Evidence for phosphorylation and oligomeric assembly of presenilin 1

Presenilin: A Multi-Functional Molecule in the Pathogenesis of Alzheimer's Disease and Other Neurodegenerative Diseases

Presenilin, a transmembrane protein primarily known for its role in Alzheimer's disease (AD) as part of the γ-secretase complex, has garnered increased attention due to its multifaceted functions in various cellular processes. Recent investigations have unveiled a plethora of functions beyond its amyloidogenic role. This review aims to provide a comprehensive overview of presenilin's diverse roles in AD and other neurodegenerative disorders. It includes a summary of well-known substrates of presenilin, such as its involvement in amyloid precursor protein (APP) processing and Notch signaling, along with other functions. Additionally, it highlights newly discovered functions, such as trafficking function, regulation of ferritin expression, apolipoprotein E (ApoE) secretion, the interaction of ApoE and presenilin, and the Aβ42-to-Aβ40-converting activity of ACE. This updated perspective underscores the evolving landscape of presenilin research, emphasizing its broader impact beyond established pathways. The incorporation of these novel findings accentuates the dynamic nature of presenilin's involvement in cellular processes, further advancing our comprehension of its multifaceted roles in neurodegenerative disorders. By synthesizing evidence from a range of studies, this review sheds light on the intricate web of presenilin functions and their implications in health and disease.

Structure-activity relationship of presenilin in γ-secretase-mediated intramembrane cleavage

Genetic research on familial cases of Alzheimer disease have identified presenilin (PS) as an important membrane protein in the pathomechanism of this disease. PS is the catalytic subunit of γ-secretase, which is responsible for the generation of amyloid-β peptide deposited in the brains of Alzheimer disease patients. γ-Secretase is an atypical protease composed of four membrane proteins (i.e., presenilin, nicastrin, anterior pharynx defective-1 (Aph-1), and presenilin enhancer-2 (Pen-2)) and mediates intramembrane proteolysis. Numerous investigations have been conducted toward understanding the structural features of γ-secretase components as well as the cleavage mechanism of γ-secretase. In this review, we summarize our current understanding of the structure and activity relationship of the γ-secretase complex.

Unraveling the complexity of γ-secretase

γ-Secretase was initially defined as a proteolytic activity that cleaves within the transmembrane of the amyloid precursor protein (APP) to produce the amyloid β-peptide of Alzheimer's disease. The discovery of mutations in APP and the presenilins associated with familial Alzheimer's disease and their effects on APP processing dovetailed with pharmacological studies on γ-secretase, leading to the revelation that presenilins are unprecedented membrane-embedded aspartyl proteases. Other members of what became known as the γ-secretase complex were subsequently identified. In parallel with these advances, connections between presenilins and Notch receptors essential to metazoan development became evident, resulting in the concurrent realization that γ-secretase also carries out intramembrane proteolysis of Notch as part of its signaling mechanism. Substantial progress has been made toward elucidating how γ-secretase carries out complex processing of transmembrane domains, how it goes awry in familial Alzheimer's disease, the scope of its substrates, and the atomic details of its structure. Critical questions remain for future study, toward further unraveling the complexity of this unique membrane-embedded proteolytic machine and its roles in biology and disease.

Structure and Function of the γ-Secretase Complex

γ-Secretase is a membrane-embedded protease complex, with presenilin as the catalytic component containing two transmembrane aspartates in the active site. With more than 90 known substrates, the γ-secretase complex is considered "the proteasome of the membrane", with central roles in biology and medicine. The protease carries out hydrolysis within the lipid bilayer to cleave the transmembrane domain of the substrate multiple times before releasing secreted products. For many years, elucidation of γ-secretase structure and function largely relied on small-molecule probes and mutagenesis. Recently, however, advances in cryo-electron microscopy have led to the first detailed structures of the protease complex. Two new reports of structures of γ-secretase bound to membrane protein substrates provide great insight into the nature of substrate recognition and how Alzheimer's disease-causing mutations in presenilin might alter substrate binding and processing. These new structures offer a powerful platform for elucidating enzyme mechanisms, deciphering effects of disease-causing mutations, and advancing Alzheimer's disease drug discovery.

Presenilins and γ-Secretase in Membrane Proteostasis

The presenilin (PS) proteins exert a crucial role in the pathogenesis of Alzheimer disease (AD) by mediating the intramembranous cleavage of amyloid precursor protein (APP) and the generation of amyloid β-protein (Aβ). The two homologous proteins PS1 and PS2 represent the catalytic subunits of distinct γ-secretase complexes that mediate a variety of cellular processes, including membrane protein metabolism, signal transduction, and cell differentiation. While the intramembrane cleavage of select proteins by γ-secretase is critical in the regulation of intracellular signaling pathways, the plethora of identified protein substrates could also indicate an important role of these enzyme complexes in membrane protein homeostasis. In line with this notion, PS proteins and/or γ-secretase has also been implicated in autophagy, a fundamental process for the maintenance of cellular functions and homeostasis. Dysfunction in the clearance of proteins in the lysosome and during autophagy has been shown to contribute to neurodegeneration. This review summarizes the recent knowledge about the role of PS proteins and γ-secretase in membrane protein metabolism and trafficking, and the functional relation to lysosomal activity and autophagy.

Review : Gene Therapy: Applications to the Neurosciences and to Neurological Disease

Gene expression is involved in some way in every human disease. As our knowledge of gene structure and function has blossomed in the last 2 decades, so too has our potential genetically-based repertoire for combating disease. Gene therapy refers to the manipulation of gene expression, either by augmenting the expression of therapeutic genes or by diminishing the expression of deleterious genes. In some neurological diseases, such as trauma, ischemia, and neurodegenerative disorders, gene therapy might be used to express genes for such substances as growth factors or neurotransmitters to prevent neuronal degeneration or to compensate for lost function, respectively. In other cases, gene therapy could be used to block the expression of genes that cause disease such as β-amyloid precursor protein or the Huntingtin gene. In inherited diseases of the nervous system such as muscular dystrophy, normal gene copies could be placed into the nervous system to compensate for lost function resulting from abnormal gene expression. The tools for achieving well-targeted, sustained, and safe gene delivery in the nervous system are now becoming available, and this technology is likely to substantially alter the nature of neurological therapy in the future. NEUROSCIENTIST 4:398-407, 1998

