Abstract
Background: In cardiac ventricular muscle cells, the presence of voltage-gated sodium channels Nav1.5 at the lateral membrane depends in part on the interaction between the dystrophin–syntrophin complex and the Nav1.5 C-terminal PDZ-domain-binding sequence Ser-Ile-Val (SIV motif). α1-Syntrophin, a PDZ-domain adaptor protein, mediates the interaction between Nav1.5 and dystrophin at the lateral membrane of cardiac cells. Using the cell-attached patch-clamp approach on cardiomyocytes expressing Nav1.5 in which the SIV motif is deleted (ΔSIV), sodium current (INa) recordings from the lateral membrane revealed a SIV-motif-independent INa. Since immunostaining has suggested that Nav1.5 is expressed in transverse (T-) tubules, this remaining INa might be carried by channels in the T-tubules. Of note, a recent study using heterologous expression systems showed that α1-syntrophin also interacts with the Nav1.5 N-terminus, which may explain the SIV-motif independent INa at the lateral membrane of cardiomyocytes.Aim: To address the role of α1-syntrophin in regulating the INa at the lateral membrane of cardiac cells.Methods and Results: Patch-clamp experiments in cell-attached configuration were performed on the lateral membranes of wild-type, α1-syntrophin knockdown, and ΔSIV ventricular mouse cardiomyocytes. Compared to wild-type, a reduction of the lateral INa was observed in myocytes from α1-syntrophin knockdown hearts. Similar to ΔSIV myocytes, a remaining INa was still recorded. In addition, cell-attached INa recordings from lateral membrane did not differ significantly between non-detubulated and detubulated ΔSIV cardiomyocytes. Lastly, we obtained evidence suggesting that cell-attached patch-clamp experiments on the lateral membrane cannot record currents carried by channels in T-tubules such as calcium channels.Conclusion: Altogether, these results suggest the presence of a sub-pool of sodium channels at the lateral membrane of cardiomyocytes that is independent of α1-syntrophin and the PDZ-binding motif of Nav1.5, located in membrane domains outside of T-tubules. The question of a T-tubular pool of Nav1.5 channels, however, remains open.
Highlights
The sarcolemma of rod-shaped adult ventricular cardiomyocytes can be divided into two principal regions
The remaining INa at the lateral membrane of Nav1.5 SIV mouse cardiomyocytes (Shy et al, 2014) raised the question which molecular determinants are involved in this lateral membrane pool of Nav1.5 channels
Immunostaining experiments using the pan-syntrophin antibody showed a significant decrease of syntrophin expression in isolated ventricular cardiomyocytes Snta1 fl/fl+ compared to Snta1 fl/fl− cardiac cells (Figures 2A,B)
Summary
The sarcolemma of rod-shaped adult ventricular cardiomyocytes can be divided into two principal regions. Matamoros and colleagues, have shown in heterologous expression systems that α1-syntrophin cannot only interact with the C-terminal SIV motif and with the sequence comprising three N-terminal amino acids Ser, Leu, Ala (SLA) of Nav1.5 channels (Matamoros et al, 2016) Based on this observation, the remaining INa observed on the lateral membrane of SIV mice could depend on α1-syntrophin interacting with Nav1.5 through the N-terminal SLA motif. In cardiac ventricular muscle cells, the presence of voltage-gated sodium channels Nav1.5 at the lateral membrane depends in part on the interaction between the dystrophin–syntrophin complex and the Nav1.5 C-terminal PDZ-domain-binding sequence Ser-Ile-Val (SIV motif). A recent study using heterologous expression systems showed that α1-syntrophin interacts with the Nav1.5 N-terminus, which may explain the SIV-motif independent INa at the lateral membrane of cardiomyocytes
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