OBJECTIVE:Endochondral ossification is vital for bone healing, with neutrophils playing a crucial role in the osteoimmune system. While N1 (pro-inflammatory) and N2 (regenerative) neutrophils are documented in other contexts, their role in endochondral ossification remains unclear. This study investigates their effects on ATDC5 cells. METHODS:Neutrophils from five healthy volunteers were isolated and polarized into N0 (unstimulated), N1, or N2 phenotypes. After culturing for 4 h, neutrophil-conditioned media was mixed (20 % v/v) with chondrogenic differentiation media (DM). ATDC5 cells were cultured with mixture or DM alone for 24 h, followed by continued DM culturing. On days 7 and 14, several gene expressions, ALP activity, and TGF-β3 levels were assessed. RESULTS:SOX9 peaked in the ATDC5 (A)/N2 group on day 7, while the A/N1 group showed the highest levels on day 14. COL2A1 was elevated in the A/N0 group on day 7. RUNX2 was higher in A/N1 and A/N2 on day 7, with A/N1 remaining elevated on day 14. MMP-13 was significantly higher in A/N1 on day 7. COL10A1 expression showed no significant changes. COL1A1 and COX2 were continually elevated in the A/N1 group. ALP activity was consistently enhanced in the A/N1 group, and TGF-β3 was lower in both A/N1 and A/N2 on day 14. CONCLUSIONS:This study indicates that N1 neutrophils may promote chondrocyte maturation and osteogenic differentiation, while N2 neutrophils may support proliferation, hypertrophy, and maturation, providing a cell reservoir for ossification. These findings highlight their distinct roles in directing chondrogenic progenitors toward bone rather than cartilage formation.

Osteoarthritis is a highly prevalent, age-associated joint disease characterized by cartilage degeneration, joint dysfunction, and chronic pain. We previously developed a bone morphogenetic protein 7 derived peptide p[63–82], which may be a novel disease-modifying treatment option for OA. In this study we aimed to optimize the bioactivity and biostability of this peptide in the intra-articular environment to evaluate the therapeutic potential of these peptides to treat osteoarthritis. 33 peptide modifications of p[63–82] were custom-designed and synthesized to optimize the bioactivity. Chondrocytes and synovial fluid were collected from end-stage osteoarthritic patients at total knee arthroplasty surgery. To validate improvements in bioactivity, gene expression analysis, glycosaminoglycan content, matrix metalloproteinase-13 protein levels and alkaline phosphatase activity was measured. Several biochemical approaches were used to explore optimization of the original p[63–82] peptide. One cyclized peptide (C2) was able to significantly increase the expression of collagen type 2 and decrease expression of collagen type 10, matrix metalloproteinase-13 and prostaglandin-endoperoxide synthase 2. The linear p[63–82] peptide and the cyclic peptide variant C2 in the same concentration were effective in suppressing the osteoartritic phenotype in SW1353 cells, despite the presence of interleukin-1β or osteoarthritic-synovial fluid. However, peptide variant C2 had a significantly more favorable bioactivity as compared to p[63–82] in reducing matrix metalloproteinase-13 protein levels in the osteoarthritic-synovial fluid exposed condition. At lower concentrations, the cyclic peptide C2 showed a higher bioactivity as compared to the linear p[63–82] peptide. When the activity of both peptides on primary human articular chondrocytes was evaluated, we found that the linear p[63–82] peptide as well as peptide C2 counteract the hypertrophic and inflammatory state of primary OA chondrocytes. This study demonstrates that among various tested modifications of p[63–82], one cyclic variant (C2) showed similar results in bioactivity as compared to the linear peptide p[63–82], whilst the other modified peptide variants had inactive bioactive properties as compared to the original p[63–82] peptide. This highlights the challenge in enhancing peptide properties without compromising their biological activity and emphasises the need for a cautious approach in peptide modification for therapeutic use. This research underscores that while cyclization and other structural changes may offer benefits, they should be carefully evaluated on a case-by-case basis.

