SummaryIt has been proposed that cumulative somatic mutations contribute to the aging process by disrupting the transcriptional networks that regulate cell structure and function. Experimental support for this model emerged from a recent study of cardiomyocytes that showed a dramatic increase in the transcriptional heterogeneity of these long-lived postmitotic cells with age. To determine if regulatory instability is a hallmark of aging in renewing tissues, we evaluated gene expression noise in four hematopoietic cell types: stem cells, granulocytes, naïve B cells and naïve T cells. We used flow cytometry to purify phenotypically equivalent cells from young and old mice, and applied multiplexed quantitative reverse transcription-polymerase chain reaction to measure the copy number of six different mRNA transcripts in 324 individual cells. There was a trend toward higher transcript levels in cells isolated from old animals, but no significant increase in transcriptional heterogeneity with age was found in the surveyed populations. Flow cytometric analysis of membrane protein expression also indicated that cellto-cell variability was unaffected by age. We conclude that large-scale regulatory destabilization is not a universal concomitant of aging, and may be of significance as an aging mechanism primarily in nonrenewing tissues.Key words: aging, FACS, gene regulation, hematopoiesis, single-cell analysis, stem cells.
IntroductionFor years, theoreticians were divided on whether aging results from a regulated developmental program, analogous to growth and sexual maturation, or constitutes an essentially passive, wear-and-tear process. Arguments grounded in evolutionary theory have now largely settled this debate in favor of the wear-and-tear model. The idea that natural selection favors 'planned obsolescence', to prevent the aged from competing for resources with their own offspring, appears untenable on close examination, at least in the general case (Kirkwood & Austad, 2000;Kirkwood, 2005). However, as even robust animals have a limited half-life in the wild state, owing to accident, disease and predation, selection pressure for mutations whose benefits accrue in old age is weak and ultimately falls below countervailing, entropic forces such as genetic drift. In addition, mutations that bring early-life benefits along with late-life deficits tend to accumulate, the costs being discounted by the attrition curve. Hopes for the discovery of a cellular or endocrine 'death clock' have faded with the recognition that aging is not a programmed process, and it now seems likely that a multiplicity of factors contribute to the gradual loss of fitness we call aging. If aging has many causes, then the problem for aging research is to identify and characterize the most important drivers of decline at the different levels of physiology: cellular, tissue, organ and systemic. For the most part, this project remains a work in progress.Within the cell, the fundamental mechanism of homeostasis is the network that regulates the coordi...