Interaction between the zinc (II) and the heparin binding site of the Alzheimer's disease beta A4 amyloid precursor protein (APP)

Dimethylargininase, a nitric oxide regulatory protein, in Alzheimer disease

In this study, we show that dimethylargininase, a zinc protein involved in the regulation of nitric oxide synthase, is specifically elevated in neurons displaying cytoskeletal abnormalities and oxidative stress in Alzheimer disease (AD) while none of this enzyme was found in neurons in age-matched control cases. Seen in the context of earlier studies showing widespread nitric oxide related damage in AD and the role of dimethylargininase to activate nitric oxide synthetase, through catalytic removal of its endogenous inhibitors, these findings indicate major alterations in nitric oxide regulation in AD. Further, that low levels of zinc specifically inhibit dimethylargininase may provide a link between the numerous studies showing specific abnormalities in zinc and oxidative stress. Finally, our results provide additional evidence that oxidative stress- and nitric oxide-mediated events play important roles in the pathogenesis of AD.

Increased density of metallothionein I/II-immunopositive cortical glial cells in the early stages of Alzheimer's disease

We have examined the possible role of metallothionein I/II (MT I/II) in Alzheimer's disease (AD), with a focus on the cellular localization of MT I/II relative to the astrocyte marker, glial fibrillary acidic protein (GFAP). In AD and preclinical AD cases, MT I/II immunolabeling was present in glial cells and did not show a spatial relationship with beta-amyloid plaques or neurofibrillary pathology. There was a six- to sevenfold increase in both MT I/II- and GFAP-labeled cells in the gray matter of AD cases, relative to non-AD cases. However, there was a threefold increase in MT I/II-immunoreactive cells, but not GFAP-labeled cells, in the gray matter of preclinical AD cases compared to non-AD cases. Therefore, the specific increase in MT I/II is associated with the initial stages of the disease process, perhaps due to oxidative stress or the mismetabolism of heavy metals.

Effects of the amyloid protein precursor of Alzheimer's disease and other ligands of the LDL receptor-related protein on neurite outgrowth from sympathetic neurons in culture

The amyloid protein precursor (APP) of Alzheimer's disease can stimulate neurite outgrowth in vitro. The receptor responsible for this effect has not been identified. Kunitz protease inhibitor (KPI)-containing forms of APP bind to the low-density lipoprotein receptor-related protein (LRP). As LRP may regulate neurite outgrowth, we examined whether the effects of APP are mediated by LRP. Inhibitors of LRP decreased neurite outgrowth from chick sympathetic neurons. Most LRP ligands (alpha2-macroglobulin, lactoferrin, and lipoprotein lipase) stimulated outgrowth. However, in soluble form, the KPI-containing APP751 was a weak inhibitor of outgrowth. In substrate-bound form, both APP751 and APP695 (which does not bind to LRP) stimulated outgrowth. Thus the effect of substrate-bound APP on neurite outgrowth is not mediated by LRP.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Zinc metabolism in the brain: relevance to human neurodegenerative disorders

Zinc is an important trace element in biology. An important pool of zinc in the brain is the one present in synaptic vesicles in a subgroup of glutamatergic neurons. In this form it can be released by electrical stimulation and may serve to modulate responses at receptors for a number of different neurotransmitters. These include both excitatory and inhibitory receptors, particularly the NMDA and GABA(A) receptors. This pool of zinc is the only form of zinc readily stained histochemically (the chelatable zinc pool), but constitutes only about 8% of the total zinc content in the brain. The remainder of the zinc is more or less tightly bound to proteins where it acts either as a component of the catalytic site of enzymes or in a structural capacity. The metabolism of zinc in the brain is regulated by a number of transport proteins, some of which have been recently characterized by gene cloning techniques. The intracellular concentration may be mediated both by efflux from the cell by the zinc transporter ZrT1 and by complexing with apothionein to form metallothlonein. Metallothionein may serve as the source of zinc for incorporation into proteins, including a number of DNA transcription factors. However, zinc is readily released from metallothionein by disulfides, increasing concentrations of which are formed under oxidative stress. Metallothionein is a very good scavenger of free radicals, and zinc itself can also reduce oxidative stress by binding to thiol groups, decreasing their oxidation. Zinc is also a very potent inhibitor of nitric oxide synthase. Increased levels of chelatable zinc have been shown to be present in cell cultures of immune cells undergoing apoptosis. This is very reminiscent of the zinc staining of neuronal perikarya dying after an episode of ischemia or seizure activity. Thus a possible role of zinc in causing neuronal death in the brain needs to be fully investigated. intraventricular injections of calcium EDTA have already been shown to reduce neuronal death after a period of ischemia. Pharmacological doses of zinc cause neuronal death, and some estimates indicate that extracellular concentrations of zinc could reach neurotoxic levels under pathological conditions. Zinc is released in high concentrations from the hippocampus during seizures. Unfortunately, there are contrasting observations as to whether this zinc serves to potentiate or decrease seizure activity. Zinc may have an additional role in causing death in at least some neurons damaged by seizure activity and be involved in the sprouting phenomenon which may give rise to recurrent seizure propagation in the hippocampus. In Alzheimer's disease, zinc has been shown to aggregate beta-amyloid, a form which is potentially neurotoxic. The zinc-dependent transcription factors NF-kappa B and Sp1 bind to the promoter region of the amyloid precursor protein (APP) gene. Zinc also inhibits enzymes which degrade APP to nonamyloidogenic peptides and which degrade the soluble form of beta-amyloid. The changes in zinc metabolism which occur during oxidative stress may be important in neurological diseases where oxidative stress is implicated, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Zinc is a structural component of superoxide dismutase 1, mutations in which give rise to one form of familiar ALS. After HIV infection, zinc deficiency is found which may be secondary to immune-induced cytokine synthesis. Zinc is involved in the replication of the HIV virus at a number of sites. These observations should stimulate further research into the role of zinc in neuropathology.

