Cell polarity and cytoskeletons-Lesson from the testis

Katanin regulatory subunit B1 (KATNB1) regulates BTB dynamics through changes in cytoskeletal organization

In this study, we have explored the role of the KATNB1 gene, a microtubule-severing protein, in the seminiferous epithelium of the rat testis. Our data have shown that KATNB1 expressed in rat brain, testes, and Sertoli cells. KATNB1 was found to co-localize with α-tubulin showing a unique stage-specific distribution across the seminiferous epithelium. Knockdown of KATNB1 by RNAi led to significant disruption of the tight junction (TJ) permeability barrier function in primary Sertoli cells cultured in vitro with an established functional TJ-barrier, as well as perturbations in the microtubule and actin cytoskeleton organization. The disruption in these cytoskeletal structures, in turn, led to improper distribution of TJ and basal ES proteins essential for maintaining the Sertoli TJ function. More importantly, overexpression of KATNB1 in the testis in vivo was found to block cadmium-induced blood-testis barrier (BTB) disruption and testis injury. KATNB1 exerted its promoting effects on BTB and spermatogenesis through corrective spatiotemporal expression of actin- and microtubule-based regulatory proteins by maintaining the proper organization of cytoskeletons in the testis, illustrating its plausible therapeutic implication. In summary, Katanin regulatory subunit B1 (KATNB1) plays a crucial role in BTB and spermatogenesis through its effects on the actin- and microtubule-based cytoskeletons in Sertoli cells and testis, providing important insights into male reproductive biology.

SPAG6 c.900 T>C affects boar semen quality and blood-testis barrier function by creating a new splice acceptor site

Sperm associated antigen 6 (SPAG6) acts as a scaffolding protein in the center of the flagellar axoneme and has an impact on the maturation of the motility of mammalian sperm flagella and the maintenance of sperm structure. In our previous research, SPAG6 c.900 T>C in exon 7 and exon 7 skipped transcript was identified by analyzing RNA-seq data of testicular tissues from 60 day (sexually immature) and 180 day (sexually mature) Large White boars. Herein, we found porcine SPAG6 c.900 T>C to be associated with semen quality traits in Duroc, Large White and Landrace pigs. SPAG6 c.900 C can generate a new splice acceptor site, inhibit the occurrence of SPAG6 exon 7 skipping to a certain extent, thereby promote the growth of Sertoli cells and maintain the normal blood-testis barrier function. This study provides new insights into the molecular regulation of spermatogenesis and a new genetic marker for the improvement of semen quality in pigs.

Single-cell transcriptomic dissection of the toxic impact of di(2-ethylhexyl) phthalate on immature testicular development at the neonatal stage

Di(2-ethylhexyl) phthalate (DEHP) early exposure leads to immature testicular injury, and we aimed to utilize single-cell RNA (scRNA) sequencing to comprehensively assess the toxic effect of DEHP on testicular development. Therefore, we gavaged pregnant C57BL/6 mice with 750 mg/kg body weight DEHP from gestational day 13.5 to delivery and performed scRNA sequencing of neonatal testes at postnatal day 5.5. The results revealed the gene expression dynamics in testicular cells. DEHP disrupted the developmental trajectory of germ cells and the balance between the self-renewal and differentiation of spermatogonial stem cells. Additionally, DEHP caused an abnormal developmental trajectory, cytoskeletal damage and cell cycle arrest in Sertoli cells; disrupted the metabolism of testosterone in Leydig cells; and disturbed the developmental trajectory in peritubular myoid cells. Elevated oxidative stress and excessive apoptosis mediated by p53 were observed in almost all testicular cells. The intercellular interactions among four cell types were altered, and biological processes related to glial cell line-derived neurotrophic factor (GDNF), transforming growth factor-β (TGF-β), NOTCH, platelet-derived growth factor (PDGF) and WNT signaling pathways were enriched after DEHP treatment. These findings systematically describe the damaging effects of DEHP on the immature testes and provide substantial novel insights into the reproductive toxicity of DEHP.

