Formin 1 Regulates Ectoplasmic Specialization in the Rat Testis Through Its Actin Nucleation and Bundling Activity

Diverse functions of myosin VI in spermiogenesis

Spermiogenesis is the final stage of spermatogenesis, a differentiation process during which unpolarized spermatids undergo excessive remodeling that results in the formation of sperm. The actin cytoskeleton and associated actin-binding proteins play crucial roles during this process regulating organelle or vesicle delivery/segregation and forming unique testicular structures involved in spermatid remodeling. In addition, several myosin motor proteins including MYO6 generate force and movement during sperm differentiation. MYO6 is highly unusual as it moves towards the minus end of actin filaments in the opposite direction to other myosin motors. This specialized feature of MYO6 may explain the many proposed functions of this myosin in a wide array of cellular processes in animal cells, including endocytosis, secretion, stabilization of the Golgi complex, and regulation of actin dynamics. These diverse roles of MYO6 are mediated by a range of specialized cargo-adaptor proteins that link this myosin to distinct cellular compartments and processes. During sperm development in a number of different organisms, MYO6 carries out pivotal functions. In Drosophila, the MYO6 ortholog regulates actin reorganization during spermatid individualization and male KO flies are sterile. In C. elegans, the MYO6 ortholog mediates asymmetric segregation of cytosolic material and spermatid budding through cytokinesis, whereas in mice, this myosin regulates assembly of highly specialized actin-rich structures and formation of membrane compartments to allow the formation of fully differentiated sperm. In this review, we will present an overview and compare the diverse function of MYO6 in the specialized adaptations of spermiogenesis in flies, worms, and mammals.

Microtubule Cytoskeleton and Spermatogenesis-Lesson From Studies of Toxicant Models

Studies have shown that mammalian testes, in particular the Sertoli cells, are highly susceptible to exposure of environmental toxicants, such as cadmium, perfluorooctanesulfonate, phthalates, 2,5-hexanedione and bisphenol A. However, important studies conducted by reproductive toxicologists and/or biologists in the past have been treated as toxicology reports per se. Yet, many of these studies provided important mechanistic insights on the toxicant-induced testis injury and reproductive dysfunction, relevant to the biology of the testis and spermatogenesis. Furthermore, recent studies have shown that findings obtained from toxicant models are exceedingly helpful tools to unravel the biology of testis function in particular spermatogenesis, including specific cellular events associated with spermatid transport to support spermiogenesis and spermiation. In this review, we critically evaluate some recent data, focusing primarily on the molecular structure and role of microtubules in cellular function, illustrating the importance of toxicant models to unravel the biology of microtubule cytoskeleton in supporting spermatogenesis, well beyond information on toxicology. These findings have opened up some potential areas of research which should be carefully evaluated in the years to come.

Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins

Cell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that the motor protein myosin-Va works with dynamic actin tracks to drive long-range organelle dispersion in opposition to microtubules. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. Here, we show that the SPIRE-type actin nucleators (predominantly SPIRE1) are Rab27a effectors that co-operate with formin-1 to generate actin tracks required for myosin-Va-dependent transport in melanocytes. Thus, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIREs to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Based on this, we suggest a model in which organelles and force generators (motors and track assemblers) are linked, forming an organelle-based, cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm.

JMY expression by Sertoli cells contributes to mediating spermatogenesis in mice

Sertoli cells are crucial for spermatogenesis in the seminiferous epithelium because their actin cytoskeleton supports vesicular transport, cell junction formation, protein anchoring, and spermiation. Here, we show that a junction-mediating and actin-regulatory protein (JMY) affects the blood-tissue barrier (BTB) function through remodeling of the Sertoli cell junctional integrity and it also contributes to controlling endocytic vesicle trafficking. These functions are critical for the maintenance of sperm fertility since loss of Sertoli cell-specific Jmy function induced male subfertility in mice. Specifically, these mice have (a) impaired BTB integrity and spermatid adhesion in the seminiferous tubules; (b) high incidence of sperm structural deformity; and (c) reduced sperm count and poor sperm motility. Moreover, the cytoskeletal integrity was compromised along with endocytic vesicular trafficking. These effects impaired junctional protein recycling and reduced Sertoli cell BTB junctional integrity. In addition, JMY interaction with actin-binding protein candidates α-actinin1 and sorbin and SH3 domain containing protein 2 was related to JMY activity, and in turn, actin cytoskeletal organization. In summary, JMY affects the control of spermatogenesis through the regulation of actin filament organization and endocytic vesicle trafficking in Sertoli cells.

