Molecular genetics of long-QT syndrome

Potassium channels: new targets in cancer therapy

BACKGROUND

Potassium channels (KCh) are the most diverse and ubiquitous class of ion channels. KCh control membrane potential and contribute to nerve and cardiac action potentials and neurotransmitter release. KCh are also involved in insulin release, differentiation, activation, proliferation, apoptosis, and several other physiological functions. The aim of this review is to provide an updated overview of the KCh role during the cell growth. Their potential use as pharmacological targets in cancer therapies is also discussed.

METHODS

We searched PubMed (up to 2005) and identified relevant articles. Reprints were mainly obtained by on line subscription. Additional sources were identified through cross-referencing and obtained from Library services.

RESULTS

KCh are responsible for some neurological and cardiovascular diseases and for a new medical discipline, channelopathies. Their role in congenital deafness, multiple sclerosis, episodic ataxia, LQT syndrome and diabetes has been proven. Furthermore, a large body of information suggests that KCh play a role in the cell cycle progression, and it is now accepted that cells require KCh to proliferate. Thus, KCh expression has been studied in a number of tumours and cancer cells.

CONCLUSIONS

Cancer is far from being considered a channelopathy. However, it seems appropriate to take into account the involvement of KCh in cancer progression and pathology when developing new strategies for cancer therapy.

Late sodium current in the pathophysiology of cardiovascular disease: consequences of sodium-calcium overload

Late sodium current in cardiac cells is very small compared with the fast component, but as it flows throughout the action potential it may make a substantial contribution to sodium loading during each cardiac cycle. Late sodium current may contribute to triggering arrhythmia in two ways: by causing repolarisation failure (early after depolarisations); and by triggering late after depolarisations attributable to calcium oscillations in sodium-calcium overload conditions. Reduction of late sodium current would therefore be expected to have therapeutic benefits, particularly in disease states such as ischaemia in which sodium-calcium overload is a major feature.

Effects of I(Ks) channel inhibitors in insulin-secreting INS-1 cells

Potassium channels regulate insulin secretion. The closure of K(ATP) channels leads to membrane depolarisation, which triggers Ca(2+) influx and stimulates insulin secretion. The subsequent activation of K(+) channels terminates secretion. We examined whether KCNQ1 channels are expressed in pancreatic beta-cells and analysed their functional role. Using RT/PCR cellular mRNA of KCNQ1 but not of KCNE1 channels was detected in INS-1 cells. Effects of two sulfonamide analogues, 293B and HMR1556, inhibitors of KCNQ1 channels, were examined on voltage-activated outwardly rectifying K(+) currents using the patch-clamp method. It was found that 293B inhibited 60% of whole-cell outward currents induced by voltage pulses from -70 to +50 mV with a concentration for half-maximal inhibition (IC(50)) of 37 microM. The other sulfonamide analogue HMR1556 inhibited 48% of the outward current with an IC(50) of 7 microM. The chromanol 293B had no effect on tolbutamide-sensitive K(ATP) channels. Action potentials induced by current injections were broadened and after-repolarisation was attenuated by 293B. Insulin secretion in the presence but not in the absence of tolbutamide was significantly increased by 293B. These results suggest that 293B- and HMR1556-sensitive channels, probably in concert with other voltage-activated K(+) channels, influence action potential duration and frequency and thus insulin secretion.

Acute and chronic hypoxic regulation of recombinant hNa(v)1.5 alpha subunits

Acute and chronic hypoxic regulation of ion channels is involved in both cell physiology and pathology. Voltage-dependent Na(+) channels play a dominant role in the upstroke of the action potential in excitable cells, while non-inactivating (persistent or sustained) Na(+) currents contribute to action potential shape and duration. In cardiac myocytes, hypoxic augmentation of persistent Na(+) currents has been proposed to underlie cardiac arrhythmias via prolonging action potential duration. Here, we demonstrate that acute hypoxia (P(O2), 20mm Hg) augmented persistent Na(+) currents in HEK293 cells stably expressing human Na(v)1.5 alpha subunits. Hypoxia also inhibited peak Na(+) currents in a voltage-dependent manner, and the kinetics of activation and inactivation of Na(+) currents were significantly slowed during hypoxia. We further demonstrate that exposure to chronic hypoxia (6% O(2) for 24h) augmented peak Na(+) channel current, which given the exogenous promoter driving expression of the channel occurs most probably via a post-transcriptional mechanism. These effects of acute and chronic hypoxia likely play an arrhythmogenic role during both short- and long-term hypoxic/ischaemic episodes. The HEK293 expression system provides a useful paradigm in which to examine the mechanisms of O(2) sensing by the Na(+) channel.

