Release of amino-terminal fragments from amyloid precursor protein reporter and mutated derivatives in cultured cells

Multidisciplinary approaches for targeting the secretase protein family as a therapeutic route for Alzheimer's disease

The continual increase of the aging population worldwide renders Alzheimer's disease (AD) a global prime concern. Several attempts have been focused on understanding the intricate complexity of the disease's development along with the on- andgoing search for novel therapeutic strategies. Incapability of existing AD drugs to effectively modulate the pathogenesis or to delay the progression of the disease leads to a shift in the paradigm of AD drug discovery. Efforts aimed at identifying AD drugs have mostly focused on the development of disease-modifying agents in which effects are believed to be long lasting. Of particular note, the secretase enzymes, a group of proteases responsible for the metabolism of the β-amyloid precursor protein (βAPP) and β-amyloid (Aβ) peptides production, have been underlined for their promising therapeutic potential. This review article attempts to comprehensively cover aspects related to the identification and use of drugs targeting the secretase enzymes. Particularly, the roles of secretases in the pathogenesis of AD and their therapeutic modulation are provided herein. Moreover, an overview of the drug development process and the contribution of computational (in silico) approaches for facilitating successful drug discovery are also highlighted along with examples of relevant computational works. Promising chemical scaffolds, inhibitors, and modulators against each class of secretases are also summarized herein. Additionally, multitarget secretase modulators are also taken into consideration in light of the current growing interest in the polypharmacology of complex diseases. Finally, challenging issues and future outlook relevant to the discovery of drugs targeting secretases are also discussed.

Shedding of APP limits its synaptogenic activity and cell adhesion properties

The amyloid precursor protein (APP) plays a central role in Alzheimer's disease (AD) and has essential synapse promoting functions. Synaptogenic activity as well as cell adhesion properties of APP presumably depend on trans-cellular dimerization via its extracellular domain. Since neuronal APP is extensively processed by secretases, it raises the question if APP shedding affects its cell adhesion and synaptogenic properties. We show that inhibition of APP shedding using cleavage deficient forms of APP or a dominant negative α-secretase strongly enhanced its cell adhesion and synaptogenic activity suggesting that synapse promoting function of APP is tightly regulated by α-secretase mediated processing, similar to other trans-cellular synaptic adhesion molecules.

An Alzheimer's disease-specific beta-amyloid fragment signature in cerebrospinal fluid

Pathogenic events in Alzheimer's disease (AD) involve an imbalance between the production and clearance of the neurotoxic beta-amyloid peptide (Abeta), especially the 42 amino acid peptide Abeta1-42. While much is known about the production of Abeta1-42, many questions remain about how the peptide is degraded. To investigate the degradation pattern, we developed a method based on immunoprecipitation combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry that determines the Abeta degradation fragment pattern in cerebrospinal fluid (CSF). We found in total 18 C-terminally and 2 N-terminally truncated Abeta peptides and preliminary data indicated that there were differences in the detected Abeta relative abundance pattern between AD and healthy controls. Here, we provide direct evidence that an Abeta fragment signature consisting of Abeta1-16, Abeta1-33, Abeta1-39, and Abeta1-42 in CSF distinguishes sporadic AD patients from non-demented controls with an overall accuracy of 86%.

Transgenic mouse models of Alzheimer's disease and amyotrophic lateral sclerosis

Over the past several years, there has been enormous progress in generating transgenic mice that model aspects of human neurodegenerative diseases. These studies build upon the efforts of molecular geneticists who have identified a number of genes that, when mutated, cause familial forms of these diseases. In this review, we focus on the mutations that cause familial forms of Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS), and transgenic mouse models that develop clinical and pathological abnormalities resembling those occurring in the human diseases.

The proteolytic processing of the amyloid precursor protein gene family members APLP-1 and APLP-2 involves alpha-, beta-, gamma-, and epsilon-like cleavages: modulation of APLP-1 processing by n-glycosylation

