CaV1.2 channelopathies: from arrhythmias to autism, bipolar disorder, and immunodeficiency

Mutations of human Ca_V1.2 channel gene were identified only recently. The gain-of-function mutations were found at two mutually exclusive exons in patients with Timothy syndrome (TS). These patients exhibit prolonged QT interval and lethal cardiac arrhythmias. In contrast, the loss-of-function mutations of Ca_V1.2 channel in patients with Brugada syndrome produce short QT interval that could result in sudden cardiac death. TS patients also suffer from multi-organ dysfunction that includes neurological disorder such as autism and mental retardation reflecting the wide tissue distribution of Ca_V1.2 channel. Mutations found on different mutually exclusive exons determine the severity of the disease. Unexpectedly, TS patients may develop recurrent infections and bronchitis that suggests a role of Ca_V1.2 channel in the immune system. Furthermore, recent reports revealed a linkage of Ca_V1.2 channel polymorphism with multiple central nervous system disorders including bipolar disorder, depression, and schizophrenia. Here, we will discuss how alternative splicing modulates Ca_V1.2 channelopathy and the role of Ca_V1.2 channel in both excitable and non-excitable tissues.     

Cholinergic control of firing pattern and neurotransmission in rat neostriatal projection neurons: role of CaV2.1 and CaV2.2 Ca2+ channels

Besides a reduction of L-type Ca2+-currents (Ca(V)1), muscarine and the peptidic M1-selective agonist, MT-1, reduced currents through Ca(V)2.1 (P/Q) and Ca(V)2.2 (N) Ca2+ channel types. This modulation was strongly blocked by the peptide MT-7, a specific muscarinic M1-type receptor antagonist but not significantly reduced by the peptide MT-3, a specific muscarinic M4-type receptor antagonist. Accordingly, MT-7, but not MT-3, blocked a muscarinic reduction of the afterhyperpolarizing potential (AHP) and decreased the GABAergic inhibitory postsynaptic currents (IPSCs) produced by axon collaterals that interconnect spiny neurons. Both these functions are known to be dependent on P/Q and N types Ca2+ channels. The action on the AHP had an important effect in increasing firing frequency. The action on the IPSCs was shown to be caused presynaptically as it coursed with an increase in the paired-pulse ratio. These results show: first, that muscarinic M1-type receptor activation is the main cholinergic mechanism that modulates Ca2+ entry through voltage-dependent Ca2+ channels in spiny neurons. Second, this muscarinic modulation produces a postsynaptic facilitation of discharge together with a presynaptic inhibition of the GABAergic control mediated by axon collaterals. Together, both effects would tend to recruit more spiny neurons for the same task.     

HCN-related channelopathies

HCN channels are the molecular subunits of native funny (f-) channels of cardiac pacemaker cells and neurons. Although funny channels were first functionally described in cardiac cells in the late 1970s, cloning of HCN channels, of which four subunits are known today (HCN1-4), had to wait some 20 years to be accomplished, which delayed the investigation of HCN-related channelopathies. In cardiac pacemaker cells, the main function of f-channels is to contribute substantially to the generation of spontaneous activity of pacemaker cells and control of heart rate. Given this role in cardiac rhythm, it is natural to expect that defective f-channels (or their molecular correlates HCN4 channels) might be responsible for inheritable forms of cardiac arrhythmogenic diseases. Indeed, the recent search for HCN4-related inheritable arrhythmias has resulted in the finding of four different mutations of the hHcn4 gene, which have been reported to be associated with bradycardia and/or more complex arrhythmic conditions. In neurons, HCN channels display a variety of functions including the regulation of excitability, dendritic integration, plasticity, motor learning, generation of repetitive firing, and others. Defective HCN channels may therefore in principle also contribute to pathological conditions in the nervous system. While full evidence for neuronal HCN channelopathies is not yet available, several indications point to a link between temporal lobe and absence epilepsies and altered distribution of HCN1/HCN2 isoforms. Here we briefly review the current knowledge of HCN-related channelopathies in the heart and the brain.     

