Calmodulin mutations associated with long QT syndrome prevent inactivation of cardiac L-type Ca(2+) currents and promote proarrhythmic behavior in ventricular myocytes

Spatial Coupling Tunes NMDA Receptor Responses via Ca2+ Diffusion

In the CNS, NMDA receptors generate large and highly regulated Ca2+ signals, which are critical for synaptic development and plasticity. They are highly clustered at postsynaptic sites and along dendritic arbors, but whether this spatial arrangement affects their output is unknown. Synaptic NMDA receptor currents are subject to Ca2+-dependent inactivation (CDI), a type of activity-dependent inhibition that requires intracellular Ca2+ and calmodulin (CaM). We asked whether Ca2+ influx through a single NMDA receptor influences the activity of nearby NMDA receptors, as a possible coupling mechanism. Using cell-attached unitary current recordings from GluN1-2a/GluN2A receptors expressed in human HEK293 cells and from NMDA receptors native to hippocampal neurons from male and female rats, we recorded unitary currents from multichannel patches and used a coupled Markov model to determine the extent of signal coupling (κ). In the absence of extracellular Ca2+, we observed no cooperativity (κ < 0.1), whereas in 1.8 mm external Ca2+, both recombinant and native channels showed substantial negative cooperativity (κ = 0.27). Intracellular Ca2+ chelation or overexpression of a Ca2+-insensitive CaM mutant, reduced coupling, which is consistent with CDI representing the coupling mechanism. In contrast, cooperativity increased substantially (κ = 0.68) when overexpressing the postsynaptic scaffolding protein PSD-95, which increased receptor clustering. Together, these new results demonstrate that NMDA receptor currents are negatively coupled through CDI, and the degree of coupling can be tuned by the distance between receptors. Therefore, channel clustering can influence the activity-dependent reduction in NMDA receptor currents.SIGNIFICANCE STATEMENT At central synapses, NMDA receptors are a major class of excitatory glutamate-gated channels and a source of activity-dependent Ca2+ influx. In turn, fluxed Ca2+ ions bind to calmodulin-primed receptors and reduce further entry, through an autoinhibitory mechanism known as Ca2+ -dependent inactivation (CDI). Here, we show that the diffusion of fluxed Ca2+ between active channels situated within submicroscopic distances amplified receptor inactivation. Thus, calmodulin-mediated gating modulation, an evolutionarily conserved regulatory mechanism, endows synapses with sensitivity to both the temporal sequence and spatial distribution of Ca2+ signals. Perturbations in this mechanism, which coordinates the activity of NMDA receptors within a cluster, may cause signaling alterations that contribute to neuropsychiatric conditions.

Genetic Mosaicism in Calmodulinopathy

BACKGROUND

CaM (calmodulin) mutations are associated with congenital arrhythmia susceptibility (calmodulinopathy) and are most often de novo. In this report, we sought to broaden the genotype-phenotype spectrum of calmodulinopathies with 2 novel calmodulin mutations and to investigate mosaicism in 2 affected families.

METHODS

CaM mutations were identified in 4 independent cases by DNA sequencing. Biochemical and electrophysiological studies were performed to determine functional consequences of each mutation.

RESULTS

Genetic studies identified 2 novel CaM variants (CALM3-E141K in 2 cases; CALM1-E141V) and one previously reported CaM pathogenic variant (CALM3-D130G) among 4 probands with shared clinical features of prolonged QTc interval (range 505-725 ms) and documented ventricular arrhythmia. A fatal outcome occurred for 2 of the cases. The parents of all probands were asymptomatic with normal QTc duration. However, 2 of the families had multiple affected offspring or multiple occurrences of intrauterine fetal demise. The mother from the family with recurrent intrauterine fetal demise exhibited the CALM3-E141K mutant allele in 25% of next-generation sequencing reads indicating somatic mosaicism, whereas CALM3-D130G was present in 6% of captured molecules of the paternal DNA sample, also indicating mosaicism. Two novel mutations (E141K and E141V) impaired Ca²⁺ binding affinity to the C-domain of CaM. Human-induced pluripotent stem cell-derived cardiomyocytes overexpressing mutant or wild-type CaM showed that both mutants impaired Ca²⁺-dependent inactivation of L-type Ca²⁺ channels and prolonged action potential duration.

CONCLUSIONS

We report 2 families with somatic mosaicism associated with arrhythmogenic calmodulinopathy, and demonstrate dysregulation of L-type Ca²⁺ channels by 2 novel CaM mutations affecting the same residue. Parental mosaicism should be suspected in families with unexplained fetal arrhythmia or fetal demise combined with a documented CaM mutation.

Diminished inhibition and facilitated activation of RyR2-mediated Ca2+ release is a common defect of arrhythmogenic calmodulin mutations

A number of calmodulin (CaM) mutations cause severe cardiac arrhythmias, but their arrhythmogenic mechanisms are unclear. While some of the arrhythmogenic CaM mutations have been shown to impair CaM-dependent inhibition of intracellular Ca²⁺ release through the ryanodine receptor type 2 (RyR2), the impact of a majority of these mutations on RyR2 function is unknown. Here, we investigated the effect of 14 arrhythmogenic CaM mutations on the CaM-dependent RyR2 inhibition. We found that all the arrhythmogenic CaM mutations tested diminished CaM-dependent inhibition of RyR2-mediated Ca²⁺ release and increased store-overload induced Ca²⁺ release (SOICR) in HEK293 cells. Moreover, all the arrhythmogenic CaM mutations tested either failed to inhibit or even promoted RyR2-mediated Ca²⁺ release in permeabilized HEK293 cells with elevated cytosolic Ca²⁺ , which was markedly different from the inhibitory action of CaM wild-type. The CaM mutations also altered the Ca²⁺ -dependency of CaM binding to the RyR2 CaM-binding domain. These results demonstrate that diminished inhibition, and even facilitated activation, of RyR2-mediated Ca²⁺ release is a common defect of arrhythmogenic CaM mutations.

Ca2+-Binding Proteins of the EF-Hand Superfamily: Diagnostic and Prognostic Biomarkers and Novel Therapeutic Targets

A multitude of Ca²⁺-sensor proteins containing the specific Ca²⁺-binding motif (helix-loop-helix, called EF-hand) are of major clinical relevance in a many human diseases. Measurements of troponin, the first intracellular Ca-sensor protein to be discovered, is nowadays the "gold standard" in the diagnosis of patients with acute coronary syndrome (ACS). Mutations have been identified in calmodulin and linked to inherited ventricular tachycardia and in patients affected by severe cardiac arrhythmias. Parvalbumin, when introduced into the diseased heart by gene therapy to increase contraction and relaxation speed, is considered to be a novel therapeutic strategy to combat heart failure. S100 proteins, the largest subgroup with the EF-hand protein family, are closely associated with cardiovascular diseases, various types of cancer, inflammation, and autoimmune pathologies. The intention of this review is to summarize the clinical importance of this protein family and their use as biomarkers and potential drug targets, which could help to improve the diagnosis of human diseases and identification of more selective therapeutic interventions.

Calcium Sensors in Neuronal Function and Dysfunction

Calcium signaling in neurons as in other cell types can lead to varied changes in cellular function. Neuronal Ca²⁺ signaling processes have also become adapted to modulate the function of specific pathways over a wide variety of time domains and these can have effects on, for example, axon outgrowth, neuronal survival, and changes in synaptic strength. Ca²⁺ also plays a key role in synapses as the trigger for fast neurotransmitter release. Given its physiological importance, abnormalities in neuronal Ca²⁺ signaling potentially underlie many different neurological and neurodegenerative diseases. The mechanisms by which changes in intracellular Ca²⁺ concentration in neurons can bring about diverse responses is underpinned by the roles of ubiquitous or specialized neuronal Ca²⁺ sensors. It has been established that synaptotagmins have key functions in neurotransmitter release, and, in addition to calmodulin, other families of EF-hand-containing neuronal Ca²⁺ sensors, including the neuronal calcium sensor (NCS) and the calcium-binding protein (CaBP) families, play important physiological roles in neuronal Ca²⁺ signaling. It has become increasingly apparent that these various Ca²⁺ sensors may also be crucial for aspects of neuronal dysfunction and disease either indirectly or directly as a direct consequence of genetic variation or mutations. An understanding of the molecular basis for the regulation of the targets of the Ca²⁺ sensors and the physiological roles of each protein in identified neurons may contribute to future approaches to the development of treatments for a variety of human neuronal disorders.

