Fluorescent protein lifetimes report densities and phases of nuclear condensates during embryonic stem-cell differentiation

Fluorescent proteins can report on many cellular variables. Here, authors develop a method for reporting high local densities, and use it to show that density distribution of heterochromatin in mouse embryonic stem cells are not in a liquid phase. Fluorescent proteins (FP) are frequently used for studying proteins inside cells. In advanced fluorescence microscopy, FPs can report on additional intracellular variables. One variable is the local density near FPs, which can be useful in studying densities within cellular bio-condensates. Here, we show that a reduction in fluorescence lifetimes of common monomeric FPs reports increased levels of local densities. We demonstrate the use of this fluorescence-based variable to report the distribution of local densities within heterochromatin protein 1 & alpha; (HP1 & alpha;) in mouse embryonic stem cells (ESCs), before and after early differentiation. We find that local densities within HP1 & alpha; condensates in pluripotent ESCs are heterogeneous and cannot be explained by a single liquid phase. Early differentiation, however, induces a change towards a more homogeneous distribution of local densities, which can be explained as a liquid-like phase. In conclusion, we provide a fluorescence-based method to report increased local densities and apply it to distinguish between homogeneous and heterogeneous local densities within bio-condensates.

Automated summary

The authors developed a method to report high local densities and used it to show that the density distribution of heterochromatin in mouse embryonic stem cells is not in a liquid phase. They demonstrated the use of this method to distinguish between homogeneous and heterogeneous local densities within bio-condensates by measuring fluorescence lifetimes of common monomeric fluorescent proteins.