An interferometric analysis of nucleoli in cultured mesothelial cells

Cycles of protein condensation and discharge in nuclear organelles studied by fluorescence lifetime imaging

Nuclear organelles are viscous droplets, created by concentration-dependent condensation and liquid–liquid phase separation of soluble proteins. Nuclear organelles have been actively investigated for their role in cellular regulation and disease. However, these studies are highly challenging to perform in live cells, and therefore, their physico-chemical properties are still poorly understood. In this study, we describe a fluorescence lifetime imaging approach for real-time monitoring of protein condensation in nuclear organelles of live cultured cells. This approach unravels surprisingly large cyclic changes in concentration of proteins in major nuclear organelles including nucleoli, nuclear speckles, Cajal bodies, as well as in the clusters of heterochromatin. Remarkably, protein concentration changes are synchronous for different organelles of the same cells. We propose a molecular mechanism responsible for synchronous accumulations of proteins in the nuclear organelles. This mechanism can serve for general regulation of cellular metabolism and contribute to coordination of gene expression.

Studying the condensation of proteins into membraneless organelles in live cells is highly challenging. Here the authors develop a fluorescence lifetime imaging approach to monitor the condensation of proteins in nuclear organelles and report coordinated and cyclic changes in several nuclear organelles.

The crowded nucleus

The principles that determine the organization of the nucleus have become clearer in recent years, largely because of new insights into polymer, colloid, and soft-matter science. Macromolecules, together with the giant linear polymers that form the chromosomes, are confined at high concentrations within the nuclear envelope and their interactions are influenced strongly by short-range depletion or entropic forces which are negligible in dilute systems, in addition to the more familiar van der Waals, electrostatic, steric, hydrogen bonding, and hydrophobic forces. The studies described in this volume are consistent with the model that this complex and concentrated mixture of macromolecules is maintained in a delicate equilibrium by quite simple although unsuspected physicochemical principles. The sensitivity of this equilibrium to perturbation may underlie the controversies about the existence of a nuclear matrix or scaffold. In this volume, we underline the importance for cell biologists of being familiar with current work in colloid, polymer, soft matter, and nanoscience. This chapter presents a brief background to the aspects of the nucleus that are considered in detail in subsequent chapters.

Packing of the polynucleosome chain in interphase chromosomes: evidence for a contribution of crowding and entropic forces

In the crowded intranuclear environment, entropic depletion forces between macromolecules are expected to be strong. A review of simulations of linear polymers leads to several predictions about probable conformations of a polynucleosome chain in these conditions. These include a globular conformation, variable compaction due to different local rigidity or curvature of the mosaic of isochores, satellite sequences, and nucleosomes containing different histone variants, and the possibility that chromosomes represent separate phases like those seen in heterogeneous particle mixtures by experiment and simulation. Experimental results which show that macromolecular crowding alone, in the absence of exogenous cations, can stabilise interphase chromosomes and cause self-association of polynucleosome chains are presented. Together, these considerations suggest that macromolecular crowding and entropic forces are major factors in packing polynucleosome chains in vivo.