Membrane Dynamics of γ-Secretase Provides a Molecular Basis for β-Amyloid Binding and Processing

γ-Secretase produces β-amyloid (Aβ) within its presenilin (PS1) subunit, mutations in which cause Alzheimer's disease, and current therapies thus seek to modulate its activity. While the general structure is known from recent electron microscopy studies, direct loop and membrane interactions and explicit dynamics relevant to substrate processing remain unknown. We report a modeled structure utilizing the optimal multitemplate information available, including loops and missing side chains, account of maturation cleavage, and explicit all-atom molecular dynamics in the membrane. We observe three distinct conformations of γ-secretase (open, semiopen, and closed) that remarkably differ by tilting of helices 2 and 3 of PS1, directly controlling active site availability. The large hydrophilic loop of PS1 where maturation occurs reveals a new helix segment that parallels the likely helix character of other substrates. The semiopen conformation consistently shows the best fit of Aβ peptides, that is, longer residence before release and by inference more trimming. In contrast, the closed, hydrophobic conformation is largely inactive and the open conformation is active but provides fewer optimal interactions and induces shorter residence time and by inference releases Aβ peptides of longer lengths. Our simulations thus provide a molecular basis for substrate processing and changes in the Aβ₄₂/Aβ₄₀ ratio. Accordingly, selective binding to protect the semiopen "innocent" conformation provides a molecular recipe for effective γ-secretase modulators; we provide the full atomic structures for these states that may play a key role in developing selective γ-secretase modulators for treatment of Alzheimer's disease.

Substrate processing in intramembrane proteolysis by γ-secretase - the role of protein dynamics

Intramembrane proteases comprise a number of different membrane proteins with different types of catalytic sites. Their common denominator is cleavage within the plane of the membrane, which usually results in peptide bond scission within the transmembrane helices of their substrates. Despite recent progress in the determination of high-resolution structures, as illustrated here for the γ-secretase complex and its substrate C99, it is still unknown how these enzymes function and how they distinguish between substrates and non-substrates. In principle, substrate/non-substrate discrimination could occur at the level of substrate binding and/or cleavage. Focusing on the γ-secretase/C99 pair, we will discuss recent observations suggesting that global motions within a substrate transmembrane helix may be much more important for defining a substrate than local unraveling at cleavage sites.

Therapeutic peptides for CNS indications: Progress and challenges

Attacking neurodegeneration and promoting neuroprotection have been the holy grail in neurology for almost 20years and represent an area of high unmet medical need. However, indications like Alzheimer's disease and stroke are areas in drug development fraught with failure. This review will highlight three CNS peptide programs which are tackling targets and indications in which traditional small molecule approaches have been difficult and challenging. The targets for these potential peptide therapeutics include the NMDA receptor, γ-secretase, and cyclin-dependent kinase in which direct inhibition has resulted in on-target (not compound related) problems. For example, direct inhibition of γ-secretase has resulted in gastrointestinal abnormalities and inhibition of the NMDA receptor can result in hallucinations, dizziness, out-of-body sensations, and nightmares. When confronted with show-stopping side effects, the CNS peptide programs profiled in this review strike the problem with intervention and disruption of selective protein-protein interactions. The goal of these peptide programs is to produce selective therapeutics with a better safety profile.

Inhibition of γ-Secretase Leads to an Increase in Presenilin-1

γ-Secretase inhibitors (GSIs) are potential therapeutic agents for Alzheimer’s disease (AD); however, trials have proven disappointing. We addressed the possibility that γ-secretase inhibition can provoke a rebound effect, elevating the levels of the catalytic γ-secretase subunit, presenilin-1 (PS1). Acute treatment of SH-SY5Y cells with the GSI LY-374973 (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester, DAPT) augments PS1, in parallel with increases in other γ-secretase subunits nicastrin, presenilin enhancer 2, and anterior pharynx-defective 1, yet with no increase in messenger RNA expression. Over-expression of the C-terminal fragment (CTF) of APP, C99, also triggered an increase in PS1. Similar increases in PS1 were evident in primary neurons treated repeatedly (4 days) with DAPT or with the GSI BMS-708163 (avagacestat). Likewise, rats examined after 21 days administered with avagacestat (40 mg/kg/day) had more brain PS1. Sustained γ-secretase inhibition did not exert a long-term effect on PS1 activity, evident through the decrease in CTFs of APP and ApoER2. Prolonged avagacestat treatment of rats produced a subtle impairment in anxiety-like behavior. The rebound increase in PS1 in response to GSIs must be taken into consideration for future drug development.

