Megakaryocytes promote osteoclastogenesis in aging

Abstract

Megakaryocytes (MKs) support bone formation by stimulating osteoblasts (OBs) and inhibiting osteoclasts (OCs). Aging results in higher bone resorption, leading to bone loss. Whereas previous studies showed the effects of aging on MK-mediated bone formation, the effects of aging on MK-mediated OC formation is poorly understood. Here we examined the effect of thrombopoietin (TPO) and MK-derived conditioned media (CM) from young (3-4 months) and aged (22-25 months) mice on OC precursors. Our findings showed that aging significantly increased OC formation in vitro. Moreover, the expression of the TPO receptor, Mpl, and circulating TPO levels were elevated in the bone marrow cavity. We previously showed that MKs from young mice secrete factors that inhibit OC differentiation. However, rather than inhibiting OC development, we found that MKs from aged mice promote OC formation. Interestingly, these age-related changes in MK functionality were only observed using female MKs, potentially implicating the sex steroid, estrogen, in signaling. Further, RANKL expression was highly elevated in aged MKs suggesting MK-derived RANKL signaling may promote osteoclastogenesis in aging. Taken together, these data suggest that modulation in TPO-Mpl expression in bone marrow and age-related changes in the MK secretome promote osteoclastogenesis to impact skeletal aging.

Neonatal Osteomacs and Bone Marrow Macrophages Differ in Phenotypic Marker Expression and Function

“…The TPO concentration was based on our previous studies demonstrating increased osteoclast formation in BM Mφ. ( 34,35 ) In agreement with our published report, ( 34 ) TPO led to an increase in osteoclast formation by neonatal BM Mφ at two different plating densities (Fig. 5 B , C ).…”

“…( 31,35 ) We also reported that unsorted macrophages from bone marrow stimulated with TPO form more osteoclasts than controls. ( 34,35 ) In the current study, we demonstrated that although neonatal OM express higher levels of CD110 than neonatal BM Mφ (Figs. 1 and 2D and Supplemental Table S3), OM did not undergo TPO‐stimulated increase in osteoclast formation, which was observed in the BM Mφ cultures (Fig.…”

“…We previously showed that thrombopoietin (TPO), which is the ligand for CD110 (c‐Mpl), stimulates osteoclast formation of BM Mφ. ( 34,35 ) However, the effect of TPO on OM function has not been explored. Given CD110 expression was signficantly higher in neonatal OM versus neonatal BM Mφ (Figs.…”

S100 Calcium-Binding Protein P Secreted from Megakaryocytes Promotes Osteoclast Maturation

“…K562 cells were differentiated into mature MKs in phorbol 12-myristate 13-acetate (PMA)-supplemented medium for 7 days [ 26 ]; differentiation was determined by morphological changes, such as an increased nuclear/cytoplasmic ratio, large and multilobulated nuclei, and strong adhesion. Based on several reports that MKs may indirectly contribute to skeletal homeostasis [ 12 , 27 ], we investigated the effects of MK CM on osteoclast differentiation. Using tartrate-resistant acid phosphatase (TRAP) staining, TRAP-positive osteoclasts can be distinguished from TRAP-negative MKs [ 28 ].…”

“…With aging, the number of MKs increases, and the direct effect of MKs on promoting bone formation in osteoblasts is suppressed [ 34 ]. Increased MKs in aged mice activate RANKL expression and signaling, thereby promoting osteoclastogenesis and bone resorption activity [ 27 ]. Another study showed that MK CM regulates osteoblast and osteoclast differentiation and increases bone mass in ovariectomized mouse models, although the MK-secreted factors in MK CM have not been identified [ 12 ].…”