Dual-target ligand discovery for Alzheimer's disease: triphenylphosphoranylidene derivatives as inhibitors of acetylcholinesterase and β-amyloid aggregation

Alzheimer disease (AD) is one of the major neurodegenerative diseases that could not be prevented or completely cured and may lead to death. Here, we target AChE and β-amyloid proteins. Synthesising new triphenylphosphporanylidene derivatives based on the surveyed literature and testing their biological activity revealed promising results especially for the acetyl triphenylphosphoranylidene derivative 8c, which showed good inhibitor activity against AChE enzyme with IC₅₀ in the nanomolar range (97.04 nM); on the other hand, it showed poor selectivity for AChE versus butyrylcholinesterase but with some futural structural modification, this selectivity can be improved. 8c showed MMP-2 IC₅₀ of 724.19 nM and Aβ_1-42 aggregation IC₅₀ of 302.36 nM. A kinetic study demonstrated that compound 8c uncompetitively inhibited AChE. Moreover, derivative 8c showed low cytotoxicity, good in vivo behavioural studies including Y-maze and passive avoidance tests with activity similar to that of donepezil. Finally, in silico studies for 8c predict its good penetration into BBB and good binding affinity in the AChE binding site.

Cathepsin B - A Neuronal Death Mediator in Alzheimer’s Disease Leads to Neurodegeneration

Abstract:

The lysosomal cysteine protease enzyme, named Cathepsin B, mainly degrades the protein and manages its average turnover in our body. The Cathepsin B active form is mostly present inside the lysosomal part at a cellular level, providing the slightly acidic medium for its activation. Multiple findings on Cathepsin B reveal its involvement in neurons' degeneration and a possible role as a neuronal death mediator in several neurodegenerative diseases. In this review article, we highlight the participation of Cathepsin B in the etiology/progress of AD, along with various other factors. The enzyme is involved in producing neurotoxic Aβ amyloid in the AD brain by acting as the β-secretase enzyme in the regulated secretory pathways responsible for APP processing. Aβ amyloid accumulation and amyloid plaque formation lead to neuronal degeneration, one of the prominent pathological hallmarks of AD. Cathepsin B is also involved in the production of PGlu-Aβ, which is a truncated and highly neurotoxic form of Aβ. Some of the findings also revealed that Cathepsin B specific gene deletion decreases the level of PGlu-Aβ inside the brain of experimental mice. Therefore, neurotoxicity might be considered a new pathological indication of AD due to the involvement of Cathepsin B. It also damages neurons present in the CNS region by producing inflammatory responses and generating mitochondrial ROS. However, Cathepsin B inhibitors, i.e., CA-074, can prevent neuronal death in AD patients. The other natural inhibitors are also equally effective against neuronal damage with higher selectivity. Its synthetic inhibitors are specific for their target; however, they lose their selectivity in the presence of quite a few reducing agents. Therefore, a humanized monoclonal antibody is used as a selective Cathepsin B inhibitor to overcome the problem experienced. The use of Cathepsin B for the treatment of AD and other neurodegenerative diseases could be considered a rational therapeutic target.

Increased expression of GAPDH in cynomolgus monkeys with spontaneous cognitive decline and amyloidopathy reminiscent of an Alzheimer's-type disease is reflected in the circulation

Previous studies of aging cynomolgus monkeys from our group identified spontaneous age-associated cognitive declines associated with biomarkers and brain lesions reminiscent of Alzheimer's Disease (AD), in a proportion of aged monkeys. However, the molecular mechanisms that underlie the spontaneous amyloid disorders and cognitive declines observed in these affected monkeys have yet to be investigated in detail. Using reverse transcriptase quantitative real time PCR techniques, normalized to the ACTB housekeeping gene, we analyzed the expression patterns of a number of genes which have been implicated in amyloid and tau abnormalities, in well-characterized aged cynomolgus monkeys with cognitive decline. A significantly increased expression of the genes coding for glyceraldehyde 3-phosphate dehydrogenase (GAPDH), was found in aged-cognitive decline monkeys compared to age-matched healthy controls. GAPDH has been implicated in several neurodegenerative diseases and interacts with beta amyloid precursor proteins. These findings provide support for the utilization of cynomolgus macaques in translational preclinical research as valid spontaneous models in experimental investigations of the relationships among aging, cognitive decline, and the neuropathy of AD.

Nanomedicine against Alzheimer's and Parkinson's Disease

Alzheimer's and Parkinson's are the two most rampant neurodegenerative disorders worldwide. Existing treatments have a limited effect on the pathophysiology but are unable to fully arrest the progression of the disease. This is due to the inability of these therapeutic molecules to efficiently cross the blood-brain barrier. We discuss how nanotechnology has enabled researchers to develop novel and efficient nano-therapeutics against these diseases. The development of nanotized drug delivery systems has permitted an efficient, site-targeted, and controlled release of drugs in the brain, thereby presenting a revolutionary therapeutic approach. Nanoparticles are also being thoroughly studied and exploited for their role in the efficient and precise diagnosis of neurodegenerative conditions. We summarize the role of different nano-carriers and RNAi-conjugated nanoparticle-based therapeutics for their efficacy in pre-clinical studies. We also discuss the challenges underlying the use of nanomedicine with a focus on their route of administration, concentration, metabolism, and any toxic effects for successful therapeutics in these diseases.

