Wnt/beta-catenin signaling is sufficient and necessary for synovial joint formation

Comparison of Iroquois gene expression in limbs/fins of vertebrate embryos

In Drosophila, Iroquois (Irx) genes have various functions including the specification of the identity of wing veins. Vertebrate Iroquois (Irx) genes have been reported to be expressed in the developing digits of mouse limbs. Here we carry out a phylogenetic analysis of vertebrate Irx genes and compare expression in developing limbs of mouse, chick and human embryos and in zebrafish pectoral fin buds. We confirm that the six Irx gene families in vertebrates are well defined and that Clusters A and B are duplicates; in contrast, Irx1 and 3, Irx2 and 5, and Irx4 and 6 are paralogs. All Irx genes in mouse and chick are expressed in developing limbs. Detailed comparison of the expression patterns in mouse and chick shows that expression patterns of genes in the same cluster are generally similar but paralogous genes have different expression patterns. Mouse and chick Irx1 are expressed in digit condensations, whereas mouse and chick Irx6 are expressed interdigitally. The timing of Irx1 expression in individual digits in mouse and chick is different. Irx1 is also expressed in digit condensations in developing human limbs, thus showing conservation of expression of this gene in higher vertebrates. In zebrafish, Irx genes of all but six of the families are expressed in early stage pectoral fin buds but not at later stages, suggesting that these genes are not involved in patterning distal structures in zebrafish fins.

Hox11 paralogous genes are required for formation of wrist and ankle joints and articular surface organization

Limb skeletal elements are connected by distinct synovial joints, but the mechanisms regulating joint formation, diversity, and organization remain unclear. Previous studies showed that Hox11 mouse mutants have severe developmental defects in radius and ulna and tibia and fibula, but wrist and ankle joint formation and characteristics were not examined in detail. We now find that E11.5 and E12.5 triple Hox11aaccdd mutants exhibit a significant reduction in prospective carpal and tarsal mesenchyme. Although the mesenchyme became segmented into individual carpal and tarsal skeletal elements with further development, the elements were ill defined and the more proximal elements (radiale, ulnare, talus, and calcaneous) actually underwent involution and/or fusion. Wild-type carpal and tarsal elements displayed a thick articulating superficial zone at their outer perimeter that expressed genes typical of developing joint interzones and articulating cells, including Gdf5, Erg, Gli3, collagen IIA, and lubricin, and defined each element anatomically. In mutant wrists and ankles, the superficial zone around each element was thin and ill defined, and expression of several of those genes was low and often interrupted. These and other data provide novel and clear evidence that Hox11 paralogous genes regulate wrist and ankle joint organization and are essential for establishing carpal and tarsal element boundary and maintaining their articulating surface tissue.

Conditional ablation of the heparan sulfate-synthesizing enzyme Ext1 leads to dysregulation of bone morphogenic protein signaling and severe skeletal defects

Increasing evidence indicates that heparan sulfate (HS) is an integral component of many morphogen signaling pathways. However, its mechanisms of action appear to be diverse, depending on the type of morphogen and the developmental contexts. To define the function of HS in skeletal development, we conditionally ablated Ext1, which encodes an essential glycosyltransferase for HS synthesis, in limb bud mesenchyme using the Prx1-Cre transgene. These conditional Ext1 mutant mice display severe limb skeletal defects, including shortened and malformed limb bones, oligodactyly, and fusion of joints. In developing limb buds of mutant mice, chondrogenic differentiation of mesenchymal condensations is delayed and impaired, whereas the area of differentiation is diffusely expanded. Correspondingly, the distribution of both bone morphogenic protein (BMP) signaling domains and BMP2 immunoreactivity in the mutant limb mesenchyme is broadened and diffuse. In micromass cultures, chondrogenic differentiation of mutant chondrocytes is delayed, and the responsiveness to exogenous BMPs is attenuated. Moreover, the segregation of the pSmad1/5/8-expressing chondrocytes and fibronectin-expressing perichondrium-like cells surrounding chondrocyte nodules is disrupted in mutant micromass cultures. Together, our results show that HS is essential for patterning of limb skeletal elements and that BMP signaling is one of the major targets for the regulatory role of HS in this developmental context.

Versican facilitates chondrocyte differentiation and regulates joint morphogenesis

Versican/PG-M is a large chondroitin sulfate proteoglycan in the extracellular matrix, which is transiently expressed in mesenchymal condensation areas during tissue morphogenesis. Here, we generated versican conditional knock-out mice Prx1-Cre/Vcan(flox/flox), in which Vcan is pruned out by site-specific Cre recombinase driven by the Prx1 promoter. Although Prx1-Cre/Vcan(flox/flox) mice are viable and fertile, they develop distorted digits. Histological analysis of newborn mice reveals hypertrophic chondrocytic nodules in cartilage, tilting of the joint, and a slight delay of chondrocyte differentiation in digits. By immunostaining, whereas the joint interzone of Prx1-Cre/Vcan(+/+) shows an accumulation of TGF-beta, concomitant with versican, that of Prx1-Cre/Vcan(flox/flox) without versican expression exhibits a decreased incorporation of TGF-beta. In a micromass culture system of mesenchymal cells from limb bud, whereas TGF-beta and versican are co-localized in the perinodular regions of developing cartilage in Prx1-Cre/Vcan(+/+), TGF-beta is widely distributed in Prx1-Cre/Vcan(flox/flox). These results suggest that versican facilitates chondrogenesis and joint morphogenesis, by localizing TGF-beta in the extracellular matrix and regulating its signaling.

Identification of SOX9 interaction sites in the genome of chondrocytes

BACKGROUND

Our previous work has provided strong evidence that the transcription factor SOX9 is completely needed for chondrogenic differentiation and cartilage formation acting as a "master switch" in this differentiation. Heterozygous mutations in SOX9 cause campomelic dysplasia, a severe skeletal dysmorphology syndrome in humans characterized by a generalized hypoplasia of endochondral bones. To obtain insights into the logic used by SOX9 to control a network of target genes in chondrocytes, we performed a ChIP-on-chip experiment using SOX9 antibodies.

