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Emily Okoren, Rose Mathew, Miriam Merad, Daniel Saban; Microglia are maintained by retinal ganglion cells . Invest. Ophthalmol. Vis. Sci. 2017;58(8):5380.
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© ARVO (1962-2015); The Authors (2016-present)
We recently reported that retinal microglia in adult mice are maintained locally without significant contribution from hematopoiesis. An important emerging question is whether local paracrine factors in the retina are needed to help maintain normal microglia. We addressed this problem by focusing on CSF1R – a receptor required for microglia development and survival. By focusing on CSF1 and IL34, the known cognate ligands for CSF1R, we were able to expose important clues on how microglia are sustained under normal physiologic conditions.
RNA was extracted from retina, skin, and lymph nodes of C57Bl/6 mice and used to measure mRNA levels of CSF1, IL34, and GAPDH. Separately, retinas from IL34 deficient mice (IL34LacZ/LacZ) and littermate controls (IL34LacZ/+) were harvested and fixed/frozen for immunofluorescence analysis. Cryosections were stained for Iba-1 (microglia) or Brn3a (retinal ganglion cells) and β-gal. Confocal images were collected for analysis.
For mRNA levels, IL34 was >10-fold compared to CSF1 in the skin, as previously reported. The inverse relationship was observed in the lymph node where IL34 was <5-fold compared to CSF1. Strikingly, we report that mRNA levels of IL34 were >5-fold compared to CSF1 in the retina. To identify the cell type producing IL34, we stained for β-gal in retinal cross-sections from IL34LacZ/+ mice. Interestingly, β-gal staining was confined to the nerve fiber layer and, more specifically, Brn3a+ retinal ganglion cells. This localization suggests that retinal ganglion cells are the sole producers of IL34 in the normal retina. To determine the effect on microglia, we stained for Iba-1 in IL34 deficient mice. Remarkably, we observed a very striking reduction of microglia in IL34 deficient mice as compared to heterozygous mice.
Our data identify IL34 expression in the retina and the key role for this factor in local maintenance of microglia, which is consistent with recent work shown in the brain. Furthermore, our data suggest that IL34 expression is restricted to retinal ganglion cells. Collectively our data implicates a novel neuronal-microglia crosstalk mechanism that preserves immune homeostasis of the retina.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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