Gap junctions are clusters of intercellular channels connecting adjacent cells and permitting the direct exchange of ions and small molecules between cells. These channels are composed of two hemichannels, or connexons, one located on each of the two neighboring cells. Each connexon is composed of 6 trans-membrane protein subunits of the connexin (Cx) family. A gap of approximately 3 nm remains between the adjacent cell membranes, but two connexons interact and dock head-to-head in the extra-cellular space forming a tightly sealed, double-membrane intercellular channel (see Segretain and Falk, 2004). The activity of these intercellular channels is regulated, particularly by intramolecular modifications such as phosphorylation which appears to regulate connexin turnover, gap junction assembly and the opening and closure (gating) of gap junction channels.
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Bruzzone R.; ''Learning the language of cell-cell communication through connexin channels.''; PubMedEurope PMCScholia
Martin PE, Mambetisaeva ET, Archer DA, George CH, Evans WH.; ''Analysis of gap junction assembly using mutated connexins detected in Charcot-Marie-Tooth X-linked disease.''; PubMedEurope PMCScholia
Huang S, Dudez T, Scerri I, Thomas MA, Giepmans BN, Suter S, Chanson M.; ''Defective activation of c-Src in cystic fibrosis airway epithelial cells results in loss of tumor necrosis factor-alpha-induced gap junction regulation.''; PubMedEurope PMCScholia
Segretain D, Falk MM.; ''Regulation of connexin biosynthesis, assembly, gap junction formation, and removal.''; PubMedEurope PMCScholia
Giepmans BN, Hengeveld T, Postma FR, Moolenaar WH.; ''Interaction of c-Src with gap junction protein connexin-43. Role in the regulation of cell-cell communication.''; PubMedEurope PMCScholia
Martin PE, Blundell G, Ahmad S, Errington RJ, Evans WH.; ''Multiple pathways in the trafficking and assembly of connexin 26, 32 and 43 into gap junction intercellular communication channels.''; PubMedEurope PMCScholia
Spinella F, Rosanò L, Di Castro V, Nicotra MR, Natali PG, Bagnato A.; ''Endothelin-1 decreases gap junctional intercellular communication by inducing phosphorylation of connexin 43 in human ovarian carcinoma cells.''; PubMedEurope PMCScholia
Kelsell DP, Dunlop J, Hodgins MB.; ''Human diseases: clues to cracking the connexin code?''; PubMedEurope PMCScholia
Ahmad S, Diez JA, George CH, Evans WH.; ''Synthesis and assembly of connexins in vitro into homomeric and heteromeric functional gap junction hemichannels.''; PubMedEurope PMCScholia
Falk MM, Buehler LK, Kumar NM, Gilula NB.; ''Cell-free synthesis and assembly of connexins into functional gap junction membrane channels.''; PubMedEurope PMCScholia
Falk MM, Kumar NM, Gilula NB.; ''Membrane insertion of gap junction connexins: polytopic channel forming membrane proteins.''; PubMedEurope PMCScholia
Piehl M, Lehmann C, Gumpert A, Denizot JP, Segretain D, Falk MM.; ''Internalization of large double-membrane intercellular vesicles by a clathrin-dependent endocytic process.''; PubMedEurope PMCScholia
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Simon AM, Goodenough DA.; ''Diverse functions of vertebrate gap junctions.''; PubMedEurope PMCScholia
Lauf U, Giepmans BN, Lopez P, Braconnot S, Chen SC, Falk MM.; ''Dynamic trafficking and delivery of connexons to the plasma membrane and accretion to gap junctions in living cells.''; PubMedEurope PMCScholia
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GJ plaques, clusters of GJ channels, can be internalized to form large, double-membrane vesicles (aka AGJs). Internalized AGJ vesicles subdivide into smaller vesicles that are subsequently degraded by endo/lysosomal pathways (Piehl et al., 2007).
One mechanism of transport of connexon-containing vesicles involves movement along microtubules (Segretain and Falk, 2004). Such a transport system has been described for similar secretory vesicles (Toomre et al., 1999). Direct microtubule-dependent transport of connexons to GJ-assembly sites has recently been reported as well (Shaw et al., 2007).
Connexin-interacting proteins appear to function in regulating gap junction formation and communication. ZO-1 has been shown to alter the membrane localization of Cx43 and plays a role in regulating Cx43-mediated gap junctional communication in osteoblastic cells (Laing et al. 2005). ZO-1 may function in the delivery of Cx43 from a lipid raft domain to gap junctional plaques, which may be an important regulatory step in gap junction formation.
A study using cultured cells demonstrated connexon oligomerization from Cx43 subunits inside the Trans-Golgi Network after exit from the ER (Musil and Goodenough 1993).
Studies using microsomes have revealed that oligomerization of connexins Cx26, Cx43, and Cx32 can occur after insertion of connexins in the ER membrane (Falk et al. 1997; Ahmad et al. 1999, Ahmad and Evans, 2002).
Connexins (Cxs) are encoded by a large gene family predicted to include at least 20 isoforms in humans. Most mammalian Cx genes consist of two exons. The first consists of untranslated sequence, and the second contains the entire coding sequence. Exceptionally, Cx36 and Cx45 contain 3 exons and 2 introns and the third exon contains the coding sequence (Belluardo et al. 1999 ; Jacob and Beyer 2001). Connexins have been divided in two major subgroups, alpha and beta, according to their amino acid sequence similarity (see Bruzzone et al., 2001; Willecke et al., 2002). Alternative names and additional subgroups have been suggested as well. Cx are synthesized by ribosomes in the endoplasmic reticulum (ER) membrane. All Cx proteins contain four trans-membrane domains (TM1 to TM4), two extracellular loops (E1 and E2) and one cytoplasmic loop. The amino- and carboxyl termini are located in the cytosol (reviewed in Segretain and Falk, 2004). After targeting to the ER, connexins are checked by a quality control system to prevent misfolded forms from progressing through the secretory pathway. Aberrant proteins are removed by endoplasmic-reticulum-associated degradation (ERAD).
