Nicastrin functions as a gamma-secretase-substrate receptor

Alzheimer's disease: cholesterol, membrane rafts, isoprenoids and statins

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disorder and the most prevalent form of dementia worldwide. AD is characterized pathologically by amyloid-β plaques, neurofibrillary tangles and neuronal loss, and clinically by a progressive loss of cognitive abilities. At present, the fundamental molecular mechanisms underlying the disease are unclear and no treatment for AD is known. Epidemiological evidence continues to mount linking vascular diseases, such as hypertension and diabetes, and hypercholesterolaemia with an increased risk for developing AD. A growing amount of evidence suggests a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques in AD development. Cholesterol and statins clearly modulate β-amyloid precursor protein (βAPP) processing in cell culture and animal models. Statins not only reduce endogenous cholesterol synthesis but also exert other various pleiotrophic effects, such as the reduction in protein isoprenylation. Through these effects statins modulate a variety of cellular functions involving both cholesterol (and membrane rafts) and isoprenylation. Although clearly other factors, such as vascular inflammation, oxidative stress and genetic factors, are intimately linked with the progression of AD, this review focuses on the present research findings describing the effect of cholesterol, membrane rafts and isoprenylation in regulating βAPP processing and in particular γ-secretase complex assembly and function and AD progression, along with consideration for the potential role statins may play in modulating these events.

A family of membrane proteins associated with presenilin expression and gamma-secretase function

Presenilin 1 (PS1) forms the gamma-secretase complex with at least three components: nicastrin, APH-1, and PEN-2. This complex mediates intramembrane cleavage of amyloid precursor protein (APP) to generate beta-amyloid protein (Abeta) as well as other type 1 transmembrane proteins. Although PS1 mutations linked to familial Alzheimer's disease influence these cleavages, their biological consequences have not been fully understood. In this study, we used mRNA differential display analysis to identify a gene, denoted adoplin-1/ORMDL-1, which displays significantly reduced expression in association with PS1 mutations. Adoplin-1 and two highly homologous genes (adoplin-2, -3) constitute a gene family that encodes transmembrane proteins. The mRNA and protein levels of adoplins (particularly adoplin-1, -2) were markedly elevated in PS-deficient fibroblasts, compared to wild-type cells. Moreover, knockdown of the three adoplins by RNA interference affected maturation of nicastrin and its association with PS1. Adoplin knockdown additionally resulted in elevated levels of APP C-terminal fragments and decreased Abeta production, suggestive of reduced gamma-secretase activity. Our data collectively indicate that adoplins are unique molecules with PS-related expression and functions that may play important role(s) in the maturation and activity of the gamma-secretase complex.

Active gamma-secretase complexes contain only one of each component

Gamma-secretase is an intramembrane aspartyl protease complex that cleaves type I integral membrane proteins, including the amyloid beta-protein precursor and the Notch receptor, and is composed of presenilin, Pen-2, nicastrin, and Aph-1. Although all four of these membrane proteins are essential for assembly and activity, the stoichiometry of the complex is unknown, with the number of presenilin molecules present being especially controversial. Here we analyze functional gamma-secretase complexes, isolated by immunoprecipitation from solubilized membrane fractions and able to produce amyloid beta-peptides and amyloid beta-protein precursor intracellular domain. We show that the active isolated protease contains only one presenilin per complex, which excludes certain models of the active site that require aspartate dyads formed between two presenilin molecules. We also quantified components in the isolated complexes by Western blot using protein standards and found that the amounts of Pen-2 and nicastrin were the same as that of presenilin. Moreover, we found that one Aph-1 was not co-immunoprecipitated with another in active complexes, evidence that Aph-1 is likewise present as a monomer. Taken together, these results demonstrate that the stoichiometry of gamma-components presenilin:Pen-2:nicastrin:Aph-1 is 1:1:1:1.

Protein-sphingolipid interactions within cellular membranes

Each intracellular organelle critically depends on maintaining its specific lipid composition that in turn contributes to the biophysical properties of the membrane. With our knowledge increasing about the organization of membranes with defined microdomains of different lipid compositions, questions arise regarding the molecular mechanisms that underlie the targeting to/segregation from microdomains of a given protein. In addition to specific lipid-transmembrane segment interactions as a basis for partitioning, the presence in a given microdomain may alter the conformation of proteins and, thus, the activity and availability for regulatory modifications. However, for most proteins, the specific lipid environment of transmembrane segments as well as its relevance to protein function and overall membrane organization are largely unknown. To help fill this gap, we have synthesized a novel photoactive sphingolipid precursor that, together with a precursor for phosphoglycerolipids and with photo-cholesterol, was investigated in vivo with regard to specific protein transmembrane span-lipid interactions. As a proof of principle, we show specific labeling of the ceramide transporter with the sphingolipid probe and describe specific in vivo interactions of lipids with caveolin-1, phosphatidylinositol transfer protein beta, and the mature form of nicastrin. This novel photolabile sphingolipid probe allows the detection of protein-sphingolipid interactions within the membrane bilayer of living cells.

Amyloid beta-protein precursor juxtamembrane domain regulates specificity of gamma-secretase-dependent cleavages

Amyloid beta-protein (Abeta), the major component of cerebral plaques associated with Alzheimer disease, is derived from amyloid beta-protein precursor (APP) through sequential proteolytic cleavage involving beta- and gamma-secretase. The intramembrane cleavage of APP by gamma-secretase occurs at two major sites, gamma and epsilon, although the temporal and/or mechanistic relationships between these cleavages remain unknown. In our attempt to address this issue, we uncovered an important regulatory role for the APP luminal juxtamembrane domain. We demonstrated in cell-based assays that domain replacements in this region can greatly reduce secreted Abeta resulting from gamma-cleavage without affecting the epsilon-cleavage product. This Abeta reduction is likely due to impaired proteolysis at the gamma-cleavage site. Further analyses with site-directed mutagenesis identified two juxtamembrane residues, Lys-28 and Ser-26 (Abeta numbering), as the critical determinants for efficient intramembrane proteolysis at the gamma-site. Consistent with the growing evidence that epsilon-cleavage of APP precedes gamma-processing, longer Abeta species derived from the gamma-cleavage-deficient substrates were detected intracellularly. These results indicate that the luminal juxtamembrane region of APP is an important regulatory domain that modulates gamma-secretase-dependent intramembrane proteolysis, particularly in differentiating gamma- and epsilon-cleavages.

Loss-of-function presenilin mutations in Alzheimer disease. Talking Point on the role of presenilin mutations in Alzheimer disease

Presenilin mutations are the main cause of familial Alzheimer disease. From a genetic point of view, these mutations seem to result in a gain of toxic function; however, biochemically, they result in a partial loss of function in the gamma-secretase complex, which affects several downstream signalling pathways. Consequently, the current genetic terminology is misleading. In fact, the available data indicate that several clinical presenilin mutations also lead to a decrease in amyloid precursor protein-derived amyloid beta-peptide generation, further implying that presenilin mutations are indeed loss-of-function mutations. The loss of function of presenilin causes incomplete digestion of the amyloid beta-peptide and might contribute to an increased vulnerability of the brain, thereby explaining the early onset of the inherited form of Alzheimer disease. In this review, I evaluate the implications of this model for the amyloid-cascade hypothesis and for the efficacy of presenilin/gamma-secretase as a drug target.

