Spermatogonial stem cells alone are not sufficient to re-initiate spermatogenesis in the rat testis following adjudin-induced infertility

Filamin A: A regulator of blood-testis barrier assembly during post-natal development

Filamins are a family of actin-binding proteins composed of filamin A, B and C. Besides of their ability to induce perpendicular branching of F-actin filaments via their actin binding domains near the N-terminus, filamins can regulate multiple cellular functions because of their unique ability to recruit more than 90 protein binding partners to their primary sequences which are having highly diversified cellular functions. However, this family of proteins has not been examined in the testis until recently. Herein, we highlight recent findings in the field regarding the role of these proteins in cell epithelia, and based on recent data in the testis regarding their role on spermatogenesis, this review provides the basis for future functional studies.

The blood-testis barrier and its implications for male contraception

The blood-testis barrier (BTB) is one of the tightest blood-tissue barriers in the mammalian body. It divides the seminiferous epithelium into the basal and the apical (adluminal) compartments. Meiosis I and II, spermiogenesis, and spermiation all take place in a specialized microenvironment behind the BTB in the apical compartment, but spermatogonial renewal and differentiation and cell cycle progression up to the preleptotene spermatocyte stage take place outside of the BTB in the basal compartment of the epithelium. However, the BTB is not a static ultrastructure. Instead, it undergoes extensive restructuring during the seminiferous epithelial cycle of spermatogenesis at stage VIII to allow the transit of preleptotene spermatocytes at the BTB. Yet the immunological barrier conferred by the BTB cannot be compromised, even transiently, during the epithelial cycle to avoid the production of antibodies against meiotic and postmeiotic germ cells. Studies have demonstrated that some unlikely partners, namely adhesion protein complexes (e.g., occludin-ZO-1, N-cadherin-β-catenin, claudin-5-ZO-1), steroids (e.g., testosterone, estradiol-17β), nonreceptor protein kinases (e.g., focal adhesion kinase, c-Src, c-Yes), polarity proteins (e.g., PAR6, Cdc42, 14-3-3), endocytic vesicle proteins (e.g., clathrin, caveolin, dynamin 2), and actin regulatory proteins (e.g., Eps8, Arp2/3 complex), are working together, apparently under the overall influence of cytokines (e.g., transforming growth factor-β3, tumor necrosis factor-α, interleukin-1α). In short, a "new" BTB is created behind spermatocytes in transit while the "old" BTB above transiting cells undergoes timely degeneration, so that the immunological barrier can be maintained while spermatocytes are traversing the BTB. We also discuss recent findings regarding the molecular mechanisms by which environmental toxicants (e.g., cadmium, bisphenol A) induce testicular injury via their initial actions at the BTB to elicit subsequent damage to germ-cell adhesion, thereby leading to germ-cell loss, reduced sperm count, and male infertility or subfertility. Moreover, we also critically evaluate findings in the field regarding studies on drug transporters in the testis and discuss how these influx and efflux pumps regulate the entry of potential nonhormonal male contraceptives to the apical compartment to exert their effects. Collectively, these findings illustrate multiple potential targets are present at the BTB for innovative contraceptive development and for better delivery of drugs to alleviate toxicant-induced reproductive dysfunction in men.

Adjudin disrupts spermatogenesis via the action of some unlikely partners: Eps8, Arp2/3 complex, drebrin E, PAR6 and 14-3-3

Adjudin, 1-(2,4-dichlorobenzyl)-1H-indazole-3-carbohydrazide (formerly called AF-2364), is a potent analog of lonidamine [1-(2,4-dichlorobenzyl)-1H-indazole-3-carboxylic acid] known to disrupt germ cell adhesion, most notably elongating and elongated spermatids, in the seminiferous epithelium of adult rat testes and thus, leads to infertility in rats. Since the population of spermatogonia and spermatogonial stem cells (SSCs) in the seminiferous tubules is not significantly reduced by the treatment of rats with adjudin, adjudin-induced infertility is highly reversible, which enables reinitiation of spermatogenesis and germ cell re-population of the voided seminiferous epithelium. Furthermore, adjudin appears to exert its effects at the testis-specific atypical adherens junction (AJ) type known as ectoplasmic specialization (ES), most notably the apical ES at the Sertoli cell-spermatid interface. Thus, the hypothalamic-pituitary-gonadal axis is not unaffected and systemic side-effects are minimal. This also makes adjudin a potential candidate for male contraceptive development. Herein, we critically evaluate recent findings in the field and provide an updated model regarding the mechanism underlying adjudin-induced apical ES disruption. In short, adjudin targets actin filament bundles at the apical ES, the hallmark ultrastructure of this testis-specific junction type not found in any other epithelia/endothelia in mammals, by suppressing the expression of Eps8 (epidermal growth factor receptor pathway substrate 8), an actin capping protein that also plays a role in actin bundling, so that actin filament bundles can no longer be maintained at the apical ES. This is concomitant with a mis-localization of Arp3 (actin-related protein 3, a component of the Arp2/3 complex that induces actin nucleation/branching) recruited by drebrin E, causing "unwanted" actin branching, further destabilizing actin filament bundles at the apical ES. Additionally, adjudin blocks the expression of PAR6 (partitioning defective protein 6) and 14-3-3 (also known as PAR5) considerably at the apical ES, disrupting the homeostasis of endocytic vesicle-mediated protein trafficking, which in turn leads to an increase in protein endocytosis. The net result of these changes destabilizes cell adhesion and induces degeneration of the apical ES, causing premature release of spermatids, mimicking spermiation.

