Nanosims Tools and Probes for Imaging Intracellular Structures

Abstract

In eukaryotic cells, intracellular lipid membranes act as barriers that segregate specific chemical reactions into organelles for increased biochemical efficiency. Organelle efficiency is maintained by the regulation of cholesterol and different lipid distributions within these organelles and is tightly controlled by intracellular transport and lipid metabolism. Disturbances in intracellular lipid trafficking, biosynthesis and degradation are hallmarks of fatal diseases involving an accumulation of cholesterol or lipids in endosomes and lysosomes, such as Niemann Pick Type II disease. Efforts to elucidate the mechanisms for lipid-mediated cellular (dys)function would benefit from an imaging technique that enables assessing the relative abundances of cholesterol and lipid species of interest between organelles. Towards this aim, we developed a secondary ion mass spectrometry (SIMS) approach to image the sphingolipid and cholesterol distributions within intact cells in three dimensions with a NanoSIMS instrument. Because the components of interest must produce secondary ions that contain distinctive elements or isotopes to permit their detection with a NanoSIMS instrument, organelle-specific labels must be incorporated into the cells to enable identifying specific organelles. This work describes a strategy that utilizes small protein tags and functionalized substrates that permit identify organelles of interest with NanoSIMS, and the development of data processing tools that produce images of these organelles at the proper depth in the 3D SIMS images. We created a transfected cell line that stably expresses enzymatic proteins fused to organelle-specific proteins. These proteins covalently attach to functionalized small-molecule substrates which can be detected with NanoSIMS and fluorescence microscopy. By combining these tools for organelle detection with lipid-specific metabolic isotope incorporation, semi-quantitative information concerning the abundance of cholesterol or specific lipids within organelles of interest may be obtained.