Age-Related Changes in the Circadian System Unmasked by Constant Conditions

Takahiro J. Nakamura,Takahiro Ishikawa,Takashi Kudo,Isao T. Tokuda,Gene D. Block,Christopher S. Colwell,Wataru Nakamura

doi:10.1523/eneuro.0064-15.2015

Abstract

Circadian timing systems, like most physiological processes, cannot escape the effects of aging. With age, humans experience decreased duration and quality of sleep. Aged mice exhibit decreased amplitude and increased fragmentation of the activity rhythm, and lengthened circadian free-running period in both light-dark (LD) and constant dark (DD) conditions. Several studies have shown that aging impacts neural activity rhythms in the central circadian clock in the suprachiasmatic nucleus (SCN). However, evidence for age-related disruption of circadian oscillations of clock genes in the SCN has been equivocal. We hypothesized that daily exposure to LD cycles masks the full impact of aging on molecular rhythms in the SCN. We performed ex vivo bioluminescent imaging of cultured SCN slices of young and aged PER2::luciferase knock-in (PER2::LUC) mice housed under LD or prolonged DD conditions. Under LD conditions, the amplitude of PER2::LUC rhythms differed only slightly between SCN explants from young and aged animals; under DD conditions, the PER2::LUC rhythms of aged animals showed markedly lower amplitudes and longer circadian periods than those of young animals. Recordings of PER2::LUC rhythms in individual SCN cells using an electron multiplying charge-coupled device camera revealed that aged SCN cells showed longer circadian periods and that the rhythms of individual cells rapidly became desynchronized. These data suggest that aging degrades the SCN circadian ensemble, but that recurrent LD cycles mask these effects. We propose that these changes reflect a decline in pacemaker robustness that could increase vulnerability to environmental challenges, and partly explain age-related sleep and circadian disturbances.

Highlights

In mammals, daily behavioral and physiological rhythms are generated by a complex endogenous timing system involving cell, tissue, and organ-level mechanisms (Mohawk et al, 2012)
The wheel-running activity of young and aged mice housed under LD cycles was monitored for at least 10 d, and the mice were killed at ZT10-11 (Fig. 1A)
We examined PER2::luciferase knock-in (PER2)::LUC rhythms in the suprachiasmatic nucleus (SCN) of mice housed under normal LD cycles

Summary

Introduction

Daily behavioral and physiological rhythms are generated by a complex endogenous timing system involving cell-, tissue-, and organ-level mechanisms (Mohawk et al, 2012). Circadian rhythms are generated within mammalian cells by interlocking transcriptional–translational feedback loops involving a family of clock genes (Partch et al, 2014). Transcription of Period (Per) and Cryptochrome (Cry) is driven by a complex of basic helix-loophelix transcriptional factors, including circadian locomotor output cycles kaput (CLOCK) and Aryl hydrocarbon receptor nuclear translocator-like 1 (BMAL1), and repressed by PER protein-mediated negative feedback. The SCN comprises a heterogeneous population of cells that differ in endogenous period, pacemaking ability, and expression of clock genes and neuropeptides (Honma et al, 2012; Hastings et al, 2014). In the SCN, a large population of different cells is synchronized to produce a coherent, robust rhythm

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