Altered inflammatory responses are associated with impaired bone and nerve healing in diabetes. However, the detailed mechanisms of these responses and their contribution to neuropathy and poor fracture healing remain unclear. We previously identified a cell population expressing factors associated with brown adipocytes (uncoupling proteins and the β3-adrenergic receptor (ADRβ3)), which is activated by neuroinflammation and contributes to de novo bone formation. Single cell RNA-sequencing revealed these cells are macrophages. In this study, we investigated the role of this newly identified macrophage population during bone formation in wild type and diabetic mice. De novo bone formation was induced by intramuscular delivery of cells producing bone morphogenetic protein 2. After four days, ADRβ3+ cells were isolated by FACS to evaluate expression of different macrophage markers and macrophage polarization by qPCR. These cells expressed a unique set of markers similar to CNS macrophages, including TMEM119. Polarization assays to induce either a pro-inflammatory (M1) or reparative (M2) macrophage phenotype revealed these cells possess plasticity but are distinct from bone marrow-derived macrophages and microglia. In vivo addition of LPS to promote M1 polarization during de novo bone formation led to a reduction in bone volume. When a unicortical fracture was introduced to the femur, ADRβ3+ macrophages were observed within the fracture callus after seven days, indicating a role for these cells in the reparative process. Analysis of de novo bone formation and fracture in C57BL/6- ob/ob mice showed reduced numbers of ADRβ3+ macrophages. Initial histological analysis of human nerves indicates expansion of these ADRβ3+ cells in type 2 diabetes. These data suggest alterations to a novel macrophage population with reparative potential in diabetes. Further characterization of these cells may provide new approaches to direct their action for desired bone and nerve repair.
E.A. Salisbury: None. J.A. Mejia: None. Z. Gugala: None. A.R. Davis: None. E. Olmsted-Davis: None.
National Institute of Arthritis and Musculoskeletal and Skin Diseases (R01R066556); U.S. Department of Defense (W81XWH-16-1-0649, W81XWH-17-1-0628)