Expression and analysis of heparin-binding regions of the amyloid precursor protein of Alzheimer's disease

Deletion mutagenesis studies have suggested that there are two domains within APP which bind heparan sulphate. These domains have been cloned and expressed in the yeast Pichia pastoris. Both recombinant proteins bound to heparin. One domain (APP316-447) was further characterised by binding studies with peptides encompassing this region. Peptides homologous to APP316-346 and APP416-447 were found to bind heparin. Circular dichroism studies show that APP416-447 shifted towards an alpha-helical conformation in the presence of heparin. This study suggests that heparin-binding domains may lie within regions high in alpha-helical structure.

Down-regulation of the amyloid protein precursor of Alzheimer's disease by antisense oligonucleotides reduces neuronal adhesion to specific substrata

The hallmark of Alzheimer's disease is the cerebral deposition of amyloid which is derived from the amyloid precursor protein (APP). The function of APP is unknown but there is increasing evidence for the role of APP in cell-cell and/or cell-matrix interactions. Primary cultures of murine neurons were treated with antisense oligonucleotides to down-regulate APP. This paper presents evidence that APP mediates a substrate-specific interaction between neurons and extracellular matrix components collagen type I, laminin and heparan sulphate proteoglycan but not fibronectin or poly-L-lysine. It remains to be determined whether this effect is the direct result of APP-matrix interactions, or whether an intermediatry pathway is involved.

Reactive oxygen species and Alzheimer's disease

Although a consensus that Alzheimer's disease (AD) is a single disease has not been reached yet, the involvement of the amyloid precursor protein (APP) and betaA4 (A beta) in the pathologic changes advances our understanding of the underlying molecular alterations. Increasing evidence implicates oxidative stress in the neurodegenerative process of AD. This hypothesis is based on the toxicity of betaA4 in cell cultures, and the findings that aggregation of betaA4 can be induced by metal-catalyzed oxidation and that free oxygen radicals may be involved in APP metabolism. Another neurological disorder, familial amyotrophic lateral sclerosis (FALS), supports our view that AD and FALS may be linked through a common mechanism. In FALS, SOD-Cu(I) complexes are affected by hydrogen peroxide and free radicals are produced. In AD, the reduction of Cu(II) to Cu(I) by APP involves an electron-transfer reaction and could also lead to a production of hydroxyl radicals. Thus, copper-mediated toxicity of APP-Cu(II)/(I) complexes may contribute to neurodegeneration in AD.

Individual isoforms of the amyloid beta precursor protein demonstrate differential adhesive potentials to constituents of the extracellular matrix

The amyloid beta precursor protein (AbetaPP) can exist as a membrane-bound glycoprotein which modulates neural cell adhesion. The adhesion of clones of the AtT20 mouse pituitary cell line, transfected with cDNA coding for the 695 (AbetaPP695) and 751 (AbetaPP751) amino acid forms of the protein, to individual components of the extracellular matrix was determined using a centrifugal shear assay. On laminin, poly-L-lysine, fibronectin, and uncoated glass substrata, the cells transfected with AbetaPP695 (6A1 cells) demonstrated a 50% increase in adhesivity over nontransfected cells, while those transfected with AbetaPP751 (7A1 cells) showed a significant decrease in adhesion. There was, however, a significant increase in the adhesive strength of the 7A1 cells to collagen type IV with no change in the adhesivity of the 6A1 cells when compared with control. These changes in adhesivity could be attributed to changes in the levels of the membrane-bound protein and were not due to the interaction of soluble AbetaPP with elements of the extracellular matrix. These studies provide evidence for differential adhesivities of the constituent AbetaPP isoforms and the possible role of the Kunitz protease inhibitor (KPI) domain in influencing the adhesive properties of the protein backbone.

