APP dimer formation is initiated in the endoplasmic reticulum and differs between APP isoforms

The amyloid precursor protein (APP) is part of a larger gene family, which has been found to form homo- or heterotypic complexes with its homologues, whereby the exact molecular mechanism and origin of dimer formation remains elusive. In order to assess the cellular location of dimerization, we have generated a cell culture model system in CHO-K1 cells, stably expressing human APP, harboring dilysine-based organelle sorting motifs [KKAA-endoplasmic reticulum (ER); KKFF-Golgi], accomplishing retention within early secretory compartments. We show that APP exists as disulfide-bonded dimers upon ER retention after it was isolated from cells, and analyzed by SDS-polyacrylamide gel electrophoresis under non-reducing conditions. In contrast, strong denaturing and reducing conditions, or deletion of the E1 domain, resulted in the disappearance of those dimers. Thus we provide first evidence that a fraction of APP can associate via intermolecular disulfide bonds, likely generated between cysteines located in the extracellular E1 domain. We particularly visualize APP dimerization itself and identified the ER as subcellular compartment of its origin using biochemical or split GFP approaches. Interestingly, we also found that minor amounts of SDS-resistant APP dimers were located to the cell surface, revealing that once generated in the oxidative environment of the ER, dimers remained stably associated during transport. In addition, we show that APP isoforms encompassing the Kunitz-type protease inhibitor (KPI) domain exhibit a strongly reduced ability to form cis-directed dimers in the ER, whereas trans-mediated cell aggregation of Drosophila Schneider S2-cells was isoform independent. Thus, suggesting that steric properties of KPI-APP might be the cause for weaker cis-interaction in the ER, compared to APP695. Finally, we provide evidence that APP/APLP1 heterointeractions are likewise initiated in the ER.     

Monogenetic determinants of Alzheimer's disease: APP mutations

Mutations within exons 16 and 17 of the β-amyloid precursor protein (APP) gene were the first known cause of familial Alzheimer's disease. These mutations are rare and have been reported in a handful of families exhibiting autosomal dominant inheritance of Alzheimer's disease with age of onset around 50 years. In vitro and in vivo studies have demonstrated that each of these mutations alters proteolytic processing of APP, resulting in an increase in the production of Aβ42, a highly fibrillogenic peptide, that spontaneously aggregates and deposits in the brain. Transgenic mice carrying a mutant human APP gene also show age-dependent β-amyloid (Aβ) deposition in the brain. The rate of deposition in these mice can be modified by apolipoprotein E expression.     

Monogenic determinants of familial Alzheimer's disease: presenilin-2 mutations

Presenilin-2 (PS2) is one of three genes [amyloid precursor protein (APP), presenilin-1 (PS1) and PS2] shown to cause familial Alzheimer's disease (FAD), and is highly homologous to PS1. Currently demonstrated functions of PS2 include interactions with APP and Aβ, and participation in apoptotic pathways. PS2 FAD mutations influence APP processing in a manner predicted to promote amyloid formation and also enhance the proapoptotic effect of wild-type PS2. Other possible functions of PS2 are related to its homology to Notch pathway genes in Caenorhabditis elegans, suggesting it may have a developmental role. PS2-associated AD is the most reminiscent of the sporadic form of the disease in terms of older age of onset and longer disease duration. Since PS2 mutations are incompletely penetrant and age of onset in carriers is highly variable (40 – 88 years), elucidation of PS2 mechanisms may reveal factors which modify AD and are therapeutically relevant to sporadic AD.     

Human and yeast ζ-crystallins bind AU-rich elements in RNA

ζ-crystallins constitute a family of proteins with NADPH:quinone reductase activity found initially in mammalian lenses but now known to be present in many other organisms and tissues. Few proteins from this family have been characterized, and their function remains unclear. In the present work, ζ-crystallins from human and yeast (Zta1p) were expressed, purified and characterized. Both enzymes are able to reduce ortho-quinones in the presence of NADPH but are not active with 2-alkenals. Deletion of the ZTA1 gene makes yeast more sensitive to menadione and hydrogen peroxide, suggesting a role in the oxidative stress response. The human and yeast enzymes specifically bind to adenine-uracil rich elements (ARE) in RNA, indicating that both enzymes are ARE-binding proteins and that this property has been conserved in ζ-crystallins throughout evolution. This supports a role for ζ-crystallins as trans-acting factors that could regulate the turnover of certain mRNAs. Received 21 February 2007; received after revision 16 April 2007; accepted 23 April 2007 M. R. Fernández, S. Porté: These authors contributed equally to this work.     

