Distinct sites of intracellular production for Alzheimer's disease A beta40/42 amyloid peptides

Protein kinase C-dependent alpha-secretase competes with beta-secretase for cleavage of amyloid-beta precursor protein in the trans-golgi network

The release of amyloidogenic amyloid-beta peptide (Abeta) from amyloid-beta precursor protein (APP) requires cleavage by beta- and gamma-secretases. In contrast, alpha-secretase cleaves APP within the Abeta sequence and precludes amyloidogenesis. Regulated and unregulated alpha-secretase activities have been reported, and the fraction of cellular alpha-secretase activity regulated by protein kinase C (PKC) has been attributed to the ADAM (a disintegrin and metalloprotease) family members TACE and ADAM-10. Although unregulated alpha-secretase cleavage of APP has been shown to occur at the cell surface, we sought to identify the intracellular site of PKC-regulated alpha-secretase APP cleavage. To accomplish this, we measured levels of secreted ectodomains and C-terminal fragments of APP generated by alpha-secretase (sAPPalpha) (C83) versus beta-secretase (sAPPbeta) (C99) and secreted Abeta in cultured cells treated with PKC and inhibitors of TACE/ADAM-10. We found that PKC stimulation increased sAPPalpha but decreased sAPPbeta levels by altering the competition between alpha- versus beta-secretase for APP within the same organelle rather than by perturbing APP trafficking. Moreover, data implicating the trans-Golgi network (TGN) as a major site for beta-secretase activity prompted us to hypothesize that PKC-regulated alpha-secretase(s) also reside in this organelle. To test this hypothesis, we performed studies demonstrating proteolytically mature TACE intracellularly, and we also showed that regulated alpha-secretase APP cleavage occurs in the TGN using an APP mutant construct targeted specifically to the TGN. By detecting regulated alpha-secretase APP cleavage in the TGN by TACE/ADAM-10, we reveal ADAM activity in a novel location. Finally, the competition between TACE/ADAM-10 and beta-secretase for intracellular APP cleavage may represent a novel target for the discovery of new therapeutic agents to treat Alzheimer's disease.

The role of presenilin-1 in the gamma-secretase cleavage of the amyloid precursor protein of Alzheimer's disease

Presenilin-1 (PS1) is required for the release of the intracellular domain of Notch from the plasma membrane as well as for the cleavage of the amyloid precursor protein (APP) at the gamma-secretase cleavage site. It remains to be demonstrated whether PS1 acts by facilitating the activity of the protease concerned or is the protease itself. PS1 could have a gamma-secretase activity by itself or could traffic APP and Notch to the appropriate cellular compartment for processing. Human APP 695 and PS1 were coexpressed in Sf9 insect cells, in which endogenous gamma-secretase activity is not detected. In baculovirus-infected Sf9 cells, PS1 undergoes endoproteolysis and interacts with APP. However, PS1 does not cleave APP in Sf9 cells. In CHO cells, endocytosis of APP is required for Abeta secretion. Deletion of the cytoplasmic sequence of APP (APPDeltaC) inhibits both APP endocytosis and Abeta production. When APPDeltaC and PS1 are coexpressed in CHO cells, Abeta is secreted without endocytosis of APP. Taken together, these results conclusively show that, although PS1 does not cleave APP in Sf9 cells, PS1 allows the secretion of Abeta without endocytosis of APP by CHO cells.

Guinea-pig primary cell cultures provide a model to study expression and amyloidogenic processing of endogenous amyloid precursor protein

Until now guinea-pigs have been rarely used to investigate formation and deposition of Alzheimer's disease-associated amyloid beta peptides despite the sequence identity of human and guinea-pig amyloid beta peptides being known, and the overall similarity of human and guinea-pig amyloid precursor protein. We now describe a primary cell culture system of mixed fetal guinea-pig brain cells, which we have applied to characterize endogenous amyloid precursor protein processing and amyloid beta formation. These cell cultures were established at embryonic day 24 of guinea-pigs after comparison of selected stages of guinea-pig ontogenetic development with the known ontogeny of rats, and were characterized by immunocytochemical detection of neuronal and glial marker proteins. Amyloid precursor protein expression, processing and amyloid beta formation increased in parallel with cellular maturation during cultivation and reached a stable phase after approximately 14 days in vitro therefore providing a suitable time for analysis. Aged cultures display strong neuronal amyloid precursor protein immunoreactivity and an altered profile of amyloid precursor protein isoform messenger RNA expression due to glial proliferation as single neurons were shown to retain their typical pattern of amyloid precursor protein expression. We show that amyloid precursor protein in guinea-pig cells is processed by different protease activities which most likely represent alpha- and beta-secretase, leading to the generation of soluble amyloid precursor protein derivatives. Furthermore, endogenous amyloid precursor protein processing leads to production of substantial amounts of amyloid beta-peptides which accumulate in conditioned culture medium. Amyloid beta was readily detectable by western blot analysis and was shown to consist of approximately 80-90% amyloid beta(1-40). We suggest that primary guinea-pig cell cultures provide a valuable tool in amyloid research that resembles amyloid precursor protein processing under physiological concentrations and, therefore, the situation in humans more closely than current rodent models. It should be especially useful in screening experiments for secretase inhibiting compounds.

A novel carboxypeptidase B that processes native beta-amyloid precursor protein is present in human hippocampus

The processing of beta-amyloid precursor protein (APP) and generation of beta-amyloid (Abeta) are associated with the pathophysiology of Alzheimer's disease (AD). As the proteases responsible for the process in the human brain have yet to be clarified, we have searched for activities capable of cleaving native brain APP in the human hippocampus. A 40-kDa protein with proteolytic activity that degrades native brain APP in vitro was purified and characterized; molecular analysis identified it as a novel protease belonging to the carboxypeptidase B (CPB) family. PC12 cells overexpressing the cDNA encoding this protease generate a major 12-kDa beta-amyloid-bearing peptide in cytosol, a peptide which has also been detected in a cell-free system using purified brain APP as substrate. Although the protease is homologous to plasma CPB synthesized in liver, it has specific domains such as C-terminal 14 amino acid residues. Western analysis, cDNA-cloning process and Northern analysis suggested a brain-specific expression of this protease. An immunohistochemical study showed that the protease is expressed in various neuronal perikarya, including those of pyramidal neurons of the hippocampus and ependymal-choroid plexus cells, and in a portion of the microglia of normal brains. In brains of patients with sporadic AD, there is decreased neuronal expression of the protease, and clusters of microglia with protease immunoreactivity associated with its extracellular deposition are detected. These findings suggest that brain CPB has a physiological function in APP processing and may have significance in AD pathophysiology.

Intraneuronal Abeta42 accumulation in human brain

Alzheimer's disease (AD) is characterized by the deposition of senile plaques (SPs) and neurofibrillary tangles (NFTs) in vulnerable brain regions. SPs are composed of aggregated beta-amyloid (Abeta) 40/42(43) peptides. Evidence implicates a central role for Abeta in the pathophysiology of AD. Mutations in betaAPP and presenilin 1 (PS1) lead to elevated secretion of Abeta, especially the more amyloidogenic Abeta42. Immunohistochemical studies have also emphasized the importance of Abeta42 in initiating plaque pathology. Cell biological studies have demonstrated that Abeta is generated intracellularly. Recently, endogenous Abeta42 staining was demonstrated within cultured neurons by confocal immunofluorescence microscopy and within neurons of PS1 mutant transgenic mice. A central question about the role of Abeta in disease concerns whether extracellular Abeta deposition or intracellular Abeta accumulation initiates the disease process. Here we report that human neurons in AD-vulnerable brain regions specifically accumulate gamma-cleaved Abeta42 and suggest that this intraneuronal Abeta42 immunoreactivity appears to precede both NFT and Abeta plaque deposition. This study suggests that intracellular Abeta42 accumulation is an early event in neuronal dysfunction and that preventing intraneuronal Abeta42 aggregation may be an important therapeutic direction for the treatment of AD.

