Cytokines and junction restructuring during spermatogenesis—a lesson to learn from the testis

Nitric oxide and cyclic nucleotides: their roles in junction dynamics and spermatogenesis

Spermatogenesis is a highly complicated process in which functional spermatozoa (haploid, 1n) are generated from primitive mitotic spermatogonia (diploid, 2n). This process involves the differentiation and transformation of several types of germ cells as spermatocytes and spermatids undergo meiosis and differentiation. Due to its sophistication and complexity, testis possesses intrinsic mechanisms to modulate and regulate different stages of germ cell development under the intimate and indirect cooperation with Sertoli and Leydig cells, respectively. Furthermore, developing germ cells must translocate from the basal to the apical (adluminal) compartment of the seminiferous epithelium. Thus, extensive junction restructuring must occur to assist germ cell movement. Within the seminiferous tubules, three principal types of junctions are found namely anchoring junctions, tight junctions, and gap junctions. Other less studied junctions are desmosome-like junctions and hemidesmosome junctions. With these varieties of junction types, testes are using different regulators to monitor junction turnover. Among the uncountable junction modulators, nitric oxide (NO) is a prominent candidate due to its versatility and extensive downstream network. NO is synthesized by nitric oxide synthase (NOS). Three traditional NOS, specified as endothelial NOS (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS), and one testis-specific nNOS (TnNOS) are found in the testis. For these, eNOS and iNOS were recently shown to have putative junction regulation properties. More important, these two NOSs likely rely on the downstream soluble guanylyl cyclase/cGMP/protein kinase G signaling pathway to regulate the structural components at the tight junctions and adherens junctions in the testes. Apart from the involvement in junction regulation, NOS/NO also participates in controlling the levels of cytokines and hormones in the testes. On the other hand, NO is playing a unique role in modulating germ cell viability and development, and indirectly acting on some aspects of male infertility and testicular pathological conditions. Thus, NOS/NO bears an irreplaceable role in maintaining the homeostasis of the microenvironment in the seminiferous epithelium via its different downstream signaling pathways.

TGF-beta3 and TNFalpha perturb blood-testis barrier (BTB) dynamics by accelerating the clathrin-mediated endocytosis of integral membrane proteins: a new concept of BTB regulation during spermatogenesis

In adult mammals such as rats, the blood-testis barrier (BTB) conferred by adjacent Sertoli cells in the seminiferous epithelium segregates post-meiotic germ cell development from the systemic circulation and is one of the tightest blood-tissue barriers. Yet it must "open" transiently at stages VIII to IX of the epithelial cycle to accommodate the migration of preleptotene/leptotene spermatocytes. While this is a vital event of spermatogenesis, the mechanism(s) that regulates BTB dynamics is virtually unknown. Recent studies have suggested that transforming growth factor-beta3 (TGF-beta3) and tumor necrosis factor alpha (TNFalpha) secreted by Sertoli and germ cells into the microenvironment of the BTB are capable of inducing reversible BTB disruption in vivo, apparently by reducing the steady-state levels of occludin and zonula occludens-1 (ZO-1) at the BTB via the p38 mitogen activated protein (MAP) kinase signaling pathway. In this study, local administration of TGF-beta3 (200 ng/testis) to the testis was shown to reversibly perturb the BTB integrity in vivo. We next sought to delineate the mechanism by which these cytokines maintain the steady-state level of integral membrane proteins: occludin, junctional adhesion molecule-A (JAM-A) and N-cadherin at the BTB. Primary Sertoli cells cultured in vitro were shown to establish intact tight junctions and functional BTB within two days when assessed by transepithelial electrical resistance (TER) measurement across the cell epithelium. Sertoli cell integral membrane protein internalization at the BTB was assessed by biotinylation of cell surface proteins, to be followed by tracking the endocytosed/biotinylated proteins by using specific antibodies. Both TGF-beta3 (3 ng/ml) and TNFalpha (10 ng/ml) were shown to significantly accelerate the kinetics of internalization of JAM-A, N-cadherin, and occludin versus controls. Treatment of cells with phenylarsine oxide (PAO) at 10 microM that blocks clathrin-mediated endocytosis was shown to inhibit the TGF-beta3-induced protein internalization. This inhibition of TGF-beta3-mediated protein endocytosis was further validated by silencing of clathrin. The specific effect of TGF-beta3 on protein internalization was further confirmed by RNAi using specific TGF-beta receptor I (TbetaR1) siRNA duplexes. When TbetaR1 was knocked down, the TGF-beta3-induced increase in the kinetics of JAM-A and occludin endocytosis was abolished, making them indistinguishable from controls, illustrating the specificity of the TGF-beta3 effects on protein endocytosis. In summary, this report demonstrates for the first time that BTB dynamics are regulated by TGF-beta3 and TNFalpha via an enhancement of protein endocytosis at the BTB.

