Unravelling the Basic Calcium Phosphate crystal-dependent chondrocyte protein secretome; a role for TGF-β signaling

OBJECTIVE

Basic Calcium Phosphate (BCP) crystals play an active role in the progression of osteoarthritis (OA). However, the cellular consequences remain largely unknown. Therefore, we characterized for the first time the changes in the protein secretome of human OA articular chondrocytes as a result of BCP stimulation using two unbiased proteomic analysis methods.

METHOD

Isolated human OA articular chondrocytes were stimulated with BCP crystals and examined by Quantitative Reverse Transcription PCR (RT-qPCR) and enzyme-linked immune sorbent assay (ELISA) after twenty-four and forty-eight hours. Forty-eight hours conditioned media were analyzed by label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) and an antibody array. The activity of BCP dependent Transforming Growth Factor Beta (TGF-β) signaling was analyzed by RT-qPCR and luciferase reporter assays. The molecular consequences regarding BCP-dependent TGF-β signaling on BCP-dependent Interleukin 6 (IL-6) were investigated using specific pathway inhibitors.

RESULTS

Synthesized BCP crystals induced IL-6 expression and secretion upon stimulation of human articular chondrocytes. Concomitant induction of catabolic gene expression was observed. Analysis of conditioned media revealed a complex and diverse response with a large number of proteins involved in TGF-β signaling, both in activation of latent TGF-β and TGF-β superfamily members, which were increased compared to non-stimulated OA chondrocytes. Activity of this BCP driven TGF-β signaling was confirmed by increased activity of expression of TGF-β target genes and luciferase reporters. Inhibition of BCP driven TGF-β signaling resulted in decreased IL-6 expression and secretion with a moderate effect on catabolic gene expression.

CONCLUSION

BCP crystal stimulation resulted in a complex and diverse chondrocyte protein secretome response. An important role for BCP-dependent TGF-β signaling was identified in development of a pro-inflammatory environment.

Ribosomal RNA-based epitranscriptomic regulation of chondrocyte translation and proteome in osteoarthritis

OBJECTIVE

Osteoarthritis-related cartilage extracellular matrix remodeling is dependent on changes in chondrocyte protein expression. Yet, the role of ribosomes in chondrocyte translation regulation is unknown. In this exploratory study, we investigated ribosomal RNA (rRNA) epitranscriptomic-based ribosome heterogeneity in human articular chondrocytes and its relevance for osteoarthritis.

METHODS

Sequencing-based rRNA 2'-O-methylation profiling analysis (RiboMethSeq) was performed on non-OA primary human articular chondrocytes (n = 5) exposed for 14 days to osteoarthritic synovial fluid (14 donors, pooled, 20% v/v). The SW1353 SNORD71 KO cell pool was generated using LentiCRISPRv2/Cas9. The mode of translation initiation and fidelity were determined by dual-luciferase reporters. The cellular proteome was analyzed by LC-MS/MS and collagen type I protein expression was evaluated by immunoblotting. Loading of COL1A1 mRNA into polysomes was determined by sucrose gradient ultracentrifugation and fractionation.

RESULTS

We discovered that osteoarthritic synovial fluid instigates site-specific changes in the rRNA 2'-O-me profile of primary human articular chondrocytes. We identified five sites with differential 2'-O-me levels. The 2'-O-me status of 5.8S-U14 (one of identified differential 2'-O-me sites; decreased by 7.7%, 95% CI [0.9-14.5%]) was targeted by depleting the level of its guide snoRNA SNORD71 (50% decrease, 95% CI [33-64%]). This resulted in an altered ribosome translation modus (e.g., CrPV IRES, FC 3, 95% CI [2.2-4.1]) and promoted translation of COL1A1 mRNA which led to increased levels of COL1A1 protein (FC 1.7, 95% CI [1.3-2.0]).

CONCLUSIONS

Our data identify a novel concept suggesting that articular chondrocytes employ rRNA epitranscriptomic mechanisms in osteoarthritis development.

Synovial fluid from end-stage osteoarthritis induces proliferation and fibrosis of articular chondrocytes via MAPK and RhoGTPase signaling

OBJECTIVES

Alterations in the composition of synovial fluid have been associated with adverse effects on cartilage integrity and function. Here, we examined the phenotypic and proliferative behavior of human articular chondrocytes when cultured in vitro for 13 days with synovial fluid derived from end-stage osteoarthritis patients.

MATERIALS AND METHODS

Chondrocyte proliferation and phenotypical changes induced by osteoarthritic synovial fluid were analyzed using DNA staining, RT-qPCR, immunostainings, and immunoblotting. The molecular mechanisms by which osteoarthritic synovial fluid induced fibrosis and proliferation were studied using a phospho-protein antibody array and luciferase-based transcription factor activity assays. Specific pathway inhibitors were used to probe the involvement of pathways in fibrosis and proliferation.

