Cytoskeleton

Protein fiber network system in eukaryotic cells

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In a narrow sense, the concept of cytoskeleton refers to Eukaryotic cell Protein fiber grid system in（ Microtubule (MT) Microfilament ( microfilament , MF ）And intermediate fiber (IF) structural system Is called“ Cytoskeleton system ”, and the genetic system in cells Biofilm system And called "three systems in the cell". Until the 1960s glutaraldehyde It was fixed at room temperature that the cytoskeleton objective existence , it is Eukaryotic cell The important structure by which to maintain its basic shape is vividly called cytoskeleton, which is also generally considered as Organelle One of. The broad concept of cytoskeleton is nuclear skeleton 、 Cytoskeleton 、 Cytoskeleton and Extracellular matrix The network system formed. Nuclear skeleton 、 Nuclear fiber layer And Intermediate fibre They are connected in structure and run through nucleus and cytoplasm The grid system of.^[1]

Chinese name: Cytoskeleton
Foreign name: cytoskeleton
Discipline: biology

Expression: Protein fiber network structure in eukaryotic cells
Composition: Microtubules, microfilaments, intermediate fibers
Evolutionary characteristics: Highly conservative

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Cytoskeleton^[5]

Cytoskeleton (Cytoskeleton) refers to the protein fiber network structure in eukaryotic cells. It was found late, mainly because the general electron microscope sample preparation is fixed at low temperature (0-4 ℃), and the cytoskeleton will be depolymerized at low temperature. Until the 1960s glutaraldehyde It was fixed at room temperature that the cytoskeleton objective existence 。 The important structure by which eukaryotic cells maintain their basic morphology is figuratively called cytoskeleton, which is also generally considered to be in a broad sense Organelle One of.

Cytoskeleton^[2]

The cytoskeleton not only plays an important role in maintaining cell morphology, withstanding external forces, and maintaining the order of cell internal structure, but also participates in many important Life activities , such as cytoskeleton traction chromosome separation in cell division Vesicle And organelles can be transported directionally along cytoskeleton; stay Muscle cell The cytoskeleton and its Binding protein form dynamic system ； stay white blood cell (Leukocyte) migration, sperm swimming nerve cell axon And the extension of dendrites are related to cytoskeleton. In addition, in the plant cell Medium cytoskeleton guidance cell wall Synthesis of.

microtubule

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microtubule （microtubule） Available in all Mammals Animal cell The diameter is greater than 12nm, except red blood cell (Red blood cell) In addition, all microtubules are composed of about 55kD α and β Tubulin （tubulin） form. When they are normal（ αβ） Dimer It exists in form and polymerizes in the way of head to tail connection to form tubulin fibrils （protofilament） Generally, 13 such fibers form a hollow microtubule with a diameter of 22~25nm. A few variant microtubules such as Nematode All of them have other numbers of raw fibers. Microtubules determine membrane properties Organelle （membrane-enclosed organelle ）Location and as vesicular transport Guide rail of. Microtubules are the backbone of cytoskeleton architecture and also the main body of some organelles, such as Centriole ( centriole ）It is a structure composed of 9 groups of 3-linked microtubules Flagellum (flagellum) is also composed of 9+2 microtubules, that is, 9 Duplex microtubule And a pair of central microtubules, wherein the duplex microtubule is composed of two AB tubes, tube A is composed of 13 fibrils, and tube B is composed of 10 fibrils, and they share 5. Pipe A extends two lines towards the adjacent pipe B Dynein arm And send a spoke to the center of the flagellum. The microtubule composition of the matrix is 9+0, and the dual microtubule is replaced by the triple microtubule.^[3] Constituent Axoneme (axoneme) is the main body.

Purified from various tissues Tubulin It can be found that there are some other protein components (5% - 20%) Microtubule associated protein （microtube associated proteins MAPs）。 These proteins have Tissue specificity , showing that from the same αβ The microtubules formed by dimer polymerization have unique properties, and many kinds of microtubules have been found in different human tissues α and β Microtubulin, and trace the expression of some microtubule genes gene family Some genes are thought to encode unique tubulin.

