Distinct presenilin-dependent and presenilin-independent ?-secretases are responsible for total cellular A? production

Are N- and C-terminally truncated Aβ species key pathological triggers in Alzheimer's disease?

The histopathology of Alzheimer's disease (AD) is characterized by neuronal loss, neurofibrillary tangles, and senile plaque formation. The latter results from an exacerbated production (familial AD cases) or altered degradation (sporadic cases) of 40/42-amino acid-long β-amyloid peptides (Aβ peptides) that are produced by sequential cleavages of Aβ precursor protein (βAPP) by β- and γ-secretases. The amyloid cascade hypothesis proposes a key role for the full-length Aβ42 and the Aβ40/42 ratio in AD etiology, in which soluble Aβ oligomers lead to neurotoxicity, tau hyperphosphorylation, aggregation, and, ultimately, cognitive defects. However, following this postulate, during the last decade, several clinical approaches aimed at decreasing full-length Aβ42 production or neutralizing it by immunotherapy have failed to reduce or even stabilize AD-related decline. Thus, the Aβ peptide (Aβ40/42)-centric hypothesis is probably a simplified view of a much more complex situation involving a multiplicity of APP fragments and Aβ catabolites. Indeed, biochemical analyses of AD brain deposits and fluids have unraveled an Aβ peptidome consisting of additional Aβ-related species. Such Aβ catabolites could be due to either primary enzymatic cleavages of βAPP or secondary processing of Aβ itself by exopeptidases. Here, we review the diversity of N- and C-terminally truncated Aβ peptides and their biosynthesis and outline their potential function/toxicity. We also highlight their potential as new pharmaceutical targets and biomarkers.

Ca2+ enhances Aβ polymerization rate and fibrillar stability in a dynamic manner

Identifying factors that affect the self-assembly of Aβ (amyloid-β peptide) is of utmost importance in the quest to understand the molecular mechanisms causing AD (Alzheimer's disease). Ca2+ has previously been shown to accelerate both Aβ fibril nucleation and maturation, and dysregulated Ca2+ homoeostasis frequently correlates with development of AD. The mechanisms regarding Ca2+ binding, as well as its effect on fibril kinetics, are not fully understood. Using a polymerization assay we show that Ca2+ in a dynamic and reversible manner enhances both the elongation rate and fibrillar stability, where specifically the ‘dock and lock’ phase mechanism is enhanced. Through NMR analysis we found that Ca2+ affects the fibrillar architecture. In addition, and unexpectedly, we found that Ca2+ does not bind the free Aβ monomer. This implies that Ca2+ binding requires an architecture adopted by assembled peptides, and consequently is mediated through intermolecular interactions between adjacent peptides. This gives a mechanistic explanation to the enhancing effect on fibril maturation and indicates structural similarities between prefibrillar structures and mature amyloid. Taken together we show how Ca2+ levels affect the delicate equilibrium between the monomeric and assembled Aβ and how fluctuations in vivo may contribute to development and progression of the disease.

p53, a pivotal effector of a functional cross-talk linking presenilins and Pen-2

The γ-secretase is a multiprotein complex responsible for the ultimate cut yielding amyloid-β peptides and their N-terminal truncated species. This complex is composed of at least four distinct entities, namely presenilin-1 (PS1) or PS2, anterior pharynx defective-1, presenilin enhancer-2 (Pen-2) and nicastrin. Very few studies examined the transcriptional regulation of this complex, and more precisely, whether some of the members functionally interact. Here, we summarize our previous data documenting the fact that Pen-2 controls cell death in a p53-dependent manner and our recent demonstration of a pivotal role of p53 as a regulator of Pen-2 transcription. As PS trigger amyloid precursor protein intracellular domain-dependent regulation of p53, our studies delineate a feedback control mechanism by which PS and Pen-2 functionally interact in a p53-dependent manner.

