Investigating the role of Seizure related gene 6 family proteins and their BACE shed products at excitatory synapses: impacts on motor and cognitive function

Abstract

Correct wiring of the brain during development is a complex process that involves the precise spatial and temporal expression of a vast number of genes and proteins. Excitatory synapses in the central nervous system are formed primarily onto specialised protrusions of the dendrite called dendritic spines. During development, synaptic contacts mature into dendritic spines through a process that requires stabilisation and strengthening of the synaptic contact. Strengthening of neuronal connections is not only important for synaptic maturation during development but is also generally assumed to be the cellular process underlying learning and memory in the adult. Seizure related gene 6 (Sez6), Sez6 Like (Sez6L) and Sez6 Like 2 (Sez6L2) comprise a family of homologous proteins widely expressed throughout the brain and Sez6 knockout (KO) mice revealed a role for Sez6 in dendritic arborisation and the development of excitatory synapses. Additionally, Sez6 family proteins have been linked to neurodevelopmental disorders, with which synaptic dysfunction is commonly associated. Therefore, the broad aim of this project was to further characterise the role of Sez6 family proteins, and their shed ectodomains, in synaptic structure and function and additionally investigate their role in motor and cognitive function in the mouse. To elucidate the functions of Sez6 family proteins in the central nervous system, wild-type (WT) and Sez6 triple knockout (TKO) mice, which lack all three Sez6 family proteins, were used. Compared to WT, Sez6 TKO mice were found to have reduced spine density in the hippocampus and dendritic spines were shifted to more immature morphologies in the somatosensory cortex. Additionally, synaptic function was found to be altered in the prelimbic cortex. Sez6 TKO mice had impaired motor learning and motor coordination from as early as 6 weeks old. Cognitive testing revealed Sez6 TKO mice had enhanced stress responsiveness, impaired working and spatial short-term memory but intact spatial long-term memory in the Morris water maze that was accompanied by a reversal deficit. Sez6 family members are substrates of the Alzheimer’s protease, beta-amyloid precursor protein cleaving enzyme 1 (BACE1), and are cleaved proximal to the transmembrane domain to produce a shed ectodomain. The contribution of the shed Sez6 family protein ectodomains to cognition, and the potential for BACE inhibitor treatment of Alzheimer’s disease to be associated with cognitive side-effects as a result of impaired processing of other non-amyloid precursor protein substrates, was assessed by chronically treating WT and Sez6 TKO mice with a BACE inhibitor. In both WT and Sez6 TKO mice, BACE inhibition did not significantly impact cognition or anxiety-related behaviour in the behavioural paradigms utilised. WT mice treated with BACE inhibitor were, however, found to be hyperactive on the elevated open field and this effect was not observed in treated Sez6 TKO mice suggesting a role for the shed ectodomains of Sez6 family proteins in regulating activity levels. The results presented in this thesis further our understanding of the role of Sez6 family proteins in synaptic structure and function and contribute to the body of evidence that links Sez6 family proteins to the cognitive and motor deficits observed in patients with neurodevelopmental disorders, such as autism and schizophrenia.