The Runx3 transcription factor regulates development and survival of TrkC dorsal root ganglia neurons

Co-activator activator (CoAA) prevents the transcriptional activity of Runt domain transcription factors

Runx proteins are essential for a number of developmental processes and are aberrantly expressed in many human cancers. Runx factors bind DNA and co-factors to activate or repress genes crucial for bone formation, hematopoiesis, and neuronal development. Co-activator activator (CoAA) is a nuclear protein that regulates gene expression, RNA splicing and is overexpressed in many human tumors. In this study, we identified CoAA as a Runx2 binding protein. CoAA repressed Runx factor-dependent activation of reporter genes in a histone deacetylase-independent manner. CoAA also blocked Runx2-mediated repression of the Axin2 promoter, a novel Runx target gene. The carboxy-terminus of CoAA is essential for binding the Runt domains of Runx1 and Runx2. In electophoretic mobility shift assays, CoAA inhibited Runx2 interactions with DNA. These data indicate that CoAA is an inhibitor of Runx factors and can negate Runx factor regulation of gene expression. CoAA is expressed at high levels in human fetal osteoblasts and osteosarcoma cell lines. Suppression of CoAA expression by RNA interference reduced osteosarcoma cell viability in vitro, suggesting that it contributes to the proliferation and/or survival of osteoblast lineage cells.

Post-translational Regulation of Runx2 in Bone and Cartilage

The Runx2 gene product is essential for mammalian bone development. In humans, Runx2 haploinsufficiency results in cleidocranial dysplasia, a skeletal disorder characterized by bone and dental abnormalities. At the molecular level, Runx2 acts as a transcription factor for genes expressed in hypertrophic chondrocytes and osteoblasts. Runx2 gene expression and protein function are regulated on multiple levels, including transcription, translation, and post-translational modification. Furthermore, Runx2 is involved in numerous protein-protein interactions, most of which either activate or repress transcription of target genes. In this review, we discuss expression of Runx2 during development as well as the post-translational regulation of Runx2 through modification by phosphorylation, ubiquitination, and acetylation.

Jab1/CSN5 induces the cytoplasmic localization and degradation of RUNX3

Runt-related (RUNX) transcription factors play pivotal roles in neoplastic development and have tissue-specific developmental roles in hematopoiesis (RUNX1), osteogenesis (RUNX2), as well as neurogenesis and thymopoiesis (RUNX3). RUNX3 is a tumor suppressor in gastric carcinoma, and its expression is frequently inactivated by DNA methylation or its protein mislocalized in many cancer types, including gastric and breast cancer. Jun-activation domain-binding protein 1 (Jab1/CSN5), a component of the COP9 signalosome (CSN), is critical for nuclear export and the degradation of several tumor suppressor proteins, including p53, p27(Kip1), and Smad4. Here, we find that Jab1 facilitates nuclear export of RUNX3 that is controlled by CSN-associated kinases. RUNX3 sequestered in the cytoplasm is rapidly degraded through a proteasome-mediated pathway. Our results identify a novel mechanism of regulating nuclear export and protein stability of RUNX3 by the CSN complex.

Molecular pathology of RUNX3 in human carcinogenesis

A major goal of molecular biology is to elucidate the mechanisms underlying cancer development and progression in order to achieve early detection, better diagnosis and staging and novel preventive and therapeutic strategies. We feel that an understanding of Runt-related transcription factor 3 (RUNX3)-regulated biological pathways will directly impact our knowledge of these areas of human carcinogenesis. The RUNX3 transcription factor is a downstream effector of the transforming growth factor-beta (TGF-beta) signaling pathway, and has a critical role in the regulation of cell proliferation and cell death by apoptosis, and in angiogenesis, cell adhesion and invasion. We previously identified RUNX3 as a major gastric tumor suppressor by establishing a causal relationship between loss of function and gastric carcinogenesis. More recently, we showed that RUNX3 functions as a bona fide initiator of colonic carcinogenesis by linking the Wnt oncogenic and TGF-beta tumor suppressive pathways. Apart from gastric and colorectal cancers, a multitude of epithelial cancers exhibit inactivation of RUNX3, thereby making it a putative tumor suppressor in human neoplasia. This review highlights our current understanding of the molecular mechanisms of RUNX3 inactivation in the context of cancer development and progression.

Identification of novel Stat6 regulated proteins in IL-4-treated mouse lymphocytes

Interleukin 4 (IL-4) has an indispensable role in the differentiation of naive T helper (Th) cells toward the Th2 phenotype and induction of B cells to produce the IgE class of Igs. By regulating these two cell types, IL-4 has a pre-eminent role in regulation of allergic inflammation. IL-4-mediated regulation of T and B cell functions is largely transmitted through signal transducer and activator of transcription 6 (Stat6). In this study, we have used metabolic labeling and 2-D electrophoresis to detect differences in the proteomes of IL-4 stimulated spleen mononuclear cells of Stat6-/- and wild type mice and MS/MS for protein identification. With this methodology, we identified 49 unique proteins from 21 protein spots to be differentially expressed. Interestingly, in Stat6-/- CD4(+) cells the expression of isoform 2 of core binding factor b (CBFb2) was enhanced. CBFb is a non-DNA binding cofactor for the Runx family of transcription factors, which have been implicated in regulation of Th cell differentiation. We also found cellular nucleic acid protein (CNBP) to be downregulated in Stat6-/- cells. None of the proteins identified in this study have previously been reported to be regulated via Stat6. The results highlight the importance of exploiting proteomics tools to complement the studies on Stat6 target genes identified through transcriptional profiling.

HeyL regulates the number of TrkC neurons in dorsal root ganglia

The basic-helix-loop-helix transcription factor HeyL is expressed at high levels by neural crest progenitor cells (NCPs) that give rise to neurons and glia in dorsal root ganglia (DRG). Since HeyL expression was observed in these NCPs during the period of neurogenesis, we generated HeyL null mutants to help examine the factor's role in ganglion neuronal specification. Homozygous null mutation of HeyL reduced the number of TrkC(+) neurons in DRG at birth including the subpopulation that expresses the ETS transcription factor ER81. Conversely, null mutation of the Hey paralog, Hey1, increased the number of TrkC(+) neurons. Null mutation of HeyL increased expression of the Hey paralogs Hey1 and Hey2, suggesting that HeyL normally inhibits their expression. Double null mutation of both Hey1 and HeyL rescued TrkC(+) neuron numbers to control levels. Thus, the balance between HeyL and Hey1 expression regulates the differentiation of a subpopulation of TrkC(+) neurons in the DRG.

