Rho GTPases and spermatogenesis

Prepubertal nutritional modulation in the bull and its impact on sperm DNA methylation

Enhanced pre-pubertal nutrition in Holstein bulls increased reproductive hormone production and sperm production potential with no negative effects on sperm quality. However, recent trends in human epigenetic research have identified pre-pubertal period to be critical for epigenetic reprogramming in males. Our objective was to evaluate the methylation changes in sperm of bulls exposed to different pre-pubertal diets. One-week-old Holstein bull calves (n = 9), randomly allocated to 3 groups, were fed either a high, medium or low diet (20%, 17% or 12.2% crude protein and 67.9%, 66% or 62.9% total digestible nutrients, respectively) from 2 to 32 weeks of age, followed by medium nutrition. Semen collected from bulls at two specific time points, i.e. 55-59 and 69-71 weeks, was diluted, cryopreserved and used for reduced representation bisulfite sequencing. Differential methylation was detected for dietary treatment, but minimal differences were detected with age. The gene ontology term, "regulation of Rho protein signal transduction", implicated in sperm motility and acrosome reaction, was enriched in both low-vs-high and low-vs-medium datasets. Furthermore, several genes implicated in early embryo and foetal development showed differential methylation for diet. Our results therefore suggest that sperm epigenome keeps the memory of diet during pre-pubertal period in genes important for spermatogenesis, sperm function and early embryo development.

The multiple roles of RAB GTPases in female and male meiosis

BACKGROUND

RAB GTPases constitute the largest family of small GTPases and are found in all eukaryotes. RAB GTPases regulate components of the endomembrane system, the nucleus and the plasma membrane, and are involved in intracellular actin/tubulin-dependent vesicle movement, membrane fusion and cell growth in mitosis.

OBJECTIVE AND RATIONALE

RAB GTPases play multiple critical roles during both female and male meiosis. This review summarizes the progress made in our understanding of the role of RAB GTPases in female and male meiosis in different species. We also discuss the potential relationship between RAB GTPases and oocyte/sperm quality, which may help in understanding the mechanisms underlying oogenesis and spermatogenesis and potential genetic causes of infertility.

SEARCH METHODS

The PubMed database was searched for articles published between 1991 and 2020 using the following terms: 'RAB', 'RAB oocyte', 'RAB sperm' and 'RAB meiosis'.

OUTCOMES

An analysis of 126 relevant articles indicated that RAB GTPases are present in all eukaryotes, and ten subfamilies (almost 70 members) are expressed in human cells. The roles of 25 RAB proteins and orthologues in female meiosis and 12 in male meiosis have been reported. RAB proteins are essential for the accurate continuity of genetic material, successful fertilization and the normal growth of offspring. Distinct and crucial functions of RAB GTPases in meiosis have been reported. In oocytes, RAB GTPases are involved in spindle organization, kinetochore-microtubule attachment, chromosome alignment, actin filament-mediated spindle migration, cytokinesis, cell cycle and oocyte-embryo transition. RAB GTPases function in mitochondrial processes and Golgi-mediated vesicular transport during female meiosis, and are critical for cortical granule transport during fertilization and oocyte-embryo transition. In sperm, RAB GTPases are vital for cytoskeletal organization and successful cytokinesis, and are associated with Golgi-mediated acrosome formation, membrane trafficking and morphological changes of sperm cells, as well as the exocytosis-related acrosome reaction and zona reaction during fertilization.

WIDER IMPLICATIONS

Abnormal expression of RAB GTPases disrupts intracellular systems, which may induce diverse diseases. The roles of RAB proteins in female and male reproductive systems, thus, need to be considered. The mechanisms underlying the function of RAB GTPases and the binding specificity of their effectors during oogenesis, spermatogenesis and fertilization remain to be studied. This review should contribute to our understanding of the molecular mechanisms of oogenesis and spermatogenesis and potential genetic causes of infertility.

Muscarinic acetylcholine receptor M5 is involved in spermatogenesis through the modification of cell-cell junctions

Muscarinic acetylcholine receptor (mAChR) antagonists have been reported to decrease male fertility; however, the roles of mAChRs in spermatogenesis and the underlying mechanisms are not understood yet. During spermatogenesis, extensive remodeling between Sertoli cells and/or germ cells interfaces takes place to accommodate the transport of developing germ cells across the blood-testis barrier (BTB) and adluminal compartment. The cell–cell junctions play a vital role in the spermatogenesis process. This study used ICR male mice and spermatogonial cells (C18-4) and Sertoli cells (TM-4). shRNA of control or M5 gene was injected into 5-week-old ICR mice testes. Ten days post-viral grafting, mice were deeply anesthetized with pentobarbital and the testes were collected. One testicle was fresh frozen for RNA-seq analysis or Western blotting (WB). The second testicle was fixed for immunofluorescence staining (IHF). C18-4 or TM-4 cells were treated with shRNA of control or M5 gene. Then, the cells were collected for RNA-seq analysis, WB, or IHF. Knockdown of mAChR M5 disrupted mouse spermatogenesis and damaged the actin-based cytoskeleton and many types of junction proteins in both Sertoli cells and germ cells. M5 knockdown decreased Phldb2 expression in both germ cells and Sertoli cells which suggested that Phldb2 may be involved in cytoskeleton and cell–cell junction formation to regulate spermatogenesis. Our investigation has elucidated a novel role for mAChR M5 in the regulation of spermatogenesis through the interactions of Phldb2 and cell–cell junctions. M5 may be an attractive future therapeutic target in the treatment of male reproductive disorders.

