Amyloid precursor protein and mitochondria

Amyloid Precursor Protein (APP) processing to amyloid beta (Aβ) is a major hallmark of Alzheimer's disease (AD). The amyloid cascade hypothesis postulates that Aβ accumulation and aggregation causes AD, however many therapeutics targeting Aβ have failed recently. Decades of research describe metabolic deficits in AD. Mitochondrial dysfunction is observed in AD subjects within the brain and systemically. APP and γ-secretase are localized to mitochondria. APP can be processed within mitochondria and its localization to mitochondria affects function. Here we discuss the evidence showing APP and γ-secretase localize to mitochondria. We also discuss the implications for the function of APP and its cleavage products in regulating mitochondrial function.     

Amyloid precursor protein is trafficked and secreted via synaptic vesicles

A large body of evidence has implicated amyloid precursor protein (APP) and its proteolytic derivatives as key players in the physiological context of neuronal synaptogenesis and synapse maintenance, as well as in the pathology of Alzheimer's Disease (AD). Although APP processing and release are known to occur in response to neuronal stimulation, the exact mechanism by which APP reaches the neuronal surface is unclear. We now demonstrate that a small but relevant number of synaptic vesicles contain APP, which can be released during neuronal activity, and most likely represent the major exocytic pathway of APP. This novel finding leads us to propose a revised model of presynaptic APP trafficking that reconciles existing knowledge on APP with our present understanding of vesicular release and recycling.     

Amyloid precursor protein expression is upregulated in adipocytes in obesity

The aim of this study was to determine whether amyloid precursor protein (APP) is expressed in human adipose tissue, dysregulated in obesity, and related to insulin resistance and inflammation. APP expression was examined by microarray expression profiling of subcutaneous abdominal adipocytes (SAC) and cultured preadipocytes from obese and nonobese subjects. Quantitative real-time PCR (QPCR) was performed to confirm differences in APP expression in SAC and to compare APP expression levels in adipose tissue, adipocytes, and stromal vascular cells (SVCs) from subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) specimens. Adipose tissue samples were also examined by western blot and immunofluorescence confocal microscopy. Microarray studies demonstrated that APP mRNA expression levels were higher in SAC (approximately 2.5-fold) and preadipocytes (approximately 1.4) from obese subjects. Real-time PCR confirmed increased APP expression in SAC in a separate group of obese compared with nonobese subjects (P=0.02). APP expression correlated to in vivo indices of insulin resistance independently of BMI and with the expression of proinflammatory genes, such as monocyte chemoattractant protein-1 (MCP-1) (R=0.62, P=0.004), macrophage inflammatory protein-1alpha (MIP-1alpha) (R=0.60, P=0.005), and interleukin-6 (IL-6) (R=0.71, P=0.0005). Full-length APP protein was detected in adipocytes by western blotting and APP and its cleavage peptides, Abeta40 and Abeta42, were observed in SAT and VAT by immunofluorescence confocal microscopy. In summary, APP is highly expressed in adipose tissue, upregulated in obesity, and expression levels correlate with insulin resistance and adipocyte cytokine expression levels. These data suggest a possible role for APP and/or Abeta in the development of obesity-related insulin resistance and adipose tissue inflammation.     

Amyloid precursor protein is required for convergent-extension movements during Zebrafish development

Amyloid precursor protein (APP) has been a focus of intense investigation because of its role in Alzheimer's disease (AD), however, its biological function remains uncertain. Loss of APP and APP-like proteins results in postnatal lethality in mice, suggesting a role during embryogenesis. Here we show that in a zebrafish model system, knock down of APP results in the generation of fish with dramatically reduced body length and a short, curly tail. In situ examination of gene expression suggests that the APP morphant embryos have defective convergent-extension movements. We also show that wild-type human APP rescues the morphant phenotype, but the Swedish mutant APP, which causes familial AD (fAD), does not rescue the developmental defects. Collectively, this work demonstrates that the zebrafish model is a powerful system to define the role of APP during embryonic development and to evaluate the functional activity of fAD mutant APP.     

