Skip to main content

Tight Junctions and the Blood-Brain Barrier

  • Chapter
Tight Junctions

Abstract

The blood-brain barrier protects the neural microenvironment from changes of the blood composition. It is located in the endothelium which is both seamless and interconnected by tight junctions. The restrictive paracellular diffusion barrier goes along with an extremely low rate of transcytosis and the expression of a high number of channels and transporters for such molecules which cannot enter or leave the brain paracellularly.

Many tight junction molecules have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin, 7H6, JAM and ESAM. Signaling pathways involved in tight junction regulation include G-proteins, serine-, threonine-, and tyrosine-kinases, extra- and intracellular calcium levels, cAMP levels, proteases, and cytokines Most of these pathways modulate the connection of the cytoskeletal elements to the tight junction transmembrane molecules. Additionally, crosstalk between components of the tight junction- and the cadherin-catenin-system of the adherens junction suggests a close functional interdependence of the two cell-cell-contact systems.

The blood-brain barrier endothelial cells are situated on top of a basal lamina which contains various molecules of the extracellular matrix. Pericytes and astrocytes directly contact this basal lamina; however, little is known about the signaling pathways between these cell types and the endothelium which possibly are mediated by components of the basal lamina. To understand the interplay between astrocytes, pericytes, the basal lamina and the endothelial cells is a big challenge for understanding the blood-brain barrier in the future.

This is a preview of subscription content, log in via an institution to check access.

Access this chapter

Chapter
USD 29.95
Price excludes VAT (USA)
  • Available as PDF
  • Read on any device
  • Instant download
  • Own it forever
eBook
USD 129.00
Price excludes VAT (USA)
  • Available as PDF
  • Read on any device
  • Instant download
  • Own it forever
Softcover Book
USD 169.99
Price excludes VAT (USA)
  • Compact, lightweight edition
  • Dispatched in 3 to 5 business days
  • Free shipping worldwide - see info
Hardcover Book
USD 169.99
Price excludes VAT (USA)
  • Durable hardcover edition
  • Dispatched in 3 to 5 business days
  • Free shipping worldwide - see info

Tax calculation will be finalised at checkout

Purchases are for personal use only

Institutional subscriptions

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. Brightman MW, Reese TS. Junctions between intimately apposed cell membranes in the vertebrate brain. J Cell Biol 1969; 40:648–677.

    PubMed  CAS  Google Scholar 

  2. Begley DJ, Brightman MW. Structural and functional aspects of the blood-brain barrier. Progr Drug Res 2003; 61:39–78.

    CAS  Google Scholar 

  3. Reese TS, Karnovsky MJ. Fine structural localization of a blood-brain barrier to exogenous peroxidase. J Cell Biol 1967; 34:207–217.

    PubMed  CAS  Google Scholar 

  4. Mollgard K, Saunders NR. The development of the human blood-brain and blood-CSF barriers. Neuropathol appl Neurobiol 1986; 12:337–358.

    PubMed  CAS  Google Scholar 

  5. Wolburg H, Lippoldt A. Tight junctions of the blood-brain barier: Development, composition and regulation. Vascular Pharmacol 2002; 38:323–337.

    CAS  Google Scholar 

  6. Vorbrodt AW, Dobrogowska DH. Molecular anatomy of intercellular junctions in brain endothelial and epithelial barriers: Electron microscopist’s view. Brain Res Rev 2003; 42:221–242.

    PubMed  CAS  Google Scholar 

  7. Engelhardt B. Development of the blood-brain barrier. Cell Tissue Res 2003; 314:119–129.

    PubMed  CAS  Google Scholar 

  8. Nagy Z, Peters H, Hüttner I. Fracture faces of cell junctions in cerebral endothelium during normal and hyperosmotic conditions. Lab Invest 1984; 50:313–322.

    PubMed  CAS  Google Scholar 

  9. Wolburg H, Neuhaus J, Kniesel U et al. Modulation of tight junction structure in blood-brain barrier ECs. Effects of tissue culture, second messengers and cocultured astrocytes. J Cell Sci 1994; 107:1347–1357.

    PubMed  CAS  Google Scholar 

  10. Kniesel U, Risau W, Wolburg H. Development of blood-brain barrier tight junctions in the rat cortex. Dev Brain Res 1996; 96:229–240.

    CAS  Google Scholar 

  11. Mühleisen H, Wolburg H, Betz E. Freeze-fracture analysis of endothelial cell membranes in rabbit carotid arteries subjected to short-term atherogenic stimuli. Virch Arch B Cell Pathol 1989; 56:413–417.

    Google Scholar 

  12. Matter K, Balda MS. Signalling to and from tight junctions. Nature Rev Mol Biol 2003; 4:225–236.

    CAS  Google Scholar 

  13. Farquhar MG, Palade GE. Junctional complexes in various epithelial. J Cell Biol 1963; 17:375–412.

    PubMed  CAS  Google Scholar 

  14. Bentzel CJ, Hainau B, Ho S et al. Cytoplasmic regulation of tight junction permeability: Effect of plant cytokinins. Am J Physiol 1980; 239:C75–C89.

    PubMed  CAS  Google Scholar 

  15. Martinez-Palomo A, Meza I, Beaty G et al. Experimental modulation of occluding junctions in a cultured transporting epithelium. J Cell Biol 1980; 87:736–745.

    PubMed  CAS  Google Scholar 

  16. Madara JL, Dharmsathaphorn K. Occluding junction structurefunctiuon relationships in a cultured epithelial monolayer. J Cell Biol 1985; 101:2124–2133.

    PubMed  CAS  Google Scholar 

  17. González-Mariscal L, Chavez de Ramirez B, Cereijido M. Tight junction formation in cultured epithelial cells (MDCK). J Membrane Biol 1985; 86:113–121.

    Google Scholar 

  18. Gumbiner B, Simons K. A functional assay for proteins involved in establishing an epithelial occluding barrier: Identification of an uvomorulin-like polypeptide. J Cell Biol 1986; 102:457–468.

    PubMed  CAS  Google Scholar 

  19. Mandel LJ, Bacallao R, Zampighi G. Uncoupling of the molecular “fence” and paracellular “gate” functions in epithelial tight junctions. Nature 1993; 361:552–555.

    PubMed  CAS  Google Scholar 

  20. Bacallao R, Garfinkel A, Monke S et al. ATP-depletion: A novel method to study junctional properties in epithelial tissues. I. Rearrangement of the actin cytoskeleton. J Cell Sci 1994; 107:3301–3313.

