Ratiometric Imaging for Quantification of Elevated Ca2+ in Neurons Using Synthetic Low-Affinity Fluorescent Probe

The availability of various calcium ion (Ca2+) fluorescent probes has contributed to revealing physiological events related to intracellular Ca2+. However, conventional probes face challenges for quantitatively and selectively visualizing high Ca2+ concentrations in cells induced by any stimuli, including biomolecules or electrical signal that disrupt Ca2+ homeostasis. In this report, we designed and synthesized a low-affinity ratiometric Ca2+ probe, KLCA-Fura, utilizing o-aminophenol-N,N-diacetate-O-methylene-methylphosphinate (APDAP) as a ligand, for which we recently demonstrated the suitability as a new low-affinity ligand for Ca2+. KLCA-Fura showed a blue shift in excitation wavelength with increasing Ca2+ concentration based on the intramolecular charge transfer (ICT). Its affinity for Ca2+ is lower than commercially available conventional Ca2+ probes. Furthermore, the selectivity for Ca2+ and the fluorescence intensity were considered sufficient to accurately detect Ca2+. The corresponding acetoxymethyl ester, KLCA-FuraAM, was synthesized for intracellular imaging and applied to Ca2+ quantification in neurons. KLCA-FuraAM enabled quantitative ratiometric monitoring of the two-step Ca2+ concentration increase induced by glutamate stimulation. While this two-step response was not clearly observed with a commercially available low-affinity ratiometric Ca2+ probe, Fura-FF, KLCA-FuraAM has demonstrated the potential to quantitatively visualize the behavior of high Ca2+ concentrations. The ratiometric low-affinity Ca2+ probe, KLCA-Fura, is expected to be a powerful tool for discovering new functions of Ca2+ in neurons.

Recent Advances in Photoswitchable Fluorescent and Colorimetric Probes

In recent years, significant advancements have been made in the research of photoswitchable probes. These probes undergo reversible structural and electronic changes upon light exposure, thus exhibiting vast potential in molecular detection, biological imaging, material science, and information storage. Through precisely engineered molecular structures, the photoswitchable probes can toggle between "on" and "off" states at specific wavelengths, enabling highly sensitive and selective detection of targeted analytes. This review systematically presents photoswitchable fluorescent and colorimetric probes built on various molecular photoswitches, primarily focusing on the types involving photoswitching in their detection and/or signal response processes. It begins with an analysis of various molecular photoswitches, including their photophysical properties, photoisomerization and photochromic mechanisms, and fundamental design concepts for constructing photoswitchable probes. The article then elaborates on the applications of these probes in detecting diverse targets, including cations, anions, small molecules, and biomacromolecules. Finally, it offers perspectives on the current state and future development of photoswitchable probes. This review aims to provide a clear introduction for researchers in the field and guidance for the design and application of new, efficient fluorescent and colorimetric probes.

Development of Phosphinate Ligand-Based Low-Affinity Ca2+ Fluorescent Probes and Application to Intracellular Ca2+ Imaging

Divalent metal cations such as calcium ion (Ca2+) and magnesium ion (Mg2+) are indispensable to the regulation of various cellular activities. In this research, we developed the KLCA series utilizing o-aminophenol-N,N-diacetate-O-methylene-methylphosphinate (APDAP) as a target binding site, which was reported recently as a highly free Mg2+-selective ligand. KLCA-301 with orange fluorescence based on a rhodamine fluorophore and KLCA-501 with near-infrared (NIR) fluorescence based on a Si-rhodamine fluorophore were synthesized, intended for application to multicolor imaging. The evaluation of the fluorescence response to Ca2+ and Mg2+ of the KLCA series indicated the applicability as low-affinity Ca2+ probes. While KLCA-301 mainly localized in the cytosol in cultured rat hippocampal neurons, KLCA-501 localized to the cytosol and granular organelles in neurons. Comparison of the fluorescence response of KLCA-301 and the high-affinity Ca2+ probe Fluo-4 upon stimulation by glutamate in stained neurons revealed that KLCA-301 could reflect the secondary large rise of intracellular Ca2+, which Fluo-4 could not detect. In addition, KLCA-501 showed a fluorescence response similar to the low-affinity Ca2+ probe Fluo-5N upon stimulation by glutamate in stained neurons, concluding that KLCA-301 and KLCA-501 could be used as low-affinity Ca2+ probes. The KLCA series offers new options for low-affinity Ca2+ probes. Moreover, KLCA-501 achieved simultaneous visualization of the change in Ca2+ and ATP concentrations and also in mitochondrial inner membrane potential in neurons. KLCA-501 is expected to be a strong tool that enables simultaneous multicolor imaging of multiple targets and elucidation of their relationship in cells.