The Mammalian Circadian Time-Keeping System

Abstract

Our physiology and behavior follow precise daily programs that adapt us to the alternating opportunities and challenges of day and night. Under experimental isolation, these rhythms persist with a period of approximately one day (circadian), demonstrating their control by an internal autonomous clock. Circadian time is created at the cellular level by a transcriptional/translational feedback loop (TTFL) in which the protein products of the Period and Cryptochrome genes inhibit their own transcription. Because the accumulation of protein is slow and delayed, the system oscillates spontaneously with a period of ∼24 hours. This cell-autonomous TTFL controls cycles of gene expression in all major tissues and these cycles underpin our daily metabolic programs. In turn, our innumerable cellular clocks are coordinated by a central pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. When isolated in slice culture, the SCN TTFL and its dependent cycles of neural activity persist indefinitely, operating as "a clock in a dish". In vivo, SCN time is synchronized to solar time by direct innervation from specialized retinal photoreceptors. In turn, the precise circadian cycle of action potential firing signals SCN-generated time to hypothalamic and brain stem targets, which co-ordinate downstream autonomic, endocrine, and behavioral (feeding) cues to synchronize and sustain the distributed cellular clock network. Circadian time therefore pervades every level of biological organization, from molecules to society. Understanding its mechanisms offers important opportunities to mitigate the consequences of circadian disruption, so prevalent in modern societies, that arise from shiftwork, aging, and neurodegenerative diseases, not least Huntington's disease.

Keywords: Astrocyte; cryptochrome; feedback loop; hypothalamus; melanopsin; neurodegeneration; neuropeptide; period; retina; suprachiasmatic nucleus.

Fig. 1

The human circadian program. Schematic representation of circadian variations of physiological and endocrine status recorded in human volunteers subject to a constant routine protocol. Grey shading represents expected sleep interval, which was denied by constant routine protocol. Based on [2].

Fig. 2

The suprachiasmatic nucleus (SCN) as central circadian pacemaker. A) Schematic sagittal view of the location of the human SCN, at the base of the hypothalamus, depicting retinal input from the retinohypothalamic tract (RHT) via the optic nerve, and outputs to neural centers controlling sleep-wake behavior, neuroendocrine and autonomic status. B) Schematic coronal view of mouse SCN to show ventral retinorecipient core and dorsal shell sub-divisions, characterized by their distinction expression of (left) neuropeptides and (right) neurotransmitter receptors, and RHT innervation of SCN core.

Fig. 3

The cell-autonomous circadian clockwork. A) Schematic view of the transcriptional/ translational negative feedback loop (TTFL) incorporating positive (CLOCK:BMAL1 proteins) and negative (PER:CRY proteins) regulators that oppose their transactivation by CLOCK:BMAL1 at E-box regulatory sequences. The core loop is stabilized by an accessory loop controlling Bmal1 expression, and its phase is regulated by signaling cascades that converge on glucocorticoid (GRE) and calcium-cAMP (CRE) regulatory elements, especially in the Per genes. B) The cell-autonomous TTFL (depicted in A) controls the circadian expression of clock-controlled genes (CCGs) that in-turn orchestrate circadian cycles of cellular metabolism. C. Demonstration of spontaneous circadian TTFL function in human fibroblasts by lentiviral (LV) transduction with a luciferase reporter based on the Bmal1 promoter (left) and subsequent bioluminescent recording for several days (right). Based on [79].

Fig. 4

Interactions between neurons and astrocytes drive circadian time-keeping in the SCN. Both neurons (blue) and astrocytes (magenta) contain a TTFL but their cellular activity rhythms, as evidenced by rhythms of calcium ([Ca^2 +]_i), are oppositely phased (neurons day-, astrocytes night-active). Neuropeptides (NPs) and GABA synchronize the SCN neuronal network, and astrocytes signal via glutamate (Glu) (and likely other astrocyte-derived signals, magenta arrow) to regulate the neuronal rhythms. Equally, neuronal cues (yet to be identified, blue, and possibly including neuropeptides, broken line) signal circadian information to astrocytes. This reciprocal communication enhances circuit-level time-keeping. Afferent signals onto neurons from outside the SCN determine network phase, and neuronal efferents broadcast circadian time to SCN targets in the brain.

Fig. 5

Putative reciprocal links between the circadian system and neurodegenerative disease. A) With circadian coherence, intra-cellular proteostatic capacity is regulated in time by the TTFL, degrading unwanted proteins efficiently and in sequence with other cellular processes (right), while sleep-dependent glymphatic activity efficiently clears extracellular proteins (left). B) The balance is lost in neurodegeneration when disease-causing factors (age- and lifestyle-related, mutations to proteins and their metabolic pathways), overwhelm the capacity of the circadian- and/or sleep-regulated systems. C) This balance is also lost when the circadian system and/ or sleep are compromised due to genetic, environmental, or age-related factors, increasing the potential for intra- and/or extracellular aggregation. D) At a systems level, circadian coherence in the SCN suppresses neurodegenerative (ND) disease progression and sustains metabolic competence in the periphery (solid line). Conversely, ND progression can reduce circadian competence. Consequently, with loss of circadian competence ND will progress, and further compromise the SCN clock and/or sleep, creating a downward spiral. This will further compromise circadian regulation of other systems in peripheral organs (broken line), leading to metabolic disturbance.