Hepatocyte growth factor modulates Sertoli-Sertoli tight junction dynamics

In mammalian testes Sertoli cells form tight junctions whose function is fundamental for the maintenance of a normal spermatogenesis. Hepatocyte growth factor (HGF) is a cytokine influencing the cellular tight junctions either in normal or in tumor cells. We have previously demonstrated that HGF is expressed in the rat testis and influences many functional activities of somatic and germ cells. We now report that HGF decreases the levels of testicular occludin and influences the position of the molecule in the tight junctions as demonstrated by confocal microscopy analysis. In fact in the presence of the factor occludin was mainly localized in the suprabasal region of the tubules whereas in its absence occludin was prevalently localized in the basal region. Occludin production is known to be regulated by different cytokines including TGFbeta. We have investigated the role of HGF in the regulation of the levels of TGFbeta and we report that HGF significantly increases the amount of the active fraction of the factor without affecting the amount of the total TGFbeta. Urokinase type plasminogen activator (uPA) is closely related with the tight junctions and is one of the molecules able to activate the inactive TGF-beta. We found that HGF significantly increases the amount of uPA present in the testis suggesting that HGF regulates the amount of active TGFbeta via uPA levels. In conclusion we report that in the testis HGF regulates Sertoli-Sertoli tight junctions inducing a reduction and redistribution of occludin possibly modulating the levels of uPA and active TGFbeta.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

Cytokine knockouts in reproduction: the use of gene ablation to dissect roles of cytokines in reproductive biology

Cytokines play many diverse and important roles in reproductive biology, and dissecting the complex interactions between these proteins and the different reproductive organs is a difficult task. One approach is to use gene ablation, or 'knockout', to analyse the effect of deletion of a single cytokine on mouse reproductive function. This review summarizes the essential roles of cytokines in reproductive biology that have been revealed by gene knockout studies, including development and regulation of the hypothalamo-pituitary-gondal axis, ovarian folliculogenesis, implantation and immune system modulation during pregnancy. However, successful utilization of this approach must consider the caveats associated with gene ablation studies, e.g. embryonic lethality, systemic effects of cytokine ablation on local reproductive processes and the limited exposure to pathogens in mice housed in laboratory conditions. New sophisticated technology that temporally or spatially regulates gene ablation can overcome some of these limitations. Discoveries on the roles of cytokines in reproductive function uncovered by gene ablation studies can now be applied to improve in vitro fertilization for infertile couples and in the development of contraceptive therapies.

Serine proteases and serine protease inhibitors in testicular physiology: the plasminogen activation system

The testis is an organ in which a series of radical remodeling events occurs during development and in adult life. These events likely rely on a sophisticated network of proteases and complementary inhibitors, including the plasminogen activation system. This review summarizes our current knowledge on the testicular occurrence and expression pattern of members of the plasminogen activation system. The various predicted functions for these molecules in the establishment and maintenance of the testicular architecture and in the process of spermatogenesis are presented.

