Imaging T-tubules: dynamic membrane structures for deep functions

Abstract

This editorial refers to ‘Emerging mechanisms of T-tubule remodelling in heart failure’ by A. Guo et al ., pp. 204–215, this issue and ‘Ultrastructural uncoupling between T-tubules and sarcoplasmic reticulum in human heart failure’ by H.-B. Zhang, et al ., pp. 269–276, this issue. Extensive intracellular contacts of the endo/sarcoplasmic reticulum (ER/SR) with transverse tubules (TTs) are fundamental for intracellular Ca2+ cycling in cardiac muscle. For this, TTs extend from the cytoplasmic surface deep into the cell, setting up a complex three-dimensional (3D) membrane network, referred to as ‘transverse axial tubular system’ (TATS) ( Figure 1 A ). The TATS extends over several orders of magnitude in size, e.g. from local TT diameters <100 nm to cell-wide continuous networks expanding over 100 μm, in order to provide thousands of subcellular TT contacts with the ER organelle via junctional SR (jSR) contacts in a typical ventricular myocyte (VM). Not surprisingly, approaches to study the TT membrane structures and properties of the TATS network encompass different imaging and physiological methods. However, until recently ultrastructural TT resolution vs. continuous representation of TATS regions had to be traded against each other. Just imagine representative EM studies (smallest field of view, possible resolution ≤1 nm) when compared with confocal fluorescence microscopy of histological samples (field of view of ∼150 × 150 µm at ∼250 nm resolution using ×63 objectives). Meanwhile this methodological gap is addressed by recent super-resolution fluorescence microscopy approaches (see below).1 Despite apparent gaps between imaging resolution and the nanometric nature of subcellular membrane compartments, conventional fluorescence microscopy is essential to characterize TATS membranes in living samples. In particular, functional imaging studies of isolated cells and two-photon imaging of intact heart tissue have related typical TATS architectures to subcellular functions like Ca2+ release events (sparks) in resting cells (for diastole) or Ca2+ transients …