RUNX factors in development: lessons from invertebrate model systems

Runt-related (RUNX) transcription factors are evolutionarily conserved regulators of cell proliferation, differentiation and stem cell maintenance. They are critical for the correct development and function of a variety of human tissues, including during haematopoiesis. RUNX genes regulate various aspects of proliferation control, stem cell maintenance, lineage commitment and regulation of differentiation; disruptions in the correct function of RUNX genes have been associated with human pathologies, most prominently cancer. Because of the high context dependency and partial redundancy of vertebrate RUNX genes, invertebrate model systems have been studied in the hope of finding an ancestral function. Here we review the progress of these studies in three invertebrate systems, the fruit fly Drosophila melanogaster, the sea urchin Strongylocentrotus purpuratus and the nematode Caenorhabditis elegans. All essential aspects of RUNX function in vertebrates have counterparts in invertebrates, confirming the usefulness of these studies in simpler organisms. The fact that not all RUNX functions are conserved in all systems, though, underscores the importance of choosing the right model to ask specific questions.

PTHrP prevents chondrocyte premature hypertrophy by inducing cyclin-D1-dependent Runx2 and Runx3 phosphorylation, ubiquitylation and proteasomal degradation

In chondrocytes, PTHrP maintains them in a proliferative state and prevents premature hypertrophy. The mechanism by which PTHrP does this is not fully understood. Both Runx2 and Runx3 are required for chondrocyte maturation. We recently demonstrated that cyclin D1 induces Runx2 protein phosphorylation and degradation. In the present studies, we tested the hypothesis that PTHrP regulates both Runx2 and Runx3 protein stability through cyclin D1. We analyzed the effects of cyclin D1 on Runx3 protein stability and function using COS cells, osteoprogenitor C3H10T1/2 cells and chondrogenic RCJ3.1C5.18 cells. We found that cyclin D1 induced Runx3 degradation in a dose-dependent manner and that both Myc-tagged Runx3 and endogenous Runx3 interact directly with CDK4 in COS and RCJ3.1C5.18 cells. A conserved CDK recognition site was identified in the C-terminal region of Runx3 by sequence analysis (residues 356-359). Pulse-chase experiments showed that the mutation of Runx3 at Ser356 to alanine (SA-Runx3) increased the half-life of Runx3. By contrast, the mutation at the same serine residue to glutamic acid (SE-Runx3) accelerated Runx3 degradation. In addition, SA-Runx3 was resistant to cyclin D1-induced degradation. GST-Runx3 was strongly phosphorylated by CDK4 in vitro. By contrast, CDK4 had no effect on the phosphorylation of SA-Runx3. Although both wild-type and SE-Runx3 were ubiquitylated, this was not the case for SA-Runx3. Runx3 degradation by cyclin D1 was completely blocked by the proteasome inhibitor PS1. In C3H10T1/2 cells, SA-Runx3 had a greater effect on reporter activity than SE-Runx3. The same was true for ALP activity in these cells. To investigate the role of cyclin D1 in chondrocyte proliferation and hypertrophy, we analyzed the growth plate morphology and expression of chondrocyte differentiation marker genes in Ccnd1-knockout mice. The proliferating and hypertrophic zones were significantly reduced and expression of chondrocyte differentiation marker genes and ALP activity were enhanced in 2-week-old Ccnd1-knockout mice. PTHrP significantly suppressed protein levels of both Runx2 and Runx3 in primary chondrocytes derived from wild-type mice. By contrast, the suppressive effect of PTHrP on Runx2 and Runx3 protein levels was completely abolished in primary chondrocytes derived from Ccnd1-knockout mice. Our findings demonstrate that the cell cycle proteins cyclin D1 and CDK4 induce Runx2 and Runx3 phosphorylation, ubiquitylation and proteasomal degradation. PTHrP suppresses Runx2 and Runx3 protein levels in chondrocytes through cyclin D1. These results suggest that PTHrP might prevent premature hypertrophy in chondrocytes, at least in part by inducing degradation of Runx2 and Runx3 in a cyclin-D1-dependent manner.

'Runxs and regulations' of sensory and motor neuron subtype differentiation: implications for hematopoietic development

Runt-related (RUNX) transcription factors are evolutionarily conserved regulators of a number of developmental mechanisms. RUNX proteins often control the balance between proliferation and differentiation and alterations of their functions are associated with different types of cancer and other human pathologies. Moreover, RUNX factors control important steps during the developmental acquisition of mature phenotypes. A number of investigations are beginning to shed light on the involvement of RUNX family members in the development of the nervous system. This review summarizes recent progress in the study of the roles of mammalian RUNX proteins during the differentiation of sensory and motor neurons in the peripheral and central nervous system, respectively. The implications of those findings for RUNX-mediated regulation of hematopoietic development will also be discussed.

RUNX1 and RUNX2 upregulate Galectin-3 expression in human pituitary tumors

Galectin-3 is expressed in a cell-type specific manner in human pituitary tumors and may have a role in pituitary tumor development. In this study, we hypothesized that Galectin-3 is regulated by RUNX proteins in pituitary tumors. Transcription factor prediction programs revealed several putative binding sites in the LGALS3 (Galectin-3 gene) promoter region. A human pituitary cell line HP75 was used as a model to study LGALS3 and RUNX interactions using Chromatin immunoprecipitation assay and electrophoresis mobility shift assay. Two binding sites for RUNX1 and one binding site for RUNX2 were identified in the LGALS3 promoter region. LGALS3 promoter was further cloned into a luciferase reporter, and the experiments showed that both RUNX1 and RUNX2 upregulated LGALS3. Knock-down of either RUNX1 or RUNX2 by siRNA resulted in a significant downregulation of Galectin-3 expression and decreased cell proliferation in the HP 75 cell line. Immunohistochemistry showed a close correlation between Galectin-3 expression and RUNX1/RUNX2 level in pituitary tumors. These results demonstrate a novel binding target for RUNX1 and RUNX2 proteins and suggest that Galectin-3 is regulated by RUNX1 and RUNX2 in human pituitary tumor cells by direct binding to the promoter region of LGALS3 and thus may contribute to pituitary tumor progression.

RUNX proteins in transcription factor networks that regulate T-cell lineage choice

Recent research has uncovered complex transcription factor networks that control the processes of T-cell development and differentiation. RUNX (runt-related transcription factor) proteins are among the many factors that have crucial roles in these networks. In this Review, we examine the mechanisms by which RUNX complexes act together with other transcription factors, such as Th-POK (T-helper-inducing POZ/Kruppel-like factor) and GATA-binding protein 3 (GATA3) in determining the CD4/CD8 lineage choice of developing thymocytes. In addition, we discuss evidence indicating that RUNX complexes are also involved in the differentiation of effector T-cell subsets and that the molecular mechanisms by which RUNX proteins regulate T-cell fate decisions are conserved between the thymus and periphery.

