AbstractAutism Spectrum Disorder (ASD) is characterised by deficits in social interaction and the presence of repetitive behaviours. The clinical symptoms and genetic architecture of ASD are highly heterogeneous, which has confounded attempts to determine the causative factors and pathological mechanism underlying the disorder. Whole-exome sequencing studies of large numbers of ASD probands have implicated de novo deleterious mutations within the gene encoding chromodomain helicase DNA-binding protein 8 (CHD8), an ATP-dependent chromatin remodeller with important roles in transcriptional regulation during neurodevelopment, as being a highly penetrant risk factor for ASD. However, the pathological mechanisms linking CHD8 mutations with ASD aetiology remain undetermined.
Improper levels of excitatory and inhibitory activity (E:I balance) within the prefrontal cortex (PFC), due to altered synaptic development and/or homeostatic plasticity responses, have been proposed to act as a point of convergence through which highly diverse genetic risk factors can manifest as ASD. Reducing CHD8 expression in neuronal cells consistently results in dysregulated expression of genes vital for synaptic development and a mouse line that is heterozygous for a deleterious Chd8 allele (Chd8+/-) displays widespread dysregulated expression of genes that are associated with ASD-risk and involved in synaptic development and function.
This thesis aims to characterise the impact of reduced Chd8 expression upon E:I balance and homeostatic synaptic plasticity within the PFC of Chd8+/- mice. Firstly, we utilise whole-cell voltage clamp recordings to assess spontaneous synaptic transmission within ex vivo cortical slices prepared from Chd8+/- and wildtype mice across several developmental time points. We findthat E:I balance is substantially reduced in Chd8+/- neurons during a critical juvenile period of neurodevelopment. Next, we assess a range of neuronal features within the PFC of Chd8+/- mice to establish how these changes to E:I balance might manifest, concluding that they arise through distinct mechanisms operating within individual neuronal subtypes that are largely cell-autonomous. In addition, we develop a protocol to induce homeostatic plasticity changes within ex vivo slices and utilise it to show that Chd8+/- neurons display an altered ability to undergo both excitatory and inhibitory synaptic homeostatic plasticity. We also conducted analyses suggesting that reduced Chd8 expression has more severe impacts on the synaptic development of male mice. Finally, we examine the interaction between Chd8 and the canonical Wnt signalling pathway by beginning experiments to assess whether the synaptic phenotype could be altered by modulating the levels of β-catenin expression in Chd8+/- mice.
|Date of Award
|1 Sept 2020
|Albert Basson (Supervisor) & Laura Andreae (Supervisor)