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Role of Ube3a in neocortical plasticity and function

Michael D. Ehlers, M.D., Ph.D.
Duke University

Ben Philpot, Ph.D.
University of North Carolina at Chapel Hill

Autism and the related Angelman syndrome are both characterized by behavioral and cognitive deficits that arise in early childhood, but neither is accompanied by overt structural changes in the brain. Michael Ehlers of Duke University and Ben Philpot at the University of North Carolina propose that disruption of a protein degradation pathway could cause these disorders by preventing the necessary remodeling of neurons during development.

As a child explores the world, neurons record his or her experiences by remodeling the signaling connections known as synapses. During this process, new synapses are built between active neurons, and unused synapses are pruned away through targeted protein degradation by the ubiquitin pathway. This cycle of construction and destruction ensures that valuable synapses are built and strengthened, while inappropriate connections are removed. Mutations in the Ube3a gene disrupt the ubiquitin pathway and have been linked to Angelman syndrome, which, like autism, is accompanied by an excess of weak synapses in the brain. Ehlers and Philpot propose that abnormal expression of Ube3a could cause both autism and Angelman syndrome because the synapses are not properly remodeled during postnatal development.

The researchers plan to test this hypothesis by manipulating the expression of Ube3a in mice and then studying the development of the visual cortex as an example of postnatal learning and synapse remodeling. Using electrophysiology, they plan to measure the signaling strength of synapses after a learning experience to ascertain the role of the ubiquitin pathway in the modification of neurons. The researchers have preliminary data that suggest the synapses are not remodeled in the absence of Ube3a, preventing the formation of the strong synapses that would be needed for memory formation. They plan to continue these studies and perform more cognitive and behavioral assays to characterize the consequences of having weak synapses. Ehlers and Philpot also propose that modified learning practices may be able to compensate for low levels of Ube3a. Preliminary experiments have shown that limiting sensory inputs helps Ube3a-deficient mice regain synaptic plasticity. The researchers plan to pursue this experimental approach to identify new ways to enrich learning strategies for children with Angelman syndrome and autism.