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.

F5-Peptide and mTORC1/rpS6 Effectively Enhance BTB Transport Function in the Testis-Lesson From the Adjudin Model

During spermatogenesis, the blood-testis barrier (BTB) undergoes cyclic remodeling that is crucial to support the transport of preleptotene spermatocytes across the immunological barrier at stage VIII to IX of the epithelial cycle. Studies have shown that this timely remodeling of the BTB is supported by several endogenously produced barrier modifiers across the seminiferous epithelium, which include the F5-peptide and the ribosomal protein S6 [rpS6; a downstream signaling molecule of the mammalian target of rapamycin complex 1 (mTORC1)] signaling protein. Herein, F5-peptide and a quadruple phosphomimetic (and constitutively active) mutant of rpS6 [i.e., phosphorylated (p-)rpS6-MT] that are capable of inducing reversible immunological barrier remodeling, by making the barrier "leaky" transiently, were used for their overexpression in the testis to induce BTB opening. We sought to examine whether this facilitated the crossing of the nonhormonal male contraceptive adjudin at the BTB when administered by oral gavage, thereby effectively improving its BTB transport to induce germ cell adhesion and aspermatogenesis. Indeed, it was shown that combined overexpression of F5-peptide and p-rpS6-MT and a low dose of adjudin, which by itself had no noticeable effects on spermatogenesis, was capable of perturbing the organization of actin- and microtubule (MT)-based cytoskeletons through changes in the spatial expression of actin- and MT-binding/regulatory proteins to the corresponding cytoskeleton. These findings thus illustrate the possibility of delivering drugs to any target organ behind a blood-tissue barrier by modifying the tight junction permeability barrier using endogenously produced barrier modifiers based on findings from this adjudin animal model.

Myosin VIIa Supports Spermatid/Organelle Transport and Cell Adhesion During Spermatogenesis in the Rat Testis

The biology of transport of spermatids and spermatid adhesion across the seminiferous epithelium during the epithelial cycle remains largely unexplored. Nonetheless, studies have implicated the role of motor proteins in these cellular events. In this article, we report findings to unravel the role of myosin VIIa, an F-actin-based barbed (+)-end-directed motor protein, to support cellular transport and adhesion in the testis. Using RNA interference to knock down myosin VIIa in Sertoli cells cultured in vitro as a study model was shown to perturb the Sertoli cell tight junction permeability barrier, mediated through disorganization of actin- or microtubule (MT)-based cytoskeletons owing to disruptive changes on the spatiotemporal expression of F-actin or MT-regulatory proteins. Consistent with these in vitro findings, knockdown of myosin VIIa in the testis in vivo also induced disorganization of the actin- and MT-based cytoskeletons across the seminiferous epithelium, mediated by disruptive changes in the spatiotemporal expression of actin- and MT-based regulatory proteins. More important, the transport of spermatids and organelles across the epithelium, as well as cell adhesion, was grossly disrupted. For instance, step 19 spermatids failed to be transported to the adluminal compartment near the tubule lumen to undergo spermiation; in this manner, step 19 spermatids were persistently detected in stage IX and XII tubules, intermingling with step 9 and 12 spermatids, respectively. Also, phagosomes were detected near the tubule lumen in stage I to III tubules when they should have been degraded near the base of the seminiferous epithelium via the lysosomal pathway. In summary, myosin VIIa motor protein was crucial to support cellular transport and adhesion during spermatogenesis.

Planar cell polarity protein Dishevelled 3 (Dvl3) regulates ectoplasmic specialization (ES) dynamics in the testis through changes in cytoskeletal organization

