A high affinity switch for cAMP in the HCN pacemaker channels

Binding of cAMP to Hyperpolarization activated cyclic nucleotide gated (HCN) channels facilitates pore opening. It is unclear why the isolated cyclic nucleotide binding domain (CNBD) displays in vitro lower affinity for cAMP than the full-length channel in patch experiments. Here we show that HCN are endowed with an affinity switch for cAMP. Alpha helices D and E, downstream of the cyclic nucleotide binding domain (CNBD), bind to and stabilize the holo CNBD in a high affinity state. These helices increase by 30-fold cAMP efficacy and affinity measured in patch clamp and ITC, respectively. We further show that helices D and E regulate affinity by interacting with helix C of the CNBD, similarly to the regulatory protein TRIP8b. Our results uncover an intramolecular mechanism whereby changes in binding affinity, rather than changes in cAMP concentration, can modulate HCN channels, adding another layer to the complex regulation of their activity.

Alkali metal cations modulate the geometry of different binding sites in HCN4 selectivity filter for permeation or block

Hyperpolarization-activated cyclic-nucleotide gated (HCN) channels are important for timing biological processes like heartbeat and neuronal firing. Their weak cation selectivity is determined by a filter domain with only two binding sites for K+ and one for Na+. The latter acts as a weak blocker, which is released in combination with a dynamic widening of the filter by K+ ions, giving rise to a mixed K+/Na+ current. Here, we apply molecular dynamics simulations to systematically investigate the interactions of five alkali metal cations with the filter of the open HCN4 pore. Simulations recapitulate experimental data like a low Li+ permeability, considerable Rb+ conductance, a block by Cs+ as well as a punch through of Cs+ ions at high negative voltages. Differential binding of the cation species in specific filter sites is associated with structural adaptations of filter residues. This gives rise to ion coordination by a cation-characteristic number of oxygen atoms from the filter backbone and solvent. This ion/protein interplay prevents Li+, but not Na+, from entry into and further passage through the filter. The site equivalent to S3 in K+ channels emerges as a preferential binding and presumably blocking site for Cs+. Collectively, the data suggest that the weak cation selectivity of HCN channels and their block by Cs+ are determined by restrained cation-generated rearrangements of flexible filter residues.

Silvio Weidmann: laying the foundations for unravelling the mechanism of heart rhythm

Silvio Weidmann laid the basis of cardiac electrophysiology and was the forerunner in the search for mechanisms governing the electrical activity of the heart in his legendary first studies of Purkinje fibres in the 1950s. His work was the cornerstone of research in this field for many generations, and countless cardiologists and electrophysiologists have based their studies on the knowledge generated by Weidmann's pioneering data. This review summarizes his key contributions from the first intracellular recordings of cardiac membrane potentials in 1949 to the publication of his monograph in 1956. That summary is followed by an imagined dialogue between the authors and Silvio Weidmann himself, in the format of a one-act play. Both of us have such good recollections of our real-life conversations with Silvio Weidmann that we decided we could achieve a better feel for the history and issues by using a dialogue format. We hope that, in that way, we may transmit the character of Silvio Weidmann better for those readers who will not have known him personally. Silvio Weidmann was an extraordinarily sensitive and conversational person as well as a great scientist, and we feel it is worth the effort to convey that fact here. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Chinese natural compound decreases pacemaking of rabbit cardiac sinoatrial cells by targeting second messenger regulation of f-channels

Tongmai Yangxin (TMYX) is a complex compound of the Traditional Chinese Medicine (TCM) used to treat several cardiac rhythm disorders; however, no information regarding its mechanism of action is available. In this study we provide a detailed characterization of the effects of TMYX on the electrical activity of pacemaker cells and unravel its mechanism of action. Single-cell electrophysiology revealed that TMYX elicits a reversible and dose-dependent (2/6 mg/ml) slowing of spontaneous action potentials rate (−20.8/–50.2%) by a selective reduction of the diastolic phase (−50.1/–76.0%). This action is mediated by a negative shift of the I_f activation curve (−6.7/–11.9 mV) and is caused by a reduction of the cyclic adenosine monophosphate (cAMP)-induced stimulation of pacemaker channels. We provide evidence that TMYX acts by directly antagonizing the cAMP-induced allosteric modulation of the pacemaker channels. Noticeably, this mechanism functionally resembles the pharmacological actions of muscarinic stimulation or β-blockers, but it does not require generalized changes in cytoplasmic cAMP levels thus ensuring a selective action on rate. In agreement with a competitive inhibition mechanism, TMYX exerts its maximal antagonistic action at submaximal cAMP concentrations and then progressively becomes less effective thus ensuring a full contribution of I_f to pacemaker rate during high metabolic demand and sympathetic stimulation.

Gating movements and ion permeation in HCN4 pacemaker channels

The HCN1-4 channel family is responsible for the hyperpolarization-activated cation current If/Ih that controls automaticity in cardiac and neuronal pacemaker cells. We present cryoelectron microscopy (cryo-EM) structures of HCN4 in the presence or absence of bound cAMP, displaying the pore domain in closed and open conformations. Analysis of cAMP-bound and -unbound structures sheds light on how ligand-induced transitions in the channel cytosolic portion mediate the effect of cAMP on channel gating and highlights the regulatory role of a Mg2+ coordination site formed between the C-linker and the S4-S5 linker. Comparison of open/closed pore states shows that the cytosolic gate opens through concerted movements of the S5 and S6 transmembrane helices. Furthermore, in combination with molecular dynamics analyses, the open pore structures provide insights into the mechanisms of K+/Na+ permeation. Our results contribute mechanistic understanding on HCN channel gating, cyclic nucleotide-dependent modulation, and ion permeation.

