Role of the Glycosylphosphatidylinositol-Anchored Protein TEX101 and Its Related Molecules in Spermatogenesis

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) on the plasma membrane are involved in several cellular processes, including sperm functions. Thus far, several GPI-APs have been identified in the testicular germ cells, and there is increasing evidence of their biological significance during fertilization. Among GPI-APs identified in the testis, this review focuses on TEX101, a germ cell-specific GPI-AP that belongs to the lymphocyte antigen 6/urokinase-type plasminogen activator receptor superfamily. This molecule was originally identified as a glycoprotein that contained the antigen epitope for a specific monoclonal antibody; it was produced by immunizing female mice with an allogenic testicular homogenate. This review mainly describes the current understanding of the biochemical, morphological, and physiological characteristics of TEX101. Furthermore, future avenues for the investigation of testicular GPI-Aps, including their potential role as regulators of ion channels, are discussed.

Identification of an anti-sperm auto-monoclonal antibody (Ts4)-recognized molecule in the mouse sperm acrosomal region and its inhibitory effect on fertilization in vitro

We previously established an anti-mouse sperm auto-monoclonal antibody, Ts4, which shows immunoreactivity against several kinds of glycoproteins in the acrosomal region of epididymal spermatozoa, testicular germ cells, and early embryo, via binding to an epitope containing a common N-linked oligosaccharide (OS) chain on the molecules. In mice, we have already demonstrated that the OS chain in the epitope for Ts4 is a fucosylated agalacto-complex-type biantennary glycan carrying bisecting N-acetylglucosamine. In the testis, one of the specific OS chain-conjugated molecules is TEX101, a germ cell-marker glycoprotein, which is expressed in spermatocytes, spermatids, and testicular spermatozoa, but not in epididymal spermatozoa. In this study, we identified a Ts4-reactive glycoprotein in mouse cauda epididymal sperm. An immunoprecipitation method together with liquid chromatography-tandem mass spectrometry showed that alpha-N-acetylglucosaminidase (Naglu; a degradation enzyme of heparan sulfate) is one of the glycoproteins recognized by Ts4 in the epididymal spermatozoa. Western blot and immunohistochemical analyses revealed that mouse Naglu exists in two forms (82 and 77kDa) and is expressed in the acrosomal region and the flagellum of cauda epididymal sperm. Of the two Naglu-forms expressed in sperm, Ts4 immunoreacted against only the 82-kDa form located on the acrosomal region. The Ts4 mAb and anti-Naglu pAb negatively affected mouse fertilization in vitro. In addition, Ts4 inhibited sperm acrosome reaction induced by heparan sulfate. The Ts4-recognized fucosylated agalactobiantennary complex-type glycan with bisecting N-acetylglucosamine and Naglu on cauda epididymal spermatozoa may play a role in the process of fertilization.

Chemical Characterization of N-Linked Oligosaccharide As the Antigen Epitope Recognized by an Anti-Sperm Auto-Monoclonal Antibody, Ts4

Ts4, an anti-sperm auto-monoclonal antibody, possesses immunoreactivity to the acrosomal region of mouse epididymal spermatozoa. In addition, the mAb shows specific immunoreactivity to reproduction-related regions such as testicular germ cells and early embryo. Our qualitative study previously showed that the antigen epitope for Ts4 contained a N-linked common oligosaccharide (OS) chain on testicular glycoproteins as determined by Western blotting for testicular glycoproteins after treatment with several glycohydrolases. Since the distribution of the Ts4-epitope is unique, the OS chain in Ts4-epitope may have role(s) in the reproductive process. The aim of this study was to clarify the molecular structure of the Ts4-epitope, particularly its OS moiety. Using Ts4 immunoprecipitation combined with liquid chromatography and multiple-stage mass spectrometry, the candidate carbohydrate structure in the Ts4-epitope is proposed to be N-linked fucosylated agalacto-biantennary with bisecting N-acetylglucosamine (GlcNAc) or with N-acetylgalactosamine-GlcNAc motif. Further binding analyses using various lectins against the mouse testicular Ts4-immunoprecipitants revealed that Phaseolus vulgaris erythroagglutinin and Pisum sativum agglutinin showed positive staining of the bands corresponding to Ts4 reactive proteins. Moreover, the immunoreactivity of Ts4 against the testicular extract was completely abrogated after digestion with β-N-acetylglucosaminidase. These results show that the Ts4-epitope contains agalacto-biantennary N-glycan with bisecting GlcNAc carrying fucose residues.

Molecular characterization and expression of dipeptidase 3, a testis-specific membrane-bound dipeptidase: complex formation with TEX101, a germ-cell-specific antigen in the mouse testis

We previously established an anti-sperm head auto-monoclonal antibody designated Ts4. The immunoreactivity of this antibody was also observed in other reproduction-related cells, such as testicular germ cells and early embryos, suggesting that the Ts4-recognized molecules might play a role in the reproductive process. However, the molecular characteristics and functions of the antigens warrant further clarification. In this study, we primarily attempted identification of the mAb-recognized molecules within the mouse testis. An immunoprecipitation method, together with liquid chromatography-tandem mass spectrometry, revealed that the testicular immunoprecipitants with Ts4 contained dipeptidase 3 (DPEP3), a member of the membrane-bound dipeptidase family. A Western blot analysis using an anti-DPEP3 polyclonal antibody established in this study showed that this molecule was glycosylated and formed a disulfide-linked homodimer within the testis. Expression of DPEP3 protein was observed in the testicular germ cells, but not in the Sertoli or interstitial cells, or in any other major organs. Although Western blot analysis of testicular proteins separated by two-dimensional SDS-PAGE failed to demonstrate binding of Ts4 to DPEP3, we found that DPEP3 forms complexes with Ts4-immunoreactive molecules, such as TEX101, on the surfaces of spermatocytes, spermatids, and testicular spermatozoa. Based on data showing in the present study, further studies concerning DPEP3 on the testicular germ cells may help to clarify the molecular mechanisms of testicular germ-cell development.