Genetic, structural, and chemical insights into the dual function of GRASP55 in germ cell Golgi remodeling and JAM-C polarized localization during spermatogenesis

Spermatogenesis is a dynamic process that is regulated by adhesive interactions between germ and Sertoli cells. Germ cells express the Junctional Adhesion Molecule-C (JAM-C, encoded by Jam3), which localizes to germ/Sertoli cell contacts. JAM-C is involved in germ cell polarity and acrosome formation. Using a proteomic approach, we demonstrated that JAM-C interacted with the Golgi reassembly stacking protein of 55 kDa (GRASP55, encoded by Gorasp2) in developing germ cells. Generation and study of Gorasp2-/- mice revealed that knock-out mice suffered from spermatogenesis defects. Acrosome formation and polarized localization of JAM-C in spermatids were altered in Gorasp2-/- mice. In addition, Golgi morphology of spermatocytes was disturbed in Gorasp2-/- mice. Crystal structures of GRASP55 in complex with JAM-C or JAM-B revealed that GRASP55 interacted via PDZ-mediated interactions with JAMs and induced a conformational change in GRASP55 with respect of its free conformation. An in silico pharmacophore approach identified a chemical compound called Graspin that inhibited PDZ-mediated interactions of GRASP55 with JAMs. Treatment of mice with Graspin hampered the polarized localization of JAM-C in spermatids, induced the premature release of spermatids and affected the Golgi morphology of meiotic spermatocytes.
Author summary Spermatogenesis defects are a common cause of male sterility. Spermatogenesis occurs in the seminiferous tubules of the testes and involves adhesive interactions between developing germ cells and Sertoli cells. Knock-out mouse models identified several adhesion molecules that are critically involved in spermatogenesis. We previously demonstrated that the Junctional Adhesion Molecule-C (JAM-C) plays a crucial role in establishing spermatids polarity. The latter involves rearrangements of the Golgi apparatus in spermatids which contribute to acrosome formation. The present study demonstrated that the C-terminal cytosolic region of JAM-C interacted with the Golgi reassembly stacking protein of 55 kDa (GRASP55) encoded by Gorasp2 and that spermatogenesis was impaired in Gorasp2-deficient mice. We developed an inhibitor of GRASP55 interaction with JAM-C and demonstrated that treatment of wild-type mice with the inhibitory compound induced germ cell loss. Therefore, the male infertility-associated pathway identified in this study is important not only from a genetic point of view, but also as a potential target for male contraception.