Claudin-11 expression and localisation is regulated by androgens in rat Sertoli cells in vitro

Claudin-11 and occludin are protein components in tight junctions (TJs) between Sertoli cells which are important for the maintenance of the blood-testis barrier. Barrier formation occurs during puberty, with evidence suggesting hormonal regulation of both claudin-11 and occludin. This study aimed to investigate the regulation of claudin-11 and occludin mRNA expression by testosterone (T) and FSH and their immunolocalisation at rat Sertoli cell TJs in vitro, and to correlate any steroid regulation with the functional capacity of TJs. Sertoli cells formed functional TJs within 3 days as assessed by transepithelial electrical resistance (TER). Both T and dihydrotestosterone significantly (P < 0.01) increased TER twofold and claudin-11 mRNA two- to threefold within 3 days. FSH partially stimulated TER and claudin-11 mRNA, but estradiol had no effect. T also promoted claudin-11 localisation into extensive intercellular contacts. In contrast to claudin-11, Tand FSH did not change occludin mRNA expression, however, T promoted localisation of occludin at cell contacts in a similar manner to claudin-11. Addition of flutamide to T-stimulated cells caused a twofold decrease in both TER and claudin-11 mRNA expression, and resulted in the loss of both proteins from cell contacts. This effect was reversible following flutamide removal. It is concluded that androgens i) co-regulate claudin-11 mRNA expression and TER, implicating claudin-11 in TJ formation and ii) promote the localisation of claudin-11 and occludin at Sertoli cell contacts. Hence, the ability of androgens to maintain spermatogenesis in vivo is partly via their effects on TJ proteins and regulation of the blood-testis barrier.

Regulation of cell junction dynamics by cytokines in the testis: a molecular and biochemical perspective

Studies in the past decade in the field have demonstrated the significance of cytokines in regulating epithelial and endothelial cell junctions including tight and anchoring junctions in multiple organs including the testis. There are mounting evidences in recent years that cytokines play a crucial role in the restructuring of junctions at the Sertoli-Sertoli and Sertoli-germ cell interface in the seminiferous epithelium during spermatogenesis. These earlier studies, however, were focused on the effects of cytokines in maintaining the steady-state protein levels of integral membrane proteins at the sites of the blood-testis barrier (BTB) and anchoring junctions at the Sertoli-Sertoli and Sertoli-germ cell interface, such as basal and apical ectoplasmic specialization, respectively. The molecular pathway(s) and/or mechanism(s) underlying these effects remained virtually unexplored until very recently. Herein, we summarize and provide some discussions on studies that focused on the role of cytokines in regulating junction restructuring events in epithelia from a molecular and biochemical perspective. Specifically, we use the adult rat or mouse testis as a model to highlight the significance of transcriptional and translational regulation. Specific areas of research that require further attentions are also highlighted.

Unraveling the molecular targets pertinent to junction restructuring events during spermatogenesis using the Adjudin-induced germ cell depletion model

During spermatogenesis, extensive restructuring takes place at the Sertoli-Sertoli and Sertoli-germ cell interface, which is regulated via intriguing interactions among cytokines, proteases, protease inhibitors, kinases, phosphatases, and transcription factors. This in turn determines the steady-state levels of integral membrane proteins at the cell junctions. We sought to further expand these observations using the Adjudin model. Adjudin is a potential male contraceptive that targets Sertoli-germ cell adhesion, causing exfoliation of spermatids and spermatocytes, but not spermatogonia, from the seminiferous epithelium. This model thus provides the means to identify crucial regulatory molecules and signaling pathways pertinent to junction restructuring events during spermatogenesis. In this study, genome-wide expression profiling of rat testes after treatment with Adjudin at the time of extensive junction restructuring was performed. Differentially regulated genes, such as cytokines, proteases, protease inhibitors, cell junction-associated proteins, and transcription factors pertinent to junction restructuring were identified. These data were consistent with earlier findings; however, much new information was obtained which has been deposited at the Gene Expression Omnibus data repository website: http://www.ncbi.nih.gov/geo/ with Accession number: GSE5131. The primary signaling events pertinent to junction restructuring in the testis induced by Adjudin were also delineated using bioinformatics. These findings were also consistent with recently published reports. The identified molecular signatures or targets pertinent to junction dynamics in the testis as reported herein, many of which have not been investigated, thus offer a framework upon which the regulation of junction restructuring events at the Sertoli-Sertoli and Sertoli-germ cell interface pertinent to spermatogenesis can be further studied.

