Alzheimer's disease amyloid precursor protein on the surface of cortical neurons in primary culture co-localizes with adhesion patch components

Up-regulation of APP endocytosis by neuronal aging drives amyloid dependent-synapse loss

Neuronal aging increases the risk of late-onset Alzheimer's disease. During normal aging, synapses decline, and β-amyloid (Aβ) accumulates intraneuronally. However, little is known about the underlying cell biological mechanisms. We studied normal neuronal aging using normal aged brain and aged mouse primary neurons that accumulate lysosomal lipofuscin and show synapse loss. We identify the up-regulation of amyloid precursor protein (APP) endocytosis as a neuronal aging mechanism that potentiates APP processing and Aβ production in vitro and in vivo. The increased APP endocytosis may contribute to the observed early endosomes enlargement in the aged brain. Mechanistically, we show that clathrin-dependent APP endocytosis requires F-actin and that clathrin and endocytic F-actin increase with neuronal aging. Finally, Aβ production inhibition reverts synaptic decline in aged neurons while Aβ accumulation, promoted by endocytosis up-regulation in younger neurons, recapitulates aging-related synapse decline. Overall, we identify APP endocytosis up-regulation as a potential mechanism of neuronal aging and, thus, a novel target to prevent late-onset Alzheimer's disease.

Dopamine promotes cathepsin B-mediated amyloid precursor protein degradation by reactive oxygen species-sensitive mechanism in neuronal cell

Recent literature suggested an important function of native amyloid precursor protein (APP) as amine oxidase implicating in protection of brain cells from catecholamine-induced toxicity. However, any role of catecholamines on regulation of native APP has not been explored. Here we report that dopamine (DA), one of the most prominent catecholamine neurotransmitters in brain, down-modulates native APP protein in several neuronal cell types. Using SH-SY5Y cells as model, we detected no alteration of transcript expression and unaffected translation suggested that DA might induce APP degradation. We actually found that DA treatment decreased the stability of APP. Lysosomal blockers inhibited DA-induced APP degradation, but specific proteasomal blocker failed to do so. We detected the role of cathepsin B in DA-induced APP degradation by using pharmacological inhibitor and specific siRNA. We also revealed that DA could increase cathepsin B expression at both transcript and protein levels. Using antioxidant N-acetyl cysteine, we detected increased level of reactive oxygen species generation that was found responsible for induced cathepsin B expression by DA and resultant APP degradation. Our study reveals the existence of reciprocal regulation of a catecholamine and an amine oxidase implicating in brain catecholamine homeostasis.

An early dysregulation of FAK and MEK/ERK signaling pathways precedes the β-amyloid deposition in the olfactory bulb of APP/PS1 mouse model of Alzheimer's disease

Olfactory dysfunction is an early event of Alzheimer's disease (AD). However, the mechanisms associated to AD neurodegeneration in olfactory areas are unknown. Here we used double-transgenic amyloid precursor protein/presenilin 1 (APPswe/PS1dE9) mice and label-free quantitative proteomics to analyze early pathological effects on the olfactory bulb (OB) during AD progression. Prior to β-amyloid plaque formation, 9 modulated proteins were detected on 3-month-old APP/PS1 mice while 16 differential expressed proteins were detected at 6months, when β-amyloid plaques appear, indicating a moderate imbalance in cytoskeletal rearrangement, and synaptic plasticity in APP/PS1 OBs. Moreover, β-amyloid induced an inactivation of focal adhesion kinase (FAK) together with a transient activation of MEK1/2, leading to inactivation of ERK1/2 in 6-months APP/PS1 OBs. In contrast, the analysis of human OBs revealed a late activation of FAK in advanced AD stages, whereas ERK1/2 activation was enhanced across AD staging respect to controls. This survival potential was accompanied by the inhibition of the proapototic factor BAD in the OB across AD phenotypes. Our data contribute to a better understanding of the early molecular mechanisms that are modulated in AD neurodegeneration, highlighting significant differences in the regulation of survival pathways between APP/PS1 mice and sporadic human AD.

SIGNIFICANCE

Loss of smell is involved in early stages of Alzheimer's disease (AD), usually preceding classic disease symptoms. However, the mechanisms governing this dysfunction are still poorly understood, losing its potential as a useful tool for clinical diagnosis. Our study characterizes potential AD-associated molecular changes in APP/PS1 mice olfactory bulb (OB) using MS-quantitative proteomics, revealing early cytoskeletal disruption and synaptic plasticity impairment. Moreover, an opposite pattern was found when comparing the activation status of specific survival pathways between APP/PS1 OBs and OBs derived from sAD subjects with different neuropathological grading. Our data reflect, in part, the progressive effect of APP overproduction and Aβ accumulation on the OB proteome during AD progression.

Yeast Two-Hybrid Screening for Proteins that Interact with the Extracellular Domain of Amyloid Precursor Protein

Alzheimer's disease (AD) is a neurodegenerative disorder in which amyloid β plaques are a pathological characteristic. Little is known about the physiological functions of amyloid β precursor protein (APP). Based on its structure as a type I transmembrane protein, it has been proposed that APP might be a receptor, but so far, no ligand has been reported. In the present study, 9 proteins binding to the extracellular domain of APP were identified using a yeast two-hybrid system. After confirming the interactions in the mammalian system, mutated PLP1, members of the FLRT protein family, and KCTD16 were shown to interact with APP. These proteins have been reported to be involved in Pelizaeus-Merzbacher disease (PMD) and axon guidance. Therefore, our results shed light on the mechanisms of physiological function of APP in AD, PMD, and axon guidance.

Comparative analysis of single and combined APP/APLP knockouts reveals reduced spine density in APP-KO mice that is prevented by APPsα expression

Synaptic dysfunction and synapse loss are key features of Alzheimer's pathogenesis. Previously, we showed an essential function of APP and APLP2 for synaptic plasticity, learning and memory. Here, we used organotypic hippocampal cultures to investigate the specific role(s) of APP family members and their fragments for dendritic complexity and spine formation of principal neurons within the hippocampus. Whereas CA1 neurons from APLP1-KO or APLP2-KO mice showed normal neuronal morphology and spine density, APP-KO mice revealed a highly reduced dendritic complexity in mid-apical dendrites. Despite unaltered morphology of APLP2-KO neurons, combined APP/APLP2-DKO mutants showed an additional branching defect in proximal apical dendrites, indicating redundancy and a combined function of APP and APLP2 for dendritic architecture. Remarkably, APP-KO neurons showed a pronounced decrease in spine density and reductions in the number of mushroom spines. No further decrease in spine density, however, was detectable in APP/APLP2-DKO mice. Mechanistically, using APPsα-KI mice lacking transmembrane APP and expressing solely the secreted APPsα fragment we demonstrate that APPsα expression alone is sufficient to prevent the defects in spine density observed in APP-KO mice. Collectively, these studies reveal a combined role of APP and APLP2 for dendritic architecture and a unique function of secreted APPs for spine density.

