Structure and properties of tight junctions of cells. Groups and types of intercellular contacts Structure and molecular composition

In multicellular organisms, due to intercellular interactions, complex cellular assemblies are formed, the maintenance of which can be carried out in different ways. In germinal, embryonic tissues, especially in the early stages of development, cells remain connected to each other due to the ability of their surfaces to stick together. This property adhesion(connection, adhesion) of cells can be determined by the properties of their surface, which specifically interact with each other. The mechanism of these connections is quite well studied; it is ensured by the interaction between glycoproteins of plasma membranes.

In addition to relatively simple adhesive (but specific) connections, there are a number of special intercellular structures, contacts or connections that perform specific functions.

Locking or tight connection characteristic of single-layer epithelia (Fig. 9). This is the zone where the outer layers of the two plasma membranes are as close as possible. The three-layer structure of the membrane at this contact is often visible: the two outer osmophilic layers of both membranes seem to merge into one common layer 2-3 nm thick.

Membrane fusion does not occur over the entire area of ​​tight contact, but represents a series of point-like approaches of membranes. With special stains, such structures can also be seen in a light microscope. They received the name from morphologists end plates. The role of the closing tight junction is not only to mechanically connect cells to each other. This contact area is poorly permeable to macromolecules and ions, and thus it locks and blocks the intercellular cavities, isolating them (and with them the internal environment of the body) from the external environment (in this case, the intestinal lumen).

Closing, or tight, contact occurs between all types of single-layer epithelium (endothelium, mesothelium, ependyma).

Simple contact, found among the majority of adjacent cells of various origins (Fig. 10). Most of the surface of contacting epithelial cells is also connected using a simple contact, where the plasma membranes of the contacting cells are separated by a space of 15-20 nm. This space represents the supramembrane components of cell surfaces. The width of the gap between cell membranes can be more than 20 nm, forming expansions and cavities, but not less than 10 nm.

On the cytoplasmic side, no special additional structures are adjacent to this zone of the plasma membrane.

Gear contact (“lock”) is a protrusion of the surface of the plasma membrane of one cell into the intussusception (invagination) of another (Fig. 11).

On a cut, this type of connection resembles a carpenter's seam. The intermembrane space and cytoplasm in the “lock” zone have the same characteristics as in the zones of simple contact. This type of intercellular connections is characteristic of many epithelia, where it connects cells into a single layer, promoting their mechanical fastening to each other.

The role of mechanical tight fastening of cells to each other is played by a number of special structured intercellular connections.

Desmosomes, structures in the form of plaques or buttons also connect cells to each other (Fig. 12). In the intercellular space, a dense layer is also visible here, represented by interacting integral membrane cadherins - desmogleins, which adhere cells to each other.

On the cytoplasmic side, a layer of the desmoplakin protein is adjacent to the plasmalemma, with which the intermediate filaments of the cytoskeleton are associated. Desmosomes are most often found in epithelia, in which case the intermediate filaments contain keratins. In cardiac muscle cells, cardiomyocytes, contain desmin fibrils as part of desmosomes. In the vascular endothelium, desmosomes contain vimentin intermediate filaments.

Hemidesmosomes, in principle, they are similar in structure to the desmosome, but represent a connection of cells with intercellular structures. Thus, in epithelia, linker glycoproteins (integrins) of desmosomes interact with proteins of the so-called. basement membrane, which includes collagen, laminin, proteoglycans, etc.

The functional role of desmosomes and hemidesmosomes is purely mechanical - they adhere cells to each other and to the underlying extracellular matrix firmly, which allows the epithelial layers to withstand large mechanical loads.

Similarly, desmosomes tightly bind cardiac muscle cells to each other, which allows them to carry out enormous mechanical loads while remaining connected into a single contractile structure.

Unlike tight contact, all types of adhesive contacts are permeable to aqueous solutions and do not play any role in limiting diffusion.

Gap contacts (nexuses) are considered communication junctions of cells; these are structures that are involved in the direct transfer of chemicals from cell to cell, which can play a major physiological role not only in the functioning of specialized cells, but also provide intercellular interactions during the development of the organism, during the differentiation of its cells (Fig. 13).

