Early-onset neuroinflammation drives neurodegeneration caused by lysosomal PI(3,5)P2 insufficiency

Phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂] is a lysosomal signaling lipid whose deficiency, caused by mutations in the PIKfyve complex subunits FIG4 or VAC14, underlies a spectrum of fatal neurologic diseases including Charcot-Marie-Tooth type 4J (CMT4J) and amyotrophic lateral sclerosis (ALS). To map the molecular consequences of PI(3,5)P₂ insufficiency in the brain, we performed quantitative proteomic and transcriptomic analyses of three mouse lines bearing distinct loss-of-function mutations in Fig4 or Vac14, examining the brain at the presymptomatic and end stages. Strikingly, profound neuroinflammation was already present at postnatal day 5 (before significant neurodegeneration), characterized by complement activation, interferon signaling, and parenchymal infiltration of peripheral myeloid cells and T-cells. Isolated mutant microglia exhibited a markedly pro-oxidative transcriptional state with elevated reactive oxygen species, a partly non-cell-autonomous phenotype, being present in microglia from mice with conditional Fig4 inactivation in just neurons and astrocytes. Comparison of early (P5) and late (P25) proteomics data revealed that PI(3,5)P₂ insufficiency impairs developmental remodeling of the brain proteome: proteins typically upregulated during postnatal maturation failed to accumulate, implicating lysosomal function in neurodevelopment. We identify coordinated elevation of p53, Fas receptor, inflammatory caspases, Gasdermin D, RIPK1, and ZBP1, consistent with multifactorial inflammatory cell death with features of apoptosis, pyroptosis, and necroptosis. Many of the dysregulated proteins are encoded by genes mutated in lysosomal storage disorders, ALS, CMT, Alzheimer's and Parkinson diseases, extending the pathogenic relevance of PI(3,5)P₂ insufficiency. Together, these findings establish that early neuroinflammation is a defining - and likely initiating - feature of neurodegeneration caused by disruption of lysosomal PI(3,5)P₂.