Characterization of the high affinity heparin binding site of the Alzheimer's disease beta A4 amyloid precursor protein (APP) and its enhancement by zinc(II)

To target Tau pathologies, we must embrace and reconstruct their complexities

The accumulation of hyperphosphorylated fibrillar Tau aggregates in the brain is one of the defining hallmarks of Tauopathy diseases, including Alzheimer's disease. However, the primary events or molecules responsible for initiation of the pathological Tau aggregation and spreading remain unknown. The discovery of heparin as an effective inducer of Tau aggregation in vitro was instrumental to enabling different lines of research into the role of Tau aggregation in the pathogenesis of Tauopathies. However, recent proteomics and cryogenic electron microscopy (cryo-EM) studies have revealed that heparin-induced Tau fibrils generated in vitro do not reproduce the biochemical and ultrastructural properties of disease-associated brain-derived Tau fibrils. These observations demand that we reassess our current approaches for investigating the mechanisms underpinning Tau aggregation and pathology formation. Our review article presents an up-to-date survey and analyses of 1) the evolution of our understanding of the interactions between Tau and heparin, 2) the various structural and mechanistic models of the heparin-induced Tau aggregation, 3) the similarities and differences between brain-derived and heparin-induced Tau fibrils; and 4) emerging concepts on the biochemical and structural determinants underpinning Tau pathological heterogeneity in Tauopathies. Our analyses identify specific knowledge gaps and call for 1) embracing the complexities of Tau pathologies; 2) reassessment of current approaches to investigate, model and reproduce pathological Tau aggregation as it occurs in the brain; 3) more research towards a better understanding of the naturally-occurring cofactor molecules that are associated with Tau brain pathology initiation and propagation; and 4) developing improved approaches for in vitro production of the Tau aggregates and fibrils that recapitulate and/or amplify the biochemical and structural complexity and diversity of pathological Tau in Tauopathies. This will result in better and more relevant tools, assays, and mechanistic models, which could significantly improve translational research and the development of drugs and antibodies that have higher chances for success in the clinic.

The molecular and cellular basis of copper dysregulation and its relationship with human pathologies

Copper (Cu) is an essential micronutrient required for the activity of redox-active enzymes involved in critical metabolic reactions, signaling pathways, and biological functions. Transporters and chaperones control Cu ion levels and bioavailability to ensure proper subcellular and systemic Cu distribution. Intensive research has focused on understanding how mammalian cells maintain Cu homeostasis, and how molecular signals coordinate Cu acquisition and storage within organs. In humans, mutations of genes that regulate Cu homeostasis or facilitate interactions with Cu ions lead to numerous pathologic conditions. Malfunctions of the Cu⁺ -transporting ATPases ATP7A and ATP7B cause Menkes disease and Wilson disease, respectively. Additionally, defects in the mitochondrial and cellular distributions and homeostasis of Cu lead to severe neurodegenerative conditions, mitochondrial myopathies, and metabolic diseases. Cu has a dual nature in carcinogenesis as a promotor of tumor growth and an inducer of redox stress in cancer cells. Cu also plays role in cancer treatment as a component of drugs and a regulator of drug sensitivity and uptake. In this review, we provide an overview of the current knowledge of Cu metabolism and transport and its relation to various human pathologies.

Deconstructing Alzheimer's Disease: How to Bridge the Gap between Experimental Models and the Human Pathology?

Discovered more than a century ago, Alzheimer's disease (AD) is not only still present in our societies but has also become the most common dementia, with 50 million people worldwide affected by the disease. This number is expected to double in the next generation, and no cure is currently available to slow down or stop the disease progression. Recently, some advances were made due to the approval of the aducanumab treatment by the American Food and Drug Administration. The etiology of this human-specific disease remains poorly understood, and the mechanisms of its development have not been completely clarified. Several hypotheses concerning the molecular mechanisms of AD have been proposed, but the existing studies focus primarily on the two main markers of the disease: the amyloid β peptides, whose aggregation in the brain generates amyloid plaques, and the abnormally phosphorylated tau proteins, which are responsible for neurofibrillary tangles. These protein aggregates induce neuroinflammation and neurodegeneration, which, in turn, lead to cognitive and behavioral deficits. The challenge is, therefore, to create models that best reproduce this pathology. This review aims at gathering the different existing AD models developed in vitro, in cellulo, and in vivo. Many models have already been set up, but it is necessary to identify the most relevant ones for our investigations. The purpose of the review is to help researchers to identify the most pertinent disease models, from the most often used to the most recently generated and from simple to complex, explaining their specificities and giving concrete examples.

