Proteins are fundamentally the most important macromolecules for biochemical, mechanical, and structural functions in living organisms. Therefore, they provide us with diverse structural building blocks for constructing various types of biomaterials, including an important class of such materials, hydrogels. Since natural peptides and proteins are biocompatible and biodegradable, they have features advantageous for their use as the building blocks of hydrogels for biomedical applications. They display constitutional and mechanical similarities with the native extracellular matrix (ECM), and can be easily bio-functionalized via genetic and chemical engineering with features such as bio-recognition, specific stimulus-reactivity, and controlled degradation. This review aims to give an overview of hydrogels made up of recombinant proteins or synthetic peptides as the structural elements building the polymer network. A wide variety of hydrogels composed of protein or peptide building blocks with different origins and compositions – including β-hairpin peptides, α-helical coiled coil peptides, elastin-like peptides, silk fibroin, and resilin – have been designed to date. In this review, the structures and characteristics of these natural proteins and peptides, with each of their gelation mechanisms, and the physical, chemical, and mechanical properties as well as biocompatibility of the resulting hydrogels are described. In addition, this review discusses the potential of using protein- or peptide-based hydrogels in the field of biomedical sciences, especially tissue engineering.

Xylose, the second most abundant sugar in lignocellulosic biomass hydrolysates, can be fermented by Saccharomyces cerevisiae expressing one of two heterologous xylose pathways: a xylose oxidoreductase pathway and a xylose isomerase pathway. Depending on the type of the pathway, its optimization strategies and the fermentation efficiencies vary significantly. In the present study, we constructed two isogenic strains expressing either the oxidoreductase pathway (XYL123) or the isomerase pathway (XI-XYL3), and delved into simple and reproducible ways to improve the resulting strains. First, the strains were subjected to the deletion of PHO13, overexpression of TAL1, and adaptive evolution, but those individual approaches were only effective in the XYL123 strain but not in the XI-XYL3 strain. Among other optimization strategies of the XI-XYL3 strain, we found that increasing the copy number of the xylose isomerase gene (xylA) is the most promising but yet preliminary strategy for the improvement. These results suggest that the oxidoreductase pathway might provide a simpler metabolic engineering strategy than the isomerase pathway for the development of efficient xylose-fermenting strains under the conditions tested in the present study.

The pandemic caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has upended healthcare systems and economies around the world. Rapid understanding of the structural biology and pathogenesis of SARS-CoV-2 has allowed the development of emergency use or FDA-approved vaccines and various candidate vaccines. Among the recently developed SARS-CoV-2 candidate vaccines, natural protein-based nanoparticles well suited for multivalent antigen presentation and enhanced immune stimulation to elicit potent humoral and cellular immune responses are currently being investigated. This mini-review presents recent innovations in protein-based nanoparticle vaccines against SARS-CoV-2. The design and strategy of displaying antigenic domains, including spike protein, receptor-binding domain (RBD), and other domains on the surface of various protein-based nanoparticles and the performance of the developed nanoparticle-based vaccines are highlighted. In the final part of this review, we summarize and discuss recent advances in clinical trials and provide an outlook on protein-based nanoparticle vaccines.

Targeted delivery of immunomodulatory molecules to the lymph nodes is an attractive means of improving the efficacy of anti-cancer immunotherapy. In this study, to improve the efficacy of PD-1 blockade-based therapy, nanocages were designed by surface engineering to decorate a programmed cell death protein 1 (PD-1) that is capable of binding against programmed death-ligand 1 (PD-L1) and -ligand 2 (PD-L2). This nanocage-mediated multivalent interaction remarkably increases the binding affinity and improves the antagonistic activity compared to free soluble PD-1. In addition, with the desirable nanocage size for optimal tumor-draining lymph node (TDLN) targeting (approximately 20 nm), rapid draining and increased accumulation into the TDLNs were observed. Moreover, the interference of the PD-1/PD-L axis with ultra-high affinity in the tumor microenvironment (effector phase) and the TDLNs (cognitive phase) significantly enhances the dendritic cell-mediated tumor-specific T cell activation. This characteristic successfully inhibited tumor growth and induced complete tumor eradication in some mice. Thus, the delivery of immunomodulatory molecules with nanocages can be a highly efficient strategy to achieve stronger anti-tumor immunity.

Tumor-specific apoptosis-inducing ligands have attracted considerable attention in cancer therapy. But, the evasion of apoptosis by tumors can cause acquired resistance to the therapy. TNF-related apoptosis-inducing ligand (TRAIL) has been investigated as an ideal antitumor agent owing to its inherent tumor cell-specific apoptotic activity. However, there are several barriers to its wider application, including the inability for stable formation of the trimeric structure, poor stability and pharmacokinetics, and differences in the sensitivity of different tumor types. Especially, almost 70% of tumor cells have acquired resistance to TRAIL, leading to failure of TRAIL-based therapeutics in clinical trials. To overcome therapeutic efficiency limitations against TRAIL-resistant tumors, we exploited the characteristic of a naturally derived nanocage that not only delivers TRAIL in its native-like trimeric structure, but also delivers a drug (doxorubicin [DOX]) that re-sensitizes TRAIL-resistant tumor cells. These TRAIL-presenting nanocages (TTPNs) showed high loading efficiency, pH-dependent release profiles, and effective intracellular delivery of the re-sensitizing agent DOX. As a result, DOX-TTPNs efficiently re-sensitized TRAIL-resistant tumor cells to TRAIL-mediated apoptosis in vitro by regulating levels of the TRAIL receptor, DR5, and anti- and pro-apoptotic proteins involved in extrinsic and intrinsic apoptosis pathways. We further demonstrated the antitumor efficacy of DOX-TTPNs in vivo, showing that even at a very low dose, the incorporated DOX successfully re-sensitized tumors to the apoptotic effects of TRAIL, underscoring the potential of this platform as an antitumor agent. Given that other homotrimeric TNF superfamily ligands and immunotherapeutic agents can be substituted for TRAIL ligand and re-sensitizing drugs on the surface and in the inner cavity of the nanocage, respectively, this platform is potentially suitable for development of a broad range of anticancer or immunotherapeutic combinations.