QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online October 22, 2003
doi: 10.1242/10.1242/jcs.00821

This Article Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JCS Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Nicholson, B. J. Search for Related Content PubMed PubMed Citation Articles by Nicholson, B. J. Social Bookmarking                         What's this?

Gap junctions – from cell to molecule

Department of Biological Sciences, University of Buffalo, SUNY, 619 Cooke Hall, NY 14260, USA

(e-mail: bjn{at}acsu.buffalo.edu)

	Introduction

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 
Gap junctions were first defined morphologically as specialized contacts between cells that mediate the direct exchange of small molecules needed to coordinate behavior of multicellular systems. As a functional entity, they have evolved independently three times, if one includes plasmodesmata in plants, which are structurally very distinct from gap junctions but mediate similar functions. The gap junctions of vertebrates and invertebrates are similar in both structure and function, but the former are composed of the connexin family (Eiberger et al., 2001; Willecke et al., 2002), while the latter are composed of members of a topologically similar but unrelated family called the innexins (Phelan and Starrich, 2001). Recently, however, three `pannexin' genes displaying distant homology to the innexins, but of as yet unknown function, have been found in vertebrates (Panchin et al., 2000).

View larger version (71K): [in this window] [in a new window]

	Diverse functions

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

	Formation and degradation

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 
Gap junctions form punctate `plaques' at the interfaces between cells, although protein is often seen trapped in the ER and Golgi in some disease-associated mutants. Although not glycosylated, most connexins do pass through the Golgi, and oligomerize late in their biosynthesis (Musil and Goodenough, 1993). There is also some evidence for non-Golgi transport of Cx26 (Martin et al., 2001). Hexameric connexons, or hemichannels, are inserted into the membrane, where they remain in a predominantly closed state (but see below) until recruited to the outside of existing plaques within the membrane (Gaietta et al., 2002). Removal of old connexins can occur through `buddingoff' of whole pieces of the plaque, to form annular gap junction vesicles within one cell (Jordan et al., 2001). Both lysosomal and proteasomal degradation pathways for connexins have been reported (Laing et al., 1998). Surprisingly, connexins themselves are turned over very rapidly, with a half-life of 1-5 hours (reviewed by Saffitz et al., 2000).

	Channel permeability properties

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 
The primary function ascribed to gap junctions has been the transfer of ions and metabolites under about 1000 Da between cells in contact. Although this may seem to be a `housekeeping' function, several recent studies have revealed a surprising diversity in the permeability properties of gap junctions composed of different connexins. Ion substitution experiments show that the anion/cation preferences of connexins can differ by up to tenfold, while larger permeants (cationic tertiary amines, neutral polyethylene glycols and anionic Alexa fluorophores), show exclusion limits that vary from 7 to 15Å (200 to over 800 in molecular weight), depending on the connexin composition of the channel (reviewed by Harris, 2001). Rates of dye flux through individual channels were 1-3 orders of magnitude higher than those predicted by diffusion, indicating a significant affinity between the dyes and the pore wall (B. J. Nicholson and J. Nitsche, unpublished). This is consistent with findings that different connexins can impart selectivity levels of 10-100-fold for natural permeants such as ATP, AMP, adenosine and glutathione (Goldberg et al., 1999), IP₃ (Niessen et al., 2000) and even different cyclic nucleotides (Bevans et al., 1998).

	Heterologous interactions between connexins

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 
The diversity of gap junction channel properties, defined above for channels composed of one connexin, is further increased by the expression of multiple connexin isotypes in most cells. Both heterotypic (different connexins in opposed cells) and heteromeric (different connexins in the same cell) channels have been documented (reviewed by Harris, 2001). Generally, heterotypic channels behave consistent with an in-series combination of homomeric channels, although rectifying channels can be produced (Suchyna et al., 1999). Interactions between connexins seem to be somewhat selective. Heterotypic coupling tends to occur, with some notable exceptions, within an homology group [i.e. or ß (Harris, 2001)], dictated by sequences, possibly as few as a couple of residues, in the second extracellular loop (H. Zhu and B. J. Nicholson, unpublished). Within a single plaque, some connexins will segregate, while others mix (Falk, 2000), which suggests that formation of heteromeric channels is also selective, probably within homology classes based on several biochemical studies (reviewed by Harris, 2001). Other than the preferential interactions just described, no common properties link the members of connexin groupings, which have been derived from sequence alignments of the connexins.