Structural Analysis of the Simultaneous Activation and Inhibition of γ-Secretase Activity in the Development of Drugs for Alzheimer's Disease

Significance: The majority of the drugs which target membrane-embedded protease γ-secretase show an unusual biphasic activation–inhibition dose-response in cells, model animals, and humans. Semagacestat and avagacestat are two biphasic drugs that can facilitate cognitive decline in patients with Alzheimer’s disease. Initial mechanistic studies showed that the biphasic drugs, and pathogenic mutations, can produce the same type of changes in γ-secretase activity. Results: DAPT, semagacestat LY-411,575, and avagacestat are four drugs that show different binding constants, and a biphasic activation–inhibition dose-response for amyloid-β-40 products in SH-SY5 cells. Multiscale molecular dynamics studies have shown that all four drugs bind to the most mobile parts in the presenilin structure, at different ends of the 29 Å long active site tunnel. The biphasic dose-response assays are a result of the modulation of γ-secretase activity by the concurrent binding of multiple drug molecules at each end of the active site tunnel. The drugs activate γ-secretase by facilitating the opening of the active site tunnel, when the rate-limiting step is the tunnel opening, and the formation of the enzyme–substrate complex. The drugs inhibit γ-secretase as uncompetitive inhibitors by binding next to the substrate, to dynamic enzyme structures which regulate processive catalysis. The drugs can modulate the production of different amyloid-β catalytic intermediates by penetration into the active site tunnel, to different depths, with different flexibility and different binding affinity. Conclusions: Biphasic drugs and pathogenic mutations can affect the same dynamic protein structures that control processive catalysis. Successful drug-design strategies must incorporate transient changes in the γ-secretase structure in the development of specific modulators of its catalytic activity.

Design and synthesis of novel methoxypyridine-derived gamma-secretase modulators

The evolution of gamma-secretase modulators (GSMs) through the introduction of novel heterocycles with the goal of aligning activity for reducing the levels of Aβ42 and properties consistent with a drug-like molecule are described. The insertion of a methoxypyridine motif within the tetracyclic scaffold provided compounds with improved activity for arresting Aβ42 production as well as improved properties, including solubility. In vivo pharmacokinetic analysis demonstrated that several compounds within the novel series were capable of crossing the BBB and accessing the therapeutic target. Treatment with methoxypyridine-derived compound 64 reduced Aβ42 levels in the plasma of J20 mice, in addition to reducing Aβ42 levels in the plasma and brain of Tg2576 mice.

NGP 555, a γ-secretase modulator, shows a beneficial shift in the ratio of amyloid biomarkers in human cerebrospinal fluid at safe doses

INTRODUCTION

Currently, there is no cure for Alzheimer's disease (AD), and it is widely accepted that AD is a complex disease with multiple approaches necessary to prevent and treat the disease.

METHODS

Using amyloid biomarkers in human cerebrospinal fluid, pharmacokinetic, safety, and metabolism studies, we investigate the properties of NGP 555, γ-secretase modulator, for the first time in human clinical trials.

RESULTS

Our preclinical and clinical studies combined show beneficial effects with NGP 555 on synaptic response and amyloid cerebrospinal fluid biomarkers while avoiding negative side effects. Importantly, pharmacokinetic and pharmacodynamic parameters combined with safety outcomes indicate that NGP 555 penetrates the blood-brain barrier and increases the ratio of amyloid-β peptide Aβ37 and Aβ38 compared with that of Aβ42, establishing a proof of target engagement in humans in a 14 day, once-daily oral dosing trial.

DISCUSSION

In humans, NGP 555 has demonstrated a beneficial shift in the production of Aβ37 and Aβ38 versus Aβ42 biomarker levels in the cerebrospinal fluid while maintaining an adequate safety profile. The overall clinical goal is to achieve an optimal balance of efficacy for altering amyloid-β peptide (Aβ) biomarkers while maintaining a safe profile so that NGP 555 can be given early in AD to prevent production of Aβ42 and accumulation of amyloid plaques, in an effort to prevent aggregation of tau and destruction of neurons and synapses resulting in cognitive decline.

Recent Developments in Metal-Based Drugs and Chelating Agents for Neurodegenerative Diseases Treatments

The brain has a unique biological complexity and is responsible for important functions in the human body, such as the command of cognitive and motor functions. Disruptive disorders that affect this organ, e.g. neurodegenerative diseases (NDDs), can lead to permanent damage, impairing the patients' quality of life and even causing death. In spite of their clinical diversity, these NDDs share common characteristics, such as the accumulation of specific proteins in the cells, the compromise of the metal ion homeostasis in the brain, among others. Despite considerable advances in understanding the mechanisms of these diseases and advances in the development of treatments, these disorders remain uncured. Considering the diversity of mechanisms that act in NDDs, a wide range of compounds have been developed to act by different means. Thus, promising compounds with contrasting properties, such as chelating agents and metal-based drugs have been proposed to act on different molecular targets as well as to contribute to the same goal, which is the treatment of NDDs. This review seeks to discuss the different roles and recent developments of metal-based drugs, such as metal complexes and metal chelating agents as a proposal for the treatment of NDDs.