Stress-associated endoplasmic reticulum protein 1 (SERP1)/Ribosome-associated membrane protein 4 (RAMP4) stabilizes membrane proteins during stress and facilitates subsequent glycosylation

Application of differential display to cultured rat astrocytes subjected to hypoxia allowed cloning of a novel cDNA, termed stress-associated endoplasmic reticulum protein 1 (SERP1). Expression of SERP1 was enhanced in vitro by hypoxia and/or reoxygenation or other forms of stress, causing accumulation of unfolded proteins in endoplasmic reticulum (ER) stress, and in vivo by middle cerebral artery occlusion in rats. The SERP1 cDNA encodes a 66-amino acid polypeptide which was found to be identical to ribosome-associated membrane protein 4 (RAMP4) and bearing 29% identity to yeast suppressor of SecY 6 protein (YSY6p), suggesting participation in pathways controlling membrane protein biogenesis at ER. In cultured 293 cells subjected to ER stress, overexpression of SERP1/RAMP4 suppressed aggregation and/or degradation of newly synthesized integral membrane proteins, and subsequently, facilitated their glycosylation when the stress was removed. SERP1/RAMP4 interacted with Sec61alpha and Sec61beta, which are subunits of translocon, and a molecular chaperon calnexin. Furthermore, Sec61alpha and Sec61beta, but not SERP1/RAMP4, were found to associate with newly synthesized integral membrane proteins under stress. These results suggest that stabilization of membrane proteins in response to stress involves the concerted action of a rescue unit in the ER membrane comprised of SERP1/RAMP4, other components of translocon, and molecular chaperons in ER.

Identification of a novel aspartic protease (Asp 2) as beta-secretase

The Alzheimer's disease beta-amyloid peptide (Abeta) is produced by excision from the type 1 integral membrane glycoprotein amyloid precursor protein (APP) by the sequential actions of beta- and then gamma-secretases. Here we report that Asp 2, a novel transmembrane aspartic protease, has the key activities expected of beta-secretase. Transient expression of Asp 2 in cells expressing APP causes an increase in the secretion of the N-terminal fragment of APP and an increase in the cell-associated C-terminal beta-secretase APP fragment. Mutation of either of the putative catalytic aspartyl residues in Asp 2 abrogates the production of the fragments characteristic of cleavage at the beta-secretase site. The enzyme is present in normal and Alzheimer's disease (AD) brain and is also found in cell lines known to produce Abeta. Asp 2 localizes to the Golgi/endoplasmic reticulum in transfected cells and shows clear colocalization with APP in cells stably expressing the 751-amino-acid isoform of APP.

Manipulating the amyloid-beta aggregation pathway with chemical chaperones

Amyloid-beta (Abeta) assembly into fibrillar structures is a defining characteristic of Alzheimer's disease that is initiated by a conformational transition from random coil to beta-sheet and a nucleation-dependent aggregation process. We have investigated the role of organic osmolytes as chemical chaperones in the amyloid pathway using glycerol to mimic the effects of naturally occurring molecules. Osmolytes such as the naturally occurring trimethylamine N-oxide and glycerol correct folding defects by preferentially hydrating partially denatured proteins and entropically stabilize native conformations and polymeric states. Trimethylamine N-oxide and glycerol were found to rapidly accelerate the Abeta random coil-to-beta-sheet conformational change necessary for fiber formation. This was accompanied by an immediate conversion of amorphous unstructured aggregates into uniform globular and possibly nucleating structures. Osmolyte-facilitated changes in Abeta hydration also affected the final stages of amyloid formation and mediated transition from the protofibrils to mature fibers that are observed in vivo. These findings suggest that hydration forces can be used to control fibril assembly and may have implications for the accumulation of Abeta within intracellular compartments such as the endoplasmic reticulum and in vitro modeling of the amyloid pathway.

Expression of beta-amyloid precursor protein-CD3gamma chimeras to demonstrate the selective generation of amyloid beta(1-40) and amyloid beta(1-42) peptides within secretory and endocytic compartments

Amyloid beta-protein (Abeta) is the main constituent of amyloid fibrils found in senile plaques and cerebral vessels in Alzheimer's disease (AD) and is derived by proteolysis from the beta-amyloid precursor protein (APP). We have analyzed the amyloidogenic processing of APP using chimeric proteins stably transfected in Chinese hamster ovary cells. The extracellular and transmembrane domains of APP were fused to the cytoplasmic region derived from the CD3 gamma chain of the T cell antigen receptor (CD3gamma). CD3gamma contains an endoplasmic reticulum (ER) retention motif (RKK), in the absence of which the protein is targeted to lysosomes without going through the cell surface (Letourneur, F., and Klausner, R.D. (1992) Cell 69, 1143-1157). We used the wild-type sequence of CD3gamma to create an APP chimera predicted to remain in the ER (gammaAPP(ER)). Deletion of the RKK motif at the C terminus directed the protein directly to the lysosomes (gammaAPP(LYS)). A third chimera was created by removing both lysosomal targeting signals in addition to RKK (gammaAPP(DeltaDelta)). This last construct does not contain known targeting signals and consequently accumulates at the cell surface. We show by immunofluorescence and by biochemical methods that all three APP chimeras localize to the predicted compartments within the cell, thus providing a useful model to study the processing of APP. We found that Abeta(1-40) is generated in the early secretory and endocytic pathways, whereas Abeta(1-42) is made mainly in the secretory pathway. More importantly, we provide evidence that, unlike in neuronal models, both ER/intermediate compartment- and endocytic-derived Abeta forms can enter the secretable pool. Finally, we directly demonstrate that lysosomal processing is not involved in the generation or secretion of either Abeta(1-40) or Abeta(1-42).

In vitro processing of amyloid precursor protein by cathepsin D

The formation of beta A4 amyloid in the brains of individuals with Alzheimer's disease requires the proteolytic cleavage of amyloid precursor protein. Several lines of evidence suggest that cathepsin D, the major lysosomal/endosomal aspartic protease, may be involved in this process. In this work, we used a sensitive in vitro method of detection to investigate the role of cathepsin D in the proteolytic processing of a 100-amino acid C-terminal fragment (C100) inclusive of beta A4 and cytoplasmic domain of APP. Digestion of C100 with cathepsin D resulted in cleavage at the amyloidogenic gamma-cleavage sites. This occurred preferentially at Thr43-Val44 and at Ala42-Thr43, generating full length beta A4 43 and beta A4 42 amyloid peptides, respectively. Cathepsin D was also found to cleave the substrate at the following nonamyloidogenic sites; Leu34-Met35, Thr48-Leu49 and Leu49-Val50. A high concentration of cathepsin D resulted in cleavage also occurring at Phe19-Phe20, Phe20-Ala21 and Phe93-Phe94 of the C100, suggesting that these sites are somewhat less sensitive to the action of cathepsin D. Digestion of C100 using different solublizing agents indicated that the cleavage of C100 by cathepsin D is greatly influenced by the structural integrity of the substrate. However, our results suggest that cathepsin D could generate the pathogenic beta A4 amyloid peptides from its precursor in vitro, which may indicate a role in the amyloidogenesis of Alzheimer's disease.

The presenilins in Alzheimer's disease--proteolysis holds the key

Alzheimer's disease (AD) research has shown that patients with an inherited form of the disease carry mutations in the presenilin proteins or the amyloid precursor protein (APP). These disease-linked mutations result in increased production of the longer form of amyloid-beta (the primary component of the amyloid deposits found in AD brains). However, it is not clear how the presenilins contribute to this increase. New findings now show that the presenilins affect APP processing through their effects on gamma-secretase, an enzyme that cleaves APP. Also, it is known that the presenilins are involved in the cleavage of the Notch receptor, hinting that they either directly regulate gamma-secretase activity or themselves are protease enzymes. These findings suggest that the presenilins may prove to be valuable molecular targets for the development of drugs to combat AD.

