Background: The cytoskeleton and associated motors play an important role in the establishment of intracellular polarity. Microtubule-based transport is required in many cell types for the asymmetric ...localization of mRNAs and organelles. A striking example is the Drosophila oocyte, where microtubule-dependent processes govern the asymmetric positioning of the nucleus and the localization to distinct cortical domains of mRNAs that function as cytoplasmic determinants. A conserved machinery for mRNA localization and nuclear positioning involving cytoplasmic Dynein has been postulated; however, the precise role of plus- and minus end-directed microtubule-based transport in axis formation is not yet understood.
Results: Here, we show that mRNA localization and nuclear positioning at mid-oogenesis depend on two motor proteins, cytoplasmic Dynein and Kinesin I. Both of these microtubule motors cooperate in the polar transport of bicoid and gurken mRNAs to their respective cortical domains. In contrast, Kinesin I-mediated transport of oskar to the posterior pole appears to be independent of Dynein. Beside their roles in RNA transport, both motors are involved in nuclear positioning and in exocytosis of Gurken protein. Dynein-Dynactin complexes accumulate at two sites within the oocyte: around the nucleus in a microtubule-independent manner and at the posterior pole through Kinesin-mediated transport.
Conclusion: The microtubule motors cytoplasmic Dynein and Kinesin I, by driving transport to opposing microtubule ends, function in concert to establish intracellular polarity within the Drosophila oocyte. Furthermore, Kinesin-dependent localization of Dynein suggests that both motors are components of the same complex and therefore might cooperate in recycling each other to the opposite microtubule pole.
Microtubules and the Kinesin heavy chain, the force-generating component of the plus end-directed microtubule motor Kinesin I are required for the localisation of oskar mRNA to the posterior pole of ...the Drosophila oocyte, an essential step in the determination of the anteroposterior axis. We show that the Kinesin heavy chain is also required for the posterior localisation of Dynein, and for all cytoplasmic movements within the oocyte. Furthermore, the KHC localises transiently to the posterior pole in an oskar mRNA-independent manner. Surprisingly, cytoplasmic streaming still occurs in kinesin light chain null mutants, and both oskar mRNA and Dynein localise to the posterior pole. Thus, the Kinesin heavy chain can function independently of the light chain in the oocyte, indicating that it associates with its cargoes by a novel mechanism.
The serine-threonine kinase LKB1 regulates cell polarity from Caenorhabditis elegans to man. Loss of lkb1 leads to a cancer predisposition, known as Peutz-Jeghers Syndrome. Biochemical analysis ...indicates that LKB1 can phosphorylate and activate a family of AMPK- like kinases, however, the precise contribution of these kinases to the establishment and maintenance of cell polarity is still unclear. Recent studies propose that LKB1 acts primarily through the AMP kinase to establish and/or maintain cell polarity. To determine whether this simple model of how LKB1 regulates cell polarity has relevance to complex tissues, we examined lkb1 mutants in the Drosophila eye. We show that adherens junctions expand and apical, junctional, and basolateral domains mix in lkb1 mutants. Surprisingly, we find LKB1 does not act primarily through AMPK to regulate cell polarity in the retina. Unlike lkb1 mutants, ampk retinas do not show elongated rhabdomeres or expansion of apical and junctional markers into the basolateral domain. In addition, nutrient deprivation does not reveal a more dramatic polarity phenotype in lkb1 photoreceptors. These data suggest that AMPK is not the primary target of LKB1 during eye development. Instead, we find that a number of other AMPK-like kinase, such as SIK, NUAK, Par-1, KP78a, and KP78b show phenotypes similar to weak lkb1 loss of function in the eye. These data suggest that in complex tissues, LKB1 acts on an array of targets to regulate cell polarity.
The specification of polar, main-body and stalk follicle cells in the germarium of the Drosophila ovary plays a key role in the formation of the egg chamber and polarisation of its anterior-posterior ...axis. High levels of Notch pathway activation, resulting from a germline Delta ligand signal, induce polar cells. Here we show that low Notch activation levels, originating from Delta expressed in the polar follicle cells, are required for stalk formation. The metalloprotease Kuzbanian-like, which cleaves and inactivates Delta, reduces the level of Delta signaling between follicle cells, thereby limiting the size of the stalk. We find that Notch activation is required in a continuous fashion to maintain the polar and stalk cell fates. We further demonstrate that mutual antagonism between the Notch and JAK/STAT signaling pathways provides a crucial facet of follicle cell patterning. Notch signaling in polar and main-body follicle cells inhibits JAK/STAT signaling by preventing STAT nuclear translocation, thereby restricting the influence of this pathway to stalk cells. Conversely, signaling by JAK/STAT reduces Notch signaling in the stalk. Thus, variations in the levels of Notch pathway activation, coupled with a continuous balance between the Notch and JAK/STAT pathways, specify the identity of the different follicle cell types and help establish the polarity of the egg chamber.
