Point mutation in kit receptor tyrosine kinase reveals essential roles for kit signaling in spermatogenesis and oogenesis without affecting other kit responses

Foxo1 is required in mouse spermatogonial stem cells for their maintenance and the initiation of spermatogenesis

Spermatogonial stem cells (SSCs) capable of self-renewal and differentiation are the foundation for spermatogenesis. Although several factors important for these processes have been identified, the fundamental mechanisms regulating SSC self-renewal and differentiation remain unknown. Here, we investigated a role for the Foxo transcription factors in mouse spermatogenesis and found that Foxo1 specifically marks mouse gonocytes and a subset of spermatogonia with stem cell potential. Genetic analyses showed that Foxo1 was required for both SSC homeostasis and the initiation of spermatogenesis. Combined deficiency of Foxo1, Foxo3, and Foxo4 resulted in a severe impairment of SSC self-renewal and a complete block of differentiation, indicating that Foxo3 and Foxo4, although dispensable for male fertility, contribute to SSC function. By conditional inactivation of 3-phosphoinositide-dependent protein kinase 1 (Pdk1) and phosphatase and tensin homolog (Pten) in the male germ line, we found that PI3K signaling regulates Foxo1 stability and subcellular localization, revealing that the Foxos are pivotal effectors of PI3K-Akt signaling in SSCs. We also identified a network of Foxo gene targets--most notably Ret--that rationalized the maintenance of SSCs by the Foxos. These studies demonstrate that Foxo1 expression in the spermatogenic lineage is intimately associated with the stem cell state and revealed what we believe to be novel Foxo-dependent mechanisms underlying SSC self-renewal and differentiation, with implications for common diseases, including male infertility and testicular cancer, due to abnormalities in SSC function.

C-KIT signaling depends on microphthalmia-associated transcription factor for effects on cell proliferation

The development of melanocytes is regulated by the tyrosine kinase receptor c-KIT and the basic-helix-loop-helix-leucine zipper transcription factor Mitf. These essential melanocyte survival regulators are also well known oncogenic factors in malignant melanoma. Despite their importance, not much is known about the regulatory mechanisms and signaling pathways involved. In this study, we therefore sought to identify the signaling pathways and mechanisms involved in c-KIT mediated regulation of Mitf. We report that c-KIT stimulation leads to the activation of Mitf specifically through the c-KIT phosphorylation sites Y721 (PI3 kinase binding site), Y568 and Y570 (Src binding site). Our study not only confirms the involvement of Ras-Erk signaling pathway in the activation of Mitf, but also establishes that Src kinase binding to Y568 and Y570 of c-KIT is required. Using specific inhibitors we observe and verify that c-KIT induced activation of Mitf is dependent on PI3-, Akt-, Src-, p38- or Mek kinases. Moreover, the proliferative effect of c-KIT is dependent on Mitf in HEK293T cells. In contrast, c-KIT Y568F and Y721F mutants are less effective in driving cell proliferation, compared to wild type c-KIT. Our results reveal novel mechanisms by which c-KIT signaling regulates Mitf, with implications for understanding both melanocyte development and melanoma.

Spermatogonial stem cells, infertility and testicular cancer

The spermatogonial stem cells (SSCs) are responsible for the transmission of genetic information from an individual to the next generation. SSCs play critical roles in understanding the basic reproductive biology of gametes and treatments of human infertility. SSCs not only maintain normal spermatogenesis, but also sustain fertility by critically balancing both SSC self-renewal and differentiation. This self-renewal and differentiation in turn is tightly regulated by a combination of intrinsic gene expression within the SSC as well as the extrinsic gene signals from the niche. Increased SSCs self-renewal at the expense of differentiation result in germ cell tumours, on the other hand, higher differentiation at the expense of self-renewal can result in male sterility. Testicular germ cell cancers are the most frequent cancers among young men in industrialized countries. However, understanding the pathogenesis of testis cancer has been difficult because it is formed during foetal development. Recent studies suggest that SSCs can be reprogrammed to become embryonic stem (ES)-like cells to acquire pluripotency. In the present review, we summarize the recent developments in SSCs biology and role of SSC in testicular cancer. We believe that studying the biology of SSCs will not only provide better understanding of stem cell regulation in the testis, but eventually will also be a novel target for male infertility and testicular cancers.

RanBPM is essential for mouse spermatogenesis and oogenesis

RanBPM is a recently identified scaffold protein that links and modulates interactions between cell surface receptors and their intracellular signaling pathways. RanBPM has been shown to interact with a variety of functionally unrelated proteins; however, its function remains unclear. Here, we show that RanBPM is essential for normal gonad development as both male and female RanBPM(-/-) mice are sterile. In the mutant testis there was a marked decrease in spermatogonia proliferation during postnatal development. Strikingly, the first wave of spermatogenesis was totally compromised, as seminiferous tubules of homozygous mutant animals were devoid of post-meiotic germ cells. We determined that spermatogenesis was arrested around the late pachytene-diplotene stages of prophase I; surprisingly, without any obvious defect in chromosome synapsis. Interestingly, RanBPM deletion led to a remarkably quick disappearance of all germ cell types at around one month of age, suggesting that spermatogonia stem cells are also affected by the mutation. Moreover, in chimeric mice generated with RanBPM(-/-) embryonic stem cells all mutant germ cells disappeared by 3 weeks of age suggesting that RanBPM is acting in a cell-autonomous way in germ cells. RanBPM homozygous mutant females displayed a premature ovarian failure due to a depletion of the germ cell pool at the end of prophase I, as in males. Taken together, our results highlight a crucial role for RanBPM in mammalian gametogenesis in both genders.

The Leydig cell MEK/ERK pathway is critical for maintaining a functional population of adult Leydig cells and for fertility

MAPK kinase (MEK)1 and MEK2 were deleted from Leydig cells by crossing Mek1(f/f);Mek2(-/-) and Cyp17iCre mice. Primary cultures of Leydig cell from mice of the appropriate genotype (Mek1(f/f);Mek2(-/-);iCre(+)) show decreased, but still detectable, MEK1 expression and decreased or absent ERK1/2 phosphorylation when stimulated with epidermal growth factor, Kit ligand, cAMP, or human choriogonadotropin (hCG). The body or testicular weights of Mek1(f/f);Mek2(-/-);iCre(+) mice are not significantly affected, but the testis have fewer Leydig cells. The Leydig cell hypoplasia is paralleled by decreased testicular expression of several Leydig cell markers, such as the lutropin receptor, steroidogenic acute regulatory protein, cholesterol side chain cleavage enzyme, 17α-hydroxylase, and estrogen sulfotransferase. The expression of Sertoli or germ cell markers, as well as the shape, size, and cellular composition of the seminiferous tubules, are not affected. cAMP accumulation in response to hCG stimulation in primary cultures of Leydig cells from Mek1(f/f);Mek2(-/-);iCre(+) mice is normal, but basal testosterone and testosterone syntheses provoked by addition of hCG or a cAMP analog, or by addition of substrates such as 22-hydroxycholesterol or pregnenolone, are barely detectable. The Mek1(f/f);Mek2(-/-);iCre(+) males show decreased intratesticular testosterone and display several signs of hypoandrogenemia, such as elevated serum LH, decreased expression of two renal androgen-responsive genes, and decreased seminal vesicle weight. Also, in spite of normal sperm number and motility, the Mek1(f/f);Mek2(-/-);iCre(+) mice show reduced fertility. These studies show that deletion of MEK1/2 in Leydig cells results in Leydig cell hypoplasia, hypoandrogenemia, and reduced fertility.

