Evidence from ITIR-FCS Diffusion Studies that the Amyloid-Beta (Aβ) Peptide Does Not Perturb Plasma Membrane Fluidity in Neuronal Cells

Quantification of membrane fluidity in bacteria using TIR-FCS

Plasma membrane fluidity is an important phenotypic feature that regulates the diffusion, function, and folding of transmembrane and membrane-associated proteins. In bacterial cells, variations in membrane fluidity are known to affect respiration, transport, and antibiotic resistance. Membrane fluidity must therefore be tightly regulated to adapt to environmental variations and stresses such as temperature fluctuations or osmotic shocks. Quantitative investigation of bacterial membrane fluidity has been, however, limited due to the lack of available tools, primarily due to the small size and membrane curvature of bacteria that preclude most conventional analysis methods used in eukaryotes. Here, we develop an assay based on total internal reflection-fluorescence correlation spectroscopy (TIR-FCS) to directly measure membrane fluidity in live bacteria via the diffusivity of fluorescent membrane markers. With simulations validated by experiments, we could determine how the small size, high curvature, and geometry of bacteria affect diffusion measurements and correct subsequent measurements for unbiased diffusion coefficient estimation. We used this assay to quantify the fluidity of the cytoplasmic membranes of the Gram-positive bacteria Bacillus subtilis (rod-shaped) and Staphylococcus aureus (coccus) at high (37°C) and low (20°C) temperatures in a steady state and in response to a cold shock, caused by a shift from high to low temperature. The steady-state fluidity was lower at 20°C than at 37°C, yet differed between B. subtilis and S. aureus at 37°C. Upon cold shock, the membrane fluidity decreased further below the steady-state fluidity at 20°C and recovered within 30 min in both bacterial species. Our minimally invasive assay opens up exciting perspectives for the study of a wide range of phenomena affecting the bacterial membrane, from disruption by chemicals or antibiotics to viral infection or change in nutrient availability.

Evaluation of functional transbilayer coupling in live cells by controlled lipid exchange and imaging fluorescence correlation spectroscopy

Biophysical coupling between the inner and outer leaflets, known as inter-leaflet or transbilayer coupling, is a fundamental organizational principle in the plasma membranes of live mammalian cells. Lipid-based interactions between the two leaflets are proposed to be a primary mechanism underlying transbilayer coupling. However, there are only a few experimental evidence supporting the existence of such interactions in live cells. This is seemingly due to the lack of experimental strategies to perturb the lipid composition in one leaflet and quantitative techniques to evaluate the biophysical properties of the opposite leaflet. The existing strategies often dependent on immobilization and clustering a component in one of the leaflets and technically demanding biophysical tools to evaluate the effects on the opposing leaflet. In the recent years, the London group developed a simple but elegant method, namely methyl-alpha-cyclodextrin catalyzed lipid exchange (LEX), to efficiently exchange outer leaflet lipids with an exogenous lipid of choice. Here, we adopted this method to perturb outer leaflet lipid composition. The corresponding changes in the inner leaflet is evaluated by comparing the diffusion of lipid probes localized in this leaflet in unperturbed and perturbed conditions. We employed highly multiplexed imaging fluorescence correlation spectroscopy (ImFCS), realized in a commercially available or home-built total internal reflection fluorescence microsocope equipped with a fast and sensitive camera, to determine diffusion coefficient of the lipid probes. Using the combination of LEX and ImFCS, we directly demonstrate lipid-based transbilayer coupling that does not require immobilization of membrane components in live mast cells in resting conditions. Overall, we present a relatively straightforward experimental strategy to evaluate transbilayer coupling quantitively in live cells.