	Channel gating and regulation

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 
Gap junction channels are unusual among ion channels in that they typically remain open at rest, and only close under specific circumstances. However, the gating of these channels is among the most complex of any ion channel. They close in response to a number of stimuli that generally reflect the poor health of a cell, including low pH, high intracellular Ca²⁺ concentration, and voltage differences between cells (V_j) (reviewed by Harris, 2001). Some channels are sensitive to transmembrane voltage (V_m) as well, but this is more common among the invertebrate innexins. Gap junction channels also close in response to a number of physiological stimuli, typically associated with increased cell division. These include growth factors such as PDGF and EGF, oncogenes such as v-Src, and PKC and Cdc2 kinases associated with mitosis that either directly, or indirectly, phosphorylate the C-terminal tail (reviewed by Lampe and Lau, 2000).

	Hemichannels

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 
Prior to incorporation into gap junctions, hemichannels are typically kept closed through a combination of extracellular Ca²⁺ and V_m gates. On docking with a compatible hemichannel in the other cell, intercellular channels are triggered to open (`loop' gating). Some hemichannels (e.g. Cx38, Cx46 and Cx50) can be opened by membrane depolarization and reduced extracellular Ca²⁺ concentration. However, significant circumstantial evidence has implicated open hemichannels of other connexins in physiological processes as varied as extracellularly propagated Ca²⁺ waves (by release of ATP or NAD⁺), inhibition of apoptosis in osteocytes (both Cx43), and center surround inhibition of cones by horizontal cells (through local extracellular currents from Cx26) (Goodenough and Paul, 2003).

	Molecular structure of the pore

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 
Improved microscopic, biophysical and mutagenic strategies have led to recent advances in our understanding of the structural basis of gap junction function. Atomic force microscopy has provided a dynamic view of conformational changes in the hexameric channels during gating, but at limited resolution (Muller et al., 2002). However, a major breakthrough in resolving the structure of the pore came with the reconstruction of electron diffraction images of isolated Cx43 gap junctions (Unger et al., 1999). The four transmembrane helices of each subunit can be seen, with one contributing most of the pore lining (red asterisk, top panel of pore detail image). The docking interface between the two hemichannels appears seamless and shows a structure consistent with a previous model deduced from mutagenesis where interdigitation of the two antiparallel ß-strand loops of each subunit from opposed hemichannels form concentric ß-barrels (Foote et al., 1998).

All connexins have four transmembrane helices, two highly conserved extracellular loops that are held rigidly together by three disulfide linkages and three highly variable cytoplasmic domains. Systematic cysteine scanning mutagenesis of Cx32 has mapped the residues that contribute to the pore lining in open and closed states (Skerrett et al., 2002), which, surprisingly, revealed that the channel is lined with hydrophobic residues predominantly in M3, with some at the cytoplasmic end of M2. Other mutagenesis studies have indicated that M1 and its flanking domains may contribute to the pore lining of hemichannels (Harris, 2001) and are likely to serve as the voltage sensor of the channel (Rubin et al., 1992). The cytoplasmic C-terminal domain serves as a `ball' to close the channel in response to pH (Morley et al., 1996) and phosphorylation (Zhou et al., 1999), and contains a variety of targets and binding sites for various kinases, scaffolding and signaling proteins that have been most extensively mapped in Cx43. The latter suggest that, like adhesive and tight junctions, gap junctions may be an important nexus for signaling beyond their role in selective exchanges of metabolites between cells.

	Acknowledgments

 
I apologize to all who have contributed to our understanding of these channels and their functions but who could not be mentioned here because of space constraints. Sincerest thanks go to Jim Stamos who pieced together the collage in the accompanying poster.