Improved Neuroprotection Provided by Drug Combination in Neurons Exposed to Cell-Derived Soluble Amyloid-β Peptide

Oligomeric amyloid-β (Aβ) peptide contributes to impaired synaptic connections and neurodegenerative processes, and as such, represents a primary therapeutic target for Alzheimer's disease (AD)-modifying approaches. However, the lack of efficacy of drugs that inhibit production of Aβ demonstrates the need for a better characterization of its toxic effects, both on synaptic and neuronal function. Here, we used conditioned medium obtained from recombinant HEK-AβPP cells expressing the human amyloid-β protein precursor (Aβ-CM), to investigate Aβ-induced neurotoxic and synaptotoxic effects. Characterization of Aβ-CM revealed that it contained picomolar amounts of cell-secreted Aβ in its soluble form. Incubation of primary cortical neurons with Aβ-CM led to significant decreases in synaptic protein levels as compared to controls. This effect was no longer observed in neurons incubated with conditioned medium obtained from HEK-AβPP cells grown in presence of the γ-secretase inhibitor, Semagacestat or LY450139 (LY-CM). However, neurotoxic and pro-apoptotic effects of Aβ-CM were only partially prevented using LY-CM, which could be explained by other deleterious compounds related to chronic oxidative stress that were released by HEK-AβPP cells. Indeed, full neuroprotection was observed in cells exposed to LY-CM by additional treatment with the antioxidant resveratrol, or with the pluripotent n-3 polyunsaturated fatty acid docosahexaenoic acid. Inhibition of Aβ production appeared necessary but insufficient to prevent neurodegenerative effects associated with AD due to other neurotoxic compounds that could exert additional deleterious effects on neuronal function and survival. Therefore, association of various types of protective agents needs to be considered when developing strategies for AD treatment.

Signal peptide peptidase and SPP-like proteases - Possible therapeutic targets?

Signal peptide peptidase (SPP) and the four homologous SPP-like proteases SPPL2a, SPPL2b, SPPL2c and SPPL3 are GxGD-type intramembrane-cleaving proteases (I-CLIPs). In addition to divergent subcellular localisations, distinct differences in the mechanistic properties and substrate requirements of individual family members have been unravelled. SPP/SPPL proteases employ a catalytic mechanism related to that of the γ-secretase complex. Nevertheless, differential targeting of SPP/SPPL proteases and γ-secretase by inhibitors has been demonstrated. Furthermore, also within the SPP/SPPL family significant differences in the sensitivity to currently available inhibitory compounds have been reported. Though far from complete, our knowledge on pathophysiological functions of SPP/SPPL proteases, in particular based on studies in mice, has been significantly increased over the last years. Based on this, inhibition of distinct SPP/SPPL proteases has been proposed as a novel therapeutic concept e.g. for the treatment of autoimmunity and viral or protozoal infections, as we will discuss in this review. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Exercise and BDNF reduce Aβ production by enhancing α-secretase processing of APP

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by aggregation of toxic forms of amyloid β peptide (Aβ). Treatment strategies have largely been focused on inhibiting the enzymes (β- and γ-secretases) that liberate Aβ from the amyloid precursor protein (APP). While evidence suggests that individuals who exercise regularly are at reduced risk for AD and studies of animal models demonstrate that running can ameliorate brain Aβ pathology and associated cognitive deficits, the underlying mechanisms are unknown. However, considerable evidence suggests that brain-derived neurotrophic factor (BDNF) mediates beneficial effects of exercise on neuroplasticity and cellular stress resistance. Here, we tested the hypothesis that BDNF promotes non-amyloidogenic APP processing. Using a transgenic mouse model of Alzheimer's disease and cultured human neural cells, we demonstrate that exercise and BDNF reduce production of toxic Aβ peptides through a mechanism involving enhanced α-secretase processing of APP. This anti-amyloidogenic APP processing involves subcellular redistribution of α-secretase and an increase in intracellular neuroprotective APP peptides capable of binding and inhibiting β-secretase. Moreover, our results suggest that BDNF's ability to promote neurite outgrowth is primarily exerted through pathways other than APP processing. Exercise and other factors that enhance BDNF signaling may therefore have both therapeutic and prophylactic value in the battle against AD. Read the Editorial Highlight for this article on page 191.

