A {gamma}-secretase-independent mechanism of signal transduction by the amyloid precursor protein

Accumulation of APP C-terminal fragments causes endolysosomal dysfunction through the dysregulation of late endosome to lysosome-ER contact sites

Neuronal endosomal and lysosomal abnormalities are among the early changes observed in Alzheimer's disease (AD) before plaques appear. However, it is unclear whether distinct endolysosomal defects are temporally organized and how altered γ-secretase function or amyloid precursor protein (APP) metabolism contribute to these changes. Inhibiting γ-secretase chronically, in mouse embryonic fibroblast and hippocampal neurons, led to a gradual endolysosomal collapse initiated by decreased lysosomal calcium and increased cholesterol, causing downstream defects in endosomal recycling and maturation. This endolysosomal demise is γ-secretase dependent, requires membrane-tethered APP cytoplasmic domains, and is rescued by APP depletion. APP C-terminal fragments (CTFs) localized to late endosome/lysosome-endoplasmic reticulum contacts; an excess of APP-CTFs herein reduced lysosomal Ca2+ refilling from the endoplasmic reticulum, promoting cholesterol accretion. Tonic regulation by APP-CTFs provides a mechanistic explanation for their cellular toxicity: failure to timely degrade APP-CTFs sustains downstream signaling, instigating lysosomal dyshomeostasis, as observed in prodromal AD. This is the opposite of substrates such as Notch, which require intramembrane proteolysis to initiate signaling.

Experience-dependent Tip60 nucleocytoplasmic transport is regulated by its NLS/NES sequences for neuroplasticity gene control

Nucleocytoplasmic transport (NCT) in neurons is critical for enabling proteins to enter the nucleus and regulate plasticity genes in response to environmental cues. Such experience-dependent (ED) neural plasticity is central for establishing memory formation and cognitive function and can influence the severity of neurodegenerative disorders like Alzheimer's disease (AD). ED neural plasticity is driven by histone acetylation (HA) mediated epigenetic mechanisms that regulate dynamic activity-dependent gene transcription profiles in response to neuronal stimulation. Yet, how histone acetyltransferases (HATs) respond to extracellular cues in the in vivo brain to drive HA-mediated activity-dependent gene control remains unclear. We previously demonstrated that extracellular stimulation of rat hippocampal neurons in vitro triggers Tip60 HAT nuclear import with concomitant synaptic gene induction. Here, we focus on investigating Tip60 HAT subcellular localization and NCT specifically in neuronal activity-dependent gene control by using the learning and memory mushroom body (MB) region of the Drosophila brain as a powerful in vivo cognitive model system. We used immunohistochemistry (IHC) to compare the subcellular localization of Tip60 HAT in the Drosophila brain under normal conditions and in response to stimulation of fly brain neurons in vivo either by genetically inducing potassium channels activation or by exposure to natural positive ED conditions. Furthermore, we found that both inducible and ED condition-mediated neural induction triggered Tip60 nuclear import with concomitant induction of previously identified Tip60 target genes and that Tip60 levels in both the nucleus and cytoplasm were significantly decreased in our well-characterized Drosophila AD model. Mutagenesis of a putative nuclear localization signal (NLS) sequence and nuclear export signal (NES) sequence that we identified in the Drosophila Tip60 protein revealed that both are functionally required for appropriate Tip60 subcellular localization. Our results support a model by which neuronal stimulation triggers Tip60 NCT via its NLS and NES sequences to promote induction of activity-dependent neuroplasticity gene transcription and that this process may be disrupted in AD.

Inhibition of CDK4/6 regulates AD pathology, neuroinflammation and cognitive function through DYRK1A/STAT3 signaling

Repurposing approved drugs is an emerging therapeutic development strategy for Alzheimer's disease (AD). The CDK4/6 inhibitor abemaciclib mesylate is an FDA-approved drug for breast cancer treatment. However, whether abemaciclib mesylate affects Aβ/tau pathology, neuroinflammation, and Aβ/LPS-mediated cognitive impairment is unknown. In this study, we investigated the effects of abemaciclib mesylate on cognitive function and Aβ/tau pathology and found that abemaciclib mesylate improved spatial and recognition memory by regulating the dendritic spine number and neuroinflammatory responses in 5xFAD mice, an Aβ-overexpressing model of AD. Abemaciclib mesylate also inhibited Aβ accumulation by enhancing the activity and protein levels of the Aβ-degrading enzyme neprilysin and the α-secretase ADAM17 and decreasing the protein level of the γ-secretase PS-1 in young and aged 5xFAD mice. Importantly, abemaciclib mesylate suppressed tau phosphorylation in 5xFAD mice and tau-overexpressing PS19 mice by reducing DYRK1A and/or p-GSK3β levels. In wild-type (WT) mice injected with lipopolysaccharide (LPS), abemaciclib mesylate rescued spatial and recognition memory and restored dendritic spine number. In addition, abemaciclib mesylate downregulated LPS-induced microglial/astrocytic activation and proinflammatory cytokine levels in WT mice. In BV2 microglial cells and primary astrocytes, abemaciclib mesylate suppressed LPS-mediated proinflammatory cytokine levels by downregulating AKT/STAT3 signaling. Taken together, our results support repurposing the anticancer drug, CDK4/6 inhibitor abemaciclib mesylate as a multitarget therapeutic for AD pathologies.

Arc Regulates Transcription of Genes for Plasticity, Excitability and Alzheimer’s Disease

The immediate early gene Arc is a master regulator of synaptic function and a critical determinant of memory consolidation. Here, we show that Arc interacts with dynamic chromatin and closely associates with histone markers for active enhancers and transcription in cultured rat hippocampal neurons. Both these histone modifications, H3K27Ac and H3K9Ac, have recently been shown to be upregulated in late-onset Alzheimer’s disease (AD). When Arc induction by pharmacological network activation was prevented using a short hairpin RNA, the expression profile was altered for over 1900 genes, which included genes associated with synaptic function, neuronal plasticity, intrinsic excitability, and signalling pathways. Interestingly, about 100 Arc-dependent genes are associated with the pathophysiology of AD. When endogenous Arc expression was induced in HEK293T cells, the transcription of many neuronal genes was increased, suggesting that Arc can control expression in the absence of activated signalling pathways. Taken together, these data establish Arc as a master regulator of neuronal activity-dependent gene expression and suggest that it plays a significant role in the pathophysiology of AD.

