Purchase this article with an account.
D. C. Beebe, L. A. Wiley, L. K. Dattilo, K. Kang, M. Giovannini; The Tumor Suppressor, Merlin, Is Required for Lens Cell Polarity, Separation of the Lens Vesicle and Withdrawal of Lens Fiber Cells From the Cell Cycle. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3480.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Patients with neurofibromatosis type 2, a tumor syndrome with a classical "two hit" genetic etiology, develop posterior subcapsular cataracts (PSCs), Schwann cell tumors and ependymomas. A previous study suggested that merlin, the protein product of the NF2 gene, is required for the formation, but not the stability of cell-cell junctions (McLaughlin, et al. PNAS 104 3261.) To provide insight about the causes of PSCs and the functions of merlin, we conditionally deleted Nf2 from the mouse lens.
Homozygous Nf2 flox; LeCre (+/-) mice were bred to delete Nf2 in the lens. Wild type (WT) (Cre-negative) and Nf2CKO lenses were examined from E10.5 through postnatal ages by BrdU and TUNEL labeling and by antibody and phalloidin staining. The specificity of anti-merlin immunostaining was confirmed by absence in knockout lens cells. Tissue sections were examined by light and confocal microscopy.
Nf2CKO lens pits had normal appearance and BrdU labeling at E10.5, but the lens vesicle failed to separate from the surface ectoderm at E11.5. Cells in the "lens stalk" lost their epithelial morphology, as indicated by their shape and loss of the apical localization of ZO-1, a marker of apical junctions. At later stages, these aberrant cells degenerated, causing expulsion of the fiber cells through the cornea. In CKO lenses beginning at E10.5, ZO-1 staining was detected on the lateral membranes of a few cells in the posterior of the lens vesicle, a phenotype not seen in WT lenses. Rather than withdrawing from the cell cycle, E12.5 fiber cells continued to proliferate, with frequent staining for Ki67 and BrdU and loss of nuclear staining for p57KIP2. Increased apoptosis was detected in the epithelium and fibers by TUNEL and activated caspase-3. Markers of epithelial or fiber cell differentiation, including FoxE3, E-cadherin, Prox1 and c-Maf were expressed normally. Clusters of proliferating cells appeared beneath the equatorial epithelial cells by E12.5, forming tumor-like masses. In WT lenses, staining with anti-merlin antibodies localized to vesicular structures in the cytoplasm, not obviously to cell junctions, as would have been expected from previous in vitro studies.
Merlin is required to maintain the apical-basal polarity of lens cells, although merlin was not detected at cell junctions in vivo. Loss of merlin also drives excess proliferation in lens fiber cells. The loss of epithelial polarity and the increased proliferation present in Nf2CKO lenses seem sufficient to explain the formation of PSCs in NF2 patients.
This PDF is available to Subscribers Only