Component Of Stem Cell Niche Research Articles

Cellular Reporter Systems for High-Throughput Screening of Interactions Between Bioactive Matrices and Human Mesenchymal Stromal Cells

Mesenchymal stromal cells (MSC) and factors secreted by them are essential components of the hematopoietic stem cell (HSC) niche within the bone marrow microenvironment. It has been shown that the extracellular matrix (ECM) can influence HSC-supportive potential of MSC and is a prerequisite for the proper signaling of morphogens. Therefore, we aimed at the identification of ECM components and candidate morphogens capable of enhancing the expression of HSC-supportive proteins in human MSC, namely, angiopoietin-1 (Ang-1) and stromal cell-derived factor 1 (SDF-1). For this purpose, highly sensitive secreted dual reporter constructs for Ang-1 and SDF-1 were established. These newly designed dual reporter systems enable continuous monitoring of the Ang-1 and SDF-1 promoter activity in an immortalized human MSC line cultured on ECM/morphogen microarrays. Reporter arrays showed that Ang-1 and SDF-1 expression can be induced by different ECM/morphogen combinations. In addition, continuous monitoring of promoter activity allows delineating time-dependent effects of the ECM and morphogens. Thus, we identified that collagen I and vitronectin in combination with Wnt3a favored SDF-1 expression over time, while only transiently inducing the expression of Ang-1. Taken together, the newly developed reporter systems allow for the monitoring of Ang-1 and SDF-1 promoter activity induced by morphogens and the ECM in a combinatorial and high-throughput manner. This technology might therefore be helpful to optimize culture conditions, which favor the activity of MSC as feeder cells for various types of stem and progenitor cells.

Investigating the interaction between hematopoietic stem cells and their niche during embryonic development: optimizing the isolation of fetal and newborn stem cells from liver, spleen, and bone marrow.

Hematopoietic stem cells (HSCs) are maintained in a particular microenvironment termed a "niche." Within the niche, a number of critical molecules and supportive cell types have been identified to play key roles in modulating adult HSC quiescence, proliferation, differentiation, and reconstitution. However, unlike in the adult bone marrow (BM), the components of stem cell niches, as well as their interactions with fetal HSC during different stages of embryonic development, are poorly understood. During embryogenesis, hematopoietic development migrates through multiple organs, each with different cellular and molecular components and hence each with a potentially unique HSC niche. As a consequence, isolating fetal HSC from each organ at the time of hematopoietic colonization is fundamental for assessing and understanding both HSC function and their interactions with specific microenvironments. Herein, we describe methodologies for harvesting cells as well as the purification of stem and progenitors from fetal and newborn liver, spleen, and BM at various developmental stages following the expansion of hematopoiesis in the fetal liver at E14.5.

Artificial extracellular matrices with oversulfated glycosaminoglycan derivatives promote the differentiation of osteoblast-precursor cells and premature osteoblasts.

Sulfated glycosaminoglycans (GAG) are components of the bone marrow stem cell niche and to a minor extent of mature bone tissue with important functions in regulating stem cell lineage commitment and differentiation. We anticipated that artificial extracellular matrices (aECM) composed of collagen I and synthetically oversulfated GAG derivatives affect preferentially the differentiation of osteoblast-precursor cells and early osteoblasts. A set of gradually sulfated chondroitin sulfate and hyaluronan derivatives was used for the preparation of aECM. All these matrices were analysed with human bone marrow stromal cells to identify the most potent aECM and to determine the influence of the degree and position of sulfate groups and the kind of disaccharide units on the osteogenic differentiation. Oversulfated GAG derivatives with a sulfate group at the C-6 position of the N-acetylglycosamine revealed the most pronounced proosteogenic effect as determined by tissue nonspecific alkaline phosphatase activity and calcium deposition. A subset of the aECM was further analysed with different primary osteoblasts and cell lines reflecting different maturation stages to test whether the effect of sulfated GAG derivatives depends on the maturation status of the cells. It was shown that the proosteogenic effect of aECM was most prominent in early osteoblasts.

