Proliferative potential of human CD8+ cells senescing in vivo

The budding yeast Saccharomyces cerevisiae has become a popular model in aging studies, on the basis of the assumption that the limited capacity for budding of individual yeast cells is a consequence of their accumulation of a “senescence factor“. However, the molecular nature of the yeast “senescence factor“ has not been unequivocally elucidated, and results of yeast aging studies do not correspond to expectations with respect to this model of aging. The group of Vaupel has questioned recently (2005) the methodology of studies of yeast aging, showing that yeast cells which ceased to divide cannot be treated as dead. These authors pointed to the necessity of introducing time as a measure of yeast longevity, instead of the dimensionless replicative lifespan. Our studies question another basic assumption of yeast gerontology, i. e. that the cell division limit is due to accumulation of a “senescence factor“ in the yeast mother cell. We demonstrate that, instead, reaching a critical volume is the factor limiting the number of yeast cell divisions. The questioning of the two basic assumptions of yeast aging studies cast doubt on the validity of linking the term “replicative lifespan“ to aging since a phenomenon not connected to aging is the causative factor of the limitation of budding. Basic arguments challenging the usefulness of the yeast as a model organism in aging studies, as well as proposals of new classification of terms used for description of yeast aging, providing their compatibility with terminology employed in aging of metazoans, will be presented.