Imaging the distributions of lipids and proteins in the plasma membrane with high-resolution secondary ion mass spectrometry

Characterizing the distributions of distinct lipid and protein species within biological membranes is an important step toward elucidating the roles of these molecules in membrane function. Ideally, the lipids and proteins of interest could be imaged under native conditions with high spatial resolution and without labels that might alter their distributions and functions. While no single technique meets all these criteria, a variety of complementary imaging modalities, each with its own unique benefits and limitations, have yielded new insight into biological membrane structure. In this chapter, we review one of these techniques, highresolution secondary ion mass spectrometry (SIMS) performed on a commercial Cameca NanoSIMS 50(L) instrument. High-resolution SIMS reveals the elemental and isotopic distributions at the surface of a sample with a lateral resolution better than 100 nm and a depth resolution as good as a few nanometers. First, we explain the fundamental principles of this technique, emphasizing the aspects that affect the application of SIMS to imaging the distributions of distinct lipid and protein species in the plasma membranes of intact mammalian cells. Then we describe the sample preparation, imaging conditions, and data analysis strategies that have been developed for cell membrane analysis. The data analysis strategies include statistical approaches for quantitatively defining membrane domains enriched with a component of interest, and quantifying co-localization between different membrane species. We also discuss the potential artifacts that may arise when imaging component distributions in the membranes of mammalian cells with high-resolution SIMS, control experiments for detecting these artifacts, and strategies for avoiding them. Examples drawn from our reported results are provided to illustrate key concepts in this chapter and demonstrate how this technique may be used to improve understanding of plasma membrane organization. Finally, we discuss future directions for using high-resolution SIMS to elucidate membrane structure-function relationships.