Morphology, differentiation and adhesion molecule expression changes of bone marrow mesenchymal stem cells from acute myeloid leukemia patients

Bone marrow mesenchymal stem cells (BMSCs) have been identified as an important component of the hematopoietic stem cell (HSC) niche, which is essential for the maintenance of HSCs. HSC niche alternation has been considered to be the main cause of acute myeloid leukemia (AML). However, little is known with regard to BMSC alteration in AML patients. BMSCs were collected from 10AML patients and 13controls in order to examine the morphology, differentiation and adhesion molecule expression changes. It was observed that primary BMSCs from AML patients exhibited aberrant morphologies compared with those from the controls. Prior to adipogenic differentiation, the mRNA and protein levels of the lipid marker gene lipoprotein lipase, from the BMSCs of AML patients, were significantly higher. lipid drops were present early during differentiation in the BMSCs of AML patients and exhibited greater numbers later. Following adipogenic differentiation, the mRNA level of E-cadherin in the BMSCs of AML patients was significantly lower than that identified in the BMSCs of the control groups. Following osteogenic induction, the mRNA level of E-cadherin in the BMSCs of AML patients was significantly higher than in the controls. Therefore BMSCs from the AML patients exhibited irregular morphology, tendency to pre-differentiate to adipocytes and different adhesion molecule expression following differentiation. These differences may further our understanding of the HSC niche in the pathological condition.

Adipocytic Cells Augment the Support of Primitive Hematopoietic Cells In Vitro But Have No Effect in the Bone Marrow Niche Under Homeostatic Conditions

Mesenchymal stem cells (MSCs), as well as osteoblastic cells derived from these MSCs, have been shown to be key components of the hematopoietic stem cell (HSC) niche. In this study, we wished to examine whether other cell types that are known to differentiate from MSCs similarly regulate the stem cell niche, namely cells of the adipocyte lineage. Recent studies have examined the role that adipocytes play in the biology of the HSCs in different bone locations and in transplantation settings; however, none have examined their role under homeostatic conditions. We compared the ability of adipocytic and nonadipocytic cell lines to support primitive hematopoietic cells in vitro. Preadipocytic cell lines demonstrated enhanced support of hematopoietic cells. Similarly, primary bone marrow (BM) cells treated with troglitazone, a drug that enhances adipogenesis, also demonstrated augmented support over control-treated stromal cells. We further examined the effects of increased adipocyte number in vivo under homeostatic conditions using troglitazone treatment and found that these alterations had no effect on HSC frequency. Taken together, we demonstrate that cells of the adipocyte lineage promote the ability of stromal cells to support primitive hematopoietic cells in vitro, yet alterations of adipocyte number and volume in vivo have no effect. These data suggest that adipocytes are not a component of the adult BM HSC niche under homeostatic conditions.

Presence of native limbal stromal cells increases the expansion efficiency of limbal stem/progenitor cells in culture

Niche factors are important in the maintenance and regulation of stem cells. Limbal stromal cells are potentially a component of limbal stem cell (LSC) niche. We investigated the role of the limbal stromal cells in the ex vivo expansion of limbal stem/progenitor cells. Limbal epithelial cells were cultured as single-cell suspension and cell clusters from dispase II or collagenase A (ColA), or tissue explant. ColA isolated limbal stromal cells along with limbal epithelial cells. In the presence of limbal stromal cells, a higher absolute number of p63αbright cells (p < 0.05) and a higher proportion of K14 positive epithelial cells were obtained from both ColA and explant tissue cultures. Expansion of the stem/progenitor population from dispase isolation was more efficient in the form of cell clusters than single cell suspension based on the absolute number of p63αbright cells. Expansion of the stem cell population is similar in the single cell and cell cluster cultures that are derived from ColA isolation. Our finding suggests that limbal stromal cells and an intact cell–cell contact help to maintain LSCs in an undifferentiated state in vitro during expansion.

