Analysis of Msx1; Msx2 double mutants reveals multiple roles for Msx genes in limb development

The homeobox-containing genes Msx1 and Msx2 are highly expressed in the limb field from the earliest stages of limb formation and, subsequently, in both the apical ectodermal ridge and underlying mesenchyme. However, mice homozygous for a null mutation in either Msx1 or Msx2 do not display abnormalities in limb development. By contrast, Msx1; Msx2 double mutants exhibit a severe limb phenotype. Our analysis indicates that these genes play a role in crucial processes during limb morphogenesis along all three axes. Double mutant limbs are shorter and lack anterior skeletal elements (radius/tibia, thumb/hallux). Gene expression analysis confirms that there is no formation of regions with anterior identity. This correlates with the absence of dorsoventral boundary specification in the anterior ectoderm, which precludes apical ectodermal ridge formation anteriorly. As a result, anterior mesenchyme is not maintained, leading to oligodactyly. Paradoxically, polydactyly is also frequent and appears to be associated with extended Fgf activity in the apical ectodermal ridge, which is maintained up to 14.5 dpc. This results in a major outgrowth of the mesenchyme anteriorly, which nevertheless maintains a posterior identity, and leads to formation of extra digits. These defects are interpreted in the context of an impairment of Bmp signalling.     

Conditional deletion of Msx homeobox genes in the uterus inhibits blastocyst implantation by altering uterine receptivity

An effective bidirectional communication between an implantation-competent blastocyst and the receptive uterus is a prerequisite for mammalian reproduction. The blastocyst will implant only when this molecular cross-talk is established. Here we show that the muscle segment homeobox gene (Msh) family members Msx1 and Msx2, which are two highly conserved genes critical for epithelial-mesenchymal interactions during development, also play crucial roles in embryo implantation. Loss of Msx1/Msx2 expression correlates with altered uterine luminal epithelial cell polarity and affects E-cadherin/β-catenin complex formation through the control of Wnt5a expression. Application of Wnt5a in vitro compromised blastocyst invasion and trophoblast outgrowth on cultured uterine epithelial cells. The finding that Msx1/Msx2 genes are critical for conferring uterine receptivity and readiness to implantation could have clinical significance, because compromised uterine receptivity is a major cause of pregnancy failure in IVF programs.     

Ectopic expression of cyclin D3 corrects differentiation of DM1 myoblasts through activation of RNA CUG-binding protein, CUGBP1

Differentiation of myocytes is impaired in patients with myotonic dystrophy type 1, DM1. CUG repeat binding protein, CUGBP1, is a key regulator of translation of proteins that are involved in muscle development and differentiation. In this paper, we present evidence that RNA-binding activity of CUGBP1 and its interactions with initiation translation complex eIF2 are differentially regulated during myogenesis by specific phosphorylation and that this regulation is altered in DM1. In normal myoblasts, Akt kinase phosphorylates CUGBP1 at Ser28 and increases interactions of CUGBP1 with cyclin D1 mRNA. During differentiation, CUGBP1 is phosphorylated by cyclinD3-cdk4/6 at Ser302, which increases CUGBP1 binding with p21 and C/EBPbeta mRNAs. While cyclin D3 and cdk4 are elevated in normal myotubes; DM1 differentiating cells do not increase these proteins. In normal myotubes, CUGBP1 interacts with cyclin D3/cdk4/6 and eIF2; however, interactions of CUGBP1 with eIF2 are reduced in DM1 differentiating cells and correlate with impaired muscle differentiation in DM1. Ectopic expression of cyclin D3 in DM1 cells increases the CUGBP1-eIF2 complex, corrects expression of differentiation markers, myogenin and desmin, and enhances fusion of DM1 myoblasts. Thus, normalization of cyclin D3 might be a therapeutic approach to correct differentiation of skeletal muscle in DM1 patients.     

Up-regulation of cyclin D1 by HBx is mediated by NF-kappaB2/BCL3 complex through kappaB site of cyclin D1 promoter

Cyclin D1 is frequently overexpressed in hepatocellular carcinoma (HCC) exhibiting increased malignant phenotypes. It has also been known that the hepatitis Bx (HBx) protein is strongly associated with HCC development and progression. Although overexpression of both proteins is related to HCC, the relationship between the two has not been well studied. Here we show that HBx up-regulates cyclin D1 and that this process is mediated by the NF-kappaB2(p52)/BCL-3 complex. Our experiments indicate that HBx up-regulates BCL-3 in the mRNA level, which subsequently results in the up-regulation of the NF-kappaB2(p52)/BCL-3 complex in the nucleus. Moreover, impaired HBx-mediated BCL-3 up-regulation by small interfering RNA for BCL-3 reduced HBx-mediated cyclin D1 up-regulation. Down-regulation of the HBx protein level by p53 also reduced HBx-mediated cyclin D1 up-regulation. From these results, we conclude that the up-regulation of cyclin D1 by HBx is mediated by the up-regulation of NF-kappaB2(p52)/BCL-3 in the nucleus. This HBx-mediated-cyclin D1 up-regulation might play an important role in the HBx-mediated HCC development and progression.     

