Abstract
Blood-testis barrier dynamics are regulated by testosterone and cytokines via their differential effects on the kinetics of protein endocytosis and recycling in Sertoli cells
Yan,Helen H.N.; Mruk,Dolores D.; Lee,Will M.; Cheng,Chuen-yan
FASEB Journal 22(6): 1945-1959
Publication date: 2008
During spermatogenesis in the mammalian testis, preleptotene/leptotenespermatocytes differentiate from type B spermatogonia and traversethe blood-testis barrier (BTB) at stage VIII of the seminiferousepithelial cycle for further development. This timely movementof germ cells involves extensive junction restructuring at theBTB. Previous studies have shown that these events are regulatedby testosterone (T) and cytokines [e.g., the transforming growthfactor (TGF) -ßs], which promote and disrupt the BTB assembly,respectively. However, the mechanisms underlying the "opening"of the BTB above a migrating preleptotene/leptotene spermatocyteand the "resealing" of the barrier underneath this cell remainobscure. We now report findings on a novel mechanism utilizedby the testes to regulate these events. Using cell surface proteinbiotinylation coupled with immunoblotting and immunofluorescentmicroscopy, we assessed the kinetics of endocytosis and recyclingof BTB-associated integral membrane proteins: occludin, JAM-A,and N-cadherin. It was shown that these proteins were continuouslyendocytosed and recycled back to the Sertoli cell surface viathe clathrin-mediated but not the caveolin-mediated pathway.When T or TGF-ß2 was added to Sertoli cell cultures withestablished functional BTB, both factors accelerated the kineticsof internalization of BTB proteins from the cell surface, perhapsabove the migrating preleptotene spermatocyte, thereby openingthe BTB. Likewise, T also enhanced the kinetics of recyclingof internalized biotinylated proteins back to the cell surface,plausibly relocating these proteins beneath the migrating spermatocyteto reassemble the BTB. In contrast, TGF-ß2 targeted internalizedbiotinylated proteins to late endosomes for degradation, destabilizingthe BTB. In summary, the transient opening of the BTB that facilitatesgerm cell movement is mediated via the differential effectsof T and cytokines on the kinetics of endocytosis and recyclingof integral membrane proteins at the BTB. The net result ofthese interactions, in turn, determines the steady-state proteinlevels at the Sertoli-Sertoli cell interface at the BTB.
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