Cellular mechanisms of γ-secretase substrate selection, processing and toxicity

Amyloid-β Pathology Is the Common Nominator Proteinopathy of the Primate Brain Aging

Senile plaques, mainly diffuse, and cerebral amyloid-β (Aβ) angiopathy are prevalent in the aging brain of non-human primates, from lemurs to non-human Hominidae. Aβ but not hyper-phosphorylated tau (HPtau) pathology is the common nominator proteinopathy of non-human primate brain aging. The abundance of Aβ in the aging primate brain is well tolerated, and the impact on cognitive functions is usually limited to particular tasks. In contrast, human brain aging is characterized by the early appearance of HPtau pathology, mainly forming neurofibrillary tangles, dystrophic neurites of neuritic plaques, and neuropil threads, preceding Aβ deposits by several decades and by its severity progressing from selected nuclei of the brain stem, entorhinal cortex, and hippocampus to the limbic system, neocortex, and other brain regions. Neurofibrillary tangles correlate with cognitive impairment and dementia in advanced cases. Aβ pathology is linked in humans to altered membrane protein and lipid composition, particularly involving lipid rafts. Although similar membrane alterations are unknown in non-human primates, membrane senescence is postulated to cause the activated β-amyloidogenic pathway, and Aβ pathology is the prevailing signature of non-human and human primate brain aging.

Preferential Regulation of Γ-Secretase-Mediated Cleavage of APP by Ganglioside GM1 Reveals a Potential Therapeutic Target for Alzheimer's Disease

A hallmark of Alzheimer's disease (AD) is the senile plaque, which contains β-amyloid peptides (Aβ). Ganglioside GM1 is the most common brain ganglioside. However, the mechanism of GM1 in modulating Aβ processing is rarely known. Aβ levels are detected by using Immunohistochemistry (IHC) and enzyme-linked immune-sorbent assay (ELISA). Cryo-electron microscopy (Cryo-EM) is used to determine the structure of γ-secretase supplemented with GM1. The levels of the cleavage of amyloid precursor protein (APP)/Cadherin/Notch1 are detected using Western blot analysis. Y maze, object translocation, and Barnes maze are performed to evaluate cognitive functions. GM1 leads to conformational change of γ-secretase structure and specifically accelerates γ-secretase cleavage of APP without affecting other substrates including Notch1, potentially through its interaction with the N-terminal fragment of presenilin 1 (PS1). Reduction of GM1 levels decreases amyloid plaque deposition and improves cognitive dysfunction. This study reveals the mechanism of GM1 in Aβ generation and provides the evidence that decreasing GM1 levels represents a potential strategy in AD treatment. These results provide insights into the detailed mechanism of the effect of GM1 on PS1, representing a step toward the characterization of its novel role in the modulation of γ-secretase activity and the pathogenesis of AD.

Notch Signaling Plays a Dual Role in Regulating the Neuron-to-Oligodendrocyte Switch in the Developing Dorsal Forebrain

Neural progenitor cells in the developing dorsal forebrain generate excitatory neurons followed by oligodendrocytes (OLs) and astrocytes. However, the specific mechanisms that regulate the timing of this neuron-glia switch are not fully understood. In this study, we show that the proper balance of Notch signaling in dorsal forebrain progenitors is required to generate oligodendrocytes during late stages of embryonic development. Using ex vivo and in utero approaches in mouse embryos of both sexes, we found that Notch inhibition reduced the number of oligodendrocyte lineage cells in the dorsal pallium. However, Notch overactivation also prevented oligodendrogenesis and maintained a progenitor state. These results point toward a dual role for Notch signaling in both promoting and inhibiting oligodendrogenesis, which must be fine-tuned to generate oligodendrocyte lineage cells at the right time and in the right numbers. We further identified the canonical Notch downstream factors HES1 and HES5 as negative regulators in this process. CRISPR (clustered regularly interspaced short palindromic repeat)/Cas9-mediated knockdown of Hes1 and Hes5 caused increased expression of the pro-oligodendrocyte factor ASCL1 and led to precocious oligodendrogenesis. Conversely, combining Notch overactivation with ASCL1 overexpression robustly promoted oligodendrogenesis, indicating a separate mechanism of Notch that operates synergistically with ASCL1 to specify an oligodendrocyte fate. We propose a model in which Notch signaling works together with ASCL1 to specify progenitors toward the oligodendrocyte lineage but also maintains a progenitor state through Hes-dependent repression of Ascl1 so that oligodendrocytes are not made too early, thus contributing to the precise timing of the neuron-glia switch.SIGNIFICANCE STATEMENT Neural progenitors make oligodendrocytes after neurogenesis starts to wind down, but the mechanisms that control the timing of this switch are poorly understood. In this study, we identify Notch signaling as a critical pathway that regulates the balance between progenitor maintenance and oligodendrogenesis. Notch signaling is required for the oligodendrocyte fate, but elevated Notch signaling prevents oligodendrogenesis and maintains a progenitor state. We provide evidence that these opposing functions are controlled by different mechanisms. Before the switch, Notch signaling through Hes factors represses oligodendrogenesis. Later, Notch signaling through an unknown mechanism promotes oligodendrogenesis synergistically with the transcription factor ASCL1. Our study underscores the complexity of Notch and reveals its importance in regulating the timing and numbers of oligodendrocyte production.

