The nectin-2 N-terminal signal peptide will likely target the resulting nectin-2-LacZ fusion protein to the cell surface of cells in which the nectin-2 gene promoter is active. Staining for LacZ protein expression thus provided us with an alternative means of assessing the activity of the nectin-2 gene in the absence of secondary posttranslational events of the nectin-2 protein (e.g., protein-protein interactions). We found LacZ staining to outline the Sertoli cells, whereas no LacZ protein expression was associated with spermatid heads (Fig. 6, C, arrowheads, and D inset, asterisk). This indicates to us that LacZ was expressed by Sertoli cells. It can thus be inferred that the nectin-2 gene is active only in Sertoli cells and not in spermatids. This result supports those of our previous ultra-structural studies and germ cell transplantation experiments. so
Expression of nectin-3, the only known heterophilic binding partner of nectin-2, remained associated with spermatids in nectin-2LacZ/LacZ testes (Fig. 6, D and D inset). However, nectin-3 protein localization was severely disturbed. Rather than evenly decorating the convex aspect of elongated spermatid heads, nectin-3 protein was found to be concentrated mainly in a single, dense speck of immunore-activity located to the anterior on the spermatid head (Fig. 6D inset, asterisk). The absence of Sertoli cell-expressed nectin-2 appears to disrupt proper targeting of nectin-3 to Sertoli-spermatid junctions, or it may even indicate a defect in the formation of such junctions altogether. The present data support our previous finding of a heterophilic adhesion system maintained by nectin-2 on Sertoli cells and its binding partner, nectin-3, on heads of elongated spermatids.