Protease nexin-II, a potent antichymotrypsin, shows identity to amyloid beta-protein precursor

Pathogenic factors in vascular dementia and Alzheimer's disease. Multiple actions of heparin that probably are beneficial

The following areas are discussed in this review: atherogenesis; cerebrovascular factors; hypoperfusion; beta-amyloid production; beta-amyloid fibril formation; beta-sheets; metal cations; reactive oxygen species/free radicals; chronic inflammatory factors; endogenous plasma heparin; lipoprotein lipase; polyamines; protein kinase C; casein kinases; phospholipase A2; serine proteases; myeloperoxidase; cyclooxygenase 2; cysteine proteases; caspases; proprotein convertases; aspartic proteases; cyclin proteinases; thrombin; tau hyperphosphorylation; advanced glycosylation end products; activator protein 1; calcium; apolipoprotein E epsilon4; histamine; blood-brain barrier; glutamate; transglutaminase; insulin-like growth factor 1.

Early-onset and robust cerebral microvascular accumulation of amyloid beta-protein in transgenic mice expressing low levels of a vasculotropic Dutch/Iowa mutant form of amyloid beta-protein precursor

Cerebrovascular deposition of amyloid beta-protein (Abeta) is a common pathological feature of Alzheimer's disease and related disorders. In particular, the Dutch E22Q and Iowa D23N mutations in Abeta cause familial cerebrovascular amyloidosis with abundant diffuse amyloid plaque deposits. Both of these charge-altering mutations enhance the fibrillogenic and pathogenic properties of Abeta in vitro. Here, we describe the generation of several transgenic mouse lines (Tg-SwDI) expressing human neuronal Abeta precursor protein (AbetaPP) harboring the Swedish K670N/M671L and vasculotropic Dutch/Iowa E693Q/D694N mutations under the control of the mouse Thy1.2 promoter. Tg-SwDI mice expressed transgenic human AbetaPP only in the brain, but at levels below those of endogenous mouse AbetaPP. Despite the paucity of human AbetaPP expression, quantitative enzyme-linked immunosorbent assay measurements revealed that Tg-SwDI mice developed early-onset and robust accumulation of Abeta in the brain with high association with isolated cerebral microvessels. Tg-SwDI mice exhibited striking perivascular/vascular Abeta deposits that markedly increased with age. The vascular Abeta accumulations were fibrillar, exhibiting strong thioflavin S staining, and occasionally presented signs of microhemorrhage. In addition, numerous largely diffuse, plaque-like structures were observed starting at 3 months of age. In vivo transport studies demonstrated that Dutch/Iowa mutant Abeta was more readily retained in the brain compared with wild-type Abeta. These results with Tg-SwDI mice demonstrate that overexpression of human AbetaPP is not required for early-onset and robust accumulation of both vascular and parenchymal Abeta in mouse brain.

Cerebrovascular requirement for sealant, anti-coagulant and remodeling molecules that allow for the maintenance of vascular integrity and blood supply

The integrity of the vasculature and the maintenance of the blood supply to the brain are crucial for the survival of higher vertebrates. However, peripheral mechanisms of sealing the vasculature that rely on the clotting of blood and platelet aggregation around the site of a 'leak' would lead to decreased cerebral perfusion and compromise the viability of terminally differentiated and irreplaceable neurons. Therefore, in higher organisms it is likely that a sealant/anti-coagulant system that maintains vascular supply has evolved as a necessity to life. We propose that one such system involves the amyloid-beta precursor protein (AbetaPP) and its cleavage product Abeta since (1) both AbetaPP/Abeta are known to deposit in the media of the cerebrovasculature wall following localized injury, (2) Abeta is generated from AbetaPP, a known acute phase reactant, (3) Abeta's physiochemical properties allow it to span between the extracellular matrix and the (endothelial) cell membrane and under inflammatory conditions aggregate to form an intracranial 'scab', thereby maintaining structural integrity of the blood brain barrier, (4) AbetaPP/Abeta together act as an anti-coagulant, (5) Abeta promotes vascular/neuronal remodeling, and (6) Abeta deposits resolve after injury. These properties are consistent with the acute phase generation and rapid cortical deposition of AbetaPP/Abeta following injury (either sustained by trauma or stresses associated with aging) that would be an important compensatory response aimed at limiting the loss of terminally differentiated neurons. Such a system would allow the maintenance of blood supply to the brain by sealing vascular lesions, preventing hemorrhagic stroke while at the same time inhibiting the coagulation cascade from blocking capillaries. Obviously, strategies to remove Abeta would have serious consequences for the integrity of the blood-brain barrier. Indeed, recent in vivo evidence demonstrates that the removal of deposited Abeta from the vasculature leads to increased cerebral microhemorrhage and strongly support the above mentioned functions of AbetaPP/Abeta. These insights also explain the root cause of the encephalitis and meningitis suffered by individuals in immunotherapy trials as being directly associated with the removal of Abeta from the vasculature, i.e. immunological responses to Abeta vaccination do not discriminate between physiologically purposive deposits of Abeta (vascular deposits) and pathological deposits of Abeta (senile plaques).

The tissue plasminogen activator-plasminogen proteolytic cascade accelerates amyloid-beta (Abeta) degradation and inhibits Abeta-induced neurodegeneration

Accumulation of the amyloid-beta (Abeta) peptide depends on both its generation and clearance. To better define clearance pathways, we have evaluated the role of the tissue plasminogen activator (tPA)-plasmin system in Abeta degradation in vivo. In two different mouse models of Alzheimer's disease, chronically elevated Abeta peptide in the brain correlates with the upregulation of plasminogen activator inhibitor-1 (PAI-1) and inhibition of the tPA-plasmin system. In addition, Abeta injected into the hippocampus of mice lacking either tPA or plasminogen persists, inducing PAI-1 expression and causing activation of microglial cells and neuronal damage. Conversely, Abeta injected into wild-type mice is rapidly cleared and does not cause neuronal degeneration. Thus, the tPA-plasmin proteolytic cascade aids in the clearance of Abeta, and reduced activity of this system may contribute to the progression of Alzheimer's disease.

Regulated hyperaccumulation of presenilin-1 and the "gamma-secretase" complex. Evidence for differential intramembranous processing of transmembrane subatrates

Intramembranous "gamma-secretase" processing of beta-amyloid precursor protein (APP) and other transmembrane proteins, including Notch, is mediated by a macromolecular complex consisting of presenilins (PSs), nicastrin (NCT), APH-1, and PEN-2. We now demonstrate that in cells coexpressing PS1, APH-1, and NCT, full-length PS1 accumulates to high levels and is fairly stable. Upon expression of PEN-2, the levels of PS1 holoprotein are significantly reduced, commensurate with an elevation in levels of PS1 fragments. These findings suggest that APH-1 and NCT are necessary for stabilization of full-length PS1 and that PEN-2 is critical for the proteolysis of stabilized PS1. In N2a and 293 cell lines that stably overexpress PS1, APH-1, NCT, and PEN-2, PS1 fragment levels are elevated by up to 10-fold over endogenous levels. In these cells, we find a marked accumulation of the APP-CTF gamma (AICD) fragment and a concomitant reduction in levels of both APP-CTF beta and CTF alpha. Moreover, the production of the gamma-secretase-generated Notch S3/NICD derivative is modestly elevated. However, we failed to observe a corresponding increase in levels of secreted A beta peptides in the medium of these cells. These results lead us to conclude that, although the PS1, APH-1, NCT, and PEN-2 are essential for gamma-secretase activity, the proteolysis of APP-CTF and Notch S2/NEXT are differentially regulated and require the activity of additional cofactors that promote production of AICD, NICD, and A beta.

