The parathyroid hormone/parathyroid hormone-related peptide receptor coordinates endochondral bone development by directly controlling chondrocyte differentiation

The effect of parathyroid hormone and teriparatide on fracture healing

INTRODUCTION

Daily subcutaneous injections of parathyroid hormone (PTH), and its synthetic peptide fragment, teriparatide (PTH 1-34, TPTD), have a net anabolic effect on bone and prevent osteoporotic fractures. TPTD is currently approved for this indication worldwide. Because of the anabolic effect, there is an interest in a role for TPTD (and, where available, human PTH 1-84) in improving bone healing after a fracture. PTH has been studied in animal fracture healing models and in a limited number of human trials. We have reviewed current literature regarding possible mechanisms and efficacy for PTH and TPTD to improve the healing process in the setting of various types of fractures.

AREAS COVERED

Our review focuses first on the role of PTH in normal bone. We then discuss mechanisms of normal bone healing as well as delayed and impaired fracture healing. We summarize pertinent animal data and then review human studies utilizing PTH or TPTD for fracture healing. In particular, we examine unique situations including osteoporotic fractures, diabetes, stress fractures, delayed or poor healing and combination with bisphosphonate therapy.

EXPERT OPINION

Available data indicate there is likely an important role for TPTD and PTH in promoting fracture healing in selected patients, but more clinical trial data are needed.

Recent research on the growth plate: Advances in fibroblast growth factor signaling in growth plate development and disorders

Skeletons are formed through two distinct developmental actions, intramembranous ossification and endochondral ossification. During embryonic development, most bone is formed by endochondral ossification. The growth plate is the developmental center for endochondral ossification. Multiple signaling pathways participate in the regulation of endochondral ossification. Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling has been found to play a vital role in the development and maintenance of growth plates. Missense mutations in FGFs and FGFRs can cause multiple genetic skeletal diseases with disordered endochondral ossification. Clarifying the molecular mechanisms of FGFs/FGFRs signaling in skeletal development and genetic skeletal diseases will have implications for the development of therapies for FGF-signaling-related skeletal dysplasias and growth plate injuries. In this review, we summarize the recent advances in elucidating the role of FGFs/FGFRs signaling in growth plate development, genetic skeletal disorders, and the promising therapies for those genetic skeletal diseases resulting from FGFs/FGFRs dysfunction. Finally, we also examine the potential important research in this field in the future.

Expression of Wnt signaling skeletal development genes in the cartilaginous fish, elephant shark (Callorhinchus milii)

Jawed vertebrates (Gnasthostomes) are broadly separated into cartilaginous fishes (Chondricthyes) and bony vertebrates (Osteichthyes). Cartilaginous fishes are divided into chimaeras (e.g. ratfish, rabbit fish and elephant shark) and elasmobranchs (e.g. sharks, rays and skates). Both cartilaginous fish and bony vertebrates are believed to have a common armoured bony ancestor (Class Placodermi), however cartilaginous fish are believed to have lost bone. This study has identified and investigated genes involved in skeletal development in vertebrates, in the cartilaginous fish, elephant shark (Callorhinchus milii). Ctnnb1 (β-catenin), Sfrp (secreted frizzled protein) and a single Sost or Sostdc1 gene (sclerostin or sclerostin domain-containing protein 1) were identified in the elephant shark genome and found to be expressed in a number of tissues, including cartilage. β-catenin was also localized in several elephant shark tissues. The expression of these genes, which belong to the Wnt/β-catenin pathway, is required for normal bone formation in mammals. These findings in the cartilaginous skeleton of elephant shark support the hypothesis that the common ancestor of cartilaginous fishes and bony vertebrates had the potential for making bone.

Autocrine and Paracrine Actions of IGF-I Signaling in Skeletal Development

Insulin-like growth factor-I (IGF-I) regulates cell growth, survival, and differentiation by acting on the IGF-I receptor, (IGF-IR)-a tyrosine kinase receptor, which elicits diverse intracellular signaling responses. All skeletal cells express IGF-I and IGF-IR. Recent studies using tissue/cell-specific gene knockout mouse models and cell culture techniques have clearly demonstrated that locally produced IGF-I is more critical than the systemic IGF-I in supporting embryonic and postnatal skeletal development and bone remodeling. Local IGF-I/IGF-IR signaling promotes the growth, survival and differentiation of chondrocytes and osteoblasts, directly and indirectly, by altering other autocrine/paracrine signaling pathways in cartilage and bone, and by enhancing interactions among these skeletal cells through hormonal and physical means. Moreover, local IGF-I/IGF-IR signaling is critical for the anabolic bone actions of growth hormone and parathyroid hormone. Herein, we review evidence supporting the actions of local IGF-I/IGF-IR in the above aspects of skeletal development and remodeling.

The extracellular calcium-sensing receptor, CaSR, in fetal development

In fetal mammals, serum levels of both total and ionized calcium significantly exceed those in the adult. This relative fetal hypercalcemia is crucial for skeletal development and is maintained irrespectively of maternal serum calcium levels. Elegant studies by Kovacs and Kronenberg have previously addressed the role of the CaSR in creating and maintaining this relative fetal hypercalcemia, through the regulation of parathyroid hormone-related peptide secretion. More recently we have shown that the CaSR is widely distributed throughout the developing fetus, where the receptor plays major, unexpected roles in ensuring growth and maturation of several organs. In this article, we present evidence for a role of the CaSR in the control of skeletal development, and how fetal hypercalcemia, acting through the CaSR, regulates lung development.

Anabolic effects of IGF-1 signaling on the skeleton

This review focuses on the anabolic effects of IGF-1 signaling on the skeleton, emphasizing the requirement for IGF-1 signaling in normal bone formation and remodeling. We first discuss the genomic context, splicing variants, and species conservation of the IGF-1 locus. The modulation of IGF-1 action by growth hormone (GH) is then reviewed while also discussing the current model which takes into account the GH-independent actions of IGF-1. Next, the skeletal phenotypes of IGF-1-deficient animals are described in both embryonic and postnatal stages of development, which include severe dwarfism and an undermineralized skeleton. We then highlight two mechanisms by which IGF-1 exerts its anabolic action on the skeleton. Firstly, the role of IGF-1 signaling in the modulation of anabolic effects of parathyroid hormone (PTH) on bone will be discussed, presenting in vitro and in vivo studies that establish this concept and the proposed underlying molecular mechanisms involving Indian hedgehog (Ihh) and the ephrins. Secondly, the crosstalk of IGF-1 signaling with mechanosensing pathways will be discussed, beginning with the observation that animals subjected to skeletal unloading by hindlimb elevation are unable to mitigate cessation of bone growth despite infusion with IGF-1 and the failure of IGF-1 to activate its receptor in bone marrow stromal cell cultures from unloaded bone. Disrupted crosstalk between IGF-1 signaling and the integrin mechanotransduction pathways is discussed as one of the potential mechanisms for this IGF-1 resistance. Next, emerging paradigms on bone-muscle crosstalk are examined, focusing on the potential role of IGF-1 signaling in modulating such interactions. Finally, we present a future outlook on IGF research.