Age-Related Changes in Cardiac Autonomic Modulation and Heart Rate Variability in Mice

Objective

The aim of this study was to assess age-related changes in cardiac autonomic modulation and heart rate variability (HRV) and their association with spontaneous and pharmacologically induced vulnerability to cardiac arrhythmias, to verify the translational relevance of mouse models for further in-depth evaluation of the link between autonomic changes and increased arrhythmic risk with advancing age.

Methods

Heart rate (HR) and time- and frequency-domain indexes of HRV were calculated from Electrocardiogram (ECG) recordings in two groups of conscious mice of different ages (4 and 19 months old) (i) during daily undisturbed conditions, (ii) following peripheral β-adrenergic (atenolol), muscarinic (methylscopolamine), and β-adrenergic + muscarinic blockades, and (iii) following β-adrenergic (isoprenaline) stimulation. Vulnerability to arrhythmias was evaluated during daily undisturbed conditions and following β-adrenergic stimulation.

Results

HRV analysis and HR responses to autonomic blockades revealed that 19-month-old mice had a lower vagal modulation of cardiac function compared with 4-month-old mice. This age-related autonomic effect was not reflected in changes in HR, since intrinsic HR was lower in 19-month-old compared with 4-month-old mice. Both time- and frequency-domain HRV indexes were reduced following muscarinic, but not β-adrenergic blockade in younger mice, and to a lesser extent in older mice, suggesting that HRV is largely modulated by vagal tone in mice. Finally, 19-month-old mice showed a larger vulnerability to both spontaneous and isoprenaline-induced arrhythmias.

Conclusion

The present study combines HRV analysis and selective pharmacological autonomic blockades to document an age-related impairment in cardiac vagal modulation in mice which is consistent with the human condition. Given their short life span, mice could be further exploited as an aged model for studying the trajectory of vagal decline with advancing age using HRV measures, and the mechanisms underlying its association with proarrhythmic remodeling of the senescent heart.

Human iPSC modelling of a familial form of atrial fibrillation reveals a gain of function of If and ICaL in patient-derived cardiomyocytes

AIMS

Atrial fibrillation (AF) is the most common type of cardiac arrhythmias, whose incidence is likely to increase with the aging of the population. It is considered a progressive condition, frequently observed as a complication of other cardiovascular disorders. However, recent genetic studies revealed the presence of several mutations and variants linked to AF, findings that define AF as a multifactorial disease. Due to the complex genetics and paucity of models, molecular mechanisms underlying the initiation of AF are still poorly understood. Here we investigate the pathophysiological mechanisms of a familial form of AF, with particular attention to the identification of putative triggering cellular mechanisms, using patient's derived cardiomyocytes (CMs) differentiated from induced pluripotent stem cells (iPSCs).

METHODS AND RESULTS

Here we report the clinical case of three siblings with untreatable persistent AF whose whole-exome sequence analysis revealed several mutated genes. To understand the pathophysiology of this multifactorial form of AF we generated three iPSC clones from two of these patients and differentiated these cells towards the cardiac lineage. Electrophysiological characterization of patient-derived CMs (AF-CMs) revealed that they have higher beating rates compared to control (CTRL)-CMs. The analysis showed an increased contribution of the If and ICaL currents. No differences were observed in the repolarizing current IKr and in the sarcoplasmic reticulum calcium handling. Paced AF-CMs presented significantly prolonged action potentials and, under stressful conditions, generated both delayed after-depolarizations of bigger amplitude and more ectopic beats than CTRL cells.

CONCLUSIONS

Our results demonstrate that the common genetic background of the patients induces functional alterations of If and ICaL currents leading to a cardiac substrate more prone to develop arrhythmias under demanding conditions. To our knowledge this is the first report that, using patient-derived CMs differentiated from iPSC, suggests a plausible cellular mechanism underlying this complex familial form of AF.

A circadian clock in the sinus node mediates day-night rhythms in Hcn4 and heart rate

Background

Heart rate follows a diurnal variation, and slow heart rhythms occur primarily at night.

Objective

The lower heart rate during sleep is assumed to be neural in origin, but here we tested whether a day-night difference in intrinsic pacemaking is involved.

Methods

In vivo and in vitro electrocardiographic recordings, vagotomy, transgenics, quantitative polymerase chain reaction, Western blotting, immunohistochemistry, patch clamp, reporter bioluminescence recordings, and chromatin immunoprecipitation were used.

Results

The day-night difference in the average heart rate of mice was independent of fluctuations in average locomotor activity and persisted under pharmacological, surgical, and transgenic interruption of autonomic input to the heart. Spontaneous beating rate of isolated (ie, denervated) sinus node (SN) preparations exhibited a day-night rhythm concomitant with rhythmic messenger RNA expression of ion channels including hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4). In vitro studies demonstrated 24-hour rhythms in the human HCN4 promoter and the corresponding funny current. The day-night heart rate difference in mice was abolished by HCN block, both in vivo and in the isolated SN. Rhythmic expression of canonical circadian clock transcription factors, for example, Brain and muscle ARNT-Like 1 (BMAL1) and Cryptochrome (CRY) was identified in the SN and disruption of the local clock (by cardiomyocyte-specific knockout of Bmal1) abolished the day-night difference in Hcn4 and intrinsic heart rate. Chromatin immunoprecipitation revealed specific BMAL1 binding sites on Hcn4, linking the local clock with intrinsic rate control.

Conclusion

The circadian variation in heart rate involves SN local clock–dependent Hcn4 rhythmicity. Data reveal a novel regulator of heart rate and mechanistic insight into bradycardia during sleep.

