Cell autonomy and synchrony of suprachiasmatic nucleus circadian oscillators

Abstract

The suprachiasmatic nucleus (SCN) of the hypothalamus is the site of the master circadian pacemaker in mammals. The individual cells of the SCN are capable of functioning independently from one another and therefore must form a cohesive circadian network through intercellular coupling. The network properties of the SCN lead to coordination of circadian rhythms among its neurons and neuronal subpopulations. There is increasing evidence for multiple interconnected oscillators within the SCN, and in this review we will highlight recent advances in our knowledge of the complex organization and function of the cellular and network-level SCN clock. Understanding the way in which synchrony is achieved between cells in the SCN will provide insight into the means by which this important nucleus orchestrates circadian rhythms throughout the organism.

FIG 1. Expression of neuropeptides in the mammalian SCN

Photomicrographs of mouse SCN slices. The bilateral nuclei are located directly above the optic chiasm and positioned on either side of the 3^rd ventricle. Slices were obtained from transgenic mice expressing the CLOCKΔ19 mutant protein tagged with hemagglutinin (HA) in secretogranin positive cells (expression of the HA-tagged transgene protein product is shown in green). Sections were immunostained to detect expression of the neuropeptides (A) AVP (red) and (B) VIP (red). AVP expression is observed most prominently in the dorsal SCN, whereas VIP expression is strongest in the ventral SCN. Cells expressing both CLOCKΔ19-HA (expressed throughout the nucleus) and the peptide of interest appear in yellow. Reproduced, with permission, from [113].

FIG 2. Schematic of the intracellular molecular clock and model of intercellular signaling pathways

The molecular circadian clock consists of the positive elements CLOCK and BMAL1 (represented by the green and blue circles, respectively), which dimerize and activate transcription of Per and Cry genes. PER and CRY dimerize and translocate to the nucleus to suppress further transcriptional activation by CLOCK:BMAL1. CK1 regulates protein turnover, contributing to the degradation and nuclear translocation of PER. Intercellular coupling agents (CA), which may include VIP, AVP, or other neuropeptides, activate cAMP-signaling pathways [including protein kinase A (PKA)]. These signaling cascades result in CREB-dependent transcriptional activation of Per genes. Adapted, with permission, from [79].

FIG 3. Organized PER2 expression in the SCN

Assessment of luciferase bioluminescence in acute brain slices of the SCN from PER2∷LUC mice using unconstrained cluster analysis [53]. The analysis reveals a wave of oscillatory activity within the SCN, with expression of PER2 (as assessed by the bioluminescence signal of the LUC reporter gene) spreading through the nucleus. This pattern of serial activation of PER2 can be visualized as a “map” of distinct SCN subregions, defined here solely by the spatiotemporal distribution of the bioluminescent signal, without any underlying assumptions being specified by the experimenter. (A) The color coded ‘map’ indicates the location of each cluster in the slice. Note the substantial symmetry between the left and right sides of the SCN. (B) The time series graph displays the mean brightness for each distinct cluster over time [53]. Such a finding reveals that circadian oscillation is characterized by a stable daily cycle of gene expression involving orderly serial activation of specific SCN subregions, followed by a silent interval [53]. Time series for clusters in the map that cannot be discerned from scatter of light are not shown. Figure provided by the authors. Adapted, with permission, from [53].

FIG 4. Intercellular coupling compensates for genetic deficits

Uncoupled SCN cells are susceptible to arrhythmicity due to genetic defects in the molecular clock. Dispersed SCN cells from mice null for either Per1 or Cry1 display weak rhythms [5] SCN explants from these animals, however, are rhythmic, as is their locomotor activity [5]. Coupling of weakly rhythmic cellular oscillators can compensate for compromised circadian function, preserving stable rhythmicity at the level of the SCN and output rhythms. Individual cells derived from wildtype animals are rhythmic, although increased arrhythmicity and variability in period is observed in widely dispersed cultures [4, 55]. Thus, even among independent cells that are already rhythmic, coupling serves to stabilize the rhythmicity of the SCN.

FIG 5. Network properties convey temperature resistance to the SCN

(a) Bioluminescence trace from an SCN explant obtained from PER2∷LUC mice (bioluminescence represents PER2∷LUC expression). The SCN is resistant to perturbation by temperature signals. Blocking communication among SCN neurons with TTX allows a 6h temperature pulse (temperature increased to 38.5°C, indicated by yellow bar) to reset the phase of the SCN. (b) Phase transition curves (PTCs) plotting phase prior to temperature pulse (“old phase”) against phase following temperature pulse (“new phase”). PTCs are from SCN cultures from PER2∷LUC mice without drug (gray), with 1μM TTX (red), and with 5μM TTX (blue). Following TTX treatment, the SCNs display type 0 phase-resetting. Type 0 resetting is strong resetting, with oscillators resetting to a common phase, regardless of the phase at which the resetting signal is administered. (c) Heat maps demonstrating the effects of TTX on temperature resetting in individual neurons within an SCN culture. In whole SCN explants, phase and period length are stable and robust, although some variability in phase can be observed (left). Following TTX treatment, SCN neurons are uncoupled (right). Individual neurons in the TTX-treated condition free-run, resulting in a wide range of phases. TTX also renders individual SCN neurons sensitive to temperature-induced phase resetting. A 6h temperature pulse (temperature increased to 38.5°C, indicated by yellow bar) synchronizes the phases of individual neurons in the TTX-treated SCN. No effect on phase was observed when a temperature pulse was administered in the absence of TTX. Heat maps were generated by imaging single cell-sized regions of interest using a cooled charge-coupled device (CCD) camera. Maps display voxels measured from dorsal to ventral through the SCN. Red corresponds to the peak of bioluminescence and green to the trough. Reproduced, with permission, from AAAS [72].