Pleiotrophin inhibits chondrocyte proliferation and stimulates proteoglycan synthesis in mature bovine cartilage

Single cell RNA sequencing reveals emergent notochord-derived cell subpopulations in the postnatal nucleus pulposus

Intervertebral disc degeneration is a leading cause of chronic low back pain. Cell-based strategies that seek to treat disc degeneration by regenerating the central nucleus pulposus (NP) hold significant promise, but key challenges remain. One of these is the inability of therapeutic cells to effectively mimic the performance of native NP cells, which are unique amongst skeletal cell types in that they arise from the embryonic notochord. In this study, we use single cell RNA sequencing to demonstrate emergent heterogeneity amongst notochord-derived NP cells in the postnatal mouse disc. Specifically, we established the existence of progenitor and mature NP cells, corresponding to notochordal and chondrocyte-like cells, respectively. Mature NP cells exhibited significantly higher expression levels of extracellular matrix (ECM) genes including aggrecan, and collagens II and VI, along with elevated transforming growth factor-beta and phosphoinositide 3 kinase-protein kinase B signaling. Additionally, we identified Cd9 as a novel surface marker of mature NP cells, and demonstrated that these cells were localized to the NP periphery, increased in numbers with increasing postnatal age, and co-localized with emerging glycosaminoglycan-rich matrix. Finally, we used a goat model to show that Cd9+ NP cell numbers decrease with moderate severity disc degeneration, suggesting that these cells are associated with maintenance of the healthy NP ECM. Improved understanding of the developmental mechanisms underlying regulation of ECM deposition in the postnatal NP may inform improved regenerative strategies for disc degeneration and associated low back pain.

Single Cell RNA Sequencing Reveals Emergent Notochord-Derived Cell Subpopulations in the Postnatal Nucleus Pulposus

Intervertebral disc degeneration is a leading cause of chronic low back pain. Cell-based strategies that seek to treat disc degeneration by regenerating the central nucleus pulposus hold significant promise, but key challenges remain. One of these is the inability of therapeutic cells to effectively mimic the performance of native nucleus pulposus cells, which are unique amongst skeletal cell types in that they arise from the embryonic notochord. In this study we use single cell RNA sequencing to demonstrate emergent heterogeneity amongst notochord-derived nucleus pulposus cells in the postnatal mouse disc. Specifically, we established the existence of early and late stage nucleus pulposus cells, corresponding to notochordal progenitor and mature cells, respectively. Late stage cells exhibited significantly higher expression levels of extracellular matrix genes including aggrecan, and collagens II and VI, along with elevated TGF-β and PI3K-Akt signaling. Additionally, we identified Cd9 as a novel surface marker of late stage nucleus pulposus cells, and demonstrated that these cells were localized to the nucleus pulposus periphery, increased in numbers with increasing postnatal age, and co-localized with emerging glycosaminoglycan-rich matrix. Finally, we used a goat model to show the Cd9+ nucleus pulposus cell numbers decrease with moderate severity disc degeneration, suggesting that these cells are associated with maintenance of the healthy nucleus pulposus extracellular matrix. Improved understanding of the developmental mechanisms underlying regulation of ECM deposition in the postnatal NP may inform improved regenerative strategies for disc degeneration and associated low back pain.

Effects of mechanical loading on the expression of pleiotrophin and its receptor protein tyrosine phosphatase beta/zeta in a rat spinal deformity model

Mechanical loading of the spine is a major causative factor of degenerative changes and causes molecular and structural changes in the intervertebral disc (IVD) and the vertebrae end plate (EP). Pleiotrophin (PTN) is a growth factor with a putative role in bone remodeling through its receptor protein tyrosine phosphatase beta/zeta (RPTPβ/ζ). The present study investigates the effects of strain on PTN and RPTPβ/ζ protein expression in vivo. Tails of eight weeks old Sprague-Dawley rats were subjected to mechanical loading using a mini Ilizarov external apparatus. Rat tails untreated (control) or after 0 degrees of compression and 10°, 30° and 50° of angulation (groups 0, I, II and III respectively) were studied. PTN and RPTPβ/ζ expression were evaluated using immunohistochemistry and Western blot analysis. In the control group, PTN was mostly expressed by the EP hypertrophic chondrocytes. In groups 0 to II, PTN expression was increased in the chondrocytes of hypertrophic and proliferating zones, as well as in osteocytes and osteoblast-like cells of the ossification zone. In group III, only limited PTN expression was observed in osteocytes. RPTPβ/ζ expression was increased mainly in group 0, but also in group I, in all types of cells. Low intensity RPTPβ/ζ immunostaining was observed in groups II and III. Collectively, PTN and RPTPβ/ζ are expressed in spinal deformities caused by mechanical loading, and their expression depends on the type and severity of the applied strain.

