Exploring viscosity, polarity and temperature sensitivity of BODIPY-based molecular rotors

Microscopic Viscosity of Neuronal Plasma Membranes Measured Using Fluorescent Molecular Rotors: Effects of Oxidative Stress and Neuroprotection

Molecular mobility in neuronal plasma membranes is a crucial factor in brain function. Microscopic viscosity is an important parameter that determines molecular mobility. This study presents the first direct measurement of the microviscosity of plasma membranes of live neurons. Microviscosity maps were obtained using fluorescence lifetime imaging of environment-sensing dyes termed "molecular rotors". Neurons were investigated both in the basal state and following common neurodegenerative stimuli, excitotoxicity, or oxidative stress. Both types of neurotoxic challenges induced microviscosity decrease in cultured neurons, and oxidant-induced membrane fluidification was counteracted by the wide-spectrum neuroprotectant, the H3 peptide. These results provide new insights into molecular mobility in neuronal membranes, paramount for basic brain function, and suggest that preservation of membrane stability may be an important aspect of neuroprotection in brain insults and neurodegenerative disorders.

Expanding the Scope of Reporting Nanoparticles: Sensing of Lipid Phase Transitions and Nanoviscosities in Lipid Membranes

Biological membrane fluidity and thus the local viscosity in lipid membranes are of vital importance for many life processes and implicated in various diseases. Here, we introduce a novel viscosity sensor design for lipid membranes based on a reporting nanoparticle, a sulfated dendritic polyglycerol (dPGS), conjugated to a fluorescent molecular rotor, indocarbocyanine (ICC). We show that dPGS-ICC provides high affinity to lipid bilayers, enabling viscosity sensing in the lipid tail region. The systematic characterization of viscosity- and temperature-dependent photoisomerization properties of ICC and dPGS-ICC allowed us to determine membrane viscosities in different model systems and in living cells using fluorescence lifetime imaging (FLIM). dPGS-ICC distinguishes between ordered lipids and the onset of membrane defects in small unilamellar single lipid vesicles and is highly sensitive in the fluid phase to small changes in viscosity introduced by cholesterol. In microscopy-based viscosity measurements of large multilamellar vesicles, we observed an order of magnitude more viscous environments by dPGS-ICC, lending support to the hypothesis of heterogeneous nanoviscosity environments even in single lipid bilayers. The existence of such complex viscosity structures could explain the large variation in the apparent membrane viscosity values found in the literature, depending on technique and probe, both for model membranes and live cells. In HeLa cells, a tumor-derived cell line, our nanoparticle-based viscosity sensor detects a membrane viscosity of ∼190 cP and is able to discriminate between cell membrane and intracellular vesicle localization. Thus, our results show the versatility of the dPGS-ICC nano-conjugate in physicochemical and biomedical applications by adding a new analytical functionality to its medical properties.

Computational Characterization of a Cholesterol-Based Molecular Rotor in Lipid Membranes

Biophysical properties of cellular membranes critically depend on their content of cholesterol and its interaction with various other lipid species. Cholesterol-dependent friction at the nanoscale can be studied with molecular rotors, whose quantum yield depends on rotational dynamics of functional groups during their excited state lifetime. Here, we present a detailed computational analysis of a phenyl-BODIPY-linked cholesterol based molecular rotor in direct comparison with the well-known TopFluor-cholesterol. We describe a new parametrization strategy of force field parameters for the BODIPY moiety and carry out extensive molecular dynamics simulations of the probe in membranes in the absence or presence of cholesterol. Our study quantifies the extent of membrane perturbation by these probes, analyzes their tilting resistance in the bilayer and derives dynamic properties directly related to the rotor propensity. We show that phenyl-BODIPY-cholesterol bears potential as a cholesterol-dependent molecular rotor to report about microviscosity of sterol-containing model and cell membranes.

Enhancing the Viscosity-Sensitive Range of a BODIPY Molecular Rotor by Two Orders of Magnitude

Molecular rotors are a class of fluorophores that enable convenient imaging of viscosity inside microscopic samples such as lipid vesicles or live cells. Currently, rotor compounds containing a boron-dipyrromethene (BODIPY) group are among the most promising viscosity probes. In this work, it is reported that by adding heavy-electron-withdrawing -NO₂ groups, the viscosity-sensitive range of a BODIPY probe is drastically expanded from 5-1500 cP to 0.5-50 000 cP. The improved range makes it, to our knowledge, the first hydrophobic molecular rotor applicable not only at moderate viscosities but also for viscosity measurements in highly viscous samples. Furthermore, the photophysical mechanism of the BODIPY molecular rotors under study has been determined by performing quantum chemical calculations and transient absorption experiments. This mechanism demonstrates how BODIPY molecular rotors work in general, why the -NO₂ group causes such an improvement, and why BODIPY molecular rotors suffer from undesirable sensitivity to temperature. Overall, besides reporting a viscosity probe with remarkable properties, the results obtained expand the general understanding of molecular rotors and show a way to use the knowledge of their molecular action mechanism for augmenting their viscosity-sensing properties.

