Planar cell polarity (PCP) proteins and spermatogenesis

Regulation of sertoli cell function by planar cell polarity (PCP) protein Fjx1

Four-jointed box kinase 1 (Fjx1) is a planar cell protein (PCP) and a member of the Fat (FAT atypical cadherin 1)/Dchs (Dachsous cadherin-related protein)/Fjx1 PCP complex. Fjx1 is also a non-receptor Ser/Thr protein kinase capable of phosphorylating Fat1 at is extracellular cadherin domains when it is transport across the Golgi system. As such, Fjx1 is a Golgi-based regulator of Fat1 function by determining its extracellular deposition. Herein, Fjx1 was found to localize across the Sertoli cell cytoplasm, partially co-localized with the microtubules (MTs) across the seminiferous epithelium. It was most notable at the apical ES (ectoplasmic specialization) and basal ES, displaying distinctive stage-specific expression. The apical ES and basal ES are the corresponding testis-specific cell adhesion ultrastructures at the Sertoli-elongated spermatid and Sertoli cell-cell interface, respectively, consistent with the role of Fjx1 as a Golgi-associated Ser/Thr kinase that modulates the Fat (and/or Dchs) integral membrane proteins. Its knockdown (KD) by RNAi using specific Fjx1 siRNA duplexes versus non-targeting negative control siRNA duplexes was found to perturb the Sertoli cell tight junction function, as well as perturbing the function and organization of MT and actin. While Fjx1 KD did not affect the steady-state levels of almost two dozens of BTB-associated Sertoli cell proteins, including structural and regulatory proteins, its KD was found to down-regulate Fat1 (but not Fat2, 3, and 4) and to up-regulate Dchs1 (but not Dchs2) expression. Based on results of biochemical analysis, Fjx1 KD was found to be capable of abolishing phosphorylation of its putative substrate Fat1 at its Ser/Thr sites, but not at its Tyr site, illustrating an intimate functional relationship of Fjx1 and Fat1 in Sertoli cells.

Developmental exposure to real-life environmental chemical mixture programs a testicular dysgenesis syndrome-like phenotype in prepubertal lambs

Current declines in male reproductive health may, in part, be driven by anthropogenic environmental chemical (EC) exposure. Using a biosolids treated pasture (BTP) sheep model, this study examined the effects of gestational exposure to a translationally relevant EC mixture. Testes of 8-week-old ram lambs from mothers exposed to BTP during pregnancy contained fewer germ cells and had a greater proportion of Sertoli-cell-only seminiferous tubules. This concurs with previous published data from fetuses and neonatal lambs from mothers exposed to BTP. Comparison between the testicular transcriptome of biosolids lambs and human testicular dysgenesis syndrome (TDS) patients indicated common changes in genes involved in apoptotic and mTOR signalling. Gene expression data and immunohistochemistry indicated increased HIF1α activation and nuclear localisation in Leydig cells of BTP exposed animals. As HIF1α is reported to disrupt testosterone synthesis, these results provide a potential mechanism for the pathogenesis of this testicular phenotype, and TDS in humans.

Spermiation: Insights from Studies on the Adjudin Model

Spermatogenesis is comprised of a series of cellular events that lead to the generation of haploid sperm. These events include self-renewal of spermatogonial stem cells (SSC), proliferation of spermatogonia by mitosis, differentiation of spermatogonia and spermatocytes, generation of haploid spermatids via meiosis I/II, and spermiogenesis. Spermiogenesis consists of a series of morphological events in which spermatids are being transported across the apical compartment of the seminiferous epithelium while maturing into spermatozoa, which include condensation of the genetic materials, biogenesis of acrosome, packaging of the mitocondria into the mid-piece, and elongation of the sperm tail. However, the biology of spermiation remains poorly understood. In this review, we provide in-depth analysis based on the use of bioinformatics tools and an animal model that mimics spermiation through treatment of adult rats with adjudin, a non-hormonal male contraceptive known to induce extensive germ cell exfoliation across the seminiferous epithelium, but nost notably elongating/elongated spermatids. These analyses have shed insightful information regaridng the biology of spermiation.

