Dissociated phenotypes in presenilin transgenic mice define functionally distinct gamma-secretases

Unravelling of molecular biomarkers in synaptic plasticity of Alzheimer's disease: Critical role of the restoration of neuronal circuits

Learning and memory storage are the fundamental activities of the brain. Aberrant expression of synaptic molecular markers has been linked to memory impairment in AD. Aging is one of the risk factors linked to gradual memory loss. It is estimated that approximately 13 million people worldwide will have AD by 2050. A massive amount of oxidative stress is kept under control by a complex network of antioxidants, which occasionally fails and results in neuronal oxidative stress. Increasing evidence suggests that ROS may affect many pathological aspects of AD, including Aβ accumulation, tau hyperphosphorylation, synaptic plasticity, and mitochondrial dysfunction, which may collectively result in neurodegeneration in the brain. Further investigation into the relationship between oxidative stress and AD may provide an avenue for effective preservation and pharmacological treatment of this neurodegenerative disease. In this review, we briefly summarize the cellular mechanism underlying Aβ induced synaptic dysfunction. Since oxidative stress is common in the elderly and may contribute to the pathogenesis of AD, we also shed light on the role of antioxidant and inflammatory pathways in oxidative stress adaptation, which has a potential therapeutic target in neurodegenerative diseases.

An Alternative View of Familial Alzheimer's Disease Genetics

Probabilistic and parsimony-based arguments regarding available genetics data are used to propose that Hardy and Higgin's amyloid cascade hypothesis is valid but is commonly interpreted too narrowly to support, incorrectly, the primacy of the amyloid-β peptide (Aβ) in driving Alzheimer's disease pathogenesis. Instead, increased activity of the βCTF (C99) fragment of AβPP is the critical pathogenic determinant altered by mutations in the APP gene. This model is consistent with the regulation of APP mRNA translation via its 5' iron responsive element. Similar arguments support that the pathological effects of familial Alzheimer's disease mutations in the genes PSEN1 and PSEN2 are not exerted directly via changes in AβPP cleavage to produce different ratios of Aβ length. Rather, these mutations likely act through effects on presenilin holoprotein conformation and function, and possibly the formation and stability of multimers of presenilin holoprotein and/or of the γ-secretase complex. All fAD mutations in APP, PSEN1, and PSEN2 likely find unity of pathological mechanism in their actions on endolysosomal acidification and mitochondrial function, with detrimental effects on iron homeostasis and promotion of "pseudo-hypoxia" being of central importance. Aβ production is enhanced and distorted by oxidative stress and accumulates due to decreased lysosomal function. It may act as a disease-associated molecular pattern enhancing oxidative stress-driven neuroinflammation during the cognitive phase of the disease.

Expression and proteolytic processing of the amyloid precursor protein is unaffected by the expression of the three human apolipoprotein E alleles in the brains of mice

The 3 human apolipoprotein E (APOE) gene alleles modify an individual's risk of developing Alzheimer's disease (AD): compared to the risk-neutral APOE ε3 allele, the ε4 allele (APOE4) is strongly associated with increased AD risk while the ε2 allele is protective. Multiple mechanisms have been shown to link APOE4 expression and AD risk, including the possibility that APOE4 increases the expression of the amyloid precursor protein (APP) (Y-W.A. Huang, B. Zhou, A.M. Nabet, M. Wernig, T.C. Südhof, 2019). In this study, we investigated the impact of APOE genotype on the expression, and proteolytic processing of endogenously expressed APP in the brains of mice humanized for the 3 APOE alleles. In contrast to prior studies using neuronal cultures, we found in the brain that both App gene expression, and the levels of APP holoprotein were not affected by APOE genotype. Additionally, our analysis of APP fragments showed that APOE genotype does not impact APP processing in the brain: the levels of both α- and β-cleaved soluble APP fragments (sAPPs) were similar across genotypes, as were the levels of the membrane-associated α- and β-cleaved C-terminal fragments (CTFs) of APP. Lastly, APOE genotype did not impact the level of soluble amyloid beta (Aβ). These findings argue that the APOE-allele-dependent AD risk is independent of the brain expression and processing of APP.

In-Frame and Frameshift Mutations in Zebrafish Presenilin 2 Affect Different Cellular Functions in Young Adult Brains

BACKGROUND

Mutations in PRESENILIN 2 (PSEN2) cause early onset familial Alzheimer's disease (EOfAD) but their mode of action remains elusive. One consistent observation for all PRESENILIN gene mutations causing EOfAD is that a transcript is produced with a reading frame terminated by the normal stop codon-the "reading frame preservation rule". Mutations that do not obey this rule do not cause the disease. The reasons for this are debated.

OBJECTIVE

To predict cellular functions affected by heterozygosity for a frameshift, or a reading frame-preserving mutation in zebrafish psen2 using bioinformatic techniques.

METHODS

A frameshift mutation (psen2 N140fs ) and a reading frame-preserving (in-frame) mutation (psen2 T141 _ L142delinsMISLISV ) were previously isolated during genome editing directed at the N140 codon of zebrafish psen2 (equivalent to N141 of human PSEN2). We mated a pair of fish heterozygous for each mutation to generate a family of siblings including wild type and heterozygous mutant genotypes. Transcriptomes from young adult (6 months) brains of these genotypes were analyzed.

RESULTS

The in-frame mutation uniquely caused subtle, but statistically significant, changes to expression of genes involved in oxidative phosphorylation, long-term potentiation and the cell cycle. The frameshift mutation uniquely affected genes involved in Notch and MAPK signaling, extracellular matrix receptor interactions and focal adhesion. Both mutations affected ribosomal protein gene expression but in opposite directions.

CONCLUSION

A frameshift and an in-frame mutation at the same position in zebrafish psen2 cause discrete effects. Changes in oxidative phosphorylation, long-term potentiation and the cell cycle may promote EOfAD pathogenesis in humans.

Spastic paraplegia preceding PSEN1-related familial Alzheimer's disease

INTRODUCTION

We investigated the frequency, neuropathology, and phenotypic characteristics of spastic paraplegia (SP) that precedes dementia in presenilin 1 (PSEN1) related familial Alzheimer's disease (AD).