The Functionally Unannotated Proteome of Human Male Tissues: A Shared Resource to Uncover New Protein Functions Associated with Reproductive Biology

In the context of the Human Proteome Project, we built an inventory of 412 functionally unannotated human proteins for which experimental evidence at the protein level exists (uPE1) and which are highly expressed in tissues involved in human male reproduction. We implemented a strategy combining literature mining, bioinformatics tools to collate annotation and experimental information from specific molecular public resources, and efficient visualization tools to put these unknown proteins into their biological context (protein complexes, tissue and subcellular location, expression pattern). The gathered knowledge allowed pinpointing five uPE1 for which a function has recently been proposed and which should be updated in protein knowledge bases. Furthermore, this bioinformatics strategy allowed to build new functional hypotheses for five other uPE1s in link with phenotypic traits that are specific to male reproductive function such as ciliogenesis/flagellum formation in germ cells (CCDC112 and TEX9), chromatin remodeling (C3orf62) and spermatozoon maturation (CCDC183). We also discussed the enigmatic case of MAGEB proteins, a poorly documented cancer/testis antigen subtype. Tools used and computational outputs produced during this study are freely accessible via ProteoRE (http://www.proteore.org), a Galaxy-based instance, for reuse purposes. We propose these five uPE1s should be investigated in priority by expert laboratories and hope that this inventory and shared resources will stimulate the interest of the community of reproductive biology.

Characterization of biological pathways associated with semen traits in the Thai multibreed dairy population

The objective of this research was to characterize biological pathways associated with semen volume (VOL), number of sperm (NS), and sperm motility (MOT) of dairy bulls in the Thai multibreed dairy population. Phenotypes for VOL (n = 13,535), NS (n = 12,773), and MOT (n = 12,660) came from 131 bulls of the Dairy Farming Promotion Organization of Thailand. Genotypic data consisted of 76,519 imputed and actual single nucleotide polymorphisms (SNP) from 72 animals. The SNP variances for VOL, NS, and MOT were estimated using a three-trait genomic-polygenic repeatability model. Fixed effects were contemporary group, ejaculate order, age of bull, ambient temperature, and heterosis. Random effects were animal additive genetic, permanent environmental, and residual. Individual SNP explaining at least 0.001% of the total genetic variance for each trait were selected to identify associated genes in the NCBI database (UMD Bos taurus 3.1 assembly) using the R package Map2NCBI. A set of 1,999 NCBI genes associated with all three semen traits was utilized for the pathway analysis conducted with the ClueGO plugin of Cytoscape using information from the Kyoto Encyclopedia of Genes and Genomes database. The pathway analysis revealed seven significant biological pathways involving 127 genes that explained 1.04% of the genetic variance for VOL, NS, and MOT. These genes were known to affect cell structure, motility, migration, proliferation, differentiation, survival, apoptosis, signal transduction, oxytocin release, calcium channel, neural development, and immune system functions related to sperm morphology and physiology during spermatogenesis.

Bactrocera dorsalis male sterilization by targeted RNA interference of spermatogenesis: empowering sterile insect technique programs

RNA interference (RNAi) is a genetic technique which has novel application for sustainable pest control. The Sterile Insect Technique (SIT) uses releases of mass-produced, sterile male insects to out-compete wild males for mates to reduce pest populations. RNAi sterilization of SIT males would have several advantages over radiation sterilization, but to achieve this appropriate target genes must first be identified and then targeted with interference technology. With this goal, eight spermatogenesis related candidate genes were cloned and tested for potential activity in Bactrocera dorsalis. The knockdown of candidate genes by oral delivery of dsRNAs did not influence the mating of male flies, but significantly affected the daily average number of eggs laid by females, and reduced egg hatching rate by 16-60%. RNAi negatively affected spermatozoa quantitatively and qualitatively. Following the mating of lola-/topi-/rac-/rho-/upd-/magu-silenced males, we recorded a significant decrease in number and length of spermatozoa in female spermatheca compared to gfp-silenced control group. In a greenhouse trial, the number of damaged oranges and B. dorsalis larvae were significantly reduced in a dsrho-treated group compared with the dsgfp group. This study provides strong evidence for the use RNAi in pest management, especially for the improvement of SIT against B. dorsalis and other species.

p70 S6 kinase and actin dynamics: A perspective

p70 S6 kinase (p70^S6K), a member of the AGC serine/threonine kinase family, was initially identified as a key player, together with its downstream effector S6, in the regulation of cellular growth and survival. The p70^S6K protein has emerged in recent years as a multifunctional protein which also regulates the actin cytoskeleton and thus plays a role in cell migration. This new function is through two important activities of p70^S6K, namely actin cross-linking and Rac1 and Cdc42 activation. The testis is critically dependent on an intricate balance of fundamental cellular processes such as adhesion, migration, and differentiation. It is increasingly evident that Rho GTPases and actin binding proteins play fundamental roles in regulating spermatogenesis within the testis. In this review, we will discuss current findings of p70^S6K in the control of actin cytoskeleton dynamics. In addition, the potential role of p70^S6K in spermatogenesis and testicular function will be highlighted.