METHODOLOGY/PRINCIPAL FINDINGS

The ChIP DNA was hybridized to a microarray, which covered 80 genes, many of which are involved in chondrocyte differentiation. Hybridization peaks were detected in a series of cartilage extracellular matrix (ECM) genes including Col2a1, Col11a2, Aggrecan and Cdrap as well as in genes for specific transcription factors and signaling molecules. Our results also showed SOX9 interaction sites in genes that code for proteins that enhance the transcriptional activity of SOX9. Interestingly, a strong SOX9 signal was also observed in genes such as Col1a1 and Osx, whose expression is strongly down regulated in chondrocytes but is high in osteoblasts. In the Col2a1 gene, in addition to an interaction site on a previously identified enhancer in intron 1, another strong interaction site was seen in intron 6. This site is free of nucleosomes specifically in chondrocytes suggesting an important role of this site on Col2a1 transcription regulation by SOX9.

CONCLUSIONS/SIGNIFICANCE

Our results provide a broad understanding of the strategies used by a "master" transcription factor of differentiation in control of the genetic program of chondrocytes.

Synovial joint morphogenesis requires the chondrogenic action of Sox5 and Sox6 in growth plate and articular cartilage

The mechanisms underlying synovial joint development remain poorly understood. Here we use complete and cell-specific gene inactivation to identify the roles of the redundant chondrogenic transcription factors Sox5 and Sox6 in this process. We show that joint development aborts early in complete mutants (Sox5(-/-)6(-/-)). Gdf5 and Wnt9a expression is punctual in articular progenitor cells, but Sox9 downregulation and cell condensation in joint interzones are late. Joint cell differentiation is unsuccessful, regardless of lineage, and cavitation fails. Sox5 and Sox6 restricted expression to chondrocytes in wild-type embryos and continued Erg expression and weak Ihh expression in Sox5(-/-)6(-/-) growth plates suggest that growth plate failure contribute to this Sox5(-/-)6(-/-) joint morphogenesis block. Sox5/6 inactivation in specified joint cells and chondrocytes (Sox5(fl/fl)6(fl/fl)Col2Cre) also results in a joint morphogenesis block, whereas Sox5/6 inactivation in specified joint cells only (Sox5(fl/fl)6(fl/fl)Gdf5Cre) results in milder joint defects and normal growth plates. Sox5(fl/fl)6(fl/fl)Gdf5Cre articular chondrocytes remain undifferentiated, as shown by continued Gdf5 expression and pancartilaginous gene downregulation. Along with Prg4 downregulation, these defects likely account for joint tissue overgrowth and incomplete cavitation in adult mice. Together, these data suggest that synovial joint morphogenesis relies on essential roles for Sox5/6 in promoting both growth plate and articular chondrocyte differentiation.

Wnt inhibitors Dkk1 and Sost are downstream targets of BMP signaling through the type IA receptor (BMPRIA) in osteoblasts

The bone morphogenetic protein (BMP) and Wnt signaling pathways both contribute essential roles in regulating bone mass. However, the molecular interactions between these pathways in osteoblasts are poorly understood. We recently reported that osteoblast-targeted conditional knockout (cKO) of BMP receptor type IA (BMPRIA) resulted in increased bone mass during embryonic development, where diminished expression of Sost as a downstream effector of BMPRIA resulted in increased Wnt/beta-catenin signaling. Here, we report that Bmpr1a cKO mice exhibit increased bone mass during weanling stages, again with evidence of enhanced Wnt/beta-catenin signaling as assessed by Wnt reporter TOPGAL mice and TOPFLASH luciferase. Consistent with negative regulation of the Wnt pathway by BMPRIA signaling, treatment of osteoblasts with dorsomorphin, an inhibitor of Smad-dependent BMP signaling, enhanced Wnt signaling. In addition to Sost, Wnt inhibitor Dkk1 also was downregulated in cKO bone. Expression levels of Dkk1and Sost were upregulated by BMP2 treatment and downregulated by Noggin. Moreover, expression of a constitutively active Bmpr1a transgene in mice resulted in the upregulation of both Dkk1 and Sost and partially rescued the Bmpr1a cKO bone phenotype. These effectors are differentially regulated by mitogen-activated protein kinase (MAPK) p38 because pretreatment of osteoblasts with SB202190 blocked BMP2-induced Dkk1 expression but not Sost. These results demonstrate that BMPRIA in osteoblasts negatively regulates endogenous bone mass and Wnt/beta-catenin signaling and that this regulation may be mediated by the activities of Sost and Dkk1. This study highlights several interactions between BMP and Wnt signaling cascades in osteoblasts that may be amenable to therapeutic intervention for the modification of bone mass density.

Mapping of Wnt, frizzled, and Wnt inhibitor gene expression domains in the avian otic primordium

Wnt signaling activates at least three different pathways involved in development and disease. Interactions of secreted ligands and inhibitors with cell-surface receptors result in the activation or regulation of particular downstream intracellular cascades. During the developmental stages of otic vesicle closure and beginning morphogenesis, the forming inner ear transcribes a plethora of Wnt-related genes. We report expression of 23 genes out of 25 tested in situ hybridization probes on tissue serial sections. Sensory primordia and Frizzled gene expression share domains, with Fzd1 being a continuous marker. Prospective nonsensory domains express Wnts, whose transcripts mainly flank prosensory regions. Finally, Wnt inhibitor domains are superimposed over both prosensory and nonsensory otic regions. Three Wnt antagonists, Dkk1, SFRP2, and Frzb are prominent. Their gene expression patterns partly overlap and change over time, which adds to the diversity of molecular microenvironments. Strikingly, prosensory domains express Wnts transiently. This includes: 1) the prosensory otic region of high proliferation, neuroblast delamination, and programmed cell death at stage 20/21 (Wnt3, -5b, -7b, -8b, -9a, and -11); and 2) sensory primordia at stage 25 (Wnt7a and Wnt9a). In summary, robust Wnt-related gene expression shows both spatial and temporal tuning during inner ear development as the otic vesicle initiates morphogenesis and prosensory cell fate determination.

