Fe65 and the protein network centered around the cytosolic domain of the Alzheimer's beta-amyloid precursor protein

Cholesterol and Lipid Rafts in the Biogenesis of Amyloid-β Protein and Alzheimer's Disease

Cholesterol has been conjectured to be a modulator of the amyloid cascade, the mechanism that produces the amyloid-β (Aβ) peptides implicated in the onset of Alzheimer's disease. We propose that cholesterol impacts the genesis of Aβ not through direct interaction with proteins in the bilayer, but indirectly by inducing the liquid-ordered phase and accompanying liquid-liquid phase separations, which partition proteins in the amyloid cascade to different lipid domains and ultimately to different endocytotic pathways. We explore the full process of Aβ genesis in the context of liquid-ordered phases induced by cholesterol, including protein partitioning into lipid domains, mechanisms of endocytosis experienced by lipid domains and secretases, and pH-controlled activation of amyloid precursor protein secretases in specific endocytotic environments. Outstanding questions on the essential role of cholesterol in the amyloid cascade are identified for future studies.

HSPA12A targets the cytoplasmic domain and affects the trafficking of the Amyloid Precursor Protein receptor SorLA

SorLA and Sortilin are multifunctional receptors involved in endocytosis and intracellular sorting of different and unrelated ligands. SorLA has recently attracted much attention as a novel strong risk gene for Alzheimer’s disease, and much effort is currently being put into understanding the underlying molecular mechanism. Trafficking of SorLA and Sortilin are mediated by interacting with AP-1, AP-2, GGA 1-3 and the retromer complex. Although these cytosolic adaptor proteins all bind to both SorLA and Sortilin, a large fraction of intracellular Sortilin and SorLA are located in different subcellular vesicles. This indicates that unknown specialised adaptor proteins targeting SorLA for trafficking are yet to be discovered. We have identified HSPA12A as a new adaptor protein that, among Vps10p-D receptors, selectively binds to SorLA in an ADP/ATP dependent manner. This is the first described substrate of HSPA12A, and we demonstrate that the binding, which affects both endocytic speed and subcellular localisation of SorLA, is mediated by specific acidic residues in the cytosolic domain of SorLA. The identification of the relatively unknown HSPA12A as a SorLA specific interaction partner could lead to novel insight into the molecular mechanism of SorLA, and re-emphasises the role of heat shock proteins in neurodegenerative diseases.

Structure of APP-C991-99 and implications for role of extra-membrane domains in function and oligomerization

The 99 amino acid C-terminal fragment of Amyloid Precursor Protein APP-C99 (C99) is cleaved by γ-secretase to form Aβ peptide, which plays a critical role in the etiology of Alzheimer's Disease (AD). The structure of C99 consists of a single transmembrane domain flanked by intra and intercellular domains. While the structure of the transmembrane domain has been well characterized, little is known about the structure of the flanking domains and their role in C99 processing by γ-secretase. To gain insight into the structure of full-length C99, REMD simulations were performed for monomeric C99 in model membranes of varying thickness. We find equilibrium ensembles of C99 from simulation agree with experimentally-inferred residue insertion depths and protein backbone chemical shifts. In thin membranes, the transmembrane domain structure is correlated with extra-membrane structural states and the extra-membrane domain structural states become less correlated to each other. Mean and variance of the transmembrane and G₃₇G₃₈ hinge angles are found to increase with thinning membrane. The N-terminus of C99 forms β-strands that may seed aggregation of Aβ on the membrane surface, promoting amyloid formation. In thicker membranes the N-terminus forms α-helices that interact with the nicastrin domain of γ-secretase. The C-terminus of C99 becomes more α-helical as the membrane thickens, forming structures that may be suitable for binding by cytoplasmic proteins, while C-terminal residues essential to cytotoxic function become α-helical as the membrane thins. The heterogeneous but discrete extra-membrane domain states analyzed here open the path to new investigations of the role of C99 structure and membrane in amyloidogenesis. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

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.

Fe65 negatively regulates Jagged1 signaling by decreasing Jagged1 protein stability through the E3 ligase Neuralized-like 1

Fe65 is a highly conserved adaptor protein that interacts with several binding partners. Fe65 binds proteins to mediate various cellular processes. But the interacting partner and the regulatory mechanisms controlled by Fe65 are largely unknown. In this study, we found that Fe65 interacts with the C-terminus of Jagged1. Furthermore, Fe65 negatively regulates AP1-mediated Jagged1 intercellular domain transactivation in a Tip60-independent manner. We found that Fe65 triggers the degradation of Jagged1, but not the Jagged1 intracellular domain (JICD), through both proteasome and lysosome pathways. We also showed that Fe65 promotes recruitment of the E3 ligase Neuralized-like 1 (Neurl1) to membrane-tethered Jagged1 and monoubiquitination of Jagged1. These three proteins form a stable trimeric complex, thereby decreasing Jagged1 targeting by ubiquitin-mediated degradation. Consequently, Jagged1 is a novel binding partner of Fe65, and Fe65 may act as a novel effector of Jagged1 signaling.

Tyrosine phosphorylation of WW proteins

A number of key regulatory proteins contain one or two copies of the WW domain known to mediate protein-protein interaction via proline-rich motifs, such as PPxY. The Hippo pathway components take advantage of this module to transduce tumor suppressor signaling. It is becoming evident that tyrosine phosphorylation is a critical regulator of the WW proteins. Here, we review the current knowledge on the involved tyrosine kinases and their roles in regulating the WW proteins.

Use of Caenorhabditis elegans as a model to study Alzheimer's disease and other neurodegenerative diseases

Advances in research and technology has increased our quality of life, allowed us to combat diseases, and achieve increased longevity. Unfortunately, increased longevity is accompanied by a rise in the incidences of age-related diseases such as Alzheimer’s disease (AD). AD is the sixth leading cause of death, and one of the leading causes of dementia amongst the aged population in the USA. It is a progressive neurodegenerative disorder, characterized by the prevalence of extracellular Aβ plaques and intracellular neurofibrillary tangles, derived from the proteolysis of the amyloid precursor protein (APP) and the hyperphosphorylation of microtubule-associated protein tau, respectively. Despite years of extensive research, the molecular mechanisms that underlie the pathology of AD remain unclear. Model organisms, such as the nematode, Caenorhabditis elegans, present a complementary approach to addressing these questions. C. elegans has many advantages as a model system to study AD and other neurodegenerative diseases. Like their mammalian counterparts, they have complex biochemical pathways, most of which are conserved. Genes in which mutations are correlated with AD have counterparts in C. elegans, including an APP-related gene, apl-1, a tau homolog, ptl-1, and presenilin homologs, such as sel-12 and hop-1. Since the neuronal connectivity in C. elegans has already been established, C. elegans is also advantageous in modeling learning and memory impairments seen during AD. This article addresses the insights C. elegans provide in studying AD and other neurodegenerative diseases. Additionally, we explore the advantages and drawbacks associated with using this model.

