Roles and regulation of the cardiac sodium channel Nav1.5: Recent insights from experimental studies

During the past decade, Nav1.5, the main voltage-gated Na+ channel in the heart, has been shown to be involved in many cardiac diseases. Genetic variants in the gene SCN5A, encoding Nav1.5, have been linked to various cardiac phenotypes, such as the congenital and acquired long QT syndromes, Brugada syndrome, conduction slowing, sick sinus syndrome, atrial fibrillation, and even cases of dilated cardiomyopathy. This unexpected phenotypic diversity may reflect that Nav1.5 is not only restricted to the initiation of the action potential and rapid cardiac conduction, but may also be involved in other, not-yet elucidated, functions. Despite the fact that our understanding of the regulation of expression, localization, and function of Nav1.5 is deepening, we are still far from a comprehensive view. Much of our current knowledge has been obtained by carrying out experiments using "cellular expression systems”, e.g. host cells expressing exogenous Nav1.5. Although very informative, these techniques are limited, in that Nav1.5 is not expressed in the physiological cellular environment of a cardiac cell. Recently, however, there have been several studies published which used approaches closer to "normal” or pathological physiology. In an attempt to summarize recently published data, this article will review the phenotypes of genetically-modified mouse strains where Nav1.5 expression and activity are directly or indirectly modified, as well as the regulation of Nav1.5 function using native cardiac myocytes. Despite obvious limitations, the reviewed studies provide an overview of the complex multi-factorial and multi-protein regulation of Nav1.

The NCCR TransCure: An Incubator for Interdisciplinary Research

The National Center of Competence in Research (NCCR) TransCure, funded by the Swiss National Science Foundation and the University of Bern, was active from 2010 to 2022. It provided unique research and educational framework in the membrane transporter and ion channel field. Thanks to an interdisciplinary approach comprising physiology, structural biology, and chemistry, in parallel to a rich offer in complementary areas such as education and technology transfer, the network achieved outstanding scientific results and contributed to the education of young scientists. In this review, we present the main features and milestones of the NCCR TransCure

Exploring the Oligomerization of Nav1.5 and Its Implication for the Dominant-Negative Effect

Clusters of the α-subunit of voltage-gated sodium (Nav) channels have been observed in various tissues and are recognized as key regulators of cellular excitability and action potential propagation. In cardiomyocytes, the most abundant Nav α-subunit, Nav1.5, is expressed at specialized membrane microdomains within the intercalated disk and lateral membrane. Although Nav1.5 remodeling within these microdomains could cause abnormal cardiac phenotypes, the molecular mechanisms underlying single-molecule redistribution and biophysical cooperativity of Nav1.5 remain not fully understood. This review summarizes the current knowledge on the oligomerization of Nav1.5. In particular, direct α–α-subunit interactions and oligomerization through intermediary proteins such as Navβ-subunits and 14–3–3 proteins are discussed. The possible implication of Nav1.5 oligomerization in the coupled gating in cis and trans conformations as well as in the dominant-negative effect is reviewed

Regulation of the cardiac sodium channel Nav1.5 by utrophin in dystrophin-deficient mice

Aims Duchenne muscular dystrophy (DMD) is a severe striated muscle disease due to the absence of dystrophin. Dystrophin deficiency results in dysfunctional sodium channels and conduction abnormalities in hearts of mdx mice. Disease progression in the mdx mouse only modestly reflects that of DMD patients, possibly due to utrophin up-regulation. Here, we investigated mice deficient in both dystrophin and utrophin [double knockout (DKO)] to assess the role of utrophin in the regulation of the cardiac sodium channel (Nav1.5) in mdx mice. Methods and results Co-immunoprecipitation studies in HEK293 cells showed that utrophin interacts with Nav1.5 via syntrophin proteins, an interaction abolished by deletion of the PDZ (PSD-95, Dlg, and Zona occludens) domain-binding motif of Nav1.5. We also provide evidence for such interaction in mouse heart using Nav1.5 C-terminus fusion proteins. In hearts of DKO mice, Nav1.5 protein levels were decreased by 25 ± 8%, together with a 42 ± 12% reduction of syntrophins compared with mdx, where utrophin was up-regulated by 52 ± 9% compared with C57BL/10 control mice. Sodium current was found to be reduced by 41 ± 5% in DKO cardiomyocytes compared with mdx, representing a loss of 63 ± 3% when compared with C57BL/10 wild-type control mice. Decreased Nav1.5 protein and current in DKO were reflected in a significant slowing of 27 ± 6% of maximal upstroke velocity of the cardiac action potential compared with mdx. Conclusion Utrophin plays a central role in the regulation of Nav1.5 in mdx mice. These findings provide support for therapeutic strategies aimed at overexpressing utrophin in the hopes of reducing cardiac pathology in DMD patient

Electrophysiological properties of mouse and epitope-tagged human cardiac sodium channel Nav1.5 expressed in HEK293 cells

Background: The pore-forming subunit of the cardiac sodium channel, Nav1.5, has been previously found to be mutated in genetically determined arrhythmias. Nav1.5 associates with many proteins that regulate its function and cellular localisation. In order to identify more in situ Nav1.5 interacting proteins, genetically-modified mice with a high-affinity epitope in the sequence of Nav1.5 can be generated. Methods: In this short study, we (1) compared the biophysical properties of the sodium current (INa) generated by the mouse Nav1.5 (mNav1.5) and human Nav1.5 (hNav1.5) constructs that were expressed in HEK293 cells, and (2) investigated the possible alterations of the biophysical properties of the human Nav1.5 construct that was modified with specific epitopes. Results: The biophysical properties of mNav1.5 were similar to the human homolog. Addition of epitopes either up-stream of the N-terminus of hNav1.5 or in the extracellular loop between the S5 and S6 transmembrane segments of domain 1, significantly decreased the amount of INa and slightly altered its biophysical properties. Adding green fluorescent protein (GFP) to the N-terminus did not modify any of the measured biophysical properties of hNav1.5. Conclusions: These findings have to be taken into account when planning to generate genetically-modified mouse models that harbour specific epitopes in the gene encoding mNav1.5