Structural and biochemical differences between the Notch and the amyloid precursor protein transmembrane domains

γ-Secretase cleavage of the Notch receptor transmembrane domain is a critical signaling event for various cellular processes. Efforts to develop inhibitors of γ-secretase cleavage of the amyloid-β precursor C99 protein as potential Alzheimer's disease therapeutics have been confounded by toxicity resulting from the inhibition of normal cleavage of Notch. We present biochemical and structural data for the combined transmembrane and juxtamembrane Notch domains (Notch-TMD) that illuminate Notch signaling and that can be compared and contrasted with the corresponding traits of C99. The Notch-TMD and C99 have very different conformations, adapt differently to changes in model membrane hydrophobic span, and exhibit different cholesterol-binding properties. These differences may be exploited in the design of agents that inhibit cleavage of C99 while allowing Notch cleavage.

Insight into the Dissociation of Behavior from Histology in Synucleinopathies and in Related Neurodegenerative Diseases

Recent clinical trials using immunization approaches against Alzheimer's disease (AD) have failed to demonstrate improved cognitive functions in patients, despite potent suppression in the formation of both senile plaques and other amyloid-β deposits in postmortem brains. Similarly, we observed that treatment with ibuprofen, a non-steroidal anti-inflammatory drug, was effective in improving the histopathology, such as reducing both protein aggregation and glial activation, in the brains of transgenic mice expressing dementia with Lewy bodies-linked P123H β-synuclein. In contrast, only a small improvement in cognitive functions was observed in these mice. Collectively, it is predicted that histology does not correlate with behavior that is resilient and resistant to therapeutic stimuli. Notably, such a 'discrepancy between histology and behavior' is reminiscent of AD-like pathologies and incidental Lewy bodies, which are frequently encountered in postmortem brains of the elderly who had been asymptomatic for memory loss and Parkinsonism during their lives. We suggest that 'the discrepancy between histology and behavior' may be a universal feature that is associated with various aspects of neurodegenerative diseases. Furthermore, given that the cognitive reserve is specifically observed in human brains, human behavior may be evolutionally distinct from that in other animals, thus, contributing to the differential efficiency of therapy between human and lower animals, an important issue in the therapy of neurodegenerative diseases. Overall, it is important to better understand 'the discrepancy between histology and behavior' in the mechanism of neurodegeneration for the development of effective therapies against neurodegenerative diseases.

Future Therapeutics in Alzheimer's Disease: Development Status of BACE Inhibitors

Alzheimer's disease (AD) is the primary cause of dementia in the elderly. It remains incurable and poses a huge socio-economic challenge for developed countries with an aging population. AD manifests by progressive decline in cognitive functions and alterations in behaviour, which are the result of the extensive degeneration of brain neurons. The AD pathogenic mechanism involves the accumulation of amyloid beta peptide (Aβ), an aggregating protein fragment that self-associates to form neurotoxic fibrils that trigger a cascade of cellular events leading to neuronal injury and death. Researchers from academia and the pharmaceutical industry have pursued a rational approach to AD drug discovery and targeted the amyloid cascade. Schemes have been devised to prevent the overproduction and accumulation of Aβ in the brain. The extensive efforts of the past 20 years have been translated into bringing new drugs to advanced clinical trials. The most progressed mechanism-based therapies to date consist of immunological interventions to clear Aβ oligomers, and pharmacological drugs to inhibit the secretase enzymes that produce Aβ, namely β-site amyloid precursor-cleaving enzyme (BACE) and γ-secretase. After giving an update on the development and current status of new AD therapeutics, this review will focus on BACE inhibitors and, in particular, will discuss the prospects of verubecestat (MK-8931), which has reached phase III clinical trials.

