Increased cell diameter precedes chondrocyte terminal differentiation, whereas cell-matrix attachment complex proteins appear constant

Boosting in vitro cartilage tissue engineering through the fabrication of polycaprolactone-gelatin 3D scaffolds with specific depth-dependent fiber alignments and mechanical stimulation

Due to the limited self-healing ability of natural cartilage, several tissue engineering strategies have been explored to develop functional replacements. Still, most of these approaches do not attempt to recreate in vitro the anisotropic organization of its extracellular matrix, which is essential for a suitable load-bearing function. In this work, different depth-dependent alignments of polycaprolactone-gelatin electrospun fibers were assembled into three-dimensional scaffold architectures to assess variations on chondrocyte response under static, unconfined compressed and perfused culture conditions. The in vitro results confirmed that not only the 3D scaffolds specific depth-dependent fiber alignments potentiated chondrocyte proliferation and migration towards the fibrous systems, but also the mechanical stimulation protocols applied were able to enhance significantly cell metabolic activity and extracellular matrix deposition, respectively.

Triptolide Upregulates Myocardial Forkhead Helix Transcription Factor p3 Expression and Attenuates Cardiac Hypertrophy

The forkhead/winged helix transcription factor (Fox) p3 can regulate the expression of various genes, and it has been reported that the transfer of Foxp3-positive T cells could ameliorate cardiac hypertrophy and fibrosis. Triptolide (TP) can elevate the expression of Foxp3, but its effects on cardiac hypertrophy remain unclear. In the present study, neonatal rat ventricular myocytes (NRVM) were isolated and stimulated with angiotensin II (1 μmol/L) to induce hypertrophic response. The expression of Foxp3 in NRVM was observed by using immunofluorescence assay. Fifty mice were randomly divided into five groups and received vehicle (control), isoproterenol (Iso, 5 mg/kg, s.c.), one of three doses of TP (10, 30, or 90 μg/kg, i.p.) for 14 days, respectively. The pathological morphology changes were observed after Hematoxylin and eosin, lectin and Masson's trichrome staining. The levels of serum brain natriuretic peptide (BNP) and troponin I were determined by enzyme-linked immunosorbent assay and chemiluminescence, respectively. The mRNA and protein expressions of α- myosin heavy chain (MHC), β-MHC and Foxp3 were determined using real-time PCR and immunohistochemistry, respectively. It was shown that TP (1, 3, 10 μg/L) treatment significantly decreased cell size, mRNA and protein expression of β-MHC, and upregulated Foxp3 expression in NRVM. TP also decreased heart weight index, left ventricular weight index and, improved myocardial injury and fibrosis; and decreased the cross-scetional area of the myocardium, serum cardiac troponin and BNP. Additionally, TP markedly reduced the mRNA and protein expression of myocardial β-MHC and elevated the mRNA and protein expression of α-MHC and Foxp3 in a dose-dependent manner. In conclusion, TP can effectively ameliorate myocardial damage and inhibit cardiac hypertrophy, which is at least partly related to the elevation of Foxp3 expression in cardiomyocytes.

Epithelial membrane protein 1 inhibits human spinal chondrocyte differentiation

The molecular mechanisms underlying human spinal chondrocyte differentiation remain unclear. We recently demonstrated that epithelial membrane protein 1 (EMP1) is highly expressed in degenerative intervertebral discs. EMP1 is involved in the differentiation of multiple cell types, including progenitor/pre-B cells, neurons, and podocytes. Therefore, we hypothesize that EMP1 may participate in the differentiation of spinal chondrocytes. We cultured chondrocytes from human nucleus pulposus. Through lentivirus-mediated knockdown and overexpression of EMP1, we find that EMP1 promotes cell proliferation and survival, alters cell morphology and cell cycle, reduces cell condensation, and inhibits cell hypertrophy and the expression of chondrocyte maturation markers such as collagen X, aggrecan, sex-determining region Y (SRY)-box 9, and runt-related transcription factor 2. We also show that EMP1 is not expressed in the ossification center of vertebrae but is highly expressed in the nucleus pulposus and growth plate, where chondrocytes are immature and endochondral ossification has not occurred. These results suggest that EMP1 inhibits human spinal chondrocyte differentiation.

