Activation of the endoplasmic reticulum unfolded protein response by lipid disequilibrium without disturbed proteostasis in vivo

Abstract

The Mediator is a conserved transcriptional coregulator complex required for eukaryotic gene expression. In Caenorhabditis elegans, the Mediator subunit mdt-15 is essential for the expression of genes involved in fatty acid metabolism and ingestion-associated stress responses. mdt-15 loss of function causes defects in reproduction and mobility and shortens lifespan. In the present study, we find that worms with mutated or depleted mdt-15 (mdt-15 worms) exhibit decreased membrane phospholipid desaturation, especially in phosphatidylcholine. Accordingly, mdt-15 worms exhibit disturbed endoplasmic reticulum (ER) homeostasis, as indicated by a constitutively activated ER unfolded protein response (UPR ER ). Activation of this stress response is only partially the consequence of reduced membrane lipid desaturation, implicating other mdt-15-regulated processes in maintaining ER homeostasis. Interestingly, mdt-15 inactivation or depletion of the lipid metabolism enzymes stearoyl-CoA-desaturases (SCD) and S-adenosyl methionine synthetase (sams-1) activates the UPR ER without promoting misfolded protein aggregates. Moreover, these worms exhibit wild-type sensitivity to chemically induced protein misfolding, and they do not display synthetic lethality with mutations in UPR ER genes, which cause protein misfolding. Therefore, the constitutively activated UPR ER in mdt-15, SCD, and sams-1 worms is not the consequence of proteotoxic stress but likely is the direct result of changes in ER membrane fluidity and composition. Together, our data suggest that the UPR ER is induced directly upon membrane disequilibrium and thus monitors altered ER homeostasis.

Membrane lipids and the endoplasmic reticulum unfolded protein response: An interesting relationship

“…1,2,13 However, the ER is also a major site of lipid synthesis, particularly of phospholipids and sterols. Taken together with the discovery of UPR ER activation without accompanying proteostatic imbalance, 8,12 this hints at a broader potential regulatory role for the UPR ER ( Fig. 1).…”

“…12 In contrast, the model we used as a proxy of reduced PC synthesis (worms depleted of the S-Adenosyl methionine synthetase sams-1, which are unable to synthesize the universal methyl-group donor SAM) 26 displayed an activated UPR mito . 12 Mutation or depletion of sams-1 reduces PC synthesis, and this defect as well as UPR ER and UPR mito activation can be rescued by dietary choline, pinpointing altered PC metabolism, not methylation in general as the culprit of these phenotypes. 12,26 Thus, normal PC levels are apparently essential for mitochondria and ER homeostasis, which is supported by the fact that PC is the most abundance PL in both organelles.…”

“…1,13,14 As the UPR ER is apparently capable of sensing lipid disequilibrium independently of sensing proteostatic imbalance, it will be interesting to assess whether the compensatory changes to lipid disequilibrium similarly serve to alleviate the specific upstream stressors, namely abnormal lipid compositions. For example, in our worm models of increased fatty acid saturation (leading to increased PC saturation) or reduced PC production, 12 are relevant biosynthetic pathways modulated to alleviate this membrane stress? Studies using metabolic labeling approaches to quantitate the flux through lipid synthesis, modification, and turnover pathways 15 should be particularly insightful in this context.…”

“…12 At least for the IRE-1 branch, this activation reflected canonical signaling, as xbp-1 -the key effector of activated IRE-1 -was required for the activation of the downstream target hsp-4. Such canonical activation might reflect a necessity to activate a protective response upon the physiological disturbances in the models we employed, but surprisingly, ire-1, pek-1/PERK, and atf-6 inactivation all failed to synergize with lipid disequilibrium.…”

“…For example, induction of HSP-4/BiP is usually interpreted as an indicator of compromised protein folding in the ER, yet Volmer et al and we showed that such activation need not strictly be accompanied by compromised proteostasis. 8,12 Gene expression profiling in worms under lipid disequilibrium might help identify genes that are specifically activated by such changes but not by altered proteostasis. Alternatively, protein folding and processing may be adapted even if the initial insult affected lipid levels or composition, potentially to preemptively prevent further aggregation of overall ER health.…”