Contribution of syndecans to cellular internalization and fibrillation of amyloid-β(1-42)

Intraneuronal accumulation of amyloid-β(1-42) (Aβ1-42) is one of the earliest signs of Alzheimer's disease (AD). Cell surface heparan sulfate proteoglycans (HSPGs) have profound influence on the cellular uptake of Aβ1-42 by mediating its attachment and subsequent internalization into the cells. Colocalization of amyloid plaques with members of the syndecan family of HSPGs, along with the increased expression of syndecan-3 and -4 have already been reported in postmortem AD brains. Considering the growing evidence on the involvement of syndecans in the pathogenesis of AD, we analyzed the contribution of syndecans to cellular uptake and fibrillation of Aβ1-42. Among syndecans, the neuron specific syndecan-3 isoform increased cellular uptake of Aβ1-42 the most. Kinetics of Aβ1-42 uptake also proved to be fairly different among SDC family members: syndecan-3 increased Aβ1-42 uptake from the earliest time points, while other syndecans facilitated Aβ1-42 internalization at a slower pace. Internalized Aβ1-42 colocalized with syndecans and flotillins, highlighting the role of lipid-rafts in syndecan-mediated uptake. Syndecan-3 and 4 also triggered fibrillation of Aβ1-42, further emphasizing the pathophysiological relevance of syndecans in plaque formation. Overall our data highlight syndecans, especially the neuron-specific syndecan-3 isoform, as important players in amyloid pathology and show that syndecans, regardless of cell type, facilitate key molecular events in neurodegeneration.

APLP1 is endoproteolytically cleaved by γ-secretase without previous ectodomain shedding

Regulated intramembrane proteolysis of the amyloid precursor protein (APP) and its homologs, the APP like proteins APLP1 and APLP2, is typically a two-step process, which is initiated by ectodomain-shedding of the substrates by α- or β-secretases. Growing evidence, however, indicates that the cleavage process for APLP1 is different than for APP. Here, we describe that full-length APLP1, but not APP or APLP2, is uniquely cleaved by γ-secretase without previous ectodomain shedding. The new fragment, termed sAPLP1γ, was exclusively associated with APLP1, not APP, APLP2. We provide an exact molecular analysis showing that sAPLP1γ was uniquely generated by γ-secretase from full-length APLP1. Mass spectrometry analysis showed that the sAPLP1γ fragment and the longest Aβ-like peptide share the C-terminus. This novel mechanism of γ-secretase action is consistent with an ϵ-cut based upon the nature of the reaction in APP. We further demonstrate that the APLP1 transmembrane sequence is the critical determinant for γ-shedding and release of full-length APLP1. Moreover, the APLP1 TMS is sufficient to convert larger type-I membrane proteins like APP into direct γ-secretase substrates. Taken together, the direct cleavage of APLP1 is a novel feature of the γ-secretase prompting a re-thinking of γ-secretase activity modulation as a therapeutic strategy for Alzheimer disease.

The Neuroprotective Properties of the Amyloid Precursor Protein Following Traumatic Brain Injury

Despite the significant health and economic burden that traumatic brain injury (TBI) places on society, the development of successful therapeutic agents have to date not translated into efficacious therapies in human clinical trials. Injury to the brain is ongoing after TBI, through a complex cascade of primary and secondary injury events, providing a valuable window of opportunity to help limit and prevent some of the severe consequences with a timely treatment. Of note, it has been suggested that novel treatments for TBI should be multifactorial in nature, mimicking the body's own endogenous repair response. Whilst research has historically focused on the role of the amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease, recent advances in trauma research have demonstrated that APP offers considerable neuroprotective properties following TBI, suggesting that APP is an ideal therapeutic candidate. Its acute upregulation following TBI has been shown to serve a beneficial role following trauma and has lead to significant advances in understanding the neuroprotective and neurotrophic functions of APP and its metabolites. Research has focused predominantly on the APP derivative sAPPα, which has consistently demonstrated neuroprotective and neurotrophic functions both in vitro and in vivo following various traumatic insults. Its neuroprotective activity has been narrowed down to a 15 amino acid sequence, and this region is linked to both heparan binding and growth-factor-like properties. It has been proposed that APP binds to heparan sulfate proteoglycans to exert its neuroprotective action. APP presents us with a novel therapeutic compound that could overcome many of the challenges that have stalled development of efficacious TBI treatments previously.

