Insights into E6AP regulation and function

Abstract

Angelman Syndrome is a severe neurological disorder caused by mutations in the UBE3A gene, which encodes the HECT E3 ubiquitin ligase E6AP. Since loss-of-function of E6AP is the monogenetic cause for Angelman Syndrome development, the identification of E6AP interaction partners and substrates is crucial to understand thephysiology of this disease. Although several E6AP substrates have been identified in the meantime, their stabilization does not or only partially provide explanations for the symptoms observed in Angelman patients. Therefore, the identification of pathways, which are deregulated in the absence of E6AP, is a prerequisite in order to understand E6AP function and its role in Angelman Syndrome physiology. Several approaches have been performed in our lab over the past few years to identify new substrates and interaction partners of E6AP. Among others, we found the giant HECT E3 ligase Herc2 as a potential new E6AP interaction partner. In the course of this project, it was possible to confirm the interaction of E6AP with Herc2 and furthermore map the binding sites required for interaction on both proteins. Moreover, E6AP ubiquitin ligase activity was stimulated upon binding to Herc2, indicating that this interaction has functional consequences. In conclusion, we were able to show that Herc2 does not serve as a substrate of E6AP, but rather acts as an allosteric activator of E6AP, which could probably be important for Angelman Syndrom physiology.In addition to the identification of E6AP interaction partners, previous proteomics-based approaches performed in our group identified several proteins that showed increased protein levels in the absence of E6AP, therefore representing putative E6AP substrates. Two of the candidates that have been identified in this manner are the proteins Arc and SFPQ. However, E6AP-mediated ubiquitination and degradation of these two proteins could not be demonstrated, indicating that Arc and SFPQ protein levels are regulated via pathways other than the ubiquitin-proteasome system. The facts that E6AP is a known modulator of steroid receptor signaling and that the promoters of Arc and SFPQ, respectively, contain putative estrogen-responsive elements, led us to investigate the role of E6AP in the transcription of estrogen responsive genes. Indeed, E6AP displayed a negative effect on estrogen receptor activity in luciferase reporter assays, indicating that E6AP acts as a repressor in this type of assay. Moreover, estrogen receptor signaling stimulated transcription of endogenous Arc and SFPQ genes and thus both could be classified as estrogen responsive genes. In addition, E6AP knockdown resulted in increased transcription of the Arc and SFPQ genes. Based on these results a model is proposed in this thesis, in which E6AP acts as a repressor of estrogen receptor mediated transcription. This offers a new perspective for the physiology of Angelman Syndrome, as global deregulation of estrogen responsive genes can potentially explain a variety of symptoms observed in Angelman patients.