F-actin does not modulate the initial steps of the protein kinase C activation process in living nerve cells

Actin is a major substrate for protein kinase C (PKC) and PKC is considered a modulator of the actin network. In addition in vitro studies (Biochemistry 39 (2000) 271) have suggested that all PKC isoforms bind to actin during the process of activation of the enzyme. To test the physiological significance of such a coupling we used living PC12 cells and primary cultures of cerebellar granule cells. When PC12 cells were treated with either latrunculin B, which impairs actin polymerization, or phalloidin, which stabilizes actin filaments, we observed a significant reduction of the [Ca2+](i) response revealed by Fura-2 fluorescence, while the PKC conformational changes followed by Fim-1 fluorescence were unaffected. The responses induced either by cell depolarization or muscarinic receptor activation were similarly affected by the toxin treatment of PC12 cells. In cerebellar granule cells the [Ca2+](i) response induced by KCl depolarization was increased by latrunculin treatment, whereas no effect was observed on the PKC response. Latrunculin had no effect on the NMDA-induced responses in these cells. Finally we also show that the response induced by a long-lasting depolarization, which mimics stimulation leading to neuronal plasticity, was not significantly altered by latrunculin or phalloidin treatment of the cells. These results suggest that the actin network is not involved in the initial steps of the PKC activation process in living nerve cells. (C) 2003 Elsevier Inc. All rights reserved.