A Fluorescent Probe for Stimulated Emission Depletion Super-Resolution Imaging of Vicinal-Dithiol-Proteins on Mitochondrial Membrane

Mitochondria-targeted BODIPY dyes for small molecule recognition, bio-imaging and photodynamic therapy

Mitochondria are essential for a diverse array of biological functions. There is increasing research focus on developing efficient tools for mitochondria-targeted detection and treatment. BODIPY dyes, known for their structural versatility and excellent spectroscopic properties, are being actively explored in this context. Numerous studies have focused on developing innovative BODIPYs that utilize optical signals for imaging mitochondria. This review presents a comprehensive overview of the progress made in this field, aiming to investigate mitochondria-related biological events. It covers key factors such as design strategies, spectroscopic properties, and cytotoxicity, as well as mechanism to facilitate their future application in organelle imaging and targeted therapy. This work is anticipated to provide valuable insights for guiding future development and facilitating further investigation into mitochondria-related biological sensing and phototherapy.

Organic fluorescent probes for live-cell super-resolution imaging

The development of super-resolution technology has made it possible to investigate the ultrastructure of intracellular organelles by fluorescence microscopy, which has greatly facilitated the development of life sciences and biomedicine. To realize super-resolution imaging of living cells, both advanced imaging systems and excellent fluorescent probes are required. Traditional fluorescent probes have good availability, but that is not the case for probes for live-cell super-resolution imaging. In this review, we first introduce the principles of various super-resolution technologies and their probe requirements, then summarize the existing designs and delivery strategies of super-resolution probes for live-cell imaging, and finally provide a brief conclusion and overview of the future.

Recent progress in analytical strategies of arsenic-binding proteomes in living systems

Arsenic (As) is one of the most concerning elements due to its high exposure risks to organisms and ecosystems. The interaction between arsenicals and proteins plays a pivotal role in inducing their biological effects on living systems, e.g., arsenicosis. In this review article, the recent advances in analytical techniques and methods of As-binding proteomes were well summarized and discussed, including chromatographic separation and purification, biotin-streptavidin pull-down probes, in situ imaging using novel fluorescent probes, and protein identification. These analytical technologies could provide a growing body of knowledge regarding the composition, level, and distribution of As-binding proteomes in both cells and biological samples, even at the organellar level. The perspectives on analysis of As-binding proteomes are also proposed, e.g., isolation and identification of minor proteins, in vivo targeted protein degradation (TPD) technologies, and spatial As-binding proteomics. The application and development of sensitive, accurate, and high-throughput methodologies of As-binding proteomics would enable us to address the key molecular mechanisms underlying the adverse health effects of arsenicals.

Xanthene, cyanine, oxazine and BODIPY: the four pillars of the fluorophore empire for super-resolution bioimaging

In the realm of biological research, the invention of super-resolution microscopy (SRM) has enabled the visualization of ultrafine sub-cellular structures and their functions in live cells at the nano-scale level, beyond the diffraction limit, which has opened up a new window for advanced biomedical studies to unravel the complex unknown details of physiological disorders at the sub-cellular level with unprecedented resolution and clarity. However, most of the SRM techniques are highly reliant on the personalized special photophysical features of the fluorophores. In recent times, there has been an unprecedented surge in the development of robust new fluorophore systems with personalized features for various super-resolution imaging techniques. To date, xanthene, cyanine, oxazine and BODIPY cores have been authoritatively utilized as the basic fluorophore units in most of the small-molecule-based organic fluorescent probe designing strategies for SRM owing to their excellent photophysical characteristics and easy synthetic acquiescence. Since the future of next-generation SRM studies will be decided by the availability of advanced fluorescent probes and these four fluorescent building blocks will play an important role in progressive new fluorophore design, there is an urgent need to review the recent advancements in designing fluorophores for different SRM methods based on these fluorescent dye cores. This review article not only includes a comprehensive discussion about the recent developments in designing fluorescent probes for various SRM techniques based on these four important fluorophore building blocks with special emphasis on their effective integration into live cell super-resolution bio-imaging applications but also critically evaluates the background of each of the fluorescent dye cores to highlight their merits and demerits towards developing newer fluorescent probes for SRM.