Intermittent parathyroid hormone administration converts quiescent lining cells to active osteoblasts

Intermittent administration of parathyroid hormone (PTH) increases bone mass, at least in part, by increasing the number of osteoblasts. One possible source of osteoblasts might be conversion of inactive lining cells to osteoblasts, and indirect evidence is consistent with this hypothesis. To better understand the possible effect of PTH on lining cell activation, a lineage tracing study was conducted using an inducible gene system. Dmp1-CreERt2 mice were crossed with ROSA26R reporter mice to render targeted mature osteoblasts and their descendents, lining cells and osteocytes, detectable by 5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside (X-gal) staining. Dmp1-CreERt2(+):ROSA26R mice were injected with 0.25 mg 4-OH-tamoxifen (4-OHTam) on postnatal days 3, 5, 7, 14, and 21. The animals were euthanized on postnatal day 23, 33, or 43 (2, 12, or 22 days after the last 4-OHTam injection). On day 43, mice were challenged with a subcutaneous injection of human PTH (1-34, 80 µg/kg) or vehicle once daily for 3 days. By 22 days after the last 4-OHTam injection, most X-gal (+) cells on the periosteal surfaces of the calvaria and the tibia were flat. Moreover, bone formation rate and collagen I(α1) mRNA expression were decreased at day 43 compared to day 23. After 3 days of PTH injections, the thickness of X-gal (+) cells increased, as did their expression of osteocalcin and collagen I(α1) mRNA. Electron microscopy revealed X-gal-associated chromogen particles in thin cells prior to PTH administration and in cuboidal cells following PTH administration. These data support the hypothesis that intermittent PTH treatment can increase osteoblast number by converting lining cells to mature osteoblasts in vivo.

Inhibitory function of parathyroid hormone-related protein on chondrocyte hypertrophy: the implication for articular cartilage repair

Cartilage repair tissue is usually accompanied by chondrocyte hypertrophy and osseous overgrowths, and a role for parathyroid hormone-related protein (PTHrP) in inhibiting chondrocytes from hypertrophic differentiation during the process of endochondral ossification has been demonstrated. However, application of PTHrP in cartilage repair has not been extensively considered. This review systemically summarizes for the first time the inhibitory function of PTHrP on chondrocyte hypertrophy in articular cartilage and during the process of endochondral ossification, as well as the process of mesenchymal stem cell chondrogenic differentiation. Based on the literature review, the strategy of using PTHrP for articular cartilage repair is suggested, which is instructive for clinical treatment of cartilage injuries as well as osteoarthritis.

Bioactive factors for tissue regeneration: state of the art

THERE ARE THREE COMPONENTS FOR THE CREATION OF NEW TISSUES: cell sources, scaffolds, and bioactive factors. Unlike conventional medical strategies, regenerative medicine requires not only analytical approaches but also integrative ones. Basic research has identified a number of bioactive factors that are necessary, but not sufficient, for organogenesis. In skeletal development, these factors include bone morphogenetic proteins (BMPs), transforming growth factor β TGF-β, Wnts, hedgehogs (Hh), fibroblast growth factors (FGFs), insulin-like growth factors (IGFs), SRY box-containing gene (Sox) 9, Sp7, and runt-related transcription factors (Runx). Clinical and preclinical studies have been extensively performed to apply the knowledge to bone and cartilage regeneration. Given the large number of findings obtained so far, it would be a good time for a multi-disciplinary, collaborative effort to optimize these known factors and develop appropriate drug delivery systems for delivering them.

Indian Hedgehog signalling triggers Nkx3.2 protein degradation during chondrocyte maturation

The Ihh (Indian Hedgehog) pathway plays an essential role in facilitating chondrocyte hypertrophy and bone formation during skeletal development. Nkx3.2 (NK3 homeobox 2) is initially induced in chondrocyte precursor cells, maintained in early-stage chondrocytes and down-regulated in terminal-stage chondrocytes. Consistent with these expression patterns, Nkx3.2 has been shown to enhance chondrocyte differentiation and cell survival, while inhibiting chondrocyte hypertrophy and apoptosis. Thus, in the present study, we investigated whether Nkx3.2, an early-stage chondrogenic factor, can be regulated by Ihh, a key regulator for chondrocyte hypertrophy. We show that Ihh signalling can induce proteasomal degradation of Nkx3.2. In addition, we found that Ihh can suppress levels of Lrp (low-density-lipoprotein-receptor-related protein) (Wnt co-receptor) and Sfrp (secreted frizzled-related protein) (Wnt antagonist) expression, which, in turn, may selectively enhance Lrp-independent non-canonical Wnt pathways in chondrocytes. In agreement with these findings, Ihh-induced Nkx3.2 degradation requires Wnt5a, which is capable of triggering Nkx3.2 degradation. Finally, we found that Nkx3.2 protein levels in chondrocytes are remarkably elevated in mice defective in Ihh signalling by deletion of either Ihh or smoothened. Thus these results suggest that Ihh/Wnt5a signalling may play a role in negative regulation of Nkx3.2 for appropriate progression of chondrocyte hypertrophy during chondrogenesis.

Gsα enhances commitment of mesenchymal progenitors to the osteoblast lineage but restrains osteoblast differentiation in mice

The heterotrimeric G protein subunit Gsα stimulates cAMP-dependent signaling downstream of G protein-coupled receptors. In this study, we set out to determine the role of Gsα signaling in cells of the early osteoblast lineage in vivo by conditionally deleting Gsα from osterix-expressing cells. This led to severe osteoporosis with fractures at birth, a phenotype that was found to be the consequence of impaired bone formation rather than increased resorption. Osteoblast number was markedly decreased and osteogenic differentiation was accelerated, resulting in the formation of woven bone. Rapid differentiation of mature osteoblasts into matrix-embedded osteocytes likely contributed to depletion of the osteoblast pool. In addition, the number of committed osteoblast progenitors was diminished in both bone marrow stromal cells (BMSCs) and calvarial cells of mutant mice. In the absence of Gsα, expression of sclerostin and dickkopf1 (Dkk1), inhibitors of canonical Wnt signaling, was markedly increased; this was accompanied by reduced Wnt signaling in the osteoblast lineage. In summary, we have shown that Gsα regulates bone formation by at least two distinct mechanisms: facilitating the commitment of mesenchymal progenitors to the osteoblast lineage in association with enhanced Wnt signaling; and restraining the differentiation of committed osteoblasts to enable production of bone of optimal mass, quality, and strength.

IGF-1R signaling in chondrocytes modulates growth plate development by interacting with the PTHrP/Ihh pathway

Systemic derangements and perinatal death of generalized insulin-like growth factor 1 (IGF-1) and IGF-1 receptor (IGF-1R) knockout mice preclude definitive assessment of IGF-1R actions in growth-plate (GP) chondrocytes. We generated cartilage-specific Igf1r knockout ((Cart) Igf1r(-/-)) mice to investigate local control of chondrocyte differentiation in the GP by this receptor. These mice died shortly after birth and showed disorganized chondrocyte columns, delayed ossification and vascular invasion, decreased cell proliferation, increased apoptosis, and increased expression of parathyroid hormone-related protein (Pthrp) RNA and protein in their GPs. The increased Pthrp expression in the knockout GPs likely was due to an increase in gene transcription, as determined by the increased activity of a LacZ reporter that was inserted downstream of the endogenous PTHrP promoter and bred into the knockout mice. To circumvent the early death of (Cart) Igf1r(-/-) mice and investigate the role of IGF-1R during postnatal growth, we made tamoxifen (Tam)-inducible, cartilage-specific Igf1r knockout ((TamCart) Igf1r(-/-)) mice. At 2 weeks of age and 7 to 8 days after Tam injection, the (TamCart) Igf1r(-/-) mice showed growth retardation with a disorganized GP, reduced chondrocyte proliferation, decreased type 2 collagen and Indian Hedgehog (Ihh) expression, but increased expression of PTHrP. Consistent with in vivo observations, in vitro knockout of the Igf1r gene by adenoviral expression of Cre recombinase suppressed cell proliferation, promoted apoptosis, and increased Pthrp expression. Our data indicate that the IGF-1R in chondrocytes controls cell growth, survival, and differentiation in embryonic and postnatal GPs in part by suppression of Pthrp expression.

