Functional and cooperative interactions between the homeodomain PDX1, Pbx, and Prep1 factors on the somatostatin promoter

Directed differentiation of pancreatic δ cells from human pluripotent stem cells

Dysfunction of pancreatic δ cells contributes to the etiology of diabetes. Despite their important role, human δ cells are scarce, limiting physiological studies and drug discovery targeting δ cells. To date, no directed δ-cell differentiation method has been established. Here, we demonstrate that fibroblast growth factor (FGF) 7 promotes pancreatic endoderm/progenitor differentiation, whereas FGF2 biases cells towards the pancreatic δ-cell lineage via FGF receptor 1. We develop a differentiation method to generate δ cells from human stem cells by combining FGF2 with FGF7, which synergistically directs pancreatic lineage differentiation and modulates the expression of transcription factors and SST activators during endoderm/endocrine precursor induction. These δ cells display mature RNA profiles and fine secretory granules, secrete somatostatin in response to various stimuli, and suppress insulin secretion from in vitro co-cultured β cells and mouse β cells upon transplantation. The generation of human pancreatic δ cells from stem cells in vitro would provide an unprecedented cell source for drug discovery and cell transplantation studies in diabetes.

A poly-histidine motif of HOXA1 is involved in regulatory interactions with cysteine-rich proteins

Homopolymeric amino acid repeats are found in about 24 % of human proteins and are over-represented in transcriptions factors and kinases. Although relatively rare, homopolymeric histidine repeats (polyH) are more significantly found in proteins involved in the regulation of embryonic development. To gain a better understanding of the role of polyH in these proteins, we used a bioinformatic approach to search for shared features in the interactomes of polyH-containing proteins in human. Our analysis revealed that polyH protein interactomes are enriched in cysteine-rich proteins and in proteins containing (a) cysteine repeat(s). Focusing on HOXA1, a HOX transcription factor displaying one long polyH motif, we identified that the polyH motif is required for the HOXA1 interaction with such cysteine-rich proteins. We observed a correlation between the length of the polyH repeat and the strength of the HOXA1 interaction with one Cys-rich protein, MDFI. We also found that metal ion chelators disrupt the HOXA1-MDFI interaction supporting that such metal ions are required for the interaction. Furthermore, we identified three polyH interactors which down-regulate the transcriptional activity of HOXA1. Taken together, our data point towards the involvement of polyH and cysteines in regulatory interactions between proteins, notably transcription factors like HOXA1.

Delineating mouse β-cell identity during lifetime and in diabetes with a single cell atlas

Although multiple pancreatic islet single-cell RNA-sequencing (scRNA-seq) datasets have been generated, a consensus on pancreatic cell states in development, homeostasis and diabetes as well as the value of preclinical animal models is missing. Here, we present an scRNA-seq cross-condition mouse islet atlas (MIA), a curated resource for interactive exploration and computational querying. We integrate over 300,000 cells from nine scRNA-seq datasets consisting of 56 samples, varying in age, sex and diabetes models, including an autoimmune type 1 diabetes model (NOD), a glucotoxicity/lipotoxicity type 2 diabetes model (db/db) and a chemical streptozotocin β-cell ablation model. The β-cell landscape of MIA reveals new cell states during disease progression and cross-publication differences between previously suggested marker genes. We show that β-cells in the streptozotocin model transcriptionally correlate with those in human type 2 diabetes and mouse db/db models, but are less similar to human type 1 diabetes and mouse NOD β-cells. We also report pathways that are shared between β-cells in immature, aged and diabetes models. MIA enables a comprehensive analysis of β-cell responses to different stressors, providing a roadmap for the understanding of β-cell plasticity, compensation and demise.

HOXA1 Is an Antagonist of ERα in Breast Cancer

Breast cancer is a heterogeneous disease and the leading cause of female cancer mortality worldwide. About 70% of breast cancers express ERα. HOX proteins are master regulators of embryo development which have emerged as being important players in oncogenesis. HOXA1 is one of them. Here, we present bioinformatic analyses of genome-wide mRNA expression profiles available in large public datasets of human breast cancer samples. We reveal an extremely strong opposite correlation between HOXA1 versus ER expression and that of 2,486 genes, thereby supporting a functional antagonism between HOXA1 and ERα. We also demonstrate in vitro that HOXA1 can inhibit ERα activity. This inhibition is at least bimodal, requiring an intact HOXA1 DNA-binding homeodomain and involving the DNA-binding independent capacity of HOXA1 to activate NF-κB. We provide evidence that the HOXA1-PBX interaction known to be critical for the transcriptional activity of HOXA1 is not involved in the ERα inhibition. Finally, we reveal that HOXA1 and ERα can physically interact but that this interaction is not essential for the HOXA1-mediated inhibition of ERα. Like other HOX oncoproteins interacting with ERα, HOXA1 could be involved in endocrine therapy resistance.

Regulatory Mechanisms of Somatostatin Expression

Somatostatin is a peptide hormone, which most commonly is produced by endocrine cells and the central nervous system. In mammals, somatostatin originates from pre-prosomatostatin and is processed to a shorter form, i.e., somatostatin-14, and a longer form, i.e., somatostatin-28. The two peptides repress growth hormone secretion and are involved in the regulation of glucagon and insulin synthesis in the pancreas. In recent years, the processing and secretion of somatostatin have been studied intensively. However, little attention has been paid to the regulatory mechanisms that control its expression. This review provides an up-to-date overview of these mechanisms. In particular, it focuses on the role of enhancers and silencers within the promoter region as well as on the binding of modulatory transcription factors to these elements. Moreover, it addresses extracellular factors, which trigger key signaling pathways, leading to an enhanced somatostatin expression in health and disease.

HOXA2 activity regulation by cytoplasmic relocation, protein stabilization and post-translational modification

HOX proteins are homeodomain transcription factors critically involved in patterning animal embryos and controlling organogenesis. While the functions of HOX proteins and the processes under their control begin to be well documented, the modalities of HOX protein activity regulation remain poorly understood. Here we show that HOXA2 interacts with PPP1CB, a catalytic subunit of the Ser/Thr PP1 phosphatase complex. This interaction co-localizes in the cytoplasm with a previously described HOXA2 interactor, KPC2, which belongs to the KPC E3 ubiquitin ligase complex. We provide evidence that HOXA2, PPP1CB and KPC2 define a molecularly and functionally interacting complex. Collectively, our experiments support that PPP1CB and KPC2 together inhibit the activity of HOXA2 by activating its nuclear export, but favored HOXA2 de-ubiquitination and stabilization thereby establishing a store of HOXA2 in the cytoplasm.

The O-GlcNAc transferase OGT interacts with and post-translationally modifies the transcription factor HOXA1

HOXA1 belongs to the HOX family of transcription factors which are key regulators of animal development. Little is known about the molecular pathways controlling HOXA1. Recent data from our group revealed distinct partner proteins interacting with HOXA1. Among them, OGT is an O-linked N-acetylglucosamine (O-GlcNAc) transferase modifying a variety of proteins involved in different cellular processes including transcription. Here, we confirm OGT as a HOXA1 interactor, we characterise which domains of HOXA1 and OGT are required for the interaction, and we provide evidence that OGT post-translationally modifies HOXA1. Mass spectrometry experiments indeed reveal that HOXA1 can be phosphorylated on the AGGTVGSPQYIHHSY peptide and that upon OGT expression, the phosphate adduct is replaced by an O-GlcNAc group.