    PubMed  CAS  Google Scholar 

  21. Stevenson BR, Anderson JM, Goodenough DA et al. Tight junction structure and ZO-1 content are identical in two strains of Madin-Darby canine kidney cells which differ in transepithelial resistance. J Cell Biol 1988; 107:2401–2408.

    PubMed  CAS  Google Scholar 

  22. Furuse M, Furuse K, Sasaki H et al. Conversion of Zonulae occludentes from tight to leaky strand type by introducing claudin-2 into Madin-Darby canine kidney I cells. J Cell Biol 2001; 153:263–272.

    PubMed  CAS  Google Scholar 

  23. Simionescu M, Simionescu N, Palade GE. Segmental differentiations of cell junctions in the vascular endothelium. Arteries and veins. J Cell Biol 1976; 68:705–723.

    PubMed  CAS  Google Scholar 

  24. Lippoldt A, Kniesel U, Liebner S et al. Structural alterations of tight junctions are associated with loss of polarity in stroke-prone spontaneously hypertensive rat blood-brain barrier endothelial cells. Brain Res 2000; 885:251–261.

    PubMed  CAS  Google Scholar 

  25. Wolburg H, Wolburg-Buchholz K, Kraus J et al. Localization of claudin-3 in tight junctions of the blood-brain barrier is selectively lost during experimental autoimmune encephalomyelitis and human glioblastoma multiforme. Acta Neuropathol 2003; 105:586–592.

    PubMed  CAS  Google Scholar 

  26. Nag S. Role of the endothelial cytoskeleton in blood-brain barrier permeability to protein. Acta Neuropathol 1995; 90:454–460.

    PubMed  CAS  Google Scholar 

  27. Méresse S, Dehonk M-P, Delorme P et al. Bovine brain ECs express tight junctions and monoamine oxidase activity in long-term culture. J Neurochem 1989; 53:1363–1371.

    PubMed  Google Scholar 

  28. Rubin LL, Hall DE, Porter S et al. A cell culture model of the blood-brain barrier. J Cell Biol 1991; 115:1725–1736.

    PubMed  CAS  Google Scholar 

  29. Tontsch U, Bauer HC. Glial cells and neurons induce blood brain barrier related enzymes in cultured cerebral ECs. Brain Res 1991; 539:247–253.

    PubMed  CAS  Google Scholar 

  30. Abbott NJ, Hughes CCW, Revest PA et al. Development and characterization of a rat capillary endothelial culture. Towards an in vitro BBB. J Cell Sci 1992; 103:23–38.

    PubMed  CAS  Google Scholar 

  31. Pekny M, Stanness KA, Eliasson C et al. Impaired induction of blood-brain barrier properties in aortic endothelial cells by astrocytes from GFAP-deficient mice. Glia 1998; 22:390–400.

    PubMed  CAS  Google Scholar 

  32. Stanness KA, Neumaier JF, Sexton TJ et al. A new model of the blood-brain barrier: Coculture of neuronal, endothelial and glial cells under dynamic conditions. NeuroReport 1999; 10:3725–3731.

    PubMed  CAS  Google Scholar 

  33. Franke H, Galla H-J, Beuckmann CT. Primary cultures of brain microvessel endothelial cells: A valid and flexible model to study drug transport through the blood-brain barrier in vitro. Brain Res Protocols 2000; 5:248–256.

    CAS  Google Scholar 

  34. Gaillard PJ, Voorwinden LH, Nielsen JL et al. Establishment and functional characterization of an in vitro model of the blood-brain barrier, comprising a coculture of brain capillary endothelial cells and astrocytes. Europ J Pharmac Sci 2001; 12:215–222.

    CAS  Google Scholar 

  35. Cucullu L, McAllister MS, Kight K et al. A new dynamic in vitro model for the multidimensional study of astrocyte-endothelial cell interactions at the blood-brain barrier. Brain Res 2002; 951:243–254.

    Google Scholar 

  36. Nitz T, Eisenblätter T, Psathaki K et al. Serum-derived weaken the barrier properties of cultured porcine brain capillary endothelial cells in vitro. Brain Res 2003; 981:30–40.

    PubMed  CAS  Google Scholar 

  37. Parkinson FE, Friesen J, Krizanac-Bengez L et al. Use of a three-dimensional in vitro model of the rat blood-brain barrier to assay nucleoside efflux from brain. Brain Res 2003; 980:233–241.

    PubMed  CAS  Google Scholar 

  38. Hamm S, Dehouck B, Kraus J et al. Astrocyte mediated modulation of blood-brain barrier permeability does not correlate with a loss of tight junction proteins from the cellular contacts. Cell Tissue Res 2004; 315:157–166.

    PubMed  Google Scholar 

  39. Arthur FE, Shivers RR, Bowman PD. Astrocyte-mediated induction of tight junctions in brain capillary endothelium: An efficient in vitro model. Dev Brain Res 1987; 36:155–159.

    Google Scholar 

  40. Dehouck B, Dehouck M-P, Fruchart J-C et al. Upregulation of the low density lipoprotein receptor at the blood-brain barrier: Intercommunications between brain capillary ECs and astrocytes. J Cell Biol 1994; 126:465–474.

    PubMed  CAS  Google Scholar 

  41. Igarashi Y, Utsumi H, Chiba H et al. Glial cell line-derived neurotrophic factor (GDNF) enhances barrier function of endothelial cells forming the blood-brain barrier. Biochem Biophys Res Comm 1999; 261:108–112.

    PubMed  CAS  Google Scholar 

  42. Utsumi H, Chiba H, Kamimura Y et al. Expression of GFRa-1, receptor for GDNF, in rat brain capillary during postnatal development of the BBB. Am J Physiol 2000; 279:C361–C368.

    CAS  Google Scholar 

  43. Yagi T, Jikihara I, Fukumura M et al. Rescue of ischemic brain injury by adenoviral gene transfer of glial cell line-derived factor after transient global ischemia in gerbils. Brain Res 2000; 885:273–282.

    PubMed  CAS  Google Scholar 

  44. Rieckmann P, Engelhardt B. Building up the blood-brain barrier. Nature Med 2003; 9:828–829.

    PubMed  CAS  Google Scholar 

  45. Lee S-W, Kim W-J, Choi YK et al. SSeCKS regulates angiogenesis and tight junction formation in blood-brain barrier. Nature Med 2003; 9:900–906.

