Beta-1 integrin is important for the structural maintenance and homeostasis of differentiating fiber cells

Abstract

β1-integrin is a heterodimeric transmembrane protein that has roles in both cell-extracellular matrix and cell-cell interactions. Conditional deletion of β1-integrin from all lens cells during embryonic development results in profound lens defects, however, it is less clear whether this reflects functions in the lens epithelium alone or whether this protein plays a role in lens fibers. Thus, a conditional approach was used to delete β1-integrin solely from the lens fiber cells. This deletion resulted in two distinct phenotypes with some lenses exhibiting cataracts while others were clear, albeit with refractive defects. Analysis of “clear” conditional knockout lenses revealed that they had profound defects in fiber cell morphology associated with the loss of the F-actin network. Physiological measurements found that the lens fiber cells had a two-fold increase in gap junctional coupling, perhaps due to differential localization of connexins 46 and 50, as well as increased water permeability. This would presumably facilitate transport of ions and nutrients through the lens, and may partially explain how lenses with profound structural abnormalities can maintain transparency. In summary, β1-integrin plays a role in maintaining the cellular morphology and homeostasis of the lens fiber cells.

Compound mouse mutants of bZIP transcription factors Mafg and Mafk reveal a regulatory network of non-crystallin genes associated with cataract

“…Scanning electron microscopy (SEM) was performed on 2–7 month old Mafg+/+:Mafk+/+ and Mafg+/−:Mafk+/− controls and Mafg−/−:Mafk+/− , Mafg+/−:Mafk−/− mutant lenses as previously described (Scheiblin et al 2014). Briefly, eyes were enucleated from mice and lenses were removed and transferred to a fixative (0.08M sodium cacodylate pH 7.4, 1.25% glutaraldehyde, 1% paraformaldehyde) (Electron Microscopy Sciences) for 48 hours.…”

The molecular mechanisms underlying lens fiber elongation

“…Removal of β1–integrin, another transmembrane protein able to serve as a scaffold for actin assembly, from either the entire lens or lens fibers exclusively, attenuates F-actin levels, and results in profound defects in lens fiber cell structure, although fiber cell elongation/differentiation per se proceeds normally (Scheiblin et al, 2014; Simirskii et al, 2007). Similarly, removal and/or inhibition of numerous regulators of actin cytoskeletal dynamics/organization such as Rho GTPases (Maddala et al, 2004), Rac1 GTPase (Maddala et al, 2011), tropomodulin (Nowak et al, 2009), and dystrophin (Fort et al, 2014), lead to disruptions in lens fiber cell structure without a profound effect on fiber cell elongation per se.…”

β1‐integrin controls cell fate specification in early lens development

“…Prior work on integrin function in the lens demonstrated that β1-integrins are required for both lens epithelial cell (LEC) phenotype/survival (Simirskii et al, 2007) as well as lens fiber cell structure at later stages of lens development (Scheiblin et al, 2014). However, the previous studies did not address the role of β1-integrins during early lens development.…”

“…While Le-Cre activity is first detected in the lens placode (Ashery-Padan et al, 2000), β1 integrin protein is known to have a long half-life in the lens and other tissues (Li et al, 2005; Raghavan et al, 2000; Scheiblin et al, 2014; Simirskii et al, 2007). Thus, the timing of β1-integrin protein loss in β1LE mice was determined by immunofluorescence (Figure 2A–F).…”

“…Prior studies suggest that the functions of β1-integrins in the lens are complex, with distinct roles in lens cell/lens capsule adhesion, lens cell survival (Samuelsson et al, 2007; Simirskii et al, 2007), lens fiber cell differentiation and fiber morphology (Hayes et al, 2012; Scheiblin et al, 2014), as well as maintenance of the lens epithelial cell phenotype (Simirskii et al, 2007) and its pathological transition to a myofibroblast fate (Barbour et al, 2004; Hay and Zuk, 1995). However, the function of β1-integrins during the early stages of lens development, when the invaginating lens placode forms a lens vesicle, has not been previously reported.…”