PRDM14, a putative histone methyl-transferase, interacts with and decreases the stability and activity of the HOXA1 transcription factor

Understanding how the activity of transcription factors like HOX proteins is regulated remains a widely open question. In a recent screen for proteins interacting with HOXA1, we identified a PRDM protein family member, PRDM14, which is known to be transiently co-expressed with HOXA1 in epiblast cells before their specification towards somatic versus germ cell fate. Here, we confirm PRDM14 is an interactor of HOXA1 and we identify the homeodomain of HOXA1 as well as the PR domain and Zinc fingers of PRDM14 to be required for the interaction. An 11-His repeat of HOXA1 previously highlighted to contribute to HOXA1-mediated protein-protein interactions is also involved. At a functional level, we provide evidence that HOXA1 displays an unexpectedly long half-life and demonstrate that PRDM14 can reduce the stability and affect the transcriptional activity of HOXA1.

Prep1, A Homeodomain Transcription Factor Involved in Glucose and Lipid Metabolism

The three-amino acid loop extension (TALE) homeodomain proteins are a family of transcription factor including the mammalian Pbx, MEIS and Prep proteins. TALE proteins can bind other transcription factors such as Pdx-1 and play an important role in the regulation of glucose metabolism. Experiments performed in mutant mice have shown that while the single Pbx1 or Pdx-1 knockout mice feature pancreatic islet malformations, impaired glucose tolerance and hypoinsulinemia, the trans-heterozygous Pbx1^+/-Pdx1^+/- mice develop age-dependent overt diabetes mellitus. In contrast, Prep1 plays a different role with respect to these proteins. Indeed, Prep1 hypomorphic mice, expressing low levels of protein, feature pancreatic islet hypoplasia accompanied by hypoinsulinemia similar to Pbx1 or Pdx1. Nevertheless, these animals show increased insulin sensitivity in skeletal muscle, liver and adipose tissue accompanied by protection from streptozotocin-induced diabetes. In addition, Prep1 hypomorphic mice feature reduced triglyceride synthesis and do not develop steatohepatitis after a methionine and coline deficient diet. In this review we have underlined how important metabolic functions are controlled by TALE proteins, in particular by Prep1, leading to hypothesis that its suppression might represent beneficial effect in the care of metabolic diseases.

HOXA1 binds RBCK1/HOIL-1 and TRAF2 and modulates the TNF/NF-κB pathway in a transcription-independent manner

HOX proteins define a family of key transcription factors regulating animal embryogenesis. HOX genes have also been linked to oncogenesis and HOXA1 has been described to be active in several cancers, including breast cancer. Through a proteome-wide interaction screening, we previously identified the TNFR-associated proteins RBCK1/HOIL-1 and TRAF2 as HOXA1 interactors suggesting that HOXA1 is functionally linked to the TNF/NF-κB signaling pathway. Here, we reveal a strong positive correlation between expression of HOXA1 and of members of the TNF/NF-κB pathway in breast tumor datasets. Functionally, we demonstrate that HOXA1 can activate NF-κB and operates upstream of the NF-κB inhibitor IκB. Consistently, we next demonstrate that the HOXA1-mediated activation of NF-κB is non-transcriptional and that RBCK1 and TRAF2 influences on NF-κB are epistatic to HOXA1. We also identify an 11 Histidine repeat and the homeodomain of HOXA1 to be required both for RBCK1 and TRAF2 interaction and NF-κB stimulation. Finally, we highlight that activation of NF-κB is crucial for HOXA1 oncogenic activity.

The Role of ARX in Human Pancreatic Endocrine Specification

The in vitro differentiation of human embryonic stem cells (hESCs) offers a model system to explore human development. Humans with mutations in the transcription factor Aristaless Related Homeobox (ARX) often suffer from the syndrome X-linked lissencephaly with ambiguous genitalia (XLAG), affecting many cell types including those of the pancreas. Indeed, XLAG pancreatic islets lack glucagon and pancreatic polypeptide-positive cells but retain somatostatin, insulin, and ghrelin-positive cells. To further examine the role of ARX in human pancreatic endocrine development, we utilized genomic editing in hESCs to generate deletions in ARX. ARX knockout hESCs retained pancreatic differentiation capacity and ARX knockout endocrine cells were biased toward somatostatin-positive cells (94% of endocrine cells) with reduced pancreatic polypeptide (rarely detected), glucagon (90% reduced) and insulin-positive (65% reduced) lineages. ARX knockout somatostatin-positive cells shared expression patterns with human fetal and adult δ-cells. Differentiated ARX knockout cells upregulated PAX4, NKX2.2, ISL1, HHEX, PCSK1, PCSK2 expression while downregulating PAX6 and IRX2. Re-expression of ARX in ARX knockout pancreatic progenitors reduced HHEX and increased PAX6 and insulin expression following differentiation. Taken together these data suggest that ARX plays a key role in pancreatic endocrine fate specification of pancreatic polypeptide, somatostatin, glucagon and insulin positive cells from hESCs.

Molecular Analysis of the HOXA2-Dependent Degradation of RCHY1

The homeodomain transcription factor Hoxa2 interacts with the RING-finger type E3 ubiquitin ligase RCHY1 and induces its proteasomal degradation. In this work, we dissected this non-transcriptional activity of Hoxa2 at the molecular level. The Hoxa2-mediated decay of RCHY1 involves both the 19S and 20S proteasome complexes. It relies on both the Hoxa2 homeodomain and C-terminal moiety although no single deletion in the Hoxa2 sequence could disrupt the RCHY1 interaction. That the Hoxa2 homeodomain alone could mediate RCHY1 binding is consistent with the shared ability all the Hox proteins we tested to interact with RCHY1. Nonetheless, the ability to induce RCHY1 degradation although critically relying on the homeodomain is not common to all Hox proteins. This identifies the homeodomain as necessary but not sufficient for what appears to be an almost generic Hox protein activity. Finally we provide evidence that the Hoxa2-induced degradation of RCHY1 is evolutionarily conserved among vertebrates. These data therefore support the hypothesis that the molecular and functional interaction between Hox proteins and RCHY1 is an ancestral Hox property.

KPC2 relocalizes HOXA2 to the cytoplasm and decreases its transcriptional activity

Regulation of transcription factor activity relies on molecular interactions or enzymatic modifications which influence their interaction with DNA cis-regulatory sequences, their transcriptional activation or repression, and stability or intracellular distribution of these proteins. Regarding the well-conserved Hox protein family, a restricted number of activity regulators have been highlighted thus far. In the framework of a proteome-wide screening aiming at identifying proteins interacting with Hoxa2, KPC2, an adapter protein constitutive of the KPC ubiquitin-ligase complex, was identified. In this work, KPC2 was confirmed as being a genuine interactor of Hoxa2 by co-precipitation and bimolecular fluorescence complementation assays. At functional level, KPC2 diminishes the transcriptional activity and induces the nuclear exit of Hoxa2. Gene expression analyses revealed that Kpc2 is active in restricted areas of the developing mouse embryo which overlap with the Hoxa2 expression domain. Together, our data support that KPC2 regulates Hoxa2 by promoting its relocation to the cytoplasm.

