Abstract
Notch signaling activity governs widespread cellular differentiation in higher animals, including humans, and is involved in several congenital diseases and different forms of cancer. Notch signals are mediated by the transcriptional regulator RBPJ in a complex with activated Notch (NICD). Analysis of Notch pathway regulation in humans is hampered by a partial redundancy of the four Notch receptor copies, yet RBPJ is solitary, allowing its study in model systems. In Drosophila melanogaster, the RBPJ orthologue is encoded by Suppressor of Hairless [Su(H)]. Using genome engineering, we replaced Su(H) by murine RBPJ in order to study its function in the fly. In fact, RBPJ largely substitutes for Su(H)’s function, yet subtle phenotypes reflect increased Notch signaling activity. Accordingly, the binding of RBPJ to Hairless (H) protein, the general Notch antagonist in Drosophila, was considerably reduced compared to that of Su(H). An H-binding defective RBPJLLL mutant matched the respective Su(H)LLL allele: homozygotes were lethal due to extensive Notch hyperactivity. Moreover, RBPJLLL protein accumulated at lower levels than wild type RBPJ, except in the presence of NICD. Apparently, RBPJ protein stability depends on protein complex formation with either H or NICD, similar to Su(H), demonstrating that the murine homologue underlies the same regulatory mechanisms as Su(H) in Drosophila. These results underscore the importance of regulating the availability of RBPJ protein to correctly mediate Notch signaling activity in the fly.
Highlights
A small number of highly conserved signaling cascades governs the spatiotemporal development of multicellular organisms, including humans
It comes as little surprise that various diseases, as well as different cancers, are associated with a dysregulation of Notch signal transduction
Notch signal transduction is initialized by the binding of ligands to the receptor Notch, resulting in the cleavage and release of the intracellular Notch domain NICD from the membrane
Summary
A small number of highly conserved signaling cascades governs the spatiotemporal development of multicellular organisms, including humans. The Notch signaling pathway is one of those, being involved in a multitude of cell fate decisions by mediating direct cell–cell interactions. It comes as little surprise that various diseases, as well as different cancers, are associated with a dysregulation of Notch signal transduction (reviewed in [1,2]). No further intermediate steps are needed to transmit the Notch signal to the nucleus, as NICD itself is involved in the transcriptional response of Notch target genes. This response is realized by the association of NICD with the nuclear effector CSL and the co-activator Mastermind (Mam): Cells 2019, 8, 1252; doi:10.3390/cells8101252 www.mdpi.com/journal/cells
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