Screening and Characterization Strategies for Nanobodies Targeting Membrane Proteins

The study of membrane protein function and structure requires their successful detection, expression, solubilization, and/or reconstitution, which poses a challenging task and relies on the availability of suitable tools. Several research groups have successfully applied Nanobodies in the purification, as well as the functional and structural characterization of membrane proteins. Nanobodies are small, single-chain antibody fragments originating from camelids presenting on average a longer CDR3 which enables them to bind in cavities and clefts (such as active and allosteric sites). Notably, Nanobodies generally bind conformational epitopes making them very interesting tools to stabilize, dissect, and characterize specific protein conformations. In the clinic, several Nanobodies are under evaluation either as potential drug candidates or as diagnostic tools. In recent years, we have successfully generated high-affinity, conformation-sensitive anti-γ-secretase Nanobodies. γ-Secretase is a multimeric membrane protease involved in processing of the amyloid precursor protein with high clinical relevance as mutations in its catalytic subunit (Presenilin) cause early-onset Alzheimer's disease. Advancing our knowledge on the mechanisms governing γ-secretase intramembrane proteolysis through various strategies may lead to novel therapeutic avenues for Alzheimer's disease. In this chapter, we present the strategies we have developed and applied for the screening and characterization of anti-γ-secretase Nanobodies. These protocols could be of help in the generation of Nanobodies targeting other membrane proteins.

Intramembrane proteolysis within lysosomes

Regulated intramembrane proteolysis is of pivotal importance in a diverse set of developmental and physiological processes. Altered intramembrane substrate turnover may be associated with neurodegeneration, cancer and impaired immune function. In this review we will focus on the intramembrane proteases which have been localized in the lysosomal membrane. Members of the γ-secretase complex and γ-secretase activity are found in the lysosomal membrane and are discussed to contribute to intracellular amyloid β production. Mutant or deficient γ-secretase may cause disturbed lysosomal function. The signal peptide peptidase-like (SPPL) protease 2a is a lysosomal membrane component and cleaves CD74, the invariant chain of the MHC II complex, as well as FasL, TNF, ITM2B and TMEM106, type II transmembrane proteins involved in the regulation of immunity and neurodegeneration. Therefore, it can be concluded, that not only proteolysis within the lysosomal lumen but also within lysosomal membranes regulates important cellular functions and contributes essentially to proteostasis of membrane proteins what may become increasingly compromised in the aged individual.

Overcoming translational barriers impeding development of Alzheimer's disease modifying therapies

It has now been ~ 30 years since the Alzheimer's disease (AD) research entered what may be termed the 'molecular era' that began with the identification of the amyloid β protein (Aβ) as the primary component of amyloid within senile plaques and cerebrovascular amyloid and the microtubule-associated protein tau as the primary component of neurofibrillary tangles in the AD brain. These pivotal discoveries and the subsequent genetic, pathological, and modeling studies supporting pivotal roles for tau and Aβ aggregation and accumulation have provided firm rationale for a new generation of AD therapies designed not to just provide symptomatic benefit, but as disease modifying agents that would slow or even reverse the disease course. Indeed, over the last 20 years numerous therapeutic strategies for disease modification have emerged, been preclinically validated, and advanced through various stages of clinical testing. Unfortunately, no therapy has yet to show significant clinical disease modification. In this review, I describe 10 translational barriers to successful disease modification, highlight current efforts addressing some of these barriers, and discuss how the field could focus future efforts to overcome barriers that are not major foci of current research efforts. Seminal discoveries made over the past 25 years have provided firm rationale for a new generation of Alzheimer's disease (AD) therapies designed as disease modifying agents that would slow or even reverse the disease course. Unfortunately, no therapy has yet to show significant clinical disease modification. In this review, I describe 10 translational barriers to successful AD disease modification, highlight current efforts addressing some of these barriers, and discuss how the field could focus future efforts to overcome these barriers. This article is part of the 60th Anniversary special issue.

