Homing of mouse spermatogonial stem cells to germline niche depends on beta1-integrin. Cell Stem Cell. 2008;3:533-542. [PMID: 18983968 DOI: 10.1016/j.stem.2008.08.002]

Cadherin 22 participates in the self-renewal of mouse female germ line stem cells via interaction with JAK2 and β-catenin

The self-renewal capacity of the stem cell pool determines tissue function and health. Cadherin-22 (Cdh22), a member of the cadherin superfamily, has two splicing patterns in rats, and the short type that lacks a catenin binding domain is closely related to spermatogonial stem cell self-renewal. Previously, we reported that CDH22 was highly expressed in mouse ovary germ cells, especially in female germ line stem cells (FGSCs). However, its underlying function in FGSCs is still not clear. Here, we found that Cdh22 encodes only one type of protein product in mice and demonstrated that CDH22 was required for FGSC self-renewal. In addition, JAK2 and β-catenin were found to interact with CDH22 and be involved in CDH22 signaling in mouse FGSCs. Moreover, extrinsic CDH22 was identified as a potential molecule that participates in FGSC adhesion and is pivotal for FGSC maintenance and self-renewal. These results reveal that CDH22 functions as an essential molecule in FGSC maintenance and self-renewal via different mechanisms, including interaction with the JAK-STAT signaling pathway and β-catenin.

Expression patterns and role of SDF-1/CXCR4 axis in boar spermatogonial stem cells

The signaling of chemokine stromal cell-derived factor (SDF)-1 and its receptor C-X-C motif chemokine receptor 4 (CXCR4) is involved in the cellular proliferation, survival, and migration of various cell types. Although SDF-1/CXCR4 has been implicated in the maintenance of the spermatogonial population during mouse testis development, their expression patterns and functions in boar testis remain unclear. In the present study, the expression pattern of SDF-1 and CXCR4 was determined during pre-pubertal and post-pubertal stage boar testes and in vitro cultured porcine spermatogonial stem cells (pSSCs). The role of these proteins in colony formation in cultured pSSCs was also investigated. Interestingly, SDF-1 expression was observed in PGP 9.5-positve spermatogonia in all developing stages of boar testis; however, CXCR4 expression was only detected in spermatogonia from 5-day-old boar testis. In addition, SDF-1 and CXCR4 expression was observed in cultured pSSCs from 5-day-old boar testes, and inhibition of the CXCR4 receptor signaling pathway by AMD3100 significantly decreased the colony formation of pSSCs. These results suggest that SDF-1 and CXCR4 are useful markers for detecting stage-specific spermatogonia in boar testis. Our results reveal the role of the SDF-1/CXCR4 axis in pSSC in vitro culture.

Cancer treatment in childhood and testicular function: the importance of the somatic environment

Testicular function and future fertility may be affected by cancer treatment during childhood. Whilst survival of the germ (stem) cells is critical for ensuring the potential for fertility in these patients, the somatic cell populations also play a crucial role in providing a suitable environment to support germ cell maintenance and subsequent development. Regulation of the spermatogonial germ-stem cell niche involves many signalling pathways with hormonal influence from the hypothalamo-pituitary-gonadal axis. In this review, we describe the somatic cell populations that comprise the testicular germ-stem cell niche in humans and how they may be affected by cancer treatment during childhood. We also discuss the experimental models that may be utilized to manipulate the somatic environment and report the results of studies that investigate the potential role of somatic cells in the protection of the germ cells in the testis from cancer treatment.

Spermatogonial stem cell transplantation and male infertility: Current status and future directions

Objective

To summarise the current state of research into spermatogonial stem cell (SSC) therapies with a focus on future directions, as SSCs show promise as a source for preserving or initiating fertility in otherwise infertile men.

Materials and methods

We performed a search for publications addressing spermatogonial stem cell transplantation in the treatment of male infertility. The search engines PubMed and Google Scholar were used from 1990 to 2017. Search terms were relevant for spermatogonial stem cell therapies. Titles of publications were screened for relevance; abstracts were read, if related and full papers were reviewed for directly pertinent original research.

Results

In all, 58 papers were found to be relevant to this review, and were included in appropriate subheadings. This review discusses the various techniques that SSCs are being investigated to treat forms of male infertility.

Conclusions

Evidence does not yet support clinical application of SSCs in humans. However, significant progress in the in vitro and in vivo development of SSCs, including differentiation into functional germ cells, gives reason for cautious optimism for future research.

Tracing and Characterizing the Development of Transplanted Female Germline Stem Cells In Vivo

It has long been believed that most female mammalian species lose the ability to generate oocytes in postnatal ovaries. Recent evidence has demonstrated the isolation and culture of female germline stem cells (FGSCs) from adult mice and humans. However, the process and mechanisms of FGSC differentiation in vivo following transplantation have not yet been studied. Here, we isolated and characterized FGSCs from a single EGFP-transgenic mouse, and traced the development and behavior of transplanted FGSCs (F-TFs) in vivo. Comparisons of folliculogenesis between recipients with FGSC transplantation and wild-type (WT) mice were performed by single follicle RNA-sequencing (RNA-seq). Results showed that FGSCs exhibited a homing ability and began to differentiate into early-stage oocytes only when they reached the edge of the ovarian cortex. The F-TFs restored function of premature ovarian failure (gdf9iCre; Pten^loxP/loxP genotype) and generated offspring. Furthermore, results demonstrated that the developmental mechanisms of follicles derived from F-TFs were similar to that of WT follicles. Weighted gene co-expression network analysis identified two potential sub-networks and core genes that played a critical role in follicular development. These findings provide a theoretical basis and lay a technology platform for specific or personalized medical treatment of ovarian failure or other ovarian diseases.

IL-27 triggers IL-10 production in Th17 cells via a c-Maf/RORγt/Blimp-1 signal to promote the progression of endometriosis

Endometriosis is an estrogen-dependent inflammatory disease. The anti-inflammatory cytokine IL-10 is also increased in endometriosis. IL-10 production by Th17 cells is critical for limiting autoimmunity and inflammatory responses. However, the mechanism of inducing IL-10-producing Th17 cells is still largely unknown. The present study investigated the differentiation mechanism and role of IL-10-producing Th17 cells in endometriosis. Here, we report that IL-10⁺Th17 cells are significantly increased in the peritoneal fluid of women with endometriosis, along with an elevation of IL-27, IL-6 and TGF-β. Compared with peripheral CD4⁺ T cells, endometrial CD4⁺ T cells highly expressed IL-27 receptors, especially the ectopic endometrium. Under external (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) and local (estrogen, IL-6 and TGF-β) environmental regulation, IL-27 from macrophages and endometrial stromal cells (ESCs) induces IL-10 production in Th17 cells in vitro and in vivo. This process may be mediated through the interaction between c-musculoaponeurotic fibrosarconna (c-Maf) and retinoic acid-related orphan receptor gamma t (RORγt), and associated with the upregulation of downstream B lymphocyte-induced maturation protein-1 (Blimp-1). IL-10⁺Th17 cells, in turn, stimulate the proliferation and implantation of ectopic lesions and accelerate the progression of endometriosis. These results suggest that IL-27 is a pivotal regulator in endometriotic immune tolerance by triggering Th17 cells to produce IL-10 and promoting the rapid growth and implantation of ectopic lesions. This finding provides a scientific basis for potential therapeutic strategies aimed at preventing the development of endometriosis, especially for patients with high levels of IL-10⁺Th17 cells.

