Cell Competition During Growth and Regeneration

Tissue growth and regeneration are autonomous, stem-cell-mediated processes in which stem cells within the organ self-renew and differentiate to create new cells, leading to new tissue. The processes of growth and regeneration require communication and interplay between neighboring cells. In particular, cell competition, which is a process in which viable cells are actively eliminated by more competitive cells, has been increasingly implicated to play an important role. Here, we discuss the existing literature regarding the current landscape of cell competition, including classical pathways and models, fitness fingerprint mechanisms, and immune system mechanisms of cell competition. We further discuss the clinical relevance of cell competition in the physiological processes of tissue growth and regeneration, highlighting studies in clinically important disease models, including oncological, neurological, and cardiovascular diseases.

Transcriptional and translational regulators of gene expression in germ cell development

Germ cell development is a highly coordinated process driven, in part, by regulatory mechanisms that control gene expression. Not only transcription, but also translation, is under regulatory control to direct proper germ cell development. In this dissertation, I have focused on two regulators of germ cell development. One is the homeobox protein RHOX10, which has the potential to be both a transcriptional and translational regulator in mouse male germ cell development. The other is the RNA-binding protein, Hermes, which functions as a translational regulator in Xenopus laevis female germ cell development. ^ Rhox10 is a member of reproductive homeobox gene X-(linked (Rhox) gene cluster, of which expression is developmentally regulated in developing mouse testes. To identify the cell types and developmental stages in which Rhox10 might function, I characterized its temporal and spatial expression pattern in mouse embryonic, neonatal, and adult tissues. Among other things, this analysis revealed that both the level and the subcellular localization of RHOX10 are regulated during germ cell development. To understand the role of Rhox10 in germ cell development, I generated transgenic mice expressing an artificial microRNA (miRNA) targeting Rhox10. While this artificial miRNA robustly downregulated RHOX10 protein expression in vitro, it did not significantly reduce RHOX10 expression in vivo. So I next elected to knockdown RHOX10 levels in spermatogonial stem cells (SSCs), which I found highly express both Rhox10 mRNA and RHOX10 protein. Using a recently developed in vitro culture system for SSCs combined with a short-hairpin RNA (shRNA) approach, I strongly depleted RHOX10 expression in SSCs. These RHOX10-depleted cells exhibited a defect in the ability to form stem cell clusters in vitro. Expression profiling analysis revealed many genes regulated by Rhox10, including many meiotic genes, which could be downstream of Rhox10 in a molecular pathway that controls SSC differentiation. ^ RNA recognition motif (RRM) containing protein, Hermes is localized in germ plasm, where dormant mRNAs are also located, of Xenopus oocytes, which implicates its role in translational regulator. To understand the function of Hermes in oocyte meiosis, I used a morpholino oligonucleotide (MO) based knockdown approach. Microinjection of Hermes MO into fully grown oocytes, which are arrested in meiotic prophase, caused acceleration of oocytes reentry into meiosis (i.e., maturation) upon progesterone induction. Using a candidate approach, I identified at least three targets of Hermes: Ringo/Spy, Xcat2, and Mos. Ringo/Spy and Mos are known to have functions in oocyte maturation, while Ringo/Spy, Xcat2 mRNA are localized in the germ plasm of oocytes, which drives germ cell specification after fertilization. This led me to propose that Hermes functions in both oocyte maturation and germ cell development through its ability to regulate 3 crucial target mRNAs. ^

