Developmental Regulation of Neuronal KCa Channels by TGF 1: An Essential Role for PI3 Kinase Signaling and Membrane Insertion

Lhuillier, Loic, and Stuart E. Dryer. Developmental regulation of neuronal KCa channels by TGF 1: an essential role for PI3 kinase signaling and membrane insertion. J Neurophysiol 88: 954–964, 2002; 10.1152/jn.00976.2001. TGF 1 is a target-derived factor responsible for the developmental expression of large-conductance Ca -activated K (KCa) channels in ciliary neurons of the chick ciliary ganglion. The acute effects of TGF 1 on KCa channels are mediated by posttranslational events and require activation of the MAP kinase Erk. Here we show that TGF 1 evokes robust phosphorylation of Akt/PKB, a protein kinase dependent on the products of phosphatidylinositol 3-OH kinase (PI3K). TGF 1-evoked stimulation of KCa channels is blocked by the PI3K inhibitors wortmannin and LY294002. These drugs also inhibit TGF 1 effects on Akt/PKB phosphorylation but have no effect on TGF 1-evoked Erk activation. Application of the MEK1 inhibitor PD98059 blocked TGF 1 effects on Erk but had no effect on Akt/PKB phosphorylation. These results indicate that PI3K and Erk represent parallel signaling cascades activated by TGF 1 in ciliary neurons. The effects of TGF 1 on functional expression of KCa are blocked by the microtubule inhibitors colchicine and nocodazole, by botulinum toxins A and E, and by brefeldin-A, an agent that disrupts the Golgi apparatus. These data indicate that translocation of a membrane protein, possibly Slowpoke (SLO), is required for the acute posttranslational effects of TGF 1 on KCa channels. Confocal immunofluorescence studies with three different SLO antisera showed robust expression of SLO in multiple intracellular compartments of embryonic day 9–13 ciliary neurons, including the cell nucleus. These data suggest that TGF 1 evokes insertion of SLO channels into the plasma membrane as a result of signaling cascades that entail activation of Erk and PI3K.