Amyloid precursor-like protein 1 (APLP1) exhibits stronger zinc-dependent neuronal adhesion than amyloid precursor protein and APLP2

The amyloid precursor protein (APP) and its paralogs, amyloid precursor-like protein 1 (APLP1) and APLP2, are metalloproteins with a putative role both in synaptogenesis and in maintaining synapse structure. Here, we studied the effect of zinc on membrane localization, adhesion, and secretase cleavage of APP, APLP1, and APLP2 in cell culture and rat neurons. For this, we employed live-cell microscopy techniques, a microcontact printing adhesion assay and ELISA for protein detection in cell culture supernatants. We report that zinc induces the multimerization of proteins of the amyloid precursor protein family and enriches them at cellular adhesion sites. Thus, zinc facilitates the formation of de novo APP and APLP1 containing adhesion complexes, whereas it does not have such influence on APLP2. Furthermore, zinc-binding prevented cleavage of APP and APLPs by extracellular secretases. In conclusion, the complexation of zinc modulates neuronal functions of APP and APLPs by (i) regulating formation of adhesion complexes, most prominently for APLP1, and (ii) by reducing the concentrations of neurotrophic soluble APP/APLP ectodomains. Earlier studies suggest a function of the amyloid precursor protein (APP) family proteins in neuronal adhesion. We report here that adhesive function of these proteins is tightly regulated by zinc, most prominently for amyloid precursor-like protein 1 (APLP1). Zinc-mediated APLP1 multimerization, which induced formation of new neuronal contacts and decreased APLP1 shedding. This suggests that APLP1 could function as a zinc receptor processing zinc signals to stabilized or new neuronal contacts.

Novel zinc-binding site in the E2 domain regulates amyloid precursor-like protein 1 (APLP1) oligomerization

The amyloid precursor protein (APP) and the APP-like proteins 1 and 2 (APLP1 and APLP2) are a family of multidomain transmembrane proteins possessing homo- and heterotypic contact sites in their ectodomains. We previously reported that divalent metal ions dictate the conformation of the extracellular APP E2 domain (Dahms, S. O., Könnig, I., Roeser, D., Gührs, K.-H., Mayer, M. C., Kaden, D., Multhaup, G., and Than, M. E. (2012) J. Mol. Biol. 416, 438-452), but unresolved is the nature and functional importance of metal ion binding to APLP1 and APLP2. We found here that zinc ions bound to APP and APLP1 E2 domains and mediated their oligomerization, whereas the APLP2 E2 domain interacted more weakly with zinc possessing a less surface-exposed zinc-binding site, and stayed monomeric. Copper ions bound to E2 domains of all three proteins. Fluorescence resonance energy transfer (FRET) analyses examined the effect of metal ion binding to APP and APLPs in the cellular context in real time. Zinc ions specifically induced APP and APLP1 oligomerization and forced APLP1 into multimeric clusters at the plasma membrane consistent with zinc concentrations in the blood and brain. The observed effects were mediated by a novel zinc-binding site within the APLP1 E2 domain as APLP1 deletion mutants revealed. Based upon its cellular localization and its dominant response to zinc ions, APLP1 is mainly affected by extracellular zinc among the APP family proteins. We conclude that zinc binding and APP/APLP oligomerization are intimately linked, and we propose that this represents a novel mechanism for regulating APP/APLP protein function at the molecular level.

Combined deletions of amyloid precursor protein and amyloid precursor-like protein 2 reveal different effects on mouse brain metal homeostasis

Alterations to the expression of the Amyloid Precursor Protein (APP) and its paralogue Amyloid Precursor-Like Protein 2 (APLP2) affect metal homeostasis in vitro and in vivo. Analysis of the in vivo effects of the APP and APLP2 knockouts on metal homeostasis has been restricted to APP and APLP2 single knockout mice, and up to12 month old animals. To define the redundancy and inter-relationship between the APP and APLP2 genes as regulators of metal homeostasis, and how this is influenced by aging, we investigated copper, iron, zinc and manganese levels in APP and APLP2 single knockout mice as well as homozygous:hemizygous knockout mice at 3, 12 and 18 plus months of age. These studies identified age and genotype dependent changes in metal levels, and established differences in the relative roles played by APP and APLP2 in modulating metal homeostasis.

