The in vivo response of stem and other undifferentiated spermatogonia to the reversible inhibition of glial cell line-derived neurotrophic factor signaling in the adult

Low nephron endowment increases susceptibility to renal stress and chronic kidney disease

Preterm birth results in low nephron endowment and increased risk of acute kidney injury (AKI) and chronic kidney disease (CKD). To understand the pathogenesis of AKI and CKD in preterm humans, we generated potentially novel mouse models with a 30%-70% reduction in nephron number by inhibiting or deleting Ret tyrosine kinase in the developing ureteric bud. These mice developed glomerular and tubular hypertrophy, followed by the transition to CKD, recapitulating the renal pathological changes seen in humans born preterm. We injected neonatal mice with gentamicin, a ubiquitous nephrotoxic exposure in preterm infants, and detected more severe proximal tubular injury in mice with low nephron number compared with controls with normal nephron number. Mice with low nephron number had reduced proliferative repair with more rapid development of CKD. Furthermore, mice had more profound inflammation with highly elevated levels of MCP-1 and CXCL10, produced in part by damaged proximal tubules. Our study directly links low nephron endowment with postnatal renal hypertrophy, which in this model is maladaptive and results in CKD. Underdeveloped kidneys are more susceptible to gentamicin-induced AKI, suggesting that AKI in the setting of low nephron number is more severe and further increases the risk of CKD in this vulnerable population.

Unraveling the effect of the inflammatory microenvironment in spermatogenesis progression

Experimental autoimmune orchitis (EAO) is a chronic inflammatory disorder that causes progressive spermatogenic impairment. EAO is characterized by high intratesticular levels of nitric oxide (NO) and tumor necrosis factor alpha (TNFα) causing germ cell apoptosis and Sertoli cell dysfunction. However, the impact of this inflammatory milieu on the spermatogenic wave is unknown. Therefore, we studied the effect of inflammation on spermatogonia and preleptotene spermatocyte cell cycle progression in an EAO context and through the intratesticular DETA-NO and TNFα injection in the normal rat testes. In EAO, premeiotic germ cell proliferation is limited as a consequence of the undifferentiated spermatogonia (CD9⁺) cell cycle arrest in G2/M and the reduced number of differentiated spermatogonia (c-kit⁺) and preleptotene spermatocytes that enter in the meiotic S-phase. Although inflammation disrupts spermatogenesis in EAO, it is maintained in some seminiferous tubules at XIV and VII-VIII stages of the epithelial cell cycle, thereby guaranteeing sperm production. We found that DETA-NO (2 mM) injected in normal testes arrests spermatogonia and preleptotene spermatocyte cell cycle; this effect reduces the number of proliferative spermatogonia and the number of preleptotene spermatocytes in meiosis S-phase (36 h after). The temporal inhibition of spermatogonia clonal amplification delayed progression of the spermatogenic wave (5 days after) finally altering spermatogenesis. TNFα (0.5 and 1 µg) exposure did not affect premeiotic germ cell cycle or spermatogenic wave. Our results show that in EAO the inflammatory microenvironment altered spermatogenesis kinetics through premeiotic germ cell cycle arrest and that NO is a sufficient factor contributing to this phenomenon.

The Regulation of Spermatogonial Stem Cells in an Adult Testis by Glial Cell Line-Derived Neurotrophic Factor

This review focuses on the in vivo regulation of spermatogonial stem cells (SSCs) in adult testes by glial cell line-derived neurotrophic factor (GDNF). To study adult mouse testes, we reversibly inhibited GDNF stimulation of SSCs via a chemical-genetic approach. This inhibition diminishes replication and increases differentiation of SSCs, and inhibition for 9 days reduces transplantable SSC numbers by 90%. With more sustained inhibition, all SSCs are lost, and testes eventually resemble human testes with Sertoli cell-only (SCO) syndrome. This resemblance prompted us to ask if GDNF expression is abnormally low in these infertile human testes. It is. Expression of FGF2 and FGF8 is also reduced, but some SCO testes contain SSCs. To evaluate the possible rebuilding of an SSC pool depleted due to inadequate GDNF signaling, we inhibited and then restored signaling to mouse SSCs. Partial rebuilding occurred, suggesting GDNF as therapy for men with SCO syndrome.

Sperm-carried IGF2 downregulated the expression of mitogens produced by Sertoli cells: A paracrine mechanism for regulating spermatogenesis?

INTRODUCTION

Insulin-like growth factor 2 (IGF2) mRNA has been found in human and mouse spermatozoa. It is currently unknown whether the IGF2 protein is expressed in human spermatozoa and, if so, its possible role in the cross-talk between germ and Sertoli cells (SCs) during spermatogenesis.