Total body 100-mGy X-irradiation does not induce Alzheimer's disease-like pathogenesis or memory impairment in mice

The cause and progression of Alzheimer's disease (AD) are poorly understood. Possible cognitive and behavioral consequences induced by low-dose radiation are important because humans are exposed to ionizing radiation from various sources. Early transcriptional response in murine brain to low-dose X-rays (100 mGy) has been reported, suggesting alterations of molecular networks and pathways associated with cognitive functions, advanced aging and AD. To investigate acute and late transcriptional, pathological and cognitive consequences of low-dose radiation, we applied an acute dose of 100-mGy total body irradiation (TBI) with X-rays to C57BL/6J Jms mice. We collected hippocampi and analyzed expression of 84 AD-related genes. Mouse learning ability and memory were assessed with the Morris water maze test. We performed in vivo PET scans with (11)C-PIB, a radiolabeled ligand for amyloid imaging, to detect fibrillary amyloid beta peptide (Aβ) accumulation, and examined characteristic AD pathologies with immunohistochemical staining of amyloid precursor protein (APP), Aβ, tau and phosphorylated tau (p-tau). mRNA studies showed significant downregulation of only two of 84 AD-related genes, Apbb1 and Lrp1, at 4 h after irradiation, and of only one gene, Il1α, at 1 year after irradiation. Spatial learning ability and memory were not significantly affected at 1 or 2 years after irradiation. No induction of amyloid fibrillogenesis or changes in APP, Aβ, tau, or p-tau expression was detected at 4 months or 2 years after irradiation. TBI induced early or late transcriptional alteration in only a few AD-related genes but did not significantly affect spatial learning, memory or AD-like pathological change in mice.

Phosphorylation of Tip60 Tyrosine 327 by Abl Kinase Inhibits HAT Activity through Association with FE65

The transfer of acetyl groups from acetyl coenzyme A to the ε amino group of internal lysine residues is catalyzed by Tip60, which is in the MYST family of nuclear histone acetyltransferases (HATs). The tyrosine phosphorylation of Tip60 seems to be a unique modification. We present evidence that Tip60 is modified on tyrosine 327 by Abl kinase. We show that this causes functional changes in HAT activity and the subcellular localization of TIP60, which forms a complex with Abl kinase. The Tip60 mutation Y327F abolished tyrosine phosphorylation, reduced the inhibition of Tip60 HAT activity, and caused G0-G1 arrest and association with FE65. Thus, our findings for the first time suggested a novel regulation mechanism of Tip60. Regulation was through phosphorylation of tyrosine 327 by Abl tyrosine kinase and depended on environmental conditions, suggesting that the tyrosine residue of Tip60 is important for the activation process.

Phosphorylation of Tip60 Tyrosine 327 by Abl Kinase Inhibits HAT Activity through Association with FE65

The transfer of acetyl groups from acetyl coenzyme A to the ε amino group of internal lysine residues is catalyzed by Tip60, which is in the MYST family of nuclear histone acetyltransferases (HATs). The tyrosine phosphorylation of Tip60 seems to be a unique modification. We present evidence that Tip60 is modified on tyrosine 327 by Abl kinase. We show that this causes functional changes in HAT activity and the subcellular localization of TIP60, which forms a complex with Abl kinase. The Tip60 mutation Y327F abolished tyrosine phosphorylation, reduced the inhibition of Tip60 HAT activity, and caused G0-G1 arrest and association with FE65. Thus, our findings for the first time suggested a novel regulation mechanism of Tip60. Regulation was through phosphorylation of tyrosine 327 by Abl tyrosine kinase and depended on environmental conditions, suggesting that the tyrosine residue of Tip60 is important for the activation process.

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.

Amyloid precursor protein (APP) regulates synaptic structure and function

The amyloid precursor protein (APP) plays a critical role in Alzheimer's disease (AD) pathogenesis. APP is proteolytically cleaved by β- and γ-secretases to generate the amyloid β-protein (Aβ), the core protein component of senile plaques in AD. It is also cleaved by α-secretase to release the large soluble APP (sAPP) luminal domain that has been shown to exhibit trophic properties. Increasing evidence points to the development of synaptic deficits and dendritic spine loss prior to deposition of amyloid in transgenic mouse models that overexpress APP and Aβ peptides. The consequence of loss of APP, however, is unsettled. In this study, we investigated whether APP itself plays a role in regulating synaptic structure and function using an APP knock-out (APP-/-) mouse model. We examined dendritic spines in primary cultures of hippocampal neurons and CA1 neurons of hippocampus from APP-/- mice. In the cultured neurons, there was a significant decrease (~35%) in spine density in neurons derived from APP-/- mice compared to littermate control neurons that were partially restored with sAPPα-conditioned medium. In APP-/- mice in vivo, spine numbers were also significantly reduced but by a smaller magnitude (~15%). Furthermore, apical dendritic length and dendritic arborization were markedly diminished in hippocampal neurons. These abnormalities in neuronal morphology were accompanied by reduction in long-term potentiation. Strikingly, all these changes in vivo were only seen in mice that were 12-15 months in age but not in younger animals. We propose that APP, specifically sAPP, is necessary for the maintenance of dendritic integrity in the hippocampus in an age-associated manner. Finally, these age-related changes may contribute to AD pathology independent of Aβ-mediated synaptic toxicity.

Functional and molecular interactions between Rac1 and FE65

FE65 is reported to act as an adaptor protein with several protein-interaction domains, including one WW domain and two phosphotyrosine interaction/binding domains. Through these binding domains, FE65 was considered to recruit various binding partners together to form functional complexes in a certain cellular compartment. In this study, we demonstrated that Rac1, a member of the Rho family GTPases, bound with FE65. We also elucidated that Rac1 inhibitor significantly suppressed FE65 expression, and Rac1 small interfering RNA transduction significantly decreased FE65 expression. FE65 small interfering RNA, however, did not influence Rac1 expression and its activity. Taken together, our results reveal that Rac1 interacts with FE65, and Rac1 activity regulates FE65 expression.

Dual regulation of notch1 signaling pathway by adaptor protein fe65

Notch1 receptor functions as a critical controller of cell fate decisions and also as a key regulator of cell growth, differentiation, and proliferation in invertebrates and vertebrates. In this study, we have demonstrated that the adaptor protein Fe65 attenuates Notch1 signaling via the accelerated degradation of the membrane-tethered Notch1 in the cytoplasm. Fe65 also suppresses Notch1 transcriptional activity via the dissociation of the Notch1-IC-recombining binding protein suppressor of hairless (RBP)-Jk complex within the nucleus. Fe65 is capable of forming a trimeric complex with Itch and membrane-tethered Notch1, and Fe65 enhances the protein degradation of membrane-tethered Notch1 via an Itch-dependent proteasomal pathway. Collectively, our results demonstrate that Fe65 carries out different functions depending on its location in the regulation of Notch1 signaling.

