A hematopoietic growth factor, thrombopoietin, has a proapoptotic role in the brain

Abstract

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...

Widespread Expression of Erythropoietin Receptor in Brain and Its Induction by Injury

“…In addition, EPO and EPOR mRNA are expressed in brain tissue (21), and specific binding sites for EPO in the brain have been demonstrated in mouse and humans by means of radiolabeled EPO (22)(23). In cell culture, mRNA expression combined with functional assays, for example, altered phosphorylation of second messenger pathways induced by EPO in microglia, served to prove specific EPOR expression in the absence of reliable EPOR-AB (24).…”

Thrombopoietin Contributes to Neuronal Damage in Experimental Bacterial Meningitis

“…Proapoptotic effects of Tpo on neurons in vivo were first reported in a rat model of hypoxic brain ischemia (12). Under the conditions used in that study, newly formed neurons in the dentate gyrus were especially vulnerable to the effects of endogenous and recombinant Tpo.…”

“…Under the conditions used in that study, newly formed neurons in the dentate gyrus were especially vulnerable to the effects of endogenous and recombinant Tpo. Thus, a physiological role of the Tpo/c-Mpl system was postulated by which excess neurons are removed from the maturing brain (12). In bacterial meningitis, net loss of neurons and atrophy of the hippocampus may in part result from negative effects on neurogenesis (16), which could also involve Tpo signaling.…”

“…Tpo and c-Mpl are widely transcribed in the developing human and rodent brain (11,12,20,26). However, the exact cellular sources of Tpo in the brain have not been established in vivo.…”

“…Importantly, both Tpo expression and cMpl expression have also been detected in nonhematopoietic tissues, including the brain (9,11,20,26). Proaptotic effects of Tpo on neurons were reported in the context of hypoxia and ischemia (12), suggesting a neurotoxic effect of Tpo in brain diseases.…”