Asymmetry of hippocampal function in mice: left-right differences in memory processing and vulnerability to amyloid beta

Amyloid beta (ABeta) and tau protein are both implicated in memory impairment in early Alzheimer’s disease, but whether and how they interact to cause synaptic dysfunction are unknown. Consequently, I firstly investigated whether tau protein is required for the robust phenomenon of ABeta-induced impairment of hippocampal long-term potentiation (LTP), a widely accepted cellular model of memory. I demonstrate that the absence of tau prevents the ABeta-induced impairment of LTP; moreover, a specific inhibitor of the tau kinase glycogen synthase kinase 3 blocks both an ABeta-induced increase in tau phosphorylation and the ABeta-induced LTP impairment. Thus, tau protein, likely in its phosphorylated form, is required for ABeta to impair LTP. Secondly, I investigated the underlying mechanisms for this ABeta-induced impairment and find that ABeta changes the balance between the two major types of glutamate receptors involved in plasticity processes, with a specific effect on GluN2B subunit-containing NMDA receptors. Since the distribution of these receptors is asymmetric between the left and right mouse hippocampus, I accessed these different types of synapses optogenetically and found that only the GluN2B-rich synapses receiving left CA3 input show ABeta-induced changes in the balance of glutamate receptors, suggesting an asymmetry in synaptic vulnerability to ABeta. Moreover, there was a left-right difference in tetanus-induced LTP and therefore, thirdly, I investigated whether mice have a hemispheric dissociation in memory processing using acute optogenetic silencing of left or right CA3 during hippocampus-dependent memory tasks. Unilateral silencing of either the left or the right CA3 caused a deficit in short-term memory, but only left CA3 silencing impaired performance on a spatial long-term memory task. Together, these results suggest that memory may be routed via distinct left-right pathways within the mouse hippocampus, and that neural pathways subserving distinct functions may also be differentially vulnerable to pathological changes at the synaptic level.This thesis is not currently available in ORA