Developmental Expression of Wild-Type and Mutant Presenilin-1 in Hippocampal Neurons from Transgenic Mice: Evidence for Novel Species-Specific Properties of Human Presenilin-1

Axonal Transport and Alzheimer's Disease

In contrast to most eukaryotic cells, neurons possess long, highly branched processes called axons and dendrites. In large mammals, such as humans, some axons reach lengths of over 1 m. These lengths pose a major challenge to the movement of proteins, vesicles, and organelles between presynaptic sites and cell bodies. To overcome this challenge axons and dendrites rely upon specialized transport machinery consisting of cytoskeletal motor proteins generating directed movements along cytoskeletal tracks. Not only are these transport systems crucial to maintain neuronal viability and differentiation, but considerable experimental evidence suggests that failure of axonal transport may play a role in the development or progression of neurological diseases such as Alzheimer's disease.

Transient abundance of presenilin 1 fragments/nicastrin complex associated with synaptogenesis during development in rat cerebellum

Immunolocalization and expression of endogenous nicastrin (NCT) and presenilin 1 (PS1) fragments during postnatal development of rat cerebellum were investigated with fragment-specific antibodies. Immunoblotting for NCT revealed the expected mature and immature species, which gradually declined during development. In contrast, the expression of PS1 N-terminal fragment exhibited a peak at postnatal day 14 (P14) and declined thereafter. This chronological change was similarly observed with PS1 C-terminal fragment. Immunoprecipitation of NCT indicated its physical association with PS1 fragments. Colocalization of these molecules to the endoplasmic reticulum in cerebellar Purkinje cells indicates that they are organized into a complex in developing neurons. In addition, active sites of synaptogenesis, the base of the external granular layer and glomeruli, contained PS1 fragments and smaller amount of NCT. Isolated synaptic fraction contained both PS1 and NCT, suggesting their functional association within synapses. Transient abundance of NCT and PS1 fragments as a complex, when (P14) and where synaptogenesis is active, is consistent with intracellular trafficking of this complex in developing neurons and suggests its role as gamma-secretase in synaptogenesis.

Ryanodine receptor-mediated interference of neuronal cell differentiation by presenilin 2 mutation

Neuronal cell differentiation alterations induced by mutant presenilin 2 (PS2) were investigated in transgenic mice expressing wild-type or mutant-type PS2. Progressive increases in differentiation and marker protein expression were found in neuronal cells expressing wild-type PS2, whereas these processes were much perturbed in mutant-type PS2 with elevated ryanodine-receptor (RyR) expression and intracellular calcium levels. Moreover, dantrolene, a blocker of RyR reduced the PS2 mutation-induced interference of cell differentiation and calcium release, but caffeine, an activator of RyR, exacerbated PS2 mutation-induced interference with cell differentiation. Our results indicate that mutant PS2 inhibits normal neuronal cell differentiation and that RyR-mediated calcium overrelease may be a significant factor.

Autophagic vacuoles are enriched in amyloid precursor protein-secretase activities: implications for β-amyloid peptide over-production and localization in Alzheimer’s disease

In Alzheimer's disease (AD), the neuropathologic hallmarks of beta-amyloid deposition and neurofibrillary degeneration are associated with early and progressive pathology of the endosomal-lysosomal system. Abnormalities of autophagy, a major pathway to lysosomes for protein and organelle turnover, include marked accumulations of autophagy-related vesicular compartments (autophagic vacuoles or AVs) in affected neurons. Here, we investigated the possibility that AVs contain the proteases and substrates necessary to cleave the amyloid precursor protein (APP) to A beta peptide that forms beta-amyloid, a key pathogenic factor in AD. AVs were highly purified using a well-established metrizamide gradient procedure from livers of transgenic YAC mice overexpressing wild-type human APP. By Western blot analysis, AVs contained APP, beta CTF - the beta-cleaved carboxyl-terminal domain of APP, and BACE, the protease-mediating beta-cleavage of APP. beta-Secretase activity measured against a fluorogenic peptide was significantly enriched in the AV fraction relative to whole-liver lysate. Compared to other recovered subcellular fractions, AVs exhibited the highest specific activity of gamma-secretase based on a fluorogenic assay and inhibition by a specific inhibitor of gamma-secretase, DAPT. AVs were also the most enriched subcellular fraction in levels of the gamma-secretase components presenilin and nicastrin. Immunoelectron microscopy demonstrated selective immunogold labeling of AVs with antibodies specific for the carboxyl termini of human A beta 40 and A beta 42. These data indicate that AVs are a previously unrecognized and potentially highly active compartment for A beta generation and suggest that the abnormal accumulation of AVs in affected neurons of the AD brain contributes to beta-amyloid deposition.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.

