Depletion of border-associated macrophages impairs CSF flow and exacerbates extranuclear mutant Huntingtin aggregation in the zQ175DN mouse model of Huntington disease

Huntington disease (HD) is a fatal, neurodegenerative disorder driven by a CAG trinucleotide repeat expansion in the huntingtin (HTT) gene, resulting in neuronal toxicity and brain atrophy, initially in the most vulnerable brain region striatum and expanding to the cortex and other brain regions along disease progression. While cell-autonomous mechanisms within neurons have been extensively characterized, how brain waste clearance pathways at the brain's borders change during disease progression remains poorly understood. In this study, we investigated the structural and functional adaptations of border-associated macrophages (BAMs) in the zQ175DN knock-in mouse model of HD. Immunohistochemical analysis revealed an age-dependent, selective reduction in the CD206 Lyve1 subset of leptomeningeal BAMs in 16-month-old zQ175DN mice, which tightly correlated with a profound impairment in cerebrospinal fluid (CSF) flow velocity and perivascular influx. To determine whether BAMs actively drives this CSF flow, we selectively depleted these populations of macrophages in 10-month-old zQ175DN mice via intracerebroventricular (i.c.v.) injection of clodronate liposomes (CLO). Chemical ablation of BAMs significantly exacerbated global CSF dynamic impairment, indicted by decreasing CSF fluorescent tracer distribution across perivascular parenchymal channels. Crucially, this CSF flow failure resulted in a marked accumulation of extranuclear mutant huntingtin (mHTT) aggregates in the zQ175DN HD mouse brain, while intranuclear inclusions were not significantly altered. Furthermore, BAMs depletion did not alter microglial or astrocytic gliosis in the brain, demonstrating that the exacerbation of mHTT load is a byproduct of brain waste clearance dysfunction. Together, our results indicate that BAMs are non-redundant regulators of CSF flow dynamics at the brain borders, offering a unique non-neuronal therapeutic target to combat HD.