Spectral shifts of BODIPY derivatives: a simple continuous model

The BODIPY dyes are a versatile family of chromophores that have found use in fluorescence based bioimaging and other applications. The BODIPY core can be substituted in a vast number of ways, but the photophysical changes, such as shifts in absorption spectra, are not always immediately obvious from the molecular structure. We introduce a simple model that let you vary the electron withdrawing or electron donating character of each substituent continuously to get an overview of the landscape of possible spectral shifts. The features of substituted BODIPY cores are compared to the corresponding linear system, giving a new perspective on BODIPY photophysics. Using the model, we are able to rationalize the trend seen in a family of BODIPY, with chalcogen-containing substituents, as being due to a change in electronegativity.

Förster Resonance Energy Transfer-Based Stability Assessment of PLGA Nanoparticles in Vitro and in Vivo

The knowledge of in vitro and in vivo stability of polymeric nanoparticles is vital for the development of clinical formulations for drug delivery and cell labeling applications. Förster resonance energy transfer (FRET)-based fluorescence labeling approaches are promising tools to study nanoparticle stability under different physiological conditions. Here, we present the FRET-based stability assessment of poly(lactic-co-glycolic acid) (PLGA) nanoparticles encapsulating BODIPY-FL12 and Nile Red as the donor and acceptor, respectively. The stability of PLGA nanoparticles is studied via monitoring the variations of fluorescence emission characteristics along with colloidal characterization. Accordingly, PLGA nanoparticles are colloidally stable for more than 2 weeks when incubated in aqueous buffers in situ, whereas in vitro particle degradation starts in between 24 and 48 h, reaching a complete loss of FRET at 72 h as shown with fluorescence microscopy imaging and flow cytometry analysis. PLGA nanoparticles systemically administered to mice predominantly accumulate in the liver, in which FRET no longer takes place at time points as early as 24 h postadministration as determined by ex vivo organ imaging and flow cytometry analysis. The results of this study expand our knowledge on drug release and degradation behavior of PLGA nanoparticles under different physiological conditions, which will prove useful for the rational design of PLGA-based formulations for various applications that can be translated into clinical practice.

Targeted fluorescence lifetime probes reveal responsive organelle viscosity and membrane fluidity

The only way to visually observe cellular viscosity, which can greatly influence biological reactions and has been linked to several human diseases, is through viscosity imaging. Imaging cellular viscosity has allowed the mapping of viscosity in cells, and the next frontier is targeted viscosity imaging of organelles and their microenvironments. Here we present a fluorescent molecular rotor/FLIM framework to image both organellar viscosity and membrane fluidity, using a combination of chemical targeting and organelle extraction. For demonstration, we image matrix viscosity and membrane fluidity of mitochondria, which have been linked to human diseases, including Alzheimer's Disease and Leigh's syndrome. We find that both are highly dynamic and responsive to small environmental and physiological changes, even under non-pathological conditions. This shows that neither viscosity nor fluidity can be assumed to be fixed and underlines the need for single-cell, and now even single-organelle, imaging.

Solid-state emissive O-BODIPY dyes with bimodal emissions across red and near infrared region

Fluorescent compounds with solid-state emission are expected to have broad applications in the development of optoelectronic devices. In this study, we develop O-BODIPY based fluorescent dyes which exhibit strong bimodal solid-state emissions across red and NIR regions. After one pot synthesis, samples are characterized by X-ray diffraction, cyclic voltammetry, UV-vis absorption, and fluorescence spectra. All the experimental data reveal the multiple excitation and efficient emission features in the aggregation states. Furthermore, the two produced probes can be successfully applied for tracking lysosomes in HeLa cells with low cytotoxicity.

We develop O-BODIPY based fluorescent probes which exhibit strong bimodal solid-state emissions across red and NIR regions, tracking lysosomes in HeLa cells with low cytotoxicity.

A fluorogenic BODIPY molecular rotor as an apoptosis marker

Based on a BODIPY molecular rotor, we designed a probe that lights up its green fluorescence in apoptotic cells and distinguishes between early and late apoptosis.

