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Cell adhesion is the process by which cells interact and attach to neighbouring cells through specialised molecules of the cell surface. This process can occur either through direct contact between cell surfaces such as cell junctions or indirect interaction, where cells attach to surrounding extracellular matrix, a gel-like structure containing molecules released by cells into spaces between them. [1] Cells adhesion occurs from the interactions between cell-adhesion molecules (CAMs), [2] transmembrane proteins located on the cell surface. Cell adhesion links cells in different ways and can be involved in signal transduction for cells to detect and respond to changes in the surroundings. [1] [3] Other cellular processes regulated by cell adhesion include cell migration and tissue development in multicellular organisms. [4] Alterations in cell adhesion can disrupt important cellular processes and lead to a variety of diseases, including cancer [5] [6] and arthritis. [7] Cell adhesion is also essential for infectious organisms, such as bacteria or viruses, to cause diseases. [8] [9]
CAMs are classified into four major families: integrins, immunoglobulin (Ig) superfamily, cadherins, and selectins. [2] Cadherins and IgSF are homophilic CAMs, as they directly bind to the same type of CAMs on another cell, while integrins and selectins are heterophilic CAMs that bind to different types of CAMs. [2] [ citation needed ] Each of these adhesion molecules has a different function and recognizes different ligands. Defects in cell adhesion are usually attributable to defects in expression of CAMs.
In multicellular organisms, bindings between CAMs allow cells to adhere to one another and creates structures called cell junctions. According to their functions, the cell junctions can be classified as: [1]
Alternatively, cell junctions can be categorised into two main types according to what interacts with the cell: cell–cell junctions, mainly mediated by cadherins, and cell–matrix junctions, mainly mediated by integrins.
Cell–cell junctions can occur in different forms. In anchoring junctions between cells such as adherens junctions and desmosomes, the main CAMs present are the cadherins. This family of CAMs are membrane proteins that mediate cell–cell adhesion through its extracellular domains and require extracellular Ca2+ ions to function correctly. [2] Cadherins forms homophilic attachment between themselves, which results in cells of a similar type sticking together and can lead to selective cell adhesion, allowing vertebrate cells to assemble into organised tissues. [1] Cadherins are essential for cell–cell adhesion and cell signalling in multicellular animals and can be separated into two types: classical cadherins and non-classical cadherins. [2]
Adherens junctions mainly function to maintain the shape of tissues and to hold cells together. In adherens junctions, cadherins between neighbouring cells interact through their extracellular domains, which share a conserved calcium-sensitive region in their extracellular domains. When this region comes into contact with Ca2+ ions, extracellular domains of cadherins undergo a conformational change from the inactive flexible conformation to a more rigid conformation in order to undergo homophilic binding. Intracellular domains of cadherins are also highly conserved, as they bind to proteins called catenins, forming catenin-cadherin complexes. These protein complexes link cadherins to actin filaments. This association with actin filaments is essential for adherens junctions to stabilise cell–cell adhesion. [10] [11] [12] Interactions with actin filaments can also promote clustering of cadherins, which are involved in the assembly of adherens junctions. This is since cadherin clusters promote actin filament polymerisation, which in turn promotes the assembly of adherens junctions by binding to the cadherin–catenin complexes that then form at the junction.[ citation needed ]
Desmosomes are structurally similar to adherens junctions but composed of different components. Instead of classical cadherins, non-classical cadherins such as desmogleins and desmocollins act as adhesion molecules and they are linked to intermediate filaments instead of actin filaments. [13] No catenin is present in desmosomes, as intracellular domains of desmosomal cadherins interact with desmosomal plaque proteins, which form the thick cytoplasmic plaques in desmosomes and link cadherins to intermediate filaments. [14] Desmosomes provides strength and resistance to mechanical stress by unloading forces onto the flexible but resilient intermediate filaments, something that cannot occur with the rigid actin filaments. [13] This makes desmosomes important in tissues that encounter high levels of mechanical stress, such as heart muscle and epithelia, and explains why it appears frequently in these types of tissues.
