Ágnes K Mike

Voltage-Gated Ion Channel Dysfunction Precedes Cardiomyopathy Development in the Dystrophic Heart

Background Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is associated with severe cardiac complications including cardiomyopathy and cardiac arrhythmias. Recent research suggests that impaired voltage-gated ion channels in dystrophic cardiomyocytes accompany cardiac pathology. It is, however, unknown if the ion channel defects are primary effects of dystrophic gene mutations, or secondary effects of the developing cardiac pathology. Methodology/Principal Findings To address this question, we first investigated sodium channel impairments in cardiomyocytes derived from dystrophic neonatal mice prior to cardiomyopahty development, by using the whole cell patch clamp technique. Besides the most common model for DMD, the dystrophin-deficient mdx mouse, we also used mice additionally carrying an utrophin mutation. In neonatal cardiomyocytes, dystrophin-deficiency generated a 25% reduction in sodium current density. In addition, extra utrophin-deficiency significantly altered sodium channel gating parameters. Moreover, also calcium channel inactivation was considerably reduced in dystrophic neonatal cardiomyocytes, suggesting that ion channel abnormalities are universal primary effects of dystrophic gene mutations. To assess developmental changes, we also studied sodium channel impairments in cardiomyocytes derived from dystrophic adult mice, and compared them with the respective abnormalities in dystrophic neonatal cells. Here, we found a much stronger sodium current reduction in adult cardiomyocytes. The described sodium channel impairments slowed the upstroke of the action potential in adult cardiomyocytes, and only in dystrophic adult mice, the QRS interval of the electrocardiogram was prolonged. Conclusions/Significance Ion channel impairments precede pathology development in the dystrophic heart, and may thus be considered potential cardiomyopathy triggers.

Anti-addiction drug ibogaine inhibits voltage-gated ionic currents: A study to assess the drug's cardiac ion channel profile

The plant alkaloid ibogaine has promising anti-addictive properties. Albeit not licenced as a therapeutic drug, and despite hints that ibogaine may perturb the heart rhythm, this alkaloid is used to treat drug addicts. We have recently reported that ibogaine inhibits human ERG (hERG) potassium channels at concentrations similar to the drugs affinity for several of its known brain targets. Thereby the drug may disturb the hearts electrophysiology. Here, to assess the drugs cardiac ion channel profile in more detail, we studied the effects of ibogaine and its congener 18-Methoxycoronaridine (18-MC) on various cardiac voltage-gated ion channels. We confirmed that heterologously expressed hERG currents are reduced by ibogaine in low micromolar concentrations. Moreover, at higher concentrations, the drug also reduced human Nav1.5 sodium and Cav1.2 calcium currents. Ion currents were as well reduced by 18-MC, yet with diminished potency. Unexpectedly, although blocking hERG channels, ibogaine did not prolong the action potential (AP) in guinea pig cardiomyocytes at low micromolar concentrations. Higher concentrations (≥ 10 μM) even shortened the AP. These findings can be explained by the drugs calcium channel inhibition, which counteracts the AP-prolonging effect generated by hERG blockade. Implementation of ibogaines inhibitory effects on human ion channels in a computer model of a ventricular cardiomyocyte, on the other hand, suggested that ibogaine does prolong the AP in the human heart. We conclude that therapeutic concentrations of ibogaine have the propensity to prolong the QT interval of the electrocardiogram in humans. In some cases this may lead to cardiac arrhythmias.

