Evidence for a six-transmembrane domain structure of presenilin 1

Molecular insight of DREAM and presenilin 1 C-terminal fragment interactions

Interactions between downstream regulatory element antagonist modulator (DREAM) and presenilin 1 (PS1) are related to numerous neuronal processes. We demonstrate that association of PS1 carboxyl peptide (residues 445-467, HL9) with DREAM is calcium dependent and stabilized by a cluster of three aromatic residues: F462 and F465 from PS1 and F252 from DREAM. Additional stabilization is provided by residues in a loop connecting α helices 7 and 8 in DREAM and residues of PS1, namely cation-π interactions between R200 in DREAM and F465 in PS1 and the salt bridges formed by R207 in DREAM and D450 and D458 in PS1.

Peptides of presenilin-1 bind the amyloid precursor protein ectodomain and offer a novel and specific therapeutic approach to reduce ß-amyloid in Alzheimer's disease

β-Amyloid (Aβ) accumulation in the brain is widely accepted to be critical to the development of Alzheimer's disease (AD). Current efforts at reducing toxic Aβ40 or 42 have largely focused on modulating γ-secretase activity to produce shorter, less toxic Aβ, while attempting to spare other secretase functions. In this paper we provide data that offer the potential for a new approach for the treatment of AD. The method is based on our previous findings that the production of Aβ from the interaction between the β-amyloid precursor protein (APP) and Presenilin (PS), as part of the γ-secretase complex, in cell culture is largely inhibited if the entire water-soluble NH2-terminal domain of PS is first added to the culture. Here we demonstrate that two small, non-overlapping water-soluble peptides from the PS-1 NH2-terminal domain can substantially and specifically inhibit the production of total Aβ as well as Aβ40 and 42 in vitro and in vivo in the brains of APP transgenic mice. These results suggest that the inhibitory activity of the entire amino terminal domain of PS-1 on Aβ production is largely focused in a few smaller sequences within that domain. Using biolayer interferometry and confocal microscopy we provide evidence that peptides effective in reducing Aβ give a strong, specific and biologically relevant binding with the purified ectodomain of APP 695. Finally, we demonstrate that the reduction of Aβ by the peptides does not affect the catalytic activities of β- or γ-secretase, or the level of APP. P4 and P8 are the first reported protein site-specific small peptides to reduce Aβ production in model systems of AD. These peptides and their derivatives offer new potential drug candidates for the treatment of AD.

Novel presenilin mutations within Moroccan patients with Early-Onset Alzheimer's Disease

Alzheimer's disease (AD) is a progressive brain disorder that causes gradual and irreversible loss of higher brain functions and is the most common cause of dementia in the elderly, as assessed by autopsy and clinical series. Furthermore, it has an annual incidence of approximately 3% in the 65-74-year-old age group. This incidence rate doubles with every increment of 5 years above the age of 65. In Morocco, AD affects almost 30,000 individuals and this number will possibly increase to 75,000 by 2020 (projections of the World Health Organization (WHO)). Genetically, AD is caused by a mutation in one of at least 3 genes: presenilin 1 (PS1), presenilin 2 (PS2) and the amyloid precursor protein (APP). Most cases are late onset and apparently sporadic, most likely as a result of a combination of environmental and non-dominant genetic factors. In Morocco, the genes predisposing individuals to AD and predicting disease incidence remain elusive. The purpose of the present study was to evaluate the genetic contribution of mutations in PS1 and PS2 genes to familial early-onset AD cases and sporadic late-onset AD cases. Seventeen sporadic late-onset AD cases and eight familial early-onset AD cases were seen at the memory clinic of the University of Casablanca Neurology Department. These patients underwent standard somatic neurological examination, cognitive function assessment, brain imaging and laboratory tests. Direct sequencing of each exon in PS1 and PS2 genes was performed on genomic DNA of AD patients. Further, we identified 1 novel frameshift mutation in the PS1 gene and 2 novel frameshift mutations in the PS2 gene. Our mutational analysis reports a correlation between clinical symptoms and genetic factors in our cases of Early-Onset Alzheimer's Disease (EOAD). These putative mutations cosegregate with affected family members suggesting a direct mutagenic effect.

Structural and Functional Determinants of gamma-Secretase, an Intramembrane Protease Implicated in Alzheimer's Disease

Alzheimer’s disease is the most common form of neurodegenerative diseases in humans, characterized by the progressive accumulation and aggregation of amyloid-β peptides (Aβ) in brain regions subserving memory and cognition. These 39-43 amino acids long peptides are generated by the sequential proteolytic cleavages of the amyloid-β precursor protein (APP) by β- and γ-secretases, with the latter being the founding member of a new class of intramembrane-cleaving proteases (I-CliPs) characterized by their intramembranous catalytic residues hydrolyzing the peptide bonds within the transmembrane regions of their respective substrates. These proteases include the S2P family of metalloproteases, the Rhomboid family of serine proteases, and two aspartyl proteases: the signal peptide peptidase (SPP) and γ-secretase. In sharp contrast to Rhomboid and SPP that function as a single component, γ-secretase is a multi-component protease with complex assembly, maturation and activation processes. Recently, two low-resolution three-dimensional structures of γ-secretase and three high-resolution structures of the GlpG rhomboid protease have been obtained almost simultaneously by different laboratories. Although these proteases are unrelated by sequence or evolution, they seem to share common functional and structural mechanisms explaining how they catalyze intramembrane proteolysis. Indeed, a water-containing chamber in the catalytic cores of both γ-secretase and GlpG rhomboid provides the hydrophilic environment required for proteolysis and a lateral gating mechanism controls substrate access to the active site. The studies that have identified and characterized the structural determinants critical for the assembly and activity of the γ-secretase complex are reviewed here.

