Non-fouling materials that resist cell adhesion are very important in fundamental research on cell–biomaterial interactions and for practical applications. Thus, they have been extensively investigated during the last decade. Natural biomacromolecules such as bovine serum albumin (BSA) have conventionally been used to block cell adhesion. Zhao and Ding (Fudan University, Shanghai, China) recently reported that the purple membrane (PM) containing a natural photoresponsive protein, bacteriorhodopsin (BR), can serve as a new basic substance that resists adhesion of mammalian cells.
BR is a retinal-containing bacterial protein that is present in the PM region of the bacterial membrane. The PM is the simplest biological system for energy conversion. Excitation of BR with a photon causes a photocycle, resulting in uptake of a proton from the cytoplasmic side of the PM and release of another proton to the extracellular side. BR and similar retinal proteins have attracted much attention as potential optical materials for applications in 2-D or 3-D storage, holographic storage, optical filtering, light switching, neural networks, super-fast photo detection, motion detection, and artificial retinas, etc., and also as model proteins in fundamental research.
Nevertheless, the PM had not been recognized as a non-fouling material for preventing cell adhesion. Adhesion is the first cellular event that occurs when a cell comes into contact with a material surface, and it influences subsequent cellular events such as proliferation and differentiation. Non-fouling surfaces are useful for studies on cell–material interactions for generating patterned surfaces of contrasting adhesion, and they are also useful for controlling cellular and bacterial adhesion in medical applications and/or in complex bio-related environments. A number of non-fouling substances have been reported, including polyethylene glycol (PEG) or oligoethylene glycol (OEG), PEG hydrogels, poly(N-isopropylacrylamide) or its copolymers, and the globular protein, BSA. This study affords a new choice of non-fouling substance, because the PM containing the membrane protein BR was shown to resist cell adhesion.
Our results reveal that the natural PM has significant resistance to cell adhesion. This non-fouling property is very beneficial for potential applications of BR materials, for instance, as biosensors in complex environments. The finding also affords an excellent model substance that resists cell adhesion, which could be useful in surface modification of biomaterials for regenerative medicine. The photoresponsive property of BR distinguishes it from all of the currently known anti-fouling substances.
It is also worth noting that many other retinal proteins such as rhodopsin, halorhodopsin, xanthorhodopsin, and archaerhodopsin, are light-driven ion pumps. Therefore, a direct extension of this work would be to examine possible non-fouling properties of other retinal proteins. Further studies are required examine the resistance to adhesion of other types of mammalian and non-mammalian cells. Hence, while the mechanism by which the PM prevents non-specific protein absorption remains an open question, this report paves the way for further investigations and new applications of BR and possibly other light-sensitive proteins.