Analysis of RhoE expression in the testis, epididymis and ductus deferens, and the effects of its deficiency in mice

Rho proteins are a large family of GTPases involved in the control of actin cytoskeleton dynamics, proliferation and survival. Rnd1, Rnd2 and RhoE/Rnd3 form a subfamily of Rho proteins characterized by being constitutively active. The role of these proteins has been studied during the last years in several systems; however, little is known about their expression and functions in the reproductive organs. In this work we analysed the localization and the effect of RhoE deficiency in the testes using mice lacking RhoE expression (RhoE gt/gt), and our research shows some unexpected and relevant results. First, we have observed that RhoE is only expressed in Leydig cells within the testicular parenchyma and it is absent of seminiferous tubules. In addition, RhoE is expressed in the excurrent ducts of the testis, including the ductuli efferentes, epididymis and ductus deferens. Moreover, the testes of postnatal 15-day-old RhoE null mice are smaller, both in absolute values and in relation to the body weight. Furthermore, the dimensions of their seminiferous tubules are also reduced compared with wild-types. In order to study the role of RhoE in the adult, we analysed heterozygous animals as RhoE null mice die early postnatally. Our results show that the testes of adult RhoE heterozygous mice are also smaller than those of the wild-types, with a 17% decrease in the ratio testis weight/body weight. In addition, their seminiferous tubules have reduced tubular diameter (12%) and a thinner epithelial wall (33%) that appears disorganized and with a swollen lumen. Finally, and probably as a consequence of those alterations, the sperm concentration of heterozygous animals was found to be lower than in the wild-types. These results indicate that accurate levels of RhoE in the testes are necessary for a correct development and function of male gonads, and suggest novel and unexpected roles of Rnd GTPases in the reproductive physiology.

Interleukin-6 disrupts blood-testis barrier through inhibiting protein degradation or activating phosphorylated ERK in Sertoli cells

It has been recently ascribed to several inflammatory cytokines (i.e. TGF-β3, TNF-α, and IL-1) a functional role in regulating Sertoli cell blood-testis barrier (BTB) dynamics. In the testis, IL-6 inhibits meiotic DNA synthesis during the seminiferous epithelium cycle, reduces sperm motility and influences the secretion of transferrin and inhibin B by Sertoli cells. Also, it has been shown that IL-6 affects tight junction permeability in Sertoli cells, but, little is known about its role in regulating the BTB. The aim of this study was to investigate the molecular mechanisms by which IL-6 affects BTB dynamics. We show that IL-6 perturbs the integrity of the BTB, and alters the normal localization and steady-state levels of BTB integral membrane proteins. We demonstrated that IL-6 regulates the BTB by inhibiting the degradation of BTB constitutive proteins and activating ERK-MAPK pathways. Our results provide mechanistic insight into the roles of IL-6 in regulating BTB dynamics.

Phosphatidylinositol 4,5-bisphosphate regulates CapZβ1 and actin dynamics in response to mechanical strain

Mechanical stress causes filament remodeling leading to myocyte hypertrophy and heart failure. The actin capping protein Z (CapZ) tightly binds to the barbed end of actin filaments, thus regulating actin assembly. The hypothesis is that the binding between CapZ and the actin filament is modulated through phosphatidylinositol 4,5-bisphosphate (PIP2) and how the COOH-terminus of CapZβ1 regulates this binding. Primary neonatal rat ventricular myocytes (NRVMs) were strained at 10% amplitude and 1-Hz frequency. Dot blotting measured the PIP2 amount, and affinity precipitation assay assessed the direct interaction between PIP2 and CapZβ1. Fluorescence recovery after photobleaching of green fluorescent protein-CapZβ1 and actin-green fluorescent protein after 1 h of strain shows the dynamics significantly increased above the unstrained group. The increases in CapZ and actin dynamics were blunted by neomycin, suggesting PIP2 signaling is involved. The amount of PIP2 dramatically increased in NRVMs strained for 1 h. With a ROCK or RhoA inhibitor, changes were markedly reduced. Subcellular fractionation and antibody localization showed PIP2 distributed to the sarcomeres. More PIP2-bound CapZβ1 was found in strained NRVMs. Less PIP2 bound to the CapZβ1 with its COOH-terminus intact than in the COOH-terminal mutant of CapZβ1, suggesting some inhibitory role for the COOH-terminus. Myocyte hypertrophy normally induced by 48 h of cyclic strain was blunted by dominant negative RhoA or neomycin. This suggests that after many hours of cyclic strain, a possible mechanism for cell hypertrophy is the accumulation of thin filament assembly triggered partially by the increased PIP2 level and its binding to CapZ.

Characterization of gonadal transcriptomes from Nile tilapia (Oreochromis niloticus) reveals differentially expressed genes

Four pairs of XX and XY gonads from Nile tilapia were sequenced at four developmental stages, 5, 30, 90, and 180 days after hatching (dah) using Illumina Hiseq(TM) technology. This produced 28 Gb sequences, which were mapped to 21,334 genes. Of these, 259 genes were found to be specifically expressed in XY gonads, and 69 were found to be specific to XX gonads. Totally, 187 XX- and 1,358 XY-enhanced genes were identified, and 2,978 genes were found to be co-expressed in XX and XY gonads. Almost all steroidogenic enzymes, including cyp19a1a, were up-regulated in XX gonads at 5 dah; but in XY gonads these enzymes, including cyp11b2, were significantly up-regulated at 90 dah, indicating that, at a time critical to sex determination, the XX fish produced estrogen and the XY fish did not produce androgens. The most pronounced expression of steroidogenic enzyme genes was observed at 30 and 90 dah for XX and XY gonads, corresponding to the initiation of germ cell meiosis in the female and male gonads, respectively. Both estrogen and androgen receptors were found to be expressed in XX gonads, but only estrogen receptors were expressed in XY gonads at 5 dah. This could explain why exogenous steroid treatment induced XX and XY sex reversal. The XX-enhanced expression of cyp19a1a and cyp19a1b at all stages suggests an important role for estrogen in female sex determination and maintenance of phenotypic sex. This work is the largest collection of gonadal transcriptome data in tilapia and lays the foundation for future studies into the molecular mechanisms of sex determination and maintenance of phenotypic sex in non-model teleosts.