METHODS

To accomplish this, we analyzed sperm samples from four consecutive Caucasian men. Furthermore, to understand its role during the spermatogenetic process, porcine SCs were incubated with increasing concentrations (0.33, 3.33, and 10 ng/mL) of recombinant human IGF2 (rhIGF2) for 48 hours. Subsequently, the experiments were repeated by pre-incubating SCs with the non-competitive insulin-like growth factor 1 receptor (IGF1R) inhibitor NVP-AEW541. The following outcomes were evaluated: 1) Gene expression of the glial cell-line derived neurotrophic factor (GDNF), fibroblast growth factor 2 (FGF2), and stem cell factor (SCF) mitogens; 2) gene and protein expression of follicle-stimulating hormone receptor (FSHR), anti-Müllerian hormone (AMH), and inhibin B; 3) SC proliferation.

RESULTS

We found that the IGF2 protein was present in each of the sperm samples. IGF2 appeared as a cytoplasmic protein localized in the equatorial and post-acrosomal segment and with a varying degree of expression in each cell. In SCs, IGF2 significantly downregulated GDNF gene expression in a concentration-dependent manner. FGF2 and SCF were downregulated only by the highest concentration of IGF2. Similarly, IGF2 downregulated the FSHR gene and FSHR, AMH, and inhibin B protein expression. Finally, IGF2 significantly suppressed the SC proliferation rate. All these findings were reversed by pre-incubation with NVP-AEW541, suggesting an effect mediated by the interaction of IGF2 with the IGFR.

CONCLUSION

In conclusion, sperm IGF2 seems to downregulate the expression of mitogens, which are known to be physiologically released by the SCs to promote gonocyte proliferation and spermatogonial fate adoption. These findings suggest the presence of paracrine regulatory mechanisms acting on the seminiferous epithelium during spermatogenesis, by which germ cells can influence the amount of mitogens released by the SCs, their sensitivity to FSH, and their rate of proliferation.

Spermatogonial Stem Cell Numbers Are Reduced by Transient Inhibition of GDNF Signaling but Restored by Self-Renewing Replication when Signaling Resumes

One cause of human male infertility is a scarcity of spermatogonial stem cells (SSCs) in testes with Sertoli cells that neither produce adequate amounts of GDNF nor form the Sertoli-Sertoli junctions that form the blood-testis barrier (BTB). These patients raise the issue of whether a pool of SSCs, depleted due to inadequate GDNF stimulation, will expand if normal signaling is restored. Here, we reduce adult mouse SSC numbers by 90% using a chemical-genetic approach that reversibly inhibits GDNF signaling. Signal resumption causes all remaining SSCs to replicate immediately, but they primarily form differentiating progenitor spermatogonia. Subsequently, self-renewing replication restores SSC numbers. Testicular GDNF levels are not increased during restoration. However, SSC replication decreases as numbers of SSCs and progenitors increase, suggesting important regulatory interactions among these cells. Finally, sequential loss of SSCs and then pachytene spermatocytes causes dissolution of the BTB, thereby recapitulating another important characteristic of some infertile men.

Low retinoic acid levels mediate regionalization of the Sertoli valve in the terminal segment of mouse seminiferous tubules

In mammalian testes, undifferentiated spermatogonia (A_undiff) undergo differentiation in response to retinoic acid (RA), while their progenitor states are partially maintained by fibroblast growth factors (FGFs). Sertoli valve (SV) is a region located at the terminal end of seminiferous tubule (ST) adjacent to the rete testis (RT), where the high density of A_undiff is constitutively maintained with the absence of active spermatogenesis. However, the molecular and cellular characteristics of SV epithelia still remain unclear. In this study, we first identified the region-specific AKT phosphorylation in the SV Sertoli cells and demonstrated non-cell autonomous specialization of Sertoli cells in the SV region by performing a Sertoli cell ablation/replacement experiment. The expression of Fgf9 was detected in the RT epithelia, while the exogenous administration of FGF9 caused ectopic AKT phosphorylation in the Sertoli cells of convoluted ST. Furthermore, we revealed the SV region-specific expression of Cyp26a1, which encodes an RA-degrading enzyme, and demonstrated that the increased RA levels in the SV region disrupt its pool of A_undiff by inducing their differentiation. Taken together, RT-derived FGFs and low levels of RA signaling contribute to the non-cell-autonomous regionalization of the SV epithelia and its local maintenance of A_undiff in the SV region.

FSH regulates RA signaling to commit spermatogonia into differentiation pathway and meiosis

BACKGROUND

Spermatogenesis is a complex process that is controlled by interactions between germ cells and somatic cells. The commitment of undifferentiated spermatogonia to differentiating spermatogonia and normal spermatogenesis requires the action of gonadotropins. Additionally, numerous studies revealed the role of retinoic acid signaling in induction of germ cell differentiation and meiosis entry.

