Isoenzyme-specific regulation of cardiac Kv1.5/Kvβ1.2 ion channel complex by protein kinase C: central role of PKCβII

Kv Channel Ancillary Subunits: Where Do We Go from Here?

Voltage-gated potassium (Kv) channels each comprise four pore-forming α-subunits that orchestrate essential duties such as voltage sensing and K+ selectivity and conductance. In vivo, however, Kv channels also incorporate regulatory subunits-some Kv channel specific, others more general modifiers of protein folding, trafficking, and function. Understanding all the above is essential for a complete picture of the role of Kv channels in physiology and disease.

Kv1.5 channels are regulated by PKC-mediated endocytic degradation

The voltage-gated potassium channel Kv1.5 plays important roles in the repolarization of atrial action potentials and regulation of the vascular tone. While the modulation of Kv1.5 function has been well studied, less is known about how the protein levels of Kv1.5 on the cell membrane are regulated. Here, through electrophysiological and biochemical analyses of Kv1.5 channels heterologously expressed in HEK293 cells and neonatal rat ventricular myocytes, as well as native Kv1.5 in human induced pluripotent stem cell (iPSC)-derived atrial cardiomyocytes, we found that activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA, 10 nM) diminished Kv1.5 current (I_Kv1.5) and protein levels of Kv1.5 in the plasma membrane. Mechanistically, PKC activation led to monoubiquitination and degradation of the mature Kv1.5 proteins. Overexpression of Vps24, a protein that sorts transmembrane proteins into lysosomes via the multivesicular body (MVB) pathway, accelerated, whereas the lysosome inhibitor bafilomycin A1 completely prevented PKC-mediated Kv1.5 degradation. Kv1.5, but not Kv1.1, Kv1.2, Kv1.3, or Kv1.4, was uniquely sensitive to PMA treatment. Sequence alignments suggested that residues within the N terminus of Kv1.5 are essential for PKC-mediated Kv1.5 reduction. Using N-terminal truncation as well as site-directed mutagenesis, we identified that Thr15 is the target site for PKC that mediates endocytic degradation of Kv1.5 channels. These findings indicate that alteration of protein levels in the plasma membrane represents an important regulatory mechanism of Kv1.5 channel function under PKC activation conditions.

PKC and AMPK regulation of Kv1.5 potassium channels

The voltage-gated Kv1.5 potassium channel, conducting the ultra-rapid rectifier K(+) current (IKur), is regulated through several pathways. Here we investigate if Kv1.5 surface expression is controlled by the 2 kinases PKC and AMPK, using Xenopus oocytes, MDCK cells and atrial derived HL-1 cells. By confocal microscopy combined with electrophysiology we demonstrate that PKC activation reduces Kv1.5 current, through a decrease in membrane expressed channels. AMPK activation was found to decrease the membrane expression in MDCK cells, but not in HL-1 cells and was furthermore shown to be dependent on co-expression of Nedd4-2 in Xenopus oocytes. These results indicate that Kv1.5 channels are regulated by both kinases, although through different molecular mechanisms in different cell systems.

Deletion of Kvβ1.1 subunit leads to electrical and haemodynamic changes causing cardiac hypertrophy in female murine hearts

NEW FINDINGS

What is the central question of this study? The goal of this study was to evaluate sex differences and the role of the potassium channel β1 (Kvβ1) subunit in the heart. What is the main finding and its importance? Genetic ablation of Kvβ1.1 in females led to cardiac hypertrophy characterized by increased heart size, prolonged monophasic action potentials, elevated blood pressure and increased myosin heavy chain α (MHCα) expression. In contrast, male mice showed only electrical changes. Kvβ1.1 binds the MHCα isoform at the protein level, and small interfering RNA targeted knockdown of Kvβ1.1 upregulated MHCα. Cardiovascular disease is the leading cause of death and debility in women in the USA, and cardiac arrhythmias are a major concern. Voltage-gated potassium (Kv) channels along with the binding partners; Kvβ subunits are major regulators of the action potential (AP) shape and duration (APD). The regulation of Kv channels by the Kvβ1 subunit is unknown in female hearts. In the present study, we hypothesized that the Kvβ1 subunit is an important regulator of female cardiac physiology. To test this hypothesis, we ablated (knocked out; KO) the KCNAB1 isoform 1 (Kvβ1.1) subunit in mice and evaluated cardiac function and electrical activity by using ECG, monophasic action potential recordings and echocardiography. Our results showed that the female Kvβ1.1 KO mice developed cardiac hypertrophy, and the hearts were structurally different, with enlargement and increased area. The electrical derangements caused by Kvβ1.1 KO in female mice included long QTc and QRS intervals along with increased APD (APD20-90% repolarization). The male Kvβ1.1 KO mice did not develop cardiac hypertrophy, but they showed long QTc and prolonged APD. Molecular analysis showed that several genes that support cardiac hypertrophy were significantly altered in Kvβ1.1 KO female hearts. In particular, myosin heavy chain α expression was significantly elevated in Kvβ1.1 KO mouse heart. Using a small interfering RNA strategy, we identified that knockdown of Kvβ1 increases myosin heavy chain α expression in H9C2 cells. Collectively, changes in molecular and cell signalling pathways clearly point towards a distinct electrical and structural remodelling consistent with cardiac hypertrophy in the Kvβ1.1 KO female mice.