Cellular Adhesion (cellular junctions) | Sintesi del corso di Biologia

Cell adhesion!

From single cell to multicellularity!

Cell-cell adhesion!

Cell-ECM adhesion (cell matrix)!

In vertebrates, cells organize themselves to form 4 types of tissues:!

• The epithelial tissue: covers the surface of the body; it lines the body cavities and forms the

glands.!

• The connective tissue: protects and supports the body and its organs; fills the interior spaces;

constitutes the body's energy reserves and provides immune protection.!

• Muscle tissue: it is specialized for contraction; constitutes all the muscles of the body including

the heart muscle and the muscular lining of the internal organs.!

• Nervous tissue: recognizes internal and external changes in the body; transfers information and

maintains homeostasis.!

The formation of a fabric requires:!

Cell-to-cell recognition (transmembrane adhesive receptors)!

Cell-cell adhesion (transmembrane adhesive receptors and cell junctions)!

Cell-extracellular matrix adhesion (transmembrane adhesive receptors and cell-ECM junctions)!

Cells belonging to the same tissue in an adequate culture medium are able to recognize,

aggregate and form agglomerates. Cell-to-cell recognition occurs thanks to transmembrane

adhesion proteins or adhesive receptors.!

Adhesive receptors belong to 4 classes:!

The CAM (Cell Adhesion Molecule) of the IgSF immunoglobulin superfamily.!

Caderine!

Selectine!

Integrins!

Cell junctions!

Cells that are in close contact with each other normally develop specialized intercellular junctions.

Such structures can make it possible to form very strong connections, prevent the passage of

materials or establish rapid communication between adjacent cells. Animal cells are connected by

diﬀerent types of junctions, which include anchoring junctions, tight junctions, and

communicating junctions. Plant cells are connected to each other by plasmodesms.!

Anchoring joints!

Adjacent epithelial cells, such as those in the surface layer of the skin, are so strongly bonded

together by anchoring junctions that very strong mechanical forces are required to separate them.

Cadherins, transmembrane proteins, are important components of these junctions. The anchoring

junctions do not aﬀect the passage of materials between adjacent cells and are of two types:

desmosomes and adherent junctions.!

Desmosomes are attachment points between cells that hold the cells together like a rivet.

Desmosomes allow cells to form a resistant layer, which however leaves spaces between the

membranes through which substances can still pass. Each desmosome is made up of regions of

dense material associated with the cytosolic side of the two membranes, in addition to the protein

filaments that cross the narrow intercellular space; moreover, the desmosomes are anchored to

the intracellular system of the intermediate filaments. In this way, the network of intermediate

filaments of adjacent cells is connected, allowing mechanical stresses to be distributed

throughout the tissue.!

Adherent junctions cement the cells together. Cadherins form a continuous adhesion belt around

each cell, binding it to neighboring cells. These junctions connect with the microfilaments of the

cytoskeleton. The cadherins of the adherent junctions represent a potential system for the

transmission of signals from the external environment to the cytoplasm.!

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Cell adhesion From single cell to multicellularity Cell-cell adhesion Cell-ECM adhesion (cell matrix) In vertebrates, cells organize themselves to form 4 types of tissues:

The epithelial tissue: covers the surface of the body; it lines the body cavities and forms the glands.
The connective tissue: protects and supports the body and its organs; fills the interior spaces; constitutes the body's energy reserves and provides immune protection.
Muscle tissue: it is specialized for contraction; constitutes all the muscles of the body including the heart muscle and the muscular lining of the internal organs.
Nervous tissue: recognizes internal and external changes in the body; transfers information and maintains homeostasis. The formation of a fabric requires: Cell-to-cell recognition (transmembrane adhesive receptors) Cell-cell adhesion (transmembrane adhesive receptors and cell junctions) Cell-extracellular matrix adhesion (transmembrane adhesive receptors and cell-ECM junctions) Cells belonging to the same tissue in an adequate culture medium are able to recognize, aggregate and form agglomerates. Cell-to-cell recognition occurs thanks to transmembrane adhesion proteins or adhesive receptors. Adhesive receptors belong to 4 classes: The CAM (Cell Adhesion Molecule) of the IgSF immunoglobulin superfamily. Caderine Selectine Integrins Cell junctions Cells that are in close contact with each other normally develop specialized intercellular junctions. Such structures can make it possible to form very strong connections, prevent the passage of materials or establish rapid communication between adjacent cells. Animal cells are connected by different types of junctions, which include anchoring junctions, tight junctions, and communicating junctions. Plant cells are connected to each other by plasmodesms. Anchoring joints Adjacent epithelial cells, such as those in the surface layer of the skin, are so strongly bonded together by anchoring junctions that very strong mechanical forces are required to separate them. Cadherins, transmembrane proteins, are important components of these junctions. The anchoring junctions do not affect the passage of materials between adjacent cells and are of two types: desmosomes and adherent junctions. Desmosomes are attachment points between cells that hold the cells together like a rivet. Desmosomes allow cells to form a resistant layer, which however leaves spaces between the membranes through which substances can still pass. Each desmosome is made up of regions of dense material associated with the cytosolic side of the two membranes, in addition to the protein filaments that cross the narrow intercellular space; moreover, the desmosomes are anchored to the intracellular system of the intermediate filaments. In this way, the network of intermediate filaments of adjacent cells is connected, allowing mechanical stresses to be distributed throughout the tissue. Adherent junctions cement the cells together. Cadherins form a continuous adhesion belt around each cell, binding it to neighboring cells. These junctions connect with the microfilaments of the cytoskeleton. The cadherins of the adherent junctions represent a potential system for the transmission of signals from the external environment to the cytoplasm.

Tight joints Tight (tight) junctions are areas of connection between the membranes of adjacent cells. These connections are so tight that the spaces around the cell disappear completely; in this way, the passage of some substances through the cell layer can be prevented. Transmission electron microscope photographs of the tight junctions show that in the junction area the membranes of two cells are held together by protein connections and are actually in contact with each other. However, the tight junctions are distributed discontinuously, so that the plasma membranes of the two cells are not fused along the entire surface. Cells connected by tight junctions seal body cavities. For example, the tight junctions existing between the cells lining the intestinal wall prevent the substances found in the intestine from entering the bloodstream passing around the cells. The cell layer thus functions as a selective barrier: nutrients must be transported across the plasma membranes and cross the interior of the intestinal cells to reach the bloodstream. This organization helps to prevent toxins and other unwanted substances from entering the bloodstream and nutrients from escaping from the intestine. Tight junctions are also present between the cells and lining the capillaries in the brain; they contribute to the formation of the blood-brain barrier, which prevents various substances from passing from the blood to the brain. Communicating junctions A communicating junction (gap) resembles a desmosome in that it is not continuous, although it is narrower than it. Communicating junctions also differ in that they not only join membranes, but also contain channels that connect the cytoplasm of adjacent cells. The communicating junctions are composed of connexin, an integral membrane protein. Groups of six connexin molecules form a cylinder that crosses the plasma membrane, and the connexin cylinders of adjacent cells are firmly joined together to form a channel approximately 1.5 nm in diameter. Small inorganic molecules (eg ions) and some regulatory molecules can pass through these channels; larger molecules, on the other hand, are excluded. If appropriate markers are injected into one of the cells connected by this type of junction, they quickly pass into adjacent cells but do not enter the space between cells. Communicating junctions ensure very fast chemical and electrical communications between cells. Cells can control the passage of material through communicating junctions by opening and closing channels. The cells of the pancreas, for example, are held together by communicating junctions, so that if a group of cells is stimulated to secrete insulin, the signal passes through the junctions. This mechanism ensures a coordinated response to the initial signal. The communicating junctions allow the electrical coupling of some nerve cells. Heart muscle cells are also connected by communicating junctions, so as to ensure the flow of ions necessary for the synchronized contraction of the cells. Plasmodesms Plant cells do not need desmosomes to bulk up because they are equipped with a cell wall. Plant cells need connections that are functionally equivalent to the communicating junctions of animal cells. Plasmodesms are channels 20 to 40 nanometers wide that pass through the walls of adjacent plant cells, connecting their cytoplasm. The plasma membranes of adjacent cells are therefore continuous with each other thanks to plasmodesms. Many of them contain a cylindrical membranous structure, the desmotubule, which passes through the opening and connects the endoplasmic reticulum of two adjacent cells. Plasmodesms generally allow molecules and ions, but not organelles, to pass from one cell to another through the openings. The movement of ions through the plasmodesms causes weak electrical signals in plants. While the channels of the communicating junctions have a fixed diameter, the plant cells are able to dilate the channels of the plasmodesms. Some proteins and RNAs can pass through plasmodesms. Some plant viruses spread the infection by passing through this type of junction. Structural fibers

