Purpose: Recent studies have reported neuroprotective effects of erythropoietin (EPO) and vascular endothelial growth factor (VEGF). The purpose of the present study was to clarify their influence on neurite outgrowth and regeneration of rat retinal ganglion cells (RGCs) in vitro and to elucidate the expression of corresponding receptors in the rat retina in vivo.
Methods: Retinal explants from postnatal rats were stimulated with VEGF alone; VEGF in combination with anti-VEGF-receptor (VEGF-R)-2 antibody or T-type Ca2+ channel blocker ethosuximide (ESX); EPO alone; or EPO in combination with anti-EPO-receptor antibody or ESX. The presence of the corresponding receptors in the rat retina was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and by immunohistochemistry.
Results: EPO induced a stable improvement of neurite outgrowth of RGCs in a dose-dependent manner (5 x 10(-15) M to 5 x 10(-13) M) up to 169% (P < 0.05). Treatment of the explants with anti-EPO-R antibody (1:80 dilution) and with ESX (5 microM) totally inhibited EPO-mediated effects on RGCs. In comparison, VEGF (50 ng/mL), induced neurite outgrowth of retina explants up to 167% (P < 0.05), which again was inhibited in the presence of anti-VEGF-R2 antibody or ESX. Transcripts of EPO-R, VEGF-R1, and VEGF-R2 were detected by RT-PCR. Intense immunoreactivity for VEGF-R1, VEGF-R2, and EPO-R were found in the RGC layer of the retina.
Conclusions: The data demonstrate for the first time that EPO and VEGF have a significant and specific biological effect on neurite regrowth of axotomized RGCs. Therefore, these results imply that EPO and VEGF have not only a neuroprotective but also a neuroregenerative role in ischemic retinal conditions.