Zinc and Alzheimer's disease: is there a direct link?

Zinc is an essential trace element in human biology, but is neurotoxic at high concentrations. Several studies show that zinc promotes aggregations of beta-amyloid protein, the main component of the senile plaques typically found in Alzheimer's disease brains. In other neurological disorders where neurons appear to be dying by apoptosis (gene-directed cell death), chelatable zinc accumulates in the perikarya of neurons before, or during degeneration. As there is evidence for apoptotic death of neurons in Alzheimer's disease, an involvement of zinc in this process needs to be investigated. Zinc interacts with enzymes and proteins, including transcription factors, which are critical for cell survival and could be linked to apoptotic processes. While controversial, some studies indicate that total tissue zinc is markedly reduced in several brain regions of Alzheimer's patients. At face value, it seems that a paradox exists between reports of a decrease in zinc in the Alzheimer's brain and the putative link to aberrant high zinc levels promoting plaque formation. An hypothesis to explain this inconsistency is presented. Neuropathological changes mediated by endogenous or exogenous stressors may be relevant factors affecting abnormal zinc metabolism. This paper reviews current investigations that suggest a role of zinc in the etiology of Alzheimer's disease.

Amyloid precursor protein, copper and Alzheimer's disease

Although a consensus that Alzheimer's disease (AD) is a single disease has not yet been reached, the involvement of the amyloid precursor protein (APP) and beta A4 (A beta) in the pathologic changes advances our understanding of the underlying molecular alterations. Increasing evidence implicates oxidative stress in the neurodegenerative process of AD. This hypothesis is based on the toxicity of beta A4 in cell cultures, and the findings that aggregation of beta A4 can be induced by metal-catalyzed oxidation and that free oxygen radicals might be involved in APP metabolism. Another neurological disorder, familial amyotrophic lateral sclerosis (FALS), supports our view that AD and FALS might be linked through a common mechanism. In FALS, SOD-Cu(I) complexes are affected by hydrogen peroxide and free radicals are produced. In AD, the reduction of Cu(II) to Cu(I) by APP involves an electron-transfer reaction and could also lead to a production of hydroxyl radicals. Thus, copper-mediated toxicity of APP-Cu(II)/(I) complexes may contribute to neurodegeneration in AD.

Identification of candidate proteins binding to prion protein

Prion diseases are disorders of protein conformation that produce neurodegeneration in humans and animals. Studies of transgenic (Tg) mice indicate that a factor designated protein X is involved in the conversion of the normal cellular prion protein (PrPC) into the scrapie isoform (PrPSc); protein X appears to interact with PrPC but not with PrPSc. To search for PrPC binding proteins, we fused PrP with alkaline phosphatase (AP) to produce a soluble, secreted probe. PrP-AP was used to screen a lambdagt11 mouse brain cDNA library, and six clones were isolated. Four cDNAs are novel while two clones are fragments of Nrf2 (NF-E2 related factor 2) transcription factor and Aplp1 (amyloid precursor-like protein 1). The observation that PrP binds to a member of the APP (amyloid precursor protein) gene family is intriguing, in light of possible relevance to Alzheimer's disease. Four of the isolated clones are expressed preferentially in the mouse brain and encode a similar motif.

Alzheimer's disease amyloid precursor protein on the surface of cortical neurons in primary culture co-localizes with adhesion patch components

Immunofluorescence on primary dissociated rat neuronal cultures (cortical, hippocampal, and cerebellar) and organotypic hippocampal cultures was used to investigate the pattern of distribution of cell-surface amyloid protein (APP). Antibodies directed against the extracellular (N-terminal) portion of APP or against the entire molecule, but not against the C-terminal portion, revealed a striking segmental pattern of immunoreactivity along both axons and dendrites of all neuronal types tested. The pattern first developed between 24 and 48 h in culture. The segments showed co-localization with beta 1-integrin and talin immunoreactivities, but not with GAP-43 or clathrin, indicating that they may mark adhesion patches. Confocal laser microscopy supported a surface location for the APP responsible for the segmented pattern on neurites, as did the reduction of segmental immunoreactivity after exposure to mu-calpain or trypsin. It is conjectured that APP may have a role in cell-substratum interactions in the medium term, during such events as synaptic plasticity and neurite stability during extension.