    PubMed  CAS  Google Scholar 

  46. Ando-Akatsuka Y, Saitou M, Hirase T et al. Interspecies diversity of the occludin sequence: cDNA cloning of human, mouse, dog, and rat-kangaroo homologues. J Cell Biol 1996; 133:43–48.

    PubMed  CAS  Google Scholar 

  47. Furuse M, Hirase T, Itoh M et al. Occludin: A novel integral membrane protein localizing at tight junctions. J Cell Biol 1993; 123:1777–1788.

    PubMed  CAS  Google Scholar 

  48. Furuse M, Fujita K, Hiiragi T et al. Claudin-1 and-2: Novel integral membrane proteins localizing at tight junctions. J Cell Biol 1998; 141:1539–1550.

    PubMed  CAS  Google Scholar 

  49. Morita K, Furuse M, Fujimoto K et al. Claudin multigene family encoding four-transmembrane domain protein components of tight junction strands. Proc Natl Acad Sci USA 1999; 96:511–516.

    PubMed  CAS  Google Scholar 

  50. Balda MS, Matter K. Transmembrane proteins of tight junctions. Sem Cell Develop Biol 2000; 11:281–289.

    CAS  Google Scholar 

  51. Tsukita S, Furuse M. Occludin and claudins in tight-junction strands: Leading or supporting players? Trends Cell Biol 1999; 9:268–273.

    PubMed  CAS  Google Scholar 

  52. Tsukita S, Furuse M. Pores in the wall: Claudins constitute tight junction strands containing aqueous pores. J Cell Biol 2000; 149:13–16.

    PubMed  CAS  Google Scholar 

  53. Tsukita S, Furuse M, Itoh M. Multifunctional strands in tight junctions. Nature Reviews Mol Cell Biol 2001; 2:285–293.

    CAS  Google Scholar 

  54. Heiskala M, Peterson PA, Yang Y. The roles of claudin superfamily proteins in paracellular transport. Traffic 2001; 2:92–98.

    CAS  Google Scholar 

  55. Huber JD, Egleton RD, Davis TP. Molecular physiology and pathophysiology of tight junctions in the blood-brain barrier. Trends Neurosci 2001; 24:719–725.

    PubMed  CAS  Google Scholar 

  56. D’Atri F, Citi S. Molecular complexity of vertebrate tight junctions. Mol Membr Biol 2002; 19:103–112.

    PubMed  CAS  Google Scholar 

  57. González-Mariscal L, Betanzos A, Nava P et al. Tight junction proteins. Progr Biophys Mol Biol 2003; 81:1–44.

    Google Scholar 

  58. Hein M, Madefessel C, Haag B et al. Reversible modulation of transepithelial resistance in high and low resistance MDCK-cells by basic amino acids, Ca2+, protamine and protons. Chem Phys Lipids 1992; 63:223–233.

    PubMed  CAS  Google Scholar 

  59. Grebenkämper K, Galla H-J. Translational diffusion measurements of a fluorescent phospholipids between MDCK-1 cells support the lipid model of the tight junctions. Chem Phys Lipids 1994; 71:133–143.

    PubMed  Google Scholar 

  60. Wolburg H, Liebner S, Lippoldt A. Freeze-fracture studies of cerebral endothelial tight junctions. Meth Mol Med 2003; 89:51–66.

    CAS  Google Scholar 

  61. Lacaz-Vieira F, Jaeger MMM, Farshori P et al. Small synthetic peptides homologous to segments of the first loop of occludin impair tight junction resealing. J Membrane Biol 1999; 168:289–297.

    CAS  Google Scholar 

  62. Wong V, Gumbiner BM. A synthetic peptide corresponding to the extracellular domain of occluding perturbs the tight junctioin permeability barrier. J Cell Biol 1997; 136:399–409.

    PubMed  CAS  Google Scholar 

  63. Colegio OR, Van Itallie C, Rahner C et al. Claudin extracellular domains determine paracellular charge selectivity and resistance but not tight junction fibril architecture. Am J Physiol Cell Physiol 2003; 284:C1346–C1354.

    PubMed  CAS  Google Scholar 

  64. Kan FWK. Cytochemical evidence for the presence of phospholipids in epithelial tight junction strands. J Histochem Cytochem 1993; 41:649–656.

    PubMed  CAS  Google Scholar 

  65. Yamagata K, Tagami M, Takenaga F et al. Polyunsaturated fatty acids induce tight junctions to form in brain capillary endothelial cells. Neuroscience 2003; 116:649–656.

    PubMed  CAS  Google Scholar 

  66. Saitou M, Furuse M, Sasaki H et al. Complex phenotype of mice lacking occludin, a component of tight junction strands. Mol Biol Cell 2000; 11:4131–4142.

    PubMed  CAS  Google Scholar 

  67. Traweger A, Fang D, Liu Y-C et al. The tight junction specific protein occludin is a functional target of the E3 ubiquitin-protein ligase Itch. J Biol Chem 2002; 277:10201–10208.

    PubMed  CAS  Google Scholar 

  68. Sakakibara A, Furuse M, Saitou M et al. Possible involvement of phosphorylation of occludin in tight junction formation. J Cell Biol 1997; 137:1393–1401.

    PubMed  CAS  Google Scholar 

  69. Chen Y, Merzdorf C, Paul DL et al. COOH terminus of occludin is required for tight junction barrier function in early Xenopus embryos. J Cell Biol 1997; 138:891–899.

    PubMed  CAS  Google Scholar 

  70. Hirase T, Kawashima S, Wong EYM et al. Regulation of tight junction permeability and occluding phosphorylation by RhoA-p160ROCK-dependent and-independent mechanisms. J Biol Chem 2001; 276:10423–10431.

    PubMed  CAS  Google Scholar 

  71. Balda MS, Flores-Maldonado C, Cereijido M et al. Multiple domains of occludin are involved in the regulation of paracellular permeability. J Cell Biochem 2000; 78:85–96.

    PubMed  CAS  Google Scholar 

  72. Huber D, Balda MS, Matter K. Occludin modulates transepithelial migration of neutrophils. J Biol Chem 2000; 275:5773–5778.