Preferential Involvement of BRCA1/BARD1, Not Tip60/Fe65, in DNA Double-Strand Break Repair in Presenilin-1 P117L Alzheimer Models

Recently, we showed that DNA double-strand breaks (DSBs) are increased by the Aβ₄₂-amyloid peptide and decreased by all-trans retinoic acid (RA) in SH-SY5Y cells and C57BL/6J mice. The present work was aimed at investigating DSBs in cells and murine models of Alzheimer's disease carrying the preseniline-1 (PS1) P117L mutation. We observed that DSBs could hardly decrease following RA treatment in the mutated cells compared to the wild-type cells. The activation of the amyloidogenic pathway is proposed in the former case as Aβ₄₂- and RA-dependent DSBs changes were reproduced by an α-secretase and a γ-secretase inhibitions, respectively. Unexpectedly, the PS1 P117L cells showed lower DSB levels than the controls. As the DSB repair proteins Tip60 and Fe65 were less expressed in the mutated cell nuclei, they do not appear to contribute to this difference. On the contrary, full-length BRCA1 and BARD1 proteins were significantly increased in the chromatin compartment of the mutated cells, suggesting that they decrease DSBs in the pathological situation. These Western blot data were corroborated by in situ proximity ligation assays: the numbers of BRCA1-BARD1, not of Fe65-Tip60 heterodimers, were increased only in the mutated cell nuclei. RA also enhanced the expression of BARD1 and of the 90 kDa BRCA1 isoform. The increased BRCA1 expression in the mutated cells can be related to the enhanced difficulty to inhibit this pathway by BRCA1 siRNA in these cells. Overall, our study suggests that at earlier stages of the disease, similarly to PS1 P117L cells, a compensatory mechanism exists that decreases DSB levels via an activation of the BRCA1/BARD1 pathway. This supports the importance of this pathway in neuroprotection against Alzheimer's disease.

Nuclear Localization Is Not Required for Tip60 Tumor Suppressor Activity in Breast and Lung Cancer Cells

The Tip60 lysine acetyltransferase is a tumor suppressor in most cancers but an oncogene in prostate and gastric cancer. Tip60 is commonly found in the nucleus, where it acetylates proteins involved in transcription, DNA repair, and chromatin; however, it has also been shown to acetylate cytoplasmic targets. In this study, we investigated the relationship between Tip60 localization and breast and lung cancer. In cell fractionation experiments, cancer-derived cell lines showed a shift from nuclear to cytoplasmic endogenous Tip60 compared with cell lines derived from normal cells. With immunofluorescence, we observed four different localization patterns of overexpressed Tip60 and found that cancer cells had increased cytoplasmic localization of Tip60 compared with HEK-293 cells. The addition of a nuclear localization signal (NLS) increased the number of cells containing nuclear Tip60, whereas mutation of a putative endogenous NLS increased the number of cells with cytoplasmic Tip60. Overexpression of Tip60 increased cancer cell line sensitivity to paclitaxel regardless of changes in localization. These results suggest that dysregulation of Tip60 in breast and lung cancer is not limited to reduced expression but may also involve subcellular localization.

The cellular expression and proteolytic processing of the amyloid precursor protein is independent of TDP-43

Alzheimer's disease (AD) is a neurodegenerative condition, of which one of the cardinal pathological hallmarks is the extracellular accumulation of amyloid β (Aβ) peptides. These peptides are generated via proteolysis of the amyloid precursor protein (APP), in a manner dependent on the β-secretase, BACE1 and the multicomponent γ-secretase complex. Recent data also suggest a contributory role in AD of transactive response DNA binding protein 43 (TDP-43). There is little insight into a possible mechanism linking TDP-43 and APP processing. To this end, we used cultured human neuronal cells to investigate the ability of TDP-43 to interact with APP and modulate its proteolytic processing. Immunocytochemistry showed TDP-43 to be spatially segregated from both the extranuclear APP holoprotein and its nuclear C-terminal fragment. The latter (APP intracellular domain) was shown to predominantly localise to nucleoli, from which TDP-43 was excluded. Furthermore, neither overexpression of each of the APP isoforms nor siRNA-mediated knockdown of APP had any effect on TDP-43 expression. Doxycycline-stimulated overexpression of TDP-43 was explored in an inducible cell line. Overexpression of TDP-43 had no effect on expression of the APP holoprotein, nor any of the key proteins involved in its proteolysis. Furthermore, increased TDP-43 expression had no effect on BACE1 enzymatic activity or immunoreactivity of Aβ1-40, Aβ1-42 or the Aβ1-40:Aβ1-42 ratio. Also, siRNA-mediated knockdown of TDP-43 had no effect on BACE1 immunoreactivity. Taken together, these data indicate that TDP-43 function and/or dysfunction in AD is likely independent from dysregulation of APP expression and proteolytic processing and Aβ generation.

Ablation of amyloid precursor protein increases insulin-degrading enzyme levels and activity in brain and peripheral tissues

The amyloid precursor protein (APP) is a type I transmembrane glycoprotein widely studied for its role as the source of β-amyloid peptide, accumulation of which is causal in at least some cases of Alzheimer's disease (AD). APP is expressed ubiquitously and is involved in diverse biological processes. Growing bodies of evidence indicate connections between AD and somatic metabolic disorders related to type 2 diabetes, and App^-/- mice show alterations in glycemic regulation. We find that App^-/- mice have higher levels of insulin-degrading enzyme (IDE) mRNA, protein, and activity compared with wild-type controls. This regulation of IDE by APP was widespread across numerous tissues, including liver, skeletal muscle, and brain as well as cell types within neural tissue, including neurons, astrocytes, and microglia. RNA interference-mediated knockdown of APP in the SIM-A9 microglia cell line elevated IDE levels. Fasting levels of blood insulin were lower in App^-/- than App^+/+ mice, but the former showed a larger increase in response to glucose. These low basal levels may enhance peripheral insulin sensitivity, as App^-/- mice failed to develop impairment of glucose tolerance on a high-fat, high-sucrose ("Western") diet. Insulin levels and insulin signaling were also lower in the App^-/- brain; synaptosomes prepared from App^-/- hippocampus showed diminished insulin receptor phosphorylation compared with App^+/+ mice when stimulated ex vivo. These findings represent a new molecular link connecting APP to metabolic homeostasis and demonstrate a novel role for APP as an upstream regulator of IDE in vivo.

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.

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.

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.

Insights into the physiological function of the β-amyloid precursor protein: beyond Alzheimer's disease

The β-amyloid precursor protein (APP) has been extensively studied for its role as the precursor of the β-amyloid protein (Aβ) of Alzheimer's disease. However, the normal function of APP remains largely unknown. This article reviews studies on the structure, expression and post-translational processing of APP, as well as studies on the effects of APP in vitro and in vivo. We conclude that the published data provide strong evidence that APP has a trophic function. APP is likely to be involved in neural stem cell development, neuronal survival, neurite outgrowth and neurorepair. However, the mechanisms by which APP exerts its actions remain to be elucidated. The available evidence suggests that APP interacts both intracellularly and extracellularly to regulate various signal transduction mechanisms.

This article reviews studies on the structure, expression and post-translational processing of β-amyloid precursor protein (APP), as well as studies on the effects of APP in vitro and in vivo. We conclude that the published data provide strong evidence that APP has a trophic function. APP is likely to be involved in neural stem cell development, neuronal survival, neurite outgrowth and neurorepair. However, the mechanisms by which APP exerts its actions remain to be elucidated. The available evidence suggests that APP interacts both intracellularly and extracellularly to regulate various signal transduction mechanisms.

