Molecular Changes in the Cardiac RyR2 With Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)

Mfn2 regulates calcium homeostasis and suppresses PASMCs proliferation via interaction with IP3R3 to mitigate pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is a chronic disorder characterized by the excessive proliferation of pulmonary arterial smooth muscle cells (PASMCs). Recent studies indicate that Mitochondrial fusion protein 2 (Mfn2) maintains intracellular calcium (Ca2+) homeostasis via the mitochondria-associated endoplasmic reticulum membranes (MAMs) pathway, thereby inhibiting PASMCs proliferation and reducing pulmonary artery pressure. However, the precise mechanisms remain unclear.

METHODS

This study explored the roles of Mfn2 and IP3R3 in PAH progression by assessing their expression in lung tissues of a monocrotaline (MCT)-induced PAH rat model. Immunoprecipitation assays were performed to confirm the interaction between Mfn2 and IP3R3. PASMCs were treated with either silenced or overexpressed Mfn2 and exposed to TNF-ɑ to observe effects on ER stress, IP3R3 expression, mitochondrial Ca2+ transport, and mitochondrial integrity. We also evaluated the effects of 4-phenylbutyric acid (4-PBA) and cistanche phenylethanol glycosides (CPGs) on the Mfn2-IP3R3 interaction in a TNF-α-induced PAH cell model, focusing on Ca2+ transport and mitochondrial structure.

RESULTS

Mfn2 expression was significantly down-regulated in the MCT-induced PAH rat model. Inhibition of ER stress upregulated Mfn2 expression, downregulated IP3R3 expression, increased mitochondrial Ca2+ concentration, and reduced autophagy, improving pulmonary hemodynamics and vascular remodeling. Overexpression of Mfn2 reduced ER stress, decreased IP3R3 expression, decreased mitochondrial Ca2+ transport, and restored mitochondrial integrity. Immunoprecipitation assays confirmed the interaction between Mfn2 and IP3R3. Inhibition of IP3R3 elevated Mfn2 levels, yielding similar beneficial effects as Mfn2 overexpression. 4-PBA and CPGs modulated the Mfn2-IP3R3 signaling axis, effectively inhibiting PAH progression.

CONCLUSIONS

Mfn2 mediates mitochondrial Ca2+ transport via IP3R3, suppressing PASMCs proliferation and pulmonary vascular remodeling, underscoring Mfn2's potential in regulating metabolic processes and vascular remodeling in PAH. These findings provide new insights for developing PAH-targeted therapeutics and establish a theoretical basis for traditional Chinese medicine in PAH prevention and treatment.

Extracorporeal membrane oxygenation for catecholaminergic polymorphic ventricular tachycardia: a case report and literature review

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited ion channelopathy characterized by a structurally normal heart sensitive to catecholamines. It primarily presents as Bidirectional ventricular tachycardia (BiVT) and is a significant cause of sudden cardiac death in children.

CASE PRESENTATION

We report our experience with central Extracorporeal Membrane Oxygenation (ECMO) therapy in a 4-year-old boy with CPVT. Despite these measures, his CPVT was refractory to standard medical treatment and mechanical ventilatory support, with symptom progression. Consequently, ECMO support was initiated in addition to existing treatment. The patient was successfully weaned off ECMO on the 10th day of therapy and was discharged in a good condition. Follow-up after discharge showed favorable outcomes.

CONCLUSIONS

The successful outcome in this case was attributed to the application of ECMO, which helped maintain the patient's circulatory status and address progressively worsening cardiogenic shock and uncontrolled ventricular arrhythmia. In such situations, the early use of ECMO can provide essential circulatory support and stability for patients, as demonstrated in this case.

[Catecholaminergic polymorphic ventricular tachycardia (CPVT): an insidious disease that can often lead to sudden cardiac death in young people]

The first symptoms of catecholaminergic polymorphic ventricular tachycardia (CPVT) usually occur in childhood and adolescence. 60% of patients experience syncope before the age of 40. Sudden cardiac death (SCD) is the first symptom of the disease in 30-50% of patients with CPVT. Early diagnosis is therefore crucial for the patient's prognosis. The diagnosis of CPVT is confirmed by a normal resting ECG, exclusion of structural heart disease, detection of bidirectional or polymorphic ventricular tachycardia (VT) in the stress ECG and/or detection of a pathogenic mutant in a gene associated with CPVT. Up to 60% of CPVT patients carry changes in the RYR2 gene. This gene encodes the cardiac ryanodine receptor, the most important Ca2+-releasing channel of the sarcoplasmic reticulum, which plays a central role in the contraction and relaxation of the heart muscle. If the function of the ryanodine receptor is impaired, too much calcium enters the cells, which triggers life-threatening arrhythmias. The overactive ryanodine receptor is therefore the main target for gene therapy methods. Even though the development of gene therapy is progressing, there is still no causal therapy available and it is all the more important to make a diagnosis as early as possible, which enables appropriate behavior and adequate symptomatic therapy. The decisive factor here is the evaluation of the genetic analysis in the context of the clinical findings. Based on this, recommendations can be made for preventive measures and the avoidance of specific triggers that could lead to life-threatening arrhythmias.

