Developmental abnormalities in Steel17H mice result from a splicing defect in the steel factor cytoplasmic tail

Extramedullary hematopoiesis in cancer

Hematopoiesis can occur outside of the bone marrow during inflammatory stress to increase the production of primarily myeloid cells at extramedullary sites; this process is known as extramedullary hematopoiesis (EMH). As observed in a broad range of hematologic and nonhematologic diseases, EMH is now recognized for its important contributions to solid tumor pathology and prognosis. To initiate EMH, hematopoietic stem cells (HSCs) are mobilized from the bone marrow into the circulation and to extramedullary sites such as the spleen and liver. At these sites, HSCs primarily produce a pathological subset of myeloid cells that contributes to tumor pathology. The EMH HSC niche, which is distinct from the bone marrow HSC niche, is beginning to be characterized. The important cytokines that likely contribute to initiating and maintaining the EMH niche are KIT ligands, CXCL12, G-CSF, IL-1 family members, LIF, TNFα, and CXCR2. Further study of the role of EMH may offer valuable insights into emergency hematopoiesis and therapeutic approaches against cancer. Exciting future directions for the study of EMH include identifying common and distinct EMH mechanisms in cancer, infectious diseases, and chronic autoimmune diseases to control these conditions.

Stem cells, in vitro gametogenesis and male fertility

Reconstitution in culture of biological processes, such as differentiation and organization, is a key challenge in regenerative medicine, and one in which stem cell technology plays a central role. Pluripotent stem cells and spermatogonial stem cells are useful materials for reconstitution of germ cell development in vitro, as they are capable of differentiating into gametes. Reconstitution of germ cell development, termed in vitro gametogenesis, will provide an experimental platform for a better understanding of germ cell development, as well as an alternative source of gametes for reproduction, with the potential to cure infertility. Since germ cells are the cells for 'the next generation', both the culture system and its products must be carefully evaluated. In this issue, we summarize the progress in in vitro gametogenesis, most of which has been made using mouse models, as well as the future challenges in this field.

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

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

Expression and localisation of c-kit and KITL in the adult human ovary

BACKGROUND

The c-kit/kit ligand (KITL) signalling axis is an essential component of ovarian folliculogenesis in mammals, but little is known about expression and localisation of its key components in the ovaries of reproductive age women. This study aimed to characterise mRNA expression of c-kit and KITL isoforms and the localisation of c-kit and KITL proteins in adult human premenopausal ovaries.

METHODS

This study utilised granulosa cells obtained from the preovulatory follicles of women undergoing assisted reproduction, pieces of ovarian tissue obtained from premenopausal women undergoing gynaecological surgeries and archival paraffin-embedded premenopausal ovarian tissues. Methodology included PCR for gene expression and Western blot or immunohistochemistry for protein expression.

RESULTS

Both c-kit mRNA isoforms, known as GNNK+ and GNNK-, were detected in human ovarian cortex, while KITL protein isoforms (KITL1 and KITL2) were present in ovarian cortex and human granulosa cells. Immunohistochemistry showed expression of KITL and c-kit protein in multiple cell types within follicles throughout development, from primordial follicles to large antral follicles, in addition to atretic follicles. Oocytes of all follicle stages expressed c-kit protein exclusively. Interestingly, unlike animal models, expression of both proteins displayed a less cell-type specific distribution with immunostaining present in granulosa, theca and stromal cells, suggesting that autocrine signalling occurs within the human ovary.

CONCLUSION

The results of this study indicate that c-kit/KITL signalling also occurs in the human ovary, as established in various animal models, and may involve previously unknown autocrine signalling.

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.

Dimerization of Kit-ligand and efficient cell-surface presentation requires a conserved Ser-Gly-Gly-Tyr motif in its transmembrane domain

Kit-ligand (Kitl), also known as stem cell factor, is a membrane-anchored, noncovalently bound dimer signaling via the c-kit receptor tyrosine kinase, required for migration, survival, and proliferation of hematopoietic stem and germ cells, melanocytes, and mastocytes. Despite its fundamental role in morphogenesis and stem cell biology, the mechanisms that regulate Kitl dimerization are not well understood. By employing cell-permeable cross-linker and quantitative bimolecular fluorescence complementation of wild-type and truncated forms of Kitl, we determined that Kitl dimerization is initiated in the endoplasmic reticulum and mediated to similar levels by the transmembrane and the extracellular growth factor domain. Further biochemical and mutational analysis revealed a conserved Ser-Gly-Gly-Tyr-containing motif that is required for transmembrane domain dimerization and efficient cell-surface expression of Kitl. A novel intracellular capture assay with the Kitl transmembrane domain as bait revealed specific interactions with Kitl, but not with unrelated transmembrane proteins. During evolution, the transmembrane dimerization motif appeared in Kitl at the transition from teleosts to tetrapods, which correlates with the emergence of Kitl as a supporter of stem cell populations. Thus, transmembrane-mediated association of membrane-anchored growth factors consists of a novel mechanism to improve paracrine signaling and morphogenesis.

Mechanism of action of l-CDB-4022, a potential nonhormonal male contraceptive, in the seminiferous epithelium of the rat testis

The present study was conducted to elucidate the possible molecular mechanisms involved in the antispermatogenic activity of l-CDB-4022, an indenopyridine. In this study 45-d-old male Sprague-Dawley rats were treated with a single oral dose of l-CDB-4022 (2.5 mg/kg) or vehicle, and blood and testes were collected at various time points. The rate of body weight gain was not affected, but a significant loss of testes weight was induced by l-CDB-4022. Serum hormones were assayed using specific RIAs or ELISAs, and testicular protein and RNA were analyzed by Western blotting and RT-PCR, respectively. There was a significant decrease in inhibin B and concomitant increase in FSH in serum from l-CDB-4022-treated rats, but serum levels of activin A, testosterone, and LH were unchanged. Western analysis of testicular lysates from l-CDB-4022-treated rats exhibited phosphorylation of ERK1/2 at 4 h and later time points. Loss of nectin/afadin complex occurred at 48 h, but there was an increase in levels of integrin-beta1, N-cadherin, alpha-catenin, and beta-catenin protein at 24 h and later time points. Increase in expression of Fas ligand and Fas receptor was detected 8 and 24 h after l-CDB-4022 treatment. The ratio of the membrane to soluble form of stem cell factor mRNA was decreased. Immunohistochemical analysis of testicular sections indicated a dramatic disruption of the Sertoli cell microtubule network in l-CDB-4022-treated rats. Collectively, these results suggest that l-CDB-4022 activates the MAPK pathway, reduces expression of prosurvival factors such as the membrane form of stem cell factor, alters expression of Sertoli-germ cell adherens junction proteins, disrupts Sertoli cell microtubule structure, and induces the proapoptotic factor, Fas, culminating in germ cell loss from the seminiferous epithelium.

