T cells missing Rac GTPases

James Briscoe group project:

Dynamics of Shh signalling and gene expression

In many developing tissues, gradients of extracellular signalling molecules – often termed morphogens – act as patterning cues by dividing the tissue into a series of molecularly distinct territories. This is the case in the ventral neural tube where a gradient of Shh signalling divides neural progenitors into a series of domains arrayed along the dorsal-ventral axis. Most models of morphogen interpretation are based on the idea that thresholds of morphogen concentration determine cell identity. In this view, increasing concentrations of morphogen generate increasing levels of intracellular signalling with the result that distinct target genes are activated at different levels of signalling. However, we found that the duration, as well as the level, of Shh signalling is important for morphogen interpretation in the neural tube. Using a mechanism that we termed ‘temporal adaptation’, cells are progressively desensitized to ongoing Shh signalling. The result is that different extracellular concentrations of Shh sustain intracellular signalling for different periods of time, such that the duration of signalling is proportional to Shh concentration. Consistent with this model, we found that the domains of ventral progenitors are established sequentially, with progressively more ventral identities requiring correspondingly higher levels and longer periods of Shh signalling. Moreover, cells remain sensitive to changes in Shh signalling for an extended time, reverting to antecedent identities if signalling levels fall below a threshold. Thus, the duration of signalling is important not only for the assignment but also for the refinement and maintenance of positional identity.

Level and duration of signalling contribute to ventral neural tube patterning

Level and duration of signalling contribute to ventral neural tube patterning

Analysing the gene regulatory logic of gradient interpretation

To understand how the level and duration of Shh signalling control the timing and position of target gene activation we are combining in vivo and in vitro experiments in mouse and chick with mathematical modeling. Our focus is on the gene regulatory circuit controlled by Shh signalling and understanding how cross-regulatory interactions between the transcription factors in this circuit provide a regulatory logic for morphogen interpretation. Together with our collaborators from the Mathematics Department, UCL London, we aim to refine and extend a dynamical systems model of the transcriptional circuit and experimentally test predictions of the model.

Establishing a dynamic molecular map of spinal cord development

The importance of the dynamics of signalling and gene expression to neural tube development has led us to begin generating a spatially and temporally quantitative molecular map of spinal cord development. To do this, we are developing, in collaboration with image analysis specialists at NIMR a pipeline to automate data collection and analysis of spinal cord growth and gene expression data. Using this pipeline we are collecting gene expression time series from chick, mouse and zebrafish. These data serve as the foundation for (i) generating mechanistic hypothesis for the coordination of growth with gene expression; (ii) understanding positional precision of patterning; and (iii) providing insight into the evolutionary scaling between species of pattern with neural tube size.

An atlas of neural tube patterning

An atlas of neural tube patterning

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Visualising signalling and transcriptional activity

Key to understanding embryonic development is an insight into how extrinsic signalling is interpreted by cells to control gene expression and how this produces the necessary precision despite the noise inherent to signalling and gene expression. In order to link patterning to signalling we are quantifying the distribution and signalling activity of Shh in vivo. To measure Shh ligand we are using established antibody techniques. For Shh signalling activity, we have developed fluorescent reporters that respond to Shh and allow the spatial distribution of Shh signalling activity to be quantified in mouse, chick and zebrafish. These data offer a single cell resolution view of the signalling dynamics in the spinal cord and provide measurements of cell-to-cell and embryo-to-embryo variability. Based on these data, we are developing testable models of the conversion of the Shh ligand gradient into a profile of signalling activity in vivo and the influence of noise on this process.

Quantitation of graded Shh and intracellular signaling

Quantitation of graded Shh and intracellular signaling

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Selected publications

Briscoe group

Recent publications

Research projects

Briscoe biography

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