METHODS

We performed whole exome sequencing (WES) in 60 probands with hereditary spastic paraplegia (HSP) phenotype that was negative for variants in known HSP-related genes. Where PSEN1 mutation was identified, brain biopsy was performed. We investigated the link between HSP and AD with PSEN1 in silico pathway analysis and measured in vivo the stability of PSEN1 mutant γ-secretase.

RESULTS

We identified a PSEN1 variant (p.Thr291Pro) in an individual presenting with pure SP at 30 years of age. Three years later, SP was associated with severe, fast cognitive decline and amyloid deposition with diffuse cortical plaques on brain biopsy. Biochemical analysis of p.Thr291Pro PSEN1 revealed that although the mutation does not alter active γ-secretase reconstitution, it destabilizes γ-secretase-amyloid precursor protein (APP)/amyloid beta (Aβn) interactions during proteolysis, enhancing the production of longer Aβ peptides. We then extended our analysis to all 226 PSEN1 pathogenic variants reported and show that 7.5% were associated with pure SP onset followed by cognitive decline later in the disease. We found that PSEN1 cases manifesting initially as SP have a later age of onset, are associated with mutations located beyond codon 200, and showed larger diffuse, cored plaques, amyloid-ring arteries, and severe CAA.

DISCUSSION

We show that pure SP can precede dementia onset in PSEN1-related familial AD. We recommend PSEN1 genetic testing in patients presenting with SP with no variants in known HSP-related genes, particularly when associated with a family history of cognitive decline.

Knock-in rats with homozygous PSEN1L435F Alzheimer mutation are viable and show selective γ-secretase activity loss causing low Aβ40/42 and high Aβ43

Familial forms of Alzheimer's disease (FAD) are caused by mutations in the gene encoding amyloid precursor protein, whose processing can result in formation of β-amyloid (Aβ). FAD can also result from mutations in the presenilin 1/2 (PSEN1/2) genes, whose protein products partially compose the γ-secretase complex that cleaves Aβ from amyloid precursor protein fragments. Psen1 KO mice and knock-in (KI) mice with homozygous FAD-associated L435F mutations (Psen1LF/LF ) are embryonic and perinatally lethal, precluding a more rigorous examination of the effect of Alzheimer's disease-causing Psen1 mutations on neurodegeneration. Given that the rat is a more suitable model organism with regard to surgical interventions and behavioral testing, we generated a rat KI model of the Psen1LF mutation. In this study, we focused on young Psen1LF rats to determine potential early pathogenic changes caused by this mutation. We found that, unlike Psen1LF/LF mice, Psen1LF/LF rats survive into adulthood despite loss of γ-secretase activity. Consistent with loss of γ-secretase function, Psen1LF/LF rats exhibited low levels of Aβ38, Aβ40, and Aβ42 peptides. In contrast, levels of Aβ43, a longer and potentially more amyloidogenic Aβ form, were significantly increased in Psen1LF/LF and Psen1LF/w rats. The longer survival of these KI rats affords the opportunity to examine the effect of homozygous Psen1 Alzheimer's disease-associated mutations on neurodegeneration in older animals.

MMP13 inhibition rescues cognitive decline in Alzheimer transgenic mice via BACE1 regulation

MMP13 (matrix metallopeptidase 13) plays a key role in bone metabolism and cancer development, but has no known functions in Alzheimer's disease. In this study, we used high-throughput small molecule screening in SH-SY5Y cells that stably expressed a luciferase reporter gene driven by the BACE1 (β-site amyloid precursor protein cleaving enzyme 1) promoter, which included a portion of the 5' untranslated region (5'UTR). We identified that CL82198, a selective inhibitor of MMP13, decreased BACE1 protein levels in cultured neuronal cells. This effect was dependent on PI3K (phosphatidylinositide 3-kinase) signalling, and was unrelated to BACE1 gene transcription and protein degradation. Further, we found that eukaryotic translation initiation factor 4B (eIF4B) played a key role, as the mutation of eIF4B at serine 422 (S422R) or deletion of the BACE1 5'UTR attenuated MMP13-mediated BACE1 regulation. In APPswe/PS1E9 mice, an animal model of Alzheimer's disease, hippocampal Mmp13 knockdown or intraperitoneal CL82198 administration reduced BACE1 protein levels and the related amyloid-β precursor protein processing, amyloid-β load and eIF4B phosphorylation, whereas spatial and associative learning and memory performances were improved. Collectively, MMP13 inhibition/CL82198 treatment exhibited therapeutic potential for Alzheimer's disease, via the translational regulation of BACE1.

A pathogenic PSEN2 p.His169Asn mutation associated with early-onset Alzheimer's disease

Background

Autosomal dominant early-onset Alzheimer’s disease (EOAD) is genetically heterogeneous and has been associated with mutations in 3 different genes, coding for amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2). Most frequent cases are associated with mutations in the PSEN1 gene, whereas mutations in the APP and PSEN2 genes are rare.

Methods

Patient who presented progressive memory decline in her 50s was enrolled in this study. A broad battery of neuropsychological tests and neuroimaging was applied to make the diagnosis. Genetic tests were performed in the patient to evaluate possible mutations using next-generation sequencing (NGS). The pathogenic nature of missense mutation and its 3D protein structure prediction were performed by in silico prediction programs.

Results

A pathogenic mutation in the PSEN2 gene in a Korean patient associated with EOAD was identified. Targeted Next-generation sequencing and Sanger sequencing revealed a heterozygous C to A transition at position 505 (c.505C>A), resulting in a probably missense mutation at codon 169 (p.His169Asn) in PSEN2. PolyPhen-2 and SIFT software analyses predicted this mutation to be a probable damaging variant. This hypothesis was supported by the results of 3D in silico modelling analyses that predicted the p.His169Asn may result in major helix torsion due to histidine to asparagine substitution. Mutation may cause additional stresses with hydrophobic residues on the surface that interact inside the transmembrane domain III, which is a conserved domain in PSEN2 His169.

Conclusion

These findings revealed that the p.His169Asn might be an important residue in PSEN2, which may alter the functions of PSEN2, suggesting its potential involvement with AD phenotype. Future functional studies are needed to evaluate the role of PSEN2 p.His169Asn mutation in AD disease progression.