Amyloid precursor protein (APP) processing is of major interest in Alzheimer's disease research, since sequential cleavages by beta- and gamma-secretase lead to the formation of the 4-kDa amyloid Abeta protein peptide that accumulates in Alzheimer's disease brain. The processing of APP involves proteolytic conversion by different secretases leading to alpha-, beta-, gamma-, delta-, and epsilon-cleavages. Since modulation of these cleavages represents a rational therapeutic approach to control amyloid formation, its interference with the processing of the members of the APP gene family is of considerable importance. By using C-terminally tagged constructs of APLP-1 and APLP-2 and the untagged proteins, we have characterized their proteolytic C-terminal fragments produced in stably transfected SH-SY5Y cells. Pharmacological manipulation with specific protease inhibitors revealed that both homologues are processed by alpha- and gamma-secretase-like cleavages, and that their intracellular domains can be released by cleavage at epsilon-sites. APLP-2 processing appears to be the most elaborate and to involve alternative cleavage sites. We show that APLP-1 is the only member of the APP gene family for which processing can be influenced by N-glycosylation. Additionally, we were able to detect p3-like fragments of APLP-1 and p3-like and Abeta-like fragments of APLP-2 in the media of stably transfected SH-SY5Y cells.

Regulation of Alzheimer beta-amyloid precursor trafficking and metabolism

Alzheimer's disease (AD) is characterized by the intracranial accumulation of the 4 kDa amyloid-beta peptide (Abeta), following proteolysis of a approximately 700-amino acid, integral membrane precursor, the Alzheimer amyloid precursor protein (APP). The best evidence causally linking APP to AD has been provided by the discovery of mutations within the APP coding sequence that segregate with disease phenotypes in autosomal dominant forms of familial AD (FAD). Though FAD is rare ( < 10% of all AD), the hallmark features (amyloid plaques, neurofibrillary tangles, synaptic and neuronal loss, neurotransmitter deficits and dementia) are indistinguishable when FAD is compared with typical, common, 'non-familial', or sporadic, AD (SAD). Studies of some clinically relevant mutant APP molecules from FAD families have yielded evidence that APP mutations can lead to the enhanced generation or aggregability of Abeta, consistent with a pathogenic role in AD. Other genetic loci for FAD have been discovered which are distinct from the immediate regulatory and coding regions of the APP gene, indicating that defects in molecules other than APP can also specify cerebral amyloidogenesis and FAD. To date, all APP and non-APP FAD mutations can be demonstrated to have the common feature of promoting amyloidogenesis of Abeta. Epidemiological studies indicate that postmenopausal women on estrogen replacement therapy (ERT) have their relative risk of developing SAD diminished by about one third as compared with age-matched women not receiving ERT [M.X. Tang, D. Jacobs, Y. Stern, K. Marder, P. Schofield, B. Gurland, H. Andrews, R. Mayeux, Effect of estrogen during menopause on risk and age at onset of Alzheimer's disease, Lancet 348 (2000) 429432]. Because of the key role of cerebral Abeta accumulation in initiating AD pathology, it is most attractive that estradiol might modulate SAD risk or age-at-onset by inhibiting Abeta accumulation. A possible mechanistic basis for such a scenario is reviewed here.

Neurohormonal Signaling Pathways and the Regulation of Alzheimer beta-Amyloid Precursor Metabolism

Postmenopausal women on estrogen replacement appear to have their relative risk of developing Alzheimer's disease diminished by about one half. Because brain amyloid accumulation plays a key role in initiating Alzheimer's pathology, it is attractive to postulate that estrogen might modulate Alzheimer's risk by inhibiting amyloid accumulation. Data and cell biological models supporting such a scenario are reviewed here.

Cleavage of Alzheimer's amyloid precursor protein by alpha-secretase occurs at the surface of neuronal cells

The amyloid precursor protein (APP) is proteolytically processed predominantly by alpha-secretase to release the ectodomain (sAPPalpha). In this study, we have addressed the cellular location of the constitutive alpha-secretase cleavage of endogenous APP in a neuronal cell line. Incubation of the neuroblastoma cell line IMR32 at 20 degrees C prevented the secretion into the medium of soluble wild-type APP cleaved by alpha-secretase as revealed by both immunoelectrophoretic blot analysis with a site-specific antibody and immunoprecipitation following metabolic labeling of the cells. No sAPPalpha was detected in the cell lysates following incubation of the cells at 20 degrees C, indicating that alpha-secretase does not cleave APP in the secretory pathway prior to or within the trans-Golgi network. Parallel studies using an antibody that recognizes specifically the neoepitope revealed on soluble APP cleaved by beta-secretase indicated that this enzyme was acting intracellularly. alpha-Secretase is a zinc metalloproteinase susceptible to inhibition by hydroxamate-based compounds such as batimastat [Parvathy, S., et al. (1998) Biochemistry 37, 1680-1685]. Incubation of the cells with a cell-impermeant, biotinylated hydroxamate inhibitor inhibited the release of sAPPalpha by >92%, indicating that alpha-secretase is cleaving APP almost exclusively at the cell surface. The observation that alpha-secretase cleaves APP at the cell surface, while beta-secretase can act earlier in the secretory pathway within the neuronal cell line indicates that there must be strict control mechanisms in place to ensure that APP is normally cleaved primarily by alpha-secretase in the nonamyloidogenic pathway to produce the neuroprotective sAPPalpha.

