Testis developmental phenotypes in neurotropin receptor trkA and trkC null mutations: role in formation of seminiferous cords and germ cell survival

From Gene to Protein: Unraveling the Reproductive Blueprint of Male Grey Squirrels via Nerve Growth Factor (NGF) and Cognate Receptors

The grey squirrel, an invasive species, threatens the Eurasian red squirrel's conservation, particularly in Umbria, Italy. Understanding its reproductive biology is essential to limiting its reproductive success. This study investigates the NGF system and its receptors (NTRK1 and p75NTR) in the testes of male grey squirrels, following prior research on female reproductive biology. NGF plays a role in testicular morphogenesis and spermiogenesis in animals and humans. As part of the LIFE Project U-SAVEREDS, eighteen squirrels were captured and classified into three morphotypes (immature, pubertal, and active spermatogenesis). NGF and its receptors were analyzed using real-time PCR, western blotting, immunohistochemistry, and plasma levels measured via ELISA. NGF qPCR expression levels were significantly higher during puberty compared to the immature and spermatogenesis stages (p < 0.01). Immunohistochemistry revealed NGF in Leydig cells, with stronger staining in pubertal and mature squirrels, while NTRK1 was found in Leydig cells in immature squirrels and germ cells in pubertal and mature ones. NGF receptors were observed in Sertoli cells in pubertal and mature squirrels. Plasma NGF levels showed a significant upregulation in pubertal squirrels (135.80 ± 12 pg/mL) compared to those in the immature (25.60 ± 9.32 pg/mL) and spermatogenesis stages (34.20 ± 6.06 pg/mL), with a p value < 0.01. The co-localization of NGF and its receptors suggests that NGF, produced by Leydig cells, regulates testis development and reproductive success through autocrine or paracrine mechanisms, potentially involving an unidentified pathway.

Seasonal variation of NGF in seminal plasma and expression of NGF and its cognate receptors NTRK1 and p75NTR in the sex organs of rams

Nerve growth factor (NGF) has long been known as the main ovulation-inducing factor in induced ovulation species, however, recent studies suggested the NGF role also in those with spontaneous ovulation. The first aim of this study was to evaluate the presence and gene expression of NGF and its cognate receptors, high-affinity neurotrophic tyrosine kinase 1 receptor (NTRK1) and low-affinity p75 nerve growth factor receptor (p75NTR), in the ram genital tract. Moreover, the annual trend of NGF seminal plasma values was investigated to evaluate the possible relationship between the NGF production variations and the ram reproductive seasonality. The presence and expression of the NGF/receptors system was evaluated in the testis, epididymis, vas deferens ampullae, seminal vesicles, prostate, and bulbourethral glands through immunohistochemistry and real-time PCR (qPCR), respectively. Genital tract samples were collected from 5 adult rams, regularly slaughtered at a local abattoir. Semen was collected during the whole year weekly, from 5 different adult rams, reared in a breeding facility, with an artificial vagina. NGF seminal plasma values were assessed through the ELISA method. NGF, NTRK1 and p75NTR immunoreactivity was detected in all male organs examined. NGF-positive immunostaining was observed in the spermatozoa of the germinal epithelium, in the epididymis and the cells of the secretory epithelium of annexed glands, NTRK1 receptor showed a localization pattern like that of NGF, whereas p75NTR immunopositivity was localized in the nerve fibers and ganglia. NGF gene transcript was highest (p < 0.01) in the seminal vesicles and lowest (p < 0.01) in the testis than in the other tissues. NTRK1 gene transcript was highest (p < 0.01) in the seminal vesicles and lowest (p < 0.05) in all the other tissues examined. Gene expression of p75NTR was highest (p < 0.01) in the seminal vesicles and lowest (p < 0.01) in the testis and bulbourethral glands. NGF seminal plasma concentration was greater from January to May (p < 0.01) than in the other months. This study highlighted that the NGF system was expressed in the tissues of all the different genital tracts examined, confirming the role of NGF in ram reproduction. Sheep are short-day breeders, with an anestrus that corresponds to the highest seminal plasma NGF levels, thus suggesting the intriguing idea that this factor could participate in an inhibitory mechanism of male reproductive activity, activated during the female anestrus.