In the mammalian testes, such as in rats, the directional alignment of polarized elongating/elongated spermatids, in particular step 17-19 spermatids, across the plane of seminiferous epithelium resembles planar cell polarity (PCP) found in hair cells of the cochlea. It is obvious that spermatid PCP is necessary to support the simultaneous development of maximal number of elongating/elongated spermatids to sustain the daily production of > 50 million sperm per adult rat. Studies have shown that the testis indeed expresses multiple PCP proteins necessary to support spermatid PCP. Herein, using physiological and biochemical assays, and morphological analysis, and with the technique of RNA interference (RNAi) to knockdown PCP protein Dishevelled (Dvl) 1 (Dvl1), Dvl2, Dvl3, or Dvl1/2/3, Dvl proteins, in particular Dvl3, it was shown that Dvl3 played a crucial role of support Sertoli cell tight junction (TJ)-permeability barrier function through changes in the organization of actin- and microtubule (MT)-based cytoskeletons. More important, an in vivo knockdown of Dvl1/2/3 in the testis, defects of spermatid polarity were remarkably noted across the seminiferous epithelium, concomitant with defects of spermatid adhesion and spermatid transport, leading to considerably defects in spermatogenesis. More important, Dvl1/2/3 triple knockdown in the testis also impeded the organization of actin- and MT-based cytoskeletons owing to disruptive spatial expression of actin- and MT-regulatory proteins. In summary, PCP Dishevelled proteins, in particular, Dvl3 is a regulator of Sertoli cell blood-testis barrier (BTB)  and also spermatid PCP function through its effects on the actin- and MT-based cytoskeletons in Sertoli cells.

YAP Partially Reprograms Chromatin Accessibility to Directly Induce Adult Cardiogenesis In Vivo

Specialized adult somatic cells, such as cardiomyocytes (CMs), are highly differentiated with poor renewal capacity, an integral reason underlying organ failure in disease and aging. Among the least renewable cells in the human body, CMs renew approximately 1% annually. Consistent with poor CM turnover, heart failure is the leading cause of death. Here, we show that an active version of the Hippo pathway effector YAP, termed YAP5SA, partially reprograms adult mouse CMs to a more fetal and proliferative state. One week after induction, 19% of CMs that enter S-phase do so twice, CM number increases by 40%, and YAP5SA lineage CMs couple to pre-existing CMs. Genomic studies showed that YAP5SA increases chromatin accessibility and expression of fetal genes, partially reprogramming long-lived somatic cells in vivo to a primitive, fetal-like, and proliferative state.

Regulation of Blood-Testis Barrier (BTB) Dynamics, Role of Actin-, and Microtubule-Based Cytoskeletons

The blood-testis barrier (BTB) is an important ultrastructure in the testis that supports meiosis and postmeiotic spermatid development since a delay in the establishment of a functional Sertoli cell barrier during postnatal development in rats or mice by 17-20 day postpartum (dpp) would lead to a delay of the first wave of meiosis. Furthermore, irreversible disruption of the BTB by toxicants also induces infertility in rodents. Herein, we summarize recent findings that BTB dynamics (i.e., disassembly, reassembly, and stabilization) are supported by the concerted efforts of the actin- and microtubule (MT)-based cytoskeletons. We focus on the role of two actin nucleation protein complexes, namely, the Arp2/3 (actin-related protein 2/3) complex and formin 1 (or the formin 1/spire 1 complex) known to induce actin nucleation, respectively, by conferring plasticity to actin cytoskeleton. We also focus on the MT plus (+)-end tracking protein (+TIP) EB1 (end-binding protein 1) which is known to confer MT stabilization. Furthermore, we discuss in particular how the interactions of these proteins modulate BTB dynamics during spermatogenesis. These findings also yield a novel hypothetical concept regarding the molecular mechanism that modulates BTB function.

Dynein 1 supports spermatid transport and spermiation during spermatogenesis in the rat testis

In the mammalian testis, spermatogenesis is dependent on the microtubule (MT)-specific motor proteins, such as dynein 1, that serve as the engine to support germ cell and organelle transport across the seminiferous epithelium at different stages of the epithelial cycle. Yet the underlying molecular mechanism(s) that support this series of cellular events remain unknown. Herein, we used RNAi to knockdown cytoplasmic dynein 1 heavy chain (Dync1h1) and an inhibitor ciliobrevin D to inactivate dynein in Sertoli cells in vitro and the testis in vivo, thereby probing the role of dynein 1 in spermatogenesis. Both treatments were shown to extensively induce disruption of MT organization across Sertoli cells in vitro and the testis in vivo. These changes also perturbed the transport of spermatids and other organelles (such as phagosomes) across the epithelium. These changes thus led to disruption of spermatogenesis. Interestingly, the knockdown of dynein 1 or its inactivation by ciliobrevin D also perturbed gross disruption of F-actin across the Sertoli cells in vitro and the seminiferous epithelium in vivo, illustrating there are cross talks between the two cytoskeletons in the testis. In summary, these findings confirm the role of cytoplasmic dynein 1 to support the transport of spermatids and organelles across the seminiferous epithelium during the epithelial cycle of spermatogenesis.

