Final Report Summary - SARA ("Endosomal trafficking during morphogenetic signaling and asymmetric cell division")
How a growing, developing organ knows when to stop growing when it is completed and reached its final size and shape is a key, unsolved question in biology. In the Sara advanced ERC project, we have established that during the growth of imaginal discs in Drosophila, growth is controlled by a mechanism in which cells compute neither the absolute concentration of the Dpp morphogen or the signaling activity elicited by this ligand, but instead by the time derivatives of signaling.
Cells compute the speed at which the signaling levels increase and their growth and proliferation rates are therefore proportional to the rate of signaling increase. This was shown by means of a series of novel assays. First, this correlation (i.e. the growth rate and the rate of signaling increase are proportional) was established quantitatively in wildtype. Second, it was confirmed in a number of mutant conditions in which the final size of the imaginal disc, the signaling dynamics or the scaling properties of the gradient (see below) were manipulated: the correlation holds. Third, beyond correlation, the causal relationship between the time derivative of signaling and the rate of growth was established by exogenously manipulating the time derivative of Dpp signaling. Finally, the model was tested in a completely different scenario, growth in the eye disc. In the eye disc, the source of transcription of Dpp is not static, but swipes from anterior to posterior through the disc. In this situation, cells perceived an increase of signaling over time as the source moves towards them. The principle of growth by time derivatives also hold in this unusual scenario with quantitative detail, serving as an orthogonal proof of the concept.
The Sara project also addressed the mechanism of directed signalling during other mode of proliferation: asymmetric cell division, including the asymmetric division of stem cells. We have unravelled the role of the Sara endosomes during asymmetric cell division in the sensory organ precursors of in Drosophila. We have established three key issues: i) how are the endosomes biased to one of the daughter cells? ii) what is the machinery controlling the asymmetric motility of the Sara endosomes? and iii) is the asymmetric dispatch of Sara endosomes during asymmetric division a wide spread mechanism for asymmetric fate decisions?
Is the asymmetric segregation of Sara endosomes in asymmetric division a wide spread mechanism? We showed that the mechanism is used during asymmetric division of adult intestinal stem cells in Drosophila. We imaged life the asymmetric division of intestinal stem cells, showed the asymmetric segregation of Sara endosomes in this system and established the key role of Sara endosomes for the self-renewal of the stem cells. Furthermore, in Zebrafish, we showed that neural precursors of the spinal cord also segregate Sara endosomes asymmetrically. We generated Zebrafish mutants for Sara and showed that Sara is essential for asymmetric cell fate assignation in the spinal cord of a vertebrate.
Cells compute the speed at which the signaling levels increase and their growth and proliferation rates are therefore proportional to the rate of signaling increase. This was shown by means of a series of novel assays. First, this correlation (i.e. the growth rate and the rate of signaling increase are proportional) was established quantitatively in wildtype. Second, it was confirmed in a number of mutant conditions in which the final size of the imaginal disc, the signaling dynamics or the scaling properties of the gradient (see below) were manipulated: the correlation holds. Third, beyond correlation, the causal relationship between the time derivative of signaling and the rate of growth was established by exogenously manipulating the time derivative of Dpp signaling. Finally, the model was tested in a completely different scenario, growth in the eye disc. In the eye disc, the source of transcription of Dpp is not static, but swipes from anterior to posterior through the disc. In this situation, cells perceived an increase of signaling over time as the source moves towards them. The principle of growth by time derivatives also hold in this unusual scenario with quantitative detail, serving as an orthogonal proof of the concept.
The Sara project also addressed the mechanism of directed signalling during other mode of proliferation: asymmetric cell division, including the asymmetric division of stem cells. We have unravelled the role of the Sara endosomes during asymmetric cell division in the sensory organ precursors of in Drosophila. We have established three key issues: i) how are the endosomes biased to one of the daughter cells? ii) what is the machinery controlling the asymmetric motility of the Sara endosomes? and iii) is the asymmetric dispatch of Sara endosomes during asymmetric division a wide spread mechanism for asymmetric fate decisions?
Is the asymmetric segregation of Sara endosomes in asymmetric division a wide spread mechanism? We showed that the mechanism is used during asymmetric division of adult intestinal stem cells in Drosophila. We imaged life the asymmetric division of intestinal stem cells, showed the asymmetric segregation of Sara endosomes in this system and established the key role of Sara endosomes for the self-renewal of the stem cells. Furthermore, in Zebrafish, we showed that neural precursors of the spinal cord also segregate Sara endosomes asymmetrically. We generated Zebrafish mutants for Sara and showed that Sara is essential for asymmetric cell fate assignation in the spinal cord of a vertebrate.