A role for presenilin 1 in regulating the delivery of amyloid precursor protein to the cell surface

Release of a membrane-bound death domain by gamma-secretase processing of the p75NTR homolog NRADD

Neurotrophin receptor alike death domain protein (NRADD) is a death-receptor-like protein with a unique ectodomain and an intracellular domain homologous to p75(NTR). Expression of NRADD results in apoptosis, but only in certain cell types. This paper characterizes the expression and proteolytic processing of the mature 55 kDa glycoprotein. N-terminally truncated NRADD is processed by a gamma-secretase activity that requires presenilins and has the same susceptibility to gamma-secretase inhibitors as the secretion of amyloid beta (A beta). The ectodomain of endogenous NRADD is shed by activation of metalloproteinases. Inhibitor studies provide evidence that NRADD is cleaved in two steps typical of regulated intramembrane proteolysis (RIP). Inhibition of gamma-secretase abrogates both the production of the soluble intracellular domain of NRADD and the appearance of NRADD in subnuclear structures. Thus, solubilized death domains with close homology to p75(NTR) might have a nuclear function. Furthermore, presenilin deficiency leads to abnormally glycosylated NRADD and overexpression of presenilin 2 inhibits NRADD maturation, which is dependent on the putative active site residue D366 but not on gamma-secretase activity. Our results demonstrate that NRADD is an additional gamma-secretase substrate and suggest that drugs against Alzheimer's disease will need to target gamma-secretase in a substrate-specific manner.

Self-assembly of the cytoskeletal glial fibrillary acidic protein is inhibited by an isoform-specific C terminus

The predominant isoform of glial fibrillary acidic protein (GFAP), GFAPalpha, is the characteristic building block of the cytoskeletal intermediate filaments in astrocytes. Isoform GFAPepsilon, produced by alternative splicing of the GFAP gene, includes a new tail domain that confers a presenilin binding capacity. We here show that the GFAPepsilon tail prevents GFAPepsilon homodimerization and homomeric filament formation, whereas the ability to form heterodimers and filaments with GFAPalpha is retained. Furthermore, GFAPepsilon shows decreased affinity for several GFAPalpha-interacting proteins. A GFAPepsilon tail mutation that results in gain of GFAPepsilon dimerization and filament formation abolishes presenilin binding. This mutation also abolishes interaction between the tail and the coiled-coil domain of GFAPepsilon. Together, this indicates that direct interaction between the coiled-coil and tail domains may serve as an inhibitory mechanism for homomeric dimerization and filament formation. We propose that the GFAPepsilon isoform represents a new functionally distinct component of GFAP intermediate filaments.

Presenilins and gamma-secretase inhibitors affect intracellular trafficking and cell surface localization of the gamma-secretase complex components

The intramembranous cleavage of Alzheimer beta-amyloid precursor protein and the signaling receptor Notch is mediated by the presenilin (PS, PS1/PS2)-gamma-secretase complex, the components of which also include nicastrin, APH-1, and PEN-2. In addition to its essential role in gamma-secretase activity, we and others have reported that PS1 plays a role in intracellular trafficking of select membrane proteins including nicastrin. Here we examined the fate of PEN-2 in the absence of PS expression or gamma-secretase activity. We found that PEN-2 is retained in the endoplasmic reticulum and has a much shorter half-life in PS-deficient cells than in wild type cells, suggesting that PSs are required for maintaining the stability and proper subcellular trafficking of PEN-2. However, the function of PS in PEN-2 trafficking is distinct from its contribution to gamma-secretase activity because inhibition of gamma-secretase activity by gamma-secretase inhibitors did not affect the PEN-2 level or its egress from the endoplasmic reticulum. Instead, membrane-permeable gamma-secretase inhibitors, but not a membrane-impermeable derivative, markedly increased the cell surface levels of PS1 and PEN-2 without affecting that of nicastrin. In support of its role in PEN-2 trafficking, PS1 was also required for the gamma-secretase inhibitor-induced plasma membrane accumulation of PEN-2. We further showed that gamma-secretase inhibitors specifically accelerated the Golgi to the cell surface transport of PS1 and PEN-2. Taken together, we demonstrate an essential role for PSs in intracellular trafficking of the gamma-secretase components, and that selective gamma-secretase inhibitors differentially affect the trafficking of the gamma-secretase components, which may contribute to an inactivation of gamma-secretase.

