ABSTRACT
Schmid metaphyseal chondrodysplasia (MCDS) involves dwarfism and growth plate cartilage hypertrophic zone expansion resulting from dominant mutations in the hypertrophic zone collagen, Col10a1. Mouse models phenocopying MCDS through the expression of an exogenous misfolding protein in the endoplasmic reticulum (ER) in hypertrophic chondrocytes have demonstrated the central importance of ER stress in the pathology of MCDS. The resultant unfolded protein response (UPR) in affected chondrocytes involved activation of canonical ER stress sensors, IRE1, ATF6, and PERK with the downstream effect of disrupted chondrocyte differentiation. Here, we investigated the role of the highly conserved IRE1/XBP1 pathway in the pathology of MCDS. Mice with a MCDS collagen X p.N617K knock-in mutation (ColXN617K) were crossed with mice in which Xbp1 was inactivated specifically in cartilage (Xbp1CartΔEx2), generating the compound mutant, C/X. The severity of dwarfism and hypertrophic zone expansion in C/X did not differ significantly from ColXN617K, revealing surprising redundancy for the IRE1/XBP1 UPR pathway in the pathology of MCDS. Transcriptomic analyses of hypertrophic zone cartilage identified differentially expressed gene cohorts in MCDS that are pathologically relevant (XBP1-independent) or pathologically redundant (XBP1-dependent). XBP1-independent gene expression changes included large-scale transcriptional attenuation of genes encoding secreted proteins and disrupted differentiation from proliferative to hypertrophic chondrocytes. Moreover, these changes were consistent with disruption of C/EBP-ß, a master regulator of chondrocyte differentiation, by CHOP, a transcription factor downstream of PERK that inhibits C/EBP proteins, and down-regulation of C/EBP-ß transcriptional co-factors, GADD45-ß and RUNX2. Thus we propose that the pathology of MCDS is underpinned by XBP1 independent UPR-induced dysregulation of C/EBP-ß-mediated chondrocyte differentiation. Our data suggest that modulation of C/EBP-ß activity in MCDS chondrocytes may offer therapeutic opportunities.
Subject(s)
Bone Diseases/pathology , CCAAT-Enhancer-Binding Protein-beta/antagonists & inhibitors , Cell Differentiation/physiology , Chondrocytes/pathology , DNA-Binding Proteins/physiology , Endoplasmic Reticulum Stress/physiology , Transcription Factors/physiology , Unfolded Protein Response/physiology , Animals , CCAAT-Enhancer-Binding Protein-beta/physiology , DNA-Binding Proteins/genetics , Gene Expression Profiling , Mice , Mice, Transgenic , Regulatory Factor X Transcription Factors , Transcription Factors/genetics , X-Box Binding Protein 1ABSTRACT
Familial digital arthropathy-brachydactyly (FDAB) is a dominantly inherited condition that is characterized by aggressive osteoarthropathy of the fingers and toes and consequent shortening of the middle and distal phalanges. Here we show in three unrelated families that FDAB is caused by mutations encoding p.Gly270Val, p.Arg271Pro and p.Phe273Leu substitutions in the intracellular ankyrin-repeat domain of the cation channel TRPV4. Functional testing of mutant TRPV4 in HEK-293 cells showed that the mutant proteins have poor cell-surface localization. Calcium influx in response to the synthetic TRPV4 agonists GSK1016790A and 4αPDD was significantly reduced, and mutant channels did not respond to hypotonic stress. Others have shown that gain-of-function TRPV4 mutations cause skeletal dysplasias and peripheral neuropathies. Our data indicate that TRPV4 mutations that reduce channel activity cause a third phenotype, inherited osteoarthropathy, and show the importance of TRPV4 activity in articular cartilage homeostasis. Our data raise the possibility that TRPV4 may also have a role in age- or injury-related osteoarthritis.
