The beta-amyloid precursor protein APP is tyrosine-phosphorylated in cells expressing a constitutively active form of the Abl protoncogene

Receptor- and non-receptor tyrosine kinases induce processing of the amyloid precursor protein: role of the low-density lipoprotein receptor-related protein

The Alzheimer's beta-amyloid peptides derive from the proteolytic processing of the beta-amyloid precursor protein, APP, by beta- and gamma-secretases. The regulation of this processing is not fully understood. Experimental evidence suggests that the activation of pathways involving protein tyrosine kinases, such as PDGFR and Src, could induce the cleavage of APP and in turn the generation of amyloid peptides. In this paper we addressed the effect of receptor and nonreceptor protein tyrosine kinases on the cleavage of APP and the mechanisms of their action. To this aim, we developed an in vitro system based on the APP-Gal4 fusion protein stably transfected in SHSY5Y neuroblastoma cell line. The cleavage of this molecule, induced by various stimuli, results in the activation of the transcription of the luciferase gene under the control of Gal4 cis-elements. By using this experimental system we demonstrated that, similarly to Src, three tyrosine kinases, TrkA, Ret and EGFR, induced the cleavage of APP-Gal4. We excluded that this effect was mediated by the activation of Ras-MAPK, PI3K-Akt and PLC-gamma pathways. Furthermore, the direct phosphorylation of the APP cytosolic domain does not affect Abeta peptide generation. On the contrary, experiments in cells lacking the LDL-receptor related protein LRP support the hypothesis that the interaction of APP with LRP is required for the induction of APP cleavage by tyrosine kinases.

Structural basis for polyproline recognition by the FE65 WW domain

The neuronal protein FE65 functions in brain development and amyloid precursor protein (APP) signaling through its interaction with the mammalian enabled (Mena) protein and APP, respectively. The recognition of short polyproline sequences in Mena by the FE65 WW domain has a central role in axon guidance and neuronal positioning in the developing brain. We have determined the crystal structures of the human FE65 WW domain (residues 253-289) in the apo form and bound to the peptides PPPPPPLPP and PPPPPPPPPL, which correspond to human Mena residues 313-321 and 347-356, respectively. The FE65 WW domain contains two parallel ligand-binding grooves, XP (formed by residues Y269 and W280) and XP2 (formed by Y269 and W271). Both Mena peptides adopt a polyproline helical II conformation and bind to the WW domain in a forward (N-C) orientation through selection of the PPPPP motif by the XP and XP2 grooves. This mode of ligand recognition is strikingly similar to polyproline interaction with SH3 domains. Importantly, comparison of the FE65 WW structures in the apo and liganded forms shows that the XP2 groove is formed by an induced-fit mechanism that involves movements of the W271 and Y269 side-chains upon ligand binding. These structures elucidate the molecular determinants underlying polyproline ligand selection by the FE65 WW domain and provide a framework for the design of small molecules that would interfere with FE65 WW-ligand interaction and modulate neuronal development and APP signaling.

Amyloid Precursor Protein and Presenilin1 Interact with the Adaptor GRB2 and Modulate ERK 1,2 Signaling

The amyloid precursor protein (APP) and the presenilins 1 and 2 are genetically linked to the development of familial Alzheimer disease. APP is a single-pass transmembrane protein and precursor of fibrillar and toxic amyloid-beta peptides, which are considered responsible for Alzheimer disease neurodegeneration. Presenilins are multipass membrane proteins, involved in the enzymatic cleavage of APP and other signaling receptors and transducers. The role of APP and presenilins in Alzheimer disease development seems to be related to the formation of amyloid-beta peptides; however, their physiological function, reciprocal interaction, and molecular mechanisms leading to neurodegeneration are unclear. APP and presenilins are also involved in multiple interactions with intracellular proteins, the significance of which is under investigation. Among the different APP-interacting proteins, we focused our interest on the GRB2 adaptor protein, which connects cell surface receptors to intracellular signaling pathways. In this study we provide evidence by co-immunoprecipitation experiments, confocal and electron microscopy, and by fluorescence resonance energy transfer experiments that both APP and presenilin1 interact with GRB2 in vesicular structures at the centrosome of the cell. The final target for these interactions is ERK1,2, which is activated in mitotic centrosomes in a PS1- and APP-dependent manner. These data suggest that both APP and presenilin1 can be part of a common signaling pathway that regulates ERK1,2 and the cell cycle.

Amyloid precursor protein modulates ERK-1 and -2 signaling

The amyloid precursor protein (APP) is a transmembrane protein with a short cytoplasmic tail whose physiological function is unclear, although it is well documented that the proteolytic processing of APP could influence the development of Alzheimer's disease (AD) through the formation of membrane-bound C-terminal fragments (CTFs) and of beta-amyloid peptides (Abeta). We have recently shown that tyrosine-phosphorylated APP and CTFs may interact with Grb2 and ShcA adaptor proteins and that this coupling occurs at a higher extent in AD subjects only. To study the interaction between APP or CTFs and ShcA/Grb2 and to investigate their molecular target we have used as experimental model two different cell lines: H4 human neuroglioma cells and APP/APLP null mouse embryonic fibroblast cells (MEFs). Here we show that in H4 cells APP interacts with Grb2; conversely in APP/APLP-null MEF cells this interaction is possible only after the reintroduction of human APP by transfection. We have also shown that in MEF cells the transfection of a plasmid encoding for human APP wild-type enhances the phosphorylation of ERK-1 and -2 as revealed by Western blotting and immunofluorescence experiments. Finally, also in H4 cells the overexpression of APP upregulates the levels of phospho-ERK-1 and -2. In summary our data suggest that APP may influence phospho-ERK-1 and -2 signaling through its binding with Grb2 and ShcA adaptors. The meaning of this event is not clear, but APP interaction with these adaptors could be relevant to regulate mitogenic pathway.

Tyr687 dependent APP endocytosis and abeta production

The neurotoxic Abeta peptide is derived by proteolytic processing from the Alzheimer's amyloid precursor protein (APP), whose short cytoplasmic domain contains several phosphorylatable amino acids. The latter can be phosphorylated 'in vitro' and 'in vivo,' and in some cases phosphorylation appears to be associated with the disease condition. Using APP-GFP fusion proteins to monitor APP processing, the role of Tyr687 was addressed by mimicking its constitutive phosphorylation (Y687E) and dephosphorylation (Y687F). Contrasting effects on subcellular APP distribution were observed. Y687E-APP-GFP was targeted to the membrane but could not be detected in transferrin containing vesicular structures, and exhibited a concomitant and dramatic decrease in Abeta production. In contrast, Y687F-APP-GFP was endocytosed similarly to wild type APP, but was relatively favoured for beta-secretase cleavage. Overall, Tyr687 appears to be a critical residue determining APP targeting and processing via different pathways, including endocytosis and retrograde transport. Significantly, from a disease perspective, mimicking Tyr687 phosphorylation resulted in a hitherto undescribed inhibition of Abeta production. Our results provide novel insights into the role of direct APP phosphorylation on APP targeting, processing and Abeta production.

