Nuclear deformation and expression change of cartilaginous genes during in vitro expansion of chondrocytes

Cartilaginous gene expression decreased when chondrocytes were expanded on cell-culture plates. Understanding the dedifferentiation mechanism may provide valuable insight into cartilage tissue engineering. Here, we demonstrated the relationship between the nuclear shape and gene expression during in vitro expansion culture of chondrocytes. Specifically, the projected nuclear area increased and cartilaginous gene expressions decreased during in vitro expansion culture. When the nuclear deformation was recovered by cytochalasin D treatment, aggrecan expression was up-regulated and type I collagen (Col1a2) expression was down-regulated. These results suggest that nuclear deformation may be one of the mechanisms for chondrocyte dedifferentiation during in vitro expansion culture.     

Dedifferentiation alters chondrocyte nuclear mechanics during in vitro culture and expansion

The biophysical features of a cell can provide global insights into diverse molecular changes, especially in processes like the dedifferentiation of chondrocytes. Key biophysical markers of chondrocyte dedifferentiation include flattened cellular morphology and increased stress-fiber formation. During cartilage regeneration procedures, dedifferentiation of chondrocytes during in vitro expansion presents a critical limitation to the successful repair of cartilage tissue. Our study investigates how biophysical changes of chondrocytes during dedifferentiation influence the nuclear mechanics and gene expression of structural proteins located at the nuclear envelope. Through an experimental model of cell stretching and a detailed spatial intranuclear strain quantification, we identified that strain is amplified and the distribution of strain within the chromatin is altered under tensile loading in the dedifferentiated state. Further, using a confocal microscopy image-based finite element model and simulation of cell stretching, we found that the cell shape is the primary determinant of the strain amplification inside the chondrocyte nucleus in the dedifferentiated state. Additionally, we found that nuclear envelope proteins have lower gene expression in the dedifferentiated state. This study highlights the role of cell shape in nuclear mechanics and lays the groundwork to design biophysical strategies for the maintenance and enhancement of the chondrocyte phenotype during cell expansion with a goal of successful cartilage tissue engineering.     

Effects of extracellular matrix proteins in chondrocyte‐derived matrices on chondrocyte functions

Loss of cartilaginous phenotype during in vitro expansion culture of chondrocytes is a major barrier to the application of chondrocytes for tissue engineering. In previous study, we showed that dedifferentiation of chondrocytes during the passage culture was delayed by matrices formed by primary chondrocytes (P0‐ECM). In this study, we investigated bovine chondrocyte functions when being cultured on isolated extracellular matrix (ECM) protein‐coated substrata and P0‐ECM. Low chondrocyte attachment was observed on aggrecan‐coated substratum and P0‐ECM. Cell proliferation on aggrecan‐ and type II collagen/aggrecan‐coated substrata and P0‐ECM was lower than that on the other ECM protein (type I collagen and type II collagen)‐coated substrata. When chondrocytes were subcultured on aggrecan‐coated substratum, decline of cartilaginous gene expression was delayed, which was similar to the cells subcultured on P0‐ECM. These results indicate that aggrecan plays an important role in the regulation of chondrocyte functions and P0‐ECM may be a good experimental control for investigating the role of each ECM protein in cartilage ECM. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1331–1336, 2013     

P38 mitogen‐activated protein kinase promotes dedifferentiation of primary articular chondrocytes in monolayer culture

