The antigen-binding fragment of human gamma immunoglobulin prevents amyloid β-peptide folding into β-sheet to form oligomers

// Victoria Valls-Comamala 1 , Biuse Guivernau 1 , Jaume Bonet 2 , Marta Puig 1 , Alex Peralvarez-Marin 3 , Ernest Palomer 1 , Xavier Fernandez-Busquets 4, 5 , Xavier Altafaj 6 , Marta Tajes 7 , Albert Puig-Pijoan 8 , Ruben Vicente 1 , Baldomero Oliva 2 and Francisco J. Munoz 1 1 Laboratory of Molecular Physiology, Faculty of Health and Life Sciences, Universitat Pompeu Fabra, Barcelona, Spain 2 Laboratory of Structural Bioinformatics (GRIB), Faculty of Health and Life Sciences, Universitat Pompeu Fabra, Barcelona, Spain 3 Unitat de Biofisica, Departament de Bioquimica i de Biologia Molecular, Facultat de Medicina, Universitat Autonoma de Barcelona, Barcelona, Spain 4 Institute for Bioengineering of Catalonia (IBEC), Barcelona, Spain 5 ISGlobal, Barcelona Centre for International Health Research, Hospital Clinic-Universitat de Barcelona, Barcelona, Spain 6 Bellvitge Biomedical Research Institute (IDIBELL) - Unit of Neuropharmacology and Pain, University of Barcelona, Barcelona, Spain 7 Heart Diseases Biomedical Research Group, IMIM-Hospital del Mar Medical Research Institute, Barcelona, Spain 8 Servei de Neurologia, Hospital del Mar-IMIM-Parc de Salut Mar, Barcelona, Spain Correspondence to: Francisco J. Munoz, email: paco.munoz@upf.edu Keywords: Alzheimer’s disease, amyloid, immunoglobulin, Fab, oligomers Received: December 22, 2016      Accepted: March 22, 2017      Published: April 13, 2017 ABSTRACT The amyloid beta-peptide (Aβ) plays a leading role in Alzheimer’s disease (AD) physiopathology. Even though monomeric forms of Aβ are harmless to cells, Aβ can aggregate into β-sheet oligomers and fibrils, which are both neurotoxic. Therefore, one of the main therapeutic approaches to cure or delay AD onset and progression is targeting Aβ aggregation. In the present study, we show that a pool of human gamma immunoglobulins (IgG) protected cortical neurons from the challenge with Aβ oligomers, as assayed by MTT reduction, caspase-3 activation and cytoskeleton integrity. In addition, we report the inhibitory effect of IgG on Aβ aggregation, as shown by Thioflavin T assay, size exclusion chromatography and atomic force microscopy. Similar results were obtained with Palivizumab, a human anti-sincitial virus antibody. In order to dissect the important domains, we cleaved the pool of human IgG with papain to obtain Fab and Fc fragments. Using these cleaved fragments, we functionally identified Fab as the immunoglobulin fragment inhibiting Aβ aggregation, a result that was further confirmed by an in silico structural model. Interestingly, bioinformatic tools show a highly conserved structure able to bind amyloid in the Fab region. Overall, our data strongly support the inhibitory effect of human IgG on Aβ aggregation and its neuroprotective role.