The Influence of Autism Spectrum Disorder Associated Shank2 Mutations on Excitatory Glutamatergic Synapses

Abstract

Autism Spectrum Disorders (ASDs) are neurodevelopmental disorders characterised by deficits in social communication and interactions, and restricted repetitive behaviours. ASDs have a strong genetic basis with mutations in hundreds of genes identified in ASD patients. Many of these are involved in the development and function of neural circuitry, and code for synaptic proteins. In excitatory glutamatergic synapses, Shank proteins are essential scaffolding proteins and act as master regulators of the postsynaptic density. Isoform specific Shank mutations have been found in ASD patients, and animal models expressing Shank deletions cause behavioural and neurological deficits characteristic of ASDs. This thesis investigates the impact of ASD-associated Shank2 mutations at the synaptic level. More specifically, the effects of ASD-associated S557N, V717F, A729T, R818H, G1170R, D1535N and L1722P Shank2 point mutations on excitatory glutamatergic synapses in dissociated hippocampal cultures are examined. In comparison to wild type Shank2, the density of synapses is significantly reduced in neurons expressing ASD-associated Shank2 mutations – a phenotype that could adversely affect the basal synaptic transmission and alter the excitation-inhibition balance within the hippocampal circuitry. Chronic zinc treatment had the potential to prevent the development of synaptic deficits in neurons expressing some ASD-associated Shank2 mutations and restore synaptic density back to levels present in wild type Shank2 expressing neurons. Furthermore, ASD-associated Shank2 mutations significantly increased postsynaptic and presynaptic protein expression, an effect that was not seen with the wild type protein. It is hypothesised that this may be a compensation for the reduction in synaptic density; but this could only be functionally significant if the increased protein level is translated to the receptor level. Data from this study revealed a reduction in the density of NMDA receptor expressing synapses in neurons with ASD-associated Shank2 mutations. Furthermore, the S557N Shank2 point mutation decreased the density as well as the intensity of surface AMPA receptor expression in hippocampal neurons. These ASD-associated reductions in synapse density accompanied with reduced surface receptor levels may have adverse consequences on synapse function and the overall hippocampal circuitry, causing impairments that may underlie the intellectual disabilities and behavioural deficits characteristic of ASDs.