Armet/Manf and Creld2 are components of a specialized ER stress response provoked by inappropriate formation of disulphide bonds: implications for genetic skeletal diseases.

Mutant matrilin-3 (V194D) forms non-native disulphide bonded aggregates in the rER of chondrocytes from cell and mouse models of multiple epiphyseal dysplasia (MED). Intracellular retention of mutant matrilin-3 causes endoplasmic reticulum (ER) stress and induces an unfolded protein response (UPR) including the upregulation of two genes recently implicated in ER stress: Armet and Creld2. Nothing is known about the role of Armet and Creld2 in human genetic diseases. In this study, we used a variety of cell and mouse models of chondrodysplasia to determine the genotype-specific expression profiles of Armet and Creld2. We also studied their interactions with various mutant proteins and investigated their potential roles as protein disulphide isomerases (PDIs). Armet and Creld2 were up-regulated in cell and/or mouse models of chondrodysplasias caused by mutations in Matn3 and Col10a1, but not Comp. Intriguingly, both Armet and Creld2 were also secreted into the ECM of these disease models following ER stress. Armet and Creld2 interacted with mutant matrilin-3, but not with COMP, thereby validating the genotype-specific expression. Substrate-trapping experiments confirmed Creld2 processed PDI-like activity, thus identifying a putative functional role. Finally, alanine substitution of the two terminal cysteine residues from the A-domain of V194D matrilin-3 prevented aggregation, promoted mutant protein secretion and reduced the levels of Armet and Creld2 in a cell culture model. We demonstrate that Armet and Creld2 are genotype-specific ER stress response proteins with substrate specificities, and that aggregation of mutant matrilin-3 is a key disease trigger in MED that could be exploited as a potential therapeutic target.