Fate of the hypertrophic chondrocyte: microenvironmental perspectives on apoptosis and survival in the epiphyseal growth plate

FGFR3/fibroblast growth factor receptor 3 inhibits autophagy through decreasing the ATG12-ATG5 conjugate, leading to the delay of cartilage development in achondroplasia

FGFR3 (fibroblast growth factor receptor 3) is a negative regulator of endochondral ossification. Gain-of-function mutations in FGFR3 are responsible for achondroplasia, the most common genetic form of dwarfism in humans. Autophagy, an evolutionarily conserved catabolic process, maintains chondrocyte viability in the growth plate under stress conditions, such as hypoxia and nutritional deficiencies. However, the role of autophagy and its underlying molecular mechanisms in achondroplasia remain elusive. In this study, we found activated FGFR3 signaling inhibited autophagic activity in chondrocytes, both in vivo and in vitro. By employing an embryonic bone culture system, we demonstrated that treatment with autophagy inhibitor 3-MA or chloroquine led to cartilage growth retardation, which mimics the effect of activated-FGFR3 signaling on chondrogenesis. Furthermore, we found that FGFR3 interacted with ATG12-ATG5 conjugate by binding to ATG5. More intriguingly, FGFR3 signaling was found to decrease the protein level of ATG12-ATG5 conjugate. Consistently, using in vitro chondrogenic differentiation assay system, we showed that the ATG12-ATG5 conjugate was essential for the viability and differentiation of chondrocytes. Transient transfection of ATG5 partially rescued FGFR3-mediated inhibition on chondrocyte viability and differentiation. Our findings reveal that FGFR3 inhibits the autophagic activity by decreasing the ATG12-ATG5 conjugate level, which may play an essential role in the pathogenesis of achondroplasia.

The chondrocytic journey in endochondral bone growth and skeletal dysplasia

The endochondral bones of the skeleton develop from a cartilage template and grow via a process involving a cascade of chondrocyte differentiation steps culminating in formation of a growth plate and the replacement of cartilage by bone. This process of endochondral ossification, driven by the generation of chondrocytes and their subsequent proliferation, differentiation, and production of extracellular matrix constitute a journey, deviation from which inevitably disrupts bone growth and development, and is the basis of human skeletal dysplasias with a wide range of phenotypic severity, from perinatal lethality to progressively deforming. This highly coordinated journey of chondrocyte specification and fate determination is controlled by a myriad of intrinsic and extrinsic factors. SOX9 is the master transcription factor that, in concert with varying partners along the way, directs the different phases of the journey from mesenchymal condensation, chondrogenesis, differentiation, proliferation, and maturation. Extracellular signals, including bone morphogenetic proteins, wingless-related MMTV integration site (WNT), fibroblast growth factor, Indian hedgehog, and parathyroid hormone-related peptide, are all indispensable for growth plate chondrocytes to align and organize into the appropriate columnar architecture and controls their maturation and transition to hypertrophy. Chondrocyte hypertrophy, marked by dramatic volume increase in phases, is controlled by transcription factors SOX9, Runt-related transcription factor, and FOXA2. Hypertrophic chondrocytes mediate the cartilage to bone transition and concomitantly face a live-or-die situation, a subject of much debate. We review recent insights into the coordination of the phases of the chondrocyte journey, and highlight the need for a systems level understanding of the regulatory networks that will facilitate the development of therapeutic approaches for skeletal dysplasia.

TGFβ signaling in cartilage development and maintenance

Members of the transforming growth factor beta (TGFβ) superfamily of secreted factors play essential roles in nearly every aspect of cartilage formation and maintenance. However, the mechanisms by which TGFβs transduce their effects in cartilage in vivo remain poorly understood. Mutations in several TGFβ family members, their receptors, extracellular modulators, and intracellular transducers have been described, and these usually impact the development of the cartilaginous skeleton. Furthermore, genome-wide association studies have linked components of the (TGFβ) superfamily to susceptibility to osteoarthritis. This review focuses on recent discoveries from genetic studies in the mouse regarding the regulation of TGFβ signaling in developing growth plate and articular cartilage, as well as the different modes of crosstalk between canonical and noncanonical TGFβ signaling. These new insights into TGFβ signaling in cartilage may open new prospects for therapies that maintain healthy articular cartilage.

Comparative proteomic analysis of hypertrophic chondrocytes in osteoarthritis

Background

Osteoarthritis (OA) is a multi-factorial disease leading progressively to loss of articular cartilage and subsequently to loss of joint function. While hypertrophy of chondrocytes is a physiological process implicated in the longitudinal growth of long bones, hypertrophy-like alterations in chondrocytes play a major role in OA. We performed a quantitative proteomic analysis in osteoarthritic and normal chondrocytes followed by functional analyses to investigate proteome changes and molecular pathways involved in OA pathogenesis.

Methods

Chondrocytes were isolated from articular cartilage of ten patients with primary OA undergoing knee replacement surgery and six normal donors undergoing fracture repair surgery without history of joint disease and no OA clinical manifestations. We analyzed the proteome of chondrocytes using high resolution mass spectrometry and quantified it by label-free quantification and western blot analysis. We also used WebGestalt, a web-based enrichment tool for the functional annotation and pathway analysis of the differentially synthesized proteins, using the Wikipathways database. ClueGO, a Cytoscape plug-in, is also used to compare groups of proteins and to visualize the functionally organized Gene Ontology (GO) terms and pathways in the form of dynamical network structures.

Results

The proteomic analysis led to the identification of a total of ~2400 proteins. 269 of them showed differential synthesis levels between the two groups. Using functional annotation, we found that proteins belonging to pathways associated with regulation of the actin cytoskeleton, EGF/EGFR, TGF-β, MAPK signaling, integrin-mediated cell adhesion, and lipid metabolism were significantly enriched in the OA samples (p ≤10⁻⁵). We also observed that the proteins GSTP1, PLS3, MYOF, HSD17B12, PRDX2, APCS, PLA2G2A SERPINH1/HSP47 and MVP, show distinct synthesis levels, characteristic for OA or control chondrocytes.

Conclusion

In this study we compared the quantitative changes in proteins synthesized in osteoarthritic compared to normal chondrocytes. We identified several pathways and proteins to be associated with OA chondrocytes. This study provides evidence for further testing on the molecular mechanism of the disease and also propose proteins as candidate markers of OA chondrocyte phenotype.

Electronic supplementary material

The online version of this article (doi:10.1186/s12014-015-9085-6) contains supplementary material, which is available to authorized users.

