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Developmental microRNA Plays Role in C. elegans Aging; Role in Mammals Remains Speculative


The microRNA lin-4, which was previously found to play a role in developmental timing in C. elegans, also regulates lifespan in the worm, according to research from the lab of Yale University's Frank Slack.

The findings, published in the Dec. 23 issue of Science, suggest the existence of a "developmental clock" controlling aging, and may ultimately lead to human therapies for various age-associated disorders.

Lin-4 — the first miRNA to be discovered — is initially expressed in C. elegans during the first larval stage and acts as a sort of timing switch to initiate a cascade of developmental changes in the somatic cells between various larval stages, Slack told RNAi News this week. In the absence of lin-4, cells in a developing C. elegans larva continuously divide as they would during the first stage of development, never progressing to the next stages.

"You end up with these strange looking worms at the end of the day because they never [progress] to their adult fates," he said. But any defects related to lin-4 suppression can be avoided by also knocking out the putative transcription factor lin-14, "so it appears that lin-4 works by repressing lin-14."

The notion to explore the possible roles of miRNAs beyond developmental timing stemmed from Slack's graduate student Michelle Boehm, he said. "We sat down and said, 'We work on all these developmental timing mutants and we know that these genes are expressed [in both] the larval stages [of C. elegans as well as] the adults. What are they doing in the adults?'"

"it's possible that evolution selects for developmental genes, which then have a secondary role in aging, which is what we think has happened here."

Boehm began testing the hypothesis that miRNAs affecting developmental timing in C. elegans also control the timing of aging and life span in the adult worm, Slack said. To do so, "we synchronized a population of animals so that you know they are all adults at the same stage — they begin what we call day 1 of adulthood all at the same time. Basically, every day you observe the entire plate of animals and score how many of them are alive each day … over the course of two weeks or so," he said. "Using standard procedures in the field and standard statistical analyses, you can determine whether the population of animals of one mutant, versus a wild-type or another mutant, lives shorter or longer."

After testing a number of "heterochronic mutants to see if they had any effect on the timing of death, [Boehm] discovered that a subset of them did, [and that] the most dramatic effects were seen with the lin-4 microRNA and its target lin-14," Slack said.

"Just like in developmental timing, where the lin-4 microRNA negatively regulates lin-14 and therefore they give opposite phenotypes when you knock them out, we saw a similar correlation between those two genes in aging — the lin-4 mutant lived much shorter than normal, and a lin-14 mutant lived much longer than normal," he said.

"The thing that was interesting was that the lin-4 mutant lived shorter, but if you over-expressed the lin-4 RNA you got an extended lifespan" beyond what is normal, Slack noted. "So it wasn't that the lin-4 mutants were dying because they were sick or because of some unrelated pathology. It appeared that lin-4 was actually promoting a regular lifespan. In the absence of lin-4 you lose this lifespan promoter and the animals die sooner than they should, [but if] you over-express lin-4 you can allow the animals to live longer than they would normally."

In order to ensure that their findings weren't the result of the developmental effects of the miRNAs, they knocked out lin-14 only in adult worms and still observed an extended lifespan, indicating that the miRNA is acting independently of its developmental role in the adult worm to extend lifespan.

"This a controversial issue — it's probably the one we took the most flak on in trying to get this paper published," Slack noted. "We're trying to make the point that there might be a developmental clock that's regulating aging just like there's a clock regulating development.

"One of the [paper's] reviewers pointed out … that evolution doesn't usually select for genes that control aging because in general once an animal is post-reproductive, that animal is essentially competing with its young for scarce resources," he said. "However, as we pointed out in the paper, it's possible that evolution selects for developmental genes, which then have a secondary role in aging, which is what we think has happened here."

According to Slack, his lab's findings could have broad implications in the field of human therapeutics should lin-4 homologs prove to have similar effects in mammals.

"There are [three] lin-4 homologs that can be detected" in humans, collectively known as miR-125, although they do not exactly match the microRNA in C. elegans, he said. "Maybe [the lin-4 homologs] could have a … role in humans in promoting a regular lifespan.

"We don't have any evidence of that right now," he pointed out, but said that his lab is beginning to examine the role of mir-125 in the mouse and whether its over-expression could boost lifespan.

This work is expected to take up to three years, and could lead someday to the development of "an anti-aging therapy or [a method of] controlling diseases of aging" in humans, he said. "This is something that is, of course, completely speculative right now, [and] we would end up having to partner with some sort of company [eventually] because … it would be too daunting for us" alone.

— Doug Macron ([email protected])

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