The Transcription Factors Six3 and Six6 have Different Functions in Circadian Regulation of the Adult Mouse SCN

Circadian rhythms are processes that repeat approximately every 24 hours and are regulated by the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain. The regulation of circadian rhythms and downstream processes is highly dependent on the proper development and function of the SCN. Six3 and Six6 are homeodomain transcription factors that are both required for SCN development. Six3 and Six6 remain expressed in the adult SCN where their roles are unknown. To determine the roles of Six3 and Six6 in the adult SCN, we conditionally knocked out either Six3 or Six6 from cells that express neuromedin-S (NMS), a neuropeptide expressed in a majority of neurons within the SCN, that is known to be involved in SCN circadian output and reproductive function. As NMS is not expressed until after completion of SCN neuron proliferation, the Nms-cre allele allows the investigation of the role of Six3 or Six6 in NMS neurons from around birth onward. We hypothesize that Six3 and Six6 in NMS neurons regulate SCN circadian output and reproductive function in males and females. We found that deletion of Six6 had no impact on pubertal onset in males or females, estrous cyclicity (female), and sperm counts/motility (males). However, deletion of Six3 from NMS neurons resulted in significantly decreased sperm motility. We next examined the roles of Six3 and Six6 in locomotor rhythms by placing the mice in constant darkness. No differences in behavioral activity onset, period, or overall activity were identified in Six6 conditional knockout mice. However, both male and female mice with Six3 conditional knockout had shortened wheel-running periods in constant darkness, indicating a shortening of the rhythm within the SCN in males and females. Together, these data suggest differing roles of Six3 and Six6 in the adult SCN’s regulation of reproduction and locomotor rhythms. B.V.L. T32HD087166 H.H. K99/R00 HD084759 from NICHD; United States Department of Agriculture National Institute of Food and Agriculture Hatch project MICL1018024 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.