| Popis: |
Diffusion is the major short-range transport mechanism in living cells. Within individual compartments of a eukaryotic cell, such as the nucleus, mitochondria or the cytosol, biological macromolecules find their targets mostly by thermally driven random motion. For instance, specific access of DNA-binding proteins to their target sequences in the genome occurs through a sequence of three-dimensional diffusion, DNA-binding and one-dimensional search events on the DNA. The DNA/chromatin network in the cell nucleus thus has two effects on protein diffusion: obstruction due to crowding and accelerated association to specific sequences through guided diffusion along the DNA chain. The problem of target finding of proteins in the cell nucleus is only one example of diffusion-controlled reactions in a dense polymer network. Outside the direct relevance for molecular and cellular biology, the study of diffusing particles in viscoelastic media has important applications in many fields of physics. By recording fast image series of two-dimensional sections of live cells, we monitor these diffusion processes in real time and gain better understanding of the underlying physics. The method used is light sheet fluorescence microscopy followed by auto (-cross) correlation analysis. We particularly studied the random motion of chromatin and its interconnection with nucleoplasmic A-type lamins. Utilizing this method, we find that 1. Nucleoplasmic lamin As and chromatin show significant co-mobility, indicating that their motions are interconnected in the nucleus. 2. The random motion of histones H2A within the chromatin network is subdiffusive, i.e. the effective diffusion coefficient decreases for slow timescales. Knocking out lamin A changes the diffusion back to normal. Thus, lamin A influences the dynamics of the entire chromatin network. 3. A-type lamins affect the spatial organisation of chromatin inside the cellular interior. We have also attempted to develop a modelling framework that describes chromatin dynamics within the cell nucleus in the presence and absence of nucleoplasmic A-type lamins. Our conclusion is that lamin A plays a central role in determining the viscoelasticity of the chromatin network and helping to maintain local ordering of interphase chromosomes. These findings enabled us to derive a qualitative description of diffusion based on the viscoelasticity of the cellular environment. |
