Parathyroid hormone-related peptide (PTHrP) inhibits Runx2 expression through the PKA signaling pathway

A PTHrP Gradient Drives Mandibular Condylar Chondrogenesis via Runx2

The mandibular condylar cartilage (MCC) is an essential component of the temporomandibular joint, which orchestrates the vertical growth of the mandibular ramus through endochondral ossification with distinctive modes of cell differentiation. Parathyroid hormone-related protein (PTHrP) is a master regulator of chondrogenesis; in the long bone epiphyseal growth plate, PTHrP expressed by resting zone chondrocytes promotes chondrocyte proliferation in the adjacent layer. However, how PTHrP regulates chondrogenesis in the MCC remains largely unclear. In this study, we used a Pthrp-mCherry knock-in reporter strain to map the localization of PTHrP+ cells in the MCC and define the function of PTHrP in the growing mandibular condyle. In the postnatal MCC of PthrpmCherry/+ mice, PTHrP-mCherry was specifically expressed by cells in the superficial layer immediately adjacent to RUNX2-expressing cells in the polymorphic layer. PTHrP ligands diffused across the polymorphic and chondrocyte layers where its cognate receptor PTH1R was abundantly expressed. We further analyzed the mandibular condyle of PthrpmCherry/mCherry mice lacking functional PTHrP protein (PTHrP-KO). At embryonic day (E) 18.5, the condylar process and MCC were significantly truncated in the PTHrP-KO mandible, which was associated with a significant reduction in cell proliferation across the polymorphic layer and a loss of SOX9+ cells in the chondrocyte layers. The PTHrP-KO MCC showed a transient increase in the number of Col10a1+ hypertrophic chondrocytes at E15.5, followed by a significant loss of these cells at E18.5, indicating that superficial layer-derived PTHrP prevents premature chondrocyte exhaustion in the MCC. The expression of Runx2, but not Sp7, was significantly reduced in the polymorphic layer of the PTHrP-KO MCC. Therefore, PTHrP released from cells in the superficial layer directly acts on cells in the polymorphic layer to promote proliferation of chondrocyte precursor cells and prevent their premature differentiation by maintaining Runx2 expression, revealing a unique PTHrP gradient-directed mechanism that regulates MCC chondrogenesis.

Parathyroid Hormone Related Protein (PTHrP)-Associated Molecular Signatures in Tissue Differentiation and Non-Tumoral Diseases

Parathyroid-hormone-related protein (PTHrP) is encoded by the PTHLH gene which, via alternative promoter usage and splicing mechanisms, can give rise to at least three isoforms of 139, 141, and 173 amino acids with distinct C-terminals. PTHrP is subjected to different post-translational processing that generates smaller bioactive forms, comprising amino terminus, mid-region (containing a nuclear/nucleolar targeting signal), and carboxy terminus peptides. Both the full-length protein and the discrete peptides are key controllers of viability, proliferation, differentiation, and apoptosis in diverse normal and pathological biological systems via the reprogramming of gene expression and remodulation of PKA or PKC-mediated signalization mechanisms. The aim of this review is to pick up selected studies on PTHrP-associated signatures as revealed by molecular profiling assays, focusing on the available data about exemplary differentiating, differentiated, or nontumoral cell and tissue models. In particular, the data presented relate to adipose, bone, dental, cartilaginous, and skin tissues, as well as intestinal, renal, hepatic, pulmonary, and pancreatic epithelia, with a focus on hepatic fibrosis-, pancreatitis-, and diabetes-related changes as diseased states. When reported, the biochemical and/or physiological aspects associated with the specific molecular modulation of gene expression and signal transduction pathways in the target model systems under examination are also briefly described.

Abaloparatide and teriparatide enhance mandibular growth in adolescent rats with site-specific and mechano-related effects

OBJECTIVE

Teriparatide (TPTD) and abaloparatide (ABL) are two osteoanabolic drugs targeting parathyroid hormone (PTH)1R signalling. This study aimed to investigate the effects of TPTD and ABL on the adolescent mandibular growth.

METHOD

In total, 70 4-week-old male Sprague-Dawley rats were randomly divided into 14 groups, treated with intermittent TPDT or ABL at various doses, accompanied by mandibular advancement (MA) or not. 3D printing was used to fabricate an innovative splint for MA. After a 4-week treatment, morphological measurement, histological and immunohistochemical analysis were performed. Mandibular condylar chondrocytes (MCCs) were treated with TPTD or ABL, followed by CCK-8 assay, alcian blue staining, real time-PCR and immunofluorescent staining.

RESULT

In vivo, TPTD or ABL alone increased the condylar length and cartilage thickness, with up-regulated SOX9 and COL II, whilst down-regulated COL X; however, when combined with MA, the promotive effects were attenuated. TPTD or ABL alone increased the mandibular body height and mandibular angle width, whilst increased the mandibular body length and alveolar bone width when combined with MA. In vitro, TPTD or ABL enhanced the MCC proliferation, glycosaminoglycan synthesis, COL II and SOX9 expression, whilst down-regulated COL X, Ihh and PTH1R expression.

CONCLUSION

Both ABL and TPTD enhance mandibular growth in adolescent rats with site-specific and mechano-related effects, including propelling chondrogenesis at the condylar cartilage and promoting bone apposition at other mechano-responsive sites. They behave as promising drugs for mandibular growth modification, and in general ABL seems more potent than TPTD in this context.

Chondrocyte Hypertrophy in Osteoarthritis: Mechanistic Studies and Models for the Identification of New Therapeutic Strategies

Articular cartilage shows limited self-healing ability owing to its low cellularity and avascularity. Untreated cartilage defects display an increased propensity to degenerate, leading to osteoarthritis (OA). During OA progression, articular chondrocytes are subjected to significant alterations in gene expression and phenotype, including a shift towards a hypertrophic-like state (with the expression of collagen type X, matrix metalloproteinases-13, and alkaline phosphatase) analogous to what eventuates during endochondral ossification. Present OA management strategies focus, however, exclusively on cartilage inflammation and degradation. A better understanding of the hypertrophic chondrocyte phenotype in OA might give new insights into its pathogenesis, suggesting potential disease-modifying therapeutic approaches. Recent developments in the field of cellular/molecular biology and tissue engineering proceeded in the direction of contrasting the onset of this hypertrophic phenotype, but knowledge gaps in the cause-effect of these processes are still present. In this review we will highlight the possible advantages and drawbacks of using this approach as a therapeutic strategy while focusing on the experimental models necessary for a better understanding of the phenomenon. Specifically, we will discuss in brief the cellular signaling pathways associated with the onset of a hypertrophic phenotype in chondrocytes during the progression of OA and will analyze in depth the advantages and disadvantages of various models that have been used to mimic it. Afterwards, we will present the strategies developed and proposed to impede chondrocyte hypertrophy and cartilage matrix mineralization/calcification. Finally, we will examine the future perspectives of OA therapeutic strategies.