A comparative analysis of resonance energy transfer methods for Alzheimer related protein-protein interactions in living cells

Fluorescence resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) are extensively used to analyze protein interactions occurring in living cells. Although these two techniques are broadly applied in cellular biology, comparative analysis of their strengths and limitations is lacking. To this end, we analyzed a small network of proteins involved in the amyloidogenic processing of the Alzheimer β-amyloid precursor using FRET based cytometry, BRET, and fluorescence lifetime imaging microscopy (FLIM). Using all three methods, we were able to detect the interactions of the amyloid precursor protein with APBB1, APBB2, and APP itself. And we found an unreported interacting pair, APP-APH1A. In addition, we show that these four interacting pairs exhibit a strong FRET correlation with the acceptor/donor expression ratios. Overall the FRET based cytometry was the most sensitive and reliable approach to screen for new interacting proteins. Therefore, we applied FRET based cytometry to study competitive binding of two proteins, APBB1 and APBB2, with the same APP target.

Exploiting publicly available biological and biochemical information for the discovery of novel short linear motifs

The function of proteins is often mediated by short linear segments of their amino acid sequence, called Short Linear Motifs or SLiMs, the identification of which can provide important information about a protein function. However, the short length of the motifs and their variable degree of conservation makes their identification hard since it is difficult to correctly estimate the statistical significance of their occurrence. Consequently, only a small fraction of them have been discovered so far. We describe here an approach for the discovery of SLiMs based on their occurrence in evolutionarily unrelated proteins belonging to the same biological, signalling or metabolic pathway and give specific examples of its effectiveness in both rediscovering known motifs and in discovering novel ones. An automatic implementation of the procedure, available for download, allows significant motifs to be identified, automatically annotated with functional, evolutionary and structural information and organized in a database that can be inspected and queried. An instance of the database populated with pre-computed data on seven organisms is accessible through a publicly available server and we believe it constitutes by itself a useful resource for the life sciences (http://www.biocomputing.it/modipath).

Similarities and differences in structure, expression, and functions of VLDLR and ApoER2

Very Low Density Lipoprotein Receptor (VLDLR) and Apolipoprotein E Receptor 2 (ApoER2) are important receptors in the brain for mediating the signaling effects of the extracellular matrix protein Reelin, affecting neuronal function in development and in the adult brain. VLDLR and ApoER2 are members of the low density lipoprotein family, which also mediates the effects of numerous other extracellular ligands, including apolipoprotein E. Although VLDLR and ApoER2 are highly homologous, they differ in a number of ways, including structural differences, expression patterns, alternative splicing, and binding of extracellular and intracellular proteins. This review aims to summarize important aspects of VLDLR and ApoER2 that may account for interesting recent findings that highlight the unique functions of each receptor.

Biomarkers associated with diffuse traumatic axonal injury: exploring pathogenesis, early diagnosis, and prognosis

BACKGROUND

Diffuse traumatic axonal injury (dTAI) is a significant pathologic feature of traumatic brain injury and is associated with substantial mortality and morbidity. It is still a challenge for clinicians to make an early diagnosis of dTAI and generate accurate prognosis and direct therapeutic decisions because most patients rapidly progress to coma after trauma and because specific neurologic symptoms and focal lesions detectable with current routine imaging techniques are absent. To address these issues, many investigations have sought to identify biomarkers of dTAI.

METHODS

This article is a review of the pertinent medical literature.

RESULTS

From the perspective of the pathophysiology of dTAI, we reviewed several biomarkers that are associated with structural damage and biochemical cascades in the secondary injury or repair response to traumatic brain injury. Although some biomarkers are not specific to dTAI, they are nevertheless useful in elucidating its pathogenesis, making early diagnosis possible, predicting outcomes, and providing candidate targets for novel therapeutic strategies.

CONCLUSIONS

The availability of biomarker data, clinical case histories, and radiologic information can improve our current ability to diagnose and monitor pathogenic conditions and predict outcomes in patients with dTAI.

Tyr(682) in the intracellular domain of APP regulates amyloidogenic APP processing in vivo

BACKGROUND

The pathogenesis of Alzheimer's disease is attributed to misfolding of Amyloid-β (Aβ) peptides. Aβ is generated during amyloidogenic processing of Aβ-precursor protein (APP). Another characteristic of the AD brain is increased phosphorylation of APP amino acid Tyr(682). Tyr(682) is part of the Y(682)ENPTY(687) motif, a docking site for interaction with cytosolic proteins that regulate APP metabolism and signaling. For example, normal Aβ generation and secretion are dependent upon Tyr(682) in vitro. However, physiological functions of Tyr(682) are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To this end, we have generated an APP Y682G knock-in (KI) mouse to help dissect the role of APP Tyr(682) in vivo. We have analyzed proteolytic products from both the amyloidogenic and non-amyloidogenic processing of APP and measure a profound shift towards non-amyloidogenic processing in APP KI mice. In addition, we demonstrate the essential nature of amino acid Tyr(682) for the APP/Fe65 interaction in vivo.

CONCLUSIONS/SIGNIFICANCE

Together, these observations point to an essential role of APP intracellular domain for normal APP processing and function in vivo, and provide rationale for further studies into physiological functions associated with this important phosphorylation site.

Targeting NMNAT1 to axons and synapses transforms its neuroprotective potency in vivo

Axon and synapse degeneration are common components of many neurodegenerative diseases, and their rescue is essential for effective neuroprotection. The chimeric Wallerian degeneration slow protein (Wld(S)) protects axons dose dependently, but its mechanism is still elusive. We recently showed that Wld(S) acts at a non-nuclear location and is present in axons. This and other recent reports support a model in which Wld(S) protects by extranuclear redistribution of its nuclear NMNAT1 portion. However, it remains unclear whether cytoplasmic NMNAT1 acts locally in axons and synapses or at a non-nuclear site within cell bodies. The potency of axon protection by non-nuclear NMNAT1 relative to Wld(S) also needs to be established in vivo. Because the N-terminal portion of Wld(S) (N70) localized to axons, we hypothesized that it mediates the trafficking of the NMNAT1 portion. To test this, we substituted N70 with an axonal targeting peptide derived from amyloid precursor protein, and fused this to NMNAT1 with disrupted nuclear targeting. In transgenic mice, this transformed NMNAT1 from a molecule unable to inhibit Wallerian degeneration, even at high expression levels, into a protein more potent than Wld(S), able to preserve injured axons for several weeks at undetectable expression levels. Preventing NMNAT1 axonal delivery abolished its protective effect. Axonally targeted NMNAT1 localized to vesicular structures, colocalizing with extranuclear Wld(S), and was cotransported at least partially with mitochondria. We conclude that axonal targeting of NMNAT activity is both necessary and sufficient to delay Wallerian degeneration, and that promoting axonal and synaptic delivery greatly enhances the effectiveness.