β2-adrenergic receptors inhibit the expression of collagen type II in growth plate chondrocytes by stimulating the AP-1 factor Jun-B

The sympathetic nervous system can regulate both osteoblast and chondrocyte growth and activity through β(2)-adrenergic receptors (β(2)-AR). We have shown previously that β(2)-AR activate both adenylyl cyclase and mitogen-activated protein kinases ERK1/2 in growth plate chondrocytes prepared from ribs of embryonic E18.5 mice. Here we examined β(2)-AR inhibition of collagen type II (Col II) expression in growth plate chondrocytes and the molecular pathways involved. Stimulation of β(2)-AR by isoproterenol inhibited Col II mRNA and protein levels by ∼50% beginning at 2 h, with both remaining suppressed over 24 h. This inhibition was blocked by propranolol and inhibitors of either MEK1 or PKA. Isoproterenol stimulated an AP-1-luciferase reporter and increased the expression of AP-1 factors c-Fos, Fra-1, Fra-2, c-Jun, and Jun-B but had no effect on Jun-D. Stimulation of AP-1 activity was blocked by inhibitors of MEK1 or PKA. siRNA inhibition of AP-1 factors showed that depletion of only Jun-B attenuated isoproterenol-mediated inhibition of Col II. Transfection with jun-B or c-fos showed selective inhibition of Col II mRNA and a Col II luciferase reporter construct by jun-B. Isoproterenol as well as jun-B overexpression in the chondrocytes also inhibited the expression of Sox-6 mRNA and protein, and depletion of Jun-B abrogated β(2)-AR inhibition of Sox-6. Collectively, these findings demonstrate regulation of chondrocyte differentiation through β(2)-AR mediated by ERK1/2 and PKA stimulation of the AP-1 factor Jun-B that inhibits the expression of Sox-6 and Col II.

The zinc transporter SLC39A14/ZIP14 controls G-protein coupled receptor-mediated signaling required for systemic growth

Aberrant zinc (Zn) homeostasis is associated with abnormal control of mammalian growth, although the molecular mechanisms of Zn's roles in regulating systemic growth remain to be clarified. Here we report that the cell membrane-localized Zn transporter SLC39A14 controls G-protein coupled receptor (GPCR)-mediated signaling. Mice lacking Slc39a14 (Slc39a14-KO mice) exhibit growth retardation and impaired gluconeogenesis, which are attributable to disrupted GPCR signaling in the growth plate, pituitary gland, and liver. The decreased signaling is a consequence of the reduced basal level of cyclic adenosine monophosphate (cAMP) caused by increased phosphodiesterase (PDE) activity in Slc39a14-KO cells. We conclude that SLC39A14 facilitates GPCR-mediated cAMP-CREB signaling by suppressing the basal PDE activity, and that this is one mechanism for Zn's involvement in systemic growth processes. Our data highlight SLC39A14 as an important novel player in GPCR-mediated signaling. In addition, the Slc39a14-KO mice may be useful for studying the GPCR-associated regulation of mammalian systemic growth.

Molecular basis of parathyroid hormone receptor signaling and trafficking: a family B GPCR paradigm

The parathyroid hormone (PTH) receptor type 1 (PTHR), a G protein-coupled receptor (GPCR), transmits signals to two hormone systems-PTH, endocrine and homeostatic, and PTH-related peptide (PTHrP), paracrine-to regulate different biological processes. PTHR responds to these hormonal stimuli by activating heterotrimeric G proteins, such as G(S) that stimulates cAMP production. It was thought that the PTHR, as for all other GPCRs, is only active and signals through G proteins on the cell membrane, and internalizes into a cell to be desensitized and eventually degraded or recycled. Recent studies with cultured cell and animal models reveal a new pathway that involves sustained cAMP signaling from intracellular domains. Not only do these studies challenge the paradigm that cAMP production triggered by activated GPCRs originates exclusively at the cell membrane but they also advance a comprehensive model to account for the functional differences between PTH and PTHrP acting through the same receptor.

TGF-beta promotion of Gli2-induced expression of parathyroid hormone-related protein, an important osteolytic factor in bone metastasis, is independent of canonical Hedgehog signaling

Breast cancer frequently metastasizes to bone, in which tumor cells receive signals from the bone marrow microenvironment. One relevant factor is TGF-β, which upregulates expression of the Hedgehog (Hh) signaling molecule, Gli2, which in turn increases secretion of important osteolytic factors such as parathyroid hormone-related protein (PTHrP). PTHrP inhibition can prevent tumor-induced bone destruction, whereas Gli2 overexpression in tumor cells can promote osteolysis. In this study, we tested the hypothesis that Hh inhibition in bone metastatic breast cancer would decrease PTHrP expression and therefore osteolytic bone destruction. However, when mice engrafted with human MDA-MB-231 breast cancer cells were treated with the Hh receptor antagonist cyclopamine, we observed no effect on tumor burden or bone destruction. In vitro analyses revealed that osteolytic tumor cells lack expression of the Hh receptor, Smoothened, suggesting an Hh-independent mechanism of Gli2 regulation. Blocking Gli signaling in metastatic breast cancer cells with a Gli2-repressor gene (Gli2-rep) reduced endogenous and TGF-β-stimulated PTHrP mRNA expression, but did not alter tumor cell proliferation. Furthermore, mice inoculated with Gli2-Rep-expressing cells exhibited a decrease in osteolysis, suggesting that Gli2 inhibition may block TGF-β propagation of a vicious osteolytic cycle in this MDA-MB-231 model of bone metastasis. Accordingly, in the absence of TGF-β signaling, Gli2 expression was downregulated in cells, whereas enforced overexpression of Gli2 restored PTHrP activity. Taken together, our findings suggest that Gli2 is required for TGF-β to stimulate PTHrP expression and that blocking Hh-independent Gli2 activity will inhibit tumor-induced bone destruction.

Coordination of chondrogenesis and osteogenesis by hypertrophic chondrocytes in endochondral bone development

Mammalian bones have three distinct origins (paraxial mesoderm, lateral plate mesoderm, and neural crest) and undergo two different modes of formation (intramembranous and endochondral). Bones derived from the paraxial mesoderm and lateral plate mesoderm mainly form through the endochondral process. During this process, hypertrophic chondrocytes play a vital role in inducing osteogenesis. So far, a number of published papers have provided evidence that chondrocyte hypertrophy and osteoblast differentiation are controlled by a variety of signaling pathways and factors; however, little is known about their hierarchy (which are upstream? which are most potent?). In this review, we discuss the signaling pathways and transcriptional factors regulating chondrocyte hypertrophy and osteoblast differentiation based on the evidence that has been reported and confirmed by multiple independent groups. We then discuss which factor would provide the most coherent evidence for its role in endochondral ossification.