Rab5-stimulated up-regulation of the endocytic pathway increases intracellular beta-cleaved amyloid precursor protein carboxyl-terminal fragment levels and Abeta production

We previously identified abnormalities of the endocytic pathway in neurons as the earliest known pathology in sporadic Alzheimer's disease (AD) and Down's syndrome brain. In this study, we modeled aspects of these AD-related endocytic changes in murine L cells by overexpressing Rab5, a positive regulator of endocytosis. Rab5-transfected cells exhibited abnormally large endosomes immunoreactive for Rab5 and early endosomal antigen 1, resembling the endosome morphology seen in affected neurons from AD brain. The levels of both Abeta40 and Abeta42 in conditioned medium were increased more than 2.5-fold following Rab5 overexpression. In Rab5 overexpressing cells, the levels of beta-cleaved amyloid precursor protein (APP) carboxyl-terminal fragments (betaCTF), the rate-limiting proteolytic intermediate in Abeta generation, were increased up to 2-fold relative to APP holoprotein levels. An increase in beta-cleaved soluble APP relative to alpha-cleaved soluble APP was also observed following Rab5 overexpression. BetaCTFs were co-localized by immunolabeling to vesicular compartments, including the early endosome and the trans-Golgi network. These results demonstrate a relationship between endosomal pathway activity, betaCTF generation, and Abeta production. Our findings in this model system suggest that the endosomal pathology seen at the earliest stage of sporadic AD may contribute to APP proteolysis along a beta-amyloidogenic pathway.

Upregulation and antiapoptotic role of endogenous Alzheimer amyloid precursor protein in dorsal root ganglion neurons

The amyloid precursor protein (APP) is a transmembrane protein whose abnormal processing is associated with the pathogenesis of Alzheimer's disease. In this study, we examined the expression and role of cell-associated APP in primary dorsal root ganglion (DRG) neurons. When dissociated DRG cells prepared from mouse embryos were treated with nerve growth factor (NGF), neuronal APP levels were transiently elevated. DRG neurons treated with an antibody against cell surface APP failed to mature and underwent apoptosis. When NGF was withdrawn from the cultures after a 36-h NGF treatment, virtually all neurons underwent apoptosis by 48 h. During the course of apoptosis, some neurons with intact morphology contained increased levels of APP immunoreactivity, whereas the APP levels were greatly reduced in apoptotic neurons. Furthermore, affected neurons contained immunoreactivities for activated caspase-3, a caspase-cleaved APP fragment (APPDeltaC31), and Abeta. Downregulation of endogenous APP expression by treatment with an APP antisense oligodeoxynucleotide significantly increased the number of apoptotic neurons in NGF-deprived DRG cultures. Furthermore, overexpression of APP by adenovirus vector-mediated gene transfer reduced the number of apoptotic neurons deprived of NGF. These results suggest that endogenous APP is upregulated to exert an antiapoptotic effect on neurotrophin-deprived DRG neurons and subsequently undergoes caspase-dependent proteolysis.

Activation of calpain in cultured neurons overexpressing Alzheimer amyloid precursor protein

We have previously reported that overexpression of wild-type amyloid precursor protein (APP) in postmitotic neurons induces cleavage-dependent activation of caspase-3 both in vivo and in vitro. In this study, we investigated the mechanism underlying APP-induced caspase-3 activation using adenovirus-mediated gene transfer into postmitotic neurons derived from human embryonal carcinoma NT2 cells. Overexpression of wild-type APP significantly increased intracellular (45)Ca(2+) content prior to the activation of caspase-3 in NT2-derived neurons. Chelation of intracellular Ca(2+) markedly suppressed APP-induced activation of caspase-3. Furthermore, calpain, a Ca(2+)-dependent cysteine protease, was activated in neurons overexpressing APP as assessed by increased levels of calpain-cleaved alpha-fodrin and autolytic mu-calpain fragments. Neither calpain nor caspase-3 was activated in neurons expressing an APP mutant defective in the Abeta(1-20) domain. Calpain inhibitors almost completely suppressed APP-induced activation of neuronal caspase-3. E64d, a membrane permeable inhibitor of calpain, significantly suppressed APP-induced neuronal death. These results suggest that overexpression of wild-type APP activates calpain that mediates caspase-3 activation in postmitotic neurons.

Age-dependent differential regulation of genes encoding APP and alpha-synuclein in hippocampal synaptic plasticity

We investigated the modulation of the messenger RNA encoding the amyloid precursor protein (APP) and alpha-synuclein following induction of long-term potentiation (LTP) in the dentate gyrus of young and aged rats. Three hours after tetanic stimulation, LTP induced in the young rats was maintained; the aged rats, however, fell into two subgroups: those in which LTP was maintained, and those in which LTP had declined to basal levels. In young rats, the global expression of mRNAs of all isoforms of APP and in particular that of the isoform lacking the KPI domain were significantly upregulated. In aged rats, the global expression of mRNAs of all isoforms of APP was not modified, regardless of whether LTP was maintained or not. The level of mRNA encoding the Kunitz protease-inhibitory (KPI)-minus isoform of APP, however, was increased in aged rats in which LTP was maintained, suggesting that the gene of this isoform may be more specifically regulated by synaptic plasticity. In contrast, we found that the gene encoding alpha-synuclein showed a trend towards being downregulated at the mRNA level in young rats following LTP, and significantly so in aged rats in which LTP was maintained, whereas it was not downregulated in aged rats with decremental LTP. These data suggest that the regulated expression of APP isoforms is part of the tanscriptional response associated with the enduring forms of synaptic plasticity and is altered with age. Whereas the level of alpha-synuclein mRNA is not apparently modified in normal LTP, it may reflect a mechanism of apoptotic cell death in aging that is in part responsible for decremental synaptic plasticity.

Calpain activity regulates the cell surface distribution of amyloid precursor protein. Inhibition of calpains enhances endosomal generation of beta-cleaved C-terminal APP fragments

In murine L cells, treatment with calpeptin or calpain inhibitor III increased Abeta42, but not Abeta40, secretion in a dose-dependent fashion. This correlated with an increase in the levels of amyloid precursor protein (APP) carboxyl-terminal fragments (CTFs). Immunoprecipitation with novel mAbs directed against the carboxyl-terminus of APP or specific for the beta-cleaved CTF showed that generation of both alpha- and beta-cleaved CTFs increase proportionately following inhibition of calpains. Pulse-chase metabolic labeling confirmed that inhibiting calpains increases the production of alpha- and beta-cleaved APP metabolites. Immunolabeling showed greater betaCTF signal in calpeptin-treated cells, primarily in small vesicular compartments that were shown to be predominantly endosomal by colocalization with early endosomal antigen 1. A second mAb, which recognizes an extracellular/luminal epitope found on both APP and betaCTFs, gave more cell surface labeling of calpeptin-treated cells than control cells. Quantitative binding of this antibody confirmed that inhibiting calpains caused a partial redistribution of APP to the cell surface. These results demonstrate that 1) calpain inhibition results in a partial redistribution of APP to the cell surface, 2) this redistribution leads to an increase in both alpha- and beta-cleavage without changing the ratio of alphaCTFs/betaCTFs, and 3) the bulk of the betaCTFs in the cell are within early endosomes, confirming the importance of this compartment in APP processing.