■ REVIEW : The Presenilins

Presenilin-1 and presenilin-2 are highly homologous genes located on chromosomes 14 and 1, respectively, that have recently been linked to some cases of early-onset autosomal dominant inherited forms of Alzhei mer's disease (AD). Presenilins are integral membrane proteins localized in the endoplasmic reticulum of neurons throughout the nervous system. Studies of presenilin-1 knockout mice, and of invertebrate homo logues of presenilins and their interacting proteins, suggest major roles for presenilins in normal develop ment. Presenilin-1 mutant knockin mice do not exhibit developmental abnormalities, which indicates that the pathogenic mechanism of presenilin mutations involves gain of an adverse property of the mutant protein. Expression of presenilin mutations in cultured neurons and transgenic mice results in increased sensitivity to apoptosis induced by trophic factor withdrawal and exposure to oxidative and metabolic insults, and also alters gene expression. The pathogenic mechanism of presenilin mutations may involve perturbed endo plasmic reticulum calcium homeostasis resulting in enhanced oxidative stress, altered proteolytic processing of the amyloid precursor protein (APP), and increased neuronal vulnerability to excitotoxicity. Studies of presenilins are rapidly increasing our understanding the molecular and cellular underpinnings of AD and are also elucidating novel roles of the endoplasmic reticulum in neuronal plasticity and cell death. NEURO SCIENTIST 5:112-124, 1999

Function of Integrin-Linked Kinase in Modulating the Stemness of IL-6-Abundant Breast Cancer Cells by Regulating γ-Secretase-Mediated Notch1 Activation in Caveolae

Interleukin-6 (IL-6) and Notch signaling are important regulators of breast cancer stem cells (CSCs), which drive the malignant phenotype through self-renewal, differentiation, and development of therapeutic resistance. We investigated the role of integrin-linked kinase (ILK) in regulating IL-6–driven Notch1 activation and the ability to target breast CSCs through ILK inhibition. Ectopic expression/short hairpin RNA-mediated knockdown of ILK, pharmacological inhibition of ILK with the small molecule T315, Western blot analysis, immunofluorescence, and luciferase reporter assays were used to evaluate the regulation of IL-6–driven Notch1 activation by ILK in IL-6–producing triple-negative breast cancer cell lines (MDA-MB-231, SUM-159) and in MCF-7 and MCF-7^IL-6 cells. The effects of ILK on γ-secretase complex assembly and cellular localization were determined by immunofluorescence, Western blots of membrane fractions, and immunoprecipitation. In vivo effects of T315-induced ILK inhibition on CSCs in SUM-159 xenograft models were assessed by mammosphere assays, flow cytometry, and tumorigenicity assays. Results show that the genetic knockdown or pharmacological inhibition of ILK suppressed Notch1 activation and the abundance of the γ-secretase components presenilin-1, nicastrin, and presenilin enhancer 2 at the posttranscriptional level via inhibition of caveolin-1-dependent membrane assembly of the γ-secretase complex. Accordingly, knockdown of ILK inhibited breast CSC-like properties in vitro and the breast CSC subpopulation in vivo in xenograft tumor models. Based on these findings, we propose a novel function of ILK in regulating γ-secretase–mediated Notch1 activation, which suggests the targeting of ILK as a therapeutic approach to suppress IL-6–induced breast CSCs.

Twenty Years of Presenilins--Important Proteins in Health and Disease

Alzheimer's disease (AD) is characterized by progressive decline in cognitive functions associated with depositions of aggregated proteins in the form of extracellular plaques and neurofibrillary tangles in the brain. Extracellular plaques contain characteristic fibrils of amyloid β peptides (Aβ); tangles consist of paired helical filaments of the microtubuli-associated protein tau. Although AD manifests predominantly at ages above 65 years, rare cases show a much earlier onset of disease symptoms with very similar neuropathological characteristics. In 1995, two homologous genes were identified, in which mutations are associated with dominantly inherited familial forms of early onset AD. The genes therefore were dubbed presenilins (PS) and encode polytopic transmembrane proteins. At this time the role of these proteins in the pathogenesis of AD and their biological function in general were completely unknown. However, individuals carrying PS mutations showed alterations in the composition of different length variants of Aβ peptides in blood and cerebrospinal fluid, which indicated the potential involvement of presenilins in the metabolism of Aβ. After 20 years of intense research, the roles of presenilins in Aβ generation as well as important functions in biological processes have been identified. Presenilins represent the catalytic components of protease complexes that directly cleave the amyloid precursor protein (APP) but also many other proteins with important physiological functions. Here, the progress in presenilin research from basic characterization of their cellular functions to the targeting in clinical trials for AD therapy, and potential future directions, will be discussed.

The γ-secretase complex: from structure to function

One of the most critical pathological features of Alzheimer’s disease (AD) is the accumulation of β-amyloid (Aβ) peptides that form extracellular senile plaques in the brain. Aβ is derived from β-amyloid precursor protein (APP) through sequential cleavage by β- and γ-secretases. γ-secretase is a high molecular weight complex minimally composed of four components: presenilins (PS), nicastrin, anterior pharynx defective 1 (APH-1), and presenilin enhancer 2 (PEN-2). In addition to APP, γ-secretase also cleaves many other type I transmembrane (TM) protein substrates. As a crucial enzyme for Aβ production, γ-secretase is an appealing therapeutic target for AD. Here, we summarize current knowledge on the structure and function of γ-secretase, as well as recent progress in developing γ-secretase targeting drugs for AD treatment.

Genetic studies in Alzheimer's disease

Alzheimer's disease (AD), the most common cause of dementia in aged populations, is believed to be caused by both environmental factors and genetic variations. Extensive linkage and association studies have established that a broad range of loci are associated with AD, including both causative and susceptibility (risk factor) genes. So far, at least three genes, APP, PS1, and PS2, have been identified as causative genes. Mutations in these genes have been found to cause mainly early-onset AD. On the other hand, APOE has been identified to be the most common high genetic risk factor for late-onset AD. Polymorphisms in the coding region, intron, and promoter region of certain genes constitute another kind of genetic variation associated with AD. A number of other genes or loci have been reported to have linkage with AD, but many show only a weak linkage or the results are not well reproduced. Currently, the measurable genetic associations account for about 50% of the population risk for AD. It is believed that more new loci will be found to associate with AD, either as causative genes or genetic risk factors, and that eventually the understanding of genetic factors in the pathogenesis of AD will be important for our efforts to cure this illness.

Protein phosphatases and Alzheimer's disease

Alzheimer's Disease (AD) is characterized by progressive loss of cognitive function, linked to marked neuronal loss. Pathological hallmarks of the disease are the accumulation of the amyloid-β (Aβ) peptide in the form of amyloid plaques and the intracellular formation of neurofibrillary tangles (NFTs). Accumulating evidence supports a key role for protein phosphorylation in both the normal and pathological actions of Aβ as well as the formation of NFTs. NFTs contain hyperphosphorylated forms of the microtubule-binding protein tau, and phosphorylation of tau by several different kinases leads to its aggregation. The protein kinases involved in the generation and/or actions of tau or Aβ are viable drug targets to prevent or alleviate AD pathology. However, it has also been recognized that the protein phosphatases that reverse the actions of these protein kinases are equally important. Here, we review recent advances in our understanding of serine/threonine and tyrosine protein phosphatases in the pathology of AD.