Common binding sites for beta-amyloid fibrils and fibroblast growth factor-2 in heparan sulfate from human cerebral cortex

Heparan sulfate found in the cerebral plaques of Alzheimer's disease binds to beta-amyloid (Abeta) fibrils. This interaction has been proposed to enhance fibril deposition and mediate Abeta-induced glia activation and neurotoxicity. On the other hand, heparan sulfate augments signaling of fibroblast growth factor-2 (FGF-2), a neuroprotective factor that antagonizes the neurotoxic effects of Abeta. We defined structures in heparan sulfate from human cerebral cortex that bind Abeta fibrils. The minimal binding site is found in N-sulfated hexasaccharide domains and contains critical 2-O-sulfated iduronic acid residues. By contrast, binding of Abeta monomers requires, in addition, 6-O-sulfate groups on glucosamine residues. The binding specificity of fibrillar Abeta is shared by FGF-2, and we here show that cerebral heparan sulfate domains selected for binding to Abeta-(1-40) fibrils bind also to FGF-2. These data suggest that neurotoxic and neuroprotective signals may converge by competing for the same binding sites on the heparan sulfate chain.

Presenilin 1 controls gamma-secretase processing of amyloid precursor protein in pre-golgi compartments of hippocampal neurons

Mutations of presenilin 1 (PS1) causing Alzheimer's disease selectively increase the secretion of the amyloidogenic betaA4(1-42), whereas knocking out the gene results in decreased production of both betaA4(1-40) and (1-42) amyloid peptides (De Strooper et al. 1998). Therefore, PS1 function is closely linked to the gamma-secretase processing of the amyloid precursor protein (APP). Given the ongoing controversy on the subcellular localization of PS1, it remains unclear at what level of the secretory and endocytic pathways PS1 exerts its activity on APP and on the APP carboxy-terminal fragments that are the direct substrates for gamma-secretase. Therefore, we have reinvestigated the subcellular localization of endogenously expressed PS1 in neurons in vitro and in vivo using confocal microscopy and fine-tuned subcellular fractionation. We show that uncleaved PS1 holoprotein is recovered in the nuclear envelope fraction, whereas the cleaved PS fragments are found mainly in post-ER membranes including the intermediate compartment (IC). PS1 is concentrated in discrete sec23p- and p58/ERGIC-53-positive patches, suggesting its localization in subdomains involved in ER export. PS1 is not found to significant amounts beyond the cis-Golgi. Surprisingly, we found that APP carboxy-terminal fragments also coenrich in the pre-Golgi membrane fractions, consistent with the idea that these fragments are the real substrates for gamma-secretase. Functional evidence that PS1 exerts its effects on gamma-secretase processing of APP in the ER/IC was obtained using a series of APP trafficking mutants. These mutants were investigated in hippocampal neurons derived from transgenic mice expressing PS1wt or PS1 containing clinical mutations (PS1(M146L) and PS1(L286V)) at physiologically relevant levels. We demonstrate that the APP-London and PS1 mutations have additive effects on the increased secretion of betaA4(1-42) relative to betaA4(1-40), indicating that both mutations operate independently. Overall, our data clearly establish that PS1 controls gamma(42)-secretase activity in pre-Golgi compartments. We discuss models that reconcile this conclusion with the effects of PS1 deficiency on the generation of betaA4(1-40) peptide in the late biosynthetic and endocytic pathways.

Presenilins: molecular switches between proteolysis and signal transduction

Mis-sense mutations of presenilin 1 increase the release of amyloidogenic peptide from amyloid precursor protein (APP) and are a major cause of familial Alzheimer's Disease. Loss-of-function mutations of presenilins in the mouse, Caenorhabditis elegans and Drosophila result in severe developmental defects caused by disturbed Notch signalling. Recent studies suggest that the diverse biological roles of presenilin 1 can be explained at the molecular level by its role in the proteolytic cleavage of the integral membrane domains of Notch and APP. This cleavage is a central switch in Notch signalling, while, for APP, its physiological role remains elusive. Evidence that presenilin 1 itself has catalytic properties could explain many of the biological and biochemical alterations caused by presenilin-1 deficiency or clinical mutations in presenilin 1. However, as presenilins reside in the endoplasmic reticulum and the cleavage of Notch and APP is believed to occur close to the cell membrane, the scientific field now faces a 'spatial paradox'.

Cellular mechanisms of beta-amyloid production and secretion

The major constituent of senile plaques in Alzheimer's disease is a 42-aa peptide, referred to as beta-amyloid (Abeta). Abeta is generated from a family of differentially spliced, type-1 transmembrane domain (TM)-containing proteins, called APP, by endoproteolytic processing. The major, relatively ubiquitous pathway of APP metabolism in cell culture involves cleavage by alpha-secretase, which cleaves within the Abeta sequence, thus precluding Abeta formation and deposition. An alternate secretory pathway, enriched in neurons and brain, leads to cleavage of APP at the N terminus of the Abeta peptide by beta-secretase, thus generating a cell-associated beta-C-terminal fragment (beta-CTF). A pathogenic mutation at codons 670/671 in APP (APP "Swedish") leads to enhanced cleavage at the beta-secretase scissile bond and increased Abeta formation. An inhibitor of vacuolar ATPases, bafilomycin, selectively inhibits the action of beta-secretase in cell culture, suggesting a requirement for an acidic intracellular compartment for effective beta-secretase cleavage of APP. beta-CTF is cleaved in the TM domain by gamma-secretase(s), generating both Abeta 1-40 (90%) and Abeta 1-42 (10%). Pathogenic mutations in APP at codon 717 (APP "London") lead to an increased proportion of Abeta 1-42 being produced and secreted. Missense mutations in PS-1, localized to chromosome 14, are pathogenic in the majority of familial Alzheimer's pedigrees. These mutations also lead to increased production of Abeta 1-42 over Abeta 1-40. Knockout of PS-1 in transgenic animals leads to significant inhibition of production of both Abeta 1-40 and Abeta 1-42 in primary cultures, indicating that PS-1 expression is important for gamma-secretase cleavages. Peptide aldehyde inhibitors that block Abeta production by inhibiting gamma-secretase cleavage of beta-CTF have been discovered.

Intracellular and cell-surface distribution of amyloid precursor protein in cortical astrocytes

Amyloid peptides that aggregate to form plaques in Alzheimer's disease are derived from secretory processing of the amyloid precursor protein (APP). Transport of APP to the cell surface may be prerequisite for non-amyloidogenic APP processing and the secretion of soluble APP (APPs), while missorting or reinternalization of APP to intracellular compartments can promote amyloid formation. In cultured astrocytes, APP mRNA and holoprotein are increased by elevations in cAMP levels, and 8-Bromo-cAMP promotes process formation on these cells. We now report that treatment of cultured astrocytes with 8-Bromo-cAMP increased intracellular and cell surface APP in the soma and perinuclear region as detected by immunolabeling with monoclonal antibody 22C11 and polyclonal antibody Kunitz-type protease inhibitor (KPI) (against the N-terminus and KPI domain of APP, respectively) and led to intense but discontinuous labelling of APP on the surface of astrocytic processes. Northern and Western blot analyses confirmed that 8-Bromo-cAMP treatment of cultured astrocytes also increased APP mRNA and KPI-containing APP holoprotein, implying that the intense APP immunolabeling observed in 8-Bromo-cAMP treated astrocytes was not derived from truncated forms of APP (e.g., APPs), but reflected high levels of APP holoprotein containing intact amyloid peptides. Discontinuous cell surface staining in process-bearing astrocytes may be caused by miscompartmentalization of APP related to rearrangement of the cytoskeleton. Inasmuch as intracellular APP is not accessible for non-amyloidogenic processing, we suggest that the increased immunoreactivity of intracellular APP in process-bearing astrocytes may predispose the cells to increased amyloid production.

Immunolocalization of caspase proteolysis in situ: evidence for widespread caspase-mediated apoptosis of neurons and glia in the postnatal rat brain

Activation of a member of the caspase family of cysteine proteases is thought to be required for the execution of apoptosis in neurons and other cell types. We describe here an antibody (Ab127) reactive with a neoantigenic site on caspase substrate proteins degraded during apoptosis, and its characterization as a biochemical and histochemical probe for apoptosis-associated proteolysis in growth factor-deprived neural cells in vitro and the developing postnatal rat brain. Neuronally differentiated PC12 cells became strongly Ab127 immunoreactive only during apoptosis following nerve growth factor withdrawal. Apoptosis-associated caspase proteolysis was detectable on western blots as markedly increased immunoreactivity of a approximately 46,000 mol. wt polypeptide, a product also generated by caspase-3 treatment of cell-free extracts. In the postnatal rat brain, intense immunoreactivity indicative of caspase activation was exhibited by small proportions of neurons and glia in distinct regional and temporal patterns. The degenerating nature of these cells was confirmed by their argyrophilia, cytoplasmic immunoreactivity for c-jun and fragmented processes. Combined immunofluorescence and Hoechst 33342 staining demonstrated that cells immunopositive for caspase activation have apoptotic nuclear morphologies. Caspase proteolysis was observed throughout the neuraxis in a minority of progenitor cells in germinal zones, postmitotic neurons in the parenchyma, and glia in the corpus callosum and other white matter tracts, but was observed rarely in the adult brain. These data characterize a new approach for evaluating apoptosis in physiological and pathological neurodegeneration, and demonstrate that caspase-associated apoptosis is a widespread mechanism for the programmed death of neurons and glia in the postnatal rat brain.

