Cholesterol manipulation switches the production of “toxic Aβ” to “protective Aβ”

The amyloid hypothesis maintains that the pathogenesis of Alzheimer’s disease (AD) is driven by the production and accumulation of amyloid-β (Aβ) peptides in the brain. Consequently, many therapeutic strategies to reduce Aβ production have been examined. However, as Aβ peptides self-aggregate they are found in multiple forms, consisting of different sizes, conformations and biological activities. Critically there exist both toxic and non-toxic forms of Aβ. Therefore, it is possible for therapies to substantially reduce total Aβ concentrations without affecting the concentrations of toxic Aβ, and consequently have no effect on the disease process. For this reason, our recent study sought to differentiate between toxic and non-toxic forms of Aβ.

Fig. 1. Cholesterol ester hydrolase inhibitors alter the Aβ oligomer: monomer ratio. There was a significant inverse correlation between the concentrations of Aβ42 oligomers and Aβ42 monomers in supernatants from 7PA2 cells incubated with CEH inhibitors (diethylumbelliferyl phosphate (○), Pearson’s coefficient = -0.92.

Firstly, we defined “toxic Aβ”. Since the loss of memory that is characteristic of AD is attributed to synapse failure  we sought to identify forms of Aβ that affected synapses. Small soluble Aβ oligomers have been identified as key causes of synapse damage. These are produced by 7PA2 cells (a cell line expressing the human amyloid precursor protein (APP) that is the source of Aβ peptides). Supernatants from untreated 7PA2 cells reduced synaptic density in cultured neurons (measured by a loss of synaptic proteins)indicating they contained synaptotoxic forms of Aβ.  

A key observation from this work was that supernatants from 7PA2 cells treated with some drugs contained high concentrations of Aβ but did not cause synapse damage. This unexpected observation was explained by a switch in the nature of the Aβ produced, drug-treated 7PA2 cells increased production of Aβ monomers and reduced production of Aβ oligomers. Notably there was a significant inverse correlation between concentrations of Aβ monomers and Aβ oligomers in supernatants from treated cells, Fig. 1). While Aβ oligomers cause synapse degeneration, Aβ monomers were neuroprotective. Indeed, our study showed that supernatants from treated 7PA2 cells containing high concentrations of Aβ monomers reduced synapse degeneration caused by Aβ oligomers. Critically these results suggest that synapse damage is dependent upon the ratio of Aβ monomers and Aβ oligomers, rather than the total Aβ concentration. Furthermore,these results illustrate that it is possible for a drug to have a beneficial clinical effect, by selectively reducing toxic Aβ, without affecting total Aβ concentrations.

These effects were seen when 7PA2 cells were treated with a cholesterol synthesis inhibitor, or with cholesterol ester hydrolase (CEH) inhibitors. The CEHs are part of the cholesterol ester cycle that controls cholesterol concentrations/cholesterol distribution within cell membranes. Cholesterol affects the function of membrane micro-domains called lipid rafts  in which APP and many of the enzymes involved in Aβ generation are found. It is possible that the drug-induced changes in cholesterol concentrations affect the processing of APP to Aβ oligomers or the formation of Aβ oligomers from Aβ monomers.

In summary, these results indicate that in 7PA2 cells the production of toxic and neuroprotective forms of Aβ can be pharmacologically modified. More specifically, the Aβ monomer: oligomer ratio was sensitive to drugs that altered membrane cholesterol concentrations. This study suggests that measuring Aβ concentrations alone can be a poor indicator of biological activity and highlights the need to understand the biological activity of the forms of Aβ measured.

Harriet McHale-Owen, Clive Bate
Department ofPathology and Pathogen Biology, Royal Veterinary College, Hawkshead Lane, NorthMymms, Herts, AL9 7TA, United Kingdom

Publication

Cholesterol ester hydrolase inhibitors reduce the production of synaptotoxic amyloid-β oligomers.
McHale-Owen H, Bate C
Biochim Biophys Acta Mol Basis Dis. 2018 Mar
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