A RNA-Targeted Two-Photon Bioprobe with High Selective Permeability into Nuclear Pore Complexes for Dynamically Tracking the Autophagy Process among Multi-Organelles

Real-Time Fluorescence Visualization of the Dynamic Distribution of Zn2+ Ions during Osteoblast Differentiation

The differentiation and maturation of osteoblasts are essential for bone formation. Zn2+ plays a crucial role in cell differentiation and is involved in osteogenic differentiation. The concentration and distribution of Zn2+ in the nucleus and cytoplasm indicate the differentiation states of osteoblasts. However, there is an absence of a real-time method for monitoring the dynamic fluctuations of endogenous Zn2+ within the nucleus. Here, a novel Zn2+ fluorescent probe (NTAD-N1) with nuclear membrane permeability was designed and developed, allowing for distribution throughout the entire cell, including the nucleus. The NTAD-N1 probe successfully showed the dynamic distribution and concentration changes of Zn2+ in the nucleus and cytoplasm of preosteoblast MC3T3-E1 during the 21-day differentiation period. The results showed that free Zn2+ increased significantly during differentiation of osteoblasts (2-21 days). Importantly, after 4 days of differentiation, osteoblasts are mainly distributed in the nucleus, which is confirmed by metallothionein expression. Subsequently, the level of free Zn2+ in the cytoplasm remained at a high level, which promoted the increase in alkaline phosphatase activity and inhibited the activity of cis-aconitase in the tricarboxylic acid cycle, resulting in the accumulation of citric acid. This series of events promotes the formation of mineralized nodules. In the process of osteoblast differentiation, the detection time of Zn2+ (≤7 days) is ahead of the late marker of alkaline phosphatase (14 days) and mineralized nodules (14-21 days). This indicates that Zn2+ can be used as a biomarker and an intervention point for early differentiation of osteoblasts.

Triphenylphosphine-bonded coumaranone dyes realize dual color imaging of mitochondria and nucleoli

Probing intracellular organelles with fluorescent dyes offers opportunities to understand the structures and functions of these cellular compartments, which is attracting increasing interests. Normally, the design principle varies for different organelle targets as they possess distinct structural and functional profiles against each other. Therefore, developing a probe with dual intracellular targets is of great challenge. In this work, a new sort of donor-π-bridge-acceptor (D-π-A) type coumaranone dyes (CMO-1/2/3/4) have been prepared. Four fluorescent probes (TPP@CMO-1/2/3/4) were then synthesized by linking these coumaranone dyes with an amphiphilic cation triphenylphosphonium (TPP). Interestingly, both TPP@CMO-1 and TPP@CMO-2 exhibited dual color emission upon targeting to two different organelles, respectively. The green emission is well localized in mitochondria, while, the red emission realizes nucleoli imaging. RNA is the target of TPP@CMOs, which was confirmed by spectroscopic analysis and computational calculation. More importantly, the number and morphology changes of nucleoli under drug stress have been successfully evaluated using TPP@CMO-1.

Julolidine-based small molecular probes for fluorescence imaging of RNA in live cells

Intracellular RNA imaging with organic small molecular probes has been an intense topic, although the number of such reported dyes, particularly dyes with high quantum yields and long wavelength excitation/emission, is quite limited. The present work reports the design and synthesis of three cationic julolidine-azolium conjugates (OX-JLD, BTZ-JLD and SEZ-JLD) as turn-on fluorescent probes with appreciably high quantum yields and brightness upon interaction with RNA. A structure-efficiency relationship has been established for their potential for the interaction and imaging of intracellular RNA. Given their chemical structure, the free rotation between the donor and the acceptor gets restricted when the probes bind with RNA resulting in strong fluorescence emission towards a higher wavelength upon photoexcitation. A detailed investigation revealed that the photophysical properties and the optical responses of two probes, viz. BTZ-JLD and SEZ-JLD, towards RNA are very promising and qualify them to be suitable candidates for biological studies, particularly for cellular imaging applications. The probes allow imaging of intracellular RNA with prominent staining of nucleoli in live cells under a range of physiological conditions. The results of the cellular digest test established the appreciable RNA selectivity of BTZ-JLD and SEZ-JLD inside the cellular environment. Moreover, a comparison between the relative intensity profile of SEZ-JLD before and after the RNA-digestion test inside the cellular environment indicated that the interference of cellular viscosity in fluorescence enhancement is insignificant, and hence, SEZ-JLD can be used as a cell membrane permeable cationic molecular probe for deep-red imaging of intracellular RNA with a good degree of selectivity.

