The fibril forming region of the beta-amyloid precursor differs from that of the amyloid A precursor in its interaction with lipids

Oxidation of cholesterol by amyloid precursor protein and beta-amyloid peptide

Alzheimer's disease (AD) is characterized by accumulation of the neurotoxic peptide beta-amyloid, which is produced by proteolysis of amyloid precursor protein (APP). APP is a large membrane-bound copper-binding protein that is essential in maintaining synaptic function and may play a role in synaptogenesis. beta-Amyloid has been shown to contribute to the oxidative stress that accompanies AD. Later stages of AD are characterized by neuronal apoptosis. However, the biochemical function of APP and the mechanism of the toxicity of beta-amyloid are still unclear. In this study, we show that both beta-amyloid and APP can oxidize cholesterol to form 7beta-hydroxycholesterol, a proapoptotic oxysterol that was neurotoxic at nanomolar concentrations. 7beta-Hydroxycholesterol inhibited secretion of soluble APP from cultured rat hippocampal H19-7/IGF-IR neuronal cells and inhibited tumor necrosis factor-alpha-converting enzyme alpha-secretase activity but had no effect on beta-site APP-cleaving enzyme 1 activity. 7beta-Hydroxycholesterol was also a potent inhibitor of alpha-protein kinase C, with a K(i) of approximately 0.2 nm. The rate of reaction between cholesterol and beta-amyloid was comparable to the rates of cholesterol-metabolizing enzymes (k(cat) = 0.211 min(-)1). The rate of production of 7beta-hydroxycholesterol by APP was approximately 200 times lower than by beta-amyloid. Oxidation of cholesterol was accompanied by stoichiometric production of hydrogen peroxide and required divalent copper. The results suggest that a function of APP may be to produce low levels of 7-hydroxycholesterol. Higher levels produced by beta-amyloid could contribute to the oxidative stress and cell loss observed in Alzheimer's disease.

Function of beta-amyloid in cholesterol transport: a lead to neurotoxicity

Amyloid beta-peptide (Abeta), Abeta precursor protein (APP), apolipoprotein E (apoE), and elevated cholesterol levels have been linked to Alzheimer's disease (AD) pathology. High cholesterol levels increase APP and apoE expression in human NT2 neuron progenitor cells. A cholesterol-rich environment also induces processing of APP, leading to the formation of Abeta and Abeta peptide fragments. Using a novel method, we determined that 1) cholesterol binds to Abeta at alpha-secretase cleavage site; 2) Abeta17-40 rather than Abeta1-40 prevents cholesterol from binding to apoE; 3) Abeta1-40 inhibits cholesterol from binding to low-density lipoprotein (LDL), leading to decrease cholesterol influx and intracellular cholesterol levels; 4) the binding of cholesterol to apoE or LDL was abolished completely in presence of Abeta1-42. Increased extracellular free cholesterol levels are toxic to neurons; this toxicity is prevented by specific lipoproteins, such as high-density lipoproteins, which maintain their ability to bind cholesterol in the presence of Abeta. We propose that one of the physiological functions of Abeta and APP is to control cholesterol transport. AD is associated with increased Abeta production. High cholesterol levels also lead to overproduction of Abeta. Abeta blocks cholesterol trafficking and changes cholesterol homeostasis leading to neurodegeneration and the onset and/or progression of AD pathology.

Cholesterol-dependent localization of NAP-22 on a neuronal membrane microdomain (raft)

A membrane microdomain called raft has been under extensive study since the assembly of various signal-transducing molecules into this region has been envisaged. This domain is isolated as a low buoyant membrane fraction after the extraction with a nonionic detergent such as Triton X-100. The characteristic low density of this fraction is ascribed to the enrichment of several lipids including cholesterol. To clear the molecular mechanism of raft formation, several extraction methods were applied to solubilize raft components. Cholesterol extraction using methyl-beta-cyclodextrin was found to be effective to solubilize NAP-22, a neuron-enriched Ca(2+)-dependent calmodulin-binding protein as well as one of the main protein components of brain raft. Purified NAP-22 bound to the liposomes that were made from phosphatidylcholine and cholesterol. This binding was dependent on the amount of cholesterol in liposomes. Calmodulin inhibited this binding in a dose-dependent manner. These results suggest that the presence of a calcium-dependent regulatory mechanism works on the assembly of raft within the neuron.

