Visualizing intraorganellar ultrastructures, dynamics, and interactions with open-access background-free Lock-in-SIM.

Structured illumination microscopy (SIM) is a powerful method for fast and gentle live-cell super-resolution imaging. However, its susceptibility to reconstruction artifacts from out-of-focus blur and background imposes substantial barriers to analyze the dynamics of densely packed volumetric intraorganellar ultrastructures that are typically in a size range of SIM's spatial resolution. To address this limitation, we have developed Lock-in-SIM, an open-access two-dimensional SIM framework that eliminates background and maximizes the recovery of sub-diffraction information with the highest possible frequency extraction. By leveraging the intrinsic modulation differences of volumetric sample structures, Lock-in-SIM enables efficient optical sectioning, extends imaging depth, and improves data fidelity and quantifiability. We demonstrate the superiority of Lock-in-SIM by visualizing various challenging intraorganellar ultrastructures in live cells. Our investigations uncover mechanisms of mitochondrial fission and endoplasmic reticulum-lysosome interactions and provide insights into the intricate yet highly regulated structural remodeling of organelles.