Supercharged protein-surfactant bioconjugates for next-generation cell therapies

Cell membrane engineering is a rapidly emerging field with significant potential to impact on cell therapies, as the introduction of exogenous proteins into the cell membrane is highly advantageous for in vivo site-directed tissue repair. This is because there exist a number of barriers to the widespread uptake of cell therapies, including the lack of engraftment of transplanted cells, which results in limited functional integration. Several cell therapy studies have shown that intravenous or intra-arterial infusion of stem cells leads to undesirable accumulation in the lungs, which reduces the efficiency of systemic delivery and increases the likelihood of producing lethal microemboli. Even when implanted directly into the organ of interest, the number of cells required for therapeutic benefit can be prohibitively high. Accordingly, this research programme describes the rational design of a new class of artificial membrane binding proteins with chemotrophic homing properties that will help overcome these limitations and have far-reaching implications in cell therapies for disease and regenerative medicine.

The new approach involves cell functionalisation with protein-polymer surfactant bioconjugates, which circumvents the need for direct cell-surface chemistry, and offers a high degree of flexibility, as the approach can be readily applied to a diverse range of proteins for use on potentially any cell type. The synthetic methodology recently pioneered by the PI involves the re-engineering of a protein surface in two key steps: (i) amplification of the positive charge density on the protein surface (supercharging) (ii) electrostatic coupling of anionic polymer surfactant chains to the cationic sites displayed on the protein surface. Significantly, by constructing this polymer surfactant corona, the cell membrane affinity can be systematically tuned to facilitate spontaneous insertion of the bioconjugate into a stem cell membrane, whilst retaining the native function of the protein linked to the cell surface. The PI recently applied this methodology to the oxygen-binding protein myoglobin and demonstrated that the bioconjugates rapidly inserted into the cytoplasmic membranes of adult bone-marrow derived human mesenchymal stem cells (hMSCs).

The research programme has a strong (but not exclusive) focus on developing the bioconjugates for cell homing for cardiac stem cell therapy, as cardiovascular disease (CVD) is the leading cause of death globally (2015 WHO estimate is 18 million deaths p.a.). Here, the conceptual advance is centred on hijacking the homing properties of infectious bacteria by immobilising the fibronectin (Fn) binding domain of the bacterial adhesion protein CshA on the cytoplasmic membranes of stem cells. The CshA-Fn interaction has been proposed to be responsible for directing and immobilising S. gordonii colonisation at the cardiac endothelium, promoting the onset of infective endocarditis. Accordingly, the ability to display multiple copies of this binding motif, which has been evolutionarily optimised to recognise and bind specific molecular targets in the cardiac endothelium, on the membrane of stem cells has the potential to advance cardiac cell therapy.

Significantly, the cell membrane modification platform has the potential to have clinical impact beyond cell therapies for CVD, as it could be readily applied to other cell types and vesicles (e.g., monocytes, blood outgrowth endothelial cells, natural killer cells or exosomes) and involve other homing protein- or peptide-based molecules (e.g., integrins, antibodies, or other bacterial adhesins). The research programme describes a scientific approach that combines in-house techniques for biophysics and regenerative medicine, as well as cutting-edge techniques available at large-scale facilities. As there is a strong medical focus within the programme, the applicant has engaged clinical scientist partners to aid with medical translation.

Scientific innovation is essential for the international competitiveness of the UK and the Knowledge Economy, and convergent interdisciplinary science will be a major driver in the development of new technologies that will have significant impact on world economics. Significantly, the research described in the proposal fits within two of the Eight Great Technologies (Synthetic Biology and Regenerative Medicine) in which the UK is set to be a global leader. Within this emerging paradigm, the development of new classes of smart biological materials and the associated synthetic methodologies will create new opportunities for interdisciplinary scientists working at the interface of physical sciences and medicine. Moreover, the scientific components of the research, which probe fundamental processes such as protein supercharging, electrostatic self-assembly, cell membrane engineering and cell homing, will provide valuable insight which will be made accessible to the commercial, defence and public sectors.

With respect to societal impact, cardiovascular disease (CVD) is the world's number 1 killer, causing more than a quarter (26 %) of all deaths in the UK. This is approximately 160,000 deaths per year, with ~7 million people living with cardiovascular disease in the UK (British Heart Foundation). The ability to effectively target autologous cell therapies to sites of damage will be at the forefront of next-generation approaches in the treatment of CVD, and will also impact on other cell therapy treatments. This is because improved site-specific delivery of cells increases the success rate of re-implantation to damaged tissues, providing a potential step change in efficacy of cell therapies - not only by improving clinical success rates, but also by improving the therapeutic index of these approaches by reducing off-target delivery (typically membrane targeting to the lung). Through the selection of tissue-specific "homing proteins" and the ability to embed these targeting moieties into stem/progenitor cell membranes, there is a significant opportunity to reduce off-target delivery whilst significantly improving site-specific delivery and, importantly, retention at the damaged site whilst the cells adapt to the surrounding microenvironment. In essence, this approach will result in a disruptive technology for the manufacture of autologous cell therapies and provides clinical benefit via targeted delivery, reduction in off-target risks and an improvement in therapeutic index which will likely be linked to clinical success rates.

The PI's recent work on liquid proteins attracted significant publicity and was featured in a number of applied engineering and pharmaceutical journals, as well as in the popular media. Similarly, the new science generated by the discovery and exploration of this new cell membrane re-engineering platform will be published in top-tier journals, which will promote the dissemination of knowledge to beneficiaries across multiple fields. Participation at international conferences is also anticipated in order to communicate the potential technology to the private sector. It is also likely that the research will impact internationally upon policy-makers and international funding bodies by demonstrating the value of synthetic biology and regenerative medicine research. The University of Bristol's Research News website will also act as a vehicle to promote the research, and this will raise the university's profile within the UK and internationally.