Pyk2 regulates megakaryocyte-induced increases in osteoblast number and bone formation

Activated PyK2 and Its Associated Molecules Transduce Cellular Signaling from the Cancerous Milieu for Cancer Metastasis

PyK2 is a member of the proline-rich tyrosine kinase and focal adhesion kinase families and is ubiquitously expressed. PyK2 is mainly activated by stimuli, such as activated Src kinases and intracellular acidic pH. The mechanism of PyK2 activation in cancer cells has been addressed extensively. The up-regulation of PyK2 through overexpression and enhanced phosphorylation is a key feature of tumorigenesis and cancer migration. In this review, we summarized the cancer milieu, including acidification and cancer-associated molecules, such as chemical reagents, interactive proteins, chemokine-related molecules, calcium channels/transporters, and oxidative molecules that affect the fate of PyK2. The inhibition of PyK2 leads to a beneficial strategy to attenuate cancer cell development, including metastasis. Thus, we highlighted the effect of PyK2 on various cancer cell types and the distribution of molecules that affect PyK2 activation. In particular, we underlined the relationship between PyK2 and cancer metastasis and its potential to treat cancer cells.

Inhibition of Osteoblast Differentiation by JAK2V617F Megakaryocytes Derived From Male Mice With Primary Myelofibrosis

Past studies described interactions between normal megakaryocytes, the platelet precursors, and bone cell precursors in the bone marrow. This relationship has also been studied in context of various mutations associated with increased number of megakaryocytes. The current study is the first to examine the effects of megakaryocytes from transgenic mice carrying the most common mutation that causes primary myelofibrosis (PMF) in humans (JAK2^V617F) on bone cell differentiation. Organ level assessments of mice using micro-computed tomography showed decreased bone volume in JAK2^V617F males, compared to matching controls. Tissue level histology revealed increased deposition of osteoid (bone matrix prior mineralization) in these mutated mice, suggesting an effect on osteoblast differentiation. Mechanistic studies using a megakaryocyte-osteoblast co-culture system, showed that both wild type or JAK2^V617F megakaryocytes derived from male mice inhibited osteoblast differentiation, but JAK2^V617F cells exerted a more significant inhibitory effect. A mouse mRNA osteogenesis array showed increased expression of Noggin, Chordin, Alpha-2-HS-glycoprotein, Collagen type IV alpha 1 and Collagen type XIV alpha 1 (mostly known to inhibit bone differentiation), and decreased expression of alkaline phosphatase, Vascular cell adhesion molecule 1, Sclerostin, Distal-less homeobox 5 and Collagen type III alpha 1 (associated with osteogenesis) in JAK2^V617F megakaryocytes, compared to controls. This suggested that the mutation re-programs megakaryocytes to express a cluster of genes, which together could orchestrate greater suppression of osteogenesis in male mice. These findings provide mechanistic insight into the effect of JAK2^V617F mutation on bone, encouraging future examination of patients with this or other PMF-inducing mutations.

Circulating platelet concentration is associated with bone mineral density in women

In this cross-sectional study, enrollment included 818 female adults undergoing bone mineral density (BMD) assessment during the health examination. Subjects with osteoporosis had the lowest circulating platelet concentrations. The circulating platelet concentration was positively correlated with BMD. A high platelet concentration had independently low odds of osteoporosis.

PURPOSE

Platelets play an important role in bone metabolism. However, the association between circulating platelet counts and bone mineral density (BMD) has been inconsistently reported. We aimed to investigate the relationship between platelet counts and osteoporosis in Chinese women.

METHODS

In this cross-sectional study, a total of 818 female adults who underwent BMD assessment during the health examination were enrolled. Blood cell counts and biochemistry data were recorded.

