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J. Biol. Chem., Vol. 280, Issue 16, 15544-15552, April 22, 2005

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Misfolding of Collagen X Chains Harboring Schmid Metaphyseal Chondrodysplasia Mutations Results in Aberrant Disulfide Bond Formation, Intracellular Retention, and Activation of the Unfolded Protein Response^*

Richard Wilson, Susanna Freddi, Danny Chan, Kathryn S. E. Cheah, and John F. Bateman¶

From the Cell and Matrix Biology Research Unit, Murdoch Children's Research Institute, and the Department of Pediatrics, University of Melbourne, Royal Children's Hospital, Parkville, Victoria 3052, Australia and the Department of Biochemistry, University of Hong Kong, Faculty of Medicine, Hong Kong, China

Collagen X is a short chain collagen expressed specifically by the hypertrophic chondrocytes of the cartilage growth plate during endochondral bone formation. Accordingly, COL10A1 mutations disrupt growth plate function and cause Schmid metaphyseal chondrodysplasia (SMCD). SMCD mutations are almost exclusively located in the NC1 domain, which is crucial for both trimer formation and extracellular assembly. Several mutations are expected to reduce the level of functional collagen X due to NC1 domain misfolding or exclusion from stable trimer formation. However, other mutations may be tolerated within the structure of the assembled NC1 trimer, allowing mutant chains to exert a dominant-negative impact within the extracellular matrix. To address this, we engineered SMCD mutations that are predicted either to prohibit subunit folding and assembly (NC1del10 and Y598D, respectively) or to allow trimerization (N617K and G618V) and transfected these constructs into 293-EBNA and SaOS-2 cells. Although expected to form stable trimers, G618V and N617K chains (like Y598D and NC1del10 chains) were secreted very poorly compared with wild-type collagen X. Interestingly, all mutations resulted in formation of an unusual SDS-stable dimer, which dissociated upon reduction. As the NC1 domain sulfhydryl group is not solvent-exposed in the correctly folded NC1 monomer, disulfide bond formation would result only from a dramatic conformational change. In cells expressing mutant collagen X, we detected significantly increased amounts of the spliced form of X-box DNA-binding protein mRNA and up-regulation of BiP, two key markers for the unfolded protein response. Our data provide the first clear evidence for misfolding of SMCD collagen X mutants, and we propose that solvent exposure of the NC1 thiol may trigger the recognition and degradation of mutant collagen X chains.

Received for publication, September 20, 2004 , and in revised form, January 20, 2005.

^* This work was supported by Grant 114118 from the National Health and Medical Research Council of Australia. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed. Tel.: 61-3-9345-6367; Fax: 61-3-9345-7997; E-mail: john.bateman{at}mcri.edu.au.

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