γ-Secretase and its modulators: Twenty years and beyond

Twenty years ago, Wolfe, Xia, and Selkoe identified two aspartate residues in Alzheimer's presenilin protein that constitute the active site of the γ-secretase complex. Mutations in the genes encoding amyloid precursor protein (APP) or presenilin (PS) cause early onset familial Alzheimer's disease (AD), and sequential cleavages of the APP by β-secretase and γ-secretase/presenilin generate amyloid β protein (Aβ), the major component of pathological hallmark, neuritic plaques, in brains of AD patients. Therapeutic strategies centered on targeting γ-secretase/presenilin to reduce amyloid were implemented and led to several high profile clinical trials. This review article focuses on the studies of γ-secretase and its inhibitors/modulators since the discovery of presenilin as the γ-secretase. While a lack of complete understanding of presenilin biology renders failure of clinical trials, the lessons learned from some γ-secretase modulators, while premature for human testing, provide new directions to develop potential therapeutics. Imbalanced Aβ homeostasis is an upstream event of neurodegenerative processes. Exploration of γ-secretase modulators for their roles in these processes is highly significant, e.g., decreasing neuroinflammation and levels of phosphorylated tau, the component of the other AD pathological hallmark, neurofibrillary tangles. Agents with excellent human pharmacology hold great promise in suppressing neurodegeneration in pre-symptomatic or early stage AD patients.

Ultrasound-Assisted Facile Synthesis of Nanostructured Hybrid Vesicle for the Nasal Delivery of Indomethacin: Response Surface Optimization, Microstructure, and Stability

This work is devoted to design a novel nanostructured hybrid vesicle (NHV) made of lecithin and an acrylate/C10-C30 alkyl acrylate for the nasal delivery of a model active indomethacin (IND), and further to probe its microstructure, intermolecular interactions, drug release behavior, ex vivo permeation, and stability. NHVs were prepared by cavitation technology employing RSM-based central composite design (CCD). Amount of lecithin (X₁), power of ultrasound (X₂), and sonication time (X₃) were selected as three independent variables while the studied response included Z-Avg (nm), polydispersity index (PDI), and zeta potential (mV). The designed system (NHV) was investigated through dynamic (DLS) and electrophoretic light scattering (ELS), attenuated total reflectance (ATR-FTIR), oscillatory measurement (stress and frequency sweep), and transmission electron microscopy (TEM). CCD was found useful in optimizing NHV. An optimized formulation (S6) had Z-Avg 80 nm, PDI 0.2, and zeta potential of - 43.26 mV. Morphology investigation revealed spherical vesicles with smaller TEM diameters (the largest particle being 52.26 nm). ATR analysis demonstrated significant intermolecular interactions among the drug (IND) and the components of vesicles. The designed vesicles had an elastic predominance and displayed supercase II (n > 1) type of drug release. Besides, the vesicles possessed potential to transport IND across the nasal mucosa with the steady-state flux (μg/cm²/h) and permeability coefficient (cm/h) of 26.61 and 13.30 × 10^-3, respectively. NHV exhibited an exceptional stability involving a combination of electrostatic and steric interactions while the histopathology investigation confirmed their safety for nasal administration.

Anti-Aβ drug candidates in clinical trials and plasmonic nanoparticle-based drug-screen for Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of neurodegenerative disorder in elderly people, and has become a social problem in aging societies globally. Amyloid-β (Aβ) aggregates (i.e., Aβ fibrils and plaques) present in the brains of AD patients are hallmarks of AD. Although various promising anti-Aβ drugs have been tested in pre-clinical and randomized controlled trials, the trial results have not yet been translated into clinical practice due to increasing time and cost of drug development. Recent investigations have addressed how the formation of Aβ aggregates is influenced by the surface of gold nanoparticles (AuNPs) to obtain a detailed understanding of the in vivo process of amyloid formation. Particularly, AuNPs catalytically provide nucleation sites to accelerate the formation of Aβ aggregates. Moreover, AuNPs have great potential as a sensing tool due to their optical property. Employing this dual function (i.e., catalytic and optical property), AuNP-based colorimetry is highlighted as a simple and innovative method for monitoring the efficacy of anti-Aβ reagents. In this review, we briefly survey important developments and designs of anti-Aβ drugs. The significance and perspectives of AuNP-based drug-screening in pharmacologic research are also discussed.

Recruitment of the amyloid precursor protein by γ-secretase at the synaptic plasma membrane

Γ-secretase is a membrane-embedded protease that cleaves single transmembrane helical domains of various integral membrane proteins. The amyloid precursor protein (APP) is an important substrate due to its pathological relevance to Alzheimer's disease. The mechanism of the cleavage of APP by γ-secretase that leads to accumulation of Alzheimer's disease causing amyloid-β (Aβ) is still unknown. Coarse-grained molecular dynamics simulations in this study reveal initial lipids raft formation near the catalytic site of γ-secretase as well as changes in dynamic behavior of γ-secretase once interacting with APP. The results suggest a precursor of the APP binding mode and hint at conformational changes of γ-secretase in the nicastrin (NCT) domain upon APP binding.

