Galectin-3 shapes microglial phenotype through endogenous and exogenous mechanisms.

Galectin-3 (Gal3) is a multifunctional lectin expressed and released by microglia, where it influences diverse processes in both homeostasis and disease. To dissect its intracellular and extracellular roles, we generated Gal3-deficient BV2 microglial cells and systematically assessed how genetic deletion and exogenously added recombinant Gal3 shape microglial physiology. Gal3 deletion increased cell area, mitochondrial activity, and motility without affecting proliferation, linking endogenous Gal3 to microglial energetic control and dynamic cellular physiology. Endogenous Gal3 was required to maintain CD11b surface levels, and restrains TREM2 and Clec7a expression, whereas exogenous Gal3 promoted CD45 internalization and drove a paracrine TNFα release. Endogenous and exogenous Gal3 are synergistically needed for Syk phosphorylation and NOX2 expression. Internalization assays demonstrated that endogenous Gal3 constrained phagocytosis and endocytosis, while exogenous Gal3 enhanced endocytosis in a paracrine manner. In the Alzheimer's disease 5xFAD mouse model, where Gal3 deletion was reported to lower amyloid plaque burden, the absence of Gal3 does not affect microgliosis but elevates Clec7a levels around plaques. Together, these findings reveal Gal3 as a critical regulator of microglial homeostasis, uptake pathways, receptor expression, and inflammatory signaling. We have defined a novel microglial regulation based on endogenous and exogenous pools of Gal3. By identifying a novel Gal3-Clec7a interaction, this work highlights Gal3 as a key modulator of microglial phenotype and a potential target for therapeutic modulation of neuroinflammation.