It is produced in the form of an inactive precursor in the endomembrane system such as collagen. Proteoglycans The structural fibers are found immersed in a matrix rich in very complex structural glycoproteins, molecular aggregates called proteoglycans. They remind the bacterial peptidoglycan, and they take on the role of carrier unit to bind oligosaccharide chains, the glycosaminoglycans. The latter are made up of a repeating disaccharide fundamental unit, of which one component is always an amino sugar. These structures draw a lot of water as they have several hydroxyl groups and can make hydrogen bonds. They also possess negatively charged groups such as sulfate and carboxyl groups. The most common are chondroitin sulfate, keratan sulfate and hyaluronic acid. The main functions of the proetoglycan-glycosaminoglycan complex are to recall water and therefore are reserves of water and mineral salts for the various tissues and create a turgor pressure, supporting the tissue surrounded by the extracellular matrix. In fact, they play a very important role in regulating cell functions, conserving metabolites that are important for tissues, as signal molecules for tissue cells. They are donors of oligosaccharide units when it is required for the formation of glycopeptides, for example The proteoglycan can join with other proteoglycans to form complex molecular aggregates. These can be bound to a carrier polysaccharide which usually, as in the case of cartilage, is hyaluronic acid, forming a gelatinous structure. Variations in the components of the extracellular matrix also occur during our life. For example, in a young skin there is an abundance of collagen (especially type), the right content of elastin and an abundance of proteoglycans associated with hyaluronic acid, which attract water and create turghours. With aging, collagen begins to deconstruct, so there are fewer hydrogen bonds, a decrease in elastin and also the amount of proteoglycans and hyaluronic acid decrease and the skin is less hydrated, less elastic, etc. Fibronectin / laminin These fibrous elements are connected to the cells of our tissues via fibrous glycoproteins. In the case of proteoglycans, they can insert directly inside the plasma membranes and bind membrane phospholipids, so these proteins are not necessary. One of these proteins is fibronectin, formed by two polypeptide chains (dimeric protein) with covalent bonds determined by sulfur atoms that form disulfide bridges. They are fabric specific. The amino acid sequence of the two polypeptide chains is divided into functional domains. Fibronectin also binds fibrin involved in coagulation. A fibronectin molecule consists of two nearly polypeptide chains identical linked by two disulfide bonds at the carboxy terminal end. Each polypeptide chain is structured in a series of globular domains spaced by short flexible tracts. Globular domains contain binding sites for components of the ECM or for specific cell surface receptors. The receptor-binding domain contains the tripeptide sequence RGD (argininaglycine aspartate), recognized by the fibronectin receptor. In addition to the known binding activities, fibronectin can also bind heparan sulfate, hyaluronate and gangliosides (glycosphingolipids that contain residues of sialic acid). The adhesion glycoprotein present in the basal lamina is laminin, formed by three polypeptide chains in a cross-like structure. The laminin molecule consists of three large polypeptides - alpha, beta and gamma - linked together by disulfide bridges to form a cross structure. Part of the long arm is made up of the three wrapped chains. The functional domains at the ends of the a-chain bind organospecific surface receptors, while those at the ends of the two arms of the cross are specific for type IV collagen. The crossing region of the arms also contains laminin- laminin binding sites, thus allowing the formation of laminin aggregates. Laminin also contains binding sites for leparin, leparan sulfate and lentactin.

