Dresden, Techn. Univ., Diss., 2005.

Decapentaplegic (Dpp), a member of the TGF-[Beta] superfamily, acts as a morphogen to direct cell differentiation, determine cell fate and promote cell survival and proliferation in Drosophila wing development. To investigate the role of Dpp in Drosophila wing development, three aspects of the patterning role of Dpp have been analyzed. First, I investigated the cellular responses to Dpp signaling by a loss of function strategy. The consequences of lacking Dpp signal transduction on cell morphology and tissue integrity were analyzed. Second, I investigated whether Dpp signaling is down-stream of Hh signaling to maintain the normal cell segregation at the A/P boundary by clonal analysis. Third, I investigated whether cross talk among the Hh, Dpp and Wg signaling pathways exists and what its relevance for wing patterning is. To investigate the role of Dpp in Drosophila wing development, the general strategies are to look at the phenotypes of loss-of-function and gain-of-function. Mutant clones lacking Dpp signal transduction by knock down Dpp receptor Thick veins (Tkv) do not survive in wing blade due to JNK dependent apoptosis. To get larger mutant clones for analysis, JNK pathway was inhibited by knock down bsk (encodes JNK) in mutant clones lacking Dpp signaling using FLP-FRT system. Clones double mutant for tkv and bsk did not undergo apoptosis, but recovered at very low frequencies compared to sibling clones. Here, I showed that the low recovery of tkv bsk double mutant clones are due to the extrusion of mutant cells. The extrusion of tkv bsk double mutant cells correlated with changes in the actin cytoskeleton and a dramatic loss of the apical microtubule web normally present in these cells. These results suggest that Dpp signaling is required for cell morphogenesis in Drosophila wing development. We propose that Dpp acts as a survival factor in the wing disc epithelium by orchestrating proper cytoskeletal organization and maintaining normal cell-cell contact. Drosophila wing is subdivided into anterior (A) and posterior (P) compartments. This developing into adjacent compartments is crucial for the patterning of Drosophila wing. Previous study has shown that Hedgehog (Hh) signaling is required in A cells to maintain the A/P boundary and is sufficient to specify A type cell sorting. A previous study has in addition implicated the signaling molecule Decapentaplegic (Dpp) in maintaining the A/P boundary. However, this study did not address whether and in which cells, A and/or P, Dpp signal transduction was required to maintain this boundary. Here, I have analyzed the role of components of the Dpp signal transduction pathway and the relation of Dpp and Hh signaling in maintaining the A/P boundary by clonal analysis. I showed that Dpp signaling mediated by the Dpp target gene, T-box protein Optomotor-blind (Omb), is required in A cells, but not in P cells, to maintain the normal position of the A/P boundary. During patterning formation, it is essential for cells to receive precise positional information to pattern the tissue. It has been proposed for a long time that different signaling pathways such as Hedgehog (Hh), Dpp and Wingless (Wg) signaling pathways provide positional information for tissue patterning in an integrated manner. Recently, evidence of interactions between Hh and Dpp as well as Wg and Hh signaling pathways has been reported in Drosophila wing. Here, I have identified additional interactions among Hh, Dpp and Notch/Wg signaling. We propose that the selector gene engrailed, Hh and Dpp signaling interact with each other to regulate target genes expression and thus to pattern the wing along the A/P axis. Further more, I showed that Dpp signaling is also participating in the patterning along the D/V axis by interaction with the selector gene apterous and Notch/Wg signaling.

Morphogens are signaling molecules that play a key role in animal development. They spread from a restricted source into an adjacent target tissue forming a concentration gradient. The fate of cells in the target tissue is determined by the local concentration of such morphogens. Morphogen transport through the tissue has been studied in experiments which lead to the suggestion of several transport mechanisms. While diffusion in the extracellular space contributes to transport, recent experiments on the morphogen Decapentaplegic (Dpp) in the fruit fly Drosophila provide evidence for the importance of a cellular transport mechanism that was termed "planar transcytosis". In this mechanism, morphogens are transported through cells by repeated rounds of internalization and externalization. Starting from a microscopic theoretical description of these processes, we derive systems of nonlinear transport equations which describe the interplay of transcytosis and passive diffusion. We compare the results of numerical calculations based on this theoretical description of morphogen transport to recent experimental data on the morphogen Dpp in the Drosophila wing disk. Agreement with the experimental data is only achieved if the parameters entering the theoretical description are chosen such that transcytosis contributes strongly to transport. Analyzing the derived transport equations, we find that transcytosis leads to an increased robustness of the created gradients with respect to morphogen over-expression. Indications for this kind of robustness have been found in experiments. Furthermore, we theoretically investigate morphogen gradient formation in disordered systems. Here, an important question is how the position of concentration thresholds can be defined with high precision in the noisy environment present in typical developing tissues. Among other things, we find that the dimensionality of the system in which the gradient is formed plays an important role for the precision. Comparing gradients formed by transcytosis to those formed by extracellular diffusion, we find substantial differences that may result in a higher precision of gradients formed by transcytosis. Finally, we suggest several experiments to test the theoretical predictions of this work.