Neprilysin and Aβ Clearance: Impact of the APP Intracellular Domain in NEP Regulation and Implications in Alzheimer's Disease

One of the characteristic hallmarks of Alzheimer's disease (AD) is an accumulation of amyloid β (Aβ) leading to plaque formation and toxic oligomeric Aβ complexes. Besides the de novo synthesis of Aβ caused by amyloidogenic processing of the amyloid precursor protein (APP), Aβ levels are also highly dependent on Aβ degradation. Several enzymes are described to cleave Aβ. In this review we focus on one of the most prominent Aβ degrading enzymes, the zinc-metalloprotease Neprilysin (NEP). In the first part of the review we discuss beside the general role of NEP in Aβ degradation the alterations of the enzyme observed during normal aging and the progression of AD. In vivo and cell culture experiments reveal that a decreased NEP level results in an increased Aβ level and vice versa. In a pathological situation like AD, it has been reported that NEP levels and activity are decreased and it has been suggested that certain polymorphisms in the NEP gene result in an increased risk for AD. Conversely, increasing NEP activity in AD mouse models revealed an improvement in some behavioral tests. Therefore it has been suggested that increasing NEP might be an interesting potential target to treat or to be protective for AD making it indispensable to understand the regulation of NEP. Interestingly, it is discussed that the APP intracellular domain (AICD), one of the cleavage products of APP processing, which has high similarities to Notch receptor processing, might be involved in the transcriptional regulation of NEP. However, the mechanisms of NEP regulation by AICD, which might be helpful to develop new therapeutic strategies, are up to now controversially discussed and summarized in the second part of this review. In addition, we review the impact of AICD not only in the transcriptional regulation of NEP but also of further genes.

Physiological functions of APP family proteins

Biochemical and genetic evidence establishes a central role of the amyloid precursor protein (APP) in Alzheimer disease (AD) pathogenesis. Biochemically, deposition of the β-amyloid (Aβ) peptides produced from proteolytic processing of APP forms the defining pathological hallmark of AD; genetically, both point mutations and duplications of wild-type APP are linked to a subset of early onset of familial AD (FAD) and cerebral amyloid angiopathy. As such, the biological functions of APP and its processing products have been the subject of intense investigation, and the past 20+ years of research have met with both excitement and challenges. This article will review the current understanding of the physiological functions of APP in the context of APP family members.

Systematic evaluation of candidate ligands regulating ectodomain shedding of amyloid precursor protein

Despite intense interest in the proteolysis of the β-Amyloid Precursor Protein (APP) in Alzheimer's disease, how the normal processing of this type I receptor-like glycoprotein is physiologically regulated remains ill-defined. In recent years, several candidate protein ligands for APP, including F-spondin, Reelin, β1 Integrin, Contactins, Lingo-1, and Pancortin, have been reported. However, a cognate ligand for APP that regulates its processing by α- or β-secretase has yet to be widely confirmed in multiple laboratories. Here, we developed new assays in an effort to confirm a role for one or more of these candidate ligands in regulating APP ectodomain shedding in a biologically relevant context. A comprehensive quantification of APPsα and APPsβ, the immediate products of secretase processing, in both non-neuronal cell lines and primary neuronal cultures expressing endogenous APP yielded no evidence that any of these published candidate ligands stimulate ectodomain shedding. Rather, Reelin, Lingo-1, and Pancortin-1 emerged as the most consistent ligands for significantly inhibiting ectodomain shedding. These findings led us to conduct further detailed analyses of the interactions of Reelin and Lingo-1 with APP.

Fe65 matters: new light on an old molecule

The discovery that the main constituents of amyloid deposits, characteristic of Alzheimer neuropathology, derive from the proteolytic processing of the membrane precursor amyloid precursor protein (APP) is one of the milestones of the research history of this disease. Despite years of intense studies, the functions of APP and of its amyloidogenic processing are still under debate. One focus of these studies was the complex network of protein-protein interactions centered at the cytosolic domain of APP, which suggests the involvement of APP in a lively signaling pathway. Fe65 was the first protein to be demonstrated to interact with the APP cytodomain. Starting from this observation, a large body of data has been gathered, indicating that Fe65 is an adaptor protein, which binds numerous proteins, further than APP. Among these proteins, the crosstalk with Mena, mDab, and Abl suggested the involvement of the Fe65-APP complex in the regulation of cell motility, with a relevant role in differentiation and development. Other partners, like the histone acetyltransferase Tip60, indicated the possibility that the nuclear fraction of Fe65 could be involved in gene regulation and/or DNA repair.

Immunoreactivity of the amino-terminal portion of the amyloid-beta precursor protein in the nucleolus

The functioning and metabolic pathway of the amyloid β-precursor protein (APP) have not been fully elucidated. To fill this research gap, this study immunocytochemically investigated the intracellular localization of APP in the neuroblastoma cell line SK-N-SH and in normal primary cells. Using antibodies against the amino-terminal portion of the APP molecule, immunoreactivity was detected not only in the cytoplasm but also in the nucleus and nucleolus. Further analysis revealed the co-localization of amino acids 44-63 of the APP molecule with fibrillarin, a nucleolus marker. These findings indicate that a fraction of APP, including its amino-terminal portion, may be localized in the nucleus as well as in the nucleolus, suggesting an important role of APP in RNA metabolism and other intra-nucleolus functions.

Stress-induced cell-cycle activation in Tip60 haploinsufficient adult cardiomyocytes

Background

Tat-interactive protein 60 (Tip60) is a member of the MYST family of histone acetyltransferases. Studies using cultured cells have shown that Tip60 has various functions including DNA repair, apoptosis and cell-cycle regulation. We globally ablated the Tip60 gene (Htatip), observing that Tip60-null embryos die at the blastocyst stage (Hu et al. Dev.Dyn.238:2912;2009). Although adult heterozygous (Tip60^+/−) mice reproduce normally without a haploinsufficient phenotype, stress caused by Myc over-expression induced B-cell lymphoma in Tip60^+/− adults, suggesting that Tip60 is a tumor suppressor (Gorrini et al. Nature 448:1063;2007). These findings prompted assessment of whether Tip60, alternative splicing of which generates two predominant isoforms termed Tip60α and Tip60β, functions to suppress the cell-cycle in adult cardiomyocytes.

Methodology/Principal Findings

Western blotting revealed that Tip60α is the predominant Tip60 isoprotein in the embryonic heart, transitioning at neonatal stages to Tip60β, which is the only isoprotein in the adult heart wherein it is highly enriched. Over-expression of Tip60β, but not Tip60α, inhibited cell proliferation in NIH3T3 cells; and, Tip60-haploinsufficient cultured neonatal cardiomyocytes exhibited increased cell-cycle activity. To address whether Tip60β suppresses the cardiomyocyte cell-cycle in the adult heart, hypertrophic stress was induced in Tip60^+/+ and Tip^+/− littermates via two methods, Myc over-expression and aortic banding. Based on immunostaining cell-cycle markers and western blotting cyclin D, stress increased cardiomyocyte cell-cycle mobilization in Tip60^+/− hearts, in comparison with Tip60^+/+ littermates. Aortic-banded Tip60^+/− hearts also exhibited significantly decreased apoptosis.

