Purchase this article with an account.
R. Rajagopal, L. K. Dattilo, V. Kaartinen, D. C. Beebe; Signaling Through the Type I BMP Receptor, Alk2, Regulates Withdrawal of Lens Fiber Cells From the Cell Cycle. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4232.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To determine the effects of deleting the type-I bone morphogenetic protein (BMP) receptor, Alk2, in the lens.
Lenses lacking Alk2, Smad4, Smad1, Smad5 or Alk2 and p53 were generated by mating the respectivefloxed mice with mice expressing Cre recombinase in the lens (LeCre). Postnatal day 3 (P3) mice or pregnant females were injected with BrdU (50 mg /kg) and sacrificed 1 hour later. Whole embryos or heads were fixed in 10% formalin, embedded in paraffin, sectioned, treated for antigen retrieval and labeled with antibodies to Ki67, BrdU, phospho-Histone H3, p57 or c-maf or using the TUNEL assay.
In wild-type lens epithelia, the percentage of proliferating cells declines from ~40% at embryonic day 12.5 to ~18% by P3. Most of this decline occurs in the central epithelium and in the transition zone, a region of the epithelium that lies near the lens equator between the germinative zone and the lens fiber cells. As a result, cell proliferation is progressively restricted to the germinative zone. Alk2CKO lenses appeared smaller at E12.5 but were otherwise morphologically normal. In later development, cells in the transition zone failed to decrease their rate of proliferation, as measured by the BrdU (cells in S-phase) or Ki67 (a marker of all cycling cells) labeling index. A few Alk2CKO fibers also expressed Ki67, incorporated BrdU, expressed phospho-Histone H3 (a marker for mitotic cells) and failed to express the Cdk inhibitor, p57, which is present in all wild-type fiber cells as they exit from cell cycle. The failure of fiber cells and cells in the transition zone to exit from cell cycle was also seen in lenses lacking Co-Smad, Smad4, or R-Smads, Smad5 or Smad1, suggesting that the ability of Alk2 signaling to promote withdrawal from the cell cycle is mediated by these Smads. Although some Alk2CKO fibers failed to withdraw from cell cycle, they expressed c-maf, a transcription factor that is present in differentiated fiber cells. Alk2CKO lenses also showed increased cell death in the transition zone and in the fiber compartment, suggesting that failure of these cells to exit from cell cycle triggers apoptosis. This apoptosis is mediated by p53 as it is rescued in the Alk2; p53DCKO lenses.
BMP signaling through Alk2 and the canonical Smad signaling pathway contributes to cessation of cell proliferation during lens fiber cell terminal differentiation. Failure of appropriate cell cycle exit triggers apoptosis in a p53-dependent manner.
This PDF is available to Subscribers Only