Ire1 membrane stress responses support cell growth upon disruptions in inter-organelle contacts
Disruptions in inter-organelle contacts result in membrane lipid homeostasis defects that ultimately impair cellular function and viability. Yet, essential responses to membrane lipid imbalances remain poorly understood. In this study, we demonstrate that Ire1-dependent Membrane Stress Responses (MSR), distinct from the canonical Unfolded Protein Response (UPR), sustain the growth of yeast cells lacking inter-organelle contacts. Comprehensive lipidomics reveal that the Ire1-mediated MSR compensates for glycerophospholipid synthesis defects by modulating sphingolipid metabolism. Quantitative imaging further indicates that Ire1 mediates these effects, at least in part, by elevating cytoplasmic Ca2+ which in turn stimulates calcineurin activity necessary for cellular homeostasis. Accordingly, inhibition of calcineurin results in severe endoplasmic reticulum stress in yeast cells depleted of inter-organelle contacts. Thus, the Ire1 MSR directs Ca2+-dependent calcineurin activity and lipid metabolism upon disruptions in membrane contact sites to maintain cellular homeostasis. Alterations in inter-organelle contacts are associated with several diseases, including neurodegenerative disorders. Our findings in yeast suggest that evoking the MSR may be a means to ameliorate neuronal degeneration and delay the progression of neurodegenerative disorders.