The physiology of the β-amyloid precursor protein intracellular domain AICD

The amyloid-β precursor protein (βAPP) undergoes several cleavages by enzymatic activities called secretases. Numerous studies aimed at studying the biogenesis and catabolic fate of Aβ peptides, the proteinaceous component of the senile plaques that accumulate in Alzheimer's disease-affected brains. Relatively recently, another secretase-mediated β-APP-derived catabolite called APP IntraCellular Domain (AICD) entered the game. Whether AICD corresponded to a biologically inert by-pass product of βAPP processing or whether it could harbor its own function remained questionable. In this study, we review the mechanisms by which AICD is generated and how its production is regulated. Furthermore, we discuss the degradation mechanism underlying its rapid catabolic fate. Finally, we review putative AICD-related functions and more particularly, the numerous studies indicating that AICD could translocate to the nucleus and control at a transcriptional level, the expression of a series of proteins involved in various functions including the control of cell death and Aβ degradation.

Importance of Shank3 protein in regulating metabotropic glutamate receptor 5 (mGluR5) expression and signaling at synapses

Shank3/PROSAP2 gene mutations are associated with cognitive impairment ranging from mental retardation to autism. Shank3 is a large scaffold postsynaptic density protein implicated in dendritic spines and synapse formation; however, its specific functions have not been clearly demonstrated. We have used RNAi to knockdown Shank3 expression in neuronal cultures and showed that this treatment specifically reduced the synaptic expression of the metabotropic glutamate receptor 5 (mGluR5), but did not affect the expression of other major synaptic proteins. The functional consequence of Shank3 RNAi knockdown was impaired signaling via mGluR5, as shown by reduction in ERK1/2 and CREB phosphorylation induced by stimulation with (S)-3,5-dihydroxyphenylglycine (DHPG) as the agonist of mGluR5 receptors, impaired mGluR5-dependent synaptic plasticity (DHPG-induced long-term depression), and impaired mGluR5-dependent modulation of neural network activity. We also found morphological abnormalities in the structure of synapses (spine number, width, and length) and impaired glutamatergic synaptic transmission, as shown by reduction in the frequency of miniature excitatory postsynaptic currents (mEPSC). Notably, pharmacological augmentation of mGluR5 activity using 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)-benzamide as the positive allosteric modulator of these receptors restored mGluR5-dependent signaling (DHPG-induced phosphorylation of ERK1/2) and normalized the frequency of mEPSCs in Shank3-knocked down neurons. These data demonstrate that a deficit in mGluR5-mediated intracellular signaling in Shank3 knockdown neurons can be compensated by 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)-benzamide; this raises the possibility that pharmacological augmentation of mGluR5 activity represents a possible new therapeutic approach for patients with Shank3 mutations.

p53-dependent control of transactivation of the Pen2 promoter by presenilins

The senile plaques found in the brains of patients with Alzheimer's disease are mainly due to the accumulation of amyloid beta-peptides (A beta) that are liberated by gamma-secretase, a high molecular weight complex including presenilins, PEN-2, APH-1 and nicastrin. The depletion of each of these proteins disrupts the complex assembly into a functional protease. Here, we describe another level of regulation of this multimeric protease. The depletion of both presenilins drastically reduces Pen2 mRNA levels and its promoter transactivation. Furthermore, overexpression of presenilin-1 lowers Pen2 promoter transactivation, a phenotype abolished by a double mutation known to prevent presenilin-dependent gamma-secretase activity. PEN-2 expression is decreased by depletion of beta-amyloid precursor protein (APP) and increased by the APP intracellular domain (AICD). We show that AICD and APP complement for Pen2 mRNA levels in APP/APLP1-2 knockout fibroblasts. Interestingly, overexpression of presenilin-2 greatly increases Pen2 promoter transactivation. The opposite effect triggered by both presenilins was reminiscent of our previous study, which showed that these two proteins elicit antagonistic effects on p53. Therefore, we examined the contribution of p53 on Pen2 transcription. Pen2 promoter transactivation, and Pen2 mRNA and protein levels were drastically reduced in p53(-/-) fibroblasts. Furthermore, PEN-2 expression could be rescued by p53 complementation in p53- and APP-deficient cells. Interestingly, PEN-2 expression was also reduced in p53-deficient mouse brain. Overall, our study describes a p53-dependent regulation of PEN-2 expression by other members of the gamma-secretase complex, namely presenilins.

