How miRNA can cause Sequence Non-Specific Effects (Essay Sample)

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How miRNA can cause Sequence Non-Specific Effects and Trigger RNAi
The micro-RNA are small RNA molecules of about 20-30 nucleotide base pairs that function as guide molecules in the RNA-induced silencing complex (RISC) (Cloonan 379; Umbach & Cullen 1151). The miRNA can cause sequence non-specific effects such as immune responses, besides the intended sequence-specific silencing effects. The effects mainly result from the interferon (IFN) induction and cell death from the activation of cellular sensors of foreign DNA (Olejniczak 5). The initiation of the interferon response and eventual cell death emanates from the induction of pattern-recognition receptors (PRRs), which are cellular sensors located in the cytoplasm that identify the miRNA in the viral molecules (Umbach & Cullen 1156). The TLR3 and RIG-1 are some of the PRR that recognize the viral dsRNA, and upon activation, initiate a signal cascade that leads to the expression and secretion of interferon. The TLR3. The interferon response is then amplified and spreads to neighboring cells, aided by a positive feedback loop from the JAK/STAT pathway and resulting in the generation of the antiviral state (Olejniczak 7).
The binding of interferons to their cell-surface receptors leads to the generation of many interferons stimulated genes like OAS/RNAse L and PKR (Umbach & Cullen 1156). The OAS proteins recognize the dsRNA of the virus and tag them to the 2'-5'-adenosine oligomers, which in turn activate the RNAse L to cause the degradation of the miRNA from the virus. The degraded products can further stimulate the RIG-1 proteins, leading to interferon expression via a positive feedback loop (Umbach & Cullen 1157). The TLR7 and TLR8 are mainly involved in the recognition of the ssRNA of the virus though they can also slightly recognize the dsRNA. For the case of the PKR, after recognizing the viral miRNA, it undergoes dimerization and autophosphorylation, resulting in the phosphorylation of the α-subunit of the translational initiation factor eIF2 and IkB proteins (Olejniczak 7). This results in the regulation of the p38, MAP/INK, STAT 1 proteins leading to the accelerated transcription of the IFN-stimulated genes. Furthermore, the regulation of the p38, MAP/INK, and STAT 1 proteins results in the stimulation of inflammatory cytokines through the initiation of the transcription factors c-Jun and ATF2 (Olejniczak 8).


Figure 1: Cellular sensors and responders to ds-RNA and ss-RNA. Adapted from Nucleic Acid Respiration, 38(1), 1-16. Jan 2010.
RNA interference is a biological process involving the inhibition of gene expression through the destruction of specific mRNA molecules (Hammond 5822). At the heart of the RNAi pathway are two ribonuclease machinery; the Dicer and the Ribonuclease enzyme, which result in the generation of short interfering RNA triggers (Dang et al. 1149). The siRNA of 19 base pairs or lower do not result in the activation of PKR and TLR3, which modulate the sequence non-specific effects of the double-stranded miRNA (Olejniczak 6). The TLR7 and TLR8, which are involved in the generation of IFN-stimulated genes for single-stranded miRNA are also not involved.
Umbach and Cullen (1152) contend that the miRNAs are transcribed as long primary miRNAs, which may be located in the non-coding RNAs or an intron. After the molecule's excision by the Drosha, the pre-miRNA hairpins of the RNA molecules get exported to the cytoplasm for further processing. The one strand of the resultant miRNA intermediate duplex is assimilated into the RISC and serves as a guide RNA to in making the RISC to partially or completely interact with the complementary strands of the mRNA leading to translational inhibition or mRNA degradation in that order (Cloonan 379). The proces...