Occludin is a protein that in humans is encoded by the OCLNgene. Occludin is a 65-kDa (522-amino acid polypeptide -human) integral plasma-membrane protein located at the tight junctions, described for the first time in 1993 by Shoichiro Tsukita. Together with the Claudin group of proteins, it is the main component of the tight junctions.
Occludin's structure can be broken down into 9 domains. These domains are separated into two groups. 5 of the domains are located intracellularly and extracellularly. These 5 domains are separated by the 4 transmembrane domains of the protein. The nine domains are as follows:
N-terminus domain (66 aa)
transmembrane domain 1 (23 aa)
extracellular loop 1 (46 aa)
transmembrane domain 2 (25 aa)
intracellular loop (10 aa)
transmembrane domain 3 (25 aa)
extracellular domain 2 (48 aa)
transmembrane domain 4 (22 aa)
C-terminus domain (257 aa)
The C-terminus domain has been shown experimentally to be required for correct assembly of tight junction barrier function. The C-terminus also interacts with several cytoplasmic proteins of the junctional plaque and interacts with signaling molecules responsible for cell survival. The N-terminus of occludin experimentally has been linked to involvement in tight junction sealing/barrier properties. The extracellular loops are thought to be involved in the regulation of paracellualr permeability and the second extracellular has been shown to be involved in the localization of occludin at the tight junction.
Occludin is an important protein in tight junction function. Studies have shown that rather than being important in tight junction assembly, occludin is important in tight junction stability and barrier function. Furthermore, studies in which mice were deprived of occludin expression showed morphological stability in several epithelial tissues but also found chronic inflammation and hyperplasia in the gastric epithelium, calcification in the brain, testicular atrophy, loss of cytoplasmic granules in straited duct cells of salivary gland, and thinning of the compact bone. The phenotypical response of these mice to the lack of occludin suggest that the function of occludin is more complex than thought and requires more work.
Occludin plays a critical role in maintaining the barrier properties of a tight junction. Thus, mutation or absence of occludin increases epithelial leakiness which is an important barrier in preventing metastasis of cancer. Loss of occludin or abnormal expression of occludin has been shown to cause increased invasion, reduced adhesion and significantly reduced tight junction function in breast cancer tissues. Furthermore, patients with metastatic disease displayed significantly lower levels of occludin suggesting that the loss of occludin and thereby loss of tight junction integrity is important in metastatic development of breast cancer.
Occludin also plays an important role in the apoptosis. The C-terminus of occludin is important in receiving and transmitting cell survival signals. In standard cells, loss or disruption of occludin and other tight junction proteins leads to initiation of apoptosis through extrinsic pathways. Studies involving high levels of expression of occludin in cancer cells have shown that occludin mitigates several important cancer proliferation properties. The presence of occludin decreased cellular invasiveness and motility, enhanced cellular sensitivity to apoptogenic factors and lowered tumorigenesis and metastasis of the cancer cells. Specifically, occludin has a strong inhibitory effect on Raf1-induced tumorigenesis. Still, the exact mechanism of how occludin prevents the progression of cancer is not known but it has been shown that cancer progression is linked to the loss of occludin or the silencing of the OCLN gene.
Disruption of occludin regulation is an important aspect of a number of diseases. Strategies to prevent and/or reverse occludin downregulation may be an important therapeutic target. Mutation of occludin are thought to be a cause of band-like calcification with simple gyration and polymicrogyria (BLC-PMG). BLC-PMG is an autosomal recessive neurologic disorder.
^Chen Y, Merzdorf C, Paul DL, Goodenough DA (1997). "COOH terminus of occludin is required for tight junction barrier function in early Xenopus embryos". J. Cell Biol. 138: 891–899. doi:10.1083/jcb.138.4.891.
^ abcFeldman Gemma J., Mullin James M., Ryan Michael P. (2005). "Occludin: structure, function and regulation". Advanced drug delivery reviews. 57 (6): 883–917. doi:10.1016/j.addr.2005.01.009.CS1 maint: Multiple names: authors list (link)
^Saitou M.; et al. (2000). "Complex phenotype of micelacking occludin, a component of tight junctionstrands". Mol. Biol. Cell. 11: 4131–4142. doi:10.1091/mbc.11.12.4131.
^Martin Tracey A., Mansel Robert E., Jiang Wen G. (2010). "Loss of occludin leads to the progression of human breast cancer". International journal of molecular medicine. 26 (5): 723–734. doi:10.3892/ijmm_00000519.CS1 maint: Multiple names: authors list (link)
^Beeman, N., P. G. Webb, and H. K. Baumgartner. "Occludin is required for apoptosis when claudin–claudin interactions are disrupted." Cell death & disease 3.2 (2012): e273.
^Osanai Makoto; et al. (2006). "Epigenetic silencing of occludin promotes tumorigenic and metastatic properties of cancer cells via modulations of unique sets of apoptosis-associated genes". Cancer Research. 66 (18): 9125–9133. doi:10.1158/0008-5472.can-06-1864.CS1 maint: Explicit use of et al. (link)
^Peng, Bi-Hung; Lee J Ching; Campbell Gerald A (Dec 2003). "In vitro protein complex formation with cytoskeleton-anchoring domain of occludin identified by limited proteolysis". J. Biol. Chem. United States. 278 (49): 49644–49651. ISSN0021-9258. PMID14512431. doi:10.1074/jbc.M302782200.
