Fluorescence Lifetime Imaging of Tetracycline-Stained Retinal Hydroxyapatite: an Early Biomarker for Age-Related Macular Degeneration?

Age-related macular degeneration (AMD) is the most common cause of loss of vision in the elderly in the developed world, affecting over ten million in the United States alone. It is widely held that the atrophy of the light-sensitive cells and their associated neurons in the retina has its origin in protein- and lipid-rich deposits between the retinal pigment epithelium (RPE) and Bruch's membranes; the best known of these deposits are called drusen. Recently, we found that drusen contained microscopic spherules of hydroxyapatite (HAP; Ca5(PO4)3OH), the hard form of calcium phosphate found in bones and teeth, and that these spherules became coated in vivo with proteins characteristic of drusen such as amyloid beta, vitronectin, and complement factor H, a variant of which is the strongest genetic risk factor for AMD (Thompson, et al., PNAS 2015 PMID: 25605911). These findings and others led us to propose that the growth of drusen is nucleated by the HAP spherules, and thus they might be an early biomarker to be used for screening for AMD. We found that the HAP stained by certain tetracyclines exhibited substantial increases in fluorescence lifetime upon binding, significantly longer than the lifetime of the retina background fluorescence imaged by Schweitzer, Zinkernagel, and their colleagues. Thus we were able to image tetracycline-stained drusen in vitro from aged human and macaque retinas using FLIM. Recent results will be shown.