	References

Top Introduction Diverse functions Formation and degradation Channel permeability properties Heterologous interactions... Channel gating and regulation Hemichannels Molecular structure of the... References

 

Bevans, C. G., Kordel, M., Rhee, S. K., Harris, A. L. (1998). Isoform composition of connexin channels determines selectivity among second messengers and uncharged molecules. J. Biol. Chem. 273, 2808-2816.[Abstract/Free Full Text]

Eiberger, J., Degen, J., Roumaldi, A., Deutsch, U., Willecke, K. and Sohl, G. (2001). Connexin genes in the mouse and human genome. Cell Commun. Adhes. 8, 163-165.[Medline]

Falk, M. M. (2000). Connexin-specific distribution within gap junctions revealed in living cells. J. Cell Sci. 113, 4109-4120.[Abstract]

Foote, C. I., Zhou, L., Zhu, X. and Nicholson, B. J. (1998). Pattern of disulfide linkages in the extracellular loop regions of connexin 32: a model of the docking interface of gap junctions. J. Cell Biol. 140, 1187-1197.[Abstract/Free Full Text]

Gaietta, G., Deerinck, T. J., Adams, S. R., Bouwer, J., Tour, O., Laird, D. W., Sosinsky, G. E., Tsien, R. Y. and Ellisman, M. H. (2002). Multicolor and electron microscopic imaging of connexin trafficking. Science. 296, 503-507.[Abstract/Free Full Text]

Goldberg, G. S., Lampe, P. D. and Nicholson, B. J. (1999). Selective transfer of endogenous metabolites through gap junctions composed of different connexins Nat. Cell Biol. 1, 457-459.[CrossRef][Medline]

Goodenough, D. A. and Paul, D. L. (2003). Beyond the gap: functions of unpaired connexon hemichannels. Nat. Rev. Mol. Cell. Biol. 4, 1-10.

Harris, A. L. (2001). Emerging issues of connexin channels: biophysics fills the gap. Q. Rev. Biophys. 34, 325-472. [Erratum in: Q. Rev. Biophys. 35, 109][Medline]

Jordan, K., Chodock, R., Hand, A. R. and Laird, D. W. (2001). The origin of annular junctions: a mechanism of gap junction internalization. J. Cell Sci. 114, 763-773.[Abstract]

Kelsell, D. P., Dunlop, J., Stevens, H. P., Lench, N. J., Liang, J. N., Parry, G., Mueller, R. F. and Leigh, I. M. (1997). Connexin26 mutations in hereditarry non-syndromic sensoneurial deafness. Nature 387, 80-83.[CrossRef][Medline]

Laing, J. G., Tadros, P. N., Green, K., Saffitz, J. E. and Beyer, E. C. (1998). Proteolysis of connexin43-containing gap junctions in normal and heat-stressed cardiac myocytes. Cardiovasc. Res. 38, 711-718.[Abstract/Free Full Text]

Lampe, P. D. and Lau, A. F. (2000). Regulation of gap junctions by phosphorylation of connexins. Arch. Biochem. Biophys. 384, 205-215,[CrossRef][Medline]

Martin, P. E., Blundell, G., Ahmad, S., Errington, R. J. and Evans, W. H. (2001). Multiple pathways in the trafficking and assembly of connexins 26, 32 and 43 into gap junction intercellular communication channels. J. Cell Sci. 114, 3845-3855.[Abstract/Free Full Text]

Morley, G. E., Taffet, S. M. and Delmar, M. (1996). Intramolecular interactions mediate pH regulation of connexin43 channels. Biophys. J. 70, 1294-1302.[Medline]

Muller, D. J., Hand, G. M., Engel, A. and Sosinsky, G. E. (2002). Conformational changes in surface structures of isolated connexin 26 gap junctions. EMBO J. 21, 3598-3607[CrossRef][Medline]

Musil, L. S. and Goodenough, D. A. (1993). Multisubunit assembly of an integral plasma membrane channel protein, gap junction connexin43, occurs after exit from the ER. Cell 74, 1065-1077.[CrossRef][Medline]

Niessen, H., Harz, H., Bedner, P., Kramer, K. and Willecke, K. (2000). Selective permeability of different connexin channels to the second messenger inositol 1,4,5-trisphosphate. J. Cell Sci. 113, 1365-1372.[Abstract]

Panchin, Y., Kelmanson, I., Matz, M., Lukyanov, K., Usman, N. and Lukyanov, S. (2000). A ubiquitous family of putative gap junction molecules. Curr. Biol. 10, R473-R474.[CrossRef][Medline]

Phelan, P. and Starrich, T. (2001). Innexins get into the gap. Bioessays 23, 388-396.[CrossRef][Medline]

Rubin, J. B., Verselis, V. K., Bennett, M. V. L. and Bargiello, T. A. (1992). A domain substitution procedure and its use to analyze voltage dependence of homotypic gap junctions formed by connexins 26 and 32. Proc. Natl. Acad. Sci. USA 89, 3820-3824.[Abstract/Free Full Text]