RESULTS

Subjects with osteoporosis had the lowest platelet counts (238 ± 59 × 10⁹/L) compared with subjects with osteopenia (256 ± 64 × 10⁹/L) and a normal BMD (269 ± 76 × 10⁹/L, P < 0.001). The circulating platelet concentration was positively correlated with the BMD of the lumbar spine (r = 0.195, P < 0.001), left hip (r = 0.145, P < 0.001), and right hip (r = 0.149, P < 0.001). According to the receiver operating characteristic curve, the cutoff platelet concentration for differentiating osteoporosis was 260 × 10⁹/L. A high platelet concentration had significantly low odds of osteoporosis after adjusting for other covariates (odds ratio = 0.574, 95% confidence interval: 0.346‒0.953, P = 0.032).

CONCLUSION

The circulating platelet concentration was significantly correlated with BMD in Chinese women.

Integrins and their role in megakaryocyte development and function

Mature megakaryocytes, the platelet precursors, originate from hematopoietic stem cell progenitors, which, once committed to this lineage, undergo endomitosis leading to polyploidization. The process entails repeated rounds of DNA replication without cell division, yielding polyploid cells. Supporting the cell's developmental process and various cellular functions are integrin receptors, a conduit of communication between the extracellular environment and the cell actin cytoskeleton. Integrins are heterodimers of α and β subunits, where different combinations of the known 18 α and 8 β subunits confer specificity to the receptor. Integrin ligands range from extracellular matrices through soluble ligands, infectious agents, and counterreceptors, to cells. In this review, we describe the different integrins expressed on bone marrow megakaryocytes and their attributed roles in lineage development and cellular functions, including adhesion, spreading, proplatelet formation, and functional interaction with other cells. Pathologies associated with dysregulated megakaryocyte integrin expression are also reviewed.

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.

Analysis of the effects of spaceflight and local administration of thrombopoietin to a femoral defect injury on distal skeletal sites

With increased human presence in space, bone loss and fractures will occur. Thrombopoietin (TPO) is a recently patented bone healing agent. Here, we investigated the systemic effects of TPO on mice subjected to spaceflight and sustaining a bone fracture. Forty, 9-week-old, male, C57BL/6 J were divided into 4 groups: (1) Saline+Earth; (2) TPO + Earth; (3) Saline+Flight; and (4) TPO + Flight (n = 10/group). Saline- and TPO-treated mice underwent a femoral defect surgery, and 20 mice were housed in space ("Flight") and 20 mice on Earth for approximately 4 weeks. With the exception of the calvarium and incisor, positive changes were observed in TPO-treated, spaceflight bones, suggesting TPO may improve osteogenesis in the absence of mechanical loading. Thus, TPO, may serve as a new bone healing agent, and may also improve some skeletal properties of astronauts, which might be extrapolated for patients on Earth with restraint mobilization and/or are incapable of bearing weight on their bones.

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.

Pyk2 deficiency enhances bone mass during midpalatal suture expansion

OBJECTIVE

To determine if Pyk2 deficiency increases midpalatal suture bone mass and preserves sutural integrity after maxillary expansion.

SETTING AND SAMPLE

Thirty-six male Pyk2 knockout (KO) and control (WT) mice at 6 weeks of age.

MATERIALS AND METHODS

Mice received nickel-titanium spring expanders delivering 0 g (no intervention control), 10 or 20 g force for 14 days. High-resolution micro-CT was used to determine bone volume/tissue volume (BV/TV), sutural width and intermolar width. Effects on osteoclasts, chondrocytes and suture morphology were determined by histomorphometry.

RESULTS

Pyk2-KO controls (0 g) had 7% higher BV/TV compared with WT controls. Expanded Pyk2-KO maxillae also exhibited 12% (10 g) and 18% (20 g) higher BV/TV than WT mice. Although bone loss following expansion occurred in both genotypes, BV/TV was decreased to a greater extent in WT maxillae (-10% at 10g; -22% at 20 g) compared with Pyk2-KO maxillae (-11% only at 20 g). Expanded WT maxillae also showed a greater increase in sutural width, intermolar width and fibrous connective tissue width compared with expanded Pyk2-KO maxillae. Moreover, osteoclast number was increased 77% (10 g) and 132% (20 g) in expanded WT maxillae, but remained unchanged in expanded Pyk2-KO, compared to their respective controls. Cartilage area and chondrocyte number were increased to the same extent in expanded WT and Pyk2-KO sutures.

CONCLUSIONS

These findings suggest that midpalatal suture expansion increases osteoclast formation in WT but not Pyk2-KO mice, leading to higher BV/TV in expanded Pyk2-KO maxillae. These studies suggest Pyk2-targeted strategies may be beneficial to increase bone density and preserve sutural integrity during maxillary expansion.

Aging negatively impacts the ability of megakaryocytes to stimulate osteoblast proliferation and bone mass

Osteoblast number and activity decreases with aging, contributing to the age-associated decline of bone mass, but the mechanisms underlying changes in osteoblast activity are not well understood. Here, we show that the age-associated bone loss critically depends on impairment of the ability of megakaryocytes (MKs) to support osteoblast proliferation. Co-culture of osteoblast precursors with young MKs is known to increase osteoblast proliferation and bone formation. However, co-culture of osteoblast precursors with aged MKs resulted in significantly fewer osteoblasts compared to co-culture with young MKs, and this was associated with the downregulation of transforming growth factor beta. In addition, the ability of MKs to increase bone mass was attenuated during aging as transplantation of GATA1^low/low hematopoietic donor cells (which have elevated MKs/MK precursors) from young mice resulted in an increase in bone mass of recipient mice compared to transplantation of young wild-type donor cells, whereas transplantation of GATA1^low/low donor cells from old mice failed to enhance bone mass in recipient mice compared to transplantation of old wild-type donor cells. These findings suggest that the preservation or restoration of the MK-mediated induction of osteoblast proliferation during aging may hold the potential to prevent age-associated bone loss and resulting fractures.

Effects of mechanobiological signaling in bone marrow on skeletal health

Bone marrow is a cellular tissue that forms within the pore space and hollow diaphysis of bones. As a tissue, its primary function is to support the hematopoietic progenitor cells that maintain the populations of both erythroid and myeloid lineage cells in the bone marrow, making it an essential element of normal mammalian physiology. However, bone's primary function is load bearing, and deformations induced by external forces are transmitted to the encapsulated marrow. Understanding the effects of these mechanical inputs on marrow function and adaptation requires knowledge of the material behavior of the marrow at multiple scales, the loads that are applied, and the mechanobiology of the cells. This paper reviews the current state of knowledge of each of these factors. Characterization of the marrow mechanical environment and its role in skeletal health and other marrow functions remains incomplete, but research on the topic is increasing, driven by interest in skeletal adaptation and the mechanobiology of cancer metastasis.

Effect of calcium glucoheptonate on proliferation and osteogenesis of osteoblast-like cells in vitro

Calcium is the key macromineral having a role in skeletal structure and function, muscle contraction, and neurotransmission. Bone remodeling is maintained through a constant balance between calcium resorption and deposition. Calcium deficiency is resolved through calcium supplementation, and among the supplements, water-soluble organic molecules attracted great pharmaceutical interest. Calcium glucoheptonate is a highly water-soluble organic calcium salt having clinical use; however, detailed investigations on its biological effects are limited. We assessed the effects of calcium glucoheptonate on cell viability and proliferation of osteoblast-like MG-63 cells. Calcium uptake and mineralization were evaluated using Alizarin red staining of osteoblast-like MG-63 cells treated with calcium glucoheptonate. Expression of osteogenic markers were monitored by western blotting, immunofluorescence, and qRT-PCR assays. Increased proliferation and calcium uptake were observed in the MG-63 cells treated with calcium glucoheptonate. The treatment also increased the expression of osteopontin and osteogenic genes such as collagen-1, secreted protein acidic and cysteine rich (SPARC), and osteocalcin. Calcium glucoheptonate treatment did not exert any cytotoxicity on colorectal and renal epithelial cells, indicating the safety of the treatment. This is the first report with evidence for its beneficial effect for pharmaceutical use in addressing calcium deficiency conditions.

A Pyk2 inhibitor incorporated into a PEGDA-gelatin hydrogel promotes osteoblast activity and mineral deposition

Pyk2 is a non-receptor tyrosine kinase that belongs to the family of focal adhesion kinases. Studies from our laboratory and others demonstrated that mice lacking the Pyk2 gene (Ptk2B) have high bone mass, which was due to increased osteoblast activity, as well as decreased osteoclast activity. It was previously reported that a chemical inhibitor that targets both Pyk2 and its homolog FAK, led to increased bone formation in ovariectomized rats. In the current study, we developed a hydrogel containing poly(ethylene glycol) diacrylate (PEGDA) and gelatin which was curable by visible-light and was suitable for the delivery of small molecules, including a Pyk2-targeted chemical inhibitor. We characterized several critical properties of the hydrogel, including viscosity, gelation time, swelling, degradation, and drug release behavior. We found that a hydrogel composed of PEGDA1000 plus 10% gelatin (P1000:G10) exhibited Bingham fluid behavior that can resist free flowing before in situ polymerization, making it suitable for use as an injectable carrier in open wound applications. The P1000:G10 hydrogel was cytocompatible and displayed a more delayed drug release behavior than other hydrogels we tested. Importantly, the Pyk2-inhibitor-hydrogel retained its inhibitory activity against the Pyk2 tyrosine kinase, and promoted osteoblast activity and mineral deposition in vitro. Overall, our findings suggest that a Pyk2-inhibitor based hydrogel may be suitable for the treatment of craniofacial and appendicular skeletal defects and targeted bone regeneration.

Enhancement of osteoblast activity on nanostructured NiTi/hydroxyapatite coatings on additive manufactured NiTi metal implants by nanosecond pulsed laser sintering

Background

The osteoinductive behaviors of nitinol (NiTi)-based metal implants for bone regeneration are largely dependent on their surface composition and topology. Continuous-mode laser sintering often results in complete melting of the materials and aggregation of particles, which lack control of heat transfer, as well as microstructural changes during sintering of the nanocomposite materials.

Methods

In the current study, in situ direct laser deposition was used to additively manufacture three-dimensional NiTi structures from Ni and Ti powders. The mechanical property of NiTi has been shown to be similar to bone. Nanosecond pulsed laser sintering process was then utilized to generate a nanoporous composite surface with NiTi alloy and hydroxyapatite (HA) by ultrafast laser heating and cooling of Ni, Ti, and HA nanoparticles mixtures precoated on the 3D NiTi substrates; HA was added in order to improve the biocompatibility of the alloy. We then studied the underlying mechanism in the formation of NiTi/HA nanocomposite, and the synergistic effect of the sintered HA component and the nanoporous topology of the composite coating. In addition, we examined the activity of bone-forming osteoblasts on the NiTi/HA surfaces. For this, osteoblast cell morphology and various biomarkers were examined to evaluate cellular activity and function.

Results

We found that the nanoscale porosity delivered by nanosecond pulsed laser sintering and the HA component positively contributed to osteoblast differentiation, as indicated by an increase in the expression of collagen and alkaline phosphatase, both of which are necessary for osteoblast mineralization. In addition, we observed topological complexities which appeared to boost the activity of osteoblasts, including an increase in actin cytoskeletal structures and adhesion structures.

Conclusion

These findings demonstrate that the pulsed laser sintering method is an effective tool to generate biocompatible coatings in complex alloy-composite material systems with desired composition and topology. Our findings also provide a better understanding of the osteoinductive behavior of the sintered nanocomposite coatings for use in orthopedic and bone regeneration applications.

Pyk2 deficiency potentiates osteoblast differentiation and mineralizing activity in response to estrogen or raloxifene

Bone remodeling is controlled by the actions of bone-degrading osteoclasts and bone-forming osteoblasts (OBs). Aging and loss of estrogen after menopause affects bone mass and quality. Estrogen therapy, including selective estrogen receptor modulators (SERMs), can prevent bone loss and increase bone mineral density in post-menopausal women. Although investigations of the effects of estrogen on osteoclast activity are well advanced, the mechanism of action of estrogen on OBs is still unclear. The proline-rich tyrosine kinase 2 (Pyk2) is important for bone formation and female mice lacking Pyk2 (Pyk2-KO) exhibit elevated bone mass, increased bone formation rate and reduced osteoclast activity. Therefore, in the current study, we examined the role of estrogen signaling on the mechanism of action of Pyk2 in OBs. As expected, Pyk2-KO OBs showed significantly higher proliferation, matrix formation, and mineralization than WT OBs. In addition we found that Pyk2-KO OBs cultured in the presence of either 17β-estradiol (E2) or raloxifene, a SERM used for the treatment of post-menopausal osteoporosis, showed a further robust increase in alkaline phosphatase (ALP) activity and mineralization. We examined the possible mechanism of action and found that Pyk2 deletion promotes the proteasome-mediated degradation of estrogen receptor α (ERα), but not estrogen receptor β (ERβ). As a consequence, E2 signaling via ERβ was enhanced in Pyk2-KO OBs. In addition, we found that Pyk2 deletion and E2 stimulation had an additive effect on ERK phosphorylation, which is known to stimulate cell differentiation and survival. Our findings suggest that in the absence of Pyk2, estrogen exerts an osteogenic effect on OBs through altered ERα and ERβ signaling. Thus, targeting Pyk2, in combination with estrogen or raloxifene, may be a novel strategy for the prevention and/or treatment of bone loss diseases.

Proline-Rich Protein Tyrosine Kinase 2 in Inflammation and Cancer

Focal adhesion kinase (FAK) and its homologous FAK-related proline-rich tyrosine kinase 2 (Pyk2) contain the same domain, exhibit high sequence homology and are defined as a distinct family of non-receptor tyrosine kinases. This group of kinases plays critical roles in cytoskeletal dynamics and cell adhesion by regulating survival and growth signaling. This review summarizes the physiological and pathological functions of Pyk2 in inflammation and cancers. In particular, overexpression of Pyk2 in cancerous tissues is correlated with poor outcomes. Pyk2 stimulates multiple oncogenic signaling pathways, such as Wnt/β-catenin, PI3K/Akt, MAPK/ERK, and TGF-β/EGFR/VEGF, and facilitates carcinogenesis, migration, invasion, epithelial⁻mesenchymal transition and metastasis. Therefore, Pyk2 is a high-value therapeutic target and has clinical significance.

The high bone mass phenotype of Lrp5-mutant mice is not affected by megakaryocyte depletion

Bone remodeling is a continuously ongoing process mediated by bone-resorbing osteoclasts and bone-forming osteoblasts. One key regulator of bone formation is the putative Wnt co-receptor Lrp5, where activating mutations in the extracellular domain cause increased bone formation in mice and humans. We have previously reported that megakaryocyte numbers are increased the bone marrow of mice carrying a high bone mass mutation (HBM) of Lrp5 (Lrp5^G170V). Since megakaryocytes can promote bone formation, we addressed the question, if the bone remodeling phenotype of Lrp5^G170V mice is affected by megakaryocyte depletion. For that purpose we took advantage of a mouse model carrying a mutation of the Mpl gene, encoding the thrombopoietin receptor. These mice (Mpl^hlb219) were crossed with Lrp5^G170V mice to generate animals carrying both mutations in a homozygous state. Using μCT, undecalcified histology and bone-specific histomorphometry of 12 weeks old littermates we observed that megakaryocyte number was remarkably decreased in Mpl^hlb219/Lrp5^G170V mice, yet the high bone mass phenotype of Lrp5^G170V mice was not significantly affected by the homozygous Mpl mutation. Finally, when we analyzed 24 weeks old wildtype and Mpl^hlb219 mice we did not observe a statistically significant alteration of bone remodeling in the latter ones. Taken together, our results demonstrate that an increased number of bone marrow megakaryocytes does not contribute to the increased bone formation caused by Lrp5 activation.

Osteomacs interact with megakaryocytes and osteoblasts to regulate murine hematopoietic stem cell function

Networking between hematopoietic stem cells (HSCs) and cells of the hematopoietic niche is critical for stem cell function and maintenance of the stem cell pool. We characterized calvariae-resident osteomacs (OMs) and their interaction with megakaryocytes to sustain HSC function and identified distinguishing properties between OMs and bone marrow (BM)-derived macrophages. OMs, identified as CD45⁺F4/80⁺ cells, were easily detectable (3%-5%) in neonatal calvarial cells. Coculture of neonatal calvarial cells with megakaryocytes for 7 days increased OM three- to sixfold, demonstrating that megakaryocytes regulate OM proliferation. OMs were required for the hematopoiesis-enhancing activity of osteoblasts, and this activity was augmented by megakaryocytes. Serial transplantation demonstrated that HSC repopulating potential was best maintained by in vitro cultures containing osteoblasts, OMs, and megakaryocytes. With or without megakaryocytes, BM-derived macrophages were unable to functionally substitute for neonatal calvarial cell-associated OMs. In addition, OMs differentiated into multinucleated, tartrate resistant acid phosphatase-positive osteoclasts capable of bone resorption. Nine-color flow cytometric analysis revealed that although BM-derived macrophages and OMs share many cell surface phenotypic similarities (CD45, F4/80, CD68, CD11b, Mac2, and Gr-1), only a subgroup of OMs coexpressed M-CSFR and CD166, thus providing a unique profile for OMs. CD169 was expressed by both OMs and BM-derived macrophages and therefore was not a distinguishing marker between these 2 cell types. These results demonstrate that OMs support HSC function and illustrate that megakaryocytes significantly augment the synergistic activity of osteoblasts and OMs. Furthermore, this report establishes for the first time that the crosstalk between OMs, osteoblasts, and megakaryocytes is a novel network supporting HSC function.

Osteoblasts support megakaryopoiesis through production of interleukin-9

Osteoblast-produced IL-9 supports megakaryopoiesis and platelet formation. IL-9 is a promising therapeutic agent for treatment of thrombocytopenia.

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.

Pyk2 and Megakaryocytes Regulate Osteoblast Differentiation and Migration Via Distinct and Overlapping Mechanisms

Osteoblast differentiation and migration are necessary for bone formation during bone remodeling. Mice lacking the proline-rich tyrosine kinase Pyk2 (Pyk2-KO) have increased bone mass, in part due to increased osteoblast proliferation. Megakaryocytes (MKs), the platelet-producing cells, also promote osteoblast proliferation in vitro and bone-formation in vivo via a pathway that involves Pyk2. In the current study, we examined the mechanism of action of Pyk2, and the role of MKs, on osteoblast differentiation and migration. We found that Pyk2-KO osteoblasts express elevated alkaline phosphatase (ALP), type I collagen and osteocalcin mRNA levels as well as increased ALP activity, and mineralization, confirming that Pyk2 negatively regulates osteoblast function. Since Pyk2 Y402 phosphorylation is important for its catalytic activity and for its protein-scaffolding functions, we expressed the phosphorylation-mutant (Pyk2(Y402F) ) and kinase-mutant (Pyk2(K457A) ) in Pyk2-KO osteoblasts. Both Pyk2(Y402F) and Pyk2(K457A) reduced ALP activity, whereas only kinase-inactive Pyk2(K457A) inhibited Pyk2-KO osteoblast migration. Consistent with a role for Pyk2 on ALP activity, co-culture of MKs with osteoblasts led to a decrease in the level of phosphorylated Pyk2 (pY402) as well as a decrease in ALP activity. Although, Pyk2-KO osteoblasts exhibited increased migration compared to wild-type osteoblasts, Pyk2 expression was not required necessary for the ability of MKs to stimulate osteoblast migration. Together, these data suggest that osteoblast differentiation and migration are inversely regulated by MKs via distinct Pyk2-dependent and independent signaling pathways. Novel drugs that distinguish between the kinase-dependent or protein-scaffolding functions of Pyk2 may provide therapeutic specificity for the control of bone-related diseases.

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.

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

Emerging data suggest that megakaryocytes (MKs) play a significant role in skeletal homeostasis. Indeed, osteosclerosis observed in several MK-related disorders may be a result of increased numbers of MKs. In support of this idea, we have previously demonstrated that MKs increase osteoblast (OB) proliferation by a direct cell-cell contact mechanism and that MKs also inhibit osteoclast (OC) formation. As MKs and OCs are derived from the same hematopoietic precursor, in these osteoclastogenesis studies we examined the role of the main MK growth factor, thrombopoietin (TPO) on OC formation and bone resorption. Here we show that TPO directly increases OC formation and differentiation in vitro. Specifically, we demonstrate the TPO receptor (c-mpl or CD110) is expressed on cells of the OC lineage, c-mpl is required for TPO to enhance OC formation in vitro, and TPO activates the mitogen-activated protein kinases, Janus kinase/signal transducer and activator of transcription, and nuclear factor-kappaB signaling pathways, but does not activate the PI3K/AKT pathway. Further, we found TPO enhances OC resorption in CD14+CD110+ human OC progenitors derived from peripheral blood mononuclear cells, and further separating OC progenitors based on CD110 expression enriches for mature OC development. The regulation of OCs by TPO highlights a novel therapeutic target for bone loss diseases and may be important to consider in the numerous hematologic disorders associated with alterations in TPO/c-mpl signaling as well as in patients suffering from bone disorders.

The secret life of a megakaryocyte: emerging roles in bone marrow homeostasis control

Megakaryocytes are rare cells found in the bone marrow, responsible for the everyday production and release of millions of platelets into the bloodstream. Since the discovery and cloning, in 1994, of their principal humoral factor, thrombopoietin, and its receptor c-Mpl, many efforts have been directed to define the mechanisms underlying an efficient platelet production. However, more recently different studies have pointed out new roles for megakaryocytes as regulators of bone marrow homeostasis and physiology. In this review we discuss the interaction and the reciprocal regulation of megakaryocytes with the different cellular and extracellular components of the bone marrow environment. Finally, we provide evidence that these processes may concur to the reconstitution of the bone marrow environment after injury and their deregulation may lead to the development of a series of inherited or acquired pathologies.

Signaling pathways involved in megakaryocyte-mediated proliferation of osteoblast lineage cells

Recent studies suggest that megakaryocytes (MKs) may play a significant role in skeletal homeostasis, as evident by the occurrence of osteosclerosis in multiple MK related diseases (Lennert et al., 1975; Thiele et al., 1999; Chagraoui et al., 2006). We previously reported a novel interaction whereby MKs enhanced proliferation of osteoblast lineage/osteoprogenitor cells (OBs) by a mechanism requiring direct cell-cell contact. However, the signal transduction pathways and the downstream effector molecules involved in this process have not been characterized. Here we show that MKs contact with OBs, via beta1 integrin, activate the p38/MAPKAPK2/p90RSK kinase cascade in the bone cells, which causes Mdm2 to neutralizes p53/Rb-mediated check point and allows progression through the G1/S. Interestingly, activation of MAPK (ERK1/2) and AKT, collateral pathways that regulate the cell cycle, remained unchanged with MK stimulation of OBs. The MK-to-OB signaling ultimately results in significant increases in the expression of c-fos and cyclin A, necessary for sustaining the OB proliferation. Overall, our findings show that OBs respond to the presence of MKs, in part, via an integrin-mediated signaling mechanism, activating a novel response axis that de-represses cell cycle activity. Understanding the mechanisms by which MKs enhance OB proliferation will facilitate the development of novel anabolic therapies to treat bone loss associated with osteoporosis and other bone-related diseases.