Meis2, a novel mouse Pbx-related homeobox gene induced by retinoic acid during differentiation of P19 embryonal carcinoma cells

MEIS2 suppresses breast cancer development by downregulating IL10

BACKGROUND

Breast cancer (BC) is the most commonly diagnosed female cancer. Homeobox protein MEIS2, a key transcription factor, is involved in the regulation of many developmental and cellular processes. However, the role of MEIS2 in the development of breast cancer is still unclear.

AIMS

We aimed to examine the role of myeloid ecotropic insertion site (MEIS2) in breast cancer and the association of MEIS2 with breast cancer clinical stages and pathological grades. We revealed the underlying mechanism by which MEIS2 affected breast cancer cell growth and tumor development.

METHODS AND RESULTS

Using human BC cell lines, clinical samples and animal xenograft model, we reveal that MEIS2 functions as a tumor suppressor in breast cancer. The expression of MEIS2 is inversely correlated with BC clinical stages and pathological grades. MEIS2 knockdown (MEIS2-KD) promotes while MEIS2 overexpression suppresses breast cancer cell proliferation and tumor development in vitro and in animal xenograft models, respectively. To determine the biological function of MEIS2, we screen the expression of a group of MEIS2 potential targeting genes in stable-established cell lines. Results show that the knockdown of MEIS2 in breast cancer cells up-regulates the IL10 expression, but MEIS2 overexpression opposed the effect on IL10 expression. Furthermore, the suppressive role of MEIS2 in breast cancer cell proliferation is associated with the IL10 expression and myeloid cells infiltration.

CONCLUSION

Our study demonstrates that the tumor suppressor of MEIS2 in breast cancer progression is partially via down regulating the expression of IL10 and promoting myeloid cells infiltration. Targeting MEIS2 would be a potentially therapeutic avenue for BC.

How "Neuronal" Are Human Skin Mast Cells?

Mast cells are evolutionarily old cells and the principal effectors in allergic responses and inflammation. They are seeded from the yolk sac during embryogenesis or are derived from hematopoietic progenitors and are therefore related to other leukocyte subsets, even though they form a separate clade in the hematopoietic system. Herein, we systematically bundle information from several recent high-throughput endeavors, especially those comparing MCs with other cell types, and combine such information with knowledge on the genes’ functions to reveal groups of neuronal markers specifically expressed by MCs. We focus on recent advances made regarding human tissue MCs, but also refer to studies in mice. In broad terms, genes hyper-expressed in MCs, but largely inactive in other myelocytes, can be classified into subcategories such as traffic/lysosomes (MLPH and RAB27B), the dopamine system (MAOB, DRD2, SLC6A3, and SLC18A2), Ca²⁺-related entities (CALB2), adhesion molecules (L1CAM and NTM) and, as an overall principle, the transcription factors and modulators of transcriptional activity (LMO4, PBX1, MEIS2, and EHMT2). Their function in MCs is generally unknown but may tentatively be deduced by comparison with other systems. MCs share functions with the nervous system, as they express typical neurotransmitters (histamine and serotonin) and a degranulation machinery that shares features with the neuronal apparatus at the synapse. Therefore, selective overlaps are plausible, and they further highlight the uniqueness of MCs within the myeloid system, as well as when compared with basophils. Apart from investigating their functional implications in MCs, a key question is whether their expression in the lineage is due to the specific reactivation of genes normally silenced in leukocytes or whether the genes are not switched off during mastocytic development from early progenitors.

In vitro atlas of dorsal spinal interneurons reveals Wnt signaling as a critical regulator of progenitor expansion

Restoring sensation after injury or disease requires a reproducible method for generating large quantities of bona fide somatosensory interneurons. Toward this goal, we assess the mechanisms by which dorsal spinal interneurons (dIs; dI1-dI6) can be derived from mouse embryonic stem cells (mESCs). Using two developmentally relevant growth factors, retinoic acid (RA) and bone morphogenetic protein (BMP) 4, we recapitulate the complete in vivo program of dI differentiation through a neuromesodermal intermediate. Transcriptional profiling reveals that mESC-derived dIs strikingly resemble endogenous dIs, with the correct molecular and functional signatures. We further demonstrate that RA specifies dI4-dI6 fates through a default multipotential state, while the addition of BMP4 induces dI1-dI3 fates and activates Wnt signaling to enhance progenitor proliferation. Constitutively activating Wnt signaling permits the dramatic expansion of neural progenitor cultures. These cultures retain the capacity to differentiate into diverse populations of dIs, thereby providing a method of increasing neuronal yield.

Dlx1/2-dependent expression of Meis2 promotes neuronal fate determination in the mammalian striatum

The striatum is a central regulator of behavior and motor function through the actions of D1 and D2 medium-sized spiny neurons (MSNs), which arise from a common lateral ganglionic eminence (LGE) progenitor. The molecular mechanisms of cell fate specification of these two neuronal subtypes are incompletely understood. Here, we found that deletion of murine Meis2, which is highly expressed in the LGE and derivatives, led to a large reduction in striatal MSNs due to a block in their differentiation. Meis2 directly binds to the Zfp503 and Six3 promoters and is required for their expression and specification of D1 and D2 MSNs, respectively. Finally, Meis2 expression is regulated by Dlx1/2 at least partially through the enhancer hs599 in the LGE subventricular zone. Overall, our findings define a pathway in the LGE whereby Dlx1/2 drives expression of Meis2, which subsequently promotes the fate determination of striatal D1 and D2 MSNs via Zfp503 and Six3.

Placental Expression of the Forelimb Patterning Transcription Factor MEIS2 in Trisomy 15

BackgroundObservations of first trimester human trisomy 15 (T15) embryos have identified meromelic changes in the upper limbs. These changes are similar to those observed in animal studies investigating the effects of overexpression of Meis2, a signaling transcription factor expressed during forelimb development. Although it would be exceedingly difficult to assess MEIS2 expression in the human embryonic arm, MEIS2 has been reported as consistently expressed in first trimester placental villus stroma. Methods: This study addresses whether gene dosage effect might underlie meromelia in T15 by comparing MEIS2 expression in placentas from T15 and euploid spontaneous abortions employing manual and automated assessment of MEIS2 immunohistochemical scoring. Results: Average MEIS2 expression is increased in T15 first trimester placental tissue compared to euploid controls but that the difference is marginal. Manual and automated scoring showed moderately strong correlation. Conclusion: Extrapolation of these results suggests MEIS2 overexpression may not be required for meromelia in T15.

