Gating of the HypoPP-1 mutations: I. Mutant-specific effects and cooperativity

Memory in Ion Channel Kinetics

Ion channels are transport proteins present in the lipid bilayers of biological membranes. They are involved in many physiological processes, such as the generation of nerve impulses, hormonal secretion, and heartbeat. Conformational changes in the ion channel-forming protein allow the opening or closing of pores to control the ionic flux through the cell membranes. The opening and closing of the ion channel have been classically treated as a random kinetic process, known as a Markov process. Here the time the channel remains in a given state is assumed to be independent of the condition it had in the previous state. More recently, however, several studies have shown that this process is not random but a deterministic one, where both the open and closed dwell-times and the ionic current flowing through the channel are history-dependent. This property is called long memory or long-range correlation. However, there is still much controversy regarding how this memory originates, which region of the channel is responsible for this property, and which models could best reproduce the memory effect. In this article, we provide a review of what is, where it is, its possible origin, and the mathematical methods used to analyze the long-term memory present in the kinetic process of ion channels.

Channelopathies of skeletal muscle excitability

Familial disorders of skeletal muscle excitability were initially described early in the last century and are now known to be caused by mutations of voltage-gated ion channels. The clinical manifestations are often striking, with an inability to relax after voluntary contraction (myotonia) or transient attacks of severe weakness (periodic paralysis). An essential feature of these disorders is fluctuation of symptoms that are strongly impacted by environmental triggers such as exercise, temperature, or serum K(+) levels. These phenomena have intrigued physiologists for decades, and in the past 25 years the molecular lesions underlying these disorders have been identified and mechanistic studies are providing insights for therapeutic strategies of disease modification. These familial disorders of muscle fiber excitability are "channelopathies" caused by mutations of a chloride channel (ClC-1), sodium channel (NaV1.4), calcium channel (CaV1.1), and several potassium channels (Kir2.1, Kir2.6, and Kir3.4). This review provides a synthesis of the mechanistic connections between functional defects of mutant ion channels, their impact on muscle excitability, how these changes cause clinical phenotypes, and approaches toward therapeutics.

Pathophysiological role of omega pore current in channelopathies

In voltage-gated cation channels, a recurrent pattern for mutations is the neutralization of positively charged residues in the voltage-sensing S4 transmembrane segments. These mutations cause dominant ion channelopathies affecting many tissues such as brain, heart, and skeletal muscle. Recent studies suggest that the pathogenesis of associated phenotypes is not limited to alterations in the gating of the ion-conducting alpha pore. Instead, aberrant so-called omega currents, facilitated by the movement of mutated S4 segments, also appear to contribute to symptoms. Surprisingly, these omega currents conduct cations with varying ion selectivity and are activated in either a hyperpolarized or depolarized voltage range. This review gives an overview of voltage sensor channelopathies in general and focuses on pathogenesis of skeletal muscle S4 disorders for which current knowledge is most advanced.

Hypokalemic periodic paralysis; two different genes responsible for similar clinical manifestations

Primary hypokalemic periodic paralysis (HOKPP) is an autosomal dominant disorder manifesting as recurrent periodic flaccid paralysis and concomitant hypokalemia. HOKPP is divided into type 1 and type 2 based on the causative gene. Although 2 different ion channels have been identified as the molecular genetic cause of HOKPP, the clinical manifestations between the 2 groups are similar. We report the cases of 2 patients with HOKPP who both presented with typical clinical manifestations, but with mutations in 2 different genes (CACNA1Sp.Arg528His and SCN4A p.Arg672His). Despite the similar clinical manifestations, there were differences in the response to acetazolamide treatment between certain genotypes of SCN4A mutations and CACNA1S mutations. We identified p.Arg672His in the SCN4A gene of patient 2 immediately after the first attack through a molecular genetic testing strategy. Molecular genetic diagnosis is important for genetic counseling and selecting preventive treatment.

Recent advances in the pathogenesis and drug action in periodic paralyses and related channelopathies

The periodic paralysis (PP) are rare autosomal-dominant disorders associated to mutations in the skeletal muscle sodium, calcium, and potassium channel genes characterized by muscle fiber depolarization with un-excitability, episodes of weakness with variations in serum potassium concentrations. Recent advances in thyrotoxic PP and hypokalemic PP (hypoPP) confirm the involvement of the muscle potassium channels in the pathogenesis of the diseases and their role as target of action for drugs of therapeutic interest. The novelty in the gating pore currents theory help to explain the disease symptoms, and open the possibility to more specifically target the disease. It is now known that the fiber depolarization in the hypoPP is due to an unbalance between the novel identified depolarizing gating pore currents (I_gp) carried by protons or Na⁺ ions flowing through aberrant alternative pathways of the mutant subunits and repolarizing inwardly rectifying potassium channel (Kir) currents which also includes the ATP-sensitive subtype. Abnormal activation of the I_gp or deficiency in the Kir channels predispose to fiber depolarization. One pharmacological strategy is based on blocking the I_gp without affecting normal channel gating. It remains safe and effective the proposal of targeting the K_ATP, Kir channels, or BK channels by drugs capable to specifically open at nanomolar concentrations the skeletal muscle subtypes with less side effects.

Skeletal muscle channelopathies: new insights into the periodic paralyses and nondystrophic myotonias

PURPOSE OF REVIEW

To summarize advances in our understanding of the clinical phenotypes, genetics, and molecular pathophysiology of the periodic paralyses, the nondystrophic myotonias, and other muscle channelopathies.

