Pbx1 regulates self-renewal of long-term hematopoietic stem cells by maintaining their quiescence

ΔNp63α promotes cellular quiescence via induction and activation of Notch3

Genetic analysis of TP63 indicates that ΔNp63 isoforms are required for preservation of self-renewing capacity in the stem cell compartments of diverse epithelial structures; however, the underlying cellular and molecular mechanisms remain incompletely defined. Cellular quiescence is a common feature of adult stem cells that may account for their ability to retain long-term replicative capacity while simultaneously limiting cellular division. Similarly, quiescence within tumor stem cell populations may represent a mechanism by which these populations evade cytotoxic therapy and initiate tumor recurrence. Here, we present evidence that ΔNp63α, the predominant TP63 isoform in the regenerative compartment of diverse epithelial structuresm, promotes cellular quiescence via activation of Notch signaling. In HC11 cells, ectopic ΔNp63α mediates a proliferative arrest in the 2N state coincident with reduced RNA synthesis characteristic of cellular quiescence. Additionally, ΔNp63α and other quiescence-inducing stimuli enhanced expression of Notch3 in HC11s and breast cancer cell lines, and ectopic expression of the Notch3 intracellular domain (N3 (ICD) ) was sufficient to cause accumulation in G 0/G 1 and increased expression of two genes associated with quiescence, Hes1 and Mxi1. Pharmacologic inhibition of Notch signaling or shRNA-mediated suppression of Notch3 were sufficient to bypass quiescence induced by ΔNp63α and other quiescence-inducing stimuli. These studies identify a novel mechanism by which ΔNp63α preserves long-term replicative capacity by promoting cellular quiescence and identify the Notch signaling pathway as a mediator of multiple quiescence-inducing stimuli, including ΔNp63α expression.

The p53 tumor suppressor protein regulates hematopoietic stem cell fate

The p53 tumor suppressor protein is a key transcription factor that regulates several signaling pathways involved in the cell's response to stress. Through stress-induced activation, p53 accumulates and triggers the expression of target genes that protect the genetic integrity of all cells including hematopoietic stem cells (HSCs). These protective mechanisms include cell-cycle arrest, DNA repair, induction of apoptosis, or initiation of senescence. In addition to its function under stress conditions, p53 has important functions during steady-state hematopoiesis, regulating HSC quiescence and self-renewal. In addition, it appears that p53 levels affect HSC competition for the hematopoietic niche, with the less p53 activated HSCs preferentially surviving. The specific genes and precise mechanisms underlying p53's effects on normal HSCs are slowly being clarified. p53 also plays an important role in leukemia stem cell (LSC) behavior, with p53 loss affecting drug resistance and disease progression. Pharmacologic activation of p53 function could overcome the adverse impact of p53 inactivation in LSCs. Thus, understanding the p53 regulatory mechanisms active in HSCs and LSCs may promote the development of new therapeutic strategies that could eliminate the population of largely quiescent LSCs.

The critical role of agrin in the hematopoietic stem cell niche

Hematopoiesis is the process leading to the sustained production of blood cells by hematopoietic stem cells (HSCs). Growth, survival, and differentiation of HSCs occur in specialized microenvironments called "hematopoietic niches," through molecular cues that are only partially understood. Here we show that agrin, a proteoglycan involved in the neuromuscular junction, is a critical niche-derived signal that controls survival and proliferation of HSCs. Agrin is expressed by multipotent nonhematopoietic mesenchymal stem cells (MSCs) and by differentiated osteoblasts lining the endosteal bone surface, whereas Lin(-)Sca1(+)c-Kit(+) (LSK) cells express the α-dystroglycan receptor for agrin. In vitro, agrin-deficient MSCs were less efficient in supporting proliferation of mouse Lin(-)c-Kit(+) cells, suggesting that agrin plays a role in the hematopoietic cell development. These results were indeed confirmed in vivo through the analysis of agrin knockout mice (Musk-L;Agrn(-/-)). Agrin-deficient mice displayed in vivo apoptosis of CD34(+)CD135(-) LSK cells and impaired hematopoiesis, both of which were reverted by an agrin-sufficient stroma. These data unveil a crucial role of agrin in the hematopoietic niches and in the cross-talk between stromal and hematopoietic stem cells.

