In recent years, soluble amyloid beta oligomers (AßO) have emerged as key elements in the cascade leading to synaptic dysfunction and neurodegeneration in Alzheimer's disease (AD). Previous work from our lab has shown that synaptic zinc released during excitatory neurotransmission increases the formation and accumulation of AßO at synaptic sites, and that the AßO-zinc interaction accelerates oligomer formation. Other recent research has shown that sequestration of synaptic zinc by AßO disrupts synaptic function, a significant finding as zinc has been demonstrated to modulate seizure activity and signaling pathways, has a high affinity for AßO and accumulates in Aß plaques in AD brain.
We sought to investigate markers of seizure activity in zinc transporter ZnT3 knockout (ZnT3KO) mice, which lack synaptic zinc, and the effects of treatment with an anti-seizure drug on the cognitive impairment demonstrated by aged ZnT3KO mice. Hippocampus tissue collected from age-matched cohorts of wild type and ZnT3KO were assayed for markers of seizure activity, finding age-dependent alterations in levels of these markers consistent with seizure activity in ZnT3KO. To study the effects of seizure activity on cognition, memory was assessed after acute treatment with an anti-seizure drug, finding no significant improvement in six month old ZnT3KO.
We also investigated alterations in neurotrophic signaling pathways in acute hippocampal slices from ZnT3KO mice. Neurotrophic protein expression and phosphorylation were assessed, finding reduction in basal and activity-dependent levels of Erk1/2 and p-Erk1/2, reduction in AKT, age-dependent reduction in BDNF, and reduction in activity-dependent BDNF mRNA expression. Pharmacological treatments of acute hippocampus sections were performed to investigate the effects of Zn2+ on activation of Erk1/2 through NMDA receptors and receptor subunits NR2A and NR2B, finding that Zn2+ inhibits NR2B-mediated activation of Erk1/2. Neurodegeneration was assessed through histochemistry of age-matched hippocampus sections, demonstrating an increase in neurodegeneration in aged ZnT3KO.
Collectively, the results discussed in this dissertation support the hypothesis that dysregulation of synaptic zinc results in excessive excitatory neuronal activity and neurodegenerative alterations in signaling pathways. Consequently, therapeutics targeting maintenance of zinc homeostasis and reduction of AßO interference with zinc neurotransmission may prove beneficial in managing neuronal hyperactivity and neurodegenerative changes in AD.