^Itoh, M; Morita K; Tsukita S (Feb 1999). "Characterization of ZO-2 as a MAGUK family member associated with tight as well as adherens junctions with a binding affinity to occludin and alpha catenin". J. Biol. Chem. UNITED STATES. 274 (9): 5981–5986. ISSN0021-9258. PMID10026224. doi:10.1074/jbc.274.9.5981.
^Wittchen, E S; Haskins J; Stevenson B R (Dec 1999). "Protein interactions at the tight junction. Actin has multiple binding partners, and ZO-1 forms independent complexes with ZO-2 and ZO-3". J. Biol. Chem. UNITED STATES. 274 (49): 35179–35185. ISSN0021-9258. PMID10575001. doi:10.1074/jbc.274.49.35179.
^Fanning, A S; Jameson B J; Jesaitis L A; Anderson J M (Nov 1998). "The tight junction protein ZO-1 establishes a link between the transmembrane protein occludin and the actin cytoskeleton". J. Biol. Chem. UNITED STATES. 273 (45): 29745–29753. ISSN0021-9258. PMID9792688. doi:10.1074/jbc.273.45.29745.
Fanning AS, Jameson BJ, Jesaitis LA, Anderson JM (1998). "The tight junction protein ZO-1 establishes a link between the transmembrane protein occludin and the actin cytoskeleton". J. Biol. Chem. 273 (45): 29745–29753. PMID9792688. doi:10.1074/jbc.273.45.29745.
Itoh M, Morita K, Tsukita S (1999). "Characterization of ZO-2 as a MAGUK family member associated with tight as well as adherens junctions with a binding affinity to occludin and alpha catenin". J. Biol. Chem. 274 (9): 5981–5986. PMID10026224. doi:10.1074/jbc.274.9.5981.
Wittchen ES, Haskins J, Stevenson BR (2000). "Protein interactions at the tight junction. Actin has multiple binding partners, and ZO-1 forms independent complexes with ZO-2 and ZO-3". J. Biol. Chem. 274 (49): 35179–35185. PMID10575001. doi:10.1074/jbc.274.49.35179.
Kojima T, Sawada N, Chiba H, et al. (2000). "Induction of tight junctions in human connexin 32 (hCx32)-transfected mouse hepatocytes: connexin 32 interacts with occludin". Biochem. Biophys. Res. Commun. 266 (1): 222–229. PMID10581193. doi:10.1006/bbrc.1999.1778.
Burns AR, Bowden RA, MacDonell SD, et al. (2000). "Analysis of tight junctions during neutrophil transendothelial migration". J. Cell. Sci. 113 (1): 45–57. PMID10591624.
Singh U, Van Itallie CM, Mitic LL, et al. (2000). "CaCo-2 cells treated with Clostridium perfringens enterotoxin form multiple large complex species, one of which contains the tight junction protein occludin". J. Biol. Chem. 275 (24): 18407–18417. PMID10749869. doi:10.1074/jbc.M001530200.
Marzioni D, Banita M, Felici A, et al. (2001). "Expression of ZO-1 and occludin in normal human placenta and in hydatidiform moles". Mol. Hum. Reprod. 7 (3): 279–285. PMID11228248. doi:10.1093/molehr/7.3.279.
Andreeva AY, Krause E, Müller EC, et al. (2001). "Protein kinase C regulates the phosphorylation and cellular localization of occludin". J. Biol. Chem. 276 (42): 38480–38486. PMID11502742. doi:10.1074/jbc.M104923200.
Papadopoulos MC, Saadoun S, Woodrow CJ, et al. (2001). "Occludin expression in microvessels of neoplastic and non-neoplastic human brain". Neuropathol. Appl. Neurobiol. 27 (5): 384–395. PMID11679090. doi:10.1046/j.0305-1846.2001.00341.x.
Schmidt A, Utepbergenov DI, Krause G, Blasig IE (2001). "Use of surface plasmon resonance for real-time analysis of the interaction of ZO-1 and occludin". Biochem. Biophys. Res. Commun. 288 (5): 1194–1199. PMID11700038. doi:10.1006/bbrc.2001.5914.
Pummi K, Malminen M, Aho H, et al. (2001). "Epidermal tight junctions: ZO-1 and occludin are expressed in mature, developing, and affected skin and in vitro differentiating keratinocytes". J. Invest. Dermatol. 117 (5): 1050–1058. PMID11710912. doi:10.1046/j.0022-202x.2001.01493.x.
Traweger A, Fang D, Liu YC, et al. (2002). "The tight junction-specific protein occludin is a functional target of the E3 ubiquitin-protein ligase itch". J. Biol. Chem. 277 (12): 10201–10208. PMID11782481. doi:10.1074/jbc.M111384200.