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Velia M Fowler, Catherine Cheng, Roberta B Nowak, Sondip K Biswas, Woo-Kuen Lo, Paul G FitzGerald; Tropomodulin 1 regulation of actin is required for the formation of large paddle protrusions between mature lens fiber cells. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3073.
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© 2017 Association for Research in Vision and Ophthalmology.
The complex interface between mature lens fiber cell membranes is characterized by specialized interlocking small protrusions and larger paddle domains, which are hypothesized to be important for lens mechanical integrity. We previously demonstrated that tropomodulin 1 (Tmod1), an F-actin pointed-end capping protein, plays an important role in fiber cell packing and lens stiffness. We hypothesize Tmod1 is needed for formation of complex fiber cell interdigitations that promote cell-cell interactions.
Using wild-type (WT) and Tmod1(-/-) mice, we characterized mature lens fiber cell structure using electron microscopy (EM) and immunostaining of single fiber cells for F-actin and F-actin-binding proteins.
EM and immunostaining of WT lenses reveals rows of fiber cells with coordinated F-actin-rich paddle protrusions that are decorated by smaller protrusions of equal size and spacing. In contrast, Tmod1(-/-) lens fibers have disorganized paddles with irregular protrusions. In WT lens fibers, Tmod1 and β2-spectrin is localized in large puncta at the base of paddles. Alpha-actinin, an F-actin cross-linker, is localized on Tmod1(-/-) fiber membranes, while large α-actinin puncta are observed at the base of WT paddles. At the base of small protrusions, Arp3, fimbrin and adducin are enriched in both WT and Tmod1(-/-) lens fibers. Ezrin co-localizes with F-actin along the fiber cell membrane, including in small protrusions, while N-cadherin is found along the membrane, but is excluded from small protrusions.
These results suggest Tmod1 is required for normal formation of large paddle domains between mature fiber cells. Tmod1 may stabilize the actin-spectrin network and α-actinin cross-linked anti-parallel F-actin bundles at the base of large paddles to maintain their structure. Formation of small protrusions may be facilitated by Arp3-nucleated actin networks, fimbrin-cross-linked parallel F-actin bundles and short actin filaments capped by adducin. This is the first work to reveal proteins required for the formation of paddles between lens fibers and suggests that paddles are needed for lens integrity.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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