Pin1 in Alzheimer's disease: multiple substrates, one regulatory mechanism?

Presence of neuritic plaques and neurofibrillary tangles in the brain are two neuropathological hallmarks of Alzheimer's disease (AD), although the molecular basis of their coexistence remains elusive. The neurofibrillary tangles are composed of microtubule binding protein Tau, whereas neuritic plaques consist of amyloid-beta peptides derived from amyloid precursor protein (APP). Recently, the peptidyl-prolyl cis/trans isomerase Pin1 has been identified to regulate the function of certain proteins after phosphorylation and to play an important role in cell cycle regulation and cancer development. New data indicate that Pin1 also regulates the function and processing of Tau and APP, respectively, and is important for protecting against age-dependent neurodegeneration. Furthermore, Pin1 is the only gene known so far that, when deleted in mice, can cause both Tau and Abeta-related pathologies in an age-dependent manner, resembling many aspects of human Alzheimer's disease. Moreover, in the human AD brain Pin1 is downregulated or inhibited by oxidative modifications and/or genetic changes. These results suggest that Pin1 deregulation may provide a link between formation of tangles and plaques in AD.

Structure and functions of the human amyloid precursor protein: the whole is more than the sum of its parts

The amyloid precursor protein (APP) is a transmembrane protein that plays major roles in the regulation of several important cellular functions, especially in the nervous system, where it is involved in synaptogenesis and synaptic plasticity. The secreted extracellular domain of APP, sAPPalpha, acts as a growth factor for many types of cells and promotes neuritogenesis in post-mitotic neurons. Alternative proteolytic processing of APP releases potentially neurotoxic species, including the amyloid-beta (Abeta) peptide that is centrally implicated in the pathogenesis of Alzheimer's disease (AD). Reinforcing this biochemical link to neuronal dysfunction and neurodegeneration, APP is also genetically linked to AD. In this review, we discuss the biological functions of APP in the context of tissue morphogenesis and restructuring, where APP appears to play significant roles both as a contact receptor and as a diffusible factor. Structural investigation of APP, which is necessary for a deeper understanding of its roles at a molecular level, has also been advancing rapidly. We summarize recent progress in the determination of the structure of isolated APP fragments and of the conformations of full-length sAPPalpha, in both monomeric and dimeric states. The potential role of APP dimerization for the regulation of its biological functions is also discussed.

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 actin-depolymerizing factor/cofilin by LIM-kinase mediates amyloid beta-induced degeneration: a potential mechanism of neuronal dystrophy in Alzheimer's disease

Deposition of fibrillar amyloid beta (fAbeta) plays a critical role in Alzheimer's disease (AD). We have shown recently that fAbeta-induced dystrophy requires the activation of focal adhesion proteins and the formation of aberrant focal adhesion structures, suggesting the activation of a mechanism of maladaptative plasticity in AD. Focal adhesions are actin-based structures that provide a structural link between the extracellular matrix and the cytoskeleton. To gain additional insight in the molecular mechanism of neuronal degeneration in AD, here we explored the involvement of LIM kinase 1 (LIMK1), actin-depolymerizing factor (ADF), and cofilin in Abeta-induced dystrophy. ADF/cofilin are actin-binding proteins that play a central role in actin filament dynamics, and LIMK1 is the kinase that phosphorylates and thereby inhibits ADF/cofilin. Our data indicate that treatment of hippocampal neurons with fAbeta increases the level of Ser3-phosphorylated ADF/cofilin and Thr508-phosphorylated LIMK1 (P-LIMK1), accompanied by a dramatic remodeling of actin filaments, neuritic dystrophy, and neuronal cell death. A synthetic peptide, S3 peptide, which acts as a specific competitor for ADF/cofilin phosphorylation by LIMK1, inhibited fAbeta-induced ADF/cofilin phosphorylation, preventing actin filament remodeling and neuronal degeneration, indicating the involvement of LIMK1 in Abeta-induced neuronal degeneration in vitro. Immunofluorescence analysis of AD brain showed a significant increase in the number of P-LIMK1-positive neurons in areas affected with AD pathology. P-LIMK1-positive neurons also showed early signs of AD pathology, such as intracellular Abeta and pretangle phosphorylated tau. Thus, LIMK1 activation may play a key role in AD pathology.

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.

Essential roles for the FE65 amyloid precursor protein-interacting proteins in brain development

Targeted deletion of two members of the FE65 family of adaptor proteins, FE65 and FE65L1, results in cortical dysplasia. Heterotopias resembling those found in cobblestone lissencephalies in which neuroepithelial cells migrate into superficial layers of the developing cortex, aberrant cortical projections and loss of infrapyramidal mossy fibers arise in FE65/FE65L1 compound null animals, but not in single gene knockouts. The disruption of pial basal membranes underlying the heterotopias and poor organization of fibrillar laminin by isolated meningeal fibroblasts from double knockouts suggests that FE65 proteins are involved in basement membrane assembly. A similar phenotype is observed in triple mutant mice lacking the APP family members APP, APLP1 and APLP2, all of which interact with FE65 proteins, suggesting that this phenotype may be caused by decreased transmission of an APP-dependent signal through the FE65 proteins. The defects observed in the double knockout may also involve the family of Ena/Vasp proteins, which participate in actin cytoskeleton remodeling and interact with the WW domains of FE65 proteins.

The amyloid precursor protein and postnatal neurogenesis/neuroregeneration

The amyloid precursor protein (APP) is the source of amyloid-beta (Abeta) peptide, produced via its sequential cleavage beta- and gamma-secretases. Various biophysical forms of Abeta (and the mutations of APP which results in their elevated levels) have been implicated in the etiology and early onset of Alzheimer's disease. APP's evolutionary conservation and the existence of APP-like isoforms (APLP1 and APLP2) which lack the Abeta sequence, however, suggest that these might have important physiological functions that are unrelated to Abeta production. Soluble N-terminal fragments of APP have been known to be neuroprotective, and the interaction of its cytoplasmic C-terminus with a myriad of proteins associates it with diverse processes such as axonal transport and transcriptional regulation. The notion for an essential postnatal function of APP has been demonstrated genetically, as mice deficient in both APP and APLP2 or all three APP isoforms exhibit early postnatal lethality and neuroanatomical abnormalities. Recent findings have also brought to light two possible functions of the APP family in the brain-regulation of neural progenitor cell proliferation and axonal outgrowth after injury. Interestingly, these two apparently related neurogenic/neuroregenerative functions of APP involve two separate domains of the molecule.

