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Probing protein dynamics in vivo using non-canonical amino acid labeling
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The cellular protein pool exists in a state of dynamic equilibrium, such that a balance between protein synthesis and degradation is maintained to sustain protein homeostasis. This equilibrium is essential for normal cellular functions and hence alteration in protein dynamics has several pathological implications in developing and adult tissues. Recent progress in mass spectrometry (MS) and metabolic labeling techniques has advanced our understanding of the mechanisms of protein regulation in cultured cells and less complicated multicellular organisms. However, methods for the analysis of the dynamics of intra- and extra-cellular proteins in embryonic and adult tissues remain lacking.
To address this gap, we developed a metabolic labeling technique that enables labeling the nascent murine proteome via injection of non-canonical amino acids (ncAAs), which can be selectively enriched by “clickable” tags for identification and quantification. Using this technique, we developed a MS-based method for the selective identification and quantification of the intra- and extra-cellular newly synthesized proteins in developing murine tissues. We then applied this technique to study the dynamic regulation of extracellular matrix (ECM) proteins during embryonic and adolescent musculoskeletal development. We show that the applied technique enables resolving differences in the nascent proteome of different developmental time points with high temporal resolution. The technique can also reveal protein dynamic information that cannot be captured by the traditional proteomic techniques. Additionally, we identified key ECM components that play roles in musculoskeletal development to provide insights into the mechanisms of musculoskeletal tissue regeneration.
To fully characterize our labeling technique, we developed a mathematical model to describe the biodistribution kinetics of azidohomoalanine (Aha), the most widely used ncAA, in murine tissues. The model enabled measuring the relative rates of protein synthesis and turnover in different tissues and predicting the effect of different dosing regimens of Aha on the degree of protein labeling. Finally, we analyzed the plasma metabolome of Aha-injected mice to investigate the impact of Aha incorporation on normal physiology. The analysis revealed that Aha administration into mice does not significantly perturb metabolic functions. Taken together, the findings presented in this dissertation demonstrate the utility of the ncAA labeling technique in mapping protein dynamics in mammalian tissues. This will ultimately have a significant impact on our understanding of protein regulation in health and disease.