Micro-organization and visco-elasticity of the interphase nucleus revealed by particle nanotracking

Abstract

The microstructure of the nucleus, one of the most studied but least understood cellular organelles, is the subject of much debate. Through the use of particle nanotracking, we detect and quantify the micro-organization as well as the viscoelastic properties of the intranuclear region in single, live, interphase somatic cells. We find that the intranuclear region is much stiffer than the cytoplasm; it is also more elastic than viscous, which reveals that the intranuclear region displays an unexpectedly strong solid-like behavior. The mean shear viscosity and elasticity of the intranuclear region of Swiss 3T3 fibroblasts are 520 Poise (P) and 180 dyn/cm2, respectively. These measurements determine a lower bound of the propulsive forces (3-15 picoNewton) required for nuclear organelles such as promyelocytic-leukemia bodies to undergo processive transport within the nucleus by overcoming friction forces set by the intranuclear viscosity. Dynamic analysis of the spontaneous movements of nanospheres embedded in the nucleus reveals the presence of putative transient nuclear microdomains of mean size 290±50 nm, which are mostly absent in the cytoplasm. The strong elastic character and micro-organization of the intranuclear region revealed by particle nanotracking analysis may help the nucleus to preserve its structural coherence. These studies also highlight the difference between the low interstitial nucleoplasmic viscosity, which controls the transport of nuclear proteins and molecules, and the much higher mesoscale viscosity, which affects the diffusion and directed transport of nuclear organelles and re-organization of interphase chromosomes.

Chromosome dynamics, molecular crowding, and diffusion in the interphase cell nucleus: a Monte Carlo lattice simulation study

“…We verify the relaxation time scales for interphase chromosomes recently estimated by Rosa and Everaers (2008) by a continuum model and show that molecular crowding by diffusing macromolecules can lead to anomalous diffusion but to a much lesser degree than crowding by the chromatin network. We also find that the stiffness of the network probed with diffusing particles is much lower than detected earlier in microrheology experiments (Tseng et al 2004).…”

“…12a). In microrheological experiments with viscoelastic media such as gels, elastic trapping can usually be detected for diffusing particles by the appearance of a trapping plateau on shorter time scales, while on longer time scales, the particles break free and diffuse normally (Biehl et al 2004;Mason and Weitz 1995;Tseng et al 2004;Wong et al 2004). For our chromatin networks, no characteristic time scale for the transition from being trapped to free diffusion can be determined, suggesting that the self-diffusion of the chromatin network is too rapid to keep molecules trapped.…”

“…16, however, the viscous modulus G (ω) dominates on all frequencies, which means that the network is rather viscous than elastic. Tseng et al (2004) measured a high-frequency plateau of 18 Pa of the storage modulus G (ω) in the intranuclear region of cells. This is significantly more elastic than what we obtained from our simulations.…”

Density effects in entangled solutions of linear and ring polymers

“…Pointing out on the analogy between entangled ring polymers and branched trees, Rubinstein and coworkers suggested [7] that ring polymers relax instead owing to mass flowing along the ring contour length, a mechanism leading to the faster relaxation time ∼ L 2.5 . We now discuss the micro-rheological properties of polymer solutions by studying the diffusive motion of hard-sphere colloid particles [44,45] of diameter d = 5σ. This size was specifically chosen because, being at the crossover between the solution mesh size of ≈ 8σ for ρσ 3 = 0.1 and ≈ 4σ for ρσ 3 = 0.…”

In‐Cell NMR Spectroscopy

“…While the viscosity in endosomes, [47] for example, is only slightly greater than the viscosity of the cytoplasm, the viscosity of the nucleus seems significantly increased. [48,49] In addition, the viscosity of the organelles and the cytoplasm can also change during different states of the cell, for example, different phases of the cell cycle.…”