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 Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.

Amyloid precursor protein promotes post-developmental neurite arborization in the Drosophila brain

The mechanisms regulating the outgrowth of neurites during development, as well as after injury, are key to the understanding of the wiring and functioning of the brain under normal and pathological conditions. The amyloid precursor protein (APP) is involved in the pathogenesis of Alzheimer's disease (AD). However, its physiological role in the central nervous system is not known. Many physical interactions between APP and intracellular signalling molecules have been described, but their functional relevance remains unclear. We show here that human APP and Drosophila APP-Like (APPL) can induce postdevelopmental axonal arborization, which depends critically on a conserved motif in the C-terminus and requires interaction with the Abelson (Abl) tyrosine kinase. Brain injury induces APPL upregulation in Drosophila neurons, correlating with increased post-traumatic mortality in appl(d) mutant flies. Finally, we also found interactions between APP and the JNK stress kinase cascade. Our findings suggest a role for APP in axonal outgrowth after traumatic brain injury.

Fasciclin II signals new synapse formation through amyloid precursor protein and the scaffolding protein dX11/Mint

Cell adhesion molecules (CAMs) have been universally recognized for their essential roles during synapse remodeling. However, the downstream pathways activated by CAMs have remained mostly unknown. Here, we used the Drosophila larval neuromuscular junction to investigate the pathways activated by Fasciclin II (FasII), a transmembrane CAM of the Ig superfamily, during synapse remodeling. We show that the ability of FasII to stimulate or to prevent synapse formation depends on the symmetry of transmembrane FasII levels in the presynaptic and postsynaptic cell and requires the presence of the fly homolog of amyloid precursor protein (APPL). In turn, APPL is regulated by direct interactions with the PDZ (postsynaptic density-95/Discs large/zona occludens-1)-containing protein dX11/Mint/Lin-10, which also regulates synapse expansion downstream of FasII. These results provide a novel mechanism by which cell adhesion molecules are regulated and provide fresh insights into the normal operation of APP during synapse development.

The amyloid precursor protein and its network of interacting proteins: physiological and pathological implications

The amyloid precursor protein (APP) is an ubiquitous receptor-like molecule involved in the pathogenesis of Alzheimer's disease that generates beta-amyloid peptides and causes plaque formation. APP and some of its C-terminal proteolytic fragments (CTFs) have also been shown to be in the center of a complex protein-protein network, where selective phosphorylation of APP C-terminus may regulate the interaction with cytosolic phosphotyrosine binding (PTB) domain or Src homology 2 (SH2) domain containing proteins involved in cell signaling. We have recently described an interaction between tyrosine-phosphorylated CTFs and ShcA adaptor protein which is highly enhanced in AD brain, and a new interaction between APP and the adaptor protein Grb2 both in human brain and in neuroblastoma cultured cells. These data suggest a possible role in cell signaling for APP and its CTFs, in a manner similar to that previously reported for other receptors, through a tightly regulated coupling with intracellular adaptors to control the signaling of the cell. In this review, we discuss the significance of these novel findings for AD development.

Interaction of Tau with Fe65 links tau to APP

The beta-amyloid precursor protein APP and the microtubule-associated protein Tau play a crucial role in the pathogenesis of Alzheimer's disease (AD). However, the possible molecular events linking these two proteins are still unknown. Here, we show that Fe65, one of the ligands of the APP cytodomain, is associated with Tau in vivo and in vitro, as demonstrated by co-immunoprecipitation, co-localization, and FRET experiments. Deletion studies indicated that the N-terminal domain of Tau and the PTB1 domain of Fe65 are required for this association. This interaction is regulated by the phosphorylation of Tau at selected sites, by glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase 5 (Cdk5), and requires an intact microtubule network. Furthermore, laser scanner microscopy and co-immunoprecipitation experiments provide preliminary evidence of possible complex(es) involving Tau, Fe65, APP. These findings open new perspectives for the study of the possible crosstalk between these proteins in the pathogenesis of AD.

Apoptotic Cell Death and Amyloid Precursor Protein Signaling in Neuroblastoma SH-SY5Y Cells

We have recently shown that the amyloid precursor protein (APP) and a subset of its C-terminal fragments (CTFs) are tyrosine phosphorylated in human brain and in cultured cells. Tyrosine phosphorylation generates a substrate that is sequentially bound by the adaptor proteins ShcA and Grb2, and this interaction is significantly enhanced in Alzheimer's disease brains. Here we have studied the APP/CTFs phosphorylation and ShcA activation in a human neuroblastoma cell line, SH-SY5Y, under basal and apoptotic conditions. To commit these cells to apoptosis, we used staurosporin, a well-known apoptotic inducer and protein kinase C blocker. Our data suggest the following: (1) in normally proliferating SH-SY5Y cells, full-length APP is complexed with Grb2[Q3], likely through its SH2 domain; (2) upon induction of apoptosis, APP is degraded and ShcA-Grb2 coimmunoprecipitates with CTFs recognized by anti-APP antibodies; and (3) caspase inhibitors partially block the degradation of APP and the coprecipitation of CTFs with ShcA-Grb2 adaptors. In summary, our data suggest that in SH-SY5Y cells, tyrosine-phosphorylated APP is involved in a complex with ShcA-Grb2 adaptors that is disrupted during apoptosis. The abnormal degradation of APP and consequent increased levels of CTFs (as has been observed in Alzheimer's disease and Down's syndrome) generate a complex between tyrosine-phosphorylated CTFs and intracellular adaptors. The signaling through APP and its CTFs may have significant relevance for apoptotic cell death in Alzheimer's disease.