Up-regulation of soluble amyloid precursor protein fragment secretion in the rat retina in vivo by metabotropic glutamate receptor stimulation

Using the novel rat retinal-vitreal model we have investigated the effect of metabotropic glutamate receptor activation on amyloid precursor protein (APP) metabolism. The release of low mol. wt fragments of APP, at 15-23 kDa in particular, was markedly up-regulated by the metabotropic glutamate receptor agonist (1S,3R)-1-amino-1,3-cyclopentane dicarboxylic acid ((1S,3R)-ACPD) in a concentration- and time-dependent manner, and this response was blocked by the receptor antagonist (S)-alpha-methyl-4-caboxyphenylglycine ((S)-MCPG). These results, together with the observation of a lack of deleterious effects of (1S,3R)-ACPD on the retinal neurons, support a physiological role of metabotropic glutamate receptors in mediating the release of soluble APP fragments, an action which may have important functional and therapeutic implications for Alzheimer's disease.

Presenilin-1 and intracellular calcium stores regulate neuronal glutamate uptake

Glutamate uptake by high affinity glutamate transporters is essential for preventing excitotoxicity and maintaining normal synaptic function. We have discovered a novel role for presenilin-1 (PS1) as a regulator of glutamate transport. PS1-deficient neurons showed a decrease in glutamate uptake of approximately 50% compared to wild-type neurons. Gamma-secretase inhibitor treatment mimicked the effects of PS1 deficiency on glutamate uptake. PS1 loss-of-function, accomplished by PS1 deficiency or gamma-secretase inhibitor treatment, caused a corresponding decrease in cell surface expression of the neuronal glutamate transporter, EAAC1. PS1 deficiency is known to reduce intracellular calcium stores. To explore the possibility that PS1 influences glutamate uptake via regulation of intracellular calcium stores, we examined the effects of treating neurons with caffeine, thapsigargin, and SKF-96365. These compounds depleted intracellular calcium stores by distinct means. Nonetheless, each treatment mimicked PS1 loss-of-function by impairing glutamate uptake and reducing EAAC1 expression at the cell surface. Blockade of voltage-gated calcium channels, activation and inhibition of protein kinase C (PKC), and protein kinase A (PKA) all had no effect on glutamate uptake in neurons. Taken together, these findings indicate that PS1 and intracellular calcium stores may play a significant role in regulating glutamate uptake and therefore may be important in limiting glutamate toxicity in the brain.

Modulation of β-amyloid metabolism by non-steroidal anti-inflammatory drugs in neuronal cell cultures

Alzheimer disease (AD) is characterized by cerebral deposits of beta-amyloid (Abeta) peptides, which are surrounded by neuroinflammatory cells. Epidemiological studies have shown that prolonged use of non-steroidal anti-inflammatory drugs (NSAIDs) reduces the risk of developing AD. In addition, biological data indicate that certain NSAIDs specifically lower Abeta42 levels in cultures of peripheral cells independently of cyclooxygenase (COX) activity and reduce cerebral Abeta levels in AD transgenic mice. Whether other NSAIDs, including COX-selective compounds, modulate Abeta levels in neuronal cells remains unexploited. Here, we investigated the effects of compounds from every chemical class of NSAIDs on Abeta40 and Abeta42 secretion using both Neuro-2a cells and rat primary cortical neurons. Among non-selective NSAIDs, flurbiprofen and sulindac sulfide concentration-dependently reduced the secretion not only of Abeta42 but also of Abeta40. Surprisingly, both COX-2 (celecoxib; sc-125) or COX-1 (sc-560) selective compounds significantly increased Abeta42 secretion, and either did not alter (sc-560; sc-125) or reduced (celecoxib) Abeta40 levels. The levels of betaAPP C-terminal fragments and Notch cleavage were not altered by any of the NSAIDs, indicating that gamma-secretase activity was not overall changed by these drugs. The present findings show that only a few non-selective NSAIDs possess Abeta-lowering properties and therefore have a profile potentially relevant to their clinical use in AD.

