Cholesterol ester hydrolase inhibitors reduce the production of synaptotoxic amyloid-ß oligomers.

The production of amyloid-ß (Aß) is the key factor driving pathogenesis in Alzheimer's disease (AD). Increasing concentrations of Aß within the brain cause synapse degeneration and the dementia that is characteristic of AD. Here the factors that affect the release of disease-relevant forms Aß were studied in a cell model. 7PA2 cells expressing the human amyloid precursor protein released soluble Aß oligomers that caused synapse damage in cultured neurons. Supernatants from 7PA2 cells treated with the cholesterol synthesis inhibitor squalestatin contained similar concentrations of Aß42 to control cells but did not cause synapse damage in neuronal cultures. These supernatants contained reduced concentrations of Aß42 oligomers and increased concentrations of Aß42 monomers. Treatment of 7PA2 cells with platelet-activating factor (PAF) antagonists had similar effects; it reduced concentrations of Aß42 oligomers and increased concentrations of Aß42 monomers in cell supernatants. PAF activated cholesterol ester hydrolases (CEH), enzymes that released cholesterol from stores of cholesterol esters. Inhibition of CEH also reduced concentrations of Aß42 oligomers and increased concentrations of Aß42 monomers in cell supernatants. The Aß monomers produced by treated cells protected neurons against Aß oligomer-induced synapse damage. These studies indicate that pharmacological manipulation of cells can alter the ratio of Aß monomer:oligomer released and consequently their effects on synapses.

Sialylated glycosylphosphatidylinositols suppress the production of toxic amyloid-ß oligomers.

The production of amyloid-ß (Aß) is a key factor driving pathogenesis in Alzheimer's disease (AD). Increasing concentrations of soluble Aß oligomers within the brain lead to synapse degeneration and the progressive dementia characteristic of AD. Since Aß exists in both disease-relevant (toxic) and non-toxic forms, the factors that affected the release of toxic Aß were studied in a cell model. 7PA2 cells expressing the human amyloid precursor protein released Aß oligomers that caused synapse damage when incubated with cultured neurones. These Aß oligomers had similar potency to soluble Aß oligomers derived from the brains of Alzheimer's patients. Although the conditioned media from 7PA2 cells treated with the cellular prion protein (PrPC) contained Aß, it did not cause synapse damage. The loss of toxicity was associated with a reduction in Aß oligomers and an increase in Aß monomers. The suppression of toxic Aß release was dependent on the glycosylphosphatidylinositol (GPI) anchor attached to PrPC, and treatment of cells with specific GPIs alone reduced the production of toxic Aß. The efficacy of GPIs was structure-dependent and the presence of sialic acid was critical. The conditioned medium from GPI-treated cells protected neurones against Aß oligomer-induced synapse damage; neuroprotection was mediated by Aß monomers. These studies support the hypothesis that the ratio of Aß monomers to Aß oligomers is a critical factor that regulates synapse damage.

Sialic Acid within the Glycosylphosphatidylinositol Anchor Targets the Cellular Prion Protein to Synapses.

Although the cellular prion protein (PrP(C)) is concentrated at synapses, the factors that target PrP(C) to synapses are not understood. Here we demonstrate that exogenous PrP(C) was rapidly targeted to synapses in recipient neurons derived from Prnp knock-out((0/0)) mice. The targeting of PrP(C) to synapses was dependent upon both neuronal cholesterol concentrations and the lipid and glycan composition of its glycosylphosphatidylinositol (GPI) anchor. Thus, the removal of either an acyl chain or sialic acid from the GPI anchor reduced the targeting of PrP(C) to synapses. Isolated GPIs (derived from PrP(C)) were also targeted to synapses, as was IgG conjugated to these GPIs. The removal of sialic acid from GPIs prevented the targeting of either the isolated GPIs or the IgG-GPI conjugate to synapses. Competition studies showed that pretreatment with sialylated GPIs prevented the targeting of PrP(C) to synapses. These results are consistent with the hypothesis that the sialylated GPI anchor attached to PrP(C) acts as a synapse homing signal.

Glimepiride protects neurons against amyloid-ß-induced synapse damage.

Alzheimer's disease is associated with the accumulation within the brain of amyloid-ß (Aß) peptides that damage synapses and affect memory acquisition. This process can be modelled by observing the effects of Aß on synapses in cultured neurons. The addition of picomolar concentrations of soluble Aß derived from brain extracts triggered the loss of synaptic proteins including synaptophysin, synapsin-1 and cysteine string protein from cultured neurons. Glimepiride, a sulphonylurea used for the treatment of diabetes, protected neurons against synapse damage induced by Aß. The protective effects of glimepiride were multi-faceted. Glimepiride treatment was associated with altered synaptic membranes including the loss of specific glycosylphosphatidylinositol (GPI)-anchored proteins including the cellular prion protein (PrP(C)) that acts as a receptor for Aß42, increased synaptic gangliosides and altered cell signalling. More specifically, glimepiride reduced the Aß-induced increase in cholesterol and the Aß-induced activation of cytoplasmic phospholipase A2 (cPLA2) in synapses that occurred within cholesterol-dense membrane rafts. Aß42 binding to glimepiride-treated neurons was not targeted to membrane rafts and less Aß42 accumulated within synapses. These studies indicate that glimepiride modified the membrane micro-environments in which Aß-induced signalling leads to synapse damage. In addition, soluble PrP(C), released from neurons by glimepiride, neutralised Aß-induced synapse damage. Such observations raise the possibility that glimepiride may reduce synapse damage and hence delay the progression of cognitive decline in Alzheimer's disease.