Conclusions/Significance

These findings provide evidence that Tip60 may function in a tumor suppressor pathway(s) to maintain adult cardiomyocytes in replicative senescence.

The amyloid precursor protein intracellular domain-fe65 multiprotein complexes: a challenge to the amyloid hypothesis for Alzheimer's disease?

Since its proposal in 1994, the amyloid cascade hypothesis has prevailed as the mainstream research subject on the molecular mechanisms leading to the Alzheimer's disease (AD). Most of the field had been historically based on the role of the different forms of aggregation of β-amyloid peptide (Aβ). However, a soluble intracellular fragment termed amyloid precursor protein (APP) intracellular domain (AICD) is produced in conjunction with Aβ fragments. This peptide had been shown to be highly toxic in both culture neurons and transgenic mice models. With the advent of this new toxic fragment, the centerpiece for the ethiology of the disease may be changed. This paper discusses the potential role of multiprotein complexes between the AICD and its adapter protein Fe65 and how this could be a potentially important new agent in the neurodegeneration observed in the AD.

Neurodegeneration in Alzheimer disease: role of amyloid precursor protein and presenilin 1 intracellular signaling

Alzheimer disease (AD) is a heterogeneous neurodegenerative disorder characterized by (1) progressive loss of synapses and neurons, (2) intracellular neurofibrillary tangles, composed of hyperphosphorylated Tau protein, and (3) amyloid plaques. Genetically, AD is linked to mutations in few proteins amyloid precursor protein (APP) and presenilin 1 and 2 (PS1 and PS2). The molecular mechanisms underlying neurodegeneration in AD as well as the physiological function of APP are not yet known. A recent theory has proposed that APP and PS1 modulate intracellular signals to induce cell-cycle abnormalities responsible for neuronal death and possibly amyloid deposition. This hypothesis is supported by the presence of a complex network of proteins, clearly involved in the regulation of signal transduction mechanisms that interact with both APP and PS1. In this review we discuss the significance of novel finding related to cell-signaling events modulated by APP and PS1 in the development of neurodegeneration.

The physiology of the β-amyloid precursor protein intracellular domain AICD

The amyloid-β precursor protein (βAPP) undergoes several cleavages by enzymatic activities called secretases. Numerous studies aimed at studying the biogenesis and catabolic fate of Aβ peptides, the proteinaceous component of the senile plaques that accumulate in Alzheimer's disease-affected brains. Relatively recently, another secretase-mediated β-APP-derived catabolite called APP IntraCellular Domain (AICD) entered the game. Whether AICD corresponded to a biologically inert by-pass product of βAPP processing or whether it could harbor its own function remained questionable. In this study, we review the mechanisms by which AICD is generated and how its production is regulated. Furthermore, we discuss the degradation mechanism underlying its rapid catabolic fate. Finally, we review putative AICD-related functions and more particularly, the numerous studies indicating that AICD could translocate to the nucleus and control at a transcriptional level, the expression of a series of proteins involved in various functions including the control of cell death and Aβ degradation.

Proteolytic processing of Alzheimer's β-amyloid precursor protein

β-Amyloid precursor protein (APP) is a critical factor in the pathogenesis of Alzheimer's disease (AD). APP undergoes post-translational proteolysis/processing to generate the hydrophobic β-amyloid (Aβ) peptides. Deposition of Aβ in the brain, forming oligomeric Aβ and plaques, is identified as one of the key pathological hallmarks of AD. The processing of APP to generate Aβ is executed by β- and γ-secretase and is highly regulated. Aβ toxicity can lead to synaptic dysfunction, neuronal cell death, impaired learning/memory and abnormal behaviors in AD models in vitro and in vivo. Aside from Aβ, proteolytic cleavages of APP can also give rise to the APP intracellular domain, reportedly involved in multiple types of cellular events such as gene transcription and apoptotic cell death. In addition to amyloidogenic processing, APP can also be cleaved by α-secretase to form a soluble or secreted APP ectodomain (sAPP-α) that has been shown to be mostly neuro-protective. In this review, we describe the mechanisms involved in APP metabolism and the likely functions of its various proteolytic products to give a better understanding of the patho/physiological functions of APP.

Amyloid precursor protein binding protein Fe65 is cleaved by caspases during DNA damage-induced apoptosis

Caspases cleave several cellular proteins to execute cell death by apoptosis. The identification of novel substrates of caspases could provide an important clue for elucidation of new apoptosis signaling pathways. In this study, we tested whether an amyloid precursor protein (APP) binding protein Fe65 is proteolytically degraded in neuronal cell death by apoptosis, using a neuron-like cell line, human neuroblastoma SH-SY5Y cells. When treated with DNA damaging agents, etoposide (ETP) and camptothecin (CPT), SH-SY5Y cells underwent apoptosis in a dose-dependent manner. Interestingly, Fe65 (97 kDa) was cleaved to a 65 kDa product during DNA damage-induced apoptosis. Furthermore, the cleavage of Fe65 was accompanied by activation of caspases-9 and -3. The restriction cleavage of Fe65 was completely suppressed by the treatment with a pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(OMe) fluoromethylketone (z-VAD-fmk). These results reveal the restriction cleavage of Fe65 by caspases during DNA damage-induced apoptosis. Since Fe65 has been shown to suppress APP processing to amyloid β (Aβ) production, our findings may provide a new insight into the molecular mechanism by which DNA damage induces Aβ production and subsequent neuronal cell death in Alzheimer's disease (AD).

Biology and pathophysiology of the amyloid precursor protein

The amyloid precursor protein (APP) plays a central role in the pathophysiology of Alzheimer's disease in large part due to the sequential proteolytic cleavages that result in the generation of β-amyloid peptides (Aβ). Not surprisingly, the biological properties of APP have also been the subject of great interest and intense investigations. Since our 2006 review, the body of literature on APP continues to expand, thereby offering further insights into the biochemical, cellular and functional properties of this interesting molecule. Sophisticated mouse models have been created to allow in vivo examination of cell type-specific functions of APP together with the many functional domains. This review provides an overview and update on our current understanding of the pathobiology of APP.

