Novel insights into IKur modulation by Lgi3-4: Implications in atrial fibrillation

BackgroundPatients with atrial fibrillation (AF) exhibit a reduction in the ultrarapid outward potassium current (IKur) conducted by KV1.5 channels. Ion channels are closely modulated by regulatory subunits, forming macromolecular complexes known as channelosomes. One such regulatory family is the leucine-rich glioma-inactivated protein family (Lgi1-4), which has been shown to interact with KV1, modifying their trafficking and/or biophysical properties in neurons. However, the expression and impact of these proteins in the heart is still unknown. We investigated the role of Lgi3-4 proteins in cardiac electrophysiology, focusing specifically on IKur, and their potential contribution to the pathophysiology of AF. MethodsWe used three complementary biological systems, including heterologous COS-7, HEK297 and CHO cells, AAV-mediated cardiac-specific Lgi4 gene transfer in mice (Lgi4 mice), and human samples from patients in sinus rhythm and AF. Our multidisciplinary approach included immunolocalization, patch clamping, surface ECG, transvenous catheter-mediated intracardiac stimulation, and molecular biology techniques. ResultsOnly Lgi3 and Lgi4 were expressed in the human heart. In human atrial tissue and heterologous cells, Lgi3 and Lgi4 interacted with KV1.5 channels. In HEK293 cells, Lgi3-4 impaired KV1.5/KV{beta} association, partially reversing the KV{beta}-induced N-type inactivation and reducing IKur amplitude. On surface ECG, the QRS interval was prolonged, and impulse conduction was impaired in cardiac-specific Lgi4 mice compared with control. In isolated ventricular cardiomyocytes from Lgi4 mice, early action potential repolarization was prolonged compared to control cardiomyocytes. These results correlated with the reduced KV1.5 membrane expression and IKur density observed in Lgi4 cardiomyocytes and HEK293 cells. Notably, Lgi4 protein expression was lower in atrial tissue from patients with AF than sinus rhythm patients. The reduction in Lgi4 protein levels in AF was also associated with an altered colocalization with KV1.5 channels, suggesting potential disruptions in their functional interactions. ConclusionsLgi3-4 proteins are new components of the KV1.5 channelosome. They modulate IKur by interfering with KV1.5 interaction with the KV{beta} subunit. Importantly, Lgi4 is dysregulated differently in paroxysmal versus permanent AF. The results improved the understanding of this most common type of arrhythmia and identified Lgi proteins as a new potential target for treatment. NOVELTY AND SIGNIFICANCEWhat is known? O_LILeucine-rich glioma-inactivated protein family (Lgi1-4) exert an important role in the nervous system and neurological diseases. In neurons, certain Lgi proteins interact with KV1 channels, modifying their trafficking and/or biophysical properties. C_LIO_LIIn cardiomyocytes, the activation of KV1.5 channels generates the ultrarapid outward potassium current (IKur), which is essential for the initial phase of human atrial repolarization, and it is dysregulated in AF. C_LIO_LIChanges in the properties or functional expression of some KV1.5 interacting proteins have crucial pathophysiological consequences. C_LI What new information does this article contribute? O_LIWe demonstrate that Lgi3-4 are novel components of KV1.5 channelosome, modulating IKur and hence human atrial electrophysiology. Lgi3-4 proteins decrease IKur by interfering with the interaction between KV1.5 and KV{beta} subunits. C_LIO_LIThe decrease in IKur in cardiac-specific mouse model expressing Lgi4 slows the early repolarization in the action potential, as well as produce electrophysiological changes in the surface ECG and the cardiac conduction system. C_LIO_LILgi4 is dysregulated differently in paroxysmal (PX) versus permanent (PM) AF, thus shedding light into the mechanisms underlying this cardiac arrhythmia. C_LI