Effect of PKD1 gene missense mutations on polycystin-1 membrane topogenesis

Polycystin-1 (PC1), the product of the polycystic kidney disease-1 (PKD1) gene, has a number of reported missense mutations whose pathogenicity is indeterminate. Previously, we utilized N-linked glycosylation reporter tags along with membrane insertion and topology assays to define the 11 membrane-spanning domains (I-XI) of PC1. In this report, we utilize glycosylation assays to determine whether two reported human polymorphisms/missense mutations within transmembrane (TM) domains VI and X affect the membrane topology of PC1. M3677T within TM VI had no effect on the topology of this TM domain as shown by the ability of two native N-linked glycosylation sites within the extracellular loop following TM VI to be glycosylated. In contrast, G4031D, within TM X, decreased the glycosylation of TM X reporter constructs, demonstrating that the substitution affected the C-terminal translocating activity of TM X. Furthermore, G4031D reduced the membrane association of TM X and XI together. These results suggest that G4031D affects the membrane insertion and topology of the C-terminal portion of polycystin-1 and represents a bona fide pathogenic mutation.

Participation of transmembrane domain 1 of presenilin 1 in the catalytic pore structure of the γ-secretase

γ-Secretase is an intramembrane-cleaving protease that is responsible for the generation of amyloid-β peptides linked to the pathogenesis of Alzheimer's disease. Using a substituted cysteine accessibility method, we have previously shown that the hydrophilic "catalytic pore" structure of γ-secretase is formed by the transmembrane domains (TMDs) 6, 7, and 9 of presenilin 1 (PS1), the catalytic subunit of γ-secretase, within the membrane. Here, we analyzed the structure in and around the first hydrophobic region, the putative TMD1, of PS1, of which the precise function as well as three-dimensional location within γ-secretase remained unknown. We found that TMD1 is located in proximity to the catalytic GxGD and PAL motifs within the C-terminal fragment of PS1, facing directly the catalytic pore. Competition experiments using known γ-secretase inhibitors suggested that the N-terminal region of TMD1 functions as a subsite during proteolytic action of the γ-secretase. Intriguingly, binding of inhibitors affected water accessibility of residues at the membrane border of TMD1, suggesting the possibility of a dynamic motion of TMD1 during the catalytic process. Our results provide mechanistic insights into the functional role of TMD1 of PS1 in the intramembrane-cleaving activity of the γ-secretase.

Structural investigation of the C-terminal catalytic fragment of presenilin 1

The gamma-secretase complex has a decisive role in the development of Alzheimer's disease, in that it cleaves a precursor to create the amyloid beta peptide whose aggregates form the senile plaques encountered in the brains of patients. Gamma-secretase is a member of the intramembrane-cleaving proteases which process their transmembrane substrates within the bilayer. Many of the mutations encountered in early onset familial Alzheimer's disease are linked to presenilin 1, the catalytic component of gamma-secretase, whose active form requires its endoproteolytic cleavage into N-terminal and C-terminal fragments. Although there is general agreement regarding the topology of the N-terminal fragment, studies of the C-terminal fragment have yielded ambiguous and contradictory results that may be difficult to reconcile in the absence of structural information. Here we present the first structure of the C-terminal fragment of human presenilin 1, as obtained from NMR studies in SDS micelles. The structure reveals a topology where the membrane is likely traversed three times in accordance with the more generally accepted nine transmembrane domain model of presenilin 1, but contains unique structural features adapted to accommodate the unusual intramembrane catalysis. These include a putative half-membrane-spanning helix N-terminally harboring the catalytic aspartate, a severely kinked helical structure toward the C terminus as well as a soluble helix in the assumed-to-be unstructured N-terminal loop.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

Characterization of presenilin-amyloid precursor interaction using bacterial expression and two-hybrid systems for human membrane proteins

An Escherichia coli system was used to produce the human membrane proteins presenilin 1 and amyloid precursor protein and to analyse their interaction. Our data indicate that the main binding site for amyloid precursor protein is located in the N-terminal three-transmembrane segments of presenilin and not in the proposed active site containing the two conserved aspartate residues. The data also suggest the presence of an additional segment of sufficient hydrophobicity at the C-terminus of PS1 to act potentially as a transmembrane segment. The implications of these findings for the function of gamma-secretase are discussed.

Accurate determination of carboxyl-terminal fragment of presenilin 1 in various tissues from rat and cell lines

Presenilin 1 (PS1) has been identified as a causative gene for the early-onset of familial Alzheimer's disease, and it is mainly localized in the endoplasmic reticulum and the Golgi membrane as a multiple membrane-spanning protein. In the cell, PS1 is proteolytically processed to a 30-kDa N-terminal fragment and a 20-kDa C-terminal fragment (CTF), both of which exist as a stable high-molecular-weight protein complex, together with other components of gamma-secretase. However, as there has been no report about the precise amount of PS1 expressed in mammalian tissues, the aim of this study was to quantitatively determine PS1-CTF amounts in various tissues such as liver, kidney, brain and heart of rat by western blotting using a [(35)S]-methionine-labelled PS1-CTF as a standard synthesized in a wheat germ cell-free protein synthesizing system. PS1-CTF contents in kidney, liver, brain and heart were 17.0, 6.6, 6.4 and 0.2 fmol/mg protein, respectively. PS1-CTF contents were also determined in cultured cell lines such as HeLa, HEK293 and COS-1.

Cholesterol-like effects of selective cyclooxygenase inhibitors and fibrates on cellular membranes and amyloid-beta production

Strong evidence suggests a mechanistic link between cholesterol metabolism and the formation of amyloid-beta peptides, the principal constituents of senile plaques found in the brains of patients with Alzheimer's disease. Here, we show that several fibrates and diaryl heterocycle cyclooxygenase inhibitors, among them the commonly used drugs fenofibrate and celecoxib, exhibit effects similar to those of cholesterol on cellular membranes and amyloid precursor protein (APP) processing. These drugs have the same effects on membrane rigidity as cholesterol, monitored here by an increase in fluorescence anisotropy. The effect of the drugs on cellular membranes was also reflected in the inhibitory action on the sarco(endo)plasmic reticulum Ca(2+)-ATPase, which is known to be inhibited by excess ordering of membrane lipids. The drug-induced decrease of membrane fluidity correlated with an increased association of APP and its beta-site cleaving enzyme BACE1 with detergent-resistant membranes (DRMs), which represent membrane clusters of substantial rigidity. DRMs are hypothesized to serve as platforms for the amyloidogenic processing of APP. According to this hypothesis, both cholesterol and the examined compounds stimulated the beta-secretase cleavage of APP, resulting in a massive increase of secreted amyloid-beta peptides. The membrane-ordering potential of the drugs was observed in a cell-free assay, suggesting that the amyloid-beta promoting effect was analog to cholesterol due to primary effect on membrane rigidity. Because fenofibrate and celecoxib are widely used in humans as hypolipidemic drugs for prevention of atherosclerosis and as anti-inflammatory drugs against arthritis, possible side effects should be considered upon long-term clinical application.