Small GTPase protein Rac-1 is activated with maturation and regulates cell morphology and function in chondrocytes

During maturation, chondrocytes undergo changes in morphology, matrix production, and gene expression; however, it remains unclear whether these are interrelated. In this study, we examined whether Rho GTPases were involved in these regulatory interplays. Levels of active Rho GTPases were assayed in immature and mature primary chondrocytes. We found that activation of Rac-1 and Cdc42 increased with maturation, whereas RhoA levels remained unchanged. GFP-tagged Rho GTPases tracked cellular localization. Rac-1 was enriched at the cell membrane where it co-localized with cortical actin, while RhoA and Cdc42 were cytoplasmic. To test the roles of Rac-1 in chondrocyte maturation, we force-expressed constitutively active or dominant negative forms of Rac-1 and assessed phenotypic consequences in primary chondrocytes. Activated Rac-1 expression induced chondrocyte enlargement and increased matrix metalloproteinase expression, which are characteristic of mature chondrocytes. Conversely, Rac-1 inactivation diminished adhesion, decreased alkaline phosphatase activity, and stimulated functions typical of immature chondrocytes. Exposure to a pro-maturation factor, Wnt3A, induced a flattened and enlarged morphology accompanied by peripheral Rac-1 re-arrangement. Wnt3A stimulated Tiam1 expression and Rac-1 activation, while DN-Rac-1 inhibited Wnt3A-induced cell spreading. Our data provide strong evidence that Rac-1 coordinates changes in chondrocyte phenotype and function and stimulates the maturation process essential for skeletal development.

Morphological regulation of rabbit chondrocytes on glucose-displayed surface

A culture surface was designed to regulate morphology of rabbit chondrocytes by changing the ratio of D- and L-glucose isomers displayed on a glass plate. With increasing ratio of d-glucose displayed on the surfaces, the efficiency of cell attachment improved, meaning that the attachment exclusively occurred via mediation of an affinity between D-glucose displayed and glucose transporter on cell membrane. At 0% and 100% D-glucose display, the round-shaped cells appeared dominantly, and most of cells became stretched in shape at 50% d-glucose display, indicating that the frequency of round-shaped cells depicted a concave profile against the ratio of D-glucose displayed. From the cytoskeletal staining of F-actin and vinculin, the immature stress fibers with fewer focal contacts were recognized in both the round shaped cells and those stretched in shape on 100% D-glucose-displayed surface. The time-lapse observation revealed that the cells on 100% D-glucose-displayed surface conducted active migration and aggregation with formation of collagen type II. These results suggest that 100% D-glucose-displayed surface can offer culture environment to maintain the chondrocytic phenotype of cells, similarly to the conditions achieved in three-dimensional (3-D) culture.

Articular cartilage and growth plate defects are associated with chondrocyte cytoskeletal abnormalities in Tg737orpk mice lacking the primary cilia protein polaris

Primary cilia are highly conserved organelles found on almost all eukaryotic cells. Tg737(orpk) (orpk) mice carry a hypomorphic mutation in the Tg737 gene resulting in the loss of polaris, a protein essential for ciliogenesis. Orpk mice have an array of skeletal patterning defects and show stunted growth after birth, suggesting defects in appositional and endochondral development. This study investigated the association between orpk tibial long bone growth and chondrocyte primary cilia expression using histomorphometric and immunohistochemical analysis. Wild-type chondrocytes throughout the developing epiphysis and growth plate expressed primary cilia, which showed a specific orientation away from the articular surface in the first 7-10 cell layers. In orpk mice, primary cilia were identified on very few cells and were significantly shorter. Orpk chondrocytes also showed significant increases in cytoplasmic tubulin, a likely result of failed ciliary assembly. The growth plates of orpk mice were significantly smaller in length and width, with marked changes in cellular organization in the presumptive articular cartilage, proliferative and hypertrophic zones. Cell density at the articular surface and in the hypertrophic zone was significantly altered, suggesting defects in both appositional and endochondral growth. In addition, orpk hypertrophic chondrocytes showed re-organization of the F-actin network into stress fibres and failed to fully undergo hypertrophy, while there was a marked reduction in type X collagen sequestration. These data suggest that failure to form a functional primary cilium affects chondrocyte differentiation and results in delayed chondrocyte hypertrophy within the orpk growth plate.

