Alleles at the Nicastrin locus modify presenilin 1- deficiency phenotype

Dysregulation of hypoxia-inducible factor by presenilin/γ-secretase loss-of-function mutations

Presenilin (PSEN) 1 and 2 are the catalytic components of the γ-secretase complex, which cleaves a variety of proteins, including the amyloid precursor protein (APP). Proteolysis of APP leads to the formation of the APP intracellular domain (AICD) and amyloid β that is crucially involved in the pathogenesis of Alzheimer's disease. Prolyl-4-hydroxylase-domain (PHD) proteins regulate the hypoxia-inducible factors (HIFs), the master regulators of the hypoxic response. We previously identified the FK506 binding protein 38 (FKBP38) as a negative regulator of PHD2. Genetic ablation of PSEN1/2 has been shown to increase FKBP38 protein levels. Therefore, we investigated the role of PSEN1/2 in the oxygen sensing pathway using a variety of genetically modified cell and mouse lines. Increased FKBP38 protein levels and decreased PHD2 protein levels were found in PSEN1/2-deficient mouse embryonic fibroblasts and in the cortex of forebrain-specific PSEN1/2 conditional double knock-out mice. Hypoxic HIF-1α protein accumulation and transcriptional activity were decreased, despite reduced PHD2 protein levels. Proteolytic γ-secretase function of PSEN1/2 was needed for proper HIF activation. Intriguingly, PSEN1/2 mutations identified in Alzheimer patients differentially affected the hypoxic response, involving the generation of AICD. Together, our results suggest a direct role for PSEN in the regulation of the oxygen sensing pathway via the APP/AICD cleavage cascade.

Immunization targeting a minor plaque constituent clears β-amyloid and rescues behavioral deficits in an Alzheimer's disease mouse model

Although anti-human β-amyloid (Aβ) immunotherapy clears brain β-amyloid plaques in Alzheimer's disease (AD), targeting additional brain plaque constituents to promote clearance has not been attempted. Endogenous murine Aβ is a minor Aβ plaque component in amyloid precursor protein (APP) transgenic AD models, which we show is ∼3%-8% of the total accumulated Aβ in various human APP transgenic mice. Murine Aβ codeposits and colocalizes with human Aβ in amyloid plaques, and the two Aβ species coimmunoprecipitate together from brain extracts. In the human APP transgenic mouse model Tg2576, passive immunization for 8 weeks with a murine-Aβ-specific antibody reduced β-amyloid plaque pathology, robustly decreasing both murine and human Aβ levels. The immunized mice additionally showed improvements in two behavioral assays, odor habituation and nesting behavior. We conclude that passive anti-murine Aβ immunization clears Aβ plaque pathology--including the major human Aβ component--and decreases behavioral deficits, arguing that targeting minor endogenous brain plaque constituents can be beneficial, broadening the range of plaque-associated targets for AD therapeutics.

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.

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.

Dissection of a QTL hotspot on mouse distal chromosome 1 that modulates neurobehavioral phenotypes and gene expression

A remarkably diverse set of traits maps to a region on mouse distal chromosome 1 (Chr 1) that corresponds to human Chr 1q21-q23. This region is highly enriched in quantitative trait loci (QTLs) that control neural and behavioral phenotypes, including motor behavior, escape latency, emotionality, seizure susceptibility (Szs1), and responses to ethanol, caffeine, pentobarbital, and haloperidol. This region also controls the expression of a remarkably large number of genes, including genes that are associated with some of the classical traits that map to distal Chr 1 (e.g., seizure susceptibility). Here, we ask whether this QTL-rich region on Chr 1 (Qrr1) consists of a single master locus or a mixture of linked, but functionally unrelated, QTLs. To answer this question and to evaluate candidate genes, we generated and analyzed several gene expression, haplotype, and sequence datasets. We exploited six complementary mouse crosses, and combed through 18 expression datasets to determine class membership of genes modulated by Qrr1. Qrr1 can be broadly divided into a proximal part (Qrr1p) and a distal part (Qrr1d), each associated with the expression of distinct subsets of genes. Qrr1d controls RNA metabolism and protein synthesis, including the expression of approximately 20 aminoacyl-tRNA synthetases. Qrr1d contains a tRNA cluster, and this is a functionally pertinent candidate for the tRNA synthetases. Rgs7 and Fmn2 are other strong candidates in Qrr1d. FMN2 protein has pronounced expression in neurons, including in the dendrites, and deletion of Fmn2 had a strong effect on the expression of few genes modulated by Qrr1d. Our analysis revealed a highly complex gene expression regulatory interval in Qrr1, composed of multiple loci modulating the expression of functionally cognate sets of genes.

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.