RESULTS

Prolonged stimulation with osteoarthritic synovial fluid sustained chondrocyte proliferation and induced profound phenotypic changes, favoring a fibrotic over a chondrogenic or hypertrophic phenotype. A clear loss of chondrogenic markers at both the transcriptional and protein level was observed, while expression of several fibrosis-associated markers were upregulated over time. Phospho-kinase analysis revealed activation of MAPK and RhoGTPase signaling pathways by osteoarthritic synovial fluid, which was confirmed by elevated transcriptional activity of Elk-1 and SRF. Inhibitor studies revealed that ERK played a central role in the loss of chondrocyte phenotype, while EGFR and downstream mediators p38, JNK and Rac/Cdc42 were essential for fibrosis-associated collagen expression. Finally, we identified EGF signaling as a key activator of chondrocyte proliferation.

CONCLUSIONS

Osteoarthritic synovial fluid promoted chondrocyte fibrosis and proliferation through EGF receptor activation and downstream MAPK and RhoGTPase signaling.

Reporter gene comparison demonstrates interference of complex body fluids with secreted luciferase activity

Reporter gene assays are widely used to study cellular signaling and transcriptional activity. Few studies describe the use of reporter genes for studying cellular responses on complex body fluids, such as urine and blood. Selection of the optimal reporter gene is crucial for study outcome. Here, we compared the characteristics of five reporter genes (Firefly luciferase, stable- and unstable Nano luciferase, secretable Gaussia luciferase and Red Fluorescent Protein) to study complex body fluids. For this comparison, the NFκB Response Element (NFκB-RE) and Smad Binding Element (SBE) were identically cloned into the five different reporter vectors. Reporter characteristics were evaluated by kinetic and concentration–response measurements in SW1353 and HeLa cell lines. Finally, reporter compatibility with complex body fluids (fetal calf serum, knee joint synovial fluid and human serum) and inter-donor variation were evaluated. Red Fluorescent Protein demonstrated poor inducibility as a reporter gene and slow kinetics compared to luciferases. Intracellularly measured luciferases, such as Firefly luciferase and Nano luciferase, revealed good compatibility with complex body fluids. Secreted Gaussia luciferase appeared to be incompatible with complex body fluids, due to variability in inter-donor signal interference. Unstable Nano luciferase demonstrated clear inducibility, high sensitivity and compatibility with complex body fluids and therefore can be recommended for cellular signaling studies using complex body fluids.

Identification of tissue-dependent proteins in knee OA synovial fluid

OBJECTIVE

For many proteins from osteoarthritic synovial fluid, their intra-articular tissue of origin remains unknown. In this study we performed comparative proteomics to identify osteoarthritis-specific and joint tissue-dependent secreted proteins that may serve as candidates for osteoarthritis biomarker development on a tissue-specific basis.

DESIGN

Protein secretomes of cartilage, synovium, Hoffa's fat pad and meniscus from knee osteoarthritis patients were determined using liquid chromatography tandem mass spectrometry, followed by label-free quantification. Validation of tissue-dependent protein species was conducted by ELISA on independent samples. Differential proteomes of osteoarthritic and non-osteoarthritic knee synovial fluids were obtained via similar proteomics approach, followed by ELISA validation.

RESULTS

Proteomics revealed 64 proteins highly secreted from cartilage, 94 from synovium, 37 from Hoffa's fat pad and 21 from meniscus. Proteomic analyses of osteoarthritic vs non-osteoarthritic knee synovial fluid revealed 70 proteins with a relatively higher abundance and 264 proteins with a relatively lower abundance in osteoarthritic synovial fluid. Of the 70 higher abundance proteins, 23 were amongst the most highly expressed in the secretomes of a specific intra-articular tissue measured. Tissue-dependent release was validated for SLPI, C8, CLU, FN1, RARRES2, MATN3, MMP3 and TNC. Abundance in synovial fluid of tissue-dependent proteins was validated for IGF2, AHSG, FN1, CFB, KNG and C8.

CONCLUSIONS

We identified proteins with a tissue-dependent release from intra-articular human knee OA tissues. A number of these proteins also had an osteoarthritis-specific abundance in knee synovial fluid. These proteins may serve as novel candidates for osteoarthritis biomarker development on a tissue-specific basis.

Dysplasia epiphysealis hemimelica: a histological comparative study with osteochondromas

PURPOSE

Dysplasia epiphysealis hemimelica (DEH) is a rare developmental disorder resulting in epiphyseal overgrowth. Based on histological appearance, it is often described as an osteochondroma or osteochondroma-like lesion, although clinical differences exist between DEH and osteochondromas. The aim of this study was to test whether DEH and osteochondromas are histologically identical diseases.

