Mitosis, Cytokinesis and the Cytoskeleton CS6

When observed under time-lapse photography, the rounding up of the cell, the break up of the nuclear membrane, the nuclear condensation of the “line-dance” of the chromosomes, and the eventual splitting of the cells seems directed by an unseen intelligence. Yet of course the whole program is scripted by genes. Most animal cells divide by forming a constriction ring that progressively tightens to pinch the daughter cells apart; plant cells, fungi and fission yeast form a septum between the two new cells. Budding yeast do something in between by producing a bud off the parent cell that expands and is then pinched off. The cytoskeleton has a very large part to play in all of this, and it is likely that the original purpose in having a cytoskeleton was to direct the distribution of genomes at mitosis.

Figure 37. The Cell Cycle. 1.The centrosome divides. 2.The nuclear envelope breaks down, chromosomes condense, spindle poles form. 3. Cleavage furrow (contractile ring) assembles. 4. Cleavage furrow contracts as chromosomes move off towards the poles. 5. Mid-body forms as the cleavage furrow narrows further, the chromosomes de-condense and the nuclear envelope reforms. 6. Cytokinesis is now complete, sometimes the mid-body is left behind. Each daughter cell grows in size to complete the cycle.

The Centrosome

The centrosome (or microtubule organising centre, MTOC) is so called because it usually occupies the centre of the cell or close by. It is a very complex organelle, capable of replication. The structure of the centrosome is not yet fully characterised (Figure 38). Two short bundles (centrioles) containing odd triple microtubule-like arrays are evident, these are at approximately 90^o w.r.t. each other. Vague filamentous material connects these

Figure 38

Microtubule

Pericentriolar material

Centrioles

.

Figure 39. Centrosome duplication.

CdcK2-cyclin E is required for centrosome duplication that starts in G1 (2). In the S phase each centriole grows a daughter centriole close to its base at about 90^O to it (3). This is complete by G2 and two complete centrosomes are formed by growth of the accessory protein structures such as the pericentriolar region and filaments. At M phase, (4) the two centrosomes move apart to occupy opposite ends of the spindle. Cytoskinesis then follows bringing the now single centrosome back to the start of the cycle (1).

Cdk2-cyclin E activity is required for the splitting of the two centrioles, these then form nuclei onto which the other components add at subsequent stages. The other cyclins active at the other stages are shown, but it is not yet known how or even if, these co-ordinate centrosome duplication. Under certain circumstances, centrosomes can originate spontaneously in the cytoplasm, so there no absolute requirement for pre-existing centrioles. This is unlike most organelles such as mitochondrial where a cell can only grow new mitochondria from an old one. The centrosome is only half way from being an organelle and a protein complex.

Microtubule-based motors co-operate to form, maintain and regulate bipolar spindles.

In addition to binding membranes, many microtubule motors also bind other microtubules with the non-motor end and so can bind two microtubules. Minus end directed motors such as dyneins can therefore gather minus ends together (Figure 40a). These simple “rules” if applied at the correct time give rise to fairly complex geometries.

A

Likewise, plus end directed motors (Figure40b) can sort out microtubules arrays bound at their minus ends by dyneins into anti-parallel arrays:-

B

Figure 40

The position of the cleavage furrow is determined by anti-parallel microtubule arrays (usually the spindle). Classic experiments by Rappaport demonstrated the importance of microtubule arrays in the determination of the position of the cleavage furrow.

Figure 41. 1. A glass needle was used to create a hole in a cell that was about to divide. 2. A cleavage furrow appeared between the separated chromosome masses as usual but the cell did not separate because of its shape. 3. At the subsequent round of division spindle poles were created at the two chromosome sets but an addition “spindle” was created by the interaction of the back to back astral microtubules. 4. This extra spindle now dictates the formation of an extra cleavage furrow. Note that this results in four cells as would have arisen without the glass rod intervention.

What is the signal from the spindle that stimulates furrow formation?

Several candidates have been proposed to constitute this signal. These include the protein INCENP (Eckley et al, 1997) a chromosome passenger inner centromere protein that is one of the very earliest (if not the earliest) components of the cleavage furrow, before any accumulation of myosin II. The properties that such a protein would need are to be able to recognise anti-parallel microtubules (in order to locate the spindle pole), and some sort of regulatory activity, possibly a kinase, that would result in the accumulation of active components of the contractile ring?

Constituents of the cleavage furrow.