Inactivation of γ-secretases leads to accumulation of substrates and non-Alzheimer neurodegeneration

γ-Secretases are a family of intramembrane cleaving aspartyl proteases and important drug targets in Alzheimer's disease. Here, we generated mice deficient for all γ-secretases in the pyramidal neurons of the postnatal forebrain by deleting the three anterior pharynx defective 1 (Aph1) subunits (Aph1abc cKO Cre⁺). The mice show progressive cortical atrophy, neuronal loss, and gliosis. Interestingly, this is associated with more than 10-fold accumulation of membrane-bound fragments of App, Aplp1, Nrg1, and Dcc, while other known substrates of γ-secretase such as Aplp2, Lrp1, and Sdc3 accumulate to lesser extents. Despite numerous reports linking neurodegeneration to accumulation of membrane-bound App fragments, deletion of App expression in the combined Aph1 knockout does not rescue this phenotype. Importantly, knockout of only Aph1a- or Aph1bc-secretases causes limited and differential accumulation of substrates. This was not associated with neurodegeneration. Further development of selective Aph1-γ-secretase inhibitors should be considered for treatment of Alzheimer's disease.

Time-dependent reversal of synaptic plasticity induced by physiological concentrations of oligomeric Aβ42: an early index of Alzheimer's disease

The oligomeric amyloid-β (Aβ) peptide is thought to contribute to the subtle amnesic changes in Alzheimer’s disease (AD) by causing synaptic dysfunction. Here, we examined the time course of synaptic changes in mouse hippocampal neurons following exposure to Aβ₄₂ at picomolar concentrations, mimicking its physiological levels in the brain. We found opposite effects of the peptide with short exposures in the range of minutes enhancing synaptic plasticity, and longer exposures lasting several hours reducing it. The plasticity reduction was concomitant with an increase in the basal frequency of spontaneous neurotransmitter release, a higher basal number of functional presynaptic release sites, and a redistribution of synaptic proteins including the vesicle-associated proteins synapsin I, synaptophysin, and the post-synaptic glutamate receptor I. These synaptic alterations were mediated by cytoskeletal changes involving actin polymerization and p38 mitogen-activated protein kinase. These in vitro findings were confirmed in vivo with short hippocampal infusions of picomolar Aβ enhancing contextual memory and prolonged infusions impairing it. Our findings provide a model for initiation of synaptic dysfunction whereby exposure to physiologic levels of Aβ for a prolonged period of time causes microstructural changes at the synapse which result in increased transmitter release, failure of synaptic plasticity, and memory loss.

Probable novel PSEN2 Pro123Leu mutation in a Chinese Han family of Alzheimer's disease

We describe a probably novel mutation in exon 5 of the presenilin 2 gene (Pro123Leu) in a Chinese familial early-onset Alzheimer's disease, which clinically manifests as progressive memory loss, cognitive impairment, parkinsonism, and myoclonic jerks. Clinical and neuroimaging examination, target region capture, and high-throughput sequencing were performed in a family of 4 generations. Cerebral perfusion and glucose metabolism were evaluated using arterial spin labeling perfusion magnetic resonance imaging and (18)F-fludeoxyglucose positron emission tomography, respectively. Target region capture sequencing yielded a novel missense mutation at codon 123 (P123L) which is a heterozygous C to T point mutation at position 368 (c.368C>T) in exon 5 of the presenilin 2 leading to a proline-to-leucine substitution. The results were also identified by Sanger sequencing in 7 family members but not in the other 9 unaffected family members and 100 control subjects. This mutation is probably pathogenic and is the first of its kind reported in an early-onset familial AD associated with atypical symptom presentation.

Neuregulin-ErbB4 signaling in the developing lung alveolus: a brief review

Lung immaturity is the major cause of morbidity and mortality in premature infants, especially those born <28 weeks gestation. Proper lung development from 23-28 weeks requires coordinated cell proliferation and differentiation. Infants born at this age are at high risk for respiratory distress syndrome (RDS), a lung disease characterized by insufficient surfactant production due to immaturity of the alveoli and its constituent cells in the lung. The ErbB4 receptor and its stimulation by neuregulin (NRG) plays a critical role in surfactant synthesis by alveolar type II epithelial cells. In this review, we first provide an introduction to normal human alveolar development, followed by a discussion of the neuregulin and ErbB4-mediated mechanisms regulating alveolar development and surfactant production.

Dissociated presenilin-1 and TACE processing of ErbB4 in lung alveolar type II cell differentiation

Neuregulin (NRG) stimulation of ErbB4 signaling is important for type II cell surfactant synthesis. ErbB4 may mediate gene expression via a non-canonical pathway involving enzymatic cleavage releasing its intracellular domain (4ICD) for nuclear trafficking and gene regulation. The accepted model for release of 4ICD is consecutive cleavage by Tumor necrosis factor alpha Converting Enzyme (TACE) and γ-secretase enzymes. Here, we show that 4ICD mediates surfactant synthesis and its release by γ-secretase is not dependent on previous TACE cleavage. We used siRNA to silence Presenilin-1 (PSEN-1) expression in a mouse lung type II epithelial cell line (MLE12 cells), and both siRNA knockdown and chemical inhibition of TACE. Knockdown of PSEN-1 significantly decreased baseline and NRG-stimulated surfactant phospholipid synthesis, expression of the surfactant proteins SP-B and SP-C, as well as 4ICD levels, with no change in ErbB4 ectodomain shedding. Neither siRNA knockdown nor chemical inhibition of TACE inhibited 4ICD release or surfactant synthesis. PSEN-1 cleavage of ErbB4 for non-canonical signaling through 4ICD release does not require prior cleavage by TACE.