Regulated intramembrane proteolysis--lessons from amyloid precursor protein processing

Regulated intramembrane proteolysis (RIP) controls the communication between cells and the extracellular environment. RIP is essential in the nervous system, but also in other tissues. In the RIP process, a membrane protein typically undergoes two consecutive cleavages. The first one results in the shedding of its ectodomain. The second one occurs within its transmembrane domain, resulting in secretion of a small peptide and the release of the intracellular domain into the cytosol. The proteolytic cleavage fragments act as versatile signaling molecules or are further degraded. An increasing number of membrane proteins undergo RIP. These include growth factors, cytokines, cell adhesion proteins, receptors, viral proteins and signal peptides. A dysregulation of RIP is found in diseases, such as leukemia and Alzheimer's disease. One of the first RIP substrates discovered was the amyloid precursor protein (APP). RIP processing of APP controls the generation of the amyloid β-peptide, which is believed to cause Alzheimer's disease. Focusing on APP as the best-studied RIP substrate, this review describes the function and mechanism of the APP RIP proteases with the goal to elucidate cellular mechanisms and common principles of the RIP process in general.

Cdk5: multitasking between physiological and pathological conditions

Cyclin-dependent kinase 5 (Cdk5) is a peculiar proline-directed serine/threonine kinase. Unlike the other members of the Cdk family, Cdk5 is not directly involved in cell cycle regulation, being normally associated with neuronal processes such as migration, cortical layering and synaptic plasticity. This kinase is present mainly in post-mitotic neurons and its activity is tightly regulated by the interaction with the specific activators, p35 and p39. Despite its pivotal role in CNS development, Cdk5 dysregulation has been implicated in different pathologies, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and, most recently, prion-related encephalopathies (PRE). In these neurodegenerative conditions, Cdk5 overactivation and relocalization occurs upon association with p25, a truncated form of the normal activator p35. This activator switching will cause a shift in the phosphorylative pattern of Cdk5, with an alteration both in targets and activity, ultimately leading to neuronal demise. In AD and PRE, two disorders that share clinical and neuropathological features, Cdk5 dysregulation is a linking event between the major neuropathological markers: amyloid plaques, tau hyperphosphorylation and synaptic and neuronal loss. Moreover, this kinase was shown to be involved in abortive cell cycle re-entry, a feature recently proposed as a possible step in the neuronal apoptosis mechanism of several neurological diseases. This review focuses on the role of Cdk5 in neurons, namely in the regulation of cytoskeletal dynamics, synaptic function and cell survival, both in physiological and in pathological conditions, highlighting the relevance of Cdk5 in the main mechanisms of neurodegeneration in Alzheimer's disease and other brain pathologies.

HPRT deficiency coordinately dysregulates canonical Wnt and presenilin-1 signaling: a neuro-developmental regulatory role for a housekeeping gene?

We have used microarray-based methods of global gene expression together with quantitative PCR and Western blot analysis to identify dysregulation of genes and aberrant cellular processes in human fibroblasts and in SH-SY5Y neuroblastoma cells made HPRT-deficient by transduction with a retrovirus stably expressing an shRNA targeted against HPRT. Analysis of the microarray expression data by Gene ontology (GO) and Gene Set Enrichment Analysis (GSEA) as well as significant pathway analysis by GeneSpring GX10 and Panther Classification System reveal that HPRT deficiency is accompanied by aberrations in a variety of pathways known to regulate neurogenesis or to be implicated in neurodegenerative disease, including the canonical Wnt/β-catenin and the Alzheimer's disease/presenilin signaling pathways. Dysregulation of the Wnt/β-catenin pathway is confirmed by Western blot demonstration of cytosolic sequestration of β-catenin during in vitro differentiation of the SH-SY5Y cells toward the neuronal phenotype. We also demonstrate that two key transcription factor genes known to be regulated by Wnt signaling and to be vital for the generation and function of dopaminergic neurons; i.e., Lmx1a and Engrailed 1, are down-regulated in the HPRT knockdown SH-SY5Y cells. In addition to the Wnt signaling aberration, we found that expression of presenilin-1 shows severely aberrant expression in HPRT-deficient SH-SY5Y cells, reflected by marked deficiency of the 23 kDa C-terminal fragment of presenilin-1 in knockdown cells. Western blot analysis of primary fibroblast cultures from two LND patients also shows dysregulated presenilin-1 expression, including aberrant proteolytic processing of presenilin-1. These demonstrations of dysregulated Wnt signaling and presenilin-1 expression together with impaired expression of dopaminergic transcription factors reveal broad pleitropic neuro-regulatory defects played by HPRT expression and suggest new directions for investigating mechanisms of aberrant neurogenesis and neuropathology in LND and potential new targets for restoration of effective signaling in this neuro-developmental defect.

Protein kinase C and rho activated coiled coil protein kinase 2 (ROCK2) modulate Alzheimer's APP metabolism and phosphorylation of the Vps10-domain protein, SorL1

Background

Generation of the amyloid β (Aβ) peptide of Alzheimer's disease (AD) is differentially regulated through the intracellular trafficking of the amyloid β precursor protein (APP) within the secretory and endocytic pathways. Protein kinase C (PKC) and rho-activated coiled-coil kinases (ROCKs) are two "third messenger" signaling molecules that control the relative utilization of these two pathways. Several members of the Vps family of receptors (Vps35, SorL1, SorCS1) play important roles in post-trans-Golgi network (TGN) sorting and generation of APP derivatives, including Aβ at the TGN, endosome and the plasma membrane. We now report that Vps10-domain proteins are candidate substrates for PKC and/or ROCK2 and act as phospho-state-sensitive physiological effectors for post-TGN sorting of APP and its derivatives.

Results

Analysis of the SorL1 cytoplasmic tail revealed multiple consensus sites for phosphorylation by protein kinases. SorL1 was subsequently identified as a phosphoprotein, based on sensitivity of its electrophoretic migration pattern to calf intestine alkaline phosphatase and on its reaction with anti-phospho-serine antibodies. Activation of PKC resulted in increased shedding of the ectodomains of both APP and SorL1, and this was paralleled by an apparent increase in the level of the phosphorylated form of SorL1. ROCK2, the neuronal isoform of another protein kinase, was found to form complexes with SorL1, and both ROCK2 inhibition and ROCK2 knockdown enhanced generation of both soluble APP and Aβ.

Conclusion

These results highlight the potential importance of SorL1 in elucidating phospho-state sensitive mechanisms in the regulation of metabolism of APP and Aβ by PKC and ROCK2.