Hypoxic induction of vascular endothelial growth factor regulates erythropoiesis but not hematopoietic stem cell function in the fetal liver

Hypoxia is an important factor in the hematopoietic stem cell (HSC) niche in the bone marrow (BM) but whether it also plays a role in the regulation of fetal liver (FL) HSCs is unclear. Definitive HSCs first arise in the AGM region and the placenta during development and later migrate to the fetal liver (FL). FL HSCs undergo extensive proliferation in the liver and migrate to the bone marrow (BM) shortly before birth, where they become quiescent. The molecular mechanisms behind this switch in proliferation remain elusive. While many components of the BM stem cell niche have been identified, relatively little is known about the FL HSC niche. Vascular endothelial growth factor A (VEGFA) is essential for adult HSC survival and hypoxic induction of Vegfa in adult HSC is required for proper function. Mice with defective hypoxia-induced Vegfa expression (Vegfaδ/δ) show a partial embryonic lethality and increases the numbers of phenotypically defined hematopoietic stem and progenitor cells in the FL, but whether stem cell function is affected in FL HSCs has not been examined. To investigate whether hypoxia-induced Vegfa is involved in regulation of fetal HSC function and to identify the reason for the embryonic lethality we have examined FL hematopoiesis in the Vegfaδ/δ model. We show that fetal erythropoiesis is severely impaired when hypoxic induction of Vegfa is lacking. FL HSCs deficient for hypoxia-induced Vegfa expression have normal HSC function arguing against a hypoxic FL HSC niche. However, after adaptation of FL HSCs to the BM microenvironment, FL HSCs lose their function as measured by serial transplantation. Hypoxia is an important factor in the hematopoietic stem cell (HSC) niche in the bone marrow (BM) but whether it also plays a role in the regulation of fetal liver (FL) HSCs is unclear. Definitive HSCs first arise in the AGM region and the placenta during development and later migrate to the fetal liver (FL). FL HSCs undergo extensive proliferation in the liver and migrate to the bone marrow (BM) shortly before birth, where they become quiescent. The molecular mechanisms behind this switch in proliferation remain elusive. While many components of the BM stem cell niche have been identified, relatively little is known about the FL HSC niche. Vascular endothelial growth factor A (VEGFA) is essential for adult HSC survival and hypoxic induction of Vegfa in adult HSC is required for proper function. Mice with defective hypoxia-induced Vegfa expression (Vegfaδ/δ) show a partial embryonic lethality and increases the numbers of phenotypically defined hematopoietic stem and progenitor cells in the FL, but whether stem cell function is affected in FL HSCs has not been examined. To investigate whether hypoxia-induced Vegfa is involved in regulation of fetal HSC function and to identify the reason for the embryonic lethality we have examined FL hematopoiesis in the Vegfaδ/δ model. We show that fetal erythropoiesis is severely impaired when hypoxic induction of Vegfa is lacking. FL HSCs deficient for hypoxia-induced Vegfa expression have normal HSC function arguing against a hypoxic FL HSC niche. However, after adaptation of FL HSCs to the BM microenvironment, FL HSCs lose their function as measured by serial transplantation.

α-Tanycytes of the adult hypothalamic third ventricle include distinct populations of FGF-responsive neural progenitors

Emerging evidence suggests that new cells, including neurons, can be generated within the adult hypothalamus, suggesting the existence of a local neural stem/progenitor cell niche. Here, we identify α-tanycytes as key components of a hypothalamic niche in the adult mouse. Long-term lineage tracing in vivo using a GLAST::CreER(T2) conditional driver indicates that α-tanycytes are self-renewing cells that constitutively give rise to new tanycytes, astrocytes and sparse numbers of neurons. In vitro studies demonstrate that α-tanycytes, but not β-tanycytes or parenchymal cells, are neurospherogenic. Distinct subpopulations of α-tanycytes exist, amongst which only GFAP-positive dorsal α2-tanycytes possess stem-like neurospherogenic activity. Fgf-10 and Fgf-18 are expressed specifically within ventral tanycyte subpopulations; α-tanycytes require fibroblast growth factor signalling to maintain their proliferation ex vivo and elevated fibroblast growth factor levels lead to enhanced proliferation of α-tanycytes in vivo. Our results suggest that α-tanycytes form the critical component of a hypothalamic stem cell niche, and that local fibroblast growth factor signalling governs their proliferation.

