An Aggregation-Induced Emission-Based Dual Emitting Nanoprobe for Detecting Intracellular pH and Unravelling Metabolic Variations in Differentiating Lymphocytes

Monitoring T lymphocyte differentiation is essential for understanding T cell fate regulation and advancing adoptive T cell immunotherapy. However, current biomarker analysis methods necessitate cell lysis, leading to source depletion. Intracellular pH (pHi) can be affected by the presence of lactic acid (LA), a metabolic mediator of T cell activity such as glycolysis during T cell activation; therefore, it is a potentially a good biomarker of T cell state. In this work, a dual emitting enhancement-based nanoprobe, namely, AIEgen@F127-AptCD8, was developed to accurately detect the pHi of T cells to "read" the T cell differentiation process. The nanocore of this probe comprises a pair of AIE dyes, TPE-AMC (pH-sensitive moiety) and TPE-TCF, that form a donor-acceptor pair for sensitive detection of pHi by dual emitting enhancement analysis. The nanoprobe exhibits a distinctly sensitive narrow range of pHi values (from 6.0 to 7.4) that can precisely distinguish the differentiated lymphocytes from naïve ones based on their distinct pHi profiles. Activated CD8+ T cells demonstrate lower pHi (6.49 ± 0.09) than the naïve cells (7.26 ± 0.11); Jurkat cells exhibit lower pHi (6.43 ± 0.06) compared to that of nonactivated ones (7.29 ± 0.09) on 7 days post-activation. The glycolytic product profiles in T cells strongly correlate with their pHi profiles, ascertaining the reliability of probing pHi for predicting T cell states. The specificity and dynamic detection capabilities of this nanoprobe make it a promising tool for indirectly and noninvasively monitoring T cell activation and differentiation states.

Aggregation-Induced Emission Luminogens: A New Possibility for Efficient Visualization of RNA in Plants

RNAs play important roles in regulating biological growth and development. Advancements in RNA-imaging techniques are expanding our understanding of their function. Several common RNA-labeling methods in plants have pros and cons. Simultaneously, plants' spontaneously fluorescent substances interfere with the effectiveness of RNA bioimaging. New technologies need to be introduced into plant RNA luminescence. Aggregation-induced emission luminogens (AIEgens), due to their luminescent properties, tunable molecular size, high fluorescence intensity, good photostability, and low cell toxicity, have been widely applied in the animal and medical fields. The application of this technology in plants is still at an early stage. The development of AIEgens provides more options for RNA labeling. Click chemistry provides ideas for modifying AIEgens into RNA molecules. The CRISPR/Cas13a-mediated targeting system provides a guarantee of precise RNA modification. The liquid-liquid phase separation in plant cells creates conditions for the enrichment and luminescence of AIEgens. The only thing that needs to be looked for is a specific enzyme that uses AIEgens as a substrate and modifies AIEgens onto target RNA via a click chemical reaction. With the development and progress of artificial intelligence and synthetic biology, it may soon be possible to artificially synthesize or discover such an enzyme.

Tetraphenylethylene-based AIE nanoprobes for labeling lysosome by two-photon imaging in living cells

Lysosomes are essential cellular organelles, serving vital functions in cellular metabolism and degradation. The design of specifically targeting lysosomes probes with aggregation-induced emission (AIE) characteristics using two-photon excitation techniques is significance and challenging work. Here we designed and synthesized two tetraphenylethylene (TPE)-based AIE fluorescence probes, naming TPE-Ma and TPE-Py, with TPE as the matrix and morpholine (Ma) or pyrrolidone (Py) as the targeting group. These probes exhibit a significant Stokes shift, low cytotoxicity, two-photo fluorescence imaging and lysosome-specific targeting capability ensuring their suitability for fluorescence imaging applications. To enhance the water solubility and cellular accumulation of TPE-Ma and TPE-Py in tumor cells, we employed a biocompatible polymer 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-mPEG2000) as a nanocarrier. By encapsulating TPE-Ma and TPE-Py within DSPE-mPEG2000, we successfully developed two AIE fluorescent nanoprobes known as DSPE@ TPE-Ma and DSPE@ TPE-Py. The results demonstrated that fluorescent nanoprobes DSPE@ TPE-Ma and DSPE@ TPE-Py possess excellent cell permeability, biocompatibility, superior photostability and specific targeting towards lysosomes in MCF-7 cells. Our findings highlight the potential of these fluorescent nanoprobes as effective tools for two-photon fluorescence imaging and targeted detection of lysosomes in cancer cells.