NECL2 regulates blood-testis barrier dynamics in mouse testes

The adhesion protein nectin-like molecule 2 (NECL2) is involved in spermatogenesis and participates in the connections between Sertoli cells and germ cells. Necl2 deficiency leads to infertility in male mice. We found that NECL2 is relatively highly expressed on the cell membranes of preleptotene spermatocytes. It is known that preleptotene spermatocytes pass through the blood-testis barrier (BTB) from the base of the seminiferous tubules to the lumen to complete meiosis. We hypothesized that the NECL2 protein on the surfaces of preleptotene spermatocytes has an effect on the BTB when crossing the barrier. Our results showed that Necl2 deficiency caused the levels of proteins in the BTB to be abnormal, such as those of Claudin 3, claudin 11, and Connexin43. NECL2 interacted and colocalized with adhesion proteins forming the BTB, such as Connexin43, Occludin, and N-cadherin. NECL2 regulated BTB dynamics when preleptotene spermatocytes passed through the barrier, and Necl2 deficiency caused BTB damage. Necl2 deletion significantly affected the testicular transcriptome, especially the expression of spermatogenesis-related genes. These results suggest that before meiosis and spermatid development occur, BTB dynamics regulated by NECL2 are necessary for spermatogenesis.

Characteristics of piRNAs and their comparative profiling in testes of sheep with different fertility

As a novel class of small RNAs, piRNAs are highly expressed in the animal gonads and their main known role is to inhibit transposon activity for ensuring the correctness and integrity of genome. In order to explore the characteristics of piRNAs in sheep testis and their possible regulatory roles on male reproduction, deep sequencing technology was used to sequence small RNAs and identify piRNAs in testes of sheep. The length of piRNAs in sheep testes showed a unimodal distribution between 26 and 31 nt, with a peak at 29 nt. These piRNAs exhibited obvious ping-pong signature and strand specificity. In the genome, they were mainly aligned to CDS, intron, repetitive sequence regions and unannotated regions. Furthermore, in transposon analysis, piRNAs were aligned predominantly to LINE, SINE, and LTR types of retrotransposon in sheep testes, and the piRNAs derived from each type showed obvious ping-pong signature. The piRNA clusters identified in sheep testes were mainly distributed on chromosomes 3, 7, 15, 17, 18 and 20. The results combining semen determination with pathway enrichment analysis implied that differentially expressed piRNAs between the testes of rams with different fertility might participate in spermatogenesis by regulating multiple pathways closely related to stabilization of blood-testis barrier and renewal and differentiation of spermatogonial stem cell. Taken together, the study provided new insights into the characteristics, origin and expression patterns of piRNAs in sheep testes tissue, which would help us better understand the role of piRNAs in sheep reproduction.

BCL2-associated athanogene 6 exon24 contributes to testosterone synthesis and male fertility in mammals

Objectives

BCL2‐associated athanogene 6 (BAG6) plays critical roles in spermatogenesis by maintaining testicular cell survival. Our previous data showed porcine BAG6 exon24‐skipped transcript is highly expressed in immature testes compared with mature testes. The objective of this study is to reveal the functional significance of BAG6 exon24 in mammalian spermatogenesis.

Materials and Methods

CRISPR/Cas9 system was used to generate Bag6 exon24 knockout mice. Testes and cauda epididymal sperm were collected from mice. TMT proteomics analysis was used to discover the protein differences induced by Bag6 exon24 deletion. Testosterone enanthate was injected into mice to generate a high‐testosterone mice model. H&E staining, qRT‐PCR, western blotting, vector/siRNA transfection, immunofluorescence, immunoprecipitation, transmission electron microscopy, TUNEL and ELISA were performed to investigate the phenotypes and molecular basis.