Wnt/beta-catenin and retinoic acid receptor signaling pathways interact to regulate chondrocyte function and matrix turnover

Activation of the Wnt/beta-catenin and retinoid signaling pathways is known to tilt cartilage matrix homeostasis toward catabolism. Here, we investigated possible interactions between these pathways. We found that all-trans-retinoic acid (RA) treatment of mouse epiphyseal chondrocytes in culture did increase Wnt/beta-catenin signaling in the absence or presence of exogenous Wnt3a, as revealed by lymphoid enhancer factor/T-cell factor/beta-catenin reporter activity and beta-catenin nuclear accumulation. This stimulation was accompanied by increased gene expression of Wnt proteins and receptors and was inhibited by co-treatment with Dickkopf-related protein-1, an extracellular inhibitor of Wnt/beta-catenin signaling, suggesting that RA modulates Wnt signaling at Wnt cell surface receptor level. RA also enhanced matrix loss triggered by Wnt/beta-catenin signaling, whereas treatment with a retinoid antagonist reduced it. Interestingly, overexpression of retinoic acid receptor gamma (RARgamma) strongly inhibited Wnt/beta-catenin signaling in retinoid-free cultures, whereas small interfering RNA-mediated silencing of endogenous RARgamma expression strongly increased it. Small interfering RNA-mediated silencing of RARalpha or RARbeta had minimal effects. Co-immunoprecipitation and two-hybrid assays indicated that RARgamma interacts with beta-catenin and induces dissociation of beta-catenin from lymphoid enhancer factor in retinoid-free cultures. The N-terminal domain (AF-1) of RARgamma but not the C-terminal domain (AF-2) was required for association with beta-catenin, whereas both AF-1 and AF-2 were necessary for inhibition of beta-catenin transcriptional activity. Taken together, our data indicate that the Wnt and retinoid signaling pathways do interact in chondrocytes, and their cross-talks and cross-regulation play important roles in the regulation of cartilage matrix homeostasis.

Vertebrate skeletogenesis

Vertebrate skeletogenesis consists in elaborating an edifice of more than 200 pieces of bone and cartilage. Each skeletal piece is crafted at a distinct location in the body, is articulated with others, and reaches a specific size, shape, and tissue composition according to both species instructions and individual determinants. This complex, customized body frame fulfills multiple essential tasks. It confers morphological features, allows controlled postures and movements, protects vital organs, houses hematopoiesis, stores minerals, and adsorbs toxins. This review provides an overview of the multiple facets of this ingenious process for experts as well as nonexperts of skeletogenesis. We explain how the developing vertebrate uses both specific and ubiquitously expressed genes to generate multipotent mesenchymal cells, specify them to a skeletogenic fate, control their survival and proliferation, and direct their differentiation into cartilage, bone, and joint cells. We review milestone discoveries made toward uncovering the intricate networks of regulatory factors that are involved in these processes, with an emphasis on signaling pathways and transcription factors. We describe numerous skeletal malformation and degeneration diseases that occur in humans as a result of mutations in regulatory genes, and explain how these diseases both help and motivate us to further decipher skeletogenic processes. Upon discussing current knowledge and gaps in knowledge in the control of skeletogenesis, we highlight ultimate research goals and propose research priorities and approaches for future endeavors.

Wnt signalling and cancer stem cells

Intracellular signalling mediated by secreted Wnt proteins is essential for the establishment of cell fates and proper tissue patterning during embryo development and for the regulation of tissue homeostasis and stem cell function in adult tissues. Aberrant activation of Wnt signalling pathways has been directly linked to the genesis of different tumours. Here, the components and molecular mechanisms implicated in the transduction of Wnt signal, along with important results supporting a central role for this signalling pathway in stem cell function regulation and carcinogenesis will be briefl y reviewed.

Promising bone-related therapeutic targets for rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic, debilitating autoimmune disease that results in inflammation and structural destruction of the joints. A hallmark of RA pathogenesis is an imbalance of the osteoblast-osteoclast axis driven by inflammatory processes, resulting in elevated bone resorption by osteoclasts. Current therapies used to treat this disease have focused on inhibition of synovitis, but such treatments do not adequately repair damaged bone. A key pathway of osteoclast formation involves the receptor activator of nuclear factor kappaB ligand (RANKL) pathway acting on myeloid progenitor cells. The Wnt pathway has been shown to be important for the differentiation of osteoblasts from mesenchymal lineage precursors, and endogenous Wnt inhibitors, such as Dickkopf1 and sclerostin, might have important roles in osteoclast dysregulation in RA. Inhibition of the RANKL pathway, or blockade of Dickkopf1 and sclerostin, might serve to restore the osteoblast-osteoclast balance and repair bone erosion in RA joints. Such treatments, in combination with anti-inflammatory therapies, could stabilize and repair damaged joints and have the potential to be valuable additions to the armory of RA treatments.

SoxE factors as multifunctional neural crest regulatory factors

Neural crest cells are the primary innovation that led to evolution of the vertebrates, and transcription factors of the SoxE family (Sox8, Sox9 and Sox10) are among the central players regulating the development of these cells. In all vertebrates examined to date, one or more SoxE proteins are required for the formation of neural crest cells, the maintenance of their multipotency, and their survival. Later, SoxE proteins drive the formation of multiple neural crest derivatives including chondrocytes, melanocytes, and cells of the peripheral nervous system, particularly Schwann cells/peripheral glia. Given their multiple diverse roles in the development of the neural crest, it is important to understand how the activity of SoxE factors is controlled such that they direct the correct developmental outcome. While combinatorial control with other regulatory factors is clearly one mechanism for generating such functional versatility, modulation of SoxE activity, both by SoxD family factors and by post-translational modification, also appears to be important. Elucidating the mechanisms that control SoxE function is essential to understand the evolutionary origin of the vertebrates, as well as a host of SoxE-linked syndromes and diseases, and may prove crucial for developing stem cell based therapies that target SoxE-regulated cell types.

Canonical Wnt signaling activity during synovial joint development

Wnt signaling plays important roles in skeletal development. However, the activation and function of canonical Wnt signaling in joint development remains unclear. We analyzed the lineage identity and developmental changes of the Wnt-responsive cells during synovial joint formation as well as adulthood in the Wnt signaling reporter TOPgal transgenic mice. At embryonic day (E) 12.5, we found that the TOPgal was inactivated in the presumptive joint forming interzone, but it was intensively activated in the cartilage anlage of developing long bones and digits. At E14.5, the TOPgal activity was found in a subgroup of the articular chondrocyte lineage cells, which were co-immunolabeled with Doublecortin intensively and with Vinculin weakly. At E18.5, the TOPgal/Doublecortin co-immunolabeled cells were found in the superficial layer of the developing articular cartilage. During postnatal development, the TOPgal(+) articular chondrocytes were abundant at P7 and decreased from P10. A small number of TOPgal(+) articular chondrocytes were also found in adult joints. Our study suggests an age- and lineage-specific role of canonical Wnt signaling in joint development and maintenance.

Control of BMP gene expression by long-range regulatory elements

Much evidence suggests that "developmental regulator" genes, like those encoding transcription factors and signaling molecules, are typically controlled by many modular, tissue-specific cis-regulatory elements that function during embryogenesis. These elements are often far from gene coding regions and promoters. Bone morphogenetic proteins (BMPs) drive many processes in development relating to organogenesis and differentiation. Four BMP family members, Bmp2, Bmp4, Bmp5, and Gdf6, are now known to be under the control of distant cis-regulatory elements. BMPs are thus firmly placed in the category of genes prone to this phenomenon. The analysis of distant BMP regulatory elements has provided insight into the many pleiotropic effects of BMP genes, and underscores the biological importance of non-coding genomic DNA elements.

