Actin binding proteins and spermiogenesis: Some unexpected findings

Optimization of spermatozoa analysis in mice: A comprehensive protocol

Sperm quality is critical to predict reproductive alterations caused by immunological factors or toxicant agents. Yet, no detailed protocol has been published focusing on analyses of sperm parameters in mice. Our aim was to evaluate the most efficient diluent for mice sperm analyses and to optimize the sperm morphology classification, through the comparison of different staining methods. The diluents assessed were PBS (baseline), HTF, DMEM, 1 % BSA in PBS and 9 % skimmed powdered milk diluted in PBS. Spermatozoa were evaluated for vitality, motility, and morphology, smears were stained with Papanicolaou, HE, Giemsa, and Rapid staining. Sperm vitality and total motility reached better scores in milk based and DMEM diluents. HE raised up as an effective option since its combination with any of the diluents we tested, resulted in a fair staining, which was appropriated to evaluate mice spermatozoa. Finally, based on WHO manual, we have updated the current morphological classification for mice sperm, since we have detailed the head defects as well as included midpiece and tail defects on it. Taken together, we presented a useful, low cost, and reliable method to assess sperm morphology that could be employed worldwide by laboratories dedicated to study reproductive biology on mice model.

Overexpression of PD-L1 causes germ cells to slough from mouse seminiferous tubules via the PD-L1/PD-L1 interaction

Spermatogenesis is a cyclical process in which different generations of spermatids undergo a series of developmental steps at a fixed time and finally produce spermatids. Here, we report that overexpression of PD‐L1 (B7 homolog1) in the testis causes sperm developmental disorders and infertility in male mice, with severe malformation and sloughing during spermatid development, characterized by disorganized and collapsed seminiferous epithelium structure. PD‐L1 needs to be simultaneously expressed on Sertoli cells and spermatogonia to cause spermatogenesis failure. After that, we excluded the influence of factors such as the PD‐L1 receptor and humoral regulation, confirming that PD‐L1 has an intrinsic function to interact with PD‐L1. Studies have shown that PD‐L1 not only serves as a ligand but also plays a receptor‐like role in signal transduction. PD‐L1 interacts with PD‐L1 to affect the adhesive function of germ cells, causing malformation and spermatid sloughing. Taken together, these results indicate that PD‐L1 can interact with PD‐L1 to cause germ cell detachment and male infertility.

The dynamics and regulation of microfilament during spermatogenesis

Spermatogenesis is a highly complex physiological process which contains spermatogonia proliferation, spermatocyte meiosis and spermatid morphogenesis. In the past decade, actin binding proteins and signaling pathways which are critical for regulating the actin cytoskeleton in testis had been found. In this review, we summarized 5 actin-binding proteins that have been proven to play important roles in the seminiferous epithelium. Lack of them perturbs spermatids polarity and the transport of spermatids. The loss of Arp2/3 complex, Formin1, Eps8, Palladin and Plastin3 cause sperm release failure suggesting their irreplaceable role in spermatogenesis. Actin regulation relies on multiple signal pathways. The PI3K/Akt signaling pathway positively regulate the mTOR pathway to promote actin reorganization in seminiferous epithelium. Conversely, TSC1/TSC2 complex, the upstream of mTOR, is activated by the LKB1/AMPK pathway to inhibit cell proliferation, differentiation and migration. The increasing researches focus on the function of actin binding proteins (ABPs), however, their collaborative regulation of actin patterns and potential regulatory signaling networks remains unclear. We reviewed ABPs that play important roles in mammalian spermatogenesis and signal pathways involved in the regulation of microfilaments. We suggest that more relevant studies should be performed in the future.

Multiple aspects of male germ cell development and interactions with Sertoli cells require inositol hexakisphosphate kinase-1

Inositol hexakisphosphate kinase-1 (IP6K1) is required for male fertility, but the underlying mechanisms have been elusive. Here, we report that IP6K1 is required for multiple aspects of male germ cell development. This development requires selective interactions between germ cells and Sertoli cells, namely apical ectoplasmic specialization. Spermiation (sperm release) requires tubulobulbar complexes. We found that the apical ectoplasmic specialization and tubulobulbar complexes were poorly formed or disrupted in IP6K1 KOs. Deletion of IP6K1 elicited several aberrations, including: 1, sloughing off of round germ cells; 2, disorientation and malformation of elongating/elongated spermatids; 3, degeneration of acrosomes; 4, defects in germ-Sertoli cell interactions and 5, failure of spermiation. Eventually the sperm cells were not released but phagocytosed by Sertoli cells leading to an absence of sperm in the epididymis.

