Cátia Marques: Down to single-cell metabolomics: Developments of direct infusion tools for electrospray ionization mass spectrometry

Abstract

Metabolomics provides a snapshot of the metabolic status of biological samples. This knowledge can give insights into several biological processes, including understanding the etiology and development of diseases such as cancer or type 2 diabetes. Mass spectrometry is the golden standard technique for metabolite profiling due to its high sensitivity and comprehensive analyte coverage. Although several sampling techniques are available for routine metabolomics studies, these usually require sample sizes of around 1 million cells. The work entailed in this thesis focuses on developing sampling strategies to perform metabolomic studies on challenging samples, namely, low cell number and minute volume samples, down to single cells. 

The direct infusion probe (DIP) that I developed enables the analysis of metabolites and lipids in minute sample volumes with low cell numbers. Specifically, it enables analysis of samples containing as low as 20 cells/µL in 5 µL volumes. The DIP has a user-friendly and simple design that allows for rapidly switching samples. Furthermore, it provides minimal carryover between samples by simply washing with the next sample for 8 seconds, which enhances the analysis throughput by eliminating extra washing steps. I utilized the DIP to conduct metabolite and lipid profiling of insulin-releasing cells (INS-1) exposed to high glucose. I observed that high glucose exposure causes changes in the abundance of amino acids, lipids, and other small molecules. Our results are linked with pathways, such as glycolysis, fatty acid biosynthesis, and glutamate and sphingolipid metabolisms that are altered in type 2 diabetes. 

Another analytically challenging task that I address in this thesis is the chemical analysis of individual cells. Single-cell metabolomics is crucial to decoding cellular heterogeneity and identifying phenotype variations within a cell population. I redesigned the pneumatically assisted nanospray desorption electrospray ionization (PA nano-DESI) probe to enable the analysis of metabolites and lipids from single cells. This was accomplished by reducing the inner diameter of capillaries to enhance ionization and tapering the capillaries of the probe to establish a miniaturized liquid bridge. Then, I utilized the tapered PA nano-DESI probe to profile the metabolome and lipidome of single INS-1 cells and senescent IMR-90 cells. The results show alterations in glutamate and arachidonic acid levels, characteristic of insulin release and aging processes, respectively. 

Overall, the DIP and the tapered PA nano-DESI probes represent significant analytical advances in conducting metabolomic studies of samples containing low cell numbers down to single cells.