Chromatin Mechanically Buffers Cytoskeletal Forces at the Nuclear Envelope

Abstract

Building tissues from individual cells requires mechanisms to communicate mechanical properties between cells and organelles, as well as the ability to alter the physical properties of cellular materials. The LINC (Linker of Nucleoskeleton and Cytoskeleton) complex, which bridges both membranes of the nuclear envelope, may provide one such mechanism. Cytoskeletal forces are delivered onto the nucleus via LINC complexes, which physically couple the cytoplasmic cytoskeleton to the nuclear interior and its associated chromatin. These complexes can mechanically transmit cytoskeletal forces to chromatin, suggesting the possibility that they may participate in mechanotransduction. While our previous work suggests an additional role for chromatin and its connections to the LINC complex and the nuclear envelope in maintaining nuclear integrity and buffering cytoskeletal forces, a quantitative analysis of these properties has been lacking.We explicitly address the individual contributions of chromatin and chromatin-binding membrane proteins to nuclear integrity in the genetic organism, fission yeast (Schizosaccharomyces pombe). We have developed a combined optical tweezers/epifluorescence assay that allows us to manipulate, exert forces on and image isolated nuclei. Deriving nuclei from the genetically facile fission yeast model, we have interrogated how the presence of individual nuclear components and their crosslinking contributes to the ensemble physical behavior of the nucleus. Using genetic perturbations, we implicate integral inner nuclear membrane proteins that couple chromatin to the nuclear envelope in defining nuclear stiffness and supporting nuclear elasticity. The relevance of these findings to in vivo nuclear mechanics has been established by measuring nuclear envelope fluctuations in living S. pombe cells. These results provide new insights into how the mechanical properties of nuclei can be modulated. In addition, our work suggests that cytoskeletal forces coupled to the nuclear interior by the LINC complex contribute to overall chromatin mobility.