RECENT FINDINGS

The number of pathogenic mutations causing periodic paralysis, nondystrophic myotonias, and ryanodinopathies continues to grow with the advent of exon hierarchy analysis strategies for genetic screening and better understanding and recognition of disease phenotypes. Recent studies have expanded and clarified the role of gating pore current in channelopathy pathogenesis. It has been shown that the gating pore current can account for the molecular and phenotypic diseases observed in the muscle sodium channelopathies, and, given that homologous residues are affected in mutations of calcium channels, it is possible that pore leak represents a pathomechanism applicable to many channel diseases. Improvements in treatment of the muscle channelopathies are on the horizon. A randomized controlled trial has been initiated for the study of mexiletine in nondystrophic myotonias. The class IC antiarrhythmia drug flecainide has been shown to depress ventricular ectopy and improve exercise capacity in patients with Andersen-Tawil syndrome.

SUMMARY

Recent studies have expanded our understanding of gating pore current as a disease-causing mechanism in the muscle channelopathies and have allowed new correlations to be drawn between disease genotype and phenotype.

A critical GxxxA motif in the gamma6 calcium channel subunit mediates its inhibitory effect on Cav3.1 calcium current

The eight members of the calcium channel gamma subunit family are integral membrane proteins that regulate the expression and behaviour of voltage and ligand gated ion channels. While a subgroup consisting of gamma(2), gamma(3), gamma(4) and gamma(8) (the TARPs) modulate AMPA receptor localization and function, the gamma(1) and gamma(6) subunits conform to the original description of these proteins as regulators of voltage gated calcium channels. We have previously shown that the gamma(6) subunit is highly expressed in atrial myocytes and that it is capable of acting as a negative modulator of low voltage activated calcium current. In this study we extend our understanding of gamma(6) subunit modulation of low voltage activated calcium current. Using engineered chimeric constructs, we demonstrate that the first transmembrane domain (TM1) of gamma(6) is necessary for its inhibitory effect on Cav3.1 current. Mutational analysis is then used to identify a unique GxxxA motif within TM1 that is required for the function of the subunit strongly suggesting the involvement of helix-helix interactions in its effects. Results from co-immunoprecipitation experiments confirm a physical association of gamma(6) with the Cav3.1 channel in both HEK cells and atrial myocytes. Single channel analysis reveals that binding of gamma(6) reduces channel availability for activation. Taken together, the results of this study provide both a molecular and a mechanistic framework for understanding the unique ability of the gamma(6) calcium channel subunit to modulate low voltage activated (Cav3.1) calcium current density.

Skeletal-muscle channelopathies: periodic paralysis and nondystrophic myotonias

PURPOSE OF REVIEW

To provide a current review of clinical phenotypes, genetics, molecular pathophysiology, and electro-diagnostic testing strategies of periodic paralysis and nondystrophic myotonias.

RECENT FINDINGS

The number of pathogenic mutations causing periodic paralysis and nondystrophic myotonias continues to increase. Important insight into the molecular pathogenesis of muscle sodium channelopathies has been revealed by the finding of 'leaky' closed sodium channels. Previously, alterations in sodium-channel activation or inactivation have been identified as important disease mechanisms. The recent discovery that substitutions of key arginine residues in the voltage-sensing segment of the channel may lead to a 'pore leak' when the channel is closed suggests a new mechanism. Since similar mutations exist in corresponding positions of other channels, this mechanism may apply to other channel diseases. The recognition of different electrophysiological patterns that are specific to muscle ion-channel genotypes will be useful in diagnosis and in guiding genetic testing. Recent studies demonstrate that magnetic resonance imaging may be used to detect intramuscular accumulation of sodium during episodes of weakness.

SUMMARY

Recent advances have refined our ability to make a precise molecular diagnosis in muscle channelopathies. The description of a pore leak with voltage-sensor mutations may represent a new disease mechanism.

Gating of the HypoPP-1 mutations: II. Effects of a calcium-channel agonist BayK 8644

L-type calcium-channel mutations causing hypokalemic periodic paralysis type 1 (HypoPP-1) have pronounced "loss-of-function" features and stabilize the less-selective second open state O(2), as we demonstrated in the companion paper. Here, we compared the effects of the L-type calcium-channel activator (+/-)BayK 8644 (BayK) on the heterologously expressed wild-type (WT) calcium channel, rabbit Cav1.2 HypoPP-1 analogs, and two double mutants (R650H/R1362H, R650H/R1362G). Our goal was to elucidate (1) whether the "loss-of-function" in HypoPP-1 can be compensated by BayK application, (2) how the less-selective open state is affected by BayK in WT and HypoPP-1 mutants, as well as (3) to gain an insight into BayK mechanism of action. Ionic currents were examined by whole-cell patch-clamp and analyzed by the global-fitting procedure. Our results imply that (1) BayK promotes channel activation, but equalized the differences among the WT and mutants, thus attenuating HypoPP-related effects on activation and deactivation; (2) BayK binds to the first open state O(1), and then serves as a catalyst for O(2) formation; (3) binding of BayK is impaired in the HypoPP mutants, thus affecting the formation of the less-selective second open state; (4) BayK affects cooperativity between the single HypoPP-1 mutations at all stages of the channel gating; and (5) BayK favoring of O(2) lowers calcium-channel selectivity.