Co-stimulation of the bone-related Runx2 P1 promoter in mesenchymal cells by SP1 and ETS transcription factors at polymorphic purine-rich DNA sequences (Y-repeats)

Transcriptional control of Runx2 gene expression through two alternative promoters (P1 and P2) is critical for the execution of its function as an osteogenic cell fate determining factor. In all vertebrates examined to date, the bone related P1 promoter contains a purine-rich region (-303 to -128 bp in the rat) that separates two regulatory domains. The length of this region differs dramatically between species even within the same order. Using deletion analysis, we show that part of this purine-rich region (-200 to -128) containing a duplicated element (Y-repeat) positively regulates Runx2 P1 transcription. Electrophoretic mobility assays and chromatin immunoprecipitations reveal that Y-repeat binds at least two different classes of transcription factors related to GC box binding proteins (e.g. SP1 and SP7/Osterix) and ETS-like factors (e.g. ETS1 and ELK1). Forced expression of SP1 increases Runx2 P1 promoter activity through the Y-repeats, and small interfering RNA depletion of SP1 decreases Runx2 expression. Similarly, exogenous expression of wild type ELK1, but not a defective mutant that cannot be phosphorylated, enhances Runx2 gene expression. SP1 is most abundant in proliferating cells, and ELK1 is most abundant in postconfluent cells; during MC3T3-E1 osteoblast differentiation, both proteins are transiently co-expressed when Runx2 expression is enhanced. Taken together, our data suggest that basal Runx2 gene transcription is regulated by dynamic interactions between SP1 and ETS-like factors during progression of osteogenesis.

Runx3 expression in gastrointestinal tract epithelium: resolving the controversy

We reported earlier that RUNX3 is expressed in human and mouse gastrointestinal tract (GIT) epithelium and that it functions as a tumor suppressor in gastric and colorectal tissues. However, there have been conflicting reports describing the absence of Runx3 in GIT epithelial cells. A part of the controversy may be derived from the use of a specific antibody by other groups (referred to as G-poly). Here, we show further evidence to support our earlier observations and provide a possible explanation for this apparent controversy. We generated multiple anti-RUNX3 monoclonal antibodies and found that RUNX3 antibodies recognizing the RUNX3 N-terminal region (residues 1-234) react with RUNX3 in gastric epithelial cells, whereas those recognizing the C-terminal region (beyond residue 234) did not. G-poly primarily recognizes the region beyond 234 and hence, is unable to detect Runx3 in this tissue.

Subnuclear targeting of the Runx3 tumor suppressor and its epigenetic association with mitotic chromosomes

Runx proteins are tissue-specific transcriptional scaffolds that organize and assemble regulatory complexes at strategic sites of target gene promoters and at intranuclear foci to govern activation or repression. During interphase, fidelity of intranuclear targeting supports the biological activity of Runx1 and Runx2 proteins. Both factors regulate genes involved in cell cycle control and cell growth (e.g., rRNA genes), as well as lineage commitment. Here, we have examined the subcellular regulatory properties of the third Runx member, the tumor suppressor protein Runx3, during interphase and mitosis. Using in situ cellular and biochemical approaches we delineated a subnuclear targeting signal that directs Runx3 to discrete transcriptional foci that are nuclear matrix associated. Chromatin immunoprecipitation results show that Runx3 occupies rRNA promoters during interphase. We also find that Runx3 remains associated with chromosomes during mitosis and localizes with nucleolar organizing regions (NORs), reflecting an interaction with epigenetic potential. Taken together, our study establishes that common mechanisms control the subnuclear distribution and activities of Runx1, Runx2, and Runx3 proteins to support RNA polymerase I and II mediated gene expression during interphase and mitosis.

Runx expression is mitogenic and mutually linked to Wnt activity in blastula-stage sea urchin embryos

Background

The Runt homology domain (Runx) defines a metazoan family of sequence-specific transcriptional regulatory proteins that are critical for animal development and causally associated with a variety of mammalian cancers. The sea urchin Runx gene SpRunt-1 is expressed throughout the blastula stage embryo, and is required globally during embryogenesis for cell survival and differentiation.

Methodology/Principal Findings

Depletion of SpRunt-1 by morpholino antisense-mediated knockdown causes a blastula stage deficit in cell proliferation, as shown by bromodeoxyuridine (BrdU) incorporation and direct cell counts. Reverse transcription coupled polymerase chain reaction (RT-PCR) studies show that the cell proliferation deficit is presaged by a deficit in the expression of several zygotic wnt genes, including wnt8, a key regulator of endomesoderm development. In addition, SpRunt-1-depleted blastulae underexpress cyclinD, an effector of mitogenic Wnt signaling. Blastula stage cell proliferation is also impeded by knockdown of either wnt8 or cyclinD. Chromatin immunoprecipitation (ChIP) indicates that Runx target sites within 5′ sequences flanking cyclinD, wnt6 and wnt8 are directly bound by SpRunt-1 protein at late blastula stage. Furthermore, experiments using a green fluorescent protein (GFP) reporter transgene show that the blastula-stage operation of a cis-regulatory module previously shown to be required for wnt8 expression (Minokawa et al., Dev. Biol. 288: 545–558, 2005) is dependent on its direct sequence-specific interaction with SpRunt-1. Finally, inhibitor studies and immunoblot analysis show that SpRunt-1 protein levels are negatively regulated by glycogen synthase kinase (GSK)-3.

Conclusions/Significance

These results suggest that Runx expression and Wnt signaling are mutually linked in a feedback circuit that controls cell proliferation during development.

E1A physically interacts with RUNX3 and inhibits its transactivation activity

The adenoviral gene, termed early region 1A (E1A), is crucial for transformation and has been used very effectively as a tool to determine the molecular mechanisms that underlie the basis of cellular transformation. pRb, p107, p130, p300/CBP, p400, TRRAP, and CtBP were identified to be E1A-binding proteins and their roles in cellular transformation have been established. Although the major function of E1A is considered to be the regulation of gene expression that is critical for differentiation and cell cycle exit, one of the most significant questions relating to E1A transformation is how E1A mediates this regulation. RUNX3 is a transcription factor that was first described as a gastric cancer tumor suppressor but is now known to be involved in many different cancers. Exogenous expression of RUNX3 strongly inhibits the growth of cells. Here, we show that the adenovirus oncoprotein E1A interacts with RUNX3 in vitro and in vivo. RUNX3 interacts with the N-terminus (amino acids 2-29) of E1A, which is known to interact with p300/CBP, p400, and TRRAP. E1A interacts directly with the Runt domain of RUNX3 but does not interfere with CBFbeta-RUNX3 interactions. In addition, E1A inhibits the transactivation activity of RUNX3 on the p21(WAF1/CIP1) promoter. Consistent with these observations, the growth inhibition induced by RUNX3 is reduced by E1A. These results demonstrate that E1A specifically binds to RUNX3 and inactivates its transactivation activity. We propose that one of the mechanisms for the oncogenic activity of E1A is the inhibition of RUNX3, similar to that of RB and p300/CBP.

