Dynamics of Passive and Active Particles in the Cell Nucleus

Abstract

Inspite of being embedded in a dense meshwork of nuclear chromatin, gene loci and large nuclear components are highly dynamic at C. To understand this apparent unfettered movement in an overdense environment, we study the dynamics of a passive micron size bead in live cell nuclei at two different temperatures ( and C) with and without external force. In the absence of a force, the beads are caged over large time scales. On application of a threshold uniaxial force (about 10 pN), the passive beads appear to hop between cages; this large scale movement is absent upon ATP-depletion, inhibition of chromatin remodeling enzymes and RNAi of lamin B1 proteins. Our results suggest that the nucleus behaves like an active solid with a finite yield stress when probed at a micron scale. Spatial analysis of histone fluorescence anisotropy (a measure of local chromatin compaction, defined as the volume fraction of tightly bound chromatin) shows that the bead movement correlates with regions of low chromatin compaction. This suggests that the physical mechanism of the observed yielding is the active opening of free-volume in the nuclear solid via chromatin remodeling. Enriched transcription sites at C also show caging in the absence of the applied force and directed movement beyond a yield stress, in striking contrast with the large scale movement of transcription loci at C in the absence of a force. This suggests that at physiological temperatures, the loci behave as active particles which remodel the nuclear mesh and reduce the local yield stress.

Soft inclusion in a confined fluctuating active gel

“…In this paper, we study the positioning and shape dynamics of a deformable inclusion embedded in a confined active gel. Formulated in this general way, our study is applicable to a variety of in-vivo and in-vitro contexts -(i) the positioning and shape fluctuations of the nucleus (or other localized organelles) within a cell [4][5][6][7][8], (ii) the dynamics of large colloidal particles embedded in an active medium [9][10][11][12], (iii) the positioning and dynamics of nuclei in multi-nucleated cells [13], (iv) the positioning and segregation of chromosomes within a nucleus [14][15][16][17] and (v) the fluctuations of a cell / cell-junction within a developing tissue [18,19].…”

Nuclear Positioning and Its Translational Dynamics Are Regulated by Cell Geometry

“…Together, these mechanisms have crucial effects on the positioning and dynamics of various organelles in the cell. There have been significant advances in experimental techniques and theory on the use of optically trapped or injected beads to probe the mechanical properties of cells and the intracellular dynamics (1)(2)(3)(4)(5). In a number of microrheological studies of living cells that used microinjected particles as probes (6), the large-scale properties of the cytoplasm were estimated using multiparticle correlation studies due to the difficulty of introducing large particles into the cell.…”

Maximal Fluctuations of Confined Actomyosin Gels: Dynamics of the Cell Nucleus

“…Contrary to expectations, recent experiments have shown that fluctuations of nuclear components (eg. membrane or DNA loci) are not governed by intra-nuclear activity, but rather by the extra-nuclear actomyosin contractility [4][5][6][7][8][9].In this Letter, we introduce a simple model based on active gel theory [10-12] which illustrates how variation of active stress noise can induce a maximum in the fluctuation spectrum of the nuclear shape. We discuss the rather unexpected experimental result of Ref.[7] which, combining drug treatments and geometric constraints, shows that the nuclear area fluctuations are maximal for an intermediate level of contractility.…”