Expression and purification of a recombinant transmembrane domain amyloid precursor protein associated with Alzheimer’s disease

More than half of the mutations of the amyloid precursor protein (APP) discovered in familiar forms of Alzheimer’s disease are located in the transmembrane domain. The pathogenic mutations presumably affect the lateral dimerization of the APP transmembrane domain in the membrane and change the dimer conformation and/or stability. Thus, the mutations cause an alternative APP digestion pattern in the membrane and neurotoxic amyloid β-peptide generation. For the detailed study of the specific protein-protein and protein-lipid interactions of the APP transmembrane domain, an E. coli recombinant expression construct was made. The recombinant protein contains an APP transmembrane domain (APPtm(686–726)) with adjacent extramembrane N and C ends. Here, we report the method of isotope-labeled APPtm expression and purification in quantities necessary for a heteronuclear NMR spectroscopy structure and dynamics study. On the basis of the 1H-15N-HSQC spectra, we developed APPtm(686–726) solubilization conditions in the membrane-emulated milieu detergent micelles and lipid bicelles.     

Dimeric structure of transmembrane domain of amyloid precursor protein in micellar environment

Some pathogenic mutations associated with Alzheimer's disease are thought to affect structural-dynamic properties and the lateral dimerization of amyloid precursor protein (APP) in neuron membrane. Dimeric structure of APP transmembrane fragment Gln(686)-Lys(726) was determined in membrane-mimicking dodecylphosphocholine micelles using high-resolution NMR spectroscopy. The APP membrane-spanning α-helix Lys(699)-Lys(724) self-associates in a left-handed parallel dimer through extended heptad repeat motif I(702)X(3)M(706)X(2)G(709)X(3)A(713)X(2)I(716)X(3)I(720)X(2)I(723), whereas the juxtamembrane region Gln(686)-Val(695) constitutes the nascent helix, also sensing the dimerization. The dimerization mechanism of APP transmembrane domain has been described at atomic resolution for the first time and is important for understanding molecular events of APP sequential proteolytical cleavage resulting in amyloid-β peptide.     

FKBP12 regulates the localization and processing of amyloid precursor protein in human cell lines

One of the pathological hallmarks of Alzheimer’s disease is the presence of insoluble extracellular amyloid plaques. These plaques are mainly constituted of amyloid beta peptide (Aβ), a proteolytic product of amyloid precursor protein (APP). APP processing also generates the APP intracellular domain (AICD). We have previously demonstrated that AICD interacts with FKBP12, a peptidyl-prolyl cis-trans isomerase (PPIase) ubiquitous in nerve systems. This interaction was interfered by FK506, a clinically used immunosuppressant that has recently been reported to be neuroprotective. To elucidate the roles of FKBP12 in the pathogenesis of Alzheimer’s disease, the effect of FKBP12 overexpression on APP processing was evaluated. Our results revealed that APP processing was shifted towards the amyloidogenic pathway, accompanied by a change in the subcellular localization of APP, upon FKBP12 overexpression. This FKBP12-overexpression-induced effect was reverted by FK506. These findings support our hypothesis that FKBP12 may participate in the regulation of APP processing. FKBP12 overexpression may lead to the stabilization of a certain isomer (presumably the cis form) of the Thr668-Pro669 peptide bond in AICD, therefore change its affinity to flotillin-1 or other raft-associated proteins, and eventually change the localization pattern and cause a shift in the proteolytic processing of APP.     

The Distinct Role of ADAM17 in APP Proteolysis and Microglial Activation Related to Alzheimer’s Disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disease with the symptom of cognitive impairment. The deposition of amyloid β (Aβ) peptide is believed to be the primary cause to neuronal dystrophy and eventually dementia. Aβ is the proteolytic product from its precursor amyloid precursor protein (APP) by β- and γ- secretase. An optional cleavage by α-secretase happens inside the Aβ domain. ADAM17 is supposed to be the regulated α-secretase of APP. Enhanced activity of ADAM17 leads to the increasing secretion of neuroprotective soluble APP α fragment and reduction of Aβ generation, which may be benefit to the disease. ADAM17 is then considered the potential therapeutic target for AD. Microglia activation and neuroinflammation is another important event in AD pathogenesis. Interestingly, ADAM17 also participates in the cleavage of many other membrane-bound proteins, especially some inflammatory factors related to microglia activation. The facilitating role of ADAM17 in inflammation and further neuronal damage has also been illustrated. In results, the activation of ADAM17 as the solution to AD may be a tricky task. The comprehensive consideration and evaluation has to be carried out carefully before the final treatment. In the present review, the distinct role of ADAM17 in AD-related APP shedding and neuroinflammatory microglial activation will be carefully discussed.     