A pathway to bone: signaling molecules and transcription factors involved in chondrocyte development and maturation

Decades of work have identified the signaling pathways that regulate the differentiation of chondrocytes during bone formation, from their initial induction from mesenchymal progenitor cells to their terminal maturation into hypertrophic chondrocytes. Here, we review how multiple signaling molecules, mechanical signals and morphological cell features are integrated to activate a set of key transcription factors that determine and regulate the genetic program that induces chondrogenesis and chondrocyte differentiation. Moreover, we describe recent findings regarding the roles of several signaling pathways in modulating the proliferation and maturation of chondrocytes in the growth plate, which is the 'engine' of bone elongation.

Dual pathways to endochondral osteoblasts: a novel chondrocyte-derived osteoprogenitor cell identified in hypertrophic cartilage

According to the general understanding, the chondrocyte lineage terminates with the elimination of late hypertrophic cells by apoptosis in the growth plate. However, recent cell tracking studies have shown that murine hypertrophic chondrocytes can survive beyond “terminal” differentiation and give rise to a progeny of osteoblasts participating in endochondral bone formation. The question how chondrocytes convert into osteoblasts, however, remained open. Following the cell fate of hypertrophic chondrocytes by genetic lineage tracing using BACCol10;Cre induced YFP-reporter gene expression we show that a progeny of Col10Cre-reporter labelled osteoprogenitor cells and osteoblasts appears in the primary spongiosa and participates – depending on the developmental stage – substantially in trabecular, endosteal, and cortical bone formation. YFP⁺ trabecular and endosteal cells isolated by FACS expressed Col1a1, osteocalcin and runx2, thus confirming their osteogenic phenotype. In searching for transitory cells between hypertrophic chondrocytes and trabecular osteoblasts we identified by confocal microscopy a novel, small YFP⁺Osx⁺ cell type with mitotic activity in the lower hypertrophic zone at the chondro-osseous junction. When isolated from growth plates by fractional enzymatic digestion, these cells termed CDOP (chondrocyte-derived osteoprogenitor) cells expressed bone typical genes and differentiated into osteoblasts in vitro. We propose the Col10Cre-labeled CDOP cells mark the initiation point of a second pathway giving rise to endochondral osteoblasts, alternative to perichondrium derived osteoprogenitor cells. These findings add to current concepts of chondrocyte-osteocyte lineages and give new insight into the complex cartilage-bone transition process in the growth plate.

Pubertal growth and epiphyseal fusion

The complex networks of nutritional, cellular, paracrine, and endocrine factors are closely related with pubertal growth and epiphyseal fusion. Important influencing factors include chondrocyte differentiation capacity, multiple molecular pathways active in the growth plate, and growth hormone-insulin-like growth factor-I axis activation and epiphyseal fusion through estrogen and its receptors. However, the exact mechanisms of these phenomena are still unclear. A better understanding of the detailed processes involved in the pubertal growth spurt and growth plate closure in longitudinal bone growth will help us develop methods to efficiently promote pubertal growth and delay epiphyseal fusion with fewer adverse effects.

Fate of growth plate hypertrophic chondrocytes: death or lineage extension?

The vertebrate growth plate is an essential tissue that mediates and controls bone growth. It forms through a multistep differentiation process in which chondrocytes differentiate, proliferate, stop dividing and undergo hypertrophy, which entails a 20-fold increase in size. Hypertrophic chondrocytes are specialized cells considered to be the end state of the chondrocyte differentiation pathway, and are essential for bone growth. They are characterized by expression of type X collagen encoded by the Col10a1 gene, and synthesis of a calcified cartilage matrix. Whether hypertrophy marks a transition preceding osteogenesis, or it is the terminal differentiation stage of chondrocytes with cell death as the ultimate fate has been the subject of debate for over a century. In this review, we revisit this debate in the light of new findings arising from genetic-mediated lineage tracing studies showing that hypertrophic chondrocytes can survive at the chondro-osseous junction and further make the transition to become osteoblasts and osteocytes. The contribution of chondrocytes to the osteoblast lineage has important implications in bone development, disease and repair.

The multifaceted role of the vasculature in endochondral fracture repair

Fracture healing is critically dependent upon an adequate vascular supply. The normal rate for fracture delayed or non-union is estimated to be between 10 and 15%, and annual fracture numbers are approximately 15 million cases per year. However, when there is decreased vascular perfusion to the fracture, incidence of impaired healing rises dramatically to 46%. Reduction in the blood supply to the fracture can be the result of traumatic injuries that physically disrupt the vasculature and damage supportive soft tissue, the result of anatomical location (i.e., distal tibia), or attributed to physiological conditions such as age, diabetes, or smoking. The role of the vasculature during repair is multifaceted and changes during the course of healing. In this article, we review recent insights into the role of the vasculature during fracture repair. Taken together these data highlight the need for an updated model for endochondral repair to facilitate improved therapeutic approaches to promote bone healing.

Automated Cell Detection and Morphometry on Growth Plate Images of Mouse Bone

Microscopy imaging of mouse growth plates is extensively used in biology to understand the effect of specific molecules on various stages of normal bone development and on bone disease. Until now, such image analysis has been conducted by manual detection. In fact, when existing automated detection techniques were applied, morphological variations across the growth plate and heterogeneity of image background color, including the faint presence of cells (chondrocytes) located deeper in tissue away from the image's plane of focus, and lack of cell-specific features, interfered with identification of cell. We propose the first method of automated detection and morphometry applicable to images of cells in the growth plate of long bone. Through ad hoc sequential application of the Retinex method, anisotropic diffusion and thresholding, our new cell detection algorithm (CDA) addresses these challenges on bright-field microscopy images of mouse growth plates. Five parameters, chosen by the user in respect of image characteristics, regulate our CDA. Our results demonstrate effectiveness of the proposed numerical method relative to manual methods. Our CDA confirms previously established results regarding chondrocytes' number, area, orientation, height and shape of normal growth plates. Our CDA also confirms differences previously found between the genetic mutated mouse Smad1/5^CKO and its control mouse on fluorescence images. The CDA aims to aid biomedical research by increasing efficiency and consistency of data collection regarding arrangement and characteristics of chondrocytes. Our results suggest that automated extraction of data from microscopy imaging of growth plates can assist in unlocking information on normal and pathological development, key to the underlying biological mechanisms of bone growth.

