Sall1 Regulates Microglial Morphology Cell Autonomously in the Developing Retina.

Retinal degeneration often accompanies microglial activation and infiltration of monocyte-derived macrophages into the retina, resulting in the coexistence of microglia and monocyte-derived macrophages in the retina. We previously showed that the Sall1 zinc-finger transcriptional factor is expressed specifically in microglia within the retinal phagocyte pool, and analyses of Sall1 knockout mice revealed that microglial morphology changed from a ramified to a more amoeboid appearance in the developing retina. To investigate further whether Sall1 functions autonomously in microglia, we generated Sall1 conditional knockout mice, in which Sall1 was depleted specifically in the Cx3cr1+ microglial compartment of the developing retina. Sall1-deficient microglia exhibited morphological abnormalities on embryonic day 18 that strikingly resembled the phenotype observed in Sall1 knockout mice, demonstrating that Sall1 regulates microglial morphology cell autonomously. Analysis of the postnatal retina revealed that Sall1-deficient microglia extended their processes and their morphology became comparable to that of wild-type microglia on postnatal day 21, indicating that Sall1 is essential for microglial ramification in the developing retina, but not in the postnatal retina.