Beyond pathology: APP, brain development and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia among the elderly. Research in the AD field has been mostly focused on the biology of the Aβ peptide but increasing evidence is shifting attention toward the physiological role of APP as key to understanding AD pathology. It is becoming apparent that APP plays a central role in the mechanisms that guarantee the accuracy and the robustness of brain wiring. In the present review we explore APP functions with focus on some of the underlying molecular mechanisms.

Amyloid precursor protein is an autonomous growth cone adhesion molecule engaged in contact guidance

Amyloid precursor protein (APP), a transmembrane glycoprotein, is well known for its involvement in the pathogenesis of Alzheimer disease of the aging brain, but its normal function is unclear. APP is a prominent component of the adult as well as the developing brain. It is enriched in axonal growth cones (GCs) and has been implicated in cell adhesion and motility. We tested the hypothesis that APP is an extracellular matrix adhesion molecule in experiments that isolated the function of APP from that of well-established adhesion molecules. To this end we plated wild-type, APP-, or β1-integrin (Itgb1)- misexpressing mouse hippocampal neurons on matrices of either laminin, recombinant L1, or synthetic peptides binding specifically to Itgb1 s or APP. We measured GC adhesion, initial axonal outgrowth, and substrate preference on alternating matrix stripes and made the following observations: Substrates of APP-binding peptide alone sustain neurite outgrowth; APP dosage controls GC adhesion to laminin and APP-binding peptide as well as axonal outgrowth in Itgb1- independent manner; and APP directs GCs in contact guidance assays. It follows that APP is an independently operating cell adhesion molecule that affects the GC's phenotype on APP-binding matrices including laminin, and that it is likely to affect axon pathfinding in vivo.

Crystal structure of the E2 domain of amyloid precursor protein-like protein 1 in complex with sucrose octasulfate

Missense mutations in the amyloid precursor protein (APP) gene can cause familial Alzheimer disease. It is thought that APP and APP-like proteins (APLPs) may play a role in adhesion and signal transduction because their ectodomains interact with components of the extracellular matrix. Heparin binding induces dimerization of APP and APLPs. To help explain how these proteins interact with heparin, we have determined the crystal structure of the E2 domain of APLP1 in complex with sucrose octasulfate (SOS). A total of three SOS molecules are bound to the E2 dimer. Two SOSs are bound inside a narrow intersubdomain groove, and the third SOS is bound near the two-fold axis of the protein. Mutational analyses show that most residues interacting with SOS also contribute to heparin binding, although in varying degrees; a deep pocket, defined by His-376, Lys-422, and Arg-429, and an interfacial site between Lys-314 and its symmetry mate are most important in the binding of the negatively charged polysaccharide. Comparison with a lower resolution APP structure shows that all key heparin binding residues are conserved and identically positioned, suggesting that APLP1 and APP may bind heparin similarly. In transfected HEK-293 cells, mutating residues responsible for heparin binding causes little change in the proteolysis of APP by the secretases. However, mutating a pair of conserved basic residues (equivalent to Arg-414 and Arg-415 of APLP1) immediately adjacent to the heparin binding site affects both the maturation and the processing of APP.

A domain level interaction network of amyloid precursor protein and Abeta of Alzheimer's disease

The primary constituent of the amyloid plaque, beta-amyloid (Abeta), is thought to be the causal "toxic moiety" of Alzheimer's disease. However, despite much work focused on both Abeta and its parent protein, amyloid precursor protein (APP), the functional roles of APP and its cleavage products remain to be fully elucidated. Protein-protein interaction networks can provide insight into protein function, however, high-throughput data often report false positives and are in frequent disagreement with low-throughput experiments. Moreover, the complexity of the CNS is likely to be under represented in such databases. Therefore, we curated the published work characterizing both APP and Abeta to create a protein interaction network of APP and its proteolytic cleavage products, with annotation, where possible, to the level of APP binding domain and isoform. This is the first time that an interactome has been refined to domain level, essential for the interpretation of APP due to the presence of multiple isoforms and processed fragments. Gene ontology and network analysis were used to identify potentially novel functional relationships among interacting proteins.

Characterization of the beta amyloid precursor protein-like gene in the central nervous system of the crab Chasmagnathus. Expression during memory consolidation

Background

Human β-amyloid, the main component in the neuritic plaques found in patients with Alzheimer's disease, is generated by cleavage of the β-amyloid precursor protein. Beyond the role in pathology, members of this protein family are synaptic proteins and have been associated with synaptogenesis, neuronal plasticity and memory, both in vertebrates and in invertebrates. Consolidation is necessary to convert a short-term labile memory to a long-term and stable form. During consolidation, gene expression and de novo protein synthesis are regulated in order to produce key proteins for the maintenance of plastic changes produced during the acquisition of new information.

Results

Here we partially cloned and sequenced the beta-amyloid precursor protein like gene homologue in the crab Chasmagnathus (cappl), showing a 37% of identity with the fruit fly Drosophila melanogaster homologue and 23% with Homo sapiens but with much higher degree of sequence similarity in certain regions. We observed a wide distribution of cappl mRNA in the nervous system as well as in muscle and gills. The protein localized in all tissues analyzed with the exception of muscle. Immunofluorescence revealed localization of cAPPL in associative and sensory brain areas. We studied gene and protein expression during long-term memory consolidation using a well characterized memory model: the context-signal associative memory in this crab species. mRNA levels varied at different time points during long-term memory consolidation and correlated with cAPPL protein levels

Conclusions

cAPPL mRNA and protein is widely distributed in the central nervous system of the crab and the time course of expression suggests a role of cAPPL during long-term memory formation.

Structural characterization of the E2 domain of APL-1, a Caenorhabditis elegans homolog of human amyloid precursor protein, and its heparin binding site

The amyloid beta-peptide deposit found in the brain tissue of patients with Alzheimer disease is derived from a large heparin-binding protein precursor APP. The biological function of APP and its homologs is not precisely known. Here we report the x-ray structure of the E2 domain of APL-1, an APP homolog in Caenorhabditis elegans, and compare it to the human APP structure. We also describe the structure of APL-1 E2 in complex with sucrose octasulfate, a highly negatively charged disaccharide, which reveals an unexpected binding pocket between the two halves of E2. Based on the crystal structure, we are able to map, using site-directed mutagenesis, a surface groove on E2 to which heparin may bind. Our biochemical data also indicate that the affinity of E2 for heparin is influenced by pH: at pH 5, the binding appears to be much stronger than that at neutral pH. This property is likely caused by histidine residues in the vicinity of the mapped heparin binding site and could be important for the proposed adhesive function of APL-1.