Understanding the role of the microenvironment during definitive hemopoietic development

Hemopoietic stem cells (HSCs) are sustained in a specific microenvironment known as the stem cell niche. Recent studies in adult bone marrow have identified osteoblasts and endothelial cells as two important supportive cell types within the niche and demonstrated that interactions between HSCs and cellular and extracellular components within the endosteal and perivascular regions are critical for HSC regulation. However, the understanding of the role of the microenvironment in definitive HSC establishment, expansion, and maintenance during embryonic development is extremely limited. This review focuses on what is known about the components of each HSC microenvironment at various developmental stages and their known functional roles.

Abstract 1518: Microenvironment regulation of acute lymphoblastic leukemia miRNA profiles.

Abstract It is well established that the bone marrow microenvironment provides cues to hematopoietic tumors that initiate and progress in this unique anatomical site that likely mimics, to some degree, cell signaling that directs steady state hematopoiesis. We have previously documented diverse changes in gene and protein expression in acute lymphoblastic leukemia (ALL) cells in contact with either primary bone marrow stromal cells (BMSC obtained from consenting donors) or human derived osteoblasts (HOB); two components of the marrow stem cell niche (Wang 2007). Importantly, many of these changes are coincident with chemoresistance. In the current study we have expanded our investigation to include investigation of miRNA profile comparisons of the human Ph+ ALL cell line Sup-B15 (ATCC # CRL-1929) grown in media versus tumor cells grown in long term co-culture (greater than 3 months) with adherent BMSC or HOB. We have previously described the stem cell-like characteristics of Sup-B15, making it an intriguing cell line for continued study (Wang 2007). In addition, comparisons were completed for the cell lines Nalm-27 and Nalm-30 (Fujisaki Cancer Center, Okayama, Japan) for evaluation of tumors at diagnosis versus relapse of disease. Total RNA was isolated using Ambion miRVana RNA extraction kit and a microarray assay was performed using a service provider (LC Sciences, Houston TX) and μParafloTM MicroRNA Microarray Assays as described (Wei 2009). Preliminary observations identified over 15 different microRNAs expressed at baseline in all three lines evaluated which met the medium to high relative threshold according to the analysis. Profiles of Nalm-27 and Nalm-30 were highly conserved with the exception of miR-7-1-3p and miR-99b-5p which were upregulated and miR-142-3p, miR-1260b, and miR-19a-3p which were down regulated when the cell line established at relapse (Nalm-30) was compared to that from diagnosis (Nalm-27). Consistent with microRNAs as critical regulators of diverse cellular processes including proliferation, survival, and differentiation, several targets were noted to be significantly influenced by LTCC of Sup-B15 ALL cells with BMSC or HOB. Preliminary evaluation suggests miR-4254 expression was increased by BMSC and miR-4484, miR-34b-3p and miR-1237-5p were increased when tumor cells were provided continual access to HOB. In contrast, miR-221-3p, miR-222-3p, miR-4521, levels were decreased during BMSC co-culture as well as miR-221-3p, miR-222-3p, miR-199b-5p, miR-185-5p, miR-4521 during co-culture with HOB. Collectively these changes highlight the dynamic expression of microRNAs and emphasize their responsiveness to microenvironment derived signals. These observations will contribute to identification of novel pathways that include miRNAs that warrant consideration as therapeutic targets in efforts to eradicate residual disease in the setting of ALL. Citation Format: Blake Moses, Sriganesh G. Sharma, J. Michael Ruppert, Laura F. Gibson. Microenvironment regulation of acute lymphoblastic leukemia miRNA profiles. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1518. doi:10.1158/1538-7445.AM2013-1518

Release of matrix metalloproteinase-8 during physiological trafficking and induced mobilization of human hematopoietic stem cells.

