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

Background

Patients with atrial fibrillation (AF) exhibit a reduction in the ultrarapid outward potassium current ( I _Kur ) conducted by K _V 1.5 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 K _V 1, 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 I _Kur , and their potential contribution to the pathophysiology of AF.

Methods

We 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.

Results

Only Lgi3 and Lgi4 were expressed in the human heart. In human atrial tissue and heterologous cells, Lgi3 and Lgi4 interacted with K _V 1.5 channels. In HEK293 cells, Lgi3-4 impaired K _V 1.5/K _V β association, partially reversing the K _V β-induced N-type inactivation and reducing I _Kur 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 K _V 1.5 membrane expression and I _Kur 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 K _V 1.5 channels, suggesting potential disruptions in their functional interactions.

Conclusions

Lgi3-4 proteins are new components of the K _V 1.5 channelosome. They modulate I _Kur by interfering with K _V 1.5 interaction with the K _V β 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 SIGNIFICANCE

What is known?

• Leucine-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 K _V 1 channels, modifying their trafficking and/or biophysical properties.

• In cardiomyocytes, the activation of K _V 1.5 channels generates the ultrarapid outward potassium current ( I _Kur ), which is essential for the initial phase of human atrial repolarization, and it is dysregulated in AF.

• Changes in the properties or functional expression of some K _V 1.5 interacting proteins have crucial pathophysiological consequences.

• We demonstrate that Lgi3-4 are novel components of K _V 1.5 channelosome, modulating I _Kur and hence human atrial electrophysiology. Lgi3-4 proteins decrease I _Kur by interfering with the interaction between K _V 1.5 and K _V β subunits.

• The decrease in I _Kur 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.

• Lgi4 is dysregulated differently in paroxysmal (PX) versus permanent (PM) AF, thus shedding light into the mechanisms underlying this cardiac arrhythmia.