The novel intestinal filament organizer IFO-1 contributes to epithelial integrity in concert with ERM-1 and DLG-1

Abstract

SUMMARYThe nematode Caenorhabditis elegans is an excellent model system in which to study in vivo organization and function of the intermediate filament (IF) system for epithelial development and function. Using a transgenic ifb-2::cfp reporter strain, a mutagenesis screen was performed to identify mutants with aberrant expression patterns of the IF protein IFB-2, which is expressed in a dense network at the subapical endotube just below the microvillar brush border of intestinal cells. Two of the isolated alleles (kc2 and kc3) were mapped to the same gene, which we refer to as ifo-1 (intestinal filament organizer). The encoded polypeptide colocalizes with IF proteins and F-actin in the intestine. The apical localization of IFO-1 does not rely on IFB-2 but is dependent on LET-413, a basolateral protein involved in apical junction assembly and maintenance of cell polarity. In mutant worms, IFB-2 and IFC-2 are mislocalized in cytoplasmic granules and accumulate in large aggregates at the C. elegans apical junction (CeAJ) in a DLG-1-dependent fashion. Electron microscopy reveals loss of the prominent endotube and disordered but still intact microvilli. Semiquantitative fluorescence microscopy revealed a significant decrease of F-actin, suggesting a general role of IFO-1 in cytoskeletal organization. Furthermore, downregulation of the cytoskeletal organizer ERM-1 and the adherens junction component DLG-1, each of which leads to F-actin reduction on its own, induces a novel synthetic phenotype in ifo-1 mutants resulting in disruption of the lumen. We conclude that IFO-1 is a multipurpose linker between different cytoskeletal components of the C. elegans intestinal terminal web and contributes to proper epithelial tube formation.

Intermediate filaments EXC-2 and IFA-4 Maintain Luminal Structure of the Tubular Excretory Canals inCaenorhabditis elegans

“…In genetic screens, mutations in nine “ exc ” genes were discovered to affect canal structure to allow fluid-filled cysts to accumulate during canal extension during late embryogenesis and early first larval stage (BUECHNER et al 1999). Other studies found similar mutations that affect additional tubular structures in the nematode, including the seamed single-cell excretory duct cell (JONES and BAILLIE 1995; MANCUSO et al 2012; GILL et al 2016; PU et al 2017) and the multicellular intestine (BOSSINGER et al 2004; ZHANG et al 2011; CARBERRY et al 2012; KHAN et al 2013; ZHU et al 2015; GEISLER et al 2016). In the canal cell, proteins implementing tubule structure comprise apical cytoskeletal elements (MCKEOWN et al 1998; PRAITIS et al 2005; KHAN et al 2013; KOLOTUEV et al 2013; SHAYE and GREENWALD 2015), vesicular trafficking and exocyst proteins (TONG and BUECHNER 2008; MATTINGLY and BUECHNER 2011; ARMENTI et al 2014; LANT et al 2015; GRUSSENDORF et al 2016), and ion and lipid transporters (BERRY et al 2003; KHAN et al 2013), among others.…”

A novel function for the MAP kinase SMA-5 in intestinal tube stability

“…In the wild-type situation ( n = 3), the intestinal lumen, in transverse section, has a regular ellipsoid shape and is surrounded by evenly sized and regularly arranged microvilli of the brush border (Figure 9, A–C; see also Carberry et al. , 2012; www.wormatlas.org).…”

“…Further identification of the mutated gene was done by single-nucleotide polymorphism mapping (Davis et al. , 2005; Carberry et al. , 2012) and RNAi targeting of the remaining 94 candidate genes within the mapped region.…”

Luminal matrices: An inside view on organ morphogenesis

“…PCP appears to control the spatially oriented cell rearrangements and the orientation of cell divisions along the renal tube axis, and defects in this process have been proposed to cause cyst formation in polycystic kidney disease (PKD) [8]. The stability and proper dimensions of lumina in epithelial tubes are further supported by subapical actin and the intermediate filamentbased terminal web [9,10].…”