Aging is the progressive loss of cellular homeostasis - including protein quality control. The endoplasmic reticulum (ER) is the site of protein synthesis for all secreted and membrane proteins. ER quality control depends on N-glycosylation of nascent peptide chains and on chaperones as well as degradation mechanisms such as ER associated degradation (ERAD) and autophagy. ER function is critical for normal C. elegans lifespan and is implicated as an output of longevity pathways. We hypothesized that novel mutants with improved ER protein quality control might show improved cellular homeostasis and longevity. After EMS mutagenesis, we selected for developmental resistance to the N-glycosylation inhibitor tunicamycin (TM), and analyzed lifespans of TM resistant mutants. We found that gain-of-function (gof) mutations in the key enzyme of the hexosamine pathway (HP), glucosamine-fructose 6-phosphate aminotransferase (
gfat-1, F07A11.2), result in ER stress resistance and lifespan extension. The HP provides UDP-N-acetylglucosamine (UDP-GlcNAc) that is required in the first step of N-glycan synthesis. Notably, feeding wild type C. elegans with GlcNAc resulted in TM resistance and extended lifespan in a dose dependent manner. We tested the activity of the unfolded protein response (UPR), but could detect no activation of UPR target genes, even during ER stress. To address the link between N-glycosylation and lifespan extension we used an aggregation-prone point mutant of SRP-2 as a fluorescent sensor of ER protein quality control.
gfat-1 gof significantly reduced SRP-2 aggregation, which was dependent on ERAD and on autophagy. We detected induced autophagosome formation in
gfat-1 gof mutants and further found that the lifespan extension required autophagy. We next tested cytosolic protein aggregation using polyglutamine expansion and detected a reduction of polyQ toxicity that was likewise autophagy-dependent. This could be mimicked by GlcNAc supplementation. Together, we demonstrate a genetic or pharmacological intervention that links N-glycoslyation and ER protein quality control with autophagy and lifespan extension. This makes the HP a potential target for the treatment of age-related proteotoxic diseases.