A Brief History of Pacemaking

Cardiac pacemaking is a most fundamental cardiac function, thoroughly investigated for decades with a multiscale approach at organ, tissue, cell and molecular levels, to clarify the basic mechanisms underlying generation and control of cardiac rhythm. Understanding the processes involved in pacemaker activity is of paramount importance for a basic physiological knowledge, but also as a way to reveal details of pathological dysfunctions useful in the perspective of a therapeutic approach. Among the mechanisms involved in pacemaking, the "funny" (I_f) current has properties most specifically fitting the requirements for generation and control of repetitive activity, and has consequently received the most attention in studies of the pacemaker function. Present knowledge of the basic mechanisms of pacemaking and the properties of funny channels has led to important developments of clinical relevance. These include: (1) the successful development of heart rate-reducing agents, such as ivabradine, able to control cardiac rhythm and useful in the treatment of diseases such as coronary artery disease, heart failure and tachyarrhythmias; (2) the understanding of the genetic basis of disorders of cardiac rhythm caused by HCN channelopathies; (3) the design of strategies to implement biological pacemakers based on transfer of HCN channels or of stem cell-derived pacemaker cells expressing I_f, with the ultimate goal to replace electronic devices. In this review, I will give a brief historical account of the discovery of the funny current and the development of the concept of I_f-based pacemaking, in the context of a wider, more complex model of cardiac rhythmic function.

Comparing pathways for long-term heart rate modulation by the funny current

Heart rate control by the funny current (I_f) involves both fast, cAMP-dependent, and slow, membrane expression–based mechanisms to adapt to different needs.

Abstract 808: Functional Characterization of a Novel Scn5a Mutation Associated With the Brugada Syndrome

Background: Brugada syndrome (BrS) is a cardiac disorder characterized by conduction abnormalities that can lead to sudden death; syncope and cardiac arrest are clinical manifestations which are often associated with an enhancement of the vagal activity. Mutations in the SCN5A gene (Na v 1.5 channel) are the most common cause of the inherited forms of BrS.

Objective: To characterize the functional behavior of mutant Na v 1.5 channels expressing a novel heterozygous mutation (S805L) recently identified in an Italian family affected by the BrS.

Methods: HEK cells were used as experimental model to express both the wild-type (WT) and the mutated S805L channels (alone, Homo or in combination, Hetero) and the accessory β-subunit (SCN1B). Patch-clamp and western blot experiments were carried out to assess the dysfunctional role of the mutation.

Results: When compared to the WT current, the S508L mutation significantly (P&It0.05) decreases the peak current density by about 65% for the Homo condition (WT: -120.2±10.2, n=28); Homo: -40.3±4.2, n=16) and by 35% for the Hetero condition (Hetero: -78.2±8.3, n=27). Densitometric analysis carried out on western blot data further support the conclusion that S805L channels are less abundant in the plasma membrane. We also observed that the S805L mutation positively shifts the V½ values of the voltage dependence of the inactivation of both Homo and Hetero currents (V½: WT -85.5±0.2 mV, n=55; Homo -80.9±0.3 mV, n=22; Hetero -81.9±0.2 mV, n=25; P&It0.05); a positive shift of the V½ of the activation was also observed but only in the Homo condition (V½: WT -33.0±0.4 mV, n=28; Homo -30.0±0.5, n=16, P&It0.05). The kinetics of recovery from inactivation and the amplitude of the late sodium current were also evaluated but they were unaffected by the mutation.

Conclusion: When expressed in the Hetero condition, the S805L mutation causes a reduction in the channel expression, however, the positive shift of the inactivation curve suggests an increase in Na channel availability. We thus believe that the precise quantitative balance between these two phenomena and their relation with vagal activity may underlie the clinical manifestation of the disease.

HCN1 mutation spectrum: from neonatal epileptic encephalopathy to benign generalized epilepsy and beyond

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels control neuronal excitability and their dysfunction has been linked to epileptogenesis but few individuals with neurological disorders related to variants altering HCN channels have been reported so far. In 2014, we described five individuals with epileptic encephalopathy due to de novo HCN1 variants. To delineate HCN1-related disorders and investigate genotype-phenotype correlations further, we assembled a cohort of 33 unpublished patients with novel pathogenic or likely pathogenic variants: 19 probands carrying 14 different de novo mutations and four families with dominantly inherited variants segregating with epilepsy in 14 individuals, but not penetrant in six additional individuals. Sporadic patients had epilepsy with median onset at age 7 months and in 36% the first seizure occurred during a febrile illness. Overall, considering familial and sporadic patients, the predominant phenotypes were mild, including genetic generalized epilepsies and genetic epilepsy with febrile seizures plus (GEFS+) spectrum. About 20% manifested neonatal/infantile onset otherwise unclassified epileptic encephalopathy. The study also included eight patients with variants of unknown significance: one adopted patient had two HCN1 variants, four probands had intellectual disability without seizures, and three individuals had missense variants inherited from an asymptomatic parent. Of the 18 novel pathogenic missense variants identified, 12 were associated with severe phenotypes and clustered within or close to transmembrane domains, while variants segregating with milder phenotypes were located outside transmembrane domains, in the intracellular N- and C-terminal parts of the channel. Five recurrent variants were associated with similar phenotypes. Using whole-cell patch-clamp, we showed that the impact of 12 selected variants ranged from complete loss-of-function to significant shifts in activation kinetics and/or voltage dependence. Functional analysis of three different substitutions altering Gly391 revealed that these variants had different consequences on channel biophysical properties. The Gly391Asp variant, associated with the most severe, neonatal phenotype, also had the most severe impact on channel function. Molecular dynamics simulation on channel structure showed that homotetramers were not conducting ions because the permeation path was blocked by cation(s) strongly complexed to the Asp residue, whereas heterotetramers showed an instantaneous current component possibly linked to deformation of the channel pore. In conclusion, our results considerably expand the clinical spectrum related to HCN1 variants to include common generalized epilepsy phenotypes and further illustrate how HCN1 has a pivotal function in brain development and control of neuronal excitability.

