The unfolded protein response and its relevance to

Authors: Raymond P BootHandford Michael D Briggs

Publish Date: 2009/10/23

Volume: 339, Issue: 1, Pages: 197-

PDF Link

Abstract

The unfolded protein response UPR has evolved to counter the stresses that occur in the endoplasmic reticulum ER as a result of misfolded proteins This sophisticated quality control system attempts to restore homeostasis through the action of a number of different pathways that are coordinated in the first instance by the ER stresssenor proteins IRE1 ATF6 and PERK However prolonged ERstressrelated UPR can have detrimental effects on cell function and in the longer term may induce apoptosis Connective tissue cells such as fibroblasts osteoblasts and chondrocytes synthesise and secrete large quantities of proteins and mutations in many of these gene products give rise to heritable disorders of connective tissues Until recently these mutant gene products were thought to exert their effect through the assembly of a defective extracellular matrix that ultimately disrupted tissue structure and function However it is now becoming clear that ER stress and UPR because of the expression of a mutant gene product is not only a feature of but may be a key mediator in the initiation and progression of a whole range of different connective tissue diseases This review focuses on ER stress and the UPR that characterises an increasing number of connective tissue diseases and highlights novel therapeutic opportunities that may ariseThe endoplasmic reticulum ER is a distinct compartment of eukaryotic cells and plays major roles in the synthesis folding and trafficking of proteins entering the secretory pathway In addition the ER is involved in Ca2+ storage and controlling various aspects of lipid and sterol synthesis see Schröder 2008 The general area of molecular chaperones ER stress and the unfolded protein response UPR has been extensively reviewed recently Bukau et al 2006 Szegezdi et al 2006 Bernales et al 2006 Malhotra and Kaufman 2007 Ron and Walter 2007 Lin et al 2008 In this review we will therefore give a brief description of these areas and focus upon the impact that misfolding mutant forms of extracellular matrix ECM proteins have on ER homeostasis in human or mammalian cells and the possible downstream cellular consequences as they are related to disease pathogenesisProteins destined for the secretory pathway of the cell are synthesised in the cytoplasm on ribosomes bound to the ER forming the rough ER and cotranslationally translocated into the lumen of the ER via the Sec61 translocon Within the lumen of the ER newly synthesised proteins undergo a series of posttranslational modifications such as signal peptide cleavage asparagine Nlinked glycosylation disulphidebond formation and collagenspecific modifications such as hydroxylation of prolyl and lysyl residues and glycosylation of hydroxylysyl residues Bateman et al 2009 These posttranslational modifications occur as the protein is folding to attain its mature form and many of the modifications are a prerequisite for correct folding Folding is assisted by a host of chaperones cochaperones foldases and oxidoreductases Chaperones such as BiP/Grp78 and Grp98 prevent unfolded proteins from aggregating by means of their exposed hydrophobic sequences and provide an environment conducive to protein folding Enzymes such as protein disulphide isomerases PDI catalyse disulphidebond formation and exchange Many ancillary proteins are involved in securing and delivering the energy input required to drive the energyconsuming process of protein folding and in maintaining the correct redox state within the ER lumen to permit the reversible formation of disulphide bondsThe ER has a sophisticated quality control system for ensuring that misfolded proteins do not accumulate or pass further long the secretory pathway but are instead retained within the ER and targeted for degradation Anelli and Sitia 2008 For proteins that undergo Nlinked glycosylation the addition of a high mannose oligosaccharide from a dolicolpyrophosphate donor presages a series of trimming and reglucosylation reactions that govern the interaction of the folding glycoproteins with the lectin chaperones calnexin and calreticulin The extent of carbohydrate trimming serves as a molecular clock and misfolded or slowly folding proteins are targeted for degradation Molinari 2007 Nonglycosylated proteins that misfold or fold too slowly are recognised by their sequential interactions with chaperones such as BiP Grp98 and PDI and are targeted for degradation Schröder 