Substantial evidence suggests that lifespan is increased if an organism restricts its daily calorie intake, a spartan regime that some say works by just making life seem longer. A team of scientists from the Salk Institute of Biological Studies has discovered a molecular switch flipped by hunger that could not only make longevity more appetizing but identify drug targets for patients with aging-related diseases such as type II diabetes or cancer.
In the February 17, 2011 issue of Nature Howard Hughes Medical Institute investigator Andrew Dillin, Ph.D., an associate professor in the Molecular and Cell Biology Laboratory, and Howard Hughes Medical Institute early career scientist Reuben Shaw, Ph.D., an assistant professor in the Molecular and Cell Biology Laboratory and the Dulbecco Laboratory for Cancer Research, report for the first time that deactivation of a protein called CRTC1 in roundworms increases their lifespan, most likely mediating the effects of calorie restriction.
Previously, researchers knew hunger promoted longevity by activating an enzyme called AMPK, which senses that food is scarce and pushes cells into a low energy state. "We knew AMPK was a major energy sensor but didn't know what it was talking to," says Dillin, one of two senior authors of the study. "Our goal was to understand the genetic circuitry that registered that response."
To define the circuitry, Dillin, who studies aging using the roundworm Ceanorhabditis elegans as a model system, joined forces with Shaw, who had a long-term interest in AMPK's role in mammalian metabolism. "It was clear that one pathway that coordinated metabolism with growth in response to nutrients was AMPK signaling," says Shaw. "Studies had also suggested that AMPK might regulate lifespan in worms. What was not known was what factors downstream of AMPK mediated those effects."
Together they searched the genome of Caenorhabditis elegans for likely AMPK targets, and identified one suspect encoding a protein called CRTC1, which was expressed at the same time and place as AMPK.
To determine if CRTC1 played any role in lifespan, the team fed worms an inhibitory RNA engineered to deplete them of CRTC1 protein. When they measured the worms' lifespan-normally about 3 weeks-they found that worms fed the anti-CRTC1 RNA lived a whopping 40% longer, suggesting that AMPK retards aging by antagonizing CRTC1 activity.
The group then showed how AMPK silences CRTC1. AMPK is a kinase, an enzyme that modifies the activity of other proteins by decorating them with chemical phosphate groups. The team found that AMPK deactivated CRTC1 by adding phosphates to a specific region of the CRTC1 protein, an effect equivalent to eliminating CRTC1 altogether.
Likewise, when the worms were fed an inhibitory RNA depleting them of an enzyme that lops off the CRTC1 phosphates, they lived longer, showing that AMPK and the lopper-known to scientists as calcineurin-determine lifespan by controlling the extent to which CRTC1 is phosphorylated. In fact, Dillin's lab had previously identified calcineurin as a regulator of aging in an earlier study but they did not know what calcineurin's key target was.
"What we have identified is a binary switch that turns the aging process on and off," says Dillin, referring to the push-pull effects of AMPK and calcineurin on CRTC1. "Aging is a risk factor for a number of pathological conditions-if you could find a way to control this switch you could ameliorate a plethora of age-related diseases."
The good news for the burger and fries crowd is that the entire pathway----AMPK, calcineurin, and CRTC1----and a host of interacting factors may operate similarly in worms and humans. In fact, one well-characterized protein partner of CRTC1 is the gene regulator CREB. The group found that worms lacking the worm version of CREB lived longer, similar to worms lacking CRTC1, suggesting that both factors conspire to antagonize longevity.
"CREB is involved in a suite of physiological processes-from memory to drug addiction, to energy homeostasis," says William Mair, Ph.D., a postdoctoral fellow in the Dillin lab and the study's first author. "CRTC factors may regulate CREB's ability to activate targets specifically involved in aging."
Circumstantial evidence already suggests that factors downstream of AMPK impact aging-related human diseases: both metformin, widely used to treat type II diabetes, and age-retardant resveratrol, red wine drinkers' best excuse for having just one more glass, are activated by AMPK.
"This pathway is evolutionarily conserved biochemically-that single phosphorylation site on the CRTC1 protein, which is critical for longevity in worms, is conserved as an AMPK target site in CRTC1-like genes from worms to mammals," says Shaw, suggesting that inducing that site pharmacologically was a goal worth going after. This study also dovetails nicely with a number of key studies on the function of the CRTC family in mammals by Marc Montminy, M.D., Ph.D, a professor in the Clayton Foundation Laboratories for Peptide Biology and a co-author on the current study.
"Whether you are talking about yeast, worms, Labradors, or rhesus monkeys-dietary restriction is the best intervention we have so far against age-related conditions like neurodegeneration, cancer and diabetes," says Mair. "Our goal now is to use information we have derived from worm studies to find a way to treat many of these diseases with one magic bullet." With any luck, that magic bullet will be maximally effective when taken on a full stomach.
Deep learning techniques transform protein structure prediction