C9orf72 -associated G4C2 hexanucleotide repeat expression in Drosophila mushroom bodies causes age dependent TDP-43 pathology and dementia relevant phenotypes mediated in part by the glypican Dlp/GPC6

Hexanucleotide repeat expansions (HREs) in C9orf72 are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), yet the age-, sex-, repeat-length-, and circuit-specific influence on the pathology of neurons remains incompletely understood. Here, we established a Drosophila model of C9orf72 -associated dementia by expressing G4C2 repeats in mushroom body neurons (MBNs), a brain region critical for memory, locomotion, and sleep. Expression of 44X G4C2 repeats ((G4C2) _44X ) led to progressive axonal thinning, age-dependent accumulation of Repeat Associated Non-AUG (RAN) translated GR-GFP dipeptide repeat (DPR) puncta, premature nuclear-to-cytoplasmic mislocalization of endogenous TDP-43, increased caspase, reduced lifespan and a loss of presynaptic active zones. Behaviorally, (G4C2) _44X expression caused locomotor hyperactivity, altered spatial working memory, and fragmentation of sleep architecture in an age- and sex-dependent manner, recapitulating core features of FTD. Surprisingly, the shorter (G4C2) _12X repeat, traditionally considered a control, also produced detectable RAN translation and intermediate phenotypes in aging MBNs, suggesting that length- and tissue-associated factors modulate repeat toxicity. We further identified a repeat-length- and age-dependent reduction of the glypican Dally-like protein (Dlp) in (G4C2) _44X consistent with disrupted Wnt-related signaling linked to TDP-43 proteinopathies. Restoring Dlp expression in MBNs mitigated locomotor and working-memory alterations, and loss of presynaptic active zones. In contrast, axonal degeneration, TDP-43 mislocalization, and lifespan were not significantly improved by restoring Dlp, suggesting that multiple mechanisms contribute to G4C2-induced toxicity. Supporting our findings in Drosophila MBNs, a CRISPRi screen in TDP-43 knock-down iNeurons identified GPC6, a human ortholog of Dlp, as a significant contributor to TDP-43 dependent synaptic loss. Together, our findings reveal an aging-sensitive, circuit-specific model of C9orf72 -associated neurodegeneration and highlight roles for DPR accumulation and Dlp/GPC6 dependent synaptic loss in FTD pathomechanisms.