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

Expression of Mammalian BM88/CEND1 in Drosophila Affects Nervous System Development by Interfering with Precursor Cell Formation

We used Drosophila melanogaster as an experimental model to express mouse and pig BM88/CEND1 (cell cycle exit and neuronal differentiation 1) in order to investigate its potential functional effects on Drosophila neurogenesis. BM88/CEND1 is a neuron-specific protein whose function is implicated in triggering cells to exit from the cell cycle and differentiate towards a neuronal phenotype. Transgenic flies expressing either mouse or pig BM88/CEND1 in the nervous system had severe neuronal phenotypes with variable expressivity at various stages of embryonic development. In early embryonic stage 10, BM88/CEND1 expression led to an increase in the neural-specific antigenicity of neuroectoderm at the expense of precursor cells [neuroblasts (Nbs) and ganglion mother cells (GMCs)] including the defective formation and differentiation of the MP2 precursors, whereas at later stages (12-15), protein accumulation induced gross morphological defects primarily in the CNS accompanied by a reduction of Nb and GMC markers. Furthermore, the neuronal precursor cells of embryos expressing BM88/CEND1 failed to carry out proper cell-cycle progression as revealed by the disorganized expression patterns of specific cell-cycle markers. BM88/CEND1 accumulation in the Drosophila eye affected normal eye disc development by disrupting the ommatidia. Finally, we demonstrated that expression of BM88/CEND1 modified/reduced the levels of activated MAP kinase indicating a functional effect of BM88/CEND1 on the MAPK signaling pathway. Our findings suggest that the expression of mammalian BM88/CEND1 in Drosophila exerts specific functional effects associated with neuronal precursor cell formation during embryonic neurogenesis and proper eye disc development. This study also validates the use of Drosophila as a powerful model system in which to investigate gene function and the underlying molecular mechanisms.

Failure of the PTEN/aPKC/Lgl Axis Primes Formation of Adult Brain Tumours in Drosophila

Different regions in the mammalian adult brain contain immature precursors, reinforcing the concept that brain cancers, such as glioblastoma multiforme (GBM), may originate from cells endowed with stem-like properties. Alterations of the tumour suppressor gene PTEN are very common in primary GBMs. Very recently, PTEN loss was shown to undermine a specific molecular axis, whose failure is associated with the maintenance of the GBM stem cells in mammals. This axis is composed of PTEN, aPKC, and the polarity determinant Lethal giant larvae (Lgl): PTEN loss promotes aPKC activation through the PI3K pathway, which in turn leads to Lgl inhibition, ultimately preventing stem cell differentiation. To find the neural precursors responding to perturbations of this molecular axis, we targeted different neurogenic regions of the Drosophila brain. Here we show that PTEN mutation impacts aPKC and Lgl protein levels also in Drosophila. Moreover, we demonstrate that PI3K activation is not sufficient to trigger tumourigenesis, while aPKC promotes hyperplastic growth of the neuroepithelium and a noticeable expansion of the type II neuroblasts. Finally, we show that these neuroblasts form invasive tumours that persist and keep growing in the adult, leading the affected animals to untimely death, thus displaying frankly malignant behaviours.

Drosophila and the hallmarks of cancer

: Cancer was the disease of the twentieth century. Today it is still a leading cause of death worldwide despite being intensively investigated. Abundant knowledge exists regarding the pathological and molecular mechanisms that drive healthy cells to become malignant and form metastatic tumors. The relation of oncogenes and tumor suppressors to the genetic trigger of carcinogenesis is unquestionable. However, the development of the disease requires many characteristics that due to their proven role in cancer are collectively described as the "hallmarks of cancer." We highlight here the historic discoveries made using the model organism Drosophila melanogaster and its contributions to biomedical and cancer research. Flies are utilized as a model organism for the investigation of each and every aspect of cancer hallmarks. Due to the significant conservation between flies and mammals at the signaling and tissue physiology level it is possible to explore the genes and mechanisms responsible for cancer pathogenesis in flies. Recent Drosophila studies suggest novel aspects of therapeutic intervention and are expected to guide cancer research in the twenty-first century.