Expression and localization of myosin VI in developing mouse spermatids

Myosin VI (MVI) is a versatile actin-based motor protein that has been implicated in a variety of different cellular processes, including endo- and exocytic vesicle trafficking, Golgi morphology, and actin structure stabilization. A role for MVI in crucial actin-based processes involved in sperm maturation was demonstrated in Drosophila. Because of the prominence and importance of actin structures in mammalian spermiogenesis, we investigated whether MVI was associated with actin-mediated maturation events in mammals. Both immunofluorescence and ultrastructural analyses using immunogold labeling showed that MVI was strongly linked with key structures involved in sperm development and maturation. During the early stage of spermiogenesis, MVI is associated with the Golgi and with coated and uncoated vesicles, which fuse to form the acrosome. Later, as the acrosome spreads to form a cap covering the sperm nucleus, MVI is localized to the acroplaxome, an actin-rich structure that anchors the acrosome to the nucleus. Finally, during the elongation/maturation phase, MVI is associated with the actin-rich structures involved in nuclear shaping: the acroplaxome, manchette, and Sertoli cell actin hoops. Since this is the first report of MVI expression and localization during mouse spermiogenesis and MVI partners in developing sperm have not yet been identified, we discuss some probable roles for MVI in this process. During early stages, MVI is hypothesized to play a role in Golgi morphology and function as well as in actin dynamics regulation important for attachment of developing acrosome to the nuclear envelope. Next, the protein might also play anchoring roles to help generate forces needed for spermatid head elongation. Moreover, association of MVI with actin that accumulates in the Sertoli cell ectoplasmic specialization and other actin structures in surrounding cells suggests additional MVI functions in spermatid movement across the seminiferous epithelium and in sperm release.

Adjudin--A Male Contraceptive with Other Biological Activities

BACKGROUND

Adjudin has been explored as a male contraceptive for the last 15 years since its initial synthesis in the late 1990s. More than 50 papers have been published and listed in PubMed in which its mechanism that induces exfoliation of germ cells from the seminiferous epithelium, such as its effects on actin microfilaments at the apical ES (ectoplasmic specialization, a testis-specific actin-rich anchoring junction) has been delineated.

OBJECTIVE

Recent studies have demonstrated that, besides its activity to induce germ cell exfoliation from the seminiferous epithelium to cause reversible infertility in male rodents, adjudin possesses other biological activities, which include anti-cancer, anti-inflammation in the brain, and anti-ototoxicity induced by gentamicin in rodents. Results of these findings likely spark the interest of investigators to explore other medical use of this and other indazole-based compounds, possibly mediated by the signaling pathway(s) in the mitochondria of mammalian cells following treatment with adjudin. In this review, we carefully evaluate these recent findings.

METHODS

Papers published and listed at www.pubmed.org and patents pertinent to adjudin and its related compounds were searched. Findings were reviewed and critically evaluated, and summarized herein.

RESULTS

Adjudin is a novel compound that possesses anti-spermatogenetic activity. Furthermore, it possesses anti-cancer, anti-inflammation, anti-neurodegeneration, and anti-ototoxicity activities based on studies using different in vitro and in vivo models.

CONCLUSION

Studies on adjudin should be expanded to better understand its biological activities so that it can become a useful drug for treatment of other ailments besides serving as a male contraceptive.