BACE1 Overexpression Regulates Amyloid Precursor Protein Cleavage and Interaction with the ShcA Adapter

The amyloid precursor protein (APP) is a cell surface protein with a large extracellular N-terminal domain, a single transmembrane segment, and a short cytoplasmic tail. Its location and structural features are characteristic of a receptor for signal transduction. Yet, the physiological function of APP is unclear, although it is well documented that APP's proteolytic processing, through the formation of membrane-bound C-terminal fragments (CTFs) and of beta-amyloid peptides, likely influences the development of Alzheimer's disease (AD). There is evidence that BACE1 is the enzyme responsible for beta-site cleavage of the APP and for the generation of CTFs. BACE1 expression is upregulated in AD brain, and we have recently shown in human brain and in vitro that BACE product CTFs, when phosphorylated in tyrosine residues, interact with the adaptor proteins ShcA and Grb2, which usually are involved in signal transduction pathways. We investigated the interaction between ShcA, APP, and CTFs in the H4 human cell line that overexpresses BACE1 to clarify the significance of such interactions in vitro and for AD generation. Our result show that the APP, CTF, and ShcA interaction is induced only upon overexpression of BACE1 either transiently or in stable cell lines. In particular, although BACE1 drives the formation of C99 and C89 CTFs, only C99 interacts with the ShcA adaptor protein. Therefore, our data suggest that BACE1 activity influences APP processing and its intracellular signaling through the ShcA adaptor protein.

Mutation of the feh-1 gene, the Caenorhabditis elegans orthologue of mammalian Fe65, decreases the expression of two acetylcholinesterase genes

The molecular adaptor Fe65 is one of the cytosolic ligands of the Alzheimer's beta-amyloid precursor protein (APP), and this complex is believed to play important roles in mammalian cells. Upon cleavage of APP by specific processing activities, the complex between Fe65 and the APP intracellular domain (AICD) translocates to the nucleus. Experimental evidence suggests that the Fe65-AICD complex regulates gene transcription. In Caenorhabditis elegans the orthologue of the Fe65 gene, feh-1, regulates pharyngeal activity. In fact, the rate of pharyngeal contraction is increased following transient or stable suppression of the feh-1 gene expression. Here we show that the increased contraction rate of the pharynx in feh-1 mutant worms is associated to decreased acetylcholinesterase activity. The decreased activity is accompanied by reduced expression of ace-1 and ace-2 transcripts, coding for the two major acetylcholinesterase activities in the nematode. These results indicate a target of the regulatory mechanisms based on the Fe65-APP complex that could be relevant for the pathogenesis of Alzheimer's disease.

The c-Abl Tyrosine Kinase Phosphorylates the Fe65 Adaptor Protein to Stimulate Fe65/Amyloid Precursor Protein Nuclear Signaling

The amyloid precursor protein (APP) is proteolytically processed to release a C-terminal domain that signals to the nucleus to regulate transcription of responsive genes. The APP C terminus binds to a number of phosphotyrosine binding (PTB) domain proteins and one of these, Fe65, stimulates APP nuclear signaling. Fe65 is an adaptor protein that contains a number of protein-protein interaction domains. These include two PTB domains, the second of which binds APP, and a WW domain that binds proline-rich ligands. One ligand for the Fe65WW domain is the tyrosine kinase c-Abl. Here, we show that active c-Abl stimulates APP/Fe65-mediated gene transcription and that this effect is mediated by phosphorylation of Fe65 on tyrosine 547 within its second PTB domain. The homologous tyrosine within the motif Tyr-(Leu/Met)-Gly is conserved in a variety of PTB domains, and this suggests that PTB tyrosine phosphorylation occurs in other proteins. As such, PTB domain phosphorylation may represent a novel mechanism for regulating the function of this class of protein.

Amyloid Precursor Protein Mediates Proinflammatory Activation of Monocytic Lineage Cells

Alzheimer's disease is a progressive neurodegenerative disorder characterized by extracellular deposition of beta-amyloid (Abeta) peptide containing neuritic plaques. Abeta peptides are proteolytically derived from the membrane-bound amyloid precursor protein (APP). Although the function of APP is not entirely clear, previous studies demonstrate that neuronal APP colocalizes with beta(1) integrin receptors at sites of focal adhesion, suggesting that APP is involved in mediating neuronal process adhesion. Integrin-dependent adhesion is also a well-characterized component of immune cell proinflammatory activation. Using primary mouse microglia and the human monocytic cell line, THP-1, we have begun investigating the role of APP in integrin-dependent activation. Co-immunoprecipitation studies demonstrate that APP is recruited into a multi-receptor signaling complex during beta(1) integrin-mediated adhesion of monocytes. Stimulation induces a subsequent, specific recruitment of tyrosine phosphorylated proteins to APP, including Lyn and Syk. Antibody cross-linking of cell surface APP leads to a similar response characterized by activation and recruitment of tyrosine kinases to APP as well as subsequent activation of mitogen-activated protein kinases and increased proinflammatory protein levels. These data demonstrate that APP can act as a proinflammatory receptor in monocytic lineage cells and provide insight into the contribution of this protein to the inflammatory conditions described in Alzheimer's disease.

Growth factor receptor-bound protein 2 interaction with the tyrosine-phosphorylated tail of amyloid beta precursor protein is mediated by its Src homology 2 domain

The sequential processing of the familial disease gene product amyloid beta precursor protein (AbetaPP) by beta- and gamma-secretases generates amyloid beta, which is considered to be the pathogenic factor of Alzheimer's disease, and the AID peptide (AbetaPP intracellular domain). The AID peptide acts as a positive regulator of apoptosis and modulates transcription and calcium release. To gain clues about the molecular mechanisms regulating the function of AbetaPP and AID, proteins interacting with the AID region of AbetaPP have been isolated using the yeast two-hybrid system. Recent evidence indicates that AbetaPP undergoes post-translational modification events in the AID region and that phosphorylation might regulate its affinity for interacting proteins. To test this possibility and to uncover AbetaPP-binding partners whose interaction depends on AbetaPP phosphorylation, we used a proteomic approach. Here we describe a protein, growth factor receptor-bound protein 2 (Grb2), that specifically binds AbetaPP, phosphorylated in Tyr(682). Furthermore, we show that this interaction is direct and that Grb2 binds to phospho-AbetaPP via its Src homology 2 region. Together with the evidence that Grb2 is in complex with AbetaPP in human brains and that these complexes are augmented in brains from Alzheimer's cases, our data indicate that Grb2 may mediate some biological and possibly pathological AbetaPP-AID function.

Adaptor protein interactions: modulators of amyloid precursor protein metabolism and Alzheimer's disease risk?