Digital gene expression tag profiling analysis of the gene expression patterns regulating the early stage of mouse spermatogenesis

Detailed characterization of the gene expression patterns in spermatogonia and primary spermatocytes is critical to understand the processes which occur prior to meiosis during normal spermatogenesis. The genome-wide expression profiles of mouse type B spermatogonia and primary spermatocytes were investigated using the Solexa/Illumina digital gene expression (DGE) system, a tag based high-throughput transcriptome sequencing method, and the developmental processes which occur during early spermatogenesis were systematically analyzed. Gene expression patterns vary significantly between mouse type B spermatogonia and primary spermatocytes. The functional analysis revealed that genes related to junction assembly, regulation of the actin cytoskeleton and pluripotency were most significantly differently expressed. Pathway analysis indicated that the Wnt non-canonical signaling pathway played a central role and interacted with the actin filament organization pathway during the development of spermatogonia. This study provides a foundation for further analysis of the gene expression patterns and signaling pathways which regulate the molecular mechanisms of early spermatogenesis.

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.

Inferring quantitative trait pathways associated with bull fertility from a genome-wide association study

Whole-genome association studies typically focus on genetic markers with the strongest evidence of association. However, single markers often explain only a small component of the genetic variance and hence offer a limited understanding of the trait under study. As such, the objective of this study was to perform a pathway-based association analysis in Holstein dairy cattle in order to identify relevant pathways involved in bull fertility. The results of a single-marker association analysis, using 1,755 bulls with sire conception rate data and genotypes for 38,650 single nucleotide polymorphisms (SNPs), were used in this study. A total of 16,819 annotated genes, including 2,767 significantly associated with bull fertility, were used to interrogate a total of 662 Gene Ontology (GO) terms and 248 InterPro (IP) entries using a test of proportions based on the cumulative hypergeometric distribution. After multiple-testing correction, 20 GO categories and one IP entry showed significant overrepresentation of genes statistically associated with bull fertility. Several of these functional categories such as small GTPases mediated signal transduction, neurogenesis, calcium ion binding, and cytoskeleton are known to be involved in biological processes closely related to male fertility. These results could provide insight into the genetic architecture of this complex trait in dairy cattle. In addition, this study shows that quantitative trait pathways inferred from single-marker analyses could enhance our interpretations of the results of genome-wide association studies.

Global proteomics analysis of testis and ovary in adult zebrafish (Danio rerio)

The molecular mechanisms controlling sex determination and differentiation in zebrafish (Danio rerio) are largely unknown. A genome-wide analysis may provide comprehensive insights into the processes involved. The mRNA expression in zebrafish gonads has been fairly well studied, but much less data on the corresponding protein expression are available, although the proteins are considered to be more relevant markers of gene function. Because mRNA and protein abundances rarely correlate well, mRNA profiles need to be complemented with the information on protein expression. The work presented here analyzed the proteomes of adult zebrafish gonads by a multidimensional protein identification technology, generating the to-date most populated lists of proteins expressed in mature zebrafish gonads. The acquired proteomics data partially confirmed existing transcriptomics information for several genes, including several novel transcripts. However, disagreements between mRNA and protein abundances were often observed, further stressing the necessity to assess the expression on different levels before drawing conclusions on a certain gene's expression and function. Several gene groups expressed in a sexually dimorphic way in zebrafish gonads were identified. Their potential importance for gonad development and function is discussed. The data gained in the current study provide a basis for further work on elucidating processes occurring during zebrafish development with use of high-throughput proteomics.

Cytoskeletal dynamics and spermatogenesis

Different cellular events occur during spermatogenesis, and these include (i) mitosis for self-renewal of spermatogonia, (ii) differentiation of type A spermatogonia into type B and commitment of type B spermatogonia to develop into preleptotene primary spermatocytes, (iii) transit of preleptotene/leptotene spermatocytes across the blood-testis barrier in coordination with germ cell cycle progression and meiosis, (iv) spermiogenesis and spermiation. These events also associate with extensive changes in cell shape and size, and germ cell movement. The cytoskeleton, which comprises actin, microtubules and intermediate filaments, is believed to function in these cellular events. However, few studies have been conducted by investigators in the past decades to unfold the role of the cytoskeleton during spermatogenesis. This review summarizes recent advances in the field relating to cytoskeletal dynamics in the testis, and highlights areas of research that require additional emphasis so that new approaches for male contraception, as well as therapeutic approaches to alleviate environmental toxicant-induced reproductive dysfunction in men, can possibly be developed.

Polarity proteins and cell-cell interactions in the testis

In mammalian testes, extensive junction restructuring takes place in the seminiferous epithelium at the Sertoli-Sertoli and Sertoli-germ cell interface to facilitate the different cellular events of spermatogenesis, such as mitosis, meiosis, spermiogenesis, and spermiation. Recent studies in the field have shown that Rho GTPases and polarity proteins play significant roles in the events of cell-cell interactions. Furthermore, Rho GTPases, such as Cdc42, are working in concert with polarity proteins in regulating cell polarization and cell adhesion at both the blood-testis barrier (BTB) and apical ectoplasmic specialization (apical ES) in the testis of adult rats. In this chapter, we briefly summarize recent findings on the latest status of research and development regarding Cdc42 and polarity proteins and how they affect cell-cell interactions in the testis and other epithelia. More importantly, we provide a new model in which how Cdc42 and components of the polarity protein complexes work in concert with laminin fragments, cytokines, and testosterone to regulate the events of cell-cell interactions in the seminiferous epithelium via a local autocrine-based regulatory loop known as the apical ES-BTB-basement membrane axis. This new functional axis coordinates various cellular events during different stages of the seminiferous epithelium cycle of spermatogenesis.

