The hemosteoblast: friend or foe?

Identification of circulating murine CD34+OCN+ cells

BACKGROUND AIMS

Previous studies identified a circulating human osteoblastic population that expressed osteocalcin (OCN), increased following fracture and pubertal growth, and formed mineralized colonies in vitro and bone in vivo. A subpopulation expressed CD34, a hematopoietic/endothelial marker. These findings led to our hypothesis that hematopoietic-derived CD34⁺OCN⁺ cells exist in the circulation of mice and are modulated after fracture.

METHODS

Flow cytometry was used to identify CD34⁺OCN⁺ cells in male B6.SJL-Ptprc^aPepc^b/BoyJ and Vav-Cre/mTmG (VavR) mice. Non-stabilized tibial fractures were created by three-point bend. Fractures were longitudinally imaged by micro-computed tomography, and immunofluorescent staining was used to evaluate CD34⁺OCN⁺ cells within fracture callus. AMD3100 (10 mg/kg) was injected subcutaneously for 3 days and the CD34⁺OCN⁺ population was evaluated by flow cytometry.

RESULTS

Circulating CD34⁺OCN⁺ cells were identified in mice and confirmed to be of hematopoietic origin (CD45⁺; Vav1⁺) using two mouse models. Both circulating and bone marrow-derived CD34⁺OCN⁺ cells peaked three weeks post-non-stabilized tibial fracture, suggesting association with cartilage callus transition to bone and early mineralization. Co-expression of CD34 and OCN in the fracture callus at two weeks post-fracture was observed. By three weeks, there was 2.1-fold increase in number of CD34⁺OCN⁺ cells, and these were observed throughout the fracture callus. AMD3100 altered CD34⁺OCN⁺ cell levels in peripheral blood and bone marrow.

DISCUSSION

Together, these data demonstrate a murine CD34⁺OCN⁺ circulating population that may be directly involved in fracture repair. Future studies will molecularly characterize CD34⁺OCN⁺ cells, determine mechanisms regulating their contribution, and examine if their number correlates with improved fracture healing outcomes.

Gadolinium contrast agent-induced CD163+ ferroportin+ osteogenic cells in nephrogenic systemic fibrosis

Gadolinium-based contrast agents are linked to nephrogenic systemic fibrosis in patients with renal insufficiency. The pathology of nephrogenic systemic fibrosis is characterized by abnormal tissue repair: fibrosis and ectopic ossification. The mechanisms by which gadolinium could induce fibrosis and ossification are not known. We examined in vitro the effect of a gadolinium-based contrast agent on human peripheral blood mononuclear cells for phenotype and function relevant to the pathology of nephrogenic systemic fibrosis using immunofluorescence, flow cytometry, real-time PCR, and osteogenic assays. We also examined tissues from patients with nephrogenic systemic fibrosis, using IHC to identify the presence of cells with phenotype induced by gadolinium. Gadolinium contrast induced differentiation of human peripheral blood mononuclear cells into a unique cellular phenotype--CD163(+) cells expressing proteins involved in fibrosis and bone formation. These cells express fibroblast growth factor (FGF)23, osteoblast transcription factors Runt-related transcription factor 2, and osterix, and show an osteogenic phenotype in in vitro assays. We show in vivo the presence of CD163(+)/procollagen-1(+)/osteocalcin(+) cells in the fibrotic and calcified tissues of nephrogenic systemic fibrosis patients. Gadolinium contrast-induced CD163(+)/ferroportin(+)/FGF23(+) cells with osteogenic potential may play a role in systemic fibrosis and ectopic ossification in nephrogenic systemic fibrosis.