Editing Gap junction (section)

==Discovery==
===Form to function===
Well before the demonstration of the gap in gap junctions, they were seen at the junction of neighboring nerve cells. The close proximity of the neighboring cell membranes at the gap junction led researchers to speculate that they had a role in intercellular communication, in particular the transmission of electrical signals.<ref name="robertson" /><ref>{{Cite book|editor1-first=Michael |editor1-last=Locke |year=1963 |title=Cellular membranes in development |publisher=Academic Press |location=New York |oclc=261587041 |last1=Robertson |first1=J. D. }}{{Page needed|date=September 2010}}</ref><ref>{{Cite journal|pmid=7033238 |pmc=2112820 |title=Membrane structure |journal=The Journal of Cell Biology |year=1981 |volume=91 |issue=3 |author=Robertson |pages=189s–204s |jstor=1609517 |doi=10.1083/jcb.91.3.189s}}</ref> Gap junctions were also found to be electrically rectifying in the early studies and referred to as an [[electrical synapse]]<ref>{{Cite journal|doi=10.1038/180342a0 |title=Mechanism of Nerve-Impulse Transmission at a Crayfish Synapse |year=1957 |vauthors=Furshpan EJ, Potter DD |journal=Nature |volume=180 |pages=342–343 |pmid=13464833 |issue=4581 |bibcode=1957Natur.180..342F |s2cid=4216387}}</ref><ref>{{Cite journal |pmid=13642302 |pmc=1356828 |year=1959 |vauthors=Furshpan EJ, Potter DD |title=Transmission at the giant motor synapses of the crayfish |volume=145 |issue=2 |pages=289–325 |journal=The Journal of Physiology |doi=10.1113/jphysiol.1959.sp006143}}</ref> but are now known to be bidirectional in general.<ref name=":1">{{Cite book |title=Neuroscience |date=2001 |publisher=Sinauer Associates |isbn=978-0-87893-742-4 |editor-last=Purves |editor-first=Dale |edition=2nd |location=Sunderland, Mass |editor-last2=Williams |editor-first2=Stephen Mark}}</ref><ref name=":0">{{Cite book |title=Principles of neural science |date=2013 |publisher=McGraw-Hill medical |isbn=978-0-07-139011-8 |edition=5th |location=New York}}</ref> Later, it was found that chemicals could also be transported between cells through gap junctions.<ref>{{cite journal |pmid=5360587 |volume=166 |issue=3913 |title=Permeability and structure of junctional membranes at an electrotonic synapse |date=December 1969 |journal=Science |pages=1641–1643 |doi=10.1126/science.166.3913.1641 |last1=Payton |first1=B. W. |last2=Bennett |first2=M. V. L. |last3=Pappas |first3=G. D. |bibcode=1969Sci...166.1641P |s2cid=24701801}}</ref>

Implicit or explicit in most of the early studies is that the area of the gap junction was different in structure to the surrounding membranes in a way that made it look different. The gap junction had been shown to create a micro-environment between the two cells in the extracellular space or ''gap''. This portion of extracellular space was somewhat isolated from the surrounding space and also bridged by what we now call connexon pairs, which form even more tightly sealed bridges that cross the gap junction gap between two cells. When viewed in the plane of the membrane by freeze-fracture techniques, higher-resolution distribution of connexons within the gap junction plaque is possible.<ref>{{cite journal | pmid = 4935338 | doi=10.1083/jcb.47.1.49 | pmc=2108397 | volume=47 | issue=1 | title=An interpretation of liver cell membrane and junction structure based on observation of freeze-fracture replicas of both sides of the fracture |date=October 1970 | journal=J. Cell Biol. | pages=49–60 | last1 = Chalcroft | first1 = J. P. | last2 = Bullivant | first2 = S}}</ref>

