Unravelling Lgi3-4 role in cardiac electrophysiology

Résumé

The heart's function as a rhythmic electromechanical pump depends on the action potential (AP) generation and propagation in individual cardiac cells. APs are determined by the sequential activation and inactivation of ion channels. The ultrarapid and the transient outward potassium currents (IKur and Ito), crucial for the initial phase of the human atrial repolarization, are generated after the activation of KV1.5 and KV4.3 channels. Ion channels associate with regulatory proteins that modulate its trafficking and biophysical properties, forming signalling complexes or channelosomes. However, the exact composition of these channelosomes remains undeciphered. KV1.5 assemble with several KVß subunits (KVß1.2, KVß1.3 and KVß2.1), but also with other modulatory subunits. IKur and Ito are altered in cardiac pathologies such as atrial fibrillation (AF), being IKur a potential target in this disease. Changes in the expression levels of some interacting proteins of KV1.5 and KV4.3 channels may have crucial pathophysiological consequences. Thus, deciphering the composition of these channelosomes is essential. Possible candidates for the modulation of IKur and Ito are the leucine-rich glioma inactivated protein family (Lgi1-4). Previous studies show that Lgi1 interacts with KV1, KVß1 and KV4 channels in neurons, modifying their trafficking and biophysical properties. These findings led us to hypothesise that Lgi proteins could exert similar effects in the heart. Our results indicate that Lgi3 and Lgi4 are present in human heart tissue, and that they interact with KV1.5 and KV4.3 channels. Lgis interaction impairs KV1.5/KVß association, partially reverting the KVß-induced effects on KV1.5 currents, producing a decrease in: 1) the current amplitude and membrane expression; and 2) the N- type (with KVß1.x) and C-type (KVß2.1) inactivation. Moreover, Lgi3 and Lgi4 increase the current amplitude of KV4.3, but not that of KV4.2. To deepen into the physiological role of these proteins, we generated a cardiac specific mice model expressing Lgi4. In these mice, the QRS interval in the surface electrocardiogram was prolonged, along with the duration of the early repolarization in the APs recorded from Lgi4 ventricular myocytes compared to Control ones. This can be explained by the reduction in IKur and KV1.5 membrane expression observed in Lgi4 cardiomyocytes, similar to the results obtained in heterologous systems. Furthermore, the conduction system of Lgi4 mice is altered, maybe due to a decrease in IKur in the cells of this cardiac structure. Lastly, we analysed the possible changes in the expression of some components of these channelosomes in atrial samples from patients in AF compared to sinus rhythm, demonstrating a decrease in Lgi4 both at the mRNA and protein levels, and Lgi3 only in mRNA, in AF. In summary, this Doctoral Thesis provides insight into IKur modulation by Lgi3-4, claiming them as components of the KV1.5 channelosome, as well as possible new therapeutic targets towards AF.