Stem cell factor affects fate determination of human gonocytes in vitro

The stem cell factor (SCF), binding its tyrosine kinase receptor c-Kit, has been shown to play essential roles in the proliferation, differentiation, and survival of germline cells. However, few reports are available about the effect of SCF on the development of human gonocytes within the fetal testis. The objective of this study was to investigate whether SCF affects the biological behaviors of human gonocytes before or after they enter the mitotic arrest stage. Employing an organ culture system, we observed that addition of exogenous SCF could influence the morphology of human gonocytesin vitro. Moreover, SCF was able to trigger the colony formation of round gonocytes, which were characterized positive for alkaline phosphatase activity, Oct-4, SSEA-4, and c-Kit as well. We found that SCF exerted actions in a dose- and age-dependent manner, although the stimulatory effect lasted no more than 14 days. We also showed that SCF played a role in suppressing the apoptosis of human gonocytes. Blocking of SCF signaling with either phosphatidylinositol 3-kinase or mitogen-activated protein kinase inhibitor resulted in similar apoptotic features as well as the SCF-withdrawal cultures. Taken together, we report that SCF acts as a potent regulator in the fate determination of human gonocytes. Our studies should form the basis forin vitrostudies and facilitate investigation of the molecular mechanisms underlying this unique stage.

Embryonic germ cell lines and their derivation from mouse primordial germ cells

When primordial germ cells of the mouse are cultured on feeder layers with the addition of the polypeptide signalling molecules leukaemia inhibitory factor, Steel factor and basic fibroblast growth factor they give rise to cells that resemble undifferentiated blastocyst-derived embryonic stem cells. These primordial germ cell-derived embryonic germ cells (EG cells) can be induced to differentiate extensively in culture and also form teratocarcinomas when injected into nude mice. Additionally, they contribute to chimeras when injected into host blastocysts. We have derived multiple EG cell lines from 8.5 days post coitum (dpc) embryos of C57BL/6 inbred mice. Four independent EG cell lines with normal male karyotypes have formed chimeras (up to 70% coat colour chimerism) when injected into BALB/c host blastocysts. Chimeric mice from all four cell lines are fertile, but only those from one line have transmitted coat colour markers through the germline. Studies have also been carried out to determine whether gonadal primordial germ cells can give rise to pluripotent EG cells. Germ cells from gonads of 15.5 dpc C57BL/6 embryos and newborn mice failed to produce EG cell lines. EG cell lines capable of forming teratocarcinomas and coat colour chimeras have been established from primordial germ cells of 12.5 dpc genital ridges. We are currently testing the genomic imprinting status of the insulin-like growth factor type 2 receptor gene (Igf2r) in our different EG cell lines.

KIT/KIT Ligand in Mammalian Oogenesis and Folliculogenesis: Roles in Rabbit and Murine Ovarian Follicle Activation and Oocyte Growth1

In rodent ovaries Kit ligand (KITL) and its receptor KIT have diverse roles, including the promotion of primordial follicle activation, oocyte growth, and follicle survival. Studies were undertaken to determine whether KITL and KIT carry out similar activities in rabbits. KitlandKitmRNA and protein were localized to oocytes and granulosa cells, respectively, in the rabbit ovary. Ovarian cortical explants from juvenile rabbits and neonatal mouse ovaries were subsequently cultured with recombinant mouse KITL and/or KITL neutralizing antibody. Indices of follicle growth initiation were compared with controls and between treatment groups for each species. Recombinant mouse KITL had no stimulatory effect on primordial follicle recruitment in cultured rabbit ovarian explants. However, the mean diameter of oocytes from primordial, early primary, primary, and growing primary follicles increased significantly in recombinant mouse KITL-treated explants compared with untreated tissues. In contrast, recombinant mouse KITL promoted both primordial follicle activation and an increase in the diameter of oocytes from primordial and early primary follicles in the mouse, and these effects were inhibited by coculture with KITL-neutralizing antibody. Recombinant mouse KITL had no effect on follicle survival for either species. These data demonstrate that KITL promotes the growth of rabbit and mouse oocytes and stimulates primordial follicle activation in the mouse but not in the rabbit. We propose that the physiologic roles of KITL and KIT may differ between species, and this has important implications for the design of in vitro culture systems for folliculogenesis in mammals, including the human.

Kit ligand and c-Kit have diverse roles during mammalian oogenesis and folliculogenesis

Paracrine signalling between the oocyte and its surrounding somatic cells is fundamental to the processes of oogenesis and folliculogenesis in mammals. The study of animal models has revealed that the interaction of granulosa cell-derived kit ligand (KL) with oocyte and theca cell-derived c-Kit is important for multiple aspects of oocyte and follicle development, including the establishment of primordial germ cells within the ovary, primordial follicle activation, oocyte survival and growth, granulosa cell proliferation, theca cell recruitment and the maintenance of meiotic arrest. Though little is known about the specific roles of KL and c-Kit during human oogenesis, the expression profiles for KL and c-Kit within the human ovary suggest that they are also functionally relevant to female fertility. This review details our current understanding of the roles of KL and c-Kit within the mammalian ovary, with a particular focus on the functional diversity of this receptor-ligand interaction at different stages of oocyte and follicle development.

Analysis of Hypomorphic KitlSl Mutants Suggests Different Requirements for KITL in Proliferation and Migration of Mouse Primordial Germ Cells1

Germ cell development in mice is initiated when a small number of primordial germ cells (PGCs) are set aside from somatic cells during gastrulation. In the subsequent 4 to 5 days, PGCs enter the hindgut, undergo a directed migration away from the hindgut into the developing gonads, and undergo a massive increase in cell number. It is well established that Kit ligand (KITL, also known as stem cell factor and mast cell growth factor) is required for the survival and proliferation of PGCs. However, there is little information on a direct role for KITL in PGC migration. By comparing the effects of multiple Kitl mutations, including two N-ethyl-N-nitrosourea-induced hypomorphic mutations, we were able to distinguish stages of PGC development that are preferentially affected by certain mutations. We provide evidence that the requirements for KITL in proliferation are different in PGCs before and after they start migrating, and different levels of KITL function are required to support PGC proliferation and migration. This study illustrates the usefulness of an allelic series of mutations to dissect developmental processes and suggests that these mutants may be useful for further studies of molecular mechanisms of KITL functions in gametogenesis.

Role of retinoid signaling in the regulation of spermatogenesis

While the need for vitamin A for the normal progression of male germ cell differentiation has been known for many years, the molecular mechanisms underlying this requirement are poorly understood. This review will explore the aspects of the effects on spermatogenesis of dietary deprivation of vitamin A, in particular as to how they compare to the male sterility that results from the genetic ablation of function of the retinoid receptor RARalpha. The effects of other genes involved with retinoid synthesis, transport, and degradation are also considered. The possible cellular mechanisms that may be affected by the lack of retinoid signaling are discussed, in particular, cell cycle regulation and cell-cell interaction, both of which are critical for normal spermatogenesis.

Role of c-Kit and erythropoietin receptor in erythropoiesis

Erythropoiesis is regulated by a number of growth factors, among which stem cell factor (SCF) and erythropoietin (Epo) play a non-redundant function. Viable mice with mutations in the SCF gene (encoded by the Steel (Sl) locus), or its receptor gene c-Kit (encoded by the White spotting (W) locus) develop a hypoplastic macrocytic anemia. Mutants of W or Sl that are completely devoid of c-Kit or SCF expression die in utero of anemia between days 14 and 16 of gestation and contain reduced numbers of erythroid progenitors in the fetal liver. Likewise, Epo and Epo receptor (Epo-R)-deficient mice die in utero due to a marked reduction in the number of committed fetal liver derived erythroid progenitors. Thus, committed erythroid progenitors require both c-Kit and Epo-R signal transduction pathways for their survival, proliferation and differentiation. In vitro, Epo alone is capable of generating mature erythroid progenitors; however, a combined treatment of Epo and SCF results in synergistic proliferation and expansion of developing erythroid progenitors. This review summarizes recent advances made towards understanding the signaling mechanisms by which Epo-R and c-Kit regulate growth, survival, and differentiation of erythroid progenitors alone and cooperatively.

