Secreted form of amyloid beta protein precursor is involved in the growth regulation of fibroblasts

The survival of rat cerebral cortical neurons in the presence of trophic APP peptides

One function of Alzheimer amyloid protein precursor (APP) is the regulation of growth and differentiation in several types of cells, including fibroblasts, PC12 cells, and neurons. This activity is represented by a small stretch of amino acids in the center of the molecule around RERMS. The APP 17-mer peptide containing the RERMS domain supported survival and neurite extension of rat cortical neurons in a dose-dependent and sequence-specific manner. The APP fragment synthesized in Escherichia coli supported the survival and neurite extension of rat cortical neurons, whereas the mutant APP fragment lacking the 30 amino acids around the RERMS domain had drastically reduced activity to support the survival and neurite extension. The current study established APP as a neuron survival factor and determined that the sequence around RERMS is important for this function.

Heat shock alters Alzheimer's beta amyloid precursor protein expression in human endothelial cells

One of the pathological lesions in Alzheimer's disease (AD) is the amyloid or senile plaque. The plaque core is predominantly made up of amyloid beta peptide (A beta), a 42-43 amino acid peptide derived from amyloid precursor protein (APP). APP is a membrane bound glycoprotein which is expressed ubiquitously in many cells. Although normal or pathological functions for APP are not well understood, several observations suggest that APP may play a role in cellular stress and inflammation at the endothelial cell/vascular barrier. APP is found in platelets and endothelial cells, it can inhibit a blood coagulation factor, and secreted APP can be neuroprotective. Changes in expression of APP during cellular stress or inflammation may contribute to pathological deposition of A beta. In the present studies, expression of APP in human endothelial cells was examined following heat shock. In human umbilical vein endothelial cells (HUVECs) exposed to 42 degrees C for 30 min, there was a five- to eight-fold increase in APP mRNA levels which peaked at 4 hr. The increase in APP mRNA was followed by an increase in APP protein immunoreactivity in the cytoplasm in a perinuclear Golgi-like region, and in discrete granular cytoplasmic structures. Immunoblot analysis of APP in the cell media found a transient increase in APP which peaked at 1 hr after heat shock. These results suggest that cellular stress induces the secretion of APP from endothelial cells followed by a subsequent increase in APP mRNA and protein synthesis. The upregulation of APP mRNA and protein supports a cellular stress role for APP.

Differential distribution of amyloid protein precursor immunoreactivity in the rat brain studied by using five different antibodies

The beta-amyloid or A4 protein is found deposited in neuritic plaques and neurofibrillary tangles in Alzheimer's disease (AD) affected brains and in the brains of adults with Down's Syndrome. The precursor to this 42 amino acid protein is the 695 amino acid long amyloid protein precursor (APP-695). Two additional APP species, APP-751 and APP-770, each contain a 56-amino-acid insert sequence that is analogous to Kunitz protease inhibitors. APP mRNA is widely distributed in both the human and rat brain, although the adult rat does not develop mature amyloid pathology. In this study we used antibodies against the N-terminus, junction site (unique to APP-695) insert sequence (unique to APP-751,-770), A4 region, and C-terminus of APP to immunolabel sections from throughout the young adult rat brain. From these results we constructed maps of the staining pattern of each antibody. We found that APP is widely distributed throughout the brain, that labelling is predominantly neuronal in character, and that there is marked variation among the antibodies in the extent of labelling, the particular cell populations stained, and the structures labelled within individual cells. The differential staining patterns observed with the five different antibodies suggest that the way APP is processed differs from one region to another and within different compartments in the cell. The specificity of the antibodies was established by Western blot analysis, in which APP species of approximately 95 and 110 kD were found. Our findings on the distribution of APP provide a foundation for further investigations into the normal role of APP and the pathogenesis of AD.

Novel amyloid precursor protein gene mutation (codon 665Asp) in a patient with late-onset Alzheimer's disease

Amyloid plaques in Alzheimer's disease contain beta-amyloid, encoded by portions of exons 16 and 17 of the amyloid precursor protein. The specific association of rare amyloid precursor protein mutations with some kindreds with early-onset familial Alzheimer's disease suggests that specific abnormalities in amyloid precursor protein may contribute to the pathogenesis of Alzheimer's disease. Until now, there has been no evidence suggesting that amyloid precursor protein mutations could be involved in late-onset or sporadic Alzheimer's disease. We used reverse transcription-polymerase chain reaction, denaturing gradient gel electrophoresis, and direct DNA sequencing to analyze amyloid precursor protein exons 16 and 17 from postmortem cerebellar samples from patients with histologically confirmed Alzheimer's disease and control subjects. We found a novel point mutation, substitution of cytosine for guanine, at nucleotide 2119 (amyloid precursor protein 770 messenger RNA transcript) in a patient with late-onset Alzheimer's disease. This substitution deletes a BglII site and substitutes aspartate for glutamine at codon 665. Denaturing gradient gel electrophoresis analysis showed that this mutation was absent in 40 control subjects and 127 dementia patients. Whether this mutation is a rare but normal variant or contributes to the development of Alzheimer's disease is not known. The BglII restriction fragment length polymorphism enables investigators to determine the frequency of this polymorphism in normal subjects and Alzheimer's disease patients.