The pathological relevance of articular cartilage calcification in osteoarthritis (OA) is becoming increasingly evident. We are only beginning to understand the pathobiological mechanisms that contribute to articular cartilage calcification in OA. How molecular environmental factors interact with calcification mechanisms is poorly explored. In this study, we developed an in vitro phosphate-driven calcification model for human OA articular chondrocytes, in which these cells are cultured in the presence of calcification medium containing adenosine triphosphate (ATP) and β-glycerophosphate (BGP). We employed this model to investigate the role of transforming growth factor β (TGF-β) in chondrocyte calcification. Chondrocyte culture in calcification medium resulted in mineral nodule formation over a time course of 7 days. The presence of calcium and phosphate deposition in these nodules was validated with von Kossa staining, scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDX), and colorimetric calcium and phosphate assays. Supplementation of calcification medium with TGF-β resulted in enhanced nodule formation with a different morphology and changed the expression of extracellular matrix-related genes such as collagen type I and III. In conclusion, we developed a new in vitro model for human OA articular chondrocyte calcification, in which we demonstrated a pro-calcifying role for TGF-β. This in vitro model may be used as a basis to aid the investigation of the influence of environmental factors on chondrocyte calcification and the development of new anti-calcification disease-modifying osteoarthritis drugs.

PurposeKnee joint distraction is a surgical procedure with cartilage-regenerating properties. The composition of joint distraction-regenerated cartilage in human patients is poorly documented. In this case-study, provided a unique opportunity to biomolecularly characterize the regenerated tissue from a patient who underwent bilateral distraction and later knee replacements.MethodsKnee joint distraction was conducted using an external fixation frame and total knee arthroplasty was performed several years later. Radiographic imaging was performed to assess the status of the knee joint prior, during and after clinical interventions. Following total knee replacement, cartilage biopsies were collected and processed for tissue sectioning and histochemical staining. Tandem mass-spectrometry proteomics analysis was used to characterize and compare the proteomic composition.ResultsBoth knee joints showed joint-space improvement pre- and post-knee joint distraction. Regenerated cartilage was white with an irregular surface, while native (lateral) cartilage had a yellow appearance and smooth surface. Histochemical staining showed higher Safranin-O positivity in native cartilage compared to regenerated cartilage, and differences in collagen structure. Proteomic analysis did not reveal major differences in cartilage extracellular matrix protein abundance. Bioinformatic analyses revealed enrichment in ribosomal proteins (regenerated cartilage) and RNA Polymerase II Transcription Termination (native cartilage).ConclusionHistologically, knee joint distraction-regenerated cartilage showed less glycosaminoglycans and disorganized collagen compared to native cartilage. However, mass-spectrometry has no major differences in extracellular matrix protein abundance, with proteomic clues suggesting protein translation regulation as a potential mechanism for regeneration.

Alterations in cell fate are often attributed to (epigenetic) regulation of gene expression. An emerging paradigm focuses on specialized ribosomes within a cell. However, little evidence exists for the dynamic regulation of ribosome composition and function. Here, we stimulated a chondrocytic cell line with transforming growth factor beta (TGF-β2) and mapped changes in ribosome function, composition and ribosomal RNA (rRNA) epitranscriptomics. 35S Met/Cys incorporation was used to evaluate ribosome activity. Dual luciferase reporter assays were used to assess ribosomal modus. Ribosomal RNA expression and processing were determined by RT-qPCR, while RiboMethSeq and HydraPsiSeq were used to determine rRNA modification profiles. Label-free protein quantification of total cell lysates, isolated ribosomes and secreted proteins was done by LC-MS/MS. A three-day TGF-β2 stimulation induced total protein synthesis in SW1353 chondrocytic cells and human articular chondrocytes. Specifically, TGF-β2 induced cap-mediated protein synthesis, while IRES-mediated translation was not (P53 IRES) or little affected (CrPv IGR and HCV IRES). Three rRNA post-transcriptional modifications (PTMs) were affected by TGF-β2 stimulation (18S-Gm1447 downregulated, 18S-ψ1177 and 28S-ψ4598 upregulated). Proteomic analysis of isolated ribosomes revealed increased interaction with eIF2 and tRNA ligases and decreased association of eIF4A3 and heterogeneous nuclear ribonucleoprotein (HNRNP)s. In addition, thirteen core ribosomal proteins were more present in ribosomes from TGF-β2 stimulated cells, albeit with a modest fold change. A prolonged stimulation of chondrocytic cells with TGF-β2 induced ribosome activity and changed the mode of translation. These functional changes could be coupled to alterations in accessory proteins in the ribosomal proteome.