Ubiquitylation of Fe65 adaptor protein by neuronal precursor cell expressed developmentally down regulated 4-2 (Nedd4-2) via the WW domain interaction with Fe65

Fe65 has been characterized as an adaptor protein, originally identified as an expressed sequence tag (EST) corresponding to an mRNA expressed at high levels in the rat brain. It contains one WW domain and two phosphotyrosine interaction/phosphotyrosine binding domains (PID1/PID2). As the neuronal precursor cell expressed developmentally down regulated 4-2 (Nedd4-2) has a putative WW domain binding motif ((72)PPLP(75)) in the N-terminal domain, we hypothesized that Fe65 associates with Nedd4-2 through a WW domain interaction, which has the characteristics of E3 ubiquitin-protein ligase. In this paper, we present evidence for the interaction between Fe65 WW domain and Nedd4-2 through its specific motif, using a pull down approach and co-immunoprecipitation. Additionally, the co-localization of Fe65 and Nedd4-2 were observed via confocal microscopy. Co-localization of Fe65 and Nedd4-2 was disrupted by either the mutation of Fe65 WW domain or its putative binding motif of Nedd4-2. When the ubiquitin assay was performed, the interaction of Nedd4-2 (wt) with Fe65 is required for the cell apoptosis and the ubiquitylation of Fe65. We also observed that the ubiquitylation of Fe65 (wt) was augmented depending on Nedd4-2 expression levels, whereas the Fe65 WW domain mutant (W243KP245K) or the Nedd4-2 AL mutant ((72)PPLP(75) was changed to (72)APLA(75)) was under-ubiquitinated significantly. Thus, our observations implicated that the protein-protein interaction between the WW domain of Fe65 and the putative binding motif of Nedd4-2 down-regulates Fe65 protein stability and subcellular localization through its ubiquitylation, to contribute cell apoptosis.

Abnormal neuronal networks and seizure susceptibility in mice overexpressing the APP intracellular domain

Alterations in the processing of the amyloid precursor protein (APP) lead to familial Alzheimer's disease (AD). AD patients exhibit increased seizure susceptibility and alterations in their EEGs, which suggests that APP and its metabolites may modulate neuronal networks. Here we demonstrate that transgenic mice overexpressing APP intracellular domain (AICD) and its binding partner Fe65 exhibit abnormal spiking events and a susceptibility to induced seizures. These abnormalities are not observed in PDAPP(D664A) mice, which express high Aβ levels but harbor a mutation in the APP intracellular domain. These data suggest that alterations in the levels of AICD contribute to network dysfunction in AD.

Structural studies of the transmembrane C-terminal domain of the amyloid precursor protein (APP): does APP function as a cholesterol sensor?

The amyloid precursor protein (APP) is subject to alternative pathways of proteolytic processing, leading either to production of the amyloid-beta (Abeta) peptides or to non-amyloidogenic fragments. Here, we report the first structural study of C99, the 99-residue transmembrane C-terminal domain of APP liberated by beta-secretase cleavage. We also show that cholesterol, an agent that promotes the amyloidogenic pathway, specifically binds to this protein. C99 was purified into model membranes where it was observed to homodimerize. NMR data show that the transmembrane domain of C99 is an alpha-helix that is flanked on both sides by mostly disordered extramembrane domains, with two exceptions. First, there is a short extracellular surface-associated helix located just after the site of alpha-secretase cleavage that helps to organize the connecting loop to the transmembrane domain, which is known to be essential for Abeta production. Second, there is a surface-associated helix located at the cytosolic C-terminus, adjacent to the YENPTY motif that plays critical roles in APP trafficking and protein-protein interactions. Cholesterol was seen to participate in saturable interactions with C99 that are centered at the critical loop connecting the extracellular helix to the transmembrane domain. Binding of cholesterol to C99 and, most likely, to APP may be critical for the trafficking of these proteins to cholesterol-rich membrane domains, which leads to cleavage by beta- and gamma-secretase and resulting amyloid-beta production. It is proposed that APP may serve as a cellular cholesterol sensor that is linked to mechanisms for suppressing cellular cholesterol uptake.

Structure of the C-terminal phosphotyrosine interaction domain of Fe65L1 complexed with the cytoplasmic tail of amyloid precursor protein reveals a novel peptide binding mode

Fe65L1, a member of the Fe65 family, is an adaptor protein that interacts with the cytoplasmic domain of Alzheimer amyloid precursor protein (APP) through its C-terminal phosphotyrosine interaction/phosphotyrosine binding (PID/PTB) domain. In the present study, the solution structures of the C-terminal PID domain of mouse Fe65L1, alone and in complex with a 32-mer peptide (DAAVTPEERHLSKMQQNGYENPTYKFFEQMQN) derived from the cytoplasmic domain of APP, were determined using NMR spectroscopy. The C-terminal PID domain of Fe65L1 alone exhibits a canonical PID/PTB fold, whereas the complex structure reveals a novel mode of peptide binding. In the complex structure, the NPTY motif forms a type-I beta-turn, and the residues immediately N-terminal to the NPTY motif form an antiparallel beta-sheet with the beta5 strand of the PID domain, the binding mode typically observed in the PID/PTB.peptide complex. On the other hand, the N-terminal region of the peptide forms a 2.5-turn alpha-helix and interacts extensively with the C-terminal alpha-helix and the peripheral regions of the PID domain, representing a novel mode of peptide binding that has not been reported previously for the PID/PTB.peptide complex. The indispensability of the N-terminal region of the peptide for the high affinity of the PID-peptide interaction is consistent with NMR titration and isothermal calorimetry data. The extensive binding features of the PID domain of Fe65L1 with the cytoplasmic domain of APP provide a framework for further understanding of the function, trafficking, and processing of APP modulated by adapter proteins.

Regulation of FE65 nuclear translocation and function by amyloid beta-protein precursor in osmotically stressed cells

FE65, a neural adaptor protein, interacts with amyloid beta-protein precursor (APP) and is known to regulate amyloid beta generation from APP. FE65 also associates with nuclear proteins; however, its physiological function in the nucleus remains unclear. A fixed population of cytoplasmic FE65 is tethered to membranes by binding APP. This membrane-tethered FE65 is liberated from membranes by APP phosphorylation, which is facilitated by a stress-activated protein kinase in sorbitol-treated cells. Here we show that liberated FE65, which is distinct from "virgin" FE65 in the cytoplasm, translocates into the nucleus and accumulates in the nuclear matrix forming a patched structure. Targeting of FE65 into the nuclear matrix was suppressed by the APP intracellular domain fragment, which is generated by consecutive cleavages of APP. Thus, nuclear translocation of FE65 is under the regulation of APP. In the nucleus, FE65 induced gammaH2AX, which plays an important role in DNA repair as a cellular response by stress-damaged cells. These observations suggest that APP-regulated FE65 plays an important role in the early stress response of cells and that FE65 deregulated from APP induces apoptosis.

