Apelin play an important role in angiogenesis of mouse retina

Aim: Apelin is upregulated in the vitreous of patients with diabetic retinopathy. Apelin and its receptor APJ are primarily expressed in endothelial cells contributing to angiogenesis. Apelin stimulates the proliferation of retinal cells in vitro. However, whether apelin is involved in the regulation of retinal vascularization remains unknown.

Methods: Quantitative RT-PCR was used to assess apelin expression on postnatal day 5 (p5) to 17 (p17) in C57Bl/6 mice. Recombinant apelin-13 was injected intraperitoneally at p4/p5/p6 or p7/p8/p9. The vasculature of the superficial and the deep capillary layer were analyzed at p7 and p10, respectively. Tip cells, sprout length, vessel outgrowth and vessel density were quantified using immunofluorescence staining of whole mount preparations for lectin and collagenIV. The oxygen induced retinopathy (OIR) model was used to study its role in pathological angiogenesis. Retinal cryosections were stained to detect sites of apelin expression.

Results: During physiological angiogenesis apelin expression showed a peak value at p14 with a 52.6 ± 9.2 folds higher expression compared to its lowest level at p5. Apelin treated animals showed an increase in tip cells (37 ± 6.2 vs. 24.4 ± 7.6), extension of sprout length (26 ± 2.5 µm vs. 16.8 ± 1.9 µm) and higher vessel density (51.3 ± 5.1 vs. 40.6 ± 4.5%) compared to controls. In the OIR model apelin was upregulated 2.2 ± 0.7 folds at p12+6h compared to physiological conditions. The cryosections showed that apelin is not only expressed peri-capillary but also in the GCL, OPL, and the layer of rods and cones at p17.

Conclusion: Apelin promotes physiological angiogenesis during retinal vessel development and is upregulated in pathological angiogenesis.