Age and damage induced changes in amyloid protein precursor immunohistochemistry in the rat brain

Alzheimer's disease (AD) is characterized by the extensive deposition of the 42-amino-acid beta-amyloid or A4 protein in neuritic plaques and neurofibrillary tangles within the brain. This protein is liberated from the much larger amyloid protein precursor (APP). Multiple species of APP have been proposed, including several forms that contain a 56 amino acid insert sequence analogous to the Kunitz protease inhibitors. Although expression of APP mRNA is reportedly altered in AD brain and various roles for APP have been proposed, the pathogenesis of amyloid deposition and AD remains unclear. AD is also characterized by specific memory impairments associated with decreased cholinergic activity. While aging rats do not develop mature amyloid pathology, behaviorally impaired aged rats demonstrate an analogous cholinergic decline. In this study, we examined behaviorally characterized aged rats and normal young controls for changes in APP immunohistochemistry by using anti-APP antibodies, which detect N- or C-terminal regions and which distinguish APP species with or without the Kunitz protease inhibitor domain. The results show specific age- and behavior-related changes in cortical APP immunoreactivity as well as limited numbers of APP immunoreactive deposits in the aged rats. Additionally, we found that lesions of the fimbria-fornix pathway, which in part mimic the memory impairments and loss of cholinergic activity seen in AD, result in the marked accumulation of APP immunoreactive material in the region of cholinergic fiber degeneration in the hippocampus. These findings are discussed in relation to the pathogenesis of AD in humans.

Secreted forms of beta-amyloid precursor protein modulate dendrite outgrowth and calcium responses to glutamate in cultured embryonic hippocampal neurons

In addition to being the major excitatory neurotransmitter in the mammalian brain, glutamate is believed to play a key role in the regulation of neurite outgrowth and synaptogenesis during development. In cultured embryonic hippocampal pyramidal neurons, glutamate inhibits dendrite outgrowth by a mechanism involving elevation of intracellular-free calcium levels ([Ca2+]i). In the present study, secreted forms of the beta-amyloid precursor protein (APPss) counteracted the inhibitory effect of glutamate on dendrite outgrowth in cultured embryonic hippocampal neurons. The prolonged elevation of [Ca2+]i normally induced by glutamate was significantly attenuated in neurons that had been pretreated with 2-10 nM of APPs695 or APPs751. Immunocytochemistry with beta-amyloid precursor protein antibodies showed that immunoreactivity was concentrated in axons and, particularly, in their growth cones. Because beta-amyloid precursor proteins are axonally transported, and APPss can be released from axon terminals/growth cones in response to electrical activity, the present findings suggest that APPss may play a role in developmental and synaptic plasticity by modulating dendritic responses to glutamate.

beta-Amyloid precursor protein fragments and lysosomal dense bodies are found in rat brain neurons after ventricular infusion of leupeptin

Infusion of the serine and thiol protease inhibitor, leupeptin, is known to cause a reduction of fast axoplasmic transport, and accumulation of lysosomal dense bodies in neuronal perikarya. We have found these dense bodies in hippocampal and cerebellar neurons contain ubiquitin conjugated proteins. We now demonstrate that these accumulated neuronal lysosomes are labeled by antisera to the cytoplasmic, transmembrane and extracellular domains of beta-amyloid precursor protein (APP) and also that lysosomal APP is fragmented. This in vivo model confirms that neurons can process APP via a lysosomal pathway and that neuronal lysosomes in vivo contain both N-terminal and potentially amyloidogenic C-terminal fragments of APP. We also show that increased APP immunoreactivity after leupeptin treatment is seen first in neurons and later in astrocytes. On recovery from infusion, APP N-terminal immunoreactivity diminishes whilst C-terminal reactivity remains in neurons. These findings are consistent with production in whole brain of potentially amyloidogenic fragments of APP within neuronal lysosomes in perikarya and dendrites implying that neurons may play a role in forming the beta-amyloid of plaques.

Chromaffin cells express Alzheimer amyloid precursor protein in the same manner as brain cells

Amyloid precursor protein (APP) 695 is remarkably expressed in the brain as compared with APP751 and APP770 which are dominant in other tissues. This study showed that human and bovine adrenal medullae dominantly expressed mRNA of APP695 as do brain nerve cells, while the adrenal cortexes expressed mRNAs of APP751 and APP770 as in other non-neural tissues. In immunohistochemistry, chromaffin cells of young rat adrenal medullae and primary cultured bovine chromaffin cells were significantly stained with a monoclonal antibody (mAb) against the common domain of the amino-terminal side of human APPs. At higher magnification, the immunostained cells revealed that APP was granularly distributed not only in the perikaryon but also in the cell processes. These results suggest that primary cultured chromaffin cells representing the state of adrenal medulla in vivo are a useful model for studying the pathophysiological functions of APPs and the mechanism of processing of APPs as a model of neuronal systems.

Polarized secretion of beta-amyloid precursor protein and amyloid beta-peptide in MDCK cells

The beta-amyloid precursor protein (beta APP) is a widely expressed integral membrane protein that is proteolytically processed to yield several secreted derivatives, including soluble APP (APPs), the 4-kDa amyloid beta-peptide (A beta), and a related 3-kDa peptide (p3). To understand beta APP trafficking and processing, we analyzed the sorting of beta APP in Madin-Darby canine kidney (MDCK) cells, an epithelial cell known to possess physiologically distinct apical and basolateral plasma membranes. Processing of beta APP resulted in highly polarized secretion of APPs. More than 90% of APPs was detected in the basolateral compartment, and less than 10% was found in the apical compartment. This was associated with a preferential localization of beta APP on the basolateral cell surface. Activation of protein kinase C, which is known to enhance the secretion of APPs, did not change the polarity of APPs release but significantly increased the amount secreted. A beta and p3 peptides were also secreted predominantly basolaterally. In addition, MDCK cells secreted a truncated form of A beta beginning at Arg-5. These data show that the proteolytic processing products of beta APP undergo polarized secretion. Moreover, the results suggest that the amyloidogenic A beta peptide is generated following the polarized sorting of beta APP. The polarized basolateral secretion of A beta in these epithelial cells provides a potential mechanism for the accumulation of A beta in the abluminal basement membrane of brain microvessels during Alzheimer disease.

