The Role And Mechanism Of Norepinephrine On Thrombocytopoiesis In Mice With Radiation-induced Injury

Nuclear energy has been widely utilized in the fields of military,industry,agriculture,medical equipments,etc,and that leads to generate increasingly risk of ionizing radiation-induced injury.The ionizing radiation could damage the DNA structure,interrupt gene expression and transcription,and change the intercellular signal transduction pathways.The bone marrow is one of the main target organs.Most of the hematopoietic stem cells are quiescent in steady state,especially for long-term hematopoietic stem cells,which are essentially dormant in the bone marrow.They are,therefore,not sensitive to ionizing radiation,which is the main reason for the hematopoietic restoration after radiation damage.However,hematopoietic progenitor cells and immature hematopoietic cells with high proliferation activity are sensitive to radiation,and the hematopoiesis in bone marrow could be heavily disrupted which has been exposed to more than a certain dose of ionizing radiation.The dysfunction of megakaryocytopoiesis is the primary pathological manifestation.As well known that,platelets are generated by megakaryocytes,which are large polyploidy hematopoietic cells that reside primarily in the bone marrow.Like all mature hematopoietic cells,megakaryocytes are derived from hematopoietic stem cells.The number of platelets is seriously reduced after being exposed to large dose of ionizing radiation,that damaged the hematopoietic progenitors and megakaryocytes in the bone marrow.Thrombocytopenia is one of the main manifestations of acute radiation injury,and it will bring about some severe consequences,such as hemorrhage,infection,and the hemorrhage of important organs could be life-threatening.Therefore,it is important to research the effects and mechanisms of various factors on thrombocytopenia induced by irradiation.Previous findings have shown that the level of norepinephrine increased after physiological or pathological stimulus that leaded to generate and activate more platelets.It is demonstrated that NE is the primary transmitter of the sympathetic nervous system(SNS),which is synthesized and secreted in sympathetic postganglionic neurons and brain adrenergic nerve endings.It is noteworthy that many researches have revealed that the bone marrow is innervated by sympathetic nerves that it is an important component of the hematopoietic stem cell microenvironment.The SNS plays an vital role in regulating the mobilization,migration,homing of hematopoietic stem cells.However,the role and mechanism of norepinephrine on thrombopoiesis in mice with ionizing radiation induced injury are unclear.On the basis of previous findings,here we experimented on M07 e,Sca1+ cells,CD34+ cells in vitro,C57 BL / 6 mice in vivo.The main results and conclusions were in the following:1.Norepinephrine,which was administered on C57BL/6 mice before ionizing radiation,had detrimental effects that it aggravated acute radiation injury:⑴ We observed the dynamic change of peripheral blood platelets level when C57BL/6 mice were administrated NE 24 hours before irradiation.As results,the peripheral blood platelet level in mice subjected to pretreatment of NE was lower than those exposed to irradiation only at day 7~25 after irradiation.⑵ Proportion of mice HSCs as measured by flow cytometry.It showed that NE induced HSCs proliferation.Compared with irradiation only,the proportion of HSCs,however,decreased seriously in mice of pretreatment with NE,and the level of HSCs failed to back to normality till day 30.It implied that NE treatment in vivo increased sensitivity of irradiation by promoting proliferative capacity of HSCs.⑶ HE staining of bone marrow showed that the number of nucleated cells including megakaryocytes increased markedly in NE treatment mice.Nevertheless,the number of bone marrow nucleated cells including megakaryocytes decreased seriously in mice of pretreatment with NE compared with those exposed to irradiation alone.Besides,rarefaction and vacuoles of the bone marrow structure,severe hyperaemia and hemorrhage could be seen.In addition,the apoptotic rate was higher in megakaryocytic progenitors M07 e of pretreatment with NE compared with those exposed to irradiation only.Those findings above suggested that pretreatment with NE in vivo or in vitro before irradiation increased sensitivity of irradiation in megakaryocytes.2.NE administration post irradiation could obviously promote the recovery of peripheral blood platelets level.⑴ We observed the dynamic change of peripheral blood platelets level in C57BL/6 mice with severe thrombocytopenia induced by irradiation at the same dose.NE was then administered for 7 days starting on the 10 th day when the platelets level arrived at the nadir.As a result,the number of peripheral blood platelets after NE treatment significantly increased compared with those exposed to irradiation only.It indicated that NE treatment in vivo significantly increased the recovery of thrombocytopenia in mice.⑵ Transwell and HE staining showed that NE could promote the adhesion and migration of megakaryocytes.It implied that NE could accelerate the recovery of platelets by promoting the adhesion and migration of the regenerative megakaryocytes.In summary,it was the first time to revealed that NE administration before or after irradiation could generate completely different effects.On one side,NE increased radiosensitivity by triggering HSCs and megakaryocytes into cell cycle or shortening their cell cycles before irradiation and caused more severe radiation-induced injury with more severe apoptosis of HSCs and megakaryocyes.On the other side,NE administration after irradiation accelerated the recovery of peripheral platelet level by promoting proliferation of HSCs and adhesion and migration of megakaryocytes.These findings could provide a reference for optimizing clinical treatments of ionizing irradiation induced injury.