A Specific Endoplasmic Reticulum Export Signal Drives Transport of Stem Cell Factor (Kitl) to the Cell Surface

Stem cell factor, also known as Kit ligand (Kitl), belongs to the family of dimeric transmembrane growth factors. Efficient cell surface presentation of Kitl is essential for the migration, proliferation, and survival of melanocytes, germ cells, hemopoietic stem cells, and mastocytes. Here we demonstrate that intracellular transport of Kitl to the cell surface is driven by a motif in the cytoplasmic tail that acts independently of the previously described basolateral sorting signal. Transport of Kitl to the cell surface is controlled at the level of the endoplasmic reticulum (ER) and requires a C-terminal valine residue positioned at a distance of 19-36 amino acids from the border between the transmembrane and cytoplasmic domains. Deletion or substitution of the valine with other hydrophobic amino acids results in ER accumulation and reduced cell surface transport of Kitl at physiological expression levels. When these mutant proteins are overexpressed in the ER, they are transported by bulk flow to the cell surface albeit at lower efficiency. A fusion construct between Kitl and the green fluorescent protein-labeled extracellular domain of a temperature-sensitive mutant of vesicular stomatitis virus G protein revealed the valine-dependent recruitment into coat protein complex II-coated ER exit sites and vesicular ER to Golgi transport in living cells. Thus the C-terminal valine defines a specific ER export signal in Kitl. It is responsible for the capture of Kitl at coat protein complex II-coated ER exit sites, leading to subsequent cell surface transport under physiological conditions.

The mouse juvenile spermatogonial depletion (jsd) phenotype is due to a mutation in the X-derived retrogene, mUtp14b

The recessive juvenile spermatogonial depletion (jsd) mutation results in a single wave of spermatogenesis, followed by failure of type A spermatogonia to differentiate, resulting in adult male sterility. We have identified a jsd-specific rearrangement in the mouse homologue of the Saccharomyces cerevisiae gene UTP14, termed mUtp14b. Confirmation that mUtp14b underlies the jsd phenotype was obtained by transgenic bacterial artificial chromosome (BAC) rescue. We also identified a homologous gene on the Mus musculus X chromosome (MMUX) (mUtp14a) that is the strict homologue of the yeast gene, from which the intronless mUtp14b has been derived by retrotransposition. Expression analysis showed that mUtp14b is predominantly expressed in the germ line of the testis from zygotene through round spermatids, whereas mUtp14a, although well expressed in all somatic tissues, could be detected only in the germ line in round spermatids. In yeast, depletion of the UTP proteins impedes production of 18S rRNA, leading to cell death. We propose that the retroposed autosomal copy mUtp14b, having acquired a testis-specific expression pattern, could have provided a mechanism for increasing the efficiency and/or numbers of germ cells produced by meeting the need for more 18S rRNA and protein. Such a mechanism would be of obvious reproductive advantage and be strongly selected for in evolution. Consistent with this hypothesis is the finding of a similar X-autosome retroposition of UTP14 in human which seems to have arisen independently of that in rodents. In jsd homozygotes, which lack a functional copy of Utp14b, insufficient production of rRNA quickly leads to a cessation of spermatogenesis.

In vivo and in vitro constant expression of GATA-4 in mouse postnatal Sertoli cells

In the mammalian postnatal testis, the biochemical and structural features of Sertoli cells change, depending on developmental stage and spermatogenic cycle, to support efficient spermatogenesis. Consequently, basic transcription factors that determine fundamental properties should be strictly maintained in postnatal Sertoli cells. We have confirmed that GATA-4 expression is kept at a constant level in mouse Sertoli cells during postnatal development, and is also maintained at a constant level in primary cultures, independent of treatment with hormones or the addition of germ cell fractions. In transient transfection assays with the testicular cell line TM3, established from Leydig cells, GATA-4 induced several Sertoli cell-specific genes. In the Sertoli cell line TM4, and in Sertoli cells in primary culture, GATA-4 slightly up-regulated these genes. These results suggest that GATA-4 plays an important role in the regulation of Sertoli cell function, and is exactly regulated in these cells.

Cyclic adenosine monophosphate (cAMP) stimulation of the kit ligand promoter in sertoli cells requires an Sp1-binding region, a canonical TATA box, and a cAMP-induced factor binding to an immediately downstream GC-rich element

Expression of Kit ligand (KL) mRNA is induced in primary prepuberal Sertoli cells by FSH and by other agents that increase cAMP levels. The cAMP effect is exerted at the transcriptional level and appears to be cell type specific, since it is not observed in other KL-expressing primary cells or cell lines. Deletion analysis of the 5'-flanking region of the mouse KL gene shows that the proximal promoter sequence between -88 and +8 from the transcriptional start site is necessary and sufficient to obtain the full cAMP responsiveness of the promoter in primary mouse Sertoli cells. In the -88/+8 promoter region, several cis-acting elements play a role in cAMP response. The -88/-56 sequence is necessary for full induction of the gene, since its removal causes a drastic decrease in cAMP responsiveness; however, cAMP-stimulated expression is still observed with the minimal promoter region between -55 and +8. A more detailed mutational analysis of the minimal promoter region shows that mutations in the canonical TATA box sequence and in an immediately downstream GC-rich element completely abolish cAMP responsiveness. DNA-binding experiments show that transcription factor Sp1 binds to the -88/-56 fragment of the KL proximal promoter in both control and cAMP-stimulated cells, whereas a new cAMP-induced complex is observed when the -55/+8 minimal promoter region is used as probe. The canonical TATA box sequence is essential for formation of the latter complex. We also show that the binding of an unknown nuclear factor (different from Sp1, Egr-1, Rnf6, and AP-2) to a GC-rich element between -19 and +8 increases after cAMP treatment, and this effect seems to be specific of primary Sertoli cells. Thus, cAMP-induced transcription from the KL gene promoter in primary mouse Sertoli cells is mediated by a complex interaction among a Sp1-binding region, factors recognizing the canonical TATA box sequence, and a not yet identified cAMP-induced factor binding a GC-rich sequence just downstream from it.

