Chromatin Dynamics are Controlled by Nuclear Lamin A: Light Sheet Microscopy - FCS Studies

Abstract

The viscoelastic properties of the cell nucleus and their connection with gene function have become a recent focus of interest. Microscopy techniques allow vizualizing the dynamics of chromatin and proteins in the cell nucleus, and constitute valuable tools for understanding the mechanism of genome function.Dynamics of intracellular motion can conveniently be characterized by fluorescence correlation spectroscopy (FCS), which offers sub-millisecond time resolution, but so far was mostly limited to single-point measurements in a laser focus. This is time-consuming and inconvenient for live cell measurements.SPIM-FCS is a new method that allows the acquisition of mobility data on entire two-dimensional cross-sections of cells, providing diffusion coefficients, flow velocities and concentrations in an imaging mode. Two-color fluorescence cross-correlation spectroscopy (SPIM-FCCS) in addition allows imaging of molecular interactions.Here we studied the influence of the intermediate filament protein lamin A on the dynamics of interphase chromatin by SPIM-FCS of fluorescently labeled histones and lamin A in HeLa cell nuclei. We find that a) lamin A-mRFP and histone H2A-eGFP show significant co-mobility, indicating strong interaction inside the nucleus, and b) that the random motion of the chromatin network is subdiffusive, i.e., the effective diffusion coefficient decreases for slow time scales. Knocking out lamin A changes the diffusion back to normal. Recently, Bronshtein et al. ((2015) Nat.Comm. 6,8044) showed a similar effect by single particle tracking of telomers and isolated gene loci; however, our data show that lamin A influences the dynamics of the entire chromatin network.Our conclusion is that lamin A plays a central role for determining the elasticity of the chromatin network and to help maintaining local ordering of interphase chromosomes.