Microglial Lag3 Drives α-Synuclein–induced Neurotoxic Activated (A1) Astrocytes and Neurodegeneration

Background: Neuroinflammation and pathologic α-synuclein (α-syn) aggregation cooperate to drive dopaminergic neurodegeneration in Parkinson's disease, but the glial receptors that couple extracellular α-syn to inflammatory cascades remain incompletely defined. Microglia express higher levels of lymphocyte activation gene 3 (Lag3) than neurons, yet the contribution of microglial Lag3 to α-syn recognition, glial crosstalk, and neurodegeneration is unknown. Methods: ) and littermate controls received unilateral intrastriatal α-syn PFF injections, followed by histological, biochemical, and behavioral assessments of α-syn pathology, gliosis, nigrostriatal integrity, and motor performance. Results: α-syn PFFs bound microglial Lag3 with high specificity and nanomolar affinity and required Lag3 for efficient fibril internalization and induction of proinflammatory cytokines. Microglial Lag3 deficiency markedly blunted α-syn PFF-evoked microglial activation, prevented cytokine-driven conversion of astrocytes into neurotoxic reactive A1 astrocytes, and abolished astrocyte-dependent neuronal death in vitro. In vivo, microglia-specific Lag3 deletion reduced cortical, striatal, and substantia nigra pS129 α-syn pathology, suppressed microgliosis and A1 astrocyte induction, preserved substantia nigra dopaminergic neurons and striatal dopamine transporter/tyrosine hydroxylase expression, and ameliorated α-syn PFF-induced motor deficits. Conclusions: This study identifies microglial Lag3 as a key receptor linking extracellular α-syn PFF recognition to inflammatory amplification, neurotoxic reactive A1 astrocyte conversion, and dopaminergic neurodegeneration. Together with prior work on neuronal Lag3, these findings support a cell-type-specific dual-axis model in which neuronal Lag3 mediates α-syn propagation while microglial Lag3 drives glia-dependent neurotoxicity, positioning Lag3 as a promising precision therapeutic target in α-synucleinopathies.