Thapsigargin affects presenilin-2 but not presenilin-1 regulation in SK-N-BE cells

Presenilin-1 (PS1) and presenilin-2 (PS2) are transmembrane proteins widely expressed in the central nervous system, which function as the catalytic subunits of γ-secretase, the enzyme that releases amyloid-β protein (Aβ) from ectodomain cleaved amyloid precursor protein (APP) by intramembrane proteolysis. Mutations in PS1, PS2, and Aβ protein precursor are involved in the etiology of familial Alzheimer's disease (FAD), while the cause of the sporadic form of AD (SAD) is still not known. However, since similar neuropathological changes have been observed in both FAD and SAD, a common pathway in the etiology of the disease has been suggested. Given that age-related deranged Ca(2+) regulation has been hypothesized to play a role in SAD pathogenesis via PS gene regulation and γ-secretase activity, we studied the in vitro regulation of PS1 and PS2 in the human neuron-like SK-N-BE cell line treated with the specific endoplasmic reticulum (ER) calcium ATPase inhibitor Thapsigargin (THG), to introduce intracellular Ca(2+) perturbations and mimic the altered Ca(2+) homeostasis observed in AD. Our results showed a consistent and significant down-regulation of PS2, while PS1 appeared to be unmodulated. These events were accompanied by oxidative stress and a number of morphological alterations suggestive of the induction of apoptotic machinery. The administration of the antioxidant N-acetylcysteine (NAC) did not revert the THG-induced effects reported, while treatment with the Ca(2+)-independent ER stressor Brefeldin A did not modulate basal PS1 and PS2 expression. Collectively, these results suggest that Ca(2+) fluctuation rather than ER stress and/or oxidative imbalance seems to play an essential role in PS2 regulation and confirm that, despite their strong homology, PS1 and PS2 could play different roles in AD.

Signature amyloid β profiles are produced by different γ-secretase complexes

γ-Secretase complexes are involved in the generation of amyloid-β (Aβ) in the brain. Therefore, γ-secretase has been proposed as a potential therapeutic target in Alzheimer disease (AD). Targeting γ-secretase activity in AD requires the pharmacological dissociation of the processing of physiological relevant substrates and the generation of "toxic" Aβ. Previous reports suggest the differential targeting of γ-secretase complexes, based on their subunit composition, as a valid strategy. However, little is known about the biochemical properties of the different complexes, and key questions regarding their Aβ product profiles should be first addressed. Here, we expressed, purified, and analyzed, under the same conditions, the endopeptidase and carboxypeptidase-like activities of the four γ-secretase complexes present in humans. We find that the nature of the catalytic subunit in the complex affects both activities. Interestingly, PSEN2 complexes discriminate between the Aβ40 and Aβ38 production lines, indicating that Aβ generation in one or the other pathway can be dissociated. In contrast, the APH1 subunit mainly affects the carboxypeptidase-like activity, with APH1B complexes favoring the generation of longer Aβ peptides. In addition, we determined that expression of a single human γ-secretase complex in cell lines retains the intrinsic attributes of the protease while present in the membrane, providing validation for the in vitro studies. In conclusion, our data show that each γ-secretase complex produces a characteristic Aβ signature. The qualitative and quantitative differences between different γ-secretase complexes could be used to advance drug development in AD and other disorders.

From soluble aβ to progressive aβ aggregation: could prion-like templated misfolding play a role?

Accumulation, aggregation and deposition of Aβ peptides are pathological hallmarks in the brains of individuals affected by Alzheimer's disease (AD) or by cerebral β-amyloid angiopathy (Aβ-CAA). While Aβ is a peptide of yet largely unknown function, it is constantly produced in the human brain where it normally remains in a soluble state. However, Aβ peptides are aggregation prone by their intrinsic ability to adopt alternative conformations rich in β-sheet structure that aggregate into oligomeric as well as fibrillar formations. This transition from soluble to aggregated state has been hypothesized to initiate the pathological cascade and is therefore subject to intensive research. Mounting evidence suggests prion-like templated misfolding as the biochemical phenomenon responsible for promoting progressive Aβ aggregation. Here, we review studies in vitro and in vivo that suggest that cerebral Aβ aggregation may indeed progress via prion-like templated misfolding. The implications of these findings are discussed with respect to understanding initiation and progression of the disease and to developing therapeutics.

Biological function of Presenilin and its role in AD pathogenesis

Presenilins (PSs) are the catalytic core of γ-secretase complex. However, the mechanism of FAD-associated PS mutations in AD pathogenesis still remains elusive. Here we review the general biology and mechanism of γ-secretase and focus on the catalytic components – presenilins and their biological functions and contributions to the AD pathogenesis. The functions of presenilins are divided into γ-secretase dependent and γ-secretase independent ones. The γ-secretase dependent functions of presenilins are exemplified by the sequential cleavages in the processing of APP and Notch; the γ-secretase independent functions of presenilins include stabilizing β-catenin in Wnt signaling pathway, regulating calcium homeostasis and their interaction with synaptic transmission.

Interactome analyses of mature γ-secretase complexes reveal distinct molecular environments of presenilin (PS) paralogs and preferential binding of signal peptide peptidase to PS2

γ-Secretase plays a pivotal role in the production of neurotoxic amyloid β-peptides (Aβ) in Alzheimer disease (AD) and consists of a heterotetrameric core complex that includes the aspartyl intramembrane protease presenilin (PS). The human genome codes for two presenilin paralogs. To understand the causes for distinct phenotypes of PS paralog-deficient mice and elucidate whether PS mutations associated with early-onset AD affect the molecular environment of mature γ-secretase complexes, quantitative interactome comparisons were undertaken. Brains of mice engineered to express wild-type or mutant PS1, or HEK293 cells stably expressing PS paralogs with N-terminal tandem-affinity purification tags served as biological source materials. The analyses revealed novel interactions of the γ-secretase core complex with a molecular machinery that targets and fuses synaptic vesicles to cellular membranes and with the H⁺-transporting lysosomal ATPase macrocomplex but uncovered no differences in the interactomes of wild-type and mutant PS1. The catenin/cadherin network was almost exclusively found associated with PS1. Another intramembrane protease, signal peptide peptidase, predominantly co-purified with PS2-containing γ-secretase complexes and was observed to influence Aβ production.