Analysis of Wnt/beta-catenin signaling in kidney tubulogenesis

The β-catenin/Lef/Tcf-mediated Wnt pathway is central to the developmental of all animals, stem cell renewal, and cancer progression. Prior studies in frogs and mice have indicated that the ligand Wnt-4 is essential for the mesenchyme to epithelial transition that generates tubules in the context of kidney organogenesis. More recently, Wnt-9b in mice, was likewise found to be required. Yet despite the importance of Wnt signals in renal development, the corresponding Frizzled receptor(s) and downstream signaling mechanim(s) are unclear. My work addresses these knowledge gaps using in vitro (Madin-Darby Canine Kidney cells) and in vivo (Xenopus laevis and zebrafish pronephros) tubulogenic kidney model systems. Employing established reporter constructs of Wnt/β-catenin pathway activity, I have determined that MDCK cells are highly responsive to Wnt-4, -1, and -3A, but not to Wnt-5A and control conditions. I have confirmed that Wnt-4's canonical signaling activity in MDCK cells is mediated by downstream effectors of the Wnt/β-catenin pathway using β-Engrailed and dnTCF-4, constructs that suppress this pathway. I have further found that MDCK cells express the Frizzled-6 receptor, and that Wnt-4 forms a biochemical complex with Frizzled-6, yet does not appear to transduce Wnt-4's canonical signal. Additionally, I demonstrate that standard Hepatocyte Growth Factor (HGF)-mediated (non-physiologic) induction of MDCK tubulogenesis in collagen matrices is not altered by activation or suppression of β-catenin signaling activity; however, β-catenin signaling maintains cell survival in this in vitro system. Using a Wnt/β-catenin signaling reporter in Xenopus laevis, I detect β-catenin signaling activity in the early pronephric epithelial kidney tubules. By inhibiting the Wnt/β-catenin signaling pathway in both zebrafish and Xenopus , a significant loss of kidney tubulogenesis is observed with little or no effect on adjoining axis or somite development. This inhibition also leads to the appearance of severe edema that phenocopies embryos depleted for Wnt-4. Tubulogenic loss does not appear to be caused by increased cell death in the Xenopus pronephric field, but rather by lessened expression of tubule epithelium genes associated with cellular differentiation. Together, my results show that Wnt/β-catenin signaling is required for renal tubule development and that Wnt-4 is a strong candidate for activating this pathway. ^

TAZ AS A REGULATOR OF MESENCHYMAL TRANSFORMATION AND CLINICAL AGGRESSIVENESS IN GLIOMAS

Glioblastoma multiforme (GBM) is an aggressive, high grade brain tumor. Microarray studies have shown a subset of GBMs with a mesenchymal gene signature. This subset is associated with poor clinical outcome and resistance to treatment. To establish the molecular drivers of this mesenchymal transition, we correlated transcription factor expression to the mesenchymal signature and identified transcriptional co-activator with PDZ-binding motif (TAZ) to be highly associated with the mesenchymal shift. High TAZ expression correlated with worse clinical outcome and higher grade. These data led to the hypothesis that TAZ is critical to the mesenchymal transition and aggressive clinical behavior seen in GBM. We investigated the expression of TAZ, its binding partner TEAD, and the mesenchymal marker FN1 in human gliomas. Western analyses demonstrated increased expression of TAZ, TEAD4, and FN1 in GBM relative to lower grade gliomas. We also identified CpG islands in the TAZ promoter that are methylated in most lower grade gliomas, but not in GBMs. TAZ-methylated glioma stem cell (GSC) lines treated with a demethylation agent showed an increase in mRNA and protein TAZ expression; therefore, methylation may be another novel way TAZ is regulated since TAZ is epigenetically silenced in tumors with a better clinical outcome. To further characterize the role of TAZ in gliomagenesis, we stably silenced or over-expressed TAZ in GSCs. Silencing of TAZ decreased invasion, self-renewal, mesenchymal protein expression, and tumor-initiating capacity. Over-expression of TAZ led to an increase in invasion, mesenchymal protein expression, mesenchymal differentiation, and tumor-initiating ability. These actions are dependent on TAZ interacting with TEAD since all these effects were abrogated with TAZ could not bind to TEAD. We also show that TAZ and TEAD directly bind to mesenchymal gene promoters. Thus, TAZ-TEAD interaction is critically important in the mesenchymal shift and in the aggressive clinical behavior of GBM. We identified TAZ as a regulator of the mesenchymal transition in gliomas. TAZ could be used as a biomarker to both estimate prognosis and stratify patients into clinically relevant subgroups. Since mesenchymal transition is correlated to tumor aggressiveness, strategies to target and inhibit TAZ-TEAD and the downstream gene targets may be warranted in alternative treatment.

MicroRNA Regulation of Prostate Cancer Stem/Progenitor Cells and Prostate Cancer Development

