siRNA molecules are approximately twenty-two (22) nucleotides in length and are double-stranded. Initially, siRNA precursors are recognized by Dicer RNase and incorporated into the RNA-induced silencing complex (RISC). The siRNA-RISC complex binds the targeting site of mRNA, resulting in a sequence-specific cleavage by endonuclease Argonaute-2 (AGO2). The result is a decreased expression of a targeted protein.
(Elbashir SM, Lendeckel W, Tuschl T.; RNA interference is mediated by 21-and 22-nucleotide RNAs. Genes Dev. 2001;15:188-200).
MicroRNAs are cellular RNA molecules that prevent the production of proteins by degrading the messenger (mRNA) of the proteins. MicroRNAs are short regulatory noncoding RNAs that block gene expression by binding to target sites in the 3'-untranslated regions (UTR) of protein-coding transcripts. The primary microRNAs (pri-microRNA) with a characteristic hairpin structure is recognized and processed by enzymes in Drosha and DGCR8 into ∼70 nucleotide long precursor microRNAs (pre-microRNAs). Resulting pre-microRNAs are further cleaved by Dicer RNase, forming mature dsRNAs (microRNAs). Finally, mature microRNAs are incorporated into RISC to induce cleavage of targeted mRNAs, such as siRNAs, or translational repression, resulting in a decrease of targeted proteins. Frequently, target sequences of microRNAs are found in the 3' UTR of mRNA, often within noncoding or intronic regions. Each microRNA can target hundreds of different mRNAs, thereby inducing regulation of the transcriptome.
In summary, microRNAs target multiple mRNAs, whereas siRNAs only possess a specific binding activity targeting only one mRNA at a time.
(Bartel DP. MicroRNAs: target recognition and regulatory functions. Cell. 2009;136:215-33.)
Messenger RNA (mRNA) delivery upregulates the expression of targeted proteins. mRNA delivered into cells has a low risk for insertional mutagenesis. Furthermore, the delivery of mRNAs into cells is also more consistent. Also, the kinetics of protein expression are more predictable, and in-vitro synthesis is more convenient. The transfection efficiency with mRNA is higher than that of DNA, especially in immune cells. Each mRNA has an open reading frame (ORF) that includes two untranslated regions (UTRs) located at the 5' and 3' ends of mRNA. The 5' methyl cap and the 3' poly(adenosine) tail are crucial for efficient translation. The translational system (the ribosome) recognizes mRNAs during protein synthesis.
(Tan L, Sun X.; Recent advances in mRNA vaccine delivery. Nano Res. 2018;11:5338-54; Van Tendeloo VFI, Ponsaerts P, Lardon F, Nijs G, Lenjou M, Van Broeckhoven C. et al.; Highly efficient gene delivery by mRNA electroporation in human hematopoietic cells: superiority to lipofection and passive pulsing of mRNA and to electroporation of plasmid cDNA for tumor antigen loading of dendritic cells. Blood. 2001;98:49-56. ; Matsui A, Uchida S, Ishii T, Itaka K, Kataoka K.; Messenger RNA-based therapeutics for the treatment of apoptosis-associated diseases. Sci Rep. 2015;5:15810. ; Xiong QQ, Lee GY, Ding JX, Li WL, Shi JJ.; Biomedical applications of mRNA nanomedicine. Nano Res. 2018;11:5281-309. )