mDia1/3 generate cortical F-actin meshwork in Sertoli cells that is continuous with contractile F-actin bundles and indispensable for spermatogenesis and male fertility

Formin is one of the two major classes of actin binding proteins (ABPs) with nucleation and polymerization activity. However, despite advances in our understanding of its biochemical activity, whether and how formins generate specific architecture of the actin cytoskeleton and function in a physiological context in vivo remain largely obscure. It is also unknown how actin filaments generated by formins interact with other ABPs in the cell. Here, we combine genetic manipulation of formins mammalian diaphanous homolog1 (mDia1) and 3 (mDia3) with superresolution microscopy and single-molecule imaging, and show that the formins mDia1 and mDia3 are dominantly expressed in Sertoli cells of mouse seminiferous tubule and together generate a highly dynamic cortical filamentous actin (F-actin) meshwork that is continuous with the contractile actomyosin bundles. Loss of mDia1/3 impaired these F-actin architectures, induced ectopic noncontractile espin1-containing F-actin bundles, and disrupted Sertoli cell-germ cell interaction, resulting in impaired spermatogenesis. These results together demonstrate the previously unsuspected mDia-dependent regulatory mechanism of cortical F-actin that is indispensable for mammalian sperm development and male fertility.

Actin nucleator Spire 1 is a regulator of ectoplasmic specialization in the testis

Germ cell differentiation during the epithelial cycle of spermatogenesis is accompanied by extensive remodeling at the Sertoli cell-cell and Sertoli cell-spermatid interface to accommodate the transport of preleptotene spermatocytes and developing spermatids across the blood-testis barrier (BTB) and the adluminal compartment of the seminiferous epithelium, respectively. The unique cell junction in the testis is the actin-rich ectoplasmic specialization (ES) designated basal ES at the Sertoli cell-cell interface, and the apical ES at the Sertoli-spermatid interface. Since ES dynamics (i.e., disassembly, reassembly and stabilization) are supported by actin microfilaments, which rapidly converts between their bundled and unbundled/branched configuration to confer plasticity to the ES, it is logical to speculate that actin nucleation proteins play a crucial role to ES dynamics. Herein, we reported findings that Spire 1, an actin nucleator known to polymerize actins into long stretches of linear microfilaments in cells, is an important regulator of ES dynamics. Its knockdown by RNAi in Sertoli cells cultured in vitro was found to impede the Sertoli cell tight junction (TJ)-permeability barrier through changes in the organization of F-actin across Sertoli cell cytosol. Unexpectedly, Spire 1 knockdown also perturbed microtubule (MT) organization in Sertoli cells cultured in vitro. Biochemical studies using cultured Sertoli cells and specific F-actin vs. MT polymerization assays supported the notion that a transient loss of Spire 1 by RNAi disrupted Sertoli cell actin and MT polymerization and bundling activities. These findings in vitro were reproduced in studies in vivo by RNAi using Spire 1-specific siRNA duplexes to transfect testes with Polyplus in vivo-jetPEI as a transfection medium with high transfection efficiency. Spire 1 knockdown in the testis led to gross disruption of F-actin and MT organization across the seminiferous epithelium, thereby impeding the transport of spermatids and phagosomes across the epithelium and perturbing spermatogenesis. In summary, Spire 1 is an ES regulator to support germ cell development during spermatogenesis.

Sperm Release at Spermiation Is Regulated by Changes in the Organization of Actin- and Microtubule-Based Cytoskeletons at the Apical Ectoplasmic Specialization-A Study Using the Adjudin Model

The mechanism that regulates sperm release at spermiation is unknown. Herein, we used an animal model wherein rats were treated with adjudin, 1-(2,4-dichlorobenzyl)-1H-indazole-3-carbohydrazide, via oral gavage to induce premature release of elongating/elongated spermatids, followed by round spermatids and spermatocytes. Spermatid release mimicking spermiation occurred within 6 to 12 hours following adjudin treatment and, by 96 hours, virtually all tubules were devoid of elongating/elongated spermatids. Using this model, we tracked the organization of F-actin and microtubules (MTs) by immunofluorescence microscopy, and the association of actin or MT regulatory proteins that either promote or demolish cytoskeletal integrity through changes in the organization of actin microfilaments or MTs by coimmunoprecipitation. Adjudin treatment induced an increase in the association of (1) epidermal growth factor receptor pathway substrate 8 (an actin barbed-end capping and bundling protein) or formin 1 (an actin nucleator) with actin and (2) end-binding protein 1 (an MT stabilizing protein) with MT shortly after adjudin exposure (at 6 hours), in an attempt to maintain spermatid adhesion to the Sertoli cell at the apical ectoplasmic specialization (ES). However, this was followed by a considerable decline of their steady-state protein levels, replacing with an increase in association of (1) actin-related protein 3 (a branched actin nucleator that converts actin filaments into a branched/unbundled network) with actin and (2) MT affinity-regulating kinase 4 (an MT destabilizing protein kinase) with MTs by 12 hours after adjudin treatment. These latter changes thus promoted actin and MT disorganization, leading to apical ES disruption and the release of elongating/elongated spermatids, mimicking spermiation. In summary, spermiation is a cytoskeletal-dependent event, involving regulatory proteins that modify cytoskeletal organization.