The Alzheimer's disease-associated gamma-secretase complex: functional domains in the presenilin 1 protein

Alzheimer's disease is neuropathologically characterized by the presence of neurofibrillary tangles and amyloid plaques in the brain. Amyloid plaques are extracellular deposits primarily composed of the amyloid beta-peptide, which is derived from the amyloid beta-precursor protein (APP) by sequential cleavages at the beta-secretase and gamma-secretase sites. gamma-Secretase cleavage is performed by a high molecular weight protein complex containing presenilin (PS), nicastrin, Aph-1 and Pen-2. The gamma-secretase complex is an unusual transmembrane aspartyl protease that cleaves APP within the transmembrane domain. In addition to APP, a large number of other single membrane-spanning proteins have been shown to be cleaved within their transmembrane domains by the gamma-secretase complex in a process referred to as regulated intramembrane proteolysis. Here we review recent research leading to the identification and understanding of the gamma-secretase complex components with emphasis on PS, which harbors the catalytic site. In addition, we summarize our own work focused on identifying and studying domains in PS1 that are critical for mediating gamma-secretase activity. Biochemical understanding of the gamma-secretase complex is important from a basic biological and physiological point of view, and could help in the development of small molecules that modulate gamma-secretase processing in an APP-specific manner.

Rer1p competes with APH-1 for binding to nicastrin and regulates gamma-secretase complex assembly in the early secretory pathway

The γ-secretase complex, consisting of presenilin, nicastrin, presenilin enhancer-2 (PEN-2), and anterior pharynx defective-1 (APH-1) cleaves type I integral membrane proteins like amyloid precursor protein and Notch in a process of regulated intramembrane proteolysis. The regulatory mechanisms governing the multistep assembly of this “proteasome of the membrane” are unknown. We characterize a new interaction partner of nicastrin, the retrieval receptor Rer1p. Rer1p binds preferentially immature nicastrin via polar residues within its transmembrane domain that are also critical for interaction with APH-1. Absence of APH-1 substantially increased binding of nicastrin to Rer1p, demonstrating the competitive nature of these interactions. Moreover, Rer1p expression levels control the formation of γ-secretase subcomplexes and, concomitantly, total cellular γ-secretase activity. We identify Rer1p as a novel limiting factor that negatively regulates γ-secretase complex assembly by competing with APH-1 during active recycling between the endoplasmic reticulum (ER) and Golgi. We conclude that total cellular γ-secretase activity is restrained by a secondary ER control system that provides a potential therapeutic value.

Enzymatic analysis of a rhomboid intramembrane protease implicates transmembrane helix 5 as the lateral substrate gate

Intramembrane proteolysis is a core regulatory mechanism of cells that raises a biochemical paradox of how hydrolysis of peptide bonds is accomplished within the normally hydrophobic environment of the membrane. Recent high-resolution crystal structures have revealed that rhomboid proteases contain a catalytic serine recessed into the plane of the membrane, within a hydrophilic cavity that opens to the extracellular face, but protected laterally from membrane lipids by a ring of transmembrane segments. This architecture poses questions about how substrates enter the internal active site laterally from membrane lipid. Because structures are static glimpses of a dynamic enzyme, we have taken a structure-function approach analyzing >40 engineered variants to identify the gating mechanism used by rhomboid proteases. Importantly, our analyses were conducted with a substrate that we show is cleaved at two intramembrane sites within the previously defined Spitz substrate motif. Engineered mutants in the L1 loop and active-site region of the GlpG rhomboid protease suggest an important structural, rather than dynamic, gating function for the L1 loop that was first proposed to be the substrate gate. Conversely, three classes of mutations that promote transmembrane helix 5 displacement away from the protease core dramatically enhanced enzyme activity 4- to 10-fold. Our functional analyses have identified transmembrane helix 5 movement to gate lateral substrate entry as a rate-limiting step in intramembrane proteolysis. Moreover, our mutagenesis also underscores the importance of other residue interactions within the enzyme that warrant further scrutiny.

The Nicastrin-like Protein Nicalin Regulates Assembly and Stability of the Nicalin-Nodal Modulator (NOMO) Membrane Protein Complex

The assembly of the gamma-secretase complex, an Alzheimer disease-related protease required for beta-amyloid generation, is tightly regulated and predominantly limited by the stoichiometrical availability of its components. We have identified a novel endoplasmic reticulum-located protein complex that is regulated in a similar fashion. It contains the recently identified Nodal signaling antagonists Nicalin (a distant homolog of the gamma-secretase component Nicastrin) and NOMO (Nodal modulator). Using an RNA interference approach, we found that Nicalin and NOMO became unstable in the absence of the respective binding partner, suggesting that complex formation has a stabilizing effect. Overexpression of Nicalin resulted in an increase in NOMO, whereas endogenous Nicalin was reduced below the detection limit. Both effects were shown to occur at a post-transcriptional level. Thus, NOMO is most likely produced in excess amounts and either stabilized by Nicalin or rapidly degraded. In contrast, Nicalin levels are limited independently of NOMO. We, therefore, propose that Nicalin controls the assembly and stability of the Nicalin-NOMO complex.

The role of presenilin and its interacting proteins in the biogenesis of Alzheimer's beta amyloid

The biogenesis and accumulation of the beta amyloid protein (Abeta) is a key event in the cascade of oxidative and inflammatory processes that characterises Alzheimer's disease. The presenilins and its interacting proteins play a pivotal role in the generation of Abeta from the amyloid precursor protein (APP). In particular, three proteins (nicastrin, aph-1 and pen-2) interact with presenilins to form a large multi-subunit enzymatic complex (gamma-secretase) that cleaves APP to generate Abeta. Reconstitution studies in yeast and insect cells have provided strong evidence that these four proteins are the major components of the gamma-secretase enzyme. Current research is directed at elucidating the roles that each of these protein play in the function of this enzyme. In addition, a number of presenilin interacting proteins that are not components of gamma-secretase play important roles in modulating Abeta production. This review will discuss the components of the gamma-secretase complex and the role of presenilin interacting proteins on gamma-secretase activity.

