Abstract
Purpose: :
Among the many tyrosine kinase receptor (TKR) activating ligands that have been expressed in transgenic mouse lenses, only FGFs induce widespread premature lens epithelial differentiation in vivo. Of the many different TKRs expressed by lens epithelial cells, FGFRs are the only ones that mediate signalling through Frs2α. Lens epithelial cells do not express TrkC, a TKR for NT3 (neurotrophin 3), but TrkC activation is known to activate Frs2α in other tissues. The purpose of these experiments was to test the hypothesis that Frs2α activation is sufficient to induce a lens fiber cell differentiation response in lens epithelial cells independent of ectopic FGFR activation.
Methods: :
We used 2 lines of transgenic mice that express NT3 in the lens and created 3 lines of transgenic mice that express TrkC in the lens. All combinations of NT3 and TrkC transgenic lines were mated to obtain transgenic mice expressing both NT3 and TrkC. E15.5 lenses were tested for proliferation with BrdU incorporation, for apoptosis by TUNEL analysis and for β-crystallin expression.
Results: :
NT3-expressing transgenic lenses exhibit normal morphology. TrkC transgenic lenses exhibit slightly elongated epithelial cells (approximately 40% longer than the wildtype), but no ectopic β-crystallin expression. The TrkC/NT3 double transgenics exhibited significantly elongated lens epithelial cells (approximately 100% longer than wildtype), fiber cell dysmorphology and epithelial multilayering. We also observed defects in fiber cell denucleation. β-crystallin expression was seen in clusters of lens epithelial cells in the double transgenics indicating areas of premature fiber cell differentiation. BrdU incorporation rates among the TrkC/NT3, TrkC and NT3 transgenic lenses were not significantly higher than in wildtype lenses. However, apoptosis was much higher in the TrkC/NT3 lenses, specifically in the prematurely elongating nucleated fiber cells.
Conclusions: :
NT3 is capable of inducing epithelial cell elongation and β-crystallin expression in lens epithelial cells ectopically expressing TrkC. TrkC and FGFRs both activate Frs2α, while other TKRs in the lens do not. Our data indicate that the lens phenotypes exhibited by the TrkC/NT3 double transgenics are similar to those of Fgf1 over-expressing mice. The induction of lens fiber differentiation characteristics through TrkC stimulation suggests that Frs2α-mediated signal transduction may play a unique and critical role in lens fiber differentiation.
Keywords: signal transduction • receptors • genetics