Pleiotrophin commits human bone marrow mesenchymal stromal cells towards hypertrophy during chondrogenesis

Pleiotrophin (PTN) is a growth factor present in the extracellular matrix of the growth plate during bone development and in the callus during bone healing. Bone healing is a complicated process that recapitulates endochondral bone development and involves many cell types. Among those cells, mesenchymal stromal cells (MSC) are able to differentiate toward chondrogenic and osteoblastic lineages. We aimed to determine PTN effects on differentiation properties of human bone marrow stromal cells (hBMSC) under chondrogenic induction using histological analysis and quantitative reverse transcription polymerase chain reaction. PTN dramatically potentiated chondrogenic differentiation as indicated by a strong increase of collagen 2 protein, and cartilage-related gene expression. Moreover, PTN increased transcription of hypertrophic chondrocyte markers such as MMP13, collagen 10 and alkaline phosphatase and enhanced calcification and the content of collagen 10 protein. These effects are dependent on PTN receptors signaling and PI3 K pathway activation. These data suggest a new role of PTN in bone regeneration as an inducer of hypertrophy during chondrogenic differentiation of hBMSC.

Effects of oxygen on zonal marker expression in human articular chondrocytes

Articular cartilage is organized in depth zones with phenotypically distinct subpopulations of chondrocytes that are exposed to different oxygen tensions. Despite growing evidence of the critical role for oxygen in chondrogenesis, little is known about its effect on chondrocytes from different zones. This study evaluates zonal marker expression of human articular chondrocytes from different zones under various oxygen tensions. Chondrocytes isolated from full-thickness, superficial, and middle/deep cartilage from knee replacement surgeries were expanded and redifferentiated under hypoxic (5% O(2)) or normoxic (20% O(2)) conditions. Differentiation under hypoxia increased expression of hypoxia-inducible factors 1alpha and 2alpha and accumulation of extracellular matrix, particularly in middle/deep chondrocytes, and favored re-expression of proteoglycan 4 by superficial chondrocytes compared with middle/deep cells. Zone-dependent expression of clusterin varied with culture duration. These results demonstrate that zonal chondrocytes retain important phenotypic differences during in vitro cultivation, and that these characteristics can be improved by altering the oxygen environment. However, transcript levels for pleiotrophin, cartilage intermediate layer protein, and collagen type X were similar between zones, challenging their reliability as zonal markers for tissue-engineered cartilage from osteoarthritis patients. Key factors including oxygen tension and cell source should be considered to prescribe zone-specific properties to tissue-engineered cartilage.

Alterations of overused supraspinatus tendon: a possible role of glycosaminoglycans and HARP/pleiotrophin in early tendon pathology

Supraspinatus tendon overuse injuries lead to significant pain and disability in athletes and workers. Despite the prevalence and high social cost of these injuries, the early pathological events are not well known. We analyzed the potential relation between glycosaminoglycan (GAG) composition and phenotypic cellular alteration using a rat model of rotator cuff overuse. Total sulfated GAGs increased after 4 weeks of overuse and remained elevated up to 16 weeks. GAG accumulation was preceded by up-regulation of decorin, versican, and aggrecan proteoglycans (PGs) mRNAs and proteins and biglycan PG mRNA after 2 weeks. At 2 weeks, collagen 1 transcript decreased whereas mRNAs for collagen 2, collagen 3, collagen 6, and the transcription factor Sox9 were increased. Protein levels of heparin affine regulatory peptide (HARP)/pleiotrophin, a cytokine known to regulate developmental chondrocyte formation, were enhanced especially at 4 weeks, without up-regulation of HARP/pleiotrophin mRNA. Further results suggest that the increased GAGs present in early lesions may sequester HARP/pleiotrophin, which could contribute to a loss of tenocyte's phenotype. All these modifications are characteristic of a shift towards the chondrocyte phenotype. Identification of these early changes in the extra-cellular matrix may help to prevent the progression of the pathology to more disabling, degenerative alterations.

Recombinant human midkine stimulates proliferation of articular chondrocytes

OBJECTIVES

Midkine, a heparin-binding growth factor, promotes population growth, survival and migration of several cell types, but its effect on articular chondrocytes remains unknown. The aim of this study was to investigate its role on proliferation of articular chondrocytes in vitro and in vivo.

MATERIALS AND METHODS

Bromodeoxyuridine incorporation and MTT assays were performed to examine the proliferative effect of recombinant human midkine (rhMK) on primary articular chondrocytes. Activation of extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K) was analysed using western blot analysis. Systemic and local delivery of rhMK into mice and rats was preformed to investigate the proliferative effect of rhMK in vivo, respectively. Histological evaluation, including measurement of articular cartilage thickness, cell density, matrix staining and immunostaining of proliferating cell nuclear antigen was carried out.

