A transcriptionally [correction of transcriptively] active complex of APP with Fe65 and histone acetyltransferase Tip60

Potential Link Between Proprotein Convertase Subtilisin/Kexin Type 9 and Alzheimer's Disease

Alzheimer's disease [AD] is not only the most common neurodegenerative disease but is also currently incurable. Proprotein convertase subtilisin/kexin-9 [PCSK9] is an indirect regulator of plasma low density lipoprotein [LDL] levels controlling LDL receptor expression at the plasma membrane. PCSK9 also appears to regulate the development of glucose intolerance, insulin resistance, abdominal obesity, inflammation, and hypertension, conditions that have been identified as risk factors for AD. PCSK9 levels also depend on age, sex, and ethnic background, factors associated with AD. Herein, we will review indirect evidence that suggests a link between PCSK9 levels and AD.

Role of Amyloid-β and Tau Proteins in Alzheimer's Disease: Confuting the Amyloid Cascade

The "Amyloid Cascade Hypothesis" has dominated the Alzheimer's disease (AD) field in the last 25 years. It posits that the increase of amyloid-β (Aβ) is the key event in AD that triggers tau pathology followed by neuronal death and eventually, the disease. However, therapeutic approaches aimed at decreasing Aβ levels have so far failed, and tau-based clinical trials have not yet produced positive findings. This begs the question of whether the hypothesis is correct. Here we have examined literature on the role of Aβ and tau in synaptic dysfunction, memory loss, and seeding and spreading of AD, highlighting important parallelisms between the two proteins in all of these phenomena. We discuss novel findings showing binding of both Aβ and tau oligomers to amyloid-β protein precursor (AβPP), and the requirement for the presence of this protein for both Aβ and tau to enter neurons and induce abnormal synaptic function and memory. Most importantly, we propose a novel view of AD pathogenesis in which extracellular oligomers of Aβ and tau act in parallel and upstream of AβPP. Such a view will call for a reconsideration of therapeutic approaches directed against Aβ and tau, paving the way to an increased interest toward AβPP, both for understanding the pathogenesis of the disease and elaborating new therapeutic strategies.

Cystic Fibrosis, Cystic Fibrosis Transmembrane Conductance Regulator and Drugs: Insights from Cellular Trafficking

The eukaryotic cell is organized into membrane-delineated compartments that are characterized by specific cadres of proteins sustaining biochemically distinct cellular processes. The appropriate subcellular localization of proteins is key to proper organelle function and provides a physiological context for cellular processes. Disruption of normal trafficking pathways for proteins is seen in several genetic diseases, where a protein's absence for a specific subcellular compartment leads to organelle disruption, and in the context of an individual, a disruption of normal physiology. Importantly, several drug therapies can also alter protein trafficking, causing unwanted side effects. Thus, a deeper understanding of trafficking pathways needs to be appreciated as novel therapeutic modalities are proposed. Despite the promising efficacy of novel therapeutic agents, the intracellular bioavailability of these compounds has proved to be a potential barrier, leading to failures in treatments for various diseases and disorders. While endocytosis of drug moieties provides an efficient means of getting material into cells, the subsequent release and endosomal escape of materials into the cytosol where they need to act has been a barrier. An understanding of cellular protein/lipid trafficking pathways has opened up strategies for increasing drug bioavailability. Approaches to enhance endosomal exit have greatly increased the cytosolic bioavailability of drugs and will provide a means of investigating previous drugs that may have been shelved due to their low cytosolic concentration.

Stem Cell Therapies for Neurodegenerative Diseases

Stem cell therapies have been proposed as a treatment option for neurodegenerative diseases, but the best stem cell source and therapeutic efficacy for neuroregeneration remain uncertain. Embryonic stem cells (ESCs) and neural stem cells (NSCs), which can efficiently generate neural cells, could be good candidates but they pose ethical and practical issues. Not only difficult to find the good source of those cells but also they alway pose immunorejection problem since they may not be an autologous cells. Even if we overcome the immunorejection problem, it has also been reported that transplantation of ESCs develop teratoma. Although adult stem cells are more accessible, they have a limited developmental potential. We developed technologies to increase potency of mesenchymal stem cells, which allow them to develop into neural cells, by over expression of the ESC gene, nanog. We also developed a small molecule compound, which significantly increases endogenous NSCs by peripheral administration, eliminating even the necessity of stem cell injection to the brain. These novel technologies may offer neuroregenerative therapies for Alzheimers disease (AD). However, we found that AD pathological condition prevent neurogenesis from NSCs. This chapter discusses how to overcome the problem associated stem cell therapy under AD pathology and introduces exosome as a tool to improve the modification of adult stem cells. These new technologies may open a door for the new era for AD therapy.

APP upregulation contributes to retinal ganglion cell degeneration via JNK3

Axonal injury is a common feature of central nervous system insults. Upregulation of amyloid precursor protein (APP) is observed following central nervous system neurotrauma and is regarded as a marker of central nervous system axonal injury. However, the underlying mechanism by which APP mediates neuronal death remains to be elucidated. Here, we used mouse optic nerve axotomy (ONA) to model central nervous system axonal injury replicating aspects of retinal ganglion cell (RGC) death in optic neuropathies. APP and APP intracellular domain (AICD) were upregulated in retina after ONA and APP knockout reduced Tuj1⁺ RGC loss. Pathway analysis of microarray data combined with chromatin immunoprecipitation and a luciferase reporter assay demonstrated that AICD interacts with the JNK3 gene locus and regulates JNK3 expression. Moreover, JNK3 was found to be upregulated after ONA and to contribute to Tuj1⁺ RGC death. APP knockout reduced the ONA-induced enhanced expression of JNK3 and phosphorylated JNK (pJNK). Gamma-secretase inhibitors prevented production of AICD, reduced JNK3 and pJNK expression similarly, and protected Tuj1⁺ RGCs from ONA-induced cell death. Together these data indicate that ONA induces APP expression and that gamma-secretase cleavage of APP releases AICD, which upregulates JNK3 leading to RGC death. This pathway may be a novel target for neuronal protection in optic neuropathies and other forms of neurotrauma.

Omega-3 fatty acids, lipids, and apoE lipidation in Alzheimer's disease: a rationale for multi-nutrient dementia prevention

In the last decade, it has become obvious that Alzheimer's disease (AD) is closely linked to changes in lipids or lipid metabolism. One of the main pathological hallmarks of AD is amyloid-β (Aβ) deposition. Aβ is derived from sequential proteolytic processing of the amyloid precursor protein (APP). Interestingly, both, the APP and all APP secretases are transmembrane proteins that cleave APP close to and in the lipid bilayer. Moreover, apoE4 has been identified as the most prevalent genetic risk factor for AD. ApoE is the main lipoprotein in the brain, which has an abundant role in the transport of lipids and brain lipid metabolism. Several lipidomic approaches revealed changes in the lipid levels of cerebrospinal fluid or in post mortem AD brains. Here, we review the impact of apoE and lipids in AD, focusing on the major brain lipid classes, sphingomyelin, plasmalogens, gangliosides, sulfatides, DHA, and EPA, as well as on lipid signaling molecules, like ceramide and sphingosine-1-phosphate. As nutritional approaches showed limited beneficial effects in clinical studies, the opportunities of combining different supplements in multi-nutritional approaches are discussed and summarized.