The cytoplasmic C-terminus of APP plays critical roles in its cellular trafficking and delivery to proteases. Adaptor proteins with phosphotyrosine-binding (PTB) domains, including those in the X11, Fe65, and c-Jun N-terminal kinase (JNK)-interacting protein (JIP) families, bind specifically to the absolutely conserved -YENPTY- motif in the APP C-terminus to regulate its trafficking and processing. Compounds that modulate APP-adaptor protein interactions may inhibit Abeta generation by specifically targeting the substrate (APP) instead of the enzyme (beta- or gamma-secretase). Genetic polymorphisms in (or near) adaptor proteins may influence risk of sporadic AD by interacting with APP in vivo to modulate its trafficking and processing to Abeta.

Fe65 is not involved in the platelet-derived growth factor-induced processing of Alzheimer's amyloid precursor protein, which activates its caspase-directed cleavage

The proteolytic processing of the precursor of the beta-amyloid peptides (APP) is believed to be a key event in the pathogenesis of Alzheimer's disease. This processing is activated through a pathway involving the PDGF receptor, Src, and Rac1. In this paper, we demonstrate that this pathway specifically acts on APP and requires the YENPTY motif present in the APP cytosolic domain. Considering that several results indicate that the adaptor proteins interacting with this domain affect the processing of APP, we examined their possible involvement in the PDGF-induced pathway. By using an APP-Gal4 reporter system, we observed that the overexpression of Fe65 activates APP-Gal4 cleavage, whereas X11 stabilizes APP. Although mDab1 and Jip1 have no effect, Shc induces a strong activation of APP cleavage, and the contemporary exposure of cells to PDGF causes a dramatic cooperative effect. The analysis of point mutations of the APP YENPTY motif indicates that Fe65 and PDGF function through different mechanisms. In fact, Fe65 requires the integrity of APP695 Tyr682 residue, whereas PDGF effect is dependent upon the integrity of Asn684. Furthermore, the mutation of Asp664 of APP, which is the target site for the caspase-directed APP cleavage, strongly decreases the effect of Fe65. This suggests that Fe65 activates the cleavage of APP by caspases, and in fact, caspase inhibitor Z-VEVD decreases the APP cleavage induced by Fe65. On the contrary, the effects of Shc overexpression, like those of PDGF, are completely absent in the presence of compound X and require the integrity of the Asn684 residue of APP695. The involvement of Shc in the pathway regulating APP processing is confirmed by the effects of constitutively active and dominant negative mutants of Src and Rac1.

X11α impairs γ- but not β-cleavage of amyloid precursor protein

The phosphotyrosine binding domain of the neuronal protein X11alpha/mint-1 binds to the C-terminus of amyloid precursor protein (APP) and inhibits catabolism to beta-amyloid (Abeta), but the mechanism of this effect is unclear. Coexpression of X11alpha or its PTB domain with APPswe inhibited secretion of Abeta40 but not APPsbetaswe, suggesting inhibition of gamma- but not beta-secretase. To further probe cleavage(s) inhibited by X11alpha, we coexpressed beta-secretase (BACE-1) or a component of the gamma-secretase complex (PS-1Delta9) with APP, APPswe, or C99, with and without X11alpha, in HEK293 cells. X11alpha suppressed the PS-1Delta9-induced increase in Abeta42 secretion generated from APPswe or C99. However, X11alpha did not impair BACE-1-mediated proteolysis of APP or APPswe to C99. In contrast to impaired gamma-cleavage of APPswe, X11alpha or its PTB domain did not inhibit gamma-cleavage of NotchDeltaE to NICD (the Notch intracellular domain). The X11alpha PDZ-PS.1Delta9 interaction did not affect gamma-cleavage activity. In a cell-free system, X11alpha did not inhibit the catabolism of APP C-terminal fragments. These data suggest that X11alpha may inhibit Abeta secretion from APP by impairing its trafficking to sites of active gamma-secretase complexes. By specifically targeting substrate instead of enzyme X11alpha may function as a relatively specific gamma-secretase inhibitor.

The neuronal adaptor protein Fe65 is phosphorylated by mitogen-activated protein kinase (ERK1/2)

Fe65 is a neuronal adaptor protein that binds a number of ligands and which functions in both gene transcription/nuclear signalling and in the regulation of cell migration and motility. These different functions within the nucleus and at the cell surface are mediated via Fe65's different binding partners. An Fe65/APP/TIP60 complex is transcriptionally active within the nucleus and an Fe65/APP/Mena complex probably regulates actin dynamics in lamellipodia. The mechanisms that regulate these different Fe65 functions are unclear. Here, we demonstrate that Fe65 is a phosphoprotein and, using mass spectrometry sequencing, identify for the first time in vivo phosphorylation sites in Fe65. We also show that Fe65 is a substrate for phosphorylation by the mitogen-activated protein kinases ERK1/2. Our results provide a mechanism by which Fe65 function may be modulated to fulfil its various roles.

Protein phosphorylation and APP metabolism

Numerous lines of evidence place signal transduction cascades at the core of many processes having a direct role in neurodegeneration and associated disorders. Key players include neurotransmitters, growth factors, cytokines, hormones, and even binding and targeting proteins. Indeed, abnormal phosphorylation of key control proteins has been detected in many cases and is thought to underlie the associated cellular dysfunctions. Several signaling cascades have been implicated, affecting processes as varied as protein processing, protein expression, and subcellular protein localization, among others. The Alzheimer's amyloid precursor protein (APP) is a phosphoprotein, with well-defined phosphorylation sites but whose function is not clearly understood. The factors and pathways regulating the processing of APP have been particularly elusive, both in normal ageing and the Alzheimer's disease (AD) condition. Not surprisingly, the physiological function(s) of the protein remain(s) to be elucidated, although many hypotheses have been advanced. Nonetheless, considerable data has accumulated over the last decade, placing APP in key positions to be modulated both directly and indirectly by phosphorylation and phosphorylation-dependent events. The pathological end product of APP processing is the main proteinaceous component of the hallmark senile plaques found in the brains of AD patients, that is, a toxic peptide termed Abeta. In this minireview we address the importance of phosphorylation and signal transduction cascades in relation to APP processing and Abeta production. The possible use of the identified molecular alterations as therapeutic targets is also addressed.