Transport of connexins along the secretory pathway (including transit from the Golgi to the TGN where Cx43 is predicted to oligomerize) occurs in vesicular transport containers.
Oligomerization of connexins Cx32 and Cx26 has also been observed in the ER-Golgi-intermediate compartment (ERGIC) (Diez et al. 1999). Heteromeric connexons containing both Cx32 and Cx26 have been observed. For the sake of simplicity, the connexon here is described as containing equal numbers of Cx26 and Cx32 subunits, although the ratio may vary.
Transport of connexins along the secretory pathway (including transit from the ER to the ERGIC where Cx32 is predicted to oligomerize) occurs in vesicular transport containers.
Junctional channels are an assembly of two docked connexons on adjacent cells that permits direct communication of the cytoplasm in the two cells as shown below. Proteins associated with GJs such as catenins (Wu et al., 2003, Shaw et al., 2007) and L-CAM (Musil et al., 1990) might be required for connexon docking. Docking occurs through a tight interaction of the extracellular loops (Unger et al., 1999; Sosinsky and Nicholson, 2005). Intramolecular disulfide bridges between the two extracellular loops (E1 and E2) of connexin polypeptides are important for the correct three-dimensional structure of the extracellular loops (Foote et al., 1998)
Once transported to the plasma membrane, junctional channels aggregate into clusters forming gap junction plaques that may contain a few to many thousands of individual channels and that vary in size from a few square nanometers to many square micrometers (Bruzzone et al. 1996; Falk 2000; Severs et al. 2001). Gap junction plaques are involved in numerous processes including growth and differentiation (Loewenstein and Rose 1992), pathological cell proliferation (Roger et al. 2004; Segretain et al. 2003) and spermatogenesis (Juneja et al. 1999; Plum et al. 2000). The physiological importance of gap junction plaques is underscored by the diverse pathologies associated with connexin gene mutations (De Maio et al. 2002). An arbitrary number (10) of channels is shown as aggregating in this reaction but the actual number may be hundreds to thousands.
Internalized GJ plaques are degraded by lysosomes. Lysosomal degradation appears to be the most common pathway of GJ degradation. (Qin et al., 2003; Grinzberg and Gilula., 1979 ; Berthoud et al., 2004 ; and Leithe et al., 2006).
Hemi-channels appear to play a role in isosmotic cell volume regulation (Quist et al., 2000), in apoptosis regulation (Contreras et al. 2002; John et al. 1999), and in the differentiation of different cell types (Boucher and Bennett 2003). Individual connexons have been observed dispersed around gap junction plaques. This observation suggests that there is a pool of connexons dispersed in the plasma membrane which can migrate to gap junction plaques (Benedetti et al. 2000; Hulser et al. 1997; Lauf et al. 2002; Gaietta et al. 2002).
It has been observed, however, that the oligomerization of Cx43 into connexons does not occur before the Trans-Golgi network (Musil and Goodenough, 1993; Koval, 2006).
c-src has been shown to interact with Cx43 (Giepmans et al., 2001). Models describing v-src mediated Cx43 channel gating propose that the initial interaction between v-src and Cx43 may occur via a SH3 domain interaction (see Lau 2005).
Docking of Cx43 at the plasma membrane may involve ZO-1 as well as alpha- and beta-catenin (Shaw et al., 2007). The role of ZO-1 in regulating gap junction biology is unclear. Recent results indicate a role for ZO-1 in regulating gap junction plaque size (Hunter et al., 2007).
The GTPase dynamin, which functions in the completion of vesicle budding localizes in Cx43-based GJs and especially invaginating plaques and AGJ vesicles (Piehl et al., 2007).
Dab2 is recruited to Cx43-based GJs possibly through a direct interaction between its N-terminal phosphotyrosine binding (PTB) domain and a putative XPXY internalization motif found in the C-terminal tail of Cx43 as well as a number of other connexin family members (Piehl et al., 2007).The distal portion of Dab2 on its opposite end binds the globular N-terminal domain of clathrin heavy chains (Piehl et al., 2007).
Connexons may also traffic using a microtubule-independent mechanism. A few studies suggest that rough ER membranes can directly transfer connexons to the plasma membrane (Martin et al. 2001; Bloom and Goldstein 1998). Other cytoskeletal components, such as actin filaments, might be involved in the delivery of connexons to gap junction plaques (Thomas et al. 2001; Gilleron et al. 2006).
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DataNodes
in Golgi transport
vesicleCx43-containing
transport vesiclesplaques associated with Dab2 and
DynaminCx43 junctional
channeltransport vesicle docked to
microtubulesCx43:ZO-1 gap
junctionplaques associated
with Dab2Annotated Interactions
in Golgi transport
vesiclein Golgi transport
vesiclein Golgi transport
vesicleCx43-containing
transport vesiclesCx43-containing
transport vesiclesplaques associated with Dab2 and
Dynaminplaques associated with Dab2 and
DynaminCx43 junctional
channeltransport vesicle docked to
microtubulestransport vesicle docked to
microtubulesCx43:ZO-1 gap
junctionplaques associated
with Dab2plaques associated
with Dab2