Role of the testis interstitial compartment in spermatogonial stem cell function

Intricate cellular and molecular interactions ensure that spermatogonial stem cells (SSCs) proceed in a step-wise differentiation process through spermatogenesis and spermiogenesis to produce sperm. SSCs lie within the seminiferous tubule compartment, which provides a nurturing environment for the development of sperm. Cells outside of the tubules, such as interstitial and peritubular cells, also help direct SSC activity. This review focuses on interstitial (interstitial macrophages, Leydig cells and vasculature) and peritubular (peritubular macrophages and peritubular myoid cells) cells and their role in regulating the SSC self-renewal and differentiation in mammals. Leydig cells, the major steroidogenic cells in the testis, influence SSCs through secreted factors, such as insulin growth factor 1 (IGF1) and colony-stimulating factor 1 (CSF1). Macrophages interact with SSCs through various potential mechanisms, such as CSF1 and retinoic acid (RA), to induce the proliferation or differentiation of SSCs respectively. Vasculature influences SSC dynamics through CSF1 and vascular endothelial growth factor (VEGF) and by regulating oxygen levels. Lastly, peritubular myoid cells produce one of the most well-known factors that is required for SSC self-renewal, glial cell line-derived neurotrophic factor (GDNF), as well as CSF1. Overall, SSC interactions with interstitial and peritubular cells are critical for SSC function and are an important underlying factor promoting male fertility.

Chemokine (C-X-C) Ligand 12 Facilitates Trafficking of Donor Spermatogonial Stem Cells

The chemokine (C-X-C) receptor type 4 (CXCR4) is an early marker of primordial germ cells (PGCs) essential for their migration and colonization of the gonads. In spermatogonial stem cells (SSCs), the expression of CXCR4 is promoted by the self-renewal factor, glial cell line-derived neurotrophic factor (GDNF). Here, we demonstrate an important role of CXCR4 during donor mouse SSCs reoccupation of the endogenous niche in recipient testis. Silencing of CXCR4 expression in mouse SSCs dramatically reduced the number of donor stem cell-derived colonies, whereas colony morphology and spermatogenesis were comparable to controls. Inhibition of CXCR4 signaling using a small molecule inhibitor (AMD3100) during the critical window of homing also significantly lowered the efficiency of donor-derived SSCs to establish spermatogenic colonies in recipient mice; however, the self-renewal of SSCs was not affected by exposure to AMD3100. Rather, in vitro migration assays demonstrate the influence of CXCR4-CXCL12 signaling in promoting germ cell migration. Together, these studies suggest that CXCR4-CXCL12 signaling functions to promote homing of SSCs towards the stem cell niche and plays a critical role in reestablishing spermatogenesis.

Genetics of gonadal stem cell renewal

Stem cells are necessary for the maintenance of many adult tissues. Signals within the stem cell microenvironment, or niche, regulate the self-renewal and differentiation capability of these cells. Misregulation of these signals through mutation or damage can lead to overgrowth or depletion of different stem cell pools. In this review, we focus on the Drosophila testis and ovary, both of which contain well-defined niches, as well as the mouse testis, which has become a more approachable stem cell system with recent technical advances. We discuss the signals that regulate gonadal stem cells in their niches, how these signals mediate self-renewal and differentiation under homeostatic conditions, and how stress, whether from mutations or damage, can cause changes in cell fate and drive stem cell competition.

Stage-specific control of niche positioning and integrity in the Drosophila testis

A fundamental question is how complex structures are maintained after their initial specification. Stem cells reside in a specialized microenvironment, called niche, which provides essential signals controlling stem cell behavior. We addressed this question by studying the Drosophila male stem cell niche, called the hub. Once specified, the hub cells need to maintain their position and architectural integrity through embryonic, larval and pupal stages of testis organogenesis and during adult life. The Hox gene Abd-B, in addition to its described role in male embryonic gonads, maintains the architecture and positioning of the larval hub from the germline by affecting integrin localization in the neighboring somatic cyst cells. We find that the AbdB-Boss/Sev cascade affects integrin independent of Talin, while genetic interactions depict integrin as the central downstream player in this system. Focal adhesion and integrin-adaptor proteins within the somatic stem cells and cyst cells, such as Paxillin, Pinch and Vav, also contribute to proper hub integrity and positioning. During adult stages, hub positioning is controlled by Abd-B activity in the outer acto-myosin sheath, while Abd-B expression in adult spermatocytes exerts no effect on hub positioning and integrin localization. Our data point at a cell- and stage-specific function of Abd-B and suggest that the occurrence of new cell types and cell interactions in the course of testis organogenesis made it necessary to adapt the whole system by reusing the same players for male stem cell niche positioning and integrity in an alternative manner.

