Temporally chimeric mice reveal flexibility of circadian period-setting in the suprachiasmatic nucleus

Abstract

The suprachiasmatic nucleus (SCN) is the master circadian clock controlling daily behavior in mammals. It consists of a heterogeneous network of neurons, in which cell-autonomous molecular feedback loops determine the period and amplitude of circadian oscillations of individual cells. In contrast, circuit-level properties of coherence, synchrony, and ensemble period are determined by intercellular signals and are embodied in a circadian wave of gene expression that progresses daily across the SCN. How cell-autonomous and circuit-level mechanisms interact in timekeeping is poorly understood. To explore this interaction, we used intersectional genetics to create temporally chimeric mice with SCN containing dopamine 1a receptor (Drd1a) cells with an intrinsic period of 24 h alongside nonDrd1a cells with 20-h clocks. Recording of circadian behavior in vivo alongside cellular molecular pacemaking in SCN slices in vitro demonstrated that such chimeric circuits form robust and resilient circadian clocks. It also showed that the computation of ensemble period is nonlinear. Moreover, the chimeric circuit sustained a wave of gene expression comparable to that of nonchimeric SCN, demonstrating that this circuit-level property is independent of differences in cell-intrinsic periods. The relative dominance of 24-h Drd1a and 20-h non-Drd1a neurons in setting ensemble period could be switched by exposure to resonant or nonresonant 24-h or 20-h lighting cycles. The chimeric circuit therefore reveals unanticipated principles of circuit-level operation underlying the emergent plasticity, resilience, and robustness of the SCN clock. The spontaneous and light-driven flexibility of period observed in chimeric mice provides a new perspective on the concept of SCN pacemaker cells.aily rhythms of behavior and physiology adapt organisms to the solar cycle. In mammals, these rhythms are coordinated by a central circadian pacemaker: the suprachiasmatic nucleus (SCN) of the hypothalamus (1). The SCN consists of 10,000 neurons that work together to ensure that rhythms are tuned appropriately to anticipate day and night. Circadian dysfunction is increasingly linked to a variety of psychological and metabolic disorders (2). The principal subdivisions of the SCN are the shell, characterized by neurons expressing arginine vasopressin (AVP), and the retinorecipient core, containing vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP) neurons. The core mediates entrainment to light after birth (3), whereas neurons expressing the dopamine 1a receptor (Drd1a) mediate transplacental dopaminergic entrainment of the SCN in utero (4). Drd1a cells are, therefore, a functionally distinct subpopulation that spans elements of both core and shell.Timekeeping in individual SCN neurons involves a molecular clockwork based on a transcriptional-translational feedback loop (TTFL) whereby PERIOD and CRYPTOCHROME proteins inhibit their own transactivation by CLOCK/BMAL1 heterodimers (5). Almost all of the cells in the body also have this TTFL, b...

Regulating the Suprachiasmatic Nucleus (SCN) Circadian Clockwork: Interplay between Cell-Autonomous and Circuit-Level Mechanisms

“…The waves of [Ca 2þ ] i and gene expression may reflect a release from inhibition, rather than activation, and so the source of the wave may arise in inhibitory inputs to the dorsomedial lip projecting from elsewhere in the SCN circuit. The second caveat is that recent studies with temporally chimeric mice, in which the SCN contains a spatially heterogeneous mixture of neurons with either 24 h or 20 h genetically specified cell-autonomous periods, nevertheless show spatiotemporal waves of PER2::LUC expression no different from WT and CK11 Tau mutants (Smyllie et al 2016). This suggests that the generation of the daily wave remains intact despite a bimodal distribution of intracellular periods.…”

“…6A) Mieda et al 2015;Smyllie et al 2016). Two types of strategies have been used to generate temporally chimeric animals.…”

“…The SCN from such mice contain cells that are either Tau-competent or Taudeleted, and so express periods of either ca. 20 h or 24 h, respectively (Smyllie et al 2016). Tau deletion was targeted by the gene encoding the dopamine 1a receptor (Drd1a), a population that covers 60% of SCN neurons, spanning both core and shell, and likely overlapping with the NMS cell population.…”

“…This remarkable flexibility thus begs the question of whether "absolute" pacemaker cells exist, or whether in fact pace setting reflects both the environmental and circuit context. Nevertheless, under normal 24 h environmental conditions, given that the NMS cells constitute both AVP and VIP neurons, and deletion of BMAL1 in VIP cells had very little effect on circadian behavior and cellular rhythms, the combination of these investigations Mieda et al 2015;Smyllie et al 2016) points toward an important role for both AVP neurons and non-VIP NMS and Drd1a neurons as the dominant pacemaking elements in a nonlinear, hierarchical SCN circuit (Fig. 6B).…”

G-protein-coupled receptor signaling through Gpr176, Gz, and RGS16 tunes time in the center of the circadian clock [Review]

“…Moreover, SCN transplants from wild-type mice restore circadian rhythmicity in global clock-gene knockout mice, where no functional clocks operate throughout the body [13], providing evidence that the transplanted SCN alone is sufficient to produce organismal rhythmicity. Recent cell-type-specific conditional clock-gene knockout studies also support the assertion that the SCN is the central clock [14][15][16].…”