Alzheimer disease: amyloidogenesis, the presenilins and animal models

Alzheimer's disease is the most prevalent form of dementia. Neuropathogenesis is proposed to be a result of the accumulation of amyloid beta peptides in the brain together with oxidative stress mechanisms and neuroinflammation. The presenilin proteins are central to the gamma-secretase cleavage of the amyloid prescursor protein (APP), releasing the amyloid beta peptide. Point mutations in the presenilin genes lead to cases of familial Alzheimer's disease by increasing APP cleavage resulting in excess amyloid beta formation. This review discusses the molecular mechanism of Alzheimer's disease with a focus on the presenilin genes. Alternative splicing of transcripts from these genes and how these may function in several disease states is discussed. There is an emphasis on the importance of animal models in elucidating the molecular mechanisms behind the development of Alzheimer's disease and how the zebrafish, Danio rerio, can be used as a model organism for analysis of presenilin function and Alzheimer's disease pathogenesis.

Insertion and topology of normal and mutant bestrophin-1 in the endoplasmic reticulum membrane

The vitelliform macular dystrophy type 2 (VMD2) gene mutated in Best macular dystrophy encodes a 585-amino acid putative transmembrane protein termed bestrophin-1. The vast majority of known disease-associated alterations are of the missense type, which cluster near predicted transmembrane domains (TMDs). To investigate bestrophin-1 membrane topology and to assess consequences of point mutations on membrane integration, we have analyzed the insertion of putative TMDs into the endoplasmic reticulum (ER) membrane. Out of six potential TMDs, our data suggest a topological model of bestrophin-1 with four transmembrane-spanning segments and one large cytoplasmatic loop between putative TMD2 and TMD5. Consequently, a relatively hydrophobic segment containing putative TMD3 (aa 130-149) and TMD4 (aa 179-201) is located within the cytoplasm. Furthermore, we show that three out of 18 disease-associated alterations investigated (I73N, Y85H, F281del) reveal measurable effects on membrane insertion suggesting that defective membrane integration of bestrophin-1 may represent a potential disease mechanism for a small subset of Best macular dystrophy-related mutations.

Presenilin-1 Maintains a Nine-Transmembrane Topology throughout the Secretory Pathway

Presenilin-1 is a polytopic membrane protein that assembles with nicastrin, PEN-2, and APH-1 into an active gamma-secretase complex required for intramembrane proteolysis of type I transmembrane proteins. Although essential for a correct understanding of structure-function relationships, its exact topology remains an issue of strong controversy. We revisited presenilin-1 topology by inserting glycosylation consensus sequences in human PS1 and expressing the obtained mutants in a presenilin-1 and 2 knock-out background. Based on the glycosylation status of these variants we provide evidence that presenilin-1 traffics through the Golgi after a conformational change induced by complex assembly. Based on our glycosylation variants of presenilin-1 we hypothesize that complex assembly occurs during transport between the endoplasmic reticulum and the Golgi apparatus. Furthermore, our data indicate that presenilin-1 has a nine-transmembrane domain topology with the COOH terminus exposed to the lumen/extracellular surface. This topology is independently underscored by lysine mutagenesis, cell surface biotinylation, and cysteine derivation strategies and is compatible with the different physiological functions assigned to presenilin-1.

Inhibition of gamma-secretase as a therapeutic intervention for Alzheimer's disease: prospects, limitations and strategies

Genetic and experimental evidence points to amyloid-beta (Abeta) peptide as the culprit in Alzheimer's disease pathogenesis. This protein fragment abnormally accumulates in the brain cortex and hippocampus of patients with Alzheimer's disease, and self-aggregates to form toxic oligomers causing neurodegeneration.Abeta is heterogeneous and produced from a precursor protein (amyloid precursor protein [APP]) by two sequential proteolytic cleavages that involve beta- and gamma-secretases. This latter enzyme represents a potentially attractive drug target since it dictates the solubility of the generated Abeta fragment by creating peptides of various lengths, namely Abeta(40) and Abeta(42), the longest being the most aggregating. gamma-Secretase comprises a molecular complex of four integral membrane proteins - presenilin, nicastrin, APH-1 and PEN-2 - and its molecular mechanism remains under extensive scrutiny. The ratio of Abeta(42) over Abeta(40) is increased by familial Alzheimer's disease mutations occurring in the presenilin genes or in APP, near the gamma-secretase cleavage site. Potent gamma-secretase inhibitors have been identified by screening drug libraries or by designing aspartyl protease transition-state analogues based on the APP substrate cleavage site. Most of these compounds are not specific for gamma-secretase cleavage of APP, and equally inhibit the processing of other gamma-secretase substrates, such as Notch and a subset of cell-surface receptors and proteins involved in embryonic development, haematopoiesis, cell adhesion and cell/cell contacts. Therefore, current research aims at finding compounds that show selectivity for APP cleavage, and particularly that inhibit the formation of the aggregating form, Abeta(42). Compounds that target the substrate docking site rather than the enzyme active site are also being investigated as an alternative strategy. The finding that some NSAID analogues preferentially inhibit the formation of Abeta(42) over Abeta(40) and do not affect Notch processing has opened a new therapeutic window. The progress in design of selective inhibitors as well as recent results obtained in animal studies prove that gamma-secretase remains among the best targets for the therapeutic control of amyloid build-up in Alzheimer's disease. The full understanding of gamma-secretase regulation may yet uncover new therapeutic leads.