The many substrates of presenilin/γ-secretase

The Alzheimer's disease (AD)-associated amyloid-β protein precursor (AβPP) is cleaved by α-, β-, and presenilin (PS)/γ-secretases through sequential regulated proteolysis. These proteolytic events control the generation of the pathogenic amyloid-β (Aβ) peptide, which excessively accumulates in the brains of individuals afflicted by AD. A growing number of additional proteins cleaved by PS/γ-secretase continue to be discovered. Similarly to AβPP, most of these proteins are type-I transmembrane proteins involved in vital signaling functions regulating cell fate, adhesion, migration, neurite outgrowth, or synaptogenesis. All the identified proteins share common structural features, which are typical for their proteolysis. The consequences of the PS/γ-secretase-mediated cleavage on the function of many of these proteins are largely unknown. Here, we review the current literature on the proteolytic processing mediated by the versatile PS/γ-secretase complex. We begin by discussing the steps of AβPP processing and PS/γ-secretase complex composition and localization, which give clues to how and where the processing of other PS/γ-secretase substrates may take place. Then we summarize the typical features of PS/γ-secretase-mediated protein processing. Finally, we recapitulate the current knowledge on the possible physiological function of PS/γ-secretase-mediated cleavage of specific substrate proteins.

Molecular consequences of amyloid precursor protein and presenilin mutations causing autosomal-dominant Alzheimer's disease

Mutations in both the amyloid precursor protein (APP) and the presenilin (PSEN) genes cause familial Alzheimer's disease (FAD) with autosomal dominant inheritance and early onset of disease. The clinical course and neuropathology of FAD and sporadic Alzheimer's disease are highly similar, and patients with FAD constitute a unique population in which to conduct treatment and, in particular, prevention trials with novel pharmaceutical entities. It is critical, therefore, to exactly defi ne the molecular consequences of APP and PSEN FAD mutations. Both APP and PSEN mutations drive amyloidosis in FAD patients through changes in the brain metabolism of amyloid-β (Aβ) peptides that promote the formation of pathogenic aggregates. APP mutations do not seem to impair the physiological functions of APP. In contrast, it has been proposed that PSEN mutations compromise γ-secretase-dependent and -independent functions of PSEN. However, PSEN mutations have mostly been studied in model systems that do not accurately refl ect the genetic background in FAD patients. In this review, we discuss the reported cellular phenotypes of APP and PSEN mutations, the current understanding of their molecular mechanisms, the need to generate faithful models of PSEN mutations, and the potential bias of APP and PSEN mutations on therapeutic strategies that target Aβ.

Lessons from genome-wide association studies findings in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder with a complex genetic background. Recent genome-wide association studies (GWAS) have placed important new contributors into the genetic framework of early- and late-onset forms of this dementia. Besides confirming the major role of classic allelic variants (e.g. apolipoprotein E) in the development of AD, GWAS have thus far implicated over 20 single nucleotide polymorphisms in AD. In this review, we summarize the findings of 16 AD-based GWAS performed to date whose public registries are available at the National Human Genome Research Institute, with an emphasis on understanding whether the polymorphic markers under consideration support functional implications to the pathophysiological role of the major genetic risk factors unraveled by GWAS.

Aβ measurement by enzyme-linked immunosorbent assay

The neuritic plaque in the brain of Alzheimer's disease patients consists of an amyloid composed primarily of Aβ, an approximately 4-kDa peptide derived from the amyloid precursor protein. Multiple lines of evidence suggest that Aβ plays a key role in the pathogenesis of the disease, and potential treatments that target Aβ production and/or Aβ accumulation in the brain as β-amyloid are being aggressively pursued. Methods to quantitate the Aβ peptide are, therefore, invaluable to most studies aimed at a better understanding of the molecular etiology of the disease and in assessing potential therapeutics. Although other techniques have been used to measure Aβ in the brains of AD patients and β-amyloid-depositing transgenic mice, the enzyme-linked immunosorbent assay (ELISA) is one of the most commonly used, reliable, and sensitive methods for quantitating the Aβ peptide. Here we describe methods for the recovery of both soluble and deposited Aβ from brain tissue and the subsequent quantitation of the peptide by sandwich ELISA.

Tissue processing prior to analysis of Alzheimer's disease associated proteins and metabolites, including Aβ

Amyloid-containing tissue, whether from human patients or an animal model of a disease, is typically characterized by various biochemical and immunohistochemical techniques, many of which are described in detail in this volume. In this chapter, we describe a straightforward technique for the homogenization of tissue prior to these analyses. The technique is particularly well suited for performing a large number of different biochemical analyses on a single mouse brain hemisphere. Starting with this homogenate multiple characterizations can be done, including western blot analysis and isolation of membrane-associated proteins, both of which are described here. Additional analyses can readily be performed on the tissue homogenate, including the ELISA quantitation of Aβ in the brain of a transgenic mouse model of β-amyloid deposition. The ELISA technique is described in detail in Chapter 34.

Down-regulation of Shadoo in prion infections traces a pre-clinical event inversely related to PrP(Sc) accumulation

During prion infections of the central nervous system (CNS) the cellular prion protein, PrP(C), is templated to a conformationally distinct form, PrP(Sc). Recent studies have demonstrated that the Sprn gene encodes a GPI-linked glycoprotein Shadoo (Sho), which localizes to a similar membrane environment as PrP(C) and is reduced in the brains of rodents with terminal prion disease. Here, analyses of prion-infected mice revealed that down-regulation of Sho protein was not related to Sprn mRNA abundance at any stage in prion infection. Down-regulation was robust upon propagation of a variety of prion strains in Prnp(a) and Prnp(b) mice, with the exception of the mouse-adapted BSE strain 301 V. In addition, Sho encoded by a TgSprn transgene was down-regulated to the same extent as endogenous Sho. Reduced Sho levels were not seen in a tauopathy, in chemically induced spongiform degeneration or in transgenic mice expressing the extracellular ADan amyloid peptide of familial Danish dementia. Insofar as prion-infected Prnp hemizygous mice exhibited accumulation of PrP(Sc) and down-regulation of Sho hundreds of days prior to onset of neurologic symptoms, Sho depletion can be excluded as an important trigger for clinical disease or as a simple consequence of neuronal damage. These studies instead define a disease-specific effect, and we hypothesize that membrane-associated Sho comprises a bystander substrate for processes degrading PrP(Sc). Thus, while protease-resistant PrP detected by in vitro digestion allows post mortem diagnosis, decreased levels of endogenous Sho may trace an early response to PrP(Sc) accumulation that operates in the CNS in vivo. This cellular response may offer new insights into the homeostatic mechanisms involved in detection and clearance of the misfolded proteins that drive prion disease pathogenesis.

