Osmotic Pressure Enables High-Yield Assembly of Giant Vesicles in Solutions of Physiological Ionic Strengths

Fabrication and characterization of phosphoinositide containing asymmetric vesicles in physiological salt

Phosphoinositide (PIPs) lipids mediate a broad range of physiological functions by attracting proteins at specific time points to distinct cellular sites. Many of these processes are associated with the local accumulation of PIPs and PIP/protein signaling platform formation. Studies aimed at determining the physicochemical underpinnings of PIP domain formation have been limited to model systems that exhibited the same lipid composition in both bilayer leaflets. However, biological membranes are asymmetric, and it is desirable to develop an experimental approach that allows for the fabrication of lipid model systems with a non-symmetric lipid bilayer, i.e., a membrane mimic that exhibits a PIP containing lipid mixture in one leaflet and a different lipid composition in the opposing leaflet. We adapted the previously introduced hemifusion method for the fabrication of asymmetric Giant Unilamellar Vesicles (aGUVs) for the fabrication of aGUVs with phosphatidylinositol-(4,5)-bisphosphate (PI(4,5)P2) in a physiological ionic strength buffer solution. The general method involved the bivalent cation-initiated fusion of a symmetric GUV (sGUV) with solid-supported lipid bilayers, which leads to the exchange of the outer leaflet of the sGUV. We find that initiating the hemifusion with 6 mM Ca2+ leads to a low yield and quality of the aGUVs. We attribute this to macroscopic Ca2+/PI(4,5)P2 domain formation of the solid support lipid bilayer (SLB), which leads to the interaction of the sGUVs with regions enriched in PI(4,5)P2 (domain) and other areas that are void of the PIP lipid. Using 6 mM Mg2+ as the initiator instead led to an improvement in terms of yield and aGUV quality. The best results were obtained when using 1 mM Mg2+. We are introducing several data analysis approaches that allow for the identification of aGUVs that exhibit high quality in terms of the outer leaflet exchange and composition of the two aGUV leaflets.

Ultrahigh yields of giant vesicles obtained through mesophase evolution and breakup

Self-assembly of dry amphiphilic lipid films on surfaces upon hydration is a crucial step in the formation of cell-like giant unilamellar vesicles (GUVs). GUVs are useful as biophysical models, as soft materials, as chassis for bottom-up synthetic biology, and in biomedical applications. Here via combined quantitative measurements of the molar yield and distributions of sizes and high-resolution imaging of the evolution of thin lipid films on surfaces, we report the discovery of a previously unknown pathway of lipid self-assembly which can lead to ultrahigh yields of GUVs of >50%. This yield is about 60% higher than any GUV yield reported to date. The "shear-induced fragmentation" pathway occurs in membranes containing 3 mol% of the poly(ethylene glycol) modified lipid PEG2000-DSPE (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000]), when a lipid-dense foam-like mesophase forms upon hydration. The membranes in the mesophase fragment and close to form GUVs upon application of fluid shear. Experiments with varying mol% of PEG2000-DSPE and with lipids with partial molecular similarity to PEG2000-DSPE show that ultrahigh yields are only achievable under conditions where the lipid-dense mesophase forms. The increased yield of GUVs compared to mixtures without PEG2000-DSPE was general to flat supporting surfaces such as stainless steel sheets and to various lipid mixtures. In addition to increasing their accessibility as soft materials, these results demonstrate a route to obtaining ultrahigh yields of cell-sized liposomes using longstanding clinically-approved lipid formulations that could be useful for biomedical applications.

In Situ Self-Inflating-Modeled Giant-Vesicle-Like Quantum Dot Assembly for Biomimetic Artificial Photosynthesis

The study of biomimetic self-assembly is crucial for scientists aiming to understand the origin of life and construct biomimetic functional structures. In our endeavor to create a biomimetic photosynthetic assembly, we discover a self-inflation behavior that drives the components, MPA-CdSe quantum dots (QDs) and a solid cationic polyelectrolyte, CPPA, to form a giant-vesicle-like (GVL) architecture, termed GVL-QDs@CPPA. The in situ generation of osmotic pressure during the self-assembly of QDs onto swollen CPPA in water was found to cause this self-inflation process. The resulting vesicle-like structure exhibits spatial characteristics similar to those of natural photosynthetic cells, with QDs acting as pigments uniformly distributed on the CPPA membranes, which have embedded cobalt catalytic centers. This architecture ensures optimal absorption of visible light and facilitates efficient electron transfer between the QDs and catalytic centers. As a result, GVL-QDs@CPPA assemblies efficiently harness photogenerated electrons and holes to convert protons and isopropanol into hydrogen (H2) and acetone, respectively, achieving a nearly 1:1 ratio of the reduction product (H2) to the oxidation product (acetone).

Dynamics of giant vesicle assembly from thin lipid films

MOTIVATION

Giant unilamellar vesicles (GUVs), cell-like synthetic micrometer size structures, assemble when thin lipid films are hydrated in aqueous solutions. Quantitative measurements of static yields and distribution of sizes of GUVs obtained from thin film hydration methods were recently reported. Dynamic data such as the time evolution of yields and distribution of sizes, however, is not known. Dynamic data can provide insights into the assembly pathway of GUVs and guidelines for choosing conditions to obtain populations with desired size distributions.

APPROACH

We develop the 'stopped-time' technique to characterize the time evolution of the distribution of sizes and molar yields of populations of free-floating GUVs. We additionally capture high resolution time-lapse images of surface-attached GUV buds on the lipid films. We systematically study the dynamics of assembly of GUVs from three widely used thin film hydration methods, PAPYRUS (Paper-Abetted amPhiphile hYdRation in aqUeous Solutions), gentle hydration, and electroformation.

FINDINGS

We find that the molar yield versus time curves of GUVs demonstrate a characteristic sigmoidal shape, with an initial yield, a transient, and then a steady state plateau for all three methods. The population of GUVs showed a right-skewed distribution of diameters. The variance of the distributions increased with time. The systems reached steady state within 120 min. We rationalize the dynamics using the thermodynamically motivated budding and merging (BNM) model. These results further the understanding of lipid dynamics and provide for the first-time practical parameters to tailor the production of GUVs of specific sizes for applications.