Two novel presenilin 1 gene mutations connected with frontotemporal dementia-like clinical phenotype: Genetic and bioinformatic assessment

Mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) genes are associated with early-onset familial Alzheimer's disease (EOAD). There are several reports describing mutations in PSEN1 in cases with frontotemporal dementia (FTD). We identified two novel mutations in the PSEN1 gene: L226F and L424H. The first mutation was detected in a patient with a clinical diagnosis of FTD and a post-mortem diagnosis of AD. The second mutation is connected with a clinical phenotype of variant AD with strong FTD signs. In silico modeling revealed that the mutations, as well as mutations used for comparison (F177L and L424R), change the local structure, stability and/or properties of the transmembrane regions of the presenilin 1 protein (PS1). In contrast, a silent non-synonymous substitution F175S is eclipsed by external residues and has no influence on PS1 interfacial surface. We suggest that in silico analysis of PS1 substitutions can be used to characterize novel PSEN1 mutations, to discriminate between silent polymorphisms and a potential disease-causing mutation. We also propose that PSEN1 mutations should be considered in FTD patients with no MAPT mutations.

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.

C-terminal PAL motif of presenilin and presenilin homologues required for normal active site conformation

The Alzheimer's disease-associated beta-amyloid peptide is produced through cleavage of amyloid precursor protein by beta-secretase and gamma-secretase. gamma-Secretase is a complex containing presenilin (PS) as the catalytic component and three essential cofactors: Nicastrin, anterior pharynx defective (APH-1) and presenilin enhancer-2 (PEN-2). PS and signal peptide peptidase (SPP) define a novel family of aspartyl proteases that cleave substrates within the transmembrane domain presumptively using two membrane-embedded aspartic acid residues for catalysis. Apart from the two aspartate-containing active site motifs, the only other region that is conserved between PS and SPP is a PAL sequence at the C-terminus. Although it has been well documented that this motif is essential for gamma-secretase activity, the mechanism underlying such a critical role is not understood. Here we show that mutations in this motif affect the conformation of the active site of gamma-secretase resulting in a complete loss of PS binding to a gamma-secretase transition state analog inhibitor, Merck C. Analogous mutations in SPP significantly inhibit its enzymatic activity. Furthermore, these mutations also abolish SPP binding to Merck C, indicating that SPP and gamma-secretase share a similar active site conformation, which is dependent on the PAL motif. Exploring the amino acid requirements within this motif reveals a very small side chain requirement, which is conserved during evolution. Together, these observations strongly support the hypothesis that the PAL motif contributes to the active site conformation of gamma-secretase and of SPP.

Pen-2 is incorporated into the gamma-secretase complex through binding to transmembrane domain 4 of presenilin 1

gamma-Secretase is a multimeric membrane protein complex comprised of presenilin (PS), nicastrin (Nct), Aph-1, and Pen-2. It is a member of an atypical class of aspartic proteases that hydrolyzes peptide bonds within the membrane. During the biosynthetic process of the gamma-secretase complex, Nct and Aph-1 form a heterodimeric intermediate complex and bind to the C-terminal region of PS, serving as a stabilizing scaffold for the complex. Pen-2 is then recruited into this trimeric complex and triggers endoproteolysis of PS, conferring gamma-secretase activity. Although the Pen-2 accumulation depends on PS, the binding partner of Pen-2 within the gamma-secretase complex remains unknown. We reconstituted PS1 in Psen1/Psen2 deficient cells by expressing a series of PS1 mutants in which one of the N-terminal six transmembrane domains (TMDs) was swapped with those of CD4 (a type I transmembrane protein) or CLAC-P (a type II transmembrane protein). We report that the proximal two-thirds of TMD4 of PS1, including the conserved Trp-Asn-Phe sequence, are required for its interaction with Pen-2. Using a chimeric CD4 molecule harboring PS1 TMD4, we further demonstrate that the PS1 TMD4 bears a direct binding motif to Pen-2. Pen-2 may contribute to the activation of the gamma-secretase complex by directly binding to the TMD4 of PS1.

Assembly of the N-cadherin complex during synapse formation involves uncoupling of p120-catenin and association with presenilin 1

N-cadherin is an adhesion receptor that participates in both interaction between immature pre- and postsynaptic neurons and in the stabilization and function of matured neuron-neuron synapses. To better understand how the N-cadherin complex contributes to synapse formation, we examined its distribution and composition during synapse formation in the chick ciliary neurons. It was found that at early phases of synaptogenesis, N-cadherin is distributed in small clusters on the cell surface and primarily associates with p120-catenin and beta-catenin. In contrast, as synaptic contacts matured, larger N-cadherin clusters were found localized adjacent to the active zone and associated with PS1 and gamma-catenin, while p120- and beta-catenin were dispersed among other cell regions, including axons. As it is known that PS1 binds gamma-catenin and that uncoupled p120-catenin can alter the cytoskeleton via its effect on Rho GTPases, these changes in the molecular composition of the N-cadherin complex (represented by the uncoupling of p120-catenin and association with PS1) may correspond to distinct functional states of the complex involved in synaptic maturation.

Importance of N-glycosylation positioning for cell-surface expression, targeting, affinity and quality control of the human AT1 receptor

GPCRs (G-protein-coupled receptors) are preferentially N-glycosylated on ECL2 (extracellular loop 2). We previously showed that N-glycosylation of ECL2 was crucial for cell-surface expression of the hAT1 receptor (human angiotensin II receptor subtype 1). Here, we ask whether positioning of the N-glycosylation sites within the various ECLs of the receptor is a vital determinant in the functional expression of hAT(1) receptor at the cell surface. Artificial N-glycosylation sequons (Asn-Xaa-Ser/Thr) were engineered into ECL1, ECL2 and ECL3. N-glycosylation of ECL1 caused a very significant decrease in affinity and cell surface expression of the resulting receptor. Shifting the position of the ECL2 glycosylation site by two residues led to the synthesis of a misfolded receptor which, nevertheless, was trafficked to the cell surface. The misfolded nature of this receptor is supported by an increased interaction with the chaperone HSP70 (heat-shock protein 70). Introduction of N-glycosylation motifs into ECL3 yielded mutant receptors with normal affinity, but low levels of cell surface expression caused by proteasomal degradation. This behaviour differed from that observed for the aglycosylated receptor, which accumulated in the endoplasmic reticulum. These results show how positioning of the N-glycosylation sites altered many properties of the AT1 receptor, such as targeting, folding, affinity, cell surface expression and quality control.

