Rifampicin inhibits the toxicity of pre-aggregated amyloid peptides by binding to peptide fibrils and preventing amyloid-cell interaction

Dual binding modes of Congo red to amyloid protofibril surface observed in molecular dynamics simulations

Congo red has been used to identify amyloid fibrils in tissues for more than 80 years and is also a weak inhibitor to both amyloid-beta fibril formation and toxicity. However, the specificity of the binding and its inhibition mechanism remain unclear. Using all-atom molecular dynamics simulations with the explicit solvent model, we have identified and characterized two specific binding modes of Congo red molecules to a protofibril formed by an amyloidogenic fragment (GNNQQNY) of the yeast prion protein Sup35. The observation of dual-mode was consistent with the experimentally observed dual-mode binding to Abeta fibrils by a series of compounds similar to Congo red. In the primary mode, Congo red bound to a regular groove formed by the first three residues (GNN) of the beta-strands along the beta-sheet extension direction. Comparative simulations demonstrated that Thioflavin T also bound to the grooves on KLVFFAE protofibril surface. Because of the ubiquitous long grooves on the amyloid fibril surface, we propose that this binding interaction could be a general recognition mode of amyloid fibrils by Congo red, Thioflavin T, and other long flat molecules. In the secondary mode, Congo red bound parallel to the beta-strands on the edge or in the middle of a beta-sheet. The primary binding mode of Congo red and GNNQQNY protofibril was more stable than the secondary mode by -5.7 kcal/mol as estimated by the MM-GBSA method. Detailed analysis suggests that the hydrophobic interactions play important roles for burial of the hydrophobic part of the Congo red molecules. Two potential inhibition mechanisms of disrupting beta-sheet stacking were inferred from the primary mode, which could be exploited for the development of non-peptidic amyloid-specific inhibitors.

Inhibition of human IAPP fibril formation does not prevent beta-cell death: evidence for distinct actions of oligomers and fibrils of human IAPP

Type 2 diabetes mellitus (T2DM) is characterized by an approximately 60% deficit in beta-cell mass, increased beta-cell apoptosis, and islet amyloid derived from islet amyloid polypeptide (IAPP). Human IAPP (hIAPP) forms oligomers, leading to either amyloid fibrils or toxic oligomers in an aqueous solution in vitro. Either application of hIAPP on or overexpression of hIAPP in cells induces apoptosis. It remains controversial whether the fibrils or smaller toxic oligomers induce beta-cell apoptosis. Rifampicin prevents hIAPP amyloid fibril formation and has been proposed as a potential target for prevention of T2DM. We examined the actions of rifampicin on hIAPP amyloid fibril and toxic oligomer formation as well as its ability to protect beta-cells from either application of hIAPP or endogenous overexpression of hIAPP (transgenic rats and adenovirus-transduced beta-cells). We report that rifampicin (Acocella G. Clin Pharmacokinet 3: 108-127, 1978) prevents hIAPP fibril formation, but not formation of toxic hIAPP oligomers (Bates G. Lancet 361: 1642-1644, 2003), and does not protect beta-cells from apoptosis induced by either overexpression or application of hIAPP. These data emphasize that toxic hIAPP oligomers, rather than hIAPP fibrils, initiate beta-cell apoptosis and that screening tools to identify inhibitors of amyloid fibril formation are likely to be less useful than those that identify inhibitors of toxic oligomer formation. Finally, rifampicin and related molecules do not appear to be useful as candidates for prevention of T2DM.

Phenol red interacts with the protofibril-like oligomers of an amyloidogenic hexapeptide NFGAIL through both hydrophobic and aromatic contacts

Amyloid-associated diseases affect millions of people worldwide. Phenol red exhibits modest inhibition toward fibril formation of human Islet amyloid polypeptide (hIAPP) and its toxicity, which is associated with type II diabetes mellitus. However, the molecular level mechanisms of interactions remain elusive. The binding of phenol red molecules to the protofibrils of an amyloidogenic fragment (NFGAIL) of hIAPP has been investigated by molecular dynamics simulations with explicit solvent. The phenol red molecules were observed to bind primarily along either beta-sheet stacking or beta-strand directions. Through its three aromatic rings, the phenol red molecule preferentially interacted with the hydrophobic side chains of Phe, Leu, and Ile; and the polar sulfone and hydroxyl groups were mainly exposed in solvent. Thus, phenol red improves the solubility of the early protofibrils and represses further growth. Interestingly, there was no obvious preference toward the aromatic Phe residue in comparison to the hydrophobic Leu or Ile residues. The lack of binding along the hydrogen bond direction indicates that phenol red does not directly block the beta-sheet extension. Further free energy analysis suggested that a phenol red analog may potentially improve the binding affinity.