Temporomandibular joint formation requires two distinct hedgehog-dependent steps

We conducted a genetic analysis of the developing temporo-mandibular or temporomandi-bular joint (TMJ), a highly specialized synovial joint that permits movement and function of the mammalian jaw. First, we used laser capture microdissection to perform a genome-wide expression analysis of each of its developing components. The expression patterns of genes identified in this screen were examined in the TMJ and compared with those of other synovial joints, including the shoulder and the hip joints. Striking differences were noted, indicating that the TMJ forms via a distinct molecular program. Several components of the hedgehog (Hh) signaling pathway are among the genes identified in the screen, including Gli2, which is expressed specifically in the condyle and in the disk of the developing TMJ. We found that mice deficient in Gli2 display aberrant TMJ development such that the condyle loses its growth-plate-like cellular organization and no disk is formed. In addition, we used a conditional strategy to remove Smo, a positive effector of the Hh signaling pathway, from chondrocyte progenitors. This cell autonomous loss of Hh signaling allows for disk formation, but the resulting structure fails to separate from the condyle. Thus, these experiments establish that Hh signaling acts at two distinct steps in disk morphogenesis, condyle initiation, and disk-condyle separation and provide a molecular framework for future studies of the TMJ.

Wif-1 is expressed at cartilage-mesenchyme interfaces and impedes Wnt3a-mediated inhibition of chondrogenesis

Wnt factors are involved in the regulation of all steps of cartilage development. The activity of Wnt factors is generally regulated at the extracellular level by factors like the Dkk family, sFRPs, Cerberus and Wnt inhibitory factor 1 (Wif-1). Here we report that Wif-1 is highly expressed at cartilage-mesenchyme interfaces of the early developing skeleton. In fetal and postnatal skeletal development, Wif-1 is expressed in a sharply restricted zone in the upper hyaline layer of epiphyseal and articular cartilage and in trabecular bone. Coimmunoprecipitation and pull-down assays using recombinant Wif-1 and Wnt factors show specific binding of Wif-1 to Wnt3a, Wnt4, Wnt5a, Wnt7a, Wnt9a and Wnt11. Moreover, Wif-1 was able to block Wnt3a-mediated activation of the canonical Wnt signalling pathway. Consequently, Wif-1 impaired growth of mesenchymal precursor cells and neutralised Wnt3a-mediated inhibition of chondrogenesis in micromass cultures of embryonic chick limb-bud cells. These results identify Wif-1 as a novel extracellular Wnt modulator in cartilage biology.

Chromatin protein HMGB2 regulates articular cartilage surface maintenance via beta-catenin pathway

The superficial zone (SZ) of articular cartilage is critical in maintaining tissue function and homeostasis and represents the site of the earliest changes in osteoarthritis. Mechanisms that regulate the unique phenotype of SZ chondrocytes and maintain SZ integrity are unknown. We recently demonstrated that expression of the chromatin protein high mobility group box (HMGB) protein 2 is restricted to the SZ in articular cartilage suggesting a transcriptional regulation involving HMGB2 in SZ. Here, we show that an interaction between HMGB2 and the Wnt/beta-catenin pathway regulates the maintenance of the SZ. We found that the Wnt/beta-catenin pathway is active specifically in the SZ in normal mouse knee joints and colocalizes with HMGB2. Both Wnt signaling and HMGB2 expression decrease with aging in mouse joints. Our molecular studies show that HMGB2 enhances the binding of Lef-1 to its target sequence and potentiates transcriptional activation of the Lef-1-beta-catenin complex. The HMG domain within HMGB2 is crucial for interaction with Lef-1, suggesting that both HMGB2 and HMGB1 may be involved in this function. Furthermore, conditional deletion of beta-catenin in cultured mouse chondrocytes induced apoptosis. These findings define a pathway where protein interactions of HMGB2 and Lef-1 enhance Wnt signaling and promote SZ chondrocyte survival. Loss of the HMGB2-Wnt signaling interaction is a new mechanism in aging-related cartilage pathology.

Extracellular matrix genes as hypoxia-inducible targets

Low oxygen tension, i.e., hypoxia, is a pathophysiological component involved in many human disorders but is also a critically important phenomenon in normal development and differentiation. The ability of cells to survive under hypoxia or to adapt to it depends on a family of hypoxia-inducible transcription factors (HIFs) that induce the expression of a number of genes involved in hematopoiesis, angiogenesis, iron transport, glucose utilization, resistance to oxidative stress, cell proliferation, survival and apoptosis, and extracellular matrix homeostasis. We introduce here the recently identified molecular mechanisms responsible for the oxygen-dependent stability and activity of HIF, after which we focus on extracellular matrix genes as HIF targets. The vital role of the hypoxia response pathway in chondrogenesis and joint development is then discussed.

beta-Catenin expression in the bone marrow microenvironment is required for long-term maintenance of primitive hematopoietic cells

Hematopoiesis is dependent upon the bone marrow microenvironment, which is comprised of multiple mesenchymal cell types, including fibroblasts, endothelial cells, osteoblasts, and stroma progenitors. The canonical Wnt signaling pathway, which relies on the beta-catenin protein to mediate its signal, is necessary for the normal development of mesenchymal tissue. We hypothesized that canonical Wnt signaling regulates the cellular composition and function of the bone marrow microenvironment. We observed that a beta-catenin-deficient bone marrow microenvironment maintained hematopoietic stem cells but exhibited a decreased capacity to support primitive hematopoietic cells. These results correlated with decreased numbers of osteoblasts and with decreased production of basic fibroblast growth factor, stem cell factor, and vascular cell adhesion molecule-1. From these data, we propose a model in which beta-catenin in the microenvironment is required noncell autonomously for long-term maintenance of hematopoietic progenitors.

Inflammation induction of Dickkopf-1 mediates chondrocyte apoptosis in osteoarthritic joint

OBJECTIVE

Dysregulated Wnt signaling appears to modulate chondrocyte fate and joint disorders. Dickkopf-1 (DKK1) regulates the pathogenesis of skeletal tissue by inhibiting Wnt actions. This study examined whether DKK1 expression is linked to chondrocyte fate in osteoarthritis (OA).

