Occludin Explained

Occludin is a transmembrane protein that regulates the permeability of epithelial and endothelial barriers. It was first identified in epithelial cells as a 65 kDa integral plasma-membrane protein localized at the tight junctions.[1] Together with Claudins, and zonula occludens-1 (ZO-1), occludin has been considered a staple of tight junctions, and although it was shown to regulate the formation, maintenance, and function of tight junctions, its precise mechanism of action remained elusive and most of its actions were initially attributed to conformational changes following selective phosphorylation,[2] and its redox-sensitive dimerization.[3] [4] However, mounting evidence demonstrated that occludin is not only present in epithelial/endothelial cells, but is also expressed in large quantities in cells that do not have tight junctions but have very active metabolism: pericytes,[5] neurons and astrocytes,[6] oligodendrocytes,[7] dendritic cells,[8] monocytes/macrophages[9] lymphocytes,[10] and myocardium.[11] Recent work, using molecular modeling, supported by biochemical and live-cell experiments in human cells demonstrated that occludin is a NADH oxidase that influences critical aspects of cell metabolism like glucose uptake, ATP production and gene expression.[12] Furthermore, manipulation of occludin content in human cells is capable of influencing the expression of glucose transporters, and the activation of transcription factors like NFkB, and histone deacetylases like sirtuins, which proved capable of diminishing HIV replication rates in infected human macrophages under laboratory conditions.

Gene location

In humans is encoded by the OCLN gene[13] [14] located on the long (q) arm of chromosome 5 at position q13.1. The canonical gene is 65,813 base pairs long, spanning base pairs 69,492,292 to 69,558,104.[15] Its product is 522 amino acids long.

Protein structure

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:

The C-terminus domain has been shown experimentally to be required for correct assembly of tight junction barrier function.[16] The C-terminus also interacts with several cytoplasmic proteins of the junctional plaque and interacts with signaling molecules responsible for cell survival.[17] 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.

Function

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. Indeed, MDCK cells lacking occludin and its homolog tricellulin exhibit less complex tight junction strand network and impaired barrier function.[18] 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.[19]

Role in cancer

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.[20]

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.[21] 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.[22]

Disease linkage

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.

Interactions

Occludin has been shown to interact with Tight junction protein 2,[23] [24] [25] YES1[26] and Tight junction protein 1 (ZO-1).[27] [28]