Regulation of chondrocyte differentiation by actin-severing protein adseverin

The importance of actin organization in controlling the chondrocyte phenotype is well established, but little is known about the cytoskeletal components regulating chondrocyte differentiation. Previously, we have observed up-regulation of an actin-binding gelsolin-like protein in hypertrophic chondrocytes. We have now identified it as adseverin (scinderin). Adseverin is drastically up-regulated during chondrocyte maturation, as shown by Northern blot analysis, in situ hybridization, and real-time RT-PCR. Its expression is positively regulated by PKC and MEK signaling as shown by inhibitory analyses. Over-expression of adseverin in non-hypertrophic chondrocytes causes rearrangement of the actin cytoskeleton, a change in cell morphology, a dramatic (3.5-fold) increase in cell volume, and up-regulation of Indian hedgehog (Ihh) and of collagen type X--all indicative of chondrocyte differentiation. These changes are mediated by ERK1/2 and p38 kinase pathways. Thus, adseverin-induced rearrangements of the actin cytoskeleton may mediate the PKC-dependent activation of p38 and Erk1/2 signaling pathways necessary for chondrocyte hypertrophy, as evidenced by changes in cell morphology, increase in cell size and expression of the chondrocyte maturation markers. These results demonstrate that interdependence of cytoskeletal organization and chondrogenic gene expression is regulated, at least in part, by actin-binding proteins such as adseverin.

Localization of extracellular matrix receptors on the chondrocyte primary cilium

A single primary cilium is found in chondrocytes and other connective tissue cells. We have previously shown that extracellular matrix (ECM) macromolecules such as collagen fibers closely associate with chondrocyte primary cilia, and their points of contact are characterized by electron-opaque plaques suggesting a direct link between the ECM and the cilium. This study examines the expression of receptors for ECM molecules on chondrocyte primary cilia. Embryonic chick sterna were fluorescently labeled with antibodies against alpha and beta integrins, NG2, CD44, and annexin V. Primary cilia were labeled using acetylated alpha-tubulin antibody. Expression of ECM receptors was examined on chondrocyte plasma membranes and their primary cilia using immunofluorescence and confocal microscopy. All receptors examined showed a punctate distribution on the plasma membrane. alpha2, alpha3, and beta1 integrins and NG2 were also present on primary cilia, whereas annexin V and CD44 were excluded. The number of receptor-positive cilia varied from 8/50 for NG2 to 43/50 for beta1 integrin. This is the first study to demonstrate the expression of integrins and NG2 on chondrocyte primary cilia. The data strongly suggest that chondrocyte primary cilia have the necessary machinery to act as mechanosensors, linking the ECM to cytoplasmic organelles responsible for matrix production and secretion.

Chondrocyte terminal differentiation, apoptosis, and type X collagen expression are downregulated by parathyroid hormone

Parathyroid hormone (PTH) regulates calcium and phosphate homeostasis through the endocrine system. Parathyroid hormone-related peptide (PTHrP) is a heterogeneous polypeptide with sequence homology to PTH in its first 13 amino acid residues. Both bind and activate a common receptor, the type 1 PTH/PTHrP receptor (PTH1R). Activation of this G-protein-coupled receptor by PTHrP has been shown to regulate chondrogenesis in a manner that attenuates chondrocyte hypertrophy. Here, we report the dose-response (10(-7) to 10(-15) M) effects of PTH on chondrogenesis using an avian sternal organ culture model. PTH increased cartilaginous tissue length and downregulated the deposition of type X collagen and its mRNA expression. In addition, PTH increased chondrocyte cell diameter in prehypertrophic and proliferative regions while decreasing chondrocyte apoptosis in the hypertrophic zone. In conclusion, these experiments demonstrate that PTH regulates cartilage growth, chondrocytic apoptosis, deposition of type X collagen protein, and expression of type X collagen mRNA. Type X collagen mRNA expression was downregulated by PTH in this organ culture model, but cell size, another marker for terminal differentiation, increased.

Hyaline cartilage engineered by chondrocytes in pellet culture: histological, immunohistochemical and ultrastructural analysis in comparison with cartilage explants

Cartilage engineering is a strategic experimental goal for the treatment of multiple joint diseases. Based on the process of embryonic chondrogenesis, we hypothesized that cartilage could be engineered by condensing chondrocytes in pellet culture and, in the present study, examined the quality of regenerated cartilage in direct comparison with native cartilage. Chondrocytes isolated from the sterna of chick embryos were cultured in pellets (4 x 10(6) cells per pellet) for 2 weeks. Cartilage explants from the same source were cultured as controls. After 2 weeks, the regenerated cartilage from pellet culture had a disc shape and was on average 9 mm at the longest diameter. The chondrocyte phenotype was stabilized in pellet culture as shown by the synthesis of type II collagen and aggrecan, which was the same intensity as in the explant after 7 days in culture. During culture, chondrocytes also continuously synthesized type IX collagen. Type X collagen was negatively stained in both pellets and explants. Except for fibril orientation, collagen fibril diameter and density in the engineered cartilage were comparable with the native cartilage. In conclusion, hyaline cartilage engineered by chondrocytes in pellet culture, without the transformation of cell phenotypes and scaffold materials, shares similarities with native cartilage in cellular distribution, matrix composition and density, and ultrastructure.

