Cytoskeleton
Animal cell diagram | |
---|---|
The cytoskeleton is a complex, dynamic network of interlinking
A multitude of functions can be performed by the cytoskeleton. Its primary function is to give the cell its shape and mechanical resistance to deformation, and through association with extracellular
A large-scale example of an action performed by the cytoskeleton is muscle contraction. This is carried out by groups of highly specialized cells working together. A main component in the cytoskeleton that helps show the true function of this muscle contraction is the microfilament. Microfilaments are composed of the most abundant cellular protein known as actin.[10] During contraction of a muscle, within each muscle cell, myosin molecular motors collectively exert forces on parallel actin filaments. Muscle contraction starts from nerve impulses which then causes increased amounts of calcium to be released from the sarcoplasmic reticulum. Increases in calcium in the cytosol allows muscle contraction to begin with the help of two proteins, tropomyosin and troponin.[10] Tropomyosin inhibits the interaction between actin and myosin, while troponin senses the increase in calcium and releases the inhibition.[11] This action contracts the muscle cell, and through the synchronous process in many muscle cells, the entire muscle.
History
In 1903, Nikolai K. Koltsov proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton. The concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was proposed by Rudolph Peters in 1929[12] while the term (cytosquelette, in French) was first introduced by French embryologist Paul Wintrebert in 1931.[13]
When the cytoskeleton was first introduced, it was thought to be an uninteresting gel-like substance that helped organelles stay in place.[14] Much research took place to try to understand the purpose of the cytoskeleton and its components.
Initially, it was thought that the cytoskeleton was exclusive to eukaryotes but in 1992 it was discovered to be present in prokaryotes as well. This discovery came after the realization that bacteria possess proteins that are homologous to tubulin and actin; the main components of the eukaryotic cytoskeleton.[15]
Eukaryotic cytoskeleton
Research into
Accessory proteins including motor proteins regulate and link the filaments to other cell compounds and each other and are essential for controlled assembly of cytoskeletal filaments in particular locations.[26]
A number of small-molecule cytoskeletal drugs have been discovered that interact with actin and microtubules. These compounds have proven useful in studying the cytoskeleton, and several have clinical applications.
Microfilaments
Microfilaments, also known as actin filaments, are composed of linear polymers of
Functions include:
- Muscle contraction
- Cell movement
- Intracellular transport/trafficking
- Maintenance of eukaryotic cell shape
- Cytokinesis
- Cytoplasmic streaming[27]
Intermediate filaments
Intermediate filaments are a part of the cytoskeleton of many
Intermediate filaments are most commonly known as the support system or "scaffolding" for the cell and nucleus while also playing a role in some cell functions. In combination with proteins and desmosomes, the intermediate filaments form cell-cell connections and anchor the cell-matrix junctions that are used in messaging between cells as well as vital functions of the cell. These connections allow the cell to communicate through the desmosome of multiple cells to adjust structures of the tissue based on signals from the cells environment. Mutations in the IF proteins have been shown to cause serious medical issues such as premature aging, desmin mutations compromising organs, Alexander Disease, and muscular dystrophy.[5]
Different intermediate filaments are:
- made of vimentins. Vimentin intermediate filaments are in general present in mesenchymal cells.
- made of keratin. Keratin is present in general in epithelial cells.
- neurofilaments of neural cells.
- made of lamin, giving structural support to the nuclear envelope.
- made of desmin, play an important role in structural and mechanical support of muscle cells.[30]
Microtubules
Microtubules are hollow cylinders about 23 nm in diameter (lumen diameter of approximately 15 nm), most commonly comprising 13 protofilaments that, in turn, are polymers of alpha and beta tubulin. They have a very dynamic behavior, binding GTP for polymerization. They are commonly organized by the centrosome.
In nine triplet sets (star-shaped), they form the
- intracellular transport (associated with dyneins and vesicles).
- the axoneme of cilia and flagella.
- the mitotic spindle.
- synthesis of the cell wall in plants.
