The role of N-cadherin in neural stem cells

The human brain develops from a pool of special cells called neural stem cells. These cells divide throughout life, generating the new nerve cells required for the growth of the brain during development and for its maintenance in adult life. At the same time, the stem cells are not depleted, and this is achieved by a process called 'asymmetrical division', where the two cells formed by a dividing stem cell (daughter cells) have different fates / one becomes a nerve cell while the other remains a stem cell so renewing the stem cell population. Clearly, then, the mechanisms that enable these asymmetrical divisions are absolutely fundamental to the correct development of the brain, and we know that abnormalities cause mental retardation syndromes in children. We also know that the difference in cell fates is achieved by two mechanisms; first, the precise location to one part of the cell of special molecules that control fate ('fate determinants') and second, control of exactly where the cell splits apart during division so that these fate determinants all end up in only one of the two daughter cells. We don't know, however, the details of these two mechanisms and how they so precisely locate fate determinants and cell splitting. Here, we will ask whether a group of molecules called cadherins, known to stick the neural stem cells together on the outside so keeping them in the correct place in the developing brain, also bind to the fate determinants and cell-splitting molecules on the inside and keep them in the right place. We will do this by using molecular biology to alter the location of cadherins on the neural stem cells, using slices of rat brain to model the human brain and enable these experiments do be done in tissue culture. We will then see what happens to the dividing neural stem cells and so prove or disprove our ideas as the function of these cadherins. As stem cells throughout the body use quite similar mechanisms to divide, our results will be valuable not only for a better understanding of the brain but also for other tissues where stem cells might provide a means of repair following disease.

The mammalian central nervous system (CNS) develops from neural stem cells (NSC) that initially undergo symmetric divisions, expanding the stem cell pool before initiating asymmetric divisions that generate a committed neuronal precursor cell as well as another NSC. Based largely on studies in invertebrates, the key factors enabling symmetrical and asymmetrical division in vertebrate NSC are known to be the localization of fate determinants to the apical region of the cell and precise regulation of the plane of cleavage. At present we have an incomplete understanding of how these are achieved and in particular how they are orientated appropriately for the 3D environment of the developing CNS. A number of different studies suggest the hypothesis that cadherins in adherens junctions (AJ) play a major role by anchoring the astral microtubules that define the position of the centrosome and/or binding fate determinants, in addition to their adhesive role in maintaining the apical pole of the NSC within the ventricular zone. This extended role of cadherins in the regulation of NSC has not previously been examined directly, and the aim of this project is therefore to test this hypothesis. To do this, we will use slice cultures of embryonic CNS to determine the effect on NSC of disrupting or mislocalizing N-cadherin. Using confocal microscopy, we will examine the consequent changes in both division axis and the location of fate determinants, as well as in the fate of the daughter cells. The results will identify novel mechanisms in the development of the mammalian CNS and may suggest strategies for the manipulation of neural stem cells so as to promote repair.