Role of adrenergic signaling in fracture healing of non-osteoporotic and osteoporotic bone

骨质疏松症是最常见的代谢性骨骼疾病，其特征是骨量低和骨折风险增加，几乎一半的女性和四分之一的60岁以上的男性受到影响。 由于骨质疏松性骨折患者骨折愈合明显延迟，因此骨质疏松性骨折给患者带来巨大痛苦、住院时间长和经济负担高。实验研究表明，在早期骨折血肿中炎性中性粒细胞的存在增加，并且在骨折小鼠中延迟了软骨内骨化过程。因此，迫切需要新的治疗策略来改善骨质疏松性骨愈合。为此，需要更深入地了解骨折愈合的生物学。近年来，骨的感觉神经和交感神经纤维支配在骨折愈合过程中起着至关重要的作用。也有越来越多的证据表明，局部儿茶酚胺信号，特别是肾上腺素能信号，可能在骨质疏松症的病理机制中发挥作用。在该提议的初步工作中，我们将肾上腺素能信号传导确定为来自非骨化小鼠与骨化小鼠的骨折骨痂组织的RNASeq分析中最丰富的途径之一。此外，我们的研究结果表明，中性粒细胞招聘到早期骨折血肿是依赖于β-肾上腺素能信号和软骨细胞到成骨细胞的转分化减少了儿茶酚胺治疗。因此，该项目的主要假设是肾上腺素能信号传导，主要是β 2-肾上腺素受体信号传导，参与了炎性中性粒细胞向骨折血肿的募集，并在软骨内骨化过程中起着至关重要的作用。我们进一步假设肾上腺素能信号传导参与了骨质疏松小鼠骨折愈合受损的发病机制。开放的问题，应解决的拟议项目是如何全身和局部的儿茶酚胺水平变化后，骨折事件和骨折愈合过程中，在非骨质疏松和骨质疏松小鼠。我们将进一步确定哪些肾上腺素受体（AR）可能在骨折后免疫细胞的募集和激活过程中发挥最重要的作用，以及中性粒细胞是否是β 2-AR信号传导的直接靶细胞。此外，我们将分析特定的AR阻断剂对软骨内骨形成的影响，以及在非骨质疏松和骨质疏松的骨折愈合过程中，软骨细胞是否是β 2-AR信号传导的直接靶细胞。在分子水平上，我们将确定骨愈合过程中肾上腺素能信号的下游通路。拟议项目的结果可能与临床相关，以建立骨折愈合并发症的新治疗方案。

Osteoporosis is the most common metabolic bone disease characterized by low bone mass and increased fracture risk with almost half of all women and a quarter of all man above the age of 60 affected. Osteoporotic fractures are associated with immense suffering for the patient, longer hospitalization and high economic burden, because fracture healing is significantly delayed in osteoporotic patients. Experimental studies demonstrated an increased presence of inflammatory neutrophils in the early fracture hematoma and a delayed endochondral ossification process in osteoporotic mice. Therefore, there is the urgent need for new therapeutic strategies to improve osteoporotic bone healing. For that, a deeper understanding of the biology of fracture healing is required. During recent years, it became evident that the innervation of bone with sensory and sympathetic nerve fibers plays a crucial role during fracture healing. There is also growing evidence that local catecholamine signaling, especially adrenergic signaling, might play a role in the pathomechanisms of osteoporosis. In preliminary work for this proposal, we identified adrenergic signaling as one of the most enriched pathways in RNASeq analysis from fracture callus tissue of non-osteoporotic versus osteoporotic mice. Further, our results suggest that neutrophil recruitment to the early fracture hematoma is dependent on beta-adrenergic signaling and that chondrocyte-to-osteoblast transdifferentiation is diminished by catecholamine treatment. Therefore, the main hypothesis of the proposed project is that adrenergic signaling, mainly beta2-adrenoreceptor signaling, is involved in recruitment of inflammatory neutrophils to the fracture hematoma and plays a crucial role during the endochondral ossification process. We further hypothesize that adrenergic signaling is involved in the pathogenesis of compromised fracture healing in osteoporotic mice. Open questions which should be addressed by the proposed project are how systemic and local catecholamine levels change after a fracture event and during the course of fracture healing in non-osteoporotic and osteoporotic mice. We will further determine which adrenoreceptor (AR) might play the most important role during recruitment and activation of immune cells after fracture and if neutrophils are direct target cells of beta2-AR signaling. Additionally, we will analyze the influence of specific AR blockers on endochondral bone formation and if chondrocytes are direct target cells of beta2-AR signaling during fracture healing in non-osteoporotic and osteoporotic bone. On the molecular level, we will identify downstream pathways of adrenergic signaling during bone healing. The results of the proposed project might be clinically relevant to establish new treatment options for fracture healing complications.