Whole Aspect of Runx2 Functions in Skeletal Development

Runt-related transcription factor 2 (Runx2) is a fundamental transcription factor for bone development. In endochondral ossification, Runx2 induces chondrocyte maturation, enhances chondrocyte proliferation through Indian hedgehog (Ihh) induction, and induces the expression of vascular endothelial growth factor A (Vegfa), secreted phosphoprotein 1 (Spp1), integrin-binding sialoprotein (Ibsp), and matrix metallopeptidase 13 (Mmp13) in the terminal hypertrophic chondrocytes. Runx2 inhibits the apoptosis of the terminal hypertrophic chondrocytes and induces their transdifferentiation into osteoblasts and osteoblast progenitors. The transdifferentiation is required for trabecular bone formation during embryonic and newborn stages but is dispensable for acquiring normal bone mass in young and adult mice. Runx2 enhances the proliferation of osteoblast progenitors and induces their commitment to osteoblast lineage cells through the direct regulation of the expressions of a hedgehog, fibroblast growth factor (Fgf), Wnt, and parathyroid hormone-like hormone (Pthlh) signaling pathway genes and distal-less homeobox 5 (Dlx5), which all regulate Runx2 expression and/or protein activity. Runx2, Sp7, and Wnt signaling further induce osteoblast differentiation. In immature osteoblasts, Runx2 regulates the expression of bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and bone gamma carboxyglutamate protein (Bglap)/Bglap2, and induces osteoblast maturation. Osteocalcin (Bglap/Bglap2) is required for the alignment of apatite crystals parallel to the collagen fibers; however, it does not physiologically work as a hormone that regulates glucose metabolism, testosterone synthesis, or muscle mass. Thus, Runx2 exerts multiple functions essential for skeletal development.

Pthlha and mechanical force control early patterning of growth zones in the zebrafish craniofacial skeleton

Secreted signals in patterning systems often induce repressive signals that shape their distributions in space and time. In developing growth plates (GPs) of endochondral long bones, Parathyroid hormone-like hormone (Pthlh) inhibits Indian hedgehog (Ihh) to form a negative-feedback loop that controls GP progression and bone size. Whether similar systems operate in other bones and how they arise during embryogenesis remain unclear. We show that Pthlha expression in the zebrafish craniofacial skeleton precedes chondrocyte differentiation and restricts where cells undergo hypertrophy, thereby initiating a future GP. Loss of Pthlha leads to an expansion of cells expressing a novel early marker of the hypertrophic zone (HZ), entpd5a, and later HZ markers, such as ihha, whereas local Pthlha misexpression induces ectopic entpd5a expression. Formation of this early pre-HZ correlates with onset of muscle contraction and requires mechanical force; paralysis leads to loss of entpd5a and ihha expression in the pre-HZ, mislocalized pthlha expression and no subsequent ossification. These results suggest that local Pthlh sources combined with force determine HZ locations, establishing the negative-feedback loop that later maintains GPs.

Hypoxia-inducible factor 2α is a novel inhibitor of chondrocyte maturation

Hypoxic environment is essential for chondrocyte maturation and longitudinal bone growth. Although hypoxia-inducible factor 1 alpha (Hif-1α) has been known as a key player for chondrocyte survival and function, the function of Hif-2α in cartilage is mechanistically and clinically relevant but remains unknown. Here we demonstrated that Hif-2α was a novel inhibitor of chondrocyte maturation through downregulation of Runx2 stability. Mechanistically, Hif-2α binding to Runx2 inhibited chondrocyte maturation by Runx2 degradation through disrupting Runx2/Cbfβ complex formation. The Hif-2α-mediated-Runx2 degradation could be rescued by Cbfβ transfection due to the increase of Runx2/Cbfβ complex formation. Consistently, mesenchymal cells derived from Hif-2α heterozygous mice were more rapidly differentiated into hypertrophic chondrocytes than those of wild-type mice in a micromass culture system. Collectively, these findings demonstrate that Hif-2α is a novel inhibitor for chondrocyte maturation by disrupting Runx2/Cbfβ complex formation and consequential regulatory activity.

Paroxetine-mediated GRK2 inhibition is a disease-modifying treatment for osteoarthritis

Osteoarthritis (OA) is a debilitating joint disease characterized by progressive cartilage degeneration, with no available disease-modifying therapy. OA is driven by pathological chondrocyte hypertrophy (CH), the cellular regulators of which are unknown. We have recently reported the therapeutic efficacy of G protein-coupled receptor kinase 2 (GRK2) inhibition in other diseases by recovering protective G protein-coupled receptor (GPCR) signaling. However, the role of GPCR-GRK2 pathway in OA is unknown. Thus, in a surgical OA mouse model, we performed genetic GRK2 deletion in chondrocytes or pharmacological inhibition with the repurposed U.S. Food and Drug Administration (FDA)-approved antidepressant paroxetine. Both GRK2 deletion and inhibition prevented CH, abated OA progression, and promoted cartilage regeneration. Supporting experiments with cultured human OA cartilage confirmed the ability of paroxetine to mitigate CH and cartilage degradation. Our findings present elevated GRK2 signaling in chondrocytes as a driver of CH in OA and identify paroxetine as a disease-modifying drug for OA treatment.

New Insights Into Cranial Synchondrosis Development: A Mini Review

The synchondroses formed via endochondral ossification in the cranial base are an important growth center for the neurocranium. Abnormalities in the synchondroses affect cranial base elongation and the development of adjacent regions, including the craniofacial bones. In the central region of the cranial base, there are two synchondroses present—the intersphenoid synchondrosis and the spheno-occipital synchondrosis. These synchondroses consist of mirror image bipolar growth plates. The cross-talk of several signaling pathways, including the parathyroid hormone-like hormone (PTHLH)/parathyroid hormone-related protein (PTHrP), Indian hedgehog (Ihh), Wnt/β-catenin, and fibroblast growth factor (FGF) pathways, as well as regulation by cilium assembly and the transcription factors encoded by the RUNX2, SIX1, SIX2, SIX4, and TBX1 genes, play critical roles in synchondrosis development. Deletions or activation of these gene products in mice causes the abnormal ossification of cranial synchondrosis and skeletal elements. Gene disruption leads to both similar and markedly different abnormalities in the development of intersphenoid synchondrosis and spheno-occipital synchondrosis, as well as in the phenotypes of synchondroses and skeletal bones. This paper reviews the development of cranial synchondroses, along with its regulation by the signaling pathways and transcription factors, highlighting the differences between intersphenoid synchondrosis and spheno-occipital synchondrosis.

Inhibition of Y1 Receptor Promotes Osteogenesis in Bone Marrow Stromal Cells via cAMP/PKA/CREB Pathway

Inhibition of neuropeptide Y1 receptor stimulates osteogenesis in vitro and in vivo. However, the underlying mechanisms involved in these effects remain poorly understood. Here we identify the effects of Y1 receptor deficiency on osteogenic differentiation in human bone marrow stromal cells (BMSCs) by using genetic and pharmacological regulation, and to explore the pathways mediating these effects. In BMSCs, inhibition of Y1 receptor stimulates osteogenesis and upregulates the expression levels of the master transcriptional factor RUNX2. Mechanistically, Y1 receptor deficiency increases the levels of intracellular cAMP, which via protein kinase A (PKA) mediated pathways results in activation of phospho-CREB (p-CREB). We find RUNX2 activation induced by Y1 receptor deficiency is reversed by H-89, a PKA inhibitor. These results indicate Y1 receptor deficiency activates PKA-mediated phosphorylation of CREB, leading to activation of RUNX2 and enhances osteogenic differentiation in BMSCs. In conclusion, these data indicate that Y1 receptor deficiency promotes osteogenic differentiation by RUNX2 stimulation through cAMP/PKA/CREB pathway.

miR-195 contributes to human osteoarthritis via targeting PTHrP

The dysregulated expression of the osteoarthritis (OA)-related genes in cartilage, such as matrix metalloproteinase 13 (MMP-13) and type X collagen (Col X), facilitates the onset and progression of OA. Reduced parathyroid hormone-related protein (PTHrP) may also accelerate this progression. Furthermore, miRNAs, endogenous regulators of mRNAs, are thought to play key roles in the pathogenesis of OA. In this study, we found that miR-195 levels were significantly upregulated in OA tissue, while PTHrP mRNA/protein expression was substantially downregulated, and there was a negative correlation between miR-195 and PTHrP. Upregulated miR-195 strongly inhibited Aggrecan, type II collagen (Col II) mRNA/protein expression, while it enhanced the expression of MMP-13 and Col X at mRNA/protein level; conversely, downregulated miR-195 significantly increased Col II mRNA/protein expression, while it decreased the expression of MMP-13 and Col X mRNA/protein. Moreover, our study demonstrated that PTHrP is a novel target of miR-195 using dual luciferase reporter assay. Finally, miR-195-mediated changes of Col II and OA-related genes were substantially attenuated by siRNA_PTHrP treatment. These results suggested that miR-195 is involved in the pathogenesis of OA via PTHrP.