Interaction of aging-associated signaling cascades: Inhibition of NF-κB signaling by longevity factors FoxOs and SIRT1

Research on aging in model organisms has revealed different molecular mechanisms involved in the regulation of the lifespan. Studies on Saccharomyces cerevisiae have highlighted the role of the Sir2 family of genes, human Sirtuin homologs, as the longevity factors. In Caenorhabditis elegans, the daf-16 gene, a mammalian homolog of FoxO genes, was shown to function as a longevity gene. A wide array of studies has provided evidence for a role of the activation of innate immunity during aging process in mammals. This process has been called inflamm-aging. The master regulator of innate immunity is the NF-κB system. In this review, we focus on the several interactions of aging-associated signaling cascades regulated either by Sirtuins and FoxOs or NF-κB signaling pathways. We provide evidence that signaling via the longevity factors of FoxOs and SIRT1 can inhibit NF-κB signaling and simultaneously protect against inflamm-aging process. Received 4 October 2007; received after revision 7 November 2007; accepted 9 November 2007     

DyP-type peroxidases comprise a novel heme peroxidase family

Dye-decolorizing peroxidase (DyP) is produced by a basidiomycete (Thanatephorus cucumeris Dec 1) and is a member of a novel heme peroxidase family (DyP-type peroxidase family) that appears to be distinct from general peroxidases. Thus far, 80 putative members of this family have been registered in the PeroxiBase database (http://peroxibase.isbsib.ch/) and more than 400 homologous proteins have been detected via PSI-BLAST search. Although few studies have characterized the function and structure of these proteins, they appear to be bifunctional enzymes with hydrolase or oxygenase, as well as typical peroxidase activities. DyP-type peroxidase family suggests an ancient root compared with other general peroxidases because of their widespread distribution in the living world. In this review, firstly, an outline of the characteristics of DyP from T. cucumeris is presented and then interesting characteristics of the DyP-type peroxidase family are discussed. Received 14 October 2008; received after revision 12 November 2008; accepted 17 November 2008     

The BAG proteins: a ubiquitous family of chaperone regulators

The BAG (Bcl-2 associated athanogene) family is a multifunctional group of proteins that perform diverse functions ranging from apoptosis to tumorigenesis. An evolutionarily conserved group, these proteins are distinguished by a common conserved region known as the BAG domain. BAG genes have been found in yeasts, plants, and animals, and are believed to function as adapter proteins forming complexes with signaling molecules and molecular chaperones. In humans, a role for BAG proteins has been suggested in carcinogenesis, HIV infection, and Parkinson’s disease. These proteins are therefore potential therapeutic targets, and their expression in cells may serve as a predictive tool for such diseases. In plants, the Arabidopsis thaliana genome contains seven homologs of the BAG family, including four with domain organization similar to animal BAGs. Three members contain a calmodulin-binding domain possibly reflecting differences between plant and animal programmed cell death. This review summarizes current understanding of BAG proteins in both animals and plants. Received 21 November 2007; received after revision 17 December 2007; accepted 2 January 2008     

The role of thrombospondin-1 in apoptosis

The thrombospondins are a family of extracellular proteins that participate in cell-to-cell and cell-to-matrix communication. They regulate cellular phenotype during tissue genesis and repair. Five family members, each representing a separate gene product, probably exist in most vertebrate species. Like most extracellular proteins, the thrombospondins are composed of several structural domains that are responsible for the numerous biological functions that have been described for this protein family. Considerable progress has been made towards understanding the function of thrombospondins. The role of thrombospondin in the process of apoptosis or programmed cell death has recently come into focus. In this review we will concentrate on the role of thrombospondin-1 in the broad field of apoptotis research. Received 5 December 2001; received after revision 28 March 2002; accepted 28 March 2002     

