The ryanodine receptor stabilizer S107 ameliorates contractility of adult Rbm20 knockout rat cardiomyocytes

Alternative splicing factors and cardiac disease: more than just missplicing?

Alternative splicing (AS) is the process wherein the exons from a single gene are joined in different combinations to produce nonidentical, albeit related, RNA transcripts. This process is important for the development and physiological function of many organs and is particularly important in the heart. Notably, AS has been implicated in cardiac disease and failure, and a growing number of genetic variants in AS factors have been identified in association with cardiac malformation and/or disease. With the field poised to interrogate how these variants affect cardiac development and disease, an understandable point of emphasis will undoubtedly be on downstream target gene missplicing. In this Perspective article, we would like to encourage consideration not only of the potential for novel disease mechanisms, but also for contributions from disruption of the ever-expanding list of nonsplicing functions ascribed to many AS factors. We discuss the emergence of a novel cardiac disease mechanism based on pathogenic RNA granules and speculate on the generality of such a mechanism among localization-disrupting AS factor genetic variants. We also highlight emerging nonsplicing functions attributed to several AS factors with cardiac disease-associated genetic variants in the hopes of pointing to avenues for exploration of mechanisms that may contribute to disease alongside target gene missplicing.

Mechanisms of RBM20 Cardiomyopathy: Insights From Model Systems

RBM20 (RNA-binding motif protein 20) is a vertebrate- and muscle-specific RNA-binding protein that belongs to the serine-arginine-rich family of splicing factors. The RBM20 gene was first identified as a dilated cardiomyopathy-linked gene over a decade ago. Early studies in Rbm20 knockout rodents implicated disrupted splicing of RBM20 target genes as a causative mechanism. Clinical studies show that pathogenic variants in RBM20 are linked to aggressive dilated cardiomyopathy with early onset heart failure and high mortality. Subsequent studies employing pathogenic variant knock-in animal models revealed that variants in a specific portion of the arginine-serine-rich domain in RBM20 not only disrupt splicing but also hinder nucleocytoplasmic transport and lead to the formation of RBM20 biomolecular condensates in the sarcoplasm. Conversely, mice harboring a disease-associated variant in the RRM (RNA recognition motif) do not show evidence of adverse remodeling or exhibit sudden death despite disrupted splicing of RBM20 target genes. Thus, whether disrupted splicing, biomolecular condensates, or both contribute to dilated cardiomyopathy is under debate. Beyond this, additional questions remain, such as whether there is sexual dimorphism in the presentation of RBM20 cardiomyopathy. What are the clinical features of RBM20 cardiomyopathy and why do some individuals develop more severe disease than others? In this review, we summarize the reported observations and discuss potential mechanisms of RBM20 cardiomyopathy derived from studies employing in vivo animal models and in vitro human-induced pluripotent stem cell-derived cardiomyocytes. Potential therapeutic strategies to treat RBM20 cardiomyopathy are also discussed.

The Role of Ryanodine Receptor 2 in Drug-Associated Learning

Type-2 ryanodine receptor (RyR2) ion channels facilitate the release of Ca 2+ from stores and serve an important function in neuroplasticity. The role for RyR2 in hippocampal-dependent learning and memory is well established and chronic hyperphosphorylation of RyR2 (RyR2P) is associated with pathological calcium leakage and cognitive disorders, including Alzheimer's disease. By comparison, little is known about the role of RyR2 in the ventral medial prefrontal cortex (vmPFC) circuitry important for working memory, decision making, and reward seeking. Here, we evaluated the basal expression and localization of RyR2 and RyR2P in the vmPFC. Next, we employed an operant model of sucrose, cocaine, or morphine self-administration (SA) followed by a (reward-free) recall test, to reengage vmPFC neurons and reactivate reward-seeking and re-evaluated the expression and localization of RyR2 and RyR2P in vmPFC. Under basal conditions, RyR2 was expressed in pyramidal cells but not regularly detected in PV/SST interneurons. On the contrary, RyR2P was rarely observed in PFC somata and was restricted to a different subcompartment of the same neuron - the apical dendrites of layer-5 pyramidal cells. Chronic SA of drug (cocaine or morphine) and nondrug (sucrose) rewards produced comparable increases in RyR2 protein expression. However, recalling either drug reward impaired the usual localization of RyR2P in dendrites and markedly increased its expression in somata immunoreactive for Fos, a marker of highly activated neurons. These effects could not be explained by chronic stress or drug withdrawal and instead appeared to require a recall experience associated with prior drug SA. In addition to showing the differential distribution of RyR2/RyR2P and affirming the general role of vmPFC in reward learning, this study provides information on the propensity of addictive drugs to redistribute RyR2P ion channels in a neuronal population engaged in drug-seeking. Hence, focusing on the early impact of addictive drugs on RyR2 function may serve as a promising approach to finding a treatment for substance use disorders.