Synthesis of 6-[2-(Benzoxazol-2-ylmethylamino)ethoxy]-1-Alkyl-1H-lndole-2-Carboxylic acid and inhibitory activity on β-Amyloid aggregation

6-[2-(benzoxazol-2-ylmethylamino)ethoxy]-1-alkyl-1H-indole-2-carboxylic acids were designed and synthesized as beta-amyloid (Abeta) fibril assembly inhibitors. Their inhibitory activity on Abeta, aggregation was evaluated by thioflavin T assay although their activities were insignificant.

Inhibition of hIAPP amyloid-fibril formation and apoptotic cell death by a designed hIAPP amyloid- core-containing hexapeptide

The pathogenesis of type II diabetes is associated with the aggregation of the 37-residue human islet amyloid polypeptide (hIAPP) into cytotoxic beta sheet aggregates and fibrils. We have recently shown that introduction of two N-methyl rests in the beta sheet- and amyloid-core-containing sequence hIAPP(22-27), or NFGAIL converted this amyloidogenic and cytotoxic sequence into nonamyloidogenic and noncytotoxic NF(N-Me)GA(N-Me)IL. Here, we show that NF(N-Me)GA(N-Me)IL is able to bind with high-affinity full-length hIAPP and to inhibit its fibrillogenesis. NF(N-Me)GA(N-Me)IL also inhibits hIAPP-mediated apoptotic beta cell death. By contrast, unmodified NFGAIL does not inhibit hIAPP amyloidogenesis and cytotoxicity, suggesting that N-methylation conferred on NFGAIL the properties of NF(N-Me)GA(N-Me)IL. These results support the concept that rational N-methylation of hIAPP amyloid-core sequences may be a valuable strategy to design pancreatic-amyloid diagnostics and therapeutics for type II diabetes.

Rifampicin inhibits alpha-synuclein fibrillation and disaggregates fibrils

The aggregation of alpha-synuclein in dopaminergic neurons of the substantia nigra is a critical step in the pathogenesis of Parkinson's disease. We show that the antibiotic rifampicin inhibited alpha-synuclein fibrillation and disaggregated existing fibrils in a concentration-dependent manner. Size-exclusion chromatography data indicated that rifampicin stabilized alpha-synuclein as both a monomer and soluble oligomers comprised of partially folded alpha-synuclein. Experiments using aged samples of rifampicin indicated that the most active species in inhibiting fibrillation and disaggregating fibrils is an oxidation product of rifampicin, which was confirmed in experiments under anaerobic conditions. These results indicate that rifampicin-mediated inhibition of alpha-synuclein fibrillation and disaggregation of fibrils involves preferential stabilization of monomeric and soluble oligomeric forms, and that rifampicin potentially may have therapeutic application for Parkinson's disease.

Targeting alpha-synuclein in Parkinson's disease

alpha-Synuclein aggregation into fibrils is associated with the pathogenesis of Parkinson's disease (PD). Li et al. provide strong evidence that rifampicin interacts with alpha-synuclein and inhibits its fibrillization. Rifampicin could be a promising candidate for therapeutic application for PD.

Melittin as model system for probing interactions between proteins and cyclodextrins

Cylcodextrin sugars are cyclic sugars that have a hydrophilic exterior and a hydrophobic center. This enables cyclodextrins to solubilize hydrophobic molecules in aqueous media. Cyclodextrins may inhibit aggregation by intercalating surface aromatic residues and competing with interprotein aromatic clusters (pi-pi interactions). In order to investigate this concept, the interaction of hydroxypropyl-beta-cyclodextrin (HPBCD) with melittin is studied with steady-state and time-resolved fluorescence, fluorescence polarization, circular dichroism, and IR spectroscopy. HPBCD inhibits the aggregation of melittin. This inhibition and the spectroscopic results are consistent with the lone aromatic tryptophan of the peptide being intercalated within HPBCD.