Regulation of amyloid beta-protein precursor by phosphorylation and protein interactions

Amyloid beta-protein precursor (APP), a type I membrane protein, is cleaved by primary alpha-or beta-secretase and secondary gamma-secretase. Cleavage of APP by beta- and gamma-secretases generates amyloid beta-protein, the main constituent of the cerebrovascular amyloid that accompanies Alzheimer disease. The generation and aggregation of amyloid beta-protein in the brain are believed to be a primary cause of Alzheimer disease pathogenesis, and indeed, early onset Alzheimer disease is genetically linked to APP and also to presenilins 1 and 2, which are components of gamma-secretase. Proteolytic cleavage of APP has been investigated as a candidate target for Alzheimer disease therapy, but the mechanisms regulating APP metabolism are still unclear. APP is a type I membrane protein with a short cytoplasmic region consisting of 47 amino acids. Recent research has elucidated the significance of the cytoplasmic region in the metabolism, trafficking, and physiological function of APP. The structure and function of the APP cytoplasmic domain can be modified by phosphorylation and through interaction with cytoplasmic proteins. This minireview summarizes a large body of recent information on the regulation of APP by phosphorylation and protein interaction, along with some of the physiological functions of APP. Recent findings regarding the regulation of APP processing contribute to the development of novel drugs and/or therapies for Alzheimer disease.     

The Interplay of Tau Protein and β-Amyloid: While Tauopathy Spreads More Profoundly Than Amyloidopathy,Both Processes Are Almost Equally Pathogenic

Cellular and Molecular Neurobiology - Alzheimer’s disease (AD) is a neurodegenerative disorder, in which amyloid precursor protein (APP) misprocessing and tau protein hyperphosphorylation are...     

Trafficking regulation of proteins in Alzheimer’s disease

The β-amyloid (Aβ) peptide has been postulated to be a key determinant in the pathogenesis of Alzheimer’s disease (AD). Aβ is produced through sequential cleavage of the β-amyloid precursor protein (APP) by β- and γ-secretases. APP and relevant secretases are transmembrane proteins and traffic through the secretory pathway in a highly regulated fashion. Perturbation of their intracellular trafficking may affect dynamic interactions among these proteins, thus altering Aβ generation and accelerating disease pathogenesis. Herein, we review recent progress elucidating the regulation of intracellular trafficking of these essential protein components in AD.     

Regulation of Neurotransmitter Release by Amyloid Precursor Protein Through Synapsin Phosphorylation

Neurochemical Research - Abnormal processing of amyloid precursor protein (APP) and aggregation of the Aβ peptide are known to play a key role in the pathogenesis of Alzheimer disease, but the...     

Cross-talk of membrane lipids and Alzheimer-related proteins

Alzheimer’s disease (AD) is neuropathologically characterized by the combined occurrence of extracellular β-amyloid plaques and intracellular neurofibrillary tangles in the brain. While plaques contain aggregated forms of the amyloid β-peptide (Aβ), tangles are formed by fibrillar forms of the microtubule associated protein tau. All mutations identified so far to cause familial forms of early onset AD (FAD) are localized close to or within the Aβ domain of the amyloid precursor protein (APP) or in the presenilin proteins that are essential components of a protease complex involved in the generation of Aβ. Mutations in the tau gene are not associated with FAD, but can cause other forms of dementia. The genetics of FAD together with biochemical and cell biological data, led to the formulation of the amyloid hypothesis, stating that accumulation and aggregation of Aβ is the primary event in the pathogenesis of AD, while tau might mediate its toxicity and neurodegeneration.The generation of Aβ involves sequential proteolytic cleavages of the amyloid precursor protein (APP) by enzymes called β-and γ-secretases. Notably, APP itself as well as the secretases are integral membrane proteins. Thus, it is very likely that membrane lipids are involved in the regulation of subcellular transport, activity, and metabolism of AD related proteins.Indeed, several studies indicate that membrane lipids, including cholesterol and sphingolipids (SLs) affect Aβ generation and aggregation. Interestingly, APP and other AD associated proteins, including β-and γ-secretases can, in turn, influence lipid metabolic pathways. Here, we review the close connection of cellular lipid metabolism and AD associated proteins and discuss potential mechanisms that could contribute to initiation and progression of AD.     

