Semaphorin-dependent GABAergic synapse formation: A novel approach to increasing inhibition in the intact brain

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We propose to harness the synaptogenic potential of Sema4D signaling to increase GABAergic synapse number, thus enhancing inhibition in neural circuits and suppressing seizures. This approach could be beneficial to preventing the establishment of epilepsy, halting its progression, or suppressing hyperexcitability during a seizure event. Previously our lab discovered that the extracellular domain of transmembrane protein Semaphorin 4D (Sema4D) drives inhibitory synapse formation on a remarkably fast time scale (i.e. minutes) in hippocampal neurons and slice cultured from the pre-natal and neonatal hippocampus. We also demonstrated that intra-hippocampal infusion of purified, Sema4D extracellular domain into the adult hippocampus rapidly promotes the formation of new GABAergic synapses. Further, we reported that Sema4D treatment protects against seizures induced by electrical stimulation of the dentate gyrus or by intravenous infusion of the proconvulsant drug pentylenetetrazol. Given the success of these studies, we undertook a new experimental direction to determine if Sema4D treatment has therapeutic potential for human epilepsies. The first aim of this proposal is to investigate Sema4D treatment as an acute, anti-seizure therapeutic in rodent models of status epilepticus (SE). SE is a medical emergency requiring immediate intervention in humans; approximately 30% of patients with SE are refractory to treatment with current medications including benzodiazepines. Preliminary data presented in this proposal shows that Sema4D treatment restored the efficacy of diazepam in a rodent model of refractory SE. This result is consistent with our hypothesis that Sema4D treatment acutely increases the number of GABAergic synapses in hippocampus, which maintains or re-establishes benzodiazepine sensitivity in the brain. The second aim of our proposal is to determine the effect of chronic exposure of Sema4D (via viral delivery to the CNS) on inhibitory synapse formation and ultimately, seizure frequency and severity in rodent models of chronic epilepsy. As a first step towards this goal we will explore the efficacy and time-course of alternative methods of administering Sema4D (e.g. intracerebral injection of virus encoding Sema4D) to mice. In the third aim, we will begin to address the translatability of our findings with Sema4D in rodents by asking if Sema4D promotes inhibitory synapse formation and network activity in cultures of human neurons.