Abstracts

Rationale: Hippocampal sclerosis is a pathological hallmark of mesial temporal lobe epilepsy. Excessive levels of reactive oxygen species (ROS) are produced during status epilepticus and thought to contribute to seizure induced hippocampal neuronal injury and sclerosis. Therefore, understanding the molecular signaling mechanism by which excess ROS impact life and death of cortical neurons may provide novel targets that could be exploited to protect neurons from seizure insults. While the neurotoxic consequences of ROS in the central nervous system are widely recognized, neuroprotective consequences of ROS are less well appreciated.Our laboratory has been studying the role of the brain derived neurotrophic factor (BDNF) receptor tyrosine kinase, TrkB, in limbic epileptogenesis. In the course of these studies, we discovered that the divalent cation, zinc, can activate TrkB in the absence of BDNF. Several facts led us to explore the effects of zinc on neuronal survival and how such effects might be related to ROS and TrkB. Enhanced TrkB activation promotes survival of CNS neurons. Vesicular zinc is present in axon terminals of hippocampal granule cells and thought to contribute to death of CA3 pyramids during status epilepticus. Methods: Using cultured embryonic rat cortical neurons as a model, we performed a series of biochemical, live cell imaging, and cell death assays. Results: We discovered that: 1) addition of ROS in the form of exogenous H2O2 to cortical neurons enhances cytosolic zinc content by promoting its release from a zinc binding protein, metallothionin-3 (MT-3); 2) both exogenous and endogenous ROS-induced release of zinc activates TrkB signaling by a Src family kinase dependent but BDNF-independent mechanism; 3) to our surprise, while ROS result in neurotoxicity, the ROS-triggered activation of TrkB exerts neuroprotective effects because inhibition of TrkB kinase activity or uncoupling Shc signaling from TrkB exacerbates neuronal cell death induced by H2O2. Conclusions: Thus we propose a molecular signaling event whereby ROS induce release of zinc from cytosolic MT-3, the increased cytosolic zinc promotes activation of TrkB, and the enhanced TrkB signaling promotes pro-survival effects. We suggest that such neuroprotective effects are operative in diverse acute and chronic neurological disorders, including temporal lobe epilepsy.