Pharmacological evidences for DFK167-sensitive presenilin-independent gamma-secretase-like activity

Amyloid-beta (Abeta) peptides production is thought to be a key event in the neurodegenerative process ultimately leading to Alzheimer's disease (AD) pathology. A bulk of studies concur to propose that the C-terminal moiety of Abeta is released from its precursor beta-amyloid precursor protein by a high molecular weight enzymatic complex referred to as gamma-secretase, that is composed of at least, nicastrin (NCT), Aph-1, Pen-2, and presenilins (PS) 1 or 2. They are thought to harbor the gamma-secretase catalytic activity. However, several lines of evidence suggest that additional gamma-secretase-like activities could potentially contribute to Abeta production. By means of a quenched fluorimetric substrate (JMV2660) mimicking the beta-amyloid precursor protein sequence targeted by gamma-secretase, we first show that as expected, this probe allows monitoring of an activity detectable in several cell systems including the neuronal cell line telencephalon specific murine neurons (TSM1). This activity is reduced by DFK167, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT), and LY68458, three inhibitors known to functionally interact with PS. Interestingly, JMV2660 but not the unrelated peptide JMV2692, inhibits Abeta production in an in vitrogamma-secretase assay as expected from a putative substrate competitor. This activity is enhanced by PS1 and PS2 mutations known to be responsible for familial forms of AD and reduced by aspartyl mutations inactivating PS or in cells devoid of PS or NCT. However, we clearly establish that residual JMV2660-hydrolysing activity could be recovered in PS- and NCT-deficient fibroblasts and that this activity remained inhibited by DFK167. Overall, our study describes the presence of a proteolytic activity displaying gamma-secretase-like properties but independent of PS and still blocked by DFK167, suggesting that the PS-dependent complex could not be the unique gamma-secretase activity responsible for Abeta production and delineates PS-independent gamma-secretase activity as a potential additional therapeutic target to fight AD pathology.

Aminopeptidase A contributes to the N-terminal truncation of amyloid beta-peptide

Several lines of data previously indicated that N-terminally truncated forms of amyloid-beta (Abeta) peptides are likely the earliest and more abundant species immunohistochemically detectable in Alzheimer's disease-affected brains. It is noteworthy that the free N-terminal residue of full-length Abeta (fl-Abeta) is an aspartyl residue, suggesting that Abeta could be susceptible to exopeptidasic attack by aminopeptidase A (APA)-like proteases. In this context, we have examined whether APA could target Abeta peptides in both cell-free and cellular models. We first show that the general aminopeptidase inhibitor amastatin as well as two distinct aminopeptidase A inhibitors EC33 and pl302 both significantly increase the recovery of genuine fl-Abeta peptides generated by cells over-expressing Swedish-mutated beta amyloid precursor protein (APP) while the aminopeptidase N blocker pl250 did not modify fl-Abeta recovery. In agreement with this observation, we establish that over-expressed APA drastically reduces, in a calcium dependent manner, fl-Abeta but not APP IntraCellular Domain in a cell-free model of Abeta production. In agreement with the above data, we show that recombinant APA degrades fl-Abeta in a pl302-sensitive manner. Interestingly, we also show that EC33 and pl302 lower staurosporine-stimulated activation of caspase-3 in wild-type fibroblasts but not in betaAPP/beta-amyloid precursor protein-like protein 2 (APLP2) double knockout fibroblasts, suggesting that protecting endogenous fl-Abeta physiological production triggers neuroprotective phenotype. By contrast, EC33 does not modify staurosporine-induced caspase-3 activation in wild-type and Swedish-mutated betaAPP-HEK293 expressing cells that display exacerbated production of Abeta. Overall, our data establish that APA contributes to the N-terminal truncation of Abeta and suggest that this cleavage is likely abrogating a protective function associated with physiological but not supraphysiological levels of genuine fl-Abeta peptides.

