Amyloid beta-protein precursor (APP) of cultured cells: secretory and non-secretory forms of APP

Expression of the gene encoding the beta-amyloid precursor protein APP in Xenopus laevis

The beta-amyloid precursor protein APP is generally accepted to be directly or indirectly involved in the neurodegenerative disorder Alzheimer's disease and has been extensively studied in a number of mammalian systems. Its normal function remains, however, still elusive. We have used the clawed toad, Xenopus laevis, to study the first non-mammalian APP protein. Screening of a Xenopus laevis intermediate pituitary cDNA library led to the identification of two structurally different APP gene transcripts presumably resulting from duplicated genes. Sequence comparison between the Xenopus and human APP proteins revealed at the amino acid sequence level an identity of 92%. Both Xenopus genes were found to be expressed in all tissues examined, but their expression levels differed among tissues. In addition, as in mammals, alternative splicing was observed and the alternatively spliced APP(695) mRNA variant was expressed predominantly in the brain and the oocyte, while the longer isoforms (APP(751-770)) were predominant in the other tissues examined. Of special interest is the finding that, like human but unlike mouse or rat beta-amyloid (Abeta), the Xenopus peptide contains all amino acid residues implicated in amyloidogenesis. We conclude that Xenopus APP mRNA is ubiquitously expressed and alternatively spliced, and that the highly conserved Xenopus APP protein contains an Abeta peptide with amyloidogenic potency.

Secretory cleavage of beta-amyloid precursor protein in the cerebral white matter produces amyloidogenic carboxyl-terminal fragments

To elucidate the metabolic process generating amyloid-beta protein (A beta) from beta-amyloid precursor protein (APP) in human brain, we partially purified secretory forms and carboxyl-terminal fragments (CTFs) of APP from the white matter of a Down's syndrome brain. We obtained secretory forms of APP which lack the entire A beta sequence and CTFs which contain the full-length A beta from the cerebral white matter. Some A beta-lacking secretory APP isoforms in the white matter were derived from APP695. These results suggest that amyloidogenic CTFs can be produced by secretory cleavage of APP which is anterogradely transported through the axon in human brain.

Accumulation of amyloid precursor protein in brain lesions of patients with Pick disease

The expression of amyloid precursor protein (APP) was examined immunohistochemically in Pick disease brain using several antibodies to various segments of APP. Some neurons, including ballooned neurons, were intensely stained with antibodies to all but the N-terminal APP segment. However, Pick bodies were labeled with the antibody to that segment as well as the antibody to intermediate segment of APP. The pattern was similar to that previously observed for Alzheimer neurofibrillary tangles. These data provide additional evidence that Pick bodies and neurofibrillary tangles share some immunohistochemical characteristics.

Evidence for excitoprotective and intraneuronal calcium-regulating roles for secreted forms of the beta-amyloid precursor protein

The beta-amyloid precursor protein (beta APP) is a membrane-spanning glycoprotein that is the source of the beta-amyloid peptide (beta AP) which accumulates as senile plaques in the brains of patients with Alzheimer's disease. beta APP is normally processed such that a cleavage occurs within the beta AP, liberating secreted forms of beta APP (APPss) from the cell. The neuronal functions of these forms are unknown. We now report that APPss have a potent neuroprotective action in cultured rat hippocampal and septal neurons and in human cortical neurons. APPs695 and APPs751 protected neurons against hypoglycemic damage, and the neuroprotection was abolished by antibodies to a specific region common to both APPs695 and APPs751. APPss caused a rapid and prolonged reduction in [Ca2+]i and prevented the rise in [Ca2+]i that normally mediated hypoglycemic damage. APPss also protected neurons against glutamate neurotoxicity, effectively raising the excitotoxic threshold. APPss may normally play excitoprotective and neuromodulatory roles. Alternative processing of APPss in Alzheimer's disease may contribute to neuronal degeneration by compromising the normal function of APPss and by promoting the deposition of beta AP.

Expression of a carboxy-terminal region of the beta-amyloid precursor protein in a heterogeneous culture of neuroblastoma cells: evidence for altered processing and selective neurotoxicity

Six independent clonal isolates from a morphologically heterogeneous human neuroblastoma cell line stably expressed several products of the human amyloid precursor protein (APP) from an introduced DNA construct; the "substrate-adherent" phenotype (fibroblast-like cells) predominated in all 6; these displayed immunoreactivity of vimentin, but little to no reactivity of neuron-specific enolase. A stably transfected isolate which did not show any expression from the identical construct (presumably because of a position effect) exhibited the predominantly neuronal phenotype of the parental cells (neuron-specific enolase positive). These results suggest selective neurotoxicity of the expressed products. Two of the 6 stably expressing cell lines showed a decrease of native mRNA for APP to levels that were 1/4-1/3 that of the parental cells and a decrease of their growth rates to half that of the parental cells; these decreased growth rates were improved by conditioned medium from the parental cell line. Western blot analysis revealed at least four distinct fragments of the COOH-terminus of APP in the isolate which expressed protein and mRNA in greatest abundance, suggesting that overexpression of APP in a human neural cell line leads to aberrant cleavage of APP.

