Circadian Rhythms in a Petri Dish: Synchronizing Mouse Hypothalamic mHypoE-42 Cells for Circadian Clock Gene Expression Analysis.

Circadian rhythms are endogenous oscillations of approximately 24 h that regulate a wide range of cellular and physiological processes, including gene expression, metabolism, and behavior. These rhythms arise from interconnected transcriptional-translational feedback loops that respond to temporal and environmental cues. Because circadian regulation is highly dynamic, even minor experimental variations can influence phase, amplitude, and rhythmicity, making standardized experimental workflows essential for generating reliable and reproducible results. The goal of the present protocol is to provide a practical and reproducible workflow for synchronizing cultured cells, performing time-course sampling, and analyzing circadian clock gene expression under standard laboratory conditions. The protocol describes serum shock-based synchronization, staggered sample collection over 24-72 h to avoid overnight sampling, ribonucleic acid extraction, complementary deoxyribonucleic acid synthesis, quantitative real-time polymerase chain reaction, and circadian rhythm analysis using appropriate statistical approaches. The workflow also highlights critical experimental considerations, including synchronization conditions, sample quality assessment, reference gene selection, and data analysis, to improve reproducibility across experiments. This method provides an accessible approach for investigating molecular circadian mechanisms and evaluating rhythmic gene expression in cultured cells, facilitating studies of circadian regulation in physiological and disease-related experimental models.