METHODS

Tissue samples of two age- and gender-matched cases with DEH and hereditary multiple exostoses were histologically compared. Sections were stained with Safranin-O for detection of proteoglycans and immunohistochemistry was performed for detection of collagen type II, collagen type X as a marker of hypertrophic chondrocytes and Sox9 as a marker of proliferative chondrocytes. Due to the rarity, descriptions of the included DEH patients were outlined.

RESULTS

Histologically, chondrocyte clusters in a fibrillary matrix, a thick disorganised cartilage cap and ossification centres with small amounts of unabsorbed cartilage, were observed in DEH. In contrast, cartilage organisation of osteochondromas displays characteristics of the normal growth plate. Collagen type II was clearly detected in the cartilaginous extracellular matrix in osteochondromas, while weak expression was observed in DEH. Collagen type X was not detected in DEH, while expressed in the matrix surrounding hypertrophic chondrocytes in osteochondromas. Sox9 staining was positive in hypertrophic chondrocytes in osteochondromas, while expressed in nuclei of chondrocyte clusters in DEH.

CONCLUSION

Both morphological and immunohistological differences were observed in histological sections of DEH and osteochondromas. These results support the previously identified clinical, radiological and genetic differences and imply a different aetiology between DEH and osteochondroma formation.

Hypertrophic differentiation during chondrogenic differentiation of progenitor cells is stimulated by BMP-2 but suppressed by BMP-7

OBJECTIVE

Bone morphogenic protein (BMP)-2 and BMP-7 are clinically approved and their recombinant proteins are used for bone tissue regenerative purposes and widely evaluated for cartilage regeneration. Previous comparison of the in vitro chondrogenic characteristics of BMP-2 vs BMP-7 did not address hypertrophic differentiation and characterizing their chondrogenic properties with a focus in on chondrocyte hypertrophy was topic of investigation in this study.

DESIGN

Equimolar concentrations of BMP-2 or BMP-7 were added to chondrogenic differentiating ATDC5, human bone marrow stem cells or rabbit periosteal explants. Expression of Col2a1, Sox9, Acan, Col10a1, Runx2, ALP, Mmp13, Mef2c and Bapx1/Nkx3.2 was determined by reverse transcription-quantitative PCR (RT-qPCR) and immunoblotting. Glycosaminoglycan content, cell proliferation capacity and ALP activity were analysed by colourimetric analyses. Expression of Bapx1/Nkx3.2 and Sox9 was targeted by transfection of target specific siRNA duplexes.

RESULTS

BMP-2 dose-dependently increased chondrocyte hypertrophy during chondrogenic differentiation of progenitor cells, whereas BMP-7 acted hypertrophy-suppressive and chondro-promotive. Both BMPs did not influence cell proliferation, but they did increase total glycosaminoglycan content. In a candidate approach Bapx1/Nkx3.2 was found to be involved in the BMP-7 mediated suppression of chondrocyte hypertrophy in ATDC5 cells.

CONCLUSIONS

BMP-2 and BMP-7 display opposing actions on the chondrogenic outcome of differentiating progenitor cells: BMP-2 acts a specific inducer of chondrocyte hypertrophy, while BMP-7 appears to increase or maintain chondrogenic potential and prevent chondrocyte hypertrophy. Our results pave the way for an application-dependent differential use of BMP-2 or BMP-7.

Redifferentiation of dedifferentiated human articular chondrocytes: comparison of 2D and 3D cultures

OBJECTIVE

Three-dimensional (3D) cultures are widely used to redifferentiate chondrocytes. However, the rationale behind the choice for 3D above two-dimensional (2D) cultures is poorly systematically investigated and mainly based on mRNA expression and glycosaminoglycan (GAG) content. The objective was to determine the differential redifferentiation characteristics of human articular chondrocytes (HACs) in monolayer, alginate beads and pellet culture by investigating mRNA expression, protein expression, GAG content and cell proliferation.

DESIGN

Dedifferentiated HACs from six individuals were redifferentiated in identical medium conditions for 7 days in monolayer, alginate beads or pellet culture. Read-out parameters were expression of chondrogenic and hypertrophic mRNAs and proteins, GAG content and cell proliferation.

RESULTS

3D cultures specifically expressed chondrogenic mRNAs [collagen type II (COL2A1), SRY (sex determining region Y)-box 9 (SOX9), aggrecan (ACAN)), whereas 2D cultures did not. Hypertrophic mRNAs (collagen type X (COL10A1), runt-related transcription factor 2 (RUNX2), matrix metalloproteinase 13 (MMP13), vascular endothelial growth factor A (VEGFA), osteopontin (OPN), alkaline phosphatase (ALP)) were highly increased in 2D cultures and lower in 3D cultures. Collagen type I (COL1A1) mRNA expression was highest in 3D cultures. Protein expression supports most of the mRNA data, although an important discrepancy was found between mRNA and protein expression of COL2A1 and SOX9 in monolayer culture, stressing on the importance of protein expression analysis. GAG content was highest in 3D cultures, whereas chondrocyte proliferation was almost specific for 2D cultures.

