Breaching the Barrier

Science Signaling  24 Feb 2009:
Vol. 2, Issue 59, pp. ec70
DOI: 10.1126/scisignal.259ec70

Unlike the blood-brain barrier, the blood-testis barrier (BTB), which is formed by tight junctions between Sertoli cells of the seminiferous epithelium, opens transiently during each round of spermatogenesis to allow developing spermatocytes to pass from the basal compartment of the seminiferous tubule into the lumen. Transforming growth factor–β3 (TGF-β3) and tumor necrosis factor–α (TNFα) are present in these tissues, and each induces reversible opening of the BTB. Given the importance of integral membrane protein dynamics in modulating cellular junctions, Xia et al. investigated the effect of TGF-β3 and TNFα on endocytosis of junctional adhesion molecule A (JAM-A), the tight junction component occludin, and the adhesion molecule N-cadherin in an in vitro model in which Sertoli cells formed tight junctions and a functional BTB in culture. Treatment with TGF-β3, TNFα, or germ cell–conditioned medium increased the amounts of these proteins that were internalized, as measured by immunoblotting cell extracts after surface protein biotinylation. Pharmacological inhibition of clathrin-mediated endocytosis reduced internalization of occludin, JAM-A, and N-cadherin in response to TGF-β3 or TNFα. TGF-β3–induced endocytosis of JAM-A required dynamins and clathrin, because inhibiting either with small interfering RNAs (siRNAs) reduced internalization of JAM-A in response to TGF-β3 treatment. Neither TGF-β3 nor TNFα was required for steady-state endocytosis of these integral membrane proteins, only for the increased endocytosis associated with opening of the BTB. Although it has not yet been determined whether these endocytosed proteins are targeted for degradation or recycling, this change in their localization may be the mechanism by which BTB permeability is modified.

W. Xia, E. W. P. Wong, D. D. Mruk, C. Y. Cheng, TGF-β3 and TNFα perturb blood–testis barrier (BTB) dynamics by accelerating the clathrin-mediated endocytosis of integral membrane proteins: A new concept of BTB regulation during spermatogenesis. Dev. Biol. 327, 48–61 (2009). [PubMed]