Molecular mechanisms mediating asymmetric subcellular localisation of the core planar polarity pathway proteins

Planar polarity refers to cellular polarity in an orthogonal plane to apicobasal polarity, and is seen across scales from molecular distributions of proteins to tissue patterning. In many contexts it is regulated by the evolutionarily conserved ‘core' planar polarity pathway that is essential for normal organismal development. Core planar polarity pathway components form asymmetric intercellular complexes that communicate polarity between neighbouring cells and direct polarised cell behaviours and the formation of polarised structures. The core planar polarity pathway consists of six structurally different proteins. In the fruitfly Drosophila melanogaster, where the pathway is best characterised, an intercellular homodimer of the seven-pass transmembrane protein Flamingo interacts on one side of the cell junction with the seven-pass transmembrane protein Frizzled, and on the other side with the four-pass transmembrane protein Strabismus. The cytoplasmic proteins Diego and Dishevelled are co-localised with Frizzled, and Prickle co-localises with Strabismus. Between these six components there are myriad possible molecular interactions, which could stabilise or destabilise the intercellular complexes and lead to their sorting into polarised distributions within cells. Post-translational modifications are key regulators of molecular interactions between proteins. Several post-translational modifications of core proteins have been reported to be of functional significance, in particular phosphorylation and ubiquitination. In this review, we discuss the molecular control of planar polarity and the molecular ecology of the core planar polarity intercellular complexes. Furthermore, we highlight the importance of understanding the spatial control of post-translational modifications in the establishment of planar polarity.

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.

Gene Polymorphisms in Boar Spermatozoa and Their Associations with Post-Thaw Semen Quality

Genetic markers have been used to assess the freezability of semen. With the advancement in molecular genetic techniques, it is possible to assess the relationships between sperm functions and gene polymorphisms. In this study, variant calling analysis of RNA-Seq datasets was used to identify single nucleotide polymorphisms (SNPs) in boar spermatozoa and to explore the associations between SNPs and post-thaw semen quality. Assessment of post-thaw sperm quality characteristics showed that 21 boars were considered as having good semen freezability (GSF), while 19 boars were classified as having poor semen freezability (PSF). Variant calling demonstrated that most of the polymorphisms (67%) detected in boar spermatozoa were at the 3’-untranslated regions (3’-UTRs). Analysis of SNP abundance in various functional gene categories showed that gene ontology (GO) terms were related to response to stress, motility, metabolism, reproduction, and embryo development. Genomic DNA was isolated from sperm samples of 40 boars. Forty SNPs were selected and genotyped, and several SNPs were significantly associated with motility and membrane integrity of frozen-thawed (FT) spermatozoa. Polymorphism in SCLT1 gene was associated with significantly higher motility and plasma membrane integrity of FT spermatozoa from boars of the GSF group compared with those of the PSF group. Likewise, polymorphisms in MAP3K20, MS4A2, and ROBO1 genes were significantly associated with reduced cryo-induced lipid peroxidation and DNA damage of FT spermatozoa from boars of the GSF group. Candidate genes with significant SNP associations, including APPL1, PLBD1, FBXO16, EML5, RAB3C, OXSR1,PRICKLE1, and MAP3K20 genes, represent potential markers for post-thaw semen quality, and they might be relevant for future improvement in the selection procedure of boars for cryopreservation. The findings of this study provide evidence indicating that polymorphisms in genes expressed in spermatozoa could be considered as factors associated with post-thaw semen quality.

Combover interacts with the axonemal component Rsp3 and is required for Drosophila sperm individualization

Gamete formation is key to survival of higher organisms. In male animals, spermatogenesis gives rise to interconnected spermatids that differentiate and individualize into mature sperm, each tightly enclosed by a plasma membrane. In Drosophila melanogaster, individualization of sister spermatids requires the formation of specialized actin cones that synchronously move along the sperm tails, removing inter-spermatid bridges and most of the cytoplasm. Here, we show that Combover (Cmb), originally identified as an effector of planar cell polarity (PCP) under control of Rho kinase, is essential for sperm individualization. cmb mutants are male sterile, with actin cones that fail to move in a synchronized manner along the flagella, despite being correctly formed and polarized initially. These defects are germline autonomous, independent of PCP genes, and can be rescued by wild-type Cmb, but not by a version of Cmb in which known Rho kinase phosphorylation sites are mutated. Furthermore, Cmb binds to the axonemal component Radial spoke protein 3, knockdown of which causes similar individualization defects, suggesting that Cmb coordinates the individualization machinery with the microtubular axonemes.

