Erythropoietin fosters both intrinsic and extrinsic neuronal protection through modulation of microglia, Akt1, Bad, and caspase-mediated pathways.

Erythropoietin (EPO) plays a significant role in the hematopoietic system, but the function of EPO as a neuroprotectant and anti-inflammatory mediator requires further definition. We therefore examined the cellular mechanisms that mediate protection by EPO during free radical injury in primary neurons and cerebral microglia. Neuronal injury was evaluated by trypan blue, DNA fragmentation, phosphatidylserine (PS) exposure, Akt1 phosphorylation, Bad phosphorylation, mitochondrial membrane potential, and cysteine protease activity. Microglial activation was assessed through proliferating cell nuclear antigen and PS receptor expression. EPO provides intrinsic neuronal protection that is both necessary and sufficient to prevent acute genomic DNA destruction and subsequent membrane PS exposure, since protection by EPO is completely abolished by cotreatment with an anti-EPO neutralizing antibody. Extrinsic protection by EPO is offered through the inhibition of cerebral microglial activation and the suppression of microglial PS receptor expression for the prevention of neuronal phagocytosis. In regards to microglial chemotaxis, EPO modulates neuronal poptotic membrane PS exposure necessary for microglial activation primarily through the regulation of caspase 1. EPO increases Akt1 activity, phosphorylates Bad, and maintains neuronal nuclear DNA integrity through the downstream modulation of mitochrondrial membrane potential, cytochrome c release, and caspase 1, 3, and 8-like activities. Elucidating the intrinsic and extrinsic protective pathways of EPO that mediate both neuronal integrity and inflammatory microglial activation may enhance the development of future therapies directed against acute neuronal injury. Keywords: Apoptosis, Bad, cytochrome c, cysteine proteases, erythropoietin, microglial activation, mitochondrial membrane potential, phosphatidylserine exposure, protein kinase B Introduction Initially considered to mediate primarily the proliferation and differentiation of erythroid progenitors, erythropoietin (EPO) has emerged as a versatile growth factor that may play a significant role in the nervous system. Both EPO and the erythropoietin receptor EPOR are expressed throughout the nervous system in neurons, endothelial cells, and astrocytes in the cerebral cortex, hippocampus, and the amygdala (Morishita et al., 1997; Nagai et al., 2001; Chong et al., 2002b). In neuronal injury paradigms, EPO has been shown to provide protection against toxic insults, such as ischemia and free radical injury (Bernaudin et al., 1999; Chong et al., 2002a; Wen et al., 2002). As a result, EPO has been identified as a possible candidate in the formulation of therapeutic strategies against neurodegenerative diseases. To further the development of EPO as a novel neuroprotectant against both acute and chronic neurodegenerative disease, it is first critical to understand the cellular pathways that may mediate neuronal injury and are subsequently susceptible to modulation by EPO. Oxygen-free radicals, such as nitric oxide (NO), have been established as significant precipitants of neuronal degeneration (Maiese & Vincent, 2000; Anderson et al., 2001). NO can trigger the induction of two independent apoptotic pathways that consist of nuclear DNA degradation and the exposure of membrane phosphatidylserine (PS) residues (Maiese & Vincent, 2000; Dumont et al., 2001; Lin & Maiese, 2001). Although DNA degradation in neurons may immediately impact cellular integrity (Jessel et al., 2002), the exposure of membrane PS residues in neurons can precipitate a latent cellular inflammation (Dombroski et al., 2000) and microglial phagocytosis of viable neurons (Maiese & Vincent, 2000; Hoffmann et al., 2001). Several downstream cellular pathways may ultimately determine the protective role of EPO. In particular, the serine/threonine kinase Akt1, a key determinant of cell survival, appears to be necessary for EPO to prevent apoptosis of erythroid progenitors (Uddin et al., 2000). Once activated through phosphorylation, Akt1 can inhibit the activity of several substrates that promote apoptosis, such as Bad (Blume-Jensen et al., 1998), IκB kinase α (IKKβ) (Romashkova & Makarov, 1999), the forkhead transcription factor (FHKRL1) (Brunet et al., 1999), and the glycogen synthase kinase-3β (Cross et al., 1995). The protective role of Akt1 also may be dependent upon the preservation of mitochondrial membrane integrity and the modulation of cysteine protease activity through cytochrome c. The protein Akt1 may serve to stabilize mitochondrial membrane potential and prevent the release of cytochrome c (Kennedy et al., 1999). Cellular release of NO can directly lead to mitochondrial membrane depolarization and the opening of mitochondrial permeability transition pores (Bal-Price & Brown, 2000; Chong et al., 2002a). As a result, cytochrome c is released from mitochondria and subsequently leads to the activation of a family of cysteine proteases (caspases) that include caspase 8, caspase 1, and caspase 3. Together, these cysteine proteases can lead to both DNA fragmentation and membrane PS exposure (Lin & Maiese, 2001; Mandal et al., 2002). Given the strong neuroprotective potential of EPO, we investigated the underlying cellular mechanisms controlled by EPO that may determine both the maintenance of neuronal cellular integrity and the inhibition of microglial activation to gain greater insight for the development of future neurodegenerative therapeutic strategies.