Dysregulation of Ca2+ signaling in astrocytes from mice lacking amyloid precursor protein

The relationship between altered metabolism of the amyloid-β precursor protein (APP) and Alzheimer's disease is well established but the physiological roles of APP still remain unclear. Here, we studied Ca(2+) signaling in primary cultured and freshly dissociated cortical astrocytes from APP knockout (KO) mice and from Tg5469 mice overproducing by five- to sixfold wild-type APP. Resting cytosolic Ca(2+) (measured with fura-2) was not altered in cultured astrocytes from APP KO mice. The stored Ca(2+) evaluated by measuring peak amplitude of cyclopiazonic acid [CPA, endoplasmic reticulum (ER) Ca(2+) ATPase inhibitor]-induced Ca(2+) transients in Ca(2+)-free medium was significantly smaller in APP KO astrocytes than in wild-type cells. Store-operated Ca(2+) entry (SOCE) activated by ER Ca(2+) store depletion with CPA was also greatly reduced in APP KO astrocytes. This reflected a downregulated expression in APP KO astrocytes of TRPC1 (C-type transient receptor potential) and Orai1 proteins, essential components of store-operated channels (SOCs). Indeed, silencer RNA (siRNA) knockdown of Orai1 protein expression in wild-type astrocytes significantly attenuated SOCE. SOCE was also essentially reduced in freshly dissociated APP KO astrocytes. Importantly, knockdown of APP with siRNA in cultured wild-type astrocytes markedly attenuated ATP- and CPA-induced ER Ca(2+) release and extracellular Ca(2+) influx. The latter correlated with downregulation of TRPC1. Overproduction of APP in Tg5469 mice did not alter, however, the stored Ca(2+) level, SOCE, and expression of TRPC1/4/5 in cultured astrocytes from these mice. The data demonstrate that the functional role of APP in astrocytes involves the regulation of TRPC1/Orai1-encoded SOCs critical for Ca(2+) signaling.

Reciprocal relationship between APP positioning relative to the membrane and PS1 conformation

Background

Several familial Alzheimer disease (FAD) mutations within the transmembrane region of the amyloid precursor protein (APP) increase the Aβ_42/40 ratio without increasing total Aβ production. In the present study, we analyzed the impact of FAD mutations and γ-secretase modulators (GSMs) that alter the Aβ_42/40 ratio on APP C-terminus (CT) positioning relative to the membrane, reasoning that changes in the alignment of the APP intramembranous domain and presenilin 1 (PS1) may impact the PS1/γ-secretase cleavage site on APP.

Results

By using a Förster resonance energy transfer (FRET)-based technique, fluorescent lifetime imaging microscopy (FLIM), we show that Aβ_42/40 ratio-modulating factors which target either APP substrate or PS1/γ-secretase affect proximity of the APP-CT to the membrane and change PS1 conformation.

Conclusions

Thus, we propose that there is a reciprocal relationship between APP-CT positioning relative to the membrane and PS1 conformation, suggesting that factors that modulate either APP positioning in the membrane or PS1 conformation could be exploited therapeutically.

The tale of protein lysine acetylation in the cytoplasm

Reversible posttranslational modification of internal lysines in many cellular or viral proteins is now emerging as part of critical signalling processes controlling a variety of cellular functions beyond chromatin and transcription. This paper aims at demonstrating the role of lysine acetylation in the cytoplasm driving and coordinating key events such as cytoskeleton dynamics, intracellular trafficking, vesicle fusion, metabolism, and stress response.

Phosphorylation of APP-CTF-AICD domains and interaction with adaptor proteins: signal transduction and/or transcriptional role--relevance for Alzheimer pathology

In recent decades, the study of the amyloid precursor protein (APP) and of its proteolytic products carboxy terminal fragment (CTF), APP intracellular C-terminal domain (AICD) and amyloid beta has been mostly focussed on the role of APP as a producer of the toxic amyloid beta peptide. Here, we reconsider the role of APP suggesting, in a provocative way, the protein as a central player in a putative signalling pathway. We highlight the presence in the cytosolic tail of APP of the YENPTY motif which is typical of tyrosine kinase receptors, the phosphorylation of the tyrosine, serine and threonine residues, the kinases involved and the interaction with intracellular adaptor proteins. In particular, we examine the interaction with Shc and Grb2 regulators, which through the activation of Ras proteins elicit downstream signalling events such as the MAPK pathway. The review also addresses the interaction of APP, CTFs and AICD with other adaptor proteins and in particular with Fe65 for nuclear transcriptional activity and the importance of phosphorylation for sorting the secretases involved in the amyloidogenic or non-amyloidogenic pathways. We provide a novel perspective on Alzheimer's disease pathogenesis, focussing on the perturbation of the physiological activities of APP-CTFs and AICD as an alternative perspective from that which normally focuses on the accumulation of neurotoxic proteolytic fragments.

Cell-type dependent modulation of Notch signaling by the amyloid precursor protein

The amyloid precursor protein is a ubiquitously expressed transmembrane protein that has been long implicated in the pathogenesis of Alzheimer's disease but its normal biological function has remained elusive despite extensive effort. We have previously reported the identification of Notch2 as an amyloid precursor protein interacting protein in E18 rat neurons. Here, we sought to reveal the physiologic consequences of this interaction. We report a functional relationship between amyloid precursor protein and Notch1, which does not affect Delta ligand binding. First, we observed interactions between the amyloid precursor protein and Notch in mouse embryonic stem cells lacking both presenilin 1 and presenilin 2, the active proteolytic components of the gamma-secretase complex, suggesting that these two transmembrane proteins can interact in the absence of presenilin. Next, we demonstrated that the amyloid precursor protein affects Notch signaling by using Notch-dependent luciferase assays in two cell lines, the human embryonic kidney 293 and the monkey kidney, COS7. We found that the amyloid precursor protein exerts opposing effects on Notch signaling in human embryonic kidney 293 vs. COS7 cells. Finally, we show that more Notch Intracellular Domain is found in the nucleus in the presence of exogenous amyloid precursor protein or its intracellular domain, suggesting the mechanism by which the amyloid precursor protein affects Notch signaling in certain cells. Our results provide evidence of potentially important communications between the amyloid precursor protein and Notch.

Structural and functional characterization of a novel FE65 protein product up-regulated in cognitively impaired FE65 knockout mice

FE65 is a multi-modular adaptor protein that binds the cytoplasmic tail of the beta-amyloid precursor protein (APP). Genetic evidence suggests that APP is intimately involved in the pathogenesis of dementias of the Alzheimer type, neurodegenerative disorders that affect multiple cognitive domains, including learning and memory. Evidence from p97FE65-specific knockout mice (lacking the 97 kDa full-length FE65 protein, p97FE65) suggests an important role for FE65 in learning and memory. Interpretation of the learning and memory phenotype, however, is complicated by the up-regulation (compared with wild-type mice) of a novel 60 kDa FE65 isoform (p60FE65). Here, we report an evidence that p60FE65 is translated from an alternative methionine, M261, on the p97FE65 transcript. Thus, p60FE65 has a shortened N-terminus, lacking part of the WW domain that is considered important for nuclear translocation and transactivation of gene expression. Consistently, p60FE65 exhibits an attenuated ability for APP-Gal4-mediated transcription as compared with p97FE65. Similar to p97FE65, however, both transfected and endogenous p60FE65 are able to translocate to the nucleus in cultured cells and in neurons. These results are consistent with earlier evidence from our laboratory that reduced FE65 nuclear signaling may contribute, in part, to the phenotypes observed in p97FE65 knockout mice.