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.

Modulation of amyloid precursor protein processing by compounds with various mechanisms of action: detection by liquid phase electrochemiluminescent system

beta-Amyloid peptide (A beta) is a major component of senile plaques formed in the brain of Alzheimer disease patients. We describe here a new method of quantitating A beta in biological material using liquid phase electrochemiluminescent system (LPECL). We used both enzyme-linked immunosorbent assay (ELISA) and LPECL methods to measure A beta and APPs alpha levels in conditioned medium of beta-amyloid precursor protein (APP)-transfected CHO cells treated with known modulators of APP processing, and in CSF and plasma of guinea pigs. Our results indicate that while maintaining the accuracy and sensitivity of ELISA, LPECL is a significantly taster and less labor-intensive method for measuring of A beta and APPs alpha levels in biological fluids.

Mutant genes in familial Alzheimer's disease and transgenic models

The most common cause of dementia occurring in mid- to late-life is Alzheimer's disease (AD). Some cases of AD, particularly those of early onset, are familial and inherited as autosomal dominant disorders linked to the presence of mutant genes that encode the amyloid precursor protein (APP) or the presenilins (PS1 or PS2). These mutant gene products cause dysfunction/death of vulnerable populations of nerve cells important in memory, higher cognitive processes, and behavior. AD affects 7-10% of individuals > 65 years of age and perhaps 40% of individuals > 80 years of age. For the late-onset cases, the principal risk factors are age and apolipoprotein (apoE) allele type, with apoE4 allele being a susceptibility factor. In this review, we briefly discuss the clinical syndrome of AD and the neurobiology/neuropathology of the disease and then focus attention on mutant genes linked to autosomal dominant familial AD (FAD), the biology of the proteins encoded by these genes, and the recent exciting progress in investigations of genetically engineered animal models that express these mutant genes and develop some features of AD.

Characterization of endogenous APP processing in a cell-free system

We have developed a simple in vitro assay using tissue homogenates that allows detection and characterization of several endogenous proteolytic activities which convert Alzheimer's amyloid precursor protein (APP) to the smaller, carboxy-terminal fragments, postulated to be intermediates in the formation of β-amyloid peptide (Aβ). Incubation at 37°C results in the degradation of transmembrane APP and formation of a mixture of carboxy-terminal containing peptides with mass values of 9-12 kDa. Epitope mapping and electrophoretic comparison with a truncated APP standard showed one of these peptides to contain the entire Aβ sequence. Analysis of pH dependence shows that formation of this carboxy-terminal product as well as another fragment, that is the likely product of 'secretase' activity, requires acidic pH. This suggests that cleavage of full-length APP to secreted forms may take place in an acidic intracellular compartment.

A beta-amyloid peptide variant related with familial Alzheimer's disease and hereditary cerebral hemorrhage with amyloidosis is poorly eliminated by cathepsin D

The cerebral deposition of 40-42 residue amyloid beta-protein (Abeta) is a characteristic of Alzheimer's disease. Cathepsin D is possibly involved in the intracellular clearance of Abeta (Hamazaki, H. (1996) FEBS Lett., in press). The present work shows that cathepsin D hydrolyzes wild-type Abeta 20 times faster than a variant Abeta with a substitution at residue 21 from Ala to Gly. Since the substitution has been linked to familial Alzheimer's disease and hereditary cerebral hemorrhage with amyloidosis (Hendriks et al. (1992) Nature Genet. 1, 218-221), the present observations suggest that the inefficient elimination of Abeta by cathepsin D is capable of being one of causes of the amyloid fibril formation.