Retinoic Acid-Regulated Target Genes During Development: Integrative Genomics Analysis

Retinoic acid (RA), a major natural active metabolite of vitamin A (VA) is well known to play critical roles in embryonic development. The effects of RA are mediated by nuclear receptors (RARs), which regulate the expression of gene batteries involved in cell growth and differentiation. Since the early 1990s several laboratories have focused on understanding how RA-regulated genes and RAR binding sites operate by studying the differentiation of embryonal carcinoma cells and embryonic stem cells. The development of hybridization-based microarray technology and high performance software analysis programs has allowed the characterization of thousands of RA-regulated genes. During the two last decades, publication of the genome sequence of various organisms has allowed advances in massive parallel sequencing and bioinformatics analysis of genome-wide data sets. These new generation sequencing (NGS) technologies have revolutionized the field by providing a global integrated picture of RA-regulated gene networks and the regulatory programs involved in cell fate decisions during embryonal carcinoma and embryonic stem cells differentiation. Now the challenge is to reconstruct the RA-regulated gene networks at the single cell level during the development of specialized embryonic tissues.

RA Signaling in Limb Development and Regeneration in Different Species

This chapter brings together data on the role of retinoic acid (RA) in the embryonic development of fins in zebrafish , limbs in amphibians , chicks , and mice, and regeneration of the amphibian limb . The intention is to determine whether there is a common set of principles by which we can understand the mode of action of RA in both embryos and adults. What emerges from this synthesis is that there are indeed commonalities in the involvement of RA in processes that ventralize, posteriorize, and proximalize the developing and regenerating limb . Different axes of the limb have historically been studied independently; as for example, the embryonic development of the anteroposterior (AP) axis of the chick limb bud versus the regeneration of the limb bud proximodistal (PD) axis . But when we take a broader view, a unifying principle emerges that explains why RA administration to embryos and regenerating limbs results in the development of multiple limbs in both cases. As might be expected, different molecular pathways govern the development of different systems and model organisms, but despite these differences, the pathways involve similar RA signaling genes, such as tbx5, meis, shh, fgfs and hox genes. Studies of developing and regenerating systems have highlighted that RA acts by being synthesized in one embryonic location while acting in another one, exactly as embryonic morphogens do, although there is no evidence for the presence of an RA gradient within the limb . What also emerges is that there is a paucity of information on the involvement of RA in development of the dorsoventral (DV) axis . A molecular explanation as to how RA establishes and alters positional information in all three axes is the most important area of study for the future.

Dental Epithelial Stem Cells Express the Developmental Regulator Meis1

MEIS1 is a key developmental regulator of several organs and participates in stem cell maintenance in different niches. However, despite the murine continuously growing incisor being a well described model for the study of adult stem cells, Meis1 has not been investigated in a dental context. Here, we uncover that Meis1 expression in the tooth is confined to the epithelial compartment. Its expression arises during morphogenesis and becomes restricted to the mouse incisor epithelial stem cell niche, the labial cervical loop. Meis1 is specifically expressed by Sox2⁺ stem cells, which give rise to all dental epithelial cell lineages. Also, we have found that Meis1 in the incisor is coexpressed with potential binding partner Pbx1 during both embryonic and adult stages. Interestingly, Meis2 is present in different areas of the forming tooth and it is not expressed by dental epithelial stem cells, suggesting different roles for these two largely homologous genes. Additionally, we have established the expression patterns of Meis1 and Meis2 during tongue, hair, salivary gland and palate formation. Finally, analysis of Meis1-null allele mice indicated that, similarly, to SOX2, MEIS1 is not essential for tooth initiation, but might have a role during adult incisor renewal.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

Meis2 as a critical player in MN1-induced leukemia

Meningioma 1 (MN1) is an independent prognostic marker for normal karyotype acute myeloid leukemia (AML), with high expression linked to all-trans retinoic acid resistance and poor survival. MN1 is also a potent and sufficient oncogene in murine leukemia models, strongly dependent on the MEIS1/AbdB-like HOX protein complex to transform common myeloid progenitors, block myeloid differentiation, and promote leukemic stem cell self-renewal. To identify key genes and pathways underlying leukemic activity, we functionally assessed MN1 cell phenotypic heterogeneity, revealing leukemic and non-leukemic subsets. Using gene expression profiling of these subsets combined with previously published comparisons of full-length MN1 and mutants with varying leukemogenic activity, we identified candidate genes critical to leukemia. Functional analysis identified Hlf and Hoxa9 as critical to MN1 in vitro proliferation, self-renewal and impaired myeloid differentiation. Although critical to transformation, Meis1 knockdown had little impact on these properties in vitro. However, we identified Meis2 as critical to MN1-induced leukemia, with essential roles in proliferation, self-renewal, impairment of differentiation and disease progression in vitro and in vivo. Here, we provide evidence of phenotypic and functional hierarchy in MN1-induced leukemic cells, characterise contributions of Hlf, Hoxa9 and Meis1 to in vitro leukemic properties, and reveal Meis2 as a novel player in MN1-induced leukemogenesis.

Epigenetic silencing of downstream genes mediated by tandem orientation in lung cancer

Epigenetic deregulation is of importance in tumorigenesis. In particular CpG islands (CGI), are frequently hypermethylated. Here, genome-wide DNA-methylation profiles of 480,000 CpGs in lung cancer cells were generated. It was observed that intra- and intergenic CGI exhibited higher methylation compared to normal cells. The functional annotation of hypermethylated CGI revealed that the hypermethylation was associated with homeobox domain genes and targets marked by repressive histone modifications. The strongest methylation variation was observed in transitional areas of CGI, termed shores. 5'-shores of promoter-associated CGI in lung cancer cell lines were higher methylated than 3'-shores. Within two tandem-oriented genes, a significant hypermethylation of the downstream-located CGI promoters was revealed. Hypermethylation correlates with the length of the intergenic region between such tandem genes. As the RASSF1A tumor suppressor gene represents such a downstream tandem gene, its silencing was analyzed using an inducible system. It was determined that the induction of an upstream gene led to a repression of RASSF1A through a process involving histone deacetylases and CPSF1. A tumor-specific increase in expression of histone deacetylases and CPSF1 was detected in lung cancer. Our results suggest that the downstream gene could be susceptible to epigenetic silencing when organized in a tandem orientation.

A Gene Regulatory Network Balances Neural and Mesoderm Specification during Vertebrate Trunk Development

Transcriptional networks, regulated by extracellular signals, control cell fate decisions and determine the size and composition of developing tissues. One example is the network controlling bipotent neuromesodermal progenitors (NMPs) that fuel embryo elongation by generating spinal cord and trunk mesoderm tissue. Here, we use single-cell transcriptomics to identify the molecular signature of NMPs and reverse engineer the mechanism that regulates their differentiation. Together with genetic perturbations, this reveals a transcriptional network that integrates opposing retinoic acid (RA) and Wnt signals to determine the rate at which cells enter and exit the NMP state. RA, produced by newly generated mesodermal cells, provides feedback that initiates NMP generation and induces neural differentiation, thereby coordinating the production of neural and mesodermal tissue. Together, the data define a regulatory network architecture that balances the generation of different cell types from bipotential progenitors in order to facilitate orderly axis elongation.

