ZnR Regulates Neuronal Excitability

Regulation of KCC2 function by the metabotropic zinc receptor mZnR; # 2011126 PIs: Elias Aizenman and Michal Hershfinkel EXECUTIVE SUMMARY While it has been suggested that Zn2+ can regulate neuronal excitability and seizure activity, a mechanism underlying this process has heretofore remained unknown. Our collaborative work has showed that zinc receptor (mZnR/GPR39) signaling triggers a homeostatic mechanism for Zn2+-dependent regulation of neuronal excitability. Our results indicate that the synaptic metabotropic zinc receptor mZnR/GPR39 is the mechanism that regulates physiological adaptation to epileptic seizures. We showed that following a single intraperitoneal injection of the seizure inducer kainic acid mice deficient of the mZnR/GPR39 have dramatically enhanced susceptibility to seizures compared to wild type (WT) littermates that express a functional mZnR/GPR39. Importantly, we showed that application of kainic acid to brain slices induced synaptic Zn2+ release, and that this resulted in enhanced membrane expression of the K+/Cl- cotransporter KCC2 in WT, but not mZnR/GPR39 deficient hippocampal neurons. As such, kainic acid, via this mechanism, enhances the KCC2-dependent transport activity of Cl- out of the cell and enhances the inhibitory drive. Therefore, mZnR/GPR39 may provide a novel pharmacological target for dampening epileptic seizure activity. Under pathological conditions, however, there are changes in pH that may affect the signaling triggered by Zn2+ and mZnR/GPR39. We therefore addressed the role of changes in extracellular and intracellular pH in Zn2+-dependent signaling. We report that upregulation of a major H+ extrusion pathway in neurons, the Na+/H+ exchanger (NHE), is induced by mZnR/GPR39 activation that relieves intracellular acidification in hippocampal neurons in vitro. Yet, we also observed that changes in extracellular pH, pHe, can modulate the neuronal mZnR/GPR39-dependent signaling, resulting in reduced activity at pHe 8 or 6.5. This mechanism may serve to protect neurons from excessive decreases in pHe.

Altogether, this study established the important physiological role of synaptic Zn2+ inducing a self-dampening drive during excessive neuronal activity. These results reveal the signaling pathways activated by synaptic Zn2+ and the mZnR/GPR39 and the cellular mechanism for regulating KCC2 activity. The identification of the pathway and its regulation by extracellular pH will pave the way for new therapeutic approaches to relieve the burden of seizures.