Vps35 p. D620N causes Lrrk2 kinase hyperactivity, chronic microglial activation and inflammation

Pathogenic variants in <i>leucine-rich repeat kinase 2 (LRRK2), vacuolar protein sorting 35 (VPS35</i> ), and <i>RAB32</i> cause dominantly inherited parkinsonism, indistinguishable from idiopathic late-onset Parkinson's disease (PD). All three causes constitutively activate LRRK2 kinase activity to augment immune responses, enhancing immunity to fight pathogens, but similar mechanisms in the brain increase the vulnerability of dopaminergic neurons to degeneration. Although <i>VPS35 p.D620N</i> possess the highest constitutive increase in LRRK2 kinase activity among known variants in <i>LRRK2</i> or <i>RAB32,</i> its effects on the immune system remain poorly understood. LRRK2 and Rab32 are highly expressed in myeloid cells including microglia; thus we examined the transcriptomic and functional consequences of Vps35 p.D620N in knock-in mice (VKI). Microglia were isolated from brains of six-month-old VKI mice and were analyzed via single-cell RNA sequencing. Differential gene expression highlighted pathways involved in antimicrobial humoral immune response, lysosomal stress sensing, and phagocytosis. Notably, genes of S100 family proteins, along with <i>lipocalin 2</i> ( <i>Lcn2</i> ), were significantly upregulated, and those measures were complimented by immunohistochemistry and quantitative PCR. In contrast, pathways involved in synaptic transmission, neuronal development, and homeostatic immune signaling were downregulated. Peripheral stimulation with lipopolysaccharide amplified microglial activation and phagocytic markers in wildtype mice, and VKI mice also display enhanced morphological activation and increased synaptic engulfment. Collectively, Vps35 p.D620N drives a chronic pro-inflammatory microglial phenotype characterized by heightened innate immune signaling, lysosomal stress, and enhanced phagocytic activity. VKI microglia are sensitized to peripheral immune challenges and may promote synaptic remodeling and neurodegenerative vulnerability in PD. These results provide mechanistic insight into how retromer dysfunction and LRRK2 kinase hyperactivity intersect with microglial biology to influence PD pathogenesis.