Gamma-secretase catalyzes sequential cleavages of the AbetaPP transmembrane domain

The biogenesis of the amyloid-beta peptide (Abeta) is a central issue in Alzheimer's disease (AD) research. Abeta is produced by beta- and gamma-secretases from the amyloid-beta protein precursor (AbetaPP). These proteases are targets for the development of therapeutic compounds to downregulate Abeta production. gamma-secretase has received more attention 1) because it generates the C-terminus of Abeta, which is important in the pathogenesis of AD because the longer Abeta species are more amyloidogenic, and 2) because it cleaves AbetaPP within its transmembrane domain. In the understanding the mechanism of gamma-secretase cleavage, three major cleavage sites have been identified, namely, gamma-cleavage site at Abeta(40/42), zeta-cleavage site at Abeta(46), and epsilon-cleavage site at Abeta(49). Moreover, the novel finding that some of the known gamma-secretase inhibitors inhibit the formation of secreted Abeta(40) and Abeta(42), but cause an intracellular accumulation of long Abeta(46), provided information extremely important for the development of strategies aimed at the design of gamma-secretase inhibitors to prevent and treat AD. In addition, it has been established that the C-terminus of Abeta is generated by a series of sequential cleavages: first, epsilon-cleavage, followed by zeta-cleavage and finally by gamma-cleavage, commencing from the membrane boundary to the middle of the AbetaPP membrane domain.

Examining requirement for formation of functional Presenilin proteins and their processing events in vivo

Presenilin (Psn) is a multipass transmembrane protein that functions as the catalytic subunit of gamma-secretase for mediating intramembrane cleavage of type 1 transmembrane proteins. Normally active Psn is in the form of a heterodimer composed by its N-terminal and C-terminal fragments that are generated from a Presenilinase-mediated endoproteolytic cleavage within its large cytosolic loop during assembly of the protease complex. Using the Psn forms that either bypass or disable Presenilinase-mediated endoproteolysis, and a Psn form that has most of the large cytosolic loop deleted, we have established an in vivo system to enable investigations of Psn functional domains in Drosophila. We show that the Presenilinase-mediated endoproteolytic event is not essential for producing Psn activity during animal development, and is regulated by integrity of the large cytosolic loop of Psn in Drosophila. The Psn transgenic flies described here could be applied to a broad range of studies on Psn functioning and its related gamma-secretase activity at any developmental stage.

Flavonoid-mediated presenilin-1 phosphorylation reduces Alzheimer's disease beta-amyloid production

Glycogen synthase kinase 3 (GSK-3) dysregulation is implicated in the two Alzheimer's disease (AD) pathological hallmarks: beta-amyloid plaques and neurofibrillary tangles. GSK-3 inhibitors may abrogate AD pathology by inhibiting amyloidogenic gamma-secretase cleavage of amyloid precursor protein (APP). Here, we report that the citrus bioflavonoid luteolin reduces amyloid-beta (Abeta) peptide generation in both human 'Swedish' mutant APP transgene-bearing neuron-like cells and primary neurons. We also find that luteolin induces changes consistent with GSK-3 inhibition that (i) decrease amyloidogenic gamma-secretase APP processing, and (ii) promote presenilin-1 (PS1) carboxyl-terminal fragment (CTF) phosphorylation. Importantly, we find GSK-3alpha activity is essential for both PS1 CTF phosphorylation and PS1-APP interaction. As validation of these findings in vivo, we find that luteolin, when applied to the Tg2576 mouse model of AD, decreases soluble Abeta levels, reduces GSK-3 activity, and disrupts PS1-APP association. In addition, we find that Tg2576 mice treated with diosmin, a glycoside of a flavonoid structurally similar to luteolin, display significantly reduced Abeta pathology. We suggest that GSK-3 inhibition is a viable therapeutic approach for AD by impacting PS1 phosphorylation-dependent regulation of amyloidogenesis.

Distinct pharmacological effects of inhibitors of signal peptide peptidase and gamma-secretase

Signal peptide peptidase (SPP) and gamma-secretase are intramembrane aspartyl proteases that bear similar active site motifs but with opposite membrane topologies. Both proteases are inhibited by the same aspartyl protease transition-state analogue inhibitors, further evidence that these two enzymes have the same basic cleavage mechanism. Here we report that helical peptide inhibitors designed to mimic SPP substrates and interact with the SPP initial substrate-binding site (the "docking site") inhibit both SPP and gamma-secretase, but with submicromolar potency for SPP. SPP was labeled by helical peptide and transition-state analogue affinity probes but at distinct sites. Nonsteroidal anti-inflammatory drugs, which shift the site of proteolysis by SPP and gamma-secretase, did not affect the labeling of SPP or gamma-secretase by the helical peptide or transition-state analogue probes. On the other hand, another class of previously reported gamma-secretase modulators, naphthyl ketones, inhibited SPP activity as well as selective proteolysis by gamma-secretase. These naphthyl ketones significantly disrupted labeling of SPP by the helical peptide probe but did not block labeling of SPP by the transition-state analogue probe. With respect to gamma-secretase, the naphthyl ketone modulators allowed labeling by the transition-state analogue probe but not the helical peptide probe. Thus, the naphthyl ketones appear to alter the docking sites of both SPP and gamma-secretase. These results indicate that pharmacological effects of the four different classes of inhibitors (transition-state analogues, helical peptides, nonsteroidal anti-inflammatory drugs, and naphthyl ketones) are distinct from each other, and they reveal similarities and differences with how they affect SPP and gamma-secretase.

Tumor necrosis factor-alpha-elicited stimulation of gamma-secretase is mediated by c-Jun N-terminal kinase-dependent phosphorylation of presenilin and nicastrin

Gamma-secretase is a multiprotein complex composed of presenilin (PS), nicastrin (NCT), Aph-1, and Pen-2, and it catalyzes the final proteolytic step in the processing of amyloid precursor protein to generate amyloid-beta. Our previous results showed that tumor necrosis factor-alpha (TNF-alpha) can potently stimulate gamma-secretase activity through a c-Jun N-terminal kinase (JNK)-dependent pathway. Here, we demonstrate that TNF-alpha triggers JNK-dependent serine/threonine phosphorylation of PS1 and NCT to stimulate gamma-secretase activity. Blocking of JNK activity with a potent JNK inhibitor (SP600125) reduces TNF-alpha-triggered phosphorylation of PS1 and NCT. Consistent with this, we show that activated JNKs can be copurified with gamma-secretase complexes and that active recombinant JNK2 can promote the phosphorylation of PS1 and NCT in vitro. Using site-directed mutagenesis and a synthetic peptide, we clearly show that the Ser(319)Thr(320) motif in PS1 is an important JNK phosphorylation site that is critical for the TNF-alpha-elicited regulation of gamma-secretase. This JNK phosphorylation of PS1 at Ser(319)Thr(320) enhances the stability of the PS1 C-terminal fragment that is necessary for gamma-secretase activity. Together, our findings strongly suggest that JNK is a critical intracellular mediator of TNF-alpha-elicited regulation of gamma-secretase and governs the pivotal step in the assembly of functional gamma-secretase.

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.

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.

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.

