Phagocytic glia are obligatory intermediates in transmission of mutant huntingtin aggregates across neuronal synapses

Abstract

14Emerging evidence supports the hypothesis that pathogenic protein aggregates associated with 15 many neurodegenerative diseases spread from cell to cell through the brain in a manner akin to 16 infectious prions. Here, we show that mutant huntingtin (mHtt) aggregates associated with Huntington 17 disease transfer anterogradely from presynaptic to postsynaptic neurons in the adult Drosophila 18 olfactory system. Trans-synaptic transmission of mHtt aggregates is inversely correlated with neuronal 19 activity and blocked by caspase inhibition in presynaptic neurons, implicating synaptic dysfunction and 20 cell death in aggregate spreading. Remarkably, mHtt aggregate transmission across synapses requires 21 the scavenger receptor Draper and involves a transient visit to the glial cytoplasm, indicating that 22 phagocytic glia act as obligatory intermediates in aggregate spreading between synaptically-connected 23 neurons. These findings expand our understanding of phagocytic glia as double-edged players in 24 neurodegeneration-they clear neurotoxic protein aggregates, but also provide opportunities for prion-25 like seeds to evade phagolysosomal degradation and propagate further in the brain. 26 93 5 RESULTS 94Anterograde transfer of prion-like mHtt aggregates across synapses in the adult fly olfactory system 95 Aggregates formed by N-terminal fragments of mHtt generated by aberrant splicing (e.g., exon 96 1; Httex1) (Sathasivam et al., 2013) or caspase cleavage (e.g., exon 1-12; Httex1-12) (Graham et al., 2006) 97 (Fig. 1A) accumulate in HD patient brains, are highly cytotoxic, and spread between cells in culture and 98 in vivo (Babcock and Ganetzky, 2015; Costanzo et al., 2013; Pearce et al., 2015; Pecho-Vrieseling et 99 al., 2014; Ren et al., 2009). We have previously established transgenic Drosophila that employ binary 100 expression systems [e.g., Gal4-UAS, QF-QUAS, or LexA-LexAop (Riabinina and Potter, 2016)] to 101 express fluorescent protein (FP) fusions of Httex1 in non-overlapping cell populations in order to monitor 102 cell-to-cell transfer of mHttex1 aggregates in intact brains (Donnelly and Pearce, 2018; Pearce et al., 103 2015). Our experimental approach (Fig. 1B) exploits the fact that wtHttex1 proteins only aggregate upon 104 encountering mHttex1 aggregate seeds (Chen et al., 2001; Preisinger et al., 1999), such that transfer of 105 mHttex1 aggregates from "donor" cells is reported by conversion of cytoplasmic wtHttex1 from its normally 106 soluble, diffuse state to a punctate, aggregated state in "acceptor" cells. To confirm that mHttex1 107 nucleates the aggregation of wtHttex1 in fly neurons, we co-expressed FP-fusions of these two proteins 108 using pan-neuronal elav[C155]-Gal4. In flies expressing only mCherry-tagged mHttex1 (Httex1Q91-109 mCherry) pan-neuronally, aggregates were visible as discrete mCherry+ puncta throughout neuronal 110 cell bodies and neuropil regions of adult fly brains ( Fig. 1-figure supplement 1A). By contrast, GFP-111 tagged wtHttex1 (Httex1Q25-GFP) was expressed di...

Reactive astrocytes promote proteostasis in Huntington’s disease through the JAK2-STAT3 pathway

“…In a landmark study in Drosophila , it was shown that mHTT exon 1 tagged with mCherry shuttles from neurons to neighbouring phagocytic glia and forms aggregates with endogenous HTT in glia 56 . Other studies showed that mHTT can be exchanged between brain cells in Drosophila , in mice 57–59 , and even in humans as mHTT aggregates were detected in healthy embryonic neurons grafted in the brain of Huntington’s disease patients 60 . mHTT can be packaged in exosomes of different cell types 61, 62 and be taken up by neighbouring cells, including neurons.…”

Modulating glial genes involved in synaptic function mitigates pathogenesis and behavioral deficits in aDrosophilamodel of Huntington’s Disease

“…Astrogliosis, or the proliferation of immune active astrocytes, is typically observed at later stages of HD (Al-Dalahmah et al, 2020;Buscemi et al, 2017). These immune-activated glia not only eliminate synapses (Liddelow et al, 2017;Sofroniew, 2009) but can also transmit mHTT aggregates through the synapse (Donnelly et al, 2020). In Drosophila, knockdown of draper prevents astrocytic phagocytosis and stops the spread of mHTT protein aggregates from pre-synaptic neurons to the post-synaptic compartment (Donnelly et al, 2020;Pearce et al, 2015).…”

“…These immune-activated glia not only eliminate synapses (Liddelow et al, 2017;Sofroniew, 2009) but can also transmit mHTT aggregates through the synapse (Donnelly et al, 2020). In Drosophila, knockdown of draper prevents astrocytic phagocytosis and stops the spread of mHTT protein aggregates from pre-synaptic neurons to the post-synaptic compartment (Donnelly et al, 2020;Pearce et al, 2015). mHTT protein can also enter the synaptic space by endosomal/lysosomal secretion mediated by Syt7 (Trajkovic et al, 2017).…”

“…Drosophila have been used to elucidate glial biology (Freeman and Doherty, 2006;Olsen and Feany, 2019;Pearce et al, 2015;Ziegenfuss et al, 2012) and are a tractable model system for studying HD and other neurodegenerative diseases (Al-Ramahi et al, 2018;Bondar et al, 2018;Donnelly et al, 2020;Fernandez-Funez et al, 2000;Filimonenko et al, 2010;Goodman et al, 2019;Ochaba et al, 2014;Olsen and Feany, 2019;O'Rourke et al, 2013;Rousseaux et al, 2018;Yuva-Aydemir et al, 2018), we decided to generate flies that express mHTT solely in glia so that we could compare their transcriptomic signature with that of flies expressing mHTT in neurons. We took an unbiased approach, first establishing the repertoire of evolutionarily conserved genes that show concordant expression changes across HD human and mouse striata and HD fly brains.…”

C. elegans Models to Study the Propagation of Prions and Prion-Like Proteins

“…They are now not only recognized as victims, but also as a source of non-cell autonomous toxicity. Microglia, the CNS residing macrophages, were shown to be involved in the spreading of several prion-like proteins [ 149 , 150 , 151 , 152 , 153 ]. Moreover, the uptake of toxic protein species might facilitate microglia activation and lead to neuroinflammation, which is implicated in several neurodegenerative diseases [ 154 ].…”