Overexpression of plastin 3 in Sertoli cells disrupts actin microfilament bundle homeostasis and perturbs the tight junction barrier

Throughout the epithelial cycle of spermatogenesis, actin microfilaments arranged as bundles near the Sertoli cell plasma membrane at the Sertoli cell-cell interface that constitute the blood-testis barrier (BTB) undergo extensive re-organization by converting between bundled and unbundled/branched configuration to give plasticity to the F-actin network. This is crucial to accommodate the transport of preleptotene spermatocytes across the BTB. Herein, we sought to examine changes in the actin microfilament organization at the Sertoli cell BTB using an in vitro model since Sertoli cells cultured in vitro is known to establish a functional tight junction (TJ)-permeability barrier that mimics the BTB in vivo. Plastin 3, a known actin microfilament cross-linker and bundling protein, when overexpressed in Sertoli cells using a mammalian expression vector pCI-neo was found to perturb the Sertoli cell TJ-barrier function even though its overexpression increased the overall actin bundling activity in these cells. Furthermore, plastin 3 overexpression also perturbed the localization and distribution of BTB-associated proteins, such as occludin-ZO1 and N-cadherin-β-catenin, this thus destabilized the barrier function. Collectively, these data illustrate that a delicate balance of actin microfilaments between organized in bundles vs. an unbundled/branched configuration is crucial to confer the homeostasis of the BTB and its integrity.

Actin-bundling protein plastin 3 is a regulator of ectoplasmic specialization dynamics during spermatogenesis in the rat testis

Ectoplasmic specialization (ES) is an actin-rich adherens junction in the seminiferous epithelium of adult mammalian testes. ES is restricted to the Sertoli-spermatid (apical ES) interface, as well as the Sertoli cell-cell (basal ES) interface at the blood-testis barrier (BTB). ES is typified by the presence of an array of bundles of actin microfilaments near the Sertoli cell plasma membrane. These actin microfilament bundles require rapid debundling to convert them from a bundled to branched/unbundled configuration and vice versa to confer plasticity to support the transport of 1) spermatids in the adluminal compartment and 2) preleptotene spermatocytes at the BTB while maintaining cell adhesion. Plastin 3 is one of the plastin family members abundantly found in yeast, plant and animal cells that confers actin microfilaments their bundled configuration. Herein, plastin 3 was shown to be a component of the apical and basal ES in the rat testis, displaying spatiotemporal expression during the epithelial cycle. A knockdown (KD) of plastin 3 in Sertoli cells by RNA interference using an in vitro model to study BTB function showed that a transient loss of plastin 3 perturbed the Sertoli cell tight junction-permeability barrier, mediated by changes in the localization of basal ES proteins N-cadherin and β-catenin. More importantly, these changes were the result of an alteration of the actin microfilaments, converting from their bundled to branched configuration when examined microscopically, and validated by biochemical assays that quantified actin-bundling and polymerization activity. Moreover, these changes were confirmed by studies in vivo by plastin 3 KD in the testis in which mis-localization of N-cadherin and β-catenin was also detected at the BTB, concomitant with defects in the transport of spermatids and phagosomes and a disruption of cell adhesion most notably in elongated spermatids due to a loss of actin-bundling capability at the apical ES, which in turn affected localization of adhesion protein complexes at the site. In summary, plastin 3 is a regulator of actin microfilament bundles at the ES in which it dictates the configuration of the filamentous actin network by assuming either a bundled or unbundled/branched configuration via changes in its spatiotemporal expression during the epithelial cycle.

Germ cell transport across the seminiferous epithelium during spermatogenesis

Transport of germ cells across the seminiferous epithelium is crucial to spermatogenesis. Its disruption causes infertility. Signaling molecules, such as focal adhesion kinase, c-Yes, c-Src, and intercellular adhesion molecules 1 and 2, are involved in these events by regulating actin-based cytoskeleton via their action on actin-regulating proteins, endocytic vesicle-mediated protein trafficking, and adhesion protein complexes. We critically evaluate these findings and provide a hypothetical framework that regulates these events.

