Superresolution Imaging—Caveolae, Caveolins, Mitochondria, and Function in Heart

Our ability to investigate questions improves as methods advance. Such advances may permit some issues to be resolved while other questions develop a new complexion and engender new and/or refined questions. Here experiments that combine superresolution optical imaging (or stochastic optical reconstruction microscopy, STORM (1,2)) and electron microscopy (EM) tomography are presented in a study on cardiac structures by Wong et al. (3). The focus is primarily on the spatial organization of the cardiac transverse tubular (TT) system, also called the transverse-axial tubular system (TATS).

While the transverse character of the TT/TATS system clearly dominates in healthy mammalian adult ventricular myocytes, heart failure leads to disorganization and dedifferentiation of this tubular system (4). In addition, the organization of caveolin-3 and the ryanodine receptors (RyR2 or the Ca²⁺ release channel in the junctional sarcoplasmic reticulum) are presented and discussed. The work seeks to link dynamic cellular structures and nanoscopic organization to cellular function. This work is also a very nice demonstration of the value of combining superresolution optical microscopy (STORM, PALM, and STED—see below (1,2,5–7)) and EM tomography (8,9).

The name of the protein caveolin comes from the name of structures with which they are reported to be associated: the caveolae (10). The caveolae are omega-shaped invaginations of the surface membrane (10,11) of cells with localized function and organization (12). In cardiac and skeletal muscle cells, the caveolae are thought to be associated with both the external sarcolemmal membrane and the membranes of the TATS. In some cases, caveolae are associated with mitochondria (13) and in adipocytes with lipid metabolism (14). It is thus often thought that the spatial organization of the caveolae as well as the specific array of proteins and lipids in the very small omega-shaped structure underlie caveolar function. There is, however, a distinction of the concept between caveolin-rich membranes and caveolae as presented in the work by Wong et al. (3).

The authors show that the majority of the surface membrane (external SL and TATS membranes) containing caveolin-3 does not include caveolar invaginations. Although there are TT dilations in the region of the junctional sarcoplasmic reticulum, they are not caveolae. This suggests that some quantitative reexamination of a number of hypotheses may be in order. Perhaps the exciting recent findings by Makarewich et al. (15), for example, would be recast if caveolae were not present. Nevertheless, the findings of Wong et al. (3) serve mainly to draw attention to a critical question: What exactly do the caveolins do? In addition, the work demonstrates both the power and utility of superresolution optical imaging combined with EM tomography.