Exfoliation syndrome (XFS) is an age-related, generalized disorder of the extracellular matrix, which is characterized by the excessive production and accumulation of elastic microfibrils and their aggregation into mature XFS fibrils. These abnormal fibrils contain elastic fiber components, such as elastin, latent transforming growth factor binding proteins (LTBP): LTBP-1, LTBP-2, and predominantly fibrillin-1, but also enzymatically active compounds, such as LOXL1, and the extracellular chaperone clusterin. XFS fiber components are excessively cross-linked and glycosylated, and therefore resistant to enzymatic degradation.1
The main genetic risk factor for XFS, LOXL1, encodes lysyl oxidase-like 1, a key enzyme of elastogenesis and elastic fiber homeostasis that catalyzes, in concert with other LOX isoenzymes, the formation of regular elastic fiber networks.2,3 At present, the mechanisms by which genetic variants in the LOXL1 gene confer risk for the disease still remain elusive. Current models of pathogenesis include an altered expression, abnormal splicing events, or an altered substrate specificity of LOXL1.
For this study, XFS eyes were classified as early-stage or late-stage XFS according to a semiquantitative grading score of the amount of macroscopically visible XFS deposits on ocular structures. As XFS deposits can be detected earliest on zonular fibers, early stages were defined by a frosted appearance of the zonules, whereas late stages revealed prominent XFS deposits on lens, iris, and ciliary processes in addition. The macroscopic evidence of XFS was further confirmed by electron microscopic analysis of ciliary and iridal tissue sections.
In tissues of the anterior segment, such as the trabecular meshwork, the iris, and the ciliary body, the mRNA, and protein expression of LOXL1 was significantly increased in early stages of XFS, but markedly decreased in late stages with and without glaucoma, compared with normal donor eyes.4 Other members of the LOX family, such as LOX, or LOXL2 were not affected by the exfoliation process in their expression levels (Fig. 1A). Therefore, it appears that LOXL1 is selectively upregulated in the initial phase of the XFS process and downregulated in advanced stages. In contrast to other LOX isoforms, LOXL1 was found to represent a major component of XFS deposits (Fig. 2B),4,5 suggesting a distinct role of LOXL1 for the cross-linking of XFS fibrils. Furthermore, LOXL1 staining of XFS deposits colocalized, with tropoelastin, fibrillin-1, LTBP-1, LTBP-2, and fibulin-4, which also displayed increased expression levels in early and late XFS stages (Fig. 1A). The specific colocalization of these elastic proteins with LOXL1 in exfoliation deposits may raise the possibility that, fibrillin-1, LTBP-1, LTBP-2, and fibulin-4 are potential substrates for LOXL1, in addition to the known substrate tropoelastin.4
As XFS is a complex genetic disease, genetic variants of the LOXL1 gene are a prerequisite, but are not sufficient for the manifestation of the disease. Further genetic and/or external factors appear to be necessary for the initiation of the abnormal matrix process. Increased oxidative stress conditions, a pronounced anterior chamber hypoxia, elevated homocysteine levels, elevated transforming growth factor (TGF)-ß1 levels, and a transient upregulation of proinflammatory cytokines, such as interleukin (IL): IL-6 and IL-8, are well-documented characteristics of XFS eyes and reasonable candidate triggering factors of the abnormal matrix process in tissues of the anterior segment.1 Indeed, using in vitro cell culture models, all of these factors present in XFS eyes, have been shown to upregulate LOXL1 and elastic components on the mRNA as well as on the protein level in a direct manner or through the action of secondary mediators.6,7
However, at present it is not clear which of these pathogenetic factors initiate the pathologic cascade. Moreover, in XFS eyes, these factors seem to be intensified by impaired cytoprotective mechanisms, including antioxidant defense, proteasome function, endoplasmic reticulum-related stress response, extracellular chaperone function, and DNA repair in anterior segment tissues.8 Increased cellular stress combined with weakened cytoprotective mechanisms could lead to a prolonged proinflammatory state, also termed “molecular inflammation,” which has been suggested as a major event underlying the onset of many fibrotic disorders and age-related chronic degenerative diseases, such as Alzheimer disease, and cardiovascular disorders.9,10 At the moment it is still unclear how a prolonged proinflammatory state, reflected by elevated aqueous IL-6 and IL-8 levels in early XFS stages, may participate in the molecular events underlying XFS. On the basis of their known activities, IL-6 and IL-8 may act by mechanisms such diverse as stimulation of extracellular matrix synthesis, induction of the fibrogenic growth factor TGF-ß1, or enhancement of vascular permeability in the iris.6,11 Taken together, chronically elevated stress conditions in combination with impaired stress defense mechanisms and a specific genetic background may participate in the onset of fibrosis by upregulating proinflammatory cytokines, which in turn may upregulate TGF-ß1 and elastic matrix proteins, and may exert pathologic tissue effects such as chronic blood-aqueous barrier compromise. Further studies are required to elucidate the sequence of events leading to the development of XFS, as it would identify potential pharmacological targets to interfere with this sight-threatening matrix process on an early stage.
In contrast to anterior segment tissues, most tissues of the posterior segment, such as the retina and choroid, displayed no differential expression of LOXL1 and elastic proteins in XFS eyes, with the exception of the lamina cribrosa, which revealed a selective downregulation of LOXL1 and elastic proteins on the mRNA as well as on the protein level.12 This downregulation of LOXL1 and elastic proteins appears stage independent, as it was present in early and late stages with and without glaucoma (Fig. 1B), suggesting a primary dysregulation of LOXL1 and elastic protein expression. The deficiency of LOXL1 and elastic proteins was further associated with alterations of the laminar elastic fiber network in XFS eyes. By immunolabeling, the elastic fiber system of the lamina cribrosa of nonglaucomatous XFS eyes appeared disorganized and fragmented, which was particularly prominent for elastin-positive elastic fibers and less prominent for elastic microfibrils with positive staining for fibrillin-1 (Figs. 2C–F). On the electron microscopic level, the lamina cribrosa revealed regularly structured collagen fibers, but strikingly abnormal elastic fibers in the laminar beams of XFS eyes as compared with controls. The abnormal elastic fibers displayed a moth-eaten, fragmented appearance and a diminished elastin core.
Therefore, it may be reasonable, that reduced cross-linking of elastin by LOXL1 may lead to the fragmentation of the elastic fiber network and subsequently to a reduced stability and altered biomechanical properties of the lamina cribrosa in XFS eyes, rendering the lamina cribrosa more vulnerable to pressure-induced optic nerve damage. This XFS-specific susceptibility factor might be implicated in the particularly rapid progression and poor prognosis of XFS glaucoma as compared with other types of glaucoma.
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Keywords:© 2014 by Lippincott Williams & Wilkins.
exfoliation syndrome; exfoliation glaucoma; lysyl oxidase-like 1; elastin