A nine-transmembrane domain topology for presenilin 1

Presenilin (PS) provides the catalytic core of the gamma-secretase complex. Gamma-secretase activity leads to generation of the amyloid beta-peptide, a key event implicated in the pathogenesis of Alzheimer disease. PS has ten hydrophobic regions, which can all theoretically form membrane-spanning domains. Various topology models have been proposed, and the prevalent view holds that PS has an eight-transmembrane (TM) domain organization; however, the precise topology has not been unequivocally determined. Previous topological studies are based on non-functional truncated variants of PS proteins fused to reporter domains, or immunocytochemical staining. In this study, we used a more subtle N-linked glycosylation scanning approach, which allowed us to assess the topology of functional PS1 molecules. Glycosylation acceptor sequences were introduced into full-length human PS1, and the results showed that the first hydrophilic loop is oriented toward the lumen of the endoplasmic reticulum, whereas the N terminus and large hydrophilic loop are in the cytosol. Although this is in accordance with most current models, our data unexpectedly revealed that the C terminus localized to the luminal side of the endoplasmic reticulum. Additional studies on the glycosylation pattern after TM domain deletions, combined with computer-based TM protein topology predictions and biotinylation assays of different PS1 mutants, led us to conclude that PS1 has nine TM domains and that the C terminus locates to the lumen/extracellular space.

Presenilin function and gamma-secretase activity

Alzheimer's disease (AD) is the most common form of dementia and is characterized pathologically by the accumulation of beta-amyloid (Abeta) plaques and neurofibrillary tangles in the brain. Genetic studies of AD first highlighted the importance of the presenilins (PS). Subsequent functional studies have demonstrated that PS form the catalytic subunit of the gamma-secretase complex that produces the Abeta peptide, confirming the central role of PS in AD biology. Here, we review the studies that have characterized PS function in the gamma-secretase complex in Caenorhabditis elegans, mice and in in vitro cell culture systems, including studies of PS structure, PS interactions with substrates and other gamma-secretase complex members, and the evidence supporting the hypothesis that PS are aspartyl proteases that are active in intramembranous proteolysis. A thorough knowledge of the mechanism of PS cleavage in the context of the gamma-secretase complex will further our understanding of the molecular mechanisms that cause AD, and may allow the development of therapeutics that can alter Abeta production and modify the risk for AD.

A novel transmembrane topology of presenilin based on reconciling experimental and computational evidence

The transmembrane topology of presenilins is still the subject of debate despite many experimental topology studies using antibodies or gene fusions. The results from these studies are partly contradictory and consequently several topology models have been proposed. Studies of presenilin-interacting proteins have produced further contradiction, primarily regarding the location of the C-terminus. It is thus impossible to produce a topology model that agrees with all published data on presenilin. We have analyzed the presenilin topology through computational sequence analysis of the presenilin family and the homologous presenilin-like protein family. Members of these families are intramembrane-cleaving aspartyl proteases. Although the overall sequence homology between the two families is low, they share the conserved putative active site residues and the conserved 'PAL' motif. Therefore, the topology model for the presenilin-like proteins can give some clues about the presenilin topology. Here we propose a novel nine-transmembrane topology with the C-terminus in the extracytosolic space. This model has strong support from published data on gamma-secretase function and presenilin topology. Contrary to most presenilin topology models, we show that hydrophobic region X is probably a transmembrane segment. Consequently, the C-terminus would be located in the extracytosolic space. However, the last C-terminal amino acids are relatively hydrophobic and in conjunction with existing experimental data we cannot exclude the possibility that the extreme C-terminus could be buried within the gamma-secretase complex. This might explain the difficulties in obtaining consistent experimental evidence regarding the location of the C-terminal region of presenilin.

Presenilin endoproteolysis is an intramolecular cleavage

Mutations in the presenilin genes (PS) account for most cases of familial Alzheimer's disease. PS contain the active site of the gamma-secretase complex that cleaves within the transmembrane domain of beta-amyloid precursor protein (APP). Full-length PS undergoes regulated endoproteolysis to produce fragments that comprise the active form of PS. The "presenilinase" responsible for endoproteolysis is unknown but may be the same presenilin-dependent gamma-secretase activity that cleaves APP. To investigate the mechanism of endoproteolysis, we examined sequence specificity at the cleavage site and tested whether PS dimers are important for endoproteolysis as well as gamma-secretase activity. No single point mutation, or a double mutation M292D/V293K, was able to completely abolish endoproteolysis and all mutants supported gamma-secretase activity. When wtPS1 was co-expressed with either M292D/V293K or D257A, it was unable to restore normal endoproteolysis to either mutant. Lack of transcleavage by wtPS1 suggests that PS1 endoproteolysis occurs via intramolecular cleavage and does not require dimerization.

Evidence that the COOH terminus of human presenilin 1 is located in extracytoplasmic space

The polytopic membrane protein presenilin 1 (PS1) is a component of the gamma-secretase complex that is responsible for the intramembranous cleavage of several type I transmembrane proteins, including the beta-amyloid precursor protein (APP). Mutations of PS1, apparently leading to aberrant processing of APP, have been genetically linked to early-onset familial Alzheimer's disease. PS1 contains 10 hydrophobic regions (HRs) sufficiently long to be alpha-helical membrane spanning segments. Most topology models for PS1 place its COOH terminal approximately 40 amino acids, which include HR 10, in the cytosolic space. However, several recent observations suggest that HR 10 may be integrated into the membrane and involved in the interaction between PS1 and APP. We have applied three independent methodologies to investigate the location of HR 10 and the extreme COOH terminus of PS1. The results from these methods indicate that HR 10 spans the membrane and that the COOH terminal amino acids of PS1 lie in the extracytoplasmic space.