Connexin free islands are observed in some junctions. The observation was largely without explanation until vesicles were shown by Peracchia using [[transmission electron microscopy]] (TEM) thin sections to be systematically associated with gap junction plaques.<ref>{{cite journal | pmid = 4120610 | pmc=2108965 | volume=57 | issue=1 | title=Low resistance junctions in crayfish. II. Structural details and further evidence for intercellular channels by freeze-fracture and negative staining |date=April 1973 | journal=J. Cell Biol. | pages=54–65 | last1 = Peracchia | first1 = C | doi = 10.1083/jcb.57.1.54}}</ref> Peracchia's study was probably also the first study to describe paired connexon structures, which he called a ''globule''. Studies showing vesicles associated with gap junctions and proposing the vesicle contents may move across the junction plaques between two cells were rare, as most studies focused on connexons rather than vesicles. A later study using a combination of microscopy techniques confirmed the early evidence of a probable function for gap junctions in intercellular vesicle transfer. Areas of vesicle transfer were associated with connexin free islands within gap junction plaques.<ref name ="Gruijters-vesicles"/> Connexin 43 has been shown to be necessary for the transfer of whole mitochondrias to neighboring cells, though whether the mitochondria is transferred directly through the membrane or within a vesicle has not been determined <ref>{{cite journal | last1 =Islam | first1 = M.| last2 = Das | first2 = S. |last3=Emin | first3=M. |display-authors= et al. | title=Mitochondrial transfer from bone-marrow–derived stromal cells to pulmonary alveoli protects against acute lung injury. | journal=Nat Med | volume=18 | pages=759–765 | year=2012 | issue = 5| doi=10.1038/nm.2736| pmid = 22504485| pmc = 3727429}}</ref>

===Electrical and chemical synapses===
Because of the widespread occurrence of gap junctions in cell types other than nerve cells, the term ''gap junction'' became more generally used than terms such as electrical synapse or nexus. Another dimension in the relationship between nerve cells and gap junctions was revealed by studying chemical synapse formation and gap junction presence. By tracing nerve development in leeches with gap junction expression suppressed it was shown that the bidirectional gap junction (electrical nerve synapse) needs to form between two cells before they can grow to form a unidirectional ''chemical nerve synapse''.<ref>{{cite journal |vauthors=Todd KL, Kristan WB, French KA |title=Gap junction expression is required for normal chemical synapse formation |journal=J. Neurosci. |volume=30 |issue=45 |pages=15277–85 |date=November 2010 |pmid=21068332 |pmc=3478946 |doi=10.1523/JNEUROSCI.2331-10.2010 }}</ref> The chemical nerve synapse is the synapse most often truncated to the more ambiguous term ''nerve synapse''.

===Composition===

====Connexins====
The purification<ref>{{Cite journal|doi=10.1083/jcb.54.3.646 |title=The isolation of mouse hepatocyte gap junctions : Preliminary Chemical Characterization and X-Ray Diffraction |year=1972 |last1=Goodenough |first1=D. A. |last2=Stoeckenius |first2=W. |journal=The Journal of Cell Biology |volume=54 |issue=3 |pages=646–56 |pmid=4339819 |pmc=2200277}}</ref><ref>{{Cite journal|doi=10.1083/jcb.61.2.557 |title=Bulk isolation of mouse hepatocyte gap junctions : Characterization of the Principal Protein, Connexin |year=1974 |last1=Goodenough |first1=D. A. |journal=The Journal of Cell Biology |volume=61 |pages=557–63 |pmid=4363961 |issue=2 |pmc=2109294}}</ref> of the intercellular gap junction plaques enriched in the channel forming protein ([[connexin]]) showed a protein forming hexagonal arrays in [[x-ray diffraction]]. Because of this, the systematic study and identification of the predominant [[gap junction protein]]<ref>{{Cite journal|doi=10.1083/jcb.103.3.767 |title=Cloning and characterization of human and rat liver cDNAs coding for a gap junction protein |year=1986 |last1=Kumar |first1=N. M. |journal=The Journal of Cell Biology |volume=103 |pages=767–76 |pmid=2875078 |first2=NB |issue=3 |pmc=2114303 |last2=Gilula}}</ref> became possible. 
Refined ultrastructural studies by TEM<ref>{{cite journal |vauthors=McNutt NS, Weinstein RS |title=The ultrastructure of the nexus. A correlated thin-section and freeze-cleave study |journal=J. Cell Biol. |volume=47 |issue=3 |pages=666–88 |date=December 1970 |pmid=5531667 |pmc=2108148 |doi=10.1083/jcb.47.3.666}}</ref><ref>{{Cite journal|doi=10.1083/jcb.47.1.49 |title=An interpretation of liver cell membrane and junction structure based on observation of freeze-fracture replicas of both sides of the fracture |year=1970 |last1=Chalcroft |first1=J. P. |journal=The Journal of Cell Biology |volume=47 |pages=49–60 |pmid=4935338 |first2=S |issue=1 |pmc=2108397 |last2=Bullivant}}</ref> showed protein occurred in a complementary fashion in both cells participating in a gap junction plaque. The gap junction plaque is a relatively large area of membrane observed in TEM [[thin section]] and freeze fracture (FF) seen filled with transmembrane proteins in both tissues and more gently treated gap junction preparations. With the apparent ability for one protein alone to enable intercellular communication seen in gap junctions<ref>{{Cite journal|pmid=3815522 |year=1987 |author1=Young |first2=ZA |first3=NB |title=Functional assembly of gap junction conductance in lipid bilayers: demonstration that the major 27 kd protein forms the junctional channel |volume=48 |issue=5 |pages=733–43 |journal=Cell |doi=10.1016/0092-8674(87)90071-7 |last2=Cohn |last3=Gilula|s2cid=39342230 }}</ref> the term gap junction tended to become synonymous with a group of assembled connexins though this was not shown in vivo. Biochemical analysis of gap junction isolated from various tissues demonstrated a family of connexins.<ref>{{Cite journal |year=1985 |vauthors=Nicholson BJ, Gros DB, Kent SB, Hood LE, Revel JP |title=The Mr 28,000 gap junction proteins from rat heart and liver are different but related |volume=260 |issue=11 |pages=6514–6517 |journal=The Journal of Biological Chemistry |doi=10.1016/S0021-9258(18)88810-X |doi-access=free |pmid=2987225 }}</ref><ref>{{Cite journal |doi=10.1083/jcb.105.6.2621 |title=Connexin43: a protein from rat heart homologous to a gap junction protein from liver |year=1987 |vauthors=Beyer EC, Paul DL, Goodenough DA |journal=The Journal of Cell Biology |volume=105 |pages=2621–2629 |pmid=2826492 |issue=6 Pt 1 |pmc=2114703}}</ref><ref>{{Cite journal |pmid=3891760 |year=1985 |vauthors=Kistler J, Kirkland B, Bullivant S |title=Identification of a 70,000-D protein in lens membrane junctional domains |volume=101 |issue=1 |pages=28–35 |pmc=2113615 |journal=The Journal of Cell Biology |doi=10.1083/jcb.101.1.28}}</ref>