Articular cartilage is an avascular tissue with poor regenerative capacity following injury, a contributing factor to joint degenerative disease. Cell‐based therapies for cartilage tissue regeneration have rapidly advanced; however, expansion of autologous chondrocytes in vitro using standard methods causes ‘dedifferentiation’ into fibroblastic cells. Mitogen‐activated protein kinase (MAPK) signalling is crucial for chondrocyte metabolism and matrix production, and changes in MAPK signals can affect the phenotype of cultured cells. We investigated the effects of inhibition of MAPK signalling on chondrocyte dedifferentiation during monolayer culture. Blockade of extracellular signal‐regulated kinase (ERK) and c‐Jun N‐terminal kinase (JNK) signalling caused a significant increase in cartilage gene expression, however, also caused up‐regulation of fibrotic gene expression. Inhibition of p38 MAPK (p38) caused a significant up‐regulation of collagen type II while suppressing collagen type I expression. P38 inhibition also resulted in consistently more organized secretion of collagen type II protein deposits on cell culture surfaces. Follow‐on pellet culture of treated cells revealed that MAPK inhibition reduced cell migration from the pellet. ERK and JNK inhibition caused more collagen type I accumulation in pellets versus controls while p38 inhibition strongly promoted collagen type II accumulation with no effect on collagen type I. Blockade of all three MAPKs caused increased GAG content in pellets. These results indicate a role for MAPK signalling in chondrocyte phenotype loss during monolayer culture, with a strong contribution from p38 signalling. Thus, blockade of p38 enhances chondrocyte phenotype in monolayer culture and may promote more efficient cartilage tissue regeneration for cell‐based therapies.     

Cartilage–Specific Over-Expression of CCN Family Member 2/Connective Tissue Growth Factor (CCN2/CTGF) Stimulates Insulin-Like Growth Factor Expression and Bone Growth

Previously we showed that CCN family member 2/connective tissue growth factor (CCN2) promotes the proliferation, differentiation, and maturation of growth cartilage cells in vitro. To elucidate the specific role and molecular mechanism of CCN2 in cartilage development in vivo, in the present study we generated transgenic mice overexpressing CCN2 and analyzed them with respect to cartilage and bone development. Transgenic mice were generated expressing a ccn2/lacZ fusion gene in cartilage under the control of the 6 kb-Col2a1-enhancer/promoter. Changes in cartilage and bone development were analyzed histologically and immunohistologically and also by micro CT. Primary chondrocytes as well as limb bud mesenchymal cells were cultured and analyzed for changes in expression of cartilage–related genes, and non-transgenic chondrocytes were treated in culture with recombinant CCN2. Newborn transgenic mice showed extended length of their long bones, increased content of proteoglycans and collagen II accumulation. Micro-CT analysis of transgenic bones indicated increases in bone thickness and mineral density. Chondrocyte proliferation was enhanced in the transgenic cartilage. In in vitro short-term cultures of transgenic chondrocytes, the expression of col2a1, aggrecan and ccn2 genes was substantially enhanced; and in long-term cultures the expression levels of these genes were further enhanced. Also, in vitro chondrogenesis was strongly enhanced. IGF-I and IGF-II mRNA levels were elevated in transgenic chondrocytes, and treatment of non-transgenic chondrocytes with recombinant CCN2 stimulated the expression of these mRNA. The addition of CCN2 to non-transgenic chondrocytes induced the phosphorylation of IGFR, and ccn2-overexpressing chondrocytes showed enhanced phosphorylation of IGFR. Our data indicates that the observed effects of CCN2 may be mediated in part by CCN2-induced overexpression of IGF-I and IGF-II. These findings indicate that CCN2-overexpression in transgenic mice accelerated the endochondral ossification processes, resulting in increased length of their long bones. Our results also indicate the possible involvement of locally enhanced IGF-I or IGF-II in this extended bone growth.     

Identification and characterization of arginase II as a chondrocyte phenotype-specific gene

We have previously shown that activation of extracellular signal-regulated protein kinase-1 and -2 (ERK1/2) causes chondrocyte dedifferentiation, which contributes to the destruction of arthritic cartilage. In the present study, we identified genes involved in the ERK1/2 regulation of chondrocyte dedifferentiation. Several genes were identified by subtractive hybridization, and, of these, arginase II was selected for further functional characterization. Similar to the pattern of type II collagen expression, which is a hallmark of chondrocyte differentiation, arginase II expression was increased during chondrogenesis of mesenchymal cells. The high expression level of arginase II was decreased during dedifferentiation of chondrocytes, whereas its expression was restored during redifferentiation of the dedifferentiated chondrocytes. Inhibition of ERK1/2 signaling in chondrocytes enhanced type II collagen expression with a concomitant increase in expression and activity of arginase II. However, ectopic expression of arginase II or inhibition of its activity did not affect chondrocyte differentiation. The results collectively indicate that expression of arginase II is specific to the chondrocyte phenotype, although the expression of arginase II alone is not sufficient for articular chondrocytes to maintain a differentiated phenotype.     

