2011 Group 2 Project

From CellBiology


Direct interactions of cells with their neighbouring cells or extracellular matrix at particular site of contact are called cell junctions. [1]

Cell junctions can be categorized functionally into three groups:

1. Occluding junctions

2. Anchoring junctions

3. Communicating junctions

Cell junctions.jpg

Occluding junctions (Tight junctions found only in vertebrates) fasten the adjacent epithelial sheets to avoid small molecules leakage from one side of the sheet to the other.

Anchoring junctions link the cytoskeleton of a cell to cytoskeleton of its neighbours or to the extracellular matrix. Main types of anchoring junctions in vertebrate tissues are Adherens junctions, Focal adhesions, Desmosomes and Hemidesmosomes. [2]

Gap junctions or communicating junctions are clusters of intercellular membrane channels that directly interact with the cytoplasm of adjoining cells. The topic of interest for this project is on Gap Junctions thus, the remainder of the discussion on this page will mainly be focused upon gap junctions.

In vertebrates, gap junction channels are composed of connexins, a large family of proteins containing approximately more than 20 members. Connexins (Cx) are named after their molecular weight and they are tissue-specific and even cell-specific which represent that some connexins are prominently expressed only in a few tissues and some, like Cx43 being more prevalent. [3] Six connexins oligomerize to form a connexon or hemichannel. [4] Connexons contain either a single type of connexin (homomeric) or several connexins (heteromeric). In addition, adjacent cells may comprise of identically or differently composed connexons hence forming a homotypic or heterotypic intercellular channels. [5]

The structure of gap junctions is to be discussed in detail after a brief history on gap junctions.

Functionally, gap junctions are important in generating electrical impulses in electrically excitable tissues such as heart, smooth muscle and some neurons. In non-excitable cells and other general aspects, they are reviewed to play a key role in electrical coupling and provide a pathway for sharing and selective exchange of metabolites. [6]

Likewise, the functional role of gap junctions will be elaborated further followed by the distribution of gap junctions in different tissues.

Following the discussion on location of gap junctions, a table comparing between different junctions will be constructed as a summary of information concerning various junctions.

Lastly, we will attempt to discuss some of the diseases that may be associated with the mutations or disruptions of different connexin genes and recent research with regard to gap junctions.

Cell adhesion summary.png


1959: Furshpan and Potter report that electrical stimulation that is insufficient to generate and action potential still allows current transfer between some nerve cells. [7]

1962: Dewey and Barr coin the term “nexus” to describe an intercellular connection between smooth muscle cells. [8] This term is now used interchangeably with the term “gap junction”

1963: Loewenstein and Kanno use microelectrodes and flurescent tracers to analyze the membrane permeability of epithelial cell junctions on Drosophilla salivary glands. The find that the junctional membrane surface is highly permeable so that small ions and fluorescent markers can move freely from one cell to the next.[9]

1963: Using thin slices of permanganate and osmium-fixed material, David Robinson discovers an array of hexagonal subunits in electrical synapses of Mauthner cells of goldfish. [10]

1963-1967: Difference between gap junction and tight junction remains unclear and confusing.[11]

1967: Karnovsky and Revel use a lanthanum salt preparation on tissues to reveal the existence of hexagonal intercellular junctions in sections of mouse heart and mouse liver.[12] The lanthanum tracer penetrates a 2-3 nm gap between cell membranes for gap junctions, yet is not able to penetrate the firmly fused tight junctions.[13]

1970: Freeze cleaving electron microscopy techniques conducted by McNutt and Weinstein revealed that each of the two membranes which formed the gap junction could be split into lamellae, one neighbouring the cytoplasm, the other neighbouring the extracellular space.[14] Goodenough and Revel showed that the gap junction was characterized by 2 hexagonal subunits. [15]

1972: A method for isolating gap junctions from mouse liver using the detergent sarkosyl and X-ray diffraction is developed by Goodenough and Stoeckenius. [16]

1974: Connexin is chosen as the family name for the major gap junction proteins that were related but not identical in different tissues.[17]

