Editing Cell adhesion

{{Short description|Process of cell attachment}}
{{More citations needed|date=September 2014}}

[[Image:Cell Adhesion.png|thumb|300px|Schematic of cell adhesion]]

'''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_junction|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.<ref name="Alberts2014">{{cite book| last1=Alberts| first1=Bruce|last2=Johnson|first2=Alexander|last3=Lewis|first3=Julian| last4=Morgan|first4=David|last5=Raff| first5=Martin|last6=Roberts| first6=Keith|last7=Walter|first7=Peter|title=Molecular Biology of the Cell.|date=2014|publisher=Garland Science|isbn=9780815344322|edition= 6th}}</ref> Cells adhesion occurs from the interactions between [[cell adhesion molecules|cell-adhesion molecules]] (CAMs),<ref name="Lodish2003">{{cite book|last1=Lodish|first1=Harvey|last2=Berk|first2=Arnold|last3=Matsudaira|first3=Paul|last4=Kaiser|first4=Chris A.|last5=Krieger|first5=Monty|last6=Scott|first6=Matthew P.|last7=Zipursky|first7=Lawrence|last8=Darnell|first8=James|title=Molecular Cell Biology|date=2003|publisher=W.H. Freeman|isbn=978-0716743668|edition=5th|url=https://archive.org/details/molecularcellbio00harv}}</ref> 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.<ref name="Alberts2014" /><ref name="Gumbiner1996">{{Cite journal | last1 = Gumbiner | first1 = Barry M. | title = Cell Adhesion: The Molecular Basis of Tissue Architecture and Morphogenesis| doi = 10.1016/S0092-8674(00)81279-9 | journal = Cell | volume = 84 | issue = 3 | pages = 345–357 | year = 1996 | pmid =  8608588| s2cid = 13443584 | doi-access = free }}</ref> Other cellular processes regulated by cell adhesion include [[cell migration]] and tissue development in [[multicellular organisms]].<ref name="Sumigray2015">{{Cite book|last1=Sumigray|first1=Kaelyn D.|last2=Lechler|first2=Terry|title=Cell Adhesion in Epidermal Development and Barrier Formation|journal=Current Topics in Developmental Biology|date=2015|volume=112|pages=383–414|doi=10.1016/bs.ctdb.2014.11.027|pmid=25733147|pmc=4737682|isbn=9780124077584}}</ref> Alterations in cell adhesion can disrupt important cellular processes and lead to a variety of diseases, including cancer<ref name="Okegawa2004">{{cite journal|last1=Okegawa|first1=T|last2=Pong|first2=RC|last3=Li|first3=Y|last4=Hsieh|first4=JT|title=The role of cell adhesion molecule in cancer progression and its application in cancer therapy.|journal=Acta Biochimica Polonica|date=2004|volume=51|issue=2|pages=445–57|pmid=15218541|doi=10.18388/abp.2004_3583|doi-access=free}}</ref><ref name="Hirohashi2003">{{cite journal|last1=Hirohashi|first1=Setsuo|last2=Kanai|first2=Yae|title=Cell adhesion system and human cancer morphogenesis|journal=Cancer Science|date=2003|volume=94|issue=7|pages=575–581|doi=10.1111/j.1349-7006.2003.tb01485.x|pmid=12841864|s2cid=22154824|doi-access=free|pmc=11160151}}</ref> and [[arthritis]].<ref name="Szekanecz2000">{{cite journal|last1=Szekanecz|first1=Zoltan|last2=Koch|first2=Alisa E|title=Cell–cell interactions in synovitis: Endothelial cells and immune cell migration|journal=Arthritis Research|date=2000|volume=2|issue=5|pages=368–373|doi=10.1186/ar114|pmid=11094450|pmc=130138 |doi-access=free }}</ref> Cell adhesion is also essential for infectious organisms, such as [[bacteria]] or [[Virus|viruses]], to cause [[Disease|diseases]].<ref name="Pizarro2006">{{cite journal|last1=Pizarro-Cerdá|first1=Javier|last2=Cossart|first2=Pascale|title=Bacterial Adhesion and Entry into Host Cells|journal=Cell|date=2006|volume=124|issue=4|pages=715–727|doi=10.1016/j.cell.2006.02.012|pmid=16497583|s2cid=5769387|doi-access=free}}</ref><ref name="Mateo2015">{{cite journal|last1=Mateo|first1=M.|last2=Generous|first2=A.|last3=Sinn|first3=P. L.|last4=Cattaneo|first4=R.|title=Connections matter - how viruses use cellcell adhesion components|journal=Journal of Cell Science|date=2015|volume=128|issue=3|pages=431–439|doi=10.1242/jcs.159400|pmid=26046138|pmc=4311127}}</ref>

