Mapping the Embryological Origin of the Radius and Ulna

Supervisors: Megan Davey, Neil Vargesson

Project description:

Correct posterior-anterior digit patterning (little finger to thumb) in the developing limb bud has been a central paradigm for understanding pattering during embryogenesis.  The morphogen which patterns the digits is Sonic hedgehog (SHH), which acts in a time and concentration dependent manner.  The autoregulatory control of SHH expression has been the focus of The Davey lab  (“Direct functional consequences of ZRS enhancer mutation combine with secondary long range SHH signalling effects to cause preaxial polydactyly” Johnson et al. Dev Bio. 2014, “Evolutionary digit loss in ratites is caused by a loss of SHH” Johnson et al. in preparation). As well as the digits, the Davey lab have also shown that the radius can be lost through manipulation of the Hedgehog pathway (“Insights into the Embryogenesis of Pre-Axial and Forearm Duplication Conditions Based on Experimental Manipulations of Sonic Hedgehog Signalling” Lam et al. in submission). In humans, Radial dysplasia is a rare and mostly unexplained congenital malformation of the arm in which the radius is reduced or lost entirely. Although experimental evidence from the Davey lab suggests that the patterning of the radius is Hedgehog dependent, limb manipulation studies in chicken and mice have also shown that additional Hedgehog signalling can lead to either a duplicated ulna in the place of the radius (ulna dimelia) or to a loss of the radius (radial dysplasia). There is no current model for the role of SHH in ulna/radius specification and no understanding of the dichotomous outcome of Hedgehog signalling manipulation (ulna dimelia Vs. radial dysplasia). Further complicating our understanding of the causes of Radial dysplasia and the influence SHH has on the radius/ulna during development, is that we currently have a very poor idea where these bones originate from. Thus it is of central importance that the embryological origin of the radius and ulna are ‘fate mapped’.

The Roslin Institute has recently developed an as yet unpublished novel transgenic line, the mini-Cytbow (CPX) line which integrates a multi-fluorescent protein encoding-cassette (Brainbow) and which is TAT-Cre inducible. This transgenic line represent a new method to permanently labelling of cells during embryonic development and thus to map distal regions of the limb bud. This project will fate map the origin of the radius and ulna. Furthermore this project will then investigate the role of SHH in ulna/radius patterning with the aim to particularly understand the cause of radial dysplasia. We will use models of radial dysplasia, combined with new transgenic technology to assess and generate understanding of the anatomy of the limb. Finally we will combine fate mapping of the normal and manipulated limb with single cell gene expression analysis to explore the transcriptomic signature of the radius and ulna. With collaborators we will mathematically model the role of SHH in zeugopod patterning.

Objective1-Map the origin of the zeugopod

Objective 2-Determine the role of Hedgehog signal in zeugopod patterning

Objective3- Uncover the transcriptional signature of the presumptive radius/ulna tissue

Objective4- Mathematically model zeugopod patterning