Mass spectrometry–based methodologies for single-cell metabolite detection and identification

Metabolomic studies strive to determine the small molecule composition (e.g., sugars, lipids, and amino acids, but not peptides, proteins, DNA, and RNA) of a biological sample in a given state. Information generated from the sample, which is often complementary to gene expression and proteomic data, serves to link chemical content to the observed phenotype. Metabolomics is often used to examine metabolic differences between a control and an altered state, such as normal and diseased (Sreekumar et al., 2009; Bathen et al., 2010; Madsen et al., 2010). Metabolites are typically measured in complex, multicellular biological sample matrices – blood, serum, urine, and tissue biopsies. Diverse cell types can be present within these specimens; for instance, brain tissue consists of neurons, glia, and other cell types. Furthermore, cell-to-cell heterogeneity is known to exist, even within the same cell type (Raj and van Oudenaarden, 2008; Altschuler and Wu, 2010; Lidstrom and Konopka, 2010; Paszek et al., 2010; Spiller et al., 2010), especially in the nervous system (Chan-Palay et al., 1981; Kamme et al., 2003; Zhang and Barres, 2010). When performed at single-cell levels, chemical analysis is beneficial for improving the fundamental understanding of the underlying biochemistry of biological systems. Significant technological advances in single-cell analysis and understanding the crucial role of this technology in answering biological and medical questions have resulted in widespread applications in the field of metabolomics (Borland et al., 2008; Schmid et al., 2010; Wang and Bodovitz, 2010; Rubakhin et al., 2011) (for examples of nuclear magnetic resonance [NMR]-based methods, see Chapter 6). Electrochemical detection is a highly sensitive technique that has been applied to monitoring cellular exocytosis and secretion (Leszczyszyn et al., 1991; Pihel et al., 1994; Schulte and Schuhmann, 2007; Amatore et al., 2008; Ge et al., 2010). Fluorescence detection is also suitable for single-cell investigations and has been implemented to study the metabolism of particular pathways by fluorescently tagging an analyte of interest within a cell and observing the resulting enzymatic products (Cohen et al., 2008). Single-cell separations have also been accomplished using capillary-scale separations with electrochemical and fluorescence detection, enabling characterization of multiple analytes (Kennedy et al., 1989; Olefirowicz and Ewing, 1990).