Decapentaplegic (Dpp), a member of the TGF-[Beta] superfamily, acts as a morphogen to direct cell differentiation, determine cell fate and promote cell survival and proliferation in Drosophila wing development. To investigate the role of Dpp in Drosophila wing development, three aspects of the patterning role of Dpp have been analyzed. First, I investigated the cellular responses to Dpp signaling by a loss of function strategy. The consequences of lacking Dpp signal transduction on cell morphology and tissue integrity were analyzed. Second, I investigated whether Dpp signaling is down-stream of Hh signaling to maintain the normal cell segregation at the A/P boundary by clonal analysis. Third, I investigated whether cross talk among the Hh, Dpp and Wg signaling pathways exists and what its relevance for wing patterning is. To investigate the role of Dpp in Drosophila wing development, the general strategies are to look at the phenotypes of loss-of-function and gain-of-function. Mutant clones lacking Dpp signal transduction by knock down Dpp receptor Thick veins (Tkv) do not survive in wing blade due to JNK dependent apoptosis. To get larger mutant clones for analysis, JNK pathway was inhibited by knock down bsk (encodes JNK) in mutant clones lacking Dpp signaling using FLP-FRT system. Clones double mutant for tkv and bsk did not undergo apoptosis, but recovered at very low frequencies compared to sibling clones. Here, I showed that the low recovery of tkv bsk double mutant clones are due to the extrusion of mutant cells. The extrusion of tkv bsk double mutant cells correlated with changes in the actin cytoskeleton and a dramatic loss of the apical microtubule web normally present in these cells. These results suggest that Dpp signaling is required for cell morphogenesis in Drosophila wing development. We propose that Dpp acts as a survival factor in the wing disc epithelium by orchestrating proper cytoskeletal organization and maintaining normal cell-cell contact. Drosophila wing is subdivided into anterior (A) and posterior (P) compartments. This developing into adjacent compartments is crucial for the patterning of Drosophila wing. Previous study has shown that Hedgehog (Hh) signaling is required in A cells to maintain the A/P boundary and is sufficient to specify A type cell sorting. A previous study has in addition implicated the signaling molecule Decapentaplegic (Dpp) in maintaining the A/P boundary. However, this study did not address whether and in which cells, A and/or P, Dpp signal transduction was required to maintain this boundary. Here, I have analyzed the role of components of the Dpp signal transduction pathway and the relation of Dpp and Hh signaling in maintaining the A/P boundary by clonal analysis. I showed that Dpp signaling mediated by the Dpp target gene, T-box protein Optomotor-blind (Omb), is required in A cells, but not in P cells, to maintain the normal position of the A/P boundary. During patterning formation, it is essential for cells to receive precise positional information to pattern the tissue. It has been proposed for a long time that different signaling pathways such as Hedgehog (Hh), Dpp and Wingless (Wg) signaling pathways provide positional information for tissue patterning in an integrated manner. Recently, evidence of interactions between Hh and Dpp as well as Wg and Hh signaling pathways has been reported in Drosophila wing. Here, I have identified additional interactions among Hh, Dpp and Notch/Wg signaling. We propose that the selector gene engrailed, Hh and Dpp signaling interact with each other to regulate target genes expression and thus to pattern the wing along the A/P axis. Further more, I showed that Dpp signaling is also participating in the patterning along the D/V axis by interaction with the selector gene apterous and Notch/Wg signaling.

Morphogens are signaling molecules that play a key role in animal development. They spread from a restricted source into an adjacent target tissue forming a concentration gradient. The fate of cells in the target tissue is determined by the local concentration of such morphogens. Morphogen transport through the tissue has been studied in experiments which lead to the suggestion of several transport mechanisms. While diffusion in the extracellular space contributes to transport, recent experiments on the morphogen Decapentaplegic (Dpp) in the fruit fly Drosophila provide evidence for the importance of a cellular transport mechanism that was termed "planar transcytosis". In this mechanism, morphogens are transported through cells by repeated rounds of internalization and externalization. Starting from a microscopic theoretical description of these processes, we derive systems of nonlinear transport equations which describe the interplay of transcytosis and passive diffusion. We compare the results of numerical calculations based on this theoretical description of morphogen transport to recent experimental data on the morphogen Dpp in the Drosophila wing disk. Agreement with the experimental data is only achieved if the parameters entering the theoretical description are chosen such that transcytosis contributes strongly to transport. Analyzing the derived transport equations, we find that transcytosis leads to an increased robustness of the created gradients with respect to morphogen over-expression. Indications for this kind of robustness have been found in experiments. Furthermore, we theoretically investigate morphogen gradient formation in disordered systems. Here, an important question is how the position of concentration thresholds can be defined with high precision in the noisy environment present in typical developing tissues. Among other things, we find that the dimensionality of the system in which the gradient is formed plays an important role for the precision. Comparing gradients formed by transcytosis to those formed by extracellular diffusion, we find substantial differences that may result in a higher precision of gradients formed by transcytosis. Finally, we suggest several experiments to test the theoretical predictions of this work.