The presenilins turned inside out: implications for their structures and functions

The presenilin (PS) proteins are polytopic integral membrane proteins that are critically involved in the development of Alzheimer's disease. The topography of the PS molecule in the endoplasmic reticulum membrane is widely accepted as exhibiting eight-hydrophobic-transmembrane (8-TM) helices. We have previously provided evidence, however, that the intact PS molecule is also present in the cell surface where it exhibits exclusively a 7-TM topography, which differs in significant structural features from the 8-TM model. This evidence, however, has been disparaged and generally rejected by researchers in Alzheimer's disease. The 7-TM model is definitively demonstrated in the present study for PS-1 at the surfaces of PS-1-transfected cells and for endogenous PS-1 at the surfaces of untransfected cells, by immunofluorescence studies using mAbs. These studies force substantial revision of current views of the structural and functional properties of the PS proteins.

PEN-2 and APH-1 coordinately regulate proteolytic processing of presenilin 1

Presenilin (PS, PS1/PS2) complexes are known to be responsible for the intramembranous gamma-secretase cleavage of the beta-amyloid precursor protein and signaling receptor Notch. PS holoprotein undergoes endoproteolysis by an unknown enzymatic activity to generate NH(2)- and COOH-terminal fragments, a process that is required for the formation of the active and stable PS/-gamma-secretase complex. Biochemical and genetic studies have recently identified nicastrin, APH-1, and PEN-2 as essential cofactors that physically interact with PS1 and are necessary for the gamma-secretase activity. However, their precise function in regulating the PS complex and gamma-secretase activity remains unknown. Here, we demonstrate that endogenous PEN-2 preferentially interacts with PS1 holoprotein. Down-regulation of PEN-2 expression by small interfering RNA (siRNA) abolishes the endoproteolysis of PS1, whereas overexpression of PEN-2 promotes the production of PS1 fragments, indicating a critical role for PEN-2 in PS1 endoproteolysis. Interestingly, accumulation of full-length PS1 resulting from down-regulation of PEN-2 is alleviated by additional siRNA down-regulation of APH-1. Furthermore, overexpression of APH-1 facilitates PEN-2-mediated PS1 proteolysis, resulting in a significant increase in PS1 fragments. Our data reveal a direct role of PEN-2 in proteolytic cleavage of PS1 and a regulatory function of APH-1, in coordination with PEN-2, in the biogenesis of the PS1 complex.

Association between presenilin-1 Glu318Gly mutation and familial Alzheimer's disease in the Australian population

Mutations in the presenilin-1 (PS-1) gene on chromosome 14 account for the majority of early-onset familial Alzheimer's disease (FAD) cases. To date, more than 90 mutations have been identified and, while most of these mutations are completely penetrant, the Glu318Gly mutation has been suggested to be partially penetrant. These findings indicate that it may play a similar role to apolipoprotein E (APOE)-epsilon4 by acting as a genetic risk factor for AD. In the current study, a total of 682 subjects were tested to assess the frequency of the Glu318Gly mutation in AD in the Australian population. The Glu318Gly mutation was identified in six sporadic late-onset AD patients, four FAD patients (unrelated) and in nine control subjects. The frequency of this mutation was highest in the familial AD group (8.7%) and lowest in control subjects (2.2%). When the mutation frequencies were compared, we found a statistically significant difference between the latter two groups (Fisher's exact test, P < 0.05). The genotype frequency of the Glu318Gly mutation in all AD cases and controls in the Australian population was 2.8%. This frequency is comparable to that observed for the Dutch population (3.2%), but not for the Finnish population (6.8% and 6.0%) or the Spanish population (5.3%). These findings show that the frequency of the Glu318Gly mutation is increased in FAD patients, suggesting a potential role as a genetic risk factor contributing to the pathogenesis of familial AD.

Overexpression of wild type but not an FAD mutant presenilin-1 promotes neurogenesis in the hippocampus of adult mice

Mutations in the presenilin-1 (PS-1) gene are one cause of familial Alzheimer's disease (FAD). However, the functions of the PS-1 protein as well as how PS-1 mutations cause FAD are incompletely understood. Here we investigated if neuronal overexpression of wild-type or FAD mutant PS-1 in transgenic mice affects neurogenesis in the hippocampus of adult animals. We show that either a wild-type or an FAD mutant PS-1 transgene reduces the number of neural progenitors in the dentate gyrus. However, the wild-type, but not the FAD mutant PS-1 promoted the survival and differentiation of progenitors leading to more immature granule cell neurons being generated in PS-1 wild type expressing animals. These studies suggest that PS-1 plays a role in regulating neurogenesis in adult hippocampus and that FAD mutants may have deleterious properties independent of their effects on amyloid deposition.

Presenilin-1 protects against neuronal apoptosis caused by its interacting protein PAG

Mutations in the presenilin-1 (PS-1) gene account for a significant fraction of familial Alzheimer's disease. The biological function of PS-1 is not well understood. We report here that the proliferation-associated gene (PAG) product, a protein of the thioredoxin peroxidase family, interacts with PS-1. Microinjection of a plasmid expressing PAG into superior cervical ganglion (SCG) sympathetic neurons in primary cultures led to apoptosis. Microinjection of plasmids expressing wild-type PS-1 or a PS-1 mutant with a deletion of exon 10 (PS1dE10) by themselves had no effect on the survival of primary SCG neurons. However, co-injection of wild-type PS-1 with PAG prevented neuronal death, whereas co-injection with the mutant PS-1 did not affect PAG-induced apoptosis. Furthermore, overexpression of PAG accelerated SCG neuronal death induced by nerve growth factor deprivation. This sensitizing effect was also blocked by wild-type PS-1, but not by PS1dE10. These results establish an assay for studying the function of PS-1 in primary neurons, reveal the neurotoxicity of a thioredoxin peroxidase, demonstrate a neuroprotective activity of the wild-type PS-1, and suggest possible involvement of defective neuroprotection by PS-1 mutants in neurodegeneration.