Subcellular localization and dimerization of APLP1 are strikingly different from APP and APLP2

The molecular association between APP and its mammalian homologs has hardly been explored. In systematically addressing this issue, we show by live cell imaging that APLP1 mainly localizes to the cell surface, whereas APP and APLP2 are mostly found in intracellular compartments. Homo- and heterotypic cis interactions of APP family members could be detected by FRET and co-immunoprecipitation analysis and occur in a modular mode. Only APLP1 formed trans interactions, supporting the argument for a putative specific role of APLP1 in cell adhesion. Deletion mutants of APP family members revealed two highly conserved regions as important for the protein crosstalk. In particular, the N-terminal half of the ectodomain was crucial for APP and APLP2 interactions. By contrast, multimerization of APLP1 was only partially dependent on this domain but strongly on the C-terminal half of the ectodomain. We further observed that coexpression of APP with APLP1 or APLP2 leads to diminished generation of Abeta42. The current data suggest that this is due to the formation of heteromeric complexes, opening the way for novel therapeutic strategies targeting these complexes.

Secreted APP regulates the function of full-length APP in neurite outgrowth through interaction with integrin beta1

BACKGROUND

Beta-amyloid precursor protein (APP) has been reported to play a role in the outgrowth of neurites from cultured neurons. Both cell-surface APP and its soluble, ectodomain cleavage product (APPs-alpha) have been implicated in regulating the length and branching of neurites in a variety of assays, but the mechanism by which APP performs this function is not understood.

RESULTS

Here, we report that APP is required for proper neurite outgrowth in a cell autonomous manner, both in vitro and in vivo. Neurons that lack APP undergo elongation of their longest neurite. Deletion of APLP1 or APLP2, homologues of APP, likewise stimulates neurite lengthening. Intriguingly, wild-type neurons exposed to APPs-alpha, the principal cleavage product of APP, also undergo neurite elongation. However, APPs-alpha is unable to stimulate neurite elongation in the absence of cellular APP expression. The outgrowth-enhancing effects of both APPs-alpha and the deletion of APP are inhibited by blocking antibodies to Integrin beta1 (Itgbeta1). Moreover, full length APP interacts biochemically with Itgbeta1, and APPs-alpha can interfere with this binding.

CONCLUSION

Our findings indicate that APPs-alpha regulates the function of APP in neurite outgrowth via the novel mechanism of competing with the binding of APP to Itgbeta1.

Gamma-secretase and metalloproteinase activity regulate the distribution of endoplasmic reticulum to hippocampal neuron dendritic spines

The neuronal endoplasmic reticulum (ER) contributes to many physiological and pathological processes in the brain. A subset of dendritic spines on hippocampal neurons contains ER that may contribute to synapse-specific intracellular signaling. Distribution of ER to spines is dynamic, but knowledge of the regulatory mechanisms is lacking. In live cell imaging experiments we now show that cultured hippocampal neurons rapidly lost ER from spines after phorbol ester treatment. ER loss was reduced by inhibiting gamma-secretase (DAPT at 2 microM) and metalloproteinase (TAPI-0 and GM6001 at 4 microM) activity. Inhibition of protein kinase C also diminished loss of ER by preventing exit of ER from spines. Furthermore, gamma-secretase and metalloproteinase inhibition, in the absence of phorbol ester, triggered a dramatic increase in spine ER content. Metalloproteinases and gamma-secretase cleave several transmembrane proteins. Many of these substrates are known to localize to adherens junctions, a structural specialization with which spine ER interacts. One interesting possibility is thus that ER content within spines may be regulated by proteolytic activity affecting adherens junctions. Our data demonstrate a hitherto unknown role for these two proteolytic activities in regulating dynamic aspects of cellular ultrastructure, which is potentially important for cellular calcium homeostasis and several intracellular signaling pathways.

Morphoregulation by acetylcholinesterase in fibroblasts and astrocytes

Acetylcholinesterase (AChE) terminates neurotransmission at cholinergic synapses by hydrolysing acetylcholine, but also has non-enzymatic morphoregulatory effects on neurons such as stimulation of neurite outgrowth. It is widely expressed outside the nervous system, but its function in non-neuronal cells is unclear. Here we have investigated the distribution and function of AChE in fibroblasts and astrocytes. We show that these cells express high levels of AChE protein that co-migrates with recombinant AChE but contains little catalytic activity. Fibroblasts express transcripts encoding the synaptic AChE-T isoform and its membrane anchoring peptide PRiMA-I. AChE is strikingly distributed in arcs, rings and patches at the leading edge of spreading and migrating fibroblasts and astrocytes, close to the cell-substratum interface, and in neuronal growth cones. During in vivo healing of mouse skin, AChE becomes highly expressed in re-epithelialising epidermal keratinocytes 1 day after wounding. AChE appears to be functionally important for polarised cell migration, since an AChE antibody reduces substratum adhesion of fibroblasts, and slows wound healing in vitro as effectively as a beta1-integrin antibody. Moreover, elevation of AChE expression increases fibroblast wound healing independently of catalytic activity. Interestingly, AChE surface patches precisely co-localise with amyloid precursor protein and the extracellular matrix protein perlecan, but not focal adhesions or alpha-dystroglycan, and contain a high concentration of tyrosine phosphorylated proteins in spreading cells. These findings suggest that cell surface AChE, possibly in a novel signalling complex containing APP and perlecan, contributes to a generalised mechanism for polarised membrane protrusion and migration in all adherent cells.

beta-amyloid precursor protein can be transported independent of any sorting signal to the axonal and dendritic compartment

In neurons, amyloid precursor protein (APP) is localized to the dendritic and axonal compartment. Changes in subcellular localization affect secretase cleavage of APP, altering the generation of Abeta, and presumably also its pathogenic features. It was reported that APP is sorted initially to the axon and transcytosed subsequently to the somatodendritic compartment. This may be carried out by a recessive dendritic sorting signal in the cytoplasmic C-terminus, possibly the tyrosine based basolateral sorting signal (BaSS), and an axonal sorting motif within the extracellular juxtamembraneous domain. We investigated whether the C- or N-terminal domain of APP contains an independent dendritic or axonal sorting signal. We generated different APP deletion mutants, and produced chimeric proteins of APP and a non-related Type I transmembrane protein. Quantitative immunocytochemical analyses of transfected primary neurons showed that similar amounts of all APP mutants, lacking either the N- or C-terminus, were transported to the axonal and dendritic compartment. Investigations of the chimeric proteins showed that neither the N- nor the C-terminus of APP functions as independent sorting signal, whereas another tyrosine based dendritic sorting signal was sufficient to prevent axonal entry of APP. This data shows that, under steady state conditions, Heterologously expressed APP is transported equally to axons and dendrites irrespective of any putative sorting signal in its N- or C-terminus. This shows that APP can enter the axon in absence of the initial axonal sorting motif, indicating the existence of an alternative pathway allowing axonal entry of APP.