A dual-salt fluorescent probe for specific recognition of mitochondrial NADH and potential cancer diagnosis

Reduced nicotinamide adenine dinucleotide (NADH) is a kind of coenzyme and widely works as a biomarker in cancer cells. It plays a crucial role in many cellular metabolic processes, especially NADH in mitochondria is indispensable for the mitochondrial respiration chain that produces ATP. Herein, we designed a fluorescent probe Mito-FCC based on an ethylene-bridging dual-salt structure, in which benzo[e]indolium fluorophore was used as the mitochondria-targeting group and 1-methylquinolinium moiety as the NADH recognition unit. Mito-FCC exhibited high sensitivity and selectivity for NADH with a rapid "turn-on" fluorescence signal. The dual-salt structure endowed the probe with a reliable mitochondria-targeted ability even after the recognition unit was reduced by NADH. With the help of the probe, the fluctuations of endogenous NADH induced by glucose or pyruvate were imaged. Besides, Mito-FCC had a capability to make a distinction between cancer cells and normal cells due that the content of NADH in cancer cells was distinctly higher than that in normal ones. Notably, the visualization of tumor in vivo through monitoring NADH using Mito-FCC was realized successfully. These experimental results showed that Mito-FCC hold a great perspective in study of mitochondrial function and potential diagnosis of cancer diseases.

Molecular and Aggregate Synergistic Engineering of Aggregation-Induced Emission Luminogens to Manipulate Optical/Electronic Properties for Efficient and Diversified Functions

The optical/electronic properties of organic luminescent materials can be regulated by molecular structure modification, which not only requires sophisticated and time-consuming synthesis but also is unable to accurately afford the optical properties of materials in the aggregate state. Herein, a facile strategy of molecular and aggregate synergistic engineering is proposed to manipulate the optical/electronic properties of a luminogen, ACIK, in the solid state for efficient and diversified functions. ACIK is facilely synthesized and exhibits three polymorphic states (ACIK-Y, ACIK-R, and ACIK-N) with a large emission difference of 102 nm from yellow to near-infrared (NIR). Their structure-property relationships were investigated by crystallographic analyses and computational studies. ACIK-Y, with the most twisted structure, exhibits an intriguing color-tuned fluorescence between yellow and NIR in the solid state in response to multiple stimuli. Shuttle-like ACIK-R microcrystals exhibit an optical waveguide property with a low optical loss coefficient of 19 dB mm^-1. ACIK dots display bright NIR-I emission, large Stokes shift, and strong NIR-II two-photon absorption. ACIK dots show specific lipid droplets-targeting capability and can be successfully applied for two-photon fluorescence imaging of mouse brain vasculature with deep penetration and high spatial resolution. This study will inspire more insights in developing advanced optical/electronic materials based on a single chromophore for practical applications.

Polarity-Ultrasensitive and Lipophilicity-Enhanced Structurally Modified Hemicyanine for Two-Color Staining to Reveal Cell Apoptosis during Chemotherapy

Programmed cell death (PCD) is a precisely controlled physiological process to sustain tissue homeostasis. Even though the PCD pathways have been explicitly subdivided, the individual cell death process seems to synergistically operate to eliminate cells rather than separately execute signal transduction. Apoptosis is the dominant intracellular PCD subtype, which is intimately regulated and controlled by mitochondria, thus tracing mitochondrial actions could reveal the dynamic changes of apoptosis, which may provide important tools for screening preclinical therapeutic agents. Herein, we exploited an innovative fluorophore Cy496 based on the light-initiated cleavage reaction. Cy496 bears the typical D-π-A structure and serves as a versatile building block for chemosensor construction through flexible side chains. By regulating lipophilicity and basicity through bis-site substitution, we synthesized a series of fluorescence probes and screened a novel mitochondria-targeted ratiometric probe Cy1321, which can real-time evaluate the dynamic changes of mitochondrial micropolarity mediated by bis-cholesterol anchoring. Cy1321 has realized two-color quantification and real-time visualization of polarity fluctuations on chemotherapy agent (cisplatin)-induced apoptosis through flow cytometry and confocal imaging and also achieved the purpose of detecting mitochondria-related apoptosis at the level of tissues. It is envisioned that Cy1321 has sufficient capability as a promising and facile tool for the evaluation of apoptosis and contributing to therapeutic drug screening.

