Inherited dysfunction of sarcoplasmic reticulum Ca2+ handling and arrhythmogenesis

Abstract

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disease occurring in patients with a structurally normal heart: the disease is characterized by life-threatening arrhythmias elicited by stress and emotion. In 2001, the ryanodine receptor was identified as the gene that is linked to CPVT; shortly thereafter, cardiac calsequestrin was implicated in the recessive form of the same disease. It became clear that abnormalities in intracellular Ca(2+) regulation could profoundly disrupt the electrophysiological properties of the heart. In this article, we discuss the molecular basis of the disease and the pathophysiological mechanisms that are impacting clinical diagnosis and management of affected individuals. As of today, the interaction between basic scientists and clinicians to understand CPVT and identify new therapeutic strategies is one of the most compelling examples of the importance of translational research in cardiology.

Figure. 1. Ca²⁺ induced Ca²⁺ release (CICR), store-overload-induced Ca²⁺ release (SOICR) and triggered arrhythmia

The left part of the diagram (in blue) depicts the mechanism of CICR, in which an action potential activates the voltage-dependent L-type Ca²⁺ channel, leading to a small Ca²⁺ influx. This Ca²⁺ entry opens the RyR2 channel in the sarcoplasmic reticulum (SR), resulting in SR Ca²⁺ release and muscle contraction. The right part of the diagram (in red) denotes the mechanism of SOICR, in which spontaneous SR Ca²⁺ release or Ca²⁺ spillover occurs under conditions of SR Ca²⁺ overload caused, for example, by stress via the beta-adrenergic receptor (b-AR)/protein kinase A (PKA)/phospholamban (PLB) signaling pathway. SOICR can activate the Na+/Ca²⁺ exchanger (Na/CaX), which, in turn, can lead to delayed afterdepolarizations (DADs) and triggered activities. (Illustration Credit: Cosmocyte/Ben Smith).

Figure 2

Example of typical bidirectional Ventricular tachycardia in a CPVT patient during exercise stress test

Figure 3

A plot of the accumulation of cardiac ryanodine receptor mutations (RyR2) reported in CPVT and IVF against amino acids sequence of the human RyR2 protein. A total of 152 mutations are reported in the plot (source www.fsm.it/cardmoc as of December 01, 2010). Each dot represents a mutation. Gray-shaded areas represent the mutations clusters; the number in the gray area is the relative percentage of mutations occurring that cluster. The linear correlation slope (m), which is an index of mutation density, is reported for each cluster. Amino acid numbers (human RyR2 protein sequence) for cluster boundaries are reported above the line together with the cluster ordinal numbering.

Figure 4

Cartoon (upper panel) and schematic representation (lower panel) of the RyR2 protein. Clusters with frequent mutations are depicted with their location along RyR2 amino acid (AA) sequence. Percentage of known (published) mutation for each cluster is also reported (lower panel). Mutation outside the canonical clusters are depicted as dots (upper panel). Each dot represents a unique mutation.

Figure 5

Ca²⁺ transients (1) and action potential (2) in R4496C knock-in mouse. 1) Ca²⁺ transients at stimulation rates from 1Hz to 5Hz during isoproterenol superfusion; last five stimulated transients of each train followed by diastolic pause are shown. Spontaneous diastolic releases occurs with increasing frequency at faster rates where triggered activations are also evident. 2) Action potential recording show DAD and triggered beats (a); triggered activity is greatly enhanced by isoproterenol (30nM) (B) and completely suppressed by ryanodine (10uM)

Figure 6. A unifying theory for CPVT?

The SOICR thresholds and free luminal Ca²⁺ levels in normal SR (B) and abnormal SR associated with CASQ2 mutations (A) or RyR2 mutations (C) in the resting state (top panels) and under the conditions of SR Ca²⁺ overload (bottom panels) are shown. The normal SOICR threshold is depicted as a dashed red bar, whereas the mutation-lowered SOICR threshold is depicted as a solid red bar. The blue area represents the free SR luminal Ca²⁺ concentration, while the yellow area represents the increased free SR luminal Ca²⁺ level during a sudden increase in SR Ca²⁺ loading. The RyR2 channel complex is depicted as a black structure and the Ca²⁺-CASQ2 complexes are shown as pink diamond structures. Mutations in RyR2 lower the SOICR threshold (C), while mutations in CASQ2 reduce the level of CASQ2 protein and/or Ca²⁺ buffering capability (A). The R33Q CASQ2 mutation or a reduction in the CASQ2 protein level has also been shown to lower the SOICR threshold (A). During SR Ca²⁺ overload, the free luminal Ca²⁺ level is more likely to exceed the RyR2 mutation-lowered SOICR threshold (panel C, bottom) or the normal or reduced SOICR threshold in the absence or lack of SR Ca²⁺ buffering as a result of CASQ2 mutations (panel A, bottom), leading to SOICR that can produce DADs and triggered arrhythmias (adapted from MacLennan, D. H., and Chen, S. R. W. (2009) J. Physiol. 587:3113-3115). (Illustration Credit: Cosmocyte/Ben Smith).