Binding polymorphism in the DNA bound state of the Pdx1 homeodomain

The subtle effects of DNA-protein recognition are illustrated in the homeodomain fold. This is one of several small DNA binding motifs that, in spite of limited DNA binding specificity, adopts crucial, specific roles when incorporated in a transcription factor. The homeodomain is composed of a 3-helix domain and a mobile N-terminal arm. Helix 3 (the recognition helix) interacts with the DNA bases through the major groove, while the N-terminal arm becomes ordered upon binding a specific sequence through the minor groove. Although many structural studies have characterized the DNA binding properties of homeodomains, the factors behind the binding specificity are still difficult to elucidate. A crystal structure of the Pdx1 homeodomain bound to DNA (PDB 2H1K) obtained previously in our lab shows two complexes with differences in the conformation of the N-terminal arm, major groove contacts, and backbone contacts, raising new questions about the DNA recognition process by homeodomains. Here, we carry out fully atomistic Molecular Dynamics simulations both in crystal and aqueous environments in order to elucidate the nature of the difference in binding contacts. The crystal simulations reproduce the X-ray experimental structures well. In the absence of crystal packing constraints, the differences between the two complexes increase during the solution simulations. Thus, the conformational differences are not an artifact of crystal packing. In solution, the homeodomain with a disordered N-terminal arm repositions to a partially specific orientation. Both the crystal and aqueous simulations support the existence of different stable binding conformers identified in the original crystallographic data with different degrees of specificity. We propose that protein-protein and protein-DNA interactions favor a subset of the possible conformations. This flexibility in DNA binding may facilitate multiple functions for the same transcription factor.

Genome-wide reprogramming of the chromatin landscape underlies endocrine therapy resistance in breast cancer

The estrogen receptor (ER)α drives growth in two-thirds of all breast cancers. Several targeted therapies, collectively termed endocrine therapy, impinge on estrogen-induced ERα activation to block tumor growth. However, half of ERα-positive breast cancers are tolerant or acquire resistance to endocrine therapy. We demonstrate that genome-wide reprogramming of the chromatin landscape, defined by epigenomic maps for regulatory elements or transcriptional activation and chromatin openness, underlies resistance to endocrine therapy. This annotation reveals endocrine therapy-response specific regulatory networks where NOTCH pathway is overactivated in resistant breast cancer cells, whereas classical ERα signaling is epigenetically disengaged. Blocking NOTCH signaling abrogates growth of resistant breast cancer cells. Its activation state in primary breast tumors is a prognostic factor of resistance in endocrine treated patients. Overall, our work demonstrates that chromatin landscape reprogramming underlies changes in regulatory networks driving endocrine therapy resistance in breast cancer.

PDX1 regulation of FABP1 and novel target genes in human intestinal epithelial Caco-2 cells

The transcription factor pancreatic and duodenal homeobox 1 (PDX1) plays an essential role in pancreatic development and in maintaining proper islet function via target gene regulation. Few intestinal PDX1 targets, however, have been described. We sought to define novel PDX1-regulated intestinal genes. Caco-2 human intestinal epithelial cells were engineered to overexpress PDX1 and gene expression profiles relative to control cells were assessed. Expression of 80 genes significantly increased while that of 49 genes significantly decreased more than 4-fold following PDX1 overexpression in differentiated Caco-2 cells. Analysis of the differentially regulated genes with known functional annotations revealed genes encoding transcription factors, growth factors, kinases, digestive glycosidases, nutrient transporters, nutrient binding proteins, and structural components. The gene for fatty acid binding protein 1, liver, FABP1, is repressed by PDX1 in Caco-2 cells. PDX1 overexpression in Caco-2 cells also results in repression of promoter activity driven by the 0.6kb FABP1 promoter. PDX1 regulation of promoter activity is consistent with the decrease in FABP1 RNA abundance resulting from PDX1 overexpression and identifies FABP1 as a candidate PDX1 target. PDX1 repression of FABP1, LCT, and SI suggests a role for PDX1 in patterning anterior intestinal development.

Research resource: the pdx1 cistrome of pancreatic islets

The homeodomain transcription factor pancreas duodenal homeobox 1 (Pdx1, also known as insulin promoter factor 1) is a master regulator of pancreas development, as mice or humans lacking Pdx1 function are a pancreatic. Importantly, heterozygous mutations in Pdx1 cause early and late onset forms of diabetes in humans. Despite these central roles in development and adult β-cell function, we have only rudimentary knowledge of the transcriptome targets of Pdx1 that mediate these phenotypes. Therefore, we performed global location analysis of Pdx1 occupancy in pancreatic islets. We used evolutionary conservation of target genes to identify the most relevant Pdx1 targets by performing chromatin immunoprecipitation sequencing on both human and mouse islets. Remarkably, the conserved target set is highly enriched for genes annotated to function in endocrine system and metabolic disorders, various signaling pathways, and cell survival, providing a molecular explanation for many of the phenotypes resulting from Pdx1 deficiency.

The Pbx interaction motif of Hoxa1 is essential for its oncogenic activity

Hoxa1 belongs to the Hox family of homeodomain transcription factors involved in patterning embryonic territories and governing organogenetic processes. In addition to its developmental functions, Hoxa1 has been shown to be an oncogene and to be overexpressed in the mammary gland in response to a deregulation of the autocrine growth hormone. It has therefore been suggested that Hoxa1 plays a pivotal role in the process linking autocrine growth hormone misregulation and mammary carcinogenesis. Like most Hox proteins, Hoxa1 can interact with Pbx proteins. This interaction relies on a Hox hexapeptidic sequence centred on conserved Tryptophan and Methionine residues. To address the importance of the Hox-Pbx interaction for the oncogenic activity of Hoxa1, we characterized here the properties of a Hoxa1 variant with substituted residues in the hexapeptide and demonstrate that the Hoxa1 mutant lost its ability to stimulate cell proliferation, anchorage-independent cell growth, and loss of contact inhibition. Therefore, the hexapeptide motif of Hoxa1 is required to confer its oncogenic activity, supporting the view that this activity relies on the ability of Hoxa1 to interact with Pbx.

C-terminal domain of MEIS1 converts PKNOX1 (PREP1) into a HOXA9-collaborating oncoprotein

The three-amino-acid loop extension (TALE) class homeodomain proteins MEIS1 and PKNOX1 (PREP1) share the ability to interact with PBX and HOX family members and bind similar DNA sequences but appear to play opposing roles in tumor development. Elevated levels of MEIS1 accelerate development of HOX- and MLL-induced leukemias, and this pro-tumorigenic property has been associated with transcriptional activity of MEIS1. In contrast, reduction of PKNOX1 levels has been linked with cancer development despite the absence of an identifiable transactivating domain. In this report, we show that a chimeric protein generated by fusion of the MEIS1 C-terminal region encompassing the transactivating domain with the full-length PKNOX1 (PKNOX1-MC) acquired the ability to accelerate the onset of Hoxa9-induced leukemia in the mouse bone marrow transduction/transplantation model. Gene expression profiling of primary bone marrow cells transduced with Hoxa9 plus Meis1, or Hoxa9 plus Pknox1-MC revealed perturbations in overlapping functional gene subsets implicated in DNA packaging, chromosome organization, and in cell cycle regulation. Together, results presented in this report suggest that the C-terminal domain of MEIS1 confers to PKNOX1 an ectopic transactivating function that promotes leukemogenesis by regulating expression of genes involved in chromatin accessibility and cell cycle progression.

