Mechanical stretch increases Kv1.5 current through interaction between the S1-S2 linker and N-terminus of the channel
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The voltage-gated potassium channel Kv1.5 mediates the ultra-rapid delayed rectifier current (IKur) in the heart, important for human atrial repolarization. In physiological conditions cardiomyocytes continuously undergo mechanical load, a parameter influenced by blood pressure and fluid retention. Dysfunction of Kv1.5 has been associated with atrial fibrillation, an arrhythmia linked to hypertension and atrial dilation, risk factors that involve cardiomyocyte stretch. In the current project, the effects of mechanical stretch on Kv1.5 channels were investigated. Mechanical stretch was induced by centrifuging cells at 70 × g for 5 minutes or subjecting HEK 293 cells and neonatal rat ventricular myocytes expressing Kv1.5 to low osmolarity cell culture medium (LO) leading to a 33% increase in cell size. The results demonstrated mechanical stretch induced by LO increased Kv1.5 current (IKv1.5) at 20 mV and above (P < 0.01) without altering membrane expression and such an increase required the long, proline-rich S1-S2 linker of the Kv1.5 channel to be intact. The LO effect also required the N-terminus, which contains the binding domain for endogenous Src tyrosine kinase that mediates IKv1.5 inhibition. Disrupting the Src binding domain of Kv1.5 through N-terminal deletion or point mutations abolished the mechanical stretch-mediated effect on IKv1.5. The results further revealed that the S1-S2 linker is coupled with the N-terminus; deletion of the N-terminus up to amino acid residue 209 altered the conformation of the S1-S2 linker such that S1-S2 linker was no longer susceptible to proteinase K (PK) cleavage. The Kv1.5 voltage dependence of inactivation which is regulated by the N-terminus was also altered by PK cleavage of the S1-S2 linker, revealing an indirect link between the N-terminus and S1-S2 linker. It is concluded that the S1-S2 linker of Kv1.5 represents a mechanosensor; when targeted by mechanical stretch, a change in N-terminal conformation occurs, relieving Src-mediated tonic channel inhibition, resulting in an increase in IKv1.5. This study reveals a signalling pathway linking mechanical stretch and Kv1.5 activity, providing a potential mechanism for stretch-induced atrial fibrillation.