Intracellular pH imaging in cancer cells in vitro and tumors in vivo using the new genetically encoded sensor SypHer2

• We developed a method for pHi mapping in living cancer cells in vitro and in tumors in vivo.
• The novel genetically encoded indicator, SypHer2, was used.
• Intracellular pH was measured in HeLa cells in monolayer and tumor spheroids.
• We obtained fluorescence ratio maps, representing the pHi distribution, for HeLa tumors in vivo and ex vivo.
• A correspondence of the zones with higher pHi to the necrotic and hypoxic areas was demonstrated.

BackgroundMeasuring intracellular pH (pHi) in tumors is essential for the monitoring of cancer progression and the response of cancer cells to various treatments. The purpose of the study was to develop a method for pHi mapping in living cancer cells in vitro and in tumors in vivo, using the novel genetically encoded indicator, SypHer2.MethodsA HeLa Kyoto cell line stably expressing SypHer2 in the cytoplasm was used, to perform ratiometric (dual excitation) imaging of the probe in cell culture, in 3D tumor spheroids and in tumor xenografts in living mice.ResultsUsing SypHer2, pHi was demonstrated to be 7.34 ± 0.11 in monolayer HeLa cells in vitro under standard cultivation conditions. An increasing pHi gradient from the center to the periphery of the spheroids was displayed. We obtained fluorescence ratio maps for HeLa tumors in vivo and ex vivo. Comparison of the map with the pathomorphology and with hypoxia staining of the tumors revealed a correspondence of the zones with higher pHi to the necrotic and hypoxic areas.ConclusionsOur results demonstrate that pHi imaging with the genetically encoded pHi indicator, SypHer2, can be a valuable tool for evaluating tumor progression in xenograft models.General significanceWe have demonstrated, for the first time, the possibility of using the genetically encoded sensor SypHer2 for ratiometric pH imaging in cancer cells in vitro and in tumors in vivo. SypHer2 shows great promise as an instrument for pHi monitoring able to provide high accuracy and spatiotemporal resolution.