The fruitfly Drosophila melanogaster is one of the most studied genetic systems. Many genetic pathways have been well characterized in this organism and it is accessible to a variety of genetic manipulations. Not only that a large number of genes are highly conserved between flies and human, several developmental events, pathways, cell and tissue organization etc are similar between the two systems. Using Drosophila as a model system, we are addressing two fundamental questions in biology: What is the mechanism by which various cells/tissues/organs are positioned in their respective places in our body? How are shape and size of different organs determined? Our approach to address these questions is studying molecular and morphogenic events downstream to Hox genes, which play critical roles in the elaboration of segmental identities along the antero-posterior axis in all bilaterian animals.
In Drosophila, wings and halteres are the dorsal appendages of the second and third thoracic segments, respectively. In the third thoracic segment, wing development is suppressed by the Hox gene Ultrabithorax (Ubx) to mediate haltere development (Lewis, 1978). Loss of Ubx function from developing haltere discs induces haltere-to-wing transformations, whereas ectopic expression of Ubx in developing wing discs leads to wing-to-haltere transformations (Lewis, 1978). Thus, the differential development of wings and halteres constitutes a good genetic system to study cell fate determination at different levels such as growth, cell shape, size and its biochemical and physiological properties . They also represent the evolutionary trend that has established the differences between fore and hind wings in insects, wings and legs in birds and fore and hind limbs in mammals.
One way to approach the mechanism of Ubx function is to reconstruct a wing appendage in the third thoracic segment without altering the patterns/levels of Ubx expression. This necessitates identification of genes that are differentially expressed between wing and haltere during development and reverse-engineer the expression of one or more of those genes during haltere development. We have employed two complementary approaches in our studies, the first one being the examination of genetic interactions between Ubx and certain other genes that are already shown to be functional during wing and haltere development and the second approach is identification of downstream targets of Ubx function by highthroughput techniques such as microarray and ChIP-chip.
Observations from our lab suggest that Ubx down regulates activities of the signaling centers, such as anterior-posterior (A/P) and dorso-ventral (D/V) organizers, to specify haltere fate. Our observations also suggest a mechanism by which Ubx dampens organizing activities of compartment boundaries and thereby, repress the wing fate. Our work has, thus, opened up new avenues to study genetic mechanisms that help in fine-tuning signal transduction pathways. We have demonstrated that d ifferential development of wing and haltere discs is a good assay system to identify, hitherto unknown, regulators or mechanism of regulation of key signal transduction pathways, such as Wnt, TGF- b and Egfr/Ras pathways, which are implicated in many cancers .
Our future plan in this direction involves identifying those genes that have come under the influence of Ubx specifically during dipteran evolution. This involves extensive bioinformatics analyses and identifying direct targets of Ubx from different insect groups such as Apis, butterflies, silkworm, Tribolium, mosquito and different species of Drosophila.
We have further extended our work on the mechanism of specification of the proximal part of the wing appendage as against the distal part, by studying signaling interactions between squamous and comumnar epithelia of the developing wing. This finding has led to the development of a novel assay system to study the mechanism for the diffusion of important signal molecules such as Hedgehog. We have also employed central nervous system to study the mechanism of Hox function at the cellular level as against the organ level discussed above. Our work has led to the identification of a novel and important mechanism of cell-fate determination wherein Cyclin E, a regulator of the cell cycle, is an effector of Hox gene function in generating neuronal diversity in the central nervous system.
Our current and future work also involves functional genomics using Drosophila as a genetic system to study human genes, to develop fly models of human diseases and the application of the same in drug screening. In this direction, we have generated transgenic flies expressing human colon cancer gene, Adenamatous polyposis Coli (APC). We have demonstrated that human APC induced phenotypes in transgenic Drosophila constitute a novel, fast and inexpensive assay system for developing therapeutics for the prevention and treatment of cancer. We have generated several transgenic flies expressing different truncated and mutant versions of human APC (normally found in colon cancer patients) to study structure-function relationship of various biochemical domains and to further improve drug-screening efficiency. In addition, we are working to determine “mode-of-action” of certain drugs and plant-based insecticides using Drosophila genetics.
LS Shashidhara Pavan Aggarwal V Bharathi TTS Harsha Naveen Prasad Palaparthi Ramesh Myneni Sudha N Usha
Ruchi Bajpai, Poonam Bhandari, Prafulla Chandrika, Tripura Chaturvedula, Starling Emerald, Ramakrishnan Kannan, Jeevan Karloss, Karedla Kavitha, Pallavi Kshetrapal, Ashish Lal, Kalpana Makhijani, T Mrudula, Vadivel Prabahar, Mohit Prasad, Prashanth Ramesh Rao, Nagraj Sambrani, Bernd Stadelmayer, VNL Sushmita, Ramesh Yelagandula.
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