Therapeutic strategies for Alzheimer's disease

Therapeutic approaches for Alzheimer's disease (AD) are guided by four disease characteristics: amyloid plaques, neurofibrillar tangles (NFT), neurodegeneration, and dementia. Amyloid plaques are composed largely of 4 kDa beta-amyloid (Abeta) peptides, with the more amyloidogenic, 42 amino acid form (Abeta42) as the primary species. Because multiple, rare mutations that cause early-onset, familial AD lead to increased production or aggregation of Abeta42, amyloid therapeutics aim to reduce the amount of toxic Abeta42 aggregates. Amyloid-based therapies include gamma-secretase inhibitors and modulators, BACE inhibitors, aggregation blockers, catabolism inducers, and anti-Abeta biologics. Tangles are composed of paired helical filaments of hyperphosphorylated tau protein. Tau-based therapeutics include kinase inhibitors, microtubule stabilizers, and catabolism inducers. Therapeutic strategies for neurodegeneration target multiple mechanisms, including excitotoxicity, mitochondrial dysfunction, oxidative damage, and inflammation or stimulation of neuronal viability. Although not disease modifying, cognition enhancers are important to treat the symptom of dementia. Strategies for cognition enhancement include cholinesterase inhibitors, and other approaches to enhance the signaling of cholinergic and glutamatergic neurons. In summary, plaques, tangles, neurodegeneration and dementia guide the development of multiple therapeutic approaches for AD and are the subject of this review.

TMP21 regulates Abeta production but does not affect caspase-3, p53, and neprilysin

The presenilin (PS)-dependent gamma-secretase activity refers to a high molecular mass-complex including, besides PS1 or PS2, three other proteins recently identified, namely nicastrin, Aph-1, and Pen-2. This proteolytic complex has been shown to contribute to both gamma- and epsilon-cleavages of the beta-amyloid precursor protein (betaAPP), thereby generating beta-amyloid peptides (Abeta) and the APP intracellular domain (AICD), respectively. TMP21, a member of the p24 cargo protein family, was recently shown to interact with PS complexes. Interestingly, TMP21 modulates gamma-secretase-mediated Abeta production but does not regulate epsilon-secretase-derived AICD formation [F. Chen, H. Hasegawa, G. Schmitt-ulms, T. Kawarai, C. Bohm, T. Katayama, Y. Gu, N. Sanjo, M. Glista, E. Rogaeva, Y. Wakutami, R. Pardossi-Piquard, X. Ruan, A. Tandon, F. Checler, P. Marambaud, K. Hansen, D. Westaway, P. St. George-Hyslop, P. Fraser, TMP21 is a presenilin complex component that modulates gamma- but not epsilon-secretase activities, Nature 440 (2006) 1208-1212]. Here we investigate the functional incidence of the over-expression or depletion of TMP21 on both intracellular and secreted Abeta recoveries and AICD-associated phenotypes. First we confirm that TMP21 depletion yields increased levels of secreted Abeta40. However, we demonstrate that both staurosporine-stimulated caspase-3 activation, p53 and neprilysin expression and activity were not affected by TMP21 over-expression or depletion. Overall, our functional data further reinforce the view that TMP21 behaves as a regulator of gamma- but not epsilon-cleavages generated by PS-dependent gamma-secretase complex.

Protection against beta-amyloid-induced apoptosis by peptides interacting with beta-amyloid

beta-Amyloid peptide produces apoptosis in neurons at micromolar concentrations, but the mechanism by which beta-amyloid exerts its toxic effect is unknown. The normal biological function of beta-amyloid is also unknown. We used phage display, co-precipitation, and mass spectrometry to examine the protein-protein interactions of beta-amyloid in normal rabbit brain in order to identify the biochemical receptors for beta-amyloid. beta-Amyloid was found to bind primarily to proteins involved in low density lipoprotein and cholesterol transport and metabolism, including sortilin, endoplasmic reticulum-Golgi intermediate compartment 2 (ERGIC2), ERGIC-53, steroid 5alpha-reductase, and apolipoprotein B. beta-Amyloid also bound to the C-reactive protein precursor, a protein involved in inflammation, and to 14-3-3, a protein that regulates glycogen synthase kinase-3beta, the kinase involved in tau phosphorylation. Of eight synthetic peptides identified as targets of beta-amyloid, three were found to be effective blockers of the toxic effect of beta-amyloid on cultured neuronal cells. These peptides bound to the hydrophobic region (residues 17-21) or to the nearby protein kinase C pseudo-phosphorylation site (residues 26-30) of beta-amyloid, suggesting that these may be the most critical regions for beta-amyloid effector action and for aggregation. Peptides or other small molecules that bind to this region may protect against beta-amyloid toxic effect by competitively blocking its ability to bind beta-amyloid effector proteins such as sortilin and 14-3-3.

