Recombinant BRICHOS domains delivered over the blood-brain barrier : a possible way to treat Alzheimer’s disease

Abstract

One of the current major challenges to treat neurological diseases is the ability of drug candidates to cross the blood-brain barrier (BBB). Alzheimer´s disease (AD) is the most common form of dementia worldwide, affecting more people every year due to a world where the average lifespan is increasing. During the last decade, there have been many drug candidates in AD clinical trials. However, all of them have failed and poor permeability over the BBB is probably a contributing factor. The aggregation of the amyloid-b (Ab) peptide is considered one of the trigger mechanisms of the disease, and Ab constitutes the core of the amyloid plaques in AD brains. Thus, targeting Ab seems like a relevant therapeutic approach. Previous studies have demonstrated the ability of recombinant human (rh) BRICHOS domain to efficiently delay the fibril formation of Ab peptide and to reduce its neurotoxic effects, both in vitro and in vivo.

The general aim of the present thesis was to test the potential of rh BRICHOS domain as a therapeutic candidate for AD. In paper I we tested the ability of rh Bri2 and proSP-C BRICHOS domains to cross the BBB and their permeability over the blood-cerebrospinal fluid (CSF) barrier after systemic administration in the tail vein of wild type (wt) mice. We thus uncovered qualitatively different BBB permeabilities of rh Bri2 and proSP-C BRICHOS domains. Only rh Bri2 BRICHOS reaches the brain parenchyma while both proteins are detected in the CSF. In order to enhance the passage into the brain of intravenously administered rh BRICHOS domains, in paper II we used focused ultrasound (FUS) in combination with microbubbles (MBs) to transiently open the BBB in a specific region in the mouse brain. This led to efficient delivery of rh proSP-C BRICHOS over the BBB in the targeted hemisphere and, unexpectedly, to detection of rh Bri2 BRICHOS domains in both targeted and non-targeted brain hemispheres. Both rh proSP-C and Bri2 BRICHOS domains were internalized by a subset of neurons in the hippocampus and cortex. Based on the observations from paper II, in paper III we explored the effects of MBs without FUS on the BBB permeability of rh Bri2 BRICHOS domain fused to a tag or to a globular protein. MBs facilitate the passage of rh Bri2 BRICHOS domain and the fusion partners to the brain parenchyma. Finally, we evaluated treatment effects of a designed monomeric mutant, rh Bri2 BRICHOS R221E, in Ab precursor protein (APP) knock-in mouse models of AD (paper IV). Our results showed an improvement in working and recognition memory, a decrease in the plaque load, reduced astrogliosis and microgliosis, as well as a reduction in the amounts of astrocytes surrounding the Ab plaques after repeated injections of rh Bri2 BRICHOS R221E.

These findings highlight that rh Bri2 BRICHOS domain, can enter the brain parenchyma and have positive effects in an AD model with advanced pathology. These results support continued investigations of Bri2 BRICHOS domain as a treatment strategy for AD.