A novel AIE fluorescence probe featuring with high quantum yield for high-fidelity lysosomal targeting and tracking

High-fidelity imaging and long-term visualization of lysosomes are pivotal factors in the functional assessment of lysosomes, which perform an instrumental role in the physiological activity of cells. However, commercial probes have great limitations in lysosome exploration resulting from the aggregation-caused quenching effect as well as photobleaching instability and small Stokes shift. Therefore, we constructed a novel probe named TTAM with triphenylamine as the matrix and morpholine ring as the targeting group. In contrast with commonly accessible Lyso-tracker Red, TTAM has the merits of aggregation-induced emission effect, extremely high quantum yields (51.57 % solid-state) as well as fluorescence intensity, significant photostability, and high resolution. These properties make it ideal for imaging and activity monitoring lysosomes, which provides a powerful condition for bio-imaging.

Pitfalls in methods to study colocalization of nanoparticles in mouse macrophage lysosomes

Background

In the field of nanoscience there is an increasing interest to follow dynamics of nanoparticles (NP) in cells with an emphasis on endo-lysosomal pathways and long-term NP fate. During our research on this topic, we encountered several pitfalls, which can bias the experimental outcome. We address some of these pitfalls and suggest possible solutions. The accuracy of fluorescence microscopy methods has an important role in obtaining insights into NP interactions with lysosomes at the single cell level including quantification of NP uptake in a specific cell type.

Methods

Here we use J774A.1 cells as a model for professional phagocytes. We expose them to fluorescently-labelled amorphous silica NP with different sizes and quantify the colocalization of fluorescently-labelled NP with lysosomes over time. We focus on confocal laser scanning microscopy (CLSM) to obtain 3D spatial information and follow live cell imaging to study NP colocalization with lysosomes.

Results

We evaluate different experimental parameters that can bias the colocalization coefficients (i.e., Pearson’s and Manders’), such as the interference of phenol red in the cell culture medium with the fluorescence intensity and image post-processing (effect of spatial resolution, optical slice thickness, pixel saturation and bit depth). Additionally, we determine the correlation coefficients for NP entering the lysosomes under four different experimental set-ups. First, we found out that not only Pearson’s, but also Manders’ correlation coefficient should be considered in lysosome-NP colocalization studies; second, there is a difference in NP colocalization when using NP of different sizes and fluorescence dyes and last, the correlation coefficients might change depending on live-cell and fixed-cell imaging set-up.

Conclusions

The results summarize detailed steps and recommendations for the experimental design, staining, sample preparation and imaging to improve the reproducibility of colocalization studies between the NP and lysosomes.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01670-9.

Antrodia Camphorata Polysaccharide activates autophagy and regulates NLRP3 degradation to improve liver injury-related inflammatory response

This study illustrated the liver protection mechanism of ACP from the perspective of autophagy activation. ACP suppressed the inflammatory injury of KCs, and decreased the cell apoptosis rate. After LTG and LC3 staining, ACP promoted lysosomal production, increased LC3 expression, activated autophagy, and suppressed the expression of NLRP3 and inflammatory factors. Under the electron microscope, ACP accelerated the production of autophagosomes. After simultaneous treatment with 3-MA and ACP, the effect of ACP on resisting KC injury decreased, the expression of NLRP3 was up-regulated, and autophagy was suppressed. As discovered in the mouse model of liver injury, ACP inhibited the ALT and AST levels, promoted the occurrence of autophagy, reduced NLRP3 expression and alleviated liver injury. ACP activates autophagy to induce NLRP3 degradation, thus suppressing inflammatory response in liver injury and exerting the liver protection effect, which is one of the mechanisms of action of ACP.