Alterations in the ryanodine receptor calcium release channel correlate with Alzheimer's disease neurofibrillary and beta-amyloid pathologies

Investigation of the integrity of the ryanodine receptor in Alzheimer's disease is important because it plays a critical role in the regulation of calcium release from the endoplasmic reticulum in brain, impairment of which is believed to contribute to the pathogenesis of Alzheimer's disease. The present study compared ryanodine receptor levels and their functional modulation in particulate fractions from control and Alzheimer's disease temporal cortex, occipital cortex and putamen. Relationships between ryanodine receptor changes and the progression of Alzheimer's disease pathology were determined by examining autoradiographic [3H]ryanodine binding in entorhinal cortex/anterior hippocampus sections from 22 cases that had been staged for neurofibrillary changes and beta-amyloid deposition. A significant (P < 0.02) 40% decrease in the Bmax for [3H]ryanodine binding and significantly higher IC50 values for both magnesium and Ruthenium Red inhibition of [3H]ryanodine binding were detected in Alzheimer's disease temporal cortex particulate fractions compared to controls. Immunoblot analyses showed Type 2 ryanodine receptor holoprotein levels to be decreased by 20% (P < 0.05) in these Alzheimer's disease cases compared to controls. No significant differences were detected in [3H]ryanodine binding comparing control and Alzheimer's disease occipital cortex or putamen samples. The autoradiography study detected increased [3H]ryanodine binding in the subiculum, CA2 and CA1 regions in cases with early (stage I-II) neurofibrillary pathology when compared to Stage 0 cases. Analysis of variance of data with respect to the different stages of neurofibrillary pathology revealed significant stage-related declines of [3H]ryanodine binding in the subiculum (P < 0.02) with trends towards significant decreases in CA1, CA2 and CA4. Post-hoc testing with Fisher's PLSD showed significant reductions (74-94%) of [3H]ryanodine binding in the subiculum, and CA1-CA4 regions of the late isocortical stage (V-VI) cases compared to the early entorhinal stage I-II cases. [3H]Ryanodine binding also showed significant declines with staging for beta-amyloid deposition in the entorhinal cortex (P < 0.01) and CA4 (P < 0.05) with trends towards a significant decrease in the dentate gyrus. We conclude that alterations in ryanodine receptor binding and function are very early events in the pathogenesis of Alzheimer's disease, and may be fundamental to the progression of both neurofibrillary and beta-amyloid pathologies.

Rank-order of potencies for inhibition of the secretion of abeta40 and abeta42 suggests that both are generated by a single gamma-secretase

The Alzheimer's disease amyloid peptide Abeta has a heterogeneous COOH terminus, as variants 40 and 42 residues long are found in neuritic plaques and are secreted constitutively by cultured cells. The proteolytic activity that liberates the Abeta COOH terminus from the beta-amyloid precursor protein is called gamma-secretase. It could be one protease with dual specificity or two distinct enzymes. By using enzyme-linked immunosorbent assays selective for Abeta40 or Abeta42, we have measured Abeta secretion by a HeLa cell line, and we have examined the dose responses for a panel of five structurally diverse gamma-secretase inhibitors. The inhibitors lowered Abeta and p3 secretion and increased levels of the COOH-terminal 99-residue beta-amyloid precursor protein derivative that is the precursor for Abeta but did not alter secretion of beta-amyloid precursor protein derivatives generated by other secretases, indicating that the inhibitors blocked the gamma-secretase processing step. The dose-dependent inhibition of Abeta42 was unusual, as the compounds elevated Abeta42 secretion at sub-inhibitory doses and then inhibited secretion at higher doses. A compound was identified that elevated Abeta42 secretion at a low concentration without inhibiting Abeta42 or Abeta40 at high concentrations, demonstrating that these phenomena are separable pharmacologically. Using either of two methods, IC50 values for inhibition of Abeta42 and Abeta40 were found to have the same rank-order and fall on a trend line with near-unit slope. These results favor the hypothesis that Abeta variants ending at residue 40 or 42 are generated by a single gamma-secretase.

Mutagenesis identifies new signals for beta-amyloid precursor protein endocytosis, turnover, and the generation of secreted fragments, including Abeta42

It has long been assumed that the C-terminal motif, NPXY, is the internalization signal for beta-amyloid precursor protein (APP) and that the NPXY tyrosine (Tyr743 by APP751 numbering, Tyr682 in APP695) is required for APP endocytosis. To evaluate this tenet and to identify the specific amino acids subserving APP endocytosis, we mutated all tyrosines in the APP cytoplasmic domain and amino acids within the sequence GYENPTY (amino acids 737-743). Stable cell lines expressing these mutations were assessed for APP endocytosis, secretion, and turnover. Normal APP endocytosis was observed for cells expressing Y709A, G737A, and Y743A mutations. However, Y738A, N740A, and P741A or the double mutation of Y738A/P741A significantly impaired APP internalization to a level similar to that observed for cells lacking nearly the entire APP cytoplasmic domain (DeltaC), arguing that the dominant signal for APP endocytosis is the tetrapeptide YENP. Although not an APP internalization signal, Tyr743 regulates rapid APP turnover because half-life increased by 50% with the Y743A mutation alone. Secretion of the APP-derived proteolytic fragment, Abeta, was tightly correlated with APP internalization, such that Abeta secretion was unchanged for cells having normal APP endocytosis but significantly decreased for endocytosis-deficient cell lines. Remarkably, secretion of the Abeta42 isoform was also reduced in parallel with endocytosis from internalization-deficient cell lines, suggesting an important role for APP endocytosis in the secretion of this highly pathogenic Abeta species.

beta-Amyloid immunoreactivity in astrocytes in Alzheimer's disease brain biopsies: an electron microscope study

The deposition of amyloid beta (A beta) protein plays a central role in the neuropathology of Alzheimer's disease (AD) and it constitutes the core of classical senile plaques. However, little is known about its intracellular distribution. An immunogold electron microscope study was therefore carried out on biopsies of brain tissue from patients with AD using a monoclonal antibody raised against residues 8 to 17 of the A beta protein. Specific A beta immunogold labeling was observed over extracellular amyloid fibrils associated with senile plaques. In addition, widespread intracellular A beta immunolabeling was observed adjacent to granular structures (30-40 nm in diameter) within membrane-bound processes. Pretreatment of some sections with amylase or omission of lead citrate staining from others strongly suggests that the electron-dense granular structures associated with A beta immunoreactivity are glycogen. Some of the A beta-immunolabeled processes contained gliofilaments and immunolabeling of alternate sections for glial fibrillary acidic protein confirmed that the A beta-immunolabeled processes were astrocytic. A beta immunolabeling was not observed over neuronal or microglial processes. Whether the presence of A beta protein in astrocytes is the result of synthetic or degradation processes requires further investigation.

Alzheimer's disease: correlation of the suppression of beta-amyloid peptide secretion from cultured cells with inhibition of the chymotrypsin-like activity of the proteasome

Peptide aldehyde inhibitors of the chymotrypsin-like activity of the proteasome (CLIP) such as N-acetyl-Leu-Leu-Nle-H (or ALLN) have been shown previously to inhibit the secretion of beta-amyloid peptide (A beta) from cells. To evaluate more fully the role of the proteasome in this process, we have tested the effects on A beta formation of a much wider range of peptide-based inhibitors of CLIP than published previously. The inhibitors tested included several peptide boronates, some of which proved to be the most potent peptide-based inhibitors of beta-amyloid production reported so far. We found that the ability of the peptide aldehyde and boronate inhibitors to suppress A beta formation from cells correlated extremely well with their potency as CLIP inhibitors. Thus, we conclude that the proteasome may be involved either directly or indirectly in A beta formation.