X11alpha modulates secretory and endocytic trafficking and metabolism of amyloid precursor protein: mutational analysis of the YENPTY sequence

The neuronal adaptor X11alpha interacts with the conserved -GYENPTY- sequence in the C-terminus of amyloid precursor protein (APP) or its Swedish mutation (APPswe) to inhibit Abeta40 and Abeta42 secretion. We hypothesized that the -YENP- motif essential for APP endocytosis is also essential for X11alpha-mediated effects on APP trafficking and metabolism, and that X11alpha modulates APP metabolism in both secretory and endocytic pathways. X11alpha failed to interact with the endocytic-defective APPswe mutants Y738A, N740A, or P741A, and thus did not modulate their trafficking or metabolism. However, endocytic-competent APPswe Y743A had unique trafficking and metabolism including a prolonged half-life and increased secretion of catabolites compared with APPswe. In contrast to endocytic-defective mutants, X11alpha interacted with APPswe Y743A as well as with APPswe. Thus, similar to APPswe, coexpression of X11alpha with APPswe Y743A retarded its maturation, prolonged its half-life, and inhibited APPs, Abeta40, and Abeta42 secretion. Collectively, these data suggest that by direct interaction with the APPswe -YENP- motif in the cytoplasmic tail, X11alpha modulated its trafficking and processing in both secretory and endocytic compartments, and may reduce secretion of Abeta generated in either pathway.

Chronic myelogenous leukemia as a paradigm of early cancer and possible curative strategies

The chronological history of the important discoveries leading to our present understanding of the essential clinical, biological, biochemical, and molecular features of chronic myelogenous leukemia (CML) are first reviewed, focusing in particular on abnormalities that are responsible for the massive myeloid expansion. CML is an excellent target for the development of selective treatment because of its highly consistent genetic abnormality and qualitatively different fusion gene product, p210(bcr-abl). It is likely that the multiple signaling pathways dysregulated by p210(bcr-abl) are sufficient to explain all the initial manifestations of the chronic phase of the disease, although understanding of the circuitry is still very incomplete. Evidence is presented that the signaling pathways that are constitutively activated in CML stem cells and primitive progenitors cooperate with cytokines to increase the proportion of stem cells that are activated and thereby increase recruitment into the committed progenitor cell pool, and that this increased activation is probably the primary cause of the massive myeloid expansion in CML. The cooperative interactions between Bcr-Abl and cytokine-activated pathways interfere with the synergistic interactions between multiple cytokines that are normally required for the activation of stem cells, while at the same time causing numerous subtle biochemical and functional abnormalities in the later progenitors and precursor cells. The committed CML progenitors have discordant maturation and reduced proliferative capacity compared to normal committed progenitors, and like them, are destined to die after a limited number of divisions. Thus, the primary goal of any curative strategy must be to eliminate all Philadelphia positive (Ph+) primitive cells that are capable of symmetric division and thereby able to expand the Ph+ stem cell pool and recreate the disease. Several highly potent and moderately selective inhibitors of Bcr-Abl kinase have recently been discovered that are capable of killing the majority of actively proliferating early CML progenitors with minimal effects on normal progenitors. However, like their normal counterparts, most of the CML primitive stem cells are quiescent at any given time and are relatively invulnerable to the Bcr-Abl kinase inhibitors as well as other drugs. We propose that survival of dormant Ph+ stem cells may be the most important reason for the inability to cure the disease during initial treatment, while resistance to the inhibitors and other drugs becomes increasingly important later. An outline of a possible curative strategy is presented that attempts to take advantage of the subtle differences in the proliferative behavior of normal and Ph+ stem cells and the newly discovered selective inhibitors of Bcr-Abl. Leukemia (2003) 17, 1211-1262. doi:10.1038/sj.leu.2402912

Expression of the Fe65 adapter protein in adult and developing mouse brain

Fe65 is a multimodular adaptor protein expressed mainly in the nervous system. Fe65 binds to the Alzheimer's disease amyloid precursor protein (APP) and the interaction is mediated via a phosphotyrosine binding domain in Fe65 and the carboxy-terminal cytoplasmic domain of APP. Fe65 modulates trafficking and processing of APP, including production of the beta-amyloid peptide that is believed to be central to the pathogenesis of Alzheimer's disease. Fe65 also facilitates translocation of a carboxy-terminal fragment of APP to the nucleus and is required for APP-mediated transcription events. In addition, Fe65 functions in regulation of the actin cytoskeleton and cell movement. Here we report the distribution profile of Fe65 immunoreactivity in adult mouse brain. Fe65 expression was found to be widespread in neurones in adult brain. The areas of highest expression included regions of the hippocampus in which the earliest abnormalities of Alzheimer's disease are detectable. Fe65 was also highly expressed in the cerebellum, thalamus and selected brain stem nuclei. Fe65 was evident in a sub-set of astrocytes within the stratum oriens and radiatum in the hippocampus. Expression of Fe65 was found to be developmentally regulated with levels reducing after embryonic day 15 and increasing again progressively from post-partum day 10 up to adulthood, a developmental pattern that partially parallels that of APP. These data indicate a widespread distribution of Fe65 in neurones throughout mouse brain and also suggest that Fe65 may have functions independent of APP and any potential role in the pathogenesis of Alzheimer's disease.

Platelet-derived growth factor induces the beta-gamma-secretase-mediated cleavage of Alzheimer's amyloid precursor protein through a Src-Rac-dependent pathway

The beta-amyloid peptide (Abeta) present in the senile plaques of Alzheimer's disease derives from the cleavage of a membrane protein, named APP, driven by two enzymes, known as beta- and gamma-secretases. The mechanisms regulating this cleavage are not understood. We have developed an experimental system to identify possible extracellular signals able to trigger the cleavage of an APP-Gal4 fusion protein, which is detected by measuring the expression of the CAT gene transcribed under the control of the Gal4 transcription factor, which is released from the membrane upon the cleavage of APP-Gal4. By using this assay, we purified a protein contained in the C6 cell-conditioned medium, which activates the cleavage of APP-Gal4 and which we demonstrated to be PDGF-BB. The APP-Gal4 processing induced by PDGF is dependent on the gamma-secretase activity, being abolished by an inhibitor of this enzyme, and is the consequence of the activation of a pathway downstream of the PDGF-receptor, which includes the non-receptor tyrosine kinase Src and the small G-protein Rac1. These findings are confirmed by the observation that a constitutively active form of Src increases Abeta generation and that, in cells stably expressing APP, the generation of A is strongly decreased by the Src tyrosine kinase inhibitor PP2.