A Loss-of-Function HCN4 Mutation Associated With Familial Benign Myoclonic Epilepsy in Infancy Causes Increased Neuronal Excitability

HCN channels are highly expressed and functionally relevant in neurons and increasing evidence demonstrates their involvement in the etiology of human epilepsies. Among HCN isoforms, HCN4 is important in cardiac tissue, where it underlies pacemaker activity. Despite being expressed also in deep structures of the brain, mutations of this channel functionally shown to be associated with epilepsy have not been reported yet. Using Next Generation Sequencing for the screening of patients with idiopathic epilepsy, we identified the p.Arg550Cys (c.1648C>T) heterozygous mutation on HCN4 in two brothers affected by benign myoclonic epilepsy of infancy. Functional characterization in heterologous expression system and in neurons showed that the mutation determines a loss of function of HCN4 contribution to activity and an increase of neuronal discharge, potentially predisposing to epilepsy. Expressed in cardiomyocytes, mutant channels activate at slightly more negative voltages than wild-type (WT), in accordance with borderline bradycardia. While HCN4 variants have been frequently associated with cardiac arrhythmias, these data represent the first experimental evidence that functional alteration of HCN4 can also be involved in human epilepsy through a loss-of-function effect and associated increased neuronal excitability. Since HCN4 appears to be highly expressed in deep brain structures only early during development, our data provide a potential explanation for a link between dysfunctional HCN4 and infantile epilepsy. These findings suggest that it may be useful to include HCN4 screening to extend the knowledge of the genetic causes of infantile epilepsies, potentially paving the way for the identification of innovative therapeutic strategies.

A novel de novo HCN1 loss-of-function mutation in genetic generalized epilepsy causing increased neuronal excitability

The causes of genetic epilepsies are unknown in the majority of patients. HCN ion channels have a widespread expression in neurons and increasing evidence demonstrates their functional involvement in human epilepsies. Among the four known isoforms, HCN1 is the most expressed in the neocortex and hippocampus and de novo HCN1 point mutations have been recently associated with early infantile epileptic encephalopathy. So far, HCN1 mutations have not been reported in patients with idiopathic epilepsy. Using a Next Generation Sequencing approach, we identified the de novo heterozygous p.Leu157Val (c.469C > G) novel mutation in HCN1 in an adult male patient affected by genetic generalized epilepsy (GGE), with normal cognitive development. Electrophysiological analysis in heterologous expression model (CHO cells) and in neurons revealed that L157V is a loss-of-function, dominant negative mutation causing reduced HCN1 contribution to net inward current and responsible for an increased neuronal firing rate and excitability, potentially predisposing to epilepsy. These data represent the first evidence that autosomal dominant missense mutations of HCN1 can also be involved in GGE, without the characteristics of epileptic encephalopathy reported previously. It will be important to include HCN1 screening in patients with GGE, in order to extend the knowledge of the genetic causes of idiopathic epilepsies, thus paving the way for the identification of innovative therapeutic strategies.

A synthetic peptide that prevents cAMP regulation in mammalian hyperpolarization-activated cyclic nucleotide-gated (HCN) channels

Binding of TRIP8b to the cyclic nucleotide binding domain (CNBD) of mammalian hyperpolarization-activated cyclic nucleotide-gated (HCN) channels prevents their regulation by cAMP. Since TRIP8b is expressed exclusively in the brain, we envisage that it can be used for orthogonal control of HCN channels beyond the central nervous system. To this end, we have identified by rational design a 40-aa long peptide (TRIP8b_nano) that recapitulates affinity and gating effects of TRIP8b in HCN isoforms (hHCN1, mHCN2, rbHCN4) and in the cardiac current I_f in rabbit and mouse sinoatrial node cardiomyocytes. Guided by an NMR-derived structural model that identifies the key molecular interactions between TRIP8b_nano and the HCN CNBD, we further designed a cell-penetrating peptide (TAT-TRIP8b_nano) which successfully prevented β-adrenergic activation of mouse I_f leaving the stimulation of the L-type calcium current (I_CaL) unaffected. TRIP8b_nano represents a novel approach to selectively control HCN activation, which yields the promise of a more targeted pharmacology compared to pore blockers.

The expression of the rare caveolin-3 variant T78M alters cardiac ion channels function and membrane excitability

Aims

Caveolinopathies are a family of genetic disorders arising from alterations of the caveolin-3 (cav-3) gene. The T78M cav-3 variant has been associated with both skeletal and cardiac muscle pathologies but its functional contribution, especially to cardiac diseases, is still controversial. Here, we evaluated the effect of the T78M cav-3 variant on cardiac ion channel function and membrane excitability.

Methods and results

We transfected either the wild type (WT) or T78M cav-3 in caveolin-1 knock-out mouse embryonic fibroblasts and found by immunofluorescence and electron microscopy that both are expressed at the plasma membrane and form caveolae. Two ion channels known to interact and co-immunoprecipitate with the cav-3, hKv1.5 and hHCN4, interact also with T78M cav-3 and reside in lipid rafts. Electrophysiological analysis showed that the T78M cav-3 causes hKv1.5 channels to activate and inactivate at more hyperpolarized potentials and the hHCN4 channels to activate at more depolarized potentials, in a dominant way. In spontaneously beating neonatal cardiomyocytes, the expression of the T78M cav-3 significantly increased action potential peak-to-peak variability without altering neither the mean rate nor the maximum diastolic potential. We also found that in a small cohort of patients with supraventricular arrhythmias, the T78M cav-3 variant is more frequent than in the general population. Finally, in silico analysis of both sinoatrial and atrial cell models confirmed that the T78M-dependent changes are compatible with a pro-arrhythmic effect.

Conclusion

This study demonstrates that the T78M cav-3 induces complex modifications in ion channel function that ultimately alter membrane excitability. The presence of the T78M cav-3 can thus generate a susceptible substrate that, in concert with other structural alterations and/or genetic mutations, may become arrhythmogenic.