Functional Amyloids and their Possible Influence on Alzheimer Disease

Amyloids play critical roles in human diseases but have increasingly been recognized to also exist naturally. Shared physicochemical characteristics of amyloids and of their smaller oligomeric building blocks offer the prospect of molecular interactions and crosstalk amongst these assemblies, including the propensity to mutually influence aggregation. A case in point might be the recent discovery of an interaction between the amyloid β peptide (Aβ) and somatostatin (SST). Whereas Aβ is best known for its role in Alzheimer disease (AD) as the main constituent of amyloid plaques, SST is intermittently stored in amyloid-form in dense core granules before its regulated release into the synaptic cleft. This review was written to introduce to readers a large body of literature that surrounds these two peptides. After introducing general concepts and recent progress related to our understanding of amyloids and their aggregation, the review focuses separately on the biogenesis and interactions of Aβ and SST, before attempting to assess the likelihood of encounters of the two peptides in the brain, and summarizing key observations linking SST to the pathobiology of AD. While the review focuses on Aβ and SST, it is to be anticipated that crosstalk amongst functional and disease-associated amyloids will emerge as a general theme with much broader significance in the etiology of dementias and other amyloidosis.

Phosphorylation of FE65 at threonine 579 by GSK3β stimulates amyloid precursor protein processing

Excessive generation of amyloid-β peptide (Aβ) by aberrant proteolysis of amyloid precursor protein (APP) is a key event in Alzheimer's disease (AD) pathogenesis. FE65 is a brain-enriched phospho-adaptor protein that interacts with APP and has been shown to modulate APP processing. However, the mechanism(s) that FE65 alters APP processing is still not fully understood. In the present study, we demonstrate that FE65 is phosphorylated at threonine 579 (T579) by glycogen synthase kinase 3β (GSK3β). Moreover, FE65 T579 phosphorylation potentiates γ- and β-secretases-mediated APP processing and Aβ liberation. Additionally, the phosphorylation suppresses FE65 PTB2 intermolecular dimerization but enhances FE65/APP complex formation. Hence, our findings reveal a novel mechanism that GSK3β stimulates amyloidogenic processing of APP by phosphorylation of FE65 at T579.

Phosphorylation of amyloid precursor protein by mutant LRRK2 promotes AICD activity and neurotoxicity in Parkinson's disease

Mutations in LRRK2, which encodes leucine-rich repeat kinase 2, are the most common genetic cause of familial and sporadic Parkinson's disease (PD), a degenerative disease of the central nervous system that causes impaired motor function and, in advanced stages, dementia. Dementia is a common symptom of another neurodegenerative disease, Alzheimer's disease, and research suggests that there may be pathophysiological and genetic links between the two diseases. Aggregates of β amyloid [a protein produced through cleavage of amyloid precursor protein (APP)] are seen in both diseases and in PD patients carrying G2019S-mutant LRRK2. Using patient-derived cells, brain tissue, and PD model mice, we found that LRRK2 interacted with and phosphorylated APP at Thr⁶⁶⁸ within its intracellular domain (AICD). Phosphorylation of APP at Thr⁶⁶⁸ promoted AICD transcriptional activity and correlated with increased nuclear abundance of AICD and decreased abundance of a dopaminergic neuron marker in cultures and brain tissue. The AICD regulates the transcription of genes involved in cytoskeletal dynamics and apoptosis. Overexpression of AICD, but not a phosphodeficient mutant (AICD^T668A), increased the loss of dopaminergic neurons in older mice expressing LRRK2^G2019S Moreover, the amount of Thr⁶⁶⁸-phosphorylated APP was substantially greater in postmortem brain tissue and dopaminergic neurons (generated by reprogramming skin cells) from LRRK2^G2019S patients than in those from healthy individuals. LRRK2 inhibitors reduced the phosphorylation of APP at Thr⁶⁶⁸ in the patient-derived dopaminergic neurons and in the midbrains of LRRK2^G2019S mice. Thus, APP is a substrate of LRRK2, and its phosphorylation promotes AICD function and neurotoxicity in PD.

Cerebrospinal Fluid Biomarkers in Alzheimer's Disease-From Brain Starch to Bench and Bedside

Alzheimer’s disease is the most common cause of dementia. Over the last three decades, research has advanced dramatically and provided a detailed understanding of the molecular events underlying the pathogenesis of Alzheimer’s disease. In parallel, assays for the detection of biomarkers that reflect the typical Alzheimer’s disease-associated pathology have been developed and validated in myriads of clinical studies. Such biomarkers complement clinical diagnosis and improve diagnostic accuracy. The use of biomarkers will become even more important with the advent of disease-modifying therapies. Such therapies will likely be most beneficial when administered early in the disease course. Here, we summarise the development of the core Alzheimer’s disease cerebrospinal fluid biomarkers: amyloid-β and tau. We provide an overview of their role in cellular physiology and Alzheimer’s disease pathology, and embed their development as cerebrospinal fluid biomarkers into the historical context of Alzheimer’s disease research. Finally, we summarise recommendations for their use in clinical practice, and outline perspectives for novel cerebrospinal fluid candidate biomarkers.

LTP and memory impairment caused by extracellular Aβ and Tau oligomers is APP-dependent

The concurrent application of subtoxic doses of soluble oligomeric forms of human amyloid-beta (oAβ) and Tau (oTau) proteins impairs memory and its electrophysiological surrogate long-term potentiation (LTP), effects that may be mediated by intra-neuronal oligomers uptake. Intrigued by these findings, we investigated whether oAβ and oTau share a common mechanism when they impair memory and LTP in mice. We found that as already shown for oAβ, also oTau can bind to amyloid precursor protein (APP). Moreover, efficient intra-neuronal uptake of oAβ and oTau requires expression of APP. Finally, the toxic effect of both extracellular oAβ and oTau on memory and LTP is dependent upon APP since APP-KO mice were resistant to oAβ- and oTau-induced defects in spatial/associative memory and LTP. Thus, APP might serve as a common therapeutic target against Alzheimer's Disease (AD) and a host of other neurodegenerative diseases characterized by abnormal levels of Aβ and/or Tau.

