High-resolution spatial mapping of diffusion coefficient and compaction states of chromatin in living cells

Yi-Teng Hsiao^1*, I-Hsin Liao¹, Chia-Lung Hsieh¹

¹Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan

* Presenter:Yi-Teng Hsiao, email:tonyhsian@gmail.com

Organization and mobility of biomolecules play essential roles in various cell activities. As an example, DNA is stored in the nucleus by wrapped on histone proteins, known as chromatin. The spatial compaction state of chromatin and its dynamics are closely connected to the gene activity. The chromatin is difficult to be detected in cells without labeling because of their small size and the complex cellular environment. Current observations of chromatin are mainly based on fluorescence microscopy through labeling. However, fluorescence imaging faces the challenges of labeling artifacts and photobleaching effects. Recent advancement of optical interference microscopy enables the detection of linear scattering signals of unlabeled chromatin in living cells. In this work, we used high-speed coherent brightfield microscopy (COBRI) to directly record the chromatin fluctuation. Through temporal analysis and system calibration, we estimated the microscopic diffusion coefficient of chromatin based on the correlation rate of the dynamic scattering signal in the sub-millisecond timescale. We infer the chromatin compaction state based on the diffusion coefficient because the chromatin condensation affects the nanoscopic chromatin structures and thus chromatin diffusion. We interrogate the measured diffusion coefficient by correlating it with the strength of linear scattering signal. We found that, in the live cell nuclei, the diffusion coefficient and scattering strength follow a power law relationship, indicating that the chromatin compaction states are responsible for the spatially heterogeneous diffusion coefficient and scattering strength of cell nuclei. Finally, we validate our methods by examining the chromatin compaction change induced by chemical drug treatments. The chromatin decondensation was induced by adding the histone deacetylase inhibitor of sodium butyrate, whereas the chromatin condensation was induced by depleting adenosine triphosphate. As expected, both the diffusion coefficient and scattering strength changed upon the chemical treatment. The addition of mobility analysis to the label-free bioimaging offers valuable information for the characterization of molecular configuration at the nanoscale.

Keywords: coherent brightfield microscopy, label-free bioimaging , diffusion coefficient, chromatin