Immunohistological profile of the Ras homologous B protein (RhoB) in human testes showing normal spermatogenesis, spermatogenic arrest and Sertoli cell only syndrome

Ras homologous B protein (RhoB) belongs to the Ras homologous subfamily which consists of low molecular weight (21 kDa) GTP-binding proteins. Rho proteins are regulatory molecules associated with various kinases and as such they mediate changes in cell shape, contractility, motility and gene expression. To date, no data are available about the expression pattern of RhoB protein in the human testis showing normal and abnormal spermatogenesis. The present study addresses these issues. Human testicular biopsy specimens were obtained from patients suffering from post-testicular infertility (testis showing normal spermatogenesis, 10 cases) and testicular infertility (testis showing Sertoli cell only syndrome and spermatogenic arrest, 10 patients each). The expression of RhoB was examined using in situ immunofluorescent staining methods. In testes showing normal spermatogenesis, RhoB had a strong expression in the seminiferous epithelium (cytoplasm of Sertoli-cells, spermatogonia and spermatocytes) and in the interstitium (Leydig cells). RhoB expression was weak in the myofibroblasts and absent in the spermatids and sperms. In the testes showing abnormal spermatogenesis, RhoB expression was moderate in the seminiferous epithelium (cytoplasm of Sertoli cells, spermatogonia and spermatocytes) and was completely absent in the Leydig cells, myofibroblasts, spermatids and sperms. To the best of our knowledge, this study provides the first morphological indication that RhoB protein is expressed in human testis and its expression undergoes testicular infertility associated changes. These findings suggest the involvement of RhoB in the process of spermatogenesis in human and their possible therapeutic ramifications in testicular infertility are open for further investigations.

Epidermal growth factor receptor pathway substrate 8 (Eps8) is a novel regulator of cell adhesion and the blood-testis barrier integrity in the seminiferous epithelium

In the seminiferous epithelium, Eps8 is localized to actin-based cell junctions at the blood-testis barrier (BTB) and the apical ectoplasmic specialization (ES) in stage V-VI tubules but is considerably diminished in stage VIII tubules. Eps8 down-regulation coincides with the time of BTB restructuring and apical ES disassembly, implicating the role of Eps8 in cell adhesion. Its involvement in Sertoli-germ cell adhesion was substantiated in studies using an in vivo animal model by treating rats with 1-(2,4-dichlorobenzy)-1H-indazole-3-carbohydrazide (adjudin) to induce anchoring junction restructuring, during which Eps8 disappeared at the apical ES before germ cell departure. In Sertoli cell cultures with established permeability barrier mimicking the BTB in vivo, the knockdown of Eps8 by RNAi led to F-actin disorganization and the mislocalization of the tight junction proteins occludin and ZO-1, suggesting the function of Eps8 in maintaining BTB integrity. In vivo knockdown of Eps8 in the testis caused germ cell sloughing and BTB damage, concomitant with occludin mislocalization, further validating that Eps8 is a novel regulator of cell adhesion and BTB integrity in the seminiferous epithelium.

Gastrointestinal maturation and feeding

The intestine serves not only as a digestive absorptive organ, it is also one of the largest immune organs of the body, has a huge endocrine and exocrine role, and also encompasses neural tissue equivalent to that of the entire spinal cord. The microbial microenvironment also plays a critical role in development. This brief overview of developmental aspects of these intestinal functions will be related to clinical problems in the neonatal period and subsequent health.

Mitogen-activated protein kinases, adherens junction dynamics, and spermatogenesis: a review of recent data

Mitogen-activated protein kinases (MAPKs) are important regulators of many cellular processes. In mammalian testes, these kinases are involved in controlling cell division, differentiation, survival and death, and are therefore critical to spermatogenesis. Recent studies have also illustrated their involvement in junction restructuring in the seminiferous epithelium, especially at the ectoplasmic specialization (ES), a testis-specific adherens junction (AJ) type. ES contributes to the adhesion between Sertoli cells at the blood-testis barrier, as well as between Sertoli and developing spermatids (step 9 and beyond) at the adluminal compartment. MAPKs regulate AJ dynamics in the testis via their effects on the turnover of junction-associated protein complexes, the production of proteases and protease inhibitors, and the cytoskeleton structure. In this review, roles of the three major MAPK members, namely extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, in ES dynamics are critically discussed. An integrated model of how these three MAPKs regulate adhesion function in the seminiferous epithelium is also presented. This model will serve as the framework for future investigation in the field.

cAMP perturbs inter-Sertoli tight junction permeability barrier in vitro via its effect on proteasome-sensitive ubiquitination of occludin

Throughout spermatogenesis, inter-Sertoli tight junctions (TJs) that constitute the blood-testis barrier must be disassembled and reassembled to permit the timely movement of preleptotene and leptotene spermatocytes from the basal to the adluminal compartment of the seminiferous epithelium. However, the mechanism and the participating molecules that regulate the bioavailability of TJ proteins are entirely unknown. Using Sertoli cell culture, it was shown that there was an increase in occludin level, concomitant with a reduction of an E3 ubiquitin ligase, Itch, at the time when inter-Sertoli TJs were assembled. By co-immunoprecipitation, occludin was shown to associate with Itch at the TJs. A novel interaction between Itch and UBC4 (an ubiquitin-conjugating enzyme) was identified. When TJs were disrupted by dibutyryl-cAMP (db-cAMP), an increase in protein levels of Itch and UBC4 along with a significant reduction in endogenous occludin was detected. These results seemingly suggest that the interaction of Itch and UBC4 on occludin is potentially involved in regulating Sertoli TJ dynamics. Addition of a proteasome inhibitor, MG-132, into Sertoli cells cultured with db-cAMP blocked the db-cAMP-induced occludin loss in vitro. Accumulations of ubiquitin-conjugated and Itch-conjugated occludin were detected in Sertoli cells cultured in the presence of both MG-132 and db-cAMP. These results suggest that MG-132 prevented db-cAMP-induced TJ disruption by altering the rate of occludin degradation. Taken collectively, the results reported herein support the notion that db-cAMP-induced TJ disruption was mediated by an induction of Itch protein expression, which in turn triggered the ubiquitination of occludin resulting in TJ disruption.

