gms | German Medical Science

Objectives: It is known that catecholamines have a strong influence on bone as catecholamine-releasing tumors promote the development of osteoporosis and fractures. Besides adrenal and sympathetic catecholamine synthesis, myeloid cells have also been shown to produce catecholamines. However, it is unknown if myeloid cell-derived catecholamines influence bone and fracture healing. Therefore, the aim of this study was to investigate the effects of a specific knockout (KO) of tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis, in myeloid cells on bone growth, bone metabolism and fracture healing in mice. Additionally, β₂-adrenoceptor signaling in chondrocytes was investigated as one possible pathway through which catecholamines might influence bone.

Methods: Male TH^flox/flox/CD11b-Cre mice, which lack TH in CD11b⁺ myeloid cells (TH-KO), and TH^flox/flox control mice were used. Harvested intact femurs underwent bone length measurement, micro-computed tomography (μCT) and histological analysis. Another subset of animals received a standardized femur osteotomy stabilized by an external fixator. After 3 and 21 days, mice were sacrificed and the fracture calli were assessed by immunohistochemistry and μCT. Additionally, intact femurs of male Adrb2^flox/flox/Col2a1-Cre mice with a specific KO of the β₂-adrenoceptor in chondrocytes (Adrb2-KO) and Adrb2^flox/flox control mice were analyzed by bone length measurement, μCT and histology. Statistical analysis: Student’s t-test. p<0.05. n=6-8.

Results and conclusion: TH-KO mice displayed shorter femurs (15.8 vs 16.2 mm, p=0.003) compared to control mice, whereas growth plate thickness was unaltered. μCT analysis revealed a reduced trabecular tissue mineral density (698 vs 741 mgHA/cm³, p=0.001) and a decreased relative bone volume (17 vs 23%, p=0.0001) in TH-KO mice. Besides, number (-27%, p=0.04) and active surface (-31%, p=0.03) of osteoblasts were reduced. TH-KO mice, which underwent femur osteotomy, showed an increased recruitment of macrophages (2.8-fold, p=0.001) and T cells (3.9-fold, p=0.004) to the fracture hematoma on day 3 after fracture. On day 21, fracture calli of TH-KO mice displayed a reduced relative bone volume (20 vs 25%, p=0.03) and a decreased tissue mineral density (848 vs 876 mgHA/cm³, p=0.047). Adrb2-KO mice displayed a reduced femur length (14.6 vs 15.7 mm, p<0.0001), while growth plate thickness, tissue mineralization, relative bone volume and osteoblasts were unaffected.

We could show that catecholamines released by myeloid cells influence bone growth, bone metabolism and fracture healing, possibly by changes in the immune cell-bone cell interactions. A balanced concentration of myeloid cell-derived catecholamines seems to be essential for bone health. Besides, we identified the β₂-adrenoceptor in chondrocytes as mediator of catecholaminergic effects on bone growth. Further research is necessary to unravel the receptors involved in the influence of catecholamines on bone metabolism and fracture healing.