Efficient expression of E. coli dihydrofolate reductase gene by an in vitro translation system using phosphorothioate mRNA

The Pivotal Role of Chemical Modifications in mRNA Therapeutics

After over a decade of development, mRNA has recently matured into a potent modality for therapeutics. The advantages of mRNA therapeutics, including their rapid development and scalability, have been highlighted due to the SARS-CoV-2 pandemic, in which the first two clinically approved mRNA vaccines have been spotlighted. These vaccines, as well as multiple other mRNA therapeutic candidates, are modified to modulate their immunogenicity, stability, and translational efficiency. Despite the importance of mRNA modifications for harnessing the full efficacy of mRNA drugs, the full breadth of potential modifications has yet to be explored clinically. In this review, we survey the field of mRNA modifications, highlighting their ability to tune the properties of mRNAs. These include cap and tail modifications, nucleoside substitutions, and chimeric mRNAs, each of which represents a component of mRNA that can be exploited for modification. Additionally, we cover clinical and preclinical trials of the modified mRNA platform not only to illustrate the promise of modified mRNAs but also to call attention to the room for diversifying future therapeutics.

Phosphorothioate Modification of mRNA Accelerates the Rate of Translation Initiation to Provide More Efficient Protein Synthesis

Messenger RNAs (mRNAs) with phosphorothioate modification (PS-mRNA) to the phosphate site of A, G, C, and U with all 16 possible combinations were prepared, and the translation reaction was evaluated using an E. coli cell-free translation system. Protein synthesis from PS-mRNA increased in 12 of 15 patterns when compared with that of unmodified mRNA. The protein yield increased 22-fold when the phosphorothioate modification at A/C sites was introduced into the region from the 5'-end to the initiation codon. Single-turnover analysis of PS-mRNA translation showed that phosphorothioate modification increases the number of translating ribosomes, thus suggesting that the rate of translation initiation (rate of ribosome complex formation) is positively affected by the modification. The method provides a new strategy for improving translation by using non-natural mRNA.

Recent advances in producing and selecting functional proteins by using cell-free translation

Prokaryotic and eukaryotic in vitro translation systems have recently become the focus of increasing interest for tackling fundamental problems in biochemistry. Cell-free systems can now be used to study the in vitro assembly of membrane proteins and viral particles, rapidly produce and analyze protein mutants, and enlarge the genetic code by incorporating unnatural amino acids. Using in vitro translation systems, display techniques of great potential have been developed for protein selection and evolution. Furthermore, progress has been made to efficiently produce proteins in batch or continuous cell-free translation systems and to elucidate the molecular causes of low yield and find possible solutions for this problem.

Cell-free protein synthesis systems

Cell-free protein synthesis systems enable the direct in vitro expression of proteins from template DNA or RNA. Use of biochemical and bioengineering techniques has greatly improved the yields and productivities of cell-free systems. In some cases, the yields approach the in vivo levels. Moreover, in vitro systems are capable of rapidly providing artificial polypeptides that greatly facilitate protein engineering. Post-translational modification steps in cell-free systems also offer exciting possibilities as reviewed here.

Dihydrofolate reductase synthesis in the presence of immobilized methotrexate. An approach to a continuous cell-free protein synthesis system

Dihydrofolate reductase was synthesized in a batch system in the presence of the affinity ligand methotrexate, bound to various matrices. Two types of gel were used: commercial methotrexate-agarose with pores inaccessible for translation machinery and methotrexate-POROS with pores easily accessible for translation reaction mixture components. The transcription/translation reaction was not inhibited by either the immobilized methotrexate or the matrix. The enzyme was synthesized with a high yield and could simultaneously be removed from the reaction mixture by the affinity matrix during the synthesis. With methotrexate-POROS present the reaction probably proceeded mainly in the pores of the gel. Kinetic limitations to the reaction in the presence of the gel were not observed. Active dihydrofolate reductase was eluted from methotrexate-POROS. The activity recovered was higher than dihydrofolate reductase activity synthesized in free solution system. The influence of the presence of immobilized methotrexate on dihydrofolate reductase synthesis will be further studied in a novel type of a continuous protein synthesis system.