Date Published: February 4, 2013
Publisher: Public Library of Science
Author(s): Nigel I. Wood, Catherine J. McAllister, Marc Cuesta, Juliet Aungier, Eloise Fraenkel, A. Jennifer Morton, Shin Yamazaki. http://doi.org/10.1371/journal.pone.0055036
The R6/2 transgenic mouse model of Huntington’s disease (HD) shows a disintegration of circadian rhythms that can be delayed by pharmacological and non-pharmacological means. Since the molecular machinery underlying the circadian clocks is intact, albeit progressively dysfunctional, we wondered if light phase shifts could modulate the deterioration in daily rhythms in R6/2 mice. Mice were subjected to four x 4 hour advances in light onset. R6/2 mice adapted to phase advances, although angles of entrainment increased with age. A second cohort was subjected to a jet-lag paradigm (6 hour delay or advance in light onset, then reversal after 2 weeks). R6/2 mice adapted to the original shift, but could not adjust accurately to the reversal. Interestingly, phase shifts ameliorated the circadian rhythm breakdown seen in R6/2 mice under normal LD conditions. Our previous finding that the circadian period (tau) of 16 week old R6/2 mice shortens to approximately 23 hours may explain how they adapt to phase advances and maintain regular circadian rhythms. We tested this using a 23 hour period light/dark cycle. R6/2 mice entrained to this cycle, but onsets of activity continued to advance, and circadian rhythms still disintegrated. Therefore, the beneficial effects of phase-shifting are not due solely to the light cycle being closer to the tau of the mice. Our data show that R6/2 mice can adapt to changes in the LD schedule, even beyond the age when their circadian rhythms would normally disintegrate. Nevertheless, they show abnormal responses to changes in light cycles. These might be caused by a shortened tau, impaired photic re-synchronization, impaired light detection and/or reduced masking by evening light. If similar abnormalities are present in HD patients, they may suffer exaggerated jet-lag. Since the underlying molecular clock mechanism remains intact, light may be a useful treatment for circadian dysfunction in HD.
It is now well established that sleep disruption and changes to circadian cycles of activity are important symptoms of neurodegenerative diseases such as Alzheimer’s disease , Parkinson’s disease , and Huntington’s disease (HD) . We have shown previously that the sleep/wake dysfunction seen in HD patients is recapitulated in the R6/2 mouse, a transgenic mouse model of HD . The R6/2 transgenic mouse was one of the first, and is the best characterized, model of HD. It expresses exon 1 of the HD gene, with an expanded CAG repeat . The R6/2 mouse displays many features of the symptoms that are seen in human HD patients, including motor, cognitive, emotional and social impairments –. R6/2 mice also display a disintegration of their daily rhythms of rest and activity, and disruption to the temporal expression of both clock genes and clock-controlled genes in vivo, –. However, when the suprachiasmatic nuclei (SCN) are removed from dysrhythmic mice and cultured in vitro, the endogenous rhythm of clock gene expression is normal . This suggests that the molecular machinery underlying circadian rhythms generated by the SCN is intact, and raises the possibility that the circadian dysfunction in R6/2 mice may respond to treatments aimed at activating the deficient pathways afferent to and/or efferent from the SCN. In support of this, we have demonstrated that the pharmacological imposition of sleep through administration of the sedative alprazolam can produce improvements in rest/activity cycles, and also in cognitive behavior . In addition, improvements in daily rhythms were obtained through the imposition of a regime of time-restricted feeding . Since the circadian rhythms of R6/2 mice can be improved through both pharmacological and non-pharmacological means, we were interested in determining whether or not R6/2 mice were capable of responding appropriately to a challenge to their circadian mechanisms invoked by alterations in the light/dark cycle. It is known that R6/2 mice have retinal degeneration , which may cause visual defects. Although it has not been studied, it is possible that these histological abnormalities extend to the intrinsically photosensitive retinal ganglion cells; thus retinal degeneration may contribute to the loss of circadian rhythmicity. We planned to subject the mice to repeated light phase-shifts of 4–6 hours, and monitor how well (and for how long) their daily activity rhythms adapted to the new time of light onset. However, there is a potential difficulty in using this approach, since we would, in effect, be inducing jet-lag in the mice. It is well known that the disruption to the circadian system that is a consequence of jet-lag can impact on cognitive performance and health –, as a result of de-synchronization between Zeitgebers and the endogenous clockwork. It has also been found that inducing experimental jet-lag in mice has deleterious effects on age at death, and that advances in the light cycle have a more profound effect than delays . Therefore, in addition to testing a set of serial advances in light onset, we tested the response of R6/2 mice to a second experimental jet-lag regime (a shift/reversal paradigm that comprised 6 hours phase-advance followed by 6 hours phase-delay, and vice versa). As well as providing information as to whether or not R6/2 mice could adapt to these changes in their light cycles, we anticipated that this study could also provide clues as to how well HD patients might cope with jet-lag.
As expected , –, under normal 12∶12 LD conditions, R6/2 mice start to show a breakdown in their daily cycling from approximately 12–15 weeks of age (shown by the absence of a robust circadian rhythm and the increased ratio of light/dark activity). By 16 weeks of age they showed no clear daily rhythm. However, when the mice were subjected to a series of 4 hour light phase-shifts at 2 week intervals, R6/2 mice adjusted to the new onsets of light and dark, and retained a significant daily rhythmicity at 16 weeks of age. These data suggest not only that R6/2 mice are capable of adapting to changes in the light/dark cycle, but also that the process of doing so may halt the disintegration of daily behavioral rhythmicity. When we challenged the mice with a shift/reversal paradigm, we found that mice of both genotypes adjusted to the phase-advance as quickly as they did to the phase-delay. It was particularly interesting to see that the R6/2 mice clearly entrained to a 6 hour phase-shift at an age where their daily cycling of light/dark activity was typically breaking down (16 weeks). Indeed, when challenged at 17–18 weeks of age by a reversal of 6 hour phase-shifts back to the “normal” LD cycle, the R6/2 mice showed evidence of re-entraining. These results strongly support our hypothesis that although R6/2 mice exhibit disrupted daily rhythms, the underlying molecular machinery is intact and responsive to change.