Early retinal blood vessel growth in normal and growth restricted rat pups raised in oxygen and room air

Pathophysiology of Retinopathy of Prematurity

Retinopathy of prematurity (ROP) is a complex disease involving development of the neural retina, ocular circulations, and other organ systems of the premature infant. The external stresses of the ex utero environment also influence the pathophysiology of ROP through interactions among retinal neural, vascular, and glial cells. There is variability among individual infants and presentations of the disease throughout the world, making ROP challenging to study. The methods used include representative animal models, cell culture, and clinical studies. This article describes the impact of maternal-fetal interactions; stresses that the preterm infant experiences; and biologic pathways of interest, including growth factor effects and cell-cell interactions, on the complex pathophysiology of ROP phenotypes in developed and emerging countries.

Retinopathy of prematurity protection conferred by uteroplacental insufficiency through erythropoietin signaling in an experimental Murine Model

BACKGROUND

Recent clinical studies suggest that preeclampsia, characterized by uteroplacental insufficiency (UPI) and infant intrauterine growth restriction (IUGR), may be protective against retinopathy of prematurity (ROP) in preterm infants. Experimental models of UPI/IUGR have found an association of erythropoietin (EPO) with less severe oxygen-induced retinopathy (OIR); however, it is unclear if EPO/EPO receptor (EPOR) signaling was involved. We hypothesized that maternal UPI and resultant infant IUGR would protect against features of ROP through EPO/EPOR signaling.

METHODS

We compared transgenic mice with hypoactive EPOR signaling (hWtEPOR) to littermate wild-type mice (mWtEpoR) in a novel combined model of IUGR and ROP. Thromboxane A2 (TXA2) was infused into pregnant C57Bl/6J dams to produce UPI/IUGR; postnatal pups and their foster dams were subjected to a murine OIR model.

RESULTS

Following hyperoxia, hematocrits were similar between littermate wild-type (mWtEpoR) TXA2/OIR and vehicle/OIR pups. mWtEpoR TXA2/OIR had increased serum EPO, retinal EPO and VEGF, and decreased avascular retinal area (AVA) compared to vehicle/OIR pups. In comparison to the mWtEpoR TXA2/OIR pups, AVA was not reduced in hWtEPOR TXA2/OIR pups.

CONCLUSION

Our findings provide biologic evidence that UPI/OIR-induced endogenous EPOR signaling confers protection against hyperoxia-induced vascular damage that may be related to pathophysiology in ROP.

IMPACT

Maternal preeclampsia and infant growth restriction confer retinovascular protection against high oxygen-induced damage through endogenous erythropoietin signaling.

Pharmacotherapy and ROP: Going Back to the Basics

Retinopathy of prematurity (ROP) is a leading cause of blindness in preterm infants around the world. Through the development of animal models and clinical trials our understanding of the pathophysiology of this disease and approach to therapy has evolved significantly since ROP was first described in the 1940s in the United States. The mainstay of treatment in ROP remains ablative laser therapy to the avascular retina but pharmacologic agents are being more and more commonly used with new targets for pharmacotherapy emerging. This paper summarizes our current understanding of the pathophysiology of ROP based on the data gleaned from animal models and discusses current approaches to pharmacotherapy.

Protective effect of maternal uteroplacental insufficiency on oxygen-induced retinopathy in offspring: removing bias of premature birth

To address the hypothesis that maternal uteroplacental insufficiency (UPI) increases severity of retinopathy of prematurity, we developed a composite rat model of UPI and oxygen-fluctuations and removed premature birth as a confounding factor. Timed-pregnant Sprague-Dawley dams underwent bilateral uterine artery ligation or anesthesia (control) at e19.5. Full-term pups developed in room air (RA) or an oxygen-induced retinopathy (OIR) model. Isolectin-stained retinal flat-mounts were analyzed for percent of areas of avascular/total retina (AVA) and of intravitreal neovascular/total retina (IVNV). Pup weights and serum and mRNA of liver and kidney VEGF, IGF-1, and erythropoietin (EPO) were determined. Multivariable mixed effects linear regressions and Pearson correlations were performed using STATA14. Postnatal growth restriction occurred in pups in UPI/RA, but not in UPI/OIR. Weight gain was similar between UPI/OIR and control/OIR pups. AVA was reduced and a trend toward reduced IVNV was seen in UPI/OIR compared to control/OIR. No difference in birth weights of UPI/OIR vs. control/OIR pups occurred. Serum and renal IGF-1 and EPO were significantly increased in UPI/OIR compared to control/OIR pups. In the absence of prematurity, UPI increased angiogenic factors in association with reduced OIR severity, suggesting that ischemia from UPI could yield protective angiogenic effects by offspring.

