Are amyloid-degrading enzymes viable therapeutic targets in Alzheimer's disease?

: The amyloid cascade hypothesis of Alzheimer's disease envisages that the initial elevation of amyloid ß-peptide (Aß) levels, especially of Aß(1-42) , is the primary trigger for the neuronal cell death specific to onset of Alzheimer's disease. There is now substantial evidence that brain amyloid levels are manipulable because of a dynamic equilibrium between their synthesis from the amyloid precursor protein and their removal by amyloid-degrading enzymes (ADEs) providing a potential therapeutic strategy. Since the initial reports over a decade ago that two zinc metallopeptidases, insulin-degrading enzyme and neprilysin (NEP), contributed to amyloid degradation in the brain, there is now an embarras de richesses in relation to this category of enzymes, which currently number almost 20. These now include serine and cysteine proteinases, as well as numerous zinc peptidases. The experimental validation for each of these enzymes, and which to target, varies enormously but up-regulation of several of them individually in mouse models of Alzheimer's disease has proved effective in amyloid and plaque clearance, as well as cognitive enhancement. The relative status of each of these enzymes will be critically evaluated. NEP and its homologues, as well as insulin-degrading enzyme, remain as principal ADEs and recently discovered mechanisms of epigenetic regulation of NEP expression potentially open new avenues in manipulation of AD-related genes, including ADEs.

The impact of amyloid precursor protein signalling and histone deacetylase inhibition on neprilysin expression in human prostate cells.

The zinc metallopeptidase, neprilysin (NEP), is an endothelin-1 degrading enzyme whose expression is extensively downregulated in prostate cancer. The expression of NEP in neuronal cells is regulated by intramembrane proteolysis of the amyloid precursor protein (APP) through its intracellular domain (AICD) facilitating histone acetylation of the NEP promoter and gene transcription. The present study has examined whether similar mechanisms operate in prostate cell lines. The expression of APP and its processing enzymes (ß- and γ-secretases) was examined in a number of prostate cell lines, and the effect of γ-secretase inhibition was explored on NEP expression and activity. The potential interaction of AICD with the NEP promoter was examined by chromatin immunoprecipitation. Our results indicated that all key components involved in APP processing were expressed in prostate cancer cell lines but suppression of AICD production using a γ-secretase inhibitor did not decrease NEP expression and activity, and no direct AICD-NEP promoter interaction could be detected. However, histone deacetylase inhibitors (valproate and trichostatin A) caused a 2- to 3-fold increase in NEP expression in PC-3 cells, and combinatorial treatment with the DNA demethylating agent, AzaC, further increased NEP expression levels. Although AICD is detectable in prostate cell lines, it does not appear to regulate NEP by AICD-mediated signalling. Apart from promoter de-methylation, the data suggest that histone acetylation may facilitate partial re-activation of NEP expression in advanced prostate cancer cells. Upregulation of this tumour-suppressing protein may provide a novel therapeutic strategy in prostate cancer.

The transcriptionally active amyloid precursor protein (APP) intracellular domain is preferentially produced from the 695 isoform of APP in a {beta}-secretase-dependent pathway.

Amyloidogenic processing of the amyloid precursor protein (APP) by ß- and γ-secretases generates several biologically active products, including amyloid-ß (Aß) and the APP intracellular domain (AICD). AICD regulates transcription of several neuronal genes, especially the Aß-degrading enzyme, neprilysin (NEP). APP exists in several alternatively spliced isoforms, APP(695), APP(751), and APP(770). We have examined whether each isoform can contribute to AICD generation and hence up-regulation of NEP expression. Using SH-SY5Y neuronal cells stably expressing each of the APP isoforms, we observed that only APP(695) up-regulated nuclear AICD levels (9-fold) and NEP expression (6-fold). Increased NEP expression was abolished by a ß- or γ-secretase inhibitor but not an α-secretase inhibitor. This correlated with a marked increase in both Aß(1-40) and Aß(1-42) in APP(695) cells as compared with APP(751) or APP(770) cells. Similar phenomena were observed in Neuro2a but not HEK293 cells. SH-SY5Y cells expressing the Swedish mutant of APP(695) also showed an increase in Aß levels and NEP expression as compared with wild-type APP(695) cells. Chromatin immunoprecipitation revealed that AICD was associated with the NEP promoter in APP(695), Neuro2a, and APP(Swe) cells but not APP(751) nor APP(770) cells where AICD was replaced by histone deacetylase 1 (HDAC1). AICD occupancy of the NEP promoter was replaced by HDAC1 after treatment of the APP(695) cells with a ß- but not an α-secretase inhibitor. The increased AICD and NEP levels were significantly reduced in cholesterol-depleted APP(695) cells. In conclusion, Aß and functional AICD appear to be preferentially synthesized through ß-secretase action on APP(695).

