The pathogenic consequences of chondrocyte endoplasmic reticulum stress

Ray Boot-Handford

Several chondrodysplasias are caused by mutations in cartilage extracellular matrix (ECM) proteins. The mechanisms by which these mutations cause the associated pathology was until recently thought to involve either a deficiency in the extracellular matrix if the mutant protein was retained intracellularly, or alternatively, due to the assembly of a defect cartilage if secreted. Work from several laboratories has now demonstrated that the synthesis of mutant cartilage proteins is, in many instances, associated with increased endoplasmic reticulum (ER) stress and an unfolded protein response (UPR). We have recently demonstrated in vivo using mouse models that the targeted induction of ER stress in hypertrophic chondrocytes is sufficient to phenocopy metaphyseal chondrodysplasia type Schmid (MCDS) demonstrating the central role played by increased ER stress in the disease mechanism for this chondrodysplasia (Rajpar et al. 2009). COMP and matrilin-3 mutations causing pseudoachondroplasia (PSACH) / multiple epiphyseal dysplasia (MED) are often associated with increased ER stress and a UPR in proliferative zone chondrocytes (Nundall et al. 2010 and papers therein). In order to determine the effects of increased ER stress in proliferative zone chondrocytes, we targeted the expression of an ER stress-inducing protein (Tgcog) to these cells using the collagen II promoter. Proliferative zone chondrocytes of mice carrying the Col2Tgcog transgenic construct expressed and retained the Tgcog protein and exhibited increased ER stress and a UPR based on elevated levels of the ER chaperone BiP (Grp78). Mice expressing the ER stress-inducing transgene exhibited significant decreases in bone growth rates compared to their non-stressed controls. The decreases in bone growth rate related to decreased rates of chondrocyte proliferation as assessed by BrdU incorporation. The Col2Tgcog mouse did not exhibit any abnormalities in apoptosis within the growth plate or in growth plate organisation. Microarray analyses are being performed to determine the effects of increased ER stress upon chondrocyte gene expression. These investigations illustrate that increased levels of ER stress in proliferative zone chondrocytes are sufficient to reduce growth bone growth rates but do not induce all of the phenotypic changes seen in the growth plate of MED mice expressing a matrilin-3 mutation such as dysregulated apoptosis and changes in cell shape accompanying defects in column formation. Initial attempts to pharmacologically reduce ER stress caused by the collagen X and matrillin-3 mutations in cellulo will also be described.

Rajpar et al. 2009, PLoS Genet 5(10): e1000691

Nundall et al. 2010, Cell Stress Chaperones15(6):835-49