Amyloid Precursor Protein Aβ Research Articles

Altered subcellular localization of fragile X mental retardation signaling partners and targets in superior frontal cortex of individuals with schizophrenia

Schizophrenia is a severe, debilitating, neurodevelopmental disorder that affects 1% of the world's population. Recent findings from our laboratory have identified reduced levels of fragile X mental retardation protein (FMRP) and several downstream FMRP targets in superior frontal cortex of individuals with schizophrenia. We hypothesized that altered subcellular expression of FMRP and its signaling partners may explain these changes. In the current study we employed subcellular fractionation and western blotting to determine levels of FMRP, phosphorylated-FMRP as well as selected signaling partners [protein phosphatase 2A catalytic subunit (PP2AC), p70 S6 kinase (p70 S6K), and amyloid-β A4 precursor protein (APP)] in the total homogenate, nuclear, and rough endoplasmic reticulum fractions in superior frontal cortex of individuals with schizophrenia versus controls (N=12/group). In total homogenate of individuals with schizophrenia, we identified significantly lower levels of FMRP, phosphorylated-FMRP, and PP2AC. In the nuclear fraction of individuals with schizophrenia we found significantly higher levels of PP2AC, p70 S6K, APP 120 kDa, and APP 88 kDa proteins. Finally, in rough endoplasmic reticulum of individuals with schizophrenia, we identified significantly lower protein levels of p70 S6K and APP 120 kDa. These results provide evidence for a potential mechanism to explain altered FMRP expression in schizophrenia.

Influence of Coding Variability in APP-Aβ Metabolism Genes in Sporadic Alzheimer's Disease.

The cerebral deposition of Aβ42, a neurotoxic proteolytic derivate of amyloid precursor protein (APP), is a central event in Alzheimer’s disease (AD)(Amyloid hypothesis). Given the key role of APP-Aβ metabolism in AD pathogenesis, we selected 29 genes involved in APP processing, Aβ degradation and clearance. We then used exome and genome sequencing to investigate the single independent (single-variant association test) and cumulative (gene-based association test) effect of coding variants in these genes as potential susceptibility factors for AD, in a cohort composed of 332 sporadic and mainly late-onset AD cases and 676 elderly controls from North America and the UK. Our study shows that common coding variability in these genes does not play a major role for the disease development. In the single-variant association analysis, the main hits, none of which statistically significant after multiple testing correction (1.9e-4<p-value<0.05), were found to be rare coding variants (0.009%<MAF<1.4%) with moderate to strong effect size (1.84<OR<Inf) that map to genes mainly involved in Aβ extracellular degradation (TTR, ACE), clearance (LRP1) and APP trafficking and recycling (SORL1). These results were partially replicated in the gene-based analysis (c-alpha and SKAT tests), that reports ECE1, LYZ and TTR as nominally associated to AD (1.7e-3 <p-value <0.05). In concert with previous studies, we suggest that 1) common coding variability in APP-Aβ genes is not a critical factor for AD development and 2) Aβ degradation and clearance, rather than Aβ production, may play a key role in the etiology of sporadic AD.

Appoptosin is a Novel Pro-Apoptotic Protein and Mediates Cell Death in Neurodegeneration

Apoptosis is an essential cellular process in multiple diseases and a major pathway for neuronal death in neurodegeneration. The detailed signaling events/pathways leading to apoptosis, especially in neurons, require further elucidation. Here we identify a β-amyloid precursor protein (APP)-interacting protein, designated as appoptosin, whose levels are upregulated in brain samples from Alzheimer's disease and infarct patients, and in rodent stroke models, as well as in neurons treated with β-amyloid (Aβ) and glutamate. We further demonstrate that appoptosin induces reactive oxygen species release and intrinsic caspase-dependent apoptosis. The physiological function of appoptosin is to transport/exchange glycine/5-amino-levulinic acid across the mitochondrial membrane for heme synthesis. Downregulation of appoptosin prevents cell death and caspase activation caused by glutamate or Aβ insults. APP modulates appoptosin-mediated apoptosis through interaction with appoptosin. Our study identifies appoptosin as a crucial player in apoptosis and a novel pro-apoptotic protein involved in neuronal cell death, providing a possible new therapeutic target for neurodegenerative disorders.