A Possible Role for Nerve Growth Factor and Its Receptors in Human Sperm Pathology

Nerve growth factor (NGF) signalling affects spermatogenesis and mature sperm traits. In this paper, we aimed to evaluate the distribution and the role of NGF and its receptors (p75NTR and TrKA) on the reproductive apparatus (testis and epididymis) and sperm of fertile men (F) and men with different pathologies, namely varicocele (V) and urogenital infections (UGIs). We collected semen samples from 21 individuals (31-40 years old) subdivided as follows: V (n = 7), UGIs (n = 7), and F (n = 7). We submitted the semen samples to bacteriological analysis, leucocyte identification, and analysis of sperm parameters (concentration, motility, morphology, and viability). We determined the seminal plasma levels of NGF, interleukin 1β (IL-1β), and F2-isoprostanes (F2-IsoPs), and the gene and protein expression of NGF receptors on sperm. We also used immunofluorescence to examine NGF receptors on ejaculated sperm, testis, and epididymis. As expected, fertile men showed better sperm parameters as well as lower levels of NGF, F2-IsoPs, and IL-1β compared with men with infertility. Notably, in normal sperm, p75NTR and TrKA were localised throughout the entire tail. TrKA was also found in the post-acrosomal sheath. This localisation appeared different in patients with infertility: in particular, there was a strong p75NTR signal in the midpiece and the cytoplasmic residue or coiled tails of altered ejaculated sperm. In line with these findings, NGF receptors were intensely expressed in the epididymis and interstitial tissue of the testis. These data suggest the distinctive involvement of NGF and its receptors in the physiology of sperm from fertile men and men with infertility, indicating a possible role for new targeted treatment strategies.

Epigenetic transgenerational inheritance, gametogenesis and germline development†

One of the most important developing cell types in any biological system is the gamete (sperm and egg). The transmission of phenotypes and optimally adapted physiology to subsequent generations is in large part controlled by gametogenesis. In contrast to genetics, the environment actively regulates epigenetics to impact the physiology and phenotype of cellular and biological systems. The integration of epigenetics and genetics is critical for all developmental biology systems at the cellular and organism level. The current review is focused on the role of epigenetics during gametogenesis for both the spermatogenesis system in the male and oogenesis system in the female. The developmental stages from the initial primordial germ cell through gametogenesis to the mature sperm and egg are presented. How environmental factors can influence the epigenetics of gametogenesis to impact the epigenetic transgenerational inheritance of phenotypic and physiological change in subsequent generations is reviewed.

Molecular Characterization of XX Maleness

Androgens and anti-Müllerian hormone (AMH), secreted by the foetal testis, are responsible for the development of male reproductive organs and the regression of female anlagen. Virilization of the reproductive tract in association with the absence of Müllerian derivatives in the XX foetus implies the existence of testicular tissue, which can occur in the presence or absence of SRY. Recent advancement in the knowledge of the opposing gene cascades driving to the differentiation of the gonadal ridge into testes or ovaries during early foetal development has provided insight into the molecular explanation of XX maleness.

The transcriptional regulator CBX2 and ovarian function: A whole genome and whole transcriptome approach

The chromobox homolog 2 (CBX2) was found to be important for human testis development, but its role in the human ovary remains elusive. We conducted a genome-wide analysis based on DNA adenine methyltransferase identification (DamID) and RNA sequencing strategies to investigate CBX2 in the human granulosa cells. Functional analysis revealed that CBX2 was upstream of genes contributing to ovarian function like folliculogenesis and steroidogenesis (i.e. ESR1, NRG1, AKR1C1, PTGER2, BMP15, BMP2, FSHR and NTRK1/2). We identified CBX2 regulated genes associated with polycystic ovary syndrome (PCOS) such as TGFβ, MAP3K15 and DKK1, as well as genes implicated in premature ovarian failure (POF) (i.e. POF1B, BMP15 and HOXA13) and the pituitary deficiency (i.e. LHX4 and KISS1). Our study provided an excellent opportunity to identify genes surrounding CBX2 in the ovary and might contribute to the understanding of ovarian physiopathology causing infertility in women.

NGF promotes mitochondrial function by activating PGC-1α in TM4 Sertoli cells

Nerve growth factor (NGF), which is required for the survival and differentiation of the nervous system, has been proved to play important roles in male reproductive physiology. Several studies have focused on the roles of NGF in the testes. However, no study has reported on the mechanism of paracrine and autocrine actions of NGF in Sertoli cells. This study showed that NGF stimulated mitochondrial activity and biogenesis in TM4 Sertoli cells. Moreover, our results demonstrated that peroxisome proliferator-activated receptor-gamma coactivator-1α is a possible downstream key target of the NGF signalling pathway. In a 3-nitropropionic acid cell model, NGF treatment attenuated mitochondrial activity defect and depolarisation. This NGF-triggered signalling may help in discovering new therapeutic targets for certain male infertility disorders.

