BRICHOS interactions with amyloid proteins and implications for Alzheimer disease

Abstract

To date, about 30 diseases, in which amyloid fibrils form extracellular deposits, have been identified in humans. It is not known if the fibrils have a function, like storage of misfolded proteins, or if they just reflect failure of the cell to manage misfolded proteins. There is no treatment for the majority of the amyloid diseases and therefore disease modifying therapies are sought for. The work in this thesis is focused on studying the BRICHOS domain, which is expressed as part of proproteins found in several protein families involved in a wide range of functions, and some of them are associated with amyloid disease, e.g. interstitial lung disease (proSP-C) and dementia (Bri2). BRICHOS is suggested to have a role in preventing amyloid aggregation of its proproteins. Alzheimer disease (AD) is the most common form of dementia, and aggregation of the amyloid-β peptide (Aβ) is widely considered as the causative event. Aβ is derived by sequential cleavages of the Aβ precursor protein AβPP. Previous studies have shown that proSP-C BRICHOS reduces Aβ aggregation, and suggested that the monomer is the active form.

In Paper I we studied ways to increase the monomer/trimer ratio of proSP-C BRICHOS expressed in E. coli, and how this affects its activity against Aβ fibrillation. We found that treatment with amphipathic agents increased proSP-C BRICHOS monomer/trimer ratio and its activity. We also determined that proSP-C BRICHOS is monomeric in mammalian cells. ProSP-C BRICHOS is only expressed in alveolar type II cells where it facilitates folding of the extremely aggregation prone transmembrane region of proSP-C. In Paper II we studied whether proSP-C BRICHOS could reduce amyloid aggregation of a designed amyloidogenic protein in the secretory pathway of mammalian cells. We found that co-expression of BRICHOS led to reduced amyloid aggregation, and prevented subsequent inhibition of proteasomal degradation. This suggests that BRICHOS has generic anti-amyloid properties.

The BRICHOS containing Bri2 and Bri3 proteins are expressed in the central nervous system and have been proposed to be involved in AβPP processing. In Paper III we studied interactions between Bri2 and Bri3 BRICHOS and endogenous neuronal AβPP and Aβ. We found that Bri2 BRICHOS is shed from cells, and interacts with intracellular Aβ and AβPP. Bri3 BRICHOS was not shed into the extracellular space, showed abundant interactions with intracellular Aβ, and exhibited reduced hippocampal and cortical levels in AD.

In Paper IV we studied proSP-C and Bri2 BRICHOS effects on Aβ aggregation in vivo in a mouse model overexpressing mutant AβPP and presenilin1 (PS1). Both proSP-C and Bri2 BRICHOS reduced Aβ levels and aggregation without affecting AβPP processing. Mice co- expressing BRICHOS and AβPP/PS1 showed improved memory and reduced neuroinflammation compared to AβPP/PS1 control animals.

The results in this thesis show that BRICHOS reduces amyloid aggregation in vitro, in cells and in a mouse AD model, and indicate a potential physiological relationship between BRICHOS and Aβ. These findings together support that BRICHOS and its properties are worth continuing to study in relation to amyloid aggregation and AD.