Arrhythmogenic calmodulin E105A mutation alters cardiac RyR2 regulation leading to cardiac dysfunction in zebrafish

Calmodulin (CaM) is a universal calcium (Ca²⁺ )-binding messenger that regulates many vital cellular events. In cardiac muscle, CaM associates with ryanodine receptor 2 (RyR2) and regulates excitation-contraction coupling. Mutations in human genes CALM1, CALM2, and CALM3 have been associated with life-threatening heart disorders, such as long QT syndrome (LQTS) and catecholaminergic polymorphic ventricular tachycardia. A novel de novo LQTS-associated missense CaM mutation (E105A) was recently identified in a 6-year-old boy, who experienced an aborted first episode of cardiac arrest. Herein, we report the first molecular characterization of the CaM E105A mutation. Expression of the CaM E105A mutant in zebrafish embryos resulted in cardiac arrhythmia and increased heart rate, suggestive of ventricular tachycardia. In vitro biophysical and biochemical analysis revealed that E105A confers a deleterious effect on protein stability and a reduced Ca²⁺ -binding affinity due to loss of cooperativity. Finally, the CaM E105A mutation resulted in reduced CaM-RyR2 interaction and defective modulation of ryanodine binding. Our findings suggest that the CaM E105A mutation dysregulates normal cardiac function by a complex mechanism involving alterations in both CaM-Ca²⁺ and CaM-RyR2 interactions.

Long QT syndrome caveolin-3 mutations differentially modulate Kv 4 and Cav 1.2 channels to contribute to action potential prolongation

KEY POINTS

Mutations in the caveolae scaffolding protein, caveolin-3 (Cav3), have been linked to the long QT type 9 inherited arrhythmia syndrome (LQT9) and the cause of underlying action potential duration prolongation is incompletely understood. In the present study, we show that LQT9 Cav3 mutations, F97C and S141R, cause mutation-specific gain of function effects on Ca_v 1.2-encoded L-type Ca²⁺ channels responsible for I_Ca,L and also cause loss of function effects on heterologously expressed K_v 4.2 and K_v 4.3 channels responsible for I_to . A computational model of the human ventricular myocyte action potential suggests that the major ionic current change causing action potential duration prolongation in the presence of Cav3-F97C is the slowly inactivating I_Ca,L but, for Cav3-S141R, both increased I_Ca,L and increased late Na⁺ current contribute equally to action potential duration prolongation. Overall, the LQT9 Cav3-F97C and Cav3-S141R mutations differentially impact multiple ionic currents, highlighting the complexity of Cav3 regulation of cardiac excitability and suggesting mutation-specific therapeutic approaches.

ABSTRACT

Mutations in the CAV3 gene encoding caveolin-3 (Cav3), a scaffolding protein integral to caveolae in cardiomyocytes, have been associated with the congenital long-QT syndrome (LQT9). Initial studies demonstrated that LQT9-associated Cav3 mutations, F97C and S141R, increase late sodium current as a potential mechanism to prolong action potential duration (APD) and cause LQT9. Whether these Cav3 LQT9 mutations impact other caveolae related ion channels remains unknown. We used the whole-cell, patch clamp technique to characterize the effect of Cav3-F97C and Cav3-S141R mutations on heterologously expressed Ca_v 1.2+Ca_v β_2cN4 channels, as well as K_v 4.2 and K_v 4.3 channels, in HEK 293 cells. Expression of Cav3-S141R increased I_Ca,L density without changes in gating properties, whereas expression of Cav3-F97C reduced Ca²⁺ -dependent inactivation of I_Ca,L without changing current density. The Cav3-F97C mutation reduced current density and altered the kinetics of I_Kv4.2 and I_Kv4.3 and also slowed recovery from inactivation. Cav3-S141R decreased current density and also slowed activation kinetics and recovery from inactivation of I_Kv4.2 but had no effect on I_Kv4.3 . Using the O'Hara-Rudy computational model of the human ventricular myocyte action potential, the Cav3 mutation-induced changes in I_to are predicted to have negligible effect on APD, whereas blunted Ca²⁺ -dependent inactivation of I_Ca,L by Cav3-F97C is predicted to be primarily responsible for APD prolongation, although increased I_Ca,L and late I_Na by Cav3-S141R contribute equally to APD prolongation. Thus, LQT9 Cav3-associated mutations, F97C and S141R, produce mutation-specific changes in multiple ionic currents leading to different primary causes of APD prolongation, which suggests the use of mutation-specific therapeutic approaches in the future.

The Crossroad of Ion Channels and Calmodulin in Disease

Calmodulin (CaM) is the principal Ca²⁺ sensor in eukaryotic cells, orchestrating the activity of hundreds of proteins. Disease causing mutations at any of the three genes that encode identical CaM proteins lead to major cardiac dysfunction, revealing the importance in the regulation of excitability. In turn, some mutations at the CaM binding site of ion channels cause similar diseases. Here we provide a summary of the two sides of the partnership between CaM and ion channels, describing the diversity of consequences of mutations at the complementary CaM binding domains.

Calmodulinopathy: Functional Effects of CALM Mutations and Their Relationship With Clinical Phenotypes

In spite of the widespread role of calmodulin (CaM) in cellular signaling, CaM mutations lead specifically to cardiac manifestations, characterized by remarkable electrical instability and a high incidence of sudden death at young age. Penetrance of the mutations is surprisingly high, thus postulating a high degree of functional dominance. According to the clinical patterns, arrhythmogenesis in CaM mutations can be attributed, in the majority of cases, to either prolonged repolarization (as in long-QT syndrome, LQTS phenotype), or to instability of the intracellular Ca2+ store (as in catecholamine-induced tachycardias, CPVT phenotype). This review discusses how mutations affect CaM signaling function and how this may relate to the distinct arrhythmia phenotypes/mechanisms observed in patients; this involves mechanistic interpretation of negative dominance and mutation-specific CaM-target interactions. Knowledge of the mechanisms involved may allow critical approach to clinical manifestations and aid in the development of therapeutic strategies for "calmodulinopathies," a recently identified nosological entity.

Calmodulinopathy: A Novel, Life-Threatening Clinical Entity Affecting the Young

Sudden cardiac death (SCD) in the young may often be the first manifestation of a genetic arrythmogenic disease that had remained undiagnosed. Despite the significant discoveries of the genetic bases of inherited arrhythmia syndromes, there remains a measurable fraction of cases where in-depth clinical and genetic investigations fail to identify the underlying SCD etiology. A few years ago, 2 cases of infants with recurrent cardiac arrest episodes, due to what appeared to be as a severe form of long QT syndrome (LQTS), came to our attention. These prompted a number of clinical and genetic research investigations that allowed us to identify a novel, closely associated to LQTS but nevertheless distinct, clinical entity that is now known as calmodulinopathy. Calmodulinopathy is a life-threatening arrhythmia syndrome, affecting mostly young individuals, caused by mutations in any of the 3 genes encoding calmodulin (CaM). Calmodulin is a ubiquitously expressed Ca²⁺ signaling protein that, in the heart, modulates several ion channels and participates in a plethora of cellular processes. We will hereby provide an overview of CaM's structure and function under normal and disease states, highlighting the genetic etiology of calmodulinopathy and the related disease mechanisms. We will also discuss the phenotypic spectrum of patients with calmodulinopathy and present state-of-the art approaches with patient-derived induced pluripotent stem cells that have been thus far adopted in order to accurately model calmodulinopathy in vitro, decipher disease mechanisms and identify novel therapies.

Calcium Channelopathies: Structural Insights into Disorders of the Muscle Excitation–Contraction Complex

Ion channels are membrane proteins responsible for the passage of ions down their electrochemical gradients and across biological membranes. In this, they generate and shape action potentials and provide secondary messengers for various signaling pathways. They are often part of larger complexes containing auxiliary subunits and regulatory proteins. Channelopathies arise from mutations in the genes encoding ion channels or their associated proteins. Recent advances in cryo-electron microscopy have resulted in an explosion of ion channel structures in multiple states, generating a wealth of new information on channelopathies. Disease-associated mutations fall into different categories, interfering with ion permeation, protein folding, voltage sensing, ligand and protein binding, and allosteric modulation of channel gating. Prime examples of these are Ca2+-selective channels expressed in myocytes, for which multiple structures in distinct conformational states have recently been uncovered. We discuss the latest insights into these calcium channelopathies from a structural viewpoint.

