The amyloid proteins of Alzheimer's disease as potential targets for drug therapy

Inhibitory mechanism of an antifungal drug, caspofungin against amyloid β peptide aggregation: Repurposing via neuroinformatics and an experimental approach

The multifactorial neurological condition called Alzheimer's disease (AD) primarily affects elderly individuals. Despite the calamitous consequences of AD, curative strategies for a regimen to apply remain inadequate as several factors contribute to AD etiology. Drug repurposing is an advance strategy prior to drug discovery as various effective drugs perform through alteration of multiple targets, and the present "poly-pharmacology" can be a curative approach to complex disorders. AD's multifactorial behavior actively encourages the hypothesis for a drug design approach focused on drug repurposing. In this study, we discovered that an antifungal drug, Caspofungin (CAS) is a potent Aβ aggregation inhibitor that displays significantly reduced toxicity associated with AD. Drug reprofiling and REMD simulations demonstrated that CAS interacts with the β-sheet section, known as Aβ amyloid fibrils hotspot. CAS leads to destabilization of β-sheet and, conclusively, in its devaluation. Later, in vitro experiments were acquired in which the fibrillar volume was reduced for CAS-treated Aβ peptide. For the first time ever, this study has determined an antifungal agent as the Aβ amyloid aggregation's potent inhibitor. Several efficient sequence-reliant potent inhibitors can be developed in future against the amyloid aggregation for different amyloid peptide by the processing and conformational optimization of CAS.

Peptide-Peptide Co-Assembly: A Design Strategy for Functional Detection of C-peptide, A Biomarker of Diabetic Neuropathy

Diabetes-related neuropathy is a debilitating condition that may be averted if it can be detected early. One possible way this can be achieved at low cost is to utilise peptides to detect C-peptide, a biomarker of diabetic neuropathy. This depends on peptide-peptide co-assembly, which is currently in a nascent stage of intense study. Instead, we propose a bead-based triple-overlay combinatorial strategy that can preserve inter-residue information during the screening process for a suitable complementary peptide to co-assemble with C-peptide. The screening process commenced with a pentapeptide general library, which revealed histidine to be an essential residue. Further screening with seven tetrapeptide focused libraries led to a table of self-consistent peptide sequences that included tryptophan and lysine at high frequencies. Three complementary nonapeptides (9mer com-peptides), wpkkhfwgq (Trp-D), kwkkhfwgq (Lys-D), and KWKKHFWGQ (Lys-L) (as a negative control) were picked from this table for co-assembly studies with C-peptide. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) and circular dichroism (CD) spectroscopies were utilized to study inter-peptide interactions and changes in secondary structures respectively. ATR-FTIR studies showed that there is indeed inter-peptide interaction between C-peptide and the tryptophan residues of the 9mer com-peptides. CD studies of unaggregated and colloidal C-peptide with the 9mer com-peptides suggest that the extent of co-assembly of C-peptide with Trp-D is greatest, followed by Lys-D and Lys-L. These results are promising and indicate that the presented strategy is viable for designing and evaluating longer complementary peptides, as well as complementary peptides for co-assembly with other polypeptides of interest and importance. We discuss the possibility of designing complementary peptides to inhibit toxic amyloidosis with this approach.

Structure-Based Peptide Inhibitor Design of Amyloid-β Aggregation

Many human neurodegenerative diseases are associated with amyloid fibril formation. Inhibition of amyloid formation is of importance for therapeutics of the related diseases. However, the development of selective potent amyloid inhibitors remains challenging. Here based on the structures of amyloid β (Aβ) fibrils and their amyloid-forming segments, we designed a series of peptide inhibitors using RosettaDesign. We further utilized a chemical scaffold to constrain the designed peptides into β-strand conformation, which significantly improves the potency of the inhibitors against Aβ aggregation and toxicity. Furthermore, we show that by targeting different Aβ segments, the designed peptide inhibitors can selectively recognize different species of Aβ. Our study developed an approach that combines the structure-based rational design with chemical modification for the development of amyloid inhibitors, which could be applied to the development of therapeutics for different amyloid-related diseases.

