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Extracellular domain of V-set and immunoglobulin domain containing 1 (VSIG1) interacts with Sertoli cell membrane protein, while its PDZ-binding motif forms a complex with ZO-1
Mammalian spermatogenesis takes place in the seminiferousepithelium, which is composed of Sertoli cells and germ cells. The interaction between spermatogenic and Sertoli cells as well as elongated spermatids and Sertoli cells is tightly regulated by junctional adhesion molecules (JAMs). JAMs, which are cell adhesion molecules, are known to play roles in various biological processes such as fertilization, neurogenesis, cancer progression, and spermatogenesis. Members of the JAM family have a unique structure: they contain an N-terminal signal peptide domain, immunoglobulin (Ig)-like domains, transmembrane and cytoplasmic tail domains, each of which has distinct functions. The extracellular Ig-like domains interact in a homophilic or heterophilic manner, whereas cytoplasmic tail domain mediates the tight junction assembly. Although members of the JAM family are exclusively present in or restricted to the testis, their precise roles in spermatogenesis and fertilization have not yet been completely explored. The functional roles of Nectin-2, Nectin-3, JAM-C, cell adhesion molecule1 (CADM1), coxsackie and adenovirus receptor (CAR) have been evaluated by analysis of null mutant mice. Unfortunately, CAR-deficient mice had an embryonic lethal phenotype; this demonstrates the importance of CAR in development, but its physiological role in spermatogenesis is not known. The loss of CADM1, Nectin-3 and JAM-C resulted in male infertility caused by loss of adhesion between germ and Sertoli cells. A variety of JAMs participate in the interaction between germ and Sertoli cells. Recently, human VSIG1 has been characterized, which was originally known as A34, as a new member of the JAM family; VSIG1 is composed of two extracellular Ig-like domains, a transmembrane domain, and a cytoplasmic domain. However, this molecule has not been functionally characterized, so this was one of the aims of our present study. RT-PCR and immunoblot analyses were used to study VSIG1 expression, VSIG1 was specifically expressed in testicular germ cells but not in sperm. Pull-down assay with glutathione S-transferase (GST) or His-fused first Ig and second Ig domains of VSIG1 and SDS-PAGE under mild non-reducing conditions demonstrated that VSIG1 functions as an in vitro homophilic adhesion molecule. Furthermore, cells expressing a deletion of the C-terminus of VSIG1 failed to interact with ZO-1, the central structural protein of the tight junction. These findings suggest mouse VSIG1 interacts with an unknown molecule in Sertoli cells via its extracellular domain, while its cytoplasmic domain is needed for binding to ZO-1. Thus, we suggest mouse VSIG1 may play an important role in spermatogenesis rather than fertilization by forming heterophilic complex with a molecule similar to JAM family.
