Three-dimensional electron microscopy reveals new details of membrane systems for Ca2+ signaling in the heart

TLDR

The study proposes that the spatial arrangement of dyadic clefts and adjacent membrane structures governs local cardiac Ca²⁺ signaling, shaping the quantal nature of SR Ca²⁺ releases such as sparks. Using electron tomography, the authors mapped the three‑dimensional topologies of dyadic clefts, transverse tubules, junctional sarcoplasmic reticulum, and nearby mitochondria in mouse ventricular myocardium to characterize their morphology and distribution. They found that T‑tubules exhibit diameter variation and frequent branching, dyadic clefts are small and irregularly spaced, many can accommodate fewer than 15 ryanodine receptor tetramers, and electron‑dense linkers frequently connect mitochondria to SR or T‑tubules.

Abstract

In the current study, the three-dimensional (3D) topologies of dyadic clefts and associated membrane organelles were mapped in mouse ventricular myocardium using electron tomography. The morphological details and the distribution of membrane systems, including transverse tubules (T-tubules), junctional sarcoplasmic reticulum (SR) and vicinal mitochondria, were determined and presumed to be crucial for controlling cardiac Ca(2+) dynamics. The geometric complexity of T-tubules that varied in diameter with frequent branching was clarified. Dyadic clefts were intricately shaped and remarkably small (average 4.39x10(5) nm(3), median 2.81x10(5) nm(3)). Although a dyadic cleft of average size could hold maximum 43 ryanodine receptor (RyR) tetramers, more than one-third of clefts were smaller than the size that is able to package as many as 15 RyR tetramers. The dyadic clefts were also adjacent to one another (average end-to-end distance to the nearest dyadic cleft, 19.9 nm) and were distributed irregularly along T-tubule branches. Electron-dense structures that linked membrane organelles were frequently observed between mitochondrial outer membranes and SR or T-tubules. We, thus, propose that the topology of dyadic clefts and the neighboring cellular micro-architecture are the major determinants of the local control of Ca(2+) in the heart, including the establishment of the quantal nature of SR Ca(2+) releases (e.g. Ca(2+) sparks).