Unique Matrix Structure in the Rough Endoplasmic Reticulum Cisternae of Pseudoachondroplasia Chondrocytes

Abstract

Mutations in cartilage oligomeric matrix protein (COMP) cause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED/EDM1). Because COMP exists as a homopentamer, only one mutant COMP subunit may result in an abnormal complex that is accumulated in expanded rough endoplasmic reticulum (rER) cisternae, a hallmark of PSACH. Type IX collagen and matrilin-3 (MATN3), also accumulate in the rER cisternae of PSACH chondrocytes, but it is unknown how mutant COMP interacts with these proteins. The studies herein focus on defining the organization of these intracellularly retained proteins using fluorescence deconvolution microscopy. A unique matrix organization was identified in which type II procollagen formed a central core surrounded by a protein network of mutant COMP, type IX collagen, and MATN3. This pattern of matrix organization was found in multiple cisternae from single chondrocytes and in chondrocytes with different COMP mutations, indicating a common pattern of interaction. This suggests that stalling of mutant COMP and an interaction between mutant COMP and type II procollagen are initiating events in the assembly of matrix in the rER, possibly explaining why the material is not readily cleared from the rER. Altogether, these data suggest that mutant COMP initiates and perhaps catalyzes premature intracellular matrix assembly.

Skeletal Abnormalities in Mice Lacking Extracellular Matrix Proteins, Thrombospondin-1, Thrombospondin-3, Thrombospondin-5, and Type IX Collagen

“…[22][23][24][25][26][27][28] The presence of an ordered intracellular matrix network in pseudoachondroplasia chondrocytes and colocalization of these proteins in the normal ECM suggest that TSP5 has a significant role in the ECM. 20 These observations support a model in which TSP5, type IX collagen, and MATN3 interact and function to establish and maintain the homeostasis of the ECM. 29 Furthermore, TSP1 and TSP3 are present in the growth plate and may play similar and/or redundant roles with TSP5 in ECM.…”

“…15,18 -20 Recent studies show that these ECM proteins associate with TSP5 to form a structured intracellular matrix network. 20 Interestingly, mutations in MATN3 (MED/EDM5) and all three type IX collagen (Col9) genes also cause MED phenotypes (MED/EDM2, -3, and -6) that are typically milder than the MED condition caused by TSP5 mutations. 21 Different approaches have attempted to define the role of TSP5.…”

Ribozyme-mediated reduction of wild-type and mutant cartilage oligomeric matrix protein (COMP) mRNA and protein

“…The disease pathology of these conditions results from the massive retention of misfolded COMP and inappropriate formation of intracellular matrix in the rER (Merritt et al 2007). In contrast, COMP null mice are normal, suggesting that loss of COMP does not compromise normal growth, development, or longevity.…”

Antisense Reduction of Mutant COMP Reduces Growth Plate Chondrocyte Pathology

“…4,5 Functionally, COMP facilitates type II collagen fibril assembly, enhances chondrocyte attachment in vitro, and interacts with other ECM proteins, including type II and IX collagens, matrilin 3, and SPARC. [6][7][8][9][10][11][12] In contrast, mutations in COMP cause misfolding of the protein, preventing export from the chondrocyte and resulting in massive retention within the endoplasmic reticulum (ER). 10,[13][14][15] Although this finding has long been appreciated, there was little understanding of the pathologic molecular mechanisms, which are critical to develop mechanism-driven therapeutics.…”

“…[6][7][8][9][10][11][12] In contrast, mutations in COMP cause misfolding of the protein, preventing export from the chondrocyte and resulting in massive retention within the endoplasmic reticulum (ER). 10,[13][14][15] Although this finding has long been appreciated, there was little understanding of the pathologic molecular mechanisms, which are critical to develop mechanism-driven therapeutics. To circumvent this problem, we generated the mutant (MT)-COMP mouse with the common D469del PSACH mutation that expresses human MT-COMP in chondrocytes in the presence of the inducing agent doxycycline (DOX).…”