Most human tumors contain a population of cells with stem cell properties, called cancer stem cells (CSCs), which are believed to be responsible for tumor establishment, metastasis, and resistance to clinical therapy. It’s crucial to understand the regulatory mechanisms unique to CSCs, so that we may design CSC-specific therapeutics. Recent discoveries of microRNA (miRNA) have provided a new avenue in understanding the regulatory mechanisms of cancer. However, how miRNAs may regulate CSCs is still poorly understood. Here, we present miRNA expression profiling in six populations of prostate cancer (PCa) stem/progenitor cells that possess distinct tumorigenic properties. Six miRNAs were identified to be commonly and differentially expressed, namely, four miRNAs (miR-34a, let-7b, miR-106a and miR-141) were under-expressed, and two miRNAs (miR-301 and miR-452) were over-expressed in the tumorigenic subsets compared to the corresponding marker-negative subpopulations. Among them, the expression patterns of miR-34, let-7b, miR-141 and miR-301 were further confirmed in the CD44+ human primary prostate cancer (HPCa) samples. We then showed that miR-34a functioned as a critical negative regulator in prostate CSCs and PCa development and metastasis. Over-expression of miR-34a in either bulk or CD44+ PCa cells significantly suppressed clonal expansion, tumor development and metastasis. Systemic delivery of miR-34a in tumor-bearing mice demonstrated a potent therapeutic effect again tumor progression and metastasis, leading to extended animal survival. Of great interest, we identified CD44 itself as a direct and relevant downstream target of miR-34a in mediating its tumor-inhibitory effects. Like miR-34a, let-7 manifests similar tumor suppressive effects in PCa cells. In addition, we observed differential mechanisms between let-7 and miR-34a on cell cycle, with miR-34a mainly inducing G1 cell-cycle arrest followed by cell senescence and let-7 inducing G2/M arrest. MiR-301, on the other hand, exerted a cell type dependent effect in regulating prostate CSC properties and PCa development. In summary, our work reveals that the prostate CSC populations display unique miRNA expression signatures and different miRNAs distinctively and coordinately regulate various aspects of CSC properties. Altogether, our results lay a scientific foundation for developing miRNA-based anti-cancer therapy.

REGULATION OF FOXC2 EXPRESSION AND FUNCTION DURING EPITHELIAL-MESENCHYMAL TRANSITION BY GSK-3β

Metastasis is the ultimate cause for the majority of cancer-related deaths. The forkhead box transcription factor FOXC2 is known to be involved in regulating metastasis as well as a variety of developmental processes, including the formation of lymphatic and cardiovascular systems. Previous studies have shown that FOXC2 protein is localized either in the nucleus and/or in the cytoplasm of human breast tumor cells. This pattern of localization is similar to that of another forkhead family member, FOXO3a. Additionally, localization of FOXO3a is known to be differentially regulated by upstream kinase AKT. Therefore, I investigated whether FOXC2 localization could also be regulated by upstream kinases. Analysis of FOXC2 protein sequence revealed two potential phosphorylation sites for GSK-3β. Furthermore, inhibition of GSK-3βsignificantly reduces FOXC2 protein. In addition, exposure of HMLE Twist cells expressing endogenous FOXC2 to the GSK-3β inhibitor, TWS119, results in accumulation of FOXC2 protein in the cytoplasm with concomitant decrease in the nucleus in a time-dependent manner. Furthermore, continued treatment with TWS119 eventually induces epithelial morphology and decreased stem cell properties including sphere formation in these cells. Further characterization of FOXC2- GSK-3β interaction and the associated signaling cascade are necessary to determine the effect of FOXC2 phosphorylation by GSK-3β on EMT and metastasis.

LOW REACTIVE OXYGEN SPECIES AND HIGH GLYCOLYSIS IN GLIOBLASTOMA STEM CELLS:MECHANISMS AND THERAPEUTIC IMPLICATIONS

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain tumor with poor prognosis due in part to drug resistance and high incidence of tumor recurrence. The drug resistant and cancer recurrence phenotype may be ascribed to the presence of glioblastoma stem cells (GSCs), which seem to reside in special stem-cell niches in vivo and require special culture conditions including certain growth factors and serum-free medium to maintain their stemness in vitro. Exposure of GSCs to fetal bovine serum (FBS) can cause their differentiation, the underlying mechanism of which remains unknown. Reactive oxygen species (ROS) play an important role in normal stem cell differentiation, but their role in affecting cancer stem cell fate remains unclear. Whether the metabolic characteristics of GSCs are different from other glioblastoma cells and can be targeted are also unknown. In this study, we used several stem-like glioblastoma cell lines derived from clinical tissues by typical neurosphere culture system or orthotopic xenografts, and showed that addition of fetal bovine serum to the medium induced an increase of ROS, leading to aberrant differentiation and decreases of stem cell markers such as CD133. We found that exposure of GSCs to serum induced their differentiation through activation of mitochondrial respiration, leading to an increase in superoxide (O2-) generation and a profound ROS stress response manifested by upregulation of oxidative stress response pathway. This increase in mitochondrial ROS led to a down-regulation of molecules including SOX2, and Olig2, and Notch1 that are important for stem cell function and an upregulation of mitochondrial superoxide dismutase SOD2 that converts O2- to H2O2. Neutralization of ROS by antioxidant N-acetyl-cysteine in the serum-treated GSCs suppressed the increase of superoxide and partially rescued the expression of SOX2, Olig2, and Notch1, and prevented the serum-induced differentiation phenotype. Additionally, GSCs showed high dependence on glycolysis for energy production. The combination of a glycolytic inhibitor 3-BrOP and a chemotherapeutic agent BCNU depleted cellular ATP and inhibited the repair of BCNU-induced DNA damage, achieving strikingly synergistic killing effects in drug resistant GSCs. This study uncovers the metabolic properties of glioblastoma stem cells and suggests that mitochondrial function and cellular redox status may profoundly affect the fates of glioblastoma stem cells via a ROS-mediated mechanism, and that the active glycolytic metabolism in cancer stem cells may provide a biochemical basis for developing novel therapeutic strategies to effectively eliminate GSCs.

