An in-cell NMR study of monitoring stress-induced increase of cytosolic Ca2+ concentration in HeLa cells

Time-resolved measurements of 2D 1H-15N SOFAST-HMQC experiments of calbindin D9k (P47M + C80) in HeLa cells showed time-dependent changes in the cross-peak patterns in the spectra. Comparison with in vitro assignments revealed that calbindin D9k (P47M + C80) is initially in the Mg2+-bound state, and then gradually converted to the Ca2+-bound state. This conversion process initiates after NMR sample preparation. These results showed, for the first time, that cells inside the NMR tube were stressed, presumably because of cell precipitation, the lack of oxygen and nutrients, etc., thereby releasing Ca2+ into cytosol during the measurements. The results demonstrated that in-cell NMR can monitor the state transitions of stimulated cells through the observation of proteins involved in the intracellular signalling systems. Our method provides a very useful tool for in situ monitoring of the “healthiness” of the cells in various in-cell NMR studies.