Surviving Endoplasmic Reticulum Stress Is Coupled to Altered Chondrocyte Differentiation and Function

Kwok Yeung Tsang,Wilson C W Chan,John F Bateman,Deborah Cheslett,Ian G Melhado,Tori W Y Chan,Danny Chan,Kin Ming Kwan,Kenneth M C Cheung,Yoshihiko Yamada,Kathryn S E Cheah,Ernst B Hunziker,Chi Leong So,Marianne Bronner-Fraser

doi:10.1371/journal.pbio.0050044

Abstract

In protein folding and secretion disorders, activation of endoplasmic reticulum (ER) stress signaling (ERSS) protects cells, alleviating stress that would otherwise trigger apoptosis. Whether the stress-surviving cells resume normal function is not known. We studied the in vivo impact of ER stress in terminally differentiating hypertrophic chondrocytes (HCs) during endochondral bone formation. In transgenic mice expressing mutant collagen X as a consequence of a 13-base pair deletion in Col10a1 (13del), misfolded α1(X) chains accumulate in HCs and elicit ERSS. Histological and gene expression analyses showed that these chondrocytes survived ER stress, but terminal differentiation is interrupted, and endochondral bone formation is delayed, producing a chondrodysplasia phenotype. This altered differentiation involves cell-cycle re-entry, the re-expression of genes characteristic of a prehypertrophic-like state, and is cell-autonomous. Concomitantly, expression of Col10a1 and 13del mRNAs are reduced, and ER stress is alleviated. ERSS, abnormal chondrocyte differentiation, and altered growth plate architecture also occur in mice expressing mutant collagen II and aggrecan. Alteration of the differentiation program in chondrocytes expressing unfolded or misfolded proteins may be part of an adaptive response that facilitates survival and recovery from the ensuing ER stress. However, the altered differentiation disrupts the highly coordinated events of endochondral ossification culminating in chondrodysplasia.

Highlights

Development and growth require the ability to detect, respond to, and survive stresses that compromise the normal state
Chondrodysplasias caused by mutations that affect protein assembly and secretion are characterized by a disorganization of bony growth plates and distension of the endoplasmic reticulum (ER)
By investigating the impact of ER stress on the cell fate of hypertrophic chondrocytes (HCs) in transgenic mice expressing mutations in collagen that prevent proper folding, we revealed a novel adaptive mechanism that helps alleviate the unfolded protein load

Summary

Introduction

Development and growth require the ability to detect, respond to, and survive stresses that compromise the normal state. To cope with ER stress, ER-resident sensors detect misfolded or unfolded proteins and elicit the ER stress signaling (ERSS), which includes the induction of the highly conserved ‘‘unfolded protein response’’ (UPR). ERSS involves the activation of at least three independent ER stress sensors: inositol-requiring 1 (IRE1), PKR-like ER kinase (PERK), and membrane-tethered activating transcription factor 6 (ATF6) [1]. Their activation represses protein synthesis via phosphorylation of the translation initiation factor eIF2a and activates signaling pathways that up-regulate the expression of ER-resident molecular chaperones and translation regulators. Activation of IRE1, PERK, and ATF6 depends on their dissociation from the molecular chaperone, binding Ig protein (BiP), a master regulator of ERSS. Induction of ERSS means that the amount of new protein translocated into the ER

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