dsRNA - Revolutionizing Gene Silencing and Beyond

How dsRNA Mediates Gene Silencing

Double-stranded RNA (dsRNA) is composed of two complementary RNA strands that create a double helix shape. It is closely related to the majority of viral infections, either as genetic material in dsRNA viruses or as a byproduct of viral replication cycles. It serves a vital role in immunological control, as practically all species can recognize and respond to dsRNA.

dsRNA is one of the key components of RNA interference (RNAi). RNAi is a gene-silencing mechanism mediated by dsRNA or microRNA (miRNA). After a series of reactions, these molecules bind to specific messenger RNA (mRNA) targets in the host cell, inhibiting their translation or inducing mRNA degradation, thereby preventing normal gene expression. As a naturally occurring and highly conserved mechanism, RNAi has important applications in biological research, biotechnology, and therapeutic studies.

Upon viral infection or artificial introduction into cells, long dsRNA is recognized and cleaved by an endonuclease called Dicer, producing small fragments known as small interfering RNA (siRNA). siRNA typically consists of 19-25 nucleotide sequences and activates the RNA-induced silencing complex (RISC). Within RISC, the sense strand of siRNA is cleaved, while the antisense strand guides RISC to bind complementarily with the target mRNA. Exogenous siRNA is usually designed to perfectly complement a single mRNA's coding sequence, thereby conservatively and stably achieving cleavage and degradation of the target mRNA. In research, a complementary short RNA sequence of 19-22 nucleotides, known as short hairpin RNA (shRNA), can replace long dsRNA to achieve the same effect, but its expression requires viral vectors, which may introduce safety concerns.

On the other side, miRNA mediates gene silence in a slightly different mechanism to dsRNA, but they involve similar enzymes and pathways. miRNA production requires numerous precursor stages. Following the transcription of miRNA genes in the nucleus, primary miRNA (pri-miRNA) is generated. The microprocessor complex subsequently cleaves this miRNA to make precursor microRNA (pre-miRNA). Pre-miRNA is delivered to the cytoplasm, where Dicer converts it into mature miRNA. MiRNA creates a complex with RISC in which the sense strand is discarded rather than cleaved, while the antisense strand directs the complex's target mRNA binding and control. Endogenous miRNA and mRNA sequence match can range from partial to virtually perfect, allowing for control of target mRNA expression by cleavage, degradation, and/or protein translation inhibition.

Figure 1. The Mechanisms of Gene Silencing by siRNA and miRNA
(Source: Lam JKW, et al. 2015)

Novel pest control methods based on dsRNA

In the past two decades, dsRNA and its mediated RNAi have received widespread attention for their potential applications in agricultural production. The precise manipulation of target gene expression by dsRNA allows it to be used for protecting crops and as a species-specific insecticide to control field pests, without affecting beneficial insects. Furthermore, dsRNA has a half-life of less than 3 days and degrades easily in the environment, without persisting in aquatic systems, making it a safe, non-toxic, and environmentally friendly pest control solution.

Introducing dsRNA into crops and ensuring its correct expression can improve crop traits, enhance the production efficiency of natural products, prolong flowering duration, increase disease resistance, and cultivate flowers and fruits with longer shelf life and higher nutritional value. Achieving these goals does not necessarily require complete gene knockout and can substitute traditional genetic modification methods.

As an efficient insecticide, dsRNA has been or is about to be applied to control common field pests such as aphids, red fire ants, leafhoppers (one of the rice pests), fruit flies, and moths. Typically, critical genes involved in pest development and reproduction are selected as optimal target genes. Silencing these genes can lead to developmental stagnation, significantly reduce egg-hatching rates, or directly kill the insects. It is important to note that the configuration and length of introduced dsRNA/shRNA and their target mRNA can affect the efficacy of gene silencing and off-target effects, resulting in varying RNAi effects among different insect species and potentially within the same species. The efficacy of RNAi is also influenced by the physiological stage and gender of insects, with larvae often exhibiting high sensitivity to RNAi.

Figure 2. The connection between RNAi in antiviral defense and as a pest control strategy
(Source: Niu J, et al. 2023)

Methods for applying dsRNA include soaking larvae in solutions, spraying, feeding adults with sugar solutions or microorganisms expressing iRNA, and microinjection. Spraying may not penetrate the insect's cuticle layer, and naked dsRNA in the environment degrades quickly, so oral ingestion of sufficient dsRNA is the preferred method for production applications. Complexing dsRNA with lipid nanoparticles is also a primary technique currently used, enhancing dsRNA stability in the environment and enabling effective penetration through various barriers to reach insect cells.

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