Mapping neuronal network in Angelman syndrome – the role of ubiquitin-protein ligase e3a in shaping synapses

Neuronal maturation and plasticity are manifested through dynamic changes in synaptic function and structure. Morphological changes of the pre- and postsynapse reflect its activity and strength. In excitatory synapses, size and shape of dendritic spines are finely regulated during development and experience-dependent remodelling of the neuronal network. Indeed, early and late onset neurological conditions arise as consequences of mutations in genes accounting for synapse maturation and plasticity. Genomic alterations leading to unbalanced expression of the Ubiquitin Ligase E3A (UBE3A) cause defects in brain development, leading to neurodevelopmental disorders. Increased expression/activity of UBE3A in neurons is associated to the appearance of autistic-like phenotypes, while its reduction due to the loss of the maternal copy of UBE3A gene (paternally imprinted in neurons) is causing Angelman Syndrome (AS). AS is a rare neurological disorder affecting around 1:20.000 newborn. Features of AS include delayed development, severe learning difficulties, little or no speech, movement and balance impairments, seizures. Most of these symptoms are likely originating from synaptic defects. To clarify whether neuronal dysfunctions and synaptic alterations in AS are due to a disrupted neuronal network wiring or if they have a cell-autonomous origin, we took advantage of two different mouse models: 1) UBE3A-KO mice in which all neurons are lacking the expression of the ligase, and 2) in utero-electroporated mice in which a subset of pyramidal neurons of layer 2/3 of the somatosensory cortex is Ube3adepleted with CRISPR/Cas9 mediated genome editing. Synaptic connectivity of excitatory neuronal circuits in AS models were characterized and mapped using advanced imaging technologies as volume Electron Microscopy (vEM) and volume Correlative Light and Electron Microscopy (vCLEM)