Enhanced Brain Retention of Aβ4‐x Proteoforms and its Contribution to Amyloid Deposits in Alzheimer’s Disease
Authors: Ghiso Jorge , Cabrera Erwin , Lashley Tammaryn , Rostagno Agueda 
Published: May 2022

The deposition of amyloid‐β (Aβ) in brain parenchyma and cerebral vasculature together with intraneuronal neurofibrillary tangles and the gradual loss of synapsis are central neuropathological hallmarks of Alzheimer’s disease (AD). Although it remains unclear what primarily triggers and drives the progression of AD, different lines of investigation point out to a central role of oligomeric Aβ conformations for the disease pathogenesis. Recent studies revealed that the molecular heterogeneity of Aβ deposits is significantly more complex than originally anticipated, extending well beyond the classic Aβ1‐40/Aβ1‐42 dichotomy and being substantially expanded by the presence of multiple post‐translational modifications that increase the proteome diversity. Although N‐terminal truncations at glutamates 3 and 11 and their subsequent cyclation to pyroglutamate are perhaps the most extensively studied modifications, many others have been reported, with species beginning at phenylalanine 4 and bearing an intact C‐terminus being especially relevant. Indeed, Aβ4‐42 peptides were reported as major components of amyloid plaque cores and the limited studies available seem to indicate their relative abundance in patients with AD, Down’s syndrome, and vascular dementia. In this work we assessed the topographic distribution of these species with in‐house‐generated anti‐Aβ4‐x monoclonal 18H6 – specific for Aβ species starting at position 4 and blind for the full‐length Aβ40 and Aβ42 – and used in conjunction with thioflavin S and antibodies specific for the C‐terminus 40 and 42 in confirmed AD cases. All types of Aβ deposits – parenchymal plaques, pre‐amyloid lesions and CAA – contained the Aβ epitopes tested, albeit expressed in different proportions. These co‐localizations were further stressed by combining double immunohistochemical analysis with Thioflavin S staining, showing the preferential location of Aβ4‐x species in CAA and cored plaques with fibrillar amyloid conformation, strongly suggesting poor clearance characteristics. To investigate the brain clearance of Aβ4‐x species in comparison with full‐length Aβ40 and Aβ42 homologues, peptides were synthesized, biochemically and biophysically characterized, radiolabeled with [125I]Na and utilized for in vivo brain clearance experiments in 5–6‐week‐old C57Bl/6 mice. Brain efflux of all monomeric Aβ species tested was fast, with >80% cleared within the 60 min duration of the experiment whereas retention of oligomeric preparations was consistently higher than that of monomeric forms. Notably, clearance of Aβ4‐40 and Aβ4‐42 was significantly lower (<45%) than the full‐length counterparts (61% ‐ 68%), further supporting a pathogenic role for Aβ oligomerization in AD and specifically highlighting the importance of oligomeric Aβ4‐x species exhibiting higher brain retention compared to their full‐length counterparts. Since the process of multimerization is concentration dependent, failing to remove these oligomeric forms of Aβ from the brain will further contribute to exacerbate the amyloidogenic loop and the persistence of amyloid deposits.