Endogenous Aβ causes cell death via early tau hyperphosphorylation

Alzheimer's disease (AD) is characterized by Aβ overproduction and tau hyperphosphorylation. We report that an early, transient and site-specific AD-like tau hyperphosphorylation at Ser262 and Thr231 epitopes is temporally and causally related with an activation of the endogenous amyloidogenic pathway that we previously reported in hippocampal neurons undergoing cell death upon NGF withdrawal [Matrone, C., Ciotti, M.T., Mercanti, D., Marolda, R., Calissano, P., 2008b. NGF and BDNF signaling control amyloidogenic route and Ab production in hippocampal neurons. Proc. Natl. Acad. Sci. 105, 13138-13143]. Such tau hyperphosphorylation, as well as apoptotic death, is (i) blocked by 4G8 and 6E10 Aβ antibodies or by specific β and/or γ-secretases inhibitors; (ii) temporally precedes tau cleavage mediated by a delayed (6-12h after NGF withdrawal) activation of caspase-3 and calpain-I; (iii) under control of Akt-GSK3β-mediated signaling. Finally, we show that such site-specific tau hyperphosphorylation causes tau detachment from microtubules and an impairment of mitochondrial trafficking. These results depict, for the first time, a rapid interplay between endogenous Aβ and tau post-translational modifications which act co-ordinately to compromise neuronal functions in the same neuronal system, under physiological conditions as seen in AD brain.

FE65 binds Teashirt, inhibiting expression of the primate-specific caspase-4

The Alzheimer disease (AD) amyloid protein precursor (APP) can bind the FE65 adaptor protein and this complex can regulate gene expression. We carried out yeast two-hybrid studies with a PTB domain of FE65, focusing on those genes that might be involved in nuclear signaling, and identified and validated Teashirt proteins as FE65 interacting proteins in neurons. Using reporter systems, we observed that FE65 could simultaneously recruit SET, a component of the inhibitor of acetyl transferase, and Teashirt, which in turn recruited histone deacetylases, to produce a powerful gene-silencing complex. We screened stable cell lines with a macroarray focusing on AD-related genes and identified CASP4, encoding caspase-4, as a target of this silencing complex. Chromatin immunoprecipitation showed a direct interaction of FE65 and Teashirt3 with the promoter region of CASP4. Expression studies in postmortem samples demonstrated decreasing expression of Teashirt and increasing expression of caspase-4 with progressive cognitive decline. Importantly, there were significant increases in caspase-4 expression associated with even the earliest neuritic plaque changes in AD. We evaluated a case-control cohort and observed evidence for a genetic association between the Teashirt genes TSHZ1 and TSHZ3 and AD, with the TSHZ3 SNP genotype correlating with expression of Teashirt3. The results were consistent with a model in which reduced expression of Teashirt3, mediated by genetic or other causes, increases caspase-4 expression, leading to progression of AD. Thus the cell biological, gene expression and genetic data support a role for Teashirt/caspase-4 in AD biology. As caspase-4 shows evidence of being a primate-specific gene, current models of AD and other neurodegenerative conditions may be incomplete because of the absence of this gene in the murine genome.

Loss of gamma-secretase function impairs endocytosis of lipoprotein particles and membrane cholesterol homeostasis

Presenilins (PSs) are components of the gamma-secretase complex that mediates intramembranous cleavage of type I membrane proteins. We show that gamma-secretase is involved in the regulation of cellular lipoprotein uptake. Loss of gamma-secretase function decreased endocytosis of low-density lipoprotein (LDL) receptor. The decreased uptake of lipoproteins led to upregulation of cellular cholesterol biosynthesis by increased expression of CYP51 and enhanced metabolism of lanosterol. Genetic deletion of PS1 or transgenic expression of PS1 mutants that cause early-onset Alzheimer's disease led to accumulation of gamma-secretase substrates and mistargeting of adaptor proteins that regulate endocytosis of the LDL receptor. Consistent with decreased endocytosis of these receptors, PS1 mutant mice have elevated levels of apolipoprotein E in the brain. Thus, these data demonstrate a functional link between two major genetic factors that cause early-onset and late-onset Alzheimer's disease.

Presenilin: RIP and beyond

Over the years the presenilins (PSENs), a family of multi-transmembrane domain proteins, have been ascribed a number of diverse potential functions. Recent in vivo evidence has supported the existence of PSEN functions beyond its well-established role in regulated intramembrane proteolysis. In this review, we will briefly discuss the ability of PSEN to modulate cellular signaling pathways through gamma-secretase cleavage of transmembrane proteins. Additionally, we will critically examine the proposed roles of PSEN in the regulation of beta-catenin function, protein trafficking, calcium regulation, and apoptosis.

Dexras1 interacts with FE65 to regulate FE65-amyloid precursor protein-dependent transcription

FE65 is an adaptor protein that binds to and forms a transcriptionally active complex with the gamma-secretase-derived amyloid precursor protein (APP) intracellular domain. The regulatory mechanisms of FE65-APP-mediated transcription are still not clear. In this report, we demonstrate that Dexras1, a Ras family small G protein, binds to FE65 PTB2 domain and potently suppresses the FE65-APP-mediated transcription. The suppression is not via competition for binding of FE65 between Dexras1 and APP because the two proteins can simultaneously bind to the FE65 PTB2 domain. Phosphorylation of FE65 tyrosine 547 within the PTB2 domain has been shown to enhance FE65-APP-mediated transcription but not to influence binding to APP. Here we find that this phosphorylation event reduces the binding between Dexras1 and FE65. We also demonstrate that Dexras1 inhibits the FE65-APP-mediated transcription of glycogen synthase kinase 3beta (GSK3 beta). Moreover, small interfering RNA knockdown of Dexras1 enhances GSK3 beta expression and increases phosphorylation of Tau, a GSK3 beta substrate. Thus, Dexras1 functions as a suppressor of FE65-APP-mediated transcription, and FE65 tyrosine 547 phosphorylation enhances FE65-APP-mediated transcription, at least in part, by modulating the interaction between FE65 and Dexras1. These findings reveal a novel regulatory mechanism for FE65-APP-mediated signaling.

SERCA pump activity is physiologically regulated by presenilin and regulates amyloid beta production

In addition to disrupting the regulated intramembraneous proteolysis of key substrates, mutations in the presenilins also alter calcium homeostasis, but the mechanism linking presenilins and calcium regulation is unresolved. At rest, cytosolic Ca²⁺ is maintained at low levels by pumping Ca²⁺ into stores in the endoplasmic reticulum (ER) via the sarco ER Ca²⁺-ATPase (SERCA) pumps. We show that SERCA activity is diminished in fibroblasts lacking both PS1 and PS2 genes, despite elevated SERCA2b steady-state levels, and we show that presenilins and SERCA physically interact. Enhancing presenilin levels in Xenopus laevis oocytes accelerates clearance of cytosolic Ca²⁺, whereas higher levels of SERCA2b phenocopy PS1 overexpression, accelerating Ca²⁺ clearance and exaggerating inositol 1,4,5-trisphosphate–mediated Ca²⁺ liberation. The critical role that SERCA2b plays in the pathogenesis of Alzheimer's disease is underscored by our findings that modulating SERCA activity alters amyloid β production. Our results point to a physiological role for the presenilins in Ca²⁺ signaling via regulation of the SERCA pump.

