Synaptic function in the hippocampus in neurodegenerative disease

Abstract

At glutamatergic synapses in the brain the ionotropic glutamate receptors N-Methyl-D-Aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, are bound to membrane-associated guanylate kinases (MAGUKs). Together they regulate important brain functions such as synaptic transmission, plasticity and learning and memory. MAGUKs are the central organisers of excitatory synapses where they are responsible for the architecture of the postsynaptic density (PSD), glutamate receptor trafficking and activating downstream signalling molecules. Neurodegenerative diseases are known to directly affect synapses (Wishart et al., 2006, Raymond et al., 2011) and studying the early changes that occur at the level of the synapse is of the essence for future therapeutic strategies. The work in this thesis has examined the changes that occur in glutamate receptor subunits and MAGUK expression in postmortem human brain tissue from Huntington’s disease (HD) and Parkinson’s disease (PD) patients as well as synaptic function in an animal and cell model of HD. The first part of this thesis aimed to investigate the changes that occur in glutamate receptor subunit and MAGUK expression in the human brain in HD and PD in both the hippocampus and striatum. We specifically investigated changes that occur in the expression of synapse associated protein 97 kDa (SAP97), postsynaptic density protein 95 kDa (PSD95), the GluA2 subunit of the AMPA receptor and the GluN1 subunit of the NMDA receptor. Our immunohistochemical data revealed that these synaptic proteins are significantly and differentially altered in the hippocampus vs. striatum of HD and PD cases. We hypothesise that the hippocampal changes in synaptic protein expression underlie the cognitive deficits in HD and PD patients (Diamond et al., 1992, Emre, 2003, Ziemssen and Reichmann, 2007, Paulsen et al., 2008). Alongside these changes in the human brain we show no changes in any of the glutamate receptor subunits or MAGUKs in the hippocampus of the YAC128 animal model of HD at 12 months of age. These data indicated that unique changes are occurring in the hippocampus in the human HD brain compared with the YAC128 animal model of HD, correlating with the relative sparing of the hippocampus in the YAC128 model (Slow et al., 2003, Van Raamsdonk et al., 2005a) as opposed to the reported cell loss in the human hippocampus (Spargo et al., 1993, Rosas et al., 2003). The second part of this thesis aimed to examine synaptic vs. extrasynaptic NMDA receptor signalling and synaptic plasticity in HD in the hippocampus. These experiments utilised acute brain slices from the YAC128 animal model of HD as well as a hippocampal cell culture model of HD. We studied the YAC128 animal model at the early age of 1 month to detect early changes in synaptic function at an age where we have shown significant cognitive decline to occur (Kim, J., Fourie, C. et al, manuscript in preparation) before classic motor symptom onset. Our electrophysiology data show for the first time that hippocampal synaptic transmission through AMPA and NMDA receptors are normal in the YAC128 slices as compared to slices from their wild type (WT) littermates. Electrophysiology in hippocampal slices further revealed that extrasynaptic NMDA receptor currents were normal in YAC128 slices unlike that reported in the striatum (Milnerwood et al., 2010, Milnerwood et al., 2012). Furthermore, we established a dissociated hippocampal culture model system of HD by transfecting hippocampal neurons with mutant or WT huntingtin. Similarly, electrophysiology experiments using this model also revealed normal synaptic and extrasynaptic glutamate receptor signalling unlike in dissociated striatal neurons from the YAC128 animal model (Milnerwood et al., 2012). These results indicate that extrasynaptic NMDA receptor signalling is unaltered by mutant huntingtin in the hippocampus in our study as compared to increased signalling reported in the striatum. These data correlate with the relative sparing of hippocampal neurons in the YAC128 animal model in contrast to striatal neurons in animal and cell models of HD (Slow et al., 2003, Van Raamsdonk et al., 2005a, Milnerwood et al., 2010, Milnerwood et al., 2012). Furthermore, we also show for the first time a significant deficit in long term potentiation (LTP) in YAC128 hippocampal slices at the young age of 1 month and suggest that this underlies the cognitive and memory impairment. Our results indicate that this is not due to altered presynaptic function or an increase in the threshold for LTP but involves postsynaptic changes. Altogether our results have important consequences for future studies of aberrant synaptic function in neurodegenerative disease and future therapeutic strategies.