SERS nanosensors that report pH of endocytic compartments during FcεRI transit

Unraveling Eumelanin Radical Formation by Nanodiamond Optical Relaxometry in a Living Cell

Defect centers in a nanodiamond (ND) allow the detection of tiny magnetic fields in their direct surroundings, rendering them as an emerging tool for nanoscale sensing applications. Eumelanin, an abundant pigment, plays an important role in biology and material science. Here, for the first time, we evaluate the comproportionation reaction in eumelanin by detecting and quantifying semiquinone radicals through the nitrogen-vacancy color center. A thin layer of eumelanin is polymerized on the surface of nanodiamonds (NDs), and depending on the environmental conditions, such as the local pH value, near-infrared, and ultraviolet light irradiation, the radicals form and react in situ. By combining experiments and theoretical simulations, we quantify the local number and kinetics of free radicals in the eumelanin layer. Next, the ND sensor enters the cells via endosomal vesicles. We quantify the number of radicals formed within the eumelanin layer in these acidic compartments by applying optical relaxometry measurements. In the future, we believe that the ND quantum sensor could provide valuable insights into the chemistry of eumelanin, which could contribute to the understanding and treatment of eumelanin- and melanin-related diseases.

Synergistic Phenomena between Iron-Doped ZnO Nanoparticles and Shock Waves Exploited against Pancreatic Cancer Cells

We propose the use of iron-doped zinc oxide nanoparticles (Fe:ZnO NPs) showing theranostic capabilities and being synergistically active against pancreatic ductal adenocarcinoma once combined with mechanical pressure waves, such as shock waves. Fe:ZnO NPs are synthesized by employing oleic acid as a capping agent and are functionalized with amino-propyl groups. We first report their superior characteristics with respect to undoped ZnO NPs in terms of magnetic properties, colloidal stability, cytocompatibility, and internalization into BxPC-3 pancreatic cancer cells in vitro. These Fe:ZnO NPs are also cytocompatible toward normal pancreatic cells. We then perform a synergistic cell treatment with both shock waves and Fe:ZnO NPs once internalized into cells. We also evaluate the contribution to the synergistic activity of the NPs located in the extracellular space. Results show that both NPs and shock waves, when administered separately, are safe to cells, while their combination provokes an enhanced cell death after 24 h. Various mechanisms are then considered, such as dissolution of NPs, production of free radicals, and cell membrane disruption or permeation. It is understood so far that iron-doped ZnO NPs can degrade intracellularly into zinc cations, while the use of shock waves produce cell membrane permeabilization and possible rupture. In contrast, the production of reactive oxygen species is here ruled out. The provoked cell death can be recognized in both apoptotic and necrotic events. The proposed work is thus a first proof-of-concept study enabling promising future applications to deep-seated tumors such as pancreatic cancer, which is still an unmet clinical need with a tremendous death rate.

Covalent organic framework-based fluorescent nanoprobe for intracellular pH sensing and imaging

Lysosomes are the acidic organelles in the cells that play an important role in intracellular degradation and other various cellular functions. The pH disturbance of lysosomes will result in the lysosomal dysfunction and many lysosomal related diseases. In this work, we reported a methoxy-based covalent organic framework (TAPB-DMTP-COF) that a novel pH-responsive fluorescent probe for lysosomal pH imaging in cells. The prepared TAPB-DMTP-COF presented regular crystal structure, low toxicity and good pH responsive property. The rich imine structure in the material enabled pH-responsive properties of the TAPB-DMTP-COF and made it exhibited pH-dependent fluorescence response. Good detection linearity for pH measurements in aqueous solution was achieved by this probe. Moreover, the TAPB-DMTP-COF can be used for the selective lysosomal pH imaging. Confocal fluorescence imaging results demonstrated that the pH fluctuations (from 4.0 to 7.4) and the pH changes in lysosomes can be effectively monitored in situ by the developed probe. This study may provide a new avenue for the intracellular pH sensing, deep study and understanding about the mechanism of diseases related to abnormal lysosomal pH.

