Evidence that Mothers-against-dpp-related 1 (Madr1) plays a role in the initiation and maintenance of spermatogenesis in the mouse

Expression of the TGF-beta1 system in human testicular pathologies

BACKGROUND

In non-obstructive azoospermia, histological patterns of Sertoli cell-only Syndrome (SCO) and hypospermatogenesis (H) are commonly found. In these pathologies, Leydig cell hyperplasia (LCH) is detected in some patients. Since TGF-β1 is involved in cellular proliferation/development, the aim of this work was to analyze the expression of TGF-β1, its receptors TGFBRII, TGFBRI (ALK-1 and ALK-5), and the co-receptor endoglin in human biopsies from patients with idiopathic infertility.

METHODS

Specific immunostaining of TGF-β1, its receptors TGFBRII, TGFBRI (ALK-1 and ALK-5), co-receptor endoglin and Smads proteins, were carried out in testicular biopsies from normal and infertile men with SCO or H. Gene expression of TGF-β1 system were made in biopsies from infertile patients with semi-quantitative and quantitative PCR.

RESULTS

Immunohistochemical studies revealed that TGF-β1 and its specific receptors are present in Leydig cells in biopsies from normal tissue or patients with SCO or H with or without LCH. Smad proteins, which are involved in TGF-β1 signaling, are also detected in both their phosphorylated (activated) and dephosphorylated form in all samples TGF-β1, ALK-1 and endoglin gene expression are stronger in human biopsies with LCH than in those with SCO or H. Neither TGFBRII nor ALK-5 gene expression showed significant differences between pathologies. A significant correlation between ALK-1 and endoglin expression was observed.

CONCLUSIONS

In conclusion, the high levels of mRNA and protein expression of the TGF-β1 system in patients with LCH, particularly ALK1 and its correlation with endoglin, suggest that these proteins acting in concert might be, at least in part, committed actors in the Leydig cell hyperplasia.

Alteration of transforming growth factor-beta signaling system expression in adult rat germ cells with a chronic apoptotic cell death process after fetal androgen disruption

In utero exposure to chemicals with antiandrogen activity induces undescended testis, hypospadias, and sub- or infertility. The hypospermatogenesis observed in the adult rat testis exposed in utero to the antiandrogen flutamide has been reported to be a result of a long-term apoptotic cell death process in mature germ cells. However, little if anything is known about the upstream signaling mechanisms controlling this apoptosis. In the present study, we have investigated the possibility that the TGF-beta signaling pathway may be at play in this control of the apoptotic germ cell death process. By using a model of adult rat exposed in utero to 0, 0.4, 2, or 10 mg/kg.d flutamide, we observed that pro-TGF-beta signaling members, such as the three isoforms of TGF-beta ligands (TGF-beta1-3), the two TGF-beta receptors (TGF-betaRI and -RII) and the R-Smads Smad 1, Smad 2, Smad 3, and Smad 5 were inhibited at the mRNA and protein levels, whereas the anti-TGF-beta signaling member Smad 7 was overexpressed. Furthermore, we report that the overexpression of Smad 7 mRNA could induce an activation of c-Jun N-terminal kinase, because of the observed c-Jun overexpression, activation, and nuclear translocation leading to an increase in the transcription of the proapoptotic factor Fas-L. Together, the alterations of TGF-beta signaling may represent upstream mechanisms underlying the adult germ cell apoptotic process evidenced in adult rat testis exposed in utero to antiandrogenic compounds such as flutamide.