During the development of multicellular organisms, organs grow to well-defined shapes and sizes. The proper size and patterning of tissues are ensured by signaling molecules as e.g. morphogens. Secreted from a restricted source, morphogens spread into the adjacent target tissue where they form a graded concentration profile. Upon binding of the morphogens to receptors on the cell surfaces, the morphogenetic signal is transduced inside the cell via the phosphorylation of transcription factors, which subsequently regulate the expression of different target genes. Thus, cell fates are determined by the local concentration of such morphogens. In this work, we investigate three key aspects of morphogenetic signaling processes in growing tissues. First, we study the mechanics of tissue growth via cell division and cell death. We examine the rearrangements of cells on large scales and times by developing a continuum theory which describes the growing tissue as an active complex fluid. In our description we include anisotropic stresses generated by oriented cell division, and we show that average cellular trajectories exhibit anisotropic scaling behaviors. Our description is used to study experimentally measured shape changes of the developing wing disk of the fruit fly Drosophila melanogaster. Second, we focus on the spreading of morphogens in growing tissues. We show that the flow field of cell movements due to oriented cell division and cell death causes a drift term in the morphogen transport equation, which captures the stretching and dilution of the concentration profile. Comparing our theoretical results to recent experiments in the Drosophila wing disk, we find that the transport of the morphogen Dpp is mainly intracellular. We moreover show that the decay length of the Dpp gradient increases during development as a result of changing degradation rate and diffusion coefficient, whereas the drift of molecules due to growth is negligible. Thus growth does not affect the decay length of the gradient, but the decay length of the gradient might affect the tissue growth rate as discussed in this work. Finally, we develop a microscopic theoretical description of the intracellular transduction machinery of morphogenetic signals within an individual cell. Our description captures the kinetics of the trafficking of proteins between different cellular compartments in response to receptor-bound signaling molecules. Analyzing experimental data at the Drosophila neuromuscular junction, we show that the morphogenetic signaling is modulated by synaptic signaling via neuronal action potentials.

No description available.

No description available.

During the development of multicellular organisms, organs grow to well-defined shapes and sizes. The proper size and patterning of tissues are ensured by signaling molecules as e.g. morphogens. Secreted from a restricted source, morphogens spread into the adjacent target tissue where they form a graded concentration profile. Upon binding of the morphogens to receptors on the cell surfaces, the morphogenetic signal is transduced inside the cell via the phosphorylation of transcription factors, which subsequently regulate the expression of different target genes. Thus, cell fates are determined by the local concentration of such morphogens. In this work, we investigate three key aspects of morphogenetic signaling processes in growing tissues. First, we study the mechanics of tissue growth via cell division and cell death. We examine the rearrangements of cells on large scales and times by developing a continuum theory which describes the growing tissue as an active complex fluid. In our description we include anisotropic stresses generated by oriented cell division, and we show that average cellular trajectories exhibit anisotropic scaling behaviors. Our description is used to study experimentally measured shape changes of the developing wing disk of the fruit fly Drosophila melanogaster. Second, we focus on the spreading of morphogens in growing tissues. We show that the flow field of cell movements due to oriented cell division and cell death causes a drift term in the morphogen transport equation, which captures the stretching and dilution of the concentration profile. Comparing our theoretical results to recent experiments in the Drosophila wing disk, we find that the transport of the morphogen Dpp is mainly intracellular. We moreover show that the decay length of the Dpp gradient increases during development as a result of changing degradation rate and diffusion coefficient, whereas the drift of molecules due to growth is negligible. Thus growth does not affect the decay length of the gradient, but the decay length of the gradient might affect the tissue growth rate as discussed in this work. Finally, we develop a microscopic theoretical description of the intracellular transduction machinery of morphogenetic signals within an individual cell. Our description captures the kinetics of the trafficking of proteins between different cellular compartments in response to receptor-bound signaling molecules. Analyzing experimental data at the Drosophila neuromuscular junction, we show that the morphogenetic signaling is modulated by synaptic signaling via neuronal action potentials.

No description available.