Regulation of self-renewal and differentiation in the Drosophila nervous system

Function of Proneural Genes Ascl1 and Asense in Neurogenesis: How Similar Are They?

Proneural genes were initially identified in Drosophila, where pioneer work on these important regulators of neural development was performed, and from which the term proneural function was coined. Subsequently, their counterparts in vertebrates were identified, and their function in neural development extensively characterized. The function of proneural transcription factors in flies and vertebrates is, however, very distinct. In flies, proneural genes play an early role in neural induction, by endowing neural competence to ectodermal cells. In contrast, vertebrate proneural genes are expressed only after neural specification, in neural stem and progenitor cells, where they play key regulatory functions in quiescence, proliferation, and neuronal differentiation. An exception to this scenario is the Drosophila proneural gene asense, which has a late onset of expression in neural stem cells of the developing embryo and larvae, similar to its vertebrate counterparts. Although the role of Asense remains poorly investigated, its expression pattern is suggestive of functions more in line with those of vertebrate proneural genes. Here, we revise our current understanding of the multiple activities of Asense and of its closest vertebrate homologue Ascl1 in neural stem/progenitor cell biology, and discuss possible parallels between the two transcription factors in neurogenesis regulation.

Optix defines a neuroepithelial compartment in the optic lobe of the Drosophila brain

BACKGROUND

During early brain development, the organisation of neural progenitors into a neuroepithelial sheet maintains tissue integrity during growth. Neuroepithelial cohesion and patterning is essential for orderly proliferation and neural fate specification. Neuroepithelia are regionalised by the expression of transcription factors and signalling molecules, resulting in the formation of distinct developmental, and ultimately functional, domains.

RESULTS

We have discovered that the Six3/6 family orthologue Optix is an essential regulator of neuroepithelial maintenance and patterning in the Drosophila brain. Six3 and Six6 are required for mammalian eye and forebrain development, and mutations in humans are associated with severe eye and brain malformation. In Drosophila, Optix is expressed in a sharply defined region of the larval optic lobe, and its expression is reciprocal to that of the transcription factor Vsx1. Optix gain- and loss-of-function affects neuroepithelial adhesion, integrity and polarity. We find restricted cell lineage boundaries that correspond to transcription factor expression domains.

CONCLUSION

We propose that the optic lobe is compartmentalised by expression of Optix and Vsx1. Our findings provide insight into the spatial patterning of a complex region of the brain, and suggest an evolutionarily conserved principle of visual system development.

Specific polar subpopulations of astral microtubules control spindle orientation and symmetric neural stem cell division

Mitotic spindle orientation is crucial for symmetric vs asymmetric cell division and depends on astral microtubules. Here, we show that distinct subpopulations of astral microtubules exist, which have differential functions in regulating spindle orientation and division symmetry. Specifically, in polarized stem cells of developing mouse neocortex, astral microtubules reaching the apical and basal cell cortex, but not those reaching the central cell cortex, are more abundant in symmetrically than asymmetrically dividing cells and reduce spindle orientation variability. This promotes symmetric divisions by maintaining an apico-basal cleavage plane. The greater abundance of apical/basal astrals depends on a higher concentration, at the basal cell cortex, of LGN, a known spindle-cell cortex linker. Furthermore, newly developed specific microtubule perturbations that selectively decrease apical/basal astrals recapitulate the symmetric-to-asymmetric division switch and suffice to increase neurogenesis in vivo. Thus, our study identifies a novel link between cell polarity, astral microtubules, and spindle orientation in morphogenesis.

DOI:http://dx.doi.org/10.7554/eLife.02875.001

A stem cell can divide in two ways. Either it can split symmetrically into two identical daughter stem cells, or it can split asymmetrically into a stem cell and a specialist cell. The structure that forms inside the dividing cell to separate pairs of chromosomes—called the mitotic spindle—also partitions the molecules that determine what kind of cell each daughter cell will become.

The mitotic spindle is made up of protein microtubules. Astral microtubules connect the spindle to a structure found at the inner face of the cell membrane called the cell cortex. This helps the spindle to orient itself correctly and control the plane of cell division. This is particularly important in cells that are different at their top and bottom, like polarized neural stem cells.

To divide symmetrically, these cells need to split vertically from top to bottom. Then, to divide asymmetrically they tilt the cell division plane off-vertical. Classical studies on neuroblasts from the fruit fly Drosophila have shown that a big, 90° reorientation, from vertical to horizontal underlies this change. However, in the primary stem cells of the mammalian brain, subtle off-vertical tilting suffices for asymmetric divisions to occur. This tilting must be finely regulated: if not, neurodevelopmental disorders, such as microcephaly and lissencephaly, may arise.

Mora-Bermúdez et al. investigated how mammalian cortical stem cells control such subtle spindle orientation changes by taking images of developing brain tissue from genetically modified mice. These show that not all astral microtubules affect whether the spindle reorients, as was previously thought. Instead, only those connecting the spindle to the cell cortex at the top and bottom of the cell—the apical/basal astrals—are involved.

A decrease in the number of apical/basal astrals enables the spindle to undergo small reorientations. Mora-Bermúdez et al. therefore propose a model in which the spindle becomes less strongly anchored when the number of apical/basal astrals is reduced. This makes the spindle easier to tilt, allowing neural stem cells to undergo asymmetric divisions to produce neurons.