Pre-transplantational Control of the Post-transplantational Fate of Human Pluripotent Stem Cell-Derived Cartilage

Cartilage pellets generated from ectomesenchymal progeny of human pluripotent stem cells (hPSCs) in vitro eventually show signs of commitment of chondrocytes to hypertrophic differentiation. When transplanted subcutaneously, most of the surviving pellets were fully mineralized by 8 weeks. In contrast, treatment with the adenylyl cyclase activator, forskolin, in vitro resulted in slightly enlarged cartilage pellets containing an increased proportion of proliferating immature chondrocytes that expressed very low levels of hypertrophic/terminally matured chondrocyte-specific genes. Forskolin treatment also enhanced hyaline cartilage formation by reducing type I collagen gene expression and increasing sulfated glycosaminoglycan accumulation in the developed cartilage. Chondrogenic mesoderm from hPSCs and dedifferentiated nasal chondrocytes responded similarly to forskolin. Furthermore, forskolin treatment in vitro increased the frequency at which the cartilage pellets maintained unmineralized chondrocytes after subcutaneous transplantation. Thus, the post-transplantational fate of chondrocytes originating from hPSC-derived chondroprogenitors can be controlled during their genesis in vitro.

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Forskolin/cAMP suppresses/delays BMP-induced chondrocyte maturation in vitro

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Forskolin supports chondrocyte proliferation and hyaline chondrogenesis in vitro

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Forskolin suppresses osteogenesis and BMP signaling gene expression in cartilage

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In vitro forskolin treatment improves in vivo maintenance of uncalcified cartilage

Pseudoshikonin I enhances osteoblast differentiation by stimulating Runx2 and Osterix

Pseudoshikonin I (PSI), a novel biomaterial isolated from Lithospermi radix, has been recognized as an herbal medicine for the treatment of infectious and inflammatory diseases. Bone remodeling maintains a balance through bone resorption (osteoclastogenesis) and bone formation (osteoblastogenesis). Bone formation is generally attributed to osteoblasts. However, the effects of PSI on the bone are not well known. In this study, we found that the ethanol extracts of PSI induced osteoblast differentiation by increasing the expression of bone morphogenic protein 4 (BMP 4). PSI positively regulates the transcriptional expression and osteogenic activity of osteoblast-specific transcription factors such as Runx2 and Osterix. To identify the signaling pathways that mediate PSI-induced osteoblastogenesis, we examined the effects of serine-threonine kinase inhibitors that are known regulators of Osterix and Runx2. PSI-induced upregulation of Osterix and Runx2 was suppressed by treatment with AKT and PKA inhibitors. These results suggest that PSI enhances osteoblast differentiation by stimulating Osterix and Runx2 via the AKT and PKA signaling pathways. Thus, the activation of Runx2 and Osterix is modulated by PSI, thereby demonstrating its potential as a treatment target for bone disease.

The association between parathyroid hormone 1‑34 and the Wnt/β‑catenin signaling pathway in a rat model of osteoarthritis

The aim of the current study was to determine the effect of parathyroid hormone (PTH) 1‑34 on cartilage degeneration, and the association between PTH 1‑34 and factors associated with the Wnt/β‑catenin pathway following anterior cruciate ligament and medial meniscectomy‑induced osteoarthritis (OA) in rats. A total of 64 Sprague‑Dawley rats were randomly divided into the following four groups: Sham‑operated rats with normal saline (NS)‑treatment (n=16); anterior cruciate ligament transection with partial medial meniscectomy (ACLT + MMx) rats with NS‑treatment (n=16); sham‑operated rats treated with PTH 1‑34 (n=16); and ACLT + MMx rats treated with PTH 1‑34 (n=16). PTH (15 µg/kg/day) was administered via subcutaneous injection 5 days per week from the first postoperative day for 2 or 6 weeks. Staining with hematoxylin and eosin and safranin O, and a scoring system modified by Mankin were used to assess the histopathological features of cartilage. The present study detected the expression of PTH 1 receptor (PTH1R), sclerostin, dickkopf Wnt signaling pathway inhibitor 1 (DKK1), β‑catenin and runt‑related transcription factor 2 (RUNX2) in cartilage by immunohistochemical analysis to determine the association between PTH 1‑34 and factors associated with the Wnt/β‑catenin pathway. Reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) was performed to detect the mRNA expression levels of PTH1R and β‑catenin in cartilage. Histological analysis demonstrated that cartilage degeneration was present post‑surgery and gradually increased over time. PTH 1‑34 reduced the Mankin scores in ACLT + MMx rats compared with the NS‑treated ACLT + MMx rats. Immunohistochemistry and RT‑qPCR analysis demonstrated that, in cartilage, PTH 1‑34 treatment increased the mRNA expression and protein levels of PTH1R and β‑catenin, and decreased protein levels of sclerostin, DKK1 and RUNX2 in ACLT + MMx rats compared with the NS‑treated ACLT + MMx group. The present study demonstrated that PTH 1‑34 upregulated the Wnt/β‑catenin signaling pathway and that PTH1‑34 downregulated RUNX2 through an alternative pathway to the Wnt/β‑catenin signaling pathway, in a rat model of OA.

Cell Death in Chondrocytes, Osteoblasts, and Osteocytes

Cell death in skeletal component cells, including chondrocytes, osteoblasts, and osteocytes, plays roles in skeletal development, maintenance, and repair as well as in the pathogenesis of osteoarthritis and osteoporosis. Chondrocyte proliferation, differentiation, and apoptosis are important steps for endochondral ossification. Although the inactivation of P53 and RB is involved in the pathogenesis of osteosarcomas, the deletion of p53 and inactivation of Rb are insufficient to enhance chondrocyte proliferation, indicating the presence of multiple inhibitory mechanisms against sarcomagenesis in chondrocytes. The inflammatory processes induced by mechanical injury and chondrocyte death through the release of danger-associated molecular patterns (DAMPs) are involved in the pathogenesis of posttraumatic osteoarthritis. The overexpression of BCLXL increases bone volume with a normal structure and maintains bone during aging by inhibiting osteoblast apoptosis. p53 inhibits osteoblast proliferation and enhances osteoblast apoptosis, thereby reducing bone formation, but also exerts positive effects on osteoblast differentiation through the Akt–FoxOs pathway. Apoptotic osteocytes release ATP, which induces the receptor activator of nuclear factor κ-B ligand (Rankl) expression and osteoclastogenesis, from pannexin 1 channels. Osteocyte death ultimately results in necrosis; DAMPs are released to the bone surface and promote the production of proinflammatory cytokines, which induce Rankl expression, and osteoclastogenesis is further enhanced.