Proteolytic cleavage of Notch: "HIT and RUN"

The Notch pathway is a highly conserved signaling pathway in multicellular eukaryotes essential in controlling spatial patterning, morphogenesis and homeostasis in embryonic and adult tissues. Notch proteins coordinate cell-cell communication through receptor-ligand interactions between adjacent cells. Notch signaling is frequently deregulated by oncogenic mutation or overexpression in many cancer types. Notch activity is controlled by three sequential cleavage steps leading to ectodomain shedding and transcriptional activation. Here we review the key regulatory steps in the activation of Notch, from receptor maturation to receptor activation (HIT) via a rate-limiting proteolytic cascade (RUN) in the context of species-specific differences.

Amyloid-β as a modulator of synaptic plasticity

Alzheimer's disease is associated with synapse loss, memory dysfunction, and pathological accumulation of amyloid-β (Aβ) in plaques. However, an exclusively pathological role for Aβ is being challenged by new evidence for an essential function of Aβ at the synapse. Aβ protein exists in different assembly states in the central nervous system and plays distinct roles ranging from synapse and memory formation to memory loss and neuronal cell death. Aβ is present in the brain of symptom-free people where it likely performs important physiological roles. New evidence indicates that synaptic activity directly evokes the release of Aβ at the synapse. At physiological levels, Aβ is a normal, soluble product of neuronal metabolism that regulates synaptic function beginning early in life. Monomeric Aβ40 and Aβ42 are the predominant forms required for synaptic plasticity and neuronal survival. With age, some assemblies of Aβ are associated with synaptic failure and Alzheimer's disease pathology, possibly targeting the N-methyl-D-aspartic acid receptor through the nicotinic acetylcholine receptor, mitochondrial Aβ alcohol dehydrogenase, and cyclophilin D. But emerging data suggests a distinction between age effects on the target response in contrast to the assembly state or the accumulation of the peptide. Both aging and Aβ independently decrease neuronal plasticity. Our laboratory has reported that Aβ, glutamate, and lactic acid are each increasingly toxic with neuron age. The basis of the age-related toxicity partly resides in age-related mitochondrial dysfunction and an oxidative shift in mitochondrial and cytoplasmic redox potential. In turn, signaling through phosphorylated extracellular signal-regulated protein kinases is affected along with an age-independent increase in phosphorylated cAMP response element-binding protein. This review examines the long-awaited functional impact of Aβ on synaptic plasticity.

Ca2+ dysfunction in neurodegenerative disorders: Alzheimer's disease

More than one century ago "a peculiar disorder of the cerebral cortex" was noticed in a middle-aged patient who had been affected by dementia in the last years of his life. The postmortem hallmarks of his brain were protein plaques, neurofibrillary tangles, and atherosclerotic changes: the neuropathologist who found these alterations and gave his name to the disease that underlied them was Alois Alzheimer (Alzheimer et al., Clin Anat 1995;8:429-431). Following its discovery, the disease has been studied with a vigor that went parallel to the increase of its social importance. The amount of information amassed in the literature is impressive, but knowledge on the mechanism underlying its onset and its progression is still very limited. Numerous hypotheses on the molecular pathogenesis of the Alzheimer's disease (AD) have been proposed and two have gradually gained wide consensus: (i) the amyloid cascade hypothesis, first proposed on the basis of the toxicity evoked by the deposition of amyloid β (Aβ) aggregates; (ii) the Ca(2+) hypothesis, which focuses on the correlation between the dysfunction of Ca(2+) homeostasis and the neurodegeneration process. This succinct review will discuss the essential aspects of the role of Ca(2+) homeostasis dysregulation in the onset and development of AD.

Presenilin 2 is the predominant γ-secretase in microglia and modulates cytokine release

Presenilin 1 (PS1) and Presenilin 2 (PS2) are the enzymatic component of the γ-secretase complex that cleaves amyloid precursor protein (APP) to release amyloid beta (Aβ) peptide. PS deficiency in mice results in neuroinflammation and neurodegeneration in the absence of accumulated Aβ. We hypothesize that PS influences neuroinflammation through its γ-secretase action in CNS innate immune cells. We exposed primary murine microglia to a pharmacological γ-secretase inhibitor which resulted in exaggerated release of TNFα and IL-6 in response to lipopolysaccharide. To determine if this response was mediated by PS1, PS2 or both we used shRNA to knockdown each PS in a murine microglia cell line. Knockdown of PS1 did not lead to decreased γ-secretase activity while PS2 knockdown caused markedly decreased γ-secretase activity. Augmented proinflammatory cytokine release was observed after knockdown of PS2 but not PS1. Proinflammatory stimuli increased microglial PS2 gene transcription and protein in vitro. This is the first demonstration that PS2 regulates CNS innate immunity. Taken together, our findings suggest that PS2 is the predominant γ-secretase in microglia and modulates release of proinflammatory cytokines. We propose PS2 may participate in a negative feedback loop regulating inflammatory behavior in microglia.

Presenilin-1 processing of ErbB4 in fetal type II cells is necessary for control of fetal lung maturation

Maturation of pulmonary fetal type II cells to initiate adequate surfactant production is crucial for postnatal respiratory function. Little is known about specific mechanisms of signal transduction controlling type II cell maturation. The ErbB4 receptor and its ligand neuregulin (NRG) are critical for lung development. ErbB4 is cleaved at the cell membrane by the γ-secretase enzyme complex whose active component is either presenilin-1 (PSEN-1) or presenilin-2. ErbB4 cleavage releases the 80kDa intracellular domain (4ICD), which associates with chaperone proteins such as YAP (Yes-associated protein) and translocates to the nucleus to regulate gene expression. We hypothesized that PSEN-1 and YAP have a development-specific expression in fetal type II cells and are important for ErbB4 signaling in surfactant production. In primary fetal mouse E16, E17, and E18 type II cells, PSEN-1 and YAP expression increased at E17 and E18 over E16. Subcellular fractionation showed a strong cytosolic and a weaker membrane location of both PSEN-1 and YAP. This was enhanced by NRG stimulation. Co-immunoprecipitations showed ErbB4 associated separately with PSEN-1 and with YAP. Their association, phosphorylation, and co-localization were induced by NRG. Confocal immunofluorescence and nuclear fractionation confirmed these associations in a time-dependent manner after NRG stimulation. Primary ErbB4-deleted E17 type II cells were transfected with a mutant ErbB4 lacking the γ-secretase binding site. When compared to transfection with wild-type ErbB4, the stimulatory effect of NRG on surfactant protein mRNA expression was lost. We conclude that PSEN-1 and YAP have crucial roles in ErbB4 signal transduction during type II cell maturation.

