Near-Infrared Fluorescent Probes for Imaging of Intracellular Mg2+ and Application to Multi-Color Imaging of Mg2+, ATP, and Mitochondrial Membrane Potential

Fluorescence-Encoded Time-Domain Coherent Raman Spectroscopy in the Visible Range

Fluorescence-encoded vibrational spectroscopy has attracted increasing attention by virtue of its high sensitivity and high chemical specificity. We recently demonstrated fluorescence-encoded time-domain coherent Raman spectroscopy (FLETCHERS), which enables low-frequency vibrational spectroscopy of low-concentration fluorophores using near-infrared (800-900 nm) light excitation. However, the feasibility of this study was constrained by the scarcity of excitable molecules in the near-infrared range. Consequently, the broader applicability of FLETCHERS has not been investigated. Here we extend the capabilities of FLETCHERS into the visible range by employing a noncollinear optical parametric amplifier as a light source, significantly enhancing its versatility. Specifically, we use the method, which we refer to as visible FLETCHERS (vFLETCHERS), to individually acquire Raman spectra from five visible fluorophores that have absorption peaks in the 600-700 nm region. These results not only confirm the versatility of vFLETCHERS for a wide range of molecules but also allude to its widespread applicability in biological research through highly sensitive supermultiplexed imaging.

Sensitive monitoring of NAD(P)H levels within cancer cells using mitochondria-targeted near-infrared cyanine dyes with optimized electron-withdrawing acceptors

A series of near-infrared fluorescent probes, labeled A to E, were developed by combining electron-rich thiophene and 3,4-ethylenedioxythiophene bridges with 3-quinolinium and various electron deficient groups, enabling the sensing of NAD(P)H. Probes A and B exhibit absorptions and emissions in the near-infrared range, offering advantages such as minimal interference from autofluorescence, negligible photo impairment in cells and tissues, and exceptional tissue penetration. These probes show negligible fluorescence when NADH is not present, and their absorption maxima are at 438 nm and 470 nm, respectively. In contrast, probes C-E feature absorption maxima at 450, 334 and 581 nm, respectively. Added NADH triggers the transformation of the electron-deficient 3-quinolinium units into electron-rich 1,4-dihydroquinoline units resulting in fluorescence responses which were established at 748, 730, 575, 625 and 661 for probes AH-EH, respectively, at detection limits of 0.15 μM and 0.07 μM for probes A and B, respectively. Optimized geometries based on theoretical calculations reveal non-planar geometries for probes A-E due to twisting of the 3-quinolinium and benzothiazolium units bonded to the central thiophene group, which all attain planarity upon addition of hydride resulting in absorption and fluorescence in the near-IR region for probes AH and BH in contrast to probes CH-EH which depict fluorescence in the visible range. Probe A has been successfully employed to monitor NAD(P)H levels in glycolysis and specific mitochondrial targeting. Furthermore, it has been used to assess the influence of lactate and pyruvate on the levels of NAD(P)H, to explore how hypoxia in cancer cells can elevate levels of NAD(P)H, and to visualize changes in levels of NAD(P)H under hypoxic conditions with CoCl2 treatment. Additionally, probe A has facilitated the examination of the potential impact of chemotherapy drugs, namely gemcitabine, camptothecin, and cisplatin, on metabolic processes and energy generation within cancer cells by affecting NAD(P)H levels. Treatment of A549 cancer cells with these drugs has been shown to increase NAD(P)H levels, which may contribute to their anticancer effects ultimately leading to programmed cell death or apoptosis. Moreover, probe A has been successfully employed in monitoring NAD(P)H level changes in D. melanogaster larvae treated with cisplatin.

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.

A novel polyhedral oligomeric silsesquioxane nanohybrid fluorescent sensor designed based on an osmotic mechanism for specific detection and intelligent scavenging of magnesium ions

BACKGROUND

Mg2+ has long been recognized as one of the most vital cations due to its diverse physiological and pathological roles, making it indispensable in both biomedical and biological research. Organic fluorescent sensors are commonly employed for Mg2+ detection, but they often lack high selectivity and exhibit poor hydrophilicity, limiting their biomedical applications.

RESULTS

Herein, we introduced a novel organic-inorganic hybrid fluorescence sensor, PFHBS, constructed on the POSS nanoplatforms. The efficient connection between PEGylated POSS and the small molecule sensor FHBS through Click chemistry enhances the selectivity and reduces interference, making this chemical sensor ideal for the accurate detection of Mg2+. Furthermore, the incorporation of POSS amplifies the ligand field effect of FHBS, making it more conducive to Mg2+ capture. The modification of PEG chains enhances the sensor's amphiphilicity, facilitating efficient cell penetration and effective Mg2+ detection at the biological level.

SIGNIFICANCE

Finally, relying on spontaneous permeation, coupled with its strong ligand field effect and excellent cell permeability, the chemosensor demonstrates the capability to intelligently remove excess Mg2+ from the body. It has been successfully applied to mitigate renal overload resulting from acute Mg2+ poisoning.

Lanthanide-based luminescent probes for biological magnesium: accessing polyphosphate-bound Mg2

Biomolecule-bound Mg2+ species, particularly polyphosphate complexes, represent a large and dynamic fraction of the total cellular magnesium that is essential for cellular function but remains invisible to most indicators. Here we report a new family of Eu(III)-based indicators, the MagQEu family, functionalized with a 4-oxo-4H-quinolizine-3-carboxylic acid metal recognition group/sensitization antenna for turn-on, luminescence-based detection of biologically relevant Mg2+ species.

