Erythropoietin (EPO) is a candidate compound for neuroprotection in human brain disease capable of combating a spectrum of pathophysiological processes operational during the progression of schizophrenic psychosis. The purpose of the present study was to prepare the ground for its application in a first neuroprotective add-on strategy in schizophrenia, aiming at improvement of cognitive brain function as well as prevention/slowing of degenerative processes. Using rodent studies, primary hippocampal neurons in culture, immunohistochemical analysis of human post-mortem brain tissue and nuclear imaging technology in man, we demonstrate that: (1) peripherally applied recombinant human (rh) EPO penetrates into the brain efficiently both in rat and humans, (2) rhEPO is enriched intracranially in healthy men and more distinctly in schizophrenic patients, (3) EPO receptors are densely expressed in hippocampus and cortex of schizophrenic subjects but distinctly less in controls, (4) rhEPO attenuates the haloperidol-induced neuronal death in vitro, and (4) peripherally administered rhEPO enhances cognitive functioning in mice in the context of an aversion task involving cortical and subcortical pathways presumably affected in schizophrenia. These observations, together with the known safety of rhEPO, render it an interesting compound for neuroprotective add-on strategies in schizophrenia and other human diseases characterized by a progressive decline in cognitive performance.

Central nervous and hematopoietic systems share developmental features. We report that thrombopoietin (TPO), a stimulator of platelet formation, acts in the brain as a counterpart of erythropoietin (EPO), a hematopoietic growth factor with neuroprotective properties. TPO is most prominent in postnatal brain, whereas EPO is abundant in embryonic brain and decreases postnatally. Upon hypoxia, EPO and its receptor are rapidly reexpressed, whereas neuronal TPO and its receptor are down-regulated. Unexpectedly, TPO is strongly proapoptotic in the brain, causing death of newly generated neurons through the Ras-extracellular signal-regulated kinase 1͞2 pathway. This effect is not only inhibited by EPO but also by neurotrophins. We suggest that the proapoptotic function of TPO helps to select for neurons that have acquired targetderived neurotrophic support.astrocytes ͉ erythropoietin ͉ neurons ͉ differentiation ͉ development I n the hematopoietic system, survival, proliferation, and differentiation of cells are regulated by a plethora of growth factors (1-4). The effect of erythropoietin (EPO) on the generation of red blood cells is well known. The hematopoietic growth factor thrombopoietin (TPO) stimulates megakaryopoiesis and thrombocyte formation (1-3, 5, 6). During hematopoiesis, EPO and TPO can interact in a synergistic and an antagonistic fashion (1-3, 7).EPO and TPO exhibit significant homology in their receptorbinding domain (20% identity and 25% similarity). Likewise, they bind to receptors, erythropoietin receptor (EPOR) and thrombopoietin receptor (TPOR), respectively, that belong to the same cytokine receptor superfamily (1)(2)(3)(8)(9)(10). Previous studies reported a neurotrophin-like motif in the N-terminal receptor binding region of the TPO molecule, with conflicting data about the presence of TPO in the brain (5,6,(11)(12)(13).For EPO, it is well established that the gene is expressed in the embryonic CNS. EPO has a marked effect as a survival factor for neurons and their progenitors (14, 15), presumably to overcome phases of physiological hypoxia (16, 17). The widespread but rather ''unspecific'' neuroprotective potential of EPO is regained in the adult CNS upon distress or injury. This finding has been confirmed in rodent models of cerebral ischemia (18-23), brain trauma (18), and neurodegenerative disease (18), as well as in a clinical study with stroke patients (24).Here we show that TPO plays a previously unrecognized proapoptotic role in the brain. Materials and MethodsAll experiments were approved by and conducted in accordance with the regulations of the local Animal Care and Use Committee. For detailed information on all methods see Supporting Materials and Methods, which is published as supporting information on the PNAS web site.Cell Culture. Primary hippocampal neuronal cultures were prepared from newborn Wistar-Imamichi rats, cultured under serum-free conditions (25, 26), and used for experiments after five days (purity: Ͼ95% neurons). Neuronal cell number and viability was assessed by try...

Correct timing and spatial location of growth factor expression is critical for undisturbed brain development and functioning. In terminally differentiated cells distinct biological responses to growth factors may depend on cell type specific activation of signalling cascades. We show that the hematopoietic growth factors thrombopoietin (TPO) and granulocyte colony-stimulating factor (GCSF) exert cell type specific effects on survival, proliferation and the degree of phosphorylation of Akt1, ERK1/2 and STAT3 in rat hippocampal neurons and cortical astrocytes. In neurons, TPO induced cell death and selectively activated ERK1/2. GCSF protected neurons from TPO- and hypoxia-induced cell death via selective activation of Akt1. In astrocytes, neither TPO nor GCSF had any effect on cell viability but inhibited proliferation. This effect was accompanied by activation of ERK1/2 and inhibition of STAT3 activity. A balance between growth factors, their receptors and signalling proteins may play an important role in regulation of neural cell survival.

Endothelin (ETB)-receptors mediate anti-apoptotic actions. Lack of functional ETB-receptors leads to increased neuronal apoptosis in the hippocampus. The increased apoptosis must be compensated by other mechanisms, however, as ETB-deficient rats display normal overall brain morphology. To illuminate on brain plasticity in ETB-receptor deficiency, we studied the expression and function of another neuroprotective system, the cannabinoid CB1-receptors, in ETB-deficient hippocampus. We show that CB1 expression in hippocampus increases postnatally in all rats but that the increase in CB1-receptor expression is significantly higher in ETB-deficient compared to wildtype littermates. Neuronal apoptosis decreases during brain maturation but remains on a significantly higher level in the ETB-deficient compared to wildtype dentate. When investigating survival of hippocampal neurons in culture, we found significant protection against hypoxia-induced cell death with CB1-analogs (noladin, (9-tetrahydrocannabinol) only in ETB-deficient neurons. We suggest that CB1-receptor upregulation in the ETB-mutant hippocampus reflects an attempt to compensate for the lack of ETB-receptors.