Prefoldin and Pins synergistically regulate asymmetric division and suppress dedifferentiation

Prefoldin is a molecular chaperone complex that regulates tubulin function in mitosis. Here, we show that Prefoldin depletion results in disruption of neuroblast polarity, leading to neuroblast overgrowth in Drosophila larval brains. Interestingly, co-depletion of Prefoldin and Partner of Inscuteable (Pins) leads to the formation of gigantic brains with severe neuroblast overgrowth, despite that Pins depletion alone results in smaller brains with partially disrupted neuroblast polarity. We show that Prefoldin acts synergistically with Pins to regulate asymmetric division of both neuroblasts and Intermediate Neural Progenitors (INPs). Surprisingly, co-depletion of Prefoldin and Pins also induces dedifferentiation of INPs back into neuroblasts, while depletion either Prefoldin or Pins alone is insufficient to do so. Furthermore, knocking down either α-tubulin or β-tubulin in pins^- mutant background results in INP dedifferentiation back into neuroblasts, leading to the formation of ectopic neuroblasts. Overexpression of α-tubulin suppresses neuroblast overgrowth observed in prefoldin pins double mutant brains. Our data elucidate an unexpected function of Prefoldin and Pins in synergistically suppressing dedifferentiation of INPs back into neural stem cells.

The GAL4 System: A Versatile System for the Manipulation and Analysis of Gene Expression

Since its introduction in 1993, the GAL4 system has become an essential part of the Drosophila geneticist's toolkit. Widely used to drive gene expression in a multitude of cell- and tissue-specific patterns, the system has been adapted and extended to form the basis of many modern tools for the manipulation of gene expression in Drosophila and other model organisms.

EAG2 potassium channel with evolutionarily conserved function as a brain tumor target

Over 20% of the drugs for treating human diseases target ion channels, but no cancer drug approved by the US Food and Drug Administration (FDA) is intended to target an ion channel. We found that the EAG2 (Ether-a-go-go 2) potassium channel has an evolutionarily conserved function for promoting brain tumor growth and metastasis, delineate downstream pathways, and uncover a mechanism for different potassium channels to functionally cooperate and regulate mitotic cell volume and tumor progression. EAG2 potassium channel was enriched at the trailing edge of migrating medulloblastoma (MB) cells to regulate local cell volume dynamics, thereby facilitating cell motility. We identified the FDA-approved antipsychotic drug thioridazine as an EAG2 channel blocker that reduces xenografted MB growth and metastasis, and present a case report of repurposing thioridazine for treating a human patient. Our findings illustrate the potential of targeting ion channels in cancer treatment.

Zinc finger X-chromosomal protein promotes growth and tumorigenesis in human osteosarcoma cells

Objective: To investigate the role of Zinc finger X-chromosomal protein (ZFX) in oncogenesis of Osteosarcoma tumor.

Methods: Here, we first conducted an expression analysis of ZFX in Osteosarcoma cell lines. Then, we constructed ZFX-specific small interfering RNA (siRNA)-lentiviral vector that is capable of effectively inhibiting the expression of ZFX gene in human Osteosarcoma Saos-2 cells, and investigated systemically the impacts of ZFX silence on the growth and invasive ability of the cancer cells in vitro. Furthermore, we determined the effects of ZFX knockdown on the cell cycle distribution and apoptosis of Saos-2 cells.

Results: We found that ZFX inhibition resulted in significantly impaired proliferation and colony formation as well as mitigated invasiveness of Saos-2 cells. Importantly, si-ZFX infected cells exhibited a greater portion of cells at G1 phase, but a minor portion of S and G2/M phase cells. Moreover, a greater portion of sub-G1 apoptotic cells was observed in si-ZFX infected cells.

