Microtubules are an essential component of the cytoskeleton present in all eukaryotic cells. These dynamic filamentous structures play crucial roles in various cellular processes, such as cell division, intracellular transport, and maintenance of cell shape. Microtubule inhibitors are a class of drugs that target and disrupt the normal dynamics of microtubules within cells. By interfering with the microtubule polymerization and depolymerization processes, these inhibitors can inhibit cell division and induce cell death. A well-known example of a microtubule inhibitor is paclitaxel, which stabilizes microtubules and prevents their disassembly. Another example is colchicine, which binds to tubulin and inhibits microtubule formation.

What is a Microtubule?

Microtubules (MT) are long tubular organelle structures assembled by tubulin in all eukaryotic cells, with an average outer diameter of 24 nm. The length can be changed by the assembly and disassembly of its subunits. It is sensitive to low temperature, high pressure and colchicine. Intracellular microtubules are distributed in a network or bundle, and can be assembled with other proteins into structures such as spindles, basal bodies, centrioles, flagella, cilia, axons, neural tubes, etc., and participate in the maintenance of cell morphology, cell movement and cell division.

Structure of a Microtubule

Microtubules are composed of two types of tubulin subunits, namely α-tubulin and β-tubulin. Their amino acid sequences have been determined. α-tubulin contains 450 amino acid residues and its molecular weight is 50 kD, β-tubulin contains 455 amino acids. Both α- and β-tubulin contain acidic C-terminal sequences. With very few exceptions, such as human red blood cells, microtubules are present in the cytoplasm of almost all eukaryotic cells from amoeba to higher animals and plants, while there are no microtubules in all prokaryotes. Tubulin molecules may be one of the most stable protein molecules in biological evolution. α-tubulin and β-tubulin form tubulin heterodimers, which are the basic units of microtubule assembly. Tubulin dimers contain two binding sites for guanine nucleotides, and divalent cations can also bind to tubulin dimers. In addition, tubulin dimers have a colchicine binding site and a vinblastine binding site.

Microtubules are long tubular organelles assembled from tubulin dimers, with an average outer diameter of 24 nm and an inner diameter of 15 nm. The microtubule wall is composed of 13 arranged protofibrils. In cross section, microtubules are hollow, and the microtubule wall is composed of 13 arranged protofibrils. Microtubules can be assembled into single tubes, double tubes (in cilia and flagella), and triple tubes (in centrioles and matrix). There are also some microtubule-attached structures in the cell, such as the motor protein arms in cilia or flagella. The functions of microtubule-attached structures are: stabilizing microtubules; forming connections between microtubules so that microtubules are arranged in a certain way; connecting microtubules with other structures, mainly membrane structures.

Microtubule Inhibitor

Microtubule inhibitors are a class of drugs that target microtubules. These inhibitors disrupt the dynamic assembly and disassembly of microtubules, thereby interfering with key cellular functions such as cell division, intracellular transport, and maintenance of cell shape. As a result, microtubule inhibitors have become important chemotherapeutic drugs for the treatment of various cancers and other diseases characterized by uncontrolled cell growth.

The most well-known class of microtubule inhibitors are taxanes, which include drugs such as paclitaxel and docetaxel. Taxanes bind to microtubules and promote their assembly, thereby stabilizing microtubule structure and inhibiting their dynamic behavior. This disrupts normal mitotic spindle function during cell division, ultimately leading to cell cycle arrest and cell death. Taxanes are commonly used to treat breast cancer, ovarian cancer, and lung cancer, among others. Another class of microtubule inhibitors are vinca alkaloids, which include drugs such as vincristine and vinblastine. Vinca alkaloids bind to tubulin subunits in microtubules and prevent their polymerization, leading to microtubule depolymerization and disruption of mitotic spindle formation. This leads to cell cycle arrest and ultimately cell death. Vinca alkaloids are used to treat a variety of cancers, including leukemias, lymphomas, and solid tumors such as breast and lung cancer. In addition to the taxanes and vinca alkaloids, there are other classes of microtubule inhibitors with varying mechanisms of action. For example, colchicine and its derivatives bind to tubulin and inhibit microtubule assembly by promoting the formation of tubulin dimers that are unable to polymerize. This disrupts cell division and other microtubule-dependent processes, leading to cell death. Colchicine derivatives are used to treat gout and familial Mediterranean fever, as well as in research as experimental anticancer agents.

Microtubule Function

In eukaryotic cells, actin forms a unique microfilament skeleton structure under the cooperation of microfilament binding proteins, which is related to many important functional activities in cells, such as muscle contraction, deformation movement, cytokinesis, etc. In recent years, it has been found that the microfilament skeleton network system is related to cell signal transmission, and some microfilament binding proteins, such as vinculin, are substrates for protein kinases and oncogene products. The relationship between polyribosomes and protein synthesis and microfilaments has also begun to attract attention.

• Maintaining Cell Morphology

Maintaining cell morphology is the earliest confirmed function of microtubules. Treating cells with colchicine destroyed microtubules and caused the cells to become round, indicating that microtubules are important for maintaining the asymmetric shape of cells. Microtubules also play a key role in the formation and maintenance of cell protrusions such as cilia, flagella, and axons.