RESULTS

rhMK promoted proliferation of articular chondrocytes cultured in a monolayer, which was mediated by activation of ERK and PI3K. The proliferative role of rhMK was not coupled to dedifferentiation of culture-expanded cells. Consistent with its action in vitro, rhMK stimulated proliferation of articular chondrocytes in vivo when it was administered subcutaneously and intra-articularly in mice and rats, respectively.

CONCLUSION

Our results demonstrate that rhMK stimulates proliferation of primary articular chondrocytes in vitro and in vivo. The results of this study warrant further examination of rhMK for treatment of animal models of articular cartilage defects.

Variations in gene and protein expression in human nucleus pulposus in comparison with annulus fibrosus and cartilage cells: potential associations with aging and degeneration

OBJECTIVE

Regardless of recent progress in the elucidation of intervertebral disc (IVD) degeneration, the basic molecular characteristics that define a healthy human IVD are largely unknown. Although work in different animal species revealed distinct molecules that might be used as characteristic markers for IVD or specifically nucleus pulposus (NP) cells, the validity of these markers for characterization of human IVD cells remains unknown.

DESIGN

Eleven potential marker molecules were characterized with respect to their occurrence in human IVD cells. Gene expression levels of NP were compared with annulus fibrosus (AF) and articular cartilage (AC) cells, and potential correlations with aging were assessed.

RESULTS

Higher mRNA levels of cytokeratin-19 (KRT19) and of neural cell adhesion molecule-1 were noted in NP compared to AF and AC cells. Compared to NP cytokeratin-18 expression was lower in AC, and alpha-2-macroglobulin and desmocollin-2 lower in AF. Cartilage oligomeric matrix protein (COMP) and glypican-3 expression was higher in AF, while COMP, matrix gla protein (MGP) and pleiotrophin expression was higher in AC cells. Furthermore, an age-related decrease in KRT19 and increase in MGP expression were observed in NP cells. The age-dependent expression pattern of KRT19 was confirmed by immunohistochemistry, showing the most prominent KRT19 immunoreaction in the notochordal-like cells in juvenile NP, whereas MGP immunoreactivity was not restricted to NP cells and was found in all age groups.

CONCLUSIONS

The gene expression of KRT19 has the potential to characterize human NP cells, whereas MGP cannot serve as a characteristic marker. KRT19 protein expression was only detected in NP cells of donors younger than 54 years.

Pleiotrophin is expressed in avian somites and tendon anlagen

Pleiotrophin (Ptn) is a secreted, developmentally regulated growth factor associated with the extracellular matrix. During mammalian embryogenesis, Ptn has been suggested to play a role in the development of various embryonic structures including nervous system and skeleton. In the avian embryo, Ptn has been proposed to be involved in limb cartilage development, but embryonic Ptn expression has not been comprehensively studied. We isolated a cDNA fragment containing the full-length coding sequence of chick Ptn and studied the expression of Ptn in detail until embryonic day 10. We, furthermore, isolated a 6,385-bp phage clone containing the Ptn cDNA of 2,551 bp and additional 3,787 bp downstream of the published Ptn cDNA sequence classifying a yet Ptn-unrelated chEST clone as the 3' untranslated region of Ptn. Our studies revealed novel expression domains in developing somites and during limb formation. We found prominent expression in the somitocoel cells of epithelial somites, and in a sclerotomal subcompartment, the syndetome, which gives rise to the axial tendons in the vertebral motion segment. In the limbs, Ptn was markedly expressed in tendon anlagen and in phalangeal joints. Our results introduce Ptn as a novel marker gene in avian somite and tendon development.

The Potential of N-Rich Plasma-Polymerized Ethylene (PPE:N) Films for Regulating the Phenotype of the Nucleus Pulposus

We recently developed a nitrogen-rich plasma-polymerized biomaterial, designated “PPE:N” (N-doped plasma-polymerized ethylene) that is capable of suppressing cellular hypertrophy while promoting type I collagen and aggrecan expression in mesenchymal stem cells from osteoarthritis patients. We then hypothesized that these surfaces would form an ideal substrate on which the nucleus pulposus (NP) phenotype would be maintained. Recent evidence using microarrays showed that in young rats, the relative mRNA levels of glypican-3 (GPC3) and pleiotrophin binding factor (PTN) were significantly higher in nucleus pulposus (NP) compared to annulus fibrosus (AF) and articular cartilage. Furthermore, vimentin (VIM) mRNA levels were higher in NP versus articular cartilage. In contrast, the levels of expression of cartilage oligomeric matrix protein (COMP) and matrix gla protein precursor (MGP) were lower in NP compared to articular cartilage. The objective of this study was to compare the expression profiles of these genes in NP cells from fetal bovine lumbar discs when cultured on either commercial polystyrene (PS) tissue culture dishes or on PPE:N with time. We found that the expression of these genes varies with the concentration of N ([N]). More specifically, the expression of several genes of NP was sensitive to [N], with a decrease of GPC3, VIM, PTN, and MGP in function of decreasing [N]. The expression of aggrecan, collagen type I, and collagen type II was also studied: no significant differences were observed in the cells on different surfaces with different culture time. The results support the concept that PPE:N may be a suitable scaffold for the culture of NP cells. Further studies are however necessary to better understand their effects on cellular phenotypes.