Beta-amyloid precursor protein. Location of transmembrane domain and specificity of gamma-secretase cleavage

The formation of beta-amyloid by processing of its precursor protein is a characteristic of Alzheimer's disease. Two proteolytic cleavages produce the amino and carboxyl termini of beta-amyloid, with the latter cleavage site located within the transmembrane domain. Using DNA mutagenesis, we investigated the membrane position and sequence requirements for carboxyl-terminal processing of the beta-amyloid domain. Substitution of negatively charged residues across positions 40-46 of the beta-amyloid domain precluded both beta-amyloid formation and precursor maturation associated with secretory protein transport. In contrast, identical substitutions from positions 48-50 had no adverse effects. Since charged residues typically prevent protein membrane insertion, these data define the membrane boundary to position 46/47, a location allowing greater access to carboxyl-terminal processing of beta-amyloid, possibly without membrane destruction. Deletions within the carboxyl-terminal domain, including 4 residues spanning positions 39-42 of beta-amyloid, resulted in formation of the beta-amyloid peptide. Substituting residues 38-47 or 39-56 of the beta-amyloid domain in the precursor with a transmembrane sequence from another protein yielded a approximately 4-kDa beta-amyloid peptide, reflecting a loose residue specificity for carboxyl-terminal processing to beta-amyloid.

Proteolytic release of membrane-bound angiotensin-converting enzyme: role of the juxtamembrane stalk sequence

Many structurally and functionally diverse membrane proteins are solubilized by a specific proteolytic cleavage in the stalk sequence adjacent to the membrane anchor, with release of the extracellular domain. Examples are the amyloid precursor protein, membrane-bound growth factors, and angiotensin-converting enzyme (ACE). The identities and characteristics of the responsible proteases remain elusive. We have studied this process in Chinese hamster ovary (CHO) cells stably expressing wild-type ACE (WT-ACE; human testis isozyme) or one of four juxtamembrane (stalk) mutants containing either deletions of 17, 24, and 47 residues (ACE-JM delta 17, -JM delta 24, and -JM delta 47, respectively) or a substitution of 26 stalk residues with a 20-residue sequence from the stalk of the low-density lipoprotein receptor (ACE-JMLDL). The C termini of released, soluble WT-ACE and ACE-JM delta 17 and -JMLDL were determined by MALDI-TOF mass spectrometry analyses of C-terminal peptides generated by CNBr cleavage. Observed masses of 4264 (WT-ACE) and 4269 (ACE-JM delta 17) are in good agreement with an expected mass of 4262 for the C-terminal CNBr peptide ending at Arg-627, indicating cleavage at the Arg-627/Ser-628 bond in both WT-ACE and ACE-JM delta 17, at distances of 24 and 10 residues from the membrane, respectively. Data for ACE-JM delta 24 are also consistent with cleavage at or near Arg-627. For ACE-JMLDL, in which the native cleavage site is absent, observed masses of 4372 and 4542 are in close agreement with expected masses of 4371 and 4542 for peptides ending at Ala-628 and Gly-630, respectively, indicating cleavages at 17 or 15 residues from the membrane. These data indicate that the membrane-protein-solubilizing protease (MPSP) in CHO cells is not constrained by a particular cleavage site motif or by a specific distance from the membrane but instead may position itself with respect to the putative proximal, folded extracellular domain adjacent to the stalk. Nevertheless, cleavage at a distance of 10 residues from the membrane is more favorable, as ACE-JM delta 17 is cleaved 12-fold faster than WT-ACE. In contrast, ACE-JM delta 24 is released 17-fold slower, suggesting that a minimum distance from the membrane must be preserved. This is supported by results with the ACE-JM delta 47 mutant, which is membrane-bound but not cleaved, likely because the entire stalk has been deleted. Finally, soluble full-length (anchor-plus) WT-ACE is not cleaved when incubated with various CHO cell fractions or intact CHO cells. On the basis of these and other data, we propose that the CHO cell MPSP that solubilizes ACE (1) only cleaves proteins embedded in a membrane; (2) requires an accessible stalk and cleaves at a minimum distance from both the membrane and proximal extracellular domain; (3) positions itself primarily with respect to the proximal extracellular domain; and (4) may have a weak preference for cleavage at Arg/Lys-X bonds.

