Cellular mechanisms underlying the cognition-enhancing properties of metal-complexes

© 2014 Dr. Laura BicaAlzheimer’s disease (AD) is caused by a myriad of complex pathological factors that contribute to Amyloid-beta (Aβ) accumulation and oxidative stress as well as synaptic damage and dysfunction that result in cognitive decline. Metal dyshomeostasis is a key factor in these processes and is therefore an attractive therapeutic target. Clioquinol (CQ) was initially investigated as an AD therapeutic due to its copper- and zinc-chelating properties. It inhibited Aβ accumulation and enhanced cognitive performance in an AD mouse model as well as humans with AD in a clinical trial; however, issues with purification led to development of an alternative candidate, PBT2. PBT2 has been tested in AD mouse models and trialled in humans with AD, it improved cognition and reduced cerebrospinal fluid (CSF) Aβ levels. Bis(thiosemicarbazonato) metal-complexes (mII(btsc)s), previously used in applications such as cancer imaging, have been examined as potential treatments for AD as well as other neurodegenerative disorders such as Parkinson’s disease and Amyotrophic Lateral Sclerosis. Their structure contains a Cu or Zn molecule and is able to cross the cell membrane into the cytosol where metals are released, making them bioavailable. Like CQ and PBT2, the mII(btsc) CuII(gtsm) enhanced cognition while lowering CSF Aβ in a mouse model of AD. CQ and PBT2 don’t introduce more metals into the body or cell but may help remove excess metals from outside cells and redistribute them into a metal-depleted intracellular environment. However, mII(btsc)s allow a more controlled delivery of bioavailable metals that has proven to have therapeutic effects in models of AD as well as other neurodegenerative disorders. Despite these advances, little is known of the cellular metal delivery and neurotherapeutic action of these metal-binding compounds. This thesis investigated the potential mechanisms of action of metal delivery agents PBT2 and CuII(gtsm), potential therapeutic compounds for AD. While CQ is not currently being pursued as an AD therapeutic, its effects were also examined. PBT2 enhanced dendritic spine density of Tg2576 mice compared to sham treated controls and had no effect on wild-type controls. Several biomarkers of synaptic plasticity were examined and found to be increased with PBT2 treatment. In vitro, neurite elongation was also increased by exposure to PBT2 with a significantly stronger effect with the addition of equimolar Cu or Zn. When a chelator with high affinity for Cu and Zn was present, the effect of PBT2 on neurite elongation was blocked, indicating that bioavailable Cu and Zn is necessary for this effect. CuII(gtsm) also enhanced neurite elongation in vitro and similar compounds with different Cu-binding affinity or with a Zn molecule instead of Cu required 10-fold higher concentrations to elicit a similar neurogenerative effect. The neurogenerative effect of CuII(gtsm) was examined further and found to require JNK phosphorylation and was associated with inhibition of cellular phosphatase activity, in particular, calcineurin. Furthermore, specific inhibition of calcineurin with FK506 enhanced neurite elongation. While there were subtle differences in the effects of the metal delivery agents examined, this thesis supports the use of metal delivery agents as a potential AD therapeutic and demonstrated that the effects of these agents involves neurogenerative actions requiring the activity of phosphatases such as calcineurin