Atrial fibrillation (AF) is a type of sustained arrhythmia in humans often characterized by devastating alterations to the cardiac conduction system as well as the structure of the atria. AF can lead to decreased cardiac function, heart failure, and other complications. Long non-coding RNAs (lncRNAs) have been shown to play important roles in the cardiovascular system, including AF; however, a large group of lncRNAs is not conserved between mouse and human. Furthermore, AF has complex networks showing variations in mechanisms in different species, making it challenging to utilize conventional animal models to investigate the functional roles and potential therapeutic benefits of lncRNAs for AF. Fortunately, pluripotent stem cell (PSC)-derived cardiomyocytes (CMs) offer a reliable platform to study lncRNA functions in AF because of certain electrophysiological and molecular similarities with native human CMs. In this review, we first summarize the broad aspects of lncRNAs in various heart disease settings, then focus on their potential roles in AF development and pathophysiology. We also discuss current uses of PSCs in AF research and describe how these studies could be developed into novel therapeutics for AF and other cardiovascular diseases.

The regulation of the informational flow from the mitochondria to the nucleus (mitonuclear communication) is not fully characterized in the heart. We have determined that mitochondrial ribosomal protein S5 (MRPS5/uS5m) can regulate cardiac function and key pathways to coordinate this process during cardiac stress. We demonstrate that loss of Mrps5 in the developing heart leads to cardiac defects and embryonic lethality while postnatal loss induces cardiac hypertrophy and heart failure. The structure and function of mitochondria is disrupted in Mrps5 mutant cardiomyocytes, impairing mitochondrial protein translation and OXPHOS. We identify Klf15 as a Mrps5 downstream target and demonstrate that exogenous Klf15 is able to rescue the overt defects and re-balance the cardiac metabolome. We further show that Mrps5 represses Klf15 expression through c-myc, together with the metabolite L-phenylalanine. This critical role for Mrps5 in cardiac metabolism and mitonuclear communication highlights its potential as a target for heart failure therapies.

Actin is integral to eukaryotic physiology as a biomechanical polymer and as a structural barrier for cell-autonomous defense against infection. Some microbial pathogens exploit the actin cytoskeleton, however, to evade cell-autonomous immunity. Subversion of actin to enter host cells and for actin-based motility are often employed by intracellular pathogens to spread from cell-to-cell. Using RNA-sequencing and computational data mining, we identify the host actin-binding protein XIRP1 as commonly induced during infection. XIRP1 is expressed by fibroblasts and macrophages in response to immune cytokines such as interferon-gamma (IFN-γ) and infection with bacteria such as Listeria, Shigella, and Salmonella. Confocal and super-resolution structured illumination microscopy (SIM) found XIRP1 localizes to fibroblast focal adhesions and macrophages podosomes. Within human macrophages, XIRP1 is recruited to cytosolic Listeria monocytogenes in an ActA-dependent manner as it replicates and uses actin-based motility for host cell escape. Chromosomal removal of XIRP1 in mice impaired this dissemination and rendered them more resistant to Listeria infection than C57BL/6NJ wildtype controls in vivo. We propose that professional cytosolic pathogens like Listeria can co-opt XIRP1 to escape the hostile intracellular environment of IFN-γ-activated macrophages as part of the host-pathogen arms race during cell-autonomous immunity.

Alanyl-transfer RNA synthetase 2 (AARS2) is a nuclear encoded mitochondrial tRNA synthetase that is responsible for charging of tRNA-Ala with alanine during mitochondrial translation. Homozygous or compound heterozygous mutations in the Aars2 gene, including those affecting its splicing, are linked to infantile cardiomyopathy in humans. However, how Aars2 regulates heart development, and the underlying molecular mechanism of heart disease remains unknown. Here, we found that poly(rC) binding protein 1 (PCBP1) interacts with the Aars2 transcript to mediate its alternative splicing and is critical for the expression and function of Aars2. Cardiomyocyte-specific deletion of Pcbp1 in mice resulted in defects in heart development that are reminiscent of human congenital cardiac defects, including noncompaction cardiomyopathy and a disruption of the cardiomyocyte maturation trajectory. Loss of Pcbp1 led to an aberrant alternative splicing and a premature termination of Aars2 in cardiomyocytes. Additionally, Aars2 mutant mice with exon-16 skipping recapitulated heart developmental defects observed in Pcbp1 mutant mice. Mechanistically, we found dysregulated gene and protein expression of the oxidative phosphorylation pathway in both Pcbp1 and Aars2 mutant hearts; these date provide further evidence that the infantile hypertrophic cardiomyopathy associated with the disorder oxidative phosphorylation defect type 8 (COXPD8) is mediated by Aars2. Our study therefore identifies Pcbp1 and Aars2 as critical regulators of heart development and provides important molecular insights into the role of disruptions in metabolism on congenital heart defects.

In this study, we sought to determine the role of tRNA‐derived fragments in the regulation of gene expression during skeletal muscle cell proliferation and differentiation. We employed cell culture to examine the function of mt‐Ty 5’ tiRNAs. Northern blotting, RT‐PCR as well as RNA‐Seq, were performed to determine the effects of mt‐Ty 5’ tiRNA loss and gain on gene expression. Standard and transmission electron microscopy (TEM) were used to characterize cell and sub‐cellular structures. mt‐Ty 5’tiRNAs were found to be enriched in mouse skeletal muscle, showing increased levels in later developmental stages. Gapmer‐mediated inhibition of tiRNAs in skeletal muscle C2C12 myoblasts resulted in decreased cell proliferation and myogenic differentiation; consistent with this observation, RNA‐Seq, transcriptome analyses, and RT‐PCR revealed that skeletal muscle cell differentiation and cell proliferation pathways were also downregulated. Conversely, overexpression of mt‐Ty 5’tiRNAs in C2C12 cells led to a reversal of these transcriptional trends. These data reveal that mt‐Ty 5’tiRNAs are enriched in skeletal muscle and play an important role in myoblast proliferation and differentiation. Our study also highlights the potential for the development of tiRNAs as novel therapeutic targets for muscle‐related diseases.