Protein-Folding Homeostasis in the Endoplasmic Reticulum and Nutritional Regulation

Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press Protein-Folding Homeostasis in the Endoplasmic Reticulum and Nutritional Regulation David Ron and Heather P. Harding Metabolic Research Laboratories, University of Cambridge, and NIHR Cambridge Biomedical Research Centre, Addenbrookes Hospital, Cambridge CB2 0QQ, United Kingdom Correspondence: [email protected] The flux of newly synthesized proteins entering the endoplasmic reticulum (ER) is under negative regulation by the ER-localized PKR-like ER kinase (PERK). PERK is activated by unfolded protein stress in the ER lumen and inhibits new protein synthesis by the phosphorylation of translation initiation factor eIF2a. This homeostatic mechanism, shared by all animal cells, has proven to be especially important to the well-being of professional secretory cells, notably the endocrine pancreas. PERK, its downstream effectors, and the allied branch- es of the unfolded protein response intersect broadly with signaling pathways that regulate nutrient assimilation, and ER stress and the response to it have been implicated in the development of the metabolic syndrome accompanying obesity in mammals. Here we review our current understanding of the cell biology underlying these relationships. nsulin was among the first proteins to be se- through its amino-terminal signal sequence Iquenced, among the first to have its structure (Mandon et al. 2013). Oxidative folding and solved, and therefore among the first to provide signal sequence removal yield mature pro-insu- clues to the diversity of modifications that affect lin, whose tertiary structure is stabilized by three secreted proteins. The b cell of the pancreas, disulfide bonds (Bulleid 2012). Folded pro-in- which produces insulin, is one of the best-stud- sulin clears ER quality control (Braakman and ied secretory cells, and the role of the secretory Hebert 2013) and traffics distally (Lord et al. pathway in insulin biosynthesis has been recog- 2013). nized from the dawn of modern cell biology. The peptidase involved in post-ER steps of Years later, when the stress pathways that con- pro-insulin maturation has long been recog- tribute to protein-folding homeostasis in the nized as playing a key role in its secretion, but endoplasmic reticulum (the unfolded protein the sensitivity of insulin biosynthesis to integri- response, UPR) came under scrutiny (Gardner ty of ER steps was not recognized until later. An et al. 2013; Olzmann et al. 2013), it was revealed early clue came from study of a naturally occur- that their integrity is important to insulin ring mutation in mouse Ins2. The Akita muta- metabolism and to the function of b cells. tion results in a Cys-92!Tyr substitution, dis- The precursor of insulin, prepro-insulin, is rupting an essential disulfide bond and leading recruited to the ER membrane cotranslationally to misfolding of proinsulin 2 (Wanget al. 1999). Editors: Susan Ferro-Novick, Tom A. Rapoport, and Randy Schekman Additional Perspectives on The Endoplasmic Reticulum available at www.cshperspectives.org Copyright # 2012 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a013177 Cite this article as Cold Spring Harb Perspect Biol 2012;4:a013177 1 Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press D. Ron and H.P. Harding Interestingly, a single copy of the mutation is ogy of the common, type II form of diabetes sufficient to compromise b cells, whereas ho- mellitus by limiting both the production of in- mozygosity for a null mutation in Ins2 is with- sulin and the body’s sensitivity to it. out an obvious phenotype in mice (because of redundancy between a rodent’s two insulin genes) (Duvillie et al. 1997). The biochemical CONTROLLING THE FLUX OF CLIENT PROTEINS INTO THE ANIMAL CELL ER (and phenotypic) dominance of the Akita mutation in mice (Colombo et al. 2008) fit well In unicellular eukaryotes and plants, the UPR is with retention of the mutant pro-insulin in predominantly atranscriptional program. IRE1, the ER, high levels of UPR signaling, and with an ER stress sensor that responds to an imbal- a progressive decline in b-cell mass and insulin ance between unfolded client protein load and stores as the mutant mice age. Thus, a pertur- chaperone reserve, activates a downstream tran- bation to ER protein-folding homeostasis in- scription factor by an unconventional splicing duced by the misfolding-prone mutant pro-in- event (Gardneret al. 2013). Metazoans evolved a sulin has a long-term negative effect on b-cell translational strand to their UPR, whereby ER function. stress attenuates new protein synthesis. Thus, Unbiased human genetics provided