Copper in the Suprachiasmatic Nucleus: Copper Signaling, Homeostasis, and Circadian Rhythms and Trace Metals in the Master Clock

we present data supporting a role for copper (Cu) and Cu homeostasis in the suprachiasmatic nucleus (SCN), the location of the mammalian circadian pacemaker. Although many studies have investigated the function and effects of Cu in synaptic function and receptor signaling in the brain and other tissues, these results are the first to directly link Cu with the SCN master clock and circadian neuronal activity rhythms. Previous work using extracellular recordings of SCN neurons in ex vivo hypothalamic slices has demonstrated that resetting the circadian clock, e.g. by glutamate (Glu) treatment during the night, induces shifts in the phase of SCN neuronal activity rhythms that correspond to phase shifts in circadian behavioral activity (Albers et al., 2017; Golombek and Rosenstein, 2010; Herzog et al., 2017; Lindsay et al., 2014; Prosser, 1998). Here we first have demonstrated that both Cu chelation and Cu application are able to induce night-time phase shifts in neuronal activity rhythms in vitro. Second, we have shown that these two treatments affect N-methyl-D-aspartate receptor (NMDAR) and Glu neurotransmission differently. Since Glu phase-shifts the SCN clock through NMDAR-mediated, calcium-dependent signaling pathways and activation of other pathways, we pharmacologically tested several of these pathways to investigate how application of Cu or the Cu chelator, tetrathiomolybdate (TTM), induces NMDAR-independent and dependent phase shifts, respectively. Our results demonstrate that Cu induces mitogen-activated protein kinase (MAPK)-dependent phase shifts in the absence of NMDAR-mediated calcium influx. On the other hand, the specific extra- and intracellular mechanisms by which Cu removal induces phase shifts remain unclear. Lastly, we have preliminary results indicating that concentrations of Cu in the SCN are comparable to other brain regions, and show day-night expression of two Cu transporters, copper transporter 1 (CTR1) and ATPase-7A (ATP7A) in the SCN. We discuss these findings in light of the existing literature and current models of SCN circadian oscillator mechanisms. Our results together with published findings suggest that Cu homeostasis is tightly regulated in the SCN, and that changes in Cu levels serves as a time cue for the circadian clock. Future research can elucidate how Cu (dys)regulation interacts with oscillations in SCN neuronal firing and signaling activity and whether Cu or other trace elements influence SCN metabolic and redox activity.