Model of selective neurodegeneration driven by a Ccp1 mutation leads to atypical microglia with an increased response to pathological stimuli.

Microglia are the primary immune cells of the central nervous system and maintain tissue homeostasis through phagocytosis and regulation of inflammatory signalling. Although these functions are well established, the molecular mechanisms that control microglial activation during neurodegeneration remain poorly understood. We focused on the Purkinje Cell Degeneration (PCD) mouse, which carries a loss-of-function mutation in Ccp1 that disrupts tubulin post-translational modifications essential for cytoskeletal stability. Because cytoskeletal dynamics are fundamental for microglial motility, phagocytosis, and proliferation, the Ccp1 mutation offers a model to directly examine how intrinsic cytoskeletal defects alter microglial behaviour and how these alterations manifest within regions undergoing distinct patterns of neurodegeneration. To this end, we combined in vitro and in vivo approaches. Microglia were isolated from neonatal cortex and adult cerebellum and olfactory bulb, and microglia-like cells were generated from bone marrow-derived haematopoietic stem cells. In vivo microglial depletion was achieved with the CSF1R inhibitor PLX5622. Immunohistochemistry quantified microglial density, morphology, and marker expression; transcriptomic profiling assessed identity and functional pathways; and functional assays evaluated phagocytosis, motility, and proliferation. Motor behaviour tests were performed to determine whether microglial dysfunction contributes to circuit-level impairments. Statistical analyses used parametric or non-parametric tests according to distribution. Ccp1-deficient microglia exhibited intrinsic deficits in phagocytosis, motility, and proliferation, independent of overt neuronal loss. These impairments were amplified in degenerating regions, where microglia adopted a predominantly anti-inflammatory rather than pro-inflammatory activation profile. This atypical state suggests a maladaptive response that may compromise tissue homeostasis and intensify disease progression. Consistent with this, animals showed altered motor behaviour, indicating functional consequences of microglial dysfunction. Together, these findings identify Ccp1 as a key regulator of microglial homeostasis and demonstrate how cytoskeletal disruption can reshape microglial responses in neurodegenerative environments, providing mechanistic insight and potential therapeutic targets.