Previous studies indicate that the release of proteases, including the gelatinase matrix metalloproteinase (MMP)-9, from mature granulocytes plays a crucial role in cytokine-induced hematopoietic stem and progenitor cell (HSPC) mobilization. However, studies with MMP-9-deficient mice revealed that HSPC mobilization was normal in these animals, suggesting that additional proteases must be active at clinically relevant cytokine concentrations. In the present study, we provide evidence that the collagenase MMP-8 is involved in stem cell mobilization. A rapid release of MMP-8 from isolated neutrophil granulocytes can be observed during an in vitro culture. During granulocyte colony-stimulating factor-induced HSPC mobilization, highly elevated serum concentrations of MMP-8 were observed on days 4 to 6 of the mobilization regimen, concomitantly with elevated MMP-9 serum levels and higher numbers of circulating CD34(+) cells. Elevated serum concentrations of both proteases were also found in umbilical cord blood serum. In functional assays, adhesion of HSPC to osteoblasts as an essential component of the endosteal stem cell niche is negatively influenced by MMP-8. The chemokine CXCL12, which is critically involved in stem cell trafficking, can be proteolytically processed by MMP-8 treatment. This degradation has a strong inhibitory influence on HSPC migration. Taken together, our data strongly suggest that MMP-8 can be directly involved in hematopoietic stem cell mobilization and trafficking.

Mesenchymal stem cells as an essential hematopoietic stem cell niche component

Hematopoietic stem cells (HSCs) are capable of self-renewal and multi-lineage differentiation, which are characteristics that enable them to maintain the stem cell population and produce large numbers of blood cells. Quiescent mammalian adult HSCs localize within the bone marrow niche, primarily in the endosteal region of the trabecular bone. These cells scarcely participate in the cell cycle, but are instead maintained by assorted niche cells within the bone marrow, including mesenchymal stem cells (MSCs). This review summarizes the crucial role(s) that MSCs play as niche cells for the support of HSCs.

Extracellular Matrix is an Important Component of Limbal Stem Cell Niche.

Extracellular matrix plays an important role in stem cell niche which maintains the undifferentiated stem cell phenotype. Human corneal epithelial stem cells are presumed to reside mainly at the limbal basal epithelium. Efforts have been made to characterize different components of the extracellular matrix that are preferentially expressed at the limbus. Mounting evidence from experimental data suggest that these components are part of the stem cell niche and play a role in the homeostasis of limbal stem cells. The extracellular matrix provides a mechanical and structural support as well as regulates cellular functions such as adhesion, migration, proliferation, self-renewal and differentiation. Optimization of the extracellular matrix components might be able to recreate an ex vivo stem cell niche to expand limbal stem cells.

Mesenchymal stromal cells: radio‐resistant members of the bone marrow

Mesenchymal stromal cells (MSCs) are multi-potent adult stem cells located in various tissues, including the bone marrow. MSCs are key components of the haematopoietic stem cell (HSC) niche within the bone marrow where they function to maintain haematopoietic homoeostasis by regulating HSC self-renewal and function. Bone marrow exposure to ionising radiation causes rapid depletion of radio-sensitive HSCs and their progenitors, leading to haematopoietic failure. However, host-/patient-derived MSCs can survive radiation doses lethal to the haematopoietic system. The mechanisms underlying MSC radio-resistance are currently under intense investigation. Here, we review the current knowledge of MSC radio-biology. The DNA damage response (DDR) represents an orchestrated network of signalling pathways that enable cells to respond to genotoxic damage. We discuss in detail the emerging importance of the DDR in mediating MSC radio-resistance and examine the DDR of MSCs in the context of other stem cell types. Finally, we examine future advances in understanding MSC radio-resistance and discuss the potential impact of the radio-resistance of these stem cells for the clinic.