Purpose (the aim of the study): Characterization of osteoarthritis (OA) patient-specific molecular processes is essential for OA endotyping. Several studies stratified OA in a number of molecular endotypes, based on transcriptomic profiles of joint tissues. While joint tissue sampling provides insight into the underlying disease mechanisms, clinical implementation of such an approach is challenging due to invasiveness and potential damage to joint tissues. Alternatively, synovial fluid (SF) aspiration is considered a less invasive approach and more suitable for diagnostic purposes. Considering the compositional complexity of synovial fluid, we aimed to stratify OA based on SF-induced differential cellular signaling patterns.

Objective Osteoarthritis (OA) is characterized by articular cartilage erosion, pathological subchondral bone changes, and signs of synovial inflammation and pain. We previously identified p[63-82], a bone morphogenetic protein 7 (BMP7)-derived bioactive peptide that attenuates structural cartilage degeneration in the rat medial meniscal tear-model for posttraumatic OA. This study aimed to evaluate the cartilage erosion-attenuating activity of p[63-82] in a different preclinical model for OA (anterior cruciate ligament transection—partial medial meniscectomy [anterior cruciate ligament transection (ACLT)-pMMx]). The disease-modifying action of the p[63-82] was followed-up in this model for 5 and 10 weeks. Design Skeletally mature male Lewis rats underwent ACLT-pMMx surgery. Rats received weekly intra-articular injections with either saline or 500 ng p[63-82]. Five and 10 weeks postsurgery, rats were sacrificed, and subchondral bone characteristics were determined using microcomputed tomography (µCT). Histopathological evaluation of cartilage degradation and Osteoarthritis Research Society International (OARSI)-scoring was performed following Safranin-O/Fast Green staining. Pain-related behavior was measured by incapacitance testing and footprint analysis. Results Histopathological evaluation at 5 and 10 weeks postsurgery showed reduced cartilage degeneration and a significantly reduced OARSI score, whereas no significant changes in subchondral bone characteristics were found in the p[63-82]-treated rats compared to the saline-treated rats. ACLT-pMMx-induced imbalance of static weightbearing capacity in the p[63-82] group was significantly improved compared to the saline-treated rats at weeks 5 postsurgery. Footprint analysis scores in the p[63-82]-treated rats demonstrated improvement at week 10 postsurgery. Conclusions Weekly intra-articular injections of p[63-82] in the rat ACLT-pMMx posttraumatic OA model resulted in reduced degenerative cartilage changes and induced functional improvement in static weightbearing capacity during follow-up.

Purpose (the aim of the study): The emergence of single-cell transcriptomics has led to the identification of functionally distinct chondrocyte subpopulations in osteoarthritic (OA) articular cartilage. However, few chondrocyte subtypes have been characterized at the protein level and their role in OA pathobiology is poorly understood. In this study, we aimed to quantify OA chondrocyte subpopulations at the protein level in a high throughput manner and to characterize interpatient variation and functional differences.

Purpose (the aim of the study): The pathobiological role of calcium-containing crystals has become inseparable from osteoarthritis (OA). The crystals found in OA articular cartilage and synovial fluid mainly comprise basic calcium phosphate (BCP) crystals, where hydroxyapatite (HA) is the most prominent species. Proteins such as alpha-2 HS glycoprotein (AHSG) bind amorphous calcium phosphate particles to allow clearance. These protein-bound calcium phosphate species can be categorized into primary (pCPP) and secondary calciprotein particles (sCPP), based on their distinct maturation characteristics.

