The genetics of mammalian circadian order and disorder: implications for physiology and disease

Abstract

Circadian cycles affect a variety of physiological processes, and disruptions of normal circadian biology therefore have the potential to influence a range of disease-related pathways. The genetic basis of circadian rhythms is well studied in model organisms and, more recently, studies of the genetic basis of circadian disorders has confirmed the conservation of key players in circadian biology from invertebrates to humans. In addition, important advances have been made in understanding how these molecules influence physiological functions in tissues throughout the body. Together, these studies set the scene for applying our knowledge of circadian biology to the understanding and treatment of a range of human diseases, including cancer and metabolic and behavioural disorders.

Figure 1. A schematic diagram of the SCN and its input and output pathways

The mammalian circadian pacemaker in the hypothalamic suprachiasmatic nucleus (SCN) is organized into two major subdivisions, the core and the shell. The core region of the SCN receives information about the daily light cycle via the retinohypothalamic tract (RHT). Both neuronal and humoral signals act as output signals from the SCN to other regions of the brain and the periphery. The SCN output pathways are responsible for proper timing of diverse physiological functions, including hormone release, sleep-wake cycle, feeding behavior, and thermoregulation. The SCN output to the subparaventricular zone (sPVz) is relayed to the medial preoptic region (MPO) to control circadian rhythms of body temperature; and a separate projection through the dorsomedial nucleus of the hypothalamus (DMH) controls daily hormone secretion (via paraventricular nucleus, PVN) and sleep-wake cycles (via lateral hypothalamus, LH, and ventrolateral preoptic nucleus, VLPO). Figure modified from [permission obtained].

Figure 2. The mammalian circadian clock is composed of a transcriptional–translational feedback network

The circadian clock mechanism involves transcription-translation feedback loops comprised of a set of core clock genes. In mammals, the circadian clock is composed of a primary negative feedback loop involving the genes, Clock/Npas2, Bmal1, Period1 (Per1), Per2, Cryptochrome1 (Cry1) and Cry2. CLOCK/NPAS2 and BMAL1 are basic helix-loop-helix (bHLH) PAS-domain containing transcription factors that activate transcription of the Per and Cry genes. The resulting PER and CRY proteins heterodimerize, translocate to the nucleus and interact with the CLOCK:BMAL1 complex to inhibit their own transcription. With time, the PER-CRY repressor complex is degraded, and CLOCK:BMAL1 can then activate a new cycle of transcription. The secondary autoregulatory feedback loop is composed of REV-ERBα that is a direct target of CLOCK:BMAL1 transcription activator complex. REV-ERBα feeds back to repress Bmal1 transcription and competes with RORa to bind retinoic acid-related orphan receptor response elements (RREs) in the Bmal1 promoter. In addition to the transcriptional activators and repressors, post-translational modification and degradation of circadian clock proteins are critical steps for determining circadian periodicity. Key kinases for PER (and CRY) phosphorylation are Casein kinase 1δ (CK1δ) and CK1ε. One of the roles for phosphorylation of clock proteins is to target them for polyubiquitination and degradation by the 26S proteosomal pathway. Both β-TCRP1 and FBXL3 E3 ubiquitin ligase complexes have been implicated in targeting the PER and CRY proteins, respectively, for degradation. Figure modified from [permission sought].

Figure 3. Central and peripheral oscillators

The expression of the core circadian clock genes is ubiquitous and reflects the presence of circadian oscillators in virtually every tissue and cell in the body. The SCN expresses robust oscillations of PER2::Luciferase activity when isolated in explant culture ex vivo, as do virtually every tissue in the mouse, such as the lung and the liver. Data from .

Figure 4. Interactions between circadian and metabolic systems

Fundamental metabolic pathways such as glycolysis, fatty acid metabolism, cholesterol biosynthesis, and xenobiotic and intermediate metabolism are all under circadian regulation. Metabolism itself can also have effects on circadian clocks: these include NADH and NADPH which can modulate CLOCK:BMAL1 and NPAS2:BMAL1 DNA binding; the PAS domains of NPAS2 which can bind heme and can be modulated by carbon monoxide; and, heme can also be a ligand for the circadian nuclear receptor, Rev-erbα.

Figure 5. Circadian control of cell division cycles

Circadian system is linked to the cell division cycle via circadian control of gene expression and posttranslational mechanisms. The transcription of c-Myc and Wee1 is circadian and appears to be a direct target of the CLOCK:BMAL1. The expression of Wee1 is co-regulated with Per and the entry of the cell cycle into M phase is suppressed during the day time when the transcription of Per (and Wee1) is high. In addition, the PER1 protein interacts with the check-point proteins, ATM and Chk2; while related work has linked the TIMELESS (TIM) and CRY proteins with Chk1. Activation of the DNA damage pathway can also reset the phase of the circadian clock.