Some structures formed by microtubules are relatively stable, because microtubule-binding protein And the reason of enzyme modification. Such as nerve cell axon Microtubule fibers in cilia and flagella. Most microtubule fibers are in a state of dynamic polymerization and cataclysm (a sudden, rapid and generally irreversible decomposition), which is necessary for realizing their functions (such as Spindle ）。 And Colchicine (colchicine) bound tubulin can be added to microtubules and prevent other tubulin monomers from continuing to add, thereby destroying the structure of the spindle, Vinblastine It has similar functions. Taxol (taxol) can promote the polymerization of microtubules and stabilize the formed microtubules, but this stability will destroy the normal function of microtubules. These drugs can destroy the microtubule function to prevent cell division and become cancer New hope for treatment.

At least two distinct α- Tubulin and three distinct β- Microtubule genes, which produce tubulin with specific functions mRNA Because these codes are very similar to protein molecules in structural components, how many proteins exist in different tissues Specificity Of has differential expression The microtubulin isoforms of.

except α- And β- Tubulin There are different variants with similar codes, and a variety of new tubulins with greater coding differences have been found, forming different gene family 。 In which gamma tubulin is located in the Microtubule Organization Center (microtubule organizing center, MTOC) is used to provide α and β Tubulin polymerization It forms the starting core of microtubule. Delta and epsilon are considered to be related to Centrosome ( centrosome ）Its structure is related to its formation. Other variants include eta, zeta, theta, etc., but there are only a few Eukaryote Unicellular organism as protozoan or a ciliate It may be related to the unique structure and physiological habits of these organisms, and further details still need to be studied.

Microfilament

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Microfilament （ microfilament ）It also exists in all Eukaryotic cell It is a solid fiber with a diameter of 4nm-7nm. The content of general cells accounts for 1% - 2% of the total protein in cells, but it can account for 20% - 30% in cells with strong activity. In general, cells are mainly distributed on the surface of cells, which directly affects the shape of cells. The microfilament has multiple functions, and its performance is different in different cells muscle cells form Thick myofilament 、 Filamentous muscle It can contract (contractile protein) and mainly plays a supporting role, non muscular movement and information transmission role in non muscle cells.

The microfilaments are mainly composed of Actin (action) composition, and Myosin (myosin, a kind of Molecular motor Proteins) together to make Cell movement 。 They are involved in cellular Amoeba Sports plant cell Of cytoplasmic streaming And Muscle cell Shrinkage of:

Plant cellular cytoplasmic streaming Actin and myosin in microfilaments（ Myosin ）A three-dimensional network system is formed in the cytoplasm. Actin is located in the ectoplasm and myosin is located in the endoplasm. Myosin connects cytoplasmic granules ATP When energy is supplied, the combination of myosin and cytoplasmic particles slides along the actin filament, thus driving the circulation of the whole cytoplasm.

Amoeboid movememt, Amoeba movement ）: Contraction of muscle cells:

like Tubulin , actin gene constitutes a Superfamily And form a variety of very similar structures. For example, various Muscle cell There are different actins: ① skeletal muscle Stripe fiber; ② Striped fibers of myocardium; ③ Vascular wall Of smooth muscle ；④ Smooth muscle of gastrointestinal tract wall. They are amino acid There are slight differences in composition (about 400 Amino acid residue 4-6 variations in the sequence), which still exists in both muscle and non muscle cells β and γ Actins, which are associated with striated α Actin can have 25 amino acid differences.

G-actin monomer (containing ATP) can be polymerized into fibrous F-actin (containing ADP), which can be induced by Mg2+and high concentration of K+or Na+to polymerize ATP hydrolysis It is the monomer of ADP and C-actin ADP, constituting F-actin. In skeletal muscle Filamentous muscle （ thin Filaments, composed of actin) and thick myofilaments（ thick Filiment, by Myosin Composition) Muscle contraction (Myosin can act as ATPase activated by actin). Myosin is also present in mammal In non muscle cells (but in non polymerized state).

