Expression pattern of the thrombopoietin receptor (TpoR) in the developing and adult central nervous system

Background: Thrombopoietin (Tpo) is the primary regulator of megakaryopoiesis and platelet production. As other hematopoietic growth factors like erythropoietin (Epo), Tpo is also expressed in the central nervous system (CNS). Although initial studies suggested a neurotrophic and/or neuroprotective function of Tpo in the CNS, recent data indicate its proapoptotic effect on neuronal cells. We have reported increased Tpo concentrations in the cerebrospinal fluid in about 40% of neonates with sepsis/meningitis or posthemorrhagic hydrocephalus. To understand a role of Tpo in the CNS, the expression pattern of its receptor is of particular interest, but current data are available only for Tpo receptor (TpoR) mRNA expression in the human adult brain. The aim of our study was to identify and characterize TpoR expressing cells in the murine CNS. Methods: TpoR expression has been analysed by immunohistochemistry (hTpoR, R&D Systems) in the murine CNS at embryonic day e12.5, e15.5, e18.5, at birth (p0), p4, p8 and in the adult. We performed single or double labelling with Tpo (monoclonal anti-human Tpo, Kirin), NeuroD (N19, Santa-Cruz), GFAP (DAKO), and BrdU (Roche), using DAPI for counterstaining. To detect dividing cells, we injected pregnant dams or pups with BrdU (200mg/kg body weight, Roche) and sacrificed the animals 3 hours later. Results: At e12-e15 hTpoR antibody labelled meninges and some cells scattered among the neuroepithelium. Later in gestation, TpoR was detected in the medulla, hypothalamus, ventro-caudal cortex and the olfactory bulb. In the neonatal CNS, TpoR has been detected in the subventricular zone (SVZ) of the lateral ventricles, cortical plate, ventral striatum, hippocampus and cerebellar white matter. The perinatal increase of TpoR expression by resident cells of the brain was followed by a decline. At p8 TpoR was detected only in the SVZ of the lateral ventricles and in the hippocampus. In the adult brain TpoR was found only in a single cell cluster in dorsal thalamus. Although the expression pattern suggested possible TpoR expression in immature granular cells, there was no overlap with NeuroD. Some TpoR-positive cells in the dorsal thalamus, medulla and rostral migratory stream were double-labelled with astrocytic marker GFAP. Since none of TpoR+ cells incorporated BrdU within 3 hours, they are rather postmitotic than self-renewing. Conclusions: TpoR has a distinct expression pattern in the murine CNS. We have indications that TpoR is expressed in neurons and astrocytes. A strong up-regulation of TpoR between day e18 and p4 suggests an active involvement of the Tpo/TpoR system in perinatal brain development.