Transforming Growth Factor-Beta Signaling in the Neural Stem Cell Niche: A Therapeutic Target for Huntington's Disease

The neural stem cell niches possess the regenerative capacity to generate new functional neurons in the adult brain, suggesting the possibility of endogenous neuronal replacement after injury or disease. Huntington disease (HD) is a neurodegenerative disease and characterized by neuronal loss in the basal ganglia, leading to motor, cognitive, and psychological disabilities. Apparently, in order to make use of the neural stem cell niche as a therapeutic concept for repair strategies in HD, it is important to understand the cellular and molecular composition of the neural stem cell niche under such neurodegenerative conditions. This paper mainly discusses the current knowledge on the regulation of the hippocampal neural stem cell niche in the adult brain and by which mechanism it might be compromised in the case of HD.

Computational integration of homolog and pathway gene module expression reveals general stemness signatures

The stemness hypothesis states that all stem cells use common mechanisms to regulate self-renewal and multi-lineage potential. However, gene expression meta-analyses at the single gene level have failed to identify a significant number of genes selectively expressed by a broad range of stem cell types. We hypothesized that stemness may be regulated by modules of homologs. While the expression of any single gene within a module may vary from one stem cell type to the next, it is possible that the expression of the module as a whole is required so that the expression of different, yet functionally-synonymous, homologs is needed in different stem cells. Thus, we developed a computational method to test for stem cell-specific gene expression patterns from a comprehensive collection of 49 murine datasets covering 12 different stem cell types. We identified 40 individual genes and 224 stemness modules with reproducible and specific up-regulation across multiple stem cell types. The stemness modules included families regulating chromatin remodeling, DNA repair, and Wnt signaling. Strikingly, the majority of modules represent evolutionarily related homologs. Moreover, a score based on the discovered modules could accurately distinguish stem cell-like populations from other cell types in both normal and cancer tissues. This scoring system revealed that both mouse and human metastatic populations exhibit higher stemness indices than non-metastatic populations, providing further evidence for a stem cell-driven component underlying the transformation to metastatic disease.

TALE homeodomain proteins regulate site-specific terminal differentiation, LCE genes and epidermal barrier

The epidermal barrier varies over the body surface to accommodate regional environmental stresses. Regional skin barrier variation is produced by site-dependent epidermal differentiation from common keratinocyte precursors and often manifests as site-specific skin disease or irritation. There is strong evidence for body-site-dependent dermal programming of epidermal differentiation in which the epidermis responds by altering expression of key barrier proteins, but the underlying mechanisms have not been defined. The LCE multigene cluster encodes barrier proteins that are differentially expressed over the body surface, and perturbation of LCE cluster expression is linked to the common regional skin disease psoriasis. LCE subclusters comprise genes expressed variably in either external barrier-forming epithelia (e.g. skin) or in internal epithelia with less stringent barriers (e.g. tongue). We demonstrate here that a complex of TALE homeobox transcription factors PBX1, PBX2 and Pknox (homologues of Drosophila Extradenticle and Homothorax) preferentially regulate external rather than internal LCE gene expression, competitively binding with SP1 and SP3. Perturbation of TALE protein expression in stratified squamous epithelia in mice produces external but not internal barrier abnormalities. We conclude that epidermal barrier genes, such as the LCE multigene cluster, are regulated by TALE homeodomain transcription factors to produce regional epidermal barriers.

