HERG- and IRK-like Inward Rectifier Currents are Sequentially Expressed During Neuronal Development of Neural Crest Cells and their Derivatives

Quail neural crest cells were cultured in a differentiative medium to study the inward K⁺ channel profile in neuronal precursors at various stages of maturation. Between 12 and 24 h of culture, neural crest-derived neurons displayed, in addition to the previously described outward depolarization-activated K⁺ currents, an inward current enhanced in high K⁺ medium. A biophysical and pharmacological analysis led us to conclude that this inward K⁺ current is identical to that previously demonstrated in mouse and human neuroblastoma cell lines (I_IR). This current (quail I_IR or ql_lR), which is active at membrane potentials positive to -35 mV, was blocked by Cs⁺ and by class Ill antiarrhythmic drugs, thus resembling the K⁺ current encoded by the human ether-a-go-go-related gene (HERG). At later stages of incubation (>48 h), neural crest-derived neurons underwent morphological and biochemical differentiation and expressed fast Na⁺ currents. At this stage the cells lost qll_R, displaying instead a classical inward rectifier K⁺ (IRK) current (quail I_IRK= qI_IRK). This substitution was reflected in the resting potential (V_REST), which became hyperpolarized by >20 mV compared with the 24 h cells. Neurons were also harvested from peripheral ganglia and other derivatives originating from the migration of neural crest cells, viz. ciliary ganglia, dorsal root ganglia, adrenal medulla and sympathetic chain ganglia. After brief culture following harvesting from young embryos, ganglionic neurons always expressed qi_lR. On the other hand, when ganglia were explanted from older embryos (7–12 days), briefly cultured neurons displayed the IRK-like current. Again, in all the above derivatives the ql_lR substitution by ql_lRK was accompanied by a 20 mV hyperpolarization of V_REST. Together, these data indicate that the V_REST of normal neuronal precursors is sequentially regulated by HERG- and IRK-like currents, suggesting that HERG-like channels mark an immature and transient stage of neuronal differentiation, probably the same stage frozen in neuroblastomas by neoplastic transformation.