Circadian alignment and physiology

Entrainment enables circadian clock systems to anticipate and adapt to reoccurring environmental changes by daily alignment of endogenous and exogenous rhythms.

Under normal conditions photic timing information, received by the SCN, is passed on to the periphery in order to establish stable phase relationships among various tissue clocks and phase align their physiological outputs with external requirements. SCN lesion has been shown to result in behavioral arrhythmicity and phase dispersion of peripheral tissue clocks [33], [35]. In addition to the photic cues, non-photic entrainment signals can be integrated in a tissue-specific manner and may induce desynchrony between peripheral circadian clocks and the SCN if occurring in

dissonance with the light-dark cycle, e.g. feeding-fasting, sleep-wake, rest-activity, and temperature cycles [82], [85], [111].

Moreover, body clocks have been found to regulate transcriptional programs of clock-controlled genes (CCG) with little overlap between tissues [44], [46], [81], [246]. Thus, expression of these tissue-specific CCGs must be coordinated systemically to generate coherent circadian rhythms on the level of the organism. For example, it has been demonstrated that disruption of liver clocks, in otherwise wildtype animals, results in the loss of a majority of rhythmic hepatic transcripts leading to aberrant systemic glucose homeostasis and metabolic disruption [247]. Therefore, it appears that the circadian system must maintain a delicate balance between external or SCN driven synchronization and tissue-specific regulation of circadian organ functions in order to guarantee the correct temporal coordination of physiological processes in accordance with rhythmic environmental demands (Figure 1-8).

Figure 1-8: States of circadian (de)synchrony

(A) Aligned circadian clocks exists in a state of resonance between internal rhythms, environmental cycles, and behavior: the SCN is synchronized by the daily light-dark cycle and transmits timing information to the rest of the body via direct neuronal projections or the regulation of rhythmic hormone release, body temperature, and feeding. Subordinate clocks are entrained by the SCN but may consolidate system-level synchrony by rhythmic physiology and production of secreted/humoral factors (B) Disrupted or falsely timed external and internal cycles can induce disruption of the circadian system by constant (mistimed) phase resetting leading to internal desynchrony. This prevents the anticipation of rhythmic environmental changes, enables only passive responsiveness or induces complete misalignment of endogenous and exogenous rhythms. (adapted from [248])

1.5.1 The synchronized state: circadian physiology

In healthy organisms, phase relationships between exogenous Zeitgeber and endogenous circadian cycles, as well as among individual tissue clocks are stable, resulting in mutual reinforcement [248]. Phase coherence is achieved by daily phase resetting of the SCN via the light-dark cycle, converting external to central timing information. This time information is further transmitted to other body clocks either via direct neuronal projections or indirect regulation of physiological processes that impinge on the core clock machinery. Tissue-specific regulation of rhythmic biological functions has been shown take place on the level of genes, transcripts [41], [249]–

[251], proteins [252]–[254], and metabolites [255]–[258]. Thus, in order to drive the temporal coordination of diverse organ functions in such a way that they align with external demands and reinforce synchronized rhythmicity on the system-level, synchronizing signals must arrive at their target sites during the right time of the circadian cycle (Figure 1-8 A) [86]. One example for complex feedback regulations of the circadian system are glucose homeostasis and hunger regulation [119], [259]–

[262]. Involved hormone rhythms are driven by feeding-fasting cycles, generated by the SCN or the FEO, or by cellular oscillators in liver, pancreas, and adipose tissue themselves. Therefore, the same signals serve as input signals to tissue clocks, while at the same time functioning as feedback signals among body clocks to give information about the metabolic state of the organism. As a result of such feedback regulations, as well as of fluctuating environmental conditions, the circadian system has to be able to differentially integrate timing information coming from the external environment, the central pacemaker, and other tissue clocks. How body clocks are able to distinguish between origin, nature, and strength of the input signals, as well as how they are able to respond in a time- and tissue-specific manner to established synchronized physiological outputs remains elusive. Some studies suggest that tissue and phase specificity is achieved on the transcript level through distinct sets of circadian enhancer elements [263], histone modification [264], [265], chromatin landscape [266], or alternative TTFL usage [81]. Additionally, post-translational regulations or metabolic and redox states of the cell have been suggested to be involved in the generation and maintenance of tissue-specific oscillations and functions [267], [268].

1.5.2 The desynchronize state: circadian pathology

Modern lifestyle is commonly associated with behavior leading to the disruption of the synchronized circadian state, e.g. artificial lighting, use of electronic devices, shift work, long distance travel, social responsibilities, abnormal mealtimes, and food excess. As a consequence, external Zeitgeber or SCN derived synchronization signals are transmitted to the circadian system at times when they do not induce phase alignment but rather phase dispersion between internal and external or among internal clocks.

Consequently, “circadian misalignment” is expected to result in non-resonating feedback regulations, which further contribute to the incoherence of rhythmic physiological processes, temporal instability of the circadian system (Figure 1-8 B), as well as development of associated pathologies [248]. For example, studies with clock deficient animals have shown that disruption of the circadian system results in serious health issues including metabolic disruption [122], [269], [270], cardiovascular disease [271], [272], obesity [273], premature aging [274], as well as cancer [275]. Moreover, while resonating endogenous and exogenous cycles appear to provide a selective advantage [276], discrepancy between the two has been associated with reduced survival in lower organisms [277], [278]. Also for humans, it has been shown that forced desynchrony protocols (keeping subjects in artificially short or long days resulting in non-resonating endogenous and behavioral cycles) lead to cognitive, cardiac and metabolic malfunctions [279]–[283], as well as alterations of the rhythmic transcriptome [284]. Underlying mechanisms of pathologies associated with circadian misalignment are suggested to derive from altered circadian oscillations on transcript [285], protein [286], and metabolite level [287], [288]. Moreover, disruption of VIP dependent coupling in the SCN has been demonstrated to impact entrainment to external Zeitgeber cycles (on behavioral and peripheral clock level) [53], [289], [290], suggesting that intercellular coupling constitutes and additional layer of system-level synchronization of the mammalian circadian system. Additionally, since tissue clocks regulate circadian processes in a tissue-specific manner, it has been suggested that circadian desynchrony results in organ-specific pathologies [291]. Therefore, it is possible that also intercellular coupling among peripheral circadian oscillators plays an important role for entrainment of peripheral tissue clocks, as well as the temporal coordination of tissue-specific circadian physiology.