C-type natriuretic peptide regulates blood-testis barrier dynamics in adult rat testes

In adult rat testes, the blood-testis barrier (BTB) in the seminiferous epithelium must "open" (or "disassemble") to accommodate the migration of preleptotene spermatocytes from the basal to the adluminal compartment that occurs at stage VIII of the epithelial cycle. However, the molecule(s) and/or mechanism(s) that regulate this event are unknown. In this report, C-type natriuretic peptide (CNP) was shown to be a regulator of BTB dynamics. Although Sertoli and germ cells contributed to the pool of CNP in the seminiferous epithelium, its receptor, natriuretic peptide receptor B, resided almost exclusively in Sertoli cells. CNP also expressed stage-specifically and localized predominantly at the BTB in the seminiferous epithelium at stage VIII of the epithelial cycle. A synthetic CNP-22 peptide, when added to Sertoli cell cultures, was shown to perturb Sertoli cell tight junction in vitro, causing disappearance of BTB-associated proteins (JAM-A, occludin, N-cadherin, and beta-catenin) from the cell-cell interface. This inhibitory effect of CNP on the tight junction was confirmed by transient overexpression of CNP in these cells, which was mediated, at least in part, by accelerating the internalization of BTB integral membrane proteins. To validate these in vitro findings, CNP-22 was administered to testes at a dose of 0.35 or 3.5 mug per testis, which was shown to perturb the BTB integrity In vivo when the barrier function was assessed by monitoring the diffusion of a small molecular probe across the BTB. In summary, CNP secreted by Sertoli and germ cells into the BTB microenvironment regulates BTB dynamics during spermatogenesis.

Densin is a novel cell membrane protein of sertoli cells in the testis

Cell-cell interactions between Sertoli cells and germ cells are crucial for the maturation of germ cells in spermatogenesis but the structural and functional aspects of the interactions remain to be fully elucidated. Densin is a junction protein suggested to play a role in establishment of specific cell-cell contacts in the post-synaptic densities of the brain and the slit diaphragm of the kidney podocyte. In the present study, densin was discovered to be expressed in the testis of the man and the mouse. Expression of densin at the gene and the protein level was studied by using RT-PCR and Western blotting analyses, and the localization of densin was explored with immunofluorescence staining. RT-PCR and Western blotting analyses showed that densin is expressed at the gene and the protein levels. Immunofluorescence staining localized the expression of densin to the cell membranes of Sertoli cells suggesting that densin may be an adherens junction protein between Sertoli cells and developing germ cells. Densin is a novel testicular protein expressed in the cell membranes of Sertoli cells. Its functional role remains to be assessed.

Perturbation of spermatogenesis by androgen antagonists directly injected into seminiferous tubules of live mice

Natural and artificial substances present in the environment can affect our health. Testicular toxicants in particular are troublesome, because they disturb gonadal function of males. Translocation of substances into the seminiferous epithelium where sperm production proceeds is restricted due to the blood–testis barrier, but this permeability barrier temporarily disappears under physiological and sub-physiological conditions. This means that any substance could enter the seminiferous epithelium and disturb sperm production. To reduce the risk posed by such toxins, it is important to accurately determine which substances possess the toxicity. However, existing assay systems are not satisfactory in terms of both accuracy and sensitivity. Here, we report the establishment of such a system. We injected the androgen antagonists, flutamide and vinclozolin, directly into seminiferous tubules of live mice, which had been treated with busulfan for a temporal arrest of spermatogenesis, and the testes were histologically examined to see the effect of the injected materials on spermatogenesis that was in the process of recovery. The injection of either substance brought about a severe impairment of spermatogenesis at an amount over a million times smaller than that used in the previous assay systems where animals are administered with test substances outside of the testis. In contrast, these androgen antagonists at the same doses showed lesser effects when intratubularly or intraperitoneally administered into mice that had not been pretreated with busulfan. We propose that the method adopted in this study is a novel assay system to identify potential testicular toxicants.

