Numb Suppresses Notch-Dependent Activation of Enhancer of split during Lateral Inhibition in the Drosophila Embryonic Nervous System

The role of <i>Drosophila numb</i> in regulating Notch signaling and neurogenesis has been extensively studied, with a particular focus on its effects on the peripheral nervous system (PNS). Previous studies based on a single loss-of-function allele of <i>numb</i>, <i>numb¹</i>, showed an antineurogenic effect on the peripheral nervous system (PNS), which revealed that the wild-type <i>numb</i> suppresses Notch signaling. In the current study, we examined whether this phenotype is consistently observed in loss-of-function mutations of <i>numb.</i> Two more <i>numb</i> alleles, <i>numb^EY03840</i> and <i>numb^EY03852</i>, were shown to have an antineurogenic phenotype in the PNS. We also found that introducing a wild-type <i>numb</i> genomic fragment into <i>numb¹</i> homozygotes rescued their antineurogenic phenotype. These results demonstrated that loss-of-function mutations of <i>numb</i> universally induce this phenotype. Many components of Notch signaling are encoded by maternal effect genes, but no maternal effect of <i>numb</i> was observed in this study. The antineurogenic phenotype of <i>numb</i> was found to be dependent on the <i>Enhancer of split</i> (<i>E</i>(<i>spl</i>)), a downstream gene of Notch signaling. We found that the combination of <i>E</i>(<i>spl</i>) homozygous and <i>numb¹</i> homozygous suppressed the neurogenic phenotype of the embryonic central nervous system (CNS) associated with the <i>E</i>(<i>spl</i>) mutation. In the <i>E(spl)</i> allele, genes encoding basic helix-loop-helix proteins, such as <i>m5</i>, <i>m6</i>, <i>m7</i>, and m8, remain. Thus, in the <i>E(spl)</i> allele, derepression of Notch activity by <i>numb</i> mutation can rescue the neurogenic phenotype by increasing the expression of the remaining genes in the <i>E(spl)</i> complex. We also uncovered a role for <i>numb</i> in regulating neuronal projections. Our results further support an important role for <i>numb</i> in the suppression of Notch signaling during embryonic nervous system development.