Cartilage comprises a single cell type, the chondrocyte, embedded in a highly complex extracellular matrix. Disruption to the cartilage growth plate leads to reduced bone growth and results in a clinically diverse group of conditions known as genetic skeletal diseases (GSDs). Similarly, long-term degradation of articular cartilage can lead to osteoarthritis (OA), a disease characterised by joint pain and stiffness. As professionally secreting cells, chondrocytes are particularly susceptible to endoplasmic reticulum (ER) stress and this has been identified as a core disease mechanism in a group of clinically and pathologically related GSDs. If unresolved, ER stress can lead to chondrocyte cell death. Recent interest has focused on ER stress as a druggable target for GSDs and this has led to the first clinical trial for a GSD by repurposing an antiepileptic drug. Interestingly, ER stress markers have also been associated with OA in multiple cell and animal models and there is increasing interest in it as a possible therapeutic target for treatment. In summary, chondrocyte ER stress has been identified as a core disease mechanism in GSDs and as a contributory factor in OA. Thus, chondrocyte ER stress is a unifying factor for both common and rare cartilage-related diseases and holds promise as a novel therapeutic target.

COMP (cartilage oligomeric matrix protein) is a member of the thrombospondin family and forms homopentamers as well as mixed heterooligomers with its closely related family member TSP-4. COMP is long known to bind to collagens and to influence collagen fibril formation. Recent work indicates that already intracellular interaction with collagen is important for collagen secretion. However, the exact binding site of COMP on the collagen triple helix has not been described up to now. In this study we have identified a GXKGHR motif on the collagen II helix to bind to COMP, using a recombinantly expressed collagen II peptide library. This binding sequence is conserved throughout evolution and we demonstrate that TSP-4 binds to the same sequence. The identified binding motif overlaps with the recognition sites of many other collagen-binding partners (e.g. PEDF, Heparin) and also spans the lysine residues, which form collagen cross-links. COMP might thereby protect collagen helices from premature modification and cross-linking. Interestingly, this motif is only found in classical fibrillar collagens, although COMP is known to also bind other types. This might indicate that COMP has a unique interface for fibrillar collagens, thus making it an interesting target for the development of antifibrotic drugs.

The intracellular retention of misfolded extracellular matrix proteins is a common disease mechanism in various rare skeletal diseases. This discovery has driven the study of ER stress and the unfolded protein response (UPR) as a promising therapeutic target in several skeletal dysplasias. In the case of COL10A1 mutations, targeting the UPR resulted in a clinical trial of the repurposed drug carbamazepine; however, for other closely related skeletal disorders, treatment with carbamazepine was ineffective, indicating the need for suitable markers for in vitro screenings of potential drug treatments. Mutations in cartilage oligomeric matrix protein (COMP), a cartilage structural protein, cause both multiple epiphyseal dysplasia (MED) and pseudoachondroplasia (PSACH); together referred to as the COMPopathies, which result from the intracellular retention of mutant COMP to varying degrees. In contrast to other closely related skeletal disorders, caused by mutations in cartilage structural proteins, the involvement of the UPR is less clear, and so far, no common COMPopathy marker has been identified. Here, using cell models of COMPopathies, we identified MMP9 upregulation as a common feature of six pathogenic COMP variants that do not induce a prominent UPR. We further show that the archetypal p.V194D matrilin-3 MED variant (which causes MED) does not induce MMP9 expression, suggesting that MMP9 upregulation could serve as a specific marker of COMPopathies in vitro.

The intracellular retention of misfolded extracellular matrix proteins is a common disease mechanism in various individually rare skeletal diseases. This discovery has driven the study of ER-stress and the unfolded protein response (UPR) as a promising therapeutic target in several skeletal dysplasias. In the case of COL10A1 mutations, targeting the UPR resulted in a clinical trial of the repurposed drug carbamazepine; however, for other closely related skeletal disorders treatment with carbamazepine was ineffective, indicating the need for suitable markers for in vitro screenings of potential drug treatments. Mutations in cartilage oligomeric matrix protein (COMP), a cartilage structural protein, causes both multiple epiphyseal dysplasia (MED) and pseudoachondroplasia (PSACH); together referred to as COMPopathies, and result in intracellular retention of mutant COMP protein to varying degrees. In contrast to other closely related skeletal disorders, caused by mutations in cartilage structural proteins, the involvement of the UPR is less clear, and so far, no common COMPopathy marker has been identified. Here, using cell models of COMPopathies we identified MMP9 upregulation as a common feature of six pathogenic COMP variants that do not induce a prominent UPR. We further show that the archetypal p.V194D matrilin-3 MED variant (which causes MED), does not induce MMP9 expression, suggesting that MMP9 upregulation could serve as a specific marker of COMPopathies in vitro.

Background: Endochondral instead of chondral differentiation hinders mesenchymal stromal cell (MSC) application for clinical cartilage regeneration. We previously showed that heparin–polyethylene glycol (PEG) hydrogels loaded with transforming growth factor TGF-β instructed stable chondral MSC development in vivo. Here, we assessed this approach in vitro, utilizing heparin–PEG hydrogels or the pellet culture system with soluble heparin supplementation of chondrogenic medium. Methods: Human MSCs were cultured in heparin–PEG hydrogels (22.4 mg/mL crosslinked heparin, 120 ng TGF-β1) or as pellet cultures treated with soluble heparin (0, 10, 100, 700 μg/mL) in TGF-β1-containing (10 ng/mL) chondrogenic medium. Chondral and endochondral signaling (1–3 h, 4 weeks) and cartilage matrix formation (4 weeks) were analyzed using western blot, histology, quantitative polymerase chain reaction (qPCR), enzyme-linked immunosorbent assay (ELISA), and enzyme activity. Results: Unlike in vivo, human MSCs differentiated in heparin–PEG hydrogels into type X collagen and alkaline phosphatase-positive hypertrophic chondrocytes in vitro. Interestingly, treating MSC pellets with soluble heparin (10–700 µg/mL) revealed reduced TGF-β-SMAD3 but not SMAD2 activation at unaffected type II collagen and proteoglycan/DNA levels. We propose that the stimulation of the insulin-AKT pathway by heparin aided in maintaining SMAD2 activation, which apparently plays a more prominent role than SMAD3 for MSC chondrogenesis. Heparin treatment inhibited the pro-hypertrophic WNT/β-catenin pathway in vitro but insufficiently silenced TGF-β-SMAD1/5/9 activation and unfortunately reduced antihypertrophic prostaglandin PGE2 levels. Ultimately, treatment with 10 µg/mL heparin reduced the upregulation of several hypertrophy markers ( MEF2C , IHH , IBSP messenger RNAs [mRNAs], alkaline phosphatase activity) below control levels, but type X collagen remained unresponsive. Thus, soluble heparin treatment was similarly selective and effective as previous antihypertrophic interventions (PTHrP pulses, WNT inhibition), while offering technical simplicity, reduced cost, and solvent-free formulation. Conclusions: Taken together, heparin-TGF-β showed a novel dichotomous SMAD2/3 inhibition at maintained chondrogenic differentiation and context-dependent lineage-instructive properties—permitting endochondral commitment in vitro but chondral development in vivo. Thus, environmental contributions are mandatory to allow heparin–PEG-guided chondral versus endochondral lineage commitment of MSCs in vivo, potentially involving SMAD1/5/9 suppressors and PGE2 sources. Graphical