CONCLUSIONS

For redifferentiation of dedifferentiated HACs, 3D cultures exhibit the most potent chondrogenic potential, whereas a hypertrophic phenotype is best achieved in 2D cultures. This is the first human study that systematically evaluates the differences between proliferation, GAG content, protein expression and mRNA expression of commonly used 2D and 3D chondrocyte culture techniques.

Tissue-engineered constructs: the effect of scaffold architecture in osteochondral repair

Cartilage has a poor regenerative capacity. Tissue-engineering approaches using porous scaffolds seeded with chondrocytes may improve cartilage repair. The aim of this study was to examine the effect of pore size and pore interconnectivity on cartilage repair in osteochondral defects treated with different scaffolds seeded with allogenic chondrocytes. Scaffolds consisting of 55 wt% poly(ethylene oxide terephthalate) and 45 wt% poly(butylene terephthalate) (PEOT/PBT) with different pore sizes and interconnectivities were made, using a compression moulding (CM) and a three-dimensional fibre (3DF) deposition technique. In these scaffolds, allogenic chondrocytes were seeded, cultured for 3 weeks and implanted in osteochondral defects of skeletally mature rabbits. At 3 weeks no difference in cartilage repair between an empty osteochondral defect, CM or 3DF scaffolds was found. Three months post-implantation, cartilage repair was significantly improved after implantation of a 3DF scaffold compared to a CM scaffold. Although not significant, Mankin scores for osteoarthritis (OA) indicated less OA in the 3DF scaffold group compared to empty defects and CM-treated defects. It is concluded that scaffold pore size and pore interconnectivity influences osteochondral repair and a decreased pore interconnectivity seems to impair osteochondral repair.

Inhibition of cyclooxygenase-2 impacts chondrocyte hypertrophic differentiation during endochondral ossification

Skeletogenesis and bone fracture healing involve endochondral ossification, a process during which cartilaginous primordia are gradually replaced by bone tissue. In line with a role for cyclooxygenase-2 (COX-2) in the endochondral ossification process, non-steroidal anti-inflammatory drugs (NSAIDs) were reported to negatively affect bone fracture healing due to impaired osteogenesis. However, a role for COX-2 activity in the chondrogenic phase of endochondral ossification has not been addressed before. We show that COX-2 activity fulfils an important regulatory function in chondrocyte hypertrophic differentiation. Our data reveal essential cross-talk between COX-2 and bone morphogenic protein-2 (BMP-2) during chondrocyte hypertrophic differentiation. BMP-2 mediated chondrocyte hypertrophy is associated with increased COX-2 expression and pharmacological inhibition of COX-2 activity by NSAIDs (e.g., Celecoxib) decreases hypertrophic differentiation in various chondrogenic models in vitro and in vivo, while leaving early chondrogenic development unaltered. Our findings demonstrate that COX-2 activity is a novel factor partaking in chondrocyte hypertrophy in the context of endochondral ossification and these observations provide a novel etiological perspective on the adverse effects of NSAIDs on bone fracture healing and have important implications for the use of NSAIDs during endochondral skeletal development.

Chimeric arteriviruses generated by swapping of the M protein ectodomain rule out a role of this domain in viral targeting

Arteriviruses are enveloped, positive-strand RNA viruses for which the two major envelope proteins GP(5) and M occur as disulfide-linked heterodimers. These were assumed to serve the viral targeting functions, but recent ectodomain swapping studies with equine arteritis virus (EAV) indicate that the GP(5) protein does not determine arteriviral tropism. Here, we focused on the short, 13- to 18-residue ectodomain of the M protein. Using an infectious cDNA clone of the Lelystad virus isolate of porcine reproductive and respiratory syndrome virus (PRRSV), we substituted the genomic sequence encoding the M ectodomain by that of murine lactate dehydrogenase-elevating virus, EAV, and the US PRRSV-isolate, VR2332. Viable viruses with a chimeric M protein were obtained in all three cases, but for the latter two only after removal of the genomic overlap between the M and GP(5) genes. Characterization of the chimeric viruses revealed that they could be distinguished immunologically from wild-type virus, that they were genetically stable in vitro, but that they were impaired in their growth, reaching lower titers than the parental virus. The latter appeared to be due to an increased particle-to-infectivity ratio of the chimeric virus particles. Interestingly, the chimeric viruses had retained their ability to infect porcine cells and had not acquired tropism for cells susceptible to the viruses from which the foreign ectodomains were derived. We conclude that the surface structures composed by the arterivirus M and GP(5) ectodomains do not determine viral tropism.