Fractone-heparan sulfates mediate BMP-7 inhibition of cell proliferation in the adult subventricular zone

Bone morphogenetic protein-7 (BMP-7) is a heparin-binding growth factor that inhibits cell proliferation in the subventricular zone (SVZ) of the lateral ventricle, the primary neurogenic niche in the adult brain. However, the physiological mechanisms regulating the activity of BMP-7 in the SVZ are unknown. Here, we report the inhibitory effect of BMP-7 on cell proliferation through the anterior SVZ after intracerebroventricular injection in the adult mouse. To determine whether the inhibition of cell proliferation induced by BMP-7 is dependant on heparin-binding, heparitinase-1 was intracerebroventricularly injected to N-desulfate heparan sulfate proteoglycans before BMP-7 was injected. Heparatinase-1 drastically reduced the inhibitory effect of BMP-7 on cell proliferation in the SVZ. To determine where BMP-7 binds within the niche, we visualized biotinylated-BMP-7 after intracerebroventricular injection, using streptavidin Texas red on frozen brain sections. BMP-7 binding was seen as puncta in the SVZ at the location of fractones, the particulate specialized extracellular matrix of the SVZ, which have been identified primarily by N-sulfated heparan sulfate immunoreactivity (NS-HS+). BMP binding was also seen in NS-HS+ blood vessels of the SVZ. Injection of heparitinase-1 prior to biotinylated BMP-7 resulted in the absence of signal for biotinylated-BMP-7 in the fractones and blood vessels, indicating that the binding is heparan sulfate dependant. These results indicate that BMP-7 requires heparan sulfates to bind and inhibit cell proliferation in the SVZ neurogenic niche. Heparan sulfates concentrated in fractones and SVZ blood vessels emerge as a functional stem cell niche component involved in growth factor activity.

Abstract 77: Fibronectin Signals Through Pim-1 and β1 Integrin in Cardiac Progenitor Cells to Induce Proliferation and Protection

Background: Cardiac Progenitor Cells (CPC) are pivotally involved in cardiac repair. Fibronectin (FN), an extracellular matrix protein, is highly expressed during cardiac development. In adult heart FN is a component of the cardiac stem cell niche and re-appears after myocardial infarction (MI). The role of FN signaling in CPC function and cardiac remodeling following MI has not been elucidated. We demonstrate here proliferative and protective effects of FN signaling in CPC as mediated by β1 integrin receptor and cardioprotective serine/threonine kinase Pim1. Methods: Cell death and proliferation of CPCs was measured using propidium iodide and CyQuant assays. Signaling pathways were analyzed by immunoblotting, qRT-PCR and siRNA depletion of targets. FN was localized in heart sections by immunohistochemistry and cardiac function assessed by echocardiography in control and conditional FN knockout (KO) mouse hearts following MI. Results: FN inhibits starvation and staurosporine induced cell death in CPCs and promotes proliferation in conjunction with induction of Pim1 expression. Protective and pro-proliferative effects of FN are abrogated by inhibition of Pim1 or deletion of β1 integrin receptor. In vivo CPC expansion correlates with FN expression following MI, and CPC localize to regions of up-regulated FN protein in the infarct. Cardiac function in control and KO mice remains equivalent up to 2 weeks post MI, however by 4 weeks KO heart function worsens compared to control as evidenced by ejection fraction values measuring 16.4 +/- 1.5% vs 26.6 +/- 1.8% at 12 weeks post MI in KO and control hearts, respectively. Conclusion: FN provides pro-survival and pro-proliferative effects to CPCs in a Pim1 kinase and β1 integrin dependent manner. CPCs and FN colocalize in vivo in an infarction injury model, while conditional KO of FN in mice leads to further impairment of cardiac function after MI. Taken together these results indicate previously unidentified cardioprotective and regenerative roles for FN in pathologically challenged heart.

