pH-Assisted multichannel heat shock monitoring in the endoplasmic reticulum with a pyridinium fluorophore.

Heat shock is a global health concern as it causes permanent damage to living cells and has a relatively high mortality rate. Therefore, diagnostic tools that facilitate a better understanding of heat shock damage and the defense mechanism at the sub-cellular level are of great importance. In this report, we have demonstrated the use of a pyridinium-based fluorescent molecule, PM-ER-OH, as a 'multichannel' imaging probe to monitor the pH change associated with a heat shock in the endoplasmic reticulum. Among the three pyridinium derivatives synthesized, PM-ER-OH was chosen for study due to its excellent biocompatibility, good localization in the endoplasmic reticulum, and intracellular pH response signaled by a yellow fluorescence ( λ max = 556 nm) at acidic pH and a far red fluorescence ( λ max = 660 nm) at basic pH. By changing the excitation wavelength, we could modulate the fluorescence signal in 'turn-ON', single excitation ratiometric and 'turn-OFF' modes, making the fluorophore a 'multichannel' probe for both ex vitro and in vitro pH monitoring in the endoplasmic reticulum. The probe could efficiently monitor the pH change when heat shock was applied to cells either directly or in a pre-heated manner, which gives insight on cellular acidification caused by heat stress.