Results

Bag6 exon24 knockout mice show sub‐fertility along with partially impaired blood‐testis barrier, increased apoptotic testicular cell rate and abnormal sperm morphology. Endoplasmic reticulum stress occurs in Bag6 exon24‐deficient testes and sterol regulatory element‐binding transcription factor 2 is activated; as a result, cytochrome P450 family 51 subfamily A member 1 expression is up‐regulated, which causes a high serum testosterone level. Additionally, serine/arginine‐rich splicing factor 1 down‐regulates BAG6 exon24‐skipped transcripts in porcine Sertoli cells by binding to 35–51 nt on BAG6 exon24 via its N‐terminal RNA‐recognition domain.

Conclusions

Our findings reveal the critical roles of BAG6 exon24 in testosterone biosynthesis and male fertility, which provides new insights into the regulation of spermatogenesis and pathogenesis of subfertility in mammals.

Long-term exposure to low concentrations of MC-LR induces blood-testis barrier damage through the RhoA/ROCK pathway

Microcystin-leucine arginine (MC-LR), an emerging water pollutant, produced by cyanobacteria, has an acute testicular toxicity. However, little is known about the chronic toxic effects of MC-LR exposure on the testis at environmental concentrations and the underlying molecular mechanisms. In this study, C57BL/6 J mice were exposed to different low concentrations of MC-LR for 6, 9 and 12 months. The results showed that MC-LR could cause testis structure loss, cell abscission and blood-testis barrier (BTB) damage. Long-term exposure of MC-LR also activated RhoA/ROCK pathway, which was accompanied by the rearrangement of α-Tubulin. Furthermore, MC-LR reduced the levels of the adherens junction proteins (N-cadherin and β-catenin) and the tight junction proteins (ZO-1 and Occludin) in a dose- and time-dependent way, causing BTB damage. MC-LR also reduced the expressions of Occludin, ZO-1, β-catenin, and N-cadherin in TM4 cells, accompanied by a disruption of cytoskeletal proteins. More importantly, the RhoA inhibitor Rhosin ameliorated these MC-LR-induced changes. Together, these new findings suggest that long-term exposure to MC-LR induces BTB damage through RhoA/ROCK activation: involvement of tight junction and adherens junction changes and cytoskeleton disruption. This study highlights a new mechanism for MC-LR-induced BTB disruption and provides new insights into the cause and treatment of BTB disruption.

Transcriptional Control of Apical-Basal Polarity Regulators

Cell polarity is essential for many functions of cells and tissues including the initial establishment and subsequent maintenance of epithelial tissues, asymmetric cell division, and morphogenetic movements. Cell polarity along the apical-basal axis is controlled by three protein complexes that interact with and co-regulate each other: The Par-, Crumbs-, and Scrib-complexes. The localization and activity of the components of these complexes is predominantly controlled by protein-protein interactions and protein phosphorylation status. Increasing evidence accumulates that, besides the regulation at the protein level, the precise expression control of polarity determinants contributes substantially to cell polarity regulation. Here we review how gene expression regulation influences processes that depend on the induction, maintenance, or abolishment of cell polarity with a special focus on epithelial to mesenchymal transition and asymmetric stem cell division. We conclude that gene expression control is an important and often neglected mechanism in the control of cell polarity.

Preliminary Investigation on the Involvement of Cytoskeleton-Related Proteins, DAAM1 and PREP, in Human Testicular Disorders

Herein, for the first time, the potential relationships between the cytoskeleton-associated proteins DAAM1 and PREP with different testicular disorders, such as classic seminoma (CS), Leydig cell tumor (LCT), and Sertoli cell-only syndrome (SOS), were evaluated. Six CS, two LCT, and two SOS tissue samples were obtained during inguinal exploration in patients with a suspect testis tumor based on clinical examination and ultrasonography. DAAM1 and PREP protein levels and immunofluorescent localization were analyzed. An increased DAAM1 protein level in CS and SOS as compared to non-pathological (NP) tissue was observed, while LCT showed no significant differences. Conversely, PREP protein level increased in LCT, while it decreased in CS and SOS compared to NP tissue. These results were strongly supported by the immunofluorescence staining, revealing an altered localization and signal intensity of DAAM1 and PREP in the analyzed samples, highlighting a perturbed cytoarchitecture. Interestingly, in LCT spermatogonia, a specific DAAM1 nuclear localization was found, probably due to an enhanced testosterone production, as confirmed by the increased protein levels of steroidogenic enzymes. Finally, although further studies are needed to verify the involvement of other formins and microtubule-associated proteins, this report raised the opportunity to indicate DAAM1 and PREP as new potential markers, supporting the cytoskeleton dynamics changes occurring during normal and/or pathological cell differentiation.

