Mechanisms of mutant huntingtin aggregate engulfment and spreading by phagocytic glia - PROJECT SUMMARY Neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease (PD), and Huntington’s disease (HD), are rapidly rising in prevalence as our population ages. Despite extensive research, no disease- modifying treatments are currently available to slow or stop progression of these fatal disorders, underscoring an urgent need for new strategies. Protein misfolding and aggregation in the central nervous system are pathognomonic features of all neurodegenerative diseases and have been heavily explored as potential therapeutic targets. A growing body of evidence suggests that pathological protein aggregates exhibit “prion- like” behavior: they template the aggregation of natively-folded versions of the same protein and spread between cells in the brain. Recent findings suggest that phagocytic glia, such as astrocytes and microglia, contribute to prion-like spreading, perhaps due to age-related decline in the degradative capacity of the glial phagolysosomal system. However, the mechanisms that drive prion-like transmission of aggregates between neurons and glia and their potential for therapeutic intervention, remain poorly understood. The overall goal of this R15 renewal application is to elucidate how phagocytic glia recognize and engulf neuronal mutant huntingtin (mHTT) aggregates associated with HD and how this process can both alleviate and exacerbate neurodegeneration. Our prior work identified the scavenger receptor Draper/MEGF10 and the small GTPase Rab10 as modifiers of mHTT aggregate transfer from axons to glia in adult Drosophila brains. Interestingly, Rab10 is phosphorylated by LRRK2, a well-known risk factor for familial PD, on stressed or damaged lysosomes. Thus, the central hypothesis of the proposed work is that the Draper/MEGF10-Rab10 phagocytic pathway mediates spreading of mHTT between cells by promoting escape of prion-like aggregates from the phagolysosomal vesicle network. The central hypothesis will be tested in 3 Specific Aims, which will be carried out in complementary Drosophila and primary astrocyte cell culture experimental models. In Aim 1, we will define Rab10’s roles in Draper/MEGF10-dependent phagocytosis in the presence or absence of mHTT aggregates. In Aim 2, we will determine if LRRK2/Lrrk-mediated phosphorylation of Rab10 regulates mHTT aggregate spreading. In Aim 3, we will identify signals that trigger phagocytic glial recognition of N-terminal fragments or full-length mHTT proteins expressed in neurons. The proposed research is significant in that it will uncover new knowledge about interactions between neurons and phagocytic glia that contribute to neurodegeneration. The proposed research is innovative as it investigates novel mechanistic relationships between Draper/MEGF10, Rab10, LRRK2, and pathogenic protein aggregates. Our findings are therefore likely to inform about common mechanisms underlying HD, PD, and perhaps other neurodegenerative diseases. Further, our complementary in vivo and in vitro experimental models enable a rigorous examination of conserved, disease-relevant pathways in intact brains and isolated primary cells.
