Hyperammonemia alters membrane expression of GluA1 and GluA2 subunits of ampa receptors in hippocampus by enhancing activation of the il-1 receptor: underlying mechanisms

Abstract

Background: Hyperammonemic rats reproduce the cognitive alterations of patients with hepatic encephalopathy, including altered spatial memory, attributed to altered membrane expression of AMPA receptor subunits in hippocampus. Neuroinflammation mediates these cognitive alterations. We hypothesized that hyperammonemia-induced increase in IL-1β in hippocampus would be responsible for the altered GluA1 and GluA2 membrane expression. The aims of this work were to (1) assess if increased IL-1β levels and activation of its receptor are responsible for the changes in GluA1 and/or GluA2 membrane expression in hyperammonemia and (2) identify the mechanisms by which activation of IL-1 receptor leads to altered membrane expression of GluA1 and GluA2. Methods: We analyzed in hippocampal slices from control and hyperammonemic rat membrane expression of AMPA receptors using the BS3 cross-linker and phosphorylation of the GluA1 and GluA2 subunits using phosphor-specific antibodies. The IL-1 receptor was blocked with IL-Ra, and the signal transduction pathways involved in modulation of membrane expression of GluA1 and GluA2 were analyzed using inhibitors of key steps. Results: Hyperammonemia reduces GluA1 and increases GluA2 membrane expression and reduces phosphorylation of GluA1 at Ser831 and of GluA2 at Ser880. Hyperammonemia increases IL-1β, enhancing activation of IL-1 receptor. This leads to activation of Src. The changes in membrane expression of GluA1 and GluA2 are reversed by blocking the IL-1 receptor with IL-1Ra or by inhibiting Src with PP2. After Src activation, the pathways for GluA2 and GluA1 diverge. Src increases phosphorylation of GluN2B at Tyr14721 and membrane expression of GluN2B in hyperammonemic rats, leading to activation of MAP kinase p38, which binds to and reduces phosphorylation at Thr560 and activity of PKCζ, resulting in reduced phosphorylation at Ser880 and enhanced membrane expression of GluA2. Increased Src activity in hyperammonemic rats also activates PKCδ which enhances phosphorylation of GluN2B at Ser1303, reducing membrane expression of CaMKII and phosphorylation at Ser831 and membrane expression of GluA1. Conclusions: This work identifies two pathways by which neuroinflammation alters glutamatergic neurotransmission in hippocampus. The steps of the pathways identified could be targets to normalize neurotransmission in hyperammonemia and other pathologies associated with increased IL-1β by acting, for example, on p38 or PKCδ.

Peripheral inflammation induces neuroinflammation that alters neurotransmission and cognitive and motor function in hepatic encephalopathy: Underlying mechanisms and therapeutic implications

“…74 Increased Src activity in the hippocampus of hyperammonaemic rats also activated PKCδ, which enhanced phosphorylation of GluN2B at Ser1303, reduced membrane expression of CaMKII and phosphorylation of GluA1 at Ser831, which reduced membrane expression of GluA1. 74 This report clearly showed the mechanisms by which neuroinflammation, through increased IL-1β content and activation of its receptor, altered the membrane surface expression of the subunits of AMPA receptors, which was responsible for impaired spatial learning and memory. This work identified two pathways by which neuroinflammation altered glutamatergic neurotransmission in the hippocampus.…”

“…AMPA receptors are key in the modulation of spatial learning and memory [67][68][69] and it has been reported that IL-1β could alter membrane expression 70 and cognitive function. [71][72][73] Taking this into account, Taoro-Gonzalez et al 74 proposed that, in hyperammonaemic rats, increased IL-1β levels in the hippocampus would activate their receptors and associated signal transduction pathways, which would alter GluA1 and GluA2 membrane surface expression. They showed that blocking the IL-1 receptor with the endogenous antagonist IL-1Ra reversed the alterations in membrane surface expression of AMPA receptor subunits in hippocampal slices from rats with 4-5 weeks of chronic hyperammonaemia.…”

“…The calcium entering through this receptor activated MAP kinase p38, which bound to PKCζ, reducing its phosphorylation at Thr560 and its activity. Reduced PKCζ activity resulted in reduced phosphorylation of GluA2 at Ser880, which increased GluA2 membrane expression …”

Differential role of interleukin‐1β in neuroinflammation‐induced impairment of spatial and nonspatial memory in hyperammonemic rats

“…Male Wistar rats (120-140 g; Charles River Laboratories, Wilmington, MA, USA) were made hyperammonemic by feeding them an ammonium-enriched diet as established by Felipo et al (38) with the modification introduced by Taoro-González et al (22). The experiments were approved by the Comité deÉtica de Experimentación Animal (Prince Felipe Research Center, Ministry of Agriculture, Valencia, Spain) and performed in accordance with the European Communities Council Directive (86/609/EEC).…”

“…IL-1b, a proinflammatory cytokine, acts as a neuromodulator in the healthy CNS. However, chronic low-grade elevation of IL-1b levels in the hippocampus (and other brain regions) is primarily involved in the pathogenesis of neurologic and psychiatric disorders, such as stroke, epilepsy, Parkinson's disease, Alzheimer's disease, or HE (19)(20)(21)(22).…”