Planar cell polarity (PCP) proteins support spermatogenesis through cytoskeletal organization in the testis

Few reports are found in the literature regarding the role of planar cell polarity (PCP) in supporting spermatogenesis in the testis. Yet morphological studies reported decades earlier have illustrated the directional alignment of polarized developing spermatids, most notably step 17-19 spermatids in stage V-early VIII tubules in the testis, across the plane of the epithelium in seminiferous tubules of adult rats. Such morphological features have unequivocally demonstrated the presence of PCP in developing spermatids, analogous to the PCP noted in hair cells of the cochlea in mammals. Emerging evidence in recent years has shown that Sertoli and germ cells express numerous PCP proteins, mostly notably, the core PCP proteins, PCP effectors and PCP signaling proteins. In this review, we discuss recent findings in the field regarding the two core PCP protein complexes, namely the Van Gogh-like 2 (Vangl2)/Prickle (Pk) complex and the Frizzled (Fzd)/Dishevelled (Dvl) complex. These findings have illustrated that these PCP proteins exert their regulatory role to support spermatogenesis through changes in the organization of actin and microtubule (MT) cytoskeletons in Sertoli cells. For instance, these PCP proteins confer PCP to developing spermatids. As such, developing haploid spermatids can be aligned and orderly packed within the limited space of the seminiferous tubules in the testes for the production of sperm via spermatogenesis. Thus, each adult male in the mouse, rat or human can produce an upward of 30, 50 or 300 million spermatozoa on a daily basis, respectively, throughout the adulthood. We also highlight critical areas of research that deserve attention in future studies. We also provide a hypothetical model by which PCP proteins support spermatogenesis based on recent studies in the testis. It is conceivable that the hypothetical model shown here will be updated as more data become available in future years, but this information can serve as the framework by investigators to unravel the role of PCP in spermatogenesis.

Bisphenol S perturbs Sertoli cell junctions in male rats via alterations in cytoskeletal organization mediated by an imbalance between mTORC1 and mTORC2

Bisphenol S (BPS) is now used as an alternative of bisphenol A (BPA), but has been implicated in male reproductive dysfunction-including diminished sperm number and quality and altered hormonal concentrations. However, the mechanisms of action subserving these effects remains unclear. In the present study, BPS at doses of 50 mg/kg bw and 100 mg/kg bw caused defects in the integrity of the blood-testis barrier (BTB) and apical ectoplasmic specialization (ES), and we also delineated an underlying molecular mechanism of action. BPS induced F-actin and α-tubulin disorganization in seminiferous tubules, which in turn led to the truncation of actin filaments and microtubules. Additionally, BPS was found to perturb the expression of the actin-binding proteins Arp3 and Eps8, which are critical for the organization of the actin filaments. mTORC1 and mTORC2 manifest opposing roles in Sertoli cell junctional function, and we demonstrated that mTORC1/rpS6/Akt/MMP9 signaling was increased and that mTORC2/rictor activity was also attenuated. In summary, we showed that BPS-induced disruption of the BTB and apical ES perturbed normal spermatogenic function that was mediated by mTORC1 and mTORC2. The imbalance in mTORC1 and mTORC2, in turn, altered the expression of actin-binding proteins, resulting in the impairment of F-actin and MT organization, and inhibited the expression of junctional proteins at the BTB and apical ES.