Actin cross-linking protein palladin and spermatogenesis

In the seminiferous epithelium of the mammalian testis, the most distinctive ultrastructure is the extensive bundles of actin filaments that lie near the Sertoli-spermatid interface and the Sertoli-Sertoli cell interface known as the apical ectoplasmic specialization (apical ES) and the basal ES, respectively. These actin filament bundles not only confer strong adhesion at these sites, they are uniquely found in the testis. Recent studies have shown that ES also confers spermatid and Sertoli cell polarity in the seminiferous epithelium during the epithelial cycle. While these junctions were first described in the 1970s, there are few functional studies in the literature to examine the regulation of these actin filament bundles. It is conceivable that these actin filament bundles at the ES undergo extensive re-organization to accommodate changes in location of developing spermatids during spermiogenesis as spermatids are transported across the seminiferous epithelium. Additionally, these actin filaments are rapidly reorganized during BTB restructuring to accommodate the transit of preleptotene spermatocytes across the barrier at stage VIII of the epithelial cycle. Thus, actin binding and regulatory proteins are likely involved in these events to confer changes in F-actin organization at these sites. Interestingly, there are no reports in the field to study these regulatory proteins until recently. Herein, we summarize some of the latest findings in the field regarding a novel actin cross-linker and actin-bundling protein called palladin. We also discuss in this opinion article the likely role of palladin in regulating actin filament bundles at the ES during spermatogenesis, highlighting the significant of palladin and how this protein is plausibly working in concert with other actin-binding/regulatory proteins and components of polarity proteins to regulate the cyclic events of actin organization and re-organization during the epithelial cycle of spermatogenesis. We also propose a hypothetic model by which palladin regulates ES restructuring during the epithelial cycle of spermatogenesis.

Actin binding proteins, spermatid transport and spermiation

The transport of germ cells across the seminiferous epithelium is composed of a series of cellular events during the epithelial cycle essential to the completion of spermatogenesis. Without the timely transport of spermatids during spermiogenesis, spermatozoa that are transformed from step 19 spermatids in the rat testis fail to reach the luminal edge of the apical compartment and enter the tubule lumen at spermiation, thereby arriving the epididymis for further maturation. Step 19 spermatids and/or sperms that remain in the epithelium beyond stage VIII of the epithelial cycle will be removed by the Sertoli cell via phagocytosis to form phagosomes and be degraded by lysosomes, leading to subfertility and/or infertility. However, the biology of spermatid transport, in particular the final events that lead to spermiation remain elusive. Based on recent data in the field, we critically evaluate the biology of spermiation herein by focusing on the actin binding proteins (ABPs) that regulate the organization of actin microfilaments at the Sertoli-spermatid interface, which is crucial for spermatid transport during this event. The hypothesis we put forth herein also highlights some specific areas of research that can be pursued by investigators in the years to come.

Focal adhesion kinase is a regulator of F-actin dynamics: New insights from studies in the testis

During spermatogenesis, spermatogonia (2n, diploid) undergo a series of mitotic divisions as well as differentiation to become spermatocytes, which enter meiosis I to be followed by meiosis II to form round spermatids (1n, haploid), and then differentiate into spermatozoa (1n, haploid) via spermiogenesis. These events take place in the epithelium of the seminiferous tubule, involving extensive junction restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface to allow the transport of developing germ cells across the epithelium. Although structural aspects of these cell-cell junctions have been studied, the underlying mechanism(s) that governs these events has yet to be explored. Earlier studies have shown that a non-receptor protein tyrosine kinase known as focal adhesion kinase (FAK) is a likely regulator of these events due to the stage-specific and spatiotemporal expression of its various phosphorylated/activated forms at the testis-specific anchoring junctions in the testis, as well as its association with actin regulatory proteins. Recent studies have shown that FAK, in particular its two activated phosphorylated forms p-FAK-Tyr⁴⁰⁷ and p-FAK-Tyr³⁹⁷, are crucial regulators in modulating junction restructuring at the Sertoli cell-cell interface at the blood-testis barrier (BTB) known as the basal ectoplasmic specialization (basal ES), as well as at the Sertoli-spermatid interface called apical ES during spermiogenesis via its effects on the filamentous (F)-actin organization at the ES. We herein summarize and critically evaluate the current knowledge regarding the physiological significance of FAK in regulating BTB and apical ES dynamics by governing the conversion of actin filaments at the ES from a “bundled” to a “de-bundled/branched” configuration and vice versa. We also provide a molecular model on the role of FAK in regulating these events based on the latest findings in the field.