Regulation of CCL2 expression by an upstream TALE homeodomain protein-binding site that synergizes with the site created by the A-2578G SNP

CC Chemokine Ligand 2 (CCL2) is a potent chemoattractant produced by macrophages and activated astrocytes during periods of inflammation within the central nervous system. Increased CCL2 expression is correlated with disease progression and severity, as observed in pulmonary tuberculosis, HCV-related liver disease, and HIV-associated dementia. The CCL2 distal promoter contains an A/G polymorphism at position -2578 and the homozygous -2578 G/G genotype is associated with increased CCL2 production and inflammation. However, the mechanisms that contribute to the phenotypic differences in CCL2 expression are poorly understood. We previously demonstrated that the -2578 G polymorphism creates a TALE homeodomain protein binding site (TALE binding site) for PREP1/PBX2 transcription factors. In this study, we identified the presence of an additional TALE binding site 22 bp upstream of the site created by the -2578 G polymorphism and demonstrated the synergistic effects of the two sites on the activation of the CCL2 promoter. Using chromatin immunoprecipitation (ChIP) assays, we demonstrated increased binding of the TALE proteins PREP1 and PBX2 to the -2578 G allele, and binding of IRF1 to both the A and G alleles. The presence of TALE binding sites that form inverted repeats within the -2578 G allele results in increased transcriptional activation of the CCL2 distal promoter while the presence of only the upstream TALE binding site within the -2578 A allele exerts repression of promoter activity.

Endoplasmic reticulum oxidoreductin-1-like β (ERO1lβ) regulates susceptibility to endoplasmic reticulum stress and is induced by insulin flux in β-cells

Hyperglycemia increases insulin flux through the endoplasmic reticulum (ER) of pancreatic β-cells, and the unfolded protein response pathway is required to enhance insulin processing. Pancreatic and duodenal homeobox 1 (PDX1), a key pancreatic transcription factor, regulates insulin along with targets involved in insulin processing and secretion. Here we find that PDX1 is a direct transcriptional regulator of ER oxidoreductin-1-like β (Ero1lβ), which maintains the oxidative environment of the ER to facilitate disulfide bond formation. PDX1 deficiency reduced Ero1lβ transcript levels in mouse islets and mouse insulinoma (MIN6) cells; moreover, PDX1 occupied the Ero1lβ promoter in β-cells. ERO1lβ levels were induced by high glucose concentrations and by the reducing agent dithiothreitol, indicating potential roles in adaptation to increased oxidative protein folding load in the β-cell ER. In MIN6 cells, small interfering RNA-mediated silencing of Ero1lβ decreased insulin content and increased susceptibility to ER stress-induced apoptosis. These findings demonstrate roles for the PDX1 target ERO1lβ in maintaining insulin content and regulating cell survival during ER stress.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

A novel Gli3 enhancer controls the Gli3 spatiotemporal expression pattern through a TALE homeodomain protein binding site

The zinc finger transcription factor Gli3 is an essential mediator of hedgehog signaling. Gli3 has a dynamic expression pattern during embryonic development. In the neural tube, Gli3 transcripts are patterned along the anteroposterior and dorsoventral axes such that the initial broad expression in the posterior neural tube becomes dorsally restricted as neurogenesis takes place. Little is known about the molecular mechanisms that regulate this dynamic expression. Here, we report on a phylogenetic analysis of the Gli3 locus that uncovered a novel regulatory element, HCNE1. HCNE1 contains a compound Pbx/Meis binding site that binds Pbx and Meis/Prep proteins in vitro and in vivo. We show that HCNE1 recapitulates Gli3 expression in the developing neural tube and that mutations in the Pbx/Meis binding site affect the spatiotemporal control of HCNE1 transcriptional activity. Ectopic expression or loss of function of Pbx and Meis/Prep proteins in the chick and mouse embryo results in aberrant expression of endogenous Gli3 transcripts. We propose a novel role for TALE proteins in establishing the correct spatiotemporal expression pattern of Gli3 in the vertebrate spinal cord, thus implicating TALE transcription factors in early embryonic patterning events controlled by Sonic hedgehog signaling.

Pentapeptide insertion mutagenesis of the Hoxa1 protein: mapping of transcription activation and DNA-binding regulatory domains

The mode of action of Hoxa1, like that of most Hox proteins, remains poorly characterized. In an effort to identify functional determinants contributing to the activity of Hoxa1 as a transcription factor, we generated 18 pentapeptide insertion mutants of the Hoxa1 protein and we assayed them in transfected cells for their activity on target enhancers from the EphA2 and Hoxb1 genes known to respond to Hoxa1 in the developing hindbrain. Only four mutants displayed a complete loss-of-function. Three of them contained an insertion in the homeodomain of Hoxa1, whereas the fourth loss-of-function mutant harbored an insertion in the very N-terminal end of the protein. Transcription activation assays in yeast further revealed that the integrity of both the N-terminal end and homeodomain is required for Hoxa1-mediated transcriptional activation. Furthermore, an insertion in the serine-threonine-proline rich C-terminal extremity of Hoxa1 induced an increase in activity in mammalian cells as well as in the yeast assay. The C-terminal extremity thus modulates the transcriptional activation capacity of the protein. Finally, electrophoretic mobility shift assays revealed that the N-terminal extremity of the protein also exerts a modulatory influence on DNA binding by Hoxa1-Pbx1a heterodimers.

CK2 phosphorylation of Pdx-1 regulates its transcription factor activity

The duodenal homeobox-1 protein Pdx-1 is one of the regulators for the transcription of the insulin gene. Pdx-1 is a phosphoprotein, and there is increasing evidence for the regulation of some of its functions by phosphorylation. Here, we asked whether protein kinase CK2 might phosphorylate Pdx-1 and how this phosphorylation could be implicated in the functional regulation of Pdx-1. We used fragments of Pdx-1 as well as phosphorylation mutants for experiments with protein kinase CK2. Transactivation was measured by reporter assays using the insulin promoter. Our data showed that Pdx-1 is phosphorylated by protein kinase CK2 at amino acids thr(231) and ser(232), and this phosphorylation was implicated in the regulation of the transcription factor activity of Pdx-1. Furthermore, inhibition of protein kinase CK2 by specific inhibitors led to an elevated release of insulin from pancreatic beta-cells. Thus, these findings identify CK2 as a novel mediator of the insulin metabolism.

Pdx1 inactivation restricted to the intestinal epithelium in mice alters duodenal gene expression in enterocytes and enteroendocrine cells

Null mutant mice lacking the transcription factor pancreatic and duodenal homeobox 1 (Pdx1) are apancreatic and survive only a few days after birth. The role of Pdx1 in regulating intestinal gene expression has therefore yet to be determined in viable mice with normal pancreatic development. We hypothesized that conditional inactivation of Pdx1 restricted to the intestinal epithelium would alter intestinal gene expression and cell differentiation. Pdx1(flox/flox);VilCre mice with intestine-specific Pdx1 inactivation were generated by crossing a transgenic mouse strain expressing Cre recombinase, driven by a mouse villin 1 gene promoter fragment, with a mutant mouse strain homozygous for loxP site-flanked Pdx1. Pdx1 protein is undetectable in all epithelial cells in the intestinal epithelium of Pdx1(flox/flox);VilCre mice. Goblet cell number and mRNA abundance for mucin 3 and mucin 13 genes in the proximal small intestine are comparable between Pdx1(flox/flox);VilCre and control mice. Similarly, Paneth cell number and expression of Paneth cell-related genes Defa1, Defcr-rs1, and Mmp7 in the proximal small intestine remain statistically unchanged by Pdx1 inactivation. Although the number of enteroendocrine cells expressing chromogranin A/B, gastric inhibitory polypeptide (Gip), or somatostatin (Sst) is unaffected in the Pdx1(flox/flox);VilCre mice, mRNA abundance for Gip and Sst is significantly reduced in the proximal small intestine. Conditional Pdx1 inactivation attenuates intestinal alkaline phosphatase (IAP) activity in the duodenal epithelium, consistent with an average 91% decrease in expression of the mouse enterocyte IAP gene, alkaline phosphatase 3 (a novel Pdx1 target candidate), in the proximal small intestine following Pdx1 inactivation. We conclude that Pdx1 is necessary for patterning appropriate gene expression in enterocytes and enteroendocrine cells of the proximal small intestine.