Aβ42 Overproduction Associated with Structural Changes in the Catalytic Pore of γ-Secretase

gamma-Secretase is an atypical aspartyl protease that cleaves amyloid beta-precursor protein to generate Abeta peptides that are causative for Alzheimer disease. gamma-Secretase is a multimeric membrane protein complex composed of presenilin (PS), nicastrin, Aph-1, and Pen-2. Pen-2 directly binds to transmembrane domain 4 of PS and confers proteolytic activity on gamma-secretase, although the mechanism of activation and its role in catalysis remain unknown. Here we show that an addition of amino acid residues to the N terminus of Pen-2 specifically increases the generation of Abeta42, the longer and more aggregable species of Abeta. The effect of the N-terminal elongation of Pen-2 on Abeta42 generation was independent of the amino acid sequences, the expression system and the presenilin species. In vitro gamma-secretase assay revealed that Pen-2 directly affects the Abeta42-generating activity of gamma-secretase. The elongation of Pen-2 N terminus caused a reduction in the water accessibility of the luminal side of the catalytic pore of PS1 in a similar manner to that caused by an Abeta42-raising gamma-secretase modulator, fenofibrate, as determined by substituted cysteine accessibility method. These data suggest a unique mechanism of Abeta42 overproduction associated with structural changes in the catalytic pore of presenilins caused commonly by the N-terminal elongation of Pen-2 and fenofibrate.

GxxxG motifs within the amyloid precursor protein transmembrane sequence are critical for the etiology of Abeta42

Processing of the amyloid precursor protein (APP) by beta- and gamma-secretases leads to the generation of amyloid-beta (Abeta) peptides with varying lengths. Particularly Abeta42 contributes to cytotoxicity and amyloid accumulation in Alzheimer's disease (AD). However, the precise molecular mechanism of Abeta42 generation has remained unclear. Here, we show that an amino-acid motif GxxxG within the APP transmembrane sequence (TMS) has regulatory impact on the Abeta species produced. In a neuronal cell system, mutations of glycine residues G29 and G33 of the GxxxG motif gradually attenuate the TMS dimerization strength, specifically reduce the formation of Abeta42, leave the level of Abeta40 unaffected, but increase Abeta38 and shorter Abeta species. We show that glycine residues G29 and G33 are part of a dimerization site within the TMS, but do not impair oligomerization of the APP ectodomain. We conclude that gamma-secretase cleavages of APP are intimately linked to the dimerization strength of the substrate TMS. The results demonstrate that dimerization of APP TMS is a risk factor for AD due to facilitating Abeta42 production.

Presenilin diversifies its portfolio

Presenilin, the catalytic member of the gamma-secretase proteolytic complex, was discovered through its roles in generating Alzheimer's-disease-associated amyloid-beta peptides from the amyloid-beta precursor protein and in releasing the transcriptionally active domain of the receptor Notch. Recent work has revealed many additional cleavage substrates and interacting proteins, suggesting a diversity of roles for presenilin during development and adult life, some of which might contribute to Alzheimer's disease progression. Although many of these functions depend on the proteolytic activity of gamma-secretase, others are independent of its role as a protease. Here, we review recent data on candidate functions for presenilin and its interactors and on their potential significance in disease.

Degradation of nicastrin involves both proteasome and lysosome

The glycoprotein nicastrin (NCT) is an essential component of the gamma-secretase complex, a high molecular weight complex which also contains the presenilin proteins, Aph-1 and Pen-2. The gamma-secretase complex is not only involved in APP processing but also in the processing of an increasing number of other type I integral membrane proteins. As the largest subunit of the gamma-secretase complex, NCT plays a crucial role in its activation. Considerable information exists on the distribution, structure and function of NCT; however, little is known of its proteolysis. The present study is aimed at exploring the molecular mechanism of NCT degradation. We found that either proteasomal or lysosomal inhibition can significantly increase the levels of both endogenous and exogenous NCT in various cell lines, and the effect of these inhibitions on NCT was time- and dose-dependent. Immunofluorescent microscopic analysis revealed that NCT accumulates in the ER and Golgi apparatus after proteasomal inhibition, while lysosomal inhibition leads to the accumulation of NCT in the lysosomal apparatus. Co-immunoprecipitation can pull down both NCT and ubiquitin. Taken together, our results demonstrate that NCT degradation involves both the proteasome and the lysosome.

Regulated intramembrane proteolysis of the interleukin-1 receptor II by alpha-, beta-, and gamma-secretase

Ectodomain shedding and intramembrane proteolysis of the amyloid precursor protein (APP) by alpha-, beta- and gamma-secretase are involved in the pathogenesis of Alzheimer disease (AD). Increased proteolytic processing and secretion of another membrane protein, the interleukin-1 receptor II (IL-1R2), have also been linked to the pathogenesis of AD. IL-1R2 is a decoy receptor that may limit detrimental effects of IL-1 in the brain. At present, the proteolytic processing of IL-1R2 remains little understood. Here we show that IL-1R2 can be proteolytically processed in a manner similar to APP. IL-1R2 expressed in human embryonic kidney 293 cells first undergoes ectodomain shedding in an alpha-secretase-like manner, resulting in secretion of the IL-1R2 ectodomain and the generation of an IL-1R2 C-terminal fragment. This fragment undergoes further intramembrane proteolysis by gamma-secretase, leading to the generation of the soluble intracellular domain of IL-1R2. Intramembrane cleavage of IL-1R2 was abolished by a highly specific inhibitor of gamma-secretase and was absent in mouse embryonic fibroblasts deficient in gamma-secretase activity. Surprisingly, the beta-secretase BACE1 and its homolog BACE2 increased IL-1R2 secretion resulting in C-terminal fragments nearly identical to the ones generated by the alpha-secretase-like cleavage. This suggests that both proteases may act as alternative alpha-secretase-like proteases. Importantly, BACE1 and BACE2 did not cleave several other membrane proteins, demonstrating that both proteases do not contribute to general membrane protein turnover but only cleave specific proteins. This study reveals a similar proteolytic processing of IL-1R2 and APP and may provide an explanation for the increased IL-1R2 secretion observed in AD.

Excess of nicastrin in brain results in heterozygosity having no effect on endogenous APP processing and amyloid peptide levels in vivo

Nicastrin is an integral member of PS-complexes that perform gamma-secretase cleavage of numerous type I membrane proteins including amyloid precursor protein that underlies Alzheimer's disease; thus, diminishing gamma-secretase activity by reducing levels of functional PS-complexes is suggested as a possible preventative/therapeutic avenue for the disease. One means of reducing PS-complex activity entails decreasing the levels of one or more of its components, such as nicastrin, which is fundamental to its assembly. Two previous studies detailing the effects of decreased nicastrin on gamma-secretase cleavage of APP in nicastrin heterozygous mouse fibroblast, which express relatively low levels of endogenous nicastrin compared to neurons, were contradictory. One report documented a 50% reduction in gamma-secretase cleavage of APP while the second showed markedly higher levels of this activity. Here we report that brains of heterozygous nicastrin mice show no difference in levels of APP gamma-secretase cleavage, APP C-terminal fragments or beta-amyloid peptides, compared to wild-type. This result is explained by the levels of nicastrin protein and functional presenilin complexes being similar between the heterozygous and wild-type brains, though nicastrin mRNA levels were diminished appropriately in the former. These in vivo results indicate that nicastrin mRNA and its immature protein are likely in overabundance in neurons and not limiting for assembly of PS-complexes, and that a 50% reduction of its mRNA or protein production would not affect APP processing, in contrast to fibroblast. Thus, partial reduction (maintaining a level above 50% of normal) of brain nicastrin would likely not be efficacious in reducing functional PS-complexes and gamma-secretase activity as a therapeutic strategy for Alzheimer's disease.

