Osteoblasts express types I and II activin receptors during early intramembranous and endochondral bone formation

Dose-response effect of Montelukast on post-extraction dental socket repair and skeletal phenotype of mice

Bone metabolism and repair are directly regulated by arachidonic acid metabolites. At present, we analyzed the dose-response effects of a selective cysteinyl leukotriene receptor type-1 antagonist during bone repair after tooth extraction and on non-injured skeleton. Sixty-three 129 Sv/Ev male mice composed the groups: C-Control (saline solution); MTK2-2 mg/Kg of Montelukast (MTK) and MTK4-4 mg/Kg of MTK, daily administered by mouth throughout all experimental periods set at 7, 14, and 21 days post-operative. Dental sockets were analyzed by computed microtomography (microCT), histopathology, and immunohistochemistry. Femurs, L5 vertebra and organs were also removed for observation. Blood was collected for plasma bone and liver markers. Histopathology and microCT analysis revealed early socket repair of MTK2 and MTK4 animals, with significant increased BV/TV at days 14 and 21 compared to C. Higher plasma calcium was detected at days 7 and 21 in MTK4 in comparison to C, while phosphate was significantly increased in MTK2 in the same periods in comparison to C and MTK4. No significant differences were found regarding plasma ALP and TRAP, neither for local TRAP and Runx2 immunolabeling at the healing sockets. Organs did not present histological abnormalities. Increased AST levels have been detected in distinct groups and periods. In general, femur phenotype was improved in MTK treated animals. Collectively, MTK promoted early bone formation after tooth extraction and increased bone quality of femurs and vertebra in a time-dose-dependent manner, and should be considered as an alternative therapy when improved post-extraction socket repair or skeleton preservation is required.

Association of activins, follistatins and inhibins with incident hip fracture in women with postmenopausal osteoporosis: a proof of concept, case-control study

PURPOSE

The activins-follistatins-inhibins (AFI) hormonal system is considered to regulate muscle and bone mass. We aimed to evaluate AFI in postmenopausal women with an incident hip fracture.

METHODS

In this post-hoc analysis of a hospital based case-control study, we evaluated circulating levels of the AFI system in postmenopausal women with a low-energy hip fracture admitted for fixation compared with postmenopausal women with osteoarthritis scheduled for arthroplasty.

RESULTS

Circulating levels of follistatin (p = 0.008), FSTL3 (p = 0.013), activin B and AB (both p < 0.001), as well as activin AB/follistatin and activin AB/FSTL3 ratios (p = 0.008 and p = 0.029, respectively) were higher in patients than controls in unadjusted models. Differences for activins B and AB remained after adjustment for age and BMI (p = 0.006 and p = 0.009, respectively) and for FRAX-based risk for hip fracture (p = 0.008 and p = 0.012, respectively) but were lost when 25OHD was added to the regression models.

CONCLUSIONS

Our data indicate no major changes in the AFI system in postmenopausal women at the time of hip fracture compared to postmenopausal women with osteoarthritis except for higher activin B and AB levels, whose significance, however, was lost when 25OHD was added to the adjustment models.

CLINICAL TRIALS

Clinical Trials identifier: NCT04206618.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Administration of an activin receptor IIB ligand trap protects male juvenile rhesus macaques from simian immunodeficiency virus-associated bone loss

HIV-infected individuals are at an increased risk of osteoporosis despite effective viral suppression. Observations that myostatin null mice have increased bone mass led us to hypothesize that simian immunodeficiency virus (SIV)-associated bone loss may be attenuated by blocking myostatin/TGFβ signaling. In this proof-of-concept study, pair-housed juvenile male rhesus macaques were inoculated with SIVmac239. Four weeks later, animals were treated with vehicle or Fc-conjugated soluble activin receptor IIB (ActR2B·Fc, iv. 10mg∗kg^-1∗week^-1) - an antagonist of myostatin and related members of TGFβ superfamily. Limb and trunk bone mineral content (BMC) and density (BMD) using dual-energy X-Ray absorptiometry, circulating markers of bone growth and turnover, and serum testosterone levels were measured at baseline and during the 12-week intervention period. The increase in BMC was significantly greater in the ActRIIB.Fc-treated group (+8g) than in the placebo group (-4g) (p<0.05). BMD also increased significantly more in the ActRIIB.Fc-treated macaques (+0.03g/cm²) than in the placebo-treated animals (+0g/cm²) (p<0.005). Serum osteocalcin was about two-fold higher in the ActRIIB.Fc-treated group than in the placebo group (p<0.05), but serum C-terminal telopeptide and testosterone levels did not differ significantly between groups. The expression levels of TNFalpha (p<0.05), GADD45 (p<0.005), and sclerostin (p<0.038) in the bone-marrow were significantly lower in the ActRIIB.Fc-treated group than in the placebo group.