    PubMed  CAS  Google Scholar 

  73. DeMaio L, Chang YS, Gardner TW et al. Shear stress regulates occludin content and phosphorylation. Am J Physiol 2001; 281:H105–113.

    CAS  Google Scholar 

  74. Antonetti DA, Barber AJ, Hollinger LA et al. Vascular endothelial growth factor induces rapid phosphorylation of tight junction proteins occludin and zonula occludens 1. A potential mechanism for vascular permeability in diabetic retinopathy and tumors. J Biol Chem 1999; 174:23463–23467.

    Google Scholar 

  75. Furuse M, Sasaki H, Tsukita S. Manner of interaction of heterogenous claudin species within and between tight junction strands. J Cell Biol 1999; 147:891–903.

    PubMed  CAS  Google Scholar 

  76. Morita K, Furuse M, Fujimoto K et al. Claudin multigene family encoding four-transmembrane domain protein components of tight junction strands. Proc Nad Acad Sci USA 1999; 96:511–516.

    CAS  Google Scholar 

  77. Mitic LC, Van Itallie CM, Anderson JM. Molecular physiology and pathophysiology of tight junctions. I. Tight junction structure and function: Lessons from mutant animals and proteins. Am J Physiol 2000; 279:G250–G254.

    CAS  Google Scholar 

  78. Rahner C, Mitic LL, Anderson JM. Heterogeneity in expression and subcellular localization of claudins 2, 3, 4, and 5 in rat liver, pancreas, and gut. Gastroenterology 2001; 120:411–422.

    PubMed  CAS  Google Scholar 

  79. Simon DB, Lu Y, Choate KA et al. Paracellin-1, a renal tight junction protein required for paracellular Mg2+ resorption. Science 1999; 285:103–106.

    PubMed  CAS  Google Scholar 

  80. Hirase T, Staddon JM, Saitou M et al. Occludin as a possible determinant of tight junction permeability in ECs. J Cell Sci 1997; 110:1603–1613.

    PubMed  CAS  Google Scholar 

  81. Inai T, Kobayashi J, Shibata Y. Claudin-1 contributes to the epithelial barrier function in MDCK cells. Europ J Cell Biol 1999; 78:849–855.

    PubMed  CAS  Google Scholar 

  82. Sonoda N, Furuse M, Sasaki H et al. Clostridium perfringens enterotoxin fragment removes specific claudins from tight junction strands: Evidence for direct involvement of claudins in tight junction barrier. J Cell Biol 1999; 147:195–204.

    PubMed  Google Scholar 

  83. Morita K, Sasaki H, Furuse M et al. Endothelial claudin: Claudin-5/TMVCF constitutes tight junction strands in endothelial cells. J Cell Biol 1999; 147:185–194.

    PubMed  CAS  Google Scholar 

  84. Liebner S, Fischmann A, Rascher G et al. Claudin-1 expression and tight junction morphology are altered in blood vessels of human glioblastoma multiforme. Acta Neuropathol 2000; 100:323–331.

    PubMed  CAS  Google Scholar 

  85. Liebner S, Kniesel U, Kalbacher H et al. Correlation of tight junction morphology with the expression of tight junction proteins in blood-brain barrier endothelial cells. Europ J Cell Biol 2000; 79:707–717.

    PubMed  CAS  Google Scholar 

  86. Nitta T, Hata M, Gotoh S et al. Size-selective loosening of the blood-brain barrier in claudin-5-deficient mice. J Cell Biol 2003; 161:653–660.

    PubMed  CAS  Google Scholar 

  87. Martin-Padura I, Lostaglio S, Schneemann M et al. Junctional adhesion molecule, a novel member of the immunoglobulin superfamily that distributes at intercellular junctions and modulates monocyte transmigration. J Cell Biol 1998; 142:117–127.

    PubMed  CAS  Google Scholar 

  88. Muller WA. Leukocyte-endothelial-cell interactions in leukocyte transmigration and the inflammatory response. Trends Immunol 2003; 24:327–334.

    PubMed  CAS  Google Scholar 

  89. Liu Y, Nusrat A, Schnell FJ et al. Human junction adhesion molecule regulates tight junction resealing in epithelia. J Cell Sci 2000; 113:2363–2374.

    PubMed  CAS  Google Scholar 

  90. Palmeri D, van Zante A, Huang CC et al. Vascular endothelial junction-associated molecule, a novel member of the immunoglobulin superfamily, is localized to intercellular boundaries of endothelial cells. J Biol Chem 2000; 275:19139–19145.

    PubMed  CAS  Google Scholar 

  91. Cunningham SA, Arrate MP, Rodriguez JM et al. A novel protein with homology to the Junctional Adhesion Molecule: Characterization of leukocyte interactions. J Biol Chem 2000; 275:34750–34756.

    PubMed  CAS  Google Scholar 

  92. Aurrand-Lions MA, Duncan L, Du Pasquier L et al. Cloning of JAM-2 and JAM-3: An emerging junctional adhesion molecular family? Curr Top Microbiol Immunol 2000; 251:91–98.

    PubMed  CAS  Google Scholar 

  93. Aurrand-Lions MA, Duncan L, Ballestrem C et al. JAM-2, a novel Immunoglobulin Superfamily Molecule, expressed by endothelial and lymphatic cells. J Biol Chem 2001; 276:2733–2741.

    PubMed  CAS  Google Scholar 

  94. Arrate MP, Rodriguez JM, Tran TM et al. Cloning of human junctional adhesion molecule 3 (JAM3) and its identification as the JAM2 counter-receptor. J Biol Chem 2001; 276:45826–45832.

    PubMed  CAS  Google Scholar 

  95. Bazzoni G. The JAM family of junctional adhesion molecules. Curr Opin Cell Biol 2003; 15:525–530.

    PubMed  CAS  Google Scholar 

  96. Ebnet K, Suzuki A, Ohno S et al. Junctional adhesion molecules (JAMs): More molecules with dual functions? J Cell Sci 2004; 117:19–29.

    PubMed  CAS  Google Scholar 

  97. Liang TW, DeMarco RA, Mrsny RJ et al. Characterization of huJAM: Evidence for involvement in cell-cell contact and tight junction regulation. Am J Physiol Cell Physiol 2000; 279:C1733–C1743.

    PubMed  CAS  Google Scholar 

  98. Bazzoni G, Martinez-Estrada OM, Orsenigo F et al. Interaction of junctional adhesion molecule with the tight junction components ZO-1, cingulin, and occludin. J Biol Chem 2000; 275:20520–20526.