Reinterpretation of evidence advanced for neo-oogenesis in mammals, in terms of a finite oocyte reserve

The central tenet of ovarian biology, that the oocyte reserve in adult female mammals is finite, has been challenged over recent years by proponents of neo-oogenesis, who claim that germline stem cells exist in the ovarian surface epithelium or the bone marrow. Currently opinion is divided over these claims, and further scrutiny of the evidence advanced in support of the neo-oogenesis hypothesis is warranted - especially in view of the enormous implications for female fertility and health. This article contributes arguments against the hypothesis, providing alternative explanations for key observations, based on published data. Specifically, DNA synthesis in germ cells in the postnatal mouse ovary is attributed to mitochondrial genome replication, and to DNA repair in oocytes lagging in meiotic progression. Lines purported to consist of germline stem cells are identified as ovarian epithelium or as oogonia, from which cultures have been derived previously. Effects of ovotoxic treatments are found to negate claims for the existence of germline stem cells. And arguments are presented for the misidentification of ovarian somatic cells as de novo oocytes. These clarifications, if correct, undermine the concept that germline stem cells supplement the oocyte quota in the postnatal ovary; and instead comply with the theory of a fixed, unregenerated reserve. It is proposed that acceptance of the neo-oogenesis hypothesis is erroneous, and may effectively impede research in areas of ovarian biology. To illustrate, a novel explanation that is consistent with orthodox theory is provided for the observed restoration of fertility in chemotherapy-treated female mice following bone marrow transplantation, otherwise interpreted by proponents of neo-oogenesis as involving stimulation of endogenous germline stem cells. Instead, it is proposed that the chemotherapeutic regimens induce autoimmunity to ovarian antigens, and that the haematopoietic chimaerism produced by bone marrow transplantation circumvents activation of an autoreactive response, thereby rescuing ovarian function. The suggested mechanism draws from animal models of autoimmune ovarian disease, which implicate dysregulation of T cell regulatory function; and from a surmised role for follicular apoptosis in the provision of ovarian autoantigens, to sustain self-tolerance during homeostasis. This interpretation has direct implications for fertility preservation in women undergoing chemotherapy.

Follicular somatic cell factors and follicle development

Considerable attention is currently paid to oocyte-derived secreted factors that act upon cumulus and granulosa cells. Also important for follicle development are somatic cell-derived secreted factors. This is illustrated by the ability of granulosa cell-derived Kit ligand (KITL) to promote primordial follicle activation, and the loss of follicle development that accompanies KITL gene disruption. This review summarises our current understanding of somatic cell factors during both preantral and antral follicle growth, involving not only signalling from granulosa cells to the oocyte, but also signalling between granulosa and theca cells. Principal granulosa cell-derived factors include activin, anti-Müllerian hormone (AMH), bone morphogenetic proteins (BMPs) and fibroblast growth factors (FGFs). Theca cells also secrete BMPs and FGFs. The interplay between these factors is equally important for follicle growth as the activity of oocyte-derived factors.

Inhibition of PIK3 signaling pathway members by the ovotoxicant 4-vinylcyclohexene diepoxide in rats

4-Vinylcyclohexene diepoxide (VCD), an occupational chemical that specifically destroys primordial and small primary follicles in the ovaries of rats and mice, is thought to target an oocyte-expressed tyrosine kinase receptor, Kit. This study compared the temporal effect of VCD on protein distribution of KIT and its downstream PIK3-activated proteins, AKT and FOXO3. Postnatal Day 4 Fischer 344 rat ovaries were cultured in control media ± VCD (30 μM) for 2-8 days (d2-d8). KIT, AKT, phosphorylated AKT, FOXO3, and pFOXO3 protein levels were assessed by Western blotting and/or immunofluorescence staining with confocal microscopy. Phosphorylated AKT was decreased (P < 0.05) in oocyte nuclei in primordial (39% decrease) and small primary (37% decrease) follicles within 2 days of VCD exposure. After d4, VCD reduced (P < 0.05) oocyte staining for KIT (primordial, 44% decrease; small primary, 39% decrease) and FOXO3 (primordial, 40% decrease; small primary, 36% decrease) protein. Total AKT and pFOXO3 were not affected by VCD at any time. Akt1 mRNA, as measured by quantitative RT-PCR, was reduced (P < 0.05) by 23% on d4 of VCD exposure, but returned to control levels on d6 and d8. VCD exposure reduced Foxo3a mRNA by 26% on d6 (P < 0.05) and by 23% on d8 (P < 0.1). These results demonstrate that the earliest observed effect of VCD is an inhibition of phosphorylation and nuclear localization of AKT in the oocyte of primordial and small primary follicles. This event is followed by reductions in KIT and FOXO3 protein subcellular distribution prior to changes in mRNA. Thus, these findings further support that VCD induces ovotoxicity by directly targeting the oocyte through posttranslational inhibition of KIT-mediated signaling components.

Bidirectional communication between oocytes and follicle cells: ensuring oocyte developmental competence

Female fertility is determined to a large extent by the quality (developmental competence) of the oocyte as reflected in its ability to undergo meiosis, be fertilized, and give rise to a healthy embryo. Growth of the mammalian oocyte is coordinated with that of the follicle that encloses it by the actions of signals that pass in both directions between the germline and somatic components. This review summarizes what is known about the roles played by 2 different modes of intrafollicular signalling in oogenesis: paracrine factors activating receptors on the opposite cell type, and direct sharing of small molecules throughout the follicle via gap junction channels. Recent evidence indicates that these 2 modes of signalling interact to regulate oocyte growth and granulosa cell proliferation and that defects in either can contribute to female infertility.

The use of transgenic mouse models in the study of male infertility

Over the past few decades with the rapid advances in embryo and embryonic stem cell manipulation techniques, transgenic mouse models have emerged as a powerful tool for the study of gene function and complex diseases including male infertility. In this review we give a brief history of the development of tools for the production of transgenic mouse models. This spans the advances from early pronuclear injection to the use of targeted embryonic stem cells to produce gene targeted, conditional, and inducible knockout mouse models. Lastly we provide a few examples to illustrate the utility of mouse models in the study of male infertility.