Aggregation-Induced Emission Luminogens for Cell Death Research

Cell death is closely related to various diseases, and monitoring and controlling cell death is a promising strategy to develop efficient therapy. Aggregation-induced emission luminogens (AIEgens) are ideal candidates for developing novel theranostic agents because of their intriguing properties in the aggregate state. The rational application of AIE materials in cell death-related research is still in its infancy but has shown great clinical potential. This review discussed the research frontier and our understanding of AIE materials in various subroutines of cell death, including apoptosis, necrosis, immunogenic cell death, pyroptosis, autophagy, lysosome-dependent cell death, and ferroptosis. We hope that the new insights can be offered to this growing field and attract more researchers to provide valuable contributions.

A bifunctional sensitive fluorescence probe based on pyrene for the detection of pH and viscosity in lysosome

Lysosome is one of the important organelles in intracellular transport. It plays a significant role in the physiological process. The lysosomal microenvironment affects the functions of lysosome. When the original acidic environment of lysozyme is destroyed or the fluid viscosity increases gradually, various diseases are easily induced. However, most fluorescent probes can only locate in cells. The fewer probes of subcellular organelles were found and their functions are often single. So, it is of great importance to design multifunctional fluorescent probes with the capable of localizing in lysosome. In this study, a novel lysosome probe, 4-(4-Pyren-1-yl-but-3-enyl)-morpholine (PIM), was synthesized using pyrene as a fluorescent group and morpholine as a target group. The introduction of morpholine group made PIM localize in lysosome with high selectivity. The fluorescence will be enhanced with the increased viscosity because of restricting the rotation of CC bond and CN in PIM, and the detecting linear range is from 4.05 cP to 393.48 cP, which qualified the requirement of the viscosity monitoring in body. Meanwhile, the fluorescence intensity of PIM declines with the decrease of pH because the Schiff base of PIM is hydrolyzed, which was affirmed by ¹H NMR, LC-MS and fluorescence spectra. Moreover, cell imaging and MTT experiments confirmed that PIM as a novel bifunctional probe can be used to detect pH and endogenous viscosity in lysosome.

Synthesis and mechanism of biological action of morpholinyl-bearing arylsquaramides as small-molecule lysosomal pH modulators

Lysosomal pH is an important modulator for many cellular processes. An agent that is capable of regulating lysosomal pH may find a wide range of potential applications in the field of biomedicine. In this study, we describe the synthesis of a family of morpholinyl-bearing arylsquaramides as small-molecule lysosomal pH modulators. These compounds are able to efficiently facilitate the transmembrane transport of chloride anions as mobile carriers across vesicular and cellular phospholipid membranes. They are capable of specifically alkalizing liposomes, disrupting the homeostasis of lysosomal pH and inactivivating lysosomal Cathepsin B enzyme. Anion transport is considered as the probable mechanism of action for the high efficiency of these compounds to modulate lysosomal pH. The present findings present a novel means to efficiently regulate lysosomal pH, which is in contrast to the methods shown by conventional lysosomal pH modulators that generally function by either acting as a weak base/acid, or releasing a basic/acidic component in lysosomal environments to change lysosomal pH.

A family of small-molecule arylsquaramides were synthesized as transmembrane anion transporters for modulating lysosomal pH.

The diversity of the coordination bond generated a POSS-based fluorescent probe for the reversible detection of Cu(II), Fe(III) and amino acids