Study on the Putative Contribution of Caspases and the Proteasome to the Degradation of Aph-1a and Pen-2

The presenilin-dependent gamma-secretase complex is mainly composed of four distinct proteins, namely presenilin 1 or presenilin 2, nicastrin, anterior pharynx defective-1 (Aph-1) and presenilin enhancer (Pen-2). The mechanisms by which the complex is assembled, how its stoichiometry is controlled and how its catalytic activity is regulated are poorly understood. Recent studies indicated that Aph-1 and Pen-2 undergo proteolysis by the proteasome. We have examined the susceptibility of endogenous and overexpressed Aph-1a and Pen-2 to proteolysis by endogenous and purified proteasome as well as by recombinant caspases. We show that endogenous Aph-1a and Pen-2 resist proteolysis by caspases and by the proteasome. Furthermore, we show that unexpected interference of proteasome inhibitors with the cmv promoter region driving expression of Aph-1a and Pen-2 led to artifactual enhancement of overexpressed Aph-1a and Pen-2-like immunoreactivities but that these proteins also resist to in vitro degradation by endogenous and purified proteasome.

Biogenesis and processing of the amyloid precursor protein in the early secretory pathway

The beta-amyloid peptide is an aggregation-prone peptide that is released from the amyloid precursor protein (APP) after cleavage by the beta- and gamma-secretase. A number of studies have suggested that processing of APP by beta- and gamma-secretase occurs not only at the cell surface and in the endosomal compartments but also in the endoplasmic reticulum (ER) and Golgi complex. Here, we studied the role of the early secretory pathway in processing of APP. For this purpose, APP was in vitro translated in semi-permeabilized cells, which have a functionally intact ER and Golgi complex but lack a functional plasma membrane. We show that the beta-secretase cleavage product C99 is generated in the early secretory pathway. Moreover, nicastrin and presenilin, two components of the gamma-secretase complex, interacted with newly synthesized APP. Administration of the gamma-secretase inhibitor L685,458 caused accumulation of full length APP and C99. Full length APP but not C99 interacted with several protein quality control ER chaperones including the thiol oxidoreductase ERp57. Our in vitro study suggests that newly synthesized APP is subject to amyloidogenic processing during the initial phases of the secretory pathway.

Unique neuronal functions of cathepsin L and cathepsin B in secretory vesicles: biosynthesis of peptides in neurotransmission and neurodegenerative disease

Proteases are required for the production of peptide neurotransmitters and toxic peptides in neurodegenerative diseases. Unique roles of the cysteine proteases cathepsin L and cathepsin B in secretory vesicles for the production of biologically active peptides have been demonstrated in recent studies. Secretory vesicle cathepsin L participates in the proteolytic conversion of proenkephalin into the active enkephalin, an opioid peptide neurotransmitter that mediates pain relief. Moreover, recent findings provide evidence that cathepsin B in regulated secretory vesicles participates in the production of toxic beta-amyloid peptides that are known to accumulate extracellularly in Alzheimer's disease brains. The neurobiological functions of cathepsins L and B demonstrate that these secretory vesicle cysteine proteases produce biologically active peptides. These results demonstrate newly identified roles for cathepsins L and B in neurosecretory vesicles in the production of biologically active peptides.