Characteristic of this type of contact is the bringing together of the plasma membranes of two neighboring cells to a distance of 2-3 nm. It was precisely this circumstance that for a long time did not allow distinguishing this type of contact from a dense separating (closing) contact on ultrathin sections. When using lanthanum hydroxide, it was observed that some tight junctions would allow the contrast agent to pass through. In this case, lanthanum filled a thin gap about 3 nm wide between the close plasma membranes of neighboring cells. This gave rise to the term gap contact. Further progress in deciphering its structure was achieved using the freezing-cleavage method. It turned out that on the cleaved membranes, the zones of gap contacts (sizes from 0.5 to 5 μm) are dotted with hexagonally arranged particles with a period of 8-10 nm, 7-8 nm in diameter, having a channel about 2 nm wide in the center. These particles are called connexons.

In gap junction zones there can be from 10-20 to several thousand connexons, depending on the functional characteristics of the cells. Connectons were isolated preparatively; they consist of six subunits of connectin - a protein with a molecular weight of about 30 thousand. By combining with each other, connectins form a cylindrical aggregate - a connecton, in the center of which there is a channel.

Individual connexons are embedded in the plasma membrane so that they pierce it right through. One connexon on the plasma membrane of a cell is precisely opposed by a connexon on the plasma membrane of an adjacent cell, so that the channels of the two connexons form a single unit. Connexons play the role of direct intercellular channels through which ions and low molecular weight substances can diffuse from cell to cell. It was discovered that connexons can close, changing the diameter of the internal channel, and thereby participate in the regulation of the transport of molecules between cells.

The functional significance of gap junctions was understood from the study of giant cells of the salivary glands of Diptera. Due to their size, microelectrodes can easily be introduced into such cells in order to study the electrical conductivity of their membranes. If you insert electrodes into two neighboring cells, their plasma membranes exhibit low electrical resistance and current flows between the cells. This ability of gap junctions to serve as a site for the transport of low-molecular compounds is used in those cellular systems where rapid transmission of an electrical impulse (excitation wave) from cell to cell without the participation of a nerve transmitter is needed. Thus, all muscle cells of the myocardium of the heart are connected using gap junctions (in addition, the cells there are also connected by adhesive junctions). This creates the condition for the synchronous reduction of a huge number of cells.

With the growth of a culture of embryonic cardiac muscle cells (cardiomyocytes), some cells in the layer begin to spontaneously contract independently of each other at different frequencies, and only after the formation of gap junctions between them do they begin to beat synchronously as a single contracting layer of cells. In the same way, the joint contraction of smooth muscle cells in the uterine wall is ensured.

Synaptic contact(synapses). This type of contact is characteristic of nervous tissue and occurs both between two neurons and between a neuron and some other element - a receptor or effector (for example, a neuromuscular ending) (Fig. 14).

Synapses are areas of contact between two cells specialized for the unilateral transmission of excitation or inhibition from one element to another. In principle, this kind of functional load and impulse transmission can be carried out by other types of contacts (for example, a gap junction in the cardiac muscle), but in synaptic communication high efficiency in the implementation of a nerve impulse is achieved.

Synapses are formed on the processes of nerve cells - these are the terminal sections of dendrites and axons. Interneuron synapses usually have pear-shaped extensions, plaques at the end of the nerve cell process. Such a terminal extension of the process of one of the nerve cells can contact and form a synaptic connection both with the body of another nerve cell and with its processes. Peripheral processes of nerve cells (axons) form specific contacts with effector cells or receptor cells. Therefore, a synapse is a structure formed between regions of two cells (just like a desmosome). The membranes of these cells are separated by an intercellular space - a synaptic cleft about 20-30 nm wide. Often, in the lumen of this gap, a fine-fibrous material located perpendicular to the membranes is visible. The membrane in the area of ​​synaptic contact of one cell is called presynaptic, the other, which receives the impulse, is called postsynaptic. In an electron microscope, both membranes look dense and thick. Near the presynaptic membrane, a huge number of small vacuoles, synaptic vesicles filled with transmitters are detected. Synaptic vesicles, at the moment of passage of a nerve impulse, release their contents into the synaptic cleft. The postsynaptic membrane often appears thicker than normal membranes due to the accumulation of many thin fibrils near it on the cytoplasmic side.