Hyaluronan synthase-2 upregulation protects smpd3-deficient fibroblasts against cell death induced by nutrient deprivation, but not against apoptosis evoked by oxidized LDL

The neutral type 2 sphingomyelinase (nSMase2) hydrolyzes sphingomyelin and generates ceramide, a major bioactive sphingolipid mediator, involved in growth arrest and apoptosis. The role of nSMase2 in apoptosis is debated, and apparently contradictory results have been observed on fibroblasts isolated from nSMase2-deficient fragilitas ossium (homozygous fro/fro) mice. These mice exhibit a severe neonatal dysplasia, a lack of long bone mineralization and delayed apoptosis patterns of hypertrophic chondrocytes in the growth plate. We hypothesized that apoptosis induced by nutrient deprivation, which mimics the environmental modifications of the growth plate, requires nSMase2 activation. In this study, we have compared the resistance of fro/fro fibroblasts to different death inducers (oxidized LDL, hydrogen peroxide and nutrient starvation). The data show that nSMase2-deficient fro/fro cells resist to apoptosis evoked by nutrient starvation (fetal calf serum/glucose/pyruvate-free DMEM), whereas wt fibroblasts die after 48 h incubation in this medium. In contrast, oxidized LDL and hydrogen peroxide are similarly toxic to fro/fro and wt fibroblasts, indicating that nSMase2 is not involved in the mechanism of toxicity evoked by these agents. Interestingly, wt fibroblasts treated with the SMase inhibitor GW4869 were more resistant to starvation-induced apoptosis.

The resistance of fro/fro cells to starvation-induced apoptosis is associated with an increased expression of hyaluronan synthase 2 (HAS2) mRNAs and protein, which is inhibited by ceramide. In wt fibroblasts, this HAS2 rise and its protective effect did not occur, but exogenously added HA exhibited a protective effect against starvation-induced apoptosis.

The protective mechanism of HAS2 involves an increased expression of the heat-shock protein Hsp72, a chaperone with antiapoptotic activity. Taken together, these results highlight the role of nSMase2 in apoptosis evoked by nutrient starvation that could contribute to the delayed apoptosis of hypertrophic chondrocytes in the growth plate, and emphasize the antiapoptotic properties of HAS2.

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Apoptosis evoked by oxidized LDL and H₂O₂ is comparable in fro/fro and wt fibroblasts.

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fro/fro fibroblasts resist to apoptosis evoked by nutrient starvation.

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HAS2 increased expression protects fro/fro fibroblasts against apoptosis.

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HAS2 regulates the expression of the antiapoptotic heat-shock protein HsP72.

Chondrocytes transdifferentiate into osteoblasts in endochondral bone during development, postnatal growth and fracture healing in mice

One of the crucial steps in endochondral bone formation is the replacement of a cartilage matrix produced by chondrocytes with bone trabeculae made by osteoblasts. However, the precise sources of osteoblasts responsible for trabecular bone formation have not been fully defined. To investigate whether cells derived from hypertrophic chondrocytes contribute to the osteoblast pool in trabecular bones, we genetically labeled either hypertrophic chondrocytes by Col10a1-Cre or chondrocytes by tamoxifen-induced Agc1-CreERT2 using EGFP, LacZ or Tomato expression. Both Cre drivers were specifically active in chondrocytic cells and not in perichondrium, in periosteum or in any of the osteoblast lineage cells. These in vivo experiments allowed us to follow the fate of cells labeled in Col10a1-Cre or Agc1-CreERT2 -expressing chondrocytes. After the labeling of chondrocytes, both during prenatal development and after birth, abundant labeled non-chondrocytic cells were present in the primary spongiosa. These cells were distributed throughout trabeculae surfaces and later were present in the endosteum, and embedded within the bone matrix. Co-expression studies using osteoblast markers indicated that a proportion of the non-chondrocytic cells derived from chondrocytes labeled by Col10a1-Cre or by Agc1-CreERT2 were functional osteoblasts. Hence, our results show that both chondrocytes prior to initial ossification and growth plate chondrocytes before or after birth have the capacity to undergo transdifferentiation to become osteoblasts. The osteoblasts derived from Col10a1-expressing hypertrophic chondrocytes represent about sixty percent of all mature osteoblasts in endochondral bones of one month old mice. A similar process of chondrocyte to osteoblast transdifferentiation was involved during bone fracture healing in adult mice. Thus, in addition to cells in the periosteum chondrocytes represent a major source of osteoblasts contributing to endochondral bone formation in vivo.

Defective autophagy in chondrocytes with Kashin-Beck disease but higher than osteoarthritis

OBJECTIVE

This study was undertaken to monitor autophagy in chondrocytes with Kashin-Beck disease (KBD) and osteoarthritis (OA).

METHODS

The identification and quantification of autophagy were morphologically visualized by transmission electron microscopy (TEM), together with immunohistochemical localization of Beclin1 and LC3 in cartilage, and immunoblotting of cellular Beclin1, LC3 and p62/SQSTM1 in the normal, KBD and OA groups. Sequentially, regulated-autophagy genes (ATG) were analyzed by IPA software and validated by quantitative real-time polymerase chain reaction (qRT-PCR). Cytotoxicity of cell death was measured by fluorescence detection and flow cytometry (FCM). The co-localization and measurement of autophagy and mitochondria/reactive oxygen species (ROS) were carried out.

RESULTS

KBD chondrocytes exhibited a variety of abnormal cellular contents including nuclei, mitochondrial, glycogen deposits and microfilaments, and OA chondrocytes mainly presented swelled endocytoplasmic reticulum (ER). Beclin1 and LC3 were reduced both in KBD and OA compared with normal controls; however, the two proteins and p62 in KBD were in a higher level than OA. Simultaneously, KBD chondrocytes showed 45 genes that were different from normal controls and 92 genes different from OA, whose functions were mainly involved in cell morphology, cellular functions, cell death and survival. Autophagy was negatively correlated with apoptosis in the three kinds of chondrocytes, and the rates decreased when autophagy was induced by rapamycin. Similarly, KBD and OA chondrocytes showed lower autophagy and higher ROS production compared with the normal chondrocytes.

CONCLUSION

Autophagy was defective in KBD chondrocytes, but it was higher than in OA. The insufficient autophagy may be associated with apoptosis and mitochondrial change in the pathogenesis of KBD and OA.

