Conditional rod photoreceptor ablation reveals Sall1 as a microglial marker and regulator of microglial morphology in the retina

Abstract

Neurodegeneration has been shown to induce microglial activation and the infiltration of monocyte-derived macrophages into the CNS, resulting in the coexistence of these two populations within the same lesion, though their distinct features remain elusive. To investigate the impact of rod photoreceptor degeneration on microglial activation, we generated a toxin-mediated genetic model of rod degeneration. Rod injury induced microglial proliferation and migration toward the photoreceptors. Bone marrow transplantation revealed the invasion of monocyte-derived macrophages into the retina, with microglia and the infiltrating macrophages showing distinct distribution patterns in the retina. By comparing the gene expression profiles of the activated microglia and infiltrating macrophages, we identified microglia-specific genes, including Ak1, Ctsf, Sall1, Phlda3, and Spns2. An analysis of Sall1gfp knock-in mice showed GFP expression in the microglia of developing and mature healthy retinas. DTA injury induced the expansion of Sall1gfp(+) microglia, whereas Ly6C(+) monocyte-derived macrophages were mostly Sall1gfp(-) , supporting the idea that Sall1 is exclusively expressed in microglia within the retinal phagocyte pool. We evaluated the contribution of microglia to the phagocyte pool in rd1 mutant retinas and found that Sall1gfp(+) microglia constituted the majority of phagocytes. A Sall1 deficiency did not affect microglial colonization of the retina and the cortex, but it did change their morphology from a ramified to a more amoeboid appearance. The morphological defects observed in Sall1-deficient microglia were not rescued by the presence of wild-type non-microglial cells, suggesting that Sall1 functions cell-autonomously in microglia. Taken together, our data indicate that Sall1 regulates microglial morphology during development. GLIA 2016;64:2005-2024.

A systems-level framework for drug discovery identifies Csf1R as an anti-epileptic drug target

“…Of these three modules, however, only module 18 was enriched (FDR <0.01) for genes down-regulated by PLX3397 (Supplementary Figure 9), suggesting a selective effect on module 18 expression by PLX3397 as opposed to a global down-regulation of microglial modules due to gross microglial depletion. Consistent with this interpretation, we observed that the expression of the microglia-specific gene Sall1 35 was up-regulated by PLX3397 (Log 2 fold change = 0.17, FDR = 2.16x10 −6 ).…”

Transcriptional control of microglia phenotypes in health and disease

“…Early microglia, like EMPs, express genes associated with cell cycle signaling pathways and the TF E2f6 and DNA methyltransferase-1 (Dnmt1), which may help them populate the brain. The genes expressed by premicroglia, including the TFs early growth response-1 (Egr1) and spalt-like TF-1 (Sall1), are associated with neural maturation and synaptic pruning (29,30). Later, as the brain …”

“…Human and mouse microglia uniquely and highly express SALL1 as compared with other macrophage populations, suggesting that it functions as an LDTF (34,35,37). Sall1-deficient animals have normal microglia colonization in the brain, but the microglia exhibit an abnormal ameboid morphology (30). Moreover, in the absence of Sall1, …”

Comprehensive analysis of mouse retinal mononuclear phagocytes

“…In fact, most recent in situ analyses of ramified and reactive retinal microglia rely on Iba1 staining [26][27][28][29][30] as the marker allows consistent and reliable staining of microglia somata and their processes in a broad spectrum of different activation phenotypes [31][32][33] . In general, a combined in situ analysis of retinal microglia using sections and flat mounts as presented in our protocol is recommended.…”