Secretases as targets for drug design in Alzheimer's disease

Alzheimer's disease accounts for the majority of dementia in the elderly. Worldwide, approximately 20 million people are suffering from this devastating disease, with no effective treatment currently available. For efficient drug design, it is important to identify the molecular mechanisms underlying the pathology of the disease. An invariant feature in the pathology of Alzheimer's disease is the amyloid-beta peptide. Amyloid-beta is produced by endoproteolytic cleavage of the amyloid precursor protein by beta- and gamma-secretase. In the past 2 years, the protein responsible for beta-secretase activity has been isolated and researchers are close to identifying gamma-secretase. These recent achievements in Alzheimer's disease research have provided helpful tools for the development of therapeutics.

Membrane topology of the murine fatty acid transport protein 1

The murine fatty acid transport protein (FATP1) was identified in an expression cloning screen for proteins that facilitate transport of fatty acids across the plasma membranes of mammalian cells. Hydropathy analysis of this protein suggests a model in which FATP1 has multiple membrane-spanning domains. To test this model, we inserted a hemagglutinin epitope tag at the amino terminus or a FLAG tag at the carboxyl terminus of the FATP1 cDNA and expressed these constructs in NIH 3T3 cells. Both tagged constructs produce proteins of the expected molecular masses and are functional in fatty acid import assays. Indirect immunofluorescence studies with selective permeabilization conditions and protease protection studies of sealed membrane vesicles from cells expressing epitope-tagged FATP1 were performed. These experiments show that the extreme amino terminus of tagged FATP1 is oriented toward the extracellular space, whereas the carboxyl terminus faces the cytosol. Additionally, enhanced green fluorescent protein fusion constructs containing predicted membrane-associated or soluble portions of FATP1 were expressed in Cos7 cells and analyzed by immunofluorescence and subcellular fractionation. These experiments demonstrate that amino acids 1-51, 52-100, and 101-190 contain signals for integral association with the membrane, whereas residues 258-313 and 314-475 are only peripherally membrane-associated. Amino acid residues 191-257 and 476-646 do not direct membrane association and likely face the cytosol. Taken together, these data support a model of FATP1 as a polytopic membrane protein with at least one transmembrane and multiple membrane-associated domains. This study provides the first experimental evidence for topology of a member of the family of plasma membrane fatty acid transport proteins.

Detection of the presenilin 1 COOH-terminal fragment in the extracellular compartment: a release enhanced by apoptosis

Mutations in gene encoding presenilin 1 (PS1) are responsible for the majority of familial Alzheimer's disease (FAD) cases. We studied PS1 localization in HEK293 cells and in primary neurons obtained from rat cortex and hippocampus. We first demonstrated that PS1-CTF, but neither PS1-FL nor PS1-NTF, is released into the medium as a soluble and membrane-associated form. After induction of apoptosis with staurosporine (Sts), we observed a dramatic increase in the level of PS1-CTF in the medium, both in HEK293 and in primary neurons. Immunocytochemical analysis suggested that the release of PS1-CTF might occur via membrane shedding. Abeta(1-42) treatment reduced PS1-CTF extracellular levels. This decrease was strongly associated to an impaired secretion of sAPP fragments, thus suggesting a role of PS1-CTF in the control of trafficking and generation of APP fragments.

Presenilin 1 negatively regulates beta-catenin/T cell factor/lymphoid enhancer factor-1 signaling independently of beta-amyloid precursor protein and notch processing

In addition to its documented role in the proteolytic processing of Notch-1 and the beta-amyloid precursor protein, presenilin 1 (PS1) associates with beta-catenin. In this study, we show that this interaction plays a critical role in regulating beta-catenin/T Cell Factor/Lymphoid Enhancer Factor-1 (LEF) signaling. PS1 deficiency results in accumulation of cytosolic beta-catenin, leading to a beta-catenin/LEF-dependent increase in cyclin D1 transcription and accelerated entry into the S phase of the cell cycle. Conversely, PS1 specifically represses LEF-dependent transcription in a dose-dependent manner. The hyperproliferative response can be reversed by reintroducing PS1 expression or overexpressing axin, but not a PS1 mutant that does not bind beta-catenin (PS1Deltacat) or by two different familial Alzheimer's disease mutants. In contrast, PS1Deltacat restores Notch-1 proteolytic cleavage and Abeta generation in PS1-deficient cells, indicating that PS1 function in modulating beta-catenin levels can be separated from its roles in facilitating gamma-secretase cleavage of beta-amyloid precursor protein and in Notch-1 signaling. Finally, we show an altered response to Wnt signaling and impaired ubiquitination of beta-catenin in the absence of PS1, a phenotype that may account for the increased stability in PS1-deficient cells. Thus, PS1 adds to the molecules that are known to regulate the rapid turnover of beta-catenin.

Neurotoxic traffic: uncovering the mechanics of amyloid production in Alzheimer's disease

Alzheimer's disease (AD) is thought by many to result from the accumulation of the neurotoxic amyloid-beta (A beta) peptide in brain parenchyma. The process by which A beta is proteolytically derived from the larger amyloid precursor protein (APP) has been the focus of much attention in the AD research field over the past decade. Recently, several of the proteins directly involved in the generation of A beta have been identified and characterized providing a number of viable therapeutic targets for the treatment of AD. However, the cellular mechanisms by which these proteins interact in the proteolytic processing of APP have not been well defined, nor are they readily apparent when one considers what is known about the intracellular localization and trafficking of the various participants. This article will review the underlying cell biology of A beta production and discuss the mechanistic options for APP processing given the current knowledge of the proteases involved.

The role of Alzheimer's disease-related presenilin 1 in intercellular adhesion

Most cases of familial early-onset Alzheimer's disease are caused by mutations in the presenilin 1 (PS1) gene. However, the cellular functions of PS1 are unknown. We showed predominant localization of PS1 to cell-cell contacts of the plasma membrane in human prostate epithelial tissue and in a human epithelial cell line HEp2 stably transfected with an inducible PS1 construct. PS1 co-immunoprecipitated with beta-catenin from cell lysates of stable transfectants. Conversely, PS1 lacking the PS1-beta-catenin interaction site did not co-immunoprecipitate with beta-catenin and was not recruited to the cell-cell contacts. L cells, which do not form tight intercellular contacts, formed clusters of adhered cells after stable transfection with GFP-PS1 cDNA and demonstrated a clear preference for independent aggregation in the mixed cultures. However, L cells transfected with mutant GFP-PS1 constructs, which had a truncated N-terminus of PS1 or deleted PS1-beta-catenin interaction site, failed to form intercellular contacts. In addition, in primary cultures of mouse cortical neurons PS1 was highly concentrated on the surface of extended growth cones. Taken together, our results suggest an important role of PS1 in intercellular adhesion in epithelial cells and neurons.

