Static SIMS Imaging of Liposomes and Cells

Sara G. Ostrowski, Thomas P. Roddy, Christopher W. Szakal, Nicholas Winograd, Andrew G. Ewing

A wide variety of interesting biological processes occur at cellular membranes. Regulated exocytosis is one such membrane-mediated event and involves the release of a cell’s vesicular content to the extracellular space. A better understanding of the driving forces behind exocytosis is of importance because irregularities in exocytosis have been associated with various diseases, like Parkinson’s Disease. Although membrane proteins have been implicated as key players in exocytosis, it is highly probable that membrane lipids have an important role as well. Static time-of-flight secondary ion mass spectrometry (ToF-SIMS) can capture molecule-specific images of cryogenically preserved liposome and single cell membranes. With this capability, we have been developing ToF-SIMS imaging with the long-term goal being to elucidate the suggested involvement of lipid domains in exocytosis.

At the current time, SIMS spectra for various cellular lipids have been characterized using thick film standard samples and the unique mass spectral peaks have allowed certain lipids to be imaged across the membranes of single cell. Rat pheochromocytoma (PC12) cells are a particularly useful cell system to study because they model neurons. Using PC12 cells, chemical signatures have been identified which determine the membrane leaflet being probed. Molecule-specific images of other cell systems, such as paramecia and tetrahymena, have demonstrated the single cell resolution of static SIMS. In spite of these advances, improvements in secondary ion yield will be crucial to further investigate lipid domains with SIMS. Polyatomic primary ion sources have been shown to increase the signal of high mass secondary ions by several orders of magnitude. Therefore, the benefit of a buckminsterfullerene (C₆₀⁺) primary ion source for biological samples is being explored and shows great promise.