Heparanase expression and activity influences chondrogenic and osteogenic processes during endochondral bone formation

Endochondral bone formation is a highly orchestrated process involving coordination among cell-cell, cell-matrix and growth factor signaling that eventually results in the production of mineralized bone from a cartilage template. Chondrogenic and osteogenic differentiation occur in sequence during this process, and the temporospatial patterning clearly requires the activities of heparin binding growth factors and their receptors. Heparanase (HPSE) plays a role in osteogenesis, but the mechanism by which it does so is incompletely understood. We used a combination of ex vivo and in vitro approaches and a well described HPSE inhibitor, PI-88 to study HPSE in endochondral bone formation. In situ hybridization and immunolocalization with HPSE antibodies revealed that HPSE is expressed in the peri-chondrium, peri-osteum, and at the chondro-osseous junction, all sites of key signaling events and tissue morphogenesis. Transcripts encoding Hpse also were observed in the pre-hypertrophic zone. Addition of PI-88 to metatarsals in organ culture reduced growth and suggested that HPSE activity aids the transition from chondrogenic to osteogenic processes in growth of long bones. To study this, we used high density cultures of ATDC5 pre-chondrogenic cells grown under conditions favoring chondrogenesis or osteogenesis. Under chondrogenic conditions, HPSE/Hpse was expressed at high levels during the mid-culture period, at the onset of terminal chondrogenesis. PI-88 addition reduced chondrogenesis and accelerated osteogenesis, including a dramatic up-regulation of osteocalcin levels. In normal growth medium, addition of PI-88 reduced migration of ATDC-5 cells, suggesting that HPSE facilitates cartilage replacement by bone at the chondro-osseous junction by removing the HS component of proteoglycans, such as perlecan/HSPG2, that otherwise prevent osteogenic cells from remodeling hypertrophic cartilage.

A phenotypic comparison of intervertebral disc and articular cartilage cells in the rat

The basic molecular characteristics of intervertebral disc cells are still poorly defined. This study compared the phenotypes of nucleus pulposus (NP), annulus fibrosus (AF) and articular cartilage (AC) cells using rat coccygeal discs and AC from both young and aged animals and a combination of microarray, real-time RT-PCR and immunohistochemistry. Microarray analysis identified 63 genes with at least a fivefold difference in fluorescence intensity between the NP and AF cells and 41 genes with a fivefold or greater difference comparing NP cells and articular chondrocytes. In young rats, the relative mRNA levels, assessed by real-time RT-PCR, of annexin A3, glypican 3 (gpc3), keratin 19 (k19) and pleiotrophin (ptn) were significantly higher in NP compared to AF and AC samples. Furthermore, vimentin (vim) mRNA was higher in NP versus AC, and expression levels of cartilage oligomeric matrix protein (comp) and matrix gla protein (mgp) were lower in NP versus AC. Higher NP levels of comp and mgp mRNA and higher AF levels of gpc3, k19, mgp and ptn mRNA were found in aged compared to young tissue. However, the large differences between NP and AC expression of gpc3 and k19 were obvious even in the aged animals. Furthermore, the differences in expression levels of gpc3 and k19 were also evident at the protein level, with intense immunostaining for both proteins in NP and non-existent immunoreaction in AF and AC. Future studies using different species are required to evaluate whether the expression of these molecules can be used to characterize NP cells and distinguish them from other chondrocyte-like cells.

Targeting pleiotropin to treat osteoarthritis

Pleiotropin (PTN) is a secreted heparin-binding peptide expressed in mesodermal and neuroectodermal cells during development, but rarely in adult tissues. Although PTN is abundant in fetal or juvenile cartilage, it is undectable in mature cartilage. However, PTN is re-expressed in chondrocytes in early stages of osteoarthritis where it is detectable in situ and in synovial fluids from patients. PTN enhances chondrogenesis by stimulation of extra-cellular matrix synthesis, reduction of degrading matrix metalloproteases and induction of their inhibitors; PTN also slightly reduces pro-inflammatory factors, such as nitric oxide and vascular endothelial growth factor. Furthermore, PTN stimulates chondrocyte clustering and proliferation. Thus, PTN appears to mediate repair and protective processes in osteoarthritic cartilage and appears to be a promising factor to treat osteoarthritis.