Membrane trafficking and proteolytic activity of γ-secretase in Alzheimer’s disease

γ-Secretase is an intramembrane-cleaving protease that generates various forms of amyloid-β peptides (Aβ) that accumulate in the brains of Alzheimer’s disease (AD) patients. The intracellular trafficking and subcellular localization of γ-secretase are linked to both qualitative and quantitative changes in Aβ production. However, the precise intracellular localization of γ-secretase as well as its detailed regulatory mechanisms have remained elusive. Recent genetic studies on AD provide ample evidence that alteration of the subcellular localization of γ-secretase contributes to the pathogenesis of AD. Here we review our current understanding of the intracellular membrane trafficking of γ-secretase, the association between its localization and proteolytic activity, and the possibility of γ-secretase as a therapeutic target against AD.

Alzheimer's disease due to loss of function: A new synthesis of the available data

Alzheimer's Disease (AD) is a highly complex disease involving a broad range of clinical, cellular, and biochemical manifestations that are currently not understood in combination. This has led to many views of AD, e.g. the amyloid, tau, presenilin, oxidative stress, and metal hypotheses. The amyloid hypothesis has dominated the field with its assumption that buildup of pathogenic β-amyloid (Aβ) peptide causes disease. This paradigm has been criticized, yet most data suggest that Aβ plays a key role in the disease. Here, a new loss-of-function hypothesis is synthesized that accounts for the anomalies of the amyloid hypothesis, e.g. the curious pathogenicity of the Aβ42/Aβ40 ratio, the loss of Aβ caused by presenilin mutation, the mixed phenotypes of APP mutations, the poor clinical-biochemical correlations for genetic variant carriers, and the failure of Aβ reducing drugs. The amyloid-loss view accounts for recent findings on the structure and chemical features of Aβ variants and their coupling to human patient data. The lost normal function of APP/Aβ is argued to be metal transport across neuronal membranes, a view with no apparent anomalies and substantially more explanatory power than the gain-of-function amyloid hypothesis. In the loss-of-function scenario, the central event of Aβ aggregation is interpreted as a loss of soluble, functional monomer Aβ rather than toxic overload of oligomers. Accordingly, new research models and treatment strategies should focus on remediation of the functional amyloid balance, rather than strict containment of Aβ, which, for reasons rationalized in this review, has failed clinically.

Molecular medicine - To be or not to be

Molecular medicine is founded on the synergy between Chemistry, Physics, Biology and Medicine, with the ambitious goal of tackling diseases from a molecular perspective. This Review aims at retracing a personal outlook of the birth and development of molecular medicine, as well as at highlighting some of the most urgent challenges linked to aging and represented by incurable neurodegenerative diseases caused by protein misfolding. Furthermore, we emphasize the emerging role of the retromer dysfunctions and improper protein sorting in Alzheimer's disease and other important neurological disordered.

Substrate recruitment of γ-secretase and mechanism of clinical presenilin mutations revealed by photoaffinity mapping

Intramembrane proteases execute fundamental biological processes ranging from crucial signaling events to general membrane proteostasis. Despite the availability of structural information on these proteases, it remains unclear how these enzymes bind and recruit substrates, particularly for the Alzheimer's disease-associated γ-secretase. Systematically scanning amyloid precursor protein substrates containing a genetically inserted photocrosslinkable amino acid for binding to γ-secretase allowed us to identify residues contacting the protease. These were primarily found in the transmembrane cleavage domain of the substrate and were also present in the extramembranous domains. The N-terminal fragment of the catalytic subunit presenilin was determined as principal substrate-binding site. Clinical presenilin mutations altered substrate binding in the active site region, implying a pathogenic mechanism for familial Alzheimer's disease. Remarkably, PEN-2 was identified besides nicastrin as additional substrate-binding subunit. Probing proteolysis of crosslinked substrates revealed a mechanistic model of how these subunits interact to mediate a stepwise transfer of bound substrate to the catalytic site. We propose that sequential binding steps might be common for intramembrane proteases to sample and select cognate substrates for catalysis.