Stem cell autotomy and niche interaction in different systems

The best known cases of cell autotomy are the formation of erythrocytes and thrombocytes (platelets) from progenitor cells that reside in special niches. Recently, autotomy of stem cells and its enigmatic interaction with the niche has been reported from male germline stem cells (GSCs) in several insect species. First described in lepidopterans, the silkmoth, followed by the gipsy moth and consecutively in hemipterans, foremost the milkweed bug. In both, moths and the milkweed bug, GSCs form finger-like projections toward the niche, the apical cells (homologs of the hub cells in Drosophila). Whereas in the milkweed bug the projection terminals remain at the surface of the niche cells, in the gipsy moth they protrude deeply into the singular niche cell. In both cases, the projections undergo serial retrograde fragmentation with progressing signs of autophagy. In the gipsy moth, the autotomized vesicles are phagocytized and digested by the niche cell. In the milkweed bug the autotomized vesicles accumulate at the niche surface and disintegrate. Autotomy and sprouting of new projections appears to occur continuously. The significance of the GSC-niche interactions, however, remains enigmatic. Our concept on the signaling relationship between stem cell-niche in general and GSC and niche (hub cells and cyst stem cells) in particular has been greatly shaped by Drosophila melanogaster. In comparing the interactions of GSCs with their niche in Drosophila with those in species exhibiting GSC autotomy it is obvious that additional or alternative modes of stem cell-niche communication exist. Thus, essential signaling pathways, including niche-stem cell adhesion (E-cadherin) and the direction of asymmetrical GSC division - as they were found in Drosophila - can hardly be translated into the systems where GSC autotomy was reported. It is shown here that the serial autotomy of GSC projections shows remarkable similarities with Wallerian axonal destruction, developmental axon pruning and dying-back degeneration in neurodegenerative diseases. Especially the hypothesis of an existing evolutionary conserved “autodestruction program” in axons that might also be active in GSC projections appears attractive. Investigations on the underlying signaling pathways have to be carried out. There are two other well known cases of programmed cell autotomy: the enucleation of erythroblasts in the process of erythrocyte maturation and the segregation of thousands of thrombocytes (platelets) from one megakaryocyte. Both progenitor cell types - erythroblasts and megakaryocytes - are associated with a niche in the bone marrow, erythroblasts with a macrophage, which they surround, and the megakaryocytes with the endothelial cells of sinusoids and their extracellular matrix. Although the regulatory mechanisms may be specific in each case, there is one aspect that connects all described processes of programmed cell autotomy and neuronal autodestruction: apoptotic pathways play always a prominent role. Studies on the role of male GSC autotomy in stem cell-niche interaction have just started but are expected to reveal hitherto unknown ways of signal exchange. Spermatogenesis in mammals advance our understanding of insect spermatogenesis. Mammal and insect spermatogenesis share some broad principles, but a comparison of the signaling pathways is difficult. We have intimate knowledge from Drosophila, but of almost no other insect, and we have only limited knowledge from mammals. The discovery of stem cell autotomy as part of the interaction with the niche promises new general insights into the complicated stem cell-niche interdependence.

AIP1-mediated actin disassembly is required for postnatal germ cell migration and spermatogonial stem cell niche establishment

In mammals, spermatogonial stem cells (SSCs) arise from early germ cells called gonocytes, which are derived from primordial germ cells during embryogenesis and remain quiescent until birth. After birth, these germ cells migrate from the center of testicular cord, through Sertoli cells, and toward the basement membrane to form the SSC pool and establish the SSC niche architecture. However, molecular mechanisms underlying germ cell migration and niche establishment are largely unknown. Here, we show that the actin disassembly factor actin interacting protein 1 (AIP1) is required in both germ cells and Sertoli cells to regulate this process. Germ cell-specific or Sertoli cell-specific deletion of Aip1 gene each led to significant defects in germ cell migration after postnatal day 4 or 5, accompanied by elevated levels of actin filaments (F-actin) in the affected cells. Furthermore, our data demonstrated that interaction between germ cells and Sertoli cells, likely through E-cadherin-mediated cell adhesion, is critical for germ cells' migration toward the basement membrane. At last, Aip1 deletion in Sertoli cells decreased SSC self-renewal, increased spermatogonial differentiation, but did not affect the expression and secretion levels of growth factors, suggesting that the disruption of SSC function results from architectural changes in the postnatal niche.

Mechanobiology of mesenchymal stem cells: Perspective into mechanical induction of MSC fate

Bone marrow-derived mesenchymal stem and stromal cells (MSCs) are promising candidates for cell-based therapies in diverse conditions including tissue engineering. Advancement of these therapies relies on the ability to direct MSCs toward specific cell phenotypes. Despite identification of applied forces that affect self-maintenance, proliferation, and differentiation of MSCs, mechanisms underlying the integration of mechanically induced signaling cascades and interpretation of mechanical signals by MSCs remain elusive. During the past decade, many researchers have demonstrated that external applied forces can activate osteogenic signaling pathways in MSCs, including Wnt, Ror2, and Runx2. Besides, recent advances have highlighted the critical role of internal forces due to cell-matrix interaction in MSC function. These internal forces can be achieved by the materials that cells reside in through its mechanical properties, such as rigidity, topography, degradability, and substrate patterning. MSCs can generate contractile forces to sense these mechanical properties and thereby perceive mechanical information that directs broad aspects of MSC functions, including lineage commitment. Although many signaling pathways have been elucidated in material-induced lineage specification of MSCs, discovering the mechanisms by which MSCs respond to such cell-generated forces is still challenging because of the highly intricate signaling milieu present in MSC environment. However, bioengineers are bridging this gap by developing platforms to control mechanical cues with improved throughput and precision, thereby enabling further investigation of mechanically induced MSC functions. In this review, we discuss the most recent advances that how applied forces and cell-generated forces may be engineered to determine MSC fate, and overview a subset of the operative signal transduction mechanisms and experimental platforms that have emerged in MSC mechanobiology research. Our main goal is to provide an up-to-date view of MSC mechanobiology that is relevant to both mechanical loading and mechanical properties of the environment, and introduce these emerging platforms for tissue engineering use.

Extracellular, stem cells and regenerative ophthalmology

Retinal degenerative diseases, including retinitis pigmentosa, age-related macular degeneration, and glaucoma, still lack effective medical treatments. The stem cell-based regenerative approach has been proposed to treat these degenerative diseases. The major challenge for regenerative ophthalmology is to produce enough desirable retinal neurons in vitro from various stem cell types. Extracellular matrix proteins are important for stem cell self-renewal and differentiation in various systems. They have also been used in combination with various growth factors to expand retinal stem cells and produce desirable retinal neuronal types. This review summarizes our current understanding of how extracellular matrix proteins regulate stem cell function and discusses their application in regenerative ophthalmology.

Importance of the stem cell microenvironment for ophthalmological cell-based therapy

Cell therapy is a promising treatment for diseases that are caused by cell degeneration or death. The cells for clinical transplantation are usually obtained by culturing healthy allogeneic or exogenous tissue in vitro. However, for diseases of the eye, obtaining the adequate number of cells for clinical transplantation is difficult due to the small size of tissue donors and the frequent needs of long-term amplification of cells in vitro, which results in low cell viability after transplantation. In addition, the transplanted cells often develop fibrosis or degrade and have very low survival. Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPS) are also promising candidates for cell therapy. Unfortunately, the differentiation of ESCs can bring immune rejection, tumorigenicity and undesired differentiated cells, limiting its clinical application. Although iPS cells can avoid the risk of immune rejection caused by ES cell differentiation post-transplantation, the low conversion rate, the risk of tumor formation and the potentially unpredictable biological changes that could occur through genetic manipulation hinder its clinical application. Thus, the desired clinical effect of cell therapy is impaired by these factors. Recent research findings recognize that the reason for low survival of the implanted cells not only depends on the seeded cells, but also on the cell microenvironment, which determines the cell survival, proliferation and even reverse differentiation. When used for cell therapy, the transplanted cells need a specific three-dimensional structure to anchor and specific extra cellular matrix components in addition to relevant cytokine signaling to transfer the required information to support their growth. These structures present in the matrix in which the stem cells reside are known as the stem cell microenvironment. The microenvironment interaction with the stem cells provides the necessary homeostasis for cell maintenance and growth. A large number of studies suggest that to explore how to reconstruct the stem cell microenvironment and strengthen its combination with the transplanted cells are key steps to successful cell therapy. In this review, we will describe the interactions of the stem cell microenvironment with the stem cells, discuss the importance of the stem cell microenvironment for cell-based therapy in ocular diseases, and introduce the progress of stem cell-based therapy for ocular diseases.