Engulfment adapter PTB domain containing 1 interacts with and affects processing of the amyloid-β precursor protein

Previous studies identified engulfment adapter phosphotyrosine binding (PTB) domain containing 1 (GULP1) as an NPXY-motif interactor of low-density lipoprotein receptor-related protein 1 (LRP1) and suggested a potential relevance in Alzheimer's disease (AD). Since AD associated proteins amyloid-β A4 precursor protein (APP) and LRP1 were shown to interact with the PTB domain of Fe65 and several other adapters via their intracellular NPXY-motifs, we examined a possible interaction of GULP1 PTB domain with the YENPTY-motif of APP. Here we demonstrate that GULP1 is present in human hippocampal and neocortical neurons. Confocal live cell imaging revealed that coexpressed and endogenous GULP1 colocalizes with APP in the Golgi and endoplasmic reticulum. Analysis of the interacting domains by co-immunoprecipitation of point and deletion mutants revealed that the interaction depends on the PTB domain of GULP1 and the YENPTY-motif of APP. Coexpression of GULP1 affected APP cell surface localization and suppressed generation of Aβ40/42 and sAPPα. Taken together, these data identify GULP1 as a novel neuronal APP interacting protein that alters trafficking and processing of APP.

Nogo-A is involved in secondary axonal degeneration of thalamus in hypertensive rats with focal cortical infarction

We investigate whether Nogo-A is involved in the secondary axonal degeneration in the thalamus after distal middle cerebral artery occlusion (MCAO) in stroke-prone renovascular hypertensive rats (RHRSP). The expression of Nogo-A in ipsilateral ventroposterior nucleus (VPN) of the thalamus in RHRSP was observed at 1, 2 and 4 weeks after distal MCAO. In addition, intracerebroventricular infusion of NEP1-40, a Nogo-66 receptor (NgR) antagonist peptide, was administered starting 24 h after MCAO and continued for 1, 2 and 4 weeks, respectively. Axonal damage and regeneration were evaluated by analysis of the immunoreactivity (IR) of amyloid βA4 precursor protein (APP), growth associated protein 43 (GAP-43) and microtubule associated protein 2 (MAP-2) in ipsilateral VPN of the thalamus at 1, 2 and 4 weeks after distal MCAO. Following ischemia, the expression of Nogo-A in oligodendrocytes increased persistently and its localization became redistributed around damaged axons and dendrites. Administration of NEP1-40 downregulated the expression of Nogo-A, reduced axonal injury and enhanced axonal regeneration. Our data suggest that Nogo-A is involved in secondary axonal degeneration and that inhibition of Nogo-A can reduce neuronal damage in the thalamus after distal MCAO.

Proteomic analysis of brain tissue from an Alzheimer's disease mouse model by two-dimensional difference gel electrophoresis

We used a β-amyloid precursor protein (APP) transgenic (Tg) mouse model that displays some of the typical Alzheimer-associated pathological features to study the brain proteoma associated with amyloid plaque deposition. Two groups (male and female) of 14-month-old Tg mice were compared with their wild type littermates. We used differential 2D electrophoresis coupled with mass spectrometry to generate one of the first complete image of changes in brain protein expression occurring in this well-recognized model of Alzheimer's disease (AD). We identified 15 different proteins, which are significantly regulated in this pathology ( p < 0.05, ≥1.5-fold variation in expression comparing with the wild type samples). These comprise a number of proteins that were already known to be implicated in AD and neurodegeneration, as well as several proteins which relationship with AD had not been shown before. Identified proteins were grouped according to their biological key pathways. Results obtained are discussed in view of existing bibliographic data on human AD transcriptoma and proteoma.

Axonal damage and demyelination in the white matter after chronic cerebral hypoperfusion in the rat

Cerebral white matter (WM) lesions are observed frequently in human ischemic cerebrovascular disease and have been thought to contribute to cognitive impairment. This type of lesion can be experimentally induced in rat brains under chronic cerebral hypoperfusion by the permanent occlusion of both common carotid arteries. However, it remains uncertain whether chronic ischemia can damage both the gray and white matter, and whether it can induce demyelination with or without axonal damage. Therefore, we examined axonal damage using immunohistochemistry for the amyloid β/A4 precursor protein (APP), chromogranin A (CgA) and demyelination using immunohistochemistry for the encephalitogenic peptide (EP) in this model. Severe WM lesions such as vacuolation and the loss of nerve fibers appeared in the optic nerve and optic tract after 3 days of ligation, and less intense changes were observed in the corpus callosum, internal capsule, and fiber bundles of the caudoputamen after 7 days with Klüver–Barrera and Bielschowsky staining. These WM lesions persisted even after 30 days. The APP, CgA, and EP-immunopositive fibers increased in number from 1 to 30 days after the ligation in the following WM regions: the optic nerve, optic tract, corpus callosum, internal capsule, and fiber bundles of the caudoputamen. In contrast, only a few APP, CgA, or EP-immunopositive fibers were detected in the gray matter regions, including the cerebral cortex and hippocampus. These results indicate that the WM is more susceptible to chronic cerebral hypoperfusion than the gray matter, with an involvement of both axonal and myelin components. Furthermore, immunohistochemistry for APP, CgA, and EP is far superior to routine histological staining in sensitivity and may become a useful tool to investigate WM lesions caused by various pathoetiologies.

