# Influence of circadian rhythm on effects induced by mechanical strain in periodontal ligament cells

## TL;DR

This study shows that mechanical stress affects clock genes in periodontal ligament cells, and the timing of stress within the circadian rhythm influences gene expression.

## Contribution

The novel finding is that the timing of mechanical stress within the circadian rhythm alters gene expression in periodontal ligament cells.

## Key findings

- Mechanical strain significantly altered the expression of core clock genes in periodontal ligament cells.
- The expression of Col-1α and IL1-β was influenced by the timing of mechanical stress within the circadian rhythm.
- RUNX2 expression was not significantly affected by the timing of mechanical stress.

## Abstract

The aim of this study was to investigate the influence of mechanical strain on clock gene function in periodontal ligament (PDL) cells. Furthermore, we wanted to analyze whether effects induced by mechanical stress vary in relation to the circadian rhythm.

Human PDL fibroblasts were synchronized in their circadian rhythm with dexamethasone and stretched over 24 h. Unstretched cells served as controls. Gene expression of the core clock genes were analyzed at 4 h intervals by quantitative real-time polymerase chain reaction (qRT-PCR). Time points 0 h (group SI1) and 12 h (group SI2) after synchronization served as starting points of a 4 h force application period. Collagen-1α (COL-1α/Col-1α), interleukin-1β (IL1-β), and runt-related transcription factor 2 (RUNX2/Runx2) were assessed by qRT-PCR and enzyme-linked immunosorbent assay (ELISA) after 2 and 4 h. Statistical analysis comprised one-way analysis of variance (ANOVA) and post hoc tests.

After synchronization, the typical pattern for clock genes was visible in control cells over the 24 h period. This pattern was significantly altered by mechanical strain. Under tensile stress, ARNTL gene expression was reduced, while Per1 and 2 gene expression were upregulated. In addition, mechanical stress had a differential effect on the expression of Col-1α and IL1‑β depending on its initiation within the circadian rhythm (group SI1 vs group SI2). For RUNX2, no significant differences in the two groups were observed.

Our results suggest that mechanical stress affects the molecular peripheral oscillator of PDL cells. Vice versa, the circadian rhythm also seems to partially influence the effects that mechanical stress exerts on PDL cells.