Thermal epiphysiodesis performed with radio frequency in a porcine model

BACKGROUND AND PURPOSE

Current techniques for epiphysiodesis involve opening of cortical windows; use of staples, screws, and tension devices; and fusion with curettes or drills. Complications may have serious consequences. There is a need for a more reliable, precise, and less traumatic procedure that overcomes the known complications from existing techniques. We analyzed a new epiphysiodesis technique using radio-frequency ablation (RFA) in a porcine model.

METHODS

Six 35-kg and two 25-kg immature pigs were used. 1 hind leg of each animal was randomly selected and the proximal tibia growth plate was ablated laterally and medially. The contralateral leg was used as a control. MR images were obtained immediately after the ablation and 12 weeks later for 6 animals, and 24 weeks later for the other 2 animals. CT was done for the 2 animals that were followed for 24 weeks for proof of bone bridges.

RESULTS

Both tibias were equal in length initially. At the 12-week follow-up, there was an average leg length discrepancy of 3.9 mm (95% CI: 3.0-4.8), and at 24 weeks the difference was 8.4 mm and 7.5 mm. No damage to the adjacent tissue was found. Bone bridges and physeal closure were found after 24 weeks. The pigs showed no discomfort after the intervention.

INTERPRETATION

We found RFA to be feasible for epiphysiodesis in a pig model. The method is minimally invasive and recovery may be quick compared to conventional methods. We recommend that the method should be tested in larger-scale safety studies before clinical application.

Hypertrophic chondrocytes in the rabbit growth plate can proliferate and differentiate into osteogenic cells when capillary invasion is interposed by a membrane filter

The fate of hypertrophic chondrocytes during endochondral ossification remains controversial. It has long been thought that the calcified cartilage is invaded by blood vessels and that new bone is deposited on the surface of the eroded cartilage by newly arrived cells. The present study was designed to determine whether hypertrophic chondrocytes were destined to die or could survive to participate in new bone formation. In a rabbit experiment, a membrane filter with a pore size of 1 µm was inserted in the middle of the hypertrophic zone of the distal growth plate of ulna. In 33 of 37 animals, vascular invasion was successfully interposed by the membrane filter. During 8 days, the cartilage growth plate was enlarged, making the thickness 3-fold greater than that of the nonoperated control side. Histological examination demonstrated that the hypertrophic zone was exclusively elongated. At the terminal end of the growth plate, hypertrophic chondrocytes extruded from their territorial matrix into the open cavity on the surface of the membrane filter. The progenies of hypertrophic chondrocytes (PHCs) were PCNA positive and caspase-3 negative. In situ hybridization studies demonstrated that PHCs did not express cartilage matrix proteins anymore but expressed bone matrix proteins. Immunohistochemical studies also demonstrated that the new matrix produced by PHCs contained type I collagen, osteonectin, and osteocalcin. Based on these results, we concluded that hypertrophic chondrocytes switched into bone-forming cells after vascular invasion was interposed in the normal growth plate.

Hypertrophic chondrocytes can become osteoblasts and osteocytes in endochondral bone formation

According to current dogma, chondrocytes and osteoblasts are considered independent lineages derived from a common osteochondroprogenitor. In endochondral bone formation, chondrocytes undergo a series of differentiation steps to form the growth plate, and it generally is accepted that death is the ultimate fate of terminally differentiated hypertrophic chondrocytes (HCs). Osteoblasts, accompanying vascular invasion, lay down endochondral bone to replace cartilage. However, whether an HC can become an osteoblast and contribute to the full osteogenic lineage has been the subject of a century-long debate. Here we use a cell-specific tamoxifen-inducible genetic recombination approach to track the fate of murine HCs and show that they can survive the cartilage-to-bone transition and become osteogenic cells in fetal and postnatal endochondral bones and persist into adulthood. This discovery of a chondrocyte-to-osteoblast lineage continuum revises concepts of the ontogeny of osteoblasts, with implications for the control of bone homeostasis and the interpretation of the underlying pathological bases of bone disorders.

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

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

Autophagy impairment aggravates the inhibitory effects of high glucose on osteoblast viability and function

Autophagy is a highly regulated homoeostatic process involved in the lysosomal degradation of damaged cell organelles and proteins. This process is considered an important pro-survival mechanism under diverse stress conditions. A diabetic milieu is known to hamper osteoblast viability and function. In the present study, we explored the putative protective role of autophagy in osteoblastic cells exposed to an HG (high glucose) medium. HG was found to increase protein oxidation and triggered autophagy by a mechanism dependent on reactive oxygen species overproduction in osteoblastic MC3T3-E1 cells. MC3T3-E1 cell survival was impaired by HG and worsened by chemical or genetic inhibition of autophagy. These findings were mimicked by H2O2-induced oxidative stress in these cells. Autophagy impairment led to both defective mitochondrial morphology and decreased bioenergetic machinery and inhibited further osteoblast differentiation in MC3T3-E1 cells upon exposure to HG. These novel findings indicate that autophagy is an essential mechanism to maintain osteoblast viability and function in an HG environment.

Notch signaling in skeletal stem cells

The skeleton originates from stem cells residing in the sclerotome and neural crest that undergo proliferation, migration, and commitment. The development of the skeletal stem cells is influenced by many signaling pathways that govern cell fate determination, proliferation, differentiation, and apoptosis. This review will focus on Notch signaling functions in regulating the different cell types that form the skeletal system as well as the interplay between them to maintain homeostasis. Osteochondroprogenitors require Notch signaling to maintain multipotency and to prevent premature differentiation into osteoblasts. Subsequently, overactivation of Notch signaling suppresses osteoblast maturation. Moreover, Notch signaling in osteochondroprogenitors is required for chondrocyte proliferation and hypertrophy and suppresses terminal differentiation. Translational studies demonstrated a crucial role of Notch signaling in osteosarcoma and osteoarthritis, where concepts derived from developmental pathways are often recapitulated. This brings hope of taking advantage of the molecular mechanisms learned from development to approach the pathological processes underlying abnormal bone/cartilage metabolism or tumorigenesis. Pharmacological agents that target Notch receptors or ligands in a tissue-specific fashion would offer new opportunities for treating bone/cartilage diseases caused by dysregulation of Notch signaling.

