Ratiometric biological nanosensors

In-situ quantification of lipids in live cells through imaging approaches

Lipids are important molecules that are widely distributed within the cell, and they play a crucial role in several biological processes such as cell membrane formation, signaling, cell motility and division. Monitoring the spatiotemporal dynamics of cellular lipids in real-time and quantifying their concentrations in situ is crucial since the local concentration of lipids initiates various signaling pathways that regulate cellular processes. In this review, we first introduced the historical background of lipid quantification methods. We then delve into the current state of the art of in situ lipid quantification, including the establishment and utility of fluorescence imaging techniques based on sensors of lipid-binding domains labeled with organic dyes or fluorescent proteins, and Raman and magnetic resonance imaging (MRI) techniques that do not require lipid labeling. Next, we highlighted the biological applications of live-cell lipid quantification techniques in the study of in situ lipid distribution, lipid transformation, and lipid-mediated signaling pathways. Finally, we discussed the technical challenges and prospects for the development of lipid quantification in live cells, with the aim of promoting the development of in situ lipid quantification in live cells, which may have a profound impact on the biological and medical fields.

Triplet-Triplet Annihilation Upconversion-Based Oxygen Sensors to Overcome the Limitation of Autofluorescence

Autofluorescence is one of the many challenges in bioimaging as it can mask the emission from fluorescent probes or markers, a limitation that can be overcome via upconversion. Herein, we have developed a nanosensor that uses triplet-triplet annihilation upconversion to optically report changes in the dissolved oxygen concentration. Using a sensitizer-annihilator dye pairing of platinum(II) octaethylporphyrin and 9,10-diphenylanthracene, we monitored the oxygen consumption (as a proxy for metabolic activity) over time in a biological system─Saccharomyces cerevisiae (brewing yeast). The nanosensor demonstrated good reversibility over multiple cycles and showed good signal and colloidal stability when tested over the course of 7 days, and it was sensitive to dissolved oxygen from 0.00 to 3.17 mg/L O2. Additionally, there was no signal overlap between the nanosensor emission and S. cerevisiae autofluorescence, thus underscoring the utility of upconversion as a facile and economical means of overcoming autofluorescence.

Photostable and Orthogonal Solvatochromic Fluorophores for Simultaneous In Situ Quantification of Multiple Cellular Signaling Molecules

Ratiometric fluorescence sensors are powerful tools for direct quantification of diverse biological analytes. To overcome a shortage of solvatochromic fluorophores crucial for in situ ratiometric imaging of biological targets, we prepared and characterized a small library of modular fluorophores with diverse spectral properties. Among them, WCB and WCR showed excellent spectral properties, including high photostability, brightness, and solvatochromism, and are ideally suited for dual ratiometric imaging due to their spectral orthogonality. By conjugating WCB and WCR with protein-based lipid sensors, we were able to achieve robust simultaneous in situ quantitative imaging of two metabolically linked signaling lipids, phosphatidylinositol-4,5-bisphosphate and phosphatidylinositol-3,4,5-trisphosphate in live cells. This study shows that any combination of signaling molecules can be simultaneously quantified in a spatiotemporally resolved manner by ratiometric imaging with finely tuned solvatochromic fluorophores.

Human cystic fibrosis monocyte derived macrophages display no defect in acidification of phagolysosomes when measured by optical nanosensors

BACKGROUND

Defective macrophage phagolysosomal acidification is implicated in numerous lung diseases including Cystic Fibrosis (CF) and may contribute to defective pathogen killing. Conflicting reports relating to phagolysosomal pH in CF macrophages have been published, in part related to the use of pH-sensitive fluorescent probes where potential inadequacies in experimental design can be a contributing factor (e.g. employing probes with incorrect pKa for the cellular compartment of interest). We developed a reliable method to quantify macrophage phagolysosomal pH using surface-enhanced Raman spectroscopy-based nanosensors.

METHODS

Monocyte-derived macrophages from CF and healthy control participants were incubated with nanosensors. Live cell imaging identified phagocytosed nanosensors, and surface-enhanced Raman spectroscopy was performed using para-mercaptobenzoic acid functionalised gold nanoparticles which produce Raman spectra that change predictably with their environmental pH. Conventional fluorescence spectroscopy was carried out in comparison. Nanosensor localisation to phagolysosomes was confirmed by transmission electron microscopy.

RESULTS

Nanosensors were actively phagocytosed by macrophages into phagolysosomes and acidification occurred rapidly and remained stable for at least 60 min. There was no difference in phagolysosomal pH between healthy control and CF macrophages (5.41 ± 0.11 vs. 5.41 ± 0.20, p > .9999), further confirmed by inhibiting Cystic Fibrosis Transmembrane Conductance Regulator in healthy control monocyte-derived macrophages.

CONCLUSIONS

Optical nanosensors accurately measure macrophage phagolysosomal pH and demonstrate no phagolysosomal acidification defect in human CF monocyte-derived macrophages. Further studies using alveolar macrophages could extend the impact of our findings. Nanosensors represent a novel and precise means to measure organelle functions with widespread potential for the study and monitoring of several lung diseases.

Nanoparticle-Based and Bioengineered Probes and Sensors to Detect Physiological and Pathological Biomarkers in Neural Cells

Nanotechnology, a rapidly evolving field, provides simple and practical tools to investigate the nervous system in health and disease. Among these tools are nanoparticle-based probes and sensors that detect biochemical and physiological properties of neurons and glia, and generate signals proportionate to physical, chemical, and/or electrical changes in these cells. In this context, quantum dots (QDs), carbon-based structures (C-dots, grapheme, and nanodiamonds) and gold nanoparticles are the most commonly used nanostructures. They can detect and measure enzymatic activities of proteases (metalloproteinases, caspases), ions, metabolites, and other biomolecules under physiological or pathological conditions in neural cells. Here, we provide some examples of nanoparticle-based and genetically engineered probes and sensors that are used to reveal changes in protease activities and calcium ion concentrations. Although significant progress in developing these tools has been made for probing neural cells, several challenges remain. We review many common hurdles in sensor development, while highlighting certain advances. In the end, we propose some future directions and ideas for developing practical tools for neural cell investigations, based on the maxim "Measure what is measurable, and make measurable what is not so" (Galileo Galilei).