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Review
. 2016 Jan:142:92-101.
doi: 10.1016/j.exer.2015.02.004. Epub 2015 May 21.

Fibrosis in the lens. Sprouty regulation of TGFβ-signaling prevents lens EMT leading to cataract

Affiliations
Review

Fibrosis in the lens. Sprouty regulation of TGFβ-signaling prevents lens EMT leading to cataract

F J Lovicu et al. Exp Eye Res. 2016 Jan.

Abstract

Cataract is a common age-related condition that is caused by progressive clouding of the normally clear lens. Cataract can be effectively treated by surgery; however, like any surgery, there can be complications and the development of a secondary cataract, known as posterior capsule opacification (PCO), is the most common. PCO is caused by aberrant growth of lens epithelial cells that are left behind in the capsular bag after surgical removal of the fiber mass. An epithelial-to-mesenchymal transition (EMT) is central to fibrotic PCO and forms of fibrotic cataract, including anterior/posterior polar cataracts. Transforming growth factor β (TGFβ) has been shown to induce lens EMT and consequently research has focused on identifying ways of blocking its action. Intriguingly, recent studies in animal models have shown that EMT and cataract developed when a class of negative-feedback regulators, Sprouty (Spry)1 and Spry2, were conditionally deleted from the lens. Members of the Spry family act as general antagonists of the receptor tyrosine kinase (RTK)-mediated MAPK signaling pathway that is involved in many physiological and developmental processes. As the ERK/MAPK signaling pathway is a well established target of Spry proteins, and overexpression of Spry can block aberrant TGFβ-Smad signaling responsible for EMT and anterior subcapsular cataract, this indicates a role for the ERK/MAPK pathway in TGFβ-induced EMT. Given this and other supporting evidence, a case is made for focusing on RTK antagonists, such as Spry, for cataract prevention. In addition, and looking to the future, this review also looks at possibilities for supplanting EMT with normal fiber differentiation and thereby promoting lens regenerative processes after cataract surgery. Whilst it is now known that the epithelial to fiber differentiation process is driven by FGF, little is known about factors that coordinate the precise assembly of fibers into a functional lens. However, recent research provides key insights into an FGF-activated mechanism intrinsic to the lens that involves interactions between the Wnt-Frizzled and Jagged/Notch signaling pathways. This reciprocal epithelial-fiber cell interaction appears to be critical for the assembly and maintenance of the highly ordered three-dimensional architecture that is central to lens function. This information is fundamental to defining the specific conditions and stimuli needed to recapitulate developmental programs and promote regeneration of lens structure and function after cataract surgery.

Keywords: EMT; Fibrosis; Lens epithelium; Lens regeneration; Myofibroblasts; RTK antagonists; Sprouty; TGFβ.

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Figures

Figure 1
Figure 1
Complications following cataract surgery primarily lead to posterior capsular opacification (PCO). PCO results from residual lens epithelial cells (a), left behind following fiber cell extraction, that undergo an epithelial to mesenchymal transition (b) and/or aberrant differentiation into fiber cells (c) more commonly referred to as Soemmering's ring and Elschnig's pearls. The resultant myofibroblasts (d), also migrate posteriorly to populate and cover the posterior capsule, invading the visual axis as they further lay aberrant extracellular matrix (e) and modulate the underlying capsule, causing it to fold and wrinkle (f).
Figure 2
Figure 2
TGFß induces an epithelial to mesenchymal phenotype in lens epithelial cells. In the process, cells lose many of their normal epithelial markers and characteristics as they dissociate from each other to acquire a more myofibroblastic, migratory phenotype. Some of the specific markers now expressed include alpha smooth muscle actin (α-SMA), as well as excessive levels of extracellular matrix molecules, including fibronectin and collagens type I and III. It is this cellular process that contributes to lens pathology, especially the fibrotic changes leading to polar cataracts and posterior capsular opacification. Included are representative lens epithelial explants prepared from postnatal-day-15 murine lens exposed to either no TGFß (A) or 50pg/ml TGFß2 (B) for up to 5 days, immunolabeled for α-SMA (green), with cell nuclei counterstained with propidium iodide (red). With TGFß, lens epithelial cells undergo an EMT, highlighted by α-SMA-labeled myofibroblasts. Scale bar; 20μm.
Figure 3
Figure 3
TGFß-signaling via the Smad/Snai molecules results in a lens EMT that contributes to fibrotic forms of cataract. This process also involves (either directly or indirectly) MAPK/ERK1/2 signaling, possibly by downregulating specific RTK antagonists, such as Spry that are normally expressed in the lens epithelium. In Spry-deficient lenses, phosphorylated ERK1/2 levels are elevated and pSmads and Snai1 and Snai2 are translocated to the cell nuclei, leading to an EMT/cataract, similar to that induced by TGFß. Overexpression of Spry in lens cells can effectively block TGFß-induced lens EMT (see Shin et al., 2012).
Figure 4
Figure 4
Whilst complications following cataract surgery may lead to PCO (A), an alternative approach to preventing PCO (B) is to promote normal lens architecture and growth. This latter approach would first require blocking aberrant TGFß-signaling at the time of surgery (that may maintain Spry levels in the lens), hence maintain the normal phenotype of lens epithelia (a). The normal in vivo ocular environment could then regenerate the lens fiber mass (b) by promoting the coordinated assembly and alignment of the differentiating secondary fiber cells. This partial regeneration of the equatorial region of the lens may also serve to hold the intraocular lens in place and in addition, unlike the progressive fibrosis that leads to PCO and capsular wrinkling, maintain lens clarity (c).

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