Plasmodesmata. This type of intercellular communication is found in plants. Plasmodesmata are thin tubular cytoplasmic channels connecting two adjacent cells (Fig. 15). The diameter of these channels is usually 20-40 nm. The membrane limiting these channels directly passes into the plasma membranes of neighboring cells.

Plasmodesmata pass through the cell wall that separates the cells. Thus, in some plant cells, plasmodesmata connect the hyaloplasm of neighboring cells, so formally there is no complete demarcation, separation of the body of one cell from another, it rather represents a syncytium: the union of many cellular territories with the help of cytoplasmic bridges.

Membranous tubular elements connecting the cisterns of the endoplasmic reticulum of neighboring cells can penetrate inside the plasmodesmata. Plasmodesmata are formed during cell division, when the primary cell membrane is built. In newly divided cells, the number of plasmodesmata can be very large (up to 1000 per cell); as cells age, their number decreases due to ruptures with increasing cell wall thickness.

The functional role of plasmodesmata is very great: with their help, intercellular circulation of solutions containing nutrients, ions and other compounds is ensured.

Tight junctions are formed by point-to-point connections between the membranes of neighboring cells through transmembrane proteins claudin And occludine, built in rows that can intersect so that they form a kind of lattice or network on the surface of the chip.
Tight junctions block the movement of macromolecules, liquids and ions between cells, thereby ensuring the barrier function of the epithelium and regulation of the transport of substances through the epithelial layer.
Tight junctions connect cells of single-layer epithelium, especially glandular and intestinal (cells lining the gastrointestinal tract and respiratory system). Tight junction occurs between all types of single-layer epithelium (endothelium, mesothelium, ependyma).

Tight junctions prevent the free movement and mixing of functionally different intramembrane proteins localized in the plasmalemma of the apical and basolateral surfaces of the cell, which helps maintain its polarity.
Tight junctions look like a belt 0.1-0.5 µm wide, surrounding the cell along the perimeter (usually at its apical pole).
To maintain the integrity of these compounds, divalent cations Mg 2+ and Ca 2+ are required. Contacts can be dynamically rearranged (due to changes in the expression and degree of polymerization of occludin) and temporarily opened (for example, for the migration of leukocytes through intercellular spaces).

Focal contacts

They are found in many cells and have been especially well studied in fibroblasts.
They are built according to the general plan with adhesive tapes, but are expressed in the form of small sections - plaques on the plasmalemma. In this case, transmembrane integrin linker proteins specifically bind to extracellular matrix proteins (for example, fibronectin). On the cytoplasmic side, these same glycoproteins are associated with near-membrane proteins, which include vinculin, which in turn is associated with a bundle of actin filaments. The functional significance of focal contacts is both to anchor cells to extracellular structures and to provide a mechanism that allows cells to move.

Slot contacts

Slot contacts– the gap is 2-4 nm, consists of protein channels that allow the passage of substances up to 1 kDa.
Connexons– adjustable channels, consisting of 6 connexins– protein subunits M=26-54 kDa.

The channels close when Ca 2+ enters the cell due to damage. The exchange of thymine through connexons during the selection of hybrid cells complicates the selection - because two connexons of neighboring cells form a channel through which thymine is transferred from cell to cell.
Electrical signals and small regulatory molecules (for example, cAMP, InsP 3, adenosine, ADP and ATP) are transmitted through connexons. Connexins are unstable proteins that live for several hours.
Present in almost all cells.
There are 20 different connexins in mice and 21 in humans. Many cells form several types of connexins, which are capable of polymerizing in various combinations. For example, keratinocytes express Cx26, Cx30, Cx30.3, Cx31, Cx31.1, and Cx43; hepatocytes - Cx26 and Cx32; cardiomyocytes - Cx40, Cx43 and Cx45. Some connexins can replace others when mutated. Heteromeric connexons (consisting of different connexins) Cx26/Cx32 in liver cells, Cx46/Cx50 in the lens, and Cx26/Cx30 connexons in the cochlea.