CONCLUSION

The administration of ActRIIB.FC in SIV-infected juvenile macaques significantly increases BMC and BMD in association with reduced expression levels of markers of bone marrow inflammation.

Does Activin Receptor Blockade by Bimagrumab (BYM338) Pose Detrimental Effects on Bone Healing in a Rat Fibula Osteotomy Model?

Bimagrumab (BYM338) is a novel fully human monoclonal antibody that exerts strong promyogenic effects on skeletal muscle by blocking activin type II receptors (ActRII). We investigated whether such blockade of ActRII by bimagrumab manifests any detrimental effect on outcomes of bone healing in a rat fibula osteotomy model. Animals (n = 150) were divided into 11 groups and received weekly treatment with either bimagrumab (10 or 100 mg/kg) or vehicle. Progression and outcomes of bone healing were assessed by lateral radiographs in vivo as well as by peripheral quantitative computed tomography (pQCT), 4-point bending test, and microscopic examination of the excised fibula at Day 29 or later. The radiographic progression of bone healing showed no significant differences between treatment groups in any comparative setting. In 3-month-old animals, pQCT revealed slightly reduced immature callus size and bone mineral content in bimagrumab-treated animals compared with vehicle-treated animals at Day 29 (p < 0.05). There were, however, no differences in mature callus size, bone mineral density, or biomechanical competency. The aforementioned effects on immature callus size were not present when the treatment was initiated 4 weeks post osteotomy or when treating 6-month-old animals. In summary, these findings suggest that there is no major impact of ActRII blockade on overall fracture healing, and delayed treatment initiation can bypass the small and transient effect of the therapy on immature callus formation observed in younger animals. Verification of these findings in humans is the subject of an ongoing clinical trial on elderly hip fracture patients.

Myostatin regulates tissue potency and cardiac calcium-handling proteins

Attenuating myostatin enhances striated muscle growth, reduces adiposity, and improves cardiac contractility. To determine whether myostatin influences tissue potency in a manner that could control such pleiotropic actions, we generated label-retaining mice with wild-type and mstn(-/-) (Jekyll) backgrounds in which slow-cycling stem, transit-amplifying, and progenitor cells are preferentially labeled by histone 2B/green fluorescent protein. Jekyll mice were born with fewer label-retaining cells (LRCs) in muscle and heart, consistent with increased stem/progenitor cell contributions to embryonic growth of both tissues. Cardiac LRC recruitment from noncardiac sources occurred in both groups, but lasted longer in Jekyll hearts, whereas heightened β-adrenergic sensitivity of mstn(-/-) hearts was explained by elevated SERCA2a, phospholamban, and β2-adrenergic receptor levels. Jekyll mice were also born with more adipose LRCs despite significantly smaller tissue weights. Reduced adiposity in mstn(-/-) animals is therefore due to reduced lipid deposition as adipoprogenitor pools appear to be enhanced. By contrast, increased bone densities of mstn(-/-) mice are likely compensatory to hypermuscularity because LRC counts were similar in Jekyll and wild-type tibia. Myostatin therefore significantly influences the potency of different tissues, not just muscle, as well as cardiac Ca²⁺-handling proteins. Thus, the pleiotropic phenotype of mstn(-/-) animals may not be due to enhanced muscle development per se, but also to altered stem/progenitor cell pools that ultimately influence tissue potency.