A central role for Islet1 in sensory neuron development linking sensory and spinal gene regulatory programs

We have used conditional knockout strategies in mice to determine the developmental events and gene expression program regulated by the LIM-homeodomain factor Islet1 in developing sensory neurons. Early development of the trigeminal and dorsal root ganglia are grossly normal in the absence of Islet1. However, from E12.5 onward, Islet1 mutant embryos exhibit loss of the nociceptive markers TrkA and Runx1 and a near absence of cutaneous innervation. Proprioceptive neurons characterized by the expression of TrkC/Runx3/Etv1 are relatively spared. Microarray analysis of Islet1 mutant ganglia reveals prolonged expression of developmental regulators normally restricted to early sensory neurogenesis, and ectopic expression of transcription factors normally found in the CNS but not in sensory ganglia. Later excision of Islet1 does not reactivate early genes, but results in decreased expression of transcripts related to specific sensory functions. Together these results establish a central role for Islet1 in the transition from sensory neurogenesis to subtype specification.

Cancer genes hypermethylated in human embryonic stem cells

Developmental genes are silenced in embryonic stem cells by a bivalent histone-based chromatin mark. It has been proposed that this mark also confers a predisposition to aberrant DNA promoter hypermethylation of tumor suppressor genes (TSGs) in cancer. We report here that silencing of a significant proportion of these TSGs in human embryonic and adult stem cells is associated with promoter DNA hypermethylation. Our results indicate a role for DNA methylation in the control of gene expression in human stem cells and suggest that, for genes repressed by promoter hypermethylation in stem cells in vivo, the aberrant process in cancer could be understood as a defect in establishing an unmethylated promoter during differentiation, rather than as an anomalous process of de novo hypermethylation.

ThPOK acts late in specification of the helper T cell lineage and suppresses Runx-mediated commitment to the cytotoxic T cell lineage

The transcription factor ThPOK has been shown to be required and sufficient for CD4⁺CD8⁻ thymocyte generation, yet the mechanism through which ThPOK orchestrates CD4 helper T cell lineage differentiation remains unclear. Here we utilized reporter mice to track expression of transcription factors in developing thymocytes. Distal promoter-driven Runx3 (Runx3d) expression was restricted to MHC class I-selected thymocytes. In ThPOK-deficient mice, Runx3d expression was de-repressed in MHCII-selected thymocytes, contributing to their redirection to the CD8 T cell lineage. In the absence of both ThPOK and Runx, redirection was prevented and cells potentially belonging to the CD4 lineage, presumably specified independently of ThPOK, were generated. Our results suggest that MHCII-selected thymocytes are directed towards the CD4 lineage independently of ThPOK, but require ThPOK to prevent Runx-dependent differentiation towards the CD8 lineage.

Runx transcription factors in neuronal development

Runt-related (Runx) transcription factors control diverse aspects of embryonic development and are responsible for the pathogenesis of many human diseases. In recent years, the functions of this transcription factor family in the nervous system have just begun to be understood. In dorsal root ganglion neurons, Runx1 and Runx3 play pivotal roles in the development of nociceptive and proprioceptive sensory neurons, respectively. Runx appears to control the transcriptional regulation of neurotrophin receptors, numerous ion channels and neuropeptides. As a consequence, Runx contributes to diverse aspects of the sensory system in higher vertebrates. In this review, we summarize recent progress in determining the role of Runx in neuronal development.

The evolutionary origin of the Runx/CBFbeta transcription factors--studies of the most basal metazoans

Background

Members of the Runx family of transcriptional regulators, which bind DNA as heterodimers with CBFβ, are known to play critical roles in embryonic development in many triploblastic animals such as mammals and insects. They are known to regulate basic developmental processes such as cell fate determination and cellular potency in multiple stem-cell types, including the sensory nerve cell progenitors of ganglia in mammals.

Results

In this study, we detect and characterize the hitherto unexplored Runx/CBFβ genes of cnidarians and sponges, two basal animal lineages that are well known for their extensive regenerative capacity. Comparative structural modeling indicates that the Runx-CBFβ-DNA complex from most cnidarians and sponges is highly similar to that found in humans, with changes in the residues involved in Runx-CBFβ dimerization in either of the proteins mirrored by compensatory changes in the binding partner. In situ hybridization studies reveal that Nematostella Runx and CBFβ are expressed predominantly in small isolated foci at the base of the ectoderm of the tentacles in adult animals, possibly representing neurons or their progenitors.

Conclusion

These results reveal that Runx and CBFβ likely functioned together to regulate transcription in the common ancestor of all metazoans, and the structure of the Runx-CBFβ-DNA complex has remained extremely conserved since the human-sponge divergence. The expression data suggest a hypothesis that these genes may have played a role in nerve cell differentiation or maintenance in the common ancestor of cnidarians and bilaterians.

Gene array analysis reveals a common Runx transcriptional programme controlling cell adhesion and survival

The Runx genes are important in development and cancer, where they can act either as oncogenes or tumour suppressors. We compared the effects of ectopic Runx expression in established fibroblasts, where all three genes produce an indistinguishable phenotype entailing epithelioid morphology and increased cell survival under stress conditions. Gene array analysis revealed a strongly overlapping transcriptional signature, with no examples of opposing regulation of the same target gene. A common set of 50 highly regulated genes was identified after further filtering on regulation by inducible RUNX1-ER. This set revealed a strong bias toward genes with annotated roles in cancer and development, and a preponderance of targets encoding extracellular or surface proteins, reflecting the marked effects of Runx on cell adhesion. Furthermore, in silico prediction of resistance to glucocorticoid growth inhibition was confirmed in fibroblasts and lymphoid cells expressing ectopic Runx. The effects of fibroblast expression of common RUNX1 fusion oncoproteins (RUNX1-ETO, TEL-RUNX1 and CBFB-MYH11) were also tested. Although two direct Runx activation target genes were repressed (Ncam1 and Rgc32), the fusion proteins appeared to disrupt the regulation of downregulated targets (Cebpd, Id2 and Rgs2) rather than impose constitutive repression. These results elucidate the oncogenic potential of the Runx family and reveal novel targets for therapeutic inhibition.

Enhancer of zeste homologue 2 (EZH2) down-regulates RUNX3 by increasing histone H3 methylation

Overexpression of enhancer of zeste homologue 2 (EZH2) occurs in various malignancies and is associated with a poor prognosis, especially because of increased cancer cell proliferation. In this study we found an inverse correlation between EZH2 and RUNX3 gene expression in five cancer cell lines, i.e. gastric, breast, prostate, colon, and pancreatic cancer cell lines. Chromatin immunoprecipitation assay showed an association between EZH2 bound to the RUNX3 gene promoter, and trimethylated histone H3 at lysine 27, and HDAC1 (histone deacetylase 1) bound to the RUNX3 gene promoter in cancer cells. RNA interference-mediated knockdown of EZH2 resulted in a decrease in H3K27 trimethylation and unbound HDAC1 and an increase in expression of the RUNX3 gene. Restoration of RUNX3 expression was not associated with any change in DNA methylation status in the RUNX3 promoter region. RUNX3 was repressed by histone deacetylation and hypermethylation of a CpG island in the promoter region and restored by trichostatin A or/and 5-aza-2'-deoxycytidine. Immunofluorescence staining confirmed restoration of expression of the RUNX3 protein after knockdown of EZH2 and its restoration resulted in decreased cell proliferation. In vivo, an inverse relationship between expression of the EZH2 and RUNX3 proteins was observed at the individual cell level in gastric cancer patients in the absence of DNA methylation in the RUNX3 promoter region. The results showed that RUNX3 is a target for repression by EZH2 and indicated an underlying mechanism of the functional role of EZH2 overexpression on cancer cell proliferation.