Channels formed with a mutant prion protein PrP(82-146) homologous to a 7-kDa fragment in diseased brain of GSS patients

A major prion protein (PrP) mutant that forms amyloid fibrils in the diseased brain of patients with Gerstmann-Sträussler-Scheinker syndrome (GSS) is a fragment of 7 kDa spanning from residues 81-82 to 144-153 of PrP. Analysis of ionic membrane currents, recorded with a lipid bilayer technique, revealed that the wild-type fragment PrP(82-146) WT and the partially scrambled PrP(82-146) (127-146) SC are capable of forming heterogeneous ion channels that are similar to those channels formed with PrP(106-126). In contrast, PrP(82-146) peptides in which the region from residue 106 to 126 had been scrambled (SC) showed a reduction in interaction with lipid membranes and did not form channels. The PrP(82-146) WT- and PrP(82-146) (127-146) SC-formed cation channels with fast kinetics are Cu2+ sensitive and rifampicin (RIF) insensitive, whereas the time-dependent inactivating channels formed by these same peptides are both Cu2+ and RIF insensitive. The presence of RIF in the solution before the addition of PrP(82-146) WT or PrP(82-146) (127-146) SC affected their incorporation into the lipid bilayers. PrP(82-146) WT and PrP(82-146) (127-146) SC fast cation channels formed in the presence of RIF appeared in an electrically semisilent state or an inactivated state. Increasing [Cd2+]cis enhanced the incorporation of PrP(82-146) WT and PrP(82-146) (127-146) SC channels formed in the presence of RIF. We conclude that the major PrP mutant fragment in the diseased brain of GSS patients is prone to form channels in neuronal membranes, causing their dysfunction. We propose that Cd2+ may accentuate the neurotoxicity of this channel-forming PrP fragment by enhancing its incorporation into the membrane.

Amyloidogenicity and cytotoxicity of islet amyloid polypeptide

Insoluble amyloid formation by islet amyloid polypeptide (IAPP) in the islets of Langerhans of the pancreas is a major pathophysiological feature of noninsulin dependent diabetes mellitus (NIDDM) or type II diabetes. Because in vivo formed amyloid colocalizes with areas of cell degeneration and IAPP amyloid aggregates are cytotoxic per se, the process of IAPP amyloid formation has been strongly associated with the progressive pancreatic cell degeneration and thus much of the pathology of type II diabetes. IAPP is a pancreatic polypeptide of 37 residues that, in its soluble form, is believed to play a role as a regulator of glucose homeostasis. The molecular cause and mechanism of the conversion of soluble IAPP into insoluble amyloid aggregates in vivo and its role in disease progress still remain to be clarified. Nevertheless, in the past few years significant progress has been made in understanding the amyloidogenesis pathway of IAPP in vitro and gaining insight into the structural and conformational "requirements" of IAPP amyloidogenesis and related cytotoxic effects. Importantly, several of the studies have revealed significant similarities of the above features of IAPP to other amyloidogenic polypeptides such as the beta-amyloid polypeptide Abeta. This suggests that, at the molecular level, amyloidogenesis, and possibly related cell degeneration and disease pathogenesis by completely different polypeptide sequences, may obey to common structural and conformational "rules" and follow similar molecular pathways. This review describes studies on the structural and conformational features of IAPP amyloid formation and cytotoxicity, and the application of the obtained knowledge for the understanding of the molecular mechanism of the IAPP amyloidogenesis pathway and the related cytotoxicity.

Immunization treatment approaches in Alzheimer's and prion diseases

There is growing realization that many neurodegenerative conditions have the same underlying pathogenetic mechanism: a change in protein conformation, where the beta-sheet content is increased. In Alzheimer's disease (AD), amyloid deposition in the form of neuritic plaques and congophilic angiopathy is driven by the conversion of normal soluble amyloid beta (sAbeta) to Abeta plaques, whereas in the prionoses the critical event is the conversion of normal prion protein, PrP(C), to PrP(Sc). This common theme in the pathogenesis of these disorders and the extracellular localization of the accumulating abnormal protein make them highly amenable to therapeutic approaches based on experimental manipulation of protein conformation and clearance. Different approaches under development include drugs that affect the processing of the precursor proteins, enhance clearance of the amyloidogenic protein, and inhibit or prevent the conformation change. Particularly interesting are recent studies of immune system activation, which appear to increase the clearance of the disease-associated protein. These immunologically based approaches are highly effective in animal models of these disorders, and in these model systems are associated with no obvious side effects. In transgenic mice with AD-related pathology, immunization has also been shown to prevent age-related cognitive impairment. However, the first clinical trial of this approach in AD patients was associated with unacceptable toxicity. These immune-based treatment approaches have great potential as rational therapies for this devastating group of disorders, but additional development is needed before they can be safely applied to humans.

