Correlative Raman and immunofluorescence imaging reveals different protein abundance between stress granules induced by oxidative damage

Heavy metal toxicity generates reactive oxygen species (ROS) that can contribute to neurodegeneration. Oxidative damage from exposure to metals such as sodium arsenite will activate the integrated stress response and may result in the cytosolic formation of stress granules (SGs), which have been implicated in neurodegenerative disorders such as amyotrophic lateral sclerosis. Here, two different ROS sources, sodium arsenite and hydrogen peroxide, under acute (1 h) and chronic (24 h) conditions, were used to induce SG formation in human osteosarcoma (U-2 OS) cells and investigate if characteristics of SGs could depend on the induction. Specifically, correlative Raman and immunofluorescence imaging (CRIFI) was developed to evaluate the relative protein abundance found in SGs to ascertain their potential as loci for protein accumulation. Interestingly, while there are differences in the punctate-staining phenotypes for different stressors, two types of puncta visualized by CRIFI were common to all treatment conditions, where notably a subset exhibited protein concentration above cytosolic background, indicating that only some SGs are composed of protein-rich, dense phases. Differences in protein abundance between SGs were also observed within a single cell, suggesting that individual SGs can develop differently. These results demonstrate the versatility and the strength of pairing Raman spectroscopy, which allows for probe-free detection of different chemical functional groups, with specific protein localization granted by immunofluorescence, providing new cellular insights unattainable by either modality alone.