Gap junctions are membrane channels between adjacent cells that allow the direct exchange of cytoplasmic substances.[1] Substances exchanged include small molecules, substrates, and metabolites.[1]
Gap junctions were first described as close appositions as other tight junctions, but following electron microscopy studies in 1967, they were renamed gap junctions to distinguish them from tight junctions.[2] They bridge a 2-4 nm gap between cell membranes.[3]
Gap junctions use protein complexes known as connexons to connect one cell to another. The proteins are called connexins. Gap junction proteins include the more than 26 types of connexin, and at least 12 non-connexin components that make up the gap junction complex or nexus.[4] These components include the tight junction protein ZO-1—a protein that holds membrane content together and adds structural clarity to a cell,[5]sodium channels,[6] and aquaporin.[7][8]
More gap junction proteins have become known due to the development of next-generation sequencing. Connexins were found to be structurally homologous between vertebrates and invertebrates but different in sequence.[9] As a result, the term innexin is used to differentiate invertebrate connexins.[10] There are more than 20 known innexins,[11] along with unnexins in parasites and vinnexins in viruses.
An electrical synapse is a gap junction that can transmit action potentials between neurons. Such synapses create bidirectional continuous-time electrical coupling[12][13] between neurons. Connexon pairs act as generalized regulated gates for ions and smaller molecules between cells. Hemichannel connexons form channels to the extracellular environment.[14][15][16][17]
A gap junction or macula communicans is different from an ephaptic coupling that involves electrical signals external to the cells.[18][19]
^Cite error: The named reference Gruijters, WTM 1989 509–13 was invoked but never defined (see the help page).
^Phelan, Pauline; Stebbings, Lucy A.; Baines, Richard A.; Bacon, Jonathan P.; Davies, Jane A.; Ford, Chris (January 1998). "Drosophila Shaking-B protein forms gap junctions in paired Xenopus oocytes". Nature. 391 (6663): 181–184. Bibcode:1998Natur.391..181P. doi:10.1038/34426. PMID9428764. S2CID205003383.
^Lampe, Paul D.; Lau, Alan F. (2000). "Regulation of gap junctions by phosphorylation of connexins". Archives of Biochemistry and Biophysics. 384 (2): 205–15. doi:10.1006/abbi.2000.2131. PMID11368307.