Targeted Induction of Endoplasmic Reticulum Stress Induces Cartilage Pathology

Abstract

Pathologies caused by mutations in extracellular matrix proteins are generally considered to result from the synthesis of extracellular matrices that are defective. Mutations in type X collagen cause metaphyseal chondrodysplasia type Schmid (MCDS), a disorder characterised by dwarfism and an expanded growth plate hypertrophic zone. We generated a knock-in mouse model of an MCDS–causing mutation (COL10A1 p.Asn617Lys) to investigate pathogenic mechanisms linking genotype and phenotype. Mice expressing the collagen X mutation had shortened limbs and an expanded hypertrophic zone. Chondrocytes in the hypertrophic zone exhibited endoplasmic reticulum (ER) stress and a robust unfolded protein response (UPR) due to intracellular retention of mutant protein. Hypertrophic chondrocyte differentiation and osteoclast recruitment were significantly reduced indicating that the hypertrophic zone was expanded due to a decreased rate of VEGF–mediated vascular invasion of the growth plate. To test directly the role of ER stress and UPR in generating the MCDS phenotype, we produced transgenic mouse lines that used the collagen X promoter to drive expression of an ER stress–inducing protein (the cog mutant of thyroglobulin) in hypertrophic chondrocytes. The hypertrophic chondrocytes in this mouse exhibited ER stress with a characteristic UPR response. In addition, the hypertrophic zone was expanded, gene expression patterns were disrupted, osteoclast recruitment to the vascular invasion front was reduced, and long bone growth decreased. Our data demonstrate that triggering ER stress per se in hypertrophic chondrocytes is sufficient to induce the essential features of the cartilage pathology associated with MCDS and confirm that ER stress is a central pathogenic factor in the disease mechanism. These findings support the contention that ER stress may play a direct role in the pathogenesis of many connective tissue disorders associated with the expression of mutant extracellular matrix proteins.

ER Stress During the Pubertal Growth Spurt Results in Impaired Long-Bone Growth in Chondrocyte-Specific ERp57 Knockout Mice

“…Examples are transgenic mouse lines expressing a mutant and poorly secreted form of collagen X (COL10A1 p.Asn617Lys) or animals expressing an exogenous, ER stress-inducing protein (cog mutant of thyroglobulin [Tg cog ]) in hypertrophic chondrocytes. (29) Independent of the mechanisms of ER stress generation in these different mouse strains, a shortening of long bones with expanded hypertrophic zones ensues. Moreover, the osteoclast number is reduced, indicating that the hypertrophic zone is expanded due to a delayed remodeling of cartilage tissue in the vascularization zone.…”

“…The fact that the expression of Tg cog alone is sufficient to induce MCDS-related changes indicates that ER stress itself is a pathogenic mechanism in MCDS which directly induces the observed skeletal changes. (29) Likewise, the collagen Xmutation p.Asn617Lys, corrupting secretion of the mutated protein, also entails ER stress, and the clinical phenotype of the patients harboring this mutation shares critical features with MCDS. In addition, other skeletal abnormalities, such as pseudoachondrodysplasia (PSACH) or multiple epiphyseal dysplasia (MED) are caused by mutations in cartilage ECM proteins like collagens II, IX, X, and XI, or aggrecan, COMP, and matrilin-3.…”

“…In addition, other skeletal abnormalities, such as pseudoachondrodysplasia (PSACH) or multiple epiphyseal dysplasia (MED) are caused by mutations in cartilage ECM proteins like collagens II, IX, X, and XI, or aggrecan, COMP, and matrilin-3. (26)(27)(28)(29)34,(37)(38)(39)(40)(41)(42)(43) ER stress is thought to constitute the underlying common disease mechanism due to retention of abnormal, improperly folded proteins within the ER. The cells can be rescued after the ER stress-induced unfolded protein response, which arrests the cell cycle, reduces the general protein synthesis, and enhances the production of new chaperones facilitating additional folding processes.…”

Abnormal bone quality in cartilage oligomeric matrix protein and matrilin 3 double‐deficient mice caused by increased tissue inhibitor of metalloproteinases 3 deposition and delayed aggrecan degradation

“…Alterations in the number or distribution of osteoclasts have been shown to lead to abnormalities in cartilage degradation (34). However, using TRAP staining, we found no differences in the number or distribution of osteoclasts in mutant animals as compared to controls ( Figure 3A).…”

Changes in the Chondrocyte and Extracellular Matrix Proteome during Post-natal Mouse Cartilage Development

“…Accumulation of these complexes in dilated ER cisternae is a hallmark of multiple epiphyseal dysplasia and pseudoachondroplasia (60 -62), and the resulting ER stress affects normal chondrocyte differentiation and organization in the growth plate (63)(64)(65). ER stress induced by misfolding of collagen X in hypertrophic chondrocytes also contributes to the pathology of Schmid metaphyseal chondrodysplasia, type Schmid (66,67), but the role of specific ER chaperones is not known. Although ERp72 was significantly reduced at P21, the master regulator of the unfolded protein response BiP (Hspa5) was detected at approximately equal levels in P3 and P21 cartilage.…”