Investigating Lipid Heterogeneity in Single Cells Using Secondary Ion Time-of-Flight Mass Spectrometry

Open Access
Author:
Piehowski, Paul David
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 11, 2009
Committee Members:
  • Andrew Ewing, Dissertation Advisor
  • Andrew Ewing, Committee Chair
  • Nicholas Winograd, Committee Member
  • Christine Dolan Keating, Committee Member
  • Carlo G Pantano, Committee Member
  • Michael Heien, Committee Member
Keywords:
  • Cholesterol
  • ToF-SIMS
  • Single Cell Analysis
  • Imaging Mass Spectrometry
  • Cell Membrane
Abstract:
Imaging time-of-flight secondary ion mass spectrometry (ToF-SIMS) can be utilized to map the spatial distribution of small molecules on a surface with potentially submicron resolution. Due to the inherent characteristics of this technique and its potential to provide higher spatial resolution than light microscopy based techniques without the use of chemical labels, it has been utilized to study the distribution of phospholipid species in the cell membrane. It is now known that many cell membranes contain transient compositional heterogeneities, colloquially referred to as domains, which participate in vital physiological processes such as exocytosis and signal transduction. Because of their size and lifetime, much remains unknown about the nature of these heterogeneities. ToF-SIMS imaging combined with cryogenic sample preparation techniques is a promising analytical platform poised to contribute greatly to this growing field of study. Sample preparation is crucial to obtaining quality lipid distribution maps, especially when dealing with single biological cells. To achieve this end the Winograd and Ewing groups have developed a freeze-fracture methodology adapted from cryo-SEM studies. Freeze-etching, the practice of removing excess surface water from a sample through sublimation into the vacuum of the analysis environment, has also been extensively used in conjunction with electron microscopy. This technique has been applied to ToF-SIMS imaging of cryogenically preserved single cells. 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 topmost surface layers. In addition, I found that water can be controllably re-deposited onto the sample at temperatures below -100 oC in vacuum. The re-deposited water increases the ionization of characteristic fragments of biologically interesting molecules 2-fold without loss of spatial resolution. The utilization of freeze-etch methodology will increase the reliability of cryogenic sample preparations for SIMS analysis by providing greater control of the surface environment. Using these procedures, high quality spectra with both atomic bombardment as well as C60+ cluster ion bombardment, have been obtained. To date, many cell imaging studies have concentrated on phosphatidylcholine distributions, owing to its abundance and high ionization efficiency. However, cholesterol is a particularly interesting molecule due to its involvement in numerous biological processes. For many studies, the effectiveness of chemical mapping is limited by low signal intensity from various bio-molecules. Due to the high energy nature of the SIMS ionization process, many molecules are identified by detection of characteristic fragments. Commonly, fragments of a molecule are identified using standard samples, and those fragments are used to map the location of the molecule. MS/MS data obtained from a prototype C60+/ quadrupole time-of-flight mass spectrometer was used in conjunction with indium LMIG imaging to map previously unrecognized cholesterol fragments in single cells. A model system of J774 macrophages doped with cholesterol was used to show that these fragments are derived from cholesterol in cell imaging experiments. Examination of relative quantification experiments reveals that m/z 147 is the most specific diagnostic fragment and offers a 3-fold signal enhancement. These findings greatly increase the prospects for cholesterol mapping experiments in biological samples, particularly with single cell experiments. In addition, these findings demonstrate the wealth of information that is hidden in the traditional ToF-SIMS spectrum. In order for this technique to provide insight into biological processes, it is critical to characterize the figures of merit. Because a SIMS instrument counts individual events, the precision of the measurement is controlled by counting statistics. As the analysis area decreases, the number of molecules available for analysis diminishes. This becomes critical when imaging sub-cellular features; it limits the information obtainable, resulting in images with only a few counts of interest per pixel. Many features observed in low intensity images are artifacts of counting statistics, making validation of these features crucial to arriving at accurate conclusions. With ToF-SIMS imaging, the experimentally attainable spatial resolution is a function of the molecule of interest, sample matrix, concentration, primary ion, instrument transmission, and spot size of the primary ion beam. A model, based on Poisson statistics, has been developed to validate SIMS imaging data when signal is limited. This model can be used to estimate the effective spatial resolution and limits of detection prior to analysis, making it a powerful tool for tailoring future investigations. In addition, the model allows for pixel-to-pixel intensity comparisons and can be used to validate the significance of observed image features. The implications and capabilities of the model are demonstrated here by imaging the cell membrane of resting RBL-2H3 mast cells. Mass spectrometry imaging has been used to demonstrate that changes in membrane structure drive lipid domain formation in mating single-cell organisms. Chemical studies of lipid bilayers in both living and model systems have revealed that chemical composition is coupled to localized membrane structure. However, it is not clear if the lipids that compose the membrane actively modify membrane structure or if structural changes cause heterogeneity in the surface chemistry of the lipid bilayer. ToF-SIMS images of mating Tetrahymena thermophila, acquired at various stages during mating, can be used to demonstrate that lipid domain formation follows rather than precedes structural changes in the membrane. Domains are formed in response to structural changes that occur during cell-to-cell conjugation. This observation has wide implications in all membrane processes. There is considerable interest in the unique properties of cluster ion projectiles and investigations of how they may be utilized to improve biological imaging. A C60+ cluster ion projectile was employed for sputter cleaning biological surfaces to reveal spatio-chemical information obscured by contamination overlayers. This protocol is used as a supplemental sample preparation method for time of flight secondary ion mass spectrometry (ToF-SIMS) imaging of frozen and freeze dried biological materials. 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 damage to the surface produced by freeze-drying the tissue sample was found to have a greater effect on the loss of cholesterol signal than the sputter-induced damage. In addition to maintaining the chemical information, sputtering is not found to alter the spatial distribution of molecules on the surface. This approach removes artifacts that may obscure the surface chemistry of the sample and are common to many biological sample preparation schemes for ToF-SIMS imaging. In general, out results show that by removing these artifacts, the number of analyzable samples for SIMS imaging is greatly expanded. Although imaging with sub-cellular spatial resolution has been demonstrated, it is clear that the success of future experiments is limited by the ionization efficiency of the lipids, as well as limitations imposed by a coaxial ToF geometry. Considerable work has been done in the lab, to address these limitations. This effort has resulted in the development of a hybrid quadrupole orthogonal ToF instrument equipped with a C60+ primary ion source. The capabilities and potential of this new platform will greatly increase the contributions of SIMS to the biological sciences.