    PubMed  CAS  Google Scholar 

  99. Ebnet K, Schulz CU, Meyer Zu et al. Junctional adhesion molecule interacts with the PDZ domain-containing proteins AF-6 and ZO-1. J Biol Chem 2000; 275:27979–27988.

    PubMed  CAS  Google Scholar 

  100. Ebnet K, Aurrand-Lions M, Kuhn A et al. The junctional adhesion molecule (JAM) family members JAM-2 and JAM-3 associate with the cell polarity protein PAR-3: A possible role for JAMs in endothelial cell polarity. J Cell Sci 2003; 116:3879–3891.

    PubMed  CAS  Google Scholar 

  101. Ebnet K, Suzuki A, Horikoshi Y et al. The cell polarity protein ASIP/PAR-3 directly associates with junctional adhesion molecule (JAM). EMBO J 2001; 20:3738–3748.

    PubMed  CAS  Google Scholar 

  102. Itoh M, Sasaki H, Furuse M et al. Junctional adhesion molecule (JAM) binds to PAR-3: A possible mechanism for the recruitment of PAR-3 to tight junctions. J Cell Biol 2001; 154:491–498.

    PubMed  CAS  Google Scholar 

  103. Ohno S. Intercellular junctions and cellular polarity: The PAR-aPKC complex, a conserved core cassette playing fundamental roles in cell polarity. Curr Opin Cell Biol 2001; 13:641–648.

    PubMed  CAS  Google Scholar 

  104. Suzuki A, Yamanaka T, Hirose T et al. Atypical protein kinase C is involved in the evolutionary conserved PAR protein complex and plays a critical role in establishing epithelia-specific junctional structures. J Cell Biol 2001; 152:1183–1196.

    PubMed  CAS  Google Scholar 

  105. Yamanaka T, Horikoshi Y, Suzuki A et al. Par-6 regulates aPKC activity in a novel way and mediates cell-cell contact-induced formation of epithelial junctional complex. Genes Cells 2001; 6:721–731.

    PubMed  CAS  Google Scholar 

  106. Gao L, Joberty G, Macara IG. Assembly of epithelial tight junctions is negatively regulated by Par6. Curr Biol 2002; 12:221–225.

    PubMed  CAS  Google Scholar 

  107. Nagai-Tamai Y, Mizuno K, Hirose T et al. Regulated protein-protein interaction between aPKC and PAR-3 plays an essential role in the polarization of epithelial cells. Genes Cells 2002; 7:1161–1171.

    PubMed  CAS  Google Scholar 

  108. Suzuki A, Ishiyama C, Hashiba K et al. aPKC kinase activity is required for the asymmetric differentiation of the premature junctional complex during epithelial cell polarization. J Cell Sci 2002; 115:3565–3573.

    PubMed  CAS  Google Scholar 

  109. Cohen CJ, Shieh JT, Pickles RJ et al. The coxsackievirus and adenovirus receptor is a transmembrane component of the tight junction. Proc Natl Acad Sci USA 2001; 98:15191–15196.

    PubMed  CAS  Google Scholar 

  110. Nasdala I, Wolburg-Buchholz K, Wolburg H et al. A transmembrane tight junction protein selectively expressed on endothelial cells and platelets. J Biol Chem 2002; 277:16294–16303.

    PubMed  CAS  Google Scholar 

  111. Hirabayashi S, Tajima M, Yao I et al. JAM4, a junctional cell adhesion molecule interacting with a tight junction protein, MAGI-1. Mol Cell Biol 2003; 23:4267–4282.

    PubMed  CAS  Google Scholar 

  112. Raschperger E, Engstrom U, Pettersson RF et al. CLMP, a novel member of the CTX family and a new component of epithelial tight junctions. J Biol Chem 2004; 279:796–804.

    PubMed  CAS  Google Scholar 

  113. Hirata K, Ishida T, Penta K et al. Cloning of an immunoglobulin family adhesion molecule selectively expressed by endothelial cells. J Biol Chem 2001; 276:16223–16231.

    CAS  Google Scholar 

  114. Ishida T, Kundu RK, Yang E et al. Targeted disruption of endothelial cell-selective adhesion molecule inhibits angiogenic processes in vitro and in vivo. J Biol Chem 2003; 278:34598–34604.

    PubMed  CAS  Google Scholar 

  115. Harris BZ, Lim WA. Mechanism and role of PDZ domains in signaling complex assembly. J Cell Sci 2001; 114:3219–3231.

    PubMed  CAS  Google Scholar 

  116. Pawson T, Nash P. Assembly of cell regulatory systems through protein interaction domains. Science 2003; 300:445–452.

    PubMed  CAS  Google Scholar 

  117. Anderson JM. Cell signalling: MAGUK magic. Curr Biol 1996; 6:382–384.

    PubMed  CAS  Google Scholar 

  118. Dobrosotskaya I, Guy RK, James GL. MAGI-1, a membrane-associated guanylate kinase with a unique arrangement of protein-protein interaction domains. J Biol Chem 1997; 272:31589–31597.

    PubMed  CAS  Google Scholar 

  119. Roh MH, Margolis B. Composition and function of PDZ protein complexes during cell polarization. Am J Physiol Renal Physiol 2003; 285:F377–387.

    PubMed  Google Scholar 

  120. Roh MH, Makarova O, Liu CJ et al. The Maguk protein, Pals1, functions as an adapter, linking mammalian homologues of Crumbs and Discs Lost. J Cell Biol 2002; 157:161–172.

    PubMed  CAS  Google Scholar 

  121. Ullmer C, Schmuck K, Figge A et al. Cloning and characterization of MUPP1, a novel PDZ domain protein. FEBS Lett 1998; 424:63–68.

    PubMed  CAS  Google Scholar 

  122. Hopkins AM, Li D, Mrsny RJ et al. Modulation of tight junction function by G protein-coupled events. Adv Drug Deliv Rev 2000; 41:329–340.

    PubMed  CAS  Google Scholar 

  123. Benais-Pont G, Punn A, Flores-Maldonado C et al. Identification of a tight junction-associated guanine nucleotide exchange factor that activates Rho and regulates paracellular permeability. J Cell Biol 2003; 160:729–740.