METHOD

Articular cartilage specimens harvested from nine patients with knee OA and from six controls with femoral neck fracture were assessed for DKK1, interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), Bad, Bax, Bcl2 and caspase-3 expression by real time-polymerase chain reaction (RT-PCR) and immunohistochemistry. Apoptotic chondrocytes were detected by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labelling (TUNEL) and 4', 6-dianidino-2-phenylindole dihydrochloride (DAPI) staining. Human chondrocyte cultures were treated with recombinant IL-1beta and monoclonal DKK1 antibody to determine whether DKK1 impairs chondrocyte survival.

RESULTS

Expression of DKK1 correlated with inflammatory cytokine levels (IL-1beta and TNF-alpha expressions), proapoptosis regulators (Bad and caspase-3 expressions) and TUNEL staining in OA cartilage tissues. The IL-1beta induced expressions of DKK1, Bax, Bad and caspase-3-dependent apoptosis of chondrocyte cultures. Neutralization of DKK1 by monoclonal DKK1 antibody significantly abrogated IL-1beta-mediated caspase-3 cleavage and apoptosis and reversed chondrocyte proliferation. Recombinant DKK1 treatment impaired chondrocyte growth and promoted apoptosis. By suppressing nuclear beta-catenin accumulation and Akt phosphorylation, DKK1 mediated IL-1beta promotion of chondrocyte apoptosis.

CONCLUSION

Chondrocyte apoptosis correlates with joint OA. Expression of DKK1 contributes to cartilage deterioration and is a potent factor in OA pathogenesis. Attenuating DKK1 may reduce cartilage deterioration in OA.

The Wnt/beta-catenin pathway interacts differentially with PTHrP signaling to control chondrocyte hypertrophy and final maturation

Sequential proliferation, hypertrophy and maturation of chondrocytes are required for proper endochondral bone development and tightly regulated by cell signaling. The canonical Wnt signaling pathway acts through β-catenin to promote chondrocyte hypertrophy whereas PTHrP signaling inhibits it by holding chondrocytes in proliferating states. Here we show by genetic approaches that chondrocyte hypertrophy and final maturation are two distinct developmental processes that are differentially regulated by Wnt/β-catenin and PTHrP signaling. Wnt/β-catenin signaling regulates initiation of chondrocyte hypertrophy by inhibiting PTHrP signaling activity, but it does not regulate PTHrP expression. In addition, Wnt/β-catenin signaling regulates chondrocyte hypertrophy in a non-cell autonomous manner and Gdf5/Bmp signaling may be one of the downstream pathways. Furthermore, Wnt/β-catenin signaling also controls final maturation of hypertrophic chondrocytes, but such regulation is PTHrP signaling-independent.

Muscle contraction is necessary to maintain joint progenitor cell fate

During embryogenesis, organ development is dependent upon maintaining appropriate progenitor cell commitment. Synovial joints develop from a pool of progenitor cells that differentiate into various cell types constituting the mature joint. The involvement of the musculature in joint formation has long been recognized. However, the mechanism by which the musculature regulates joint formation has remained elusive. In this study, we demonstrate, utilizing various murine models devoid of limb musculature or its contraction, that the contracting musculature is fundamental in maintaining joint progenitors committed to their fate, a requirement for correct joint cavitation and morphogenesis. Furthermore, contraction-dependent activation of beta-catenin, a key modulator of joint formation, provides a molecular mechanism for this regulation. In conclusion, our findings provide the missing link between progenitor cell fate determination and embryonic movement, two processes shown to be essential for correct organogenesis.

Tubulin polymerization-promoting protein family member 3, Tppp3, is a specific marker of the differentiating tendon sheath and synovial joints

Tppp3, a member of the Tubulin polymerization-promoting protein family, is an intrinsically unstructured protein that induces tubulin polymerization. We show that Tppp3 is a distinct marker in the developing musculoskeletal system. In tendons, Tppp3 is expressed in cells at the circumference of the developing tendons, likely the progenitors of connective tissues that surround tendons: the tendon sheath, epitenon, and paratenon. These tissues form an elastic sleeve around tendons and provide lubrication to minimize friction between tendons and surrounding tissues. Tppp3 is the first molecular marker of the tendon sheath, opening the door for direct examination of these tissues. Tppp3 is also expressed in forming synovial joints. The onset of Tppp3 expression in joints coincides with cavitation, representing a molecular marker that can be used to indicate this stage in joint transition in joint differentiation. In late embryonic stages, Tppp3 expression highlights other demarcation lines that surround differentiating tissues in the forelimb.

Involvement of the Wnt signaling pathway in experimental and human osteoarthritis: prominent role of Wnt-induced signaling protein 1

OBJECTIVE

Wnt signaling pathway proteins are involved in embryonic development of cartilage and bone, and, interestingly, developmental processes appear to be recapitulated in osteoarthritic (OA) cartilage. The present study was undertaken to characterize the expression pattern of Wnt and Fz genes during experimental OA and to determine the function of selected genes in experimental and human OA.

METHODS

Longitudinal expression analysis was performed in 2 models of OA. Levels of messenger RNA for genes from the Wnt/beta-catenin pathway were determined in synovium and cartilage, and the results were validated using immunohistochemistry. Effects of selected genes were assessed in vitro using recombinant protein, and in vivo by adenoviral overexpression.

RESULTS

Wnt-induced signaling protein 1 (WISP-1) expression was strongly increased in the synovium and cartilage of mice with experimental OA. Wnt-16 and Wnt-2B were also markedly up-regulated during the course of disease. Interestingly, increased WISP-1 expression was also found in human OA cartilage and synovium. Stimulation of macrophages and chondrocytes with recombinant WISP-1 resulted in interleukin-1-independent induction of several matrix metalloproteinases (MMPs) and aggrecanase. Adenoviral overexpression of WISP-1 in murine knee joints induced MMP and aggrecanase expression and resulted in cartilage damage.

CONCLUSION

This study included a comprehensive characterization of Wnt and Frizzled gene expression in experimental and human OA articular joint tissue. The data demonstrate, for the first time, that WISP-1 expression is a feature of experimental and human OA and that WISP-1 regulates chondrocyte and macrophage MMP and aggrecanase expression and is capable of inducing articular cartilage damage in models of OA.

