Fast monitoring of in-vivo conformational changes in myosin using single scan polarization-SHG microscopy

Fast imaging of molecular changes under high-resolution and label-free conditions are essential for understanding in-vivo processes, however, current techniques are not able to monitor such changes in real time. Polarization sensitive second harmonic generation (PSHG) imaging is a minimally invasive optical microscopy technique capable of quantifying molecular conformational changes occurring below the diffraction limit. Up to now, such information is generally retrieved by exciting the sample with different linear polarizations. This procedure requires the sample to remain static during measurements (from a few second to minutes), preventing the use of PSHG microscopy from studying moving samples or molecular dynamics in living organisms. Here we demonstrate an imaging method that is one order of magnitude faster than conventional PSHG. Based on circular polarization excitation and instantaneous polarimetry analysis of the second harmonic signal generated in the tissue, the method is able to instantaneously obtain molecular information within a pixel dwell time. As a consequence, a single scan is only required to retrieve all the information. This allowed us to perform PSHG imaging in moving C. elegans, monitoring myosin's dynamics during the muscular contraction and relaxation. Since the method provides images of the molecular state, an unprecedented global understanding of the muscles dynamics is possible by correlating changes in different regions of the sample.