==General mechanism==
[[File:Cell junctions types shown on epithelial cells including cell-cell and cell-matrix junctions.jpeg|thumb|600px|Overview diagram of different types of cell junctions present in epithelial cells, including cell–cell junctions and cell–matrix junctions.]]

CAMs are classified into four major families: [[integrins]], [[immunoglobulin superfamily|immunoglobulin (Ig) superfamily]], [[cadherins]], and [[selectins]].<ref name="Lodish2003" />  '''[[Cadherin|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.<ref name="Lodish2003" /> {{Citation needed|reason= The section 'Adhesion mediated by members of the immunoglobulin superfamily' on this page appears to detail this is true for neural IgSF members but not intercellular or vascular IgSF members.|date=March 2021}} 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:<ref name=Alberts2014 />

*Anchoring junctions ([[adherens junctions]], [[desmosomes]] and [[hemidesmosomes]]), which maintain cells together and strengthens contact between cells.
*Occluding junctions ([[tight junctions]]), which seal gaps between cells through cell–cell contact, making an impermeable barrier for diffusion
*Channel-forming junctions ([[gap junctions]]), which links cytoplasm of adjacent cells allowing transport of molecules to occur between cells
*Signal-relaying junctions, which can be synapses in the nervous system

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===
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 Ca<sup>2+</sup> ions to function correctly.<ref name="Lodish2003" /> 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.<ref name="Alberts2014" /> 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.<ref name="Lodish2003" />

====Adherens junctions====
[[File:Adheren junction showing homophilic binding between cadherins.jpg|thumb|350px|Adheren junction showing homophilic binding between cadherins and how catenin links it to actin filaments]]
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 Ca<sup>2+</sup> ions, extracellular domains of cadherins undergo a [[conformational change]] from the inactive [[protein dynamics | 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.<ref name="Meng2009">{{cite journal|last1=Meng|first1=W.|last2=Takeichi|first2=M.|title=Adherens Junction: Molecular Architecture and Regulation|journal=Cold Spring Harbor Perspectives in Biology|date=2009|volume=1|issue=6|page=a002899|doi=10.1101/cshperspect.a002899|pmid=20457565|pmc=2882120}}</ref><ref name="pmid30037495">{{cite journal | vauthors = Nicholl ID, Matsui T, Weiss TM, Stanley CB, Heller WT, Martel A, Farago B, Callaway DJ, Bu Z | title = Alpha-catenin structure and nanoscale dynamics in solution and in complex with F-actin | journal = Biophysical Journal | volume = 115 | issue = 4  | pages = 642–654 | date =  Aug 21, 2018 | pmid = 30037495 | pmc = 6104293 | doi = 10.1016/j.bpj.2018.07.005 | bibcode = 2018BpJ...115..642N | url = }}</ref><ref name="Harris2010">{{cite journal|last1=Harris|first1=Tony J. C.|last2=Tepass|first2=Ulrich|title=Adherens junctions: from molecules to morphogenesis|journal=Nature Reviews Molecular Cell Biology|date=2010|volume=11|issue=7|pages=502–514|doi=10.1038/nrm2927|pmid=20571587|s2cid=13638902}}</ref> 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.{{cn|date=August 2019}}