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Single-cell RNA-seq reveals a signature of neuromesodermal progenitors

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In vitro NMPs resemble and differentiate similar to their in vivo counterparts

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Dual role for retinoic acid signaling in NMP induction and neural differentiation

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A transcriptional network regulates neural versus mesodermal allocation

Retinoic acid-independent expression of Meis2 during autopod patterning in the developing bat and mouse limb

BACKGROUND

The bat has strikingly divergent forelimbs (long digits supporting wing membranes) and hindlimbs (short, typically free digits) due to the distinct requirements of both aerial and terrestrial locomotion. During embryonic development, the morphology of the bat forelimb deviates dramatically from the mouse and chick, offering an alternative paradigm for identifying genes that play an important role in limb patterning.

RESULTS

Using transcriptome analysis of developing Natal long-fingered bat (Miniopterus natalensis) fore- and hindlimbs, we demonstrate that the transcription factor Meis2 has a significantly higher expression in bat forelimb autopods compared to hindlimbs. Validation by reverse transcriptase and quantitative polymerase chain reaction (RT-qPCR) and whole mount in situ hybridisation shows that Meis2, conventionally known as a marker of the early proximal limb bud, is upregulated in the bat forelimb autopod from CS16. Meis2 expression is localised to the expanding interdigital webbing and the membranes linking the wing to the hindlimb and tail. In mice, Meis2 is also expressed in the interdigital region prior to tissue regression. This interdigital Meis2 expression is not activated by retinoic acid (RA) signalling as it is present in the retained interdigital tissue of Rdh10 (trex/trex) mice, which lack RA. Additionally, genes encoding RA-synthesising enzymes, Rdh10 and Aldh1a2, and the RA nuclear receptor Rarβ are robustly expressed in bat fore- and hindlimb interdigital tissues indicating that the mechanism that retains interdigital tissue in bats also occurs independently of RA signalling.

CONCLUSIONS

Mammalian interdigital Meis2 expression, and upregulation in the interdigital webbing of bat wings, suggests an important role for Meis2 in autopod development. Interdigital Meis2 expression is RA-independent, and retention of interdigital webbing in bat wings is not due to the suppression of RA-induced cell death. Rather, RA signalling may play a role in the thinning (rather than complete loss) of the interdigital tissue in the bat forelimb, while Meis2 may interact with other factors during both bat and mouse autopod development to maintain a pool of interdigital cells that contribute to digit patterning and growth.

Hoxa2 selectively enhances Meis binding to change a branchial arch ground state

Hox transcription factors (TFs) are essential for vertebrate development, but how these evolutionary conserved proteins function in vivo remains unclear. Because Hox proteins have notoriously low binding specificity, they are believed to bind with cofactors, mainly homeodomain TFs Pbx and Meis, to select their specific targets. We mapped binding of Meis, Pbx, and Hoxa2 in the branchial arches, a series of segments in the developing vertebrate head. Meis occupancy is largely similar in Hox-positive and -negative arches. Hoxa2, which specifies second arch (IIBA) identity, recognizes a subset of Meis prebound sites that contain Hox motifs. Importantly, at these sites Meis binding is strongly increased. This enhanced Meis binding coincides with active enhancers, which are linked to genes highly expressed in the IIBA and regulated by Hoxa2. These findings show that Hoxa2 operates as a tissue-specific cofactor, enhancing Meis binding to specific sites that provide the IIBA with its anatomical identity.

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Meis provides a ground state that is common to all the branchial arches

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Hoxa2 recognizes Meis prebound sites in the second arch that contain Hox motifs

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Hoxa2 enhances Meis binding, which coincides with active enhancers, at these sites

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Hoxa2 modulates the ground-state binding of Meis to instruct second arch identity

Identification of novel retinoic acid target genes

Retinoic acid is required for diverse ontogenic processes and as such identification of the genes and pathways affected by retinoic acid is critical to understanding these pleiotropic effects. The presomitic mesoderm of the E8.5 mouse embryo is composed of undifferentiated cells that are depleted of retinoic acid, yet are competent to respond to the retinoid signal. We have exploited these properties to use this tissue to identify novel retinoic acid-responsive genes, including candidate target genes, by treating E8.5 embryos with retinoic acid and assessing changes in gene expression in the presomitic mesoderm by microarray analysis. This exercise yielded a cohort of genes that were differentially expressed in response to exogenous retinoic acid exposure. Among these were a number of previously characterized retinoic acid targets, validating this approach. In addition, we recovered a number of novel candidate target genes which were confirmed as retinoic acid-responsive by independent analysis. Chromatin immunoprecipitation assays revealed retinoic acid receptor occupancy of the promoters of certain of these genes. We further confirmed direct retinoic acid regulation of the F11r gene, a new RA target, using tissue culture models. Our results reveal a significant number of potential RA targets implicated in embryonic development and offer a novel in vivo system for better understanding of retinoid-dependent transcription.

Differential expression of Meis2, Mab21l2 and Tbx3 during limb development associated with diversification of limb morphology in mammals

Bats are the only mammals capable of self-powered flight using wings. Differing from mouse or human limbs, four elongated digits within a broad wing membrane support the bat wing, and the foot of the bat has evolved a long calcar that spread the interfemoral membrane. Our recent mRNA sequencing (mRNA-Seq) study found unique expression patterns for genes at the 5' end of the Hoxd gene cluster and for Tbx3 that are associated with digit elongation and wing membrane growth in bats. In this study, we focused on two additional genes, Meis2 and Mab21l2, identified from the mRNA-Seq data. Using whole-mount in situ hybridization (WISH) we validated the mRNA-Seq results for differences in the expression patterns of Meis2 and Mab21l2 between bat and mouse limbs, and further characterize the timing and location of the expression of these two genes. These analyses suggest that Meis2 may function in wing membrane growth and Mab21l2 may have a role in AP and DV axial patterning. In addition, we found that Tbx3 is uniquely expressed in the unique calcar structure found in the bat hindlimb, suggesting a role for this gene in calcar growth and elongation. Moreover, analysis of the coding sequences for Meis2, Mab21l2 and Tbx3 showed that Meis2 and Mab21l2 have high sequence identity, consistent with the functions of genes being conserved, but that Tbx3 showed accelerated evolution in bats. However, evidence for positive selection in Tbx3 was not found, which would suggest that the function of this gene has not been changed. Together, our findings support the hypothesis that the modulation of the spatiotemporal expression patterns of multiple functional conserved genes control limb morphology and drive morphological change in the diversification of mammalian limbs.