Substrate specificity of gamma-secretase and other intramembrane proteases

Gamma-Secretase is a promiscuous protease that cleaves bitopic membrane proteins within the lipid bilayer. Elucidating both the mechanistic basis of gamma-secretase proteolysis and the precise factors regulating substrate identification is important because modulation of this biochemical degradative process can have important consequences in a physiological and pathophysiological context. Here, we briefly review such information for all major classes of intramembranously cleaving proteases (I-CLiPs), with an emphasis on gamma-secretase, an I-CLiP closely linked to the etiology of Alzheimer's disease. A large body of emerging data allows us to survey the substrates of gamma-secretase to ascertain the conformational features that predispose a peptide to cleavage by this enigmatic protease. Because substrate specificity in vivo is closely linked to the relative subcellular compartmentalization of gamma-secretase and its substrates, we also survey the voluminous body of literature concerning the traffic of gamma-secretase and its most prominent substrate, the amyloid precursor protein.

Amyloid-beta precursor protein mediates neuronal toxicity of amyloid beta through Go protein activation

Amyloid beta (Abeta) is a metabolic product of amyloid-beta precursor protein (APP). Deposition of Abeta in the brain and neuronal degeneration are characteristic hallmarks of Alzheimer's disease (AD). Abeta induces neuronal degeneration, but the mechanism of neurotoxicity remains elusive. Here we show that overexpression of APP renders hippocampal neurons vulnerable to Abeta toxicity. Deletion of the extracellular Abeta sequence of APP prevents binding of APP to Abeta, and abolishes toxicity. Abeta toxicity is also abrogated by deletion of the cytoplasmic domain of APP, or by deletions comprising the Go protein-binding sequence of APP. Treatment with Pertussis toxin (PTX) abrogates APP-dependent toxicity of Abeta. Overexpression of PTX-insensitive Galpha-o subunit, but not Galpha-i subunit, of G protein restores Abeta toxicity in the presence of PTX, and this requires the integrity of APP-binding site for Go protein. Altogether, these experiments indicate that interaction of APP with toxic Abeta-species promotes toxicity in hippocampal neurons by a mechanism that involves APP-mediated Go protein activation, revealing an Abeta-receptor-like function of APP directly implicated in neuronal degeneration in AD.

Miniature protein ligands for EVH1 domains: interplay between affinity, specificity, and cell motility

Dynamic rearrangements of the actin cytoskeleton power cell motility in contexts ranging from intracellular microbial pathogenesis to axon guidance. The Ena/VASP family proteins-Mena, VASP, and Evl-are believed to control cell motility by serving as a direct link between signaling events and the actin cytoskeleton. It has previously been reported that a novel miniature protein, pGolemi, binds with high affinity to the EVH1 domain of Mena (Mena1-112) but not to those of VASP (VASP1-115) or Evl (Evl1-115) and also causes an unusual defect in actin-driven Listeria monocytogenes motility. Here, scanning mutagenesis was used to examine the effects of single amino acid changes within pGolemi on EVH1 domain affinity and specificity, miniature protein secondary structure, and L. monocytogenes motility. The data suggest that pGolemi contains the expected aPP-like fold and binds Mena1-112 in a manner highly analogous to the proline-rich repeat region of L. monocytogenes ActA protein. Residues throughout pGolemi contribute to both EVH1 domain affinity and paralog specificity. Moreover, the affinities of pGolemi variants for Mena1-112 correlate with selectivity against the EVH1 domains of VASP and Evl. In L. monocytogenes motility assays, speed and speed variability correlate strongly with EVH1 paralog specificity, suggesting that the Ena/VASP paralogs do not play equivalent roles in the process of L. monocytogenes actin tail maturation.

Human NIMA-related kinase 6 is one of the Fe65 WW domain binding proteins

The Aspergillus nidulans protein NIMA (never in mitosis, gene A) is a protein kinase required for initiation of mitosis, whereas its inactivation is necessary for mitotic exit. Here, we present evidence that human Nek6 is associated with Fe65. Based on the presence of Fe65 WW domain binding motifs ((267)PPLP(270)) in the Nek6 catalytic domain, we observed that Nek6 interacts physically with Fe65 both in vivo and in vitro, using a pull-down approach. Additionally, we detected co-localization of Nek6 and Fe65 via confocal microscopy. Co-localization of Nek6 and Fe65 was disrupted by mutation of the WW domain binding motifs ((267)PPLP(270)). Finally, when transient transfection assays were performed, interaction of Nek6 (wt) with Fe65 induced substantial cell apoptosis, whereas interaction using the Nek6 pplp mutant ((267)PPLP(270) changes (267)APVA(270)) did not. Thus, our observations indicated that Nek6 binds to Fe65 through its (267)PPLP(270) motif and that the protein-protein interaction between Nek6 and Fe65 regulates their subcellular localization and cell apoptosis.

ArhGAP9, a novel MAP kinase docking protein, inhibits Erk and p38 activation through WW domain binding

We have identified human ArhGAP9 as a novel MAP kinase docking protein that interacts with Erk2 and p38alpha through complementarily charged residues in the WW domain of ArhGAP9 and the CD domains of Erk2 and p38alpha. This interaction sequesters the MAP kinases in their inactive states through displacement of MAP kinase kinases targeting the same sites. While over-expression of wild type ArhGAP9 caused MAP kinase activation by the epidermal growth factor receptor (EGFR) to be suppressed and preserved the actin stress fibres in quiescent Swiss 3T3 fibroblasts, over-expression of an ArhGAP9 mutant defective in MAP kinase binding restored EGFR-induced MAP kinase activation and resulted in significant disruption of the stress fibres, consistent with the role of Erk activation in disassembly of actin stress fibres. The interaction between ArhGAP9 and the MAP kinases represents a novel mechanism of cross-talk between Rho GTPase and MAP kinase signaling.

Essential Roles for Fe65, Alzheimer Amyloid Precursor-binding Protein, in the Cellular Response to DNA Damage

Fe65 interacts with the cytosolic domain of the Alzheimer amyloid precursor protein (APP). The functions of the Fe65 are still unknown. To address this point we generated Fe65 knockout (KO) mice. These mice do not show any obvious phenotype; however, when fibroblasts (mouse embryonic fibroblasts), isolated from Fe65 KO embryos, were exposed to low doses of DNA damaging agents, such as etoposide or H2O2, an increased sensitivity to genotoxic stress, compared with wild type animals, clearly emerged. Accordingly, brain extracts from Fe65 KO mice, exposed to non-lethal doses of ionizing radiations, showed high levels of gamma-H2AX and p53, thus demonstrating a higher sensitivity to X-rays than wild type mice. Nuclear Fe65 is necessary to rescue the observed phenotype, and few minutes after the exposure of MEFs to DNA damaging agents, Fe65 undergoes phosphorylation in the nucleus. With a similar timing, the proteolytic processing of APP is rapidly affected by the genotoxic stress: in fact, the cleavage of the APP COOH-terminal fragments by gamma-secretase is induced soon after the exposure of cells to etoposide, in a Fe65-dependent manner. These results demonstrate that Fe65 plays an essential role in the response of the cells to DNA damage.

