A Distinct Pool of Nav1.5 Channels at the Lateral Membrane of Murine Ventricular Cardiomyocytes

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). a1-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 (1SIV), 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 a1-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 a1-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, a1-syntrophin knockdown, and
1SIV ventricular mouse cardiomyocytes. Compared to wild-type, a reduction of the
lateral INa was observed in myocytes from a1-syntrophin knockdown hearts. Similar
to 1SIV 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 1SIV 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 a1-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.