Tunnelling nanotubes (TNTs) are actin-based cell protrusions that connect neighbouring cells and enable cytoplasm continuity and communication through cargo transfer, including ions, proteins and organelles. TNTs are predominantly studied in cell culture, and their characterization in vivo is challenging owing to the lack of TNT-specific markers to distinguish them from other cell protrusions (such as cytonemes or filopodia). Korenkova, Liu et al. now characterize TNTs in live zebrafish embryos.

The actin-binding protein Eps8, which regulates actin activity by capping actin barbed ends and cross-linking actin filaments, is known to induce TNTs in vitro, through its bundling activity. Overexpression of Eps8 in the embryo did not affect the formation of TNT connections, but decreased their average length, potentially owing to its role in actin capping. To eliminate this effect, a mutant form that lacked capping activity was overexpressed in zebrafish embryos. This caused a substantial increase in the number of TNT-like connections, without affecting the number of Cep55-positive cytokinetic bridges or Wnt8a-positive cytonemes. Interestingly, overexpression of the Eps8 non-capping mutant led to a significant increase in developmental defects, indicating that the ectopic formation of TNTs is detrimental to normal embryonic development, although an effect based on other actin-based structures cannot be excluded.