The amyloid cascade hypothesis: an updated critical review

Results from recent clinical trials of antibodies that target amyloid-β (Aβ) for Alzheimer's disease have created excitement and have been heralded as corroboration of the amyloid cascade hypothesis. However, while Aβ may contribute to disease, genetic, clinical, imaging and biochemical data suggest a more complex aetiology. Here we review the history and weaknesses of the amyloid cascade hypothesis in view of the new evidence obtained from clinical trials of anti-amyloid antibodies. These trials indicate that the treatments have either no or uncertain clinical effect on cognition. Despite the importance of amyloid in the definition of Alzheimer's disease, we argue that the data point to Aβ playing a minor aetiological role. We also discuss data suggesting that the concerted activity of many pathogenic factors contribute to Alzheimer's disease and propose that evolving multi-factor disease models will better underpin the search for more effective strategies to treat the disease.

Presenilin and APP Regulate Synaptic Kainate Receptors

Kainate receptors (KARs) form a family of ionotropic glutamate receptors that regulate the activity of neuronal networks by both presynaptic and postsynaptic mechanisms. Their implication in pathologies is well documented for epilepsy. The higher prevalence of epileptic symptoms in Alzheimer's disease (AD) patients questions the role of KARs in AD. Here we investigated whether the synaptic expression and function of KARs was impaired in mouse models of AD. We addressed this question by immunostaining and electrophysiology at synapses between mossy fibers and CA3 pyramidal cells, in which KARs are abundant and play a prominent physiological role. We observed a decrease of the immunostaining for GluK2 in the stratum lucidum in CA3, and of the amplitude and decay time of synaptic currents mediated by GluK2-containing KARs in an amyloid mouse model (APP/PS1) of AD. Interestingly, a similar phenotype was observed in CA3 pyramidal cells in male and female mice with a genetic deletion of either presenilin or APP/APLP2 as well as in organotypic cultures treated with γ-secretase inhibitors. Finally, the GluK2 protein interacts with full-length and C-terminal fragments of APP. Overall, our data suggest that APP stabilizes KARs at synapses, possibly through a transsynaptic mechanism, and this interaction is under the control the γ-secretase proteolytic activity of presenilin.SIGNIFICANCE STATEMENT Synaptic impairment correlates strongly with cognitive deficits in Alzheimer's disease (AD). In this context, many studies have addressed the dysregulation of AMPA and NMDA ionotropic glutamate receptors. Kainate receptors (KARs), which form the third family of iGluRs, represent an underestimated actor in the regulation of neuronal circuits and have not yet been examined in the context of AD. Here we provide evidence that synaptic KARs are markedly impaired in a mouse model of AD. Additional experiments indicate that the γ-secretase activity of presenilin acting on the amyloid precursor protein controls synaptic expression of KAR. This study clearly indicates that KARs should be taken into consideration whenever addressing synaptic dysfunction and related cognitive deficits in the context of AD.

APP accumulates with presynaptic proteins around amyloid plaques: A role for presynaptic mechanisms in Alzheimer's disease?

In Alzheimer's disease (AD), the distribution of the amyloid precursor protein (APP) and its fragments other than amyloid beta, has not been fully characterized. Here, we investigate the distribution of APP and its fragments in human AD brain samples and in mouse models of AD in reference to its proteases, synaptic proteins, and histopathological features characteristic of the AD brain, by combining an extensive set of histological and analytical tools. We report that the prominent somatic distribution of APP observed in control patients remarkably vanishes in human AD patients to the benefit of dense accumulations of extra-somatic APP, which surround dense-core amyloid plaques enriched in APP-Nter. These features are accentuated in patients with familial forms of the disease. Importantly, APP accumulations are enriched in phosphorylated tau and presynaptic proteins whereas they are depleted of post-synaptic proteins suggesting that the extra-somatic accumulations of APP are of presynaptic origin. Ultrastructural analyses unveil that APP concentrates in autophagosomes and in multivesicular bodies together with presynaptic vesicle proteins. Altogether, alteration of APP distribution and its accumulation together with presynaptic proteins around dense-core amyloid plaques is a key histopathological feature in AD, lending support to the notion that presynaptic failure is a strong physiopathological component of AD.

Evaluation of Virtual Screening Strategies for the Identification of γ-Secretase Inhibitors and Modulators

γ-Secretase is an intramembrane aspartyl protease that is important in regulating normal cell physiology via cleavage of over 100 transmembrane proteins, including Amyloid Precursor Protein (APP) and Notch family receptors. However, aberrant proteolysis of substrates has implications in the progression of disease pathologies, including Alzheimer's disease (AD), cancers, and skin disorders. While several γ-secretase inhibitors have been identified, there has been toxicity observed in clinical trials associated with non-selective enzyme inhibition. To address this, γ-secretase modulators have been identified and pursued as more selective agents. Recent structural evidence has provided an insight into how γ-secretase inhibitors and modulators are recognized by γ-secretase, providing a platform for rational drug design targeting this protease. In this study, docking- and pharmacophore-based screening approaches were evaluated for their ability to identify, from libraries of known inhibitors and modulators with decoys with similar physicochemical properties, γ-secretase inhibitors and modulators. Using these libraries, we defined strategies for identifying both γ-secretase inhibitors and modulators incorporating an initial pharmacophore-based screen followed by a docking-based screen, with each strategy employing distinct γ-secretase structures. Furthermore, known γ-secretase inhibitors and modulators were able to be identified from an external set of bioactive molecules following application of the derived screening strategies. The approaches described herein will inform the discovery of novel small molecules targeting γ-secretase.