Anti-inflammatory lipoxin A4 is an endogenous allosteric enhancer of CB1 cannabinoid receptor

Allosteric modulation of G-protein-coupled receptors represents a key goal of current pharmacology. In particular, endogenous allosteric modulators might represent important targets of interventions aimed at maximizing therapeutic efficacy and reducing side effects of drugs. Here we show that the anti-inflammatory lipid lipoxin A(4) is an endogenous allosteric enhancer of the CB(1) cannabinoid receptor. Lipoxin A(4) was detected in brain tissues, did not compete for the orthosteric binding site of the CB(1) receptor (vs. (3)H-SR141716A), and did not alter endocannabinoid metabolism (as opposed to URB597 and MAFP), but it enhanced affinity of anandamide at the CB1 receptor, thereby potentiating the effects of this endocannabinoid both in vitro and in vivo. In addition, lipoxin A(4) displayed a CB(1) receptor-dependent protective effect against β-amyloid (1-40)-induced spatial memory impairment in mice. The discovery of lipoxins as a class of endogenous allosteric modulators of CB(1) receptors may foster the therapeutic exploitation of the endocannabinoid system, in particular for the treatment of neurodegenerative disorders.

Current drugs and future hopes in the treatment of Alzheimer's disease

In spite of several years of experience with the use of cholinesterase inhibitors for treatment of symptoms of Alzheimer's disease their influence on disease progression remains still unclear. New cholinesterase inhibitors should provide an additional neuroprotective activity, because only substances which stop neuronal death can influence disease progression. New treatment strategies are focusing on amyloid processing, preventing the occurrence of toxic A beta(1-42) peptide. These procedures include the vaccination trials, but their clinical usefulness has to be proven. Also strategies focussing on neurofibrillary pathologies should be explored in detail. Drug development for Alzheimer's disease should include all pathological events associated with neurodegeneration, like oxidative stress, neuroinflammation or disturbances in growth factor signaling. Abnormal protein aggregation as a common feature of different neurodegenerative diseases might also be a promising drug target. Beside beta sheet breakers directed against beta-amyloid deposition the endogenous protein beta-synuclein or derivatives of it might be able to counteract aggregation of alpha-synuclein as well as of amyloid beta protein. Interaction with alpha-synuclein deserves special attention because it might be an early step of synaptic degeneration. Due to the complexity of the disease combination of different drugs might be the most promising way to go. The parallel development of early biological markers should enable intervention in pre-symptomatic disease stages.

Identification of microtubule-associated protein tau isoforms in Alzheimer's paired helical filaments

AD66 proteins derived from sodium dodecylsulfate (SDS) insoluble paired helical filaments (PHF) were isolated from Alzheimer's brain using a purification procedure developed previously in this laboratory, and characterized by immunologic and chemical cleavage methods. AD66 proteins were immunoreactive with antibodies that recognize the amino terminal, tubulin-binding, and carboxy terminal domains of microtubule-associated protein tau indicating the presence of the entire tau sequence in AD66 proteins. These proteins were reactive with antibody 423 that binds to PHF but not human adult tau. Immunologic and chemical cleavage studies indicated that only two of the six tau isoforms were present in these proteins. AD66 proteins were comprised of tau proteins containing only three tubulin binding domains with either a 29 amino acid insert or no amino terminal insert. For comparative purposes, SDS soluble PHF-tau (A68 proteins) was purified from Alzheimer's brains and normal adult tau purified from control brains. Antibody Alz-50 was immunoreactive with PHF-tau or normal tau regardless of alkaline phosphatase treatment while immunoreactivity was only observed with dephosphorylated AD66 proteins. A second phosphorylated epitope on AD66 proteins but not PHF-tau or normal tau proteins was demonstrated with antibody PHF9. These data suggest that AD66 proteins represent a more phosphorylated form of tau than PHF-tau or normal tau proteins. Two-dimensional gel electrophoresis demonstrated that AD66 proteins have higher apparent molecular weights and lower pI values than normal tau, differences possibly due to the greater phosphorylation observed in these proteins.