Neural Transmembrane Protease and Endothelial Gs Protein Activation in Cell Contact-dependent Signaling between Neural Stem/Progenitor Cells and Brain Endothelial Cells

Vasculature is an important component of the neural stem cell niche in brain. It regulates neural stem/progenitor (NS/P) cell self-renewal, differentiation, and migration. In the neurogenic niches of adult brain, NS/P cells lie close to blood vessels, and proliferating NS/P cells frequently contact the vasculature. In the present study we showed that NS/P cells in co-culture with brain endothelial (bEND) cells activated endothelial G proteins and p38 mitogen-activated protein kinase (p38 MAPK) and stimulated cytokine/chemokine expression. These NS/P cell-induced endothelial responses took place during NS/P cell and bEND cell direct contact and were critically dependent on the expression of the type II transmembrane serine protease matriptase (MTP) by NS/P cells, because knocking down of MTP in NS/P cells impaired and re-expression of MTP restored their ability to induce endothelial cytokine/chemokine expression, p38 MAPK, or G protein activation. Cholera toxin blocked NS/P cell-induced endothelial responses, suggesting that the endothelial G protein activated by NS/P MTP is in the G(s) subfamily. The addition of p38 MAPK inhibitor impaired NS/P cell-induced endothelial cytokine/chemokine expression. The known G protein-coupled receptor substrate of MTP, protease-activated receptor 2, was not involved in this system. These results revealed a novel signaling pathway in neural stem cell vascular niches that is mediated by neural MTP and endothelial G(s) protein signaling at the cell-cell interface. This is the first report of direct cell-cell signaling between NS/P and bEND cells.

FGF9 and FGF20 Maintain the Stemness of Nephron Progenitors in Mice and Man

The identity of niche signals necessary to maintain embryonic nephron progenitors is unclear. Here we provide evidence that Fgf20 and Fgf9, expressed in the niche, and Fgf9, secreted from the adjacent ureteric bud, are necessary and sufficient to maintain progenitor stemness. Reduction in the level of these redundant ligands in the mouse led to premature progenitor differentiation within the niche. Loss of FGF20 in humans, or of both ligands in mice, resulted in kidney agenesis. Sufficiency was shown invitro where Fgf20 or Fgf9 (alone or together with Bmp7) maintained isolated metanephric mesenchyme or sorted nephron progenitors that remained competent to differentiate in response to Wnt signals after 5 or 2days in culture, respectively. These findings identify a long-sought-after critical component of the nephron stem cell niche and hold promise for long-term culture and utilization of these progenitors invitro.

HSCs play a distinct role in different phases of oval cell‐mediated liver regeneration

Hepatic stem cell niche plays an important role in hepatic oval cell-mediated liver regeneration. As a component of hepatic stem cell niche, the role of hepatic stellate cells (HSCs) in oval cell proliferation needs further studies. In the present study, we isolated HSCs from rats at indicated time point after partial hepatectomy (PH) in 2-acetylaminofluorene/PH oval cell proliferation model. Conditional medium (CM) from HSCs were collected to detect their effects on proliferation and the mitogen-activated protein kinase pathway activation of two oval cell lines. We found that CM collected from HSCs at early phase of liver regeneration (4 and 9 days group) contained high levels of hepatocyte growth factor (HGF) and stimulated oval cell proliferation via extracellular signal-regulated kinase and p38 pathway. CM collected from HSCs at terminal phase of liver regeneration (12 and 15 days group) contained high levels of transforming growth factor (TGF)-β1, which suppressed DNA synthesis of oval cells. The shift between these two distinct effects depended on the balance between HGF and TGF-β1 secreted by HSCs. Our study demonstrated that HSCs acted as a positive regulator at the early phase and a negative regulator at the terminal phase of the oval cell-mediated liver regeneration.

To be or not to be a stem cell: dissection of cellular and molecular components of haematopoietic stem cell niches

Recent studies have identified multiple cell types that regulate haematopoietic stem cells (HSCs); however, proof that a specific cell type produces a specific factor important for HSC function and maintenance is largely lacking. Ding et al (2012) reported recently that conditional deletion of stem cell factor (SCF) in Leptin receptor ( Lepr ) expressing perivascular cells or endothelial and haematopoietic cells resulted in significant reductions in number but less profound reduction in function of HSCs. Although the long‐term fate of HSCs in these models is largely unexplored and an underlying mechanism for reduction in HSCs not yet reported, these findings further implicate the vascular niche in the functional maintenance of HSCs in vivo and also raise intriguing questions for future studies in this field.