Boning up on autophagy: the role of autophagy in skeletal biology

From an evolutionary perspective, the major function of bone is to provide stable sites for muscle attachment and affording protection of vital organs, especially the heart and lungs (ribs) and spinal cord (vertebrae and intervertebral discs). However, bone has a considerable number of other functions: serving as a store for mineral ions, providing a site for blood cell synthesis and participating in a complex system-wide endocrine system. Not surprisingly, bone and cartilage cell homeostasis is tightly controlled, as is the maintenance of tissue structure and mass. While a great deal of new information is accruing concerning skeletal cell homeostasis, one relatively new observation is that the cells of bone (osteoclasts osteoblasts and osteocytes) and cartilage (chondrocytes) exhibit autophagy. The focus of this review is to examine the significance of this process in terms of the functional demands of the skeleton in health and during growth and to provide evidence that dysregulation of the autophagic response is involved in the pathogenesis of diseases of bone (Paget disease of bone) and cartilage (osteoarthritis and the mucopolysaccharidoses). Delineation of molecular changes in the autophagic process is uncovering new approaches for the treatment of diseases that affect the axial and appendicular skeleton.

Cartilage to bone transitions in health and disease

Aberrant redeployment of the 'transient' events responsible for bone development and postnatal longitudinal growth has been reported in some diseases in what is otherwise inherently 'stable' cartilage. Lessons may be learnt from the molecular mechanisms underpinning transient chondrocyte differentiation and function, and their application may better identify disease aetiology. Here, we review the current evidence supporting this possibility. We firstly outline endochondral ossification and the cellular and physiological mechanisms by which it is controlled in the postnatal growth plate. We then compare the biology of these transient cartilaginous structures to the inherently stable articular cartilage. Finally, we highlight specific scenarios in which the redeployment of these embryonic processes may contribute to disease development, with the foresight that deciphering those mechanisms regulating pathological changes and loss of cartilage stability will aid future research into effective disease-modifying therapies.

Deletion of beta catenin in hypertrophic growth plate chondrocytes impairs trabecular bone formation

In order to elucidate the role of β-catenin in hypertrophic cartilage zone of the growth plate, we deleted the β-catenin gene ctnnb1specifically from hypertrophic chondrocytes by mating ctnnb1(fl/fl) mice with BAC-Col10a1-Cre-deleter mice. Surprisingly, this resulted in a significant reduction of subchondral trabecular bone formation in BACCol10Cre; ctnnb1(Δ/Δ) (referred to as Cat-ko) mice, although Cre expression was restricted to hypertrophic chondrocytes. The size of the Col10a1 positive hypertrophic zone was normal, but qRT-PCR revealed reduced expression of Mmp13, and Vegfa in Cat-ko hypertrophic chondrocytes, indicating impaired terminal differentiation. Immunohistological and in situ hybridization analysis revealed the substantial deficiency of collagen I positive mature osteoblasts, but equal levels of osterix-positive cells in the subchondral bone marrow space of Cat-ko mice, indicating that the supply of osteoblast precursor cells was not reduced. The fact that in Cat-ko mice subchondral trabeculae were lacking including their calcified cartilage core indicated a strongly enhanced osteoclast activity. In fact, TRAP staining as well as in situ hybridization analysis of Mmp9 expression revealed denser occupation of the cartilage erosion zone with enlarged osteoclasts as compared to the control growth plate, suggesting increased RANKL or reduced osteoprotegerin (Opg) activity in this zone. This notion was confirmed by qRT-PCR analysis of mRNA extracted from cultured hypertrophic chondrocytes or from whole epiphyses, showing increased Rankl mRNA levels in Cat-ko as compared to control chondrocytes, whereas changes in OPG levels were not significant. These results indicate that β-catenin levels in hypertrophic chondrocytes play a key role in regulating osteoclast activity and trabecular bone formation at the cartilage-bone interface by controlling RANKL expression in hypertrophic chondrocytes.

The Transient Chondrocyte Phenotype in Human Osteophytic Cartilage: A Role of Pigment Epithelium-Derived Factor?

OBJECTIVE

To identify factors that are responsible for the phenotypic differences between transient chondrocytes within human osteophytes prone to endochondral ossification and permanent chondrocytes within articular cartilage persisting for decades.

METHODS

Differential gene expression of chondrocytes from human osteophytes or from articular cartilage was detected by cDNA microarray analysis. The expression of pigment epithelium-derived factor (PEDF), one of the most impressively differentially expressed genes, was validated by quantitative reverse transcriptase polymerase chain reaction as well as immunohistochemistry. The mode of action of PEDF was explored by cell viability assays and by detecting target genes.

RESULTS

PEDF mRNA expression was upregulated by 118.5-fold (P = 0.01) in human osteophytic cartilage compared with articular cartilage, which was reflected by strong immunostaining for PEDF in the cartilaginous layer of osteophytes but largely negative staining in articular cartilage. Elevated levels of PEDF in osteophytes were associated with enhanced apoptosis. PEDF increased the expression of the proapoptotic factor FasL and induced cell death in cell culture. Osteochondral progenitor cells were more responsive to PEDF than differentiated articular chondrocytes.

CONCLUSIONS

The induction of the proapoptotic factor PEDF within the osteophyte cartilage suggests a molecular concept for the transient chondrocyte phenotype that arises from progenitor cells and is prone to terminal differentiation and cell death.

Association of cartilage-specific deletion of peroxisome proliferator-activated receptor γ with abnormal endochondral ossification and impaired cartilage growth and development in a murine model

OBJECTIVE

Long bones develop through the strictly regulated process of endochondral ossification within the growth plate, resulting in the replacement of cartilage by bone. Defects in this process can result in skeletal abnormalities and a predisposition to degenerative joint diseases such as osteoarthritis (OA). Studies suggest that activation of the transcription factor peroxisome proliferator-activated receptor γ (PPARγ) is an important therapeutic target in OA. To devise PPARγ-related therapies in OA, it is critical to identify the role of this transcription factor in cartilage biology. Therefore, this study sought to determine the in vivo role of PPARγ in endochondral ossification and cartilage development, using cartilage-specific PPARγ-knockout (KO) mice.

METHODS

Cartilage-specific PPARγ-KO mice were generated using the Cre/loxP system. Histomorphometric and immunohistochemical analyses were performed to assess the patterns of ossification, proliferation, differentiation, and hypertrophy of chondrocytes, skeletal organization, bone density, and calcium deposition in the KO mice.

