Labelled Tableaux For Non-Normal Modal Logics

In this paper we show how to extend KEM, a tableaux-like proof system for normal modal logic, in order to deal with classes of non-normal modal logic, such as monotonic and regular, in a uniform and modular way

Extracellular interface between APP and Nicastrin regulates Aβ length and response to γ-secretase modulators

γ‐Secretase complexes (GSECs) are multimeric membrane proteases involved in a variety of physiological processes and linked to Alzheimer's disease (AD). Presenilin (PSEN, catalytic subunit), Nicastrin (NCT), Presenilin Enhancer 2 (PEN‐2), and Anterior Pharynx Defective 1 (APH1) are the essential subunits of GSECs. Mutations in PSEN and the Amyloid Precursor Protein (APP) cause early‐onset AD. GSECs successively cut APP to generate amyloid‐β (Aβ) peptides of various lengths. AD‐causing mutations destabilize GSEC‐APP/Aβ_{n} interactions and thus enhance the production of longer Aβs, which elicit neurotoxic events underlying pathogenesis. Here, we investigated the molecular strategies that anchor GSEC and APP/Aβ_{n} during the sequential proteolysis. Our studies reveal that a direct interaction between NCT ectodomain and APP_{C99} influences the stability of GSEC‐Aβn assemblies and thereby modulates Aβ length. The data suggest a potential link between single‐nucleotide variants in NCSTN and AD risk. Furthermore, our work indicates that an extracellular interface between the protease (NCT, PSEN) and the substrate (APP) represents the target for compounds (GSMs) modulating Aβ length. Our findings may guide future rationale‐based drug discovery efforts

Export-deficient monoubiquitinated PEX5 triggers peroxisome removal in SV40 large T antigen-transformed mouse embryonic fibroblasts

Peroxisomes are ubiquitous cell organelles essential for human health. To maintain a healthy cellular environment, dysfunctional and superfluous peroxisomes need to be selectively removed. Although emerging evidence suggests that peroxisomes are mainly degraded by pexophagy, little is known about the triggers and molecular mechanisms underlying this process in mammalian cells. In this study, we show that PEX5 proteins fused to a bulky C-terminal tag trigger peroxisome degradation in SV40 large T antigen-transformed mouse embryonic fibroblasts. In addition, we provide evidence that this process is autophagy-dependent and requires monoubiquitination of the N-terminal cysteine residue that marks PEX5 for recycling. As our findings also demonstrate that the addition of a bulky tag to the C terminus of PEX5 does not interfere with PEX5 monoubiquitination but strongly inhibits its export from the peroxisomal membrane, we hypothesize that such a tag mimics a cargo protein that cannot be released from PEX5, thus keeping monoubiquitinated PEX5 at the membrane for a sufficiently long time to be recognized by the autophagic machinery. This in turn suggests that monoubiquitination of the N-terminal cysteine of peroxisomeassociated PEX5 not only functions to recycle the peroxin back to the cytosol, but also serves as a quality control mechanism to eliminate peroxisomes with a defective protein import machinery.This work was supported by grants from the ’Fonds voor Wetenschappelijk Onderzoek-Vlaanderen (Onderzoeksprojecten G.0754.09 and G095315N)’ (to MF and PVV), the KU Leuven (OT/09/045, OT/14/100, and DBOF/10/059) (to MF and PVV), and by FEDER funds through the Operational Competi-tiveness Program, COMPETE, and by national funds through FCT, Fundação para a Ciência e a Tecnologia, under the projects FCOMP-01–0124-FEDER-019731 (PTDC/BIA-BCM/118577/2010) and FCOMP-01–0124-FEDER-022718 (PEst-C/SAU/LA0002/2011) (to JEA). MN was supported by a FLOF fellow-ship from the Department of Cellular and Molecular Medicine (KU Leuven). TF was supported by Fundação para a Ciência e aTecnologia, Programa Operacional Potencial Humano do QREN, and Fundo Social Europeu

Mitochondrial fission factor (MFF) is a critical regulator of peroxisome maturation