Physiological and pathological roles of the γ-secretase complex

Gamma-secretase (GS) is an enzyme complex that cleaves numerous substrates, and it is best known for cleaving amyloid precursor protein (APP) to form amyloid-beta (Aβ) peptides. Aberrant cleavage of APP can lead to Alzheimer's disease, so much research has been done to better understand GS structure and function in hopes of developing therapeutics for Alzheimer's. Therefore, most of the attention in this field has been focused on developing modulators that reduce pathogenic forms of Aβ while leaving Notch and other GS substrates intact, but GS provides multiple avenues of modulation that could improve AD pathology. GS has complex regulation, through its essential subunits and other associated proteins, providing other targets for AD drugs. Therapeutics can also alter GS trafficking and thereby improve cognition, or move beyond Aβ entirely, effecting Notch and neural stem cells. GS also cleaves substrates that affect synaptic morphology and function, presenting another window by which GS modulation could improve AD pathology. Taken together, GS presents a unique cross road for neural processes and an ideal target for AD therapeutics.

Immunotherapies in Alzheimer's disease: Too much, too little, too late or off-target?

Years of research have highlighted the importance of the immune system in Alzheimer's disease (AD), a system that, if manipulated during strategic time windows, could potentially be tackled to treat this disorder. However, to minimize adverse effects, it is essential to first grasp which exact aspect of it may be targeted. Several clues have been collected over the years regarding specific immune players strongly modulated during different stages of AD progression. However, the inherent complexity of the immune system as well as conflicting data make it quite challenging to pinpoint a specific immune target in AD. In this review, we discuss immune-related abnormalities observed in the periphery as well as in the brain of AD patients, in relation to known risk factors of AD such as genetics, type-2 diabetes or obesity, aging, physical inactivity and hypertension. Although not investigated yet in clinical trials, C5 complement system component, CD40/CD40L interactions and the CXCR2 pathway are altered in AD patients and may represent potential therapeutic targets. Immunotherapies tested in a clinical context, those aiming to attenuate the innate immune response and those used to facilitate the removal of pathological proteins, are further discussed to try and understand the causes of the limited success reached. The prevailing eagerness to move basic research data to clinic should not overshadow the fact that a careful preclinical characterization of a drug is still required to ultimately improve the chance of clinical success. Finally, specific elements to consider prior to initiate large-scale trials are highlighted and include the replication of preclinical data, the use of small-scale human studies, the sub-typing of AD patients and the determination of pharmacokinetic and pharmacodynamics parameters such as brain bioavailability and target engagement.

Traditional Chinese Nootropic Medicine Radix Polygalae and Its Active Constituent Onjisaponin B Reduce β-Amyloid Production and Improve Cognitive Impairments

Decline of cognitive function is the hallmark of Alzheimer’s disease (AD), regardless of the pathological mechanism. Traditional Chinese medicine has been used to combat cognitive impairments and has been shown to improve learning and memory. Radix Polygalae (RAPO) is a typical and widely used herbal medicine. In this study, we aimed to follow the β-amyloid (Aβ) reduction activity to identify active constituent(s) of RAPO. We found that Onjisaponin B of RAPO functioned as RAPO to suppress Aβ production without direct inhibition of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) and γ-secretase activities. Our mechanistic study showed that Onjisaponin B promoted the degradation of amyloid precursor protein (APP). Further, oral administration of Onjisaponin B ameliorated Aβ pathology and behavioral defects in APP/PS1 mice. Taken together, our results indicate that Onjisaponin B is effective against AD, providing a new therapeutic agent for further drug discovery.

ACAT1/SOAT1 as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia with no cure at present. Cholesterol metabolism is closely associated with AD at several stages. ACAT1 converts free cholesterol to cholesteryl esters, and plays important roles in cellular cholesterol homeostasis. Recent studies show that in a mouse model, blocking ACAT1 provides multiple beneficial effects on AD. Here we review the current evidence that implicates ACAT1 as a therapeutic target for AD. We also discuss the potential usage of various ACAT inhibitors currently available to treat AD.

