Multimodal Probes: Superresolution and Transmission Electron Microscopy Imaging of Mitochondria, and Oxygen Mapping of Cells, Using Small-Molecule Ir(III) Luminescent Complexes

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

Probing the dynamic crosstalk of lysosomes and mitochondria with structured illumination microscopy

Structured illumination microscopy (SIM) is a super-resolution technology for imaging living cells and has been used for studying the dynamics of lysosomes and mitochondria. Recently, new probes and analyzing methods have been developed for SIM imaging, enabling the quantitative analysis of these subcellular structures and their interactions. This review provides an overview of the working principle and advances of SIM, as well as the organelle-targeting principles and types of fluorescence probes, including small molecules, metal complexes, nanoparticles, and fluorescent proteins. Additionally, quantitative methods based on organelle morphology and distribution are outlined. Finally, the review provides an outlook on the current challenges and future directions for improving the combination of SIM imaging and image analysis to further advance the study of organelles. We hope that this review will be useful for researchers working in the field of organelle research and help to facilitate the development of SIM imaging and analysis techniques.

Small-molecule fluorogenic probes for mitochondrial nanoscale imaging

Mitochondria are inextricably linked to the development of diseases and cell metabolism disorders. Super-resolution imaging (SRI) is crucial in enhancing our understanding of mitochondrial ultrafine structures and functions. In addition to high-precision instruments, super-resolution microscopy relies heavily on fluorescent materials with unique photophysical properties. Small-molecule fluorogenic probes (SMFPs) have excellent properties that make them ideal for mitochondrial SRI. This paper summarizes recent advances in the field of SMFPs, with a focus on the chemical and spectroscopic properties required for mitochondrial SRI. Finally, we discuss future challenges in this field, including the design principles of SMFPs and nanoscopic techniques.

Organelle Interaction and Drug Discovery: Towards Correlative Nanoscopy and Molecular Dynamics Simulation

The inter-organelle interactions, including the cytomembrane, endoplasmic reticulum, mitochondrion, lysosome, dictyosome, and nucleus, play the important roles in maintaining the normal function and homeostasis of cells. Organelle dysfunction can lead to a range of diseases (e.g., Alzheimer's disease (AD), Parkinson's disease (PD), and cancer), and provide a new perspective for drug discovery. With the development of imaging techniques and functional fluorescent probes, a variety of algorithms and strategies have been developed for the ever-improving estimation of subcellular structures, organelle interaction, and organelle-related drug discovery with accounting for the dynamic structures of organelles, such as the nanoscopy technology and molecular dynamics (MD) simulations. Accordingly, this work summarizes a series of state-of-the-art examples of the recent progress in this rapidly changing field and uncovering the drug screening based on the structures and interactions of organelles. Finally, we propose the future outlook for exciting applications of organelle-related drug discovery, with the cooperation of nanoscopy and MD simulations.

Unimolecular cooperative metallaphotocatalysis with conjugately bridged Ir-Ni complexes and its applications in organic coupling reactions

Recent advances successfully upgraded the unique cooperative partnership between the two distinct metals in photocatalysis. Herein we are reporting design, synthesis and comprehensive study of series of heteroleptic Ir(III) complexes...

Photostable Red-Emitting Fluorescent Rhein-Magnesium(Ⅱ) Coordination Polymer Nanodot-Based Nanostructures With a Large Stokes Shift for Imaging Mitochondria in Cancer Cell

The mitochondria play a significant role in many cellular processes and are recognized as one of the most important therapeutic targets in cancer. Direct long-term imaging of the mitochondria is very crucial for treating cancer. However, the development of a red-emitting mitochondrial probe with a large Stokes shift and photostability remains highly challenging. Fluorescent metal complexes with superior physicochemical property have emerged as new fluorescent nanomaterials due to their increasing advantages in bioimaging. Herein, a luminescent pitaya-type nanostructure based on rhein-magnesium(II) (Rh-Mg) coordination polymer nanodots was used as a fluorescent nanoprobe to selectively image the mitochondria benefiting from the introduction of triphenylphosphine. The as-prepared Rh-Mg nanodot-based nanoprobe showed red emission peaking at 620 nm, a large Stokes shift (100 nm), and excellent photostability as compared with commercial mitochondrial probes. Due to these extraordinary features, this fluorescent nanoprobe was successfully used for mitochondrial targeting imaging of live cancer cell line Neuro-2a (mouse neuroblastoma) and BV2 microglial cells. Therefore, our results pave a new way for the design of fluorescent nanoprobes for imaging mitochondria in cancer cell.