an addi- the UPR in animal cells is bipartite, with an tional clue to the importance of protein-folding acute program that attenuates the load on the homeostasis in the ER; the Wolcott–Rallison ER and a more latent transcriptional pro- syndrome is a rare recessive monogenic form gram—mediated jointly by IRE1, PERK, and of hypoinsulinaemic neonatal diabetes asso- ATF6—that builds ER capacity. ciated with bone dysplasia and episodic liver failure (Julier and Nicolino 2010). Positional a cloning revealed that the causative mutations eIF2 PHOSPHORYLATION—AN ANCIENT MECHANISM FOR REGULATING in EIF2AK3 severely disrupted the expression TRANSLATION or function of PERK (Delepine et al. 2000), an ER-localized stress-activated kinase that Translational regulation in the animal cell UPR tunes rates of new protein synthesis to the un- is achieved by coupling ER stress to the phos- folded protein load in the ER (Harding et al. phorylation of the a-subunit of translation 1999). Although known to be enriched in b initiation factor 2 (eIF2a). The key to this inno- cells, PERK expression is ubiquitous (Shi et al. vation is PERK, a type I ER-localized transmem- 1998). Therefore, the prominence of diabetes brane protein that fuses a stress-sensing luminal in the phenotype associated with loss-of-func- domain that is functionally interchangeable tion mutations in a ubiquitous component of with IRE1, with an eIF2a kinase cytosolic effec- the unfolded protein response (UPR) pointed tor domain that most certainly arose by dupli- to a special role for ER homeostasis in b-cell cation of an ancestral eIF2a kinase gene (likely health. GCN2) (Harding et al. 1999). More surprising has been the link between The signaling pathway downstream from chronic ER stress and the ability of insulin tar- PERKwas already in existence in the unicellular get tissues to respond to the hormone; it has ancestor of animals. Known as the general con- emerged that nutrient excess and obesity are trol response, this signaling pathway comprises associated with higher levels of UPR signaling eIF2, a translation initiation factor that escorts in the liver and fat and that steps that mitigate the charged initiator methionyl-tRNA to the ER stress in these tissues ameliorate the insulin small ribosomal subunit, and, as a ternary com- resistance that is part of the metabolic syndrome plex of eIF2–GTP–tRNAmet, contributes to the linked to nutrient excess. Thus, ER stress and 43S-ribosomes’ ability to recognize AUG start the response to it affect both the insulin-pro- codons and initiate translation (Hinnebusch ducing b cell and the insulin-responsive tissues 2000). AUG codon recognition and forma- and may therefore influence the pathophysiol- tion of the first peptide bond leads to GTP 2 Cite this article as Cold Spring Harb Perspect Biol 2012;4:a013177 Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press ER Stress, PERK, and Insulin hydrolysis. Replenishment of ternary complex selectivity of PERK action for pools of translat- pools requires exchange of GDP for GTP on ing ribosomes thus awaits the development of eIF2. This is a highly regulated step, with an more refined tools to measure its effects. exchange factor, eIF2B, and an anti-exchange factor, eIF5. eIF2(aP), phosphorylated on ser- eIF2(aP) DEPHOSPHORYLATION ine 51 of its a-subunit, inhibits the exchange factor eIF2B and thus attenuates the availability In animals, eIF2(aP) dephosphorylation is per- of ternary complexes for translation. Because formed by two phosphatase complexes. These protein synthesis has a major impact on energy consist of a common catalytic subunit, PP1, and nutrient stores, it is not surprising that sig- and a regulatory subunit, encoded either by naling by eIF2a phosphorylation has evolved to PPP1R15a (GADD34) or PPP1R15b (CReP). influence intermediary metabolism broadly. The regulatory subunit avidly binds the catalyt- Protein-folding homeostasis in the ER ben- ic PP1 subunit and endows the complex with efits from PERK-mediated eIF2a phosphory- specificity toward phosphorylated eIF2(aP). lation, because translational attenuation by ER In mammals, PPP1R15b (the ancestral metazo- stress contributes to the balance between un- an protein) is constitutively expressed (Jousse folded proteins entering the ER and chaperone et al. 2003), whereas expression of PPP1R15a, reserve of the compartment. The PERK-me- a mammalian invention, is tightly regulated by diated translational arm of the UPR gained stress, being induced by conditions that pro- importance with the rising complexity of the mote eIF2a phosphorylation. Thus, PPP1R15a secretory agenda of metazoans. Whereas worms contributes to a negative-feedback loop con-

Load more