1975: the term connexon is coined to describe a hexagonal subunit that spans the plasma membrane and forms half of the gap junction, along with a second connexon from an adjacent cell. [18]

1977: Crystallographic analysis of X-ray diffraction images make it possible to obtain a relatively detailed image of connexon structure. [19]

1979: Spray et al. show that gap junction channels in early amphibian embryos are voltage gated. [20]

Early 1980s: Development of ultra rapid freezing techniques allowed us to directly freeze cells and thus view gap junctions instantly at the moment of freezing.[21]

1983: Immunocytochemical localization is used to identify a specific type of gap junction protein. Anti 26K antibodies were localized to the liver gap junction protein (26K)[22].

1985: Manjunath and Page use optical diffraction with negatively stained isolated rabbit heart gap junctions to show six protein subunits of identical molecular weight forming the connexon. [23] These subunits each correspond to a connexin molecule. [24]

1986: Kumar and Gilula develop, clone, and characterize two recombinant cDNA probes coding for gap junction proteins, one for rat liver and one for human liver. [25] Different connexins continue to be discovered using cDNA clones from many tissues.[26] [27]

1987: At the Gap Junciton meeting held in Asilomar, scientists decide to adopt a nomenclature system for connexins which distinguishes connexins on the basis of species of origin and molecular mass in kiloDaltons. [28]

1991: Stauffer et. al develop a method involving cDNA which is able to isolate and purify intact connexons[29]

1995: 13 members of the connexin family in rodents have been identified. [30]

1995: The first connexin defect is described by Reaume et al. who observed that neonatal mice lacking connexin 43 suffered from cardiac malformation. [31] [32]

1997: Kelsell et al. identify a mutation in a gene encoding a the gap junction connexin protein connexin 26 which causes autosomal dominant deafness. [33]

1999: Unger et al. determine the structure of a recombinant cardiac gap junction using electron crystallography. [34]

2002: After screening both mouse and human genomic databases, Willecke et al. determine that there are 19 connexin genes in the mouse genome and 20 in the human genome. [35]

2004: New methodologies that use the activation of certain protein kinases show that phosphorylation of certain amino acids in the C terminal region of a connexin can lead to changes in gap junction communication, as well as regulate connexin trafficking, assembly, disassembly, degradation, and the opening and closing of the channel. [36]

2005: Valiunas et al. show that short interfering RNAs (siRNA) can move from one cell to an adjacent cell via a gap junction if the correct connexin is a part of that particular gap junction. [37]

2006: Bugiani et al. demonstrate that mutations in the GJA12 gene which cause loss of function in connexin 47 cause Pelizaues-Merzbacher-like disease, a disease which manifests itself as a permanent lack of myelin deposition in the brain. [38]

2008: Mutations in the gene GJA1 which codes for connexin 43 can cause Oculodentodigital syndrome (ODD) which is characterized by abnormal development of the face, eyes, limbs, and teeth. Immunofluorescence analysis of adult tissues show that these areas affected by ODD are high in the expression on connexion 43. [39]

2010: The connexin gene family is made up of 20 genes in the mouse genome and 21 genes in the human genome.[40]

2010-2011: Research relating to gap junctions continues to exist, with over 1300 articles relating to gap junctions turning up on an April 22, 2011 Pubmed search for “gap junction”. [41]



Gap junction1.jpg

ZA tangential section through a mouse heart reveals the hexagonal gap junctions..jpg A tangential section through a mouse heart reveals the hexagonal gap junctions.


Gap junction communication plays a principal role in maintaining cellular homeostasis by providing a pathway for exchanging small, hydrophilic molecules including glucose, glutathione, glutamate, cyclic adenosine monophosphate (cAMP), adenosine triphosphate (ATP), inositol trisphosphate (IP3), and ions like calcium, sodium and potassium. [42]

The permeability of gap junctions depends on the composition of different connexin types. For instance, it was reviewed that Cx 43 channels are 120-160 fold more permeable to ADP and/or ATP than Cx 32 channels. Additionally, heterotypic gap junctions allow for special permeability characteristics compared to homotypic gap junctions. [43]

[44]Gap junction is also important for:

1) Transmission of excitation in cardiac muscle, smooth muscle and central nervous system (CNS) neurons;

2) Signalling in avascular and/or uninnervated tissues;

3) Control of cell growth and oncogenic transformation and

4) Regulation of early developmental events

The mechanisms of the functional role of gap junctions described above are explained further below.