Small Molecule Cardiogenol C Upregulates Cardiac Markers and Induces Cardiac Functional Properties in Lineage-Committed Progenitor Cells

Background/Aims: Cell transplantation into the heart is a new therapy after myocardial infarction. Its success, however, is impeded by poor donor cell survival and by limited transdifferentiation of the transplanted cells into functional cardiomyocytes. A promising strategy to overcome these problems is the induction of cardiomyogenic properties in donor cells by small molecules. Methods: Here we studied cardiomyogenic effects of the small molecule compound cardiogenol C (CgC), and structural derivatives thereof, on lineage-committed progenitor cells by various molecular biological, biochemical, and functional assays. Results: Treatment with CgC up-regulated cardiac marker expression in skeletal myoblasts. Importantly, the compound also induced cardiac functional properties: first, cardiac-like sodium currents in skeletal myoblasts, and secondly, spontaneous contractions in cardiovascular progenitor cell-derived cardiac bodies. Conclusion: CgC induces cardiomyogenic function in lineage-committed progenitor cells, and can thus be considered a promising tool to improve cardiac repair by cell therapy.

VUT-MK142 : a new cardiomyogenic small molecule promoting the differentiation of pre-cardiac mesoderm into cardiomyocytes

Intra-cardiac cell transplantation is a new therapy after myocardial infarction. Its success, however, is impeded by the limited capacity of donor cells to differentiate into functional cardiomyocytes in the heart. A strategy to overcome this problem is the induction of cardiomyogenic function in cells prior to transplantation. Among other approaches, recently, synthetic small molecules were identified, which promote differentiation of stem cells of various origins into cardiac-like cells or cardiomyocytes. The aim of this study was to develop and characterise new promising cardiomyogenic synthetic low-molecular weight compounds. Therefore, the structure of the known cardiomyogenic molecule cardiogenol C was selectively modified, and the effects of the resulting compounds were tested on various cell types. From this study, VUT-MK142 was identified as the most promising candidate with respect to cardiomyogenic activity. Treatment using this novel agent induced the strongest up-regulation of expression of the cardiac marker ANF in both P19 embryonic carcinoma cells and C2C12 skeletal myoblasts. The activity of VUT-MK142 on this marker superseded CgC; moreover, the novel compound significantly up-regulated the expression of other cardiac markers, and promoted the development of beating cardiomyocytes from cardiovascular progenitor cells. We conclude that VUT-MK142 is a potent new cardiomyogenic synthetic agent promoting the differentiation of pre-cardiac mesoderm into cardiomyocytes, which may be useful to differentiate stem cells into cardiomyocytes for cardiac repair. Additionally, an efficient synthesis of VUT-MK142 is reported taking advantage of continuous flow techniques superior to classical batch reactions both in yield and reaction time.

The anti-addictive drug ibogaine modulates voltage-gated ion channels and may trigger cardiac arrhythmias

Background Ibogaine is an alkaloid derived from the African shrub Tabernanthe iboga. Psychoactive properties of ibogaine have been known for decades, but more recently the drug has received much attention because of its promising “anti-addictive” actions. Thus, ibogaine and its derivatives are being studied as potential treatment for opioid and stimulant abuse, as well as for alcoholism and smoking. Because ibogaine has a complex pharmacology and is known to interact with numerous different cellular targets, its potential to generate adverse effects is significant. Besides the expected neurotoxic actions, ibogaine may e.g. also affect the heart. Thus, several cases of sudden death after ibogaine use were reported, which have been hypothesised to be related to cardiac arrhythmias. In accordance, a severely prolonged QT interval of the electrocardiogram and ventricular tachyarrhythmias were observed in a woman after she had taken ibogaine.

The anti-addiction drug ibogaine inhibits cardiac ion channels: a study to assess the drug’s proarrhythmic potential

Background The plant alkaloid ibogaine has shown promising antiaddictive properties in animals and humans. Although not licensed as a therapeutic drug, and despite evidence that ibogaine may disturb the rhythm of the heart, this alkaloid is used as an anti-addiction drug in alternative medicine. We have recently reported that therapeutic concentrations of ibogaine inhibit human ERG (hERG) potassium channels, and thereby uncovered a mechanism by which the drug may induce life-threatening cardiac arrhythmias.

Impaired L-type Ca2+ channel function in the dystrophic heart

Background Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is an inherited disease characterized by progressive muscle weakness and degeneration. Besides the relatively well described skeletal muscle degenerative processes, DMD is associated with cardiovascular complications including cardiomyopathy and cardiac arrhythmias. The current understanding of the patho-mechanisms is still very limited, but recent research suggests, that dysfunctional ion channels in dystrophic cardiomyocytes considerably contribute to the cardiovascular complications.

