Traumatic Brain Injury Stimulates Sympathetic Tone-mediated Bone Marrow Myelopoiesis To Favor Fracture Healing

Objective: High energy fractures are always associated with traumatic brain injury(TBI).It has been observed that the fracture healing rate is significantly accelerated in patients with TBI.Previous studies have shown that TBI leads to increased callus formation with higher mineral density than normal bone healing.However,the underlying mechanism remains largely unknown.The central nervous system(CNS)plays a key role in regulating the immune system and bones.Long-range interactions between CNS and immune system permit the nervous system to regulate immune cells in bone marrow.Notably,sympathetic nerves penetrate the bone marrow tightly and precisely regulate skeletal and hematopoietic homeostasis.Skeletal interoception maintains bone homeostasis and differentiation of bone marrow cells by regulating sympathetic nervous system.The sympathetic nerves orchestrate the mobilization and egress of HSCs through adrenergic signaling in the stem cell niche.However,the effect of TBI on hematopoietic function has been neglected for a long time.The purpose of this study was to explore whether TBI could accelerate fracture healing by regulating hematopoiesis through sympathetic nerve.Methods: Patients with TBI and extremities fracture were selected,and the serum was obtained by venous blood drawing on the first day after TBI to examine the level of noradrenaline.In order to explore whether TBI promote fracture healing by regulating sympathetic nerve activity,chemical sympathectomy was introdeced with 6-hydroxydopamine(6OHDA).The fracture mice were divided into four groups: sham operation group,TBI group,6OHDA group and TBI+6OHDA group.In addition,Adrb2 and Adrb3 gene knockout mice were constructed.Placebo,norepinephrine,clenbuterol(β2-adrenergic agonist)or BRL37344(β3-adrenergic agonists)were used in the treatment of fracture.Brain tissues were collected to detect the expression of tyrosine hydroxylase in central nervous system,serum and bone marrow interstitial fluid were collected to detect the level of noradrenaline,callus tissues were collected to detect the fracture healing rate,and bone marrow cells were collected to analyze the differentiation of hematopoietic stem cells and their progeny cells.Results: We found that TBI significantly increases the sympathetic tone,and the serum norepinephrine level in TBI patients and TBI mice was elevated.The expression level of tyrosine hydroxylase in hypothalamic paraventricular nucleus of TBI mice increased,and the fracture healing rate was accelerated in TBI group;while chemical sympathectomy blocked TBI-accelerated fracture healing.Importantly,the hemopoietic progenitor cells of TBI mice were expanded,and the expanded hemopoietic progenitor cells differentiate to anti-inflammatory myeloid immune cells.Chemical sympathectomy hindered TBI-induced acute hematopoietic mobilization and myeloid bias.Noradrenaline simulates the hematopoiesis bias caused by TBI.Knockout β3-or β2-adrenergic receptor(ARs)eliminate TBI-mediated expansion of anti-inflammatory macrophages and accelerated-fracture healing.β3-and β2-ARs agonists synergistically promote the infiltration of M2 macrophages in callus tissue and accelerate the process of bone healing.Conclusion: The injured CNS elevates sympathetic tone to mobilize HSCs lineage cells and to promotes myeloid-bias.These expanded myeloid cells suppress the inflammation in callus to accelerate fracture healing.Recognition of this mechanism will contribute to developing fracture healing therapies by harnessing beneficial hematopoiesis.The sympathetic nervous system is a potential target and can be used to treat fractures.