Cytoskeleton, ECM, and Intercellular Junctions PDF

Cytoskeleton, ECM, Intercellular Junctions Dr. Hilal Eren Gözel [email protected] Faculty of Medicine, Department of Medical Biology & Genetics  Cytoskeleton Objectives Types of fibers and their functions EC...

Cytoskeleton, ECM, Intercellular Junctions Dr. Hilal Eren Gözel [email protected] Faculty of Medicine, Department of Medical Biology & Genetics  Cytoskeleton Objectives Types of fibers and their functions ECM Intercellular Junctions Clinical Correlation  Cytoskeleton The cytoskeleton is a network of fibers extending throughout the cytoplasm. M icrotubule M icrofilaments 0.25 µm  Cytoskeleton The cytoskeleton helps to support the cell and maintain its shape It interacts with motor proteins to produce motility Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton The cytoskeleton provides anchorage for many organelles and even cytosolic enzyme molecules.  Vesicle ATP Receptor for motor protein M otor protein M icrotubule (ATP powered) of cytoskeleton (a) M icrotubule Vesicles 0.25 µm (b)  Cytoskeleton It is composed of three types of molecular structures: Microtubules are the thickest of the three components of the cytoskeleton Microfilaments, also called actin filaments, are the thinnest components Intermediate filaments are fibers with diameters in a middle range  Cytoskeleton Video Link: https://www.youtube.com/watch?v=YTv9ItGd050 Microtubule Microtubules have a crucial organizing role in all eukaryotic cells. These long and relatively stiff hollow tubes of protein can rapidly disassemble in one location and reassemble in another. In a typical animal cell, microtubules grow out from a small structure near the center of the cell called the centrosome.  Microtubules Extending out toward the cell periphery, they create a system of tracks within the cell, along which vesicles, organelles, and other cell components can be transported. They are mainly responsible for transporting and positioning membrane-enclosed organelles within the cell and for guiding the intracellular transport of various cytosolic macromolecules. When a cell enters mitosis, the cytoplasmic microtubules disassemble and then reassemble into an intricate structure called the mitotic spindle. The mitotic spindle provides the machinery that will segregate the chromosomes equally into the two daughter cells just before a cell divides.  Microtubules Microtubules can also form stable structures, such as rhythmically beating cilia and flagella.  10 µm Column of tubulin dimers 25 nm a b Tubulin dimer  Cytoskeleton – Micro filaments Microfilaments are thin, solid, and flexible rods about 7 nm in diameter, built as a twisted double chain of actin subunits. The structural role of microfilaments is to bear tension, resisting pulling forces within the cell. Actin filaments are linked by actin-binding proteins into a meshwork that supports the plasma membrane and gives it mechanical strength. Thus, they help support the cell’s shape. (Stable structure) They are essential for many of the cell’s movements (Temporary structure). In human red blood cells, a simple and regular network of fibrous proteins (including actin and spectrin filaments) attaches to the plasma membrane, providing the support necessary for the cells to maintain their simple discoid shape. Bundles of microfilaments make up the core of microvilli of intestinal cells.  Cytoskeleton – Micro filaments Microfilaments that function in cellular motility contain the protein myosin in addition to actin. In muscle cells, thousands of actin filaments are arranged parallel to one another. Thicker filaments composed of myosin interdigitate with the thinner actin fibers.  Cytoskeleton – Micro filaments Localized contraction brought about by actin and myosin also drives amoeboid movement. Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments.  Cytoskeleton – Micro filaments Cytoplasmic streaming is a circular flow of cytoplasm within cells. This streaming speeds distribution of materials within the cell.  10 µm Actin subunit 7 nm  Intermediate Filaments Intermediate filaments have great tensile strength, and their main function is to enable cells to withstand the mechanical stress that occurs when cells are stretched. Intermediate filaments are the toughest and most durable of the cytoskeletal filaments (anchorage!) There they are often anchored to the plasma membrane at cell– cell junctions called desmosomes where the plasma membrane is connected to that of another cell. Inside nucleus, they form a meshwork called the nuclear lamina, which underlies and strengthens the nuclear envelope.  Intermediate Filaments An intermediate filament is like a rope in which many long strands are twisted together to provide tensile strength.  Intermediate Filaments Intermediate filaments are particularly prominent in the cytoplasm of cells that are subject to mechanical stress. They are also abundant in muscle cells and in epithelial cells such as those of the skin. In all these cells, intermediate filaments distribute the effect of locally applied forces, thereby keeping cells and their membranes from tearing in response to mechanical shear.  Intermediate Filaments Intermediate filaments can be grouped into four classes: (1) keratin filaments in epithelial cells; (2) vimentin and vimentin-related filaments in connective-tissue cells, muscle cells, and supporting cells of the nervous system (glial cells); (3) neurofilaments in nerve cells; and (4) nuclear lamins, which strengthen the nuclear envelope. The first three filament types are found in the cytoplasm, whereas the fourth is found in the nucleus. Clinical Correlation – Epidermolysis bullosa simplex EBS is an inherited genetic (Autosomal dominant) disorder resulting from mutations in the genes encoding keratin 5 or keratin 14. These genetic mutations prevent the proper formation of keratin structures in the skin’s epidermis which cause the skin to become very fragile. Any trauma or friction to the skin can cause painful blisters and bleedings. Blisters especially occurs on the hands and soles of the feet! Thickened skin that may be scarred or change colour over time.  5 µm Keratin proteins Fibrous subunit (keratins coiled together) 8–12 nm  Borders of the Cell and Beyond Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures include: The extracellular matrix (ECM) of animal cells Intercellular junctions  Extracellular Matrix (ECM) Since animal cells have no cell wall, ECM provides biochemical and structural support to surrounding cells. The composition of ECM varies between multicellular structures; however, cell adhesion, cell-to-cell communication and differentiation are common functions of the ECM. The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin ECM proteins bind to receptor proteins in the plasma membrane called integrins ECM Intercellular Junctions  Intercellular Junctions Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact Intercellular junctions facilitate this contact There are several types of intercellular junctions Tight junctions Desmosomes Gap junctions  Intercellular Junctions At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid  Intercellular Junctions Desmosomes (anchoring junctions) fasten cells together into strong sheets Intercellular Junctions Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells References &Thank You