CRAC channelopathies

Store-operated Ca²⁺ entry (SOCE) is an important Ca²⁺ influx pathway in many non-excitable and some excitable cells. It is regulated by the filling state of intracellular Ca²⁺ stores, notably the endoplasmic reticulum (ER). Reduction in [Ca²⁺]_ER results in activation of plasma membrane Ca²⁺ channels that mediate sustained Ca²⁺ influx which is required for many cell functions as well as refilling of Ca²⁺ stores. The Ca²⁺ release activated Ca²⁺ (CRAC) channel is the best characterized SOC channel with well-defined electrophysiological properties. In recent years, the molecular components of the CRAC channel, long mysterious, have been defined. ORAI1 (or CRACM1) acts as the pore-forming subunit of the CRAC channel in the plasma membrane. Stromal interaction molecule (STIM) 1 is localized in the ER, senses [Ca²⁺]_ER, and activates the CRAC channel upon store depletion by binding to ORAI1. Both proteins are widely expressed in many tissues in both human and mouse consistent with the widespread prevalence of SOCE and CRAC channel currents in many cells types. CRAC channelopathies in human patients with mutations in STIM1 and ORAI1 are characterized by abolished CRAC channel currents, lack of SOCE and—clinically—immunodeficiency, congenital myopathy, and anhydrotic ectodermal dysplasia. This article reviews the role of ORAI and STIM proteins for SOCE and CRAC channel function in a variety of cell types and tissues and compares the phenotypes of ORAI1 and STIM1-deficient human patients and mice with targeted deletion of Orai and Stim genes.     

HERG1 channelopathies

Human ether a go-go-related gene type 1 (hERG1) K⁺ channels conduct the rapid delayed rectifier K⁺ current and mediate action potential repolarization in the heart. Mutations in KCNH2 (the gene that encodes hERG1) causes LQT2, one of the most common forms of long QT syndrome, a disorder of cardiac repolarization that predisposes affected subjects to ventricular arrhythmia and increases the risk of sudden cardiac death. Hundreds of LQT2-associated mutations have been described, and most cause a loss of function by disrupting subunit folding, assembly, or trafficking of the channel to the cell surface. Loss-of-function mutations in hERG1 channels have also recently been implicated in epilepsy. A single gain-of-function mutation has been described that causes short QT syndrome and cardiac arrhythmia. In addition, up-regulation of hERG1 channel expression has been demonstrated in specific tumors and has been associated with skeletal muscle atrophy in mice.     

Ryanodine receptor channelopathies

Ryanodine receptors (RyR) are intracellular Ca²⁺-permeable channels that provide the sarcoplasmic reticulum Ca²⁺ release required for skeletal and cardiac muscle contractions. RyR1 underlies skeletal muscle contraction, and RyR2 fulfills this role in cardiac muscle. Over the past 20 years, numerous mutations in both RyR isoforms have been identified and linked to skeletal and cardiac diseases. Malignant hyperthermia, central core disease, and catecholaminergic polymorphic ventricular tachycardia have been genetically linked to mutations in either RyR1 or RyR2. Thus, RyR channelopathies are both of interest because they cause significant human diseases and provide model systems that can be studied to elucidate important structure–function relationships of these ion channels.     

KV7 channelopathies

K_V7 voltage-gated potassium channels, encoded by the KCNQ gene family, have caught increasing interest of the scientific community for their important physiological roles, which are emphasized by the fact that four of the five so far identified members are related to different hereditary diseases. Furthermore, these channels prove to be attractive pharmacological targets for treating diseases characterized by membrane hyperexcitability. K_V7 channels are expressed in brain, heart, thyroid gland, pancreas, inner ear, muscle, stomach, and intestines. They give rise to functionally important potassium currents, reduction of which results in pathologies such as long QT syndrome, diabetes, neonatal epilepsy, neuromyotonia, or progressive deafness. Here, we summarize some key traits of K_V7 channels and review how their molecular deficiencies could explain diverse disease phenotypes. We also assess the therapeutic potential of K_V7 channels; in particular, how the activation of K_V7 channels by the compounds retigabine and R-L3 may be useful for treatment of epilepsy or cardiac arrhythmia.     