In recent years, it has been found that Cu²⁺, Fe³⁺, and amino acids play an irreplaceable and subtle role in organisms and have attracted the considerable attention of many researchers. Therefore, it is vital to design visual indicators to reveal the relationships between metal ions and amino acids. However, there have been few reports on this vigorous subject. Fortunately, based on the different coordination effects between metal ions and boron groups, we have designed an accessible fluorescent probe (PSI-A). Borane was introduced as an ion-sensitive group to form a novel POSS-based fluorescent probe, which achieves fascinating performance, in situ dynamic multiple detection, excellent photostability, and enervative biological toxicity. PSI-A exhibited predominant selectivity and sensitivity to Cu²⁺/amino acids and Fe³⁺/amino acids sequence reactions in HepG2 cells and zebrafish. The fluorescence of PSI-A was quenched by Cu²⁺, which can be recovered by adding Asp, Ser, Arg, Ace or Trp. Additionally, the fluorescence of PSI-A quenched by Fe³⁺ can be restored after adding Asp. PSI-A is available to monitor Cu²⁺/amino acids and Fe³⁺/amino acids sequence reactions and can be repeated for at least three consecutive cycles without a fatigued performance. Therefore, this multifunctional fluorescent probe may have prospective application potentials in the biological field.

Practicable Applications of Aggregation-Induced Emission with Biomedical Perspective

Considerable efforts have been made into developing aggregation-induced emission fluorogens (AIEgens)-containing nano-therapeutic systems due to the excellent properties of AIEgens. Compared to other fluorescent molecules, AIEgens have advantages including low background, high signal-to-noise ratio, good sensitivity, and resistance to photobleaching, in addition to being exempt from concentration quenching or aggregation-caused quenching effects. The present review outlines the major developments in the biomedical applications of AIEgens-containing systems. From a literature survey, the recent AIE works are reviewed and the reasons why AIEgens are chosen in various biomedical applications are highlighted. The research activities on AIEgens-containing systems are increasing rapidly, therefore, the present review is timely.

AIE materials for lysosome imaging

The aggregation-induced emission (AIE) active bioprobes are known for their high photostability and extraordinary signal to noise ratio. In view of this, research efforts to synthesize new AIE bioimaging probes are at an incredible speed. In this chapter, we have summarized the various lysosome specific AIE active "turn-on" bioprobes having applications in lysosome imaging, monitoring of lysosome bioactivity and evaluation of their therapeutic effects. By discussing their design and operational mechanisms, we hope to provide more insight into designing new AIE bioprobes for specific sensing and imaging of lysosome having flexibility for broad range of biomedical applications.

Small molecule based fluorescent chemosensors for imaging the microenvironment within specific cellular regions

The microenvironment (local environment), including viscosity, temperature, polarity, hypoxia, and acidic-basic status (pH), plays indispensable roles in cellular processes. Significantly, organelles require an appropriate microenvironment to perform their specific physiological functions, and disruption of the microenvironmental homeostasis could lead to malfunctions of organelles, resulting in disorder and disease development. Consequently, monitoring the microenvironment within specific organelles is vital to understand organelle-related physiopathology. Over the past few years, many fluorescent probes have been developed to help reveal variations in the microenvironment within specific cellular regions. Given that a comprehensive understanding of the microenvironment in a particular cellular region is of great significance for further exploration of life events, a thorough summary of this topic is urgently required. However, there has not been a comprehensive and critical review published recently on small-molecule fluorescent chemosensors for the cellular microenvironment. With this review, we summarize the recent progress since 2015 towards small-molecule based fluorescent probes for imaging the microenvironment within specific cellular regions, including the mitochondria, lysosomes, lipid drops, endoplasmic reticulum, golgi, nucleus, cytoplasmic matrix and cell membrane. Further classifications at the suborganelle level, according to detection of microenvironmental factors by probes, including polarity, viscosity, temperature, pH and hypoxia, are presented. Notably, in each category, design principles, chemical synthesis, recognition mechanism, fluorescent signals, and bio-imaging applications are summarized and compared. In addition, the limitations of the current microenvironment-sensitive probes are analyzed and the prospects for future developments are outlined. In a nutshell, this review comprehensively summarizes and highlights recent progress towards small molecule based fluorescent probes for sensing and imaging the microenvironment within specific cellular regions since 2015. We anticipate that this summary will facilitate a deeper understanding of the topic and encourage research directed towards the development of probes for the detection of cellular microenvironments.