A bilobal model of Ca2+-dependent inactivation to probe the physiology of L-type Ca2+ channels

L-type calcium channels undergo Ca²⁺-dependent inactivation (CDI) in order to precisely control the entry of Ca²⁺ into cells such as cardiomyocytes. Limpitikul et al. develop a bilobal model of CDI and use it to understand the pathogenesis of arrhythmias associated with mutations in CaM.

L-type calcium channels (LTCCs) are critical elements of normal cardiac function, playing a major role in orchestrating cardiac electrical activity and initiating downstream signaling processes. LTCCs thus use feedback mechanisms to precisely control calcium (Ca²⁺) entry into cells. Of these, Ca²⁺-dependent inactivation (CDI) is significant because it shapes cardiac action potential duration and is essential for normal cardiac rhythm. This important form of regulation is mediated by a resident Ca²⁺ sensor, calmodulin (CaM), which is comprised of two lobes that are each capable of responding to spatially distinct Ca²⁺ sources. Disruption of CaM-mediated CDI leads to severe forms of long-QT syndrome (LQTS) and life-threatening arrhythmias. Thus, a model capable of capturing the nuances of CaM-mediated CDI would facilitate increased understanding of cardiac (patho)physiology. However, one critical barrier to achieving a detailed kinetic model of CDI has been the lack of quantitative data characterizing CDI as a function of Ca²⁺. This data deficit stems from the experimental challenge of uncoupling the effect of channel gating on Ca²⁺ entry. To overcome this obstacle, we use photo-uncaging of Ca²⁺ to deliver a measurable Ca²⁺ input to CaM/LTCCs, while simultaneously recording CDI. Moreover, we use engineered CaMs with Ca²⁺ binding restricted to a single lobe, to isolate the kinetic response of each lobe. These high-resolution measurements enable us to build mathematical models for each lobe of CaM, which we use as building blocks for a full-scale bilobal model of CDI. Finally, we use this model to probe the pathogenesis of LQTS associated with mutations in CaM (calmodulinopathies). Each of these models accurately recapitulates the kinetics and steady-state properties of CDI in both physiological and pathological states, thus offering powerful new insights into the mechanistic alterations underlying cardiac arrhythmias.

Human Calmodulin Mutations

Fluxes of calcium (Ca²⁺) across cell membranes enable fast cellular responses. Calmodulin (CaM) senses local changes in Ca²⁺ concentration and relays the information to numerous interaction partners. The critical role of accurate Ca²⁺ signaling on cellular function is underscored by the fact that there are three independent CaM genes (CALM1-3) in the human genome. All three genes are functional and encode the exact same CaM protein. Moreover, CaM has a completely conserved amino acid sequence across all vertebrates. Given this degree of conservation, it was long thought that mutations in CaM were incompatible with life. It was therefore a big surprise when the first CaM mutations in humans were identified six years ago. Today, more than a dozen human CaM missense mutations have been described, all found in patients with severe cardiac arrhythmias. Biochemical studies have demonstrated differential effects on Ca²⁺ binding affinities for these CaM variants. Moreover, CaM regulation of central cardiac ion channels is impaired, including the voltage-gated Ca²⁺ channel, Ca_V1.2, and the sarcoplasmic reticulum Ca²⁺ release channel, ryanodine receptor isoform 2, RyR2. Currently, no non-cardiac phenotypes have been described for CaM variant carriers. However, sequencing of large human cohorts reveals a cumulative frequency of additional rare CaM mutations that raise the possibility of CaM variants not exclusively causing severe cardiac arrhythmias. Here, we provide an overview of the identified CaM variants and their known consequences for target regulation and cardiac disease phenotype. We discuss experimental data, patient genotypes and phenotypes as well as which questions remain open to understand this complexity.

Arrhythmia mutations in calmodulin cause conformational changes that affect interactions with the cardiac voltage-gated calcium channel

Calmodulin (CaM) represents one of the most conserved proteins among eukaryotes and is known to bind and modulate more than a 100 targets. Recently, several disease-associated mutations have been identified in the CALM genes that are causative of severe cardiac arrhythmia syndromes. Although several mutations have been shown to affect the function of various cardiac ion channels, direct structural insights into any CaM disease mutation have been lacking. Here we report a crystallographic and NMR investigation of several disease mutant CaMs, linked to long-QT syndrome, in complex with the IQ domain of the cardiac voltage-gated calcium channel (Ca_V1.2). Surprisingly, two mutants (D95V, N97I) cause a major distortion of the C-terminal lobe, resulting in a pathological conformation not reported before. These structural changes result in altered interactions with the Ca_V1.2 IQ domain. Another mutation (N97S) reduces the affinity for Ca²⁺ by introducing strain in EF hand 3. A fourth mutant (F141L) shows structural changes in the Ca²⁺-free state that increase the affinity for the IQ domain. These results thus show that different mechanisms underlie the ability of CaM disease mutations to affect Ca²⁺-dependent inactivation of the voltage-gated calcium channel.

Allosteric regulators selectively prevent Ca2+-feedback of CaV and NaV channels

Calmodulin (CaM) serves as a pervasive regulatory subunit of Ca_V1, Ca_V2, and Na_V1 channels, exploiting a functionally conserved carboxy-tail element to afford dynamic Ca²⁺-feedback of cellular excitability in neurons and cardiomyocytes. Yet this modularity counters functional adaptability, as global changes in ambient CaM indiscriminately alter its targets. Here, we demonstrate that two structurally unrelated proteins, SH3 and cysteine-rich domain (stac) and fibroblast growth factor homologous factors (fhf) selectively diminish Ca²⁺/CaM-regulation of Ca_V1 and Na_V1 families, respectively. The two proteins operate on allosteric sites within upstream portions of respective channel carboxy-tails, distinct from the CaM-binding interface. Generalizing this mechanism, insertion of a short RxxK binding motif into Ca_V1.3 carboxy-tail confers synthetic switching of CaM regulation by Mona SH3 domain. Overall, our findings identify a general class of auxiliary proteins that modify Ca²⁺/CaM signaling to individual targets allowing spatial and temporal orchestration of feedback, and outline strategies for engineering Ca²⁺/CaM signaling to individual targets.

Long QT Syndrome and Sinus Bradycardia-A Mini Review

Congenital long-QT syndrome (LQTS) is an inherited cardiac disorder characterized by the prolongation of ventricular repolarization, susceptibility to Torsades de Pointes (TdP), and a risk for sudden death. Various types of congenital LQTS exist, all due to specific defects in ion channel-related genes. Interestingly, almost all of the ion channels affected by the various types of LQTS gene mutations are also expressed in the human sinoatrial node (SAN). It is therefore not surprising that LQTS is frequently associated with a change in basal heart rate (HR). However, current data on how the LQTS-associated ion channel defects result in impaired human SAN pacemaker activity are limited. In this mini-review, we provide an overview of known LQTS mutations with effects on HR and the underlying changes in expression and kinetics of ion channels. Sinus bradycardia has been reported in relation to a large number of LQTS mutations. However, the occurrence of both QT prolongation and sinus bradycardia on a family basis is almost completely limited to LQTS types 3 and 4 (LQT3 and Ankyrin-B syndrome, respectively). Furthermore, a clear causative role of this sinus bradycardia in cardiac events seems reserved to mutations underlying LQT3.

Protein phenotype diagnosis of autosomal dominant calmodulin mutations causing irregular heart rhythms

The life-threatening group of irregular cardiac rhythmic disorders also known as Cardiac Arrhythmias (CA) are caused by mutations in highly conserved Calmodulin (CALM/CaM) genes. Herein, we present a multidimensional approach to diagnose changes in phenotypic, stability, and Ca²⁺ ion binding properties of CA-causing mutations. Mutation pathogenicity was determined by diverse computational machine learning approaches. We further modeled the mutations in 3D protein structure and analyzed residue level phenotype plasticity. We have also examined the influence of torsion angles, number of H-bonds, and free energy dynamics on the stability, near-native simulation dynamic potential of residue fluctuations in protein structures, Ca²⁺ ion binding potentials, of CaM mutants. Our study recomends to use M-CAP method for measuring the pathogenicity of CA causing CaM variants. Interestingly, most CA-causing variants we analyzed, exists in either third (V/H-96, S/I-98, V-103) or fourth (G/V-130, V/E/H-132, H-134, P-136, G-141, and L-142) EF-hands located in carboxyl domains of the CaM molecule. We observed that the minor structural fluctuations caused by these variants are likely tolerable owing to the highly flexible nature of calmodulin's globular domains. However, our molecular docking results supports that these variants disturb the affinity of CaM toward Ca²⁺ ions and corroborate previous findings from functional studies. Taken together, these computational findings can explain the molecular reasons for subtle changes in structure, flexibility, and stability aspects of mutant CaM molecule. Our comprehensive molecular scanning approach demonstrates the utility of computational methods in quick preliminary screening of CA- CaM mutations before undertaking time consuming and complicated functional laboratory assays.

