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The Many Facets of bicoid Gradient Formation in Drosophila

Cai, Xiaoli LU (2017)
Abstract
In Drosophila, the Bicoid protein servers as paradigm as the first identified morphogen, whose concentration gradient provides the initial positional information in the anterior half of the embryo. There, it differentially activates segmentation genes, in particular the gap genes. Insofar, there are two mainly prevailing models to explain how the Bicoid protein concentration gradient forms: 1) the SDD (Synthesis, Diffusion and Degradation) model proposing that the bicoid mRNA is located at the anterior pole of the embryo at all times. The mRNA then serves as a source for translation of the Bicoid protein, followed by diffusion of the protein to the posterior, combined with uniform degradation. 2) the ARTS (Active RNA Transport and... (More)
In Drosophila, the Bicoid protein servers as paradigm as the first identified morphogen, whose concentration gradient provides the initial positional information in the anterior half of the embryo. There, it differentially activates segmentation genes, in particular the gap genes. Insofar, there are two mainly prevailing models to explain how the Bicoid protein concentration gradient forms: 1) the SDD (Synthesis, Diffusion and Degradation) model proposing that the bicoid mRNA is located at the anterior pole of the embryo at all times. The mRNA then serves as a source for translation of the Bicoid protein, followed by diffusion of the protein to the posterior, combined with uniform degradation. 2) the ARTS (Active RNA Transport and Synthesis) model proposing that the mRNA is transported at the cortex along microtubules to form the mRNA gradient, which serves as template for the production of Bicoid. Hence, there are clear differences between the SDD and the ARTS model.
To corroborate the ARTS model, we used several approaches: 1) we investigated on the transport mechanism of bicoid mRNA. To this end, we detected a cortical microtubular network that was present in the anterior half of the early Drosophila embryos, which was only active during metaphase and early anaphase. We found that αTubulin67C is crucial for establishing the cortical microtubular network and that cortical bicoid mRNA transport is compromised in αTubulin67C mutants. We defined a motor protein, nonclaret disjunctional (ncd) to be a critical motor for bicoid mRNA transport and demonstrated that ncd interacts genetically with αTubulin67C. This data suggested that ncd required αTubulin67C for cortical bicoid mRNA transport, also demonstrated by colocalization of αTubulin67C with Ncd. 2) we chose one special fly stock that expresses 3 times more Bicoid and treated the embryos with hypoxia to challenge the validity of the SDD model. Our data showed that under hypoxic conditions, the Bicoid protein did not move into the interior, but rather moved along the cortex, even during long exposures. 3) Combining hypoxia with drugs that disturb the formation of microtubules, we could observe interior movement of Bicoid, while the mRNA strictly remained at the tip. When actin was compromised, little cortical Bicoid movement was observed. This data suggested that Bicoid requires an intact cytoarchitecture for cortical movement. Finally, we revealed several factors that played distinct roles in bicoid mRNA gradient formation, including trans-Golgi proteins, the poly(A) polymerase Wispy, CyclinB and egg-activation genes.
Apart from studying the mechanism of bicoid gradient transport, we explored the expression patterns of bicoid-downstream genes in Bactrocera dorsalis, which is the oriental fruit fly with high relatedness to Drosophila, however lacking bicoid. When comparing the segmentation gene expression patterns between Drosophila and Bactrocera, bicoid downstream genes showed a strong shift of expression towards the posterior suggesting that the positioning of the segmental anlagen along the anterior-posterior axis changed during evolution.
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author
supervisor
opponent
  • docent Uv, Anne, Göteborgs University, Sweden
organization
publishing date
type
Thesis
publication status
published
keywords
Drosophila, bicoid, gradient formation, ARTS model, cortical microtubule, drug
pages
54 pages
publisher
Lund University, Faculty of Medicine
defense location
Segerfalksalen, BMC A10, Sölvegatan 17 i Lund
defense date
2017-12-05 13:00
ISBN
978-91-7619-560-4
language
English
LU publication?
yes
id
ed11767e-6617-4e23-ac4c-7e3812b79733
date added to LUP
2017-11-13 17:21:02
date last changed
2017-11-16 08:18:33
@phdthesis{ed11767e-6617-4e23-ac4c-7e3812b79733,
  abstract     = {In Drosophila, the Bicoid protein servers as paradigm as the first identified morphogen, whose concentration gradient provides the initial positional information in the anterior half of the embryo. There, it differentially activates segmentation genes, in particular the gap genes. Insofar, there are two mainly prevailing models to explain how the Bicoid protein concentration gradient forms: 1) the SDD (Synthesis, Diffusion and Degradation) model proposing that the bicoid mRNA is located at the anterior pole of the embryo at all times. The mRNA then serves as a source for translation of the Bicoid protein, followed by diffusion of the protein to the posterior, combined with uniform degradation. 2) the ARTS (Active RNA Transport and Synthesis) model proposing that the mRNA is transported at the cortex along microtubules to form the mRNA gradient, which serves as template for the production of Bicoid. Hence, there are clear differences between the SDD and the ARTS model.<br/>To corroborate the ARTS model, we used several approaches: 1) we investigated on the transport mechanism of bicoid mRNA. To this end, we detected a cortical microtubular network that was present in the anterior half of the early Drosophila embryos, which was only active during metaphase and early anaphase. We found that αTubulin67C is crucial for establishing the cortical microtubular network and that cortical bicoid mRNA transport is compromised in αTubulin67C mutants. We defined a motor protein, nonclaret disjunctional (ncd) to be a critical motor for bicoid mRNA transport and demonstrated that ncd interacts genetically with αTubulin67C. This data suggested that ncd required αTubulin67C for cortical bicoid mRNA transport, also demonstrated by colocalization of αTubulin67C with Ncd. 2) we chose one special fly stock that expresses 3 times more Bicoid and treated the embryos with hypoxia to challenge the validity of the SDD model. Our data showed that under hypoxic conditions, the Bicoid protein did not move into the interior, but rather moved along the cortex, even during long exposures. 3) Combining hypoxia with drugs that disturb the formation of microtubules, we could observe interior movement of Bicoid, while the mRNA strictly remained at the tip. When actin was compromised, little cortical Bicoid movement was observed. This data suggested that Bicoid requires an intact cytoarchitecture for cortical movement. Finally, we revealed several factors that played distinct roles in bicoid mRNA gradient formation, including trans-Golgi proteins, the poly(A) polymerase Wispy, CyclinB and egg-activation genes. <br/>Apart from studying the mechanism of bicoid gradient transport, we explored the expression patterns of bicoid-downstream genes in Bactrocera dorsalis, which is the oriental fruit fly with high relatedness to Drosophila, however lacking bicoid. When comparing the segmentation gene expression patterns between Drosophila and Bactrocera, bicoid downstream genes showed a strong shift of expression towards the posterior suggesting that the positioning of the segmental anlagen along the anterior-posterior axis changed during evolution.<br/>},
  author       = {Cai, Xiaoli},
  isbn         = {978-91-7619-560-4},
  keyword      = {Drosophila,bicoid,gradient formation,ARTS model,cortical microtubule,drug},
  language     = {eng},
  pages        = {54},
  publisher    = {Lund University, Faculty of Medicine},
  school       = {Lund University},
  title        = {The Many Facets of bicoid Gradient Formation in Drosophila},
  year         = {2017},
}