gamma-Secretase-mediated release of the low density lipoprotein receptor-related protein 1B intracellular domain suppresses anchorage-independent growth of neuroglioma cells

The low density lipoprotein receptor related protein 1B (LRP1B) is a large endocytic receptor that was first identified as a candidate tumor suppressor gene. In the current investigation we demonstrate that LRP1B undergoes regulated intramembrane proteolysis in a gamma-secretase-dependent process. The released intracellular domain (ICD) then translocates to the nucleus via a nuclear localization signal that is present within this domain. ICD release first requires shedding of the LRP1B ectodomain, which appears to be catalyzed by a member of the metalloproteinase family. Employing site-directed mutagenesis studies, we identified lysine residues 4432 and 4435 and arginine 4442 as key amino acids important for ectodomain shedding of LRP1B. We also demonstrate that an LRP1B minireceptor as well as the ICD domain alone suppresses anchorage-independent growth of LRP1B-deficient neuroglioma cells (H4 cells). Interestingly, abrogating ectodomain shedding resulted in a loss of the ability of LRP1B minireceptors to suppress anchorage-independent growth. Together, these studies reveal that LRP1B has tumor suppression function that is mediated by proteolytic processing of the receptor resulting in ICD release.

Cellular distribution of gamma-secretase subunit nicastrin in the developing and adult rat brains

Nicastrin and presenilin 1 are integral components of the high molecular weight gamma-secretase complexes that regulate proteolytic processing of various type I membrane proteins including amyloid precursor protein and Notch. At present, there is little information regarding the cellular distribution of nicastrin in the developing or adult rat brain. We report here, using immunoblotting and immunohistochemical methods, that nicastrin in the adult rat brain is widely expressed and co-localized with presenilin 1 in select neuronal populations within all major areas, including the basal forebrain, striatum, cortex, hippocampus, amygdala, thalamus, hypothalamus, cerebellum and brainstem. We also observed dense neuropil labeling in many regions in the brain, suggesting that nicastrin gets transported to dendrites and/or axon terminals in the central nervous system. The levels of nicastrin are found to be relatively high at the early stages of postnatal development and then declined gradually to reach the adult profile. At the cellular level, nicastrin is localized predominantly in neuronal cell bodies at early postnatal stages, but is apparent both in cell bodies and dendrites/neuropil in all brain regions at the later stages. The regulation of nicastrin expression and localization during development and its distribution in a wide spectrum of neurons in the postnatal and adult rat brains provide an anatomical basis to suggest a multifunctional role for the gamma-secretase complex in the developing and adult rat brains.

Cellular functions of gamma-secretase-related proteins

Amyloid-beta peptide (Abeta) is generated by gamma-secretase, a membrane protein complex with an unusual aspartyl protease activity consisting of the four components presenilin, nicastrin, APH-1 and PEN-2. Presenilin is considered the catalytic subunit of this complex since it represents the prototype of the new family of intramembrane-cleaving GxGD-type aspartyl proteases. Recently, five novel members of this family and a nicastrin-like protein were identified. Whereas one of the GxGD-type proteins was shown to be identical with signal peptide peptidase (SPP), the function of the others, now called SPP-like proteins (SPPLs), is not known. We therefore analyzed SPPL2b and SPPL3 and demonstrated that they localize to different subcellular compartments suggesting nonredundant functions. This was supported by different phenotypes obtained in knockdown studies in zebrafish embryos. In addition, these phenotypes could be phenocopied by ectopic expression of putative active site mutants, providing strong evidence for a proteolytic function of SPPL2b and SPPL3. We also identified and characterized the nicastrin-like protein nicalin which, together with the 130-kDa protein NOMO (Nodal modulator), forms a membrane protein complex different from gamma-secretase. We found that during zebrafish embryogenesis this complex is involved in the patterning of the axial mesendoderm, a process controlled by the Nodal signaling pathway.

Assembly, trafficking and function of gamma-secretase

Gamma-secretase catalyzes the final cleavage of the beta-amyloid precursor protein to generate amyloid-beta peptide, the principal component of amyloid plaques in the brains of patients suffering from Alzheimer's disease. Here, we review the identification of gamma-secretase as a protease complex and its assembly and trafficking to its site(s) of cellular function. In reconstitution experiments, gamma-secretase was found to be composed of four integral membrane proteins, presenilin (PS), nicastrin (NCT), PEN-2 and APH-1 that are essential and sufficient for gamma-secretase activity. PS, which serves as a catalytic subunit of gamma-secretase, was identified as a prototypic member of novel aspartyl proteases of the GxGD type. In human cells, gamma-secretase could be further defined as a heterogeneous activity consisting of distinct complexes that are composed of PS1 or PS2 and APH-1a or APH-1b homologues together with NCT and PEN-2. Using green fluorescent protein as a reporter we localized PS and gamma-secretase activity at the plasma membrane and endosomes. Investigation of gamma-secretase complex assembly in knockdown and knockout cells of the individual subunits allowed us to develop a model of complex assembly in which NCT and APH-1 first stabilize PS before PEN-2 assembles as the last component. Furthermore, we could map domains in PS and PEN-2 that govern assembly and trafficking of the complex. Finally, Rer1 was identified as a PEN-2-binding protein that serves a role as an auxiliary factor for gamma-secretase complex assembly.

New developments in mild cognitive impairment and Alzheimer's disease

PURPOSE OF REVIEW

In this paper, we review current concepts of Alzheimer's disease, recent progress in diagnosis and treatment and important developments in our understanding of its pathogenesis with a focus on beta-amyloid both as culprit and therapeutic target.

RECENT FINDINGS

The amyloid cascade hypothesis of Alzheimer's disease pathogenesis continues to predominate with evidence suggesting that small oligomeric forms of Abeta-42 rather than fibrils or senile plaques are the key pathological substrates. The concept of mild cognitive impairment continues to be refined to define better those patients who will progress to Alzheimer's disease. Structural and functional imaging techniques and cerebrospinal fluid biomarkers are gaining acceptance as diagnostic markers of Alzheimer's disease, with a potentially exciting advance being the ability to image amyloid in vivo using novel positron emission tomography ligands. Whilst available treatments afford only symptomatic benefits, disease-modifying treatments may be within reach. Despite the halting of the first amyloid beta-vaccination trial due to adverse effects, amyloid immunotherapy continues to show promise, with new approaches already entering clinical trials. Other therapeutic strategies under investigation include inhibition of beta -and gamma-secretase, key enzymes implicated in Alzheimer's disease pathogenesis.