Preferential myofibroblast differentiation of cardiac mesenchymal progenitor cells in the presence of atrial fibrillation

Atrial fibrillation (AF) is characterized by electrical, contractile, and structural remodeling mediated by interstitial fibrosis. It has been shown that human cardiac mesenchymal progenitor cells (CMPCs) can be differentiated into endothelial, smooth muscle, and fibroblast cells. Here, we have investigated, for the first time, the contribution of CMPCs in the fibrotic process occurring in AF. As expected, right auricolae samples displayed significantly higher fibrosis in AF vs control (CTR) patients. In tissue samples of AF patients only, double staining for c-kit and the myofibroblast marker α-smooth muscle actin (α-SMA) was detected. The number of c-kit-positive CMPC was higher in atrial subepicardial regions of CTR than AF cells. AF-derived CMPC (AF-CMPC) and CTR-derived CMPC (Ctr-CMPC) were phenotypically similar, except for CD90 and c-kit, which were significantly more present in AF and CTR cells, respectively. Moreover, AF showed a lower rate of population doubling and fold enrichment vs Ctr-CMPC. When exogenously challenged with the profibrotic transforming growth factor-β1 (TGF-β1), AF-CMPC showed a significantly higher nuclear translocation of SMAD2 than Ctr-CMPC. In addition, TGF-β1 treatment induced the upregulation of COL1A1 and COL1A2 in AF-CMPC only. Further, both a marked production of soluble collagen and α-SMA upregulation have been observed in AF-CMPC only. Finally, electrophysiological studies showed that the inwardly rectifying potassium current (I_K1) was evenly present in AF- and Ctr-CMPC in basal conditions and similarly disappeared after TGF-β1 exposure. All together, these data suggest that AF steers the resident atrial CMPC compartment toward an electrically inert profibrotic phenotype.

Mammalian γ2 AMPK regulates intrinsic heart rate

AMPK is a conserved serine/threonine kinase whose activity maintains cellular energy homeostasis. Eukaryotic AMPK exists as αβγ complexes, whose regulatory γ subunit confers energy sensor function by binding adenine nucleotides. Humans bearing activating mutations in the γ2 subunit exhibit a phenotype including unexplained slowing of heart rate (bradycardia). Here, we show that γ2 AMPK activation downregulates fundamental sinoatrial cell pacemaker mechanisms to lower heart rate, including sarcolemmal hyperpolarization-activated current (I f) and ryanodine receptor-derived diastolic local subsarcolemmal Ca2+ release. In contrast, loss of γ2 AMPK induces a reciprocal phenotype of increased heart rate, and prevents the adaptive intrinsic bradycardia of endurance training. Our results reveal that in mammals, for which heart rate is a key determinant of cardiac energy demand, AMPK functions in an organ-specific manner to maintain cardiac energy homeostasis and determines cardiac physiological adaptation to exercise by modulating intrinsic sinoatrial cell behavior.

Anacardic acid and thyroid hormone enhance cardiomyocytes production from undifferentiated mouse ES cells along functionally distinct pathways

The epigenetics of early commitment to embryonal cardiomyocyte is poorly understood. In this work, we compared the effect of thyroid hormone and that of anacardic acid, a naturally occurring histone acetylase inhibitor, or both in combination, on mouse embryonic stem cells (mES) differentiating into embryonal cardiomyocyte by embryoid bodies (EBs) formation. Although the results indicated that anacardic acid (AA) and thyroid hormone were both efficient in promoting cardiomyocyte differentiation, we noticed that a transient exposure of mES to AA alone was sufficient to enlarge the beating areas of EBs compared to those of untreated controls. This effect was associated with changes in the chromatin structure at the promoters of specific cardiomyogenic genes. Among them, a rapid induction of the transcription factor Castor 1 (CASZ1), important for cardiomyocytes differentiation and maturation during embryonic development, was observed in the presence of AA. In contrast, thyroid hormone (T 3) was more effective in stimulating spontaneous firing, thus suggesting a role in the production of a population of cardiomyocyte with pacemaker properties. In conclusion, AA and thyroid hormone both enhanced cardiomyocyte formation along in apparently distinct pathways.

Mutation in S6 domain of HCN4 channel in patient with suspected Brugada syndrome modifies channel function

Diseases such as the sick sinus and the Brugada syndrome are cardiac abnormalities, which can be caused by a number of genetic aberrances. Among them are mutations in HCN4, a gene, which encodes the hyperpolarization-activated, cyclic nucleotide-gated ion channel 4; this pacemaker channel is responsible for the spontaneous activity of the sinoatrial node. The present genetic screening of patients with suspected or diagnosed Brugada or sick sinus syndrome identified in 1 out of 62 samples the novel mutation V492F. It is located in a highly conserved site of hyperpolarization-activated cyclic nucleotide-gated (HCN)4 channel downstream of the filter at the start of the last transmembrane domain S6. Functional expression of mutant channels in HEK293 cells uncovered a profoundly reduced channel function but no appreciable impact on channel synthesis and trafficking compared to the wild type. The inward rectifying HCN4 current could be partially rescued by an expression of heteromeric channels comprising wt and mutant monomers. These heteromeric channels were responsive to cAMP but they required a more negative voltage for activation and they exhibited a lower current density than the wt channel. This suggests a dominant negative effect of the mutation in patients, which carry this heterozygous mutation. Such a modulation of HCN4 activity could be the cause of the diagnosed cardiac abnormality.

Current understanding of the pathophysiological mechanisms responsible for inappropriate sinus tachycardia: role of the If "funny" current

BACKGROUND

Together with the afferent branches of the autonomic nervous system, the sinoatrial node (SAN) forms a functional unit whose function is to fire rhythmic action potentials at a rate optimal for coping with the metabolic needs of the body. Dysfunctional behavior of this complex unit may thus result in SAN rhythm disorders. Among these disorders, there is the inappropriate sinus tachycardia (IST) which occurs when an unjustified fast SAN rate is present.

METHODS

We here present a critical review of the role of pacemaker f/HCN channels in cardiac rhythm generation and modulation and their involvement in IST.

