TGF-β signaling protects retinal neurons from programmed cell death during development of the mammalian eye

Abstract

Purpose: To investigate the role of TGF-β signaling for programmed cell death in the retina.

Methods: Tgfbr2flox/flox mice were crossed with α-Cre mice to generate a conditional inactivation of TβRII in optic cup-derived cells of the neural retina. As control, Smad7flox/flox; α-Cre mice were used. Apoptotic cell death of retinal neurons was analyzed by TUNEL labeling and an ELISA for free nucleosomes. MRNA expression was analyzed by quantitative RT-PCR. Dissociated retinal cell cultures were treated with TGF- β2.

Results: Conditional TβRII-deficient mice showed a significant decrease in the amounts of phosphorylated Smad3 in the retina indicating diminished activity of TGF-β signaling. During postnatal synaptogenesis, apoptotic death of retinal neurons was significantly increased in TβRII-deficient pups. Similarly, apoptosis increased in retinal progenitors of TβRII-deficient embryos, while no effects on their proliferation were observed. In contrast, treatment with TGF-β2 inhibited cell death of retinal ganglion cells in dissociated retinal cell cultures, an effect that was blocked upon inhibition of Smad3 phosphorylation. The increase in developmental apoptosis resulted in a significant reduction of retinal neurons in adult animals; an effect that was most pronounced for retinal ganglion cells and resulted in functional deficits as evidenced by electroretinography. In contrast, conditional deletion of the inhibitory Smad7 in cells derived from the inner layer of the optic cup enhanced Smad3 phosphorylation, but decreased apoptosis of retinal neurons in embryos and pups. Conditional TβRII-deficient pups expressed lower amounts of nerve growth factor (NGF) in the retina, while higher amounts were observed in conditional Smad7-deficient pups.

Conclusions: TGF-β signaling protects retinal neurons from apoptotic cell death during development; an effect that may depend on NGF-signaling.

Keywords: 698 retinal development • 615 neuroprotection • 695 retinal degenerations: cell biology