Glaucoma is an incurable disease that leads to partial or total loss of vision in 80 million people worldwide, 100,000 – 200,000 of whom in Sweden. The three major risk factors are age, high intraocular pressure, and genetic predisposition. The only treatment strategies currently available target the pressure in the eye using eye drops or surgery; despite this, the risk of blindness in one eye is still high.
Microglia are an important type of immune cell that support the neurons of the retina. Microglia can be triggered, or activated, to respond to and remove pathogens or native dead and dying cells. In neurodegenerative disease, this response can sometimes be too extreme and worsen the degenerative process, creating an environment of neuroinflammation.
Identify neuroinflammatory responses
The study aimed to rigorously identify and quantify changes to microglia cells in glaucoma in rodents, with a focus on how the neuroinflammatory responses varied with increasing intraocular pressure and neurodegenerative load.
Photo: Stefan Zimmerman
"We identified a broad range of microglial structural changes, showing that the response of individual microglia can vary within the same eye. Globally, microglia shifted towards a morphology consistent with increased activation and inflammation," says James Tribble, a postdoctoral researcher in the Williams laboratory at St. Erik Eye Hospital and Karolinska Institutet, who led the study.
The reseach team also demonstrated that microglia in the retina, optic nerve and the first visual centre of the brain displayed increasingly activated morphologies. In the retina, this was accompanied by infiltrating immune cells, called monocytes, entering from blood vessels.
"We also detected a number of inflammatory signalling proteins that were increased in glaucoma. Crucially, in cases where glaucoma was only in one eye, we still detected significant inflammatory changes in the other non-glaucoma eye and brain, suggesting a global response to glaucoma," James Tribble says, and continues:
"This study furthers the evidence of an inflammatory immune component to glaucoma and identifies new targets to follow up as potential therapies which could benefit future patients."
Using high resolution fluorescent microscopy
To study microglia responses the reseachers used fluorescent labels applied to retina, optic nerve and brain samples, making it possible to acquire images of the microglia with high resolution fluorescent microscopy. Inflammatory proteins were identified with an antibody screen, allowing the team to probe for many potential changes.
The researchers need to further understand when these changes begin and how they progress; to do that they need earlier time point in disease. Earlier time points may offer more promising therapeutic targets.
"We have built good tools to look at these earlier time points, and have plans in place to start to investigate some of the inflammatory signalling proteins in further detail," James Tribble concludes.
The research was financed by the Swedish Research Council, Karolinska Institutet, Fight for Sight, St. Erik Eye Hospital philanthropic donations, BRIDGE – Translational Excellence Programme, and the University of Copenhagen.
When is a control not a control? Reactive microglia occur throughout the control contralateral pathway of retinal ganglion cell projections in experimental glaucoma, James R Tribble, Eirini Kokkali, Amin Otmani, Flavia Plastino, Emma Lardner, Rupali Vohra, Miriam Kolko, Helder André, James E Morgan and Pete A Williams, Translational Vision Science & Technology (TVST), online 12 January, 2021, doi: 10.1167/tvst.10.1.22.