Selective ectodomain phosphorylation and regulated cleavage of beta-amyloid precursor protein

The beta-amyloid precursor protein (beta APP) is a highly conserved integral membrane protein expressed in most mammalian tissues and found at highest levels in the nervous system. Cerebral deposition of the amyloid beta-peptide (A beta), derived by proteolysis of beta APP, is an early and invariant feature of Alzheimer's disease. Protein phosphorylation by protein kinase C (PKC) has been found to regulate the metabolism of beta APP into nonamyloidogenic and amyloidogenic derivatives, but both the mechanism of these effects and the nature of beta APP phosphorylation are unknown. When labeled in vivo with [32P]orthophosphate, beta APP was phosphorylated only on serine residues in the N-terminal half of the extracellular domain, resulting in the secretion of phosphorylated soluble beta APP. PKC-mediated stimulation of beta APP secretion and concurrent inhibition of A beta release did not involve enhanced phosphorylation of beta APP and proceeded in the absence of cytoplasmic or extracellular phosphorylation of the precursor. The region of beta APP required for this indirect regulation by PKC was largely restricted to a 64 amino acid stretch around the secretory cleavage site. Moreover, in a truncated molecule designed to release soluble beta APP without the need for proteolytic cleavage, secretion was no longer regulated by PKC. Our data indicate that PKC-mediated pathways play a pivotal role in the control of beta APP metabolism and amyloid formation. However, in contrast to current postulates, this regulation is independent of beta APP phosphorylation and instead involves phosphorylation of other substrates that alter beta APP processing, such as beta APP-cleaving proteases.

The expression of amyloid beta protein precursor protects nerve cells from beta-amyloid and glutamate toxicity and alters their interaction with the extracellular matrix

The biological function of amyloid beta protein precursor (ABPP) in nerve cells is examined by transfecting it into the B103 clonal nerve cell line which makes no ABPP and asking if a variety of biological activities are altered. Although ABPP is expressed by the transfected cells at high levels, there are no detectable differences between the ABPP negative clones and the clones expressing ABPP with respect to growth rate in high serum medium, dibutyryl-cyclic AMP-induced differentiation, or morphology on plastic culture dishes. There are, however, significant differences in a number of properties between the two groups of cell lines. Cells expressing ABPP adhere more rapidly and have an enhanced rate of neurite outgrowth on collagen substrata. They also grow more rapidly in low serum medium. Finally, the expression of ABPP weakly but significantly protects the nerve cells from amyloid beta protein and glutamate toxicity.

Molecular cloning of cDNA encoding an unrecognized component of amyloid in Alzheimer disease

A neuropathological hallmark of Alzheimer disease (AD) is a widespread amyloid deposition. We analyzed the entire amino acid sequences in an amyloid preparation and found, in addition to the major beta/A4-protein (A beta) fragment, two unknown peptides. We raised antibodies against synthetic peptides using subsequences of these peptides. These antibodies immunostained amyloid in neuritic and diffuse plaques as well as vascular amyloid. Electron microscopic analysis demonstrated that the immunostaining was localized on amyloid fibrils. We have isolated an apparently full-length cDNA encoding a 140-amino-acid protein within which two previously unreported amyloid sequences are encoded in tandem in the most hydrophobic domain. We tentatively named this 35-amino acid peptide NAC (non-A beta component of AD amyloid) and its precursor NACP. NAC is the second component, after A beta, identified chemically in the purified AD amyloid preparation. Secondary structure predictions indicate that the NAC peptide sequence has a strong tendency to form beta-structures consistent with its association with amyloid. NACP is detected as a M(r) 19,000 protein in the cytosolic fraction of brain homogenates and comigrates on immunoblots with NACP synthesized in Escherichia coli from NACP cDNA. NACP mRNA is expressed principally in brain but is also expressed in low concentrations in all tissues examined except in liver, suggesting its ubiquitous and brain-specific functions. The availability of the cDNA encoding full-length NACP should help to elucidate the mechanisms of amyloidosis in AD.

Using the subcortically lesioned rat cortex to understand the physiological role of amyloid precursor protein

Alzheimer's disease pathology is characterized by the presence of neuritic plaques and neurofibrillary tangles and specific neurotransmitter deficits in the cortex and hippocampus. Advances in the understanding of Alzheimer's disease have been hampered by the absence of appropriate animal model systems. Most in vivo rodent models have turned to aged animals, animals with experimentally induced lesions of various neurotransmitter systems, animals with pharmacologically induced neurotransmitter perturbations, and mice made transgenic for genes related to amyloid precursor protein. These models have been useful for the investigation of some discrete aspects of Alzheimer's disease, including deficits in forebrain cholinergic activity and the resulting cognitive deficits. However, none of these models have led to the development of the principal neuropathological hallmarks of Alzheimer's disease, neuritic plaques and neurofibrillary tangles. Furthermore, the relationship, if any, between the reduction of neurotransmitter activity and the formation of neuritic plaques and neurofibrillary tangles is unknown. The subcortically lesioned rat model system which we have used approximates the cortical neurotransmitter and the cognitive deficits of Alzheimer's disease. We have recently found that these same subcortical neurotransmitter system lesions alter the expression of amyloid precursor protein, the precursor of beta amyloid peptide, which is the principal component of neuritic plaques. Loss of functional subcortical innervation by either permanent lesions or transient inhibition of cortical neurotransmitter (acetylcholine) release resulted in the induction of amyloid precursor protein in the cortex. The induction was rapid and persistent with the permanent lesions or reversible with the transient inhibition. Lesions cholinergic, serotonergic,and adrenergic neurotransmitter systems all resulted in the induction.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distribution of amyloid beta-protein precursor, growth-associated phosphoprotein-43 and microtubule-associated protein 2 messenger RNAs in the nigrostriatal system of normal and Weaver mutant mice and effects of ventral mesencephalic grafts