A myostatin and activin decoy receptor enhances bone formation in mice

Myostatin is a member of the bone morphogenetic protein/transforming growth factor-β (BMP/TGFβ) super-family of secreted differentiation factors. Myostatin is a negative regulator of muscle mass as shown by increased muscle mass in myostatin deficient mice. Interestingly, these mice also exhibit increased bone mass suggesting that myostatin may also play a role in regulating bone mass. To investigate the role of myostatin in bone, young adult mice were administered with either a myostatin neutralizing antibody (Mstn-mAb), a soluble myostatin decoy receptor (ActRIIB-Fc) or vehicle. While both myostatin inhibitors increased muscle mass, only ActRIIB-Fc increased bone mass. Bone volume fraction (BV/TV), as determined by microCT, was increased by 132% and 27% in the distal femur and lumbar vertebrae, respectively. Histological evaluation demonstrated that increased BV/TV in both locations was attributed to increased trabecular thickness, trabecular number and bone formation rate. Increased BV/TV resulted in enhanced vertebral maximum compressive force compared to untreated animals. The fact that ActRIIB-Fc, but not Mstn-mAb, increased bone volume suggested that this soluble decoy receptor may be binding a ligand other than myostatin, that plays a role in regulating bone mass. This was confirmed by the significant increase in BV/TV in myostatin deficient mice treated with ActRIIB-Fc. Of the other known ActRIIB-Fc ligands, BMP3 has been identified as a negative regulator of bone mass. However, BMP3 deficient mice treated with ActRIIB-Fc showed similar increases in BV/TV as wild type (WT) littermates treated with ActRIIB-Fc. This result suggests that BMP3 neutralization is not the mechanism responsible for increased bone mass. The results of this study demonstrate that ActRIIB-Fc increases both muscle and bone mass in mice. Therefore, a therapeutic that has this dual activity represents a potential approach for the treatment of frailty.

Expression of muscle anabolic and metabolic factors in mechanically loaded MLO-Y4 osteocytes

Lack of physical activity results in muscle atrophy and bone loss, which can be counteracted by mechanical loading. Similar molecular signaling pathways are involved in the adaptation of muscle and bone mass to mechanical loading. Whether anabolic and metabolic factors regulating muscle mass, i.e., insulin-like growth factor-I isoforms (IGF-I Ea), mechano growth factor (MGF), myostatin, vascular endothelial growth factor (VEGF), or hepatocyte growth factor (HGF), are also produced by osteocytes in bone in response to mechanical loading is largely unknown. Therefore, we investigated whether mechanical loading by pulsating fluid flow (PFF) modulates the mRNA and/or protein levels of muscle anabolic and metabolic factors in MLO-Y4 osteocytes. Unloaded MLO-Y4 osteocytes expressed mRNA of VEGF, HGF, IGF-I Ea, and MGF, but not myostatin. PFF increased mRNA levels of IGF-I Ea (2.1-fold) and MGF (2.0-fold) at a peak shear stress rate of 44Pa/s, but not at 22Pa/s. PFF at 22 Pa/s increased VEGF mRNA levels (1.8- to 2.5-fold) and VEGF protein release (2.0- to 2.9-fold). Inhibition of nitric oxide production decreased (2.0-fold) PFF-induced VEGF protein release. PFF at 22 Pa/s decreased HGF mRNA levels (1.5-fold) but increased HGF protein release (2.3-fold). PFF-induced HGF protein release was nitric oxide dependent. Our data show that mechanically loaded MLO-Y4 osteocytes differentially express anabolic and metabolic factors involved in the adaptive response of muscle to mechanical loading (i.e., IGF-I Ea, MGF, VEGF, and HGF). Similarly to muscle fibers, mechanical loading enhanced expression levels of these growth factors in MLO-Y4 osteocytes. Although in MLO-Y4 osteocytes expression levels of IGF-I Ea and MGF of myostatin were very low or absent, it is known that the activity of osteoblasts and osteoclasts is strongly affected by them. The abundant expression levels of these factors in muscle cells, in combination with low expression in MLO-Y4 osteocytes, provide a possibility that growth factors expressed in muscle could affect signaling in bone cells.

Immunolocalization of myostatin (GDF-8) following musculoskeletal injury and the effects of exogenous myostatin on muscle and bone healing

The time course and cellular localization of myostatin expression following musculoskeletal injury are not well understood; therefore, the authors evaluated the temporal and spatial localization of myostatin during muscle and bone repair following deep penetrant injury in a mouse model. They then used hydrogel delivery of exogenous myostatin in the same injury model to determine the effects of myostatin exposure on muscle and bone healing. Results showed that a "pool" of intense myostatin staining was observed among injured skeletal muscle fibers 12-24 hr postsurgery and that myostatin was also expressed in the soft callus chondrocytes 4 days following osteotomy. Hydrogel delivery of 10 or 100 µg/ml recombinant myostatin decreased fracture callus cartilage area relative to total callus area in a dose-dependent manner by 41% and 80% (p<0.05), respectively, compared to vehicle treatment. Myostatin treatment also decreased fracture callus total bone volume by 30.6% and 38.8% (p<0.05), with the higher dose of recombinant myostatin yielding the greatest decrease in callus bone volume. Finally, exogenous myostatin treatment caused a significant dose-dependent increase in fibrous tissue formation in skeletal muscle. Together, these findings suggest that early pharmacological inhibition of myostatin is likely to improve the regenerative potential of both muscle and bone following deep penetrant musculoskeletal injury.