Osteoarthritis is the most common degenerative joint disorder. MicroRNAs are gene expression regulators that act post-transcriptionally to control tissue homeostasis. Microarray analysis was undertaken in osteoarthritic intact, lesioned and young intact cartilage. Principal component analysis showed that young intact cartilage samples were clustered together; osteoarthritic samples had a wider distribution; and osteoarthritic intact samples were separated into two subgroups, osteoarthritic-Intact-1 and osteoarthritic-Intact-2. We identified 318 differentially expressed microRNAs between young intact and osteoarthritic lesioned cartilage, 477 between young intact and osteoarthritic-Intact-1 cartilage and 332 between young intact and osteoarthritic-Intact-2 cartilage samples. For a selected list of differentially expressed microRNAs, results were verified in additional cartilage samples using qPCR. Of the validated DE microRNAs, four-miR-107, miR-143-3p, miR-361-5p and miR-379-5p-were selected for further experiments in human primary chondrocytes treated with IL-1β. Expression of these microRNAs decreased in human primary chondrocytes treated with IL-1β. For miR-107 and miR-143-3p, gain- and loss-of-function approaches were undertaken and associated target genes and molecular pathways were investigated using qPCR and mass spectrometry proteomics. Analyses showed that WNT4 and IHH, predicted targets of miR-107, had increased expression in osteoarthritic cartilage compared to young intact cartilage and in primary chondrocytes treated with miR-107 inhibitor, and decreased expression in primary chondrocytes treated with miR-107 mimic, suggesting a role of miR-107 in chondrocyte survival and proliferation. In addition, we identified an association between miR-143-3p and EIF2 signalling and cell survival. Our work supports the role of miR-107 and miR-143-3p in important chondrocyte mechanisms regulating proliferation, hypertrophy and protein translation.

Eukaryotic ribosomes are complex molecular nanomachines translating genetic information from mRNAs into proteins. There is natural heterogeneity in ribosome composition. The pseudouridylation (ψ) of ribosomal RNAs (rRNAs) is one of the key sources of ribosome heterogeneity. Nevertheless, the functional consequences of ψ-based ribosome heterogeneity and its relevance for human disease are yet to be understood. Using HydraPsiSeq and a chronic disease model of non-osteoarthritic primary human articular chondrocytes exposed to osteoarthritic synovial fluid, we demonstrated that the disease microenvironment is capable of instigating site-specific changes in rRNA ψ profiles. To investigate one of the identified differential rRNA ψ sites (28S-ψ4966), we generated SNORA22 and SNORA33 KO SW1353 cell pools using LentiCRISPRv2/Cas9 and evaluated the ribosome translational capacity by 35S-Met/Cys incorporation, assessed the mode of translation initiation and ribosomal fidelity using dual luciferase reporters, and assessed cellular and ribosomal proteomes by LC-MS/MS. We uncovered that the depletion of SNORA33, but not SNORA22, reduced 28S-ψ4966 levels. The resulting loss of 28S-ψ4966 affected ribosomal protein composition and function and led to specific changes in the cellular proteome. Overall, our pioneering findings demonstrate that cells dynamically respond to disease-relevant changes in their environment by altering their rRNA pseudouridylation profiles, with consequences for ribosome function and the cellular proteome relevant to human disease.

