Modulation of neural progenitor proliferation and differentiation through external contacts

'It's the ecology stupid' begins a recent News Feature in Nature magazine (Powell (2005). Nature 435: 268-70). But this is about stem cells and their environment, not rainforests and wellies, and reflects the growing interest in the concept that stem cells occupy 'niches' in our tissues. The idea is similar to ecologists thinking of plants and animals living in niches in the environment that surrounds them. Most importantly, it presumes that the way stem cells behave depends on information and support they receive from their niche. Understanding how this information and support is imparted is critically important to current attempts to manipulate stem cells to produce specific cell types that could be used to treat, particularly, degenerative diseases such as Alzheimer's or Parkinson's. Our work has identified molecules expressed on the surfaces of nerve cells as they develop which seem to be able to trigger nerve cell precursors - which are not the stem cells themselves but direct descendents of those cells - to become mature nerve cells. In this project we aim to find out what signals are sent to the cells when these events are triggered, and we also aim to find out whether these molecules, or their close relatives, can also trigger stem cells to begin the process of making nerve cells or their precursors.

Much has recently been discovered about the soluble factors required to maintain neural stem cells in vitro. Also clear from this research is the importance of ordered cell contacts, both in stem cell maintenance and in the control of the differentiation of the stem and their progenitor descendents. However, much less is known about the molecular nature of the contacts involved and their precise effect on these processes. Moreover, our current ability to control these contacts in vitro is somewhat primitive and variable. This project builds on our recent work demonstrating that L1-like neural cell adhesion molecules (L1nCAMs) can control cerebellar granule neuron progenitor (GNP) proliferation in vivo and in vitro. These molecules are expressed in a highly ordered sequence during GNP differentiation. Here our first aim is to dissect further the role of F3/contactin in this process, focussing specifically on which receptors and signalling pathways are used and how they relate to the sonic hedgehog signalling which drives proliferation in this system. However, our central hypothesis is that L1nCAMs will be used to control each stage of stem cell and progenitor differentiation and therefore further aims are to determine which other members of the family are expressed on these cells in vivo and in vitro, and to determine their effect on these cells in GNP and neurosphere assays in vitro. Of particular interest is whether the spatial arrangement of L1nCAMs makes a difference to the quality and quantity of the responses elicited; in vivo, neural progenitors develop in ordered arrays, often with cells at different stages of differentiation stacked on top of one another. In such arrays cells may have contacts with multiple cell surfaces, each of which may present a different combination of surface molecules. A particularly good example is that in development, radial glia, now thought to be the neural stem cells, because of their extension from the ventricle to the overlying pial surface of the neuroepithelium, are in simultaneous contact with their immediate descendents (proliferating progenitors) in the germinal zone, but also with post-mitotic differentiated cells that have migrated superficially. Since the complement of L1nCAMs on immature vs mature neurons is quite different, these cells may be in contact with completely different sets of these molecules in different parts of their cell surface. To this explore this, the project includes a collaboration with a chemist who specialises in the production of arrays of biomolecules at cellular and subcellular (nanometer) scales. A major goal of the project is to determine whether localised contact with different L1nCAMs can elicit specific differentiation or proliferation responses from specific neural progenitors. Since such cells also differentially adhere to these molecules, we also aim to determine whether ordered substrates can be used to organise these different progenitors into ordered arrays of cells in vitro that mimic arrays seen in vivo. The ultimate aim is that this may allow us to better control the environment necessary to produce specific cells types in vitro.