Structure and dynamics of human cardiac fibroblast nanotubes

Efficient and dynamic interactions between cardiac fibroblasts and their environment are essential for the maintenance of tissue homeostasis in healthy hearts and play an important role during pathological remodelling. Here, we investigate a relatively obscure mechanism through which human atrial fibroblasts communicate with each other, with other cells, and with the extracellular matrix (ECM) - nanotubes (NT). We investigated NT structure and dynamics in primary right atrial fibroblasts isolated from patients in sinus rhythm (SR) and atrial fibrillation (AF), in an immortalised human atrial fibroblasts cell line, and in intact human tissue, using a wide range of imaging approaches (including confocal microscopy, label-free reflection microscopy, rotating coherent scattering microscopy, and cryo-electron tomography). We show that fibroblasts maintain continuous NT activity in vitro, with numerous protrusions constantly probing the surrounding environment. NT structure and activity change during AF and following pharmacological (transforming growth factor-{beta}, latrunculin B) and environmental (hypoxia) interventions. We also show that cardiac fibroblast NT mediate intercellular organelle exchange and dynamically interact with ECM. Finally, we present evidence for the presence of fibroblast-borne NT in human atrial tissue. Our results advance our understanding of how cardiac fibroblasts interact with their environment. NT are versatile structures capable of both sensory and actuating functions, and offer a dynamic and rapid communication conduit that facilitates cell-cell and cell-extracellular matrix interactions.