Introduction In addition to the well-known cartilage extracellular matrix-related expression of Sox9, we demonstrated that chondrogenic differentiation of progenitor cells is driven by a sharply defined bi-phasic expression of Sox9: an immediate early and a late (extracellular matrix associated) phase expression. In this study, we aimed to determine what biological processes are driven by Sox9 during this early phase of chondrogenic differentiation. Materials Sox9 expression in ATDC5 cells was knocked down by siRNA transfection at the day before chondrogenic differentiation or at day 6 of differentiation. Samples were harvested at 2 h and 7 days of differentiation. The transcriptomes (RNA-seq approach) and proteomes (Label-free proteomics approach) were compared using pathway and network analyses. Total protein translational capacity was evaluated with the SuNSET assay, active ribosomes were evaluated with polysome profiling, and ribosome modus was evaluated with bicistronic reporter assays. Results Early Sox9 knockdown severely inhibited chondrogenic differentiation weeks later. Sox9 expression during the immediate early phase of ATDC5 chondrogenic differentiation regulated the expression of ribosome biogenesis factors and ribosomal protein subunits. This was accompanied by decreased translational capacity following Sox9 knockdown, and this correlated to lower amounts of active mono- and polysomes. Moreover, cap- versus IRES-mediated translation was altered by Sox9 knockdown. Sox9 overexpression was able to induce reciprocal effects to the Sox9 knockdown. Conclusion Here, we identified an essential new function for Sox9 during early chondrogenic differentiation. A role for Sox9 in regulation of ribosome amount, activity, and/or composition may be crucial in preparation for the demanding proliferative phase and subsequent cartilage extracellular matrix production of chondroprogenitors in the growth plate in vivo.

BMP7 is a morphogen capable of counteracting the OA chondrocyte hypertrophic phenotype via NKX3-2. NKX3-2 represses expression of RUNX2, an important transcription factor for chondrocyte hypertrophy. Since RUNX2 has previously been described as an inhibitor for 47S pre-rRNA transcription, we hypothesized that BMP7 positively influences 47S pre-rRNA transcription through NKX3-2, resulting in increased protein translational capacity. Therefor SW1353 cells and human primary chondrocytes were exposed to BMP7 and rRNA (18S, 5.8S, 28S) expression was determined by RT-qPCR. NKX3-2 knockdown was achieved via transfection of a NKX3-2-specific siRNA duplex. Translational capacity was assessed by the SUNsET assay, and 47S pre-rRNA transcription was determined by transfection of a 47S gene promoter-reporter plasmid. BMP7 treatment increased protein translational capacity. This was associated by increased 18S and 5.8S rRNA and NKX3-2 mRNA expression, as well as increased 47S gene promotor activity. Knockdown of NKX3-2 led to increased expression of RUNX2, accompanied by decreased 47S gene promotor activity and rRNA expression, an effect BMP7 was unable to restore. Our data demonstrate that BMP7 positively influences protein translation capacity of SW1353 cells and chondrocytes. This is likely caused by an NKX3-2-dependent activation of 47S gene promotor activity. This finding connects morphogen-mediated changes in cellular differentiation to an aspect of ribosome biogenesis via key transcription factors central to determining the chondrocyte phenotype.

Treatment of osteoarthritis (OA) is mainly symptomatic by alleviating pain to postpone total joint replacement. Bone morphogenetic protein 7 (BMP7) is a candidate morphogen for experimental OA treatment that favorably alters the chondrocyte and cartilage phenotype. Intra-articular delivery and sustained release of a recombinant growth factor for treating OA are challenging, whereas the use of peptide technology potentially circumvents many of these challenges. In this study, we screened a high-resolution BMP7 peptide library and discovered several overlapping peptide sequences from two regions in BMP7 with nanomolar bioactivity that attenuated the pathological OA chondrocyte phenotype. A single exposure of OA chondrocytes to peptides p[63-82] and p[113-132] ameliorated the OA chondrocyte phenotype for up to 8 days, and peptides were bioactive on chondrocytes in OA synovial fluid. Peptides p[63-82] and p[113-132] required NKX3-2 for their bioactivity on chondrocytes and provoke changes in SMAD signaling activity. The bioactivity of p[63-82] depended on specific evolutionary conserved sequence elements common to BMP family members. Intra-articular injection of a rat medial meniscal tear (MMT) model with peptide p[63-82] attenuated cartilage degeneration. Together, this study identified two regions in BMP7 from which bioactive peptides are able to attenuate the OA chondrocyte phenotype. These BMP7-derived peptides provide potential novel disease-modifying treatment options for OA.