PAX genes in childhood oncogenesis: developmental biology gone awry?

Childhood solid tumors often arise from embryonal-like cells, which are distinct from the epithelial cancers observed in adults, and etiologically can be considered as 'developmental patterning gone awry'. Paired-box (PAX) genes encode a family of evolutionarily conserved transcription factors that are important regulators of cell lineage specification, migration and tissue patterning. PAX loss-of-function mutations are well known to cause potent developmental phenotypes in animal models and underlie genetic disease in humans, whereas dysregulation and/or genetic modification of PAX genes have been shown to function as critical triggers for human tumorigenesis. Consequently, exploring PAX-related pathobiology generates insights into both normal developmental biology and key molecular mechanisms that underlie pediatric cancer, which are the topics of this review.

Recruitment of Tube and Pelle to signaling sites at the surface of the Drosophila embryo.

A signaling pathway initiated by activation of the transmembrane receptor Toll defines dorsoventral polarity in the Drosophila embryo. Toll, which is present over the entire surface of the embryo, is activated ventrally by interaction with a spatially restricted, extracellular ligand. Tube and Pelle then transduce the signal from activated Toll to a complex of Dorsal and Cactus. Here we demonstrate by an mRNA microinjection assay that targeting of either Tube or Pelle to the plasma membrane by myristylation is sufficient to activate the signal transduction pathway that leads to Dorsal nuclear translocation. Using confocal immunofluorescence microscopy we also show that activated Toll induces a localized recruitment of Tube and Pelle to the plasma membrane. Together, these results strongly support the hypothesis that intracellular signaling requires the Toll-mediated formation of a membrane-associated complex containing both Tube and Pelle.

A role for CKII phosphorylation of the cactus PEST domain in dorsoventral patterning of the Drosophila embryo.

Regulated proteolysis of Cactus, the cytoplasmic inhibitor of the Rel-related transcription factor Dorsal, is an essential step in patterning of the Drosophila embryo. Signal-induced Cactus degradation frees Dorsal for nuclear translocation on the ventral and lateral sides of the embryo, establishing zones of gene expression along the dorsoventral axis. Cactus stability is regulated by amino-terminal serine residues necessary for signal responsiveness, as well as by a carboxy-terminal PEST domain. We have identified Drosophila casein kinase II (CKII) as a Cactus kinase and shown that CKII specifically phosphorylates a set of serine residues within the Cactus PEST domain. These serines are phosphorylated in vivo and are required for wild-type Cactus activity. Conversion of these serines to alanine or glutamic acid residues differentially affects the levels and activity of Cactus in embryos, but does not inhibit the binding of Cactus to Dorsal. Taken together, these data indicate that wild-type axis formation requires CKII-catalyzed phosphorylation of the Cactus PEST domain.

A gradient of cactus protein degradation establishes dorsoventral polarity in the Drosophila embryo.

Dorsoventral polarity in the Drosophila embryo is established by a signaling pathway active on the ventral and ventrolateral surfaces of the embryo. Signal transduction via the protein kinase Pelle frees the Rel-related protein Dorsal from its cytoplasmic inhibitor Cactus, allowing Dorsal to translocate into ventral and ventrolateral nuclei and direct gene expression. Here, we show by immunochemical analyses that Pelle-mediated signaling induces the spatially graded degradation of Cactus. Using a tissue culture system which reconstitutes Pelle-dependent Cactus degradation, we show that a motif in Cactus resembling the sites of signal-dependent phosphorylation in the vertebrate homologs IkappaB-alpha and IkappaB-beta is essential for Pelle-induced Cactus degradation. Substitution of four serines within this motif with nonphosphorylatable alanine residues generated a mutant Cactus that still functions as a Dorsal inhibitor but is resistant to induced degradation. Injection of RNA encoding this altered form of Cactus has a dominant negative effect on establishment of dorsoventral polarity in the embryo. We conclude that dorsoventral signaling results in a Cactus concentration gradient and propose that signal-dependent phosphorylation directs the spatially regulated proteolysis of Cactus protein.

Interaction of the pelle kinase with the membrane-associated protein tube is required for transduction of the dorsoventral signal in Drosophila embryos.

Within the Drosophila embryo, tube and the protein kinase pelle transduce an intracellular signal generated by the transmembrane receptor Toll. This signal directs import of the rel-related protein dorsal into ventral and ventrolateral nuclei, thereby establishing dorsoventral polarity. We show by immunolocalization that tube protein associates with the plasma membrane during interphase. We also find that tube sequences required for signaling interact with pelle in a yeast two-hybrid assay. We demonstrate that fusion of the pelle catalytic domain to the transmembrane receptor torso is sufficient to induce ventral fates; this activity is independent of Toll or tube. Lastly, we find that fusion of the tube protein to torso also induces ventral fates, but only in the presence of functional pelle. We propose a model wherein tube activates pelle by recruiting it to the plasma membrane, thereby propagating the axis-determining signal.