Chondrocytes and osteoblasts are two primary cell types in the skeletal system that are differentiated from common mesenchymal progenitors. It is believed that osteoblast differentiation is controlled by distinct mechanisms in intramembranous and endochondral ossification. We have found that ectopic canonical Wnt signaling leads to enhanced ossification and suppression of chondrocyte formation. Conversely, genetic inactivation of beta-catenin, an essential component transducing the canonical Wnt signaling, causes ectopic formation of chondrocytes at the expense of osteoblast differentiation during both intramembranous and endochondral ossification. Moreover, inactivation of beta-catenin in mesenchymal progenitor cells in vitro causes chondrocyte differentiation under conditions allowing only osteoblasts to form. Our results demonstrate that beta-catenin is essential in determining whether mesenchymal progenitors will become osteoblasts or chondrocytes regardless of regional locations or ossification mechanisms. Controlling Wnt/beta-catenin signaling is a common molecular mechanism underlying chondrocyte and osteoblast differentiation and specification of intramembranous and endochondral ossification.

A critical step in skeletal morphogenesis is the formation of synovial joints, which define the relative size of discrete skeletal elements and are required for the mobility of vertebrates. We have found that several Wnt genes, including Wnt4, Wnt14, and Wnt16, were expressed in overlapping and complementary patterns in the developing synovial joints, where ␤-catenin protein levels and transcription activity were up-regulated. Removal of ␤-catenin early in mesenchymal progenitor cells promoted chondrocyte differentiation and blocked the activity of Wnt14 in joint formation. Ectopic expression of an activated form of ␤-catenin or Wnt14 in early differentiating chondrocytes induced ectopic joint formation both morphologically and molecularly. In contrast, genetic removal of ␤-catenin in chondrocytes led to joint fusion. These results demonstrate that the Wnt/␤-catenin signaling pathway is necessary and sufficient to induce early steps of synovial joint formation. Wnt4, Wnt14, and Wnt16 may play redundant roles in synovial joint induction by signaling through the ␤-catenin-mediated canonical Wnt pathway.[Keywords: Wnt; ␤-catenin; joint formation; skeletal development] Supplemental material is available at http://www.genesdev.org. Formation of synovial joints between different skeletal elements is essential for the mobility of vertebrates. The number and position of joints also determine characteristic skeletal patterns in each vertebrate species by defining the size and shape of skeletal elements. As alterations of early patterning signals often lead to changes in the position and number of joints in the developing limb (Dahn and Fallon 2000;Suzuki et al. 2004), understanding the regulation of joint formation in the limb will also provide critical insights into how early-limb patterning is linked to later skeletal morphogenesis at the molecular level.In the developing limb, studies of descriptive embryology have shown that skeletal elements form through temporally and spatially regulated processes that include mesenchymal condensation, elongation, branching, and/ or segmentation (Shubin and Alberch 1986). Most of the synovial joints in the limb form through segmentation of a pre-existing cartilage rod. For instance, in the developing forelimb, the initial de novo mesenchymal condensation forms the cartilage anlagen of the humerus, the growth and branching of which then produce a Y-shaped bifurcation. It is the segmentation of this Y-shaped cartilage primordium that forms the elbow joint that separates the radius and ulna from the humerus (Shubin and Alberch 1986).Synovial joint formation starts from the differentiation of newly differentiated chondrocytes into flattened and densely packed interzone cells (for review, see Archer et al. 2003), which express joint-specific markers such as Gdf5 and lose the expression of chondrocytespecific markers such as ColII (Craig et al. 1987;Nalin et al. 1995;Morrison et al. 1996;Storm and Kingsley 1996). Later in development, the interzone cells differentiate and form three laye...

Zygomorphic flowers, with bilateral (dorsoventral) symmetry, are considered to have evolved several times independently in flowering plants. In Antirrhinum majus, floral dorsoventral symmetry depends on the activity of two TCP-box genes, CYCLOIDEA (CYC) and DICHOTOMA (DICH). To examine whether the same molecular mechanism of floral asymmetry operates in the distantly related Rosid clade of eudicots, in which asymmetric flowers are thought to have evolved independently, we investigated the function of a CYC homologue LjCYC2 in a papilionoid legume, Lotus japonicus. We showed a role for LjCYC2 in establishing dorsal identity by altering its expression in transgenic plants and analyzing its mutant allele squared standard 1 (squ1). Furthermore, we identified a lateralizing factor, Keeled wings in Lotus 1 (Kew1), which plays a key role in the control of lateral petal identity, and found LjCYC2 interacted with Kew1, resulting in a double mutant that bore all petals with ventralized identity to some extents. Thus, we demonstrate that CYC homologues have been independently recruited as determinants of petal identities along the dorsoventral axis in two distant lineages of flowering plants, suggesting a common molecular origin for the mechanisms controlling floral zygomorphy.dorsoventral axis ͉ floral development ͉ keeled wings in Lotus ͉ LjCYC2 ͉ squared standard

A hallmark of aged mesenchymal stem/progenitor cells (MSCs) in bone marrow is the pivot of differentiation potency from osteoblast to adipocyte coupled with a decrease in self-renewal capacity. However, how these cellular events are orchestrated in the aging progress is not fully understood. In this study, we have used molecular and genetic approaches to investigate the role of forkhead box P1 (FOXP1) in transcriptional control of MSC senescence. In bone marrow MSCs, FOXP1 expression levels declined with age in an inverse manner with those of the senescence marker p16INK4A. Conditional depletion of Foxp1 in bone marrow MSCs led to premature aging characteristics, including increased bone marrow adiposity, decreased bone mass, and impaired MSC self-renewal capacity in mice. At the molecular level, FOXP1 regulated cell-fate choice of MSCs through interactions with the CEBPβ/δ complex and recombination signal binding protein for immunoglobulin κ J region (RBPjκ), key modulators of adipogenesis and osteogenesis, respectively. Loss of p16INK4A in Foxp1-deficient MSCs partially rescued the defects in replication capacity and bone mass accrual. Promoter occupancy analyses revealed that FOXP1 directly represses transcription of p16INK4A. These results indicate that FOXP1 attenuates MSC senescence by orchestrating their cell-fate switch while maintaining their replicative capacity in a dose- and age-dependent manner.

