Super-resolution polarization imaging on commercial SIM systems

What is it about?

Polarization is a fundamental physical dimension of light, which has been exploited in the fields of light field modulation, quantum optics, stereoscopic filming, etc. In biology, the dipole orientation of fluorophores measured by polarization imaging reveals the orientation of targeted proteins. To study the protein orientation in subcellular structures, super-resolution polarization imaging has received increasing attention in recent years, while most techniques are still limited to labs with expertise. Recently, Peng Xi at Peking University, Qionghai Dai at Tsinghua University, and their colleagues developed polarized structured illumination microscopy (pSIM), which enables super-resolution polarization imaging on commercial SIM systems. Because of its high resolution and fast imaging speed, structured illumination microscopy (SIM) has been widely applied in live cell imaging. In most SIM systems, high-frequency structured light is generated by the interference between two linearly polarized light (similar to Young's double slit interference); the superposition of structured light and the fine structure of the sample can produce moire fringes. By detecting the Moire fringes at low frequency, the fine structure of the sample can be extracted. Borrowing this principle, Peng Xi and collaborators take the modulation of polarized light to dipoles as "angular structured light illumination", thus constructing a spatial-angular hyper-space, and extracting the azimuth angle of fluorescent dipoles in a similar way as SIM. Compared with other polarization imaging methods, pSIM has the following three advantages at the same time: high spatial resolution for ultrastructure; resolving dipole orientation of biomolecules; fast dynamics imaging in live cells.

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Why is it important?

To demonstrate the compatibility of pSIM with commercial SIM super-resolution systems, the researchers have deomstrated the polarization super-resolution imaging with 2D-SIM, 3D-SIM, and TIRF-SIM imaging capabilities. They also delivers many biological experiments to demonstrate its broad applicability, such as λ-DNA, the actin filaments in BAPE cells and mouse kidney tissue, the interaction between actin and myosin, and GFP-stained microtubules in live U2OS cells. Furthermore, the team studied the membrane-associated periodic skeleton (MPS) in neurons. pSIM overturned the conventional textbook “end-to-end” actin ring structure model, and revealed the new “side-by-side” assembly model of actin ring in MPS, taking advantage of the high spatial resolution and accurate polarization detection capability of pSIM. With the high spatial-temporal resolution inherited from SIM and the unique dipole orientation information, pSIM is highly promising in the application of answering a wide variety of biological questions in the future.

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