The amyloid precursor protein of Alzheimer's disease is found on the surface of static but not activity motile portions of neurites

We have previously found that the amyloid precursor protein (APP) of Alzheimer's disease is present on the surface of rat cortical neurons in culture, in a segmental pattern which first becomes evident after 24 hours and is fully developed by five days. As APP has previously been reported to have a short half-life in neuronal cell lines, and has been shown to contain binding sites for various extracellular matrix components within its extracellular domain, we hypothesized that APP would be associated with portions of neurites undergoing rapid structural change, such as growth cones. To test this hypothesis, we observed selected neurons by video time-lapse differential interference microscopy on 24-hour-old primary rat neuronal cultures for up to 45 minutes, followed by fixation and immunocytochemistry to ascertain surface APP distribution on those same neurons. In contrast to our predictions, surface APP was not found on active portions of neurites, even if the activity produced no net translational movement. This result indicates that surface APP is actually associated with stable portions of neurites, a conclusion that tallies with other recent results showing that neuronal surface APP has a longer half-life than general cellular APP, and is associated with markers of adhesion patches, which themselves are relatively stable structures.

The possible place of cathepsins and cystatins in the puzzle of Alzheimer disease: a review

Lysosomal proteinases (cathepsins) and their endogenous inhibitors (cystatins) have been found to be closely associated with senile plaques, cerebrovascular amyloid deposits, and neurofibrillary tangles in Alzheimer disease (AD). Further, profound changes in the lysosomal system seem to be an early event in "at-risk" neurons of AD brains. There is an ongoing controversy as to whether lysosome-associated proteolytic mechanisms are causally related to the development and/or further progression of the disease. The present article deals with some arguments "pro" and "contra" an involvement of the endosomal/lysosomal pathway in amyloidogenesis as a cardinal process in AD. Other putative targets of acidic proteinases and their natural inhibitors in the pathogenesis of AD (such as formation of neurofibrillary tangles and regulation of apolipoprotein E) are also discussed.

The amyloid precursor protein of Alzheimer's disease in the reduction of copper(II) to copper(I)

The transition metal ion copper(II) has a critical role in chronic neurologic diseases. The amyloid precursor protein (APP) of Alzheimer's disease or a synthetic peptide representing its copper-binding site reduced bound copper(II) to copper(I). This copper ion-mediated redox reaction led to disulfide bond formation in APP, which indicated that free sulfhydryl groups of APP were involved. Neither superoxide nor hydrogen peroxide had an effect on the kinetics of copper(II) reduction. The reduction of copper(II) to copper(I) by APP involves an electron-transfer reaction and could enhance the production of hydroxyl radicals, which could then attack nearby sites. Thus, copper-mediated toxicity may contribute to neurodegeneration in Alzheimer's disease.

Regulation of amyloid protein precursor (APP) binding to collagen and mapping of the binding sites on APP and collagen type I

The specific binding of the amyloid precursor protein (APP) to extracellular matrix molecules suggests that APP regulates cell interactions and has a function as a cell adhesion molecule and/or substrate adhesion molecule. On the molecular level APP has binding sites for collagen, laminin, and glycosaminoglycans which is a characteristic feature of cell adhesion molecules. We have examined the interactions between the APP and collagen types I and IV and identified the corresponding binding sites on APP and collagen type I. We show that APP bound most efficiently to collagen type I in a concentration-dependent and specific manner in the native and heat-denatured states, suggesting an involvement of a contiguous binding site on collagen. This binding site was identified on the cyanogen bromide fragment alpha 1(I)CB6 of collagen type I, which also binds heparin. APP did not bind to collagen type I-heparin complexes, which suggests that there are overlapping binding sites for heparin and APP on collagen. We localized the site of APP that mediates collagen binding within residues 448-465 of APP695, which are encoded by the ubiquitously expressed APP exon 12, whereas the high affinity heparin binding site of APP is located in exon 9. Since a peptide encompassing this region binds to collagen type I and inhibits APP-collagen type I binding in nanomolar concentrations, this region may comprise the major part of the collagen type I binding site of APP. Moreover, our data also indicate that the collagen binding site is involved in APP-APP interaction that can be modulated by Zn(II) and heparin. Taken together, the data suggest that the regulation of APP binding to collagen type I by heparin occurs through the competitive binding of heparin and APP to collagen.

Regulation of APP expression, biogenesis and metabolism by extracellular matrix and cytokines

We have identified and characterized the ligand binding properties of the Alzheimer's disease (AD) beta A4 amyloid protein precursor (APP), mapped the APP ligand binding sites and analyzed the regulation of APP expression, biogenesis and metabolism by components of the extracellular matrix (ECM) and cytokines.