Kit-Shp2-Kit signaling acts to maintain a functional hematopoietic stem and progenitor cell pool

The stem cell factor (SCF)/Kit system has served as a classic model in deciphering molecular signaling events in the hematopoietic compartment, and Kit expression is a most critical marker for hematopoietic stem cells (HSCs) and progenitors. However, it remains to be elucidated how Kit expression is regulated in HSCs. Herein we report that a cytoplasmic tyrosine phosphatase Shp2, acting downstream of Kit and other RTKs, promotes Kit gene expression, constituting a Kit-Shp2-Kit signaling axis. Inducible ablation of PTPN11/Shp2 resulted in severe cytopenia in BM, spleen, and peripheral blood in mice. Shp2 removal suppressed the functional pool of HSCs/progenitors, and Shp2-deficient HSCs failed to reconstitute lethally irradiated recipients because of defects in homing, self-renewal, and survival. We show that Shp2 regulates coordinately multiple signals involving up-regulation of Kit expression via Gata2. Therefore, this study reveals a critical role of Shp2 in maintenance of a functional HSC/progenitor pool in adult mammals, at least in part through a kinase-phosphatase-kinase cascade.

Prdm16 is a physiologic regulator of hematopoietic stem cells

Fetal liver and adult bone marrow hematopoietic stem cells (HSCs) renew or differentiate into committed progenitors to generate all blood cells. PRDM16 is involved in human leukemic translocations and is expressed highly in some karyotypically normal acute myeloblastic leukemias. As many genes involved in leukemogenic fusions play a role in normal hematopoiesis, we analyzed the role of Prdm16 in the biology of HSCs using Prdm16-deficient mice. We show here that, within the hematopoietic system, Prdm16 is expressed very selectively in the earliest stem and progenitor compartments, and, consistent with this expression pattern, is critical for the establishment and maintenance of the HSC pool during development and after transplantation. Prdm16 deletion enhances apoptosis and cycling of HSCs. Expression analysis revealed that Prdm16 regulates a remarkable number of genes that, based on knockout models, both enhance and suppress HSC function, and affect quiescence, cell cycling, renewal, differentiation, and apoptosis to various extents. These data suggest that Prdm16 may be a critical node in a network that contains negative and positive feedback loops and integrates HSC renewal, quiescence, apoptosis, and differentiation.

Quiescence regulators for hematopoietic stem cell

Hematopoietic stem cell (HSC) either stays in quiescence or proliferates toward differentiation for the production of mature blood cells, or toward self-renewal for giving rise to itself. In order to both maintain a supply of mature blood cells and not exhaust HSCs throughout the lifetime of an individual, under steady state, most HSCs remain quiescent and only a small number enter the cell cycle. Quiescence of HSCs is not only critical for protecting the stem cell compartment and sustaining stem cell pools over long periods, but it is also critical for protecting stem cells by minimizing their accumulation of replication-associated mutations. The balance between quiescence and proliferation is tightly controlled by both HSC-intrinsic and -extrinsic mechanisms. In recent years, through reductionistic strategies, a wide variety of molecules or pathways critical for HSC quiescence regulation have been identified. This regulation network involves both positive and negative regulators. Understanding quiescence regulation in HSC is of great importance not only for understanding the physiological foundation of HSCs, but also for understanding the pathophysiological origins of many related disorders. In this article, I will briefly review the current advance in the quiescence regulators for the HSCs.

A Myc network accounts for similarities between embryonic stem and cancer cell transcription programs

c-Myc (Myc) is an important transcriptional regulator in embryonic stem (ES) cells, somatic cell reprogramming, and cancer. Here, we identify a Myc-centered regulatory network in ES cells by combining protein-protein and protein-DNA interaction studies and show that Myc interacts with the NuA4 complex, a regulator of ES cell identity. In combination with regulatory network information, we define three ES cell modules (Core, Polycomb, and Myc) and show that the modules are functionally separable, illustrating that the overall ES cell transcription program is composed of distinct units. With these modules as an analytical tool, we have reassessed the hypothesis linking an ES cell signature with cancer or cancer stem cells. We find that the Myc module, independent of the Core module, is active in various cancers and predicts cancer outcome. The apparent similarity of cancer and ES cell signatures reflects, in large part, the pervasive nature of Myc regulatory networks.

