A Gq-Ca2+ Axis Controls Circuit-Level Encoding of Circadian Time in the Suprachiasmatic Nucleus

Abstract

SummaryThe role of intracellular transcriptional/post-translational feedback loops (TTFL) within the circadian pacemaker of the suprachiasmatic nucleus (SCN) is well established. In contrast, contributions from G-coupled pathways and cytosolic rhythms to the intercellular control of SCN pacemaking are poorly understood. We therefore combined viral transduction of SCN slices with fluorescence/bioluminescence imaging to visualize GCaMP3-reported circadian oscillations of intracellular calcium [Ca2+]i alongside activation of Ca2+/cAMP-responsive elements. We phase-mapped them to the TTFL, in time and SCN space, and demonstrated their dependence upon G-coupled vasoactive intestinal peptide (VIP) signaling. Pharmacogenetic manipulation revealed the individual contributions of Gq, Gs, and Gi to cytosolic and TTFL circadian rhythms. Importantly, activation of Gq-dependent (but not Gs or Gi) pathways in a minority of neurons reprogrammed [Ca2+]i and TTFL rhythms across the entire SCN. This reprogramming was mediated by intrinsic VIPergic signaling, thus revealing a Gq/[Ca2+]i-VIP leitmotif and unanticipated plasticity within network encoding of SCN circadian time.

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

“…One likely avenue is via increases in intracellular calcium levels ([Ca 2þ ] i ) because of the opening of voltage-gated calcium channels and release from intracellular stores (see Allen et al 2016). Consistent with this, fluorescent imaging using a genetically encoded calcium reporter has revealed a pronounced circadian cycle of [Ca 2þ ] i in SCN neurons that peaks at CT06, coincident with high firing rates and increasing Per expression, as revealed by simultaneous bioluminescent imaging (Brancaccio et al 2013). Moreover, although this rhythm was initially thought to be independent of action potential firing and reach its maximum about 4 h in advance of peak electrical activity (Ikeda et al 2003), more recent analysis using genetically encoded reporters indicates simultaneous peaks of cytosolic [Ca 2þ ] i and electrical activity.…”

“…This structure is lost in CRY-deficient SCN (right). (Redrawn from data in Edgar et al 2012, Brancaccio et al 2013, and Edwards et al 2016 observed in SCN neurons. For example, circadian changes in the activation of BK channels regulate excitability of SCN neurons (Whitt et al 2016).…”

Ganglionic GFAP+ glial Gq-GPCR signaling enhances heart functions in vivo

“…However, the ability of sympathetic SGCs to regulate ganglionic neuronal activity has never been directly tested, likely due to the inability to selectively activate SGCs without also activating neurons. To overcome this obstacle, we took advantage of DREADD technology and generated Gfap-hM3Dq transgenic mice (16).The engineered GPCR approach has been used by others to identify novel signaling pathways that play critical physiological roles in vivo (52)(53)(54). For example, Jain et al used hM3Dq transgenic mice to demonstrate that Gq-GPCR signaling in pancreatic β cells led to activation of ERK1/2 and IRS2 signaling; this activation led to markedly improved β cell function (55).…”

Membrane Currents, Gene Expression, and Circadian Clocks

“…Within the SCN, there is clear evidence for a daily rhythm in the levels of cAMP and Ca 2þ (reviewed in Colwell 2011). More recent work used fluorescence/bioluminescence imaging to visualize GCaMP3-reported circadian oscillations of intracellular Ca 2þ alongside activation of Ca 2þ /cAMP-responsive elements (Brancaccio et al 2013). This work illustrates that key intracellular signaling pathways that are known to alter neural excitability are rhythmically regulated within the SCN.…”