Microtubule structure and dynamics

By Seán Mac Fhearraigh

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Microtubules are key components of the cytoskeleton, are composed of α-tubulin and β-tubulin and dimerize in a head to tail fashion to form linear protofilaments that associate initially into sheets and subsequently into hollow tubeswith a diameter of roughly 25 nm (1).

Each Microtubule is composed of 13 protofilaments of α-tubulin and β-tubulin heterodimers. Microtubules are highly dynamic polymers that continuously grow and shrink during interphase and mitosis, with the rate of microtubule polymerisation to depolymerisation called the ‘catastrophe rate’ (2). In many cell types, the microtubule network organises in a radial manner, with the minus end of the microtubule embedded close to nucleus and the positive end exploring the cytoplasm (1). During microtubule polymerisation GTP-loaded β-tubulin is added to the plus end of the microtubule. Shortly after the addition GTP is hydrolysed to GDP. Stabilisation of microtubules requires the maintenance of a GTP-bound β-tubulin ‘cap’ to prevent depolymerisation. Removal of the ‘cap’ results in GDP-bound tubulin subunits adopting a curved conformation, the breaking of lateral bonds and microtubule catastrophe (1).

Microtubules are required for the accurate separation of chromosomes during mitosis and the maintenance of genetic integrity. During mitosis microtubules form the mitotic spindle through the formation of a bipolar array of microtubules emanating from the centrosomes or the spindle pole body (3). During prometaphase microtubules emanate from the microtubule organising centres (MTOCs) and extend and shorten until they become amphitelically attached to chromosomes at their kinetochores. Activation of the spindle assembly checkpoint (SAC) during prometaphase prevents cells transitioning to anaphase until correct bipolar attachment of microtubules to chromosome kinetochores occurs (4). However, if the correct microtubule kinetochore attachment is not achieved in a timely manner due to monotelic, syntelic or merotelic attachment of microtubules to kinetochores, the cells arrest in pro-metaphase/metaphase-like state. Failure to resolve defective microtubule-kinetochore attachment can eventually result in cell death (5). 

Completion of metaphase and the onset of anaphase occur following satisfaction of the SAC following correct attachment of microtubules to chromosomes. Finally, during anaphase chromosomes are distributed evenly to polar spindles, allowing each daughter cell to receive the correct genomic information.

Microtubule targeting agents (MTAs)

Taxol (Paclitaxel) was originally isolated from the bark of a yew tree (Taxus brevifola) by Monroe Wall and Mansukh Wani in 1967 (6). Taxol was approved for clinical use in 1995 and is now used to treat a range of malignancies such as breast cancer, kaposi’s sarcoma, ovarian cancer and non small cell lung cancer (7). Taxol binds tubulin heterodimers with a 1:1 stoichiometry along the surface of microtubules, thus, stabilising microtubules and resulting in microtubule bundle formation (8,9)

Due to the highly dynamic nature of microtubules and the key regulatory roles they play in mitosis, compounds that alter microtubule function have been highly successful in cancer treatment. Microtubule targeting agents can be divided into two main groups based on their ability to depolymerise or hyperpolymerise microtubule filaments. Such families of compounds include the taxanes and the vinca alkaloids which were first identified over 40 years ago and members of both drug families are currently in clinical use to treat a range of cancers (7).

The microtubule depolymerising agents, which are capable of inhibiting microtubule polymerisation at high concentrations, include members of the vinca alkaloid family vincristine, vinblastine, vindesine, colchicines, combrestatins and nocodazole. Microtubule destabilisation by depolymerising agents occurs through the binding of the drugs to either the vinca domain or the colchicine domain of β-tubulin, resulting in the inhibition of microtubule dynamics and the destruction of the mitotic spindle (6). 

The suppression of microtubule dynamics by MTAs results in the inhibition of microtubule spindle formation, kinetochore-microtubule attachment and prevents chromosome bi-orientation. This in turn leads to the activation of the SAC, prolonged mitotic arrest and subsequent cell death (10). However, to date the link between prolonged mitotic arrest and the initiation of cell death is poorly understood.

References

• 1. Plus-end-tracking proteins and their interactions at microtubule ends. Galjart N. [Curr Biol. 2010;20(12):R528-37]
• 2. Cell biology. Microtubule catastrophe. McIntosh JR. [Nature. 1984;312(5991):196-7]
• 3. Cell biology. A gradient signal orchestrates the mitotic spindle. Clark PR. [Science. 2005;309(5739):1334-5]
• 4. The spindle-assembly checkpoint in space and time. Musachio A, Salmon ED. [Nat Rev Mol Cell Biol. 2007;8(5):379-93]
• 5. Cyclin B1 interacts with the BH3-only protein Bim and mediates its phosphorylation by Cdk1 during mitosis. Mac Fhearraigh S, Mc Gee MM. [Cell Cycle. 2011;10(22):3886-96]
• 6. Microtubules as a target for anitcancer drugs. Jordan MA, Wilson L. [Nat Rev Cancer. 2004;4(4):253-65]
• 7. Microtubule-binding agents: a dynamic field of cancer therapeutics. Dumontet C, Jordan A. [Nat Rev Drug Discov. 2010;9(10):790-803]
• 8. Resistance to Taxol in lung cancer cells associated with increased microtubule dynamics. Gonçalves A, Braguer D, Kamath K, Martello L, Briand C, Horwitz S, Wilson L, Jordan MA. [Proc Natl Acad Sci USA. 2001;98(20):11737-42]
• 9. Structure of tubulin at 6.5 A and location of the taxol-binding site. Nogales E, Wolf SG, Khan IA, Ludueña RF, Downing KH. [Nature. 1995;375(6530):424-7]
• 10. Cancer cells display profound intra and interline variation following prolonged exposure to antimitotic drugs. Gascoigne KE, Taylor SS. [Cancer Cell. 2008;14(2):111-22]