Functional imaging of retinal photoreceptors

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Project summary: This is a R01 renewal to develop functional intrinsic optical signal (IOS) imaging for physiological assessment of retinal photoreceptors. Retinal photoreceptors are known as the primary target of both age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Early detection of eye diseases and objective assessment of therapeutic outcomes are essential steps to prevent vision loss and blindness. Structural only biomarkers cannot provide enough information for evaluating physiological condition of retinal photoreceptors, and combined functional test is frequently required for disease detection and treatment assessment. However, it is time-consuming and costly inefficient to conduct separate structural and functional measurements. Functional IOS imaging, also termed as optoretinography (ORG) or optophysiology, is based on near infrared (NIR) light mapping of stimulus-evoked physiological activities in the retina. Because IOS imaging is based on dynamic processing of retinal images, it can naturally provide structural information offered in traditional fundus photography. During the first grant period, we have demonstrated stimulus evoked IOS response at the outer segment of retinal photoreceptors. The fast photoreceptor-IOS occurs immediately after the onset of the retinal stimulation, differentiating itself from timely delayed IOS changes at the inner retina. The fast photoreceptor-IOS provides a unique marker for objective ORG of photoreceptor physiology, without signal contamination of post-photoreceptor layers. We propose here to characterize biophysical mechanism of the fast photoreceptor-IOS (aim 1); and validate fast photoreceptor-IOS imaging for objective ORG of photoreceptor function in human subjects (aim 2). The first aim is to use animal models to verify the correlation of the fast photoreceptor-IOS to the activation phase of phototransduction. A custom-designed hybrid confocal-OCT ophthalmoscope will be used for in vivo characterization of fast photoreceptor-IOS in WT and rd10 mice. In vitro time-lapse light microscopy will be conducted to characterize transient outer segment response in individual photoreceptors. Comparative electron microscopy of dark- and light-adapted retinal tissues will be implemented to verify light-driven outer segment shrinkage at sub-disc level. The second aim is to verify the feasibility of clinical translation of using fast photoreceptor-IOS imaging for in vivo ORG of human photoreceptors. We have recently demonstrated virtually structured detection (VSD) based super-resolution imaging of individual rods and cones in awake human. During this project, the VSD based super-resolution ophthalmoscopy will be refined to achieve µm level spatial-resolution and ms level temporal-resolution for imaging fast photoreceptor-IOS changes in human photoreceptors. Success of this study will pave the way towards pursuing clinical application of objective ORG of retinal photoreceptors, enabling early disease diagnosis and prompt treatment assessment of AMD, RP and other eye problems which can cause photoreceptor dysfunctions.