====Desmosomes====
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.<ref name="Johnson2014">{{cite journal|last1=Johnson|first1=J. L.|last2=Najor|first2=N. A.|last3=Green|first3=K. J.|title=Desmosomes: Regulators of Cellular Signaling and Adhesion in Epidermal Health and Disease|journal=Cold Spring Harbor Perspectives in Medicine|date=2014|volume=4|issue=11|page=a015297|doi=10.1101/cshperspect.a015297|pmid=25368015|pmc=4208714}}</ref> 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.<ref name="Delva2009">{{cite journal|last1=Delva|first1=E.|last2=Tucker|first2=D. K.|last3=Kowalczyk|first3=A. P.|title=The Desmosome|journal=Cold Spring Harbor Perspectives in Biology|date=2009|volume=1|issue=2|page=a002543|doi=10.1101/cshperspect.a002543|pmid=20066089|pmc=2742091}}</ref> 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.<ref name="Johnson2014" /> This makes desmosomes important in tissues that encounter high levels of mechanical stress, such as heart muscle and [[epithelium|epithelia]], and explains why it appears frequently in these types of tissues.

====Tight junctions====
Tight junctions are normally present in [[Epithelium|epithelial]] and [[endothelial]] tissues, where they seal gaps and regulate [[paracellular transport]] of solutes and extracellular fluids in these tissues that function as barriers.<ref name="Steed2010">{{cite journal|last1=Steed|first1=Emily|last2=Balda|first2=Maria S.|last3=Matter|first3=Karl|title=Dynamics and functions of tight junctions|journal=Trends in Cell Biology|date=2010|volume=20|issue=3|pages=142–149|doi=10.1016/j.tcb.2009.12.002|pmid=20061152}}</ref> Tight junction is formed by transmembrane proteins, including [[claudins]], [[occludin]]s and tricellulins, that bind closely to each other on adjacent membranes in a homophilic manner.<ref name="Alberts2014" /> Similar to anchoring junctions, intracellular domains of these tight junction proteins are bound with [[scaffold protein]]s that keep these proteins in clusters and link them to actin filaments in order to maintain structure of the tight junction.<ref name="Niessen2007">{{cite journal|last1=Niessen|first1=Carien M.|title=Tight Junctions/Adherens Junctions: Basic Structure and Function|journal=Journal of Investigative Dermatology|date=2007|volume=127|issue=11|pages=2525–2532|doi=10.1038/sj.jid.5700865|pmid=17934504|doi-access=free}}</ref> 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.<ref name="Steed2010" />

====Gap junctions====
[[File:Gap junctions showing connexons and connexins.jpg|thumb|400px|Gap junctions showing connexons and connexins]]
Gap junctions are composed of channels called [[connexons]], which consist of transmembrane proteins called [[connexins]] clustered in groups of six.<ref name="Goodenough2009">{{cite journal|last1=Goodenough|first1=D. A.|last2=Paul|first2=D. L.|title=Gap Junctions|journal=Cold Spring Harbor Perspectives in Biology|date=2009|volume=1|issue=1|page=a002576|doi=10.1101/cshperspect.a002576|pmid=20066080|pmc=2742079}}</ref> 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.<ref name="Alberts2014" /> 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 [[biochemical cascade|signalling cascades]].<ref name="Mese2007">{{cite journal|last1=Meşe|first1=Gülistan|last2=Richard|first2=Gabriele|last3=White|first3=Thomas W.|title=Gap Junctions: Basic Structure and Function|journal=Journal of Investigative Dermatology|date=2007|volume=127|issue=11|pages=2516–2524|doi=10.1038/sj.jid.5700770|pmid=17934503|doi-access=free}}</ref> Channels can respond to many different stimuli and are regulated dynamically either by rapid mechanisms, such as [[voltage-gated ion channel|voltage gating]], or by slow mechanism, such as altering numbers of channels present in gap junctions.<ref name="Goodenough2009" />