Mesonephric Cell Migration into the Gonads and Vascularization Are Processes Crucial for Testis Development

Testis morphogenesis requires the integration and reorganization of multiple cell types from several sources, one of the more notable being the mesonephric-derived cell population. One of the earliest sex-specific morphogenetic events in the gonad is a wave of endothelial cell migration from the mesonephros that is crucial for (1) partitioning the gonad into domains for testis cords, (2) providing the vasculature of the testis, and (3) signaling to cells both within the gonad and beyond it to coordinately regulate testis development. In addition to endothelial cell migration, there is evidence that precursors of peritubular myoid cells migrate from the mesonephros, an event which is also important for testis cord architecture. Investigation of the mesonephric cell migration event has utilized histology, lineage tracing with mouse genetic markers, and many studies of the signaling molecules/pathways involved. Some of the more well-studied signaling molecules involved include vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and neurotrophins. In this chapter, the morphogenetic events, relevant signaling pathways, mechanisms underlying the migration, and the role of the migratory cells within the testis will be discussed. Overall, the migration of mesonephric cells into the early testis is indispensable for its development and future functionality.

Vascular endothelial growth factor A: just one of multiple mechanisms for sex-specific vascular development within the testis?

Testis development from an indifferent gonad is a critical step in embryogenesis. A hallmark of testis differentiation is sex-specific vascularization that occurs as endothelial cells migrate from the adjacent mesonephros into the testis to surround Sertoli-germ cell aggregates and induce seminiferous cord formation. Many in vitro experiments have demonstrated that vascular endothelial growth factor A (VEGFA) is a critical regulator of this process. Both inhibitors to VEGFA signal transduction and excess VEGFA isoforms in testis organ cultures impaired vascular development and seminiferous cord formation. However, in vivo models using mice which selectively eliminated all VEGFA isoforms: in Sertoli and germ cells (pDmrt1-Cre;Vegfa(-/-)); Sertoli and Leydig cells (Amhr2-Cre;Vegfa(-/-)) or Sertoli cells (Amh-Cre;Vegfa(-/-) and Sry-Cre;Vegfa(-/-)) displayed testes with observably normal cords and vasculature at postnatal day 0 and onwards. Embryonic testis development may be delayed in these mice; however, the postnatal data indicate that VEGFA isoforms secreted from Sertoli, Leydig or germ cells are not required for testis morphogenesis within the mouse. A Vegfa signal transduction array was employed on postnatal testes from Sry-Cre;Vegfa(-/-) versus controls. Ptgs1 (Cox1) was the only upregulated gene (fivefold). COX1 stimulates angiogenesis and upregulates, VEGFA, Prostaglandin E2 (PGE2) and PGD2. Thus, other gene pathways may compensate for VEGFA loss, similar to multiple independent mechanisms to maintain SOX9 expression. Multiple independent mechanism that induce vascular development in the testis may contribute to and safeguard the sex-specific vasculature development responsible for inducing seminiferous cord formation, thus ensuring appropriate testis morphogenesis in the male.

Building the mammalian testis: origins, differentiation, and assembly of the component cell populations

Development of testes in the mammalian embryo requires the formation and assembly of several cell types that allow these organs to achieve their roles in male reproduction and endocrine regulation. Testis development is unusual in that several cell types such as Sertoli, Leydig, and spermatogonial cells arise from bipotential precursors present in the precursor tissue, the genital ridge. These cell types do not differentiate independently but depend on signals from Sertoli cells that differentiate under the influence of transcription factors SRY and SOX9. While these steps are becoming better understood, the origins and roles of many testicular cell types and structures-including peritubular myoid cells, the tunica albuginea, the arterial and venous blood vasculature, lymphatic vessels, macrophages, and nerve cells-have remained unclear. This review synthesizes current knowledge of how the architecture of the testis unfolds and highlights the questions that remain to be explored, thus providing a roadmap for future studies that may help illuminate the causes of XY disorders of sex development, infertility, and testicular cancers.

Vascular contributions to early ovarian development: potential roles of VEGFA isoforms

Vascularisation is an essential component of ovarian morphogenesis; however, little is known regarding factors regulating the establishment of vasculature in the ovary. Angiogenesis involving extensive endothelial cell migration is a critical component of vessel formation in the embryonic testis but vasculogenic mechanisms appear to play a prominent role in ovarian vascularisation. Vasculature has a strong influence on the formation of ovarian structures, and the early developmental processes of ovigerous cord formation, primordial follicle assembly and follicle activation are all initiated in regions of the ovary that are in close association with the highly vascular medulla. The principal angiogenic factor, vascular endothelial growth factor A (VEGFA), has an important role in both endothelial cell differentiation and vascular pattern development. Expression of VEGFA has been localised to ovigerous cords and follicles in developing ovaries and an increased expression of pro-angiogenic Vegfa isoform mRNA in relation to anti-angiogenic isoform mRNA occurs at the same time-point as the peak of primordial follicle assembly in perinatal rats. Elucidation of specific genes that affect vascular development within the ovary may be critical for determining not only the normal mechanisms of ovarian morphogenesis, but also for understanding certain ovarian reproductive disorders.

