Evidence of Decreased Activity in Intermediate-Conductance Calcium-Activated Potassium Channels During Retinoic Acid–Induced Differentiation in Motor Neuron–Like NSC-34 Cells

Abstract

Background/Aims: Intermediate-conductance Ca²⁺-activated K⁺ (IK_Ca; K_Ca3.1 or KCNN4) channels affect the behaviors of central neurons including motor neurons. The mechanism through which neuronal differentiation is related to the activity of these channels remains largely unclear. Methods: By using various molecular biology tools and electrophysiological measurements, we investigated possible changes in the activity of IK_Ca channels in a retinoic acid (RA)-induced differentiation process in motor neuron-like NSC-34 cells. Results: The protein and messenger RNA expression of K_Ca3.1 substantially diminished as NSC-34 cells were differentiated with low serum (1%) and 1 µM RA. In whole-cell current recordings, the density of delayed-rectifier K⁺ currents obtained from differentiated cells was elevated. However, the density of a ramp pulse-elicited K⁺ current that was sensitive to blockage by 1-((2-chlorophenyl) (diphenyl)methyl)-1H-pyrazole (TRAM-34)—an inhibitor of IK_Ca channels—was significantly higher in undifferentiated NSC-34 cells than in differentiated cells. In undifferentiated cells, the activity of IK_Ca channels was readily detected and the probability of channel openings was resistant to stimulation by diazoxide or suppression by verruculogen. Furthermore, this probability was increased by 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one or 9-phenanthrol and reduced by TRAM-34. The channel-opening probability decreased in RA-induced differentiated cells, whereas the single-channel conductance of IK_Ca channels did not differ between undifferentiated and differentiated cells. Moreover, the slow component of the mean closed time in these channels was significantly shorter in undifferentiated cells than in differentiated cells; however, the mean open time in the channel remained unchanged as cells were differentiated. Conclusion: RA-induced differentiation in neurons could exert a suppressive effect on the activity of IK_Ca channels.