Regulation and Synchronization of the Master Circadian Clock by Purinergic Signaling from Suprachiasmatic Nucleus Astrocytes
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Molecular, cellular, and physiological processes within an organism are set to occur at specific times throughout the day. The timing of these processes is under control of a biological clock. Nearly all organisms on Earth have biological clocks, ranging from unicellular bacteria and fungi to multicellular plants, insects, reptiles, fish, birds, and mammals. The biological clock is an endogenous time-keeping mechanism that generates the onset of many processes and coordinates the phases of processes over 24 hours. While the biological clock allows these organisms to maintain roughly 24-hour, or circadian, timing in daily processes, many organisms have the ability to set their clocks, or entrain them, to changes in light. In mammals, the suprachiasmatic nucleus (SCN) is the master biological clock that entrains daily physiological and behavioral rhythms to the appropriate times of day and night. The SCN is located in the hypothalamus and contains thousands of neurons and glia that function in coordinating system-level physiological rhythms that are entrained to environmental light cues. Many of these neurons and glia are individual circadian oscillators, and the cellular mechanisms that couple them into ensemble oscillations are emerging. Adenosine triphosphate (ATP) is a transmitter involved in local communication among astrocytes and between astrocytes and neurons. ATP released from astrocytes may play a role in SCN cellular communication and synchrony. Extracellular ATP accumulated rhythmically in the rat SCN in vivo, and ATP released from rat SCN astrocytes in vitro was rhythmic, with a periodicity near 24 hours. ATP released from mouse SCN astrocytes was circadian, and disruption of the molecular clock abolished rhythmic extracellular ATP accumulation. SCN astrocyte cultures with disrupted molecular clocks also had marked reductions in total ATP accumulation compared to SCN astrocyte cultures with functional biological clocks. Furthermore, ATP-induced calcium transients were rhythmic, and this rhythmic purinergic sensitivity was abolished in clock mutant astrocytes. Pharmacological blockade of purinergic signaling, with antagonists of both the P2X7 and P2Y1 receptors, led to a gradual reduction in the amplitude of coordinated ATP accumulation over three days. These purinergic receptor antagonists, as expected, led to a reduction in calcium responses of SCN astrocytes to ATP and led to a dampening of clock gene expression rhythms as determined by PER2::LUC bioluminescence reporting in SCN astrocytes. These data demonstrate that astrocytes of the mammalian SCN rhythmically release ATP and are rhythmically sensitive to ATP in a manner dependent on their intrinsic molecular clock. Ensemble rhythmicity of SCN astrocytes is, in turn, dependent on that rhythmic purinergic signaling via both P2X and P2Y classes of ATP receptors. These results are indicative of a functional role for ATP accumulation within the SCN, with astrocytes releasing ATP every 24 hours for continual signaling onto astrocytes and neurons to maintain daily coordinated synchrony of the clocks in these cells.
Womac, Alisa Diane (2012). Regulation and Synchronization of the Master Circadian Clock by Purinergic Signaling from Suprachiasmatic Nucleus Astrocytes. Doctoral dissertation, Texas A&M University. Available electronically from