The amyloid precursor protein and its homologues: structural and functional aspects of native and pathogenic oligomerization

Over the last 25 years, remarkable progress has been made not only in identifying key molecules of Alzheimer's disease but also in understanding their meaning in the pathogenic state. One hallmark of Alzheimer pathology is the amyloid plaque. A major component of the extracellular deposit is the amyloid-β (Aβ) peptide which is generated from its larger precursor molecule, i.e., the amyloid precursor protein (APP) by consecutive cleavages. Processing is exerted by two enzymes, i.e., the β-secretase and the γ-secretase. We and others have found that the self-association of the amyloid peptide and the dimerization and oligomerization of these proteins is a key factor under native and pathogenic conditions. In particular, the Aβ homodimer represents a nidus for plaque formation and a well defined therapeutic target. Further, dimerization of the APP was reported to increase generation of toxic Aβ whereas heterodimerization with its homologues amyloid precursor like proteins (APLP1 and APLP2) decreased Aβ formation. This review mainly focuses on structural features of the homophilic and heterophilic interactions among APP family proteins. The proposed contact sites are described and the consequences of protein dimerization on their functions and in the pathogenesis of Alzheimer's disease are discussed.

Novel drug targets based on metallobiology of Alzheimer's disease

IMPORTANCE OF THE FIELD

Increased localization of Zn, Fe, Cu and Al within the senile plaques (SP) exacerbates amyloid beta (Aβ)-mediated oxidative damage, and acts as catalyst for Aβ aggregation in Alzheimer's disease (AD). Thus, disruption of aberrant metal-peptide interactions via chelation therapy holds considerable promise as a rational therapeutic strategy against Alzheimer's amyloid pathogenesis.

AREAS COVERED IN THIS REVIEW

The complexities of metal-induced genesis of SP are reviewed. The recent advances in the molecular mechanism of action of metal chelating agents are discussed with critical assessment of their potential to become drugs.

WHAT THE READER WILL GAIN

Taking into consideration the interaction of metals with the metal-responsive elements on the Alzheimer's amyloid precursor protein (APP), readers will gain understanding of several points to bear in mind when developing a screening campaign for AD-therapeutics.

TAKE HOME MESSAGE

A functional iron-responsive element (IRE) RNA stem loop in the 5' untranslated region (UTR) of the APP transcript regulates neural APP translation. Desferrioxamine, clioquinol, tetrathiolmolybdate, dimercaptopropanol, VK-28, and natural antioxidants, such as curcumin and ginko biloba need critical evaluation as AD therapeutics. There is a necessity for novel screens (related to metallobiology) to identify therapeutics effective in AD.

Is BACE1 a suitable therapeutic target for the treatment of Alzheimer's disease? Current strategies and future directions

Alzheimer's disease (AD) is characterized by the extracellular deposition of the β-amyloid protein (Aβ). Aβ is a fragment of a much larger precursor protein, the amyloid precursor protein (APP). Sequential proteolytic cleavage of APP by β-secretase and γ-secretase liberates Aβ from APP. The aspartyl protease BACE1 (β-site APP-cleaving enzyme 1) catalyses the rate-limiting step in the production of Aβ, and as such it is considered to be a major target for drug development in Alzheimer's disease. However, the development of a BACE1 inhibitor therapy is problematic for two reasons. First, BACE1 has been found to have important physiological roles. Therefore, inhibition of the enzyme could have toxic consequences. Second, the active site of BACE1 is relatively large, and many of the bulky compounds that are needed to inhibit BACE1 activity are unlikely to cross the blood-brain barrier. This review focuses on the structure BACE1, current therapeutic strategies based on developing active-site inhibitors, and new approaches to therapy involving targeting the expression or post-translational regulation of BACE1.