Proliferation and differentiation of spermatogonial stem cells in the w/wv mutant mouse testis

Mutations in the dominant-white spotting (W; c-kit) and stem cell factor (Sl; SCF) genes, which encode the transmembrane tyrosine kinase receptor and its ligand, respectively, affect both the proliferation and differentiation of many types of stem cells. Almost all homozygous W or Sl mutant mice are sterile because of the lack of differentiated germ cells or spermatogonial stem cells. To characterize spermatogenesis in c-kit/SCF mutants and to understand the role of c-kit signal transduction in spermatogonial stem cells, the existence, proliferation, and differentiation of spermatogonia were examined in the W/Wv mutant mouse testis. In the present study, some of the W/Wv mutant testes completely lacked spermatogonia, and many of the remaining testes contained only a few spermatogonia. Examination of the proliferative activity of the W/Wv mutant spermatogonia by transplantation of enhanced green fluorescent protein (eGFP)-labeled W/Wv spermatogonia into the seminiferous tubules of normal SCF (W/Wv) or SCF mutant (Sl/Sld) mice demonstrated that the W/Wv spermatogonia had the ability to settle and proliferate, but not to differentiate, in the recipient seminiferous tubules. Although the germ cells in the adult W/Wv testis were c-kit-receptor protein-negative undifferentiated type A spermatogonia, the juvenile germ cells were able to differentiate into spermatogonia that expressed the c-kit-receptor protein. Furthermore, differentiated germ cells with the c-kit-receptor protein on the cell surface could be induced by GnRH antagonist treatment, even in the adult W/Wv testis. These results indicate that all the spermatogonial stem cell characteristics of settlement, proliferation, and differentiation can be demonstrated without stimulating the c-kit-receptor signal. The c-kit/SCF signal transduction system appears to be necessary for the maintenance and proliferation of differentiated c-kit receptor-positive spermatogonia but not for the initial step of spermatogonial cell differentiation.

A rare complex DNA rearrangement in the murine Steel gene results in exon duplication and a lethal phenotype

Kit ligand (Kitl), encoded by the Steel (Sl) locus, plays an essential role in hematopoiesis, gametogenesis, and melanogenesis during both embryonic and adult life. We have characterized a new spontaneous mutant of the Sl locus in mice designated KitlSl-20J that arose in the breeding colony at Jackson Laboratories. Heterozygous KitlSl-20J mice display a white belly spot and intercrossing results in an embryonic lethal phenotype in the homozygous state. Analysis of homozygous embryos demonstrated a significant reduction in fetal liver cellularity, colony forming unit-erythroid (CFU-E) progenitors, and a total absence of germ cells. Although expressed in vivo, recombinant mutant protein demonstrated loss of bioactivity that was correlated with lack of receptor binding. Analysis of the Sl gene transcripts in heterozygous KitlSl-20J mice revealed an in-frame tandem duplication of exon 3. A long-range polymerase chain reaction (PCR) strategy using overlapping primers in exon 3 amplified an approximately 7-kilobase (kb) product from DNA isolated from heterozygous KitlSl-20J mice but not from wild-type DNA that contained sequences from both introns 2 and 3 and an inverted intron 2 sequence, suggesting a complex rearrangement as the mechanism of the mutation. "Complexity analysis" of the sequence of the amplified product strongly suggests that local DNA motifs may have contributed to the generation of this spontaneous KitlSl-20J allele, likely mediated by a 2-step process. The KitlSl-20J mutation is a unique KitlSl allele and represents an unusual mechanism of mutation.

2,5-hexanedione-induced testicular injury

Now in its third decade of mechanistic investigation, testicular injury caused by 2,5-hexanedione (2,5-HD) exposure is a well-studied model with a rich database. The development of this model reflects the larger changes that have moved biology from a branch of chemistry into the molecular age. Critically examined in this review is the proposed mechanism for 2,5-HD-induced testicular injury in which germ cell maturation is disrupted owing to alterations in Sertoli cell microtubule-mediated functions. The goal is to evaluate the technical and conceptual approaches used to assess 2,5-HD-induced testicular injury, to highlight unanswered questions, and to identify fruitful avenues of future research.

A role for kit receptor signaling in Leydig cell steroidogenesis

Kit and its ligand, Kitl, function in hematopoiesis, melanogenesis, and gametogenesis. In the testis, Kitl is expressed by Sertoli cells and Kit is expressed by spermatogonia and Leydig cells. Kit functions are mediated by receptor autophosphorylation and subsequent association with signaling molecules, including phosphoinositide (PI) 3-kinase. We previously characterized the reproductive consequences of blocking Kit-mediated PI 3-kinase activation in KitY(719F)/Kit(Y719F) knockin mutant male mice. Only gametogenesis was affected in these mice, and males are sterile because of a block in spermatogenesis during the spermatogonial stages. In the present study, we investigated effects of the Kit(Y719F) mutation on Leydig cell development and steroidogenic function. Although the seminiferous tubules in testes of mutant animals are depleted of germ cells, the testes contain normal numbers of Leydig cells and the Leydig cells in these animals appear to have undergone normal differentiation. Evaluation of steroidogenesis in mutant animals indicates that testosterone levels are not significantly reduced in the periphery but that LH levels are increased 5-fold, implying an impairment of steroidogenesis in the mutant animals. Therefore, a role for Kit signaling in steroidogenesis in Leydig cells was sought in vitro. Purified Leydig cells from C57Bl6/J male mice were incubated with Kitl, and testosterone production was measured. Kitl-stimulated testosterone production was 2-fold higher than that in untreated controls. The Kitl-mediated testosterone biosynthesis in Leydig cells is PI 3-kinase dependent. In vitro, Leydig cells from mutant mice were steroidogenically more competent in response to LH than were normal Leydig cells. In contrast, Kitl-mediated testosterone production in these cells was comparable to that in normal cells. Because LH levels in mutant males are elevated and LH is known to stimulate testosterone biosynthesis, we proposed a model in which serum testosterone levels are controlled by elevated LH secretion. Leydig cells of mutant males, unable to respond effectively to Kitl stimulation, initially produce lower levels of testosterone, reducing testosterone negative feedback on the hypothalamic-pituitary axis. The consequent secretion of additional LH, under this hypothesis, causes a restoration of normal levels of serum testosterone. Kitl, acting via PI 3-kinase, is a paracrine regulator of Leydig cell steroidogenic function in vivo.

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

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

GATA-1 testis activation region is essential for Sertoli cell-specific expression of GATA-1 gene in transgenic mouse

BACKGROUND

The erythroid transcription factor GATA-1 is also expressed in Sertoli cells of the testis. The testicular expression of GATA-1 is regulated in a developmental and spermatogenic stage-specific manner. To further clarify the regulatory mechanisms of testicular GATA-1 gene expression, we carried out transgenic reporter gene expression analyses.

RESULTS

We found that GATA-1 expression in Sertoli cells is markedly decreased concomitant with the emergence of elongated spermatids in the seminiferous tubules. Transgenic reporter mouse analyses revealed that a 15 kb GATA-1 genomic region is sufficient to recapitulate the gene expression profile in Sertoli cells. While the GATA-1 haematopoietic enhancer and the proximal first exon are included within the 15 kb genomic region, these regulatory elements are not essential for GATA-1 expression in Sertoli cells. Further analyses using deletion constructs revealed that a 1.5 kb region 5' to the GATA-1 haematopoietic enhancer is essential for gene expression in Sertoli cells and this region is referred to as the GATA-1 testis activation region.

CONCLUSION

These results thus demonstrated that the GATA-1 testis activation region is essential for Sertoli cell-specific expression of GATA-1 gene. The 15 kb genomic region is applicable and useful for the expression vector system specific for adult Sertoli cells in stage VII to IX.