====Adhesion mediated by [[Selectin|selectins]]====
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.<ref name="McEver2015">{{cite journal|last1=McEver|first1=Rodger P.|title=Selectins: initiators of leucocyte adhesion and signalling at the vascular wall|journal=Cardiovascular Research|date=2015|volume=107|issue=3|pages=331–339|doi=10.1093/cvr/cvv154|pmid=25994174|pmc=4592324}}</ref> Selectins undergo heterophilic bindings, as its extracellular domain binds to carbohydrates on adjacent cells instead of other selectins, while it also require Ca<sup>2+</sup> ions to function, same as cadherins.<ref name="Alberts2014" /> 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.<ref name="Barthel2007">{{cite journal|last1=Barthel|first1=Steven R|last2=Gavino|first2=Jacyln D|last3=Descheny|first3=Leyla|last4=Dimitroff|first4=Charles J|title=Targeting selectins and selectin ligands in inflammation and cancer|journal=Expert Opinion on Therapeutic Targets|date=2007|volume=11|issue=11|pages=1473–1491|doi=10.1517/14728222.11.11.1473|pmid=18028011|pmc=2559865}}</ref> 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.<ref name="Barthel2007" />

====Adhesion mediated by members of the immunoglobulin superfamily====
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 domain|immunoglobulin-like domains]] in their extracellular domains and undergo calcium-independent binding with ligands on adjacent cells.<ref name="Wong2012">{{cite journal|last1=Wong|first1=Chee Wai|last2=Dye|first2=Danielle E.|last3=Coombe|first3=Deirdre R.|title=The Role of Immunoglobulin Superfamily Cell Adhesion Molecules in Cancer Metastasis|journal=International Journal of Cell Biology|date=2012|volume=2012|pages=340296|doi=10.1155/2012/340296|pmid=22272201|pmc=3261479|doi-access=free}}</ref> Some IgSF CAMs, such as [[neural cell adhesion molecules]] (NCAMs), can perform homophilic binding while others, such as [[intercellular adhesion molecule|intercellular cell adhesion molecules]] (ICAMs) or [[VCAM-1|vascular cell adhesion molecules]] (VCAMs) undergo heterophilic binding with molecules like carbohydrates or integrins.<ref name="Arisescu2007">{{cite journal|last1=Aricescu|first1=A Radu|last2=Jones|first2=E Yvonne|title=Immunoglobulin superfamily cell adhesion molecules: zippers and signals|journal=Current Opinion in Cell Biology|date=2007|volume=19|issue=5|pages=543–550|doi=10.1016/j.ceb.2007.09.010|pmid=17935964}}</ref> 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.<ref name="Arisescu2007" />

===Cell–matrix junctions===
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]].<ref name="Alberts2014" /> 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.<ref name="Lodish2003" />
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.<ref name="Takada2007">{{cite journal|last1=Takada|first1=Yoshikazu|last2=Ye|first2=Xiaojing|last3=Simon|first3=Scott|title=The integrins|journal=Genome Biology|date=2007|volume=8|issue=5|pages=215|doi=10.1186/gb-2007-8-5-215|pmid=17543136|pmc=1929136 |doi-access=free }}</ref> 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.<ref name="Takada2007" /> 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]].<ref name="Lodish2000">{{cite book|last1=Lodish|first1=Harvey|last2=Berk|first2=Arnold|last3=Zipursky|first3=S Lawrence|last4=Matsudaira|first4=Paul|last5=Baltimore|first5=David|last6=Darnell|first6=James|title=Molecular cell biology|date=2000|publisher=W.H. Freeman|isbn=978-0-7167-3136-8|edition=4th|url=https://archive.org/details/molecularcellbio00lodi}}</ref>
[[File:Hemidesmosomes showing interaction between integrins and laminin.jpg|thumb|500px|Hemidesmosomes diagram showing interaction between integrins and laminin, including how integrins are linked to keratin intermediate filaments]]