Amyloid beta protein precursor is phosphorylated by JNK-1 independent of,yet facilitated by,JNK-interacting protein (JIP)-1

Alzheimer's disease (AD) is genetically linked to the processing of amyloid beta protein precursor (AbetaPP). Aside from being the precursor of the amyloid beta (Abeta) found in plaques in the brains of patients with AD, little is known regarding the functional role of AbetaPP. We have recently reported biochemical evidence linking AbetaPP to the JNK signaling cascade by finding that JNK-interacting protein-1 (JIP-1) binds AbetaPP. In order to study the functional implications of this interaction we assayed the carboxyl-terminal of AbetaPP for phosphorylation. We found that the threonine 668 within the AbetaPP intracellular domain (AID or elsewhere AICD) is indeed phosphorylated by JNK1. We surprisingly found that although JIP-1 can facilitate this phosphorylation, it is not required for this process. We also found that JIP-1 only facilitated phosphorylation of AbetaPP but not of the two other family members APLP1 (amyloid precursor-like protein 1) and APLP2. Understanding the connection between AbetaPP phosphorylation and the JNK signaling pathway, which mediates cell response to stress may have important implications in understanding the pathogenesis of Alzheimer's disease.     

Amyloid precursor protein is a restriction factor that protects against Zika virus infection in mammalian brains

Zika virus (ZIKV) is a neurotropic flavivirus that causes several diseases including birth defects such as microcephaly. Intrinsic immunity is known to be a frontline defense against viruses through host anti-viral restriction factors. Limited knowledge is available on intrinsic immunity against ZIKV in brains. Amyloid precursor protein (APP) is predominantly expressed in brains and implicated in the pathogenesis of Alzheimer''s diseases. We have found that ZIKV interacts with APP, and viral infection increases APP expression via enhancing protein stability. Moreover, we identified the viral peptide, HGSQHSGMIVNDTGHETDENRAKVEITPNSPRAEATLGGFGSLGL, which is capable of en-hancing APP expression. We observed that aging brain tissues with APP had protective effects on ZIKV infection by reducing the availability of the viruses. Also, knockdown of APP expression or blocking ZIKV-APP interactions enhanced ZIKV replication in human neural progenitor/stem cells. Finally, intracranial infection of ZIKV in APP-null neonatal mice resulted in higher mortality and viral yields. Taken together, these findings suggest that APP is a restriction factor that protects against ZIKV by serving as a decoy receptor, and plays a protective role in ZIKV-mediated brain injuries.     

Amyloid beta protein precursor (AbetaPP), but not AbetaPP-like protein 2, is bridged to the kinesin light chain by the scaffold protein JNK-interacting protein 1

Proteolytic processing of amyloid beta protein precursor (AbetaPP) generates peptides that regulate normal cell signaling and are implicated in Alzheimer's disease pathogenesis. AbetaPP processing also occurs in nerve processes where AbetaPP is transported from the cell body by kinesin-I, a microtubule motor composed of two kinesin heavy chain and two kinesin light chain (Klc) subunits. AbetaPP transport is supposedly mediated by the direct AbetaPP-Klc1 interaction. Here we demonstrate that the AbetaPP-Klc1 interaction is not direct but is mediated by JNK-interacting protein 1 (JIP1). The phosphotyrosine binding domain of JIP1 binds the cytoplasmic tail of AbetaPP, whereas the JIP1 C-terminal region interacts with the tetratrico-peptide repeats of Klc1. We also show that JIP1 does not bridge the AbetaPP gene family member AbetaPP-like protein 2, APLP2, to Klc1. These results support a model where JIP1 mediates the interaction of AbetaPP to the motor protein kinesin-I and that this JIP1 function is unique for AbetaPP relative to its family member APLP2. Our data suggest that kinesin-I-dependent neuronal AbetaPP transport, which controls AbetaPP processing, may be regulated by JIP1.     

