Background: The receptor protein Notch and its ligand Delta are expressed throughout proneural regions yet non-neural precursor cells are defined by Notch activity and neural precursor cells by Notch inactivity. Not even Delta overexpression activates Notch in neural precursor cells. It is possible that future neural cells are protected by cis-inactivation, in which ligands block activation of Notch within the same cell. The Delta-ubiquitin ligase Neuralized has been proposed to antagonize cis-inactivation, favoring Notch activation. Cis-inactivation and role of Neuralized have not yet been studied in tissues where neural precursor cells are resistant to nearby Delta, however, such as the R8 cells of the eye or the bristle precursor cells of the epidermis.
Results: Overexpressed ligands could block Notch signal transduction cell-autonomously in non-neural cells of the epidermis and retina, but did not activate Notch nonautonomously in neural cells. High ligand expression levels were required for cis-inactivation, and Serrate was more effective than Delta, although Delta is the ligand normally regulating neural specification. Differences between Serrate and Delta depended on the extracellular domains of the respective proteins. Neuralized was found to act cell nonautonomously in signal-sending cells during eye development, inconsistent with the view that Neuralized antagonizes cis-inactivation in non-neural cells.
Conclusions: Delta and Neuralized contribute cell nonautonomously to Notch signaling in neurogenesis, and the model that Neuralized antagonizes cis-inactivation to permit Notch activity and specification of non-neural cells is refuted. The molecular mechanism rendering Notch insensitive to paracrine activation in neural precursor cells remains uncertain.