Advancing capillary electrophoresis-mass spectrometry for top-down proteomics : technical development and biological applications

Top-down proteomics (TDP) enables the proteome profiling of biological subjects at the proteoform level and understanding of differential functions associated with proteoform heterogeneity, such as sequence variation, post-translational modifications (PTMs), etc. Drastic advances on TDP technologies (e.g. sample preparation, separation/fractionation, fragmentation, bioinformatics, etc.) have been achieved in the past decades. Further improvements in separation remain desired for better analysis throughput and deeper proteome coverage. Capillary electrophoresis (CE), including capillary zone electrophoresis (CZE) and capillary isoelectric focusing (cIEF), provide superior separation performance for proteoforms. This dissertation focuses on the advancement of CE-MS-based tools on throughput, separation resolution, and capacity for TDP and utility of these tools for biological applications.In Chapter 2, we developed high-throughput and high-capacity cIEF-MS/MS platforms. The high-throughput platform enables efficient identification and quantification of proteoforms (less than one hour per run), whereas the high-capacity cIEF-MS/MS provides large number of proteoform identifications (IDs, more than 700 proteoforms in a single shot analysis) which is valuable for deep TDP. In Chapter 3, we further improved the stability and robustness of cIEF-MS platform using optimized linear polyacrylamide (LPA) capillary coating and catholyte with lower pH (pH~10). The work achieved high-resolution characterization and accurate isoelectric point (pI) determination of charge variants (~0.1 pI difference) of monoclonal antibodies (mAbs). In Chapter 4, we developed a nondenaturing cIEF-MS platform for ultrahigh resolution characterization of microheterogeneity of a variety of protein complexes. Typically, pI determinations of variants in protein complexes allow us to decipher how sequence or PTM variations modulate the pIs of the protein complexes. In Chapter 5, while CZE-MS/MS is a well-developed approach, for the first time, we coupled FAIMS to CZE-MS/MS to facilitate online gas-phase fractionation of proteoforms. The FAIMS greatly enhanced the sensitivity of the system and expanded the number of proteoform IDs, especially large proteoform IDs. The work renders CZE-FAIMS-MS/MS as a new powerful multidimensional platform for deep TDP.In Chapters 6 and 7, we applied cIEF-MS/MS and CZE-MS/MS for studying the sexual dimorphism of zebrafish brains and proteoform-level differences between metastatic and nonmetastatic colorectal cancer (CRC) cells, respectively. In Chapter 6, quantitative TDP of thousands of proteoforms from male and female zebrafish brains by cIEF-MS/MS based approach discovered various overexpressed proteoforms in male or female brains that are closely associated with hormone activity. In Chapter 7, We performed deep TDP study of non-metastatic and metastatic CRC cells (SW480 and SW620) using CZE-MS/MS based multidimensional platform and identified more than 20,000 proteoforms of over 2,000 proteins from the two cell lines, which presents around 5-folds higher number of proteoform IDs in comparison with previous TDP studies of human cancer cells. The work revealed significant discrepancies between the two isogenic cell lines regarding proteoform and single amino acid variant (SAAV) profiles. Quantitative data disclosed differentially expressed proteoforms between the two cell lines and their corresponding genes were connected to cancer pathways and networks.