FIP200 is required for the cell-autonomous maintenance of fetal hematopoietic stem cells

Little is known about whether autophagic mechanisms are active in hematopoietic stem cells (HSCs) or how they are regulated. FIP200 (200-kDa FAK-family interacting protein) plays important roles in mammalian autophagy and other cellular functions, but its role in hematopoietic cells has not been examined. Here we show that conditional deletion of FIP200 in hematopoietic cells leads to perinatal lethality and severe anemia. FIP200 was cell-autonomously required for the maintenance and function of fetal HSCs. FIP200-deficient HSC were unable to reconstitute lethally irradiated recipients. FIP200 ablation did not result in increased HSC apoptosis, but it did increase the rate of HSC proliferation. Consistent with an essential role for FIP200 in autophagy, FIP200-null fetal HSCs exhibited both increased mitochondrial mass and reactive oxygen species. These data identify FIP200 as a key intrinsic regulator of fetal HSCs and implicate a potential role for autophagy in the maintenance of fetal hematopoiesis and HSCs.

Stem cell quiescence in the hippocampal neurogenic niche is associated with elevated transforming growth factor-beta signaling in an animal model of Huntington disease

Cellular proliferation, differentiation, integration, and survival within the adult neural stem cell niche are altered under pathological conditions, but the molecular cues regulating the biology of this niche are mostly unknown. We examined the hippocampal neural stem cell niche in a transgenic rat model of Huntington disease. In this model, progressive cognitive deficits develop at the age of 9 months, suggesting possible hippocampal dysfunction. We found a disease-associated progressive decline in hippocampal progenitor cell proliferation accompanied by an expansion of the pool of 5-bromo-2-deoxyuridine label-retaining Sox-2-positive quiescent stem cells in the transgenic animals. Increments in quiescent stem cells occurred at the expense of cAMP-responsive element-binding protein-mediated neuronal differentiation and survival. Because elevated levels of transforming growth factor-beta1 (TGF-beta1) impair neural progenitor proliferation, we investigated hippocampal TGF-beta signaling and determined that TGF-beta1 induces the neural progenitors to exit the cell cycle. Although phospho-Smad2, an effector of TGF-beta signaling, is normally absent in subgranular stem cells, it accumulated progressively in Sox2/glial fibrillary acidic protein-expressing cells of the subgranular zone in the transgenic rats. These results indicate that alterations in neurogenesis in transgenic Huntington disease rats occur in successive phases that are associated with increasing TGF-beta signaling. Thus, TGF-beta1 signaling seems to be a crucial modulator of neurogenesis in Huntington disease and may represent a target for future therapy.

Cyclin C regulates human hematopoietic stem/progenitor cell quiescence

Hematopoietic stem cells (HSCs) can remain quiescent or they can enter the cell cycle, and either self-renew or differentiate. Although cyclin C and cyclin dependent kinase (cdk3) are essential for the transition from the G(0) to the G(1) phase of the cell cycle in human fibroblasts, the role of cyclin C in hematopoietic stem/progenitor cells (HSPCs) is not clear. We have identified an important role of cyclin C (CCNC) in regulating human HSPC quiescence, as knocking down CCNC expression in human cord blood CD34(+) cells resulted in a significant increase in quiescent cells that maintain CD34 expression. CCNC knockdown also promotes in vitro HSPC expansion and enhances their engraftment potential in sublethally irradiated immunodeficient mice. Our studies establish cyclin C as a critical regulator of the G(0)/G(1) transition of human HSPCs and suggest that modulating cyclin C levels may be useful for HSC expansion and more efficient engraftment.

Pbx1 represses osteoblastogenesis by blocking Hoxa10-mediated recruitment of chromatin remodeling factors

