Neuron-selective erythropoietin (Epo) overexpression protects against transient focal ischemia and retinal ganglion cell axotomy in mice – role of Akt, Erk-1/2 and Jnk signaling

Background: It has previously been demonstrated that erythropoietin (Epo) protects against focal brain ischemia. Thus, clinical trials in human stroke patients have recently been initiated, which are currently performed. Unfortunately, the underlying mechanisms of action of Epo in vivo are still poorly understood. In order to elucidate this issue, transgenic mice overexpressing Epo in the central nervous system but not the rest of the body have recently been produced (so-called tg21 mice). We here examined the effects of Epo overexpression on neuronal injury after transient focal ischemia and retinal ganglion cell (RGC) axotomy.

Methods: Adult male or female tg21 mice as well as their non-transgenic littermates were subjected to transient focal ischemias, as induced by 90 or 30 minutes episodes of intraluminal middle cerebral artery (MCA) occlusions, followed by 1 or 3 days of reperfusion, or exposed to RGC axotomy, followed by 14 days survival.

Results: Epo overexpression markedly reduced infarct size (from 61.5±8.1 to 4.9±3.0 mm3 in male/from 58.1±15.2 to 23.5±15.4 mm3 in female mice (means±S.D.)) and diminished brain swelling (from 28±3% to 7±6% in male/from 25±8% to 8±3% in female mice) after focal ischemias leading to secondary brain infarction (90min MCA occlusion) and also attenuated disseminate neuronal injury after mild focal ischemia (30min). Besides, Epo overexpression considerably ameliorated RGC survival after optic nerve transection (from 21.3±7.6 to 61.4±20.6%). Western blots revealed that Epo overexpression was associated with elevated constitutive levels of phospho-Stat-5, phospho-Akt and diphospho-Erk-1/2 in non-lesioned retinae. During neuronal degeneration, levels of Jak-2 and phospho-Stat-5 markedly decreased, while di-phospho-Erk-1/2 levels increased. Phospho-Akt levels remained high and di-phospho-Jnk levels dropped. Bcl-XL levels, on the other hand, were not elevated by Epo overexpression, in contrast to previous in vitro studies.

Conclusion: Neuron-selective Epo overexpression exerts pronounced neuroprotection after focal brain ischemia and optic nerve transection in mice. The beneficial effects of Epo involve multiple signaling pathways, which make this factor an interesting candidate for stroke therapy.