The redox chemistry of the Alzheimer's disease amyloid beta peptide

There is a growing body of evidence to support a role for oxidative stress in Alzheimer's disease (AD), with increased levels of lipid peroxidation, DNA and protein oxidation products (HNE, 8-HO-guanidine and protein carbonyls respectively) in AD brains. The brain is a highly oxidative organ consuming 20% of the body's oxygen despite accounting for only 2% of the total body weight. With normal ageing the brain accumulates metals ions such iron (Fe), zinc (Zn) and copper (Cu). Consequently the brain is abundant in antioxidants to control and prevent the detrimental formation of reactive oxygen species (ROS) generated via Fenton chemistry involving redox active metal ion reduction and activation of molecular oxygen. In AD there is an over accumulation of the Amyloid beta peptide (Abeta), this is the result of either an elevated generation from amyloid precursor protein (APP) or inefficient clearance of Abeta from the brain. Abeta can efficiently generate reactive oxygen species in the presence of the transition metals copper and iron in vitro. Under oxidative conditions Abeta will form stable dityrosine cross-linked dimers which are generated from free radical attack on the tyrosine residue at position 10. There are elevated levels of urea and SDS resistant stable linked Abeta oligomers as well as dityrosine cross-linked peptides and proteins in AD brain. Since soluble Abeta levels correlate best with the degree of degeneration [C.A. McLean, R.A. Cherny, F.W. Fraser, S.J. Fuller, M.J. Smith, K. Beyreuther, A.I. Bush, C.L. Masters, Soluble pool of Abeta amyloid as a determinant of severity of neurodegeneration in Alzheimer's disease, Ann. Neurol. 46 (1999) 860-866] we suggest that the toxic Abeta species corresponds to a soluble dityrosine cross-linked oligomer. Current therapeutic strategies using metal chelators such as clioquinol and desferrioxamine have had some success in altering the progression of AD symptoms. Similarly, natural antioxidants curcumin and ginkgo extract have modest but positive effects in slowing AD development. Therefore, drugs that target the oxidative pathways in AD could have genuine therapeutic efficacy.

The cytosolic domain of APP induces the relocalization of dynamin 3 in hippocampal neurons

Amyloid precursor protein (APP) has been the subject of intense research to uncover its implication in Alzheimer's disease. Its physiological function is, however, still poorly understood. Herein, we investigated its possible influence on the development of cultured hippocampal neurons. A peptide corresponding to the APP intracellular domain linked to a cell-penetrating peptide was used to alter the interactions of APP with its cytosolic partners. This treatment promoted the concentration of the cytosolic GTPase dynamin 3 (Dyn3) in neurite segments when most untreated cells displayed a homogenous punctate distribution of Dyn3. The Dyn3-labelled segments were excluded from those revealed by APP staining after aldehyde fixation. Interestingly, after aldehyde fixation MAP2 also labelled segments excluded from APP-stained segments. Thus APP is also a marker for the spacing pattern of neurites demonstrated by Taylor & Fallon (2006)J. Neurosci., 26, 1154-4463.

Amyloid precursor protein: from synaptic plasticity to Alzheimer's disease

The amyloid precursor protein (APP) has been shown to be implicated in age-associated plastic changes at synapses that might contribute to memory loss in Alzheimer's disease. As APP has previously been reported to have multiple functions during normal development, and as human and avian APP share 95% homology in amino acid sequence, we have employed a one-trial passive avoidance task in day-old chicks to study its role in the process of memory formation. Administration of anti-APP antibodies, raised against human APP, APP-antisense, and Abeta during pre-training, prevented memory formation without effects on general behavior or initial acquisition. Amnesia is apparent by 30 min post-training and lasts for at least 24 hours. Injection of APP-derived peptides RERMS (APP(328-332)) and RER (APP(328-330)) homologous to the short stretches of amino acids in the Kang sequence (APP(319-335)), rescue the memory in animals rendered amnestic by previous (anti-APP antibody, antisense, and Abeta pretreatments. The protected form of RER, with a prolonged half-life (acetylated RER), proved to be effective when injected intracranially and peripherally. The tripeptide RER exerts its biological activity by binding to two neuronal plasma membrane proteins (60 and 110 kDa). The results obtained in this study suggest that RER alleviates memory deficits via receptor-mediated events, and that short APP-derived peptides might represent a novel group of therapeutically active molecules for the alleviation of memory deficits in age-related dementias.

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.

Solution Conformation and Heparin-induced Dimerization of the Full-length Extracellular Domain of the Human Amyloid Precursor Protein

Proteolytic cleavage of the amyloid precursor protein (APP) by beta and gamma-secretases gives rise to the beta-amyloid peptide, considered to be a causal factor in Alzheimer's disease. Conversely, the soluble extracellular domain of APP (sAPPalpha), released upon its cleavage by alpha-secretase, plays a number of important physiological functions. Several APP fragments have been structurally characterized at atomic resolution, but the structures of intact APP and of full-length sAPPalpha have not been determined. Here, ab initio reconstruction of molecular models from high-resolution solution X-ray scattering (SAXS) data for the two main isoforms of sAPPalpha (sAPPalpha(695) and sAPPalpha(770)) provided models of sufficiently high resolution to identify distinct structural domains of APP. The fragments for which structures are known at atomic resolution were fitted within the solution models of full-length sAPPalpha, allowing localization of important functional sites (i.e. glycosylation, protease inhibitory and heparin-binding sites). Furthermore, combined results from SAXS, analytical ultracentrifugation (AUC) and size-exclusion chromatography (SEC) analysis indicate that both sAPPalpha isoforms are monomeric in solution. On the other hand, SEC, bis-ANS fluorescence, AUC and SAXS measurements showed that sAPPalpha forms a 2:1 complex with heparin. A conformational model for the sAPPalpha:heparin complex was also derived from the SAXS data. Possible implications of such complex formation for the physiological dimerization of APP and biological signaling are discussed in terms of the structural models proposed.