Regulation of BTB dynamics in spermatogenesis - insights from the adjudin toxicant model

During spermatogenesis, cell organelles and germ cells, most notably haploid spermatids, are transported across the seminiferous epithelium so that fully developed spermatids line-up at the edge of the tubule lumen to undergo spermiation at stage VIII of the cycle. Studies have suggested that the microtubule (MT)-based cytoskeleton is necessary to support these cellular events. However, the regulatory molecule(s) and underlying mechanism(s) remain poorly understood. Herein, we sought to better understand this event by using an adjudin-based animal model. Adult rats were treated with adjudin at low-dose (10 mg/kg b.w.) which by itself had no notable effects on spermatogenesis. Rats were also treated with low-dose adjudin combined with overexpression of two endogenously produced blood-testis barrier (BTB) modifiers, namely rpS6 [ribosomal protein S6, the downstream signaling protein of mammalian target of rapamycin complex 1 (mTORC1)] and F5-peptide (a biological active peptide released from laminin-γ3 chain at the Sertoli-spermatid interface) versus the two BTB modifiers alone. Overexpression of these two BTB modifiers in the testis was shown to enhance delivery of adjudin to the testis, effectively inducing disruptive changes in MT cytoskeletons, causing truncation of MT conferred tracks that led to their collapse across the epithelium. The net result was massive germ cell exfoliation in the tubules, disrupting germ cell transport and cell adhesion across the seminiferous epithelium that led to aspermatogenesis. These changes were the result of disruptive spatial expression of several MT-based regulatory proteins. In summary, MT cytoskeleton supported by the network of MT regulatory proteins is crucial to maintain spermatogenesis.

Vangl2 regulates spermatid planar cell polarity through microtubule (MT)-based cytoskeleton in the rat testis

During spermatogenesis, developing elongating/elongated spermatids are highly polarized cells, displaying unique apico-basal polarity. For instance, the heads of spermatids align perpendicular to the basement membrane with their tails pointing to the tubule lumen. Thus, the maximal number of spermatids are packed within the limited space of the seminiferous epithelium to support spermatogenesis.  Herein, we reported findings that  elongating/elongated spermatids displayed planar cell polarity (PCP) in adult rat testes in which the proximal end of polarized spermatid heads were aligned uniformly across the plane of the seminiferous epithelium based on studies  using confocal microscopy and 3-dimensional (D) reconstruction of the seminiferous tubules.  We also discovered  that spermatid PCP was regulated by PCP protein Vangl2 (Van Gogh-like protein 2) since Vangl2 knockdown by RNAi was found to perturb spermatid PCP. More important, Vangl2 exerted its regulatory effects through changes in the organization of the microtubule (MT)-based cytoskeleton in the seminiferous epithelium. These changes were mediated via the downstream signaling proteins atypical protein kinase C ξ (PKCζ) and MT-associated protein (MAP)/microtubule affinity-regulating kinase 2 (MARK2). These findings thus provide new insights regarding the biology of spermatid PCP during spermiogenesis.

Monitoring the Integrity of the Blood-Testis Barrier (BTB): An In Vivo Assay

The blood-testis barrier is a unique ultrastructure in the mammalian testis, located near the basement membrane of the seminiferous tubule that segregates the seminiferous epithelium into the basal and the adluminal (apical) compartment. Besides restricting paracellular and transcellular passage of biomolecules (e.g., paracrine factors, hormones), water, electrolytes, and other substances including toxicants and/or drugs to enter the adluminal compartment of the epithelium, the BTB is an important ultrastructure that supports spermatogenesis. As such, a sensitive and reliable assay to monitor its integrity in vivo is helpful for studying testis biology. This assay is based on the ability of an intact BTB to exclude the diffusion of a small molecule such as sulfo-NHS-LC-biotin (C₂₀H₂₉N₄NaO₉S₂, Mr. 556.59, a water-soluble and membrane-impermeable biotinylation reagent) from the basal to the apical compartment of the seminiferous epithelium. Herein, we summarize the detailed procedures on performing the assay and to obtain semiquantitative data to assess the extent of BTB damage when compared to positive controls, such as treatment of rats with cadmium chloride (CdCl₂) which is known to compromise the BTB integrity.

Regulation of cellular and PCP signalling by the Scribble polarity module

Since the first identification of the Scribble polarity module proteins as a new class of tumour suppressors that regulate both cell polarity and proliferation, an increasing amount of evidence has uncovered a broader role for Scribble, Dlg and Lgl in the control of fundamental cellular functions and their signalling pathways. Here, we review these findings as well as discuss more specifically the role of the Scribble module in PCP signalling.

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.