Individuals of the same species can differ widely in size, but their organs have reproducible proportions and patterns of cell types. We are interested in understanding how this reproducibility of tissue patterning is achieved in the developing spinal cord. Our focus is on secreted signalling molecules, called morphogens. Morphogens control what type of neuron a neural progenitor cell will become. At the same time they control the growth of the tissue by influencing the decisions of cells to divide or exit the cell cycle. Our goal is to better understand how morphogen signalling levels are controlled and how cells interpret morphogen signalling to determine their cell fate and cell cycle progression.
We use mainly the mouse and chick model systems, as well as ES cell differentiation. Our aim is to obtain quantitative and dynamic data using live imaging, and we work in close collaboration with biophysicists.
Integration of opposing morphogen gradients
In the spinal cord, 14 transcriptionally distinct neural progenitor types are arrayed along the dorsoventral axis. This pattern is controlled by the morphogen gradients of BMP and Shh, which are secreted from the opposite poles of the tissue. Cells measure the relative levels of Shh and BMP signalling, but it is unclear how they do it. Our goal is to establish how the Shh and BMP signal transduction pathways cross-interact and how cell integrate the information from the two pathways. With the help of mathematical modelling, we will further examine the implications of the cross-talk mechanism for pattern reproducibility. We will use both mouse and chick embryos and develop reagents where we can monitor the levels of signalling with single cell resolution.
The image on the left shows a section through a mouse neural tube, expressing a Shh signalling reporter (green) and stained for pSmad1/5/8, a readout of BMP signalling (red). On the right is a set of signalling profiles that have been quantified from such images.
Morphogen control of tissue growth
In the neural tube, the morphogens Shh and Wnt promote proliferation. However, the rate of proliferation is not proportional to the level of morphogen signalling, instead it is uniform across the tissue. Our goal is to find out how cells interpret Shh and Wnt signalling to tune their proliferation rate. We will use and develop an vitro system based on chick neural tube explants, where we can monitor with live imaging the response of individual cells to defined levels of morphogens. We will further delve into the molecular mechanism behind the dynamics that we observe.
Snapshots from a time-lapse of chick neural tube explant, which is exposed to 4nM Shh at time 0h. The explant expresses a Shh reporter (GBS-H2B-Venus) and the fluorescence intensity shown is normalized to a control plasmid. Even though the concentration of Shh in the medium is constant throughout the experiment, the downstream signaling activity decreases over time – a phenomenon known as temporal adaptation.
Morphogen gradient formation
The formation of morphogen concentration profiles with specific shape and temporal dynamics is key for tissue patterning and size control. Cells are not passive substrates over which morphogen gradients form – they actively produce, degrade and transport morphogen, while at the same time they divide and dilute the morphogen concentration. We will study how these different properties contribute to the formation of the Shh morphogen gradient in the neural tube. Shh is produced and secreted by the notochord and floor plate, which are located ventrally, and makes a ventral-to-dorsal gradient of concentration. The shape of the gradient changes dynamically over time. One particular question that we will address is what is the relationship between the growing size of the Shh source (notochord and floor plate) during development and the shape of the gradient. We will use mouse imaging and genetics, ES cell differentiation and mathematical modeling in this project.
Figures from Cohen et al, Nature Comm. (2015): 6, 6709. In this study, we showed how the Shh gradient shape changes during neural tube development.