p120 catenin recruits HPV to γ-secretase to promote virus infection

During internalization and trafficking, human papillomavirus (HPV) moves from the cell surface to the endosome where the transmembrane protease γ-secretase promotes insertion of the viral L2 capsid protein into the endosome membrane. Protrusion of L2 through the endosome membrane into the cytosol allows the recruitment of cytosolic host factors that target the virus to the Golgi en route for productive infection. How endosome-localized HPV is delivered to γ-secretase, a decisive infection step, is unclear. Here we demonstrate that cytosolic p120 catenin, likely via an unidentified transmembrane protein, interacts with HPV at early time-points during viral internalization and trafficking. In the endosome, p120 is not required for low pH-dependent disassembly of the HPV L1 capsid protein from the incoming virion. Rather, p120 is required for HPV to interact with γ-secretase-an interaction that ensures the virus is transported along a productive route. Our findings clarify an enigmatic HPV infection step and provide critical insights into HPV infection that may lead to new therapeutic strategies against HPV-induced diseases.

Role of EGFR in the Nervous System

Epidermal growth factor receptor (EGFR) is the first discovered member of the receptor tyrosine kinase superfamily and plays a fundamental role during embryogenesis and in adult tissues, being involved in growth, differentiation, maintenance and repair of various tissues and organs. The role of EGFR in the regulation of tissue development and homeostasis has been thoroughly investigated and it has also been demonstrated that EGFR is a driver of tumorigenesis. In the nervous system, other growth factors, and thus other receptors, are important for growth, differentiation and repair of the tissue, namely neurotrophins and neurotrophins receptors. For this reason, for a long time, the role of EGFR in the nervous system has been underestimated and poorly investigated. However, EGFR is expressed both in the central and peripheral nervous systems and it has been demonstrated to have specific important neurotrophic functions, in particular in the central nervous system. This review discusses the role of EGFR in regulating differentiation and functions of neurons and neuroglia. Furthermore, its involvement in regeneration after injury and in the onset of neurodegenerative diseases is examined.

Presenilin1 familial Alzheimer disease mutants inactivate EFNB1- and BDNF-dependent neuroprotection against excitotoxicity by affecting neuroprotective complexes of N-methyl-d-aspartate receptor

Excitotoxicity is thought to play key roles in brain neurodegeneration and stroke. Here we show that neuroprotection against excitotoxicity by trophic factors EFNB1 and brain-derived neurotrophic factor (called here factors) requires de novo formation of 'survival complexes' which are factor-stimulated complexes of N-methyl-d-aspartate receptor with factor receptor and presenilin 1. Absence of presenilin 1 reduces the formation of survival complexes and abolishes neuroprotection. EPH receptor B2- and N-methyl-d-aspartate receptor-derived peptides designed to disrupt formation of survival complexes also decrease the factor-stimulated neuroprotection. Strikingly, factor-dependent neuroprotection and levels of the de novo factor-stimulated survival complexes decrease dramatically in neurons expressing presenilin 1 familial Alzheimer disease mutants. Mouse neurons and brains expressing presenilin 1 familial Alzheimer disease mutants contain increased amounts of constitutive presenilin 1-N-methyl-d-aspartate receptor complexes unresponsive to factors. Interestingly, the stability of the familial Alzheimer disease presenilin 1-N-methyl-d-aspartate receptor complexes differs from that of wild type complexes and neurons of mutant-expressing brains are more vulnerable to cerebral ischaemia than neurons of wild type brains. Furthermore, N-methyl-d-aspartate receptor-mediated excitatory post-synaptic currents at CA1 synapses are altered by presenilin 1 familial Alzheimer disease mutants. Importantly, high levels of presenilin 1-N-methyl-d-aspartate receptor complexes are also found in post-mortem brains of Alzheimer disease patients expressing presenilin 1 familial Alzheimer disease mutants. Together, our data identify a novel presenilin 1-dependent neuroprotective mechanism against excitotoxicity and indicate a pathway by which presenilin 1 familial Alzheimer disease mutants decrease factor-depended neuroprotection against excitotoxicity and ischaemia in the absence of Alzheimer disease neuropathological hallmarks which may form downstream of neuronal damage. These findings have implications for the pathogenic effects of familial Alzheimer disease mutants and therapeutic strategies.

Presynaptic failure in Alzheimer's disease

Synaptic loss is the best correlate of cognitive deficits in Alzheimer's disease (AD). Extensive experimental evidence also indicates alterations of synaptic properties at the early stages of disease progression, before synapse loss and neuronal degeneration. A majority of studies in mouse models of AD have focused on post-synaptic mechanisms, including impairment of long-term plasticity, spine structure and glutamate receptor-mediated transmission. Here we review the literature indicating that the synaptic pathology in AD includes a strong presynaptic component. We describe the evidence indicating presynaptic physiological functions of the major molecular players in AD. These include the amyloid precursor protein (APP) and the two presenilin (PS) paralogs PS1 or PS2, genetically linked to the early-onset form of AD, in addition to tau which accumulates in a pathological form in the AD brain. Three main mechanisms participating in presynaptic functions are highlighted. APP fragments bind to presynaptic receptors (e.g. nAChRs and GABA_B receptors), presenilins control Ca²⁺ homeostasis and Ca²⁺-sensors, and tau regulates the localization of presynaptic molecules and synaptic vesicles. We then discuss how impairment of these presynaptic physiological functions can explain or forecast the hallmarks of synaptic impairment and associated dysfunction of neuronal circuits in AD. Beyond the physiological roles of the AD-related proteins, studies in AD brains also support preferential presynaptic alteration. This review features presynaptic failure as a strong component of pathological mechanisms in AD.