Conservation of cardiac L-type Ca2+ channels and their regulation in Drosophila: A novel genetically-pliable channelopathic model

Dysregulation of L-type Ca2+ channels (LTCCs) underlies numerous cardiac pathologies. Understanding their modulation with high fidelity relies on investigating LTCCs in their native environment with intact interacting proteins. Such studies benefit from genetic manipulation of endogenous channels in cardiomyocytes, which often proves cumbersome in mammalian models. Drosophila melanogaster, however, offers a potentially efficient alternative as it possesses a relatively simple heart, is genetically pliable, and expresses well-conserved genes. Fluorescence in situ hybridization confirmed an abundance of Ca-α1D and Ca-α1T mRNA in fly myocardium, which encode subunits that specify hetero-oligomeric channels homologous to mammalian LTCCs and T-type Ca2+ channels, respectively. Cardiac-specific knockdown of Ca-α1D via interfering RNA abolished cardiac contraction, suggesting Ca-α1D (i.e. A1D) represents the primary functioning Ca2+ channel in Drosophila hearts. Moreover, we successfully isolated viable single cardiomyocytes and recorded Ca2+ currents via patch clamping, a feat never before accomplished with the fly model. The profile of Ca2+ currents recorded in individual cells when Ca2+ channels were hypomorphic, absent, or under selective LTCC blockage by nifedipine, additionally confirmed the predominance of A1D current across all activation voltages. T-type current, activated at more negative voltages, was also detected. Lastly, A1D channels displayed Ca2+-dependent inactivation, a critical negative feedback mechanism of LTCCs, and the current through them was augmented by forskolin, an activator of the protein kinase A pathway. In sum, the Drosophila heart possesses a conserved compendium of Ca2+ channels, suggesting that the fly may serve as a robust and effective platform for studying cardiac channelopathies.

Effect of Ca2+ on the promiscuous target-protein binding of calmodulin

Calmodulin (CaM) is a calcium sensing protein that regulates the function of a large number of proteins, thus playing a crucial part in many cell signaling pathways. CaM has the ability to bind more than 300 different target peptides in a Ca2+-dependent manner, mainly through the exposure of hydrophobic residues. How CaM can bind a large number of targets while retaining some selectivity is a fascinating open question. Here, we explore the mechanism of CaM selective promiscuity for selected target proteins. Analyzing enhanced sampling molecular dynamics simulations of Ca2+-bound and Ca2+-free CaM via spectral clustering has allowed us to identify distinct conformational states, characterized by interhelical angles, secondary structure determinants and the solvent exposure of specific residues. We searched for indicators of conformational selection by mapping solvent exposure of residues in these conformational states to contacts in structures of CaM/target peptide complexes. We thereby identified CaM states involved in various binding classes arranged along a depth binding gradient. Binding Ca2+ modifies the accessible hydrophobic surface of the two lobes and allows for deeper binding. Apo CaM indeed shows shallow binding involving predominantly polar and charged residues. Furthermore, binding to the C-terminal lobe of CaM appears selective and involves specific conformational states that can facilitate deep binding to target proteins, while binding to the N-terminal lobe appears to happen through a more flexible mechanism. Thus the long-ranged electrostatic interactions of the charged residues of the N-terminal lobe of CaM may initiate binding, while the short-ranged interactions of hydrophobic residues in the C-terminal lobe of CaM may account for selectivity. This work furthers our understanding of the mechanism of CaM binding and selectivity to different target proteins and paves the way towards a comprehensive model of CaM selectivity.

Cardiovascular Precision Medicine in the Genomics Era

Precision medicine strives to delineate disease using multiple data sources-from genomics to digital health metrics-in order to be more precise and accurate in our diagnoses, definitions, and treatments of disease subtypes. By defining disease at a deeper level, we can treat patients based on an understanding of the molecular underpinnings of their presentations, rather than grouping patients into broad categories with one-size-fits-all treatments. In this review, the authors examine how precision medicine, specifically that surrounding genetic testing and genetic therapeutics, has begun to make strides in both common and rare cardiovascular diseases in the clinic and the laboratory, and how these advances are beginning to enable us to more effectively define risk, diagnose disease, and deliver therapeutics for each individual patient.

Bilobal architecture is a requirement for calmodulin signaling to CaV1.3 channels

Calmodulin (CaM) regulation of voltage-gated calcium (Ca_V) channels is a powerful Ca²⁺ feedback mechanism that adjusts Ca²⁺ influx, affording rich mechanistic insights into Ca²⁺ decoding. CaM possesses a dual-lobed architecture, a salient feature of the myriad Ca²⁺-sensing proteins, where two homologous lobes that recognize similar targets hint at redundant signaling mechanisms. Here, by tethering CaM lobes, we demonstrate that bilobal architecture is obligatory for signaling to Ca_V channels. With one lobe bound, Ca_V carboxy tail rearranges itself, resulting in a preinhibited configuration precluded from Ca²⁺ feedback. Reconstitution of two lobes, even as separate molecules, relieves preinhibition and restores Ca²⁺ feedback. Ca_V channels thus detect the coincident binding of two Ca²⁺-free lobes to promote channel opening, a molecular implementation of a logical NOR operation that processes spatiotemporal Ca²⁺ signals bifurcated by CaM lobes. Overall, a unified scheme of Ca_V channel regulation by CaM now emerges, and our findings highlight the versatility of CaM to perform exquisite Ca²⁺ computations.

Resident Calmodulin Primes NMDA Receptors for Ca2+-Dependent Inactivation

N-methyl-d-aspartate (NMDA) receptors are glutamate- and glycine-gated channels that flux Na+ and Ca2+ into postsynaptic neurons during synaptic transmission. The resulting intracellular Ca2+ transient is essential to physiological and pathological processes related to synaptic development, plasticity, and apoptosis. It also engages calmodulin (CaM) to reduce subsequent NMDA receptor activity in a process known as Ca2+-dependent inactivation (CDI). Here, we used whole-cell electrophysiology to measure CDI and computational modeling to dissect the sequence of events that underlies it. With these approaches, we estimate that CaM senses NMDA receptor Ca2+ influx at ∼9 nm from the channel pore. Further, when we controlled the frequency of Ca2+ influx through individual channels, we found that a kinetic model where apoCaM associates with channels before their activation best predicts the measured CDI. These results provide, to our knowledge, novel functional evidence for CaM preassociation to NMDA receptors in living cells. This particular mechanism for autoinhibitory feedback reveals strategies and challenges for Ca2+ regulation in neurons during physiological synaptic activity and disease.

Allele-specific ablation rescues electrophysiological abnormalities in a human iPS cell model of long-QT syndrome with a CALM2 mutation

Calmodulin is a ubiquitous Ca2+ sensor molecule encoded by three distinct calmodulin genes, CALM1-3. Recently, mutations in CALM1-3 have been reported to be associated with severe early-onset long-QT syndrome (LQTS). However, the underlying mechanism through which heterozygous calmodulin mutations lead to severe LQTS remains unknown, particularly in human cardiomyocytes. We aimed to establish an LQTS disease model associated with a CALM2 mutation (LQT15) using human induced pluripotent stem cells (hiPSCs) and to assess mutant allele-specific ablation by genome editing for the treatment of LQT15. We generated LQT15-hiPSCs from a 12-year-old boy with LQTS carrying a CALM2-N98S mutation and differentiated these hiPSCs into cardiomyocytes (LQT15-hiPSC-CMs). Action potentials (APs) and L-type Ca2+ channel (LTCC) currents in hiPSC-CMs were analyzed by the patch-clamp technique and compared with those of healthy controls. Furthermore, we performed mutant allele-specific knockout using a CRISPR-Cas9 system and analyzed electrophysiological properties. Electrophysiological analyses revealed that LQT15-hiPSC-CMs exhibited significantly lower beating rates, prolonged AP durations, and impaired inactivation of LTCC currents compared with control cells, consistent with clinical phenotypes. Notably, ablation of the mutant allele rescued the electrophysiological abnormalities of LQT15-hiPSC-CMs, indicating that the mutant allele caused dominant-negative suppression of LTCC inactivation, resulting in prolonged AP duration. We successfully recapitulated the disease phenotypes of LQT15 and revealed that inactivation of LTCC currents was impaired in CALM2-N98S hiPSC model. Additionally, allele-specific ablation using the latest genome-editing technology provided important insights into a promising therapeutic approach for inherited cardiac diseases.