Signalling pathways in trophic skeletal development and morphogenesis: Insights from studies on teleost fish

During the development of the vertebrate feeding apparatus, a variety of complicated cellular and molecular processes participate in the formation and integration of individual skeletal elements. The molecular mechanisms regulating the formation of skeletal primordia and their development into specific morphological structures are tightly controlled by a set of interconnected signalling pathways. Some of these pathways, such as Bmp, Hedgehog, Notch and Wnt, are long known for their pivotal roles in craniofacial skeletogenesis. Studies addressing the functional details of their components and downstream targets, the mechanisms of their interactions with other signals as well as their potential roles in adaptive morphological divergence, are currently attracting considerable attention. An increasing number of signalling pathways that had previously been described in different biological contexts have been shown to be important in the regulation of jaw skeletal development and morphogenesis. In this review, I provide an overview of signalling pathways involved in trophic skeletogenesis emphasizing studies of the most species-rich group of vertebrates, the teleost fish, which through their evolutionary history have undergone repeated episodes of spectacular trophic diversification.

Pannexin 3 is required for late stage bone growth but not for initiation of ossification in avian embryos

BACKGROUND

Pannexin 3 (PANX3) is a channel-forming protein capable of stimulating osteogenesis in vitro. Here, we studied the in vivo roles of PANX3 in the chicken embryo using the RCAS retroviral system to over-express and knockdown expression during endochondral bone formation.

RESULTS

In the limbs, PANX3 RNA was first detected in the cartilage condensations and became restricted to the prehypertrophic cartilage of the epiphyses, diaphysis, and perichondrium. The increase in PANX3 was not sufficient to alter osteogenesis; however, knockdown with a virus containing an interference RNA construct caused a 20% reduction in bone volume. The control virus containing an shEGFP cassette did not affect development. Interestingly, the phenotype was restricted to later stages rather than to proliferation of the skeletogenic mesenchyme, formation of the cartilage condensation, or creation of the hypertrophic zones. In addition, there was also no change in readouts of Hedgehog, WNT, fibroblast growth factor, or bone morphogenetic protein signaling using either quantitative real-time polymerase chain reaction or radioactive in situ hybridization.

CONCLUSIONS

Based on the normal expression domains of PANX3 and the relatively late manifestation of the phenotype, it is possible that PANX3 hemichannels may be required to facilitate the transition of hypertrophic chondrocytes to osteoblasts, thereby achieving final bone size. Developmental Dynamics 245:913-924, 2016. © 2016 Wiley Periodicals, Inc.

Developmental Regulation of the Growth Plate and Cranial Synchondrosis

Long bones and the cranial base are both formed through endochondral ossification. Elongation of long bones is primarily through the growth plate, which is a cartilaginous structure at the end of long bones made up of chondrocytes. Growth plate chondrocytes are organized in columns along the longitudinal axis of bone growth. The cranial base is the growth center of the neurocranium. Synchondroses, consisting of mirror-image growth plates, are critical for cranial base elongation and development. Over the last decade, considerable progress has been made in determining the roles of the parathyroid hormone-related protein, Indian hedgehog, fibroblast growth factor, bone morphogenetic protein, and Wnt signaling pathways in various aspects of skeletal development. Furthermore, recent evidence indicates the important role of the primary cilia signaling pathway in bone elongation. Here, we review the development of the growth plate and cranial synchondrosis and the regulation by the above-mentioned signaling pathways, highlighting the similarities and differences between these 2 structures.

PACAP and VIP signaling in chondrogenesis and osteogenesis

Skeletal development is a complex process regulated by multifactorial signaling cascades that govern proper tissue specific cell differentiation and matrix production. The influence of certain regulatory peptides on cartilage or bone development can be predicted but are not widely studied. In this review, we aimed to assemble and overview those signaling pathways which are modulated by PACAP and VIP neuropeptides and are involved in cartilage and bone formation. We discuss recent experimental data suggesting broad spectrum functions of these neuropeptides in osteogenic and chondrogenic differentiation, including the canonical downstream targets of PACAP and VIP receptors, PKA or MAPK pathways, which are key regulators of chondro- and osteogenesis. Recent experimental data support the hypothesis that PACAP is a positive regulator of chondrogenesis, while VIP has been reported playing an important role in the inflammatory reactions of surrounding joint tissues. Regulatory function of PACAP and VIP in bone development has also been proved, although the source of the peptides is not obvious. Crosstalk and collateral connections of the discussed signaling mechanisms make the system complicated and may obscure the pure effects of VIP and PACAP. Chondro-protective properties of PACAP during oxidative stress observed in our experiments indicate a possible therapeutic application of this neuropeptide.

Strategies to minimize hypertrophy in cartilage engineering and regeneration

Due to a blood supply shortage, articular cartilage has a limited capacity for self-healing once damaged. Articular chondrocytes, cartilage progenitor cells, embryonic stem cells, and mesenchymal stem cells are candidate cells for cartilage regeneration. Significant current attention is paid to improving chondrogenic differentiation capacity; unfortunately, the potential chondrogenic hypertrophy of differentiated cells is largely overlooked. Consequently, the engineered tissue is actually a transient cartilage rather than a permanent one. The development of hypertrophic cartilage ends with the onset of endochondral bone formation which has inferior mechanical properties. In this review, current strategies for inhibition of chondrogenic hypertrophy are comprehensively summarized; the impact of cell source options is discussed; and potential mechanisms underlying these strategies are also categorized. This paper aims to provide guidelines for the prevention of hypertrophy in the regeneration of cartilage tissue. This knowledge may also facilitate the retardation of osteophytes in the treatment of osteoarthritis.

Role of G-proteins in the differentiation of epiphyseal chondrocytes

Herein, we review the regulation of differentiation of the growth plate chondrocytes by G-proteins. In connection with this, we summarize the current knowledge regarding each family of G-protein α subunit, specifically, Gα(s), Gα(q/11), Gα(12/13), and Gα(i/o). We discuss different mechanisms involved in chondrocyte differentiation downstream of G-proteins and different G-protein-coupled receptors (GPCRs) activating G-proteins in the epiphyseal chondrocytes. We conclude that among all G-proteins and GPCRs expressed by chondrocytes, Gα(s) has the most important role and prevents premature chondrocyte differentiation. Receptor for parathyroid hormone (PTHR1) appears to be the major activator of Gα(s) in chondrocytes and ablation of either one leads to accelerated chondrocyte differentiation, premature fusion of the postnatal growth plate, and ultimately short stature.

Ser/Thr-phosphoprotein phosphatases in chondrogenesis: neglected components of a two-player game

Protein phosphorylation plays a determining role in the regulation of chondrogenesis in vitro. While signalling pathways governed by protein kinases including PKA, PKC, and mitogen-activated protein kinases (MAPK) have been mapped in great details, published data relating to the specific role of phosphoprotein phosphatases (PPs) in differentiating chondroprogenitor cells or in mature chondrocytes is relatively sparse. This review discusses the known functions of Ser/Thr-specific PPs in the molecular signalling pathways of chondrogenesis. PPs are clearly equally important as protein kinases to counterbalance the effect of reversible protein phosphorylation. Of the main Ser/Thr PPs, some of the functions of PP1, PP2A and PP2B have been characterised in the context of chondrogenesis. While PP1 and PP2A appear to negatively regulate chondrogenic differentiation and maintenance of chondrocyte phenotype, calcineurin is an important stimulatory mediator during chondrogenesis but becomes inhibitory in mature chondrocytes. Furthermore, PPs are implicated to be mediators during the pathogenesis of osteoarthritis that makes them potential therapeutic targets to be exploited in the close future. Among the many yet unexplored targets of PPs, modulation of plasma membrane ion channel function and participation in mechanotransduction pathways are emerging novel aspects of signalling during chondrogenesis that should be further elucidated. Besides the regulation of cellular ion homeostasis, other potentially significant novel roles for PPs during the regulation of in vitro chondrogenesis are discussed.