Abdominal-class homeodomain-containing (Hox) factors form multimeric complexes with TALE-class homeodomain proteins (Pbx, Meis) to regulate tissue morphogenesis and skeletal development. Here we have established that Pbx1 negatively regulates Hoxa10-mediated gene transcription in mesenchymal cells and identified components of a Pbx1 complex associated with genes in osteoblasts. Expression of Pbx1 impaired osteogenic commitment of C3H10T1/2 multipotent cells and differentiation of MC3T3-E1 preosteoblasts. Conversely, targeted depletion of Pbx1 by short hairpin RNA (shRNA) increased expression of osteoblast-related genes. Studies using wild-type and mutated osteocalcin and Bsp promoters revealed that Pbx1 acts through a Pbx-binding site that is required to attenuate gene activation by Hoxa10. Chromatin-associated Pbx1 and Hoxa10 were present at osteoblast-related gene promoters preceding gene expression, but only Hoxa10 was associated with these promoters during transcription. Our results show that Pbx1 is associated with histone deacetylases normally linked with chromatin inactivation. Loss of Pbx1 from osteoblast promoters in differentiated osteoblasts was associated with increased histone acetylation and CBP/p300 recruitment, as well as decreased H3K9 methylation. We propose that Pbx1 plays a central role in attenuating the ability of Hoxa10 to activate osteoblast-related genes in order to establish temporal regulation of gene expression during osteogenesis.

Downstream targets of HOXB4 in a cell line model of primitive hematopoietic progenitor cells

Enforced expression of the homeobox transcription factor HOXB4 has been shown to enhance hematopoietic stem cell self-renewal and expansion ex vivo and in vivo. To investigate the downstream targets of HOXB4 in hematopoietic progenitor cells, HOXB4 was constitutively overexpressed in the primitive hematopoietic progenitor cell line EML. Two genome-wide analytical techniques were used: RNA expression profiling using microarrays and chromatin immunoprecipitation (ChIP)-chip. RNA expression profiling revealed that 465 gene transcripts were differentially expressed in KLS (c-Kit(+), Lin(-), Sca-1(+))-EML cells that overexpressed HOXB4 (KLS-EML-HOXB4) compared with control KLS-EML cells that were transduced with vector alone. In particular, erythroid-specific gene transcripts were observed to be highly down-regulated in KLS-EML-HOXB4 cells. ChIP-chip analysis revealed that the promoter region for 1910 genes, such as CD34, Sox4, and B220, were occupied by HOXB4 in KLS-EML-HOXB4 cells. Side-by-side comparison of the ChIP-chip and RNA expression profiling datasets provided correlative information and identified Gp49a and Laptm4b as candidate "stemness-related" genes. Both genes were highly ranked in both dataset lists and have been previously shown to be preferentially expressed in hematopoietic stem cells and down-regulated in mature hematopoietic cells, thus making them attractive candidates for future functional studies in hematopoietic cells.

Hematopoietic stem cell function requires 12/15-lipoxygenase-dependent fatty acid metabolism

Fatty acid metabolism governs multiple intracellular signaling pathways in many cell types, but its role in hematopoietic stem cells (HSCs) is largely unknown. Herein, we establish a critical role for 12/15-lipoxygenase (12/15-LOX)-mediated unsaturated fatty acid metabolism in HSC function. HSCs from 12/15-LOX-deficient mice are severely compromised in their capacity to reconstitute the hematopoietic compartment in competitive and serial reconstitution assays. Furthermore, we demonstrate that 12/15-LOX is required for the maintenance of long-term HSC quiescence and number. The defect in HSCs is cell-autonomous and associated with a selective reduction in 12/15-LOX-mediated generation of bioactive lipid mediators and reactive oxygen species and with a decrease in canonical Wnt signaling as measured by nuclear beta-catenin staining. These results have implications for development, aging, and transformation of the hematopoietic compartment.

Molecular signatures of quiescent, mobilized and leukemia-initiating hematopoietic stem cells

Hematopoietic stem cells (HSC) are rare, multipotent cells capable of generating all specialized cells of the blood system. Appropriate regulation of HSC quiescence is thought to be crucial to maintain their lifelong function; however, the molecular pathways controlling stem cell quiescence remain poorly characterized. Likewise, the molecular events driving leukemogenesis remain elusive. In this study, we compare the gene expression profiles of steady-state bone marrow HSC to non-self-renewing multipotent progenitors; to HSC treated with mobilizing drugs that expand the HSC pool and induce egress from the marrow; and to leukemic HSC in a mouse model of chronic myelogenous leukemia. By intersecting the resulting lists of differentially regulated genes we identify a subset of molecules that are downregulated in all three circumstances, and thus may be particularly important for the maintenance and function of normal, quiescent HSC. These results identify potential key regulators of HSC and give insights into the clinically important processes of HSC mobilization for transplantation and leukemic development from cancer stem cells.