Effect of parathyroid hormone on type X and type II collagen expression in mesenchymal stem cells from osteoarthritic patients

A major drawback of current cartilage and intervertebral disc tissue engineering is that human mesenchymal stem cells (MSCs) from osteoarthritic (OA) patients express type X collagen (COL10), a marker of late-stage chondrocyte hypertrophy (associated with endochondral ossification). Parathyroid hormone (PTH) regulates endochondral ossification by inhibiting chondrocyte differentiation toward hypertrophy. In this study, we investigated the effect of PTH on expression of COL10 in MSCs from OA patients and analyzed the potential mechanisms related to its effect. MSCs were obtained from aspirates from the intramedullary canal of donors undergoing total hip replacement for OA. Expanded cells were then incubated for 0-48  h without (control) or with 100  nM PTH (1-34). Protein expression and phosphorylation were measured by Western blot. Results showed that PTH (1-34) inhibited expression of COL10 in MSCs from OA patients in a time-dependent manner. In parallel, PTH (1-34) stimulated expression of COL2, a marker of chondrogenic differentiation. Results also showed that PTH (1-34) inhibited in a sustained manner the phosphorylation of p38 and AKT protein kinase signaling pathways. Interestingly, the modulation of COL2 and COL10 gene expression was significant as rapidly as after 1  h in the presence of PTH (1-34); changes in the phosphorylation of p38 and AKT were significant only after 6  h. This suggests that while p38 and AKT protein kinase signaling pathways may not be required to initiate the regulation of expression of COL2 and COL10 by PTH (1-34), these pathways may modulate later events necessary for preventing precocious MSC hypertrophy.

Developmental engineering: a new paradigm for the design and manufacturing of cell-based products. Part II: from genes to networks: tissue engineering from the viewpoint of systems biology and network science

The field of tissue engineering is moving toward a new concept of "in vitro biomimetics of in vivo tissue development." In Part I of this series, we proposed a theoretical framework integrating the concepts of developmental biology with those of process design to provide the rules for the design of biomimetic processes. We named this methodology "developmental engineering" to emphasize that it is not the tissue but the process of in vitro tissue development that has to be engineered. To formulate the process design rules in a rigorous way that will allow a computational design, we should refer to mathematical methods to model the biological process taking place in vitro. Tissue functions cannot be attributed to individual molecules but rather to complex interactions between the numerous components of a cell and interactions between cells in a tissue that form a network. For tissue engineering to advance to the level of a technologically driven discipline amenable to well-established principles of process engineering, a scientifically rigorous formulation is needed of the general design rules so that the behavior of networks of genes, proteins, or cells that govern the unfolding of developmental processes could be related to the design parameters. Now that sufficient experimental data exist to construct plausible mathematical models of many biological control circuits, explicit hypotheses can be evaluated using computational approaches to facilitate process design. Recent progress in systems biology has shown that the empirical concepts of developmental biology that we used in Part I to extract the rules of biomimetic process design can be expressed in rigorous mathematical terms. This allows the accurate characterization of manufacturing processes in tissue engineering as well as the properties of the artificial tissues themselves. In addition, network science has recently shown that the behavior of biological networks strongly depends on their topology and has developed the necessary concepts and methods to describe it, allowing therefore a deeper understanding of the behavior of networks during biomimetic processes. These advances thus open the door to a transition for tissue engineering from a substantially empirical endeavor to a technology-based discipline comparable to other branches of engineering.

Recent advances toward the clinical application of PTH (1-34) in fracture healing

PTH 1-34, an active form of parathyroid hormone, has been shown to enhance osteoblastic bone formation when administered as a daily subcutaneous injection. The effect of the intermittent administration of PTH (1-34) is an uncoupling of bone turnover with an increase in bone mass and density and decrease in risk of vertebral and nonvertebral fractures. While PTH (1-34) has been used clinically to increase bone mass and reduce fracture risk in postmenopausal women with osteoporosis, there is increasing evidence that PTH (1-34) may promote fracture healing. Animal studies have demonstrated accelerated callus formation with enhanced remodeling and biomechanical properties of the healing fracture. Given these effects, PTH (1-34) will likely be used clinically to enhance fracture union in poor healing situations such as osteoporosis and recalcitrant nonunions.

The transcriptional cofactor Lbh regulates angiogenesis and endochondral bone formation during fetal bone development

Lbh is thought to act as a transcriptional cofactor and is highly conserved among species. Here we show that Lbh is expressed in chondrocytes, cells of the perichondrium, and the primary spongiosa in fetal growth plates of mice and chickens. Lbh overexpression in chick wings, using the RCAS-retroviral vector strategy, results in shortened skeletal elements and delayed hypertrophic chondrocyte maturation and bone formation. Additionally, osteoclast and endothelial cell invasion are delayed in the Lbh-overexpressing bones. Finally, we find a dramatic suppression of Runx2 and VEGF mRNAs in chondrocytes and osteoblasts that overexpress Lbh. Strikingly, this abnormal bone development in infected limbs can be rescued by concurrent overexpression of Runx2. These results suggest that during endochondral bone formation, Lbh may negatively regulate vascular invasion and formation of the early ossification center at least in part by interfering with Runx2 and/or VEGF expression.

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.

Predominant promotion by tacrolimus of chondrogenic differentiation to proliferating chondrocytes

Tacrolimus (FK506) has been used as a therapeutic drug beneficial for the treatment of rheumatoid arthritis in humans. In this study, we investigated the effects of FK506 on cellular differentiation in cultured chondrogenic cells. Culture with FK506 led to a significant and concentration-dependent increase in Alcian blue staining for matrix proteoglycan at 0.1 to 1,000 ng/ml, but not in alkaline phosphatase (ALP) activity, in ATDC5 cells, a mouse pre-chondrogenic cell line, cultured for 7 to 28 days, while the non-steroidal anti-inflammatory drug indomethacin significantly decreased Alcian blue staining in a concentration-dependent manner, without altering ALP activity. FK506 significantly increased the expression of mRNA for both type II and type X collagen, but not for osteopontin, in ATDC5 cells. Similar promotion was seen in chondrogenic differentiation in both mouse metatarsals and chondrocytes cultured with FK506. However, FK506 failed to significantly affect transcriptional activity of the reporter construct for either sry-type HMG box 9 (Sox9) or runt-related transcription factor-2 (Runx2), which are both transcription factors responsible for chondrocytic maturation as a master regulator. These results suggest that FK506 may predominantly promote cellular differentiation into proliferating chondrocytes through a mechanism not relevant to the transactivation by either Sox9 or Runx2 in chondrogenic cells.

BMP canonical Smad signaling through Smad1 and Smad5 is required for endochondral bone formation

Bone morphogenetic protein (BMP) signaling is required for endochondral bone formation. However, whether or not the effects of BMPs are mediated via canonical Smad pathways or through noncanonical pathways is unknown. In this study we have determined the role of receptor Smads 1, 5 and 8 in chondrogenesis. Deletion of individual Smads results in viable and fertile mice. Combined loss of Smads 1, 5 and 8, however, results in severe chondrodysplasia. Smad1/5(CKO) (cartilage-specific knockout) mutant mice are nearly identical to Smad1/5(CKO);Smad8(-/-) mutants, indicating that Smads 1 and 5 have overlapping functions and are more important than Smad8 in cartilage. The Smad1/5(CKO) phenotype is more severe than that of Smad4(CKO) mice, challenging the dogma, at least in chondrocytes, that Smad4 is required to mediate Smad signaling through BMP pathways. The chondrodysplasia in Smad1/5(CKO) mice is accompanied by imbalances in cross-talk between the BMP, FGF and Ihh/PTHrP pathways. We show that Ihh is a direct target of BMP pathways in chondrocytes, and that FGF exerts antagonistic effects on Ihh expression. Finally, we tested whether FGF exerts its antagonistic effects directly through Smad linker phosphorylation. The results support the alternative conclusion that the effects of FGFs on BMP signaling are indirect in vivo.