Localization of a fibrillar amyloid beta-protein binding domain on its precursor

Deposition of fibrillar amyloid-beta protein (Abeta) in senile plaques and in the walls of cerebral blood vessels is a key pathological feature of Alzheimer's disease and certain related disorders. Fibrillar Abeta deposition is intimately associated with neuronal and cerebrovascular cell death both in vivo and in vitro. Similarly, accumulation of the Abeta protein precursor (AbetaPP) is also observed at sites of fibrillar Abeta deposition. Recently, we reported that fibrillar Abeta, but not unassembled Abeta, promotes the specific binding of AbetaPP through its cysteine-rich, amino-terminal region (Melchor, J. P., and Van Nostrand, W. E. (2000) J. Biol. Chem. 275, 9782-9791). In the present study we sought to determine the precise site on AbetaPP that facilitates its binding to fibrillar Abeta. A series of synthesized overlapping peptides spanning the cysteine-rich, amino-terminal region of AbetaPP were used as competitors for AbetaPP binding to fibrillar Abeta. A peptide spanning residues 105-119 of AbetaPP competitively inhibited AbetaPP binding to fibrillar Abeta in a solid-phase binding assay and on the surface of cultured human cerebrovascular smooth muscle cells. Alanine-scanning mutagenesis of residues 105-117 within glutathione S-transferase (GST)-AbetaPP-(18-119) revealed that His(110), Val(112), and Ile(113) are key residues that facilitate AbetaPP binding to fibrillar Abeta. These specific residues belong to a common beta-strand within this region of AbetaPP. Wild-type GST-AbetaPP-(18-119) protected cultured human cerebrovascular smooth muscle cells from Abeta-induced toxicity whereas H110A mutant GST-AbetaPP-(18-119) did not. Wild-type GST-AbetaPP-(18-119) bound to different isoforms of fibrillar Abeta and fibrillar amylin peptides whereas H110A mutant and I113A mutant GST-AbetaPP-(18-119) were substantially less efficient binding to each fibrillar peptide. We conclude that His(110), Val(112), and Ile(113), residing in a common beta-strand region within AbetaPP-(18-119), comprise a domain that mediates the binding of AbetaPP to fibrillar peptides.

Accumulation of the amyloid-beta precursor protein in multivesicular body-like organelles

It has been suggested that the successive proteolytic events leading to the production of the amyloid-beta protein from its precursor may take place at different intracellular locations. Using cultured human leptomeningeal smooth muscle cells and brain pericytes, we modulated the intracellular localization of the amyloid-beta precursor protein (APP) to study possible effects on its processing. By using immunofluorescence and immunoelectron microscopy we demonstrated that, under normal conditions, the APP is found in small intracellular vesicles, some of which were characterized as lysosomes. Both the cytokine interferon-gamma and the lysosomotropic drug chloroquine, but not the cytokines interleukin (IL)-1, IL-6, or tumor necrosis factor-alpha (TNF-alpha), induced an accumulation of APP in newly formed multivesicular body-like organelles. The secretion of the amyloid-beta precursor protein was slightly reduced by interferon-gamma or chloroquine. Double-labeling and tracer molecule uptake experiments showed that the multivesicular body-like organelles were part of the endocytic pathway. Our findings suggest that the multivesicular body-like organelles function as an intermediate organelle in the intracellular trafficking of the APP. Accumulation of the APP in this organelle is reflected by its reduced secretion from the cell.

Alzheimer's disease-related overexpression of the cation-dependent mannose 6-phosphate receptor increases Abeta secretion: role for altered lysosomal hydrolase distribution in beta-amyloidogenesis

Prominent endosomal and lysosomal changes are an invariant feature of neurons in sporadic Alzheimer's disease (AD). These changes include increased levels of lysosomal hydrolases in early endosomes and increased expression of the cation-dependent mannose 6-phosphate receptor (CD-MPR), which is partially localized to early endosomes. To determine whether AD-associated redistribution of lysosomal hydrolases resulting from changes in CD-MPR expression affects amyloid precursor protein (APP) processing, we stably transfected APP-overexpressing murine L cells with human CD-MPR. As controls for these cells, we also expressed CD-MPR trafficking mutants that either localize to the plasma membrane (CD-MPRpm) or to early endosomes (CD-MPRendo). Expression of CD-MPR resulted in a partial redistribution of a representative lysosomal hydrolase, cathepsin D, to early endosomal compartments. Turnover of APP and secretion of sAPPalpha and sAPPbeta were not altered by overexpression of any of the CD-MPR constructs. However, secretion of both human Abeta40 and Abeta42 into the growth media nearly tripled in CD-MPR- and CD-MPRendo-expressing cells when compared with parental or CD-MPRpm-expressing cells. Comparable increases were confirmed for endogenous mouse Abeta40 in L cells expressing these CD-MPR constructs but not overexpressing human APP. These data suggest that redistribution of lysosomal hydrolases to early endocytic compartments mediated by increased expression of the CD-MPR may represent a potentially pathogenic mechanism for accelerating Abeta generation in sporadic AD, where the mechanism of amyloidogenesis is unknown.

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.

Expression of hepatocyte growth factor activator and hepatocyte growth factor activator inhibitor type 1 in human hepatocellular carcinoma

Hepatocyte growth factor activator inhibitor type 1 (HAI-1), a Kunitz-type serine protease inhibitor for hepatocyte growth factor activator (HGFA), is responsible for proteolytic activation of hepatocyte growth factor. We examined the expression of HGFA and HAI-1 in liver tissues of chronic liver diseases including hepatocellular carcinoma (HCC). HGFA expression was detected not only in the liver tissues of chronic hepatitis and cirrhosis and in the nontumorous liver tissues surrounding HCC, but also in HCC tissues. On the other hand, none of the liver tissues of hepatitis and cirrhosis and none of the nontumorous tissues surrounding HCC were stained with anti-HAI-1. However, 35% of HCC tissues were stained with anti-HAI-1, and HAI-1 positivity increased as the histological grade decreased and as serum alpha-fetoprotein increased. Transduction of antisense HAI-1 inhibited the growth of human hepatoma cells. These results suggest the possibility that HAI-1 plays an important role in the progression of HCC.

Multiple effects of aspartate mutant presenilin 1 on the processing and trafficking of amyloid precursor protein

PS1 deficiency and expression of PS1 with substitutions of two conserved transmembrane aspartate residues ("PS1 aspartate variants") leads to the reduction of Abeta peptide secretion and the accumulation of amyloid precursor protein (APP) C-terminal fragments. To define the nature of the "dominant negative" effect of the PS1 aspartate variants, we stably expressed PS1 harboring aspartate to alanine substitutions at codons 257 (D257A) or 385 (D385A), singly or in combination (D257A/D385A), in mouse neuroblastoma, N2a cells. Expression of the PS1 aspartate variants resulted in marked accumulation of intracellular and cell surface APP C-terminal fragments. While expression of the D385A PS1 variant reduced the levels of secreted Abeta peptides, we now show that neither the PS1 D257A nor D257A/D385A variants impair Abeta production. Surprisingly, the stability of both immature and mature forms of APP is dramatically elevated in cells expressing PS1 aspartate variants, commensurate with an increase in the cell surface levels of APP. These findings lead us to conclude that the stability and trafficking of APP can be profoundly modulated by coexpression of PS1 with mutations at aspartate 257 and aspartate 385.