Impaired spermatogenesis and male fertility defects in CIZ/Nmp4-disrupted mice

CIZ (Cas interacting zinc finger protein), also called Nmp4 (nuclear matrix protein 4), is a nucleo-cytoplasmic shuttling transcription factor that regulates the expression of collagen and matrix metalloproteinases. CIZ/Nmp4 was originally cloned by its binding to p130(Cas), a focal adhesion protein, and was recently shown to suppress BMP2 (bone mophogenetic protein 2) signalling. To explore the physiological role of CIZ/Nmp4, we disrupted CIZ/Nmp4-gene by inserting beta-galactosidase and neomycin resistance genes into the 2nd exon of CIZ/Nmp4-gene, which is utilized by all the sequenced alternative forms. CIZ-/- mice were born and grew to adulthood. Although they tend to be smaller than wild-type mice, no pathological abnormality was observed except in the testis. Histological analysis of the testes revealed variable degrees of spermatogenic cell degeneration within the seminiferous tubules of CIZ-/- mice, resembling the histology of the 'Germinal-cell aplasia with focal spermatogenesis'. Some of the CIZ-/- male mice developed infertility. TUNEL assay on testis sections revealed an increased occurrence of apoptosis of spermatogenic cells in the testes of CIZ-/- mice. CIZ/Nmp4 was co-localized with Smad1 in the testis, suggesting that a disregulation of BMP signalling could cause these phenotypes. These results suggest that CIZ/Nmp4 plays roles in the progress and the maintenance of spermatogenesis.

Developmental expression and function of Bmp4 in spermatogenesis and in maintaining epididymal integrity

Bone morphogenetic proteins (BMPs) play essential roles in many aspects of developmental biology. We have previously shown that Bmp7, Bmp8a, and Bmp8b of the 60A class of Bmp genes have additive effects in spermatogenesis and in maintaining the epididymal integrity of the caput and caudal regions. Here we report that Bmp4 of the Dpp class has a unique expression pattern in the developing testis and epididymis. Bmp4 heterozygous males on a largely C57BL/6 background show compromised fertility due to degeneration of germ cells, reduced sperm counts, and decreased sperm motility. More interestingly, some of these males show extensive degeneration of the epididymal epithelium in the corpus region, rather than in the caput and cauda regions as for Bmp7 and Bmp8 mutants. Thus, these genetic data reveal a region-specific requirement of different classes of BMPs for epididymal epithelium to survive and have significant implications on male reproductive health and perhaps birth control.

The bone morphogenetic protein system in mammalian reproduction

Using molecular, cellular, and genetic approaches, recent studies examining the role of the bone morphogenetic protein (BMP) family of growth factors in the reproductive system have led to significant breakthroughs in our understanding of mammalian reproduction and fertility. Gene expression studies have revealed that key components of the BMP system (ligands, receptors, signaling molecules, and binding proteins) exhibit coordinated spatial and temporal expression patterns in fundamental cell types throughout the reproductive system. Availability of recombinant BMPs has enabled functional studies that have demonstrated important biological activities of BMPs in controlling cellular proliferation, differentiation, and apoptosis in reproductive tissues. The physiological importance of the BMP system for mammalian reproduction has been further highlighted by the elucidation of the aberrant reproductive phenotypes of animals with naturally occurring mutations or targeted deletions of certain BMP family genes. Collectively, these studies have established the concept that the BMP system plays a crucial role in fertility in female and male mammals. The purpose of this article is to review the evidence underpinning the importance of the BMP system in mammalian reproduction.

Developmental expression of BMP4/ALK3/SMAD5 signaling pathway in the mouse testis: a potential role of BMP4 in spermatogonia differentiation

It is well established that the c-kit gene plays an essential role in the proliferation of differentiating spermatogonia in prepuberal mice. However, the mechanisms that regulate the onset of spermatogenesis, i.e. differentiation of spermatogonial stem cells and c-kit expression, are poorly understood. Here we identify a novel signal transduction system in mouse prepuberal testis regulating this developmental event, involving bone morphogenetic protein 4 (BMP4) and its transduction machinery. BMP4 is produced by Sertoli cells very early in the postnatal life and is successively down regulated in peri-puberal Sertoli cells. Its receptor Alk3 and the R-Smad Smad5 are specifically expressed both in proliferating primordial germ cells and in postnatal spermatogonia. BMP4 stimulation of cultured spermatogonia induces Smad4/5 nuclear translocation and the formation of a DNA-binding complex with the transcriptional coactivator p300/CBP. In vitro exposure of undifferentiated spermatogonia to BMP4 exerts both mitogenic and differentiative effects, inducing [3H]thymidine incorporation and Kit expression. As a result of the latter event, Kit-negative spermatogonia acquire sensitivity to Stem Cell Factor.