Wnt signaling and osteoarthritis

Osteoarthritis is a common disease, clinically manifested by joint pain, swelling and progressive loss of function. The severity of disease manifestations can vary but most of the patients only need intermittent symptom relief without major interventions. However, there is a group of patients that shows fast progression of the disease process leading to disability and ultimately joint replacement. Apart from symptom relief, no treatments have been identified that arrest or reverse the disease process. Therefore, there has been increasing attention devoted to the understanding of the mechanisms that are driving the disease process. Among these mechanisms, the biology of the cartilage-subchondral bone unit has been highlighted as key in osteoarthritis, and pathways that involve both cartilage and bone formation and turnover have become prime targets for modulation, and thus therapeutic intervention. Studies in developmental, genetic and joint disease models indicate that Wnt signaling is critically involved in these processes. Consequently, targeting Wnt signaling in a selective and tissue specific manner is an exciting opportunity for the development of disease modifying drugs for osteoarthritis.

Mechanoactive tenogenic differentiation of human mesenchymal stem cells

A mesenchymal stem cell (MSC)-seeded collagen gel under static or dynamic tension is a well-established model to study the potential of MSCs in regenerating a tendon- or ligament-like tissue. Using this model, upregulation of fibrillar collagen mRNA expression and protein production has been demonstrated in response to cyclic tensile mechanical stimulation. However, the mechanisms driving MSC tenogenesis (differentiation into tendon or ligament fibroblasts) have not been elucidated. This study investigated the mechanisms of tenogenesis of human bone marrow-derived MSCs in a dynamic, three-dimensional (3D) tissue-engineering model by investigating the effects of cyclic stretching on matrix production and gene expression of candidate tendon and ligament markers. The 3D MSC tenogenesis culture system upregulated scleraxis, but cyclic stretching was required to maintain expression of this putative tendon marker over time. Enhanced tendinous neo-tissue development demonstrated with extracellular matrix staining was largely due to changes in matrix deposition and remodeling activity under dynamic loading conditions, as evidenced by differential regulation of matrix metalloproteinases at a transcriptional level with minimal changes in collagen mRNA levels. Regulation of Wnt gene expression with cyclic stimulation suggested a similar role for Wnt4 versus Wnt5a in tenogenesis as in cartilage development. This first report of the potential involvement of matrix remodeling and Wnt signaling during tenogenesis of human MSCs in a dynamic, 3D tissue-engineering model provides insights into the mechanisms of tenogenesis in a mechanoactive environment and supports the therapeutic potential of adult stem cells.

Sox9 inhibits Wnt signaling by promoting beta-catenin phosphorylation in the nucleus

Chondrocyte fate determination and maintenance requires Sox9, an intrinsic transcription factor, but is inhibited by Wnt/beta-catenin signaling activated by extrinsic Wnt ligands. Here we explored the underlying molecular mechanism by which Sox9 antagonizes the Wnt/beta-catenin signaling in chondrocyte differentiation. We found that Sox9 employed two distinct mechanisms to inhibit Wnt/beta-catenin signaling: the Sox9 N terminus is necessary and sufficient to promote beta-catenin degradation, whereas the C terminus is required to inhibit beta-catenin transcriptional activity without affecting its stability. Sox9 binds to beta-catenin and components of the beta-catenin "destruction complex," glycogen synthase kinase 3 and beta-transducin repeat containing protein, to promote their nuclear localization. Independent of its DNA binding ability, nuclear localization of Sox9 is both necessary and sufficient to enhance beta-catenin phosphorylation and its subsequent degradation. Thus, one mechanism whereby Sox9 regulates chondrogenesis is to promote efficient beta-catenin phosphorylation in the nucleus. This mechanism may be broadly employed by other intrinsic cell fate determining transcription factors to promptly turn off extrinsic inhibitory Wnt signaling mediated by beta-catenin.

The role of Wnt proteins in arthritis

Wnt proteins regulate organ development, tumorigenesis and bone homeostasis, among other functions. The binding of Wnt proteins to plasma membrane receptors on mesenchymal cells induces the differentiation of these cells into the osteoblast lineage and thereby supports bone formation. Wnts are also key signaling proteins in joint remodeling processes. Active Wnt signaling contributes to osteophyte formation and might have an essential role in the anabolic pattern of joint remodeling that is observed in ankylosing spondylitis and osteoarthritis. By contrast, blockade of Wnt signaling facilitates bone erosion and contributes to catabolic joint remodeling, a process that is observed in rheumatoid arthritis. This Review summarizes current knowledge of the molecular regulation of joint remodeling associated with chronic arthritis, focusing on the role of the Wnt proteins and their inhibitors. It also addresses the role of Wnt in determining the differences in clinical presentation of inflammatory arthropathies and discusses implications for future therapy.

BMP signaling negatively regulates bone mass through sclerostin by inhibiting the canonical Wnt pathway

Bone morphogenetic proteins (BMPs) are known to induce ectopic bone. However, it is largely unknown how BMP signaling in osteoblasts directly regulates endogenous bone. This study investigated the mechanism by which BMP signaling through the type IA receptor (BMPR1A) regulates endogenous bone mass using an inducible Cre-loxP system. When BMPR1A in osteoblasts was conditionally disrupted during embryonic bone development, bone mass surprisingly was increased with upregulation of canonical Wnt signaling. Although levels of bone formation markers were modestly reduced, levels of resorption markers representing osteoclastogenesis were severely reduced, resulting in a net increase in bone mass. The reduction of osteoclastogenesis was primarily caused by Bmpr1a-deficiency in osteoblasts, at least through the RANKL-OPG pathway. Sclerostin (Sost) expression was downregulated by about 90% and SOST protein was undetectable in osteoblasts and osteocytes, whereas the Wnt signaling was upregulated. Treatment of Bmpr1a-deficient calvariae with sclerostin repressed the Wnt signaling and restored normal bone morphology. By gain of Smad-dependent BMPR1A signaling in mice, Sost expression was upregulated and osteoclastogenesis was increased. Finally, the Bmpr1a-deficient bone phenotype was rescued by enhancing BMPR1A signaling, with restoration of osteoclastogenesis. These findings demonstrate that BMPR1A signaling in osteoblasts restrain endogenous bone mass directly by upregulating osteoclastogenesis through the RANKL-OPG pathway, or indirectly by downregulating canonical Wnt signaling through sclerostin, a Wnt inhibitor and a bone mass mediator.

