MGlu5 Dependent Mitochondrial Translocation of PKCδ: A Mechanism Raising Astrocytic Oxidative Metabolism in Response to Extracellular Glutamate

Abstract

Synaptic activity imposes high demands of local energy production on astrocytes. However, the (an)aerobic pathways and fuel for generation of energy equivalents in astrocytes are still debated. Also, mechanisms to ensure rapid metabolic adaptation to bouts of neuronal activity have not been sufficiently explored. Here, we show a mechanism in astrocytes linking extracellular glutamate to upregulation of oxidative phosphorylation. We stimulated primary astrocytes with glutamate, and applied fluorescent immunocytochemistry with anti-protein kinase Cδ (PKCδ), anti-pyruvate dehydrogenase (PDH) and anti-phospho-PDH antibodies, and object oriented image analysis. Glutamate induces mitochondrial translocation of PKCδ and subsequent activation of the mitochondrial enzyme PDH—the point-of-no-return in the utilization of carbohydrates. Using the specific mGlu5 antagonist 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), the metabotropic glutamate receptor 5 (mGlu5) was identified as the key receptor inducing mitochondrial PKCδ translocation and PDH activation. We demonstrate by luminometric ATP assay and subtype-specific inhibitors of PKC and mGlu5 that the distinct initial drop in intracellular ATP following glutamate application is counteracted by the mGlu5/PKCδ-dependent mitochondrial activation. mGlu5 inhibition decreases ATP production also in astrocytes in the acute brain slice. Collectively, these findings reveal that astrocytes possess a potential for oxidative phosphorylation that can be stimulated by extracellular glutamate and the mGlu5/PKCδ/PDH axis, suggesting targets for pathologies involving excess glutamate. This also focuses the issue of activity-induced glia-neuronal metabolic interaction on perisynaptic energetics and the glia-synaptic microenvironment. Up-regulation of astrocytic metabolism via the mGlu5/PKCδ/PDH axis may affect only those perisynaptic astrocyte processes (PAPs) close to the active synapse(s), leaving other astrocyte domains and the whole cell unchanged.