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  • It has already been shown that PKA

    2019-07-30

    It has already been shown that PKA may activate PLC that, in turn, activates PKC [49]. The higher levels of cAMP in hyperammonemic rats lead to increased PKA activity, which activates PLC and, subsequently PKC, which increases phosphorylation of Ser880 of GluA2, thus reducing GluA2 membrane expression (Fig. 6). This is supported by the fact that inhibiting PKA, PLC or PKC reduces GluA2 phosphorylation at Ser880 in hyperammonemic rats to levels similar to control rats and also normalizes membrane expression of GluA2. A similar result is obtained by blocking NMDA receptors with MK-801, thus confirming that enhanced activation of NMDA receptors in hyperammonemia is triggering the PKA-PLC-PKC pathway summarized in Fig. 6 which results in increased GluA2 phosphorylation at Ser880. It has been already shown that increased phosphorylation of Ser880 by PKC leads to reduced membrane expression of GluA2 [24], [25]. Activation of the NMDAR-PKA-PLC-PKC pathway would be therefore responsible for reduced membrane expression of GluA2 in (-)-Bicuculline methiodide of hyperammonemic rats. The results from all experiments performed support this idea. There is only one exception. We show that inhibiting PLC normalizes phosphorylation of Ser880 of GluA2 in hyperammonemic rats, thus supporting the role of the NMDAR-PKA-PLC-PKC pathway in modulating this phosphorylation. However, in the case of the inhibitor of PLC, reduced phosphorylation of GluA2 at Ser880 in hyperammonemic rats is not associated with increased membrane expression of the GluA2 subunit. We suppose that this unusual dissociation of phosphorylation/membrane expression could be due to some interference by the PLC inhibitor used in the process by which reducing phosphorylation leads to translocation of GluA2.This dissociation does not occur for GluA1, for which the PLC inhibitor normalizes both phosphorylation and membrane expression. Although this dissociation is unusual, it occurs in some specific situations. For example, sensitizing exposure to amphetamine increases AMPA receptor phosphorylation without increasing cell surface expression in the rat nucleus accumbens [50]. The fact that the PLC inhibitor induces this dissociation between phosphorylation/membrane expression does not affect the utility of the experiments to proof the hypothesis that increased activation of the NMDAR-PKA-PLC -PKC pathway induces the alterations in phosphorylation and membrane expression of the GluA1 and GluA2 subunits in hyperammonemia. Concerning GluA1, its membrane expression is modulated mainly by phosphorylation at Ser831 by PKC or by CaMKII [21], [22] and/or by phosphorylation at Ser845, mainly by PKA [23]. Both phosphorylation at Ser831 and at Ser845 enhance membrane expression of GluA1 [21], [22], [23]. In contrast to Ser880 in GluA2, which phosphorylation reduces membrane expression, phosphorylation of Ser831 of GluA1 enhances its membrane expression [19]. Ser831 of GluA1 may be also phosphorylated by PKC. The results reported support that the same NMDAR-PKA-PLC-PKC pathway modulating GluA2 subunit would be responsible for the increased phosphorylation of GluA1 at Ser831 in hyperammonemic rats (Fig. 6). This is supported by the fact that blocking NMDA receptors with MK-801, or inhibiting PKA, PLC or PKC reduces phosphorylation of GluA1 at Ser831 in hyperammonemic rats to levels similar to controls. In summary, the results reported show that chronic moderate hyperammonemia, similar to that present in patients with liver cirrhosis and hepatic encephalopathy, reduces membrane expression of the GluA2 (Ca-impermeable) subunit of AMPA receptors and enhances membrane expression of the GluA1 (Ca-permeable) subunit. Moreover, we show that the changes in membrane expression of both GluA2 and GluA1 subunits in hyperammonemia would be due to enhanced activation of NMDA receptors which reduces cGMP levels and activity of PDE2, resulting in increased cAMP levels. This leads to increased PKA activity which activates PLC and PKC thus increasing phosphorylation of GluA2 in (-)-Bicuculline methiodide Ser880, which reduces GluA2 membrane expression, and also increases phosphorylation of GluA1 sin Ser831, which increases GluA1 membrane expression (Fig. 6). Blocking NMDA receptors or inhibiting PKA, PLC or PKC normalizes phosphorylation and membrane expression of both GluA2 and GluA1 in hyperammonemic rats. The altered membrane expression of GluA2 and GluA1 subunits of AMPA receptors in hyperammonemia would result in altered signal transduction which may contribute to the cognitive and motor alterations in hyperammonemia and hepatic encephalopathy.