Aribosome (depicted in green) creates a protein (depicted here as a string of beads representingamino acids)encoded in an mRNA (depicted as a ribbon ofnucleotides) that may be modified to reduceinflammation in the cell.
To induce cells to make proteins that they do not normally produce, it is possible to introduceheterologous mRNA into thecytoplasm of the cell, bypassing the need for transcription. In other words, a blueprint for foreign proteins is "smuggled" into the cells. To achieve this goal, however, one must bypass cellular systems that prevent the penetration and translation of foreign mRNA. There are nearly-ubiquitousenzymes calledribonucleases (also called RNAses) that break down unprotected mRNA.[5] There are also intracellular barriers against foreign mRNA, such asinnate immune system receptors,toll-like receptor (TLR) 7 andTLR8, located inendosomal membranes. RNA sensors like TLR7 and TLR8 can dramatically reduce protein synthesis in the cell, trigger release ofcytokines such asinterferon andTNF-alpha, and when sufficiently intense lead toprogrammed cell death.[6]
The inflammatory nature of exogenous RNA can be masked by modifying the nucleosides in mRNA.[7] For example, uridine can be replaced with a similar nucleoside such aspseudouridine (Ψ) orN1-methyl-pseudouridine (m1Ψ),[8] andcytosine can be replaced by5-methylcytosine.[9] Some of these, such as pseudouridine and 5-methylcytosine, occur naturally ineukaryotes,[10] while m1Ψ occurs naturally inarchaea.[11] Inclusion of these modified nucleosides alters thesecondary structure of the mRNA, which can reduce recognition by the innate immune system while still allowing effective translation.[9]
A normal mRNA starts and ends with sections that do not code for amino acids of the actual protein. These sequences at the5′ and 3′ ends of an mRNA strand are calleduntranslated regions (UTRs). The two UTRs at their strand ends are essential for the stability of an mRNA and also of a modRNA as well as for the efficiency of translation, i.e. for the amount of protein produced. By selecting suitable UTRs during the synthesis of a modRNA, the production of the target protein in the target cells can be optimised.[5][12]
Various difficulties are involved in the introduction of modRNA into certain target cells. First, the modRNA must be protected fromribonucleases.[5] This can be accomplished, for example, by wrapping it inliposomes. Such "packaging" can also help to ensure that the modRNA is absorbed into the target cells. This is useful, for example, when used invaccines, asnanoparticles are taken up bydendritic cells andmacrophages, both of which play an important role in activating the immune system.[13]
Furthermore, it may be desirable that the modRNA applied is introduced into specific body cells. This is the case, for example, ifheart muscle cells are to be stimulated to multiply. In this case, the packaged modRNA can be injected directly into anartery such as acoronary artery.[14]
Replacinguridine withpseudouridine to evade the innate immune system was pioneered by Karikó and Weissman in 2005.[15][16] They won the 2023 Nobel Prize in Physiology or Medicine as a result of their work.[17]
Another milestone was achieved by demonstrating the life-saving efficacy of nucleoside modified mRNA in a mouse model of a lethal lung disease by the team of Kormann and others in 2011.[18]
Other possible uses of modRNA include the regeneration of damaged heart muscle tissue,[36][37] an enzyme-replacement tool[38] and cancer therapy.[39][40]
^Verbeke R, Lentacker I, Wayteck L, Breckpot K, Van Bockstal M, Descamps B, et al. (November 2017). "Co-delivery of nucleoside-modified mRNA and TLR agonists for cancer immunotherapy: Restoring the immunogenicity of immunosilent mRNA".Journal of Controlled Release.266:287–300.doi:10.1016/j.jconrel.2017.09.041.PMID28987878.S2CID20794075.
