CHANGE OF GANGLIOSIDE ACCESSIBILITY AT THE PLASMA-MEMBRANE SURFACE OFCULTURED NEURONS, FOLLOWING PROTEIN-KINASE-C ACTIVATION

While the mechanism of signal transduction across the plasma membrane from the exo- to the endoplasmic side has been extensively investigate d, the possible return of messages back to the outer layer is less kno wn. We studied the effect of protein kinase C activation on the gangli oside accessibility at the exoplasmic face of intact rat cerebellar gr anule cells in culture, using the enzyme sialidase as the probing mole cule. Under the experimental conditions (1 milliunit/mL enzyme, 2 min incubation at 37 degrees C), only GT1b and GD1a gangliosides were part ially affected by the enzyme (28.6 and 25.7% hydrolysis, respectively) . After cell treatment with phorbol 12-myristate 13-acetate, inducing protein kinase C activation, GT1b and GD1a ganglioside susceptibility to sialidase was strongly decreased (8.6 and 15.9% hydrolysis, respect ively). A reduction of ganglioside hydrolysis was also observed when p rotein kinase C activation was induced by cell treatment for 15 min wi th 100 mu M glutamate. On the contrary, accessibility did not vary whe n protein kinase C translocation was not effective (either in the abse nce of Ca2+ in the medium or using 1 mu M glutamate) or when the kinas e activity was inhibited by staurosporine. These data suggest that fol lowing PKC activation, a key step of inbound transmembrane signaling, cell may dispatch outbound messages to the plasma membrane outer layer , changing the selective recognition and crypticity of glycolipids at the cell surface, possibly through a modulation of their segregation s tate.