Tight junctions are normally present in epithelial and endothelial tissues, where they seal gaps and regulate paracellular transport of solutes and extracellular fluids in these tissues that function as barriers. [15] Tight junction is formed by transmembrane proteins, including claudins, occludins and tricellulins, that bind closely to each other on adjacent membranes in a homophilic manner. [1] Similar to anchoring junctions, intracellular domains of these tight junction proteins are bound with scaffold proteins that keep these proteins in clusters and link them to actin filaments in order to maintain structure of the tight junction. [16] Claudins, essential for formation of tight junctions, form paracellular pores which allow selective passage of specific ions across tight junctions making the barrier selectively permeable. [15]
Gap junctions are composed of channels called connexons, which consist of transmembrane proteins called connexins clustered in groups of six. [17] Connexons from adjacent cells form continuous channels when they come into contact and align with each other. These channels allow transport of ions and small molecules between cytoplasm of two adjacent cells, apart from holding cells together and provide structural stability like anchoring junctions or tight junctions. [1] Gap junction channels are selectively permeable to specific ions depending on which connexins form the connexons, which allows gap junctions to be involved in cell signalling by regulating the transfer of molecules involved in signalling cascades. [18] Channels can respond to many different stimuli and are regulated dynamically either by rapid mechanisms, such as voltage gating, or by slow mechanism, such as altering numbers of channels present in gap junctions. [17]
Selectins are a family of specialised CAMs involved in transient cell–cell adhesion occurring in the circulatory system. They mainly mediate the movement of white blood cells (leukocytes) in the bloodstream by allowing the white blood cells to "roll" on endothelial cells through reversible bindings of selections. [19] Selectins undergo heterophilic bindings, as its extracellular domain binds to carbohydrates on adjacent cells instead of other selectins, while it also require Ca2+ ions to function, same as cadherins. [1] Cell–cell adhesion of leukocytes to endothelial cells is important for immune responses as leukocytes can travel to sites of infection or injury through this mechanism. [20] At these sites, integrins on the rolling white blood cells are activated and bind firmly to the local endothelial cells, allowing the leukocytes to stop migrating and move across the endothelial barrier. [20]
The immunoglobulin superfamily (IgSF) is one of the largest superfamily of proteins in the body and it contains many diverse CAMs involved in different functions. These transmembrane proteins have one or more immunoglobulin-like domains in their extracellular domains and undergo calcium-independent binding with ligands on adjacent cells. [21] Some IgSF CAMs, such as neural cell adhesion molecules (NCAMs), can perform homophilic binding while others, such as intercellular cell adhesion molecules (ICAMs) or vascular cell adhesion molecules (VCAMs) undergo heterophilic binding with molecules like carbohydrates or integrins. [22] Both ICAMs and VCAMs are expressed on vascular endothelial cells and they interact with integrins on the leukocytes to assist leukocyte attachment and its movement across the endothelial barrier. [22]
Cells create extracellular matrix by releasing molecules into its surrounding extracellular space. Cells have specific CAMs that will bind to molecules in the extracellular matrix and link the matrix to the intracellular cytoskeleton. [1] Extracellular matrix can act as a support when organising cells into tissues and can also be involved in cell signalling by activating intracellular pathways when bound to the CAMs. [2] Cell–matrix junctions are mainly mediated by integrins, which also clusters like cadherins to form firm adhesions. Integrins are transmembrane heterodimers formed by different α and β subunits, both subunits with different domain structures. [23] Integrins can signal in both directions: inside-out signalling, intracellular signals modifying the intracellular domains, can regulate affinity of integrins for their ligands, while outside-in signalling, extracellular ligands binding to extracellular domains, can induce conformational changes in integrins and initiate signalling cascades. [23] Extracellular domains of integrins can bind to different ligands through heterophilic binding while intracellular domains can either be linked to intermediate filaments, forming hemidesmosomes, or to actin filaments, forming focal adhesions. [24]
In hemidesmosomes, integrins attach to extracellular matrix proteins called laminins in the basal lamina, which is the extracellular matrix secreted by epithelial cells. [1] Integrins link extracellular matrix to keratin intermediate filaments, which interacts with intracellular domain of integrins via adapter proteins such as plectins and BP230. [25] Hemidesmosomes are important in maintaining structural stability of epithelial cells by anchoring them together indirectly through the extracellular matrix.
