NIR-II Conjugated Electrolytes as Biomimetics of Lipid Bilayers for In Vivo Liposome Tracking

Liposomes serve as promising and versatile vehicles for drug delivery. Tracking these nanosized vesicles, particularly in vivo, is crucial for understanding their pharmacokinetics. This study introduces the design and synthesis of three new conjugated electrolyte (CE) molecules, which emit in the second near-infrared window (NIR-II), facilitating deeper tissue penetration. Additionally, these CEs, acting as biomimetics of lipid bilayers, demonstrate superior compatibility with lipid membranes compared to commonly used carbocyanine dyes like DiR. To counteract the aggregation-caused quenching effect, CEs employ a twisted backbone, as such their fluorescence intensities can effectively enhance after a fluorophore multimerization strategy. Notably, a "passive" method was employed to integrate CEs into liposomes during the liposome formation, and membrane incorporation efficiency was significantly promoted to nearly 100%. To validate the in vivo tracking capability, the CE-containing liposomes were functionalized with cyclic arginine-glycine-aspartic acid (cRGD) peptides, serving as tumor-targeting ligands. Clear fluorescent images visualizing tumor site in living mice were captured by collecting the NIR-II emission. Uniquely, these CEs exhibit additional emission peak in visible region, enabling in vitro subcellular analysis using routine confocal microscopy. These results underscore the potential of CEs as a new-generation of membrane-targeting probes to facilitate the liposome-based medicine research.

Nanoscopic lipid domains determined by microscopy and neutron scattering

Biological membranes are highly complex supramolecular assemblies, which play central roles in biology. However, their complexity makes them challenging to study their nanoscale structures. To overcome this challenge, model membranes assembled using reduced sets of membrane-associated biomolecules have been found to be both excellent and tractable proxies for biological membranes. Due to their relative simplicity, they have been studied using a range of biophysical characterization techniques. In this review article, we will briefly detail the use of fluorescence and electron microscopies, and X-ray and neutron scattering techniques used over the past few decades to study the nanostructure of biological membranes.

Electric-driven membrane poration: A rationale for water role in the kinetics of pore formation

Electroporation is a well-established technique used to stimulate cells, enhancing membrane permeability by inducing reversible membrane pores. In the absence of experimental observation of the dynamics of pore creation, molecular dynamics studies provide the molecular-level evidence that the electric field promotes pore formation. Although single steps in the pore formation process are well assessed, a kinetic model representing the mathematical description of the electroporation process, is lacking. In the present work we studied the basis of the pore formation process, providing a rationale for the definition of a first-order kinetic scheme. Here, authors propose a three-state kinetic model for the process based on the assessed mechanism of water defects intruding at the water/lipid interface, when applying electric field intensities at the edge of the linear regime. The methodology proposed is based on the use of two robust biophysical quantities analyzed for the water molecules intruding at the water/lipid interface: (i) number of hydrogen bonds; (ii) number of contacts. The final model, sustained by a robust statistical sampling, provides kinetic constants for the transitions from the intact bilayer state to the hydrophobic pore state.

Endoplasmosis and exoplasmosis: the evolutionary principles underlying endocytosis, exocytosis, and vesicular transport

Eukaryotic cells are characterized by a multicompartmental structure with a variety of organelles. Vesicular transport between these compartments requires membrane fusion events. Based on a membrane topology view, we conclude that there are two basic mechanisms of membrane fusion, namely where the membranes first come in contact with the cis-side (the plasmatic phase of the lipid bilayer) or with the trans-side (the extra-plasmatic face). We propose to designate trans-membrane fusion processes as “endoplasmosis” as they lead to uptake of a compartment into the plasmatic phase. Vice versa we suggest the term “exoplasmosis” (as already suggested in a 1964 publication) for cis-membrane fusion events, where the interior of a vesicle is released to an extraplasmatic environment (the extracellular space or the lumen of a compartment). This concept is supported by the fact that all cis- and all trans-membrane fusions, respectively, exhibit noticeable similarities implying that they evolved from two functionally different mechanisms.

Stabilization of the conductive conformation of a voltage-gated K+ (Kv) channel: the lid mechanism

Voltage-gated K(+) (Kv) channels are molecular switches that sense membrane potential and in response open to allow K(+) ions to diffuse out of the cell. In these proteins, sensor and pore belong to two distinct structural modules. We previously showed that the pore module alone is a robust yet dynamic structural unit in lipid membranes and that it senses potential and gates open to conduct K(+) with unchanged fidelity. The implication is that the voltage sensitivity of K(+) channels is not solely encoded in the sensor. Given that the coupling between sensor and pore remains elusive, we asked whether it is then possible to convert a pore module characterized by brief openings into a conductor with a prolonged lifetime in the open state. The strategy involves selected probes targeted to the filter gate of the channel aiming to modulate the probability of the channel being open assayed by single channel recordings from the sensorless pore module reconstituted in lipid bilayers. Here we show that the premature closing of the pore is bypassed by association of the filter gate with two novel open conformation stabilizers: an antidepressant and a peptide toxin known to act selectively on Kv channels. Such stabilization of the conductive conformation of the channel is faithfully mimicked by the covalent attachment of fluorescein at a cysteine residue selectively introduced near the filter gate. This modulation prolongs the occupancy of permeant ions at the gate. It is this longer embrace between ion and gate that we conjecture underlies the observed stabilization of the conductive conformation. This study provides a new way of thinking about gating.

Molecular Dynamics Simulations of DOPC Lipid Bilayers: The Effect of Lennard-Jones Parameters of Hydrocarbon Chains

The current Chemistry at Harvard Molecular Mechanics (CHARMM) force field cannot accurately describe the properties of unsaturated phospholipid membranes. In this paper, a series of simulations was performed in which the Lennard-Jones (L-J) parameters of lipid acyl chains of dioleoylphosphatidylcholine (DOPC) were systematically adjusted. The results showed that adjustment of the L-J parameters in lipid acyl chains can significantly improve the current CHARMM force field. It was found that the L-J parameters have different influences on the order parameters of the top half and bottom half of the chain, separated by the cis double bond. The order parameters of the top half and the bottom half of the chain are related to the area/lipid and the length of the chain, respectively.