Summary:The growth factor erythropoietin (EPO) and erythropoietin receptors (EPOR) are expressed in the nervous system. Neuronal expression of EPO and EPOR peaks during brain development and is upregulated in the adult brain after injury. Peripherally administered EPO, and at least some of its variants, cross the blood-brain barrier, stimulate neurogenesis, neuronal differentiation, and activate brain neurotrophic, antiapoptotic, anti-oxidant and anti-inflammatory signaling. These mechanisms underlie their tissue protective effects in nervous system disorders. As the tissue protective functions of EPO can be separated from its stimulatory action on hematopoiesis, novel EPO derivatives and mimetics, such as asialo-EPO and carbamoylated EPO have been developed. While the therapeutic potential of the novel EPO derivatives continues to be characterized in preclinical studies, the experimental findings in support for the use of recombinant human (rh)EPO in human brain disease have already been translated to clinical studies in acute ischemic stroke, chronic schizophrenia, and chronic progressive multiple sclerosis. In this review article, we assess the studies on EPO and, in particular, on its structural or functional variants in experimental models of nervous system disorders, and we provide a short overview of the completed and ongoing clinical studies testing EPO as neuroprotective/neuroregenerative treatment option in neuropsychiatric disease.

The transcription factors signal transducer and activator of transcription 5a and 5b (Stat5) are activated by the neuroprotective and neurotrophic cytokines, erythropoietin (EPO) and growth hormone (GH). Here, we show a dissociation of the intracellular pathway mediating the protective effect of EPO against glutamate toxicity from that needed for its neurotrophic activity using hippocampal neuronal cultures from Stat5a/b-knockout (Stat5 À/À ) mouse fetuses. Both pretreatment and post-treatment with EPO counteracted glutamate-induced cell death in Stat5 þ / þ and Stat5 À/À neurons. Acute pharmacological inhibition of Janus kinase 2 (JAK2)/Stat signalling had no effect on EPO neuroprotection, whereas inhibition of phosphatidylinositol-3 0 kinase (PI3K)/Akt pathway abolished the protective effect of EPO in both Stat5 þ / þ and Stat5 À/À neurons. GH effectively protected Stat5 þ / þ cells against glutamate toxicity but had no effect in Stat5 À/À neurons or in Stat5 þ / þ neurons treated with JAK2/Stat or PI3K inhibitor. EPO and GH stimulated neurite outgrowth and branching of Stat5 þ / þ neurons by activating PI3K/Akt signalling but had no trophic effect in Stat5 À/À cells. We conclude that in hippocampal neurons, Stat5 is not required for neuroprotection by EPO but is together with Akt essential for its neurotrophic activity. Both Stat5 and Akt are needed for neuroprotective and neurotrophic signalling of GH in neurons. The transcription factors signal transducer and activator of transcription 5a and 5b (Stat5) control cell fate decisions such as differentiation, proliferation and apoptosis. 1 Stat5 mediates cellular response to cytokines, growth factors and hormones. 1 Upon binding to their membrane receptors, growth factors such as erythropoietin (EPO) and growth hormone (GH) activate Janus kinase 2 (JAK2), causing activation of Stat5 by its phosphorylation, dimerization and translocation to the nucleus, where Stat5 induces expression of various antiapoptotic genes. 1 Indicative of an impaired functioning of EPO and GH receptor-associated signalling, Stat5a/b-knockout mice (Stat5 À/À ) are severely anaemic and growth retarded and the vast majority die perinatally. 2 Stat5 is expressed in the developing nervous system. 3,4 Recent studies suggest a role for Stat5 in neuronal migration and axon guidance in the developing brain 3 and a novel therapeutic potential for constitutively active Stat5 in reducing axonal outgrowth defects in spinal muscular atrophy-like motor neurons. 5 Since activation of Stat5 in neurons accompanies the antiapoptotic and neuroprotective effects of EPO in vitro and in vivo, 6-12 Stat5 has been suggested to mediate the protective effects of EPO in neuronal cells. Nevertheless, the molecular mechanisms of EPO receptor (EPOR) activation in neurons are complex, as multiple neuroprotective signalling pathways are activated downstream of EPOR/JAK2. In addition to Stat5, best characterized from these are the phosphatidylinositol-3 0 kinase (PI3K)/ Akt, nuclear factor-kB and Ras/mitogen-activated p...