Intermittent PTHrP(1-34) exposure augments chondrogenesis and reduces hypertrophy of mesenchymal stromal cells

Phenotype instability and premature hypertrophy prevent the use of human mesenchymal stromal cells (MSCs) for cartilage regeneration. Aim of this study was to investigate whether intermittent supplementation of parathyroid hormone-related protein (PTHrP), as opposed to constant treatment, can beneficially influence MSC chondrogenesis and to explore molecular mechanisms below catabolic and anabolic responses. Human MSCs subjected to chondrogenic induction in high-density culture received PTHrP(1-34), forskolin, dbcAMP, or PTHrP(7-34) either constantly or via 6-h pulses (three times weekly), before proteoglycan, collagen type II, and X deposition; gene expression; and alkaline phosphatase (ALP) activity were assessed. While constant application of PTHrP(1-34) suppressed chondrogenesis of MSCs, pulsed application significantly increased collagen type 2 (COL2A1) gene expression and the collagen type II, proteoglycan, and DNA content of pellets after 6 weeks. Collagen type 10 (COL10A1) gene expression was little affected but Indian hedgehog (IHH) expression and ALP activity were significantly downregulated by pulsed PTHrP. A faster response to PTHrP exposure was recorded for ALP activity over COL2A1 regulation, suggesting that signal duration is critical for catabolic versus anabolic reactions. Stimulation of cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling by forskolin reproduced major effects of both treatment modes, whereas application of PTHrP(7-34) capable of protein kinase C (PKC) signaling was ineffective. Pulsed PTHrP exposure of MSCs stimulated chondrogenesis and reduced endochondral differentiation apparently uncoupling chondrogenic matrix deposition from hypertrophic marker expression. cAMP/PKA was the major signaling pathway triggering the opposing effects of both treatment modes. Intermittent application of PTHrP represents an important novel means to improve chondrogenesis of MSCs and may be considered as a supporting clinical-treatment mode for MSC-based cartilage defect regeneration.

Redundancy in regulation of chondrogenesis in MIA/CD-RAP-deficient mice

Recent in vitro analysis of MIA/CD-RAP-deficient (MIA(-/-)) mesenchymal stem cells revealed altered chondrogenic differentiation, characterised by enhanced proliferation and delayed differentiation. However, adult MIA(-/-) mice develop normally and show only ultrastructural defects of the cartilage but no major abnormalities. We therefore focused, in this study, on chondrogenesis in vivo in MIA(-/-) mouse embryos to reveal potential molecular changes during embryogenesis and possible redundant mechanisms, which explain the almost normal phenotype despite MIA/CD-RAP loss. In situ hybridisation analysis revealed larger expression areas of Col2a1 and Sox9 positive, proliferating chondrocytes at day 15.5 and 16.5 of embryogenesis in MIA(-/-) mice. The initially diminished zone of Col10a1-expressing hypertrophic chondrocytes at day 15.5 was compensated at day 16.5 in MIA(-/-) embryos. Supported by in vitro studies using mesenchymal stem cells, we discovered that chondrogenesis in MIA(-/-) mice is modified by enhanced Sox9, Sox6 and AP-2α expression. Finally, we identified reduced AP1 and CRE activity, analysed by reporter gene- and electrophoretic mobility shift assays, important for redundancy mechanism which rescued delayed hypertrophic differentiation and allows normal development of MIA(-/-) mice. In summary, as observed in other knockout models of molecules important for cartilage development and differentiation, viability and functional integrity is reached by remarkable molecular redundancy in MIA/CD-RAP knockout mice.

Parathyroid hormone-related protein is induced by hypoxia and promotes expression of the differentiated phenotype of human articular chondrocytes

PTHrP (parathyroid hormone-related protein) is crucial for normal cartilage development and long bone growth and acts to delay chondrocyte hypertrophy and terminal differentiation in the growth plate. After growth plate closure adult HACs (human articular chondrocytes) still produce PTHrP, suggesting a possible role for this factor in the permanent articular cartilage. However, the expression regulation and function of PTHrP in the permanent articular cartilage is unknown. Human articular cartilage is an avascular tissue and functions in a hypoxic environment. The resident chondrocytes have adapted to hypoxia and use it to drive their tissue-specific functions. In the present study, we explored directly in normal articular chondrocytes isolated from a range of human donors the effect of hypoxia on PTHrP expression and whether PTHrP can regulate the expression of the permanent articular chondrocyte phenotype. We show that in HACs PTHrP is up-regulated by hypoxia in a HIF (hypoxia-inducible factor)-1α and HIF-2α-dependent manner. Using recombinant PTHrP, siRNA-mediated depletion of endogenous PTHrP and by blocking signalling through its receptor [PTHR1 (PTHrP receptor 1)], we show that hypoxia-induced PTHrP is a positive regulator of the key cartilage transcription factor SOX9 [SRY (sex determining region on the Y chromosome)-box 9], leading to increased COL2A1 (collagen type II, α1) expression. Our findings thus identify PTHrP as a potential factor for cartilage repair therapies through its ability to promote the differentiated HAC phenotype.

A novel adipokine C1q/TNF-related protein 1 (CTRP1) regulates chondrocyte proliferation and maturation through the ERK1/2 signaling pathway

Adipose tissue-derived adipokines play important roles as regulators of skeletal growth and development. CTRP1, a paralog of adiponectin, is a member of the C1q and tumor necrosis factor (TNF)-related protein (CTRP) superfamily. It is expressed at high levels in adipose tissue and has recently emerged as a novel adipokine. In the present study, we provide the first evidence for a physiological role of the CTRP1 in chondrocyte proliferation and maturation using a mouse chondrocytic cell line, N1511. The CTRP1 protein was strongly expressed and predominantly distributed in the reserve and proliferative chondrocytes in the fetal growth plate and its mRNA decreased during the maturation of N1511 chondrocytes. Recombinant CTRP1 promoted proliferation of immature proliferating N1511 chondrocytes in a dose-dependent manner, whereas it inhibited maturation of maturing N1511 chondrocytes. The stimulatory effect of CTRP1 on chondrocyte proliferation was associated with activation of the extracellular signal-regulated kinases 1/2 (ERK1/2) signaling pathway. On the other hand, the inhibitory effect of CTRP1 on chondrocyte maturation is associated with suppression of the ERK1/2 pathway. These results suggest a novel physiological role for CTRP1 in endochondral ossification.

Effect of parathyroid hormone-related protein in an in vitro hypertrophy model for mesenchymal stem cell chondrogenesis

PURPOSE

Mesenchymal stem cells (MSCs) express markers of hypertrophic chondrocytes during chondrogenic differentiation. We tested the suitability of parathyroid hormone-related protein (PTHrP), a regulator of chondrocyte hypertrophy in embryonic cartilage development, for the suppression of hypertrophy in an in vitro hypertrophy model of chondrifying MSCs.

METHODS

Chondrogenesis was induced in human MSCs in pellet culture for two weeks and for an additional two weeks cultures were either maintained in standard chondrogenic medium or transferred to a hypertrophy-enhancing medium. PTHrP(1-40) was added to the medium throughout the culture period at concentrations from 1 to 1,000 pM. Pellets were harvested on days one, 14 and 28 for biochemical and histological analysis.

RESULTS

Hypertrophic medium clearly enhanced the hypertrophic phenotype, with increased cell size, and strong alkaline phosphatase (ALP) and type X collagen staining. In chondrogenic medium, 1-100 pM PTHrP(1-40) did not inhibit chondrogenic differentiation, whereas 1,000 pM PTHrP(1-40) significantly reduced chondrogenesis. ALP activity was dose-dependently reduced by PTHrP(1-40) at 10-1,000 pM in chondrogenic conditions. Under hypertrophy-enhancing conditions, PTHrP(1-40) did not inhibit the induction of the hypertrophy. At the highest concentration (1,000 pM) in the hypertrophic group, aggregates were partially dedifferentiated and differentiated areas of these aggregates maintained their hypertrophic appearance.

