Tumor microtubes connect pancreatic cancer cells in an Arp2/3 complex-dependent manner

Abstract

Actin-based tubular connections between cells have been observed in many cell types. Termed "tunneling nanotubes (TNTs)", "membrane nanotubes", "tumor microtubes (TMTs)", or "cytonemes", these protrusions interconnect cells in dynamic networks. Structural features in these protrusions vary between cellular systems, including tubule diameter and presence of microtubules. We find tubular protrusions, which we classify as TMTs, in a pancreatic cancer cell line, DHPC-018. TMTs are present in DHPC-018-derived tumors in mice, as well as in a mouse model of pancreatic cancer and a sub-set of primary human tumors. DHPC-018 TMTs have heterogeneous diameter (0.39 - 5.85 m, median 1.92 m) and contain actin filaments, microtubules, and cytokeratin 19-based intermediate filaments. The actin filaments are cortical within the protrusion, as opposed to TNTs, in which filaments run down the center of the tube. TMTs are dynamic in length, but are long-lived (median > 60 min). Inhibition of actin polymerization, but not microtubules, results in TMT loss. A second class of tubular protrusion, which we term cell-substrate protrusion (CSP), has similar width range and cytoskeletal features but make contact with the substratum as opposed to another cell. Similar to previous work on TNTs, we find two assembly mechanisms for TMTs, which we term "pull-away" and "search-and-capture". Inhibition of the Arp2/3 complex, an actin assembly factor, inhibits TMT assembly by both mechanisms. This work demonstrates that the actin architecture of TMTs is fundamentally different from that of TNTs, as well as demonstrating the role of Arp2/3 complex on TMT assembly.