Cytoskeleton PDF

CYTOSKELETON Nuriye Ezgi BEKTUR AYKANAT Department of Histology and Embryology Faculty of Medicine ATILIM UNIVERSITY 1  The cytoplasm of animal cells contains a cytoskeleton, an intricate three- dimensional meshwork of prote...

CYTOSKELETON Nuriye Ezgi BEKTUR AYKANAT Department of Histology and Embryology Faculty of Medicine ATILIM UNIVERSITY 1  The cytoplasm of animal cells contains a cytoskeleton, an intricate three- dimensional meshwork of protein filaments that are responsible for the maintenance of cellular morphology. Additionally, the cytoskeleton is an active participant in cellular motion, whether of organelles or vesicles within the cytoplasm, regions of the cell, or the entire cell. The cytoskeleton has three components: Thin filaments (microfilaments), Intermediate filaments, and Microtubules. 2  Thin filaments Thin filaments (microfilaments) are composed of two chains of globular subunits, G-actin, coiled around each other to form a filamentous protein, F-actin. 3  Actin constitutes about 15% of the total protein content of non-muscle cells. Only about half of their total actin is in the filamentous form because the monomeric G-actin form is bound by small proteins, such as profilin and thymosin, which prevent their polymerization. Actin molecules, present in the cells of many different vertebrate and invertebrate species, are very similar to each other in their amino acid sequence, attesting to their 4 highly conserved nature.  Thin filaments are 7 nm thick and possess a faster growing plus end (barbed end) and a slower- growing minus end (pointed end). When the actin filament reaches its desired length, members of a family of small proteins, capping proteins, attach to the plus end, terminating the lengthening of the filament. The process of shortening of actin filaments is regulated in the presence of ATP, ADP, and Ca2+ by capping proteins, such as gelsolin, which prevent polymerization of the filament. 5  There are three classes of actin: α-actin of muscle, and β-actin, and γ-actin of non-muscle cells. Although actin participates in the formation of various cellular extensions as well as basic composition is unaltered. It is capable of fulfilling its many roles via its association with different actin binding proteins. The most commonly known of these proteins is myosin, but numerous other proteins, such as α-actinin, spectrin, fimbrin, filamin, gelsolin, and talin, also bind to actin to perform essential cellular functions. 6  Actin filaments form bundles of varied lengths, depending on the function that they perform in non-muscle cells. These bundles form three types of associations:  Contractile bundles  Gel-like networks  Parallel bundles 7  The cell cortex is the main determinant of cell shape and therefore plays a fundamental role in processes such as cell division, migration, and tissue morphogenesis. Stress fibers are contractile actin bundles found in non-muscle cells. Lamellapodim can show differentiate to filopodia for cell movement/migration. The contractile ring is a ring-shaped structure located just beneath the plasma membrane at the future division site in many eukaryotic cell types. 8  Contractile bundles are usually Contractile bundles associated with myosin. Their actin filaments are arranged loosely, parallel to each other, with the plus and minus ends alternating in direction. These assemblies are responsible for movement not only of organelles and vesicles within the cell but also for cellular activities, such as exocytosis and endocytosis, as well as the extension of filopodia and cell migration. The myosin associated with these contractile bundles may be one of several types: myosin-I through myosin IX. 9  Actin is also important in the establishment and maintenance of focal contacts of the cell with extracellular matrix. The mode of attachment involves integrin (a transmembrane protein) binding to fibronectin and talin, which contacts both vinculin and the actin filament. 10  Gel-like networks Gel-like networks provide the structural foundation of much of the cell cortex. Their stiffness is due to the protein filamin, which assists in the establishment of a loosely organized network of actin filaments. 11 Parallel bundles The proteins fimbrin and villin are responsible for forming actin filaments into closely packed parallel bundles that form the core of microspikes and microvilli, respectively. These bundles of actin filaments are anchored in the terminal web, a region of the cell cortex composed of a network of intermediate filaments and the protein spectrin. 