The combination of six connexins of two types can form 14 variants of connexons, from which 142=196 different variants of channels can be formed!

Different types of human and mouse connexins.

Connexins are polytopic integral membrane proteins that cross the membrane 4 times, have two extracellular loops (EL-1 and EL-2), a cytoplasmic loop (CL) with the N-terminus (AT) and C-terminus (CT) protruding into the cytoplasm.
Specific N- and E-cadherins ensure cell adhesion, which promotes the formation of channels between neighboring cells.
Proteins interacting with Cx43 connexons: v-, c-src kinases, kinase C, MAP kinase, Cdc2 kinase, casein kinase 1, kinase A, ZO-2, ZO-1, b-catenin, Drebrin, a-, b-tubulin , caveolin-1, NOV, CIP85.
Various proteins can interact with connexons, for example, kinases that phosphorylate connexins and change their properties, which can regulate the operation of the channel. With connexons
Tubulins (microtubule proteins) also interact, which may facilitate the transport of various substances along microtubules directly to the channel. The drebrin protein interacts with connexins and microfilaments, which indicates the relationship between channels and the organization of the cell cytoskeleton.
Connexons can close under the influence of current, pH, membrane voltage, Ca2+.

Desmosomes

Desmosomes– connect the cell membrane with intermediate filaments, forming a network that is resistant to stretching.
Cytokeratin filaments
Desmin filaments
Attachment plate It has a disc-shaped shape (diameter ~0.5 μm and thickness 15 nm) and serves as an attachment site for intermediate filaments to the plasma membrane.
Consists of proteins - desmoplakin, plakoglobin, desmocalmine.
Intercellular gap in the region of the desmosome, 25 nm wide, it is filled with proteins desmocollins and desmogleins - Ca 2+-binding adhesive proteins that interact with the attachment plates.

Hemidesmosomes

Hemidesmosomes– attach the basal part of the plasma membrane of epithelial cells to the basement membrane.

Septated contacts

Plasmodesmata

This type of intercellular communication is found in plants. Plasmodesmata are thin tubular cytoplasmic channels that connect two adjacent cells. The diameter of these channels is usually 20-40 nm. The membrane limiting these channels directly passes into the plasma membranes of neighboring cells. Plasmodesmata pass through the cell wall that separates the cells.

Membranous tubular elements connecting the cisterns of the endoplasmic reticulum of neighboring cells can penetrate inside the plasmodesmata.
Plasmodesmata are formed during cell division, when the primary cell membrane is built. In newly divided cells, the number of plasmodesmata can be very large (up to 1000 per cell); as cells age, their number decreases due to ruptures with increasing cell wall thickness.
With the help of plasmodesmata, intercellular circulation of solutions containing nutrients, ions and other compounds is ensured. Lipid droplets can move along plasmodesmata. Through plasmodesmata, cells are infected with plant viruses. However, experiments show that free transport through plasmodesmata is limited to particles with a mass of no more than 800 Da.

Interdigitation

Interdigitation- intercellular connections formed by protrusions of the cytoplasm of some cells, protruding
into the cytoplasm of others. Due to interdigitation, the strength of the cell connection and the area of ​​their contact increases.

Tight junctions are a type of junction complex that forms between adjacent epithelial or endothelial cells

Tight junctions regulate particle transport between epithelial cells

Tight junctions maintain epithelial cell polarity by acting as a “fence” that prevents the migration of plasma membrane proteins between the apical and basal regions

Intercellular contacts play a critical role in the formation of multicellular organisms and in ensuring their viability. Three different types of contact complexes can form along the lateral surfaces of adjacent epithelial and endothelial cells. In vertebrates these are tight junctions, adherens junctions and desmosomes. In invertebrates, instead of a tight contact, a septate contact often occurs. The relative positions of the contacts are shown schematically in the figure below.

