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

The role of Drosophila numb 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 numb , numb ¹ , showed an antineurogenic effect on the peripheral nervous system (PNS), which revealed that the wild-type numb suppresses Notch signaling. In the current study, we examined whether this phenotype is consistently observed in loss-of-function mutations of numb. Two more numb alleles, numb ^EY03840 and numb ^EY03852 , were shown to have an antineurogenic phenotype in the PNS. We also found that introducing a wild-type numb genomic fragment into numb ¹ homozygotes rescued their antineurogenic phenotype. These results demonstrated that loss-of-function mutations of numb universally induce this phenotype. Many components of Notch signaling are encoded by maternal effect genes, but no maternal effect of numb was observed in this study. The antineurogenic phenotype of numb was found to be dependent on the Enhancer of split ( E ( spl )), a downstream gene of Notch signaling. We found that the combination of E ( spl ) homozygous and numb ¹ homozygous suppressed the neurogenic phenotype of the embryonic central nervous system (CNS) associated with the E ( spl ) mutation. In the E(spl) allele, genes encoding basic helix-loop-helix proteins, such as m5 , m6 , m7 , and m8, remain. Thus, in the E(spl) allele, derepression of Notch activity by numb mutation can rescue the neurogenic phenotype by increasing the expression of the remaining genes in the E(spl) complex. We also uncovered a role for numb in regulating neuronal projections. Our results further support an important role for numb in the suppression of Notch signaling during embryonic nervous system development.