Breast cancer resistance protein (Bcrp) and the testis--an unexpected turn of events

Breast cancer resistance protein (Bcrp) is an ATP-dependent efflux drug transporter. It has a diverse spectrum of hydrophilic and hydrophobic substrates ranging from anticancer, antiviral and antihypertensive drugs, to organic anions, antibiotics, phytoestrogens (e.g., genistein, daidzein, coumestrol), xenoestrogens and steroids (e.g., dehydroepiandrosterone sulfate). Bcrp is an integral membrane protein in cancer and normal cells within multiple organs (e.g., brain, placenta, intestine and testis) that maintains cellular homeostasis by extruding drugs and harmful substances from the inside of cells. In the brain, Bcrp is a major component of the blood-brain barrier located on endothelial cells near tight junctions (TJs). However, Bcrp is absent at the Sertoli cell blood-testis barrier (BTB); instead, it is localized almost exclusively to the endothelial TJ in microvessels in the interstitium and the peritubular myoid cells in the tunica propria. Recent studies have shown that Bcrp is also expressed stage specifically and spatiotemporally by Sertoli and germ cells in the seminiferous epithelium of rat testes, limited only to a testis-specific cell adhesion ultrastructure known as the apical ectoplasmic specialisation (ES) in stage VI-early VIII tubules. These findings suggest that Bcrp is equipped by late spermatids and Sertoli cells to protect late-stage spermatids completing spermiogenesis. Furthermore, Bcrp was found to be associated with F (filamentous)-actin and several actin regulatory proteins at the apical ES and might be involved in the organisation of actin filaments at the apical ES in stage VII-VIII tubules. These findings will be carefully evaluated in this brief review.

The apical ES-BTB-BM functional axis is an emerging target for toxicant-induced infertility

Testes are sensitive to toxicants, such as cadmium and phthalates, which disrupt a local functional axis in the seminiferous epithelium known as the 'apical ectoplasmic specialization (apical ES)-blood-testis barrier (BTB)-basement membrane (BM)'. Following exposure, toxicants contact the basement membrane and activate the Sertoli cell, which perturbs its signaling function. Thus, toxicants can modulate signaling and/or cellular events at the apical ES-BTB-BM axis, perturbing spermatogenesis without entering the epithelium. Toxicants also enter the epithelium via drug transporters to potentiate their damaging effects, and downregulation of efflux transporters by toxicants impedes BTB function such that toxicants remain in the epithelium and efficiently disrupt spermatogenesis. These findings support a novel model of toxicant-induced disruption of spermatogenesis that could be interfered with using small molecules.

Palladin is a regulator of actin filament bundles at the ectoplasmic specialization in adult rat testes

In rat testes, the ectoplasmic specialization (ES) at the Sertoli-Sertoli and Sertoli-spermatid interface known as the basal ES at the blood-testis barrier and the apical ES in the adluminal compartment, respectively, is a testis-specific adherens junction. The remarkable ultrastructural feature of the ES is the actin filament bundles that sandwiched in between the cisternae of endoplasmic reticulum and apposing plasma membranes. Although these actin filament bundles undergo extensive reorganization to switch between their bundled and debundled state to facilitate blood-testis barrier restructuring and spermatid adhesion/transport, the regulatory molecules underlying these events remain unknown. Herein we report findings of an actin filament cross-linking/bundling protein palladin, which displayed restrictive spatiotemporal expression at the apical and the basal ES during the epithelial cycle. Palladin structurally interacted and colocalized with Eps8 (epidermal growth factor receptor pathway substrate 8, an actin barbed end capping and bundling protein) and Arp3 (actin related protein 3, which together with Arp2 form the Arp2/3 complex to induce branched actin nucleation, converting bundled actin filaments to an unbundled/branched network), illustrating its role in regulating actin filament bundle dynamics at the ES. A knockdown of palladin in Sertoli cells in vitro with an established tight junction (TJ)-permeability barrier was found to disrupt the TJ function, which was associated with a disorganization of actin filaments that affected protein distribution at the TJ. Its knockdown in vivo also perturbed F-actin organization that led to a loss of spermatid polarity and adhesion, causing defects in spermatid transport and spermiation. In summary, palladin is an actin filament regulator at the ES.

