Maintaining youth in Drosophila neural progenitors

An important question is how dividing stem cells maintain competence to generate multiple cell types, whereas most other cells become progressively restricted during development. The molecular basis for progenitor competence--or how competence is progressively restricted--has remained mysterious. Recent work has shown that Drosophila neuroblasts and mammalian neural progenitors are more similar than previously appreciated, and provide an excellent model system for using Drosophila genetics to unravel the molecular nature of progenitor competence and how it becomes progressively restricted during development.     

Keeping neural progenitor cells on a short leash during Drosophila neurogenesis

The developmental potential of stem cells and progenitor cells must be functionally distinguished to ensure the generation of diverse cell types while maintaining the stem cell pool throughout the lifetime of an organism. In contrast to stem cells, progenitor cells possess restricted developmental potential, allowing them to give rise to only a limited number of post-mitotic progeny. Failure to establish or maintain restricted progenitor cell potential can perturb tissue development and homeostasis, and probably contributes to tumor initiation. Recent studies using the developing fruit fly Drosophila larval brain have provided molecular insight into how the developmental potential is restricted in neural progenitor cells.     

Asymmetric cell division

Asymmetric cell division is a conserved mechanism for partitioning information during mitosis. Over the past several years, significant progress has been made in our understanding of how cells establish polarity during asymmetric cell division and how determinants, in the form of localized proteins and mRNAs, are segregated. In particular, genetic studies in Drosophila and Caenorhabditis elegans have linked cell polarity, G protein signaling and regulation of the cytoskeleton to coordination of mitotic spindle orientation and localization of determinants. Also, several new studies have furthered our understanding of how asymmetrically localized cell fate determinants, such as the Numb, a negative regulator Notch signaling, functions in biasing cell fates in the developing nervous system in Drosophila. In vertebrates, analysis of dividing neural progenitor cells by in vivo imaging has raised questions about the role of asymmetric cell divisions during neurogenesis.     

Cortical neuron specification: it has its time and place

Cortical neurogenesis is a highly stereotyped process in which progenitor cells generate neurons destined for specific cortical layers depending on the timing of cell cycle exit. Previous work has shown that during corticogenesis, progenitors become progressively restricted in their developmental potential. Recent work has uncovered some of the intrinsic mechanisms that underlie this fate restriction. In addition to timing, new studies suggest that the location of cell cycle exit in the cortical germinal zone may also contribute to cortical neuron specification.     

Inactivation of Numb and Numblike in embryonic dorsal forebrain impairs neurogenesis and disrupts cortical morphogenesis

Numb and Numblike, conserved homologs of Drosophila Numb, have been implicated in cortical neurogenesis; however, analysis of their involvement in later stages of cortical development has been hampered by early lethality of double mutants in previous studies. Using Emx1(IREScre) to induce more restricted inactivation of Numb in the dorsal forebrain of numblike null mice beginning at E9.5, we have generated viable double mutants that displayed striking brain defects. It was thus possible to examine neurogenesis during the later peak phase (E12.5-E16.5). Loss of Numb and Numblike in dorsal forebrain resulted in neural progenitor hyperproliferation, delayed cell cycle exit, impaired neuronal differentiation, and concomitant defects in cortical morphogenesis. These findings reveal novel and essential function of Numb and Numblike during the peak period of cortical neurogenesis. Further, these double mutant mice provide an unprecedented viable animal model for severe brain malformations due to defects in neural progenitor cells.     

Temporal and epigenetic regulation of neurodevelopmental plasticity

The anticipated therapeutic uses of neural stem cells depend on their ability to retain a certain level of developmental plasticity. In particular, cells must respond to developmental manipulations designed to specify precise neural fates. Studies in vivo and in vitro have shown that the developmental potential of neural progenitor cells changes and becomes progressively restricted with time. For in vitro cultured neural progenitors, it is those derived from embryonic stem cells that exhibit the greatest developmental potential. It is clear that both extrinsic and intrinsic mechanisms determine the developmental potential of neural progenitors and that epigenetic, or chromatin structural, changes regulate and coordinate hierarchical changes in fate-determining gene expression. Here, we review the temporal changes in developmental plasticity of neural progenitor cells and discuss the epigenetic mechanisms that underpin these changes. We propose that understanding the processes of epigenetic programming within the neural lineage is likely to lead to the development of more rationale strategies for cell reprogramming that may be used to expand the developmental potential of otherwise restricted progenitor populations.     

The adult Drosophila malpighian tubules are maintained by multipotent stem cells

All animals must excrete the waste products of metabolism. Excretion is performed by the kidney in vertebrates and by the Malpighian tubules in Drosophila. The mammalian kidney has an inherent ability for recovery and regeneration after ischemic injury. Stem cells and progenitor cells have been proposed to be responsible for repair and regeneration of injured renal tissue. In Drosophila, the Malpighian tubules are thought to be very stable and no stem cells have been identified. We have identified multipotent stem cells in the region of lower tubules and ureters of the Malpighian tubules. Using lineage tracing and molecular marker labeling, we demonstrated that several differentiated cells in the Malpighian tubules arise from the stem cells and an autocrine JAK-STAT signaling regulates the stem cells' self-renewal. Identifying adult kidney stem cells in Drosophila may provide important clues for understanding mammalian kidney repair and regeneration during injury.     