Elucidating arrhythmogenic mechanisms of long-QT syndrome CALM1-F142L mutation in patient-specific induced pluripotent stem cell-derived cardiomyocytes

Aims

Calmodulin (CaM) is a small protein, encoded by three genes (CALM1-3), exerting multiple Ca2+-dependent modulatory roles. A mutation (F142L) affecting only one of the six CALM alleles is associated with long QT syndrome (LQTS) characterized by recurrent cardiac arrests. This phenotypic severity is unexpected from the predicted allelic balance. In this work, the effects of heterozygous CALM1-F142L have been investigated in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) obtained from a LQTS patient carrying the F142L mutation, i.e. in the context of native allelic ratio and potential gene modifiers.

Methods and Results

Skin fibroblasts of the mutation carrier and two unrelated healthy subjects (controls) were reprogrammed to hiPSC and differentiated into hiPSC-CMs. Scanty IK1 expression, an hiPSC-CMs feature potentially biasing repolarization, was corrected by addition of simulated IK1 (Dynamic-Clamp). Abnormalities in repolarization rate-dependency (in single cells and cell aggregates), membrane currents and intracellular Ca2+ dynamics were evaluated as putative arrhythmogenic factors. CALM1-F142L prolonged repolarization, altered its rate-dependency and its response to isoproterenol. This was associated with severe impairment of Ca2+-dependent inactivation (CDI) of ICaL, resulting in augmented inward current during the plateau phase. As a result, the repolarization of mutant cells failed to adapt to high pacing rates, a finding well reproduced by using a recent hiPSC-CM action potential model. The mutation failed to affect IKs and INaL and changed If only marginally. Intracellular Ca2+ dynamics and Ca2+ store stability were not significantly modified. Mutation-induced repolarization abnormalities were reversed by verapamil.

Conclusion

The main functional derangement in CALM1-F142L was prolonged repolarization with altered rate-dependency and sensitivity to β-adrenergic stimulation. Impaired CDI of ICaL underlined the electrical abnormality, which was sensitive to ICaL blockade. High mutation penetrance was confirmed in the presence of the native genotype, implying strong dominance of effects.

Astragaloside IV Inhibits Membrane Ca[Formula: see text] Current but Enhances Sarcoplasmic Reticulum Ca[Formula: see text] Release

Astragaloside IV (AS-IV) is one of the active ingredients in Astragalus membrananceus (Huangqi), a traditional Chinese medicine. The present study investigated the effects of AS-IV on Ca[Formula: see text] handling in cardiac myocytes to elucidate its possible mechanism in the treatment of cardiac disease. The results showed that AS-IV at 1 and 10[Formula: see text][Formula: see text]M reduced KCl-induced [Ca[Formula: see text]]_i increase ([Formula: see text] from 1.33[Formula: see text][Formula: see text][Formula: see text]0.04 (control, [Formula: see text] 28) to 1.22[Formula: see text][Formula: see text][Formula: see text]0.02 ([Formula: see text], [Formula: see text] 29) and 1.22[Formula: see text][Formula: see text][Formula: see text]0.02 ([Formula: see text] 0.01, [Formula: see text]), but it enhanced Ca[Formula: see text] release from SR ([Formula: see text] from 1.04[Formula: see text][Formula: see text][Formula: see text]0.01 (control, [Formula: see text]) to 1.44[Formula: see text][Formula: see text][Formula: see text]0.03 ([Formula: see text], [Formula: see text]) and 1.60[Formula: see text][Formula: see text][Formula: see text]0.04 ([Formula: see text] 0.01, [Formula: see text]0), in H9c2 cells. Similar results were obtained in native cardiomyocytes. AS-IV at 1 and 10[Formula: see text][Formula: see text]M inhibited L-type Ca[Formula: see text] current ([Formula: see text] from [Formula: see text]4.42[Formula: see text][Formula: see text][Formula: see text]0.58 pA/pF of control to [Formula: see text]2.25[Formula: see text][Formula: see text][Formula: see text]0.12 pA/pF ([Formula: see text] 0.01, [Formula: see text] 5) and [Formula: see text]1.78[Formula: see text][Formula: see text][Formula: see text]0.28 pA/pF ([Formula: see text] 0.01, [Formula: see text] 5) respectively, when the interference of [Ca[Formula: see text]]_i was eliminated due to the depletion of SR Ca[Formula: see text] store by thapsigargin, an inhibitor of Ca[Formula: see text] ATPase. Moreover, when BAPTA, a rapid Ca[Formula: see text] chelator, was used, CDI (Ca[Formula: see text]-dependent inactivation) of [Formula: see text] was eliminated, and the inhibitory effects of AS-IV on I_CaL were significantly reduced at the same time. These results suggest that AS-IV affects Ca[Formula: see text] homeostasis through two opposite pathways: inhibition of Ca[Formula: see text] influx through L-type Ca[Formula: see text] channel, and promotion of Ca[Formula: see text] release from SR.

The emerging role of calmodulin regulation of RyR2 in controlling heart rhythm, the progression of heart failure and the antiarrhythmic action of dantrolene

Cardiac output and rhythm depend on the release and the take-up of calcium from the sarcoplasmic reticulum (SR). Excessive diastolic calcium leak from the SR due to dysfunctional calcium release channels (RyR2) contributes to the formation of delayed after-depolarizations, which underlie the fatal arrhythmias that occur in heart failure and inherited syndromes. Calmodulin (CaM) is a calcium-binding protein that regulates target proteins and acts as a calcium sensor. CaM is comprised of two calcium-binding EF-hand domains and a flexible linker. CaM is an accessory protein that partially inhibits RyR2 channel activity. CaM is critical for normal cardiac function, and altered CaM binding and efficacy may contribute to defects in SR calcium release. The present paper reviews CaM binding to RyR2 and how it regulates RyR2 channel activity. It then goes on to review how mutations in the CaM amino acid sequence give rise to inherited syndromes such as Catecholaminergic Polymorphic Ventricular Tachychardia (CPVT) and long QT syndrome (LQTS). In addition, the role of reduced CaM binding to RyR2 that results from RyR2 phosphorylation or from oxidation of either RyR2 or CaM contributes to the progression of heart failure is reviewed. Finally, this manuscript reviews recent evidence that CaM binding to RyR2 is required for the inhibitory action of a pharmaceutical agent (dantrolene) on RyR2. Dantrolene is a clinically used muscle relaxant that has recently been found to exert antiarrhythmic effects against SR Ca²⁺ overload arrhythmias.

Treatment of calmodulinopathy with verapamil

Pathological variants in genes encoding calmodulin are associated with severe clinical presentations, including recurrent ventricular fibrillation and sudden death. Beta-receptor antagonists (beta-blockers) and sodium-channel antagonists have been reported as pharmacotherapies in these disorders; however, recent data have demonstrated the importance of derangements in calcium channel inactivation. We report a sustained attempt to use calcium-channel antagonists to treat calmodulinopathy and review the treatment strategies reported in the literature to date.

Cryo-EM Structure of a KCNQ1/CaM Complex Reveals Insights into Congenital Long QT Syndrome

KCNQ1 is the pore-forming subunit of cardiac slow-delayed rectifier potassium (I_Ks) channels. Mutations in the kcnq1 gene are the leading cause of congenital long QT syndrome (LQTS). Here, we present the cryoelectron microscopy (cryo-EM) structure of a KCNQ1/calmodulin (CaM) complex. The conformation corresponds to an "uncoupled," PIP₂-free state of KCNQ1, with activated voltage sensors and a closed pore. Unique structural features within the S4-S5 linker permit uncoupling of the voltage sensor from the pore in the absence of PIP₂. CaM contacts the KCNQ1 voltage sensor through a specific interface involving a residue on CaM that is mutated in a form of inherited LQTS. Using an electrophysiological assay, we find that this mutation on CaM shifts the KCNQ1 voltage-activation curve. This study describes one physiological form of KCNQ1, depolarized voltage sensors with a closed pore in the absence of PIP₂, and reveals a regulatory interaction between CaM and KCNQ1 that may explain CaM-mediated LQTS.