Surface plasmon resonance for the analysis of beta-amyloid interactions and fibril formation in Alzheimer's disease research

Alzheimer's disease (AD) is a neurodegenerative disorder characterised by the accumulation of amyloid deposits, the major component of which is a 4 kDa polypeptide known as beta-amyloid protein (ABeta). Identifying the mechanism underlying the formation of Abeta and the pathways that lead to its toxicity is crucial to understanding the mechanism of AD and addressing the urgent need for new and effective treatments for AD. The accumulation of ABeta is the result of a complex interplay between genetic and environmental factors that affect the generation, clearance and aggregation of the peptide. Because of its propensity to aggregate, ABeta builds up in the brain and assembles into amyloid fibrils, ultimately creating amyloid plaques (APs) and cerebral amyloid angiopathy (CAA). Abeta has been shown to interact with a number of intracellular and extracellular molecules, but the relative contribution of these interactions to the toxicity of Abeta is not well understood. A critical step in characterising the importance of these interactions is the ability to measure both the affinity and kinetics of these interactions. Surface plasmon resonance (SPR) spectroscopy has become a widely used technique to study molecular interactions such as antibody-antigen, DNA-DNA, DNA-protein, protein-protein, receptor-ligand and peptide- and protein-membrane interactions. This article reviews the application of SPR to the study of the molecular interactions associated with AD and how this information enhances our molecular understanding of ABeta -mediated toxicity.

Amyloid-beta metal interaction and metal chelation

Alzheimer's disease (AD) is associated with the abnormal aggregation of amyloid-beta (Abeta) protein. Abeta and its precursor protein (APP) interact with metal ions such as zinc, copper and iron. Evidence shows that these metals play a role in the precipitation and cytotoxicity of Abeta. Despite recent advances in AD research, there is a lack of therapeutic agents to hinder the apparent aggregation and toxicity of Abeta. Recent studies show that drugs with metal chelating properties could produce a significant reversal of amyloid-beta plaque deposition in vitro and in vivo. Here we discuss the interaction of Abeta with metals, metal dyshomeostasis in the CNS of patients with AD, and the potential therapeutic effects of metal chelators.

The X-ray structure of an antiparallel dimer of the human amyloid precursor protein E2 domain

Amyloid beta-peptide, which forms neuronal and vascular amyloid deposits in Alzheimer's disease, is derived from an integral membrane protein precursor. The biological function of the precursor is currently unclear. Here we describe the X-ray structure of E2, the largest of the three conserved domains of the precursor. The structure of E2 consists of two coiled-coil substructures connected through a continuous helix and bears an unexpected resemblance to the spectrin family of protein structures. E2 can reversibly dimerize in the solution, and the dimerization occurs along the longest dimension of the molecule in an antiparallel orientation, which enables the N-terminal substructure of one monomer to pack against the C-terminal substructure of a second monomer. Heparan sulfate proteoglycans, the putative ligand for the precursor present in extracellular matrix, bind to E2 at a conserved and positively charged site near the dimer interface.

Heparan sulfate regulates amyloid precursor protein processing by BACE1, the Alzheimer's beta-secretase

Cleavage of amyloid precursor protein (APP) by the Alzheimer's β-secretase (BACE1) is a key step in generating amyloid β-peptide, the main component of amyloid plaques. Here we report evidence that heparan sulfate (HS) interacts with β-site APP-cleaving enzyme (BACE) 1 and regulates its cleavage of APP. We show that HS and heparin interact directly with BACE1 and inhibit in vitro processing of peptide and APP substrates. Inhibitory activity is dependent on saccharide size and specific structural characteristics, and the mechanism of action involves blocking access of substrate to the active site. In cellular assays, HS specifically inhibits BACE1 cleavage of APP but not alternative cleavage by α-secretase. Endogenous HS immunoprecipitates with BACE1 and colocalizes with BACE1 in the Golgi complex and at the cell surface, two of its putative sites of action. Furthermore, inhibition of cellular HS synthesis results in enhanced BACE1 activity. Our findings identify HS as a natural regulator of BACE1 and suggest a novel mechanism for control of APP processing.

Biogenesis and metabolism of Alzheimer's disease Abeta amyloid peptides

Biochemical and genetic evidence indicates the balance of biogenesis/clearance of Abeta amyloid peptides is altered in Alzheimer's disease. Abeta is derived, by two sequential cleavages, from the receptor-like amyloid precursor protein (APP). The proteases involved are beta-secretase, identified as the novel aspartyl protease BACE, and gamma-secretase, a multimeric complex containing the presenilins (PS). Gamma-secretase can release either Abeta40 or the more aggregating and cytotoxic Abeta42. Secreted Abeta peptides become either degraded by the metalloproteases insulin-degrading enzyme (IDE) and neprilysin or metabolized through receptor uptake mediated by apolipoprotein E. Therapeutic approaches based on secretase inhibition or amyloid clearance are currently under development.