Intermediate fibre

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The third kind of cytoskeleton Fibre structure call Intermediate fibre (intermediate filament, IF), also known as intermediate filament Medium fibre , diameter between microtubule and Microfilament (8nm-10nm), its chemical composition is relatively complex. There are as many as 5 kinds of proteins, including Vimentin (vimentin)、 Keratin (keratin)、 Desmin 、 Neurofibril , Nerve Glial fibre 。 In different cells, the composition changes greatly. Intermediate fibers make cells have tension and resistance Shear force 。 The intermediate fibers have the same Basic structure That is to build a central α screw The rod like region is flanked by terminal regions of different size and chemical composition. The diversity of the terminal region determines the difference and specificity of the shape and nature of intermediate fibers.

Above all of these. Structural unit It is not invariable, but highly developed with the life activities of cells Dynamic They are all composed of monomer proteins Noncovalent bond Combine to form fiber type POLYMER , it is easy to assemble and disassemble, which is the necessary feature to realize its function.

prokaryote

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For a long time, it was believed that the cytoskeleton Eukaryote Its unique structure, but recent studies have found that it also exists in bacteria prokaryote Medium.

FtsZ, MreB and CreS that have been found in bacteria are in turn Eukaryotic cell Skeletal protein In Tubulin 、 Actin filament and Intermediate filament similar. FtsZ can cell division Site assembly forms a Z-ring structure and participates in the regulation of cell division through this structure; MreB can form spiral filament structure, and its main functions are to maintain cell morphology and regulate Chromosome segregation Etc; CreS exists in caulobacter crescentus In the concave side of the cell cell membrane Curved filamentous or spiral filamentous structure is formed below, which plays an important role in maintaining the morphology of Bacillus lunaticus cells.

Diseases and hazards

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Cytoskeleton is indispensable in cell life activities Cellular structure , which formed complex network Change and maintenance of cell morphology, cell division and differentiation, intracellular material transport, cell information transfer 、 gene expression Are of great significance. tumour , many Hereditary disease , some Nervous system diseases The occurrence of cytoskeleton is related to the abnormality of cytoskeleton. Clinically, cytoskeleton Specificity The distribution characteristics can be used to diagnose some difficult diseases, and based on the relationship between cytoskeleton and diseases, we can design and guide drug use, and carry out disease prevention and treatment.^[6]

Cells often appear under pathological conditions Cytoskeleton system Exception. For example, in patients with Alzheimer's disease neuron A large number of Distortion Of microtubule And a lot of damaged Intermediate fibre ； In malignant transformed cells, microtubules are often reduced and depolymerized, and cytoskeleton abnormalities can be enhanced cancer cell Of Athletic ability 。 The research shows that the microfilament bundle and its end Adhesive spot And the appearance of actin bodies tumor cell The infiltration and metastasis characteristics are related.^[4]

In addition, the distribution of intermediate fibers has a strict Tissue specificity The vast majority of tumor cells still show the intermediate fiber type of their primary tumors after metastasis, so it can be used as a criterion for differential diagnosis of clinical tumors and whether tumor cells are metastatic. Intermediate fibre Microscopy And amnion Piercing combination can be used for congenital Fetal malformation Diagnosis of, for example, if amniotic fluid With Neurofibril and Neuroglia Fibroblast , it indicates that the fetus may have central nervous system Deformities.

tumour

（1） Changes of cytoskeleton in tumor cells

Each Histiocyte The structure and function of cytoskeleton are closely related. If cytoskeleton changes in assembly or distribution, it will affect the function of cells. In malignant transformed cells, the destruction of cytoskeleton structure, abnormal assembly and distribution, and depolymerization of microtubules are common manifestations.