Kainate Receptor Activation Enhances Amyloidogenic Processing of APP in Astrocytes

Kainic acid (KA) is an analogue of the excitatory neurotransmitter glutamate that, when injected systemically into adult rats, can trigger seizures and progressive neuronal loss in a manner that mirrors the neuropathology of human mesial temporal lobe epilepsy. However, biomolecular mechanisms responsible for the neuronal loss that occurs as a consequence of this treatment remains elusive. We have recently reported that toxicity induced by KA can partly be mediated by astrocyte-derived amyloid β (Aβ) peptides, which are critical in the development of Alzheimer's disease (AD). Nonetheless, little is known how KA can influence amyloid precursor protein (APP) levels and processing in astrocytes. Thus, in the present study using human U-373 astrocytoma and rat primary astrocytes, we evaluated the role of KA on APP metabolism. Our results revealed that KA treatment increased the levels of APP and its cleaved products (α-/β-CTFs) in cultured U-373 astrocytoma and primary astrocytes, without altering the cell viability. The cellular and secretory levels of Aβ_1-40/Aβ_1-42 were markedly increased in KA-treated astrocytes. We also demonstrated that the steady-state levels of APP-secretases were not altered but the activity of γ-secretase is enhanced in KA-treated U-373 astrocytoma. Furthermore, using selective receptor antagonists, we showed that the effects of KA is mediated by activation of kainate receptors and not NMDA or AMPA receptors. These results suggest that KA can enhance amyloidogenic processing of APP by activating its own receptor leading to increased production/secretion of Aβ-related peptides from activated astrocytes which may contribute to the pathogenesis of temporal lobe epilepsy.

Presenilin-mediated cleavage of APP regulates synaptotagmin-7 and presynaptic plasticity

Mutations of the intramembrane protease presenilin (PS) or of its main substrate, the amyloid precursor protein (APP), cause early-onset form of Alzheimer disease. PS and APP interact with proteins of the neurotransmitter release machinery without identified functional consequences. Here we report that genetic deletion of PS markedly decreases the presynaptic levels of the Ca²⁺ sensor synaptotagmin-7 (Syt7) leading to impaired synaptic facilitation and replenishment of synaptic vesicles. The regulation of Syt7 expression by PS occurs post-transcriptionally and depends on γ-secretase proteolytic activity. It requires the substrate APP as revealed by the combined genetic invalidation of APP and PS1, and in particular the APP-Cterminal fragments which interact with Syt7 and accumulate in synaptic terminals under pharmacological or genetic inhibition of γ-secretase. Thus, we uncover a role of PS in presynaptic mechanisms, through APP cleavage and regulation of Syt7, that highlights aberrant synaptic vesicle processing as a possible new pathway in AD.

Mutations in presenilin, which cleaves amyloid precursor protein, cause familial Alzheimer’s Disease. Here, the authors show that loss of presenilin leads to loss of synaptotagmin 7, leading to impaired presynaptic release.

The product of the γ-secretase processing of ephrinB2 regulates VE-cadherin complexes and angiogenesis

Presenilin-1 (PS1) gene encodes the catalytic component of γ-secretase, which proteolytically processes several type I transmembrane proteins. We here present evidence that the cytosolic peptide efnB2/CTF2 produced by the PS1/γ-secretase cleavage of efnB2 ligand promotes EphB4 receptor-dependent angiogenesis in vitro. EfnB2/CTF2 increases endothelial cell sprouting and tube formation, stimulates the formation of angiogenic complexes that include VE-cadherin, Raf-1 and Rok-α, and increases MLC2 phosphorylation. These functions are mediated by the PDZ-binding domain of efnB2. Acute downregulation of PS1 or inhibition of γ-secretase inhibits the angiogenic functions of EphB4 while absence of PS1 decreases the VE-cadherin angiogenic complexes of mouse brain. Our data reveal a mechanism by which PS1/γ-secretase regulates efnB2/EphB4 mediated angiogenesis.

Perspectives on the discovery of NOTCH2-specific inhibitors

The Notch pathway is a cell-cell communication system where membrane-bound ligands interact with the extracellular region of Notch receptors to induce intracellular, downstream effects on gene expression. Aberrant Notch signaling promotes tumorigenesis, and the Notch pathway has tremendous potential for novel targeting strategies in cancer treatment. While γ-secretase inhibitors as Notch-inhibiting agents are already promising in clinical trials, they are highly non-specific with adverse side-effects. One of the underlying challenges is that two of the four known human Notch paralogs, NOTCH1 and 2, share very high structural similarity but play opposing roles in some tumorigenesis pathways. This perspective explores the feasibility of developing Notch-specific small molecule inhibitors targeting the anti-NOTCH2 antibody-binding epitopes or the "S2-Leu-plug-binding site" using a computer-aided drug discovery approach.

Can Co-Activation of Nrf2 and Neurotrophic Signaling Pathway Slow Alzheimer's Disease?