SUMMARY

Current research demonstrates the potential for diagnostic strategies and disease modifying treatments to follow from an ever more detailed understanding of the molecular mechanisms underlying the pathogenesis of Alzheimer's disease.

Ubiquitin?proteasome pathway mediates degradation of APH-1

Gamma-secretase catalyzes intramembraneous proteolysis of several type I transmembrane proteins, including beta-amyloid precursor protein (APP), to generate amyloid beta protein (Abeta), a key player in the pathogenesis of Alzheimer's disease (AD). The critical components of the gamma-secretase complex include presenilin (PS), nicastrin (NCT), presenilin enhancer-2 (PEN-2) and anterior pharynx defective-1 (APH-1). Abnormalities of the ubiquitin-proteasome pathway have been implicated in the pathogenesis of AD; while PS and PEN-2 turnover is regulated by this pathway, it is unknown whether the ubiquitin-proteasome pathway is also involved in the degradation of APH-1 protein. In this study, we found that the expression of endogenous and exogenous APH-1 significantly increased in cells treated with proteasome-specific inhibitors. The effect of the proteasome inhibitors on APH-1 was dose- and time-dependent. APH-1 protein was ubiquitinated. Pulse-chase metabolic labeling experiments showed that the degradation of newly synthesized radiolabeled APH-1 proteins was inhibited by lactacystin. Disruption of the PS1 and PS2 genes did not affect the degradation of APH-1 by the ubiquitin-proteasome pathway. Furthermore, over-expression of APH-1 and inhibition of proteasomal APH-1 degradation facilitated gamma-secretase cleavage of APP to generate Abeta. These results demonstrate that the degradation of APH-1 protein is mediated by the ubiquitin-proteasome pathway.

Presenilins form ER Ca2+ leak channels, a function disrupted by familial Alzheimer's disease-linked mutations

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder. Mutations in presenilins 1 and 2 (PS1 and PS2) account for approximately 40% of familial AD (FAD) cases. FAD mutations and genetic deletions of presenilins have been associated with calcium (Ca(2+)) signaling abnormalities. We demonstrate that wild-type presenilins, but not PS1-M146V and PS2-N141I FAD mutants, can form low-conductance divalent-cation-permeable ion channels in planar lipid bilayers. In experiments with PS1/2 double knockout (DKO) mouse embryonic fibroblasts (MEFs), we find that presenilins account for approximately 80% of passive Ca(2+) leak from the endoplasmic reticulum. Deficient Ca(2+) signaling in DKO MEFs can be rescued by expression of wild-type PS1 or PS2 but not by expression of PS1-M146V or PS2-N141I mutants. The ER Ca(2+) leak function of presenilins is independent of their gamma-secretase activity. Our data suggest a Ca(2+) signaling function for presenilins and provide support for the "Ca(2+) hypothesis of AD."

Trafficking of Alzheimer's disease-related membrane proteins and its participation in disease pathogenesis

Alzheimer's disease (AD) is a common neurodegenerative disorder that causes senile dementia. The pathological characteristics are the appearance of neurofibrillary tangles comprising abnormally phosphorylated tau and senile plaques composed of amyloid beta-protein depositions. Amyloid beta-protein precursor (APP) and presenilin (PS) are known to be causative genes of familial AD. Recent analyses have documented that APP functions in the axonal transport of vesicles and PS regulates intracellular protein trafficking. Dystrophic neurites, in which APP and Alcadein accumulate in swollen axons, are also observed in AD brain. These pathological characteristics and the features of AD-related proteins suggest that AD is a disease of the vesicular transport system. Here we review recent progress of research on AD pathogenesis from the viewpoint of membrane trafficking.

Quantitative dissection of the Notch:CSL interaction: insights into the Notch-mediated transcriptional switch

Complex formation between the intracellular domain of the Notch receptor (NICD) and the transcription factor CSL is indispensable for transcriptional activation. To understand how NICD displaces CSL-associated co-repressors, we have quantified the binding of different Notch1 ICD regions to a key interaction domain (the beta trefoil domain, or BTD) of human CSL. Electrophoresis, scattering, and titration calorimetry indicate that NICD and BTD combine to form a 1:1 heterodimer. Neither the Notch1 ankyrin domain (ANK) nor C-terminal region contributes binding energy towards BTD. In contrast, binding energy is attributed largely to a short segment including the conserved WFP sequence motif within the RAM region (the approximately 140 residue polypeptide segment N-terminal to the ANK domain); substitution of this motif substantially reduces affinity. Short (< or =25 residues) WFP-containing peptides encoded by the four mammalian Notch genes have similar affinities to BTD; thus, activity differences between paralogues either result from other regions of NICD and CSL or from differences in interaction with downstream components. The importance of RAM was demonstrated by the ability of a short RAM peptides to dissociate NICD:CSL interaction in cellular lysates. These results support an emerging molecular mechanism for the displacement of co-repressors from DNA-bound CSL by NICD.

Identification of a conserved negative regulatory sequence that influences the leukemogenic activity of NOTCH1

NOTCH1 is a large type I transmembrane receptor that regulates normal T-cell development via a signaling pathway that relies on regulated proteolysis. Ligand binding induces proteolytic cleavages in NOTCH1 that release its intracellular domain (ICN1), which translocates to the nucleus and activates target genes by forming a short-lived nuclear complex with two other proteins, the DNA-binding factor CSL and a Mastermind-like (MAML) coactivator. Recent work has shown that human T-ALL is frequently associated with C-terminal NOTCH1 truncations, which uniformly remove sequences lying between residues 2524 and 2556. This region includes the highly conserved sequence WSSSSP (S4), which based on its amino acid content appeared to be a likely site for regulatory serine phosphorylation events. We show here that the mutation of the S4 sequence leads to hypophosphorylation of ICN1; increased NOTCH1 signaling; and the stabilization of complexes containing ICN1, CSL, and MAML1. Consistent with these in vitro studies, mutation of the WSSSSP sequence converts nonleukemogenic weak gain-of-function NOTCH1 alleles into alleles that cause aggressive T-ALLs in a murine bone marrow transplant model. These studies indicate that S4 is an important negative regulatory sequence and that the deletion of S4 likely contributes to the development of human T-ALL.

Pathogenic mechanisms in Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disorder associated with aging and characterized by neurofibrillary tangles and amyloid plaques that deposit in the brain, triggering the neurodegenerative phenomena and leading to neuronal death. Amyloid plaques are primarily composed of beta-amyloid peptides, which derive from the Amyloid Precursor Protein (APP) upon the consequential action of beta- and gamma-secretase. This review discusses recent literature on beta- and gamma-secretase, and on those cellular factors, like cholesterol and phosphorylation of APP, that are involved in aging and may affect the function of both beta- and gamma-secretase.