DOI:http://dx.doi.org/10.7554/eLife.26991.001

Physiological and pathophysiological control of synaptic GluN2B-NMDA receptors by the C-terminal domain of amyloid precursor protein

The amyloid precursor protein (APP) harbors physiological roles at synapses and is central to Alzheimer’s disease (AD) pathogenesis. Evidence suggests that APP intracellular domain (AICD) could regulate synapse function, but the underlying molecular mechanisms remain unknown. We addressed AICD actions at synapses, per se, combining in vivo AICD expression, ex vivo AICD delivery or APP knock-down by in utero electroporation of shRNAs with whole-cell electrophysiology. We report a critical physiological role of AICD in controlling GluN2B-containing NMDA receptors (NMDARs) at immature excitatory synapses, via a transcription-dependent mechanism. We further show that AICD increase in mature neurons, as reported in AD, alters synaptic NMDAR composition to an immature-like GluN2B-rich profile. This disrupts synaptic signal integration, via over-activation of SK channels, and synapse plasticity, phenotypes rescued by GluN2B antagonism. We provide a new physiological role for AICD, which becomes pathological upon AICD increase in mature neurons. Thus, AICD could contribute to AD synaptic failure.

DOI:http://dx.doi.org/10.7554/eLife.25659.001

Chronic treatment with cisplatin induces chemoresistance through the TIP60-mediated Fanconi anemia and homologous recombination repair pathways

The Fanconi anemia pathway in coordination with homologous recombination is essential to repair interstrand crosslinks (ICLs) caused by cisplatin. TIP60 belongs to the MYST family of acetyltransferases and is involved in DNA repair and regulation of gene transcription. Although the physical interaction between the TIP60 and FANCD2 proteins has been identified that is critical for ICL repair, it is still elusive whether TIP60 regulates the expression of FA and HR genes. In this study, we found that the chemoresistant nasopharyngeal carcinoma cells, derived from chronic treatment of cisplatin, show elevated expression of TIP60. Furthermore, TIP60 binds to the promoters of FANCD2 and BRCA1 by using the chromatin immunoprecipitation experiments and promote the expression of FANCD2 and BRCA1. Importantly, the depletion of TIP60 significantly reduces sister chromatid exchange, a measurement of HR efficiency. The similar results were also shown in the FNACD2-, and BRCA1-deficient cells. Additionally, these TIP60-deficient cells encounter more frequent stalled forks, as well as more DNA double-strand breaks resulting from the collapse of stalled forks. Taken together, our results suggest that TIP60 promotes the expression of FA and HR genes that are important for ICL repair and the chemoresistant phenotype under chronic treatment with cisplatin.

Fe65-PTB2 Dimerization Mimics Fe65-APP Interaction

Physiological function and pathology of the Alzheimer’s disease causing amyloid precursor protein (APP) are correlated with its cytosolic adaptor Fe65 encompassing a WW and two phosphotyrosine-binding domains (PTBs). The C-terminal Fe65-PTB2 binds a large portion of the APP intracellular domain (AICD) including the GYENPTY internalization sequence fingerprint. AICD binding to Fe65-PTB2 opens an intra-molecular interaction causing a structural change and altering Fe65 activity. Here we show that in the absence of the AICD, Fe65-PTB2 forms a homodimer in solution and determine its crystal structure at 2.6 Å resolution. Dimerization involves the unwinding of a C-terminal α-helix that mimics binding of the AICD internalization sequence, thus shielding the hydrophobic binding pocket. Specific dimer formation is validated by nuclear magnetic resonance (NMR) techniques and cell-based analyses reveal that Fe65-PTB2 together with the WW domain are necessary and sufficient for dimerization. Together, our data demonstrate that Fe65 dimerizes via its APP interaction site, suggesting that besides intra- also intermolecular interactions between Fe65 molecules contribute to homeostatic regulation of APP mediated signaling.

The Role of APP in Structural Spine Plasticity

Amyloid precursor protein (APP) is a transmembrane protein highly expressed in neurons. The full-length protein has cell-adhesion and receptor-like properties, which play roles in synapse formation and stability. Furthermore, APP can be cleaved by several proteases into numerous fragments, many of which affect synaptic function and stability. This review article focuses on the mechanisms of APP in structural spine plasticity, which encompasses the morphological alterations at excitatory synapses. These occur as changes in the number and morphology of dendritic spines, which correspond to the postsynaptic compartment of excitatory synapses. Both overexpression and knockout (KO) of APP lead to impaired synaptic plasticity. Recent data also suggest a role of APP in the regulation of astrocytic D-serine homeostasis, which in turn regulates synaptic plasticity.

Altered neurogenesis in mouse models of Alzheimer disease

Amyloid-β (Aβ) peptides, as well as a variety of other protein fragments, are derived from proteolytical cleavage of the amyloid precursor protein (APP) and have been demonstrated to play a key role in the pathological changes underlying Alzheimer disease (AD). In AD mouse models, altered neurogenesis has been repeatedly reported to be associated with further AD-typical pathological hallmarks such as extracellular plaque deposition, behavioral deficits or neuroinflammation. While a toxic role of Aβ in neurodegeneration and impaired neuronal progenitor proliferation is likely and well-accepted, recent findings also suggest an important influence of APP-derived proteolitical fragments like the APP intracellular domain (AICD), as well as of APP itself.

Emerging roles for the amyloid precursor protein and derived peptides in the regulation of cellular and systemic metabolism

The amyloid precursor protein (APP) is a transmembrane protein that can be cleaved by proteases through two different pathways to yield a number of small peptides, each with distinct physiological properties and functions. It has been extensively studied in the context of Alzheimer's disease, with the APP-derived amyloid β (Aβ) peptide being a major constituent of the amyloid plaques observed in this disease. It has been known for some time that APP can regulate neuronal metabolism; however, the present review examines the evidence indicating that APP and its peptides can also regulate key metabolic processes such as insulin action, lipid synthesis and storage and mitochondrial function in peripheral tissues. This review presents the hypothesis that amyloidogenic processing of APP in peripheral tissues plays a key role in the response to nutrient excess and that this could contribute to the pathogenesis of metabolic diseases such as obesity and type 2 diabetes (T2D).

The Amyloid Precursor Protein Is a Conserved Receptor for Slit to Mediate Axon Guidance

The amyloid precursor protein (APP) is a receptor-like membrane protein. Although APP processing and β-amyloid production play a central role in Alzheimer's disease (AD) pathogenesis, the physiological function of APP remains elusive. Here, we identify APP as a novel receptor for Slit that mediates axon guidance and neural circuit formation. APP deficiency abolishes the Slit repulsive effect in a 3D olfactory explant culture, consistent with its callosal projection deficit in vivo and reminiscent of Slit loss. Inactivation of APP ortholog APL-1 in Caenorhabditis elegans results in pioneer axon mistargeting and genetic analysis places APL-1 in the SLT-1 (Slit)/SAX-3 (Robo) repulsive pathway. Slit binds to APP through the E1 domain, which triggers APP ectodomain shedding and recruitment of the intracellular FE65 and Pak1 complex and associated Rac1 GTPase activation. Our study establishes APP as a novel receptor for Slit ligand mediating axon guidance and neural circuit formation.

Not just amyloid: physiological functions of the amyloid precursor protein family

Amyloid precursor protein (APP) gives rise to the amyloid-β peptide and thus has a key role in the pathogenesis of Alzheimer disease. By contrast, the physiological functions of APP and the closely related APP-like proteins (APLPs) remain less well understood. Studying these physiological functions has been challenging and has required a careful long-term strategy, including the analysis of different App-knockout and Aplp-knockout mice. In this Review, we summarize these findings, focusing on the in vivo roles of APP family members and their processing products for CNS development, synapse formation and function, brain injury and neuroprotection, as well as ageing. In addition, we discuss the implications of APP physiology for therapeutic approaches.