Developmental and stage-specific expression of Smad2 and Smad3 in rat testis

Members of the transforming growth factor beta type (TGFbeta) superfamily and their receptors are expressed in the testis, and are believed to play important paracrine and autocrine roles during testicular development and spermatogenesis. The Smad proteins are downstream mediators for the family of TGFbeta growth factors. Smad2 and Smad3 are associated with both TGFbeta and activin signaling. However, very little is known about the expression and regulation of the Smad signaling proteins in the testis. In the present study, we have determined that Smad2 and Smad3 proteins are expressed in the postnatal testes of rats from 5 days to 60 days of age. Expression levels for both proteins are higher in young rats than in sexually mature rats. Smad2 and Smad3 messenger RNA levels parallel protein expression. Smad2 and Smad3 proteins are mainly localized in the cytoplasm of meiotic germ cells, Sertoli cells, and Leydig cells. Smad3 protein is localized to the nucleus of preleptotene to zygotene primary spermatocytes in young rats. Both proteins are expressed throughout all stages of the cycle of seminiferous tubules but are expressed at their lowest levels at stages VII-VIII in the seminiferous epithelium of adult rats. The presence of these downstream mediators in these cell types supports a role for TGFbeta and activin during spermatogenesis. The difference between the expression of Smad2 and Smad3 suggests that they may have different functions within the testis.

Consequences of knocking out BMP signaling in the mouse

During the past two decades, a significant amount of data has been accumulated revealing the intriguing functions of bone morphogenetic proteins (BMPs) in all aspects of embryonic development and organogenesis. Numerous genes encoding BMPs, BMP receptors, and their downstream signal transducers have been mutated in the mouse through targeted mutagenesis. This review focuses on what is known about the role of BMP signaling in gastrulation, mesoderm formation, left-right asymmetry, neural patterning, skeletal and limb development, organogenesis, and gametogenesis as revealed by BMP-signaling mutants.

Bone morphogenetic protein signaling in prostate cancer cell lines

Prostate cancer is the most commonly diagnosed malignancy in men and is often associated with bone metastases. Prostate cancer bone lesions can be lytic or schlerotic, with the latter predominating. Bone morphogenetic proteins (BMPs) are a family of growth factors, which may play a role in the formation of prostate cancer osteoblastic bone metastases. This study evaluated the effects of BMPs on prostate cancer cell lines. We observed growth inhibitory effects of BMP-2 and -4 on LNCaP, while PC-3 was unaffected. Flow cytometric analysis determined that LNCaP cell growth was arrested in G(1) after bone morphogenetic protein-2 treatment. Treatment of LNCaP and PC-3 with BMP-2 and -4 activated downstream signaling pathways involving SMAD-1, up-regulation of p21(CIP1/WAF1) and changes in retinoblastoma (Rb) phosphorylation. Interestingly, bone morphogenetic protein-2 treatment stimulated a 2.7-fold increase in osteoprotegerin (OPG), a molecule, which inhibits osteoclastogenesis, production in PC-3.

Identification of mZnf8, a mouse Krüppel-like transcriptional repressor, as a novel nuclear interaction partner of Smad1

To identify novel genes that play critical roles in mediating bone morphogenetic protein (BMP) signal pathways, we performed a yeast two-hybrid screen using Smad1 as bait. A novel mouse Krüppel-type zinc finger protein, mZnf8, was isolated. Interactions between mZnf8 and Smad proteins were further analyzed with various in vitro and in vivo approaches, including mammalian two-hybrid, in vitro glutathione S-transferase pulldown, and copurification assays. Results from functional analysis indicate that mZnf8 is a nuclear transcriptional repressor. Overexpression of mZnf8 represses activity of BMP and transforming growth factor beta (TGF-beta) reporters. Silencing the expression of endogenous mZnf8 with an RNA interference approach caused a significant increase in the expression of one BMP reporter. These results suggest that mZnf8 negatively regulates the TGF-beta/BMP signaling pathway in vivo. Transcription of mZnf8 is ubiquitous in mouse embryos, but high levels are specifically observed in adult mouse testes, with the same cell- and stage-specific transcription pattern as Smad1. Our data support the hypothesis that mZnf8 plays critical roles in mediating BMP signaling during spermatogenesis.