PBX1 is dispensable for neural commitment of RA-treated murine ES cells

Experimentation with PBX1 knockout mice has shown that PBX1 is necessary for early embryogenesis. Despite broad insight into PBX1 function, little is known about the underlying target gene regulation. Utilizing the Cre–loxP system, we targeted a functionally important part of the homeodomain of PBX1 through homozygous deletion of exon-6 and flanking intronic regions leading to exon 7 skipping in embryonic stem (ES) cells. We induced in vitro differentiation of wild-type and PBX1 mutant ES cells by aggregation and retinoic acid (RA) treatment and compared their profiles of gene expression at the ninth day post-reattachment to adhesive media. Our results indicate that PBX1 interactions with HOX proteins and DNA are dispensable for RA-induced ability of ES to express neural genes and point to a possible involvement of PBX1 in the regulation of imprinted genes.

On the origin of the beta cell

The major forms of diabetes are characterized by pancreatic islet beta-cell dysfunction and decreased beta-cell numbers, raising hope for cell replacement therapy. Although human islet transplantation is a cell-based therapy under clinical investigation for the treatment of type 1 diabetes, the limited availability of human cadaveric islets for transplantation will preclude its widespread therapeutic application. The result has been an intense focus on the development of alternate sources of beta cells, such as through the guided differentiation of stem or precursor cell populations or the transdifferentiation of more plentiful mature cell populations. Realizing the potential for cell-based therapies, however, requires a thorough understanding of pancreas development and beta-cell formation. Pancreas development is coordinated by a complex interplay of signaling pathways and transcription factors that determine early pancreatic specification as well as the later differentiation of exocrine and endocrine lineages. This review describes the current knowledge of these factors as they relate specifically to the emergence of endocrine beta cells from pancreatic endoderm. Current therapeutic efforts to generate insulin-producing beta-like cells from embryonic stem cells have already capitalized on recent advances in our understanding of the embryonic signals and transcription factors that dictate lineage specification and will most certainly be further enhanced by a continuing emphasis on the identification of novel factors and regulatory relationships.

Cellular origins of adult human islet in vitro dedifferentiation

Cultured human islets can be dedifferentiated to duct-like structures composed mainly of cytokeratin+ and nestin+ cells. Given that these structures possess the potential to redifferentiate into islet-like structures, we sought to elucidate their specific cellular origins. Adenoviral vectors were engineered for beta-, alpha-, delta- or PP-cell-specific GFP expression. A double-stranded system was designed whereby cultures were infected with two vectors: one expressed GFP behind the cumate-inducible promoter sequence, and the other expressed the requisite transactivator behind the human insulin, glucagon, somatostatin or pancreatic polypeptide promoter. This system labels hormone+ cells in the islet in a cell-specific manner, allowing these cells to be tracked during the course of transformation from islet to duct-like structure. Post-infection, islets were cultured to induce dedifferentiation. Fluorescence microscopy demonstrated that alpha-, delta- and PP-cells contributed equally to the cytokeratin+ population, with minimal beta-cell contribution, whereas the converse was true for nestin+ cells. Complementary targeted cell ablation studies, using streptozotocin or similar adenoviral expression of the Bax (Bcl2-associated X protein) toxigene, validated these findings and suggested a redundancy between alpha-, delta- and PP-cells with respect to cytokeratin+ cell derivation. These results call into question the traditional understanding of islet cells as being terminally differentiated and provide support for the concept of adult islet morphogenetic plasticity.

Expression of zebrafish pax6b in pancreas is regulated by two enhancers containing highly conserved cis-elements bound by PDX1, PBX and PREP factors

Background

PAX6 is a transcription factor playing a crucial role in the development of the eye and in the differentiation of the pancreatic endocrine cells as well as of enteroendocrine cells. Studies on the mouse Pax6 gene have shown that sequences upstream from the P0 promoter are required for expression in the lens and the pancreas; but there remain discrepancies regarding the precise location of the pancreatic regulatory elements.

Results

Due to genome duplication in the evolution of ray-finned fishes, zebrafish has two pax6 genes, pax6a and pax6b. While both zebrafish pax6 genes are expressed in the developing eye and nervous system, only pax6b is expressed in the endocrine cells of the pancreas. To investigate the cause of this differential expression, we used a combination of in silico, in vivo and in vitro approaches. We show that the pax6b P0 promoter targets expression to endocrine pancreatic cells and also to enteroendocrine cells, retinal neurons and the telencephalon of transgenic zebrafish. Deletion analyses indicate that strong pancreatic expression of the pax6b gene relies on the combined action of two conserved regulatory enhancers, called regions A and C. By means of gel shift assays, we detected binding of the homeoproteins PDX1, PBX and PREP to several cis-elements of these regions. In constrast, regions A and C of the zebrafish pax6a gene are not active in the pancreas, this difference being attributable to sequence divergences within two cis-elements binding the pancreatic homeoprotein PDX1.

Conclusion

Our data indicate a conserved role of enhancers A and C in the pancreatic expression of pax6b and emphasize the importance of the homeoproteins PBX and PREP cooperating with PDX1, in activating pax6b expression in endocrine pancreatic cells. This study also provides a striking example of how adaptative evolution of gene regulatory sequences upon gene duplication progressively leads to subfunctionalization of the paralogous gene pair.

Prep1/Pbx2 complexes regulate CCL2 expression through the -2578 guanine polymorphism

CC-chemokine ligand 2 (CCL2) is the major chemoattractant protein that recruits monocytes to sites of inflammation and increased expression of CCL2 is associated with numerous inflammatory diseases including human immunodeficiency virus-associated dementia (HIV-D). The -2578 guanine polymorphism in the CCL2 promoter has been associated with increased expression of CCL2 as well as pathogenesis of HIV-D; however, the molecular mechanism of regulation is unknown. We propose a molecular model for -2578 G-regulated CCL2 expression in astrocytes, which are major producers of CCL2 in the brain. The -2578 G polymorphism creates a consensus-binding site for the transcriptional regulator Prep1, which along with binding partner Pbx2, preferentially binds the -2578 G allele. CCL2 promoters harboring the G allele under unstimulated conditions exhibit a lower basal activity compared to the ancestral A allele. Upon interleukin-1 beta stimulation, Prep1/Pbx2 complexes maintain the ability to bind -2578 G alleles, yet transcription levels from promoters that harbor the A or G allele are equally activated, suggesting that the -2578 region does not influence CCL2 transcription under proinflammatory conditions. Therefore, promoters that harbor the -2578 G allele undergo a higher fold induction and by extension, individuals homozygous for -2578 G would be expected to exhibit hyper-responsive CCL2 phenotypes during periods of inflammation.

An ultraconserved Hox-Pbx responsive element resides in the coding sequence of Hoxa2 and is active in rhombomere 4

The Hoxa2 gene has a fundamental role in vertebrate craniofacial and hindbrain patterning. Segmental control of Hoxa2 expression is crucial to its function and several studies have highlighted transcriptional regulatory elements governing its activity in distinct rhombomeres. Here, we identify a putative Hox–Pbx responsive cis-regulatory sequence, which resides in the coding sequence of Hoxa2 and is an important component of Hoxa2 regulation in rhombomere (r) 4. By using cell transfection and chromatin immunoprecipitation (ChIP) assays, we show that this regulatory sequence is responsive to paralogue group 1 and 2 Hox proteins and to their Pbx co-factors. Importantly, we also show that the Hox–Pbx element cooperates with a previously reported Hoxa2 r4 intronic enhancer and that its integrity is required to drive specific reporter gene expression in r4 upon electroporation in the chick embryo hindbrain. Thus, both intronic as well as exonic regulatory sequences are involved in Hoxa2 segmental regulation in the developing r4. Finally, we found that the Hox–Pbx exonic element is embedded in a larger 205-bp long ultraconserved genomic element (UCE) shared by all vertebrate genomes. In this respect, our data further support the idea that extreme conservation of UCE sequences may be the result of multiple superposed functional and evolutionary constraints.