CONCLUSIONS

PTHrP(1-40) treatment dose-dependently reduced ALP expression in MSC pellets cultured under standard chondrogenic conditions and is thus beneficial for the maintenance of the chondrogenic phenotype in this medium condition. When cultured under hypertrophy-enhancing conditions, PTHrP(1-40) could not diminish the induced enhancement of hypertrophy in the MSC pellets.

SOX9 gene transfer via safe, stable, replication-defective recombinant adeno-associated virus vectors as a novel, powerful tool to enhance the chondrogenic potential of human mesenchymal stem cells

Introduction

Transplantation of genetically modified human bone marrow-derived mesenchymal stem cells (hMSCs) with an accurate potential for chondrogenic differentiation may be a powerful means to enhance the healing of articular cartilage lesions in patients. Here, we evaluated the benefits of delivering SOX9 (a key regulator of chondrocyte differentiation and cartilage formation) via safe, maintained, replication-defective recombinant adeno-associated virus (rAAV) vector on the capability of hMSCs to commit to an adequate chondrocyte phenotype compared with other mesenchymal lineages.

Methods

The rAAV-FLAG-hSOX9 vector was provided to both undifferentiated and lineage-induced MSCs freshly isolated from patients to determine the effects of the candidate construct on the viability, biosynthetic activities, and ability of the cells to enter chondrogenic, osteogenic, and adipogenic differentiation programs compared with control treatments (rAAV-lacZ or absence of vector administration).

Results

Marked, prolonged expression of the transcription factor was noted in undifferentiated and chondrogenically differentiated cells transduced with rAAV-FLAG-hSOX9, leading to increased synthesis of major extracellular matrix components compared with control treatments, but without effect on proliferative activities. Chondrogenic differentiation (SOX9, type II collagen, proteoglycan expression) was successfully achieved in all types of cells but strongly enhanced when the SOX9 vector was provided. Remarkably, rAAV-FLAG-hSOX9 delivery reduced the levels of markers of hypertrophy, terminal and osteogenic/adipogenic differentiation in hMSCs (type I and type X collagen, alkaline phosphatise (ALP), matrix metalloproteinase 13 (MMP13), and osteopontin (OP) with diminished expression of the osteoblast-related transcription factor runt-related transcription factor 2 (RUNX2); lipoprotein lipase (LPL), peroxisome proliferator-activated receptor gamma 2 (PPARG2)), as well as their ability to undergo proper osteo-/adipogenic differentiation. These effects were accompanied with decreased levels of β-catenin (a mediator of the Wnt signaling pathway for osteoblast lineage differentiation) and enhanced parathyroid hormone-related protein (PTHrP) expression (an inhibitor of hypertrophic maturation, calcification, and bone formation) via SOX9 treatment.

Conclusions

This study shows the potential benefits of rAAV-mediated SOX9 gene transfer to propagate hMSCs with an advantageous chondrocyte differentiation potential for future, indirect therapeutic approaches that aim at restoring articular cartilage defects in the human population.

Duplicated zebrafish co-orthologs of parathyroid hormone-related peptide (PTHrP, Pthlh) play different roles in craniofacial skeletogenesis

In mammals, parathyroid hormone-related peptide (PTHrP, alias PTH-like hormone (Pthlh)) acts as a paracrine hormone that regulates the patterning of cartilage, bone, teeth, pancreas, and thymus. Beyond mammals, however, little is known about the molecular genetic mechanisms by which Pthlh regulates early development. To evaluate conserved pathways of craniofacial skeletogenesis, we isolated two Pthlh co-orthologs from the zebrafish (Danio rerio) and investigated their structural, phylogenetic, and syntenic relationships, expression, and function. Results showed that pthlh duplicates originated in the teleost genome duplication. Zebrafish pthlha and pthlhb were maternally expressed and showed overlapping and distinct zygotic expression patterns during skeletal development that mirrored mammalian expression domains. To explore the regulation of duplicated pthlh genes, we studied their expression patterns in mutants and found that both sox9a and sox9b are upstream of pthlha in arch and fin bud cartilages, but only sox9b is upstream of pthlha in the pancreas. Morpholino antisense knockdown showed that pthlha regulates both sox9a and sox9b in the pharyngeal arches but not in the brain or otic vesicles and that pthlhb does not regulate either sox9 gene, which is likely related to its highly degraded nuclear localization signal. Knockdown of pthlha but not pthlhb caused runx2b overexpression in craniofacial cartilages and premature bone mineralization. We conclude that in normal cartilage development, sox9 upregulates pthlh, which downregulates runx2, and that the duplicated nature of all three of these genes in zebrafish creates a network of regulation by different co-orthologs in different tissues.

Osteogenic and chondrogenic potential of biomembrane cells from the PMMA-segmental defect rat model

A layer of cells (the "biomembrane") has been identified in large segmental defects between bone and surgically placed methacrylate spacers or antibiotic-impregnated cement beads. We hypothesize that this contains a pluripotent stem cell population with potential valuable applications in orthopedic tissue engineering. Objectives using biomembranes harvested from rat segmental defects were to: (1) Culture biomembrane cells in specialized media to direct progenitor cells along bone or cartilage cell differentiation lineages; (2) evaluate harvested biomembranes for mesenchymal stem cell markers, and (3) define relevant gene expression patterns in harvested biomembranes using microarray analysis. Culture in osteogenic media produced mineralized nodules; culture in chondrogenic media produced masses containing chondroitin sulfate/sulfated proteoglycans. Molecular analysis of biomembrane cells versus control periosteum showed significant upregulation of key genes functioning in mesenchymal stem cell differentiation, development, maintenance, and proliferation. Results identified significant upregulation of WNT receptor signaling pathway genes and significant upregulation of BMP signaling pathway genes. Findings confirm that the biomembrane has a pluripotent stem cell population. The ability to heal large bone defects is clinically challenging, and novel tissue engineering uses of the biomembrane hold great promise in treating non-unions, open fractures with large bone loss and/or infections, and defects associated with tumor resection.

Functional characterization and evolution of PTH/PTHrP receptors: insights from the chicken

BACKGROUND

The parathyroid hormone (PTH)-family consists of a group of structurally related factors that regulate calcium and bone homeostasis and are also involved in development of organs such as the heart, mammary gland and immune system. They interact with specific members of family 2 B1 G-protein coupled receptors (GPCRs), which have been characterised in teleosts and mammals. Two PTH/PTHrP receptors, PTH1R and PTH2R exist in mammals and in teleost fish a further receptor PTH3R has also been identified. Recently in chicken, PTH-family members involved in calcium transport were characterized and specific PTHRs are suggested to exist although they have not yet been isolated or functionally characterized. The aim of this study is to further explore the evolution and function of the vertebrate PTH/PTHrP system through the isolation, phylogenetic analysis and functional characterization of the chicken receptors.