MAIN TEXT

Recent studies have shown that expression of several RA signaling molecules including Rdh10, Aldh1a2, Crabp1/2 are influenced by changes in gonadotropin levels. Components of signaling pathways that are regulated by FSH signaling such as GDNF, Sohlh1/2, c-Kit, DMRT, BMP4 and NRGs along with transcription factors that are important for proliferation and differentiation of spermatogonia are also affected by retinoic acid signaling.

CONCLUSION

According to all studies that demonstrate the interface between FSH and RA signaling, we suggest that RA may trigger spermatogonia differentiation and initiation of meiosis through regulation by FSH signaling in testis. Therefore, to the best of our knowledge, this is the first time that the correlation between FSH and RA signaling in spermatogenesis is highlighted.

CARF promotes spermatogonial self-renewal and proliferation through Wnt signaling pathway

Collaborator of ARF (CARF) regulates cell proliferative fate through both p53-dependent and -independent mechanisms. Recently, we reported a new function of CARF as a positive regulator of Wnt signaling. Despite these findings, the physiological function of CARF has not been well studied. Here, we generated CARF knockout mice and found that male CARF^-/- mice exhibited significantly impaired fertility and Sertoli-cell-only (SCO) syndrome phenotypes. Further studies revealed that loss of CARF in Sertoli cells led to decreased GDNF expression, which hindered spermatogonial stem cells (SSCs) self-renewal. Meanwhile, CARF loss in undifferentiated spermatogonia impaired their proliferation. These two mechanisms together led to SCO syndrome phenotypes, which could be functionally rescued by pharmacological or genetic reactivation of Wnt signaling. Finally, we identified CARF^S351F as a potential pathogenic mutation in an SCO patient. Overall, our findings reveal important roles of CARF in spermatogonial self-renewal and proliferation through the Wnt signaling pathway.

CDK2 kinase activity is a regulator of male germ cell fate

The ability of men to remain fertile throughout their lives depends upon establishment of a spermatogonial stem cell (SSC) pool from gonocyte progenitors, and thereafter balancing SSC renewal versus terminal differentiation. Here, we report that precise regulation of the cell cycle is crucial for this balance. Whereas cyclin-dependent kinase 2 (Cdk2) is not necessary for mouse viability or gametogenesis stages prior to meiotic prophase I, mice bearing a deregulated allele (Cdk2Y15S ) are severely deficient in spermatogonial differentiation. This allele disrupts an inhibitory phosphorylation site (Tyr15) for the kinase WEE1. Remarkably, Cdk2Y15S/Y15S mice possess abnormal clusters of mitotically active SSC-like cells, but these are eventually removed by apoptosis after failing to differentiate properly. Analyses of lineage markers, germ cell proliferation over time, and single cell RNA-seq data revealed delayed and defective differentiation of gonocytes into SSCs. Biochemical and genetic data demonstrated that Cdk2Y15S is a gain-of-function allele causing elevated kinase activity, which underlies these differentiation defects. Our results demonstrate that precise regulation of CDK2 kinase activity in male germ cell development is crucial for the gonocyte-to-spermatogonia transition and long-term spermatogenic homeostasis.

The production of glial cell line-derived neurotrophic factor by human sertoli cells is substantially reduced in sertoli cell-only testes

STUDY QUESTION

Do human Sertoli cells in testes that exhibit the Sertoli cell-only (SCO) phenotype produce substantially less glial cell line-derived neurotrophic factor (GDNF) than Sertoli cells in normal testes?

SUMMARY ANSWER

In human SCO testes, both the amounts of GDNF mRNA per testis and the concentration of GDNF protein per Sertoli cell are markedly reduced as compared to normal testes.

WHAT IS KNOWN ALREADY

In vivo, GDNF is required to sustain the numbers and function of mouse spermatogonial stem cells (SSCs) and their immediate progeny, transit-amplifying progenitor spermatogonia. GDNF is expressed in the human testis, and the ligand-binding domain of the GDNF receptor, GFRA1, has been detected on human SSCs. The numbers and/or function of these stem cells are markedly reduced in some infertile men, resulting in the SCO histological phenotype.

STUDY DESIGN, SIZE, AND DURATION

We determined the numbers of human spermatogonia per mm2 of seminiferous tubule surface that express GFRA1 and/or UCHL1, another marker of human SSCs. We measured GFRA1 mRNA expression in order to document the reduced numbers and/or function of SSCs in SCO testes. We quantified GDNF mRNA in testes of humans and mice, a species with GDNF-dependent SSCs. We also compared GDNF mRNA expression in human testes with normal spermatogenesis to that in testes exhibiting the SCO phenotype. As controls, we also measured transcripts encoding two other Sertoli cell products, kit ligand (KITL) and clusterin (CLU). Finally, we compared the amounts of GDNF per Sertoli cell in normal and SCO testes.