Diversity of Cl− Channels

Cl⁻ channels are widely found anion pores that are regulated by a variety of signals and that play various roles. On the basis of molecular biologic findings, ligand-gated Cl⁻ channels in synapses, cystic fibrosis transmembrane conductors (CFTRs) and ClC channel types have been established, followed by bestrophin and possibly by tweety, which encode Ca²⁺-activated Cl⁻ channels. The ClC family has been shown to possess a variety of functions, including stabilization of membrane potential, excitation, cellvolume regulation, fluid transport, protein degradation in endosomal vesicles and possibly cell growth. The molecular structure of Cl⁻ channel types varies from 1 to 12 transmembrane segments. By means of computer-based prediction, functional Cl⁻ channels have been synthesized artificially, revealing that many possible ion pores are hidden in channel, transporter or unidentified hydrophobic membrane proteins. Thus, novel Cl⁻-conducting pores may be occasionally discovered, and evidence from molecular biologic studies will clarify their physiologic and pathophysiologic roles. Received 28 July 2005; received after revision 25 August 2005; accepted 21 September 2005     

Bardet-Biedl syndrome: an emerging pathomechanism of intracellular transport

From a handful of uncloned genetic loci 6 years ago, great strides have been made in understanding the genetic and molecular aetiology of Bardet-Biedl syndrome (BBS), a rare pleiotropic disorder characterised by a multitude of symptoms, including obesity, retinal degeneration and cystic kidneys. Presently, 11 BBS genes have been cloned, with the likelihood that yet more BBS genes remain undiscovered. In 2003, a major breakthrough was made when it was shown that BBS is likely caused by defects in basal bodies and/or primary cilia. Since then, studies in numerous animal models of BBS have corroborated the initial findings and, in addition, have further refined the specific functions of BBS proteins. These include roles in establishing planar cell polarity (noncanonical Wnt signaling) in mice and zebrafish, modulating intraflagellar transport and lipid homeostasis in worms, and regulating intracellular trafficking and centrosomal functions in zebrafish and human tissue culture cells. From these discoveries, a common theme has emerged, namely that the primary function of BBS proteins may be to mediate and regulate microtubule-based intracellular transport processes. Received 20 April 2006; received after revision 30 May 2006; accepted 15 June 2006     

RNA editing in plant organelles: machinery, physiological function and evolution

In plants, RNA editing is a process for converting a specific nucleotide of RNA from C to U and less frequently from U to C in mitochondria and plastids. To specify the site of editing, the cis-element adjacent to the editing site functions as a binding site for the trans-acting factor. Genetic approaches using Arabidopsis thaliana have clarified that a member of the protein family with pentatricopeptide repeat (PPR) motifs is essential for RNA editing to generate a translational initiation codon of the chloroplast ndhD gene. The PPR motif is a highly degenerate unit of 35 amino acids and appears as tandem repeats in proteins that are involved in RNA maturation steps in mitochondria and plastids. The Arabidopsis genome encodes approximately 450 members of the PPR family, some of which possibly function as trans-acting factors binding the cis-elements of the RNA editing sites to facilitate access of an unidentified RNA editing enzyme. Based on this breakthrough in the research on plant RNA editing, I would like to discuss the possible steps of co-evolution of RNA editing events and PPR proteins. Received 30 September 2005; received after revision 5 November 2005; accepted 28 November 2005     

Biological activity and pathological implications of misfolded proteins

The physiological metabolism of proteins guarantees that different cellular compartments contain the appropriate concentration of proteins to perform their biological functions and, after a variable period of wear and tear, mediates their natural catabolism. The equilibrium between protein synthesis and catabolism ensures an effective turnover, but hereditary or acquired abnormalities of protein structure can provoke a premature loss of biological function, an accelerated catabolism and diseases caused by the loss of an irreplaceable function. In certain proteins, abnormal structure and metabolism are associated with a strong tendency to self-aggregation into a polymeric fibrillar structure, and in these cases the disease is not principally caused by the loss of an irreplaceable function but by the action of this new biological entity. Amyloid fibrils are an apparently inert, insoluble, mainly extracellular protein polymer that kills the cell without tissue necrosis but by activation of the apoptotic mechanism. We analyzed the data reported so far on the structural and functional properties of four prototypic proteins with well-known biological functions (lysozyme, transthyretin, β2-microglobulin and apolipoprotein AI) that are able to create amyloid fibrils under certain conditions, with the perspective of evaluating whether the achievement of biological function favors or inhibits the process of fibril formation. Furthermore, studying the biological functions carried out by amyloid fibrils reveals new types of protein-protein interactions in the transmission of messages to cells and may provide new ideas for effective therapeutic strategies. Received 9 November 1998; received after revision 15 January 1999; accepted 15 January 1999     