Claudin-8 expression in Sertoli cells and putative spermatogonial stem cells in the bovine testis

Adhesion molecules are expressed by both adult and embryonic stem cells, with different classes of adhesion molecules involved in cell-membrane and intercellular contacts. In this study the expression of the adhesion molecule claudin-8 (CLDN8), a tight-junction protein, was investigated as a potential marker for undifferentiated spermatogonia in the bovine testis. We found that CLDN8 was expressed by both spermatogonia and a subset of Sertoli cells in the bovine testis. We also showed co-expression of GFRα1 in testis cells with CLDN8 and with Dolichos biflorus agglutinin-fluorescein isothiocyanate (DBA-FITC) staining. We observed co-enrichment of spermatogonia and CLDN8-expressing Sertoli cells in DBA-FITC-assisted magnetic-activated cell sorting (MACS), an observation supported by results from fluorescence-activated cell sorting analysis, which showed CLDN8-expressing cells were over-represented in the MACS-positive cell fraction, leading to the hypothesis that CLDN8 may play a role in the spermatogonial stem-cell niche.

Stage-specific control of stem cell niche architecture in the Drosophila testis by the posterior Hox gene Abd-B

A fundamental question in biology is how complex structures are maintained after their initial specification. We address this question by reviewing the role of the Hox gene Abd-B in Drosophila testis organogenesis, which proceeds through embryonic, larval and pupal stages to reach maturation in adult stages. The data presented in this review highlight a cell- and stage-specific function of Abd-B, since the mechanisms regulating stem cell niche positioning and architecture at different stages seem to be different despite the employment of similar factors. In addition to its described role in the male embryonic gonads, sustained activity of Abd-B in the pre-meiotic germline spermatocytes during larval stages is required to maintain the architecture of the stem cell niche by regulating βPS-integrin localization in the neighboring somatic cyst cells. Loss of Abd-B is associated with cell non-autonomous effects within the niche, leading to a dramatic reduction of pre-meiotic cell populations in adult testes. Identification of Abd-B target genes revealed that Abd-B mediates its effects by controlling the activity of the sevenless ligand Boss via its direct targets Src42A and Sec63. During adult stages, when testis morphogenesis is completed with the addition of the acto-myosin sheath originating from the genital disc, stem cell niche positioning and integrity are regulated by Abd-B activity in the acto-myosin sheath whereas integrin acts in an Abd-B independent way. It seems that the occurrence of new cell types and cell interactions in the course of testis organogenesis made it necessary to adapt the system to the new cellular conditions by reusing the same players for testis stem cell niche positioning in an alternative manner.

Xenotransplantation exposes the etiology of azoospermia factor (AZF) induced male sterility

Ramathal et al. have employed an elegant xenotransplantation technique to study the fate of human induced pluripotent stem cells (hiPSCs) from fertile males and from males carrying Y chromosome deletions of the azoospermia factor (AZF) region. When placed in a mouse testis niche, hiPSCs from fertile males differentiate into germ cell-like cells (GCLCs). Highlighting the crucial role of cell autonomous factors in male sterility, hiPSCs derived from azoospermic males prove to be less successful under similar circumstances. Their studies argue that the agametic "Sertoli cell only" phenotype of two of the AZF deletions likely arises from a defect in the maintenance of germline stem cells (GSCs) rather than from a defect in their specification. These observations underscore the importance of the dialogue between the somatic niche and its inhabitant stem cells, and open up interesting questions concerning the functioning of the somatic niche and how it communicates to the GSCs.

New insights into germ cell migration and survival/apoptosis in spermatogenesis: Lessons from CD147

CD147, also named basigin (Bsg) or extracellular matrix (ECM) metalloproteinase inducer (EMMPRIN), is a highly glycosylated protein first identified as a tumor cell surface molecule. In cancer, it is well established that CD147 promotes metastasis by stimulating the production of MMPs. Recent studies have also suggested that it may be associated with tumor growth and angiogenesis. Interestingly, CD147 is expressed in germ cells of different development stages in the testis and its knockout mice are infertile, indicating an essential role of CD147 in spermatogenesis. While the detailed involvement of CD147 in spermatogenesis remains elusive, our recent findings have revealed a dual role of CD147 in germ cell development. On the one hand, it regulates the migration of spermatogonia and spermatocytes via the induction of MMP-2 production; on the other hand, it specifically regulates the survival/apoptosis of spermatocytes but not spermatogonia through a p53-independent pathway. In this review, we aim to provide an overview on the functions of CD147, comparing its roles in cancer and the testis, thereby providing new insights into the regulatory mechanisms underlying the process of spermatogenesis.

Adult stem cell niches: cellular and molecular components

As stem cells (SCs) in adult organs continue to be identified and characterized, it becomes clear that their survival, quiescence, and activation depend on specific signals in their microenvironment, or niche. Although adult SCs of diverse tissues differ by their developmental origin, cycling activity, and regenerative capacity, there appear to be conserved similarities regarding the cellular and molecular components of the SC niche. Interestingly, many organs house both slow-cycling and fast-cycling SC populations, which rely on the coexistence of quiescent and inductive niches for proper regulation. In this review we present a general definition of adult SC niches in the most studied mammalian systems. We further focus on dissecting their cellular organization and on highlighting recently identified key molecular regulators. Finally, we detail the potential involvement of the SC niche in tissue degeneration, with a particular emphasis on aging and cancer.

