ALS-linked P56S-VAPB, an aggregated loss-of-function mutant of VAPB, predisposes motor neurons to ER stress-related death by inducing aggregation of co-expressed wild-type VAPB

Authors

  • Hiroaki Suzuki,

    1. Department of Cell Biology and Neuroscience, KEIO University School of Medicine, Tokyo, Japan
    2. Department of Anatomy, KEIO University School of Medicine, Tokyo, Japan
    3. Japan Society for the Promotion of Science Research Fellow, Japan
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    • 1

      These two authors equally contributed to this study.

  • Kohsuke Kanekura,

    1. Department of Cell Biology and Neuroscience, KEIO University School of Medicine, Tokyo, Japan
    2. Department of Anatomy, KEIO University School of Medicine, Tokyo, Japan
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    • 1

      These two authors equally contributed to this study.

  • Timothy P. Levine,

    1. University College London Institute of Ophthalmology, London EC1V 9EL, England, UK
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  • Kenji Kohno,

    1. Graduate School of Biological Sciences, Nara Institute of Science and Technology (NAIST), Nara, Japan
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  • Vesa M. Olkkonen,

    1. National Public Health Institute and FIMM, Institute for Molecular Medicine Finland, Biomedicum, Helsinki, Finland
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  • Sadakazu Aiso,

    1. Department of Anatomy, KEIO University School of Medicine, Tokyo, Japan
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  • Masaaki Matsuoka

    1. Department of Cell Biology and Neuroscience, KEIO University School of Medicine, Tokyo, Japan
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Address correspondence and reprint requests to Drs K. Kanekura and M.Matsuoka, Department of Cell Biology and Neuroscience, KEIO University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. E-mail: kanekura@sc.itc.keio.ac.jp, sakimatu@sc.itc.keio.ac.jp

Abstract

A point mutation (P56S) in the vapb gene encoding an endoplasmic reticulum (ER)-integrated membrane protein [vesicle-associated membrane protein-associated protein B (VAPB)] causes autosomal-dominant amyotrophic lateral sclerosis. In our earlier study, we showed that VAPB may be involved in the IRE1/XBP1 signaling of the unfolded protein response, an ER reaction to inhibit accumulation of unfolded/misfolded proteins, while P56S-VAPB formed insoluble aggregates and lost the ability to mediate the pathway (loss-of-function), and suggested that P56S-VAPB promoted the aggregation of co-expressed wild-type (wt)-VAPB. In this study, a yeast inositol-auxotrophy assay has confirmed that P56S-VAPB is functionally a null mutant in vivo. The interaction between P56S-VAPB and wt-VAPB takes place with a high affinity through the major sperm protein domain in addition to the interaction through the C-terminal transmembrane domain. Consequently, wt-VAPB is speculated to preferentially interact with co-expressed P56S-VAPB, leading to the recruitment of wt-VAPB into cytosolic aggregates and the attenuation of its normal function. We have also found that expression of P56S-VAPB increases the vulnerability of NSC34 motoneuronal cells to ER stress-induced death. These results lead us to hypothesize that the total loss of VAPB function in unfolded protein response, induced by one P56S mutant allele, may contribute to the development of P56S-VAPB-induced amyotrophic lateral sclerosis.

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