β Amyloid peptide (Aβ₄₂) is internalized via the G‐protein‐coupled receptor FPRL1 and forms fibrillar aggregates in macrophages¹

Abstract

The 42 amino acid form of beta amyloid (Abeta42) plays a pivotal role in neurotoxicity and the activation of mononuclear phagocytes in Alzheimer's disease (AD). Our recent study revealed that FPRL1, a G-protein-coupled receptor, mediates the chemotactic and activating effect of Abeta42 on mononuclear phagocytes (monocytes and microglia), suggesting that FPRL1 may be involved in the proinflammatory responses in AD. We investigated the role of FPRL1 in cellular uptake and the subsequent fibrillar formation of Abeta42 by using fluorescence confocal microscopy. We found that upon incubation with macrophages or HEK293 cells genetically engineered to express FPRL1, Abeta42 associated with FPRL1 and the Abeta42/FPRL1 complexes were rapidly internalized into the cytoplasmic compartment. The maximal internalization of Abeta42/FPRL1 complexes occurred by 30 min after incubation. Removal of free Abeta42 from culture supernatants at 30 min resulted in a progressive recycling of FPRL1 to the cell surface and degradation of the internalized Abeta42. However, persistent exposure of the cells to Abeta42 over 24 h resulted in retention of Abeta42/FPRL1 complexes in the cytoplasmic compartment and the formation of Congo red positive fibrils in macrophages but not in HEK 293 cell transfected with FPRL1. These results suggest that besides mediating the proinflammatory activity of Abeta42, FPRL1 is also involved in the internalization of Abeta42, which culminates in the formation of fibrils only in macrophages.

Contribution of glial cells to the development of amyloid plaques in Alzheimer’s disease

“…This model is based on the assumption that microglia can detect nonfibrillar A␤, and concentrate receptor-attached monomers/oligomers to a point above the threshold required to drive fibrillization. This hypothesis is supported by studies suggesting that the FPRL1 receptor might be the critical factor in A␤ detection, concentration, and fibrillization [27,59]. The A␤ fibrillar product accumulates within narrow extracellular spaces provided by deep cell membrane invaginations, which form long channels that appear to penetrate the cytoplasm of microglia and are filled with bundles of parallel fibers (Fig.…”

“…Recent studies suggest that FPRL1 is involved in the proinflammatory response in AD [59]. FPRL1 plays a fundamental role in the detection of chemoattractant by cells of monocyte-microglial lineage, migration of activated cells to areas of high concentration of chemoattractant, and cell polarization with fast local concentration of this receptor [29].…”

“…A␤42 is internalized through FPRL1 and forms Congo red-positive fibrils in macrophages but not in epithelial cells transfected with FPRL1. This suggests that only cells of macrophage lineage are able to generate a microenvironment supporting A␤ internalization and fibrillization [59]. Elevated FPRL1 gene expression in CD11b-positive mononuclear phagocytes in amyloid plaques in individuals with AD [27] may correspond to microglial polarization and the formation of a FPRL1-positive interface with A␤ deposits.…”

“…PrP 106-126 also induces directional migration of human monocytes, FPRL1-mediated internalization, and uptake of prion protein [27]. The FPRL1 may act as a pattern recognition receptor that interacts with many factors including A␤, prion protein (PrP) and serum amyloid A [27,59].…”

Amyloid beta peptide directly inhibits PKC activation

“…These results indicate that treatment of Ah inhibited activation of PKCa and PKCq in the cell. We tested whether extracellular Ah entered the cell, as was previously reported (Bahr et al, 1998;Bi et al, 2002;Ida et al, 1996;Knauer et al, 1992;Nagele et al, 2002;Yang et al, 1995;Yazawa et al, 2001). Immunocytochemical and Western blot analyses confirmed localization of Ah 1 -42 peptides inside the cells that were treated with exogenous Ah 1 -42, but not in cells that were not treated with Ah 1 -42.…”