I787 provides signals for c-Kit receptor internalization and functionality that control mast cell survival and development

Mast cell (MC) differentiation, survival, and activation are controlled by the membrane tyrosine kinase c-Kit upon interaction with stem cell factor (SCF). Here we describe a single point mutation induced by N-ethyl-N-nitrosurea (ENU) mutagenesis in C57BL/6J mice-an A to T transversion at position 2388 (exon 17) of the c-Kit gene, resulting in the isoleucine 787 substitution by phenylalanine (787F), and analyze the consequences of this mutation for ligand binding, signaling, and MC development. The Kit(787F/787F) mice carrying the single amino acid exchange of c-Kit lacks both mucosal and connective tissue-type MCs. In bone marrow-derived mast cells (BMMCs), the 787F mutation does not affect SCF binding and c-Kit receptor shedding, but strongly impairs SCF-induced cytokine production, degranulation enhancement, and apoptosis rescue. Interestingly, c-Kit downstream signaling in 787F BMMCs is normally initiated (Erk1/2 and p38 activation as well as c-Kit autophosphorylation) but fails to be sustained thereafter. In addition, 787F c-Kit does not efficiently mediate Cbl activation, leading to the absence of subsequent receptor ubiquitination and impaired c-Kit internalization. Thus, I787 provides nonredundant signals for c-Kit internalization and functionality.

TACE/ADAM17 is involved in germ cell apoptosis during rat spermatogenesis

The pathways leading to male germ cell apoptosis in vivo are poorly understood, but are highly relevant for the comprehension of sperm production regulation by the testis. In this work, we show the evidence of a mechanism where germ cell apoptosis is induced through the inactivation and shedding of the extracellular domain of KIT (c-kit) by the protease TACE/a disintegrin and metalloprotease 17 (ADAM17) during the first wave of spermatogenesis in the rat. We show that germ cells undergoing apoptosis lacked the extracellular domain of the KIT receptor. TACE/ADAM17, a membrane-bound metalloprotease, was highly expressed in germ cells undergoing apoptosis as well. On the contrary, cell surface presence of ADAM10, a closely related metalloprotease isoform, was not associated with apoptotic germ cells. Pharmacological inhibition of TACE/ADAM17, but not ADAM10, significantly prevented germ cell apoptosis in the male pubertal rat. Induction of TACE/ADAM17 by the phorbol-ester phorbol 12-myristate 13-acetate (PMA) induced germ cell apoptosis, which was prevented when an inhibitor of TACE/ADAM17 was present in the assay. Ex-vivo rat testis culture showed that PMA induced the cleavage of the KIT extracellular domain. Isolation of apoptotic germ cells showed that even though protein levels of TACE/ADAM17 were higher in apoptotic germ cells than in nonapoptotic cells, the contrary was observed for ADAM10. These results suggest that TACE/ADAM17 is one of the elements triggering physiological germ cell apoptosis during the first wave of spermatogenesis.

Nodal signaling via an autocrine pathway promotes proliferation of mouse spermatogonial stem/progenitor cells through Smad2/3 and Oct-4 activation

Spermatogenesis is the process that involves the division and differentiation of spermatogonial stem cells into spermatozoa. However, the autocrine molecules and signaling pathways controlling their fate remain unknown. This study was designed to identify novel growth factors and signaling pathways that regulate proliferation, differentiation, and survival of spermatogonial stem/progenitor cells. To this end, we have for the first time explored the expression, function, and signaling pathway of Nodal, a member of the transforming growth factor-beta superfamily, in mouse spermatogonial stem/progenitor cells. We demonstrate that both Nodal and its receptors are present in these cells and in a spermatogonial stem/progenitor cell line (C18-4 cells), whereas Nodal is undetected in Sertoli cells or differentiated germ cells, as assayed by reverse transcription-polymerase chain reaction, Western blots, and immunocytochemistry. Nodal promotes proliferation of spermatogonial stem/progenitor cells and C18-4 cells, whereas Nodal receptor inhibitor SB431542 blocks their propagation as shown by proliferation and bromodeoxyuridine incorporation assays. Nodal knockdown by RNA interference results in a marked increase of cell apoptosis and a reduction of cell division as indicated by terminal deoxynucleotidyl transferase dUTP nick-end labeling and proliferation assays. Conversely, overexpression of Nodal leads to an increase of cell proliferation. Nodal activates Smad2/3 phosphorylation, Oct-4 transcription, cyclin D1, and cyclin E expression, whereas SB431542 completely abolishes their increase. Together, Nodal was identified as the first autocrine signaling molecule that promotes proliferation of mouse spermatogonial stem/progenitor cells via Smad2/3 and Oct-4 activation. This study thus provides novel and important insights into molecular mechanisms regulating proliferation and survival of spermatogonial stem/progenitor cells.

Microgravity promotes differentiation and meiotic entry of postnatal mouse male germ cells

A critical step of spermatogenesis is the entry of mitotic spermatogonia into meiosis. Progresses on these topics are hampered by the lack of an in vitro culture system allowing mouse spermatogonia differentiation and entry into meiosis. Previous studies have shown that mouse pachytene spermatocytes cultured in simulated microgravity (SM) undergo a spontaneous meiotic progression. Here we report that mouse mitotic spermatogonia cultured under SM with a rotary cell culture system (RCCS) enter into meiosis in the absence of any added exogenous factor or contact with somatic cells. We found that isolated Kit-positive spermatogonia under the RCCS condition enter into the prophase of the first meiotic division (leptotene stage), as monitored by chromosomal organization of the synaptonemal complex 3 protein (Scp3) and up-regulation of several pro-meiotic genes. SM was found to activate the phosphatidyl inositol 3 kinase (PI3K) pathway and to induce in Kit-positive spermatogonia the last round of DNA replication, typical of the preleptotene stage. A PI3K inhibitor abolished Scp3 induction and meiotic entry stimulated by RCCS conditions. A positive effect of SM on germ cell differentiation was also observed in undifferentiated (Kit-negative) spermatogonia, in which RCCS conditions stimulate the expression of Kit and Stra8. In conclusion, SM is an artificial environmental condition which promotes postnatal male germ cell differentiation and might provide a tool to study the molecular mechanisms underlying the switch from mitosis to meiosis in mammals.

Tumour microvessel endothelial cell KIT and stem cell factor expression in human solid tumours

AIMS

To assess KIT receptor tyrosine kinase and stem cell factor (SCF, KIT ligand) expression in tumour microvessel endothelial cells.

METHODS AND RESULTS

KIT and SCF expression were studied in 248 human tumours consisting of 15 different histological types using immunohistochemistry and in situ hybridization for KIT messenger RNA. Moderate to strong intratumoral endothelial cell KIT expression was present in 11 of the 15 tumour types, and was most common in glioblastoma (58%), mixed embryonal carcinoma with teratoma (33%) and renal clear cell carcinoma (29%). Results of in situ hybridization were in line with those obtained with immunohistochemistry in the cases studied (n = 9). Marked SCF expression was uncommon in tumour endothelial cells, but frequent in perinecrotic tumour cells. Patients with glioblastoma with moderate to strong endothelial cell KIT expression had more favourable survival than those whose tumour showed little or no expression (P = 0.024). Glioblastoma patients whose tumour expressed SCF had an unfavourable outcome compared with those with tumour with weak or no expression (P = 0.034).