Identification of Lmo1 as part of a Hox-dependent regulatory network for hindbrain patterning

The embryonic functions of Hox proteins have been extensively investigated in several animal phyla. These transcription factors act as selectors of developmental programmes, to govern the morphogenesis of multiple structures and organs. However, despite the variety of morphogenetic processes Hox proteins are involved in, only a limited set of their target genes has been identified so far. To find additional targets, we used a strategy based upon the simultaneous overexpression of Hoxa2 and its cofactors Pbx1 and Prep in a cellular model. Among genes whose expression was upregulated, we identified LMO1, which codes for an intertwining LIM-only factor involved in protein-DNA oligomeric complexes. By analysing its expression in Hox knockout mice, we show that Lmo1 is differentially regulated by Hoxa2 and Hoxb2, in specific columns of hindbrain neuronal progenitors. These results suggest that Lmo1 takes part in a Hox paralogue 2-dependent network regulating anteroposterior and dorsoventral hindbrain patterning.

Purification of the Prep1 interactome identifies novel pathways regulated by Prep1

Prep1 homeodomain transcription factor interacts with Pbx proteins to regulate oculogenesis, angiogenesis, and hematopoiesis in mice. To isolate new Prep1 interactors competing or copurifying with Pbx, we identified proteins copurified with Prep1-TAP by tandem affinity purification (TAP). Prep1-TAP was fully functional and allowed the isolation of a Prep1 proteome from cytoplasm and nucleus, but most interactors were nuclear. The Prep1-TAP complex included Pbx1b, Pbx2, and other nonhomeodomain proteins: p160 Myb-binding protein (p160), beta-actin, NMMHCIIA.

p160 Myb-binding protein interacts with Prep1 and inhibits its transcriptional activity

Prep1 is known to interact in vivo with Pbx1 to regulate development and organogenesis. We have identified a novel Prep1-interacting protein, p160 c-Myb binding protein (p160). p160 and Pbx1 compete for Prep1 in vitro, and p160 inhibits Prep1-dependent HoxB2 expression in retinoic acid-treated NT2-D1 cells. The N-terminal physiologically truncated form of p160, p67, binds the sequence 63LFPLL67 in the HR1 domain of Prep1. Mutation of both L63 and L66 impairs the binding of Prep1 to both p160/p67 and Pbx1. The sequences required to bind Prep1 are mainly located in residues 51 to 151. Immunofluorescence colocalization and coimmunoprecipitation of endogenous p160 and Prep1 are induced by ActD, which translocates p160 from the nucleolus to the nucleoplasm. These data therefore show that p160 is a novel regulator of Prep1-Pbx1 transcriptional activity.

Identification of a new type of PBX1 partner that contains zinc finger motifs and inhibits the binding of HOXA9-PBX1 to DNA

PBX1 belongs to the TALE-class of homeodomain protein and has a wide functional diversity during development. Indeed, PBX1 is required for haematopoiesis as well as for multiple developmental processes such as skeletal patterning and organogenesis. It has furthermore been shown that PBX1 functions as a HOX cofactor during development. More recent data suggest that PBX1 may act even more broadly by modulating the activity of non-homeodomain transcription factors. To better understand molecular mechanisms triggered by PBX1 during female genital tract development, we searched for additional PBX1 partners that might be involved in this process. Using a two hybrid screen, we identified a new PBX1 interacting protein containing several zinc finger motifs that we called ZFPIP for Zinc Finger PBX1 Interacting Protein. We demonstrated that ZFPIP is expressed in embryonic female genital tract but also in other PBX1 expression domains such as the developing head and the limb buds. We further showed that ZFPIP is able to bind physically and in vivo to PBX1 and moreover, that it prevents the binding of HOXA9/PBX complexes to their consensus DNA site. We suggest that ZFPIP is a new type of PBX1 partner that could participate in PBX1 function during several developmental pathways.

Structural Basis for Induced Fit Mechanisms in DNA Recognition by the Pdx1 Homeodomain,

Pancreatic and duodenal homeobox 1 (Pdx1) is a homeodomain transcription factor belonging to the ParaHox family. Pdx1 plays an essential role in pancreatic endocrine and exocrine cell development and maintenance of adult islet beta-cell function. Mutations in the human pdx1 gene are linked to an early onset form of non-insulin-dependent diabetes mellitus, MODY-4. We demonstrate that the homeodomain reproduces the binding specificity of the full-length protein. We report the 2.4 A resolution crystal structure of the homeodomain bound to a target DNA. The two Pdx1/DNA complexes in the asymmetric unit display conformational differences: in the DNA curvature, the orientation of the homeodomain in the major groove, and the order of the N-terminal arm. Comparing the two complexes indicates invariant protein-DNA contacts, and variant contacts that are unique to each binding orientation. An induced fit model is proposed that depends on the DNA conformation and provides a mechanism for nonlocal contributions to binding specificity.

Amino acid 1-209 is essential for PDX-1-mediated repression of human CMV IE promoter activity

AIM

To explore the different roles of pancreatic duodenal homeobox factors-1 (PDX-1) domains in PDX-1 mediated repression of human cytomegalovirus immediately early (CMV IE) promoter.

METHODS

A series of truncated PDX-1 mutants were constructed. The binding of PDX-1 and CMV IE promoter was identified by electrophoretic mobility shift assay (EMSA). The dual-reporter assay was applied to examine the repression activities of PDX-1 mutants on CMV IE promoter. In addition, RNAi technology was used to specifically knock down the endogenous PDX-1 expression.

RESULTS

The reporter assay indicated that compared to the mock controls (pEGFP-N2), overexpression of PDX-1 resulted in a 41% decrease of CMV IE promoter activity in the 293 cells (P< 0.05) and 43% decrease in HeLa cells (P< 0.05), and the repression levels of various truncated mutants played on CMV IE promoter were different. Specific knock down of the endogenous PDX-1 expression significantly restored the activity of CMV IE promoter. EMSA demonstrated that domain 3 is necessary for nuclear localization and DNA binding activity of PDX-1. However, binding of PDX-1 alone to CMV IE promoter was not sufficient to inhibit its transcriptional activity, and other domains of PDX-1 presented were also required.

CONCLUSION

Our data suggested that the DNA binding activity of PDX-1 domain 3 and the cooperative binding of PDX-1 domain 1/2 with other proteins were required for PDX-1 mediated repression of CMV IE promoter.