Cell polarity and planar cell polarity (PCP) in spermatogenesis

In adult mammalian testes, spermatids, most notably step 17-19 spermatids in stage IV-VIII tubules, are aligned with their heads pointing toward the basement membrane and their tails toward the tubule lumen. On the other hand, these polarized spermatids also align across the plane of seminiferous epithelium, mimicking planar cell polarity (PCP) found in other hair cells in cochlea (inner ear). This orderly alignment of developing spermatids during spermiogenesis is important to support spermatogenesis, such that the maximal number of developing spermatids can be packed and supported by a fixed population of differentiated Sertoli cells in the limited space of the seminiferous epithelium in adult testes. In this review, we provide emerging evidence to demonstrate spermatid PCP in the seminiferous epithelium to support spermatogenesis. We also review findings in the field regarding the biology of spermatid cellular polarity (e.g., head-tail polarity and apico-basal polarity) and its inter-relationship to spermatid PCP. Furthermore, we also provide a hypothetical concept on the importance of PCP proteins in endocytic vesicle-mediated protein trafficking events to support spermatogenesis through protein endocytosis and recycling.

Cell polarity, cell adhesion, and spermatogenesis: role of cytoskeletons

In the rat testis, studies have shown that cell polarity, in particular spermatid polarity, to support spermatogenesis is conferred by the coordinated efforts of the Par-, Crumbs-, and Scribble-based polarity complexes in the seminiferous epithelium. Furthermore, planar cell polarity (PCP) is conferred by PCP proteins such as Van Gogh-like 2 (Vangl2) in the testis. On the other hand, cell junctions at the Sertoli cell–spermatid (steps 8–19) interface are exclusively supported by adhesion protein complexes (for example, α6β1-integrin-laminin-α3,β3,γ3 and nectin-3-afadin) at the actin-rich apical ectoplasmic specialization (ES) since the apical ES is the only anchoring device in step 8–19 spermatids. For cell junctions at the Sertoli cell–cell interface, they are supported by adhesion complexes at the actin-based basal ES (for example, N-cadherin-β-catenin and nectin-2-afadin), tight junction (occludin-ZO-1 and claudin 11-ZO-1), and gap junction (connexin 43-plakophilin-2) and also intermediate filament-based desmosome (for example, desmoglein-2-desmocollin-2). In short, the testis-specific actin-rich anchoring device known as ES is crucial to support spermatid and Sertoli cell adhesion. Accumulating evidence has shown that the Par-, Crumbs-, and Scribble-based polarity complexes and the PCP Vangl2 are working in concert with actin- or microtubule-based cytoskeletons (or both) and these polarity (or PCP) protein complexes exert their effects through changes in the organization of the cytoskeletal elements across the seminiferous epithelium of adult rat testes. As such, there is an intimate relationship between cell polarity, cell adhesion, and cytoskeletal function in the testis. Herein, we critically evaluate these recent findings based on studies on different animal models. We also suggest some crucial future studies to be performed.

Transport of germ cells across the seminiferous epithelium during spermatogenesis-the involvement of both actin- and microtubule-based cytoskeletons

