title: Calcium binding to gatekeeper residues flanking aggregation-prone segments underlies non-fibrillar amyloid traits in superoxide dismutase 1 (SOD1).
abstract: Calcium deregulation is a central feature among neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Calcium accumulates in the spinal and brain stem motor neurons of ALS patients triggering multiple pathophysiological processes which have been recently shown to include direct effects on the aggregation cascade of superoxide dismutase 1 (SOD1). SOD1 is a Cu/Zn enzyme whose demetallated form is implicated in ALS protein deposits, contributing to toxic gain of function phenotypes. Here we undertake a combined experimental and computational study aimed at establishing the molecular details underlying the regulatory effects of Ca(2+) over SOD1 aggregation potential. Isothermal titration calorimetry indicates entropy driven low affinity association of Ca(2+) ions to apo SOD1, at pH7.5 and 37°C. Molecular dynamics simulations denote a noticeable loss of native structure upon Ca(2+) association that is especially prominent at the zinc-binding and electrostatic loops, whose decoupling is known to expose the central SOD1 β-barrel triggering aggregation. Structural mapping of the preferential apo SOD1 Ca(2+) binding locations reveals that among the most frequent ligands for Ca(2+) are negatively-charged gatekeeper residues located in boundary positions with respect to segments highly prone to edge-to-edge aggregation. Calcium interactions thus diminish gatekeeping roles of these residues, by shielding repulsive interactions via stacking between aggregating β-sheets, partly blocking fibril formation and promoting amyloidogenic oligomers such as those found in ALS inclusions. Interestingly, many fALS mutations occur at these positions, disclosing how Ca(2+) interactions recreate effects similar to those of genetic defects, a finding with relevance to understand sporadic ALS pathomechanisms.
authors.affiliation: Centro de Física da Matéria Condensada, Universidade de Lisboa, Lisboa, Portugal; Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal. Electronic address: silvia@cii.fc.ul.pt.
authors.forename: Sílvia G
authors.initials: SG
authors.lastname: Estácio
authors.affiliation: Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal.
authors.forename: Sónia S
authors.initials: SS
authors.lastname: Leal
authors.affiliation: Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal.
authors.forename: Joana S
authors.initials: JS
authors.lastname: Cristóvão
authors.affiliation: Centro de Física da Matéria Condensada, Universidade de Lisboa, Lisboa, Portugal; Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal. Electronic address: patricia.fn.faisca@gmail.com.
authors.forename: Patrícia F N
authors.initials: PF
authors.lastname: Faísca
authors.affiliation: Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal. Electronic address: gomes@itqb.unl.pt.
authors.forename: Cláudio M
authors.initials: CM
authors.lastname: Gomes
mesh_terms: D000690:Amyotrophic Lateral Sclerosis / Q000209:etiology / Q000378:metabolism* / Q000473:pathology; D002118:Calcium / Q000378:metabolism*; D019277:Entropy; D006801:Humans; D056004:Molecular Dynamics Simulation; D009046:Motor Neurons / Q000737:chemistry / Q000473:pathology; D009154:Mutation; D066263:Protein Aggregation, Pathological / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D017433:Protein Structure, Secondary; D013482:Superoxide Dismutase / Q000737:chemistry* / Q000235:genetics; D000072105:Superoxide Dismutase-1
chemical_list: C000606290:SOD1 protein, human; D013482:Superoxide Dismutase; D000072105:Superoxide Dismutase-1; D002118:Calcium
keywords: Electrostatic interaction; Gatekeeping residue; Isothermal titration calorimetry; Molecular dynamics; Protein aggregation; Protein dynamics