Summary of Molecular Networks Linked by Moesin Drive Remodeling of the Cell Cortex during Mitosis - Report Example

INTRODUCTION Despite decades of research, and over a century since the first of the process of mitosis, there are many details and permutations of the cell division- mitosis process, a conserved feature of all animal cells which has not yet been entirely elucidated. There are molecular networks within the cell, governing the process that still merit detailed study. The structure of cells during the mitotic process, and the transformations in cell shape as the division continues have been observed by numerous investigators, but our understanding of the precise mechanisms remains incomplete. The Roubinet paper identified two networks that are part of what might be termed a mitotic cortex. They also describe a membrane linkage protein, Moesin provides a facilitating role that integrates the networks of the mitotic cortex, allowing a cooperative interaction vital to the process of mitosis itself. In fruit flies, (Drosophila melanogaster) is a vital cytoskeletal regulator. Study of Moesin, otherwise known as Moe, should prove to be important in the understanding of the bridging between the actin complex and of the cell membrane. The association with the mitosis cortex is also mediated by two enzymes designated; Pten and Skittles. The raw mechanics of cell division, the process by which daughter cells take their shape, during the process of mitosis depends upon the action of actin. The Roubinet paper also describes a coupling of the actomyosin cortex. (similar to the activity of muscular contraction) the beginning of mitosis, the prophase – is characterized by increased hydrostatic pressure and cortical contractility, the combination of which forces a rounded shape to the dividing cell, prior to the metaphase. After which, inequality in the tension of the fibers driving the forces of the metaphase causes greater stress at the sides of the cell, less at the poles of the sphere. This leads to an elongation effect, as the equator of the body contracts and the poles relax, during anaphase. Mitosis discussions often revolve around the ERM. (Erzin, Radixin, and Moesin) ERM is a family of cytoskeletal regulators that dictate the mitosis process. The activation of the ERM complex, typically requires phosphorylation, by way of the phosphoinositol 4,5-bisphosphate (PI(4,5)P2) enzyme. As well as the phosphorylation of a conserved threonine residue (T559). Though phosphorylation is essential, there are still further interactions required involving P I for regulatory purposes. The Roubinet paper uses arguably the most common tool of cellular investigations, mutational dysfunction of the structure under analysis. When Moe in fruit flies is disabled, cells are unable to round properly, and this dysfunction at the beginning of mitosis creates distortions throughout the cell division process. These mutations also impair other functions of the whole cell, such as proper positioning within the tissues, as well as the organization and the segregation of chromosomes. Describing this process becomes essential for understanding how cells arrive at their shapes, how they are organized within tissues, and how dividing cells are coordinated during embryonic growth. METHODS Subsequent studies of the function of Moe, as well as high resolution live image gathering in fruit fly cells demonstrates a process by which Moe controls tension within the mitotic cortex thereby controlling the shape of cells during the cycle of cell division. The first of the aforementioned mitotic networks that form this cell division cortex depends on phosphatase control of kinases to restrict the greatest levels of Moe activity to the earlier phases of cell division. The Pten enzyme is then needed to adjust activity of Moe by producing the PI enzyme, needed for Moe to be appropriately associated with the mitotic cortex and subsequently to be phosphorylated. This burst of Moe activation during early mitosis becomes sort of a linchpin in the rounded shape necessary for proper cell division during metaphase. During anaphase, the activity of Moe adjusts the equatorial sections of the cell, allowing for the elongated shape. Investigators used time lapsed microscopy to document the localization of Moe throughout the cell cycle. Between cycles it remains mostly in the cytoplasm, before relocating to the cell cortex at the beginnings of prophase. Once that the cortex Moe is lost from the poles and gathers around the cells equator. The effects of Moe regulation were also studied through the manipulation of the Slik enzyme, allowing it to be over, or under expressed. The investigators wanted more information on the adaptive system responsible for the localization of Moe. In order to test the regulatory mechanisms that allow the enzyme to function when it is needed and where, the normally functioning enzyme was substituted with a hyperactive mutant MOE-TD GFP. This was done through the insertion of a double-stranded RNA segment targeting the 3` region of the desired cells. It is known that phosphorylation can control Moe activity. To gain more information on which phosphatase is directly responsible for Moe in activation, catalytic subunits in fruit fly genes were inactivated. The results were studied with Western blotting. Two of the catalytic subunits were found to increase phosphorylation, before Pp1-87B was found to down-regulate during Interphase. By charting the formations, and failures of cytokinesis, more data was acquired confirming that the down regulation of Moe at the polar cortex is necessary for proper cell division. We can make this inference based on the observations that Moe, when localized to the equatorial edges of the cell causes contraction, therefore the polar bulges must eventually yield in order for the cell to squeeze itself in two. The distribution of