One Scaffold, Different Organelle Sensors: pH‐Activable Fluorescent Probes for Targeting Live Microglial Cell Organelles**

Abstract

Targeting live cell organelles is essential for imaging, understanding, and controlling specific biochemical processes. Typically, fluorescent probes with distinct structural scaffolds are used to target specific cell organelles. Here, we have designed a modular one‐step synthetic strategy using a common reaction intermediate to develop new lysosomal, mitochondrial, and nucleus‐targeting pH‐activable fluorescent probes that are all based on a single boron dipyrromethane scaffold. The divergent cell organelle targeting was achieved by synthesizing probes with specific functional group changes to the central scaffold resulting in differential fluorescence and pKa. Specifically, we show that the functional group transformation of the same scaffold influences cellular localization and specificity of pH‐activable fluorescent probes in live primary microglial cells with pKa values ranging from ∼3.2–6.0. We introduce a structure‐organelle‐relationship (SOR) framework to target nuclei (NucShine), lysosomes (LysoShine), and mitochondria (MitoShine) in live microglia. This work will result in future applications of SOR beyond imaging to target and control organelle‐specific biochemical processes in disease‐specific models.

Ratiometric fluorescent probes for pH mapping in cellular organelles

“…47,48,55,56 Several fluorescent probes have been developed over the last few decades to monitor the ΔpHi inside living systems. 55–62 However, much less probes have been developed based on the ratiometric concept. Therefore, it is essential to summarize the recent developments in ratiometric fluorescent probes and critically evaluate the lack of efficient probes.…”

“…Therefore, it is essential to summarize the recent developments in ratiometric fluorescent probes and critically evaluate the lack of efficient probes. Although a few review articles focused on organelle-targeting bio-analyte sensing, 56,58,60,62–64 no review article is available on organelle-specific ratiometric pH probes for bioimaging to date. Thus, herein, we summarize the developed organelle-specific ratiometric fluorescent probes, emphasizing their design, photophysical properties, and biological application, including precise subcellular pH mapping and quantification of ΔpHi (Table 1).…”

Mitochondrial Targeting and Imaging with Small Organic Conjugated Fluorophores: A Review

“…[107] This observation could be in fact attributed to known large uncertainties in the relative determination of this photophysical parameter and not to arise from structural contributions. [108] Given the length of the linker, it was demonstrated in 56 and 57 that whilst substitution of quinolinum in 55 for pyridinium (56) and benzothiazolium (57) ensured mitochondrial accumulation, it bears little effect on the photophysical properties. [109,99] Selective mitochondrial targeting in coumarin systems has also been facilitated by means of pyrrolidinium cation.…”

“…Synthetically, the way this class of materials is afforded is dictated by the overall symmetry of the molecule. Symmetric systems[ 34 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 ] can be accessed by reaction of the desired substituted pyrrole with an aldehyde or an acid chloride/anhydride. [ 68 , 69 , 70 , 71 , 72 ] In both cases, the first synthetic step is the generation of the relevant dipyrromethene intermediate.…”

“… [58] Similarly, increments in Stokes shifts were accessed for 31 and 32 , both bearing terminal triphenylphosphonium units. [ 56 , 77 ] These systems indicate that greater Stokes shifts can also be accessed via larger peripheral substitutions on other than the meso position. Utilization of trifluoromethyl substitution on neutral BODIPY based architectures ( 33 and 34 ) resulted in subcellular accumulation within the mitochondria but also in the lysosomes.…”