PARTICIPANTS/MATERIALS SETTING METHODS

Normal human testes were obtained from beating heart organ donors. Biopsies of testes from men who were infertile due to maturation arrest or the SCO phenotype were obtained as part of standard care during micro-testicular surgical sperm extraction. Cells expressing GFRA1, UCHL1 or both on whole mounts of normal human seminiferous tubules were identified by immunohistochemistry and confocal microscopy and their numbers were determined by image analysis. Human GDNF mRNA and GFRA1 mRNA were quantified by use of digital PCR and Taqman primers. Transcripts encoding mouse GDNF and human KITL, CLU and 18 S rRNA, used for normalization of data, were quantified by use of real-time PCR and Taqman primers. Finally, we used two independent methods, flow cytometric analysis of single cells and ELISA assays of homogenates of whole testis biopsies, to compare amounts of GDNF per Sertoli cell in normal and SCO testes.

MAIN RESULTS AND THE ROLE OF CHANCE

Normal human testes contain a large population of SSCs that express GFRA1, the ligand-binding domain of the GDNF receptor. In human SCO testes, GFRA1 mRNA was detected but at markedly reduced levels. Expression of GDNF mRNA and the amount of GDNF protein per Sertoli cell were also significantly reduced in SCO testes. These results were observed in multiple, independent samples, and the reduced amount of GDNF in Sertoli cells of SCO testes was demonstrated using two different analytical approaches.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

There currently are no approved protocols for the in vivo manipulation of human testis GDNF concentrations. Thus, while our data suggest that insufficient GDNF may be the proximal cause of some cases of human male infertility, our results are correlative in nature.

WIDER IMPLICATIONS OF THE FINDINGS

We propose that insufficient GDNF expression may contribute to the infertility of some men with an SCO testicular phenotype. If their testes contain some SSCs, an approach that increases their testicular GDNF concentrations might expand stem cell numbers and possibly sperm production.

STUDY FUNDING/COMPETING INTEREST(S)

This research was funded by the Eunice Kennedy Shriver National Institute for Child Health and Human Development, National Centers for Translational Research in Reproduction and Infertility Program (NCTRI) Grant 1R01HD074542-04, as well as grants R01 HD076412-02 and P01 HD075795-02 and the U.S.-Israel Binational Science Foundation. Support for this research was also provided by NIH P50 HD076210, the Robert Dow Foundation, the Frederick & Theresa Dow Wallace Fund of the New York Community Trust and the Brady Urological Foundation. There are no competing interests.

Temporal-Spatial Establishment of Initial Niche for the Primary Spermatogonial Stem Cell Formation Is Determined by an ARID4B Regulatory Network

During neonatal testis development, centrally located gonocytes migrate to basement membrane of the seminiferous cords, where physical contact with a niche established by Sertoli cells is essential for transition of gonocytes into spermatogonial stem cells (SSCs). To provide structural support and signaling stimuli for the gonocyte-to-SSC transition that occurs at a specific location during a finite phase, temporal-spatial establishment of the niche is critical. To date, the factors that guide Sertoli cells to establish the initial stem cell niche remain largely unknown. Using the Sertoli cell-specific Arid4b knockout (Arid4bSCKO) mice, we demonstrated that ablation of AT-rich interaction domain 4B (ARID4B) resulted in abnormal detachment of Sertoli cells from the basement membrane of seminiferous cords during the gonocyte-to-SSC transition phase, suggesting failure to establish a niche for the SSC formation. Without support by a niche environment, gonocytes showed disarranged cell distribution in the Arid4bSCKO testes and underwent apoptosis. The commitment of gonocytes to differentiate into the spermatogonial lineage was broken and the capability of SSCs to self-renew and differentiate was also impaired. Gene expression profiling revealed the molecular mechanisms responsible for the phenotypic changes in the Arid4bSCKO testes, by identifying genes important for stem cell niche function as downstream effectors of ARID4B, including genes that encode gap junction protein alpha-1, KIT ligand, anti-Müllerian hormone, Glial cell-line derived neurotrophic factor, inhibin alpha, inhibin beta, and cytochrome P450 family 26 subfamily b polypeptide 1. Our results identified ARID4B as a master regulator of a signaling network that governs the establishment of a niche during the critical gonocyte-to-SSC transition phase to control the fate of gonocytes and SSCs. Stem Cells 2017;35:1554-1565.