    PubMed  CAS  Google Scholar 

  124. Avila-Flores A, Rendon-Huerta E, Moreno E et al. Tight-junction protein zonula occludens 2 is a target of phophorylation by protein kinase C. Biochem J 2001; 360:295–304.

    PubMed  CAS  Google Scholar 

  125. Izumi Y, Hirose T, Tamai Y et al. An atypical PKC directly associates and colocalizes at the epithelial tight junction with ASIP, a mammalian homologue of Caenorhabditis elegans polarity protein PAR-3. J Cell Biol 1998; 143:95–106.

    PubMed  CAS  Google Scholar 

  126. Nunbhakdi-Craig V, Machleidt T, Ogris E et al. Protein phosphatase 2A associates with and regulates atypical PKC and the epithelial tight junction complex. J Cell Biol 2002; 158:967–978.

    PubMed  CAS  Google Scholar 

  127. Wu Y, Dowbenko D, Spencer S et al. Interaction of the tumor suppressor PTEN/MMAC with a PDZ domain of MAGI3, a novel membrane-associated guanylate kinase. J Biol Chem 2000; 275:21477–21485.

    PubMed  CAS  Google Scholar 

  128. Balda MS, Garrett MD, Matter K. The ZO-1-associated Y-box factor ZONAB regulates epithelial cell proliferation and cell density. J Cell Biol 2003; 160:423–432.

    PubMed  CAS  Google Scholar 

  129. Nakamura T, Blechman J, Tada S et al. huASH1 protein, a putative transcription factor encoded by a human homologue of the Drosophila ash1 gene, localizes to both nuclei and cell-cell tight junctions. Proc Natl Acad Sci USA 2000; 97:7284–7289.

    PubMed  CAS  Google Scholar 

  130. Betanzos A, Huerta M, Lopez-Bayghen E et al. The tight junction protein ZO-2 associates with Jun, Fos and C/EBP transcription factors in epithelial cells. Exp Cell Res 2004; 292:51–66.

    PubMed  CAS  Google Scholar 

  131. Sheth P, Basuroy, Li C et al. Role of phosphatidylinositol 3-kinase in oxidative stress-induced disruption of tight junctions. J Biol Chem 2003; 278:49239–49245.

    PubMed  CAS  Google Scholar 

  132. Denker BM, Saha C, Khawaja S et al. Involvement of a heterotrimeric G protein alpha subunit in tight junction biogenesis. J Biol Chem 1996; 271:25750–25753.

    PubMed  CAS  Google Scholar 

  133. Saha C, Nigam SK, Denker BM. Expanding role of G proteins in tight junction regulation: Gas stimulates tight junction assembly. Biochem Biophys Res Commun 2001; 285:250–256.

    PubMed  CAS  Google Scholar 

  134. Dodane V, Kachar B. Identification of isoforms of G proteins and PKC that colocalize with tight junctions. J Membr Biol 1996; 149:199–209.

    PubMed  CAS  Google Scholar 

  135. Denker BM, Nigam SK. Molecular structure and assembly of the tight junction. Am J Physiol 1998; 274:F1–F9.

    PubMed  CAS  Google Scholar 

  136. Klingler C, Kniesel U, Bamforth S et al. Disruption of epithelial tight junctions is prevented by cyclic nucleotide-dependent protein kinase inhibitors. Histochem Cell Biol 2000; 113:349–361.

    PubMed  CAS  Google Scholar 

  137. Balda MS, Gonzalez-Mariscal L, Contreras RG et al. Assembly and sealing of tight junctions: Possible participation of G-proteins, phospholipase C, protein kinase C and calmodulin. J Membr Biol 1991; 122:193–202.

    PubMed  CAS  Google Scholar 

  138. Raub TJ. Signal transduction and glial cell modulation of cultured brain microvessel endothelial cell tight junctions. Am J Physiol 1996; 271:C495–C503.

    PubMed  CAS  Google Scholar 

  139. Brückener KE, El Baya A, Galla H-J et al. Permeabilization in a cerebral endothelial barrier model by pertussis toxin involves the PKC effector pathay and is abolished by elevated levels of cAMP. J Cell Sci 2003; 116:1837–1846.

    PubMed  Google Scholar 

  140. Garcia JGN, Wang P, Liu F et al. Pertussis toxin directly activates endothelial cell p42/p44 MAP kinases via a novel signaling pathway. Am J Physiol 2001; 280:C1233–C1241.

    CAS  Google Scholar 

  141. De Vries L, Farquahr MG. RGS proteins: More than just GAPs for heterotrimeric G proteins. Trends Cell Biol 1999; 9:138–144.

    PubMed  Google Scholar 

  142. Li JY, Boado RJ, Pardridge WM. Blood-brain barrier genomics. J Cerebr Blood Flow Metab 2001; 21:61–68.

    CAS  Google Scholar 

  143. Kirsch T, Wellner M, Haller H et al. Altered gene expression in cerebral capillaries of stroke-prone spontaneously hypertensive rats. Brain Res 2001; 910:106–115.

    PubMed  CAS  Google Scholar 

  144. Adamson RH, Curry FE, Adamson G et al. Rho and rho kinase modulation of barrier properties: Cultured endothelial cells and intact microvessels of rats and mice. J Physiol 2002; 539:295–308.

    PubMed  CAS  Google Scholar 

  145. Van Hinsbergh VWM, Van Nieuw Amerongen GP. Intracellular signalling involved in modulating human endothelial barrier function. J Anat 2002; 200:549–560.

    PubMed  Google Scholar 

  146. Jou TS, Nelson WJ. Effects of regulated expression of mutant rhoA and rac1 small GTPases on the development of epithelial (MDCK) cell polarity. J Cell Biol 1998; 142:85–100.

    PubMed  CAS  Google Scholar 

  147. Jou TS, Schneeberger EE, Nelson WJ. Structural and functional regulation of tight junctions by rhoA and racl small GTPases. J Cell Biol 1998; 142:101–115.

    PubMed  CAS  Google Scholar 

  148. Nusrat A, Giry M, Turner JR et al. Rho protein regulates tight junctions and perijunctional actin organization in polarized epithelia. Proc Natl Acad Sci USA 1995; 92:10629–10633.

    PubMed  CAS  Google Scholar 

  149. Hopkins AM, Walsh SV, Verkade P et al. Constitutive activation of Rho proteins by CNF-1 influences tight junction structure and epithelial barrier function. J Cell Sci 2002; 116:725–742.