Phosphorylation of amyloid precursor protein (APP) at Thr668 regulates the nuclear translocation of the APP intracellular domain and induces neurodegeneration

Amyloid precursor protein (APP) has eight potential phosphorylation sites in its cytoplasmic domain. Recently, it has demonstrated that the constitutive phosphorylation of APP at T668 (APP695 isoform numbering) was observed specifically in the brain. Neuron-specific phosphorylation of APP at T668 is thought to be important for neuronal functions of APP, although its exact physiological significance remains to be clarified. In this study, we show that the phosphorylation of the APP intracellular domain (AICD) at T668 is essential for its binding to Fe65 and its nuclear translocation and affects the resultant neurotoxicity, possibly mediated through the induction of glycogen synthase kinase 3beta and tau phosphorylation by enhancing the formation of a ternary complex with Fe65 and CP2 transcription factor. Taken together, these results suggest that the phosphorylation of AICD at T668 contributes to the neuronal degeneration in Alzheimer's disease (AD) by regulating its translocation into the nucleus and then affects neurodegeneration; therefore, the specific inhibitor of T668 phosphorylation might be the target of AD therapy.

Intracellular domains of amyloid precursor-like protein 2 interact with CP2 transcription factor in the nucleus and induce glycogen synthase kinase-3β expression

Amyloid precursor protein (APP) is a member of a gene family that includes two APP-like proteins, APLP1 and 2. Recently, it has been reported that APLP1 and 2 undergo presenilin-dependent gamma-secretase cleavage, as does APP, resulting in the release of an approximately 6 kDa intracellular C-terminal domain (ICD), which can translocate into the nucleus. In this study, we demonstrate that the APLP2-ICDs interact with CP2/LSF/LBP1 (CP2) transcription factor in the nucleus and induce the expression of glycogen synthase kinase 3beta (GSK-3beta), which has broad-ranged substrates such as tau- and beta-catenin. The significance of this finding is substantiated by the in vivo evidence of the increase in the immunoreactivities for the nuclear C-terminal fragments of APLP2, and for GSK-3beta in the AD patients' brain. Taken together, these results suggest that APLP2-ICDs contribute to the AD pathogenesis, by inducing GSK-3beta expression through the interaction with CP2 transcription factor in the nucleus.

Characterization of human Enah gene

Enabled homolog (Enah) is a mammalian ortholog of Drosophila Enabled (Ena), which is genetically linked to the Drosophila Abl tyrosine phosphorylation signaling cascade and is required for normal neural development. Vertebrates have three Ena-related genes: Enah, VASP (vasodilator-stimulated phosphoprotein) and Ena/VASP like (EVL). These genes play an important role in linking signal transduction pathways to localized remodeling of the actin cytoskeleton. We isolated and sequenced a cDNA encoding human Enah. Comparison of the amino acid sequences of mouse (Mus musculus) and human (Homo sapiens) species shows 86.6% identity. The human protein appears longer than the mouse and additional amino acids are concentrated in a region containing repeats of the amino acid sequence LERER. The complete gene is about 157 kb and consists of 14 exons. Analysis of multiple tissue northern blot revealed a major transcript of about 4.8 kb in all tissue examined. Alternatively spliced isoforms were isolated by RT-PCR. The gene is differentially expressed and to gain insight factors affecting its expression we cloned and preliminarily characterized human Enah gene promoter.

A dominant role for FE65 (APBB1) in nuclear signaling

FE65 has been described as an adaptor protein; its partners include the beta-amyloid precursor protein (APP) and Tip60 (a histone acetyltransferase). Recent evidence suggests that APP may function in a nuclear signaling pathway via formation of APP-FE65-Tip60 complexes. The evidence is largely based on experiments in which APP/Tip60 is fused to the DNA binding domain of a yeast transcriptional factor Gal4 (Gal4DB) that can activate a reporter gene only when FE65 is coexpressed. One interpretation of published experiments has not yet been tested; however, there is the possibility that FE65 itself is the dominant transcriptional activator, whereas APP and Tip60 serve merely as positive/negative modulators or bridges for connecting FE65 to Gal4DB. To test this possibility, we fused Gal4DB directly to either end of FE65 and assessed their nuclear signaling in the presence or absence of exogenous APP/Tip60 or after knockdown of endogenous APP/Tip60. We found that FE65-Gal4DB by itself was able to trigger robust reporter activities. Increasing levels of APP could not further augment the reporter activity, but knocking down endogenous APP or interrupting FE65-APP binding reduced the signaling by up to 2-fold. The magnitudes of the reporter activities did not correlate with relative FE65 affinities for APP. Both overexpression and knockdown experiments showed that Tip60 plays a negative role. The results are consistent with the notion that FE65 is the key agent of Gal4DB-mediated transcriptional transactivation, whereas Tip60 is an FE65-associated repressor. Although APP may have modest stimulating effects, apparently there is no absolute requirement for that molecule for the nuclear signaling pathway.

Lipids as modulators of proteolytic activity of BACE: involvement of cholesterol, glycosphingolipids, and anionic phospholipids in vitro

The beta-secretase, BACE, is a membrane spanning aspartic protease, which cleaves the amyloid precursor protein (APP) in the first step of proteolytic processing leading to the formation of the neurotoxic beta-amyloid peptide (Abeta). Previous results have suggested that the regulation of beta-secretase and BACE access to APP is lipid dependent, and involves lipid rafts. Using the baculovirus expression system, we have expressed recombinant human full-length BACE in insect cells and purified milligram amounts to homogeneity. We have studied partitioning of fluorophor-conjugated BACE between the liquid ordered and disordered phases in giant (10-150 mum) unilamellar vesicles, and found approximately 20% to associate with the raft-like, liquid-ordered phase; the fraction associated with liquid-ordered phase increased upon cross-linking of raft lipids. To examine involvement of individual lipid species in modulating BACE activity, we have reconstituted the purified BACE in large ( approximately 100 nm) unilamellar vesicles, and determined its specific activity in vesicles of various lipid compositions. We have identified 3 groups of lipids that stimulate proteolytic activity of BACE: 1) neutral glycosphingolipids (cerebrosides), 2) anionic glycerophospholipids, and 3) sterols (cholesterol).

Biological roles of APP in the epidermis

The amyloid precursor protein (APP) was initially detected in cells of the central nervous system where it is considered to be involved in the pathogenesis of Alzheimer's disease. However, APP is also found in peripheral organs with exceptionally strong expression in the mammalian epidermis where it fulfils a variety of distinct biological roles. Full length APP appears to facilitate keratinocyte adhesion due to its ability to interact with the extracellular matrix. The C-terminus of APP also serves as adapter protein for binding the motor protein kinesin thereby mediating the centripetal transport of melanosomes in epidermal melanocytes. By the action of alpha-secretase sAPPalpha, the soluble N-terminal portion of APP, is released. sAPPalpha has been shown to be a potent epidermal growth factor thus stimulating proliferation and migration of keratinocytes as well as the exocytic release of melanin by melanocytes. The release of sAPPalpha can be almost completely blocked by inhibiting alpha-secretase with hydroxamic acid-based zinc metalloproteinase inhibitors. In hyperproliferative keratinocytes from psoriatic skin this inhibition results in normalized growth.