• Intracellular Transport

The interior of eukaryotic cells is a highly regionalized system. The synthesis site and functional site of substances in cells are often different, and they must go through the intracellular transport process. The transport of nerve axons and the transport of pigment granules in pigment cells are two of the most intuitive examples, indicating that the cytoskeleton, especially microtubules, plays a key role in intracellular transport.

• Neuronal axonal transport: The proteins and membrane structures in neuronal axons are transported after synthesis by the cell body. On the one hand, cytoskeleton fibers move through axons as slow transport components, and on the other hand, they serve as tracks for fast transport. Microtubules can serve as tracks for the transport of Golgi bodies and other vesicles and particles, with a transport speed of up to 2 μm/s.
• Transport of pigment granules: Many amphibians contain specialized pigment cells in their skin and fish scales. Under neuromuscular control, the pigment granules in these cells can be quickly distributed to all parts of the cell within a few seconds, thereby darkening the skin color; they can also be quickly transported back to the center of the cell, making the skin color lighter to adapt to changes in the environment. Studies have found that the transport of pigment granules is microtubule-dependent, and pigment granules are actually transported along microtubules.

• Flagellar Movement and Ciliary Movement

Cilia and flagella are specialized structures on the cell surface that have movement functions. The structures of cilia and flagella are basically the same. The ciliary axis contains a bundle of parallel microtubules arranged in a "9+2" pattern. The central microtubules are all complete microtubules. The peripheral doublet microtubules are composed of A and B sub-fibers. The A sub-fiber is a complete microtubule surrounded by 13 spherical subunits. The B sub-fiber is composed of only 10 subunits, and the other 3 subunits are shared with the A sub-fiber.

• Spindle and Chromosome Movement

When cells enter the mitosis phase from the interphase, the cytoplasmic microtubule network of interphase cells collapses, microtubules depolymerize into tubulin, and reassemble to form the spindle, mediating the movement of chromosomes. At the end of the mitosis phase, the spindle microtubules depolymerize into tubulin, and reassemble to form the cytoplasmic microtubule network. Spindle microtubules can be classified as follows: (1) centromere microtubules: microtubules connecting the centromere to the two poles; (2) polar microtubules: continuous microtubules from one pole to the other; (3) midbody microtubules: microtubules between daughter chromosomes; (4) astral microtubules: microtubules that make up the astral body.

• Basal Body and Centriole

The centrosome is the main microtubule organizing center in animal cells. Spindle microtubules and cytoplasmic microtubules radiate from the centrosome. The centrosome is composed of a pair of mutually perpendicular centrioles. The root of flagella and cilia is called the basal body. Both the basal body and the centriole are microtubule structures, cylindrical in shape, with an average size of 0.2-0.5 μm. Its wall is composed of 9 groups of microtubule triplets, subfiber A is a complete microtubule, and subfibers B and C are incomplete microtubules.

Subfibers A and B cross the ciliary plate and continue with the corresponding subfibers in the ciliary axis, and subfiber C ends near the ciliary plate or basal plate. Centrioles and basal bodies are homologous and can transform into each other at certain times. Both centrioles and basal bodies have self-replication properties. The basal body contains a DNA molecule with a length of 6000-9000 kb, which encodes several proteins necessary for the function of the basal body. Whether centrioles contain DNA remains to be confirmed. Generally, new centrioles are replicated from the original centrioles during the S phase, and in some cells centrioles can self-generate.

Microfilament vs Microtubule

Microfilaments, also known as actin filaments, are thin, flexible filaments composed of actin monomers arranged in a helical structure. These filaments are typically about 7 nanometers in diameter, making them the thinnest components of the cytoskeleton. Microfilaments are highly dynamic and can be assembled and disassembled rapidly in response to cellular signals, allowing cells to quickly change shape and move. They are involved in various cellular processes, including cell division, cell migration, cytokinesis, and cell shape maintenance. One of the key functions of microfilaments is to generate the forces required for cell movement. Actin filaments interact with myosin motor proteins to generate contractile forces that power processes such as muscle contraction and cell migration. Microfilaments also form structures such as filopodia and lamellipodia, which are essential for cell movement and sensing the environment. In addition, microfilaments play a vital role in intracellular transport, acting as tracks for motor proteins to transport cargo within the cell.

With a diameter of roughly 25 nanometers, microtubules are bigger and more rigid than microfilaments. Tubulin subunits are stacked in a configuration like a hollow tube to form microtubules. Microtubules are polar, with one end referred to as the "plus end" and the other as the "minus end," unlike microfilaments. The directionality of intracellular transport and other cellular functions depend on this polarity. Microtubules play a role in the organization of the cell's internal architecture, creating a network that supports the structure and keeps the cell shaped. They also act as pathways for motor proteins, such dyneins and kinesins, which move vesicles, organelles, and other cell cargo to particular parts of the cell. Microtubules are essential for intracellular transport as well as cell division, as they create the mitotic spindle, a structure that divides chromosomes during the process.

Although they both play a crucial role in intracellular transport, shape preservation, and cell mobility, microfilaments and microtubules are distinct structures, functions, and entities within the cytoskeleton. Whereas microtubules are larger, more rigid structures that organize the internal structure of the cell, provide structural support, and act as channels for intracellular transport and cell division, microfilaments are thin, flexible filaments that are involved in producing the contractile forces for cell movement. Together, these two parts enable the myriad dynamic processes that take place in eukaryotic cells.