Transcriptional profile of hippocampal dentate granule cells in four rat epilepsy models.

Our goal is to elucidate the cellular and molecular mechanisms underlying epileptogenesis, the process by which a normal brain becomes epileptic.  The epilepsies constitute a group of common, serious neurological disorders, among which temporal lobe epilepsy (TLE) is the most prevalent and devastating. Many patients with severe TLE experienced an episode of prolonged seizures (status epilepticus, SE) years prior to the onset of TLE. Because induction of SE alone is sufficient to induce TLE in diverse mammalian species, the occurrence of de novo SE is thought to contribute to development of TLE in humans.  Elucidating the molecular mechanisms by which an episode of SE induces lifelong TLE in an animal model will provide targets for preventive and/or disease modifying therapies.   Using a chemical-genetic method, we discovered a molecular mechanism required for induction of TLE by an episode of SE, namely, the excessive activation of the BDNF receptor tyrosine kinase, TrkB (Liu et al., 2013).  We subsequently discovered that phospholipase Cg1 is the dominant signaling effector by which excessive activation of TrkB promotes epilepsy (Gu et al., 2015).  We designed a novel peptide (pY816) that uncouples TrkB from phospholipase Cg1.  Treatment with pY816 following status epilepticus prevented TLE (Gu et al., 2015).  In addition to prevention, we have now shown that partial reversal of  epileptogenesis with pY816(Krishnamurthy et al., 2019), raising the possibility of ameliorating TLE after it has developed.   Collectively, these findings provide proof-of-concept evidence for a novel strategy targeting receptor tyrosine kinase signaling and identify a novel therapeutic for prevention and disease modification of TLE.   

There are two major objectives of our current work.    1.  We are developing peptide and small molecule inhibitors of TrkB signaling for advancement to the clinic. 2.  We seek to understand the cellular consequences of TrkB activation that transform the brain from normal to epileptic.  We have identified the sites within hippocampus at which SE-induced activation of TrkB occurs (Helgager et al 2013).  One is the spines of apical dendrites of CA1 pyramidal cells.  We are utilizing an in vitro model in which we mimic the enhanced synaptic release of glutamate during SE.  Using two photon uncaging microscopy, exquisitely localized high concentrations of glutamate are generated over a spine of an apical dendrite of a CA1 pyramidal cell in cultured hippocampus, resulting in long term potentiation. We have developed novel sensors to dynamically image activation of TrkB within a single spine. We have discovered that induction of long term potentiation requires activation of TrkB, mediated in part by uncaging induced release of BDNF from the same spine (Harward et al 2016).  This provides a valuable model with which to elucidate the mechanisms mediating activation of TrkB and the downstream signaling pathways by which its activation promotes long term potentiation (Hedrick et al 2016).

Helgager J, Liu G, McNamara JO.  The cellular and synaptic location of activated TrkB in mouse hippocampus during limbic epileptogenesis. J Comp Neurol. 521(3):499-521. 2013. (PMCID: PMC3527653)

Liu, G., Gu, B, He, X., Joshi, R.B., Wackerle, H.D., Rodriguiz, R.M., Wetsel, W.C., and McNamara, J.O. Transient Inhibition of TrkB Kinase after Status Epilepticus Prevents Development of Temporal Lobe Epilepsy. Neuron 79:31-38, 2013. (PMCID: PMC3744583).*

Gu, B., Huang,  Yang Zhong Huang, He, Xiao-Ping He, Joshi, R. B., Jang, Wonjo,  & McNamara, J.O.  A Peptide Uncoupling BDNF Receptor TrkB from Phospholipase Cγ1 Prevents Epilepsy Induced by Status Epilepticus.  Neuron 88(3):484-491, 2015.  PMID:26481038. PMCID: pending

Harward, S. C., Hedrick, N. G., Hall, C. E., Parra-bueno, P., Milner, T. A., Pan, E., … Yasuda, R., McNamara J.O. (2016). Autocrine BDNF-TrkB signalling within a single dendritic spine, 13–16. doi:10.1038/nature19766

Hedrick, N. G., Harward, S. C., Hall, C. E., Murakoshi, H., McNamara, J. O., & Yasuda, R. (2016). Rho GTPase complementation underlies BDNF-dependent homo- and heterosynaptic plasticity. Nature. doi:10.1038/nature19784

Krishnamurthy K, Huang YZ, Harward SC, Sharma KK, Tamayo DL, McNamara J.O.  Regression of Epileptogenesis by Inhibiting Tropomyosin B Signaling Following a Seizure. Annals of Neurology 86(6): 939-950, 2019.

Our publications can be found at: http://www.ncbi.nlm.nih.gov/sites/myncbi/1rMG926fr2ikx/bibliography/48320844/public/?sort=date&direction=ascending

Research in this laboratory focuses primarily on mechanisms of epileptogenesis, that is, the process by which normal brain tissue becomes prone to seizures. Additional studies concern the mechanisms of excitatory synaptic transmission in the mammalian brain. We use an interdisciplinary approach that involves diverse methodologies, including cellular electrophysiology, immunocytochemistry, electron microscopy, confocal microscopy and high speed imaging.

In persons with temporal lobe epilepsy, the most common form of epilepsy in adults, mossy fibers in the hippocampus form a reverberating excitatory circuit that probably contributes to seizure development. In addition, neurons generated as a result of seizures migrate to aberrant locations, become incorporated into the reverberating excitatory circuit and fire spontaneously. We are studying the physiology and pharmacology of this circuit and its role in epileptogenesis with use of brain tissue from animals that have been made epileptic. Properties of recurrent mossy fiber synapses and ectopically-located hyperexcitable neurons may be exploited to develop novel approaches toward the treatment of temporal lobe epilepsy.

In addition, we are investigating the mechanism and significance of aspartate release. Aspartate is co-released with glutamate from some excitatory terminals in the brain. Recent data indicate that aspartate and glutamate are released by distinct mechanisms. However, the role of aspartate in excitatory transmission is currently unknown. One possibility suggested by our findings is that aspartate serves a paracrine function, targeting extrasynaptic NMDA receptors. Extrasynaptic and synaptic NMDA receptors couple to different signaling mechanisms and have opposite effects on cell survival. Thus the aspartate release/extrasynaptic NMDA receptor pathway offers a new target for pharmacological intervention in neuropsychiatric disease.