Isoform-specific knockout ofFE65 leads to impaired learning and memory

FE65 is a multimodular adapter protein that is expressed predominantly in brain. Its C-terminal phosphotyrosine interaction domain (PID) binds to the intracellular tail of the beta-amyloid precursor protein (betaPP), a protein of central importance to the pathogenesis of dementias of the Alzheimer type. To study the physiological functions of FE65, we generated a line of FE65 knockout mice via gene targeting. By Western analysis with a panel of FE65-specific antibodies, we demonstrate that the 97-kDa full-length FE65 (p97) was ablated in the mutant mice, and that a previously undescribed FE65 isoform with apparent molecular mass of 60 kDa (p60) was expressed in both wild-type and mutant mice. p60 had a truncated N-terminus and was likely to be generated through alternative translation. Expressions of the two isoforms appeared to be brain region distinct and age dependent. The p97FE65(-/-) mice were viable and showed no obvious physical impairments or histopathological abnormalities. However, p97FE65(-/-) and p97FE65(+/-) mice exhibited poorer performances than wild-type mice on a passive avoidance task when tested at 14 months (P <.05). p97FE65(-/-) mice at 14 months also exhibited impaired hidden-platform acquisition (P <.05) and a severe reversal-learning deficit (P <.002) but normal visual-platform acquisition in the Morris water maze tests. Probe trials confirmed impairments in p97FE65(-/-) mice in relearning of new spatial information, suggesting a hippocampus-dependent memory-extinction deficit. Reduced secretion of Abeta peptides was observed in primary neuronal cultures of hybrids of p97FE65(-/-)/betaPP transgenic (Tg2576) mice. These studies suggest an important and novel function of FE65 in learning and memory.

Heme deficiency may be a factor in the mitochondrial and neuronal decay of aging

Heme, a major functional form of iron in the cell, is synthesized in the mitochondria by ferrochelatase inserting ferrous iron into protoporphyrin IX. Heme deficiency was induced with N-methylprotoporphyrin IX, a selective inhibitor of ferrochelatase, in two human brain cell lines, SHSY5Y (neuroblastoma) and U373 (astrocytoma), as well as in rat primary hippocampal neurons. Heme deficiency in brain cells decreases mitochondrial complex IV, activates nitric oxide synthase, alters amyloid precursor protein, and corrupts iron and zinc homeostasis. The metabolic consequences resulting from heme deficiency seem similar to dysfunctional neurons in patients with Alzheimer's disease. Heme-deficient SHSY5Y or U373 cells die when induced to differentiate or to proliferate, respectively. The role of heme in these observations could result from its interaction with heme regulatory motifs in specific proteins or secondary to the compromised mitochondria. Common causes of heme deficiency include aging, deficiency of iron and vitamin B6, and exposure to toxic metals such as aluminum. Iron and B6 deficiencies are especially important because they are widespread, but they are also preventable with supplementation. Thus, heme deficiency or dysregulation may be an important and preventable component of the neurodegenerative process.

Signal transduction through tyrosine-phosphorylated carboxy-terminal fragments of APP via an enhanced interaction with Shc/Grb2 adaptor proteins in reactive astrocytes of Alzheimer's disease brain

The processing of the amyloid precursor protein (APP) through the formation of C-terminal fragments (CTFs) and the production of beta-amyloid, are events likely to influence the development and the progression of Alzheimer's disease (AD). APP is a transmembrane protein similar to a cell-surface receptor with the intraluminal NPTY motif in the cytosolic C terminus. Although APP holoprotein can be bound to intracellular proteins like Fe65, X11, and mDab, the ultimate function and the mechanisms through which this putative receptor transfers its message are unclear. Here it is shown that in human brain, a subset of tyrosine-phosphorylated CTFs represent docking sites for the adaptor protein ShcA. ShcA immunoreactivity is greatly enhanced in Alzheimer's patients; it is mainly localized to glial cells and occurs at reactive astrocytes surrounding cerebral vessels and amyloid plaques. Grb2 also is involved in complexes with ShcA and tyrosine-phosphorylated CTFs, and in AD brain the interaction between Grb2-ShcA and CTFs is enhanced. Also, a higher amount of phospho-ERK1,2 is present in AD brain in comparison with control cases, likely as a result of the ShcA activation. In vitro experiments show that the ShcA-CTFs interaction is strictly confined to glial cells when treated with thrombin, which is a well-known ShcA and ERK1,2 activator, mitogen, and regulator of APP cleavage. In untreated cells ShcA does not interact with either APP or CTFs, although they are normally produced. Altogether these data suggest that CTFs are implicated in cell signaling via Shc transduction machinery, likely influencing MAPK activity and glial reaction in AD patients.

Signal transduction through tyrosine-phosphorylated C-terminal fragments of amyloid precursor protein via an enhanced interaction with Shc/Grb2 adaptor proteins in reactive astrocytes of Alzheimer's disease brain

The proteolytic processing of amyloid precursor protein (APP) through the formation of membrane-bound C-terminal fragments (CTFs) and of soluble beta-amyloid peptides likely influences the development of Alzheimer's disease (AD). We show that in human brain a subset of CTFs are tyrosine-phosphorylated and form stable complexes with the adaptor protein ShcA. Grb2 is also part of these complexes, which are present in higher amounts in AD than in control brains. ShcA immunoreactivity is also greatly enhanced in patients with AD and occurs at reactive astrocytes surrounding cerebral vessels and amyloid plaques. A higher amount of phospho-ERK1,2, likely as result of the ShcA activation, is present in AD brains. In vitro experiments show that the ShcA-CTFs interaction is strictly confined to glial cells when treated with thrombin, which is a well known ShcA and ERK1,2 activator and a regulator of APP cleavage. In untreated cells ShcA does not interact with either APP or CTFs, although they are normally generated. Altogether these data suggest that CTFs are implicated in cell signaling via Shc transduction machinery, likely influencing MAPK activity and glial reaction in AD patients.

Fe65, a ligand of the Alzheimer's beta-amyloid precursor protein, blocks cell cycle progression by down-regulating thymidylate synthase expression

The functions of the Alzheimer's beta-amyloid precursor protein (APP) and of its complex with the adaptor protein Fe65 are still unknown. We have demonstrated that Fe65 is also a nuclear protein and APP functions as an extranuclear anchor, thus preventing Fe65 nuclear translocation. According to this finding, it was also demonstrated that Fe65 could play a role in the regulation of transcription. In the present paper we show that the overexpression of Fe65 prevents G(1) --> S cell cycle progression of serum-stimulated fibroblasts and that the contemporary overexpression of APP abolishes this effect of Fe65. The overexpression of Fe65 completely abolishes the activation of a key S phase gene, the thymidylate synthase (TS) gene, driven by the transcription factor LSF/CP2/LBP1 (LSF). This phenomenon is observed only in experimental conditions leading to the accumulation of Fe65 in the nucleus. Similarly, the two other members of the Fe65 protein family, Fe65L1 and Fe65L2, have been found to translocate into the nucleus and to prevent the activation of the TS gene promoter induced by LSF. Two results support the hypothesis that the inhibitory effect of Fe65 on cell cycle progression in fibroblasts is the result of the inhibition of TS gene expression: (i) Fe65 overexpression, but not Fe65 and APP co-expression, prevents the accumulation of endogenous TS upon the exposure of cells to serum, and (ii) thymidine addition to the culture medium completely overcomes the growth arrest caused by Fe65. In neuronal PC12 cells, the overexpression of Fe65 or of Fe65L1 and Fe65L2 blocks cell cycle, as observed in fibroblasts, but thymidine supplementation to culture medium does not revert this block, thus suggesting that Fe65 proteins induce in neuronal cells a gene expression program different from that activated in fibroblasts.