RESULTS

Two PTHRs were isolated in chicken and sequence comparison and phylogenetic analysis indicate that the chicken receptors correspond to PTH1R and PTH3R, which emerged prior to the teleost/tetrapod divergence since they are present in cartilaginous fish. The vertebrate PTH2R receptor and its ligand TIP39 have been lost from bird genomes. Chicken PTH1R and PTH3R have a divergent and widespread tissue expression and are also evident in very early embryonic stages of development. Receptor stimulation studies using HEK293 cells stably expressing the chicken PTH1R and PTH3R and monitoring cAMP production revealed they are activated by chicken 1-34 N-terminal PTH-family peptides in a dose dependent manner. PTH-L and PTHrP were the most effective peptides in activating PTH1R (EC(50) = 7.7 nM and EC(50) = 22.7 nM, respectively). In contrast, PTH-L (100 nM) produced a small cAMP accumulation on activation of PTH3R but PTHrP and PTH (EC(50) = 2.5 nM and EC(50) = 22.1 nM, respectively) readily activated the receptor. PTHrP also stimulated intracellular Ca(2+) accumulation on activation of PTH1R but not PTH3R.

CONCLUSION

Two PTHR homologues of the vertebrate PTH1R and PTH3R were isolated and functionally characterized in chicken. Their distinct pattern of expression during embryo development and in adult tissues, together with their ligand preference, suggests that they have acquired specific functions, which have contributed to their maintenance in the genome. PTH2R and its activating ligand, TIP39, are absent from bird genomes. Nonetheless identification of putative PTH2R and TIP39 in the genome of an ancient agnathan, lamprey, suggests the PTH/PTHrP ligand and receptor family was already present in an early basal paraphyletic group of vertebrates and during the vertebrate radiation diverged via gene/genome duplication and deletion events. Knowledge of the role PTH/PTHrP system in early vertebrates will help to establish evolution of function.

Effects of intermittent versus continuous parathyroid hormone administration on condylar chondrocyte proliferation and differentiation

Endochondral ossification is a complex process involving chondrogenesis and osteogenesis regulated by many hormones and growth factors. Parathyroid hormone (PTH), one of the key hormones regulating bone metabolism, promotes osteoblast differentiation and osteogenesis by intermittent administration, whereas continuous PTH administration inhibits bone formation. However, the effects of PTH on chondrocyte proliferation and differentiation are still unclear. In this study, intermittent PTH administration presented enhanced effects on condylar chondrocyte differentiation and bone formation, as demonstrated by increased mineral nodule formation and alkaline phosphatase (ALP) activity, up-regulated runt-related transcription factor 2 (RUNX2), ALP, collagen type X (COL10a1), collagen type I (COL1a1), osteocalcin (OCN), bone sialoprotein (BSP), bone morphogenetic protein 2 (BMP2) and osterix (OSX) mRNA and/or protein expression. On the contrary, continuous PTH administration promoted condylar chondrocyte proliferation and suppressed its differentiation, as demonstrated by up-regulated collagen type II (COL2a1) mRNA expression, reduced mineral nodule formation and down-regulated expression of the mRNAs and/or proteins mentioned above. Our data suggest that PTH can regulate condylar chondrocyte proliferation and differentiation, depending on the type of PTH administration. These results provide new insight into the effects of PTH on condylar chondrocytes and new evidence for using local PTH administration to cure mandibular asymmetry.

Signaling networks in RUNX2-dependent bone development

RUNX2 is an essential transcription factor for osteoblast differentiation and chondrocyte maturation. SP7, another transcription factor, is required for osteoblast differentiation. Major signaling pathways, including FGF, Wnt, and IHH, also play important roles in skeletal development. RUNX2 regulates Sp7 expression at an early stage of osteoblast differentiation. FGF2 upregulates Runx2 expression and activates RUNX2, and gain-of-function mutations of FGFRs cause craniosynostosis and limb defect with upregulation of Runx2 expression. Wnt signaling upregulates Runx2 expression and activates RUNX2, and RUNX2 induces Tcf7 expression. IHH is required for Runx2 expression in osteoprogenitor cells during endochondral bone development, and RUNX2 directly regulates Ihh expression in chondrocytes. Thus, RUNX2 regulates osteoblast differentiation and chondrocyte maturation through the network with SP7 and with FGF, Wnt, and IHH signaling pathways during skeletal development.

Calcium/calmodulin-dependent protein kinase II activity regulates the proliferative potential of growth plate chondrocytes

For tissues that develop throughout embryogenesis and into postnatal life, the generation of differentiated cells to promote tissue growth is at odds with the requirement to maintain the stem cell/progenitor cell population to preserve future growth potential. In the growth plate cartilage, this balance is achieved in part by establishing a proliferative phase that amplifies the number of progenitor cells prior to terminal differentiation into hypertrophic chondrocytes. Here, we show that endogenous calcium/calmodulin-dependent protein kinase II (CamkII, also known as Camk2) activity is upregulated prior to hypertrophy and that loss of CamkII function substantially blocks the transition from proliferation to hypertrophy. Wnt signaling and Pthrp-induced phosphatase activity negatively regulate CamkII activity. Release of this repression results in activation of multiple effector pathways, including Runx2- and β-catenin-dependent pathways. We present an integrated model for the regulation of proliferation potential by CamkII activity that has important implications for studies of growth control and adult progenitor/stem cell populations.

Developmental mechanisms in articular cartilage degradation in osteoarthritis

Osteoarthritis is the most common arthritic condition, which involves progressive degeneration of articular cartilage. The most recent accomplishments have significantly advanced our understanding on the mechanisms of the disease development and progression. The most intriguing is the growing evidence indicating that extracellular matrix destruction in osteoarthritic articular cartilage resembles that in the hypertrophic zone of fetal growth plate during endochondral ossification. This suggests common regulatory mechanisms of matrix degradation in OA and in the development and can provide new approaches for the treatment of the disease by targeting reparation of chondrocyte phenotype.

Zfp521 is a target gene and key effector of parathyroid hormone-related peptide signaling in growth plate chondrocytes

In the growth plate, the interplay between parathyroid hormone-related peptide (PTHrP) and Indian hedgehog (Ihh) signaling tightly regulates chondrocyte proliferation and differentiation during longitudinal bone growth. We found that PTHrP increases the expression of Zfp521, a zinc finger transcriptional coregulator, in prehypertrophic chondrocytes. Mice with chondrocyte-targeted deletion of Zfp521 resembled PTHrP(-/-) and chondrocyte-specific PTHR1(-/-) mice, with decreased chondrocyte proliferation, early hypertrophic transition, and reduced growth plate thickness. Deleting Zfp521 increased expression of Runx2 and Runx2 target genes, and decreased Cyclin D1 and Bcl-2 expression while increasing Caspase-3 activation and apoptosis. Zfp521 associated with Runx2 in chondrocytes, antagonizing its activity via an HDAC4-dependent mechanism. PTHrP failed to upregulate Cyclin D1 and to antagonize Runx2, Ihh, and collagen X expression when Zfp521 was absent. Thus, Zfp521 is an important PTHrP target gene that regulates growth plate chondrocyte proliferation and differentiation.

Neural crest-specific loss of Prkar1a causes perinatal lethality resulting from defects in intramembranous ossification

The cranial neural crest (CNC) undergoes complex molecular and morphological changes during embryogenesis in order to form the vertebrate skull, and nearly three quarters of all birth defects result from defects in craniofacial development. The molecular events leading to CNC differentiation have been extensively studied; however, the role of the cAMP-dependent protein kinase [protein kinase A (PKA)] during craniofacial development has only been described in palate formation. Here, we provide evidence that strict PKA regulation in postmigratory CNC cells is essential during craniofacial bone development. Selective inactivation of Prkar1a, a regulatory subunit of the PKA holoenzyme, in the CNC results in perinatal lethality caused by dysmorphic craniofacial development and subsequent asphyxiation. Additionally, aberrant differentiation of CNC mesenchymal cells results in anomalous intramembranous ossification characterized by formation of cartilaginous islands in some areas and osteolysis of bony trabeculae with fibrous connective tissue stabilization in others. Genetic interaction studies revealed that genetic reduction of the PKA catalytic subunit C(alpha) was able to rescue the phenotype, whereas reduction in Cbeta had no effect. Overall, these observations provide evidence of the essential role of proper regulation of PKA during the ossification of the bones of the skull. This knowledge may have implications for the understanding and treatment of craniofacial birth defects.