Amyloid precursor protein and amyloid beta peptide in human platelets. Role of cyclooxygenase and protein kinase C

The main component of Alzheimer's disease (AD) senile plaques is amyloid-beta peptide (Abeta), a proteolytic fragment of the amyloid precursor protein (APP). Platelets contain both APP and Abeta and may contribute to the perivascular amyloid deposition seen in AD. However, no data are available concerning the biochemical mechanism(s) involved in their formation and release by these cells. We found that human platelets released APP and Abeta following activation with collagen or arachidonic acid. Inhibition of platelet cyclooxygenase (COX) reduced APP but not Abeta release following those stimuli. In contrast, activation of platelets by thrombin and calcium ionophore caused release of both APP and Abeta in a COX-independent fashion. Ex vivo studies showed that, despite suppression of COX activity, administration of aspirin did not modify Abeta or APP levels in serum or plasma, suggesting that this enzyme plays only a minor role in vivo. We examined the regulation of APP cleavage and release from activated platelets and found that cleavage requires protein kinase C (PKC) activity and is regulated by the intracellular second messengers phosphatidylinositol 2-phosphate and Ca(2+). Our data provide the first evidence that in human platelets COX is a minor component of APP secretion whereas PKC plays a major role in the secretory cleavage of APP. By contrast, Abeta release may represent secretion of preformed peptide and is totally independent of both COX and PKC activity.     

Amyloid precursor protein cross-linking stimulates beta amyloid production and pro-inflammatory cytokine release in monocytic lineage cells

Beta amyloid peptide-containing neuritic plaques are a defining feature of Alzheimer's disease pathology. Beta amyloid are 38-43 residue peptides derived by proteolytic cleavage of amyloid precursor protein. Although much attention has focused on the proteolytic events leading to beta amyloid generation, the function of amyloid precursor protein remains poorly described. Previously, we reported that amyloid precursor protein functions as a pro-inflammatory receptor on monocytic lineage cells and defined a role for amyloid precursor protein in adhesion by demonstrating that beta(1) integrin-mediated pro-inflammatory activation of monocytes is amyloid precursor protein dependent. We demonstrated that antibody-induced cross-linking of amyloid precursor protein in human THP-1 monocytes and primary mouse microglia stimulates a tyrosine kinase-based pro-inflammatory signaling response leading to acquisition of a reactive phenotype. Here, we have identified pro-inflammatory mediators released upon amyloid precursor protein-dependent activation of monocytes and microglia. We show that amyloid precursor protein cross-linking stimulated tyrosine kinase-dependent increases in pro-inflammatory cytokine release and a tyrosine kinase-independent increase in beta amyloid 1-42 generation. These data provide much needed insight into the function of amyloid precursor protein and provide potential therapeutic targets to limit inflammatory changes associated with the progression of Alzheimer's disease.     

Amyloid precursor protein is involved in staurosporine induced glial differentiation of neural progenitor cells

Staurosporine (STS) has been reported as not only a pro-apoptotic agent, but also a terminal differentiation inducer in several neuroblastoma cell lines. Here, we report involvement of amyloid precursor protein (APP) in a STS induced astrocytic differentiation of human neural progenitor cells (NT-2/D1). We found that STS-treated NT-2/D1 cells expressed astrocyte-specific glial fibrillary acidic protein (GFAP), aspartate transporter, and glutamate transporter-1 with a distinctive astrocytic morphology. STS treatment increased GFAP promoter activity and increased expression and secretion of APP in NT-2/D1 cell culture. Overexpressed APP enhanced GFAP promoter activity and expression of GFAP, while gene silencing of APP by RNA interference decreased GFAP expression. These results indicate involvement of APP in STS induced astrocytic differentiation of NT-2/D1 cells. Furthermore, suppression of ERK1/2 phosphorylation, which is known to regulate APP expression by a MEK1 inhibitor, PD098059, reduced both APP and GFAP expression in STS treated NT-2/D1 cells. Thus, STS may induce astrocytic differentiation of NT-2/D1 by increasing APP levels associate with activation of ERK pathway.     

