The Cell by Cell Chemical Characterization of the Brain: from New Tools to New Insights
In the postgenomic era, one expects the suite of chemical players in a brain region to be known and their functions uncovered. However, many cell-to-cell signaling molecules remain poorly characterized and for those that are known, their localization and dynamics are oftentimes unknown.
A suite of bioanalytical approaches are described that allow the investigation of individual neurons and small brain regions; these approaches include capillary scale separations coupled to mass spectrometry and direct mass spectrometric-based profiling and imaging. Several applications of single cell analysis are highlighted, including investigating novel serotonin neurochemistry, determining the role of d-amino acids in the brain, and characterizing the peptides in single cells.
Specifically, new serotonin-related compounds and literally hundreds of new neuropeptides have been characterized in well-defined neuronal networks, and in several cases, the functional roles of these molecules described. Imaging mass spectrometry and dynamic sampling of the extracellular environment are used for elucidating novel cell-to-cell signaling molecules in a range of neuronal model systems. To characterize rare cells, presorting approaches such as flow cytometry or other high-throughput methods that can perform large number of direct single cell measurements have advantages.
For high-throughput mass spectrometry (MS) measurements, we perform optical microscopy-guided single cell profiling. The cells are deposited onto a microscope slide, their locations on the slide determined using microscopy, and these locations guide subsequent MS analysis to each cell. We have validated these approaches by assaying populations of tens of thousands of cells including neurons and endocrine cells.
Intriguingly, our laser desorption MS does not use most of a single cell contents in a measurement, so the rare cells can be located and used for follow-up profiling such as immunohistochemical staining, metabolomics, and potentially even transcriptomic measurements. Hence MS becomes a high-throughput screening approach to locate rare cells that can be used for follow-up studies.
Our overarching goal is to uncover the complex chemical mosaic of the brain and pinpoint key cellular players in physiological and pathological processes. Several additional examples of neuropeptide and neuromodulator discovery are described across a range of metazoan life.
Sweedler has published more than 400 manuscripts and presented 400 invited lectures. He has received numerous awards including the Merck Prize and the ACS Award in analytical chemistry. He is a fellow of both the American Association for the Advancement of Science and the American Chemical Society. He is currently the editor-in-chief for the journal Analytical Chemistry, the highest ranked journal in the field of analytical chemistry.