Alzheimer’s disease (AD) is a multifaceted disease that is hard to treat by single-modal treatment. AD starts with amyloid peptides, mitochondrial dysfunction, and oxidative stress and later is accompanied with chronic endoplasmic reticulum (ER) stress and autophagy dysfunction, resulting in more complicated pathogenesis. Currently, few treatments can modify the complicated pathogenic progress of AD. Compared to the treatment with exogenous antioxidants, the activation of global antioxidant defense system via Nrf2 looks more promising in attenuating oxidative stress in AD brains. Accompanying the activation of the Nrf2-mediated antioxidant defense system that reduce the AD-causative factor, oxidative stress, it is also necessary to activate the neurotrophic signaling pathway that replaces damaged organelles and molecules with new ones. Thus, the dual actions to activate both the Nrf2 antioxidant system and neurotrophic signaling pathway are expected to provide a better strategy to modify AD pathogenesis. Here, we review the current understanding of AD pathogenesis and neuronal defense systems and discuss a possible way to co-activate the Nrf2 antioxidant system and neurotrophic signaling pathway with the hope of helping to find a better strategy to slow AD.

Epigenetics in Alzheimer's Disease: Perspective of DNA Methylation

Research over the years has shown that causes of Alzheimer's disease are not well understood, but over the past years, the involvement of epigenetic mechanisms in the developing memory formation either under pathological or physiological conditions has become clear. The term epigenetics represents the heredity of changes in phenotype that are independent of altered DNA sequences. Different studies validated that cytosine methylation of genomic DNA decreases with age in different tissues of mammals, and therefore, the role of epigenetic factors in developing neurological disorders in aging has been under focus. In this review, we summarized and reviewed the involvement of different epigenetic mechanisms especially the DNA methylation in Alzheimer's disease (AD), late-onset Alzheimer's disease (LOAD), familial Alzheimer's disease (FAD), and autosomal dominant Alzheimer's disease (ADAD). Down to the minutest of details, we tried to discuss the methylation patterns like mitochondrial DNA methylation and ribosomal DNA (rDNA) methylation. Additionally, we mentioned some therapeutic approaches related to epigenetics, which could provide a potential cure for AD. Moreover, we reviewed some recent studies that validate DNA methylation as a potential biomarker and its role in AD. We hope that this review will provide new insights into the understanding of AD pathogenesis from the epigenetic perspective especially from the perspective of DNA methylation.

Presenilin 1 promotes trypsin-induced neuroprotection via the PAR2/ERK signaling pathway. Effects of presenilin 1 FAD mutations

Mutants of presenilin 1 (PS1) increase neuronal cell death causing autosomal-dominant familial Alzheimer's disease (FAD). Recent literature shows that treatment of neuronal cultures with low concentrations of trypsin, a member of the serine family of proteases, protects neurons from toxic insults by binding to the proteinase-activated receptor 2 and stimulating survival kinase extracellular signal-regulated kinase (ERK 1/2). Other studies show that PS1 is necessary for the neuroprotective activity of specific neurotrophic factors, such as brain-derived neurotrophic factor, against excitotoxicity and oxidative stress. Here, we show that treatment of mouse cortical neuronal cultures with trypsin activates ERK1/2 and protects neurons against glutamate excitoxicity. The trypsin-dependent ERK activation and neuroprotection requires both alleles of PS1 because neither PS1 knockout nor PS1 hemizygous neuronal cultures can use exogenous trypsin to activate ERK1/2 or increase neuronal survival. The protective effect of PS1 does not depend on its γ-secretase activity because inhibitors of γ-secretase have no effect on trypsin-mediated neuroprotection. Importantly, cortical neuronal cultures either heterozygous or homozygous for PS1 FAD mutants are unable to use trypsin to activate ERK1/2 and rescue neurons from excitotoxicity, indicating that FAD mutants inhibit trypsin-dependent neuroprotection in an autosomal-dominant manner. Furthermore, our data support the theory that PS FAD mutants increase neurodegeneration by inhibiting the ability of neurons to use cellular factors as protective agents against toxic insults.

Hippocampal Injections of Oligomeric Amyloid β-peptide (1-42) Induce Selective Working Memory Deficits and Long-lasting Alterations of ERK Signaling Pathway

Increasing evidence suggests that abnormal brain accumulation of soluble rather than aggregated amyloid-β1-42 oligomers (Aβo(1-42)) plays a causal role in Alzheimer's disease (AD). However, as yet, animal's models of AD based on oligomeric amyloid-β1-42 injections in the brain have not investigated their long-lasting impacts on molecular and cognitive functions. In addition, the injections have been most often performed in ventricles, but not in the hippocampus, in spite of the fact that the hippocampus is importantly involved in memory processes and is strongly and precociously affected during the early stages of AD. Thus, in the present study, we investigated the long-lasting impacts of intra-hippocampal injections of oligomeric forms of Aβo(1-42) on working and spatial memory and on the related activation of ERK1/2. Indeed, the extracellular signal-regulated kinase (ERK) which is involved in memory function had been found to be activated by amyloid peptides. We found that repeated bilateral injections (1injection/day over 4 successive days) of oligomeric forms of Aβo(1-42) into the dorsal hippocampus lead to long-lasting impairments in two working memory tasks, these deficits being observed 7 days after the last injection, while spatial memory remained unaffected. Moreover, the working memory deficits were correlated with sustained impairments of ERK1/2 activation in the medial prefrontal cortex (mPFC) and the septum, two brain areas tightly connected with the hippocampus and involved in working memory. Thus, our study is first to evidence that sub-chronic injections of oligomeric forms of Aβo(1-42) into the dorsal hippocampus produces the main sign of cognitive impairments corresponding to the early stages of AD, via long-lasting alterations of an ERK/MAPK pathway in an interconnected brain networks.

