ABSTRACT
Retinopathy of prematurity (ROP) is characterized by pathologic angiogenesis in retina, and remains a leading cause of blindness in children. Although enhanced extracellular adenosine is markedly increased in response to retinal hypoxia, adenosine acting at the A1 and A2A receptors has the opposite effect on pathologic angiogenesis. Herein, the oxygen-induced retinopathy (OIR) model of ROP was used to demonstrate that pharmacologic and genetic inactivation of CD73 (the key 5'-ectonucleotidase for extracellular generation of adenosine) did not affect normal retinal vasculature development but exacerbated intravitreal neovascularization at postnatal day (P) 17 and delayed revascularization at P21 of OIR. This exacerbated damage to retinal vessels by CD73 inactivation was associated with increased cellular apoptosis and microglial activation but decreased astrocyte function at P17 of OIR. Furthermore, pharmacologic blockade of equilibrative nucleoside transporter 1/2 (ENT1/2; bidirectional transport for controlling the balance of intracellular and extracellular adenosine) by 6-nitrobenzylthioinosine aggravated pathologic angiogenesis at P17 of OIR. Pharmacologic blockade of ENT1/2 and genetic inactivation of CD73 also aggravated avascular areas at the hyperoxia phase (P12) of OIR. Thus, disruption of CD73-derived extracellular adenosine or ENT1/2-mediated transport of adenosine flux across membrane aggravated the damage to retinal vessels. These findings support the role of adenosine as an endogenous protective regulator that limits oxygen-induced retinopathy. Thus, enhancing extracellular adenosine signaling represents a novel neuroprotection strategy for ROP by targeting CD73 and ENT1/2 activities.
ABSTRACT
Retinopathy of prematurity (ROP) remains one of the major causes of blindness in children worldwide. While current ROP treatments are mostly disruptive to reduce proliferative neovascularization by targeting the hypoxic phase, protection against early hyperoxia-induced retinal vascular loss represents an effective therapeutic window, but no such therapeutic strategy is available. Built upon our recent demonstration that the protection against oxygen-induced retinopathy by adenosine A2A receptor (A2A R) antagonists is most effective when administered at the hyperoxia (not hypoxic) phase, we here uncovered the cellular mechanism underlying the A2A R-mediated protection against early hyperoxia-induced retinal vascular loss by reversing the inhibition of cellular proliferation via possibly multiple signaling pathways. Specifically, we revealed two distinct stages of the hyperoxia phase with greater cellular proliferation and apoptosis activities and upregulation of adenosine signaling at postnatal 9 day (P9) but reduced cellular activities and adenosine-A2A R signaling at P12. Importantly, the A2A R-mediated protection at P9 was associated with the reversal of hyperoxia-induced inhibition of progenitor cells at the peripheral retina at P9 and of retinal endothelial proliferation at P9 and P12. The critical role of cellular proliferation in the hyperoxia-induced retinal vascular loss was validated by the increased avascular areas by siRNA knockdown of the multiple signaling molecules involved in modulation of cellular proliferation, including activin receptor-like kinase 1, DNA-binding protein inhibitor 1, and vascular endothelial growth factor-A.
