Cell Junctions

Abstract

For any multicellular organism to function, it is a prerequisite that its individual cells interact with each other. These interactions are realised by cell‐to‐cell junctions that provide mechanical stability, information exchange or occlusion of extracellular diffusion pathways between neighbouring cells. The latter is an unconditional prerequisite for the formation of interfaces within the organism or between organism and environment. In order to provide these different physiological functions the corresponding cell junctions have their individual molecular architecture but share some common construction principles. For all of them transmembrane proteins are required that interact with corresponding structures on the surface of the opposing cell to form either mechanically stable interconnections, tight diffusion barriers, aqueous channels or well‐defined nanometre gaps for the fast diffusion of signal molecules. In most cases these transmembrane proteins interact with intracellular adapter proteins or the cytoskeleton, which is critical for their correct functionality.

Key Concepts:

  • More than 200 different cell types act together in the tissues and organs of the human body.

  • Cellular cooperation in multicellular organisms is based on various cellular interactions and cell junctions.

  • From a functional viewpoint cell junctions can be grouped into: (1) occluding junctions, (2) mechanical junctions and (3) communicating junctions.

  • Barrier‐forming epithelial and endothelial cells, which build up interfacial tissues between two fluid compartments, express tight junctions that occlude the diffusion pathway between adjacent cells.

  • Tight junctions are mainly composed of different transmembrane proteins (occludin, members of the claudin family, etc.) that use their extracellular domains to interact with the corresponding protein domains on the adjacent cell surface to block the intercellular cleft.

  • Mechanical stability of epithelial cell layers is based on adherens junctions and desmosomes that both provide a stable mechanical linkage between the intracellular cytoskeletons of adjacent cells.

  • Exchange of signalling molecules or metabolites smaller than 1000 g mol−1 is provided by gap junctions (electrical synapses) who form aqueous channels between neighbouring cells.

  • Plant cells show a special kind of communicating junctions, the plasmodesmata.

  • Electrical communication between two neurons or a neuron and muscle cells is provided by chemical synapses, which relay electrochemical signals by a pulse of neurotransmitters.

Keywords: tight junctions; adherens junctions; desmosomes; gap junctions; plasmodesmata; synapses

Figure 1.

(a) Diagram and (b) electron micrograph of cell‐to‐cell junctions between two adjacent epithelial cells. TJ, tight junctions; AJ, adherens junctions; DS, desmosomes; and GJ, gap junctions. On the cytoplasmic site of the membrane adherens junctions are associated with the actin cytoskeleton, whereas desmosomes are linked to intermediate filaments, for instance keratin filaments in epithelial cells. Anchorage of the cells to the extracellular matrix (ECM) is provided by focal contacts (FC) or hemidesmosomes (HD), which also differ with respect to their particular connections to the cytoskeleton. The apical membrane surface shows membrane protrusions (microvilli, Mv) that are typical of transporting epithelia. Bar, 200 nm. (b) From Tsukita et al..

Figure 2.

Morphology of tight junctions in electron micrographs. (a) High‐resolution thin‐section electron micrograph of the tight junction area. The image is a magnification of the highlighted area shown in Figure b. Arrowheads indicate points of membrane kisses. Bar, 50 nm. (b) Freeze‐fracture replicas of the tight junction area show strands on the p‐face (arrowheads) and corresponding grooves on the e‐face (arrows). Ap, apical; Bl, basolateral; and Mv, microvilli. Bar, 200 nm. From Tsukita et al..

Figure 3.

(a) Protein model of the tight junctions according to which strands, as seen in a freeze‐fracture replica, correspond to linear aggregates of transmembrane proteins. (b) Lipid model of the tight junctions according to which strands, as seen in freeze‐fracture electron micrographs, correspond to cylinders of lipids in inverted micellar arrangement.

Figure 4.

Molecular composition of adherens junctions and desmosomes. In either type of junction transmembrane proteins embedded in the plasma membrane of cell 1 associate to cis‐dimers, which then form homophilic interactions (trans‐dimers) with dimers on the surface of the opposing cell 2. The intracellular domains of the transmembrane proteins are linked to cytoskeletal filaments through a dense plaque of individual adapter proteins.

Figure 5.

Cartoon of a chemical synapse as it is formed between neurons or between neurons and muscle cells. 1, Presynaptic membrane; 2, postsynaptic membrane; 3, synaptic cleft; 4, synaptic adherens junction; 5, neurotransmitter‐loaded secretory vesicles; and 6, transmitter‐gated ion channels like the acetylcholine receptor.

Figure 6.

Gap junction channels connecting the cytoplasms of two adjacent cells. Owing to the size‐selectivity of gap junctions, only ions, amino acids, sugars and other small metabolites may pass the junction, whereas macromolecules like proteins and nucleic acids are rejected.

Figure 7.

Plasmodesma forming a water‐filled channel between the interior of two plant cells. The channel width is reduced by a vesicle (desmotubule) that originates from the endoplasmic reticulum.

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References

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Further Reading

Alberts B, Bray D, Lewis J et al. (1994) Molecular Biology of the Cell. New York: Garland.

Cereijido M (ed.) (1991) Tight Junctions. Boca Raton, FL: CRC Press.

Evans WH and Martin PEM (2002) Gap junctions: structure and function. Molecular Membrane Biology 19(2): 121–136.

Green KJ and Gaudry CA (2000) Are desmosomes more than tethers for intermediate filaments? Nature Reviews: Molecular Cell Biology 1: 208–216.

Tepass U, Truong K, Godt D et al. (2000) Cadherins in embryonic development and neural morphogenesis. Nature Reviews: Molecular Cell Biology 1: 91–100.

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How to Cite close
Wegener, Joachim(Jun 2011) Cell Junctions. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001275.pub2]