A requirement forNotchin the genesis of a subset of glial cells in theDrosophilaembryonic central nervous system which arise through asymmetric divisions

Abstract

In the Drosophila central nervous system (CNS) glial cells are known to be generated from glioblasts, which produce exclusively glia or neuroglioblasts that bifurcate to produce both neuronal and glial sublineages. We show that the genesis of a subset of glial cells, the subperineurial glia (SPGs), involves a new mechanism and requires Notch. We demonstrate that the SPGs share direct sibling relationships with neurones and are the products of asymmetric divisions. This mechanism of specifying glial cell fates within the CNS is novel and provides further insight into regulatory interactions leading to glial cell fate determination. Furthermore, we show that Notch signalling positively regulates glial cells missing (gcm) expression in the context of SPG development.

Glial cell progenitors in the Drosophila embryo

“…In case of the abdominal NB 5-6, an even number of neurons and glial cells suggests that from each GMC an neuron-glia pair is made. A similar division mode was shown for NB 2-2T and NB 1-1A, even though these lineages comprise more neurons than glial cells and thus also show a pure neurogenic phase (Udolph et al, 2001). Initiation of gcm expression in glial cells of these two NB lineages was shown to depend on the asymmetric distribution of the Notch antagonist Numb during GMC division.…”

“…The only common feature for all NBs that give rise to glial cells in Drosophila is that the master regulatory gene gcm has to be expressed in order to initiate the glial cell fate. Determinants such as activated Notch, Prospero or Huckebein have been described to influence the expression of gcm (De Iaco et al, 2006;Freeman and Doe, 2001;Griffiths et al, 2007;Udolph et al, 2001). But these do not represent common regulatory pathways for glial development but are rather restricted to individual glial cells or lineages.…”

Morphological diversity and development of glia in Drosophila

“…Glial cells of each thoracic/ abdominal segment are derived from 10 pairs of progenitors, including four glioblasts and six neuro-glioblasts. Neuro-glioblasts show two different modes of divisions in which neuronal and glial progeny are separated (Udolph et al, 2001). According to the first mode, the neuro-glioblast performs an initial division whose two daughter cells then continue as a pure neuroblast and glioblast, respectively.…”

“…To distinguish them from the ''classical'' (Type I) neuroblasts, asymmetrically dividing cells producing only ganglion mother cells (and thereby ending up with considerably smaller lineages), the novel type of neuroblast was called transient amplifying (TAN or Type II) neuroblast (Boone and Doe, 2008;Bello et al, 2008). Please note that the Type I (''classical'') and Type II (transient amplifying) neuroblasts of the larval brain are unrelated to the Type I and Type II neuro-glioblasts which were differentiated by Udolph et al (2001) in reference to the embryonic ventral nerve cord.…”

“…Glial cells are derived from a relatively small set of uniquely identifiable glioblasts and neuro‐glioblasts that delaminate from the neurectoderm of the early embryo. Glioblasts are defined as progenitors that produce only glia, whereas neuro‐glioblasts have both neurons and glia among their progeny (Beckervordersandforth et al,2008; Jacobs et al,1989; Klämbt and Goodman,1991; Klämbt et al,1991; Nambu et al,1990; Udolph et al,1993). Glial progeny can be recognized at an early stage by the expression of the genes gmc and repo , which represent the key molecular factors of glial fate and differentiation (Campbell et al,1994; Halter et al, 1995; Hosoya et al,1995; Jones et al,1995; Jones,2005; Vincent et al,1996; Xiong et al, 1995).…”