A noteworthy interface-targeting fluorescent probe for long-term tracking mitochondria and visualizing mitophagy

Biosensors; a novel concept in real-time detection of autophagy

Autophagy is an early-stage response with self-degradation properties against several insulting conditions. To date, the critical role of autophagy has been well-documented in physiological and pathological conditions. This process involves various signaling and functional biomolecules, which are involved in different steps of the autophagic response. During recent decades, a range of biochemical analyses, chemical assays, and varied imaging techniques have been used for monitoring this pathway. Due to the complexity and dynamic aspects of autophagy, the application of the conventional methodology for following autophagic progression is frequently associated with a mistake in discrimination between a complete and incomplete autophagic response. Biosensors provide a de novo platform for precise and accurate analysis of target molecules in different biological settings. It has been suggested that these devices are applicable for real-time monitoring and highly sensitive detection of autophagy effectors. In this review article, we focus on cutting-edge biosensing technologies associated with autophagy detection.

Exquisite visualization of mitophagy and monitoring the increase of lysosomal micro-viscosity in mitophagy with an unusual pH-independent lysosomal rotor

BACKGROUND

Mitophagy plays indispensable roles in maintaining intracellular homeostasis in most eukaryotic cells by selectively eliminating superfluous components or damaged organelles. Thus, the co-operation of mitochondrial probes and lysosomal probes was presented to directly monitor mitophagy in dual colors. Nowadays, most of the lysosomal probes are composed of groups sensitive to pH, such as morpholine, amine and other weak bases. However, the pH in lysosomes would fluctuate in the process of mitophagy, leading to the optical interference. Thus, it is crucial to develop a pH-insensitive probe to overcome this tough problem to achieve exquisite visualization of mitophagy.

RESULTS

In this study, we rationally prepared a pH-independent lysosome probe to reduce the optical interference in mitophagy, and thus the process of mitophagy could be directly monitored in dual color through cooperation between IVDI and MTR, depending on Förster resonance energy transfer mechanism. IVDI shows remarkable fluorescence enhancement toward the increase of viscosity, and the fluorescence barely changes when pH varies. Due to the sensitivity to viscosity, the probe can visualize micro-viscosity alterations in lysosomes without washing procedures, and it showed better imaging properties than LTR. Thanks to the inertia of IVDI to pH, IVDI can exquisitely monitor mitophagy with MTR by FRET mechanism despite the changes of lysosomal pH in mitophagy, and the reduced fluorescence intensity ratio of green and red channels can indicate the occurrence of mitophagy. Based on the properties mentioned above, the real-time increase of micro-viscosity in lysosomes during mitophagy was exquisitely monitored through employing IVDI.

SIGNIFICANCE AND NOVELTY

Compared with the lysosomal fluorescent probes sensitive to pH, the pH-inert probe could reduce the influence of pH variation during mitophagy to achieve exquisite visualization of mitophagy in real-time. Besides, the probe could monitor the increase of lysosomal micro-viscosity in mitophagy. So, the probe possesses tremendous potential in the visualization of dynamic changes related to lysosomes in various physiological processes.

Near infrared in and out: Deep imaging for scrap leather induced autophagy in vivo by an ultrasensitive two-photon polarity probe

As one of the important means for eukaryotic cells to maintain homeostasis, autophagy allows for transporting deformed biomacromolecules and damaged organelles to lysosome for digestion and degradation. The process of autophagy entails the merging of autophagosomes and lysosomes, culminating in the breakdown of biomacromolecules. This, in turn, leads to a change in lysosomal polarity. Therefore, fully understanding the changes of lysosomal polarity during autophagy is of significance to the study of membrane fluidity and enzymatic reaction. However, the shorter emission wavelength has greatly damaged the imaging depth, thus seriously limiting its biological application. Therefore, in this work, a near infrared in and out lysosome-targeted polarity-sensitive probe NCIC-Pola was developed. The fluorescence intensity of NCIC-Pola showed an approximate 1160-fold increase when the polarity decreased under two-photon excitation (TPE). In addition, the excellent fluorescence emission wavelength (692 nm) enabled the deep imaging analysis of scrap leather induced autophagy in vivo.

Development of a fluorescent probe based on a tricyano structure for the detection of PhSH in environmental and biological samples

In this study, a NIR fluorescent probe based on ICT principles was developed for the detection of phenylthiophenol. An excellent fluorescent mother nucleus is constructed with tricyano groups, and benzenesulfonate was introduced as a specific recognition site for thiophene, which can be used for rapid detection of thiophenol. The probe has a significant Stokes shift (220 nm). Meanwhile, it had rapid response to thiophene and high specificity. The fluorescence intensity of the probe at 700 nm showed a good linear relationship with thiophene concentration in the range of 0 to 100 μM, and the detection limit was as low as 45 nM. The probe had also been successfully applied to the detection of thiophene in real water samples. MTT assay showed low cytotoxicity and excellent fluorescence imaging in live cells.