In addition to the roles described above, Stuart Hameroff and Roger Penrose have proposed that microtubules function in consciousness.[32]
Comparison
Cytoskeleton type[33] |
Diameter (nm)[34] |
Structure | Subunit examples[33] |
---|---|---|---|
Microfilaments
|
6 | Double helix
|
Actin |
Intermediate filaments |
10 | Two anti-parallel helices/dimers, forming tetramers |
|
Microtubules | 23 | stathmin[35]
|
β-Tubulin
|
Septins
Septins are a group of the highly conserved
Spectrin
Spectrin is a cytoskeletal
Yeast cytoskeleton
In budding
Prokaryotic cytoskeleton
Prior to the work of Jones et al., 2001, the cell wall was believed to be the deciding factor for many bacterial cell shapes, including rods and spirals. When studied, many misshapen bacteria were found to have mutations linked to development of a cell envelope.[40] The cytoskeleton was once thought to be a feature only of eukaryotic cells, but homologues to all the major proteins of the eukaryotic cytoskeleton have been found in prokaryotes.[41] Harold Erickson notes that before 1992, only eukaryotes were believed to have cytoskeleton components. However, research in the early '90s suggested that bacteria and archaea had homologues of actin and tubulin, and that these were the basis of eukaryotic microtubules and microfilaments.[42] Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional structures and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous.[43] Three laboratories independently discovered that FtsZ, a protein already known as a key player in bacterial cytokinesis, had the "tubulin signature sequence" present in all α-, β-, and γ-tubulins.[42] However, some structures in the bacterial cytoskeleton may not have been identified as of yet.[28][44]
FtsZ
FtsZ was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of guanosine triphosphate (GTP), but these filaments do not group into tubules. During cell division, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize the new cell wall between the dividing cells.
MreB and ParM
Prokaryotic actin-like proteins, such as MreB, are involved in the maintenance of cell shape. All non-spherical bacteria have genes encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis.[45]
Some
Crescentin
The bacterium Caulobacter crescentus contains a third protein, crescentin, that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and vibrioid forms of bacteria, but the mechanism by which it does this is currently unclear.[47] Additionally, curvature could be described by the displacement of crescentic filaments, after the disruption of peptidoglycan synthesis.[48]
The cytoskeleton and cell mechanics
The cytoskeleton is a highly anisotropic and dynamic network, constantly remodeling itself in response to the changing cellular microenvironment. The network influences cell mechanics and dynamics by differentially polymerizing and depolymerizing its constituent filaments (primarily actin and myosin, but microtubules and intermediate filaments also play a role).[49] This generates forces, which play an important role in informing the cell of its microenvironment. Specifically, forces such as tension, stiffness, and shear forces have all been shown to influence cell fate, differentiation, migration, and motility.[49] Through a process called “mechanotransduction,” the cell remodels its cytoskeleton to sense and respond to these forces.
The cytoskeleton changes the mechanics of the cell in response to detected forces. For example, increasing tension within the plasma membrane makes it more likely that ion channels will open, which increases ion conductance and makes cellular change ion influx or efflux much more likely.[50] Moreover, the mechanical properties of cells determine how far and where, directionally, a force will propagate throughout the cell and how it will change cell dynamics.[51] A membrane protein that is not closely coupled to the cytoskeleton, for instance, will not produce a significant effect on the cortical actin network if it is subjected to a specifically directed force. However, membrane proteins that are more closely associated with the cytoskeleton will induce a more significant response.[50] In this way, the anisotropy of the cytoskeleton serves to more keenly direct cell responses to intra or extracellular signals.
Long-range order
The specific pathways and mechanisms by which the cytoskeleton senses and responds to forces are still under investigation. However, the
Common features and differences between prokaryotes and eukaryotes
By definition, the cytoskeleton is composed of proteins that can form longitudinal arrays (fibres) in all organisms. These filament forming proteins have been classified into 4 classes.
Tubulin-like proteins are tubulin in eukaryotes and FtsZ, TubZ, RepX in prokaryotes. Actin-like proteins are actin in eukaryotes and MreB, FtsA in prokaryotes. An example of a WACA-proteins, which are mostly found in prokaryotes, is MinD. Examples for intermediate filaments, which have almost exclusively been found in animals (i.e. eukaryotes) are the lamins, keratins, vimentin, neurofilaments, and desmin.[8]
Although tubulin-like proteins share some
Cytoskeletal proteins are usually correlated with cell shape, DNA segregation and cell division in prokaryotes and eukaryotes. Which proteins fulfill which task is very different. For example, DNA segregation in all eukaryotes happens through use of tubulin, but in prokaryotes either WACA proteins, actin-like or tubulin-like proteins can be used. Cell division is mediated in eukaryotes by actin, but in prokaryotes usually by tubulin-like (often FtsZ-ring) proteins and sometimes (Thermoproteota) ESCRT-III, which in eukaryotes still has a role in the last step of division.[8]
Cytoplasmic streaming
Cytoplasmic streaming, also known as cyclosis, is the active movement of a cell's contents along the components of the cytoskeleton. While mainly seen in plants, all cell types use this process for transportation of waste, nutrients, and organelles to other parts of the cell. [54] Plant and algae cells are generally larger than many other cells; so cytoplasmic streaming is important in these types of cells. This is because the cell's extra volume requires cytoplasmic streaming in order to move organelles throughout the entire cell.[55] Organelles move along microfilaments in the cytoskeleton driven by myosin motors binding and pushing along actin filament bundles.[54]
See also
- Nuclear matrix – Fibrillar network lying on nuclear membrane
- Cell cortex – Layer on the inner face of a cell membrane
References
- ISBN 978-1-947172-04-3.