Identification of the heparan sulfate binding sites in the cellular prion protein

Data from cell culture and animal models of prion disease support the separate involvement of both heparan sulfate proteoglycans and copper (II) ions in prion (PrP) metabolism. Though direct interactions between prion protein and heparin have been recorded, little is known of the structural features implicit in this interaction or of the involvement of copper (II) ions. Using biosensor and enzyme-linked immunosorbent assay methodology we report direct heparin and heparan sulfate-binding activity in recombinant cellular prion protein (PrP(c)). We also demonstrate that the interaction of recombinant PrP(c) with heparin is weakened in the presence of Cu(II) ions and is particularly sensitive to competition with dextran sulfate. Competitive inhibition experiments with chemically modified heparins also indicate that 2-O-sulfate groups (but not 6-O-sulfate groups) are essential for heparin recognition. We have also identified three regions of the prion protein capable of independent binding to heparin and heparan sulfate: residues 23-52, 53-93, and 110-128. Interestingly, the interaction of an octapeptide-spanning peptide motif amino acids 53-93 with heparin is enhanced by Cu(II) ions. Significantly, a peptide of this sequence is able to inhibit the binding of full-length prion molecule to heparin, suggesting a direct role in heparin recognition within the intact protein. The collective data suggest a complex interaction between prion protein and heparin/heparan sulfate and has implications for the cellular and pathological functions of prion proteins.

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.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Expression of mutant amyloid precursor proteins decreases adhesion and delays differentiation of Hep-1 cells

The amyloid precursor protein (APP) is a type I integral membrane protein and is processed to generate several intra-cellular and secreted fragments. The physiological role of APP and its processed fragments is unclear. Several mutations have been discovered in APP, which are causative of early-onset, familial, neurological disease, including Alzheimer's disease (FAD). These mutations alter the processing of APP and lead to excess production and extra-cellular deposition of A-beta peptide (Abeta). We have examined the role of APP in a cell culture model of endothelial cell function. The endothelial cell line, Hep-1, was stably transfected with wild-type (wt) and FAD mutant forms of APP (mAPP). Secretion of sAPPalpha was reduced in cell lines over-expressing mAPP when these cells were grown on several different substrates. Levels of secreted Abeta were increased as measured by ELISA in the mutant cell lines. Cell adhesion to laminin-, fibronectin-, collagen I-, and collagen IV-coated culture flasks was reduced in all mAPP-expressing cell lines, while in lines over-expressing wt-APP, adhesiveness was slightly increased. Cell lines over-expressing mAPP differentiated more slowly into capillary network-like structures on Matrigel than those expressing wt-APP. No differences were detected among all cell lines in a migration/invasion assay. The results suggest that APP may have a role in cell adhesiveness and maturation of endothelial cells into capillary-like networks. The reduction in adhesion and differentiation in mutant cell lines may be due to reduced amounts of sAPPalpha released into the culture media or toxic effects of increased extracellular Abeta.

Zinc-dependent activation of the plasma kinin-forming cascade by aggregated beta amyloid protein

Beta Amyloid proteins (Abeta) of 38, 40, and 42 amino acids long were assessed for their ability to activate the plasma kinin-forming cascade in vitro. Incubation with a mixture of Factor XII (Hageman Factor), prekallikrein, and high-molecular-weight kininogen (HK) led to conversion of prekallikrein to kallikrein that was dependent on zinc ion. No activation occurred if Factor XII was omitted. There was rapid generation of bradykinin equal to the molar HK input indicating complete cleavage. Incubation of aggregated Abeta with diluted human plasma also led to prekallikrein activation and HK cleavage. Activation of the cascade by Abeta (1-38) was dependent upon its preincubation time in buffer, suggesting that aggregation of Abeta is required, and studies with Abeta (1-40) revealed time-dependent aggregation by microscopy and augmented zinc-dependent binding of both Factor XII and HK to aggregated Abeta. These data demonstrate that aggregated Abeta can bind and activate proenzymes of the plasma kinin-forming cascade in a zinc-dependent reaction to release bradykinin and is of sufficient potency to do so at physiologic concentrations of each protein and in the presence of naturally occurring protease inhibitors.