Alzheimer's disease as a disorder of dynamic brain self-organization

Mental function is based on the dynamic organization of neuronal networks. In particular, phylogenetically young brain areas (e.g., cortical associative circuits), involved in the realization of "higher brain functions" such as learning, memory, perception, self-awareness, and consciousness, are continuously re-adjusted even after development is completed. By this life-long self-optimization process, epigenetic information remodels the cognitive, behavioral and emotional reactivity of an individual to meet the environmental demands. To organize brain structures of increasing complexity during evolution, the process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. The mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodeling according to experience, are accompanied, however, by an increasing inherent potential of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time may provide the basis for selective neuronal vulnerability. The mechanisms of synaptic plasticity, i.e., of modifiable interneuronal connectivity, are largely based on external morphoregulatory cues and internal signaling pathways that nonneuronal cells have phylogenetically acquired to sense their relationship to the local neighborhood and to control proliferation and differentiation in the process of tissue repair and regeneration after development is completed. Differentiated neurons that have withdrawn from the cell cycle use these molecular machinery alternatively to control synaptic plasticity. The existence of these alternative effector pathways within a neuron puts it on the risk to erroneously convert signals derived from plastic synaptic changes into positional cues that will activate the cell cycle. This cell cycle activation potentially links synaptic plasticity to cell death. Preventing cell cycle activation by locking neurons in a differentiated but still highly plastic phenotype will, thus, be crucial to prevent neurodegeneration.

Cortical dysplasia resembling human type 2 lissencephaly in mice lacking all three APP family members

The Alzheimer's disease beta-amyloid precursor protein (APP) is a member of a larger gene family that includes the amyloid precursor-like proteins, termed APLP1 and APLP2. We previously documented that APLP2-/-APLP1-/- and APLP2-/-APP-/- mice die postnatally, while APLP1-/-APP-/- mice and single mutants were viable. We now report that mice lacking all three APP/APLP family members survive through embryonic development, and die shortly after birth. In contrast to double-mutant animals with perinatal lethality, 81% of triple mutants showed cranial abnormalities. In 68% of triple mutants, we observed cortical dysplasias characterized by focal ectopic neuroblasts that had migrated through the basal lamina and pial membrane, a phenotype that resembles human type II lissencephaly. Moreover, at E18.5 triple mutants showed a partial loss of cortical Cajal Retzius (CR) cells, suggesting that APP/APLPs play a crucial role in the survival of CR cells and neuronal adhesion. Collectively, our data reveal an essential role for APP family members in normal brain development and early postnatal survival.

The X-ray structure of an antiparallel dimer of the human amyloid precursor protein E2 domain

Amyloid beta-peptide, which forms neuronal and vascular amyloid deposits in Alzheimer's disease, is derived from an integral membrane protein precursor. The biological function of the precursor is currently unclear. Here we describe the X-ray structure of E2, the largest of the three conserved domains of the precursor. The structure of E2 consists of two coiled-coil substructures connected through a continuous helix and bears an unexpected resemblance to the spectrin family of protein structures. E2 can reversibly dimerize in the solution, and the dimerization occurs along the longest dimension of the molecule in an antiparallel orientation, which enables the N-terminal substructure of one monomer to pack against the C-terminal substructure of a second monomer. Heparan sulfate proteoglycans, the putative ligand for the precursor present in extracellular matrix, bind to E2 at a conserved and positively charged site near the dimer interface.

Beta-secretase-cleaved amyloid precursor protein in Alzheimer brain: a morphologic study

beta-amyloid (Abeta) is the main constituent of senile plaques seen in Alzheimer's disease. Abeta is derived from the amyloid precursor protein (APP) via proteolytic cleavage by proteases beta- and gamma-secretase. In this study, we examined content and localization of beta-secretase-cleaved APP (beta-sAPP) in brain tissue sections from the frontal, temporal and occipital lobe. Strong granular beta-sAPP staining was found throughout the gray matter of all three areas, while white matter staining was considerably weaker. beta-sAPP was found to be localized in astrocytes and in axons. We found the beta-sAPP immunostaining to be stronger and more extensive in gray matter in Alzheimer disease (AD) cases than controls. The axonal beta-sAPP staining was patchy and unevenly distributed for the AD cases, indicating impaired axonal transport. beta-sAPP was also found surrounding senile plaques and cerebral blood vessels. The results presented here show altered beta-sAPP staining in the AD brain, suggestive of abnormal processing and transport of APP.

Human neurodegenerative disease modeling using Drosophila

A number of approaches have been taken to recreate and to study the role of genes associated with human neurodegenerative diseases in the model organism Drosophila. These studies encompass the polyglutamine diseases, Parkinson's disease, Alzheimer's disease, and tau-associated pathologies. The findings highlight Drosophila as an important model system in which to study the fundamental pathways influenced by these genes and have led to new insights into aspects of pathogenesis and modifier mechanisms.

Roles of amyloid precursor protein and its fragments in regulating neural activity, plasticity and memory

Amyloid-beta precursor protein (APP) is a membrane-spanning protein with a large extracellular domain and a much smaller intracellular domain. It is the source of the amyloid-beta (Abeta) peptide found in neuritic plaques of Alzheimer's disease (AD) patients. Because Abeta shows neurotoxic properties, and because familial forms of AD promote Abeta accumulation, a massive international research effort has been aimed at understanding the mechanisms of Abeta generation, catabolism and toxicity. APP, however, is an extremely complex molecule that may be a functionally important molecule in its full-length configuration, as well as being the source of numerous fragments with varying effects on neural function. For example, one fragment derived from the non-amyloidogenic processing pathway, secreted APPalpha (sAPPalpha), is neuroprotective, neurotrophic and regulates cell excitability and synaptic plasticity, while Abeta appears to exert opposing effects. Less is known about the neural functions of other fragments, but there is a growing interest in understanding the basic biology of APP as it has become recognized that alterations in the functional activity of the APP fragments during disease states will have complex effects on cell function. Indeed, it has been proposed that reductions in the level or activity of certain APP fragments, in addition to accumulation of Abeta, may play a critical role in the cognitive dysfunction associated with AD, particularly early in the course of the disease. To test and modify this hypothesis, it is important to understand the roles that full-length APP and its fragments normally play in neuronal structure and function. Here we review evidence addressing these fundamental questions, paying particular attention to the contributions that APP fragments play in synaptic transmission and neural plasticity, as these may be key to understanding their effects on learning and memory. It is clear from this literature that APP fragments, including Abeta, can exert a powerful regulation of key neural functions including cell excitability, synaptic transmission and long-term potentiation, both acutely and over the long-term. Furthermore, there is a small but growing literature confirming that these fragments correspondingly regulate behavioral learning and memory. These data indicate that a full account of cognitive dysfunction in AD will need to incorporate the actions of the full complement of APP fragments. To this end, there is an urgent need for a dedicated research effort aimed at understanding the behavioral consequences of altered levels and activity of the different APP fragments as a result of experience and disease.

