Regulating neural proliferation in the Drosophila CNS

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Neural stem and progenitor cells generate the central nervous system (CNS) in organisms as diverse as insects and mammals. In Drosophila, multipotent asymmetrically dividing progenitors called neuroblasts produce neurons and glia throughout the developing CNS. Nevertheless, the time-windows of mitotic activity, the division modes, the termination mechanisms and the lineage sizes of individual neuroblasts all vary considerably from region-to-region. Recent studies shed light on some of the mechanisms underlying this neuroblast diversity and, in particular, how proliferation is boosted in two brain regions. In the central brain, some specialised neuroblasts generate intermediate neural progenitors that can each divide multiple times, thus increasing overall lineage size. In the optic lobe, an alternative expansion strategy involves symmetrically dividing neuroepithelial cells generating large numbers of asymmetrically dividing neuroblasts. Evidence is also emerging for a cell-intrinsic timer that alters the properties of each neuroblast with increasing developmental age. The core mechanism corresponds to a series of transcription factors that coordinates temporal changes in neuronal/glial identity with transitions in neuroblast cell-cycle speed, entry into quiescence and, ultimately, with termination.

Introduction

Understanding how the rich diversity of different neuronal and glial types is generated is a central theme in developmental neurobiology (see reviews in this issue by T. Lee and H. Sawa). An equally important question relates to how the numbers of each cell type are regulated and, more generally, how overall brain size is specified. Here we review recent progress in this second area, focusing on cell proliferation in the CNS of the fruit fly, Drosophila. This provides a powerful model system in which to observe neural stem cells in action and to screen for the genes underlying their remarkable properties.

The anatomy of the developing Drosophila CNS can be divided into the optic lobes (OL), the central brain (CB) and the ventral nerve cord (VNC). The CB contains olfactory learning and memory centres known as the mushroom bodies (MB) and the VNC is further subdivided into 3 thoracic (Th) and 9 abdominal (Ab) neuromeres (Figure 1). The cells that generate the neurons and glia in all of these CNS regions are neural stem-cell-like progenitors called neuroblasts. These multipotent cells are specified from ectodermal epithelia by a process involving proneural genes and Notch signalling. The CB and VNC neuroblasts derive from the ventral neuroectoderm of the early embryo, whereas OL neuroblasts form later on, during larval life, from neuroepithelial placodes (reviewed by [1, 2]). In the forming VNC, there are ∼30 neuroblasts per hemi-neuromere, which delaminate internally in a stereotypical pattern. Neuroblasts then go through numerous asymmetric self-renewing divisions, each time producing an intermediate progenitor called a ganglion mother cell (GMC). In turn, GMCs divide only once to yield two postmitotic cells that can be neurons or glia. Each of the ∼30 VNC neuroblasts is unique in that it expresses different dorsoventral and anteroposterior patterning genes and generates a unique embryonic lineage of neurons and glia [3, 4, 5, 6, 7]. Although the short embryonic phase of neuroblast divisions is sufficient to form the functioning CNS of the larva, it only contributes ∼10% of the neurons in the adult CNS. A long postembryonic phase of progenitor divisions, involving the continued activity of most (but not all) of the original embryonic neuroblasts accounts for the remaining ∼90% of adult neurons [8, 9, 10, 11, 12]. Postembryonic neuroblast divisions are highly region specific, with a general bias for anterior domains to retain a larger fraction of the original number of embryonic neuroblasts and for each one to divide more times than in posterior domains (reviewed by [13]). This leads to dramatic anterior expansion of the CNS during larval/pupal stages, in line with the anterior locations of major adult-specific sensorimotor structures such as eyes, wings and legs.

Section snippets

The nuts and bolts of a neuroblast

Neuroblasts utilise two different molecular machines that endow them with stem-cell-like properties. The first of these corresponds to asymmetrically localised protein complexes that direct one daughter cell to self-renew whereas the other becomes a GMC. The second molecular machine comprises a set of sequentially expressed transcription factors that allow each neuroblast to make an invariant series of different GMC identities, and thus different types of neurons/glia.

Many molecular components

How many different types of postembryonic neuroblasts are there?

In contrast to embryonic stages, the repertoire of neuronal/glial types generated by each neuroblast during postembryonic development has yet to be clearly defined for most CNS regions. Exceptions are MB and antennal-lobe neuroblasts where multiple different neuronal types produced in a stereotypical order have been defined systematically [38, 39]. In addition, a systematic study of Th neuroblasts has identified 24 different lineages per hemisegment based on their position and the morphology of

Most neuroblasts undergo reversible quiescence

Late in embryogenesis, the majority of neuroblasts in the CB and VNC enter a reversible G1 arrest known as quiescence [9]. This separates the embryonic and postembryonic phases of neurogenesis. The only postembryonic neuroblasts that do not undergo quiescence are the four MB neuroblasts, which generate very large lineages of ∼500 neurons each, and one less-well characterised ventrolateral CB neuroblast [11, 12, 39].

Neuroblast quiescence is known to be regulated by several different intrinsic

Increasing progenitor numbers via a symmetric division strategy

The most dramatic cell proliferation within the postembryonic CNS occurs within the OL. The embryonic OL primordium invaginates in the early embryo as a placode of 30–40 neuroepithelial cells. This expands dramatically during postembryonic stages by symmetric divisions, segregating into two separate epithelia known as the Inner Proliferation Centre (IPC) and the Outer Proliferation Centre (OPC). Most OL neurons are generated from these two centres although smaller numbers are also generated

Increasing progenitor numbers via an asymmetric division strategy

Recently, a second strategy was discovered for expanding progenitor numbers (and thus neuronal numbers), this time utilising a modified form of asymmetric division. In the dorso-posterior medial part of the postembryonic CB, eight primary neuroblasts (probably the pl and pm subgroups) divide asymmetrically to self-renew and bud off intermediate neural progenitors (INPs) that, unlike GMCs, can undergo multiple divisions [20••, 55, 56••, 57••]. We now refer to these atypical primary progenitors

Mechanisms for terminating neuroblast divisions

The time at which neuroblasts finally and irreversibly stop dividing is crucial, not only for achieving the correct balance of early versus late-born neuronal/glial fates but also for determining the final size of the growing CNS. This neuroblast termination process occurs at very different times in different regions but is complete by the end of metamorphosis, such that there are no identifiable neuroblasts in the adult CNS [12]. We now discuss what is known about the molecular mechanisms

Conclusions and future directions

An important conclusion from recent studies is that the formation and mode of division of Drosophila neuroblasts are more diverse than previously thought. Symmetric expansion of neuroepithelial cells facilitates local increases in neuroblast number and the ‘classic’ asymmetric neuroblast lineage can be adapted to make more cells via INPs. In addition, some types of neuroblasts enter quiescence whereas others do not, some neuroblasts finally terminate their divisions via apoptosis yet others

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We apologise to those of our colleagues whose work was not cited owing to space constraints. We thank Iris Salecker, Andrea Brand and an anonymous reviewer for critical reading of the manuscript and Wai Han Yau for help with illustrations. R.S-N., L.Y.C. and A.P.G. are supported by the Medical Research Council and a grant to R.S-N. from the Portuguese Foundation for Science and Technology.

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