Fluorescent quantification of size and lamellarity of membrane nanotubes

Physical Mechanism of Amyloid-β Peptide Chain Aggregation on Fluidic Lipid Nanotubules

The question of how peptide chain aggregation is influenced by lipid membranes with varying shapes and structures is crucial for a detailed understanding of the neurotoxicity effect of the peptide chains. Not like the more usual spherical liposomes and planar lipid membranes, herein, we use lipid nanotubules as a model of important neuron synapse nanowire structures and devote particular attention to the effect of nanotubule fluidity on amyloid-β peptide (Aβ) chain aggregation. We apply single-molecule tracking (SMT) to elucidate how Aβ chains diffuse and aggregate on lipid nanotubules with different fluidities. The physical mechanism implies that fluidic lipid nanotubules facilitate the super-diffusion of two-dimensional (2D)-mobile precursor Aβ chains and promote their aggregation. This aggregation mechanism is retarded on less fluidic lipid nanotubules where the super-diffusion of 2D-mobile precursor Aβ chains is restricted by "frozen" lipids with less mobility. This work provides a mechanistic explanation for Aβ chain aggregation on fluidic lipid nanotubules.

Super-Resolution Imaging of Highly Curved Membrane Structures in Giant Vesicles Encapsulating Molecular Condensates

Molecular crowding is an inherent feature of cell interiors. Synthetic cells as provided by giant unilamellar vesicles (GUVs) encapsulating macromolecules (poly(ethylene glycol) and dextran) represent an excellent mimetic system to study membrane transformations associated with molecular crowding and protein condensation. Similarly to cells, such GUVs exhibit highly curved structures like nanotubes. Upon liquid-liquid phase separation their membrane deforms into apparent kinks at the contact line of the interface between the two aqueous phases. These structures, nanotubes, and kinks, have dimensions below optical resolution. Here, these are studied with super-resolution stimulated emission depletion (STED) microscopy facilitated by immobilization in a microfluidic device. The cylindrical nature of the nanotubes based on the superior resolution of STED and automated data analysis is demonstrated. The deduced membrane spontaneous curvature is in excellent agreement with theoretical predictions. Furthermore, the membrane kink-like structure is resolved as a smoothly curved membrane demonstrating the existence of the intrinsic contact angle, which describes the wettability contrast of the encapsulated phases to the membrane. Resolving these highly curved membrane structures with STED imaging provides important insights in the membrane properties and interactions underlying cellular activities.

Visualizing Super-Diffusion, Oligomerization, and Fibrillation of Amyloid-β Peptide Chains along Tubular Membranes

A deeper mechanistic study of peptide amyloidosis on lipid membranes with varying shapes could enhance the comprehensive understanding of the contribution of cellular structures to multiple neurodegenerative diseases, including Alzheimer's disease. We report here the direct visual observation of amyloid-β peptide (Aβ) superdiffusing along tubular lipid membranes via single-molecule tracking (SMT). Such mobility on tubular membranes is critical, as it allows Aβ chains to oligomerize and elongate into fibrils. Factors such as cholesterol that favor Aβ chains with sufficient surface residence time can promote the inter-Aβ interaction and enhance Aβ fibrillation. This study provides previously uncharacterized insights into the chain behaviors of Aβ along important biological nanowire structures, which is essential to understanding and exploring the factors of cellular shapes to manipulate peptide amyloidosis.

Inward Budding and Endocytosis of Membranes Regulated by de Novo Designed Peptides

Protein-mediated endocytosis of membrane is a key event in biological system. The mechanism, however, is still not clear. Using a de novo designed bola-type peptide KKKLLLLLLLLKKK (K₃L₈K₃) as a protein mimic, we studied how it induced giant unilamellar vesicle (GUV) to form inward buds or endocytosis at varying conditions. Results show that the inward budding is initiated as the charged lipids are neutralized by K₃L₈K₃, which results in a negative spontaneous curvature. If the charged lipids have unsaturated tails, the buddings are slim fibrils, which can further wrap into a spherical structure. In the case of saturated charged lipids, the buddings are rigid tubules, stable in the studied time period. The unsaturated lipid to saturated lipid ratio in the mother membrane is another key parameter governing the shape and dynamics of the buds. A complete endocytosis is observed when K₃L₈K₃ is attached with a hydrophobic moiety, suggesting that hydrophobic interaction helps the buds to detach from the mother membrane. The molecules in the surrounding medium, such as negatively charged oligonucleotides, are engulfed into the GUV via endocytosis pathway induced by K₃L₈K₃. Our study provides a novel strategy for illustrating the endocytosis mechanism by using peptides of simple sequence.

The flexibility and dynamics of the tubules in the endoplasmic reticulum

The endoplasmic reticulum (ER) is a single organelle in eukaryotic cells that extends throughout the cell and is involved in a large number of cellular functions. Using a combination of fixed and live cells (human MRC5 lung cells) in diffraction limited and super-resolved fluorescence microscopy (STORM) experiments, we determined that the average persistence length of the ER tubules was 3.03 ± 0.24 μm. Removing the branched network junctions from the analysis caused a slight increase in the average persistence length to 4.71 ± 0.14 μm, and provides the tubule's persistence length with a moderate length scale dependence. The average radius of the tubules was 44.1 ± 3.2 nm. The bending rigidity of the ER tubule membranes was found to be 10.9 ± 1.2 kT (17.0 ± 1.3 kT without branch points). We investigated the dynamic behaviour of ER tubules in live cells, and found that the ER tubules behaved like semi-flexible fibres under tension. The majority of the ER tubules experienced equilibrium transverse fluctuations under tension, whereas a minority number of them had active super-diffusive motions driven by motor proteins. Cells thus actively modulate the dynamics of the ER in a well-defined manner, which is expected in turn to impact on its many functions.

Dynamics of membrane nanotubes coated with I-BAR

Membrane deformation is a necessary step in a number of cellular processes such as filopodia and invadopodia formation and has been shown to involve membrane shaping proteins containing membrane binding domains from the IRSp53-MIM protein family. In reconstituted membranes the membrane shaping domains can efficiently deform negatively charged membranes into tubules without any other proteins present. Here, we show that the IM domain (also called I-BAR domain) from the protein ABBA, forms semi-flexible nanotubes protruding into Giant Unilamellar lipid Vesicles (GUVs). By simultaneous quantification of tube intensity and tubular shape we find both the diameter and stiffness of the nanotubes. I-BAR decorated tubes were quantified to have a diameter of ~50 nm and exhibit no stiffening relative to protein free tubes of the same diameter. At high protein density the tubes are immobile whereas at lower density the tubes diffuse freely on the surface of the GUV. Bleaching experiments of the fluorescently tagged I-BAR confirmed that the mobility of the tubes correlates with the mobility of the I-BAR on the GUV membrane. Finally, at low density of I-BAR the protein upconcentrates within tubes protruding into the GUVs. This implies that I-BAR exhibits strong preference for negatively curved membranes.