The ultrastructure and biochemistry of isolated gap junctions already referenced had indicated the connexins preferentially group in gap junction plaques or domains and connexins were the best characterized constituent. It has been noted that the organisation of proteins into arrays with a gap junction plaque may be significant.<ref name="ReferenceA"/><ref>{{cite journal|author1-link=Lucas Andrew Staehelin |vauthors=Staehelin LA |title=Three types of gap junctions interconnecting intestinal epithelial cells visualized by freeze-etching |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=69 |issue=5 |pages=1318–21 |date=May 1972 |pmid=4504340 |pmc=426690 |bibcode=1972PNAS...69.1318S |doi=10.1073/pnas.69.5.1318 |doi-access=free}}</ref> It is likely this early work was already reflecting the presence of more than just connexins in gap junctions. Combining the emerging fields of freeze-fracture to see inside membranes and [[immunocytochemistry]] to label cell components (Freeze-fracture replica immunolabelling or FRIL and thin section immunolabelling) showed gap junction plaques in vivo contained the connexin protein.<ref>{{Cite journal|pmid=3818793 |year=1987 |last1=Gruijters |first1=WTM |first2=J |first3=S |first4=DA |title=Immunolocalization of MP70 in lens fiber 16-17-nm intercellular junctions |volume=104 |issue=3 |pages=565–72 |pmc=2114558 |journal=The Journal of Cell Biology |doi=10.1083/jcb.104.3.565 |last2=Kistler |last3=Bullivant |last4=Goodenough}}</ref><ref name="Gruijters"/> Later studies using [[immunofluorescence]] microscopy of larger areas of tissue clarified diversity in earlier results. Gap junction plaques were confirmed to have variable composition being home to connexon and non-connexin proteins as well making the modern usage of the terms "gap junction" and "gap junction plaque" non-interchangeable.<ref name="Gruijters, WTM 1989 509–13">{{Cite journal|pmid=2691517 |journal=Journal of Cell Science |title=A non-connexon protein (MIP) is involved in eye lens gap-junction formation |volume=93 |issue=3 |pages=509–513 |last1=Gruijters |first1=WTM |year=1989|doi=10.1242/jcs.93.3.509 |url=http://jcs.biologists.org/cgi/content/abstract/93/3/509|url-access=subscription }}</ref> To summarize, in early literature the term "gap junction" referred to the regular gap between membranes in vertebrates and non-vertebrates apparently bridged by "globules". The junction correlated with the cell's ability to directly couple with its neighbors through pores in their membranes. Then for a while gap junctions were only referring to a structure that contains connexins and nothing more was thought to be involved. Later, the gap junction "plaque" was also found to contain other molecules that helped define it and make it function.