The presence of pleiotrophin in the human intervertebral disc is associated with increased vascularization: an immunohistologic study

STUDY DESIGN

An immunohistological study of surgical specimens of human intervertebral disc.

OBJECTIVE

To examine the presence of pleiotrophin in diseased or damaged intervertebral disc tissue and the association between its presence and the extent of tissue vascularization and innervation.

SUMMARY OF BACKGROUND DATA

Increased levels of pleiotrophin, a growth and differentiation factor that is active in various pathophysiologic processes, including angiogenesis, has been associated with osteoarthritic changes of human articular cartilage. The association between pleiotrophin expression and pathologic conditions of the human intervertebral disc is unknown.

METHODS

Specimens of human lumbar intervertebral discs, obtained following surgical discectomy, were divided into 3 groups: non-degenerated discs (n = 7), degenerated discs (n = 6), and prolapsed discs (n = 11). Serial tissue sections of each specimen were immunostained to determine the presence of pleiotrophin, blood vessels (CD34-positive endothelial cells), and nerves (neurofilament 200 kDa [NF200]-positive nerve fibers).

RESULTS

Pleiotrophin immunoreactivity was seen in disc cells, endothelial cells, and in the extracellular matrix in most specimens of intervertebral disc but was most prevalent in vascularized tissue in prolapsed discs. There was a significant correlation between the presence of pleiotrophin-positive disc cells and that of CD34-positive blood vessels. NF200-positive nerves were seen in vascularized areas of more degenerated discs, but nerves did not appear to codistribute with blood vessels or pleiotrophin positivity in prolapsed discs.

CONCLUSIONS

Pleiotrophin is present in pathologic human intervertebral discs, and its prevalence and distribution suggest that it may play a role in neovascularization of diseased or damaged disc tissue.

Effects of pleiotrophin, a heparin-binding growth factor, on human primary and immortalized chondrocytes

OBJECTIVE

Pleiotrophin (PTN) is a secreted heparin-binding peptide expressed in mesodermal and neuroectodermal cells during development, but rarely in adult tissues. In fetal and juvenile, but not in mature cartilage, PTN is abundant. Furthermore, PTN is re-expressed in chondrocytes in early stages of osteoarthritis (OA). Since little is known about the functions of PTN in cartilage, we investigated the occurrence of PTN receptors in human articular cartilage in situ and PTN effects on human primary and immortalized chondrocytes in vitro.

METHODS

Receptor expression and regulation was monitored by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry. PTN effects and signal transduction were studied by electrophoretic mobility shift, Boyden chamber cell migration and proliferation assays, effects on gene expression by real time RT-PCR and that on nitric oxide (NO) by the Griess reaction.

RESULTS

Of the putative PTN signaling receptors, immortalized and primary chondrocytes (pc) expressed the anaplastic lymphoma kinase (ALK), less the receptor-type protein tyrosine phosphatase zeta/beta (PTPzeta). ALK expression was upregulated upon ligand exposure. PTN stimulation activated the AP-1 (activator protein-1) transcription factor and altered gene expression. Prolonged stimulation induced PTN mRNA expression slightly, reduced vascular endothelial growth factor (VEGF) mRNA as well as NO production. Whereas mRNA expression of matrix metalloproteinases (MMPs) MMP-1 and MMP-13 was reduced, their inhibitors TIMP-1 and TIMP-2 were induced. Furthermore, PTN stimulated chondrocyte migration and proliferation.

CONCLUSIONS

These results show that PTN is an autocrine growth factor in cartilage. We suggest that PTN may be involved in the clustering and proliferation of chondrocytes observed in the early stages of OA.

Activation of genes for growth factor and cytokine pathways late in chondrogenic differentiation of ATDC5 cells

The mouse embryonal carcinoma cell line ATDC5 provides an excellent model system for chondrogenesis in vitro. To understand better the molecular mechanisms of endochondral bone formation, we investigated gene expression profiles during the differentiation course of ATDC5 cells, using an in-house microarray harboring full-length-enriched cDNAs. For 28 days following chondrogenic induction, 507 genes were up- or down-regulated at least 1.5-fold. These genes were classified into five clusters based on their expression patterns. Genes for growth factor and cytokine pathways were significantly enriched in the cluster characterized by increases in expression during late stages of chondrocyte differentiation. mRNAs for decorin and osteoglycin, which have been shown to bind to transforming growth factors-beta and bone morphogenetic proteins, respectively, were found in this cluster and were detected in hypertrophic chondrocytes of developing mouse bones by in situ hybridization analysis. Taken together with assigned functions of individual genes in the cluster, interdigitated interaction between a number of intercellular signaling molecules is likely to take place in the late chondrogenic stage for autocrine and paracrine regulation among chondrocytes, as well as for chemoattraction and stimulation of progenitor cells of other lineages.