Familial Alzheimer's disease presenilin 1 mutations cause alterations in the conformation of presenilin and interactions with amyloid precursor protein

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, an enzymatic activity that cleaves amyloid beta (Abeta) from the amyloid precursor protein (APP). More than 100 mutations spread throughout the PS1 molecule are linked to autosomal dominant familial Alzheimer's disease (FAD). All of these mutations lead to a similar phenotype: an increased ratio of Abeta42 to Abeta40, increased plaque deposition, and early age of onset. We use a recently developed microscopy approach, fluorescence lifetime imaging microscopy, to monitor the relative molecular distance between PS1 N and C termini in intact cells. We show that FAD-linked missense mutations located near the N and C termini, in the mid-region of PS1, and the exon 9 deletion mutation all change the spatial relationship between PS1 N and C termini in a similar way, increasing proximity of the two epitopes. This effect is opposite of that observed by treatment with Abeta42-lowering nonsteroidal anti-inflammatory drugs (NSAIDs) (Lleo et al., 2004b). Accordingly, treatment of M146L PS1-overexpressing neurons with high-dose NSAIDs somewhat offsets the conformational change associated with the mutation. Moreover, by monitoring the relative distance between a PS1 loop epitope and the APP C terminus, we demonstrate that the FAD PS1 mutations are also associated with a consistent change in the configuration of the PS1-APP complex. The nonpathogenic E318G PS1 polymorphism had no effect on PS1 N terminus-C terminus proximity or PS1-APP interactions. We propose that the conformational change we observed may therefore provide a shared molecular mechanism for FAD pathogenesis caused by a wide range of PS1 mutations.

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

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

A nine-transmembrane domain topology for presenilin 1

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

Presenilin function and gamma-secretase activity

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

Fast and Slow Skeletal Troponin I in Serum from Patients with Various Skeletal Muscle Disorders: A Pilot Study

Background: Detection of skeletal muscle injury is hampered by a lack of commercially available assays for serum markers specific for skeletal muscle; serum concentrations of skeletal troponin I (sTnI) could meet this need. Moreover, because sTnI exists in 2 isoforms, slow (ssTnI) and fast (fsTnI), corresponding to slow- and fast-twitch muscles, respectively, it could provide insight into differential injury/recovery of specific fiber types. The purpose of this study was to investigate whether the 2 isoforms of sTnI and their modified forms are present in the blood of patients with various skeletal muscle disorders.

Methods: Serial serum samples were obtained from 25 patients with various skeletal muscle injuries. Serum proteins were separated by a modified sodium dodecyl sulfate–polyacrylamide gel electrophoresis protocol followed by Western blotting for sTnI with monoclonal antibodies specific to ssTnI and fsTnI.

Results: We observed (a) intact and, in some cases, degraded sTnI products; (b) evidence of posttranslational modifications in addition to proteolysis; and (c) differential detectability of both skeletal isoforms in the same patient.

Conclusions: It is possible to monitor both sTnI isoforms; this could lead to the development of new diagnostic assays for skeletal muscle damage.

Aph-1 Contributes to the Stabilization and Trafficking of the γ-Secretase Complex through Mechanisms Involving Intermolecular and Intramolecular Interactions

Gamma-secretase cleaves type I transmembrane proteins, including beta-amyloid precursor protein and Notch, and requires the formation of a protein complex comprised of presenilin, nicastrin, Aph-1, and Pen-2 for its activity. Aph-1 is implicated in the stabilization of this complex, although its precise mechanistic role remains unknown. Substitution of the first glycine within the transmembrane GXXXG motif of Aph-1 causes a loss-of-function phenotype in Caenorhabditis elegans. Here, using an untranslated region-targeted RNA interference/rescue strategy in Drosophila Schneider 2 cells, we show that Aph-1 contributes to the assembly of the gamma-secretase complex by multiple mechanisms involving intermolecular and intramolecular interactions depending on or independent of the conserved glycines. Aph-1 binds to nicastrin forming an early subcomplex independent of the conserved glycines within the endoplasmic reticulum. Certain mutations in the conserved GXXXG motif affect the interaction of the Aph-1.nicastrin subcomplex with presenilin that mediates trafficking of the presenilin.Aph-1.nicastrin tripartite complex to the Golgi. The same mutations decrease the stability of Aph-1 polypeptides themselves, possibly by affecting intramolecular associations through the transmembrane domains. Our data suggest that the proper assembly of the Aph-1.nicastrin subcomplex with presenilin is the prerequisite for the trafficking as well as the enzymatic activity of the gamma-secretase complex and that Aph-1 functions as a stabilizing scaffold in the assembly of this complex.

Got RIP? Presenilin-dependent intramembrane proteolysis in growth factor receptor signaling

A number of cell surface growth factor receptors are subject to presenilin-dependent regulated intramembrane proteolysis (PS-RIP) after ligand binding and/or ectodomain cleavage. PS-RIP is mediated by a highly conserved multi-component membrane-bound protease, termed gamma-secretase, responsible for generating Alzheimer's disease (AD)-associated Abeta peptide from its membrane-bound beta-amyloid precursor protein (APP), as well as for cleaving a number of other type-I membrane receptors. PS-RIP is a conserved cellular process by which cells transmit signals from one compartment to another, including the liberation of membrane-bound transcription factors. Recent studies indicate that PS-RIP also mediates the proteolytic inactivation of heteromeric receptor complexes by removing the transmembrane domains required for receptor-receptor interaction. Thus, PS-RIP appears to regulate diverse cellular pathways either by generating soluble effectors from membrane-bound precursors, or by removing the transmembrane domain of a membrane-tethered signaling component.

Coordinated and widespread expression of gamma-secretase in vivo: evidence for size and molecular heterogeneity

Gamma-secretase is a high molecular weight protein complex composed of four subunits, namely, presenilin (PS; 1 or 2), nicastrin, anterior pharynx defective-1 (Aph-1; A or B), and presenilin enhancer-2 (Pen-2), and is responsible for the cleavage of a number of type-1 transmembrane proteins. A fundamental question is whether different gamma-secretase complexes exist in vivo. We demonstrate here by in situ hybridization and by Northern and Western blotting that the gamma-secretase components are widely distributed in all tissues investigated. The expression of the different subunits seems tightly coregulated. However, some variation in the expression of the Aph-1 proteins is observed, Aph-1A being more general and abundantly distributed than Aph-1B. The previously uncharacterized rodent-specific Aph-1C mRNA is highly expressed in the kidney and testis but not in brain or other tissues, indicating some tissue specificity for the Aph-1 component of the gamma-secretase complex. Blue-native electrophoresis revealed size heterogeneity of the mature gamma-secretase complex in various tissues. Using co-immunoprecipitations and blue-native electrophoresis at endogenous protein levels, we find evidence that several independent gamma-secretase complexes can coexist in the same cell type. In conclusion, our results suggest that gamma-secretase is a heterogeneous family of protein complexes widely expressed in the adult organism.

