The Importance of BMP Signaling in Lens Development: Targeted Deletion of BMPR–1A (Alk3).

Abstract

Abstract: : Purpose: Previous studies showed that bone morphogenetic protein (BMP) signaling is important for the formation of the lens and for the differentiation of primary lens fibers. We determined the function in lens development of one of the BMP receptors, BMPR–1A or Alk3, by selectively disrupting the Alk3 gene in the lens and ocular surface epithelia using Cre/loxP–mediated gene targeting. Methods: Mice that harbored a floxed Alk3 allele were mated to mice that expressed Cre recombinase from the Pax6 P0 promoter in the lens and ocular surface epithelia. Embryos and postnatal animals that were Alk3^flox/flox and Cre⁺ or Cre^– were analyzed by standard histological methods. Cell proliferation was measured by labeling with BrdU and cell death by TUNEL analysis. Results: Cre^– lenses appeared normal at all stages examined. Cre⁺ embryos had smaller lenses that were characterized by thinner epithelia and normal–appearing fiber cells. Abundant mesenchymal cells filled the vitreous cavity and future anterior chamber. The lens stalk persisted in a few embryos. Postnatal day 2 (P2) Cre⁺ animals had small lenses. The lens epithelium adjacent to the ciliary epithelium was often discontinuous. The BrdU labeling index of epithelial cells was approximately one–fourth that of Cre^– animals. No differences in TUNEL labeling were detected at this stage. The Cre⁺ fiber cells had clusters of vacuoles. A dense plaque of mesenchymal cells was attached to the posterior capsule and the anterior chamber was filled with abnormal, pigmented cells. A distinct corneal endothelium could not be visualized. By P9, proliferating epithelial cells had migrated to the posterior of the lens. Fiber cells were degenerating and their cytoplasm sometimes extruded through breaks in the anterior lens capsule. The eyes of young adult Cre⁺ animals were smaller than normal and the lenses appeared degenerated. The lens was often filled with cells and extracellular matrix that resembled cartilage or bone, including regions that contained bone marrow. These lenses were positively stained with the Von Kossa reagent, which stains mineralized bone matrix. Cell lineage studies will show whether the bone is derived from lens cells or from bone precursors that migrate into the lens. Conclusions: BMP signaling through Alk3 is essential for the normal development of the lens and for normal signaling by the lens to surrounding mesenchyme cells. In adult animals, cartilage and bone replace the lens tissues.

Keywords: growth factors/growth factor receptors • transgenics/knock–outs • signal transduction