APP Protein Family Signaling at the Synapse: Insights from Intracellular APP-Binding Proteins

Understanding the molecular mechanisms underlying amyloid precursor protein family (APP/APP-like proteins, APLP) function in the nervous system can be achieved by studying the APP/APLP interactome. In this review article, we focused on intracellular APP interacting proteins that bind the YENPTY internalization motif located in the last 15 amino acids of the C-terminal region. These proteins, which include X11/Munc-18-interacting proteins (Mints) and FE65/FE65Ls, represent APP cytosolic binding partners exhibiting different neuronal functions. A comparison of FE65 and APP family member mutant mice revealed a shared function for APP/FE65 protein family members in neurogenesis and neuronal positioning. Accumulating evidence also supports a role for membrane-associated APP/APLP proteins in synapse formation and function. Therefore, it is tempting to speculate that APP/APLP C-terminal interacting proteins transmit APP/APLP-dependent signals at the synapse. Herein, we compare our current knowledge of the synaptic phenotypes of APP/APLP mutant mice with those of mice lacking different APP/APLP interaction partners and discuss the possible downstream effects of APP-dependent FE65/FE65L or X11/Mint signaling on synaptic vesicle release, synaptic morphology and function. Given that the role of X11/Mint proteins at the synapse is well-established, we propose a model highlighting the role of FE65 protein family members for transduction of APP/APLP physiological function at the synapse.

Cortical cells reveal APP as a new player in the regulation of GABAergic neurotransmission

The amyloid precursor protein (APP) modulates synaptic activity, resulting from the fine tuning of excitatory and inhibitory neurotransmission. GABAergic inhibitory neurotransmission is affected by modifications in intracellular chloride concentrations regulated by Na+-K+-2Cl- cotransporter 1 (NKCC1) and neuronal K+-Cl- cotransporter 2 (KCC2), allowing entrance and efflux of chloride, respectively. Modifications in NKCC1 and KCC2 expression during maturation of cortical cells induce a shift in GABAergic signaling. Here, we demonstrated that APP affects this GABA shift. Expression of APP in cortical cells decreased the expression of KCC2, without modifying NKCC1, eliciting a less inhibitory GABA response. Downregulation of KCC2 expression by APP was independent of the APP intracellular domain, but correlated with decreased expression of upstream stimulating factor 1 (USF1), a potent regulator of Slc12a5 gene expression (encoding KCC2). KCC2 was also downregulated in vivo following APP expression in neonatal mouse brain. These results argue for a key role of APP in the regulation of GABAergic neurotransmission.

Nuclear localization of amyloid-β precursor protein-binding protein Fe65 is dependent on regulated intramembrane proteolysis

Fe65 is an adaptor protein involved in both processing and signaling of the Alzheimer-associated amyloid-β precursor protein, APP. Here, the subcellular localization was further investigated using TAP-tagged Fe65 constructs expressed in human neuroblastoma cells. Our results indicate that PTB2 rather than the WW domain is important for the nuclear localization of Fe65. Electrophoretic mobility shift of Fe65 caused by phosphorylation was not detected in the nuclear fraction, suggesting that phosphorylation could restrict nuclear localization of Fe65. Furthermore, both ADAM10 and γ-secretase inhibitors decreased nuclear Fe65 in a similar way indicating an important role also of α-secretase in regulating nuclear translocation.

APP Function and Lipids: A Bidirectional Link

Extracellular neuritic plaques, composed of aggregated amyloid-β (Aβ) peptides, are one of the major histopathological hallmarks of Alzheimer's disease (AD), a progressive, irreversible neurodegenerative disorder and the most common cause of dementia in the elderly. One of the most prominent risk factor for sporadic AD, carrying one or two aberrant copies of the apolipoprotein E (ApoE) ε4 alleles, closely links AD to lipids. Further, several lipid classes and fatty acids have been reported to be changed in the brain of AD-affected individuals. Interestingly, the observed lipid changes in the brain seem not only to be a consequence of the disease but also modulate Aβ generation. In line with these observations, protective lipids being able to decrease Aβ generation and also potential negative lipids in respect to AD were identified. Mechanistically, Aβ peptides are generated by sequential proteolytic processing of the amyloid precursor protein (APP) by β- and γ-secretase. The α-secretase appears to compete with β-secretase for the initial cleavage of APP, preventing Aβ production. All APP-cleaving secretases as well as APP are transmembrane proteins, further illustrating the impact of lipids on Aβ generation. Beside the pathological impact of Aβ, accumulating evidence suggests that Aβ and the APP intracellular domain (AICD) play an important role in regulating lipid homeostasis, either by direct effects or by affecting gene expression or protein stability of enzymes involved in the de novo synthesis of different lipid classes. This review summarizes the current literature addressing the complex bidirectional link between lipids and AD and APP processing including lipid alterations found in AD post mortem brains, lipids that alter APP processing and the physiological functions of Aβ and AICD in the regulation of several lipid metabolism pathways.

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.

Novel Insights into the Physiological Function of the APP (Gene) Family and Its Proteolytic Fragments in Synaptic Plasticity

The amyloid precursor protein (APP) is well known to be involved in the pathophysiology of Alzheimer's disease (AD) via its cleavage product amyloid ß (Aß). However, the physiological role of APP, its various proteolytic products and the amyloid precursor-like proteins 1 and 2 (APLP1/2) are still not fully clarified. Interestingly, it has been shown that learning and memory processes represented by functional and structural changes at synapses are altered in different APP and APLP1/2 mouse mutants. In addition, APP and its fragments are implicated in regulating synaptic strength further reinforcing their modulatory role at the synapse. While APLP2 and APP are functionally redundant, the exclusively CNS expressed APLP1, might have individual roles within the synaptic network. The proteolytic product of non-amyloidogenic APP processing, APPsα, emerged as a neurotrophic peptide that facilitates long-term potentiation (LTP) and restores impairments occurring with age. Interestingly, the newly discovered η-secretase cleavage product, An-α acts in the opposite direction, namely decreasing LTP. In this review we summarize recent findings with emphasis on the physiological role of the APP gene family and its proteolytic products on synaptic function and plasticity, especially during processes of hippocampal LTP. Therefore, we focus on literature that provide electrophysiological data by using different mutant mouse strains either lacking full-length or parts of the APP proteins or that utilized secretase inhibitors as well as secreted APP fragments.