Low-density lipoprotein receptor-related protein levels and endocytic function are reduced by overexpression of the FE65 adaptor protein, FE65L1

The FE65 adaptor protein family was identified in two-hybrid screens as proteins that bind the cytoplasmic domain of the amyloid precursor protein (APP). Studies have shown that FE65 binding to APP modulates APP processing. Increased levels of alpha-secretase derived secreted APP (APPsalpha) and beta-amyloid (Abeta) were recovered from conditioned media upon FE65L1 or FE65 overexpression. These effects were associated with an increase in the ratio of mature/immature APP and increased cell-surface APP. FE65 has also been reported to bind low-density lipoprotein receptor-related protein (LRP). Here we show that FE65L1 overexpression results in decreased LRP steady state levels, LRPs, and LRP endocytic receptor function. These changes in LRP protein levels are not due to decreased transcription of LRP. Furthermore, pulse/chase experiments demonstrate that changes in LRP protein only occurred 12-18 h after translation. We conclude that the decreases in LRP levels likely reflect routing of LRP away from the cell surface into a degradative pathway. Previous studies suggested that LRP plays an important role for Abeta production of Kunitz protease inhibitor forms of APP in the endocytic pathway. These data show that FE65L1 can differentially affect the metabolic fate of APP and LRP. In addition, these data suggest that the LRP decrease observed in FE65L1 overexpressing cells may in part contribute to altered APP processing.

Evidence for a role of the nerve growth factor receptor TrkA in tyrosine phosphorylation and processing of beta-APP

The cytoplasmic tail of the beta-amyloid precursor protein (APP) contains a Y(682)ENPTY(687) sequence through which APP associates with phosphotyrosine binding (PTB) domain containing proteins in a tyrosine phosphorylation-independent manner. We have recently found that tyrosine phosphorylation of APP-Y(682) promotes docking of Shc proteins that modulate growth factor signaling to the ERK and PI3K/Akt pathways. We have also shown that APP is phosphorylated on Y(682) in cells that overexpress a constitutively active form of the tyrosine kinase abl. Here we present evidence that the nerve growth factor receptor TrkA may also promote phosphorylation of APP. Overexpression of TrkA, but not of mutated, kinase inactive TrkA resulted in tyrosine phosphorylation of APP. Site-directed mutagenesis studies showed that TrkA overexpression was associated with phosphorylation of APP-Y(682). Moreover, overexpression of TrkA also affected APP processing reducing the generation of the APP intracellular domain (AID). Thus, tyrosine phosphorylation of APP may functionally link APP processing and neurotrophic signaling to intracellular pathways associated with cellular differentiation and survival.

Tyrosine phosphorylation of the beta-amyloid precursor protein cytoplasmic tail promotes interaction with Shc

beta-Amyloid precursor protein (APP) is a widely expressed transmembrane protein of unknown function that is involved in the pathogenesis of Alzheimer's disease. The cytoplasmic tail of APP interacts with phosphotyrosine binding (PTB) domain containing proteins (Fe65, X11, mDab-1, and JIP-1) and may modulate gene expression and apoptosis. We now identify Shc A and Shc C, PTB-containing adapter proteins that signal to cellular differentiation and survival pathways, as novel APP-interacting proteins. The APP cytoplasmic tail contains a PTB-binding motif (Y(682)ENPTY(687)) that, when phosphorylated on Tyr(682), precipitated the PTB domain of Shc A and Shc C, as well as endogenous full-length Shc A. APP and Shc C were physically associated in adult mouse brain homogenates. Increase in phosphorylation of APP by overexpression of the nerve growth factor receptor Trk A in 293T cells promoted the interaction of transfected APP and endogenous Shc A. Pervanadate treatment of N2a neuroblastoma cells resulted in tyrosine phosphorylation and association of endogenous APP and Shc A. Thus, APP and Shc proteins interact in vitro, in cells, and in the mouse brain. Tyrosine phosphorylation of APP may promote the interaction with Shc proteins.

feh-1 and apl-1, the Caenorhabditis elegansorthologues of mammalian Fe65 and β-amyloid precursor protein genes, are involved in the same pathway that controls nematode pharyngeal pumping

The multigenic family of mammalian Fe65s encodes three highly similar proteins with the same modular organisation: a WW domain and two phosphotyrosine-binding domains. The PTB2 domain of these proteins binds to the cytosolic domains of the Alzheimer's β-amyloid precursor protein APP and related proteins APLP1 and APLP2, generating a highly redundant system that is hard to dissect by reverse genetics. By searching potential Fe65-like genes in the nematode Caenorhabditis elegans, we identified a single gene, feh-1 (Fe65 homolog-1), encoding a protein with a high sequence similarity to mammalian Fe65s. FEH-1 is also functionally related to mammalian orthologues;in fact its PTB2 domain binds to APL-1, the product of the C. elegansorthologue of APP. Staining with specific antibodies show that the neuromuscular structures of the pharynx are the sites in which FEH-1 is present at highest levels. Expression studies with reporters indicate that the feh-1 gene is also expressed by a subset of the worm neurons.

We generated and isolated a deletion allele of feh-1, and the corresponding homozygous mutants arrest as late embryos or as L1 larvae,demonstrating for the first time an essential role for a Fe65-like gene in vivo. The pharynx of homozygous larvae does not contract and the worms cannot feed. Analysis of pharyngeal pumping in heterozygous worms and in feh-1 RNA-interfered worms indicates that dosage of feh-1function affects the rate of pharyngeal contraction in C. elegans. Interference with apl-1 double-stranded RNA showed a similar effect on pharyngeal pumping, suggesting that FEH-1 and APL-1 are involved in the same pathway. The non-redundant system of the nematode will prove useful for studying the basic biology of the Fe65-APP interaction and the molecular events regulated by this evolutionarily conserved system of interacting proteins.