Deciphering interior polarity of lysosome in live cancer and normal cells using spectral scanning microscopy

An acridone based fluorescent dye for lipid droplet tracking and cancer diagnosis

Lipid droplets (LDs) are spherical organelles that localize in the cytosol of eukaryotic cells. Different proteins are embedded on the surface of LDs, so LDs play a vital role in the physiological activities of cells. The dysregulation of LDs is associated with various human diseases, such as diabetes and obesity. Therefore, it is essential to develop a fluorescent dye that labels LDs to detect and monitor illnesses. In this study, we developed the compound BDAA12C for staining LDs in cells. BDAA12C exhibits excellent LD specificity and low toxicity, enabling us to successfully stain and observe the fusion of LDs in A549 cancer cells. Furthermore, we also successfully distinguished A549 cancer cells and MRC-5 normal cells in a co-culture experiment and in normal and tumour tissues. Interestingly, we found different localizations of BDAA12C in well-fed and starved A549 cancer cells and consequently illustrated the transfer of fatty acids (FAs) from LDs to mitochondria to supply energy for β-oxidation upon starvation. Therefore, BDAA12C is a promising LD-targeted probe for cancer diagnosis and tracking lipid trafficking within cells.

Chronological development of functional fluorophores for bio-imaging

Functional fluorophores represent an emerging research field, distinguished by their diverse applications, especially in sensing and cellular imaging. After the discovery of quinine sulfate and subsequent elucidation of the fluorescence mechanism by Sir George Stokes, research in the field of fluorescence gained momentum. Over the past few decades, advancements in sophisticated instruments, including super-resolution microscopy, have further promoted cellular imaging using traditional fluorophores. These advancements include deciphering sensing mechanisms via photochemical reactions and scrutinizing the applications of fluorescent probes that specifically target organelles. This approach elucidates molecular interactions with biomolecules. Despite the abundance of literature illustrating different classes of probe development, a concise summary of newly developed fluorophores remains inadequate. In this review, we systematically summarize the chronological discovery of traditional fluorophores along with new fluorophores. We briefly discuss traditional fluorophores ranging from visible to near-infrared (NIR) in the context of cellular imaging and in vivo imaging. Furthermore, we explore ten new core fluorophores developed between 2007 and 2022, which exhibit advanced optical properties, providing new insights into bioimaging. We illustrate the utilization of new fluorophores in cellular imaging of biomolecules, such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and proteins and microenvironments, especially pH and viscosity. Few of the fluorescent probes provided new insights into disease progression. Furthermore, we speculate on the potential prospects and significant challenges of existing fluorophores and their potential biomedical research applications. By addressing these aspects, we intend to illuminate the compelling advancements in fluorescent probe development and their potential influence across various fields.

Photoinduced Ring-Opening and Phototoxicity of an Indolin-3-one Derivative

The study of a fluorescent indolin-3-one derivative is reported that, as opposed to its previously described congeners, selectively undergoes photoactivated ring-opening in apolar solvents. The excited state involved in this photoisomerization was partially deactivated by the formation of singlet oxygen. Cell studies revealed lipid droplet accumulation and efficient light-induced cytotoxicity.

Organic Fluorescent Probes for Monitoring Micro-Environments in Living Cells and Tissues