The transport of germ cells from the base of the seminiferous epithelium toward the luminal edge of the tubule lumen in the adluminal compartment during the epithelial cycle is an essential cellular event to support spermatogenesis. Thus, fully developed elongated spermatids (i.e., spermatozoa) can be released at spermiation in late stage VIII in rodents versus late stage II in humans. Earlier studies to examine the molecular mechanism(s) that support germ cell transport, most notably the transport of preleptotene spermatocytes across the blood-testis barrier (BTB), and the transport of elongating spermatids across the adluminal compartment during spermiogenesis, is focused on the adhesion protein complexes at the cell-cell interface. It is generally accepted that cell junctions at the Sertoli cell-cell interface at the BTB, including the actin-based tight junction (TJ), basal ectoplasmic specialization (basal ES, a testis-specific adherens junction) and gap junction (GJ), as well as the intermediate filament-based desmosome undergo constant remodeling to accommodate the transport of preleptotene spermatocytes across the barrier. On the other hand, similar junction dynamics (i.e., disassembly, reassembly and stabilization/maintenance) take place at the Sertoli-spermatid interface. Emerging evidence has shown that junction dynamics at the Sertoli cell-cell vs. Sertoli-germ cell interface are supported by the 2 intriguingly coordinated cytoskeletons, namely the F-actin- and microtubule (MT)-based cytoskeletons. Herein, we provide a brief summary and critically evaluate the recent findings. We also provide an updated hypothetical concept regarding germ cell transport in the testis utilizing the MT-conferred tracks and the MT-specific motor proteins. Furthermore, this cellular event is also supported by the F-actin-based cytoskeleton.

Formin 1 Regulates Microtubule and F-Actin Organization to Support Spermatid Transport During Spermatogenesis in the Rat Testis

Formin 1 confers actin nucleation by generating long stretches of actin microfilaments to support cell movement, cell shape, and intracellular protein trafficking. Formin 1 is likely involved in microtubule (MT) dynamics due to the presence of a MT binding domain near its N terminus. Here, formin 1 was shown to structurally interact with α-tubulin, the building block of MT, and also end-binding protein 1 (a MT plus [+]-end-binding protein that stabilizes MT) in the testis. Knockdown of formin 1 in Sertoli cells with an established tight junction barrier was found to induce down-regulation of detyrosinated MT (a stabilized form of MT), and disorganization of MTs, in which MTs were retracted from the cell cortical zone, mediated through a loss of MT polymerization and down-regulation of Akt1/2 signaling kinase. An efficient knockdown of formin 1 in the testis reduced the number of track-like structures conferred by MTs and F-actin considerably, causing defects in spermatid and phagosome transport across the seminiferous epithelium. In summary, formin1 maintains MT and F-actin track-like structures to support spermatid and phagosome transport across the seminiferous epithelium during spermatogenesis.

Differential roles of estrogen receptors, ESR1 and ESR2, in adult rat spermatogenesis

Estrogens, through their receptors, play an important role in regulation of spermatogenesis. However, the precise role of the estrogen receptors (ESR1 and ESR2) has been difficult to determine as in vivo estradiol treatment would signal through both the ESRs. Hence we had developed in vivo selective ESR agonist administration models in adult male rats to decipher the individual roles of the ESRs. Treatment with both ESR1 and ESR2 agonists decreased sperm counts after 60 days of treatment. The present study aimed to delineate the precise causes of decreased sperm counts following treatment with the two ESR agonists. Treatment with ESR1 agonist causes an arrest in differentiation of round spermatids into elongated spermatids, mainly due to down-regulation of genes involved in spermiogenesis. ESR2 agonist administration reduces sperm counts due to spermiation failure and spermatocyte apoptosis. Spermiation failure observed is due to defects in tubulobulbar complex formation because of decrease in expression of genes involved in actin remodelling. The increase in spermatocyte apoptosis could be due to increase in oxidative stress and decrease in transcripts of anti-apoptotic genes. Our results suggest that the two ESRs regulate distinct aspects of spermatogenesis. ESR1 is mainly involved with regulation of spermiogenesis, while ESR2 regulates spermatocyte apoptosis and spermiation. Activation of estrogen signaling through either of the receptors can affect their respective processes during spermatogenesis and lead to low sperm output. Since many environmental estrogens can bind to the two ESRs with different affinities, these observations can be useful in understanding their potential effects on spermatogenesis.

