Separate roles for chromatin and lamins in nuclear mechanics

Abstract

The cell nucleus houses, protects, and arranges the genome within the cell. Therefore, nuclear mechanics and morphology are important for dictating gene regulation, and these properties are perturbed in many human diseases, such as cancers and progerias. The field of nuclear mechanics has long been dominated by studies of the nuclear lamina, the intermediate filament shell residing just beneath the nuclear membrane. However, a growing body of work shows that chromatin and chromatin-related factors within the nucleus are an essential part of the mechanical response of the cell nucleus to forces. Recently, our group demonstrated that chromatin and the lamina provide distinct mechanical contributions to nuclear mechanical response. The lamina is indeed important for robust response to large, whole-nucleus stresses, but chromatin dominates the short-extension response. These findings offer a clarifying perspective on varied nuclear mechanics measurements and observations, and they suggest several new exciting possibilities for understanding nuclear morphology, organization, and mechanics.

Keywords: chromatin; force; lamin; micromanipulation; nucleus.

Figure 1.

Nuclear mechanics is dictated differentially by chromatin for short extensions and lamins for long extensions through strain stiffening. Chromatin, filling the interior of the nucleus, and lamins, at the exterior, the two major mechanical components of the nucleus. In our novel micromanipulation technique we isolate a single nucleus from a living cell and attach micropipettes at each end via temporary suction and then nonspecific attachment to the pipettes. The “Pull” micropipette moves to extend the nucleus while the other “Force” micropipette's deflection provides a measure of force, due to the pipette's pre-measured spring constant (schematic of brightfield images, adapted from original article [19]). Graphs show the average force-extension data for mammalian nuclei for extensions from 0 to 3 µm (≈30% of a 10–12 µm nucleus) and from 3 to 6 µm [19], representing the chromatin-dominated short (cyan) and lamin-dominated long strain stiffening (light red) differential force regimes. The upper plot shows wild-type (WT, black line), chromatin decompaction via histone deacetylase inhibitor valproic acid (VPA, blue line), and chromatin digestion (MNase, gray line). The lower plot shows wild-type and lamin A/C knockdown (LA/C KD, red line). Error bars represent standard error. Scale bar = 10 µm.