Suppression of interneuron programs and maintenance of selected spinal motor neuron fates by the transcription factor AML1/Runx1

Individual spinal motor neuron identities are specified in large part by the intrinsic repertoire of transcription factors expressed by undifferentiated progenitors and maturing neurons. It is shown here that the transcription factor AML1/Runx1 (Runx1) is expressed in selected spinal motor neuron subtypes after the onset of differentiation and is both necessary and sufficient to suppress interneuron-specific developmental programs and promote maintenance of motor neuron characteristics. These findings show an important role for Runx1 during the consolidation of selected spinal motor neuron identities. Moreover, they suggest a requirement for a persistent suppression of interneuron genes within maturing motor neurons.

Dynamic regulation of the expression of neurotrophin receptors by Runx3

Sensory neurons in the dorsal root ganglion (DRG) specifically project axons to central and peripheral targets according to their sensory modality. However, the molecular mechanisms that govern sensory neuron differentiation and the axonal projections remain unclear. The Runt-related transcription factors, Runx1 and Runx3, are expressed in DRG neuronal subpopulations, suggesting that they might regulate the cell specification and the trajectories of specific axons. Here, we show that parvalbumin-positive DRG neurons fail to differentiate from the onset in Runx3(-/-) mice. By contrast, TrkC-positive DRG neurons differentiate normally at embryonic day (E) 11.5, but disappear by E13.5 in Runx3(-/-) mice. Subsequently, TrkC-positive DRG neurons reappear but in smaller numbers than in the wild type. In Runx3(-/-) mice, central axons of the TrkC-positive DRG neurons project to the dorsal spinal cord but not to the ventral and intermediate spinal cord, whereas the peripheral axons project to skin but not to muscle. These results suggest that Runx3 controls the acquisition of distinct proprioceptive DRG neuron identities, and that TrkC-positive DRG neurons consist of two subpopulations: Runx3-dependent early-appearing proprioceptive neurons that project to the ventral and intermediate spinal cord and muscle; and Runx3-independent late-appearing cutaneous neurons that project to the dorsal spinal cord and skin. Moreover, we show that the number of TrkA-positive DRG neurons is reduced in Runx3(-/-) mice, as compared with the wild type. These results suggest that Runx3 positively regulates the expression of TrkC and TrkA in DRG neurons.

Loss of nephrocystin-3 function can cause embryonic lethality, Meckel-Gruber-like syndrome, situs inversus, and renal-hepatic-pancreatic dysplasia

Many genetic diseases have been linked to the dysfunction of primary cilia, which occur nearly ubiquitously in the body and act as solitary cellular mechanosensory organelles. The list of clinical manifestations and affected tissues in cilia-related disorders (ciliopathies) such as nephronophthisis is broad and has been attributed to the wide expression pattern of ciliary proteins. However, little is known about the molecular mechanisms leading to this dramatic diversity of phenotypes. We recently reported hypomorphic NPHP3 mutations in children and young adults with isolated nephronophthisis and associated hepatic fibrosis or tapetoretinal degeneration. Here, we chose a combinatorial approach in mice and humans to define the phenotypic spectrum of NPHP3/Nphp3 mutations and the role of the nephrocystin-3 protein. We demonstrate that the pcy mutation generates a hypomorphic Nphp3 allele that is responsible for the cystic kidney disease phenotype, whereas complete loss of Nphp3 function results in situs inversus, congenital heart defects, and embryonic lethality in mice. In humans, we show that NPHP3 mutations can cause a broad clinical spectrum of early embryonic patterning defects comprising situs inversus, polydactyly, central nervous system malformations, structural heart defects, preauricular fistulas, and a wide range of congenital anomalies of the kidney and urinary tract (CAKUT). On the functional level, we show that nephrocystin-3 directly interacts with inversin and can inhibit like inversin canonical Wnt signaling, whereas nephrocystin-3 deficiency leads in Xenopus laevis to typical planar cell polarity defects, suggesting a role in the control of canonical and noncanonical (planar cell polarity) Wnt signaling.

Relationship between tumor suppressor gene RUNX3 expression and cell proliferation and apoptosis in colonic cancer cell line Lovo

AIM: To investigate the expression and methylation status of tumor suppressor gene RUNX3 in human colon cancer cell line Lovo and explore the effects of 5-aza-2'-deoxycytidine (5-Aza-CdR) on the proliferation and apoptosis of Lovo cells and the expression of RUNX3 gene.

METHODS: Human colon cancer cell line Lovo was treated with 5-Aza-CdR, a specific methyltransferase inhibitor, at the concentrations of 0.4, 4 and 40 μmol/L for 3 d, and then cultured in RPMI 1640 medium for 5 d. The activation of Lovo cells was respectively observed by Tetrazolium salt colorimetric (MTT) assay before and after 5-Aza-CdR treatment. The change in expression of RUNX3 mRNA was observed by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). The apoptosis was analyzed by flow cytometry. The methylation status of gene promoter was determined by methylation-specific PCR (MSP).

RESULTS: Lovo cells treated with 5-Aza-CdR (0.4, 4, 40 μmol/L) displayed a slowed growth rate in different degrees in contrast with those in the control group and their growth rates decreased accordingly with the increase of 5-Aza-CdR concentration. There were significant increases in RUNX3 mRNA expression (0.46 ± 0.06, 0.71 ± 0.06, 0.84 ± 0.07 vs 0, P < 0.01) and apoptotic rates of Lovo cells (10.95% ± 2.09%, 17.61% ± 1.51%, 26.60% ± 1.89% vs 2.92% ± 0.93%, P < 0.01) after 5-Aza-CdR treatment in comparison with those in the control group. The level of RUNX3 mRNA expression and the apoptotic rates of Lovo cells were increased in correlation with 5-Aza-CdR concentration (F = 168.4, F = 145.7, P < 0.01). Methylation of RUNX3 promoter region was confirmed in Lovo cells of control group and detected partly in 5-Aza-CdR-treated group.

CONCLUSION: 5-Aza-CdR is able to reverse the methylation status of RUNX3 promoter region. The re-expression of RUNX3 gene can inhibit Lovo cell growth and partly induce Lovo cell apoptosis.