Chinese scholars gastric cancer 、 Nasopharyngeal carcinoma 、 esophageal cancer 、 Squamous cell carcinoma of lung 、 Small cell carcinoma of lung 、 Adenocarcinoma of lung 、 Mouse sarcoma 9 strains of tumor cells were observed, and the tumor cytoplasmic immunity was found Fluorescent staining The microtubule of cancer cells is reduced or even absent, and the microtubule changes mainly occur in interval , while Fission period ， Spindle microtubule And normal cells Same. use Fluorescent antibody technology It has been proved that microtubules in cancerous cells after long-term passage are significantly reduced. Therefore, the decrease of microtubules is an important sign of malignant transformation of cells. Moreover, the original microfilament bundles in tumor cells were significantly reduced or even disappeared, and actin aggregates often appeared; The distribution of microfilaments in tumor cells is abnormal, disordered, and often inconsistent with cell membrane to be connected.

The invasion and metastasis of tumor cells is an extremely complex process. In this process, changes in some cytoskeleton components can enhance the mobility of cancer cells. Studies have shown that the destruction of microfilament bundles and their end adhesive spots and the appearance of actin bodies may be related to the characteristics of tumor invasion and metastasis. The formation of actin bodies may represent the malignant phenotype of tumor cells with high metastasis.

（2） Intermediate fiber and tumor diagnosis

The intermediate fibers are similar in shape, but have strict tissue specificity. Different types of intermediate fibers are strictly distributed in different types of cells, so they can be distinguished according to the types of intermediate fibers Dermocyte , muscle cells Interstitial cell 、 Glial cell and nerve cell 。 On the basis of these fiber types, intermediate fibers can be further divided into several subtypes. Because most tumor cells continue to maintain the type ultrastructural And immunological characteristics, such as cancer epithelial cells Of Keratin Characteristic, Myoma Yes Desmin Non sarcomas are characterized by waveform fibrin 、 Glioma Is based on nerves Glial fibrillary acidic protein , from sympathetic nerve The tumor is characterized by neurofibrillar protein. Therefore, people can identify and distinguish tumor cells from different tissue sources and tumor cell subtypes according to the types of intermediate fibers Diagnosis of tumor And treatment.

Clinically, intermediate fibrin Fluorescein labeled antibody The technology has a broad application prospect in cell classification, especially in the identification of tumor cells. If used Keratin Fluorescein labeled antibody confirmation Epithelial carcinoma , using Vimentin Identification of lymph nodes by fluorescein labeled antibody sarcoma 、 Melanoma 、 Osteosarcoma The desmin fluorescein labeled antibody was used to confirm sarcomas of muscle cells, and the desmin fluorescein labeled antibody was used to identify sarcomas of muscle cells nerve fiber Confirmation of protein fluorescein labeled antibody Neuroblastoma 、 ganglion Blastoma 、 pheochromocytoma Etc.

（3） Microtubules and filaments and Antineoplastic drugs

stay mitosis Mid/late stage of, colchicine and Vinblastine And other compounds can combine with spindle tubulin or microtubule, Suppressor cell Proliferation. Research shows that drugs with anti mitotic ability such as Paclitaxel and vincristine The main mechanism of inhibiting cell proliferation and killing tumor cells is to stabilize the spindle microtubule dynamics, rather than depolymerize or over polymerize microtubules, inhibit cell division in the middle/late stages of mitosis, and induce Apoptosis 。

Cytochalasin It's fungal Metabolites , which is also the most studied and widely used microfilament specific tool drug. It acts on actin, can bind to the positive end of the microfilament, inhibit its polymerization, depolymerize the microfilament, and lead to cell surface Cortical layer Release, cause cell surface to blister microvilli It turns into stubble, and the overall shape of the cell is dendritic, and can inhibit various microfilament dependent movements, with anti-tumor potential.^[6]

