Timing live cell cycle length in diverse tissues Abstract Cellular dynamics underly tissue homeostasis and its abnormality defines many disease states. Timing how quickly cells form tissues requires knowing the cellular generational time, or cell cycle speed, most conveniently defined as the time interval between two consecutive mitoses. Determination of cell cycle speed for cells in deep tissues has relied on S-phase activity to incorporate pulsed labels, or cell division mediated- dilution of saturated labels. Thus, cell cycle rates have only been calculatable as a population average upon fixation, or after the label has been diluted to a certain level within the detectible range, among other limitations. An instantaneous readout of cell cycle speed in individual live cells that enable their isolation has been out of reach. In response to the call of Catalytic Tool and Technology Development in Kidney, Urologic, and Hematologic Diseases, I propose to develop new genetically encoded live cell cycle speed reporter. Mouse strain expressing the new reporter will be established for detecting a wide range of cell cycle speed by fluorescence, in situ or by flow cytometry, to catalyze the research on cellular dynamics in diverse tissues. We will build on our recent success in developing a first-of-its-kind live cell cycle speed reporter by exploiting the differential half-life of a color changing protein, the fluorescent timer (FT). We chose the kinetic variant that emits blue fluorescence when newly synthesized for ~1.2 hours, before converting into a red protein permanently during maturation. Expressed as a fusion protein to core histone H2B, cell cycle length of individual cells can be determined by the ratio between the two fluorescence: the faster the cell cycle, the bluer a cell appears. While this first reporter demonstrated the proof-of-principle and exceptional performance in resolving short cell cycles, such as those of the erythroid progenitors and myeloid-committed progenitors, the vast cell types dividing at slower rates were not resolvable. Through this proposal, we will test new design features so that the live cell cycle speed relevant for most mammalian cell types in vivo can be conveniently determined with a single genetically encoded reporter construct, H2B-FTmHaloD2. Given the naturally existing wide range of cell cycle speed in the hematopoietic tissues and our own expertise in studying it, we will use the hematopoietic stem and progenitor cells at baseline and during injury as model cell types to test, calibrate and standardize workflows for how a new ratiometric reporter can be used to determine cellular dynamics and sort for live cells from diverse tissues. The new design features of the H2B-FTmHaloD2 should also readily resolve the slow cell cycles present in solid tissues such as the kidney.