Massimo Stefani

Inherent toxicity of aggregates implies a common mechanism for protein misfolding diseases.

A range of human degenerative conditions, including Alzheimers disease, light-chain amyloidosis and the spongiform encephalopathies, is associated with the deposition in tissue of proteinaceous aggregates known as amyloid fibrils or plaques. It has been shown previously that fibrillar aggregates that are closely similar to those associated with clinical amyloidoses can be formed in vitro from proteins not connected with these diseases, including the SH3 domain from bovine phosphatidyl-inositol-3′-kinase and the amino-terminal domain of the Escherichia coli HypF protein. Here we show that species formed early in the aggregation of these non-disease-associated proteins can be inherently highly cytotoxic. This finding provides added evidence that avoidance of protein aggregation is crucial for the preservation of biological function and suggests common features in the origins of this family of protein deposition diseases.

Rationalization of the effects of mutations on peptide and protein aggregation rates.

In order for any biological system to function effectively, it is essential to avoid the inherent tendency of proteins to aggregate and form potentially harmful deposits. In each of the various pathological conditions associated with protein deposition, such as Alzheimers and Parkinsons diseases, a specific peptide or protein that is normally soluble is deposited as insoluble aggregates generally referred to as amyloid. It is clear that the aggregation process is generally initiated from partially or completely unfolded forms of the peptides and proteins associated with each disease. Here we show that the intrinsic effects of specific mutations on the rates of aggregation of unfolded polypeptide chains can be correlated to a remarkable extent with changes in simple physicochemical properties such as hydrophobicity, secondary structure propensity and charge. This approach allows the pathogenic effects of mutations associated with known familial forms of protein deposition diseases to be rationalized, and more generally enables prediction of the effects of mutations on the aggregation propensity of any polypeptide chain.

Kinetic partitioning of protein folding and aggregation

We have systematically studied the effects of 40 single point mutations on the conversion of the denatured form of the α/β protein acylphosphatase (AcP) into insoluble aggregates. All the mutations that significantly perturb the rate of aggregation are located in two regions of the protein sequence, residues 16–31 and 87–98, each of which has a relatively high hydrophobicity and propensity to form β-sheet structure. The measured changes in aggregation rate upon mutation correlate with changes in the hydrophobicity and β-sheet propensity of the regions of the protein in which the mutations are located. The two regions of the protein sequence that determine the aggregation rate are distinct from those parts of the sequence that determine the rate of protein folding. Dissection of the protein into six peptides corresponding to different regions of the sequence indicates that the kinetic partitioning between aggregation and folding can be attributed to the intrinsic conformational preferences of the denatured polypeptide chain.

A causative link between the structure of aberrant protein oligomers and their toxicity

The aberrant assembly of peptides and proteins into fibrillar aggregates proceeds through oligomeric intermediates that are thought to be the primary pathogenic species in many protein deposition diseases. We describe two types of oligomers formed by the HypF-N protein that are morphologically and tinctorially similar, as detected with atomic force microscopy and thioflavin T assays, though one is benign when added to cell cultures whereas the other is toxic. Structural investigation at a residue-specific level using site-directed labeling with pyrene indicated differences in the packing of the hydrophobic interactions between adjacent protein molecules in the oligomers. The lower degree of hydrophobic packing was found to correlate with a higher ability to penetrate the cell membrane and cause an influx of Ca(2+) ions. Our findings suggest that structural flexibility and hydrophobic exposure are primary determinants of the ability of oligomeric assemblies to cause cellular dysfunction and its consequences, such as neurodegeneration.

Mutational analysis of acylphosphatase suggests the importance of topology and contact order in protein folding

Muscle acylphosphatase (AcP) is a small protein that folds very slowly with two-state behavior. The conformational stability and the rates of folding and unfolding have been determined for a number of mutants of AcP in order to characterize the structure of the folding transition state. The results show that the transition state is an expanded version of the native protein, where most of the native interactions are partially established. The transition state of AcP turns out to be remarkably similar in structure to that of the activation domain of procarboxypeptidase A2 (ADA2h), a protein having the same overall topology but sharing only 13% sequence identity with AcP. This suggests that transition states are conserved between proteins with the same native fold. Comparison of the rates of folding of AcP and four other proteins with the same topology, including ADA2h, supports the concept that the average distance in sequence between interacting residues (that is, the contact order) is an important determinant of the rate of protein folding.

Studies of the aggregation of mutant proteins in vitro provide insights into the genetics of amyloid diseases

Protein aggregation and the formation of highly insoluble amyloid structures is associated with a range of debilitating human conditions, which include Alzheimers disease, Parkinsons disease, and the Creutzfeldt–Jakob disease. Muscle acylphosphatase (AcP) has already provided significant insights into mutational changes that modulate amyloid formation. In the present paper, we have used this system to investigate the effects of mutations that modify the charge state of a protein without affecting significantly the hydrophobicity or secondary structural propensities of the polypeptide chain. A highly significant inverse correlation was found to exist between the rates of aggregation of the protein variants under denaturing conditions and their overall net charge. This result indicates that aggregation is generally favored by mutations that bring the net charge of the protein closer to neutrality. In light of this finding, we have analyzed natural mutations associated with familial forms of amyloid diseases that involve alteration of the net charge of the proteins or protein fragments associated with the diseases. Sixteen mutations have been identified for which the mechanism of action that causes the pathological condition is not yet known or fully understood. Remarkably, 14 of these 16 mutations cause the net charge of the corresponding peptide or protein that converts into amyloid deposits to be reduced. This result suggests that charge has been a key parameter in molecular evolution to ensure the avoidance of protein aggregation and identifies reduction of the net charge as an important determinant in at least some forms of protein deposition diseases.

