Synchrotron infrared measurements of protein phosphorylation in living single PC12 cells during neuronal differentiation.

Protein phosphorylation is a post-translational modification that is essential for the regulation of many important cellular activities, including proliferation and differentiation. Current techniques for detecting protein phosphorylation in single cells often involve the use of fluorescence markers, such as antibodies or genetically expressed proteins. In contrast, infrared spectroscopy is a label-free and noninvasive analytical technique that can monitor the intrinsic vibrational signatures of chemical bonds. Here, we provide direct evidence that protein phosphorylation in individual living mammalian cells can be measured with synchrotron radiation-based Fourier transform-infrared (SR-FT-IR) spectromicroscopy. We show that PC12 cells stimulated with nerve growth factor (NGF) exhibit statistically significant temporal variations in specific spectral features, correlating with changes in protein phosphorylation levels and the subsequent development of neuron-like phenotypes in the cells. The spectral phosphorylation markers were confirmed by bimodal (FT-IR/fluorescence) imaging of fluorescently marked PC12 cells with sustained protein phosphorylation activity. Our results open up new possibilities for the label-free real-time monitoring of protein phosphorylation inside cells. Furthermore, the multimolecule sensitivity of this technique will be useful for unraveling the associated molecular changes during cellular signaling and response processes.