Role of TG-interacting factor (Tgif) in lipid metabolism

TG interacting factors (Tgifs) 1 and 2 are members of the TALE (three-amino-acid loop extension) superfamily of homeodomain proteins. These two proteins bind to the same DNA sequence and share a conserved C-terminal repression domain. Mutations in TGIF1 have been linked to holoprosencephaly, which is a human genetic disease that affects craniofacial development. As these proteins can interact with the ligand binding domain of retinoid X receptor α, a common heterodimeric partner of several nuclear receptors [e.g., liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs)], Tgif1 and Tgif2 might repress other transcriptional pathways activated by lipids. In line with this, Tgif1 interacts with LXRα and Tgif1 null mice have increased expression of the two Lxrα target genes apolipoproteins (Apo) c2 and a4. Also, we have recently identified Tgif1 to function as a transcriptional repressor of the cholesterol esterifying enzyme acyl-coenzyme A:cholesterol acyltransferase 2 (gene name SOAT2). As no studies yet have shown involvement of Tgif2 in the lipid metabolism, this review will focus on the role of Tgif1 in lipid and cholesterol metabolism. This article is part of a Special Issue entitled: Linking transcription to physiology in lipodomics.

TG-interacting factor 1 acts as a transcriptional repressor of sterol O-acyltransferase 2

Acat2 [gene name: sterol O-acyltransferase 2 (SOAT2)] esterifies cholesterol in enterocytes and hepatocytes. This study aims to identify repressor elements in the human SOAT2 promoter and evaluate their in vivo relevance. We identified TG-interacting factor 1 (Tgif1) to function as an important repressor of SOAT2. Tgif1 could also block the induction of the SOAT2 promoter activity by hepatocyte nuclear factor 1α and 4α. Women have ∼ 30% higher hepatic TGIF1 mRNA compared with men. Depletion of Tgif1 in mice increased the hepatic Soat2 expression and resulted in higher hepatic lipid accumulation and plasma cholesterol levels. Tgif1 is a new player in human cholesterol metabolism.

All-trans-retinoic acid antagonizes the Hedgehog pathway by inducing patched

Male germ cell tumors (GCTs) are a model for a curable solid tumor. GCTs can differentiate into mature teratomas. Embryonal carcinomas (ECs) represent the stem cell compartment of GCTs and are the malignant counterpart to embryonic stem (ES) cells. GCTs and EC cells are useful to investigate differentiation therapy and chemotherapy response. This study explored mechanistic interactions between all-trans-retinoic acid (RA), which induces differentiation of EC and ES cells, and the Hedgehog (Hh) pathway, a regulator of self-renewal and proliferation. RA was found to induce mRNA and protein expression of Patched 1 (Ptch1), the Hh ligand receptor and negative regulator of this pathway. PTCH1 is also a target gene of Hh signaling through Smoothened (Smo) activation. Yet, this observed RA-mediated Ptch1 induction was independent of Smo. It occurred despite co-treatment with RA and Smo inhibitors. Retinoid induction of Ptch1 also occurred in other RA-responsive cancer cell lines and in normal ES cells. Notably, this enhanced Ptch1 expression was preceded by induction of the homeobox transcription factor Meis1, a direct RA target. Direct interaction between Meis1 and Ptch1 was confirmed using chromatin immunoprecipitation assays. To establish the translational relevance of this work, Ptch1 expression was shown to be deregulated in human ECs relative to mature teratoma and the normal seminiferous tubule. Taken together, these findings reveal a previously unrecognized mechanism through which RA can inhibit the Hh pathway via Ptch1 induction. Engaging this pathway is a new way to repress the Hh pathway that can be translated into the cancer clinic.

Biochemistry of the tale transcription factors PREP, MEIS, and PBX in vertebrates

TALE (three amino acids loop extension) homeodomain transcription factors are required in various steps of embryo development, in many adult physiological functions, and are involved in important pathologies. This review focuses on the PREP, MEIS, and PBX sub-families of TALE factors and aims at giving information on their biochemical properties, i.e., structure, interactors, and interaction surfaces. Members of the three sets of protein form dimers in which the common partner is PBX but they can also directly interact with other proteins forming higher-order complexes, in particular HOX. Finally, recent advances in determining the genome-wide DNA-binding sites of PREP1, MEIS1, and PBX1, and their partial correspondence with the binding sites of some HOX proteins, are reviewed. These studies have generated a few general rules that can be applied to all members of the three gene families. PREP and MEIS recognize slightly different consensus sequences: PREP prefers to bind to promoters and to have PBX as a DNA-binding partner; MEIS prefers HOX as partner, and both PREP and MEIS drive PBX to their own binding sites. This outlines the clear individuality of the PREP and MEIS proteins, the former mostly devoted to basic cellular functions, the latter more to developmental functions.

Zinc regulation of transcriptional activity during retinoic acid-induced neuronal differentiation

Zinc deficiency impairs the proliferation and differentiation of stem cells in the central nervous system that participate in neurogenesis. To examine the molecular mechanisms responsible for the role of this essential nutrient in neuronal precursor cells and neuronal differentiation, we identified zinc-dependent changes in the DNA-binding activity of zinc finger proteins and other transcription factors in proliferating human Ntera-2 neuronal precursor cells undergoing retinoic acid-stimulated differentiation into a neuronal phenotype. We found that zinc deficiency altered binding activity of 28 transcription factors including retinoid X receptor (RXR) known to participate in neuronal differentiation. Alterations in zinc finger transcription factor activity were not simply the result of removal of zinc from these proteins during zinc deficiency, as the activity of other zinc-binding transcription factors such as the glucocorticoid receptor was increased by as much as twofold over zinc-adequate conditions, and nonzinc-binding transcription factors such as nuclear factor-1 and heat shock transcription factor-1 were increased by as much as fourfold over control. Western analysis did not detect significant decreases in total RXR protein abundance in neuronal precursors, suggesting that the decrease in DNA-binding activity was not simply the result of a reduction in RXR levels in neuronal precursor cells. Rather, use of a reporter gene construct containing retinoic acid response elements upstream from a luciferase coding sequence revealed that zinc deficiency results in decreased transcriptional activity of RXR and reductions in retinoic acid-mediated gene transcription during neuronal differentiation. These results show that zinc deficiency has implications for both developmental and adult neurogenesis.

Hox regulation of transcription: more complex(es)

Hox genes encode transcription factors with important roles during embryogenesis and tissue differentiation. Genetic analyses initially demonstrated that interfering with Hox genes has profound effects on the specification of cell identity, suggesting that Hox proteins regulate very specific sets of target genes. However, subsequent biochemical analyses revealed that Hox proteins bind DNA with relatively low affinity and specificity. Furthermore, it became clear that a given Hox protein could activate or repress transcription, depending on the context. A resolution to these paradoxes presented itself with the discovery that Hox proteins do not function in isolation, but interact with other factors in complexes. The first such "cofactors" were members of the Extradenticle/Pbx and Homothorax/Meis/Prep families. However, the list of Hox-interacting proteins has continued to grow, suggesting that Hox complexes contain many more components than initially thought. Additionally, the activities of the various components and the exact mechanisms whereby they modulate the activity of the complex remain puzzling. Here, we review the various proteins known to participate in Hox complexes and discuss their likely functions. We also consider that Hox complexes of different compositions may have different activities and discuss mechanisms whereby Hox complexes may be switched between active and inactive states.

