In higher eukaryotes, certain tissues consist of a special type of cells known as stem cells. Stem cells divide to self-renew and at the same time to generate a progeny that is committed to a differentiation pathway. Although of much importance, very little is known of how a stem cell acquires its identity or how it functions. In Drosophila, the primary neuronal precursor cells (Neuroblasts, NB) function as stem cells and divide by self-renewing asymmetric mitosis. During neurogenesis, a NB self-renews and also produces a chain of ganglion mother cells (GMCs). A GMC is bipotential, it does not self-renew but divides asymmetrically to generate two distinct post-mitotic neurons. Thus, from -30 NBs in a given hemisegment, -320 distinct neurons are generated. This indicates that the ability of NBs to function as stem cells and the ability of NBs and GMCs to divide by asymmetric mitosis is crucial in generating a large number of neurons from a few precursor cells. Our long-term goal aims to explore the genetic regulation of self-renewing stem cell type of asymmetric divisions using the Drosophila CNS as our model system. In order to study the problem of self-renewing and terminal asymmetric divisions, we have selected several different CNS lineages: MP2, NB7-3, GMC-1->RP2/sib, and GMC-1->aCC/pCC lineages. During the past several years, we have identified through genetic screens mutations that show self-renewing and terminal asymmetric division in one or more of these lineages. In this grant we propose to further study these mutations. Thus, our specific aims include: (1) To investigate the role of Midline in inhibiting the self- renewing asymmetric division potential of neural precursor cells, and, 2) To determine how Neuralized-like inhibits the self-renewing asymmetric division potential of precursor cells, and 3) To determine the role of Polycomb, a chromatin re-modeling protein, in the asymmetric division of GMCs. These studies will help understand pathways that govern the self-renewing and terminal asymmetric division of precursor cells in multi-cellular organisms.
