Wiechmann Allan F, Ceresa Brian P, Howard Eric W
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.
Department of Pharmacology and Toxicology, University of Louisville School of Medicine, Louisville, Kentucky, United States of America.
PLoS One. 2014 Nov 20;9(11):e113810. doi: 10.1371/journal.pone.0113810. eCollection 2014.
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.
METHODOLOGY/PRINCIPAL FINDINGS: 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/SIGNIFICANCE: 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.
角膜上皮提供了一道抵御病原体侵入和磨损力的保护屏障，这很大程度上归因于相邻细胞间的细胞间连接复合体。在角膜表面经过规定的时间后，鳞状表面细胞之间的紧密连接必须被破坏，以使它们能够作为组织稳态更新的一部分而脱落。我们推测基质金属蛋白酶（MMPs）由角膜上皮细胞分泌，并在上皮表面细胞外裂解细胞间连接蛋白。本研究的目的是检查在昼夜节律周期的光照和黑暗阶段特定MMPs和紧密连接蛋白的表达，并评估它们在非洲爪蟾角膜上皮中的时间和空间关系。
方法/主要发现：通过共聚焦双标记免疫组织化学法，对在不同昼夜时间从非洲爪蟾获取的角膜进行检查，以检测MMP-2、MMP-2组织抑制剂（TIMP-2）、膜型1-MMP（MT1-MMP）以及紧密连接蛋白闭合蛋白和紧密连接蛋白4的表达。在白天，闭合蛋白和紧密连接蛋白4在角膜表面上皮细胞侧膜上的表达通常整体保持完整，但在夜间，小细胞簇中的表达经常被破坏。同时，MMP-2的表达在夜间常以镶嵌模式升高，并与表面脱落细胞簇相关。MMP-2结合伴侣TIMP-2和MT1-MMP在光照和黑暗阶段也定位于角膜表面上皮细胞，TIMP-2在白天往往升高。
结论/意义：在非洲爪蟾中，夜间角膜表面上皮细胞中MMP-2蛋白表达以镶嵌模式升高，可能通过破坏细胞间连接蛋白在稳态表面细胞脱屑中发挥作用。MMP分泌和激活、紧密连接蛋白裂解以及随后的表面细胞脱屑和更新的顺序可能由夜间昼夜节律信号协调。