The role of Kit-ligand in melanocyte development and epidermal homeostasis

Kit-ligand (Kitl) also known as steel factor, stem cell factor and mast cell growth factor plays a crucial role in the development and maintenance of the melanocyte lineage in adult skin. Kitl exerts permanent survival, proliferation and migration functions in Kit receptor-expressing melanocytes. A comprehensive overview over the differential roles of Kitl in melanocyte development and homeostasis is provided. I discuss species-specific differences of the Kitl/Kit signalling system, regulation at the transcriptional level and also covering the regulation of cell surface Kitl presentation by cytoplasmic targeting sequences. In addition, recent studies evoked the importance of Kitl misexpression in some hyperpigmented lesions that may open the avenue for Kitl-dependent treatment of pathological skin conditions.

Mouse GGN1 and GGN3, two germ cell-specific proteins from the single gene Ggn, interact with mouse POG and play a role in spermatogenesis

The germ cell-deficient (gcd) mutation is a recessive transgenic insertional mutation leading to a deficiency of primordial germ cells (PGCs). We have recently shown that the gene underlying this mutation is Pog, which is necessary for normal proliferation of PGCs. Here we show that Pog is also involved in spermatogenesis in that meiosis is impaired in Pog-deficient mice. Yeast two-hybrid screening revealed that POG interacted with GGN1 and GGN3, two proteins formed by alternate splicing of the same gene, gametogenetin (Ggn). Ggn had more than 10 different splice variants giving rise to three proteins, GGN1, GGN2, and GGN3. The three proteins had different subcellular localizations, with GGN1, GGN2, and GGN3 localized along the nuclear membrane, in the cytoplasm, and in the nucleus/nucleoli respectively. The expression of Ggn was confined to late pachytene spermatocytes and round spermatids, a time window concomitant with the occurrence of meiosis. Mouse Ggn and Pog were both expressed in primary spermatocytes. Co-expression of POG with GGN1 or GGN3 in HeLa cells changed the localization of POG to the perinuclear localization or the nucleoli, respectively. Our data showed that in addition to functioning in proliferation of primordial germ cells, POG also functioned in spermatogenesis. Two spatial and temporal regulated proteins, GGN1 and GGN3, interacted with POG, regulated the localization of POG, and played a role in spermatogenesis.

Developmental fate of embryonic germ cells (EGCs), in vivo and in vitro

Embryonic germ cells (EGCs) derived from mouse primordial germ cells (PGCs) are known both to colonize all cell lineages of the fetus and to make tumors in vivo. When aggregated with eight-cell embryos, EGCs from a new EGC line expressing green fluorescent protein (GFP) were found to contribute preferentially to the epiblast but unexpectedly were also capable of colonizing primary endoderm. When injected under the kidney capsule, EGCs derived from 12.5 days post coitum (dpc) PGCs formed differentiated tumors. The ability of EGCs to differentiate in an organ culture system depends upon their partners in cell culture. When EGCs, marked with a LacZ transgene, were mixed with disaggregated and reaggregated mouse fetal lung in an organ culture system, they remained undifferentiated. In urogenital ridge reaggregates on the other hand, some EGCs were capable of differentiating to form small epithelial cysts.

Electroporated transgene-rescued spermatogenesis in infertile mutant mice with a sertoli cell defect

The molecular basis of most human male infertility arising from spermatogenesis disruption is poorly understood because of a lack of useful investigation systems. To study the roles of the supporting Sertoli cells in mammalian spermatogenesis, we improved an electroporation technique for seminiferous tubules in vivo. Because Sertoli cells barely proliferate in mature testis, linear transgenes are not incorporated into the genome and quickly degrade. However, circular expression vector is stably expressed in Sertoli cells for a long period. By electrotransformation of a complete cDNA, we rescued defective spermatogenesis in infertile Sl(17H)/Sl(17H) mutant mice with partial dysfunction of stem cell factor in Sertoli cells. Application of this gene transfer system will facilitate both the understanding of spermatogenesis and the development of new gene therapies for human male infertility.

An Allelic Series of Mutations in the Kit ligand Gene of Mice. I. Identification of Point Mutations in Seven Ethylnitrosourea-Induced KitlSteel Alleles

An allelic series of mutations is an extremely valuable genetic resource for understanding gene function. Here we describe eight mutant alleles at the Steel (Sl) locus of mice that were induced with N-ethyl-N-nitrosourea (ENU). The product of the Sl locus is Kit ligand (or Kitl; also known as mast cell growth factor, stem cell factor, and Steel factor), which is a member of the helical cytokine superfamily and is the ligand for the Kit receptor tyrosine kinase. Seven of the eight ENU-induced KitlSl alleles, of which five cause missense mutations, one causes a nonsense mutation and exon skipping, and one affects a splice site, were found to contain point mutations in Kitl. Interestingly, each of the five missense mutations affects residues that are within, or very near, conserved α-helical domains of Kitl. These ENU-induced mutants should provide important information on structural requirements for function of Kitl and other helical cytokines.

Role of p38 and ERK MAP kinase in proliferation of erythroid progenitors in response to stimulation by soluble and membrane isoforms of stem cell factor

Two alternatively spliced stem cell factor (SCF) transcripts encode protein products, which differ in the duration of membrane presentation. One form, soluble SCF (S-SCF) gets rapidly processed to yield predominantly secreted protein. The other form, membrane-associated SCF (MA-SCF) lacks the primary proteolytic cleavage site but is cleaved slowly from an alternate site, and thus represents a more stable membrane form of SCF. Mutants of SCF that lack the expression of MA-SCF (Steel-dickie) or possess a defect in its presentation (Steel17H) manifest deficiencies in erythroid cell development. In this study, we have compared the consequence(s) of activating Kit, the receptor for SCF by MA-SCF with S-SCF, and an obligate membrane-restricted (MR) form of SCF (MR-SCF) on erythroid cell survival, proliferation, cell cycle progression, and the activation of p38 and ERK MAP kinase pathways. Activation of Kit by MR-SCF was associated with a significantly lower incidence of apoptosis and cell death in erythroid cells compared to either other isoform. MR- or MA-SCF–induced stimulation of erythroid cells resulted in similar and significantly greater proliferation and cell cycle progression compared to soluble SCF. The increase in proliferation and cell cycle progression via MA- or MR-SCF stimulation correlated with sustained and enhanced activation of p38 and ERK MAP kinase pathways. In addition, MR- or MA-SCF–induced proliferation was more sensitive to the inhibitory effects of ERK inhibitor compared to S-SCF–induced proliferation. In contrast, soluble SCF-induced proliferation was more sensitive to the inhibitory effects of p38 inhibitor compared with MR- or MA-SCF. These results suggest that different isoforms of SCF may use different biochemical pathways in stimulation of survival and/or proliferation of erythroid cells.

Molecular genetics of male infertility: stem cell factor/c-kit system

PROBLEM

Infertility, affects about 5% of human males and genetic factors are recognized in approximately 30% of them. The mouse represents a good model to study autosomal genes that might play a role in spermatogenesis. In mice, mutations in the c-kit gene and in the gene encoding stem cell factor (SCF) cause pleiotropic defects among which sterility. A possible involvement of the SCF/c-kit system in human spermatogenesis was investigated.