====Hemidesmosomes====
In hemidesmosomes, integrins attach to extracellular matrix proteins called [[laminins]] in the [[basal lamina]], which is the extracellular matrix secreted by epithelial cells.<ref name="Alberts2014" /> Integrins link extracellular matrix to [[keratin]] intermediate filaments, which interacts with intracellular domain of integrins via adapter proteins such as [[plectin]]s and BP230.<ref name="Borradori1999">{{cite journal|last1=Borradori|first1=Luca|last2=Sonnenberg|first2=Arnoud|title=Structure and Function of Hemidesmosomes: More Than Simple Adhesion Complexes|journal=Journal of Investigative Dermatology|date=1999|volume=112|issue=4|pages=411–418|doi=10.1046/j.1523-1747.1999.00546.x|pmid=10201522|doi-access=free}}</ref> Hemidesmosomes are important in maintaining structural stability of epithelial cells by anchoring them together indirectly through the extracellular matrix.

====Focal adhesions====
In focal adhesions, integrins attach [[fibronectins]], a component in the extracellular matrix, to actin filaments inside cells.<ref name="Lodish2000" /> Adapter proteins, such as [[Talin (protein)|talins]], [[vinculin]]s, [[α-actinin]]s and [[filamin]]s, form a complex at the intracellular domain of integrins and bind to actin filaments.<ref name="Critchley2000">{{cite journal|last1=Critchley|first1=David R|title=Focal adhesions – the cytoskeletal connection|journal=Current Opinion in Cell Biology|date=2000|volume=12|issue=1|pages=133–139|doi=10.1016/S0955-0674(99)00067-8|pmid=10679361}}</ref> 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.<ref name="Critchley2000" />

==Other organisms==

===Eukaryotes===
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.<ref name="Cilia2004">{{cite journal|last1=Cilia|first1=Michelle Lynn|last2=Jackson|first2=David|title=Plasmodesmata form and function|journal=Current Opinion in Cell Biology|date=2004|volume=16|issue=5|pages=500–506|doi=10.1016/j.ceb.2004.08.002|pmid=15363799}}</ref> Molecules that are either nutrients or signals required for growth are transported, either passively or selectively, between plant cells through plasmodesmata.<ref name="Cilia2004" />

[[Protozoans]] express multiple adhesion molecules with different specificities that bind to carbohydrates located on surfaces of their host cells.<ref name="Singh2016">{{cite journal|last1=Singh|first1=Ram Sarup|last2=Walia|first2=Amandeep Kaur|last3=Kanwar|first3=Jagat Rakesh|title=Protozoa lectins and their role in host–pathogen interactions|journal=Biotechnology Advances|date=2016|volume=34|issue=5|pages=1018–1029|doi=10.1016/j.biotechadv.2016.06.002|pmid=27268207}}</ref> cell–cell adhesion is key for pathogenic protozoans to attach en enter their host cells. An example of a pathogenic protozoan is the [[malaria]]l parasite (''[[Plasmodium falciparum]]''), which uses one adhesion molecule called the [[circumsporozoite protein]] to bind to liver cells,<ref name="Rathore2002">{{cite journal|last1=Rathore|first1=Dharmendar|last2=Sacci|first2=John B.|last3=de la Vega|first3=Patricia|last4=McCutchan|first4=Thomas F.|title=Binding and Invasion of Liver Cells by Sporozoites|journal=Journal of Biological Chemistry|date=2002|volume=277|issue=9|pages=7092–7098|doi=10.1074/jbc.M106862200|pmid=11751898|doi-access=free}}</ref> and another adhesion molecule called the [[merozoite surface protein]] to bind [[red blood cells]].<ref name="Kadekoppala2010">{{cite journal|last1=Kadekoppala|first1=Madhusudan|last2=Holder|first2=Anthony A.|title=Merozoite surface proteins of the malaria parasite: The MSP1 complex and the MSP7 family|journal=International Journal for Parasitology|date=2010|volume=40|issue=10|pages=1155–1161|doi=10.1016/j.ijpara.2010.04.008|pmid=20451527}}</ref>

