Invited Article: Visualizing protein synthesis in mice with in vivo labeling of deuterated amino acids using vibrational imaging

Abstract

Proteins are one of the major components of biological systems, and understanding their metabolism is critical to study various biochemical processes in living systems. Despite extensive efforts to study protein metabolism such as autoradiography, mass spectrometry, and fluorescence microscopy, visualizing the spatial distribution of overall protein metabolism in mammals at subcellular resolution is still challenging. A recent study from our group reported imaging newly synthesized proteins in cultured mammalian cells, tissues, or even in mice using stimulated Raman scattering (SRS) microscopy coupled with metabolic labeling of deuterated amino acids (dAA). However, our previous method of dAA administration via drinking water, albeit convenient, is insufficient for in vivo studies. This is due to poor labeling efficiency and limited access to many important organs such as the brain, pancreas, or tumor. In this study, we have significantly improved and optimized the in vivo administration method by intra-carotid arterial injection of dAA in mice and obtained imaging contrast of protein metabolic activity in many more organs and tissues, such as cerebral and cerebellar cortex and hippocampal regions in the mouse brain. We also imaged newly formed proteins in the choroid plexus and pancreas at different time points, illustrating the metabolic dynamics of proteins in these important secretory organs. In addition, we visualized the metabolic heterogeneity of protein synthesis in colon tumor xenografts, which can be used to distinguish tumor and normal tissues. In summary, this combination of a new dAA administration technique and SRS imaging platform demonstrates an effective tool for the in vivo study of complex protein metabolism in mammals, in both physiological and pathological states.