Perichondrial expression of Wdr5 regulates chondrocyte proliferation and differentiation

Wdr5 is developmentally expressed in osteoblasts and is required for osteoblast differentiation. Mice overexpressing Wdr5 under the control of the mouse alpha(1)I collagen promoter (Col I-Wdr5) display accelerated osteoblast differentiation as well as accelerated chondrocyte differentiation, suggesting that overexpression of Wdr5 in osteoblasts affects chondrocyte differentiation. To elucidate the molecular mechanism by which overexpression of Wdr5 in the perichondrium regulates chondrocyte differentiation, studies were undertaken using skeletal elements and cultured metatarsals isolated from wild-type and Col I-Wdr5 embryos. FGF18 mRNA levels were decreased in Col I-Wdr5 humeri. Furthermore, local delivery of FGF18 to the bone collar of ex vivo cultures of metatarsals attenuated the chondrocyte phenotype of the Col I-Wdr5 metatarsals. Impairing local FGF action in wild-type metatarsals resulted in a chondrocyte phenotype analogous to that of Col I-Wdr5 metatarsals implicating impaired FGF action as the cause of the phenotype observed. The expression of Twist-1, which regulates chondrocyte differentiation, was increased in Col I-Wdr5 humeri. Chromatin immunoprecipitation analyses demonstrated that Wdr5 is recruited to the Twist-1 promoter. These findings support a model in which overexpression of Wdr5 in the perichondrium promotes chondrocyte differentiation by modulating the expression of Twist-1 and FGF18.

Analyses of early events during chondrogenic repair in rat full-thickness articular cartilage defects

In this study we investigated the cellular events that occur during the onset of chondrogenic differentiation during the repair of full-thickness defects of articular cartilage. The V-shaped full-thickness cartilage defects (width 0.7 or 1.5 mm; depth 0.8 mm; length 4 mm) were created in the femoral patellar groove of rats using a custom-built twin-blade device. The time course of the repair response in these cartilage defects was examined using a semi-quantitative histological grading scale. Cartilaginous repair responses failed to occur in the larger 1.5 mm defects, which was covered only by fibrous scar tissue. In contrast, hyaline-like articular cartilage was regenerated concomitantly with the repair of the subchondral bone by 4 weeks in smaller 0.7 mm width defects. Cells in the reparative regions were then characterized by immunohistochemistry and in situ hybridization. Undifferentiated mesenchymal cells migrate into the defects and fill the cavities within 4 days of their creation. The expression of PCNA, N-cadherin, and PTH/PTHrP receptors was induced in cells at the center of the defects, where type II collagen-positive polygonal-shaped cells also begin to appear at day 7. Marrow-derived mesenchymal cells acquire higher levels of proliferative activity in induced cartilage cavities after their initial migration and filling of the smaller 0.7 mm defects. During the regenerative repair of articular cartilage in the rat, there is a distinctive step that appears to be analogous to the precartilaginous condensation that is pivotal during chondrogenesis in development.

FXIIIA and TGF-beta over-expression produces normal musculo-skeletal phenotype in TG2-/- mice

Transglutaminase (TGs) enzymes and proteins crosslinking have for long time been implicated in the formation of hard tissue development, matrix maturation and mineralization. Among the TGs family members, in the context of connective tissue formation, TG2 and Factor XIII are expressed in cartilage by hypertrophic chondrocytes. Here, we analyse the morphological consequences of TG2 deficiency, during the development of skeletal elements. When TG2 is absent, there are not gross abnormalities in the development of the skeletal system, probably from compensatory mechanisms resulting in increased expression of FXIIIA and TGF-beta 1. In vivo other TGs may be involved in promoting chondrocytes and osteoblast differentiation and matrix mineralisation.

Ca2+/Calmodulin-dependent kinase II signaling causes skeletal overgrowth and premature chondrocyte maturation

The long bones of vertebrate limbs originate from cartilage templates and are formed by the process of endochondral ossification. This process requires that chondrocytes undergo a progressive maturation from proliferating to postmitotic prehypertrophic to mature, hypertrophic chondrocytes. Coordinated control of proliferation and maturation regulates growth of the skeletal elements. Various signals and pathways have been implicated in orchestrating these processes, but the underlying intracellular molecular mechanisms are often not entirely known. Here we demonstrated in the chick using replication-competent retroviruses that constitutive activation of Calcium/Calmodulin-dependent kinase II (CaMKII) in the developing wing resulted in elongation of skeletal elements associated with premature differentiation of chondrocytes. The premature maturation of chondrocytes was a cell-autonomous effect of constitutive CaMKII signaling associated with down-regulation of cell-cycle regulators and up-regulation of chondrocyte maturation markers. In contrast, the elongation of the skeletal elements resulted from a non-cell autonomous up-regulation of the Indian hedgehog responsive gene encoding Parathyroid-hormone-related peptide. Reduction of endogenous CaMKII activity by overexpressing an inhibitory peptide resulted in shortening of the skeletal elements associated with a delay in chondrocyte maturation. Thus, CaMKII is an essential component of intracellular signaling pathways regulating chondrocyte maturation.

Stimulation of fracture-healing with systemic intermittent parathyroid hormone treatment

Over the past several years, there has been an increasing interest in the biology of bone repair and potential technologies for enhancing fracture-healing. Part of this interest is derived from the growing age of the population and the recognition that increased age carries an increased risk of complications after fracture. Although use of locally implanted or injected growth factors has received the most attention, systemic treatments for the enhancement of bone repair, especially for situations in which bone repair may be diminished or delayed, are now under investigation. Since the approval of parathyroid hormone (PTH) as an anabolic treatment for osteoporosis, there has been an increasing interest in other potential clinical uses for this compound in musculoskeletal conditions. It is now widely recognized that PTH administration is an effective therapy to increase bone mineral density and prevent fractures in patients with osteoporosis. More recently, a growing body of evidence has supported the conclusion that PTH will also be an effective anabolic therapy for the enhancement of bone repair after fracture. This review focuses on the recent research demonstrating the potential of PTH in the management of bone repair in a number of fracture models and also highlights the ongoing studies into the mechanisms of PTH actions on endochondral bone repair.