Molecular and functional characterization of ERG, KCNQ, and KCNE subtypes in rat stomach smooth muscle

Contribution of K(+) channels derived from the expression of ERG, KCNQ, and KCNE subtypes, which are responsible for rapidly and slowly activating delayed rectifier K(+) currents (I(Kr) and I(Ks), respectively) in cardiac myocytes, to membrane currents was examined in stomach circular smooth muscle cells (SMCs). The region-qualified multicell RT-PCR showed that ERG1/KCNE2 transcripts were expressed in rat stomach fundus and antrum SMCs and that KCNQ1/KCNE1 transcripts were expressed in antrum but not fundus. Western blotting and immunocytochemical analyses indicate that ERG1 proteins were substantially expressed in both regions, whereas KCNE1 proteins were faintly expressed in antrum and not in fundus. Both I(Kr)- and I(Ks)-like currents susceptible to E-4031 and indapamide, respectively, were identified in circular SMCs of antrum but only I(Kr)-like current was identified in fundus. It is strongly suggested that I(Kr)- and I(Ks)-like currents functionally identified in rat stomach SMCs are attributable to the expression of ERG1/KCNE2 and KCNQ1/KCNE1, respectively. The membrane depolarization by 1 microM E-4031 indicates the contribution of K(+) channels encoded by ERG1/KCNE2 to the resting membrane potential in stomach SMCs.

Evaluation and treatment of pediatric patients with congenital or acquired long QT interval syndromes

The long QT syndrome should be considered when evaluating patients with syncope. Prolongation of the QT interval and abnormalities of T wave morphology due to abnormal ventricular repolarization characterize the syndrome. In the past decade, molecular genetics has revealed that abnormal repolarization is the result of gene mutations encoding integral ion channels that generate the cardiac action potential. Eight subgroups of long QT syndrome associated with five genes have been described to date. The explosion in research in this area has led to a greater understanding of the clinical expression of autosomal dominant (Romano-Ward syndrome), autosomal recessive (Jervell and Lange-Nielsen syndrome) and acquired forms of the disease. This has also led to investigation in the area of genotype-specific therapy. The purpose of this review is to outline the strides made in the field of molecular genetics and update the reader on the recent advances in diagnosis and treatment of the long QT syndrome.

Role of Sodium Channel Deglycosylation in the Genesis of Cardiac Arrhythmias in Heart Failure

We investigated the cellular and molecular mechanisms underlying arrhythmias in heart failure. A genetically engineered mouse lacking the expression of the muscle LIM protein (MLP-/-) was used in this study as a model of heart failure. We used electrocardiography and patch clamp techniques to examine the electrophysiological properties of MLP-/- hearts. We found that MLP-/- myocytes had smaller Na+ currents with altered voltage dependencies of activation and inactivation and slower rates of inactivation than control myocytes. These changes in Na+ currents contributed to longer action potentials and to a higher probability of early afterdepolarizations in MLP-/- than in control myocytes. Western blot analysis suggested that the smaller Na+ current in MLP-/- myocytes resulted from a reduction in Na+ channel protein. Interestingly, the blots also revealed that the alpha-subunit of the Na+ channel from the MLP-/- heart had a lower average molecular weight than in the control heart. Treating control myocytes with the sialidase neuraminidase mimicked the changes in voltage dependence and rate of inactivation of Na+ currents observed in MLP-/- myocytes. Neuraminidase had no effect on MLP-/- cells thus suggesting that Na+ channels in these cells were sialic acid-deficient. We conclude that deficient glycosylation of Na+ channel contributes to Na+ current-dependent arrhythmogenesis in heart failure.

Screening lead compounds for QT interval prolongation

The late detection of cardiotoxic side effects, such as QT prolongation, induced by compounds of pharmacological interest can dramatically impede drug discovery and development projects, and consequently increase their cost. The launch of new drugs with undetected cardiotoxic side effects could have hazardous consequences and could trigger lethal cardiac dysrhythmias in patients. It is desirable, therefore, to test for the potential cardiotoxic side effects of compounds at an early stage of drug development. Electrophysiological test systems and cellular-based fluorometric high-throughput assays are now available for cloned human cardiac ion channels. These test systems are important tools in the preclinical safety evaluation of drugs and newly developed compounds.

Distribution of a persistent sodium current across the ventricular wall in guinea pigs

A tetrodotoxin-sensitive persistent sodium current, I(pNa), was found in guinea pig ventricular myocytes by whole-cell patch clamping. This current was characterized in cells derived from the basal left ventricular subendocardium, midmyocardium, and subepicardium. Midmyocardial cells show a statistically significant (P<0.05) smaller I(pNa) than subendocardial and subepicardial myocytes. There was no significant difference in I(pNa) current density between subepicardial and subendocardial cells. Computer modeling studies support a role of this current in the dispersion of action potential duration across the ventricular wall.