The novel presenilin-1-associated protein is a proapoptotic mitochondrial protein

Recent studies have suggested a possible role for presenilin proteins in apoptotic cell death observed in Alzheimer's disease. The mechanism by which presenilin proteins regulate apoptotic cell death is not well understood. Using the yeast two-hybrid system, we previously isolated a novel protein, presenilin-associated protein (PSAP) that specifically interacts with the C terminus of presenilin 1 (PS1), but not presenilin 2 (PS2). Here we report that PSAP is a mitochondrial resident protein sharing homology with mitochondrial carrier protein. PSAP was detected in a mitochondria-enriched fraction, and PSAP immunofluorescence was present in a punctate pattern that colocalized with a mitochondrial marker. More interestingly, overexpression of PSAP caused apoptotic death. PSAP-induced apoptosis was documented using multiple independent approaches, including membrane blebbing, chromosome condensation and fragmentation, DNA laddering, cleavage of the death substrate poly(ADP-ribose) polymerase, and flow cytometry. PSAP-induced cell death was accompanied by cytochrome c release from mitochondria and caspase-3 activation. Moreover, the general caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, which blocked cell death, did not block the release of cytochrome c from mitochondria caused by overexpression of PSAP, indicating that PSAP-induced cytochrome c release was independent of caspase activity. The mitochondrial localization and proapoptotic activity of PSAP suggest that it is an important regulator of apoptosis.

Biochemical and immunocytochemical characterization of calsenilin in mouse brain

Mutations in the presenilin 1 and 2 genes cause the majority of early onset familial forms of Alzheimer's disease. Here we describe the biochemical and immunohistochemical characterization of calsenilin, a novel calcium binding protein that we have previously shown to interact with presenilins 1 and 2, in mouse brain. The co-immunoprecipitation of endogenous calsenilin and presenilin 1 demonstrates that these proteins are physiologic binding partners. Although calsenilin has been predicted to be a soluble protein, we have found that the majority of it is tightly associated with the cytoplasmic face of intracellular membranes and that it can only be dissociated using harsh treatments such as urea. In addition, we have demonstrated that calsenilin is a developmentally regulated protein that is mainly present in the brain, where it localizes to both the hippocampus and cerebellum. Calsenilin staining co-localized with the somatodendritic marker microtubule-associated protein-2 primarily in the granular cell layer of the cerebellum, indicating that calsenilin expression is primarily neuronal. In primary cultured neurons, calsenilin immunoreactivity was observed in cell bodies as well as in some neuronal processes. Co-localization experiments using specific axonal and dendritic markers indicate that these processes were mainly axonal in nature, although a smaller subset of dendrites also appears to contain calsenilin. In summary, we have established that calsenilin and presenilin 1 can interact at physiologic levels, and that calsenilin is a developmentally regulated protein that is expressed primarily in the cerebellum and hippocampus. Although calsenilin is a soluble protein, it is tightly associated with the membrane. Finally, the expression pattern of calsenilin, which is similar to that of the presenilin(s), suggests that the common locations of these two proteins provide an opportunity for physical interaction in vivo.

Mature glycosylation and trafficking of nicastrin modulate its binding to presenilins

Nicastrin is an integral component of the high molecular weight presenilin complexes that control proteolytic processing of the amyloid precursor protein and Notch. We report here that nicastrin is most probably a type 1 transmembrane glycoprotein that is expressed at moderate levels in the brain and in cultured neurons. Immunofluorescence studies demonstrate that nicastrin is localized in the endoplasmic reticulum, Golgi, and a discrete population of vesicles. Glycosidase analyses reveal that endogenous nicastrin undergoes a conventional, trafficking-dependent maturation process. However, when highly expressed in transfected cells, there is a disproportionate accumulation of the endo-beta-N-acetylglucosaminidase H-sensitive, immature form, with no significant increase in the levels of the fully mature species. Immunoprecipitation revealed that presenilin-1 interacts preferentially with mature nicastrin, suggesting that correct trafficking and co-localization of the presenilin complex components are essential for activity. These findings demonstrate that trafficking and post-translational modifications of nicastrin are tightly regulated processes that accompany the assembly of the active presenilin complexes that execute gamma-secretase cleavage. These results also underscore the caveat that simple overexpression of nicastrin in transfected cells may result in the accumulation of large amounts of the immature protein, which is apparently unable to assemble into the active complexes capable of processing amyloid precursor protein and Notch.

