Phorbol ester modulation of both δ-mutant and subunit-omitted nicotinic receptors expressed in Xenopus oocytes

The action of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), the potent stimulator of protein kinase C (PKC), on acetylcholine-activated currents (I(ACh)) was investigated in voltage clamped Xenopus laevis oocytes injected with RNAs encoding murine embryonic nicotinic acetylcholine receptor (AChR) subunits. Comparable potentiation and acceleration of decay of I(ACh) were observed within minutes of phorbol ester application in oocytes injected with various RNA subunit combinations: (i) αβγδ; (ii) αβγ; (iii) αβδ; and (iv) αβγδ(AAA), a mutant of the δ subunit with serine residues 360-361-362 mutated to alanine. Our findings indicate that the effects on I(ACh) induced by PKC stimulation are independent of both γ and δ subunits and, accordingly, of the presence of PKC phosphorylation sites on δ subunit. It is here suggested a novel PKC-dependent modulatory mechanism of cholinergic receptor which does not involve direct phosphorylation of the AChR and requires phosphorylation of intermediate regulatory protein(s).