<Emphasis Type="Italic">O</Emphasis>-fucose modifications of epidermal growth factor-like repeats and thrombospondin type 1 repeats: unusual modifications in unusual places

Recent discoveries revealing that carbohydrate modifications play critical roles in a wide variety of biological processes have brought wide recognition to the field of glycobiology. Growing attention has focused on the function of unusual O-linked carbohydrate modifications such as O-fucose. O-fucose modifications have been described in several different protein contexts, including epidermal growth factor-like repeats and thrombospondin type 1 repeats. The O-fucose modifications on thrombospondin type 1 repeats have only recently been described, but the site of modification occurs in a region proposed to play a role in cell adhesion. O-fucose modifications on epidermal growth factor-like repeats have been described as important players in several signal transduction systems. For instance, Notch, a cell-surface signaling receptor required for many developmental events, bears multiple O-fucose saccharides on the epidermal growth factor-like repeat of its extracellular domain. The O-fucose moieties serve as a substrate for the β1,3 N-acetylglucosaminyltransferase activity of Fringe, a known modifier of Notch function. The alteration of O-fucose structures by Fringe influences the ability of Notch ligands to activate the receptor and provides a means to regulate Notch signaling. Thus, O-fucose and Fringe provide a clear example of how carbohydrate modifications can have direct functional consequences on the proteins they modify. RID="*" ID="*"Corresponding author.     

Analysis of novel endosome-to-Golgi retrieval genes reveals a role for PLD3 in regulating endosomal protein sorting and amyloid precursor protein processing

The processing of amyloid precursor protein (APP) to the neurotoxic pro-aggregatory Aβ peptide is controlled by the mechanisms that govern the trafficking and localisation of APP. We hypothesised that genes involved in endosomal protein sorting could play an important role in regulating APP processing and, therefore, analysed ~ 40 novel endosome-to-Golgi retrieval genes previously identified in a genome-wide siRNA screen. We report that phospholipase D3 (PLD3), a type II membrane protein, functions in endosomal protein sorting and plays an important role in regulating APP processing. PLD3 co-localises with APP in endosomes and loss of PLD3 function results in reduced endosomal tubules, impaired trafficking of several membrane proteins and reduced association of sortilin-like 1 with APP.     

Heat shock protein gene expression during Xenopus development

Stress-induced heat shock protein gene expression is developmentally regulated during early embryogen esis of the frog, Xenopus laevis. For example, a number of heat shock protein genes, such as hsp70, hsp90, and ubiquitin are not heat-inducible until after the midblastula stage of embryogenesis. Furthermore, the family of small heat shock protein genes, hsp30, are differentially expressed after the midblastula stage as well as being regulated at the level of mRNA stability. Many of these stress proteins are also synthesized constitutively during oogenesis and embryogenesis during which they may act as molecular chaperones as well as being involved in sequestering proteins in an inactive state until required by the developing embryo. Furthermore the induction of these stress protein genes has been correlated with enhanced thermoresistance. During stressful conditions heat shock proteins probably prevent aggregation or misfolding of damaged protei ns within the embryo.     

Integrins and cardiovascular disease

Cardiovascular diseases involve abnormal cell-cell interactions leading to the development of atherosclerotic plaque, which when ruptured causes massive platelet activation and thrombus formation. Parts of a loose thrombus may detach to form an embolus, blocking circulation at a more distant point. The integrins are a family of adhesive cell receptors interacting with adhesive proteins or with counterreceptors on other cells. There is now solid evidence that the major integrin on platelets, the fibrinogen receptor α _IIb  β ₃ , has an important role in several aspects of cardiovascular diseases and that its regulated inhibition leads to a reduction in incidence and mortality due to these disorders. The development of α _IIb  β ₃ inhibitors is an important strategy of many pharmaceutical companies which foresee a large market for the treatment of acute conditions in surgery, the symptoms of chronic conditions and, it is hoped, maybe even the successful prophylaxis of these conditions. Although all the associated problems have not been solved, the undoubted improvements in patient care resulting from the first of these treatments in the clinic have stimulated further research on the role of integrins on other vascular cells in these processes and in the search for new inhibitors. Both the development of specific inhibitors and of mice with specific integrin subunit genes ablated have contributed to a better understanding of the function of integrins in development of the cardiovascular system.