Saffitz, J. E., Laing, J. G. and Yamada, K. A. (2000). Connexin expression and turnover: implications for cardiac excitability. Circ. Res. 86, 723-728.[Abstract/Free Full Text]

Skerrett, I. M., Aronowitz, J. A., Shin, J. H., Kasparek, E., Cymes, G., Cao, F. L. and Nicholson, B. J. (2002). Identification of amino acid residues lining the pore of the gap junction channel. J. Cell Biol. 159, 349-360.[Abstract/Free Full Text]

Suchyna, T. M., Chiton, M., Nitsche, J., Harris, A. L., Veenstra, R. D. and Nicholson, B. J. (1999). Different ionic permeabilities for connexin 26 and 32 produce rectifying gap junction channels. Biophys. J. 77, 2968-2987.[Medline]

Unger, V. M., Kumar, N. M., Gilula, N. B. and Yeager, M. (1999). Three-dimensional structure of a recombinant gap junction membrane channel. Science 283, 1176-1180.[Abstract/Free Full Text]

Willecke, K., Elberger, J., Degen, J., Eckardt, D., Romualdi, A., Guldenangel, M., Deutsch, U. and Sohl, G. (2002). Structural and functional diversity of connexin genes in the mouse and human genome. Biol. Chem. 383, 725-737.[CrossRef][Medline]

Zhou, L., Kasperek, E. M. and Nicholson, B. J. (1999). Dissection of the molecular basis of pp60^v-src induced gating of connexin43 gap junctions. J. Cell Biol. 144, 1033-1045.[Abstract/Free Full Text]

CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    Twitter    What's this?

Related articles in JCS:

Mind the gap!

JCS 2003 116: 2203. [Full Text]  

This article has been cited by other articles:

				K. Maass, J. Shibayama, S. E. Chase, K. Willecke, and M. Delmar C-Terminal Truncation of Connexin43 Changes Number, Size, and Localization of Cardiac Gap Junction Plaques Circ. Res., December 7, 2007; 101(12): 1283 - 1291. [Abstract] [Full Text] [PDF]

				N. J. Oviedo and M. Levin smedinx-11 is a planarian stem cell gap junction gene required for regeneration and homeostasis Development, September 1, 2007; 134(17): 3121 - 3131. [Abstract] [Full Text] [PDF]

				R. Nickel, D. Becker, and A. Forge Molecular and functional characterization of gap junctions in the avian inner ear. J. Neurosci., June 7, 2006; 26(23): 6190 - 6199. [Abstract] [Full Text] [PDF]

				Z. Chen, W. H. Evans, S. C. Pflugfelder, and D.-Q. Li Gap Junction Protein Connexin 43 Serves as a Negative Marker for a Stem Cell-Containing Population of Human Limbal Epithelial Cells Stem Cells, May 1, 2006; 24(5): 1265 - 1273. [Abstract] [Full Text] [PDF]

				Q. Zheng-Fischhofer, A. Ghanem, J.-S. Kim, M. Kibschull, G. Schwarz, J. O. Schwab, J. Nagy, E. Winterhager, K. Tiemann, and K. Willecke Connexin31 cannot functionally replace connexin43 during cardiac morphogenesis in mice J. Cell Sci., February 15, 2006; 119(4): 693 - 701. [Abstract] [Full Text] [PDF]

				A. Lai, D.-N. Le, W. A. Paznekas, W. D. Gifford, E. W. Jabs, and A. C. Charles Oculodentodigital dysplasia connexin43 mutations result in non-functional connexin hemichannels and gap junctions in C6 glioma cells J. Cell Sci., February 1, 2006; 119(3): 532 - 541. [Abstract] [Full Text] [PDF]

				R. L. Caruso, B. L. Upham, C. Harris, and J. E. Trosko Biphasic Lindane-Induced Oxidation of Glutathione and Inhibition of Gap Junctions in Myometrial Cells Toxicol. Sci., August 1, 2005; 86(2): 417 - 426. [Abstract] [Full Text] [PDF]

This Article Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JCS Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Nicholson, B. J. Search for Related Content PubMed PubMed Citation Articles by Nicholson, B. J. Social Bookmarking                         What's this?