Novel CPVT-Associated Calmodulin Mutation in CALM3 (CALM3-A103V) Activates Arrhythmogenic Ca Waves and Sparks

BACKGROUND

Calmodulin (CaM) mutations are associated with severe forms of long QT syndrome and catecholaminergic polymorphic ventricular tachycardia (CPVT). CaM mutations are found in 13% of genotype-negative long QT syndrome patients, but the prevalence of CaM mutations in genotype-negative CPVT patients is unknown. Here, we identify and characterize CaM mutations in 12 patients with genotype-negative but clinically diagnosed CPVT.

METHODS AND RESULTS

We performed mutational analysis of CALM1, CALM2, and CALM3 gene-coding regions, in vitro measurement of CaM-Ca(2+) (Ca)-binding affinity, ryanodine receptor 2-CaM binding, Ca handling, L-type Ca current, and action potential duration. We identified a novel CaM mutation-A103V-in CALM3 in 1 of 12 patients (8%), a female who experienced episodes of exertion-induced syncope since age 10, had normal QT interval, and displayed ventricular ectopy during stress testing consistent with CPVT. A103V modestly lowered CaM Ca-binding affinity (3-fold reduction versus WT-CaM), but did not alter CaM binding to ryanodine receptor 2. In permeabilized cardiomyocytes, A103V-CaM (100 nmol/L) promoted spontaneous Ca wave and spark activity, a cellular phenotype of ryanodine receptor 2 activation. Even a 1:3 mixture of A103V-CaM:WT-CaM activated Ca waves, demonstrating functional dominance. Compared with long QT syndrome D96V-CaM, A103V-CaM had significantly less effects on L-type Ca current inactivation, did not alter action potential duration, and caused delayed afterdepolarizations and triggered beats in intact cardiomyocytes.

CONCLUSIONS

We discovered a novel CPVT mutation in the CALM3 gene that shares functional characteristics with established CPVT-associated mutations in CALM1. A small proportion of A103V-CaM is sufficient to evoke arrhythmogenic Ca disturbances via ryanodine receptor 2 dysregulation, which explains the autosomal dominant inheritance.

Calcium Signaling and Cardiac Arrhythmias

There has been a significant progress in our understanding of the molecular mechanisms by which calcium (Ca2+) ions mediate various types of cardiac arrhythmias. A growing list of inherited gene defects can cause potentially lethal cardiac arrhythmia syndromes, including catecholaminergic polymorphic ventricular tachycardia, congenital long QT syndrome, and hypertrophic cardiomyopathy. In addition, acquired deficits of multiple Ca2+-handling proteins can contribute to the pathogenesis of arrhythmias in patients with various types of heart disease. In this review article, we will first review the key role of Ca2+ in normal cardiac function-in particular, excitation-contraction coupling and normal electric rhythms. The functional involvement of Ca2+ in distinct arrhythmia mechanisms will be discussed, followed by various inherited arrhythmia syndromes caused by mutations in Ca2+-handling proteins. Finally, we will discuss how changes in the expression of regulation of Ca2+ channels and transporters can cause acquired arrhythmias, and how these mechanisms might be targeted for therapeutic purposes.

Calcium triggers reversal of calmodulin on nested anti-parallel sites in the IQ motif of the neuronal voltage-dependent sodium channel NaV1.2

Several members of the voltage-gated sodium channel family are regulated by calmodulin (CaM) and ionic calcium. The neuronal voltage-gated sodium channel NaV1.2 contains binding sites for both apo (calcium-depleted) and calcium-saturated CaM. We have determined equilibrium dissociation constants for rat NaV1.2 IQ motif [IQRAYRRYLLK] binding to apo CaM (~3nM) and (Ca2+)4-CaM (~85nM), showing that apo CaM binding is favored by 30-fold. For both apo and (Ca2+)4-CaM, NMR demonstrated that NaV1.2 IQ motif peptide (NaV1.2IQp) exclusively made contacts with C-domain residues of CaM (CaMC). To understand how calcium triggers conformational change at the CaM-IQ interface, we determined a solution structure (2M5E.pdb) of (Ca2+)2-CaMC bound to NaV1.2IQp. The polarity of (Ca2+)2-CaMC relative to the IQ motif was opposite to that seen in apo CaMC-Nav1.2IQp (2KXW), revealing that CaMC recognizes nested, anti-parallel sites in Nav1.2IQp. Reversal of CaM may require transient release from the IQ motif during calcium binding, and facilitate a re-orientation of CaMN allowing interactions with non-IQ NaV1.2 residues or auxiliary regulatory proteins interacting in the vicinity of the IQ motif.

Mechanisms of the cyclic nucleotide cross-talk signaling network in cardiac L-type calcium channel regulation

Regulation of L-type Calcium (Ca2+) Channel (LCC) gating is critical to shaping the cardiac action potential (AP) and triggering the initiation of excitation-contraction (EC) coupling in cardiac myocytes. The cyclic nucleotide (cN) cross-talk signaling network, which encompasses the β-adrenergic and the Nitric Oxide (NO)/cGMP/Protein Kinase G (PKG) pathways and their interaction (cross-talk) through distinctively-regulated phosphodiesterase isoenzymes (PDEs), regulates LCC current via Protein Kinase A- (PKA) and PKG-mediated phosphorylation. Due to the tightly-coupled and intertwined biochemical reactions involved, it remains to be clarified how LCC gating is regulated by the signaling network from receptor to end target. In addition, the large number of EC coupling-related phosphorylation targets of PKA and PKG makes it difficult to quantify and isolate changes in L-type Ca2+ current (ICaL) responses regulated by the signaling network. We have developed a multi-scale, biophysically-detailed computational model of LCC regulation by the cN signaling network that is supported by experimental data. LCCs are modeled with functionally distinct PKA- and PKG-phosphorylation dependent gating modes. The model exhibits experimentally observed single channel characteristics, as well as whole-cell LCC currents upon activation of the cross-talk signaling network. Simulations show 1) redistribution of LCC gating modes explains changes in whole-cell current under various stimulation scenarios of the cN cross-talk network; 2) NO regulation occurs via potentiation of a gating mode characterized by prolonged closed times; and 3) due to compensatory actions of cross-talk and antagonizing functions of PKA- and PKG-mediated phosphorylation of LCCs, the effects of individual inhibitions of PDEs 2, 3, and 4 on ICaL are most pronounced at low levels of β-adrenergic stimulation. Simulations also delineate the contribution of the following two mechanisms to overall LCC regulation, which have otherwise been challenging to distinguish: 1) regulation of PKA and PKG activation via cN cross-talk (Mechanism 1); and 2) LCC interaction with activated PKA and PKG (Mechanism 2). These results provide insights into how cN signals transduced via the cN cross-talk signaling network are integrated via LCC regulation in the heart.

Enhanced Late Na and Ca Currents as Effective Antiarrhythmic Drug Targets

While recent advances clarified the molecular and cellular modes of action of antiarrhythmic drugs (AADs), their link to suppression of dynamical arrhythmia mechanisms remains only partially understood. The current classifications of AADs (Classes I, III, and IV) rely on blocking peak Na, K and L-type calcium currents (I_Ca,L), with Class II with dominant beta receptor blocking activity and Class V including drugs with diverse classes of actions. The discovery that the calcium and redox sensor, cardiac Ca/calmodulin-dependent protein kinase II (CaMKII) enhances both the late Na (I_Na-L) and the late I_Ca,L in patients at high risk of VT/VF provided a new and a rational AAD target. Pathological rise of either or both of I_Na-L and late I_Ca,L are demonstrated to promote cellular early afterdepolarizations (EADs) and EAD-mediated triggered activity that can initiate VT/VF in remodeled hearts. Selective inhibition of the I_Na-L without affecting their peak transients with the highly specific prototype drug, GS-967 suppresses these EAD-mediated VT/VFs. As in the case of I_Na-L, selective inhibition of the late I_Ca,L without affecting its peak with the prototype drug, roscovitine suppressed oxidative EAD-mediated VT/VF. These findings indicate that specific blockers of the late inward currents without affecting their peaks (gating modifiers), offer a new and effective AAD class action i.e., “Class VI.” The development of safe drugs with selective Class VI actions provides a rational and effective approach to treat VT/VF particularly in cardiac conditions associated with enhanced CaMKII activity such as heart failure.