They support the existence of separate specialized areas in multicellular organisms and regulate the transport of molecules between them. They also protect cells from chemical and physical damage. We will look at each type of cell-cell interactions, starting with tight junctions.

As can be seen in the figure below, a photograph of an ultrathin section of cells in a transmission electron microscope tight junctions visible as groups of small contacts (sometimes called "kisses"). These contacts exist between the lateral membranes of neighboring cells located opposite. Proteins on the cytoplasmic side of the membrane adjacent to these contacts appear as electron-dense “clouds.” Chips of frozen cell suspensions reveal a different picture, which shows the distribution of the protein in two lipid monolayers separated in the middle of the plasma membrane.

Wherein tight junctions appear as an intertwined network of thin fibrils (or threads) if the proteins remain embedded in the membrane, or appear as a network of depressions if proteins are lost during the cleavage process.

Tight junctions have a complex molecular composition. More than 24 proteins were found in the area of ​​these contacts. Among them, three types of transmembrane proteins have been identified: claudins, occludins, and junction adhesion molecules (JAMs). Claudins are the main proteins of fibrillar tight junction structures. The pores mentioned above are formed when the extracellular domains of claudins are organized into loops that form selective channels in fibrils.

In mammals, at least 24 claudin proteins, and their various combinations form channels with different permeability to ions. Transfection of claudin genes into cells that do not normally express them results in the formation of tight junctions. Occludins laterally copolymerize with claudins along tight junction fibrils in a lateral direction, but their precise function is unknown.

Three transmembrane protein tightly bound to nine or more structural proteins, including actin. They are also capable of episodic binding of more than twelve signaling proteins. This suggests that tight junctions play an additional role as signal organizers present on the cell surface, similar to the focal adhesion complex on the basal surface of cells.

A lot others squirrels tight junction zones, such as ZO-1, belong to the membrane-associated guanylate kinase (MAGUK) family in their primary structure. These proteins contain three domains arranged in a characteristic order. Through these domains, they bind to many types of protein targets, including signaling proteins and elements of the actin cytoskeleton. Some of the tight junction proteins also contain a PDZ domain, which allows them to bind to each other. In vitro model experiments using intact and truncated forms of these proteins demonstrated the possibility of the formation of their various combinations in the contact zone.

Tight junctions play two important roles. First, they are molecular structures that regulate paracellular transport (transport of materials that occurs in the space between cells) in the epithelial and endothelial layers. (Previously, they were thought to function as barriers that block (prevent) this transport, and therefore the contact zones were called zonula occludens.) In this way, tight junctions resemble a “molecular filter” through which molecules of the cellular environment are passed as they pass through the boundaries of the epithelium and endothelium.

However, not all filters are created equal, as each type of fabric requires a filter that can remove a specific set of molecules. For example, for kidney tissue, it is not necessary to remove smoke particles. In fact, the range of particle sizes passing through tight junctions by free diffusion varies between 4 and 40 A, depending on the tissue type.

Physical barriers to transport of ions and others soluble components have a different nature: ions pass instantly, while other soluble components take minutes or even hours to pass through tight contact. How is this done? A recently proposed model postulates that the permeability barrier at the tight junction is created by layers of pores that carry selective charges and form a network structure consisting of fragile thread-like structures. Ions are able to pass through these pores, but for other soluble components to pass through, the integrity of the filaments must be disrupted.

As the threads break and rejoin, the component gradually moves through the contact barrier, as shown in the figure below.

The second role of tight junctions is that in structural and functional terms they divide the plasma membrane of polarized cells into two domains. The apical (from the Greek word apex - top) surface is a part of the plasma membrane that is oriented towards the cavity or space on one side of the epithelial layer. The basal (or inferior) surface is the area on the opposite side that is in contact with the extracellular matrix.