oskar mRNA localization to the posterior of the Drosophila oocyte defines where the abdomen and germ cells form in the embryo. Although this localization requires microtubules and the plus ...end-directed motor, kinesin, its mechanism is controversial and has been proposed to involve active transport to the posterior, diffusion and trapping, or exclusion from the anterior and lateral cortex. By following oskar mRNA particles in living oocytes, we show that the mRNA is actively transported along microtubules in all directions, with a slight bias toward the posterior. This bias is sufficient to localize the mRNA and is reversed in mago, barentsz, and Tropomyosin II mutants, which mislocalize the mRNA anteriorly. Since almost all transport is mediated by kinesin, oskar mRNA localizes by a biased random walk along a weakly polarized cytoskeleton. We also show that each component of the oskar mRNA complex plays a distinct role in particle formation and transport.
Translation of oskar messenger RNA (mRNA) is activated at the posterior of the Drosophila oocyte, producing Long Oskar, which anchors the RNA, and Short Oskar, which nucleates the pole plasm, ...containing the posterior and germline determinants. Here, we show that Oskar is phosphorylated by Par-1 and GSK-3/Shaggy to create a phosphodegron that recruits the SCF-Slimb ubiquitin ligase, which targets Short Oskar for degradation. Phosphorylation site mutations cause Oskar overaccumulation, leading to an increase in pole cell number and embryonic patterning defects. Furthermore, the nonphosphorylatable mutant produces bicaudal embryos when oskar mRNA is mislocalized. Thus, the Par-1/GSK-3/Slimb pathway plays important roles in limiting the amount of pole plasm posteriorly and in degrading any mislocalized Oskar that results from leaky translational repression. These results reveal that Par-1 controls the timing of pole plasm assembly by promoting the localization of oskar mRNA but inhibiting the accumulation of Short Oskar protein.
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•Par-1 phosphorylates Short Osk to prime it for GSK-3 phosphorylation•This marks Short Osk for degradation through recruitment of the SCFSlimb complex•Par-1 promotes oskar messenger RNA localization but inhibits short Osk accumulation•This removes mislocalized Osk and controls the timing of pole plasm assembly
Drosophila axis formation depends on the tight localization of Oskar to the oocyte posterior by messenger RNA localization and translational repression. Morais-de-Sá et al. describe another essential control mechanism in which Par-1, GSK-3, and an SCF ubiquitin ligase induce Oskar degradation to prevent ectopic or excess Oskar from disrupting patterning.
The localisation of
mRNA to the posterior of the
oocyte defines where the abdomen and germ cells form in the embryo. Kinesin 1 transports
mRNA to the oocyte posterior along a polarised microtubule ...cytoskeleton that grows from non-centrosomal microtubule organising centres (ncMTOCs) along the anterior/lateral cortex. Here, we show that the formation of this polarised microtubule network also requires the posterior regulation of microtubule growth. A missense mutation in the dynactin Arp1 subunit causes most
mRNA to localise in the posterior cytoplasm rather than cortically.
mRNA transport and anchoring are normal in this mutant, but the microtubules fail to reach the posterior pole. Thus, dynactin acts as an anti-catastrophe factor that extends microtubule growth posteriorly. Kinesin 1 transports dynactin to the oocyte posterior, creating a positive feedback loop that increases the length and persistence of the posterior microtubules that deliver
mRNA to the cortex.
During asymmetric stem cell divisions, the mitotic spindle must be correctly oriented and positioned with respect to the axis of cell polarity to ensure that cell fate determinants are appropriately ...segregated into only one daughter cell. By contrast, epithelial cells divide symmetrically and orient their mitotic spindles perpendicular to the main apical–basal polarity axis, so that both daughter cells remain within the epithelium. Work in the past 20 years has defined a core ternary complex consisting of Pins, Mud and Gαi that participates in spindle orientation in both asymmetric and symmetric divisions. As additional factors that interact with this complex continue to be identified, a theme has emerged: there is substantial overlap between the mechanisms that orient the spindle and those that establish and maintain apical–basal polarity in epithelial cells. In this review, we examine several factors implicated in both processes, namely Canoe, Bazooka, aPKC and Discs large, and consider the implications of this work on how the spindle is oriented during epithelial cell divisions.
The entry of the sperm centrosome polarizes the anterior-posterior axis of the C. elegans zygote by inducing the formation of complementary cortical Par protein domains. Recent papers from the ...Seydoux and Grill laboratories (Goehring et al., 2011b and Motegi et al., 2011) reveal how two different symmetry-breaking mechanisms produce the same final pattern through interactions between Par proteins.
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