Gamma-secretase inhibitors for Alzheimer's disease: balancing efficacy and toxicity

The amyloid hypothesis, which states that beta-amyloid (Abeta) aggregates cause the onset and progression of Alzheimer's disease (AD), is a leading proposal to explain AD aetiology. Based on this hypothesis, compounds that inhibit gamma-secretase, one of the enzymes responsible for forming Abeta, are potential therapeutics for AD. Preclinical studies clearly establish that gamma-secretase inhibitors can reduce brain Abeta in rodent models. The initial investigation of the effects of a gamma-secretase inhibitor on Abeta-induced cognitive deficits in transgenic mice showed that modest Abeta reductions (15-30%) are sufficient to reverse Abeta-induced cognitive deficits in Tg2576 mice. Extending these studies to other gamma-secretase inhibitors and other models with Abeta-induced cognitive deficits will be important. Unfortunately, gamma-secretase inhibitors also cause abnormalities in the gastrointestinal tract, thymus and spleen in rodents. These changes likely result from inhibition of Notch cleavage, a transmembrane receptor involved in regulating cell-fate decisions. Two recent studies in rodents suggest that Abeta reduction using gamma-secretase inhibitors can be partially separated from Notch inhibition. Given the uncertain Abeta reduction target and the potential for mechanism-based toxicity, biomarkers for efficacy and toxicity would be helpful in clinical trials. The first report of gamma-secretase inhibitors in clinical trials was recently published. In this study, LY-450139 reduced plasma Abeta, but not cerebrospinal fluid Abeta. Taken together, the results of studies to date suggest that gamma-secretase inhibitors have the potential to address a large unmet medical need if the technical challenges can be overcome.

Drosophilaas a Model for Human Neurodegenerative Disease

Among many achievements in the neurodegeneration field in the past decade, two require special attention due to the huge impact on our understanding of molecular and cellular pathogenesis of human neurodegenerative diseases. First is defining specific mutations in familial neurodegenerative diseases and second is modeling these diseases in easily manipulable model organisms including the fruit fly, nematode, and yeast. The power of these genetic systems has revealed many genetic factors involved in the various pathways affected, as well as provided potential drug targets for therapeutics. This review focuses on fruit fly models of human neurodegenerative diseases, with emphasis on how fly models have provided new insights into various aspects of human diseases.

A presenilin-independent aspartyl protease prefers the gamma-42 site cleavage

beta-Amyloid peptides (Abeta40 and Abeta42) are the major constituents of amyloid plaques, which are one of the hallmarks of Alzheimer's disease (AD). The Abeta is derived from sequential cleavages of amyloid precursor protein (APP) by beta- and gamma-secretases. gamma-Secretase consists of at least four proteins where presenilins (PS1 and PS2 or PS) are the catalytic subunit involved in the gamma-site cleavage of APP. Secretion of both Abeta40 and Abeta42 is significantly reduced in PS1 knock-out cells and completely abolished in cells deficient for both PS1 and PS2. Consequently, both the PS proteins play essential roles in the production of the secretory of Abeta from cells. Recent studies in primary neurons, however, suggest that PSs are not required for intracellular Abeta42 accumulation; thus the intracellular Abeta42 appears to be generated in a PS-independent manner. Here we present the first biochemical evidence indicating that Abeta, especially Abeta42, can be generated in the absence of PS based on an in vitrogamma-secretase assay employing membranes prepared from PS-deficient Blastocyst-derived (BD) cells. This PS-independent gamma-secretase (PSIG) activity is sensitive to the changes in pH and displays an optimal activity at pH 6.0. Pepstatin A is a potent inhibitor for this proteolytic activity with IC50 of 1.2 nm and 0.4 nm for Abeta40 and Abeta42 generation, respectively. These results indicate that these PS-independent gamma-site cleavages are mediated by an aspartyl protease. More importantly, the PSIG activity displays a distinct preference in mediating the 42-site cleavage over the 40-site cleavage, thereby generating Abeta42 as the predominant product.

Proteolytic mechanisms in amyloid-beta metabolism: therapeutic implications for Alzheimer's disease

The accumulation of the amyloid-beta peptide, the main constituent of the "amyloid plaque", is widely considered to be the key pathological event in Alzheimer's disease. Amyloid-beta is produced from the amyloid precursor protein through the action of the proteases beta-secretase and gamma-secretase. Alternative cleavage of amyloid precursor protein by the enzyme alpha-secretase precludes amyloid-beta production. In addition, several proteases are involved in the degradation of amyloid-beta. This review focuses on the proteolytic mechanisms of amyloid-beta metabolism. An increasingly detailed understanding of proteolysis in both amyloid-beta deposition and clearance has identified some of these proteases as potential therapeutic targets for Alzheimer's disease. A more complex knowledge of these proteases takes us one step closer to developing "disease-modifying" therapies, but these advances also emphasize that significant challenges must be overcome before clinically effective drugs to treat Alzheimer's disease become a reality.