The cleavage furrow is composed primarily of actin arranged as concentric anti-parallel filaments. Many actin bundling proteins are known to concentrate within the cleavage furrow, while others are excluded. It is important for the contractile function of the furrow to have bundling proteins that bundle filaments in an anti-parallel fashion (to allow myosin II to act on them) and to bundle with a spacing compatible with myosin II mini-filament interaction.. Other molecules in the furrow include G-protein Rho, Rho kinases and many myosin kinases. Myosin II is a crucial for contraction and so is regulated by many different kinases at different stages. When the furrow is forming protein kinase C phosphorylates the myosin preventing its activity. Then Cdc2 activates myosin and the contractile activity squeezes the furrow. The furrow does not gain in thickness as it contracts and so a net loss of actin and other constituents must take place. The cofilins are important in this activity, and cytokinesis is impaired in cofilin mutants. The actin binding/severing activity of cofilin is regulated by phosphorylation by LIMkinase1 and 2, but its not yet clear if either of these kinases is involved in the regulation of cofilin at cytokinesis. In addition to myosin and actin the following ABPs are found in the cleavage furrow:-

Protein	Function
Radixin	A member of the ERM group, radixin binds the contractile ring to the membrane.
Cofilin	Depolymerises actin filaments as contractile ring contracts. Under control of the LIM kinases.
a-actinin	Bundles filaments together. Can bundle microfilaments in a parallel or anti-parallel fashion.
Talin	Binds the contractile ring to the membrane with radixin.
Cortexillins	Bundling proteins from Dictyostelium with a PIP2 binding C-terminal region important for targeting to the cleavage furrow and cytokinesis.

Two models have been forwarded to explain how pole-ward microtubule flux drives the chromosomes towards the poles.

			Minus end directed motor protein in the kinetochore hydrolyses ATP to move along the attached MT
		Kinetochore microtubule
	An alternative view is that kinetochore behaves like a thermal ratchet (compare this with Figure CS3). This ratchet works in reverse mode, with the kinetochore “chasing” the microtubule end.

Kinetochore microtubule

Depolymerisation causes the kinetochore to move as it binds to the MT end every so often as it vibrated in a Brownian fashion.

Figure 42

An unconnected (lost) kinetochore can stop the whole process until it is found by a microtubule and pulled back into line with the others at the mid-zone. Then the next step, chromosome separation can occur. How this works is largely a mystery!

Figure 43.

The role of Myosin II in cytokinesis

The conventional, two headed myosin, myosin II, is involved at every stage of cytokinesis and all cytoskeletal changes that occur before and after the actual separation process.

	Separation of cells

Figure 44. The role of myosin II at all stages of cytokinesis. 1. As the cell rounds up at the onset of cytokinesis, myosin moves from the depolymerizing stress fibres into the cortex of the rounding cell. Myosin II helps this process by loosening the stress fibres and by contraction of the gathering cortex. 2. The main job is to contract and form the cleavage furrow. 3. On completion of cytokinesis, the cell is still attached to the substrate by focal adhesion rudiments through “retraction filaments”. 4. The cell then pulls itself back to the substrate using myosin II to work down the actin bundles within the retraction filament. Note that the filaments in the retraction filament are polar, with the pointed ends all facing the cell body, the correct orientation for a productive interaction with myosin II. 5. Myosin continues to spread the cell, and the stress fibres are re-established.

Figure 45. The Wnt Pathway. The Wnt pathway connects mitosis to the cell-cell connectivity. This is how the cell “knows” what shape it is in and how the shape affects the cell cycle. APC = Adenomatous Polyposis Coli protein.

References

Heald, R. & Walczak 1999 Microtubule-based motor function in mitosis. Curr.Op.Cell Biol. 9, 268-274.

Wolf, W.A., Chew, T.-L. & Chisolm, R.L. 1999 Regulation of cytokinesis. Cell.Mol.Life Sci. 55, 108-120.

Rieder, C.L. & Slamon, E.D. 1998 The vertebrate cell kinetochore and oits role during mitosis. Trends Cell Biol. 8, 310-318.

Eckley, D.M., Ainszein, A.M. Mackay, A.M., Goldberg, I.G. & Earnshaw, W.C. 1997. Chromosome proteins and cytokinesis Patterns of cleavage furrow formation and inner centromere positioning in mitotic heterokaryons and mid-anaphase cells. J.Cell Biol. 136, 1169-1183.

Wheatley, S.P.1999 Updates on the mechanics and regulation of cytokinesis in animal cells Cell Biol.Int. 23, 797-803.

Gumbiner, B.M 1997 Carcinogenesis: A balance between b-catenein and APC. Curr.Biol. 7, R443-R446.

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