CONCLUSIONS

Intratumoral microvessels of several types of human malignant tumours express KIT. Tumour cell SCF expression and absence of marked endothelial cell KIT expression are novel adverse prognostic features in glioblastoma.

Essential role of the p110beta subunit of phosphoinositide 3-OH kinase in male fertility

Phosphoinositide 3-kinases (PI3K) are key molecular players in male fertility. However, the specific roles of different p110 PI3K catalytic subunits within the spermatogenic lineage have not been characterized so far. Herein, we report that male mice expressing a catalytically inactive p110beta develop testicular hypotrophy and impaired spermatogenesis, leading to a phenotype of oligo-azoospermia and defective fertility. The examination of testes from p110beta-defective tubules demonstrates a widespread loss in spermatogenic cells, due to defective proliferation and survival of pre- and postmeiotic cells. In particular, p110beta is crucially needed in c-Kit-mediated spermatogonial expansion, as c-Kit-positive cells are lost in the adult testis and activation of Akt by SCF is blocked by a p110beta inhibitor. These data establish that activation of the p110beta PI3K isoform by c-Kit is required during spermatogenesis, thus opening the way to new treatments for c-Kit positive testicular cancers.

Effect of phosphatidylinositol-3 kinase inhibition on ovotoxicity caused by 4-vinylcyclohexene diepoxide and 7, 12-dimethylbenz[a]anthracene in neonatal rat ovaries

4-vinylcyclohexene diepoxide (VCD) is an ovotoxicant that specifically destroys primordial and small primary follicles in the ovaries of mice and rats. In contrast, 7,12-dimethylbenz[a]anthracene (DMBA) is ovotoxic to all ovarian follicle classes. This study investigated phosphatidylinositol-3 kinase signaling involvement in VCD- and DMBA-induced ovotoxicity. Postnatal day (PND) 4 Fischer 344 (F344) rat whole ovaries were cultured for 2-12 days in vehicle control, VCD (30 microM), or DMBA (1 microM), +/-PI3 kinase inhibitor LY294002 (20 microM) or its inactive analog LY303511 (20 microM). Following culture, ovaries were histologically evaluated, and healthy follicles were classified and counted. PI3 kinase inhibition had no effect on primordial follicle number, but reduced (P<0.05) small primary and larger follicles beginning on day 4. VCD caused primordial and small primary follicle loss (P<0.05) beginning on day 6. With PI3 kinase inhibition, VCD did not affect primordial follicles (P>0.05) at any time, but did cause loss (P<0.05) of small primary follicles. DMBA exposure caused primordial and small primary follicle loss (P<0.05) on day 6. Further, DMBA-induced primordial and small primary follicle loss was greater with PI3 kinase inhibition (P<0.05) than with DMBA alone. These results support that (1) PI3 kinase mediates primordial to small primary follicle recruitment, (2) VCD, but not DMBA, enhances ovotoxicity by increasing primordial to small primary follicle recruitment, and (3) in addition to xenobiotic-induced ovotoxicity, VCD is also a useful model chemical with which to elucidate signaling mechanisms involved in primordial follicle recruitment.

Signaling molecules and pathways regulating the fate of spermatogonial stem cells

Spermatogenesis is the process that involves the division and differentiation of spermatogonial stem cells (SSCs) into mature spermatozoa. SSCs are a subpopulation of type A spermatogonia resting on the basement membrane in the mammalian testis. Self-renewal and differentiation of SSCs are the foundation of normal spermatogenesis, and thus a better understanding of molecular mechanisms and signaling pathways in the SSCs is of paramount importance for the regulation of spermatogenesis and may eventually lead to novel targets for male contraception as well as for gene therapy of male infertility and testicular cancer. Uncovering the molecular mechanisms is also of great interest to a better understanding of SSC aging and for developing novel therapeutic strategies for degenerative diseases in view of the recent work demonstrating the pluripotent potential of the SSC. Progress has recently been made in elucidating the signaling molecules and pathways that determine cell fate decisions of SSCs. In this review, we first address the morphological features, phenotypic characteristics, and the potential of SSCs, and then we focus on the recent advances in defining the key signaling molecules and crucial signaling pathways regulating self-renewal and differentiation of SSCs. The association of aberrant expression of signaling molecules and cascades with abnormal spermatogenesis and testicular cancer are also discussed. Finally, we point out potential future directions to pursue in research on signaling pathways of SSCs.

Kit ligand cytoplasmic domain is essential for basolateral sorting in vivo and has roles in spermatogenesis and hematopoiesis

Juxtamembrane signaling via the membrane growth factor KitL is critical for Kit mediated functions. KitL has a conserved cytoplasmic domain and has been shown to possess a monomeric leucine-dependent basolateral targeting signal. To investigate the consequences in vivo of impaired basolateral KitL targeting in polarized epithelial cells, we have mutated this critical leucine to alanine using a knock-in strategy. KitL(L263A/L263A) mutant mice are pigmented normally and steady-state hematopoiesis is unaffected although peritoneal and skin mast cell numbers are significantly increased. KitL localization is affected in the Sertoli cells of the KitL(L263A/L263A) testis and testis size is reduced in these mice due to aberrant spermatogonial proliferation. Furthermore, the effect of the KitL L263A mutation on the testicular phenotype is dosage dependent. The tubules of hemizygous KitL(L263A/Sl) mice completely lack germ cells in contrast to the weaker testicular phenotype of KitL(L263A/L263A) mice. The onset of the testis phenotype coincides with the formation of tight junctions between Sertoli cells during postnatal development. Thus, the altered sorting of KitL is dispensable for hematopoietic and melanogenic lineages, yet is crucial in the testicular environment, where the basal membranes of adjacent polarized Sertoli cells form a niche for the proliferating spermatogonia.