Pathogenic [[fungi]] use [[Fungal adhesin|adhesion molecules]] present on its cell wall to attach, either through protein-protein or protein-carbohydrate interactions, to host cells<ref name="Tronchin2008">{{cite journal|last1=Tronchin|first1=Guy|last2=Pihet|first2=Marc|last3=Lopes-Bezerra|first3=Leila M.|last4=Bouchara|first4=Jean-Philippe|title=Adherence mechanisms in human pathogenic fungi|journal=Medical Mycology|date=2008|volume=46|issue=8|pages=749–772|doi=10.1080/13693780802206435|pmid=18651303|doi-access=free}}</ref> or fibronectins in the extracellular matrix.<ref name="Lima2001">{{cite journal|last1=Lima|first1=O. C.|last2=Figueiredo|first2=C. C.|last3=Previato|first3=J. O.|last4=Mendonca-Previato|first4=L.|last5=Morandi|first5=V.|last6=Lopes Bezerra|first6=L. M.|title=Involvement of Fungal Cell Wall Components in Adhesion of Sporothrix schenckii to Human Fibronectin|journal=Infection and Immunity|date=2001|volume=69|issue=11|pages=6874–6880|doi=10.1128/IAI.69.11.6874-6880.2001|pmid=11598061|pmc=100066}}</ref>

===Prokaryotes===
[[Prokaryote]]s have adhesion molecules on their cell surface termed [[bacterial adhesin]]s, apart from using its [[pilus|pili]] ([[fimbria (bacteriology)|fimbriae]]) and [[flagellum|flagella]] for cell adhesion.<ref name="Pizarro2006" /> 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.<ref>{{Cite web |last=M.Sc |first=Michael Greenwood |date=2021-10-04 |title=The Role of Flagella in Adhesion and Virulence |url=https://www.news-medical.net/life-sciences/The-Role-of-Flagella-in-Adhesion-and-Virulence.aspx |access-date=2024-01-18 |website=News-Medical |language=en}}</ref>

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.<ref name="Klemm2000">{{cite journal|last1=Klemm|first1=Per|last2=Schembri|first2=Mark A.|title=Bacterial adhesins: function and structure|journal=International Journal of Medical Microbiology|date=2000|volume=290|issue=1|pages=27–35|doi=10.1016/S1438-4221(00)80102-2|pmid=11043979}}</ref>

===Viruses===
[[Virus]]es 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.<ref name="Garman2015">{{cite journal|last1=Garman|first1=E. F.|title=Antiviral adhesion molecular mechanisms for influenza: W. G. Laver's lifetime obsession|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|date=2015|volume=370|issue=1661|pages=20140034|doi=10.1098/rstb.2014.0034|pmid=25533092|pmc=4275904}}</ref> [[HIV]] has an adhesion molecule termed [[gp120]] that binds to its ligand [[CD4]], which is expressed on [[lymphocyte]]s.<ref name="Capon1991">{{cite journal|last1=Capon|first1=D J|last2=Ward|first2=R H R|title=The CD4-gpl20 Interaction and Aids Pathogenesis|journal=Annual Review of Immunology|date=1991|volume=9|issue=1|pages=649–678|doi=10.1146/annurev.iy.09.040191.003245|pmid=1910691}}</ref> 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.<ref name="Mateo2015" />

==Clinical implications==
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.<ref name="Okegawa2004"/> 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.<ref name="Hirohashi2003" /> 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.<ref name="Bendas2012">{{cite journal|last1=Bendas|first1=Gerd|last2=Borsig|first2=Lubor|title=Cancer Cell Adhesion and Metastasis: Selectins, Integrins, and the Inhibitory Potential of Heparins|journal=International Journal of Cell Biology|date=2012|volume=2012|pages=676731|doi=10.1155/2012/676731|pmid=22505933|pmc=3296185|doi-access=free}}</ref> Due to the link between CAMs and cancer metastasis, these molecules could be potential therapeutic targets for cancer treatment.

