RNA surveillance is required for endoplasmic reticulum homeostasis

Abstract

The unfolded protein response (UPR) is an intracellular stresssignaling pathway that counteracts the accumulation of misfolded proteins in the endoplasmic reticulum (ER). Because defects in ER protein folding are associated with many pathological states, including metabolic, neurologic, genetic, and inflammatory diseases, it is important to understand how the UPR maintains ER protein-folding homeostasis. All metazoans have conserved the fundamental UPR transducers IRE1, ATF6, and PERK. In Caenorhabditis elegans, the UPR is required to prevent larval lethality and intestinal degeneration. Although ire-1-null worms are viable, they are particularly sensitive to ER stress. To identify genes that are required for development of ire-1-null worms, we performed a comprehensive RNA interference screen to find 10 genes that exhibit synthetic growth and intestinal defects with the ire-1(v33) mutant but not with atf-6(tm1153) or pek-1(ok275) mutants. The expression of two of these genes, exos-3 and F48E8.6, was induced by ER stress, and their knockdown in a wildtype strain caused ER stress. Because these genes encode subunits of the exosome complex that functions in mRNA surveillance, we analyzed other gene products required for nonsense-mediated mRNA decay (NMD). Our results demonstrate that defects in smg-1, smg-4, and smg-6 in C. elegans and SMG6 in mammalian cells cause ER stress and sensitize to the lethal effects of ER stress. Although ER stress did not activate mRNA surveillance complex assembly, ER stress did induce SMG6 expression, and NMD regulators were constitutively localized to the ER. Importantly, the findings demonstrate a unique and fundamental interaction where NMD-mediated mRNA quality control is required to prevent ER stress.endoplasmic reticulum quality control | premature termination codons T he endoplasmic reticulum (ER) is the site for the folding and modification of newly synthesized proteins that are destined for intracellular organelles, the plasma membrane, and the extracellular milieu. Protein folding in the ER is facilitated by molecular chaperones and is monitored by a stringent ER quality control system (ERQC) that allows only properly folded proteins to traffic to the Golgi apparatus. In ERQC, misfolded proteins are retained in the ER for an attempt to attain their appropriate conformation, and irreversibly misfolded proteins are targeted to ER-associated protein degradation (ERAD) or to autophagy (1).The accumulation of misfolded proteins in the ER caused by alterations in ER homeostasis initiates signaling of the unfolded protein response (UPR) that attempts to resolve the proteinfolding defect (2). In metazoans, the UPR is signaled through three ER transmembrane transducers, IRE1, ATF6, and PERK, that sense the accumulation of misfolded proteins and transmit signals to the cytosol and the nucleus. IRE1 is a protein kinase/ endoribonuclease that initiates unconventional splicing of mRNA encoding the transcription factor X box-binding protein 1 (XBP1), to create a translational frameshift ...

High-resolution profiling of NMD targets in yeast reveals translational fidelity as a basis for substrate selection

“…However, NMD also targets a significant fraction of apparently normal and physiologically functional wild-type mRNAs (Schweingruber et al 2013), indicating that it also serves as a fundamental posttranscriptional regulatory mechanism for eukaryotic gene expression. Consistent with these important roles, NMD function is linked to diverse cellular processes, including cell growth and proliferation (Weischenfeldt et al 2008;Avery et al 2011;Lou et al 2014), development and differentiation (Medghalchi et al 2001;Metzstein and Krasnow 2006;Gong et al 2009;Wittkopp et al 2009), innate immunity (Gloggnitzer et al 2014), antiviral or stress responses (Sakaki et al 2012;Balistreri et al 2014), and neuronal activity or behavior (Giorgi et al 2007;Colak et al 2013).…”

Inhibition of SMG-8, a subunit of SMG-1 kinase, ameliorates nonsense-mediated mRNA decay-exacerbated mutant phenotypes without cytotoxicity

“…We then examined whether NMD suppression causes endoplasmic reticulum (ER) stress, because it is possible that the accumulation of aberrant mRNAs by NMD suppression increases the production of unfolded proteins, which leads to ER stress (33). To test this possibility, we analyzed the expression of ER stress-related proteins, including phosphorylated eIF2α, activating transcription factor 4 (ATF4), and ER chaperone glucoseregulated protein of 78 kDa (GRP78) using NMD-suppressed Ullrich disease fibroblasts and HeLa cells.…”

Proteomic Analysis Reveals Branch-specific Regulation of the Unfolded Protein Response by Nonsense-mediated mRNA Decay

“…Of note, NMD itself is down-regulated by hypoxia or ER stress (15)(16)(17)(18). The exact mechanisms of this regulation remain unknown, although previous data suggest a role of eIF2␣ phosphorylation (15).…”

“…Interestingly, many of these genes are involved in stress response pathways, such as amino acid homeostasis (9) and the integrated stress response following hypoxia or endoplasmic reticulum (ER) stress (15)(16)(17)(18)(19). In addition to its surveillance function, NMD might therefore play a regulative role in these signaling pathways.…”