title: XBP1-Independent UPR Pathways Suppress C/EBP-β Mediated Chondrocyte Differentiation in ER-Stress Related Skeletal Disease.
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-β, 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-β transcriptional co-factors, GADD45-β and RUNX2. Thus we propose that the pathology of MCDS is underpinned by XBP1 independent UPR-induced dysregulation of C/EBP-β-mediated chondrocyte differentiation. Our data suggest that modulation of C/EBP-β activity in MCDS chondrocytes may offer therapeutic opportunities.
authors.affiliation: Murdoch Childrens Research Institute, Parkville, Victoria, Australia.
authors.forename: Trevor L
authors.initials: TL
authors.lastname: Cameron
authors.affiliation: Murdoch Childrens Research Institute, Parkville, Victoria, Australia.
authors.forename: Katrina M
authors.initials: KM
authors.lastname: Bell
authors.affiliation: Murdoch Childrens Research Institute, Parkville, Victoria, Australia.
authors.forename: Irma L
authors.initials: IL
authors.lastname: Gresshoff
authors.affiliation: Murdoch Childrens Research Institute, Parkville, Victoria, Australia.
authors.forename: Lisa
authors.initials: L
authors.lastname: Sampurno
authors.affiliation: Wellcome Trust Centre for Cell-Matrix Research, Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom.
authors.forename: Lorna
authors.initials: L
authors.lastname: Mullan
authors.affiliation: Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, Massachusetts, United States of America.
authors.forename: Joerg
authors.initials: J
authors.lastname: Ermann
authors.affiliation: Weill Cornell Medical College, Cornell University, New York, New York, United States of America.
authors.forename: Laurie H
authors.initials: LH
authors.lastname: Glimcher
authors.affiliation: Wellcome Trust Centre for Cell-Matrix Research, Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom.
authors.forename: Raymond P
authors.initials: RP
authors.lastname: Boot-Handford
authors.affiliation: Murdoch Childrens Research Institute, Parkville, Victoria, Australia; Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: John F
authors.initials: JF
authors.lastname: Bateman
mesh_terms: D000818:Animals; D001847:Bone Diseases / Q000473:pathology*; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000037:antagonists & inhibitors* / Q000502:physiology; D002454:Cell Differentiation / Q000502:physiology*; D019902:Chondrocytes / Q000473:pathology*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D059865:Endoplasmic Reticulum Stress / Q000502:physiology*; D020869:Gene Expression Profiling; D051379:Mice; D008822:Mice, Transgenic; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D056811:Unfolded Protein Response / Q000502:physiology*; D000071717:X-Box Binding Protein 1
chemical_list: D022782:CCAAT-Enhancer-Binding Protein-beta; D004268:DNA-Binding Proteins; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors; D000071717:X-Box Binding Protein 1; C000605544:Xbp1 protein, mouse