Effects of Erythropoietin on CNS Neuron Regeneration and Involvement of Signal Transduction Mechanisms in vivo and in vitro

The cytokine hormone erythropoietin (EPO) promotes differentiation, proliferation, and maintenance of erythroid progenitor cells. Recent investigations have found CNS neurons responsive to local and systemic EPO, and that EPO can prevent physiological neuronal loss, and apoptotic death under pathological CNS conditions. Further, EPO has proved to be neuroprotective in various models of CNS injury, including EAE-related inflammation, kainate toxicity, hypoxic stress, serum deprivation and glutamate excitotoxicity. The impact of EPO on the regeneration of adult neurons is still unclear. Here, we compared outgrowth capacity of adult retinal ganglion cells (RGCs) in the absence and under in vivo EPO treatment immediately after crush lesion of the optic nerve (ON). Alternatively, delayed EPO treatment was administered on cultured RGCs in vitro 10 days after ON injury. Both post-lesional in vivo conditioning, and delayed addition to the culture medium of 10–10,000 IU recombinant EPO (rhEPO) increased numbers and length of neurites regenerating from cultured RGCs in a concentration-dependent manner. As indicated by protein expression analysis, cellular EPO response was mediated by induction of the antiapoptotic Bcl-2 family member Bcl-XL, involving signal transduction by Stat-3 phosphorylation and p42/44 MAPK activation. According to its survival promoting effects observed in vitro, PI3K/Akt phosphorylation was simultaneously up-regulated. Inhibition of the Jak2/Stat3 pathway completely abolished EPO-induced growth promotion. In contrast, inhibition of PI3K reduced intrinsic growth potential, but had only minor effects on EPO-stimulated neuritogenesis. Inhibition of mTOR did not affect intrinsic or stimulated axon growth. In conclusion, we introduce the cytokine EPO as a potent stimulator of survival and regeneration promoting signal cascades in CNS neurons. Such a dual role may imply a therapeutic potential preferential to combined factor treatments and become promising as a future treatment option in cases of CNS degeneration and neurotrauma.