Pathogenic effects of D23N Iowa mutant amyloid beta -protein

Cerebral amyloid beta-protein angiopathy (CAA) is a key pathological feature of patients with Alzheimer's disease and certain related disorders. In these conditions the CAA is characterized by the deposition of Abeta within the cerebral vessel wall and, in severe cases, hemorrhagic stroke. Several mutations have been identified within the Abeta region of the Abeta protein precursor (AbetaPP) gene that appear to enhance the severity of CAA. We recently described a new mutation within the Abeta region (D23N) of AbetaPP that is associated with severe CAA in an Iowa kindred (Grabowski, T. J., Cho, H. S., Vonsattel, J. P. G., Rebeck, G. W., and Greenberg, S. M. (2001) Ann. Neurol. 49, 697-705). In the present study, we investigated the effect of this new D23N mutation on the processing of AbetaPP and the pathogenic properties of Abeta. Neither the D23N Iowa mutation nor the E22Q Dutch mutation affected the amyloidogenic processing of AbetaPP expressed in H4 cells. The A21G Flemish mutation, in contrast, resulted in a 2.3-fold increase in secreted Abeta peptide. We also tested synthetic wild-type and mutant Abeta40 peptides for fibrillogenesis and toxicity toward cultured human cerebrovascular smooth muscle (HCSM) cells. The E22Q Dutch, D23N Iowa, and E22Q,D23N Dutch/Iowa double mutant Abeta40 peptides rapidly assembled in solution to form fibrils, whereas wild-type and A21G Flemish Abeta40 peptides exhibited little fibril formation. Similarly, the E22Q Dutch and D23N Iowa Abeta40 peptides were found to induce robust pathologic responses in cultured HCSM cells, including elevated levels of cell-associated AbetaPP, proteolytic breakdown of smooth muscle cell alpha-actin, and cell death. Double mutant E22Q,D23N Dutch/Iowa Abeta40 was more potent than either single mutant form of Abeta in causing pathologic responses in HCSM cells. These data suggest that the different CAA mutations in AbetaPP may exert their pathogenic effects through different mechanisms. Whereas the A21G Flemish mutation appears to enhance Abeta production, the E22Q Dutch and D23N Iowa mutations enhance fibrillogenesis and the pathogenicity of Abeta toward HCSM cells.

Secreted forms of the amyloid-beta precursor protein are ligands for the class A scavenger receptor

Upon activation, platelets secrete a 120-kDa protein that competes for the binding and internalization of acetyl low density lipoproteins (AcLDL) by macrophages. From the amino-terminal amino acid sequence, amino acid composition, and immunoblot analysis, we identified the active factor in platelet secretion products as sAPP, an alpha-secretase cleavage product of the beta-amyloid precursor protein (APP), that contains a Kunitz-type protease inhibitor (KPI) domain. We showed that both sAPP751 (also called Nexin II) and sAPP695, which does not contain a KPI domain, are ligands for the class A scavenger receptor (SR-A). Chinese hamster ovary cells stably transfected to express the SR-A bound and internalized 4-fold more human platelet-derived sAPP than control cells. The binding and internalization of sAPP were inhibited by the SR-A antagonist fucoidin. In addition, sAPP competed as effectively as fucoidin for SR-A-mediated cell association and degradation of (125)I-AcLDL. To determine if the KPI domain is required for the binding of sAPP to the SR-A, APP751 and APP695 were expressed in Chinese hamster ovary cells, and sAPP751 and sAPP695 purified from the medium were tested for their binding to the SR-A. sAPP751 and sAPP695 were equally effective in competing for the cell association of (125)I-AcLDL by SR-A-expressing cells, demonstrating that the KPI domain is not essential for binding. We also found that sAPP751 is present in extracts of atherosclerotic lesions and that sAPP competes for the SR-A-mediated cell association of oxidized low density lipoprotein. Deletion mutagenesis indicated that a negatively charged region of APP (residues 191-264) contributes to binding to the SR-A. These results suggest that the SR-A contributes to the clearance of sAPP and that sAPP competes for the cell association of other SR-A ligands.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Characterization of a brain-related serine protease, neurosin (human kaillikrein 6), in human cerebrospinal fluid

Neurosin (also known as zyme or protease M) is a trypsin-like serine protease dominantly expressed in the human brain. According to the official nomenclature, this gene is now designated as human kallikrein 6 (KLK6) and the protein is designated hK6. To investigate the metabolism of neurosin in human brain, neurosin contained in the human cerebrospinal fluid (CSF) was analyzed. Neurosin was detected in the all CSFs tested by Western blot analysis using an anti-neurosin monoclonal antibody. We purified neurosin from CSF (CSF-neurosin) using an immunoaffinity chromatography and an anion-exchange chromatography. SDS-PAGE revealed that the purified protein has a relative mol. mass (Mr) of 25,000 Da. The observed sequence of the N-terminal amino acids, Glu-Glu-Gln-Asn-Lys, of the purified CSF-neurosin was identical to the sequence of N-terminal of the pro-enzyme form, which is presumed to have no enzyme activity. CSF-neurosin neither showed any enzyme activity to Boc-Phe-Ser-Arg-4-methylcoumaryl-7-amide, which is known to be degraded by the mature neurosin, nor cleaved gelatin. To confirm that the major portion of CSF-neurosin is present in the pro-enzyme form, Western blot analysis using antibodies specific to the pro- or mature enzyme was carried out. The antibody against the mature neurosin fragment did not react with CSF-neurosin. Only the antibody against the pro-enzyme fragment detected CSF-neurosin. Thus, our results suggest that neurosin is present as an inactive pro-enzyme in the human CSF.

A genetically engineered human Kunitz protease inhibitor with increased kallikrein inhibition in an ovine model of cardiopulmonary bypass

A recombinant human serine protease inhibitor known as Kunitz protease inhibitor (KPI) wild type has functional similarities to the bovine Kunitz inhibitor, aprotinin, and had shown a potential to reduce bleeding in an ovine model of cardiopulmonary bypass (CPB). The aim of this study was to assess KPI-185, a modification of KPI-wild type that differs from KPI-wild type in two amino acid residues and which enhances anti-kallikrein activity in a further double-blind, randomized study in an ovine model of CPB, and to compare with our previous study of KPI-wild type and aprotinin in the same ovine model. Post-operative drain losses and subjective assessment of wound 'dryness' showed no significant differences between KPI-185 and KPI-wild type, despite the significant enhancement of kallikrein inhibition using KPI-185 seen in serial kallikrein inhibition assays. These preliminary findings support the hypothesis that kallikrein inhibition is not the major mechanism by which Kunitz inhibitors such as aprotinin reduce perioperative bleeding.

Platelets as a peripheral district where to study pathogenetic mechanisms of alzheimer disease: the case of amyloid precursor protein

Alzheimer disease is a progressive neurodegenerative disease, characterised by a progressive cognitive and memory decline. From a neuropathological point of view, Alzheimer disease is defined by the presence of characteristic lesions, i.e. mature senile plaques, neurofibrillary tangles (NFTs) and amyloid angiopathy. In particular, accumulation of the amyloid beta-peptide in the brain parenchyma and vasculature is an invariant event in the pathogenesis of both sporadic and familial Alzheimer cases. Amyloid beta-peptide originates from a larger precursor, the amyloid precursor protein (APP) ubiquitously expressed. Among the different peripheral cells expressing APP forms, platelets are particularly interesting since they show concentrations of its isoforms equivalent to those found in brain. Moreover, a number of laboratories independently described alterations in APP metabolism/concentration in platelets of Alzheimer patients when compared to control subjects matched for demographic characteristics. These observations defined the frame of our work aimed to investigate if a correlation between levels of platelet APP forms and Alzheimer disease could be detected. We have reported that patients affected by Alzheimer disease show a differential level of platelet APP forms. This observation has several implications: APP processing abnormalities, believed to be a very early change in Alzheimer disease in neuronal compartment, do occur in extraneuronal tissues, such as platelets, thus, suggesting that Alzheimer disease is a systemic disorder; further, our data strongly indicate that a differential level of platelet APP isoforms can be considered a potential peripheral marker of Alzheimer disease allowing for discrimination between Alzheimer and other types of dementia.