Oxygen flux reduces Cux1 positive neurons and cortical growth in a gestational rodent model of growth restriction

BACKGROUND

The mammalian cerebral cortex forms in an inside-out manner, establishing deep cortical layers before superficial layers and is regulated by transcription factors which influence cell differentiation. Preterm birth interrupts the trajectory of normal neurodevelopment and adverse perinatal exposures have been implicated in cortical injury. We hypothesise that growth restriction (GR) and fluctuating hyperoxia (ΔO₂) impair cortical laminar development.

METHODS

Sprague-Dawley rats received 18% (non-restricted, NR) or 9% (growth restricted, GR) protein diet from E15-P7. Litters were reared in air or fluctuating hyperoxia (circa 10kPa) from P0 to P7. Cortical laminae were stained and measured. Neuronal subtypes were quantified using immunofluorescence for subtype-specific transcription factors (Satb2, Cux1, Ctip2, Tbr1).

RESULTS

ΔO₂ did not affect brain weight at P7 but reduced cortical thickness in both NR (p<0.05) and GR groups (p<0.001). ΔO₂ resulted in superficial cortical thinning in both groups and in the deep layers of GR pups (p<0.001). Cell density was preserved. ΔO₂ did not affect proportions of callosal, corticothalamic and corticospinal neurons but resulted in a reduction of neurons expressing Cux1 (p<0.01) implicated in dendritic branching and synapse formation.

CONCLUSION

Postnatal ΔO₂, a modifiable factor in neonatal care, impairs cortical development in a rodent model with preferential disadvantage to superficial neurons.

The effect of oxygen saturation targeting on retinal blood vessel growth using retinal image data from the BOOST-II UK Trial

PURPOSE

Retinopathy of prematurity (ROP) is a disorder of developing retinal blood vessels in preterm infants. The purpose of this nested study was to investigate the effects of higher (91-95%) and lower (85-89%) oxygen saturation (SpO2) targeting on retinal blood vessel growth in preterm infants.

METHODS

Retinal blood vessel growth in the higher (91-95%) and lower (85-89%) oxygen saturation (SpO2) targeting groups was compared. Suitable RetCam (Clarity, Pleasanton, CA, USA) images collected in the BOOST-II UK trial were used. The distances between the centre of the optic disc and the ROP ridge in the temporal and nasal retina were measured in pixel units.

RESULTS

Images from 38 infants were studied, 20 from the higher SpO2 target group and 18 from the lower SpO2 target group. On average, temporal blood vessels extended further from the optic disc than nasal blood vessels, mean (standard deviation (SD)) 463.39 (55.05) pixels compared with 360.13 (44.47) pixels, respectively, P<0.0001. Temporal blood vessels extended less far from the optic disc in the higher SpO2 target group than in the lower SpO2 target group: mean (SD) 449.83 (56.16) pixels compared with 480.02 (49.94), respectively, P=0.055. Nasal retinal blood vessel measurements were broadly similar in the higher and lower SpO2 target groups; mean (SD) 353.96 (41.95) compared with 370.00 (48.82) pixels, respectively, P=0.38.

CONCLUSIONS

Relatively high oxygen saturation targeting (91-95%) was associated with a trend (P=0.055) towards reduced retinal blood vessel growth in this study of preterm infants.

Influence of the difference of breastfeeding volume on a rat model of oxygen-induced retinopathy

This study aimed to investigate the effects of abundant breast milk intake on rats model of oxygen-induced retinopathy (OIR). Neonatal Sprague–Dawley rats were randomly assigned to expand litters of 7 pups/litter (7-rats group) and 14 pups/litter (14-rats group). They were exposed to 80% oxygen from postnatal day (P) 0 to P12. Body weights were measured daily. At P13 and 18, rats were sacrificed, and the blood and eyes were collected. Retinal neovascularization (NV) score, total retinal area (TRA), avascular area (AVA), and vascularized area (VA) were measured in ADPase stained retinas. Retinal vascular endothelial growth factor (VEGF) and serum insulin-like growth factor (IGF-1) were measured using ELISA. Body weight gain was significantly greater in 7-rats group from P2. Serum IGF-1 levels at P13 and 18 were significantly higher in 7-rats group. Retinal VEGF and TRA at P18 were significantly larger in 7-rats group. NV score at P18 tended to be higher in 7-rats group. There was no significant difference in VA between the 2 groups at P13 and 18. Excess breast milk intake in OIR rat pups caused body weight gain and retinal development, whereas there was less effect on retinal vascularization in our study.

