Abstract
Purpose :
To investigate the physiological role of microglial communication with inner retinal vasculature and to explore the mechanism of vasoregulation.
Methods :
Immunohistochemistry was used to characterise the extent of microglial contact with neurons and inner retinal vasculature (IB4) in Cx3cr1+/GFP and Cx3cr1GFP/GFP transgenic mice in which one or both copies of the chemokine receptor Cx3cr1 are replaced with green fluorescent protein (GFP). Live cell confocal imaging of Cx3cr1+/GFP retinal explants was used to assess process motility of microglia in close contact with superficial capillaries. Microglia-mediated capillary response was evaluated in Dark Agouti rat retinal explants by measuring capillary diameter at areas of microglial contact while perfusing fractalkine (200ng/ml), a neuronally derived chemokine and sole ligand for Cx3cr1, in the presence and absence of the Angiotensin II receptor antagonist, candesartan (.227μM).
Results :
Results: Microglia make numerous physical connections with both blood vessels and synapses. The majority of inner retinal microglia in Cx3cr1+/GFP and Cx3cr1GFP/GFP retinae make contact with capillaries of the superficial vascular plexus (83 ± 1% versus 83 ± 1%, n=5, p>0.05). However, microglial density is increased in Cx3cr1GFP/GFP tissue (21 ± 3%, n=5, p<0.05). This results in an increase in the number of vessel-microglia contacts (19 ± 4%, n=5, p<0.05). Cx3cr1GFP/GFP microglia also display fewer branches per cell (-18 ± 4%, n=5, p<0.05) and accumulate in the subretinal space. Cx3cr1+ve cells making direct contact with inner retinal capillaries displayed no process motility over a duration of 20 mins (n=6 cells from 5 animals). Perfusing retinal explants with fractalkine evoked a constriction in capillaries that was reduced when explants were incubated with candesartan (-11 ± 3% baseline versus -2 ± 2% baseline, n=5-9, p<0.05).
Conclusions :
Retinal microglia are involved in the neurovascular unit and regulate blood flow in response to fractalkine, a chemokine known to be released by neurons. Fractalkine-evoked capillary constriction is abolished in the presence of candesartan, suggesting angiotensin plays a role in microglia mediated vasoregulation.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.