# Circadian disruption reduces MUC4 expression via the clock molecule BMAL1 during dry eye development

## TL;DR

Disrupted sleep patterns can harm eye health by reducing a protective protein called MUC4, but melatonin may help reverse this damage.

## Contribution

This study reveals a new mechanism linking circadian disruption to dry eye disease through MUC4 and BMAL1.

## Key findings

- Chronic jet lag in mice caused corneal epithelial defects and inflammation due to reduced MUC4.
- BMAL1 regulates MUC4 expression, and its absence leads to dry eye disease.
- Melatonin treatment restored BMAL1 and MUC4 levels, alleviating dry eye symptoms.

## Abstract

Circadian disruption, as a result of shiftwork, jet lag, and other lifestyle factors, is a common public health problem associated with a wide range of diseases, such as metabolic disorders, neurodegenerative diseases, and cancer. In the present study, we established a chronic jet lag model using a time shift method every 3 days and assessed the effects of circadian disruption on ocular surface homeostasis. Our results indicated that jet lag increased corneal epithelial defects, cell apoptosis, and proinflammatory cytokine expression. However, the volume of tear secretion and the number of conjunctival goblet cells did not significantly change after 30 days of jet lag. Moreover, further analysis of the pathogenic mechanism using RNA sequencing revealed that jet lag caused corneal transmembrane mucin deficiency, specifically MUC4 deficiency. The crucial role of MUC4 in pathogenic progression was demonstrated by the protection of corneal epithelial cells and the inhibition of inflammatory activation following MUC4 replenishment. Unexpectedly, genetic ablation of BMAL1 in mice caused MUC4 deficiency and dry eye disease. The underlying mechanism was revealed in cultured human corneal epithelial cells in vitro, where BMAL1 silencing reduced MUC4 expression, and BMAL1 overexpression increased MUC4 expression. Furthermore, melatonin, a circadian rhythm restorer, had a therapeutic effect on jet lag-induced dry eye by restoring the expression of BMAL1, which upregulated MUC4. Thus, we generated a novel dry eye mouse model induced by circadian disruption, elucidated the underlying mechanism, and identified a potential clinical treatment.

Dry eye disease, a long-term issue causing discomfort and vision problems, impacts millions globally. In this research, scientists studied how disturbances in our internal clock contribute to DED. Researchers made the mice experience an 8-hour shift in their day-night cycle every 3 days, imitating chronic jet lag. The findings showed that chronic jet lag resulted in a significant decrease in MUC4 expression in the cornea, leading to DED symptoms. Supplementing with MUC4 or treating the mice with melatonin, eased these symptoms. This indicates that disruptions to our internal clock can directly affect eye health by impacting key protective proteins in the eye. Researchers conclude that maintaining a healthy internal clock is vital for eye health and that treatments targeting internal clock disruptions could help DED patients.

This summary was initially drafted using artificial intelligence, then revised and fact-checked by the author.