RESULTS

Recent evidence demonstrates that a familial form of IST is associated with a gain-of-function mutation in the HCN4 pacemaker channel (R524Q) which confers an increased sensitivity to the second messenger cAMP, a key mediator in sympathetic modulation.

CONCLUSIONS

This finding is consistent with the general view that hypersympathetic tone is one of the causes of IST and introduces the novel concept of defective funny channel-dependent tachyarrhythmias.

Higher cardiogenic potential of iPSCs derived from cardiac versus skin stromal cells

Prior studies have demonstrated that founder cell type could influence induced pluripotent stem cells (iPSCs) molecular and developmental properties at early passages after establishing their pluripotent state. Herein, we evaluated the persistence of a functional memory related to the tissue of origin in iPSCs from syngeneic cardiac (CStC) vs skin stromal cells (SStCs). We found that, at passages greater than 15, iPSCs from cardiac stromal cells (C-iPSCs) produced a higher number of beating embryoid bodies than iPSCs from skin stromal cells (S-iPSCs). Flow cytometry analysis revealed that dissected beating areas from C-iPSCs exhibited more Troponin-T positive cells compared to S-iPSCs. Beating areas derived from C-iPSCs displayed higher expression of cardiac markers, more hyperpolarized diastolic potentials, larger action potential amplitude and higher contractility than beaters from skin. Also, different microRNA subsets were differentially modulated in CStCs vs SStCs during the reprogramming process, potentially accounting for the higher cardiogenic potentials of C-iPSCs vs S-iPSCs. Therefore, the present work supports the existence of a founder organ memory in iPSCs obtained from the stromal component of the origin tissue.

Cell-specific Dynamic Clamp analysis of the role of funny If current in cardiac pacemaking

We used the Dynamic Clamp technique for i) comparative validation of conflicting computational models of the hyperpolarization-activated funny current, If, and ii) quantification of the role of If in mediating autonomic modulation of heart rate. Experimental protocols based on the injection of a real-time recalculated synthetic If current in sinoatrial rabbit cells were developed. Preliminary results of experiments mimicking the autonomic modulation of If demonstrated the need for a customization procedure to compensate for cellular heterogeneity. For this reason, we used a cell-specific approach, scaling the maximal conductance of the injected current based on the cell's spontaneous firing rate. The pacemaking rate, which was significantly reduced after application of Ivabradine, was restored by the injection of synthetic current based on the Severi-DiFrancesco formulation, while the injection of synthetic current based on the Maltsev-Lakatta formulation did not produce any significant variation. A positive virtual shift of the If activation curve, mimicking the Isoprenaline effects, led to a significant increase in pacemaking rate (+17.3 ± 6.7%, p < 0.01), although of lower magnitude than that induced by real Isoprenaline (+45.0 ± 26.1%). Similarly, a negative virtual shift of the activation curve significantly lowered the pacemaking rate (-11.8 ± 1.9%, p < 0.001), as did the application of real Acetylcholine (-20.5 ± 5.1%). The Dynamic Clamp approach, applied to the If study in cardiomyocytes for the first time and rate-adapted to manage intercellular variability, indicated that: i) the quantitative description of the If current in the Severi-DiFrancesco model accurately reproduces the effects of the real current on rabbit sinoatrial cell pacemaking rate and ii) a significant portion (50-60%) of the physiological autonomic rate modulation is due to the shift of the If activation curve.

HCN4, Sinus Bradycardia and Atrial Fibrillation

Based on their established role in the generation of spontaneous activity in pacemaker cells and control of cardiac rate, funny/ hyperpolarisation-activated, cyclic nucleotide gated 4 (HCN4) channels are natural candidates in the search for causes of sinus arrhythmias. Investigation of funny current-related inheritable arrhythmias has led to the identification of several mutations of the HCN4 gene associated with bradycardia and/or more complex arrhythmias. More recently, the search has been extended to include auxiliary proteins such as the minK-related peptide 1 (MiRP1) β-subunit. All mutations described so far are loss-of-function and in agreement with the role of funny channels, the predominant type of arrhythmia found is bradycardia. Funny channel-linked arrhythmias, however, also include atrioventricular (AV) block and atrial fibrillation, in agreement with an emerging new concept according to which defective funny channels have a still unexplored role in impairing AV conduction and triggering atrial fibrillation. Also, importantly, recent work shows that HCN4 mutations can be associated with cardiac structural abnormalities. In this short review I briefly address the current knowledge of funny/HCN4 channel mutations and associated sinus and more complex arrhythmias.

Dysfunctional HCN ion channels in neurological diseases

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed as four different isoforms (HCN1-4) in the heart and in the central and peripheral nervous systems. HCN channels are activated by membrane hyperpolarization at voltages close to resting membrane potentials and carry the hyperpolarization-activated current, dubbed I_f (funny current) in heart and I_h in neurons. HCN channels contribute in several ways to neuronal activity and are responsible for many important cellular functions, including cellular excitability, generation, and modulation of rhythmic activity, dendritic integration, transmission of synaptic potentials, and plasticity phenomena. Because of their role, defective HCN channels are natural candidates in the search for potential causes of neurological disorders in humans. Several data, including growing evidence that some forms of epilepsy are associated with HCN mutations, support the notion of an involvement of dysfunctional HCN channels in different experimental models of the disease. Additionally, some anti-epileptic drugs are known to modify the activity of the I_h current. HCN channels are widely expressed in the peripheral nervous system and recent evidence has highlighted the importance of the HCN2 isoform in the transmission of pain. HCN channels are also present in the midbrain system, where they finely regulate the activity of dopaminergic neurons, and a potential role of these channels in the pathogenesis of Parkinson’s disease has recently emerged. The function of HCN channels is regulated by specific accessory proteins, which control the correct expression and modulation of the neuronal I_h current. Alteration of these proteins can severely interfere with the physiological channel function, potentially predisposing to pathological conditions. In this review we address the present knowledge of the association between HCN dysfunctions and neurological diseases, including clinical, genetic, and physiopathological aspects.