METHODS OF STUDY

A group of 65 idiopathic azoospermic patients was screened for the presence of mutations in the human c-kit gene codon encoding tyrosine 721 (Y721), analogous to Y719 in the murine c-kit gene (a residue known to be essential for a normal spermatogonial proliferation). Furthermore we have used a mouse model for studying the molecular mechanisms that regulate the transcription of the endogenous SCF gene.

RESULTS

No mutations have been detected on codon encoding Y721 of the human c-kit gene, in our group of infertile patients.

CONCLUSIONS

A larger group of azoospermic patients, including preferentially patients affected by Sertoli-cell-only syndrome, should be screened in order to exclude a role of c-kit mutations in Y721 in spermatogenesis defects. In this study we also show that the murine SCF promoter is transcriptionally active and stimulated by follicle stimulating hormone (FSH), 3'-5' cyclic adenosine monophosphate (cAMP) analogs, and IBMX in primary mouse Sertoli cells, and that the cAMP effect is cell-specific, as the SCF promoter is not stimulated in other SCF-expressing cell types tested.

Use of stepwise subtraction to comprehensively isolate mouse genes whose transcription is up-regulated during spermiogenesis

We report the isolation of 153 mouse genes whose expression is dramatically up-regulated during spermiogenesis. We used a novel variation of the subtractive hybridization technique called stepwise subtraction, wherein the subtraction process is systematically repeated in a stepwise manner. We named the genes thus identified as TISP genes (transcript induced in spermiogenesis). The transcription of 80 of these TISP genes is almost completely specific to the testis. This transcription is abruptly turned on after 17 days of age, when the mice enter puberty and spermiogenesis is initiated. Considering that the most advanced cells present at these stages of spermatogenesis are the spermatids, it is likely that we could isolate most of the spermatid-specific genes. DNA sequencing revealed that about half the TISP genes are novel and uncharacterized genes, confirming the utility of the stepwise subtraction approach for gene discovery.

Adenovirus-mediated gene delivery and in vitro microinsemination produce offspring from infertile male mice

Sertoli cells play a pivotal role in spermatogenesis through their interactions with germ cells. To set up a strategy for treating male infertility caused by Sertoli cell dysfunction, we developed a Sertoli cell gene transfer system by using an adenovirus vector, which maintained long-term transgene expression in the testes of infertile mice. Introduction of an adenovirus carrying the mouse Steel (Sl) gene into Sertoli cells restored partial spermatogenesis in infertile Steel/Steel(dickie) (Sl/Sl(d)) mutant mouse testes. Although these males remained infertile, round spermatids and spermatozoa from the testes produced normal fertile offspring after intracytoplasmic injection into oocytes. None of the offspring showed evidence of germ line transmission of adenoviral DNA. Thus, we demonstrate a successful treatment for infertility by using a gene therapy vector. Therefore, adenovirus-mediated gene delivery into Sertoli cells not only provides an efficient and convenient means for studying germ cell-Sertoli cell interactions through manipulation of the germ cell microenvironment in vivo, but also is a useful method to treat male infertility resulting from a Sertoli cell defect.

Signaling through extracellular signal-regulated kinase is required for spermatogonial proliferative response to stem cell factor

In vitro addition of stem cell factor (SCF) to c-kit-expressing A(1)-A(4) spermatogonia from prepuberal mice stimulates their progression into the mitotic cell cycle and significantly reduces apoptosis in these cells. SCF addition results in a transient activation of extracellular signal-regulated kinases (Erk)1/2 as well as of phosphatidylinositol 3-kinase (PI3K)-dependent Akt kinase. These events are followed by a rapid re-distribution of cyclin D3, which becomes predominantly nuclear, whereas its total cellular amount does not change. Nuclear accumulation of cyclin D3 is coupled to transient activation of the associated kinase activity, assayed using the retinoblastoma protein (Rb) as a substrate. These events were followed by a transient accumulation of cyclin E, stimulation of the associated histone H1-kinase activity, a delayed accumulation of cyclin A2, and Rb hyper-phosphorylation. All the events associated with SCF-induced cell cycle progression are inhibited by the addition of either a PI3K inhibitor or a mitogen-activated protein-kinase kinase (MEK) inhibitor, indicating that both MEK and PI3K are essential for c-kit-mediated proliferative response. On the contrary, the anti-apoptotic effect of SCF is not influenced by the separate addition of either MEK or PI3K inhibitors. Thus, SCF effects on mitogenesis and survival in c-kit expressing spermatogonia rely on different signal transduction pathways.

Stem cell factor presentation to c-Kit. Identification of a basolateral targeting domain

Stem cell factor (also known as mast cell growth factor and kit-ligand) is a transmembrane growth factor with a highly conserved cytoplasmic domain. Basolateral membrane expression in epithelia and persistent cell surface exposure of stem cell factor are required for complete biological activity in pigmentation, fertility, learning, and hematopoiesis. Here we show by site-directed mutagenesis that the cytoplasmic domain of stem cell factor contains a monomeric leucine-dependent basolateral targeting signal. N-terminal to this motif, a cluster of acidic amino acids serves to increase the efficiency of basolateral sorting mediated by the leucine residue. Hence, basolateral targeting of stem cell factor requires a mono-leucine determinant assisted by a cluster of acidic amino acids. This mono-leucine determinant is functionally conserved in colony-stimulating factor-1, a transmembrane growth factor related to stem cell factor. Furthermore, this leucine motif is not capable of inducing endocytosis, allowing for persistent cell surface expression of stem cell factor. In contrast, the mutated cytoplasmic tail found in the stem cell factor mutant Mgf(Sl17H) induces constitutive endocytosis by a motif that is related to signals for endocytosis and lysosomal targeting. Our findings therefore present mono-leucines as a novel type of protein sorting motif for transmembrane growth factors.

Il-7 and not stem cell factor reverses both the increase in apoptosis and the decline in thymopoiesis seen in aged mice

Thymic atrophy is an age-associated decline in commitment to the T cell lineage considered to be associated with defective TCR beta-chain rearrangement. Both IL-7 and stem cell factor (SCF) have dominant roles at this stage of triple negative (TN) thymocyte development. Because there is no age-associated decrease in the number of CD44(+)CD25(-)CD3(-)CD4(-)CD8(-) cells, this study investigated whether alterations in apoptosis within the TN pathway accounted for diminishing thymocyte numbers with age. Here we show significant age-associated increases in apoptotic TN thymocytes, specifically within CD44(+)CD25(+) and CD44(-)CD25(+) subpopulations, known to be the location of TCR beta-chain rearrangement. IL-7 added to TN cultures established from old mice significantly both reduces apoptosis and increases the percentage of live cells within CD44(+)CD25(+) and CD44(-)CD25(+) subpopulations after 24 h, with prosurvival effects remaining after 5 days. SCF failed to demonstrate prosurvival effects in old or young cultures, and IL-7 and SCF together did not improve upon IL-7 alone. IL-7R expression did not decline with age, ruling out the possibility that the age-associated increase in apoptosis was attributed to reduced IL-7R expression. Compared with PBS, treatment of old mice with IL-7 produced significant increases in live TN cells. By comparison, treatment with SCF failed to increase live TN numbers, and IL-7 and SCF together failed to significantly improve thymopoiesis above that shown by IL-7 alone. Thus, treatment with IL-7 alone can reverse the age-associated defect in TN thymocyte development revealed by in vitro studies to be located at the stages of TCR beta-chain rearrangement.

