International Journal of Molecular Sciences Review A Symphony of Signals: Intercellular and Intracellular Signaling Mechanisms Underlying Circadian Timekeeping in Mice and Flies 1,2, 1,2, 1,2 1,2 Sara Hegazi y, Christopher Lowden y, Julian Rios Garcia , Arthur H. Cheng , Karl Obrietan 3, Joel D. Levine 1,2 and Hai-Ying Mary Cheng 1,2,* 1 Department of Biology, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada; [email protected] (S.H.); [email protected] (C.L.); [email protected] (J.R.G.); [email protected] (A.H.C.); [email protected] (J.D.L.) 2 Department of Cell and Systems Biology, University of Toronto, Toronto, ON M5S 3G5, Canada 3 Department of Neuroscience, Ohio State University, Columbus, OH 43210, USA; [email protected] * Correspondence: [email protected] These authors contributed equally to this work. y Received: 1 April 2019; Accepted: 10 May 2019; Published: 13 May 2019 Abstract: The central pacemakers of circadian timekeeping systems are highly robust yet adaptable, providing the temporal coordination of rhythms in behavior and physiological processes in accordance with the demands imposed by environmental cycles. These features of the central pacemaker are achieved by a multi-oscillator network in which individual cellular oscillators are tightly coupled to the environmental day-night cycle, and to one another via intercellular coupling. In this review, we will summarize the roles of various neurotransmitters and neuropeptides in the regulation of circadian entrainment and synchrony within the mammalian and Drosophila central pacemakers. We will also describe the diverse functions of protein kinases in the relay of input signals to the core oscillator or the direct regulation of the molecular clock machinery. Keywords: circadian rhythms; central pacemaker; suprachiasmatic nucleus; Drosophila; neurotransmitters; neuropeptides; entrainment; synchrony; intercellular and intracellular signaling; protein kinases 1. Introduction Circadian (from the Latin phrase “circa diem,” meaning “about a day”) clocks are biological timekeeping mechanisms that have evolved to enable organisms to coordinate behavior and physiological processes to the cyclical changes occurring within their environment [1]. In mammals, the circadian system is partitioned into a hierarchy of peripheral oscillators that are governed by a central pacemaker known as the suprachiasmatic nucleus (SCN) [2,3]. Located within the anterior hypothalamus, the SCN is a bilateral structure comprised of ~20,000 GABAergic (GABA; γ-aminobutyric acid) neurons (Figure1A) [ 4,5]. It is anatomically subdivided into the dorsomedial “shell” and ventrolateral “core” regions, where each region is characterized by a distinct functional role and expression pattern of neuropeptides [5,6]. The ventral SCN primarily expresses gastrin-releasing peptide (GRP) and vasoactive intestinal polypeptide (VIP) and is directly responsive to photic input [7–10]. In contrast, the dorsal SCN expresses arginine vasopressin (AVP) and is crucial in maintaining rhythmicity [6]. Extensive intercellular communication within and between SCN compartments occurs via direct reciprocal innervations that enable the numerous oscillating clock cells to sustain a single, uniform output. Int. J. Mol. Sci. 2019, 20, 2363; doi:10.3390/ijms20092363 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2019, 20, 2363 2 of 56 Int. J. Mol. Sci. 2019, 20, x FOR PEER REVIEW 2 of 54 Figure 1. 1. TheThe mammalian mammalian suprachiasmatic suprachiasmatic nucleus: nucleus: neurochemical neurochemical composition composition and canonical and canonicalsignaling signalingpathways pathwaysthat regulate that the regulate core oscillator the core and oscillator photic entrainment. and photic entrainment.(A) Neurochemical (A) Neurochemicalcomposition of compositionthe SCN. The ofSCN the is SCN. divided The into SCN vent isral divided (core) intoand dorsal ventral (shell) (core) regions. and dorsal Most, (shell) if not regions.all, of theMost, ~20,000 if notSCN all, cells of theare ~20,000GABAergic SCN but cells differ are in GABAergic their neuropep but ditideffer content. in their neuropeptideVIP and AVP content.delineate VIP the andcoreAVP and delineateshell regions, the corerespectively. and shell GRP- regions,expressing respectively. neurons GRP-expressing are primarily neurons localized are in primarily the medial localized core. inProkineticin the medial 2 core.is another Prokineticin SCN neuropeptide 2 is another that SCN is highly neuropeptide expressed that by is core highly and expressedshell SCN neurons. by core and (B) shellCanonical SCN signaling neurons. (pathwaysB) Canonical within signaling the