LEONARDO DA VINCI AND THE ORIGIN OF SEMEN

It is well known that Leonardo da Vinci made several drawings of the human male anatomy. The early drawings (before 1500) were incorrect in identifying the origin of semen, where he followed accepted teaching of his time. It is widely thought that he did not correct this mistake, a view that is reflected in several biographies. In fact, he made a later drawing (after 1500) in which the description of the anatomy is remarkably accurate and must have been based on careful dissection. In addition to highlighting this fact, acknowledged previously in only one other source, this article reviews the background to Leonardo's knowledge of the relevant anatomy.

Exercise training reduces resting heart rate via downregulation of the funny channel HCN4

Endurance athletes exhibit sinus bradycardia, that is a slow resting heart rate, associated with a higher incidence of sinus node (pacemaker) disease and electronic pacemaker implantation. Here we show that training-induced bradycardia is not a consequence of changes in the activity of the autonomic nervous system but is caused by intrinsic electrophysiological changes in the sinus node. We demonstrate that training-induced bradycardia persists after blockade of the autonomous nervous system in vivo in mice and in vitro in the denervated sinus node. We also show that a widespread remodelling of pacemaker ion channels, notably a downregulation of HCN4 and the corresponding ionic current, I_f. Block of I_f abolishes the difference in heart rate between trained and sedentary animals in vivo and in vitro. We further observe training-induced downregulation of Tbx3 and upregulation of NRSF and miR-1 (transcriptional regulators) that explains the downregulation of HCN4. Our findings provide a molecular explanation for the potentially pathological heart rate adaptation to exercise training.

Endurance athletes are known to have a low resting heart rate. Here, D'Souza et al. propose that training-induced bradycardia is the result of electrophysiological changes in the sinus node, challenging the classical view that training-induced bradycardia is caused by increased activity of the autonomic nervous system.

Embryonic stem cell-derived CD166+ precursors develop into fully functional sinoatrial-like cells

RATIONALE

A cell-based biological pacemaker is based on the differentiation of stem cells and the selection of a population displaying the molecular and functional properties of native sinoatrial node (SAN) cardiomyocytes. So far, such selection has been hampered by the lack of proper markers. CD166 is specifically but transiently expressed in the mouse heart tube and sinus venosus, the prospective SAN.

OBJECTIVE

We have explored the possibility of using CD166 expression for isolating SAN progenitors from differentiating embryonic stem cells.

METHODS AND RESULTS

We found that in embryonic day 10.5 mouse hearts, CD166 and HCN4, markers of the pacemaker tissue, are coexpressed. Sorting embryonic stem cells for CD166 expression at differentiation day 8 selects a population of pacemaker precursors. CD166+ cells express high levels of genes involved in SAN development (Tbx18, Tbx3, Isl-1, Shox2) and function (Cx30.2, HCN4, HCN1, CaV1.3) and low levels of ventricular genes (Cx43, Kv4.2, HCN2, Nkx2.5). In culture, CD166+ cells form an autorhythmic syncytium composed of cells morphologically similar to and with the electrophysiological properties of murine SAN myocytes. Isoproterenol increases (+57%) and acetylcholine decreases (-23%) the beating rate of CD166-selected cells, which express the β-adrenergic and muscarinic receptors. In cocultures, CD166-selected cells are able to pace neonatal ventricular myocytes at a rate faster than their own. Furthermore, CD166+ cells have lost pluripotency genes and do not form teratomas in vivo.

CONCLUSIONS

We demonstrated for the first time the isolation of a nonteratogenic population of cardiac precursors able to mature and form a fully functional SAN-like tissue.

Funny channel gene mutations associated with arrhythmias

Many diverse data support a role of the funny current (If) in pacemaking and heart rate control. Among them, clinically relevant applications have special impact, since they translate the concept of funny channel-based pacemaking into practical developments of potential therapeutic value. For example, the If role in pacemaking is the basis for a pharmacological approach to heart rate control. Ivabradine, a specific f-channel blocker acting as a 'pure heart rate reducing' agent, is used today as a therapeutic tool in chronic stable angina and heart failure. Also, the pacemaking capability of funny channels has been the main rationale behind the development of 'biological' pacemakers, whose aim is to eventually replace the electronic pacemakers implanted today. Finally, the role of If in pacemaking implies that dysfunctional funny channels can contribute to rhythm disorders. This consideration has led to the search for inheritable forms of cardiac arrhythmias potentially linked to functional changes of HCN4 channels, the molecular correlates of funny channels. This review addresses recent reports where investigation of families with arrhythmias has allowed the identification of specific HCN4 channel mutations associated with different types of rhythm disorders, specifically with sinus bradycardia. The perspective of future research in this field is also addressed.

Identification of the molecular site of ivabradine binding to HCN4 channels

Ivabradine is a specific heart rate-reducing agent approved as a treatment of chronic stable angina. Its mode of action involves a selective and specific block of HCN channels, the molecular components of sinoatrial "funny" (f)-channels. Different studies suggest that the binding site of ivabradine is located in the inner vestibule of HCN channels, but the molecular details of ivabradine binding are unknown. We thus sought to investigate by mutagenesis and in silico analysis which residues of the HCN4 channel, the HCN isoform expressed in the sinoatrial node, are involved in the binding of ivabradine. Using homology modeling, we verified the presence of an inner cavity below the channel pore and identified residues lining the cavity; these residues were replaced with alanine (or valine) either alone or in combination, and WT and mutant channels were expressed in HEK293 cells. Comparison of the block efficiency of mutant vs WT channels, measured by patch-clamp, revealed that residues Y506, F509 and I510 are involved in ivabradine binding. For each mutant channel, docking simulations correctly explain the reduced block efficiency in terms of proportionally reduced affinity for ivabradine binding. In summary our study shows that ivabradine occupies a cavity below the channel pore, and identifies specific residues facing this cavity that interact and stabilize the ivabradine molecule. This study provides an interpretation of known properties of f/HCN4 channel block by ivabradine such as the “open channel block”, the current-dependence of block and the property of "trapping" of drug molecules in the closed configuration.

