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，其特征是骨量减少，骨折风险增加，几乎一半的女性和四分之一的60岁以上男性受到影响。骨质疏松性骨折与患者巨大的痛苦、更长的住院时间和高昂的经济负担有关，因为骨质疏松性患者的骨折愈合明显延迟。实验研究表明，骨质疏松症小鼠骨折早期血肿中炎性中性粒细胞增多，软骨内骨化过程延迟。因此，迫切需要新的治疗策略来促进骨质疏松的骨愈合。为此，需要对骨折愈合的生物学有更深入的了解。近年来，骨与感觉神经纤维和交感神经纤维的神经支配在骨折愈合过程中起着至关重要的作用。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发病机制中发挥作用。在这项建议的前期工作中，我们从非骨质疏松小鼠和骨质疏松小鼠的骨折骨痂组织的RNAseq分析中发现，肾上腺素能信号通路是最丰富的途径之一。此外，我们的结果表明，中性粒细胞募集到早期骨折血肿依赖于β-肾上腺素能信号，儿茶酚胺治疗减少了软骨细胞向成骨细胞的转分化。因此，该项目的主要假设是，肾上腺素能信号，主要是β2-肾上腺素受体信号，参与了炎性中性粒细胞向骨折血肿的募集，并在软骨内成骨过程中发挥关键作用。我们进一步假设，肾上腺素能信号参与了骨质疏松小鼠骨折愈合受损的发病机制。拟议的项目应该解决的未决问题是，在非骨质疏松症和骨质疏松症小鼠的骨折愈合过程中，全身和局部儿茶酚胺水平是如何变化的。我们将进一步确定哪个肾上腺素受体(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β2-AR信号的直接靶细胞。此外，我们还将分析特定的AR阻滞剂对软骨内骨形成的影响，以及在非骨质疏松性和骨质疏松性骨骨折愈合过程中，软骨细胞是否是β2-AR信号的直接靶细胞。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对建立骨折愈合并发症的新治疗方案具有临床意义。

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.