Defect in germ cells, not in supporting cells, is the cause of male infertility in the jsd mutant mouse: proliferation of spermatogonial stem cells without differentiation

C57BL/6 (B6)-jsd/jsd male mice are sterile because of lack of spermatogenesis. To find the cause of the deficient spermatogenesis, we have examined whether the mutation phenotype is the result of a defect in germ cells or in supporting cells using germ cell transplantation. In the seminiferous tubules of B6-jsd/jsd mutant mice, donor germ cells derived from the wild type GFP transgenic mouse (B6-+/+GFP) were able to undergo complete spermatogenesis, indicating that the juvenile spermatogonial depletion (jsd/jsd) mouse possesses normal supporting cell functions. In contrast, undifferentiated spermatogonia derived from B6-jsd/jsd mice were unable to differentiate in the seminiferous tubules of W/W v mice, even if the mutant germ cells successfully settled in the tubules. These results demonstrate that the deficiency in spermatogenesis of B6-jsd/jsd mice can be ascribed to a defect in spermatogonia but not in their supporting cell environment. Furthermore, the defect in B6-jsd/jsd spermatogonia is not in their ability to proliferate, but in their differentiation and may result from their hypersensitivity to high concentrations of androgen in the testis.

The metastatic ability of Ewing's sarcoma cells is modulated by stem cell factor and by its receptor c-kit

Ewing's sarcoma is a primitive highly malignant tumor of bone and soft tissues usually metastasizing to bone, bone marrow, and lung. Growth factor receptors and their ligands may be involved in its growth and dissemination. We analyzed the expression of c-kit and its ligand stem cell factor (SCF) in a panel of six Ewing's sarcoma cell lines. All cell lines exhibited substantial levels of surface c-kit expression, and five of six displayed transmembrane SCF on the cell surface. Expression of c-kit was down-modulated in all lines by exposure to exogenous SCF. The SCF treatment was able to confer to cells a growth advantage in vitro, due both to an increase in cell proliferation and to a reduction in the apoptotic rate. When used in the lower compartment of a migration chamber, SCF acted as a strong chemoattractant for Ewing's sarcoma cells. The pretreatment of cells with SCF reduced their chemotactic response to SCF. In athymic nude mice, Ewing's sarcoma cells injected intravenously metastasized to the lung and to a variety of extrapulmonary sites, including bone and bone marrow. Metastatic sites resembled those observed in Ewing's sarcoma patients and corresponded to SCF-rich microenvironments. The in vitro pretreatment of cells with SCF strongly reduced the metastatic ability of Ewing's sarcoma cells, both to the lung and to extrapulmonary sites. This could be dependent on the down-modulation of c-kit expression observed in SCF-pretreated cells, leading to a reduced sensitivity to the chemotactic and proliferative actions of SCF. Our results indicate that the response to SCF mediated by c-kit may be involved in growth, migration, and metastatic ability of Ewing's sarcoma cells.

Role of c-kit in mammalian spermatogenesis

The tyrosine-kinase receptor c-kit and its ligand, stem cell factor (SCF), are essential for the maintenance of primordial germ cells (PGCs) in both sexes. However, c-kit and a post-meiotic-specific alternative c-kit gene product play important roles also during post-natal stages of spermatogenesis. In the adult testis, the c-kit receptor is re-expressed in differentiating spermatogonia, but not in spermatogonial stem cells, whereas SCF is expressed by Sertoli cells under FSH stimulation. SCF stimulates DNA synthesis in type A spermatogonia cultured in vitro, and injection of anti-c-kit antibodies blocks their proliferation in vivo. A point mutation in the c-kit gene, which impairs SCF-mediated activation of phosphatidylinositol 3-kinase, does not cause any significant reduction in PGCs number during embryonic development, nor in spermatogonial stem cell populations. However males are completely sterile due to a block in the initial stages of spermatogenesis, associated to abolishment of DNA-synthesis in differentiating A1-A4 spermatogonia. With the onset of meiosis c-kit expression ceases, but a truncated c-kit product, tr-kit, is specifically expressed in post-meiotic stages of spermatogenesis, and is accumulated in mature spermatozoa. Microinjection of tr-kit into mouse eggs causes their parthenogenetic activation, suggesting that it might play a role in the final function of the gametes, fertilization.

Regulation of proliferation and differentiation in spermatogonial stem cells: the role of c-kit and its ligand SCF

To study self-renewal and differentiation of spermatogonial stem cells, we have transplanted undifferentiated testicular germ cells of the GFP transgenic mice into seminiferous tubules of mutant mice with male sterility, such as those dysfunctioned at Steel (Sl) locus encoding the c-kit ligand or Dominant white spotting (W) locus encoding the receptor c-kit. In the seminiferous tubules of Sl/Sl(d) or Sl(17H)/Sl(17H) mice, transplanted donor germ cells proliferated and formed colonies of undifferentiated c-kit (−) spermatogonia, but were unable to differentiate further. However, these undifferentiated but proliferating spermatogonia, retransplanted into Sl (+) seminiferous tubules of W mutant, resumed differentiation, indicating that the transplanted donor germ cells contained spermatogonial stem cells and that stimulation of c-kit receptor by its ligand was necessary for maintenance of differentiated type A spermatogonia but not for proliferation of undifferentiated type A spermatogonia. Furthermore, we have demonstrated that their transplantation efficiency in the seminiferous tubules of Sl(17H)/Sl(17H) mice depended upon the stem cell niche on the basement membrane of the recipient seminiferous tubules and was increased by elimination of the endogenous spermatogonia of mutant mice from the niche by treating them with busulfan.

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

The Kit receptor tyrosine kinase functions in hemato- poiesis, melanogenesis and gametogenesis. Kit receptor-mediated cellular responses include proliferation, survival, adhesion, secretion and differentiation. In mast cells, Kit-mediated recruitment and activation of phosphatidylinositol 3'-kinase (PI 3-kinase) produces phosphatidylinositol 3'-phosphates, plays a critical role in mediating cell adhesion and secretion and has contributory roles in mediating cell survival and proliferation. To investigate the consequences in vivo of blocking Kit-mediated PI 3-kinase activation we have mutated the binding site for the p85 subunit of PI 3-kinase in the Kit gene, using a knock-in strategy. Mutant mice have no pigment deficiency or impairment of steady-state hematopoiesis. However, gametogenesis is affected in several ways and tissue mast cell numbers are affected differentially. While primordial germ cells during embryonic development are not affected, Kit(Y719F)/Kit(Y719F) males are sterile due to a block at the premeiotic stages in spermatogenesis. Furthermore, adult males develop Leydig cell hyperplasia. The Leydig cell hyperplasia implies a role for Kit in Leydig cell differentiation and/or steroidogenesis. In mutant females follicle development is impaired at the cuboidal stages resulting in reduced fertility. Also, adult mutant females develop ovarian cysts and ovarian tubular hyperplasia. Therefore, a block in Kit receptor-mediated PI 3-kinase signaling may be compensated for in hematopoiesis, melanogenesis and primordial germ cell development, but is critical in spermatogenesis and oogenesis.

