The regulation of M channel via phosphorylation of calmodulin by CK2 and its physiological roles

Regulation of the resting membrane potential is important for neuronal excitability. One of the currents having a key part in stabilizing neuronal excitability is the M-current, a non-inactivating potassium current which is generated by M channels encoded by the KCNQ family. After identifying the M-current in sympathetic ganglion cells 30 years ago, there have been several extensive studies trying to elucidate its physiological role and regulators. For example, it has been demonstrated that Phosphatidylinositol 4,5-bisphosphate (PIP2) and calmodulin (CaM) are essential co-factors for maintaining functional M channels. Calmodulin binding to the KCNQ2 subunit regulates channel trafficking and stabilizes channel activity. However, how calmodulin modulates the M channel is an open question. In this project, we demonstrate that phosphorylation of calmodulin by casein kinase 2 (CK2) modulates KCNQ2 channel availability and kinetics. CK2-mediated phosphorylation of calmodulin increases its binding to the channel, facilitates PIP2 binding, and augments current amplitudes. Treatment with CK2-selective inhibitors suppresses KCNQ2 current. This suppression is largely prevented by coexpression of CK2 phosphomimetic calmodulin mutants or pretreatment with calyculin A, a protein phosphatase inhibitor. We also demonstrate that protein phosphatase 1 (PP1) binds KCNQ2 protein via an N-terminal KVxF consensus site, which is required for tonic regulation of CK2-mediated calmodluin phosphorylation. In neurons, CK2-mediated M-current modulation changes the duration of sub-threshold transient potential and afterhyperpolarization. Accordingly, inhibition of CK2-mediated phosphorylation of calmodulin shortened refractory periods by suppressing the M-current. These findings suggest that CK2 mediated modulation of the KCNQ2/M-current is an important regulatory mechanism for tuning firing frequencies.