Closing in on the amyloid cascade: recent insights into the cell biology of Alzheimer's disease

Accumulation of the amyloid-beta (A beta) peptide in the central nervous system (CNS) is considered by many to be the crucial pathological insult that ultimately leads to the development of Alzheimer's disease (AD). Regulating the production and/or aggregation of A beta could therefore be of considerable benefit to patients afflicted with AD. It has long been known that A beta is derived from the proteolytic processing of the amyloid precursor protein (APP) by two enzymatic activities, beta-secretase and gamma-secretase. Recent breakthroughs have led to the identification of the aspartyl protease BACE (beta-site APP-cleaving enzyme) as beta-secretase and the probable identification of the presenilin proteins as gamma-secretases. This review discusses what is know about BACE and the presenilins, focusing on their capacity as secretases, as well as the options for therapeutic advancement the careful characterization of these proteins will provide. These findings are presented in the context of the "amyloid cascade hypothesis" and its physiological relevance in AD pathogenesis.

Charge alterations of E22 enhance the pathogenic properties of the amyloid beta-protein

Cerebral amyloid angiopathy (CAA) due to amyloid beta-protein (Abeta) is a key pathological feature of patients with Alzheimer's disease and hereditary cerebral hemorrhage with amyloidosis, Dutch-type (HCHWA-D). The CAA in these disorders is characterized by deposition of Abeta in the smooth muscle cells within the cerebral vessel wall. Recently, a new mutation in Abeta, E22K, was identified in several Italian families that, like HCHWA-D, is associated with CAA and hemorrhagic stroke. These two similar disorders, stemming from amino acid substitutions at position 22 of Abeta, implicate the importance of this site in the pathology of HCHWA. Previously we showed that HCHWA-D Abeta(1-40) containing the E22Q substitution induces robust pathologic responses in cultured human cerebrovascular smooth muscle cells (HCSM cells), including highly elevated levels of cell-associated Abeta precursor (AbetaPP) and cell death. In the present study, a series of E22 mutant Abeta(1-40) peptides were synthesized, and their pathogenic properties toward cultured HCSM cells were evaluated. Quantitative fluorescence analyses showed that mutant Abeta(1-40) peptides either containing a loss of charge (E22Q and E22A) or a change of charge (E22K) bind to the surface of HCSM cells and form amyloid fibrils. Similarly, this same group of E22 mutant Abeta(1-40) peptides caused enhanced pathologic responses in HCSM cells. In contrast, wild-type E22 or the charge-preserving E22D Abeta(1-40) peptides were devoid of any of these pathogenic properties. These data suggest that a change or loss of charge at position 22 of Abeta enhances the pathogenic effects of the peptide toward HCSM cells and may contribute to the pathogenesis of the phenotypically related HCHWA disorders.

The beta-amyloid precursor protein and its derivatives: from biology to learning and memory processes

Intensive investigation towards the understanding of the biology and physiological functions of the beta-amyloid precursor protein (APP) have been supported since it is known that a 39-43 amino acid fragment of APP, called the beta-amyloid protein (Abeta), accumulates in the brain parenchyma to form the typical lesions associated with Alzheimer's disease (AD). It emerges from extensive data that APP and its derivatives show a wide range of contrasting physiological properties and therefore might be involved in distinct physiological functions. Abeta has been shown to disrupt neuronal activity and to demonstrate neurotoxic properties in a wide range of experimental procedures. In contrast, both in vitro and in vivo studies suggest that APP and/or its secreted forms are important factors involved in the viability, growth and morphological and functional plasticity of nerve cells. Furthermore, several recent studies suggest that APP and its derivatives have an important role in learning and memory processes. Memory impairments can be induced in animals by intracerebral treatment with Abeta. Altered expression of the APP gene in aged animals or in genetically-modified animals also leads to memory deficits. By contrast, secreted forms of APP have recently been shown to facilitate learning and memory processes in mice. These interesting findings open novel perspectives to understand the involvement of APP in the development of cognitive deficits associated with AD. In this review, we summarize the current data concerning the biology and the behavioral effects of APP and its derivatives which may be relevant to the roles of these proteins in memory and in AD pathology.

Developmental regulation of amyloid precursor protein at the neuromuscular junction in mouse skeletal muscle

Amyloid precursor protein (APP), associated with Alzheimer's disease plaques, is known to be present in synapses of the brain and in the adult neuromuscular junction (NMJ). In the present study we examined protein and gene expression of APP during the development of mouse skeletal muscle. Using immunocytochemical approaches, we found that APP is first detected in myotube cytoplasm at embryonic day 16 and becomes progressively concentrated at the NMJ beginning at birth until adulthood. The colocalization between APP and acetylcholine receptors at the NMJ is only partial at birth, but becomes complete upon reaching adulthood. We observed that all APP isoforms, including the Kunitz-containing (protease inhibitor or KPI) forms, are up-regulated from birth to postnatal day 5 and then decreased to reach the low levels observed in the adult. This suggests the involvement of APP during the events which lead to a mature mono-innervated synapse. A 92-kDa band, characteristic of a cleaved APP695 isoform and not due to a new muscle-specific alternative spliced form, was observed from postnatal day 15 following completion of polyneuronal synapse elimination. Taken together, these data suggest that skeletal muscle APP may well play a role in the differentiation of skeletal muscle and in the formation and maturation of NMJs.

Fibrillar amyloid beta-protein mediates the pathologic accumulation of its secreted precursor in human cerebrovascular smooth muscle cells

Cerebrovascular deposition of the amyloid beta-protein (Abeta) is a key pathologic lesion seen in patients with Alzheimer's disease and certain related disorders, including hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D). The deposition of Abeta has pronounced deleterious effects on smooth muscle cells within the cerebral vessel wall. We have previously shown that Abeta(1-40) possessing the E22Q HCHWA-D mutation extensively assembles into fibrils on the surface of cultured human cerebrovascular smooth muscle (HCSM) cells. This cell-surface Abeta fibril formation induces a series of pathologic responses in cultured HCSM cells, including a marked increase in the levels of cell-associated amyloid beta-protein precursor (AbetaPP) and cell death. In the present study, we investigated the relationship between HCSM cell-surface Abeta fibril formation and the striking increase in cell-associated AbetaPP. Time course studies showed that cell-surface HCHWA-D Abeta(1-40) fibril formation occurred rapidly, whereas both the increase in cell-associated AbetaPP and loss of cell viability were delayed responses. Domain analysis using site-specific antibodies indicated that the vast majority of the increase in cell-associated AbetaPP was secreted AbetaPP (sAbetaPP). Localization studies showed that the sAbetaPP was present on the HCSM cell surface. This result raised the possibility that sAbetaPP may bind back to HCSM cell-surface fibrils formed by HCHWA-D Abeta(1-40). Indeed, binding of biotinylated sAbetaPP to fibrillar HCHWA-D Abeta(1-40) was demonstrated by transmission electron microscopy. Furthermore, solid-phase binding assays showed that biotinylated sAbetaPP exhibited dose-dependent, saturable binding to fibrillar (but not soluble) HCHWA-D Abeta(1-40) with k(d) approximately 28 nM. Exon deletion experiments further defined a fragment of sAbetaPP (AbetaPP(18-119)), encoded by AbetaPP exons 2 and 3, to contain the fibrillar Abeta-binding domain. In addition, AbetaPP(18-119) effectively blocked the cell-surface accumulation of sAbetaPP and subsequent cell death in HCSM cells treated with pathogenic Abeta. Together, these findings could explain the accumulation of AbetaPP in cerebrovascular Abeta deposits observed both in vitro and in vivo and may contribute to the pathologic responses evoked by pathogenic forms of Abeta in HCSM cells.

