell-to-cell adhesion is mediated by the disintegrin domain (5Yuan R. Primakoff P. Myles D.G. J. Cell Biol. 1997; 137: 105-112Crossref PubMed Scopus (205) Google Scholar, 6Linder B. Heinlein U.A.O. Dev. Growth Differ. 1997; 39: 243-247Crossref PubMed Scopus (47) Google Scholar, 7Chen H. Sampson N.S. Chem. Biol. 1999; 6: 1-10Abstract Full Text PDF PubMed Scopus (109) Google Scholar). Although many members of the ADAM family are exclusively or predominantly expressed in the testis, their roles in spermatogenesis and fertilization have not yet been completely elucidated. Mammalian fertilization is a successive, multistep process of events including the adhesion and binding of sperm to the zona pellucida (ZP), 1The abbreviations used are: ZP, zona pellucida; tACE, a testis-specific isoform of angiotensin-converting enzyme; TGC, testicular germ cells; ER, endoplasmic reticulum; UTJ, uterotubal junction. an extracellular glycoprotein matrix surrounding the egg, and the membrane fusion between sperm and egg