FKBP12 binds to the cardiac ryanodine receptor with negative cooperativity: implications for heart muscle physiology in health and disease

Cardiac ryanodine receptors (RyR2) release the Ca²⁺ from intracellular stores that is essential for cardiac myocyte contraction. The ion channel opening is tightly regulated by intracellular factors, including the FK506 binding proteins, FKBP12 and FKBP12.6. The impact of these proteins on RyR2 activity and cardiac contraction is debated, with often apparently contradictory experimental results, particularly for FKBP12. The isoform that regulates RyR2 has generally been considered to be FKBP12.6, despite the fact that FKBP12 is the major isoform associated with RyR2 in some species and is bound in similar proportions to FKBP12.6 in others, including sheep and humans. Here, we show time- and concentration-dependent effects of adding FKBP12 to RyR2 channels that were partly depleted of FKBP12/12.6 during isolation. The added FKBP12 displaced most remaining endogenous FKBP12/12.6. The results suggest that FKBP12 activates RyR2 with high affinity and inhibits RyR2 with lower affinity, consistent with a model of negative cooperativity in FKBP12 binding to each of the four subunits in the RyR tetramer. The easy dissociation of some FKBP12/12.6 could dynamically alter RyR2 activity in response to changes in in vivo regulatory factors, indicating a significant role for FKBP12/12.6 in Ca²⁺ signalling and cardiac function in healthy and diseased hearts. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Pharmacological mechanism of natural drugs and their active ingredients in the treatment of arrhythmia via calcium channel regulation

Arrhythmia is characterized by abnormal heartbeat rhythms and frequencies caused by heart pacing and conduction dysfunction. Arrhythmia is the leading cause of death in patients with cardiovascular disease, with high morbidity and mortality rates, posing a serious risk to human health. Natural drugs and their active ingredients, such as matrine(MAT), tetrandrine(TET), dehydroevodiamine, tanshinone IIA, and ginsenosides, have been widely used for the treatment of atrial fibrillation, ventricular ectopic beats, sick sinus syndrome, and other arrhythmia-like diseases owing to their unique advantages. This review summarizes the mechanism of action of natural drugs and their active ingredients in the treatment of arrhythmia via the regulation of Ca²⁺, such as alkaloids, quinones, saponins, terpenoids, flavonoids, polyphenols, and lignan compounds, to provide ideas for the innovative development of natural drugs with potential antiarrhythmic efficacy.

Molecular Aspects Implicated in Dantrolene Selectivity with Respect to Ryanodine Receptor Isoforms

Dantrolene is an intra-cellularly acting skeletal muscle relaxant used for the treatment of the rare genetic disorder, malignant hyperthermia (MH). In most cases, MH susceptibility is caused by dysfunction of the skeletal ryanodine receptor (RyR1) harboring one of nearly 230 single-point MH mutations. The therapeutic effect of dantrolene is the result of a direct inhibitory action on the RyR1 channel, thus suppressing aberrant Ca2+ release from the sarcoplasmic reticulum. Despite the almost identical dantrolene-binding sequence exits in all three mammalian RyR isoforms, dantrolene appears to be an isoform-selective inhibitor. Whereas RyR1 and RyR3 channels are competent to bind dantrolene, the RyR2 channel, predominantly expressed in the heart, is unresponsive. However, a large body of evidence suggests that the RyR2 channel becomes sensitive to dantrolene-mediated inhibition under certain pathological conditions. Although a consistent picture of the dantrolene effect emerges from in vivo studies, in vitro results are often contradictory. Hence, our goal in this perspective is to provide the best possible clues to the molecular mechanism of dantrolene’s action on RyR isoforms by identifying and discussing potential sources of conflicting results, mainly coming from cell-free experiments. Moreover, we propose that, specifically in the case of the RyR2 channel, its phosphorylation could be implicated in acquiring the channel responsiveness to dantrolene inhibition, interpreting functional findings in the structural context.

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.

Therapeutic Approaches of Ryanodine Receptor-Associated Heart Diseases

Cardiac diseases are the leading causes of death, with a growing number of cases worldwide, posing a challenge for both healthcare and research. Therefore, the most relevant aim of cardiac research is to unravel the molecular pathomechanisms and identify new therapeutic targets. Cardiac ryanodine receptor (RyR2), the Ca²⁺ release channel of the sarcoplasmic reticulum, is believed to be a good therapeutic target in a group of certain heart diseases, collectively called cardiac ryanopathies. Ryanopathies are associated with the impaired function of the RyR, leading to heart diseases such as congestive heart failure (CHF), catecholaminergic polymorphic ventricular tachycardia (CPVT), arrhythmogenic right ventricular dysplasia type 2 (ARVD2), and calcium release deficiency syndrome (CRDS). The aim of the current review is to provide a short insight into the pathological mechanisms of ryanopathies and discuss the pharmacological approaches targeting RyR2.