Transforming growth factor beta2 is a neuronal death-inducing ligand for amyloid-beta precursor protein

APP, amyloid beta precursor protein, is linked to the onset of Alzheimer's disease (AD). We have here found that transforming growth factor beta2 (TGFbeta2), but not TGFbeta1, binds to APP. The binding affinity of TGFbeta2 to APP is lower than the binding affinity of TGFbeta2 to the TGFbeta receptor. On binding to APP, TGFbeta2 activates an APP-mediated death pathway via heterotrimeric G protein G(o), c-Jun N-terminal kinase, NADPH oxidase, and caspase 3 and/or related caspases. Overall degrees of TGFbeta2-induced death are larger in cells expressing a familial AD-related mutant APP than in those expressing wild-type APP. Consequently, superphysiological concentrations of TGFbeta2 induce neuronal death in primary cortical neurons, whose one allele of the APP gene is knocked in with the V642I mutation. Combined with the finding indicated by several earlier reports that both neural and glial expression of TGFbeta2 was upregulated in AD brains, it is speculated that TGFbeta2 may contribute to the development of AD-related neuronal cell death.     

beta-Amyloid precursor is a PEST protein

The beta-amyloid peptide is generated by proteolytic processing of a family of beta-amyloid precursor proteins. Here we report that beta-amyloid precursor proteins have a primary structure motif known as a PEST sequence, which is predictive of the class of most protease-sensitive rapidly turning over proteins. Consistent with this, the precursors were extraordinarily susceptible to degradation by the calcium-dependent protease calpain I. The identification of beta-amyloid precursors as PEST sequence-containing proteins has implications for both the normal cellular function of beta-amyloid precursor proteins and the mechanisms regulating their expression and processing.     

Amyloid precursor protein trafficking, processing, and function

Intracellular trafficking and proteolytic processing of amyloid precursor protein (APP) have been the focus of numerous investigations over the past two decades. APP is the precursor to the amyloid beta-protein (Abeta), the 38-43-amino acid residue peptide that is at the heart of the amyloid cascade hypothesis of Alzheimer disease (AD). Tremendous progress has been made since the initial identification of Abeta as the principal component of brain senile plaques of individuals with AD. Specifically, molecular characterization of the secretases involved in Abeta production has facilitated cell biological investigations on APP processing and advanced efforts to model AD pathogenesis in animal models. This minireview summarizes salient features of APP trafficking and amyloidogenic processing and discusses the putative biological functions of APP.     

Amyloid precursor protein at node of Ranvier modulates nodal formation

Amyloid precursor protein (APP), commonly associated with Alzheimer disease, is upregulated and distributes evenly along the injured axons, and therefore, also known as a marker of demyelinating axonal injury and axonal degeneration. However, the physiological distribution and function of APP along myelinated axons was unknown. We report that APP aggregates at nodes of Ranvier (NOR) in the myelinated central nervous system (CNS) axons but not in the peripheral nervous system (PNS). At CNS NORs, APP expression co-localizes with tenascin-R and is flanked by juxtaparanodal potassium channel expression demonstrating that APP localized to NOR. In APP-knockout (KO) mice, nodal length is significantly increased, while sodium channels are still clustered at NORs. Moreover, APP KO and APP-overexpressing transgenic (APP TG) mice exhibited a decreased and an increased thickness of myelin in spinal cords, respectively, although the changes are limited in comparison to their littermate WT mice. The thickness of myelin in APP KO sciatic nerve also increased in comparison to that in WT mice. Our observations indicate that APP acts as a novel component at CNS NORs, modulating nodal formation and has minor effects in promoting myelination.     

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.     