Pp1-87B during a normal cell cycle was then examined, using GFP fusion. This subunit was found to interact with the Slik enzyme, to ensure that the greatest activity of the Moe enzyme was limited to the earliest phases of cell division. RESULTS The effects of this disruption was on high accumulation of Moe around the cortex between cell division cycles. The levels of the enzyme were also higher around the polar regions of the cell during elongation. Investigators also found that the substitution of wild type Moe with the hyperactive mutant impeded cytokinesis half of the time. This was explained by the necessity of anaphase elongation of the cell as essential for proper location and coordination during a growth process. It can be readily concluded that the location of Moe throughout the cortex corresponds to the sites where contraction of the mitotic complex occurs. The elongation in the mutants was found to be defective, the investigators hypothesize that the mis-localization of Moe causes extra rigidity in the mitotic cortex. The Moe was thought to become over active at the poles of the dividing cell, associating heavily with the actin complex. This makes the entire apparatus to rigid, to the extent that the elongation normal for anaphase is prevented. So the conclusion becomes that Moe activity must be reduced at the polar mitotic cortex during anaphase, to allow for the proper shape of the dividing cell – elongation during anaphase, necessary for cytokinesis. Cortical organization is also dependent upon the Moe enzyme. Examinations of dividing cells shows that the absence of the enzyme causes abnormal cytoplasmic bulges, disrupting the normal shape of dividing cells. Also, a disruption of the activation of Moe also causes excessive relaxation during anaphase. All told, the process of cell division depends upon the precision regulation of the Moe enzyme. This is necessary to ensure polar relaxation, as well as complete contraction of the equator or region of the cell. If Moe is in activated, or not localized to the proper area of the mitotic cortex during the right phase, and at the right time de-formations and blebbing will occur. The success or failure of cytokinesis appears dependent on the proper function and localization of this enzyme. DISCUSSION It is clear that through the integration of cellular actinomyosin complexes, Moe activity has a guiding influence upon cell shape, and cytokinesis – inevitable for proper cellular localization during the growth of tissues, and the maturation of animal embryos. The investigators report that the changing patterns of cell membrane, and cortical rigidity – as the cell remodels itself to prepare for fission depends upon the interaction of a Pp1-87B/Slik activation complex with the Moe enzyme. In addition, the PI enzyme, (PI(4,5)P2) acts as a form of spatial control that regulates the distribution of Moe throughout the mitotic cortical complex. The balance, and interconnectedness of these complexes are the defining mechanisms that both ensure proper cellular division, cytokinesis, and transformations in cell shape. The increasing activity of actin-complexed molecules causes the necessary rounding during the beginning of a mitosis at prophase. The ERM proteins produce this impetus; and accordingly these forces (contractions of the cortical network, and the ERM protein complex) must be down regulated at the conclusion of mitosis. Although the Slik enzyme complex homogenously bonds with the cell cortex before cell division, during interphase and early prophase mitosis, the Pp1-87B factor is cytoplasmically localized during interphase and moves to the spindle structure in pro – and metaphase. The investigators describe this as a “constitutive” cortical association of the Slik activator in interphase and pro/metaphase, followed by intracellular relocalization of the Pp1-87B inhibitor factor. This represents an effective means to prevent excessive levels of Moe phosphorylation after the beginning of mitosis. The redistribution of Slik and the Pp1-87B factor is integral to elongation, and it stimulates the concentration of the Moe at the equatorial region of the dividing cell, while down regulating it at the polar region. While this complex has been shown to be crucial, also in the regulation of elongation during anaphase is, the investigators cite other studies that reveal its importance for mitotic spindle morphogenesis; highlighting a more complex role that requires further study. The PI(4,5)P factor provides another means to regulate the Moe enzyme at the mitotic cortex during cell division. This factor is also necessary for the proper localized station of the ERM complex, of which Moe is a component – at the polar regions of the dividing cell The PI(4,5)P factor appears to act as a spatial cue also, for Moe positioning/localization. The distribution of this factor suggests that its localization to the equator of the dividing cell is common during growth and differentiation of animals. The authors also describe the ways in which Moe contributes to cell elongation by providing signals through both time and space during the mitosis process. (Chen et al. 2008) PI(4,5)P, and the Slik complex has a dual role, regulating the means by which Moe stimulates cortical contractility, the authors conclude that this regulatory apparatus insures proper cell division for all animal tissues. REFERENCES Chen, W., M. Foss, K.F. Tseng, and D. Zhang. 2008. Redundant mechanisms recruit actin into the contractile ring in silkworm spermatocytes. PLoS Biol. 6:e209.http://dx.doi.org/10.1371/journal.pbio.0060209 Roubinet, Chantal. Decelle, Barbara. Chicanne, Gaëtan. Dorn, Jonas F. Payrastre, Bernard. Payre, François. Carreno, Sébastien. 2011. Molecular networks linked by Moesin drive remodeling of the cell cortex during mitosis. J. Cell Biol. Vol. 195 No. 1 99–112 www.jcb.org/cgi/doi/10.1083/jcb.201106048 Read More