In focal adhesions, integrins attach fibronectins, a component in the extracellular matrix, to actin filaments inside cells. [24] Adapter proteins, such as talins, vinculins, α-actinins and filamins, form a complex at the intracellular domain of integrins and bind to actin filaments. [26] This multi-protein complex linking integrins to actin filaments is important for assembly of signalling complexes that act as signals for cell growth and cell motility. [26]
Plants cells adhere closely to each other and are connected through plasmodesmata, channels that cross the plant cell walls and connect cytoplasms of adjacent plant cells. [27] Molecules that are either nutrients or signals required for growth are transported, either passively or selectively, between plant cells through plasmodesmata. [27]
Protozoans express multiple adhesion molecules with different specificities that bind to carbohydrates located on surfaces of their host cells. [28] cell–cell adhesion is key for pathogenic protozoans to attach en enter their host cells. An example of a pathogenic protozoan is the malarial parasite ( Plasmodium falciparum ), which uses one adhesion molecule called the circumsporozoite protein to bind to liver cells, [29] and another adhesion molecule called the merozoite surface protein to bind red blood cells. [30]
Pathogenic fungi use adhesion molecules present on its cell wall to attach, either through protein-protein or protein-carbohydrate interactions, to host cells [31] or fibronectins in the extracellular matrix. [32]
Prokaryotes have adhesion molecules on their cell surface termed bacterial adhesins, apart from using its pili (fimbriae) and flagella for cell adhesion. [8] Prokaryotes may have a single or several flagella, either located on one or several places on the cell surface. Pathogenic species such as Escherichia coli and Vibrio cholera possess flagella to facilitate adhesion. [33]
Adhesins can recognise a variety of ligands present on the host cell surfaces and also components in the extracellular matrix. These molecules also control host specificity and regulate tropism (tissue- or cell-specific interactions) through their interaction with their ligands. [34]
Viruses also have adhesion molecules required for viral binding to host cells. For example, influenza virus has a hemagglutinin on its surface that is required for recognition of the sugar sialic acid on host cell surface molecules. [35] HIV has an adhesion molecule termed gp120 that binds to its ligand CD4, which is expressed on lymphocytes. [36] Viruses can also target components of cell junctions to enter host cells, which is what happens when the hepatitis C virus targets occludins and claudins in tight junctions to enter liver cells. [9]
Dysfunction of cell adhesion occurs during cancer metastasis. Loss of cell–cell adhesion in metastatic tumour cells allows them to escape their site of origin and spread through the circulatory system. [5] One example of CAMs deregulated in cancer are cadherins, which are inactivated either by genetic mutations or by other oncogenic signalling molecules, allowing cancer cells to migrate and be more invasive. [6] Other CAMs, like selectins and integrins, can facilitate metastasis by mediating cell–cell interactions between migrating metastatic tumour cells in the circulatory system with endothelial cells of other distant tissues. [37] Due to the link between CAMs and cancer metastasis, these molecules could be potential therapeutic targets for cancer treatment.
There are also other human genetic diseases caused by an inability to express specific adhesion molecules. An example is leukocyte adhesion deficiency-I (LAD-I), where expression of the β2 integrin subunit is reduced or lost. [38] This leads to reduced expression of β2 integrin heterodimers, which are required for leukocytes to firmly attach to the endothelial wall at sites of inflammation in order to fight infections. [39] Leukocytes from LAD-I patients are unable to adhere to endothelial cells and patients exhibit serious episodes of infection that can be life-threatening.
An autoimmune disease called pemphigus is also caused by loss of cell adhesion, as it results from autoantibodies targeting a person's own desmosomal cadherins which leads to epidermal cells detaching from each other and causes skin blistering. [40]
Pathogenic microorganisms, including bacteria, viruses and protozoans, have to first adhere to host cells in order to infect and cause diseases. Anti-adhesion therapy can be used to prevent infection by targeting adhesion molecules either on the pathogen or on the host cell. [41] Apart from altering the production of adhesion molecules, competitive inhibitors that bind to adhesion molecules to prevent binding between cells can also be used, acting as anti-adhesive agents. [42]
Integrins are transmembrane receptors that help cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, organization of the intracellular cytoskeleton, and movement of new receptors to the cell membrane. The presence of integrins allows rapid and flexible responses to events at the cell surface.
