Fumio Matsuzaki

Drosophila Pins-binding protein Mud regulates spindle-polarity coupling and centrosome organization.

The orientation of the mitotic spindle relative to the cell axis determines whether polarized cells undergo symmetric or asymmetric divisions. Drosophila epithelial cells and neuroblasts provide an ideal pair of cells to study the regulatory mechanisms involved. Epithelial cells divide symmetrically, perpendicular to the apical–basal axis. In the asymmetric divisions of neuroblasts, by contrast, the spindle reorients parallel to that axis, leading to the unequal distribution of cell-fate determinants to one daughter cell. Receptor-independent G-protein signalling involving the GoLoco protein Pins is essential for spindle orientation in both cell types. Here, we identify Mushroom body defect (Mud) as a downstream effector in this pathway. Mud directly associates and colocalizes with Pins at the cell cortex overlying the spindle pole(s) in both neuroblasts and epithelial cells. The cortical Mud protein is essential for proper spindle orientation in the two different division modes. Moreover, Mud localizes to centrosomes during mitosis independently of Pins to regulate centrosomal organization. We propose that Drosophila Mud, vertebrate NuMA and Caenorhabditis elegans Lin-5 (refs 5, 6) have conserved roles in the mechanism by which G-proteins regulate the mitotic spindle.

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.

Differential functions of G protein and Baz–aPKC signaling pathways in Drosophila neuroblast asymmetric division

Drosophila melanogaster neuroblasts (NBs) undergo asymmetric divisions during which cell-fate determinants localize asymmetrically, mitotic spindles orient along the apical–basal axis, and unequal-sized daughter cells appear. We identified here the first Drosophila mutant in the Gγ1 subunit of heterotrimeric G protein, which produces Gγ1 lacking its membrane anchor site and exhibits phenotypes identical to those of Gβ13F, including abnormal spindle asymmetry and spindle orientation in NB divisions. This mutant fails to bind Gβ13F to the membrane, indicating an essential role of cortical Gγ1–Gβ13F signaling in asymmetric divisions. In Gγ1 and Gβ13F mutant NBs, Pins–Gαi, which normally localize in the apical cortex, no longer distribute asymmetrically. However, the other apical components, Bazooka–atypical PKC–Par6–Inscuteable, still remain polarized and responsible for asymmetric Miranda localization, suggesting their dominant role in localizing cell-fate determinants. Further analysis of Gβγ and other mutants indicates a predominant role of Partner of Inscuteable–Gαi in spindle orientation. We thus suggest that the two apical signaling pathways have overlapping but different roles in asymmetric NB division.

Asymmetric division of Drosophila neural stem cells: a basis for neural diversity.

Recent studies of Drosophila neural precursor cells have unveiled the essential roles played by asymmetric cell divisions in the determination of cell fates during neural development. Our understanding now extends to the molecular nature of the cell polarity that underlies asymmetric divisions. This polarity is conserved among neural stem cells, epithelial cells and fertilized eggs.

An Ana2/Ctp/Mud Complex Regulates Spindle Orientation in Drosophila Neuroblasts

Drosophila neural stem cells, larval brain neuroblasts (NBs), align their mitotic spindles along the apical/basal axis during asymmetric cell division (ACD) to maintain the balance of self-renewal and differentiation. Here, we identified a protein complex composed of the tumor suppressor anastral spindle 2 (Ana2), a dynein light-chain protein Cut up (Ctp), and Mushroom body defect (Mud), which regulates mitotic spindle orientation. We isolated two ana2 alleles that displayed spindle misorientation and NB overgrowth phenotypes in larval brains. The centriolar protein Ana2 anchors Ctp to centrioles during ACD. The centriolar localization of Ctp is important for spindle orientation. Ana2 and Ctp localize Mud to the centrosomes and cell cortex and facilitate/maintain the association of Mud with Pins at the apical cortex. Our findings reveal that the centrosomal proteins Ana2 and Ctp regulate Mud function toxa0orient the mitotic spindle during NB asymmetric division.

Differential expression of Pax6 and Ngn2 between pair-generated cortical neurons

Progenitor cells that generate neuron pairs (“pair progenitor cells”) are implicated in mammalian cortical development, and their division has been thought to be “symmetric.” However, asymmetric growth of two sister neurons generated by the division of a pair progenitor cell would lead to more efficient generation of neuronal diversity in the cortex. To explore mechanisms by which pair progenitor cells provide neuronal diversity, we examined molecular differences between a pair of neurons generated in clonal‐density culture. Time‐course analysis for the acquisition of neuronal markers and the disappearance of Pax6 and Neurogenin2 (Ngn2) demonstrated that 1) these transcription factors are expressed transiently in some but not all young neurons and 2) some neuron pairs showed uneven/asymmetric expression of Pax6 (19.5%) or Ngn2 (23.8%), whereas other pairs were either symmetrically positive or negative. Asymmetric Pax6 distribution in neuron pairs was not associated with asymmetric distribution of Numb, which raises an intriguing possibility, that Pax6 asymmetry in neuron pairs is produced by an alternative mode of the cell autonomous mechanisms. Stage‐dependent changes were noted in the pattern of Ngn2 retention in daughter neurons, reflecting qualitative changes in the pair progenitor population. We suggest that pair progenitor cells contribute to the generation of neuronal diversity through cell‐intrinsic heterogeneity and asymmetric division.

Drosophila G-protein signalling: intricate roles for Ric-8?

Receptor-independent G-protein signalling regulates both the orientation and position of the mitotic spindle during asymmetric cell division. Several new studies show that the targeting of G-protein subunits to the membrane requires Ric-8, pointing to possible novel roles for this protein in both receptor-dependent and -independent pathways.