NGF induces adult stem Leydig cells to proliferate and differentiate during Leydig cell regeneration

Nerve growth factor (NGF) has been reported to be involved in male reproductive physiology. However, few reports have described the activity of NGF during Leydig cell development. The objective of the present study was to examine the role of NGF during stem-Leydig-cell (SLC) regeneration. We investigated the effects of NGF on Leydig-cell (LC) regeneration by measuring mRNA levels in the adult rat testis after ethane dimethanesulfonate (EDS) treatment. Furthermore, we used the established organ culture model of rat seminiferous tubules to examine the regulation of NGF during SLC proliferation and differentiation using EdU staining, real-time PCR and western blotting. Progenitor Leydig cells (PLCs) and immature Leydig cells (ILCs) were also used to investigate the effects of NGF on LCs at different developmental stages. NGF mRNA levels changed significantly during Leydig-cell regeneration in vivo. In vitro, NGF significantly promoted the proliferation of stem Leydig cells and also induced steroidogenic enzyme gene expression and 3β-HSD protein expression. The data from PLCs and ILCs showed that NGF could increase Cyclin D1 and Hsd 17b3 mRNA levels in PLCs and Cyclin D1 mRNA levels in ILCs. These results indicate that NGF may play an important role during LC regeneration by regulating the proliferation and differentiation of LCs at different developmental stages, from SLCs to PLCs and from PLCs to ILCs. The discovery of this effect of NGF on Leydig cells will provide useful information for developing new potential therapies for PADAM (Partial Androgen Deficiency in the Aging Male).

The potential roles of neurotrophins in male reproduction

Neurotrophins are a family of polypeptide growth factors that are required for the proliferation, differentiation, survival, and death of neuronal cells. A growing body of evidence suggests that they may have broader physiological roles in various non-neuronal tissues. The testicles are complex non-neuronal organs in which diverse cell types interact to achieve correct spermatogenesis. Both neurotrophins and their receptors have been detected in various cell types from mammalian testes, suggesting that neurotrophins may regulate or mediate intercellular communication within this organ. This review summarizes the existing data on the cellular distribution and possible biological roles of neurotrophins in the testes. The data reported in the literature indicate that neurotrophins affect somatic cell growth and spermatogenesis and imply that they play a role in regulating testicular development and male reproduction.

The proto-oncogene Ret is required for male foetal germ cell survival

The spermatogenic and oogenic lineages originate from bipotential primordial germ cells in response to signalling in the foetal testis or ovary, respectively. The signals required for male germ cell commitment and their entry into mitotic arrest remain largely unknown. Recent data show that the ligand GDNF is up regulated in the foetal testis indicating that it may be involved in male germ cell development. In this study genetic analysis of GDNF-RET signalling shows that RET is required for germ cell survival. Affected germ cells in Ret-/- mice lose expression of key germ cell markers, abnormally express cell cycle markers and undergo apoptosis. Surprisingly, a similar phenotype was not detected in Gdnf-/- mice indicating that either redundancy with a Gdnf related gene might compensate for its loss, or that RET operates in a GDNF independent manner in mouse foetal germ cells. Either way, this study identifies the proto-oncogene RET as a novel component of the foetal male germ cell development pathway.

A proposed role for VEGF isoforms in sex-specific vasculature development in the gonad

Many scientists have expended efforts to determine what regulates development of an indifferent gonad into either a testis or ovary. Expression of Sry and upregulation of Sox9 are factors that initiate formation of the testis-specific pathway to allow for both sex-specific vasculature and seminiferous cord formation. Migration of mesonephric precursors of peritubular myoid cells and endothelial cells into the differentiating testis is a critical step in formation of both of these structures. Furthermore, these events appear to be initiated downstream from Sry expression. Sertoli cell secretion of growth factors acts to attract these mesonephric cells. One hypothesis is that a growth factor specific for these cell linages act in concert to coordinate migration of both peritubular and endothelial cells. A second hypothesis is that several growth factors stimulate migration and differentiation of mesonephric 'stem-like' cells to result in migration and differentiation into several different cell lineages. While the specific mechanism is unclear, several growth factors have been implicated in the initiation of mesonephric cell migration. This review will focus on the proposed mechanisms of a growth factor, Vascular Endothelial Growth Factor, and how different angiogenic and inhibitory isoforms from this single gene may aid in development of testis-specific vascular development.