Calcium signaling at single mossy fiber presynaptic terminals in the rat hippocampus

We investigated internal Ca(2+) release at mossy fiber synapses on CA3 pyramidal neurons (mossy fiber terminals, MFTs) in the hippocampus. Presynaptic Ca(2+) influx was induced by giving a brief train of 20 stimuli at 100 Hz to the mossy fiber pathway. Using Ca(2+) imaging techniques, we recorded the Ca(2+) response as DeltaF/F, which increased rapidly with stimulation, but was often accompanied by a delayed peak that occurred after the train. The rise in presynaptic [Ca(2+)] could be completely blocked by application of 400 microM Cd(2+). Furthermore, the evoked Ca(2+) signals were reduced by group II mGluR agonists. Under the same experimental conditions, we investigated the effects of several agents on MFTs that disrupt regulation of intracellular Ca(2+) stores resulting in depletion of internal Ca(2+). We found that ryanodine, cyclopiazonic acid, thapsigargin, and ruthenium red all decreased both the early and the delayed increase in the Ca(2+) signals. We applied D,L-2-amino-5-phosphonovaleric acid (D,L-APV; 50 microM) and 6,7-Dinitroquinoxaline-2,3-dione (DNQX; 20 microM) to exclude the action of N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Experiments with alternative lower affinity indicators for Ca(2+) (fura-2FF and calcium green-2) and the transient K(+) channel blocker, 4-aminopyridine were performed to control for the possible saturation of fura-2. Taken together, these results strongly support the hypothesis that the recorded terminals were from the mossy fibers of the dentate gyrus and suggest that a portion of the presynaptic Ca(2+) signal in response to brief trains of stimuli is due to release of Ca(2+) from internal stores.

Mutation of conserved aspartates affect maturation of presenilin 1 and presenilin 2 complexes

Presenilin (PS1 and PS2) holoproteins are transiently incorporated into low molecular weight (MW) complexes. During subsequent incorporation into a higher MW complex, they undergo endoproteolysis to generate stable N- and C-terminal fragments (NTF/CTF). Mutation of either of two conserved aspartate residues in transmembrane domains inhibits both presenilin-endoproteolysis and the proteolytic processing of APP and Notch. We show that aspartate-mutant holoprotein presenilins are not incorporated into the high molecular weight, NTF/CTF-containing complexes. Aspartate-mutant presenilin holoproteins also preclude entry of endogenous wild-type PS1/PS2 into the high molecular weight complexes, but do not affect the incorporation of wild-type holoproteins into lower molecular weight holoprotein complexes. These data suggest that the loss-of-function aspartate-mutants cause altered PS complex maturation, and argue that the functional presenilin moieties are contained in the high molecular weight presenilin NTF/CTF-containing complexes.

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.

Carboxyl-terminal fragments of Alzheimer beta-amyloid precursor protein accumulate in restricted and unpredicted intracellular compartments in presenilin 1-deficient cells

Absence of functional presenilin 1 (PS1) protein leads to loss of gamma-secretase cleavage of the amyloid precursor protein (betaAPP), resulting in a dramatic reduction in amyloid beta peptide (Abeta) production and accumulation of alpha- or beta-secretase-cleaved COOH-terminal fragments of betaAPP (alpha- or beta-CTFs). The major COOH-terminal fragment (CTF) in brain was identified as betaAPP-CTF-(11-98), which is consistent with the observation that cultured neurons generate primarily Abeta-(11-40). In PS1(-/-) murine neurons and fibroblasts expressing the loss-of-function PS1(D385A) mutant, CTFs accumulated in the endoplasmic reticulum, Golgi, and lysosomes, but not late endosomes. There were some subtle differences in the subcellular distribution of CTFs in PS1(-/-) neurons as compared with PS1(D385A) mutant fibroblasts. However, there was no obvious redistribution of full-length betaAPP or of markers of other organelles in either mutant. Blockade of endoplasmic reticulum-to-Golgi trafficking indicated that in PS1(-/-) neurons (as in normal cells) trafficking of betaAPP to the Golgi compartment is necessary before alpha- and beta-secretase cleavages occur. Thus, although we cannot exclude a specific role for PS1 in trafficking of CTFs, these data argue against a major role in general protein trafficking. These results are more compatible with a role for PS1 either as the actual gamma-secretase catalytic activity or in other functions indirectly related to gamma-secretase catalysis (e.g. an activator of gamma-secretase, a substrate adaptor for gamma-secretase, or delivery of gamma-secretase to betaAPP-containing compartments).