====The "plaque" or "formation plaque"====
[[File:Connexin43-Modulates-Cell-Polarity-and-Directional-Cell-Migration-by-Regulating-Microtubule-Dynamics-pone.0026379.s004.ogv|right|thumb|[[Immunofluorescence]] microscopy video of connexins being moved along [[microtubule]]s to the surface of a cell at 2.7 times normal speed.<ref name="ReferenceB"/>]]
Early descriptions of ''gap junctions'', ''connexons'' or ''innexons'' did not refer to them as such; many other terms were used. It is likely that ''synaptic disks''<ref>{{cite journal | pmid = 14069795 | pmc=2106854 | volume=19 | title=The occurrence of a subunit pattern in the unit membranes of club endings in mauthner cell synapses in goldfish brains |date=October 1963 | journal=J. Cell Biol. | pages=201–21 | doi = 10.1083/jcb.19.1.201 | last1 = Robertson | first1 = JD | issue = 1}}</ref> were an accurate reference to gap junction plaques. While the detailed structure and function of the connexon was described in a limited way at the time the gross ''disk'' structure was relatively large and easily seen by various TEM techniques. Disks allowed researchers using TEM to easily locate the connexons contained within the disk like patches in vivo and in vitro. The disk or ''plaque'' appeared to have structural properties different from those imparted by the connexons/innexons alone.<ref name=Hand72>{{cite journal | pmid = 4109925 | pmc=2108629 | volume=52 | issue=2 | title=The structural organization of the septate and gap junctions of Hydra |date=February 1972 | journal=J. Cell Biol. | pages=397–408 | doi = 10.1083/jcb.52.2.397 | last1 = Hand | first1 = AR | last2 = Gobel | first2 = S}}</ref> It was thought that if the area of membrane in the plaque transmitted signals, the area of membrane would have to be sealed in some way to prevent leakage.<ref>{{cite journal |vauthors=Loewenstein WR, Kanno Y |title=Studies on an epithelial (gland) cell junction. I. Modifications of surface membrane permeability |journal=J. Cell Biol. |volume=22 |pages=565–86 |date=September 1964 |pmid=14206423 |pmc=2106478 |doi=10.1083/jcb.22.3.565 |issue=3}}</ref>
Later studies showed gap junction plaques are home to non-connexin proteins, making the modern usage of the terms "gap junction" and "gap junction plaque" non-interchangeable as the area of the gap junction plaque may contain proteins other than connexins.<ref name="Gruijters, WTM 1989 509–13"/><ref name="Gruijters-vesicles"/> Just as connexins do not always occupy the entire area of the plaque, the other components described in the literature may be only long-term or short-term residents.<ref name=pmc3156236>{{cite journal |vauthors=Ozato-Sakurai N, Fujita A, Fujimoto T|title=The distribution of phosphatidylinositol 4,5-bisphosphate in acinar cells of rat pancreas revealed with the freeze-fracture replica labeling method |journal=PLOS ONE |volume=6 |issue=8 |pages=e23567 |year=2011 |pmid=21858170 |pmc=3156236 |doi=10.1371/journal.pone.0023567 |editor1-last=Wong |editor1-first=Nai Sum|bibcode = 2011PLoSO...623567O |doi-access=free }}</ref><ref name="partners" /><ref>{{cite journal |last1=Strauss |first1=RE |last2=Gourdie |first2=RG |title=Cx43 and the Actin Cytoskeleton: Novel Roles and Implications for Cell-Cell Junction-Based Barrier Function Regulation |journal=Biomolecules |date=December 2020 |volume=10 |issue=12 |page=1656 |doi=10.3390/biom10121656 |pmid=33321985|pmc=7764618 |doi-access=free }}</ref>