The heterogeneity of spermatogonia is revealed by their topology and expression of marker proteins including the germ cell-specific proteins Nanos2 and Nanos3

Spermatogonial stem cells (SSCs) reside in undifferentiated type-A spermatogonia and contribute to continuous spermatogenesis by maintaining the balance between self-renewal and differentiation, thereby meeting the biological demand in the testis. Spermatogonia have to date been characterized principally through their morphology, but we herein report the detailed characterization of undifferentiated spermatogonia in mouse testes based on their gene expression profiles in combination with topological features. The detection of the germ cell-specific proteins Nanos2 and Nanos3 as markers of spermatogonia has enabled the clear dissection of complex populations of these cells as Nanos2 was recently shown to be involved in the maintenance of stem cells. Nanos2 is found to be almost exclusively expressed in A(s) to A(pr) cells, whereas Nanos3 is detectable in most undifferentiated spermatogonia (A(s) to A(al)) and differentiating A(1) spermatogonia. In our present study, we find that A(s) and A(pr) can be basically classified into three categories: (1) GFRalpha1(+)Nanos2(+)Nanos3(-)Ngn3(-), (2) GFRalpha1(+)Nanos2(+)Nanos3(+)Ngn3(-), and (3) GFRalpha1(-)Nanos2(+/-)Nanos3(+)Ngn3(+). We propose that the first of these groups is most likely to include the stem cell population and that Nanos3 may function in transit amplifying cells.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Gonadal dysfunction and infertility in kidney transplant patients receiving sirolimus

Sirolimus is an immunosupressor of the mammalian target of rapamycin inhibitors (mTOR-I) group. Recent studies have emphasized a potential impact of sirolimus on male gonadal function. We report our clinical experience with sirolimus-induced gonadal dysfunction and infertility in both male and female kidney transplant patients. Of the 170 kidney transplant patients, nine (5.3%) patients (six males and three females) were receiving sirolimus. Follow-up data for two male patients were not available. The one unmarried female patient developed amenorrhea post-transplantation and had resumption of her menstrual cycles after discontinuation of sirolimus. The remaining six married patients (four males and two females), who all had fathered or conceived children in the pre-transplantation period, developed gonadal dysfunction and infertility on average 5-12 months after transplantation. Sirolimus was discontinued in all four male patients with full recovery of the oligo/azospermia and restoration of fertility. Both married female patients developed amenorrhea post-transplantation. Sirolimus was discontinued in one female patient with resumption of her menstrual cycles. In this small population of patients treated with sirolimus, the prevalence rate of reversible gonadal dysfunction and infertility was significant in both males and females. Infertility secondary to sirolimus is under-diagnosed and should be studied further.

Molecular mechanisms underlying the activation of mammalian primordial follicles

In humans and other mammalian species, the pool of resting primordial follicles serves as the source of developing follicles and fertilizable ova for the entire length of female reproductive life. One question that has intrigued biologists is: what are the mechanisms controlling the activation of dormant primordial follicles. Studies from previous decades have laid a solid, but yet incomplete, foundation. In recent years, molecular mechanisms underlying follicular activation have become more evident, mainly through the use of genetically modified mouse models. As hypothesized in the 1990s, the pool of primordial follicles is now known to be maintained in a dormant state by various forms of inhibitory machinery, which are provided by several inhibitory signals and molecules. Several recently reported mutant mouse models have shown that a synergistic and coordinated suppression of follicular activation provided by multiple inhibitory molecules is necessary to preserve the dormant follicular pool. Loss of function of any of the inhibitory molecules for follicular activation, including PTEN (phosphatase and tensin homolog deleted on chromosome 10), Foxo3a, p27, and Foxl2, leads to premature and irreversible activation of the primordial follicle pool. Such global activation of the primordial follicle pool leads to the exhaustion of the resting follicle reserve, resulting in premature ovarian failure in mice. In this review, we summarize both historical and recent results on mammalian primordial follicular activation and focus on the up-to-date knowledge of molecular networks controlling this important physiological event. We believe that information obtained from mutant mouse models may also reflect the molecular machinery responsible for follicular activation in humans. These advances may provide a better understanding of human ovarian physiology and pathophysiology for future clinical applications.

Control of KIT signalling in male germ cells: what can we learn from other systems?

The KIT ligand (KITL)/KIT-signalling system is among several pathways known to be essential for fertility. In the postnatal testis, the KIT/KITL interaction is crucial for spermatogonial proliferation, differentiation, survival and subsequent entry into meiosis. Hence, identification of endogenous factors that regulate KIT synthesis is important for understanding the triggers driving germ cell maturation. Although limited information is available regarding local factors in the testicular microenvironment that modulate KIT synthesis at the onset of spermatogenesis, knowledge from other systems could be used as a basis for identifying how KIT function is regulated in germ cells. This review describes the known regulators of KIT, including transcription factors implicated in KIT promoter regulation. In addition, specific downstream outcomes in biological processes that KIT orchestrates are addressed. These are discussed in relationship to current knowledge of mammalian germ cell development.

Oncogenic signaling from the hematopoietic growth factor receptors c-Kit and Flt3

Signal transduction in response to growth factors is a strictly controlled process with networks of feedback systems, highly selective interactions and finely tuned on-and-off switches. In the context of cancer, detailed signaling studies have resulted in the development of some of the most frequently used means of therapy, with several well established examples such as the small molecule inhibitors imatinib and dasatinib in the treatment of chronic myeloid leukemia. Impaired function of receptor tyrosine kinases is implicated in various types of tumors, and much effort is put into mapping the many interactions and downstream pathways. Here we discuss the hematopoietic growth factor receptors c-Kit and Flt3 and their downstream signaling in normal as well as malignant cells. Both receptors are members of the same family of tyrosine kinases and crucial mediators of stem-and progenitor-cell proliferation and survival in response to ligand stimuli from the surrounding microenvironment. Gain-of-function mutations/alterations render the receptors constitutively and ligand-independently activated, resulting in aberrant signaling which is a crucial driving force in tumorigenesis. Frequently found mutations in c-Kit and Flt3 are point mutations of aspartic acid 816 and 835 respectively, in the activation loop of the kinase domains. Several other point mutations have been identified, but in the case of Flt3, the most common alterations are internal tandem duplications (ITDs) in the juxtamembrane region, reported in approximately 30% of patients with acute myeloid leukemia (AML). During the last couple of years, the increasing understanding of c-Kit and Flt3 signaling has also revealed the complexity of these receptor systems. The impact of gain-of-function mutations of c-Kit and Flt3 in different malignancies is well established and shown to be of clinical relevance in both prognosis and therapy. Many inhibitors of both c-Kit or Flt3 or of their downstream substrates are in clinical trials with encouraging results, and targeted therapy using a combination of such inhibitors is considered a promising approach for future treatments.

Kit signaling via PI3K promotes ovarian follicle maturation but is dispensable for primordial follicle activation

In mammals, primordial follicles are generated early in life and remain dormant for prolonged intervals. Their growth resumes via a process known as primordial follicle activation. Recent genetic studies have demonstrated that phosphoinositide 3-kinase (PI3K) is the essential signaling pathway controlling this process throughout life, acting via Akt to regulate nucleocytoplasmic shuttling of Foxo3, which functions as a downstream molecular switch. The receptor tyrosine kinase Kit has been implicated by numerous studies as the critical upstream regulator of primordial follicle activation via PI3K/Akt. Here we present a genetic analysis of the contribution of Kit in regulating primordial follicle activation and early follicle growth, employing a knock-in mutation (Kit(Y719F)) that completely abrogates signaling via PI3K. Surprisingly, homozygous Kit(Y719F) female mice undergo primordial follicle activation and are fertile, demonstrating that Kit signaling via PI3K is dispensable for this process. However, other abnormalities were identified in Kit(Y719F) ovaries, including accelerated primordial follicle depletion and accumulation of morphologically abnormal primary/secondary follicles with persistent nuclear Foxo3 localization. These findings reveal specific roles of Kit in the maintenance of the primordial follicle reserve and in the primary to secondary follicle transition, but argue that Kit is dispensable in primordial follicle activation.