There are also other human [[genetics|genetic]] diseases caused by an inability to express specific adhesion molecules. An example is [[leukocyte adhesion deficiency]]-I (LAD-I), where expression of the β<sub>2</sub> integrin subunit is reduced or lost.<ref name="Harris2012">{{cite journal|last1=Harris|first1=Estelle S.|last2=Weyrich|first2=Andrew S.|last3=Zimmerman|first3=Guy A.|title=Lessons from rare maladies: leukocyte adhesion deficiency syndromes|journal=Current Opinion in Hematology|volume=20|issue=1|date=2012|pages=16–25|doi=10.1097/MOH.0b013e32835a0091|pmid=23207660|pmc=3564641}}</ref> This leads to reduced expression of β<sub>2</sub> integrin heterodimers, which are required for leukocytes to firmly attach to the endothelial wall at sites of [[inflammation]] in order to fight infections.<ref name="Hanna2012">{{cite journal|last1=Hanna|first1=Suhair|last2=Etzioni|first2=Amos|title=Leukocyte adhesion deficiencies|journal=Annals of the New York Academy of Sciences|date=2012|volume=1250|issue=1|pages=50–55|doi=10.1111/j.1749-6632.2011.06389.x|pmid=22276660|bibcode=2012NYASA1250...50H|s2cid=33727687}}</ref> 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.<ref name="Tamgadge2011">{{cite journal|last1=Tamgadge|first1=Sandhya|last2=Bhatt|first2=DaivatM|last3=Pereira|first3=Treville|last4=Tamgadge|first4=Avinash|last5=Bhalerao|first5=Sudhir|title=Pemphigus vulgaris|journal=Contemporary Clinical Dentistry|date=2011|volume=2|issue=2|pages=134–7|doi=10.4103/0976-237X.83074|pmid=21957393|pmc=3180831 |doi-access=free }}</ref>

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.<ref name="Krachler2014">{{cite journal|last1=Krachler|first1=Anne Marie|last2=Orth|first2=Kim|author-link2=Kim Orth |title=Targeting the bacteria–host interface|journal=Virulence|date=2014|volume=4|issue=4|pages=284–294|doi=10.4161/viru.24606|pmid=23799663|pmc=3710331}}</ref> 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.<ref name="Ofek2003">{{cite journal|last1=Ofek|first1=Itzhak|last2=Hasty|first2=David L|last3=Sharon|first3=Nathan|title=Anti-adhesion therapy of bacterial diseases: prospects and problems|journal=FEMS Immunology & Medical Microbiology|date=2003|volume=38|issue=3|pages=181–191|doi=10.1016/S0928-8244(03)00228-1|pmid=14522453|citeseerx=10.1.1.320.1480}}</ref>

==See also==
* [[Cell communication (biology)]]
* [[Epithelium]]
* [[Cytoskeleton]]
* [[Differential adhesion hypothesis]]
* [[Role of cell adhesions in neural development]]

==References==
{{Reflist}}

==External links==
{{Commons category|Cell adhesion}}
*[https://www.ncbi.nlm.nih.gov/books/NBK9851/ The Cell] by G. Cooper (online textbook)
*[https://www.ncbi.nlm.nih.gov/books/NBK21599/ Molecular Cell Biology] by Lodish et al. (online textbook)
*[https://www.ncbi.nlm.nih.gov/books/NBK26937/ Molecular Biology of the Cell] by Alberts et al. (online textbook)
*[http://biochemweb.fenteany.com/adhesion_ecm.shtml Cell Adhesion and Extracellular Matrix - The Virtual Library of Biochemistry, Molecular Biology and Cell Biology]

{{Cell adhesion molecules}}

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[[Category:Cell adhesion| ]]