Intercellular junctions
Cells in vertebrate epithelia are held and linked together by cell-cell junctions.[1] These direct interactions contribute to coordinated multicellular composition; they also enable the cells to tolerate and respond to different external forces that may cause disorder in epithelial tissues like lining in gut, skin outer layer etc.[1]
There are 3 major types of intercellular junctions. 'Gap' junctions, 'Tight' junctions, and 'Anchoring' junctions which are further divided into two subdivisions: 'Adherens Junctions' and 'Desmosomes'.[1] All these intercellular structures have one main feature in common; they link and adhere two adjacent cells together on the lateral side of their membranes via extremely diverse protein-made facilities.[1]
Intercellular junctions occur in a broad range of locations in vertebrate body.[1] In humans, anchoring junctions are found in tissues that stretch a lot, like muscles, heart and skin.[2] Gap junctions are also found in heart muscle[3], and in the brain - contributing to vital processes like transferring signals[4]. They can also be found between retinal and skin cells.[3][4] Tight junctions are present in tight and leaky epithelia.[5] For example, in kidneys, liver, the blood-brain barrier and CNS myelin sheath.[6]
Intercellular junctions take up particular positions in cell-cell interfaces.[1] They're found between the apical and basal sides of epithelia.[1] While the organisation of gap junctions is less regulated, the other 3 junctions are often found bundled together as 'Junctional Complex'; where tight junctions are placed at the most apical region of epithelia, topping adherens junctions and desmosomes which are closer to the basal pole.[1]
Adherens junctions and desmosomes are the 2 types of anchoring intercellular junctions.[1] They both consist of a 'Core Complex' which is made up of 2 major protein subunits[7]: 1. ‘Cadherens’; transmembrane adhesive proteins that connect the two lateral sides of interacting membranes together via their extra cellular regions. Adherens junctions possess classical cadherens and desmosomes have non-classical cadherens.[1] Cadherens on the two sides of a junction are often the same or closely related, establishing a 'homophilic adhesion' which is the basis of their symmetric structure.[1] 2. ‘Catenins’; group of proteins that bind cytosolic sides of cadherens[7] and mediate their practical linkage to cell cytoskeleton.[1] In contrast, while in adherens junctions this linkage is made to actin cytoskeleton, catenins in desmosomes link cytosolic sides of cadherens to intermediate filaments of cytoskeleton, and that’s due to the different catenins involved.[1] α, β, γ and p120 catenins serve adherens junctions, while desmosomes recruit α, plakophilin and desmoplakin catenins.[1]
Anchoring junctions mechanically integrate interacting cells while having highly dynamic characteristics enabling them to readily react to signals from their surroundings and therefore enhancing remodelling of tissues during development. [1][8]
Tight junctions share some structural features with anchoring junctions.[1] By definition, they are continuous intercellular adhesive contact points that form barriers able to discriminate between different particles in paracellular space amid two adjacent epithelial cells.[6] Homophilic adhesion of transmembrane proteins implanted in each of the two lateral membranes in these junctions is how they're structured; which is just like the pattern by which anchoring junctions are assembled.[1] However, a completely different family of proteins called 'Claudins' play this role of cell-cell adhesion here.[1] Numerous claudin-claudin contact points make up 'Sealing Strands', establishing an extensive network to obstruct diffusion of macromolecules.[1]
Permeability of tight junctions can be directed by different claudin combinations.[1] A family of large scaffold proteins called 'Zonula Occludins' or 'ZO', which are not necessary for assembly of tight junctions, are further bound to intracellular sides of claudins to limit and specialise junctional permeability.[1]
Another structural similarity between tight and anchoring junctions is that they both require additional intracellular proteins that bind the cytosolic region of adhesion proteins (claudins in tight junctions and cadherens in anchoring junctions).[1] Catenins accomplish this task in anchoring junctions, and in tight junctions, occludins mediate linkage of tight junction transmembrane adhesive proteins to the cytoplasm of their corresponding cells.[1] The network provided by occludins ultimately links claudins to actin cytoskeleton, just like adherens junctions.[1]