Synthesis and Tetraphenylethylene-Based Aggregation-Induced Emission Probe for Rapid Detection of Nitroaromatic Compounds in Aqueous Media

Tetraphenylethylene (TPE) can be used to construct fluorescent probes with typical aggregation-induced emission (AIE) behavior for next-generation sensing applications. McMurry coupling and Suzuki cross coupling strategies provided the desired sensor thiophene-substituted tetraphenylethylene (THTPE). The synthesized TPE analogues were characterized by NMR spectroscopy and mass spectrometry. Maximum AIE of THTPE was observed in 90% water (H2O/THF) content due to extensive formation of aggregates. The AIE properties of THTPE have been utilized for facile detection of nitroaromatic compounds (NACs) (1.0 nM) through a fluorescence quenching mechanism. A paper strip adsorbed with the AIE-based THTPE fluorophore is developed for rapid and convenient detection of NAC-based analytes. Further, interaction of THTPE with analytes is also studied via Gaussian software at the DFT/B3LYP/6-31G(d) level of theory. Interaction energy, frontier molecular orbitals (FMOs), and non-covalent interaction (NCI) analyses are studied by using the same method. Computational results revealed that nitrobenzene (NB) has the strongest interaction while 1,3-dinitrobenzene (DNB) exhibits the least interaction with the sensor molecule. These computational results clearly demonstrate good agreement with experimental data.

Green facile synthesis to develop nanoscale coordination polymers as lysosome-targetable luminescent bioprobes

Three new coordination polymers (CPs), namely [{M(HL)(L)(H2O)}(ClO4)(H2O)]∞ (M = Zn for CP 1, Mn for CP 2, Cu for CP 3) were synthesized to explore their efficacy as lysosome-targetable luminescent bioprobes. The synthesized CPs were characterized by techniques including single-crystal X-ray analysis, FTIR spectroscopy and elemental analysis. Single-crystal analysis revealed the formation of iso-structural CPs displaying distorted adamantoid topology developed by bridging ligands and H-bonds connections and metals at the nodes. A green hand-grinding technique with a mortar and pestle resulted in nanoscale coordination polymers (NCPs) suitable for cell permeability and was further confirmed by SEM and DLS analyses. Two of these hand-ground nanoscale coordination polymers NCP 1 and NCP 2 showed excellent green luminescence and were explored as potential and selective long-time biotrackers towards lysosome using the human lung carcinoma cell line (A549). Strikingly, the developed bioprobe displayed excellent bio-availability, photostability and excellent selectivity towards lysosomes sustained by various in vitro cell imaging experiments. Moreover, the long-term probing ability of these NCPs turned out to be better than the commercially available lysosome tracker i.e. LysoTracker Red, indicating their potential real-life application in bio-imaging. To the best ofour knowledge, this is the first example of nonexpensive and less toxic essential transition metal-based nanoscale coordination polymers that can behave as effective lysosome-targetable luminescent bioprobes.

AIE-based nanoaggregate tracker: high-fidelity visualization of lysosomal movement and drug-escaping processes

High-fidelity imaging and long-term visualization of lysosomes are crucial for their functional evaluation, related disease detection and active drug screening. However, commercial aggregation-caused quenching probes are not conducive to precise lysosomal imaging because of their inherent drawbacks, like easy diffusion, short emission and small Stokes shift, let alone their long-term tracing due to rapid photobleaching. Herein we report a novel aggregation-induced emission (AIE)-based TCM-PI nanoaggregate tracker for direct visualization of lysosomes based on the building block of tricyano-methylene-pyridine (TCM), wherein introduced piperazine (PI) groups behave as targeting units to lysosomes upon protonation, and the self-assembled nanostructure contributes to fast endocytosis for enhanced targeting ability as well as extended retention time for long-term imaging. The piperazine-stabilized TCM-PI nanoaggregate shifts the emission maximum to 677 nm in an aqueous environment, and falls within the desirable NIR region with a large Stokes shift of 162 nm, thereby greatly reducing biological fluorescent background interference. In contrast with the commercially available LysoTracker Red, the essential AIE characteristic of high photostability can guarantee three-dimensional high-fidelity tracing with low photobleaching, and little diffusion from lysosomes, and especially overcome the AIE bottleneck to target specificity. Consequently, the AIE-based nanoaggregate tracker successfully achieves the high-fidelity and long-term tracing of lysosomal movement and even monitors the drug-escaping process from lysosomes to cell nuclei, which provides a potential tool to benefit drug screening.