RUNX genes in development and cancer: regulation of viral gene expression and the discovery of RUNX family genes

Mouse embryonal carcinoma (EC) cells, also called teratocarcinoma stem cells, are nonpermissive for polyomavirus growth, whereas differentiated derivatives of the cells are permissive. Mutant viruses capable of growing in EC cells can be isolated. They have genomic alterations within the viral enhancer, which is required for viral gene expression and DNA replication. This viral regulatory region was considered as a potential probe for mouse cell differentiation. The 24-bp-long A element within the enhancer was identified as a minimum element, which also shows a lower activity in EC cells compared with the differentiated cells. Transcription factors PEA1/AP1, PEA2/PEBP2, and PEA3/ETS were identified as A element-binding proteins. All of them are absent in EC cells and induced to be expressed when the cells are differentiated. Although PEBP2 has a weaker transactivation activity compared with other two, it is essential for the enhancer function of the A element. Purification and cDNA cloning revealed that PEBP2 has two subunits, DNA-binding alpha (PEBP2alpha) and non-DNA-binding beta (PEBP2beta). PEBP2alpha was found to be highly homologous to a Drosophila segmentation gene, runt, and a human gene AML1 that was identified as a part of the fusion gene, AML1/ETO (MTG8) generated by t(8;21) chromosome translocation associated with acute myelogenous leukemia (AML). Core-binding factor (CBF), which interacts with a murine retrovirus enhancer, was found to be identical to PEBP2. runt, PEBP2alpha and AML1 are now termed RUNX family, which are involved in cell specification during development. There are three mammalian RUNX genes, RUNX1, RUNX2, and RUNX3. RUNX1 is essential for generation of hematopoietic stem cells and is involved in human leukemia. RUNX2 is essential for skeletal development and has an oncogenic potential. RUNX3 is expressed in wider ranges of tissues and has multiple roles. Among others, RUNX3 is a major tumor suppressor of gastric and many other solid tumors.

Mechanisms of compartmentalized expression of Mrg class G-protein-coupled sensory receptors

Mrg class G-protein-coupled receptors (GPCRs) are expressed exclusively in sensory neurons in the trigeminal and dorsal root ganglia. Pharmacological activation of Mrg proteins is capable of modulating sensory neuron activities and elicits nociceptive effects. In this study, we illustrate a control mechanism that allows the Runx1 runt domain transcription factor to generate compartmentalized expression of these sensory GPCRs. Expression of MrgA, MrgB, and MrgC subclasses is confined to an "A/B/C" neuronal compartment that expresses Runx1 transiently (or does not express Runx1), whereas MrgD expression is restricted to a "D" compartment with persistent Runx1 expression. Runx1 is initially required for the expression of all Mrg genes. However, during late development Runx1 becomes a repressor for MrgA/B/C genes. As a result, MrgA/B/C expression persists only in the Runx1- "A/B/C" compartment. In delta446 mice, in which Runx1 lacks the C-terminal repression domain, expression of MrgA/B/C genes is dramatically expanded into the Runx1+ "D" compartment. MrgD expression, however, is resistant to Runx1-mediated repression in the "D" compartment. Therefore, the creation of Runx1+ and Runx1- compartments, in conjunction with different responses of Mrg genes to Runx1-mediated repression, results in the compartmentalized expression of MrgA/B/C versus MrgD genes. Within the MrgA/B/C compartment, MrgB4-expressing neurons innervate exclusively the hairy skin. Here we found that Smad4, a downstream component of bone morphological protein-mediated signaling, is required selectively for the expression of MrgB4. Our study suggests a new line of evidence that specification of sensory subtypes is established progressively during perinatal and postnatal development.

Runx genes are direct targets of Scl/Tal1 in the yolk sac and fetal liver

Transcription factors such as Scl/Tal1, Lmo2, and Runx1 are essential for the development of hematopoietic stem cells (HSCs). However, the precise mechanisms by which these factors interact to form transcriptional networks, as well as the identity of the genes downstream of these regulatory cascades, remain largely unknown. To this end, we generated an Scl(-/-) yolk sac cell line to identify candidate Scl target genes by global expression profiling after reintroduction of a TAT-Scl fusion protein. Bioinformatics analysis resulted in the identification of 9 candidate Scl target transcription factor genes, including Runx1 and Runx3. Chromatin immunoprecipitation confirmed that both Runx genes are direct targets of Scl in the fetal liver and that Runx1 is also occupied by Scl in the yolk sac. Furthermore, binding of an Scl-Lmo2-Gata2 complex was demonstrated to occur on the regions flanking the conserved E-boxes of the Runx1 loci and was shown to transactivate the Runx1 element. Together, our data provide a key component of the transcriptional network of early hematopoiesis by identifying downstream targets of Scl that can explain key aspects of the early Scl(-/-) phenotype.

Runx2 deficiency and defective subnuclear targeting bypass senescence to promote immortalization and tumorigenic potential

The osteogenic Runt-related (Runx2) transcription factor negatively regulates proliferation and ribosomal gene expression in normal diploid osteoblasts, but is up-regulated in metastatic breast and prostate cancer cells. Thus, Runx2 may function as a tumor suppressor or an oncogene depending on the cellular context. Here we show that Runx2-deficient primary osteoblasts fail to undergo senescence as indicated by the absence of beta-gal activity and p16(INK4a) tumor suppressor expression. Primary Runx2-null osteoblasts have a growth advantage and exhibit loss of p21(WAF1/CIP1) and p19(ARF) expression. Reintroduction of WT Runx2, but not a subnuclear targeting-defective mutant, induces both p21(WAF/CIP1) and p19(ARF) mRNA and protein resulting in cell-cycle inhibition. Accumulation of spontaneous phospho-H2A.X foci, loss of telomere integrity and the Mre11/Rad50/Nbs1 DNA repair complex, and a delayed DNA repair response all indicate that Runx2 deficiency leads to genomic instability. We propose that Runx2 functions as a tumor suppressor in primary diploid osteoblasts and that subnuclear targeting contributes to Runx2-mediated tumor suppression.

Shh influences cell number and the distribution of neuronal subtypes in dorsal root ganglia

The molecular mechanisms responsible for specifying the dorsal-ventral pattern of neuronal identities in dorsal root ganglia (DRG) are unclear. Here we demonstrate that Sonic hedgehog (Shh) contributes to patterning early DRG cells. In vitro, Shh increases both proliferation and programmed cell death (PCD). Increasing Shh in vivo enhances PCD in dorsal DRG, while inducing greater proliferation ventrally. In such animals, markers characteristic of ventral sensory neurons are expanded to more dorsal positions. Conversely, reducing Shh function results in decreased proliferation of progenitors in the ventral region and decreased expression of the ventral marker trkC. Later arising trkA(+) afferents make significant pathfinding errors in animals with reduced Shh function, suggesting that accurate navigation of later arising growth cones requires either Shh itself or early arising, Shh-dependent afferents. These results indicate that Shh can regulate both cell number and the distribution of cell types in DRG, thereby playing an important role in the specification, patterning and pathfinding of sensory neurons.

