Nucleolar dynamics and interactions with nucleoplasm in living cells

Abstract

Liquid-liquid phase separation (LLPS) has been recognized as one of the key cellular organizing principles and was shown to be responsible for formation of membrane-less organelles such as nucleoli. Although nucleoli were found to behave like liquid droplets, many ramifications of LLPS including nucleolar dynamics and interactions with the surrounding liquid remain to be revealed. Here, we study the motion of human nucleoli in vivo, while monitoring the shape of the nucleolus-nucleoplasm interface. We reveal two types of nucleolar pair dynamics: an unexpected correlated motion prior to coalescence and an independent motion otherwise. This surprising kinetics leads to a nucleolar volume distribution, p⁢(V)∼V-1, unaccounted for by any current theory. Moreover, we find that nucleolus-nucleoplasm interface is maintained by ATP-dependent processes and susceptible to changes in chromatin transcription and packing. Our results extend and enrich the LLPS framework by showing the impact of the surrounding nucleoplasm on nucleoli in living cells.

Chromatin Mechanics Dictates Subdiffusion and Coarsening Dynamics of Embedded Condensates

“…This saturation of the MSD may take place on very long timescales which were not captured here due to drift of the nuclei and stage; it is also possible that drift contributed to a small overestimation of the MSD even after correction by image registration. We also note that we tracked condensates and considered them to be entirely independent, while local compressive stresses could cause droplets to slow down when in the vicinity of one another but still merge rapidly when the cavities they induce in the chromatin meet, explaining correlated motions observed in endogenous organelles immediately preceding merger 19,32 .…”

Mapping the nucleolar proteome reveals a spatiotemporal organization related to intrinsic protein disorder

“…This allows for dynamic cellular responses as these structures can change in size and protein composition when needed. The size and number of nucleoli change throughout the cell cycle as they fuse together, a process recently suggested to be aided by interactions with the nucleoplasm (Caragine et al, 2019). The formation of these membrane-less yet spatially distinct structures is the result of reversible liquid-liquid phase transitions similar to oil-in-water emulsions (Brangwynne et al, 2009(Brangwynne et al, , 2011Lin et al, 2015).…”

Mapping the nucleolar proteome reveals a spatiotemporal organization related to intrinsic protein disorder

“…This allows for highly dynamic cellular responses as these structures can change in size and protein composition when needed. The size and number of nucleoli changes throughout the cell cycle as they fuse together, a process recently suggested to be aided by interactions with the nucleoplasm (Caragine, Haley and Zidovska, 2019). The formation of these membrane-less, yet spatially distinct structures is a result of reversible liquid-liquid phase transitions similar to oil in water emulsions (Brangwynne et al, 2009;Brangwynne, Mitchison and Hyman, 2011;Lin et al, 2015).…”