Microglial colonization of the developing mouse brain: the effect of CD11b deletion

Abstract

Introduction: Microglia are resident mononuclear phagocytes of the central nervous system, which colonize the brain both prenatally and after birth. It is proposed that they enter the brain initially via the surrounding mesenchyme, via ventricles and later through blood vessels, but the mechanisms of entry and signals used for migration are still to be established. Previous studies have shown that ligands for some integrin adhesion molecules expressed on blood vessels in the developing nervous system (particularly ICAM‐1 and ICAM‐2 which bind CD11a/LFA‐1 and CD11b/Mac‐1), may act as potential recruiting signals for microglial precursors. This study addressed whether CD11b is influential on the migration of microglial precursors into the developing CNS. Material and methods: Ricinus communis agglutinin‐1 (RCA‐1) lectin histochemistry was employed to anatomically map the distribution of amoeboid and ramified microglia from embryonic day 15 (E15) to birth. Embryonic mouse brains from CD11b knockout (−/−) (n = 42), and heterozygote (+/−) (n = 52) mice generated on a C57/BL6 background (Melo et al. Cell Immunol 2000; 205: 13–23) and wild‐type (+/+) (n = 37) litter mates were fixed in Bouin's solution, processed to paraffin wax and serially sectioned at 15–40 µm. To investigate further potential signals for recruiting microglial precursors, brains were immunochemically screened for integrins CD11a, CD11b, CD18, αX, VLA‐4 and the chemokine MCP‐1. Results: Microscopic analysis revealed the morphological transition of microglia from predominantly amoeboid forms at E15–E16 to a flourishing population of ramified cells at E19–E20. RCA‐1 histochemistry showed no clear differences in microglial distribution or timing of colonization between CD11b (−/−) and wild‐type mice from E15 to birth. Although CD11b deletion did not influence the timing of microglial ramification, there appeared to be fewer ramified cells in (−/−) mice within comparative brain regions. This requires further quantitative morphometric analysis. Of the integrins investigated, none were restricted to microglia and only VLA‐4 and αX showed reactivity within the CNS. However, MCP‐1 was notably localized to the cortical plate within all genotypes, consistent with previous findings in human foetal CNS (Rezaie & Male. Microsc Res Tech 1999; 45: 359–382). Conclusion: The results suggest that CD11b has little influence on the timing or regional distribution of microglia in the developing murine CNS. It is more likely that CD11b is only one of several factors that influence the migration and differentiation of these cells.

Pro-Inflammatory Role of AQP4 in Mice Subjected to Intrastriatal Injections of the Parkinsonogenic Toxin MPP+

“…To resolve whether AQP4 plays a pro-inflammatory role, we analyzed the expression levels of key genes coding for microglial membrane receptors known to be upregulated during inflammation. We specifically investigated the expression levels of Cd14, a key organizer of microglial inflammatory response [43]; Trem2, a receptor promoting microglial survival, proliferation and phagocytic activity [44]; Cx3cr1, a chemokine receptor involved in microglial inflammatory response [45,46]; and Itgam, the gene coding for the integrin receptor CD11b involved in microglial adhesion and migration [47]. In WT animals, our qPCR analysis revealed a several fold increase in the expression levels of each of these genes in midbrain samples ipsilateral to MPP+ injections.…”

Microglia in the Human Nervous System during Development

“…The use of knockout (e.g. osteopetrotic/MCSF knockout and CD11b knockout [82]) as well as transgenic mice (e.g. mice overexpressing MCP-1 [83]) will continue to represent valuable tools for determining signals driving the recruitment of microglial progenitors.…”