Regulation of Sertoli-germ cell adherens junction dynamics in the testis via the nitric oxide synthase (NOS)/cGMP/protein kinase G (PRKG)/beta-catenin (CATNB) signaling pathway: an in vitro and in vivo study

During spermatogenesis, extensive restructuring of cell junctions takes place in the seminiferous epithelium to facilitate germ cell movement. However, the mechanism that regulates this event remains largely unknown. Recent studies have shown that nitric oxide (NO) likely regulates tight junction (TJ) dynamics in the testis via the cGMP/protein kinase G (cGMP-dependent protein kinase, PRKG) signaling pathway. Due to the proximity of TJ and adherens junctions (AJ) in the testis, in particular at the blood-testis barrier, it is of interest to investigate if NO can affect AJ dynamics. Studies using Sertoli-germ cell cocultures in vitro have shown that the levels of NOS (nitric oxide synthase), cGMP, and PRKG were induced when anchoring junctions were being established. Using an in vivo model in which adult rats were treated with adjudin [a molecule that induces adherens junction disruption, formerly called AF-2364, 1-(2,4-dichlorobenzyl)-IH-indazole-3-carbohydrazide], the event of AJ disruption was also associated with a transient iNOS (inducible nitric oxide synthase, NOS2) induction. Immunohistochemistry has illustrated that NOS2 was intensely accumulated in Sertoli and germ cells in the epithelium during adjudin-induced germ cell loss, with a concomitant accumulation of intracellular cGMP and an induction of PRKG but not cAMP or protein kinase A (cAMP-dependent protein kinase, PRKA). To identify the NOS-mediated downstream signaling partners, coimmunoprecipitation was used to demonstrate that NOS2 and eNOS (endothelial nitric oxide synthase, NOS3) were structurally associated with the N-cadherin (CDH2)/beta-catenin (CATNB)/actin complex but not the nectin-3 (poliovirus receptor-related 3, PVRL 3)/afadin (myeloid/lymphoid or mixed lineage-leukemia tranlocation to 4 homolog, MLLT4) nor the integrin beta1 (ITB1)-mediated protein complexes, illustrating the spatial vicinity of NOS with selected AJ-protein complexes. Interestingly, CDH2 and CATNB were shown to dissociate from NOS during the adjudin-mediated AJ disruption, implicating the CDH2/CATNB protein complex is the likely downstream target of the NO signaling. Furthermore, PRKG, the downstream signaling protein of NOS, was shown to interact with CATNB in the rat testis. Perhaps the most important of all, pretreatment of testes with KT5823, a specific PRKG inhibitor, can indeed delay the adjudin-induced germ cell loss, further validating NOS/NO regulates Sertoli-germ cell AJ dynamics via the cGMP/PRKG pathway. These results illustrate that the CDH2/CATNB-mediated adhesion function in the testis is regulated, at least in part, via the NOS/cGMP/PRKG/CATNB pathway.

Protein kinases and adherens junction dynamics in the seminiferous epithelium of the rat testis

Earlier studies in multiple epithelia have shown that cell-cell actin-based adherens junction (AJ) dynamics are regulated, at least in part, by the interplay of kinases and phosphatases that determines the intracellular phosphoprotein content. Yet it is virtually unknown regarding the role of protein kinases in Sertoli-germ cell AJ dynamics in the seminiferous epithelium of the testis. To address this issue, an in vitro coculture system utilizing Sertoli and germ cells was used to study the regulation of several protein kinases, including c-Src (the cellular form of the v-src transforming gene of Rous Sarcoma virus, RSV), carboxyl-terminal Src kinase (Csk), and casein kinase 2 (CK2), during AJ assembly. Both Sertoli and germ cells were shown to express c-Src, Csk, and CK2 with a relative Sertoli:germ cell ratio of approximately 1:1, suggesting both cell types contributed equally to the pool of these kinases in the epithelium. c-Src and Csk were shown to be stage-specific proteins during the epithelial cycle, being highest at stages VII-VIII. Studies using immunoprecipitation have illustrated that these kinases were structurally associated with the N-cadherin/beta-catenin, but not the nectin/afadin, protein complex, implicating that the cadherin/catenin protein complex is their likely putative substrate. An induction in c-Src, Csk, and CK2 were detected during Sertoli-germ cell AJ assembly in vitro but not when Sertoli cells were cultured alone. When adult rats were treated with 1-(2,4-dichlorobenzyl)-indazole-3-carbohydrazide (AF-2364), a compound known to induce germ cell loss from the seminiferous epithelium, in particular elongating/elongate and round spermatids, by disrupting Sertoli-germ cell AJs, an induction of c-Src and Csk, but not CK2, was detected. Furthermore, a transient increase in the intrinsic kinase activities of c-Src, but not CK2, was also detected. This event was also associated with a loss of protein-protein association of N-cadherin and beta-catenin from the cadherin/catenin/c-Src/Csk/CK2 protein complex. Administration of PP1, a c-Src inhibitor, into adult rats via the jugular vein could induce the loss of spermatocytes and round spermatids, but not elongating/elongate spermatids, from the seminiferous epithelium. This result thus implicates the importance of c-Src in maintaining the integrity of AJs and possibly desmosome-like junctions between Sertoli cells and spermatocytes/round spermatids. In short, the data reported herein have shown that c-Src, Csk, and CK2 are novel protein kinases in AJ dynamics in the testis.