Calcium-independent phospholipase A2 in rabbit ventricular myocytes.

We have previously reported that the majority of phospholipase A2 (PLA2) activity in rabbit ventricular myocytes is membrane-associated, calcium-independent (iPLA2), selective for arachidonylated plasmalogen phospholipids and inhibited by the iPLA2-selective inhibitor bromoenol lactone (BEL). Here, we identified the presence of iPLA2 in rabbit ventricular myocytes, determined the full length sequences for rabbit iPLA2beta and iPLA2gamma and compared their homology to the human isoforms. Rabbit iPLA2beta encoded a protein with a predicated molecular mass of 74 kDa that is 91% identical to the human iPLA2beta short isoform. Full length iPLA2gamma protein has a predicated molecular mass of 88 kDa and is 88% identical to the human isoform. Immunoblot analysis of iPLA2beta and gamma in membrane and cytosolic fractions from rabbit and human cardiac myocytes demonstrated a similar pattern of distribution with both isoforms present in the membrane fraction, but no detectable protein in the cytosol. Membrane-associated iPLA2 activity was inhibited preferentially by the R enantiomer of bromoenol lactone [(R)-BEL], indicating that the majority of activity is due to iPLA2gamma.

Phospholipase A2-catalyzed hydrolysis of plasmalogen phospholipids in thrombin-stimulated human platelets.

In the present study, phospholipase A(2) (PLA(2))-catalyzed hydrolysis of platelet membrane phospholipids was investigated by measuring PLA(2) activity, phospholipid hydrolysis, arachidonic acid release and choline lysophospholipid production in thrombin-stimulated human platelets. Thrombin-stimulated platelets demonstrated selective hydrolysis of arachidonylated plasmenylcholine and plasmenylethanolamine, with little change in diacyl phospholipids. Accelerated plasmalogen hydrolysis was accompanied by increased arachidonic acid and thromboxane B(2) release and increased lysoplasmenylcholine production. Thrombin stimulation caused an increase in PLA(2) activity measured in the cytosolic fraction with plasmenylcholine only; no increase in activity was measured with phosphatidylcholine. No change in membrane-associated PLA(2) activity was observed with either substrate tested. Pretreatment with the Ca(2+)-independent PLA(2)-selective inhibitor, bromoenol lactone, inhibited completely any thrombin-stimulated phospholipid hydrolysis. Thus, thrombin stimulation of human platelets activates a cytosolic PLA(2) that selectively hydrolyzes arachidonylated plasmalogen phospholipids.

Genomic analyses of bacterial respiratory and cytochrome c assembly systems: Bordetella as a model for the system II cytochrome c biogenesis pathway.

An analysis of thirty-three genomes of selected bacteria for the presence of specific respiratory pathways and cytochrome c biogenesis systems has led to observations on respiration and biogenesis. A table summarizing these results is presented. The data suggested that Bordetella pertussis would be an excellent genetic model to study the System II cytochrome c biogenesis pathway. These observations are discussed and the results of genetic studies on System II biogenesis in B. pertussis are presented as a case for the power of comparative genomics. System II is present in organisms as diverse as Helicobacter, Neisseria, Porphyromonas, mycobacteria, cyanobacteria, and plants (chloroplasts), indicating this pathway's prominence and that horizontal transfer of system II (and/or System I) must have occurred on multiple occasions.

Nuclear signalling by membrane protein intracellular domains: the AICD enigma.

Alzheimer's disease (AD) is a neurodegenerative illness and the leading cause of dementia in the elderly. The accumulation of amyloid-ß peptide (Aß) is a well-known pathological hallmark associated with the disease. However, Aß is only one of several metabolites produced by ß- and γ-secretase actions on the transmembrane protein, the amyloid precursor protein (APP). A proteolytic fragment termed the APP intracellular domain (AICD) is also produced. By analogy with the Notch signalling pathway, AICD has been proposed as a transcriptional regulator although its mechanism of action and the complement of genes regulated remain controversial. This review will focus on the contributions that studies of APP processing have brought to the understanding of a novel nuclear signalling pathway that may contribute to the pathology of AD and may provide new therapeutic opportunities.