Mutant presenilins specifically elevate the levels of the 42 residue beta-amyloid peptide in vivo: evidence for augmentation of a 42-specific gamma secretase

Amyloid precursor protein (APP) is endoproteolytically processed by BACE1 and gamma-secretase to release amyloid peptides (Abeta40 and 42) that aggregate to form senile plaques in the brains of patients with Alzheimer's disease (AD). The C-terminus of Abeta40/42 is generated by gamma-secretase, whose activity is dependent upon presenilin (PS 1 or 2). Missense mutations in PS1 (and PS2) occur in patients with early-onset familial AD (FAD), and previous studies in transgenic mice and cultured cell models demonstrated that FAD-PS1 variants shift the ratio of Abeta40 : 42 to favor Abeta42. One hypothesis to explain this outcome is that mutant PS alters the specificity of gamma-secretase to favor production of Abeta42 at the expense of Abeta40. To test this hypothesis in vivo, we studied Abeta40 and 42 levels in a series of transgenic mice that co-express the Swedish mutation of APP (APPswe) with two FAD-PS1 variants that differentially accelerate amyloid pathology in the brain. We demonstrate a direct correlation between the concentration of Abeta42 and the rate of amyloid deposition. We further show that the shift in Abeta42 : 40 ratios associated with the expression of FAD-PS1 variants is due to a specific elevation in the steady-state levels of Abeta42, while maintaining a constant level of Abeta40. These data suggest that PS1 variants do not simply alter the preferred cleavage site for gamma-secretase, but rather that they have more complex effects on the regulation of gamma-secretase and its access to substrates.

Presenilins mutated at Asp-257 or Asp-385 restore Pen-2 expression and Nicastrin glycosylation but remain catalytically inactive in the absence of wild type Presenilin

The Presenilins are part of the gamma-secretase complex that is involved in the regulated intramembrane proteolysis of amyloid precursor protein and other type I integral membrane proteins. Nicastrin, Pen-2, and Aph1 are the other proteins of this complex. The Presenilins probably contribute the catalytic activity to the protease complex. However, several investigators reported normal Abeta-peptide generation in cells expressing Presenilins mutated at the putative catalytic site residue Asp-257, contradicting this hypothesis. Because endogenously expressed wild type Presenilin could contribute to residual gamma-secretase activity in these experiments, we have reinvestigated the problem by expressing mutated Presenilins in a Presenilin-negative cell line. We confirm that Presenilins with mutated Asp residues are catalytically inactive. Unexpectedly, these mutated Presenilins are still partially processed into amino- and carboxyl-terminal fragments by a "Presenilinase"-like activity. They are also able to rescue Pen-2 expression and Nicastrin glycosylation in Presenilin-negative cells and become incorporated into large approximately 440-kDa complexes as assessed by blue native gel electrophoresis. Our study demonstrates that the catalytic activity of Presenilin and its other functions in the generation, stabilization, and transport of the gamma-secretase complex can be separated and extends the concept that Presenilins are multifunctional proteins.