Expression of human FE65 in amyloid precursor protein transgenic mice is associated with a reduction in beta-amyloid load

FE65 is an adaptor protein that interacts with the cytoplasmic tail of the amyloid precursor protein (APP). In cultured non-neuronal cells, the formation of the FE65-APP complex is a key element for the modulation of APP processing, signalling and beta-amyloid (Abeta) production. The functions of FE65 in vivo, including its role in the metabolism of neuronal APP, remain to be investigated. In this study, transgenic mice expressing human FE65 were generated and crossbred with APP transgenic mice, known to develop Abeta deposits at 6 months of age. Compared with APP mice, APP/FE65 double transgenic mice exhibited a lower Abeta accumulation in the cerebral cortex as demonstrated by immunohistochemistry and immunoassay, and a lower level of APP-CTFs. The reduced accumulation of Abeta in APP/FE65 double transgenics, compared with APP mice, could be linked to the low Abeta42 level observed at 4 months of age and to the lower APP-CTFs levels. The present work provides evidence that FE65 plays a role in the regulation of APP processing in an in vivo model.

Endoproteolytic Cleavage of FE65 Converts the Adaptor Protein to a Potent Suppressor of the sAPPα Pathway in Primates

Adaptor protein FE65 (APBB1) specifically binds to the intracellular tail of the type I transmembrane protein, beta-amyloid precursor protein (APP). The formation of this complex may be important for modulation of the processing and function of APP. APP is proteolytically cleaved at multiple sites. The cleavages and their regulation are of central importance in the pathogenesis of dementias of the Alzheimer type. In cell cultures and perhaps in vivo, secretion of the alpha-cleaved APP ectodomain (sAPPalpha) is the major pathway in the most cells. Regulation of the process may require extracellular/intracellular cues. Neither extracellular ligands nor intracellular mediators have been identified, however. Here, we show novel evidence that the major isoform of FE65 (97-kDa FE65, p97FE65) can be converted to a 65-kDa N-terminally truncated C-terminal fragment (p65FE65) via endoproteolysis. The cleavage region locates immediately after an acidic residue cluster but before the three major protein-protein binding domains. The cleavage activity is particularly high in human and non-human primate cells and low in rodent cells; the activity appears to be triggered/enhanced by high cell density, presumably via cell-cell/cell-substrate contact cues. As a result, p65FE65 exhibits extraordinarily high affinity for APP (up to 40-fold higher than p97FE65) and potent suppression (up to 90%) of secretion of sAPPalpha. Strong p65FE65-APP binding is required for the suppression. The results suggest that p65FE65 may be an intracellular mediator in a signaling cascade regulating alpha-secretion of APP, particularly in primates.

Lineage-specific and ubiquitous biological roles of the mammalian transcription factor LSF

Transcriptional regulation in mammalian cells is driven by a complex interplay of multiple transcription factors that respond to signals from either external or internal stimuli. A single transcription factor can control expression of distinct sets of target genes, dependent on its state of post-translational modifications, interacting partner proteins, and the chromatin environment of the cellular genome. Furthermore, many transcription factors can act as either transcriptional repressors or activators, depending on promoter and cellular contexts [Alvarez, M., Rhodes, S.J., Bidwell, J.P., 2003. Context-dependent transcription: all politics is local. Gene 313, 43-57]. Even in this light, the versatility of LSF (Late SV40 Factor) is remarkable. A hallmark of LSF is its unusual DNA binding domain, as evidenced both by lack of homology to any other established DNA-binding domains and by its DNA recognition sequence. Although a dimer in solution, LSF requires additional multimerization with itself or partner proteins in order to interact with DNA. Transcriptionally, LSF can function as an activator or a repressor. It is a direct target of an increasing number of signal transduction pathways. Biologically, LSF plays roles in cell cycle progression and cell survival, as well as in cell lineage-specific functions, shown most strikingly to date in hematopoietic lineages. This review discusses how the unique aspects of LSF DNA-binding activity may make it particularly susceptible to regulation by signal transduction pathways and may relate to its distinct biological roles. We present current progress in elucidation of both tissue-specific and more universal cellular roles of LSF. Finally, we discuss suggestive data linking LSF to signaling by the amyloid precursor protein and to Alzheimer's disease, as well as to the regulation of latency of the human immunodeficiency virus (HIV).

Identification and characterization of four novel peptide motifs that recognize distinct regions of the transcription factor CP2

Although ubiquitously expressed, the transcriptional factor CP2 also exhibits some tissue- or stage-specific activation toward certain genes such as globin in red blood cells and interleukin-4 in T helper cells. Because this specificity may be achieved by interaction with other proteins, we screened a peptide display library and identified four consensus motifs in numerous CP2-binding peptides: HXPR, PHL, ASR and PXHXH. Protein-database searching revealed that RE-1 silencing factor (REST), Yin-Yang1 (YY1) and five other proteins have one or two of these CP2-binding motifs. Glutathione S-transferase pull-down and coimmunoprecipitation assays showed that two HXPR motif-containing proteins REST and YY1 indeed were able to bind CP2. Importantly, this binding to CP2 was almost abolished when a double amino acid substitution was made on the HXPR sequence of REST and YY1 proteins. The suppressing effect of YY1 on CP2's transcriptional activity was lost by this point mutation on the HXPR sequence of YY1 and reduced by an HXPR-containing peptide, further supporting the interaction between CP2 and YY1 via the HXPR sequence. Mapping the sites on CP2 for interaction with the four distinct CP2-binding motifs revealed at least three different regions on CP2. This suggests that CP2 recognizes several distinct binding motifs by virtue of employing different regions, thus being able to interact with and regulate many cellular partners.

FE65 constitutes the functional link between the low-density lipoprotein receptor-related protein and the amyloid precursor protein

Increasing evidence has implicated the low density lipoprotein receptor-related protein (LRP) and the adaptor protein FE65 in Alzheimer's disease pathogenesis. We have shown previously that LRP mediates beta-amyloid precursor protein (APP) processing and affects amyloid beta-protein and APP secretion and APP-c-terminal fragment generation. Furthermore, LRP mediates APP processing through its intracellular domain. Here, we set out to examine whether this interaction is of direct or indirect nature. Specifically, we asked whether adaptor proteins such as FE65 influence the LRP-mediated effect on APP processing by forming a protein complex. In coimmunoprecipitation experiments, we confirmed the postulated APP-FE65 and the LRP-FE65 interaction. However, we also showed an LRP-FE65-APP trimeric complex using pull-down techniques. Because FE65 alters APP processing, we investigated whether this effect is LRP dependent. Indeed, FE65 was only able to increase APP secretion in the presence of LRP. In the absence of LRP, APP secretion was unchanged compared with the LRP knock-out phenotype. Using RNA short interference techniques against FE65, we demonstrated that a reduction in FE65 protein mimics the LRP knock-out phenotype on APP processing. These results clearly demonstrate that FE65 acts as a functional linker between APP and LRP.