An autophagy-enhancing drug promotes degradation of mutant alpha1-antitrypsin Z and reduces hepatic fibrosis.

In the classical form of alpha1-antitrypsin (AT) deficiency, a point mutation in AT alters the folding of a liver-derived secretory glycoprotein and renders it aggregation-prone. In addition to decreased serum concentrations of AT, the disorder is characterized by accumulation of the mutant alpha1-antitrypsin Z (ATZ) variant inside cells, causing hepatic fibrosis and/or carcinogenesis by a gain-of-toxic function mechanism. The proteasomal and autophagic pathways are known to mediate degradation of ATZ. Here we show that the autophagy-enhancing drug carbamazepine (CBZ) decreased the hepatic load of ATZ and hepatic fibrosis in a mouse model of AT deficiency-associated liver disease. These results provide a basis for testing CBZ, which has an extensive clinical safety profile, in patients with AT deficiency and also provide a proof of principle for therapeutic use of autophagy enhancers.

Identification of calcium-independent phospholipase A2gamma in mitochondria and its role in mitochondrial oxidative stress.

Oxidant-induced lipid peroxidation and cell death mediate pathologies associated with ischemia-reperfusion and inflammation. Our previous work in rabbit renal proximal tubular cells (RPTC) demonstrated that inhibition of Ca(2+)-independent phospholipase A(2) (iPLA(2)) potentiates oxidant-induced lipid peroxidation and necrosis, implicating iPLA(2) in phospholipid repair. This study was conducted to identify a RPTC mitochondrial PLA(2) and determine the role of PLA(2) in oxidant-induced mitochondrial dysfunction. iPLA(2) activity was detected in Percoll-purified rabbit renal cortex mitochondria (RCM) and in isolated mitochondrial inner membrane fractions from rabbit and human RCM. Immunoblot analysis and inhibitor sensitivity profiles revealed that iPLA(2)gamma is the RCM iPLA(2) activity. RCM iPLA(2) activity was enhanced in the presence of ATP and was blocked by the PKCepsilon V1-2 inhibitor. Oxidant-induced mitochondrial lipid peroxidation and swelling were accelerated by pretreatment with R-BEL, but not S-BEL. Furthermore, oxidant treatment of isolated RCM resulted in decreased iPLA(2)gamma activity. These results reveal that RCM iPLA(2) is iPLA(2)gamma, RCM iPLA(2)gamma is regulated by phosphorylation by PKCepsilon, iPLA(2)gamma protects RCM from oxidant-induced lipid peroxidation and dysfunction, and that a strategy to preserve or enhance iPLA(2)gamma activity may be of therapeutic benefit.

Regulator of G Signaling 16 is a marker for the distinct endoplasmic reticulum stress state associated with aggregated mutant alpha1-antitrypsin Z in the classical form of alpha1-antitrypsin deficiency.

In the classical form of alpha(1)-antitrypsin deficiency, a mutant protein accumulates in a polymerized form in the endoplasmic reticulum (ER) of liver cells causing liver damage and carcinogenesis by a gain-of-toxic function mechanism. Recent studies have indicated that the accumulation of mutant alpha(1)-antitrypsin Z in the ER specifically activates the autophagic response but not the unfolded protein response and that autophagy plays a critical role in disposal of insoluble alpha(1)-antitrypsin Z. In this study, we used genomic analysis of the liver in a novel transgenic mouse model with inducible expression to screen for changes in gene expression that would potentially define how the liver responds to accumulation of this mutant protein. There was no unfolded protein response. Of several distinct gene expression profiles, marked up-regulation of regulator of G signaling (RGS16) was particularly notable. RGS16 did not increase when model systems were exposed to classical inducers of ER stress, including tunicamycin and calcium ionophore, or when a nonpolymerogenic alpha(1)-antitrypsin mutant accumulated in the ER. RGS16 was up-regulated in livers from patients with alpha(1)-antitrypsin deficiency, and the degree of up-regulation correlated with the hepatic levels of insoluble alpha(1)-antitrypsin Z protein. Taken together, these results indicate that expression of RGS16 is an excellent marker for the distinct form of "ER stress" that occurs in alpha(1)-antitrypsin deficiency, presumably determined by the aggregation-prone properties of the mutant protein that characterizes the deficiency.

