Reduced expression and abnormal localization of the K(ATP) channel subunit SUR2A in patients with familial hypokalemic periodic paralysis

SUR2A as a base for cardioprotective therapeutic strategies

BACKGROUND

ATP-sensitive K⁺ (K_ATP) channels link the metabolic state of the cell with membrane excitability and SUR2A serves as a regulatory subunit of sarcolemmal K_ATP channels. The aim of the present study was to review SUR2A-mediated cardioprotection.

METHODS AND RESULTS

A related literature search in PubMed, Scopus, Web of Science, Google Scholar, and Science direct was performed. Levels of SUR2A regulate number of fully assembled K_ATP channels in the sarcolemma. Increased numbers of sarcolemmal K_ATP channels protect cardiomyocytes against different types of stress by improving the timing of K_ATP channels opening, but, also, by catalyzing ATP production in subsarcolemmal space. Fully-assembled sarcolemmal K_ATP channels protein complex contain ATP-producing enzymes in addition to channel subunits, SUR2A and Kir6.2. An increase in the number of fully-assembled channels results in increased levels of ATP-producing enzymes and subsarcolemmal ATP, which is beneficial in ischemia. Expression of SUR2A is regulated by diverse mechanisms, including AMPK, PI3K/Akt, and ERK1/2 as well as intracellular levels of NAD+/NADH and ATP. There are many compounds and treatments that can be used to regulate SUR2A and some of them seem to be clinically viable options. The most suitable medication to use to increase SUR2A and confer cardioprotection in the clinical setting seems to be nicotinamide. It is one of the safest compounds used in clinical practice and all pre-clinical studies demonstrated that it is an efficient cardioprotective agent.

CONCLUSIONS

Taken all together, SUR2A-based cardioprotection is a likely efficient and safe cardioprotective strategy that can be quickly introduced into clinical practice.

Altered KCa3.1 expression following burn injury and the therapeutic potential of TRAM-34 in post-burn hypertrophic scar formation

Hypertrophic scars are the most common post-burn complications characterized by fibroblast proliferation and excessive extracellular matrix deposition. The intermediate-conductance Ca²⁺-activated K⁺ channel (K_Ca3.1) mediates fibroblast activation, resulting in several fibrotic diseases; however, this channel's role in the formation of post-burn hypertrophic skin scars remains unknown. Herein, we investigated the role of K_Ca3.1 and the therapeutic potential of TRAM-34, a selective inhibitor of K_Ca3.1, in hypertrophic skin scar formation following burn injury. Cytosolic Ca²⁺ levels, the expression of K_Ca3.1 and hypertrophic markers, and the proliferation of skin fibroblasts obtained directly from patients with third-degree burns who consequently developed post-burn hypertrophic scars were assessed. The anti-fibrotic effect of K_Ca3.1 inhibition by TRAM-34 was evaluated in vitro (fibroblasts) and in vivo (mouse burn models). Fibroblasts from burn wounds exhibited remarkably higher levels of cytosolic Ca²⁺ than normal cells. K_Ca3.1 expression was markedly higher in the membrane fraction but lower in the cytosolic fraction of burn wound fibroblasts than in normal cells. Selective inhibition of K_Ca3.1 by TRAM-34 markedly reduced not only the proliferation of burn wound fibroblasts but also the expression of hypertrophic markers in these cells. Anti-scarring molecular, histological, and visual effects of TRAM-34 were confirmed in murine burn models. Altered subcellular expression of K_Ca3.1 is a novel mechanism underlying the cellular response to burn injury. Our results suggest that selective inhibition of K_Ca3.1 by TRAM-34 has therapeutic potential against post-burn hypertrophic scar formation.