MiR‐140 is a microRNA specially involved in chondrogenesis and osteoarthritis pathogenesis. However, its transcriptional regulation and target genes in cartilage development are not fully understood. Here we detected that miR‐140 was uniquely expressed in chondrocyte and suppressed by Wnt/β‐catenin signalling. The miR‐140 primary transcript was an intron‐retained RNA co‐expressed with Wwp2‐C isoform, which was directly induced by Sox9 through binding to the intron 10 of Wwp2 gene. Knockdown of miR‐140 in limb bud micromass cultures resulted in arrest of chondrogenic proliferation. Sp1, the activator of the cell cycle regulator p15INK4b, was identified as a target of miR‐140 in maintaining the chondrocyte proliferation. Collectively, our findings expand our understanding of the transcriptional regulation and the chondrogenic role of miR‐140 in chondrogenesis.

Triadin is an intrinsic membrane protein first identified in the skeletal muscle junctional sarcoplasmic reticulum and is considered to play an important role in excitation-contraction coupling. Using polyclonal antibodies to skeletal muscle triadin, we have identified and characterized three isoforms in rabbit cardiac muscle. The cDNAs encoding these three isoforms of triadin have been isolated by reverse transcription-polymerase chain reaction and cDNA library screening. The deduced amino acid sequences show that these proteins are identical in their N-terminal sequences, whereas the C-terminal sequences are distinct from each other and from that of skeletal muscle triadin. Based upon both the amino acid sequences and biochemical analysis, all three triadin isoforms share similar membrane topology with skeletal muscle triadin. Immunofluorescence staining of rabbit cardiac muscle with antibodies purified from the homologous region of triadin shows that cardiac triadin is primarily confined to the I-band region of cardiac myocytes, where the junctional and corbular sarcoplasmic reticulum is located. Furthermore, we demonstrate that the conserved region of the luminal domain of triadin is able to bind both the ryanodine receptor and calsequestrin in cardiac muscle. These results suggest that triadin colocalizes with and binds to the ryanodine receptor and calsequestrin and carries out a function in the lumen of the junctional sarcoplasmic reticulum that is important for both skeletal and cardiac muscle excitation-contraction coupling.

Osteoblast induction and differentiation in developing long bones is dynamically controlled by the opposing action of transcriptional activators and repressors. In contrast to the long list of activators that have been discovered over past decades, the network of repressors is not well-defined. Here we identify the expression of Foxp1/2/4 proteins, comprised of Forkhead-box (Fox) transcription factors of the Foxp subfamily, in both perichondrial skeletal progenitors and proliferating chondrocytes during endochondral ossification. Mice carrying loss-of-function and gain-of-function Foxp mutations had gross defects in appendicular skeleton formation. At the cellular level, over-expression of Foxp1/2/4 in chondroctyes abrogated osteoblast formation and chondrocyte hypertrophy. Conversely, single or compound deficiency of Foxp1/2/4 in skeletal progenitors or chondrocytes resulted in premature osteoblast differentiation in the perichondrium, coupled with impaired proliferation, survival, and hypertrophy of chondrocytes in the growth plate. Foxp1/2/4 and Runx2 proteins interacted in vitro and in vivo, and Foxp1/2/4 repressed Runx2 transactivation function in heterologous cells. This study establishes Foxp1/2/4 proteins as coordinators of osteogenesis and chondrocyte hypertrophy in developing long bones and suggests that a novel transcriptional repressor network involving Foxp1/2/4 may regulate Runx2 during endochondral ossification.

Dauricine, a bioactive component of Asiatic Moonseed Rhizome, has been widely used to treat a large number of inflammatory diseases in traditional Chinese medicine. In our study, we demonstrated that dauricine inhibited colon cancer cell proliferation and invasion, and induced apoptosis by suppressing nuclear factor-kappaB (NF-kappaB) activation in a dose- and time-dependent manner. Addition of dauricine inhibited the phosphorylation and degradation of IkappaBalpha, and the phosphorylation and translocation of p65. Moreover, dauricine down-regulated the expression of various NF-kappaB-regulated genes, including genes involved cell proliferation (cyclinD1, COX2, and c-Myc), anti-apoptosis (survivin, Bcl-2, XIAP, and IAP1), invasion (MMP-9 and ICAM-1), and angiogenesis (VEGF). In athymic nu/nu mouse model, we further demonstrated that dauricine significantly suppressed colonic tumor growth. Taken together, our results demonstrated that dauricine inhibited colon cancer cell proliferation, invasion, and induced cell apoptosis by suppressing NF-kappaB activity and the expression profile of its downstream genes. These findings provide evidence for a novel role of dauricine in preventing or treating colon cancer through modulation of NF-kappaB singling pathway.