Light-Induced Reactive Oxygen Species (ROS) Generator for Tumor Therapy through an ROS Burst in Mitochondria and AKT-Inactivation-Induced Apoptosis

Mitochondria are identified as a valuable target for cancer therapy owing to their primary function in energy supply and cellular signal regulation. Mitochondria in tumor cells are depicted by excess reactive oxygen species (ROS), which lead to numerous detrimental results. Hence, mitochondria-targeting ROS-associated therapy is an optional therapeutic strategy for cancer. In this contribution, a light-induced ROS generator (TBTP) is developed for evaluation of the efficacy of mitochondria-targeting ROS-associated therapy and investigation of the mechanism underlying mitochondrial-injure-mediated therapy of tumors. TBTP serves as an efficient ROS generator with low cytotoxicity, favorable biocompatibility, excellent photostability, mitochondria-targeted properties, and NIR emission. In vivo and in vitro experiments reveal that TBTP exhibits effective anticancer potential. ROS generated from TBTP could destroy the integrity of mitochondria, downregulate ATP, decrease the mitochondrial membrane potential, secrete Cyt-c into cytoplasm, activate Caspase-3/9, and induce cell apoptosis. Moreover, RNA-seq analysis highlights that an ROS burst in mitochondria can kill tumor cells via inhibition of the AKT pathway. All these results prove that mitochondrial-targeted ROS-associated therapy hold great potential in cancer therapy.

Biocompatible Ir(III) Complexes as Oxygen Sensors for Phosphorescence Lifetime Imaging

Synthesis of biocompatible near infrared phosphorescent complexes and their application in bioimaging as triplet oxygen sensors in live systems are still challenging areas of organometallic chemistry. We have designed and synthetized four novel iridium [Ir(N^C)2(N^N)]+ complexes (N^C-benzothienyl-phenanthridine based cyclometalated ligand; N^N-pyridin-phenanthroimidazol diimine chelate), decorated with oligo(ethylene glycol) groups to impart these emitters' solubility in aqueous media, biocompatibility, and to shield them from interaction with bio-environment. These substances were fully characterized using NMR spectroscopy and ESI mass-spectrometry. The complexes exhibited excitation close to the biological "window of transparency", NIR emission at 730 nm, and quantum yields up to 12% in water. The compounds with higher degree of the chromophore shielding possess low toxicity, bleaching stability, absence of sensitivity to variations of pH, serum, and complex concentrations. The properties of these probes as oxygen sensors for biological systems have been studied by using phosphorescence lifetime imaging experiments in different cell cultures. The results showed essential lifetime response onto variations in oxygen concentration (2.0-2.3 μs under normoxia and 2.8-3.0 μs under hypoxia conditions) in complete agreement with the calibration curves obtained "in cuvette". The data obtained indicate that these emitters can be used as semi-quantitative oxygen sensors in biological systems.

Revisiting staining of biological samples for electron microscopy: perspectives for recent research

This review revisits essential staining protocols for electron microscopy focussing on the visualization of active sites, i.e. enzymes, metabolites or proteins, in cells and tissues, which have been developed 50 to 60 years ago, however, never were established as standard protocols being used in electron microscopy in a routine fashion. These approaches offer numerous possibilities to expand the knowledge of cellular function and specifically address the localization of active compounds of these systems. It is our conviction, that many of these techniques are still useful, in particular when applied in conjunction with correlative light and electron microscopy. Revisiting specialized classical electron microscopy staining protocols for use in correlative microscopy is particularly promising, as some of these protocols were originally developed as staining methods for light microscopy. To account for this history, rather than summarizing the most recent achievements in literature, we instead first provide an overview of techniques that have been used in the past. While some of these techniques have been successfully implemented into modern microscopy techniques during recent years already, more possibilities are yet to be re-discovered and provide exciting new perspectives for their future use.

Cyclometalated Iridium(III) Complexes as Mitochondria-targeting Photosensitizers against Cisplatin-resistant Cells†

Four iridium (III) complexes Ir1-Ir4 were synthesized and characterized. Possessing high singlet oxygen production ability and specific mitochondria-localization, Ir1 was developed as a mitochondria-targeting photosensitizer. Ir1 exhibited strong phototoxicity against cancer cell line A549 and its corresponding cisplatin-resistant one A549R. In contrast, Ir1 showed low cytotoxicity toward normal cell HLF. This selectivity resulted from the different uptake amount. With 405 nm irradiation, Ir1 induced mitochondria-mediated cell death in A549R cells, achieving the overcome of drug-resistant.

Refining a correlative light electron microscopy workflow using luminescent metal complexes

The potential for increasing the application of Correlative Light Electron Microscopy (CLEM) technologies in life science research is hindered by the lack of suitable molecular probes that are emissive, photostable, and scatter electrons well. Most brightly fluorescent organic molecules are intrinsically poor electron-scatterers, while multi-metallic compounds scatter electrons well but are usually non-luminescent. Thus, the goal of CLEM to image the same object of interest on the continuous scale from hundreds of microns to nanometers remains a major challenge partially due to requirements for a single probe to be suitable for light (LM) and electron microscopy (EM). Some of the main CLEM probes, based on gold nanoparticles appended with fluorophores and quantum dots (QD) have presented significant drawbacks. Here we present an Iridium-based luminescent metal complex (Ir complex 1) as a probe and describe how we have developed a CLEM workflow based on such metal complexes.