12 13  Intermediate Filaments Electron micrographs display a category of filaments in the cytoskeleton whose diameter of 8 to 10 nm places them between thick and thin filaments and are consequently named intermediate filaments. 14  These filaments and their associated proteins accomplish the following: Provide structural support for the cell Anchor the nucleus in place Provide an adaptable connection between the cell membrane and the cytoskeleton Furnish a structural framework for the maintenance of the nuclear envelope as well as its reorganization subsequent to mitosis. 15  The tensile forces produce distortion of the cytoskeleton, with resultant deformation of the nucleus and rearrangement of the nucleoli. Cytoskeleton, specifically the intermediate filaments, reacts to forces generated in the extracellular matrix by forcing modulations in the shape and location of cellular constituents, they protect the structural and functional integrity of the cell from external stresses and strains. 16  There are several categories of intermediate filaments that share the same morphological and structural characteristics. These rope-like intermediate filaments are constructed of eight tetramers of rod-like proteins that are tightly bundled into long helical arrays. The individual subunit, the monomer, of each tetramer differs considerably for each type of intermediate filament, but their morphology is similar in that each monomer has an N-terminus (head) and C-terminus (tail) that are both folded into globular domains, whereas its central region, the central domain, is composed of an elongated alpha helix. The categories of intermediate filaments include keratins, desmin, vimentin, glial fibrillary acidic protein, neurofilaments, and nuclear 17 lamins. Keratins (Gr. keras, horn) or cytokeratins are a diverse family of acidic and basic isoforms that compose heterodimer subunits of intermediate filaments in all epithelial cells. Intermediate filaments of keratins provide mechanical stability. 18  Intermediate filaments strengthen the cellular cytoskeleton. The expression of mutant keratin genes results in the abnormal assembly of keratin filaments, which weakens the mechanical strength of cells and causes inherited skin diseases: 1. Epidermolysis bullosa simplex (EBS), characterized by skin blisters after minor trauma. EBS is determined by keratin 5 and 14 mutant genes. 2. Epidermolytic hyperkeratosis (EH), in which patients have excessive keratinization of the epidermis owing to mutations of keratin 1 and 10 genes. 3. Epidermolytic plantopalmar keratoderma (EPPK), a skin disease producing fragmentation of the epidermis of the palms and soles, caused by a mutation of the keratin 9 gene. 19  Vimentin is the most common intermediate filament protein and is found in most cells derived from embryonic mesenchyme. Important vimentin-like proteins include desmin found in almost all muscle cells and glial fibrillar acidic protein (GFAP) found especially in astrocytes, supporting cells of central nervous system tissue. Neurofilament proteins of three distinct sizes make heterodimers that form the subunits of the major intermediate filaments of neurons. Lamins are a family of seven isoforms present in the cell nucleus, where they form a structural framework called the nuclear lamina just inside the nuclear envelope. 20  Several intermediate filament-binding proteins have been discovered. As they bind to intermediate filaments, they link them into a three-dimensional network that facilitates the formation of the cytoskeleton. The best known of these proteins are as follows: Filaggrin binds keratin filaments into bundles. Synemin and plectin bind desmin and vimentin, respectively, into three-dimensional intracellular meshworks. Plakins assist the maintenance of contact between the keratin intermediate filaments and hemidesmosomes of epithelial cells as well as actin filaments with neurofilaments of sensory neurons. 21  Microtubules There are thin tubular structures known as microtubules in the cytoplasm of all eukaryotic cells. Each microtubule is hollow, with an outer diameter of 25 nm and a wall 5-nm thick, a structure that confers significant rigidity to help maintain cell shape. The protein subunit of a microtubule is a heterodimer of α-tubulin (minus end) and β tubulin (plus end), each with a molecular mass of about 50 kDa. Under appropriate conditions the tubulin heterodimers polymerize to form the microtubules, which have a slightly spiral organization. Polymerization of tubulins is directed by microtubule organizing centers (MTOCs). 