Light Influences Time-of-Day Differences in the Expression of Clock Genes and Redox Genes Involved in Glutathione Homeostasis in the Lens.

Purpose: To determine whether light influences time-of-day differences in the expression of circadian clock and redox genes and the antioxidant glutathione (GSH) in cultured rat lenses. Methods: Rat Wistar lenses (6 weeks) were cultured over a 24-hour period either in 12 hours of light and then 12 hours of dark (12-hour light/12-hour dark) or in constant darkness (12-hour dark/12-hour dark). Lenses were collected at 0 hour, after 12 hours, and then 24 hours after culture, and either quantitative PCR was performed to examine the expression of core clock genes and GSH-related redox genes or high-performance liquid chromatography was used to measure intracellular GSH levels. Lenses were collected at 10 AM, 2 PM, 6 PM, and 10 PM, fixed, sectioned, and labeled with the antibodies against the light-sensing melanopsin protein. Results: Under 12-hour light/12-hour dark conditions, core clock genes and redox genes involved in GSH synthesis displayed time-of-day differences in their expression patterns which were not maintained under constant darkness. GSH levels remained constant under both lighting conditions. Melanopsin was localized to the epithelial cells at the anterior epithelium but absent from epithelial cells in the equatorial epithelium and fiber cells. Conclusions: The lens possesses the ability to detect light via melanopsin localized exclusively in the anterior lens epithelium. This may act as a cue for the lens to drive day and night differences in the expression of clock genes and/or redox genes, ensuring that GSH levels are maintained at normal physiological levels to provide protection from oxidative stress across the light/dark cycle.