Smad4 overexpression causes germ cell ablation and leydig cell hyperplasia in transgenic mice

Members of the transforming growth factor-beta (TGF-beta) superfamily play a variety of important roles in testicular development and function. The tumor suppressor gene, Smad4, is a common mediator of TGF-beta, activin, and bone morphogenetic protein-mediated signaling pathways. To investigate the role of the Smad4 gene during testicular development and function, transgenic mice were generated using a Flag-tagged Smad4 gene driven by 180-bp fragment of the Mullerian inhibiting substance upstream promoter sequence. Three Smad4 transgenic founders (A, B, and G) were detected by Southern blot analysis; line B showed the highest expression of the Smad4 transgene and was further studied. The fertility in F1 generation (B) and F2 generation (BB) of the Smad4 transgenic mice was not impaired. However, in the F3 generation (B2x) all animals were impacted by the overexpression of the Smad4 transgene and two kinds of phenotypes were observed. In one group animals were completely infertile, while in the other group animals were fertile and sired the normal number of pups/litter. These groups are designated as infertile and fertile in the text. Histological evaluation of the testes from the infertile group showed variable degrees of Leydig cell hyperplasia, apoptosis of germ cells, spermatogenic arrest, seminiferous tubule degeneration, and infertility. In the fertile group, there was no apparent change in the histology of the testis except for a slight increase in the number of Leydig cells. Serum follicle-stimulating hormone levels in the adult animals of both groups of Smad4 transgenic male mice were not significantly different from normal littermates; however, testosterone levels in both groups were significantly (P < 0.05) increased. These results suggest that overexpression of Smad4 leads to testicular abnormalities and infertility supporting the hypothesis that the TGF-beta signaling pathways are carefully orchestrated during testicular development. In the absence of normal levels of Smad4 testicular function is compromised.

Mutation in Bmp7 exacerbates the phenotype of Bmp8a mutants in spermatogenesis and epididymis

The specificity of bone morphogenetic proteins (BMPs) to their putative heteromeric receptor complexes in vivo is largely unclear. Closely related BMPs may use the same or different receptor complexes for signaling in a time- and space-dependent manner during development and differentiation. We have shown that Bmp7 expression in epididymal epithelium is developmentally regulated. Here, we further show that Bmp7 expression is also developmentally regulated in male germ cells. Bmp7 transcripts are detected in spermatogonia and early primary spermatocytes during early puberty and in stage-7 to -15 spermatids of the adult mice. Since Bmp7 homozygous mutants die perinatally and heterozygotes do not show obvious defects in the testis and the epididymis, the role of Bmp7 in spermatogenesis and epididymal function cannot be revealed by simply examining these mutants. Therefore, we have used a genetic approach by creating Bmp7/Bmp8a double mutants to investigate the role of Bmp7 in spermatogenesis and epididymal function. Here, we report that removal of one allele of Bmp7 exacerbates the phenotype of Bmp8a null mutants in spermatogenesis and epididymis of the adult. These indicate that, similar to Bmp8a, Bmp7 plays a role in both the maintenance of spermatogenesis and epididymal function and it further suggests that BMP8 and BMP7 signal through the same or similar receptors in these two systems.

Detection of multiple bone morphogenetic protein messenger ribonucleic acids and their signal transducer, Smad1, during mouse decidualization

Decidualization is a process characterized by morphological and functional changes in the uterine stromal cells. In addition to steroid hormones, growth factors are implicated in this process. Using in situ hybridization, we found that mRNAs for several bone morphogenetic proteins (BMPs) were detected in the decidual and vascular endothelial cells. The Bmp7 mRNA was detected in the decidualizing stromal cells surrounding the blastocyst and distributed in a gradient, with the highest levels occurring near the uterine epithelium at 4.5 days post-coitus (dpc). With the progression of decidualization, Bmp7 signals in the deciduum at the antimesometrial side decreased, but strong signals were retained in the decidual area at the mesometrial side at 7.0 dpc. In contrast, Bmp8a transcripts increased from 5.5 to 7.0 dpc in the decidual tissue, with the highest levels occurring in the secondary decidual zone at the antimesometrial side. The Bmp2, Bmp4, and Smad1 transcripts were found in the secondary decidual zone, especially at the mesometrial side. The Bmp2 signals were primarily detected in decidual cells, whereas Bmp4 and Smad1 transcripts were mainly detected in vascular endothelial cells, suggesting that they may be involved in decidual angiogenesis.

