Atf-6 regulates lifespan through ER-mitochondrial calcium homeostasis

Abstract

Functional crosstalk between organelles is critical for maintaining cellular homeostasis. Individually, dysfunction of both endoplasmic reticulum (ER) and mitochondria have been linked to cellular and organismal aging, but little is known about how mechanisms of inter-organelle communication might be targeted to extended longevity. The metazoan unfolded protein response (UPR) maintains ER health through a variety of mechanisms beyond its canonical role in proteostasis, including calcium storage and lipid metabolism. Here we provide evidence that in C. elegans, inhibition of the conserved UPR mediator, activating transcription factor (atf)-6 increases lifespan via modulation of calcium homeostasis and signaling to the mitochondria. Loss of atf-6 confers long life via downregulation of the ER calcium buffering protein, calreticulin. Function of the ER calcium release channel, the inositol triphosphate receptor (IP3R/itr-1), is required for atf-6 mutant longevity while a gain-of-function IP3R/itr-1 mutation is sufficient to extend lifespan. IP3R dysfunction leads to altered mitochondrial behavior and hyperfused morphology, which is sufficient to suppress long life in atf-6 mutants. Highlighting a novel and direct role for this inter-organelle coordination of calcium in longevity, the mitochondrial calcium import channel, mcu-1, is also required for atf-6 mutant longevity. Altogether this study reveals the importance of organellar coordination of calcium handling in determining the quality of aging, and highlights calcium homeostasis as a critical output for the UPR and atf-6 in particular.