Engineering protein nanocages as carriers for biomedical applications

Bhaskar, Sathyamoorthy; Lim, Sierin

Among myriad nature-derived nanocarriers, protein-based biological systems such as viruses have been a subject of intense study owing to their innate ability to penetrate cell membranes. The structure of viruses best represents the principles of protein assembly in nature. Structural analyses of viruses show that they consist of a protein shell comprising a definite number of subunits that surrounds and protects its genome. 4 Viral capsids are naturally programmed for host-cell targeting and cell entry. They have evolved to mediate the exchange of nucleic acids between different chemical environments. 5 Viruses are stable structures that have the ability to withstand environmental pressures but are sensitive enough to detect signals or the change in signals in cellular environment, thereby releasing the nucleic acids they carry in the target microenvironment. Viruses have thus been an inspiration in developing diverse self-assembling protein nanocages from natural sources. Important aspects of protein nanocages such as biocompatibility, functional diversity, biological fabrication and flexibility of design by protein engineering make them powerful materials for various applications. 5 Viruses have been engineered to perform specific functions. For example, bacteriophages have been used in peptide display, filamentous phages have been used as templates for nanofabrication and virus-like particles (VLPs) have been used as immunogens. With the exception of peptide-displaying filamentous phages commonly used for nucleic acid and conjugated drug delivery in vitro and in vivo, these nanoparticles are hardly suitable for celltargeted drug delivery. 6 Specific functional polypeptides can be modified to self-assemble into nanoparticles with or without caged structures with desirable nanoscale properties in terms of size and geometry. The assembly of the functional building blocks to form protein nanoparticles, as observed in natural viruses, can seldom be mimicked by general nanofabrication techniques. 6 The intricacy stems from selecting protein sequences that promote protein-protein interactions without nonspecific aggregation. These protein sequences can be derived from both natural and non-natural amino-acid sequences that lead to selfassembly in various patterns. 6 Natural protein scaffolds are structures that already exist in nature with intrinsic self-assembling properties, including viruses, ferritin and eukaryotic vaults. In contrast, synthetic protein scaffolds are designed de novo to mimic the properties of the natural scaffolds and carry out specific functions. The advantage of both natural and synthetic protein scaffolds is that they can be exploited for the design of novel functional architectures. 5 Specific applications of protein nanocages for drug delivery have been discussed in a review by Molino et al. 7 and Schoonen et al. 8 In this review, we focus on natural and synthetic protein scaffolds engineered with specific functional groups to impart non-native functions, including aiding the delivery of active molecules through targeting of malignant cells and overcoming cellular barriers.