2008 The degradation of misfolded ER proteins can take place by two mechanisms First the protein can be retrotranslocated back into the cytoplasm where it is ubiquinated and degraded by the proteasome in a process known as ERassociated degradation ERAD Brodsky 2007 For proteins that cannot be retrotranslocated perhaps because they have aggregated or assemble into multimers autophagy appears to be activated Portions of ER engorged with misfolded protein are budded off and become incorporated within a doublemembrane structure known as an autophagosome that subsequently fuses with a lysosome in which the contents are degraded Bernales et al 2007 Schröder 2008 Ishida et al 2009The unfolded protein response UPR Unfolded protein UP in the lumen of the endoplasmic reticulum ER is bound by the chaperone BiP BiP also binds the ER luminal domains of the three ER stress sensors pancreatic ER eukaryotic translation initiation factor eIF2a kinase PERK inositolrequiring enzyme1 IRE1 and activating transcription factor6 ATF6 If the concentration of unfolded protein in the ER lumen increases eg because of the expression of a mutant extracellular matrix gene BiP preferentially binds the unfolded protein thereby freeing the stress sensors PERK and IRE1 are activated by dimerisation and autophosphorylation PERK then phosphorylates eIF2α which suppresses general protein synthesis thus decreasing the entry of newly synthesised protein into the ER ATF4 mRNA is more efficiently translated when eIF2α is phosphorylated and ATF4 upregulates a set of ERstresssensitive genes including that for CCAAT/enhancerbinding protein homologous protein CHOP Activated IRE1 produces an unconventional splice in cytoplasmic Xbox binding protein 1 XBP1 mRNA and this spliced XBP1 upregulates genes encoding chaperones to increase the protein folding capacity of the ER and genes controlling endoplasmicreticulumassociated degradation ERAD a mechanism by which misfolded protein is retrotranslocated into the cytoplasm and proteasomally degraded When BiP releases ATF690 the protein is translocated to the Golgi where it is cleaved by site1 and site2 proteases S1P S2P releasing the cytoplasmic 50kDa domain ATF650 which is an active transcription factor upregulating genes involved in the UPR including chaperones XBP1 and CHOP rER rough endoplasmic reticulumThe IRE1 branch of UPR is evolutionarily the oldest found in lower eukaryotes such as yeast and throughout the animal kingdom whereas PERK and ATF6 are only found in higher animals such as nematodes insects and mammals Shen et al 2005 The 110kDa type 1 transmembrane IRE1 protein exists in two closely related forms IRE1α which is expressed ubiquitously and IRE1β whose expression is restricted to gut epithelium Bertolotti et al 2001 The IRE1 protein has a cytoplasmic domain that possesses serine/threonine kinase and endoribonuclease activities a transmembrane domain and an ER luminal domain In its inactive state the ER luminal domain of IRE1 is bound by the ER chaperone BiP Upon sensing an increase in the luminal concentration of unfolded protein Credle et al 2005 IRE1 dissociates from BiP and oligomerises in the plane of the ER membrane this leads to an autocatalytic phosphorylation of the cytoplasmic domain and activation of its ribonuclease activity The endoribonuclease domain catalyses an unconventional splice within Xbox binding protein 1 XBP1 mRNA which in mammalian cells produces an alternative carboxyl terminal domain in the translated protein termed XBP1s s for spliced which is a basic leucine zipper bZIP family transcription factor XBP1s promotes the transcription of a number of genes involved in the UPR including chaperones that increase the proteinfolding capacity of the ER and proteins involved in ERAD Todd et al 2008 There is increasing evidence that in addition to processing XBP1 mRNA IRE1α and β directly or indirectly promotes the rapid cleavage and destruction of a subset of mRNAs associated with the rough ER thereby reducing the entry of new proteins into the ER lumen Hollien and Weissman 2006 Iqbal et al 2008 IRE1 also plays a direct role in signalling The active phosphorylated form of IRE1 interacts with tumour necrosis factor receptor associated factor 2 which subsequently can promote activation of JUN Nterminal kinase JNK Activated JNK can then determine downstream consequences such as apoptosis or autophagy Ogata et al 2006 Szegezdi et al 2006 Oh and Lim 2009 IRE1 activity is modulated by its interaction with BAX inhibitor 1 and members of the BCL2 family of proteins Hetz and Glimcher 2008 Lisbona et al 2009

Keywords: