A novel role for thrombopoietin in regulating osteoclast development in humans and mice

Megakaryocyte-driven changes in bone health: lessons from mouse models of myelofibrosis and related disorders

Over the years, numerous studies demonstrated reciprocal communications between processes of bone marrow hematopoiesis and bone remodeling. Megakaryocytes, rare bone marrow cells responsible for platelet production, were demonstrated to be involved in bone homeostasis. Myelofibrosis, characterized by an increase in pleomorphic megakaryocytes in the bone marrow, commonly leads to the development of osteosclerosis. In vivo, an increase in megakaryocyte number was shown to result in osteosclerosis in GATA-1^low, NF-E2^-/-, TPO^high, Mpll^f/f;PF4cre, Lnk^-/-, Mpig6b^-/-, Mpig6b^fl/fl;Gp1ba-Cr^+/KI, Pt-vWD mouse models. In vitro, megakaryocytes stimulate osteoblast proliferation and have variable effects on osteoclast proliferation and activity through soluble factors and direct cell-cell communications. Intriguingly, new studies revealed that the ability of megakaryocytes to communicate with bone cells is affected by the age and sex of animals. This mini-review summarises changes seen in bone architecture and bone cell function in mouse models with an elevated number of megakaryocytes and the effects megakaryocytes have on osteoblasts and osteoclasts in vitro, and discusses potential molecular players that can mediate these effects.

Associations between osteoporosis and drug exposure: A post-marketing study of the World Health Organization pharmacovigilance database (VigiBase®)

BACKGROUND

Bone remodeling is a complex process, and many conditions (including drug exposure) lead to osteoporosis. Here, we sought to detect new disproportionality signals for drugs associated with osteoporosis.

METHODS

We performed a disproportionality analysis of the World Health Organization's VigiBase® pharmacovigilance database through April 12, 2020. The frequency of reports on osteoporosis for all identified drug classes was compared with that for all other drugs and quoted as the reporting odds ratio (ROR) [95% confidence interval (CI)].

RESULTS

Of the 7,594,968 cases spontaneously recorded to VigiBase®, 4758 concerned osteoporosis. New disproportionality signals with a pharmacologically plausible mechanism were found for drugs used in neurology (levodopa (ROR [95%CI]: 10.18 [4.33-25.10]), selective serotonin agonists (4.22 [2.34-7.00]) and memantine (4.10 [1.56-8.93])), hematology (romiplostim (4.93 [1.15-21.10])), pulmonology (macitentan (3.02 [1.84-4.90])), ophthalmology (ranibizumab (3.31 [1.00-10.51])) and rheumatology (tofacitinib (3.65 [3.00-4.40])). The robustness of these new results is supported by the significant RORs for the vast majority of drugs already known to induce osteoporosis and/or increase the fracture risk, namely glucocorticoids, gonadotropin-releasing hormone analogs, anti-aromatases, androgen receptor blockers, thyroid hormones, proton pump inhibitors, thiazolidinediones, vitamin K antagonists, loop diuretics, protease inhibitors, nucleoside and nucleotide reverse transcriptase inhibitors, and enzyme-inducing antiepileptics including barbiturates and derivatives, hydantoin derivatives, carboxamide derivatives and fatty acid derivatives.

CONCLUSION

We established up a comprehensive list of drugs potentially associated with osteoporosis and highlighted those with pharmacologically plausible mechanisms leading to bone fragility. Our results might pave the way for additional exploration of these mechanisms.

Craniofacial Bone Tissue Engineering: Current Approaches and Potential Therapy

Craniofacial bone defects can result from various disorders, including congenital malformations, tumor resection, infection, severe trauma, and accidents. Successfully regenerating cranial defects is an integral step to restore craniofacial function. However, challenges managing and controlling new bone tissue formation remain. Current advances in tissue engineering and regenerative medicine use innovative techniques to address these challenges. The use of biomaterials, stromal cells, and growth factors have demonstrated promising outcomes in vitro and in vivo. Natural and synthetic bone grafts combined with Mesenchymal Stromal Cells (MSCs) and growth factors have shown encouraging results in regenerating critical-size cranial defects. One of prevalent growth factors is Bone Morphogenetic Protein-2 (BMP-2). BMP-2 is defined as a gold standard growth factor that enhances new bone formation in vitro and in vivo. Recently, emerging evidence suggested that Megakaryocytes (MKs), induced by Thrombopoietin (TPO), show an increase in osteoblast proliferation in vitro and bone mass in vivo. Furthermore, a co-culture study shows mature MKs enhance MSC survival rate while maintaining their phenotype. Therefore, MKs can provide an insight as a potential therapy offering a safe and effective approach to regenerating critical-size cranial defects.

The effects of bone morphogenetic protein 2 and thrombopoietin treatment on angiogenic properties of endothelial cells derived from the lung and bone marrow of young and aged, male and female mice

With an aging world population, there is an increased risk of fracture and impaired healing. One contributing factor may be aging-associated decreases in vascular function; thus, enhancing angiogenesis could improve fracture healing. Both bone morphogenetic protein 2 (BMP-2) and thrombopoietin (TPO) have pro-angiogenic effects. The aim of this study was to investigate the effects of treatment with BMP-2 or TPO on the in vitro angiogenic and proliferative potential of endothelial cells (ECs) isolated from lungs (LECs) or bone marrow (BMECs) of young (3-4 months) and old (22-24 months), male and female, C57BL/6J mice. Cell proliferation, vessel-like structure formation, migration, and gene expression were used to evaluate angiogenic properties. In vitro characterization of ECs generally showed impaired vessel-like structure formation and proliferation in old ECs compared to young ECs, but improved migration characteristics in old BMECs. Differential sex-based angiogenic responses were observed, especially with respect to drug treatments and gene expression. Importantly, these studies suggest that NTN1, ROBO2, and SLIT3, along with angiogenic markers (CD31, FLT-1, ANGPT1, and ANGP2) differentially regulate EC proliferation and functional outcomes based on treatment, sex, and age. Furthermore, treatment of old ECs with TPO typically improved vessel-like structure parameters, but impaired migration. Thus, TPO may serve as an alternative treatment to BMP-2 for fracture healing in aging owing to improved angiogenesis and fracture healing, and the lack of side effects associated with BMP-2.

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

Osteomacs (OM) are specialized bone-resident macrophages that are a component of the hematopoietic niche and support bone formation. Also located in the niche are a second subset of macrophages, namely bone marrow-derived macrophages (BM Mφ). We previously reported that a subpopulation of OM co-express both CD166 and CSF1R, the receptor for macrophage colony-stimulating factor (MCSF), and that OM form more bone-resorbing osteoclasts than BM Mφ. Reported here are single-cell quantitative RT-PCR (qRT-PCR), mass cytometry (CyTOF), and marker-specific functional studies that further identify differences between OM and BM Mφ from neonatal C57Bl/6 mice. Although OM express higher levels of CSF1R and MCSF, they do not respond to MCSF-induced proliferation, in contrast to BM Mφ. Moreover, receptor activator of NF-κB ligand (RANKL), without the addition of MCSF, was sufficient to induce osteoclast formation in OM but not BM Mφ cultures. OM express higher levels of CD166 than BM Mφ, and we found that osteoclast formation by CD166^-/- OM was reduced compared with wild-type (WT) OM, whereas CD166^-/- BM Mφ showed enhanced osteoclast formation. CD110/c-Mpl, the receptor for thrombopoietin (TPO), was also higher in OM, but TPO did not alter OM-derived osteoclast formation, whereas TPO stimulated BM Mφ osteoclast formation. CyTOF analyses demonstrated OM uniquely co-express CD86 and CD206, markers of M1 and M2 polarized macrophages, respectively. OM performed equivalent phagocytosis in response to LPS or IL-4/IL-10, which induce polarization to M1 and M2 subtypes, respectively, whereas BM Mφ were less competent at phagocytosis when polarized to the M2 subtype. Moreover, in contrast to BM Mφ, LPS treatment of OM led to the upregulation of CD80, an M1 marker, as well as IL-10 and IL-6, known anti-inflammatory cytokines. Overall, these data reveal that OM and BM Mφ are distinct subgroups of macrophages, whose phenotypic and functional differences in proliferation, phagocytosis, and osteoclast formation may contribute physiological specificity during health and disease. © 2021 American Society for Bone and Mineral Research (ASBMR).

Megakaryocytes promote osteoclastogenesis in aging

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.

Genome-wide association study of bone mineral density trait among three pig breeds

Leg weakness (LW) issues are a great concern for pig breeding industry. And it also has a serious impact on animal welfare. To dissect the genetic architecture of limb-and-hoof firmness in commercial pigs, a genome-wide association study was conducted on bone mineral density (BMD) in three sow populations, including Duroc, Landrace and Yorkshire. The BMD data were obtained by ultrasound technology from 812 pigs (including Duroc 115, Landrace 243 and Yorkshire 454). In addition, all pigs were genotyped using genome-by-sequencing and a total of 224 162 single-nucleotide polymorphisms (SNPs) were obtained. After quality control, 218 141 SNPs were used for subsequent genome-wide association analysis. Nine significant associations were identified on chromosomes 3, 5, 6, 7, 9, 10, 12 and 18 that passed Bonferroni correction threshold of 0.05/(total SNP numbers). The most significant locus that associated with BMD (P value = 1.92e-14) was detected at approximately 41.7 Mb on SSC6 (SSC stands for Sus scrofa chromosome). CUL7, PTK7, SRF, VEGFA, RHEB, PRKAR1A and TPO that are located near the lead SNP of significant loci were highlighted as functionally plausible candidate genes for sow limb-and-hoof firmness. Moreover, we also applied a new method to measure the BMD data of pigs by ultrasound technology. The results provide an insight into the genetic architecture of LW and can also help to improve animal welfare in pigs.

Regulation of bone metabolism by megakaryocytes in a paracrine manner

Megakaryocytes (MKs) play key roles in regulating bone metabolism. To test the roles of MK-secreted factors, we investigated whether MK and promegakaryocyte (pro-MK) conditioned media (CM) may affect bone formation and resorption. K562 cell lines were differentiated into mature MKs. Mouse bone marrow macrophages were differentiated into mature osteoclasts, and MC3T3-E1 cells were used for osteoblastic experiments. Bone formation was determined by a calvaria bone formation assay in vivo. Micro-CT analyses were performed in the femurs of ovariectomized female C57B/L6 and Balb/c nude mice after intravenous injections of MK or pro-MK CM. MK CM significantly reduced in vitro bone resorption, largely due to suppressed osteoclastic resorption activity. Compared with pro-MK CM, MK CM suppressed osteoblastic differentiation, but stimulated its proliferation, resulting in stimulation of calvaria bone formation. In ovariectomized mice, treatment with MK CM for 4 weeks significantly increased trabecular bone mass parameters, such as bone volume fraction and trabecular thickness, in nude mice, but not in C57B/L6 mice. In conclusion, MKs may secrete anti-resorptive and anabolic factors that affect bone tissue, providing a novel insight linking MKs and bone cells in a paracrine manner. New therapeutic agents against metabolic bone diseases may be developed from MK-secreted factors.

Rheumatoid arthritis: Recent advances on its etiology, role of cytokines and pharmacotherapy

An autoimmune disease is defined as a clinical syndrome resulted from an instigation of both T cell and B cell or individually, in the absence of any present infection or any sort of distinguishable cause. Clonal deletion of auto reactive cells remains the central canon of immunology for decades, keeping the role of T cell and B cell aside, which are actually the guards to recognize the entry of foreign body. According to NIH, 23.5 million Americans are all together affected by these diseases. They are rare, but with the exception of RA. Rheumatoid arthritis is chronic and systemic autoimmune response to the multiple joints with unknown ethology, progressive disability, systemic complications, early death and high socioeconomic costs. Its ancient disease with an old history found in North American tribes since 1500 BCE, but its etiology is yet to be explored. Current conventional and biological therapies used for RA are not fulfilling the need of the patients but give only partial responses. There is a lack of consistent and liable biomarkers of prognosis therapeutic response, and toxicity. Rheumatoid arthritis is characterized by hyperplasic synovium, production of cytokines, chemokines, autoantibodies like rheumatoid factor (RF) and anticitrullinated protein antibody (ACPA), osteoclastogensis, angiogenesis and systemic consequences like cardiovascular, pulmonary, psychological, and skeletal disorders. Cytokines, a diverse group of polypeptides, play critical role in the pathogenesis of RA. Their involvement in autoimmune diseases is a rapidly growing area of biological and clinical research. Among the proinflammatory cytokines, IL-1α/β and TNF-α trigger the intracellular molecular signalling pathway responsible for the pathogenesis of RA that leads to the activation of mesenchymal cell, recruitment of innate and adaptive immune system cells, activation of synoviocytes which in term activates various mediators including tumour necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), interleukin-6 (IL-6) and interleukin-8 (IL-8), resulting in inflamed synovium, increase angiogenesis and decrease lymphangiogensis. Their current pharmacotherapy should focus on their three phases of progression i.e. prearthritis phase, transition phase and clinical phase. In this way we will be able to find a way to keep the balance between the pro and anti-inflammatory cytokines that is believe to be the dogma of pathogenesis of RA. For this we need to explore new agents, whether from synthetic or natural source to find the answers for unresolved etiology of autoimmune diseases and to provide a quality of life to the patients suffering from these diseases specifically RA.

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

The Lnk adapter protein negatively regulates the signaling of thrombopoietin (TPO), the main megakaryocyte (MK) growth factor. Lnk-deficient (-/-) mice have increased TPO signaling and increased MK number. Interestingly, several mouse models exist in which increased MK number leads to a high bone mass phenotype. Here we report the bone phenotype of these mice. MicroCT and static histomorphometric analyses at 20 weeks showed the distal femur of Lnk^-/- mice to have significantly higher bone volume fraction and trabecular number compared to wild-type (WT) mice. Notably, despite a significant increase in the number of osteoclasts (OC), and decreased bone formation rate in Lnk^-/- mice compared to WT mice, Lnk^-/- mice demonstrated a 2.5-fold greater BV/TV suggesting impaired OC function in vivo. Additionally, Lnk^-/- mouse femurs exhibited non-significant increases in mid-shaft cross-sectional area, yet increased periosteal BFR compared to WT femurs was observed. Lnk^-/- femurs also had non-significant increases in polar moment of inertia and decreased cortical bone area and thickness, resulting in reduced bone stiffness, modulus, and strength compared to WT femurs. Of note, Lnk is expressed by OC lineage cells and when Lnk^-/- OC progenitors are cultured in the presence of TPO, significantly more OC are observed than in WT cultures. Lnk is also expressed in osteoblast (OB) cells and in vitro reduced alkaline phosphatase activity was observed in Lnk^-/- cultures. These data suggest that both direct effects on OB and OC as well as indirect effects of MK in regulating OB contributes to the observed high bone mass. J. Cell. Biochem. 118: 2231-2240, 2017. © 2017 Wiley Periodicals, Inc.

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

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 net gain in bone volume with increases in OBs and OCs. 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.