Characterization of the mouse Smad1 gene and its expression pattern in adult mouse tissues

Smad1 belongs to a family of receptor-activated proteins which mediate signals from TGF-beta superfamily ligands, including TGF-beta and BMPs. Although much is known about the biochemistry of Smad1 signal transduction, the role of Smad1 in vivo is still unclear. Here we present the first description of the genomic structure of the mouse Smad1 gene and the characterization of its expression pattern in adult mouse tissues by immunohistochemistry. The Smad1 gene contains 7 exons and spans >42 kb of genomic DNA. Its coding region is contained within 6 exons and all introns, except intron 1, follow the GT/AG rule. Immunohistochemical analysis shows that Smad1 is widely expressed in adult mouse tissues, with a varying degree of nuclear localization in different cell types, suggesting a regulated function for this protein. This study assigns all of the exon-intron boundaries of the mouse Smad1 gene and provides the basis for assessing the functional significance of this gene using targeted gene manipulation in the mouse.

Detection of PACH1, a nuclear factor implicated in the transcriptional regulation of meiotic and early haploid stages of spermatogenesis

Spermatogenesis occurs in a series of well-defined stages and serves as an excellent model for lineage-specific cell development. Yet, little is known regarding the transcriptional mechanisms responsible for cell- and stage-dependent gene regulation in the male germ line. The rat and mouse proenkephalin genes are expressed from an alternative, spermatogenic cell-specific promoter specifically in meiotically-active pachytene spermatocytes and early post-meiotic spermatids. This promoter thus serves as an excellent model for defining transcriptional regulators involved in germ line-specific gene expression in meiotic cells. Previous transgenic studies identified a proximal, 51 bp 5'-flanking sequence containing two direct repeat elements that are absolutely required for in vivo proenkephalin promoter activity in spermatocytes and spermatids. Here, footprinting analyses were used to further delineate the specific interactions of a spermatogenic cell nuclear factor with the repeat elements within the proximal promoter region. This repeat-binding factor was also shown to be developmentally upregulated specifically in pachytene spermatocytes. Using Southwestern analysis, we have identified a unique nuclear protein enriched in pachytene spermatocytes that specifically recognizes the repeat elements within the proximal 5'-flanking sequence. We propose that this DNA binding factor, termed PACH1, is a key transcriptional regulator of the proenkephalin and potentially other gene promoters, uniquely expressed during meiosis in the male germ line.

Transforming growth factor beta signal transducer Smad2 is expressed in mouse meiotic germ cells, Sertoli cells, and Leydig cells during spermatogenesis

Although previous studies have shown that members of the transforming growth factor beta (TGFbeta) family are expressed in the seminiferous tubules, the functions of these growth factors in spermatogenesis remain elusive. In order to shed light on the mechanisms of TGFbeta action in spermatogenesis, it is crucial to determine whether and where their downstream signaling molecules are expressed in the testis. We examined the expression of Smad2, an intracellular signal transducer of the TGFbetas, in mouse testes by in situ hybridization and immunohistochemistry. Both Smad2 mRNA and protein were detected in meiotic germ cells, from preleptotene to pachytene spermatocytes, but not in postmeiotic germ cells. Smad2 expression was also observed in interstitial cells and Sertoli cells. Therefore, our data provide molecular evidence for TGFbeta signal transduction during spermatogenesis.

Germline cysts: a conserved phase of germ cell development?

Germ cells in many vertebrate and invertebrate species initiate gametogenesis by forming groups of interconnected cells known as germline cysts. Recent studies using Xenopus, mouse and Drosophila are beginning to uncover the cellular and molecular mechanisms that control germline cyst formation and, in conjunction with morphological evidence, suggest that the process is highly conserved during evolution. This article discusses these recent findings and argues that cysts play an important and general role in germ line development.

