AMPA Receptor Ubiquitination and Pathological Synaptic Hyperexcitability

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Epilepsy affects 3 million people in the United States. Despite the development of current antiepileptic drugs to raise seizure threshold, one-third of epilepsy patients either respond poorly to the drugs or remain drug- resistant. Our research aims to facilitate the understanding neuronal excitability dysregulation in epilepsy with the intention to improve therapeutic outcome. We focus on α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA receptor; AMPAR), the most abundant receptor in the nervous system and one of the well- studied excitatory synaptic proteins. Elevated levels of AMPAR have been observed in epilepsy patients, and pharmacologically inhibiting AMPAR has been used in clinical practice for alleviating epilepsy. Despite all these facts, it remains unclear how the homeostasis of AMPAR mediates brain excitability and how dysregulated AMPAR contributes to epilepsy. We recently identified a novel ubiquitin E3 ligase for the GluA1 subunit of AMPAR, neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4-2). Nedd4-2 is encoded by an epilepsy-associated gene, in which three missense mutations have been identified in patients with epilepsy. Our recent publication demonstrated that Nedd4-2 mediates neuronal and brain excitability in an AMPAR-dependent manner (Zhu et al., PLOS Genetics, 2017). However, it remains unknown (1) whether and how Nedd4-2 modulates AMPAR to affect excitatory synaptic transmission; and (2) how epilepsy-associated mutations affect the functions of Nedd4-2 in this regard. Aim 1 and Aim 2 are designed to answer these questions. Nedd4-2 is a target gene of, and transcriptionally repressed by, the tumor suppressor p53. Our work showed that inhibition of p53 reduces acute seizure susceptibility in mice in a Nedd4-2-dependent manner. This finding, together with our previous work, suggests p53-Nedd4-2 as a novel signaling axis to maintain brain excitability presumably through limiting AMPAR. Aim 3 will study the regulation of p53-Nedd4-2 signaling and AMPAR ubiquitination using a preclinical model of temporal lobe epilepsy in mice, and determine the roles of p53-Nedd4- 2 signaling in epileptogenesis in this model. Successfully accomplishing this project will improve the understanding of ion channel dysregulation in epilepsy and foster future development of therapies in treating epilepsy.