Mitochondrial targeted osmium polypyridyl probe shows concentration dependent uptake, localisation and mechanism of cell death

A symmetric osmium(ii) [bis-(4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine)] was prepared and conjugated to two mitochondrial-targeting peptide sequences; FrFKFrFK (r = d-arginine). The parent and conjugate complexes showed strong near infra-red emission centred at λmax 745 nm that was modestly oxygen dependent in the case of the parent and oxygen independent in the case of the conjugate, attributed in the latter case, surprisingly, to a shorter emission lifetime of the conjugate compared to the parent. Confocal fluorescence imaging of sub-live HeLa and MCF 7 cells showed the parent complex was cell impermeable whereas the conjugate was rapidly internalised into the cell and distributed in a concentration dependent manner. At concentrations below approximately 30 μmol, the conjugate localised to the mitochondria of both cell types where it was observed to trigger apoptosis induced by the collapse of the mitochondrial membrane potential (MPP). At concentrations exceeding 30 μmol the conjugate was similarly internalised rapidly but distributed throughout the cell, including to the nucleus and nucleolus. At these concentrations, it was observed to precipitate a caspase-dependent apoptotic pathway. The combination of concentration dependent organelle targeting, NIR emission coincident with the biological window, and distribution dependent cytotoxicity offers an interesting approach to theranostics with the possibility of eliciting site dependent therapeutic effect whilst monitoring the therapeutic effect with luminescence imaging.

A biocompatible phosphorescent Ir(iii) oxygen sensor functionalized with oligo(ethylene glycol) groups: synthesis, photophysics and application in PLIM experiments

NIR emitting Ir(iii) complexes decorated with oligo(ethylene glycol) were used to assess the degree of hypoxia in biosamples.

Enhanced three-photon activity triggered by the AIE behaviour of a novel terpyridine-based Zn(ii) complex bearing a thiophene bridge

Multiphoton bioimaging benefits from good penetration of tissue, low phototoxicity and high resolution. Hence, development of efficient multiphoton imaging agents is highly desirable but remains challenging. Herein, a novel terpyridine-based Zn(ii) complex bearing a thiophene bridge was designed rationally and fabricated. Thanks to its aggregation-induced emission (AIE), DZ1 emitted bright yellow-green fluorescence (λ em = 575 nm) under physiological conditions. The three-photon spectral changes of DZ1 when binding with RNA unambiguously reflected its RNA-specific targeting behaviour, resulting in twofold enhancement in three-photon action cross-sections located at the second near-infrared window (1700 nm).

Super-Resolution Tracking of Mitochondrial Dynamics with An Iridium(III) Luminophore

Combining luminescent transition metal complex with super-resolution microscopy is an excellent strategy for the long-term visualization of the dynamics of subcellular structures in living cells. However, it remains unclear whether iridium(III) complexes are applicable for a particular type of super-resolution technique, structured illumination microscopy (SIM), to image subcellular structures. Herein, an iridium(III) dye, to track mitochondrial dynamics in living cells under SIM is described. The dye demonstrates excellent specificity and photostability and satisfactory cell permeability. While using SIM to image mitochondria, an ≈80 nm resolution is achieved that allows the clear observation of the structure of mitochondrial cristae. The dye is used to monitor and quantify mitochondrial dynamics relative to lysosomes, including fusion involved in mitophagy, and newly discovered mitochondria-lysosome contact (MLC) under different conditions. The MLC remains intact and fusion vanishes when five receptors, p62, NDP52, OPTN, NBR1, and TAX1BP1, are knocked out, suggesting that these two processes are independent.

A Series of Zn(II) Terpyridine-Based Nitrate Complexes as Two-Photon Fluorescent Probe for Identifying Apoptotic and Living Cells via Subcellular Immigration

Two-photon active probe to label apoptotic cells plays a significant role in biological systems. However, discrimination of live/apoptotic cells at subcellular level under microscopy remains unachieved. Here, three novel Zn(II) terpyridine-based nitrate complexes (C1-C3) containing different pull/push units were designed. The structures of the ligands and their corresponding Zn(II) complexes were confirmed by single-crystal X-ray diffraction analysis. On the basis of the comprehensive comparison, C3 had a suitable two-photon absorption cross section in the near-infrared wavelength and good biocompatibility. Under two-photon confocal microscopy and transmission electron microscopy, it is found that C3 could target mitochondria in living cells but immigrate into the nucleolus during the apoptotic process. This dual-functional probe (C3) not only offers a valuable image tool but also acts as an indicator for cell mortality at subcellular level in a real-time manner.

Mitochondria-localising DNA-binding biscyclometalated phenyltriazole iridium(iii) dipyridophenazene complexes: syntheses and cellular imaging properties

New DNA-binding iridium(iii) complexes are presented.