ROLE OF STAT3 IN KERATINOCYTE STEM CELLS DURING SKIN TUMORIGENESIS

STATs play crucial roles in a wide variety of biological functions, including development, proliferation, differentiation, migration and in cancer development. In the present study, we examined the impact of Stat3 deletion or activation on behavior of keratinocytes, including keratinocyte stem cells (KSCs). Deletion of Stat3 specifically in the bulge region of the hair follicle using K15.CrePR1 X Stat3fl/fl mice led to decreased tumor development by altering survival of bulge region KSCs. To further understand the role of KSCs in skin tumorigenesis, K5.Stat3C transgenic (Tg) mice which express a constitutively active/dimerized form of Stat3 called Stat3C via the bovine keratin 5 (K5) promoter were studied. The number of CD34 and α6 integrin positive cells was significantly reduced in Tg mice as compared to non-transgenic (NTg) littermates. There was a concomitant increase in the progenitor populations (Lgr-6, Lrig-1 and Sca-1) in the Tg mice vs. the stem cell population (CD34 and Keratin15). To investigate the mechanism underlying the increase in the progenitor population at the expense of bulge region KSCs we examined if Stat3C expression was involved in inducing migration of the bulge region KSCs. There was altered β-catenin and α6-integrin expression in the hair follicles of Tg mice, which may have contributed to reduced adhesive interactions between the epithelial cells and the basement membrane facilitating migration out of the niche. To further study the effect of Stat3 on differentiation of keratinocytes we analyzed the epidermal keratinocytes in K5.Cre X Stat3fl/fl mice. There was an increase in the expression of epidermal differentiation markers in the Stat3 knockout mice. These data suggest that deletion of Stat3 in the epidermis and hair follicle induced differentiation in these cells. Preliminary studies done with the BK5.Stat3C mouse model suggests that multiple hair follicle stem/progenitor populations may be involved in skin tumor development and progression in this model of skin tumorigenesis. Overall, these data suggest that Stat3 plays an important role in differentiation as well as migration of keratinocytes and that these effects may play a role during epithelial carcinogenesis.

PROSTATE CANCER STEM CELLS: DRUG TOLERANCE, EXPERIMENTAL RECONSTITUTION AND CASTRATION RESISTANCE

Prostate cancer (PCa) is one of the leading malignancies affecting men in the Western world. Although tremendous effort has been made towards understanding PCa development and developing clinical treatments in the past decades, the exact mechanisms of PCa are still not clearly understood. Emerging evidence has postulated that a population of stem cell-like cells inside a tumor, termed ‘cancer stem cells (CSCs)’, may be the cells responsible for tumor initiation, progression, recurrence, metastasis and therapy resistance. Like CSC studies in other cancer types, it has been reported that PCa also contains CSCs. However, there remain several unresolved questions that need to be clarified. First, the relationship between prostate CSCs (PCSCs) and therapy resistance (chemo- and radio-) is not known. Herein, we have found that not all CSCs are drug-tolerant, and not all drug-tolerant cells are CSCs. Second, whether primary human PCa (HPCa) actually contain PCSCs remains unclear, due to the well-known fact that we have yet to establish a reliable assay system that can reproducibly and faithfully reconstitute tumor regeneration from single HPCa cells. Herein, after utilizing more than 114 HPCa samples we have provided evidence that immortalized bone marrow-derived stromal cells (Hs5) can help dissociated HPCa cells generate undifferentiated tumors in immunodeficient NOD/SCID-IL2Rγ-/- mice, and the undifferentiated PCa cells seem to have a survival advantage to generate tumors. Third, the evolution of PCa from androgen dependent to the lethally castration resistant (CRPC) stage remains enigmatic, and the cells responsible for CRPC development have not been identified. Herein, we have found a putative cell population, ALDH+CD44+α2β1+ PCa cells that may represent a cell-of-origin for CRPC. Taken together, our work has improved our understanding of PCSC properties, possibly highlighting a potential therapeutic target for CRPC.

