The Role of Subretinal Microglia in Protecting Vision During Aging

The Ksander lab at Boston’s Schepens Eye Research Institute has uncovered a fascinating new role for subretinal microglia, shedding light on how these immune cells might protect vision during aging. Microglia, the primary immune cells of the central nervous system, are crucial for maintaining retinal health by maintaining homeostatic states and responding to stress, injury, or disease.

Typically, microglia patrol the inner retinal layers with their long processes, scanning for signs of abnormalities. But with age, some migrate into the subretinal space, the region between photoreceptors and the retinal pigment epithelium – a previously underexplored phenomenon. Could this migration be the retina’s way of fighting back against the aging process? Or is it a sign of further trouble brewing? This study sets out to uncover the role of these subretinal microglia and their impact on retinal health and function during aging.

Retinal pigment epithelial cells change during the aging process

At the outer bounds of the retina lies the retinal pigment epithelium (RPE), an indispensable layer of cells sandwiched between photoreceptive and vascular layers. It is integral for maintaining retinal integrity and keeping photoreceptors healthy and functional. The RPE not only forms the blood-retina barrier, but it also clears away photoreceptor waste, supplies essential nutrients, and phagocytizes photoreceptor outer segments.

However, time takes its toll. With age, RPE cells lose their ability to efficiently clear waste, leading to accumulation of broken off outer segments and the buildup of lipid droplets. Structural changes, including a reduction in RPE cell numbers and alterations in cell size, further weaken the retina’s integrity. These age-related changes are central to the development of age-related macular degeneration (AMD), a leading cause of blindness worldwide.

AMD arises when the RPE fails to support photoreceptors, leading to cell death and vision loss. The exact triggers of AMD are complex, involving oxidative stress, inflammation, and genetic factors, but the progressive decline in RPE health is a hallmark for disease severity.

Microglia on the Move: From Inner Retina to Subretinal Space

While the RPE deteriorates with aging, microglia begin to migrate from the inner retinal layers into the subretinal space, the region between photoreceptors and the RPE. In younger individuals, the subretinal space does not contain microglia, indicating significant changes in older retinas.

This raises some key questions: Does RPE cell death directly trigger this migration?  Is the presence of microglia in the subretinal space beneficial or detrimental to retinal function? This study by Karg et al. aimed to uncover the role of aged subretinal microglia and their impact on RPE health and visual function, in particular whether their involvement protects from or exacerbates age-related degeneration in a murine model.

Decoding Microglial Function Through Targeted Depletion

To unravel the mystery, researchers selectively depleted microglia in mice using a CSF1R inhibitor supplied in their food. This blocked colony-stimulating factor 1 receptor signaling: a pathway critical for microglial survival. Blockage of the receptor on the microglial cell surface through this specific inhibitor prevents binding and causes microglial populations to deplete significantly. 

Mice from three age groups – young adult (3 months), aged (18 months), and very old (34 months) – were treated with the inhibitor, and the results were compared to non-depleted controls. Retinal morphology was analyzed through transmission electron microscopy (TEM) and whole-mount immunofluorescent imaging.  Visual function was assessed with electroretinography (ERG), and on the behavioral level using optomotor response testing with Striatech’s Optodrum™ .

Subretinal Microglia: The Unassumed Helpers of Aging Retinas

As mice aged, microglial migration into the subretinal space became more prevalent, particularly in the 18- and 34-month-old groups. Interestingly, an amoeboid morphology—indicative of activation and marked by enlarged cell bodies and retracted processes—was the most prevalent microglial form in aged mice. Activated microglia are immune-responsive and phagocytic as opposed to their homeostatic, so called ramified, counterparts. Their presence is a sign of disturbed homeostasis and cellular damage. 

TEM revealed that these microglia were actively phagocytizing photoreceptor outer segments and lipid droplets, effectively stepping in to support the declining functionality of RPE cells. Immunofluorescent staining confirmed that these cells were compensating for the RPE’s weakened ability to clear waste by taking over RPE cell functions.

Visual Decline Accelerates Without Microglia

To measure the impact of microglial depletion, researchers turned to Striatech’s Optodrum™ for optomotor response testing. Mice were subjected to reflexive tracking of rotating black and white stripes to assess their visual acuity. As expected, visual acuity declined with age. However, somewhat unexpected, there was no noticeable difference in visual acuity between the 34-month-old mice with or without microglia. To investigate this further, Karg et al performed a more detailed study of the optomotor response behavior and found that the non-depleted mice had better contrast sensitivity. This showed conclusively that subretinal microglia play a protective role in maintaining vision.

Electroretinography (ERG) further underlined these findings. The c-wave, which reflects RPE cell health, was significantly reduced in microglia-depleted 34-month-old mice compared to controls. Other ERG waves, such as a-waves telling of photoreceptor activity and b-waves measuring health of inner retinal layers, remained unaffected, highlighting the specific reliance of RPE health on microglial support.

Deterioration of RPE cells is accelerated without microglia

Without microglia, the aging RPE faced a steeper decline. TEM imaging revealed a decrease in RPE cell number and an increase in cell diameter, indicative of stress and loss of structural integrity. RPE cells increased in diameter, compensating for overall declining populations to ensure a proper blood-retina-barrier. This alteration in morphology and structural breakdown likely exacerbates vision loss, as the weakened RPE can no longer support photoreceptors effectively.  Accelerated RPE cell loss highlights the critical role of microglia in supporting RPE cell survival during aging.

The support of RPE cells through microglia could offer promising therapeutic applications

This study paints a compelling picture: subretinal microglia are protectors of the RPE, stepping in to support RPE cells as they falter with age. By phagocytizing lipid droplets and photoreceptor outer segments, these microglia take on RPE cell responsibilities and mitigate the damage that could otherwise spiral into severe retinal degeneration.

The implications are exciting. Enhancing the supportive functions of subretinal microglia could open new therapeutic doors for conditions like AMD. While much remains to be explored, one thing is clear – microglia in the subretinal space are allies in the fight to preserve vision in old age.

Blog author: Emilia Kawecka, Technical University of Munich, Student Assistant at Striatech

Original paper: Karg, M.M., Moorefield, M., Hoffmann, E. et al. Microglia preserve visual function in the aging retina by supporting retinal pigment epithelial health. Immun Ageing 20, 53 (2023). https://doi.org/10.1186/s12979-023-00358-4