Using in situ hybridization histochemistry with [32P]oligonucleotide probes, we studied the cellular localization of RNA transcripts for amyloid beta-protein precursor (beta APP), growth-associated phosphoprotein-43 (GAP-43) and microtubule-associated protein 2 (MAP2) in the mesostriatal system of normal (+/+) and weaver (wv/wv) mutant mice, which lose mesencephalic dopamine neurons. In addition, expression of the same messages was studied in ventral mesencephalic cell suspensions transplanted to the weaver striatum. Transcripts encoding GAP-43, MAP2 and isoforms beta APP695, beta APP714 and beta APP751 were present in normal substantia nigra and progressively reduced in weaver substantia nigra; such a reduction was correlated with dopamine neuron loss. The survival of dopamine neurons in unilateral intrastriatal grafts was documented by methamphetamine-induced rotational asymmetry tests and by tyrosine hydroxylase immunocytochemistry. High hybridization signals were obtained for GAP-43, MAP2, beta APP695, beta APP714 and beta APP751 RNA transcripts in the grafted tissue; the beta APP770 species--normally seen in striatum and not substantia nigra--was not expressed in the grafts, but it was present in the recipient striatum. Following immunocytochemical labelling with antibodies, GAP-43 and MAP2 immunoreactivities were seen in cell processes in the grafts and surrounding tissue, whereas beta APP immunoreactivity was mainly found in grafted cell bodies. These results suggest that the transplanted mesencephalic cells mature very similarly to those in the normal substantia nigra, expressing different mRNAs that are normally present in the ventral midbrain and which are reduced in the weaver mutant as a consequence of dopamine neuron loss.

Alzheimer's amyloid precursor protein mRNA without exon 15 is ubiquitously expressed except in the rat central nervous system

The expression of L-beta A4 amyloid precursor protein (L-APP) mRNA, which is a splicing product excluding exon 15 of the APP gene, was investigated in various tissues of adult rats by a polymerase chain reaction analysis of reverse-transcribed RNA (RT-PCR). L-APP mRNA was ubiquitously expressed in all the examined tissues including the liver, kidney, heart, skeletal muscle, spleen, thymus, adrenal, stomach, submandibular gland, testis and ovary, except for the central nervous system (CNS) tissues such as the brain and spinal cord. The DNA sequence analysis of the RT-PCR products from adult rat liver showed an L-APP cDNA form, in which exon 14 was spliced from exon 14 to exon 16, and exon 15 of the APP gene was excluded. In addition, regarding as the brain and liver, L-APP mRNA expression was examined during the development of the embryonic stage. In the brain, no L-APP mRNA expression was detected even in the embryonic stage, whereas L-APP mRNA expression of the liver was still found in the embryonic stage. These results suggest that the splicing event excluding exon 15, which is exactly adjacent to exon 16 and exon 17 encoding the beta A4 protein, would probably occur very rarely in the CNS and that the splicing of L-APP might already be regulated in the embryonic stage.

Co-distribution of protease nexin-1 and protease nexin-2 in brains of non-human primates

The protease nexins are protease inhibitors which regulate key blood coagulation proteases and which appear to be involved in certain physiological and pathological processes in the brain. Protease nexin-1 (PN-1), a potent inhibitor of thrombin, can regulate processes on cultured neurons and astrocytes. Protease nexin-2 (PN-2), a potent inhibitor of coagulation factor XIa, is identical to the secreted form of the Alzheimer's amyloid beta-protein precursor. In the present studies, PN-1 and PN-2 were analyzed in different tissues of monkey using monoclonal antibodies for either quantitative immunoblotting or specific [125I]protease-binding assays. PN-1 was detected only in brain. PN-2 was most abundant in brain, followed by testis and to a lesser extent kidney. Other tissues examined including spinal cord, heart, pancreas, spleen, liver, lung and muscle were essentially devoid of both PN-1 and PN-2. Within the brain, the levels of PN-1 and PN-2 were highest in the parietal cortex and lowest in the cerebellum and brainstem. The thalamus and striatum contained intermediate amounts of both proteins. Aged Cebus monkey cerebral cortical tissue contained slightly lower levels of PN-1 than did the middle-aged or young monkey tissue. The co-distribution of PN-1 and PN-2 in brain, their relative abundance in brain cortex, and previous studies on their functions suggest that in the brain they may participate in the regulation of blood coagulation and cell growth and differentiation.

Biochemistry of Alzheimer's disease amyloid plaques

One of the principal identifying features of Alzheimer's disease (AD) is the extracellular deposition of fibrous protein aggregates in the form of amyloid plaques. The major component of these deposits is the amyloid beta (A beta) protein that is a proteolytic fragment of the integral membrane amyloid precursor protein (APP). Understanding the pathways responsible for A beta formation and the mechanism by which it accumulates within the brain could provide key answers to AD pathogenesis. This review will explore the biochemistry of A beta and its precursor, the possible causal relationship between amyloid and AD-associated neuronal death, the role of additional cellular elements in amyloid formation, and the potential application of these components in clinical diagnosis.

