The Role of the E3 Ubiquitin Ligase Cul3 in Brain Development and Neurodevelopmental Disorders

Background: The past decade has seen a rapid development of technological advancements in genetics and genomics, allowing unprecedented precision in the identification of mutations underlying neurodevelopmental conditions. Neurodevelopmental disorders, such as autism spectrum disorders (ASDs) and intellectual disability (ID), affect more than 3% of children worldwide, yet for the majority of the cases there are no curative therapies and the underlying molecular causes remain elusive. Here we analyze the molecular and behavioral changes of a mouse model haploinsufficient for the high risk ASD gene Cul3. Cul3 encodes a E3 ubiquitin ligase, involved in the recognition and recruitment of target proteins for ubiquitination.

Objectives: We chose to analyze Cul3 in brain development and function, because of its strong link to the ASDs and its putative role in dynamically regulating brain protein composition. Our aim is to uncover the pathological pathways causing the observed behavioral abnormalities in Cul3^+/- animals.

Methods: We first assessed Cul3 expression throughout mouse development and in the adult brain by western blot and qPCR analysis. Furthermore, we employed a large battery of histological, molecular and behavioral assays in Cul3^+/- adult animals (2-4 month old) and their sex-matched littermates (n > 10). Additionally, we investigated the effect of forebrain specific Cul3 homozygous deletion in embryos and newborn animals, making use of Cul3^flox/Emx1-Cre double transgenic animals.

Results: Cul3 haploinsufficient animals display mild but significant motor defects, abnormal behavior in the three chamber sociability test and aberrant freezing behavior in the contextual fear conditioning task. Phenotypes that were previously also described in mouse models for ASD and ID. In addition, Cul3^flox/flox/Emx1-Cre transgenic animals present severe brain abnormalities at postnatal day 1 and do not survive into adulthood. To investigate the effects of Cul3 loss on the protein composition of the brain, whole quantitative proteomic analysis is ongoing for different timepoints and on multiple brain regions.

Conclusions: Currently, there are no effective pharmacological therapies for autism spectrum disorders. Many factors contribute to this absence of effective treatments, not least the lack of a clear understanding of their neurobiological causes, genetic and phenotype heterogeneity. Cul3^+/- adult animals present with ASD relevant phenotypes linked to abnormal brain development and regulation of protein composition in the brain. Our findings indicate a pivotal role of the E3 ubiquitin ligase Cul3 for normal brain development and function.