A KIT juxtamembrane PY567 -directed pathway provides nonredundant signals for erythroid progenitor cell development and stress erythropoiesis

OBJECTIVE

KITL/KIT can elicit diverse sets of signals within lymphoid, myeloid, mast, and erythroid lineages, and exert distinct effects on growth, survival, migration, adhesion, and secretory responses. Presently, we have applied a PY-mutant allele knockin approach to specifically assess possible roles for KIT-PY567 and KIT-PY719 sites, and coupled pathways, during erythropoiesis.

MATERIALS AND METHODS

Mouse models used to investigate this problem include those harboring knocked-in KIT(Y567F/Y567F), KIT(Y569F/Y569F), KIT(Y719F,Y719F), and KIT(Y567F/Y567F:Y569F/Y569F) alleles. The erythron was stressed by myelosuppression using 5-fluorouracil, and by phenylhydrazine-induced hemolysis. In addition, optimized systems for ex vivo analyses of bone marrow and splenic erythropoiesis were employed to more directly analyze possible stage-specific effects on erythroid cell growth, survival, development and KIT signaling events.

RESULTS

In Kit(Y567F/Y567F) mice, steady-state erythropoiesis was unperturbed while recovery from anemia due to 5-fluorouracil or phenylhydrazine was markedly impaired. Deficiencies in erythroid progenitor expansion occurred both in the bone marrow and the spleen. Responses to chronic erythropoietin dosing were also compromised. Ex vivo, Kit(Y567F/Y567F) (pro)erythroblast development was skewed from a Kit(pos)CD71(high) stage toward a subsequent Kit(neg)CD71(high) compartment. Proliferation and, to an extent, survival capacities were also compromised. Similar stage-specific defects existed for erythroid progenitors from Kit(Y567F/Y567F:Y569F/Y569F) but not KIT(Y719F/Y719F) mice. Kit(Y567F/Y567F) erythroblasts were used further to analyze KIT-PY567-dependent signals. MEK-1,2/ERK-1,2 signaling was unaffected while AKT, p70S6K, and especially JNK2/p54 pathways were selectively attenuated.

CONCLUSIONS

Nonredundant KIT-PY567-directed erythroblast-intrinsic signals are selectively critical for stress erythropoiesis. Investigations also add to an understanding of how KIT directs distinct outcomes among diverse progenitors and lineages.

Sohlh2 affects differentiation of KIT positive oocytes and spermatogonia

The differentiation programs of spermatogenesis and oogenesis are largely independent. In the early stages, however, the mechanisms partly overlap. Here we demonstrated that a germ-cell-specific basic helix-loop-helix (bHLH) transcription factor gene, Sohlh2, is required for early spermatogenesis and oogenesis. SOHLH2 was expressed in mouse spermatogonia from the undifferentiated stage through differentiation and in primordial-to-primary oocytes. Sohlh2-null mice, produced by gene targeting, showed both male and female sterility, owing to the disrupted differentiation of mature (KIT(+)) spermatogonia and oocytes. The Sohlh2-null mice also showed the downregulation of genes involved in spermatogenesis and oogenesis, including the Sohlh1 gene, which is essential for these processes. Furthermore, we showed that SOHLH2 and SOHLH1 could form heterodimers. These observations suggested that SOHLH2 might coordinate with SOHLH1 to control spermatogonial and oocyte genes, including Sohlh1, to promote the differentiation of KIT(+) germ cells in vivo. This study lays the foundation for further dissection of the bHLH network that regulates early spermatogenesis and oogenesis.

The multiple roles of phosphoinositide 3-kinase in mast cell biology

Mast cells play a central role in the initiation of inflammatory responses associated with asthma and other allergic disorders. Receptor-mediated mast cell growth, differentiation, homing to their target tissues, survival and activation are all controlled, to varying degrees, by phosphoinositide-3-kinase (PI3K)-driven pathways. It is not fully understood how such diverse responses can be differentially regulated by PI3K. However, recent studies have provided greater insight into the mechanisms that control, and those that are controlled by, different PI3K subunit isoforms in mast cells. In this review, we discuss how PI3K influences the mast cell processes described above. Furthermore, we describe how different mast cell receptors use alternative isoforms of PI3K for these functions and discuss potential downstream targets of these isoforms.

Sirolimus may reduce fertility in male renal transplant recipients

Assessment of sex hormones in organ transplant recipients suggests that sirolimus may impair testicular function. The aim of this study was to evaluate the frequency and severity of sirolimus-associated alterations in sperm parameters and their impact on fathered pregnancy rate. An observational study was carried out in male patients aged 20-40 years who received a kidney transplant during 1995-2005. Patients were sent a questionnaire by post, and sperm analysis was proposed. The fathered pregnancy rates according to the immunosuppressive regimen were estimated and compared using the Poisson model. Complete information was obtained from 95 out of 116 recipients. Patients treated with sirolimus throughout the post-transplant period had a significantly reduced total sperm count compared to patients who did not receive sirolimus (28.6 +/- 31.2 x 10(6) and 292.2 +/- 271.2 x 10(6), respectively; p = 0.006), and a decreased proportion of motile spermatozoa (22.2 +/- 12.3% and 41.0 +/- 14.5%, p = 0.01). Moreover, the fathered pregnancy rate (pregnancies/1000 patient years) was 5.9 (95% CI, 0.8-42.1) and 92.9 (95% CI, 66.4-130.0) in patients receiving sirolimus-based and sirolimus-free regimens, respectively (p = 0.007). Of six patients in whom sirolimus treatment was interrupted, only three showed a significant improvement in sperm parameters. Sirolimus is associated with impaired spermatogenesis and, as a corollary, may reduce male fertility.

Molecular pathobiology of gastrointestinal stromal sarcomas

Gastrointestinal stromal tumors (GISTs) form an interesting group of sarcomas whose unique pathobiology provides a model of how molecularly targeted therapeutics can have a major impact on patient welfare. Approximately 85% of GISTs are driven by oncogenic mutations in either of two receptor tyrosine kinases: KIT or platelet-derived growth factor receptor alpha. We review the pivotal relationship between specific mutations in these kinase genes, the origin and pathologic spectrum of GISTs, and the response of these tumors to treatment with kinase inhibitors such as imatinib and sunitinib. Mechanisms of resistance to kinase inhibitor therapy are discussed, and targets for the next generation of therapeutics are considered. The rapid evolution in our understanding of GISTs, which stems directly from the close alliance of basic and clinical researchers in the field, illustrates the growing role of the molecular classification of solid tumors in the development of modern oncological treatments.