Overview and Current Status of Alzheimer's Disease in Bangladesh

Alzheimer’s disease (AD) is a complex neurological disorder with economic, social, and medical burdens which is acknowledged as leading cause of dementia marked by the accumulation and aggregation of amyloid-β peptide and phosphorylated tau (p-tau) protein and concomitant dementia, neuron loss and brain atrophy. AD is the most prevalent neurodegenerative brain disorder with sporadic etiology, except for a small fraction of cases with familial inheritance where familial forms of AD are correlated to mutations in three functionally related genes: the amyloid-β protein precursor and presenilins 1 and 2, two key γ-secretase components. The common clinical features of AD are memory impairment that interrupts daily life, difficulty in accomplishing usual tasks, confusion with time or place, trouble understanding visual images and spatial relationships. Age is the most significant risk factor for AD, whereas other risk factors correlated with AD are hypercholesterolemia, hypertension, atherosclerosis, coronary heart disease, smoking, obesity, and diabetes. Despite decades of research, there is no satisfying therapy which will terminate the advancement of AD by acting on the origin of the disease process, whereas currently available therapeutics only provide symptomatic relief but fail to attain a definite cure and prevention. This review also represents the current status of AD in Bangladesh.

Emerging treatments for Alzheimer's disease for non-amyloid and non-tau targets

INTRODUCTION

The number of people with dementia, including Alzheimer's disease, is growing as a result of an ageing global population. Treatments available for AD only alleviate the symptoms of the disease, and are effective in some people with AD for a limited time. There is no disease-modifying treatment available, and despite research efforts, the underlying mechanisms of AD and optimal treatment targets have not been fully elucidated. Amyloid and tau are key pathological markers of AD with ongoing trials targeting both. However, there are also many trials at various stages of development that primarily target other markers and processes implicated in the disease, which are now being investigated. Areas covered: This review summarizes current treatment approaches for AD and explores both repositioned and novel therapies that target non amyloid and non tau mechanisms that are in the clinical trials pipeline. This includes treatments for cognitive and neuropsychiatric symptoms and potentially disease modifying therapies. The studies included in this review have been obtained from searches of PubMed and clinical trials databases. Expert commentary: There is a renewed energy in identifying better treatments for behavioural symptoms of AD using both novel drugs and repositioning existing drugs. Lack of success in clinical trials of drugs targeting amyloid and tau have led to a surge in targeting alternative mechanisms. Progress in the development of biomarkers will provide further tools for clinical trials of potential therapeutics for both symptomatic treatment and disease modification in AD.

Chronic Sleep Restriction Induces Cognitive Deficits and Cortical Beta-Amyloid Deposition in Mice via BACE1-Antisense Activation

AIMS

To clarify the correlation between chronic sleep restriction (CSR) and sporadic Alzheimer disease (AD), we determined in wild-type mice the impact of CSR, on cognitive performance, beta-amyloid (Aβ) peptides, and its feed-forward regulators regarding AD pathogenesis.

METHODS

Sixteen nine-month-old C57BL/6 male mice were equally divided into the CSR and control groups. CSR was achieved by application of a slowly rotating drum for 2 months. The Morris water maze test was used to assess cognitive impairment. The concentrations of Aβ peptides, amyloid precursor protein (APP) and β-secretase 1 (BACE1), and the mRNA levels of BACE1 and BACE1-antisense (BACE1-AS) were measured.

RESULTS

Following CSR, impairments of spatial learning and memory consolidation were observed in the mice, accompanied by Aβ plaque deposition and an increased Aβ concentration in the prefrontal and temporal lobe cortex. CSR also upregulated the β-secretase-induced cleavage of APP by increasing the protein and mRNA levels of BACE1, particularly the BACE1-AS.

CONCLUSIONS

This study shows that a CSR accelerates AD pathogenesis in wild-type mice. An upregulation of the BACE1 pathway appears to participate in both cortical Aβ plaque deposition and memory impairment caused by CSR. BACE1-AS is likely activated to initiate a cascade of events that lead to AD pathogenesis. Our study provides, therefore, a molecular mechanism that links CSR to sporadic AD.

NGP 555, a γ-Secretase Modulator, Lowers the Amyloid Biomarker, Aβ42, in Cerebrospinal Fluid while Preventing Alzheimer's Disease Cognitive Decline in Rodents

Introduction

Alzheimer's disease (AD) is defined by the progressive accumulation of amyloid plaques and neurofibrillary tangles in the brain which precedes cognitive decline by years.

Methods

Using amyloid biomarkers, chemical modeling, mouse behavioral models, and drug development techniques, we investigate the properties of NGP 555, a clinical-stage γ-secretase modulator.

Results

NGP 555 shifts amyloid peptide production to the smaller, nonaggregating forms of amyloid. Our preclinical studies show beneficial effects on amyloid biomarkers, pathology, and cognition. NGP 555 has successfully completed chemistry, pharmacology, toxicity, metabolism, and safety studies.

Discussion

Abundant data support Aβ₄₂ as a target for prophylactic or early-stage intervention therapies in AD. The γ-secretase modulator, NGP 555 is being actively developed in human clinical trials for the prevention of Alzheimer's disease with the overall aim to achieve an appropriate balance of potency/efficacy on reducing the toxic forms of amyloid versus safety.

