Role of Cytoskeleton in Cell Division

Some Key Points        What keeps a cell from a jelly-like melt down?
                                How do the chromosomes walk to the opposite ends of a cell?
                                Are muscles involved in the movements of chromosomes in cells?

The cytoskeleton is a system of microscopic molecular filaments, present in the cytoplasm of all nucleated eucaryotic cells. The cytoskeleton provides an architectural framework upon which the cell can organize the subcell organelles and the metabolic machinery.  It is responsible for sustaining a cell's shape, for the locomotion of a cell, and for the movement of the various organelles within the cell itself. Besides the intracellular movement of organelles, the cytoskeleton is responsible for chromosome movement during cell division and for cytokinesis. This intricate network of protein filaments, which runs throughout cytoplasm, makes up a dynamic, continuously reorganizing framework, not a static scaffolding.

Two structural components of the cytoskeleton are required to efficiently separate the replicated chromosomes during cell division:
        1) the Mitotic Spindle, an array of microtubular proteins, formed in late G-2 after the centrosomes duplicate.  One end of the mitotic spindle microtubules is anchored in the centrosome and grow outward cross-linking to microtubules from other pole's centrosome. and
        2) the Contractile Ring, an overlapping array of actin/myosin proteins (like the sliding filaments found in muscle).  The ring, which is responsible for cytokinesis, becomes smaller, as it closes like a camera diaphragm, finally dissecting the cell's cytoplasm into two separate domains.

 Three major types of filaments make up the cytoskeleton: actin filaments, microtubules, and intermediate filaments. Actin filaments (microfilaments) occur in a cell in the form of helical polymers of the protein G-actin.  They make bundles of two flexible parallel fibers with a diameter of 7nm. Actin is a highly conserved protein and even though it is universally dispersed through the cytoplasm, actin filaments are most concentrated adjacent to the plasma membrane. They aid in the establishment of shape of the cell and also help a cell adhere to the substratum. Intermediate filaments, in contrast to actin filaments and microtubules, are very stable structures that form a true skeleton in a cell. They are twisted-pair-like fibers made from an assorted complex of proteins. Running across the cytoplasm these filaments provide mechanical strength. In epithelial cells the intermediate filaments span across cell-to-cell junctions providing significant mechanical support. Intermediate filaments are especially important in places where tensile strength is critical. The variety of intermediate filaments includes keratin filaments in human skin cells, neurofilaments in nerve cells, and desmin filaments in muscle cells. The nuclear lamina is a network of intermediate filaments just below the inner nuclear membrane. Intermediate filaments anchor the nucleus and position it within the cell, and they give the cell its elastic properties and its ability to withstand tension.

Microtubules ( MT) are long, hollow filaments made of globular monomeric proteins, "alpha and beta-tubulin. MT's are more rigid than the other filaments and have a diameter of 25nm. MT's are continuously assembling and disassembling. They have one end attached to the centrosome, a Microtubule Organizing Center ( MTOC). Located near nucleus in animal cells, the MTOC anchors the non-growth end (-) of MT's and thus is the origin site of new MT's.

        MT's grow from a third type of monomer, ( alpha tubulin, which forms an entity called ring tubulin within a centrosome.  Growth and shrinkage of MT elements involves a dynamic (in/out) process called Dynamic instability, which is characterized by alternating polymerization and depolymerization of  "apha and bata-tubulin growth and shrinkage.  Tubulin subunits contain GTP hydrolytic activity [GTP --> GDP + P].  Tubulin monomer-GTP subunits rapidly bind together at their growing end (the + end). The monomer subunits assemble faster than the GTP can be split, producing a region called the GTP cap, which prevents depolymerization, thereby effecting MT growth.

        As the tubulin subunits hydrolyzes the GTP, to tubulin-GDP + P, the cap becomes unstable and the tubulin-GDP disassembles, resulting in autocatalytic shrinkage.  The net result of dynamic instability is that the MTOC (centrosome) continually forms new MT's.  Growth of MT's from the centrosome occurs in all directions (360 ^o), until a MT hits something, attaches to it, and forms the basis of the cytoskeleton, the architectural superstructure of cells and the highway of intracellular movement.

Much of our understanding of MT assembly and disassembly comes from the action of drugs that inhibit MT formation. Colchicine, is a drug obtained from the crocus and other plants, that disrupts normal cell division by stopping mitosis at the point where MT's bind to and separate chromosomes. Colchicine binds to free tubulin and prevents assembly of a MT cap. Taxol, currently used as an anticancer drug, seems to work by binds to MT's and favoring disassembly, thereby preventing cell division.

            The mitotic spindle, is the name given to the complex of centrioles, centrosomes, and entire array of MT and associated proteins that from between the poles of a cell during nuclear division. The role of the mitotic spindle is to separate the chromosomes. In  animal cell late S phase, the centrosome duplicates. In prophase, the two centrosomes separate to opposite poles of the cell and MT's grow from each centrosome. Their rate of formation during mitosis may be up to 20 times faster, than under normal cellular conditions.  MT's grow in all directions forming three types of spindle MT's. MT's growing from each pole form unattached (plain) MT's.  If the growing MT's from opposite poles interact and bind together with associated proteins, they form polar MT's. In prometaphase, MT's which bind to a kinetochore (the protein complex found in centromere constriction of a pair of chromatids), make up the kinetochore MT's.

        During anaphase, proteolytic enzymes sever the protein links between sister chromatids, and they are pulled and pushed apart by the MT's of mitotic spindle. Rates of 1:m per min have been recorded. A number of forces are at work in the movement of the chromatids to the opposite poles. One force has kinetochore MT shortening by depolymerization of tubulin subunits at the kinetochore end, thus moving chromatids poleward. Another force has polar MT's increasing in length by polymerization thus moving the spindle poles apart. Two additional forces are involved here: 1) a pushing force in which elongation of polar MT's occur by polymerize of MT subunits at their + end, and 2) a pulling motion in which the unattached MT's are depolymerizing, pulling the spindle poles toward the cell's cortex.

            The cleaving of the cytoplasm into two domains begins in anaphase and is achieved by the constriction of a contractile ring. The ring forms just below the membrane's surface and is composed of overlapping arrays of actin and myosin filaments. Through the same sliding-filament mechanism responsible for muscle contraction, the filaments contract constricting the ring, while forming the cleavage furrow. The furrow appears as a puckering in of the plasma membrane along the axis perpendicular to the mitotic spindle apparatus.

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