What the evolution of the amyloid protein precursor supergene family tells us about its function

The Alzheimer's disease amyloid protein precursor (APP) gene is part of a multi-gene super-family from which sixteen homologous amyloid precursor-like proteins (APLP) and APP species homologues have been isolated and characterised. Comparison of exon structure (including the uncharacterised APL-1 gene), construction of phylogenetic trees, and analysis of the protein sequence alignment of known homologues of the APP super-family were performed to reconstruct the evolution of the family and to assess the functional significance of conserved protein sequences between homologues. This analysis supports an adhesion function for all members of the APP super family, with specificity determined by those sequences which are not conserved between APLP lineages, and provides evidence for an increasingly complex APP superfamily during evolution. The analysis also suggests that Drosophila APPL and Caenorhabditis elegans APL-1 may be a fourth APLP lineage indicating that these proteins, while not functional homologues of human APP, are similarly likely to regulate cell adhesion. Furthermore, the betaA4 sequence is highly conserved only in APP orthologues, strongly suggesting this sequence is of significant functional importance in this lineage.

Suppression of an amyloid beta peptide-mediated calcium channel response by a secreted beta-amyloid precursor protein

Secreted isoforms of the beta-amyloid precursor protein potently enhance neuronal survival in cell cultures exposed to toxic amyloid beta peptide. Lowering of intracellular calcium levels to offset the increases in intraneuronal calcium caused by amyloid beta peptide is thought to underly this neuroprotection. Because we have shown previously that an amyloid beta peptide-mediated potentiation of calcium channel currents may contribute to this cytosolic calcium overload, the present study examined the effects of a secreted beta-amyloid precursor protein on the calcium channel response to amyloid beta peptide. When compared with untreated cultured rat hippocampal neurons, cells that underwent a 24 h preincubation with beta-amyloid precursor protein 751 displayed decreases in the relative size of the calcium channel response to amyloid beta peptide. A membrane-permeable analog of cyclic GMP, a second messenger believed to be involved in the calcium regulation process mediated by beta-amyloid precursor proteins, also attenuated the modulatory calcium channel response. Co-application of beta-amyloid precursor protein 751 with amyloid beta peptide did not alter calcium channel response to amyloid beta peptide. Taken together, these findings suggest that secreted beta-amyloid precursor proteins can suppress a calcium channel response to amyloid beta peptide that is potentially injurious to the cell, and as such, may define a neuroprotective mechanism that is specific for amyloid beta toxicity.

Activation of neuronal caspase-3 by intracellular accumulation of wild-type Alzheimer amyloid precursor protein

Forced overexpression of wild-type Alzheimer amyloid precursor protein (APP) causes postmitotic neurons to degenerate. Caspase-3 (CPP32) is a principal cell death protease involved in neuronal apoptosis during physiological development and under pathological conditions. Here, we investigated whether APP overexpression activates caspase-3 in human postmitotic neurons using adenovirus-mediated gene transfer. When a recombinant adenovirus vector expressing human wild-type APP695 was infected in vitro into neurally differentiated embryonal carcinoma NT2 cells, only postmitotic neurons underwent severe degeneration. Before neurodegeneration, full-length APP- and Abeta-immunoreactive peptides were accumulated in infected neurons, and caspase-3-like protease activity was markedly elevated. Western blot analysis revealed that activated caspase-3 subunits were generated in APP-accumulating neurons. Such neuronal caspase-3 activation was undetectable in NT2 neurons infected with beta-galactosidase-expressing adenovirus. Addition of the caspase-3 inhibitor acetyl-Asp-Glu-Val-Asp-aldehyde to the culture medium significantly reduced the severity of degeneration exhibited by APP-overexpressing neurons. Immunocytochemical analyses revealed that some APP-accumulating neurons contained activated caspase-3 subunits and exhibited the characteristics of apoptosis, such as chromatin condensation and DNA fragmentation. Activation of caspase-3 was also observed in vivo in rat hippocampal neurons infected with the APP-expressing adenovirus. These results suggest that wild-type APP is an intrinsic activator of caspase-3-mediated death machinery in postmitotic neurons.

hFE65L influences amyloid precursor protein maturation and secretion

The amyloid precursor protein (APP) is processed in the secretory and endocytic pathways, where both the neuroprotective alpha-secretase-derived secreted APP (APPs alpha) and the Alzheimer's disease-associated beta-amyloid peptide are generated. All three members of the FE65 protein family bind the cytoplasmic domain of APP, which contains two sorting signals, YTS and YENPTY. We show here that binding of APP to the C-terminal phosphotyrosine interaction domain of hFE65L requires an intact YENPTY clathrin-coated pit internalization sequence. To study the effects of the hFE65L/APP interaction on APP trafficking and processing, we performed pulse/chase experiments and examined APP maturation and secretion in an H4 neuroglioma cell line inducible for expression of the hFE65L protein. Pulse/chase analysis of endogenous APP in these cells showed that the ratio of mature to total cellular APP increased after the induction of hFE65L. We also observed a three-fold increase in the amount of APPs alpha recovered from conditioned media of cells overexpressing hFE65L compared with uninduced controls. The effect of hFE65L on the levels of APPs alpha secreted is due neither to a simple increase in the steady-state levels of APP nor to activation of the protein kinase C-regulated APP secretion pathway. We conclude that the effect of hFE65L on APP processing is due to altered trafficking of APP as it transits through the secretory pathway.

The amyloid precursor protein of Alzheimer's disease and the Abeta peptide

Alzheimer's disease is characterized by the accumulation of beta amyloid peptides in plaques and vessel walls and by the intraneuronal accumulation of paired helical filaments composed of hyperphosphorylated tau. In this review, we concentrate on the biology of amyloid precursor protein, and on the central role of amyloid in the pathogenesis of Alzheimer's disease. Amyloid precursor protein (APP) is part of a super-family of transmembrane and secreted proteins. It appears to have a number of roles, including regulation of haemostasis and mediation of neuroprotection. APP also has potentially important metal and heparin-binding properties, and the current challenge is to synthesize all these varied activities into a coherent view of its function. Cleavage of amyloid precursor protein by beta-and gamma-secretases results in the generation of the Abeta (betaA4) peptide, whereas alpha-secretase cleaves within the Abeta sequence and prevents formation from APP. Recent findings indicate that the site of gamma-secretase cleavage is critical to the development of amyloid deposits; Abeta1-42 is much more amyloidogenic than Abeta1-40. Abeta1-42 formation is favoured by mutations in the two presenilin genes (PS1 and PS2), and by the commonest amyloid precursor protein mutations. Transgenic mouse models of Alzheimer's disease incorporating various mutations in the presenilin gene now exist, and have shown amyloid accumulation and cognitive impairment. Neurofibrillary tangles have not been reproduced in these models, however. While aggregated Abeta is neurotoxic, perhaps via an oxidative mechanism, the relationship between such toxicity and neurofibrillary tangle formation remains a subject of ongoing research.