Coupling through gap junctions in electrically excitable cells such as some nerve cells allow for the action potential to spread between the cells rapidly and accurately. Similarly, gap junctions in smooth muscle cells and cardiac muscle cells are responsible for peristaltic movement of the intestines and coordinating rhythmic contractions respectively. [45]

Gap junctions in neocortex.jpg

The signalling role of gap junctions can be exemplified in the liver. In normal physiological conditions, low levels of glucose in blood amount to the release of noradrenaline from the sympathetic nerve fibres which trigger the liver cells to break down glycogen and release glucose into the bloodstream. Neural stimulation of hepatocytes give rise to generation of signalling molecules such as inositol (1,4,5) -trisphosphate and release of Ca2+ from intracellular stores. However, hepatocytes at the venous end of the lobule are not directly innervated by sympathetic nerve fibres. Thus, gap junctions provide a pathway for those signalling molecules to be transported to uninnervated hepatocytes for glucose mobilization and release. [46]

Connexin 32 (Cx32) is highly expressed in hepatocytes and the review on the experiment concerning Cx32 knockout (KO) mice shows vulnerability to mutations and increase in incidence of hepatic tumors. The rate of cell growth was also remarkably increased in those Cx32 KO mice. These findings support the idea that inhibitory signals passing through intercellular channels of gap junctions may contribute to control of cell proliferation. Therefore, connexin genes of gap junctions might act as tumor suppressors. [47]

The development of ovarian follicles and the production of fertile oocytes depend on gap junction mediated intercellular communication particularly between the oocyte and the surrounding granulosa cells. Cx37 containing gap junctions exist between the oocyte and the granulosa cells whereas granulosa cells are coupled to each other by Cx43 containing gap junctions. [48] The granulosa cells infer the loss of gap junction interaction between the oocyte and granulosa cells as equivalent to ovulation thus, granulosa cells start to luteinize. This may represent that in normal follicles, inhibitory signals are transmitted through gap junctions from the oocyte to granulosa cells to prevent luteinisation until ovulation has occurred. Thus, communication via gap junctions seems to offer bidirectional signalling system to regulate follicle growth, oogenesis, ovulation and luteinization. [49]

Gap junctions have various functions depending on different types and properties of connexins and their unique expressions, distributions and co-interactions with each other in different tissues. Hence, the role of gap junctions may not only be limited to aforementioned functions and can be explored further.



Gap Junctions are located in cardiac muscle and smooth muscle

Comparison with Other Junctions


Comparison of Cell Junctions
Name of Junction Function Location Associated Proteins Diseases Diagram
Tight Junction

Occluding Junction

Tight junctions are the most apical junctions and are play an important role in sealing neighbouring cells together. They form a transmembrane permeability barrier between individual cells which regulates the transport of solutes, immune cells and drugs.[50] Tight junctions also prevent the mixing of apical and basolateral components which helps to maintain cell polarity.[51]

Further research finds that tight junctions are important in basic cellular processes such as transcription, tumor suppression, cell proliferation [52]

Apical junction of epithelial cells and endothelial cells [53] occludin & claudins


Inflammatory Bowel Disease (IBD)

Crohn's Disease and Ulcerative Colitis associated with loss of tight junction [55]

Tight junction3.jpg
Adherens Junction cadherins

β-catenin or gamma-catenin α-catenin

Desmosomes anchors cells to each other Epethilum and cardiac muscle Cadherin Bullous Impetigo

Palmoplantar Keratoderma

Tunneling Nanotubes
Neuromuscular Junction Invloved in communcation and muscle contraction forms between motor neurons and skeletal muscle fibres [56] Myasthenia Gravis