Regulation of actin dynamics and protein trafficking during spermatogenesis--insights into a complex process

In the mammalian testis, extensive restructuring takes place across the seminiferous epithelium at the Sertoli-Sertoli and Sertoli-germ cell interface during the epithelial cycle of spermatogenesis, which is important to facilitate changes in the cell shape and morphology of developing germ cells. However, precise communications also take place at the cell junctions to coordinate the discrete events pertinent to spermatogenesis, namely spermatogonial renewal via mitosis, cell cycle progression and meiosis, spermiogenesis and spermiation. It is obvious that these cellular events are intimately related to the underlying actin-based cytoskeleton which is being used by different cell junctions for their attachment. However, little is known on the biology and regulation of this cytoskeleton, in particular its possible involvement in endocytic vesicle-mediated trafficking during spermatogenesis, which in turn affects cell adhesive function and communication at the cell-cell interface. Studies in other epithelia in recent years have shed insightful information on the intimate involvement of actin dynamics and protein trafficking in regulating cell adhesion and communications. The goal of this critical review is to provide an updated assessment of the latest findings in the field on how these complex processes are being regulated during spermatogenesis. We also provide a working model based on the latest findings in the field including our laboratory to provide our thoughts on an apparent complicated subject, which also serves as the framework for investigators in the field. It is obvious that this model will be rapidly updated when more data are available in future years.

The Scribble/Lgl/Dlg polarity protein complex is a regulator of blood-testis barrier dynamics and spermatid polarity during spermatogenesis

During spermatogenesis, spermiogenesis that releases sperm into the tubule lumen and restructuring of the blood-testis barrier (BTB) that accommodates the transit of preleptotene spermatocytes take place simultaneously, but at the opposite ends of the seminiferous epithelium. These events are tightly regulated and coordinated; however, neither the underlying mechanism(s) nor the involving molecules are known. Herein, the Scribble/Lgl (Lethal giant larvae)/Dlg (Discs large) polarity complex was shown to regulate spermatid polarity during spermiogenesis and tight junction (TJ)-permeability barrier via changes in protein distribution at the apical ectoplasmic specialization and the BTB during the epithelial cycle, respectively. Scribble, Lgl2, and Dlg1 were found to be expressed by Sertoli and germ cells. Scribble also displayed stage-specific expression at the BTB, being highest at stages VII-VIII, colocalizing with TJ proteins occludin and ZO-1. Unlike components of other polarity complex modules, such as partitioning-defective 6, the knockdown of which by RNA interference was found to impede Sertoli cell TJ barrier, a knockdown of the Scribble complex (i.e. simultaneous knockdown of Scribble, Lgl and Dlg or Lgl alone; but not Scribble or Dlg alone) both in vitro and in vivo promoted the TJ integrity. This was mediated by reorganizing actin filament network at the Sertoli cell-cell interface, which, in turn, affected changes in the localization and/or distribution of occludin and/or β-catenin at the BTB. These knockdowns also perturbed F-actin organization at the Sertoli cell-spermatid interface, thereby modulating spermatid adhesion and polarity at the apical ectoplasmic specialization. In summary, the Scribble/Lgl/Dlg complex participates in the regulation of BTB dynamics and spermatid adhesion/polarity in the testis.

Filamin A is a regulator of blood-testis barrier assembly during postnatal development in the rat testis

The blood-testis barrier (BTB) is an important ultrastructure in the testis. A delay in its assembly during postnatal development leads to meiotic arrest. Also, a disruption of the BTB by toxicants in adult rats leads to a failure in spermatogonial differentiation. However, the regulation of BTB assembly remains unknown. Herein, filamin A, an actin filament cross-linker that is known to maintain and regulate cytoskeleton structure and function in other epithelia, was shown to be highly expressed during the assembly of Sertoli cell BTB in vitro and postnatal development of BTB in vivo, perhaps being used to maintain the actin filament network at the BTB. A knockdown of filamin A by RNA interference was found to partially perturb the Sertoli cell tight junction (TJ) permeability barrier both in vitro and in vivo. Interestingly, this down-regulating effect on the TJ barrier function after the knockdown of filamin A was associated with a mis-localization of both TJ and basal ectoplasmic specialization proteins. Filamin A knockdown also induced a disorganization of the actin filament network in Sertoli cells in vitro and in vivo. Collectively, these findings illustrate that filamin A regulates BTB assembly by recruiting these proteins to the microenvironment in the seminiferous epithelium to serve as the building blocks. In short, filamin A participates in BTB assembly by regulating protein recruitment during postnatal development in the rat testis.

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.

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.