Browsing by Author "Krishnamurthy, Kamesh"
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Item Open Access Disease Modification of Epilepsy by Disruption of TrkB Signaling(2019) Krishnamurthy, KameshEpilepsy is the most common acquired neurological disorder and is characterized by spontaneous, recurrent seizures. Of the various forms of epilepsy, Temporal Lobe Epilepsy (TLE) has received intense clinical and research interest. Current therapeutic options for TLE are anti-convulsive and purely symptomatic. Improved treatments are needed that either (1) prevent epileptogenesis or (2) ameliorate existing disease. Studies suggest that TLE may be induced by a preceding episode of prolonged seizure activity (status epilepticus, or SE). Our lab previously utilized a chemical-genetic strategy to establish proof of concept that transient inhibition of the receptor tyrosine kinase TrkB following SE prevented TLE. Subsequent studies identified the downstream effector of TrkB activation by demonstrating that transient administration of a peptide (“pY816”) uncoupling TrkB from the enzyme PLCγ1 also prevented SE-induced TLE.
TLE is analogous to associative memory formation in that both involve activity-determined plasticity. Associative memories can be rendered labile following re-exposure to the inciting stimulus; during this labile period inhibition of molecular mechanisms necessary for initial learning inhibits reconsolidation and results in memory “erasure”. Given the proposed parallels between epileptogenesis and memory formation as well as the central role of TrkB-PLCγ1 signaling in the development of epilepsy, I sought to test whether the occurrence of a seizure introduces a period of lability and whether inhibition of TrkB-PLCγ1 signaling prevents subsequent reconsolidation. I demonstrate in the kindling model of TLE that the combination of an evoked seizure and chemical-genetic inhibition of TrkB kinase, but not inhibition of TrkB kinase alone, reduces the severity of subsequent evoked seizures. Combination of an evoked seizure and pY816 (but not pY816 alone) produces the same effect. These results suggest that seizures induce a period of lability in a model of TLE and perturbation of TrkB-PLCγ1 signaling inhibits reconsolidation of pathologic plasticity.
In specimens from patients who underwent surgical resection for medically refractory TLE there is a striking increase in expression of the ligand for TrkB, BDNF. In a second study, I demonstrate that this increase, as well as an increase in TrkB-PLCγ1 signaling, is also seen in a TLE model exhibiting spontaneous seizures. Given the result that TrkB-PLCγ1 inhibition prevents reconsolidation, I asked what effect treatment with pY816 has in an animal model after spontaneous seizures emerged. I demonstrate that pY816 induced a remission in seizures that persists after treatment termination.
These studies elucidate a signaling pathway (TrkB-PLCγ1) underlying epilepsy progression and persistence, connect TLE to other disorder of pathologic plasticity like PTSD and neuropathic pain, and open the door to a novel therapeutic approach for treating patients with existing epilepsy.
Item Open Access Regression of Epileptogenesis by Inhibiting Tropomyosin Kinase B Signaling following a Seizure.(Annals of neurology, 2019-12) Krishnamurthy, Kamesh; Huang, Yang Zhong; Harward, Stephen C; Sharma, Keshov K; Tamayo, Dylan L; McNamara, James OOBJECTIVE:Temporal lobe epilepsy (TLE) is a devastating disease in which seizures persist in 35% of patients despite optimal use of antiseizure drugs. Clinical and preclinical evidence implicates seizures themselves as one factor promoting epilepsy progression. What is the molecular consequence of a seizure that promotes progression? Evidence from preclinical studies led us to hypothesize that activation of tropomyosin kinase B (TrkB)-phospholipase-C-gamma-1 (PLCγ1) signaling induced by a seizure promotes epileptogenesis. METHODS:To examine the effects of inhibiting TrkB signaling on epileptogenesis following an isolated seizure, we implemented a modified kindling model in which we induced a seizure through amygdala stimulation and then used either a chemical-genetic strategy or pharmacologic methods to disrupt signaling for 2 days following the seizure. The severity of a subsequent seizure was assessed by behavioral and electrographic measures. RESULTS:Transient inhibition of TrkB-PLCγ1 signaling initiated after an isolated seizure limited progression of epileptogenesis, evidenced by the reduced severity and duration of subsequent seizures. Unexpectedly, transient inhibition of TrkB-PLCγ1 signaling initiated following a seizure also reverted a subset of animals to an earlier state of epileptogenesis. Remarkably, inhibition of TrkB-PLCγ1 signaling in the absence of a recent seizure did not reduce severity of subsequent seizures. INTERPRETATION:These results suggest a novel strategy for limiting progression or potentially ameliorating severity of TLE whereby transient inhibition of TrkB-PLCγ1 signaling is initiated following a seizure. ANN NEUROL 2019;86:939-950.Item Open Access TrkB-Shc Signaling Protects against Hippocampal Injury Following Status Epilepticus.(The Journal of neuroscience : the official journal of the Society for Neuroscience, 2019-06) Huang, Yang Zhong; He, Xiao-Ping; Krishnamurthy, Kamesh; McNamara, James OTemporal lobe epilepsy (TLE) is a common and commonly devastating form of human epilepsy for which only symptomatic therapy is available. One cause of TLE is an episode of de novo prolonged seizures [status epilepticus (SE)]. Understanding the molecular signaling mechanisms by which SE transforms a brain from normal to epileptic may reveal novel targets for preventive and disease-modifying therapies. SE-induced activation of the BDNF receptor tyrosine kinase, TrkB, is one signaling pathway by which SE induces TLE. Although activation of TrkB signaling promotes development of epilepsy in this context, it also reduces SE-induced neuronal death. This led us to hypothesize that distinct signaling pathways downstream of TrkB mediate the desirable (neuroprotective) and undesirable (epileptogenesis) consequences. We subsequently demonstrated that TrkB-mediated activation of phospholipase Cγ1 is required for epileptogenesis. Here we tested the hypothesis that the TrkB-Shc-Akt signaling pathway mediates the neuroprotective consequences of TrkB activation following SE. We studied measures of molecular signaling and cell death in a model of SE in mice of both sexes, including wild-type and TrkBShc/Shc mutant mice in which a point mutation (Y515F) of TrkB prevents the binding of Shc to activated TrkB kinase. Genetic disruption of TrkB-Shc signaling had no effect on severity of SE yet partially inhibited activation of the prosurvival adaptor protein Akt. Importantly, genetic disruption of TrkB-Shc signaling exacerbated hippocampal neuronal death induced by SE. We conclude that therapies targeting TrkB signaling for preventing epilepsy should spare TrkB-Shc-Akt signaling and thereby preserve the neuroprotective benefits.SIGNIFICANCE STATEMENT Temporal lobe epilepsy (TLE) is a common and devastating form of human epilepsy that lacks preventive therapies. Understanding the molecular signaling mechanisms underlying the development of TLE may identify novel therapeutic targets. BDNF signaling thru TrkB receptor tyrosine kinase is one molecular mechanism promoting TLE. We previously discovered that TrkB-mediated activation of phospholipase Cγ1 promotes epileptogenesis. Here we reveal that TrkB-mediated activation of Akt protects against hippocampal neuronal death in vivo following status epilepticus. These findings strengthen the evidence that desirable and undesirable consequences of status epilepticus-induced TrkB activation are mediated by distinct signaling pathways downstream of this receptor. These results provide a strong rationale for a novel therapeutic strategy selectively targeting individual signaling pathways downstream of TrkB for preventing epilepsy.