Transforming growth factor beta (TGFbeta1, TGFbeta2 and TGFbeta3) null-mutant phenotypes in embryonic gonadal development

The role transforming growth factor beta (TGFb) isoforms TGFb1, TGFb2 and TGFb3 have in the regulation of embryonic gonadal development was investigated with the use of null-mutant (i.e. knockout) mice for each of the TGFb isoforms. Late embryonic gonadal development was investigated because homozygote TGFb null-mutant mice generally die around birth, with some embryonic loss as well. In the testis, the TGFb1 null-mutant mice had a decrease in the number of germ cells at birth, postnatal day 0 (P0). In the testis, the TGFb2 null-mutant mice had a decrease in the number of seminiferous cords at embryonic day 15 (E15). In the ovary, the TGFb2 null-mutant mice had an increase in the number of germ cells at P0. TGFb isoforms appear to have a role in gonadal development, but interactions between the isoforms is speculated to compensate in the different TGFb isoform null-mutant mice.

Integration of CREB and bHLH transcriptional signaling pathways through direct heterodimerization of the proteins: role in muscle and testis development

The cAMP response element binding protein/activating transcription factor (CREB/ATF) family of transcription factors is hormone responsive and critical for nearly all mammalian cell types. The basic helix-loop-helix (bHLH) family of transcription factors is important during the development and differentiation of a wide variety of cell types. Independent studies of the role of the bHLH protein scleraxis in testicular Sertoli cells and paraxis in muscle development using yeast-2-hybrid screens provided the novel observation that bHLH proteins can directly interact with ATF/CREB family members. Analysis of the interactions demonstrated the helix-loop-helix domain of bHLH proteins directly interacts with the leucine zipper (ZIP) region of CREB2/ATF4 to form heterodimers. The direct bHLH-CREB2 binding interactions were supported using co-immunoprecipitation of recombinant proteins. Structural analysis of bHLH and ATF4 heterodimer using previous crystal structures demonstrated the heterodimer likely involves the HLH and Zip domains and has the potential capacity to bind DNA. Transfection assays demonstrated CREB2/ATF4 over-expression blocked stimulatory actions of scleraxis or paraxis. CREB1 inhibited MyoD induced myogenic conversion of C3H10T1/2 cells. CREB2/ATF4 and scleraxis are expressed throughout embryonic and postnatal testis development, with scleraxis specifically expressed in Sertoli cells. ATF4 and scleraxis null mutant mice both had similar adult testis phenotypes of reduced spermatogenic capacity. In summary, bHLH and CREB family members were found to directly heterodimerize and inhibit the actions of bHLH dimers on Sertoli cells and myogenic precursor cells. The observations suggest a mechanism for direct cross-talk between cAMP induced and bHLH controlled cellular differentiation.

Regulation of the gonadal transcriptome during sex determination and testis morphogenesis: comparative candidate genes

Gene expression profiles during sex determination and gonadal differentiation were investigated to identify new potential regulatory factors. Embryonic day 13 (E13), E14, and E16 rat testes and ovaries were used for microarray analysis, as well as E13 testis organ cultures that undergo testis morphogenesis and develop seminiferous cords in vitro. A list of 109 genes resulted from a selective analysis for genes present in male gonadal development and with a 1.5-fold change in expression between E13 and E16. Characterization of these 109 genes potentially important for testis development revealed that cytoskeletal-associated proteins, extracellular matrix factors, and signaling factors were highly represented. Throughout the developmental period (E13-E16), sex-enriched transcripts were more prevalent in the male with 34 of the 109 genes having testis-enriched expression during sex determination. In ovaries, the total number of transcripts with a 1.5-fold change in expression between E13 and E16 was similar to the testis, but none of those genes were both ovary enriched and regulated during the developmental period. Genes conserved in sex determination were identified by comparing changing transcripts in the rat analysis herein, to transcripts altered in previously published mouse studies of gonadal sex determination. A comparison of changing mouse and rat transcripts identified 43 genes with species conservation in sex determination and testis development. Profiles of gene expression during E13-E16 rat testis and ovary development are presented and candidate genes for involvement in sex determination and testis differentiation are identified. Analysis of cellular pathways did not reveal any specific pathways involving multiple candidate genes. However, the genes and gene network identified influence numerous cellular processes with cellular differentiation, proliferation, focal contact, RNA localization, and development being predominant.

