Potassium current development and its linkage to membrane expansion during growth of cultured embryonic mouse hippocampal neurons: sensitivity to inhibitors of phosphatidylinositol 3-kinase and other protein kinases.

Hippocampal pyramidal neurons express three major voltage-dependent potassium currents, IA, ID, and IK. During hippocampal development, IA, the rapidly activating and inactivating transient potassium current, is detected soon after pyramidal neurons can be morphologically identified. Appearance of IA in developing pyramidal neurons is dependent on contact with cocultured astroglial cells; cultured pyramidal neurons not in contact with astroglial cells have reduced membrane area and IA (Wu and Barish, 1994). We have examined intracellular signaling pathways that could contribute to the regulation of IA development by probing developing pyramidal neurons with kinase inhibitors. We observed that exposure to LY294002 or wortmannin, inhibitors of phosphatidylinositol (PI) 3-kinase, reduced somatic cross-sectional area, neurite outgrowth, whole-cell capacitance, IA amplitude and density (amplitude normalized to membrane area), and immunoreactivity for Kv4.2 and/or Kv4.3 (potassium channel subunits likely to be present in the channels carrying IA). In contrast, exposure to ML-9 or KN-62, inhibitors of myosin light chain kinase or Ca2+-calmodulin-dependent protein kinase II (CaMKII), reduced membrane area and IA amplitude but did not affect IA density or Kv4. 2/3 immunoreactivity to the same extent as inhibitors of PI 3-kinase. Unexpectedly, exposure to bisindolymaleimide I or calphostin C, inhibitors of protein kinase C (PKC), did not affect membrane area or potassium current development. Our data suggest that PI 3-kinases regulate both A-type potassium channel synthesis and plasmalemmal insertion of vesicles bearing these potassium channels. CaMKII appears to regulate fusion of channel-bearing vesicles with the plasmalemma and myosin light chain kinase to regulate centripetal transport of channel-bearing vesicles from the Golgi. We further suggest that astroglial cells exert their influence on pyramidal neuron development through activation of PI 3-kinases.