The apical protein Apnoia interacts with Crumbs to regulate tracheal growth and inflation

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. Here we identify 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, primarily expressed in the tracheae, which exerts its function by controlling the localization of Crumbs (Crb), 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. We conclude that Apn is a novel regulator of tracheal tube expansion in larval tracheae, the function of which is mediated by Crb.多细胞生物的绝大多数器官均由上皮管（epithelial tubes）构成。为实现其生理功能，这类管结构依赖于顶基极性（apico-basal polarity）、用于分隔内外环境的细胞连接，以及介导气体或液体运输的功能性管腔（lumen）。本研究鉴定出apnoia（apn）——一种新型果蝇基因，其在幼虫阶段对气管管的伸长与管腔稳定性不可或缺。缺失Apn的幼虫会出现气管异常充气与气道扭曲的表型，但气管成熟的早期步骤无明显缺陷。apn编码一种跨膜蛋白，主要在气管组织中表达，其通过调控进化保守的顶端决定因子Crumbs (Crb)的定位来发挥功能。Apn可与Crb发生物理相互作用，以调控其在发育中气道顶端膜的定位与维持。在apn突变体的气管细胞中，Crb无法正确定位于顶端区域，反而被困于retromer（Retromer）阳性囊泡内。鉴于Crb在顶端膜生长中的关键作用，通过RNA干扰（RNAi）介导的Crb敲低会导致气管细胞的顶端表面生长受到抑制，以及背干线（dorsal trunk）的轴向伸长受损。综上，Apn是幼虫气管中调控气管管扩张的新型调控因子，其功能通过Crb介导实现。