Aberrant activation of focal adhesion proteins mediates fibrillar amyloid beta-induced neuronal dystrophy

Neuronal dystrophy is a pathological hallmark of Alzheimer's disease (AD) that is not observed in other neurodegenerative disorders that lack amyloid deposition. Treatment of cortical neurons with fibrillar amyloid beta (Abeta) peptides induces progressive neuritic dystrophy accompanied by a marked loss of synaptophysin immunoreactivity (Grace et al., 2002). Here, we report that fibrillar Abeta-induced neuronal dystrophy is mediated by the activation of focal adhesion (FA) proteins and the formation of aberrant FA structures adjacent to Abeta deposits. In the AD brain, activated FA proteins are observed associated with the majority of senile plaques. Clustered integrin receptors and activated paxillin (phosphorylated at Tyr-31) and focal adhesion kinase (phosphorylated at Tyr-297) are mainly detected in dystrophic neurites surrounding Abeta plaque cores, where they colocalize with hyperphosphorylated tau. Deletion experiments demonstrated that the presence of the LIM domains in the paxillin C terminus and the recruitment of the protein-Tyr phosphatase (PTP)-PEST to the FA complex are required for Abeta-induced neuronal dystrophy. Therefore, both paxillin and PTP-PEST appear to be critical elements in the generation of the dystrophic response. Paxillin is a scaffolding protein to which other FA proteins bind, leading to the formation of the FA contact and initiation of signaling cascades. PTP-PEST plays a key role in the dynamic regulation of focal adhesion contacts in response to extracellular cues. Thus, in the AD brain, fibrillar Abeta may induce neuronal dystrophy by triggering a maladaptive plastic response mediated by FA protein activation and tau hyperphosphorylation.

The beta-amyloid precursor protein (APP) and Alzheimer's disease: does the tail wag the dog?

The beta-amyloid precursor protein has been the focus of much attention from the Alzheimer's disease community for the past decade and a half. The beta-amyloid precursor protein holds a pivotal position in Alzheimer's disease research because it is the precursor to the amyloid beta-protein which many believe plays a central role in Alzheimer's disease pathogenesis. It was also the first gene in which mutations associated with inherited Alzheimer's disease were found. Although the molecular details of the generation of amyloid beta-protein from beta-amyloid precursor protein are being unraveled, the actual physiological functions of beta-amyloid precursor protein are far from clear. This situation is changing as accumulating new evidence suggests that the C-terminal cytosolic tail of beta-amyloid precursor protein may have multiple biological activities, ranging from axonal transport to nuclear signaling. This article reviews the current state of knowledge about the biological functions of beta-amyloid precursor protein.

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.

Biogenesis and metabolism of Alzheimer's disease Abeta amyloid peptides

Biochemical and genetic evidence indicates the balance of biogenesis/clearance of Abeta amyloid peptides is altered in Alzheimer's disease. Abeta is derived, by two sequential cleavages, from the receptor-like amyloid precursor protein (APP). The proteases involved are beta-secretase, identified as the novel aspartyl protease BACE, and gamma-secretase, a multimeric complex containing the presenilins (PS). Gamma-secretase can release either Abeta40 or the more aggregating and cytotoxic Abeta42. Secreted Abeta peptides become either degraded by the metalloproteases insulin-degrading enzyme (IDE) and neprilysin or metabolized through receptor uptake mediated by apolipoprotein E. Therapeutic approaches based on secretase inhibition or amyloid clearance are currently under development.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Expression of mutant amyloid precursor proteins decreases adhesion and delays differentiation of Hep-1 cells

The amyloid precursor protein (APP) is a type I integral membrane protein and is processed to generate several intra-cellular and secreted fragments. The physiological role of APP and its processed fragments is unclear. Several mutations have been discovered in APP, which are causative of early-onset, familial, neurological disease, including Alzheimer's disease (FAD). These mutations alter the processing of APP and lead to excess production and extra-cellular deposition of A-beta peptide (Abeta). We have examined the role of APP in a cell culture model of endothelial cell function. The endothelial cell line, Hep-1, was stably transfected with wild-type (wt) and FAD mutant forms of APP (mAPP). Secretion of sAPPalpha was reduced in cell lines over-expressing mAPP when these cells were grown on several different substrates. Levels of secreted Abeta were increased as measured by ELISA in the mutant cell lines. Cell adhesion to laminin-, fibronectin-, collagen I-, and collagen IV-coated culture flasks was reduced in all mAPP-expressing cell lines, while in lines over-expressing wt-APP, adhesiveness was slightly increased. Cell lines over-expressing mAPP differentiated more slowly into capillary network-like structures on Matrigel than those expressing wt-APP. No differences were detected among all cell lines in a migration/invasion assay. The results suggest that APP may have a role in cell adhesiveness and maturation of endothelial cells into capillary-like networks. The reduction in adhesion and differentiation in mutant cell lines may be due to reduced amounts of sAPPalpha released into the culture media or toxic effects of increased extracellular Abeta.

Ultrastructural analyses of beta-amyloid in the aged dog brain: neuronal beta-amyloid is localized to the plasma membrane

1. An electron microscopic study was undertaken to study beta-amyloid (Abeta) deposition and neuropathology in aged dogs. 2. A positive correlation between Abeta deposits and neuropathology was found in some dogs. Massive Abeta deposition was correlated to advanced lesions. 3. By use of immunocytochemistry Abeta fibers were identified within plaques, around vessels and in association with cell membranes.