Copper-binding amyloid precursor protein undergoes a site-specific fragmentation in the reduction of hydrogen peroxide.

The extracellular domain of transmembrane Abeta amyloid precursor protein (APP) has a Cu(II) reducing activity upon Cu(II) binding associated with the formation of a new disulfide bridge. The complete assignment of the disulfide bond revealed the involvement of cysteines 144 and 158 around copper-binding histidine residues. The vulnerability of APP-Cu(I) complexes to reactive oxygen species was elaborated as a site-specific and random fragmentation of APP in a time-dependent manner and at low concentrations of H2O2. Analysis of the specific reaction revealed the generation of C-terminal polypeptides, containing the Abeta domain. APP catalyzed the reduction of H2O2 and oxidation of Cu(I) to Cu(II) in a "peroxidative" reaction in vitro. The resulting bound copper-hydroxyl radical intermediate [APP-Cu(II)(.OH)] then likely participated in a Fenton type of reaction with radical formation as a prerequisite for protein degradation. Evidence from two observations suggests that the reaction takes place in two phases. Bathocuproine, a trapping agent for Cu(I), abolished the initial fragmentation, and chelation of Cu(II) by DTPA (diethylenetriaminepentaacetic acid) interrupted the reaction cascade induced by H2O2 at later stages. Consequently, the results suggest that a cytotoxic gain-of-function of APP-Cu(I) complexes might result in a perturbation of free radical homeostasis. What significance such a perturbation may have for the pathogenesis of Alzheimer's disease remains to be determined.

Dorothy Russell Memorial Lecture. The molecular pathology of Alzheimer's disease: are we any closer to understanding the neurodegenerative process?

Alzheimer's disease, the most common cause of dementia in the elderly, is rapidly becoming epidemic in the western world, with major social and economic ramifications. Thus enormous international scientific efforts are being made to increase our understanding of the pathogenesis of this disease, with the eventual goal of developing beneficial therapy. The two major neuropathological hallmarks of Alzheimer's disease (AD) are extracellular senile plaques, the principal component of which is the A beta amyloid peptide, and intraneuronal neurofibrillary tangles, which are composed of aggregated tau protein in the form of paired helical filaments (PHF). In the past decade, since the major proteinaceous components of these pathological markers have been identified, great strides have been made in elucidating the biochemical processes which may underlie their abnormal deposition and aggregation in Alzheimer's disease. Simultaneously, extensive population genetic analyses have identified mutations in the A beta amyloid precursor protein (APP) in a small number of pedigrees with familial Alzheimer's disease (FAD) whilst other FAD cases have been linked to an, as yet, unidentified marker on chromosome 14. Most recently, inheritance of the type 4 allele of apolipoprotein E has also been identified as a risk factor in sporadic AD. The challenge facing scientists now is to incorporate this wealth of exciting new biochemical and genetic data into a coherent model which can explain the long established neurochemical and histopathological lesions characteristic of AD.

Expression and metabolism of mutant human Aβ-amyloid precursor protein (APP) in transgenic mice and cultured cells : D.R. Borchelt 1, C.A. Kitt 1, S.S. Sisodia 1, G. Carlson 2, S.B. Prusiner 3, K.K. Hsiao 2 and D.L. Price 1. 1Johns Hopkins Medical Institutions, 2University of Minnesota School of Medicine, 3McLaughlin Research Institute, and the University of California, San Francisco

Expression and metabolism of mutant human Aβ-amyloid precursor protein (APP) in transgenic mice and cultured cells : D.R. Borchelt 1, C.A. Kitt 1, S.S. Sisodia 1, G. Carlson 2, S.B. Prusiner 3, K.K. Hsiao 2 and D.L. Price 1. 1Johns Hopkins Medical Institutions, 2University of Minnesota School of Medicine, 3McLaughlin Research Institute, and the University of California, San Francisco

Localization of Alzheimer βA4 amyloid precursor protein at central and peripheral synaptic sites

We have recently shown that the amyloid βA4 precursor protein (APP) is synthesized in neurons and undergoes fast axonal transport to synaptic sites [Koo et al., Proc. Natl. Acad. Sci. U.S.A., 87 (1990) 1561–1565]. Using immunofluorescence, laser confocal microscopy and immunoelectron microscopy with simultaneous detection of APP and synaptophysin, we now report a preferential localization of APP at synaptic sits of human and rat brain and at neuromuscular junctions. APP is further found on vesicular elements of neuronal perikarya, dendrites and axons. The synaptic localization of APP implies (1) a role of APP in physiological synaptic activity and (2) a potential and early impairment of central synapses when synaptic APP is converted to βA4 amyloid during the pathological evolution of Alzheimer's disease and Down's syndrome.