Purpose: Transforming growth factor beta (TGF-beta) is a multifunctional cytokine that modulates biological events as diverse as wound healing and angiogenesis and which may be important in the pathogenesis of choroidal neovascularization. We investigated the mRNA expression of TGF-beta isoforms in a model of experimental choroidal neovascularization induced by krypton-laser photocoagulation.
Methods: Rat TGF-beta 1, mouse TGF-beta 2 or TGF-beta 3 cDNAs was inserted into the pBluescript vector to prepare antisense and sense riboprobes. Intense laser burns were applied to the posterior poles of the eyes of pigmented rats according to a protocol described for producing choroidal neovascularization in these animals. At intervals up to 4 weeks after photocoagulation, the eyes were obtained and cut into thin sections. The sections were subjected to in situ hybridization with digoxigenin (DIG)-labeled single-strand riboprobes synthesized from each TGF-beta cDNA.
Results: In normal adult rat retinas and choroids, TGF-beta 1 mRNA was found only in cells of the ganglion cell layer, TGF-beta 2 mRNA was found in cells of the ganglion cell layer and choriocapillaris endothelium, whereas TGF-beta 3 mRNA was not detected at all. During the process of neovascularization, TGF-beta 1 and TGF-beta 2 mRNAs (the latter being expressed more prominently) were detected in retinal pigment epithelial cells, fibroblast-like cells and the endothelium of the neovascular region. TGF-beta 2 was the predominant isoform of TGF-beta, and its expression was especially strong in the endothelium of the choroidal neovascularization at 2 weeks. However, TGF-beta mRNAs was decreased in cells 4 weeks after photocoagulation.
Conclusions: Our findings suggest that TGF-beta may act in the retina as a neurotrophic agent, since TGF-beta 1 is normally transcribed in ganglion cells and TGF-beta 2 is also transcribed in ganglion cells and choriocapillaris endothelium. TGF-beta 1 and TGF-beta 2 mRNA expression were increased in photocoagulated lesions from 3 days to 2 weeks after laser treatment. Therefore, it is likely that TGF-beta acts as a mediator of the neovascularization process.