Phorbol ester-induced neuritic alterations in the rat neocortex. Structural and immunocytochemical studies

In order to explore the effect of aberrant sprouting in the CNS, phorbol 12-myristate 13-acetate (PMA) was administered into the neocortex of adult rats. PMA is a growth-promoting agent that activates and eventually downregulates protein kinase C (PKC), and induces in the rat the expression of several genes, including amyloid precursor protein (APP). We found that multiple injections of 100 nM PMA into the rat neocortex promote, in the first week postinjection, a widespread vacuolization of the neuropil with a subsequent disruption of the synapses in the injection site, followed, at d 15, by the formation of abnormally distended clusters of neurites that resembled aberrant, sprouting axons. At d 30, fewer aberrant sprouts were observed, and many degenerating neurites were found. At the ultrastructural level, the PMA-induced abnormal neurites at d 7-15 resembled growth cones, whereas the dystrophic neurites at d 30 contained abundant dense and laminated bodies. Immunohistochemical analysis indicated that the abnormal neurites in the areas of denervation and PMA administration were positive with antisynaptophysin and antigrowth-associated protein 43 (GAP-43), with an increased APP immunoreactivity surrounding them. APP immunoreactivity around the injection site was mostly associated with pyramidal neurons and glial cells. Control experiments, where saline alone or 4 alpha-phorbol 12, 13-didecanoate (PDD, an inactive phorbol derivative) was injected, failed to show aberrant sprouting neurites. Further immunohistochemical analysis showed that the PMA-treated animals presented increased amyloid beta immunoreactivity in the pyramidal cells at the site of injection, when compared with control injections. These findings suggest that aberrant sprouting induced by overstimulation could be followed by neurodegeneration. Alternatively, PKC downregulation could directly induce the neurodegeneration, with a secondary sprouting response.

beta-Amyloid precursor protein metabolites and loss of neuronal Ca2+ homeostasis in Alzheimer's disease

Recent findings link altered processing of beta-amyloid precursor protein (beta APP) to disruption of neuronal Ca2+ homeostasis and an excitotoxic mechanism of cell death in Alzheimer's disease. A major pathway of beta APP metabolism results in the release of secreted forms of beta APP, APPss. These secreted forms are released in response to electrical activity and can modulate neuronal responses to glutamate, suggesting roles in developmental and synaptic plasticity. beta APP is upregulated in response to neural injury and APPss can protect neurons against excitotoxic or ischemic insults by stabilizing the intracellular Ca2+ concentration [Ca2+]i. An alternative beta APP processing pathway liberates intact beta-amyloid peptide, which can form aggregates that disrupt Ca2+ homeostasis and render neurons vulnerable to metabolic or excitotoxic insults. Genetic abnormalities (e.g. certain beta APP mutations or Down syndrome) and age-related changes in brain metabolism (e.g. reduced energy availability or increased oxidative stress) may favor accumulation of [Ca2+]i-destabilizing beta-amyloid peptide and diminish the release of [Ca2+]i-stabilizing, neuroprotective APPss.

beta-Amyloid precursor protein mismetabolism and loss of calcium homeostasis in Alzheimer's disease

The suspected involvement of the beta-amyloid precursor protein (beta APP) in the etiology of Alzheimer's disease (AD) has been strengthened by recent genetic evidence, but pursuit of the mechanisms involved will initially require basic cell biology approaches. Several studies have concentrated on toxic activities of beta-amyloid peptide (beta AP) itself, illuminating its contributions to excitotoxicity and calcium-mediated degeneration in general. We now know that generation of beta AP from beta APP also compromises the production of an important set of trophic factors: the secreted forms of beta APP (APPS), which may act--ironically--by conferring protection from calcium-mediated insults. Therefore, conditions which contribute to the formation of beta AP (possibly including ischemia) not only produce an agent which exacerbates calcium-mediated cell death, but also reduce the levels of one of the few factors able to rescue calcium homeostasis. The implications of these postulates and their relationship to the process of aging are discussed.

Biologically active domain of the secreted form of the amyloid beta/A4 protein precursor

The amyloid beta/A4 protein precursor (APP), a large transmembrane protein, is expressed ubiquitously in many organisms, as well as in a variety of cultured cells. Studies of the synthesis and processing of APP have revealed several intricate metabolic pathways for this protein. One of these pathways involves the cleavage of APP in the middle of the beta/A4 domain and results in the secretion of the large amino-terminal portion of the protein. The biological function of this secreted form of APP has been the subject of intense investigation by several groups and various activities have been described for the different domains of APP studied. Our initial approach was to create a fibroblast cell line in which APP expression is dramatically reduced. These fibroblasts, called A-1, have a very slow growth rate. Addition of exogenous APP in the medium of A-1 cells restores their growth to the level of normal parent fibroblasts, demonstrating a growth factor-like activity for the secreted form of APP. Using APP fragments made in bacteria as well as synthetic peptides, we have been able to locate the active site of APP within a domain of 17 amino-acids (Ala319-Met335). This domain of APP can stimulate neurite extension of cultured neuroblastoma cells and it is proposed that APP mediates this effect through binding to a cell surface receptor, triggering intracellular transduction mechanisms. Thus, the secreted form of APP can function as a growth and/or differentiation factor and the site involved in these activities is within a 17-mer domain in the middle of the molecule. Our current lines of research seek to further characterize the mechanisms of APP function as well as its activity in vivo.

Beta APP gene expression is increased in the rat brain after motor neuron axotomy

The response of the beta APP gene to neuronal injury was studied in the facial and hypoglossal nerve nuclei of the rat after corresponding nerve axotomy. Increased levels of beta APP 695, 714, 751 and 770 mRNAs were observed after either facial or hypoglossal nerve axotomy in the parent ipsilateral motor neurons. The increase was gradual, with maximal values 7 days after axotomy. beta APP mRNA expression returned to normal values 60 days after the lesion. Increased beta APP immunostaining was also detected in ipsilateral chromatolytic motor neurons. No change in beta APP immunoreactivity was observed in oligodendrocytes, another cell type expressing beta APP under normal conditions. A rapid increase in the expression of the GFAP gene was observed in reactive astrocytes surrounding chromatolytic neurons in the ipsilateral facial or hypoglossal nuclei. Thus, in contrast with other models of neuronal injury, where only the Kunitz protease inhibitor-containing beta APP mRNA isoforms are increased, all beta APP mRNAs are increased in the axotomy model. Furthermore, although beta APP expression has been shown to be increased in reactive astrocytes following neuronal injury, in the present study the increase was essentially found in the motor neurons reacting to axotomy.