The lateral surfaces form " sides» between these two areas. completely surround epithelial and endothelial cells along the lateral surface along the border of the apical and lateral zones. Thus, the cell is divided into two regions: apical and basolateral domains. These domains demarcate the cell surface into a “top” region and a “base” region, which play different roles in controlling the transcellular movement of metabolites. Although membrane proteins can diffuse in the plane of each domain, they do not migrate from one domain to another across tight junctions.

In this sense tight junctions as if they play the role of a “fence”, thanks to which a unique molecular composition is maintained within each of the two membrane domains.

Although the molecular mechanisms of this diffusion barrier have not yet been sufficiently studied, two have been identified individual macromolecular complex, playing an important role in the formation and maintenance of the polar distribution of plasma membrane proteins in epithelial and endothelial cells. Changes in the expression of any of these proteins cause the cell to lose polarity. These complexes are found in tight junctions and are directly associated with proteins that are part of the network structures discussed above.

Epithelial tissues perform barrier and transport functions; for this they must be able to pass some substances and retain others. Such selective permeability is successfully provided by cell membranes, however, gaps remain between the cells through which the so-called paracellular (paracellular) transport can pass. Paracellular transport). The role of tight junctions is to limit and regulate paracellular diffusion: they prevent the flow of tissue fluid through the epithelium, but, if necessary, can be permeable to ions, small hydrophilic molecules and even macromolecules. Also, tight junctions perform the so-called “fence” function; they prevent the diffusion of membrane components in its outer layer, thereby maintaining the difference in the composition of the apical and basolateral membranes. Tight junctions are involved in signaling pathways that regulate epithelial cell proliferation, polarization, and differentiation.

The analogue of tight junctions in invertebrates is septate junctions.

Encyclopedic YouTube

1 / 1

Jaundice - causes, treatment & pathology

Structure and molecular composition

Tight junctions consist of thin, intersecting bands that completely encircle the cell and contact similar bands on neighboring cells. In electron micrographs, it is noticeable that in areas of tight junctions the membranes come into contact with one another or even merge. Combining the freeze-cleavage technique with high-resolution electron microscopy revealed that tight junction films are composed of protein particles 3-4 nm in diameter that protrude from both surfaces of the membrane. Also in favor of the fact that proteins play a key role in the formation of tight junctions is evidenced by cell division under the action of the proteolytic enzyme trypsin.

In total, close junctions include about 40 different proteins, both membrane and cytoplasmic. The latter are necessary for actin filament attachment, regulation and signaling.

Membrane proteins

Membrane tight junction proteins can be divided into two groups: those that cross the membrane 4 times and those that cross it only once. The first group is significantly widespread and includes the proteins claudins, occludins and tricellulin. They have common structural features, in particular, they have four α-helical transmembrane domains, the N- and C-termini are facing the cytosol, and domains protruding into the intercellular space are involved in homo- or heterophilic interactions with similar proteins on the neighboring cell.

The main proteins of tight junctions are claudins (lat. claudo). Their role was demonstrated in mice lacking the claudin-1 gene; tight junctions do not form in the epidermis of such animals and they die within a day of birth due to dehydration due to intense evaporation. Claudins are also involved in the formation of selective channels for ion transport. There are genes for at least 24 different claudins in the human genome, which are expressed in a tissue-specific manner.

The second most common proteins in tight junctions are occludins (from the Latin occludo - to close), they regulate the transport of small hydrophilic molecules and the passage of neutrophils through the epithelium. The highest concentrations of the third protein, tricellulin, are observed at the points of contact of three cells.

Cytoplasmic proteins

The cytoplasmic plate of tight junctions is necessary for their attachment to actin filaments, regulation of cell adhesion and paracellular transport, as well as for the transmission of signals from the surface into the cell. It consists of adapter, framework and cytoskeletal proteins, as well as elements of signaling pathways (kinases, phosphatases). The most studied protein of the cytoplasmic plate is ZO-1, it has several protein-protein interaction domains, each of which provides contact with other components, including three PDZ domains (eng. PSD95–DlgA–ZO-1) - with claudins and other adapter proteins - ZO-2 and ZO -3, GUK domain (eng. guanylate kinase homology) - with occludins, and SH3 domain - with signaling proteins.