Interactions of laminin β3 fragment with β1-integrin receptor: A revisit of the apical ectoplasmic specialization-blood-testis-barrier-hemidesmosome functional axis in the testis

Recent studies have demonstrated the presence of a functional axis that coordinates the events of spermiation and blood-testis barrier (BTB) restructuring which take place simultaneously at the opposite ends of the seminiferous epithelium at stage VIII of the epithelial cycle of spermatogenesis in the rat testis. In short, the disruption of the apical ectoplasmic specialization (apical ES) at the Sertoli cell-elongated spermatid interface, which facilitates the release of sperm at spermiation near the tubule lumen, is coordinated with restructuring at the BTB to accommodate the transit of preleptotene spermatocytes across the immunological barrier near the basement membrane. These two events are likely coordinated by a functional axis involving hemidesmosome at the Sertoli cell-basement membrane interface, and it was designated the apical ES-BTB-hemidesmosome axis. It was demonstrated that fragments of laminin chains (e.g., laminin β3 or γ3 chains) derived from the α6β1-integrin-laminin333 protein complex at the apical ES, which were likely generated via the action of MMP-2 (matrix metalloprotease-2, MMP2) prior to spermiation, acted as biologically active peptides to perturb the BTB permeability function by accelerating protein endocytosis (e.g., occludin) at the site, thereby destabilizing the BTB integrity to facilitate the transit of preleptotene spermatocytes. These laminin fragments also perturbed hemidesmosome function via their action on β1-integrin, a component of hemidesmosome in the testis, which in turn, sent a signal to further destabilize the BTB function. As such, the events of spermiation and BTB restructuring are coordinated via this functional axis. Recent studies using animal models treated with toxicants, such as mono-(2-ethylhexyl) phthalate (MEHP), or adjudin, a male contraceptive under investigation, have also supported the presence of this functional axis in the mouse. In this short review, we critically evaluate the role of this local functional axis in the seminiferous epithelium in spermatogenesis. We also provide molecular modeling information on the interactions between biologically active laminin fragments and β1-integrin, which will be important to assist in the design of more potent laminin-based peptides to disrupt this axis, thereby perturbing spermatogenesis for male contraception and to understand the underlying biology that coordinates spermiation and BTB restructuring during spermatogenesis.

A study to assess the assembly of a functional blood-testis barrier in developing rat testes

The blood-testis barrier (BTB) is an important ultrastructure in the seminiferous tubule of the mammalian testis that segregates the events of spermatogenesis, in particular post-meiotic germ cell development, from the harmful substances in the environment including toxicants and drugs, as well as from the unwanted hormones and biomolecules in the systemic circulation. It is known that the BTB is assembled by ∼15-21 days postpartum (dpp) in rats coinciding with the onset of late cell cycle progression, namely the formation of zygotene and pachytene spermatocytes by day 15-18 dpp. This is to prepare for: (1) the differentiation/transformation of pachytene spermatocytes to diplotene and dictyate spermatocytes and (2) meiosis I and II, which take place by 23-26 and 26 dpp, respectively. Recent findings have shown spermatogonia/spermatogonial stem cells (SSC) in the tubules failed to re-initiate spermatogenesis by differentiating spermatogonia beyond type A spermatogonia in the absence of a functional BTB, leading to meiotic arrest. These studies thus illustrate that a functional BTB is crucial to the initiation and/or re-initiation of spermatogenesis. Herein, we sought to examine the precise time window when a functional and intact BTB is established in the developing rat testis during the final stage of cell cycle progression and meiosis. Using the techniques of: (1) dual-labeled immunofluorescence analysis to assess the distribution of integrated proteins at the tight junction (TJ), basal ectoplasmic specialization [basal ES, a testis-specific atypical adherens junction (AJ) type] and gap junction (GJ) at the BTB, (2) functional assay to assess the BTB integrity in vivo, (3) immunoblot analysis to monitor changes in steady-state levels of adhesion proteins at the BTB, and (4) co-immunoprecipitation to assess changes in protein-protein interactions at the BTB, it was shown that a BTB was being assembled by day 15-20 dpp, but a functional BTB was not fully established until day 25 dpp in Sprague-Dawley rats, tightly associated with the onset of meiosis I and II. These findings thus illustrate the significance of the BTB on cell cycle progression and the preparation for meiosis, such as germ cell differentiation beyond type A spermatogonia.