GENOME-WIDE PROFILING UNVEILS CRITICIAL FUNCTIONS OF p53 IN HUMAN EMBRYONIC STEM CELLS

Embryonic stem cells (ESCs) possess two unique characteristics: infinite self-renewal and the potential to differentiate into almost every cell type (pluripotency). Recently, global expression analyses of metastatic breast and lung cancers revealed an ESC-like expression program or signature, specifically for cancers that are mutant for p53 function. Surprisingly, although p53 is widely recognized as the guardian of the genome, due to its roles in cell cycle checkpoints, programmed cell death or senescence, relatively little is known about p53 functions in normal cells, especially in ESCs. My hypothesis is that p53 has specific transcription regulatory functions in human ESCs (hESCs) that a) oppose pluripotency and b) protect the stem cell genome in response to DNA damage and stress signaling. In mouse ESCs, these roles are believed to coincide, as p53 promotes differentiation in response to DNA damage, but this is unexplored in hESCs. To determine the biological roles of p53, specifically in hESCs, we mapped genome-wide chromatin interactions of p53 by chromatin immunoprecipitation and massively parallel tag sequencing (ChIP-Seq), and did so under three VIdifferent conditions of hESC status: pluripotency, differentiation-initiated and DNA-damage-induced. ChIP-Seq showed that p53 is enriched at distinct, induction-specific gene loci during each of these different conditions. Microarray gene expression analysis and functional annotation of the distinct p53-target genes revealed that p53 regulates specific genes encoding developmental regulators, which are expressed in differentiation-initiated but not DNA- damaged hESCs. We further discovered that, in response to differentiation signaling, p53 binds regions of chromatin that are repressed but also poised for rapid activation by core pluripotency factors OCT4 and NANOG in pluripotent hESCs. In response to DNA damage, genes associated with migration and motility are targeted by p53; whereas, the prime targets of p53 in control of cell death are conserved for p53 regulation in both differentiation and DNA damage. Our genome-wide profiling and bioinformatics analyses show that p53 occupies a special set of developmental regulatory genes during early differentiation of hESCs and functions in an induction-specific manner. In conclusion, our research unveiled previously unknown functions of p53 in ESC biology, which augments our understanding of one of the most deregulated proteins in human cancers.

Cell survival in early embryogenesis requires proper dosage of GCN5 and p300

Histone acetylation is a central event in transcriptional activation. The importance of this modification in mammalian development is highlighted by knockout studies that revealed loss of the histone acetyltransferases GCN5, p300, or CBP results in embryonic lethality. Furthermore, early embryogenesis is sensitive to the dosage of p300 and CBP since double p300 +/−CBP+/− heterozygotes die in utero, although either single heterozygote survives. PCAF and GCN5 physically interact with p300 and CBP in vitro. To determine whether these two groups of HATs interact functionally in vivo, we created mice lacking one or more allele of p300, GCN5 or PCAF. As expected, we found that mice heterozygous for any one of these null alleles are viable. The majority of GCN5 p300 double heterozygotes also survive to adulthood with no apparent abnormalities. However, a portion of these mice die prior to birth. These embryos are developmentally stunted and exhibit increased apoptosis compared to wild type or single GCN5 or p300 heterozygous littermates at E8.5. Tissue specification is unaffected in these embryos but organ formation is compromised. In contrast, no abnormalities were observed in mice harboring mutations in both PCAF and p300 , emphasizing the specificity of HAT functions in mammalian development. ^ Since GCN5 null embryos die early in embryogenesis because of a marked increase in apoptosis, studies of its function and mechanism in late development and in tissue specific differentiation are precluded. Here, we also report the establishment of a GCN5 null embryonic stem cell line and a conditional floxGCN5 mouse line, which will serve as powerful genetic tools to examine in depth the function of GCN5 in mammalian development and in adult tissues. ^

Connexin-43 in the osteogenic BM niche regulates its cellular composition and the bidirectional traffic of hematopoietic stem cells and progenitors