Deposition of amyloid fibrils promotes cell-surface accumulation of amyloid β precursor protein

Amyloid beta protein (Abeta) deposition and neuronal degeneration are characteristic pathological features of Alzheimer's disease (AD). In vitro, Abeta fibrils (fAbeta) induce neuronal degeneration reminiscent to AD, but the mechanism of neurotoxicity is unknown. Here we show that amyloid fibrils increase the level of cell-surface full-length amyloid beta precursor protein (h-AbetaPP) and secreted AbetaPP (s-AbetaPP). Pulse-chase analysis indicated that fAbeta selectively inhibited the turnover of cell-surface AbetaPP, without altering its intracellular levels. FAbeta-induced AbetaPP accumulation was not abrogated by cycloheximide, suggesting that increased protein synthesis is not critically required. Abeta fibrils sequester s-AbetaPP from the culture medium and promote its accumulation at the cell surface, indicating that binding of Abeta fibrils mediates AbetaPP accumulation. A time course analysis of Abeta treatment showed that AbetaPP level is elevated before significant cell death can be detected, while other toxic insults do not augment AbetaPP level, suggesting that AbetaPP may be specifically involved in early stages of Abeta-induced neurodegeneration. Finally, Abeta fibrils promote clustering of h-AbetaPP in abnormal focal adhesion-like (FA-like) structures that mediate neuronal dystrophy, increasing its association with the cytoskeleton. These results indicate that the interaction of Abeta fibrils with AbetaPP is an early event in the mechanism of Abeta-induced neurodegeneration that may play a significant role in AD pathogenesis.

Coordinated metabolism of Alcadein and amyloid beta-protein precursor regulates FE65-dependent gene transactivation

The Alcadeins (Alcs)/calsyntenins and the amyloid beta-protein precursor (APP) associate with each other in the brain by binding via their cytoplasmic domains to X11L (the X11-like protein). We previously reported that the formation of this APP-X11L-Alc tripartite complex suppresses the metabolic cleavages of APP. We show here that the metabolism of the Alcs markedly resembles that of APP. The Alcs are subjected to a primary cleavage event that releases their extracellular domain. Alcs then undergo a secondary presenilin-dependent gamma-cleavage that leads to the secretion of the amyloid beta-protein-like peptide and the liberation of an intracellular domain fragment (AlcICD). However, when Alc is in the tripartite complex, it escapes from these cleavages, as does APP. We also found that AlcICD suppressed the FE65-dependent gene transactivation activity of the APP intracellular domain fragment, probably because AlcICD competes with the APP intracellular domain fragment for binding to FE65. We propose that the Alcs and APP are coordinately metabolized in neurons and that their cleaved cytoplasmic fragments are reciprocally involved in the regulation of FE65-dependent gene transactivation. Any imbalance in the metabolism of Alcs and APP may influence the FE65-dependent gene transactivation, which together with increased secretion of amyloid beta-protein may contribute to neural disorders.

Novel cadherin-related membrane proteins, Alcadeins, enhance the X11-like protein-mediated stabilization of amyloid beta-protein precursor metabolism

Previously we found that X11-like protein (X11L) associates with amyloid beta-protein precursor (APP). X11L stabilizes APP metabolism and suppresses the secretion of the amyloid beta-protein (Abeta) that are the pathogenic agents of Alzheimer's disease (AD). Here we found that Alcadein (Alc), a novel membrane protein family that contains cadherin motifs and originally reported as calsyntenins, also interacted with X11L. Alc was abundant in the brain and occurred in the same areas of the brain as X11L. X11L could simultaneously associate with APP and Alc, resulting in the formation of a tripartite complex in brain. The tripartite complex stabilized intracellular APP metabolism and enhanced the X11L-mediated suppression of Abeta secretion that is due to the retardation of intracellular APP maturation. X11L and Alc also formed another complex with C99, a carboxyl-terminal fragment of APP cleaved at the beta-site (CTFbeta). The formation of the Alc.X11L.C99 complex inhibited the interaction of C99 with presenilin, which strongly suppressed the gamma-cleavage of C99. In AD patient brains, Alc and APP were particularly colocalized in dystrophic neurites in senile plaques. Deficiencies in the X11L-mediated interaction between Alc and APP and/or CTFbeta enhanced the production of Abeta, which may be related to the development or progression of AD.

The neuronal adaptor protein X11alpha reduces Abeta levels in the brains of Alzheimer's APPswe Tg2576 transgenic mice

Increased production and deposition of the 40-42-amino acid beta-amyloid peptide (Abeta) is believed to be central to the pathogenesis of Alzheimer's disease. Abeta is derived from the amyloid precursor protein (APP), but the mechanisms that regulate APP processing to produce Abeta are not fully understood. X11alpha (also known as munc-18-interacting protein-1 (Mint1)) is a neuronal adaptor protein that binds APP and modulates APP processing in transfected non-neuronal cells. To investigate the in vivo effect of X11alpha on Abeta production in the brain, we created transgenic mice that overexpress X11alpha and crossed these with transgenics harboring a familial Alzheimer's disease mutant APP that produces increased levels of Abeta (APPswe Tg2576 mice). Analyses of Abeta levels in the offspring generated from two separate X11alpha founder mice revealed a significant, approximate 20% decrease in Abeta(1-40) in double transgenic mice expressing APPswe/X11alpha compared with APPswe mice. At a key time point in Abeta plaque deposition (8 months old), the number of Abeta plaques was also deceased in APPswe/X11alpha mice. Thus, we report here the first demonstration that X11alpha inhibits Abeta production and deposition in vivo in the brain.

APP-BP1 mediates APP-induced apoptosis and DNA synthesis and is increased in Alzheimer's disease brain

APP-BP1, first identified as an amyloid precursor protein (APP) binding protein, is the regulatory subunit of the activating enzyme for the small ubiquitin-like protein NEDD8. We have shown that APP-BP1 drives the S- to M-phase transition in dividing cells, and causes apoptosis in neurons (Chen, Y., D.L. McPhie, J. Hirschberg, and R.L. Neve. 2000. J. Biol. Chem. 275:8929–8935). We now demonstrate that APP-BP1 binds to the COOH-terminal 31 amino acids of APP (C31) and colocalizes with APP in a lipid-enriched fraction called lipid rafts. We show that coexpression of a peptide representing the domain of APP-BP1 that binds to APP, abolishes the ability of overexpressed APP or the V642I mutant of APP to cause neuronal apoptosis and DNA synthesis. A dominant negative mutant of the NEDD8 conjugating enzyme hUbc12, which participates in the ubiquitin-like pathway initiated by APP-BP1, blocks neuronal apoptosis caused by APP, APP(V642I), C31, or overexpression of APP-BP1. Neurons overexpressing APP or APP(V642I) show increased APP-BP1 protein levels in lipid rafts. A similar increase in APP-BP1 in lipid rafts is observed in the Alzheimer's disease brain hippocampus, but not in less-affected areas of Alzheimer's disease brain. This translocation of APP-BP1 to lipid rafts is accompanied by a change in the subcellular localization of the ubiquitin-like protein NEDD8, which is activated by APP-BP1.