New Positive Regulators of lin-12 Activity in Caenorhabditis elegans Include the BRE-5/Brainiac Glycosphingolipid Biosynthesis Enzyme

Abstract

Screens for suppressors of lin-12 hypermorphic alleles in C. elegans have identified core components and modulators of the LIN-12/Notch signaling pathway. Here we describe the recovery of alleles of six new genes from a screen for suppressors of the egg-laying defect associated with elevated lin-12 activity. The molecular identification of one of the new suppressor genes revealed it as bre-5, which had previously been identified in screens for mutations that confer resistance to Bt toxin in C. elegans. bre-5 is the homolog of D. melanogaster brainiac. BRE-5/Brainiac catalyzes a step in the synthesis of glycosphingolipids, components of lipid rafts that are thought to act as platforms for association among certain kinds of membrane-bound proteins. Reducing the activity of several other genes involved in glycosphingolipid biosynthesis also suppresses the effects of constitutive lin-12 activity. Genetic analysis and cell ablation experiments suggest that bre-5 functions prior to ligand-induced ectodomain shedding that activates LIN-12 for signal transduction.

Expression of ceramide glucosyltransferases, which are essential for glycosphingolipid synthesis, is only required in a small subset ofC. eleganscells

“…Analysis of mutants resistant to the Bacillus thuringiensis toxin show that complex GSLs act in the intestine as toxin receptors and that the oligosaccharide chains are essential for this function (Griffitts et al, 2003;Griffitts et al, 2005). Additional studies show that mutations in genes involved in oligosaccharide addition to GSLs suppress gain-of-function mutations in lin-12/Notch, implying that complex GSLs act as positive regulators of Notch signalling (Katic et al, 2005). Recent work shows that mutations in genes required for the synthesis of C17 branched-chain fatty acids -which are normally incorporated into the GSL backbone -cause arrest at the first larval stage (L1) (Chitwood et al, 1995;Kniazeva et al, 2004;Entchev et al, 2008;Kniazeva et al, 2008).…”

Glycosylation Genes Expressed in Seam Cells Determine Complex Surface Properties and Bacterial Adhesion to the Cuticle of Caenorhabditis elegans

“…Mutants in these genes are viable but resistant to killing by certain Cry toxins produced by Bacillus thuringiensis because the glycosphingolipid on the luminal surface of intestinal cells acts as the receptor for these toxins. The bre genes also affect developmental signaling by the Notch/ LIN-12 pathway (Katic et al 2005). …”

The intracellular region of Notch ligands Dll1 and Dll3 regulates their trafficking and signaling activity

“…By providing an ordered membrane microenvironment, lipid rafts (7) or other domains may contribute to the clustering of Dll1 molecules or to the interaction with specific cofactors (4). Interestingly, a genetic screen performed in Caenorhabditis elegans identified BRE-5/Brainiac as a positive regulator of Notch signaling that acts before the ligandinduced cleavage of Lin-12 and may target the ligands (8). Brainiac is an enzyme that participates in the biosynthesis of glycosphingolipid, a component of lipid rafts.…”