Translating cell biology into therapeutic advances in Alzheimer's disease

Studies of the molecular basis of Alzheimer's disease exemplify the increasingly blurred distinction between basic and applied biomedical research. The four genes so far implicated in familial Alzheimer's disease have each been shown to elevate brain levels of the self-aggregating amyloid-beta protein, leading gradually to profound neuronal and glial alteration, synaptic loss and dementia. Progress in understanding this cascade has helped to identify specific therapeutic targets and provides a model for elucidating other neurodegenerative disorders.

Genome structure and chromosomal mapping of the gene for Fe65L2 interacting with Alzheimer's beta-amyloid precursor protein

Recently we cloned the cDNA encoding human Fe65L2, which interacts with Alzheimer's beta-amyloid precursor protein (APP). The protein has one WW domain and two PID elements (Neurosci. Lett. (1999) 261, 143-146.). Here, we report the entire genome structure and the chromosomal mapping of the Fe65L2 gene. The gene is composed of thirteen coding exons distributed over 6 kb and the genomic organization is similar to another Fe65 member, Fe65 gene. Two transcription start sites of the gene were found in a CpG island by primer extension analysis. Radiation hybrid mapping revealed that the Fe65L2 gene is on chromosome 5 between markers SHGC-9824 (D5S2374) and SHGC-8489 (D5S2569). Characterization of the Fe65L2 gene structure will be useful in the linkage analysis and search for mutations in the Fe65L2 gene in patients with Alzheimer's disease.

Beta-amyloid precursor protein is detectable on monocytes and is increased in Alzheimer's disease

Using the anti-beta-amyloid precursor protein (betaAPP) monoclonal antibodies 4G8, 6E10 and 22C11 and flow cytometry, we report that human circulating peripheral blood monocytes display surface immunoreactivity for betaAPP. In contrast, circulating lymphocytes do not possess cell surface betaAPP immunoreactivity, despite similar levels of betaAPP expression. Immunoblotting analysis showed that monocytes, but not lymphocytes, possess an 82 kDa C-terminal betaAPP fragment consistent with a processed transmembrane species. Monocyte surface betaAPP was upregulated approximately threefold by activation with lipopolysaccharide and interferon-gamma, activation did not produce detectable betaAPP on the cell surface of lymphocytes. Surface betaAPP immunoreactivity was reduced in a normal aged population compared to normal young controls (Young = 81.07 +/- 13.67 mean fluorescence units, Aged = 36.74 +/- 3.81, p < 0.01), but was significantly increased in AD subjects compared to age-matched healthy controls (AD = 60.31 +/- 7.42, p < 0.05). Our data suggest that a proportion of peripheral A beta may be derived from monocyte/macrophages, and that defects in brain cell processing of betaAPP in AD may be shared by this readily accessible peripheral cell.

Role of endoplasmic reticulum, endosomal-lysosomal compartments, and microtubules in amyloid precursor protein metabolism of human neurons

A wide interest in amyloid precursor protein (APP) metabolism stems from the fact that increased amounts of amyloid beta peptide (Abeta), arising through proteolytic processing of APP, likely play a significant role in Alzheimer's disease. As Alzheimer's disease pathology is limited almost exclusively to the human species, we established human primary neuron cultures to address the possibility of distinctive APP processing in human CNS neurons. In the present study, we investigate the role of organelles and protein trafficking in APP metabolism. Using brefeldin A, we failed to detect APP processing into Abeta in the endoplasmic reticulum. Monensin and the lysomotropic agents, NH4Cl and chloroquine, revealed a bypass pH-dependent secretory pathway in a compartment between the endoplasmic reticulum and the medial Golgi, resulting in the secretion of full-length APP. Colchicine treatment resulting in the loss of neurites inhibited processing of APP through the secretory, but not the endosomal-lysosomal, pathway of APP metabolism. The serine protease inhibitor, leupeptin, indicates a role for lysosomes in APP, Abeta, and APP C-terminal fragment turnover. These results demonstrate that the regulation of APP metabolism in human neurons differs considerably from those reported in rodent CNS primary neuron cultures or continuously dividing cell types.

gamma-Secretase, evidence for multiple proteolytic activities and influence of membrane positioning of substrate on generation of amyloid beta peptides of varying length

gamma-Secretase activity is the final cleavage event that releases the amyloid beta peptide (Abeta) from the beta-secretase cleaved carboxyl-terminal fragment of the amyloid beta protein precursor (APP). No protease responsible for this highly unusual, purportedly intramembranous, cleavage has been definitively identified. We examined the substrate specificity of gamma-secretase by mutating various residues within or adjacent to the transmembrane domain of the APP and then analyzing Abeta production from cells transfected with these mutant APPs by enzyme-linked immunosorbent assay and mass spectrometry. Abeta production was also analyzed from a subset of transmembrane domain APP mutants that showed dramatic shifts in gamma-secretase cleavage in the presence or absence of pepstatin, an inhibitor of gamma-secretase activity. These studies demonstrate that gamma-secretase's cleavage specificity is primarily determined by location of the gamma-secretase cleavage site of APP with respect to the membrane, and that gamma-secretase activity is due to the action of multiple proteases exhibiting both a pepstatin- sensitive activity and a pepstatin-insensitive activity. Given that gamma-secretase is a major therapeutic target in Alzheimer's disease these studies provide important information with respect to the mechanism of Abeta production that will direct efforts to isolate the gamma-secretases and potentially to develop effective therapeutic inhibitors of pathologically relevant gamma-secretase activities.

Cerebrospinal fluid A beta42 is increased early in sporadic Alzheimer's disease and declines with disease progression

All mutations known to cause familial Alzheimer's disease (AD) act by increasing the levels of soluble beta-amyloid peptide (A beta), especially the longer form, A beta42. However, in vivo elevation of soluble A beta in sporadic AD has so far not been shown. In the present study, we used enzyme-linked immunosorbent assays specific for A beta42 and A beta40 to investigate cerebrospinal fluid from sporadic AD at different stages of disease severity, to clarify the roles of A beta42 and A beta40 during disease progression. We also evaluated three other groups--one group of patients with mild cognitive impairment who were at risk of developing dementia, a cognitively intact, nondemented reference group diagnosed with depression, and a perfectly healthy control group. We found that A beta42 is strongly elevated in early and mid stages of AD, and thereafter it declines with disease progression. On the contrary, A beta40 levels were decreased in early and mid stages of AD. The group of cognitively impaired patients and the depression reference group had significantly higher levels of A beta42 than the healthy control group, implying that A beta42 is increased not only in AD, but in other central nervous system conditions as well. Our data also point out the importance of having thoroughly examined control material. The initial increase and subsequent decrease of A beta42 adds a new biochemical tool to follow the progression of AD and might be important in the monitoring of therapeutics.

Recombinant adenovirus is an appropriate vector for endocytotic protein trafficking studies in cultured neurons

Endocytosis of full-length beta-amyloid precursor protein (APP) from the plasma membrane contributes to beta-amyloid peptide (Abeta) secretion, and, hence, potentially contributes to the molecular pathogenesis of Alzheimer's disease. We recently have demonstrated that central neuronal APP is endocytosed in a common vesicular compartment with recycling synaptic vesicle integral membrane proteins, but is then sorted away from synaptic vesicles for retrograde transport to the neuronal soma. For this report, we explore whether recombinant adenovirus can be used to modulate APP expression in cultured central neurons to study APP processing by the endocytotic pathway in these cells. Using a replication-deficient recombinant adenovirus that expresses a lacZ reporter (Ad5/CMV-lacZ), we demonstrate high efficiency of transfection (30-35%) at low viral titer (10-20 MOI), with no significant neuronal toxicity or cytoarchitectural change. In addition, we demonstrate that infection with the control virus does not result in re-direction of endogenous neuronal APP from usual endocytotic pathways. We have prepared, using the same genomic background as the control virus, an adenoviral vector that expresses the neuronal isoform of human APP (Ad5/CMV-APP). Infection with Ad5/CMV-APP at 10-20 MOI results in significantly increased immunoreactivity for endocytosed APP with preservation of usual endocytotic trafficking. These results demonstrate that recombinant adenovirus at low titer is an appropriate and effective vector for protein trafficking/processing studies in cultured central neurons.