The FE65 proteins and Alzheimer's disease

The FE65s (FE65, FE65L1, and FE65L2) are a family of multidomain adaptor proteins that form multiprotein complexes with a range of functions. FE65 is brain-enriched, whereas FE65L1 and FE65L2 are more widely expressed. All three members contain a WW domain and two PTB domains. Through the PTB2 domain, they all interact with the Alzheimer's disease amyloid precursor protein (APP) intracellular domain (AICD) and can alter APP processing. After sequential proteolytic processing of membrane-bound APP and release of AICD to the cytoplasm, FE65 can translocate to the nucleus to participate in gene transcription events. This role is further mediated by interactions of FE65 PTB1 with the transcription factors CP2/LSF/LBP1 and Tip60 and the WW domain with the nucleosome assembly factor SET. However, FE65 target genes have not yet been confirmed. The FE65 PTB1 domain also interacts with two cell surface lipoproteins receptors, the low-density lipoprotein receptor-related protein (LRP) and ApoEr2, forming trimeric complexes with APP. The FE55 WW domain also binds to mena, through which it functions in regulation of the actin cytoskeleton, cell motility, and neuronal growth cone formation. While single knockout mice appear normal, double FE65(-/-)/FE65L1(-/-) mice have substantial neurodevelopmental defects. These include heterotopic neurons and axonal pathfinding defects, findings similar to findings in both Mena and triple APP:APLP1:APLP2 knockout mice and also lissencephalopathies in humans. Thus APPs, FE65s, and mena may act together in a developmental signalling pathway. This article reviews the known functions of the FE65 family and their role in APP function and Alzheimer's disease.

The amyloid precursor protein intracellular domain (AICD) as modulator of gene expression, apoptosis, and cytoskeletal dynamics-relevance for Alzheimer's disease

Since the discovery of the amyloid precursor protein (APP) in 1987, extensive research has been conducted analyzing the APP-derived beta-amyloid (Abeta) which is found in massive quantities in senile plaques of Alzheimer disease (AD) patients. Numerous studies over the last two decades have demonstrated the neurotoxic properties of Abeta. However, it is still unclear whether Abeta neurotoxicity is an initial cause or rather a late event in the pathophysiology of AD. The understanding of preclinical AD-related pathophysiological mechanisms is of significant interest in the identification of potential pharmacological targets. In this context another APP-derived cleavage product, the amyloid precursor protein intracellular domain (AICD), has sparked considerable research interest over the last 7 years. Different AICD levels as a result of gamma-secretase activity may contribute to early pathophysiological mechanisms in AD. However, the relevance of AICD is being discussed highly controversially amongst AD researchers. This review summarizes recent findings in terms of the origin of AICD by regulated intramembrane proteolysis; its structure, binding factors, and post-translational modifications; and its putative role in gene transcription, apoptosis, and cytoskeletal dynamics.

Interaction of amyloid precursor protein with contactins and NgCAM in the retinotectal system

The amyloid precursor protein (APP) plays a central role in Alzheimer's disease, but its actions in normal development are not well understood. Here, a tagged APP ectodomain was used to identify extracellular binding partners in developing chick brain. Prominent binding sites were seen in the olfactory bulb and on retinal axons growing into the optic tectum. Co-precipitation from these tissues and tandem mass spectrometry led to the identification of two associated proteins: contactin 4 and NgCAM. In vitro binding studies revealed direct interactions among multiple members of the APP and contactin protein families. Levels of the APP processing fragment, CTFalpha, were modulated by both contactin 4 and NgCAM. In the developing retinotectal system, APP, contactin 4 and NgCAM are expressed in the retina and tectum in suitable locations to interact. Functional assays revealed regulatory effects of both APP and contactin 4 on NgCAM-dependent growth of cultured retinal axons, demonstrating specific functional interactions among these proteins. These studies identify novel binding and functional interactions among proteins of the APP, contactin and L1CAM families, with general implications for mechanisms of APP action in neural development and disease.

A critical function for beta-amyloid precursor protein in neuronal migration revealed by in utero RNA interference

Physiological processing of the beta-amyloid precursor protein (APP) generates amyloid beta-protein, which can assemble into oligomers that mediate synaptic failure in Alzheimer's disease. Two decades of research have led to human trials of compounds that chronically target this processing, and yet the normal function of APP in vivo remains unclear. We used the method of in utero electroporation of shRNA constructs into the developing cortex to acutely knock down APP in rodents. This approach revealed that neuronal precursor cells in embryonic cortex require APP to migrate correctly into the nascent cortical plate. cDNAs encoding human APP or its homologues, amyloid precursor-like protein 1 (APLP1) or APLP2, fully rescued the shRNA-mediated migration defect. Analysis of an array of mutations and deletions in APP revealed that both the extracellular and cytoplasmic domains of APP are required for efficient rescue. Whereas knock-down of APP inhibited cortical plate entry, overexpression of APP caused accelerated migration of cells past the cortical plate boundary, confirming that normal APP levels are required for correct neuronal migration. In addition, we found that Disabled-1 (Dab1), an adaptor protein with a well established role in cortical cell migration, acts downstream of APP for this function in cortical plate entry. We conclude that full-length APP functions as an important factor for proper migration of neuronal precursors into the cortical plate during the development of the mammalian brain.

A TAG on to the neurogenic functions of APP

The proteolytic processing of amyloid beta precursor protein (APP) has long been studied because of its association with the pathology of Alzheimer's disease (AD). The ectodomain of APP is shed by alpha- or beta-secretase cleavage. The remaining membrane bound stub can then undergo regulated intramembrane proteolysis (RIP) by gamma-secretase. This cleavage can release amyloid beta (Abeta) from the stub left by beta-secretase cleavage but also releases the APP intracellular domain (AICD) after alpha- or beta-secretase cleavage. The physiological functions of this proteolytic processing are not well understood. We compare the proteolytic processing of APP to the ligand-dependent RIP of Notch. In this review, we discuss recent evidence suggesting that TAG1 is a functional ligand for APP. The interaction between TAG1 and APP triggers gamma-secretase-dependent release of AICD. TAG1, APP and Fe65 colocalise in the neurogenic ventricular zone and in fetal neural progenitor cells in vitro. Experiments in TAG1, APP and Fe65 null mice as well as TAG1 and APP double-null mice demonstrate that TAG1 induces a gamma-secretase- and Fe65-dependent suppression of neurogenesis.