Role of phosphatidylinositol clathrin assembly lymphoid-myeloid leukemia (PICALM) in intracellular amyloid precursor protein (APP) processing and amyloid plaque pathogenesis

One of the pathological hallmarks of Alzheimer disease is the accumulation of amyloid plaques in the extracellular space in the brain. Amyloid plaques are primarily composed of aggregated amyloid β peptide (Aβ), a proteolytic fragment of the transmembrane amyloid precursor protein (APP). For APP to be proteolytically cleaved into Aβ, it must be internalized into the cell and trafficked to endosomes where specific protease complexes can cleave APP. Several recent genome-wide association studies have reported that several single nucleotide polymorphisms (SNPs) in the phosphatidylinositol clathrin assembly lymphoid-myeloid leukemia (PICALM) gene were significantly associated with Alzheimer disease, suggesting a role in APP endocytosis and Aβ generation. Here, we show that PICALM co-localizes with APP in intracellular vesicles of N2a-APP cells after endocytosis is initiated. PICALM knockdown resulted in reduced APP internalization and Aβ generation. Conversely, PICALM overexpression increased APP internalization and Aβ production. In vivo, PICALM was found to be expressed in neurons and co-localized with APP throughout the cortex and hippocampus in APP/PS1 mice. PICALM expression was altered using AAV8 gene transfer of PICALM shRNA or PICALM cDNA into the hippocampus of 6-month-old APP/PS1 mice. PICALM knockdown decreased soluble and insoluble Aβ levels and amyloid plaque load in the hippocampus. Conversely, PICALM overexpression increased Aβ levels and amyloid plaque load. These data indicate that PICALM, an adaptor protein involved in clathrin-mediated endocytosis, regulates APP internalization and subsequent Aβ generation. PICALM contributes to amyloid plaque load in brain likely via its effect on Aβ metabolism.     

与阿尔兹海默症相关的G 蛋白偶联受体研究进展

G蛋白偶联受体(GPCRs)在大脑信号传递中至关重要,而在阿尔兹海默症(AD)中,G蛋白偶联受体通过调控α-、β-及γ-分泌酶分泌、淀粉样前体蛋白(APP)生成及β-淀粉样蛋白(Aβ)降解,直接影响β-淀粉样蛋白在神经系统信号级联反应;另外,阿尔兹海默症中β-淀粉样蛋白的生成可以扰乱G蛋白偶联受体功能.因此,阐明G蛋白偶联受体与阿尔兹海默症发病之间的关联有助于开发以G蛋白偶联受体为靶点的阿尔兹海默症治疗药物.     

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.     

The amyloid precursor protein and its homologues: structural and functional aspects of native and pathogenic oligomerization

Over the last 25 years, remarkable progress has been made not only in identifying key molecules of Alzheimer's disease but also in understanding their meaning in the pathogenic state. One hallmark of Alzheimer pathology is the amyloid plaque. A major component of the extracellular deposit is the amyloid-β (Aβ) peptide which is generated from its larger precursor molecule, i.e., the amyloid precursor protein (APP) by consecutive cleavages. Processing is exerted by two enzymes, i.e., the β-secretase and the γ-secretase. We and others have found that the self-association of the amyloid peptide and the dimerization and oligomerization of these proteins is a key factor under native and pathogenic conditions. In particular, the Aβ homodimer represents a nidus for plaque formation and a well defined therapeutic target. Further, dimerization of the APP was reported to increase generation of toxic Aβ whereas heterodimerization with its homologues amyloid precursor like proteins (APLP1 and APLP2) decreased Aβ formation. This review mainly focuses on structural features of the homophilic and heterophilic interactions among APP family proteins. The proposed contact sites are described and the consequences of protein dimerization on their functions and in the pathogenesis of Alzheimer's disease are discussed.     