Cardiac sodium channelopathies

Cardiac sodium channel are protein complexes that are expressed in the sarcolemma of cardiomyocytes to carry a large inward depolarizing current (I_Na) during phase 0 of the cardiac action potential. The importance of I_Na for normal cardiac electrical activity is reflected by the high incidence of arrhythmias in cardiac sodium channelopathies, i.e., arrhythmogenic diseases in patients with mutations in SCN5A, the gene responsible for the pore-forming ion-conducting α-subunit, or in genes that encode the ancillary β-subunits or regulatory proteins of the cardiac sodium channel. While clinical and genetic studies have laid the foundation for our understanding of cardiac sodium channelopathies by establishing links between arrhythmogenic diseases and mutations in genes that encode various subunits of the cardiac sodium channel, biophysical studies (particularly in heterologous expression systems and transgenic mouse models) have provided insights into the mechanisms by which I_Na dysfunction causes disease in such channelopathies. It is now recognized that mutations that increase I_Na delay cardiac repolarization, prolong action potential duration, and cause long QT syndrome, while mutations that reduce I_Na decrease cardiac excitability, reduce electrical conduction velocity, and induce Brugada syndrome, progressive cardiac conduction disease, sick sinus syndrome, or combinations thereof. Recently, mutation-induced I_Na dysfunction was also linked to dilated cardiomyopathy, atrial fibrillation, and sudden infant death syndrome. This review describes the structure and function of the cardiac sodium channel and its various subunits, summarizes major cardiac sodium channelopathies and the current knowledge concerning their genetic background and underlying molecular mechanisms, and discusses recent advances in the discovery of mutation-specific therapies in the management of these channelopathies.     

Sodium channelopathies and pain

Chronic pain often represents a severe, debilitating condition. Up to 10% of the worldwide population are affected, and many patients are poorly responsive to current treatment strategies. Nociceptors detect noxious conditions to produce the sensation of pain, and this signal is conveyed to the CNS by means of action potentials. The fast upstroke of action potentials is mediated by voltage-gated sodium channels, of which nine pore-forming α-subunits (Nav1.1–1.9) have been identified. Heterogeneous functional properties and distinct expression patterns denote specialized functions of each subunit. The Nav1.7 and Nav1.8 subunits have emerged as key molecules involved in peripheral pain processing and in the development of an increased pain sensitivity associated with inflammation and tissue injury. Several mutations in the SCN9A gene encoding for Nav1.7 have been identified as important cellular substrates for different heritable pain syndromes. This review aims to cover recent progress on our understanding of how biophysical properties of mutant Nav1.7 translate into an aberrant electrogenesis of nociceptors. We also recapitulate the role of Nav1.8 for peripheral pain processing and of additional sodium channelopathies which have been linked to disorders with pain as a significant component.     

Genetics and cardiac channelopathies

Sudden cardiac death is a major contributor to mortality in industrialized nations; in fact, it is the cause of more deaths than acquired immune deficiency syndrome, lung and breast cancer, and stroke together. Frequently, the autopsy becomes the principal diagnostic tool because macroscopic and microscopic analyses reveal the underlying cause of death. However, a significant number of sudden cardiac deaths remain unexplained. These cases are referred to as “natural” or arrhythmogenic. In the young, in up to 50% of sudden cardiac death cases, sudden death is the first and only clinical manifestation of an inherited cardiac disease that had remained undetected by conventional clinical investigations. To improve diagnosis, genetic testing has recently been added to these clinical tools. During the last two decades, there has been considerable progress in the understanding about genetics of sudden cardiac death. With that new information, the probands and their family members can make an informed decision regarding their care and know whether and to what extent they are at risk of suffering from the disease. Thus, genetic technology and expertise have become essential for the diagnosis of some forms of inherited cardiac diseases and to provide a basis for subsequent prevention strategies. This review focuses on recent advances in the understanding of cardiopathies owing to genetic investigations.     

Transient receptor potential channelopathies

In the past years, several hereditary diseases caused by defects in transient receptor potential channels (TRP) genes have been described. This review summarizes our current knowledge about TRP channelopathies and their possible pathomechanisms. Based on available genetic indications, we will also describe several putative pathological conditions in which (mal)function of TRP channels could be anticipated.     

Mitochondrial channelopathies in aging

Defects in ion channels (channelopathies) are increasingly found in a large spectrum of human pathologies including aging. Mutations in genes encoding ion channel proteins, which disrupt channel function, are the most commonly identified cause of channelopathies. Mutations in associated proteins, alterations in the expression of ion channels, or changes in the activity of non-mutated channel genes or associated proteins can also produce acquired channelopathies. Mitochondria, the powerhouse of the cells, are considered to be the most important cellular organelles to contribute to aging mainly because of their role in the production of reactive oxygen species in the initiation of apoptotic cell remodeling and in efficient ATP synthesis. During the past 50 years, multiple ion channels or transporters have been found in mitochondria, and the relationship between the activity of these channels and cellular aging, as well as the overall cellular biological function, has been intensively studied in a number of cell types and animal models. In this review, we discuss the better characterized mitochondrial ion channels whose dysfunction (mitochondrial channelopathies) may affect or accelerate the aging processes. These channels include the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)), Ca(2+) transporters, voltage-dependent anion channel, and the mitochondrial permeability transition pore (mitoPTP).     

