03 Jul 2009

Research shows that from delicate yeast to mammalian gourmands, restricting caloric intake can dramatically increase longevity. That life extension may not only depend on avoiding junk food, but on disposal of another type of junk—the unwanted debris that accumulates within cells. In the June 25 Nature, researchers at the Salk Institute for Biological Sciences, La Jolla, California, report that the benefits of caloric restriction in roundworms is lost if they tamper with the cell’s garbage disposal. Specifically, they show that wwp-1, a component of the ubiquitin-proteasome system that rids cells of unwanted protein, is necessary for the benefits of caloric restriction. “Our study uncovers for the first time, to our knowledge, a role of the ubiquitin pathway in longevity in response to dietary restriction,” write the authors. The finding may also have implications for human longevity and for age-related illnesses such as Alzheimer and Parkinson diseases, which are characterized by the accumulation of intracellular protein aggregates.

Mammalian WWP-1 and its C. elegans counterpart (wwp-1) are ubiquitin ligases, components of the complex ubiquitin-proteasome system (UPS) that selects unwanted proteins for recycling. The UPS has been implicated in the pathology of several neurodegenerative diseases: Parkin, one of the first genes linked to Parkinson disease, also encodes a ubiquitin ligase. The Salk researchers, led by Andrew Dillin and Tony Hunter, found that wwp-1 is necessary for stress resistance in roundworms, and since stress resistance has been linked to longevity (see ARF related news story) the researchers looked to see how wwp-1 affects lifespan.

First author Andrea Carrano and colleagues found that overexpressing wwp-1 by half increased worm lifespan by about 20 percent (typically worms live a maximum of about 30 days). In addition, worms carrying a mutant form of wwp-1 did not show the typical 20-30-day increase in lifespan when their caloric intake was reduced, and knocking down wwp-1 with RNAi abolished lifespan gains seen in eat-2 mutant worms, which have trouble swallowing their chow (eat-2 codes for a protein that drives pharyngeal pumping). These different lines of evidence indicate that wwp-1 is necessary for the increased longevity that comes with reduce caloric intake. Interestingly, knocking down wwp-1 had little effect on long-lived worms that have mutations in the insulin-like growth factor pathway (daf-2 and daf-16 mutants—see ARF related news story), or on worms with extended lifespan due to mutations in mitochondrial genes. These findings indicate that wwp-1mutations are not simply shortening lifespan by making the worms ill, but are specifically mediating caloric restriction mediated longevity.

To determine if lifespan changes are specifically associated with the ubiquitin ligase activity of wwp-1, the researchers used a dominant negative mutant with an alanine substituted for cysteine 762, which is essential for in wwp-1 catalysis. eat-2 mutant worm lines expressing C762A wwp-1 had significantly shorter lifespans than eat-2 mutant controls. “Therefore, the ubiquitin ligase activity of WWP-1 is essential for diet-restriction-induced longevity,” write the authors.

What substrates of wwp-1 might be involved in longevity is an interesting question. Like parkin, wwp-1 is one of many E3 ligases, which transfer ubiquitin from E2 ligases to other protein substrates. Using C. elegans extracts as a source of substrates, Carrano and colleagues found that the E2 ligase UBC-18 is necessary for wwp-1 activity, and they found that as with wwwp-1, RNAi knockdown of ubc-18 abolished lifespan gains in eat-2 mutant worms. The results indicate that the UBC-18/wwp-1 transfer of ubiquitin is necessary for caloric-restriction induced longevity. Whether that will hold true in mammals is yet to be determined, as is what substrates downstream of wwp-1 might be involved. In vitro the researchers tested two likely suspects, transcription factors PHA-4 and SKN-1B—both linked to CR-induced longevity, but found that wwp-1 ubiquitinates neither. They suggest that identification of the targets of UBC-18-WWP-1 is needed to allow precise placement of this complex in the diet-restriction, and conclude that “a detailed understanding of the pathways that mediate the benefits of dietary restriction may lead to new therapies for age-related diseases.”—Tom Fagan