Targeting protein aggregation for the treatment of degenerative diseases

The aggregation of specific proteins is hypothesized to underlie several degenerative diseases, which are collectively known as amyloid disorders. However, the mechanistic connection between the process of protein aggregation and tissue degeneration is not yet fully understood. Here, we review current and emerging strategies to ameliorate aggregation-associated degenerative disorders, with a focus on disease-modifying strategies that prevent the formation of and/or eliminate protein aggregates. Persuasive pharmacological and genetic evidence now supports protein aggregation as the cause of postmitotic tissue dysfunction or loss. However, a more detailed understanding of the factors that trigger and sustain aggregate formation and of the structure-activity relationships underlying proteotoxicity is needed to develop future disease-modifying therapies.

Opportunities and challenges of radiotherapy for treating cancer

The past 20 years have seen dramatic changes in the delivery of radiation therapy, but the impact of radiobiology on the clinic has been far less substantial. A major consideration in the use of radiotherapy has been on how best to exploit differences between the tumour and host tissue characteristics, which in the past has been achieved empirically by radiation-dose fractionation. New advances are uncovering some of the mechanistic processes that underlie this success story. In this Review, we focus on how these processes might be targeted to improve the outcome of radiotherapy at the individual patient level. This approach would seem a more productive avenue of treatment than simply trying to increase the radiation dose delivered to the tumour.

Can insulin signaling pathways be targeted to transport Aβ out of the brain?

Although the causal role of Amyloid-β (Aβ) in Alzheimer’s disease (AD) is unclear, it is still reasonable to expect that lowering concentrations of Aβ in the brain may decrease the risk of developing the neurocognitive symptoms of the disease. Brain capillary endothelial cells forming the blood-brain barrier (BBB) express transporters regulating the efflux of Aβ out of the cerebral tissue. Age-related BBB dysfunctions, that have been identified in AD patients, might impair Aβ clearance from the brain. Thus, targeting BBB outward transport systems has been suggested as a way to stimulate the clearance of Aβ from the brain. Recent data indicate that the increase in soluble brain Aβ and behavioral impairments in 3×Tg-AD mice generated by months of intake of a high-fat diet can be acutely reversed by the administration of a single dose of insulin. A concomitant increase in plasma Aβ suggests that clearance from the brain through the BBB is a likely mechanism for this rapid effect of insulin. Here, we review how BBB insulin response pathways could be stimulated to decrease brain Aβ concentrations and improve cognitive performance, at least on the short term.

Notch Transmembrane Domain: Secondary Structure and Topology

The Notch signaling pathway is critical in development, neuronal maintenance, and hematopoiesis. An obligate step in the activation of this pathway is cleavage of its transmembrane (TM) domain by γ-secretase. While the soluble domains have been extensively studied, little has been done to characterize its TM and flanking juxtamembrane (JM) segments. Here, we present the results of nuclear magnetic resonance (NMR) studies of the human Notch1 TM/JM domain. The TM domain is largely α-helical. While the flanking JM segments do not adopt regular secondary structure, they interact with the membrane surface, suggesting membrane interactions may play a role in modulating its cleavage by γ-secretase and subsequent NOTCH signaling function.

Alzheimer's disease pathology is attenuated in a CD38-deficient mouse model

OBJECTIVE

Alzheimer's disease (AD)-associated dementia is due to tissue damage caused by amyloid β (Aβ) deposition within the brain and by accompanying neuroinflammation. The nicotinamide adenine dinucleotide (NAD) glycohydrolase CD38, which is expressed by neurons, astrocytes, and microglial cells, regulates inflammatory and repair processes in the brain and other tissues by degrading NAD and repressing the activity of other NAD-consuming enzymes and by producing NAD-derived metabolites that regulate calcium signaling and migration of inflammatory cells. Given the role of CD38 in neuroinflammation and repair, we examined the effect of CD38 deletion on AD pathology.

METHODS

We crossed APPswePS1ΔE9 (APP.PS) mice with Cd38(-) (/) (-) mice to generate AD-prone CD38-deficient animals (APP.PS.Cd38(-) (/) (-) ) and examined AD-related phenotypes in both groups.