Long-term culture and analysis of cashmere goat Sertoli cells

Sertoli cells have important functions in the testis for spermatogenesis. Thus, Sertoli cell culture systems have been established in many animals, such as rat, mouse, human, dog, cow, and pig, but a goat culture has not been reported. This study describes the isolation and culture of Sertoli cells from 3- to 4-month-old cashmere goat (Capra hircus) testes. These proliferative cells were expanded for 20 passages and repeatedly cryopreserved in vitro, in contrast to previous study in human, of which maintain steady growth for up to seven passages and only passages 1 to 5 could be refrozen. The microstructure and ultrastructure of the culture were typical of Sertoli cells, bearing irregular nuclei and a cytoplasm that was rich in smooth and rough endoplasmic reticulum, mitochondria, Golgi, lysosomes, lipid drops, and glycogenosomes. By immunofluorescence analysis, the all cells expressed SRY-related HMG box gene 9 (Sox9). Growth curves and 5-bromo-2'-deoxyuridine (BrdU) incorporation were used to analyze the proliferation of the cultured cells. With increasing passage times, the proliferation of the Sertoli cells declined, but the transcription of glial cell-derived neurotrophic factor (GDNF), stem cell factor (SCF), and β1-integrin was constant. By flow cytometry, the cells retained the ability to proliferate after 5 yr of cryopreservation. Thus, cashmere goat Sertoli cells have significant proliferative potential in vitro, expressing germ cell regulatory factors and have important applications in studying Sertoli cell-germ cell interactions, spermatogenesis, reproductive toxicology, and male infertility.

In vitro culture and characterization of spermatogonial stem cells on Sertoli cell feeder layer in goat (Capra hircus)

PURPOSE

To develop an efficient protocol for isolation, purification and long-term culture of spermatogonial stem cell (SSC) in goat.

METHODS

The isolation of SSC was performed by testicular disaggregation by enzymatic digestion using collagenase IV, trypsin and DNase I. Further SSCs were enriched using Percoll density gradient centrifugation. The purity of SSCs was assessed by immunocytochemistry (ICC) using α6 integrin. The SSCs were co-cultured on Sertoli cell feeder layer. The SSC colonies were characterized by studying the expression of SSC specific markers (viz., α6 integrin and PLZF) using ICC. The abundance of mRNAs encoding the markers of SSC (viz., β1 integrin and Oct-4) and Sertoli cells (viz., vimentin) was also assayed using quantitative real-time PCR (qPCR).

RESULTS

The viability of isolated testicular cells was > 90 % and the Percoll density gradient method resulted in 3.65 folds enrichment with a purity of 82.5 %. Co-culturing of SSCs with Sertoli cell feeder layer allowed the maintenance of stable SSC colonies even after one and half months of culture. The results of ICC analysis showed the expression of α6 integrin and PLZF in almost all the SSC colonies. qPCR analysis revealed a differential expression of mRNAs encoding β1 integrin, Oct-4 and vimentin markers.

CONCLUSION

Results of this study demonstrate a simple enzymatic digestion and Percoll density gradient method for isolation and enrichment of SSCs, and suitability of Sertoli cell feeder layer for long term in vitro culture of SSC in goats. Results also suggest the possible application of non-caprine antibodies against SSC specific markers for the identification and subsequent assessment of SSCs in goats.

Fluorescence- and magnetic-activated cell sorting strategies to isolate and enrich human spermatogonial stem cells

OBJECTIVE

To determine the molecular characteristics of human spermatogonia and optimize methods to enrich spermatogonial stem cells (SSCs).

DESIGN

Laboratory study using human tissues.

SETTING

Research institute.

PATIENT(S)

Healthy adult human testicular tissue.

INTERVENTION(S)

Human testicular tissue was fixed or digested with enzymes to produce a cell suspension. Human testis cells were fractionated by fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS).

MAIN OUTCOME MEASURE(S)

Immunostaining for selected markers, human-to-nude mouse xenotransplantation assay.

RESULT(S)

Immunohistochemistry costaining revealed the relative expression patterns of SALL4, UTF1, ZBTB16, UCHL1, and ENO2 in human undifferentiated spermatogonia as well as the extent of overlap with the differentiation marker KIT. Whole mount analyses revealed that human undifferentiated spermatogonia (UCHL1+) were typically arranged in clones of one to four cells whereas differentiated spermatogonia (KIT+) were typically arranged in clones of eight or more cells. The ratio of undifferentiated-to-differentiated spermatogonia is greater in humans than in rodents. The SSC colonizing activity was enriched in the THY1dim and ITGA6+ fractions of human testes sorted by FACS. ITGA6 was effective for sorting human SSCs by MACS; THY1 and EPCAM were not.

CONCLUSION(S)

Human spermatogonial differentiation correlates with increased clone size and onset of KIT expression, similar to rodents. The undifferentiated-to-differentiated developmental dynamics in human spermatogonia is different than rodents. THY1, ITGA6, and EPCAM can be used to enrich human SSC colonizing activity by FACS, but only ITGA6 is amenable to high throughput sorting by MACS.

Drosophila perlecan regulates intestinal stem cell activity via cell-matrix attachment

Stem cells require specialized local microenvironments, termed niches, for normal retention, proliferation, and multipotency. Niches are composed of cells together with their associated extracellular matrix (ECM). Currently, the roles of ECM in regulating niche functions are poorly understood. Here, we demonstrate that Perlecan (Pcan), a highly conserved ECM component, controls intestinal stem cell (ISC) activities and ISC-ECM attachment in Drosophila adult posterior midgut. Loss of Pcan from ISCs, but not other surrounding cells, causes ISCs to detach from underlying ECM, lose their identity, and fail to proliferate. These defects are not a result of a loss of epidermal growth factor receptor (EGFR) or Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling activity but partially depend on integrin signaling activity. We propose that Pcan secreted by ISCs confers niche properties to the adjacent ECM that is required for ISC maintenance of stem cell identity, activity, and anchorage to the niche.

De novo morphogenesis of testis tissue: an improved bioassay to investigate the role of VEGF165 during testis formation

De novo formation of testis tissue from single-cell suspensions allows manipulation of different testicular compartments before grafting to study testicular development and the spermatogonial stem cell niche. However, the low percentages of newly formed seminiferous tubules supporting complete spermatogenesis and lack of a defined protocol have limited the use of this bioassay. Low spermatogenic efficiency in de novo formed tissue could result from the scarcity of germ cells in the donor cell suspension, cell damage caused by handling or from hypoxia during tissue formation in the host environment. In this study, we compared different proportions of spermatogonia in the donor cell suspension and the use of Matrigel as a scaffold to support de novo tissue formation and spermatogenesis. Then, we used the system to investigate the role of vascular endothelial growth factor 165 (VEGF165) during testicular morphogenesis on blood vessel and seminiferous tubule formation, and on presence of germ cells in the de novo developed tubules. Our results show that donor cell pellets with 10×10(6) porcine neonatal testicular cells in Matrigel efficiently formed testis tissue de novo. Contrary to what was expected, the enrichment of the cell suspension with germ cells did not result in higher numbers of tubules supporting spermatogenesis. The addition of VEGF165 did not improve blood vessel or tubule formation, but it enhanced the number of tubules containing spermatogonia. These results indicate that spermatogenic efficiency was improved by the addition of Matrigel, and that VEGF165 may have a protective role supporting germ cell establishment in their niche.