Intracellular pH - Advantages and pitfalls of surface-enhanced Raman scattering and fluorescence microscopy - A review

The value of pH in various parts of protoplasm can affect nearly all aspects of cell functions. Therefore, the determination of intracellular acid-base features is required in many areas of biological and biochemical studies. Because of a significant scientific importance of in vivo intracellular pH measurements, various groups carried out such experiments. In this review article we describe intracellular pH measurements using two the most sensitive optical spectroscopies: surface-enhanced Raman scattering (SERS) and fluorescence. It is reasonable to present these two techniques in one review article because the experimental approach in Raman and fluorescence experiments is relatively similar. The basic theoretical background explaining the mechanism of operation of fluorescence and SERS sensors are discussed and the motivations to carry out intracellular pH measurements are briefly described. Future perspectives in this field are also discussed.

Might proton pump or sodium-hydrogen exchanger inhibitors be of value to ameliorate SARs-CoV-2 pathophysiology?

Discovering therapeutics for COVID-19 is a priority. Besides high-throughput screening of compounds, candidates might be identified based on their known mechanisms of action and current understanding of the SARs-CoV-2 life cycle. Using this approach, proton pump (PPIs) and sodium-hydrogen exchanger inhibitors (NHEIs) emerged, because of their potential to inhibit the release of extracellular vesicles (EVs; exosomes and/or microvesicles) that could promote disease progression, and to directly disrupt SARs-CoV-2 pathogenesis. If EVs exacerbate SARs-CoV-2 infection as suggested for other viruses, then inhibiting EV release by PPIs/NHEIs should be beneficial. Mechanisms underlying inhibition of EV release by these drugs remain uncertain, but may involve perturbing endosomal pH especially of multivesicular bodies where intraluminal vesicles (nascent exosomes) are formed. Additionally, PPIs might inhibit the endosomal sorting complex for transport machinery involved in EV biogenesis. Through perturbing endocytic vesicle pH, PPIs/NHEIs could also impede cleavage of SARs-CoV-2 spike protein by cathepsins necessary for viral fusion with the endosomal membrane. Although pulmonary epithelial cells may rely mainly on plasma membrane serine protease TMPRSS2 for cell entry, PPIs/NHEIs might be efficacious in ACE2-expressing cells where viral endocytosis is the major or a contributing entry pathway. These pharmaceutics might also perturb pH in the endoplasmic reticulum-Golgi intermediate and Golgi compartments, thereby potentially disrupting viral assembly and glycosylation of spike protein/ACE2, respectively. A caveat, however, is that facilitation not inhibition of avian infectious bronchitis CoV pathogenesis was reported in one study after increasing Golgi pH. Envelope protein-derived viroporins contributed to pulmonary edema formation in mice infected with SARs-CoV. If similar pathogenesis occurs with SARs-CoV-2, then blocking these channels with NHEIs could ameliorate disease pathogenesis. To ascertain their potential efficacy, PPIs/NHEIs need evaluation in cell and animal models at various phases of SARs-CoV-2 infection. If they prove to be therapeutic, the greatest benefit might be realized with the administration before the onset of severe cytokine release syndrome.

Monitoring the Changes of pH in Lysosomes during Autophagy and Apoptosis by Plasmon Enhanced Raman Imaging

Lysosomes are acidic organelles that not only participate in intracellular degradation but also relate to various cellular functions. Abnormal pH in lysosomes would lead to lysosomal dysfunction, which may further result in many diseases. In this work, we statistically analyze the pH change in the lysosomes of HeLa cells model by using surface enhanced Raman scattering (SERS) imaging technique. We prepared a plasmon Raman pH probe and localized the pH probe to lysosomes via an incubation-depletion method. The pH profiles within lysosomes during the process of cellular autophagy and apoptosis were monitored in situ by SERS imaging. The pH in lysosomes decreased slightly during the process of autophagy, while the pH in lysosomes increased during apoptosis. The phenomenon, in general, is consistent with our current biological knowledge. However, we did not observe significant variation of pH between different individual cells. This information might provide an in depth understanding about the relationship of lysosomal pH with fundamental cellular functions and mechanism of diseases.