Ultrastructural evidence of fibrillar beta-amyloid associated with neuronal membranes in behaviorally characterized aged dog brains

The aged dog brain accumulates beta-amyloid in the form of diffuse senile plaques, which provides a potentially useful in vivo model system for studying the events surrounding the deposition of beta-amyloid. We used postembedding immunocytochemistry at the electron microscopic level to determine the subcellular distribution of beta-amyloid 1-40 and beta-amyloid 1-42 peptides in the prefrontal and parietal cortex of behaviorally characterized dogs ranging in age from one to 17 years. Immunogold particles signaling beta-amyloid 1-42 occurred over intracellular and extracellular fibrils that were approximately 8 nm in width. Intracellular beta-amyloid 1-42 fibrils were found in close proximity to glial fibrillary acidic protein fibers within astrocytes, but only in cells with signs of plasma membrane disruption. Neuronal labeling of beta-amyloid 1-42 appears to be associated with the plasma membrane. Membrane-bound beta-amyloid 1-42 occurs in the form of fine fibrils that are embedded in the dendritic membrane and appear to project into the extracellular space as determined by quantitative analysis of the immunogold particle distribution. Bundles of beta-amyloid 1-42 were also closely associated and/or integrated with degenerating myelin sheaths of axons. In one dog that was impaired on several cognitive tasks, extensive beta-amyloid 1-42 deposition was associated with microvacuolar changes and vascular pathology. The present findings suggest that beta-amyloid 1-42 may be generated at the dendritic plasma membrane as well as in intracellular compartments. The close association between beta-amyloid 1-42 and destroyed myelin suggests one possible new mechanism by which beta-amyloid 1-42 induces neurodegeneration.

The beta-amyloid precursor protein and its derivatives: from biology to learning and memory processes

Intensive investigation towards the understanding of the biology and physiological functions of the beta-amyloid precursor protein (APP) have been supported since it is known that a 39-43 amino acid fragment of APP, called the beta-amyloid protein (Abeta), accumulates in the brain parenchyma to form the typical lesions associated with Alzheimer's disease (AD). It emerges from extensive data that APP and its derivatives show a wide range of contrasting physiological properties and therefore might be involved in distinct physiological functions. Abeta has been shown to disrupt neuronal activity and to demonstrate neurotoxic properties in a wide range of experimental procedures. In contrast, both in vitro and in vivo studies suggest that APP and/or its secreted forms are important factors involved in the viability, growth and morphological and functional plasticity of nerve cells. Furthermore, several recent studies suggest that APP and its derivatives have an important role in learning and memory processes. Memory impairments can be induced in animals by intracerebral treatment with Abeta. Altered expression of the APP gene in aged animals or in genetically-modified animals also leads to memory deficits. By contrast, secreted forms of APP have recently been shown to facilitate learning and memory processes in mice. These interesting findings open novel perspectives to understand the involvement of APP in the development of cognitive deficits associated with AD. In this review, we summarize the current data concerning the biology and the behavioral effects of APP and its derivatives which may be relevant to the roles of these proteins in memory and in AD pathology.

What the evolution of the amyloid protein precursor supergene family tells us about its function

The Alzheimer's disease amyloid protein precursor (APP) gene is part of a multi-gene super-family from which sixteen homologous amyloid precursor-like proteins (APLP) and APP species homologues have been isolated and characterised. Comparison of exon structure (including the uncharacterised APL-1 gene), construction of phylogenetic trees, and analysis of the protein sequence alignment of known homologues of the APP super-family were performed to reconstruct the evolution of the family and to assess the functional significance of conserved protein sequences between homologues. This analysis supports an adhesion function for all members of the APP super family, with specificity determined by those sequences which are not conserved between APLP lineages, and provides evidence for an increasingly complex APP superfamily during evolution. The analysis also suggests that Drosophila APPL and Caenorhabditis elegans APL-1 may be a fourth APLP lineage indicating that these proteins, while not functional homologues of human APP, are similarly likely to regulate cell adhesion. Furthermore, the betaA4 sequence is highly conserved only in APP orthologues, strongly suggesting this sequence is of significant functional importance in this lineage.

Hemisynaptic distribution patterns of presenilins and beta-APP isoforms in the rodent cerebellum and hippocampus

Healthy brain neurons co-express Alzheimer's disease (AD) related proteins presenilins (PS) and beta-amyloid precursor protein (beta-APP). Deposition of beta-amyloid and PS in the senile plaques of AD brain and their ability to interact in vitro suggest that AD pathology could arise from a defect in the physiological interactions between beta-APP and PS within and/or between neurons. The present study compares the immunocytochemical distribution of PS (1 and 2) and beta-APP major isoforms (695 and 751/770) in the synapses of the cerebellum and hippocampus of the adult rat and mouse. In the cerebellar cortex of both species, the four molecules are immunodetected in the presynaptic or the postsynaptic compartments of synapses, suggesting that they are involved in interneuronal relationships. In contrast, PS and beta-APP are postsynaptic in almost all the immunoreactive synapses of the hippocampus. The different distribution patterns of these proteins in cerebellar and hippocampal synapses may reflect specific physiological differences, responsible for differential vulnerability of neurons to AD synaptic pathology. Defective interactions between beta-APP and PS at the synapses could impede the synaptic functions of beta-APP, inducing the selective loss of synapses that accounts for cognitive impairment in AD.

Upregulation of amyloid precursor protein messenger RNA in response to traumatic brain injury: an ovine head impact model

There is evidence that the amyloid precursor protein (APP) plays an important role in neuronal growth and synaptic plasticity and that its increased expression following traumatic brain injury represents an acute phase response to trauma. We hypothesized that the previously described increased APP expression in response to injury (Van den Heuvel et al., Acta Neurochir. Suppl. 71, 209-211) is due to increased mRNA expression and addressed this by examining the expression of APP mRNA and APP within neuronal cell bodies over time in an ovine head impact model. Twenty-five anesthetized and ventilated 2-year-old Merino ewes sustained a left temporal head impact using a humane stunner and 9 normal sheep were used as nonimpact controls. Following postimpact survival periods of 15, 30, 45, 60, and 120 min, brains were perfusion fixed in 4% paraformaldehyde and examined according to standard neuropathological protocol. APP mRNA and antigen expression were examined in 5-microm sections by nonisotopic in situ hybridization and APP immunocytochemistry. The percentage of brain area with APP immunoreactivity within neuronal cell bodies in the impacted animals increased with time from a mean of 7.5% at 15 min to 54.5% at 2 h. Control brains showed only very small numbers of weakly APP-positive neuronal cell bodies ranging from 2 to 14% (mean 7%). Increased expression of APP mRNA was first evident in impacted hemispheres at 30 min after impact and progressively increased over time to involve neurons in all sampled regions of the brain, suggesting increased transcription of APP. In contrast, APP mRNA was undetectable in tissue from nonimpacted sheep. These data show that APP mRNA and antigen expression are sensitive early indicators of neuronal injury with widespread upregulation occurring as early as 30 min after head impact.