Rational design of a fluorescent probe for specific sensing of hydrogen peroxide/glucose and intracellular imaging applications

A new type of dye with advantages of high selectivity and sensitivity is formed by using the strategy of hybridization between the luminescent unit and recognition unit. Based on this strategy, we exploit a novel dye bonding the benzopyrylium salt as a luminescent unit and phenylboronate group as a response site, which is served as a fluorescent probe 1 for specific recognition of hydrogen peroxide in biological application. Probe 1 employs a unique recognition switch, phenylboronate unit, to"turn-on"a highly specific and rapid fluorescence response toward hydrogen peroxide combined with the 1,6-rearrangement elimination reaction strategy. Meanwhile, probe 1 has the ability to glucose assay by taking advantage of glucose oxidase/glucose enzymatic reaction. What's more, the probe 1 is capable of tracking endogenous hydrogen peroxide in living cells and intracellular imaging. Therefore, the newly developed bioprobe 1 is expected to be used to monitor hydrogen peroxide and glucose levels in complex organisms.

Coumarin-based fluorescent probes toward viscosity in mitochondrion/lysosome

Viscosity is an important microenvironmental indicator that plays an important role in the process of information transmission in various regions. Herein, two coumarin-based viscosity-sensitive fluorescent probes (CHB, CHN) were synthesized and the photophysical properties of the two probes were studied. The fluorescence quantum yields of CHB and CHN in glycerol can be as high as 25.2% and 18.3% respectively. The two probes can linearly detect viscosity in the viscosity logarithm range of 0.83-2.07, which is not interfered with pH, metal ions, anions and biomolecules. Fluorescent confocal cell experiments show CHB and CHN have good targeting ability to mitochondrion, lysosome, Endoplasmic reticulum and Golgi apparatus, and can be used to detect viscosity in mitochondrion/lysosome.

Fast Imaging of Mitochondrial Thioredoxin Reductase Using a Styrylpyridinium-Based Two-Photon Ratiometric Fluorescent Probe

Thioredoxin reductase (TrxR) is a pivotal antioxidant enzyme, but there remains a challenge for its fast imaging. This work describes the combination of a hydroxyl styrylpyridinium scaffold as the push-pull fluorophore with a carbonate-bridged 1,2-dithiolane unit as the reaction site to develop a fast mitochondrial TrxR2 probe, DSMP. It manifested a plethora of excellent properties including a rapid specific response (12 min), large Stokes shift (170 nm), ratiometric two-photon imaging, favorable binding with TrxR (K_m = 12.5 ± 0.2 μM), and the ability to cross the blood-brain barrier. With the aid of DSMP, we visualized the increased mitochondrial TrxR2 activity in cancer cells compared to normal cells. This offers the direct imaging evidence of the connection between the increased TrxR2 activity and the development of cancer. Additionally, the probe allowed the visualization of the loss in TrxR2 activity in a cellular Parkinson's disease model and, more importantly, in mouse brain tissues of a middle cerebral artery occlusion model for ischemic stroke.

Organosulfur/selenium-based Highly Fluorogenic Molecular Probes for Live-Cell Nucleolus Imaging

We present rationally designed cationic organochalcogens highly selective to RNA. We have demonstrated that the conformational dynamics and subsequently the optical properties of these dyes can be controlled to facilitate efficient bioimaging. We report organoselenium and organosulfur-based cell-permeable red-emissive probes bearing favorable cyclic sidearm with potential for selective and high contrast imaging of cell nucleoli. The probes exhibit high quantum yield upon interaction with RNA in an aqueous solution. An in-depth multiscale simulation study reveals that the prominent rotational freezing of the electron-donating sidearm of the probes in the microenvironment of RNA helps in attaining more planar conformation when compared to DNA. It exerts a greater extent of intramolecular charge transfer and hence leads to enhanced fluorescence emission. A systematic structure-interaction relationship study highlighted the impact of heavy-chalcogens toward the improved emissive properties of the probes.

