Protein fluctuations in single cells and cell-to-cell variability

An important characteristic of biological cells is their ability to generate a wide spectrum of phenotypes across a genetically homogeneous population. This variability is believed to play an important role in diverse biological phenomena. We have recently found that variability in cellular protein content among genetically identical cells (used here to simply represent non-genetic phenotypic variability), displays some universal statistical properties that seem to result from the single-cell expression dynamics. In other words, temporal fluctuations in protein expression measured in a single cell exhibit similar statistical properties to fluctuations measured in a snapshot of a population (i.e. cell-to-cell variability at a given time). However, this does not imply that the dynamics of protein expression is ergodic. Our measurements show that cells exhibit distinct characteristics that are maintained over their lifetime. This individuality is observed in the average cell-size as well as the average protein content measured over tens of generations. A simple mapping of the cellular dynamics with a feedback mechanism that links consecutive generations can reproduce all statistical properties observed. It also provides valuable insight on how cell individuality can be maintained for many generations.