The testis in immune privilege

The production, differentiation, and presence of male gametes represent inimitable challenges to the immune system, as they are unique to the body and appear long after the maturation of the immune system and formation of systemic self-tolerance. Known to protect germ cells and foreign tissue grafts from autoimmune attack, the 'immune privilege' of the testis was originally, and somewhat simplistically, attributed to the existence of the blood-testis barrier. Recent research has shown a previously unknown level of complexity with a multitude of factors, both physical and immunological, necessary for the establishment and maintenance of the immunotolerance in the testis. Besides the blood-testis barrier and a diminished capability of the large testicular resident macrophage population to mount an inflammatory response, it is the constitutive expression of anti-inflammatory cytokines in the testis by immune and particularly somatic cells, that represents an essential element for local immunosuppression. The role of androgens in testicular immune regulation has long been underestimated; yet, accumulating evidence now shows that they orchestrate the inhibition of proinflammatory cytokine expression and shift cytokine balance toward a tolerogenic environment. Furthermore, the role of the testicular dendritic cells in suppressing antigen-specific immunity and T-lymphocyte activation is discussed. Finally, the active role mast cells play in the induction and amplification of immune responses, both in infertile humans and in experimental models, highlights the importance of preventing mast cell activation to maintain the immune-privileged status of the testis.

Blockage of testicular connexins induced apoptosis in rat seminiferous epithelium

Spermatogenesis, a tightly regulated developmental process of male germ cells in testis, is associated with temporal and spatial expression of gap junction proteins, such as the connexin family members. Perturbation of their expressions may lead to spermatogenic arrest as manifested by disruption of cell-cell interaction. To explore the role(s) of connexins during spermatogenesis, we utilized the small peptide antagonistic approach to specifically deplete connexin 31, connexin 33, and pan-connexin. Three connexin peptides corresponding to the extracellular binding domain of connexin 31 and connexin 33 and to the extracellular conserved domain of connexins were designed and synthesized commercially. Peptides (at single dosage of 0.5, 1, or 2 mg per animal) were injected into rat testes and testes were collected on day 0, 1, 3, 5, 10, 15, and 30 after microinjection. In situ TUNEL assay demonstrated the induction of apoptosis in the testes after pan-connexin peptide treatment in a dose-dependent manner from day 3 and onward. Unlike the pan-connexin peptide, connexin 31 and connexin 33 peptides appeared to have little effect on inducing apoptosis and germ cell loss. CD45 staining also detected the occasional presence of infiltrating lymphocytes in the seminiferous tubules. Accompanied with the apoptotic events, two apoptotic markers, NF-kappaB and caspase 3, demonstrated a general up-regulation in their expressions. In adjacent testis sections, eliminations of connexin 31, 32, and 43 were observed. However, an induction of connexin 33 expression was detected. This suggests the versatility and functional diversity of connexins in the testis. The expression of ZO-1, the only known adaptor of connexins in the testis, was reduced and remained in a low level in the seminiferous epithelium. As such, the alterations of connexins in seminiferous epithelium may induce apoptotic signaling in the testis via the caspase 3 and the NF-kappaB pathway. This demonstrates the significant role of testicular connexins to maintain the survival of germ cells by regulating inter-cellular communications among germ cells and adjacent supporting cells during spermatogenesis. In addition, the inter-relationship between connexins and other junction proteins and associated signaling protein were investigated. After pan-connexin peptide treatment, a dys-localization of N-cadherin, an adherens junction protein, and diminution of occludin, a tight junction protein, level were detected. In addition, inductions of junction regulatory protein, cathepsin L, was observed during the course of peptide-mediated germ cell loss in the testes. In summary, pan-connexin peptide treatment triggered apoptosis and germ cell loss in the testes. This event influenced the localization and expression of different junction proteins and junction-associated protein in the testes.

