Extracellular signal-regulated kinase 5 (ERK5) regulates diverse physiological responses such as proliferation, differentiation, and gene expression. Previously, we demonstrated that ERK5 is essential for neurite outgrowth and catecholamine biosynthesis in PC12 cells and sympathetic neurons. However, it remains unclear how ERK5 regulates the activity of ion channels, which are important for membrane excitability. Thus, we examined the effect of ERK5 on the ion channel activity in the PC12 cells that overexpress both ERK5 and the constitutively active MEK5 mutant. The gene and protein expression levels of voltage-dependent Ca2+ and K+ channels were determined by RT-qPCR or Western blotting. The A-type K+ current was recorded using the whole-cell patch clamp method. In these ERK5-activated cells, the gene expression levels of voltage-dependent L- and P/Q-type Ca2+ channels did not alter, but the N-type Ca2+ channel was slightly reduced. In contrast, those of Kv4.2 and Kv4.3, which are components of the A-type current, were significantly enhanced. Unexpectedly, the protein levels of Kv4.2 were not elevated by ERK5 activation, but the phosphorylation levels were increased by ERK5 activation. By electrophysiological analysis, the inactivation time constant of the A-type current was prolonged by ERK5 activation, without changes in the peak current. Taken together, ERK5 inhibits an inactivation of the A-type current by phosphorylation of Kv4.2, which may contribute to the neuronal differentiation process.

Extracellular signal-regulated kinase (ERK) 5, a member of mitogen-activated protein kinase, plays important roles in the neuronal development. In our previous studies, we demonstrated that ERK5 mediates neurite/axon outgrowth and catecholamine biosynthesis in PC12 cells and sympathetic neurons. However, the regulation of membrane excitability by ERK5 remains unclear. Thus, we examined the effect of ERK5 on Ca 2+ and K + channels in PC12 cells. In order to activate ERK5 signaling selectively, ERK5 and the constitutively active MEK5 mutant were overexpressed in PC12 cells. In these cells, the gene expression of L-, P/Q-and N-type Ca 2+ channels was not increased. In contrast, those of Kv4.2 and Kv4.3 were enhanced by ERK5 signaling. Although the protein levels of Kv4.2 were not correlated to mRNA levels, phosphorylation levels of Kv4.2 were increased by ERK5 activation. Because Kv4.2 is a pore-forming subunit of A-type K + channels, which play essential roles in membrane excitability, we measured the A-type K + current by a whole-cell patch clamp method. The electrophysiological data showed that ERK5 inhibits inactivation of the A-type current, which may be involved in the neural differentiation process by affecting membrane excitability.