TNF- alpha Downregulates Inhibitory Neurotransmission through Protein Phosphatase 1-Dependent Trafficking of GABAA Receptors

Inflammation has been implicated in the progression of neurological disease, yet precisely how inflammation affects neuronal function remains unclear. Tumor necrosis factor- alpha (TNF alpha ) is a proinflammatory cytokine that regulates synapse function by controlling neurotransmitter receptor trafficking and homeostatic synaptic plasticity. Here we characterize the mechanisms through which TNF alpha regulates inhibitory synapse function in mature rat and mouse hippocampal neurons. Acute application of TNF alpha induces a rapid and persistent decrease of inhibitory synaptic strength and downregulation of cell-surface levels of GABAARs containing alpha 1, alpha 2, beta 2/3, and gamma 2 subunits. We show that trafficking of GABAARs in response to TNF alpha is mediated by neuronally expressed TNF receptor 1 and requires activation of p38 MAPK, phosphatidylinositol 3-kinase, protein phosphatase 1 (PP1), and dynamin GTPase. Furthermore, TNF alpha enhances the association of PP1 with GABAAR beta 3 subunits and dephosphorylates a site on beta 3 known to regulate phospho-dependent interactions with the endocytic machinery. Conversely, we find that calcineurin and PP2A are not essential components of the signaling pathway and that clustering of the scaffolding protein gephyrin is only reduced after the initial receptor endocytosis. Together, these findings demonstrate a distinct mechanism of regulated GABAAR endocytosis that may contribute to the disruption of circuit homeostasis under neuroinflammatory conditions.