Medicinal Chemistry Focusing on Aggregation of Amyloid-β

The aggregation of peptides/proteins is intimately related to a number of human diseases. More than 20 have been identified which aggregate into fibrils containing extensive β-sheet structures, and species generated in the aggregation processes (i.e., oligomers and fibrils) contribute to disease development. Amyloid-β peptide (designated Aβ), related to Alzheimer's disease (AD), is the representative example. The intensive aggregation property of Aβ also leads to difficulty in its synthesis. To improve the synthetic problem, we developed an O-acyl isopeptide of Aβ1-42, in which the N-acyl linkage (amide bond) of Ser(26) was replaced with an O-acyl linkage (ester bond) at the side chain. The O-acyl isopeptide demonstrated markedly higher water-solubility than that of Aβ1-42, while it quickly converted to intact monomer Aβ1-42 via an O-to-N acyl rearrangement under physiological conditions. Inhibition of the pathogenic aggregation of Aβ1-42 might be a therapeutic strategy for curing AD. We succeeded in the rational design and identification of a small molecule aggregation inhibitor based on a pharmacophore motif obtained from cyclo[-Lys-Leu-Val-Phe-Phe-]. Moreover, the inhibition of Aβ aggregation was achieved via oxygenation (i.e., incorporation of oxygen atoms to Aβ) using an artificial catalyst. We identified a selective, cell-compatible photo-oxygenation catalyst of Aβ, a flavin catalyst attached to an Aβ-binding peptide, which markedly decreased the aggregation potency and neurotoxicity of Aβ.

Design and synthesis of aminothiazole modulators of the gamma-secretase enzyme

The design and construction of a series of novel aminothiazole-derived γ-secretase modulators is described. The incorporation of heterocyclic replacements of the terminal phenyl D-ring of lead compound 1 was conducted in order to align potency with favorable drug-like properties. γ-Secretase modulator 28 displayed good activity for in vitro inhibition of Aβ42, as well as substantial improvement in ADME and physicochemical properties, including aqueous solubility. Pharmacokinetic evaluation of compound 28 in mice revealed good brain penetration, as well as good clearance, half-life, and volume of distribution which collectively support the continued development of this class of compounds.

The role and therapeutic targeting of α-, β- and γ-secretase in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly and its prevalence is set to increase rapidly in coming decades. However, there are as yet no available drugs that can halt or even stabilize disease progression. One of the main pathological features of AD is the presence in the brain of senile plaques mainly composed of aggregated β amyloid (Aβ), a derivative of the longer amyloid precursor protein (APP). The amyloid hypothesis proposes that the accumulation of Aβ within neural tissue is the initial event that triggers the disease. Here we review research efforts that have attempted to inhibit the generation of the Aβ peptide through modulation of the activity of the proteolytic secretases that act on APP and discuss whether this is a viable therapeutic strategy for treating AD.

Thiosemicarbazone modification of 3-acetyl coumarin inhibits Aβ peptide aggregation and protect against Aβ-induced cytotoxicity

Aggregation of amyloid β peptide (Aβ) is an important event in the progression of Alzheimer's disease. Therefore, among the available therapeutic approaches to fight with disease, inhibition of Aβ aggregation is widely studied and one of the promising approach for the development of treatments for Alzheimer's disease. Thiosemicarbazone compounds are known for their variety of biological activities. However, the potential of thiosemicarbazone compounds towards inhibition of Aβ peptide aggregation and the subsequent toxicity is little explored. Herein, we report synthesis and x-ray crystal structure of novel compound 3-acetyl coumarin thiosemicarbazone and its efficacy toward inhibition of Aβ(1-42) peptide aggregation. Our results indicate that 3-acetyl coumarin thiosemicarbazone inhibits Aβ(1-42) peptide aggregation up to 80% compared to the parent 3-acetyl coumarin which inhibits 52%. Further, 3-acetyl coumarin thiosemicarbazone provides neuroprotection against Aβ-induced cytotoxicity in SH-SY5Y cell line. These findings indicate that thiosemicarbazone modification renders 3-acetyl coumarin neuroprotective properties.

Production of active glycosylation-deficient γ-secretase complex for crystallization studies

Alzheimer's disease (AD)-associated γ-secretase is a ubiquitously expressed multi-subunit protease complex embedded in the lipid bilayer of cellular compartments including endosomes and the plasma membrane. Although γ-secretase is of crucial interest for AD drug discovery, its atomic structure remains unresolved. γ-Secretase assembly and maturation is a multistep process, which includes extensive glycosylation on nicastrin (NCT), the only γ-secretase subunit having a large extracellular domain. These posttranslational modifications lead to protein heterogeneity that likely prevents the three-dimensional (3D) crystallization of the protease complex. To overcome this issue, we have engineered a Chinese hamster ovary (CHO) cell line deficient in complex sugar modifications (CHO lec1) to overexpress the four subunits of γ-secretase as a functional complex. We purified glycosylation-deficient γ-secretase from this recombinant cell line (CL1-9) and fully glycosylated γ-secretase from a recombinant CHO DG44-derived cell line (SS20). We characterized both complexes biochemically and pharmacologically in vitro. Interestingly, we found that the complex oligosaccharides, which largely decorate the extracellular domain of fully glycosylated NCT, are not involved in the proper assembly and maturation of the complex, and are dispensable for the specific generation, in physiological ratios, of the amyloid precursor protein (APP) cleavage products. In conclusion, we propose a novel bioengineering approach for the production of functional glycosylation-deficient γ-secretase, which may be suitable for crystallization studies. We expect that these findings will contribute both to solving the high-resolution 3D structure of γ-secretase and to structure-based drug discovery for AD.