Studies allowing views inside the plane of the membrane of gap junctions during formation indicated that a "formation plaque" formed between two cells prior to the connexins moving in. They were particle free areas—when observed by TEM FF, indicated very small or no [[transmembrane protein]]s were likely present. Little is known about what structures make up the formation plaque or how the formation plaque's structure changes when connexins and other components move in and out. One of the earlier studies of the formation of small gap junctions describes rows of particles and particle free halos.<ref>{{cite journal | pmid = 4135001 | pmc=2109180 | volume=62 | issue=1 | title=Assembly of gap junctions during amphibian neurulation |date=July 1974 | journal=J. Cell Biol. | pages=32–47 | doi = 10.1083/jcb.62.1.32 | last1 = Decker | first1 = RS | last2 = Friend | first2 = DS}}</ref> With larger gap junctions they were described as formation plaques with connexins moving into them. The particulate gap junctions were thought to form 4–6 hours after the formation plaques appeared.<ref>{{cite journal | pmid = 1083855 | pmc=2109697 | volume=69 | issue=3 | title=Hormonal regulation of gap junction differentiation |date=June 1976 | journal=J. Cell Biol. | pages=669–85 | doi = 10.1083/jcb.69.3.669 | last1 = Decker | first1 = RS}}</ref> How the connexins may be transported to the plaques using [[tubulin]] is becoming clearer.<ref name="ReferenceB"/><ref>{{cite journal |vauthors=Lauf U, Giepmans BN, Lopez P, Braconnot S, Chen SC, Falk MM|title=Dynamic trafficking and delivery of connexons to the plasma membrane and accretion to gap junctions in living cells |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=99 |issue=16 |pages=10446–51 |date=August 2002 |pmid=12149451 |pmc=124935 |doi=10.1073/pnas.162055899 |bibcode = 2002PNAS...9910446L |doi-access=free }}</ref>

The formation of plaque and the non-connexin part of the classical gap junction plaque have been difficult for early researchers to analyse. It appears in TEM FF and thin section to be a lipid membrane domain that can somehow form a comparatively rigid barrier to other lipids and proteins. There has been indirect evidence for certain lipids being preferentially involved with the formation plaque, however this cannot be considered definitive.<ref>{{cite journal | pmid = 1698798 | volume=96 | title=Increased gap junction assembly between cultured cells upon cholesterol supplementation |date=June 1990 | journal=J. Cell Sci. | pages=231–8 | issue=2 | last1 = Meyer | first1 = R | last2 = Malewicz | first2 = B | last3 = Baumann | first3 = WJ | last4 = Johnson | first4 = RG| doi=10.1242/jcs.96.2.231 }}</ref><ref>{{cite journal | pmid = 22049024 | doi=10.1091/mbc.E11-02-0141 | volume=23 | issue=1 | title=Gap junction assembly: roles for the formation plaque and regulation by the C-terminus of connexin43 | pmc=3248906 |date=January 2012 | journal=Mol. Biol. Cell | pages=71–86 | last1 = Johnson | first1 = R. G. | last2 = Reynhout | first2 = J. K. | last3 = Tenbroek | first3 = E. M. | last4 = Quade | first4 = B. J. | last5 = Yasumura | first5 = T. | last6 = Davidson | first6 = K. G. V. | last7 = Sheridan | first7 = J. D. | last8 = Rash | first8 = J. E.}}</ref> It is difficult to envisage breaking up the membrane to analyse membrane plaques without affecting their composition. By study of connexins still in membranes lipids associated with the connexins have been studied.<ref>{{cite journal | pmid = 19686581 | doi=10.1186/1741-7007-7-52 | volume=7 | title=Connexin channels and phospholipids: association and modulation | pmc=2733891 | year=2009 | journal=BMC Biol. | pages=52 | last1 = Locke | first1 = Darren | last2 = Harris | first2 = Andrew L | issue=1 | doi-access=free }}</ref> It was found that specific connexins tended to associate preferentially with specific phospholipids. As formation plaques precede connexins these results still give no certainty as to what is unique about the composition of plaques themselves. Other findings show connexins associate with protein scaffolds used in another junction, the zonula occludens [[Tight junction protein 1|ZO-1]].<ref name="Naomi Kamasawa PMC">{{cite journal |vauthors=Li X, Kamasawa N, Ciolofan C, etal |title=Connexin45-containing neuronal gap junctions in rodent retina also contain connexin36 in both apposing hemiplaques, forming bihomotypic gap junctions, with scaffolding contributed by zonula occludens-1 |journal=J. Neurosci. |volume=28 |issue=39 |pages=9769–89 |date=September 2008 |pmid=18815262 |pmc=2638127 |doi=10.1523/JNEUROSCI.2137-08.2008}}</ref> While this helps us understand how connexins may be moved into a gap junction formation plaque, the composition of the plaque itself is still somewhat sketchy. Some headway on the in vivo composition of the gap junction plaque is being made using TEM FRIL.<ref name=pmc3156236/><ref name="Naomi Kamasawa PMC"/>