It has been reported that different tissue or cultured cell types are variously affected by the exposure to toxic protein aggregates, however a substantial lack of information exists about the biochemical basis of cell resistance or susceptibility to the aggregates. We investigated the extent of the cytotoxic effects elicited by supplementing the media of a panel of cultured cell lines with aggregates of HypF-N, a prokaryotic domain not associated with any amyloid disease. The cell types exposed to early, pre-fibrillar aggregates (not mature fibrils) displayed variable susceptibility to damage and to apoptotic death with a significant inverse relation to membrane content in cholesterol. Susceptibility to damage by the aggregates was also found to be significantly related to the ability of cells to counteract early modifications of the intracellular free Ca2+ and redox status. Accordingly, cell resistance appeared related to the efficiency of the biochemical equipment leading any cell line to sustain the activity of Ca2+ pumps while maintaining under control the oxidative stress associated with the increased metabolic rate. Our data depict membrane destabilization and the subsequent early derangement of ion balance and intracellular redox status as key events in targeting exposed cells to apoptotic death.

The active site of ACP has been located, enabling a reaction mechanism to be suggested in which the phosphate moiety bound to Arg23 acts as a base, abstracting a proton from a nucleophilic water molecule liganded to Asn41. The transition-state intermediate is stabilized by the phosphate-binding loop. We suggest the catalysis to be substrate assisted, which probably explains why this enzyme can only hydrolyze acylphosphates.

Peptides and proteins can convert from their soluble forms into highly ordered fibrillar aggregates, giving rise to pathological conditions ranging from neurodegenerative disorders to systemic amyloidoses. It is increasingly recognized that protein oligomers forming early in the process of fibril aggregation represent the pathogenic species in protein deposition diseases. The N-terminal domain of the HypF protein from Escherichia coli (HypF-N) has previously been shown to form, under distinct conditions, two types of HypF-N oligomers with indistinguishable morphologies but distinct structural features at the molecular level. Only the oligomer type exposing hydrophobic surfaces and possessing sufficient structural plasticity is toxic (type A), whereas the other type is benign to cultured cells (type B). Here we show that only type A oligomers are able to induce a Ca2+ influx from the cell medium to the cytosol, to penetrate the plasma membrane, to increase intracellular reactive oxygen species production, lipid peroxidation and release of intracellular calcein, resulting in the activation of the apoptotic pathway. Remarkably, these oligomers can also induce a loss of cholinergic neurons when injected into rat brains. By contrast, markers of cellular stress and viability were unaffected in cultured and rat neuronal cells exposed to type B oligomers. The analysis of the time scales of such effects indicates that the difference of toxicity between the two oligomer types involve the early events of the toxicity cascade, shedding new light on the mechanism of action of protein oligomers and on the molecular targets for the therapeutic intervention against protein deposition diseases.

The role of cholesterol as a susceptibility factor or a protective agent in neurodegeneration and, more generally, in amyloid-induced cytotoxicity is still controversial. Epidemiological studies on the hypercholesterolemia-AD risk relation and some reports indicating a beneficial effect of statin therapy suggest cholesterol as a susceptibility factor in AD. The ApoE4 genotype as a prevalent genetic risk factor for AD and the function of ApoE as main cholesterol carrier in the brain also underlie a close cholesterol load-AD risk relation. Finally, cell biology evidences support a critical involvement of lipid raft cholesterol in the modulation of beta- and gamma-secretase cleavage of APP with altered Abeta production. However, little exchange does exist between circulating and brain cholesterol, the latter arising from endogenous synthesis. In addition, increasing evidence supports the idea that amyloid cytotoxicity in most cases is initiated by oligomer recruitment at the cell membrane with loss of membrane integrity, Ca(2+) ingress into the cell, oxidative stress and apoptosis. In such a scenario, increased membrane cholesterol seems to be protective by disfavouring aggregate binding to the membrane. Recent findings also indicate that a reduction of cellular cholesterol favours co-localization of BACE1 and APP in non-raft membrane domains and hinders generation of plasmin, an Abeta-degrading enzyme. Finally, recent researches on Seladin-1, involved in cholesterol biosynthesis, show that modulation of membrane cholesterol affects Abeta generation and cell resistance against Abeta oligomer toxicity. These data confirm previous findings indicating a reduction of the cholesterol/phospholipid ratio in aged and AD brains. The aim of this review is to critically discuss some of the main results reported in the recent years in this field supporting a role of cholesterol either as a susceptibility factor or as a protective agent in AD.

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