Linear patterns of Alzheimer's disease mutations along α-helices of presenilins as a tool for PS-1 model construction

We performed an analysis of mutation patterns in all 10 hydrophobic regions (HRs) of presenilin-1 (PS-1) and PS-2 using a recent database of Alzheimer's disease (AD) mutations. The linear patterns were confirmed and extended to areas spanning as many as three faces of a given HR. The complementary areas of residues free of AD mutations were identified based on the location of non-pathogenic polymorphisms and PS-1 versus PS-2 amino acid discordances. Taking into account the location of areas of AD mutations and mutation-free areas/regions, we proposed a preliminary model of PS-1 structure using a general stick-out-mutation rule. To build a molecular structure of PS-1 and preserve features of the preliminary model, we used bacteriorhodopsin template in homology/comparative modelling. Two molecular models were built differing in the location of C-terminal fragment helices. The models properly distinguish residues belonging to AD-affected sites and non-pathogenic areas, and may be used for classification purposes. They also comply with experimental results, such as differences in accessibility of the catalytic residues in uncleaved PS-1, and binding of PEN-2 by the PS-1 NF motif.

N-glycosylation of human nicastrin is required for interaction with the lectins from the secretory pathway calnexin and ERGIC-53

The gamma-secretase complex, composed of four non-covalently bound transmembrane proteins Presenilin, Nicastrin (NCT), APH-1 and PEN-2, is responsible for the intramembranous cleavage of amyloid precursor protein (APP), Notch and several other type I transmembrane proteins. gamma-Secretase cleavage of APP releases the Abeta peptides, which form the amyloid plaques characteristic of Alzheimer's disease brains, and cleavage of Notch releases an intracellular signalling peptide that is critical for numerous developmental processes. NCT, a type I membrane protein, is the only protein within the complex that is glycosylated. The importance of these glycosylation sites is not fully understood. Here, we have observed that NCT N-linked oligosaccharides mediated specific interactions with the secretory pathway lectins calnexin and ERGIC-53. In order to investigate the role played by N-glycosylation, mutation of each site was performed. All hNCT mutants interacted with calnexin and ERGIC-53, indicating that the association was not mediated by any single N-glycosylation site. Moreover, the interaction with ERGIC-53 still occurred in PS1/2 double knockout cells as detected in immunoprecipitation as well as confocal immunofluorescence microscopy studies, which indicated that NCT interacted with ERGIC-53 prior to its association with the active gamma-secretase complex.

Intra- or intercomplex binding to the gamma-secretase enzyme. A model to differentiate inhibitor classes

Gamma-secretase is one of the critical enzymes required for the generation of amyloid-beta peptides from the beta-amyloid precursor protein. Because amyloid-beta peptides are generally accepted to play a key role in Alzheimer disease, gamma-secretase inhibition holds the promise for a disease-modifying therapy for this neurodegenerative condition. Although recent progress has enhanced the understanding of the biology and composition of the gamma-secretase enzyme complex, less information is available on the actual interaction of various inhibitor classes with the enzyme. Here we show that the two principal classes of inhibitor described in the scientific and patent literature, aspartyl protease transition state analogue and small molecule non-transition state inhibitors, display fundamental differences in the way they interact with the enzyme. Taking advantage of a gamma-secretase enzyme overexpressing cellular system and different radiolabeled gamma-secretase inhibitors, we observed that the maximal binding of non-transition state gamma-secretase inhibitors accounts only for half the number of catalytic sites of the recombinant enzyme complex. This characteristic stoichiometry can be best accommodated with a model whereby the non-transition state inhibitors bind to a unique site at the interface of a dimeric enzyme. Subsequent competition studies confirm that this site appears to be targeted by the main classes of small molecule gamma-secretase inhibitor. In contrast, the non-steroidal anti-inflammatory drug gamma-secretase modulator sulindac sulfide displayed noncompetitive antagonism for all types of inhibitor. This finding suggests that non-steroidal anti-inflammatory drug-type gamma-secretase modulators target an alternative site on the enzyme, thereby changing the conformation of the binding sites for gamma-secretase inhibitors.

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.

Pore-forming scissors? A first structural glimpse of gamma-secretase

Amyloid plaques, which are composed of amyloid-beta peptide (Abeta), signify Alzheimer's disease pathology. Secretases generate Abeta by processing the beta-amyloid precursor protein. gamma-Secretase, a complex comprising four different proteins, liberates Abeta from its precursor by intramembrane proteolysis. The first impression of the shape of gamma-secretase has recently been revealed by electron microscopy. It indicates a spherical transmembrane particle with an interior chamber that, presumably, accommodates its catalytic residues, and two openings that might be exit sites for the cleavage products.

c-Myc is an important direct target of Notch1 in T-cell acute lymphoblastic leukemia/lymphoma

Human acute T-cell lymphoblastic leukemias and lymphomas (T-ALL) are commonly associated with gain-of-function mutations in Notch1 that contribute to T-ALL induction and maintenance. Starting from an expression-profiling screen, we identified c-myc as a direct target of Notch1 in Notch-dependent T-ALL cell lines, in which Notch accounts for the majority of c-myc expression. In functional assays, inhibitors of c-myc interfere with the progrowth effects of activated Notch1, and enforced expression of c-myc rescues multiple Notch1-dependent T-ALL cell lines from Notch withdrawal. The existence of a Notch1-c-myc signaling axis was bolstered further by experiments using c-myc-dependent murine T-ALL cells, which are rescued from withdrawal of c-myc by retroviral transduction of activated Notch1. This Notch1-mediated rescue is associated with the up-regulation of endogenous murine c-myc and its downstream transcriptional targets, and the acquisition of sensitivity to Notch pathway inhibitors. Additionally, we show that primary murine thymocytes at the DN3 stage of development depend on ligand-induced Notch signaling to maintain c-myc expression. Together, these data implicate c-myc as a developmentally regulated direct downstream target of Notch1 that contributes to the growth of T-ALL cells.

Leukemia-associated mutations within the NOTCH1 heterodimerization domain fall into at least two distinct mechanistic classes

The NOTCH1 receptor is cleaved within its extracellular domain by furin during its maturation, yielding two subunits that are held together noncovalently by a juxtamembrane heterodimerization (HD) domain. Normal NOTCH1 signaling is initiated by the binding of ligand to the extracellular subunit, which renders the transmembrane subunit susceptible to two successive cleavages within and C terminal to the heterodimerization domain, catalyzed by metalloproteases and gamma-secretase, respectively. Because mutations in the heterodimerization domain of NOTCH1 occur frequently in human T-cell acute lymphoblastic leukemia (T-ALL), we assessed the effect of 16 putative tumor-associated mutations on Notch1 signaling and HD domain stability. We show here that 15 of the 16 mutations activate canonical NOTCH1 signaling. Increases in signaling occur in a ligand-independent fashion, require gamma-secretase activity, and correlate with an increased susceptibility to cleavage by metalloproteases. The activating mutations cause soluble NOTCH1 heterodimers to dissociate more readily, either under native conditions (n = 3) or in the presence of urea (n = 11). One mutation, an insertion of 14 residues immediately N terminal to the metalloprotease cleavage site, increases metalloprotease sensitivity more than all others, despite a negligible effect on heterodimer stability by comparison, suggesting that the insertion may expose the S2 site by repositioning it relative to protective NOTCH1 ectodomain residues. Together, these studies show that leukemia-associated HD domain mutations render NOTCH1 sensitive to ligand-independent proteolytic activation through two distinct mechanisms.