The biology of stem cell factor and its receptor C-kit

The receptor tyrosine kinase c-Kit and its ligand Stem Cell Factor (SCF) are essential for haemopoiesis, melanogenesis and fertility. SCF acts at multiple levels of the haemopoietic hierarchy to promote cell survival, proliferation, differentiation, adhesion and functional activation. It is of particular importance in the mast cell and erythroid lineages, but also acts on multipotential stem and progenitor cells, megakaryocytes, and a subset of lymphoid progenitors. SCF exists in soluble or transmembrane forms which appear to differ in function. Multiple isoforms of c-Kit also exist as a result of alternate mRNA splicing, proteolytic cleavage and the use of cryptic internal promoters in certain cell types. This review focuses on what is known about the regulation of c-Kit expression, the functions of SCF and c-Kit isoforms, and the nature of the biological responses elicited by this receptor-ligand pair with emphasis on the haemopoietic system.

The Presence of Novel Amino Acids in the Cytoplasmic Domain of Stem Cell Factor Results in Hematopoietic Defects inSteel17H Mice

Stem cell factor (SCF) is expressed as an integral membrane growth factor that may be differentially processed to produce predominantly soluble (S) (SCF248) or membrane-associated (MA) (SCF220) protein. A critical role for membrane presentation of SCF in the hematopoietic microenvironment (HM) has been suggested from the phenotype of the Steel-dickie(Sld) mice, which lack MA SCF, and by studies performed in our laboratory (and by others) using long-term bone marrow cultures and transgenic mice expressing different SCF isoforms.Steel17H (Sl17H) is an SCF mutant that demonstrates melanocyte defects and sterility in males but not in females. The Sl17H allele contains a intronic mutation resulting in the substitution of 36 amino acids (aa’s) in the SCF cytoplasmic domain with 28 novel aa’s. This mutation, which affects virtually the entire cytoplasmic domain of SCF, could be expected to alter membrane SCF presentation. To investigate this possibility, we examined the biochemical and biologic properties of the Sl17H-encoded protein and its impact in vivo and in vitro on hematopoiesis and on c-Kit signaling. We demonstrate that compound heterozygous Sl/Sl17H mice manifest multiple hematopoietic abnormalities in vivo, including red blood cell deficiency, bone marrow hypoplasia, and defective thymopoiesis. In vitro, both S and MA Sl17H isoforms of SCF exhibit reduced cell surface expression on stromal cells and diminished biological activity in comparison to wild-type (wt) SCF isoforms. These alterations in presentation and biological activity are associated with a significant reduction in the proliferation of an SCF-responsive erythroid progenitor cell line and in the activation of phosphatidylinositol 3-Kinase/Akt and mitogen-activated protein-Kinase signaling pathways. In vivo, transgene expression of the membrane-restricted (MR) (SCFX9/D3) SCF in Sl/Sl17H mutants results in a significant improvement in peripheral red blood cell counts in comparison toSl/Sl17H mice.

The Presence of Novel Amino Acids in the Cytoplasmic Domain of Stem Cell Factor Results in Hematopoietic Defects inSteel17H Mice

Stem cell factor (SCF) is expressed as an integral membrane growth factor that may be differentially processed to produce predominantly soluble (S) (SCF248) or membrane-associated (MA) (SCF220) protein. A critical role for membrane presentation of SCF in the hematopoietic microenvironment (HM) has been suggested from the phenotype of the Steel-dickie(Sld) mice, which lack MA SCF, and by studies performed in our laboratory (and by others) using long-term bone marrow cultures and transgenic mice expressing different SCF isoforms.Steel17H (Sl17H) is an SCF mutant that demonstrates melanocyte defects and sterility in males but not in females. The Sl17H allele contains a intronic mutation resulting in the substitution of 36 amino acids (aa’s) in the SCF cytoplasmic domain with 28 novel aa’s. This mutation, which affects virtually the entire cytoplasmic domain of SCF, could be expected to alter membrane SCF presentation. To investigate this possibility, we examined the biochemical and biologic properties of the Sl17H-encoded protein and its impact in vivo and in vitro on hematopoiesis and on c-Kit signaling. We demonstrate that compound heterozygous Sl/Sl17H mice manifest multiple hematopoietic abnormalities in vivo, including red blood cell deficiency, bone marrow hypoplasia, and defective thymopoiesis. In vitro, both S and MA Sl17H isoforms of SCF exhibit reduced cell surface expression on stromal cells and diminished biological activity in comparison to wild-type (wt) SCF isoforms. These alterations in presentation and biological activity are associated with a significant reduction in the proliferation of an SCF-responsive erythroid progenitor cell line and in the activation of phosphatidylinositol 3-Kinase/Akt and mitogen-activated protein-Kinase signaling pathways. In vivo, transgene expression of the membrane-restricted (MR) (SCFX9/D3) SCF in Sl/Sl17H mutants results in a significant improvement in peripheral red blood cell counts in comparison toSl/Sl17H mice.

Arrest of spermatogonial differentiation in jsd/jsd, Sl17H/Sl17H, and cryptorchid mice

The nature of the spermatogenic arrest in cryptorchid C57Bl mice and in jsd/jsd and Sl17H/Sl17H mutant mice was identified by studying whole mounts of seminiferous tubules. In all three types of mice, virtually only A spermatogonia were found, topographically arranged in clones of 1 to 16 (rarely more) cells. These clonal sizes are typical for undifferentiated spermatogonia. The proportion of these cells lying in chains of more than 2 cells (50-70%) was comparable to that seen in epithelial stages VII-VIII in the normal epithelium. It is concluded that in all three types of mice, spermatogenesis is arrested at the point where the undifferentiated A spermatogonia, specifically A(al) spermatogonia, differentiate into the first generation of the differentiating-type spermatogonia, the A1 spermatogonia. The remaining A spermatogonia were proliferating, but no accumulation of spermatogonia was present, as spermatogonial apoptosis also took place. Spermatogonial clones of all sizes were seen to undergo apoptosis, but there were relatively many large apoptotic clones, indicating that the clones became more vulnerable when they became larger. In contrast to what is seen in the normal epithelium, odd-numbered clones, not composed of 2(n) cells, were present, as well as clumps of 2 or more spermatogonial nuclei in the same cytoplasm, in all three types of mice. This indicates a lack of integrity of spermatogonial clones, also observed in other situations with a relative paucity of cells on the basal membrane. It is concluded that the differentiation of the undifferentiated spermatogonia, affected in all three types of mice as well as in vitamin A-deficient animals, is a rather vulnerable point in the spermatogenic developmental pathway.

Hepatoma-derived growth factor-related protein (HRP)-1 gene in spermatogenesis in mice

Hepatoma-derived growth factor (HDGF)-related protein (HRP)-1, a member of the HDGF gene family, showed testis-specific expression in mice. HRP-1 expression in spermatogenesis was analyzed in the testis of normal and azoospermic mice by Northern blot and immunohistochemistry. HRP-1 gene message was not expressed in the ovary and its product was detected only in the nuclei of germ cells, not in somatic cells. The HRP-1 gene is expressed through pachytene spermatocyte to round spermatid. HRP-1 gene expression was not detected in the testis of cryptorchid mice or in some strains of mutant mice. These findings suggest that the testis-specific HRP-1 gene may play an important role in the phase around meiotic cell division.