Calpastatin-Mediated Inhibition of Calpain Ameliorates Skin Scar Formation after Burn Injury

Hypertrophic scars, the most common complication of burn injuries, are characterized by excessive deposition of fibroblast-derived extracellular matrix proteins. Calpain, a calcium-dependent protease, is involved in the fibroblast proliferation and extracellular matrix production observed in certain fibrotic diseases. However, its role in the formation of post-burn hypertrophic skin scars remains largely unknown. Here, calpain expression and activity were assessed in skin fibroblasts obtained directly from patients with third-degree burns, who consequently developed post-burn hypertrophic scars. Furthermore, the antifibrotic effect of calpastatin, an endogenous calpain inhibitor, was evaluated in human fibroblasts and a murine burn model. The activity, mRNA levels, and protein levels of calpain were markedly higher in fibroblasts from the burn wounds of patients than in normal cells. Selective calpain inhibition by calpastatin markedly reduced not only the proliferation of burn-wound fibroblasts but also the mRNA and protein expression of calpain, transforming growth factor-beta 1, α-smooth muscle actin, type I and type III collagens, fibronectin, and vimentin in burn-wound fibroblasts. The anti-scarring effects of calpastatin were validated using a murine burn model by molecular, histological, and visual analyses. This study demonstrates the pathological role of calpain and the antifibrotic effect of calpastatin via calpain inhibition in post-burn hypertrophic scar formation.

The large-conductance calcium-activated potassium channel holds the key to the conundrum of familial hypokalemic periodic paralysis

Purpose

Familial hypokalemic periodic paralysis (HOKPP) is an autosomal dominant channelopathy characterized by episodic attacks of muscle weakness and hypokalemia. Mutations in the calcium channel gene, CACNA1S, or the sodium channel gene, SCN4A, have been found to be responsible for HOKPP; however, the mechanism that causes hypokalemia remains to be determined. The aim of this study was to improve the understanding of this mechanism by investigating the expression of calcium-activated potassium (K_Ca) channel genes in HOKPP patients.

Methods

We measured the intracellular calcium concentration with fura-2-acetoxymethyl ester in skeletal muscle cells of HOKPP patients and healthy individuals. We examined the mRNA and protein expression of KCa channel genes (KCNMA1, KCNN1, KCNN2, KCNN3, and KCNN4) in both cell types.

Results

Patient cells exhibited higher cytosolic calcium levels than normal cells. Quantitative reverse transcription polymerase chain reaction analysis showed that the mRNA levels of the K_Ca channel genes did not significantly differ between patient and normal cells. However, western blot analysis showed that protein levels of the KCNMA1 gene, which encodes K_Ca1.1 channels (also called big potassium channels), were significantly lower in the membrane fraction and higher in the cytosolic fraction of patient cells than normal cells. When patient cells were exposed to 50 mM potassium buffer, which was used to induce depolarization, the altered subcellular distribution of BK channels remained unchanged.

Conclusion

These findings suggest a novel mechanism for the development of hypokalemia and paralysis in HOKPP and demonstrate a connection between disease-associated mutations in calcium/sodium channels and pathogenic changes in nonmutant potassium channels.

Channelopathies

Channelopathies are a heterogeneous group of disorders resulting from the dysfunction of ion channels located in the membranes of all cells and many cellular organelles. These include diseases of the nervous system (e.g., generalized epilepsy with febrile seizures plus, familial hemiplegic migraine, episodic ataxia, and hyperkalemic and hypokalemic periodic paralysis), the cardiovascular system (e.g., long QT syndrome, short QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia), the respiratory system (e.g., cystic fibrosis), the endocrine system (e.g., neonatal diabetes mellitus, familial hyperinsulinemic hypoglycemia, thyrotoxic hypokalemic periodic paralysis, and familial hyperaldosteronism), the urinary system (e.g., Bartter syndrome, nephrogenic diabetes insipidus, autosomal-dominant polycystic kidney disease, and hypomagnesemia with secondary hypocalcemia), and the immune system (e.g., myasthenia gravis, neuromyelitis optica, Isaac syndrome, and anti-NMDA [N-methyl-D-aspartate] receptor encephalitis). The field of channelopathies is expanding rapidly, as is the utility of molecular-genetic and electrophysiological studies. This review provides a brief overview and update of channelopathies, with a focus on recent advances in the pathophysiological mechanisms that may help clinicians better understand, diagnose, and develop treatments for these diseases.

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.