D. melanogaster

Four activating ligands, Spitz, Keren, Gurken and Vein have been identified for drosophila EGF receptors, called DER. Spitz (SPI) is the major ligand and is involved in most situations where the pathway is activated. Keren plays a minor, redundant role, while Gurken is used exclusively during oogenesis. These ligands are produced as inactive transmembrane precursors, which are retained in the endoplasmic reticulum and needed to processed by the chaperone protein Star. Processed ligands are directed into another compartment where they encounter Rhomboid (RHO) serine proteases, which cleave the ligand precursors within the transmembrane domain to release the active, secreted ligand form. RHO also cleaves and inactivates Star, attenuating the level of cleaved ligand that is produced. The fourth ligand, Vein, is produced as a secreted molecule, which is a weaker activating ligand used either to enhance signalling by other ligands or in specific situations such as muscle patterning. Binding of ligands to DER leads to dimerization and triggering of the canonical DRK/SOS/RAS/RAF/DSOR1/Rolled pathway. DRK (SH2-domain-containing protein) recruits SOS (Son of sevenless, a guanine nucleotide exchange factor) to catalyze the exchange of RAS bound GDP for GTP exchange, thereby activating RAS. RAS then promotes the activation of RAF, leading to DSOR1 activation, and eventually to Rolled (RL) activation. RL inactivates transcriptional co-repressors, such as Aop, and activates transcription factors, such as Pointed (PNT) ([1],[2]). The transcriptional activator PNT is a the major effector of the pathway. The protein Anterior open (AOP) is a constitutive repressor, which competes for PNT binding sites and can be removed from the nucleus and degraded upon phosphorylation by MAP kinases. AOS (Argos), STY (Sprouty) and KEK (Kekkon) are inducible repressive elements involved in negative feedbacks. AOS is a secreted molecule, which sequesters the ligand SPI (Spitz). STY acts downstream of DER, but upstream of RAS and RAF, by recruiting GAP1 and blocking the ability of DRK to bind to its positive effector. KEK is a transmembrane protein forming a non-functional heterodimer with the receptor. Constitutively expressed, CBL (E3 ligase) modulates DER signalling by recognizing activated, internalized receptor molecules and inducing their ubiquitination and degradation. CBL may also enhance the endocytosis of DER, following ligand binding. Modulation of DER signalling by CBL has been reported only in the follicle cells, which receive the Gurken signal from the oocyte ([3], [4], [5], [6]). Our logical model represents a cell receiving different combinations of ligands binding (SPI or Vein or both) and express/receive different levels of inhibitory inputs (Aos, Sty, Cbl, Kek). The signalling pathway is characterized by no signalling, medium or high signalling process designed by multi-valued nodes. We consider five main wild-type cellular situations: i. Cells secreting ligands but lacking Der activation (inhibition of Der), leading to no signalling. ii. Cells receiving medium signal with SPI expressed at level 1 and/or Vein expressed also at level 1, leading to medium signalling. iii. Cells receiving SPI at level of expression 1 (and/or Vein expressed at level 1) and in presence of an inhibitor (e.g. STY, AOS, or KEK), leading to no signalling. iv. Cells receiving SPI at level of expression 2 in the absence of inhibitors, leading to high signalling. v. Cells receiving SPI at level of expression 2 (and/or Vein expressed at level 1) in presence of an inhibitor (e.g. STY, AOS, or KEK,...), leading only to medium signalling.

References

This manuscript focuses on the formal analysis of the gap-gene network involved in Drosophila segmentation. The gap genes are expressed in defined domains along the anterior–posterior axis of the embryo, as a response to asymmetric maternal information in the oocyte. Though many of the individual interactions among maternal and gap genes are reasonably well understood, we still lack a thorough understanding of the dynamic behavior of the system as a whole. Based on a generalized logical formalization, the present analysis leads to the delineation of: (1) the minimal number of distinct, qualitative, functional levels associated with each of the key regulatory factors (the three maternal Bcd, Hb and Cad products, and the four gap Gt, Hb, Kr and Kni products); (2) the most crucial interactions and regulatory circuits of the earliest stages of the segmentation process; (3) the ordering of different regulatory interactions governed by each of these products according to corresponding concentration scales; and (4) the role of gap-gene cross-interactions in the transformation of graded maternal information into discrete gap-gene expression domains. The proposed model allows not only the qualitative reproduction of the patterns of gene expression characterized experimentally, but also the simulation and prediction of single and multiple mutant phenotypes.

The larval development of the Drosophila melanogaster wings is organized by the protein Wingless, which is secreted by cells adjacent to the dorsal–ventral (DV) boundary. Two signaling processes acting between the second and early third instars and between the mid- and late third instar control the expression of Wingless in these boundary cells.

A preliminary model (Apterous-dependent network) was presented in (see [1]), for the inter-cellular regulatory network activating Notch at the dorsal–ventral boundary in the wing imaginal disc of Drosophila. This model focussed on the cross-regulations between five genes (within and between two cells), which implements the dorsal–ventral border in the developing imaginal disc.

A refined model (complete network) was then published (see [2]) that integrates both signaling processes into a logical multivalued model encompassing four cells, i.e., a boundary and a flanking cell at each side of the boundary. Computer simulations of this model enable a qualitative reproduction of the main wild-type and mutant phenotypes described in the experimental literature. During the first signaling process, Notch becomes activated by the first signaling process in an Apterous-dependent manner. In silico perturbation experiments show that this early activation of Notch is unstable in the absence of Apterous. However, during the second signaling process, the Notch pattern becomes consolidated, and thus independent of Apterous, through activation of the paracrine positive feedback circuit of Wingless. Consequently, we propose that appropriate delays for Apterous inactivation and Wingless induction by Notch are crucial to maintain the wild-type expression at the dorsal–ventral boundary. Finally, another mutant simulation shows that cut expression might be shifted to late larval stages because of a potential interference with the early signaling process.

References

Initially activated by the pair-rule genes, the expression patterns of the segment polarity genes engrailed and wingless become consolidated through inter-cellular interactions between juxtaposed cells. Here, we delineate a logical model focusing on a dozen molecular components at the core of the regulatory network controlling this process. Our model analysis leads to the following conclusions: (1) the pair-rule signals, which activate engrailed and wingless genes independently of each other, need to be operative until the inter-cellular circuit involving these two genes is functional. This implies that the pair-rule pattern is instrumental both in determining the activation of the genes engrailed and wingless in rows of adjacent cells, and in consolidating these expression patterns. (2) The consolidation of engrailed and wingless expression patterns requires the proper activation of both autocrine and paracrine Wingless-pathways, as well as the full activation of the Hedgehog pathway. (3) Protein kinase A plays at least two roles through the phosphorylation of Cubitus interruptus, the effector molecule of the Hedgehog signalling pathway. (4) The roles of Sloppy-paired and Naked in the delineation of the engrailed and wingless expression domains are emphasized as being important for segmental boundary formation. Moreover, the application of an original computational method leads to the delineation of a subset of crucial regulatory circuits enabling the coexistence of specific expression states at the cellular level, as well as specific combination of cellular states inter-connected through Wingless and Hedgehog signalling. Finally, the simulation of altered expressions of segment polarity genes leads to results consistent with the published data concerning loss-of-function mutants and ectopic gene expressions. Moreover, this model makes predictions about genetic interactions that can be tested by further experiments.