Cell-type specific trafficking of expressed mutant COMP in a cell culture model for PSACH Tung-Ling L. Chen a , Jeff W. Stevens b , William G. Cole c , Jacqueline T. Hecht d , Barbara M. Vertel a, * a Department of Cell Biology and Anatomy, Rosalind Franklin University of Medicine and Science, 3333 Green Bay Road, North Chicago, IL 60064, United States b Department of Orthopaedics and Rehabilitation, The University of Iowa, Iowa City, IA, United States c Department of Orthopaedics, Hospital for Sick Children, Toronto, Canada, United States d Department of Pediatrics, University of Texas Medical School at Houston, Houston, TX, United States Received 2 August 2004; received in revised form 23 September 2004; accepted 23 September 2004 Abstract Pseudoachondroplasia (PSACH) is an autosomal dominant disease that mainly affects cartilage, resulting in skeletal dysplasias and early onset osteoarthritis. PSACH is caused by mutations in the cartilage oligomeric matrix protein (COMP) gene. PSACH chondrocytes accumulate unique COMP-containing lamellar structures in an expanded rough endoplasmic reticulum (rER). Although COMP is also present in tendon extracellular matrix (ECM), it does not accumulate in PSACH tendon cells, suggesting the disease involves a chondrocyte- specific trafficking problem. To investigate putative cell-specific trafficking differences, we generated a cell culture model utilizing expression of the common DD469 COMP mutation. In rat chondrosarcoma (RCS) cells, we find delayed secretion and ER accumulation of DD469 COMP, paralleling the altered trafficking defect in PSACH chondrocytes. Non-chondrocytic COS-1 cells, in contrast, efficiently trafficked and secreted both mutant and wild-type COMP. In chondrocytic cells, expression of DD469 COMP led to ER accumulation of type IX collagen, but did not affect aggrecan trafficking. Endogenous rat COMP accumulated in the ER along with expressed DD469 COMP in a stably expressing RCS clone, consistent with the dominant negative effect of PSACH. When these stably expressing cells were cultured to promote ECM deposition, the small amount of secreted mutant COMP disrupted assembly of the normal fibrillar meshwork and caused irregular aggregates of COMP and type IX collagen to form. Thus, in a new model that reflects the cellular pathology of PSACH, we establish trafficking differences for mutant COMP in chondrocytic and non-chondrocytic cells and demonstrate that mutant COMP interferes with assembly of a normal ECM. D 2004 Elsevier B.V./International Society of Matrix Biology. All rights reserved. Keywords: Cartilage oligomeric matrix protein (COMP); Trafficking; Pseudoachondroplasia (PSACH); Rat chondrosarcoma cells (RCS); Cartilage; Endoplasmic reticulum (ER) 1. Introduction Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasias (MED) are autosomal dominant diseases that principally affect cartilage, and result in skeletal dysplasias that are associated with early onset osteoarthritis (reviewed in Unger and Hecht, 2001; Briggs and Chapman, 2002). An unusual, extremely distended rough endoplasmic reticulum (rER) containing lamellar material in patient chondrocytes is involved in the disease pathology (Maynard et al., 1972; Stanescu et al., 1984). Both PSACH and MED/EDMI are caused by mutations in cartilage oligomeric matrix protein (COMP) (Briggs et al., 1995; Hecht et al., 1995; Chapman et al., 2001). COMP is a pentameric member of the thrombospondin family present in the cartilage extracellular 0945-053X/$ - see front matter D 2004 Elsevier B.V./International Society of Matrix Biology. All rights reserved. doi:10.1016/j.matbio.2004.09.005 Abbreviations: ST, sialyltransferase; ECM, extracellular matrix. * Corresponding author. Tel.: +1 847 578 3443; fax: +1 847 578 3253. E-mail address: Barbara.Vertel@rosalindfranklin.edu (B.M. Vertel). Matrix Biology 23 (2004) 433 – 444 www.elsevier.com/locate/matbio