Management of retinopathy of prematurity

While current management of retinopathy of prematurity (ROP) is well evidenced, the recent Neonatal Oxygenation Prospective Meta-analysis (NeoPROM) oxygen therapy trials, and the Bevacizumab Eliminates the Angiogenic Threat of Retinopathy of Prematurity (BEAT-ROP) trial of intravitreal injection bevacizumab, have reopened debate on optimal management. Early postnatal manipulation of oxygen therapy, nutrition and serum IGF 1 levels may improve early retinal blood vessel development and prevent later severe ROP. While the use of intravitreal injections of antivascular endothelial growth factor (VEGF) agents may appear to be an attractive alternative to laser ablation of the peripheral retina, caution is needed. The optimal choice of agent and dose remain unknown, and suppression of serum VEGF levels might interfere with normal angiogenesis processes in developing tissues. There is a pressing need for good Phase 1 studies of these agents, and safety trials.

Effect(s) of preterm birth on normal retinal vascular development and oxygen-induced retinopathy in the neonatal rat

PURPOSE

Maturity is a critical factor in the pathogenesis of retinopathy of prematurity (ROP). One widely used method for studying this condition is that of oxygen-induced retinopathy (OIR). The general conditions of an OIR term animal, both at the time of birth and following birth, differ from those of the preterm infant. This, to simulate preterm conditions and to provide a basis for further studies on ROP, we investigated the effect(s) of preterm birth on retinal vascularization using the neonatal rat.

MATERIALS AND METHODS

Sprague-Dawley (SD) rats were delivered preterm by caesarean section on the day 19 of gestation. Term pups were used as controls. On the day of birth, preterm and term pups were housed under conditions of room air or cyclic oxygen. Retinas of pups housed in room air on days 4, 7, 10, 14, 18 and 22, as well as pups housed in oxygen on days 14, 18, and 22 were whole-mounted and stained with isolectin-B4. On day 18, cross-sections of the retina were cut and stained with hematoxylin and eosin for the identification of preretinal neovascular tufts. Images of avascular and neovascular areas were compared using light and fluorescence microscopy.

RESULTS

Preterm pups had significantly larger avascular retinal areas than term rats on the various postnatal days. After exposure to cyclic oxygen, preterm pups demonstrated significantly larger avascular (days 14 and 18) and neovascular areas (day 18) compared with term rats. On day 22, residual retinopathy of preterm pups was greater than that of term pups.

CONCLUSIONS

Preterm birth of rats, which are comparable in their physiology to humans, had negative effects on retinal vascularization. The impaired retinal vascular development and subsequent vasoproliferation resulting from hyperoxia in preterm pups is more severe and enduring.

Association between retinal neovascularization and serial weight measurements in murine and human newborns

INTRODUCTION

Retinopathy of prematurity (ROP) is a leading cause of preventable blindness throughout the world. Several risk factors have been studied, but most studies remain inconclusive. Evidence is accumulating that one of the strongest predictors of ROP, in addition to oxygen use and low gestational age, is poor weight gain during the first postnatal weeks.


METHODS

In a prospective study, we sought to determine the importance of serial weight measurements to help predict neovascularization (NV). In a first stage, a summary of the response in each case is identified and calculated as area under the curve (AUC). In a second stage, these different AUCs are analyzed by nonparametric Mann-Whitney U test. For the murine study, pups were redistributed in smaller and larger litters. On postnatal day (P)7-12, the oxygen-induced retinopathy (OIR) model was applied. Body weight was measured on P7, P14, and P17. Retinal NV was assessed on P17. For the human study, the subjects were part of the control arm of the NIRTURE trial. Ophthalmologists screened for ROP. Birthweight was recorded. Weekly weight measurements were performed for the first 4 weeks.


RESULTS

The AUC of serial weight (gain) measurements was significantly lower in murine (14 vs 17 g; p = 0.01) and human (140 g/wk vs 240 g/wk; p = 0.0001) newborns developing retinal NV.


CONCLUSION

This prospective study supports previous findings, using a new way of statistical analysis, that early postnatal weight gain is an important indicator in the development of neovascular disease.