Contribution of presenilin transmembrane domains 6 and 7 to a water-containing cavity in the gamma-secretase complex

Gamma-secretase is a multiprotein complex responsible for the intramembranous cleavage of the amyloid precursor protein and other type I transmembrane proteins. Mutations in Presenilin, the catalytic core of this complex, cause Alzheimer disease. Little is known about the structure of the protein and even less about the catalytic mechanism, which involves proteolytic cleavage in the hydrophobic environment of the cell membrane. It is basically unclear how water, needed to perform hydrolysis, is provided to this reaction. Presenilin transmembrane domains 6 and 7 seem critical in this regard, as each bears a critical aspartate contributing to catalytic activity. Current models imply that both aspartyl groups should closely oppose each other and have access to water. This is, however, still to be experimentally verified. Here, we have performed cysteine-scanning mutagenesis of both domains and have demonstrated that several of the introduced residues are exposed to water, providing experimental evidence for the existence of a water-filled cavity in the catalytic core of Presenilin. In addition, we have demonstrated that the two aspartates reside within this cavity and are opposed to each other in the native complex. We have also identified the conserved tyrosine 389 as a critical partner in the catalytic mechanism. Several additional amino acid substitutions affect differentially the processing of gamma-secretase substrates, implying that they contribute to enzyme specificity. Our data suggest the possibility that more selective gamma-secretase inhibitors could be designed.

A γ-secretase-like intramembrane cleavage of TNFα by the GxGD aspartyl protease SPPL2b

Gamma-secretase and signal peptide peptidase (SPP) are unusual GxGD aspartyl proteases, which mediate intramembrane proteolysis. In addition to SPP, a family of SPP-like proteins (SPPLs) of unknown function has been identified. We demonstrate that SPPL2b utilizes multiple intramembrane cleavages to liberate the intracellular domain of tumor necrosis factor alpha (TNFalpha) into the cytosol and the carboxy-terminal counterpart into the extracellular space. These findings suggest common principles for regulated intramembrane proteolysis by GxGD aspartyl proteases.

Conserved residues in juxtamembrane region of the extracellular domain of nicastrin are essential for gamma-secretase complex formation

The Alzheimer's disease-linked protein, presenilin, forms the active site of the gamma-secretase enzyme complex. However, three other proteins, nicastrin (NCT), PEN-2 and APH-1, are required for enzyme activity. This complex is responsible for cleaving the beta-amyloid precursor protein to produce amyloid beta and the intracellular domain (AICD). Although much research has focused on the regions of presenilin that are important for gamma-secretase function, less is known about NCT. To further our understanding of the role of NCT in gamma-secretase activity and complex formation, we have undertaken a systematic evaluation of conserved residues in the juxtamembrane region of the extracellular domain of NCT. Two mutants, S632A and W648A, greatly reduce gamma-secretase activity, as seen by a reduction in amyloid beta and AICD levels. Several lines of evidence suggest that these mutations result in reduced gamma-secretase activity because they affect the ability of NCT to stably associate with the other gamma-secretase components. Since NCT and APH-1 must first bind in order for presenilin and PEN-2 to stably join the complex, we propose that S632 and W648 are essential for a stable interaction with APH-1.

A basolateral sorting signal directs ADAM10 to adherens junctions and is required for its function in cell migration

ADAM10 (a disintegrin and metalloprotease) initiates regulated intramembrane proteolysis by shedding the ectodomain of a number of different substrates. Shedding is followed by subsequent intramembrane proteolysis leading to the liberation of intracellular domains capable of nuclear signaling. ADAM10 substrates have been found at cell-cell contacts and are apparently involved in cell-cell interaction and cell migration. Here we have investigated the cellular mechanism that guides ADAM10 to substrates at cell-cell contacts. We demonstrate that intracellular trafficking of ADAM10 critically requires a novel sorting signal within its cytoplasmic domain. Sequential deletion of the cytoplasmic domain and site-directed mutagenesis suggest that a potential Src homology 3-binding domain is essential for ADAM10 sorting. In a polarized epithelial cell line this motif not only targets ADAM10 to adherens junctions but is also strictly required for ADAM10 function in E-cadherin processing and cell migration.

Pathological and physiological functions of presenilins

Mutations in PSEN1 and PSEN2 genes account for the majority of cases of early-onset familial Alzheimer disease. Since the first prediction of a genetic link between PSEN1 and PSEN2 with Alzheimer's disease, many research groups from both academia and pharmaceutical industry have sought to unravel how pathogenic mutations in PSEN cause presenile dementia. PSEN genes encode polytopic membrane proteins termed presenilins (PS1 and PS2), which function as the catalytic subunit of γ-secretase, an intramembrane protease that has a wide spectrum of type I membrane protein substrates. Sequential cleavage of amyloid precursor protein by BACE and γ-secretase releases highly fibrillogenic β-amyloid peptides, which accumulate in the brains of aged individuals and patients with Alzheimer's disease. Familial Alzheimer's disease-associated presenilin variants are thought to exert their pathogenic function by selectively elevating the levels of highly amyloidogenic Aβ42 peptides. In addition to Alzheimer's disease, several recent studies have linked PSEN1 to familiar frontotemporal dementia. Here, we review the biology of PS1, its role in γ-secretase activity, and discuss recent developments in the cell biology of PS1 with respect to Alzheimer's disease pathogenesis.

A faster migrating variant masquerades as NICD when performing in vitro gamma-secretase assays with bacterially expressed Notch substrates

Intramembrane proteolysis is a new and rapidly growing field. In vitro assays utilizing recombinant substrates for gamma-secretase, an intramembrane-cleaving enzyme, are critically important in order to characterize the biochemical properties of this unusual enzyme. Several recombinant Notch proteins of varying length are commonly used as in vitro substrates for CHAPSO-solubilized gamma-secretase. Here we report that several recombinant Notch constructs undergo limited or no proteolysis in vitro. Instead, upon incubation with or without gamma-secretase, variants of the intact protein migrate during SDS-PAGE at the location expected for the gamma-secretase specific cleavage products. In addition, we show that addition of aspartyl- and gamma-secretase specific protease inhibitors are able to retard the formation of these variants independent of gamma-secretase, which could lead to the erroneous conclusion that Notch cleavage by solubilized gamma-secretase was achieved in vitro even when no proteolysis occurred. In contrast, substrates produced in mammalian or insect cells are cleaved efficiently in vitro. These observations suggest that in vitro studies reliant on recombinant, bacterially produced Notch TMD should be performed with the inclusion of additional controls able to differentiate between actual cleavage and this potential artifact.