Further reading

External links

Notes and References

  1. Furuse M, Hirase T, Itoh M, Nagafuchi A, Yonemura S, Tsukita S, Tsukita S . Occludin: a novel integral membrane protein localizing at tight junctions . The Journal of Cell Biology . 123 . 6 Pt 2 . 1777–88 . December 1993 . 8276896 . 2290891 . 10.1083/jcb.123.6.1777 .
  2. Blasig IE, Bellmann C, Cording J, Del Vecchio G, Zwanziger D, Huber O, Haseloff RF . Occludin protein family: oxidative stress and reducing conditions . Antioxidants & Redox Signaling . 15 . 5 . 1195–219 . September 2011 . 21235353 . 10.1089/ars.2010.3542 .
  3. Walter JK, Castro V, Voss M, Gast K, Rueckert C, Piontek J, Blasig IE . Redox-sensitivity of the dimerization of occludin . Cellular and Molecular Life Sciences . 66 . 22 . 3655–62 . November 2009 . 19756380 . 10.1007/s00018-009-0150-z . 23090886 . 11115754 .
  4. Villela . Castro . Manuel . Víctor . vanc . 2011 . The interplay between occludin and ZO-1 is redox sensitive. 10.17169/refubium-12742 .
  5. Castro V, Bertrand L, Luethen M, Dabrowski S, Lombardi J, Morgan L, Sharova N, Stevenson M, Blasig IE, Toborek M . 6 . Occludin controls HIV transcription in brain pericytes via regulation of SIRT-1 activation . FASEB Journal . 30 . 3 . 1234–46 . March 2016 . 26601824 . 4750406 . 10.1096/fj.15-277673 . free .
  6. Bauer H, Stelzhammer W, Fuchs R, Weiger TM, Danninger C, Probst G, Krizbai IA . Astrocytes and neurons express the tight junction-specific protein occludin in vitro . Experimental Cell Research . 250 . 2 . 434–8 . August 1999 . 10413597 . 10.1006/excr.1999.4558 .
  7. Romanitan MO, Popescu BO, Winblad B, Bajenaru OA, Bogdanovic N . Occludin is overexpressed in Alzheimer's disease and vascular dementia . Journal of Cellular and Molecular Medicine . 11 . 3 . 569–79 . 2007 . 17635647 . 3922362 . 10.1111/j.1582-4934.2007.00047.x .
  8. Rescigno M, Rotta G, Valzasina B, Ricciardi-Castagnoli P . Dendritic cells shuttle microbes across gut epithelial monolayers . Immunobiology . 204 . 5 . 572–81 . December 2001 . 11846220 . 10.1078/0171-2985-00094 .
  9. Castro V, Bertrand L, Luethen M, Dabrowski S, Lombardi J, Morgan L, Sharova N, Stevenson M, Blasig IE, Toborek M . 6 . Occludin controls HIV transcription in brain pericytes via regulation of SIRT-1 activation . FASEB Journal . 30 . 3 . 1234–46 . March 2016 . 26601824 . 10.1096/fj.15-277673 . free . 4750406 .
  10. Alexander JS, Dayton T, Davis C, Hill S, Jackson TH, Blaschuk O, Symonds M, Okayama N, Kevil CG, Laroux FS, Berney SM, Kimpel D . 6 . Activated T-lymphocytes express occludin, a component of tight junctions . Inflammation . 22 . 6 . 573–82 . December 1998 . 9824772 . 10.1023/a:1022310429868 . 23713562 .
  11. Qiu L, Chen C, Ding G, Zhou Y, Zhang M . The effects of electromagnetic pulse on the protein levels of tight junction associated-proteins in the cerebral cortex, hippocampus, heart, lung, and testis of rats . Biomedical and Environmental Sciences . 24 . 4 . 438–44 . August 2011 . 22108334 . 10.3967/0895-3988.2011.04.016 . 2011BioES..24..438Q .
  12. Castro V, Skowronska M, Lombardi J, He J, Seth N, Velichkovska M, Toborek M . Occludin regulates glucose uptake and ATP production in pericytes by influencing AMP-activated protein kinase activity . Journal of Cerebral Blood Flow and Metabolism . 38 . 2 . 317–332 . February 2018 . 28718701 . 5951017 . 10.1177/0271678X17720816 .
  13. Ando-Akatsuka Y, Saitou M, Hirase T, Kishi M, Sakakibara A, Itoh M, Yonemura S, Furuse M, Tsukita S . 6 . Interspecies diversity of the occludin sequence: cDNA cloning of human, mouse, dog, and rat-kangaroo homologues . The Journal of Cell Biology . 133 . 1 . 43–7 . April 1996 . 8601611 . 2120780 . 10.1083/jcb.133.1.43 .
  14. Web site: Entrez Gene: OCLN occludin.
  15. Web site: OCLN occludin [Homo sapiens (human)] - Gene - NCBI.
  16. Chen Y, Merzdorf C, Paul DL, Goodenough DA . COOH terminus of occludin is required for tight junction barrier function in early Xenopus embryos . The Journal of Cell Biology . 138 . 4 . 891–9 . August 1997 . 9265654 . 2138038 . 10.1083/jcb.138.4.891 .
  17. Feldman GJ, Mullin JM, Ryan MP . Occludin: structure, function and regulation . Advanced Drug Delivery Reviews . 57 . 6 . 883–917 . April 2005 . 15820558 . 10.1016/j.addr.2005.01.009 .
  18. Saito AC, Higashi T, Fukazawa Y, Otani T, Tauchi M, Higashi AY, Furuse M, Chiba H . 7 . Occludin and tricellulin facilitate formation of anastomosing tight-junction strand network to improve barrier function . Molecular Biology of the Cell . 32 . 8 . 722–38 . April 2021 . 33566640 . 8108510 . 10.1091/mbc.E20-07-0464 .
  19. Saitou M, Furuse M, Sasaki H, Schulzke JD, Fromm M, Takano H, Noda T, Tsukita S . 6 . Complex phenotype of mice lacking occludin, a component of tight junction strands . Molecular Biology of the Cell . 11 . 12 . 4131–42 . December 2000 . 11102513 . 15062 . 10.1091/mbc.11.12.4131 .
  20. Martin TA, Mansel RE, Jiang WG . Loss of occludin leads to the progression of human breast cancer . International Journal of Molecular Medicine . 26 . 5 . 723–34 . November 2010 . 20878095 . 10.3892/ijmm_00000519 . free .
  21. Beeman N, Webb PG, Baumgartner HK . Occludin is required for apoptosis when claudin-claudin interactions are disrupted . Cell Death & Disease . 3 . 2 . e273 . February 2012 . 22361748 . 3288343 . 10.1038/cddis.2012.14 .
  22. Osanai M, Murata M, Nishikiori N, Chiba H, Kojima T, Sawada N . 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 . 9125–33 . September 2006 . 16982755 . 10.1158/0008-5472.CAN-06-1864 . free .
  23. Peng BH, Lee JC, Campbell GA . In vitro protein complex formation with cytoskeleton-anchoring domain of occludin identified by limited proteolysis . The Journal of Biological Chemistry . 278 . 49 . 49644–51 . December 2003 . 14512431 . 10.1074/jbc.M302782200 . 33062461 . free .
  24. Itoh M, Morita K, Tsukita S . 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 . The Journal of Biological Chemistry . 274 . 9 . 5981–6 . February 1999 . 10026224 . 10.1074/jbc.274.9.5981 . 20269381 . free .
  25. Wittchen ES, Haskins J, Stevenson BR . Protein interactions at the tight junction. Actin has multiple binding partners, and ZO-1 forms independent complexes with ZO-2 and ZO-3 . The Journal of Biological Chemistry . 274 . 49 . 35179–85 . December 1999 . 10575001 . 10.1074/jbc.274.49.35179 . 23928833 . free .
  26. Chen YH, Lu Q, Goodenough DA, Jeansonne B . Nonreceptor tyrosine kinase c-Yes interacts with occludin during tight junction formation in canine kidney epithelial cells . Molecular Biology of the Cell . 13 . 4 . 1227–37 . April 2002 . 11950934 . 102264 . 10.1091/mbc.01-08-0423 .
  27. Fanning AS, Jameson BJ, Jesaitis LA, Anderson JM . The tight junction protein ZO-1 establishes a link between the transmembrane protein occludin and the actin cytoskeleton . The Journal of Biological Chemistry . 273 . 45 . 29745–53 . November 1998 . 9792688 . 10.1074/jbc.273.45.29745 . 23935899 . free .
  28. Rao RK, Basuroy S, Rao VU, Karnaky KJ, Gupta A . Tyrosine phosphorylation and dissociation of occludin-ZO-1 and E-cadherin-beta-catenin complexes from the cytoskeleton by oxidative stress . The Biochemical Journal . 368 . Pt 2 . 471–81 . December 2002 . 12169098 . 1222996 . 10.1042/BJ20011804 .

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