A Diacylglycerol Kinase Inhibitor, R-59-022, Blocks Filovirus Internalization in Host Cells

Filoviruses, such as Ebola virus (EBOV) and Marburg virus, are causative agents of unpredictable outbreaks of severe hemorrhagic fevers in humans and non-human primates. For infection, filoviral particles need to be internalized and delivered to intracellular vesicles containing cathepsin proteases and the viral receptor Niemann-Pick C1. Previous studies have shown that EBOV triggers macropinocytosis of the viral particles in a glycoprotein (GP)-dependent manner, but the molecular events required for filovirus internalization remain mostly unknown. Here we report that the diacylglycerol kinase inhibitor, R-59-022, blocks EBOV GP-mediated entry into Vero cells and bone marrow-derived macrophages. Investigation of the mode of action of the inhibitor revealed that it blocked an early step in entry, more specifically, the internalization of the viral particles via macropinocytosis. Finally, R-59-022 blocked viral entry mediated by a panel of pathogenic filovirus GPs and inhibited growth of replicative Ebola virus. Taken together, our studies suggest that R-59-022 could be used as a tool to investigate macropinocytic uptake of filoviruses and could be a starting point for the development of pan-filoviral therapeutics.

Bioconjugation strategy for cell surface labelling with gold nanostructures designed for highly localized pH measurement

Regulation of intracellular pH is critically important for many cellular functions. The quantification of proton extrusion in different types of cells and physiological conditions is pivotal to fully elucidate the mechanisms of pH homeostasis. Here we show the use of gold nanoparticles (AuNP) to create a high spatial resolution sensor for measuring extracellular pH in proximity of the cell membrane. We test the sensor on HepG2 liver cancer cells and MKN28 gastric cancer cells before and after inhibition of Na+/H+ exchanger. The gold surface conjugation strategy is conceived with a twofold purpose: i) to anchor the AuNP to the membrane proteins and ii) to quantify the local pH from AuNP using surface enhanced Raman spectroscopy (SERS). The nanometer size of the cell membrane anchored sensor and the use of SERS enable us to visualize highly localized variation of pH induced by H+ extrusion, which is particularly upregulated in cancer cells.

Tyramine detection using PEDOT:PSS/AuNPs/1-methyl-4-mercaptopyridine modified screen-printed carbon electrode with molecularly imprinted polymer solid phase extraction

Tyramine (4-hydroxyphenethylamine), which is a monoamine metabolized by monoamine oxidase (MAO), exists widely in plants, animals, fermented foods, and salted foods. The incidence of hypertension, or "cheese effect", which is associated with a large dietary intake of tyramine while taking MAO inhibitors has been reported; therefore, the measurement of tyramine is an urgent concern. Herein, an efficient approach that integrates a molecular imprinting polymer for solid phase extraction (MISPE) technique with a sensitive electrochemical sensing platform (SPCE/PEDOT: PSS/AuNP/1-m-4-MP) for the quantification of tyramine is presented. Enhanced electrode conductivity was achieved sequentially by constructing a conductive polymer (PEDOT: PSS) on a screen-printed carbon electrode (SPCE), followed by electrodeposition with gold nanoparticles (AuNPs) and, finally, by modification with positively charged 1-methyl-4-mercaptopyridine (1-m-4-MP) using an Au-S bond. Tyramine was isolated selectively and pre-concentrated by the MISPE technique; electroanalysis that used differential pulse voltammetry (DPV) in NaOH (0.1M, pH 13) was conducted successively. Experimental parameters (such as modes of electrode modification, ratio of PEDOT: PSS, pH of electrolyte, time required for AuNP deposition, and 1-m-4-MP concentrations) that were associated with optimal detection conditions were evaluated also. We obtained a linear concentration range (5-100nM, R²=0.9939) with LOD and sensitivity at 2.31nM, and 3.11μAnM^-1cm^-2, respectively. The applicability of our technique was demonstrated by analyzing tyramine in spiked serum and milk. The feature of our newly developed analytical methods that coupled sample pre-treatment (sample clean-up and pre-concentration) with sensitive detection makes it a promising tool for quantifying of tyramine.