TSPAN8 Expression Distinguishes Spermatogonial Stem Cells in the Prepubertal Mouse Testis

Precise separation of spermatogonial stem cells (SSCs) from progenitor spermatogonia that lack stem cell activity and are committed to differentiation remains a challenge. To distinguish between these spermatogonial subtypes, we identified genes that exhibited bimodal mRNA levels at the single-cell level among undifferentiated spermatogonia from Postnatal Day 6 mouse testes, including Tspan8, Epha2, and Pvr, each of which encode cell surface proteins useful for cell selection. Transplantation studies provided definitive evidence that a TSPAN8-high subpopulation is enriched for SSCs. RNA-seq analyses identified genes differentially expressed between TSPAN8-high and -low subpopulations that clustered into multiple biological pathways potentially involved in SSC renewal or differentiation, respectively. Methyl-seq analysis identified hypomethylated domains in the promoters of these genes in both subpopulations that colocalized with peaks of histone modifications defined by ChIP-seq analysis. Taken together, these results demonstrate functional heterogeneity among mouse undifferentiated spermatogonia and point to key biological characteristics that distinguish SSCs from progenitor spermatogonia.

In vivo dynamics of GFRα1-positive spermatogonia stimulated by GDNF signals using a bead transplantation assay

In mouse testes, spermatogonial stem cells (SSCs), a subpopulation of GFRα1 (GDNF family receptor-α1)-positive spermatogonia, are widely distributed along the convoluted seminiferous tubules. The proliferation and differentiation of the SSCs are regulated in part by local expression of GDNF (glial cell-derived neurotorphic factor), one of major niche factors for SSCs. However, the in vivo dynamics of the GDNF-stimulated GFRα1-positive spermatogonia remains unclear. Here, we developed a simple method for transplanting DiI-labeled and GDNF-soaked beads into the mouse testicular interstitium. By using this method, we examined the dynamics of GFRα1-positive spermatogonia in the tubular walls close to the transplanted GDNF-soaked beads. The bead-derived GDNF signals were able to induce the stratified aggregate formation of GFRα1-positive undifferentiated spermatogonia by day 3 post-transplantation. Each aggregate consisted of tightly compacted Asingle and marginal Apaired-Aaligned GFRα1-positive spermatogonia and was surrounded by Aaligned GFRα1-negative spermatogonia at more advanced stages. These data not only provide in vivo evidence for the inductive roles of GDNF in forming a rapid aggregation of GFRα1-positive spermatogonia but also indicate the usefulness of this in vivo assay system of various growth factors for the stem/progenitor spermatogonia in mammalian spermatogenesis.

Spermatogenesis: The Commitment to Meiosis

Mammalian spermatogenesis requires a stem cell pool, a period of amplification of cell numbers, the completion of reduction division to haploid cells (meiosis), and the morphological transformation of the haploid cells into spermatozoa (spermiogenesis). The net result of these processes is the production of massive numbers of spermatozoa over the reproductive lifetime of the animal. One study that utilized homogenization-resistant spermatids as the standard determined that human daily sperm production (dsp) was at 45 million per day per testis (60). For each human that means ∼1,000 sperm are produced per second. A key to this level of gamete production is the organization and architecture of the mammalian testes that results in continuous sperm production. The seemingly complex repetitious relationship of cells termed the "cycle of the seminiferous epithelium" is driven by the continuous commitment of undifferentiated spermatogonia to meiosis and the period of time required to form spermatozoa. This commitment termed the A to A1 transition requires the action of retinoic acid (RA) on the undifferentiated spermatogonia or prospermatogonia. In stages VII to IX of the cycle of the seminiferous epithelium, Sertoli cells and germ cells are influenced by pulses of RA. These pulses of RA move along the seminiferous tubules coincident with the spermatogenic wave, presumably undergoing constant synthesis and degradation. The RA pulse then serves as a trigger to commit undifferentiated progenitor cells to the rigidly timed pathway into meiosis and spermatid differentiation.

Mechanisms Regulating Spermatogonial Differentiation

Mammalian spermatogenesis is a complex and highly ordered process by which male germ cells proceed through a series of differentiation steps to produce haploid flagellated spermatozoa. Underlying this process is a pool of adult stem cells, the spermatogonial stem cells (SSCs), which commence the spermatogenic lineage by undertaking a differentiation fate decision to become progenitor spermatogonia. Subsequently, progenitors acquire a differentiating spermatogonia phenotype and undergo a series of amplifying mitoses while becoming competent to enter meiosis. After spermatocytes complete meiosis, post-meiotic spermatids must then undergo a remarkable transformation from small round spermatids to a flagellated spermatozoa with extremely compacted nuclei. This chapter reviews the current literature pertaining to spermatogonial differentiation with an emphasis on the mechanisms controlling stem cell fate decisions and early differentiation events in the life of a spermatogonium.