Characterization of senescence- and apoptosis-dependent forms of terminin as derived from a precursor found in replicating and nonreplicating cells

Previously we have reported the production of a monoclonal antibody (Mab 1.2) which recognizes a cytoplasmic protein, terminin, in three different molecular weights: 90 (Tp90), 60 (Tp60), and 30 kDa (Tp30) forms. Further characterization shows that Tp90 is found in young growing and nongrowing quiescent fibroblasts, while Tp60 is found in permanently growth-arrested senescent fibroblasts and Tp30 in cells committed to undergo programmed cell death (apoptosis). In tissue, Tp90 is found in embryonic brain; later, in neonatal brain after terminal differentiation is completed, only Tp60 is found. Tp30 is found in crude liver fractions extracted without the protective action of protease inhibitors. In all these circumstances, Tp90 is mostly seen in the detergent-soluble fraction, while Tp60 and Tp30 are detergent-insoluble. We now report that in cultured fibroblasts, as well as in tissues such as brain and liver, Tp60 and Tp30 are derived from the Tp90 polypeptide, indicated by the fact that only the Tp90 species is identified by both immunoblotting and immunoprecipitation assays, when the cell or tissue extracts are prepared in the presence of protease inhibitors. Further evidence shows that immunoprecipitation of in vitro translation products from brain, liver, and cultured fibroblasts also present a single band of Tp90 polypeptide. Pulse-chase experiments show that during apoptosis, Tp90 is processed to Tp60, and eventually to Tp30. However, when the total protein extracts are fractionated, only Tp90 is found in the detergent-soluble fraction, with diminishing quantities during the time course of apoptosis, and Tp30, in contrast, is found as the only protein species in the insoluble fraction, with increasing quantity during the same time course. Newly processed Tp60 is not found in either of the fractions, reflecting its loss during the fractionation procedure. Limited one-dimensional peptide mapping of Tp90 yields three different bands at 30, 28, and 25 kDa, but only the one at 30 kDa is recognized by Mab 1.2. These results lead us to suggest that terminin protein is synthesized in the Tp90 form, and cleaved to lower molecular weight forms depends upon different physiologic conditions, with Tp60 processed in the terminally differentiated or senescent state and rapidly to Tp30 in apoptosis. Our findings further suggest that Tp90's processing to either Tp60 or Tp30 produces insoluble protein forms. Furthermore, the presence of Tp90 in nonapoptotic (either replicating or nonreplicating) cells may reflect the absence of necessary proteolytic action required for the execution of apoptosis. Future experiments will allow us to determine the nature of this proteolytic action, as well as whether this action is due to the autocatalytic action of Tp90 or by other endogenous proteases, and then to determine the significance of this biochemical action in cells.

Pharmacotherapy of cognitive impairment in Alzheimer's disease: a review

Experimental pharmacotherapy of cognitive impairment in Alzheimer's disease has seen a recent proliferation of drug trials involving a wide variety of drugs. Many of the earlier studies focused on cholinergic agents. However, subsequent advances in basic and biological sciences have broadened the scope of therapeutic strategies beyond the neurotransmitter approaches to include neurotrophic, metabolic-enhancing, membrane-modifying, and antitoxic agents, and have also provided rationale for developing antiamyloid and anti-infective therapies. For the clinician, it has not been easy to keep abreast of these developments. In this article, I present an overview of the cognition-enhancing drugs that have been used in the past, of those currently under investigation, and of new drugs and strategies that are likely to receive attention in the next few years.

Microvasculature in brain biopsy specimens from patients with Alzheimer's disease: an immunohistochemical and ultrastructural study

Brain biopsy specimens from five patients with Alzheimer's disease obtained in the course of a trial of intracerebroventricular bethanechol were studied by immunohistochemical (antibody to A4 peptide) and ultrastructural techniques, with particular emphasis on the microvessels. In some cases, numbers of A4-immunoreactive lesions (senile plaques) correlated well with numbers of plaques demonstrable by silver stains. Prominent A4-immunoreactive amyloid angiopathy was seen in one patient. The patient with severe cerebral amyloid angiopathy (CAA) showed extensive arteriolar deposition of amyloid filaments with apparent destruction of the media but remarkably intact endothelium. A cell of origin for amyloid filaments was not apparent, although close proximity to smooth muscle cell remnants in the arteriolar media suggested this as one possible cell of origin. Frequent vessels showed medial or adventitial collagen deposition, even when the amount of amyloid was minimal or negligible. Thus relatively severe CAA can exist in the absence of overt endothelial injury, although related studies on this tissue indicate definite abnormalities of the blood-brain barrier. Conversely, destruction of smooth muscle cells and collagen deposition in vessel walls may be the cellular correlates of arteriolar weakening that can lead to CAA-related brain hemorrhage.

