Retinopatía del prematuro y estrés oxidativo

New findings in ATP supply in rod outer segments: insights for retinopathies

BACKGROUND INFORMATION

The rod outer segment (OS) is the specialised organelle where phototransduction takes place. Our previous proteomic and biochemical analyses on purified rod disks showed the functional expression of the respiratory chain complexes I-IV and F1 Fo -ATP synthase in OS disks, as well as active soluble tricarboxylic acid cycle enzymes. Here, we focussed our study on the whole OS that contains the cytosol and plasma membrane and disks as native flattened saccules, unlike spherical osmotically intact disks.

RESULTS

OS were purified from bovine retinas and characterised for purity. Oximetry, ATP synthesis and cytochrome c oxidase (COX) assays were performed. The presence of COX and F₁F₀-ATP synthase (ATP synthase) was assessed by semi-quantitative Western blotting, immunofluorescence or confocal laser scanning microscopy on whole bovine retinas and bovine retinal sections and by immunogold transmission electron microscopy (TEM) of purified OS or bovine retinal sections. Both ATP synthase and COX are catalytically active in OS. These are able to consume oxygen (O₂) in the presence of pyruvate and malate. CLSM analyses showed that rhodopsin autofluorescence and MitoTracker Deep Red 633 fluorescence co-localise on rod OS. Data are confirmed by co-localisation studies of ATP synthase with Rh in rod OS by immunofluorescence and TEM in bovine retinal sections.

CONCLUSIONS

Our data confirm the expression and activity of COX and ATP synthase in OS, suggestive of the presence of an extra-mitochondrial oxidative phosphorylation in rod OS, meant to supply ATP for the visual transduction. In this respect, the membrane rich OS environment would be meant to absorb both light and O₂. The ability of OS to manipulate O₂ may shed light on the pathogenesis of many retinal degenerative diseases ascribed to oxidative stress, as well as on the efficacy of the treatment with dietary supplements, presently utilised as supporting therapies.

The protective effect of 17 beta-estradiol on oxygen-induced retinopathy and its relation with the changes of malondialdehyde

Objective

Retinopathy of prematurity is becoming obvious with the improvement of neonatal ambulance. However there is still not a good treatment. The present study is to observe the effect of 17 beta-estradiol (E2) on oxygen-induced retinopathy (OIR), and explore the relationship between the changes of avascular area and malondialdehyde (MDA) in retina.

Methods

Newborn oxygen-exposed mice underwent subcutaneous injections of different dose of E2 (0.1 µg, 1.0 µg, 10.0 µg ), tamoxifen or phosphate buffered saline (PBS; controls)everyday from post-natal day (p)7 to p17. At p17, retinal flat mounts were scored for the percentage of avascular/total retinal area, and pathological changes during revascularization. The MDA concentration in the retina was determined also. In the most efficacious E2 group (10.0 µg), 100.0 µg tamoxifen was also administered, and the percentage of capillary-free/total retinal area determined, and the retinal malondialdehyde concentration assayed.

Results

The mean percentage of capillary-free area over total retinal area was 0(PBS, in room air), 34.197±1.301(PBS, in hyperoxia), 23.685±0.407 (0.1 µg E2), 14.648±0.355 (1.0 µg E2), 4.693±0.450 (10.0 µg E2) and 32.240±0.654 (10.0 µg E2 +100.0 µg tamoxifen). The difference was significant (F = 2778.759, P < 0.01), and the difference between any two groups were also significant (all P value were less than 0.01). The predilection of tufts and clusters during revascularization was mainly aggregated in zones 2 and 3, but the difference of retinal neovascular clusters and tufts in fourth zone among different groups were significant [clusters (F = 44.719, P < 0.01) vs tufts (F = 39.997, P < 0.01)]. The mean MDA concentration were 0.711±0.037(PBS, in room air), 2.084±0.066 (PBS, in hyperoxia), 1.829±0.091(0.1 µg E2), 1.152±0.067(1.0 µg E2), 0.796±0.027(10.0 µg E2), 1.988±0.049(10.0 µg E2 +100.0 µg tamoxifen) (F = 628.103, P < 0.01). The difference between any two groups were also significant (all P value were less than 0.05). The close relation between the percentage of avascular/total retinal area and MDA concentration was also verified (r = 0.981, P < 0.01).

Conclusion

Oxidative stress responses play a pivotal role in OIR, by means of receptor pathway. E2 can alleviate oxidative stress reaction, and thus ameliorate the severity of oxygen induced retinopathy.

