Altered Expression of Zonula occludens-1 Affects Cardiac Na+ Channels and Increases Susceptibility to Ventricular Arrhythmias

Zonula occludens-1 (ZO-1) is an intracellular scaffolding protein that orchestrates the anchoring of membrane proteins to the cytoskeleton in epithelial and specialized tissue including the heart. There is clear evidence to support the central role of intracellular auxiliary proteins in arrhythmogenesis and previous studies have found altered ZO-1 expression associated with atrioventricular conduction abnormalities. Here, using human cardiac tissues, we identified all three isoforms of ZO-1, canonical (Transcript Variant 1, TV1), CRA_e (Transcript Variant 4, TV4), and an additionally expressed (Transcript Variant 3, TV3) in non-failing myocardium. To investigate the role of ZO-1 on ventricular arrhythmogenesis, we generated a haploinsufficient ZO-1 mouse model (ZO-1^+/-). ZO-1^+/- mice exhibited dysregulated connexin-43 protein expression and localization at the intercalated disc. While ZO-1^+/- mice did not display abnormal cardiac function at baseline, adrenergic challenge resulted in rhythm abnormalities, including premature ventricular contractions and bigeminy. At baseline, ventricular myocytes from the ZO-1^+/- mice displayed prolonged action potential duration and spontaneous depolarizations, with ZO-1^+/- cells displaying frequent unsolicited (non-paced) diastolic depolarizations leading to spontaneous activity with multiple early afterdepolarizations (EADs). Mechanistically, ZO-1 deficient myocytes displayed a reduction in sodium current density (I_Na) and an increased sensitivity to isoproterenol stimulation. Further, ZO-1 deficient myocytes displayed remodeling in I_Ca current, likely a compensatory change. Taken together, our data suggest that ZO-1 deficiency results in myocardial substrate susceptible to triggered arrhythmias.

The effects of anti-hypertensive drugs and the mechanism of hypertension in vascular smooth muscle cell-specific ATP2B1 knockout mice

ATP2B1 is a gene associated with hypertension. We reported previously that mice lacking ATP2B1 in vascular smooth muscle cells (VSMC ATP2B1 KO mice) exhibited high blood pressure and increased intracellular calcium concentration. The present study was designed to investigate whether lack of the ATP2B1 gene causes a higher response to calcium channel blockers (CCBs) than to other types of anti-hypertensive drugs. Both VSMC ATP2B1 KO and control mice were administered anti-hypertensive drugs while monitoring blood pressure shifts. We also examined the association of nitric oxide synthase (NOS) activity in those mice to investigate whether another mechanism of hypertension existed. VSMC ATP2B1 KO mice exhibited significantly greater anti-hypertensive effects with a single injection of nicardipine, but the effects of an angiotensin II receptor blocker (ARB), an α-blocker and amlodipine on blood pressure were all similar to control mice. However, long-term treatment with amlodipine, but not an ARB, significantly decreased the blood pressure of KO mice compared with control mice. Both mRNA and protein expression levels of the L-type calcium channel were significantly upregulated in KO VSMCs. There were no alterations in neural NOS protein expression of VSMCs or in urinary NO production between the two groups. VSMC ATP2B1 KO mice had a higher response to CCBs for blood pressure-lowering effects than other anti-hypertensive drugs. These results mean that increased intracellular calcium concentration in VSMCs due to lack of ATP2B1 and subsequent activation of L-type calcium channels mainly affects blood pressure and suggests increased susceptibility to CCBs in this type of hypertension.

Variants of Transient Receptor Potential Melastatin Member 4 in Childhood Atrioventricular Block

Background

Transient receptor potential melastatin member 4 (TRPM4) is a nonselective cation channel. TRPM4 mutations have been linked to cardiac conduction disease and Brugada syndrome. The mechanisms underlying TRPM4‐dependent conduction slowing are not fully understood. The aim of this study was to characterize TRPM4 genetic variants found in patients with congenital or childhood atrioventricular block.

Methods and Results

Ninety‐one patients with congenital or childhood atrioventricular block were screened for candidate genes. Five rare TRPM4 genetic variants were identified and investigated. The variants were expressed heterologously in HEK293 cells. Two of the variants, A432T and A432T/G582S, showed decreased expression of the protein at the cell membrane; inversely, the G582S variant showed increased expression. Further functional characterization of these variants using whole‐cell patch‐clamp configuration showed a loss of function and a gain of function, respectively. We hypothesized that the observed decrease in expression was caused by a folding and trafficking defect. This was supported by the observation that incubation of these variants at lower temperature partially rescued their expression and function. Previous studies have suggested that altered SUMOylation of TRPM4 may cause a gain of function; however, we did not find any evidence that supports SUMOylation as being directly involved for the gain‐of‐function variant.

Conclusions

This study underpins the role of TRPM4 in the cardiac conduction system. The loss‐of‐function variants A432T/G582S found in 2 unrelated patients with atrioventricular block are most likely caused by misfolding‐dependent altered trafficking. The ability to rescue this variant with lower temperature may provide a novel use of pharmacological chaperones in treatment strategies.

The PDZ motif of the α1C subunit is not required for surface trafficking and adrenergic modulation of CaV1.2 channel in the heart

Voltage-gated Ca(2+) channels play a key role in initiating muscle excitation-contraction coupling, neurotransmitter release, gene expression, and hormone secretion. The association of CaV1.2 with a supramolecular complex impacts trafficking, localization, turnover, and, most importantly, multifaceted regulation of its function in the heart. Several studies hint at an important role for the C terminus of the α1C subunit as a hub for multidimensional regulation of CaV1.2 channel trafficking and function. Recent studies have demonstrated an important role for the four-residue PDZ binding motif at the C terminus of α1C in interacting with scaffold proteins containing PDZ domains, in the subcellular localization of CaV1.2 in neurons, and in the efficient signaling to cAMP-response element-binding protein in neurons. However, the role of the α1C PDZ ligand domain in the heart is not known. To determine whether the α1C PDZ motif is critical for CaV1.2 trafficking and function in cardiomyocytes, we generated transgenic mice with inducible expression of an N-terminal FLAG epitope-tagged dihydropyridine-resistant α1C with the PDZ motif deleted (ΔPDZ). These mice were crossed with α-myosin heavy chain reverse transcriptional transactivator transgenic mice, and the double-transgenic mice were fed doxycycline. The ΔPDZ channels expressed, trafficked to the membrane, and supported robust excitation-contraction coupling in the presence of nisoldipine, a dihydropyridine Ca(2+) channel blocker, providing functional evidence that they appropriately target to dyads. The ΔPDZ Ca(2+) channels were appropriately regulated by isoproterenol and forskolin. These data indicate that the α1C PDZ motif is not required for surface trafficking, localization to the dyad, or adrenergic stimulation of CaV1.2 in adult cardiomyocytes.