Expression of VEGF and pleiotrophin in deer antler

Deer antlers represent a unique model of mammalian regeneration in that they cast and fully regenerate every year. The deer antler thus provides a fascinating model of both rapid angiogenesis and chondrogenesis and the opportunity to investigate unique growth regulatory processes. One such phenomenon is the presence of vascularized cartilage in the growing antler tip-unlike other cartilage, which is typically avascular. The mechanisms by which blood vessels grow in the cartilage as well as the factors that drive antler extension at approximately 1 cm a day have been hitherto largely unknown. The aim of this study was to determine the expression of VEGF and pleiotrophin within the growing antler tip. We isolated cervine VEGF121 and VEGF165 from deer antler and found that mRNA is produced for VEGF in the precartilage and cartilage regions. By in situ hybridization, we examined whether the VEGF receptors Flt-1 and KDR are present in deer antler and found only KDR mRNA within the endothelial cells of the precartilage region. This finding is compatible with VEGF having an angiogenic effect within antler. Pleiotrophin mRNA was found in the vascular smooth muscle cells of the dermis, thus supporting a possible role in vascular growth. High levels of pleiotrophin mRNA were also detected in the precartilage region with possible implications for both angiogenesis and chondrogenesis. This is the first report of cervine angiogenic growth factors within the growing antler tip.

Subchondral bone osteoblasts induce phenotypic changes in human osteoarthritic chondrocytes

OBJECTIVE

To determine the influence of osteoarthritic (OA) phenotype of subchondral osteoblasts on the phenotype of human chondrocytes.

METHODS

Human chondrocytes were isolated from OA cartilage and cultured in alginate beads for 4 or 10 days in the absence or in the presence of osteoblasts in monolayer. The osteoblasts were either isolated from non-sclerotic (N) or sclerotic (SC) zones of human subchondral bone. Before co-culture, osteoblasts were incubated for 72 h with or without 1.7 ng/ml interleukin (IL)-1beta, 100 ng/ml IL-6 with its soluble receptor (50 ng/ml) or 10 ng/ml oncostatin M. SOX9, type I, II and X collagen (COL1, COL2, COL10), osteoblasts-stimulating factor (OSF)-1, bone alkaline phosphatase (ALP), parathyroid hormone related peptide (PTHrP) and its receptor (PTH-R) messenger RNA (mRNA) levels in chondrocytes were quantified by real-time polymerase chain reaction.

RESULTS

In comparison with chondrocytes cultured alone in alginate beads, chondrocytes after 4 days in co-culture with N or SC osteoblasts expressed significantly less SOX9 and COL2 mRNA. The decrease of SOX9 and COL2 gene expression was significantly more pronounced in the presence of SC than in the presence of N osteoblasts (P<0.001). OSF-1 mRNA level in chondrocyte was increased by both N and SC osteoblasts, but to a larger extent by SC osteoblasts (P<0.001). PTHrP expression in chondrocytes was 21-fold increased by N osteoblasts but four-fold inhibited by SC osteoblasts. PTHrP secretion was also increased by N but reduced by SC osteoblasts. SC, but not N osteoblasts, induced a significant decrease of PTH-R gene expression in chondrocyte. In our experimental conditions, chondrocytes did not express COL1, COL10 or ALP, even after 10 days of co-culture with osteoblasts.

CONCLUSIONS

In co-culture, SC subchondral osteoblasts decrease SOX9, COL2, PTHrP and PTH-R gene expression by chondrocytes but increase that of OSF-1. These findings suggest that SC osteoblasts could initiate chondrocyte phenotype shift towards hypertrophic differentiation and subsequently further matrix mineralization.

Heparan sulfate proteoglycans: Coordinators of multiple signaling pathways during chondrogenesis

Heparan sulfate proteoglycans are abundantly expressed in the pericellular matrix of both developing and mature cartilage. Increasing evidence indicates that the action of numerous chondroregulatory molecules depends on these proteoglycans. This review summarizes the current understanding of the interactions of heparan sulfate chains of cartilage proteoglycans with both soluble and nonsoluble ligands during the process of chondrogenesis. In addition, the consequences of mutating genes encoding heparan sulfate biosynthetic enzymes or heparan sulfate proteoglycan core proteins on cartilage development are discussed.