Age-related changes in Drosophila midgut are associated with PVF2, a PDGF/VEGF-like growth factor

Age-associated changes in stem cell populations have been implicated in age-related diseases, including cancer. However, little is known about the underlying molecular mechanisms that link aging to the modulation of adult stem cell populations. Drosophila midgut is an excellent model system for the study of stem cell renewal and aging. Here we describe an age-related increase in the number and activity of intestinal stem cells (ISCs) and progenitor cells in Drosophila midgut. We determined that oxidative stress, induced by paraquat treatment or loss of catalase function, mimicked the changes associated with aging in the midgut. Furthermore, we discovered an age-related increase in the expression of PVF2, a Drosophila homologue of human PDGF/VEGF, which was associated with and required for the age-related changes in midgut ISCs and progenitor cell populations. Taken together, our findings suggest that PDGF/VEGF may play a central role in age-related changes in ISCs and progenitor cell populations, which may contribute to aging and the development of cancer stem cells.     

Specification of neuronal identity in the embryonic CNS

One of the earliest and most crucial steps in the development of connectivity within the CNS is the acquisition of specific identities by developing neural cells. In this review, we discuss how a neural cell may come to acquire its unique identity and some of the genes that may be involved in this process. Experimental evidence suggests that ectodermal cells may pass through several phases at which their potential fates become progressively more restricted. An initial step occurs during neural induction when ectodermal cells become restricted to either a neural or non-neural fate. A little later in development, a further set of interactions determine which of the neural cells become postmitotic and begin a programme of differentiation. The differentiation phase may itself involve several steps at which the postmitotic neuron progressively advances towards its final identity.     

NRP1 and NRP2 cooperate to regulate gangliogenesis, axon guidance and target innervation in the sympathetic nervous system

During mouse pancreas development, the transient expression of Neurogenin3 (Neurog3) in uncommitted pancreas progenitors is required to determine endocrine destiny. However it has been reported that Neurog3-expressing cells can eventually adopt acinar or ductal fates and that Neurog3 levels were important to secure the islet destiny. It is not known whether the competence of Neurog3-induced cells to give rise to non-endocrine lineages is an intrinsic property of these progenitors or depends on pancreas developmental stage. Using temporal genetic labeling approaches we examined the dynamic of endocrine progenitor differentiation and explored the plasticity of Neurog3-induced cells throughout development. We found that Neurog3(+) progenitors develop into hormone-expressing cells in a fast process taking less then 10h. Furthermore, fate-mapping studies in heterozygote (Neurog3(CreERT/+)) and Neurog3-deficient (Neurog3(CreERT/CreERT)) embryos revealed that Neurog3-induced cells have different potential over time. At the early bud stage, failed endocrine progenitors can adopt acinar or ductal fate, whereas later in the branching pancreas they do not contribute to the acinar lineage but Neurog3-deficient cells eventually differentiate into duct cells. Thus these results provide evidence that the plasticity of Neurog3-induced cells becomes restricted during development. Furthermore these data suggest that during the secondary transition, endocrine progenitor cells arise from bipotent precursors already committed to the duct/endocrine lineages and not from domain of cells having distinct potentialities.     

When timing is everything: role of cell cycle regulation in asymmetric division

Asymmetric division is a fundamental mechanism of generating cell diversity during development. One of its hallmarks is asymmetric localization during mitosis of proteins that specify daughter cell fate. Studies in Drosophila show that subcellular localization of many proteins required for asymmetric division of neuronal progenitors correlates with progression through mitosis. Yet, how cell cycle and asymmetric division machineries cooperate remains unclear. Recent data show that (1) key cell cycle regulators are required for asymmetric localization of cell fate determinants and for cell fate determination and (2) molecules that mediate asymmetric division can also act to modulate proliferation potential of progenitor cells.     

Regulation of ganglion cell production by Notch signaling during retinal development

Although progenitor cells in developing vertebrate retina are capable of producing all retinal cell types, they are competent to produce only certain cell types at a given time, and this competence changes as development progresses. We asked whether a change in progenitor cell competence is primarily responsible for ending production of a specific cell type, the retinal ganglion cell. Reducing Notch expression using an antisense oligonucleotide in vitro or in vivo increased ganglion cell genesis. The antisense treatment could reinitiate ganglion cell genesis after it had terminated in a region of the retina, but only for a brief period. The failure of the Notch antisense treatment to reinitiate ganglion cell production after this period was not due to the lack of receptor or ligand expression, as both Notch-1 and Delta-1 were still expressed. The failure of the Notch antisense treatment to reinitiate ganglion cell production is consistent with the suggestion that the intrinsic competence of progenitor cells changes as development progresses. Because reducing Notch signaling can reinitiate ganglion cell production for a brief period after ganglion cell production has normally ceased, it appears that ganglion cell production initially ends in a region of the retina because of cell-cell interactions and not because progenitor cells lose the competence to make ganglion cells. Notch signaling appears to temporarily prevent production of ganglion cells in a region, while some other signal must initiate a change in progenitor cell competence, thus permanently ending the possibility of further ganglion cell production.