The cellular pathology associated with Alzheimer beta-amyloid deposits in non-demented aged individuals

In this study, we have compared the cellular pathology associated with beta-amyloid (beta A) deposits which characterize Alzheimer's disease (AD) in demented patients with pathologically confirmed AD, with that in non-demented aged individuals. Brain sections from two severely demented AD cases, six non-demented individuals with beta A deposits, and six age-matched controls devoid of beta A deposits were double-immunostained with antibodies against beta A, and antibody markers for neurofibrillary tangles (NFT), astrocytes and microglial cells. We found that the severely demented patients displayed numerous plaques of variable morphology, most of which were associated with NFT, hypertrophied astrocytes and reactive microglial cells. In contrast, non-demented patients showed fewer plaques, few or not NFT and less astroglial and microglial reaction. The number of plaques with associated abnormal cellular elements were much lower in non-demented than in demented cases. Furthermore, classical plaques were more likely to be associated with abnormal cellular elements than diffuse plaques, which were most often devoid of any associated cellular change. These findings suggest that: (i) beta A plaques in non-demented individuals may represent an early stage of AD; (ii) beta A deposition is the first recognizable pathological abnormality of AD; and (iii) NFT, and astro- and microglial proliferation are later features, possibly secondary to the known dystrophic effects of the beta A peptide and other fragments of its precursor protein.

Amino acid sequence RERMS represents the active domain of amyloid beta/A4 protein precursor that promotes fibroblast growth

The growth of A-1 fibroblasts depends on exogenous amyloid beta/A4 protein precursor (APP), providing a simple bioassay to study the function of APP. Our preliminary study, testing the activity of a series of fragments derived from the secreted form of APP-695 (sAPP-695) on this bioassay, has shown that at least one of the active sites of sAPP-695 was localized within a 40-mer sequence (APP296-335, Kang sequence; Roch, J.-M., I. P. Shapiro, M. P. Sundsmo, D. A. C. Otero, L. M. Refolo, N. K. Robakis, and T. Saitoh. 1992. J. Biol. Chem. 267:2214-2221). In the present study, to further characterize the growth-promoting activity of sAPP-695 on fibroblasts, we applied a battery of synthetic peptides on this bioassay and found that: (a) the sequence of five amino acids, RERMS (APP328-332), was uniquely required for the growth-promoting activity of sAPP-695; (b) the activity was sequence-specific because the reverse-sequence peptide of the active domain had no activity; and (c) the four-amino-acid peptide RMSQ (APP330-333), which partially overlaps the COOH-terminal side of the active sequence RERMS, could antagonize the activity of sAPP-695. Furthermore, a recombinant protein which lacks this active domain (APP20-591 without 306-335) did not promote fibroblast cell growth, suggesting that this domain is the only site of sAPP-695 involved in the growth stimulation. The availability of these biologically active, short peptides and their antagonists should prove to be an essential step for the elucidation of APP involvement in regulation of cellular homeostasis.

An analysis of the effects of Alzheimer's plaques on living neurons

Although senile plaques represent a consistent neuropathological feature in Alzheimer's brains, it is not known what role plaques play in the etiology of the disease. Both growth-promoting and growth-inhibiting influences have been postulated. One of the major components in plaques, beta-amyloid, has been shown to affect neuron survival and neurite outgrowth in vitro. Because plaques consist of other components in addition to beta-amyloid, we undertook the present study to determine whether neuronal survival and neurite outgrowth are affected by the presence of a senile plaque. This was accomplished by using cryostat sections from the cerebral cortex of Alzheimer's patients as a substratum for cultured rat hippocampal neurons. Evaluation of these living neurons on Alzheimer's tissue demonstrated that senile plaques affect the amount, complexity, and direction of neurite outgrowth. In addition, neurons were more likely to extend processes away from plaques rather than toward a plaque. Although cell survival on plaques and in control regions was similar, cell survival was significantly reduced in the peri-plaque region. These observations suggest that senile plaques could have deleterious effects on neural organization in situ.

An antibody against the Alzheimer's disease amyloid precursor protein recognizes distinct conformational isoforms

The Alzheimer's disease amyloid precursor protein (APP) consists of several isoforms, which are extensively post-translationally modified and processed. A monoclonal antibody, MAbE1, was raised against a synthetic peptide from an extracellular domain that is common to all isoforms of APP. Immunoblots and immunolocalization studies on cells of neuronal and other origins demonstrated that this antibody recognized a subclass of APP isoforms when compared to a monoclonal antibody raised against a bacterial fusion protein of APP, MAb22C11. Prominent protein bands of 71 kDa and 120 kDa were only detected on immunoblots of cell lysates and no immunoreactivity was observed in protein samples obtained from cell conditioned media. Immunofluorescence labelling with MAbE1 revealed predominantly perinuclear staining of cells of neuronal and glial origin. The data suggest that this monoclonal antibody detects distinct conformational isoforms of APP present in intracellular compartments.

Differential regional and cellular distribution of beta-amyloid precursor protein messenger RNAs containing and lacking the Kunitz protease inhibitor domain in the brain of human, rat and mouse