- ^ ISBN 978013399939-6.
- ISBN 978-0-07-352573-0.
- ^ ISBN 978-0-8153-4105-5.
- ^ S2CID 27115011.
- ^ PMID 20110992.
- PMID 9512499.
- ^ PMID 21859859.
- PMID 11156599.
- ^ a b Cooper, Geoffrey M. (2000). "Actin, Myosin, and Cell Movement". The Cell: A Molecular Approach. 2nd Edition. Archived from the original on 2018-04-28.
- ^ Berg JM, Tymoczko JL, Stryer L (2002). "Myosins Move Along Actin Filaments". Biochemistry. 5th Edition. Archived from the original on 2018-05-02.
- ^ Peters RA. "The Harben Lectures, 1929. Reprinted in: Peters, R. A. (1963) Biochemical lesions and lethal synthesis, p. 216. Pergamon Press, Oxford".
{{cite journal}}
: Cite journal requires|journal=
(help) - S2CID 16728876.
- ISBN 978-0-321-93492-5.
- PMID 21859859.
- PMID 32580314.
- PMID 1420928.
- S2CID 17352662.
- PMID 32019166.
- PMID 27600680.
- PMID 19479823.
- PMID 19269181.
- )
- ^ Elsevier. "Discovery of Quantum Vibrations in "Microtubules" Inside Brain Neurons Corroborates Controversial 20-Year-Old Theory of Consciousness". www.elsevier.com. Archived from the original on 2016-11-07. Retrieved 2017-11-20.
- PMID 24070914.
- ISBN 978-0-8153-4464-3.
- ^ a b Cooper, Geoffrey M. (2000). "Structure and Organization of Actin Filaments". The Cell: A Molecular Approach. 2nd Edition. Archived from the original on 2018-05-02.
- ^ PMID 25788699.
- PMID 23270662.
- PMID 15501438.
- ^ a b Lodish, Harvey; Berk, Arnold; Zipursky, S. Lawrence; Matsudaira, Paul; Baltimore, David; Darnell, James (2 May 2018). "Cilia and Flagella: Structure and Movement". Archived from the original on 2 May 2018. Retrieved 2 May 2018 – via www.ncbi.nlm.nih.gov.
{{cite journal}}
: Cite journal requires|journal=
(help) - ^ Hameroff, S. and Penrose, R. Physics of Life Reviews 2014, 11, 39-78
- ^ ISBN 978-1-4160-2328-9. Page 25
- PMID 9438837.
- PMID 17029844.
- ^ S2CID 2418522.
- S2CID 85080734.
- S2CID 53270680.
- PMID 10652251.
- S2CID 14207533.
- PMID 16959967.
- ^ PMID 28137947.
- PMID 16756499.
- PMID 16987173.
- PMID 20223832.
- PMID 22514279.
- S2CID 14459851.
- PMID 20140233.
- ^ S2CID 1287523.
- ^ PMID 16399074.
- PMID 17461730.
- PMID 20110992.
- PMID 20182610.
- ^ PMID 23940314.
- PMID 26464789.
External links
- Cytoskeleton Monthly News and Blog
- MBInfo - Cytoskeleton Dynamics
- Cytoskeleton, Cell Motility and Motors - The Virtual Library of Biochemistry, Molecular Biology and Cell Biology
- Cytoskeleton database, clinical trials, recent literature, lab registry ...
- Animation of leukocyte adhesion (Animation with some images of actin and microtubule assembly and dynamics.)
- http://cellix.imba.oeaw.ac.at/ Cytoskeleton and cell motility including videos
- Open access review article on the emergent complexity of the cytoskeleton (appeared in Advances in Physics, 2013)