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Stem cell functions continue to be governed by day and night cycles (circadian rhythms) during ageing, but their rhythms become devoted to tissue repair and not to the maintenance of tissue tone — ScienceDaily

It is widely believed that, with the passage of time, stem cells cease to differentiate between day and night cycles, in other words they lose their circadian rhythm, and that this loss promotes ageing. However, this has been found not to be the case. Two studies by scientists at the Institute for Research in Biomedicine (IRB Barcelona), the Pompeu Fabra University (UPF), (both in Spain), and the University of California, Irvine (US) published on 10 August in the journal Cell reject this hypothesis. During ageing, stem cells continue to show rhythmic activity but reprogram their circadian functions.

“Aged stem cells conserve circadian rhythm but now perform another set of functions to tackle the problems that arise with age,” explains ICREA researcher Salvador Aznar Benitah, head of the Stem Cells and Cancer group and leader of the two studies. The scientists do not know how such reprogramming comes about. Aznar Benitah adds, “the problem is that as they age, stem cells lose the rhythmic functions necessary for tissue protection and maintenance, which become replaced by functions aimed at coping with stress. Loss of the previous circadian functions of stem cells during natural ageing contributes in some way to greater damage and greater ageing.”

The first authors of both studies, associate researcher Guiomar Solanas and “la Caixa” PhD student Francisca Oliveira Peixoto, both at IRB Barcelona, compared stem cells from young mice (three months old) with those of aged mice (between 18 and 22 months old) in three kinds of tissue, namely skin, muscle and liver, every four hours over one day. “The experiments have been technically highly complex and demanding, but the results are surprising,” says Solanas.

Specific reprogramming for each tissue

Gene reprogramming occurs, for example, in response to accumulated DNA damage, inflamed tissues, and an inefficient cell self-cleaning process (autophagy).

“Although ageing always involves circadian reprogramming, an…

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