SERS-based monitoring of the intracellular pH in endothelial cells: the influence of the extracellular environment and tumour necrosis factor-α

The intracellular pH plays an important role in various cellular processes. In this work, we describe a method for monitoring of the intracellular pH in endothelial cells by using surface enhanced Raman spectroscopy (SERS) and 4-mercaptobenzoic acid (MBA) anchored to gold nanoparticles as pH-sensitive probes. Using the Raman microimaging technique, we analysed changes in intracellular pH induced by buffers with acid or alkaline pH, as well as in endothelial inflammation induced by tumour necrosis factor-α (TNFα). The targeted nanosensor enabled spatial pH measurements revealing distinct changes of the intracellular pH in endosomal compartments of the endothelium. Altogether, SERS-based analysis of intracellular pH proves to be a promising technique for a better understanding of intracellular pH regulation in various subcellular compartments.

Determination of cell uptake pathways for tumor inhibitor lysyl oxidase propeptide

The lysyl oxidase propeptide (LOX-PP) is derived from pro-lysyl oxidase (Pro-LOX) by extracellular biosynthetic proteolysis. LOX-PP inhibits breast and prostate cancer xenograft tumor growth and has tumor suppressor activity. Although, several intracellular targets and molecular mechanisms of action of LOX-PP have been identified, LOX-PP uptake pathways have not been reported. Here we demonstrate that the major uptake pathway for recombinant LOX-PP (rLOX-PP) is PI3K-dependent macropinocytosis in PWR-1E, PC3, SCC9, MDA-MB-231 cell lines. A secondary pathway appears to be dynamin- and caveola dependent. The ionic properties of highly basic rLOX-PP provide buffering capacity at both high and low pHs. We suggest that the buffering capacity of rLOX-PP, which serves to limit endosomal acidification, sustains PI3K-dependent macropinocytosis in endosomes which in turn is likely to facilitate LOX-PP endosomal escape into the cytoplasm and its observed interactions with cytoplasmic targets and nuclear uptake.

BSA-coated nanoparticles for improved SERS-based intracellular pH sensing

Local microenvironment pH sensing is one of the key parameters for the understanding of many biological processes. As a noninvasive and high sensitive technique, surface-enhanced Raman spectroscopy (SERS) has attracted considerable interest in the detection of the local pH of live cells. We herein develop a facile way to prepare Au-(4-MPy)-BSA (AMB) pH nanosensor. The 4-MPy (4-mercaptopyridine) was used as the pH sensing molecule. The modification of the nanoparticles with BSA not only provides a high sensitive response to pH changes ranging from pH 4.0 to 9.0 but also exhibits a high sensitivity and good biocompatibility, stability, and reliability in various solutions (including the solutions of high ionic strength or with complex composition such as the cell culture medium), both in the aggregation state or after long-term storage. The AMB pH nanosensor shows great advantages for reliable intracellular pH analysis and has been successfully used to monitor the pH distribution of live cells and can address the grand challenges in SERS-based pH sensing for practical biological applications.