Amyloid precursor protein is localized in growing neurites of neonatal rat brain

Previous studies have indicated that amyloid precursor protein (APP) might be a trophic agent in the nervous system, possibly through the regulation of cell adhesion and the protease/protease inhibitor activity. Additionally, APP is upregulated during the development of the nervous system. In order to further study the role of APP in neuritic outgrowth, we examined the patterns of distribution of APP in the immature neonatal rat brain (P1). Laser-scanning confocal imaging of double-immunolabeled sections showed that a subpopulation of the anti-GAP43-immunoreactive outgrowing neurites contained APP immunoreactivity in the neocortex and hippocampus. These fine, long neuritic processes were also positive with antibodies against phosphorylated neurofilaments and were glial fibrillary acidic protein (GFAP) negative. In addition, anti-APP strongly immunolabeled neurons in the inner cortical layers, while GAP43 strongly immunolabeled the neuropil surrounding them. These observations are consistent with a previous study where APP was localized to aberrant sprouting neurites and suggest a possible role for APP in neuritic outgrowth in plaques of patients with Alzheimer's disease (AD), which might explain the abnormal neuritic response found in AD.

Accumulation of amyloid precursor protein in damaged neuronal processes and microglia following intracerebral administration of aluminum salts

Alzheimer's disease is characterized by neurofibrillary tangles and amyloid deposits, with the latter probably occurring because of abnormal accumulation and/or processing of amyloid precursor protein (APP). Aluminum salts are known to be neurotoxic and to be capable of inducing neurofibrillary tangles. We explored the effects of intraventricular or intrastriatal injections of AlCl3 on the immunodistribution of APP in rat brain. There was a striking and long-lasting accumulation of APP in affected neurites, as well as in activated microglia/macrophages. Abnormal neurites also showed argentophilic changes, neurofilament accumulation, and Alz50 immunoreactivity. However, no extracellular amyloid fibrils were seen. The results, taken together with previous studies on colchicine, are consistent with the hypothesis that interruption of axoplasmic flow can lead to both APP accumulation and cytoskeletal changes.

Localization of amyloid precursor protein in selective postsynaptic densities of rat cortical neurons

One of the hallmarks of Alzheimer pathology is extracellular deposition of beta-amyloid protein (BAP) which is derived from a larger glycoprotein called amyloid precursor protein (APP). Although APP has often been described as a surface membrane protein, such a localization has not previously been demonstrated at the light or electron microscopic level. We now report the results of immunoelectron microscopy using three specific antibodies against different synthetic fragments of APP. All three antibodies demonstrated a major localization to organelles such as the Golgi apparatus, endoplasmic reticulum and vesicular-like structures. A minor proportion of staining with all three was on selective postsynaptic membranes of asymmetrical synapses, whereas staining of presynaptic membranes was not observed. The morphological evidence suggests that one role of APP may be in association with the function of selective synapses.

Protease inhibitors generate cytotoxic fragments from Alzheimer amyloid protein precursor in cDNA-transfected glioma cells

A human glioma cell line (Bu-17) was stably transfected with full-length cDNA encoding beta/A4 amyloid protein precursor (APP). When the transfectants were treated with protease inhibitors (leupeptin, E-64, and antipain) and the lysosomotropic agent chloroquine, aberrantly processed fragments of APP having molecular sizes of 8-30 kDa were detected with an antibody against the carboxyl-terminal sequence of APP. Immunocytochemistry revealed that these fragments were localized in the lysosome-like organelles. Treatment of the APP cDNA transfectants with chloroquine for 3 days caused cellular degeneration, and leupeptin and E-64 enhanced chloroquine-induced cytotoxicity. These results suggest that inhibition of lysosomal hydrolases impairs intracellular APP metabolism to generate aberrantly processed fragments that induce cytotoxicity.

Increased expression of beta-amyloid protein precursor and microtubule-associated protein tau during the differentiation of murine embryonal carcinoma cells

Expression of the genes encoding the beta/A4 amyloid protein precursor (APP) and microtubule-associated protein tau was studied in an embryonal carcinoma cell line (P19) that differentiates in vitro into cholinergic neurons after treatment with retinoic acid. Expression of APP increased 34- (mRNA) and 50-fold (protein) during neuronal differentiation; APP-695 accounted for most of this increase. These remarkable increases in APP expression coincided with a proliferation of neuronal processes and with an increase in content of tau mRNA. Moreover, subsequent decreases in the levels of APP and tau mRNA coincided with the onset of the degeneration of the neuronal processes. Immunocytochemical staining suggested that greater than 85% of the P19-derived neurons are cholinergic and that APP is present in the neuronal processes and cell bodies. These results suggest that APP may play an important role in construction of neuronal networks and neuronal differentiation and also indicate that this embryonal carcinoma cell line provides an ideal model system to investigate biological functions of APP and the roles of APP and tau protein in development of Alzheimer's disease in cholinergic neurons.