Altered sodium channel function in dystrophin/utrophin-deficient cardiomyocytes

Background Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is an inherited disease characterized by progressive muscle weakness and degeneration. Besides the relatively well-described skeletal muscle degenerative processes, DMD and some other muscular dystrophy types are also associated with cardiovascular complications including cardiomyopathy and cardiac arrhythmias. The current understanding of the patho-mechanisms underlying these cardiovascular complications is still very limited, but recent research points to a contribution of dysfunctional ion channels in dystrophic cardiomyocytes. Materials and methods By using the whole cell patch-clamp technique, the functional properties of voltage-gated sodium channels were studied in cardiomyocytes derived from normal and dystrophic mice. In addition, a computer model was used to simulate the effects of altered sodium channel properties on the cardiac action potential. Besides the most common mouse model for human DMD, the dystrophin-deficient mdx mouse, we also used mice additionally carrying a mutation in the utrophin gene. The mdx-utr double mutant mouse exhibits a more severe cardiac disease phenotype than the mdx mouse, and is thought to represent a more suitable animal model for human DMD. Results We found that dystrophic cardiomyocytes show a reduced sodium current density compared to wild-type cardiomyocytes. In addition, extra utrophin deficiency significantly shifted both the sodium channel activation and inactivation curve to more depolarised potentials, which was not observed in only dystrophin-deficient mdx cardiomyocytes. Computer modelling revealed that the described sodium channel impairments in dystrophic cardiomyocytes suffice to affect the action potential. Conclusions Sodium channel dysfunction may perturb electrical impulse propagation in the dystrophic heart, and thus contribute to cardiac complications associated with the muscular dystrophies.

New structural determinants of charged local anaesthetic block of voltage-gated sodium channels

Background Some blockers of voltage-gated Na and Ca channels are assumed to pass through the membrane and then bind to amino acids in the internal vestibule by access from the internal side of the membrane. However, in the heart isoform of the voltage-gated Na channel, in L-type calcium channels and in T-type calcium channels an additional external access pathway (EAP) through the protein has been suggested. Furthermore, in voltage-gated Na channels (NaV) mutations at a specific site in the middle of the domain IV transmembrane segment 6 (site 1575 in rNaV1.4, 1760 in rNaV1.4) open an EAP for QX-222, a permanently charged, hydrophilic lidocaine analogue. Recently, the first crystal structure of a NaV was published [1]. In this bacterial channel structure (NaVAb) the side chain homologous to rNaV1.4 I1575 (I202 in NaVAb) is in close contact with a pore-loop sidechain, homologous to rNaV1.4 W1531 (W179 in NaVAb). In contrast, in all currently available structural homology models of NaV, W1531 is not in contact with I1575. If W1531 were positioned as suggested in the NaVAb structure then a reduction in the length of the side chain at this site would be predicted to open the EAP. To test this hypothesis we generated the mutations W1531A and W1531G and tested these constructs for block by external QX-222.

A conformational change of the domain IV S6 segment of the voltage-gated sodium channel during inactivation

Background In voltage-gated Na channels the S6 transmembrane segment of domain IV (DIV-S6) is part of the lining of the inner part of the pore. It is of pivotal importance for inactivation gating. We recently showed that amino acid I1581 of DIV-S6 (rNaV1.4 amino acid numbering) is extraordinarily sensitive to both local and distal mutations suggesting a unique role in coupling of voltagesensor movements to conformational changes in the pore. To date the only structural information relevant to voltage-gated Na channels can be derived from the recently crystallized bacterial channel NaVAb. In this structure the amino acid homologous to I1581 faces the lipid phase and is in close spacial relationship to the voltage-sensing apparatus. If this arrangement holds true for the eukaryotic Na channel then site 1581 should not be exposed to bulk solution.