Prefibrillar Amyloid Aggregates Could Be Generic Toxins in Higher Organisms

More than 40 human diseases are associated with fibrillar deposits of specific peptides or proteins in tissue. Amyloid fibrils, or their precursors, can be highly toxic to cells, suggesting their key role in disease pathogenesis. Proteins not associated with any disease are able to form oligomers and amyloid assemblies in vitro displaying structures and cytotoxicity comparable with those of aggregates of disease-related polypeptides. In isolated cells, such toxicity has been shown to result from increased membrane permeability with disruption of ion homeostasis and oxidative stress. Here we microinjected into the nucleus basalis magnocellularis of rat brains aggregates of an Src homology 3 domain and the N-terminal domain of the prokaryotic HypF, neither of which is associated with amyloid disease. Prefibrillar aggregates of both proteins, but not their mature fibrils or soluble monomers, impaired cholinergic neuron viability in a dose-dependent manner similar to that seen in cell cultures. Contrary to the situation with cultured cells, however, under our experimental conditions, cell stress in tissue is not followed by a comparable level of cell death, a result that is very likely to reflect the presence of protective mechanisms reducing aggregate toxicity. These findings support the hypothesis that neurodegenerative disorders result primarily from a generic cell dysfunction caused by early misfolded species in the aggregation process.

Biochemical and biophysical features of both oligomer/fibril and cell membrane in amyloid cytotoxicity

A great deal must still be learnt on the structural features of amyloid assemblies, particularly prefibrillar aggregates, and the relationship of the latter with amyloid cytotoxicity. Presently, it is recognized that the population of unstable, heterogeneous amyloid oligomers and protofibrils is mainly responsible for amyloid cytotoxicity. Conversely, mature fibrils are considered stable, harmless reservoirs of molecular species devoid of toxicity in the polymerized state. This view has been modified by recent reports showing that mature fibrils grown at different conditions can display different structural features and stabilities, possibly leading them to undergo disassembly with the leak of toxic oligomers. Fibril polymorphism is paralleled by oligomer polymorphism and both can be traced back to amyloid growth from differently destabilized monomers with distinct structural features at differing conditions. Recent research has started to unravel oligomer structural and biophysical features and the relationship between the latter and oligomer cytotoxicity. These data have led to the proposal that, together, both oligomer and membrane physical features determine the extent of oligomer–membrane interaction with the resulting disruption of membrane integrity and cell impairment. Such a view can help to explain the variable vulnerability of different cell types to the same amyloids and the lack of relationship between amyloid load and the severity of clinical symptoms. It also stresses the importance, for cell/tissue impairment, of the presence, in tissue, in addition to toxic oligomers, of fibrils conformers of reduced stability as a possible source of toxic oligomers, whose leakage can be favoured upon interaction with suitable surfaces or by other environmental conditions.

Cholesterol in Alzheimer's Disease: Unresolved Questions

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.

Structural features and cytotoxicity of amyloid oligomers: implications in Alzheimer's disease and other diseases with amyloid deposits.

Amyloid diseases display the presence, in targeted tissues and organs, of fibrillar deposits of specific peptides or proteins. Increasing efforts are presently spent in investigating the structural features and the structure-toxicity relation of the soluble oligomeric precursors arising in the path of fibrillization as well as the importance of surfaces as triggers of protein misfolding and aggregation and as possible responsible for amyloid polymorphism. Presently, it is recognized that the unstable, heterogeneous pre-fibrillar aggregates are the main responsible for amyloid toxicity. Conversely, mature fibrils are considered stable, harmless reservoirs of toxic species, although direct fibril toxicity has been reported. Recent studies show that mature fibrils grown at various conditions can display different structural features, stabilities and tendency to disassemble with leak of toxic oligomers. Fibril polymorphism can result from protein aggregation at differing conditions populating misfolded monomers and oligomers with distinct conformational characteristics. Recent research has started to unravel oligomer structural and biophysical features and their relation to cytotoxicity. Increasing information supports the notion that oligomer-membrane interaction, disruption of membrane integrity and cell impairment results from both oligomer and membrane biophysical features; accordingly, the formation of the oligomer-membrane complex, often the first step of amyloid toxicity, can be the result of the interplay of these events. This view can help explaining the variable vulnerability of different cell types to the same amyloids and the lack of relation between amyloid load and severity of clinical symptoms; it also stresses the importance, for cell/tissue impairment, of the presence of fibrils conformers of reduced stability as a possible source of oligomers resulting from leakage possibly favored by the interaction with suitable macromolecular/lipid surfaces or by other environmental conditions.