Blood–testis barrier dynamics are regulated by an engagement/disengagement mechanism between tight and adherens junctions via peripheral adaptors

Abstract

In the mammalian testis, the blood-testis barrier (BTB), unlike the blood-brain and blood-retina barriers, is composed of coexisting tight junctions (TJs) and adherens junctions (AJs). Yet these junctions must open (or disassemble) to accommodate the migration of preleptotene and leptotene spermatocytes across the BTB during spermatogenesis while maintaining its integrity. In this report, we show that the BTB utilizes a unique ''engagement'' and ''disengagement'' mechanism to permit the disruption of AJ that facilitates germ cell movement without compromising the BTB integrity. For instance, both TJ (e.g., occludin and JAM-1) and AJ (e.g., N-cadherin) integral membrane proteins were colocalized to the same site at the BTB. Although these TJ-and AJ-integral membrane proteins did not physically interact with each other, they were structurally linked by means of peripheral adaptors (e.g., ZO-1 and ␣-and ␥-catenins). As such, these proteins are structurally ''engaged'' under physiological conditions to reinforce the BTB. When rats were exposed to Adjudin to induce AJ restructuring that eventually led to germ cell loss from the epithelium, this structural interaction between occludin and N-cadherin by means of their adaptors became ''disengaged'' while their protein levels were significantly induced. In short, when the epithelium is under assault, such as by Adjudin or plausibly at the time of germ cell migration across the BTB during spermatogenesis, the TJ-and AJ-integral membrane proteins can be disengaged. Thus, this mechanism is used by the testis to facilitate AJ restructuring to accommodate germ cell migration while maintaining the BTB integrity.spermatogenesis ͉ ectoplasmic specialization ͉ Sertoli-germ cell interaction D uring spermatogenesis, preleptotene and leptotene spermatocytes that are residing outside the blood-testis barrier (BTB) in the basal compartment of the seminiferous epithelium traverse the BTB at stages VIII-IX of the epithelial cycle in mammalian testes, such as those of the rat (1). As such, extensive restructuring of tight junctions (TJs), adherens junctions (AJs), and desmosomelike junctions must take place in the epithelium to facilitate germ cell movement (for a review, see ref.2). Interestingly, unlike barriers in other mammalian organs, such as the blood-brain, blood-retina, and blood-epididymal barriers, where TJ is restricted to the apical portion of the cell epithelium and͞or endothelium, to be followed by AJ (for reviews, see refs. 3-6), the BTB is composed of coexisting TJs, AJs, and desmosome-like junctions (for reviews, see refs. 7-9). One of the main roles of the TJ barrier is to prevent small molecules from passing through the paracellular space. The endothelial TJ barrier in blood vessels, for example, is only opened occasionally to allow the passage of neutrophils and macrophages during inflammation (for a review, see ref. 10). However, in the testis, the BTB is a highly dynamic structure because it must open (or disassemble) periodically to accommodate the migration of g...

Nitric Oxide and Cyclic Nucleotides: Their Roles in Junction Dynamics and Spermatogenesis

“…23 In addition, the structural association of γ-catenin, an AJ adaptor, and ZO-1, a TJ adaptor, is weakened after treatment of rats using adjudin, which is shown to have a role in disturbing the AJ without compromising the TJ in the testes. 3,24 These findings substantiate the significance of adaptors that mediate cross-talk between different junction types in the testes. Apart from that, other functions are recently uncovered for adaptors, including their participation in cytokine signaling in the testes and immune-related activities 25,26 Based on this emerging evidence, adaptors are proteins with diversified physiological functions in maintaining spermatogenesis, in particular the events of junction restructuring.…”

Laminin α 3 Forms a Complex with β3 and γ3 Chains That Serves as the Ligand for α 6β1-Integrin at the Apical Ectoplasmic Specialization in Adult Rat Testes

“…Seminiferous tubules used for studies reported herein were isolated from adult rat testes and were shown to be contaminated with negligible Leydig cells as reported previously (31). Seminiferous tubule and germ cell lysates were prepared in an IP buffer (50 mM Tris-HCl, 150 mM NaCl, 1% Nonidet P-40 (v/v), 1 mM EGTA, 1 mM phenylmethylsulfonyl fluoride, 1 mM sodium orthovandate, 1 g/ml leupeptin and 1 g/ml aprotinin, pH 7.4) as described (32).…”

“…Co-immunoprecipitation (Co-IP) and Immunoblotting-To study the binding partners of laminin ␥3 as well as other interacting proteins in rat testes, co-IP was performed using lysates of either seminiferous tubules or germ cells as described previously (31). Seminiferous tubules used for studies reported herein were isolated from adult rat testes and were shown to be contaminated with negligible Leydig cells as reported previously (31).…”

Cross-Talk between Tight and Anchoring Junctions—Lesson from the Testis

“…Even though occludin and cadherin at the BTB have no direct protein-protein interaction as demonstrated by coimmunoprecipitation, they are linked via their peripheral adaptors, such as ZO-1 and catenins. 89 This finding is not entirely unexpected since at the early stage of TJ assembly, ZO-1 was shown to associate with catenins in MDCK cells. 90 Besides, ZO-1 also serves as a cross-linker between the cadherin/catenin protein complex and the actin-based cytoskeleton in nonepithelial cells.…”

“…In essence, ZO-1 was no longer associated with catenins, and as such, occludin and N-cadherin as well as their adaptors, ZO-1 and γ-catenin were diffusing away from the BTB site. 89 Yet, their association was reestablished by day 7 after treatment when germ cells (e.g., elongating/elongate spermatids and some round spermatids) were depleted from the epithelium. Based on such timely dissociation between TJ and AJ protein complexes in the seminiferous epithelium, an "engagement and disengagement" theory was proposed to describe the unique mechanism employed by the testis to facilitate germ cell movement pertinent to spermatogenesis 89 (Fig.…”