Polarized transport of Alzheimer amyloid precursor protein is mediated by adaptor protein complex AP1-1B

Alzheimer amyloid precursor protein (APP) is the precursor for the Abeta peptide involved in pathogenesis of Alzheimer's disease. The soluble ectodomain fragment of APP (sAPP) functions as a growth factor for epithelial cells, suggesting an important function for APP outside neuronal tissue. Previous studies have shown that in polarized epithelial cells, APP is targeted to the basolateral domain. Tyr653 within the cytoplasmic tail of APP mediates the basolateral targeting of APP, but the sorting machinery that binds to this residue has largely remained unknown. In this study, we analyzed the role of adaptor complexes in the polarized sorting of APP. We show that the medium subunit mu1B of the epithelia-specific adaptor protein (AP)-1B binds onto the cytoplasmic tail of APP in a Tyr653-dependent way. Moreover, ectopic expression of mu1B in cells lacking AP-1B resulted in correction of apical missorting of wild-type but not Tyr653Ala APP. Basolateral secretion of sAPP was found to be independent of Tyr653. We propose a model for polarized targeting of APP according to which sorting of APP to basolateral domain is dependent on binding of AP-1B on Tyr653 in basolateral endosomes. This model is in accordance with the current understanding of sorting mechanisms mediating polarized targeting of membrane proteins.     

Lysine 624 of the amyloid precursor protein (APP) is a critical determinant of amyloid β peptide length: support for a sequential model of γ-secretase intramembrane proteolysis and regulation by the amyloid β precursor protein (APP) juxtamembrane region

γ-Secretase is a multiprotein intramembrane cleaving aspartyl protease (I-CLiP) that catalyzes the final cleavage of the amyloid β precursor protein (APP) to release the amyloid β peptide (Aβ). Aβ is the primary component of senile plaques in Alzheimer's disease (AD), and its mechanism of production has been studied intensely. γ-Secretase executes multiple cleavages within the transmembrane domain of APP, with cleavages producing Aβ and the APP intracellular domain (AICD), referred to as γ and ε, respectively. The heterogeneous nature of the γ cleavage that produces various Aβ peptides is highly relevant to AD, as increased production of Aβ 1-42 is genetically and biochemically linked to the development of AD. We have identified an amino acid in the juxtamembrane region of APP, lysine 624, on the basis of APP695 numbering (position 28 relative to Aβ) that plays a critical role in determining the final length of Aβ peptides released by γ-secretase. Mutation of this lysine to alanine (K28A) shifts the primary site of γ-secretase cleavage from 1-40 to 1-33 without significant changes to ε cleavage. These results further support a model where ε cleavage occurs first, followed by sequential proteolysis of the remaining transmembrane fragment, but extend these observations by demonstrating that charged residues at the luminal boundary of the APP transmembrane domain limit processivity of γ-secretase.     

Cell death induced by a caspase-cleaved transmembrane fragment of the Alzheimer amyloid precursor protein

The Alzheimer amyloid precursor protein (APP) is a transmembrane protein whose abnormal processing is associated with the pathogenesis of Alzheimer's disease. Activated caspases cleave APP and generate its carboxyl-terminally truncated fragment (APPdeltaC31). We have previously reported that overexpression of wild-type APP induces caspase-3 activation and apoptosis in postmitotic neurons. We now report that APPdeltaC31 potentially plays pathophysiological roles in neuronal death. Adenovirus-mediated overexpression of wild-type APP695 induced activation of caspase-3 and accumulation of APPdeltaC31 in postmitotic neurons derived from human NT2 embryonal carcinoma cells, whereas an APP mutant lacking the Abeta(1-20) region induced neither caspase-3 activation nor APPdeltaC31 generation. Inhibition of caspase-3 suppressed the generation of APPdeltaC31 in APP-overexpressing neurons. Forced expression of APPdeltaC31 induced apoptotic changes of neurons and non-neuronal cells, but failed to activate caspase-3. The cytotoxicity of APPdeltaC31 was also dependent on the Abeta(1-20) region. These results suggest that accumulation of wild-type APP activates neuronal caspase-3 to generate APPdeltaC31 that mediates caspase-3-independent cell death.     

