Tomas E. Meijome,Jenna T. Baughman,R. Adam Hooker,Ying-Hua Cheng,Wendy A. Ciovacco,Sanjeev M. Balamohan,Trishya L. Srinivasan,Brahmananda R. Chitteti, Pierre P. Eleniste,Mark C. Horowitz,Edward F. Srour,Angela Bruzzaniti,Robyn K. Fuchs,Melissa A. Kacena
C-Mpl is the receptor for thrombopoietin (TPO), the main megakaryocyte (MK) growth factor, and c-Mpl is believed to be expressed on cells of the hematopoietic lineage. As MKs have been shown to enhance bone formation, it may be expected that mice in which c-Mpl was globally knocked out (c-Mpl-/- mice) would have decreased bone mass because they have fewer MKs. Instead, c-Mpl-/- mice have a higher bone mass than WT controls. Using c-Mpl-/- mice we investigated the basis for this discrepancy and discovered that c-Mpl is expressed on both osteoblasts (OBs) and osteoclasts (OCs), an unexpected finding that prompted us to examine further how c-Mpl regulates bone. Static and dynamic bone histomorphometry parameters suggest that c-Mpl deficiency results in a high bone turnover state with a net gain in bone volume. In vitro, a higher percentage of c-Mpl-/- OBs were in active phases of the cell cycle, leading to an increased number of OBs. No difference in OB differentiation was observed in vitro as examined by real-time PCR and functional assays. In co-culture systems, which allow for the interaction between OBs and OC progenitors, c-Mpl-/- OBs enhanced osteoclastogenesis. Two of the major signaling pathways by which OBs regulate osteoclastogenesis, MCSF/OPG/RANKL and EphrinB2-EphB2/B4, were unaffected in c-Mpl-/- OBs. These data provide new findings for the role of MKs and c-Mpl expression in bone and may provide insight into the homeostatic regulation of bone mass as well as bone loss diseases such as osteoporosis.