The decrease in the number of apical/basal astrals appears to be caused by a reduction in the amount of a molecule that is known to help link the microtubules to the cell cortex. This reduction occurs only in the cortex at the top of the cell. Mora-Bermúdez et al. were also able to manipulate this process by adding very low doses of a microtubule inhibitor called nocodazole, which reduced the number of only the apical/basal astrals, increasing the ability of the spindle to reorient.

DOI:http://dx.doi.org/10.7554/eLife.02875.002

Cancer stem cells: potential target for bioactive food components

Cancer stem cells often have phenotypic and functional characteristics similar to normal stem cells including the properties of self-renewal and differentiation. Recent findings suggest that uncontrolled self-renewal may explain cancer relapses and may represent a critical target for cancer prevention. It is conceivable that the loss of regulatory molecules resulting from inappropriate consumption of specific foods and their constituents may foster the aberrant self-renewal of cancer stem cells. In fact, increasing evidence points to the network delivering signals for self-renewal from extracellular compartments to the nucleus including changes in stem cell environments, inducible expression of microRNAs, hyperplastic nuclear chromatin structures, and the on/off of differentiation process as possible sites of action for bioactive food components. Diverse dietary constituents such as vitamins A and D, genistein, (-)-epigallocatechin-3-gallate (EGCG), sulforaphane, curcumin, piperine, theanine and choline have been shown to modify self-renewal properties of cancer stem cells. The ability of these bioactive food components to influence the balance between proliferative and quiescent cells by regulating critical feedback molecules in the network including dickkopf 1 (DKK-1), secreted frizzled-related protein 2 (sFRP2), B cell-specific Moloney murine leukemia virus integration site 1 (Bmi-1) and cyclin-dependent kinase 6 (CDK6) may account for their biological response. Overall, the response to food components does not appear to be tissue or organ specific, suggesting there may be common cellular mechanisms. Unquestionably, additional studies are needed to clarify the physiological role of these dietary components in preventing the resistance of tumor cells to traditional drugs and cancer recurrence.

Neural stem cells in Drosophila: molecular genetic mechanisms underlying normal neural proliferation and abnormal brain tumor formation

Neural stem cells in Drosophila are currently one of the best model systems for understanding stem cell biology during normal development and during abnormal development of stem cell-derived brain tumors. In Drosophila brain development, the proliferative activity of neural stem cells called neuroblasts gives rise to both the optic lobe and the central brain ganglia, and asymmetric cell divisions are key features of this proliferation. The molecular mechanisms that underlie the asymmetric cell divisions by which these neuroblasts self-renew and generate lineages of differentiating progeny have been studied extensively and involve two major protein complexes, the apical complex which maintains polarity and controls spindle orientation and the basal complex which is comprised of cell fate determinants and their adaptors that are segregated into the differentiating daughter cells during mitosis. Recent molecular genetic work has established Drosophila neuroblasts as a model for neural stem cell-derived tumors in which perturbation of key molecular mechanisms that control neuroblast proliferation and the asymmetric segregation of cell fate determinants lead to brain tumor formation. Identification of novel candidate genes that control neuroblast self-renewal and differentiation as well as functional analysis of these genes in normal and tumorigenic conditions in a tissue-specific manner is now possible through genome-wide transgenic RNAi screens. These cellular and molecular findings in Drosophila are likely to provide valuable genetic links for analyzing mammalian neural stem cells and tumor biology.

Transcriptome analysis of Drosophila neural stem cells

In Drosophila, the central nervous system is populated by a set of asymmetrically dividing neural stem cells called neuroblasts. Neuroblasts are derived from epithelial or neuroepithelial precursors, and divide along their apico-basal axes to produce a large apical neuroblast and a smaller basal ganglion mother cell. The ganglion mother cell will divide once again to produce two post-mitotic neurons or glia. In this chapter we outline a method for labeling different types of neural precursors in the Drosophila central nervous system, followed by their extraction and processing for transcriptome analysis. This technique has allowed us to capture and compare the expression profiles of neuroblasts and neuroepithelial cells, resulting in the identification of key genes required for the regulation of self-renewal and differentiation.

Coordinated sequential action of EGFR and Notch signaling pathways regulates proneural wave progression in the Drosophila optic lobe

During neurogenesis in the medulla of the Drosophila optic lobe, neuroepithelial cells are programmed to differentiate into neuroblasts at the medial edge of the developing optic lobe. The wave of differentiation progresses synchronously in a row of cells from medial to the lateral regions of the optic lobe, sweeping across the entire neuroepithelial sheet; it is preceded by the transient expression of the proneural gene lethal of scute [l(1)sc] and is thus called the proneural wave. We found that the epidermal growth factor receptor (EGFR) signaling pathway promotes proneural wave progression. EGFR signaling is activated in neuroepithelial cells and induces l(1)sc expression. EGFR activation is regulated by transient expression of Rhomboid (Rho), which is required for the maturation of the EGF ligand Spitz. Rho expression is also regulated by the EGFR signal. The transient and spatially restricted expression of Rho generates sequential activation of EGFR signaling and assures the directional progression of the differentiation wave. This study also provides new insights into the role of Notch signaling. Expression of the Notch ligand Delta is induced by EGFR, and Notch signaling prolongs the proneural state. Notch signaling activity is downregulated by its own feedback mechanism that permits cells at proneural states to subsequently develop into neuroblasts. Thus, coordinated sequential action of the EGFR and Notch signaling pathways causes the proneural wave to progress and induce neuroblast formation in a precisely ordered manner.