Plastid-produced interorgannellar stress signal MEcPP potentiates induction of the unfolded protein response in endoplasmic reticulum

Abstract

Cellular homeostasis in response to internal and external stimuli requires a tightly coordinated interorgannellar communication network. We recently identified methylerythritol cyclodiphosphate (MEcPP) as a novel stress-specific retrograde signaling metabolite that accumulates in response to environmental perturbations to relay information from plastids to the nucleus. We now demonstrate, using a combination of transcriptome and proteome profiling approaches, that mutant plants (ceh1) with high endogenous levels of MEcPP display increased transcript and protein levels for a subset of the core unfolded protein response (UPR) genes. The UPR is an adaptive cellular response conserved throughout eukaryotes to stress conditions that perturb the endoplasmic reticulum (ER) homeostasis. Our results suggest that MEcPP directly triggers the UPR. Exogenous treatment with MEcPP induces the rapid and transient induction of both the unspliced and spliced forms of the UPR gene bZIP60. Moreover, compared with the parent background (P), ceh1 mutants are less sensitive to the ER-stress-inducing agent tunicamycin (Tm). P and ceh1 plants treated with Tm display similar UPR transcript profiles, suggesting that although MEcPP accumulation causes partial induction of selected UPR genes, full induction is triggered by accumulation of misfolded proteins. This finding refines our perspective of interorgannellar communication by providing a link between a plastidial retrograde signaling molecule and its targeted ensemble of UPR components in ER.T he endoplasmic reticulum (ER) function is crucial to adjustment and maintenance of a balance between protein loads and folding capacity in response to frequently changing intracellular and environmental conditions. To maintain balance (homeostasis) under stressful conditions, the ER activates conserved intracellular signal transduction pathways collectively termed the unfolded protein response (UPR) (1). The UPR monitors ER protein-folding capacity and communicates the ER status to gene expression programs that up-regulate genes encoding components of the protein folding machinery or the ER-associated degradation system (1, 2). In plants, two distinct and parallel branches of the UPR signaling pathway have been identified. One pathway involves two integral membrane-bound transcription factors (bZIP17 and bZIP28). The other pathway involves an ER membrane-localized dual-functioning (kinase/ribonuclease) protein, inositol-requiring protein-1 (IRE1). IRE1 catalyzes unconventional cytoplasmic splicing of mRNA encoding basic leucine zipper 60 (bZIP60), the transcription factor responsible for the induction of ER quality control genes (3, 4). Stress causes activation and nuclear relocation of bZIP17 and bZIP28, and activation of IRE1 responsible for splicing of bZIP60 mRNA that encodes transcriptionally active nuclear localized bZIP60. These activated transcription factors induce transcription of target genes, including genes that mediate the UPR.Protein folding is coupled to many biological processes,...

Modeling of Dolichol Mass Spectra Isotopic Envelopes as a Tool to Monitor Isoprenoid Biosynthesis

“…One is also tempted to speculate that methylerythritol cyclodiphosphate, a product of the MEP pathway so far recognized as a retrograde signaling metabolite inducing the expression of selected abiotic stressresponsive genes (Xiao et al, 2012), could be involved. Notably, methylerythritol cyclodiphosphate potentiates the unfolded protein response (Walley et al, 2015) induced upon osmotic stress in plants (Moreno and Orellana, 2011), and interestingly, Dol attenuates this stress reaction (Jozwiak et al, 2015).…”

Plastidial metabolite MEcPP induces a transcriptionally centered stress-response hub via the transcription factor CAMTA3

“…for the induction of ER quality control (15). Additional molecular genetics, pharmacological, and metabolic studies provided unequivocal evidence supporting the MEcPP-mediated induction of the RSRE and RSRE containing genes such as IRE1a and bZIP60 via the transcription factor CAMTA3, in a calcium dependent manner.…”

“…The ceh1 (constitutively expressing hydroperoxide lyase1) mutant accumulates large quantities of MEcPP and displays various stress-associated phenotypes such as compromised growth, high salicylic acid (SA) content, and constitutive activation of the unfolded protein response (UPR) required for restoration of protein-folding homeostasis in the endoplasmic reticulum (ER) (13)(14)(15). We performed bioinformatic analyses of global microarray expression profiles and identified overrepresentation of the RSRE motif in the promoters of robustly induced genes in the ceh1 mutant, including key UPR-regulatory genes such as the ER membrane-localized inositol-requiring protein-1 (IRE1a) and the transcription factor basic leucine zipper 60 (bZIP60) responsible Significance A dual-function plastidial metabolite, methylerythritol cyclodiphosphate (MEcPP), an intermediate of the plastidial pathway for isoprenoids production and a retrograde signaling metabolite, transduces signals to activate general stress-response (GSR) genes by inducing a transcriptionally centered stress hub.…”

Redox- and Reactive Oxygen Species-Dependent Signaling into and out of the Photosynthesizing Chloroplast

“…Thus, the data imply that the redox state of the chloroplast is transmitted to peroxisomes via the malate valve. Information about the redox and ROS states of the chloroplast, particularly under stress, also is transmitted to the cytosol via metabolically coupled compounds such as cyclocitral (Ramel et al, 2012), 39-phosphoadenosine 59-phosphate (PAP; Estavillo et al, 2011), and methylerythritol cyclodiphosphate (MEcPP; Walley et al, 2015). PAP participates in the expressional control of a subset of genes in the high-light acclimation response.…”

“…MEcPP is a signaling metabolite released from perturbed chloroplasts. Elevated MEcPP levels up-regulate the expression of genes involved in the unfolded protein response, such as the transcription factor gene bZIP60, possibly sensitizing the cell for episodes of increased stress (Walley et al, 2015).…”