First Evidence of DAAM1 Localization During the Post-Natal Development of Rat Testis and in Mammalian Sperm

Dishevelled-associated activator of morphogenesis 1 (DAAM1) is a formin-family protein involved in nucleation of unbranched actin filaments and in cytoskeletal organization through Wnt-Dishevelled PCP pathway, which participates in essential biological processes, such as cell polarity, movement, and adhesion during morphogenesis and organogenesis. While its role has been investigated during development and in somatic cells, its potential association with the germinal compartment and reproduction is still unexplored. In this work, we assessed the possible association of DAAM1 with the morphogenesis of rat testis. We studied its expression and profiled its localization versus actin and tubulin, during the first wave of spermatogenesis and in the adult gonad (from 7 to 60 dpp). We show that, in mitotic phases, DAAM1 shares its localization with actin in Sertoli cells, gonocytes, and spermatogonia. Later, during meiosis, both proteins are found in spermatocytes, while only actin is detectable at the forming blood-testis barrier. DAAM1, then, follows the development of the acrosome system throughout spermiogenesis, and it is finally retained inside the cytoplasmic droplet in mature gametes, as corroborated by additional immunolocalization data on both rat and human sperm. Unlike the DAAM1, actin keeps its localization in Sertoli cells, and tubulin is associated with their protruding cytoplasm during the process. Our data support, for the first time, the hypothesis of a role for DAAM1 in cytoskeletal organization during Mammalian testis morphogenesis and gamete progression, while also hinting at its possible investigation as a morphological marker of germ cell and sperm physiology. J. Cell. Physiol. 231: 2172-2184, 2016. © 2016 Wiley Periodicals, Inc.

Is toxicant-induced Sertoli cell injury in vitro a useful model to study molecular mechanisms in spermatogenesis?

Sertoli cells isolated from rodents or humans and cultured in vitro are known to establish a functional tight junction (TJ)-permeability barrier that mimics the blood-testis barrier (BTB) in vivo. This model has been widely used by investigators to study the biology of the TJ and the BTB. Studies have shown that environmental toxicants (e.g., perfluorooctanesulfonate (PFOS), bisphenol A (BPA) and cadmium) that exert their disruptive effects to induce Sertoli cell injury using this in vitro model are reproducible in studies in vivo. Thus, this in vitro system provides a convenient approach to probe the molecular mechanism(s) underlying toxicant-induced testis injury but also to provide new insights in understanding spermatogenesis, such as the biology of cell adhesion, BTB restructuring that supports preleptotene spermatocyte transport, and others. Herein, we provide a brief and critical review based on studies using this in vitro model of Sertoli cell cultures using primary cells isolated from rodent testes vs. humans to monitor environmental toxicant-mediated Sertoli cell injury. In short, recent findings have shown that environmental toxicants exert their effects on Sertoli cells to induce testis injury through their action on Sertoli cell actin- and/or microtubule-based cytoskeleton. These effects are mediated via their disruptive effects on actin- and/or microtubule-binding proteins. Sertoli cells also utilize differential spatiotemporal expression of these actin binding proteins to confer plasticity to the BTB to regulate germ cell transport across the BTB.

Regulation of microtubule (MT)-based cytoskeleton in the seminiferous epithelium during spermatogenesis

In rodents and humans, testicular cells, similar to other mammalian cells, are supported by actin-, microtubule (MT)- and intermediate filament-based cytoskeletons. Although the cytoskeletal network of the testis serves an important role in regulating spermatogenesis during the epithelial cycle, most of the published findings in the literature are limited to studies that only visualize these cytoskeletons in the seminiferous epithelium. Few focus on the underlying molecular mechanism that regulates their organization in the epithelium in response to changes in the stages of the epithelial cycle. Functional studies in the last decade have begun to focus on the role of binding proteins that regulate these cytoskeletons, with some interesting findings rapidly emerging in the field. Since the actin- and intermediate filament-based cytoskeletons have been recently reviewed, herein we focus on the MT-based cytoskeleton for two reasons. First, besides serving as a structural support cytoskeleton, MTs are known to serve as the track to support and facilitate the transport of germ cells, such as preleptotene spermatocytes connected in clones and elongating/elongated spermatids during spermiogenesis, across the blood-testis barrier (BTB) and the adluminal compartment, respectively, during spermatogenesis. While these cellular events are crucial to the completion of spermatogenesis, they have been largely ignored in the past. Second, MT-based cytoskeleton is working in concert with the actin-based cytoskeleton to provide structural support for the transport of intracellular organelles across the cell cytosol, such as endosome-based vesicles, and phagosomes, which contain residual bodies detached from spermatids, to maintain the cellular homeostasis in the seminiferous epithelium. We critically evaluate some recent published findings herein to support a hypothesis regarding the role of MT in conferring germ cell transport in the seminiferous epithelium.