JAWS coordinates chondrogenesis and synovial joint positioning

Properly positioned synovial joints are crucial to coordinated skeletal movement. Despite their importance for skeletal development and function, the molecular mechanisms that underlie joint positioning are not well understood. We show that mice carrying an insertional mutation in a previously uncharacterized gene, which we have named Jaws (joints abnormal with splitting), die perinatally with striking skeletal defects, including ectopic interphalangeal joints. These ectopic joints develop along the longitudinal axis and persist at birth, suggesting that JAWS is uniquely required for the orientation and consequent positioning of interphalangeal joints within the endochondral skeleton. Jaws mutant mice also exhibit severe chondrodysplasia characterized by delayed and disorganized maturation of growth plate chondrocytes, together with impaired chondroitin sulfation and abnormal metabolism of the chondroitin sulfate proteoglycan aggrecan. Our findings identify JAWS as a key regulator of chondrogenesis and synovial joint positioning required for the restriction of joint formation to discrete stereotyped locations in the embryonic skeleton.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

WNT signaling affects gene expression in the ventral diencephalon and pituitary gland growth

We examined the role of WNT signaling in pituitary development by characterizing the pituitary phenotype of three WNT knockout mice and assessing the expression of WNT pathway components. Wnt5a mutants have expanded domains of Fgf10 and bone morphogenetic protein expression in the ventral diencephalon and a reduced domain of LHX3 expression in Rathke's pouch. Wnt4 mutants have mildly reduced cell differentiation, reduced POU1F1 expression, and mild anterior lobe hypoplasia. Wnt4, Wnt5a double mutants exhibit an additive pituitary phenotype of dysmorphology and mild hypoplasia. Wnt6 mutants have no obvious pituitary phenotype. We surveyed WNT expression and identified transcripts for numerous Wnts, Frizzleds, and downstream pathway members in the pituitary and ventral diencephalon. These findings support the emerging model that WNT signaling affects the pituitary gland via effects on ventral diencephalon signaling, and suggest additional Wnt genes that are worthy of functional studies.

Brachy-syndactyly caused by loss of Sfrp2 function

Wnt signaling pathways are regulated both at the intracellular and extracellular levels. During embryogenesis, the in vivo effects of the secreted frizzled-related protein (Sfrp) family of Wnt inhibitors are poorly understood. Here, we show that inactivation of Sfrp2 results in subtle limb defects in mice with mesomelic shortening and consistent shortening of all autopodal elements that is clinically manifested as brachydactyly. In addition, there is soft-tissue syndactyly of the hindlimb. The brachydactyly is caused by decreased chondrocyte proliferation and delayed differentiation in distal limb chondrogenic elements. These data suggest that Sfrp2 can regulate both chondrogenesis and regression of interdigital mesenchyme in distal limb. Sfrp2 can also repress canonical Wnt signaling by Wnt1, Wnt9a, and Wnt4 in vitro. Sfrp2-/- and TOPGAL/Sfrp2-/- mice have a mild increase in beta-catenin and beta-galactosidase staining, respectively, in some phalangeal elements. This however does not exclude a potential concurrent effect on non-canonical Wnt signaling in the growth plate. In combination with what is known about BMP and Wnt signaling in human brachydactylies, our data establish a critical role for Sfrp2 in proper distal limb formation and suggest SFPR2 could be a novel candidate gene for human brachy-syndactyly defects.

Multipotent mesenchymal stromal cells in articular diseases

Although cartilage defects are common features of osteoarthritis and rheumatoid arthritis, current treatments can rarely restore the full function of native cartilage. Recent studies have provided new perspectives for cartilage engineering using multipotent mesenchymal stromal cells (MSC). Moreover, MSC have been used as immunosuppressant agents in autoimmune diseases and have tested successfully in animal models of arthritis. However, the sequential events occurring during chondrogenesis must be fully understood before we can reproduce the complex molecular events that lead to MSC differentiation and long-term maintenance of cartilage characteristics in the context of chronic joint inflammation. This chapter focuses on the potential of MSC to repair cartilage, with an emphasis on the factors that are known to be required in inducing chondrogenesis and on their immunosuppressive potential.

Molecular network of cartilage homeostasis and osteoarthritis

This review article presents the current understanding of the molecular basis of articular cartilaginous homeostasis, and outlines potential areas to focus on within the developing field of therapeutics for cartilage disorders. Articular cartilage, an integral component of joints in extremities and the vertebral column, is essential for locomotion. Disturbance of joint development or cartilage homeostasis causes congenital osteocartilaginous dysplasia or osteoarthritic diseases, respectively. Symptomatic treatments and surgical replacement of joints are effective but can also be problematic in terms of quality of life over time. Recently, new insights into the molecular biological basis of chondrocyte differentiation and cartilage homeostasis have been reported. While joint formation is regulated by several growth factors such as Wnts (wingless-related MMTV integration site) and Gdfs (growth and differentiation factors), the pathology of osteoarthritis is now interpreted as the disruption of balance between anabolic and catabolic signals. Current findings in molecular biology on joint development are reviewed concisely to aid in the understanding of the molecular network that governs articular cartilage development and homeostasis.

Identification of the molecular response of articular cartilage to injury, by microarray screening: Wnt-16 expression and signaling after injury and in osteoarthritis

OBJECTIVE

To characterize the molecular response of adult human articular cartilage to acute mechanical injury.

METHODS

An established ex vivo model was used to compare gene expression of adult human articular cartilage explants 24 hours after mechanical injury with that of uninjured controls by microarray analysis of gene expression. Confirmation for selected genes was obtained by real-time polymerase chain reaction and immunohistochemical analysis. Expression of selected genes was also investigated in preserved and osteoarthritic (OA) cartilage.

RESULTS

Six hundred ninety genes were significantly regulated at least 2-fold following mechanical injury. They included genes previously reported to be differentially expressed in OA versus normal cartilage or having allelic variants genetically linked to OA. Significant functional clusters included genes associated with wound healing, developmental processes, and skeletal development. The transforming growth factor beta, fibroblast growth factor, and Wnt pathways were modulated. A systematic analysis of the Wnt signaling pathway revealed up-regulation of Wnt-16, down-regulation of FRZB, up-regulation of Wnt target genes, and nuclear localization of beta-catenin in injured cartilage. In addition, in OA, Wnt-16 and beta-catenin were barely detectable in preserved cartilage areas, but were dramatically up-regulated in areas of the same joint with moderate to severe OA damage.

CONCLUSION

Our findings indicate that mechanical injury to adult human articular cartilage results in the activation of a signaling response, with reactivation of morphogenetic pathways. Therapeutic targeting of such pathways may improve current protocols of joint surface defect repair and/or prevent the evolution of such lesions into posttraumatic OA.