Recent advances in brain cholesterol dynamics: transport, domains, and Alzheimer's disease

Major advances in understanding cholesterol dynamics and the role that cholesterol plays in vascular disease have recently been made. The brain is an organ that is highly enriched in cholesterol, but progress toward understanding brain cholesterol dynamics has been relatively limited. This review examines recent contributions to the understanding of brain cholesterol dynamics, focusing on extracellular and intracellular lipid carrier proteins, membrane cholesterol domains, and emerging evidence linking an association between cholesterol dynamics and Alzheimer's disease.

Acute inflammation, acute phase serum amyloid A and cholesterol metabolism in the mouse

Acute inflammation results in a profound change in the apolipoprotein composition of high density lipoprotein (HDL). Several isoforms of the serum amyloid A (SAA) family, SAA1 and SAA2, become major components of HDL. This structural relationship has suggested that acute phase SAA plays some as yet unidentified role in HDL function, possibly related to cholesterol transport, during the course of acute inflammation. Using subcutaneous AgNO3 to induce a sterile abscess changes in plasma cholesterol and SAA were monitored over the subsequent 144 h. Total plasma cholesterol began to increase within 12 h of the induction of inflammation and reached a peak in 24 h. Thereafter its plasma levels fell returning to normal values by 96-120 h. The bulk of the increase in plasma cholesterol was found in the free cholesterol fraction of HDL. This pattern of cholesterol increase corresponds to the established temporal changes for acute phase SAA (AP-SAA). AP-SAA levels increased within 8 h of the induction of inflammation and reached a peak at 24 h. They began to decrease by 48 h with small quantites still present 120 h later. In concert, but inversely, with the changes in AP-SAA the apoA-I, apoA-II, and apo-E, content of HDL decreased during the AP-SAA increases and increased as AP-SAA levels fell. The plasma appearance of cholesterol from the periphery, and central parts of the inflammatory site was assessed by the use of radiolabelled cholesterol. The peripherally placed cholesterol rapidly reached a peak plasma concentration within 24 h of injection. Cholesterol placed in the central part of the sterile abscess, a site relatively inaccessible to the vasculature required 48 h to reach its peak and was 5-times lower than that placed peripherally. The influence of AP-SAA on neutral cholesterol ester hydrolase (nCEH) activity in mouse liver homogenates, mouse peritoneal macrophage homogenates, and a purified porcine pancreatic enzyme with nCEH activity was also assessed. Following optimization with regard to pH, bile salt concentration, protein concentration and incubation time, mouse peritoneal macrophages had a significantly higher nCEH specific activity than that found in liver (7-8 fold). Purified AP-SAA, assessed over a concentration range of 0-10 microg/ml, enhanced nCEH activity at concentrations above 2 microg/ml. The nCEH activity, regardless of its source, increased by 3-7 fold in the presence of AP-SAA. Equivalent concentrations of apolipoprotein A-I (apo A-I) and bovine serum albumin (BSA) failed to alter the activity of nCEH. The effect of AP-SAA on a purified form of nCEH suggests that AP-SAA may have a direct effect on the activity of this enzyme. The temporal correlation of circulating AP-SAA and plasma cholesterol and the significant stimulation of nCEH by AP-SAA (but not apoA-I or BSA) provides further evidence that AP-SAA plays a role in cholesterol metabolism during the course of acute inflammation.

Evidence for local production of acute phase response apolipoprotein serum amyloid A in Alzheimer's disease brain

Acute phase serum amyloid A (A-apoSAA), but not constitutive apoSAA (C-apoSAA), was identified by Western blotting experiments in brain protein extracts from eight of nine patients with Alzheimer's disease (AD), one with a brain tumor and one with multiple sclerosis. A-apoSAA was not detected in six subjects with Pick's or Lewy Body disease or three other non-AD brain specimens. Apolipoprotein A-I and albumin were not found in any of the brain protein extracts. A-apoSAA mRNA was detected in AD brain by reverse transcription-polymerase chain reaction (RT-PCR). These data suggest that apoSAA is locally produced in AD brain and that investigation of the neuroinflammatory effects of this injury specific apolipoprotein is warranted.