Effects of growth factors on testicular morphogenesis

Since the discovery of the sex-determining gene Sry in 1990, research effort has focused on the events downstream of its expression. A range of different experimental approaches including gene expression, knocking-out and knocking-in genes of interest, and cell and tissue culture techniques have been applied, highlighting the importance of growth factors at all stages of testicular morphogenesis. Migration of primordial germ cells and the mesonephric precursors of peritubular myoid cells and endothelial cells to the gonad is under growth factor control. Proliferation of both germ cells and somatic cells within the gonadal primordium is also controlled by cytokines as is the interaction of Sertoli cells (with each other and with the extracellular matrix) to form testicular cords. Several growth factors/growth factor families (e.g., platelet-derived growth factor, fibroblast growth factor family, TGFbeta family, and neurotrophins) have emerged as key players, exerting an influence at different time points and steps in organogenesis. Although most evidence has emerged in the mouse, comparative studies are important in elucidating the variety, potential, and evolution of control mechanisms.

Effects of increased nerve growth factor plasma levels on the expression of TrkA and p75 in rat testicles

In addition to their well-known roles within the nervous system, the neurotrophins and their receptors regulate some functions in the reproductive system. In this study we used combined morphological and immunohistochemical techniques to investigate the presence and cellular localization in the rat testicle of the two receptors of nerve growth factor (NGF), i.e. TrkA and p75(NTR). Furthermore, to evaluate whether increased plasma levels of NGF affect the ageing process, 4-methylcathechol (4-MC), an inductor of NGF synthesis, was administered. Both TrkA and p75(NTR) were expressed in rat testicles, but the pattern and intensity of immunoreaction were marginally different between them. In adult rats TrkA was expressed in spermatozoa and spermatids, and p75 was expressed in spermatogonia. In newborn rats TrkA immunoreactivity was found in the Leydig cells, whereas p75 was detected in a cellular layer that surrounds the seminiferous tubules. In adult treated animals the immunoreaction for TrkA and p75(NTR) was also localized in the spermatocytes, whereas in newborn treated rats no changes in the pattern of immunoreaction was observed. The present findings suggest a role of the NGF/TrkA/p75 system in the physiology of reproduction, but the practical relevance of this remains to be established.

Vascular endothelial growth factor and kinase domain region receptor are involved in both seminiferous cord formation and vascular development during testis morphogenesis in the rat

Morphological male sex determination is dependent on migration of endothelial and preperitubular cells from the adjacent mesonephros into the developing testis. Our hypothesis is that VEGFA and its receptor KDR are necessary for both testicular cord formation and neovascularization. The Vegfa gene has 8 exons with many splice variants. Vegfa120, Vegfa164, and Vegfa188 mRNA isoforms were detected on Embryonic Day (E) 13.5 (plug date=E0) in the rat. Vegfa120, Vegfa144, Vegfa164, Vegfa188, and Vegfa205 mRNA were detected at E18 and Postnatal Day 3 (P3). Kdr mRNA was present on E13.5, whereas Fms-like tyrosine kinase 1 receptor (Flt1) mRNA was not detected until E18. VEGFA protein was localized to Sertoli cells at cord formation and KDR to germ and interstitial cells. The VEGFA signaling inhibitors SU1498 (40 microM) and VEGFR-TKI (8 microM) inhibited cord formation in E13 testis cultures with 90% reduced vascular density (P<0.01) in VEGFR-TKI-treated organs. Furthermore, Je-11 (10 microM), an antagonist to VEGFA, also perturbed cord formation and inhibited vascular density by more than 50% (P<0.01). To determine signal transduction pathways involved in VEGFA's regulation of testis morphogenesis, E13 testis were treated with LY 294002 (15 microM), a phosphoinositide 3-kinase (PI3K) pathway inhibitor, resulting in inhibition of both vascular density (46%) and cord formation. Thus, we support our hypothesis and conclude that VEGFA, secreted by the Sertoli cell, is involved in both neovascularization and cord formation and potentially acts through the PI3K pathway during testis morphogenesis to elicit its effects.

The expression of neurotrophins and their receptors in the prenatal and adult human testis: evidence for functions in Leydig cells

Previous studies have demonstrated local functions for neurotrophins in the developing and mature testis of rodents. To examine whether these signaling molecules are present and also potentially active in the human testis, we characterized immunohistochemically the expression and cellular localization of the known neurotrophins and their receptors during prenatal testicular development as well as in the adult human testis. Results obtained revealed the presence of nerve growth factor (NGF), brain-derived neurotrophic factor, neurotrophin-3 and 4, as well as neurotrophin receptors p75(NTR), TrkA, TrkB, and TrkC during testis morphogenesis. These proteins were also detectable in the adult human testis, and their local expression could be confirmed largely by immunoblot and RT-PCR analyses. Remarkably, the Leydig cells were found to represent the predominant neurotrophin/receptor expression sites within both fetal and adult human testes. Functional assays performed with a mouse tumor Leydig cell line revealed that NGF exposure increases cellular steroid production, indicating a role in differentiation processes. These findings support previously-recognized neuronal characteristics of Leydig cells, provide additional evidence for potential roles of neurotrophins during testis morphogenesis and in the mature testis, and demonstrate for the first time a neurotrophin-induced functional activity in Leydig cells.