Also associated with the cytoplasmic side of tight junctions are the PAR3/PAR6 and Pals1/PATJ protein complexes, which are necessary for establishing cell polarity and epithelial morphogenesis.

Functions

Early studies of the functions of tight junctions led to the idea that they were static, impermeable structures necessary to limit the diffusion of substances between cells. Subsequently, it was found that they are selectively permeable, and their permeability differs in different tissues and can be regulated. Another function of tight junctions has also been established: its role in maintaining cell polarity by limiting the diffusion of lipids and proteins in the outer layer of the plasma membrane. In the first decade of the 21st century, evidence has also accumulated indicating the participation of these structures in signaling pathways, in particular those regulating proliferation and polarity.

Regulation of paracellular transport

The impermeability of tight junctions in most water-soluble compounds can be demonstrated by injecting lanthanum hydroxide (an electron-dense colloidal solution) into the blood vessels of the pancreas. A few minutes after injection, acinar cells are fixed, and preparations for microscopy are prepared from them. In this case, it can be observed that lanthanum hydroxide diffuses from the blood into the space between the lateral surfaces of the cells, but cannot penetrate the tight junctions in their upper part. Other experiments have shown that tight junctions are also impermeable to salts. For example, when growing the kidneys of an MDCK dog (eng. Madin-Darby canine kidney) in an environment with a very low calcium concentration, they form a monolayer, but are not combined with each other by tight junctions. Salts and liquids can move freely through such a monolayer. If calcium is added to the culture, tight junctions are formed within an hour, and the layer becomes impermeable to liquids.

However, not all tissues have tight junctions that are completely impermeable; there are so-called leaky epithelia. For example, the epithelium of the small intestine allows 1000 times more Na+ ions to pass through than the epithelium of the kidney tubules. Ions penetrate through paracellular pores with a diameter of 4, selective for charge and particle size, which are formed by claudin proteins. Since the epithelia of different organs express different sets of claudins, their permeability to ions also differs. For example, a specific claudin, present only in the kidneys, allows the passage of magnesium ions during reabsorption.

The intercellular space of the epithelium can also be permeable for large particles, for example, when repeating the mentioned experiment with lanthanum hydroxide on the epithelial tissue of the small intestine of a rabbit, one can observe the passage of colloidal particles between cells. Large molecules are transported through special leak pathways with a diameter of more than 60 Å. This is important, for example, for the processes of absorption of amino acids and monosaccharides, the concentration of which in the small intestine increases after eating enough for their passive transport.

Maintaining distinction between apical and basolateral membranes

If liposomes containing fluorescently labeled glycoproteins are added to the medium in contact with the apical part of the MDCK cell monolayer, some of them spontaneously fuse with cell membranes. After this, fluorescence can be detected in the apical, but not in the basolateral part of the cells, provided that the tight junctions are intact. If they are destroyed by removing calcium from the medium, fluorescent proteins diffuse and are evenly distributed over the entire surface of the cell.

Diseases associated with tight junctions

Some hereditary human disorders are associated with impaired formation of tight junctions, such as mutations in the claudin-16 and claudin-19 genes, which lead to hypomagnesemia due to excessive loss of magnesium in the urine. Mutations in the claudin-13 and tricellulin genes cause hereditary deafness. Dysregulation of several tight junction proteins is associated with cancer, for example, the expression of ZO-1 and ZO-2 is reduced in many types of cancer. Components of close contacts may also be targets for oncogenic viruses.

Some viruses use membrane tight junction proteins to enter cells; in particular, claudin-1 is a coreceptor for the hepatitis C virus. Other viruses attach to tight junction proteins to disrupt the barrier separating them from their actual receptors on the basolateral layer of epithelial cells, or non-epithelial cells.

