Development and application of methods for mass spectrometry imaging of lipids across biological surfaces

In this thesis I will present the development of time of flight secondary ion mass spectrometric (ToF-SIMS) imaging methodology for biological analyses as well as applications that have yielded information about the role of lipids in membrane organization.In the first chapter, I introduce the plasma membrane and describe its fundamental role in maintaining life through the dynamic remodeling of its structure. I focus on two concepts that are believed to influence the localized chemical make up and structure of the membrane, intrinsic curvature and lipid domains.In Chapter 2 I report a protocol for the use of SIMS imaging to comparatively quantify the relative difference in cholesterol level between the plasma membranes of two cells. This development enables direct comparison of the chemical effects of different drug treatments and incubation conditions in the plasma membrane at the single-cell level. Relative, quantitative ToF-SIMS imaging was used to compare macrophage cells treated to contain elevated levels of cholesterol with respect to control cells.Chapter 3 investigates prospects for three-dimensional SIMS analysis of biological materials using model multilayer structures and single cells. Molecular depth profile studies involving dehydrated dipalmitoylphosphatidylcholine (DPPC) organic films indicate that cell membrane lipid materials do not experience significant chemical damage when bombarded with C60+ ion fluences greater than 1015 ions/cm2. Moreover, depth profile analyses of DPPC--sucrose frozen multilayer structures suggest that biomolecule information can be uncovered after the C60 + sputter removal of a 20 nm overlayer with no appreciable loss of underlying molecular signal.Two methods that were developed to increase the reproducibility of biological SIMS analysis are covered in Chapter 4. First I demonstrate the utility of the C60+ cluster ion projectile for sputter cleaning biological surfaces to reveal obscured spatio-chemical information. Following the removal of nanometers of material from the surface using sputter cleaning a frozen-patterned cholesterol film and a freeze-dried tissue sample were analyzed using ToF-SIMS imaging. In both experiments the chemical information was maintained after the sputter dose, due to the minimal chemical damage caused by C60+ bombardment. The second method covered in Chapter 4 is freeze-etching, the practice of removing excess surface water from a sample through sublimation into the vacuum of the analysis environment. This method was used to cryogenically preserve single cells for ToF-SIMS imaging analysis. By removing the excess water, which condenses onto the sample in vacuo, a uniform surface is produced that is ideal for imaging by static SIMS. I demonstrate that the conditions employed to remove deposited water do not adversely affect cell morphology and do not redistribute molecules in the top most surface layers.In Chapter 5, I describe a device which has been designed to prepare frozen, hydrated single cell cultures with a freeze fracture methodology for ToF-SIMS analysis in an ION-TOF (GmbH) TOF-SIMS IV mass spectrometer. The device reproducibly produces frozen hydrated sample surfaces for SIMS analysis. I show that SIMS analysis with the Bi32+ produces high-resolution molecular images of single PC12 cells in an ice matrix. I also show that the combination of ionization enhancements that are provided by both the ice matrix and the cluster ion source facilitates the localization of lipid ions that have not been localized in these cells previously.In Chapter 6 ToF-SIMS imaging was used to demonstrate that lipid domain formation in mating single-cell organisms is driven by changes in membrane structure. I report that time of flight secondary ion mass spectrometry images of mating Tetrahymena thermophila acquired before, during and after mating demonstrate that lipid domain formation, identified as a decrease in the lamellar lipid phosphatidylcholine, does not precede structural changes in the membrane. Rather, domains are formed in response to function during cell-to-cell conjugation. ToF-SIMS imaging has been used to collect information with wide implications in all membrane processes. (Abstract shortened by UMI.)...