A desmosome, also known as a macula adherens, is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes. Desmosomes are one of the stronger cell-to-cell adhesion types and are found in tissue that experience intense mechanical stress, such as cardiac muscle tissue, bladder tissue, gastrointestinal mucosa, and epithelia.
Cadherins (named for "calcium-dependent adhesion") are cell adhesion molecules important in forming adherens junctions that let cells adhere to each other. Cadherins are a class of type-1 transmembrane proteins, and they depend on calcium (Ca2+) ions to function, hence their name. Cell-cell adhesion is mediated by extracellular cadherin domains, whereas the intracellular cytoplasmic tail associates with numerous adaptors and signaling proteins, collectively referred to as the cadherin adhesome.
Cell adhesion molecules (CAMs) are a subset of cell surface proteins that are involved in the binding of cells with other cells or with the extracellular matrix (ECM), in a process called cell adhesion. In essence, CAMs help cells stick to each other and to their surroundings. CAMs are crucial components in maintaining tissue structure and function. In fully developed animals, these molecules play an integral role in generating force and movement and consequently ensuring that organs are able to execute their functions normally. In addition to serving as "molecular glue", CAMs play important roles in the cellular mechanisms of growth, contact inhibition, and apoptosis. Aberrant expression of CAMs may result in a wide range of pathologies, ranging from frostbite to cancer.
Cell junctions or junctional complexes are a class of cellular structures consisting of multiprotein complexes that provide contact or adhesion between neighboring cells or between a cell and the extracellular matrix in animals. They also maintain the paracellular barrier of epithelia and control paracellular transport. Cell junctions are especially abundant in epithelial tissues. Combined with cell adhesion molecules and extracellular matrix, cell junctions help hold animal cells together.
Catenins are a family of proteins found in complexes with cadherin cell adhesion molecules of animal cells. The first two catenins that were identified became known as α-catenin and β-catenin. α-Catenin can bind to β-catenin and can also bind filamentous actin (F-actin). β-Catenin binds directly to the cytoplasmic tail of classical cadherins. Additional catenins such as γ-catenin and δ-catenin have been identified. The name "catenin" was originally selected because it was suspected that catenins might link cadherins to the cytoskeleton.
In mammalian cells, vinculin is a membrane-cytoskeletal protein in focal adhesion plaques that is involved in linkage of integrin adhesion molecules to the actin cytoskeleton. Vinculin is a cytoskeletal protein associated with cell-cell and cell-matrix junctions, where it is thought to function as one of several interacting proteins involved in anchoring F-actin to the membrane.
In cell biology, focal adhesions are large macromolecular assemblies through which mechanical force and regulatory signals are transmitted between the extracellular matrix (ECM) and an interacting cell. More precisely, focal adhesions are the sub-cellular structures that mediate the regulatory effects of a cell in response to ECM adhesion.
In cell biology, adherens junctions are protein complexes that occur at cell–cell junctions and cell–matrix junctions in epithelial and endothelial tissues, usually more basal than tight junctions. An adherens junction is defined as a cell junction whose cytoplasmic face is linked to the actin cytoskeleton. They can appear as bands encircling the cell or as spots of attachment to the extracellular matrix.
In biology, juxtracrine signalling is a type of cell–cell or cell–extracellular matrix signalling in multicellular organisms that requires close contact. In this type of signalling, a ligand on one surface binds to a receptor on another adjacent surface. Hence, this stands in contrast to releasing a signaling molecule by diffusion into extracellular space, the use of long-range conduits like membrane nanotubes and cytonemes or the use of extracellular vesicles like exosomes or microvesicles. There are three types of juxtracrine signaling:
ICAM-1 also known as CD54 is a protein that in humans is encoded by the ICAM1 gene. This gene encodes a cell surface glycoprotein which is typically expressed on endothelial cells and cells of the immune system. It binds to integrins of type CD11a / CD18, or CD11b / CD18 and is also exploited by rhinovirus as a receptor for entry into respiratory epithelium.