Most pathogenic mutations do not alter the membrane topology of the prion protein

The prion protein (PrP), a glycolipid-anchored membrane glycoprotein, contains a conserved hydrophobic sequence that can span the lipid bilayer in either direction, resulting in two transmembrane forms designated (Ntm)PrP and (Ctm)PrP. Previous studies have shown that the proportion of (Ctm)PrP is increased by mutations in the membrane-spanning segment, and it has been hypothesized that (Ctm)PrP represents a key intermediate in the pathway of prion-induced neurodegeneration. To further test this idea, we have surveyed a number of mutations associated with familial prion diseases to determine whether they alter the proportions of (Ntm)PrP and (Ctm)PrP produced in vitro, in transfected cells, and in transgenic mice. For the in vitro experiments, PrP mRNA was translated in the presence of murine thymoma microsomes which, in contrast to the canine pancreatic microsomes used in previous studies, are capable of efficient glycolipidation. We confirmed that mutations within or near the transmembrane domain enhance the formation of (Ctm)PrP, and we demonstrate for the first time that this species contains a C-terminal glycolipid anchor, thus exhibiting an unusual, dual mode of membrane attachment. However, we find that pathogenic mutations in other regions of the molecule have no effect on the amounts of (Ctm)PrP and (Ntm)PrP, arguing against the proposition that transmembrane PrP plays an obligate role in the pathogenesis of prion diseases.

Intramembrane proteolysis by presenilins

Many neurodegenerative diseases involve the deposition of insoluble amyloid molecules. In Alzheimer's disease, for example, the amyloid beta-peptide (A beta) is the main component of the characteristic senile plaques. Proteolytic enzymes called secretases are involved in generating A beta, and one of these may have been identified as presenilin--a discovery that paves the way for a more complete understanding of presenilin structure and function.

Presenilin I interaction with cytoskeleton and association with actin filaments

Presenilin I (PSI) has been shown to interact with microfilament-associated proteins of the filamin family. Here, we investigated a possible association of PSI with the cytoskeleton. Immunoblotting of detergent-insoluble fractions of rat brain homogenate revealed enrichment of neuron-specific 36 and 14 kDa proteolytic fragments of PSI, whereas 30 and 20 kDa fragments were found in the detergent-soluble fraction. Specific severing of microfilaments with gelsolin in the detergent-insoluble pellet and subsequent centrifugation led to the detection of both actin and PSI fragments in the supernatant. In addition, in vitro translated PSI cosedimented with actin filaments. Our findings provide biochemical evidence for the association of PSI fragments with actin filaments.

A topological study of the human γ-glutamyl carboxylase

γ-Glutamyl carboxylase (GC), a polytopic membrane protein found in the endoplasmic reticulum (ER), catalyzes vitamin K–dependent posttranslational modification of glutamate to γ-carboxyl glutamate. In an attempt to delineate the structure of this important enzyme, in vitro translation and in vivo mapping were used to study its membrane topology. Using terminus-tagged full-length carboxylase, expressed in 293 cells, it was demonstrated that the amino-terminus of the GC is on the cytoplasmic side of the ER, while the carboxyl-terminus is on the lumenal side. In addition, a series of fusions were made to encode each predicted transmembrane domain (TMD) followed by a leader peptidase (Lep) reporter tag, as analyzed by the computer algorithm TOPPRED II. Following in vitro translation of each fusion in the presence of canine microsomes, the topological orientation of the Lep tag was determined by proteinase K digestion and endoglycosidase H (Endo H) cleavage. From the topological orientation of the Lep tag in each fusion, the GC spans the ER membrane at least 5 times, with its N-terminus in the cytoplasm and its C-terminus in the lumen.

A topological study of the human γ-glutamyl carboxylase

γ-Glutamyl carboxylase (GC), a polytopic membrane protein found in the endoplasmic reticulum (ER), catalyzes vitamin K–dependent posttranslational modification of glutamate to γ-carboxyl glutamate. In an attempt to delineate the structure of this important enzyme, in vitro translation and in vivo mapping were used to study its membrane topology. Using terminus-tagged full-length carboxylase, expressed in 293 cells, it was demonstrated that the amino-terminus of the GC is on the cytoplasmic side of the ER, while the carboxyl-terminus is on the lumenal side. In addition, a series of fusions were made to encode each predicted transmembrane domain (TMD) followed by a leader peptidase (Lep) reporter tag, as analyzed by the computer algorithm TOPPRED II. Following in vitro translation of each fusion in the presence of canine microsomes, the topological orientation of the Lep tag was determined by proteinase K digestion and endoglycosidase H (Endo H) cleavage. From the topological orientation of the Lep tag in each fusion, the GC spans the ER membrane at least 5 times, with its N-terminus in the cytoplasm and its C-terminus in the lumen.

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

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

Presenilin structure, function and role in Alzheimer disease

Numerous missense mutations in the presenilins are associated with the autosomal dominant form of familial Alzheimer disease. Presenilin genes encode polytopic transmembrane proteins, which are processed by proteolytic cleavage and form high-molecular-weight complexes under physiological conditions. The presenilins have been suggested to be functionally involved in developmental morphogenesis, unfolded protein responses and processing of selected proteins including the beta-amyloid precursor protein. Although the underlying mechanism by which presenilin mutations lead to development of Alzheimer disease remains elusive, one consistent mutational effect is an overproduction of long-tailed amyloid beta-peptides. Furthermore, presenilins interact with beta-catenin to form presenilin complexes, and the physiological and mutational effects are also observed in the catenin signal transduction pathway.