Intracellular and cell-surface distribution of amyloid precursor protein in cortical astrocytes

Amyloid peptides that aggregate to form plaques in Alzheimer's disease are derived from secretory processing of the amyloid precursor protein (APP). Transport of APP to the cell surface may be prerequisite for non-amyloidogenic APP processing and the secretion of soluble APP (APPs), while missorting or reinternalization of APP to intracellular compartments can promote amyloid formation. In cultured astrocytes, APP mRNA and holoprotein are increased by elevations in cAMP levels, and 8-Bromo-cAMP promotes process formation on these cells. We now report that treatment of cultured astrocytes with 8-Bromo-cAMP increased intracellular and cell surface APP in the soma and perinuclear region as detected by immunolabeling with monoclonal antibody 22C11 and polyclonal antibody Kunitz-type protease inhibitor (KPI) (against the N-terminus and KPI domain of APP, respectively) and led to intense but discontinuous labelling of APP on the surface of astrocytic processes. Northern and Western blot analyses confirmed that 8-Bromo-cAMP treatment of cultured astrocytes also increased APP mRNA and KPI-containing APP holoprotein, implying that the intense APP immunolabeling observed in 8-Bromo-cAMP treated astrocytes was not derived from truncated forms of APP (e.g., APPs), but reflected high levels of APP holoprotein containing intact amyloid peptides. Discontinuous cell surface staining in process-bearing astrocytes may be caused by miscompartmentalization of APP related to rearrangement of the cytoskeleton. Inasmuch as intracellular APP is not accessible for non-amyloidogenic processing, we suggest that the increased immunoreactivity of intracellular APP in process-bearing astrocytes may predispose the cells to increased amyloid production.

Amyloid precursor protein of Alzheimer's disease: evidence for a stable, full-length, trans-membrane pool in primary neuronal cultures

We and colleagues have shown that the amyloid protein precursor of Alzheimer's disease (APP) is distributed along the surface of neurites of fixed but nonpermeabilized neurons in primary culture in a segmental pattern, which shows colocalization with some markers of adhesion patches. This is in contrast to the diffuse pattern of immunoreactivity seen after permeabilization. We have also recently demonstrated that the APP in these surface patches is likely to be integral to the membrane rather than secreted and re-adsorbed, based on alkali stripping experiments and on soluble APP adsorption experiments. Total cellular APP has previously been shown to have a short half-life of approximately 30-60 min. We confirm this in neurons in primary culture in pulse-chase experiments using short labelling times. Additionally, we provide evidence that a separate, stable pool of neuronal APP can be demonstrated in pulse-chase experiments using long labelling times. Experiments involving inhibition of protein synthesis suggest that this corresponds with the surface, segmental pool. Metabolic labelling followed by surface biotinylation and two-stage precipitation demonstrates that the surface APP is trans-membrane and full-length (not carboxyl-terminal truncated), and confirms that the surface APP belongs to the stable pool. This two-stage procedure is necessary as the surface APP appears to be present in low copy number, and is difficult to detect by direct labelling. This information is consistent with a role for APP in stable cell-matrix or cell-cell interactions.

Transgenic Drosophila expressing human amyloid precursor protein show gamma-secretase activity and a blistered-wing phenotype

The importance of the amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease (AD) became apparent through the identification of distinct mutations in the APP gene, causing early onset familial AD with the accumulation of a 4-kDa peptide fragment (betaA4) in amyloid plaques and vascular deposits. However, the physiological role of APP is still unclear. In this work, Drosophila melanogaster is used as a model system to analyze the function of APP by expressing wild-type and various mutant forms of human APP in fly tissue culture cells as well as in transgenic fly lines. After expression of full-length APP forms, secretion of APP but not of betaA4 was observed in both systems. By using SPA4CT, a short APP form in which the signal peptide was fused directly to the betaA4 region, transmembrane domain, and cytoplasmic tail, we observed betaA4 release in flies and fly-tissue culture cells. Consequently, we showed a gamma-secretase activity in flies. Interestingly, transgenic flies expressing full-length forms of APP have a blistered-wing phenotype. As the wing is composed of interacting dorsal and ventral epithelial cell layers, this phenotype suggests that human APP expression interferes with cell adhesion/signaling pathways in Drosophila, independently of betaA4 generation.

Memory-enhancing effects of secreted forms of the beta-amyloid precursor protein in normal and amnestic mice

When administered intracerebroventricularly to mice performing various learning tasks involving either short-term or long-term memory, secreted forms of the beta-amyloid precursor protein (APPs751 and APPs695) have potent memory-enhancing effects and block learning deficits induced by scopolamine. The memory-enhancing effects of APPs were observed over a wide range of extremely low doses (0.05-5,000 pg intracerebroventricularly), blocked by anti-APPs antisera, and observed when APPs was administered either after the first training session in a visual discrimination or a lever-press learning task or before the acquisition trial in an object recognition task. APPs had no effect on motor performance or exploratory activity. APPs695 and APPs751 were equally effective in the object recognition task, suggesting that the memory-enhancing effect of APPs does not require the Kunitz protease inhibitor domain. These data suggest an important role for APPss on memory processes.

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

A distinctive tract of all the forms of Alzheimer's disease is the extracellular deposition of a 40-42/43 amino acid-long peptide derived from the so-called beta-amyloid precursor protein (APP). This is a membrane protein of unknown function, whose short cytosolic domain has been recently demonstrated to interact with several proteins. One of these proteins, named Fe65, has the characteristics of an adaptor protein; in fact, it possesses three protein-protein interaction domains: a WW domain and two PID/PTB domains. The interaction with APP requires the most C-terminal PID/PTB domain, whereas the WW domain is responsible for the interaction with various proteins, one of which was demonstrated to be the mammalian homolog of the Drosophila enabled protein (Mena), which in turn interacts with the cytoskeleton. The second PID/PTB domain of Fe65 binds to the CP2/LSF/LBP1 protein, which is an already known transcription factor. The other proteins interacting with the cytosolic domain of APP are the G(o) heterotrimeric protein, APP-BP1 and X11. The latter interacts with APP through a PID/PTB domain and possesses two other protein-protein interaction domains. The small size of the APP cytodomain and the overlapping of its regions involved in the binding of Fe65 and X11 suggest the existence of competitive mechanisms regulating the binding of the various ligands to this cytosolic domain. In this short review the possible functional roles of this complex protein network and its involvement in the generation of Alzheimer's phenotype are discussed.