N-terminally truncated Aβ4-x proteoforms and their relevance for Alzheimer’s pathophysiology

Background:The molecular heterogeneity of Alzheimer’s amyloid-β(Aβ)deposits extends well beyond the clas-sic Aβ1-40/Aβ1-42 dichotomy,substantially expanded by multiple post-translational modifications that increase the proteome diversity.Numerous truncated fragments consistently populate the brain Aβpeptidome,and their homeo-static regulation and potential contribution to disease pathogenesis are largely unknown.Aβ4-x peptides have been reported as major components of plaque cores and the limited studies available indicate their relative abundance in Alzheimer’s disease(AD).Methods:Immunohistochemistry was used to assess the topographic distribution of Aβ4-x species in well-char-acterized AD cases using custom-generated monoclonal antibody 18H6-specific for Aβ4-x species and blind for full-length Aβ1-40/Aβ1-42-in conjunction with thioflavin-S and antibodies recognizing Aβx-40 and Aβx-42 proteo-forms.Circular dichroism,thioflavin-T binding,and electron microscopy evaluated the biophysical and aggregation/oligomerization properties of full-length and truncated synthetic homologues,whereas stereotaxic intracerebral injections of monomeric and oligomeric radiolabeled homologues in wild-type mice were used to evaluate their brain clearance characteristics.Results:All types of amyloid deposits contained the probed Aβepitopes,albeit expressed in different proportions.Aβ4-x species showed preferential localization within thioflavin-S-positive cerebral amyloid angiopathy and cored plaques,strongly suggesting poor clearance characteristics and consistent with the reduced solubility and enhanced oligomerization of their synthetic homologues.In vivo clearance studies demonstrated a fast brain efflux of N-termi-nally truncated and full-length monomeric forms whereas their oligomeric counterparts-particularly of Aβ4-40 and Aβ4-42-consistently exhibited enhanced brain retention.Conclusions:The persistence of aggregation-prone Aβ4-x proteoforms likely contributes to the process of amyloid formation,self-perpetuating the amyloidogenic loop and exacerbating amyloid-mediated pathogenic pathways.