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordData availability: The research data supporting this publication are provided within this paper and as supplementary information.Peroxisomes are highly dynamic subcellular compartments with important functions in lipid and ROS metabolism. Impaired peroxisomal function can lead to severe metabolic disorders with developmental defects and neurological abnormalities. Recently, a new group of disorders has been identified, characterised by defects in the membrane dynamics and division of peroxisomes rather than by loss of metabolic functions. However, the contribution of impaired peroxisome plasticity to the pathophysiology of those disorders is not well understood. Mitochondrial fission factor (MFF) is a key component of both the peroxisomal and mitochondrial division machinery. Patients with MFF deficiency present with developmental and neurological abnormalities. Peroxisomes (and mitochondria) in patient fibroblasts are highly elongated as a result of impaired organelle division. The majority of studies into MFF-deficiency have focused on mitochondrial dysfunction, but the contribution of peroxisomal alterations to the pathophysiology is largely unknown. Here, we show that MFF deficiency does not cause alterations to overall peroxisomal biochemical function. However, loss of MFF results in reduced import-competency of the peroxisomal compartment and leads to the accumulation of pre-peroxisomal membrane structures. We show that peroxisomes in MFF-deficient cells display alterations in peroxisomal redox state and intra-peroxisomal pH. Removal of elongated peroxisomes through induction of autophagic processes is not impaired. A mathematical model describing key processes involved in peroxisome dynamics sheds further light into the physical processes disturbed in MFF-deficient cells. The consequences of our findings for the pathophysiology of MFF-deficiency and related disorders with impaired peroxisome plasticity are discussed.Biotechnology & Biological Sciences Research Council (BBSRC)European Union Horizon 2020Research Foundation – FlandersGerman Research Foundation (DFG)Medical Faculty Mannheim (MEAMEDMA)Medical Research Council (MRC)Wellcome TrustKU LeuvenZellweger UKSidney Perry FoundationDevon Educational Trus

Alzheimer's-Causing Mutations Shift Aβ Length by Destabilizing γ-Secretase-Aβn Interactions

Alzheimer's disease (AD)-linked mutations in Presenilins (PSEN) and the amyloid precursor protein (APP) lead to production of longer amyloidogenic Aβ peptides. The shift in Aβ length is fundamental to the disease; however, the underlying mechanism remains elusive. Here, we show that substrate shortening progressively destabilizes the consecutive enzyme-substrate (E-S) complexes that characterize the sequential γ-secretase processing of APP. Remarkably, pathogenic PSEN or APP mutations further destabilize labile E-S complexes and thereby promote generation of longer Aβ peptides. Similarly, destabilization of wild-type E-S complexes by temperature, compounds, or detergent promotes release of amyloidogenic Aβ. In contrast, E-Aβn stabilizers increase γ-secretase processivity. Our work presents a unifying model for how PSEN or APP mutations enhance amyloidogenic Aβ production, suggests that environmental factors may increase AD risk, and provides the theoretical basis for the development of γ-secretase/substrate stabilizing compounds for the prevention of AD

[An Anti-founded Universal Belief Class]

Mertens and Zamir have proposed the construction of a universal belief space which allows model games with incomplete information. We suggest here an alternative method for constructing such a universal space. Our construction is purely set theoretical and makes use of Aczel's theory of anti-founded sets. We also show that all Mertens and Zamir belief subspaces can be embedded in this alternative universe

La connaissance commune en logique modale

The problem of Common Knowledge will be considered in two classes of models: a class K* of Kripke models and a class S of Scott models. Two modal logic systems will be defined. Those systems, KC and MC, include an axiomatisation of Common Knowledge. We prove determination of each system by the corresponding class of models

Belief Closure - a Semantics of Common Knowledge for Modal Propositional Logic

The paper investigates the relations between iterate and fixed-point accounts of common belief and common knowledge, using the formal tools of epistemic modal logic. Its main logical contribution is to introduce and axiomatize the following (fixed-point) notion of common belief. We first define a proposition to be belief-closed if everybody believes it in every world where it is true. We then define a proposition to be common belief in a world if it is implied by a belief-closed proposition that everybody believes in that world. Using the belief closure semantics of common belief, the paper proves soundness and completeness theorems for modal logics of varying strength. The weakest system involves a monotonicity assumption on individual belief; the strongest system is based on S5. Axiomatizations of common knowledge are secured by adding the truth axiom to any system. The paper also discusses anticipations of the belief closure semantics in the economic and game-theoretic literatures

A Nonminimal But Very Weak Axiomatization of Common Belief

The paper introduces a modal logic system of individual and common belief which is shown to be sound and complete with respect to a version of Neighbourhood semantics. This axiomatization of common belief is the weakest of all those currently available: it dispenses with even the Monotonicity rule of individual belief. It is non-minimal in that it does not use just the Equivalence rule but the conjunction of the latter with the specially devised rule of C-Restricted Monotonicity