Notch signaling is involved in human articular chondrocytes de-differentiation during osteoarthritis

Abstract

Context: During osteoarthritis (OA), chondrocytes undergo de-differentiation, resulting in the acquisition of a fibroblast-like morphology, decreased expression of collagen type II (colII) and aggrecan, and increased expression of collagen type I (colI), metalloproteinase 13 (MMP13) and nitric oxide synthase (eNOS). Notch signaling plays a crucial role during embryogenesis. Several studies showed that Notch is expressed in adulthood. Objective: The aim of our study was to confirm the involvement of Notch signaling in human OA at in vitro and ex vivo levels. Materials and methods: Normal human articular chondrocytes were cultured during four passages either treated or not with a Notch inhibitor: DAPT. Human OA cartilage was cultured with DAPT for five days. Chondrocytes secreted markers and some Notch pathway components were analyzed using Western blotting and qPCR. Results: Passaging chondrocytes induced a decrease in the cartilage markers: colII and aggrecan. DAPT-treated chondrocytes and OA cartilage showed a significant increase in healthy cartilage markers. De-differentiation markers, colI, MMP13 and eNOS, were significantly reduced in DAPT-treated chondrocytes and OA cartilage. Notch1 expression was proportional to colI, MMP13 and eNOS expression and inversely proportional to colII and aggrecan expression in nontreated cultured chondrocytes. Notch ligand: Jagged1 increased in chondrocytes culture. DAPT treatment resulted in reduced Jagged1 expression. Notch target gene HES1 increased during chondrocyte culture and was reduced when treated with DAPT. Conclusion: Targeting Notch signaling during OA might lead to the restitution of the typical chondrocyte phenotype and even to chondrocyte redifferentiation during the pathology.     

Effect of eldecalcitol on articular cartilage through the regulation of transcription factor Erg in a murine model of knee osteoarthritis

Clinical studies have reported an association between low blood levels of 25-hydroxyvitamin D and the progression of osteoarthritis (OA), but the mechanism and effects of vitamin D signaling on articular chondrocytes and cartilage remains unclear. The purpose of this study was to investigate the effects of vitamin D on articular cartilage degeneration using eldecalcitol (ED-71), which is an active vitamin D₃ analog. Eight-week old male C57BL/6NCrSlc mice were subjected to experimental surgery to induce OA and local treatments with 10 μL ED-71 (0.5 μg/mL) were administered weekly. Four and 12 weeks after surgery, joints were evaluated using histological scoring systems. In addition, gene expression was analyzed in chondrocytes that were isolated from wildtype neonatal mice, cultured, and treated with ED-71 (10^?8 M). Joints treated with ED-71 demonstrated slowed progression of OA at 4 weeks after surgery, but few effects were observed at 12 weeks after surgery. Ets-related gene (Erg) expression was upregulated in OA articular cartilage, and further increased by ED-71 treatment. In primary chondrocytes cultured with ED-71, the gene expression of Erg and lubricin/proteoglycan 4 significantly increased, as compared to that of cells cultured without ED-71. Local treatment with ED-71 reduced degenerative changes to the articular cartilage during the early phase of experimental OA. Regulation of Erg by ED-71 in articular cartilage could confer resistance to early osteoarthritic changes.     