Fsh Stimulates Spermatogonial Proliferation and Differentiation in Zebrafish via Igf3

Growth factors modulate germ line stem cell self-renewal and differentiation behavior. We investigate the effects of Igf3, a fish-specific member of the igf family. Fsh increased in a steroid-independent manner the number and mitotic index of single type A undifferentiated spermatogonia and of clones of type A differentiating spermatogonia in adult zebrafish testis. All 4 igf gene family members in zebrafish are expressed in the testis but in tissue culture only igf3 transcript levels increased in response to recombinant zebrafish Fsh. This occurred in a cAMP/protein kinase A-dependent manner, in line with the results of studies on the igf3 gene promoter. Igf3 protein was detected in Sertoli cells. Recombinant zebrafish Igf3 increased the mitotic index of type A undifferentiated and type A differentiating spermatogonia and up-regulated the expression of genes related to spermatogonial differentiation and entry into meiosis, but Igf3 did not modulate testicular androgen release. An Igf receptor inhibitor blocked these effects of Igf3. Importantly, the Igf receptor inhibitor also blocked Fsh-induced spermatogonial proliferation. We conclude that Fsh stimulated Sertoli cell production of Igf3, which promoted via Igf receptor signaling spermatogonial proliferation and differentiation and their entry into meiosis. Because previous work showed that Fsh also released spermatogonia from an inhibitory signal by down-regulating anti-Müllerian hormone and by stimulating androgen production, we can now present a model, in which Fsh orchestrates the activity of stimulatory (Igf3, androgens) and inhibitory (anti-Müllerian hormone) signals to promote spermatogenesis.

The logic and design of analog-sensitive kinases and their small molecule inhibitors

Analog-sensitive AS Kinase technology allows for rapid, reversible, and highly specific inhibition of individual engineered kinases in cells and in mouse models of human diseases. The technique consists of two parts: a kinase containing a space-creating mutation in the ATP-binding pocket and a bulky ATP-competitive small molecule inhibitor that complements the shape of the mutant ATP pocket. This strategy enables dissection of phospho-signaling pathways, elucidation of the physiological function of individual kinases, and characterization of the pharmacology of clinical-kinase inhibitors. Here, we present an overview of AS technology and describe a stepwise approach for generating AS Kinase mutants and identifying appropriate small molecule inhibitors. We also describe commonly encountered technical obstacles and provide strategies to overcome them.

Peritubular myoid cells participate in male mouse spermatogonial stem cell maintenance

Peritubular myoid (PM) cells surround the seminiferous tubule and together with Sertoli cells form the cellular boundary of the spermatogonial stem cell (SSC) niche. However, it remains unclear what role PM cells have in determining the microenvironment in the niche required for maintenance of the ability of SSCs to undergo self-renewal and differentiation into spermatogonia. Mice with a targeted disruption of the androgen receptor gene (Ar) in PM cells experienced a progressive loss of spermatogonia, suggesting that PM cells require testosterone (T) action to produce factors influencing SSC maintenance in the niche. Other studies showed that glial cell line-derived neurotrophic factor (GDNF) is required for SSC self-renewal and differentiation of SSCs in vitro and in vivo. This led us to hypothesize that T-regulated GDNF expression by PM cells contributes to the maintenance of SSCs. This hypothesis was tested using an adult mouse PM cell primary culture system and germ cell transplantation. We found that T induced GDNF expression at the mRNA and protein levels in PM cells. Furthermore, when thymus cell antigen 1-positive spermatogonia isolated from neonatal mice were cocultured with PM cells with or without T and transplanted to the testes of germ cell-depleted mice, the number and length of transplant-derived colonies was increased considerably by in vitro T treatment. These results support the novel hypothesis that T-dependent regulation of GDNF expression in PM cells has a significant influence on the microenvironment of the niche and SSC maintenance.

