The canonical Notch signaling pathway is one of a limited group of pathway modules that transduce signals from outside the cell to alter gene expression inside the nucleus 
. These pathways together orchestrate the developmental processes that can be dauntingly complex. Yet it is the same modules that are used repeatedly, not only in different organisms, but also in vastly different processes within an organism 
. Thus, how these pathway modules are activated in a specific manner, with regard to not only space and time but also the quantity of their signaling output, represents a fundamental question in developmental biology. Here we describe a newly characterized protein, Uif, which can antagonize the canonical Notch signaling pathway in a neomorphic manner. These findings underscore the importance of the precise tuning of Notch activity in normal patterning.
EGF-like repeats are a common feature of Notch receptors, ligands and co-ligands 
. While Uif was originally characterized for its role in tracheal development, its EGF-like repeats suggest a possible role in Notch signaling. Our results are consistent with a model where ectopically expressed Uif may modulate the accessibility of the extracellular domain of Notch to its ligands during activation. It is possible that the EGF-like repeats of Uif directly interact with the extracellular domain of Notch to exert its inhibitory effect in a manner similar to the cis
inhibition by Notch ligands themselves 
. Our finding that Uif* acts on Notch through a cis
inhibitory mechanism () is supportive of this possibility. In our experiments, Uif* is more effective than wt Uif in antagonizing Notch, and this difference may be attributed to the difference in their expression levels (Figure S2
). These results suggest that ectopically expressed Uif* and wt Uif have a similar neomorphic function in regulating Notch signaling.
A proposed neomorphic function of Uif* and Uif in Notch signaling is consistent with our results of loss of function analysis of uif
. Knockdown (assayed for adult wing phenotypes and Notch target gene expression using independent RNAi lines; data not shown) or knockout (assayed for Notch target gene expression in somatic mutant clones; Figure S4
) of uif
revealed neither Notch loss of function nor gain of function phenotypes. However, it remains formally possible that the endogenous uif
gene has a native role in regulating Notch signaling in tissues or cells (other than those that we have examined) at a time during Drosophila
development. Further studies are required to investigate this possibility.
The biological activities of Uif are not restricted to regulating Notch signaling. The fact that Uif was originally characterized for its role in tracheal inflation underscores the complexity of its biological activities. In addition to the EGF-like repeats, Uif also contains several other domains that may have important biological functions. These domains include a C-type lectin-like (CLECT) domain, three CUB domains, eight complement control protein (CCP) domains, two coagulation factor 5/8 C-terminal (FA58C) domains and three hyaline repeat (HYR) domains. Both CLECT and FA58C domains are putative carbohydrate binding domains known to play important roles in many diverse processes 
. The CUB domain is an evolutionary conserved protein domain found almost exclusively in extracellular and plasma membrane-associated proteins 
. HYR is an immunoglobulin fold domain likely involved in cell adhesion 
. The CCP domains, also known as the Sushi domains or Short Consensus Repeats (SCR), exist in a wide variety of complement and adhesion proteins 
. These domains suggest that Uif may also play a role in cell adhesion. Indeed, in a recent genetic modifier screen, uif
was identified as a regulator (Mod29
) of the Drosophila
Dystroglycan-Dystrophin Complex, a specialized cell adhesion complex 
. Mod29/Uif was suggested to play roles in multiple developmental processes, including wing vein formation, muscle and photoreceptor axon development, and oogenesis 
. Although it remains to be investigated whether Uif, a large regulator with multiple conserved protein domains, may functionally connect distinct cellular processes, our own unpublished data offer some speculative insights. In particular, the blistering wing phenotype caused by knockdown of Dl
can be fully rescued by depletion of uif
(data not shown), suggesting that Uif may functionally extend the role of Notch ligands to cell adhesion. Uif is an N-glycosylated protein, a modification shared by several proteins known to play a role in the formation of large protein complexes 
. Understanding the full spectrum of the biological functions of Uif during development and, importantly, its potential role in harmonizing different cellular processes, represents future challenges.