Intracellular lumen extension requires ERM-1-dependent apical membrane expansion and AQP-8-mediated flux

Abstract

SUMMARYMany unicellular tubes such as capillaries form lumens intracellularly, a process that is not well understood. Here we show that the cortical membrane organizer ERM-1 is required to expand the intracellular apical/lumenal membrane and its actin undercoat during single-cell C.elegans excretory canal morphogenesis. We characterize AQP-8, identified in an ERM-1 overexpression (ERM-1[++]) suppressor screen, as a canalicular aquaporin that interacts with ERM-1 in lumen extension in a mercury-sensitive manner, implicating water-channel activity. AQP-8 is transiently recruited to the lumen by ERM-1, co-localizing in peri-lumenal cuffs interspaced along expanding canals. An ERM-1[++]-mediated increase in the number of lumen-associated canaliculi is reversed by AQP-8 depletion. We propose that the ERM-1-AQP-8 interaction propels lumen extension by translumenal flux, suggesting a direct morphogenetic effect of water-channel-regulated fluid pressure.

Physiological and molecular mechanisms of salt and water homeostasis in the nematodeCaenorhabditis elegans

“…Several mutant strains with aberrant excretory canal morphology have been isolated and used to investigate tubule formation during normal development (e.g., 1-2, 10 -11, 32, 37); these studies have identified cytoskeletal, signaling, and transport proteins as being essential for formation or maintenance of the canals. Recent studies have demonstrated that the excretory canals are extended when worms are shifted between high and low osmolarity (52,56), implying that osmotic pressure and water flux may play important roles in remodeling the excretory Fig. 1.…”

“…Direct transfer from high to low osmolarity causes swelling and temporary paralysis presumably because of elevated turgor pressure (42,52). After ϳ10 -15 min, swelling subsides and motility is regained by most individuals.…”

“…After ϳ10 -15 min, swelling subsides and motility is regained by most individuals. Occasionally, some adult worms actually burst with the intestine and gonads protruding through the vulva, an opening in the cuticle that is used to lay eggs (52). Aquaporins and the excretory system have been suggested to play a role in volume decrease (42,52), but the mechanism of volume recovery from swelling remains largely uncharacterized.…”

Luminal matrices: An inside view on organ morphogenesis

“…1A and 1B). Indeed, such hydrostatic pressure was proposed to play important roles in lumen formation and expansion in the gut [12], in Kupffer's vesicle [13] and in brain ventricles [14] in zebrafish, and in the excretory cell [15] and vulva [16] in C. elegans. Moreover, the cylindrical geometry of tubular structures brings along physical constraints that are distinct from those experienced by cells in planar epithelial sheets.…”

Noncentrosomal microtubules in C. elegans epithelia

“…The identification of mutants defective for axon outgrowth has suggested that extension of the basolateral membrane requires the axon outgrowth machinery (Hedgecock et al, 1987). By contrast, extension of the apical membrane appears to depend on osmotic stress and involves the fusion of the peri-apical vesicles with the lumen, which requires a physical interaction between the aquaporin AQP-8 and the apical actin-binding Ezrin-Radixin-Moesin homologue ERM-1 (Gobel et al, 2004;Khan et al, 2013;Kolotuev et al, 2013). Vesicular trafficking is important for canal morphogenesis to control the fusion of canalicular vesicles with the lumen, and/or to recycle proteins and/or lipids that remain to be defined.…”