Red-to-Near-Infrared Emitting PyrrolylBODIPY Dyes: Synthesis, Photophysical Properties and Bioimaging Application

Near-infrared (NIR) fluorophores with characteristics such as deep tissue penetration, minimal damage to the biological samples, and low background interference, are highly sought-after materials for in vivo and deep-tissue fluorescence imaging. Herein, series of 3-pyrrolylBODIPY derivatives and 3,5-dipyrrolylBODIPY derivatives have been prepared by a facile regioselective nucleophilic aromatic substitution reaction (S_N Ar) on 3,5-halogenated BODIPY derivatives (3,5-dibromo or 2,3,5,6-tetrachloroBODIPYs) with pyrroles. The installation of a pyrrolic unit onto the 3-position of the BODIPY chromophore leads to a dramatic red shift of both the absorption (up to 160 nm) and the emission (up to 260 nm) in these resultant 3-pyrrolylBODIPYs with respect to that of the BODIPY chromophore. Their further 5-positional functionalization provides a facile way to fine tune their photophysical properties, and these resulting dipyrrolylBODIPYs and functionalized pyrrolylBODIPYs show strong absorption in the deep red-to-NIR regions (595-684 nm) and intense NIR fluorescence emission (650-715 nm) in dichloromethane. To demonstrate the applicability of these functionalized pyrrolylBODIPYs as NIR fluorescent probes for cell imaging, pyrrolylBODIPY 6 a containing mitochondrion-targeting butyltriphenylphosphonium cationic species was also prepared. It selectively localized in mitochondria of HeLa cells, with low cytotoxicity and intense deep red fluorescence emission.

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.

Precise and long-term tracking of mitochondria in neurons using a bioconjugatable and photostable AIE luminogen

Tracking mitochondrial movement in neurons is an attractive but challenging research field as dysregulation of mitochondrial motion is associated with multiple neurological diseases. To realize accurate and long-term tracking of mitochondria in neurons, we elaborately designed a novel aggregation-induced emission (AIE)-active luminogen, TPAP-C5-yne, where we selected a cationic pyridinium moiety to target mitochondria and employed an activated alkyne terminus to achieve long-term tracking through bioconjugation with amines on mitochondria. For the first time, we successfully achieved the accurate analysis of the motion of a single mitochondrion in live primary hippocampal neurons and the long-term tracking of mitochondria for up to a week in live neurons. Therefore, this new AIEgen can be used as a potential tool to study the transport of mitochondria in live neurons.

Mitochondrial Toxicity of Organic Arsenicals

Arsenic is either notorious toxicant or miracle cure for acute promyelocytic leukemia and several other diseases. It interacts with mitochondria directly or indirectly, by interacting with mitochondrial enzymes, such as respiratory chain complexes and tricarboxylic acid cycle proteins, or affecting mitochondrial homeostasis via ROS or mitochondrial outer membrane permeabilization. Given the ubiquitous presence of mitochondria and indispensable role in cellular metabolism, arsenical-mitochondrial interactions may manifest clinical importance by revealing mechanism of disease curation, preventing severe side effects, and foreseeing potential health issues. Here, we described the interaction between isolated mitochondria and arsenicals.

Thiol-Mediated Uptake

This Perspective focuses on thiol-mediated uptake, that is, the entry of substrates into cells enabled by oligochalcogenides or mimics, often disulfides, and inhibited by thiol-reactive agents. A short chronology from the initial observations in 1990 until today is followed by a summary of cell-penetrating poly(disulfide)s (CPDs) and cyclic oligochalcogenides (COCs) as privileged scaffolds in thiol-mediated uptake and inhibitors of thiol-mediated uptake as potential antivirals. In the spirit of a Perspective, the main part brings together topics that possibly could help to explain how thiol-mediated uptake really works. Extreme sulfur chemistry mostly related to COCs and their mimics, cyclic disulfides, thiosulfinates/-onates, diselenolanes, benzopolysulfanes, but also arsenics and Michael acceptors, is viewed in the context of acidity, ring tension, exchange cascades, adaptive networks, exchange affinity columns, molecular walkers, ring-opening polymerizations, and templated polymerizations. Micellar pores (or lipid ion channels) are considered, from cell-penetrating peptides and natural antibiotics to voltage sensors, and a concise gallery of membrane proteins, as possible targets of thiol-mediated uptake, is provided, including CLIC1, a thiol-reactive chloride channel; TMEM16F, a Ca-activated scramblase; EGFR, the epithelial growth factor receptor; and protein-disulfide isomerase, known from HIV entry or the transferrin receptor, a top hit in proteomics and recently identified in the cellular entry of SARS-CoV-2.