Disruption of PDGFRalpha-initiated PI3K activation and migration of somite derivatives leads to spina bifida

Spina bifida, or failure of the vertebrae to close at the midline, is a common congenital malformation in humans that is often synonymous with neural tube defects (NTDs). However, it is likely that other etiologies exist. Genetic disruption of platelet-derived growth factor receptor (PDGFR) alpha results in spina bifida, but the underlying mechanism has not been identified. To elucidate the cause of this birth defect in PDGFRalpha mutant embryos, we examined the developmental processes involved in vertebrae formation. Exposure of chick embryos to the PDGFR inhibitor imatinib mesylate resulted in spina bifida in the absence of NTDs. We next examined embryos with a tissue-specific deletion of the receptor. We found that loss of the receptor from chondrocytes did not recapitulate the spina bifida phenotype. By contrast, loss of the receptor from all sclerotome and dermatome derivatives or disruption of PDGFRalpha-driven phosphatidyl-inositol 3' kinase (PI3K) activity resulted in spina bifida. Furthermore, we identified a migration defect in the sclerotome as the cause of the abnormal vertebral development. We found that primary cells from these mice exhibited defects in PAK1 activation and paxillin localization. Taken together, these results indicate that PDGFRalpha downstream effectors, especially PI3K, are essential for cell migration of a somite-derived dorsal mesenchyme and disruption of receptor signaling in these cells leads to spina bifida.

A mouse model of a human multiple GIST family with KIT-Asp820Tyr mutation generated by a knock-in strategy

Several families exhibiting multiple gastrointestinal stromal tumours (GISTs) and germline c-kit gene mutations at exons 8, 11, 13, or 17 have been reported. These patients also exhibit diffuse hyperplasia of the interstitial cells of Cajal (ICCs) as a pre-existing lesion of multiple GISTs. We generated a mouse model of a family with germline c-kit gene mutation at exon 17, and compared the phenotypes between the mice and humans. The mouse counterpart (KIT-Asp818Tyr) of the human KIT-Asp820Tyr mutation was transmitted into germline by a knock-in strategy. Mating of male and female heterozygotes (KIT-Asp818Tyr/+) resulted in the generation of homozygotes (KIT-Asp818Tyr/KIT-Asp818Tyr). Histological examination revealed that all heterozygotes had both a small KIT-positive mesenchymal tumour at the caecum, consistent with GIST, and KIT-positive diffuse spindle-shaped cell proliferation in the distal oesophagus, stomach, proximal duodenum, and colon consistent with ICC hyperplasia. All homozygotes exhibited a larger caecal tumour and more prominent spindle-shaped cell proliferation compared with the heterozygous mice, and they usually died within 10 weeks after birth, likely due to ileus. The small intestine of both genotypes showed no apparent morphological abnormality, and autonomous contraction of the ileal segments appeared normal. Western blotting demonstrated that the caecal tumours expressed phosphorylated KIT, MAPK, Stat1, and Stat5. These mutant mice are considered to be useful for further investigation of the mechanism of GIST development as a result of ICC hyperplasia and for assessment of the in vivo effects of drugs against molecular targets.

Sohlh2 knockout mice are male-sterile because of degeneration of differentiating type A spermatogonia

The spermatogenesis and oogenesis-specific transcription factor Sohlh2 is normally expressed only in premeiotic germ cells. In this study, Sohlh2 and several other germ cell transcripts were found to be induced in mouse embryonic stem cells when cultured on a feeder cell line that overexpresses bone morphogenetic protein 4. To study the function of Sohlh2 in germ cells, we generated mice harboring null alleles of Sohlh2. Male Sohlh2-deficient mice were infertile because of a block in spermatogenesis. Although normal prior to birth, Sohlh2-null mice had reduced numbers of intermediate and type B spermatogonia by postnatal day 7. By day 10, development to the preleptotene spermatocyte stage was severely disrupted, rendering seminiferous tubules with only Sertoli cells, undifferentiated spermatogonia, and degenerating colonies of differentiating spermatogonia. Degenerating cells resembled type A2 spermatogonia and accumulated in M-phase prior to death. A similar phenotype was observed in Sohlh2-null mice on postnatal days 14, 21, 35, 49, 68, and 151. In adult Sohlh2-mutant mice, the ratio of undifferentiated type A spermatogonia (DAZL+/PLZF+) to differentiating type A spermatogonia (DAZL+/PLZF-) was twice normal levels. In culture, undifferentiated type A spermatogonia isolated from Sohlh2-null mice proliferated normally but linked the mutant phenotype to aberrant cell surface expression of the receptor-tyrosine kinase cKit. Thus, Sohlh2 is required for progression of differentiating type A spermatogonia into type B spermatogonia. One conclusion originating from these studies would be that testicular factors normally regulate the viability of differentiating spermatogonia by signaling through Sohlh2. This regulation would provide a crucial checkpoint to optimize the numbers of spermatocytes entering meiosis during each cycle of spermatogenesis. Disclosure of potential conflicts of interest is found at the end of this article.

Roles for deregulated receptor tyrosine kinases and their downstream signaling molecules in hematologic malignancies

Growth, survival and differentiation of hematopoietic cells are regulated by the interactions between hematopoietic growth factors and their receptors. The defect in these interactions results in a failure of hematopoiesis, while aberrantly elevated and/or sustained activation of these signals cause hematologic malignancies. Among them, constitutively activating mutations of the receptor tyrosine kinases (RTKs), such as c-Kit, platelet-derived growth factor receptor (PDGFR) and FLT3, are often involved in the pathogenesis of various types of hematologic malignancies. Constitutive activation of RTKs is provoked by several mechanisms including chromosomal translocations and various mutations involving their regulatory regions. Chromosomal translocations commonly generate chimeric proteins consisting of the cytoplasmic domain of RTKs and the dimerization or multimerization motif of the fusion partner, resulting in the constitutive dimerization of RTKs. On the other hand, missense, insertion or deletion mutations in the regulatory regions, such as juxtamembrane domain, activation loop, and extracellular domain, also cause constitutive activation of RTKs mainly by preventing the auto-inhibitory regulation. Oncogenic RTKs activate downstream signaling molecules such as Ras/MAPK, PI3-K/Akt/mTOR, and STATs as well as ligand-activated wild type RTKs. However, their signals are quantitatively and qualitatively different from wild type RTKs. Based on these findings, several agents that target oncogenic RTKs or their downstream molecules have been developed: imatinib and FLT3 inhibitors for RTKs themselves, farnesyltransferase inhibitors, mTOR inhibitors and MEK inhibitors for the downstream signaling molecules. As promising results have been obtained in several clinical trials using these agents, the establishment of these molecular targeted agents is expected.

