Get Spliced: Uniting Alternative Splicing and Arthritis

Figures

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  Figure 1. Schematic overview of splicing outcome effects on mRNA. Schematic depiction of a pre-mRNA containing four exons (colored boxes) and three introns (black lines). Constitutive splicing results in a mature transcript that contains all four exons. Different types of alternative splicing can have different outcomes. Intron retention results in the inclusion of one or more introns in the mRNA. With mutually exclusive exons, either one exon or another exon is incorporated but not both exons simultaneously. Cassette alternative exons can either be skipped or incorporated into the mature mRNA. Alternative splicing at alternative 5′ or 3′ splice sites will result in the formation of extended or shortened exons.

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  Figure 2. Examples of alternative splicing events that are involved in the innate and adaptive immune response. (A) The presence of heterogeneous nuclear ribonucleoproteins (hnRNPs) M on poised chromatin enables immediate interaction with nascent innate immune transcripts, including IL-6. Expression of IL-6 is induced upon the kappa-light-chain-enhancer of B cell (NFκB)-signaling, and the interaction of hnRNP M with the IL-6 pre-mRNAs causes intron retention. This prevents further mRNA processing. Upon full macrophage activation, hnRNP M is phosphorylated and IL-6 transcripts are fully processed and can exit the nucleus. (B) In activated macrophages, NLRP3 engages in inflammasome formation and contributes to the cleaving of pro-IL-1β and -IL-18 into functional IL-1β and IL-18. The alternative splicing of NLRP3 results in the generation of short NLRP3 isoforms that lose their pro-inflammatory function. (C) IL-1β induces the expression of short isoforms of tenascin-C, which lack the AD2AD1 domain. In contrast to the fibrillar structure of full-length tenascin-C, the short isoform is secreted into the extracellular matrix (ECM) as soluble protein, which activates macrophages and enables…

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  Figure 3. Schematic representation of alternative splicing examples in arthritis. (A) Elevated expression levels of CD44v5 and CD44v6 were observed in patients with rheumatoid arthritis. These isoforms were subjected to proteolytic shedding, resulting in soluble isoforms that lack the intracellular signaling domain. Soluble CD44 captures galectin-8 in rheumatoid synovia without subsequently inducing apoptosis, and thus the inflammation will persist. (B) In contrast to full-length IL-6R, the delta exon 9 splice variant can be subjected to proteolytic cleaving. Both receptors are overexpressed in patients with arthritis and can induce a cytokine storm upon IL-6 binding and gp130 activation. Tocilizumab binds and inhibits both isoforms of IL-6R and thereby prevents IL-6 signaling. (C) Survivin has three different isoforms, FL, Δ3, and 2B. The function of survivin depends on the location and the isoform expressed. Nuclear Δ3 can modulate cell division, migration, and stability. Both FL and Δ3 have anti-apoptotic properties, forming a complex with XIAP and XAF1. This complex binds and inhibits caspase-3 and caspase-9, thereby preventing apoptosis. The 2B can promote cell cycle activation and…

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  Figure 4. Summarizing overview of alternative splicing events in the scope of arthritis. Known cases of alternative splicing in arthritis are shown in a word web. The gene names are displayed within the outer circle, and corresponding isoforms are described outside of the circle. The splicing isoforms were found using the search query “(arthritis) AND ((alternative splicing) OR (splicing isoform))” on 3rd of May 2023. Isoforms were included when there were at least two references. The references are displayed in a supplementary table and list; respectively, Supplementary SI and SII.

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  Figure 5. Illustrative examples of potential treatment options based on biomarker detection using 3rd generation sequencing in the scope of arthritis. (A) Antisense oligonucleotide (ASO) therapy can be used to induce the specific skipping of a detrimental exon, while maintaining a beneficial protein, depending on the original protein function. (B) Monoclonal antibodies capable of recognizing a detrimental protein epitope can be used to specifically target detrimental proteins while retaining beneficial protein isoforms. (C) Naturally existing antagonistic isoforms can be used to counter inflammation. Highly purified antagonistic isoforms can be isolated and injected to capture cytokine/receptor complexes, thus suppressing cytokine effects.