Nervous system diseases

Alzheimer's disease （Alzheimer’s disease， AD ）I.e Alzheimer's disease The disease belongs to microtubule hereditary disease. For patients cerebrospinal fluid The analysis found that in the cerebrospinal fluid of AD patients Tau protein The content is significantly higher than that of non AD patients and normal people, and a large number of injured nerve fibers can also be seen in their neurons, which are composed of paired spiral fibers (paired helical Filament (PHF) and relatively straight fiber (SF). The main component is tau protein in high phosphorylation state, and its performance is stable. Analysis of the brain of dead AD patients found that the number of tubulin in neurons was not abnormal, but microtubule aggregation was defective. Isolated tubulin and bound tubulin can be hyperphosphorylated with other Ligand binding Form stable tau protein. Because microtubules are Axial flow Necessary cytoskeleton, so microtubule aggregation defect in AD can hinder microtubule aggregation, distort microtubules, and may cause axial plasma flow blockage, leading to neuronal fibers inclusion body The formation of signal transmission Disturbance Axon The substance transportation of the neuron causes the nutrition and metabolism of the neuron to be obstructed, which leads to dementia.

The abnormality of Tau protein and other cytoskeleton proteins can also cause other nervous system diseases, such as Motor neuron disease 、 Parkinson's disease , myotonic dystrophy, etc.^[6]

Hereditary disease

some genetic Diseases are often associated with abnormalities of cytoskeleton or mutations of cytoskeleton protein genes. WAS (Wiskoff Aldrich syndrome) is a kind of heredity immunodeficiency Disease, characterized by eczema , bleeding and Repeated infection 。 Research shows that the abnormality of microfilaments is the root cause of WAS.

along with research method And continuous improvement of means, especially the use of transgenic mice or Gene knockout mice It is found that the intermediate fiber is Genetic disease of human beings Hereditary dermatosis Epidermolysis bullosa simplex （ epidermolysis Bullosa simplex (EBS) is the most typical example, which is caused by the mutation of keratin 14 (CK14) gene.^[6]

be senile

Senile disease Scientific research shows that with the increase of age, all cells in the body appear to have low function. This is related to the changes in the number, structure and function of cytoskeleton. Animal experiment It shows that the number of microtubules in neurons of aged animals decreases macrophage The number of microfilaments in the Neural signal Transmission affects the material transport of the axoplasm, affects the nutrition and metabolism of neurons, affects the immune function, and then affects the function of cells. Therefore, the aging of the elderly is manifested by the decline of brain function and the low function of multiple systems such as body immunity.^[6]

Other proteins

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In addition, cytoskeleton also contains many Structural unit Of protein , such as:

Molecular motor (molecular motors): Dynein (dynein), kinesin, myosin

Binding protein ：vinculin, cofilin, tropomyosin wait

In a broad sense, cytoskeleton also includes Nuclear matrix （nucleoskeleton）、 Nuclear fiber layer (nuclear lamina) and Extracellular matrix (extracellar matrix), forming an integration throughout the nucleus, cytoplasm and extracellular network structure 。

Discovery History

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Cytoskeleton refers to Eukaryotic cell Protein fiber network structure in the. The discovery is late, mainly because the general Electron microscope sample preparation It is fixed at low temperature (0-4 ℃), and the cytoskeleton will be depolymerized at low temperature. Until the 1960s, electron microscope samples were prepared by glutaraldehyde People gradually realized that the cytoskeleton objective existence 。 The important structure by which eukaryotic cells maintain their basic morphology is figuratively called cytoskeleton, which is also generally considered to be in a broad sense Organelle One of. The cytoskeleton not only plays an important role in maintaining cell morphology, withstanding external forces, and maintaining the order of cell internal structure, but also participates in many important Life activities , such as: cell division Medium cytoskeleton traction Chromosome segregation , in the transportation of cell materials Vesicle And organelles can be transported directionally along cytoskeleton; stay Muscle cell Cytoskeleton and its binding protein dynamic system ； stay white blood cell The migration of white blood cells, the swimming of sperm, and the extension of axons and dendrites of nerve cells are all related to the cytoskeleton. In addition, cytoskeleton guidance in plant cells cell wall Synthesis of.

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