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X. Liu, C. Ribelayga; Melanopsin-Containing Ganglion Cells Express Molecular Components of the Circadian Clock. Invest. Ophthalmol. Vis. Sci. 2010;51(13):670.
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The retina contains autonomous circadian clocks that regulate many aspects of its physiology. The core machinery of circadian clocks is a self-sustained cell-based mechanism that relies on a specific set of genes (the clock genes) and their protein products interlocked in recurrent transcriptional/post-translational feedback loops. Although clock gene expression is widespread in the mammalian retina, in particular in the inner nuclear and ganglion cell (GC) layers, the phenotype of the cells expressing the clock genes remains largely unknown. Here we sought to examine whether core clock components were expressed in the subset of GCs that contain the visual pigment melanopsin and are intrinsically photosensitive (ipRGCs).
Clock protein expression was investigated by double-label immunocytochemistry of formaldehyde-fixed mouse retinas using an anti-melanopsin antibody (1/5000, Adv. Targeting Syst.) to identify ipRGCs. Retinal sections were reacted one week with one of the following antibodies from Chemicon: anti-PER1 (1/500), anti-PER2 (1/100), anti-PER3 (1/100), anti-CLOCK (1/100), anti-CRY2 (1/100), or anti-BMAL1 (1/1000), followed by 2 days with the anti-melanopsin antibody. Secondary antibodies were conjugated to DyLight fluorophores (Jackson ImmunoRes.) and applied for 2 hrs.
Melanopsin-immunoreactivity was restricted to a small subset of cells in the ganglion cell layer with large dendritic profile terminating in the outer-most sub-layer of the IPL, characteristic of ipRGCs. Although the expression of the circadian clock proteins varied in intensity among cells of the inner nuclear and GC layers, all of the clock components tested were expressed in melanopsin-positive ganglion cells.
These findings indicate that the most important clock components are expressed in ipRGCs, suggesting that these cells contain an endogenous circadian clock. The circadian clock in ipRGCs could control the ipRGC intrinsic sensitivity, which is under circadian control (Cheng et al., 2009), drive some circadian rhythms in the SCN (Lee et al., 2003), and/or control retinal dopaminergic cell activity on a circadian basis (Zhang et al., 2008).
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