NC1-peptide derived from collagen α3 (IV) chain is a blood-tissue barrier regulator: lesson from the testis

Collagen α3 (IV) chains are one of the major constituent components of the basement membrane in the mammalian testis. Studies have shown that biologically active fragments, such as noncollagenase domain (NC1)-peptide, can be released from the C-terminal region of collagen α3 (IV) chains, possibly through the proteolytic action of metalloproteinase 9 (MMP9). NC1-peptide was shown to promote blood–testis barrier (BTB) remodeling and fully developed spermatid (e.g., sperm) release from the seminiferous epithelium because this bioactive peptide was capable of perturbing the organization of both actin- and microtubule (MT)-based cytoskeletons at the Sertoli cell–cell and also Sertoli–spermatid interface, the ultrastructure known as the basal ectoplasmic specialization (ES) and apical ES, respectively. More importantly, recent studies have shown that this NC1-peptide-induced effects on cytoskeletal organization in the testis are mediated through an activation of mammalian target of rapamycin complex 1/ribosomal protein S6/transforming retrovirus Akt1/2 protein (mTORC1/rpS6/Akt1/2) signaling cascade, involving an activation of cell division control protein 42 homolog (Cdc42) GTPase, but not Ras homolog family member A GTPase (RhoA), and the participation of end-binding protein 1 (EB1), a microtubule plus (+) end tracking protein (+TIP), downstream. Herein, we critically evaluate these findings, providing a critical discussion by which the basement membrane modulates spermatogenesis through one of its locally generated regulatory peptides in the testis.

Loss of myosin VI expression affects acrosome/acroplaxome complex morphology during mouse spermiogenesis†

During spermiogenesis in mammals, actin filaments and a variety of actin-binding proteins are involved in the formation and function of highly specialized testis-specific structures. Actin-based motor proteins, such as myosin Va and VIIa, play a key role in this complex process of spermatid transformation into mature sperm. We have previously demonstrated that myosin VI (MYO6) is also expressed in mouse testes. It is present in actin-rich structures important for spermatid development, including one of the earliest events in spermiogenesis—acrosome formation. Here, we demonstrate using immunofluorescence, cytochemical, and ultrastructural approaches that MYO6 is involved in maintaining the structural integrity of these specialized actin-rich structures during acrosome biogenesis in mouse. We show that MYO6 together with its binding partner TOM1/L2 is present at/around the spermatid Golgi complex and the nascent acrosome. Depletion of MYO6 in Snell’s waltzer mice causes structural disruptions of the Golgi complex and affects the acrosomal granule positioning within the developing acrosome. In summary, our results suggest that MYO6 plays an anchoring role during the acrosome biogenesis mainly by tethering of different cargo/membranes to highly specialized actin-related structures.

Protective effect of Dioscorea zingiberensis ethanol extract on the disruption of blood-testes barrier in high-fat diet/streptozotocin-induced diabetic mice by upregulating ZO-1 and Nrf2

Testicular injury is the primary pathogenesis of diabetes-induced male infertility. Dioscorea zingiberensis (DZ), a traditional Chinese medicine (TCM) including saponins, flavonoids and cellulose, is used to treat diseases in the reproductive system. But the protective effects of DZ on diabetes-induced testicular injury remain poorly understood. In this study, the therapeutic effects of chronic oral DZ treatment on testis impairment in a diabetic mouse model were explored by assessing sperm morphology, blood-testes barrier (BTB) integrity and testicular histological examination. Our results showed that DZ significantly reversed BTB disruption, testicular tissue injury and abnormal sperm morphology in diabetic mice. Interestingly, diabetes-induced disruption of the BTB was associated with a decrease in the tight junction (TJ) protein zonula occludens-1 (ZO-1). Dioscorea zingiberensis effectively increased ZO-1 expression in testis tissue to restore the integrity of the BTB. Moreover, DZ treatment significantly reduced hyperglycaemia-induced increases in malondialdehyde (MDA) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels. Further mechanistic studies revealed that DZ substantially enhanced the expression of Nrf2, NOQ1 and HO-1, which indicated that DZ exerts potential antioxidant effects against testicular tissue damage via the activation of Nrf2. In conclusion, the protective effects of DZ rely on repairing the integrity of the BTB and on reducing oxidative stress damage by mediating ZO-1 and Nrf2. The study contributes to discovering the DZ possible mechanism of action.