Dicer-dependent pathways regulate chondrocyte proliferation and differentiation

Small noncoding RNAs, microRNAs (miRNAs), bind to messenger RNAs through base pairing to suppress gene expression. Despite accumulating evidence that miRNAs play critical roles in various biological processes across diverse organisms, their roles in mammalian skeletal development have not been demonstrated. Here, we show that Dicer, an essential component for biogenesis of miRNAs, is essential for normal skeletal development. Dicer-null growth plates show a progressive reduction in the proliferating pool of chondrocytes, leading to severe skeletal growth defects and premature death of mice. The reduction of proliferating chondrocytes in Dicer-null growth plates is caused by two distinct mechanisms: decreased chondrocyte proliferation and accelerated differentiation into postmitotic hypertrophic chondrocytes. These defects appear to be caused by mechanisms downstream or independent of the Ihh-PTHrP signaling pathway, a pivotal signaling system that regulates chondrocyte proliferation and differentiation. Microarray analysis of Dicer-null chondrocytes showed limited expression changes in miRNA-target genes, suggesting that, in the majority of cases, chondrocytic miRNAs do not directly regulate target RNA abundance. Our results demonstrate the critical role of the Dicer-dependent pathway in the regulation of chondrocyte proliferation and differentiation during skeletal development.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

Enhanced chondrogenesis and Wnt signaling in PTH-treated fractures

Studies have shown that systemic PTH treatment enhanced the rate of bone repair in rodent models. However, the mechanisms through which PTH affects bone repair have not been elucidated. In these studies we show that PTH primarily enhanced the earliest stages of endochondral bone repair by increasing chondrocyte recruitment and rate of differentiation. In coordination with these cellular events, we observed an increased level of canonical Wnt-signaling in PTH-treated bones at multiple time-points across the time-course of fracture repair, supporting the conclusion that PTH responses are at least in part mediated through Wnt signaling.

INTRODUCTION

Since FDA approval of PTH [PTH(1-34); Forteo] as a treatment for osteoporosis, there has been interest in its use in other musculoskeletal conditions. Fracture repair is one area in which PTH may have a significant clinical impact. Multiple animal studies have shown that systemic PTH treatment of healing fractures increased both callus volume and return of mechanical competence in models of fracture healing. Whereas the potential for PTH has been established, the mechanism(s) by which PTH produces these effects remain elusive.

MATERIALS AND METHODS

Closed femoral fractures were generated in 8-wk-old male C57Bl/6 mice followed by daily systemic injections of either saline (control) or 30 microg/kg PTH(1-34) for 14 days after fracture. Bones were harvested at days 2, 3, 5, 7, 10, 14, 21, and 28 after fracture and analyzed at the tissue level by radiography and histomorphometry and at the molecular and biochemical levels level by RNase protection assay (RPA), real-time PCR, and Western blot analysis.

RESULTS

Quantitative muCT analysis showed that PTH treatment induced a larger callus cross-sectional area, length, and total volume compared with controls. Molecular analysis of the expression of extracellular matrix genes associated with chondrogenesis and osteogenesis showed that PTH treated fractures displayed a 3-fold greater increase in chondrogenesis relative to osteogenesis over the course of the repair process. In addition, chondrocyte hypertrophy occurred earlier in the PTH-treated callus tissues. Analysis of the expression of potential mediators of PTH actions showed that PTH treatment significantly induced the expression of Wnts 4, 5a, 5b, and 10b and increased levels of unphosphorylated, nuclear localized beta-catenin protein, a central feature of canonical Wnt signaling.

CONCLUSIONS

These results showed that the PTH-mediated enhancement of fracture repair is primarily associated with an amplification of chondrocyte recruitment and maturation in the early fracture callus. Associated with these cellular effects, we observed an increase in canonical Wnt signaling supporting the conclusion that PTH effects on bone repair are mediated at least in part through the activation of Wnt-signaling pathways.

A-raf and B-raf are dispensable for normal endochondral bone development, and parathyroid hormone-related peptide suppresses extracellular signal-regulated kinase activation in hypertrophic chondrocytes

Parathyroid hormone-related peptide (PTHrP) and the parathyroid hormone-PTHrP receptor increase chondrocyte proliferation and delay chondrocyte maturation in endochondral bone development at least partly through cyclic AMP (cAMP)-dependent signaling pathways. Because data suggest that the ability of cAMP to stimulate cell proliferation involves the mitogen-activated protein kinase kinase kinase B-Raf, we hypothesized that B-Raf might mediate the proliferative action of PTHrP in chondrocytes. Though B-Raf is expressed in proliferative chondrocytes, its conditional removal from cartilage did not affect chondrocyte proliferation and maturation or PTHrP-induced chondrocyte proliferation and PTHrP-delayed maturation. Similar results were obtained by conditionally removing B-Raf from osteoblasts. Because A-raf and B-raf are expressed similarly in cartilage, we speculated that they may fulfill redundant functions in this tissue. Surprisingly, mice with chondrocytes deficient in both A-Raf and B-Raf exhibited normal endochondral bone development. Activated extracellular signal-regulated kinase (ERK) was detected primarily in hypertrophic chondrocytes, where C-raf is expressed, and the suppression of ERK activation in these cells by PTHrP or a MEK inhibitor coincided with a delay in chondrocyte maturation. Taken together, these results demonstrate that B-Raf and A-Raf are dispensable for endochondral bone development and they indicate that the main role of ERK in cartilage is to stimulate not cell proliferation, but rather chondrocyte maturation.

Evc is a positive mediator of Ihh-regulated bone growth that localises at the base of chondrocyte cilia

EVC is a novel protein mutated in the human chondroectodermal dysplasia Ellis-van Creveld syndrome (EvC; OMIM: 225500). We have inactivated Evc in the mouse and show that Evc(-/-) mice develop an EvC-like syndrome, including short ribs, short limbs and dental abnormalities. lacZ driven by the Evc promoter revealed that Evc is expressed in the developing bones and the orofacial region. Antibodies developed against Evc locate the protein at the base of the primary cilium. The growth plate of Evc(-/-) mice shows delayed bone collar formation and advanced maturation of chondrocytes. Indian hedgehog (Ihh) is expressed normally in the growth plates of Evc(-/-) mice, but expression of the Ihh downstream genes Ptch1 and Gli1 was markedly decreased. Recent studies have shown that Smo localises to primary cilia and that Gli3 processing is defective in intraflagellar transport mutants. In vitro studies using Evc(-/-) cells demonstrate that the defect lies downstream of Smo. Chondrocyte cilia are present in Evc(-/-) mice and Gli3 processing appears normal by western blot analysis. We conclude that Evc is an intracellular component of the hedgehog signal transduction pathway that is required for normal transcriptional activation of Ihh target genes.

Studies of the regulation and function of the Gs alpha gene Gnas using gene targeting technology

The heterotrimeric G protein alpha-subunit G(s)alpha is ubiquitously expressed and mediates receptor-stimulated intracellular cAMP generation. Its gene Gnas is a complex imprinted gene which uses alternative promoters and first exons to generate other gene products, including the G(s)alpha isoform XL alpha s and the chromogranin-like protein NESP55, which are specifically expressed from the paternal and maternal alleles, respectively. G(s)alpha itself is imprinted in a tissue-specific manner, being biallelically expressed in most tissues but paternally silenced in a few tissues. Gene targeting of specific Gnas transcripts demonstrates that heterozygous mutation of G(s)alpha on the maternal (but not the paternal) allele leads to early lethality, perinatal subcutaneous edema, severe obesity, and multihormone resistance, while the paternal mutation leads to only mild obesity and insulin resistance. These parent-of-origin differences are the consequence of tissue-specific G(s)alpha imprinting. XL alpha s deficiency leads to a perinatal suckling defect and a lean phenotype with increased insulin sensitivity. The opposite metabolic effects of G(s)alpha and XL alpha s deficiency are associated with decreased and increased sympathetic nervous system activity, respectively. NESP55 deficiency has no metabolic consequences. Other gene targeting experiments have shown Gnas to have 2 independent imprinting domains controlled by 2 different imprinting control regions. Tissue-specific G(s)alpha knockout models have identified important roles for G(s)alpha signaling pathways in skeletal development, renal function, and glucose and lipid metabolism. Our present knowledge gleaned from various Gnas gene targeting models are discussed in relation to the pathogenesis of human disorders with mutation or abnormal imprinting of the human orthologue GNAS.