Development of a Reporter System for In Vivo Monitoring of γ-Secretase Activity in Drosophila

The γ-secretase complex represents an evolutionarily conserved family of transmembrane aspartyl proteases that cleave numerous type-I membrane proteins, including the β-amyloid precursor protein (APP) and the receptor Notch. All known rare mutations in APP and the γ-secretase catalytic component, presenilin, which lead to increased amyloid βpeptide production, are responsible for early-onset familial Alzheimer's disease. β-amyloid protein precursor-like (APPL) is the Drosophila ortholog of human APP. Here, we created Notch- and APPL-based Drosophila reporter systems for in vivo monitoring of γ-secretase activity. Ectopic expression of the Notch- and APPL-based chimeric reporters in wings results in vein truncation phenotypes. Reporter-mediated vein truncation phenotypes are enhanced by the Notch gain-of-function allele and suppressed by RNAi-mediated knockdown of presenilin. Furthermore, we find that apoptosis partly contributes to the vein truncation phenotypes of the APPL-based reporter, but not to the vein truncation phenotypes of the Notch-based reporter. Taken together, these results suggest that both in vivo reporter systems provide a powerful genetic tool to identify genes that modulate γ-secretase activity and/or APPL metabolism.

Alzheimer's Disease and Histone Code Alterations

Substantial progress has been made in identifying Alzheimer's disease (AD) risk-associated variants using genome-wide association studies (GWAS). The majority of these risk variants reside in noncoding regions of the genome making their functional evaluation difficult; however, they also infer the presence of unconventional regulatory regions that may reside at these locations. We know from these studies that rare familial cases of AD account for less than 5% of all AD cases and autosomal dominant mutations in APP, PSEN1 and PSEN2 account for less than 10% of the genetic basis of these familial cases [1]. The sporadic form of AD, while more complex, still has a substantial genetic component evidenced by observational studies where 30-48% of AD patients have a first degree relative who is also affected [2]. In addition, the strongest risk factor after age is the APOE E4 polymorphism, and more than 20 other risk variants have been identified to date, reviewed in two recent papers [3, 4]. Monozygotic twin studies have revealed a discordance for AD, implicating that a combination of epigenetic and genetic factors are likely involved in the development of AD [5].

Amyloid precursor protein modulates Nav1.6 sodium channel currents through a Go-coupled JNK pathway

Amyloid precursor protein (APP), commonly associated with Alzheimer's disease, also marks axonal degeneration. In the recent studies, we demonstrated that APP aggregated at nodes of Ranvier (NORs) in myelinated central nervous system (CNS) axons and interacted with Nav1.6. However, the physiological function of APP remains unknown. In this study, we described reduced sodium current densities in APP knockout hippocampal neurons. Coexpression of APP or its intracellular domains containing a VTPEER motif with Nav1.6 sodium channels in Xenopus oocytes resulted in an increase in peak sodium currents, which was enhanced by constitutively active Go mutant and blocked by a dominant negative mutant. JNK and CDK5 inhibitor attenuated increases in Nav1.6 sodium currents induced by overexpression of APP. Nav1.6 sodium currents were increased by APPT668E (mutant Thr to Glu) and decreased by T668A (mutant Thr to ALa) mutant, respectively. The cell surface expression of Nav1.6 sodium channels in the white matter of spinal cord and the spinal conduction velocity is decreased in APP, p35 and JNK3 knockout mice. Therefore, APP modulates Nav1.6 sodium channels through a Go-coupled JNK pathway, which is dependent on phosphorylation of APP at Thr668.

Role of Drosophila Amyloid Precursor Protein in Memory Formation

The amyloid precursor protein (APP) is a membrane protein engaged in complex proteolytic pathways. APP and its derivatives have been shown to play a central role in Alzheimer’s disease (AD), a progressive neurodegenerative disease characterized by memory decline. Despite a huge effort from the research community, the primary cause of AD remains unclear, making it crucial to better understand the physiological role of the APP pathway in brain plasticity and memory. Drosophila melanogaster is a model system well-suited to address this issue. Although relatively simple, the fly brain is highly organized, sustains several forms of learning and memory, and drives numerous complex behaviors. Importantly, molecules and mechanisms underlying memory processes are conserved from flies to mammals. The fly encodes a single non-essential APP homolog named APP-Like (APPL). Using in vivo inducible RNA interference strategies, it was shown that APPL knockdown in the mushroom bodies (MB)—the central integrative brain structure for olfactory memory—results in loss of memory. Several APPL derivatives, such as secreted and full-length membrane APPL, may play different roles in distinct types of memory phases. Furthermore, overexpression of Drosophila amyloid peptide exacerbates the memory deficit caused by APPL knockdown, thus potentiating memory decline. Data obtained in the fly support the hypothesis that APP acts as a transmembrane receptor, and that disruption of its normal function may contribute to cognitive impairment during early AD.

Amyloid Precursor Proteins Are Dynamically Trafficked and Processed during Neuronal Development

Proteolytic processing of the Amyloid Precursor Protein (APP) produces beta-amyloid (Aβ) peptide fragments that accumulate in Alzheimer's Disease (AD), but APP may also regulate multiple aspects of neuronal development, albeit via mechanisms that are not well understood. APP is a member of a family of transmembrane glycoproteins expressed by all higher organisms, including two mammalian orthologs (APLP1 and APLP2) that have complicated investigations into the specific activities of APP. By comparison, insects express only a single APP-related protein (APP-Like, or APPL) that contains the same protein interaction domains identified in APP. However, unlike its mammalian orthologs, APPL is only expressed by neurons, greatly simplifying an analysis of its functions in vivo. Like APP, APPL is processed by secretases to generate a similar array of extracellular and intracellular cleavage fragments, as well as an Aβ-like fragment that can induce neurotoxic responses in the brain. Exploiting the complementary advantages of two insect models (Drosophila melanogaster and Manduca sexta), we have investigated the regulation of APPL trafficking and processing with respect to different aspects of neuronal development. By comparing the behavior of endogenously expressed APPL with fluorescently tagged versions of APPL and APP, we have shown that some full-length protein is consistently trafficked into the most motile regions of developing neurons both in vitro and in vivo. Concurrently, much of the holoprotein is rapidly processed into N- and C-terminal fragments that undergo bi-directional transport within distinct vesicle populations. Unexpectedly, we also discovered that APPL can be transiently sequestered into an amphisome-like compartment in developing neurons, while manipulations targeting APPL cleavage altered their motile behavior in cultured embryos. These data suggest that multiple mechanisms restrict the bioavailability of the holoprotein to regulate APPL-dependent responses within the nervous system. Lastly, targeted expression of our double-tagged constructs (combined with time-lapse imaging) revealed that APP family proteins are subject to complex patterns of trafficking and processing that vary dramatically between different neuronal subtypes. In combination, our results provide a new perspective on how the regulation of APP family proteins can be modulated to accommodate a variety of cell type-specific responses within the embryonic and adult nervous system.

Immununochemical Markers of the Amyloid Cascade in the Hippocampus in Motor Neuron Diseases

BACKGROUND

Several findings suggest that the amyloid precursor protein (APP) and the amyloid cascade may play a role in motor neuron disease (MND).