The Hox gene Abd-B controls stem cell niche function in the Drosophila testis

Proper niche architecture is critical for stem cell function, yet only few upstream regulators are known. Here, we report that the Hox transcription factor Abdominal-B (Abd-B), active in premeiotic spermatocytes of Drosophila testes, is essential for positioning the niche to the testis anterior by regulating integrin in neighboring somatic cyst cells. Abd-B also non-cell-autonomously controls critical features within the niche, including centrosome orientation and division rates of germline stem cells. By using genome-wide binding studies, we find that Abd-B mediates its effects on integrin localization by directly controlling at multiple levels the signaling activity of the Sev ligand Boss via its direct targets src42A and sec63, two genes involved in protein trafficking and recycling. Our data show that Abd-B, through local signaling between adjucent cell types, provides positional cues for integrin localization, which is critical for placement of the distant stem cell niche and stem cell activity.

Novel regulators of spermatogenesis

Spermatogenesis is a multistep process that supports the production of millions of sperm daily. Understanding of the molecular mechanisms that regulate spermatogenesis has been a major focus for decades. Yet, the regulators involved in different cellular processes of spermatogenesis remain largely unknown. Human diseases that result in defective spermatogenesis have provided hints on the molecular mechanisms regulating this process. In this review, we have summarized recent findings on the function and signaling mechanisms of several genes that are known to be associated with disease or pathological processes, including CFTR, CD147, YWK-II and CT genes, and discuss their potential roles in regulating different processes of spermatogenesis.

Extracellular matrix: a dynamic microenvironment for stem cell niche

Background

Extracellular matrix (ECM) is a dynamic and complex environment characterized by biophysical, mechanical and biochemical properties specific for each tissue and able to regulate cell behavior. Stem cells have a key role in the maintenance and regeneration of tissues and they are located in a specific microenvironment, defined as niche.

Scope of review

We overview the progresses that have been made in elucidating stem cell niches and discuss the mechanisms by which ECM affects stem cell behavior. We also summarize the current tools and experimental models for studying ECM–stem cell interactions.

Major conclusions

ECM represents an essential player in stem cell niche, since it can directly or indirectly modulate the maintenance, proliferation, self-renewal and differentiation of stem cells. Several ECM molecules play regulatory functions for different types of stem cells, and based on its molecular composition the ECM can be deposited and finely tuned for providing the most appropriate niche for stem cells in the various tissues. Engineered biomaterials able to mimic the in vivo characteristics of stem cell niche provide suitable in vitro tools for dissecting the different roles exerted by the ECM and its molecular components on stem cell behavior.

General significance

ECM is a key component of stem cell niches and is involved in various aspects of stem cell behavior, thus having a major impact on tissue homeostasis and regeneration under physiological and pathological conditions. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

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Stem cells have a key role in the maintenance and regeneration of tissues.

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The extracellular matrix is a critical regulator of stem cell function.

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Stem cells reside in a dynamic and specialized microenvironment denoted as niche.

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The extracellular matrix represents an essential component of stem cell niches.

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Bioengineered niches can be used for investigating stem cell–matrix interactions.

Wt1 deficiency causes undifferentiated spermatogonia accumulation and meiotic progression disruption in neonatal mice

Spermatogenesis is a complex process involving the regulation of multiple cell types. As the only somatic cell type in the seminiferous tubules, Sertoli cells are essential for spermatogenesis throughout the spermatogenic cycle. The Wilms tumor gene, Wt1, is specifically expressed in the Sertoli cells of the mouse testes. In this study, we demonstrated that Wt1 is required for germ cell differentiation in the developing mouse testes. At 10 days post partum, Wt1-deficient testes exhibited clear meiotic arrest and undifferentiated spermatogonia accumulation in the seminiferous tubules. In addition, the expression of claudin11, a marker and indispensable component of Sertoli cell integrity, was impaired in Wt1−/flox; Cre-ERTM testes. This observation was confirmed in in vitro testis cultures. However, the basal membrane of the seminiferous tubules in Wt1-deficient testes was not affected. Based on these findings, we propose that Sertoli cells' status is affected in Wt1-deficient mice, resulting in spermatogenesis failure.

Effects of extracellular matrices and lectin Dolichos biflorus agglutinin on cell adhesion and self-renewal of bovine gonocytes cultured in vitro

Surface molecules of primitive male germ cells, gonocytes, are essential components for regulating cell adhesion and maintaining self-renewal in mammalian species. In domestic animals, the stage-specific glycan epitope α-N-acetylgalactosamine (GalNAc) is recognised by the lectin Dolichos biflorus agglutinin (DBA) and is found on the surface of gonocytes and spermatogonia. Gonocytes from bovine testis formed mouse embryonic stem-like cell colonies on plates that had been coated with DBA or extracellular matrix (ECM) components, such as gelatin (GN), laminin (LN) and poly-L-lysine (PLL). The number of colonies on the DBA-coated plate was significantly higher than that on the GN-, LN- and PLL-coated plates. Pretreating gonocytes with DBA to neutralise the terminal GalNAc residues strongly suppressed colony formation. Furthermore, expression of a germ cell-specific gene and pluripotency-related transcription factors was increased considerably on the DBA-coated plates. These results suggest that the GalNAc residues on gonocytes can recognise precoated DBA on plates and the resulting GalNAc–DBA complexes support germ cell and stem cell potentials of gonocytes in vitro. These glycan complexes, through the GalNAc epitope, may provide a suitable microenvironment for the adhesion and cell proliferation of gonocytes in culture.

Spermatogonial stem cell self-renewal and development

Spermatogenesis originates from spermatogonial stem cells (SSCs). Development of the spermatogonial transplantation technique in 1994 provided the first functional assay to characterize SSCs. In 2000, glial cell line-derived neurotrophic factor was identified as a SSC self-renewal factor. This discovery not only provided a clue to understand SSC self-renewing mechanisms but also made it possible to derive germline stem (GS) cell cultures in 2003. In vitro culture of GS cells demonstrated their potential pluripotency and their utility in germline modification. However, in vivo SSC analyses have challenged the traditional concept of SSC self-renewal and have revealed their relationship with the microenvironment. An improved understanding of SSC self-renewal through functional assays promises to uncover fundamental principles of stem cell biology and will enable us to use these cells for applications in animal transgenesis and medicine.

