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

Summary

This protocol provides a practical and reproducible workflow for synchronizing cultured cells, performing staggered time-course sampling over 24–72 h without overnight collection, and analyzing circadian clock gene expression.

Abstract

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.

Introduction

Circadian rhythms are endogenous cycles with a period of approximately 24 h that regulate a wide range of physiological, metabolic, and cellular processes¹. In mammalian systems, these rhythms are coordinated by a central clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus and by self-sustained molecular clocks at the cellular level. These clocks consist of interlocked transcriptional–translational feedback loops involving core clock genes, including circadian locomotor output cycles protein kaput (Clock), basic helix-loop-helix aryl hydrocarbon receptor nuclear translocase-like 1 (Bmal1), period ....

Protocol

The following protocol describes a validated synchronization method for the mHypoE-42 embryonic mouse hypothalamic cell line. Validate and optimize the synchronization protocol independently before applying it to other cultured cell types. See Figure 1 for an overview of the workflow and Supplementary Figure 1 for the weekly sampling schedule. Refer to the Table of Materials for all reagents, equipment, and instruments used in this protocol. Perform all cell culture procedures in a certified biosafety cabinet using sterile reagents and equipment.

Results

Successful synchronization is indicated by statistically significant circadian rhythmicity of the analyzed clock genes under the control condition (p < 0.05). In this study, three independent biological experiments were performed, with 0.1% dimethyl sulfoxide serving as the negative control. Bmal1 and Per2 were selected as representative circadian markers because they are core components of the molecular circadian clock and exhibit a well-characterized antiphasic expression pattern. Represen.......

Discussion

Circadian rhythms are endogenous cycles of approximately 24 h that regulate a wide range of physiological, metabolic, and cellular processes through conserved molecular clock mechanisms^1,2. In vitro synchronization models provide a controlled experimental environment for investigating these mechanisms and have become valuable tools for studying circadian regulation under normal and pathological conditions, including exposure to environmental contaminants.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
6-well cell culture plate	Nest Biotechnology	15140122	Cell culture plate
Agarose	NZYtech	MB02702	RNA quality assessment
Antibiotic mixture (penicillin/streptomycin)	Gibco, Thermo Fisher Scientific	D5648	Cell culture supplement
cDNA synthesis kit (first-strand)	NZYtech	MB12502	Reverse transcription
Chloroform	Sigma-Aldrich	A5256701	Phase separation reagent
DMEM, high glucose (4500 mg/L glucose, L-glutamine)	Sigma-Aldrich	26050088	Cell culture medium
Eppendorf microcentrifuge tubes	Eppendorf	30120086	Sample storage
Ethanol (96%)	Fisher Bioreagents	15552393	RNA purification
Fetal bovine serum, heat-inactivated	Gibco, Thermo Fisher Scientific	J62692.K7	Cell culture supplement
Hard-shell 96-well PCR plate	Bio-Rad Laboratories	HSP9601	qRT-PCR plate
Horse serum, heat-inactivated	Gibco, Thermo Fisher Scientific	15400054	Serum shock synchronization
mHypoE-42 cell line (CVCL_D443)	CELLutions Biosystems Inc.	MB13402	Embryonic mouse hypothalamic cell line
Oligonucleotide primers	NZYtech	MB12501	qRT-PCR primers
Phosphate-buffered saline (PBS)	Thermo Fisher Scientific	MB18502	Cell washing
qPCR Green Master Mix (2×)	NZYtech	MB22403	qRT-PCR reagent
Real-time PCR detection system	Bio-Rad Laboratories	EP0030108116	qRT-PCR instrument
RNA isolation kit	NZYtech	288306	Silica spin-column purification
RNA lysis reagent	NZYtech	MB18502	Phenol-based RNA extraction reagent
Sodium bicarbonate	Sigma-Aldrich	MB22401	Cell culture medium supplement
Sterile 35-mm culture dish	Thermo Fisher Scientific	121V	Cell culture dish
T100 thermal cycler	Bio-Rad Laboratories	1861096	cDNA synthesis
T75 tissue culture flask	Corning	430641U	Cell culture vessel
Trypan blue solution (0.4%)	Gibco, Thermo Fisher Scientific	15250061	Cell counting
Trypsin-EDTA (0.5%)	Gibco, Thermo Fisher Scientific	15400054	Cell dissociation