A Working Model of Sarcomere Maintenance in Cardiomyocytes

Rationale: The cardiomyocyte is a long living cell that is dedicated to operating and maintaining a macromolecular contractile machine - the sarcomere. Sarcomeric proteins have limited half-lives, therefore they require continuous replacement. Such maintenance is challenging since transcription and translation may be variable ‘noisy’ processes, and cardiomyocytes need to maintain all their sarcomeres simultaneously while preserving their stoichiometry in order for normal contractile activity to proceed.

Objective: Our study aims to delineate a working model of sarcomeric maintenance, addressing the current gap in knowledge of the molecular mechanisms used by cardiomyocytes to efficiently maintain all their sarcomeres simultaneously.

Methods and Results: We performed single-cell analysis of cardiomyocytes using imaging of mRNA and protein synthesis, and gene expression via RT-qPCR. These demonstrated sarcomeric mRNA localization, which is followed by localized protein synthesis at the sarcomere. We also show that sarcomeric gene transcription and protein synthesis rates vary widely between cells, yet protein content is relatively constant. Unincorporated sarcomeric proteins are rapidly degraded by the localized proteasome system, offsetting the transcriptional and translational variability.

Conclusions: Three distinct mechanisms are responsible for the maintenance of the sarcomere: mRNAs encoding for sarcomeric proteins are localized to the sarcomere, ribosomes are localized to the sarcomere with localized sarcomeric protein translation, and finally, a localized E3 ubiquitin ligase and localized ubiquitination allow efficient degradation of excess unincorporated sarcomeric proteins. We show that these mechanisms are distinct, required, and work in unison, to ensure both spatial localization, and to overcome the large variability in transcription of sarcomeric genes. Cardiomyocytes simultaneously maintain all their sarcomeres using localized translation-degradation ‘facilities’ where proteins are continuously and locally synthesized at high rates, and excess proteins are continuously and locally degraded, providing a very robust yet adaptable system.