Gap junctions mediate a direct connection between cytoplasm of two adjacent cells.[9]
A gap junction channel is formed by 2 connexon structures on each side of the junction, each consisting of 6 connexin proteins, passing through the membrane 4 times.[1] Gap junctions are composed of many of such channels forming a sort of molecular filter that can have selective permeability for and against different particles.[1]
While gap junctions share a common feature with all other junctions discussed earlier - producing a cell-cell adhesive connection-, providing the two adjacent interacting cells with a communicative channel is what makes them completely different from anchoring junctions, and even tight junctions.[1] Although tight junctions do act as selective sieves, or 'pores', they do so through 'paracellular' pathway[1], i.e. Passing down the intercellular space, located between the cells.[10]
A remarkable similarity between gap junctions and anchoring junctions is that plaques of gap junctions also have a high dynamicity and are able to promptly assemble, disassemble or proceed remodelling, for example, to remove aged connexons from the junction.[1]
Gap junctions can have customisable permeability by having different combinations of connexin proteins forming their connexon hemichannels.[1] The connexons can even be asymmetrical and have non-identical 'half channels' on each side, which is contrary to the symmetrical nature of anchoring junctions - their assembly being based on homophilic attractions and symmetry.[1]
Although tight junctions act to seal the paracellular space, they allow certain ions and water to pass through them.[1] Having different claudins in their composition leads to different types of selectivity.[11] For example, claudin -10b shows cation selectivity, while claudin -10a shows anion selectivity.[11] On the other hand, gap junctions allow a wider range of compounds to pass through; in addition to inorganic ions and water, they basically are permeable to all other water solute molecules not heavier than 1000Da, including sugars, amino acids, nucleotides and vitamins.[1]
In conclusion, despite having different functions, the fact that all the structures mentioned above provide cells they’re connected to with a form of cohesion and connection is their clearest similarity, a feature of crucial importance.[1] Similarities do occur in structure; like how anchoring and tight junctions are linked to cell cytoskeleton, and in function; like how both tight and gap junctions act as permeable surfaces. But there are also fundamental differences to consider, most notably the completely different proteins forming them, and their overall function.[1]
References
1. Alberts et. al. (2015) Molecular Biology of The Cell, 6th edition, New York: Garland Science
2. Yan HH, Mruk DD, Lee WM, Cheng CY (2008). "Cross-talk between tight and anchoring junctions-lesson from the testis". Adv. Exp. Med. Biol. 636: 234–54.
3. "Abstracts: Proceedings of the International Gap Junction Conference. August 5–9, 2007. Elsinore, Denmark". Cell Commun. Adhes. 14 (6): 275–346. 2007.
4. Wei CJ, Xu X, Lo CW (2004). "Connexins and cell signaling in development and disease". Annu. Rev. Cell Dev. Biol. 20: 811–838.
5. Department, Biology. (2013) "Tight Junctions and other cellular connections". Davidson College. Available at: http://www.bio.davidson.edu/people/kabernd/berndcv/lab/epithelialinfoweb/Tight%20Junctions.html (Last accessed November 22 2015)
6. Gonzalez-Mariscal, L (2006) Tight Junctions, Austin: Springer US
7. Harris, Tony (2012) “Aherens Junctions: From Molecular Mechanism to Tissue Development and Disease”, Subcellular Biochemistry volume 60
8. Green, K.J., Jones, J.C.R. (1996) “Desmosomes and hemidesmosomes: structure and function of molecular components” FASEB J. 10, 871-881
9. Lampe, Paul D., Lau, Alan F. (2004). "The effects of connexin phosphorylation on gap junctional communication". The international journal of biochemistry & cell biology 36 (7): 1171–1186.
10. Medical Dictionary. (2009). Available at: http://medical-dictionary.thefreedictionary.com/paracellular (Last accessed November 22 2015)
11. M. Krug, S., D. Schulzke, J., Fromm M. (2014) “Tight junction, selective permeability, and related diseases”. Seminars in Cell & Developmental Biology. Volume 36, Pages 166–176