APLP1 as a cerebrospinal fluid biomarker for γ-secretase modulator treatment

INTRODUCTION

Alzheimer's disease brains are characterized by extracellular plaques containing the aggregated amyloid β42 (Aβ42) peptide and intraneuronal tangles containing hyperphosphorylated tau. Aβ42 is produced by sequential processing of the amyloid precursor protein (APP) by β-secretase followed by γ-secretase. Substantial efforts have been put into developing pharmaceuticals preventing the production or increasing the clearance of Aβ42. However, treatments inhibiting γ-secretase have proven disappointing due to off-target effects. To circumvent these effects, γ-secretase modulators (GSMs) have been developed, which rather than inhibiting γ-secretase shift its preference into producing less aggregation-prone shorter Aβ peptides. Belonging to the same family of proteins as APP, amyloid-like protein 1 (APLP1) is also a substrate for γ-secretase. Herein we investigated whether the GSM E2012 affects APLP1 processing in the central nervous system by measuring APLP1 peptide levels in cerebrospinal fluid (CSF) before and after E2012 treatment in dogs.

METHODS

An in-house monoclonal APLP1 antibody, AP1, was produced and utilized for immunopurification of APLP1 from human and dog CSF in a hybrid immuno-affinity mass spectrometric method. Seven dogs received a single dose of 20 or 80 mg/kg of E2012 in a randomized cross-over design and CSF was collected prior to and 4, 8 and 24 hours after dosing.

RESULTS

We have identified 14 CSF APLP1 peptides in humans and 12 CSF APLP1 peptides in dogs. Of these, seven were reproducibly detectable in dogs who received E2012. We found a dose-dependent relative increase of the CSF peptides APLP1β17, 1β18 and 1β28 accompanied with a decrease of 1β25 and 1β27 in response to E2012 treatment. All peptides reverted to baseline over the time of sample collection.

CONCLUSION

We show an in vivo effect of the GSM E2012 on the processing of APLP1 which is measurable in CSF. These data suggest that APLP1 peptides may be used as biomarkers to monitor drug effects of GSMs on γ-secretase processing in clinical trials. However, this requires further investigation in larger cohorts, including studies in man.

Presenilin 1 is necessary for neuronal, but not glial, EGFR expression and neuroprotection via γ-secretase-independent transcriptional mechanisms

Epidermal growth factor receptor (EGFR) plays pivotal roles in cell proliferation, differentiation, and tissue development, while EGFs protect neurons from toxic insults by binding EGFR and stimulating survival signaling. Furthermore, recent evidence implicates this receptor in neurometabolic disorders like Alzheimer disease and aging. Here we show that absence of presenilin 1 (PS1) results in dramatic decrease (>95%) of neuronal EGFR and that PS1-null (PS1(-/-)) brains have reduced amounts of this receptor. PS1(-/-) cortical neurons contain little EGFR and show no epidermal growth factor-induced survival signaling or protection against excitotoxicity, but exogenous EGFR rescues both functions even in absence of PS1. EGFR mRNA is greatly reduced (>95%) in PS1(-/-) neurons, and PS1(-/-) brains contain decreased amounts of this mRNA, although PS1 affects the stability of neither EGFR nor its mRNA. Exogenous PS1 increases neuronal EGFR mRNA, while down-regulation of PS1 decreases this mRNA. These effects are neuron specific, as PS1 affects the EGFR of neither glial nor fibroblast cells. In addition, PS1 controls EGFR through novel mechanisms shared with neither γ-secretase nor PS2. Our data reveal that PS1 functions as a positive transcriptional regulator of neuronal EGFR controlling its expression in a cell-specific manner. Severe downregulation of EGFR may contribute to developmental abnormalities and lethal phenotype found in PS1, but not PS2, null mice. Furthermore, PS1 may affect neuroprotection and Alzheimer disease by controlling survival signaling of neuronal EGFR.

Successful therapies for Alzheimer's disease: why so many in animal models and none in humans?

Peering into the field of Alzheimer’s disease (AD), the outsider realizes that many of the therapeutic strategies tested (in animal models) have been successful. One also may notice that there is a deficit in translational research, i.e., to take a successful drug in mice and translate it to the patient. Efforts are still focused on novel projects to expand the therapeutic arsenal to “cure mice.” Scientific reasons behind so many successful strategies are not obvious. This article aims to review the current approaches to combat AD and to open a debate on common mechanisms of cognitive enhancement and neuroprotection. In short, either the rodent models are not good and should be discontinued, or we should extract the most useful information from those models. An example of a question that may be debated for the advancement in AD therapy is: In addition to reducing amyloid and tau pathologies, would it be necessary to boost synaptic strength and cognition? The debate could provide clues to turn around the current negative output in generating effective drugs for patients. Furthermore, discovery of biomarkers in human body fluids, and a clear distinction between cognitive enhancers and disease modifying strategies, should be instrumental for advancing in anti-AD drug discovery.

Therapeutic modulation of Notch signalling--are we there yet?