Syntaxin 5 interacts with presenilin holoproteins, but not with their N- or C-terminal fragments, and affects beta-amyloid peptide production

Mutations in presenilins 1 and 2 (PS1 and PS2) account for the majority of cases of early-onset familial Alzheimer's disease. However, the trafficking and interaction of PSs with other proteins in the early secretory pathways are poorly understood. Using co-immunoprecipitation, we found that PS bound to Syx5 (syntaxin 5), which is a target-soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor involved in endoplasmic reticulum (ER)-Golgi vesicular transport in vivo. Syx5 interacted only with the full-length PS holoproteins and not with the naturally occurring N- or C-terminal fragments. The PS holoproteins co-immunoprecipitated with the mutant Syx5, which localized to the ER and Golgi compartments, despite the substitution of the transmembrane region with that of syntaxin 1A. In contrast, the transmembrane deletion mutant that localized to the cytosol, but not to the ER or Golgi compartments, did not co-immunoprecipitate the PS holoproteins. The PS1 variant linked to familial Alzheimer's disease (PS1DeltaE9), lacking the region that contains the endoproteolytic cleavage site in the cytoplasmic loop, showed markedly decreased binding to Syx5. Immunofluorescence and sucrose-density-gradient fractionation analyses showed that the full-length PS holoproteins co-localized with Syx5 to the ER and cis-Golgi compartments. Furthermore, Syx5 overexpression resulted in the accumulation of PS holoproteins and the beta-amyloid precursor protein, and reduced the secretion of the Abeta (amyloid beta) peptide in COS-7 cells. In summary, these results indicate that Syx5 binds to full-length PSs and affects the processing and trafficking of beta-amyloid precursor protein in the early secretory compartments.

Functional Implications of the Presenilin Dimerization

Presenilins are the catalytic components of gamma-secretase, an intramembrane-cleaving protease whose substrates include beta-amyloid precursor protein (betaAPP) and the Notch receptors. These type I transmembrane proteins undergo two distinct presenilin-dependent cleavages within the transmembrane region, which result in the production of Abeta and APP intracellular domain (from betaAPP) and the Notch intracellular domain signaling peptide. Most cases of familial Alzheimer's disease are caused by presenilin mutations, which are scattered throughout the coding sequence. Although the underlying molecular mechanism is not yet known, the familial Alzheimer's disease mutations produce a shift in the ratio of the long and short forms of the Abeta peptide generated by the gamma-secretase. We and others have previously shown that presenilin homodimerizes and suggested that a presenilin dimer is at the catalytic core of gamma-secretase. Here, we demonstrate that presenilin transmembrane domains contribute to the formation of the dimer. In-frame substitution of the hydrophilic loop 1, located between transmembranes I and II, which modulates the interactions within the N-terminal fragment/N-terminal fragment dimer, abolishes both presenilinase and gamma-secretase activities. In addition, by reconstituting gamma-secretase activity from two catalytically inactive presenilin aspartic mutants, we provide evidence of an active diaspartyl group assembled at the interface between two presenilin monomers. Under our conditions, this catalytic group mediates the generation of APP intracellular domain and Abeta but not Notch intracellular domain, therefore suggesting that specific diaspartyl groups within the presenilin catalytic core of gamma-secretase mediate the cleavage of different substrates.

Functional Domains in Presenilin 1

Processing of the Alzheimer amyloid precursor protein (APP) into the amyloid beta-protein and the APP intracellular domain is a proteolysis event mediated by the gamma-secretase complex where presenilin (PS) proteins are key constituents. PS is subjected to an endoproteolytic cleavage, generating a stable heterodimer composed of an N-terminal and a C-terminal fragment. Here we aimed at further understanding the role of PS in endoproteolysis, in proteolytic processing of APP and Notch, and in assembly of the gamma-secretase complex. By using a truncation protocol and alanine scanning, we identified Tyr-288 in the PS1 N-terminal fragment as critical for PS-dependent intramembrane proteolysis. Further mutagenesis of the 288 site identified mutants differentially affecting endoproteolysis and gamma-secretase activity. The Y288F mutant was endoproteolyzed to the same extent as wild type PS but increased the amyloid beta-protein 42/40 ratio by approximately 75%. In contrast, the Y288N mutant was also endoproteolytically processed but was inactive in reconstituting gamma-secretase in PS null cells. The Y288D mutant was deficient in both endoproteolysis and gamma-secretase activity. All three mutant PS1 molecules were incorporated into gamma-secretase complexes and stabilized Pen-2 in PS null cells. Thus, mutations at Tyr-288 do not affect gamma-secretase complex assembly but can differentially control endoproteolysis and gamma-secretase activity.

Co-expressed presenilin 1 NTF and CTF form functional gamma-secretase complexes in cells devoid of full-length protein

The enzyme gamma-secretase catalyzes the intramembrane proteolytic cleavage that generates the amyloid beta-peptide from the beta-amyloid precursor protein. The presenilin (PS) protein is one of the four integral membrane protein components of the mature gamma-secretase complex. The PS protein is itself subjected to endoproteolytic processing, generating stable N- and C-terminal fragment (NTF and CTF, respectively) heterodimers. Here we demonstrate that coexpression of PS1 NTF and CTF functionally mimics expression of the full-length PS1 protein and restores gamma-secretase activity in PS-deficient mammalian cells. The coexpressed fragments re-associate with each other inside the cell, where they also interact with nicastrin, another gamma-secretase complex component. Analysis of gamma-secretase activity following the expression of mutant forms of NTF and CTF, under conditions bypassing endoproteolysis, indicated that the putatively catalytic Asp257 and Asp385 residues have a direct effect on gamma-secretase activity. Moreover, we demonstrate that expression of the wild-type CTF rescues endoproteolytic cleavage of C-terminally truncated PS1 molecules that are otherwise uncleaved and inactive. Recovery of cleavage is critically dependent on the integrity of Asp385. Taken together, our findings indicate that ectopically expressed NTF and CTF restore functional gamma-secretase complexes and that the presence of full-length PS1 is not a requirement for proper complex assembly.

Characterization of presenilin complexes from mouse and human brain using Blue Native gel electrophoresis reveals high expression in embryonic brain and minimal change in complex mobility with pathogenic presenilin mutations

The presenilin proteins are required for intramembrane cleavage of a subset of type 1 membrane proteins including the Alzheimer's disease amyloid precursor protein. Previous studies indicate presenilin proteins form enzymatically active high molecular mass complexes consisting of heterodimers of N- and C-terminal fragments in association with nicastrin, presenilin enhancer-2 and anterior pharynx defective-1 proteins. Using Blue Native gel electrophoresis (BN/PAGE) we have studied endogenous presenilin 1 complex mass, stability and association with nicastrin, presenilin enhancer-2 and anterior pharynx defective-1. Solubilization of mouse or human brain membranes with dodecyl-d-maltoside produced a 360-kDa species reactive with antibodies to presenilin 1. Presenilin 1 complex levels were high in embryonic brain. Complex integrity was sensitive to Triton X-100 and SDS, but stable to reducing agent. Addition of 5 M urea caused complex dissolution and nicastrin to migrate as a subcomplex. Nicastrin and presenilin enhancer-2 were detected in the presenilin 1 complex following BN/PAGE, electroelution and second-dimension analysis. Anterior pharynx defective-1 was detected as an 18-kDa form and 9-kDa C-terminal fragment by standard SDS/PAGE of mouse tissues, and as a predominant 36-kDa band after presenilin 1 complex second-dimension analysis. Membranes from brain cortex of Alzheimer's disease patients, or from cases with presenilin 1 missense mutations, indicated no change in presenilin 1 complex mobility. Higher molecular mass presenilin 1-reactive species were detected in brain containing presenilin 1 exon 9 deletion mutation. This abnormality was confirmed using cells transfected with the same presenilin deletion mutation.

