Abstract
The retina is both a sensory organ and a self-sustained circadian clock. Gene targeting studies have revealed that mammalian circadian clocks generate molecular circadian rhythms through coupled transcription/translation feedback loops which involve 6 core clock genes, namely Period (Per) 1 and 2, Cryptochrome (Cry) 1 and 2, Clock, and Bmal1 and that the roles of individual clock genes in rhythms generation are tissue-specific. However, the mechanisms of molecular circadian rhythms in the mammalian retina are incompletely understood and the extent to which retinal neural clocks share mechanisms with the suprachiasmatic nucleus (SCN), the central neural clock, is unclear. In the present study, we examined the rhythmic amplitude and period of real-time bioluminescence rhythms in explants of retina from Per1-, Per2-, Per3-, Cry1-, Cry2-, and Clock-deficient mice that carried transgenic PERIOD2::LUCIFERASE (PER2::LUC) or Period1::luciferase (Per1::luc) circadian reporters. Per1-, Cry1- and Clock-deficient retinal and SCN explants showed weakened or disrupted rhythms, with stronger effects in retina compared to SCN. Per2, Per3, and Cry2 were individually dispensable for sustained rhythms in both tissues. Retinal and SCN explants from double knockouts of Cry1 and Cry2 were arrhythmic. Gene effects on period were divergent with reduction in the number of Per1 alleles shortening circadian period in retina, but lengthening it in SCN, and knockout of Per3 substantially shortening retinal clock period, but leaving SCN unaffected. Thus, the retinal neural clock has a unique pattern of clock gene dependence at the tissue level that it is similar in pattern, but more severe in degree, than the SCN neural clock, with divergent clock gene regulation of rhythmic period.
Highlights
Numerous aspects of retinal physiology and function are under the control of an intrinsic retinal circadian clock, including rod disk shedding [1], melatonin release [2,3], dopamine synthesis [4,5,6], gamma-aminobutyric acid (GABA) turnover rate and release [7], extracellular pH [8], electroretinogram (ERG) b-wave amplitude [9], and circadian clock gene expression [10,11,12]
PER2::LUC expression in Per1+/+ retinal and suprachiasmatic nucleus (SCN) explants was robustly rhythmic for multiple circadian cycles, and media change on Day 8 partly restored the amplitude of the ongoing oscillations
Our present study revealed that the molecular circadian rhythms expressed by the neural retina exhibit distinct dependence on individual core clock genes compared to those expressed by the SCN neural clock
Summary
Numerous aspects of retinal physiology and function are under the control of an intrinsic retinal circadian clock, including rod disk shedding [1], melatonin release [2,3], dopamine synthesis [4,5,6], gamma-aminobutyric acid (GABA) turnover rate and release [7], extracellular pH [8], electroretinogram (ERG) b-wave amplitude [9], and circadian clock gene expression [10,11,12]. Mammalian tissues generate molecular circadian rhythms through self-sustaining transcription/translation feedback loops in which two transcription factors CLOCK and BMAL1 periodically drive the expression of three Period genes (Per1-3) and two Cryptochrome genes (Cry1-2), and the resulting PER and CRY protein complexes translocate back into the nucleus to suppress their own transcription [17]. The central neural circadian clock (the suprachiasmatic nuclei, SCN) can more readily compensate for loss of individual clock genes compared to peripheral tissue circadian oscillators (e.g. liver or fibroblast), possibly because of strong inter-neural communication and the expression of Npas, a Clock paralog [18,19]. In the SCN the only single clock gene knockout (KO) able to ablate rhythmicity is Bmal, whereas in peripheral tissue clocks Bmal, Per, Cry and Clock are all individually required for rhythms generation [9,18,20]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