Integrins The most widely used cell-ECM adhesive receptor present on the membrane is that of the integrin family, the same group that allows adhesion between the cells themselves. It is formed only of two polypeptide chains, divided into a transmembrane region that interacts with membrane lipids, a section that protrudes outside and one that protrudes inside, necessary to bind to the cytoskeleton. Focal adhesions Focal adhesions are typical of creeping cells, those found in artificial environments, have a firm structure but make the cells able to move. The presence of focal adhesions is fundamental because it is the signal that the cell has "taken root" in culture and is therefore ready for cell division. In fact, at first it has a rounded shape then tends to flatten and distribute itself on the rigid substrate. In the case of resistant cells, the plate must be pretreated with a matrix that affects the extracellular matrix of the cell in culture. In the case of the creeping cell at the start, this is attached to the substrate with a focal adhesion, then polymerizes g actins, consequently elongating the f actin filament and creating a pseudopod. Subsequently the myosin pulls the filament back, contracting it and bringing the body forward, reforming the focal adhesion. So this type of adhesion is dynamic, that is, it can be formed and dissolved depending on the needs of the cell. Often focal adhesions are the recruitment of signal molecules linked to cell proliferation and death. They therefore influence gene expression and the cell's response. This is also found in the case of cancer cells that take on metastatic properties. Costamer The costamer is the junction between the cell and the ecm in skeletal muscle. This is reminiscent of focal adhesions and this is because we are in a muscle fiber that even if it does not crawl is subject to movement and cannot be a static structure. The fundamental connection point is given by dystrophin, a protein that in the event of a genetic disease changes and makes the connection unstable, so the muscles do not contract adequately. Hemicellulose / extensins In the case of plant cells, hemicellulose is present in place of proteoglycans, which varies according to the type of plant. Unlike cellulose, it is a polysaccharide that has branched structure and many hydroxyl groups, so it attracts water. It then creates a gelatinous extracellular matrix that mimics that of ours or organism, in which the cellulose fibers constituting the cell wall are immersed. The cellulose microfibrils are linked to hemicelluloses and glycoproteins called extensins, to form a rigid network rich in interconnections, immersed in a matrix of pectins. Cellulose microfibrils are often wrapped around each other to form larger structures called macrofibrils. Depending on the proximity of the membrane of two plant cells, there are different stages of the extracellular matrix and therefore of the wall, since the extracellular matrix is mainly formed by the wall. The part that joins the two walls is the median lamella, richer in hemicellulose than cellulose, and therefore the hydrated and gelatinous part prevails over the more rigid part of cellulose. The second part of the wall (primary wall) which is sometimes the definitive one has a higher concentration of cellulose. Some cells aging can lead to the formation of another wall called secondary adherent wall. The cells themselves produce the cellulose necessary for the formation of the walls and the mobile enzymatic complexes on the membrane called rosettes are able to produce cellulose and deposit it on the cell wall. Plasmodesms Plasmodesms are continuity channels that are created between two contiguous plant cells and cross their plasma membrane and wall. Often they are crossed by a vesicular tube called a desmotubule which can connect the cytoplasm of the two cells and can pass very large molecules such as ions. In some cells, desmotubules are physically connected to the vesicles of

Cellular Adhesion (cellular junctions), Sintesi del corso di Biologia

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