Mass spectrometry imaging and profiling of single cells

Abstract

Mass spectrometry imaging and profiling of individual cells and subcellular structures provide unique analytical capabilities for biological and biomedical research, including determination of the biochemical heterogeneity of cellular populations and intracellular localization of pharmaceuticals. Two mass spectrometry technologies-secondary ion mass spectrometry (SIMS) and matrix assisted laser desorption/ionization mass spectrometry (MALDI MS)-are most often used in micro-bioanalytical investigations. Recent advances in ion probe technologies have increased the dynamic range and sensitivity of analyte detection by SIMS, allowing two- and three-dimensional localization of analytes in a variety of cells. SIMS operating in the mass spectrometry imaging (MSI) mode can routinely reach spatial resolutions at the submicron level; therefore, it is frequently used in studies of the chemical composition of subcellular structures. MALDI MS offers a large mass range and high sensitivity of analyte detection. It has been successfully applied in a variety of single-cell and organelle profiling studies. Innovative instrumentation such as scanning microprobe MALDI and mass microscope spectrometers enables new subcellular MSI measurements. Other approaches for MS-based chemical imaging and profiling include those based on near-field laser ablation and inductively-coupled plasma MS analysis, which offer complementary capabilities for subcellular chemical imaging and profiling.

Figure 1. High resolution dynamic SIMS imaging reveals gold complex distribution within single human breast cancer cells

The CN⁻ ion (top left) reveals overall cellular structure while P⁻ (bottom left) shows nucleic acid distribution. In other cells after treatment with a gold-containing anticancer complex, superimposed Au⁻ and P⁻ images (center and right) indicate that the Au accumulates as ~200 nm aggregates in and around the nucleus, segregated clearly from the DNA. Adapted from ref. [38]. Reproduced by permission of The Royal Society of Chemistry.

Figure 2. Static SIMS imaging of a large protein at cell-scale resolution

Thyroglobulin (660 kDa) in thyroid gland tissue is visualized using static matrix-enhanced SIMS after on-tissue trypsin digest. An Si⁺ ion image (left) of sectioned tissue reveals cell morphology since removal of colloid within the cells exposes the underlying silicon substrate. Summed signals of the detected tryptic peptides generates an ion image (middle) indicating protein localization along the epithelial cell borders. A mass spectrum (right) from the tissue on the right shows labeled tryptic peptides in the m/z 450–900 range. Images are represented in false-color scale ranging from black (low signal) through red to yellow (high signal); field of view is 500 × 500 μm. Adapted with permission from ref. [87], copyright 2010, John Wiley and Sons.

Figure 3. Single-organelle mass profiling with MALDI

Individual secretory granules from the A. californica atrial gland (left, TEM of tissue cross-section) are isolated manually using micropipette (right, video image) and prepared with matrix for MALDI-TOF analysis. Several peptides contained within the single granule are detected as shown in the mass spectrum (bottom). Scale bars are 10 μm. Adapted with permission from ref. [137], Macmillan Publishers Ltd: Nature Biotechnology, copyright 2000.

Figure 4. MALDI MSI at subcellular spatial resolution

MALDI analysis of cultured human renal cancer cells allows visualization of analytes across an extended m/z range relative to SIMS; vapor deposition of matrix permits 2 μm effective spatial resolution. Two ion images are overlaid (m/z 551 in red false color and m/z 4933 in greyscale) to reveal differences in the profiles of adjacent cells. Field of view shown is 100 × 100 μm. Adapted with permission from ref. [143], copyright 2010, John Wiley and Sons.