Regulation of blood-testis barrier dynamics by desmosome, gap junction, hemidesmosome and polarity proteins: An unexpected turn of events

The blood-testis barrier (BTB) is a unique ultrastructure in the mammalian testis. Unlike other blood-tissue barriers, such as the blood-brain barrier and the blood-ocular (or blood-retina) barrier which formed by tight junctions (TJ) between endothelial cells of the microvessels, the BTB is constituted by coexisting TJ, basal ectoplasmic specialization (basal ES), desmosomes and gap junctions between adjacent Sertoli cells near the basement membrane of the seminiferous tubule. The BTB also divides the seminiferous epithelium into the apical (or adluminal) and basal compartments so that meiosis I and II and post-meiotic germ cell development can all take place in a specialized microenvironment in the apical compartment behind the BTB. While the unusual anatomical features of the BTB have been known for decades, the physiological function of the coexisting junctions, in particular the desmosome and gap junction, that constitute the BTB was unknown until recently. Based on recently published findings, we critically evaluate the role of the desmosome and gap junction that serve as a signaling platform to coordinate the "opening" and "closing" of the TJ-permeability barrier conferred by TJ and basal ES during the seminiferous epithelial cycle of spermatogenesis. This is made possible by polarity proteins working in concert with nonreceptor protein tyrosine kinases, such as focal adhesion kinase (FAK) and c-Src, at the site to regulate endosome-mediated protein trafficking events (e.g., endocytosis, transcytosis, recycling or protein degradation). These events not only serve to destabilize the existing "old" BTB above preleptotene spermatocytes in transit in "clones" at the BTB, but also contribute to the assembly of "new" BTB below the transiting spermatocytes. Furthermore, hemidesmosomes at the Sertoli cell-basement membrane interface also contribute to the BTB restructuring events at stage VIII of the epithelial cycle. Additionally, the findings that a gap junction at the BTB provides a possible route for the passage of toxicants [e.g., bisphenol A (BPA)] and potential male contraceptives (e.g., adjudin) across the BTB also illustrate that these coexisting junctions, while helpful to maintain the immunological barrier integrity during the transit of spermatocytes, can be the "gateway" to making the BTB so vulnerable to toxicants and/or chemicals, causing male reproductive dysfunction.

The β1-integrin-p-FAK-p130Cas-DOCK180-RhoA-vinculin is a novel regulatory protein complex at the apical ectoplasmic specialization in adult rat testes

During spermatogenesis, step 1 spermatids (round spermatids) derive from spermatocytes following meiosis I and II at stage XIV of the epithelial cycle begin a series of morphological transformation and differentiation via 19 steps in rats to form spermatozoa. This process is known as spermiogenesis, which is marked by condensation of the genetic material in the spermatid head, formation of the acrosome and elongation of the tail. Since developing spermatids are lacking the robust protein synthesis and transcriptional activity, the cellular, molecular and morphological changes associated with spermiogenesis rely on the Sertoli cell in the seminiferous epithelium via desmosome and gap junction between Sertoli cells and step 1-7 spermatids. Interestingly, a unique anchoring junction type arises at the interface of step 8 spermatid and Sertoli cell known as apical ectoplasmic specialization (apical ES). Once it appears, apical ES is the only anchoring device restricted to the interface of step 8-19 spermatids and Sertoli cells to confer spermatid polarity, adhesion, signal communication and structural support, and to provide nutritional support during spermiogenesis, replacing desmosome and gap junction. While the adhesion protein complexes that constitute the apical ES are known, the signaling protein complexes that regulate apical ES dynamics, however, remain largely unknown. Herein we report the presence of a FAK (focal adhesion kinase)-p130Cas (p130 Crk-associated substrate)-DOCK180 (Dedicator of cytokinesis 180)-RhoA (Ras homolog gene family, member A)-vinculin signaling protein complex at the apical ES, which is also an integrated component of the β1-integrin-based adhesion protein complex based on co-immunoprecipitation experiment. It was also shown that besides p-FAK-Tyr(397) and p-FAK-Tyr(576), β1-integrin, p130Cas, RhoA and vinculin displayed stage-specific expression in the seminiferous epithelium during the epithelial cycle with predominant localization at the apical ES as demonstrated by immunohistochemistry. Based on these findings, functional studies can now be performed to assess the role of this β1-integrin-p-FAK-p130Cas-DOCK180-RhoA-vinculin protein complex in apical ES dynamics during spermiogenesis.