Protein disulfide isomerase PDI-6 regulates Wnt secretion to coordinate inter-tissue UPRmt activation and lifespan extension in C. elegans

Coordination of inter-tissue stress signaling is essential for organismal fitness. Neuronal mitochondrial perturbations activate the mitochondrial unfolded-protein response (UPRmt) in the intestine via the mitokine Wnt
signaling in Caenorhabditis elegans. Here, we found that the protein disulfide isomerase PDI-6 coordinates
inter-tissue UPRmt signaling via regulating the Wnt ligand EGL-20. PDI-6 is expressed in the endoplasmic reticulum (ER) and interacts with EGL-20 through disulfide bonds that are essential for EGL-20 stability and
secretion. pdi-6 deficiency results in misfolded EGL-20, which leads to its degradation via ER-associated
protein degradation (ERAD) machinery. Expression of PDI-6 declines drastically with aging, and animals
with pdi-6 deficiency have decreased lifespan. Overexpression of PDI-6 is sufficient to maintain Wnt/EGL-
20 protein levels during aging, activating the UPRmt, and significantly extending lifespan in a Wnt- and
UPRmt-dependent manner. Our study reveals that protein disulfide isomerase facilitates Wnt secretion to
coordinate the inter-tissue UPRmt signaling and organismal aging.