Structure-Based Peptide Design to Modulate Amyloid Beta Aggregation and Reduce Cytotoxicity

The deposition of Aβ peptide in the brain is the key event in Alzheimer disease progression. Therefore, the prevention of Aβ self assembly into disease-associated oligomers is a logical strategy for treatment. π stacking is known to provide structural stability to many amyloids; two phenylalanine residues within the Aβ 14–23 self recognition element are in such an arrangement in many solved structures. Therefore, we targeted this structural stacking by substituting these two phenylalanine residues with their D-enantiomers. The resulting peptides were able to modulate Aβ aggregation in vitro and reduce Aβ cytotoxicity in primary neuronal cultures. Using kinetic analysis of fibril formation, electron microscopy and dynamic light scattering characterization of oligomer size distributions, we demonstrate that, in addition to altering fibril structural characteristics, these peptides can induce the formation of larger amorphous aggregates which are protective against toxic oligomers, possibly because they are able to sequester the toxic oligomers during co-incubation. Alternatively, they may alter the surface structure of the oligomers such that they can no longer interact with cells to induce toxic pathways.

Advances in the therapy of Alzheimer's disease: targeting amyloid beta and tau and perspectives for the future

Worldwide multidisciplinary translational research has led to a growing knowledge of the genetics and molecular pathogenesis of Alzheimer's disease (AD) indicating that pathophysiological brain alterations occur decades before clinical signs and symptoms of cognitive decline can be diagnosed. Consequently, therapeutic concepts and targets have been increasingly focused on early-stage illness before the onset of dementia; and distinct classes of compounds are now being tested in clinical trials. At present, there is a growing consensus that therapeutic progress in AD delaying disease progression would significantly decrease the expanding global burden. The evolving hypothesis- and evidence-based generation of new diagnostic research criteria for early-stage AD has positively impacted the development of clinical trial designs and the characterization of earlier and more specific target populations for trials in prodromal as well as in pre- and asymptomatic at-risk stages of AD.

Potential therapeutic strategies for Alzheimer's disease targeting or beyond β-amyloid: insights from clinical trials

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with two hallmarks: β-amyloid plagues and neurofibrillary tangles. It is one of the most alarming illnesses to elderly people. No effective drugs and therapies have been developed, while mechanism-based explorations of therapeutic approaches have been intensively investigated. Outcomes of clinical trials suggested several pitfalls in the choice of biomarkers, development of drug candidates, and interaction of drug-targeted molecules; however, they also aroused concerns on the potential deficiency in our understanding of pathogenesis of AD, and ultimately stimulated the advent of novel drug targets tests. The anticipated increase of AD patients in next few decades makes development of better therapy an urgent issue. Here we attempt to summarize and compare putative therapeutic strategies that have completed clinical trials or are currently being tested from various perspectives to provide insights for treatments of Alzheimer's disease.

Decreased CALM expression reduces Aβ42 to total Aβ ratio through clathrin-mediated endocytosis of γ-secretase

A body of evidence suggests that aberrant metabolism of amyloid-β peptide (Aβ) underlies the aetiology of Alzheimer disease (AD). Recently, a single-nucleotide polymorphism in phosphatidylinositol binding clathrin assembly protein (PICALM/CALM) gene, which encodes a protein implicated in the clathrin-mediated endocytosis, was identified as a genetic protective factor for AD, although its mechanistic details have little been explored. Here we show that loss of CALM leads to the selective decrease in the production ratio of the pathogenic Aβ species, Aβ42. Active form of γ-secretase is constitutively endocytosed via the clathrin-mediated pathway in a CALM dependent manner. Alteration in the rate of clathrin-mediated endocytosis of γ-secretase causes a shift in its steady-state localization, which consequently impacts on the production ratio of Aβ42. Our study identifies CALM as an endogenous modulator of γ-secretase activity by regulating its endocytosis and also as an excellent target for Aβ42-lowering AD therapeutics.

Soluble γ-secretase modulators selectively inhibit the production of the 42-amino acid amyloid β peptide variant and augment the production of multiple carboxy-truncated amyloid β species