The Notch signalling pathway is evolutionarily conserved and is crucial for the development and homeostasis of most tissues. Deregulated Notch signalling leads to various diseases, such as T cell leukaemia, Alagille syndrome and a stroke and dementia syndrome known as CADASIL, and so strategies to therapeutically modulate Notch signalling are of interest. Clinical trials of Notch pathway inhibitors in patients with solid tumours have been reported, and several approaches are under preclinical evaluation. In this Review, we focus on aspects of the pathway that are amenable to therapeutic intervention, diseases that could be targeted and the various Notch pathway modulation strategies that are currently being explored.

[(3) H]-L685,458 binding sites are abundant in multiple peripheral organs in rats: implications for safety assessment of putative γ-secretase targeting drugs

γ-Secretase is a multimeric enzyme complex that carries out proteolytic processing to a variety of cellular proteins. It is currently explored as a therapeutic target for Alzheimer's disease (AD) and cancer. Mechanism-based toxicity needs to be thoroughly evaluated for γ-secretase inhibitory and/or modulatory drugs. This study comparatively assessed putative γ-secretase catalytic sites in rat peripheral tissues relative to brain and explored an effort of its pharmacological inhibition on hair regeneration. Using [(3) H]-labelled L685,458, a potent γ-secretase inhibitor, as probe, we found more abundant presence of γ-secretase binding sites in the liver, gastrointestinal tract, hair follicle, pituitary gland, ovary and testis, as compared to the brain. Local application of L658,458 delayed vibrissal regrowth following whisker removal. These results suggest that γ-secretase may execute important biological functions in many peripheral systems, as in the brain. The development of γ-secretase inhibitors/modulators for AD and cancer therapy should include close monitoring of toxicological panels for hepatic, gastrointestinal, endocrinal and reproductive functions.

Evidence of a novel mechanism for partial γ-secretase inhibition induced paradoxical increase in secreted amyloid β protein

BACE1 (β-secretase) and α-secretase cleave the Alzheimer's amyloid β protein (Aβ) precursor (APP) to C-terminal fragments of 99 aa (CTFβ) and 83 aa (CTFα), respectively, which are further cleaved by γ-secretase to eventually secrete Aβ and Aα (a.k.a. P3) that terminate predominantly at residues 40 and 42. A number of γ-secretase inhibitors (GSIs), such as N-[N-(3,5-Difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester (DAPT), have been developed with the goal of reducing Aβ to treat Alzheimer's disease (AD). Although most studies show that DAPT inhibits Aβ in a dose-dependent manner several studies have also detected a biphasic effect with an unexpected increase at low doses of DAPT in cell cultures, animal models and clinical trials. In this article, we confirm the increase in Aβ40 and Aβ42 in SH-SY5Y human neuroblastoma cells treated with low doses of DAPT and identify one of the mechanisms for this paradox. We studied the pathway by first demonstrating that stimulation of Aβ, a product of γ-secretase, was accompanied by a parallel increase of its substrate CTFβ, thereby demonstrating that the inhibitor was not anomalously stimulating enzyme activity at low levels. Secondly, we have demonstrated that inhibition of an Aβ degrading activity, endothelin converting enzyme (ECE), yielded more Aβ, but abolished the DAPT-induced stimulation. Finally, we have demonstrated that Aα, which is generated in the secretory pathway before endocytosis, is not subject to the DAPT-mediated stimulation. We therefore conclude that impairment of γ-secretase can paradoxically increase Aβ by transiently skirting Aβ degradation in the endosome. This study adds to the growing body of literature suggesting that preserving γ-secretase activity, rather than inhibiting it, is important for prevention of neurodegeneration.

The very many faces of presenilins and the γ-secretase complex

Presenilin is a central, catalytic component of the γ-secretase complex which conducts intramembrane cleavage of various protein substrates. Although identified and mainly studied through its role in the development of amyloid plaques in Alzheimer disease, γ-secretase has many other important functions. The complex seems to be evolutionary conserved throughout the Metazoa, but recent findings in plants and Dictyostelium discoideum as well as in archeons suggest that its evolution and functions might be much more diversified than previously expected. In this review, a selective survey of the multitude of functions of presenilins and the γ-secretase complex is presented. Following a brief overview of γ-secretase structure, assembly and maturation, three functional aspects are analyzed: (1) the role of γ-secretase in autophagy and phagocytosis; (2) involvement of the complex in signaling related to endocytosis; and (3) control of calcium fluxes by presenilins.

Allelic interference: a mechanism for trans-dominant transmission of loss of function in the neurodegeneration of familial Alzheimer's disease

Presenilins (PSs) are catalytic components of the γ-secretase complexes that promote the ε-cleavage of cell surface proteins producing cytosolic peptides shown to function in cell signaling and gene expression. In addition, secretase cleavages at γ-sites of amyloid precursor protein substrates produce the amyloid-β (Aβ) peptides found in all people. Aggregation of Aβ peptides form the amyloid fibrils found in amyloid plaques of Alzheimer's disease (AD) patients and aged individuals. A common hypothesis suggests that AD is caused by aggregated Aβ peptides, but treatments with either inhibitors of Aβ production or anti-Aβ antibodies showed no therapeutic value. Importantly, recent evidence [Marambaud et al.: Cell 2003;114:635-645] shows that PS familial AD (FAD) mutations cause a loss of γ-secretase cleavage function at the ε-site of substrates manifested by a decreased production of cytosolic peptides and an accumulation of transmembrane γ-secretase substrates. These data support the hypothesis that PS FAD mutations promote neurotoxicity by inhibiting the γ-secretase-catalyzed ε-cleavage of substrates, thus reducing cell signaling while causing accumulation of membrane-bound cytotoxic peptides. Similar mechanisms may be involved in toxicities observed in clinical trials of γ-secretase inhibitors. A model of allelic interference may explain the dominant negative transmission of neurotoxic loss of function in FAD neurodegeneration.