Seminiferous cord formation and germ-cell programming: epigenetic transgenerational actions of endocrine disruptors

The molecular and cellular control of embryonic testis development was investigated through an analysis of the embryonic testis transcriptome to identify potential regulatory factors for male sex determination and testis morphogenesis. One critical factor identified is neurotropin 3 (NT3). At the onset of male sex determination, Sertoli cells initiate differentiation and express NT3 to act as a chemotactic factor for mesonephros cells to migrate and associate with Sertoli-germ cell aggregates to promote cord formation. Promoter analysis suggests that NT3 may be an initial downstream gene to SRY and helps promote testis morphogenesis. Endocrine disruptors were used to potentially interfere with embryonic testis development and further investigate this biological process. The estrogenic pesticide methoxychlor and antiandrogenic fungicide vinclozolin were used. Previous studies have shown that methoxychlor and vinclozolin both interfere with embryonic testis cord formation and cause increased spermatogenic cell apoptosis in the adult testis. Interestingly, transient in vivo exposure to endocrine disruptors at the time of male sex determination caused a transgenerational phenotype (F1-F4) of spermatogenic cell apoptosis and subfertility. This apparent epigenetic mechanism involves altered DNA methylation and permanent re-programming of the male germ-line. A series of genes with altered DNA methylation and imprinting are being identified. Observations reviewed demonstrate that a transient embryonic in utero exposure to an endocrine disruptor influences the embryonic testis transcriptome and through epigenetic effects (e.g., DNA methylation) results in abnormal germ-cell differentiation that subsequently influences adult spermatogenic capacity and male fertility, and that this phenotype is transgenerational through the germ-line. The novel observations of transgenerational epigenetic endocrine disruptor actions on male reproduction critically impact the potential hazards of these compounds as environmental toxins. The literature reviewed provides insight into the molecular and cellular control of embryonic testis development, male sex determination, and the programming of the male germ-line.

Effects of glial cell line-derived neurotrophic factor on isolated developing mouse Sertoli cells in vitro

Cell proliferation is a key factor in sex determination where a size increase relative to the XX gonad is one of the first signs of testis differentiation. Moreover, proliferation of Sertoli cells during development is important in building up the stock of supporting cells necessary for subsequent successful fertility. Because proliferation is such an essential part of testis development, the hypothesis under long-term investigation is that it is under fail-safe control by multiple alternative growth factors. This study was undertaken to investigate the role of glial cell-derived neurotrophic factor (GDNF) on developing mouse Sertoli cells in vitro. Sertoli cells, isolated from mouse embryos at three stages of testis development, were maintained for 2-7 days in vitro (div) in the presence or absence of GDNF at 1, 10 and 100 ng mL(-1). Overall the presence of extracellular matrix gel had little effect on proliferative activity, but encouraged expression of the epithelial phenotype. A statistically significant difference in proliferation, assessed by immunocytochemical staining for proliferating cell nuclear antigen, was seen with GDNF at embryonic day (E)12.5 after 2 div (at both 10 and 100 ng mL(-1), P < 0.001) and 7 div (at both 10 and 100 ng mL(-1), P < 0.05); at E13.5 after 3 div (at both 10 and 100 ng mL(-1), P < 0.05) and at E14.5 after 7 div (100 ng mL(-1), P < 0.01), compared with controls cultured without growth factor. In conclusion, GDNF stimulates mitosis throughout this critical developmental window. The in vitro approach used here is a useful adjunct to the knockout mouse model and has been applied to show that GDNF exerts a proliferative effect on developing mouse Sertoli cells.

Embryonic mouse testis development: role of platelet derived growth factor (PDGF-BB)