Mechanisms of stem cell self-renewal

Self-renewal is the process by which stem cells divide to make more stem cells, perpetuating the stem cell pool throughout life. Self-renewal is division with maintenance of the undifferentiated state. This requires cell cycle control and often maintenance of multipotency or pluripotency, depending on the stem cell. Self-renewal programs involve networks that balance proto-oncogenes (promoting self-renewal), gate-keeping tumor suppressors (limiting self-renewal), and care-taking tumor suppressors (maintaining genomic integrity). These cell-intrinsic mechanisms are regulated by cell-extrinsic signals from the niche, the microenvironment that maintains stem cells and regulates their function in tissues. In response to changing tissue demands, stem cells undergo changes in cell cycle status and developmental potential over time, requiring different self-renewal programs at different stages of life. Reduced stem cell function and tissue regenerative capacity during aging are caused by changes in self-renewal programs that augment tumor suppression. Cancer arises from mutations that inappropriately activate self-renewal programs.

Pbx1 is a downstream target of Evi-1 in hematopoietic stem/progenitors and leukemic cells

Ecotropic viral integration site-1 (Evi-1) is a nuclear transcription factor, which is essential for the proliferation/maintenance of hematopoietic stem cells (HSCs). Aberrant expression of Evi-1 has been frequently found in myeloid leukemia, and is associated with a poor patient survival. Recently, we reported candidate target genes of Evi-1 shared in HSCs and leukemic cells using gene expression profiling analysis. In this study, we identified Pbx1, a proto-oncogene in hematopoietic malignancy, as a target gene of Evi-1. Overexpression of Evi-1 increased Pbx1 expression in hematopoietic stem/progenitor cells. An analysis of the Pbx1 promoter region revealed that Evi-1 upregulates Pbx1 transcription. Furthermore, reduction of Pbx1 levels through RNAi-mediated knockdown significantly inhibited Evi-1-induced transformation. In contrast, knockdown of Pbx1 did not impair bone marrow transformation by E2A/HLF or AML1/ETO, suggesting that Pbx1 is specifically required for the maintenance of bone marrow transformation mediated by Evi-1. These results indicate that Pbx1 is a target gene of Evi-1 involved in Evi-1-mediated leukemogenesis.

Balancing dormant and self-renewing hematopoietic stem cells

The mouse hematopoietic stem cell (HSC) is probably the best-understood somatic stem cell in higher organisms. Recent studies have shown that the highest self-renewal potential is most likely contained within an exceedingly small number of deeply dormant bone marrow HSCs. These stem cells are housed in individual niches that preserve their dormancy via signaling molecules such as Thrombopoietin, Angiopoietins, and Stem Cell Factor. In response to injury cues, dormant HSCs are efficiently activated and produce numerous progenitors and mature cells. A series of intracellular regulatory molecules including FoxOs, mTORC1, Fbw7, Egr1, Pbx1, pRb, c-Cbl, Myc, and Bmi1 mediate the processes of dormancy, cycling, self-renewal, differentiation, and survival, all of which control the behavior of HSCs.

Essential role for Pbx1 in corneal morphogenesis

PURPOSE

The Pbx TALE (three-amino-acid loop extension) homeodomain proteins interact with class 1 Hox proteins, which are master regulators of cell fate decisions. This study was performed to elucidate the role of the Pbx1 TALE protein in the corneal epithelium of mice.

METHODS

Pbx1(f/f) mice were crossed with mice containing Cre recombinase under the control of the K14 promoter. Subsequently, the eyes of these mice were dissected and prepared for histologic or molecular analysis.

RESULTS

Tissue-specific deletion of Pbx1 in the corneal epithelium of mice resulted in corneal dystrophy and clouding that was apparent in newborns and progressively worsened with age. Thickening of the cornea epithelium was accompanied by stromal infiltration with atypical basal cells, severe disorganization of stromal collagen matrix, and loss of corneal barrier function. High epithelial cell turnover was associated with perturbed expression of developmental regulators and aberrant differentiation, suggesting an important function for Pbx1 in determining corneal identity.

