Integrin is required for basement membrane crossing and branching of an invading intracellular tube

The narrowest biological tubes are formed by cells that hollow to form an intracellular lumen. Here, we examine early lumenogenesis of the C. elegans excretory cell, which branches to form an H-shaped intracellular tube spanning the length of the worm. Using genetically paralyzed embryos to freeze movement, we describe lumen initiation and branching for the first time using time-lapse fluorescence microscopy. We show that the excretory cell lumen forms through a plasma membrane invasion mechanism when a nascent lumen grows from the plasma membrane into the cytoplasm. The lumen subsequently extends along the left-right axis before branching to form anterior-posterior projections. Through a genetic screen, we identify mutations in ina-1 (α-integrin) and pat-3 (β-integrin) that block lumenogenesis at the anterior-posterior branching step, and we show that integrin function is required within the excretory cell. Finally, we find that the excretory cell crosses the epidermal basement membrane where anterior-posterior branches form, and demonstrate that basement membrane crossing fails in integrin mutant embryos. Our findings reveal how an intracellular lumen initiates and branches, and identify integrins and basement membrane as key branching regulators.