Connexin 43 reboots meiosis and reseals blood-testis barrier following toxicant-mediated aspermatogenesis and barrier disruption

Earlier studies have shown that rats treated with an acute dose of 1-(2,4-dichlorobenzyl)-1H-indazole-3-carbohydrazide (adjudin, a male contraceptive under development) causes permanent infertility due to irreversible blood-testis barrier (BTB) disruption even though the population of undifferentiated spermatogonia remains similar to normal rat testes, because spermatogonia fail to differentiate into spermatocytes to enter meiosis. Since other studies have illustrated the significance of connexin 43 (Cx43)-based gap junction in maintaining the homeostasis of BTB in the rat testis and the phenotypes of Sertoli cell-conditional Cx43 knockout mice share many of the similarities of the adjudin-treated rats, we sought to examine if overexpression of Cx43 in these adjudin-treated rats would reseal the disrupted BTB and reinitiate spermatogenesis. A full-length Cx43 cloned into mammalian expression vector pCI-neo was used to transfect testes of adjudin-treated ratsversusempty vector. It was found that overexpression of Cx43 indeed resealed the Sertoli cell tight junction-permeability barrier based on a functionalin vivoassay in tubules displaying signs of meiosis as noted by the presence of round spermatids. Thus, these findings suggest that overexpression of Cx43 reinitiated spermatogenesis at least through the steps of meiosis to generate round spermatids in testes of rats treated with an acute dose of adjudin that led to aspermatogenesis. It was also noted that the round spermatids underwent eventual degeneration with the formation of multinucleated cells following Cx43 overexpression due to the failure of spermiogenesis because no elongating/elongated spermatids were detected in any of the tubules examined. The mechanism by which overexpression of Cx43 reboots meiosis and rescues BTB function was also examined. In summary, overexpression of Cx43 in the testis with aspermatogenesis reboots meiosis and reseals toxicant-induced BTB disruption, even though it fails to support round spermatids to enter spermiogenesis.-Li, N., Mruk, D. D., Mok, K.-W., Li, M. W. M., Wong, C. K. C., Lee, W. M., Han, D., Silvestrini, B., Cheng, C. Y. Connexin 43 reboots meiosis and reseals blood-testis barrier following toxicant-mediated aspermatogenesis and barrier disruption.

Regulation of blood-testis barrier by actin binding proteins and protein kinases

The blood-testis barrier (BTB) is an important ultrastructure in the testis, since the onset of meiosis and spermiogenesis coincides with the establishment of a functional barrier in rodents and humans. It is also noted that a delay in the assembly of a functional BTB following treatment of neonatal rats with drugs such as diethylstilbestrol or adjudin also delays the first wave of spermiation. While the BTB is one of the tightest blood-tissue barriers, it undergoes extensive remodeling, in particular, at stage VIII of the epithelial cycle to facilitate the transport of preleptotene spermatocytes connected in clones across the immunological barrier. Without this timely transport of preleptotene spermatocytes derived from type B spermatogonia, meiosis will be arrested, causing aspermatogenesis. Yet the biology and regulation of the BTB remains largely unexplored since the morphological studies in the 1970s. Recent studies, however, have shed new light on the biology of the BTB. Herein, we critically evaluate some of these findings, illustrating that the Sertoli cell BTB is regulated by actin-binding proteins (ABPs), likely supported by non-receptor protein kinases, to modulate the organization of actin microfilament bundles at the site. Furthermore, microtubule-based cytoskeleton is also working in concert with the actin-based cytoskeleton to confer BTB dynamics. This timely review provides an update on the unique biology and regulation of the BTB based on the latest findings in the field, focusing on the role of ABPs and non-receptor protein kinases.

Formins: Actin nucleators that regulate cytoskeletal dynamics during spermatogenesis

Formins are a growing class of actin nucleation proteins that promote the polymerization of actin microfilaments, forming long stretches of actin microfilaments to confer actin filament bundling in mammalian cells. As such, microfilament bundles can be formed in specific cellular domains, in particular in motile mammalian cells, such as filopodia. Since ectoplasmic specialization (ES), a testis-specific adherens junction (AJ), at the Sertoli cell-cell and Sertoli-spermatid interface is constituted by arrays of actin microfilament bundles, it is likely that formins are playing a significant physiological role on the homeostasis of ES during the epithelial cycle of spermatogenesis. In this Commentary, we provide a timely discussion on formin 1 which was recently shown to be a crucial regulator of actin microfilaments at the ES in the rat testis (Li N et al. Endocrinology, 2015, in press; DOI: 10.1210/en.2015-1161, PMID:25901598). We also highlight research that is needed to unravel the functional significance of formins in spermatogenesis.