The organophosphate pesticide methamidophos opens the blood-testis barrier and covalently binds to ZO-2 in mice

Methamidophos (MET) is an organophosphate (OP) pesticide widely used in agriculture in developing countries. MET causes adverse effects in male reproductive function in humans and experimental animals, but the underlying mechanisms remain largely unknown. We explored the effect of MET on mice testes (5 mg/kg/day/4 days), finding that this pesticide opens the blood-testis barrier and perturbs spermatogenesis, generating the appearance of immature germ cells in the epididymis. In the seminiferous tubules, MET treatment changed the level of expression or modified the stage-specific localization of tight junction (TJ) proteins ZO-1, ZO-2, occludin, and claudin-3. In contrast, claudin-11 was barely altered. MET also modified the shape of claudin-11, and ZO-2 at the cell border, from a zigzag to a more linear pattern. In addition, MET diminished the expression of ZO-2 in spermatids present in seminiferous tubules, induced the phosphorylation of ZO-2 and occludin in testes and reduced the interaction between these proteins assessed by co-immunoprecipitation. MET formed covalent bonds with ZO-2 in serine, tyrosine and lysine residues. The covalent modifications formed on ZO-2 at putative phosphorylation sites might interfere with ZO-2 interaction with regulatory molecules and other TJ proteins. MET bonds formed at ZO-2 ubiquitination sites likely interfere with ZO-2 degradation and TJ sealing, based on results obtained in cultured epithelial cells transfected with ZO-2 mutated at a MET target lysine residue. Our results shed light on MET male reproductive toxicity and are important to improve regulations regarding the use of OP pesticides and to protect the health of agricultural workers.

Src family kinases (SFKs) and cell polarity in the testis

Non-receptor Src family kinases (SFKs), most notably c-Src and c-Yes, are recently shown to be expressed by Sertoli and/or germ cells in adult rat testes. Studies have shown that SFKs are involved in modulating the cell cytoskeletal function, and involved in endocytic vesicle-mediated protein endocytosis, transcytosis and/or recycling as well as intracellular protein degradation events. Furthermore, a knockdown to SFKs, in particular c-Yes, has shown to induce defects in spermatid polarity. These findings, coupled with emerging evidence in the field, thus prompt us to critically evaluate them to put forth a developing concept regarding the role of SFKs and cell polarity, which will become a basis to design experiments for future investigations.

Regulation of spermatid polarity by the actin- and microtubule (MT)-based cytoskeletons

It is conceivable that spermatid apico-basal polarity and spermatid planar cell polarity (PCP) are utmost important to support spermatogenesis. The orderly arrangement of developing germ cells in particular spermatids during spermiogenesis are essential to obtain structural and nutrient supports from the fixed number of Sertoli cells across the limited space of seminiferous epithelium in the tubules following Sertoli cell differentiation by ∼17 day postpartum (dpp) in rodents and ∼12 years of age at puberty in humans. Yet few studies are found in the literature to investigate the role of these proteins to support spermatogenesis. Herein, we briefly summarize recent findings in the field, in particular emerging evidence that supports the concept that apico-basal polarity and PCP are conferred by the corresponding polarity proteins through their effects on the actin- and microtubule (MT)-based cytoskeletons. While much research is needed to bridge our gaps of understanding cell polarity, cytoskeletal function, and signaling proteins, a critical evaluation of some latest findings as summarized herein provides some important and also thought-provoking concepts to design better functional experiments to address this important, yet largely expored, research topic.