    Google Scholar 

  150. van Leeuwen FN, Giepmans BN, van Meeteren LA et al. Lysophosphatidic acid: Mitogen and motility factor. Biochem Soc Trans 2003; 31:1209–1212.

    PubMed  Google Scholar 

  151. Schulze C, Smales C, Rubin LL et al. Lysophophatidic acid increases tight junctional permeability in cultured brain ECs. J Neurochem 1997; 68:991–1000.

    PubMed  CAS  Google Scholar 

  152. Lum H, Malik AB. Mechanisms of increased endothelial permeability. Can J Physiol Pharmacol 1996; 74:787–800.

    PubMed  CAS  Google Scholar 

  153. Wójciak-Stothard B, Entwistle A, Garg R et al. Regulation of TNF-α-induced reorganization of the actin cytoskeleton and cell-cell junctions by rho, rac, and cdc42 in human endothelial cells. J Cell Physiol 1998; 176:150–165.

    PubMed  Google Scholar 

  154. Johnson-Léger C, Aurrand-Lions M, Imhof BA. The parting of the endothelium: Miracle, or simply a junctional affair? J Cell Sci 2000; 113:921–933.

    PubMed  Google Scholar 

  155. Mayhan WG. Leukocyte adherence contriburtes to disruption of the blood-brain barrier during activation of mast cells. Brain Res 2000; 869:112–120.

    PubMed  CAS  Google Scholar 

  156. Liu F, Verin AD, Borbiev T et al. Role of cAMP-dependent protein kinase A activity in endothelial cell cytoskeleton rearrangement. Am J Physiol 2001; 280:L1309–L1317.

    CAS  Google Scholar 

  157. Petty MA, Lo EH. Junctional complexes of the blood-brain barrier: Permeability changes in neuroinflammation. Progr Neurobiol 2002; 68:311–323.

    CAS  Google Scholar 

  158. Essler M, Amano M, Kruse HJ et al. Thrombin inactivates myosin light chain phosphatase via rho and its target rho kinase in human endothelial cells. J Biol Chem 1998; 273:21867–21874.

    PubMed  CAS  Google Scholar 

  159. Essler M, Staddon JM, Weber PC et al. Cyclic AMP blocks bacterial lipopolysaccharide-induced myosin light chain phosphorylation in endothelial cells through inhibition of rho/rho kinase signaling. J Immunol 2000; 164:6543–6549.

    PubMed  CAS  Google Scholar 

  160. Adamson P, Etienne S, Couraud PO et al. Lymphocyte migration through brain endothelial cell monolayers involves signaling through endothelial ICAM-1 via a rho-dependent pathway. J Immunol 1999; 162:2964–2973.

    PubMed  CAS  Google Scholar 

  161. Etienne S, Adamson P, Greenwood J et al. ICAM-1 signaling pathways associated with rho activation in microvascular brain endothelial cells. J Immunol 1998; 161:5755–5761.

    PubMed  CAS  Google Scholar 

  162. Barber AJ, Lieth E. Agrin accumulates in the brain microvascular basal lamina during development of the blood-brain barrier. Dev Dyn 1997; 208:62–74.

    PubMed  CAS  Google Scholar 

  163. Gladson CL. The extracellular matrix of gliomas: Modulation of cell function. J Neuropathol exp Neurol 1999; 58:1029–1040.

    PubMed  CAS  Google Scholar 

  164. Savettieri G, Di Liegro I, Catania C et al. Neurons and ECM regulate occludin localization in brain endothelial cells. NeuroReport 2000; 11:1081–1084.

    PubMed  CAS  Google Scholar 

  165. Sixt M, Engelhardt B, Pausch F et al. Endothelial cell laminin isoforms, laminins 8 and 10, play decisive roles in T cell recruitment across the blood-brain barrier in experimental autoimmune encephalomyelitis. J Cell Biol 2001; 153:933–945.

    PubMed  CAS  Google Scholar 

  166. Alexander JS, Elrod JW. Extracellular matrix, junctional integrity and matrix metalloproteinase interactions in endothelial permeability regulation. J Anat 2002; 200:561–574.

    PubMed  CAS  Google Scholar 

  167. Robert AM, Robert L. Extracellular matrix and blood-brain barrier function. Pathol Biol 1998; 46:535–542.

    PubMed  CAS  Google Scholar 

  168. Sobel RA, Hinojoza JR, Maeda A et al. Endothelial cell integrin laminin receptor expression in multiple sclerosis lesions. Am J Pathol 1998; 153:405–415.

    PubMed  CAS  Google Scholar 

  169. Rascher G, Wolburg H. The blood-brain barrier in the aging brain. In: J de Vellis, ed. Neuroglia in the Aging Brain. Totowa: Humana Press, 2001:305–320.

    Google Scholar 

  170. Tilling T, Engelbertz C, Decker S et al. Expression and adhesive properties of basement membrane proteins in cerebral capillary endothelial cell cultures. Cell Tissue Res 2002; 310:19–29.

    PubMed  CAS  Google Scholar 

  171. Jiang X, Couchman JR. Perlecan and tumor angiogenesis. J Histochem Cytochem 2003; 51:1393–1410.

    PubMed  CAS  Google Scholar 

  172. Asahi M, Wang X, Mori T et al. Effects of matrix metalloproteinase-9 gene knockout on the proteolysis of blood-brain barrier and whize mater components after cerebral ischemia. J Neurosci 2001; 21:7724–7732.

    PubMed  CAS  Google Scholar 

  173. McMahon UJ. The agrin hypothesis. Cold Spring Harb Symp Quant Biol 1990; 55:407–418.

    Google Scholar 

  174. Bezakova G, Ruegg MA. New insights into the roles of agrin. Nature Rev Mol Cell Biol 2003; 4:295–308.

    CAS  Google Scholar 

  175. Berzin TM, Zipser BD, Rafii MS et al. Agrin and microvascular damage in Alzheimer’s disease. Neurobiol Aging 2000; 21:349–355.

    PubMed  CAS  Google Scholar 

  176. Smith MA, Hilgenberg LGW. Agrin in the CNS: A protein in search of a function? NeuroReport 2002; 13:1485–1495.

    PubMed  CAS  Google Scholar 

  177. Stone DM, Nikolics K. Tissue-an age-specific expression patterns of alterantively spliced agrin mRNA transcripts in embryonic rat suggest novel developmental roles. J Neurosci 1995; 15:6767–6778.

