NMDA receptor-mediated extracellular adenosine accumulation is blocked by phosphatase 1/2A inhibitors

Abstract

We have previously demonstrated that NMDA receptor-mediated extracellular adenosine accumulation in neuronal cultures is receptor-mediated and requires calcium influx. Because protein kinase C (PKC) is a calcium-dependent enzyme, we hypothesized that activation of PKC might be involved in NMDA-mediated adenosine accumulation. PKC inhibitors however, did not block NMDA-evoked adenosine accumulation, but rather, stimulated basal adenosine accumulation. These data suggested the possibility that NMDA receptor mediated adenosine accumulation involves net dephosphorylation rather than phosphorylation of one or more substrates. Thus, inhibition of kinases would be expected to increase adenosine accumulation and inhibition of phosphatases would be expected to block adenosine accumulation. To test this hypothesis, we used the phosphatase 1/2A inhibitors calyculin A and okadaic acid. Both inhibitors significantly reduced NMDA-evoked adenosine accumulation. In contrast phosphatase 2B inhibitors did not block NMDA-evoked adenosine accumulation. These data suggest that NMDAevoked adenosine accumulation is mediated by activation of phosphatase 1/2A. We have established previously that NMDA-mediated adenosine accumulation is associated with adenosine kinase inhibition. However, adenosine kinase is not a direct substrate for phosphatase 1/2A because inhibition of phosphatase 1/2A did not abolish NMDA-evoked adenosine kinase inhibition. Okadaic acid also had no effect on NO donor-evoked adenosine accumulation, which previously has been shown to be associated with adenosine kinase inhibition. Dephosphorylation of one or more proteins other than adenosine kinase as a consequence of NMDA receptor activation might play an important role in extracellular adenosine regulation, with important consequences for the regulation of excitatory synaptic transmission, plasticity, epileptogenesis, and excitotoxicity.

Modified Huang-Lian-Jie-Du Decoction Ameliorates Aβ Synaptotoxicity in a Murine Model of Alzheimer’s Disease

“…These results suggest that modified-HLJDD may improve dysfunctional ATP metabolism and ameliorate the oxidative stress and mitochondrial dysfunction in AD model, which may be partially explained by modified-HLJDD suppressing reactive microgliosis and astrogliosis in the hippocampus of AD model. We conclude that these two mechanisms may be linked, since NMDA receptors can decrease adenosine kinase activity and evoke adenosine accumulation [59, 60], and adenosine can also regulate glutamatergic transmission [61]. According to this previous study and the results of the present study, NMDA receptors were decreased in the hippocampus in AD model, and we also concluded that the downstream adenosine pathway may also be decreased.…”

Mechanism of Oxidative Stress and Synapse Dysfunction in the Pathogenesis of Alzheimer’s Disease: Understanding the Therapeutics Strategies

“…Inhibition of protein phosphatases reduces postsynaptic signaling as a major mechanism for basal synaptic transmission and memory formation [84]. It has been reported that inhibition of protein phosphatases increases tau phosphorylation and initiates neuronal cell death which includes altered Ca 2+ homeostasis and glutamate excitotoxicity that alter the memory formation [85, 86]. Previous reports suggested that accumulation and mislocalization of hyperphosphorylated tau in the somatodendritic compartment of neurons in AD disrupts glutamate receptor trafficking and synaptic function [87–89].…”

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