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R. N. Jinks, O. C. Yi, S. J. Hartman; Circadian Changes in the Axial Actin Cytoskeleton in the Rhabdomeral Microvilli of the Photoreceptor Cells of the Lateral Eye of Limulus. Invest. Ophthalmol. Vis. Sci. 2008;49(13):185.
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Transient rhabdom shedding (TRS), the invertebrate analog of rod outer segment disc shedding, is primed by a minimum of 3-5 h of overnight circadian efferent outflow and triggered by dawn in the horseshoe crab Limulus (J. Neurosci. 4:2792). Efferent input to the lateral eye is octopaminergic and activates an adenylate cyclase/cAMP-dependent protein kinase (PKA) pathway that leads to phosphorylation of the actin-binding protein myosin III (LpMyoIII) (J. Neurosci. 18:4548). Phosphorylation of LpMyoIII in lateral eye photoreceptor cells may act to decrease actin-LpMyoIII interactions in preparation for TRS at dawn. LpMyoIII immunoreactivity (IR) at the rhabdomeral microvilli decreases at night with a circadian rhythm (ASCB Abstracts 2007:185). We investigated whether the clock causes a similar decrease in the frequency of actin filaments within the axial cytoskeleton of the rhabdomeral microvilli in the lateral eye of Limulus.Methods/
Lateral eyes with intact or chronically-transected lateral optic nerves (iLON vs cLON, respectively) were excised at noon under ambient lighting or at midnight in darkness and fixed for f-actin preservation in 2% glutaraldehyde, 1% OsO4, and 10% sucrose in 0.45M sodium acetate buffer (pH 6.4). The frequency of microvillar axial actin filaments is greatest in iLON eyes at noon, and decreases by ~2 times at midnight in iLON eyes (p<0.05). The frequency of axial actin filaments in both daytime and nighttime cLON eyes is reduced to levels not significantly different from those of iLON eyes fixed at midnight (p>0.05).
These data suggest that the circadian clock drives a nighttime reduction in the frequency of the microvillar axial actin cytoskeleton that may act to prime the rhabdom for TRS at dawn. Transection of the LON "locks" the frequency of axial actin filaments into a nighttime-like state. Because cLON eyes do not display TRS at dawn, and the frequency of axial actin cores in cLON eyes is not significantly different from nighttime levels in iLON eyes, circadian priming of the lateral eye for TRS must involve a more sophisticated mechanism than depolymerization of the axial actin and/or actin-LpMyoIII decoupling.
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