Conclusion
AT junctions represent a conserved, cell-specific membrane structure for rapid excitation-contraction coupling throughout a representative spectrum of species including human. Since ATs provide the major excitable membrane network component in AMs, a new model of atrial "super-hub" Ca2+ signaling may apply across biomedically relevant species, opening avenues for future investigations about atrial disease mechanisms and therapeutic targeting.
Objective
To examine and quantitatively analyze the architecture of AT membrane structures and associated Ca2+ signaling proteins across species from mouse to human.
Results
We developed superresolution microscopy (nanoscopy) strategies for intact live AMs based on a new custom-made photostable cholesterol dye and immunofluorescence imaging of membraneous structures and membrane proteins in fixed tissue sections from human, porcine, and rodent atria. Consistently, in mouse, rat, and rabbit AMs, intact cell-wide tubule networks continuous with the surface membrane were observed, mainly composed of ATs. Moreover, co-immunofluorescence nanoscopy showed L-type Ca2+ channel clusters adjacent to extensive junctional RyR2 clusters at ATs. However, only junctional RyR2 clusters were highly phosphorylated and may thus prime Ca2+ release at ATs, locally for rapid signal amplification. While the density of the integrated L-type Ca2+ current was similar in human and mouse AMs, the intracellular Ca2+ transient showed quantitative differences. Importantly, local intracellular Ca2+ release from AT junctions occurred through instantaneous action potential propagation via transverse tubules (TTs) from the surface membrane. Hence, sparse TTs were sufficient as electrical conduits for rapid activation of Ca2+ release through ATs. Nanoscopy of atrial tissue sections confirmed abundant ATs as the major network component of AMs, particularly in human atrial tissue sections.