9.B.326.  The Transmembrane Apical Trachael Tube Apnoia (Apn) Family 

Most organs of multicellular organisms are built from epithelial tubes. To exert their functions, tubes rely on apico-basal polarity, on junctions, which form a barrier to separate the inside from the outside, and on a proper lumen, required for gas or liquid transport. Skouloudaki et al. 2019 identified apnoia (apn), a novel Drosophila gene required for tracheal tube elongation and lumen stability at larval stages. Larvae lacking Apn show abnormal tracheal inflation and twisted airway tubes, but no obvious defects in early steps of tracheal maturation. apn encodes a transmembrane protein of 137 aas and 4 TMSs in a 1 + 3 TMS arrangement, primarily expressed in the tracheae, which exerts its function by controlling the localization of Crumbs (Crb, P10040, 2,164 aas and 1 TMS with a large extracellular domain), an evolutionarily conserved apical determinant. Apn physically interacts with Crb to control its localization and maintenance at the apical membrane of developing airways. In apn mutant tracheal cells, Crb fails to localize apically and is trapped in retromer-positive vesicles. Consistent with the role of Crb in apical membrane growth, RNAi-mediated knockdown of Crb results in decreased apical surface growth of tracheal cells and impaired axial elongation of the dorsal trunk. Skouloudaki et al. 2019 concluded that Apn is a novel regulator of tracheal tube expansion in larval tracheae, the function of which is mediated by Crb.

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