An updated computational model of rabbit sinoatrial action potential to investigate the mechanisms of heart rate modulation

The cellular basis of cardiac pacemaking is still debated. Reliable computational models of the sinoatrial node (SAN) action potential (AP) may help gain a deeper understanding of the phenomenon. Recently, novel models incorporating detailed Ca(2+)-handling dynamics have been proposed, but they fail to reproduce a number of experimental data, and more specifically effects of 'funny' (I(f)) current modifications. We therefore developed a SAN AP model, based on available experimental data, in an attempt to reproduce physiological and pharmacological heart rate modulation. Cell compartmentalization and intracellular Ca(2+)-handling mechanisms were formulated as in the Maltsev-Lakatta model, focusing on Ca(2+)-cycling processes. Membrane current equations were revised on the basis of published experimental data. Modifications of the formulation of currents/pumps/exchangers to simulate I(f) blockers, autonomic modulators and Ca(2+)-dependent mechanisms (ivabradine, caesium, acetylcholine, isoprenaline, BAPTA) were derived from experimental data. The model generates AP waveforms typical of rabbit SAN cells, whose parameters fall within the experimental ranges: 352 ms cycle length, 80 mV AP amplitude, -58 mV maximum diastolic potential (MDP), 108 ms APD(50), and 7.1 Vs(-1) maximum upstroke velocity. Rate modulation by I(f) -blocking drugs agrees with experimental findings: 20% and 22% caesium-induced (5mM) and ivabradine-induced (3 μM) rate reductions, respectively, due to changes in diastolic depolarization (DD) slope, with no changes in either MDP or take-off potential (TOP). The model consistently reproduces the effects of autonomic modulation: 20% rate decrease with 10 nM acetylcholine and 28%increase with 1 μM isoprenaline, again entirely due to increase in the DD slope,with no changes in either MDP or TOP. Model testing of BAPTA effects showed slowing of rate, -26%, without cessation of beating. Our up-to-date model describes satisfactorily experimental data concerning autonomic stimulation, funny-channel blockade and inhibition of the Ca(2+)-related system by BAPTA, making it a useful tool for further investigation. Simulation results suggest that a detailed description of the intracellular Ca(2+) fluxes is fully compatible with the observation that I(f) is a major component of pacemaking and rate modulation.

Human cardiac and bone marrow stromal cells exhibit distinctive properties related to their origin

AIMS

Bone marrow mesenchymal stromal cell (BMStC) transplantation into the infarcted heart improves left ventricular function and cardiac remodelling. However, it has been suggested that tissue-specific cells may be better for cardiac repair than cells from other sources. The objective of the present work has been the comparison of in vitro and in vivo properties of adult human cardiac stromal cells (CStC) to those of syngeneic BMStC.

METHODS AND RESULTS

Although CStC and BMStC exhibited a similar immunophenotype, their gene, microRNA, and protein expression profiles were remarkably different. Biologically, CStC, compared with BMStC, were less competent in acquiring the adipogenic and osteogenic phenotype but more efficiently expressed cardiovascular markers. When injected into the heart, in rat a model of chronic myocardial infarction, CStC persisted longer within the tissue, migrated into the scar, and differentiated into adult cardiomyocytes better than BMStC.

CONCLUSION

Our findings demonstrate that although CStC and BMStC share a common stromal phenotype, CStC present cardiovascular-associated features and may represent an important cell source for more efficient cardiac repair.

The role of the funny current in pacemaker activity

Abstract: Pacemaking is a basic physiological process, and the cellular mechanisms involved in this function have always attracted the keen attention of investigators. The "funny" (I(f)) current, originally described in sinoatrial node myocytes as an inward current activated on hyperpolarization to the diastolic range of voltages, has properties suitable for generating repetitive activity and for modulating spontaneous rate. The degree of activation of the funny current determines, at the end of an action potential, the steepness of phase 4 depolarization; hence, the frequency of action potential firing. Because I(f) is controlled by intracellular cAMP and is thus activated and inhibited by beta-adrenergic and muscarinic M2 receptor stimulation, respectively, it represents a basic physiological mechanism mediating autonomic regulation of heart rate. Given the complexity of the cellular processes involved in rhythmic activity, an exact quantification of the extent to which I(f) and other mechanisms contribute to pacemaking is still a debated issue; nonetheless, a wealth of information collected since the current was first described more than 30 years ago clearly agrees to identify I(f) as a major player in both generation of spontaneous activity and rate control. I(f)- dependent pacemaking has recently advanced from a basic, physiologically relevant concept, as originally described, to a practical concept that has several potentially useful clinical applications and can be valuable in therapeutically relevant conditions. Typically, given their exclusive role in pacemaking, f-channels are ideal targets of drugs aiming to pharmacological control of cardiac rate. Molecules able to bind specifically to and block f-channels can thus be used as pharmacological tools for heart rate reduction with little or no adverse cardiovascular side effects. Indeed a selective f-channel inhibitor, ivabradine, is today commercially available as a tool in the treatment of stable chronic angina. Also, several loss-of-function mutations of HCN4 (hyperpolarization-activated, cyclic-nucleotide gated 4), the major constitutive subunit of f-channels in pacemaker cells, are known today to cause rhythm disturbances, such as for example inherited sinus bradycardia. Finally, gene- or cell-based methods for in situ delivery of f-channels to silent or defective cardiac muscle represent novel approaches for the development of biological pacemakers eventually able to replace electronic devices.