Phosphodiesterase 8 governs cAMP/PKA-dependent reduction of L-type calcium current in human atrial fibrillation: a novel arrhythmogenic mechanism

Atrial fibrillation (AF) is associated with altered cAMP/PKA signaling and an AF-promoting reduction of L-type Ca²⁺-current (I_Ca,L), the mechanisms of which are poorly understood. Cyclic-nucleotide phosphodiesterases (PDEs) degrade cAMP and regulate PKA-dependent phosphorylation of key calcium-handling proteins, including the I_Ca,L-carrying Cav1.2_α1C subunit. The aim was to assess whether altered function of PDE type-8 (PDE8) isoforms contributes to the reduction of I_Ca,L in persistent (chronic) AF (cAF) patients.

mRNA, protein levels, and localization of PDE8A and PDE8B isoforms were measured by RT-qPCR, western blot, co-immunoprecipitation and immunofluorescence. PDE8 function was assessed by FRET, patch-clamp and sharp-electrode recordings. PDE8A gene and protein levels were higher in paroxysmal AF (pAF) vs. sinus rhythm (SR) patients, whereas PDE8B was upregulated in cAF only. Cytosolic abundance of PDE8A was higher in atrial pAF myocytes, whereas PDE8B tended to be more abundant at the plasmalemma in cAF myocytes. In co-immunoprecipitation, only PDE8B2 showed binding to Cav1.2_α1C subunit which was strongly increased in cAF. Accordingly, Cav1.2_α1C showed a lower phosphorylation at Ser1928 in association with decreased I_Ca,L in cAF. Selective PDE8 inhibition increased Ser1928 phosphorylation of Cav1.2_α1C, enhanced cAMP at the subsarcolemma and rescued the lower I_Ca,L in cAF, which was accompanied by a prolongation of action potential duration at 50% of repolarization.

Both PDE8A and PDE8B are expressed in human heart. Upregulation of PDE8B isoforms in cAF reduces I_Ca,L via direct interaction of PDE8B2 with the Cav1.2_α1C subunit. Thus, upregulated PDE8B2 might serve as a novel molecular mechanism of the proarrhythmic reduction of I_Ca,L in cAF.