Cell-cycle-dependent colonization of mouse spermatogonial stem cells after transplantation into seminiferous tubules

Spermatogonial stem cells (SSCs) migrate to the niche upon introduction into the seminiferous tubules of the testis of infertile animals. However, only 5–10% of the transplanted cells colonize recipient testes. In this study, we analyzed the impact of cell cycle on spermatogonial transplantation. We used fluorescent ubiquitination-based cell cycle indicator transgenic mice to examine the influence of cell cycle on SSC activity of mouse germline stem (GS) cells, a population of cultured spermatogonia enriched for SSCs. GS cells in the G1 phase are more efficient than those in the S/G2-M phase in colonizing the seminiferous tubules of adult mice. Cells in the G1 phase not only showed higher expression levels of GFRA1, a component of the GDNF self-renewal factor receptor, but also adhered more efficiently to laminin-coated plates. Furthermore, this cell cycle-dependency was not observed when cells were transplanted into immature pup recipients, which do not have the blood-testis barrier (BTB) between Sertoli cells, suggesting that cells in the G1 phase may passage through the BTB more readily than cells in the S/G2-M phase. Thus cell cycle status is an important factor in regulating SSC migration to the niche.

Hormone suppression with GnRH antagonist promotes spermatogenic recovery from transplanted spermatogonial stem cells in irradiated cynomolgus monkeys

Hormone suppression given before or after cytotoxic treatment stimulates the recovery of spermatogenesis from endogenous and transplanted spermatogonial stem cells (SSC) and restores fertility in rodents. To test whether the combination of hormone suppression and transplantation could enhance the recovery of spermatogenesis in primates, we irradiated (7 Gy) the testes of 12 adult cynomolgus monkeys and treated six of them with gonadotropin-releasing hormone antagonist (GnRH-ant) for 8 weeks. At the end of this treatment, we transfected cryopreserved testicular cells with green fluorescent protein-lentivirus and autologously transplanted them back into one of the testes. The only significant effect of GnRH-ant treatment on endogenous spermatogenesis was an increase in the percentage of tubules containing differentiated germ cells (tubule differentiation index; TDI) in the sham-transplanted testes of GnRH-ant-treated monkeys compared with radiation-only monkeys at 24 weeks after irradiation. Although transplantation alone after irradiation did not significantly increase the TDI, detection of lentiviral DNA in the spermatozoa of one radiation-only monkey indicated that some transplanted cells colonized the testis. However, the combination of transplantation and GnRH-ant clearly stimulated spermatogenic recovery as evidenced by several observations in the GnRH-ant-treated monkeys receiving transplantation: (i) significant increases (~20%) in the volume and weight of the testes compared with the contralateral sham-transplanted testes and/or to the transplanted testes of the radiation-only monkeys; (ii) increases in TDI compared to the transplanted testes of radiation-only monkeys at 24 weeks (9.6% vs. 2.9%; p = 0.05) and 44 weeks (16.5% vs. 6.1%, p = 0.055); (iii) detection of lentiviral sequences in the spermatozoa or testes of five of the GnRH-ant-treated monkeys and (iv) significantly higher sperm counts than in the radiation-only monkeys. Thus hormone suppression enhances spermatogenic recovery from transplanted SSC in primates and may be a useful tool in conjunction with spermatogonial transplantation to restore fertility in men after cancer treatment.

Constructing stem cell microenvironments using bioengineering approaches

Within the adult organism, stem cells reside in defined anatomical microenvironments called niches. These architecturally diverse microenvironments serve to balance stem cell self-renewal and differentiation. Proper regulation of this balance is instrumental to tissue repair and homeostasis, and any imbalance can potentially lead to diseases such as cancer. Within each of these microenvironments, a myriad of chemical and physical stimuli interact in a complex (synergistic or antagonistic) manner to tightly regulate stem cell fate. The in vitro replication of these in vivo microenvironments will be necessary for the application of stem cells for disease modeling, drug discovery, and regenerative medicine purposes. However, traditional reductionist approaches have only led to the generation of cell culture methods that poorly recapitulate the in vivo microenvironment. To that end, novel engineering and systems biology approaches have allowed for the investigation of the biological and mechanical stimuli that govern stem cell fate. In this review, the application of these technologies for the dissection of stem cell microenvironments will be analyzed. Moreover, the use of these engineering approaches to construct in vitro stem cell microenvironments that precisely control stem cell fate and function will be reviewed. Finally, the emerging trend of using high-throughput, combinatorial methods for the stepwise engineering of stem cell microenvironments will be explored.

Mice stage-specific claudin 3 expression regulates progression of meiosis in early stage spermatocytes

Claudin 3 is a protein component of the tight junction strands. Tight junctions between adjacent Sertoli cells form the blood-testis barrier (BTB). During spermatogenesis, seminiferous stage-specific expression of claudin 3 is believed to regulate the migration of preleptotene/leptotene spermatocytes across the BTB. Here, we determined the cell types expressing claudin 3 in adult mouse testis and investigated spermatogenesis after testis-specific in vivo knockdown of claudin 3. The results of in situ hybridization revealed that claudin 3 mRNA was predominantly expressed in germ cells near the basal lamina of seminiferous tubules at stages VI-IX. Furthermore, claudin 3 protein was localized not only to the BTB but also to the cell membrane of STRA8-expressing preleptotene/leptotene spermatocytes in the testis of adult ICR.Cg-Tg(Stra8-EGFP)1Ysa/YsaRbrc mice. Although claudin 3 knockdown did not affect BTB integrity, it did cause a partial delay in spermatocyte migration across the BTB. Moreover, claudin 3 knockdown resulted in a prolonged preleptotene phase during spermatogenesis. These data indicate that the seminiferous stage-specific expression and localization of claudin 3 during spermatogenesis regulate the progression of meiosis by promoting germ cell migration across the BTB.

Expression of stem cell markers: OCT4, KIT, ITGA6, and ITGB1 in the male germinal epithelium

Efforts have been made for the isolation and characterization of human stem spermatogonia (SG) which would be of major interest for fertility preservation in oncologic patients. We evaluated the expression of mammalian SG stem cell markers, KIT, OCT4, integrin alpha 6 (ITGA6), and integrin beta 1 (ITGB1) as possible indicators for the isolation of those cells in humans. Two different types of SG were individually isolated by micromanipulation from testicular biopsies of men with conserved spermatogenesis. Expression of mRNA showed the absence of KIT and ITGB1 markers in SG. By immunocytochemistry (IC), protein expression for KIT and integrins revealed two types of SG populations, negative (type-1) and positive (type-2). By immunohistochemistry (IH), protein expression for KIT and ITGB1 also revealed two kinds of SG populations, negative (SG A-dark) and positive (SG A-pale). Results suggest that in humans it may be possible to obtain pure populations of stem SG by using negative KIT((-))/ITGB1((-)) sorting.