Structural requirements for the ectodomain cleavage of human cell surface macrophage colony-stimulating factor

One form of human macrophage colony-stimulating factor (CSF-1(256), M-CSFalpha) is a member of a restricted set of cell surface transmembrane proteins, which is selected to undergo proteolytic ectodomain cleavage. To determine the substrate requirements for this cleavage, we have constructed a series of mutations in the cytoplasmic tail, transmembrane domain, and juxtamembrane region of CSF-1(256) and stably expressed the mutated genes in NIH 3T3 cells. Our results demonstrate that membrane association of the CSF-1 precursor is required for cleavage of its growth factor ectodomain and furthermore that the juxtamembrane region Pro161-Gln162-Leu163-Gln164-Glu165 (PQLQE) (residues 161-165 of the ectodomain) is an essential determinant of cell surface CSF-1(256) cleavage and that the cleavage site is partially sequence-specific. Furthermore, a mechanism of steric hindrance, which likely involves interference with protease accessibility, is postulated to explain the observed decreases in the cleavage efficiency in certain CSF-1 mutants. Finally, our results strongly suggest that the CSF-1 ectodomain is cleaved at or very near the cell surface by a membrane-associated proteolytic system.

Hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D): II--A review of histopathological aspects

Cerebral amyloid-beta (A beta) angiopathy is the histopathological hallmark of hereditary cerebral hemorrhage with amyloidosis (Dutch) (HCHWA-D). A beta deposits are found mainly in the cerebral and cerebellar meningocortical blood vessels and as plaques throughout the cerebrocortical gray matter. A beta deposition in arteries and arterioles starts at the junction of media and adventitia and proceeds to involve the media causing degeneration of the vascular smooth muscle cells. Cerebrocortical arterioles often show one or two layers of radial A beta around a layer of homogenous A beta that replaces the media. Degenerating neurites, reactive astrocytes and microglial cells may surround cerebrocortical angiopathic arterioles and capillaries, probably in reaction to invasion of the perivascular neuropil by A beta fibrils. Furthermore, clusters of coarse extracellular matrix deposits may be found near A beta-laden cerebrocortical arterioles. The amyloid-associated proteins, cystatin C, and beta PP colocalize diffusely with Dutch vascular A beta, whereas HLA-DR immunoreactivity is found only in the periphery of the diseased vessel wall. The latter phenomenon may be related to the presence of perivascular cells. Angiopathic blood vessels frequently show structural changes. The relation of the described pathology to the development of hemorrhage, infarction and leukoencephalopathy needs further elucidation.

Hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D): I--A review of clinical, radiologic and genetic aspects

Hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D) is an autosomal dominant disease caused by deposition of beta-amyloid in the leptomeningeal arteries and cortical arterioles, in addition to preamyloid deposits and amyloid plaques in the brain parenchyma. The disease is due to a point mutation at codon 693 of the amyloid precursor protein (beta PP) gene at chromosome 21. Since this point mutation is diagnostic for HCHWA-D, presymptomatic testing is feasible and offered, together with genetic counselling and psychological support, to subjects at risk. HCHWA-D is clinically characterized by recurrent strokes, in addition to dementia, which can occur after the first stroke but also preceding it. Radiological studies revealed focal lesions (hemorrhages, hemorrhagic and non-hemorrhagic infarctions) and diffuse white matter damage. Diffuse white matter hyperintensities on MRI are an early symptom of HCHWA-D since they have been found on MRI scans of subjects who had not suffered a stroke. The presence of the diagnostic point mutation makes HCHWA-D a useful model to study the effects of cerebral amyloid angiopathy in vivo. The characteristic pathological abnormalities and its implications for Alzheimer's disease will be discussed in Part II of this article.

Shedding of the lymphocyte L-selectin adhesion molecule is inhibited by a hydroxamic acid-based protease inhibitor. Identification with an L-selectin-alkaline phosphatase reporter

Expression of the L-selectin adhesion molecule can be rapidly down-modulated by regulated proteolysis at a membrane-proximal site. The L-selectin secretase has remained undefined, and the secretase activity is resistant to a broad panel of common protease inhibitors. We have developed an L-selectin-alkaline phosphatase reporter, consisting of the ectodomain of human placental alkaline phosphatase fused to the membrane-proximal cleavage, transmembrane, and cytoplasmic domains of L-selectin, to aid in the screening for L-selectin secretase inhibitors. A hydroxamic acid-based metalloprotease inhibitor, KD-IX-73-4, inhibited release of the L-selectin-alkaline phosphatase reporter in a dose-dependent manner. The hydroxamic acid-based peptide was also found to inhibit wild type L-selectin down-regulation from the surfaces of phorbol myristate acetate-activated peripheral blood lymphocytes and phytohemagglutinin-stimulated lymphoblasts. Analysis of the proteolytic cleavage fragments of L-selectin confirmed that KD-IX-73-4 inhibited L-selectin proteolysis. Lymphocyte L-selectin was not down-regulated when co-cultured with formylmethionylleucylphenylalanine-stimulated neutrophils, suggesting that the putative secretase acts in cis with the membrane-bound L-selectin. These results suggest that the L-selectin secretase activity may involve a cell surface, zinc-dependent metalloprotease, although L-selectin shedding is not affected by EDTA and may be related to the recently described activity involved in processing of membrane-bound TNF-alpha.

