ERK/MAPK Pathway Regulation of GABA-A Receptor Function.

GABA-A receptor is a ligand-gated ion channel that conducts negatively charged chloride ions. Influx of this ion leads to hyperpolarization of neurons; thus, suppression of the neuronal excitability. Alterations in GABAergic neurotransmission may contribute to depression and anxiety. While neurosteroids can regulate the responsiveness of the GABAA receptor in allosteric manner, certain intracellular signaling pathways can also regulate the function of the GABA-A receptors through phosphorylation of its subunits. One pathway that is regulated by both neurotrophic factors and steroid hormones is the ERK/MAPK pathway. This pathway is involved in cell proliferation, maturation, and even, cell death. The role of this pathway in the regulation of the GABAA receptor, however, is not well studied and is the subject of my dissertation. Initial studies conducted by the Singh and Dillon laboratories showed that pharmacological inhibition of the ERK/MAPK pathway potentiated the α1β2γ2 configuration of the GABAA receptor, expressed in HEK-t cells. This suggested that the ERK/MAPK pathway was involved in the negative regulation of the GABAA receptor function. In silico analysis revealed that the Thr 375 residue within the α1 subunit was a plausible target of the ERK/MAPK pathway. As a result, I hypothesized that the activation of the ERK/MAPK pathway inhibited GABAA receptor function through the direct phosphorylation of the Thr 375 residue, resulting in receptor internalization. Supporting this hypothesis was data showing that mutation of the Thr 375 residue to Alanine prevented the enhancement of GABA-gated currents elicited by inhibiting the ERK/MAPK pathway. However, using the HEK-t cell line transfected with the α1β2γ2 configuration of the GABAA receptor, I determined that the activation of the ERK/MAPK pathway by HGF did not influence the peak amplitude of the GABA-gated currents. Further, the potentiation of the GABA-gated currents was apparently not due to internalization of the receptor. Collectively, while we believe that the Thr 375 within the α1 subunit is relevant to the effect of ERK/MAPK pathway inhibition, it was not a direct target of the ERK/MAPK pathway.