Morphological and functional changes in the retina after chronic oxygen-induced retinopathy

Abstract

The mouse model of oxygen-induced retinopathy (OIR) has been widely used for studies of retinopathy of prematurity (ROP). This disorder, characterized by abnormal vascularization of the retina, tends to occur in low birth weight neonates after exposure to high supplemental oxygen. Currently, the incidence of ROP is increasing because of increased survival of these infants due to medical progress. However, little is known about changes in the chronic phase after ROP. Therefore, in this study, we examined morphological and functional changes in the retina using a chronic OIR model. Both the a- and b-waves in the OIR model recovered in a time-dependent manner at 4 weeks (w), 6 w, and 8 w, but the oscillatory potential (OP) amplitudes remained depressed following a return to normoxic conditions. Furthermore, decrease in the thicknesses of the inner plexiform layer (IPL) and inner nuclear layer (INL) at postnatal day (P) 17, 4 w, and 8 w and hyperpermeability of blood vessels were observed in conjunction with the decrease in the expression of claudin-5 and occludin at 8 w. The chronic OIR model revealed the following: (1) a decrease in OP amplitudes, (2) morphological abnormalities in the retinal cells (limited to the IPL and INL) and blood vessels, and (3) an increase in retinal vascular permeability via the impairment of the tight junction proteins. These findings suggest that the experimental animal model used in this study is suitable for elucidating the pathogenesis of ROP and may lead to the development of potential therapeutic agents for ROP treatment.

Figure 1. Measurement of dark-adapted electroretinography (ERG) amplitudes in the oxygen-induced retinopathy (OIR) model and normal mice.

Amplitudes of a- (A) and b-waves (B) from the OIR model or from normal mice were measured at 4, 6, and 8 w. Stimulus flashes were used from −2.92 to 0.98 log cds/m². (C) Representative ERG waveforms at 4, 6, and 8 w. Values are expressed as the mean ± S.D., n = 4 to 6. ^* P<0.05, ^** P<0.01 versus Normal. OIR, oxygen-induced retinopathy model.

Figure 2. The oscillatory potentials (OPs) amplitudes in response to a light flash in the oxygen-induced retinopathy (OIR) model and normal mice.

The averaged OP amplitudes were measured at 4, 6, and 8 w. Values are expressed as the mean ± S.D., n = 4 to 6. ^* P<0.05, ^** P<0.01 versus Normal. OIR, oxygen-induced retinopathy model.

Figure 3. Retinal damage in the oxygen-induced retinopathy (OIR) model and normal mice.

Retinal cross sections were prepared at 4 w and 8 w. (A) Hematoxylin and eosin staining. Scale bar, 50 µm. Retinal damage was evaluated by counting the number of cells in the GCL (B) and measuring the thickness of the IPL (C), INL (D), and ONL (E) in mice at 4 w and 8 w. Values are expressed as the mean ± S.D., n = 5 or 6. ^* P<0.05, ^** P<0.01 versus Normal. GCL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; ONL, outer nuclear layer; OIR, oxygen-induced retinopathy model.

Figure 4. The development of retinal vascular lumen in the oxygen-induced retinopathy (OIR) model and normal mice.

(A) Flat-mounted retinas in the OIR model mice and normal mice, along with (insets) enlargements, at 3 time points (P17, 4 w, and 8 w). Scale bars, 500 µm (100 µm in insets). Quantitative analysis of retinal vascular lumen at 4 w and 8 w was performed using an imaging analyzer (the Angiogenesis Tube Formation module) on the entire retina; 4 parameters were measured: (B) length, (C) area, (D) branch points, and (E) segments. Values are expressed as the mean ± S.D., n = 4 or 5. ^** P<0.01 versus normal. OIR, oxygen-induced retinopathy model.

Figure 5. Retinal permeability in the oxygen-induced retinopathy (OIR) model and normal mice.

Each upper image is shown using Metamorph (A, B). Confocal fluorescence micrographs (C, D) show a higher magnification version of part of the corresponding upper image. Scale bars, 100 µm (B) and 50 µm (D). Retinal permeability was evaluated in the 4 areas shown in (E) (each area 0.144 mm²×4 areas; total area 0.576 mm²) at 8 w. (F) Retinal permeability rate was quantified using mathematical formulae; see Materials and Methods for further details. Western immunoblots of claudin-5 (G), occludin (H), and β-actin proteins in the retina at 8 w in the OIR model mice and in normal mice. Expression was quantified by densitometry and corrected by reference to β-actin. Values are expressed as the mean ± S.D., n = 7 or 8. ^** P<0.01 versus normal.