# Severe attenuation of circadian clock output in the heart following sustained augmentation of cardiomyocyte protein O-GlcNAcylation

## TL;DR

Elevated O-GlcNAcylation in heart cells disrupts circadian rhythms and leads to heart fibrosis.

## Contribution

This study reveals that increased O-GlcNAcylation in cardiomyocytes impairs circadian control of the heart transcriptome.

## Key findings

- A 1.5-fold increase in O-GlcNAcylation impacts 70% of core circadian clock components in the heart.
- There is a 95% loss of circadian governance of the cardiac transcriptome despite normal oscillations of core clock proteins.
- Loss of circadian governance results in interstitial fibrosis in the heart.

## Abstract

Changes in circadian-related behaviors (e.g., the timing of food intake, sleep cycles) and the environment (e.g., light-dark cycles) increase the risk of numerous cardiometabolic diseases, including diabetes mellitus and cardiac disease. Recent studies indicate a close interrelationship between circadian clocks and the posttranslational modification, protein O-GlcNAcylation. The current study was designed to investigate whether a modest elevation of protein O-GlcNAcylation in the adult mouse heart, similar to levels observed during pathologic states, influenced circadian governance of the heart.

Cardiomyocyte-specific expression of a dominant negative O-GlcNAcase (dnOGAh) for a 2-week period resulted in an approximate 1.5-fold increase in cardiac protein O-GlcNAcylation, impacting 70% of core circadian clock components in the heart at the mRNA level. Further interrogation of cardiac mRNA species in dnOGAh hearts at candidate (RT-PCR) and unbiased (RNAseq) levels revealed a 95% loss of circadian governance of the cardiac transcriptome. This was despite persistent/augmented 24 h oscillations of the core circadian clock proteins BMAL1, REVERBα, and PER2 in dnOGAh hearts. Direct comparison of dnOGAh hearts with cardiomyocyte-specific BMAL1 knockout (CBK) hearts underscored an apparent uncoupling of the core clock mechanism from clock control of downstream target genes in dnOGAh hearts, and highlighted that loss of circadian governance results in interstitial fibrosis.

Sustained protein O-GlcNAcylation in the heart causes loss of circadian governance, likely downstream of the core circadian clock mechanism. Moreover, interstitial fibrosis appears to be a universal adverse outcome following impaired circadian governance.