Expression of bone morphogenetic protein 7 in murine epididymis is developmentally regulated

Bone morphogenetic proteins (BMPs) have been shown to play a role in the functional maintenance of the adult epididymis. To begin to investigate the role of BMP signal transduction during postnatal epididymal development, we examined the expression profile of Bmp7 in murine epididymis by in situ hybridization. Our data show that during early postnatal development (younger than 3 wk of age), Bmp7 transcripts are detected uniformly in epithelial cells throughout the epididymis. As the mice aged (from 3 to 4 wk), Bmp7 expression was gradually restricted to the initial segment, with increased levels. Bmp7 expression in the rest of the caput and corpus regions became undetectable after 4 wk of age. However, after 4 wk of age, an ascending gradient of Bmp7 expression was observed in the epididymal epithelial cells in the transition from the cauda epididymal tubule to the vas deferens. Such a unique expression profile of Bmp7 strongly suggests that epididymis-produced BMP7 may play a role in the development and functional maintenance of the epididymis, and that Bmp7 expression in the epididymis is developmentally regulated.

The self-renewing mechanism of stem cells in the germline

Germline stem cells (GSCs) are a self-renewing population of germ cells that serve as the source of gametes in diverse organisms. Current research suggests that the self-renewing division of GSCs is controlled both by somatic signaling and by intracellular mechanisms such as differential gene expression, asymmetric cytoskeletal organization, and the cell cycle machinery. These findings provide a framework for the further study of GSCs and stem cell renewal in general.

Bone morphogenetic protein 8A plays a role in the maintenance of spermatogenesis and the integrity of the epididymis

The murine Bmp8a and Bmp8b genes are tightly linked on mouse chromosome 4 and have similar expression during reproduction. Previous studies have shown that targeted mutagenesis of Bmp8b causes male infertility due to germ cell degeneration. To investigate the function of Bmp8a, we have inactivated the gene by homologous recombination. Heterozygous and homozygous Bmp8a mutants reveal normal embryonic and postnatal development. Despite high levels of Bmp8a expression in the deciduum, homozygous mutant females have normal fertility, suggesting that the gene is not essential for female reproduction. Bmp8a and Bmp8b are expressed in similar patterns in male germ cells. Unlike homozygous Bmp8btm1 mutants, homozygous Bmp8atm1 males do not show obvious germ cell defects during the initiation of spermatogenesis. However, germ cell degeneration is observed in 47% of adult homozygous Bmp8atm1 males, establishing a role of Bmp8a in the maintenance of spermatogenesis. A small proportion of the mating homozygous Bmp8atm1 males also show degeneration of the epididymal epithelium, indicating a novel role for BMPs in the control of epididymal function.

The tao of stem cells in the germline

Germline stem cells (GSCs) are the self-renewing population of germ cells that serve as the source for gametogenesis. GSCs exist in diverse forms, from those that undergo strict self-renewing asymmetric divisions in Drosophila to those that maintain their population by balancing between mitosis and differentiation in Caenorhabditis elegans. Most vertebrate spermatogonial GSCs appear to adopt an intermediate strategy. In most animals, GSCs are established during preadult gonadogenesis following the proliferation and migration of embryonic primordial germ cells. GSCs produce numerous gametes throughout the sexually active period of adult life. The establishment and self-renewing division of GSCs are controlled by extracellular signals such as hormones from the hypothalamic-pituitary axis and local interactions between GSCs and their neighboring cells. These extracellular signals may then influence differential gene expression, cell cycle machinery, and cytoskeletal organization of GSCs for their formation and/or divisional asymmetry. In addition, the GSC mechanism is related to that for germline and sex determination. Current knowledge has provided a solid framework for further study of GSCs and stem cells in general.

Regulation of germ cell death in mammalian gonads

A large number of primordial germ cells (PGCs), as well as spermatogonia, undergo programmed cell death or apoptosis in the physiological context. In this process, environmental, cytoplasmic and nuclear factors are involved. Bcl-2 and its related molecules are known as general regulators of cell death, and some are important for survival of PGCs and spermatogonia. Steel factor, a ligand for c-Kit, also supports growth and survival of these cells. In addition, bone morphogenetic protein (BMP)8B and Desert Hedgehog (Dhh), which are secreted proteins, and a nuclear factor, c-Myc, play a role in spermatocyte survival. This suggests that germ cell survival or death at each stage of differentiation is precisely controlled by specific signalling pathways which consist of a number of molecules.