Regulation of body mass growth through activin type IIB receptor in teleost fish

Myostatin is a TGF-beta family member that plays a key role in regulating skeletal muscle growth. Previous studies in mammals have demonstrated that myostatin is capable of binding the two activin type II receptors. Additionally, activin type II receptors have been shown to be capable of binding a number of other TGF-beta family members besides myostatin. An injection of a soluble form of activin type IIB receptor obtained from CHO cells into wild-type mice generated up to a 60% increase in muscle mass in 2 weeks. The knowledge on the role of activin receptors in fish is limited. In the present study, we examined the growth effect of administering a recombinant, soluble form of goldfish activin type IIB receptor extracellular domain to juvenile and larval goldfish (Carassius auratus), African catfish (Clarias gariepinus) larvae and tilapia (Oreochromis aureus) larvae. We have expressed the goldfish activin type IIB receptor extracellular domain in the yeast Pichia pastoris and we have demonstrated for the first time that this recombinant molecule stimulates growth in teleost fish in a dose-dependent manner. We provide evidence that this body weight increase is achieved by an increase in muscle mass and protein content. Histological analysis of the goldfish muscle revealed that treated fish exhibited hyperplasia as compared to controls. These findings contribute to the understanding of the mechanisms that regulate growth in non-mammalian vertebrates and suggest a powerful biotechnology approach to improving fish growth in aquaculture.

Differential antagonism of activin, myostatin and growth and differentiation factor 11 by wild-type and mutant follistatin

Follistatin binds and neutralizes members of the TGFbeta superfamily including activin, myostatin, and growth and differentiation factor 11 (GDF11). Crystal structure analysis of the follistatin-activin complex revealed extensive contacts between follistatin domain (FSD)-2 and activin that was critical for the high-affinity interaction. However, it remained unknown whether follistatin residues involved with myostatin and GDF11 binding were distinct from those involved with activin binding. If so, this would allow development of myostatin antagonists that would not inhibit activin actions, a desirable feature for development of myostatin antagonists for treatment of muscle-wasting disorders. We tested this hypothesis with our panel of point and domain swapping follistatin mutants using competitive binding analyses and in vitro bioassays. Our results demonstrate that activin binding and neutralization are mediated primarily by FSD2, whereas myostatin binding is more dependent on FSD1, such that deletion of FSD2 or adding an extra FSD1 in place of FSD2 creates myostatin antagonists with vastly reduced activin antagonism. However, these mutants also bind GDF11, indicating that further analysis is required for creation of myostatin antagonists that will not affect GDF11 activity that could potentially elicit GDF11-induced side effects in vivo.

Activin subunit and receptor expression in normal and cleft human fetal palate tissues

Craniofacial malformations, such as cleft palate, present serious complications in the newborn and are often of unknown etiology. Activin BA subunit deletion leads to cleft palate in mice, but the expression of this protein in the human palate has not been explored. Our goal was to determine the spatial and temporal expression of inhibin/activin subunits; the binding protein, follistatin; and activin receptors in the human fetal palate. Residual human fetal palate tissues, with or without cleft, were collected during routine autopsy at Women and Infants Hospital. Inhibin/activin alpha and beta subunits, follistatin, and activin receptor protein and mRNA expression were studied by immunocytochemistry and reverse-transcriptase polymerase chain reaction (RT-PCR) experiments, respectively. Dimeric activin A levels were compared in cleft and normal palate tissue homogenates by immunoassay. Activin BA, follistatin, and activin receptor type IIA proteins were observed in normal and cleft palate tissues throughout pregnancy (gestational weeks 11 to 40). Proteins were predominantly found in developing bone cells, with no significant group differences. Inhibin/activin BA subunit, follistatin, and activin receptor mRNAs were also detected in normal and cleft fetal palate tissues, but inhibin alpha and BB subunit were absent. Inhibin/activin BA subunit expression was consistent with the presence of dimeric activin A, but levels did not differ significantly between cleft and control tissues. Inhibin/activin BA subunit, follistatin, and activin receptor proteins and mRNAs are present in the human fetal palate. These data suggest that activin signalling has the potential to be associated with human palate development.

