Journal of Biological Chemistry
Volume 288, Issue 35, 30 August 2013, Pages 25219-25228
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Protein Structure and Folding
Calcium Ions Promote Superoxide Dismutase 1 (SOD1) Aggregation into Non-fibrillar Amyloid: A LINK TO TOXIC EFFECTS OF CALCIUM OVERLOAD IN AMYOTROPHIC LATERAL SCLEROSIS (ALS)?*

https://doi.org/10.1074/jbc.M113.470740Get rights and content
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Imbalance in metal ion homeostasis is a hallmark in neurodegenerative conditions involving protein deposition, and amyotrophic lateral sclerosis (ALS) is no exception. In particular, Ca2+ dysregulation has been shown to correlate with superoxide dismutase-1 (SOD1) aggregation in a cellular model of ALS. Here we present evidence that SOD1 aggregation is enhanced and modulated by Ca2+. We show that at physiological pH, Ca2+ induces conformational changes that increase SOD1 β-sheet content, as probed by far UV CD and attenuated total reflectance-FTIR, and enhances SOD1 hydrophobicity, as probed by ANS fluorescence emission. Moreover, dynamic light scattering analysis showed that Ca2+ boosts the onset of SOD1 aggregation. In agreement, Ca2+ decreases SOD1 critical concentration and nucleation time during aggregation kinetics, as evidenced by thioflavin T fluorescence emission. Attenuated total reflectance FTIR analysis showed that Ca2+ induced aggregates consisting preferentially of antiparallel β-sheets, thus suggesting a modulation effect on the aggregation pathway. Transmission electron microscopy and analysis with conformational anti-fibril and anti-oligomer antibodies showed that oligomers and amyloidogenic aggregates constitute the prevalent morphology of Ca2+-induced aggregates, thus indicating that Ca2+ diverts SOD1 aggregation from fibrils toward amorphous aggregates. Interestingly, the same heterogeneity of conformations is found in ALS-derived protein inclusions. We thus hypothesize that transient variations and dysregulation of cellular Ca2+ levels contribute to the formation of SOD1 aggregates in ALS patients. In this scenario, Ca2+ may be considered as a pathogenic effector in the formation of ALS proteinaceous inclusions.

Background: SOD1-enriched protein inclusions and Ca2+ overload are hallmarks in ALS-affected motor neurons. Ca2+ burden correlates with SOD1 aggregation in cellular models.

Results: Ca2+ induces conformational changes that enhance and shift SOD1 aggregation from fibrils toward amorphous aggregates.

Conclusion: SOD1 aggregation is enhanced and modulated by Ca2+.

Significance: Ca2+ can behave as a pathogenic effector in the formation of ALS proteinaceous inclusions.

Amyloid
Amyotrophic Lateral Sclerosis (Lou Gehrig's Disease)
Biophysics
Calcium
Circular Dichroism (CD)
Electron Microscopy (EM)
Infrared Spectroscopy
Neurodegenerative Diseases
Protein Aggregation
Superoxide Dismutase (SOD)

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*

This work was supported by the Fundação para a Ciência e Tecnologia (FCT/MCTES, Portugal) through Research Grants PTDC/QUI-BIQ/117789/2010 and PTDC/EBB-BIO/117793/2010 (to C. M. G.), Postdoctoral Fellowship SFRH/BPD/47477/2008 (to S. S. L.), and Strategic Grant PEst-OE/EQB/LA0004/2011 (to the Instituto Tecnologia Química e Biológica Laboratório Associado).