A triphenylamine-based fluorescence probe for detection of hypochlorite in mitochondria

The level of HClO/ClO^- in mitochondria is essential to keep the normal function of mitochondria. Therefore, it is meaningful to accurately and quickly monitor ClO^- in mitochondria. In this work, a new triphenylamine-based fluorescence probe PDTPA was designed and synthesized, in which pyridinium salt and dicyano-vinyl group were introduced as mitochondria targeting site and reaction site for ClO^-. The probe showed high sensitivity and fast fluorescence response (<10 s) in the detection of ClO^-. Moreover, the probe PDTPA had good linearity in a wide concentration range of ClO^- and its detection limit was calculated as 10.5 μM. Confocal fluorescence images demonstrated that the probe could target mitochondria and track the fluctuations of endogenous/exogenous ClO^- levels in the mitochondria of living cells.

Fluorescence microscopic platforms imaging mitochondrial abnormalities in neurodegenerative diseases

Neurodegenerative diseases (NDs) are progressive disorders that cause the degeneration of neurons. Mitochondrial dysfunction is a common symptom in NDs and plays a crucial role in neuronal loss. Mitochondrial abnormalities can be observed in the early stages of NDs and evolve throughout disease progression. Visualizing mitochondrial abnormalities can help understand ND progression and develop new therapeutic strategies. Fluorescence microscopy is a powerful tool for dynamically imaging mitochondria due to its high sensitivity and spatiotemporal resolution. This review discusses the relationship between mitochondrial dysfunction and ND progression, potential biomarkers for imaging dysfunctional mitochondria, advances in fluorescence microscopy for detecting organelles, the performance of fluorescence probes in visualizing ND-associated mitochondria, and the challenges and opportunities for developing new generations of fluorescence imaging platforms for monitoring mitochondria in NDs.

Mitochondrial Membrane Potential Independent Near-Infrared Mitochondrial Viscosity Probes for Real-Time Tracking Mitophagy

Mitophagy is a vital cellular process playing vital roles in regulating cellular metabolism and mitochondrial quality control. Mitochondrial viscosity is a key microenvironmental index, closely associated with mitochondrial status. To monitor mitophagy and mitochondrial viscosity, three molecular rotors (Mito-1, Mito-2, and Mito-3) were developed. All probes contain a cationic quinolinium unit and a C₁₂ chain so that they can tightly bind mitochondria and are not affected by the mitochondrial membrane potential. Optical studies showed that all probes are sensitive to viscosity changes with an off-on fluorescence response, and Mito-3 shows the best fluorescence enhancement. Bioimaging studies showed that all these probes can not only tightly locate and visualize mitochondria with near-infrared fluorescence but also effectively monitor the mitochondrial viscosity changes in cells. Moreover, Mito-3 was successfully applied to visualize the mitophagy process induced by starvation, and mitochondrial viscosity was found to show an increase during mitophagy. We expect Mito-3 to become a useful imaging tool for studying mitochondrial viscosity and mitophagy.

(E)-2-styrylanthracene-9,10-dione derivatives as novel fluorescent probes: synthesis, photophysical properties and application in mitochondria imaging

Our previous work firstly reported that (E)-2-styrylanthracene-9,10-dione is a novel fluorescent core (EK01) with the ability of specific mitochondria imaging. In this effort, we mainly focused our attention on the structure-photophysical property relationship and application in cells imaging of this new fluorescent chemotype. A series of the structural derivatives (TZ series) were designed and synthesized by introducing some substituents onto the 2-styryl moiety. The structure-photophysical property relationship analysis suggested that TZ03 is an excellent fluorescent molecular building block with the property of fluorescent "turn-on" effect after the modification of acylation, and TZ07 is an excellent fluorescent dye with a series of advantages such as high fluorescence intensity (F_max = 4049.0 in CH₂Cl₂, 25.80 μM), moderate molar extinction coefficients (3.77 × 10³-5.93 × 10³ mol^-1∙L∙cm^-1), strong fluorescence quantum yield (Φ_max = 0.739 in CH₂Cl₂), large Stokes shift (99.0 nm-161.8 nm) and well biological tolerance. As a classical D-π-A structure, the ICT characteristic of TZ07 was analyzed through spectroscopy verification and DFT calculations. Furthermore, optimized compound TZ07 was successfully applied in the living cells imaging with the excellent selectivity to mitochondria in a green fluorescent form. It was also suggested that the mechanism of TZ07 targeting mitochondria is independent of mitochondrial membrane potential, but probably related to the mitochondrial complex I. These findings may provide some insights into the development of novel mitochondria-targeted fluorescent probes.

