Regulation of spermatogenesis by intercellular adhesion molecules (ICAMS) and sarcoma (SRC) family kinases

Abstract

 In rat testes, at stage VIII of the epithelial cycle of spermatogenesis, two cellular events, namely blood-testis barrier (BTB) restructuring and spermiation, take place simultaneously but at the opposite ends of the seminiferous epithelium. BTB is constituted by tight junctions (TJs), basal ectoplasmic specializations (ES), gap junctions and desmosomes, which must disassemble intermittently at stage VIII to facilitate preleptotene spermatocyte migration across the barrier. Synchronously, spermiation occurs at the luminal edge of the tubule lumen, involving the disruption of the apical ES, the only anchoring device there, and the release of sperm. The mechanism coordinating these events is not well understood. In this dissertation, I provide evidence that intercellular adhesion molecule (ICAM)-1 and -2, are working in concert with sarcoma (Src) family kinases to regulate these events.

ICAMs comprise an immunoglobulin subfamily of cell adhesion proteins expressed by hematopoietic, endothelial and epithelial cells. They are known to function in the transendothelial migration of leukocytes. In the rat testis, ICAM-1 was shown to localize to both BTB and apical ES stage-specifically, with its immunoreactivity highest at stage VIII at the BTB. Besides co-immunoprecipitation and co-localization with BTB proteins, such as occludin and N-cadherin, ICAM-1 was found to promote BTB integrity in that its over-expression (O-E) in Sertoli cells in vitro increased transepithelial electrical resistance (TER). However, O-E of a truncated form of ICAM-1 (sICAM-1) that only consisted of the extracellular domain resulted in decreased TER and down-regulation of several BTB constituent proteins, possibly via the Src/Pyk2 signaling pathway. O-E of sICAM-1 in vivo also compromised the BTB integrity. These findings illustrate that ICAM-1 is an important regulator of the BTB.

On the other hand, the localization of ICAM-2 was restricted to the Sertoli-germ cell interface and absent from the BTB, and associated with β1-integrin, nectin-3 and F-actin at the apical ES. Further, ICAM-2 was shown to interact with Src and Pyk2, as well as annexin II, a phospholipid-binding protein. Intriguingly, ICAM-2, Src and annexin II were specifically up-regulated during CdCl2-induced germ cell loss. These results reveal that ICAM-2 actively participates in the restructuring of apical ES based on studies using the cadmium model.

The function of c-Yes, a member of the Src family, was also investigated. It was found to be stage-specifically expressed at the BTB and the apical ES, and it structurally associated with BTB components (e.g., occludin and N-cadherin) and with the apical ES proteins (e.g., β1-integrin, laminin β3 and γ3). In the study, the knockdown of c-Yes by RNAi in vitro and in vivo affected BTB and apical ES function, causing changes in the distribution/localization of adhesion proteins at the BTB and the apical ES, inducing germ cell loss from the seminiferous epithelium, possibly via an interference with the F-actin network.

These findings implicate that ICAMs and c-Yes are regulatory molecules of cell adhesion at the BTB and the apical ES, and are biomarkers for male contraceptive development.