Gap junction
Cell-cell junction composed of innexins or connexins, / From Wikipedia, the free encyclopedia
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Gap junctions are one of three broad categories of intercellular connections that form between a multitude of animal cell types.[1][2] Photographed with an electron microscope circa 1952,[3][4] and first referred to as gap junctions in 1969.[5] They were named after the 2-4 nm gap they bridged between cell membranes.[6]
Gap junction | |
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Identifiers | |
MeSH | D017629 |
TH | H1.00.01.1.02024 |
FMA | 67423 |
Anatomical terminology |
This article's tone or style may not reflect the encyclopedic tone used on Wikipedia. (April 2024) |
Within a gap junction reside protein complexes referred to initially as globules, connecting one cell to another, and vesicles within a cell to the outer cell membrane.[7] A major protein component of the globules was dubbed a connexin,[8][9] and six connexins were observed to form a channel called a connexon due to the connections connexon pairs made between cells. The initial discovery of gap junctions in nerve cells lent credence to their function in transmission of electrical impulses. Experimental confirmation followed, with molecules, ions, and electrical impulses being shown to pass through the connexons; this proved them to be a generalized regulated gate between cells in gap junctions. A type of hemichannel connexon was also found to form channels to the extracellular regions as well.[10][11][12][13]
More than 26 different connexins are known to comprise gap junctions in various different tissues; there are at least 12 other components[14] that form the specialized area of membrane called the gap junction complex. These components include the tight junction protein ZO-1 that holds the membranes close together,[15] sodium channels,[16] and aquaporin.[17][18]
The increasing ability to sequence the DNA of organisms also led to an increase in the complexity of the gap junction family of proteins. The term connexin was used to describe the gap junction proteins connecting two cells with pores. Sequencing of these pore proteins showed them to be structurally similar between vertebrates and invertebrates but different in sequence.[19] As a result, the term innexin was used to differentiate invertebrate connexins from vertebrate ones.[20] While the sequencing of invertebrate species is far less complete than for vertebrates, more than 20 innexins have already been uncovered,[21] along with unnexins in parasites and vinnexins in viruses.
A gap junction has also been referred to has a nexus or macula communicans though are sometimes confused with an ephapse. While an ephapse, like a gap junction, also involves the transmission of electrical signals, the two are distinct from each other. Ephaptic coupling involves electrical signals external to the cells. Ephapses are often studied in the context of electrically induced potentials propagated among groups of nerve cell membranes, even in the absence of gap junction communication, with no discrete subcellular structures known.[22][23] Unlike gap junctions, no specific structure related to an ephapse has yet been described, so the process is often referred to as ephaptic coupling rather than as an ephapse.