Fluorescence Lifetime for Studying Ophthalmic Diseases in Animal Models

Autofluorescence of the retina features a rich landscape of endogenous fluorophores emitting in the broad spectral range from UV to far red. The emission of retinal fluorophores and their intensity distribution reflect the composition of the tissue and might be considered an indicator of the pathology. However, the intensity of the fluorophores tells only a part of the story. Potentially, more information could be obtained from fluorescence lifetime measurements. Fluorescence lifetime is an average time the fluorophore remains in the excited state. The lifetime is an intrinsic property of a fluorophore that does not depend on fluorophore concentration or methods of measurements. Each individual fluorophore in the eye, such as nicotinamide adenine dinucleotide (NADH), flavin adenine dinucleotide (FAD), lipofuscin, melanin, or collagen, has its unique fluorescent lifetime.¹ In contrast to the intensity, fluorescence lifetime does not depend on the wavelength of excitation or duration of light exposure and is not affected by photobleaching. This parameter is sensitive to a great variety of environmental factors, such as pH and presence of oxygen, proteins, other fluorophores, and quenchers. A combination of environmental sensitivity and independence of fluorescence intensity makes the fluorescence lifetime a separate yet complementary method to traditional fluorescence intensity measurements.² 

In their first publication, Dysli and colleagues³ described the design of the Fluorescence Lifetime Imaging Ophthalmoscope (FLIO) allowing reproducible measurements of fluorescence lifetimes of the macula in human subjects. In the current publication, Dysli et al.,⁴ have further modified the FLIO system for a mouse, a challenging task given that the ocular size of the mouse eye is 10 times smaller than that of the human eye. Powered by the developed technology, the authors mapped the fluorescence lifetime of a retina in several strains of mice, including mice featuring slow retinal degeneration. A unique fluorescence lifetime signature for each strain was observed, thus opening up a new direction in investigation, diagnostics, and treatment of ophthalmic diseases in animal models.