α-Catenin (alpha-catenin) functions as the primary protein link between cadherins and the actin cytoskeleton. It has been reported that the actin binding proteins vinculin and α-actinin can bind to alpha-catenin. It has been suggested that alpha-catenin does not bind with high affinity to both actin filaments and the E-cadherin-beta-catenin complex at the same time. It has been observed that when α-catenin is not in a molecular complex with β-catenin, it dimerizes and functions to regulate actin filament assembly, possibly by competing with Arp2/3 protein. α-Catenin exhibits significant protein dynamics. However, a protein complex including a cadherin, actin, β-catenin and α-catenin has not been isolated.
In immunology, leukocyte extravasation is the movement of leukocytes out of the circulatory system (extravasation) and towards the site of tissue damage or infection. This process forms part of the innate immune response, involving the recruitment of non-specific leukocytes. Monocytes also use this process in the absence of infection or tissue damage during their development into macrophages.
Receptor-type tyrosine-protein phosphatase mu is an enzyme that in humans is encoded by the PTPRM gene.
Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated actomyosin structure within non-muscle cells. Stress fibers have been shown to play an important role in cellular contractility, providing force for a number of functions such as cell adhesion, migration and morphogenesis.
A catch bond is a type of noncovalent bond whose dissociation lifetime increases with tensile force applied to the bond. Normally, bond lifetimes are expected to diminish with force. In the case of catch bonds, the lifetime of the bond actually increases up to a maximum before it decreases like in a normal bond. Catch bonds work in a way that is conceptually similar to that of a Chinese finger trap. While catch bonds are strengthened by an increase in force, the force increase is not necessary for the bond to work. Catch bonds were suspected for many years to play a role in the rolling of leukocytes, being strong enough to roll in presence of high forces caused by high shear stresses, while avoiding getting stuck in capillaries where the fluid flow, and therefore shear stress, is low. The existence of catch bonds was debated for many years until strong evidence of their existence was found in bacteria. Definite proof of their existence came shortly thereafter in leukocytes.
Cell–cell interaction refers to the direct interactions between cell surfaces that play a crucial role in the development and function of multicellular organisms. These interactions allow cells to communicate with each other in response to changes in their microenvironment. This ability to send and receive signals is essential for the survival of the cell. Interactions between cells can be stable such as those made through cell junctions. These junctions are involved in the communication and organization of cells within a particular tissue. Others are transient or temporary such as those between cells of the immune system or the interactions involved in tissue inflammation. These types of intercellular interactions are distinguished from other types such as those between cells and the extracellular matrix. The loss of communication between cells can result in uncontrollable cell growth and cancer.
Cellular adhesions can be defined as proteins or protein aggregates that form mechanical and chemical linkages between the intracellular and extracellular space. Adhesions serve several critical processes including cell migration, signal transduction, tissue development and repair. Due to this functionality, adhesions and adhesion molecules have been a topic of study within the scientific community. Specifically, it has been found that adhesions are involved in tissue development, plasticity, and memory formation within the central nervous system (CNS), and may prove vital in the generation of CNS-specific therapeutics.
Synaptic stabilization is crucial in the developing and adult nervous systems and is considered a result of the late phase of long-term potentiation (LTP). The mechanism involves strengthening and maintaining active synapses through increased expression of cytoskeletal and extracellular matrix elements and postsynaptic scaffold proteins, while pruning less active ones. For example, cell adhesion molecules (CAMs) play a large role in synaptic maintenance and stabilization. Gerald Edelman discovered CAMs and studied their function during development, which showed CAMs are required for cell migration and the formation of the entire nervous system. In the adult nervous system, CAMs play an integral role in synaptic plasticity relating to learning and memory.
Tight junction proteins are molecules situated at the tight junctions of epithelial, endothelial and myelinated cells. This multiprotein junctional complex has a regulatory function in passage of ions, water and solutes through the paracellular pathway. It can also coordinate the motion of lipids and proteins between the apical and basolateral surfaces of the plasma membrane. Thereby tight junction conducts signaling molecules, that influence the differentiation, proliferation and polarity of cells. So tight junction plays a key role in maintenance of osmotic balance and trans-cellular transport of tissue specific molecules. Nowadays is known more than 40 different proteins, that are involved in these selective TJ channels.
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