Analysis of the DNA-binding profile and function of TALE homeoproteins reveals their specialization and specific interactions with Hox genes/proteins

The interactions of Meis, Prep, and Pbx1 TALE homeoproteins with Hox proteins are essential for development and disease. Although Meis and Prep behave similarly in vitro, their in vivo activities remain largely unexplored. We show that Prep and Meis interact with largely independent sets of genomic sites and select different DNA-binding sequences, Prep associating mostly with promoters and housekeeping genes and Meis with promoter-remote regions and developmental genes. Hox target sequences associate strongly with Meis but not with Prep binding sites, while Pbx1 cooperates with both Prep and Meis. Accordingly, Meis1 shows strong genetic interaction with Pbx1 but not with Prep1. Meis1 and Prep1 nonetheless coregulate a subset of genes, predominantly through opposing effects. Notably, the TALE homeoprotein binding profile subdivides Hox clusters into two domains differentially regulated by Meis1 and Prep1. During evolution, Meis and Prep thus specialized their interactions but maintained significant regulatory coordination.

Genome-wide in silico identification of new conserved and functional retinoic acid receptor response elements (direct repeats separated by 5 bp)

The nuclear retinoic acid receptors interact with specific retinoic acid (RA) response elements (RAREs) located in the promoters of target genes to orchestrate transcriptional networks involved in cell growth and differentiation. Here we describe a genome-wide in silico analysis of consensus DR5 RAREs based on the recurrent RGKTSA motifs. More than 15,000 DR5 RAREs were identified and analyzed for their localization and conservation in vertebrates. We selected 138 elements located ±10 kb from transcription start sites and gene ends and conserved across more than 6 species. We also validated the functionality of these RAREs by analyzing their ability to bind retinoic acid receptors (ChIP sequencing experiments) as well as the RA regulation of the corresponding genes (RNA sequencing and quantitative real time PCR experiments). Such a strategy provided a global set of high confidence RAREs expanding the known experimentally validated RAREs repertoire associated to a series of new genes involved in cell signaling, development, and tumor suppression. Finally, the present work provides a valuable knowledge base for the analysis of a wider range of RA-target genes in different species.

Cooperative transcriptional activation by Klf4, Meis2, and Pbx1

The Kruppel-like factor Klf4 is implicated in tumorigenesis and maintaining stem cell pluripotency, and Klf4 can both activate and repress gene expression. We show that the Pbx1 and Meis2 homeodomain proteins interact with Klf4 and can be recruited to DNA elements comprising a Klf4 site or GC box, with adjacent Meis and Pbx sites. Meis2d and Pbx1a activate expression of p15(Ink4a) and E-cadherin, dependent on the Meis2d transcriptional activation domain. In HepG2 cells, reducing expression of endogenous Meis2 or Pbx1 decreases p15 gene expression and increases the number of cells entering S phase. Although DNA binding by all three proteins contributes to full cooperative activation, the sequence requirements for binding by Meis2 and Pbx1 are variable. In the E-cadherin promoter, a Pbx-like site is required for full activation, whereas in the p15 promoter, the Klf4 site appears to play the major role. Through a bioinformatics search we identified additional genes with conserved binding sites for Klf4, Meis2, and Pbx1 and show that at least some of these genes can be activated cooperatively by Klf4 and Meis2/Pbx1. We suggest a model in which genes with Klf4 sites can be cooperatively activated by Meis2/Pbx1 and Klf4, dependent primarily on recruitment by Klf4. This provides a mechanism to modulate transcriptional regulation by the multifunctional Klf4 transcription factor.

Contrasting 5' and 3' evolutionary histories and frequent evolutionary convergence in Meis/hth gene structures

Organisms show striking differences in genome structure; however, the functional implications and fundamental forces that govern these differences remain obscure. The intron–exon organization of nuclear genes is involved in a particularly large variety of structures and functional roles. We performed a 22-species study of Meis/hth genes, intron-rich homeodomain-containing transcription factors involved in a wide range of developmental processes. Our study revealed three surprising results that suggest important and very different functions for Meis intron–exon structures. First, we find unexpected conservation across species of intron positions and lengths along most of the Meis locus. This contrasts with the high degree of structural divergence found in genome-wide studies and may attest to conserved regulatory elements residing within these conserved introns. Second, we find very different evolutionary histories for the 5′ and 3′ regions of the gene. The 5′-most 10 exons, which encode the highly conserved Meis domain and homeodomain, show striking conservation. By contrast, the 3′ of the gene, which encodes several domains implicated in transcriptional activation and response to cell signaling, shows a remarkably active evolutionary history, with diverse isoforms and frequent creation and loss of new exons and splice sites. This region-specific diversity suggests evolutionary “tinkering,” with alternative splicing allowing for more subtle regulation of protein function. Third, we find a large number of cases of convergent evolution in the 3′ region, including 1) parallel losses of ancestral coding sequence, 2) parallel gains of external and internal splice sites, and 3) recurrent truncation of C-terminal coding regions. These results attest to the importance of locus-specific splicing functions in differences in structural evolution across genes, as well as to commonalities of forces shaping the evolution of individual genes along different lineages.

Meis gene expression patterns in the developing chicken inner ear

We are interested in stable gene network activities operating sequentially during inner ear specification. The implementation of this patterning process is a key event in the generation of functional subdivisions of the otic vesicle during early embryonic development. The vertebrate inner ear is a complex sensory structure that is a good model system for characterization of developmental mechanisms controlling patterning and specification. Meis genes, belonging to the TALE family, encode homodomain-containing transcription factors remarkably conserved during evolution, which play a role in normal and neoplastic development. To gain understanding of the possible role of homeobox Meis genes in the developing chick inner ear, we comprehensively analyzed their spatiotemporal expression patterns from early otic specification stages onwards. In the invaginating otic placode, Meis1/2 transcripts were observed in the borders of the otic cup, being absent in the portion of otic epithelium closest to the hindbrain. As development proceeds, Meis1 and Meis2 expressions became restricted to the dorsomedial otic epithelium. Both genes were strongly expressed in the entire presumptive domain of the semicircular canals, and more weakly in all associated cristae. The endolymphatic apparatus was labeled in part by Meis1/2. Meis1 was also expressed in the lateral wall of the growing cochlear duct, while Meis2 expression was detected in a few cells of the developing acoustic-vestibular ganglion. Our results suggest a possible role of Meis assigning regional identity in the morphogenesis, patterning, and specification of the developing inner ear.

