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

Abstract

Divalent metal cations such as calcium ion (Ca 2+ ) and magnesium ion (Mg 2+ ) 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 Mg 2+ -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 Ca 2+ and Mg 2+ of the KLCA series indicated the applicability as lowaffinity Ca 2+ 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 Ca 2+ probe Fluo-4 upon stimulation by glutamate in stained neurons revealed that KLCA-301 could reflect the secondary large rise of intracellular Ca 2+ , which Fluo-4 could not detect. In addition, KLCA-501 showed a fluorescence response similar to the low-affinity Ca 2+ probe Fluo-5N upon stimulation by glutamate in stained neurons, concluding that KLCA-301 and KLCA-501 could be used as low-affinity Ca 2+ probes. The KLCA series offers new options for low-affinity Ca 2+ probes. Moreover, KLCA-501 achieved simultaneous visualization of the change in Ca 2+ 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.