Advances in the pharmacotherapy of Alzheimer's disease

The authors reviewed the literature on the agents proposed for the treatment of Alzheimer's disease (AD). Different classes of drugs have been tested for this indication including psychostimulants, anticoagulants, vasodilators, hyperbaric oxygen, hormones, nootropics, cholinomimetics, monoaminergics and neuropeptides without conclusive evidence of being beneficial for the treatment of this condition. Among the cholinomimetics recent research data seems to indicate that they might produce modest benefits in mild-to-moderate AD patients. Recently, other drugs have also been proposed including neurotrophic factors, phosphatidylserine, angiotension [corrected] converting enzyme (ACE) inhibitors, calcium channel blockers, acetyl-L-carnitine, xanthine derivatives, anti-inflammatory agents, aluminum chelate agents, and D-cycloserine. Of these new strategies few hold promise of more substantial benefits for AD, with the possibility of altering the course of the disease, but these drugs await confirmatory trials.

Aggregation of tau protein by aluminum

Aluminum has been detected in Alzheimer neurofibrillary tangles, but the significance of its presence is unknown. The principal component of tangles is the paired helical filament (PHF), comprised of tau protein. We investigated whether aluminum could induce tau protein to form filaments or aggregate. When 10 microM bovine tau or non-phosphorylated recombinant human tau was combined with 400 microM or more aluminum, tau protein appeared to aggregate, observed as a dose-dependent decrease in electrophoretic mobility on SDS-PAGE. Tau appeared as a smear above the region of the expected tau bands and, at higher aluminum doses, failed to enter the gel. A tau fragment encompassing the microtubule binding domains did not show decreased mobility in the presence of aluminum, but did form aggregates that failed to electrophorese. However no fibrillar structures were observed in the aluminum-treated tau samples when observed by electron microscopy. The effect of aluminum on tau mobility was reversed by incubating with 1 mM deferoxamine. In contrast, the morphology of PHF fibrils was unaffected by deferoxamine treatment and the characteristic abnormal mobility of PHF-tau was not reduced by deferoxamine. This suggests that aluminum is not, by itself, a significant factor in maintaining the assembly of PHF-tau as fibrils or in its abnormal mobility on SDS gels. Aluminum treatment of 3T3 fibroblasts transfected with human tau resulted in toxicity, but did not change tau expression levels or induce tau aggregation. In conclusion, aluminum appears to induce isolated tau protein to aggregate in a phosphate-independent way, without the formation of fibrils.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of tau protein in tau-transfected 3T3 cells

The tau protein of Alzheimer paired helical filaments (PHFs) is aberrantly phosphorylated, as evidenced by its reactivity with several phosphate-dependent antibodies. We sought to identify whether this unusual phosphorylation state exists in tau expressed by transfected NIH 3T3 fibroblasts. Immunoblot analysis of cell clones transfected with constructs for either the 3-repeat or 4-repeat isoforms of tau revealed two tau bands, with the lower band migrating with unmodified tau in each case. Antibodies T3P and tau-1 were used to probe these bands, as they also react with PHF-tau in a phosphate-dependent manner. The epitopes for both antibodies were phosphorylated in both tau isoforms. Only the upper band was phosphorylated at the T3P site whereas phosphorylation at the tau-1 site was not always associated with a shift of tau mobility on gels. Tau in both bands was soluble, in contrast to PHF-tau, and was competent to bind to exogenously added bovine microtubules. Colchicine treatment of the cells resulted in an inhibition of phosphorylation at both sites, through an unknown mechanism. In conclusion human tau expressed in 3T3 cells was phosphorylated at the T3P and tau-1 sites as is PHF-tau, although no PHFs formed and the phosphorylated tau was competent to bind to microtubules.