Platelet-derived growth factors (PDGFs) are paracrine growth factors mediating epithelial-mesenchymal interactions and exerting multiple biological activities which include cell proliferation, motility, and differentiation. As previously demonstrated, PDGFs act during embryonic development and recently, by culturing male genital ridges, we have demonstrated that PDGF-BB is able to support in vitro testicular cord formation. In the present paper, we report that PDGF-BB is present during embryonic testis development and, in organ culture, induces cord formation although with reduced diameters compared with the cords formed in the genital ridges cultured in the presence of HGF. Moreover we have analyzed the roles exerted by this growth factor during the morphogenesis of the testis. We demonstrate by immunohistochemical experiments that PDGF-BB and its receptors are synthesized by the male UGRs isolated from 11.5 and 13.5 dpc embryos and by Western blot that the factor is secreted in a biologically active form by testicular cells isolated from 13.5 dpc embryos. The biological roles of the factor have also been studied and we demonstrate that PDGF-BB acts as a migratory factor for male mesonephric cells whose migration is a male specific event necessary for a normal testicular morphogenesis. In addition we demonstrate that during testicular development, PDGF-BB induces testicular cell proliferation being in this way responsible for the increase in size of the testis. Finally we demonstrate that PDGF-BB is able to reorganize dissociated testicular cells inducing the formation of large cellular aggregates. However the structures formed in vitro under PDGF-BB stimulation never had a cord-like morphology similar to the cord-like structures formed in the presence of HGF (Ricci et al., 2002, Mech Dev 118:19-28), suggesting that this factor does not act as a morphogenetic factor during testicular development. All together the data presented in this paper demonstrate that PDGF-BB and its receptors (alpha- and beta-subunits) are present during the crucial ages of embryonic mouse testis morphogenesis and indicate the multiple roles exerted by this factor during the development of the male gonad.

Vertebrate thymus and the neurotrophin system

An immunomodulary role has been proposed for growth factors included in the family of neurotrophins. This is supported by the presence of both neurotrophins and neurotrophin receptors in the immune organs and some immunocompetent cells, the in vitro and in vivo effects of the neurotrophins on the immune cells, and the structural changes of lymphoid organs in mice deficient in neurotrophins and their receptors. The current data strongly indicate that neurotrophins regulate the biology of thymic stromal cells and T cells, including survival, and are involved in the thymic organogenesis. This review compiles the available data about the occurrence and distribution of neurotrophins and their signaling receptors (Trk proteins and p75(NTR)) in the vertebrate thymus and the possible contribution of these molecules to the thymic microenvironment and, therefore, to the T cells differentiation.

The role of neurotrophin receptors in female germ-cell survival in mouse and human

During mammalian ovary formation, the production of ovarian follicles is accompanied by an enormous loss of germ cells. It is not known how this loss is regulated. We have investigated the role of the Trk tyrosine kinase receptors, primarily TrkB, in this process. The ovaries of TrkB-/- and TrkC-/- mice with a mixed (129Sv x C57BL/6) genetic background were examined shortly after birth. Around 50% of TrkB-/- mice had grossly abnormal ovaries that contained greatly reduced numbers of follicles. No defects were found in the ovaries of TrkC-/- mice. Congenic TrkB-/- mice were generated on 129Sv and C57BL/6 backgrounds: whereas the former had a mixed ovarian phenotype similar to that of the original colony of mice, the ovaries of all offspring of the C57BL/6 congenic line contained reduced numbers of follicles. RT-PCR showed that mRNA encoding TrkB and its two ligands, neurotrophin 4 (NT4) and brain-derived neurotrophic factor (BDNF), were present throughout the period of follicle formation in the mouse. In situ hybridisation showed that TrkB was expressed primarily in the germ cells before and after follicle formation. Mouse neonatal and fetal ovaries and human fetal ovaries were cultured in the presence of K252a, a potent inhibitor of all Trk receptors. In mice, K252a inhibited the survival of germ cells in newly formed (primordial) follicles. This effect was rescued by the addition of basic fibroblast growth factor (bFGF) to the culture medium. Combined addition of both BDNF and NT4 blocking antibodies lowered germ-cell survival, indicating that these TrkB ligands are required in this process. The results indicate that signalling through TrkB is an important component of the mechanism that regulates the early survival of female germ cells.

Chemotactic role of neurotropin 3 in the embryonic testis that facilitates male sex determination

The first morphological event after initiation of male sex determination is seminiferous cord formation in the embryonic testis. Cord formation requires migration of pre-peritubular myoid cells from the adjacent mesonephros. The embryonic Sertoli cells are the first testicular cells to differentiate and have been shown to express neurotropin-3 (NT3), which can act on high-affinity trkC receptors expressed on migrating mesonephros cells. NT3 expression is elevated in the embryonic testis during the time of seminiferous cord formation. A trkC receptor tyrophostin inhibitor, AG879, was found to inhibit seminiferous cord formation and mesonephros cell migration. Beads containing NT3 were found to directly promote mesonephros cell migration into the gonad. Beads containing other growth factors such as epidermal growth factor (EGF) did not influence cell migration. At male sex determination the SRY gene promotes testis development and the expression of downstream sex differentiation genes such as SOX-9. Inhibition of NT3 actions caused a reduction in the expression of SOX-9. Combined observations suggest that when male sex determination is initiated, the developing Sertoli cells express NT3 as a chemotactic agent for migrating mesonephros cells, which are essential to promote embryonic testis cord formation and influence downstream male sex differentiation.