Biodegradable hybrid block copolymer - lipid vesicles as potential drug delivery systems.

The anticipated benefits of nano-formulations for drug delivery are well known: for nanomedicines to achieve this potential, new materials are required with predictive and tuneable properties. Excretion of excipients following delivery is advantageous to minimise the possibility of adverse effects; biodegradability to non-toxic products is therefore desirable. With this in mind, we aim to develop tuneable hybrid lipid-block copolymer vesicle formulations where the hydrophilic polymer block is polyethylene glycol (PEG), which has accepted biocompatibility, and the hydrophobic block of the polymer is biodegradable: polycaprolactone (PCL) or polylactide (PLA). We investigate five different block copolymers for the formation of 1:1 phospholipid:polymer hybrid vesicles, compare their properties to the appropriate unitary liposome (POPC) and polymersome systems and assess their potential for future development as nanomedicine formulations. The PEG-PCL polymers under investigation do not form polymersomes and exhibit poor colloidal and/or encapsulation stability in hybrid formulations with lipids. The properties of PEG-PLA hybrid vesicles are found to be more encouraging: they have much enhanced passive loading of a hydrophilic small molecule (carboxyfluorescein) compared to their respective polymersomes and reduces serum induced lysis of the vesicle compared to the liposome. Significantly, burst release from hybrid vesicles can be substantially reduced by making the polymer components of the hybrid vesicle a mixture containing 10 mol% of PEG15-PLA25 that is intermediate in size between the phospholipid and larger PEG45-PLA54 components. We conclude that hybrid lipid/PEG-PLA vesicles warrant further assessment and development as candidate drug delivery systems.

A reconstitution method for integral membrane proteins in hybrid lipid-polymer vesicles for enhanced functional durability.

Hybrid vesicles composed of lipids and block copolymers hold promise for increasing liposome stability and providing a stable environment for membrane proteins. Recently we reported the successful functional reconstitution of the integral membrane protein cytochrome bo3 (ubiquinol oxidase) into hybrid vesicles composed of a blend of phospholipids and a block copolymer (PBd-PEO). We demonstrated that these novel membrane environments stabilise the enzymes' activity, prolonging their functional lifetime [Chem. Commun. 52 (2016) 11020-11023]. This approach holds great promise for applications of membrane proteins where enhanced durability, stability and shelf-life will be essential to creating a viable technology. Here we present a detailed account of our methods for membrane protein reconstitution into hybrid vesicles and discuss tips and challenges when using block copolymers compared to pure phospholipid systems that are more common materials for this purpose. We also extend the characterisation of these hybrid vesicles beyond what we have previously reported and show: (i) hybrid membranes are less permeable to protons than phospholipid bilayers; (ii) extended enzyme activity data is presented over a period of 500 days, which fully reveals the truly remarkable enhancement in functional lifetime that hybrid vesicles facilitate.

Effects of membrane curvature and pH on proton pumping activity of single cytochrome bo3 enzymes.

The molecular mechanism of proton pumping by heme-copper oxidases (HCO) has intrigued the scientific community since it was first proposed. We have recently reported a novel technology that enables the continuous characterisation of proton transport activity of a HCO and ubiquinol oxidase from Escherichia coli, cytochrome bo3, for hundreds of seconds on the single enzyme level (Li et al. J Am Chem Soc 137 (2015) 16055-16063). Here, we have extended these studies by additional experiments and analyses of the proton transfer rate as a function of proteoliposome size and pH at the N- and P-side of single HCOs. Proton transport activity of cytochrome bo3 was found to decrease with increased curvature of the membrane. Furthermore, proton uptake at the N-side (proton entrance) was insensitive to pH between pH6.4-8.4, while proton release at the P-side had an optimum pH of ~7.4, suggesting that the pH optimum is related to proton release from the proton exit site. Our previous single-enzyme experiments identified rare, long-lived conformation states of cytochrome bo3 where protons leak back under turn-over conditions. Here, we analyzed and found that ~23% of cytochrome bo3 proteoliposomes show ΔpH half-lives below 50s after stopping turnover, while only ~5% of the proteoliposomes containing a non-pumping mutant, E286C cytochrome bo3 exhibit such fast decays. These single-enzyme results confirm our model in which HCO exhibit heterogeneous pumping rates and can adopt rare leak states in which protons are able to rapidly flow back.

Durable vesicles for reconstitution of membrane proteins in biotechnology.

The application of membrane proteins in biotechnology requires robust, durable reconstitution systems that enhance their stability and support their functionality in a range of working environments. Vesicular architectures are highly desirable to provide the compartmentalisation to utilise the functional transmembrane transport and signalling properties of membrane proteins. Proteoliposomes provide a native-like membrane environment to support membrane protein function, but can lack the required chemical and physical stability. Amphiphilic block copolymers can also self-assemble into polymersomes: tough vesicles with improved stability compared with liposomes. This review discusses the reconstitution of membrane proteins into polymersomes and the more recent development of hybrid vesicles, which blend the robust nature of block copolymers with the biofunctionality of lipids. These novel synthetic vesicles hold great promise for enabling membrane proteins within biotechnologies by supporting their enhanced in vitro performance and could also contribute to fundamental biochemical and biophysical research by improving the stability of membrane proteins that are challenging to work with.

Durable proteo-hybrid vesicles for the extended functional lifetime of membrane proteins in bionanotechnology.

The full capabilities of membrane proteins in bionanotechnology can only be realised through improvements in their reconstitution environments that combine biocompatibility to support function and durability for long term stability. We demonstrate that hybrid vesicles composed of natural phospholipids and synthetic diblock copolymers have the potential to achieve these criteria.