Reduction of synaptojanin 1 accelerates Aβ clearance and attenuates cognitive deterioration in an Alzheimer mouse model

Recent studies link synaptojanin 1 (synj1), the main phosphoinositol (4,5)-biphosphate phosphatase (PI(4,5)P2-degrading enzyme) in the brain and synapses, to Alzheimer disease. Here we report a novel mechanism by which synj1 reversely regulates cellular clearance of amyloid-β (Aβ). Genetic down-regulation of synj1 reduces both extracellular and intracellular Aβ levels in N2a cells stably expressing the Swedish mutant of amyloid precursor protein (APP). Moreover, synj1 haploinsufficiency in an Alzheimer disease transgenic mouse model expressing the Swedish mutant APP and the presenilin-1 mutant ΔE9 reduces amyloid plaque load, as well as Aβ40 and Aβ42 levels in hippocampus of 9-month-old animals. Reduced expression of synj1 attenuates cognitive deficits in these transgenic mice. However, reduction of synj1 does not affect levels of full-length APP and the C-terminal fragment, suggesting that Aβ generation by β- and γ-secretase cleavage is not affected. Instead, synj1 knockdown increases Aβ uptake and cellular degradation through accelerated delivery to lysosomes. These effects are partially dependent upon elevated PI(4,5)P2 with synj1 down-regulation. In summary, our data suggest a novel mechanism by which reduction of a PI(4,5)P2-degrading enzyme, synj1, improves amyloid-induced neuropathology and behavior deficits through accelerating cellular Aβ clearance.

Presenilin-1/γ-secretase controls glutamate release, tyrosine phosphorylation, and surface expression of N-methyl-D-aspartate receptor (NMDAR) subunit GluN2B

Abnormally high concentrations of extracellular glutamate in the brain may cause neuronal damage via excitotoxicity. Thus, tight regulation of glutamate release is critical to neuronal function and survival. Excitotoxicity is caused mainly by overactivation of the extrasynaptic NMDA receptor (NMDAR) and results in specific cellular changes, including calcium-induced activation of calpain proteases. Here, we report that presenilin-1 (PS1) null mouse cortical neuronal cultures have increased amounts of calpain-dependent spectrin breakdown products (SBDPs) compared with WT cultures. NMDAR antagonists blocked accumulation of SBDPs, suggesting abnormal activation of this receptor in PS1 null cultures. Importantly, an increase in SBDPs was detected in cultures of at least 7 days in vitro but not in younger cultures. Conditioned medium from PS1 null neuronal cultures at 8 days in vitro contained higher levels of glutamate than medium from WT cultures and stimulated production of SBDPs when added to WT cultures. Use of glutamate reuptake inhibitors indicated that accumulation of this neurotransmitter in the media of PS1 null cultures was due to increased rates of release. PS1 null neurons showed decreased cell surface expression and phosphorylation of the GluN2B subunit of NMDAR, indicating decreased amounts of extrasynaptic NMDAR in the absence of PS1. Inhibition of γ-secretase activity in WT neurons caused changes similar to those observed in PS1 null neurons. Together, these data indicate that the PS1/γ-secretase system regulates release of glutamate, tyrosine phosphorylation, and surface expression of GluN2B-containing NMDARs.

Cell signaling abnormalities may drive neurodegeneration in familial Alzheimer disease

Presenilins (PSs) are catalytic components of the γ-secretase complex that produces Aβ peptides. Substrates of γ-secretase are membrane-bound protein fragments deriving from the cleavage of extracellular sequence of cell surface proteins. APP-derived γ-secretase substrates are cleaved at gamma (γ) sites to produce Aβ while cleavage at the epsilon (ε) site produces AICD proposed to function in transcription. In addition to APP, γ-secretase promotes the ε-cleavage of a large number of cell surface proteins producing cytosolic peptides shown to function in cell signaling. A common hypothesis suggests that Alzheimer's disease (AD) is caused by Aβ peptides or their products. Treatment of patients with inhibitors of Aβ production however, showed no therapeutic benefits while inducing cytotoxicity. Similarly, treatments with anti-Aβ antibodies yielded disappointing results. Importantly, recent evidence shows that PS familial AD (FAD) mutations cause a loss of γ-secretase cleavage activity at ε site of substrates thus inhibiting production of biologically important cell signaling peptides while promoting accumulation of membrane-bound cytotoxic substrates. These data support a hypothesis that FAD mutations may increase neurotoxicity by inhibiting the γ-secretase-catalyzed ε cleavage of substrates thus interfering with cell signaling while also promoting accumulation of cytotoxic peptides. Similar mechanisms may explain γ-secretase inhibitor-associated toxicities observed in clinical trials. Here we discuss evidence that FAD neurodegeneration may be caused by loss of γ-secretase cleavage function at ε sites of substrates.