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M. K. Duncan, V. D. Desai, Y. Wang, V. N. Simirskii; Cd44 Expression Is Developmentally Regulated in the Mouse Lens and Increases in the Lens Epithelium During Epithelial-Mesenchymal Transition. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4363.
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Hyaluronan is an oligosaccharide found in the pericellular matrix of numerous cell types. Further, during cataract surgery, high concentrations of hyaluronan are instilled into the eye as a viscoelastic to protect the corneal endothelium. Many cell types express receptors for hyaluronan and some of these such as CD44 are involved in cancer progression and fibrosis in liver and lung injury models. This work seeks to investigate possible relationships between hyaluronan, CD44 and epithelial-mesenchymal transition (EMT) following lens injury.
Hyaluronan distribution in the lens was assessed by hyaluronan affinity histochemistry. CD44 expression in the lens during the development of wildtype and β1-integrin null mice and following fiber cell extraction in living mice was assessed by confocal immunofluorescence and rt-PCR. The function of CD44 in the lens was assessed by phenotypic characterization of CD44 null lenses and the time course of -smooth muscle actin expression in CD44 null lenses following lens fiber cell removal.
In the adult mouse, hyaluronan is detected throughout the lens although stronger staining is observed in the lens epithelium as compared to fibers. CD44 expression is first detected in newly forming cortical lens fibers of 16.5 days post coitum mouse embryos and its expression remains highly fiber cell preferred into adulthood. Following surgical lens fiber cell removal, CD44 protein levels upregulate in the remaining lens epithelial cells by 12 hours after surgery while -smooth muscle actin protein levels do not upregulate until at least 24 hours after surgery in this model. In contrast, in β1-integrin null lenses, we observe an upregulation of smooth muscle actin expression in the lens epithelium at 16.5 dpc but do not observe an upregulation of CD44 expression until birth. The lenses of CD44 null mice remain transparent up until at least one year of age, although they have greatly increased levels of cell associated hyaluronan. Further, no differences in the timing of -smooth muscle actin expression were detected in the CD44 null lens after fiber cell removal.
CD44 is expressed in cortical fiber cells from late embryonic development onward in the mouse. Since its expression upregulates in the lens epithelium following fiber cell removal earlier than smooth muscle actin, CD44 may be an early marker of lens cell EMT. Despite this, the only discernable function of CD44 in the lens is to aid the turnover of cell associated hyaluronan although this increase in hyaluronan may allow other hyaluronan receptor systems to compensate for the loss of CD44 mediated signaling.
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