Notch 1 activation in the molecular pathogenesis of T-cell acute lymphoblastic leukaemia

The chromosomal translocation t(7;9) in human T-cell acute lymphoblastic leukaemia (T-ALL) results in deregulated expression of a truncated, activated form of Notch 1 (TAN1) under the control of the T-cell receptor-beta (TCRB) locus. Although TAN1 efficiently induces T-ALL in mouse models, t(7;9) is present in less than 1% of human T-ALL cases. The recent discovery of novel activating mutations in NOTCH1 in more than 50% of human T-ALL samples has made it clear that Notch 1 is far more important in human T-ALL pathogenesis than previously suspected.

Deletion of presenilin 1 hydrophilic loop sequence leads to impaired gamma-secretase activity and exacerbated amyloid pathology

gamma-Secretase processing of the amyloid precursor protein (APP) generates Abeta40 and Abeta42, peptides that constitute the principal components of the beta-amyloid plaque pathology of Alzheimer's disease (AD). The gamma-secretase activity is executed by a high-molecular-weight complex of which presenilin 1 (PS1) is an essential component. PS1 is a multi-pass membrane protein, and the large hydrophilic loop domain between transmembrane domains 6 and 7 has been shown to interact with various proteins. To determine the physiological function of the loop domain, we created a strain of PS1 knock-in mice in which the exon 10, which encodes most of the hydrophilic loop sequence, was deleted from the endogenous PS1 gene. We report here that the homozygous exon 10-deleted mice are viable but exhibit drastically reduced gamma-secretase cleavage at the Abeta40, but not the Abeta42, site. Surprisingly, this reduction of Abeta40 is associated with exacerbated plaque pathology when expressed on APP transgenic background. Thus, the PS1 loop plays a regulatory role in gamma-secretase processing, and decreased Abeta40, not increased Abeta42 is likely the cause for the accelerated plaque deposition in these animals. Our finding supports a protective role of Abeta40 against amyloid pathology and raises the possibility that impaired gamma-secretase activity could be the basis for AD pathogenesis in general.

The GxGD motif of presenilin contributes to catalytic function and substrate identification of gamma-secretase

Gamma-secretase is a multisubunit aspartyl protease complex that catalyzes intramembrane cleavage of beta-amyloid precursor protein (APP), a substrate key to Alzheimer's disease pathogenesis, and of Notch, a substrate crucial for cell differentiation. How gamma-secretase recognizes and selects substrates is currently barely understood. Recent data suggest that its subunit nicastrin serves as an initial substrate receptor, which might subsequently forward substrates to the active site domain located in its catalytic subunit presenilin (PS), where an additional substrate binding site has been proposed. We now used an active site domain swapping approach of PS1 with its most distant homolog, spermatogenesis defective (SPE-4), to identify sequence determinants in this region. Strikingly, when the active site domain of PS1 was exchanged with that of SPE-4, the chimeric protein, PS1/SPE-4(6/7), supported APP but not Notch processing. In addition, PS1/SPE-4(6/7) was strongly impaired in Caenorhabditis elegans Notch signaling in vivo. Mapping experiments identified a single amino acid at position x of the GxGD motif, which contains one of the two active site aspartates, to be responsible for the observed defect in Notch processing and signaling. Our data thus implicate a role of the GxGD motif in catalytic function and substrate identification of gamma-secretase.

Electron microscopic structure of purified, active gamma-secretase reveals an aqueous intramembrane chamber and two pores

Gamma-secretase is an intramembrane-cleaving aspartyl protease required for the normal development of metazoans because it processes Notch within cellular membranes to release its signaling domain. More than two dozen additional substrates of diverse functions have been reported, including the Notch ligands Delta and Jagged, N- and E-cadherins, and a sodium channel subunit. The protease is causally implicated in Alzheimer's disease because it releases the neurotoxic amyloid beta-peptide (Abeta) from its precursor, APP. Gamma-secretase occurs as a large complex containing presenilin (bearing the active site aspartates), nicastrin, Aph-1, and Pen-2. Because the complex contains at least 18 transmembrane domains, crystallographic approaches to its structure are difficult and remote. We recently purified the human complex essentially to homogeneity from stably expressing mammalian cells. Here, we use EM and single-particle image analysis on the purified enzyme, which produces physiological ratios of Abeta40 and Abeta42, to obtain structural information on an intramembrane protease. The 3D EM structure revealed a large, cylindrical interior chamber of approximately 20-40 A in length, consistent with a proteinaceous proteolytic site that is occluded from the hydrophobic environment of the lipid bilayer. Lectin tagging of the nicastrin ectodomain enabled proper orientation of the globular, approximately 120-A-long complex within the membrane and revealed approximately 20-A pores at the top and bottom that provide potential exit ports for cleavage products to the extra- and intracellular compartments. Our reconstructed 3D map provides a physical basis for hydrolysis of transmembrane substrates within a lipid bilayer and release of the products into distinct subcellular compartments.

Equimolar production of amyloid beta-protein and amyloid precursor protein intracellular domain from beta-carboxyl-terminal fragment by gamma-secretase

We showed previously that cells expressing wild-type (WT) beta-amyloid precursor protein (APP) or coexpressing WTAPP and WT presenilin (PS) 1/2 produced APP intracellular domains (AICD) 49-99 and 50-99, with the latter predominating. On the other hand, the cells expressing mutant (MT) APP or coexpressing WTAPP and MTPS1/2 produced a greater proportion of AICD-(49-99) than AICD-(50-99). In addition, the expression of amyloid beta-protein (Abeta) 49 in cells resulted in predominant production of Abeta40 and that of Abeta48 leads to preferential production of Abeta42. These observations suggest that epsilon-cleavage and gamma-cleavage are interrelated. To determine the stoichiometry between Abeta and AICD, we have established a 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonic acid-solubilized gamma-secretase assay system that exhibits high specific activity. By using this assay system, we have shown that equal amounts of Abeta and AICD are produced from beta-carboxyl-terminal fragment (C99) by gamma-secretase, irrespective of WT or MTAPP and PS1/2. Although various Abeta species, including Abeta40, Abeta42, Abeta43, Abeta45, Abeta48, and Abeta49, are generated, only two species of AICD, AICD-(49-99) and AICD-(50-99), are detected. We also have found that M233T MTPS1 produced only one species of AICD, AICD-(49-99), and only one for its counterpart, Abeta48, in contrast to WT and other MTPS1s. These strongly suggest that epsilon-cleavage is the primary event, and the produced Abeta48 and Abeta49 rapidly undergo gamma-cleavage, resulting in generation of various Abeta species.