Bone growth retardation in mouse embryos expressing human collagenase 1

Cellular growth and differentiation are readouts of multiple signaling pathways from the intercellular and/or extracellular milieu. The extracellular matrix through the activation of cellular receptors transmits these signals. Therefore, extracellular matrix proteolysis could affect cell fate in a variety of biological events. However, the biological consequence of inadequate extracellular matrix degradation in vivo is not clear. We developed a mouse model expressing human collagenase (matrix metalloproteinase-1, MMP-1) under the control of Col2a1 promoter. The mice showed significant growth retardation during embryogenesis and a loss of the demarcation of zonal structure and columnar array of the cartilage. Immunological examination revealed increased degradation of type II collagen and upregulation of fibronectin and alpha(5)-integrin subunit in the transgenic cartilage. The resting zone and proliferating zone of the growth plate cartilage exhibited a simultaneous increase in bromodeoxyuridine (BrdU)-incorporated proliferating cells and terminal deoxynucleotidyl transferase-mediated X-dUTP nick-end labeling-positive apoptotic cells, respectively. Chondrocyte differentiation was not disturbed in the transgenic mice as evidenced by normal expression of the Ihh and type X collagen expression. These data demonstrate that type II collagen proteolysis is an important determinant for the skeletal outgrowth through modulation of chondrocyte survival and cartilagenous growth.

Shox2 is required for chondrocyte proliferation and maturation in proximal limb skeleton

Mutations in the short stature homeobox gene SHOX lead to growth retardation associated with Turner, Leri-Weill dyschondrosteosis, and Langer mesomelic dysplasia syndromes, which marked the shortening of the forearms and lower legs. We report here that in contrast to the SHOX mutations in humans, Shox2 deficiency in mice leads to a virtual elimination of the stylopod in the developing limbs, while the zeugopod and autopod appear relatively normal. This phenotype is consistent with the restriction of the Shox2 expression to the proximal mesenchyme in the limb bud and later to chondrocytes associated with the forming stylopod. In the Shox2(-/-) embryo, the mesenchymal condensation for the stylopod initiates normally but the cartilaginous element subsequently fails in growth, chondrogenesis and endochondral ossification. A dramatic down-regulation of Runx2 and Runx3 could account for the lack of chondrocyte hypertrophy, while a down-regulation of Ihh expression may be responsible for a significant reduction in chondrocyte proliferation in the mutant stylopod. We further demonstrate that an enhanced and ectopic Bmp4 expression in the proximal limb of the Shox2 embryo may underlie the down-regulation of Runx2, as ectopically applied exogenous BMP4 represses Runx2 expression in the early limb bud. Moreover, we show that mouse Shox2, similar to human SHOX, can perform opposite roles on gene expression: either as a transcription activator or a repressor in different cell types. Our results establish a key role for Shox2 in regulating the growth of stylopod by controlling chondrocyte maturation via Runx2 and Runx3.

Mice expressing a constitutively active PTH/PTHrP receptor in osteoblasts show reduced callus size but normal callus morphology during fracture healing

BACKGROUND

The parathyroid hormone-/parathyroid hormone-related protein (PTH/PTHrP) receptor plays a crucial role in endochondral bone formation and possibly also in fracture healing. Patients with Jansen's metaphysial chondrodysplasia (JMC) have a gain-of-function mutation in the PTH/PTHrP receptor. Transgenic mice expressing JMC PTH/PTHrP receptor mutants in osteoblasts are characterized by increased trabecular bone formation and reduced osteoblastic activity at periosteal sites. We have analyzed the bone phenotype and studied the fracture healing process in this model.

METHODS

We performed bone density analysis of tibiae from 17-week-old transgenic mice and controls. Also, tibial fractures were produced in 14-week-old mice. Fracture healing was examined by radiographic and histological analysis.

RESULTS

Transgenic mice had a lower total bone mineral content (BMC), by a factor of one-third. The changes were bone compartment-specific with an increase in trabecular bone volume and a decrease in cortical thickness. The calluses in the transgenic mice were smaller, with a reduction in BMC and mean cross-sectional area by a factor of one-half. Despite the smaller size, however, the morphology and progression through the healing process were similar in both transgenic and wild-type littermates.

INTERPRETATION

We conclude that the constitutively active PTH/PTHrP receptor has compartment-specific effects on bone formation when expressed in osteoblasts. During fracture healing, however, both the periosteal and the endochondral processes are activated, leading to fracture healing that is temporally and morphologically normal, although the callus tissue is less prominent.

Upper extremity shortness in children with hemiplegic cerebral palsy

OBJECTIVE

To evaluate upper extremity shortness in patients with hemiplegic cerebral palsy (HCP) and to investigate the association between extremity shortness, motor level, and muscle tone.

DESIGN

Prospective, controlled study.

SUBJECTS

Forty-two children with HCP and 29 healthy children.

METHODS

Radiographs of the involved and the uninvolved humerus, forearm, and hands were obtained with a radiographic ruler placed adjacent to the extremity. The lengths and the diameters of both the diaphyses and metaphyses of the humerus, ulna, radius, and the second and the fifth metacarpal bones were measured in patients and the control group. The discrepancy was calculated as a percentage compared with the normal side. The Ashworth Scale was used in the evaluation of spasticity, and the Brunnstrom recovery staging was used in the motor evaluation.

RESULTS

Children with HCP had significant differences in bone lengths and diameters compared with control children. There was no significant correlation between the upper extremity Brunnstrom stagings and the differences of bone length and diameter. A significant correlation was observed between the hand Brunnstrom staging and percentage difference of the bone length and diameter. The spasticity level showed no relation to the differences in bone length and diameter.

CONCLUSIONS

Children with HCP have significant side-to-side limb-length discrepancy when compared with control children. The discrepancy increases with age. The extent of shortening did not appear to be related to upper extremity function and spasticity. The extremity shortness showed a relation to hand function.

Mutation of phenylalanine-34 of parathyroid hormone disrupts NHERF1 regulation of PTH type I receptor signaling