The beta-amyloid precursor protein is the precursor of the main component of senile plaques (the beta-amyloid peptide or beta/A4) found in the brain of aged humans and, in higher amounts, in the brain of Alzheimer's disease and Down's syndrome subjects. Four different forms of beta-amyloid precursor protein messenger RNAs have been described in humans and rodents: beta-amyloid precursor protein 695, beta-amyloid precursor protein 714, beta-amyloid precursor protein 751 and beta-amyloid precursor protein 770 messenger RNAs (numbers corresponding to the number of encoded amino acids). The two latter forms are characterized by containing in their sequence a region with high homology to the Kunitz family of serine protease inhibitors. We have used oligonucleotide probes to study the distribution of the different messenger RNAs encoding each of the four beta-amyloid precursor proteins by in situ hybridization histochemistry in human, rat and mouse brain. We found that beta-amyloid precursor protein 695, beta-amyloid precursor protein 714 and beta-amyloid precursor protein 751 messenger RNAs were widely distributed in the human, rat and mouse brain and that their distribution was roughly similar in most brain areas in these three species. The distribution of beta-amyloid precursor protein 770 messenger RNA was not so wide and differed among the three species studied. beta-amyloid precursor protein 751 and 770 messenger RNAs were the only forms present at significant levels in rodent choroid plexus and meninges, while beta-amyloid precursor protein messenger RNA isoforms containing and lacking the Kunitz domain were detected in the human choroid plexus. We also observed that the relative levels of beta-amyloid precursor protein 751 and 770 messenger RNAs in the rat cerebral white matter as well as in the mouse and human striatum were higher than those of the beta-amyloid precursor protein messenger RNAs lacking the Kunitz domain. While the most abundant beta-amyloid precursor protein messenger RNAs in the brain of all three species under study were, in descending order, beta-amyloid precursor protein 695 and beta-amyloid precursor protein 751 messenger RNAs, the least abundant form was not the same for all species: in human it was beta-amyloid precursor protein 714 messenger RNA and in rat and mouse brain it was beta-amyloid precursor protein 770 messenger RNA. Our results show differences both inter- and intraspecies of the relative abundance and distribution of four beta-amyloid precursor protein messenger RNAs in rat, mouse and human brain.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Increased levels of the Kunitz protease inhibitor-containing beta APP mRNAs in rat brain following neurotoxic damage

Deposits of beta-amyloid are one of the main pathological characteristics of Alzheimer's disease. The beta-amyloid peptide (or beta/A4) constituent of these deposits is derived from the beta-amyloid precursor protein (beta APP), which is expressed in several isoforms. It has been suggested that an imbalance in the normal ratio between the Kunitz protease inhibitor (KPI)-containing beta APPs versus the non containing forms could result in altered processing of beta APP and progressive beta/A4 deposition. We have studied the expression of four beta APP isoforms in the rat brain after intracerebroventricular application of kainic acid. Increased levels of the KPI-containing beta APP and GFAP mRNAs were observed in tissues surrounding areas of neuronal damage. A parallel increase of beta APP and GFAP immunoreactivity was observed in reactive astrocytes in these areas. These results suggest that the normal ratio of beta APP isoforms may be profoundly altered as a result of neuronal damage and that non-neuronal cells may respond to neuronal injury by increased expression of the KPI-containing beta APP isoforms.

A 109-amino-acid C-terminal fragment of Alzheimer's-disease amyloid precursor protein contains a sequence, -RHDS-, that promotes cell adhesion

Amyloid beta (A beta), the major constituent of the fibrils composing senile plaques and vascular amyloid deposits in Alzheimer's disease (AD) and related disorders, is a 39-42-residue self-aggregating degradation peptide of a larger multidomain membrane glycoprotein designated amyloid precursor protein (APP). An array of biological functions has been assigned to different APP domains, including growth regulation, neurotoxicity, inhibitory activity of serine proteinases and promotion of cell-cell and cell-matrix interactions. A beta is generated through an as-yet-unknown catabolic pathway that by-passes or inhibits the cleavage of APP within the A beta sequence. We have identified a 16 kDa intermediate APP C-terminal fragment containing A beta in leptomeningeal vessels of aged normal individuals and AD patients by means of its immunoreactivity with a panel of four different anti-(APP C-terminal) antibodies, indicating a different pathway of APP processing. Previous studies have indicated that the APP C-terminal domain is the most likely to be involved in cell-matrix interactions. A 109-amino-acid construct C109 with a sequence analogous to the C-terminal of APP (positions 587-695 of APP695), similar in length and immunoreactivity to the 16 kDa fragment, was found to promote cell adhesion. By use of synthetic peptides, this activity was initially located to the extracellular 28 residues of A beta. Inhibition studies demonstrated that the sequence RHDS (amino acids 5-8 of A beta, corresponding to residues 601-604 of APP695 was responsible for the adhesion-promoting activity. The interaction is dependent on bivalent cations and can be blocked either by the tetrapeptides RHDS and RGDS or by an anti-(beta 1 integrin) antibody. Thus, through integrin-like surface receptors, APP or its derivative proteolytic fragments containing the sequence RHDS may modulate cell-cell or cell-matrix interactions.

Amyloid precursor protein mRNA encoding the Kunitz protease inhibitor domain is increased by kainic acid-induced seizures in rat hippocampus

A 168-nucleotide exon, found in alternatively spliced amyloid precursor protein (APP) mRNAs, encodes a Kunitz protease inhibitor (KPI) domain. Kainic acid (ip) caused a selective increase of KPI mRNA in rat hippocampus. By in situ hybridization, KPI mRNA was induced in the neuronal layers of the hippocampus 11-12 h after the onset of kainate-induced seizures. The kainate-induced elevation of the KPI-containing APP-770 mRNA was blocked by pretreatment with the anticonvulsant pentobarbital. These data suggest that kainate-induced seizures cause alterations in APP RNA stability and/or processing in rat hippocampal neurons.

