In-vivo monitoring of macrophage mitochondrial pH dynamics in zebrafish using an ultrasensitive and water-soluble targeted fluorescent sensor.

Mitochondrial pH has a key role in cellular metabolism and also may be a sign of pathology. Thus, closely monitoring of minor changes in this value is critical in biological research. However, it proves difficult due to a lack of availability of appropriate probes. A water-soluble fluorescent mitochondrial targeting probe TPP-MpH is reported. The probe has a hemicyanin-based fluorophore, a triphenylphosphonium (TPP) group for mitochondrial targeting and a PEG group for increased water solubility and biocompatibility. The experimental results show that TPP-MpH reflects ultrasensitive responses to different pH values. The experimental results with cell and zebrafish models indicate that TPP-MpH is highly responsive and specific to pH levels, showing excellent stability and minimal toxicity. Confocal fluorescence imaging confirms that TPP-MpH effectively targets mitochondria and accurately monitors their pH fluctuations. To our astonishment, at pH 8.0 and 9.0, TPP-MpH showed expression in specific cells in the gallbladder and liver region of the zebrafish model, these cells are predominantly mitochondria-rich cells that are involved in the uptake of Na+ ions. Hence, these findings open the door to use this probe for further and deep biological studies.