A cluster of Drosophila homeobox genes involved in mesoderm differentiation programs

Although genes involved in common developmental programs are usually scattered throughout the metazoan genome, there are some important examples of functionally interconnected regulatory genes that display close physical linkage. In particular the homeotic genes, which determine the identities of body parts, are clustered in the Hox complexes and clustering is thought to be crucial for the proper execution of their developmental programs. Here we describe the organization and functional properties of a more recently identified cluster of six homeobox genes at 93DE on the third chromosome of Drosophila. These genes, which include tinman, bagpipe, ladybird early, ladybird late, C15, and slouch, all participate in mesodermal patterning and differentiation programs and show multiple regulatory interactions among each other. We propose that their clustering, through unknown mechanisms, is functionally significant and discuss the similarities and differences between the 93DE homeobox gene cluster and the Hox complexes.     

Hsc70 regulates accumulation of cyclin D1 and cyclin D1-dependent protein kinase

The cyclin D-dependent kinase is a critical mediator of mitogen-dependent G1 phase progression in mammalian cells. Given the high incidence of cyclin D1 overexpression in human neoplasias, the nature and complexity of cyclin D complexes in vivo have been subjects of intense interest. Besides its catalytic partner, the nature and complexity of cyclin D complexes in vivo remain ambiguous. To address this issue, we purified native cyclin D1 complexes from proliferating mouse fibroblasts by affinity chromatography and began to identify and functionally characterize the associated proteins. In this report, we describe the identification of Hsc70 and its functional importance for cyclin D1 and cyclin D1-dependent kinase maturation. We demonstrate that Hsc70 associates with newly synthesized cyclin D1 and is a component of a mature, catalytically active cyclin D1/CDK4 holoenzyme complex. Our data suggest that Hsc70 promotes stabilization of newly synthesized cyclin D1, thereby increasing its availability for assembly with CDK4. In addition, our data demonstrate that Hsc70 remains bound to cyclin D1 following its assembly with CDK4 and Cip/Kip proteins, where it ensures the formation of a catalytically active complex.     

Three knotted1-like homeobox genes in Arabidopsis

Five arabidopsis kn1-like homeobox genes were cloned through low-stringency screening of Arabidopsis cDNA libraries with the kn1 homeobox from maize. These five genes were named KNAT1-5 (for kn1-like Arabidopsis thaliana). An analysis of KNAT1 and 2 has been presented previously [19]. Here we present an analysis of the genes KNAT3, 4 and 5. On the basis of sequence and expression patterns, these three genes belong to the class II subfamily of kn1-like homeobox genes [16]. Low-stringency Southern analysis suggests several additional members of the class II genes exist in the Arabidopsis genome. The predicted amino acid sequences of the three genes share extensive homology outside of the homeodomain, including 84% between KNAT3 and KNAT4. Northern analysis shows that although all three genes are expressed in all tissues examined, the level of KNAT3 RNA is highest in young siliques, inflorescences and roots, KNAT4 RNA level is strongest in leaves and young siliques, and KNAT5 RNA level is highest in roots. The specificity of these patterns was confirmed by RNA fingerprint analysis. KNAT3 and 4 are light-regulated as they show reduced expression in etiolated seedlings and also in hy3, cop1 and det1 mutant backgrounds.     

The homeobox gene Msx in development and transdifferentiation of jellyfish striated muscle

Bilaterian Msx homeobox genes are generally expressed in areas of cell proliferation and in association with multipotent progenitor cells. Likewise, jellyfish Msx is expressed in progenitor cells of the developing entocodon, a cell layer giving rise to the striated and smooth muscles of the medusa. However, in contrast to the bilaterian homologs, Msx gene expression is maintained at high levels in the differentiated striated muscle of the medusa in vivo and in vitro. This tissue exhibits reprogramming competence. Upon induction, the Msx gene is immediately switched off in the isolated striated muscle undergoing transdifferentiation, to be upregulated again in the emerging smooth muscle cells which, in a stem cell like manner, undergo quantal cell divisions producing two cell types, a proliferating smooth muscle cell and a differentiating nerve cell. This study indicates that the Msx protein may be a key component of the reprogramming machinery responsible for the extraordinary transdifferentation and regeneration potential of striated muscle in the hydrozoan jellyfish.