Worming out the biology of Runx

Runx family transcription factors have risen to prominence over the last few years because of the increasing evidence implicating them as key regulators of the choice between cell proliferation and differentiation during development and carcinogenesis. Runx factors have been found to be involved in diverse developmental processes, ranging from hematopoiesis to neurogenesis, and are increasingly being linked with various human cancers. In this review, we examine the case for Runx factors as key regulators of cell proliferation in various developmental situations, a role that predisposes Runx mutations as causative agents in oncogenesis. We discuss the evidence that Runx factors regulate, and are regulated by, core components of the cell cycle machinery, and focus our attention on the solo Runx gene, rnt-1, in Caenorhabditis elegans, an organism that we feel has much to offer the Runx field.

Gene expression changes in children with autism

The objective of this study was to identify gene expression differences in blood differences in children with autism (AU) and autism spectrum disorder (ASD) compared to general population controls. Transcriptional profiles were compared with age- and gender-matched, typically developing children from the general population (GP). The AU group was subdivided based on a history of developmental regression (A-R) or a history of early onset (A-E without regression). Total RNA from blood was processed on human Affymetrix microarrays. Thirty-five children with AU (17 with early onset autism and 18 with autism with regression) and 14 ASD children (who did not meet criteria for AU) were compared to 12 GP children. Unpaired t tests (corrected for multiple comparisons with a false discovery rate of 0.05) detected a number of genes that were regulated more than 1.5-fold for AU versus GP (n=55 genes), for A-E versus GP (n=140 genes), for A-R versus GP (n=20 genes), and for A-R versus A-E (n=494 genes). No genes were significantly regulated for ASD versus GP. There were 11 genes shared between the comparisons of all autism subgroups to GP (AU, A-E, and A-R versus GP) and these genes were all expressed in natural killer cells and many belonged to the KEGG natural killer cytotoxicity pathway (p=0.02). A subset of these genes (n=7) was tested with qRT-PCR and all genes were found to be differentially expressed (p<0.05). We conclude that the gene expression data support emerging evidence for abnormalities in peripheral blood leukocytes in autism that could represent a genetic and/or environmental predisposition to the disorder.

Pbx3 is required for normal locomotion and dorsal horn development

The transcription cofactor Pbx3 is critical for the function of hindbrain circuits controlling respiration in mammals, but the perinatal lethality caused by constitutively null mutations has hampered investigation of other roles it may play in neural development and function. Here we report that the conditional loss of Pbx3 function in most tissues caudal to the hindbrain resulted in progressive deficits of posture, locomotion, and sensation that became apparent during adolescence. In adult mutants, the size of the dorsal horn of the spinal cord and the numbers of calbindin-, PKC-gamma, and calretinin-expressing neurons in laminae I-III were markedly reduced, but the ventral cord and peripheral nervous system appeared normal. In the embryonic dorsal horn, Pbx3 expression was restricted to a subset of glutamatergic neurons, but its absence did not affect the initial balance of excitatory and inhibitory interneuron phenotypes. By embryonic day 15 a subset of Meis(+) glutamatergic neurons assumed abnormally superficial positions and the number of calbindin(+) neurons was increased three-fold in the mutants. Loss of Pbx3 function thus leads to the incorrect specification of some glutamatergic neurons in the dorsal horn and alters the integration of peripheral sensation into the spinal circuitry regulating locomotion.

Lack of RUNX3 regulation in human gastric cancer

It has been proposed that the transcription factor RUNX3 is the product of a gastric tumour suppressor gene. We examined RUNX3 expression in gastric biopsies from 105 patients with different histological presentations. Surprisingly, immunohistochemical staining detected RUNX3 protein expression only in infiltrating leukocytes but not in the gastric epithelium. Using laser capture microdissection and quantitative reverse transcription-polymerase chain reaction, we confirmed that the level of RUNX3 mRNA expression in the gastric epithelium was very low and was influenced neither by H. pylori infection nor by neoplastic transformation. Instead, RUNX3 was highly expressed in the gastric stroma and the level of expression correlated with the magnitude of H. pylori-induced gastric inflammation. The low level of RUNX3 expression in gastric epithelium and the absence of downregulation in gastric cancer do not support the hypothesis that RUNX3 functions as a gastric tumour suppressor gene.

Nociceptors--noxious stimulus detectors

In order to deal effectively with danger, it is imperative to know about it. This is what nociceptors do--these primary sensory neurons are specialized to detect intense stimuli and represent, therefore, the first line of defense against any potentially threatening or damaging environmental inputs. By sensing noxious stimuli and contributing to the necessary reactions to avoid them--rapid withdrawal and the experience of an intensely unpleasant or painful sensation, nociceptors are essential for the maintenance of the body's integrity. Although nociceptive pain is clearly an adaptive alarm system, persistent pain is maladaptive, essentially an ongoing false alarm. Here, we highlight the genesis of nociceptors during development and the intrinsic properties of nociceptors that enable them to transduce, conduct, and transmit nociceptive information and also discuss how their phenotypic plasticity contributes to clinical pain.

Nociceptive sensory neurons derive from contralaterally migrating, fate-restricted neural crest cells

Neural crest cells (NCCs) are a transient population of multipotent progenitors that give rise to numerous cell types in the embryo. An unresolved issue is the degree to which the fate of NCCs is specified prior to their emigration from the neural tube. In chick embryos, we identified a subpopulation of NCCs that, upon delamination, crossed the dorsal midline to colonize spatially discrete regions of the contralateral dorsal root ganglia (DRG), where they later gave rise to nearly half of the nociceptor sensory neuron population. Our data indicate that before emigration, this NCC subset is phenotypically distinct, with an intrinsic lineage potential that differs from its temporally synchronized, but ipsilaterally migrating, cohort. These findings not only identify a major source of progenitor cells for the pain- and temperature-sensing afferents, but also reveal a previously unknown migratory pathway for sensory-fated NCCs that requires the capacity to cross the embryonic midline.

cDNA cloning of Runx family genes from the pufferfish (Fugu rubripes)

The Runx family genes are involved in hematopoiesis, osteogenesis and neuropoiesis, and mutations in these genes have been frequently associated with human hereditary diseases and cancers. Here we report the cDNA cloning of the full Runx gene family of the pufferfish (Fugu rubripes), which comprises frRunx1, frRunx2, frRunx3, frRunt and frCbfb. Fugu is evolutionarily distant from mammals, thus the annotation of the frRunx family genes greatly facilitates comparative genomics approaches. Protein sequence comparison revealed that the fugu genes show high conservation in the Runt domain and PY and VWRPY motifs. frRunx1 had an extra stretch of eight histidine residues, while frRunx2 lacked the poly-glutamine/-alanine stretch that is a hallmark of the mammalian Runx2 genes. Analysis of the promoter regions revealed high conservation of the binding sites for transcription factors, including Runx sites in the P1 promoters. Abundant CpG dinucleotides in the P2 promoter regions were also detected. The expression patterns of the frRunx family genes in various tissues showed high similarity to those of the mammalian Runx genes. The genomic structures of the fugu and mammalian Runx genes are largely conserved except for a split exon 2 in frRunx1 and an extra exon in the C-terminal region of frRunx3 that is missing in mammalian Runx3 genes. The similarities and differences between the Runx family genes of fugu and mammals will improve our understanding of the functions of these proteins.