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.

gamma-Secretase complexes containing N- and C-terminal fragments of different presenilin origin retain normal gamma-secretase activity

The gamma-secretase complex processes substrate proteins within membranes and consists of four proteins: presenilin (PS), nicastrin, Aph-1 and Pen-2. PS harbours the enzymatic activity of the complex, and there are two mammalian PS homologues: PS1 and PS2. PS undergoes endoproteolysis, generating the N- and C-terminal fragments, NTF and CTF, which represent the active species of PS. To characterize the functional similarity between complexes of various PS composition, we analysed PS1, PS2, and chimeric PS composed of the NTF from PS1 and CTF from PS2, or vice versa, in assembly and function of the gamma-secretase complex. Chimeric PSs, like PS1 and PS2, undergo normal endoproteolysis when introduced into cells devoid of endogenous PS. Furthermore, PS2 CTF can, at least partially, restore processing in a truncated PS1, which cannot undergo endoproteolysis. All PS forms enable maturation of nicastrin and cleave full length Notch receptors, indicating that both PS1 and PS2 are present at the cell surface. Finally, when co-introduced as separate molecules, NTF and CTF of different PS origin reconstitute gamma-secretase activity. In conclusion, these data show that endoproteolysis, NTF-CTF interactions, and the assembly and activity of gamma-secretase complexes are very conserved between PS1 and PS2.

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

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

Effect of anti-inflammatory agents on transforming growth factor beta over-expressing mouse brains: a model revised

BACKGROUND: The over-expression of transforming growth factor beta-1(TGF-beta1) has been reported to cause hydrocephalus, glia activation, and vascular amyloidbeta (Abeta) deposition in mouse brains. Since these phenomena partially mimic the cerebral amyloid angiopathy (CAA) concomitant to Alzheimer's disease, the findings in TGF-beta1 over-expressing mice prompted the hypothesis that CAA could be caused or enhanced by the abnormal production of TGF-beta1. This idea was in accordance with the view that chronic inflammation contributes to Alzheimer's disease, and drew attention to the therapeutic potential of anti-inflammatory drugs for the treatment of Abeta-elicited CAA. We thus studied the effect of anti-inflammatory drug administration in TGF-beta1-induced pathology. METHODS: Two-month-old TGF-beta1 mice and littermate controls were orally administered pioglitazone, a peroxisome proliferator-activated receptor-gamma agonist, or ibuprofen, a non steroidal anti-inflammatory agent, for two months. Glia activation was assessed by immunohistochemistry and western blot analysis; Abeta precursor protein (APP) by western blot analysis; Abeta deposition by immunohistochemistry, thioflavin-S staining and ELISA; and hydrocephalus by measurements of ventricle size on autoradiographies of brain sections. Results are expressed as means +/- SD. Data comparisons were carried with the Student's T test when two groups were compared, or ANOVA analysis when more than three groups were analyzed. RESULTS: Animals displayed glia activation, hydrocephalus and a robust thioflavin-S-positive vascular deposition. Unexpectedly, these deposits contained no Abeta or serum amyloid P component, a common constituent of amyloid deposits. The thioflavin-S-positive material thus remains to be identified. Pioglitazone decreased glia activation and basal levels of Abeta42- with no change in APP contents - while it increased hydrocephalus, and had no effect on the thioflavin-S deposits. Ibuprofen mimicked the reduction of glia activation caused by pioglitazone and the lack of effect on the thioflavin-S-labeled deposits. CONCLUSIONS: i) TGF-beta1 over-expressing mice may not be an appropriate model of Abeta-elicited CAA; and ii) pioglitazone has paradoxical effects on TGF-beta1-induced pathology suggesting that anti-inflammatory therapy may reduce the damage resulting from active glia, but not from vascular alterations or hydrocephalus. Identification of the thioflavin-S-positive material will facilitate the full appraisal of the clinical implication of the effects of anti-inflammatory drugs, and provide a more thorough understanding of TGF-beta1 actions in brain.

Presenilin 1 and presenilin 2 have differential effects on the stability and maturation of nicastrin in Mammalian brain

The presenilins and nicastrin form high molecular mass, multimeric protein complexes involved in the intramembranous proteolysis of several proteins. Post-translational glycosylation and trafficking of nicastrin is necessary for the activity of these complexes. We report here that although there are differences in the post-translational processing of nicastrin in neurons and glia, both of the presenilins are required for the physiological post-translational modification and for the correct subcellular distribution of nicastrin. Absence of presenilin 1 (PS1) is associated with dramatic reductions in the level of mature glycosylated nicastrin and with redistribution of nicastrin away from the cell surface. In contrast, absence of presenilin 2 (PS2) is associated with only modest reductions in the levels of immature nicastrin. It is notable that these differential effects parallel the differential effects of null mutations in PS1 and PS2 on APP and Notch processing. Our data therefore suggest that the differential interactions of PS1 and PS2 with nicastrin reflect different functions for the PS1 and PS2 complexes.