Connexin-43 (Cx43), a gap junction protein involved in control of cell proliferation, differentiation and migration, has been suggested to have a role in hematopoiesis. Cx43 is highly expressed in osteoblasts and osteogenic progenitors (OB/P). To elucidate the biologic function of Cx43 in the hematopoietic microenvironment (HM) and its influence in hematopoietic stem cell (HSC) activity, we studied the hematopoietic function in an in vivo model of constitutive deficiency of Cx43 in OB/P. The deficiency of Cx43 in OB/P cells does not impair the steady state hematopoiesis, but disrupts the directional trafficking of HSC/progenitors (Ps) between the bone marrow (BM) and peripheral blood (PB). OB/P Cx43 is a crucial positive regulator of transstromal migration and homing of both HSCs and progenitors in an irradiated microenvironment. However, OB/P Cx43 deficiency in nonmyeloablated animals does not result in a homing defect but induces increased endosteal lodging and decreased mobilization of HSC/Ps associated with proliferation and expansion of Cxcl12-secreting mesenchymal/osteolineage cells in the BM HM in vivo. Cx43 controls the cellular content of the BM osteogenic microenvironment and is required for homing of HSC/Ps in myeloablated animals

Hemato-lymphoid in vivo reconstitution potential of subpopulations derived from in vitro differentiated embryonic stem cells

During differentiation in vitro, embryonic stem (ES) cells generate progenitors for most hemato-lymphoid lineages. We studied the developmental potential of two ES cell subpopulations that share the fetal stem cell antigen AA4.1 but differ in expression of the lymphoid marker B220 (CD45R). Upon transfer into lymphoid deficient mice, the B220+ population generated a single transient wave of IgM+ IgD+ B cells but failed to generate T cells. In contrast, transfer of the B220− fraction achieved long-term repopulation of both T and B lymphoid compartments and restored humoral and cell-mediated immune reactions in the recipients. To assess the hemato-lymphopoietic potential of ES cell subsets in comparison to their physiological counterparts, cotransplantation experiments with phenotypically homologous subsets of fetal liver cells were performed, revealing a more potent developmental capacity of the latter. The results suggest that multipotential and lineage-committed lymphoid precursors are generated during in vitro differentiation of ES cells and that both subsets can undergo complete final maturation in vivo.

Hematopoietic potential of stem cells isolated from murine skeletal muscle

We have discovered that cells derived from the skeletal muscle of adult mice contain a remarkable capacity for hematopoietic differentiation. Cells prepared from muscle by enzymatic digestion and 5-day in vitro culture were harvested, and 18 × 103 cells were introduced into each of six lethally irradiated recipients together with 200 × 103 distinguishable whole bone marrow cells. After 6 or 12 weeks, all recipients showed high-level engraftment of muscle-derived cells representing all major adult blood lineages. The mean total contribution of muscle cell progeny to peripheral blood was 56 ± 20% (SD), indicating that the cultured muscle cells generated approximately 10- to 14-fold more hematopoietic activity than whole bone marrow. When bone marrow from one mouse was harvested and transplanted into secondary recipients, all recipients showed high-level multilineage engraftment (mean 40%), establishing the extremely primitive nature of these stem cells. We also show that muscle contains a population of cells with several characteristics of bone marrow-derived hematopoietic stem cells, including high efflux of the fluorescent dye Hoechst 33342 and expression of the stem cell antigens Sca-1 and c-Kit, although the cells lack the hematopoietic marker CD45. We propose that this population accounts for the hematopoietic activity generated by cultured skeletal muscle. These putative stem cells may be identical to muscle satellite cells, some of which lack myogenic regulators and could be expected to respond to hematopoietic signals.

Protamine-Cre recombinase transgenes efficiently recombine target sequences in the male germ line of mice, but not in embryonic stem cells

The production of subtle or conditional mutations in mice through the combined use of site-specific and homologous recombination has become an increasingly widespread experimental paradigm in mammalian genetics. Embryonic stem cells containing recombinase transgenes that were expressed in the male germ line, but not in other tissues or in the embryonic stem cells themselves, would substantially simplify the production of such alleles. Here we show that transgenes comprised of the mouse protamine 1 promoter and the Cre recombinase coding sequence mediate the efficient recombination of a Cre target transgene in the male germ line, but not in other tissues. Embryonic stem cell lines generated from one of these transgenic strains were transfected with targeting vectors that included loxP-flanked selectable markers, and homologously recombined alleles containing the marker and functional loxP sites were isolated. These results establish the potential of the system for substantially reducing the time, effort, and resources required to produce homologously recombined alleles in mice that have been secondarily rearranged by a site-specific recombinase.