Activation of MAPKs in thrombin-stimulated ventricular myocytes is dependent on Ca2+-independent PLA2.

Thrombin stimulation of isolated rabbit ventricular myocytes activates a membrane-associated, Ca(2+)-independent PLA(2) (iPLA(2)) that selectively hydrolyzes plasmalogen phospholipids and results in increased production of arachidonic acid and lysoplasmenylcholine. To determine whether MAPK regulates myocardial iPLA(2) activity, we isolated ventricular myocytes from rabbit heart by collagenase digestion and pretreated them with MAPK inhibitors before stimulating them with thrombin. Pretreatment with PD-98059 to inhibit p42/44 MAPK or SB-203580 to inhibit p38 MAPK had no significant effect on thrombin-stimulated, membrane-associated iPLA(2) activity. Thrombin stimulation resulted in significant increases in both p42/44 and p38 MAPK activity after 2 min. Pretreatment with the iPLA(2)-selective inhibitor bromoenol lactone completely inhibited thrombin-stimulated MAPK activity, suggesting that activation of MAPKs was dependent on iPLA(2) activation. Ventricular myocyte MAPK activity was increased by incubation of the myocytes with lysoplasmenylcholine, a metabolite produced by iPLA(2)-catalyzed membrane plasmalogen phospholipid hydrolysis. Altogether, these data suggest that activation of MAPKs occurs downstream of and is dependent on iPLA(2) activation in thrombin-stimulated rabbit ventricular myocytes.

Mutations in cytochrome assembly and periplasmic redox pathways in Bordetella pertussis.

Transposon mutagenesis of Bordetella pertussis was used to discover mutations in the cytochrome c biogenesis pathway called system II. Using a tetramethyl-p-phenylenediamine cytochrome c oxidase screen, 27 oxidase-negative mutants were isolated and characterized. Nine mutants were still able to synthesize c-type cytochromes and possessed insertions in the genes for cytochrome c oxidase subunits (ctaC, -D, and -E), heme a biosynthesis (ctaB), assembly of cytochrome c oxidase (sco2), or ferrochelatase (hemZ). Eighteen mutants were unable to synthesize all c-type cytochromes. Seven of these had transposons in dipZ (dsbD), encoding the transmembrane thioreduction protein, and all seven mutants were corrected for cytochrome c assembly by exogenous dithiothreitol, which was consistent with the cytochrome c cysteinyl residues of the CXXCH motif requiring periplasmic reduction. The remaining 11 insertions were located in the ccsBA operon, suggesting that with the appropriate thiol-reducing environment, the CcsB and CcsA proteins comprise the entire system II biosynthetic pathway. Antiserum to CcsB was used to show that CcsB is absent in ccsA mutants, providing evidence for a stable CcsA-CcsB complex. No mutations were found in the genes necessary for disulfide bond formation (dsbA or dsbB). To examine whether the periplasmic disulfide bond pathway is required for cytochrome c biogenesis in B. pertussis, a targeted knockout was made in dsbB. The DsbB- mutant makes holocytochromes c like the wild type does and secretes and assembles the active periplasmic alkaline phosphatase. A dipZ mutant is not corrected by a dsbB mutation. Alternative mechanisms to oxidize disulfides in B. pertussis are analyzed and discussed.

Identification and distribution of endoplasmic reticulum iPLA2.

Our laboratory demonstrated that endoplasmic reticulum iPLA2 (ER-iPLA2) activity protects renal cells from oxidant-induced cell death and lipid peroxidation. The goals of this study were to determine the PLA2 isoform(s) responsible for ER-iPLA2 activity in different species and tissues. ER-iPLA2 activity was observed in microsomes from rabbit and rat kidney, heart, and brain as well as in human kidney (Caki-1 and HEK293) and glioblastoma (A172) cell lines. Reverse transcriptase-polymerase chain reaction results demonstrated the presence of iPLA2gamma (group VIB PLA2) message in all tissues tested. Immunoblot analysis and PLA2 inhibitor studies with methyl arachidonyl fluorophosphonate and enantiomers of bromoenol lactone demonstrated that the ER-iPLA2 in rabbit kidney and heart and rat kidney is iPLA2gamma. These results demonstrate the expression of ER-iPLA2gamma (group VIB) across species and tissues, and suggest that iPLA2gamma may play critical roles in oxidant-induced cell injury.