Inhibitors can arrest the membrane activity of human islet amyloid polypeptide independently of amyloid formation

Human islet amyloid polypeptide (hIAPP), co-secreted with insulin from pancreatic beta cells, misfolds to form amyloid deposits in non-insulin-dependent diabetes mellitus (NIDDM). Like many amyloidogenic proteins, hIAPP is membrane-active: this may be significant in the pathogenesis of NIDDM. Non-fibrillar hIAPP induces electrical and physical breakdown in planar lipid bilayers, and IAPP inserts spontaneously into lipid monolayers, markedly increasing their surface area and producing Brewster angle microscopy reflectance changes. Congo red inhibits these activities, and they are completely arrested by rifampicin, despite continued amyloid formation. Our results support the idea that non-fibrillar IAPP is membrane-active, and may have implications for therapy and for structural studies of membrane-active amyloid.

The status of ongoing trials for mild cognitive impairment

Mild cognitive impairment (MCI) is a term used to describe memory decline or other specific cognitive impairment in individuals who do not have dementia or significant impairment of other cognitive functions beyond that expected for their age or education. It has been suggested that as much as 38% of the elderly population would meet criteria for MCI and although the associated memory deficits are mild, the fact that up to 15% of MCI patients, particularly those with a particular type of memory impairment, convert to Alzheimer's disease (AD) annually has prompted serious attention. Despite the high conversion rate, MCI cannot be used synonymously with early or mild AD, as patients with AD are impaired not only in memory performance but in other cognitive domains as well; they meet diagnostic criteria for dementia. However, since there is a high conversion rate from MCI to AD, it is likely many with MCI have the underlying neuropathology of AD, though they do not yet meet clinical diagnostic criteria. Therefore, treatment strategies developed for AD, specifically acetylcholinesterase inhibitors and Cox-2 inhibitors, have been among the first employed to treat MCI. It is hoped that by impeding the progression of MCI in this manner, fewer patients will convert to AD. This article will give a brief overview of the condition of mild cognitive impairment and an account of trial methodology and current treatment strategies being employed for MCI.

Animal models of Alzheimer's disease and evaluation of anti-dementia drugs

Alzheimer's disease (AD) is the most common cause of progressive decline of cognitive function in aged humans, and is characterized by the presence of numerous senile plaques and neurofibrillary tangles accompanied by neuronal loss. Some, but not all, of the neuropathological alterations and cognitive impairment in AD can be reproduced genetically and pharmacologically in animals. It should be possible to discover novel drugs that slow the progress or alleviate the clinical symptoms of AD by using these animal models. We review the recent progress in the development of animal models of AD and discuss how to use these model animals to evaluate novel anti-dementia drugs.

Mild cognitive impairment: emerging therapeutics

OBJECTIVE

To present a general overview of the etiology, definition, and prevalence of mild cognitive impairment (MCI), as well as outline possible treatment strategies.

DATA SOURCES

A MEDLINE search was conducted for relevant references generated from 1990 to 2000 concerning MCI, mild to moderate Alzheimer disease (AD), and therapeutic strategies. Several books were also used in the compilation of data for this review, as well as the authors' experience in designing and conducting MCI trials.

DATA EXTRACTION

All of the references listed were assessed, and all relevant information was included in this review.

DATA SYNTHESIS

Forgetful individuals most likely to develop AD have a condition known as MCI previous to their development of dementia. This condition is hallmarked by memory impairment that is abnormal for the individual's age and educational level. While not all individuals with MCI develop AD, it is apparent that the condition can serve as a potential marker for early onset of AD.

CONCLUSIONS

As many clinicians can attest, occasional forgetfulness is a common aspect of the aging process. Eventually, however, a large portion of forgetful individuals, especially those with MCI, will be diagnosed with AD or some other form of dementia. Indeed, many researchers have suggested that MCI should be regarded as incipient AD and that these individuals would benefit from drug therapy. Thus, MCI screening may be beneficial in terms of both early AD intervention and perhaps even AD prevention.

Ongoing trials in Alzheimer's disease

Researchers have sought to understand the underlying pathophysiology of Alzheimer's disease (AD) ever since Dr A Alzheimer first described the condition in 1907. Unfortunately however, until recently, they have done so with limited success. This lack of clarity has deterred advancements in therapeutic drug research beyond all but the purely symptomatic treatment relief currently available. However, through spatio-temporal analysis of the two types of cerebral lesions that characterise the disorder (senile plaques and neurofibrillary tangles) and the compilation of genetic data concerning familial AD, there now exists the foundation for a more comprehensive understanding of the disease. Although symptomatic cholinergic strategies have beneficial effects, their benefits are modest and current research has turned to the development of other promising strategies, including oestrogen replacement, anti-inflammatory agents, free radical scavengers, anti-oxidants and monoamine oxidase-B (MAO-B) inhibitors. Many of these strategies may have some merit, however further analysis and structured research are necessary before a definitive decision can be made about their efficacy and possible role in AD therapy. Strategies that are directed at halting the underlying biochemical changes in AD are nearing clinical testing and offer the promise for meaningful therapeutic outcomes.