Interfacing engineered nanoparticles with biological systems: anticipating adverse nano-bio interactions

The innovative use of engineered nanomaterials in medicine, be it in therapy or diagnosis, is growing dramatically. This is motivated by the current extraordinary control over the synthesis of complex nanomaterials with a variety of biological functions (e.g. contrast agents, drug-delivery systems, transducers, amplifiers, etc.). Engineered nanomaterials are found in the bio-context with a variety of applications in fields such as sensing, imaging, therapy or diagnosis. As the degree of control to fabricate customized novel and/or enhanced nanomaterials evolves, often new applications, devices with enhanced performance or unprecedented sensing limits can be achieved. Of course, interfacing any novel material with biological systems has to be critically analyzed as many undesirable adverse effects can be triggered (e.g. toxicity, allergy, genotoxicity, etc.) and/or the performance of the nanomaterial can be compromised due to the unexpected phenomena in physiological environments (e.g. corrosion, aggregation, unspecific absorption of biomolecules, etc.). Despite the need for standard protocols for assessing the toxicity and bio-performance of each new functional nanomaterial, these are still scarce or currently under development. Nonetheless, nanotoxicology and relating adverse effects to the physico-chemical properties of nanomaterials are emerging areas of the utmost importance which have to be continuously revisited as any new material emerges. This review highlights recent progress concerning the interaction of nanomaterials with biological systems and following adverse effects.

Live Cells as Dynamic Laboratories: Time Lapse Raman Spectral Microscopy of Nanoparticles with Both IgE Targeting and pH-Sensing Functions

This review captures the use of live cells as dynamic microlaboratories through implementation of labeled nanoparticles (nanosensors) that have both sensing and targeting functions. The addition of 2,4-ε-dinitrophenol-L-lysine (DNP) as a FcεRI targeting ligand and 4-mercaptopyridine (4-MPy) as a pH-sensing ligand enables spatial and temporal monitoring of FcεRI receptors and their pH environment within the endocytic pathway. To ensure reliability, the sensor is calibrated in vivo using the ionophore nigericin and standard buffer solutions to equilibrate the external [H⁺] concentration with that of the cell compartments. This review highlights the nanosensors, ability to traffic and respond to pH of receptor-bound nanosensors (1) at physiological temperature (37°C) versus room temperature (25°C), (2) after pharmacological treatment with bafilomycin, an H⁺ ATPase pump inhibitor, or amiloride, an inhibitor of Na⁺/H⁺ exchange, and (3) in response to both temperature and pharmacological treatment. Whole-cell, time lapse images are demonstrated to show the ability to transform live cells into dynamic laboratories to monitor temporal and spatial endosomal pH. The versatility of these probes shows promise for future applications relevant to intracellular trafficking and intelligent drug design.

Monitoring intracellular redox potential changes using SERS nanosensors

Redox homeostasis and signaling are critically important in the regulation of cell function. There are significant challenges in quantitatively measuring intracellular redox potentials, and in this paper, we introduce a new approach. Our approach is based on the use of nanosensors which comprise molecules that sense the local redox potential, assembled on a gold nanoshell. Since the Raman spectrum of the sensor molecule changes depending on its oxidation state and since the nanoshell allows a huge enhancement of the Raman spectrum, intracellular potential can be calculated by a simple optical measurement. The nanosensors can be controllably delivered to the cytoplasm, without any toxic effects, allowing redox potential to be monitored in a reversible, non-invasive manner over a previously unattainable potential range encompassing both superphysiological and physiological oxidative stress.

Surface-Enhanced Raman Scattering as an Emerging Characterization and Detection Technique

While surface-enhanced Raman spectroscopy (SERS) has been attracting a continuously increasing interest of scientific community since its discovery, it has enjoyed a particularly rapid growth in the last decade. Most notable recent advances in SERS include novel technological approaches to SERS substrates and innovative applications of SERS in medicine and molecular biology. While a number of excellent reviews devoted to SERS appeared in the literature over the last two decades, we will focus this paper more specifically on several promising trends that have been highlighted less frequently. In particular, we will briefly overview strategies in designing and fabricating SERS substrates using deterministic patterning and then cover most recent biological applications of SERS.