Characterisation and culture of neuromesodermal stem cells

The backbone and spinal cord are made during embryo development from a group of cells called axial progenitors that are similar to stem cells. Stem cells have two essential properties: they can divide to give more copies of themselves, and they can produce specialised cell types in a process called differentiation. Axial progenitors can be made to behave like stem cells under some experimental conditions, but normally they are only present during the time that the backbone and spinal cord are being made, and then they die. We aim to understand what molecules make them divide, differentiate and die, and this knowledge will help us to design conditions to keep them alive as stem cells in culture. Unlike Embryonic Stem cells, these cells do not form malignant tumours when transplanted into adult mice, and so axial progenitors may therefore be a useful therapeutic cell type for degenerative diseases and injuries involving the spinal cord, bone and muscle.

Embryonic stem (ES) cells are powerful tools for generating therapeutically-relevant differentiated cell types. However, two major barriers prevent their large-scale use in generating specific differentiated cell types in vitro: 1) the plethora of differentiation choices available to pluripotent ES cells, leading to mixed populations of differentiated cells, and 2) the persistence of pluripotent cells that are capable of malignant growth in differentiated populations. Foetal tissue stem cells offer a promising non-malignant alternative with more limited differentiation options. However clinically useful examples of stable cell lines that generate the spinal cord, muscle and skeleton have not been reported. During mammalian gestation, these tissues are laid down sequentially along the head-tail (rostrocaudal) axis by a population of axial progenitors in the primitive streak and tail bud. These cells, present from the beginning of axis elongation, are eliminated when elongation ceases. Through a combination of clonal and population lineage studies, analysis of in vivo potency, and gene expression analysis in the mouse, we have identified a novel stem cell type, the neuromesodermal stem cell or long-term axial progenitor , that generates the postcranial spinal cord and musculoskeletal system. We have also developed a novel in vitro assay that tests the maintenance of axial progenitors, and have made progress towards in vitro culture of these stem cells. In this proposal, we aim to use this knowledge and expertise to: 1) undertake detailed characterisation of the progenitors throughout axial elongation, including expression profiling, 2) identify the factors necessary for the continued growth of these cells, and 3) combine information from aims 1 and 2, to define conditions that allow the long-term growth in vitro of mouse neuromesodermal stem cells, either from foetal tissue or from differentiating ES cells. This information is crucial as a basis to transfer this work to human ES cells.