Antagonistic PCP Signaling Pathways in the developing Drosophila eye

Humanization for neurological disease modeling: A roadmap to increase the potential of Drosophila model systems

Neuroscience and neurology research is dominated by experimentation with rodents. Around 75% of neurology disease-associated genes have orthologs in Drosophila melanogaster, the fruit fly amenable to complex neurological and behavioral investigations. However, non-vertebrate models including Drosophila have so far been unable to significantly replace mice and rats in this field of studies. One reason for this situation is the predominance of gene overexpression (and gene loss-of-function) methodologies used when establishing a Drosophila model of a given neurological disease, a strategy that does not recapitulate accurately enough the genetic disease conditions. I argue here the need for a systematic humanization approach, whereby the Drosophila orthologs of human disease genes are replaced with the human sequences. This approach will identify the list of diseases and the underlying genes that can be adequately modeled in the fruit fly. I discuss the neurological disease genes to which this systematic humanization approach should be applied and provide an example of such an application, and consider its importance for subsequent disease modeling and drug discovery in Drosophila. I argue that this paradigm will not only advance our understanding of the molecular etiology of a number of neurological disorders, but will also gradually enable researchers to reduce experimentation using rodent models of multiple neurological diseases and eventually replace these models.

Global chromatin organizer SATB1 acts as a context-dependent regulator of the Wnt/Wg target genes

Special AT-rich binding protein-1 (SATB1) integrates higher-order chromatin architecture with gene regulation, thereby regulating multiple signaling pathways. In mammalian cells SATB1 directly interacts with β-catenin and regulates the expression of Wnt targets by binding to their promoters. Whether SATB1 regulates Wnt/wg signaling by recruitment of β-catenin and/or its interactions with other components remains elusive. Since Wnt/Wg signaling is conserved from invertebrates to humans, we investigated SATB1 functions in regulation of Wnt/Wg signaling by using mammalian cell-lines and Drosophila. Here, we present evidence that in mammalian cells, SATB1 interacts with Dishevelled, an upstream component of the Wnt/Wg pathway. Conversely, ectopic expression of full-length human SATB1 but not that of its N- or C-terminal domains in the eye imaginal discs and salivary glands of third instar Drosophila larvae increased the expression of Wnt/Wg pathway antagonists and suppressed phenotypes associated with activated Wnt/Wg pathway. These data argue that ectopically-provided SATB1 presumably modulates Wnt/Wg signaling by acting as negative regulator in Drosophila. Interestingly, comparison of SATB1 with PDZ- and homeo-domain containing Drosophila protein Defective Proventriculus suggests that both proteins exhibit limited functional similarity in the regulation of Wnt/Wg signaling in Drosophila. Collectively, these findings indicate that regulation of Wnt/Wg pathway by SATB1 is context-dependent.

HumanaFly: high-throughput transgenesis and expression of breast cancer transcripts in Drosophila eye discovers the RPS12-Wingless signaling axis

Drosophila melanogaster has been a model for multiple human disease conditions, including cancer. Among Drosophila tissues, the eye development is particularly sensitive to perturbations of the embryonic signaling pathways, whose improper activation in humans underlies various forms of cancer. We have launched the HumanaFly project, whereas human genes expressed in breast cancer patients are screened for their ability to aberrate development of the Drosophila eye, hoping to thus identify novel oncogenes. Here we report identification of a breast cancer transgene, which upon expression in Drosophila produces eye malformation similar to the famous Glazed phenotype discovered by Thomas Morgan and decades later dissected to originate from mis-expression of Wingless (Wg). Wg is the ortholog of human Wnt proteins serving as ligands to initiate the developmental/oncogenic Wnt signaling pathway. Through genetic experiments we identified that this transgene interacted with the Wg production machinery, rather than with Wg signal transduction. In Drosophila imaginal discs, we directly show that the transgene promoted long-range diffusion of Wg, affecting expression of the Wg target genes. The transgene emerged to encode RPS12—a protein of the small ribosomal subunit overexpressed in several cancer types and known to also possess extra-ribosomal functions. Our work identifies RPS12 as an unexpected regulator of secretion and activity of Wnts. As Wnt signaling is particularly important in the context of breast cancer initiation and progression, RPS12 might be implicated in tumorigenesis in this and other Wnt-dependent cancers. Continuation of our HumanaFly project may bring further discoveries on oncogenic mechanisms.

Convergent extension in mammalian morphogenesis

Convergent extension is a fundamental morphogenetic process that underlies not only the generation of the elongated vertebrate body plan from the initially radially symmetrical embryo, but also the specific shape changes characteristic of many individual tissues. These tissue shape changes are the result of specific cell behaviors, coordinated in time and space, and affected by the physical properties of the tissue. While mediolateral cell intercalation is the classic cellular mechanism for producing tissue convergence and extension, other cell behaviors can also provide similar tissue-scale distortions or can modulate the effects of mediolateral cell intercalation to sculpt a specific shape. Regulation of regional tissue morphogenesis through planar polarization of the variety of underlying cell behaviors is well-recognized, but as yet is not well understood at the molecular level. Here, we review recent advances in understanding the cellular basis for convergence and extension and its regulation.