Ultrastructure analysis of mitochondria, lipid droplet and sarcoplasmic reticulum apposition in human heart failure

Cardiomyocyte structural remodeling is reported as a causal contributor to heart failure (HF) development and progression. Growing evidence highlights the role of organelle apposition in cardiomyocyte function and homeostasis. Disruptions in organelle crosstalk, such as that between the sarcoplasmic reticulum (SR) and mitochondria, dysregulate numerous cellular processes that include calcium handling and cellular bioenergetics, two processes that are disrupted and implicated in cardiac pathophysiology. While the physical distance between organelles is thought to be essential for homeostatic cardiomyocyte function, whether the interactions and coupling of organelles are altered in human heart failure remains unclear.

Here, we utilized transmission electron microscopy to characterize the role of organelle apposition in cardiomyocytes from patients with various etiologies of HF. Subsequently, we employed molecular approaches to examine expression changes of proposed organelle tethers.

We demonstrate that cardiomyocytes from dilated cardiomyopathy, hypertrophic cardiomyopathy and ischemic cardiomyopathy hearts display smaller, rounded mitochondria as compared to nonfailing controls. Failing cardiomyocytes also exhibited disrupted SR-mitochondria juxtaposition and changes in the expression of various proposed molecular tethers. Further analysis revealed alterations in lipid droplet dynamics including decreased lipid droplet number and less lipid droplets in association with mitochondria in failing cardiomyocytes.

We observed dysregulated organelle dynamics which was conserved across various etiologies of heart failure. Our results suggest that organelle structure and apposition is a possible contributor to human HF progression.