Neurotrophic strategies for treating Alzheimer's disease: lessons from basic neurobiology and animal models

Because neurotrophic factors can prevent natural and experimental cases of neural cell death and induce and maintain differentiation, they are especially attractive agents for the treatment of neurodegenerative diseases, such as Alzheimer's disease (AD). The present report argues for the specific role of particular families of trophic factors, such as neurotrophins (e.g., nerve growth factor [NGF]) and neurokines (e.g., ciliary neurotrophic factor [CNTF]), for the promotion of the survival and phenotype of subsets of central nervous system (CNS) neurons vulnerable in AD, such as basal forebrain cholinergic neurons and cortical projection neurons. Although there is ample evidence for the therapeutic role of NGF in experimental or natural injury of cholinergic neurons, not enough progress has been made on trophic models involving cortical neurons. Further understanding of the mechanisms of cell death in AD and elucidation of the transduction cascades of trophic factors will undoubtedly refine our current concepts of a neurotrophic treatment for AD.

Protective effects of SR 57746A in central and peripheral models of neurodegenerative disorders in rodents and primates

Compounds possessing neurotrophic properties may represent a possible treatment for neurodegenerative disorders such as Alzheimer's disease. SR 57746A, 1-[2-(naphth-2-yl)ethyl]-4-(3-trifluoromethylphenyl)-1,2,5,6- tetrahydropyridine hydrochloride, is a new compound with neurotrophic activity in a number of in vitro preparations. The neurotrophic effects of this compound have been evaluated in vivo using four distinct rat models of neurodegeneration: transient global ischaemia produced by a four-vessel occlusion; septohippocampal lesion produced by injection of vincristine sulphate into the medial septum; sciatic nerve crushing; and acrylamide-induced peripheral neuropathy. Rats were administered vehicle or 2.5-10 mg/kg p.o. SR 57746A, after initiation of the degenerative process, then once daily for 10 days in the first two models, 16 days in the third and 26 days in the fourth model. Median scores for ischaemia-induced neuronal damage were reduced by 30-40% by SR 57746A treatment in hippocampal CA1, CA2, and CA3 regions, and in the dorsal striatum. Twelve days after intraseptal vincristine administration, there was a marked loss of septohippocampal cholinergic neurons, as indicated by reduced choline acetyltransferase activity in both the septum and hippocampus. SR 57746A dose-dependently reversed this reduction in both areas. These results were confirmed by histoenzymological evaluation of hippocampal acetylcholinesterase content. SR 57746A also reversed the loss of hippocampal choline acetyltransferase induced by intraseptal vincristine in marmosets. Behavioral deficits in these models (exploratory behaviour in the former and short-term social memory in the latter) were also significantly reduced by SR 57746A treatment. In the sciatic crush model, sensorimotor function improved more rapidly in rats treated with 10 mg/kg SR 57746A. In this same model, SR 57746A (10 mg/kg/day) also significantly increased the length of regenerated nerve eight days after the crush, as measured using the pinch test. Finally, SR 57746A retarded the onset, reduced the amplitude and accelerated the recovery of acrylamide-induced peripheral neuropathy. Thus, SR 57746A possesses notable neurotrophic activity in a variety of neurodegenerative models in vivo, suggesting that the compound may possess therapeutic potential for the treatment of neurodegenerative diseases.

Brain S-adenosylmethionine decarboxylase activity is increased in Alzheimer's disease

We measured the activity of S-adenosylmethionine decarboxylase (SAMDC), a key regulatory enzyme of polyamine biosynthesis, in autopsied brain from 13 patients with Alzheimer's Disease (AD). As compared with the controls, mean enzyme activity was increased by 37-96% in all seven examined brain regions with statistically significant increases in temporal cortex (+96%), frontal cortex (+69%) and hippocampus (+90%). The elevated SAMDC may have occurred as part of a generalized polyamine response to brain injury, which has been previously described in experimental animal conditions. Above-normal SAMDC activity implies increased levels/metabolism of spermidine and spermine, two polyamines which are involved in neuronal regeneration, growth factor production, and activation of excitatory N-methyl-D-aspartate preferring glutamate receptors. Our data suggest the involvement of the polyamine system in the brain reparative and/or pathogenetic mechanisms of AD.