BRI2 interacts with amyloid precursor protein (APP) and regulates amyloid beta (Abeta) production

Transmembrane proteins BRI2 and amyloid precursor protein (APP) co-localize with amyloid beta (Abeta) lesions in sporadic Alzheimer disease and mutations in both precursor proteins are linked to early-onset familial cases of cerebral amyloidosis associated with dementia and/or cerebral hemorrhage. A specific interaction between BRI2 and APP was unveiled by immunoprecipitation experiments using transfected and non-transfected cells. The use of deletion mutants further revealed that stretches 648-719 of APP751 and 46-106 of BRI2, both inclusive of the full transmembrane domains, are sufficient for the interaction. Removal of most of the APP and BRI2 extracellular domains without affecting the interaction implies that both proteins interact when are expressed on the same cell membrane (cis) rather than on adjacent cells (trans). The presence of BRI2 had a modulatory effect on APP processing, specifically increasing the levels of cellular APP as well as beta-secretase-generated COOH-terminal fragments while decreasing the levels of alpha-secretase-generated COOH-terminal fragments as well as the secretion of total APP and Abeta peptides. Determining the precise molecular pathways affected by the specific binding between APP and BRI2 could result in the identification of common therapeutic targets for these sporadic and familial neurodegenerative disorders.     

Mitochondrial targeting and a novel transmembrane arrest of Alzheimer's amyloid precursor protein impairs mitochondrial function in neuronal cells

Alzheimer's amyloid precursor protein 695 (APP) is a plasma membrane protein, which is known to be the source of the toxic amyloid beta (Abeta) peptide associated with the pathogenesis of Alzheimer's disease (AD). Here we demonstrate that by virtue of its chimeric NH2-terminal signal, APP is also targeted to mitochondria of cortical neuronal cells and select regions of the brain of a transgenic mouse model for AD. The positively charged residues at 40, 44, and 51 of APP are critical components of the mitochondrial-targeting signal. Chemical cross-linking together with immunoelectron microscopy show that the mitochondrial APP exists in NH2-terminal inside transmembrane orientation and in contact with mitochondrial translocase proteins. Mutational studies show that the acidic domain, which spans sequence 220-290 of APP, causes the transmembrane arrest with the COOH-terminal 73-kD portion of the protein facing the cytoplasmic side. Accumulation of full-length APP in the mitochondrial compartment in a transmembrane-arrested form, but not lacking the acidic domain, caused mitochondrial dysfunction and impaired energy metabolism. These results show, for the first time, that APP is targeted to neuronal mitochondria under some physiological and pathological conditions.     

Cellular cholesterol loss by DHCR24 knockdown leads to Aβ production by changing APP intracellular localization

For the past 20 years, the majority of cell culture studies reported that increasing cholesterol level increases amyloid-β (Aβ) production. Conversely, other studies and genetic evidences support that cellular cholesterol loss leads to Aβ generation. As a highly controversial issue in Alzheimer’s disease pathogenesis, the apparent contradiction prompted us to again explore the role of cellular cholesterol in Aβ production. Here, we adopted new neuronal and astrocytic cell models induced by 3β-hydroxysterol-Δ24 reductase (DHCR24), which obviously differ from the widely used cell models with overexpressing amyloid precursor protein (APP) in the majority of previous studies. In neuronal and astrocytic cell model, we found that deficiency of cellular cholesterol by DHCR24 knockdown obviously increased intracellular and extracellular Aβ generation. Importantly, in cell models with overexpressing APP, we found that APP overexpression could disrupt cellular cholesterol homeostasis and affect function of cells, coupled with the increase of APP β-cleavage product, 99-residue transmembrane C-terminal domain. Therefore, we suppose the results derived from the APP knockin models will need to be re-evaluated. One rational explanation for the discrepancy between our outcomes and the previous studies could be attributed to the two different cell models. Mechanistically, we showed that cellular cholesterol loss obviously altered APP intracellular localization by affecting cholesterol-related trafficking protein of APP. Therefore, our outcomes strongly support cellular cholesterol loss by DHCR24 knockdown leads to Aβ production.