Hypoxia-induced retinal ganglion cell damage through activation of AMPA receptors and the neuroprotective effects of DNQX

Hypoxia-induced glutamate accumulation in neural tissues results in damage to neurons through excitotoxic mechanisms via activation of glutamate receptors (GluRs). Here we examine whether hypoxia in the developing retina would cause activation of the ionotropic α-amino-3-hydroxy-5-methylisoxazole-4-propioate (AMPA) GluRs and increase in Ca(2+) influx into retinal ganglion cells (RGCs) that might ultimately lead to their death. Neonatal Wistar rats were subjected to hypoxia for 2h and then sacrificed at various time points after the exposure together with normal age matched control rats. Primary cultures of RGCs were also prepared and subjected to hypoxia. Expression of AMPA glutamate receptor (GluR) 1-4 was examined in the retina. Additionally, expression of GluRs, intracellular Ca(2+) influx, reactive oxygen species (ROS) generation and cell death were investigated in cultured RGCs. GluR1-4 mRNA and protein expression showed a significant increase (P < 0.01) over control values after the hypoxic exposure both in vivo and in vitro. Cells expressing GluR1-4 in the retina were identified as RGCs by double immunofluorescence labeling with Thy1.1. Increased intracellular Ca(2+) in cultured RGCs following hypoxic exposure was reduced (P < 0.01) by 10 μM AMPA antagonist 6, 7-dinitroquinoxaline-2,3-dione (DNQX). Our results suggest that following a hypoxic insult, an increased amount of glutamate accumulates in the neonatal retina. This would then activate AMPA receptors which may damage RGCs through increased Ca(2+) accumulation and ROS generation. The involvement of AMPA receptors in damaging the RGCs is evidenced by suppression of intracellular Ca(2+) influx by DNQX which also decreased ROS generation and cell death by 50%.

VEGF-mediated STAT3 activation inhibits retinal vascularization by down-regulating local erythropoietin expression

Avascular, hypoxic retina has been postulated to be a source of angiogenic factors that cause aberrant angiogenesis and intravitreal neovascularization (IVNV) in retinopathy of prematurity. Vascular endothelial growth factor (VEGF) is an important factor involved. However, VEGF is also required for normal retinal vascular development, which raises concerns about inhibiting its activity to treat IVNV in retinopathy of prematurity. Therefore, understanding the effects that VEGF has on other factors in the development of avascular retina is important to prevent aberrant angiogenesis and IVNV. Here, we show that STAT3 was activated by increased retinal VEGF in the rat 50/10 oxygen-induced retinopathy model. Phospho-STAT3 colocalized with glutamine synthetase-labeled Müller cells. Inhibition of STAT3 reduced avascular retina and increased retinal erythropoietin (Epo) expression. Epo administered exogenously also reduced avascular retina in the model. In an in vitro study, hypoxia-induced VEGF inhibited Epo gene expression by STAT3 activation in rat Müller cells. The mechanism by which activated STAT3 regulated Epo was by inhibition of Epo promoter activity. Together, these findings show that increased retinal VEGF contributes to avascular retina by regulating retinal Epo expression through Janus kinase/STAT signaling. Our results suggest that rescuing Epo expression in the retina before the development of IVNV may promote normal developmental angiogenesis and, therefore, reduce the stimulus for later pathologic IVNV.

Evidence of oxidative stress in relation to feeding type during early life in premature infants

Morbidity in the premature (PT) infant may reflect difficult adaptation to oxygen. We hypothesized that feeding including formula feeding (F) and feeding mother's milk (HM) with added fortifier would affect redox status. Therefore, 65 PT infants (birth weight: 1146 ± 261 g; GA: 29 ± 2.5 wk; mean ± SD) were followed biweekly, once oral feeds were introduced. Feeding groups: F (>75% total feeds) and HM (>75% total feeds) were further subdivided according to human milk fortifier (HMF) content of 0-19, 20-49, and ≥ 50%. Oxidative stress was quantified by F2-isoprostanes (F2-IsoPs) in urine, protein carbonyls, and oxygen radical absorbance capacity (ORAC) in plasma. F2-IsoPs (ng/mg creatinine): 0-2 wk, 125 ± 63; 3-4 wk, 191 ± 171; 5-6 wk, 172 ± 83; 7-8 wk, 211 ± 149; 9-10 wk, 222 ± 121; and >10 wk, 183 ± 67. Protein carbonyls from highest [2.41 ± 0.75 (n = 9)] and lowest [2.25 ± 0.89 (n = 12) pmol/μg protein] isoprostane groups did not differ. ORAC: baseline, 6778 ± 1093; discharge, 6639 ± 735 [full term 4 and 12 M, 9010 ± 600 mg (n = 12) TE]. Highest isoprostane values occurred in infants with >50% of their mother's milk fortified. Further research on HMF is warranted.

