Messenger RNA (mRNA) introduction is a promising approach to produce therapeutic proteins and peptides without any risk of insertion mutagenesis into the host genome. However, it is difficult to introduce mRNA in vivo mainly because of the instability of mRNA under physiological conditions and its strong immunogenicity through the recognition by Toll-like receptors (TLRs). We used a novel carrier based on self-assembly of a polyethylene glycol (PEG)-polyamino acid block copolymer, polyplex nanomicelle, to administer mRNA into the central nervous system (CNS). The nanomicelle with 50 nm in diameter has a core-shell structure with mRNA-containing inner core surrounded by PEG layer, providing the high stability and stealth property to the nanomicelle. The functional polyamino acids possessing the capacity of pH-responsive membrane destabilization allows smooth endosomal escape of the nanomicelle into the cytoplasm. After introduction into CNS, the nanomicelle successfully provided the sustained protein expression in the cerebrospinal fluid for almost a week. Immune responses after mRNA administration into CNS were effectively suppressed by the use of the nanomicelle compared with naked mRNA introduction. In vitro analyses using specific TLR-expressing HEK293 cells confirmed that the nanomicelle inclusion prevented mRNA from the recognition by TLRs. Thus, the polyplex nanomicelle is a promising system that simultaneously resolved the two major problems of in vivo mRNA introduction, the instability and immunogenicity, opening the door to various new therapeutic strategies using mRNA.

Osteoarthritis (OA) is a chronic degenerative joint disease and a major health problem in the elderly population. No disease-modifying osteoarthritis drug (DMOAD) has been made available for clinical use. Here we present a disease-modifying strategy for OA, focusing on messenger RNA (mRNA) delivery of a therapeutic transcription factor using polyethylene glycol (PEG)-polyamino acid block copolymer-based polyplex nanomicelles. When polyplex nanomicelles carrying the cartilage-anabolic, runt-related transcription factor (RUNX) 1 mRNA were injected into mouse OA knee joints, OA progression was significantly suppressed compared with the non-treatment control. Expressions of cartilage-anabolic markers and proliferation were augmented in articular chondrocytes of the RUNX1-injected knees. Thus, this study provides a proof of concept of the treatment of degenerative diseases such as OA by the in situ mRNA delivery of therapeutic transcription factors; the presented approach will directly connect basic findings on disease-protective or tissue-regenerating factors to disease treatment.

Chemical modification of nucleosides in mRNA is an important technology to regulate the immunogenicity of mRNA. In this study, various previously reported mRNA formulations were evaluated by analyzing in vitro protein expression and immunogenicity in multiple cell lines. For the macrophage-derived cell line, RAW 264.7, modified mRNA tended to have reduced immunogenicity and increased protein expression compared to the unmodified mRNA. In contrast, in some cell types, such as hepatocellular carcinoma cells (HuH-7) and mouse embryonic fibroblasts (MEFs), protein expression was decreased by mRNA modification. Further analyses revealed that mRNA modifications decreased translation efficiency but increased nuclease stability. Thus, mRNA modification is likely to exert both positive and negative effects on the efficiency of protein expression in transfected cells and optimal mRNA formulation should be determined based on target cell types and transfection purposes.