Phosphorylation of Presenilin 1 at the Caspase Recognition Site Regulates Its Proteolytic Processing and the Progression of Apoptosis

The Alzheimer's disease-associated presenilin (PS) 1 is intimately involved in gamma-secretase cleavage of beta-amyloid precursor protein and other proteins. In addition, PS1 plays a role in beta-catenin signaling and in the regulation of apoptosis. Here we demonstrate that phosphorylation of PS1 is regulated by two independent signaling pathways involving protein kinase (PK) A and PKC and that both kinases can directly phosphorylate the large hydrophilic domain of PS1 in vitro and in cultured cells. A phosphorylation site at serine residue 346 was identified that is selectively phosphorylated by PKC but not by PKA. This site is localized within a recognition motif for caspases, and phosphorylation strongly inhibits proteolytic processing of PS1 by caspase activity during apoptosis. Moreover, PS1 phosphorylation reduces the progression of apoptosis. Our data indicate that phosphorylation/dephosphorylation at the caspase recognition site provides a mechanism to reversibly regulate properties of PS1 in apoptosis.

Notch and Presenilin: regulated intramembrane proteolysis links development and degeneration

Intensive studies of three proteins--Presenilin, Notch, and the amyloid precursor protein (APP)--have led to the recognition of a direct intersection between early development and late-life neurodegeneration. Notch signaling mediates many different intercellular communication events that are essential for determining the fates of neural and nonneural cells during development and in the adult. The Notch receptor acts in a core pathway as a membrane-bound transcription factor that is released to the nucleus by a two-step cleavage mechanism called regulated intramembrane proteolysis (RIP). The second cleavage is effected by Presenilin, an unusual polytopic aspartyl protease that apparently cleaves Notch and numerous other single-transmembrane substrates within the lipid bilayer. Another Presenilin substrate, APP, releases the amyloid ss-protein that can accumulate over time in limbic and association cortices and help initiate Alzheimer's disease. Elucidating the detailed mechanism of Presenilin processing of membrane proteins is important for understanding diverse signal transduction pathways and potentially for treating and preventing Alzheimer's disease.

Regulated hyperaccumulation of presenilin-1 and the "gamma-secretase" complex. Evidence for differential intramembranous processing of transmembrane subatrates

Intramembranous "gamma-secretase" processing of beta-amyloid precursor protein (APP) and other transmembrane proteins, including Notch, is mediated by a macromolecular complex consisting of presenilins (PSs), nicastrin (NCT), APH-1, and PEN-2. We now demonstrate that in cells coexpressing PS1, APH-1, and NCT, full-length PS1 accumulates to high levels and is fairly stable. Upon expression of PEN-2, the levels of PS1 holoprotein are significantly reduced, commensurate with an elevation in levels of PS1 fragments. These findings suggest that APH-1 and NCT are necessary for stabilization of full-length PS1 and that PEN-2 is critical for the proteolysis of stabilized PS1. In N2a and 293 cell lines that stably overexpress PS1, APH-1, NCT, and PEN-2, PS1 fragment levels are elevated by up to 10-fold over endogenous levels. In these cells, we find a marked accumulation of the APP-CTF gamma (AICD) fragment and a concomitant reduction in levels of both APP-CTF beta and CTF alpha. Moreover, the production of the gamma-secretase-generated Notch S3/NICD derivative is modestly elevated. However, we failed to observe a corresponding increase in levels of secreted A beta peptides in the medium of these cells. These results lead us to conclude that, although the PS1, APH-1, NCT, and PEN-2 are essential for gamma-secretase activity, the proteolysis of APP-CTF and Notch S2/NEXT are differentially regulated and require the activity of additional cofactors that promote production of AICD, NICD, and A beta.

RIPped out by presenilin-dependent gamma-secretase

Presenilins (PS) constitute a fascinating family of proteins that play crucial roles in several major signalling processes involved in key cellular functions and are also closely associated with dysfunction in Alzheimer's disease (AD). Presenilin-dependent intramembrane cleavage of transmembrane proteins such as amyloid beta precursor protein (AbetaPP) and Notch resides in a high-molecular-weight gamma-secretase protein complex, of which at least five core components have now been identified. Remarkably, it has now become evident that presenilin-dependent gamma-secretase activity extends beyond its role in AbetaPP and Notch cleavages to have a generic role in the regulated intramembranous cleavage of certain membrane proteins. Actually, a new picture is emerging in which cells can relay signals from the extracellular space to their interior through presenilin-dependent proteolysis within the membrane-spanning regions of type 1 integral membrane proteins to generate potential transcriptionally active intracellular fragments. This review deals with the complex biology of presenilins and focuses more specifically on recent developments regarding the composition, assembly and regulation of the gamma-secretase protein complex, its substrates and its implications for cellular signalling.

Endoproteolysis of beta-secretase (beta-site amyloid precursor protein-cleaving enzyme) within its catalytic domain. A potential mechanism for regulation

Sequential proteolysis of the amyloid precursor protein (APP) by beta- and gamma-secretase activities yields the amyloid beta peptide that is widely deposited in the brains of individuals with Alzheimer's disease. The membrane-anchored aspartyl protease beta-site APP-cleaving enzyme (BACE) exhibits all of the characteristics of a beta-secretase and has been shown to cleave APP at its beta-site in vitro and in vivo. We found that BACE undergoes cleavage on a surface-exposed alpha-helix between amino acid residues Leu-228 and Ala-229, generating stable N- and C-terminal fragments that remain covalently associated via a disulfide bond. The efficiency of BACE endoproteolysis was observed to depend heavily on cell and tissue type. In contrast to brain where holoprotein was predominant, BACE was found primarily as endoproteolyzed fragments in pancreas, liver, and muscle. In addition, we observed a marked up-regulation of BACE endoproteolysis in C2 myoblasts upon differentiation into multinucleated myotubes, a well established model system of muscle tissue specification. As in liver, BACE exists as endoproteolyzed fragments in the hepatic cell line, HepG2. We found that HepG2 cells are capable of generating amyloid beta peptide, suggesting that endoproteolyzed BACE retains measurable beta-secretase activity. We also found that BACE endoproteolysis occurs only after export from the endoplasmic reticulum, is enhanced in the trans-Golgi network, and is sensitive to inhibitors of vesicular acidification. The membrane-bound proteases tumor necrosis factor alpha-converting enzyme and furin were not found to be responsible for this cleavage nor was BACE observed to mediate its own endoproteolysis by an autocatalytic mechanism. Thus, we characterize a specific processing event that may serve to regulate the enzymatic activity of BACE on a post-translational level.