Novel effects of FCCP [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone] on amyloid precursor protein processing

Amyloidogenic processing of the beta-amyloid precursor protein (APP) has been implicated in the pathology of Alzheimer's disease. Because it has been suggested that catabolic processing of the APP holoprotein occurs in acidic intracellular compartments, we studied the effects of the protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) and the H+-ATPase inhibitor bafilomycin A1 on APP catabolism in human embryonic kidney 293 cells expressing either wild-type or "Swedish" mutant APP. Unlike bafilomycin A1, which inhibits beta-amyloid production in cells expressing mutant but not wild-type APP, FCCP inhibited beta-amyloid production in both cell types. Moreover, the effects of FCCP were independent of alterations in total cellular APP levels or APP maturation, and the concentrations used did not alter either cellular ATP levels or cell viability. Bafilomycin A1, which had no effect on beta-amyloid production in wild-type cells, inhibited endocytosis of fluorescent transferrin, whereas concentrations of FCCP that inhibited beta-amyloid production in these cells had no effect on endosomal function. Thus, in wild-type-expressing cells it appears that the beta-amyloid peptide is not produced in the classically defined endosome. Although bafilomycin A1 decreased beta-amyloid release from cells expressing mutant APP but not wild-type APP, it altered lysosomal function in both cell types, suggesting that in normal cells beta-amyloid is not produced in the lysosome. Although inhibition of beta-amyloid production by bafilomycin A1 in mutant cells may occur via changes in endosomal/lysosomal pH, our data suggest that FCCP inhibits wild-type beta-amyloid production by acting on a bafilomycin A1-insensitive acidic compartment that is distinct from either the endosome or the lysosome.

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 role of the protein glycosylation state in the control of cellular transport of the amyloid beta precursor protein

The amyloid beta precursor protein can exist as both a membrane-bound and a secreted protein, with the former having the potential to generate the amyloid beta peptide present in the neuritic plaques which are characteristic of Alzheimer's disease. In this study, we have used a clone of the AtT20 mouse pituitary cell line which expresses high levels of the amyloid beta precursor protein to characterize the glycosylation state of the secreted and membrane-bound forms of the protein and to examine the role of post-translational modifications in protein processing. Lectin blot analysis of immunoprecipitated amyloid beta precursor protein demonstrated that the soluble form of the protein contains significant amounts of sialic acid, with the lectin staining being reduced in the particulate cellular fractions. Treatment of the cells with mannosidase inhibitors to interfere with the formation of complex-type N-linked glycans resulted in a decrease in secreted amyloid beta precursor protein and an increase in the level of the cellular form of the protein. The increase in amyloid beta precursor protein levels in the cellular fraction was accompanied by an increase in perinuclear staining. Furthermore, cells overexpressing the alpha2,6(N)-sialyltransferase enzyme also demonstrated an increase in amyloid beta precursor protein secretion. These results suggest that the presence of terminal sialic acid residues on complex-type N-glycans may be required for the optimal transport of the amyloid beta precursor protein from the Golgi to the cell membrane with the subsequent cleavage to generate the secreted form of the protein.

Unusual phenotypic alteration of beta amyloid precursor protein (betaAPP) maturation by a new Val-715 --> Met betaAPP-770 mutation responsible for probable early-onset Alzheimer's disease

We have identified a novel beta amyloid precursor protein (betaAPP) mutation (V715M-betaAPP770) that cosegregates with early-onset Alzheimer's disease (AD) in a pedigree. Unlike other familial AD-linked betaAPP mutations reported to date, overexpression of V715M-betaAPP in human HEK293 cells and murine neurons reduces total Abeta production and increases the recovery of the physiologically secreted product, APPalpha. V715M-betaAPP significantly reduces Abeta40 secretion without affecting Abeta42 production in HEK293 cells. However, a marked increase in N-terminally truncated Abeta ending at position 42 (x-42Abeta) is observed, whereas its counterpart x-40Abeta is not affected. These results suggest that, in some cases, familial AD may be associated with a reduction in the overall production of Abeta but may be caused by increased production of truncated forms of Abeta ending at the 42 position.

Mechanism of the cleavage specificity of Alzheimer's disease gamma-secretase identified by phenylalanine-scanning mutagenesis of the transmembrane domain of the amyloid precursor protein

Proteolytic processing of the amyloid precursor protein by beta-secretase yields A4CT (C99), which is cleaved further by the as yet unknown gamma-secretase, yielding the beta-amyloid (Abeta) peptide with 40 (Abeta40) or 42 residues (Abeta42). Because the position of gamma-secretase cleavage is crucial for the pathogenesis of Alzheimer's disease, we individually replaced all membrane-domain residues of A4CT outside the Abeta domain with phenylalanine, stably transfected the constructs in COS7 cells, and determined the effect of these mutations on the cleavage specificity of gamma-secretase (Abeta42/Abeta40 ratio). Compared with wild-type A4CT, mutations at Val-44, Ile-47, and Val-50 led to decreased Abeta42/Abeta40 ratios, whereas mutations at Thr-43, Ile-45, Val-46, Leu-49, and Met-51 led to increased Abeta42/Abeta40 ratios. A massive effect was observed for I45F (34-fold increase) making this construct important for the generation of animal models for Alzheimer's disease. Unlike the other mutations, A4CT-V44F was processed mainly to Abeta38, as determined by mass spectrometry. Our data provide a detailed model for the active site of gamma-secretase: gamma-secretase interacts with A4CT by binding to one side of the alpha-helical transmembrane domain of A4CT. Mutations in the transmembrane domain of A4CT interfere with the interaction between gamma-secretase and A4CT and, thus, alter the cleavage specificity of gamma-secretase.

Implications of presenilin 1 mutations in Alzheimer's disease

Mutations in presenilin 1 (PS1) and presenilin 2 (PS2) are the most common genetic factors underlying the development of early-onset familial Alzheimer's disease (FAD). To investigate the pathogenic mechanism of PS1 mutations linked to FAD, we established inducible mouse neuroblastoma (Neuro 2a) cell lines expressing the human wild-type (wt) or mutated PS1(M146L or deltaexon 10) under the control of the Lac repressor. Using this inducible PS1 system, the influence of PS1 mutations on the generation of endogenous murine Abeta species was assessed using a highly sensitive immunoblotting technique. The induction of mutated PS1 resulted in an increase in the extra- and intracellular levels of two distinct Abeta species ending at residue 42, Abeta1-42 and its N-terminally truncated variant(s), Abetax-42. In addition, the intracellular generation of these Abeta42 species was completely blocked by brefeldin A. In contrast, it exhibited differential sensitivities to monensin such that there was an increased accumulation of intracellular Abetax-42 but an inhibition of intracellular Abeta1-42 generation. These data strongly suggest that Abetax-42 is generated in a proximal Golgi compartment, whereas Abeta1-42 is generated in a distal Golgi and/or a post-Golgi compartment. Thus, it appears that PS1 mutations enhance the degree of 42-specific gamma-secretase cleavage which occurs (i) in the ER or the early Golgi apparatus prior to gamma-secretase cleavage, or (ii) in the distinct sites where Abetax-42 and Abeta1-42 are generated. To date, the site of Abeta42 generation has not been firmly established. Our data provide new information regarding the site of Abeta42 generation mediated by the FAD-linked mutant PS1.

Reductions in brain phosphatidylinositol kinase activities in Alzheimer's disease

BACKGROUND

Converging lines of evidence suggest that alterations in the intracellular trafficking of the amyloid precursor protein, its derivatives, and other relevant proteins may contribute to the pathophysiology of Alzheimer's disease (AD). Since phosphatidylinositol (PI) kinase plays a pivotal role in the sorting and transport of newly synthesized proteins to their final destinations, we explored the hypothesis that AD is associated with alterations in the specific activities of these enzymes in postmortem brain tissue.