RESULTS

PPARγ-KO mice exhibited reductions in body length, body weight, length of the long bones, skeletal growth, cellularity, bone density, calcium deposition, and trabecular bone thickness, abnormal organization of the growth plate, loss of columnar organization, shorter hypertrophic zones, and delayed primary and secondary ossification. Immunohistochemical analyses for Sox9, 5-bromo-2'-deoxyuridine, p57, type X collagen, and platelet endothelial cell adhesion molecule 1 revealed reductions in the differentiation, proliferation, and hypertrophy of chondrocytes and in vascularization of the growth plate in mutant mice. Isolated chondrocytes and cartilage explants from mutant mice showed aberrant expression of Sox9 and extracellular matrix markers, including aggrecan, type II collagen, and matrix metalloproteinase 13. In addition, chondrocytes from mutant mice exhibited enhanced phosphorylation of p38 and decreased expression of Indian hedgehog.

CONCLUSION

The presence of PPARγ is required for normal endochondral ossification and cartilage development in vivo.

Cartilage development and degeneration: a Wnt Wnt situation

The Wnt signaling pathway plays a crucial role in the development and homeostasis of a variety of adult tissues and, as such, is emerging as an important therapeutic target for numerous diseases. Factors involved in the Wnt pathway are expressed throughout limb development and chondrogenesis and have been shown to be critical in joint homeostasis and endochondral ossification. Therefore, in this review, we discuss Wnt regulation of chondrogenic differentiation, hypertrophy and cartilage function. Moreover, we detail the role of the Wnt signaling pathway in cartilage degeneration and its potential to act as a target for therapy in osteoarthritis.

Putative function of TAP63α during endochondral bone formation

P63, a member of the P53 tumor suppressor family, is known to play important functions in cancer and development. Interestingly, previous studies have shown that p63 null mice are absent or have truncated limbs, while mutations in human P63 cause several skeletal syndromes that also show limb and digit abnormalities, suggesting its essential role in long bone development. Indeed, we detected increased level of p63 transcript in hypertrophic MCT cells (an established cell model of chondrocyte maturation) than in proliferative MCT cells. To investigate the in vivo role of P63 upon endochondral bone formation, we have established transgenic mouse lines in which HA- and Flag-tagged TAP63α (the longest P63 isoform) is driven by the hypertrophic chondrocyte-specific Col10a1 regulatory elements. Skeletal staining of Col10a1-TAP63α transgenic mice at either embryonic day 17.5 (E17.5) or postnatal day 1 (P1) observed accelerated ossification in long bone, digit and tail bones compared to their wild-type littermates, suggesting a putative function of P63 during skeletal development. We also detected decreased level of Sox9 and Bcl-2 transcripts, while Alp and Ank are slightly upregulated in Col10a1-TAP63α transgenic mouse limbs. Further immunohistochemical analysis confirmed the decreased Sox9 expression in the proliferative and hypertrophic zone of these mice. Von Kossa staining suggests increased mineralization in hypertrophic zone of transgenic mice compared to littermate controls. Together, our results suggest a role of TAP63α upon skeletal development. TAP63a may promote endochondral ossification through interaction with genes relevant to matrix mineralization and chondrocyte maturation or apoptosis.

Cdc42 is required for chondrogenesis and interdigital programmed cell death during limb development

Cdc42, a member of the Rho subfamily of small GTPases, is known to be a regulator of multiple cellular functions, including cytoskeletal organization, cell migration, proliferation, and apoptosis. However, its tissue-specific roles, especially in mammalian limb development, remain unclear. To investigate the physiological function of Cdc42 during limb development, we generated limb bud mesenchyme-specific inactivated Cdc42 (Cdc42(fl/fl); Prx1-Cre) mice. Cdc42(fl/fl); Prx1-Cre mice demonstrated short limbs and body, abnormal calcification of the cranium, cleft palate, disruption of the xiphoid process, and syndactyly. Severe defects were also found in long bone growth plate cartilage, characterized by loss of columnar organization of chondrocytes, and thickening and massive accumulation of hypertrophic chondrocytes, resulting in delayed endochondral bone formation associated with reduced bone growth. In situ hybridization analysis revealed that expressions of Col10 and Mmp13 were reduced in non-resorbed hypertrophic cartilage, indicating that deletion of Cdc42 inhibited their terminal differentiation. Syndactyly in Cdc42(fl/fl); Prx1-Cre mice was caused by fusion of metacarpals and a failure of interdigital programmed cell death (ID-PCD). Whole mount in situ hybridization analysis of limb buds showed that the expression patterns of Sox9 were ectopic, while those of Bmp2, Msx1, and Msx2, known to promote apoptosis in the interdigital mesenchyme, were down-regulated. These results demonstrate that Cdc42 is essential for chondrogenesis and ID-PCD during limb development.

Generation and characterization of Col10a1-mcherry reporter mice

We report here on the generation of a new fluorescent protein reporter transgenic mouse line, Col10a1-mCherry, which can be used as a tool to study chondrocyte biology and pathology. Collagen, Type X, alpha 1 (Col10a1) is highly expressed in hypertrophic chondrocytes and commonly used as a gene marker for this cell population. The Col10a1-mCherry reporter line was generated using a bacterial recombination strategy with the mouse BAC clone RP23-192A7. To aid in the characterization of this animal model, we intercrossed Col10a1-mCherry mice with Collagen, Type II, alpha 1 (Col2a1) enhanced cyan fluorescent protein (ECFP) reporter mice and characterized the expression of both chondrocyte reporters during embryonic skeletal development from days E10.5 to E17.5. Additionally, at postnatal day 0, Col10a1-mCherry reporter expression was compared to endogenous Col10a1 mRNA expression in long bones and revealed that mCherry fluorescence extended past the Col10a1 expression domain. However, in situ hybridization for mCherry was consistent with the zone of Col10a1 mRNA expression, indicating that the persistent detection of mCherry fluorescence was a result of the long protein half life of mCherry in conjunction with a very rapid phase of skeletal growth and not due to aberrant transcriptional regulation. Taking advantage of the continued fluorescence of hypertrophic chondrocytes at the chondro-osseus junction, we intercrossed Col10a1-mCherry mice with two different Collagen, Type 1, alpha 1, (Col1a1) osteoblast reporter mice, pOBCol3.6-Topaz and pOBCol2.3-Emerald to investigate the possibility that hypertrophic chondrocytes transdifferentiate into osteoblasts. Evaluation of long bones at birth suggests that residual hypertrophic chondrocytes and osteoblasts in the trabecular zone exist as two completely distinct cell populations. genesis 49:410-418, 2011.

