Expansion microscopy : scalable and multiplexed nanoscale imaging

Abstract

Microscopy has facilitated the discovery of many biological insights by optically magnifying small structures in cells and tissues. However, the resolution of optical microscopy is limited by the diffraction of light to ~200-300 nm, comparable or larger to the size of many subcellular structures. In this thesis, we describe a suite of tools based on a novel super-resolution microscopy approach called Expansion microscopy. Expansion microscopy (ExM) physically expands tissues so that the resolution of ordinary microscopes is increased -5 times by leveraging the swelling properties of polyelectrolyte hydrogels. Ordinary microscopes used with ExM are more accessible and faster than the specialized optical systems designed to image beyond the diffraction limit (e.g., STORM/PALM, STED, SIM), while yielding similar performance. Expanded tissues are also optically clear, allowing for unprecedented super-resolution imaging in thick tissues and facile reagent diffusion into the sample. We have since developed a variant of ExM, called protein retention ExM, in which proteins are directly anchored to the swellable gel using a commercially available cross-linking molecule. This strategy enables ExM of genetically encoded fluorescent proteins and commercial fluorescently labeled secondary antibodies. With these advancements, ExM can be carried out with purely commercial reagents and represents a simple extension of standard histological methods used to prepare samples for imaging. Furthermore, we have developed a variant of the ExM technology that enables RNA molecules to be directly linked to the ExM gel network via a small molecule linker and isotropic expansion. This technology, termed ExFISH, enables visualization of RNAs with nanoscale precision and single molecule resolution. We have demonstrated that the covalent anchoring of RNA also enables robust repeated washing and probe hybridization steps, opening the door to combinatorial multiplexing strategies. By leveraging these benefits, we have further developed in situ analysis tools which allow for highly multiplexed imaging of RNA identity and location with nanoscale precision in intact tissues. Taken together, these tools allow for spatially mapping molecular information onto cell types and tissue structures which could be invaluable for spatially complex biological processes such as brain function, cancer heterogeneity and organismal development.