Cell-cycle regulator Whi5 shapes proliferative heterogeneity in clonal populations

Abstract

Clonal cultures exhibit phenotypic variation in spite of being composed of genetically identical cells under equal environmental conditions. Proliferation rate is one of the characters that have been described to show this heterogeneity, but the mechanisms underlying this phenomenon are still poorly understood. Cell cycle regulation controls proliferative capacity and previous transcriptomic studies revealed that budding yeast microcolonies with low proliferation rates display high levels of the G1-S transition inhibitor Whi5. This regulator, which, in budding yeast, plays a similar role as Retinoblastoma protein in mammalian cells, has been also linked to replicative aging in late steps of mother cells lifespan. In this work, we combined single cell microencapsulation with confocal microscopy to study heterogeneity in clonal cultures. Using this new method, we found that a significant proportion of slow-growing microcolonies are founded by mother cells with a short number of cell cycles. We also found that the reduction in the proliferation capacity of microcolonies founded by young mother cells is related to the expression levels of Whi5, which increases with mother cell age since early stages. Our results establish that stably reduced proliferation is not exclusively linked to old mother cells and indicate that cell age contributes to proliferation heterogeneity by modulating cell cycle regulation.