Differential sensitivity of mesencephalic neurons to inhibition of phosphatase 2A

Disturbance in phosphorylation/dephosphorylation can trigger apoptosis. Little is known as to its effects on mesencephalic dopamine neurons, the major neurons lost in Parkinson's disease. In this study, okadaic acid (OKA), a phosphatase 1 and 2A inhibitor, with greater potency toward 2A, was toxic to mesencephalic dopamine and gamma-aminobutyric acid (GABA) neurons, however, dopamine neurons were 4-fold more sensitive. The EC(50) for dopamine versus GABA toxicity was 1.5 versus 6.5 nM, respectively, and was consistent with an inhibition of phosphatase 2A. Dopamine neurons were also more sensitive to calyculin-A, a phosphatase inhibitor equipotent toward 1 and 2A. OKA-methyl-ester, which lacks phosphatase inhibitory activity, was without effect. DNA laddering typical of apoptosis was observed in cultures at a concentration that was specifically toxic to dopamine neurons (5 nM). In contrast to the sensitivity of mesencephalic neurons to phosphatase inhibition, inhibition of protein kinase activity with staurosporine or K252a showed little toxicity and protected neurons from OKA. Consistent with in vitro findings, infusion of 32 to 320 pmol of OKA into the left striatum of rats caused a dose-dependent loss of striatal dopamine without any loss of GABA 1 week following infusion. Acutely, OKA increased tyrosine hydroxylase activity, a phosphatase 2A substrate, and increased dopamine turnover. The above-mentioned findings demonstrate that dysregulation of phosphatase activity is detrimental to mesencephalic neurons, with dopamine neurons, in vitro and in vivo, being relatively more sensitive to phosphatase 2A inhibition. Disturbances in the phosphorylation control of proteins unique to dopamine neurons may contribute to their enhanced vulnerability to OKA exposure.