Assessment of mitochondrial dysfunction and implications in cardiovascular disorders

Mitochondria play a pivotal role in cellular function, not only acting as the powerhouse of the cell, but also regulating ATP synthesis, reactive oxygen species (ROS) production, intracellular Ca²⁺ cycling, and apoptosis. During the past decade, extensive progress has been made in the technology to assess mitochondrial functions and accumulating evidences have shown that mitochondrial dysfunction is a key pathophysiological mechanism for many diseases including cardiovascular disorders, such as ischemic heart disease, cardiomyopathy, hypertension, atherosclerosis, and hemorrhagic shock. The advances in methodology have been accelerating our understanding of mitochondrial molecular structure and function, biogenesis and ROS and energy production, which facilitates new drug target identification and therapeutic strategy development for mitochondrial dysfunction-related disorders. This review will focus on the assessment of methodologies currently used for mitochondrial research and discuss their advantages, limitations and the implications of mitochondrial dysfunction in cardiovascular disorders.

Optical/electrochemical methods for detecting mitochondrial energy metabolism

This review highlights the biological importance of mitochondrial energy metabolism and the applications of multiple optical/electrochemical approaches to determine energy metabolites. Mitochondria, the main sites of oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis, provide the majority of energy required by aerobic cells for maintaining their physiological activity. They also participate in cell growth, differentiation, information transmission, and apoptosis. Multiple mitochondrial diseases, caused by internal or external factors, including oxidative stress, intense fluctuations of the ionic concentration, abnormal oxidative phosphorylation, changes in electron transport chain complex enzymes and mutations in mitochondrial DNA, can occur during mitochondrial energy metabolism. Therefore, developing accurate, sensitive, and specific methods for the in vivo and in vitro detection of mitochondrial energy metabolites is of great importance. In this review, we summarise the mitochondrial structure, functions, and crucial energy metabolic signalling pathways. The mechanism and applications of different optical/electrochemical methods are thoroughly reviewed. Finally, future research directions and challenges are proposed.

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.

Development of Near-Infrared Fluorescent Mg2+ Probe and Application to Multicolor Imaging of Intracellular Signals

Recent extensive studies revealed that the intracellular concentration of magnesium ions (Mg²⁺) is one of the important factors to regulate cellular functions. To evaluate the impact of Mg²⁺ concentration changes on intracellular signals or events, simultaneous imaging of Mg²⁺ with those phenomena is a powerful technique. The present protocol describes the synthesis and evaluation of near-infrared (NIR) fluorescent Mg²⁺-selective probes, named KMG-500 series, and the application to simultaneous imaging of the corresponding intracellular signal transductions and molecular events. The present protocol for multicolor imaging using fluorescent probes in the NIR and visible ranges is highly useful to reveal how multiple molecular events are correlated each other in each single cell.

Long-term single cell bioluminescence imaging with C-3 position protected coelenterazine analogues

Bioluminescence is a powerful imaging modality for monitoring biological phenomena both in vitro and in vivo. Bioluminescence imagin (BLI) is becoming a seamless imaging technology covering the range from cells to organs of small animals. Long-term imaging at the single cell level would lead to a true understanding of the dynamics of life phenomena. This work presents a long-term single cell bioluminescence imaging technology accomplished with C-3 position protected furimazines (FMZs), a CTZ analogues, which generate intense blue emission when paired with a highly stable engineered luciferase, Nanoluc. Four types of FMZs protected at the C-3 position have been synthesized. The type and steric bulkiness of the protection group strongly contributed to storage stability and the kinetics of the bioluminescence reactions of the analogues in human living cells. In particular, two developed FMZ analogues resulted in significantly longer bioluminescence emission with higher S/N ratio than FMZ at single cell level. Long-term bioluminescence single cell imaging technology with the developed FMZ analogues will lead to seamless imaging in the range from cells to organs of small animals.

A General Strategy for the Construction of NIR-emitting Si-rhodamines and Their Application for Mitochondrial Temperature Visualization

Si-rhodamine (SiR) is an ideal fluorophore because it possesses bright emission in the NIR region and can be implemented flexibly in living cells. Currently, several promising approaches for synthesizing SiR are being developed. However, challenges remain in the construction of SiR containing functional groups for bioimaging application. Herein, we introduce a general and simple approach by a condensation reaction of diarylsilylether and arylaldehyde in o-dichlorobenzene to synthesize a series of SiRs bearing various functional substituents. These SiRs have moderate to high quantum efficiency, tolerance to photobleaching, and high water solubility as well as NIR emitting, and their NIR fluorescence properties can be controlled through the photoinduced electron transfer (PET) mechanism. Fluorescence OFF-ON switching effect is observed for SiR 9 in the presence of acid, which is rationalized by DFT/TDDFT calculations. Moreover, reversible stimuli response toward temperature is achieved. Since positive charge enables mitochondrial targeting ability and chloromethyl unit can covalently immobilize the dyes onto the mitochondrial via click reaction between the benzyl choride and protein sulfhydryls, SiR 8 is identified as a valuable fluorescent marker to visualize the morphology and monitor the temperature change of mitochondria with high photostability.

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.