Regulation of an inactivating potassium current (IA) by the extracellular matrix protein vitronectin in embryonic mouse hippocampal neurones

Abstract

Integrins are a class of intrinsic membrane receptors for extracellular matrix ligands. In the central nervous system, integrins and their ligands influence neuronal growth and synaptic function, but relatively little is known about their potential to regulate intrinsic excitability. To explore this area, we examined the effects of matrix components on potassium currents in developing mouse hippocampal neurones, using electrophysiological and immunochemical approaches. We tested the effects of three integrin ligands present in the hippocampus, fibronectin, laminin and vitronectin, on electrogenesis in late embryonic hippocampal pyramidal neurones. Explants cultured in serum-free medium were exposed to ligands (fibronectin at 3 microg ml-1, laminin at 5 microg ml-1, vitronectin at 10 microg ml-1) for 3-4 days, and voltage-gated potassium currents were recorded from presumptive CA3 pyramidal neurones. Of the three matrix components, only vitronectin affected potassium currents, selectively increasing the amplitude of the inactivating potassium current (IA, or A-current) by about 75 % over control levels, and its density (current per unit area) by about 40 % (measured after 3 day exposures from embryonic day 15.5). Other potassium currents were spared, except to the extent that membrane area was increased. The actions of vitronectin were sensitive to RGD (Arg-Gly-Asp)-sequence-containing peptide, indicating the involvement of integrins as vitronectin receptors. The kinetic properties of IA, including the voltage-dependence of activation and inactivation, inactivation rate and the rate of recovery from inactivation, were minimally affected by vitronectin and were consistent with enhanced functional expression of Kv4-family subunits. Analyses of Kv4.2 and Kv1.4 immunoreactivity also suggested a preferential increase in Kv4.2 levels, with lesser effects on Kv1.4 levels. These results indicate that vitronectin can selectively regulate IA, and together with other observations suggest that modulation of neuronal excitability by integrins and their ligands occurs commonly.