Antioxidant properties of breast milk in a novel in vitro digestion/enterocyte model

OBJECTIVES

There is good evidence to suggest that human breast milk has antioxidant properties. Our primary goal was to investigate the antioxidant properties of human milk in a combined in vitro digestion/cell culture model that more closely replicates conditions in the gastrointestinal system of the preterm infant.

MATERIALS AND METHODS

An in vitro digestion model was developed that incorporates both gastric and intestinal phases, based on reported luminal pH, digestive enzyme levels, and transit times observed in preterm infants. To mimic the human intestinal mucosa, 2 cell lines--Caco-2BBE and HT29-MTX--were cocultured on Matrigel, an artificial basement membrane substrate. Intracellular oxidative stress was measured with 2 broadly selective oxidant-sensitive dyes, and oxidative DNA damage was assessed by means of single-cell gel electrophoresis.

RESULTS

Enterocyte differentiation and mucin secretion were observed by 14 seeding of cultures. Direct exposure to digested milk resulted in a loss of transepithelial electrical resistance; however, exogenous mucin mitigated this loss. Data suggested that both milk and digested milk alleviated oxidative stress in the coculture, and both reduced hydrogen peroxide-induced oxidative DNA damage, as demonstrated by the comet assay.

CONCLUSIONS

Our results support the hypothesis that breast milk reduces oxidative stress in a cell culture model representative of the intestinal mucosa, and also confirmed the suitability of this combined in vitro digestion/cell culture system for investigating the physiologic effects of enteral nutrients such as breast milk, under conditions similar to those existing in the gastrointestinal system of the preterm infant.

Oxidative DNA damage in retinopathy of prematurity

PURPOSE

This study examines the levels of oxidative damage in patients with retinopathy of prematurity (ROP).

METHODS

Fifty patients were recruited with a birthweight below 1500 g or gestational age below 32 weeks. The cases were classified into those who developed ROP (n=25) and those without ROP (n=25). The authors obtained blood and urine samples from each infant, for measuring 8-hydroxy 2-deoxyguanosine (8-OHdG) and malondialdehyde (MDA) levels, at the time of the first examination at 4-6 postnatal weeks.

RESULTS

A significant difference was observed in leukocyte and urine 8-OHdG levels in patients with ROP compared to those without ROP (p<0.001 for both). Similarly, a significant difference was observed in plasma and urine MDA levels in patients with ROP compared to those without ROP (p<0.001 for both). In addition, significant correlations were found between levels of 8-OHdG in leukocyte DNA and plasma MDA (r=0.859, p<0.001), and between levels of urine 8-OHdG excretion and urine MDA (r=0.563, p<0.001).

CONCLUSIONS

8-OHdG in leukocyte DNA and urine levels in premature infants can be useful as an indicator for ROP screening.

Pathological investigation of oxidative stress in the post-genomic era

Aerobes, including humans, are consistently exposed to oxidative stress by consuming oxygen. The biological significance of oxidative stress via reactive oxygen and nitrogen species consists of two stages: reversible redox regulation and irreversible oxidative molecular damage, which are sometimes intermingled. During the past decade, many signaling cascades associated with oxidative stress have been discovered. An interaction between Keap1 and the Nrf2 transcription factor is among the most fundamental mechanisms of the defense system against oxidative or similar stress. Furthermore, it became apparent that reactive oxygen species are actively produced through enzymes such as xanthine oxidoreductase and nicotinamide adenine dinucleotide phosphate, reduced (NADPH) oxidases in non-phagocytic cells as well. The role of alpha-tocopherol solely as an anti-oxidant was also questioned. Now there is a long list of pathological states implicating oxidative stress. At the same time, genome projects on various species have been completed. These efforts convincingly led to a new era of oxidative stress investigation, contributing powerful strategies to select candidate genes or biomolecules. Herein are reviewed recent advances and novel concepts in this field, including oxygenomics. These fruitful results may lead to more accurate and useful pathological diagnosis and more efficient prophylaxis and therapeutic interventions on human diseases.