The C-terminal fragment of presenilin 2 triggers p53-mediated staurosporine-induced apoptosis, a function independent of the presenilinase-derived N-terminal counterpart

Mutations on presenilins are responsible for most of familial forms of Alzheimer's disease. These holoproteins undergo rapid maturation by presenilinase mainly in the endoplasmic reticulum, leading to the production of N- and C-terminal fragments. We show first that overexpression of the presenilinase-derived maturation product of presenilin 2 (CTF-PS2) increases Abeta recovery, the production of which is almost abolished by a caspase 3 inhibitor and increased by staurosporine. This and the observation that the apoptotic inducer staurosporine enhances CTF-PS2 degradation clearly link CTF-PS2 to apoptotic cascade effectors. This prompted us to analyze the putative ability of CTF-PS2 to modulate cell death. CTF-PS2 overexpression decreases cell viability and augments both caspase 3 activity and immunoreactivity. This is accompanied by lowered bcl2-like immunoreactivity and increased poly(ADP-ribose) polymerase cleavage and cytochrome c translocation into the cytosol. Interestingly, CTF-PS2-induced caspase 3 activation is prevented by pifithrin-alpha, a selective blocker of p53 transcriptional activity. On line with the latter data, CTF-PS2 drastically increases p53 immunoreactivity and transcriptional activity. Of most interest is our observation that CTF-PS2 expression also triggers increased caspase 3 activity and immunoreactivity in fibroblasts in which presenilins had been deleted. Therefore, CTF-PS2 could modulate cell death out of the NTF/CTF heterodimeric complex thought to correspond to the biologically functional entity. This is the first direct demonstration that CTF-PS2 could exhibit some of its functions in the absence of the presenilin 2 N-terminal fragment (NTF-PS2) counterpart derived from the presenilinase cleavage.

Presenilin-1 mutation L271V results in altered exon 8 splicing and Alzheimer's disease with non-cored plaques and no neuritic dystrophy

The mutation L271V in exon 8 of the presenilin-1 (PS-1) gene was detected in an Alzheimer's disease pedigree. Neuropathological examination of affected individuals identified variant, large, non-cored plaques without neuritic dystrophy, reminiscent of cotton wool plaques. Biochemical analysis of L271V mutation showed that it increased secretion of the 42-amino acid amyloid-beta peptide, suggesting a pathogenic mutation. Analysis of PS-1 transcripts from the brains of two mutation carriers revealed a 17-50% increase in PS-1 transcripts with deletion of exon 8 (PS-1deltaexon8) compared with unrelated Alzheimer's disease brains. Exon trapping analysis confirmed that L271V mutation enhanced the deletion of exon 8. Western blots of brain lysates indicated that PS-1deltaexon8 was overexpressed in an affected individual. Biochemical analysis of PS-1deltaexon8 in COS and BD8 cells indicate the splice isoform is not intrinsically active but interacts with wild-type PS-1 to generate amyloid-beta. Western blots of cell lysates immunoprecipitated with anti-Tau or anti-GSK-3beta antibodies indicated that PS-1deltaexon8, unlike wild-type PS-1, does not interact directly with Tau or GSK-3beta, potential modifiers of neuritic dystrophy. We postulate that variant plaques observed in this family are due in part to the effects of PS-1deltaexon8 and that interaction between PS-1 and various protein complexes are necessary for neuritic plaque formation.

Dimerization of presenilin-1 in vivo: suggestion of novel regulatory mechanisms leading to higher order complexes

A growing body of evidence indicates that presenilins could exist and be active as oligomeric complexes. Using yeast two-hybrid and cell culture analysis, we provide evidence that presenilin-1 (PS1) may self-oligomerize giving rise to specific full-length/full-length homodimers. When expressed in N2A and HEK239T cultured cells, full-length PS1-wt and 5(')myc-PS1-wt form specific homodimers corresponding to twice their molecular weight. The Alzheimer's disease-associated PS1 mutations Y115H, M146L, L392V, deltaE10(PS1(1-289/320-467)), the gamma-secretase dominant negative mutant D257A, and the PS1 polymorphism mutant E318G do not affect their ability to self-oligomerize. Under non-denaturing conditions, endogenous PS1 forms specific homo-oligomers in human cultured cells. The results obtained herein suggest that PS1 associates intramolecularly to form higher order complexes, which may be needed for endoproteolytic cleavage and/or gamma-secretase-associated activity.

The mechanism of gamma-secretase activities through high molecular weight complex formation of presenilins is conserved in Drosophila melanogaster and mammals

Mutations in presenilin 1 (PS1) and PS2 genes contribute to the pathogenesis of early onset familial Alzheimer's disease by increasing secretion of the pathologically relevant Abeta42 polypeptides. PS genes are also implicated in Notch signaling through proteolytic processing of the Notch receptor in Caenorhabditis elegans, Drosophila melanogaster, and mammals. Here we show that Drosophila PS (Psn) protein undergoes endoproteolytic cleavage and forms a stable high molecular weight (HMW) complex in Drosophila S2 or mouse neuro2a (N2a) cells in a similar manner to mammalian PS. The loss-of-function recessive point mutations located in the C-terminal region of Psn, that cause an early pupal-lethal phenotype resembling Notch mutant in vivo, disrupted the HMW complex formation, and abolished gamma-secretase activities in cultured cells. The overexpression of Psn in mouse embryonic fibroblasts lacking PS1 and PS2 genes rescued the Notch processing. Moreover, disruption of the expression of Psn by double-stranded RNA-mediated interference completely abolished the gamma-secretase activity in S2 cells. Surprisingly, gamma-secretase activity dependent on wild-type Psn was associated with a drastic overproduction of Abeta1-42 from human betaAPP in N2a cells, but not in S2 cells. Our data suggest that the mechanism of gamma-secretase activities through formation of HMW PS complex, as well as its abolition by loss-of-function mutations located in the C terminus, are highly conserved features in Drosophila and mammals.

Presenilin couples the paired phosphorylation of beta-catenin independent of axin: implications for beta-catenin activation in tumorigenesis

The Alzheimer's disease-linked gene presenilin 1 (PS1) is required for intramembrane proteolysis of APP and Notch. In addition, recent observations strongly implicate PS1 as a negative regulator of the Wnt/beta-catenin signaling pathway, although the mechanism underlying this activity is unknown. Here, we show that presenilin functions as a scaffold that rapidly couples beta-catenin phosphorylation through two sequential kinase activities independent of the Wnt-regulated Axin/CK1alpha complex. Thus, presenilin deficiency results in increased beta-catenin stability in vitro and in vivo by disconnecting the stepwise phosphorylation of beta-catenin, both in the presence and absence of Wnt stimulation. These findings highlight an aspect of beta-catenin regulation outside of the canonical Wnt-regulated pathway and a function of presenilin separate from intramembrane proteolysis.