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.

Alzheimer's beta-amyloid peptide: affinity for metal chelates

Alzheimer's amyloid peptide, A beta(1-42) and its fragments, A beta(1-28) and A beta(1-16), were chromatographed on IDA-M(II) columns (M: Cu2+, Ni2+ and Zn2+). The retention of A beta(1-42) and its fragments on IDA-Cu(II) could not be reversed in decreasing a gradient of pH, from 7.0 to 4.0. All A beta peptides were recovered from IDA-Ni(II) columns in a decreasing pH gradient from 7.0 to 4.0, within the pH range from 5.6 to 5.1. A beta(1-42) peptide was strongly retained on IDA-Zn(II) at pH 4.0, but its A beta(1-28) and A beta(1-16) were only transiently retained on IDA-Zn(II) columns when applied at pH 6.1. We submit that histidine clusters, residing both in the Alzheimer's beta-amyloid peptide and in most of the APP/APLP superfamily of proteins, constitute high-affinity binding sites for immobilized metal chelates.

Increased amount of zinc in the hippocampus and amygdala of Alzheimer's diseased brains: a proton-induced X-ray emission spectroscopic analysis of cryostat sections from autopsy material

Zinc has been implicated as a contributing cause of the neuropathology of Alzheimer's disease (AD), but consensus on the zinc content of AD brains has not yet been established. In the present study, multi-element PIXE was used to measure zinc in cryostat sections of brain tissue from AD patients and from normal control subjects. Compared to their age-matched controls, the AD patients showed an increase in zinc in the hippocampal and amygdalar regions. The instrumental PIXE assays do not show whether the zinc changes are due to altered zinc in the boutons of Zinc-ENriched (ZEN) neurons, i.e., zinc ions in synaptic vesicles, or to changes in the amount of zinc tightly bound to macromolecules. We hypothesise that the increased zinc level is caused by an increase in the amount of ZEN terminals. Such an increase could be the result of a sprout of ZEN terminals in diseased areas of the brain.

Expression of human amyloid precursor protein ectodomains in Pichia pastoris: analysis of culture conditions, purification, and characterization

We have examined the use of the yeast Pichia pastoris for expression of the human amyloid precursor protein (APP). The ectodomains of the isoforms APP695, APP751, and APP770 were expressed in both P. pastoris protease-deficient strain SMD1163 and wild-type strain GS115, using the secretion vector pHIL-S1. Expression of recombinant APP in each of these strains produced intact recombinant protein, together with a small number of breakdown products. The levels of these breakdown products were not significantly altered by expression in the protease-deficient strain compared with wild-type GS115. The effects of induction time and medium composition on recombinant APP stability were also examined. After optimization of expression and culture conditions, baffled shaker flask cultures of clones selected for high expression routinely yielded 13-24 mg/liter recombinant protein following a two-step purification procedure. The recombinant isoforms possessed the heparin binding, metal binding, and Kunitz-type protease inhibitor properties of human brain-derived APP. These data indicate that P. pastoris is an appropriate laboratory-scale expression system for production of sufficient quantities of recombinant APP for use in biological studies.

Analysis of cell-adhesion molecule interactions using surface plasmon resonance

The molecular interactions that mediate cell adhesion are often very weak, making them difficult to study. However, real-time optical biosensors based on surface plasmon resonance (SPR) are greatly facilitating the biochemical analysis of these interactions. Analysis of the T cell surface molecule CD2 has shown that adhesion molecules can interact with very low affinities (Kd approximately 100 microM) and dissociate with half lives of approximately 0.2 seconds or less. SPR has been combined with site-directed mutagenesis to delineate the interacting surfaces of CD2 and its ligand, CD48, quantify the contribution of individual residues to the binding energy, and determine the binding orientation of these surfaces in the CD2-CD48 complex. Furthermore, SPR has been combined with in situ modification of carbohydrates on purified glycoproteins to analyze the binding specificity of lectins such as CD22. Researchers have discovered the potential pitfalls of SPR, which can lead to inaccurate affinity and kinetic measurements.