CONCLUSIONS

These studies establish an essential role of the Pbx1 proto-oncogene in corneal morphogenesis.

KLF4 and PBX1 directly regulate NANOG expression in human embryonic stem cells

Insight into the regulation of core transcription factors is important for a better understanding of the molecular mechanisms that control self-renewal and pluripotency of human ESCs (hESCs). However, the transcriptional regulation of NANOG itself in hESCs has largely been elusive. We established a NANOG promoter luciferase reporter assay as a fast read-out for indicating the pluripotent status of hESCs. From the functional cDNA screens and NANOG promoter characterization, we successfully identified a zinc finger transcription factor KLF4 and a homeodomain transcription factor PBX1 as two novel transcriptional regulators that maintain the pluripotent and undifferentiated state of hESCs. We showed that both KLF4 and PBX1 mRNA and protein expression were downregulated during hESC differentiation. In addition, overexpression of KLF4 and PBX1 upregulated NANOG promoter activity and also the endogenous NANOG protein expression in hESCs. Direct binding of KLF4 on NANOG proximal promoter and PBX1 on a new upstream enhancer and proximal promoter were confirmed by chromatin immunoprecipitation and electrophoretic mobility shift assay. Knockdown of KLF4/PBX1 or mutation of KLF4/PBX1 binding motifs significantly downregulated NANOG promoter activity. We also showed that specific members of the SP/KLF and PBX family are functionally redundant at the NANOG promoter and that KLF4 and PBX1 cooperated with OCT4 and SOX2, and transactivated synergistically the NANOG promoter activity. Our results show two novel upstream transcription activators of NANOG that are functionally important for the self-renewal of hESC and provide new insights into the expanded regulatory circuitry that maintains hESC pluripotency.

Loss of the Rho GTPase activating protein p190-B enhances hematopoietic stem cell engraftment potential

Hematopoietic stem cell (HSC) engraftment is a multistep process involving HSC homing to bone marrow, self-renewal, proliferation, and differentiation to mature blood cells. Here, we show that loss of p190-B RhoGTPase activating protein, a negative regulator of Rho GTPases, results in enhanced long-term engraftment during serial transplantation. This effect is associated with maintenance of functional HSC-enriched cells. Furthermore, loss of p190-B led to marked improvement of HSC in vivo repopulation capacity during ex vivo culture without altering proliferation and multilineage differentiation of HSC and progeny. Transcriptional analysis revealed that p190-B deficiency represses the up-regulation of p16(Ink4a) in HSCs in primary and secondary transplantation recipients, providing a possible mechanism of p190-B-mediated HSC functions. Our study defines p190-B as a critical transducer element of HSC self-renewal activity and long-term engraftment, thus suggesting that p190-B is a target for HSC-based therapies requiring maintenance of engraftment phenotype.

Thioredoxin-interacting protein regulates hematopoietic stem cell quiescence and mobilization under stress conditions

Hematopoietic stem cells (HSCs) are maintained in a quiescent state in bone marrow (BM) niches by intrinsic and extrinsic signals. The mechanisms regulating the quiescence and mobilization of HSCs, however, remain unclear. In this study, we report that the expression of thioredoxin-interacting protein (TXNIP) is decreased during HSC activation. In Txnip(-/-) mice, the long-term reconstituting HSC population is decreased and exhausted, and its capacity to repopulate is rapidly lost. These effects are associated with hyperactive Wnt signaling, an active cell cycle, and reduced p21 expression under conditions of stress. TXNIP deficiency reduced the CXCL12- and osteopontin-mediated interaction between HSCs and the bone marrow, and impaired homing and retention in the osteoblastic niche, resulting in mobilized HSCs. Therefore, we propose that TXNIP is essential for maintaining HSC quiescence and the interaction between HSCs and the BM niche.

