Collective timekeeping among cells of the master circadian clock

Abstract

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the master circadian clock that coordinates daily rhythms in behavior and physiology in mammals. Like other hypothalamic nuclei, the SCN displays an impressive array of distinct cell types characterized by differences in neurotransmitter and neuropeptide expression. Individual SCN neurons and glia are able to display self-sustained circadian rhythms in cellular function that are regulated at the molecular level by a 24h transcriptional-translational feedback loop. Remarkably, SCN cells are able to harmonize with one another to sustain coherent rhythms at the tissue level. Mechanisms of cellular communication in the SCN network are not completely understood, but recent progress has provided insight into the functional roles of several SCN signaling factors. This review discusses SCN organization, how intercellular communication is critical for maintaining network function, and the signaling mechanisms that play a role in this process. Despite recent progress, our understanding of SCN circuitry and coupling is far from complete. Further work is needed to map SCN circuitry fully and define the signaling mechanisms that allow for collective timekeeping in the SCN network.

Figure 1

Daily hormone rhythms are regulated by the circadian timekeeping system. A. Hormone release fluctuates over the circadian cycle, with peak times occurring at specific times of day. Figure based on data from (Gavrila, et al. 2003; Koutkia, et al. 2005; Natalucci, et al. 2005; Russell, et al. 2008; Takahashi et al. 1968). B. The circadian timekeeping system is a hierarchical collection of tissue clocks located throughout the brain and body, many of which are endocrine tissues that regulate hormone synthesis and release across the day.

Figure 2

The molecular circadian clock is comprised of interlocking transcriptional-translational feedback loops. CLOCK and BMAL1 are transcription factors that bind to E-box elements within the promoter sequences of a variety of clock genes. The protein products of Period (Per) and Cryptochrome (Cry) genes form repressors that inhibit their own transcription. Additional feedback loops involve other clock genes that interact with the elements of the core loop to amplify and stabilize molecular clock function. For example, the protein products of Rev-erb and Ror genes compete for binding at ROR elements to influence Bmal1 and Clock transcription. Further, the protein products of clock-controlled genes, such as Dbp, can influence both downstream molecular targets and core clock genes by binding at D-box elements.

Figure 3

The SCN network is a heterogeneous population of cellular clocks. A. Organization of the mouse SCN, illustrating the compartmentalization of cell types and inputs in the coronal plane. AVP: Arginine Vasopressin, PK2: Prokineticin 2, CCK: Cholecystokinin, CALB: Calbindin, CALR: Calretinin, SP: Substance P, GRP: Gastrin-Releasing Peptide, VIP: Vasoactive Intestinal Polypeptide, 5-HT: Serotonin (from dorsal raphe), NPY: Neuropeptide Y (from intergeniculate leaflet of the thalamus, which also releases GABA, neurotensin, and enkephalin), Glut: Glutamate (from retina), PACAP: Pituitary Adenylate Cyclase-Activating Polypeptide (from retina), 3V: Third ventricle, OC: Optic Chiasm. B. The shell-core model of the SCN network, representing how each compartment is thought to contribute to pacemaker function.

Figure 4

Intercellular signaling mechanisms that alter cellular activity and clock gene expression within SCN neurons. A. SCN neurons express the VPAC2 receptor for VIP, which is a G-protein coupled receptor with seven transmembrane domains. Upon VIP binding to VPAC2, Gα_s activates adenylyl cyclase, cAMP, PKA, and CRE-dependent transcription. B. Within the SCN both GABA_A receptors and GABA_B receptors are expressed. GABA binding to the ionotropic GABA_A receptor allows for influx or efflux of Cl-, depending on [Cl-]_i. In contrast, the metabotropic GABA_B receptor is coupled to Gα_i/o, which inhibits adenylyl cyclase. C. The receptors for both AVP and GRP are Gα_q coupled receptors that stimulate phospholipase C (PLCβ), which in turn activates phosphatidylinositol 4,5-bisphosphate (PIP2) to cause diacyl glycerol (DAG)-mediated activation of protein kinase C (PKC) and inositol triphosphate (IP3)-induced release of intracellular calcium stores. Intracellular release of calcium activates Calmodulin (CaM), which stimulates Ca^2+/ calmodulin-dependent protein kinase (CaMK). Due to the expression of CRE-elements in Period genes, changes in the intracellular signaling cascades illustrates in A-C can alter the function of the molecular clock.