Presenilin-dependent regulated intramembrane proteolysis and γ-secretase activity

Inhibiting the production of amyloid-β by antagonising γ-secretase activity is currently being pursued as a therapeutic strategy for Alzheimer’s disease (AD). However, early pre-clinical studies have demonstrated that disruption of presenilin-dependent γ-secretase alters many presenilin-dependent processes, leading to early lethality in several AD model organisms. Subsequently, transgenic animal studies have highlighted several gross developmental side effects arising from presenilin deficiency. Partial knockdown or tissue-specific knockout of presenilins has identified the skin, vascular and immune systems as very sensitive to loss of presenilin functions. A more appreciative understanding of presenilin biology is therefore demanded if γ-secretase is to be pursued as a therapeutic target. Herein we review the current understanding of γ-secretase complexes; their regulation, abundance of interacting partners and diversity of substrates. We also discuss regulation of the γ-secretase complexes, with an emphasis on the functional role of presenilins in cell biology. Received 25 July 2008; received after revision 24 November 2008; accepted 10 December 2008     

The product of the γ-secretase processing of ephrinB2 regulates VE-cadherin complexes and angiogenesis

Presenilin-1 (PS1) gene encodes the catalytic component of γ-secretase, which proteolytically processes several type I transmembrane proteins. We here present evidence that the cytosolic peptide efnB2/CTF2 produced by the PS1/γ-secretase cleavage of efnB2 ligand promotes EphB4 receptor-dependent angiogenesis in vitro. EfnB2/CTF2 increases endothelial cell sprouting and tube formation, stimulates the formation of angiogenic complexes that include VE-cadherin, Raf-1 and Rok-α, and increases MLC2 phosphorylation. These functions are mediated by the PDZ-binding domain of efnB2. Acute downregulation of PS1 or inhibition of γ-secretase inhibits the angiogenic functions of EphB4 while absence of PS1 decreases the VE-cadherin angiogenic complexes of mouse brain. Our data reveal a mechanism by which PS1/γ-secretase regulates efnB2/EphB4 mediated angiogenesis.     

The molecular link between β- and γ-secretase activity on the amyloid β precursor protein

Alzheimer’s disease (AD) is characterized by an accumulation in the brain of amyloid β peptides (Aβ). The production of Aβ requires two sequential cleavages induced by β- and γ-secretases on the β-amyloid precursor protein (APP). Altered activity of these secretases is involved in the pathogenesis of AD. The expression and activity of β-secretase (BACE1) is augmented in the brain in late-onset sporadic AD. Mutant presenilin 1 (PS1), the major genetic defect of early-onset familial AD (FAD), alters the activity of γ-secretase, leading to increased production of Aβ42. Here we review the role of oxidative stress as a molecular link between the β- and the γ-secretase activities, and provide a mechanistic explanation of the pathogenesis of sporadic late-onset AD. We also discuss evidence for a role of the same mechanism in the pathogenesis of familial AD carrying PS1 mutations.     

Kinetic and structural evidence of the alkenal/one reductase specificity of human ζ-crystallin

Human ζ-crystallin is a Zn²⁺-lacking medium-chain dehydrogenase/reductase (MDR) included in the quinone oxidoreductase (QOR) family because of its activity with quinones. In the present work a novel enzymatic activity was characterized: the double bond α,β-hydrogenation of medium-chain 2-alkenals and 3-alkenones. The enzyme is especially active with lipid peroxidation products such as 4-hydroxyhexenal, and a role in their detoxification is discussed. This specificity is novel in the QOR family, and it is similar to that described in the distantly related alkenal/one reductase family. Moreover, we report the X-ray structure of ζ-crystallin, which represents the first structure solved for a tetrameric Zn²⁺-lacking MDR, and which allowed the identification of the active-site lining residues. Docking simulations suggest a role for Tyr53 and Tyr59 in catalysis. The kinetics of Tyr53Phe and Tyr59Phe mutants support the implication of Tyr53 in binding/catalysis of alkenal/one substrates, while Tyr59 is involved in the recognition of 4-OH-alkenals.     

Non-caspase proteases: triggers or amplifiers of apoptosis?

Caspases are the most important effectors of apoptosis, the major form of programmed cell death (PCD) in multicellular organisms. This is best reflected by the appearance of serious development defects in mice deficient for caspase-8, -9, and -3. Meanwhile, caspase-independent PCD, mediated by other proteases or signaling components has been described in numerous publications. Although we do not doubt that such cell death exists, we propose that it has evolved later during evolution and is most likely not designed to execute, but to amplify and speed-up caspase-dependent cell death. This review shall provide evidence for such a concept.     

