Alisa L. Katzen

Heterotrimeric G Proteins Regulate Daughter Cell Size Asymmetry in Drosophila Neuroblast Divisions

Cell division often generates unequally sized daughter cells by off-center cleavages, which are due to either displacement of mitotic spindles or their asymmetry. Drosophila neuroblasts predominantly use the latter mechanism to divide into a large apical neuroblast and a small basal ganglion mother cell (GMC), where the neural fate determinants segregate. Apically localized components regulate both the spindle asymmetry and the localization of the determinants. Here, we show that asymmetric spindle formation depends on signaling mediated by the G beta subunit of heterotrimeric G proteins. G beta 13F distributes throughout the neuroblast cortex. Its lack induces a large symmetric spindle and causes division into nearly equal-sized cells with normal segregation of the determinants. In contrast, elevated G beta 13F activity generates a small spindle, suggesting that this factor suppresses spindle development. Depletion of the apical components also results in the formation of a small symmetric spindle at metaphase. Therefore, the apical components and G beta 13F affect the mitotic spindle shape oppositely. We propose that differential activation of G beta signaling biases spindle development within neuroblasts and thereby causes asymmetric spindles. Furthermore, the multiple equal cleavages of G beta mutant neuroblasts accompany neural defects; this finding suggests indispensable roles of eccentric division in assuring the stem cell properties of neuroblasts.

MYB and CBP: physiological relevance of a biochemical interaction

Drosophila melanogaster possesses a single gene, Dm myb, that is closely related to the vertebrate family of Myb genes, which encode transcription factors involved in regulatory decisions affecting cell proliferation, differentiation and apoptosis. In proliferating cells, the Dm myb gene product, DMyb, promotes both S-phase and M-phase, and acts to preserve diploidy by suppressing endoreduplication. The CBP and p300 proteins are transcriptional co-activators that interact with a multitude of transcription factors, including Myb. In transient transfection assays, transcriptional activation by DMyb is enhanced by co-expression of the Drosophila CBP protein, dCBP. Genetic interaction analysis reveals that these genes work together to promote mitosis, thereby demonstrating the physiological relevance of the biochemical interaction between the Myb and CBP proteins within a developing organism.

The dREAM/Myb-MuvB complex and Grim are key regulators of the programmed death of neural precursor cells at the Drosophila posterior wing margin

Successful development of a multicellular organism depends on the finely tuned orchestration of cell proliferation, differentiation and apoptosis from embryogenesis through adulthood. The MYB-gene family encodes sequence-specific DNA-binding transcription factors that have been implicated in the regulation of both normal and neoplastic growth. The Drosophila Myb protein, DMyb (and vertebrate B-Myb protein), has been shown to be part of the dREAM/MMB complex, a large multi-subunit complex, which in addition to four Myb-interacting proteins including Mip130, contains repressive E2F and pRB proteins. This complex has been implicated in the regulation of DNA replication within the context of chorion gene amplification and transcriptional regulation of a wide array of genes. Detailed phenotypic analysis of mutations in the Drosophila myb gene, Dm myb, has revealed a previously undiscovered function for the dREAM/MMB complex in regulating programmed cell death (PCD). In cooperation with the pro-apoptotic protein Grim and dREAM/MMB, DMyb promotes the PCD of specified sensory organ precursor daughter cells in at least two different settings in the peripheral nervous system: the pIIIb precursor of the neuron and sheath cells in the posterior wing margin and the glial cell in the thoracic microchaete lineage. Unlike previously analyzed settings, in which the main role of DMyb has been to antagonize the activities of other dREAM/MMB complex members, it appears to be the critical effector in promoting PCD. The finding that Dm myb and grim are both involved in regulating PCD in two distinct settings suggests that these two genes may often work together to mediate PCD.

Two components of the Myb complex, DMyb and Mip130, are specifically associated with euchromatin and degraded during prometaphase throughout development

The Drosophila Myb protein, DMyb, is a transcription factor important for cell proliferation and development. Unlike the mRNAs produced by mammalian myb genes, Drosophila myb transcripts do not fluctuate substantially during the cell cycle. A comprehensive analysis of the localization and degradation of the DMyb protein has now revealed that DMyb is present in nuclei during S phase of all mitotically active tissues throughout embryogenesis and larval development. However, DMyb and Mip130, another member of the Myb complex, are not uniformly distributed throughout the nucleus. Instead, both proteins, which colocalize, appear to be specifically excluded from heterochromatic regions of chromosomes. Furthermore, DMyb and Mip130 are unstable proteins that are degraded during prometaphase of mitosis. The timing of their degradation is reminiscent of Cyclin A, but at least for DMyb, the mechanism differs; although DMyb degradation is dependent on core APC/C components, it does not depend on the Fizzy or Fizzy-related adaptor proteins. DMyb levels are also high in actively endoreplicating polyploid cells, but there is no indication of cyclical degradation. We conclude that cell cycle specific degradation of DMyb and Mip130 is likely to be utilized as a key regulatory mechanism in down-regulating their levels and the activity of the Myb complex.