Role of tau in the polymerization of peptides from beta-amyloid precursor protein

The composition of paired helical filaments (PHFs), the intracellular amyloid fibrils that accumulate in the brains of Alzheimer patients, is not completely known. We investigated whether synthetic peptides from beta-amyloid precursor protein (APP) can form PHF-like fibrils. Two peptides formed fibrils morphologically similar to PHFs. The presence of tau protein, a known PHF component, greatly enhanced the numbers of fibrils formed from one peptide, from the C-terminus of APP, and became associated with the fibrils. A tau fragment corresponding to the tubulin-binding region was sufficient to induce fibril formation. Tau did not alter fibril formation by the other peptide, which was from the beta/A4 region of APP. These results raise the possibility that a C-terminal fragment of APP, along with tau, may be involved in PHF formation. Thus the proteolytic processing of APP may generate fragments that contribute to both amyloids and both histopathologic lesions of Alzheimer's disease.

Immunological characterization of the region of tau protein that is bound to Alzheimer paired helical filaments

Tau protein is known to be present in the paired helical filaments (PHFs) of Alzheimer brains. This study investigated the fragments of tau protein that remain bound to pronase-treated PHFs and conditions that lead to the release of these tau fragments from the core structure of the PHF. Antibody 423 reacted with PHFs and with fetal rat tau but not with adult rat tau, pig tau, or recombinant human tau. Three other antibodies that react with the tubulin binding region of tau only reacted with PHFs after they were disrupted with formic acid or guanidine. Other antibodies that recognize tau sequences C terminal to the tubulin binding region also recognized pronase-treated PHFs. Antibodies SMI34 and T3P that recognize phosphorylated epitopes were reactive with pronase-treated PHFs. Tau fragments from the PHF were solubilized by acid or guanidine treatment. These findings suggest that the fragments of tau that are bound to PHFs and protected from pronase digestion include sequences from the tubulin binding region to the C terminus of tau. In addition, some of these sequences appear to be conformationally or post-translationally modified.

Treatment of Alzheimer's disease. Molecular pathology versus neurotransmitter-based therapy

Cortical inhibitory neurotransmitters, neuropeptides, dopamine and noradrenaline are probably either not selectively or not critically affected in AD. It is, however, likely that a key change is shrinkage or loss of corticocortical pyramidal neurones, which probably use glutamate as their transmitter. This depletion appears to be circumscribed and clinically relevant.

Ante mortem cerebral amino acid concentrations indicate selective degeneration of glutamate-enriched neurons in Alzheimer's disease

There is little information about major constituents of the brain in Alzheimer's disease. In the case of amino acids most of the previous data are contradictory. These have been interpreted in an anatomic and neurotransmitter as well as a metabolic context. To help clarify this, the contents of 14 amino acids and ethanolamine were determined in samples of neocortex from diagnostic craniotomies of 15 demented patients (10 with Alzheimer's disease) and other neurosurgical procedures (57 patients, 18 with intractable depression). A comprehensive survey of the effects of possible complicating factors on the concentrations of amino acids showed that artefacts were few; this was in contrast to a post mortem series of brains (16 with Alzheimer's disease and 16 controls; six regions assayed). We have used the ante mortem data to provide the basis for an accurate comparison of amino acid values between Alzheimer and control samples. In Alzheimer's disease, the mean contents of many amino acids were slightly higher (sum of the increases of those significantly affected was 15 nmol/mg protein) whereas glutamate content alone was significantly reduced (by 16 nmol/mg protein). This was not a feature of depression or a group of patients with other dementias. Glutamate content of Alzheimer samples was related to pyramidal neuron density in cortical layer III. These alterations were detected relatively early during the course of Alzheimer's disease and are considered to be due to loss of corticocortical glutamatergic association pathways.

Novel therapy for Alzheimer's disease

Recent results support the hypothesis that microglia and/or macrophages of the brain, by producing oxidants, could play a role in the local inactivation of the Kunitz protease inhibitor (KPI) domain of beta-amyloid precursor protein (APP), thereby facilitating deposition of abnormal amyloid filaments in patients with Alzheimer's disease (AD). Protease inhibitors and/or free radical scavengers might serve as therapy for the amyloidosis of AD.