Non-macrophage-tropic human immunodeficiency virus type 1 R5 envelopes predominate in blood, lymph nodes, and semen: implications for transmission and pathogenesis

Human immunodeficiency virus type 1 (HIV-1) R5 isolates that predominantly use CCR5 as a coreceptor are frequently described as macrophage tropic. Here, we compare macrophage tropism conferred by HIV-1 R5 envelopes that were derived directly by PCR from patient tissue. This approach avoids potentially selective culture protocols used in virus isolation. Envelopes were amplified (i) from blood and semen of adult patients and (ii) from plasma of pediatric patients. The phenotypes of these envelopes were compared to those conferred by an extended panel of envelopes derived from brain and lymph node that we reported previously. Our results show that R5 envelopes vary by up to 1,000-fold in their capacity to confer infection of primary macrophages. Highly macrophage-tropic envelopes were predominate in brain but were infrequent in semen, blood, and lymph node samples. We also confirmed that the presence of N283 in the C2 CD4 binding site of gp120 is associated with HIV-1 envelopes from the brain but absent from macrophage-tropic envelopes amplified from blood and semen. Finally, we compared infection of macrophages, CD4(+) T cells, and peripheral blood mononuclear cells (PBMCs) conferred by macrophage-tropic and non-macrophage-tropic envelopes in the context of full-length replication competent viral clones. Non-macrophage-tropic envelopes conferred low-level infection of macrophages yet infected CD4(+) T cells and PBMCs as efficiently as highly macrophage-tropic brain envelopes. The lack of macrophage tropism for the majority of the envelopes amplified from lymph node, blood, and semen is striking and contrasts with the current consensus that R5 primary isolates are generally macrophage tropic. The extensive variation in R5 tropism reported here is likely to have an important impact on pathogenesis and on the capacity of HIV-1 to transmit.

Differential interactions between transforming growth factor-beta3/TbetaR1, TAB1, and CD2AP disrupt blood-testis barrier and Sertoli-germ cell adhesion

The biochemical basis that regulates the timely and selective opening of the blood-testis barrier (BTB) to migrating preleptotene/leptotene spermatocytes at stage VIII of the epithelial cycle in adult rat testes is virtually unknown. Recent studies have shown that cytokines (e.g. transforming growth factor (TGF)-beta3) may play a crucial role in this event. However, much of this information relies on the use of toxicants (e.g. CdCl(2)), making it difficult to relay these findings to normal testicular physiology. Here we report that overexpression of TGF-beta3 in primary Sertoli cells cultured in vitro indeed perturbed the tight junction (TJ) barrier with a concomitant decline in the production of BTB constituent proteins as follows: occludin, N-cadherin, and ZO-1. Additionally, local administration of TGF-beta3 to testes in vivo was shown to reversibly perturb the BTB integrity and Sertoli-germ cell adhesion via the p38 MAPK and ERK signaling pathways. Most importantly, the simultaneous activation of p38 and ERK signaling pathways is dependent on the association of the TGF-beta3-TbetaR1 complex with adaptors TAB1 and CD2AP because if TbetaR1 was associated preferentially with CD2AP, only Sertoli-germ cell adhesion was perturbed without compromising the BTB. Collectively, these data illustrate that local production of TGF-beta3, and perhaps other TGF-betas and cytokines, by Sertoli and germ cells into the microenvironment at the BTB during spermatogenesis transiently perturbs the BTB and Sertoli-germ cell adhesion to facilitate germ cell migration when the activated TbetaRI interacts with adaptors TAB1 and CD2AP. However, TGF-beta3 selectively disrupts Sertoli-germ cell adhesion in the seminiferous epithelium to facilitate germ cell migration without compromising BTB when TbetaRI interacts only with adaptor CD2AP.

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

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 alpha- and gamma-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.