As a vital parameter in living cells and tissues, the micro-environment is crucial for the living organisms. Significantly, organelles require proper micro-environment to achieve normal physiological processes, and the micro-environment in organelles can reflect the state of organelles in living cells. Moreover, some abnormal micro-environments in organelles are closely related to organelle dysfunction and disease development. So, visualizing and monitoring the variation of micro-environments in organelles is helpful for physiologists and pathologists to study the mechanisms of the relative diseases. Recently, a large variety of fluorescent probes was developed to study the micro-environments in living cells and tissues. However, the systematic and comprehensive reviews on the organelle micro-environment in living cells and tissues have rarely been published, which may hinder the research progress in the field of organic fluorescent probes. In this review, we will summarize the organic fluorescent probes for monitoring the microenvironment, such as viscosity, pH values, polarity, and temperature. Further, diverse organelles (mitochondria, lysosome, endoplasmic reticulum, cell membrane) about microenvironments will be displayed. In this process, the fluorescent probes about the "off-on" and ratiometric category (the diverse fluorescence emission) will be discussed. Moreover, the molecular designing, chemical synthesis, fluorescent mechanism, and the bio-applications of these organic fluorescent probes in cells and tissues will also be discussed. Significantly, the merits and defects of current microenvironment-sensitive probes are outlined and discussed, and the development tendency and challenges for this kind of probe are presented. In brief, this review mainly summarizes some typical examples and highlights the progress of organic fluorescent probes for monitoring micro-environments in living cells and tissues in recent research. We anticipate that this review will deepen the understanding of microenvironment in cells and tissues and facilitate the studies and development of physiology and pathology.

Solvent-polarity reconfigurable fluorescent 4-piperazino-N-aryl-1,8-naphthalimide crown ether logic gates

Four compounds 1-4 were designed and synthesised, comprising a 4-amino-N-aryl-1,8-naphthalimide fluorophore, a piperazine receptor, and an aryl group, as fluorescent logic gates. At the imide position, the substituent is phenyl (1), 1,2-dimethoxyphenyl (2), benzo-15-crown-5 (3), or benzo-18-crown-6 (4). Molecules 1 and 2 are constructed according to a fluorophore-spacer-receptor format, while 3 and 4 are engineered according to a receptor1-spacer1-fluorophore-spacer2-receptor2 format based on photoinduced electron transfer and internal charge transfer mechanisms. The compounds were studied in water, water/methanol mixtures of different ratios, and methanol by UV-visible absorption and steady-state fluorescence spectroscopy, as a function of pH, metal ions and solvent polarity. The excited state of 1-4 is 8.4 ± 0.2 in water, 7.6 ± 0.1 in 1 : 1 (v/v) water/methanol, and 7.1 ± 0.3 in methanol. The of 3 in water is 0.92 and the and of 4 in water are 2.3 and 2.9. 1H NMR data in D2O and CD3OD confirm H+ interaction at the piperazine moiety, and Na+ and Ba2+ binding at the benzo-15-crown-5 and benzo-18-crown-6 moieties of 3 and 4. By altering the solvent polarity, the fluorescent logic gates can be reconfigured between TRANSFER logic and AND logic. Molecules with polarity reconfigurable logic could be useful tools for probing the microenvironment of cellular membranes and protein interfaces.

Responsive Fluorescent Coumarin–Cinnamic Acid Conjugates for α-Glucosidase Detection

α-Glucosidase is a potent drug target for treating type II diabetes mellitus. A great number of α-glucosidase inhibitors have been developed based on the molecular skeletons of bioactive natural products. However, efficient fluorescent probes for α-glucosidase detection are still limited, not to mention the probes with additional inhibitory functions. In this work, aiming for the enzyme’s highly specific detection, we designed and synthesized two environmentally sensitive fluorescent probes, namely, LD01 and LD02, respectively, based on conjugates of coumarin and cinnamic acid derivatives. We found a significant responsive emission enhancement upon LD02’s binding to α-glucosidase. These newly designed probes can act as a simple but efficient tool to evaluate the binding affinity of α-glucosidase to their inhibitors.

A Fluorogenic Far Red-Emitting Molecular Viscometer for Ascertaining Lysosomal Stress in Live Cells and Caenorhabditis elegans

The cellular physiochemical properties such as polarity, viscosity, and pH play a critical role in cellular homeostasis. The dynamic change of lysosomal viscosity in live cells associated with different environmental stress remains enigmatic and needs to be explored. We have developed a new class of Julolidine-based molecular viscometers with an extended π-conjugation to probe the lysosomal viscosity in live cells. High biocompatibility, pH tolerance, and the fluorogenic response with far red-emission (>600 nm) properties make these molecular viscometers suitable for live-cell fluorescence imaging in Caenorhabditis elegans. Among these probes, JIND-Mor is specifically designed to target lysosomes via simple modification. The real-time monitoring of lysosomal viscosity change under cellular stress was achieved. We believe that such a class of molecule viscometers has the potential to monitor lysosomal health in pathogenic conditions.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

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.