HIP/RPL29 down-regulation accompanies terminal chondrocyte differentiation

HIP is a heparin/heparan sulfate (Hp/HS) binding protein identical to ribosomal protein L29 that displays diverse biological functions. There is strong evidence that abnormal expression and quantitative deficiencies of essential molecules such as extracellular matrix (ECM) proteins, transcription factors, and ribosomal proteins can seriously impair embryonic development. As observed for HS-bearing molecules, high levels of HIP/RPL29 are found in proliferating chondrocytic precursors and chondrocytes of developing growth plate. Here, we demonstrate both in vitro and in developing mouse embryos that HIP/RPL29 is down-regulated in terminally differentiated chondrocytes corresponding to the late hypertrophic zone of the growth plate. Because cartilage serves as a template for endochondral bone formation, we hypothesize that the presence of HIP/RPL29 during early chondrogenesis is essential for normal skeletal growth and patterning. In particular, we believe that HIP/RPL29 expression is required to maintain proliferation of chondrocytes and avoid skeletal shortening. Increasing evidence suggests that multifunctional ribosomal proteins of eukaryotic cells are important regulators of cell growth and differentiation, not simply structural parts of translational machinery. To investigate the role of HIP/RPL29 normal expression during cartilage formation, we designed a ribozyme-mediated knock-down approach to partially down-regulate HIP/RPL29 expression in the multipotent mouse embryonic skin fibroblast cell line C3H/10T (1/2). This technology permitted us to avoid the insufficient expression associated with more severe consequences, such as lethality, and provided advantages similar to those obtained with mutations generating hypomorphic phenotypes. Our results show that partial reduction of HIP/RPL29 levels accelerates differentiation of C3H/10T(1/2) into cartilage-like cells. In conclusion, our data indicate that HIP/RPL29 constitutes an important novel regulator of chondrocytic growth and differentiation.

Pleiotrophin, an embryonic differentiation and growth factor, is expressed in osteoarthritis

OBJECTIVE

Pleiotrophin (PTN) is a 15.3 kDa heparin-binding peptide, which is expressed in mesodermal and neuroectodermal cells during development, but rarely in adult tissues. In fetal or juvenile cartilage, PTN is an abundant protein and appears to be involved in chondrocyte differentiation. Since developmentally regulated factors often re-appear in the disease state, we examined PTN expression in cartilage and synovial fluid of patients with osteoarthritis (OA).

METHODS

PTN mRNA and protein expression was assayed by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot, the protein was localized by immunohistochemistry and quantified by enzyme-linked immunoassay (ELISA).

RESULTS

PTN was undetectable in normal adult cartilage, but PTN mRNA and protein were found in OA. In cartilage from the tibial plateaus of OA patients, PTN could be immunostained in clusters of superficial chondrocytes. In the synovial fluids of OA patients, PTN concentrations were elevated in earlier OA stages, but rarely in late OA stages. Chondrosarcomas were PTN-immunonegative.

CONCLUSIONS

In addition to certain types of cancer, the embryonic growth and differentiation factor PTN is found also in adults in inflammatory diseases. In OA, PTN is especially expressed in early stages, and PTN concentrations in the synovial fluid could serve as a marker for the progress of the disease. PTN might be involved in cartilage repair in OA, in particular, in earlier stages.

Pleiotrophin, an angiogenic and mitogenic growth factor, is expressed in human gliomas

Pleiotrophin (PTN) is a mitogenic/angiogenic, 15.3 kDa heparin-binding peptide that is found in embryonic or early postnatal, but rarely in adult, tissues. Since developmentally regulated factors often re-appear in malignant cells, we examined PTN expression in human glioma cell lines, cell cultures derived from solid gliomas and glioma sections. PTN mRNA or protein was detected by reverse transcriptase-polymerase chain reaction, immunohistochemistry, western blot or enzyme-linked immunoassay in all WHO III and IV grade gliomas and cells analyzed in vitro or in situ. One WHO II grade glioma investigated was PTN negative. In vitro, PTN was synthesized in perinuclear regions of glioma cells, secreted into the cultivation medium, but its production varied considerably between glioma cells cultivated from different solid gliomas or glioma cell lines. In situ, PTN expression was restricted to distinct parts/cells of the tumour. PTN did not influence the proliferation of glioma cells themselves, but stimulated [3H]thymidine incorporation into DNA of microglial cells. Furthermore, in Boyden chamber assays, PTN showed a strong chemotactic effect on murine BV-2 microglial cells. PTN is supposed to be a paracrine growth/angiogenic factor that is produced by gliomas and contributes to their malignancy by targeting endothelial and microglial cells.

