"In past experiments, meddling with this versatile pathway to exploit its beneficial effects on aging and life span inevitably invited a host of problems," says Andrew Dillin, Ph.D., an assistant professor in the Molecular and Cell Biology Laboratory at the Salk Institute for Biological Studies and leader of the study, reported in the March 9 issue of the journal Cell.
The Salk scientists discovered the protein in studies with worms, a commonly used lab model in genetics; since this signaling cascade including the newly identified protein is conserved across many species, including humans, these findings raise the prospect that one day it might be possible to medically tweak this pathway to slow aging and improve the quality of life without harmful consequences to the body.
In humans and other mammals this signaling cascade is known as the insulin/IGF-1 pathway since it is prompted by insulin and the closely related insulin-like growth factor-1 or IGF-1. IGF-1 is the main trigger for childhood growth but continues to exert growth stimulating effects throughout an individual's life. Insulin, which is best know for controlling blood glucose levels, has additional and equally important regulatory functions in the body. In worms, however, the chain of signals is set in motion when a single receptor on the cell surface is matched to an insulin-like signal.
"If you were to interfere with insulin signaling in humans in order to prolong life, you would induce diabetes right away," cautioned graduate student and first author Suzanne Wolff. When this pathway had been clipped in worms, they lived longer than normal, but th eir larval development and reproduction were disrupted.
"So, the central question became whether we would be able to genetically manipulate one element of the pathway without disrupting its additional functions," said Dillin. "In this regard, we thought that the life extending function of this pathway would be elusive, whereas the developmental and reproductive functions would be more amenable. We were delighted to find that Smk-1 proved to be specific for the one function we thought the most elusive," he added.
Some of Dillin's earlier research had hinted at the possibility that "specificity" factors may control how and whether insulin and IGF-1 impact a target gene. Identifying those factors one by one would allow scientists to separate the different functions of insulin/IGF-1 signaling and to manipulate them individually without wreaking havoc on the organism's normal functioning.
Together with the lab of another Salk scientist, Tony Hunter, Ph.D., Dillin's team identified a protein in the worm Caenorhabditis elegans that allowed them to do just that. The protein is encoded by the Smk-1 gene. "Smk-1 is the first known gene that regulates longevity without affecting other vital functions of the insulin signaling pathway," said Wolff.
Under favorable conditions, a still unidentified molecule binds to DAF-2, the worms' equivalent of the insulin/IGF-1 receptor, which is located at the cell's surface. A cascade of signaling molecules relays the information deep into the cell till it reaches a protein called DAF-16. Known as a transcription factor, DAF-16 encodes a DNA-binding protein that turns on other genes but when DAF-2 is active, it is unable to enter the cell's nucleus to activate its target genes.
When environmental conditions turn harsh as a result of overcrowding and scarce nutrients, for example, DAF-2 signaling shuts down. No longer marooned outside the nucleus, DAF-16 crosses into the nucleus, and triggers all the necessary genes to help the body take care of the stressful situation. But food shortages aren't the only triggers for the stress program. Others are the heat that scrambles proteins into toxic clumps and marauding parasites. Highly reactive molecules known as free radicals also unleash DAF-16.
If the worms are having a good day in a favorable environment, but for some reason DAF-2 signaling gets turned off precociously, the worms reap the benefit of increased stress resistance and double their lifespan from an average of 20 to 40 days. Except the worms trade a trim life focused on reproduction for a long life with fewer progeny and a tendency to halt larval development and enter a dormant, hibernation-like stage in which they can hang on for months but don't reproduce.
Unless Smk-1 is missing, that is.
Without Smk-1 to provide guiding assistance, DAF-16 is unable to slow down the aging process, and the worms die young. "These mutants are shorter lived because they are unable to respond appropriately to oxidative stress, but they are just as resistant to heat stress as wild type worms whose signaling pathways have not been experimentally manipulated," explained Wolff.
"In the future, we may use this protein as a tool to study the genetic and metabolic requirements for extending longevity without causing diverse, wide-ranging effects on the development and maintenance of the physiological equilibrium in the organism," said Dillin.