C‐Mpl Is Expressed on Osteoblasts and Osteoclasts and Is Important in Regulating Skeletal Homeostasis

Abstract

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.

Lnk Deficiency Leads to TPO-Mediated Osteoclastogenesis and Increased Bone Mass Phenotype

“…The significance of these findings lends recognition of the difficulties of interpreting results of cell lineages studied in vitro, isolated from their natural tissue environment. Figure is a schematic representation illustrating the complexity of the cell‐cell interactions as described in our present findings and previously published data [Meijome et al, ]. The BM cavity contains MK, OB, OC, and TPO.…”

“…Seeded at optimal pre‐tested conditions (2 × 10 4 cells/ml), OB were maintained in αMEM supplemented with 10% fetal bovine serum (FBS), ascorbic acid (50 µg/ml added on day 0, and at all feedings), and β‐glycerophosphate (5 mM added starting on day 7, and all subsequent feedings). Cells were replenished twice per week, and some cultures were administered TPO (100 ng/ml) to stimulate MK as described [Meijome et al, ].…”

Megakaryocytes promote osteoclastogenesis in aging

“…In addition to MKs, Mpl is expressed on several other cell types within the bone marrow. For example, our previous findings confirmed that OC progenitors from 6-8 week-old C57BL/6J mice express Mpl, and robustly respond to recombinant TPO, resulting in an increase in OC formation [ 27 , 28 ]. Therefore, we examined if osteoclastogenesis associated with aging is affected by differences in Mpl-TPO signaling.…”

Regulation of bone metabolism by megakaryocytes in a paracrine manner

“…We cannot be sure why an inconsistency was observed between our results and those of previous studies, but one possibility is contamination of TPO in the previous studies, in which cells from mouse liver were differentiated into MK with TPO. In real, it was previously reported that depleting C-Mpl, the receptor for TPO, increases osteoblast proliferation 26 , suggesting that activation of TPO downstream signaling may suppress osteoblast proliferation. In contrast, we differentiated K562 cells into MKs with PMA but without TPO, and further demonstrated that PMA itself did not affect osteoblast viability.…”

“…In the present study, human K562 cells were primarily used, as the following reasons. First, it was reported that TPO itself can affect bone cell biology 26 , thus we concerned any confusion to interpret our results. Second, this study is a preceding one to find a human MK-secreting factor.…”