RESULTS

APP.PS.Cd38(-) (/) (-) mice exhibited significant reductions in Aβ plaque load and soluble Aβ levels compared to APP.PS mice, and this correlated with improved spatial learning. Although CD38 deficiency resulted in decreased microglia/macrophage (MM) accumulation, the transcription profile of the Cd38(-) (/) (-) and Cd38(+/) (+) MM was similar, suggesting that the decreased Aβ burden in APP.PS.Cd38(-) (/) (-) mice was not due to alterations in MM activation/function. Instead, APP.PS.Cd38(-) (/) (-) neuronal cultures secreted less Aβ and this reduction was mimicked when APP.PS neuronal cultures were treated with inhibitors that blocked CD38 enzyme activity or the signaling pathways controlled by CD38-derived metabolites. Furthermore, β- and γ-secretase activity was decreased in APP.PS.Cd38(-) (/) (-) mice, which correlated with decreased Aβ production.

INTERPRETATION

CD38 regulates AD pathology in the APP.PS model of AD, suggesting that CD38 may be a novel target for AD treatment.

Proteases at work: cues for understanding neural development and degeneration

Proteolytical processing of membrane bound molecules is a fundamental mechanism for the degradation of these proteins as well as for controlling cell-to-cell communication, which is at the basis of tissue development and homeostasis. Members of families of metalloproteinases and intra-membrane proteases are major effectors of these events. A recent workshop in Baeza, Spain, was devoted to discuss how this mechanism coordinates brain development and how its dysfunction leads to brain pathologies. Herein we summarize the findings presented during this workshop, which illuminate the role of metalloproteinases, including matrix metalloproteinase, A Disintegrin and Metalloproteinase-proteases and intra-membrane proteases, in the regulation of neurogenesis, axon guidance, and synaptogenesis as well as in neurodegeneration. Indeed, there is increasing evidence that proteolysis at the membrane is directly linked to neuropathologies such as Alzheimer Disease and autism spectrum or prion disorders. These proteolytic events are tightly regulated and we are just at the beginning of understanding how these processes could be exploited to design therapeutic treatments aimed at alleviating psychiatric and neurodegenerative pathologies.

Disturbed calcium signaling in spinocerebellar ataxias and Alzheimer's disease

Neurodegenerative disorders, such as spinocerebellar ataxias (SCAs) and Alzheimer's disease (AD) represent a huge scientific and medical question, but the molecular mechanisms of these diseases are still not clear. There is increasing evidence that neuronal calcium signaling is abnormal in many neurodegenerative disorders. Abnormal neuronal calcium release from the endoplasmic reticulum may result in disturbances of cell homeostasis, synaptic dysfunction, and eventual cell death. Neuronal loss is observed in most cases of neurodegenerative diseases. Recent experimental evidence supporting the role of neuronal calcium signaling in the pathogenesis of SCAs and AD is discussed in this review.

Characterization of Pterocarpus erinaceus kino extract and its gamma-secretase inhibitory properties

ETHNOPHARMACOLOGICAL RELEVANCE

The aqueous decoction of Pterocarpus erinaceus has been traditionally used in Benin against memory troubles.

AIM OF THE STUDY

New strategies are needed against Alzheimer׳s disease (AD), for, to date, AD treatment is symptomatic and consists in drugs treating the cognitive decline. An interesting target is the β-amyloid peptide (Aβ), whose accumulation and progressive deposition into amyloid plaques are key events in AD aetiology. Identifying new and more selective γ-secretase inhibitors or modulators (none of the existing has proven so far to be selective or fully efficient) appears in this respect of particular interest. We studied the activity and mechanisms of action of Pterocarpus erinaceus kino aqueous extract, after the removal of catechic tannins (KAST).

METHODS AND RESULTS

We tested KAST at non-toxic concentrations on cells expressing the human Amyloid Precursor Protein (APP695), as well as on primary neurons. Pterocarpus erinaceus extract was found to inhibit Aβ release in both models. We further showed that KAST inhibited γ-secretase activity in cell-free and in vitro assays, strongly suggesting that KAST is a natural γ-secretase inhibitor. Importantly, this extract did not inhibit the cleavage of Notch, another γ-secretase substrate responsible for major detrimental side effects observed with γ-secretase inhibitors. Epicatechin was further identified in KAST by HPLC-MS.

CONCLUSION

Pterocarpus erinaceus kino extract appears therefore as a new γ-secretase inhibitor selective towards APP processing.