OBJECTIVE

Considering that dementia is one of the most frequent non-motor symptoms in amyotrophic lateral sclerosis (ALS) and that hippocampus is one of the brain areas with greater presence of amyloid-related changes in neurodegenerative diseases, our aim was to analyze the molecular markers of the amyloid cascade of APP in pathology studies of the hippocampus of autopsied patients with ALS and ALS-frontotemporal dementia (FTD).

METHODS

We included nine patients with MND and four controls. Immunohistochemical studies and confocal microscopy were used to analyze the expression of APP, TDP-43, pho-TDP-43, Aβ, APP intracellular cytoplasmatic domain (AICD) peptide, Fe65 protein, and pho-TAU in the hippocampus of seven patients with ALS, two patients with ALS-FTD, and four controls. These findings were correlated with clinical data.

RESULTS

Patients displayed increased expression of APP and Aβ peptide. The latter was correlated with cytoplasmic pho-TDP-43 expression. We also found decreased Fe65 expression. A parallel increase in AICD expression was not found. Patients showed increased expression of pho-TAU in the hippocampus. Findings were similar in patients with ALS and those with ALS-FTD, though more marked in the latter group.

CONCLUSION

Post-mortem analyses showed that the amyloid cascade is activated in the hippocampus of patients with MND and correlated with cytoplasmic pho-TDP-43 expression. The number of intracellular or extracellular aggregates of Aβ peptides was not significant.

REM sleep and its Loss-Associated Epigenetic Regulation with Reference to Noradrenaline in Particular

Sleep is an essential physiological process, which has been divided into rapid eye movement sleep (REMS) and non-REMS (NREMS) in higher animals. REMS is a unique phenomenon that unlike other sleep-waking states is not under voluntary control. Directly or indirectly it influences or gets influenced by most of the physiological processes controlled by the brain. It has been proposed that REMS serves housekeeping function of the brain. Extensive research has shown that during REMS at least noradrenaline (NA) -ergic neurons must cease activity and upon REMS loss, there are increased levels of NA in the brain, which then induces many of the REMS loss associated acute and chronic effects. The NA level is controlled by many bio-molecules that are regulated at the molecular and transcriptional levels. Similarly, NA can also directly or indirectly modulate the synthesis and levels of many molecules, which in turn may affect physiological processes. The burgeoning field of behavioral neuroepigenetics has gained importance in recent years and explains the regulatory mechanisms underlying several behavioral phenomena. As REMS and its loss associated changes in NA modulate several pathophysiological processes, in this review we have attempted to explain on one hand how the epigenetic mechanisms regulating the gene expression of factors like tyrosine hydroxylase (TH), monoamine oxidase (MAO), noradrenaline transporter (NAT) control NA levels and on the other hand, how NA per se can affect other molecules in neural circuitry at the epigenetic level resulting in behavioral changes in health and diseases. An understanding of these events will expose the molecular basis of REMS and its loss-associated pathophysiological changes; which are presented as a testable hypothesis for confirmation.

Inhibition of NEDD8 and FAT10 ligase activities through the degrading enzyme NEDD8 ultimate buster 1: A potential anticancer approach

The capabilities of tumour cells to survive through deregulated cell cycles and evade apoptosis are hallmarks of cancer. The ubiquitin-like proteins (UBL) proteasome system is important in regulating cell cycles via signaling proteins. Deregulation of the proteasomal system can lead to uncontrolled cell proliferation. The Skp, Cullin, F-box containing complex (SCF complex) is the predominant E3 ubiquitin ligase, and has diverse substrates. The ubiquitin ligase activity of the SCF complexes requires the conjugation of neural precursor cell expressed, developmentally down-regulated 8 (NEDD8) to cullin proteins. A tumour suppressor and degrading enzyme named NEDD8 ultimate buster 1 (NUB1) is able to recruit HLA-F-adjacent transcript 10 (FAT10)- and NEDD8-conjugated proteins for proteasomal degradation. Ubiquitination is associated with neddylation and FAT10ylation. Although validating the targets of UBLs, including ubiquitin, NEDD8 and FAT10, is challenging, understanding the biological significance of such substrates is an exciting research prospect. This present review discusses the interplay of these UBLs, as well as highlighting their inhibition through NUB1. Knowledge of the mechanisms by which NUB1 is able to downregulate the ubiquitin cascade via NEDD8 conjugation and the FAT10 pathway is essential. This will provide insights into potential cancer therapy that could be used to selectively suppress cancer growth.

Extensive nuclear sphere generation in the human Alzheimer's brain

Nuclear spheres are protein aggregates consisting of FE65, TIP60, BLM, and other yet unknown proteins. Generation of these structures in the cellular nucleus is putatively modulated by the amyloid precursor protein (APP), either by its cleavage or its phosphorylation. Nuclear spheres were preferentially studied in cell culture models and their existence in the human brain had not been known. Existence of nuclear spheres in the human brain was studied using immunohistochemistry. Cell culture experiments were used to study regulative mechanisms of nuclear sphere generation. The comparison of human frontal cortex brain samples from Alzheimer's disease (AD) patients to age-matched controls revealed a dramatically and highly significant enrichment of nuclear spheres in the AD brain. Costaining demonstrated that neurons are distinctly affected by nuclear spheres, but astrocytes never are. Nuclear spheres were predominantly found in neurons that were negative for threonine 668 residue in APP phosphorylation. Cell culture experiments revealed that JNK3-mediated APP phosphorylation reduces the amount of sphere-positive cells. The study suggests that nuclear spheres are a new APP-derived central hallmark of AD, which might be of crucial relevance for the molecular mechanisms in neurodegeneration.

Tip60 HAT Action Mediates Environmental Enrichment Induced Cognitive Restoration

Environmental enrichment (EE) conditions have beneficial effects for reinstating cognitive ability in neuropathological disorders like Alzheimer's disease (AD). While EE benefits involve epigenetic gene control mechanisms that comprise histone acetylation, the histone acetyltransferases (HATs) involved remain largely unknown. Here, we examine a role for Tip60 HAT action in mediating activity- dependent beneficial neuroadaptations to EE using the Drosophila CNS mushroom body (MB) as a well-characterized cognition model. We show that flies raised under EE conditions display enhanced MB axonal outgrowth, synaptic marker protein production, histone acetylation induction and transcriptional activation of cognition linked genes when compared to their genotypically identical siblings raised under isolated conditions. Further, these beneficial changes are impaired in both Tip60 HAT mutant flies and APP neurodegenerative flies. While EE conditions provide some beneficial neuroadaptive changes in the APP neurodegenerative fly MB, such positive changes are significantly enhanced by increasing MB Tip60 HAT levels. Our results implicate Tip60 as a critical mediator of EE-induced benefits, and provide broad insights into synergistic behavioral and epigenetic based therapeutic approaches for treatment of cognitive disorder.