22  The main functions of microtubules are to Provide rigidity and maintain cell shape Regulate intracellular movement of organelles and vesicles Establish intracellular compartments Provide the capability of ciliary (and flagellar) motion 23  Microtubule structure is found in cilia, flagella and centriole as well as cytoplasm. Centrosomes are organized around two cylindrical centrioles, each approximately 0.2 µm in diameter and 0.3-0.5 µm in length. Each centriole is composed of nine highly organized microtubule triplets. 24 25  Microtubules also form part of the system for intracellular transport of membranous vesicles, macromolecular complexes and organelles. Well-studied examples include axoplasmic transport in neurons, melanin transport in pigment cells, chromosome movements by the mitotic spindle, and vesicle movements among different cell compartments. In each of these examples, if microtubules are disrupted, movement is suspended. 26  Transport along microtubules is under the control of proteins called motor proteins, which use ATP in moving the larger structures. Kinesins carry material away from the microtubule organizing centers (MTOC) near the nucleus toward the plus end of microtubules (anterograde transport); cytoplasmic dyneins carry material along microtubules in the opposite direction (retrograde transport), generally toward the nucleus. So microtubules polarity is important for the direction of vesicular transport. 27 Clinical significance: Microtubule-targeted drugs Antimitotic drugs act on microtubules: Some of them inhibit microtubule polymerization. Includes colchicine, colcemid, vincristine, and vinblastine, which bind to tubulin and inhibit microtubule polymerization, blocking mitosis. Colchicine is used clinically in the treatment of gout. Vincristine and vinblastine, from Vinca alkaloids isolated from the leaves of the periwinkle plant, have been successfully used in childhood hematologic malignancies (leukemias). 28  Some of microtubule-targeted drugs stabilizes microtubules instead of inhibiting their assembly. Paclitaxel (taxol) has been used widely to treat breast and ovarian cancers. Taxol stabilizes microtubules to treat breast and ovarian cancer. The antitumor drug Taxol stabilizes microtubules and reduces their dynamicity, promoting mitotic arrest and cell death. Taxol blocks the cell cycle in its G1 or M phases by stabilizing the microtubule cytoskeleton— the basis of its clinical use in cancer therapy. 29  Kartagener’s syndrome Kartagener’s syndrome is an autosomal recessive ciliary dyskinesia frequently associated with bronchiectasis (permanent dilation of bronchi and bronchioles) and sterility in men. Kartagener’s syndrome is the result of structural abnormalities in the axoneme (defective or absent dynein) that prevent mucociliary clearance in the airways (leading to persistent infections) and reduce sperm motility and ovarium transport in the oviduct (leading to sterility). 30 31 QUESTION- ANSWER 1 1. A 14-year-old boy is diagnosed with epidermolysis bullosa simplex (EBS). His skin blisters easily with rubbing or scratching. Blisters occur primarily on his hands and feet and heal without leaving scars. Genetic analysis shows mutations in the KRT5 and KRT14 genes, which code keratin 5 and keratin 14. What is the primary function of those proteins? a. Generate movement b. Provide mechanical stability c. Carry out nucleation of microtubules d. Stabilize microtubules against disassembly e. Transport organelles within the cell 32 QUESTION- ANSWER 2 2. Polarity in microtubules is important in determining which of the following? a. The strength of vinblastine binding to microtubules b. The velocity of transport along microtubules with myosin motors c. The overall dynamic instability of the microtubules d. The linkage of microtubules to intermediate filaments e. The direction of vesicular transport along microtubules 33 QUESTION- ANSWER 3 3. A patient who had surgery for ovarian cancer is placed on a combination of cisplatin and taxol therapy. Taxol is an antimitotic agent that: a. Affects microfilaments b. Affects intermediate filaments c. Has defective dynein d. Prevents the polymerization of microtubules e. Stabilizes microtubules 34 QUESTION- ANSWER 4 4. The extracellular matrix and the cytoskeleton communicate across the cell membrane through which of the following? a. Proteoglycans b. Integrins c. Cadherins d. Intermediate filaments e. Microtubules 35 QUESTION- ANSWER 5 5. Cytoplasm often stains acidophylic because of abundantly content of the organelles. This situation is most likely to occur with cytoplasmic regions rich in which of the following organelles? a. Free ribosomes b. Mitochondria c. Golgi apparatus d. Smooth endoplasmic reticulum e. Rough endoplasmic reticulum 36