Molecular Mechanisms Underlying Plasticity to Photoperiod in the Suprachiasmatic Nucleus

Abstract

*Portion of abstract previously published in Cox et al., 2024. Seasonal daylength, or circadian photoperiod, is a pervasive environmental signal that profoundly influences physiology and behavior. In mammals, the central circadian clock resides in the suprachiasmatic nuclei (SCN) of the hypothalamus where it receives retinal input and synchronizes, or entrains, organismal physiology and behavior to the prevailing light cycle. The process of entrainment induces sustained plasticity in the SCN, but the molecular mechanisms underlying SCN plasticity are incompletely understood. Entrainment to different photoperiods persistently alters the timing, waveform, period, and light resetting properties of the SCN clock and its driven rhythms. To elucidate novel candidate genes for molecular mechanisms of photoperiod plasticity, we performed RNAseq and preliminary DNA methylation analysis on whole SCN dissected from mice raised in Long (LD 16:8) and Short (LD 8:16) photoperiods. Further, DNA methylation analysis was conducted in SCN slice culture incubated with DNA methyltransferase inhibitors RG108 and SGI-1027. RNAseq detected fewer rhythmic genes in Long photoperiod, and in general the timing of gene expression rhythms was advanced 4-6 hours. However, a few genes showed significant delays, including Gem. There were significant changes in the expression clock-associated gene Timeless and in SCN genes related to light responses, neuropeptides, GABA, ion channels, and serotonin. Particularly striking were differences in the expression of the neuropeptide signaling genes Prokr2 and Cck, as well as convergent regulation of the expression of three SCN light response genes, Dusp4, Rasd1, and Gem. Transcriptional modulation of Dusp4 and Rasd1, and phase regulation of Gem, are compelling candidate molecular mechanisms for plasticity in the SCN light response through their modulation of the critical NMDAR-MAPK/ERK-CREB/CRE light signaling pathway in SCN neurons. Modulation of Prokr2 and Cck may critically support SCN neural network reconfiguration during photoperiodic entrainment. Overall, the RNAseq findings identify the SCN light response and neuropeptide signaling gene sets as rich substrates for elucidating novel mechanisms of photoperiod plasticity. DNA methylation analysis via sodium bisulfite pyrosequencing of Rasd1 found a significant difference in methylation at the 3’ enhancer region of the gene between photoperiods at the projected time of lights off. In SCN slice culture at baseline (no light stimulus) in the presence of DNA methyltransferase inhibitors, no differences in DNA methylation were detected via sodium bisulfite pyrosequencing in regulatory regions in the core clock genes Bmal1, Per1, and Nr1d1 or in the light-signaling gene Rasd1. Overall, the results of the DNA methylation analyses are preliminary but agree with the results from the RNAseq study in that no changes in expression are observed in the core clock genes, whereas Rasd1 is significantly differentially expressed between photoperiods