Wnt signaling and skeletal development

Wnt proteins are a family of secreted proteins that regulate many aspects of cellular functions. The discovery that mutations in low-density lipoprotein receptor-related protein 5, a putative Wnt coreceptor, could positively and negatively affect bone mass in humans generated an enormous amount of interest in the possible role of the Wnt signaling pathway in skeletal biology. Over the last decade, considerable progress has been made in determining the role of the canonical Wnt signaling pathway in various aspects of skeletal development. Furthermore, recent evidence indicates the important role of non-canonical Wnt signaling in skeletal development. In this review we discuss the current understanding of the role of Wnt signaling in chondrogenesis, osteoblastogenesis, and osteoclastogenesis.

Wnt9a secreted from the walls of hepatic sinusoids is essential for morphogenesis, proliferation, and glycogen accumulation of chick hepatic epithelium

Hepatic epithelial morphogenesis, including hepatoblast migration and proliferation in the septum transversum, requires the interaction of hepatic epithelium with the embryonic sinusoidal wall. No factors that mediate this interaction have yet been identified. As the beta-catenin pathway is active in hepatoblast proliferation, then Wnt ligands might activate the canonical Wnt pathway during liver development. Here, we investigated the role of Wnts in mediating epithelial vessel interactions in the developing chick liver. We found that Wnt9a was specifically expressed in both endothelial and stellate cells of the embryonic sinusoidal wall. Induced overexpression of Wnt9a resulted in hepatomegaly with hyperplasia of the hepatocellular cords, and in hyperproliferation of hepatocytes. Knockdown of Wnt9a caused a reduction in liver size, with hypoplasia of hepatocellular cord branching, and hypoproliferation of hepatoblasts, and also inhibited glycogen accumulation at later developmental stages. Wnt9a promoted in vivo stabilization of beta-catenin through binding with Frizzled 4, 7, and 9, and activated TOPflash reporter expression in vitro via Frizzled 7 and 9. Our results demonstrate that Wnt9a from the embryonic sinusoidal wall is required for the proper morphogenesis of chick hepatocellular cords, proliferation of hepatoblasts/hepatocytes, and glycogen accumulation in hepatocytes. Wnt9a signaling appears to be mediated by an Fzd7/9-beta-catenin pathway.

The mutation ROR2W749X, linked to human BDB, is a recessive mutation in the mouse, causing brachydactyly, mediating patterning of joints and modeling recessive Robinow syndrome

Mutations in ROR2 result in a spectrum of genetic disorders in humans that are classified, depending on the nature of the mutation and the clinical phenotype, as either autosomal dominant brachydactyly type B (BDB, MIM 113000) or recessive Robinow syndrome (RRS, MIM 268310). In an attempt to model BDB in mice, the mutation W749X was engineered into the mouse Ror2 gene. In contrast to the human situation, mice heterozygous for Ror2(W749FLAG) are normal and do not develop brachydactyly, whereas homozygous mice exhibit features resembling RRS. Furthermore, both Ror2(W749FLAG/W749FLAG) and a previously engineered mutant, Ror2(TMlacZ/TMlacZ), lack the P2/P3 joint. Absence of Gdf5 expression at the corresponding interzone suggests that the defect is in specification of the joint. As this phenotype is absent in mice lacking the entire Ror2 gene, it appears that specification of the P2/P3 joint is affected by ROR2 activity. Finally, Ror2(W749FLAG/W749FLAG) mice survive to adulthood and exhibit phenotypes (altered body composition, reduced male fertility) not observed in Ror2 knockout mice, presumably due to the perinatal lethality of the latter. Therefore, Ror2(W749FLAG/W749FLAG) mice represent a postnatal model for RRS, provide insight into the mechanism of joint specification, and uncover novel roles of Ror2 in the mouse.

Regulation of hematopoiesis and the hematopoietic stem cell niche by Wnt signaling pathways

Hematopoietic stem cells (HSCs) are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal (in which the ability to function as HSCs is retained) and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. Wnt signaling pathways play a crucial role during embryogenesis and deregulation of these pathways has been implicated in the formation of solid tumors. Wnt signaling also plays a role in the regulation of stem cells from multiple tissues, such as embryonic, epidermal, and intestinal stem cells. However, the function of Wnt signaling in HSC biology is still controversial. In this review, we will discuss the basic characteristics of the adult HSC and its regulatory microenvironment, the "niche", focusing on the regulation of the HSC and its niche by the Wnt signaling pathways.

Hoxd13 binds in vivo and regulates the expression of genes acting in key pathways for early limb and skeletal patterning

5' HoxD genes are required for the correct formation of limb skeletal elements. Hoxd13, the most 5'-located HoxD gene, is important for patterning the most distal limb region, and its mutation causes human limb malformation syndromes. The mechanisms underlying the control of developmental processes by Hoxd13, and by Hox genes in general, are still elusive, due to the limited knowledge on their direct downstream target genes. We identified by ChIP-on-chip 248 known gene loci bound invivo by Hoxd13. Genes relevant to limb patterning and skeletogenesis were further analysed. We found that Hoxd13 binds invivo, in developing limbs, the loci of Hand2, a gene crucial to limb AP axis patterning, of Meis1 and Meis2, involved in PD patterning, of the Sfrp1, Barx1, and Fbn1 genes, involved in skeletogenesis, and of the Dach1, Bmp2, Bmp4, andEmx2 genes. We show that Hoxd13 misexpression in developing chick limbs alters the expression of the majority of these genes, supporting the conclusion that Hoxd13 directly regulates their transcription. Our results indicate that 5' Hox proteins regulate directly both key genes for early limb AP and PD axis patterning and genes involved, at later stages, in skeletal patterning.

Gene expression patterns underlying proximal-distal skeletal segmentation in late-stage zebrafish, Danio rerio

Timing and pattern of expression of ten candidate segmentation genes or gene pairs were reviewed or examined in developing median fins of late-stage zebrafish, Danio rerio. We found a general correspondence in timing and pattern of expression between zebrafish fin radial segmentation and tetrapod joint development, suggesting that molecular mechanisms underlying radial segmentation have been conserved over 400 million years of evolution. Gene co-expression during segmentation (5.5-6.5 mm SL) is similar between tetrapods and zebrafish: bmp2b, bmp4, chordin, and gdf5 in interradial mesenchyme and ZS; bapx1, col2a1, noggin3, and sox9a in chondrocytes. Surprisingly, wnt9a is not expressed in the developing median fins, though wnt9b is detected. In contrast to all other candidate segmentation genes we examined, bapx1 is not expressed in the caudal fin, which does not segment. Together, these data suggest a scenario of gene interactions underlying radial segmentation based on the patterns and timing of candidate gene expression.