METHODS

The specific activities of soluble and particulate pools of PI 3-kinase and PI 4-kinase from the frontal cortex were compared between 11 cases with histopathologically confirmed AD and 11 nondemented controls matched for sex, race, age at death, and postmortem interval. Potential associations of these activities with sociodemographic and clinical features were also explored.

RESULTS

AD was associated with 43-59% reductions in the specific activities of the soluble forms of both lipid kinases; but no significant change in the specific activities of the particulate species. Associations of these specific activities with sex, age at onset or death, duration of illness, postmortem interval, or densities of morphologic lesions in the frontal cortex were not observed among the 11 AD cases.

CONCLUSIONS

In addition to regulating protein sorting and trafficking, PI kinases participate in a wide range of cellular processes including protection from apoptosis, differentiation and cell growth, regulation of the cytoskeleton, and glucose metabolism. The results of this study suggest that one or more of these alterations in AD may result from a common abnormality in PI kinase regulation.

cDNA cloning and molecular characterization of human brain metalloprotease MP100: a beta-secretase candidate?

Metalloprotease MP100 was originally isolated as a beta-secretase candidate from human brain using a beta-amyloid precursor protein (beta-APP)-derived p-nitroanilide (pNA) peptide substrate. Peptide sequences from purified MP100 were now found to resemble sequences reported for a puromycin-sensitive aminopeptidase (PSA) highly enriched in brain, and cDNA cloning revealed nearly complete homology of MP100 to PSA, with only a single bp difference resulting in an amino acid change at position 184. Another MP100 cDNA encoded a protein with a 36-amino acid deletion (positions 180-217) and a two-amino acid insertion after Val533. Purified recombinant human MP100 cleaved the original pNA substrate as well as a free beta-site-spanning amyloid beta (A beta) peptide (A beta(-10/+10)), generating A beta(1-10). The latter substrate, however, remained uncleaved, if N- and C-terminally blocked, and also purified beta-APP was not cleaved. Double immunoimaging revealed partial, patchy, colocalization of beta-APP and MP100 in doubly transfected human embryonic kidney cells (HEK cells) and in normal neuroblastoma cells, and both proteins could be coimmunoprecipitated from rat brain extracts, suggesting their close vicinity in vivo. Coexpression of MP100 and beta-APP695, however, did not boost A beta levels in HEK cells, although active enzyme was produced. Thus, MP100 does not exert true beta-secretase-like function in cells, although it may well act as a secondary exoprotease in a complex beta-APP/A beta metabolism.

Alzheimer's disease: genetic studies and transgenic models

Recent advances in a variety of areas of research, particularly in genetics and in transgenic (Tg)/gene targeting approaches, have had a substantial impact on our understanding of Alzheimer's disease (AD) and related disorders. After briefly reviewing the progress that has been made in diagnostic assessments of patients with senile dementia and in investigations of the neuropathology of AD, we discuss some of the genes/proteins that are causative or risk factors for this disease, including those encoding amyloid precursor protein, presenilin 1 and 2, and apolipoprotein E. In addition, we comment on several potential new candidate loci/genes. Subsequently, we review selected recent reports of analyses of a variety of lines of Tg mice that show several neuropathological features of AD, including A beta-amyloid deposits and dystrophic neurites. Finally, we discuss the several important issues in future investigations of Tg mice, with particular emphasis on the influences of genetic strains on phenotype, especially behavior, and strategies for making new models of neurodegenerative disorders. We believe that investigations of these Tg models will (a) enhance understanding of the relationships between impaired performance on memory tasks and the pathological/biochemical abnormalities in brain, (b) help to clarify pathogenic mechanisms in vivo, (c) lead to identification of new therapeutic targets, and (d) allow testing of new treatment strategies first in mice and then, if successful, in humans with AD.

Regulation of amyloid precursor protein cleavage

Multiple lines of evidence suggest that increased production and/or deposition of the beta-amyloid peptide, derived from the amyloid precursor protein, contributes to Alzheimer's disease. A growing list of neurotransmitters, growth factors, cytokines, and hormones have been shown to regulate amyloid precursor protein processing. Although traditionally thought to be mediated by activation of protein kinase C, recent data have implicated other signaling mechanisms in the regulation of this process. Moreover, novel mechanisms of regulation involving cholesterol-, apolipoprotein E-, and stress-activated pathways have been identified. As the phenotypic changes associated with Alzheimer's disease encompass many of these signaling systems, it is relevant to determine how altered cell signaling may be contributing to increasing brain amyloid burden. We review the myriad ways in which first messengers regulate amyloid precursor protein catabolism as well as the signal transduction cascades that give rise to these effects.

Presenilin mutations associated with Alzheimer disease cause defective intracellular trafficking of beta-catenin, a component of the presenilin protein complex

The presenilin proteins are components of high-molecular-weight protein complexes in the endoplasmic reticulum and Golgi apparatus that also contain beta-catenin. We report here that presenilin mutations associated with familial Alzheimer disease (but not the non-pathogenic Glu318Gly polymorphism) alter the intracellular trafficking of beta-catenin after activation of the Wnt/beta-catenin signal transduction pathway. As with their effect on betaAPP processing, the effect of PS1 mutations on trafficking of beta-catenin arises from a dominant 'gain of aberrant function' activity. These results indicate that mistrafficking of selected presenilin ligands is a candidate mechanism for the genesis of Alzheimer disease associated with presenilin mutations, and that dysfunction in the presenilin-beta-catenin protein complexes is central to this process.

Mapping the APP/presenilin (PS) binding domains: the hydrophilic N-terminus of PS2 is sufficient for interaction with APP and can displace APP/PS1 interaction

Mutations in presenilin 1 and presenilin 2 (PS1 and PS2, respectively) genes cause the large majority of familial forms of early-onset Alzheimer's disease. The physical interaction between presenilins and APP has been recently described using coimmunoprecipitation. With a similar technique, we confirmed this interaction and have mapped the interaction domains on both PS2 and APP. Using several carboxy-terminal truncated forms of PS2, we demonstrated that the hydrophilic amino terminus of PS2 (residues 1 to 87, PS2NT) was sufficient for interaction with APP. Interestingly, only a construct with a leader peptide for secretion (SecPS2NT) and not its cytosolic counterpart was shown to interact with APP. For APP, we could demonstrate interaction of PS2 with the last 100 but not the last 45 amino acids of APP, including therefore the A beta region. Accordingly, SecPS2NT is capable of binding to A beta-immunoreactive species in conditioned medium. In addition, a second region in the extracellular domain of APP also interacted with PS2. Comparable results with PS1 indicate that the two presenilins share similar determinants of binding to APP. Confirming these results, SecPS2NT is able to inhibit PS1/APP interaction. Such a competition makes it unlikely that the PS/APP interaction results from nonspecific aggregation of PS in transfected cells. The physical interaction of presenilins with a region encompassing the A beta sequence of APP could be causally related to the misprocessing of APP and the production of A beta1-42.

Role of ERAB/L-3-hydroxyacyl-coenzyme A dehydrogenase type II activity in Abeta-induced cytotoxicity

