專題演講

2021/01/14(Thu)     10:30 -11:30    視訊會議室(圖書館)3F Conference Room(Library)

Title

Label-free optical imaging of chromatin dynamics in live cells

Speaker

Dr. Chia-Lung Hsieh

Institute of Atomic and Molecular Sciences, Academia Sinica

Abstract

Optical microscope imaging is a unique tool to measure biological structures and dynamics at high spatiotemporal resolutions. Current bioimaging has relied on fluorescence labeling because of its superb sensitivity and its high specificity. However, it has encountered several challenges and fundamental limitations, such as photobleaching, blinking, and saturation. Immunostaining requires sample fixation, which interrupts continuous observations of cell dynamics. Recent advance in scattering-based imaging has circumvented the fundamental problems of fluorescence and thus become a powerful imaging modality. For example, by detecting linear scattering light through interference, single biomolecules can be detected without labeling. Despite the great success of scattering-based imaging, previous highly sensitive measurements were performed on purified samples in well controlled environments of clean coverglass. It remains unclear how scattering-based imaging could be operated for live cells where the sample is densely packed with a great diversity of molecules. Moreover, unlike fluorescence labeling, label-free linear scattering signal does not have specificity (every biomolecule scatters light!), which greatly complicates the data interpretation.



In this talk, I will introduce our approach to acquiring high-resolution image of cell nucleus in a label-free manner. The cell nucleus is densely packed with DNA and proteins. In particular, the DNA is wrapped on histone proteins, which together are known as chromatin. We use coherent brightfield microscopy (COBRI) developed in our lab in the past few years to measure the intrinsic scattering signal of chromatin through interference. A COBRI video was recorded at a high speed (>1000 Hz) for a few seconds that captures the dynamic scattering signal of the diffusive chromatin. We establish a model to interpret the fluctuating COBRI signal from which the local dynamic properties of chromatin can be estimated (including size, volume fraction, and diffusion constant). The model is validated by measuring the dynamic properties of nanoparticle colloidal. Using our methods, we are able to map the local chromatin dynamics at high spatiotemporal resolutions. In addition, since our COBRI imaging does not suffer photobleaching, long-term, high-speed chromatin imaging becomes possible. Our data show distinct dynamics between nucleoli and nucleoplasm. Within nucleoplasm regions, we are able to distinguish condensation states of chromatin, i.e., heterochromatin and euchromatin. Our method enables continuous observation of chromatin organization and dynamics in live cells without the need of labeling, showing promise of exploring nuclear biology over a long time and in a continuous manner.