The CYP2B2 phenobarbital response unit contains binding sites for hepatocyte nuclear factor 4, PBX-PREP1, the thyroid hormone receptor beta and the liver X receptor

A 163 bp enhancer in the CYP2B2 5' flank confers PB (phenobarbital) inducibility and constitutes a PBRU (PB response unit). The PBRU contains several transcription factor binding sites, including NR1, NR2 and NR3, which are direct repeats separated by 4 bp of the nuclear receptor consensus half-site AGGTCA, as well as an ER (everted repeat) separated by 7 bp (ER-7). Constitutive androstane receptor (CAR)-RXR (retinoic X receptor) heterodimers are known to bind to NR1, NR2 and NR3. Electrophoretic mobility-shift analysis using nuclear extracts from livers of untreated or PB-treated rats revealed binding of several other proteins to different PBRU elements. Using supershift analysis and in vitro coupled transcription and translation, the proteins present in four retarded complexes were identified as TRbeta (thyroid hormone receptor beta), LXR (liver X receptor), HNF-4 (hepatocyte nuclear factor 4) and heterodimers of PBX-PREP1 (pre-B cell homoeobox-Pbx regulatory protein 1). LXR-RXR heterodimers bound to NR3 and TRbeta bound to NR3, NR1 and ER-7, whereas the PBX-PREP1 site is contained within NR2. The HNF-4 site overlaps with NR1. A mutation described previously, GRE1m1, which decreases PB responsiveness, increased the affinity of this site for HNF-4. The PBRU also contains a site for nuclear factor 1. The PBRU thus contains a plethora of transcription factor binding sites. The profiles of transcription factor binding to NR1 and NR3 were quite similar, although strikingly different from, and more complex than, that of NR2. This parallels the functional differences in conferring PB responsiveness between NR1 and NR3 on the one hand, and NR2 on the other.

The coactivator Bridge-1 increases transcriptional activation by pancreas duodenum homeobox-1 (PDX-1)

Well-orchestrated transcriptional regulation of pancreatic beta cells is essential for insulin production and glucose homeostasis. Pancreas duodenum homeobox-1 (PDX-1) is a key regulator of glucose-dependent insulin production and glucose metabolism. We find that PDX-1 interacts with the PDZ-domain coactivator Bridge-1 in yeast interaction trap assays. Rat Bridge-1 and PDX-1 interact directly in GST pull-down assays via Bridge-1 interactions with the amino-terminal transactivation domain of PDX-1. Bridge-1 also interacts with wild-type and mutant human PDX-1 (IPF-1) proteins and strongly interacts with the amino-terminal PDX-1 P63fsdelC (MODY4) mutant protein. Transcriptional activation by PDX-1 is increased by addition of Bridge-1 in multiple contexts, including synergistic activation of a Gal4 reporter by Gal4-Bridge-1 and Gal4-PDX-1 fusion proteins, activation of the somatostatin promoter TAAT1 enhancer, and synergistic activation of the rat insulin I promoter FarFlat enhancer by PDX-1, E12, and E47. We propose that the coactivator Bridge-1 modulates PDX-1 functions in the regulation of its target genes.

The homeodomain protein PBX participates in JH-related suppressive regulation on the expression of major plasma protein genes in the silkworm, Bombyx mori

In the silkworm, Bombyx mori, major plasma proteins referred to as 30K proteins are the most abundant proteins in the hemolymph of final (fifth) instar larvae. Surgical extirpation of corpora allata, the source of a juvenile hormone (JH), causes rapid accumulation of 30K proteins in the hemolymph of fourth instar larvae. The 30K protein 6G1 (30K6G1) gene was repressed in primary cultured fat body cells treated with a JH analog (JHA), methoprene. To identify the JH response element present in the promoter region of the 30K6G1 gene, we performed transfection analyses of the 5'-deletion mutants of the 30K6G1 gene using primary cultured fat body cells, gel retardation assays and in vivo footprinting analysis. The results from those analyses revealed that a JH response element exists in the sequence between positions -147 and -140. When the promoter construct mutated at positions -143, -142, and -141 was transfected to fat body primary cultured cells, the suppression effect on the reporter gene expression caused by JHA was reduced. Gel retardation assay using specific antibody revealed that a PBX protein binds to the JH response element. Northern blot analysis revealed that the gene expression of Bombyx PBX is enhanced in the fat body cells by JHA treatment. These results indicate that PBX proteins are involved in the JH signaling pathway and play an important role in suppressing 30K protein gene expression in the fat body of B. mori.

In vitro transdifferentiation of hepatoma cells into functional pancreatic cells

We have characterised the transdifferentiation of human HepG2 (hepatoma) cells to pancreatic cells following introduction of an activated version of the pancreatic transcription factor Pdx1 (XlHbox8-VP16). The following questions are addressed: (1) are all types of pancreatic cells produced? (2) is the requirement for expression of the transgene temporary or permanent? (3) are the transdifferentiated beta-cells responsive to physiological stimuli? The results showed that both pancreatic exocrine cells (by detection of amylase protein), and endocrine cells (by detecting insulin, glucagon and somatostatin proteins) are induced after XlHbox8VP16 transfection. Moreover, the hepatic phenotype becomes suppressed during transdifferentiation of hepatocytes to pancreatic cells. Requirement for the transgene is only temporary and it is no longer required once the pancreatic differentiation program is activated. Finally, we provided results to suggest that the transdifferentiated cells are functional by detecting: (1) functional markers for pancreatic beta-cells including prohormone convertase 1/3 (PC1/3), insulin C-peptide and glucagon-like peptide 1 receptor (GLP-1R), (2) increased insulin mRNA expression after treatment of cells with GLP-1 and betacellulin, physiological stimuli that regulate pancreatic function and (3) elevated insulin secretion after glucose challenge. The transdifferentiation of hepatic to pancreatic cells represents one possible source of beta-cells for human islet transplantation and this study shows that such a transdifferentiation can be achieved in vitro.

Mutation analysis of theHOX paralogous 4-13 genes in children with acute lymphoid malignancies: Identification of a novel germline mutation ofHOXD4 leading to a partial loss-of-function

The molecular basis of susceptibility to childhood malignant hemopathy remains largely unknown. An excess of skeletal congenital anomalies has been reported among children with hematological malignancy and points towards involvement of developmental genes, like those belonging to the HOX gene family. In addition to their role in embryogenesis, HOX transcription factors are known to be regulators of proliferation and differentiation of hematopoietic cells. We aimed to explore the possibility that germline alterations of HOX genes might be involved in childhood acute lymphoid malignancies. A cohort of 86 children diagnosed with acute lymphoid malignancy was studied, 20 of them concurrently presenting a congenital anomaly of the skeleton. First, we screened for nucleotide changes throughout the HOX genes of paralogous groups 4 to 13 in the 20 patients with skeletal defects, following a skeletal phenotype-based strategy. Subsequently, we extended the HOX mutation screening to the other 66 children having a malignant lymphoproliferative disorder, but without skeletal defects. In total, 16 germline mutations were identified. While 13 changes were also observed in healthy controls, three variants were exclusively found in acute lymphoid malignancy cases. These comprised the germline c.242A>T (p.Glu81Val) missense mutation of HOXD4, detected in two children diagnosed with acute lymphoblastic leukemia (ALL). Furthermore, this mutation was found in association with other specific HOX variants of cluster D (2q31-q37), defining a unique haplotype. Functional analysis of the murine Hoxd4 homolog revealed that mutant Hoxd4 protein had lower transcriptional activity than wild-type protein in vitro. The p.Glu81Val mutation of HOXD4 thus results in a partial loss-of-function, which might be involved in childhood ALL.