Endoplasmic reticulum-associated amyloid beta-peptide (Abeta)-binding protein (ERAB)/L-3-hydroxyacyl-CoA dehydrogenase type II (HADH II) is expressed at high levels in Alzheimer's disease (AD)-affected brain, binds Abeta, and contributes to Abeta-induced cytotoxicity. Purified recombinant ERAB/HADH II catalyzed the NADH-dependent reduction of S-acetoacetyl-CoA with a Km of approximately 68 microM and a Vmax of approximately 430 micromol/min/mg. The contribution of ERAB/HADH II enzymatic activity to Abeta-mediated cellular dysfunction was studied by site-directed mutagenesis in the catalytic domain (Y168G/K172G). Although COS cells cotransfected to overexpress wild-type ERAB/HADH II and variant beta-amyloid precursor protein (betaAPP(V717G)) showed DNA fragmentation, cotransfection with Y168G/K172G-altered ERAB and betaAPP(V717G) was without effect. We thus asked whether the enzyme might recognize alcohol substrates of which the aldehyde products could be cytotoxic; ERAB/HADH II catalyzed oxidation of a variety of simple alcohols (C2-C10) to their respective aldehydes in the presence of NAD+ and NAD-dependent oxidation of 17beta-estradiol. Addition of micromolar levels of synthetic Abeta(1-40) to purified ERAB/HADH II inhibited, in parallel, reduction of S-acetoacetyl-CoA (Ki approximately 1.6 microM), as well as oxidation of 17beta-estradiol (Ki approximately 3.2 microM) and (-)-2-octanol (Ki approximately 2.6 microM). Because micromolar levels of Abeta were required to inhibit ERAB/HADH II activity, whereas Abeta binding to ERAB/HADH II occurred at much lower concentrations (Km approximately 40-70 nM), the latter more closely simulating Abeta levels within cells, Abeta perturbation of ERAB/HADH II was likely to result from mechanisms other than the direct modulation of enzymatic activity. Cells cotransfected to overexpress ERAB/HADH II and betaAPP(V717G) generated malondialdehyde-protein and 4-hydroxynonenal-protein epitopes, which were detectable only at the lowest levels in cells overexpressing either ERAB/HADH II or betaAPP(V717G) alone. Generation of such toxic aldehydes was not observed in cells contransfected to overexpress Y168G/K172G-altered ERAB and betaAPP(V717G). We conclude that the generalized alcohol dehydrogenase activity of ERAB/HADH II is central to the cytotoxicity observed in an Abeta-rich environment.

Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer beta-amyloid peptides

The excessive generation and accumulation of 40- and 42-aa beta-amyloid peptides (Abeta40/Abeta42) in selectively vulnerable brain regions is a major neuropathological feature of Alzheimer's disease. Abeta, derived by proteolytic cleavage from the beta-amyloid precursor protein (betaAPP), is normally secreted. However, recent evidence suggests that significant levels of Abeta also may remain inside cells. Here, we have investigated the subcellular compartments within which distinct amyloid species are generated and the compartments from which they are secreted. Three experimental approaches were used: (i) immunofluorescence performed in intact cortical neurons; (ii) sucrose gradient fractionation performed with mouse neuroblastoma cells stably expressing wild-type betaAPP695 (N2a695); and (iii) cell-free reconstitution of Abeta generation and trafficking from N2a695 cells. These studies demonstrate that: (i) Abeta40 (Abeta1-40 plus Abetax-40, where x is an NH2-terminal truncation) is generated exclusively within the trans-Golgi Network (TGN) and packaged into post-TGN secretory vesicles; (ii) Abetax-42 is made and retained within the endoplasmic reticulum in an insoluble state; (iii) Abeta42 (Abeta1-42 plus Abetax-42) is made in the TGN and packaged into secretory vesicles; and (iv) the amyloid peptides formed in the TGN consist of two pools (a soluble population extractable with detergents and a detergent-insoluble form). The identification of the organelles in which distinct forms of Abeta are generated and from which they are secreted should facilitate the identification of the proteolytic enzymes responsible for their formation.

Genes and susceptible loci of Alzheimer's disease

Alzheimer's disease (AD) is the most common and devastating neurodegenerative disease of the elderly. Many research findings on familial AD suggest that the mechanisms of the pathogenesis of the disorder is more complex although the overall neuropathology of all cases of AD is surprisingly very similar. Genetic studies on some families have shown that mutations in the genes encoding beta-amyloid precursor protein and presenilins 1 and 2 are responsible for early-onset AD. In addition, apolipoprotein E gene allele E4 and the bleomycin hydrolase locus are shown to be genetic risk factors for late-onset AD in certain sporadic cases. Mitochondrial dysfunctions and age-related oxidative stress may also contribute to degenerative processes in AD. Although several studies support the amyloid cascade hypothesis as the mechanism of the disease, transgenic experiments and recent findings on a variant form of an AD family suggest that A beta deposition may not be sufficient to cause AD. Identification in the future of other genetic, environmental, and age-related factors, may provide additional targets for therapies.

TNFalpha plus IFNgamma induce the production of Alzheimer beta-amyloid peptides and decrease the secretion of APPs

The appearance of inflammatory markers associated with amyloid plaques indicates a state of chronic inflammation in Alzheimer's disease (AD). Multiple epidemiological studies also suggest that patients taking anti-inflammatory drugs have a decreased risk of developing AD. Here we present evidence that inflammatory cytokines can alter the metabolism of the beta-amyloid precursor protein (betaAPP). We show that the combination of tumor necrosis factor alpha and interferon gamma triggers the production of beta-amyloid peptides and inhibits the secretion of soluble APPs by human neuronal and extraneuronal cells. The results demonstrate a new mechanism by which inflammatory components can exacerbate the fundamental pathology in AD.

Disturbances of the functioning of endoplasmic reticulum: a key mechanism underlying neuronal cell injury?

Cerebral ischemia leads to a massive increase in cytoplasmic calcium activity resulting from an influx of calcium ions into cells and a release of calcium from mitochondria and endoplasmic reticulum (ER). It is widely believed that this increase in cytoplasmic calcium activity plays a major role in ischemic cell injury in neurons. Recently, this concept was modified, taking into account that disturbances occurring during ischemia are potentially reversible: it then was proposed that after reversible ischemia, calcium ions are taken up by mitochondria, leading to disturbances of oxidative phosphorylation, formation of free radicals, and deterioration of mitochondrial functions. The current review focuses on the possible role of disturbances of ER calcium homeostasis in the pathologic process culminating in ischemic cell injury. The ER is a subcellular compartment that fulfills important functions such as the folding and processing of proteins, all of which are strictly calcium dependent. ER calcium activity is therefore relatively high, lying in the lower millimolar range (i.e., close to that of the extracellular space). Depletion of ER calcium stores is a severe form of stress to which cells react with a highly conserved stress response, the most important changes being a suppression of global protein synthesis and activation of stress gene expression. The response of cells to disturbances of ER calcium homeostasis is almost identical to their response to transient ischemia, implying common underlying mechanisms. Many observations from experimental studies indicate that disturbances of ER calcium homeostasis are involved in the pathologic process leading to ischemic cell injury. Evidence also has been presented that depletion of ER calcium stores alone is sufficient to activate the process of programmed cell death. Furthermore, it has been shown that activation of the ER-resident stress response system by a sublethal form of stress affords tolerance to other, potentially lethal insults. Also, disturbances of ER function have been implicated in the development of degenerative disorders such as prion disease and Alzheimer's disease. Thus, disturbances of the functioning of the ER may be a common denominator of neuronal cell injury in a wide variety of acute and chronic pathologic states of the brain. Finally, there is evidence that ER calcium homeostasis plays a key role in maintaining cells in their physiologic state, since depletion of ER calcium stores causes growth arrest and cell death, whereas cells in which the regulatory link between ER calcium homeostasis and protein synthesis has been blocked enter a state of uncontrolled proliferation.

Transgenic mice over-expressing the C-99 fragment of betaPP with an alpha-secretase site mutation develop a myopathy similar to human inclusion body myositis

Inclusion body myositis (IBM) is the most common muscle disease in the elderly. Amyloid-beta protein (A beta) has been shown to accumulate abnormally in the vacuolated fibers and to localize to amyloid-like fibrils in muscles from IBM patients. We studied the skeletal muscles from a line of transgenic mice over-expressing the carboxyl-terminal 99 amino acids (C99) of the beta-amyloid precursor protein (betaPP) with a substitution of lysine-612 to valine (K612V), intended to abolish alpha-secretase recognition and to preserve the A beta domain of C99. The majority (87%) of the 24-month-old transgenic mice showed myopathic changes, and approximately one-third of them had degenerating fibers with sarcoplasmic vacuoles and thioflavin-S-positive deposits. Ultrastructurally, the inclusions were aggregates of short thin amyloid-like fibrils, 6 to 8 nm in diameter. These features are similar to those of human IBM. Immunocytochemistry using an antibody against A beta showed membranous staining in most muscle fibers of transgenic mice, as well as granular or vacuolar cytoplasmic staining in the atrophic fibers. Western blots showed a high level of accumulation of carboxyl-terminal fragments of betaPP in the muscles of the transgenic mice with the most severe IBM-like lesions. The expression of IBM-like lesions was age dependent. These transgenic mice provide a model for the study of IBM and for the peripheral expression of a key element in the pathogenesis of Alzheimer disease.