The chondrocyte cytoskeleton in mature articular cartilage: structure and distribution of actin, tubulin, and vimentin filaments

We investigated the structure of the chondrocyte cytoskeleton in intact tissue sections of mature bovine articular cartilage using confocal fluorescence microscopy complemented by protein extraction and immunoblotting analysis. Actin microfilaments were present inside the cell membrane as a predominantly cortical structure. Vimentin and tubulin spanned the cytoplasm from cell to nuclear membrane, the vimentin network appearing finer compared to tubulin. These cytoskeletal structures were present in chondrocytes from all depth zones of the articular cartilage. However, staining intensity varied from zone to zone, usually showing more intense staining for the filament systems at the articular surface compared to the deeper zones. These results obtained on fluorescently labeled sections were also corroborated by protein contents extracted and observed by immunoblotting. The observed cytoskeletal structures are compatible with some of the proposed cellular functions of these systems and support possible microenvironmental regulation of the cytoskeleton, including that due to physical forces from load-bearing, which are known to vary through the depth layers of articular cartilage.

Inhibiting gap junctional intercellular communication alters expression of differentiation markers in osteoblastic cells

Gap junctional intercellular communication (GJIC) may contribute to cellular differentiation. To examine this possibility in bone cells we examined markers of cellular differentiation, including alkaline phosphatase, osteocalcin, and osteopontin, in ROS17/2.8 cells (ROS), a rat osteoblastic cell line expressing phenotypic characteristics of fully differentiated osteoblasts. We utilized ROS rendered communication deficient either by stable transfection with antisense cDNA to connexin 43 (Cx43), the predominant gap junction protein in bone (RCx16 cells), or by overexpression of Cx45, a gap junction protein not normally expressed in ROS (ROS/Cx45 cells). Both RCx16 and ROS/Cx45 cells displayed reduced dye coupling and Cx43 protein expression relative to ROS, control transfectants, and ROS/Cx45tr, ROS cells expressing carboxylterminal truncated Cx45. Steady-state mRNA levels for osteocalcin as well as alkaline phosphatase activity, two markers of osteoblastic differentiation, were also reduced in poorly coupled RCx16 and ROS/Cx45 cells. On the other hand, steady-state mRNA levels for osteopontin increased slightly in RCx16 and ROS/Cx45 cells. These results suggest that GJIC at least partly contributes to the regulation of expression of markers of osteoblastic differentiation.

Chondrocyte-matrix attachment complexes mediate survival and differentiation

Integrin mediated cell-extracellular matrix interactions are required for survival and differentiation of many cell types. In this review, the cell-matrix attachment complex (CMAX) is described for chondrocytes. The evidence that integrin-mediated signal transduction is necessary for normal chondrocyte differentiation and survival in various culture conditions and in vivo are reviewed. The possible signal transduction pathways stimulated by the extracellular matrix components are discussed with a review of current data from chondrocyte experiments. In addition, the influence of parathyroid hormone and transforming growth factor beta on chondrocyte survival has been included as they may function in concert with integrin mediated signal transduction. Finally, specific changes in gene expression preceding apoptosis are discussed. The current understanding of how integrin-mediated signals prevent apoptosis and implications of anchorage-dependent survival for development and differentiation of the chondrocyte phenotype are discussed.

The expression of alpha 2 beta 1 integrin and alpha smooth muscle actin in fibroblasts grown on collagen

The maturation of connective tissue involves the organization of collagen fibres by resident fibroblasts. Fibroblast attachment to collagen has been demonstrated to involve cell surface receptors, integrins of the beta 1 family. Integrins are associated with cytoplasmic actin of microfilaments either directly or through focal adhesions. The major actin isoform of fibroblast microfilaments is beta actin and to a lesser extent alpha smooth muscle (alpha SM) actin. Cultured human dermal fibroblasts derived from adult dermis, newborn foreskin or keloid scar were grown on either uncoated or collagen-coated surfaces. The expression and synthesis of both alpha 2 beta 1 integrin and alpha SM actin were followed by immunohistology and immunoprecipitation. Fibroblasts on uncoated surfaces expressed little alpha 2 beta 1 integrin on their surface, while 20 per cent of them demonstrated alpha SM actin within microfilaments. Fibroblasts grown on a collagen-coated surface minimally expressed alpha SM actin in microfilament structures and a majority of the cells were positive for alpha 2 beta 1 integrin on their membranes. Using [35S]-methionine incorporation and immunoprecipitation, it was shown that fibroblasts grown in uncoated dishes synthesized more alpha SM actin than fibroblasts grown on collagen-coated dishes. In contrast, fibroblasts grown on collagen coated dishes synthesized more alpha 2 beta 1 integrin compared to the same cells grown on uncoated dishes. Fibroblasts maintained on a type I collagen upregulate the expression and synthesis of alpha 2 beta 1 integrin, and downregulate the expression and synthesis of alpha SM actin.

