Fluorescence Micro-spectroscopy Assessment of the in vitro Dimerization of BACE1-GFP Fusion Protein in Cultured Cells

Abstract

Alzheimer’s Disease (AD) is a neurodegenerative disorder that results from the formation of beta-amyloid (Aβ) plaques in the brain, which according to the amyloid cascade hypothesis triggers known symptoms, such as memory loss, in AD patients. Aβ plaques are formed by the proteolytic cleavage of the amyloid precursor protein (APP) by the proteases BACE1 and γ secretase. These enzyme-facilitated cleavages lead to the production of Aβ fragments that then aggregate to form plaques, which ultimately lead to neuronal cell death. Recent detergent protein extraction studies suggest that the untreated BACE1 protein forms a dimer that has significantly higher catalytic activity than its monomeric counterpart. Currently, however, there are no studies that support the dimerization hypothesis of BACE1 in living cells. In this contribution, we describe our effort to examine the dimerization hypothesis of BACE1 in cultured HEK293 cells using complementary fluorescence spectroscopy and microscopy methods. Cells were transfected with a BACE1-EGFP fusion protein construct and imaged using confocal and differential interference contrast (DIC) microscopy to monitor labeled BACE1 localization and distribution within the cell. Subsequently, single molecule fluctuation analysis allowed us to test the dimerization hypothesis of the labeled BACE1 using fluorescence measurements of the diffusion coefficient (size-dependent observable) and the molecular brightness as a function of BACE1 substrate analog inhibitor binding. Complementary two-photon fluorescence lifetime and anisotropy imaging enabled us to monitor BACE1 conformational changes and its local environment as a function of substrate binding. These studies provide evidence that BACE1 substrate-mediated dimerization occurs in cells and may be dependent on cellular location.