Expansion Microscopy Application for Calcium Protein Clustering Imaging in Cells and Brain Tissues

Abstract

Many biological studies require high-resolution imaging and subsequent analysis of cell organelles and molecules. Some membrane proteins form tight clusters, and this process is directly linked to their function. In most studies, these small protein clusters have been investigated by total internal reflection fluorescence (TIRF) microscopy, which enables imaging with high spatial resolution within 100 nm of the membrane surface. Recently developed expansion microscopy (ExM) makes it possible to achieve nanometer resolution using a conventional fluorescence microscope by physically expanding the sample. In this article, we describe implementation of ExM for imaging of protein clusters formed by the endoplasmic reticulum (ER) calcium sensor protein STIM1. This protein translocates during ER store depletion and forms clusters that support contact with plasma membrane (PM) calcium-channel proteins. ER calcium channels such as the type 1 inositol triphosphate receptor (IP3R) also form clusters, but their investigation by TIRF microscopy is impossible due to the large distance from the PM. In this article, we demonstrate how to investigate IP3R clustering using ExM in hippocampal brain tissues. We compare IP3R clustering in the CA1 area of the hippocampus of wild-type and 5xFAD Alzheimer's disease model mice. To facilitate future applications, we describe experimental protocols and image processing guidelines for application of ExM to membrane and ER protein clustering studies in cultured cells and brain tissues.