Protein interplay in muscle tied to life span
Brown University biologists have uncovered a complicated chain of molecular events that leads from insulin to protein degradation in muscles and significantly diminished life span in fruit flies.
The new study may have broad implications across species, as it identifies the protein activin — also found in mammals — in fruit flies as the central culprit.
Fruit flies are notoriously short-lived but scientists interested in the biology of aging in all animals have begun to understand why some live longer than others. Researchers traced an insulin signaling cascade to protein quality control in muscle tissue with the result of a lengthening or shortening of the fruit fly life span.
The central feature of the study published in the November 2013 issue of PLoS Genetics is the newly discovered role of the activin complex. Activin blocks the natural mechanism in muscle cells for cleaning out misfolded proteins, leading to a decline in muscle performance.
In what scientists at Brown University think is no coincidence, blocking the activity of the activin equivalent, called dawdle, can lengthen a fly’s life span by as much as 20 percent, about 10 days.
Lead author Hua Bai: “For now this research is in fruit flies, but we think it can be extended to human aging biology.”
What excites the researchers is not that they can allow flies to stick around another week or two, but that the same fundamental proteins identified in flies are “conserved” in evolution, meaning they also operate in mammals including humans.
“The ultimate goal of our research is to understand how certain molecular signaling pathways control aging across all species in general,” said study lead author Hua Bai, a postdoctoral researcher in the ecology and evolutionary biology lab of Marc Tatar, professor of biology at Brown. “For now this research is in fruit flies, but we think it can be extended to human aging biology. This signaling is quite conserved evolutionarily.”
From insulin to muscle
Bai, Tatar, and their co-authors began the study armed with the understanding that a reduction in insulin signaling lengthens fly life span because when there is less insulin there is more dFOXO protein. Job one was to find out what genes relevant to life span dFOXO might be targeting.
Bai narrowed his search from hundreds of genes down to just three. He used interference RNA to suppress them and found that doing so increased life span in the flies. Suppressing dawdle (the fly version of activin) increased life span by 12 to 35 percent.
In flies, dawdle had been shown to affect neural development. In humans, our brain uses activin to stimulate ovarian follicles in the menstrual cycle of the female reproductive system. But when the team searched for where dawdle mattered to fly life span, their experiments showed that it was muscle.
Experiments revealed that dawdle suppresses the activity of a gene called Atg8a, which spurs the process of “autophagy” — or cleanup of misfolded proteins. A buildup of misfolded proteins weakens muscle tissue. A similar buildup of misfolded proteins in brain cells is believed to cause Alzheimer’s disease. When researchers suppressed dawdle in fruit flies, more misfolded proteins were cleared from muscle fibers.
Researchers also found that overexpressing Atg8a in the muscle of flies lengthened life span somewhat.
The life of fliesFlies in which expression of the protein dawdle was suppressed lived substantially longer than control flies.
At about 40 days, a low percentage of control flies still lived, while a high percentage of dawdle-suppressed flies were still alive.
Additionally, the team found suppressing dawdle reduces insulin secretion from IPCs, or insulin producing cells, in brains of flies.
Eventually, a reduction in system wide insulin signaling was observed. Reduced insulin signaling allowed more dFOXO to suppress dawdle, leading to continued suppression of insulin secretion from the brain. That same process also allowed for better muscle maintenance by promoting expression of Atg8a.
Bai acknowledged that the team doesn’t yet know why muscle performance due to a lack of autophagy results in reduced life span. One possibility could be that aging flies simply lose the mobility needed to compete for food. But the group has embarked on a new study of the most important muscle tissue: the heart.
The team is conducting experiments now to examine how the chain of events preventing autophagy affects the pumping of a fruit flies’ heart. In 2004, Tatar and his collaborator Rolf Bodmer connected insulin and dFOXO to fly heart performance. Eager to see how this extends to people, the team will be comparing activin signaling and autophagy in mammalian cell cultures.
“That’s potentially translational toward human biology,” Bai said. “If we have evidence from mammals, it could be useful for future therapeutic targets and drug design.”
Reduced insulin/IGF signaling increases lifespan in many animals. To understand how insulin/IGF mediates lifespan in Drosophila, we performed chromatin immunoprecipitation-sequencing analysis with the insulin/IGF regulated transcription factor dFOXO in long-lived insulin/IGF signaling genotypes. Dawdle, an Activin ligand, is bound and repressed by dFOXO when reduced insulin/IGF extends lifespan. Reduced Activin signaling improves performance and protein homeostasis in muscles of aged flies. Activin signaling through the Smad binding element inhibits the transcription of Autophagy-specific gene 8a (Atg8a) within muscle, a factor controlling the rate of autophagy. Expression of Atg8a within muscle is sufficient to increase lifespan. These data reveal how insulin signaling can regulate aging through control of Activin signaling that in turn controls autophagy, representing a potentially conserved molecular basis for longevity assurance. While reduced Activin within muscle autonomously retards functional aging of this tissue, these effects in muscle also reduce secretion of insulin-like peptides at a distance from the brain. Reduced insulin secretion from the brain may subsequently reinforce longevity assurance through decreased systemic insulin/IGF signaling.
It is widely known that reduced insulin/IGF signaling slows aging in many contexts. This process requires the forkhead transcription factor (FOXO). FOXO modulates the expression of many genes, and the list of those associated with slow aging is impressive. But there are few data indicating the mechanisms or genes through which FOXO actually slows aging. Here, we identify a novel FOXO target, dawdle, the Activin-like ligand in fruit flies. We show that down-regulation of Activin signaling in muscle, but not in adipose tissue, leads to extended lifespan. In part it does so when it alleviates the negative transcriptional repression of its Smox transcription factor (a Smad transcription factor) upon a keystone autophagy gene, Atg8a. This double signaling cascade autonomously improves muscle performance (measured at cellular and functional levels) and nonautonomously extends lifespan as it reduces the secretion of insulin peptides from the brain. The work develops the emerging model for interacting autonomous-nonautonomous roles of insulin/IGF signaling as a systems integrative mechanism of aging control.
In addition to Bai and Tatar, the paper’s other authors are Ping Kang and Ana Maria Hernandez.
The National Institute on Aging (grants: RO1AG024360 and R01AG031152) and the Ellison Medical Foundation funded the study.
Original press release: http://www.uphs.upenn.edu/news/News_Releases/2013/11/bale/