gms | German Medical Science

German Congress of Orthopedic and Trauma Surgery (DKOU 2017)

24.10. - 27.10.2017, Berlin

Complement receptors C5aR1 and C5aR2 differentially influence bone metabolism

Meeting Abstract

  • presenting/speaker Yvonne Hägele - Institute of Orthopaedic Research and Biomechanics, Ulm, Germany
  • Stephanie Bergdolt - Institute of Orthopaedic Research and Biomechanics, Ulm, Germany
  • Rebecca Matthes - Institute of Orthopaedic Research and Biomechanics, Ulm, Germany
  • Anna Kovtun - Institute of Orthopaedic Research and Biomechanics, Ulm, Germany
  • Anita Ignatius - Institute of Orthopaedic Research and Biomechanics, Ulm, Germany

Deutscher Kongress für Orthopädie und Unfallchirurgie (DKOU 2017). Berlin, 24.-27.10.2017. Düsseldorf: German Medical Science GMS Publishing House; 2017. DocGR14-359

doi: 10.3205/17dkou496, urn:nbn:de:0183-17dkou4968

Published: October 23, 2017

© 2017 Hägele et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License. See license information at http://creativecommons.org/licenses/by/4.0/.


Outline

Text

Objectives: Activation of the complement system, a crucial arm of innate immunity, leads to the generation of the anaphylatoxin C5a, acting via its receptors C5aR1 and C5aR2. C5aR1 has already been attributed an important role in bone development, regeneration and inflammation, as C5aR1 expression on bone cells is strongly upregulated during osteogenic differentiation and after bone injury. Moreover, it was shown that C5a modulates cytokine release from osteoblasts. However, data regarding the molecular mechanisms behind C5aR1 actions in bone are very limited and the role of C5aR2 in bone is completely unknown.

Methods: The bone phenotype of C5aR1-ko and C5aR2-ko mice aged 12 and 55 weeks was investigated by 3-point bending test, micro-CT and histomorphometry. Primary osteoblasts were isolated from long bones of 8-12-week-old wild type (WT), C5aR1-ko, C5aR2-ko and C5aR1-tg mice, which overexpress C5aR1 specifically in osteoblasts. We studied proliferation and differentiation potential of cells and intracellular signaling pathways upon C5a stimulation. Bone marrow of 8-12-week-old WT, C5aR1-ko and C5aR2-ko mice was used to study osteoclast formation. Statistics: Student's t-test or ANOVA, n=4-6; p<0.05.

Results and Conclusion: The skeleton of 12-week-old C5aR1-ko mice showed increased bone mass, evident by a higher tissue mineral density (TMD) of both cortical (p<0.0001) and trabecular bone (p=0.01) and an increase in bone mineral density (BMD) (p<0.0001). Moreover, femora had a higher bending stiffness (p=0.004). Aged mice displayed a normal bone phenotype, however, at both ages we found significantly decreased numbers of osteoclasts (p<0.0001; p=0.003). C5aR2-ko mice at 12 weeks of age showed increased BMD (p=0.0004), which was still elevated at 55 weeks of age (p=0.01). Aged mice also showed an increased bone formation rate (p=0.0003) and higher numbers of trabeculae (p=0.049). Osteoblast proliferation and differentiation in vitro was not affected by neither the lack of C5aR1 or C5aR2, nor C5aR1 overexpression. In WT osteoblasts and even more prominent in C5aR-tg osteoblasts, C5a induced phosphorylation of MAPK family members ERK1/2 and p38 and Akt, a member of the PI3K pathway. This effect was strongly diminished in C5aR1-ko and C5aR2-ko osteoblasts. In vitro, osteoclast formation from C5aR1-ko monocytes appeared to be impaired.

Our data demonstrate that the lack of the C5a receptors affects bone at different ages. C5aR1-ko mice showed a high bone mass phenotype at young age only, while in C5aR2-ko mice this phenotype was milder at young age, but persisted until 55 weeks of age. No cell autonomous defects were detected, when culturing C5aR-modified osteoblasts in vitro. Similar to the effect of C5a on immune cells, we could show that C5aR signaling in osteoblasts involves MAPK and PI3K pathways. Osteoclast numbers in C5aR1-ko mice were decreased in vivo and in vitro, accounting for the high bone mass. These data show that complement receptors crucially regulate bone metabolism.