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Zeina Dagher, Joseph Vaz, Michael Goodridge, Leona E. Ling, Mara Lorenzi, Chiara Gerhardinger; Constitutive Signaling Through the TGF-β Receptor I ALK5 is Required for Pericyte Survival in Adult Retinal Vessels. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3291.
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© ARVO (1962-2015); The Authors (2016-present)
To learn about mechanisms and consequences of constitutive TGF-β signaling in the homeostasis of adult retinal vessels. Undisturbed TGF-β signaling is required for normal cardiovascular development, as well as for the functional integrity of the adult microvasculature. The signaling events involved in the stabilizing role of TGF-β in the adult vasculature are not known, but are becoming of translational importance because anti-TGF-β interventions are being evaluated as adjunct therapies to prevent diabetic retinopathy and nephropathy.
We administered to adult rats for 3 weeks SM16 (Biogen Idec), an orally active compound that selectively inhibits the "classical" type I TGF-β receptor ALK5 and the phosphorylation of its effectors Smad2/3. SM16 was mixed with the chow to deliver a dose of 6 mg/Kg bw/day, which had shown efficacy and safety in rodent models of pathologies involving excess TGF-β. The measurement of Smad2/3 phosphorylation in the whole retina after intravitreal injection of exogenous TGF-β provided evidence of drug efficacy on targets beyond the blood-retinal barrier. We thus measured effects of SM16 on gene expression (Rat Endothelial Cell Biology RT2 ProfilerTM PCR Array) and apoptosis (TUNEL assay) in the cells of retinal microvessels. Fresh or fixed retinal microvascular networks were isolated to a high degree of purity, similar in treated and untreated rats.
In the retinal vessels of SM16-treated rats, three genes changed their expression level more than 2 fold with P < 0.05 when compared to untreated rats. Monocyte chemoattractant protein-1 increased, a pro-inflammatory change consistent with the immunosuppressive role of TGF-β. In contrast, interleukin7 and angiopoietin1 decreased. Angiopoietin1 is produced mostly or solely by pericytes. Another pericyte product, VEGFa, was also downregulated in the retinal vessels of SM16-treated rats (1.4-fold, P=0.05). The trypsin digests of SM16-treated rats showed an increase in the number of TUNEL+ pericytes [median 9 (range 0-27) vs 3 (range 2-11) in untreated rats, P=0.05). There was no increase in TUNEL+ endothelial cells.
Pericytes are endowed with only one type of TGF-βRI, ALK5, while the endothelium also carry ALK1. Pericytes are thus especially susceptible to ALK5 inhibitors such as SM16; an even minor deprivation of TGF-β signaling through ALK5 as induced by our dose of SM16 compromises their functionality and survival. It is reasonable to hypothesize that longer duration of SM16 treatment would have resulted also in an increased rate of endothelial apoptosis.
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