Editing Gap junction (section)

===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"/>