Function of the Zinc-Finger Transcription FactorSNAI2in Cancer and Development

Elucidation of the molecular mechanisms that underlie disease development is still a tremendous challenge for basic science, and a prerequisite to the development of new and disease-specific targeted therapies. This review focuses on the function of SNAI2, a member of the Snail family of zinc-finger transcription factors, and discusses its possible role in disease development. SNAI2 has been implicated in diseases of melanocyte development and cancer in humans. Many malignancies arise from a rare population of cells that alone have the ability to self-renew and sustain the tumor (i.e., cancer stem cells). SNAI2 controls key aspects of stem cell function in mouse and human, suggesting that similar mechanisms control normal development and cancer stem cell properties. These insights are expected to contribute significantly to the genetics of cancer and to the development of both cancer therapy and new methods for assessing treatment efficacy.

Transcriptome analysis of differentiating spermatogonia stimulated with kit ligand

Kit ligand (KL) is a survival factor and a mitogenic stimulus for differentiating spermatogonia. However, it is not known whether KL also plays a role in the differentiative events that lead to meiotic entry of these cells. We performed a wide genome analysis of difference in gene expression induced by treatment with KL of spermatogonia from 7-day-old mice, using gene chips spanning the whole mouse genome. The analysis revealed that the pattern of RNA expression induced by KL is compatible with the qualitative changes of the cell cycle that occur during the subsequent cell divisions in type A and B spermatogonia, i.e. the progressive lengthening of the S phase and the shortening of the G2/M transition. Moreover, KL up-regulates in differentiating spermatogonia the expression of early meiotic genes (for instance: Lhx8, Nek1, Rnf141, Xrcc3, Tpo1, Tbca, Xrcc2, Mesp1, Phf7, Rtel1), whereas it down-regulates typical spermatogonial markers (for instance: Pole, Ptgs2, Zfpm2, Egr2, Egr3, Gsk3b, Hnrpa1, Fst, Ptch2). Since KL modifies the expression of several genes known to be up-regulated or down-regulated in spermatogonia during the transition from the mitotic to the meiotic cell cycle, these results are consistent with a role of the KL/kit interaction in the induction of their meiotic differentiation.

Gdnf upregulates c-Fos transcription via the Ras/Erk1/2 pathway to promote mouse spermatogonial stem cell proliferation

Glial cell line-derived neurotrophic factor (GDNF) plays a crucial role in regulating the proliferation of spermatogonial stem cells (SSC). The signaling pathways mediating the function of GDNF in SSC remain unclear. This study was designed to determine whether GDNF signals via the Ras/ERK1/2 pathway in the C18-4 cells, a mouse SSC line. The identity of this cell line was confirmed by the expression of various markers for germ cells, proliferating spermatogonia, and SSC, including GCNA1, Vasa, Dazl, PCNA, Oct-4, GFRalpha1, Ret, and Plzf. Western blot analysis revealed that GDNF activated Ret tyrosine phosphorylation. All 3 isoforms of Shc were phosphorylated upon GDNF stimulation, and GDNF induced the binding of the phosphorylated Ret to Shc and Grb2 as indicated by immunoprecipitation and Western blotting. The active Ras was induced by GDNF, which further activated ERK1/2 phosphorylation. GDNF stimulated the phosphorylation of CREB-1, ATF-1, and CREM-1, and c-fos transcription. Notably, the increase in ERK1/2 phosphorylation, c-fos transcription, bromodeoxyuridine incorporation, and metaphase counts induced by GDNF, was completely blocked by pretreatment with PD98059, a specific inhibitor for MEK1, the upstream regulator of ERK1/2. GDNF stimulation eventually upregulated cyclin A and CDK2 expression. Together, these data suggest that GDNF induces CREB/ATF-1 family member phosphorylation and c-fos transcription via the Ras/ERK1/2 pathway to promote the proliferation of SSC. Unveiling GDNF signaling cascades in SSC has important implications in providing attractive targets for male contraception as well as for the regulation of stem cell renewal vs. differentiation.

Kit ligand 2 promotes murine oocyte growth in vitro

Oocyte-granulosa cell communication, mediated by paracrine factors, is essential for oocyte development. Kit ligand (KITL) is expressed in granulosa cells as soluble (KITL1) or membrane-associated (KITL2) proteins. However, the relative biopotency of each isoform during oocyte development is unknown. Our initial results showed that Kitl2 was down-regulated in cultured granulosa cells. To determine the effect of the two isoforms of KITL on oocyte growth, Kitl-deficient fibroblasts were transfected with constructs expressing either KITL1 or KITL2, and growing oocytes were isolated from 12-day-old mice and cultured on the transfected fibroblasts for 2 days. At the end of culture, oocyte diameters were measured, the incidence of spontaneous germinal vesicle breakdown (GVBD) was noted, and oocytes were analyzed for KIT receptor expression. Oocyte growth occurred only in the presence of the KITL2-producing fibroblasts, and suppression of KITL2 expression impaired oocyte growth. Up-regulation of KIT expression occurred in the presence of KITL2 but not KITL1. The presence of KITL2 inhibited spontaneous GVBD. Meiosis inhibitors did not attenuate the GVBD that occurred in the absence of KITL2, suggesting that this process reflects oocyte degeneration rather than meiotic progression. These results indicate that KITL2 is the principal KITL isoform required for oocyte growth and survival in vitro.

Genetic and pharmacologic evidence implicating the p85 alpha, but not p85 beta, regulatory subunit of PI3K and Rac2 GTPase in regulating oncogenic KIT-induced transformation in acute myeloid leukemia and systemic mastocytosis

Oncogenic activation loop KIT mutations are observed in acute myeloid leukemia (AML) and systemic mastocytosis (SM); however, unlike the KIT juxtamembrane mutants, the activation loop mutants are insensitive to imatinib mesylate. Furthermore, as prior studies primarily used heterologous cell lines, the molecular mechanism(s) underlying oncogenic KIT-induced transformation in primary cells is poorly understood. We demonstrate that expression of KITD814V in primary hematopoietic stem/progenitor cells (HSC/Ps) and mast cell progenitors (MCps) induces constitutive KIT autophosphorylation, supports ligand-independent hyperproliferation, and promotes promiscuous cooperation with multiple cytokines. Genetic disruption of p85 alpha, the regulatory subunit of class IA lipid kinase phosphoinositol-3-kinase (PI3K), but not of p85 beta, or genetic disruption of the hematopoietic cell-specific Rho GTPase, Rac2, normalizes KITD814V-induced ligand-independent hyperproliferation. Additionally, deficiency of p85 alpha or Rac2 corrects the promiscuous hyperproliferation observed in response to multiple cytokines in both KITD814V-expressing HSC/Ps and MCps. Treatment of KITD814V-expressing HSC/Ps with a Rac inhibitor (NC23766) or with rapamycin showed a dose-dependent suppression in ligand-independent growth. Taken together, our results identify p85 alpha and Rac2 as potential novel therapeutic targets for the treatment of KITD814V-bearing AML and SM.