Notch activity in neural cells triggered by a mutant allele with altered glycosylation

Abstract

The receptor protein Notch is inactive in neural precursor cells despite neighboring cells expressing ligands. We investigated specification of the R8 neural photoreceptor cells that initiate differentiation of each Drosophila ommatidium. The ligand Delta was required in R8 cells themselves, consistent with a lateral inhibitor function for Delta. By contrast, Delta expressed in cells adjacent to R8 could not activate Notch in R8 cells. The split mutation of Notch was found to activate signaling in R8 precursor cells, blocking differentiation and leading to altered development and neural cell death. split did not affect other, inductive functions of Notch. The Ile578→Thr578 substitution responsible for the split mutation introduced a new site for O-fucosylation on EGF repeat 14 of the Notch extracellular domain. The O-fucose monosaccharide did not require extension by Fringe to confer the phenotype. Our results suggest functional differences between Notch in neural and non-neural cells. R8 precursor cells are protected from lateral inhibition by Delta. The protection is affected by modifications of a particular EGF repeat in the Notch extracellular domain. These results suggest that the pattern of neurogenesis is determined by blocking Notch signaling, as well as by activating Notch signaling.

Scabrous and Gp150 are endosomal proteins that regulate Notch activity

“…By contrast, none of the well-known components of N signaling behave as such dose-sensitive genetic modifiers of spl. Special alleles of Dl were also recovered as dominant spl suppressors (Brand and CamposOrtega, 1990), consistent with the finding that in spl the N activity in neural cells is ligand dependent (Li et al, 2003).…”

“…We note, however, that gp150 homozygotes appear less robust than sca homozygotes, and that gp150 adults are often recovered at less than expected frequencies (Fetchko et al, 2002). Given that spl shows inappropriate N activity in neural cells (Li et al, 2003), this identifies sca and gp150 as two genes required for N activity in neural cells. gp150 and sca function in the same pathway gp150 mutants have defects in sensory bristles and eye development resembling those caused by sca mutations.…”

“…The accompanying paper reports that the split mutant phenotype is due to inappropriate activation of N signal transduction in neural precursor cells, where N would normally become inactive, not because of an effect on the non-neuronal cells where most N activity occurs (Li et al, 2003). The Sca or Gp150 genes are essential for the spl mutant activity of N, but less important for normal N activity in non-neural cells.…”

“…The split mutation alters glycosylation of the N extracellular domain and leads to inappropriate N activity within R8 precursor cells in the developing eye (Li et al, 2003). This suggests that factors specifically regulating the inactivity of N in neural cells contribute to the spatial pattern of neurogenesis.…”

Regulation of Notch signaling by glycosylation

“…Notch EGF 14 which has no O-fucose consensus site [32]. In the Notch split mutation, a Thr is introduced into EGF 14 and becomes O-fucosylated.…”