PTH stimulated growth and decreased Col-X deposition are phosphotidylinositol-3,4,5 triphosphate kinase and mitogen activating protein kinase dependent in avian sterna

Type X collagen (Col-X) deposition is a marker of terminal differentiation during chondrogenesis, in addition to appositional growth and apoptosis. The parathyroid hormone/parathyroid hormone related peptide (PTH/PTHrP) receptor, or PPR, is a G-Protein coupled receptor (GPCR), which activates several downstream pathways, moderating chondrocyte differentiation, including suppression of Col-X deposition. An Avian sterna model was used to analyze the PPR GPCR downstream kinase role in growth rate and extracellular matrix (ECM) including Col-II, IX, and X. Phosphatidylinositol kinase (PI3K), mitogen activating protein kinase (MAPK) and protein kinase A (PKA) were inhibited with specific established inhibitors LY294002, PD98059, and H89, respectively to test the hypothesis that they could reverse/inhibit the PTH/PTHrP pathway. Excised E14 chick sterna were PTH treated with or without an inhibitor and compared to controls. Sternal length was measured every 24 hr. Cultured sterna were immuno-stained using specific antibodies for Col-II, IX, or X and examined via confocal microscopy. Increased growth in PTH-treated sterna was MAPK, PI3K, and PKA dose dependent, suggesting growth was regulated through multiple pathways. Col-X deposition was rescued in PTH-treated sterna in the presence of PI3K or MAPK inhibitors, but not with the PKA inhibitor. All three inhibitors moderately disrupted Col-II and Col-IX deposition. These results suggest that PTH can activate multiple pathways during chondrocyte differentiation.

beta2- and beta3-, but not beta1-adrenergic receptors are involved in osteogenesis of mouse mesenchymal stem cells via cAMP/PKA signaling

The osteogenic capacity of mesenchymal stem cells (MSCs) and the importance of beta-adrenergic signals in bone formation and resorption have been well investigated. However, little is known about the development of beta-adrenergic receptor (beta-AR) systems and the role of beta-adrenergic signals in osteogenic differentiation of MSCs, which is critically important in bone physiology and pharmacology. In this study, we demonstrated that both the mRNA and protein levels of beta2- and beta3-AR are up-regulated following osteogenesis of mouse MSCs. We also established that beta-AR agonists negatively while antagonists positively affect MSC osteogenesis. Both beta2- and beta3-AR are involved in MSC osteogenesis, with beta2-AR being dominant. The effect of beta-ARs on MSC osteogenesis is partly mediated via the cAMP/PKA signaling. These findings suggest that MSC is also a target for beta-adrenergic regulation and beta-adrenergic signaling plays a role in MSC osteogenesis.

FGF2-activated ERK mitogen-activated protein kinase enhances Runx2 acetylation and stabilization

Runx2 is a key transcription factor regulating osteoblast differentiation and skeletal morphogenesis, and FGF2 is one of the most important regulators of skeletal development. The importance of the ERK mitogen-activated protein (MAP) kinase pathway in cranial suture development was demonstrated by the findings that the inhibition of FGF/FGF receptor (FGFR) signaling by a MEK blocker prevents the premature suture closure caused by an Fgfr2 mutation in mice. We previously demonstrated that ERK activation does not affect Runx2 gene expression but that it stimulates Runx2 transcriptional activity. However, the molecular mechanism underlying Runx2 activation by FGF/FGFR or ERK was still unclear. In this study, we found that FGF2 treatment increased the protein level of exogenously overexpressed Runx2 and that this increase is reversed by ERK inhibitors. In contrast, overexpression of constitutively active MEK strongly increased the Runx2 protein level, which paralleled an increase in Runx2 acetylation. As Runx2 protein phosphorylation mediated by ERK directly correlates with Runx2 protein stabilization, acetylation, and ubiquitination, we undertook to identify the ERK-dependent phosphorylation sites in Runx2. Analysis of two C-terminal Runx2 deletion constructs showed that the middle third of the protein is responsible for ERK-induced stabilization and activation. An in silico analysis of highly conserved ERK targets indicated that there are three relevant serine residues in this domain. Site-directed mutagenesis implicated Ser-301 in for ERK-mediated Runx2 stabilization and acetylation. In conclusion, the FGF2-induced ERK MAP kinase strongly increased the Runx2 protein level through an increase in acetylation and a decrease in ubiquitination, and these processes require the phosphorylation of Runx2 Ser-301 residue.

cAMP/PKA signaling inhibits osteogenic differentiation and bone formation in rodent models

We previously demonstrated that cAMP-mediated protein kinase A (PKA) activation induces in vitro osteogenesis and in vivo bone formation by human mesenchymal stem cells (hMSCs). To analyze the species-specific response of this phenomenon and to translate our findings into a clinical trial, suitable animal models and cell lines are desirable. In this report, we assessed whether PKA plays a similar proosteogenic role played by two commonly used PKA activators-N6,2'-O-dibutyryl-cAMP (db-cAMP) and 8-bromo cAMP (8b-cAMP)-in a number of model systems. To this end, we treated MC3T3-E1 cells, mouse calvarial osteoblasts, mouse MSCs, and rat MSCs with cAMP. We demonstrate that cAMP inhibits osteogenesis in rodent cell types, evidenced by inhibition of osteogenic markers such as alkaline phosphatase (ALP), osteocalcin (BGLAP), and collagen type 1 (COL1A1). In support of this, ex vivo-cultured mouse calvaria exposed to db-cAMP showed a reduction in bone volume. Interestingly, cAMP even stimulated adipogenic differentiation in rat MSCs. Taken together, our data demonstrate that cAMP inhibits osteogenesis in vitro and bone formation ex vivo in rodent models in contrast to our earlier findings in hMSCs. The species discrepancy in response to various osteogenic signals is a critical need to be tested in clinically relevant models to translate the fundamental findings in lower species level to clinical applications.

Parathyroid hormone-related peptide represses chondrocyte hypertrophy through a protein phosphatase 2A/histone deacetylase 4/MEF2 pathway

The maturation of immature chondrocytes to hypertrophic chondrocytes is regulated by parathyroid hormone-related peptide (PTHrP). We demonstrate that PTHrP or forskolin administration can block induction of collagen X-luciferase by exogenous Runx2, MEF2, and Smad1 in transfected chondrocytes. We have found that PTHrP/forskolin administration represses the transcriptional activity of MEF2 and that forced expression of MEF2-VP16 can restore expression of the collagen X reporter in chondrocytes treated with these agents. PTHrP/forskolin induces dephosphorylation of histone deacetylase 4 (HDAC4) phospho-S246, which decreases interaction of HDAC4 with cytoplasmic 14-3-3 proteins and promotes nuclear translocation of HDAC4 and repression of MEF2 transcriptional activity. We have found that forskolin increases the activity of an HDAC4 phospho-S246 phosphatase and that forskolin-induced nuclear translocation of HDAC4 was reversed by the protein phosphatase 2A (PP2A) antagonist, okadaic acid. Finally, we demonstrate that knockdown of PP2A inhibits forskolin-induced nuclear translocation of HDAC4 and attenuates the ability of this signaling molecule to repress collagen X expression in chondrocytes, indicating that PP2A is critical for PTHrP-mediated regulation of chondrocyte hypertrophy.