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Diabetic retinopathy, a major cause of blindness, is characterized by a distinct phenotype. The molecular causes of the phenotype are not sufficiently clear. Here, we report that deletion of transforming growth factor β signaling in the retinal microenvironment of newborn mice induces changes that largely mimic the phenotype of nonproliferative and proliferative diabetic retinopathy in humans. Lack of transforming growth factor β signaling leads to the formation of abundant microaneurysms, leaky capillaries, and retinal hemorrhages. Retinal capillaries are not covered by differentiated pericytes, but by a coat of vascular smooth muscle-like cells and a thickened basal lamina. Reactive microglia is found in close association with retinal capillaries. In older animals, loss of endothelial cells and the formation of ghost vessels are observed, findings that correlate with the induction of angiogenic molecules and the accumulation of retinal hypoxia-inducible factor 1α, indicating hypoxia. Consequently, retinal and vitreal neovascularization occurs, a scenario that leads to retinal detachment, vitreal hemorrhages, neuronal apoptosis, and impairment of sensory function. We conclude that transforming growth factor β signaling is required for the differentiation of retinal pericytes during vascular development of the retina. Lack of differentiated pericytes initiates a scenario of structural and functional changes in the retina that mimics those of diabetic retinopathy strongly indicating a common mechanism., (Copyright © 2015 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Similar to other parts of the central nervous system, there are two types of programmed cell death during retinal development. In early development, the neuronal progenitor population is affected. In the mouse eye, this kind of programmed cell death begins at around embryonic day (E) 12.5 and peaks between E14.5 and E16.5. The second phase of programmed cell death occurs during synaptogenesis within the first 2 postnatal weeks. Important signaling mechanisms that induce programmed cell death of retinal progenitors appear to involve nerve growth factor acting on the proapoptotic receptor to p75 neurotrophin receptor (p75(NTR)) and transforming growth factor-β.

We investigated the influence of transforming growth factor-β (TGF-β) signaling on developmental programmed cell death in the mouse retina by direct and specific molecular targeting of TGF-β type II receptor (TβRII) and Smad7 in retinal progenitor cells. Mice were generated carrying a conditional deletion of the TβRII in cells that originate from the inner layer of the optic cup. The animals showed a significant decrease of phosphorylated Smad3 in both the central and peripheral retina, which indicates the diminished activity of TGF-β signaling. TβRII deficiency significantly increased the apoptotic death of retinal neurons during embryonic and postnatal development without affecting their proliferation. In contrast, treatment with TGF-β2 inhibited cell death of retinal ganglion cells in dissociated retinal cell cultures, an effect that was blocked by inhibiting the phosphorylation of Smad3. The increase in apoptosis during development resulted in a significant reduction in the number of neurons in adult TβRII-deficient mice. The effect was most pronounced in the inner retina neurons and resulted in functional deficits as determined by electroretinography. In contrast, a conditional deletion of TGF-β-inhibiting Smad7 in retinal neurons significantly enhanced Smad3 phosphorylation and significantly decreased apoptosis of retinal neurons in embryos and pups. Moreover, the number of retinal ganglion cells was significantly higher in Smad7-deficient mice compared with control littermates. TβRII-deficient pups showed a lower level of nerve growth factor (NGF) in its mRNA; however, higher levels were observed in Smad7-deficient pups, which strongly suggests that the protective effects of TGF-β signaling on developmental cell death are mediated through NGF.

Mutations in the myocilin gene (MYOC) are causative for 10% of cases with juvenile open-angle glaucoma and 3-4% of those with primary open-angle glaucoma. Myocilin is a secreted protein with relatively ill-defined matricellular properties. Despite its high expression in the eye, myocilin-deficient mice have originally been reported to have no obvious ocular phenotype. Here we revisited the ocular phenotype of myocilin-deficient mice and detected a higher number of neurons in their inner (INL) and outer (ONL) nuclear layers, as well as a higher number of retinal ganglion cells (RGC) and their axons. The increase in retinal neurons appears to be caused by a decrease in programmed developmental cell death, as apoptosis of retinal neurons between postnatal days 4 and 10 was found to be attenuated when compared to that of wildtype littermates. In contrast, when Myoc-/- mice were crossed with βB1-crystallin-MYOC mice with ectopic overexpression of myocilin in the eye, no differences in developmental apoptosis, RGC number and INL thickness were observed when compared to wildtype littermates. The amounts of the anti-apoptotic Bcl-2-like protein 1 (BCL2L1, Bcl-xL) and its mRNA were increased in retinae of Myoc-/- mice, while lower amounts of BCL2L1 and its mRNA were detected in mixed Myoc-/-/βB1-crystallin-MYOC mice. The structural differences between Myoc-/- mice and wildtype littermates did not result in functional differences as measured by electroretinography. Noteworthy though mixed Myoc-/-/βB1-crystallin-MYOC mice with ocular overexpression of myocilin had significant cone function deficits. Myocilin appears to modulate apoptotic death of retinal neurons likely by interacting with the intrinsic apoptotic pathway. [ABSTRACT FROM AUTHOR]