Tgf-β receptor signaling cell-intrinsically controls conserved microglia homeostatic maturation during zebrafish development.

Microglia, embryonically derived tissue-resident macrophages of the central nervous system, are essential for brain development, homeostasis, and disease. Although transforming growth factor β (TGF-β) signaling is required for mammalian microglia ontogeny, its precise role and evolutionary conservation remain unclear. Using zebrafish as a vertebrate in vivo model, we find Tgf-β receptor signaling as a conserved, cell-intrinsic regulator of microglial differentiation. Pharmacological or genetic disruption of Tgf-β receptor function arrests microglial development after progenitor migration to the embryonic neuroepithelium, preventing activation of the microglial gene expression program. Genetic rescue experiments demonstrate a direct requirement for Tgf-β receptor signaling within the myeloid lineage. We identify Tgf-β1 as the key ligand driving this process and show that sustained signaling is required to maintain microglial identity. Adult zebrafish microglia, whose origin differs from that in mice, similarly fail to differentiate without Tgf-β signaling. Together, these findings establish an evolutionarily conserved TGF-β signaling axis that instructs and maintains microglial identity in vivo.