Size does matter. While endlessly debated in social circles, this is unquestionably true for the normal function of the epithelial and endothelial tubes that comprise such organs as the vascular system, lung and kidney. The enlarged tubules in a polycystic kidney, for instance, literally crush the surrounding normal tubules, while narrowed blood vessels can cause ischemic tissue injuries.

Insulin-like growth factors (IGFs) control cell and organism growth through evolutionarily conserved signaling pathways. The mammalian acid-labile subunit (ALS) is a secreted protein that complexes with IGFs to modulate their activity. Recent work has shown that a Drosophila homolog of ALS, dALS, can also complex with and modulate the activity of a Drosophila IGF. Here we report the first mutations in the gene encoding dALS. Unexpectedly, we find that these mutations are allelic to a previously described mutation in convoluted (conv), a gene required for epithelial morphogenesis. In conv mutants, the tubes of the Drosophila tracheal system become abnormally elongated without altering tracheal cell number. conv null mutations cause larval lethality, but do not disrupt several processes required for tracheal tube size control, including septate junction formation, deposition of a lumenal/apical extracellular matrix, and lumenal secretion of Vermiform and Serpentine, two putative matrix-modifying proteins. Clearance of lumenal matrix and subcellular localization of clathrin also appear normal in conv mutants. However, we show that Conv/dALS is required for the dynamic organization of the transient lumenal matrix and normal structure of the cuticle that lines the tracheal lumen. These and other data suggest that the Conv/ dALS-dependent tube size control mechanism is distinct from other known processes involved in tracheal tube size regulation. Moreover, we present evidence indicating that Conv/dALS has a novel, IGF-signaling independent function in tracheal morphogenesis.

CRISPR/Cas9 technology is ideally suited for genome-wide screening applications due to the ease of generating guide RNAs (gRNAs) and the versatility of Cas9 or Cas9 derivatives to knockout, repress, or activate expression of target genes. Several pooled lentiviral CRISPR libraries have been developed and are now publicly available, but while using CRISPR/Cas9 for genetic experiments has become widely adopted, genome-wide screening experiments remain technically challenging. This review covers the basics of CRISPR/Cas9, describes several publicly available CRISPR libraries, and provides a general protocol for conducting genome-wide screening experiments using CRISPR/Cas9. © 2016 by John Wiley & Sons, Inc.