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.

Red- and Far-Red-Emitting Zinc Probes with Minimal Phototoxicity for Multiplexed Recording of Orchestrated Insulin Secretion

Zinc biology, featuring intertwining signaling networks and critical importance to human health, witnesses exciting opportunities in the big data era of physiology. Here, we report a class of red- and far-red-emitting Zn²⁺ probes with K_d values ranging from 190 nM to 74 μM, which are particularly suitable for real-time monitoring the high concentration of Zn²⁺ co-released with insulin during vesicular secretory events. Compared to the prototypical rhodamine-based Zn²⁺ probes, the new class exploits morpholino auxochromes which eliminates phototoxicity during long-term live recording of isolated islets. A Si-rhodamine-based Zn²⁺ probe with high turn-on ratio (>100), whose synthesis was enabled by a new route featuring late-stage N-alkylation, allowed simultaneous recording of Ca²⁺ influx, mitochondrial signal, and insulin secretion in isolated mouse islets. The time-lapse multicolor fluorescence movies and their analysis, enabled by red-shifted Zn²⁺ and other orthogonal physiological probes, highlight the potential impact of biocompatible fluorophores on the fields of islet endocrinology and system biology.

Advances in imaging of understudied ions in signaling: A focus on magnesium

The study of metal ions in the context of cell signaling has historically focused mainly on Ca2+, the second messenger par excellence. But recent studies support an emerging paradigm in which other metals, including magnesium and d-block metals, play a role in signal transduction as well. Armed with the right indicators, fluorescence microscopy offers a unique combination of spatial and temporal resolution perfectly suited to reveal metal transients in real time, while also helping identify possible sources of ion mobilization and molecular targets. With a focus on Mg2+, we highlight recent advancements in the development of molecular indicators and imaging strategies for the study of metal ions in signaling. We discuss remaining conceptual and technical challenges in the field, and we illustrate through the case of Mg2+ how the study of nontraditional ions in signaling is inspiring technological developments applicable more broadly to the study of metals in biology.

Scleral Lens Sensor for Ocular Electrolyte Analysis

The quantitative analysis of tear analytes in point-of-care settings can enable early diagnosis of ocular diseases. Here, a fluorescent scleral lens sensor is developed to quantitatively measure physiological levels of pH, Na⁺ , K⁺ , Ca²⁺ , Mg²⁺ , and Zn²⁺ ions. Benzenedicarboxylic acid, a pH probe, displays a sensitivity of 0.12 pH units within pH 7.0-8.0. Crown ether derivatives exhibit selectivity to Na⁺ and K⁺ ions within detection ranges of 0-100 and 0-50 mmol L^-1 , and selectivities of 15.6 and 8.1 mmol L^-1 , respectively. A 1,2 bis(o-aminophenoxy)ethane-N,N,-N',N'-tetraacetic-acid-based probe allows Ca²⁺ ion sensing with 0.02-0.05 mmol L^-1 sensitivity within 0.50-1.25 mmol L^-1 detection range. 5-Oxazolecarboxylic acid senses Mg²⁺ ions, exhibiting a sensitivity of 0.10-0.44 mmol L^-1 within the range of 0.5-0.8 mmol L^-1 . The N-(2-methoxyphenyl)iminodiacetate Zn²⁺ ion sensor has a sensitivity of 1 µmol L^-1 within the range of 10-20 µmol L^-1 . The fluorescent sensors are subsequently multiplexed in the concavities of an engraved scleral lens. A handheld ophthalmic readout device comprising light-emitting diodes (LEDs) and bandpass filters is fabricated to excite as well as read the scleral sensor. A smartphone camera application and an user interface are developed to deliver quantitative measurements with data deconvolution. The ophthalmic system enables the assessment of dry eye severity stages and the differentiation of its subtypes.