Transgenerational effects of di-(2-ethylhexyl) phthalate on testicular germ cell associations and spermatogonial stem cells in mice

Recent evidence has linked human phthalate exposure to abnormal reproductive and hormonal effects. Phthalates are plasticizers that confer flexibility and transparency to plastics, but they readily contaminate the body and the environment. In this study, timed pregnant CD1 outbred mice were treated with di-(2-ethylhexyl) phthalate (DEHP) from Embryonic Day 7 (E7) to E14. The subsequent generation (F1) offspring were then bred to produce the F2, F3, and F4 offspring, without any further DEHP treatment. This exposure scheme disrupted testicular germ cell association and decreased sperm count and motility in F1 to F4 offspring. By spermatogonial transplantation techniques, the exposure scheme also disrupted spermatogonial stem cell (SSC) function of F3 offspring. The W/W(V) recipient testes transplanted with F3 offspring germ cells from the DEHP-treated group had a dramatically lower percentage of donor germ cell-derived spermatogenic recovery in seminiferous tubules when compared to the recipient testes transplanted with CD1 control germ cells. Further characterization showed that the major block of donor germ cell-derived spermatogenesis was before the appearance of undifferentiated spermatogonia. Interestingly, the testes transplanted with the F3 offspring germ cells from the DEHP-treated group, when regenerated, replicated testis morphology similar to that observed in the testes from the F1 to F3 offspring of the DEHP-treated group, suggesting that the germ cell disorganization phenotype originates from the stem cells of F3 offspring. In conclusion, embryonic exposure to DEHP was found to disrupt testicular germ cell organization and SSC function in a transgenerational manner.

Reconstitution of mouse spermatogonial stem cell niches in culture

Spermatogonial stem cells (SSCs) reside in specific niches within seminiferous tubules. These niches are thought to secrete chemotactic factors for SSCs, because SSCs migrate to them upon transplantation. However, the identity of these chemotactic molecules remains unknown. Here, we established a testis feeder cell culture system and used it to identify SSC chemotactic factors. When seeded on testis cells from infertile mice, SSCs migrated beneath the Sertoli cells and formed colonies with a cobblestone appearance that were very similar to those produced by hematopoietic stem cells. Cultured cells maintained SSC activity and fertility for at least 5 months. Cobblestone colony formation depended on GDNF and CXCL12, and dominant-negative GDNF receptor transfection or CXCL12 receptor deficiency reduced SSC colonization. Moreover, GDNF upregulated CXCL12 receptor expression, and CXCL12 transfection in Sertoli cells increased homing efficiency. Overall, our findings identify GDNF and CXCL12 as SSC chemotactic factors in vitro and in vivo.

Adhesion in the stem cell niche: biological roles and regulation

Stem cell self-renewal is tightly controlled by the concerted action of stem cell-intrinsic factors and signals within the niche. Niche signals often function within a short range, allowing cells in the niche to self-renew while their daughters outside the niche differentiate. Thus, in order for stem cells to continuously self-renew, they are often anchored in the niche via adhesion molecules. In addition to niche anchoring, however, recent studies have revealed other important roles for adhesion molecules in the regulation of stem cell function, and it is clear that stem cell-niche adhesion is crucial for stem cell self-renewal and is dynamically regulated. Here, we highlight recent progress in understanding adhesion between stem cells and their niche and how this adhesion is regulated.

Characterization and in vitro culture of putative spermatogonial stem cells derived from feline testicular tissue

Spermatogonial stem cells (SSCs) function to regulate the balance of self-renewal and differentiation of male gametes. SSCs have been successfully isolated and cultured in vitro in several species, but not in feline. Therefore, in this study, we aimed to culture and characterize feline SSCs. In experiment 1, testes (n=5) from different pubertal domestic cats were cryosectioned and fluorescently immunolabeled to examine the expression of SSC (GFRα-1), differentiated spermatogonium (c-kit) and germ cell (DDX-4) markers. In experiments 2 and 3, testicular cells were digested and subsequently cultured in vitro. The resultant presumptive SSC colonies were then collected for SSC identification (experiment 2), or further cultured in vitro on feeder cells (experiment 3). Morphology, gene expression and immunofluorescence were used to identify the SSCs. Experiment 1 demonstrated that varying types of spermatogenic cells existed and expressed different germ cell/SSC markers. A rare population of putative SSCs located at the basement membrane of the seminiferous tubules was specifically identified by co-expression of GFRα-1 and DDX-4. Following enzymatic digestion, grape-like colonies formed by 13-15 days of culture. These colonies expressed GFRA1 and ZBTB16, but did not express KIT. Although we successfully isolated and cultured feline SSCs in vitro, the SSCs could only be maintained for 57 days. In conclusion, this study demonstrates, for the first time, that putative SSCs from testes of pubertal domestic cats can be isolated and cultured in vitro. These cells exhibited SSC morphology and expressed SSC-specific genes. However, long-term culture of these putative SSCs was compromised.

CXCL12-CXCR4 signaling is required for the maintenance of mouse spermatogonial stem cells

Continual spermatogenesis relies on the activities of a tissue-specific stem cell population referred to as spermatogonial stem cells (SSCs). Fate decisions of stem cells are influenced by their niche environments, a major component of which is soluble factors secreted by support cells. At present, the factors that constitute the SSC niche are undefined. We explored the role of chemokine (C-X-C motif) ligand 12 (CXCL12) signaling via its receptor C-X-C chemokine receptor type 4 (CXCR4) in regulation of mouse SSC fate decisions. Immunofluorescent staining for CXCL12 protein in cross sections of testes from both pup and adult mice revealed its localization at the basement membrane of seminiferous tubules. Within the undifferentiated spermatogonial population of mouse testes, a fraction of cells were found to express CXCR4 and possess stem cell capacity. Inhibition of CXCR4 signaling in primary cultures of mouse undifferentiated spermatogonia resulted in SSC loss, in part by reducing proliferation and increasing the transition to a progenitor state primed for differentiation upon stimulation by retinoic acid. In addition, CXCL12-CXCR4 signaling in mouse SSCs was found to be important for colonization of recipient testes following transplantation, possibly by influencing homing to establish stem-cell niches. Furthermore, inhibition of CXCR4 signaling in testes of adult mice impaired SSC maintenance, leading to loss of the germline. Collectively, these findings indicate that CXCL12 is an important component of the growth factor milieu of stem cells in mammalian testes and that it signals via the CXCR4 to regulate maintenance of the SSC pool.