Accumulation of Extracellular Matrix and Cellular Material in the Transverse-Axial Tubular System of Cardiomyocytes in Fibrotic Human Heart Tissue

Introduction: Cardiomyocytes contain a complex network of deep cell membrane invaginations, termed transverse–axial tubular system (TATS). The TATS provides close coupling of surface plasma membrane to intracellular calcium stores, allowing fast and synchronised calcium-induced calcium release upon membrane depolarisation, and ultimately efficient excitation-contraction coupling. In disease, TATS organisation is disrupted, with loss in numbers and change in dimensions of individual elements. TATS disruption correlates closely with impaired cardiomyocytes contractility. Recent studies provided evidence that during fibrotic remodelling, extracellular matrix (ECM) accumulates not only around cardiomyocytes, but also deep inside TATS. The exact nature of the extracellular material accumulating in TATS in diseased tissue, and consequences for TATS structure and function, are not currently clear. 

 

Methods: We used samples of diseased left ventricular human heart tissue (dilated cardiomyopathy and transposition of the great arteries) with informed consent from patients or their legal guardians. Presence of ECM and other extracellular material in TATS was investigated using (i) confocal microscopy to quantify the extent of the accumulation of collagen types I, III, IV and VI, and to investigate the correlation between presence of ECM in TATS and the width of individual tubules; and (ii) array tomography – a volumetric electron microscopy technique – to assess TATS content and morphology on the nano-scale. All investigations conformed to the principles outlined in the Declaration of Helsinki and were approved by the ethics committee of the University of Freiburg.

 

Results: We detected the presence of all four collagen types inside TATS of diseased human heart tissue in all samples analysed. The prevalence of non-fibrillar collagen (types IV and VI) was higher than fibrillar collagen (types I and III). Enhanced presence of collagen types III, IV, and VI was associated with widening of individual TATS elements. There was no significant correlation between the extent of collagen accumulation in TATS and overall tissue fibrosis. Array tomography revealed the presence not only of fibrillary ECM, but also of cellular debris inside TATS, both near the TATS-surface sarcolemma junctions and deeper within cardiomyocyte. The source and nature of cellular debris is currently unknown.

 

Conclusion: Our results show that accumulation of ECM and cellular debris in TATS is a common phenotype in diseased human heart tissue, and that ECM accumulation correlates with changes in TATS geometry, providing first insight into a potential mechanism of fibrotic cardiac remodelling. Future studies will focus on exploring the correlation between ECM accumulation and TATS functional properties, such as speed of luminal diffusion as well as efficiency of action potential propagation and excitation-contraction coupling.