Principles of Network Organization of T-Tubule Membranes in Health and Disease

Abstract

Mechanical and functional behaviors of cardiac myocytes are largely determined by architectural arrangements of protein assemblies, membrane networks and organelles. Myofilaments and mitochondria fill most of the cell volume and are wrapped by the endomembrane systems that include the endo(sarco)plasmic reticulum (ESR) that is contiguous with the nuclear membrane system. While these membrane systems and organelles are essential for metabolic, proteomic and energetic homeostasis, the ESR further forms extensive contacts with the branches of the internal sarcolemmal membrane network, the T-tubules (TTs), and interacts with the cytoskeleton. In particular, TTs distribute electrical and chemical signals to intracellular Ca2+ release nanodomains that involve junctional ESR domains. We analyze the TT network quantitatively deep inside living myocytes based on data from confocal and super-resolution (STED) imaging. In adult murine cells, rectilinear TT elements showed a bimodal distribution of longitudinal and transversal orientations suggesting regular network properties. Furthermore, the properties of network branches corresponded with regular network architectures. In contrast, 4 weeks after myocardial infarction (post-MI) the orientations changed significantly, showing a differential increase versus decrease of longitudinal and transversal elements, respectively. Importantly, the number of branch points and oblique elements increased significantly post-MI. Thus, large-scale rectilinear network organization may support unique physiological functions, which become reorganized post-MI, leading to increased network complexity and dysfunction. It remains unclear how TTs interface with the microtubule (MT) system which is associated with junctional and other ESR structures, cortical scaffolds, and protein trafficking. Therefore, we further examined the MT network architecture which will be presented. In conclusion, analysis of membrane and protein networks identifies key properties of spatial organization versus pathological remodeling. The latter may directly contribute to Ca2+ release heterogeneity as a cause of pathological cellular signaling in heart failure and arrhythmias.