Protein Abundance in Long-lived Mutants

Tinkering with insulin signaling can dramatically slow the aging process and extend survival. The details of this effect have best been studied in C. elegans. What distinguishes long-lived insulin signaling mutants from normal animals? Many studies have tacked the transcriptional side of this question, comparing gene expression profiles by microarray analysis. A new study addresses the post-transcriptional side of this story. Continuing a recent streak of high profile papers, workers from the labs of Andy Dillin and John Yates examined protein abundance in worms made to live longer by RNAi knockdown of the insulin receptor DAF-2. Dong et al. (“Quantitative Mass Spectrometry Identifies Insulin Signaling Targets in C. elegans,” Science, August 3, 2007) differentially labeled worms with two different nitrogen isotopes and compared relative protein abundance by mass spectrometry. Of 1685 identifiable proteins, a total of 86 varied markedly between wild type and mutant animals — 47 were more abundant in daf-2-depleted animals and 39 were less abundant. These proteins were mostly from the usual laundry list of metabolic processes, but 51 of them had not been previously come to light in microarray studies.

Lowering daf-2 activity changes all kinds of random processes, only some of which are relevant to longevity. To figure out which of these proteins were involved in aging, Dong et al. knocked each of them down with RNAi and looked for an effect on diapause or lifespan. Not surprisingly, these two phenomena could be separated— depletion of a number of these proteins promoted diapause but did not affect lifespan. The surprise came from the characterization of a group of proteins involved in carbohydrate metabolism and translation initiation. These proteins, including ACO-2 (aconitase / iron response element binding protein), FBP-1 (fructose bisphosphatase) and other ostensibly boring enzymes, are up in daf-2-deficient worms, yet when directly depleted by RNAi cause daf-2 mutant animals to live yet longer. Ditto for TAX-6, the worm’s calcineurin A homolog, which these workers showed to undergo nuclear localization upon daf-2 knockdown. However, depleting each of these proteins in wild type animals didn’t affect survival one way or another.

At first, these findings seem counterintuitive. Genetically, the knockdown experiments say that these proteins ordinarily act to shorten lifespan, but only in daf-2 mutants. The authors invoke a model of compensatory regulation, wherein daf-2 loss of function promotes the accumulation of “pro-aging” proteins, tempering the beneficial effect of low insulin activity. Thus, for further increase our longevity, we should not only aim to inhibit our insulin receptors but also to shut down glycolysis. The Atkins Diet is looking better and better.