A sulfated proteoglycan aggregation factor mediates amyloid-beta peptide fibril formation and neurotoxicity

Proteoglycans are associated with senile plaques in Alzheimer's disease and may be involved in A beta fibril formation and plaque formation. In vitro, glycosaminoglycans have been shown to inhibit the proteolysis of A beta fibrils, accelerate formation and maintain their stability. To model their interaction, we investigated the binding of a sulfated proteoglycan derived from a natural source; marine sponge Microciona prolifera aggregation factor (MAF). This species-specific re-aggregation of sponge cells has two functional properties, a Ca2+ independent cell binding activity and a Ca2+ dependent self-aggregation. It has been shown that a novel sulfated disaccharide and a pyruvylated trisaccharide are important in the Ca(2+)-dependent MAF aggregation. Aggregation demonstrated by homophilic binding of MAF subunits may be chemically distinct from other heterotypic binding effects. We investigated A beta-MAF interactions and show that MAF induces a structural transition in A beta 40 and A beta 42 from random to beta-structure as detected by circular dichroism spectroscopy. Electron microscopy revealed that the structural transition correlated with an increase in the number of A beta 40 and A beta 42 aggregated that have a truncated fibrillar morphology. Finally, MAF increased A beta-induced toxicity of nerve growth factor (NGF)-differentiated PC-12 cells in the absence of Ca2+. The addition of Ca2+ to MAF-A beta incubations resulted in a moderate attenuation of toxicity possibly due to a reduction in A beta-cell interactions caused by extensive lateral aggregation of the MAF-A beta complexes. Our results indicate that A beta is generally susceptible to proteoglycan-mediated aggregation and fibril formation. We also propose that the MAF model system may be useful in delineating these interactions and represent a means to develop and examine potential inhibitors of the proteoglycan effects.

Common structural features determine the effectiveness of carvedilol, daunomycin and rolitetracycline as inhibitors of Alzheimer beta-amyloid fibril formation

One of the major pathological features of Alzheimer's disease is the deposition of beta-amyloid peptide (Abeta). Cellular toxicity has been shown to be associated with fibrillar forms of Abeta; preventing this fibril formation is therefore viewed as a possible method of slowing disease progression in Alzheimer's disease. With the use of a series of tetracyclic and carbazole-type compounds as inhibitors of Abeta fibril formation, we here describe a number of common structural features that seem to be associated with the inhibitory properties of these agents. Compounds such as carvedilol, rolitetracycline and daunomycin, which are shown to inhibit Abeta fibril formation, also prevent the formation of species of peptide that demonstrate biological activity in a human neuroblastoma cell line. Molecular modelling data suggest that these compounds have in common the ability to adopt a specific three-dimensional pharmacophore conformation that might be essential for binding to Abeta and preventing it from forming fibrils. Understanding such drug-peptide interactions might aid the development of disease-modifying agents.

Recent developments in the drug treatment of Alzheimer's disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder with an impact on public health which continues to increase with the increasing longevity of the population. The disease is characterised clinically by a progressive loss of cognitive and behavioural function. These deficits are thought to result from decreased cholinergic transmission; therefore, restoring cholinergic function has been the main focus in the development of drugs for AD. Several pharmacological approaches to enhancing cholinergic function have been developed for symptomatic or palliative therapy of AD. Although these strategies have resulted in modest cognitive and behavioural improvements in patients with AD, they do not address the underlying progression of the disease. New strategies will be required to slow, stop or reverse the effects of neurodegeneration in AD. A number of potential therapies are currently under investigation, including estrogen replacement, anti-inflammatory agents, free radical scavengers and antioxidants, and monoamine oxidase-B (MAO-B) inhibitors. The evidence for a protective effect of estrogens or nonsteroidal anti-inflammatory drugs (NSAIDs) is controversial, and largely based on retrospective studies. More controlled prospective studies are needed to definitively demonstrate the benefits of long term estrogen or NSAID use in the prevention of AD. Free radical scavengers/antioxidants such as idebenone, and selective prevention MAO-B inhibitors such as lazabemide are well tolerated, but require additional studies in order to demonstrate preventative effects. In addition, other approaches, such as anti-amyloid treatments that affect beta-amylase secretion, aggregation and toxicity, appear promising; treatments that hinder neurofibrillary tangle construction and nerve growth factor (NGF) induction are in the very early stages of development.