RNA-Binding Proteins: Emerging Therapeutics for Vascular Dysfunction

Abstract

Vascular diseases account for a significant number of deaths worldwide, with cardiovascular diseases remaining the leading cause of mortality. This ongoing, ever-increasing burden has made the need for an effective treatment strategy a global priority. Recent advances in regenerative medicine, largely the derivation and use of induced pluripotent stem cell (iPSC) technologies as disease models, have provided powerful tools to study the different cell types that comprise the vascular system, allowing for a greater understanding of the molecular mechanisms behind vascular health. iPSC disease models consequently offer an exciting strategy to deepen our understanding of disease as well as develop new therapeutic avenues with clinical translation. Both transcriptional and post-transcriptional mechanisms are widely accepted to have fundamental roles in orchestrating responses to vascular damage. Recently, iPSC technologies have increased our understanding of RNA-binding proteins (RBPs) in controlling gene expression and cellular functions, providing an insight into the onset and progression of vascular dysfunction. Revelations of such roles within vascular disease states have therefore allowed for a greater clarification of disease mechanisms, aiding the development of novel therapeutic interventions. Here, we discuss newly discovered roles of RBPs within the cardio-vasculature aided by iPSC technologies, as well as examine their therapeutic potential, with a particular focus on the Quaking family of isoforms.

Keywords: QKI; Quaking; RNA-binding proteins; iPSCs; stem cell technologies; vascular disease.

Figure 1

A schematic diagram summarising the various roles of RNA-binding proteins (RBPs) in post-transcriptional gene regulation (PTGR). PTGR encompasses the maturation, transport, stabilit, and translation of coding and non-coding RNAs. Each of these events are heavily regulated by ribonucleoprotein complexes with RBPs at their core. Typically, RBPs are first involved in the maturation of RNA through regulating processes such as RNA editing and splicing. Following this, mature RNA transcripts are exported to the cytoplasm by various RBPs. Within the cytoplasm, RBPs then coordinate the localisation of RNAs to subcellular compartments, as well as orchestrate either the degradation or stabilisation of RNA by binding the substrate RNAs. Stabilisation results in the translation and folding of RNAs into the corresponding proteins constructs.

Figure 2

The Quaking (QKI) locus expresses alternatively spliced RNA-binding proteins belonging to the signal transduction and activation of RNA (STAR family).A schematic displaying the three major isoforms of QKI, namely, QKI-5, QKI-6 and QKI-7, yielded from the alternative splicing of the QKI gene, displayed with both exons and introns.

Figure 3

A schematic overview of the critical pathological and physiological responses the (cardio)vascular system exhibits in response to the three major Quaking (QKI) isoforms. QKI expression is known to drastically increase in response to vascular injury, suggesting reparative roles of the QKI isoforms. QKI reduction results in an inhibition of angiogenic and proliferative capabilities, as well as promotes cell senescence. Reduction in atrial myocytes specifically results in atrial fibrillation, whilst a lack of QKI-5 in endothelial cells (ECs) greatly impairs cell-to-cell adhesions and barrier function. QKI expression has also been shown to be vital in numerous cellular processes, including angiogenesis, erythropoiesis and EC, vascular smooth muscle cells (SMCs) and myotubule differentiation. In addition, QKI expression can ameliorate pathogenic fibroproliferative responses to vascular injury, whilst QKI-5 and QKI-6 expression is able to improve vascular cell qualities. Overexpression of QKI-7, however, has been shown to induce apoptosis and to drastically impair EC function, indicating a pathogenic role of QKI-7 expression within the (cardio)vascular system. The QKI isoforms therefore have great promise for use in vascular therapy due to their vital roles within (cardio)vascular health and disease states.

Figure 4

A figure displaying the current potential RNA-binding protein (RBP)-based therapeutic strategies to post-transcriptionally control cellular phenotype, with the aim of restoring or preventing vascular disease states. RBP-based therapeutic strategies either focus on inhibiting the action of the RBP itself or the target RNA. For example, small molecules and Aptamers can be designed to inhibit the action of RBPs either by tagging a specific RBP for degradation by proteasome (1) or by blocking the RBP interaction with either its target RNA or functional protein partner (2A–C). In addition, inactivation of RBPs can be achieved through the prevention of post-translational modifications (3). Another direct therapeutic approach is to block the enzymatic activity of a specific RBP (4) or to knock out its gene by CRISPR/Cas9 (5). RNA-targeting therapeutics, however, could be potentially used to reverse the effects of pathogenic RBPs on mRNA stability, alternative splicing and translation of target RNAs. RNA-based therapeutic tools, such as antisense oligonucleotides (ASOs), siRNAs, miRNAs and CRISPR/Cas, can be exploited to induce degradation of the target RNA independent from RBP (6A and B), block or manipulate interaction between an RBP and its RNA target through RNA occupation only (7) or RNA edition (8). For the latter, an RNA editor such as RNA adenosine deaminase 2 (ADAR2), with the help of the CRISPR/dCas13b system, can introduce point mutations in target RNA; it thereby has the potential to recapitulate a new RBP binding site, rescue a known pathogenic binding site or create a premature stop codon to spoil functionality of target RNA, for example. Within the figure, validated examples are shown in red text. RISC, RNA-induced silencing complex; KO, knockout; PTM, post-translational modifications; ASO, antisense oligonucleotide; SMN2, survival of motor neuron 2 gene.