Chondrocyte survival and differentiation in situ are integrin mediated

Chondrocytes in specific areas of the chick sternum have different developmental fates. Cephalic chondrocytes become hypertrophic and secrete type X collagen into the extracellular matrix prior to bone deposition. Middle and caudal chondrocytes remain cartilaginous throughout development and continue to secrete collagen types II, IX, and XI. The interaction of integrin receptors with extracellular matrix molecules has been shown to affect cytoskeleton organization, proliferation, differentiation, and gene expression in other cell types. We hypothesized that chondrocyte survival and differentiation including the deposition into interstitial matrix of type X collagen may be integrin receptor mediated. To test this hypothesis, a serum-free organ culture sternal model that recapitulates normal development and maintains the three-dimensional relationships of the tissue was developed. We examined chondrocyte differentiation by five parameters: type X collagen deposition into interstitial matrix, sternal growth, actin distribution, cell shape, and cell diameter changes. Additional sterna were analyzed for apoptosis using a fragmented DNA assay. Sterna were organ cultured with blocking antibodies specific for integrin subunits (alpha2, alpha3, or beta1). In the presence of anti-beta1 integrin (25 microg/ml, clone W1B10), type X collagen deposition into interstitial matrix and sternal growth were significantly inhibited. In addition, all chondrocytes were significantly smaller, the actin was disrupted, and there was a significant increase in apoptosis throughout the specimens. Addition of anti-alpha2 (10 microg/ml, clone P1E6) or anti-alpha3 (10 microg/ml, clone P1B5) integrin partially inhibited type X collagen deposition into interstitial matrix; however, sternal growth and cell size were significantly decreased. These data are the first obtained from intact tissue and demonstrate that the interaction of chondrocytes with extracellular matrix is required for chondrocyte survival and differentiation.

Confocal analysis of the molecular heterogeneity in the pericellular microenvironment produced by adult canine chondrocytes cultured in agarose gel

Adult articular chondrocytes are each surrounded by a heterogeneous microenvironment and together form the chondron. Since little is known of chondron development, agarose gel culture, confocal immunohistochemistry and image analysis have been used to characterize the molecular anatomy and temporal development of the chondrocyte pericellular microenvironment in vitro. Two structurally distinct domains were identified during the 12-week culture period. The first comprised a narrow glycocalyx, 1-3 microns in width, which consolidated over time and was rich in collagen types II, VI, IX and XI, fibronectin, decorin and the aggrecan epitopes, 5D4 and HABR. The second region emerged after 4-6 weeks in culture and progressively developed a broad territorial region up to 12 microns wide around the chondrocyte and pericellular glycocalyx. Co-localization studies confirmed the dominance of aggrecan epitopes 2B6, EFG-4, 5D4 and HABR in the territorial domain, whereas surface density mapping with NIH image revealed two patterns of staining, one punctate and stippled, the other more uniform in distribution. The pericellular differentiation identified appeared analogous to the chondrons of adult articular cartilage, and provides an appropriate in vitro model for further studies of cell surface receptor function in the orchestration of pericellular matrix assembly.

Structural colocalisation of type VI collagen and fibronectin in agarose cultured chondrocytes and isolated chondrons extracted from adult canine tibial cartilage

Cell-matrix and matrix-matrix interactions are of critical importance in regulating the development, maintenance and repair of articular cartilage. In this study, we examined the structural colocalisation of type VI collagen and fibronectin in isolated chondrons and long-term agarose cultured chondrocytes extracted from normal adult canine articular cartilage. Using double labelling immunohistochemistry in conjunction with dual channel confocal microscopy and digital image processing we demonstrate that type VI collagen and fibronectin are distributed in a similar staining pattern and are colocalised at the surface of cultured chondrocytes and isolated chondrons. The results suggest that type VI collagen and fibronectin may play a role in both cell-matrix adhesion and matrix-matrix cohesion in the pericellular microenvironment surrounding articular cartilage chondrocytes.