    PubMed  CAS  Google Scholar 

  178. Gee SH, Montanaro F, Lindenbaum MH et al. Dystroglycan-α; A dystrophin-associated glyoprotein, is a functional agrin receptor. Cell 1994; 77:675–686.

    PubMed  CAS  Google Scholar 

  179. Blake DJ, Kröger S. The neurobiology of Duchenne muscular dystrophy: Learning lessons from muscle? Trends Neurosci 2000; 23:92–99.

    PubMed  CAS  Google Scholar 

  180. Rascher G, Fischmann A, Kröger S et al. Extracellular matrix and the blood-brain barrier in glioblastoma multiforme: Spatial segregation of tenascin and agrin. Acta Neuropathologica 2002; 104:85–91.

    PubMed  CAS  Google Scholar 

  181. Janzer RC, Raff MC. Astrocytes induce blood-brain barrier properties in endothelial cells. Nature 1997; 325:253–257.

    Google Scholar 

  182. Abbott NJ. Astrocyte-endothelial interactions and blood-brain barrier permeability. J Anat 2002; 200:629–638.

    PubMed  CAS  Google Scholar 

  183. Brillault J, Berezowski V, Cecchelli R et al. Intercommunications between brain capillary endothelial cells and glial cells increase the transcellular permeability of the blood-brain barrier during ischemia. J Neurochem 2002; 83:807–817.

    PubMed  CAS  Google Scholar 

  184. Wolburg H. Orthogonal arrays of intramembranous particles. A review with special reference to astrocytes. J Brain Res 1995; 36:239–258.

    CAS  Google Scholar 

  185. Yonezawa T, Ohtsuka A, Yoshitaka T et al. Limitrin, a novel immunoglobulin superfamily protein localized to glia limitans formed by astrocyte endfeet. Glia 2003; 44:190–204.

    PubMed  Google Scholar 

  186. Wolburg H. Glia-neuronal and glia-vascular interrelations in blood-brain barrier formation and axon regeneration in vertebrates. In: Vernadakis A, Roots B, eds. Neuro-Glial Interactions during Phylogeny. Totowa: Humana Press Inc., 1995:479–510.

    Google Scholar 

  187. Nico B, Frigeri A, Nicchia GP et al. Role of aquaporin-4 water channel in the development and integrity of the blood-brain barrier. J Cell Sci 2001; 114:1297–1307.

    PubMed  CAS  Google Scholar 

  188. Papadopoulos MC, Krishna S, Verkman AS. Aquaporin water channels and brain edema. Mount Sinai J Med 2002; 69:242–248.

    Google Scholar 

  189. Badaut J, Lasbennes F, Magistretti PJ et al. Aquaporins in brain: Distribution, physiology and pathophysiology. J Cer Blood Flow Metabol 2002; 22:367–378.

    CAS  Google Scholar 

  190. Verbavatz J-M, Ma T, Gobin R et al. Absence of orthogonal arrays in kidney, brain and muscle from transgenic knockout mice lacking water channel aquaporin-4. J Cell Sci 1997; 110:2855–2860.

    PubMed  CAS  Google Scholar 

  191. Yang B, Brown D, Verkman AS. The mercurial insensitive water channel (AQP-4) forms orthogonal arrays in stably transfected chinese hamster ovary cells. J Biol Chem 1996; 271:4577–4580.

    PubMed  CAS  Google Scholar 

  192. Rash JE, Yasumura T, Hudson CS et al. Direct immunogold labeling of aquaporin-4 in square arrays of astrocyte and ependymocyte plasma membranes in rat brain and spinal cord. Proc Natl Acad Sci USA 1998; 95:11981–11986.

    PubMed  CAS  Google Scholar 

  193. Nielsen S, Nagelhus EA, Amiry-Moghaddam M et al. Specialized membrane domains for water transport in glial cells: High-resolution immunogold cytochemistry of aquaporin-4 in rat brain. J Neurosci 1997; 17:171–180.

    PubMed  CAS  Google Scholar 

  194. Neuhaus J. Orthogonal arrays of particles in astroglial cells: Quantitative analysis of their density, size, and correlation with intramembranous particles. Glia 1990; 3:241–251.

    PubMed  CAS  Google Scholar 

  195. Saadoun S, Papadopoulos MC, Davies DC et al. Aquaporin-4 expression is increased in oedematous human brain tumours. J Neurol Neurosurg Psychiatry 2002; 72:262–265.

    PubMed  CAS  Google Scholar 

  196. Warth A, Kröger S, Wolburg H. Redistribution of aquaporin-4 in human glioblastoma correlates with loss of agrin immunoreactivity from brain capillary basal laminae. Acta Neuropathol 2004; 107:311–318.

    PubMed  CAS  Google Scholar 

  197. Neely JD, Amiry-Moghaddam M, Ottersen OP et al. Syntrophin-dependent expression and localization of aquaporin-4 water channel protein. Proc Natl Acad Sci USA 2001; 98:14108–14113.

    PubMed  CAS  Google Scholar 

  198. Ehrlich P. Das Sauerstoff-Bedürfnis des Organismus. eine farbenanalytische studie. Herschwald, Berlin: PhD thesis, 1885:69–72.

    Google Scholar 

  199. Nag S. Blood-brain barrier permeability using tracers and immunohistochemistry. Methods Mol Med Humana Press, 2003:89.

    Google Scholar 

  200. Sasaki H, Matsui C, Furuse K et al. Dynamic behavior of paired claudin strands within apposing plasma membranes. Proc Natl Acad Sci USA 2003; 100:3971–3976.

    PubMed  CAS  Google Scholar 

  201. Wolburg H, Wolburg-Buchholz K, Liebner S et al. Claudin-1, claudin-2 and claudin-11 are present in tight junctions of choroid plexus epithelium of the mouse. Neurosci Letters 2001; 307:77–80.

    CAS  Google Scholar 

Download references

Author information

Authors and Affiliations

Authors

Rights and permissions

Reprints and permissions

Copyright information

© 2006 Landes Bioscience and Springer Science+Business Media

About this chapter

Cite this chapter

Wolburg, H., Lippoldt, A., Ebnet, K. (2006). Tight Junctions and the Blood-Brain Barrier. In: Tight Junctions. Springer, Boston, MA. https://doi.org/10.1007/0-387-36673-3_13

Download citation

Publish with us

Policies and ethics