Tip60 modulates PLAGL2-mediated transactivation by acetylation

Pleiomorphic adenoma gene (PLAG) family proteins are oncogenes involved in various malignancies including lipoblastomas, hepatoblastomas, and acute myeloid leukemia. Overexpression of PLAGL2 induces cell transformation and proliferation, but little is known about how its activities are regulated. We previously showed that transcriptional activity of PLAGL2 is negatively regulated by sumoylation. Here we report that Tip60 modulates PLAGL2 functions through acetylation. Tip60 associates with PLAGL2 through its zinc finger domain and acetylates PLAGL2. Wild-type but not the histone acetyltransferase (HAT)-minus mutant form of Tip60 enhances PLAGL2-mediated transactivation. In addition, coexpression of Tip60 and PLAGL2 completely abolishes the sumoylation of PLAGL2. Both Tip60 and DN-Ubc9 increase transactivation activity of wild-type but not the sumoylation deficient form of PLAGL2 (K250, 269, 356R), indicating that Tip60 acetylates PLAGL2 and abolishes the sumoylation of PLAGL2 possibly through modification of the same lysine residues (K250, 269, 356) within PLAGL2. Tip60 effects vary between different PLAGL2 target gene promoters, suggesting that Tip60 is a novel promoter-specific coactivator of PLAGL2. This is the first demonstration that Tip60 can function as a sumoylation inhibitor in part through its intrinsic acetyltransferase activity to regulate specific gene expression.

Fe65 Stimulates Proteolytic Liberation of the β-Amyloid Precursor Protein Intracellular Domain

The beta-amyloid precursor protein (APP)-binding protein Fe65 is involved in APP nuclear signaling and several steps in APP proteolytic processing. In this study, we show that Fe65 stimulates gamma-secretase-mediated liberation of the APP intracellular domain (AICD). The mechanism of Fe65-mediated stimulation of AICD formation appears to be through enhanced production of the carboxyl-terminal fragment substrates of gamma-secretase and direct stimulation of processing by gamma-secretase. The stimulatory capacity of Fe65 is isoform-dependent, as the non-neuronal and a2 isoforms promote APP processing more effectively than the exon 9 inclusive neuronal form of Fe65. Intriguingly, Fe65 stimulation of AICD production appears to be inversely related to pathogenic beta-amyloid production as the Fe65 isoforms profoundly stimulate AICD production and simultaneously decrease Abeta42 production. Despite the capacity of Fe65 to stimulate gamma-secretase-mediated APP proteolysis, it does not rescue the loss of proteolytic function associated with the presenilin-1 familial Alzheimer disease mutations. These data suggest that Fe65 regulation of APP proteolysis may be integrally associated with its nuclear signaling function, as all antecedent proteolytic steps prior to release of Fe65 from the membrane are fostered by the APP-Fe65 interaction.

Amyloid precursor protein and presenilin involvement in cell signaling

To date the most relevant role for the amyloid precursor protein (APP) and for the presenilins (PSs) on Alzheimer's disease (AD) genesis is linked to the 'amyloid hypothesis', which considers an aberrant formation of amyloid-beta peptides the cause of neurodegeneration. In this view, APP is merely a substrate, cleaved by the gamma-secretase complex to form toxic amyloid peptides, PSs are key players in gamma-secretase complex, and corollary or secondary events are Tau-linked pathology and gliosis. A second theory, complementary to the amyloid hypothesis, proposes that APP and PSs may modulate a yet unclear cell signal, the disruption of which may induce cell-cycle abnormalities, neuronal death, eventually amyloid formation and finally dementia. This hypothesis is supported by the presence of a complex network of proteins, with a clear relevance for signal transduction mechanisms, which interact with APP or PSs. In this scenario, the C-terminal domain of APP has a pivotal role due to the presence of the 682YENPTY687 motif that represents the docking site for multiple interacting proteins involved in cell signaling. In this review we discuss the significance of novel findings related to cell signaling events modulated by APP and PSs for AD development.

Gleevec increases levels of the amyloid precursor protein intracellular domain and of the amyloid-beta degrading enzyme neprilysin

Amyloid-beta (Abeta) deposition is a major pathological hallmark of Alzheimer's disease. Gleevec, a known tyrosine kinase inhibitor, has been shown to lower Abeta secretion, and it is considered a potential basis for novel therapies for Alzheimer's disease. Here, we show that Gleevec decreases Abeta levels without the inhibition of Notch cleavage by a mechanism distinct from gamma-secretase inhibition. Gleevec does not influence gamma-secretase activity in vitro; however, treatment of cell lines leads to a dose-dependent increase in the amyloid precursor protein intracellular domain (AICD), whereas secreted Abeta is decreased. This effect is observed even in presence of a potent gamma-secretase inhibitor, suggesting that Gleevec does not activate AICD generation but instead may slow down AICD turnover. Concomitant with the increase in AICD, Gleevec leads to elevated mRNA and protein levels of the Abeta-degrading enzyme neprilysin, a potential target gene of AICD-regulated transcription. Thus, the Gleevec mediated-increase in neprilysin expression may involve enhanced AICD signaling. The finding that Gleevec elevates neprilysin levels suggests that its Abeta-lowering effect may be caused by increased Abeta-degradation.

An AICD-based functional screen to identify APP metabolism regulators

Background

A central event in Alzheimer's disease (AD) is the regulated intramembraneous proteolysis of the β-amyloid precursor protein (APP), to generate the β-amyloid (Aβ) peptide and the APP intracellular domain (AICD). Aβ is the major component of amyloid plaques and AICD displays transcriptional activation properties. We have taken advantage of AICD transactivation properties to develop a genetic screen to identify regulators of APP metabolism. This screen relies on an APP-Gal4 fusion protein, which upon normal proteolysis, produces AICD-Gal4. Production of AICD-Gal4 induces Gal4-UAS driven luciferase expression. Therefore, when regulators of APP metabolism are modulated, luciferase expression is altered.

Results

To validate this experimental approach we modulated α-, β-, and γ-secretase levels and activities. Changes in AICD-Gal4 levels as measured by Western blot analysis were strongly and significantly correlated to the observed changes in AICD-Gal4 mediated luciferase activity. To determine if a known regulator of APP trafficking/maturation and Presenilin1 endoproteolysis could be detected using the AICD-Gal4 mediated luciferase assay, we knocked-down Ubiquilin 1 and observed decreased luciferase activity. We confirmed that Ubiquilin 1 modulated AICD-Gal4 levels by Western blot analysis and also observed that Ubiquilin 1 modulated total APP levels, the ratio of mature to immature APP, as well as PS1 endoproteolysis.

Conclusion

Taken together, we have shown that this screen can identify known APP metabolism regulators that control proteolysis, intracellular trafficking, maturation and levels of APP and its proteolytic products. We demonstrate for the first time that Ubiquilin 1 regulates APP metabolism in the human neuroblastoma cell line, SH-SY5Y.