Abstract
Periodicity is a fundamental property of biological oscillators such as the mitotic cell cycle. In this context, periodicity refers to the time interval between the same phases of two consecutive cell cycles. The length of this interval, or the cell cycle speed, varies widely depending on cell type and the pathophysiological conditions. The relevance of cell cycle speed in various biological contexts has not been well-studied, partially due to the lack of experimental approaches that capture this parameter. Here, we describe a genetically encoded live-cell reporter of cell cycle speed. This reporter is based on the color-changing Fluorescent Timer (FT) protein, which emits blue fluorescence when newly synthesized before maturing into a red fluorescent protein. Its ability to report cell cycle speed exploits the different half-life of the blue vs. red form of the same molecule, as predicted by mathematical modeling. When a Histone H2B-FT fusion protein is expressed at steady-state in heterogeneously dividing cells, faster-cycling cells can be distinguished from slower-cycling ones by differences in their intracellular ratio between the blue and red fluorescent wavelengths. Cell cycle perturbation experiments demonstrate that the H2B-FT is a bona fide reporter of cell cycle speed in multiple cultured cell lines. In vivo, the blue/red profile faithfully tracked with known proliferation kinetics of various hematopoietic stem and progenitor cells, when expressed either from lentiviral vectors or from a targeted knock-in allele. As the H2B-FT is compatible with flow cytometry, it provides a strategy to physically separate subpopulations of live cells cycling at different rates for downstream analysis. We anticipate this system to be useful in diverse cell types and tissue contexts for dissecting the role of cell cycle speed in development and disease.