Genetic and molecular aspects of Alzheimer's disease shed light on new mechanisms of transcriptional regulation

Rapid advances made in biological research aimed at understanding the molecular basis of the pathogenesis of Alzheimer's disease have led to the characterization of a novel catalytic activity termed gamma-secretase. First described for its beta-amyloid-producing function, gamma-secretase is now actively studied for its role in a novel signal transduction paradigm, which implicates cell-surface receptor proteolysis and direct surface-to-nucleus signal transduction. gamma-Secretase targets numerous type I protein receptors involved in diverse functions ranging from normal development to neurodegeneration. In this Review we discuss how the study of the genetic and molecular aspects of Alzheimer's disease has revealed a dual role of gamma-secretase in transcriptional regulation and in the pathogenesis of familial Alzheimer's disease.

Analysis of Notch Function in Presomitic Mesoderm Suggests a γ-Secretase-Independent Role for Presenilins in Somite Differentiation

The role of Notch signaling in general and presenilin in particular was analyzed during mouse somitogenesis. We visualize cyclical production of activated Notch (NICD) and establish that somitogenesis requires less NICD than any other tissue in early mouse embryos. Indeed, formation of cervical somites proceeds in Notch1; Notch2-deficient embryos. This is in contrast to mice lacking all presenilin alleles, which have no somites. Since Nicastrin-, Pen-2-, and APH-1a-deficient embryos have anterior somites without gamma-secretase, presenilin may have a gamma-secretase-independent role in somitogenesis. Embryos triple homozygous for both presenilin null alleles and a Notch allele that is a poor substrate for presenilin (N1(V-->G)) experience fortuitous cleavage of N1(V-->G) by another protease. This restores NICD, anterior segmentation, and bilateral symmetry but does not rescue rostral/caudal identities. These data clarify multiple roles for Notch signaling during segmentation and suggest that the earliest stages of somitogenesis are regulated by both Notch-dependent and Notch-independent functions of presenilin.

Widespread γ-secretase activity in the cell, but do we need it at the mitochondria?

gamma-Secretase cleavage of the amyloid precursor protein already subjected to a prior beta-secretase cleavage generates beta-amyloid (Abeta) peptide fragments, which are major constituents of the amyloid plagues found in Alzheimer's disease brain tissues. gamma-Secretase activity and components of the gamma-secretase complex are found in the endoplasmic reticulum-Golgi intermediate compartment, the Golgi, the trans-Golgi network, the plasma membrane, the endosomal-lysosomal system and recently, the mitochondria. Abeta fragments have been shown to be neurotoxic, leading to mitochondrial dysfunction and enhanced apoptotic cell death. However, if Abeta fragments are indeed detrimental to neurons, the widespread presence of enzymatic activity that would result in their generation in the cell appears to make little sense. The presence of a gamma-secretase complex in the mitochondrion, an organelle that is particularly susceptible to Abeta toxicity, is even more puzzling. Emerging evidence suggests that both secreted and intracellular Abeta fragments have endogenous functions. Also, while the fibrillogenic Abeta1-42 is clearly neurotoxic, the more abundant and soluble Abeta1-40 is an antioxidant and could potentially be neuroprotective in several ways. A "physiological" amount of Abeta1-40 production by cellular gamma-secretase activity may be part of the neuron's natural counter against oxidative damage, in addition to endogenous roles in neuronal survival and modulation of synaptic transmission. In any case, whether Abeta is produced locally in the mitochondria and the function for mitochondrial Abeta, if produced, is yet unclear.

