Stability of Wake-Sleep Cycles Requires Robust Degradation of the PERIOD Protein

Abstract

Summary Robustness in biology is the stability of phenotype under diverse genetic and/or environmental perturbations. The circadian clock has the remarkable stability of period and phase which—unlike other biological oscillators—is maintained over a wide range of conditions. Here we show that the high fidelity of the circadian system stems from robust degradation of the clock protein, PERIOD. We show that PERIOD degradation is regulated by balanced ubiquitination/deubiquitination, and disruption of the balance can destabilize the clock. In mice with loss-of-function mutation of the E3 ligase gene β-Trcp2, the balance of PERIOD degradation is perturbed, and the clock becomes dramatically unstable, presenting a unique behavioral phenotype unlike other circadian mutant animal models. We believe our data provide a molecular explanation for how circadian phases such as the wake-sleep onset times can become unstable in humans and present a unique mouse model to study human circadian disorders with unstable circadian rhythm phases.

Basal leakage in oscillation: coupled transcriptional and translational control using feed-forward loops

“…(B) Experimental result of β-Trcp2 mutant cells showed unstable rhythms and reduced amplitude of PER gene oscillations. Data were obtained and redrawn from D’Alessandro et al 2017 (2). (C) The effect of changing the degradation control in model M5 (upper panel) and FFL (lower model) during our simulations.…”

“…For circadian clocks, several current models had successfully captured the essential dynamic behavior of the clock system mainly with transcriptional regulation. Previous studies have shown that the 26s (1, 2) proteasome degradation controls are important in maintaining the stability of circadian rhythms. However, how the loss-of-function or over-expression mutant of this targeted degradations lead to unstable oscillation is still unclear.…”

“…CRY protein is targeted for proteasomal degradation by two different F-box proteins, Fbxl3 and Fbxl21 (43-47). However, mutations in these two E3 ligase proteins did not alter the behavior of circadian rhythms markedly as compared with phenotypes that were caused by mutations in other clock genes (2). A recent study showed that alteration of β-Trcp1 and β-Trcp2 proteins, other F-box proteins that will trigger the degradation of PER protein, severely altered the clock’s function.…”

“…A recent study showed that alteration of β-Trcp1 and β-Trcp2 proteins, other F-box proteins that will trigger the degradation of PER protein, severely altered the clock’s function. In the β-Trcp double-mutant mice, the oscillation of many clock genes are highly unstable, as indicated by the greatly increased variability in circadian rhythm (2).…”

“…To the best of our knowledge, why this mutation can lead to high variability in the plant’s clock is still unknown. For the mammalian clock, D’Alessandro et al proposed that such unstable circadian rhythms in β-Trcp mutant mice may come from loss of nonlinear degradation of PER protein (2). However, why such loss of nonlinear degradation can lead to unstable behavior is still not clear.…”