Internalization of the PTH type I receptor (PTH1R) is regulated in a cell- and ligand-specific manner. We previously demonstrated that the sodium/proton exchanger regulatory factor type 1 (NHERF1; EBP50) is pivotal in determining the range of peptides that internalize the PTH1R. Antagonist PTH fragments can internalize the PTH1R in some kidney and bone cell models. PTH(7-34), which binds to, but does not activate, the PTH1R, internalizes the PTH1R in kidney distal tubule (DT) cells, where NHERF1 is not expressed. The effect of antagonist PTHrP peptides has not, to this point, been assessed. PTH1R internalization was measured by real-time confocal fluorescence microscopy of DT cells stably expressing 105 EGFP-tagged PTH1R/cell. PTHrP(7-34) internalized the PTH1R in a manner indistinguishable from PTH(7-34). Introduction of NHERF1 into DT cells, however, blocked PTH(7-34)-, but not PTHrP(7-34)-, induced PTH1R internalization. To delineate the sequences within PTHrP that determine whether PTH1R internalization is affected by NHERF1, chimeric PTH/PTHrP fragments were tested for their ability to induce PTH1R internalization. PTH(7-21)/PTHrP (22-34), PTH(7-32)/PTHrP(33-34), and PTH(7-33)/PTHrP(34) at 1 microM each internalized the PTH1R 50-70% in a NHERF1-independent manner. When the C terminus of PTHrP was replaced with homologous amino acids from PTH, NHERF1 inhibited PTH1R internalization. It was determined that simply mutating F34 to A in PTH induced PTH1R internalization in a NHERF1-independent manner. None of the chimeric peptides activated the PTH1R but all effectively competed for 1 nM PTH(1-34) in cyclic AMP assays. In addition, all chimeric peptides competed for radiolabeled PTH(1-34) in binding assays in DT cells. PTH(1- 34) and PTHrP(7-34), but not PTH(7-34), efficiently recruited beta-arrestin1 to plasma membrane PTH1Rs. We, therefore, conclude that PTH(1-34) and PTHrP(7-34) induce a conformational change in the PTH1R that promotes arrestin binding and dissociates NHERF1 from PTH1R internalization.

Insulin-like growth factor-I is essential for embryonic bone development

Although IGF-I has been identified as an important growth factor for the skeleton, the role of IGF-I on embryonic bone development remains unknown. Here we show that, in IGF-I-deficient (IGF-I(-/-)) mice, skeletal malformations, including short-limbed dwarfism, were evident at days post coitus (dpc) 14.5 to 18.5, accompanied by delays of mineralization in the spinal column, sternum, and fore paws. Reduced chondrocyte proliferation and increased chondrocyte apoptosis were identified in both the spinal ossification center and the growth plate of long bones. Abnormal chondrocyte differentiation and delayed initiation of mineralization was characterized by small size and fewer numbers of type X collagen expressing hypertrophic chondrocytes and lower osteocalcin expression. The Indian hedgehog-PTHrP feedback loop was altered; expression of Indian hedgehog was reduced in IGF-I(-/-) mice in long bones and in the spine, whereas expression of PTHrP was increased. Our results indicate that IGF-I plays an important role in skeletal development by promoting chondrocyte proliferation and maturation while inhibiting apoptosis to form bones of appropriate size and strength.

Expression of the PTH/PTHrP receptor in chondrogenic cells during the repair of full-thickness defects of articular cartilage

OBJECTIVE

We studied the accumulation of parathyroid hormone (PTH)/PTHrP receptor-positive mesenchymal cells using double immunohistochemistry and examined whether this correlated with the subsequent regeneration of 3-mm-diameter full-thickness defects of articular cartilage.

MATERIALS AND METHODS

Cylindrical full-thickness articular cartilage defects (3 mm) were artificially created in the femoral trochlea of male adolescent Japanese white rabbits (n = 210) with a hand-drill. Recombinant human PTH(1-84) was then administered into the defect cavities with an osmotic pump for either 2 or 4 weeks post-injury. Following PTH treatment, the repair processes in the cartilage defects were histologically examined. Double immunostaining analyses for the PTH/PTH-related peptide (PTHrP) receptor and proliferating cell nuclear antigen (PCNA) in the regenerating tissues were then performed.

RESULTS

Activation of PTH/PTHrP receptor signaling by hPTH(1-84) results in the inhibition of chondrogenic differentiation in full-thickness articular cartilage defects. At the conclusion of the 2-week PTH treatment, the defect cavities were filled with undifferentiated mesenchymal cells, which were similar to the controls. In addition, almost all of these cells localized at the center of the injuries were both PTH/PTHrP receptor- and PCNA-positive. In contrast, after prolonged PTH treatment for 4 weeks, there was no indication of a cartilaginous repair response and cells that had migrated to the defect cavities were found to have irreversibly lost expression of the PTH/PTHrP receptor.

CONCLUSIONS

The chondrogenic capacity of cells that had migrated to the area of these defect cavities is closely associated with their ability to express the PTH/PTHrP receptor. Moreover, these cells maintain their chondrogenic potential within only a limited time-span of 2 weeks.

PTHrP and skeletal development

Parathyroid hormone-related protein (PTHrP) participates in the regulation of endochondral bone development. After the cartilage mold is established in fetal life, perichondrial cells and chondrocytes at the ends of the mold synthesize PTHrP. This ligand then acts on PTH/PTHrP receptors on chondrocytes. As chondrocytes go through a program of proliferation and then further differentiation into post-mitotic, hypertrophic chondrocytes, PTHrP action keeps chondrocytes proliferating and delays their further differentiation. Indian hedgehog (Ihh) is synthesized by chondrocytes that have just stopped proliferating and is required for synthesis of PTHrP. The feedback loop between PTHrP and Ihh serves to regulate the pace of chondrocyte differentiation and the sites at which perichondrial cells first differentiate into osteoblasts. Activation of the PTH/PTHrP receptor leads to stimulation of both Gs and Gq family heterotrimeric G proteins. Genetic analyses demonstrate that Gs activation mediates the action of PTHrP to keep chondrocytes proliferating, while Gq activation opposes this action. Downstream from Gs activation, synthesis of the cyclin-cdk inhibitor, p57, is suppressed, thereby increasing the pool of proliferating chondrocytes. PTHrP's actions to delay chondrocyte differentiation are mediated by the phosphorylation of the transcription factor, SOX9, and by suppression of synthesis of mRNA encoding the transcription factor, Runx2. These pathways and undoubtedly others cooperate to regulate the pace of differentiation of growth plate chondrocytes in response to PTHrP.

Age-related expression of molecular regulators of hypertrophy and maturation in articular cartilage

The purpose of this study was to determine changes in the expression of regulatory molecules in normal equine articular cartilage throughout development up to 18 months of age. The hypothesis was that expression of these regulatory molecules would decrease from birth to postpubescence. Cartilage was harvested from normal femoropatellar or scapulohumeral joints from 34 fresh horse cadavers. Horses were placed in four age groups [prenatal (n = 5); prepubertal, 0-6 months (n = 11); pubertal, 7-14 months (n = 13); and postpubertal, 15-18 months (n = 5)]. Indian hedgehog (Ihh), Gli1, Gli3, Patched1 (Ptc1), Smoothened (Smo), Noggin, bone morphogenetic protein-6 (BMP-6), BMP-2, parathyroid hormone-related peptide (PTHrP), and PTH/PTHrP receptor mRNA expression levels were evaluated by real-time quantitative PCR. Spatial tissue mRNA and protein expression was determined by in situ hybridization and immunohistochemistry. The expression of PTHrP decreased (p = 0.002) in the pubertal group, while PTH/PTHrP receptor expression significantly increased (p = 0.001). No significant difference was found between groups for Ihh (p = 0.6) or Smo (p = 0.3) expression. In contrast, there was significantly increased expression of Ptc1 (p = 0.006), Gli1 (p = 0.04), and Gli3 (p = 0.007) in the pubertal group, and Gli3 (p = 0.007) remained elevated in the postpubertal group. The expression of BMP-6 significantly increased from prenatal to postnatal groups (p = 0.03) while BMP-2 expression increased during puberty and postpuberty (p = 0.03). The changes in expression of hedgehog and BMP signaling molecules in articular cartilage during postnatal development have not been shown previously. The increased expression of hedgehog receptor and transcription factors during puberty may indicate maturation of the deep articular layer during this time period.