Proteases and Proteolysis in Alzheimer Disease: A Multifactorial View on the Disease Process

Alzheimer disease is characterized by the accumulation of abnormally folded protein fragments, i.e., amyloid beta peptide (Aβ) and tau that precipitate in amyloid plaques and neuronal tangles, respectively. In this review we discuss the complicated proteolytic pathways that are responsible for the generation and clearance of these fragments, and how disturbances in these pathways interact and provide a background for a novel understanding of Alzheimer disease as a multifactorial disorder. Recent insights evolve from the static view that the morphologically defined plaques and tangles are disease driving towards a more dynamic, biochemical view in which the intermediary soluble Aβ oligomers and soluble tau fragments are considered as the main mediators of neurotoxicity. The relevance of proteolytic pathways, centered on the generation and clearance of toxic Aβ, on the cleavage and nucleation of tau, and on the general proteostasis of the neurons, then becomes obvious. Blocking or stimulating these pathways provide, or have the potential to provide, interesting drug targets, which raises the hope that we will be able to provide a cure for this dreadful disorder.

Neuronal death in Alzheimer's disease and therapeutic opportunities

Alzheimer’s disease (AD) is an age-related neurodegenerative disease that affects approximately 24 million people worldwide. A number of different risk factors have been implicated in AD; however, neuritic (amyloid) plaques are considered as one of the defining risk factors and pathological hallmarks of the disease. In the past decade, enormous efforts have been devoted to understand the genetics and molecular pathogenesis leading to neuronal death in AD, which has been transferred into extensive experimental approaches aimed at reversing disease progression. Modern medicine is facing an increasing number of treatments available for vascular and neurodegenerative brain diseases, but no causal or neuroprotective treatment has yet been established. Almost all neurological conditions are characterized by progressive neuronal dysfunction, which, regardless of the pathogenetic mechanism, finally leads to neuronal death. The particular emphasis of this review is on risk factors and mechanisms resulting in neuronal loss in AD and current and prospective opportunities for therapeutic interventions. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of AD.

Picomolar amyloid-beta positively modulates synaptic plasticity and memory in hippocampus

Amyloid-beta (Abeta) peptides are produced in high amounts during Alzheimer's disease, causing synaptic and memory dysfunction. However, they are also released in lower amounts in normal brains throughout life during synaptic activity. Here we show that low picomolar concentrations of a preparation containing both Abeta(42) monomers and oligomers cause a marked increase of hippocampal long-term potentiation, whereas high nanomolar concentrations lead to the well established reduction of potentiation. Picomolar levels of Abeta(42) also produce a pronounced enhancement of both reference and contextual fear memory. The mechanism of action of picomolar Abeta(42) on both synaptic plasticity and memory involves alpha7-containing nicotinic acetylcholine receptors. These findings strongly support a model for Abeta effects in which low concentrations play a novel positive, modulatory role on neurotransmission and memory, whereas high concentrations play the well known detrimental effect culminating in dementia.

Structure and function of gamma-secretase

The gamma-secretase complex is a prime target for pharmacological intervention in Alzheimer's disease and so far drug discovery efforts have yielded a large variety of potent and rather specific inhibitors of this enzymatic activity. However, as gamma-secretase is able to cleave a wide variety of physiological important substrates, the real challenge is to develop substrate-specific compounds. Therefore, obtaining structural information about gamma-secretase is indispensable. As crystal structures of the complex will be difficult to achieve, applied biochemical approaches need to be integrated with structural information obtained from other intramembrane-cleaving proteases. Here we review current knowledge about the structure and function of gamma-secretase and discuss the value of these findings for the mechanistic understanding of this unusual protease.

Presenilins: members of the gamma-secretase quartets, but part-time soloists too

The presenilins in combination with other proteins generate different gamma-secretases, which are involved in the regulated intramembrane proteolysis of a variety of proteins. Understanding the specificity and regulation of these proteases will potentially lead to novel therapeutics for Alzheimer's disease and cancer. Presenilins appear also to exert additional functions outside of the gamma-secretase quartets, which needs further investigation.

The association of Notch2 and NF-kappaB accelerates RANKL-induced osteoclastogenesis

Notch signaling plays a key role in various cell differentiation processes including bone homeostasis. However, the specific involvement of Notch in regulating osteoclastogenesis is still controversial. In the present study, we show that RANKL induces expression of Jagged1 and Notch2 in bone marrow macrophages during osteoclast differentiation. Suppression of Notch signaling by a selective gamma-secretase inhibitor or Notch2 short hairpin RNA suppresses RANKL-induced osteoclastogenesis. In contrast, induction of Notch signaling by Jagged1 or by ectopic expression of intracellular Notch2 enhances NFATc1 promoter activity and expression and promotes osteoclastogenesis. Finally, we found that Notch2 and p65 interact in the nuclei of RANKL-stimulated cells and that both proteins are recruited to the NFATc1 promoter, driving its expression. Taken together, our results show a new molecular cross talk between Notch and NF-kappaB pathways that is relevant in osteoclastogenesis.