High glucose is necessary for complete maturation of Pdx1-VP16-expressing hepatic cells into functional insulin-producing cells

Pdx1 has been shown to convert hepatocytes into both exocrine and endocrine pancreatic cells in mice, but it fails to selectively convert hepatocytes into pure insulin-producing cells (IPCs). The molecular mechanisms underlying the transdifferentiation remain unclear. In this study, we generated a stably transfected rat hepatic cell line named WB-1 that expresses an active form of Pdx1 along with a reporter gene, RIP-eGFP. Our results demonstrate that Pdx1 induces the expression of multiple genes related to endocrine pancreas development and islet function in these liver cells. We do not however find any expression of the late-stage genes (Pax4, Pax6, Isl-1, and MafA) related to beta-cell development, and the cells do not secrete insulin upon the glucose challenge. Yet when WB-1 cells are transplanted into diabetic NOD-scid mice, these genes become activated and hyperglycemia is completely reversed. Detailed comparison of gene expression profiles between pre- and posttransplanted WB-1 cells demonstrates that the WB-1 cells have similar properties as that seen in pancreatic beta-cells. In addition, in vitro culture in high-glucose medium is sufficient to induce complete maturation of WB-1 cells into functional IPCs. In summary, we find that Pdx1-VP16 is able to selectively convert hepatic cells into pancreatic endocrine precursor cells. However, complete transdifferentiation into functional IPCs requires additional external factors, including high glucose or hyperglycemia. Thus, transdifferentiation of hepatocytes into functional IPCs may serve as a viable therapeutic option for patients with type 1 diabetes.

The Hoxa2 enhancer 2 contains a critical Hoxa2 responsive regulatory element

Rhombomeres are embryonic territories arising from the transient segmentation of the hindbrain. Their identity is specified by Hox genes from paralogous groups 1-4. Hoxa2 is the only Hox gene to be expressed in the second rhombomere and the regulatory cues leading to this region-specific expression have been poorly investigated. A 2.5-kb DNA fragment overlapping with the 3' end of Hoxa2 was previously shown to specifically direct the expression of a reporter gene in the second rhombomere and the rostral somites of mouse embryos. Here, we report that this enhancer region is activated in vitro by Hoxa2 and that this activation is strictly dependent on a short 10-bp sequence matching the consensus for Hox-Pbx recognition sites.

The tandem complex of BEL and KNOX partners is required for transcriptional repression of ga20ox1

Summary Two interacting three amino acid loop extension (TALE) proteins of potato, StBEL5 (Solanum tuberosom BEL 5) and POTH1 (potato homeobox 1), mediate developmental processes by regulating phytohormone levels. Overexpression of either partner alone increased tuber yields by lowering gibberellin (GA) levels and increasing cytokinins. Gel shift assays demonstrated that StBEL5 and POTH1 bind to the regulatory region of ga20 oxidase1 (ga20ox1) from potato, a gene encoding a key enzyme in the GA biosynthetic pathway. In tandem, StBEL5 and POTH1 had a greater binding affinity for the ga20ox1 promoter than either protein alone. The StBEL5-POTH1 heterodimer bound specifically to a composite 10-bp sequence, containing two TGAC cores. Using a transcription assay, StBEL5 and POTH1 suppressed the activity of the ga20ox1 promoter by more than 50%. Dominant negative constructs containing the protein-binding domains of StBEL5 or POTH1 blocked the repression activity of StBEL5 or POTH1, respectively. The mutated ga20ox1 promoter that could be bound by the StBEL5 or POTH1 proteins individually but not by the StBEL5-POTH1 heterodimer also abolished the repression activity of StBEL5, POTH1, and the StBEL5-POTH1 heterodimer. These results indicate that the tandem interaction of StBEL5 and POTH1 is essential for regulation of the expression of their target gene.

PDX1, a Cellular Homeoprotein, Binds to and Regulates the Activity of Human Cytomegalovirus Immediate Early Promoter

Cellular homeoproteins have been shown to regulate the transcription of several viruses, including herpes simplex viruses, human papillomaviruses, and mouse mammary tumor viruses. Previous studies investigating the anti-viral mechanisms of several cyclin-dependent kinase inhibitors showed that the homeoproteins, pre B-cell leukemia transcription factor 1 (PBX1) and PBX-regulating protein-1 (PREP1), function as transcriptional activators of Moloney murine leukemia virus. Here, we examined the involvement of cellular homeoproteins in regulating the activity of the human cytomegalovirus immediate early (CMV IE) promoter. We identified a 45-bp element located at position -593 to -549 upstream of the transcription start site of the CMV IE gene, which contains multiple putative homeoprotein binding motifs. Gel shift assays demonstrated the physical association between a homeodomain protein, pancreatic-duodenal homeobox factor-1 (PDX1) and the 45-bp cytomegalovirus (CMV) region. We further determined that PDX1 represses the CMV IE promoter activity in 293 cells. Overexpression of PDX1 resulted in a decrease in transcription of the CMV IE gene. Conversely, blocking PDX1 protein synthesis and mutating the PDX1 binding sites enhanced CMV IE-dependent transcription. Collectively, our results represent the first work demonstrating that a cellular homeoprotein, PDX1, may be a repressor involved in regulation of human CMV gene expression.

PDX-1 and the pancreas

Many transcription factors are critical for ensuring proper embryonic development of the endocrine pancreas and normal islet function. The transcription factor pancreatic duodenal homeobox 1 (PDX-1) is uniformly expressed in early pancreatic buds of embryos as well as the beta and delta cells of the islets of Langerhans. PDX-1 has also been found in dispersed endocrine cells of the duodenum in adults and plays a key role in pancreas formation. It has been reported that null mutation of PDX-1 in mice results in a failure of the pancreatic bud to expand; thus, the mice die 2-3 days after birth from hyperglycemia and dehydration. Heterozygous PDX-1 mice developed a pancreas but were diabetic. It has been shown that PDX-1 is required for maintaining the pancreatic islet functions by activating gene transcriptions including insulin, somatostatin (SST), islet amyloid polypeptide, glucose transporter type 2, and glucokinase. PDX-1 serves a dual role in pancreatic development. It initially contributes to pancreatic formation during embryogenesis and subsequently regulates the pancreatic islet cell physiology in mature islet cells. Understanding the underlying molecular mechanisms of pancreas formation, especially the function of PDX-1, may contribute to the enhanced treatment and prevention of debilitating diseases such as diabetes, insulinomas, and pancreatic carcinomas.

Activin regulation of the follicle-stimulating hormone beta-subunit gene involves Smads and the TALE homeodomain proteins Pbx1 and Prep1

FSH is critical for normal reproductive function in both males and females. Activin, a member of the TGFbeta family of growth factors, is an important regulator of FSH expression, but little is known about the molecular mechanisms through which it acts. We used transient transfections into the immortalized gonadotrope cell line LbetaT2 to identify three regions (at -973/-962, -167, and -134) of the ovine FSH beta-subunit gene that are required for full activin response. All three regions contain homology to consensus binding sites for Smad proteins, the intracellular mediators of TGFbeta family signaling. Mutation of the distal site reduces activin responsiveness, whereas mutation of either proximal site profoundly disrupts activin regulation of the FSHbeta gene. These sites specifically bind LbetaT2 nuclear proteins in EMSAs, and the -973/-962 site binds Smad4 protein. Interestingly, the protein complex binding to the -134 site contains Smad4 in association with the homeodomain proteins Pbx1 and Prep1. Using glutathione S-transferase interaction assays, we demonstrate that Pbx1 and Prep1 interact with Smads 2 and 3 as well. The two proximal activin response elements are well conserved across species, and Pbx1 and Prep1 proteins bind to the mouse gene in vivo. Furthermore, mutation of either proximal site abrogates activin responsiveness of a mouse FSHbeta reporter gene as well, confirming their functional conservation. Our studies provide a basis for understanding activin regulation of FSHbeta gene expression and identify Pbx1 and Prep1 as Smad partners and novel mediators of activin action.