Alzheimer's disease (AD) is characterized pathologically by an abundance of extracellular neuritic plaques composed primarily of the 42-amino acid amyloid β peptide variant (Aβ42). In the majority of familial AD (FAD) cases, e.g., those harboring mutations in presenilin 1 (PS1), there is a relative increase in the levels of Aβ42 compared to the levels of Aβ40. We previously reported the characterization of a series of aminothiazole-bridged aromates termed aryl aminothiazole γ-secretase modulators or AGSMs [Kounnas, M. Z., et al. (2010) Neuron 67, 769-780] and showed their potential for use in the treatment of FAD [Wagner, S. L., et al. (2012) Arch. Neurol. 69, 1255-1258]. Here we describe a series of GSMs with physicochemical properties improved compared to those of AGSMs. Specific heterocycle replacements of the phenyl rings in AGSMs provided potent molecules with improved aqueous solubilities. A number of these soluble γ-secretase modulators (SGSMs) potently lowered Aβ42 levels without inhibiting proteolysis of Notch or causing accumulation of amyloid precursor protein carboxy-terminal fragments, even at concentrations approximately 1000-fold greater than their IC50 values for reducing Aβ42 levels. The effects of one potent SGSM on Aβ peptide production were verified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry, showing enhanced production of a number of carboxy-truncated Aβ species. This SGSM also inhibited Aβ42 peptide production in a highly purified reconstituted γ-secretase in vitro assay system and retained the ability to modulate γ-secretase-mediated proteolysis in a stably transfected cell culture model overexpressing a human PS1 mutation validating the potential for use in FAD.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

Binding of longer Aβ to transmembrane domain 1 of presenilin 1 impacts on Aβ42 generation

Background

Amyloid-β peptide ending at 42nd residue (Aβ42) is believed as a pathogenic peptide for Alzheimer disease. Although γ-secretase is a responsible protease to generate Aβ through a processive cleavage, the proteolytic mechanism of γ-secretase at molecular level is poorly understood.

Results

We found that the transmembrane domain (TMD) 1 of presenilin (PS) 1, a catalytic subunit for the γ-secretase, as a key modulatory domain for Aβ42 production. Aβ42-lowering and -raising γ-secretase modulators (GSMs) directly targeted TMD1 of PS1 and affected its structure. A point mutation in TMD1 caused an aberrant secretion of longer Aβ species including Aβ45 that are the precursor of Aβ42. We further found that the helical surface of TMD1 is involved in the binding of Aβ45/48 and that the binding was altered by GSMs as well as TMD1 mutation.

Conclusions

Binding between PS1 TMD1 and longer Aβ is critical for Aβ42 production.

Disease-modifying drugs in Alzheimer's disease

Alzheimer's disease (AD) is an age-dependent neurodegenerative disorder and the most common cause of dementia. The early stages of AD are characterized by short-term memory loss. Once the disease progresses, patients experience difficulties in sense of direction, oral communication, calculation, ability to learn, and cognitive thinking. The median duration of the disease is 10 years. The pathology is characterized by deposition of amyloid beta peptide (so-called senile plaques) and tau protein in the form of neurofibrillary tangles. Currently, two classes of drugs are licensed by the European Medicines Agency for the treatment of AD, ie, acetylcholinesterase inhibitors for mild to moderate AD, and memantine, an N-methyl-D-aspartate receptor antagonist, for moderate and severe AD. Treatment with acetylcholinesterase inhibitors or memantine aims at slowing progression and controlling symptoms, whereas drugs under development are intended to modify the pathologic steps leading to AD. Herein, we review the clinical features, pharmacologic properties, and cost-effectiveness of the available acetylcholinesterase inhibitors and memantine, and focus on disease-modifying drugs aiming to interfere with the amyloid beta peptide, including vaccination, passive immunization, and tau deposition.

Transcriptional regulation and its misregulation in Alzheimer's disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder characterized by loss of memory and cognitive function. A key neuropathological event in AD is the accumulation of amyloid-β (Aβ) peptide. The production and clearance of Aβ in the brain are regulated by a large group of genes. The expression levels of these genes must be fine-tuned in the brain to keep Aβ at a balanced amount under physiological condition. Misregulation of AD genes has been found to either increase AD risk or accelerate the disease progression. In recent years, important progress has been made in uncovering the regulatory elements and transcriptional factors that guide the expression of these genes. In this review, we describe the mechanisms of transcriptional regulation for the known AD genes and the misregualtion that leads to AD susceptibility.

FTY720/fingolimod, a sphingosine analogue, reduces amyloid-β production in neurons

Sphingosine-1-phosphate (S1P) is a pluripotent lipophilic mediator working as a ligand for G-protein coupled S1P receptors (S1PR), which is currently highlighted as a therapeutic target for autoimmune diseases including relapsing forms of multiple sclerosis. Sphingosine related compounds, FTY720 and KRP203 known as S1PR modulators, are phosphorylated by sphingosine kinase 2 (SphK2) to yield the active metabolites FTY720-P and KRP203-P, which work as functional antagonists for S1PRs. Here we report that FTY720 and KRP203 decreased production of Amyloid-β peptide (Aβ), a pathogenic proteins causative for Alzheimer disease (AD), in cultured neuronal cells. Pharmacological analyses suggested that the mechanism of FTY720-mediated Aβ decrease in cells was independent of known downstream signaling pathways of S1PRs. Unexpectedly, 6-days treatment of APP transgenic mice with FTY720 resulted in a decrease in Aβ40, but an increase in Aβ42 levels in brains. These results suggest that S1PR modulators are novel type of regulators for Aβ metabolisms that are active in vitro and in vivo.