Histochemical and ultrastructural analysis of pathology and cell responses in gills of channel catfish affected with proliferative gill disease

Pond-reared channel catfish Ictalurus punctatus with proliferative gill disease (PGD), caused by the myxozoan parasite Henneguya spp., were examined with light and transmission electron microscopy to better characterize the inflammatory response during infection. The early stages of disease are characterized by the destruction of collagen in the matrix of the gill filament cartilage causing weakness and breaks within the gill filaments. These early lesions lacked a notable inflammatory response around the disrupted cartilage, a chondrocyte response was not apparent, and the parasite was not present, suggesting that the cartilage breaks occur prior to inflammation and arrival of the parasite in the gill. In later lesions, a significant inflammatory response was generated in areas of disrupted cartilage, and the inflammatory infiltrate was composed of a mixed population of granulocytes including neutrophils and cells that resembled eosinophils. The majority of eosinophil-like cells demonstrated evidence of degranulation. Trophozoites of Henneguya spp. were surrounded by a uniform population of cells believed to be neutrophils. The granulocytes were infiltrated within the dense collagen layer of the gill filament cartilage and often appeared within chondrocyte lacunae in place of the chondrocyte. The gill lamellae adjacent to the lesions were fused and contained an inflammatory infiltrate containing granulocytes and cells with pericentriolar granules that resembled previous descriptions of Langerhans-like cells. These cells were abundant within damaged lamellar epithelium, but were only rarely found within the gill filament. Lesions that appeared to be recovering lacked the dense collagenous layer around the cartilage and contained hyperplastic and hypertrophic chondrocytes that formed a callus. Other chondrocytes in the lesions had ultrastructural features indicative of cell death.

The role of autophagy during development in higher eukaryotes

Autophagy is a lysosome-mediated degradation pathway used by eukaryotes to recycle cytosolic components in both basal and stress conditions. Several genes have been described as regulators of autophagy, many of them being evolutionarily conserved from yeast to mammals. The study of autophagy-defective model systems has made it possible to highlight the importance of correctly functioning autophagic machinery in the development of invertebrates as, for example, during the complex events of fly and worm metamorphosis. In vertebrates, on the other hand, autophagy defects can be lethal for the animal if the mutated gene is involved in the early stages of development, or can lead to severe phenotypes if the mutation affects later stages. However, in both lower and higher eukaryotes, autophagy seems to be crucial during embryogenesis by acting in tissue remodeling in parallel with apoptosis. An increase of autophagic cells is, in fact, observed in the embryonic stages characterized by massive cell elimination. Moreover, autophagic processes probably protect cells during metabolic stress and nutrient paucity that occur during tissue remodeling. In light of such evidence, it can be concluded that there is a close interplay between autophagy and the processes of cell death, proliferation and differentiation that determine the development of higher eukaryotes.

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.

Mast cells and hypoxia drive tissue metaplasia and heterotopic ossification in idiopathic arthrofibrosis after total knee arthroplasty

BACKGROUND

Idiopathic arthrofibrosis occurs in 3-4% of patients who undergo total knee arthroplasty (TKA). However, little is known about the cellular or molecular changes involved in the onset or progression of this condition. To classify the histomorphologic changes and evaluate potential contributing factors, periarticular tissues from the knees of patients with arthrofibrosis were analyzed for fibroblast and mast cell proliferation, heterotopic ossification, cellular apoptosis, hypoxia and oxidative stress.

RESULTS

The arthrofibrotic tissue was composed of dense fibroblastic regions, with limited vascularity along the outer edges. Within the fibrotic regions, elevated numbers of chymase/fibroblast growth factor (FGF)-expressing mast cells were observed. In addition, this region contained fibrocartilage and associated heterotopic ossification, which quantitatively correlated with decreased range of motion (stiffness). Fibrotic, fibrocartilage and ossified regions contained few terminal dUTP nick end labeling (TUNEL)-positive or apoptotic cells, despite positive immunostaining for lactate dehydrogenase (LDH)5, a marker of hypoxia, and nitrotyrosine, a marker for protein nitrosylation. LDH5 and nitrotyrosine were found in the same tissue areas, indicating that hypoxic areas within the tissue were associated with increased production of reactive oxygen and nitrogen species.

CONCLUSIONS

Taken together, we suggest that hypoxia-associated oxidative stress initiates mast cell proliferation and FGF secretion, spurring fibroblast proliferation and tissue fibrosis. Fibroblasts within this hypoxic environment undergo metaplastic transformation to fibrocartilage, followed by heterotopic ossification, resulting in increased joint stiffness. Thus, hypoxia and associated oxidative stress are potential therapeutic targets for fibrosis and metaplastic progression of idiopathic arthrofibrosis after TKA.

Pathway-based genome-wide association analysis identified the importance of regulation-of-autophagy pathway for ultradistal radius BMD

Wrist fracture is not only one of the most common osteoporotic fractures but also a predictor of future fractures at other sites. Wrist bone mineral density (BMD) is an important determinant of wrist fracture risk, with high heritability. Specific genes underlying wrist BMD variation are largely unknown. Most published genome-wide association studies (GWASs) have focused only on a few top-ranking single-nucleotide polymorphisms (SNPs)/genes and considered each of the identified SNPs/genes independently. To identify biologic pathways important to wrist BMD variation, we used a novel pathway-based analysis approach in our GWAS of wrist ultradistal radius (UD) BMD, examining approximately 500,000 SNPs genome-wide from 984 unrelated whites. A total of 963 biologic pathways/gene sets were analyzed. We identified the regulation-of-autophagy (ROA) pathway that achieved the most significant result (p = .005, q(fdr) = 0.043, p(fwer) = 0.016) for association with UD BMD. The ROA pathway also showed significant association with arm BMD in the Framingham Heart Study sample containing 2187 subjects, which further confirmed our findings in the discovery cohort. Earlier studies indicated that during endochondral ossification, autophagy occurs prior to apoptosis of hypertrophic chondrocytes, and it also has been shown that some genes in the ROA pathway (e.g., INFG) may play important roles in osteoblastogenesis or osteoclastogenesis. Our study supports the potential role of the ROA pathway in human wrist BMD variation and osteoporosis. Further functional evaluation of this pathway to determine the mechanism by which it regulates wrist BMD should be pursued to provide new insights into the pathogenesis of wrist osteoporosis.