Identification of a mouse brain cDNA that encodes a protein related to the Alzheimer disease-associated amyloid beta protein precursor

We have isolated a cDNA from a mouse brain library that encodes a protein whose predicted amino acid sequence is 42% identical and 64% similar to that of the amyloid beta protein precursor (APP). This 653-amino acid protein, which we have termed the amyloid precursor-like protein (APLP), appears to be similar to APP in overall structure as well as amino acid sequence. The amino acid homologies are concentrated within three distinct regions of the two proteins where the identities are 47%, 54%, and 56%. The APLP cDNA hybridizes to two messages of approximately 2.4 and 1.6 kilobases that are present in mouse brain and neuroblastoma cells. Polyclonal antibodies raised against a peptide derived from the C terminus of APLP stain the cytoplasm in a pattern reminiscent of Golgi staining. In addition to APP, APLP also displays significant homology to the Drosophila APP-like protein APPL and a rat testes APP-like protein. These data indicate that the APP gene is a member of a strongly conserved gene family. Studies aimed at determining the functions of the proteins encoded by this gene family should provide valuable clues to their potential role in Alzheimer disease neuropathology.

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.

Increased expression of beta-amyloid precursor protein during neuronal differentiation is not accompanied by secretory cleavage

Despite increasing evidence for a pathogenetic role for the beta-amyloid precursor protein (beta APP) in Alzheimer disease, the physiological function of the protein remains unclear. The expression of the neural-specific isoform containing 695 amino acids, beta APP695, is consistent with a role for the protein in neuronal development. In this study, we analyzed the expression of beta APP during the retinoic acid-induced neuronal differentiation of P19 murine embryonal carcinoma cells. Northern blot and RNase protection analyses show a selective increase in beta APP695 expression, concomitant with the morphologic differentiation of P19-derived neurons. Moreover, the time course of increase observed for the beta APP695 mRNA is paralleled by other neuronal-specific transcripts. A similar increase in beta APP695 is observed at the protein level. Furthermore, we show that levels of beta APP695 protein progressively increase during the in vitro differentiation of primary hippocampal neurons. The finding that beta APP695 increases selectively and progressively during neuronal differentiation in two different cell culture systems suggests that this isoform has an important cellular function during this process in the brain. Unlike beta APP in most peripheral cell types, the increased levels of beta APP found in terminally differentiated neuronal cells are not processed in significant amounts by secretory cleavage. Thus, differentiation of neurons is accompanied by increased beta APP695 expression and membrane retention of the protein as intact, full-length molecules that could serve as potential substrates for amyloidogenesis.

Amyloid precursor protein accumulates in regions of neurodegeneration following focal cerebral ischemia in the rat

The distribution of beta-amyloid precursor protein (APP) was examined immunocytochemically in rats subjected to focal cerebral ischemia by permanent occlusion of the middle cerebral artery. At 4 and 7 days post-occlusion, APP immunoreactivity was preferentially localized within axonal swellings, dystrophic neurites and neuronal perikarya all along the periphery of the infarct. Immunolabeling was observed with antibodies generated against N-terminal, midregion, and C-terminal domains of APP. No immunoreactivity was observed with antisera directed against beta-amyloid protein (beta A4) itself. This pathological accumulation of APP is consistent with alterations of APP recently described in other models of neurodegeneration and implies a role for this protein in the response to CNS injury.

Human amyloid precursor protein ameliorates behavioral deficit of flies deleted for Appl gene

Drosophila amyloid precursor protein-like (Appl) gene encodes a protein product (APPL) similar to beta-amyloid precursor protein (APP) associated with Alzheimer's disease. To understand the in vivo function of APPL protein, we have generated flies deleted for the Appl gene. These flies are viable, fertile, and morphologically normal, yet they exhibit subtle behavioral deficits. We show that a fast phototaxis defect in Appl- flies is partially rescued by transgenes expressing the wild-type, but not a mutant, APPL protein. We further demonstrate a functional homology between APPL and APP, since transgenes expressing human APP show a similar level of rescue as transgenes expressing fly APPL.

Beta-amyloid precursor protein isoforms in various rat brain regions and during brain development

To address the question of the possible functions of different Alzheimer's disease beta-amyloid precursor protein (beta-APP) isoforms in the brain, we studied their expression at different times during postnatal rat brain development and in various regions of the adult rat brain. Polyclonal antibodies directed to two peptide antigens were used. The majority of all beta-APP forms was found to be soluble as revealed by western blot analysis. The highest level of most beta-APP forms was reached in the second postnatal week, which is the time of brain maturation and completion of synaptic connections. Strikingly high concentrations of the Kunitz protease inhibitor-containing beta-APP were present in the adult olfactory bulb, where continuous synaptogenesis occurs in the adult animal. These findings support the idea of an involvement of beta-APPs in the processes of cell differentiation and, probably, in the establishment of synaptic contacts.

Changes in acetylcholinesterase and butyrylcholinesterase in Alzheimer's disease resemble embryonic development--a study of molecular forms

The pattern of molecular forms of acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BChE, EC 3.1.1.8) separated by density gradient centrifugation was investigated in the brain and cerebrospinal fluid in Alzheimer's disease (AD), in human embryonic brain and in rat brain after experimental cholinergic deafferentation of the cerebral cortex. While a selective loss of the AChE G4 form was a rather constant finding in AD, a small but significant increase of G1 for both AChE and BChE was found in the most severely affected cases. Both in normal human brain and in AD a significant relationship could be established between the AChE G4/G1 ratio in different brain regions and the activity of choline acetyltransferase (ChAT). A similar decrease of the AChE G4 form as observed in AD can be induced in rat by experimental cholinergic deafferentation of the cerebral cortex. The increase in G1 of both AChE and BChE in different brain regions in AD is quantitatively related to the local density of neuritic plaques which are histochemically reactive for both enzymes. In human embryonic brain, a high abundance of G1 and a low G4/G1 ratio for both AChE and BChE was found resembling the pattern observed in AD. Furthermore, both in embryonic brain and in AD AChE shows no substrate inhibition which is a constant feature of the enzyme in the adult human brain. It is, therefore, concluded that the degeneration of the cholinergic cortical afferentation in AD as reflected by a decrease of AChE G4 is accompanied by the process of a neuritic sprouting response involved in plaque formation which is probably associated with the expression of a developmental form of the enzyme.