## 5 pH in Cellular Organelles
During the so-called process of de-granulation, lytic granules and synaptic vesicles release their content when fusing to the plasma membrane [62]. The dynamics of secretory vesicles have been studied using DsRed-FasL-e\({}_{\mathrm{s}}\)-pHluorin for visualizing fusion pore openings [62], VGLUT1-2 xmOrange2, VGLUT1-pH, and synapto-pHluorin and SytIV-pHluorin for monitoring re-acidification of synaptic vesicles after endocytosis [11, 13, 14, 23, 24, 63, 64, 65, 66]. Since synaptophysin displays less background expression on the plasma membrane than synaptobrevin/VAMP2, synaptophysin is used for an improved sypHTomato reporter of vesicular turnover [24]. [PERSON] _et al_. [67] used e-pHluorin to report the location and pH of organelles to which the synaptotagmin isoforms are targeted. Among 15 isoforms, only syt-1 and 2 are targeted to synaptic vesicles; the other 13 are located in dendrites and axons. Spatial and temporal pH changes have been analyzed for many specialized vesicles, such as insulin secretory granules [68], large dense-core vesicles [45] and maturing endosomes [4, 50, 59]. pHluorin, ECFP, and E\({}^{1}\)GFP are targeted to vesicles through fusing the pH sensor to luminal site of proteins that are characteristic of those vesicles [4, 30, 50, 56, 59, 68, 69]. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Sensor** & **Tag** & **pH** & **Organism/Cell Type** & **Instrument\({}^{\mathrm{a}}\)** & **Reference** \\ \hline \multirow{3}{*}{_Medial/trans_} & GT-EGFP, & \(\beta\)-1, & & & & \\  & GT-ECFP, & 4-galactosyltransferase & 6.6 & HeLa CHO-K1, & & Micro & [11, 12] \\  & GT-EYFP & (N-terminal 81 aa) & & & & \\  & GT-EGFP/ & \(\beta\)-1, & & & & \\  & GT-ECFP & \(4\)-galactosyltransferase & 6.4 & HeLa cells & Micro & [61] \\  & GT-EYFP/ & \(\beta\)-1, & & & & \\  & GT-ECFP & \(4\)-galactosyltransferase & 6.4 & HeLa cells & Micro & [61] \\  & GT-EGFP & \(\beta\)-1, & & & Cancer cells MCF-7, & & \\  & GT-EGFP & \(4\)-galactosyltransferase & & & HT-29, SW-48 cells & & \\  & GT-EGFP & \(\beta\)-1, & & & COS7, CaCo2 cells, & & \\  & \(4\)-galactosyltransferase & & & & fibroblasts & & [57] \\  & \(\mathrm{\textit{Medial}}\)/trans & & 2,6-silyltransferase & & & & \\  & ST-pHERP & \( \begin{array}{c}2,6-\\ (1-70 aa)\end{array} \) & \(\begin{array}{c}6.4-6. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Sensor** & **Tag** & **pH** & **Organism/Cell Type** & **Instrument\({}^{\text{a}}\)** & **Reference** \\ \hline Perforin & & & & & & \\ containing & DsRed-FasL- & & & & & \\ granules & pHluorin & & & & & \\ BDNF & & & & & & \\ containing & & & & & & \\ vesicles and & & & & & & \\ Golgi & & & & & & \\ Synaptic & & & & & & \\ vesicles & & & & & & \\ Synaptic & & & & & & \\ vesicles & & & & & & \\ Synaptic & & & & & & \\ vesicles & & & & & & \\ Synaptic & & & & & & \\ vesicles & & & & & \\ Synaptic & & & & & & \\ Vesicles & & & & & \\ Synaptic & & & & & & \\ Vesicles & & & & & \\ Synaptic & & & & & \\ Synaptic & & & & & \\ vesicles & & & & & \\ Synaptic & & & & & \\ vesicles & & & & & \\ \hline \end{tabular}
\end{table} 
Table 6: Select examples of pH-sensitive FPs targeted to subcellular compartments of secretory and endocytic pathway.