The Interactive Fly
Zygotically transcribed genes
The skeleton of the cell can be thought of as a maze of tubes and ropes. The tubes are composed of the protein tubulin, the basis of the microtubular cytoskeleton, and the ropes are actin, a component of muscle that forms the actin based cytoskeleton. If the tubulin and actin "ropes" are like the rigging of a ship, then the centrioles are analogous to the mast of the ship, providing a central organizing element for the microtubular filaments. Additional elements of the cytoskeleton are present immediately under the cell membrane, and serve a supportive function, like the ribs of a ship, to stretch the analogy.
Cytoskeleton is an important aspect of cell motility, assuring that a motile cell has a front and back as it moves along a substratum. Dynamic changes in cytoskeleton, in this case the actin based cytoskeleton, take place during differentiation processes. Dorsal closure is one example of a developmental process involving cell motility and the actin based cytoskeleton. Hemipterous is involved in a signaling process that affects cell motility in dorsal closure. The process of dorsal closure is described at the Hemipterous site.
The region immediately beneath the cell membrane is known as the cortex. Here a completely different set of cytoskeletal elements establish and maintain cell polarity, help to maintain cell shape, and serve to anchor proteins embedded in the cell surface. Thus the cortex has a major role in cell-cell communication mediated by cell surface proteins. The cortex serves as an anchor for Oskar, a key protein in establishing oocyte polarity, and also anchors Prospero and Numb, proteins important in neural cell polarity.
Microtubules (MTs), built of tubulin, are the highways on which dynein and kinesin motors travel. Microtubules are hollow, cylindrical polymers of alpha and beta tubulin heterodimers. During polymerization, the dimers assemble head-to-tail into typically 13 protofilaments arranged parallel to the long axis of the MT. The asymmetry of the individual dimers imparts an intrinsic polarity to the MT, which is displayed as kinetic differences between the two ends. The plus end, originally defined as the end of the axon's microtubule distal to the cell body, elongates 2-3 times faster than the minus end, or the end proximal to the cell body. Dependent on cell type, MTs may be arrayed in a variety of configurations, and since motors are unidirectional, the arrangements of MTs dictates the effective direction of motor movement. Kinesins are plus end directed motors, while dyneins are minus end directed motors. In mitosis, the minus end of microtubules are associated with centrosomes. Gamma Tubulin is involved in the nucleation of mitotic microtubules. Two antiparallel, overlapping MT arrays are generated with the plus ends of each interdigitating in the overlap zone (or in some cases interacting with chromosomal kinetochores) (Walker, 1993).
One of the major functions of cytoskeleton carried out by microtubules during mitosis is the equitable distribution of chromosomes to the two poles of the cell. The cytoskeleton is also responsible for structuring the cytoplasm of the cell so that proteins and nucleic acids can be carried from one site to another. The polar distribution of Bicoid mRNA and Oskar mRNA in the oocyte is accomplished through employment of the oocyte microtubular cytoskeleton.
Cytoplasmic streaming, another function of the microtubular based cytoskeleton, takes place in egg chambers during stage 12 and stage 13. cappuccino and spire are required to repress this microtubule-based ooplasmic streaming in the oocyte and to ensure the proper partitioning of molecular determinants within the oocyte. In mutants, the bundling of the microtubules at the cortex of the oocyte and the streaming of the oocyte cytoplasm occurs prematurely, by stage 8 in oogenesis. It is thought that this movement within the oocyte is necessary to mix the oocyte cytoplasm with the cytoplasm being rapidly added from the nurse cells, by an actin based cytoskeleton mechanism (Theurkauf, 1994). It is likely that subcortical nurse-cell microfilaments play a role in cytoplasmic flow into the oocyte. A nonmuscle myosin is found associated with subcortical actin but not with cytoplasmic networks. These subcortical actin filaments are very sensitve to cytochalasin treatment. Contraction of the subcortical actin could play a role in the bulk movement of nurse cells into the oocyte (Cooley, 1992 and references).
Prior to late vitellogenesis, characterized by the bulk flow of cytoplasm into the oocyte, during previtellogenesis, a different type of intercellular transport occurs. During this phase, treatment with colchicine, which inhibits microtubular transport mechanisms, does not change transport processes through the ring canals. However, when the microfilament inhibitor cytochalasin B is applied, the transfer of particles through the ring canals is completely inhibited. When an inhibitor of myosin-driven motility is applied to follicles, all movements within the cytoplasm stop. It is therefore thought that cytoplasmic myosin and the actin based microfilament network play a decisive role in particle movements during previtellogenesis (Bohrmann, 1994).
The actin based microfilament cytoskeleton plays an additional role in a process known as cytoplasmic dumping. At stage 11, the nurse cells dump their contents into the oocyte through cytoplasmic bridges termed ring canals. Microfilament bundles form in the nurse cells during this process and are apparently required to hold the nurse cell nuclei in place so that they do not obstruct the ring canals and allow rapid flow of nurse cell cytoplasm into the oocyte. Mutants in chickadee, quail and singed affect actin bundle formation. Profilin, encoded by chickadee, is presumably required for the polymerization of the actin filaments that compose the bundles (Cooley, 1992), while a villin-related protein encoded by quail and a fascin-related protein encoded by singed are thought to be required to cross-link the actin filaments to form the bundles. Two components of the actin-lined ring canals have also been identified - an adducin-like protein encoded by hu-li tai shao and a protein containing scruin repeats encoded by kelch (Manseau, 1996 and references).
It appears that there is an interaction between the actin and tubulin based components of cytoskeleton. Profilin, encoded by chickadee, a component of the actin based cytoskeleton, physically interacts with Cappuccino, involved in the microtubule based cytoskeleton. Mutants in chickadee resemble cappuccino in that they fail to localize Staufen protein and Oskar mRNA in the posterior pole of the developing oocyte. A strong allele of cappuccino has multinucleate nurse cells, similar to those described for chickadee (Manseau, 1996).
References
Bohrmann, J. and Biber, K. (1994). Cytoskeleton-dependent transport of cytoplasmic particles in previtellogenic to mid-vitellogenic ovarian follicles of Drosophila. time-lapse analysis using video-enhanced contrast microscopy. J. Cell Sci. 107: 849-858. Medline abstract: 8056841
Cooley, L., Verheyen, E. and Ayers, K. (1992). chickadee encodes a profilin required for intercellular cytoplasm transport during Drosophila oogenesis. Cell 69: 173-84. Medline abstract: 1339308
Manseau, L., Calley, J. and Phan, H. (1996). Profilin is required for posterior patterning of the Drosophila oocyte. Development 122: 2109-16. Medline abstract: 8681792
Theurkauf, W. E. (1994). Premature microtubule-dependent cytoplasmic streaming in cappuccino and spire mutant oocytes. Science 265: 2093-96. Medline abstract: 8091233
Walker, R. A. and Sheetz, M. P. (1993). Cytoplasmic microtubule-associated motors. Annu. Rev. Biochem. 62: 429-451
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
Please e-mail comments/corrections to brodyt@codon.nih.gov