Specification of sensory neuron cell fate from the neural crest

How distinct cell fates are generated from initially homogeneous cell populations is a driving question in developmental biology. The neural crest is one such cell population that is capable of producing an incredible array of derivatives. Cells as different in function and form as the pigment cells in the skin or the neurons and glia of the peripheral nervous system are all derived from neural crest. How do these cells choose to migrate along distinct routes, populate defined regions of the embryo and differentiate into specific cell types? This chapter focuses on the development of one particular neural crest derivative, sensory neurons, as a model for studying these questions of cell fate specification. In the head, sensory neurons reside in the trigeminal and epibranchial ganglia, while in the trunk they form the spinal or dorsal root ganglia (DRG). The development of the DRG will be the main focus of this review. The neurons and glia of the DRG derive from trunk neural crest cells that coalesce at the lateral edge of the spinal cord (Fig. 1). These neural crest cells migrate along the same routes as neural crest cells that populate the autonomic sympathetic ganglia located along the dorsal aorta. Somehow DRG precursors must make the decision to stop and adopt a sensory fate adjacent to the spinal cord rather than continuing on to become part of the autonomic ganglia. Moreover, once the DRG precursors aggregate in their final positions there are still a number of fate choices to be made. The mature DRG is composed of many neurons with different morphologies and distinct biochemical properties as well as glial cells that support these neurons.

Runx3/AML2/Cbfa3 regulates early and late chondrocyte differentiation

We studied the expression and function of Runx3 during chondrogenesis and chondrocyte maturation. We found that Runx3 is essential for mediating the early stage of endochondral ossification through cooperation with other Runx family members.

INTRODUCTION

Runx proteins are spatially and temporally co-expressed during skeletal formation. A cooperative and/or redundant function between these factors was postulated, yet the mechanisms underlying these cooperative effects are unknown.

MATERIALS AND METHODS

Expression patterns of Runx3 transcripts were assessed during mouse embryonic developments and limb bud-derived mesenchymal cell differentiation into mature chondrocytes by real-time RT-PCR. Runx3 protein distribution was also determined by immunohistochemistry in mouse embryos. Runx3 gain and loss of function was performed through overexpression and siRNA knockdown of Runx3 into the limb bud-derived cell line MLB13MYC clone17, respectively. Co-transfection experiments were performed in clone 17 cells using the Runx1 promoter and Runx3 cDNA. Promoter activity was measured by luciferase reporter assay.

RESULTS

Both Runx3 isoforms are significantly upregulated at the onset of cartilage mineralization and bone formation in E15.5 mice. This upregulation follows that of Sox9 and is concomitant with that of alkaline phosphatase. Furthermore, Runx3 expression remains high during later stages of embryonic development when the levels of osteocalcin are maximal. We determined the expression patterns of Runx3 during chondrogenesis and chondrocyte maturation using mouse limb bud-derived micromass cultures between days 3 and 21. Whereas Runx3 mRNAs are progressively upregulated between days 3 and 14, it is dramatically downregulated at day 21. Markers of chondrocyte maturation alkaline phosphatase and type X collagen are upregulated and maintained throughout the 21 days of culture. Runx3 role in mediating chondrocyte terminal differentiation through gain and loss of function in MLB13MYC clone17 shows that Runx3 regulates both early and late markers of chondrocyte maturation. Finally, Runx3 transcriptionally inhibits Runx1 expression in chondrocytes.

CONCLUSIONS

We show a role for Runx3 in mediating stage-specific chondrocyte maturation. Our study clearly suggests that, whereas Runx3 may cooperate with Runx2 to induce chondrocyte terminal differentiation, it inhibits Runx1 expression during late maturation.

The C. elegans CBFbeta homolog, BRO-1, regulates the proliferation, differentiation and specification of the stem cell-like seam cell lineages

The RUNX/CBFbeta heterodimeric transcription factor plays an important role in regulating cell proliferation and differentiation in a variety of developmental contexts. Aberrant function of Runx and CBFbeta has been causally related to the development of various diseases, including acute myeloid leukemia, gastric cancer and cleidocranial dysplasia. The underlying mechanism of the RUNX/CBFbeta complex in regulation of cell proliferation is still poorly defined. In this study, we demonstrate that the Caenorhabditis elegans CBFbeta homolog, bro-1, is essential for the proliferation, differentiation and specification of a row of stem cell-like lineages, called seam cells. BRO-1 forms complex with the C. elegans RUNX homolog, RNT-1, and augments the DNA-binding activity of RNT-1. The RNT-1/BRO-1 complex directly interacts with the C. elegans Groucho homolog, UNC-37, whose loss of function mutations display similar defects in the proliferation of seam cells as those of bro-1 and rnt-1 mutants. Additionally, the defects in seam cell division in bro-1 mutants are substantially rescued by the inactivation of the negative regulators of the G1 to S phase cell cycle progression, including the lin-35 Rb, fzr-1 Cdh1 and cki-1 CIP homologs. Our studies indicate that the transcriptional repression activity of the RNT-1/BRO-1 complex regulates the G1 to S cell cycle progression during seam cell division.

Repression of interleukin-4 in T helper type 1 cells by Runx/Cbf beta binding to the Il4 silencer

Interferon γ (IFNγ) is the hallmark cytokine produced by T helper type 1 (Th1) cells, whereas interleukin (IL)-4 is the hallmark cytokine produced by Th2 cells. Although previous studies have revealed the roles of cytokine signaling and of transcription factors during differentiation of Th1 or Th2 cells, it is unclear how the exclusive expression pattern of each hallmark cytokine is established. The DNaseI hypersensitivity site IV within the mouse Il4 locus plays an important role in the repression of Il4 expression in Th1 cells, and it has been named the Il4 silencer. Using Cbfβ- or Runx3-deficient T cells, we show that loss of Runx complex function results in derepression of IL-4 in Th1 cells. Binding of Runx complexes to the Il4 silencer was detected in naive CD4⁺ T cells and Th1 cells, but not in Th2 cells. Furthermore, enforced expression of GATA-3 in Th1 cells inhibited binding of Runx complexes to the Il4 silencer. Interestingly, T cell–specific inactivation of the Cbfβ gene in mice led to elevated serum immunoglobulin E and airway infiltration. These results demonstrate critical roles of Runx complexes in regulating immune responses, at least in part, through the repression of the Il4 gene.