The kunitz protease inhibitor form of the amyloid precursor protein (KPI/APP) inhibits the proneuropeptide processing enzyme prohormone thiol protease (PTP). Colocalization of KPI/APP and PTP in secretory vesicles

Proteolytic processing of proenkephalin and proneuropeptides is required for the production of active neurotransmitters and peptide hormones. Variations in the extent of proenkephalin processing in vivo suggest involvement of endogenous protease inhibitors. This study demonstrates that "protease nexin 2 (PN2)," the secreted form of the kunitz protease inhibitor (KPI) of the amyloid precursor protein (APP), potently inhibited the proenkephalin processing enzyme known as prohormone thiol protease (PTP), with a Ki,app of 400 nM. Moreover, PTP and PN2 formed SDS-stable complexes that are typical of kunitz protease inhibitor interactions with target proteases. In vivo, KPI/APP (120 kDa), as well as a truncated form of KPI/APP that resembles PN2 in apparent molecular mass (110 kDa), were colocalized with PTP and (Met)enkephalin in secretory vesicles of adrenal medulla (chromaffin granules). KPI/APP (110-120 kDa) was also detected in pituitary secretory vesicles that contain PTP. In chromaffin cells, calcium-dependent secretion of KPI/APP with PTP and (Met)enkephalin demonstrated the colocalization of these components in functional secretory vesicles. These results suggest a role for KPI/APP inhibition of PTP in regulated secretory vesicles. In addition, these results are the first to identify an endogenous protease target of KPI/APP, which is developmentally regulated in aging and Alzheimer's disease.

Effects of PS1 deficiency on membrane protein trafficking in neurons

We have examined the trafficking and metabolism of the beta-amyloid precursor protein (APP), an APP homolog (APLP1), and TrkB in neurons that lack PS1. We report that PS1-deficient neurons fail to secrete Abeta, and that the rate of appearance of soluble APP derivatives in the conditioned medium is increased. Remarkably, carboxyl-terminal fragments (CTFs) derived from APP and APLP1 accumulate in PS1-deficient neurons. Hence, PS1 plays a role in promoting intramembrane cleavage and/or degradation of membrane-bound CTFs. Moreover, the maturation of TrkB and BDNF-inducible TrkB autophosphorylation is severely compromised in neurons lacking PS1. We conclude that PS1 plays an essential role in modulating trafficking and metabolism of a selected set of membrane and secretory proteins in neurons.

Inhibition of platelet activation by the Alzheimer's disease amyloid precursor protein

The amyloid precursor protein (APP) of Alzheimer's disease is abundantly expressed in the platelet alpha-granule where its role remains unclear. This study describes a novel function for APP in regulating human platelet activation. Preincubation of platelet-rich plasma with recombinant secreted APP (sAPP) isoforms dose-dependently inhibited platelet aggregation and secretion induced by ADP or adrenaline. Similarly, sAPP potently inhibited low-dose thrombin-induced activation in washed platelet suspensions, indicating that the activity does not require plasma cofactors. There were no functional differences between sAPP forms with or without the Kunitz protease inhibitor domain or derived from either alpha- or beta-secretase cleavage. In fact, the N-terminal cysteine-rich region of APP (residues 18-194) was as effective as the entire sAPP region in the inhibition of platelet activation. The inhibitory activity of sAPP correlated with a significant reduction in the agonist-induced production of the arachidonic acid (AA) metabolites thromboxane B2 and prostaglandin E2. However, sAPP did not affect AA-induced platelet aggregation or secretion, indicating the enzymatic conversion of AA was not inhibited. The addition of a threshold dose of AA reversed the sAPP-inhibition of agonist-induced platelet activation. This suggests that sAPP decreases the availability of free AA, although the mechanism is not yet known. These data provide evidence that the release of sAPP upon platelet degranulation may result in negative feedback regulation during platelet activation.

Memory-enhancing effects of secreted forms of the beta-amyloid precursor protein in normal and amnestic mice

When administered intracerebroventricularly to mice performing various learning tasks involving either short-term or long-term memory, secreted forms of the beta-amyloid precursor protein (APPs751 and APPs695) have potent memory-enhancing effects and block learning deficits induced by scopolamine. The memory-enhancing effects of APPs were observed over a wide range of extremely low doses (0.05-5,000 pg intracerebroventricularly), blocked by anti-APPs antisera, and observed when APPs was administered either after the first training session in a visual discrimination or a lever-press learning task or before the acquisition trial in an object recognition task. APPs had no effect on motor performance or exploratory activity. APPs695 and APPs751 were equally effective in the object recognition task, suggesting that the memory-enhancing effect of APPs does not require the Kunitz protease inhibitor domain. These data suggest an important role for APPss on memory processes.

Purification by column chromatographies of beta-amyloid precursor proteins and their association with other 95 kDa protein in rat brain

Beta-amyloid precursor proteins (APPs) in the subcellular fractions of the homogenate of rat brain were detected immunologically. They were found to be localized in both the cytosol and microsome fractions in generally equal amounts. APPs were purified from the cytosol fraction of rat brain by column chromatography in a DEAE-anion-exchanger, Blue-Sepharose, Ni-charged chelating Sepharose, and Sephacryl S-300 columns. They migrated at about 400 kDa or above in a final gel filtration column with trypsin inhibitor activity. They gave two broad protein bands of 80 and 100 kDa and several other protein bands in sodium dodecyl sulfate-polyacryl amide gel electrophoresis (SDS-PAGE). The 80 and 100 kDa bands were highly concentrated during purification. They gave the same amino terminal sequence and were identified as rat APPs without an amino terminal signal sequence. These results suggest that rat brain APPs form a complex with themselves or with other proteins and contain APP isoforms including a serine protease inhibitor domain, APP770 or APP751, or both. An antibody produced by a rabbit immunized with the final preparation of APPs reacted with a 95 kDa protein band which migrated between the 80 and 100 kDa bands of APPs in SDS-PAGE, but it did not react with the bands of APPs. The 80 and 100 kDa APP bands were coprecipitated with a 95 kDa antigen protein band by reacting this antibody with the partially purified APPs. We conclude that APPs in the rat brain are associated directly or indirectly with another protein to yield the 95 kDa band demonstrated by SDS-PAGE.

Importance of immunological and inflammatory processes in the pathogenesis and therapy of Alzheimer's disease

The contribution of autoimmune processes or inflammatory components in the etiology and pathogenesis of Alzheimer's disease (AD) has been suspected for many years. The presence of antigen-presenting, HLA-DR-positive and other immunoregulatory cells, components of complement, inflammatory cytokines and acute phase reactants have been established in tissue of AD neuropathology. Although these data do not confirm the immune response as a primary cause of AD, they indicate involvement of immune processes at least as a secondary or tertiary reaction to the preexisting pathogen and point out its driving-force role in AD pathogenesis. These processes may contribute to systemic immune response. Thus, experimental and clinical studies indicate impairments in both humoral and cellular immunity in an animal model of AD as well as in AD patients. On the other hand, anti-inflammatory drugs applied for the treatment of some chronic inflammatory diseases have been shown to reduce risk of AD in these patients. Therefore, it seems that anti-inflammatory drugs and other substances which can control the activity of immunocompetent cells and the level of endogenous immune response can be valuable in the treatment of AD patients.

Amyloid precursor protein modulates the interaction of nerve growth factor with p75 receptor and potentiates its activation of trkA phosphorylation

We have recently shown that the secreted form of amyloid precursor protein (APPs) potentiates the neurotrophic actions of nerve growth factor (NGF). The combined presence of NGF and APPs in low concentrations resulted in a synergistic potentiation of NGF neuritogenic activity on PC12 cells. Therefore, the effect of APPs on NGF receptor-binding has been examined. In the presence of APPs, the apparent affinity of NGF's low affinity binding site increased by a factor of 2.5. In addition, a 2- to 2.5-fold decrease in the number of sites was observed, although APPs did not compete with NGF for the same binding sites. These effects of APPs were not caused by direct interaction with NGF itself. In addition, APPs synergistically potentiated the tyrosine phosphorylation of trkA due to NGF. These results suggest that an increased affinity of p75 for NGF may underlie the potentiation of neurotrophic actions of NGF by APPs, and that increase may be caused by an indirect interaction between APPs and p75.