Developments in autoimmune channelopathies

Autoimmune forms of encephalopathy have become a hot topic in neurology. These conditions are now known to be associated with antibodies to neuronal or glial cell surface proteins, such as ion channels, receptors or associated proteins. The most common conditions are a form of limbic encephalitis associated with antibodies to voltage-gated potassium channel complex proteins, and a more complex encephalopathy with antibodies to the NR1 subunit of the N-methyl-D aspartate receptor, a class of glutamate receptor. In addition, a very inflammatory disease of the nervous system, neuromyelitis optica, associated with blindness as well as spinal cord damage, can be distinguished by the presence of antibodies to aquaporin-4, a water channel. Many other antibodies are now being identified, but their frequencies are less clear. Most importantly, these new antibody-mediated diseases are being identified in patients of all ages, and in the majority of cases, the patients improve substantially with immunotherapies.     

KATP channelopathies in the pancreas

Adenosine-triphosphate-sensitive potassium channels (K_ATP) are regulated by adenosine nucleotides, and, thereby, couple cellular metabolism with electrical activity in multiple tissues including the pancreatic β-cell. The critical involvement of K_ATP in insulin secretion is confirmed by the demonstration that inactivating and activating mutations in K_ATP underlie persistent hyperinsulinemia and neonatal diabetes mellitus, respectively, in both animal models and humans. In addition, a common variant in K_ATP represents a risk factor in the etiology of type 2 diabetes. This review focuses on the mechanistic basis by which K_ATP mutations underlie insulin secretory disorders and the implications of these findings for successful clinical intervention.     

Nicotinic receptor channelopathies and epilepsy

Characterized by sudden episodes called seizures, epilepsy was recognized long ago as a neurological disorder that can have multiple forms ranging from benign to life threatening depending upon its severity. Although several evidences indicated that genes play an important role in at least half of the patients, it is only with the advances in molecular biology and genetics that the puzzle about oligogenic and monogenic epilepsies slowly starts to unfold. The finding of an association between a monogenic form of epilepsy and a mutation in the gene encoding the neuronal nicotinic acetylcholine receptor subunit CHRNA4 marked, in 1995, a turning point in our understanding of epilepsy. It also marked the first step towards the today widely acknowledged concept of epilepsies as channelopathies. Several mutations in nicotinic acetylcholine receptor genes have, since then, been identified, and the functional properties of these mutated receptors were characterized. In this work, we review, in the light of the latest discoveries, the effects caused by the mutations on the physiological properties of the receptors and the impact of such mutations on neuronal network functions.     

Truncation of murine CaV1.2 at Asp 1904 increases CaV1.3 expression in embryonic atrial cardiomyocytes

Cardiac Ca_V1.2 channels play a critical role in cardiac function. It has been proposed that the carboxyl-terminal intracellular tail of the Ca_V1.2 channel is the target of Ca²⁺-dependent and Ca²⁺-independent regulation of the channel. Recent studies on C-terminal truncated forms of the Ca_V1.2 channel reported neonatal death, reduced Ca_V1.2 current, and failure of β-adrenergic stimulation of these channels in ventricular cardiomyocytes (CMs). Here, we used atrial CMs at embryonic day 18.5 that expressed a C-terminal truncated form of the Ca_V1.2 channel (Stop/Stop). Surprisingly, the atrial CMs showed robust L-type Ca²⁺ currents which could be stimulated by forskolin, an activator of adenylyl cyclase. These currents exhibited a left-ward shift in the voltage-dependent activation curve and a reduced sensitivity to the Ca²⁺ channel blocker isradipine as compared to currents in wild-type atrial CMs. RT-PCR analysis revealed normal levels of mRNA for the Ca_V1.2 channel but a twofold increase in the level of mRNA for the Ca_V1.3 channel in the Stop/Stop atrium as compared to wild-type atrium. A Western blot analysis indicated an increase of Ca_V1.3 protein in the Stop/Stop atrium. We suggest that, in contrast to Stop/Stop ventricular CMs, Stop/Stop atrial CMs can compensate the functional loss of the truncated Ca_V1.2 channel with an upregulation of the Ca_V1.3 channel.