Responses to glial cell line-derived neurotrophic factor change in mice as spermatogonial stem cells form progenitor spermatogonia which replicate and give rise to more differentiated progeny

Spermatogonial stem cells (SSCs) are the foundation of spermatogenesis. These cells are classically defined as a subset of morphologically defined A single (As) spermatogonia, which can produce more SSCs or they can give rise to nonstem As cells that, upon replication, generate A paired (Apr) and then A aligned (Aal) spermatogonia. These latter two cell types, along with the nonstem As cells, function as transit-amplifying progenitor cells. It is known that glial cell line-derived neurotrophic factor (GDNF) is essential for maintaining all of these cells, but it is unknown if or how the responses of these cells change as they progress down the pathway to differentiated type A1 spermatogonia. We address this issue by using a chemical-genetic approach to inhibit GDNF signaling in vivo and an in vitro approach to increase GDNF stimulation. We show that inhibition for 2 days suppresses replication of As, Apr, and Aal spermatogonia to an equal extent, whereas stimulation by GDNF preferentially increases replication of As and Apr spermatogonia. We also test if inhibiting GDNF signaling causes As, Apr, and Aal spermatogonia to express Kit, an essential step in their differentiation into type A1 spermatogonia. Inhibition for 3 or 7 days produces a progressive increase in the percentages of As, Apr, and Aal undergoing differentiation, with the largest increase observed in Aal spermatogonia. Finally, we demonstrate that numbers of SSCs decrease more slowly than numbers of progenitor spermatogonia when GDNF signaling is inhibited. Taken together, these data suggest that there are significant changes in the responses to GDNF as SSCs give rise to progenitor spermatogonia, which replicate and gradually differentiate into type A1 spermatogonia.

Adipocyte ALK7 links nutrient overload to catecholamine resistance in obesity

Obesity is associated with blunted β-adrenoreceptor (β-AR)-mediated lipolysis and lipid oxidation in adipose tissue, but the mechanisms linking nutrient overload to catecholamine resistance are poorly understood. We report that targeted disruption of TGF-β superfamily receptor ALK7 alleviates diet-induced catecholamine resistance in adipose tissue, thereby reducing obesity in mice. Global and fat-specific Alk7 knock-out enhanced adipose β-AR expression, β-adrenergic signaling, mitochondrial biogenesis, lipid oxidation, and lipolysis under a high fat diet, leading to elevated energy expenditure, decreased fat mass, and resistance to diet-induced obesity. Conversely, activation of ALK7 reduced β-AR-mediated signaling and lipolysis cell-autonomously in both mouse and human adipocytes. Acute inhibition of ALK7 in adult mice by a chemical-genetic approach reduced diet-induced weight gain, fat accumulation, and adipocyte size, and enhanced adipocyte lipolysis and β-adrenergic signaling. We propose that ALK7 signaling contributes to diet-induced catecholamine resistance in adipose tissue, and suggest that ALK7 inhibitors may have therapeutic value in human obesity.

DOI:http://dx.doi.org/10.7554/eLife.03245.001

Adrenaline and noradrenaline are two hormones that trigger the burst of energy and increase in heart rate and blood pressure that are needed for the ‘fight-or-flight’ response. Both belong to a group of chemicals called catecholamines. These chemicals bind to cells carrying proteins called adrenoceptors on their surface and stimulate the breakdown of fat, which releases energy. However, when nutrients are plentiful, fat cells become resistant to catecholamines and instead store fat so it can be used for energy if food becomes scarce. In the industrialized world where food is easily and constantly accessible, this resistance can cause an unhealthy increase in body fat and result in obesity.

Increasing fat metabolism by making fat cells more able to respond to catecholamines is an attractive strategy for combating obesity. Indeed, drugs that mimic the effect of catecholamines on an adrenoceptor found in mice reduce obesity caused by over-eating. However, these drugs are ineffective in humans and can cause harmful side effects to the cardiovascular system, including high blood pressure and an increased heart rate. Devising a strategy that specifically targets catecholamine resistance in fat cells is therefore desirable.

A protein called ALK7 is a cell surface receptor that is predominantly found in fat cells and tissues involved in controlling the metabolism. Mice with a mutation in ALK7 that stops this protein from working properly accumulate less fat than mice with a functional version of the protein, but it is not known why. To understand ALK7's involvement in fat metabolism, Guo et al. created mice whose fat cells lack ALK7, but whose other cells all produce ALK7 as normal. When fed a diet rich in fat, these mice are leaner than regular mice and they burn more energy.

The metabolic responses seen in ALK7 mutant mice are very similar to those seen in mice treated with drugs targeting adrenoceptors, suggesting that there may be a link between ALK7 and catecholamine resistance. Indeed, Guo et al. demonstrate that fat cells lacking ALK7 have an increased sensitivity to catecholamines when the mice are on a high fat diet, which decreases the amount of fat the mice accumulate. Conversely, increasing the activity of ALK7 reduces the ability of the cells to respond to catecholamines, and they accumulate more fat.

Guo et al. also generated a second line of mice carrying a mutation in ALK7 that does not affect its function, but renders it sensitive to inhibition by a custom-made chemical. When these animals were on a high-fat diet, administering the chemical made the mice leaner, suggesting that inhibiting the ALK7 receptor can prevent obesity in adult animals.