Dicyanisophorone-based near-infrared fluorescent probe for the detection of thiophenol and its application in living cells and actual water samples

This article reports a new type of dicyanisophorone-based near-infrared fluorescent probe for the rapid detection of mercaptophenol by introducing 2,4-dinitrobenzene sulfonate group as a specific recognition group for thiophenol. The probe has a significant large Stokes shift (185 nm). At the same time, it exhibits rapid response, high selectivity and high sensitivity to thiophene. In addition, the fluorescence of the probe at 650 nm has a good linear relationship with the concentration of thiophenol in the range of 0-100 μM, and the detection limit is as low as 65 nM. The probe has been successfully applied to the detection of thiophenol in actual water samples, and has good live cell imaging effects, and at the same time shows the superiority of its low cell toxicity.

A ratiometric pH probe for acidification tracking in dysfunctional mitochondria and tumour tissue in vivo

With an ideal pKa (7.4) for mitochondrial pH monitoring, CouDa could immobilize in mitochondria independent of MMP. Acidification tracking was realized in dysfunctional mitochondria and tumour tissue.

Mesoporous silica nanoparticles loaded with fluorescent coumarin-5-fluorouracil conjugates as mitochondrial-targeting theranostic probes for tumor cells

In this study, a nanodrug carrier (mesoporous silica nanoparticle (MSN)-SS-cysteamine hydrochloride (CS)-hyaluronic acid (HA)) for targeted drug delivery was prepared using MSNs, in which HA was used as a targeting ligand and blocking agent to control drug release. Coumarin is a fluorescent molecule that targets mitochondria. Two conjugates (XDS-DJ and 5-FUA-4C-XDS) were synthesized by chemically coupling nitrogen mustard and 5-fluorouracil with coumarin, which was further loaded into MSN-SS-CS-HA nanocarriers. MTT analysis demonstrated that the nanocomposite MSN-SS-CS@5-FUA-4C-XDS/HA displayed stronger cytotoxicity toward HCT-116 cells than HeLa or QSG-7701 cells. Furthermore, MSN-SS-CS@5-FUA-4C-XDS/HA was able to target the mitochondria of HCT-116 cells, causing decreased mitochondrial membrane potential and excessive production of reactive oxygen species. These results indicate that MSN-SS-CS@5-FUA-4C-XDS/HA has the potential to be a nanodrug delivery system for the treatment of colon cancer.

Accurate Monitoring and Multiple Evaluations of Mitophagy by a Versatile Two-Photon Fluorescent Probe

Mitophagy plays a critical role in regulating and maintaining cellular functions, particularly regulating the quantity and quality of mitochondria. In this research, a multifunctional two-photon fluorescent probe Mito-PV with improved mitochondria-anchored ability was designed. The proposed probe can track the fluctuation of polarity and viscosity in mitochondria simultaneously with two well-distinguished emissions. It can also precisely visualize the change in mitochondrial morphology (including mitochondrial form factor and length). The real-time and accurate monitoring of mitophagy under two-photon excitation was successfully achieved by utilizing probe Mito-PV through supervising the alterations of diverse mitophagy-related parameters (including colocalization coefficient, polarity, viscosity, and mitochondrial morphology). In addition, probe Mito-PV can be applied to evaluate drug bpV(phen) as an effective mitophagy inhibitor. Therefore, our work may provide a more efficient and reliable method for precisely monitoring mitophagy from multiple evaluations.

A dual-channel 'turn-on' fluorescent chemosensor for high selectivity and sensitivity detection of CN¯ based on a coumarin-Schiff base derivative in an aqueous system

Novel strategies still need to be proposed that can be used to identify and detect toxic environmental pollutants. In this paper, two channels of colorimetry and fluorescence 'turn-on' fluorescent probe 1 (7-hydroxy-8-[(2-hydroxy-phenylimino)- methyl]-4-methylbenzopyran-2-one) for the simple yet highly selective detection of CN¯ have been successfully designed and synthesized. Crystal features of probe 1 were defined using X-ray single crystal diffractometry. Probe 1 showed a strongly colorimetric and fluorescence response to CN¯ that induced obvious naked-eye colour changes in aqueous solution (DMSO/H₂ O, 3:1 v:v). In addition, probe 1 for CN¯ detection displayed low detection limits of 3.91 × 10^-8  M, which were significantly lower than the 1.9 × 10^-6  M maximum level specified by the World Health Organization (WHO) for potable water. The sensing mechanism for probe 1 was attributed to the deprotonation process as shown by ¹ H NMR titration. Moreover, based on the visible colorimetry and fluorescence change for probe 1 to CN¯, measurement was performed for simulated water samples containing CN¯. This study provides a broad prospect for solving other pollution problems and promoting the design of new fluorescent materials.