Activity-dependent regulation of T-type calcium channels by submembrane calcium ions

Voltage-gated Ca²⁺ channels are involved in numerous physiological functions and various mechanisms finely tune their activity, including the Ca²⁺ ion itself. This is well exemplified by the Ca²⁺-dependent inactivation of L-type Ca²⁺ channels, whose alteration contributes to the dramatic disease Timothy Syndrome. For T-type Ca²⁺ channels, a long-held view is that they are not regulated by intracellular Ca²⁺. Here we challenge this notion by using dedicated electrophysiological protocols on both native and expressed T-type Ca²⁺ channels. We demonstrate that a rise in submembrane Ca²⁺ induces a large decrease in T-type current amplitude due to a hyperpolarizing shift in the steady-state inactivation. Activation of most representative Ca²⁺-permeable ionotropic receptors similarly regulate T-type current properties. Altogether, our data clearly establish that Ca²⁺ entry exerts a feedback control on T-type channel activity, by modulating the channel availability, a mechanism that critically links cellular properties of T-type Ca²⁺ channels to their physiological roles.

DOI:http://dx.doi.org/10.7554/eLife.22331.001

Neurons, muscle cells and many other types of cells use electrical signals to exchange information and coordinate their behavior. Proteins known as calcium channels sit in the membrane that surrounds the cell and can generate electrical signals by allowing calcium ions to cross the membrane and enter the cell during electrical activities. Although calcium ions are needed to generate these electrical signals, and for many other processes in cells, if the levels of calcium ions inside cells become too high they can be harmful and cause disease.

Cells have a “feedback” mechanism that prevents calcium ion levels from becoming too high. This mechanism relies on the calcium ions that are already in the cell being able to close the calcium channels. This feedback mechanism has been extensively studied in two types of calcium channel, but it is not known whether a third group of channels – known as Cav3 channels – are also regulated in this way.

Cav3 channels are important in electrical signaling in neurons and have been linked with epilepsy, chronic pain and various other conditions in humans. Cazade et al. investigated whether calcium ions can regulate the activity of human Cav3 channels. The experiments show that these channels are indeed regulated by calcium ions, but using a distinct mechanism to other types of calcium channels. For the Cav3 channels, calcium ions alter the gating properties of the channels so that they are less easily activated . As a result, fewer Cav3 channels are “available” to provide calcium ions with a route into the cell.

The next steps following on from this work will be to identify the molecular mechanisms underlying this new feedback mechanism. Another challenge will be to find out what role this calcium ion-driven feedback plays in neurological disorders that are linked with altered Cav3 channel activity.

DOI:http://dx.doi.org/10.7554/eLife.22331.002

Inherited Ventricular Arrhythmias: The Role of the Multi-Subunit Structure of the L-Type Calcium Channel Complex

The normal heartbeat is conditioned by transient increases in the intracellular free Ca²⁺ concentration. Ca²⁺ influx in cardiomyocytes is regulated by the activity of the heteromeric L-type voltage-activated Ca_V1.2 channel. A complex network of interactions between the different proteins forming the ion channel supports the kinetics and the activation gating of the Ca²⁺ influx. Alterations in the biophysical and biochemical properties or in the biogenesis in any of these proteins can lead to serious disturbances in the cardiac rhythm. The multi-subunit nature of the channel complex is better comprehended by examining the high-resolution three-dimensional structure of the closely related Ca_V1.1 channel. The architectural map identifies precise interaction loci between the different subunits and paves the way for elucidating the mechanistic basis for the regulation of Ca²⁺ balance in cardiac myocytes under physiological and pathological conditions.

The Arrhythmogenic Calmodulin p.Phe142Leu Mutation Impairs C-domain Ca2+ Binding but Not Calmodulin-dependent Inhibition of the Cardiac Ryanodine Receptor

A number of point mutations in the intracellular Ca²⁺-sensing protein calmodulin (CaM) are arrhythmogenic, yet their underlying mechanisms are not clear. These mutations generally decrease Ca²⁺ binding to CaM and impair inhibition of CaM-regulated Ca²⁺ channels like the cardiac Ca²⁺ release channel (ryanodine receptor, RyR2), and it appears that attenuated CaM Ca²⁺ binding correlates with impaired CaM-dependent RyR2 inhibition. Here, we investigated the RyR2 inhibitory action of the CaM p.Phe142Leu mutation (F142L; numbered including the start-Met), which markedly reduces CaM Ca²⁺ binding. Surprisingly, CaM-F142L had little to no aberrant effect on RyR2-mediated store overload-induced Ca²⁺ release in HEK293 cells compared with CaM-WT. Furthermore, CaM-F142L enhanced CaM-dependent RyR2 inhibition at the single channel level compared with CaM-WT. This is in stark contrast to the actions of arrhythmogenic CaM mutations N54I, D96V, N98S, and D130G, which all diminish CaM-dependent RyR2 inhibition. Thermodynamic analysis showed that apoCaM-F142L converts an endothermal interaction between CaM and the CaM-binding domain (CaMBD) of RyR2 into an exothermal one. Moreover, NMR spectra revealed that the CaM-F142L-CaMBD interaction is structurally different from that of CaM-WT at low Ca²⁺. These data indicate a distinct interaction between CaM-F142L and the RyR2 CaMBD, which may explain the stronger CaM-dependent RyR2 inhibition by CaM-F142L, despite its reduced Ca²⁺ binding. Collectively, these results add to our understanding of CaM-dependent regulation of RyR2 as well as the mechanistic effects of arrhythmogenic CaM mutations. The unique properties of the CaM-F142L mutation may provide novel clues on how to suppress excessive RyR2 Ca²⁺ release by manipulating the CaM-RyR2 interaction.

The Arrhythmogenic Calmodulin Mutation D129G Dysregulates Cell Growth, Calmodulin-dependent Kinase II Activity, and Cardiac Function in Zebrafish

Calmodulin (CaM) is a Ca²⁺ binding protein modulating multiple targets, several of which are associated with cardiac pathophysiology. Recently, CaM mutations were linked to heart arrhythmia. CaM is crucial for cell growth and viability, yet the effect of the arrhythmogenic CaM mutations on cell viability, as well as heart rhythm, remains unknown, and only a few targets with relevance for heart physiology have been analyzed for their response to mutant CaM. We show that the arrhythmia-associated CaM mutants support growth and viability of DT40 cells in the absence of WT CaM except for the long QT syndrome mutant CaM D129G. Of the six CaM mutants tested (N53I, F89L, D95V, N97S, D129G, and F141L), three showed a decreased activation of Ca²⁺/CaM-dependent kinase II, most prominently the D129G CaM mutation, which was incapable of stimulating Thr²⁸⁶ autophosphorylation. Furthermore, the CaM D129G mutation led to bradycardia in zebrafish and an arrhythmic phenotype in a subset of the analyzed zebrafish.

A Precision Medicine Approach to the Rescue of Function on Malignant Calmodulinopathic Long-QT Syndrome

RATIONALE

Calmodulinopathies comprise a new category of potentially life-threatening genetic arrhythmia syndromes capable of producing severe long-QT syndrome (LQTS) with mutations involving CALM1, CALM2, or CALM3. The underlying basis of this form of LQTS is a disruption of Ca²⁺/calmodulin (CaM)-dependent inactivation of L-type Ca²⁺ channels.

OBJECTIVE

To gain insight into the mechanistic underpinnings of calmodulinopathies and devise new therapeutic strategies for the treatment of this form of LQTS.

METHODS AND RESULTS

We generated and characterized the functional properties of induced pluripotent stem cell-derived cardiomyocytes from a patient with D130G-CALM2-mediated LQTS, thus creating a platform with which to devise and test novel therapeutic strategies. The patient-derived induced pluripotent stem cell-derived cardiomyocytes display (1) significantly prolonged action potentials, (2) disrupted Ca²⁺ cycling properties, and (3) diminished Ca²⁺/CaM-dependent inactivation of L-type Ca²⁺ channels. Next, taking advantage of the fact that calmodulinopathy patients harbor a mutation in only 1 of 6 redundant CaM-encoding alleles, we devised a strategy using CRISPR interference to selectively suppress the mutant gene while sparing the wild-type counterparts. Indeed, suppression of CALM2 expression produced a functional rescue in induced pluripotent stem cell-derived cardiomyocytes with D130G-CALM2, as shown by the normalization of action potential duration and Ca²⁺/CaM-dependent inactivation after treatment. Moreover, CRISPR interference can be designed to achieve selective knockdown of any of the 3 CALM genes, making it a generalizable therapeutic strategy for any calmodulinopathy.

