title: Diurnal variation of tight junction integrity associates inversely with matrix metalloproteinase expression in Xenopus laevis corneal epithelium: implications for circadian regulation of homeostatic surface cell desquamation.
abstract: OBJECTIVE
  The corneal epithelium provides a protective barrier against pathogen entrance and abrasive forces, largely due to the intercellular junctional complexes between neighboring cells. After a prescribed duration at the corneal surface, tight junctions between squamous surface cells must be disrupted to enable them to desquamate as a component of the tissue homeostatic renewal. We hypothesize that matrix metalloproteinase (MMPs) are secreted by corneal epithelial cells and cleave intercellular junctional proteins extracellularly at the epithelial surface. The purpose of this study was to examine the expression of specific MMPs and tight junction proteins during both the light and dark phases of the circadian cycle, and to assess their temporal and spatial relationships in the Xenopus laevis corneal epithelium.
  
  
  RESULTS
  Expression of MMP-2, tissue inhibitor of MMP-2 (TIMP-2), membrane type 1-MMP (MT1-MMP) and the tight junction proteins occludin and claudin-4 were examined by confocal double-label immunohistochemistry on corneas obtained from Xenopus frogs at different circadian times. Occludin and claudin-4 expression was generally uniformly intact on the surface corneal epithelial cell lateral membranes during the daytime, but was frequently disrupted in small clusters of cells at night. Concomitantly, MMP-2 expression was often elevated in a mosaic pattern at nighttime and associated with clusters of desquamating surface cells. The MMP-2 binding partners, TIMP-2 and MT1-MMP were also localized to surface corneal epithelial cells during both the light and dark phases, with TIMP-2 tending to be elevated during the daytime.
  
  
  CONCLUSIONS
  MMP-2 protein expression is elevated in a mosaic pattern in surface corneal epithelial cells during the nighttime in Xenopus laevis, and may play a role in homeostatic surface cell desquamation by disrupting intercellular junctional proteins. The sequence of MMP secretion and activation, tight junction protein cleavage, and subsequent surface cell desquamation and renewal may be orchestrated by nocturnal circadian signals.
authors.affiliation: Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States of America; Department of Ophthalmology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States of America.
authors.forename: Allan F
authors.initials: AF
authors.lastname: Wiechmann
authors.affiliation: Department of Pharmacology and Toxicology, University of Louisville School of Medicine, Louisville, Kentucky, United States of America.
authors.forename: Brian P
authors.initials: BP
authors.lastname: Ceresa
authors.affiliation: Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States of America.
authors.forename: Eric W
authors.initials: EW
authors.lastname: Howard
mesh_terms: D000818:Animals; D002940:Circadian Rhythm; D062506:Claudin-4 / Q000378:metabolism; D019573:Epithelium, Corneal / Q000378:metabolism* / Q000473:pathology; D007150:Immunohistochemistry; D053511:Matrix Metalloproteinase 14 / Q000378:metabolism; D020778:Matrix Metalloproteinase 2 / Q000378:metabolism; D020782:Matrix Metalloproteinases / Q000378:metabolism*; D018613:Microscopy, Confocal; D062793:Occludin / Q000378:metabolism; D019108:Tight Junctions / Q000378:metabolism* / Q000473:pathology; D019716:Tissue Inhibitor of Metalloproteinase-2 / Q000378:metabolism; D014982:Xenopus laevis / Q000378:metabolism*
chemical_list: D062506:Claudin-4; D062793:Occludin; D019716:Tissue Inhibitor of Metalloproteinase-2; D020782:Matrix Metalloproteinases; D020778:Matrix Metalloproteinase 2; D053511:Matrix Metalloproteinase 14