Abstract
In pathological corneas, accumulation of fibrotic extracellular matrix is characterized by proteoglycans with altered glycosaminoglycans that contribute to the reduced transparency of scarred tissue. During wound healing, keratocytes in the corneal stroma transdifferentiate into fibroblasts and myofibroblasts. In this study, molecular markers were developed to identify keratocyte, fibroblast, and myofibroblast phenotypes in primary cultures of corneal stromal cells and the structure of glycosaminoglycans secreted by these cells was characterized. Quiescent primary keratocytes expressed abundant protein and mRNA for keratocan and aldehyde dehydrogenase class 3 and secreted proteoglycans containing macromolecular keratan sulfate. Expression of these marker compounds was reduced in fibroblasts and also in transforming growth factor-beta-induced myofibroblasts, which expressed high levels of alpha-smooth muscle actin, biglycan, and the extra domain A (EDA or EIIIA) form of cellular fibronectin. Collagen types I and III mRNAs were elevated in both fibroblasts and in myofibroblasts. Expression of these molecular markers clearly distinguishes the phenotypic states of stromal cells in vitro. Glycosaminoglycans secreted by fibroblasts and myofibroblasts were qualitatively similar to and differed from those of keratocytes. Chondroitin/dermatan sulfate abundance, chain length, and sulfation were increased as keratocytes became fibroblasts and myofibroblasts. Fluorophore-assisted carbohydrate electrophoresis analysis demonstrated increased N-acetylgalactosamine sulfation at both 4- and 6-carbons. Hyaluronan, absent in keratocytes, was secreted by fibroblasts and myofibroblasts. Keratan sulfate biosynthesis, chain length, and sulfation were significantly reduced in both fibroblasts and myofibroblasts. The qualitatively similar expression of glycosaminoglycans shared by fibroblasts and myofibroblasts suggests a role for fibroblasts in deposition of non-transparent fibrotic tissue in pathological corneas.
Highlights
The corneal stroma is a dense connective tissue with a highly organized extracellular matrix responsible for the remarkable
It has long been observed that keratan sulfate secretion is greatly reduced or absent in serially passaged corneal fibroblasts [36], and we recently demonstrated that freshly isolated primary bovine keratocytes exhibit a loss of sulfated keratan sulfate-proteoglycans and an increase in sulfated chondroitin/dermatan sulfate-containing proteoglycans during transdifferentiation from keratocytes to myofibroblasts [35]
The present study addresses this hypothesis by characterization of keratan sulfate and chondroitin/dermatan sulfate chains modifying proteoglycans secreted by stromal cells of different phenotypes
Summary
It has long been observed that keratan sulfate secretion is greatly reduced or absent in serially passaged corneal fibroblasts [36], and we recently demonstrated that freshly isolated primary bovine keratocytes exhibit a loss of sulfated keratan sulfate-proteoglycans and an increase in sulfated chondroitin/dermatan sulfate-containing proteoglycans during transdifferentiation from keratocytes to myofibroblasts [35]. Incorporation of labeled sulfate into proteoglycans, did exhibit marked differences between the two phenotypes with chondroitin/dermatan sulfate increased and keratan sulfate decreased This observation lead to the hypothesis that a major feature of the alteration in corneal proteoglycan profile during the phenotypic transition in wound healing arises via modulation of the structure of the glycosaminoglycan chains modifying the core proteins. These results establish the key link between cells observed in pathological corneas and specific alterations in biosynthesis of corneal glycosaminoglycans
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