Development of novel therapeutic approaches for treatment of Alzheimer's disease

Abstract

AD is a complex disease, involving the perturbation of multiple interrelated biological pathways. Despite being the most common form of neurodegenerative disease, with the number of patients expected to increase drastically, the aetiology of AD remains unknown. Although genetic factors such as mutations in the APP and PSEN genes, which drive the overproduction of Aβ, are known to cause early on-set familial AD (EOFAD), majority of AD patients (> 95%) who develop clinical symptoms of AD later in life (i.e. late on-set AD (LOAD)) do not carry these mutations. Contrary to earlier assumptions that occurrence of LOAD was sporadic, GWAS and exome sequencing studies have identified genetic variants, in genes that are microglia specific, or highly expressed in microglia, that are associated with increased risk of LOAD, suggesting altered regulation of microglial functions to be involved in the pathogenesis of LOAD. Using microglia isolated from the 5xFAD mouse model of AD, we identified by bulk and single cell RNA-seq that plaque phagocytosing (X04+) and non-plaque phagocytosing (X04-) microglia are distinct microglial populations separable by their transcriptomic signature. Our study suggests that X04+ microglia are a homogenous population of microglia with a distinct gene expression profile associated with amyloid uptake. In contrary, X04- microglia are associated with an age-related transcriptomic profile and may be involved in the over-pruning of synapses in 5xFAD mice. We also demonstrated that microglia can undergo transcriptomic changes upon exposure to different environmental cues.

Although therapeutic approaches that target different pathological features of AD have been evaluated, clinical translation of therapeutics has been very unsuccessful. This could be in part due to potential therapeutics targeting a single disease pathology. To identify new therapeutic candidates for the treatment of AD, this study also explored the use of metal-based compounds as a multi-targeting therapeutic strategy. Five novel metal-based compounds were screened to identify leading compounds that confer neuroprotective, metal-regulating and inflammation-modulating effects in a generic model of neuroinflammation. LM47 was identified as the only leading compound that exerted inflammatory-modulating activity through copper-associated action. Further testing of LM47 showed that the compound was well-tolerated in vivo. However, data by pharmacokinetic study and ICP-MS suggest that the copper free ligand LM46, and not LM47, acts as the active compound in vivo. Treatment of 5xFAD mice with LM46 increased the proportion of X04+ microglia in brain. However, Both LM46 and LM47 treatment of 5xFAD mice induced increased brain Aβ plaque load in the animals. Taken together, findings from our study suggest that improved targeting of specific microglia sub-population in AD could confer therapeutic outcomes.