Tight junctions can also be a target for bacterial pathogens, for example Clostridium perfringens, the causative agent of gas gangrene, secretes enterotoxin ( English), acting on the extracellular domains of membrane claudins and occludins, and causes epithelial leakage. Helicobacter pylori, the causative agent of gastritis, introduces the CagA protein into cells, which interacts with the ZO-1-JAM-A complex, it is believed that this helps the bacterium overcome the protective barrier of the gastric epithelium

Tight contact - the bilipid layers of the membranes of neighboring cells come into contact. In the area of ​​the tight junction zone, virtually no substances pass between cells.

Constant cellular contacts hold the cells in the epithelial cell layer together in such a way that even small molecules are prevented from flowing from one side of the layer to the other. The lateral mobility of many membrane proteins is limited. Limitation of mobility is achieved using barriers formed with the participation of tight junctions.

Clones of epithelial tissues (epithelium) function as selectively permeable barriers separating fluids with different chemical compositions on either side of the layer. Tight junctions play two roles in performing this function.

Transcellular transport carried out by epithelial cells (for example, nutrients from the cavity of the small intestine into the intracellular fluid on the other side of the layer) depends on two groups of membrane transport proteins: one is located on the apical (facing the cavity) surface of the cell and actively transports individual molecules into the cell; the other is located on the basolateral surface of the cell and allows the same molecules to leave the cell by facilitated diffusion. To maintain this directional transport, there must be no movement of apical transporter proteins to the basolateral surface and vice versa.

In addition, the spaces between the epithelial cells must be sealed so that transported molecules cannot diffuse back into the cavity through the intercellular spaces.

Tight junctions perform these two functions: barriers to the diffusion of membrane proteins between the apical and basolateral surfaces and holding neighboring cells together so that water-soluble molecules cannot flow to the other side of the layer. At the same time, tight junctions are impermeable to macromolecules, and their permeability to small molecules varies greatly in different epithelia. Epithelial cells can temporarily modify tight junctions to allow increased fluid flow through gaps in the junction barriers. This paracellular transport is especially important during the absorption of amino acids and monosaccharides from the small intestinal cavity.

The most important element in the structure of selectively permeable epithelial and endothelial barriers are tight junctions. Selective permeability varies from tissue to tissue, allowing either whole cells and macromolecules to pass through, or only protons and ions. The tight junction appears as a belt of interwoven fastening threads that completely surrounds the apical end of each cell of the epithelial layer. The tethering filaments are believed to consist of long rows of specific transmembrane proteins in each of the two interacting plasma membranes, which bind directly to each other, resulting in occlusion of the intercellular space. The integral membrane protein of the tight junction turned out to be occludin (it interacts with two cytoplasmic proteins, ZO-1 and ZO-2 (zonula occludence 1, 2). Their function is not completely clear. Perhaps their role is to localize occludin in sites between the apical and basolateral surfaces cells. Several cytoskeleton-associated proteins were also found in areas of tight junctions. Among them, zingulin, antigen and actin (according to electron microscopy, actin filaments consist of two chains of globular molecules, 4 nm in diameter and forming a double helix, each turn of which contains 13.5 molecules). These chains form the basis of thin filaments of skeletal muscles, which, in addition to actin, also contain several other proteins; globular actin has a molecular weight of about 42 kDa. It contains one polypeptide chain consisting of 375 or 374 amino acid residues; differences in amino acid the sequences of different actins, both within one species and between species, are extremely insignificant. They make up no more than 25 amino acid substitutions; Currently, in vertebrates, 6 actin isoforms are distinguished; depending on the isoelectric point, they are divided into 3 classes - alpha, beta and gamma; Beta and gamma actins are characteristic of non-muscle cells, and alpha actins are characteristic of muscle cells). Ras plays a role in regulating the functioning of tight junctions. Thus, cells apparently have similar mechanisms for the construction and regulation of adhesion structures, and these mechanisms are closely related to changes in the cytoskeleton. However, how cytoskeletal rearrangements affect the processes of intercellular adhesion is not yet completely clear. The mechanisms of adhesion and intercellular signaling are closely related to the long-known phenomenon of contact inhibition, the nature of which is still not fully understood.