Amyloid precursor protein proteoglycan is increased after brain damage

The beta-amyloid peptide (Abeta or A4) is produced by proteolytic cleavage from amyloid precursor protein (APP). The progressive cerebral deposition of this peptide is one of the most important features of Alzheimer's disease. From the study of normal and transfected cells, two APP processing pathways have been proposed as physiological alternatives. One of these can produce Abeta or amyloidogenic peptides, whereas the second does not. However, it is not completely clear how APPs are post-translationally modified, proteolytically processed and metabolized in the brain. We report here that APPs also exist as proteoglycan, chondroitin-sulfate (ChS). We have identified in normal rat brain a complex pool of 8 to 130 kDa ChS-core proteins. The main portion of these proteoglycan (PGs) APPs contains complete amyloidogenic sequence, suggesting a novel proteolytic processing of APP from the amino-terminal to the transmembrane region. This population appears augmented after brain damage. These findings may have significant implications in understanding the initial deposition and kinetics of amyloid aggregation in a pathological situation like Alzheimer's disease.

Caveolae, plasma membrane microdomains for alpha-secretase-mediated processing of the amyloid precursor protein

Caveolae are plasma membrane invaginations where key signaling elements are concentrated. In this report, both biochemical and histochemical analyses demonstrate that the amyloid precursor protein (APP), a source of Abeta amyloid peptide, is enriched within caveolae. Caveolin-1, a principal component of caveolae, is physically associated with APP, and the cytoplasmic domain of APP directly participates in this binding. The characteristic C-terminal fragment that results from APP processing by alpha-secretase, an as yet unidentified enzyme that cleaves APP within the Abeta amyloid sequence, was also localized within these caveolae-enriched fractions. Further analysis by cell surface biotinylation revealed that this cleavage event occurs at the cell surface. Importantly, alpha-secretase processing was significantly promoted by recombinant overexpression of caveolin in intact cells, resulting in increased secretion of the soluble extracellular domain of APP. Conversely, caveolin depletion using antisense oligonucletotides prevented this cleavage event. Our current results indicate that caveolae and caveolins may play a pivotal role in the alpha-secretase-mediated proteolysis of APP in vivo.

Degeneration in vivo of rat hippocampal neurons by wild-type Alzheimer amyloid precursor protein overexpressed by adenovirus-mediated gene transfer

In an attempt to elucidate the pathological implications of intracellular accumulation of the amyloid precursor protein (APP) in postmitotic neurons in vivo, we transferred APP695 cDNA into rat hippocampal neurons by using a replication-defective adenovirus vector. We first improved the efficiency of adenovirus-mediated gene transfer into neurons in vivo by using hypertonic mannitol. When a beta-galactosidase-expressing recombinant adenovirus suspended in 1 M mannitol was injected into a dorsal hippocampal region, a number of neurons in remote areas were positively stained, presumably owing to increased retrograde transport of the virus. When an APP695-expressing adenovirus was injected into the same site, part of the infected neurons in the hippocampal formation underwent severe degeneration in a few days, whereas astrocytes near the injection site showed no apparent degeneration. These degenerating neurons accumulated different epitopes of APP, and beta/A4 protein (Abeta)-immunoreactive materials were undetected in the extracellular space. A small number of degenerating neurons showed nuclear DNA fragmentation. Electron microscopic examinations demonstrated that degenerating neurons had shrunken perikarya along with synaptic abnormalities. Microglial cells/macrophages were often found in close proximity to degenerating neurons, and in some cases they phagocytosed these neurons. These results suggest that intracellular accumulation of wild-type APP695 causes a specific type of neuronal degeneration in vivo in the absence of extracellular Abeta deposition.

A beta amyloidogenesis: unique, or variation on a systemic theme?

For more than a century amyloid was considered to be an interesting, unique, but inconsequential pathologic entity that rarely caused significant clinical problems. We now recognize that amyloid is not one entity. In vivo it is a uniform organization of a disease, or process, specific protein co-deposited with a set of common structural components. Amyloid has been implicated in the pathogenesis of diseases affecting millions of patients. These range from Alzheimer's disease, adult-onset diabetes, consequences of prolonged renal dialysis, to the historically recognized systemic forms associated with inflammation and plasma cell disturbances. Strong evidence is emerging that even when deposited in local organ sites significant physiologic effects may ensue. With emphasis on A beta amyloid, we review the present definition, classification, and general in vivo pathogenetic events believed to be involved in the deposition of amyloids. This encompasses the need for an adequate amyloid precursor protein pool, whether precursor proteolysis is required prior to deposition, amyloidogenic amino acid sequences, fibrillogenic nucleating particles, and an in vivo microenvironment conducive to fibrillogenesis. The latter includes several components that seem to be part of all amyloids. The role these common components may play in amyloid accumulation, why amyloids tend to be associated with basement membranes, and how one may use these findings for anti-amyloid therapeutic strategies is also examined.

Profiles of amyloid precursor and presenilin 2-like proteins are correlated during development of the mouse hypothalamus

The amyloid precursor protein (APP) and APP-like (APLP) material, as visualized with the Mab22C11 antibody, have previously been shown to be associated with radial glia in hypothalamus, which are known to promote neurite outgrowth. By Northern blot analysis, APP 695 mRNA levels increased steadily over hypothalamic development, APP 770 mRNA was transiently expressed at 12 days postnatally, and APLP mRNA was only weakly expressed in the hypothalamus. The developmental pattern of APP moeities in mouse hypothalamus and in fetal hypothalamic neurons in culture was compared with a presenilin 2 (PS2) related protein using an antibody developed against the N-terminal part of PS2. By Western blot analysis, APP and PS2-like immunoreactivity were visualized as a 100-130 and 52 kDa bands, respectively. An APP biphasic increase was observed during hypothalamic development in vivo. APP immunoreactivity was equally detected in neuronal and glial cultures, while PS2-like material was more concentrated in neurons. A correlation between APP/APP-like and PS2-like levels was observed during development in vivo. While APP was mostly associated with membrane fractions, a significant portion of PS2-like material was also recovered from cytosolic fractions in vitro. In contrast to native PS2 in COS-transfected cells, the PS2-like material did not aggregate after heating for 90 s at 90 degrees C. These results indicate a close association between APP and PS2-like material during hypothalamic development in vivo, and suggest that neuronal and glial cultures may provide appropriate models to test their interactions.

Peripheral markers in testing pathophysiological hypotheses and diagnosing Alzheimer's disease

Alterations in amyloid precursor protein (APP) metabolism, calcium regulation, oxidative metabolism, and transduction systems have been implicated in Alzheimer's disease (AD). Limitations to the use of postmortem brain for examining molecular mechanisms underscore the need to develop a human tissue model representative of the pathophysiological processes that characterize AD. The use of peripheral tissues, particularly of cultured skin fibroblasts derived from AD patients, could complement studies of autopsy samples and provide a useful tool with which to investigate such dynamic processes as signal transduction systems, ionic homeostasis, oxidative metabolism, and APP processing. Peripheral cells as well as body fluids (i.e., plasma and CSF) could also provide peripheral biological markers for the diagnosis of AD. The criteria required for a definite diagnosis of AD presently include clinical criteria in association with histopathologic evidence obtained from biopsy or autopsy. Thus, the use of peripheral markers as a diagnostic tool, either to predict or at least to confirm a diagnosis, may be of great importance.