Molecular rotors report on changes in live cell plasma membrane microviscosity upon interaction with beta-amyloid aggregates

Amyloid deposits of aggregated beta-amyloid Aβ(1-42) peptides are a pathological hallmark of Alzheimer's disease. Aβ(1-42) aggregates are known to induce biophysical alterations in cells, including disruption of plasma membranes. We investigated the microviscosity of plasma membranes upon interaction with oligomeric and fibrillar forms of Aβ(1-42). Viscosity-sensing fluorophores termed molecular rotors were utilised to directly measure the microviscosities of giant plasma membrane vesicles (GPMVs) and plasma membranes of live SH-SY5Y and HeLa cells. The fluorescence lifetimes of membrane-inserting BODIPY-based molecular rotors revealed a decrease in bilayer microviscosity upon incubation with Aβ(1-42) oligomers, while fibrillar Aβ(1-42) did not significantly affect the microviscosity of the bilayer. In addition, we demonstrate that the neuroprotective peptide H3 counteracts the microviscosity change induced by Aβ(1-42) oligomers, suggesting the utility of H3 as a neuroprotective therapeutic agent in neurodegenerative disorders and indicating that ligand-induced membrane stabilisation may be a possible mechanism of neuroprotection during neurodegenerative disorders such as Alzheimer's disease.

Fluorescence lifetime of Rhodamine B in aqueous solutions of polysaccharides and proteins as a function of viscosity and temperature

Rhodamine B (RhB) is a well known dye extensively used in thermometric studies, either considering the decrease in the fluorescence intensity or the lifetime (τ) with temperature. Lifetime measurements are preferred over intensity ones as they are more robust. In order to expand microscopy thermometry to complex food fluids, the effect of solutes on the τ of RhB was studied using fluorescence lifetime imaging microscopy (FLIM) in a two-photon microscope. Polysaccharides of different molecular weights (glucose, lactose, dextran, maltodextrin, and sodium alginate), as well as whey proteins, were considered as typical model food ingredients. A linear increase in τ with the concentration is observed in most polysaccharides, highlighting that it is not due to an increase in the macroscopic viscosity, but in maltodextrins a Langmuir-like concentration dependence is observed. There are extensive interactions between RhB and whey proteins at small concentrations that quickly increase τ up to saturation at >10 wt% proteins, with τ modelled well using an adsorption Langmuir model. Therefore, the effect of solutes on RhB τ is not related to changes in the macroscopic viscosity. The temperature sensitivity of τ, quantified using apparent activation energies, decreases at high solute contents.

Surface functionalisation with viscosity-sensitive BODIPY molecular rotor

Surface functionalisation with viscosity sensitive dyes termed 'molecular rotors' can potentially open up new opportunities in sensing, for example for non-invasive biological viscosity imaging, in studying the effect of shear stress on lipid membranes and in cells, and in imaging contacts between surfaces upon applied pressure. We have functionalised microscope slides with BODIPY-based molecular rotor capable of viscosity sensing via its fluorescence lifetime. We have optimised functionalisation conditions and prepared the slides with the BODIPY rotor attached directly to the surface of glass slides and through polymer linkers of 5 kDa and 40 kDa in mass. The slides were characterised for their sensitivity to viscosity, and used to measure viscosity of supported lipid bilayers during photooxidation, and of giant unilamellar vesicles lying on the surface of the slide. We conclude that our functionalised slides show promise for a variety of viscosity sensing applications.

Subcellular Nanorheology Reveals Lysosomal Viscosity as a Reporter for Lysosomal Storage Diseases

We describe a new method to measure viscosity within subcellular organelles of a living cell using nanorheology. We demonstrate proof of concept by measuring viscosity in lysosomes in multiple cell types and disease models. The lysosome is an organelle responsible for the breakdown of complex biomolecules. When different lysosomal proteins are defective, they are unable to break down specific biological substrates, which get stored within the lysosome, causing about 70 fatal diseases called lysosomal storage disorders (LSDs). Although the buildup of storage material is critical to the pathology of these diseases, methods to monitor cargo accumulation in the lysosome are lacking for most LSDs. Using passive particle tracking nanorheology and fluorescence recovery after photobleaching, we report that viscosity in the lysosome increases significantly during cargo accumulation in several LSD models. In a mammalian cell culture model of Niemann Pick C, lysosomal viscosity directly correlates with the levels of accumulated cholesterol. We also observed increased viscosity in diverse LSD models in Caenorhabditis elegans, revealing that lysosomal viscosity is a powerful reporter with which to monitor substrate accumulation in LSDs for new diagnostics or to assay therapeutic efficacy.

Gaining insight into the photophysical properties of a coumarin STP ester with potential for bioconjugation

The photophysical properties of a coumarin 392 4-sulfotetrafluorophenyl ester, C392STP (sodium (E/Z)-4-(4-(2-(6,7-dimethoxycoumarin-3-yl)vinyl)-benzoyl)-2,3,5,6-tetrafluoro-benzenesulfonate), an amine reactive coumarine with potential for bioconjungation, have been studied in different solvents.