Decreased MORF leads to prolonged endoplasmic reticulum stress in periodontitis-associated chronic inflammation

The association between inflammation and endoplasmic reticulum (ER) stress has been described in many diseases. However, if and how chronic inflammation governs the unfolded protein response (UPR) and promotes ER homeostasis of chronic inflammatory disease remains elusive. In this study, chronic inflammation resulted in ER stress in mesenchymal stem cells in the setting of periodontitis. Long-term proinflammatory cytokines induced prolonged ER stress and decreased the osteogenic differentiation of periodontal ligament stem cells (PDLSCs). Interestingly, we showed that chronic inflammation decreases the expression of lysine acetyltransferase 6B (KAT6B, also called MORF), a histone acetyltransferase, and causes the upregulation of a key UPR sensor, PERK, which lead to the persistent activation of the UPR in PDLSCs. Furthermore, we found that the activation of UPR mediated by MORF in chronic inflammation contributes to the PERK-related deterioration of the osteogenic differentiation of PDLSCs both in vivo and in vitro. Taken together, our results suggest that chronic inflammation compromises UPR function through MORF-mediated-PERK transcription, which is a previously unrecognized mechanism that contributes to impaired ER function, prolonged ER stress and defective osteogenic differentiation of PDLSCs in periodontitis.

A High-Throughput Screen to Identify Inhibitors of Amyloid β-Protein Precursor Processing

Cerebral accumulation of the amyloid β-peptide (Aβ) is believed to play a key role in the pathogenesis of Alzheimer’s disease (AD). Because Aβ is produced from the proteolysis of amyloid β-protein precursor (APP) by β-and γ-secretases, these enzymes are considered important drug targets for AD. The authors have developed a luciferase-based reporter system that can identify new molecules that inhibit APP processing in a high-throughput manner. Such molecules can help in understanding the biology of APP and APP processing and in developing new drug prototypes for AD. In this system, APP is fused on its C-terminus with Gal4-VP16, a chimeric yeast-viral transcription activator, and luciferase is under control of the yeast Gal4 promoter. Compounds that modulate the luciferase signal may affect the secretases directly, interact with modifiers of these proteases, or interact with APP directly. The authors successfully interfaced this assay with a high-throughput screen, testing ~60,000 compounds with diverse chemical structures. In principle, this sensitive, specific, and quantitative assay may be useful for identifying both inhibitors and stimulators of APP processing.( Journal of Biomolecular Screening 2005:1-12)

Transcriptomic Analysis of Drosophila Mushroom Body Neurons Lacking Amyloid-β Precursor-Like Protein Activity

The amyloid-β protein precursor (AβPP) is subjected to sequential intramembrane proteolysis by α-, β-, andγ-secretases, producing secreted amyloid-β (Aβ) peptides and a cytoplasmically released AβPP Intracellular Domain (AICD). AICD complexes with transcription factors in the nucleus, suggesting that this AβPP fragment serves as an active signaling effector that regulates downstream genes, although its nuclear targets are poorly defined. To further understand this potential signaling mechanism mediated by AβPP, we performed a transcriptomic identification of the Drosophila genome that is regulated by the fly AβPP orthologue in fly mushroom body neurons, which control learning- and memory-based behaviors. We find significant changes in expression of 245 genes, representing approximately 1.6% of the Drosophila genome, with the changes ranging from +6 fold to -40 fold. The largest class of responsive targets corresponds to non-protein coding genes and includes microRNAs that have been previously implicated in Alzheimer's disease pathophysiology. Several genes were identified in our Drosophila microarray analyses that have also emerged as putative AβPP targets in similar mammalian transcriptomic studies. Our results also indicate a role for AβPP in cellular pathways involving the regulation of Drosophila Casein Kinase II, mitochondrial oxidative phosphorylation, RNA processing, and innate immunity. Our findings provide insights into the intracellular events that are regulated by AβPP activity in healthy neurons and that might become dysregulated as a result of abnormal AβPP proteolysis in AD.

Treatment of Alzheimer's Disease: The Legacy of the Cholinergic Hypothesis, Neuroplasticity, and Future Directions

In this issue, an article by Waring et al. provides a meta-analysis of the effects of apo-lipo-protein E (APOE) genotype on the beneficial effect of acetyl-cholinesterase inhibitors (AChEIs) in patients with Alzheimer's disease (AD). There was no significant effect found. As of 2015, AChEI medications are the mainstay of AD treatment, and APOE genotype is the most significant factor associated with AD causation. This lack of a significant effect of APOE is analyzed with respect to the "Cholinergic Hypothesis" of AD, dating from 1976, through the recognition that cholinergic neurons are not the sole target of AD, but rather that AD attacks all levels of neuroplasticity in the brain, an idea originated by Ashford and Jarvik in 1985 and which still provides the clearest explanation for AD dementia. The "Amyloid Hypothesis" is dissected back to the alpha/beta pathway switching mechanism affecting the nexin-amyloid pre-protein (NAPP switch). The NAPP switch may be the critical neuroplasticity component of all learning involving synapse remodeling and subserve all learning mechanisms. The gamma-secretase cleavage is discussed, and its normal complementary products, beta-amyloid and the NAPP intracellular domain (NAICD), appear to be involved in natural synapse removal, but the link to AD dementia may involve the NAICD rather than beta-amyloid. Understanding neuroplasticity and the critical pathways to AD dementia are needed to determine therapies and preventive strategies for AD. In particular, the effect of APOE on AD predisposition needs to be established and a means found to adjust its effect to prevent AD.

Amyloid Precursor Protein (APP) May Act as a Substrate and a Recognition Unit for CRL4CRBN and Stub1 E3 Ligases Facilitating Ubiquitination of Proteins Involved in Presynaptic Functions and Neurodegeneration

The amyloid precursor protein (APP), whose mutations cause Alzheimer disease, plays an important in vivo role and facilitates transmitter release. Because the APP cytosolic region (ACR) is essential for these functions, we have characterized its brain interactome. We found that the ACR interacts with proteins that regulate the ubiquitin-proteasome system, predominantly with the E3 ubiquitin-protein ligases Stub1, which binds the NH2 terminus of the ACR, and CRL4(CRBN), which is formed by Cul4a/b, Ddb1, and Crbn, and interacts with the COOH terminus of the ACR via Crbn. APP shares essential functions with APP-like protein-2 (APLP2) but not APP-like protein-1 (APLP1). Noteworthy, APLP2, but not APLP1, interacts with Stub1 and CRL4(CRBN), pointing to a functional pathway shared only by APP and APLP2. In vitro ubiquitination/ubiquitome analysis indicates that these E3 ligases are enzymatically active and ubiquitinate the ACR residues Lys(649/650/651/676/688) Deletion of Crbn reduces ubiquitination of Lys(676) suggesting that Lys(676) is physiologically ubiquitinated by CRL4(CRBN) The ACR facilitated in vitro ubiquitination of presynaptic proteins that regulate exocytosis, suggesting a mechanism by which APP tunes transmitter release. Other dementia-related proteins, namely Tau and apoE, interact with and are ubiquitinated via the ACR in vitro This, and the evidence that CRBN and CUL4B are linked to intellectual disability, prompts us to hypothesize a pathogenic mechanism, in which APP acts as a modulator of E3 ubiquitin-protein ligase(s), shared by distinct neuronal disorders. The well described accumulation of ubiquitinated protein inclusions in neurodegenerative diseases and the link between the ubiquitin-proteasome system and neurodegeneration make this concept plausible.

