Circadian clock control of connexin36 phosphorylation in retinal photoreceptors of the CBA/CaJ mouse strain

Aug 5, 2015Visual neuroscience

Daily rhythm controls a key protein modification in light-sensing eye cells of CBA/CaJ mice

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Abstract

Cx36 phosphorylation is higher under dark-adapted conditions at night than in the daytime.

The strength of electrical coupling between photoreceptor cells is linked to the phosphorylation of Cx36 at two specific sites.
In the C57Bl/6J mouse strain, light or dopamine decreases both tracer coupling and Cx36 phosphorylation in photoreceptors.
In contrast, darkness or a dopaminergic antagonist increases tracer coupling and Cx36 phosphorylation, irrespective of the time of day.
The CBA/CaJ mouse strain shows that photoreceptor coupling is regulated by both light and a circadian clock, with broader coupling at night compared to daytime.
No significant changes in Cx36 expression or the number of gap junctions were found across the examined conditions.

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The gap-junction-forming protein connexin36 (Cx36) represents the anatomical substrate of photoreceptor electrical coupling in mammals. The strength of coupling is directly correlated to the phosphorylation of Cx36 at two regulatory sites: Ser110 and Ser293. Our previous work demonstrated that the extent of biotinylated tracer coupling between photoreceptor cells, which provides an index of the extent of electrical coupling, depends on the mouse strain. In the C57Bl/6J strain, light or dopamine reduces tracer coupling and Cx36 phosphorylation in photoreceptors. Conversely, darkness or a dopaminergic antagonist increases tracer coupling and Cx36 phosphorylation, regardless of the daytime. In the CBA/CaJ strain, photoreceptor tracer coupling is not only regulated by light and dopamine, but also by a circadian clock, a type of oscillator with a period close to 24 h and intrinsic to the retina, so that under prolonged dark-adapted conditions tracer coupling is broader at night compared to daytime. In the current study, we examined whether the modulation of photoreceptor coupling by a circadian clock in the CBA/CaJ mouse photoreceptors reflected a change in Cx36 protein expression and/or phosphorylation. We found no significant change in Cx36 expression or in the number of Cx36 gap junction among the conditions examined. However, we found that Cx36 phosphorylation is higher under dark-adapted conditions at night than in the daytime, and is the lowest under prolonged illumination at any time of the day/night cycle. Our observations are consistent with the view that the circadian clock regulation of photoreceptor electrical coupling is mouse strain-dependent and highlights the critical position of Cx36 phosphorylation in the control of photoreceptor coupling.

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Circadian clock control of connexin36 phosphorylation in retinal photoreceptors of the CBA/CaJ mouse strain

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