@article {31557, title = {Maximal Fluctuations of Confined Actomyosin Gels: Dynamics of the Cell Nucleus.}, journal = {Phys Rev Lett}, volume = {120}, year = {2018}, month = {2018 Mar 02}, pages = {098001}, abstract = {

We investigate the effect of stress fluctuations on the stochastic dynamics of an inclusion embedded in a viscous gel. We show that, in nonequilibrium systems, stress fluctuations give rise to an effective attraction towards the boundaries of the confining domain, which is reminiscent of an active Casimir effect. We apply this generic result to the dynamics of deformations of the cell nucleus, and we demonstrate the appearance of a fluctuation maximum at a critical level of activity, in agreement with recent experiments [E. Makhija, D. S. Jokhun, and G. V. Shivashankar, Proc. Natl. Acad. Sci. U.S.A. 113, E32 (2016)PNASA60027-842410.1073/pnas.1513189113].

}, issn = {1079-7114}, doi = {10.1103/PhysRevLett.120.098001}, author = {Rupprecht, J-F and Singh Vishen, A and Shivashankar, G V and Rao, M and Prost, J} } @article {35146, title = {Soft inclusion in a confined fluctuating active gel.}, journal = {Phys Rev E}, volume = {97}, year = {2018}, month = {2018 Mar}, pages = {032602}, abstract = {

We study stochastic dynamics of a point and extended inclusion within a one-dimensional confined active viscoelastic gel. We show that the dynamics of a point inclusion can be described by a Langevin equation with a confining potential and multiplicative noise. Using a systematic adiabatic elimination over the fast variables, we arrive at an overdamped equation with a proper definition of the multiplicative noise. To highlight various features and to appeal to different biological contexts, we treat the inclusion in turn as a rigid extended element, an elastic element, and a viscoelastic (Kelvin-Voigt) element. The dynamics for the shape and position of the extended inclusion can be described by coupled Langevin equations. Deriving exact expressions for the corresponding steady-state probability distributions, we find that the active noise induces an attraction to the edges of the confining domain. In the presence of a competing centering force, we find that the shape of the probability distribution exhibits a sharp transition upon varying the amplitude of the active noise. Our results could help understanding the positioning and deformability of biological inclusions, e.g., organelles in cells, or nucleus and cells within tissues.

}, issn = {2470-0053}, doi = {10.1103/PhysRevE.97.032602}, author = {Singh Vishen, Amit and Rupprecht, J-F and Shivashankar, G V and Prost, J and Rao, Madan} } @article {896, title = {Cytoskeletal control of nuclear morphology and chromatin organization.}, journal = {J Mol Biol}, volume = {427}, year = {2015}, month = {2015 Feb 13}, pages = {695-706}, abstract = {

The nucleus is sculpted toward various morphologies during cellular differentiation and development. Alterations in nuclear shape often result in changes to chromatin organization and genome function. This is thought to be reflective of its role as a cellular mechanotransducer. Recent evidence has highlighted the importance of cytoskeletal organization in defining how nuclear morphology regulates chromatin dynamics. However, the mechanisms underlying cytoskeletal control of chromatin remodeling are not well understood. We demonstrate here the differential influence of perinuclear actin- and microtubule-driven assemblies on nuclear architecture using pharmacological inhibitors and targeted RNA interference knockdown of cytoskeleton components in Drosophila cells. We find evidence that the loss of perinuclear actin assembly results in basolateral enhancement of microtubule organization and this is reflected functionally by enhanced nuclear dynamics. Cytoskeleton reorganization leads to nuclear lamina deformation that influences heterochromatin localization and core histone protein mobility. We also show that modulations in actin-microtubule assembly result in differential gene expression patterns. Taken together, we suggest that perinuclear actin and basolateral microtubule organization exerts mechanical control on nuclear morphology and chromatin dynamics.

}, keywords = {Actins, Animals, Cell Nucleus, Chromatin Assembly and Disassembly, Cytoskeleton, Drosophila melanogaster, Gene Expression Regulation, Histones, Mechanotransduction, Cellular, Nuclear Proteins}, issn = {1089-8638}, doi = {10.1016/j.jmb.2014.09.008}, author = {Ramdas, Nisha M and Shivashankar, G V} }