How do calcium and protein build up in the eye and what’s the link to AMD?

Research details

  • Type of funding: Project Grant
  • Grant Holder: Dr Alan Stewart
  • Institute: University of St Andrews
  • Region: Scotland
  • Start date: July 2015
  • End Date: June 2017
  • Priority: Causes
  • Eye Category: AMD

Overview

A hallmark feature of age-related macular degeneration (AMD) is that deposits of protein and fat (called drusen) build up underneath the light-sensitive part of the eye. Light-induced damage to cells in the retina means that they need to be repaired constantly. But as we age, the eye becomes less able to clear away the debris.

Researchers think that when drusen build up, they could be blocking the exchange of waste and nutrients between the eye and the blood circulation. This in turn could be what leads to the death of light-sensitive cells (photoreceptors), causing sight loss.

Recently, the team discovered tiny bits of a type of calcium – called HAP, usually found in bone – at the centre of deposits in donated eye tissue. The deposits contained proteins previously linked to AMD, so in this project the team is finding out which proteins circulating in blood plasma contribute to deposits that form around HAP. They will also try to measure the amount of ‘HAP-binding’ proteins in blood plasma samples from 30 people with late-stage AMD, and learn more about how the proteins bind together with HAP.

Results from the project will tell us more about how drusen form and suggest new targets for treatment to prevent them, perhaps leading to preventing AMD altogether.

  • Scientific summary

    Identifying the hydroxyapatite interactome: clarifying the involvement of serum proteins in the formation of sub-retinal pigment epithelial deposits

    A major feature of the ageing retina is the thickening of Bruch’s membrane and the formation of subretinal pigment epithelial (RPE) deposits. These sub-RPE deposits can block metabolic exchange between the choroidal blood circulation and the retina leading to sensory retinal degeneration and eventually to age-related macular degeneration (AMD).

    The team has identified the presence of small (0.5-20 μm diameter) hydroxyapatite (HAP) spherules within sub-RPE deposits isolated from human cadaver eyes. Furthermore, protein aggregates containing the AMD-associated proteins complement factor H (CFH) and histidine-rich glycoprotein (HRG) were found to form on the surface of the HAP spherules. These new data led the team to hypothesise that the spherules provide nucleation sites for sub- RPE deposit formation, where the initiation, growth and retention of deposits are controlled by the binding of proteins present in the sub-RPE space to the spherules.

    In this study, HAP-binding proteins in the plasma of genotyped late-stage AMD patients are being isolated and quantitatively identified by SWATH mass spectrometry. The HAP binding properties of CFH, HRG and newly identified relevant proteins are being probed using an established binding assay and the presence of newly identified proteins of interest in sub-RPE deposits is being- confirmed in human cadaver eyes using immunofluorescent imaging. This study will provide insight into factors controlling the formation of sub-RPE deposits prior to the development of AMD and examines the contribution of plasma to sub-RPE deposit formation.