Ligand specific effects on aluminum incorporation and toxicity in neurons and astrocytes

Aluminum is present in many manufactured foods and medicines and is added to drinking water for purification purposes. It has been proposed that aluminum is a contributing factors to several neurodegenerative disorders such as Alzheimer's disease. However, this remains controversial primarily due to the unusual properties of aluminum and a lack of information on its cellular sites of action. To resolve some of these questions, we have examined aluminum uptake in both neuronal and astroglial cells as well as the role of metal speciation. The relative accumulation of four aluminum salts, aluminum maltolate, aluminum lactate, aluminum chloride and aluminum fluoride, was investigated and correlated with cell viability and intracellular distribution as determined by morin staining. Significant differences in aluminum incorporation and toxicity were observed in both neuronal and glia cells with the largest effects exhibited by the maltol species. This was accompanied by a nuclear accumulation in the neuronal cell line that was contrasted by the perinuclear, vesicular distribution in astrocytes that partially co-localized with cathepsin D, a lysosomal marker. These findings demonstrate differences in aluminum species and highlights the importance of these factors in modulating the toxic effect of aluminum.

Hypercholesterolemia accelerates the Alzheimer's amyloid pathology in a transgenic mouse model

Recent data suggest that cholesterol metabolism is linked to susceptibility to Alzheimer's disease (AD). However, no direct evidence has been reported linking cholesterol metabolism and the pathogenesis of AD. To test the hypothesis that amyloid beta-peptide (Abeta) deposition can be modulated by diet-induced hypercholesterolemia, we used a transgenic-mouse model for AD amyloidosis and examined the effects of a high-fat/high-cholesterol diet on central nervous system (CNS) Abeta accumulation. Our data showed that diet-induced hypercholesterolemia resulted in significantly increased levels of formic acid-extractable Abeta peptides in the CNS. Furthermore, the levels of total Abeta were strongly correlated with the levels of both plasma and CNS total cholesterol. Biochemical analysis revealed that, compared with control, the hypercholesterolemic mice had significantly decreased levels of sAPPalpha and increased levels of C-terminal fragments (beta-CTFs), suggesting alterations in amyloid precursor protein processing in response to hypercholesterolemia. Neuropathological analysis indicated that the hypercholesterolemic diet significantly increased beta-amyloid load by increasing both deposit number and size. These data demonstrate that high dietary cholesterol increases Abeta accumulation and accelerates the AD-related pathology observed in this animal model. Thus, we propose that diet can be used to modulate the risk of developing AD.

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.

Inhibiting amyloid precursor protein C-terminal cleavage promotes an interaction with presenilin 1

Presenilin 1 (PS1) plays a pivotal role in the production of the amyloid-beta protein, which is central to the pathogenesis of Alzheimer's disease. It has been demonstrated that PS1 regulates the gamma-secretase proteolysis of the amyloid precursor protein (APP) C-terminal fragment (APP-C100), which is the final step in amyloid-beta protein production. The mechanism and detailed pathway of this PS1 activity has yet to be fully resolved, but it may be due to a presenilin-controlled trafficking of the APP fragment or possibly an inherent PS1 proteolytic activity. We have investigated the possibility of a direct interaction of PS1 and the APP-C100 within the high molecular mass presenilin complex. However, the APP-C100 is rapidly degraded, and if it forms, then any PS1.APP complex is likely to be very transitory. To circumvent this problem, we have utilized the protease inhibitor N-acetyl-leucyl-norleucinal (LLnL) and the lysosomotropic agent NH(4)Cl, which inhibits the turnover of the APP-C100. Under these conditions, levels of the fragment increased appreciably, and as shown by glycerol gradient analysis, the APP-C100 shifted to a higher molecular mass complex that overlapped with PS1. Immunoprecipitation studies demonstrated that a significant population of the APP-C100 co-precipitated with PS1. These findings suggest that PS1 may mediate the shuttling of APP fragments and/or facilitate their presentation for gamma-secretase cleavage through a direct interaction.

Identification of syntaxin 1A as a novel binding protein for presenilin-1

Mutations in the presenilin 1 gene have been shown to result in Alzheimer's disease. Presenilin 1 is a multi-transmembrane protein with a large hydrophilic loop near the C-terminus. This region is required for known functions of presenilin 1. We have constrained this loop within the active site of the bacterial protein, thioredoxin, to mimic its native conformational state. This hybrid protein was used as bait in a yeast two hybrid screen in an attempt to identify presenilin binding proteins. By this method syntaxin 1A, a synaptic plasma membrane protein, was identified as a novel binding protein for presenilin 1. In vitro experiments confirm the two-hybrid results suggesting that PS1 binds syntaxin under physiological conditions.

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.