An autoinhibitory effect of the homothorax domain of Meis2

Myeloid ecotropic insertion site (Meis)2 is a homeodomain protein containing a conserved homothorax (Hth) domain that is present in all Meis and Prep family proteins and in the Drosophila Hth protein. The Hth domain mediates interaction with Pbx homeodomain proteins, allowing for efficient DNA binding. Here we show that, like Meis1, Meis2 has a strong C-terminal transcriptional activation domain, which is required for full activation of transcription by homeodomain protein complexes composed of Meis2 and Pbx1. We also show that the activity of the activation domain is inhibited by the Hth domain, and that this autoinhibition can be partially relieved by the interaction of Pbx1 with the Hth domain of Meis2. Targeting of the Hth domain to DNA suggests that it is not a portable trans-acting repression domain. However, the Hth domain can inhibit a linked activation domain, and this inhibition is not limited to the Meis2 activation domain. Database searching reveals that the Meis3.2 splice variant, which is found in several vertebrate species, disrupts the Hth domain by removing 17 codons from the 5'-end of exon 6. We show that the equivalent deletion in Meis2 derepresses the C-terminal activation domain and weakens interaction with Pbx1. This work suggests that the transcriptional activity of all members of the Meis/Prep Hth protein family is subject to autoinhibition by their Hth domains, and that the Meis3.2 splice variant encodes a protein that bypasses this autoinhibitory effect.

Fgf8b-containing spliceforms, but not Fgf8a, are essential for Fgf8 function during development of the midbrain and cerebellum

The single Fgf8 gene in mice produces eight protein isoforms (Fgf8a-h) with different N-termini by alternative splicing. Gain-of-function studies have demonstrated that Fgf8a and Fgf8b have distinct activities in the developing midbrain and hindbrain (MHB) due to their different binding affinities with FGF receptors. Here we have performed loss-of-function analyses to determine the in vivo requirement for these two Fgf8 spliceforms during MHB development. We showed that deletion of Fgf8b-containing spliceforms (b, d, f and h) leads to loss of multiple key regulatory genes, including Fgf8 itself, in the MHB region. Therefore, specific inactivation of Fgf8b-containing spliceforms, similar to the loss of Fgf8, in MHB progenitors results in deletion of the midbrain, isthmus, and cerebellum. We also created a splice-site mutation abolishing Fgf8a-containing spliceforms (a, c, e, and g). Mice lacking Fgf8a-containing spliceforms exhibit growth retardation and postnatal lethality, and the phenotype is variable in different genetic backgrounds, suggesting that the Fgf8a-containing spliceforms may play a role in modulating the activity of Fgf8. Surprisingly, no discernable defect was detected in the midbrain and cerebellum of Fgf8a-deficient mice. To determine if Fgf17, which is expressed in the MHB region and possesses similar activities to Fgf8a based on gain-of-function studies, may compensate for the loss of Fgf8a, we generated Fgf17 and Fgf8a double mutant mice. Mice lacking both Fgf8a-containing spliceforms and Fgf17 display the same defect in the posterior midbrain and anterior cerebellum as Fgf17 mutant mice. Therefore, Fgf8b-containing spliceforms, but not Fgf8a, are essential for the function of Fgf8 during the development of the midbrain and cerebellum.

HOX cofactors expression and regulation in the human ovary

BACKGROUND

HOX cofactors enhance HOX binding affinities and specificities and increase HOX's unique functional activities. The expression and the regulation of HOX cofactors in human ovaries are unknown.

METHODS

In this study, the expression of HOX cofactors, PBX1, PBX2, and MEIS1/2, were examined by using RT-PCR, immunofluorescence in cultured immortalized human granulosa (SVOG) cells. The distribution of these HOX cofactors in human ovaries was examined by immunohistochemistry. The effects of growth differentiation factor-9 (GDF-9) and follicle-stimulating hormone (FSH) on PBX2 in SVOG cells were investigated by western blot analysis. Binding activities of HOXA7 and PBX2 to the specific sequences in granulosa cells were determined by electrophoretic mobility shift assay (EMSA).

RESULTS AND CONCLUSION

In SVOG cells, PBX1, PBX2 and MEIS1/2 were expressed during cell culture. In normal human ovaries, PBX1 and MEIS1/2 were expressed in granulosa cells at essentially all stages of follicular development. These cofactors were expressed in the nuclei of the granulosa cells from the primordial to the secondary follicles, whereas beyond multilayered follicles was observed in the cytoplasm. The co-expression of PBX1 and MEIS1/2 in granulosa cells in normal human ovaries suggested that MEIS1/2 might control PBX1 sublocalization, as seen in other systems. PBX2 was not expressed or weakly expressed in the primordial follicles. From the primary follicles to the preovulatory follicles, PBX2 expression was inconsistent and the expression was found in the granulosa cell nuclei. The PBX2 expression pattern is similar to HOXA7 expression in ovarian follicular development. Furthermore, FSH down-regulated, GDF-9 did not change PBX2 expression, but co-treatment of the granulosa cells with FSH and GDF-9 up-regulated PBX2 expression. These results implicated a role for PBX2 expression in the steroidogenic activities of granulosa cells in humans. Moreover, PBX2 and HOXA7 bound together to the Pbx sequence, but not to the EMX2 promoter sequence, in SVOG cells. Our findings indicate that HOX cofactors expression in normal human ovary is temporally and spatially specific and regulated by FSH and GDF-9 in granulosa cells. HOX proteins may use different HOX cofactors, depending on DNA sequences that are specific to the granulosa cells.

Novel homeobox genes are differentially expressed in placental microvascular endothelial cells compared with macrovascular cells

Angiogenesis is fundamental to normal placental development and aberrant angiogenesis contributes substantially to placental pathologies. The complex process of angiogenesis is regulated by transcription factors leading to the formation of endothelial cells that line the microvasculature. Homeobox genes are important transcription factors that regulate vascular development in embryonic and adult tissues. We have recently shown that placental homeobox genes HLX, DLX3, DLX4, MSX2 and GAX are expressed in placental endothelial cells. Hence, the novel homeobox genes TLX1, TLX2, TGIF, HEX, PHOX1, MEIS2, HOXB7, and LIM6 were detected that have not been reported in endothelial cells previously. Importantly, these homeobox genes have not been previously reported in placental endothelial cells and, with the exception of HEX, PHOX1 and HOXB7, have not been described in any other endothelial cell type. Reverse transcriptase PCR was performed on cDNA from freshly isolated placental microvascular endothelial cells (PLEC), and the human placental microvascular endothelial cell line HPEC. cDNAs prepared from control term placentae, human microvascular endothelial cells (HMVEC) and human umbilical vein macrovascular endothelial cells (HUVEC) were used as controls. PCR analyses showed that all novel homeobox genes tested were expressed by all endothelial cells types. Furthermore, real-time PCR analyses revealed that homeobox genes TLX1, TLX2 and PHOX1 relative mRNA expression levels were significantly decreased in HUVEC compared with microvascular endothelial cells, while the relative mRNA expression levels of MEIS2 and TGIF were significantly increased in macrovascular cells compared with microvascular endothelial cells. Thus we have identified novel homeobox genes in microvascular endothelial cells and have shown that homeobox genes are differentially expressed between micro- and macrovascular endothelial cells.

