Crumbs3 Is Essential for Proper Epithelial Development and Viability

Abstract

f First identified in Drosophila, the Crumbs (Crb) proteins are important in epithelial polarity, apical membrane formation, and tight junction (TJ) assembly. The conserved Crb intracellular region includes a FERM (band 4.1/ezrin/radixin/moesin) binding domain (FBD) whose mammalian binding partners are not well understood and a PDZ binding motif that interacts with mammalian Pals1 (protein associated with lin seven) (also known as MPP5). Pals1 binds Patj (Pals1-associated tight-junction protein), a multi-PDZ-domain protein that associates with many tight junction proteins. The Crb complex also binds the conserved Par3/Par6/atypical protein kinase C (aPKC) polarity cassette that restricts migration of basolateral proteins through phosphorylation. Here, we describe a Crb3 knockout mouse that demonstrates extensive defects in epithelial morphogenesis. The mice die shortly after birth, with cystic kidneys and proteinaceous debris throughout the lungs. The intestines display villus fusion, apical membrane blebs, and disrupted microvilli. These intestinal defects phenocopy those of Ezrin knockout mice, and we demonstrate an interaction between Crumbs3 and ezrin. Taken together, our data indicate that Crumbs3 is crucial for epithelial morphogenesis and plays a role in linking the apical membrane to the underlying ezrin-containing cytoskeleton.

The transmembrane protein Crumbs displays complex dynamics during follicular morphogenesis and is regulated competitively by Moesin and aPKC

“…Moe forms a complex with Crb, actin and β-heavy spectrin (β Spectrin; Médina et al, 2002a), and has been shown to interact with Crb in tracheal morphogenesis (Kerman et al, 2008;Letizia et al, 2011). Furthermore, Crb3, a mouse Crb ortholog, has been shown to interact with ezrin (Ezr), a mammalian Moe ortholog, and Crb3-knockout mice display phenotypes similar to Ezr mutants (Whiteman et al, 2013).…”

Generation of intestinal surface: an absorbing tale

“…Ezrin is stabilized in its active conformation through phosphorylation at threonine 567 and binding to phosphatidylinositol 4,5-bisphosphate (PIP2) at the apical surface (Fehon et al, 2010;Jayasundar et al, 2012). Similarly, Crb3 is also required for proper elaboration of the brush border, as microvilli are shortened in Crb3 −/− mice, similar to Ezr −/− animals (Whiteman et al, 2014), although the underlying mechanisms are unclear.…”

“…This process was thought to be defective in mice lacking the apical ezrin/radixin/moesin (ERM) protein ezrin, leading to the formation of isolated lumens and fused villi (Saotome et al, 2004). Loss of Crumbs3, another apical protein, causes a similar fusedvillus phenotype (Whiteman et al, 2014). However, more recent analyses, utilizing three-dimensional reconstruction to resolve luminal contiguity, have established that all luminal surfaces are connected during villus morphogenesis (Grosse et al, 2011).…”

Drosophila Crumbs prevents ectopic Notch activation in developing wings by inhibiting ligand-independent endocytosis

“…In Drosophila embryos, loss of crb function results in embryonic lethality, caused by the breakdown of many epithelia (Grawe et al, 1996;Tepass, 1996;. Comparable phenotypes are observed in mouse embryos lacking Crb2 or Crb3, two of the three mammalian Crb genes (Charrier et al, 2015;Szymaniak et al, 2015;Whiteman et al, 2014;Xiao et al, 2011).…”