Presenilin 1 is involved in maturation and trafficking of N-cadherin to the plasma membrane

One pathological characteristic of Alzheimer's disease (AD) is extensive synapse loss. Presenilin 1 (PS1) is linked to the pathogenesis of early onset familial Alzheimer's disease (FAD) and is localized at the synapse, where it binds N-cadherin and modulates its adhesive activity. To elucidate the role of the PS1/N-cadherin interaction in synaptic contact, we established SH-SY5Y cells stably expressing wild-type (wt) PS1 and dominant-negative (D385A) PS1. We show that the formation of cadherin-based cell-cell contact among SH-SY5Y cells stably expressing D385A PS1 was suppressed. Conversely, wt PS1 cells exhibited enhanced cell-cell contact and colony formation. Suppression of cell-cell contact in D385A cells was accompanied by an alteration in N-cadherin subcellular localization; N-cadherin was retained mainly in the endoplasmic reticulum (ER) and cell surface expression was reduced. We conclude that PS1 is essential for efficient trafficking of N-cadherin from the ER to the plasma membrane. PS1-mediated delivery of N-cadherin to the plasma membrane is important for N-cadherin to exert its physiological function, and it may control the state of cell-cell contact.

Both raft- and non-raft proteins associate with CHAPS-insoluble complexes: some APP in large complexes

Components of caveolae and lipid rafts are characterized by their buoyancy after detergent extraction. Using flotations in density gradients, we now show that non-raft membrane molecules are also associated with detergent-insoluble, buoyant assemblies. When Triton X-100 cellular extracts were spun to equilibrium in Nycodenz, only components of classical rafts floated. In contrast, with the zwitterionic detergent CHAPS, non-raft residents such as calnexin and APP also buoyed. When CHAPS extracts were spun in non-equilibrium (velocity) conditions, some raft components rapidly exited the input fractions while other raft markers and non-raft molecules remained relatively immobile. This pointed to size heterogeneities of CHAPS-insoluble complexes. Combined velocity/equilibrium gradients broadly divided CHAPS-insoluble membrane complexes into three size categories, which all contained cholesterol and the glycosphingolipid GM1. Large complexes were enriched in caveolin and ESA. Medium size complexes were enriched in PrP, whereas small complexes contained non-raft proteins, PrP, and some ESA. While Alzheimer's APP was primarily confined to small assemblies, a portion of its glycosylated form did buoy with large complexes. Large CHAPS-insoluble complexes resemble, but are not equal to, classical rafts. These findings extend considerably the range of detergent-insoluble membranal domains.

Oligomerization of human presenilin-1 fragments

To gain insight into presenilin-1 (PS1) structural aspects, we explored the structure-function relationship of its N- and C-terminal (NTF and CTF, respectively) complexes. We demonstrated that both NTF and CTF act as independent but inter-changing binding units capable of binding each other (NTF/CTF) or their homologues (NTF/NTF; CTF/CTF). The Alzheimer's disease-associated PS1 mutations Y115H and M146L do not affect their ability to hetero- and/or homodimerize, thus conserving their basic integrity and function(s). These results suggest that PS1 associates intra-molecularly to form higher order complexes, which may be needed for endoproteolytic cleavage and/or gamma-secretase-associated activity.

Presenilin-1 regulates intracellular trafficking and cell surface delivery of beta-amyloid precursor protein

Presenilins (PS1/PS2) play a critical role in proteolysis of beta-amyloid precursor protein (beta APP) to generate beta-amyloid, a peptide important in the pathogenesis of Alzheimer's disease. Nevertheless, several regulatory functions of PS1 have also been reported. Here we demonstrate, in neuroblastoma cells, that PS1 regulates the biogenesis of beta APP-containing vesicles from the trans-Golgi network and the endoplasmic reticulum. PS1 deficiency or the expression of loss-of-function variants leads to robust vesicle formation, concomitant with increased maturation and/or cell surface accumulation of beta APP. In contrast, release of vesicles containing beta APP is impaired in familial Alzheimer's disease (FAD)-linked PS1 mutant cells, resulting in reduced beta APP delivery to the cell surface. Moreover, diminution of surface beta APP is profound at axonal terminals in neurons expressing a PS1 FAD variant. These results suggest that PS1 regulation of beta APP trafficking may represent an alternative mechanism by which FAD-linked PS1 variants modulate beta APP processing.