Tip off the HAT- Epigenetic control of learning and memory by Drosophila Tip60

Disruption of epigenetic gene control mechanisms involving histone acetylation in the brain causes cognitive impairment, a debilitating hallmark of most neurodegenerative disorders. Histone acetylation regulates cognitive gene expression via chromatin packaging control in neurons. Unfortunately, the histone acetyltransferases (HATs) that generate such neural epigenetic signatures and their mechanisms of action remain unclear. Our recent findings provide insight into this question by demonstrating that Tip60 HAT action is critical for morphology and function of the mushroom body (MB), the learning and memory center in the Drosophila brain. We show that Tip60 is robustly produced in MB Kenyon cells and extending axonal lobes and that targeted MB Tip60 HAT loss results in axonal outgrowth disruption. Functional consequences of loss and gain of Tip60 HAT levels in the MB are evidenced by defects in memory. Tip60 ChIP-Seq analysis reveals enrichment for genes that function in cognitive processes and accordingly, key genes representing these pathways are misregulated in the Tip60 HAT mutant fly brain. Remarkably, increasing levels of Tip60 in the MB rescues learning and memory deficits resulting from Alzheimer's disease associated amyloid precursor protein (APP) induced neurodegeneration. Our studies highlight the potential of HAT activators as a therapeutic option for cognitive disorders.

Fe65 Is Phosphorylated on Ser289 after UV-Induced DNA Damage

Fe65 undergoes a phosphatase-sensitive gel mobility shift after DNA damage, consistent with protein phosphorylation. A recent study identified Ser²²⁸ as a specific site of phosphorylation, targeted by the ATM and ATR protein kinases, with phosphorylation inhibiting the Fe65-dependent transcriptional activity of the amyloid precursor protein (APP). The direct binding of Fe65 to APP not only regulates target gene expression, but also contributes to secretase-mediated processing of APP, producing cytoactive proteolytic fragments including the APP intracellular domain (AICD) and cytotoxic amyloid β (Aβ) peptides. Given that the accumulation of Aβ peptides in neural plaques is a pathological feature of Alzheimer’s disease (AD), it is essential to understand the mechanisms controlling Aβ production. This will aid in the development of potential therapeutic agents that act to limit the deleterious production of Aβ peptides. The Fe65-APP complex has transcriptional activity and the complex is regulated by multiple post-translational modifications and other protein binding partners. In the present study, we have identified Ser²⁸⁹ as a novel site of UV-induced phosphorylation. Interestingly, this phosphorylation was mediated by ATM, rather than ATR, and occurred independently of APP. Neither phosphorylation nor mutation of Ser²⁸⁹ affected the Fe65-APP interaction, though this was markedly decreased after UV treatment, with a concomitant decrease in the protein levels of APP in cells. Using mutagenesis, we demonstrated that Fe65 Ser²⁸⁹ phosphorylation did not affect the transcriptional activity of the Fe65-APP complex, in contrast to the previously described Ser²²⁸ site.

FE65 and FE65L1 share common synaptic functions and genetically interact with the APP family in neuromuscular junction formation

The FE65 adaptor proteins (FE65, FE65L1 and FE65L2) bind proteins that function in diverse cellular pathways and are essential for specific biological processes. Mice lacking both FE65 and FE65L1 exhibit ectopic neuronal positioning in the cortex and muscle weakness. p97FE65-KO mice, expressing a shorter FE65 isoform able to bind amyloid precursor protein family members (APP, APLP1, APLP2), develop defective long-term potentiation (LTP) and aged mice display spatial learning and memory deficits that are absent from young mice. Here, we examined the central and peripheral nervous systems of FE65-KO, FE65L1-KO and FE65/FE65L1-DKO mice. We find spatial learning and memory deficits in FE65-KO and FE65L1-KO mice. Severe motor impairments, anxiety, hippocampal LTP deficits and neuromuscular junction (NMJ) abnormalities, characterized by decreased size and reduced apposition of pre- and postsynaptic sites, are observed in FE65/FE65L1-DKO mice. As their NMJ deficits resemble those of mutant APP/APLP2-DKO mice lacking the FE65/FE65L1 binding site, the NMJs of APLP2/FE65-DKO and APLP2/FE65L1-DKO mice were analyzed. NMJ deficits are aggravated in these mice when compared to single FE65- and FE65L1-KO mice. Together, our data demonstrate a role for FE65 proteins at central and peripheral synapses possibly occurring downstream of cell surface-associated APP/APLPs.

Molecular and cellular aspects of age-related cognitive decline and Alzheimer's disease

As the population of people aged 60 or older continues to rise, it has become increasingly important to understand the molecular basis underlying age-related cognitive decline. In fact, a better understanding of aging biology will help us identify ways to maintain high levels of cognitive functioning throughout the aging process. Many cellular and molecular aspects of brain aging are shared with other organ systems; however, certain age-related changes are unique to the nervous system due to its structural, cellular and molecular complexity. Importantly, the brain appears to show differential changes throughout the aging process, with certain regions (e.g. frontal and temporal regions) being more vulnerable than others (e.g. brain stem). Within the medial temporal lobe, the hippocampus is especially susceptible to age-related changes. The important role of the hippocampus in age-related cognitive decline and in vulnerability to disease processes such as Alzheimer's disease has prompted this review, which will focus on the complexity of changes that characterize aging, and on the molecular connections that exist between normal aging and Alzheimer's disease. Finally, it will discuss behavioral interventions and emerging insights for promoting healthy cognitive aging.

A cell-permeable tool for analysing APP intracellular domain function and manipulation of PIKfyve activity

The mechanisms for regulating PIKfyve complex activity are currently emerging. The PIKfyve complex, consisting of the phosphoinositide kinase PIKfyve (also known as FAB1), VAC14 and FIG4, is required for the production of phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2]. PIKfyve function is required for homoeostasis of the endo/lysosomal system and is crucially implicated in neuronal function and integrity, as loss of function mutations in the PIKfyve complex lead to neurodegeneration in mouse models and human patients. Our recent work has shown that the intracellular domain of the amyloid precursor protein (APP), a molecule central to the aetiology of Alzheimer's disease binds to VAC14 and enhances PIKfyve function. In the present study, we utilize this recent advance to create an easy-to-use tool for increasing PIKfyve activity in cells. We fused APP intracellular domain (AICD) to the HIV TAT domain, a cell-permeable peptide allowing proteins to penetrate cells. The resultant TAT-AICD fusion protein is cell permeable and triggers an increase in PI(3,5)P2 Using the PI(3,5)P2 specific GFP-ML1Nx2 probe, we show that cell-permeable AICD alters PI(3,5)P2 dynamics. TAT-AICD also provides partial protection from pharmacological inhibition of PIKfyve. All three lines of evidence show that the AICD activates the PIKfyve complex in cells, a finding that is important for our understanding of the mechanism of neurodegeneration in Alzheimer's disease.