In vivo analysis of the proliferative properties and morphological dynamics of radial glial cells in the Xenopus brain

Our bodies are made up of billions of cells, many of which are highly specialised in order to carry out different functions. During development all of these cells must be born and it is clear that some cells, referred to as 'stem' or 'progenitor' cells, can give birth to different types of cells by successively dividing. Understanding which cells function as progenitors, how they are regulated and what types of cells they give birth to, is fundamental to our understanding of how the body develops. Furthermore, understanding these events will reveal which processes become abnormal in developmental diseases and, potentially, will enable us to harness these cells therapeutically. This research proposal addresses these questions in the context of the developing nervous system. In our brains there are two major classes of cells, the electrically excitable neurons and the non-excitable glial cells. Originally it was believed that these two classes diverged very early during development and so represented different cell families or 'lineages'. However, recent work has indicated that an intriguing subset of glial cells in the developing nervous system, called 'radial glia', exhibit features of progenitor cells and may give birth to the majority of neurons in parts of our brains. In this series of studies we plan to examine the behaviour of radial glial cells in the context of an intact, developing nervous system. Using non-invasive, high-resolution imaging methods we will watch the behaviour of live radial glial cells and how they communicate with other cells in the immature brain. We will explore whether radial glia function as progenitor cells and will trace the identity of the types of cells that they give birth to. Finally, we will examine whether ongoing communication between radial glia and their neighbouring cells are important for the way brain cells become specialised for their different functions. We anticipate that these experiments will contribute novel and important data to the field of developmental biology by characterising the events surrounding the birth and maturation of cells in the living brain. The work aims to improve our understanding and treatment of neurodevelopmental disorders in humans, and it will benefit scientists that are investigating ways of growing nerve cells in order to repair damaged brains.

One of the most significant recent advances in the field of CNS development has been the appreciation that radial glial cells are neurogenic progenitors in the brain. Radial glia represent the major glial cell type during early CNS development and there is an emerging realisation that cell-cell interactions involving radial glia may be critical for the regulation of their proliferative behaviour, their own differentiation and the maturation of other cell types in the CNS. Here we propose to build upon our previous studies in which we have developed methods for labeling and observing, in real time, the morphological development and cell-cell signaling properties of radial glial cells in an intact vertebrate nervous system. The Xenopus Laevis tadpole offers excellent and non-invasive optical access to the brain, making in vivo time-lapse imaging feasible over the timescale of minutes to weeks. We aim to use this model system in order to examine a series of fundamental questions about radial glial cell development, proliferation and cell-cell signaling. First, we will investigate the genetic, morphological and functional identity of radial glial cells in Xenopus and compare these to the radial glial phenotype in other species. Second, we will use birth-dating and in vivo retroviral cell labeling techniques to test whether radial glial cells are major neural progenitors in Xenopus and to reveal the nature of their clonal progeny. Third, we will examine whether extracellular signaling molecules that are known to be released by differentiated neurons and glia (including glutamate, GABA and ATP) trigger intracellular Ca2+ events in radial glial cells and affect their proliferative behaviour. Finally, we will investigate whether the maturation of radial glia and newly differentiated neurons is coordinated and whether cell-cell signaling involving radial glial cells regulates the morphological dynamics and functional maturation of these cells.