We study how tubular networks develop from initially separate units. In the vascular system separate endothelial sprouts fuse to build tubular bridges (anastomoses). Despite the fundamental role of this process in organogenesis and pathology, the mechanisms that govern epithelial tube fusion are not well understood. Tube fusion events resembling vascular anastomosis formation also occur during development of the tracheal system. Tube fusion involves directed migration of cells towards the fusion point, formation of a new cell-cell junction, and finally the connection of adjacent tubes. We aim to understand the mechanism of membrane fusion during the connection of tracheal tubes. We use in vivo cell labeling techniques combined with high-resolution light and electron microscopy to define the intermediates of the fusion process at the cellular and ultrastructural level. To identify new components of the underlying cellular machinery, we characterize fusion-defective mutants, which we isolated in genetic screens (Caviglia and Luschnig 20143. Answering basic questions about lumen formation and conversion of cellular topology in the Drosophila tracheal tube fusion model can provide a conceptual framework to help elucidate similar processes, such as vascular anastomosis and pronephric duct fusion, in more complex vertebrate systems.
Membrane Dynamics During Epithelial Tube Fusion
Organs like the vertebrate vascular system and the insect tracheal system develop from separate primordia that undergo fusion events to form interconnected tubular networks. Although the correct pattern of tubular connections (anastomoses) in these organs is crucial for their normal function, the cellular and molecular mechanisms that govern tube fusion are only beginning to be understood. The process of tube fusion involves tip cell specification, cell-cell recognition and contact formation, self-avoidance, changes in cell shape and topology, lumen formation, and luminal membrane fusion. Significant insights into the underlying cellular machinery have been provided by genetic studies of tracheal tube fusion in Drosophila. Here, we summarize these findings and we highlight similarities and differences between tube fusion processes in the Drosophila tracheae and in the vertebrate vascular system. We integrate the findings from studies in vivo with the important mechanistic insights that have been gained from the analysis of tubulogenesis in cultured cells to propose a mechanistic model of tube fusion, aspects of which are likely to apply to diverse organs and organisms.
Cells at the tips of budding branches in the Drosophila tracheal system generate two morphologically different types of seamless tubes. Terminal cells (TCs) form branched lumenized extensions that mediate gas exchange at target tissues, whereas fusion cells (FCs) form ring-like connections between adjacent tracheal metameres. Each tracheal branch contains a specific set of TCs, FCs, or both, but the mechanisms that select between the two tip cell types in a branch-specific fashion are not clear. Here, we show that the ETS domain transcriptional repressor anterior open (aop) is dispensable for directed tracheal cell migration, but plays a key role in tracheal tip cell fate specification. Whereas aop globally inhibits TC and FC specification, MAPK signaling overcomes this inhibition by triggering degradation of Aop in tip cells. Loss of aop function causes excessive FC and TC specification, indicating that without Aop-mediated inhibition, all tracheal cells are competent to adopt a specialized fate. We demonstrate that Aop plays a dual role by inhibiting both MAPK and Wingless signaling, which induce TC and FC fate, respectively. In addition, the branch-specific choice between the two seamless tube types depends on the tracheal branch identity gene spalt major, which is sufficient to inhibit TC specification. Thus, a single repressor, Aop, integrates two different signals to couple tip cell fate selection with branch identity. The switch from a branching towards an anastomosing tip cell type may have evolved with the acquisition of a main tube that connects separate tracheal primordia to generate a tubular network.