Reciprocal endoderm-mesoderm interactions mediated by fgf24 and fgf10 govern pancreas development

In amniotes, the pancreatic mesenchyme plays a crucial role in pancreatic epithelium growth, notably through the secretion of fibroblast growth factors. However, the factors involved in the formation of the pancreatic mesenchyme are still largely unknown. In this study, we characterize, in zebrafish embryos, the pancreatic lateral plate mesoderm, which is located adjacent to the ventral pancreatic bud and is essential for its specification and growth. We firstly show that the endoderm, by expressing the fgf24 gene at early stages, triggers the patterning of the pancreatic lateral plate mesoderm. Based on the expression of isl1, fgf10 and meis genes, this tissue is analogous to the murine pancreatic mesenchyme. Secondly, Fgf10 acts redundantly with Fgf24 in the pancreatic lateral plate mesoderm and they are both required to specify the ventral pancreas. Our results unveil sequential signaling between the endoderm and mesoderm that is critical for the specification and growth of the ventral pancreas, and explain why the zebrafish ventral pancreatic bud generates the whole exocrine tissue.

The proneural determinant MASH1 regulates forebrain Dlx1/2expression through the I12b intergenic enhancer

Establishment of neuronal networks is an extremely complex process involving the interaction of a diversity of neuronal cells. During mammalian development, these highly organized networks are formed through the differentiation of multipotent neuronal progenitors into multiple neuronal cell lineages. In the developing forebrain of mammals, the combined function of the Dlx1, Dlx2, Dlx5 and Dlx6 homeobox genes is necessary for the differentiation of the GABAergic interneurons born in the ventricular and subventricular zones of the ventral telencephalon, as well as for the migration of these neurons to the hippocampus, cerebral cortex and olfactory bulbs. The 437 bp I12b enhancer sequence in the intergenic region of the Dlx1/2 bigene cluster is involved in the forebrain regulation of Dlx1/2. Using DNase I footprinting, we identified six regions of I12b potentially bound by transcription factors. Mutagenesis of each binding site affected the expression of reporter constructs in transgenic mice. However,the effects of impairing protein-DNA interactions were not uniform across the forebrain Dlx1/2 expression domains, suggesting that distinct regulatory interactions are taking place in the different populations of neuronal precursors. Analyses of protein-DNA interactions provide evidence of a direct role for MASH1 in Dlx1/2 regulation in the forebrain. DLX proteins play a crucial role in the maintenance of their own expression, as shown by transgenic and co-transfection experiments. These studies suggest that the seemingly continuous domains of Dlx gene expression in the telencephalon and diencephalon are in fact the combination of distinct cell populations within which different genetic regulatory interactions take place.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

Regulation of multiple dopamine signal transduction molecules by retinoids in the developing striatum

Increasing evidence based on pharmacological and genetic studies suggests that retinoid signaling plays an important role in developmental control of striatal neurons. In the present report, we screened for genes that might be regulated by retinoids in the developing striatum. We cultured tissue explants from the lateral ganglionic eminence (striatal primordium), and for regional comparison, its adjacent structures of the cerebral cortex and the medial ganglionic eminence in embryonic day 15 rat telencephalon. Using the ribonuclease protection assay, we found that both all-trans retinoic acid and 9-cis retinoic acid significantly up-regulated dopamine D1 receptor, heterotrimeric G protein olfactory, adenylyl cyclase type V and dopamine- and cyclic adenosine 3':5'-monophosphate-regulated phosphoprotein mRNAs in the lateral ganglionic eminence culture. By contrast, neither all-trans retinoic acid nor 9-cis retinoic acid significantly altered D1 receptor, heterotrimeric G protein olfactory, adenylyl cyclase type V and dopamine- and cyclic adenosine 3':5'-monophosphate-regulated phosphoprotein mRNAs in the cortical and the medial ganglionic eminence cultures except that D1 receptor mRNA was dramatically induced in the medial ganglionic eminence by retinoic acid treatments. To test whether the induction of multiple dopamine signaling molecules in the lateral ganglionic eminence was due to a general enhancement of neuronal differentiation by retinoic acid, we assayed the effects of retinoic acid on other differentiation markers, including glutamate decarboxylase 65, NR1 subunit of glutamate NMDA receptor and microtubule-associated protein-2. None of these genes were significantly altered by retinoic acid treatments in the lateral ganglionic eminence culture, indicating the specificity of gene regulation by retinoic acid signaling. As D1 receptor, heterotrimeric G protein olfactory, adenylyl cyclase type V and dopamine- and cyclic adenosine 3':5'-monophosphate-regulated phosphoprotein are important molecules involved in propagation of striatal dopamine neurotransmission, our study raises the hypothesis that retinoid signaling may coordinately activate the transcriptional program that is associated with the dopamine signaling pathway in developing striatal neurons. Such coordinate regulation by retinoids may be part of the mechanisms by which the complex yet highly organized neurochemical constituents of the striatum are established during development.

MEIS homeobox genes in neuroblastoma

The common pediatric tumor neuroblastoma originates from primitive neural crest-derived precursor cells of the peripheral nervous system. Neuroblastoma especially affects very young children, and can already be present at birth. Its early onset and cellular origin predict the involvement of developmental control genes in neuroblastoma etiology. These genes are indispensable for the tight regulation of normal embryonic development but as a consequence cause cancer and congenital diseases upon mutation or aberrant expression. To date however, the connotation of these genes in neuroblastoma pathogenesis is scant. This review recapitulates data on the MEIS homeobox control genes in cancer and focuses on neuroblastoma.

Proximodistal identity during vertebrate limb regeneration is regulated by Meis homeodomain proteins

The mechanisms by which cells obtain instructions to precisely re-create the missing parts of an organ remain an unresolved question in regenerative biology. Urodele limb regeneration is a powerful model in which to study these mechanisms. Following limb amputation, blastema cells interpret the proximal-most positional identity in the stump to reproduce missing parts faithfully. Classical experiments showed the ability of retinoic acid (RA) to proximalize blastema positional values. Meis homeobox genes are involved in RA-dependent specification of proximal cell identity during limb development. To understand the molecular basis for specifying proximal positional identities during regeneration, we isolated the axolotl Meis homeobox family. Axolotl Meis genes are RA-regulated during both regeneration and embryonic limb development. During limb regeneration, Meis overexpression relocates distal blastema cells to more proximal locations, whereas Meis knockdown inhibits RA proximalization of limb blastemas. Meis genes are thus crucial targets of RA proximalizing activity on blastema cells.