The Blood‐Testis Barrier: Its Biology, Regulation, and Physiological Role in Spermatogenesis

The blood-testis barrier (BTB) in mammals, such as rats, is composed of the tight junction (TJ), the basal ectoplasmic specialization (basal ES), the basal tubulobulbar complex (basal TBC) (both are testis-specific actin-based adherens junction [AJ] types), and the desmosome-like junction that are present side-by-side in the seminiferous epithelium. The BTB physically divides the seminiferous epithelium into basal and apical (or adluminal) compartments, and is pivotal to spermatogenesis. Besides its function as an immunological barrier to segregate the postmeiotic germ-cell antigens from the systemic circulation, it creates a unique microenvironment for germ-cell development and confers cell polarity. During spermatogenesis, the BTB in rodents must physically disassemble to permit the passage of preleptotene and leptotene spermatocytes. This occurs at late stage VII through early stage VIII of the epithelial cycle. Studies have shown that this dynamic BTB restructuring to facilitate germ-cell migration is regulated by two cytokines, namely transforming growth factor-beta3 (TGF-beta3) and tumor necrosis factor-alpha (TNFalpha), via downstream mitogen-activated protein kinases. These cytokines determine the homeostasis of TJ- and basal ES-structural proteins, proteases, protease inhibitors, and other extracellular matrix (ECM) proteins (e.g., collagen) in the seminiferous epithelium. Some of these molecules are known regulators of focal contacts between the ECM and other actively migrating cells, such as macrophages, fibroblasts, or malignant cells. These findings also illustrate that cell-cell junction restructuring at the BTB is regulated by mechanisms involved in the junction turnover at the cell-matrix interface. This review critically discusses these latest findings in the field in light of their significance in the biology and regulation of the BTB pertinent to spermatogenesis.

Sertoli-Sertoli and Sertoli-germ cell interactions and their significance in germ cell movement in the seminiferous epithelium during spermatogenesis

Spermatogenesis is the process by which a single spermatogonium develops into 256 spermatozoa, one of which will fertilize the ovum. Since the 1950s when the stages of the epithelial cycle were first described, reproductive biologists have been in pursuit of one question: How can a spermatogonium traverse the epithelium, while at the same time differentiating into elongate spermatids that remain attached to the Sertoli cell throughout their development? Although it was generally agreed upon that junction restructuring was involved, at that time the types of junctions present in the testis were not even discerned. Today, it is known that tight, anchoring, and gap junctions are found in the testis. The testis also has two unique anchoring junction types, the ectoplasmic specialization and tubulobulbar complex. However, attention has recently shifted on identifying the regulatory molecules that "open" and "close" junctions, because this information will be useful in elucidating the mechanism of germ cell movement. For instance, cytokines have been shown to induce Sertoli cell tight junction disassembly by shutting down the production of tight junction proteins. Other factors such as proteases, protease inhibitors, GTPases, kinases, and phosphatases also come into play. In this review, we focus on this cellular phenomenon, recapping recent developments in the field.

Adaptors, Junction Dynamics, and Spermatogenesis1

Adaptors are component proteins of junctional complexes in all epithelia, including the seminiferous epithelium of the mammalian testis. They recruit other regulatory and structural proteins to the site of both anchoring junctions (such as cell-cell actin-based adherens junctions [AJs], e.g., ectoplasmic specialization [ES] and tubulobulbar complex, which are both testis-specific cell-cell actin-based AJ types, and cell-cell intermediate filament-based desmosome-like junctions) and tight junctions (TJ). Furthermore, adaptors per se can be substrates and/or activators of kinases or phosphatases. As such, the integrity of cell junctions and the regulation of junction dynamics during spermatogenesis rely on adaptors for their ability to recruit and link different junctional components to the same site and to tether transmembrane proteins at both anchoring and TJ sites to the underlying cytoskeletons, such as the actin filaments, intermediate filaments, and microtubules. These protein-protein interactions are possible because adaptors are composed of conserved protein binding domains, which allow them to link to more than one structural or signaling protein, recruiting multi-protein complexes to the same site. Herein, we provide a timely review of adaptors recently found at the sites of AJ (e.g., ES) and TJ. In addition, several in vivo models that can be used to delineate the function of adaptors in the testis are described, and the role of adaptors in regulating junction dynamics pertinent to spermatogenesis is discussed.

Expression profiling of mammalian male meiosis and gametogenesis identifies novel candidate genes for roles in the regulation of fertility

We report a comprehensive large-scale expression profiling analysis of mammalian male germ cells undergoing mitotic growth, meiosis, and gametogenesis by using high-density oligonucleotide microarrays and highly enriched cell populations. Among 11,955 rat loci investigated, 1268 were identified as differentially transcribed in germ cells at subsequent developmental stages compared with total testis, somatic Sertoli cells as well as brain and skeletal muscle controls. The loci were organized into four expression clusters that correspond to somatic, mitotic, meiotic, and postmeiotic cell types. This work provides information about expression patterns of approximately 200 genes known to be important during male germ cell development. Approximately 40 of those are included in a group of 121 transcripts for which we report germ cell expression and lack of transcription in three somatic control cell types. Moreover, we demonstrate the testicular expression and transcriptional induction in mitotic, meiotic, and/or postmeiotic germ cells of 293 as yet uncharacterized transcripts, some of which are likely to encode factors involved in spermatogenesis and fertility. This group also contains potential germ cell-specific targets for innovative contraceptives. A graphical display of the data is conveniently accessible through the GermOnline database at http://www.germonline.org.