SCN. pathways The figure within depicts the SCN.the major The figuresignaling depicts pathways the major and signalingprotein kinases pathways that have and proteinso far been kinases shown that to havefunction so far within been the shown SCN. to Various function neuropeptides within the SCN. and Variousneurotransmitters neuropeptides impinging and neurotransmitters on SCN neurons impingingcan activate on receptors SCN neurons and ion can channels activate on receptors the plasma and ionmembrane channels to on trigger the plasma intracellular membrane signaling to trigger events. intracellular Activation signaling of G-protein events. coupled Activation receptors of G-protein (GPCRs) can signal via Gs, Gi, and Gq proteins to activate adenylyl cyclase (AC), inhibit AC, or activate phospholipase Cβ (PLCβ), respectively. AC stimulates the production of cAMP, which in turn activates protein kinase A (PKA) and exchange protein activated by cAMP (Epac). CREB-mediated transcription Int. J. Mol. Sci. 2019, 20, 2363 3 of 56 coupled receptors (GPCRs) can signal via Gs,Gi, and Gq proteins to activate adenylyl cyclase (AC), inhibit AC, or activate phospholipase Cβ (PLCβ), respectively. AC stimulates the production of cAMP, which in turn activates protein kinase A (PKA) and exchange protein activated by cAMP (Epac). CREB-mediated transcription can be induced by PKA and Epac. PLCβ catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG activates protein kinase C at the plasma membrane, whereas IP3 diffuses into the cytosol to induce the release of intracellular Ca2+ stores from the endoplasmic reticulum (ER). The rise in cytosolic Ca2+ levels can also be induced by ionotropic receptors, voltage-gated ion channels, G-protein gated ion channels, and receptor tyrosine kinases. Downregulation of GPCR signaling is achieved by phosphorylation of the receptor by G protein-coupled receptor kinase 2 (GRK2). Receptor-mediated Ras activation at the plasma membrane stimulates the mitogen-activated protein kinase pathway (RAF, MEK1/2, ERK1/2). PKC facilitates Ras/MAPK signaling, either by activating Ras or derepressing RKIP-mediated inhibition of Raf via phosphorylation of RKIP. MAPK/ERK activates p90 ribosomal S6 kinase 1 (RSK1) and mitogen- and stress-activated protein kinase 1 (MSK1), which in turn stimulate CREB-mediated transcription. MAPK/ERK is also an upstream activator of mammalian target of rapamycin (mTOR), which promotes translation by activating p70 S6 kinase (p70S6K) and inhibiting eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1)-mediated repression of eIF4E. In terms of Ca2+ signaling, Ca2+-induced activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) can couple to cyclic guanosine monophosphate (cGMP) production through the nitric oxide synthase (NOS)/guanylyl cyclase (GC) pathway. cGMP activates protein kinase G (PKG), which promotes CREB-mediated transcription. The NOS pathway also activates the small G protein, Dexras1, which inhibits GPCR-mediated Gi activation and indirectly inhibits AC through ligand-independent activation of Gi. Many upstream signaling events converge on the MAPK/ERK pathway, a pivotal player in photic entrainment via its effects on CREB. Salt-inducible kinase 1 (SIK1) is a CRE-inducible gene that acts as a feedback inhibitor of CREB signaling through suppression of CREB-dependent transcription coactivator 1 (CRTC1). Finally, a number of protein kinases have been shown to phosphorylate clock proteins, PERIOD and CRY: these include casein kinase 1 and 2 (CK1 and CK2, respectively), glycogen synthase kinase 3 (GSK3), SIK3, and GRK2. Dashed lines denote indirect interactions. A core clock mechanism of transcription-translation feedback loops (TTFLs) drives rhythms in each of the clock neurons of the SCN. Within the TTFL, the transcription factors, circadian locomotor output cycles kaput (CLOCK) and brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein 1 (BMAL1), bind E-box elements within the promoter regions of the Period (Per) and Cryptochrome (Cry) clock genes to drive their expression. In the cytoplasm, several post-transcriptional, translational and post-translational mechanisms act to regulate rhythmic protein expression in accordance with the day/night cycle. The loop eventually closes when the PER and CRY proteins translocate to the nucleus and inhibit the transcriptional activity of the CLOCK-BMAL1 complex (Reviewed in [11]). In Drosophila, the circadian system is made up of a central oscillator network in the brain, and several peripheral oscillators. Daily rhythms in behavior are dictated by the central pacemaker circuit, which has recently been modeled as a network of multiple,