Oligomers of Cyclic Oligochalcogenides for Enhanced Cellular Uptake

Monomeric cyclic oligochalcogenides (COCs) are emerging as attractive transporters to deliver substrates of interest into the cytosol through thiol-mediated uptake. The objective of this study was to explore COC oligomers. We report a systematic evaluation of monomers, dimers, and trimers of asparagusic, lipoic, and diselenolipoic acid as well as their supramolecular monomers, dimers, trimers, and tetramers. COC dimers were more than twice as active as the monomers on both the covalent and noncovalent levels, whereas COC trimers were not much better than dimers. These trends might suggest that thiol-mediated uptake of COCs is synergistic over both short and long distances, that is, it involves more than two COCs and more than one membrane protein, although other interpretations cannot be excluded at this level of complexity. These results thus provide attractive perspectives for structural evolution as well as imminent use in practice. Moreover, they validate automated HC-CAPA as an invaluable method to collect comprehensive data on cytosolic delivery within a reasonable time at a level of confidence that is otherwise inconceivable.

Nucleolus-Targeted Red Emissive Carbon Dots with Polarity-Sensitive and Excitation-Independent Fluorescence Emission: High-Resolution Cell Imaging and in Vivo Tracking

Red-emitting carbon dots (CDs) have attracted tremendous attention due to their wide applications in areas including imaging, sensing, drug delivery, and cancer therapy. However, it is still highly challenging for red-emitting CDs to simultaneously achieve high quantum yields (QYs), nucleus targeting, and super-resolution fluorescence imaging (especially the stimulated emission depletion (STED) imaging). Here, it is found that the addition of varied metal ions during the hydrothermal treatment of p-phenylenediamine (pPDA) leads to the formation of fluorescent CDs with emission wavelengths up to 700 nm. Strikingly, although metal ions play a crucial role in the synthesis of CDs with varied QYs, they are absent in the formed CDs, that is, the obtained CDs are metal-free, and the metal ions play a role similar to a "catalyst" during the CD formation. Besides, using pPDA and nickel ions (Ni²⁺) as raw materials, we prepare Ni-pPCDs which have the highest QY and exhibit various excellent fluorescence properties including excitation-independent emission (at ∼605 nm), good photostability, polarity sensitivity, and ribonucleic acid responsiveness. In vitro and in vivo experiments demonstrate that Ni-pPCDs are highly biocompatible and can realize real-time, wash-free, and high-resolution imaging of cell nuclei and high-contrast imaging of tumor-bearing mice and zebrafish. In summary, the present work may hold great promise in the synthesis and applications of red emissive CDs.

Pharmacological modulation of mitochondrial ion channels

The field of mitochondrial ion channels has undergone a rapid development during the last three decades, due to the molecular identification of some of the channels residing in the outer and inner membranes. Relevant information about the function of these channels in physiological and pathological settings was gained thanks to genetic models for a few, mitochondria-specific channels. However, many ion channels have multiple localizations within the cell, hampering a clear-cut determination of their function by pharmacological means. The present review summarizes our current knowledge about the ins and outs of mitochondrial ion channels, with special focus on the channels that have received much attention in recent years, namely, the voltage-dependent anion channels, the permeability transition pore (also called mitochondrial megachannel), the mitochondrial calcium uniporter and some of the inner membrane-located potassium channels. In addition, possible strategies to overcome the difficulties of specifically targeting mitochondrial channels versus their counterparts active in other membranes are discussed, as well as the possibilities of modulating channel function by small peptides that compete for binding with protein interacting partners. Altogether, these promising tools along with large-scale chemical screenings set up to identify new, specific channel modulators will hopefully allow us to pinpoint the actual function of most mitochondrial ion channels in the near future and to pharmacologically affect important pathologies in which they are involved, such as neurodegeneration, ischaemic damage and cancer. LINKED ARTICLES: This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.