Curcumin: a natural substance with potential efficacy in Alzheimer's disease

Curcumin is a component of turmeric, a spice used in many types of cooking. Epidemiological evidence suggesting that populations that eat food with a substantial amount of curcumin were at lower risk of Alzheimer’s disease (AD) led to the idea that this compound might have a neuroprotective effect. Curcumin has substantial antioxidant and anti-inflammatory effects, and is being used as a potential preventative agent or treatment for many types of cancer. There is evidence to suggest that the addition of curcumin to cultured neuronal cells decreases brain inflammation and protects against β-amyloid-induced neurotoxicity. Curcumin also protects against toxicity when β-amyloid is administered to produce animal models of AD. Curcumin decreases β-amyloid formation from amyloid precursor protein, and also inhibits aggregation of β-amyloid into pleated sheets. Studies in transgenic mice with overproduction of β-amyloid demonstrate a neuroprotective effect of curcumin as well. Cognitive function was also improved in these animal models. Clinical trials of curcumin in AD have not been very promising. It is possible that this is due to poor oral bioavailability of curcumin in humans, and thus several approaches are being developed to improve delivery systems or to create analogs that will mimic the neuroprotective effects and easily reach the brain. The lack of efficacy of curcumin in humans with AD may also result from treating for too short a time or starting treatment too late in the course of the disease, where substantial neuronal death has already occurred and cannot be reversed. Curcumin may be beneficial in protecting against development or progression of AD if taken over the long term and started before symptoms of AD become apparent.

Structural biology of presenilins and signal peptide peptidases

Presenilin and signal peptide peptidase are multispanning intramembrane-cleaving proteases with a conserved catalytic GxGD motif. Presenilin comprises the catalytic subunit of γ-secretase, a protease responsible for the generation of amyloid-β peptides causative of Alzheimer disease. Signal peptide peptidase proteins are implicated in the regulation of the immune system. Both protease family proteins have been recognized as druggable targets for several human diseases, but their detailed structure still remains unknown. Recently, the x-ray structures of some archaeal GxGD proteases have been determined. We review the recent progress in biochemical and biophysical probing of the structure of these atypical proteases.

Important functional role of residue x of the presenilin GxGD protease active site motif for APP substrate cleavage specificity and substrate selectivity of γ-secretase

γ-Secretase plays a central role in the generation of the Alzheimer disease-causing amyloid β-peptide (Aβ) from the β-amyloid precursor protein (APP) and is thus a major Alzheimer's disease drug target. As several other γ-secretase substrates including Notch1 and CD44 have crucial signaling functions, an understanding of the mechanism of substrate recognition and cleavage is key for the development of APP selective γ-secretase-targeting drugs. The γ-secretase active site domain in its catalytic subunit presenilin (PS) 1 has been implicated in substrate recognition/docking and cleavage. Highly critical in this process is its GxGD active site motif, whose invariant glycine residues cannot be replaced without causing severe functional losses in substrate selection and/or cleavage efficiency. Here, we have investigated the contribution of the less well characterized residue x of the motif (L383 in PS1) to this function. Extensive mutational analysis showed that processing of APP was overall well-tolerated over a wide range of hydrophobic and hydrophilic mutations. Interestingly, however, most L383 mutants gave rise to reduced levels of Aβ37-39 species, and several increased the pathogenic Aβ42/43 species. Several of the Aβ42/43 -increasing mutants severely impaired the cleavages of Notch1 and CD44 substrates, which were not affected by any other L383 mutation. Our data thus establish an important, but compared with the glycine residues of the motif, overall less critical functional role for L383. We suggest that L383 and the flanking glycine residues form a spatial arrangement in PS1 that is critical for docking and/or cleavage of different γ-secretase substrates.

Modulators of γ-secretase activity can facilitate the toxic side-effects and pathogenesis of Alzheimer's disease

Background

Selective modulation of different Aβ products of an intramembrane protease γ-secretase, could be the most promising strategy for development of effective therapies for Alzheimer's disease. We describe how different drug-candidates can modulate γ-secretase activity in cells, by studying how DAPT affects changes in γ-secretase activity caused by gradual increase in Aβ metabolism.

Results

Aβ 1–40 secretion in the presence of DAPT shows biphasic activation-inhibition dose-response curves. The biphasic mechanism is a result of modulation of γ-secretase activity by multiple substrate and inhibitor molecules that can bind to the enzyme simultaneously. The activation is due to an increase in γ-secretase's kinetic affinity for its substrate, which can make the enzyme increasingly more saturated with otherwise sub-saturating substrate. The noncompetitive inhibition that prevails at the saturating substrate can decrease the maximal activity. The synergistic activation-inhibition effects can drastically reduce γ-secretase's capacity to process its physiological substrates. This reduction makes the biphasic inhibitors exceptionally prone to the toxic side-effects and potentially pathogenic. Without the modulation, γ-secretase activity on it physiological substrate in cells is only 14% of its maximal activity, and far below the saturation.

Significance

Presented mechanism can explain why moderate inhibition of γ-secretase cannot lead to effective therapies, the pharmacodynamics of Aβ-rebound phenomenon, and recent failures of the major drug-candidates such as semagacestat. Novel improved drug-candidates can be prepared from competitive inhibitors that can bind to different sites on γ-secretase simultaneously. Our quantitative analysis of the catalytic capacity can facilitate the future studies of the therapeutic potential of γ-secretase and the pathogenic changes in Aβ metabolism.