Defining the Neuropathological Aggresome across in Silico, in Vitro, and ex Vivo Experiments

The loss of proteostasis over the life course is associated with a wide range of debilitating degenerative diseases and is a central hallmark of human aging. When left unchecked, proteins that are intrinsically disordered can pathologically aggregate into highly ordered fibrils, plaques, and tangles (termed amyloids), which are associated with countless disorders such as Alzheimer's disease, Parkinson's disease, type II diabetes, cancer, and even certain viral infections. However, despite significant advances in protein folding and solution biophysics techniques, determining the molecular cause of these conditions in humans has remained elusive. This has been due, in part, to recent discoveries showing that soluble protein oligomers, not insoluble fibrils or plaques, drive the majority of pathological processes. This has subsequently led researchers to focus instead on heterogeneous and often promiscuous protein oligomers. Unfortunately, significant gaps remain in how to prepare, model, experimentally corroborate, and extract amyloid oligomers relevant to human disease in a systematic manner. This Review will report on each of these techniques and their successes and shortcomings in an attempt to standardize comparisons between protein oligomers across disciplines, especially in the context of neurodegeneration. By standardizing multiple techniques and identifying their common overlap, a clearer picture of the soluble neuropathological aggresome can be constructed and used as a baseline for studying human disease and aging.

Interactive effects between cadmium stabilized by palygorskite and mobilized by siderophores from Pseudomonas fluorescens

The application of palygorskite (PAL) for potentially toxic trace elements (Cd²⁺, Ni²⁺, etc.) remediation in polluted soil can substantially reduce the bioavailability and toxicity of these hazard materials. However, the secretion of organic acids and siderophores by microorganisms might result in the re-mobilization of cadmium (Cd) in PAL-bound forms (PAL-Cd). In this study, the interactive effects between Cd stabilized by PAL and mobilized by siderophores from Pseudomonas fluorescens were performed with four flask-shaking experimental treatments, namely, strain with or without an ability of siderophores production respectively associated with or without PAL-Cd. The GC-MS and UHPLC-MS test methods were used to analyze the concentrations of metabolites. Results showed that the Cd mobilized by strain with siderophores production was 22.1% higher than that of strain without the ability of siderophores production (p < 0.05). The mobilization of Cd in PAL in turn significantly reduced the siderophores production of Pseudomonas fluorescens by 25.1% (p < 0.05). The numbers of metabolites significantly up-regulated and down-regulated were 9 and 22 in strain groups with PAL-Cd addition compared with the groups without PAL-Cd, respectively. Metabolomics analysis revealed that the mobilized Cd affects the signal transduction pathway and primary metabolic processes, reduces the metabolic capacity of pentose phosphate pathway, glycolysis and tricarboxylic acid cycle pathway. These changes inhibit the ability of strain to biosynthesize amino acids during the mobilization processes, further reducing the capacity of Pseudomonas fluorescens to produce siderophores. This study provides a useful information on how to select soil Cd-stabilizing materials in a targeted manner and how to avoid Cd re-mobilization by siderophores.

Effects of in vivo conditions on amyloid aggregation

One of the grand challenges of biophysical chemistry is to understand the principles that govern protein misfolding and aggregation, which is a highly complex process that is sensitive to initial conditions, operates on a huge range of length- and timescales, and has products that range from protein dimers to macroscopic amyloid fibrils. Aberrant aggregation is associated with more than 25 diseases, which include Alzheimer's, Parkinson's, Huntington's, and type II diabetes. Amyloid aggregation has been extensively studied in the test tube, therefore under conditions that are far from physiological relevance. Hence, there is dire need to extend these investigations to in vivo conditions where amyloid formation is affected by a myriad of biochemical interactions. As a hallmark of neurodegenerative diseases, these interactions need to be understood in detail to develop novel therapeutic interventions, as millions of people globally suffer from neurodegenerative disorders and type II diabetes. The aim of this review is to document the progress in the research on amyloid formation from a physicochemical perspective with a special focus on the physiological factors influencing the aggregation of the amyloid-β peptide, the islet amyloid polypeptide, α-synuclein, and the hungingtin protein.