Isolation of differentially expressed genes from wild-type and Twist mutant mouse limb buds

In the mouse, Twist is required for normal limb and craniofacial development. We show that the aristaless-like transcription factors, Alx3 and Alx4 are downregulated in the Twist(-/-) mutant and may be potential targets of Twist. By suppression subtractive hybridization we isolated 31 and 18 unique clones representing mRNAs that are putatively downregulated and upregulated respectively in Twist(-/-) forelimb buds. These included genes encoding cytoskeletal components, metabolic enzymes, hemoglobin molecules, membrane transport proteins, components of transcription and translation complexes, protein modification enzymes and proteins related to cell proliferation and apoptosis. Differential expression of selected clones was validated by whole mount in situ hybridization to E10.5 wild-type and Twist(-/-) embryos. We show that four novel clones are expressed in the Twist-expressing craniofacial tissues and paraxial mesoderm and downregulated in Twist(-/-) embryos, raising the possibility that they are, in addition to genes of the Alx family, downstream targets of Twist.

Microarray analysis of Tbx2-directed gene expression: a possible role in osteogenesis

Tbx2 is a member of the developmentally important transcriptional regulatory T-box gene family, whose target genes have not been well characterized. In an attempt to identify genes that may be regulated by Tbx2, mouse cDNA microarrays were used to analyze differential gene expression profiles, comparing stably transfected NIH3T3 cells overexpressing Tbx2 and vector-transfected controls. Among 8734 genes, 107 genes were up-regulated by 2-fold or greater, and 66 genes were down-regulated by 2-fold or greater. Caveolin, pleiotrophin (osf-1), osteoblast-specific factor-2 (osf-2) and collagen type I alpha were among the genes upregulated in the Tbx2-overexpressing cells, whereas cadherin 3, tenascin C, and insulin-like growth factor binding protein 10/CYR61 (IBP10) were among the genes downregulated. Northern blot analysis confirmed the correlation of expression of several genes, including IBP10 and osf-2, in fibroblast NIH3T3 and rat osteosarcoma ROS17/2.8 cells differentially expressing Tbx2. In ROS17/2.8 cells transfected with antisense Tbx2, osf-2 was downregulated, whereas transfection of sense Tbx2 upregulated this gene. Interestingly, the expression of pleiotrophin (osf-1) and collagen I alpha with Tbx2 transfection showed an inverse regulatory correlation between NIH3T3 and ROS17/2.8 cells. Thus, Tbx2 can act as both a repressor and activator, and the cellular context can influence the effect on gene expression. Although the data do not address whether Tbx2 directly mediates the transcriptional effect, a number of candidate genes possess putative T-box gene regulatory elements. The results support the hypothesis that Tbx2 may be an important modulator of bone development. Further functional cluster analysis indicates that Tbx2 might also be involved in the regulation of cell cycle and cell adhesion.

Pleiotrophin induces angiogenesis: involvement of the phosphoinositide-3 kinase but not the nitric oxide synthase pathways

Pleiotrophin (PTN) is a developmentally regulated protein that has been shown to be involved in tumor growth and metastasis presumably by activating tumor angiogenesis. To clarify the potential angiogenic activity of PTN and to analyze the signaling pathways involved in this process, we used an in vitro model of Human Umbilical Vein Endothelial Cells (HUVEC). We show that PTN was mitogenic toward a variety of endothelial cells including HUVEC, stimulated HUVEC migration across a reconstituted basement membrane and induced the formation of capillary-like structures by HUVEC grown as 3D-cultures in Matrigel or collagen. The signaling pathways triggered following endothelial cell stimulation by PTN were studied by using pharmacological inhibitors of the Phosphoinositide-3 kinase (PI3K) and endothelial Nitric Oxide Synthase (eNOS), two enzymes that have been shown to be crucial in the angiogenic response to Vascular Endothelial Growth Factor (VEGF). Whereas wortmannin (a PI3K inhibitor) and L-NAME (an eNOS inhibitor) dramatically reduced HUVEC growth induced by VEGF, only the former inhibitor reduced the growth induced by PTN and to a lesser extent that stimulated by basic Fibroblast Growth Factor. Thus, our results indicate that PTN induces angiogenesis and utilizes PI3K- but not eNOS-dependent pathways for its angiogenic activity.

Chondromodulin I and pleiotrophin gene expression in bovine cartilage and epiphysis

Pleiotrophin and chondromodulin-I are low molecular weight proteins that are abundant (20 microg/g tissue) in fetal cartilage and difficult to detect in adult cartilage. We characterized their gene and protein expression patterns to gain a better understanding of their roles in the regulation of limb development and growth. In order to compare and contrast the relative amounts of the respective mRNA species within the developing epiphysis, a competitive PCR assay was developed. The results showed that the mRNAs for both proteins were abundant in fetal cartilage and while present in adult cartilage, were at 20-60-fold lower levels. Northern blotting revealed gradients of mRNA for both of these proteins in growth plate cartilage, with the highest levels in the resting zone, and the lowest in the hypertrophic zone. In contrast to pleiotrophin, chondromodulin-1 is down-regulated by retinoic acid with a pattern of expression similar to collagen type II and link protein, and may play a more specific role than pleiotrophin in modulating the chondrocyte phenotype.