Dual site-controlled two-photon fluorescent probe for the imaging of lysosomal pH in living cells

Abnormal lysosomal pH is closely associated with many diseases, and real-time monitoring of lysosomal pH is important for understanding the lysosome physiological nature. Here, we present a novel lysosome-targeting two-photon fluorescent probe (MP-lys) for monitoring pH changes in living cells. As a dual site-controlled probe, MP-lys employed morpholine and piperazine groups as the lysosome-targeting groups and pH response sites. MP-lys showed rapid, reversible and sensitive fluorescence response to pH. MP-lys possessed lysosome-targeting properties, and could be used for two-photon imaging of chloroquine-induced pH variation in living cells.

Progress and Trends in AIE-Based Bioprobes: A Brief Overview

Luminescent bioprobes are powerful analytical means for biosensing and optical imaging. Luminogens featured with aggregation-induced emission (AIE) attributes have emerged as ideal building blocks for high-performance bioprobes. Bioprobes constructed with AIE luminogens have been identified to be a novel class of FL light-up probing tools. In contrast to conventional bioprobes based on the luminophores with aggregation-caused quenching (ACQ) effect, the AIE-based bioprobes enjoy diverse superiorities, such as lower background, higher signal-to-noise ratio and sensitivity, better accuracy, and more outstanding resistance to photobleaching. AIE-based bioprobes have been tailored for a vast variety of purposes ranging from biospecies sensing to bioimaging to theranostics (i.e., image-guided therapies). In this review, recent five years' advances in AIE-based bioprobes are briefly overviewed in a perspective distinct from other reviews, focusing on the most appealing trends and progresses in this flourishing research field. There are altogether 11 trends outlined, which have been classified into four aspects: the probe composition and form (bioconjugtes, nanoprobes), the output signal of probe (far-red/near-infrared luminescence, two/three-photon excited fluorescence, phosphorescence), the modality and functionality of probing system (dual-modality, dual/multifunctionality), the probing object and application outlet (specific organelles, cancer cells, bacteria, real samples). Typical examples of each trend are presented and specifically demonstrated. Some important prospects and challenges are pointed out as well in the hope of intriguing more interests from researchers working in diverse areas into this exciting research field.

Functionalized chitosan with self-assembly induced and subcellular localization-dependent fluorescence ‘switch on’ property

A chitosan-based probe was developed that offers a self-assembly-induced and subcellular localization-dependent fluorescence ‘switch on’ property.

A pH responsive AIE probe for enzyme assays

By combining leucine (Leu) and tetraphenylethene (TPE), a pH-sensitive aggregation induced emission (AIE) probe TPE-Leu was developed.

A tumor-targeting and lysosome-specific two-photon fluorescent probe for imaging pH changes in living cells

Lysosomal pH is closely related to the metastasis and apoptosis of cancer cells. Detecting lysosomal pH changes in cancer cells could be helpful for analyzing tumor progressions and in-depth study of the roles of lysosomes in tumor invasion and metastasis. Herein, we describe a novel tumor-targeting and lysosome-specific two-photon fluorescent probe (BN-lys) for imaging pH changes for the first time. Biotin was employed as the tumor-targeting module, and morpholine was selected as the lysosome-specific group and the pH site to control the fluorescence by photoinduced electron transfer (PET) mechanism. With a pK_a value of 5.36, BN-lys showed a fast and reversible fluorescence response to pH. Under the guidance of the biotin group, BN-lys displayed strong one-photon and two-photon fluorescence responses to lysosomal pH in cancer cells, while it displayed weak fluorescence in normal cells. Furthermore, BN-lys could be applied for the imaging of chloroquine-stimulated lysosomal pH changes in living cells. These features demonstrate that this probe could have practical applications in biological research.