Conclusions: These results strongly suggest that ZFX is a novel proliferation regulator that promotes growth of Osteosarcoma cells, and downregulation of ZFX expression induces growth suppression of Saos-2 cells via arrested G0/G1 phase cell cycle and apoptosis pathways, thereby indicating that ZFX may serve as a new molecular target for Osteosarcoma tumor therapy.

A multipotent transit-amplifying neuroblast lineage in the central brain gives rise to optic lobe glial cells in Drosophila

The neurons and glial cells of the Drosophila brain are generated by neural stem cell-like progenitors during two developmental phases, one short embryonic phase and one more prolonged postembryonic phase. Like the bulk of the adult-specific neurons, most of glial cells found in the adult central brain are generated postembryonically. Five of the neural stem cell-like progenitors that give rise to glial cells during postembryonic brain development have been identified as type II neuroglioblasts that generate neural and glial progeny through transient amplifying INPs. Here we identify DL1 as a novel multipotent neuroglial progenitor in the central brain and show that this type II neuroblast not only gives rise to neurons that innervate the central complex but also to glial cells that contribute exclusively to the optic lobe. Immediately following their generation in the central brain during the second half of larval development, these DL1 lineage-derived glia migrate into the developing optic lobe, where they differentiate into three identified types of optic lobe glial cells, inner chiasm glia, outer chiasm glia and cortex glia. Taken together, these findings reveal an unexpected central brain origin of optic lobe glial cells and central complex interneurons from one and the same type II neuroglioblast.

Neural stem cell survival factors

Neural stem and progenitor cells (NSCs and NPs) give rise to the central nervous system (CNS) during embryonic development. NSCs and NPs differentiate into three main cell-types of the CNS; astrocytes, oligodendrocytes, and neurons. NSCs are present in the adult CNS and are important in maintenance and repair. Adult NSCs hold great promise for endogenous or self-repair of the CNS. Intriguingly, NSCs have been implicated as the cells that give rise to brain tumors. Thus, the balance between survival, growth and differentiation is a critical aspect of NSC biology, during development, in the adult, and in disease processes. In this review, we survey what is known about survival factors that control both embryonic and adult NSCs. We discuss the neurosphere culture system as this is widely used to measure NSC activity and behavior in vitro and emphasize the importance of clonality. We define here NSC survival factors in their broadest sense to include any factor that influences survival and proliferation of NSCs and NPs. NSC survival factors identified to date include growth factors, morphogens, proteoglycans, cytokines, hormones, and neurotransmitters. Understanding NSC and NP interaction in response to these survival factors will provide insight to CNS development, disease and repair.

ASYMMETRIC CELL DIVISION: IMPLICATIONS FOR GLIOMA DEVELOPMENT AND TREATMENT

Glioma is a heterogeneous disease process with differential histology and treatment response. It was previously thought that the histological features of glial tumors indicated their cell of origin. However, the discovery of continuous neuro-gliogenesis in the normal adult brain and the identification of brain tumor stem cells within glioma have led to the hypothesis that these brain tumors originate from multipotent neural stem or progenitor cells, which primarily divide asymmetrically during the postnatal period. Asymmetric cell division allows these cell types to concurrently self-renew whilst also producing cells for the differentiation pathway. It has recently been shown that increased symmetrical cell division, favoring the self-renewal pathway, leads to oligodendroglioma formation from oligodendrocyte progenitor cells. In contrast, there is some evidence that asymmetric cell division maintenance in tumor stem-like cells within astrocytoma may lead to acquisition of treatment resistance. Therefore cell division mode in normal brain stem and progenitor cells may play a role in setting tumorigenic potential and the type of tumor formed. Moreover, heterogeneous tumor cell populations and their respective cell division mode may confer differential sensitivity to therapy. This review aims to shed light on the controllers of cell division mode which may be therapeutically targeted to prevent glioma formation and improve treatment response.