Fluorogenic probes for mitochondria and lysosomes via intramolecular photoclick reaction

The tetrazole-based photoclick chemistry has attracted considerable attention in virtue of its good biocompatibility, exclusive molecular reaction, and spatiotemporally controllable properties. Using this photoclick reaction, we designed an in situ, real-time fluorescence imaging system that targeted mitochondria and lysosomes in a spatiotemporally controllable manner. Upon irradiation, the pyrazoline fluorophore was generated in situ by the intramolecular tetrazole-alkene cycloaddition reaction ("photo-click chemistry"). This strategy exhibits features such as fast response, high efficiency, strong fluorescence intensity without background and superior stability. In addition, by integrating with an organelle-specific group, it has a good application for subcellular targeting imaging. Furthermore, the photo-responsive moiety Tet facilitates the probes, Mt-Tet and Ly-Tet, for the super-resolution imaging of subcellular structures.

The fluorescence toolbox for visualizing autophagy

Autophagy is an adaptive catabolic process functioning to promote cell survival in the event of inappropriate living conditions such as nutrient shortage and to cope with diverse cytotoxic insults. It is regarded as one of the key survival mechanisms of living organisms. Cells undergo autophagy to accomplish the lysosomal digestion of intracellular materials including damaged proteins, organelles, and foreign bodies, in a bulk, non-selective or a cargo-specific manner. Studies in the past decades have shed light on the association of autophagy pathways with various diseases and also highlighted the therapeutic value of autophagy modulation. Hence, it is crucial to develop effective approaches for monitoring intracellular autophagy dynamics, as a comprehensive account of methodology establishment is far from complete. In this review, we aim to provide an overview of the major current fluorescence-based techniques utilized for visualizing, sensing or measuring autophagic activities in cells or tissues, which are categorized firstly by targets detected and further by the types of fluorescence tools. We will mainly focus on the working mechanisms of these techniques, put emphasis on the insight into their roles in biomedical science and provide perspectives on the challenges and future opportunities in this field.

A NIR-I light-responsive superoxide radical generator with cancer cell membrane targeting ability for enhanced imaging-guided photodynamic therapy

Photodynamic therapy (PDT), as an emerging treatment modality, which takes advantage of reactive oxygen species (ROS) generated upon light illumination to ablate tumours, has suffered from a limited treatment depth, strong oxygen dependence and short ROS lifespan. Herein, we developed a highly efficient NIR-I light (808 nm laser) initiated theranostic system based on a fluorescent photosensitizer (EBD-1) with cancer cell membrane targeting ability, which can realize large penetration depth in tissue, generate superoxide radicals (O2 -˙) to relieve the oxygen-dependence, confine the ROS oxidation at the cell membrane, and self-report the cell viability during the PDT process. In vivo experiments demonstrated that EBD-1 under 808 nm light successfully accomplished remarkable cancer ablation. This work will be beneficial for the design of novel photosensitizers for PDT-based theranostic systems.

In Situ Monitoring of Mitochondria Regulating Cell Viability by the RNA-Specific Fluorescent Photosensitizer

Cell viability is greatly affected by external stimulus eliciting correlated dynamical physiological processes for cells to choose survival or death. A few fluorescent probes have been designed to detect whether the cell is in survival state or apoptotic state, but monitoring the regulation process of the cell undergoing survival to death remains a long-standing challenge. Herein, we highlight the in situ monitor of mitochondria regulating the cell viability by the RNA-specific fluorescent photosensitizer L. At normal conditions, L anchored mitochondria and interacted with mito-RNA to light up the mitochondria with red fluorescence. With external light stimulus, L generated reactive oxide species (ROS) and cause damage to mitochondria, which activated mitochondrial autophagy to prevent death, during which the red fluorescence of L witnessed dynamical distribution in accordance with the evolution of vacuole structures containing damaged mitochondria into autophagosomes. However, with ROS continuously increasing, the mitochondrial apoptosis was eventually commenced and L with red fluorescent was gradually accumulated in the nucleoli, indicating the programmed cell death. This work demonstrated how the delicate balance between survival and death are regulated by mitochondria.