Aph-1 interacts at the cell surface with proteins in the active γ-secretase complex and membrane-tethered Notch

The activity of the gamma-secretase complex is critical for the processing of a number of transmembrane proteins, including Notch. Functional gamma-secretase activity can be reconstituted from four proteins--presenilin, nicastrin, Pen-2 and Aph-1--but the role of the individual proteins remains unclear. In this report we describe the cellular localization and protein interactions of Aph-1, with particular regard to Notch receptor processing. We found that Aph-1 is present at the cell surface, where it interacts with Pen-2, the mature forms of presenilin and nicastrin, and full-length Notch. Aph-1 also interacts with a truncated form of Notch, which is a direct substrate for gamma-secretase, but not with the Notch intracellular domain. Immunoprecipitation data for Notch and Aph-1 showed that the Notch-containing gamma-secretase complexes most likely form a small subset of the total number of gamma-secretase complexes. In conclusion, these data demonstrate that Aph-1 is present at the cell surface, presumably in active gamma-secretase complexes, and interacts with the Notch receptor, both before and after ligand activation.

The metabolism of β-amyloid converting enzyme and β-amyloid precursor protein processing

Herein we investigated the processing of beta-secretase (BACE), implicated in Alzheimer's disease through processing of beta-amyloid precursor protein (betaAPP), into smaller metabolites. Four products of approximately 34, approximately 12, approximately 8, and approximately 5kDa were identified, none of which were generated autocatalytically. The approximately 34 and approximately 12kDa forms are held together by disulfide bridges. The approximately 34kDa form results from two cleavages: an N-terminal processing at RLPR(45) downward arrow by furin/PC5, and a C-terminal cleavage at SQDD(379) downward arrow by an unknown enzyme that also releases the C-terminal approximately 12kDa product. Microsequencing of the approximately 8 and approximately 5kDa fragments showed that they are the result of processing at VVFD(407) downward arrow and DMED(442) downward arrow, respectively. Mutagenesis of the identified cleavage sites revealed that the mutants D379A, D379L or D379E block the degradation of BACE into the approximately 12kDa product, confirming the importance of Asp(379). Notably, the D379E mutant results in higher betaAPP derived C99 levels. In contrast, D442A or D442E did not affect the production of the approximately 8 or approximately 5kDa products. The levels of the approximately 8 and approximately 5kDa products are significantly lower in the mutant D407A but less so D407E, likely due to the low efficacy of ER exit of the D407A mutant. Indeed, while co-expression of betaAPP with BACE results in enhanced production of Abeta(11-40), the D407A mutant produces mostly Abeta(40).

Dynamics of {beta}-amyloid reductions in brain, cerebrospinal fluid, and plasma of {beta}-amyloid precursor protein transgenic mice treated with a {gamma}-secretase inhibitor

gamma-Secretase inhibitors are one promising approach to the development of a therapeutic for Alzheimer's disease (AD). gamma-Secretase inhibitors reduce brain beta-amyloid peptide (Abeta), which is believed to be a major contributor in the etiology of AD. Transgenic mice overexpressing the human beta-amyloid precursor protein (APP) are valuable models to examine the dynamics of Abeta changes with gamma-secretase inhibitors in plaque-free and plaque-bearing animals. BMS-299897 2-[(1R)-1-[[(4-chlorophenyl)sulfony](2,5-difluorophenyl)amino]ethyl]-5-fluorobenzenepropanoic acid, a gamma-secretase inhibitor, showed dose- and time dependent reductions of Abeta in brain, cerebrospinal fluid (CSF), and plasma in young transgenic mice, with a significant correlation between brain and CSF Abeta levels. Because CSF and brain interstitial fluid are distinct compartments in composition and location, this correlation could not be assumed. In contrast, aged transgenic mice with large accumulations of Abeta in plaques showed reductions in CSF Abeta in the absence of measurable changes in plaque Abeta in the brain after up to 2 weeks of treatment. Hence, CSF Abeta levels were a valuable measure of gamma-secretase activity in the central nervous system in either the presence or absence of plaques. Transgenic mice were also used to examine potential side effects due to Notch inhibition. BMS-299897 was 15-fold more effective at preventing the cleavage of APP than of Notch in vitro. No changes in the maturation of CD8(+) thymocytes or of intestinal goblet cells were observed in mice treated with BMS-299897, showing that it is possible for gamma-secretase inhibitors to reduce brain Abeta without causing Notch-mediated toxicity.