Dynamic Excimer (DYNEX) Imaging of Lipid Droplets

Unraveling cellular physiological processes via luminescent probes that target specific cellular microenvironments is quite challenging due to the uneven distribution of probes. Herein, we designed a new dynamic excimer (DYNEX) imaging method that involves the sensitive detection of nanosecond-scale dynamic molecular contacts of a fluorescent acridone derivative and reveals the cell microenvironment polarity. Using our method, we specifically tracked cell lipid droplets in fibroblast colon carcinoma cells. These organelles play a central role in metabolic pathways, acting as energy reservoirs in regulatory processes. DYNEX imaging provides the inner polarity of cell lipid droplets, which can be related to lipid contents and metabolic dysfunctions. This new methodology will inspire development of novel multidimensional fluorescent sensors that are able to provide target-specific and orthogonal information at the nanosecond scale.

Strategic engineering of alkyl spacer length for a pH-tolerant lysosome marker and dual organelle localization

Long-term visualization of lysosomal properties is extremely crucial to evaluate diseases related to their dysfunction. However, many of the reported lysotrackers are less conducive to imaging lysosomes precisely because they suffer from fluorescence quenching and other inherent drawbacks such as pH-sensitivity, polarity insensitivity, water insolubility, slow diffusibility, and poor photostability. To overcome these limitations, we have utilized an alkyl chain length engineering strategy and synthesized a series of lysosome targeting fluorescent derivatives namely NIMCs by attaching a morpholine moiety at the peri position of the 1,8-naphthalimide (NI) ring through varying alkyl spacers between morpholine and 1,8-naphthalimide. The structural and optical properties of the synthesized NIMCs were explored by 1H-NMR, single-crystal X-ray diffraction, UV-Vis, and fluorescence spectroscopy. Afterward, optical spectroscopic measurements were carefully performed to identify a pH-tolerant, polarity sensitive, and highly photostable fluoroprobes for further live-cell imaging applications. NIMC6 displayed excellent pH-tolerant and polarity-sensitive properties. Consequently, all NIMCs were employed in kidney fibroblast cells (BHK-21) to investigate their applicability for lysosome targeting and probing lysosomal micropolarity. Interestingly, a switching of localization from lysosomes to the endoplasmic reticulum (ER) was also achieved by controlling the linker length and this phenomenon was subsequently applied in determining ER micropolarity. Additionally, the selected probe NIMC6 was also employed in BHK-21 cells for 3-D spheroid imaging and in Caenorhabditis elegans (C. elegans) for in vivo imaging, to evaluate its efficacy for imaging animal models.

An Enumerated Outlook of Intracellular Micropolarity Using Solvatochromic Organic Fluorescent Probes

The spatiotemporal distribution of intracellular physical parameters of a live cell is heterogeneous and complex. Measuring physical properties inside given cellular compartments (organelles) is challenging and important for therapy and diagnostics. The tiny volume of a single cell and even tinier organelles are not accessible by classical measuring devices. The microenvironment inside an organelle vastly controls the outcome of any biochemical and biophysical processes taking place inside it, which is crucial for the overall cellular health. Therefore, it is very important to understand the microenvironmental physical properties inside cellular organelles. Moreover, specific alterations of such microenvironmental properties were observed in the disease condition, making them a diagnostic hallmark. With this high demand, small-molecule organic fluorophores are emerging as the most successful tool due to their small relative size, bioavailability, and ease of functionalization. In this mini-review, the development of micropolarity-sensitive small organic fluorophore with the capability of targeting a specific cellular organelle has been discussed. Here, we have highlighted the strategies of targeting a specific organelle, the micropolarity, and the challenges and prospects of the field.