Lambert-Eaton Myasthenic syndrome - defective neurotransmitter release [57]

Diseases Associated with Gap Junctions


Current Research






  1. Alberts B, Johnson A, Lewis J, et al. 2002 Molecular Biology of the Cell, 4th edn, Garland Science, New York
  2. Alberts B, Johnson A, Lewis J, et al. 2002 Molecular Biology of the Cell, 4th edn, Garland Science, New York
  3. Joell L. Solan & Paul D. Lampe, 2005, ‘Connexin phosphorylation as a regulatory event linked to gap junction channel assembly’, Biochimica et Biophysica Acta (BBA)- Biomembranes, vol. 1711, no. 2, pp. 154-163.
  4. Dale W. Laird, 2005, ‘Connexin phosphorylation as a regulatory event linked to gap junction internalization and degradation’, Biochimica et Biophysica Acta (BBA)- Biomembranes, vol. 1711, no. 2, pp. 172-182.
  5. <pubmed> 9861669</pubmed>
  6. Peracchia C., 1977, ‘Gap junction structure and function’, Trends in Biochemical Sciences, vol. 2, no. 2, pp. 26-31.
  7. <pubmed> 15940850</pubmed>
  8. <pubmed> 17770946</pubmed>
  9. <pubmed> 14206423</pubmed>
  10. <pubmed> 14069795</pubmed>
  11. <pubmed> 14206423</pubmed>
  12. <pubmed> 6036535</pubmed>
  13. <pubmed> 8534895</pubmed>
  14. <pubmed> 5531667</pubmed>
  15. <pubmed> 4105112</pubmed>
  16. <pubmed> 4339819</pubmed>
  17. <pubmed> 4363961</pubmed>
  18. <pubmed> 885916</pubmed>
  19. <pubmed> 889612</pubmed>
  20. <pubmed>312530</pubmed>
  21. <pubmed>8534895</pubmed>
  22. <pubmed>11894941 </pubmed>
  23. <pubmed>2408491</pubmed>
  24. <pubmed>8534895</pubmed>
  25. <pubmed>2875078 </pubmed>
  26. <pubmed>2987225</pubmed>
  27. <pubmed>2557354</pubmed>
  28. <pubmed>8665925</pubmed>
  29. <pubmed>1655801</pubmed>
  30. <pubmed>8665925</pubmed>
  31. <pubmed>7892609</pubmed>
  32. <pubmed>12108537</pubmed>
  33. <pubmed>9139825</pubmed>
  34. <pubmed>10024245</pubmed>
  35. <pubmed>12108537</pubmed>
  36. <pubmed>15109565</pubmed>
  37. <pubmed>16037090</pubmed>
  38. <pubmed>16707726</pubmed>
  39. <pubmed>18946008</pubmed>
  40. <pubmed>20234156</pubmed>
  41. [1] Pubmed Gap Junction Search
  42. <pubmed> 20801193</pubmed>
  43. <pubmed> 15695607</pubmed>
  44. David L. Paul, 1995, ‘New functions in gap junctions’, Current Opinion in Cell Biology, vol. 7, pp. 665-672.
  45. Alberts B, Johnson A, Lewis J, et al. 2002 Molecular Biology of the Cell, 4th edn, Garland Science, New York
  46. <pubmed> 9861669</pubmed>
  47. <pubmed> 9861669</pubmed>
  48. <pubmed>12006089</pubmed>
  49. <pubmed> 9861669</pubmed>
  50. <pubmed> 19538280 </pubmed>
  51. <pubmed> 18415116 </pubmed>
  52. <pubmed> 15151915 </pubmed>
  53. <pubmed> 18415116 </pubmed>
  54. <pubmed> 15151915 </pubmed>
  55. <pubmed> 19632896 </pubmed>
  56. <pubmed> 20215342 </pubmed>
  57. <pubmed> 10616683 </pubmed>

2011 Projects: Synaptic Junctions | Gap Junctions | Tight Junctions | Desmosomes | Adherens Junctions | Neuromuscular Junction