CONCLUSIONS

Overall, this therapeutic strategy holds great promise for calmodulinopathy patients as it represents a generalizable intervention capable of specifically altering CaM expression and potentially attenuating LQTS-triggered cardiac events, thus initiating a path toward precision medicine.

Towards a Unified Theory of Calmodulin Regulation (Calmodulation) of Voltage-Gated Calcium and Sodium Channels

Voltage-gated Na and Ca(2+) channels represent two major ion channel families that enable myriad biological functions including the generation of action potentials and the coupling of electrical and chemical signaling in cells. Calmodulin regulation (calmodulation) of these ion channels comprises a vital feedback mechanism with distinct physiological implications. Though long-sought, a shared understanding of the channel families remained elusive for two decades as the functional manifestations and the structural underpinnings of this modulation often appeared to diverge. Here, we review recent advancements in the understanding of calmodulation of Ca(2+) and Na channels that suggest a remarkable similarity in their regulatory scheme. This interrelation between the two channel families now paves the way towards a unified mechanistic framework to understand vital calmodulin-dependent feedback and offers shared principles to approach related channelopathic diseases. An exciting era of synergistic study now looms.

Novel calmodulin mutations associated with congenital long QT syndrome affect calcium current in human cardiomyocytes

BACKGROUND

Calmodulin (CaM) mutations are associated with cardiac arrhythmia susceptibility including congenital long QT syndrome (LQTS).

OBJECTIVE

The purpose of this study was to determine the clinical, genetic, and functional features of 2 novel CaM mutations in children with life-threatening ventricular arrhythmias.

METHODS

The clinical and genetic features of 2 congenital arrhythmia cases associated with 2 novel CaM gene mutations were ascertained. Biochemical and functional investigations were conducted on the 2 mutations.

RESULTS

A novel de novo CALM2 mutation (D132H) was discovered by candidate gene screening in a male infant with prenatal bradycardia born to healthy parents. Postnatal course was complicated by profound bradycardia, prolonged corrected QT interval (651 ms), 2:1 atrioventricular block, and cardiogenic shock. He was resuscitated and was treated with a cardiac device. A second novel de novo mutation in CALM1 (D132V) was discovered by clinical exome sequencing in a 3-year-old boy who suffered a witnessed cardiac arrest secondary to ventricular fibrillation. Electrocardiographic recording after successful resuscitation revealed a prolonged corrected QT interval of 574 ms. The Ca(2+) affinity of CaM-D132H and CaM-D132V revealed extremely weak binding to the C-terminal domain, with significant structural perturbations noted for D132H. Voltage-clamp recordings of human induced pluripotent stem cell-derived cardiomyocytes transiently expressing wild-type or mutant CaM demonstrated that both mutations caused impaired Ca(2+)-dependent inactivation of voltage-gated Ca(2+) current. Neither mutant affected voltage-dependent inactivation.

CONCLUSION

Our findings implicate impaired Ca(2+)-dependent inactivation in human cardiomyocytes as the plausible mechanism for long QT syndrome associated with 2 novel CaM mutations. The data further expand the spectrum of genotype and phenotype associated with calmodulinopathy.

An autism-associated mutation in CaV1.3 channels has opposing effects on voltage- and Ca(2+)-dependent regulation

CaV1.3 channels are a major class of L-type Ca(2+) channels which contribute to the rhythmicity of the heart and brain. In the brain, these channels are vital for excitation-transcription coupling, synaptic plasticity, and neuronal firing. Moreover, disruption of CaV1.3 function has been associated with several neurological disorders. Here, we focus on the de novo missense mutation A760G which has been linked to autism spectrum disorder (ASD). To explore the role of this mutation in ASD pathogenesis, we examined the effects of A760G on CaV1.3 channel gating and regulation. Introduction of the mutation severely diminished the Ca(2+)-dependent inactivation (CDI) of CaV1.3 channels, an important feedback system required for Ca(2+) homeostasis. This reduction in CDI was observed in two major channel splice variants, though to different extents. Using an allosteric model of channel gating, we found that the underlying mechanism of CDI reduction is likely due to enhanced channel opening within the Ca(2+)-inactivated mode. Remarkably, the A760G mutation also caused an opposite increase in voltage-dependent inactivation (VDI), resulting in a multifaceted mechanism underlying ASD. When combined, these regulatory deficits appear to increase the intracellular Ca(2+) concentration, thus potentially disrupting neuronal development and synapse formation, ultimately leading to ASD.

Obesity alters molecular and functional cardiac responses to ischemia/reperfusion and glucagon-like peptide-1 receptor agonism

This study tested the hypothesis that obesity alters the cardiac response to ischemia/reperfusion and/or glucagon like peptide-1 (GLP-1) receptor activation, and that these differences are associated with alterations in the obese cardiac proteome and microRNA (miRNA) transcriptome. Ossabaw swine were fed normal chow or obesogenic diet for 6 months. Cardiac function was assessed at baseline, during a 30-minutes coronary occlusion, and during 2 hours of reperfusion in anesthetized swine treated with saline or exendin-4 for 24 hours. Cardiac biopsies were obtained from normal and ischemia/reperfusion territories. Fat-fed animals were heavier, and exhibited hyperinsulinemia, hyperglycemia, and hypertriglyceridemia. Plasma troponin-I concentration (index of myocardial injury) was increased following ischemia/reperfusion and decreased by exendin-4 treatment in both groups. Ischemia/reperfusion produced reductions in systolic pressure and stroke volume in lean swine. These indices were higher in obese hearts at baseline and relatively maintained throughout ischemia/reperfusion. Exendin-4 administration increased systolic pressure in lean swine but did not affect the blood pressure in obese swine. End-diastolic volume was reduced by exendin-4 following ischemia/reperfusion in obese swine. These divergent physiologic responses were associated with obesity-related differences in proteins related to myocardial structure/function (e.g. titin) and calcium handling (e.g. SERCA2a, histidine-rich Ca(2+) binding protein). Alterations in expression of cardiac miRs in obese hearts included miR-15, miR-27, miR-130, miR-181, and let-7. Taken together, these observations validate this discovery approach and reveal novel associations that suggest previously undiscovered mechanisms contributing to the effects of obesity on the heart and contributing to the actions of GLP-1 following ischemia/reperfusion.

Arrhythmogenic calmodulin mutations impede activation of small-conductance calcium-activated potassium current

BACKGROUND

Apamin-sensitive small-conductance calcium-activated potassium (SK) channels are gated by intracellular Ca(2+) through a constitutive interaction with calmodulin.

OBJECTIVE

We hypothesize that arrhythmogenic human calmodulin mutations impede activation of SK channels.

METHODS

We studied 5 previously published calmodulin mutations (N54I, N98S, D96V, D130G, and F90L). Plasmids encoding either wild-type or mutant calmodulin were transiently transfected into human embryonic kidney 293 cells that stably express subtype 2 of SK protein channels (SK2 cells). Whole-cell voltage-clamp recording was used to determine apamin-sensitive current densities. We also performed optical mapping studies in normal murine hearts to determine the effects of apamin in hearts with (n=7) or without (n=3) pretreatment with sea anemone toxin.

RESULTS

SK2 cells transfected with wild-type calmodulin exhibited an apamin-sensitive current density of 33.6 pA/pF (31.4-36.5 pA/pF) (median and confidence interval 25th-75th percentile), which was significantly higher than that observed for cells transfected with N54I (17.0 pA/pF [14.0-27.7 pA/pF]; P = .016), F90L (22.6 pA/pF [20.3-24.3 pA/pF]; P = .011), D96V (13.0 pA/pF [10.9-15.8 pA/pF]; P = .003), N98S (13.7 pA/pF [8.8-20.4 pA/pF]; P = .005), and D130G (17.6 pA/pF [13.8-24.6 pA/pF]; P = .003). The decrease in SK2 current densities was not associated with a decrease in membrane protein expression or intracellular distribution of the channel protein. Apamin increased the ventricular action potential duration at 80% repolarization (from 79.6 ms [63.4-93.3 ms] to 121.8 ms [97.9-127.2 ms]; P = .010) in hearts pretreated with anemone toxin but not in control hearts.

CONCLUSION

Human arrhythmogenic calmodulin mutations impede the activation of SK2 channels in human embryonic kidney 293 cells.