Immunohistochemical localization of beta-amyloid precursor protein sequences in Alzheimer and normal brain tissue by light and electron microscopy

Immunohistochemical staining with antibodies directed against four segments of the amyloid precursor protein (APP) was studied by light and electron microscopy in normal and Alzheimer (AD) brain tissue. The segments according to the Kang et al. sequence were: 18-38 (T97); 527-540 (R36); 597-620 (1-24 of beta-amyloid protein [BAP], R17); and 681-695 (R37) (Kang et al. [1987]: Nature 325:733-736). The antibodies recognized full length APP in Western blots of extracts of APP transfected cells. They stained cytoplasmic granules in some pyramidal neurons in normal appearing tissue from control and AD cases. In AD affected tissue, the antibodies to amino terminal sections of APP stained tangled neurons and neuropil threads, and intensely stained dystrophic neurites in senile plaques. By electron microscopy, this staining was localized to abnormal filaments. The antibody to the carboxy terminal segment failed to stain neurofibrillary tangles or neuropil threads; it did stain some neurites with globular swellings. It also stained globular and elongated deposits in senile plaque areas. The antibody against the BAP intensely stained extracellular material in senile plaques and diffuse deposits. By electron microscopy, the antibodies all stained intramicroglial deposits. Some of the extracellular and intracellular BAP-positive deposits were fibrillary. Communication between intramicroglial and extracellular fibrils was detected in plaque areas. These data suggest the following sequence of events. APP is normally concentrated in intraneuronal granules. In AD, it accumulates in damaged neuronal fibers. The amino terminal portion binds to abnormal neurofilaments. Major fragments of APP are phagocytosed and processed by microglia with the BAP portion being preserved. The preserved BAP is then extruded and accumulates in extracellular tissue.

Reactive microglia/macrophages phagocytose amyloid precursor protein produced by neurons following neural damage

Kainic acid lesions of rat striatum caused an elevation of amyloid precursor protein (APP) immunoreactivity in neurons and neurites, some of which were then phagocytosed by reactive microglia/macrophages. Immunoexpression of APP was observed in neurites and neurons 1 day after the kainic injection. Four days after lesioning, immunoreactivity was still concentrated in thick and distorted neurites, but it began to appear in microglia/macrophages and in the tissue matrix. The cells were identified as microglia/macrophages by the phenotypic markers Ia (OX6), leukocyte common antigen (OX1), C3bi receptor (OX42), and macrophage marker (ED1). They were negative for the astrocytic marker glial fibrillary acidic protein (GFAP). APP immunoreactivity in these phagocytic cells was most prominent between 1 week and 1 month postlesioning. No extracellular amyloid fibrils were detectable. These results suggest that APP production is rapidly upregulated in damaged neurons and accumulates in degenerating axons. However, phagocytosis of APP by reactive microglia/macrophages in this rat model does not result in production of Alzheimer type amyloid deposits.

Expression patterns of beta-amyloid precursor protein (beta-APP) in neural and nonneural human tissues from Alzheimer's disease and control subjects

Both neural and nonneural human tissues from patients with or without Alzheimer's disease (AD) were surveyed to detect the presence of the beta-amyloid protein and its precursors. This was accomplished using polyclonal and monoclonal antibodies to epitopes in the 695 amino acid long beta-APP (i.e., beta-APP695), as well as in related beta-APPs. Immunoreactivity in beta-APP in brain was prominent in senile plaques, extraneuronal tangles, and neurons. Outside the brain, beta-APP staining was seen in neurons and satellite glial cells of the dorsal root, enteric and trigeminal ganglia, the adeno- and neurohypophysis, megakaryocytes, and adrenal gland in samples from patients with AD and those without AD. Western blots of neocortex revealed three major proteins with apparent molecular masses of 105, 115, and 125 kDa in the insoluble membrane-associated fractions, while two broad bands with a molecular weight centered at about 100 and 120 kDa were detected in soluble fractions. In addition, the pituitary and adrenal glands as well as cardiac muscle revealed prominent immunobands in membrane-associated fractions. Notably, other nonneural tissues were devoid of beta-APP immunoreactivity. Thus, the beta-APPs are detectable only in a limited number of nonneural tissues. Taken together, these data suggest that beta-APPs produced in the brain are sources of beta-APP peptides that accumulate as senile plaques in AD.