Development of γ-glutamylcysteine synthetase and oxoprolinase in rat kidney

Summary -Glutamylcysteine synthetase is present in barely detectable amounts in foetal kidney. Its activity starts to increase in postnatal life. In contrast, oxoproline is already found in significant quantities in the foetal tissue. Both enzymes show marked elevation in activities during the weaning period.Acknowledgment. The work was supported by a research grant awarded by the Committee on Research and Conference Grants, University of Hong Kong.     

OCIAD2 activates γ-secretase to enhance amyloid β production by interacting with nicastrin

The gamma (γ)-secretase holoenzyme is composed of four core proteins and cleaves APP to generate amyloid beta (Aβ), a key molecule that causes major neurotoxicity during the early stage of Alzheimer’s disease (AD). However, despite its important role in Aβ production, little is known about the regulation of γ-secretase. OCIAD2, a novel modulator of γ-secretase that stimulates Aβ production, and which was isolated from a genome-wide functional screen using cell-based assays and a cDNA library comprising 6,178 genes. Ectopic expression of OCIAD2 enhanced Aβ production, while reduction of OCIAD2 expression suppressed it. OCIAD2 expression facilitated the formation of an active γ-secretase complex and enhanced subcellular localization of the enzyme components to lipid rafts. OCIAD2 interacted with nicastrin to stimulate γ-secretase activity. OCIAD2 also increased the interaction of nicastrin with C99 and stimulated APP processing via γ-secretase activation, but did not affect Notch processing. In addition, a cell-permeable Tat-OCIAD2 peptide that interfered with the interaction of OCIAD2 with nicastrin interrupted the γ-secretase-mediated AICD production. Finally, OCIAD2 expression was significantly elevated in the brain of AD patients and PDAPP mice. This study identifies OCIAD2 as a selective activator of γ-secretase to increase Aβ generation.     

The effects of γ-aminobutyric acid and picrotoxin on the junctional potential and the contraction of crayfish muscle

Résumé L'acide-amino-butyrique et la-alanine reproduisent l'action du transmetteur inhibiteur du système neuromusculaire de l'écrevisse. Ces deux acids aminés diminuent la contraction musculaire. L'acide-amino-butyrique réduit la différence de potentiel et augmente sa décomposition. L'action de ces acides aminés est bloquée par la picrotoxine comme l'est celle du transmetteur inhibiteur.

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The study was supported by Grants B-1089 and B-31 from the National Institute of Neurological Diseases and Blindness, United States Public Health Service. Some of this work was done at the Department of Zoology, Cornell University. We are grateful to Prof.S. C. Wang, Columbia University, for generous assistance.     

Human Genome and Diseases:¶Cellular and molecular aspects of Zellweger syndrome and other peroxisome biogenesis disorders

Peroxisomes are single-membrane-bound organelles present in virtually all eukaryotic cells. They are involved in numerous metabolic processes, both catabolic and anabolic, including β-oxidation of very long chain fatty acids, metabolism of hydrogen peroxide, plasmalogen biosynthesis and bile acid synthesis. In several genetic diseases, there is either isolated deficiency of a specific peroxisomal protein (single-protein deficiencies) or a defect in the formation of the organelle with loss of multiple peroxisomal functions (peroxisome biogenesis disorders). X-linked adrenoleukodystrophy is an example of the former, and the Zellweger spectrum of the latter. Peroxisome biogenesis disorders are inherited in an autosomal recessive manner and result from mutations in any of at least 12 PEX genes that encode peroxins. This article reviews the peroxisomal system, the clinical, biochemical and molecular aspects of peroxisomal disorders, and discusses recent scientific advances in the understanding of peroxisome biogenesis. Received 16 October 2001; received after revision 2 January 2002; accepted 3 January 2002     

Substrate binding and catalytic mechanism of class B β-lactamases: a molecular modelling study

Increased resistance to β-lactam antibiotics is mainly due to β-lactamases whose production by pathogenic bacteria makes their broad activity spectrum especially frightening. X-ray structures of several zinc β-lactamases have revealed the coordination of the two metal ions, but their mode of action remains unclear. Geometry optimisation of stable complexes along the reaction pathway of benzylpenicillin hydrolysis highlighted a proton shuttle occurring from D120 of the Bacillus cereus β-lactamase to the β-lactam nitrogen via Zn2 which is central to the network. First, the Zn1 ion has a structural role maintaining Zn-bound waters, WAT1 and WAT2, either directly or through the Zn1 tetrahedrally coordinated histidine ligands. The Zn2 ion has a more catalytic role, stabilising the tetrahedral intermediate, accepting the β-lactam nitrogen atom as a ligand. The role of Zn2 and the flexibility in the coordination geometry of both Zn ions is of crucial importance for catalysis. Received 14 August 2001; received after revision 19 October 2001; accepted 30 October 2001     

Properties of γ-glutamyltranspeptidase,and glutathione levels in rat mammary gland

Summary -Glutamyltranspeptidase activity and glutathione levels were studied in rat mammary gland during the lactogenic cycle; both increased during mid-lactation. The enzyme's specific activity with several amino acids showed that glutamine and methionine were the best substrates. Maleate decreased the transpeptidation reaction and increased the hydrolytic activity. These results suggest that -glutamyltranspeptidase from the mammary gland is similar to the enzyme described in other tissues in relation to these properties and the physiological role proposed in amino acids transport.     

