Neural stem cells (NSCs), like other stem cells, maintain their undifferentiated proliferative status while undergoing many rounds of cell division to produce a diverse array of neurons and glia. Proliferating NSCs are maintained through symmetric divisions, which expand the NSC pool, as well as self-renewing asymmetric cell divisions, which produce one daughter that becomes a NSC and another daughter with limited proliferative potential that will produce differentiated progeny. Maintaining the self-renewal of NSCs is critical for the proper formation and homeostasis of the nervous system. Premature termination of NSC self-renewal can lead to the reduction or loss of particular cell types, whereas continued or increased NSC self-renewal can lead to tumor formation. Hence, deciphering the mechanisms underlying the balance between NSC self-renewal and neuronal differentiation is important for understanding normal neurogenesis as well as the pathology of diseases that result from perturbations in NSC self-renewal. However, the mechanisms maintaining NSC self-renewal and how NSC proliferation and differentiation are balanced are just beginning to be understood.
larval NSCs (called neuroblasts, or NBs) are an excellent simple model system for studying the basic, conserved biology of NSCs. In the developing fly central brain, there are two types of NBs (type I and type II). These fly NBs, especially the newly identified type II NBs (also called p
egative [PAN] or Dorsomedial [DM] NBs), are analogous to mammalian radial glial cells, which function as NSCs in the developing mammalian brain 
. The type II NBs produce an intermediate neural progenitor (INP) that undergoes several self-renewing divisions to amplify the number of progeny produced by each NB () 
. Thus, fly INPs are comparable to the mammalian intermediate progenitor, particularly the self-renewing outer-subventricular zone radial glia-like (oRG) cells that amplify the number of progeny descended from radial glial cells as reported in human embryos and ferrets 
. In each round of self-renewing divisions, individual INPs produce a ganglion mother cell (GMC), which divides once to produce two neurons. The developing fly brain, however, is mostly populated by type I NBs that give rise directly to GMCs instead of self-renewing INPs (). Drosophila
larval NBs are derived from embryonic NBs. After several rounds of self-renewing divisions, most embryonic NBs become mitotically quiescent at late embryonic stages, except for the four mushroom body NBs and a basal anterior NB. These quiescent NBs re-enter the cell cycle during 1st
instar larval stages and undergo repeated self-renewing divisions until early pupal stages 
Loss of Dpn leads to a complete loss of type II NBs and a dramatic reduction of type I NBs at the late 3rd instar larval stage.
Studies of Drosophila
NBs have identified a number of genes that regulate NB self-renewal. The majority of these genes, such as aPKC
, Partner of inscuteable
), the NuMA-related Mushroom body defect
, and protein phosphatase 2A
), are involved in asymmetric cell division (reviewed in 
. These genes ensure the proper segregation to a single daughter cell of cell fate determinants, such as Prospero (Pros), Numb, and Brat. These cell fate determinants promote cell cycle exit of GMCs or maturation of INPs 
. Defects in asymmetric cell division or loss of cell fate determinants perturb the normal pattern of NB self-renewal, leading to an increased number of NBs or, conversely, differentiation. In the type II neuroblast lineage, the self-renewal potential of INPs is limited by the transcription factor Earmuff (Erm), which positively regulates pros
. However, the proteins that act within NBs to promote NB self-renewal remain largely unknown.
To understand how the self-renewal of NBs is regulated, we investigated the function of the Drosophila
bHLH transcriptional repressor Deadpan (Dpn), a member of the Hairy and Enhancer-of-Split (Hes) family that is expressed in all neural precursors 
. We reasoned that genes that are required for maintaining NBs might be specifically expressed in NBs, but not in their progeny. Dpn is specifically expressed in all the larval NBs as well as in the self-renewing INPs 
, making Dpn a good candidate for maintaining NB self-renewal. Our work demonstrates that Dpn is both necessary and sufficient for maintaining NB self-renewal. Furthermore, we show that Dpn and Notch function in separable pathways to suppress Ase expression in type II NBs and maintain the self-renewal of type II NBs.