Insights into neural stem cell biology from flies

Abstract

Drosophila neuroblasts are similar to mammalian neural stem cells in their ability to self-renew and to produce many different types of neurons and glial cells. In the past two decades, great advances have been made in understanding the molecular mechanisms underlying embryonic neuroblast formation, the establishment of cell polarity and the temporal regulation of cell fate. It is now a challenge to connect, at the molecular level, the different cell biological events underlying the transition from neural stem cell maintenance to differentiation. Progress has also been made in understanding the later stages of development, when neuroblasts become mitotically inactive, or quiescent, and are then reactivated postembryonically to generate the neurons that make up the adult nervous system. The ability to manipulate the steps leading from quiescence to proliferation and from proliferation to differentiation will have a major impact on the treatment of neurological injury and neurodegenerative disease.

Figure 1

Neuroectoderm specification and neuroblast formation. (a) A schematic cross-section through a Drosophila embryo. The genes ventral nervous system defective (vnd), intermediate neuroblast defective (ind) and muscle segment homeobox (msh) pattern the neuroectoderm in three columnar domains in response to antagonistic morphogenetic gradients of Dpp and Sog. (b) Single cells are selected to acquire a neuroblast fate from a proneural equivalence group of five to six cells. This is achieved by the process of lateral inhibition and is based on a molecular regulatory loop between the adjacent cells. The selected neuroblast enlarges and delaminates basally into the embryo. The remaining cells of each proneural cluster adopt an alternative epidermal fate. After delamination, each neuroblast begins to divide asymmetrically in a stem cell-like manner along the apico-basal axis. (c) A simplified scheme of lateral inhibition involving Notch, Delta and the proneural genes. Activation of the Notch signalling cascade by the Delta ligand leads to repression of proneural gene expression in the presumptive non-neural cell. Since Delta expression is regulated by proneural transcription factors, downregulation of proneural genes leads to a reduction in Notch activation in the neighbouring cell. As a result, proneural gene activity is maintained in the presumptive neuroblast and repressed in its neighbours.

Figure 2

Neural stem cell divisions. Neural stem cells undergo symmetric proliferative or asymmetric differentiative self-renewing divisions. Neural stem cells may also undergo symmetric differentiative divisions, thereby depleting the stem cell pool and producing two developmentally restricted precursors or post-mitotic progeny. Given are some examples for each division type in Drosophila. Nbs, neuroblasts.

Figure 3

Asymmetric neuroblast division. (a) The subcellular localization of several polarity proteins, cell-fate determinants and their adaptor proteins is indicated in different colours (see the figure). In the neuroectodermal cells, Baz, DmPar6 and DaPKC localize apically (yellow). As the neuroblast delaminates from the neuroectoderm, Insc is expressed and recruits Pins and Gαi to the apical cortex. The adaptor protein Mira becomes localized to the basal cortex, where it anchors the GMC determinants Pros and Brat. After segregating to the GMC, Mira is degraded and releases Pros, which then enters the GMC nucleus (adapted from Wodarz & Huttner 2003). (b) The main functions proposed for the different apical pathways. The Insc/Par pathway is crucial for the localization of basal cell determinants, whereas the Insc/Pins/Gαi pathway is necessary for mitotic spindle orientation. Loco together with the heterotrimeric G proteins are mainly involved in spindle asymmetry (adapted from Bellaiche & Gotta 2005). (c) DaPKC promotes self-renewing neuroblast divisions. In wild-type neuroblasts, Pins is necessary to localize DaPKC to the apical cortex. Basally active Lgl functions to inhibit basal localization of DaPKC and, thus, restrict its function to the apical side for neuroblast self-renewal. In pins lgl double mutants, DaPKC is delocalized and inherited by both daughter cells, and thus neuroblasts undergo continuous symmetric self-renewing divisions (adapted from Lee et al. 2006a).

Figure 4

Two division patterns in neuroglial development. (a) One type of neuroglioblast (e.g. NB6-4t) first divides to give a glioblast and a neuroblast, thereby generating precursors with restricted developmental potential that give rise to either glial cells or neurons. (b) Another type of neuroglioblast (e.g. NB1-1a) generates intermediate precursors that have the potential to generate neurons as well as glia via asymmetric cell division. Notch is used to specify the glial part of the lineage (adapted from Udolph et al. 2001).

Figure 5

Temporal neuroblast progression. (a) Neuroblasts sequentially express the transcription factors Hb, Kr, Pdm and Cas. GMCs and their neuronal/glial progeny express the transcription factor present at the time of the GMC's birth. Cytokinesis and Seven-up (Svp) are required for the Hb to Kr transition. The competence of neuroblasts to generate early-born cell fates in response to Hb expression is progressively restricted (adapted from Pearson & Doe 2003). (b) A schematic showing the effects of Hb and Kr loss (Hb⁻ and Kr⁻) and misexpression on progeny cells. The GMCs are labelled with their birth order, and transcription factor expression is colour-coded as in (a). Dashed white circles represent abnormal GMC development resulting from cell death, cell-fate skipping or transformation (adapted from Isshiki et al. 2001).

Figure 6

Postembryonic neuroblast development. (a) A schematic of a third instar larval Drosophila brain. Left, dorsal view (D); right, ventral view (V). Anterior is to the top of the page. (b) Lateral view top, with anterior to the left and dorsal to the top of the page. Bottom, cross-section of the thoracic VNC. The cell bodies of the primary neurons produced during embryogenesis surround the neuropil formed by their axons. (c) A schematic of neuroblast reactivation. Enlargement is triggered by the fat body-derived mitogen (FBDM). Anachronism (Ana)-mediated repression keeps the neuroblast in G1 until factors working through terribly reduced optic lobes (Trol) drive the neuroblast into S phase. The role of ecdysone (20E) in this process has yet to be resolved.