The N-terminus of presenilin-2 increases single channel activity of brain ryanodine receptors through direct protein-protein interaction

Presenilin-1 (PS1) and presenilin-2 (PS2) form the catalytic core in gamma-secretase complexes and mutations in these proteins result in aberrant cleavage of amyloid precursor protein leading to accumulation of the beta-amyloid in the brain of familial Alzheimer Disease patients. PS2 possesses a hydrophilic cytoplasmic N-terminal domain (PS2 NTF1-87) dispensable for gamma-secretase activity with physiological functions yet to be determined. The effects of this soluble 87 amino acid fragment of mouse PS2 on single channel activity of mouse brain ryanodine receptors (RyR) were determined. PS2 NTF1-87 application to the cytoplasmic side of the RyR significantly increased single channel activity by favoring higher sublevel openings. The Ca(2+) activation and desensitization ranges for RyRs were unchanged. We demonstrate facilitation of RyR gating by PS2 NTF1-87, which might represent a general mechanism of RyR regulation by presenilins potentially prone to be affected by mutations or external stimuli contributing to the development of neurodegenerative diseases.

gamma-Secretase: a multifaceted regulator of angiogenesis

Physiological angiogenesis is essential for development, homeostasis and tissue repair but pathological neovascularization is a major feature of tumours, rheumatoid arthritis and ocular complications. Studies over the last decade have identified γ-secretase, a presenilin-dependent protease, as a key regulator of angiogenesis through: (i) regulated intramembrane proteolysis and transmembrane cleavage of receptors (e.g. VEGFR-1, Notch, ErbB-4, IGFI-R) followed by translocation of the intracellular domain to the nucleus, (ii) translocation of full length membrane-bound receptors to the nucleus (VEGFR-1), (iii) phosphorylation of membrane bound proteins (VEGFR-1 and ErbB-4), (iv) modulation of adherens junctions (cadherin) and regulation of permeability and (v) cleavage of amyloid precursor protein to amyloid-β which is able to regulate the angiogenic process. The γ-secretase-induced translocation of receptors to the nucleus provides an alternative intracellular signalling pathway, which acts as a potent regulator of transcription. γ-secretase is a complex composed of four different integral proteins (presenilin, nicastrin, Aph-1 and Pen-2), which determine the stability, substrate binding, substrate specificity and proteolytic activity of γ-secretase. This seeming complexity allows numerous possibilities for the development of targeted γ-secretase agonists/antagonists, which can specifically regulate the angiogenic process. This review will consider the structure and function of γ-secretase, the growing evidence for its role in angiogenesis and the substrates involved, γ-secretase as a therapeutic target and future challenges in this area.

Gamma-secretase regulates VEGFR-1 signalling in vascular endothelium and RPE

In conclusion, gamma-secretase is expressed in both retinal microvascular endothelial cells and RPE cells and is able to elicit regulated intramembrane proteolysis of VEGFR1. Furthermore, gamma-secretase regulates the translocation of the intracellular domain of VEGFR1 as well as full length VEGFR1 in both cell types. Gamma-secretase is clearly able to act as a potent negative regulators of VEGFR2 induced in vitro angiogenesis but its role in the RPE has yet to be elucidated. We conclude that gamma-secretase may offer a therapeutic target for the treatment of both retinal and choroidal neovascularization.

Moonlighting activity of presenilin in plants is independent of gamma-secretase and evolutionarily conserved

Presenilins (PS) provide the catalytic activity for gamma-secretase, which cleaves physiologically relevant substrates including Notch, ErbB4, and APP. Recent genetic studies indicated that the contribution of PS1 to mouse development includes gamma-secretase-independent functions that cannot be easily explained by any of the demonstrated or hypothesized functions of this protein. To begin a nonbiased analysis of PS1 activity unencumbered by the dominant effect stemming from loss of Notch function, we characterized PS functions in the early land plant Physcomitrella patens, which lacks Notch, ErbB4, and APP. Removal of P. patens PS resulted in phenotypic abnormalities. Further assays performed to delineate the defective pathways in PS-deficient P. patens implicated improper function of the cytoskeletal network. Importantly, this characterization of a nonmetazoan PS uncovered a previously undescribed, evolutionarily conserved function (human PS1 can rescue the growth and light responses) that is gamma-secretase-independent (mutants with substitutions of the catalytic aspartyl residues retain the activity). Introduction of PpPS into PS-deficient mouse embryonic fibroblasts rescues normal growth rates, demonstrating that at least some metazoan functions of PS are evolutionarily conserved.

Monomeric Aβ and metals reduce their cytotoxicities to each other

The present study has examined the effect of metals, such as iron and copper on the cytotoxicity of amyloid beta protein 1-40 (Abeta40). First, we showed that monomeric Abeta40 has stronger cytotoxicity than various type of aggregated Abeta40. Next we showed the addition of metals into the monomeric Abeta40 reduced the cytotoxicity of either monomeric Abeta40 or metals (iron and copper) although the addition of metals into monomeric Abeta40 resulted in a marked increase of aggregated form of Abeta40, which composed of beta-sheeted Abeta40 and Abeta40 aggregation not characterized by beta-sheet fibrils (coagrated Abeta40). Taken together, the metals and monomeric Abeta40 affect on each other and cause the reduction of their cell toxicity.

Inter-individual variability in the expression of the mutated form of hPS1M146L determined the production of Abeta peptides in the PS1xAPP transgenic mice

The detection of the early phenotypic modifications of Alzheimer's disease (AD) models is fundamental to understand the progression and identify pharmacologic targets of this pathology. However, a large variability within different models and between age-matched mice from the same model has been observed. This variability could be due to heterogeneity in the Abeta production. Present results showed the existence of a large variability in the Abeta deposition in both hippocampus and cortex in 6-month-old PS1xAPP mice. This variability was not due to the expression of hAPP751SL, however, linear relationship between hPS1M146L mRNA and Abeta production was identified. The Abeta content was related to the incorporation of the hPS1M146L into functional gamma-secretase complexes, detected by the presence of the corresponding human or endogenous PS1-CTFs. Animals expressing low amount of hPS1M146L mRNA, displayed low hPS1-CTF incorporation and produced a low amount of Abeta peptides. Conversely, mice with relatively high hPS1 mRNA expression displayed high hPS1-CTF and high Abeta deposition. Furthermore, the Abeta total and Abeta1-42 content was increased dramatically by the expression of hPS1M146L (as compared with transgenic APPsl littermates). Therefore, variations in the expression of transgenic form of hPS1M146L in this model, or even between different models, influenced strongly the incorporation of the mutated PS1 into functional gamma-secretase complexes, the production of Abeta peptides and, in consequence, the detrimental effects of Abeta peptides. These data might implicate an "apparent gain-of-function" of the gamma-secretase complex by the expression of the mutated PS1M146L.