Interactions among IQGAP1, Cdc42, and the cadherin/catenin protein complex regulate Sertoli-germ cell adherens junction dynamics in the testis

The movement of developing germ cells across the seminiferous epithelium during spermatogenesis involves extensive adherens junction (AJ) restructuring between Sertoli cells, as well as between Sertoli and germ cells. In this report, we show that the intricate interactions between Cdc42 (a Rho family protein of Mr approximately 23 kDa originally identified in membranes of human platelets and placenta, and is the homolog of CDC42Sc, which is known to regulate of bud-site assembly in Saccharomyces cerevisiae) and its effector, IQ motif containing GTPase activating protein (IQGAP1, Mr approximately 189 kDa, it is also an actin-binding protein known to interact with Cdc42 and Rac1 GTPases), regulate Sertoli-germ cell, but not Sertoli-Sertoli cell, AJ dynamics. Using testis lysates for immunoprecipitation (IP), IQGAP1 was shown to associate with E-cadherin, N-cadherin, and beta-catenin (but not beta1-integrin and nectin-2), as well as with actin and vimentin (but not alpha-tubulin). Moreover, IQGAP1 was found to localize to the periphery of both Sertoli and germ cells in the seminiferous epithelium, at sites of cell-cell contacts. Using fluorescent microscopy with dual fluorescent probes, IQGAP1 was found to co-localize, at least in part, with N-cadherin in the seminiferous epithelium consistent with their localization at the basal and apical ES. Using Sertoli-germ cell cocultures, it was demonstrated that AJ assembly associated with a transient induction of Cdc42 and IQGAP1, which was not found when Sertoli cells were cultured alone. Lastly, a shift in the interactions of Cdc42, IQGAP1, beta-catenin, and N-cadherin was detected in Sertoli-germ cell cocultures using an Ca2+-induced AJ disruption model, which was used to examine AJ disassembly and its reassembly. In the presence of Ca2+, IQGAP1 bound preferentially to Cdc42 rather than to beta-catenin. However, when Ca2+ was depleted from cocultures using EGTA, a Ca2+ chelating agent, IQGAP1 lost its affinity for Cdc42 and became tightly associated with beta-catenin, destabilizing cadherin-mediated AJs between Sertoli and germ cells. Yet this shift of protein-protein interaction was not detected in Sertoli cells cultured alone. These results illustrate that the interactions among IQGAP1, Cdc42, and beta-catenin are crucial to the regulation of Sertoli-germ cell, but not Sertoli-Sertoli cell, AJ dynamics in the seminiferous epithelium.

Fer kinase/FerT and adherens junction dynamics in the testis: an in vitro and in vivo study

Fer kinase is a 94-kDa cytoplasmic cell-cell actin-based adherens junction (AJ)-associated nonreceptor protein tyrosine kinase (PTK) found in multiple epithelia including the testis, whereas FerT kinase (51 kDa) is the truncated testis-specific form of Fer kinase, lacking the Fps/Fes/Fer/CIP4 (products of oncogenes identified in avian and feline sarcoma, encoding tyrosine protein kinases) and the three coiled-coil domains versus Fer kinase. Yet the role(s) of Fer kinase in AJ dynamics in the testis remains largely unexplored. We have used an in vitro model of AJ assembly with Sertoli-germ cell cocultures and an in vivo model of AJ disassembly in which adult rats were treated with 1-(2,4-dichlorobenzyl)-indazole-3-carbohydrazide (AF-2364) to study changes in the expression and/or localization of Fer kinase during AJ restructuring. Fer kinase/FerT was expressed by Sertoli and germ cells when cultured in vitro. Using an antibody prepared against a synthetic peptide, NH2-SAPQNCPEEIFTIMMKCWDYK-COOH, corresponding to residues 779-799 of Fer kinase in the rat, which failed to cross-react with FerT kinase, for immunohistochemistry, Fer kinase was detected in the seminiferous epithelium in virtually all stages of the epithelial cycle. At stages XIII-VI, Fer kinase was associated largely with round and elongating spermatids. At stages VII-VIII, Fer kinase associated almost exclusively with round spermatids with very weak staining associated with elongated spermatids. This stage-specific localization of Fer kinase in the epithelium was confirmed by using staged tubules for semiquantitative reverse transcription-polymerase chain reaction. Studies by immunoprecipitation revealed that Fer kinase associated with N-cadherin, gamma-catenin, p120ctn, c-Src (a putative PTK and the product of the transforming, sarcoma-inducing gene of Rous sarcoma virus), Rab 8 (a GTPase), actin, vimentin, but not E-cadherin, afadin, nectin-3, and integrin beta1, suggesting Fer kinase associates not only with the actin-based cell-cell AJ structures, such as the N-cadherin/catenin complex (but not the alpha6beta1 integrin/laminin and the afadin/nectin complex), but also with intermediate filament-based cell-cell desmosomes. An induction in Fer kinase expression was detected during Sertoli-germ cell AJ assembly in vitro but not during AF-2364-induced AJ disruption in vivo. Yet this AF-2364-induced Fer kinase plummeting associated with an induction in N-cadherin, beta-catenin, and p120ctn, particularly at the base of the seminiferous epithelium. In summary, Fer kinase structurally associates with the N-cadherin/catenin protein complex in the testis and can possibly be used to mediate signaling function via the cadherin/catenin protein complex.