Guo et al. also performed experiments in human fat cells, which showed that the ALK7 receptor works in a similar way in human cells as it does in mice. As ALK7 is largely specific for fat cells and is not known to affect the cardiovascular system, drugs that inhibit ALK7 could potentially safely suppress catecholamine resistance and reduce human obesity.

DOI:http://dx.doi.org/10.7554/eLife.03245.002

Structure-guided inhibitor design expands the scope of analog-sensitive kinase technology

Engineered analog-sensitive (AS) protein kinases have emerged as powerful tools for dissecting phospho-signaling pathways, for elucidating the cellular function of individual kinases, and for deciphering unanticipated effects of clinical therapeutics. A crucial and necessary feature of this technology is a bioorthogonal small molecule that is innocuous toward native cellular systems but potently inhibits the engineered kinase. In order to generalize this method, we sought a molecule capable of targeting divergent AS-kinases. Here we employ X-ray crystallography and medicinal chemistry to unravel the mechanism of current inhibitors and use these insights to design the most potent, selective, and general AS-kinase inhibitors reported to date. We use large-scale kinase inhibitor profiling to characterize the selectivity of these molecules as well as examine the consequences of potential off-target effects in chemical genetic experiments. The molecules reported here will serve as powerful tools in efforts to extend AS-kinase technology to the entire kinome and the principles discovered may help in the design of other engineered enzyme/ligand pairs.

Age-dependent germline mosaicism of the most common noonan syndrome mutation shows the signature of germline selection

Noonan syndrome (NS) is among the most common Mendelian genetic diseases (∼1/2,000 live births). Most cases (50%-84%) are sporadic, and new mutations are virtually always paternally derived. More than 47 different sites of NS de novo missense mutations are known in the PTPN11 gene that codes for the protein tyrosine phosphatase SHP-2. Surprisingly, many of these mutations are recurrent with nucleotide substitution rates substantially greater than the genome average; the most common mutation, c.922A>G, is at least 2,400 times greater. We examined the spatial distribution of the c.922A>G mutation in testes from 15 unaffected men and found that the mutations were not uniformly distributed across each testis as would be expected for a mutation hot spot but were highly clustered and showed an age-dependent germline mosaicism. Computational modeling that used different stem cell division schemes confirmed that the data were inconsistent with hypermutation, but consistent with germline selection: mutated spermatogonial stem cells gained an advantage that allowed them to increase in frequency. SHP-2 interacts with the transcriptional activator STAT3. Given STAT3's function in mouse spermatogonial stem cells, we suggest that this interaction might explain the mutant's selective advantage by means of repression of stem cell differentiation signals. Repression of STAT3 activity by cyclin D1 might also play a previously unrecognized role in providing a germline-selective advantage to spermatogonia for the recurrent mutations in the receptor tyrosine kinases that cause Apert syndrome and MEN2B. Looking at recurrent mutations driven by germline selection in different gene families can help highlight common causal signaling pathways.

Genome-wide identification of Sox8-, and Sox9-dependent genes during early post-natal testis development in the mouse

The SOX8 and SOX9 transcription factors are involved in, among others, sex differentiation, male gonad development and adult maintenance of spermatogenesis. Sox8(-/-) mice lacking Sox9 in Sertoli cells fail to form testis cords and cannot establish spermatogenesis. Although genetic and histological data show an important role for these transcription factors in regulating spermatogenesis, it is not clear which genes depend upon them at a genome-wide level. To identify transcripts that respond to the absence of Sox8 in all cells and Sox9 in Sertoli cells we measured mRNA concentrations in testicular samples from mice at 0, 6 and 18 days post-partum. In total, 621 and 629 transcripts were found at decreased or increased levels, respectively, at different time points in the mutant as compared to the control samples. These mRNAs were categorized as preferentially expressed in Sertoli cells or germ cells using data obtained with male and female gonad samples and enriched testicular cell populations. Five candidate genes were validated at the protein level. Furthermore, we identified putative direct SOX8 and SOX9 target genes by integrating predicted SOX-binding sites present in potential regulatory regions upstream of the transcription start site. Finally, we used protein network data to gain insight into the effects on regulatory interactions that occur when Sox8 and Sox9 are absent in developing Sertoli cells. The integration of testicular samples with enriched Sertoli cells, germ cells and female gonads enabled us to broadly distinguish transcripts directly affected in Sertoli cells from others that respond to secondary events in testicular cell types. Thus, combined RNA profiling signals, motif predictions and network data identified putative SOX8/SOX9 target genes in Sertoli cells and yielded insight into regulatory interactions that depend upon these transcription factors. In addition, our results will facilitate the interpretation of genome-wide in vivo SOX8 and SOX9 DNA binding data.