Regulation of NMDA receptor surface distribution in a neuronal model of Huntington’s disease

Abstract

Huntington’s disease (HD) is a neurodegenerative genetic disorder caused by an expansion of the CAG repeat tract in the HTT gene, leading to a triad of motor, cognitive and psychiatric symptoms. N-methyl-D-aspartate-type glutamate receptors (NMDARs) that underlie excitatory synaptic transmission and plasticity were previously shown to be upregulated at extrasynaptic locations in HD. While activity of synaptic NMDARs promotes neuronal survival, activation of extrasynaptic NMDARs (ex-NMDARs) triggers cell death signalling pathways. Therefore, ex-NMDARs are currently thought to drive HD neurodegeneration. The first aim of this thesis was to determine whether the distribution of α-amino-3-hydroxy-5- methyl-4-isoxazolepropionic acid (AMPA) or NMDA receptors is altered at synapses in a cellular model of HD. Subsequently, we sought to determine whether synapse-associated protein 97 (SAP97) is a causative agent of HD synaptic dysfunction, as the β isoform was shown to direct glutamate receptors to extrasynaptic locations. Finally, we aimed to specifically target α- and βSAP97 isoforms in order to rescue normal receptor distribution in HD model neurons. A combination of super-resolution imaging and whole-cell patch-clamp electrophysiology techniques allowed us to determine that in HD model neurons expressing the mutant HTT gene, NMDAR distribution is shifted towards extrasynaptic sites, whereas AMPARs are largely unaltered. Endogenous SAP97 expression was unaffected both in the model neurons and in the YAC128 HD mouse model striatum. An overall increase in SAP97 expression could be detected in the YAC128 hippocampus, however, as it was not specific to βSAP97, this isoform is unlikely to be a part of the HD pathomechanism. Intriguingly, overexpression of either SAP97 isoform was found to maintain the normal NMDAR distribution in HD model neurons and prevent NMDAR drift to extrasynaptic sites. Together, the data presented in this thesis suggest that the number of ex-NMDARs is indeed elevated in mutant HTT expressing neurons. Furthermore, although βSAP97 does not appear to play a role in HD pathomechanism, both α and β isoforms of SAP97 were identified in this thesis as potential therapeutic targets for HD. In conclusion, our results add to the evidence for synaptic dysfunction in HD neurons and provide new clues towards designing an effective HD therapy.