Erythropoietin and its cognate receptor on bone marrow B cells play a role in the regulation of bone mass in mice

Albert Kolomansky ^1,3 Naamit Deshet-Unger ¹ Nathalie Ben-Califa ¹ Zamzam Aiwida ¹ Maria Ibrahim ¹ Tamar Liron ² Sahar Hiram-Bab ² Howard S Oster ³ Moshe Mittelman ³ Martina Rauner ⁴ Ben Wielockx ⁵ Yankel Gabet ² Drorit Neumann ¹

¹Faculty of Medicine, Department of Cell and Developmental Biology, Tel Aviv University, Israel
²Faculty of Medicine, Department of Anatomy and Anthropology, Tel Aviv University, Israel
³Department of Medicine A, Sourasky Medical Center, Israel
⁴Department of Medicine III, Technische Universität Dresden, Germany
⁵Institute for Clinical Chemistry and Laboratory Medicine, Technische Universität Dresden, Germany

It has been previously demonstrated that EPO induces substantial trabecular bone loss. Here we address the role of B cell-specific EPO-R in EPO-induced bone loss in mice and the occurrence of B cell-derived osteoclastogenesis in vivo, an issue of critical importance in the field of osteohematology.

Using female MB1-Cre;EPO-R^fl/fl mice, we found that B cell-specific EPO-R knockdown (cKD) is associated with increased bone mass phenotype in adult (11 week) female mice. Moreover, cKD attenuated the profound EPO-induced trabecular bone loss in the proximal part of the femoral distal metaphysis. Remarkably, this effect was observed despite the fact that cKD mice attained higher hemoglobin levels following EPO treatment. An EPO-induced increase in CD115⁺ (MCSF-R⁺) Pro-B cells was observed in EPO-treated control mice but was absent in mice harboring B cell-specific EPO-R knockdown. Supporting the osteoclastic potential of this specific B cell subpopulation is the fact that most of the CD115⁺ Pro-B cells also express β3 integrin (CD61), which is essential for osteoclast differentiation and function.

Using the MB1-Cre;R26R-EYFP murine model for B cell lineage tracing, we could demonstrate that some of the TRAP⁺/ β3 integrin⁺ bone lining cells were also positive for eYFP. This demonstrates the B cell origin of some of the osteoclasts in vivo.

Our work highlights an important extra-erythropoietic target of EPO-EPO-R signaling that regulates bone homeostasis. Importantly, we present here, for the first time, histological evidence for B cell-derived osteoclastogenesis in vivo, thus opening novel research avenues.