Abstract
Purpose :
Microglia are innate immune cells of the retina. Their numbers and positions are tightly controlled, so that their dendritic arbors can survail the entire plexiform layers without overlapping. In response to retinal injury microglia proliferate and migrate toward damaged tissue and dying cells. Mechanistic target of rapamycin complex 1 (mTORC1) is a central control point for cell metabolism and growth, and has been implicated in microglia proliferation. Conditional knockout (cKO) of the mTORC1 component Raptor was used to test the hypothesis that mTORC1 is necessary for maintenance of retinal microglia numbers and their proliferative response following ischemia-reperfusion (IR) injury.
Methods :
To specifically disrupt mTORC1 in microglia of adult mice, tamoxifen (TAM)-inducible Cre-driver strains, CX3CR1-CreERT2 and CX3CR1-EYFP-CreERT2, were crossed with Rptorf/f mice. Mice with no TAM treatment, or TAM-treated mice with one (het) or no (wt) disrupted Rptor allele served as controls. IR injury was unilaterally produced by injection of saline into the anterior chamber to elevate intraocular pressure and restrict retinal and choroidal blood flow for 90 min. Microglia populations were examined by flow cytometry of dissociated retinal cells using antibodies to myeloid leukocyte markers. Microglial morphologies were examined by immunofluorescence analysis of flat-mounted retinas, using antibodies to microglia markers and green fluorescent protein to detect YFP.
Results :
Disruption of mTORC1 caused significant 39% and 44% decreases in basal microglia numbers at 20 and 30 days after TAM treatment, respectively. At 30 days after TAM treatment, individual microglia arbor areas were significantly decreased by an average of 60%, leaving regions of plexiform apparently un-survailed. mTORC1-deficient microglia did not appear activated; they remained ramified and soma areas were not increased. Although microglial populations in Sham-treated eyes were progressively decreased with disruption of one or two alleles of Rptor, Rptor cKO, het and wt microglia exhibited similar 1.8-2.0 fold population increases at 4 days after IR.
Conclusions :
These results suggest that control of retinal population homeostasis and their proliferative response to injury are fundamentally different, in that mTORC1 is essential for the former but not the latter.
This is a 2020 ARVO Annual Meeting abstract.