Epiphyseal fusion in the human growth plate does not involve classical apoptosis

By the end of puberty, growth ceases and epiphyseal fusion occurs through mechanisms not yet completely understood. Human growth plate tissues were collected in various pubertal stages including a unique late pubertal growth plate, which was about to fuse. Apoptosis was studied by TUNEL staining, immunolocalization of pro- and antiapoptotic proteins, and electron microscopy (EM). Morphologic analyses of the fusing growth plate revealed disorganized, large chondrocytes surrounded by a border of dense, cortical-like bone. In the unfused growth plates, few chondrocytes were TUNEL positive. In contrast, the fusing growth plate contained no single TUNEL-positive cell. Antiapoptotic (Bcl-2 and Bcl-XL) and proapoptotic (Bax, Bad, and cleaved caspase-3) proteins were detected in all growth plate zones without change in intensity during pubertal progression. Expression of antiapoptotic proteins was found in the fusing growth plate but of the proapoptotic proteins only Bad was detected. EM revealed no typical signs of apoptosis or autophagy in any of the growth plates. In contrast, morpohological signs of hypoxia and necrosis were observed. We conclude that classical apoptosis is not likely to be involved in the process of human growth plate fusion.

Extracellular signal-regulated kinase 1 (ERK1) and ERK2 play essential roles in osteoblast differentiation and in supporting osteoclastogenesis

Osteoblasts and chondrocytes arise from common osteo-chondroprogenitor cells. We show here that inactivation of ERK1 and ERK2 in osteo-chondroprogenitor cells causes a block in osteoblast differentiation and leads to ectopic chondrogenic differentiation in the bone-forming region in the perichondrium. Furthermore, increased mitogen-activated protein kinase signaling in mesenchymal cells enhances osteoblast differentiation and inhibits chondrocyte differentiation. These observations indicate that extracellular signal-regulated kinase 1 (ERK1) and ERK2 play essential roles in the lineage specification of mesenchymal cells. The inactivation of ERK1 and ERK2 resulted in reduced beta-catenin expression, suggesting a role for canonical Wnt signaling in ERK1 and ERK2 regulation of skeletal lineage specification. Furthermore, inactivation of ERK1 and ERK2 significantly reduced RANKL expression, accounting for a delay in osteoclast formation. Thus, our results indicate that ERK1 and ERK2 not only play essential roles in the lineage specification of osteo-chondroprogenitor cells but also support osteoclast formation in vivo.

Regulation of autophagy in human and murine cartilage: hypoxia-inducible factor 2 suppresses chondrocyte autophagy

OBJECTIVE

We have previously demonstrated that the transcription factor hypoxia-inducible factor 1 (HIF-1) promotes the onset of autophagy in chondrocytes. The overall goal of this study was to test the hypothesis that another HIF family transcription factor, HIF-2, modulates the induction of autophagy by chondrocytes.

METHODS

Expression of HIF-1, HIF-2, and light chain 3 (LC3) in human and murine articular cartilage was visualized by immunohistochemistry. Suppression of HIF-2 was achieved using small interfering RNA technology. Assessments of autophagic flux and lysosomal activity, as well as ultrastructural analysis, were performed in chondrocytes in cell culture.

RESULTS

HIF-2 was expressed abundantly by cells in human and murine articular cartilage and in the cartilage of mineralizing vertebrae from neonatal mice. Protein levels were reduced in articular cartilage from older mice, in end-plate cartilage from mice, and in chondrocytes from human osteoarthritic (OA) cartilage. HIF-2 was robustly expressed in the prehypertrophic cells of mouse growth cartilage. When HIF-2alpha was silenced, the generation of reactive oxygen species was found to be elevated, with a concomitant decrease in catalase and superoxide dismutase activity. Suppression of HIF-2 was associated with decreased Akt-1 and mammalian target of rapamycin activities, reduced Bcl-xL expression, and a robust autophagic response, even under nutrient-replete conditions. In these silenced chondrocytes, HIF-1 expression was elevated. Decreased HIF-2 expression was associated with autophagy in OA tissues and aging cartilage samples. The autophagic response of chondrocytes in HIF-2alpha-knockout mouse growth plate showed an elevated autophagic response throughout the plate.

CONCLUSION

Based on these observations, we conclude that HIF-2 is a potent regulator of autophagy in maturing chondrocytes. Our data suggest that this protein acts as a brake on the autophagy-accelerator function of HIF-1.

Inflammatory cytokines and the GH/IGF-I axis: novel actions on bone growth

Longitudinal bone growth is a tightly regulated process that relies on complex synchronized mechanisms at the growth plate. Chronic paediatric inflammatory diseases are well accepted to lead to growth retardation and this is likely due to raised inflammatory cytokine levels and reduced growth hormone (GH)/insulin-like growth factor-1 (IGF-I) signalling. The precise cellular mechanisms responsible for this inhibition are unclear and therefore in this article, we will review the potential interactions between inflammatory cytokines and the GH/IGF-I axis in the regulation of bone growth. In particular, we will emphasis the potential contribution of the suppressors of cytokine signalling (SOCS) proteins, and in particular SOCS2, in mediating this process.

The fate of chondrocytes during ageing of human thyroid cartilage

Human laryngeal cartilages, especially thyroid cartilage, exhibit gender-specific ageing. In contrast to male thyroid cartilages, the ventral half of the female thyroid cartilage plate remains unmineralized until advanced age. In cartilage specimens from laryngectomies and autopsies, apoptosis was studied immunohistochemically and the oxidative mitochondrial enzyme nicotinamide adenine dinucleotide hydride tetrazolium reductase (NADH-TR) was localized histochemically. In addition, very fresh specimens from laryngectomies were fixed under addition of ruthenium hexamine trichloride or tannin to fixation solution to study cell organelles of chondrocytes by electron microscopic methods. In general, apoptotic chondrocytes decreased in thyroid cartilages of both genders, especially after the second decade. In the age group 41-60 years, thyroid cartilage from male specimens revealed a significantly higher percentage of apoptotic cells than did thyroid cartilage from women (P = 0.004), whereas in the age groups 0-20 years and 61-79 years no statistically significant gender difference was determined. In general, thyroid cartilage from women contained more living chondrocytes into advanced age than men. Chondrocytes adjacent to mineralized cartilage were partly positive for apoptosis and NADH-TR and partly negative. Apoptotic chondrocytes often were localized in areas of asbestoid fibres where vascularization and mineralization took place first. Electron microscopy revealed remnants of chondrocytes in asbestoid fibres. Taken together, it can be assumed that some chondrocytes in thyroid cartilage die by apoptosis and that these chondrocytes are characterized by absent reactivity for the mitochondrial enzyme NADH-TR. A possible influence of sexual hormones on apoptotic death of thyroid cartilage cells requires further elucidation.