Targeted resequencing and analysis of the Diamond-Blackfan anemia disease locus RPS19

Background

The Ribosomal protein S19 gene locus (RPS19) has been linked to two kinds of red cell aplasia, Diamond-Blackfan Anemia (DBA) and Transient Erythroblastopenia in Childhood (TEC). Mutations in RPS19 coding sequences have been found in 25% of DBA patients, but not in TEC patients. It has been suggested that non-coding RPS19 sequence variants contribute to the considerable clinical variability in red cell aplasia. We therefore aimed at identifying non-coding variations associated with DBA or TEC phenotypes.

Methodology/Principal Findings

We targeted a region of 19'980 bp encompassing the RPS19 gene in a cohort of 89 DBA and TEC patients for resequencing. We provide here a catalog of the considerable, previously unrecognized degree of variation in this region. We identified 73 variations (65 SNPs, 8 indels) that all are located outside of the RPS19 open reading frame, and of which 67.1% are classified as novel. We hypothesize that specific alleles in non-coding regions of RPS19 could alter the binding of regulatory proteins or transcription factors. Therefore, we carried out an extensive analysis to identify transcription factor binding sites (TFBS). A series of putative interaction sites coincide with detected variants. Sixteen of the corresponding transcription factors are of particular interest, as they are housekeeping genes or show a direct link to hematopoiesis, tumorigenesis or leukemia (e.g. GATA-1/2, PU.1, MZF-1).

Conclusions

Specific alleles at predicted TFBSs may alter the expression of RPS19, modify an important interaction between transcription factors with overlapping TFBS or remove an important stimulus for hematopoiesis. We suggest that the detected interactions are of importance for hematopoiesis and could provide new insights into individual response to treatment.

BloodExpress: a database of gene expression in mouse haematopoiesis

Haematopoiesis is the process whereby blood stem cells give rise to at least fourteen functionally distinct mature cell types, and represents the best characterized mammalian adult stem cell system. Here we introduce the BloodExpress database, the first public resource integrating mouse blood cell expression profiles. BloodExpress enables the searching of data from individual studies in a single database accessible through a user-friendly web interface. Microarray datasets have been processed uniformly to allow their comparison on the BloodExpress platform. BloodExpress covers the majority of murine blood cell types, including both progenitors and terminally differentiated cells. This allows for the identification of dynamic changes in gene expression as cells differentiate down the well-defined haematopoietic hierarchy. A gene-centric interface returns haematopoietic expression patterns together with functional annotation and a list of other genes with similar expression patterns. A cell type-centric interface allows the identification of genes expressed at specific points of blood development, with the additional and useful capability of filtering by specific gene functional categories. BloodExpress thus constitutes a platform for the discovery of novel gene functions across the haematopoietic tree. BloodExpress is freely accessible at http://hscl.cimr.cam.ac.uk/bloodexpress/.

Impaired function of primitive hematopoietic cells in mice lacking the Mixed-Lineage-Leukemia homolog MLL5

The human Mixed-Lineage-Leukemia-5 (MLL5) gene is located in a genomic region frequently deleted in patients with myeloid malignancies and encodes a widely expressed nuclear protein most closely related to MLL1, a Trithorax transcriptional regulator with established involvement in leukemogenesis. Although the physiologic function of MLL5 is completely unknown, domain structure and homology to transcriptional regulators with histone methyltransferase activity suggest a role in epigenetic gene regulation. To investigate physiologic functions of Mll5, we have generated a knockout mouse mutant using Cre/loxP technology. Adult homozygous Mll5-deficient mice are obtained at reduced frequency because of postnatal lethality. Surviving animals display a variety of abnormalities, including male infertility, retarded growth, and defects in multiple hematopoietic lineages. Interestingly, Mll5(-/-) mice die of sublethal whole-body irradiation but can be rescued with wild-type bone marrow grafts. Flow cytometric ana-lysis, bone marrow reconstitution, and in vivo BrdU-labeling experiments reveal numerical, functional, and cell-cycle defects in the lineage-negative Sca-1(+), Kit(+) (LSK) population, which contains short- and long-term hematopoietic stem cells. Together, these in vivo findings establish several nonredundant functions for Mll5, including an essential role in regulating proliferation and functional integrity of hematopoietic stem/progenitor cells.