On the glutamine and γ-aminobutyric acid contents of various regions of the cat brain

Résumé Les auteurs ont déterminé par la méthode de la chromatographie à deux dimensions, la teneur en glutamine et en acide -aminobutyrique des différentes parties du cerveau du chat. La concentration la plus élevée de l'acide -aminobutyrique a été trouvé dans l'hypothalamus, tandis que celle de la glutamine a été constatée dans le noyau caudé. Les concentrations les plus basses de ces deux protéines apparaissent dans la substance blanche du cerveau.     

Effect of hormones and cyclic AMP on γ-glutamyltranspeptidase activity of rat mammary gland explants

Summary -Glutamyl transpeptidase activity was assayed in midpregnant rat mammary gland explants at 14, 22 and 38 h, in the presence and absence of insulin, prolactin and corticosterone. With these 3 hormones the explants attained the characteristics of a secretory gland after 22 h of tissue culture, at which time the enzyme exhibited its maximal activity. The addition of dibutyryl cyclic AMP in the presence of the 3 hormones produced a significant increase in enzyme activity, which was maximal with a 1 mM concentration of the cyclic nucleotide. A similar effect was observed when theophylline or theophylline plus dibutyryl cyclic AMP were added to the culture medium.This work was supported by grant B1138-8333 from the Departamento de Desarrollo de la Investigación, Universidad de Chile.     

Structural changes of deoxyribonucleoprotein fibres following γ-irradiation under aerobic and hypoxic conditions

Summary The DNP fibres -irradiated under aerobic condition showed a reduction of their diameter, while no remarkable changes were observed in the DNP fibres irradiated under hypoxic condition by scanning electron microscopy.     

A new type of antigen induced by chemical linkage ofMycobacterium tuberculosis and γ-globulin

Zusammenfassung Mit Hilfe derPauly'schen Diazo-Reaktion wird humanes -Globulin an humanpathogene, virulenteMycobacterium tuberculosis-Stämme (M) gekoppelt. Das Zustandekommen der chemischen Bindung wurde mit gefärbten Präparaten, elektronenmikroskopischen und immunelektrophoretischen Untersuchungen der Bakterien bewiesen. Die mit-Globulin gekoppeltenM haben ihre Virulenz eingebüsst. In den Versuchstieren wurden Antikörper sowohl gegen -Globulin als auch gegenM gebildet. Durch ersteres wurde das Allergisierungsvermögen vonM gesteigert.     

Effects of γ-rays on DNA-cholesterol complex

Summary Effects of -rays on the DNA-cholesterol complex have been studied. Radiation-induced changes are found and compared with those on DNA-dye or drug complexes.Acknowledgment. Thanks are due to P. K. Roy, K. L. Bhattacharya and K. M. Khanna (Department of Physics, Birla Institute of Technology, Ranchi) for helpful discussions and the Chittaranjan Cancer Hospital (Calcutta) for irradiation facilities.     

Thyroid hormone controls carnitine status through modifications of γ-butyrobetaine hydroxylase activity and gene expression

The carnitine system plays a key role in β-oxidation of long-chain fatty acids by permitting their transport into the mitochondrial matrix. The effects of hypothyroidism and hyperthyroidism were studied on γ-butyrobetaine hydroxylase (BBH), the enzyme responsible for carnitine biosynthesis in the rat. In rat liver, BBH activity was decreased in the hypothyroid state and increased in hyperthyroid animals. The modifications in BBH activity correlated with changes in the enzyme Vmax values. These changes were shown to be related to hepatic BBH mRNA abundance. Thyroid hormones are known to interact with lipid metabolism, in particular by increasing long-chain fatty acid oxidation through activation of carnitine-dependent fatty acid import into mitochondria. Our study showed that thyroid hormones also increased carnitine bioavailability. Received 23 October 2001; received after revision 11 January 2002; accepted 15 January 2002     

Historical and other considerations regarding the crystal form of sodium-ammonium d- and l-tartrate,potassium d- and l-tartrate,potassium-ammonium d- and l-tartrate,and potassium racemate.—II

In 1914, the physics discipline had reached a very similar stage of development in Australia and Japan. A generation later the paths of development had considerably diverged. A systematic comparison of the evolution of physics in the two countries during these years identifies factors—political, economic and cultural—that led to this divergence, but it also uncovers a number of underlying parallels.