Immunophenotypic analysis of human articular chondrocytes: changes in surface markers associated with cell expansion in monolayer culture

Cartilage tissue engineering relies on in vitro expansion of primary chondrocytes. Monolayer is the chosen culture model for chondrocyte expansion because in this system the proliferative capacity of chondrocytes is substantially higher compared to non-adherent systems. However, human articular chondrocytes (HACs) cultured as monolayers undergo changes in phenotype and gene expression known as "dedifferentiation." To gain a better understanding of the cellular mechanisms involved in the dedifferentiation process, our research focused on the characterization of the surface molecule phenotype of HACs in monolayer culture. Adult HACs were isolated by enzymatic digestion of cartilage samples obtained post-mortem. HACs cultured in monolayer for different time periods were analyzed by flow cytometry for the expression of cell surface markers with a panel of 52 antibodies. Our results show that HACs express surface molecules belonging to different categories: integrins and other adhesion molecules (CD49a, CD49b, CD49c, CD49e, CD49f, CD51/61, CD54, CD106, CD166, CD58, CD44), tetraspanins (CD9, CD63, CD81, CD82, CD151), receptors (CD105, CD119, CD130, CD140a, CD221, CD95, CD120a, CD71, CD14), ectoenzymes (CD10, CD26), and other surface molecules (CD90, CD99). Moreover, differential expression of certain markers in monolayer culture was identified. Up-regulation of markers on HACs regarded as distinctive for mesenchymal stem cells (CD10, CD90, CD105, CD166) during monolayer culture suggested that dedifferentiation leads to reversion to a primitive phenotype. This study contributes to the definition of HAC phenotype, and provides new potential markers to characterize chondrocyte differentiation stage in the context of tissue engineering applications.     

Chronic exposure of bone morphogenetic protein-2 favors chondrogenic expression in human articular chondrocytes amplified in monolayer cultures

Articular cartilage is a specialized connective tissue containing chondrocytes embedded in a network of extracellular macromolecules such as type II collagen and presents poor capacity to self-repair. Autologous chondrocyte transplantation (ACT) is worldwide used for treatment of focal damage to articular cartilage. However, dedifferentiation of chondrocytes occurs during the long term culture necessary for mass cell production. The aim of this study was to investigate if addition of bone morphogenetic protein (BMP)-2, a strong inducer of chondrogenic expression, to human chondrocytes immediately after their isolation from cartilage, could help to maintain their chondrogenic phenotype in long-term culture conditions. Human articular chondrocytes were cultured according to the procedure used for ACT. Real-time PCR and Western blotting were performed to evaluate the cellular phenotype. Exogenous BMP-2 dramatically improves the chondrogenic character of knee articular chondrocytes amplified over two passages, as assessed by the BMP-2 stimulation on type II procollagen expression and synthesis. This study reveals that BMP-2 could potentially serve as a therapeutic agent for supporting the chondrogenic phenotype of human articular chondrocytes expanded in the conditions generally used for ACT.     

Modulation of collagen synthesis in normal and osteoarthritic cartilage

In osteoarthritic cartilage, chondrocytes are able to present heterogeneous cellular reactions with expression and synthesis of the (pro)collagen types characteristic of prechondrocytes (type IIA), hypertrophic chondrocytes (type X), as well as differentiated (types IIB, IX, XI, VI) and dedifferentiated (types I, III) chondrocytes. The expression of type IIA procollagen in human osteoarthritic cartilage support the assumption that OA chondrocytes reverse their phenotype towards a chondroprogenitor phenotype. Recently, we have shown that dedifferentiation of mouse chondrocytes induced by subculture was associated with the alternative splicing of type II procollagen pre-mRNA with a switch from the IIB to the IIA form. In this context, we demonstrated that BMP-2 favours expression of type IIB whereas TGF-beta1 potentiates expression of type IIA induced by subculture. These data reveal the specific capability of BMP-2 to reverse the program of chondrocyte dedifferentiation. This interesting feature needs to be tested with human chondrocytes since cell amplification is required for the currently used autologous chondrocyte transplantation.