Range of HOX/TALE superclass associations and protein domain requirements for HOXA13:MEIS interaction

AbdB-like HOX proteins form DNA-binding complexes with the TALE superclass proteins MEIS1A and MEIS1B, and trimeric complexes have been identified in nuclear extracts that include a second TALE protein, PBX. Thus, soluble DNA-independent protein-protein complexes exist in mammals. The extent of HOX/TALE superclass interactions, protein structural requirements, and sites of in vivo cooperative interaction have not been fully explored. We show that Hoxa13 and Hoxd13 expression does not overlap with that of Meis1-3 in the developing limb; however, coexpression occurs in the developing male and female reproductive tracts (FRTs). We demonstrate that both HOXA13 and HOXD13 associate with MEIS1B in mammalian and yeast cells, and that HOXA13 can interact with all MEIS proteins but not more diverged TALE superclass members. In addition, the C-terminal domains (CTDs) of MEIS1A (18 amino acids) and MEIS1B (93 amino acids) are necessary for HOXA13 interaction; for MEIS1B, this domain was also sufficient. We also show by yeast two-hybrid assay that MEIS proteins can interact with anterior HOX proteins, but for some, additional N-terminal MEIS sequences are required for interaction. Using deletion mutants of HOXA13 and HOXD13, we provide evidence for multiple HOX peptide domains interacting with MEIS proteins. These data suggest that HOX:MEIS interactions may extend to non-AbdB-like HOX proteins in solution and that differences may exist in the MEIS peptide domains utilized by different HOX groups. Finally, the capability of multiple HOX domains to interact with MEIS C-terminal sequences implies greater complexity of the HOX:MEIS protein-protein interactions and a larger role for variation of HOX amino-terminal sequences in specificity of function.

MEIS C Termini Harbor Transcriptional Activation Domains That Respond to Cell Signaling

MEIS proteins form heteromeric DNA-binding complexes with PBX monomers and PBX.HOX heterodimers. We have shown previously that transcriptional activation by PBX.HOX is augmented by either protein kinase A (PKA) or the histone deacetylase inhibitor trichostatin A (TSA). To examine the contribution of MEIS proteins to this response, we used the chromatin immunoprecipitation assay to show that MEIS1 in addition to PBX1, HOXA1, and HOXB1 was recruited to a known PBX.HOX target, the Hoxb1 autoregulatory element following Hoxb1 transcriptional activation in P19 cells. Subsequent to TSA treatment, MEIS1 recruitment lagged behind that of HOX and PBX partners. MEIS1A also enhanced the transcriptional activation of a reporter construct bearing the Hoxb1 autoregulatory element after treatment with TSA. The MEIS1 homeodomain and protein-protein interaction with PBX contributed to this activity. We further mapped TSA-responsive and CREB-binding protein-dependent PKA-responsive transactivation domains to the MEIS1A and MEIS1B C termini. Fine mutation of the 56-residue MEIS1A C terminus revealed four discrete regions required for transcriptional activation function. All of the mutations impairing the response to TSA likewise reduced activation by PKA, implying a common mechanistic basis. C-terminal deletion of MEIS1 impaired transactivation without disrupting DNA binding or complex formation with HOX and PBX. Despite sequence similarity to MEIS and a shared ability to form heteromeric complexes with PBX and HOX partners, the PREP1 C terminus does not respond to TSA or PKA. Thus, MEIS C termini possess transcriptional regulatory domains that respond to cell signaling and confer functional differences between MEIS and PREP proteins.

Retinoic acid signalling specifies intermediate character in the developing telencephalon

The organisation of the telencephalon into its major structures depends on its early regionalisation along the dorsoventral axis. Previous studies have provided evidence that sonic hedgehog (SHH) is required for the generation of telencephalic cells of ventral character, and that sequential WNT and fibroblast growth factor (FGF) signalling specifies cells of dorsal telencephalic character. However, the signalling mechanisms that specify telencephalic cells of an intermediate character remain to be defined. We provide evidence here that retinoic acid has a crucial role in specifying telencephalic progenitor cells of intermediate character.

Pre-B cell leukemia transcription factor (PBX) proteins are important mediators for retinoic acid-dependent endodermal and neuronal differentiation of mouse embryonal carcinoma P19 cells

Pre-B cell leukemia transcription factors (PBXs) act as cofactors in the transcriptional regulation mediated by Homeobox proteins during embryonic development and cellular differentition. PBX1 protein is expressed throughout murine embryonic development, and its deletion in mice disrupts chondrogenesis. PBX protein levels are also increased in mouse embryonal carcinoma P19 cells during retinoic acid (RA)-induced differentiation. To elucidate the role of PBX proteins in this process, we stably overexpressed PBX1b antisense mRNA in P19 cells (PBX1b-AS cells). PBX1b-AS cells did not differentiate to neuronal or endodermal cells following treatment with RA suggesting PBX proteins are required for both processes. Furthermore we demonstrated that PBX proteins regulate the RA-dependent induction in the mRNA levels of bone morphogenetic protein 4 (BMP4) and Decorin (DCN) in P19 cells using both PBX1b-AS cells and PBX1 small interfering RNA. Chromatin immunoprecipitation assays further demonstrated that PBX proteins directly bind to the promoter of Bmp4 and Dcn in vivo in a RA-dependent fashion. In addition, type I and type II BMP receptor mRNA levels were also increased in P19 cells following RA treatment; however, this was PBX-independent. Taken together these data demonstrate that PBX proteins are required for RA-induced differentiation of P19 cells and that PBX proteins regulate the expression of BMP4 and DCN during this differentiation process.

Retinoic acid regulates the expression of PBX1, PBX2, and PBX3 in P19 cells both transcriptionally and post-translationally

Pre-B cell leukemia transcription factors (PBXs) are important co-factors for the transcriptional regulation mediated by a number of Hox proteins during embryonic development. It was previously shown that the expression of several Pbx genes is elevated in mouse embryo limb buds and embryonal carcinoma P19 cells upon retinoic acid (RA) treatment although the mechanism of this induction is not well understood. In this report, we demonstrate that PBX1a, PBX1b, PBX2, and PBX3 mRNAs and PBX1/2/3 proteins are induced during endodermal and neuronal differentiation of P19 cells in a RAR-dependent subtype-unspecific manner following RA treatment. The increases in both PBX1 mRNA and PBX3 mRNA levels are secondary responses to RA treatment requiring new proteins synthesis while the increase in PBX2 mRNA is a primary response. The RA-dependent increases in PBX1 mRNA, PBX2 mRNA, and PBX3 mRNA levels are likely to be transcriptionally regulated since the stability of these mRNAs does not change. In addition, the half-lives of PBX1/2/3 proteins are significantly extended by RA treatment. Two possible mechanisms could contribute to the stabilization of PBX proteins: PBX proteins associate with RA-dependent increased levels of MEIS proteins, and RA may decrease the proteasome dependent degradation of PBX proteins.