Light‐induced Changes in the Suprachiasmatic Nucleus Transcriptome Regulated by the MAPK/ERK Pathway
Authors: Correa Diego Alzate , Aten Sydney , Garcia Ashley , Obrietan Karl , Hoyt Kari 
Published: November 2023

A wide range of organisms exhibit daily, 24 hour (circadian), rhythms in physiological and behavioral processes. In mammals, the brain suprachiasmatic nucleus (SCN) functions as the master oscillator that drives peripheral oscillator cell populations found throughout the body. At the molecular level, circadian rhythms are generated by an auto‐regulatory transcriptional feedback loop built around a limited number of ‘clock genes’; in turn, these ‘clock gene’ oscillations drive the rhythmic expression of many thousands of clock‐controlled genes, which, in turn underlie the biochemical, physiological and behavioral manifestation of the circadian timing process. The circadian timing system is tightly regulated by changes in the lighting cycle. Accordingly, light‐induced entrainment is initiated by the retina sending inputs to the SCN, where activation of gene expression leads to the resetting of the molecular clock timing process. Of note, light has marked time‐of‐day‐specific effects on clock entrainment, and these effects appear to be mediated at the level of kinase signaling pathways. Of particular interest here is the ERK/MAPK signaling pathway. Several studies have shown that during the night the ERK/MAPK signaling pathway is highly responsive to light input and drives the resetting of the circadian clock timing process, whereas, during the day, this signaling cascade is less responsive to light and resetting of the circadian clock is minimal. Since light entrainment is mediated by changes in gene expression, the objective of this work was to identify changes in the SCN transcriptome induced by ERK/MAPK pathway activation in response to light. To this end, we isolated the RNA from the SCN of mice exposed to a transient light pulse (10 min, 100 lux) at 3 time‐points: circadian time 4 (CT4), CT15, and CT22. These time points were chosen based on the profoundly different effects that light has on clock entrainment during the circadian day (CT4), early circadian night (CT15) and late circadian night (CT22). To assess the contribution of the ERK/MAPK pathway to light‐evoked gene expression, mice were infused with the MEK1/2 inhibitor U0126, or with drug vehicle (DMSO), 30 min prior to light stimulation. Samples from were hybridized to the ClariomTM D Mouse Transcriptome Array (Affymetrix) to evaluate transcriptome changes. Bioinformatic analysis identified 55 light‐responsive coding genes at CT4, whereas at CT15 and CT22, 128 and 413 coding genes were inducible expressed, respectively. Interestingly, in the presence of U0126 the number of light‐responsive coding genes was reduced to 23 at CT4, 75 at CT15, and 38 at CT22. For non‐coding genes, light evoked the expression of 373 transcripts at CT4, 223 transcripts at CT15, and 292 transcripts at CT22; U0126 pretreatment markedly reduced the expression non‐coding transcripts at all three time points: 72 at CT4; 159 at CT15; 84 CT22. In total, these experiments should provide important new insights into the role that ERK/MAPK signaling plays in regulating inducible gene expression within the SCN master circadian clock. To this end, current work is focused on gene network analysis of coding and non‐coding transcripts and on identifying subsets of genes through which the ERK/MAPK pathway couples light to circadian clock entrainment.Support or Funding InformationThis work was supported by NIH MH103361