Corneal epithelial basement membrane: Structure, function and regeneration

https://doi.org/10.1016/j.exer.2020.108002Get rights and content

Highlights

  • Major components of the EBM are collagens, laminins, perlecan, and nidogens.

  • Keratocytes and corneal fibroblasts contribute components during EBM regeneration.

  • The EBM regulates the localization of TGFβ, PDGF, HGF and KGF.

  • Defective regeneration of the EBM underlies stromal fibrosis.

Abstract

Basement membranes are highly specialized extracellular matrices. More than providing scaffolds, basement membranes are recognized as dynamic and versatile structures that modulate cellular responses to regulate tissue development, function, and repair. Increasing evidence suggests that, in addition to providing structural support to adjacent cells, basement membranes serve as reservoirs and modulators of growth factors that direct and fine-tune cellular functions. Since the corneal stroma is avascular and has a relatively low keratocyte density, it's likely that the corneal BM is different in composition from the BMs in other tissues. BMs are composed of a diverse assemblage of extracellular molecules, some of which are likely specific to the tissue where they function; but in general they are composed of four primary components—collagens, laminins, heparan sulfate proteoglycans, and nidogens—in addition to other components such as thrombospondin-1, matrilin-2, and matrilin-4 and fibronectin. Severe injuries to the cornea, including infection, surgery, and trauma, may trigger the development of myofibroblasts and fibrosis in the normally transparent connective tissue stroma. Ultrastructural studies have demonstrated that defective epithelial basement membrane (EBM) regeneration after injury to the cornea underlies the development of myofibroblasts from both bone marrow- and keratocyte-derived precursor cells. Defective EBM permits epithelium-derived and tear-derived transforming growth factor beta (TGF-β), platelet-derived growth factor (PDGF), and possibly other modulators, to penetrate the stroma at sustained levels necessary to drive the development and persistence of vimentin + alpha-smooth muscle actin + desmin+ (V + A + D+) mature myofibroblasts. A recent discovery that has contributed to our understanding of haze development is that keratocytes and corneal fibroblasts produce critical EBM components, such as nidogen-1, nidogen-2 and perlecan, that are essential for complete regeneration of a normal EBM once laminin secreted by epithelial cells self-polymerizes into a nascent EBM. Mature myofibroblasts that become established in the anterior stroma are a barrier to keratocyte/corneal fibroblast contributions to the nascent EBM. These myofibroblasts, and the opacity they produce, often persist for months or years after the injury. Transparency is subsequently restored if the EBM is fully regenerated, myofibroblasts are deprived of TGF-β and undergo apoptosis, and keratocytes reoccupy the anterior stroma and reabsorb the disordered extracellular matrix.

Introduction

Basement membranes (BM) are highly specialized, thin, acellular extracellular matrices underlying cells that separate them from, as well as connect them to, their associated matrix.1 Basement membranes function not only in anchoring adjacent cells and providing scaffolding during embryonic development, but also in migration, differentiation, and maintenance of the differentiated phenotype of associated epithelial, endothelial, or parenchymal cells. In addition, BM control cellular functions by binding and modulating the activation, localization and concentrations of growth factors and cytokines that control the response to corneal injury (Yurchenco, 2011; Torricelli et al., 2013b). BMs also regulate cell polarity, cell adhesion, and migration via their effects on the cytoskeleton of attached cells (Yurchenco, 2011; Torricelli et al., 2013b).

Section snippets

Structure of the corneal epithelial basement membrane

The corneal epithelial BM is positioned between the basal epithelial cells and the stroma. It is first detected at 8–9 weeks of gestation in the human, and after the fourth month of development the corneal epithelium is separated from the stroma by a continuous BM. Evidence has been provided for a stromal cellular origin for some epithelial BM components in the cornea (Hassell et al., 1992; Kabosova et al., 2007; Santhanam et al., 2017). In adult humans, rabbits, mice, and many other species,

Collagens

The presence of collagen type IV was at one time controversial, with some investigators failing in detect type IV collagen in the corneal BM. However, several immunohistochemical studies definitively localized type IV collagen beneath the human corneal epithelium. It appears the disparity between different studies arose as a result of the spatial variability (‘‘horizontal’’ heterogeneity) in the BM composition between the central cornea, limbus, and conjunctiva (Ljubimov et al., 1995). It is

Laminins

Laminins are the most abundant non-collagenous proteins in BM. Laminins are heterotrimeric glycoproteins that are composed of three chains, including one α, one β, and one γ chain. At present, five α, three β, and three γ peptides coded by different genes are known for mice and humans. The trimers were previously designated laminin-1 to −15 in order of their discovery, with no relationship to chain composition. According to the previous nomenclature, a trimer could be identified by either an

Perlecan

Perlecan is the most prevalent HSPG in the EBM. It is a complex, multi-domain protein with several discrete binding partners, including collagen type IV, nidogens and laminins (Kinsella and Wight, 2005). The protein's core consists of five domains that share homology with other molecules involved in nutrient metabolism, cell proliferation, and adhesion, including laminin, the low-density lipoprotein (LDL) receptor, epithelial growth factor (EGF), and the neural cell adhesion molecule (N-CAM) (

The epithelial basement membrane, myofibroblasts and corneal wound healing

Following severe injuries, infection or surgeries of the cornea in which the BM is damaged, large numbers of myofibroblasts are generated and persist in the corneal stroma (Wilson et al., 2017). These fibroblastic cells, and the disorganized extracellular matrix components they secrete, produce fibrosis that alters the structure and function of the corneal stroma and results in a loss of normal transparency (corneal scarring or haze). Studies using chimeric mice transplanted with bone marrow

Proprietary interest statement

The authors don't have any commercial or proprietary interests in this study.

Funding

Supported in part by US Public Health Service grants RO1EY10056 (SEW) and P30-EY025585 from the National Eye Institute, National Institutes of Health, Bethesda, MD, Department of Defense grant VR180066, and Research to Prevent Blindness, New York, NY.

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