Exchanging the extracellular domain of amyloid precursor protein for horseradish peroxidase does not interfere with alpha-secretase cleavage of the beta-amyloid region, but randomizes secretion in Madin-Darby canine kidney cells

Secretory processing and polarized sorting of horseradish peroxidase fused to the amyloid precursor protein transmembrane domain were compared with those of wild-type amyloid precursor protein in COS and polarized Madin-Darby canine kidney (MDCK) cells. The cellular and secreted forms of the chimeric protein were enzymatically active in colorimetric and cytochemical assays after reconstitution with hemin and Ca2+. The peroxidase enzyme was secreted by a proteolytic process, similar to the parent amyloid precursor protein. In polarized MDCK cells, amyloid precursor protein was secreted exclusively in the basolateral compartment, while the peroxidase chimeric protein was secreted in both compartments. The basolateral sorting determinant for secretion must therefore be located in the extracellular domain of amyloid precursor protein. On the other hand, cell surface-associated peroxidase chimeric protein was similar to cell surface-associated wild-type amyloid precursor protein, mainly expressed at the basolateral side. The basolateral cell-surface expression, in contrast to the basolateral secretion, is therefore controlled by determinants in the cytoplasmic domain. Methylamine inhibited and bafilomycin slightly increased the basolateral secretion of both proteins, but both drugs strongly increased apical secretion. The default secretory pathway of COS cells and the basolateral (but not the apical) secretory pathway of MDCK cells are therefore comparably sensitive to methylamine and not to bafilomycin.

Potential beta PP-processing proteinase activities from Alzheimer's and control brain tissues

Fluorogenic peptide substrates designed to encompass the reported alpha-secretory and amyloidogenic cleavage sites of the amyloid-beta precursor protein (beta PP) were used to analyze proteinase activities in brain extracts from control patients and those with Alzheimer's disease (AD). Activity against the secretory substrate at pH 7.5 in control and AD brains produced a major endopeptidase cleavage at the Lys687-Leu688 bond (beta PP770 numbering), consistent with the beta PP secretase cleavage. Activity in control brains against the amyloidogenic substrate at pH 7.5 produced one cleavage at the Ala673-Glu674 bond, two residues C-terminal to the amyloidogenic Met-Asp site. However, in three of four AD brains, the major cleavage was at the Asp-Ala bond, one residue from the amyloidogenic site. Both endopeptidase and carboxypeptidase activities in AD brains were lower than in control brains. Proteinase activities against the secretory substrate had a major optimum at pH 3.0-4.0 and another at pH 6.0-7.5. Proteinase activities against the amyloidogenic substrate had a major optimum at or below pH 3.0 and another at pH 6.0. Using both substrates, activities at low pH were higher in AD-brains than in controls, while at pH above 6.5, activities in control brains were higher than in AD. These results indicate that the levels of proteolytic enzymes in AD brains are altered relative to controls.

Altered cleavage and secretion of a recombinant beta-APP bearing the Swedish familial Alzheimer's disease mutation

Mutations within the beta-amyloid precursor protein gene cosegregate with the early-onset form of familial Alzheimer's Disease (FAD). It is not known how these mutations result in disease; however, one early-onset AD mutation in a Swedish kindred increases potentially amyloidogenic fragments and beta-protein production in cells expressing the mutant beta-APP. Using a novel recombinant reporter system we found a qualitative change in the secreted product, from cleavage within the beta-protein sequence to cleavage near the N-terminal region of the beta-protein, even though the total amount of secreted mutant product is similar to wild-type. The results suggest that the increased formation of potentially amyloidogenic fragments in cells expressing the Swedish FAD occurs by enzymatic cleavage in the secretory pathway. Alterations in the secretory process may predispose an individual to AD.