Role of ryanodine receptor 2 and FK506-binding protein 12.6 dissociation in pulmonary hypertension

Pulmonary hypertension (PH) is a devastating disease characterized by a progressive increase in pulmonary arterial pressure leading to right ventricular failure and death. A major cellular response in this disease is the contraction of smooth muscle cells (SMCs) of the pulmonary vasculature. Cell contraction is determined by the increase in intracellular Ca2+ concentration ([Ca2+]i), which is generated and regulated by various ion channels. Several studies by us and others have shown that ryanodine receptor 2 (RyR2), a Ca2+-releasing channel in the sarcoplasmic reticulum (SR), is an essential ion channel for the control of [Ca2+]i in pulmonary artery SMCs (PASMCs), thereby mediating the sustained vasoconstriction seen in PH. FK506-binding protein 12.6 (FKBP12.6) strongly associates with RyR2 to stabilize its functional activity. FKBP12.6 can be dissociated from RyR2 by a hypoxic stimulus to increase channel function and Ca2+ release, leading to pulmonary vasoconstriction and PH. More specifically, dissociation of the RyR2-FKBP12.6 complex is a consequence of increased mitochondrial ROS generation mediated by the Rieske iron-sulfur protein (RISP) at the mitochondrial complex III after hypoxia. Overall, RyR2/FKBP12.6 dissociation and the corresponding signaling pathway may be an important factor in the development of PH. Novel drugs and biologics targeting RyR2, FKBP12.6, and related molecules may become unique effective therapeutics for PH.

Rodent Models of Dilated Cardiomyopathy and Heart Failure for Translational Investigations and Therapeutic Discovery

Even with modern therapy, patients with heart failure only have a 50% five-year survival rate. To improve the development of new therapeutic strategies, preclinical models of disease are needed to properly emulate the human condition. Determining the most appropriate model represents the first key step for reliable and translatable experimental research. Rodent models of heart failure provide a strategic compromise between human in vivo similarity and the ability to perform a larger number of experiments and explore many therapeutic candidates. We herein review the currently available rodent models of heart failure, summarizing their physiopathological basis, the timeline of the development of ventricular failure, and their specific clinical features. In order to facilitate the future planning of investigations in the field of heart failure, a detailed overview of the advantages and possible drawbacks of each model is provided.

Rbm20 ablation is associated with changes in the expression of titin-interacting and metabolic proteins

Dilated cardiomyopathy (DCM) is a major risk factor for developing heart failure and is often associated with an increased risk for life-threatening arrhythmia. Although numerous causal genes for DCM have been identified, RNA binding motif protein 20 (Rbm20) remains one of the few splicing factors that, when mutated or genetically ablated, leads to the development of DCM. In this study we sought to identify changes in the cardiac proteome in Rbm20 knockout (KO) rat hearts using global quantitative proteomics to gain insight into the molecular mechanisms precipitating the development of DCM in these rats. Our analysis identified changes in titin-interacting proteins involved in mechanical stretch-based signaling, as well as mitochondrial enzymes, which suggests that activation of pathological hypertrophy and altered mitochondrial metabolism and/or dysfunction, among other changes, contribute to the development of DCM in Rbm20 KO rats. Collectively, our findings provide the first report on changes in the cardiac proteome associated with genetic ablation of Rbm20.