Amyloid precursor protein at node of Ranvier modulates nodal formation

Amyloid precursor protein (APP), commonly associated with Alzheimer disease, is upregulated and distributes evenly along the injured axons, and therefore, also known as a marker of demyelinating axonal injury and axonal degeneration. However, the physiological distribution and function of APP along myelinated axons was unknown. We report that APP aggregates at nodes of Ranvier (NOR) in the myelinated central nervous system (CNS) axons but not in the peripheral nervous system (PNS). At CNS NORs, APP expression co-localizes with tenascin-R and is flanked by juxtaparanodal potassium channel expression demonstrating that APP localized to NOR. In APP-knockout (KO) mice, nodal length is significantly increased, while sodium channels are still clustered at NORs. Moreover, APP KO and APP-overexpressing transgenic (APP TG) mice exhibited a decreased and an increased thickness of myelin in spinal cords, respectively, although the changes are limited in comparison to their littermate WT mice. The thickness of myelin in APP KO sciatic nerve also increased in comparison to that in WT mice. Our observations indicate that APP acts as a novel component at CNS NORs, modulating nodal formation and has minor effects in promoting myelination.     

Amyloid beta oligomerization is induced by brain lipid rafts

Detergent-resistant lipid rafts are required for the generation of Abeta as they concentrate not only amyloid precursor protein (APP), but also the beta- and gamma-secretase that convert APP to Abeta. Recently, Abeta has been shown to be oligomerized, which results in neuronal cytotoxicity and synaptic failure. In this study, we have demonstrated that Abeta oligomers appeared immediately after the incubation of Abeta with lipid rafts isolated from the brain tissues of rats, and were converted into few Abeta fibrils, even after longer periods of incubation. The oligomerization of Abeta was not abolished after the brain lipid rafts were treated with heat, or with protease K, implying that the lipid raft proteins were determined not to be prerequisites for Abeta oligomerization. The cholesterol present in the lipid rafts might not be essential to Abeta oligomerization because Abeta oligomerization was not prevented after the cholesterol was removed from the lipid rafts with methyl-beta-cyclodextrin (MbetaCD). The Abeta oligomerization was accelerated by the application of lipid rafts isolated from ganglioside-rich cells, C2C12 cells, whereas this was not observed with the lipid rafts isolated from ganglioside-poor cells SK-N-MC and HeLa cells. In addition, lipid raft-induced Abeta oligomerization was shown to be inhibited in CHO-K1 cells which were defective with regard to ganglioside biosynthesis. This indicates that Abeta oligomerization requires gangliosides that are enriched in the lipid rafts.     

Amyloid beta: Functional protein or biological junk?

During a decade there was a dogma that Alzheimer’s amyloid beta (Aβ) is produced only upon the disease, and that this protein is neurotoxic for neurons and brain tissue. Current scientific evidence demonstrates that Aβ is an essential molecule in synaptic plasticity that underlines learning and memory. Therefore, it was hypothesized that the change of Aβ biology in Alzheimer’s disease (as well as in a number of other human pathologies, including cardiovascular disease, Niemann-Pick type C disease and Down syndrome) represents a physiological mechanism serving to compensate the impaired brain structure or function. This review summarizes experimental evidence on Aβ as a functional player in synaptic plasticity and neurochemical pathways.     

Amyloid beta.

Amyloid beta (A beta) is a 39-43 residue amyloidogenic peptide that is deposited into the extracellular amyloid plaques which characterize an Alzheimer's disease (AD) brain. A beta is derived from the amyloid precursor protein (APP) and undergoes a toxic conformational change (gain of toxic function). The length of the A beta peptide dramatically influences its properties with the longer 42 and 43 residue species being more amyloidogenic. The genetics of familial AD (FAD) supports a central role for A beta in AD since mutations in the FAD causing genes APP and the presenilins (PS1 and PS2) increase the formation of A beta 42,43. Considerable activity is directed towards A beta as a therapeutic target. These strategies aim to inhibit A beta synthesis, A beta fibril formation, its toxic actions on cells or promote its clearance from the brain.