Phorbol ester receptors in cerebral cortex of cats with GM1 gangliosidosis

The pathogenesis of neuronal dysfunction in the gangliosidoses is poorly understood. Studies of the feline gangliosidoses and in vitro experiments implicate ganglioside inhibition of protein kinase C (PKC) in the pathogenesis of these neurological diseases. Therefore, in the present study, the binding of [3H]phorbol-12, 13 dibutyrate was measured to determine the levels of PKC in cerebral cortex of cats with GM1 gangliosidosis (mutant) and age matched normal siblings. This binding of ([3H]PDB) to cerebral cortex homogenates in both normal and mutant cats was highly specific. The specificity of receptors was ascertained also from displacement studies using nonradioactive phorbol ester analogues to displace [3H]PDB bound to its receptors. In both mutant and normal cat brain, phorbol 12,13-dibutyrate (PDB), 4-beta-phorbol 12,13-didecanoate (beta-PDD) and 4-beta-phorbol 12,13-dibenzoate (beta-PDBz) were highly potent (approximately to same degree) and effective in displacing [3H]PDB. On the other hand, 4-beta phorbol 12,13-diacetate (beta-PDA) was a weak displacer and 4-alpha-phorbol did not displace the bound [3H]PDB in either normal or mutant brain. Scatchard analysis of the binding data indicated a homogenous single class of binding sites in normal and mutant brain (Normal: Kd = 1.42 x 10(-7) M, Bmax = 8.40 pmoles/mg protein. Mutant: Kd = 1.60 x 10(-7) M, Bmax = 10.00 pmoles/mg protein). Sphingosine inhibited the binding to approximately the same extent in normal and mutant cortex. These studies demonstrate the presence of highly specific, homogenous, single type phorbol ester receptors in cerebral cortex of cats with GM1 gangliosidosis which are qualitatively and quantitatively similar to normal cat brain.