Loss of myostatin (GDF8) function increases osteogenic differentiation of bone marrow-derived mesenchymal stem cells but the osteogenic effect is ablated with unloading

Myostatin (GDF8) is a negative regulator of skeletal muscle growth and mice lacking myostatin show a significant increase in muscle mass and bone density compared to normal mice. In order to further define the role of myostatin in regulating bone mass we sought to determine if loss of myostatin function significantly altered the potential for osteogenic differentiation in bone marrow-derived mesenchymal stem cells in vitro and in vivo. We first examined expression of the myostatin receptor, the type IIB activin receptor (AcvrIIB), in bone marrow-derived mesenchymal stem cells (BMSCs) isolated from mouse long bones. This receptor was found to be expressed at high levels in BMSCs, and we were also able to detect AcvrIIB protein in BMSCs in situ using immunofluorescence. BMSCs isolated from myostatin-deficient mice showed increased osteogenic differentiation compared to wild-type mice; however, treatment of BMSCs from myostatin-deficient mice with recombinant myostatin did not attenuate the osteogenic differentiation of these cells. Loading of BMSCs in vitro increased the expression of osteogenic factors such as BMP-2 and IGF-1, but treatment of BMSCs with recombinant myostatin was found to decrease the expression of these factors. We investigated the effects of myostatin loss-of-function on the differentiation of BMSCs in vivo using hindlimb unloading (7-day tail suspension). Unloading caused a greater increase in marrow adipocyte number, and a greater decrease in osteoblast number, in myostatin-deficient mice than in normal mice. These data suggest that the increased osteogenic differentiation of BMSCs from mice lacking myostatin is load-dependent, and that myostatin may alter the mechanosensitivity of BMSCs by suppressing the expression of osteogenic factors during mechanical stimulation. Furthermore, although myostatin deficiency increases muscle mass and bone strength, it does not prevent muscle and bone catabolism with unloading.

Molecular cloning of the Atlantic salmon activin receptor IIB cDNA - Localization of the receptor and myostatin in vivo and in vitro in muscle cells

In mammals, the activin receptor type IIB (ActRIIB) binds with high affinity several members of the transforming growth factor-beta (TGF-beta) superfamily, including the negative muscle regulator myostatin (MSTN). In this study, an actRIIB cDNA of 1443 bp was isolated by reverse transcription (RT)-PCR from the liver of Atlantic salmon (Salmo salar) encoding almost the complete receptor. The deduced salmon ActRIIB of 481 amino acids (aa) contained the conserved catalytic domain of serine/threonine protein kinases, and showed the highest sequence identity (83-87%) to the zebrafish, chicken and goldfish ActRIIB. Salmon actRIIB mRNA was identified by RT-PCR in all the examined tissues of juvenile fish that was confirmed by in situ hybridization. In comparison, the salmon MSTN signal was less widespread, and co-expression of the receptor and this putative ligand was only demonstrated in skeletal muscle. Consistently, both ActRIIB and MSTN were immunocytologically identified in salmon myoblasts and differentiated myotubes in culture.

Plasticity of mandibular biomineralization in myostatin-deficient mice

Compared with the normal or wild-type condition, knockout mice lacking myostatin (Mstn), a negative regulator of skeletal muscle growth, develop significant increases in relative masticatory muscle mass as well as the ability to generate higher maximal muscle forces. Wild-type and myostatin-deficient mice were compared to assess the postweaning influence of elevated masticatory loads because of increased jaw-adductor muscle and bite forces on the biomineralization of mandibular cortical bone and dental tissues. Microcomputed tomography (microCT) was used to quantify bone density at a series of equidistant external and internal sites in coronal sections for two symphysis and two corpus locations. Discriminant function analyses and nonparametric ANOVAs were used to characterize variation in biomineralization within and between loading cohorts. Multivariate analyses indicated that 95% of the myostatin-deficient mice and 95% of the normal mice could be distinguished based on biomineralization values at both symphysis and corpus sections. At the corpus, ANOVAs suggest that between-group differences are due to the tendency for cortical bone mineralization to be higher in myostatin-deficient mice, coupled with higher levels of dental biomineralization in normal mice. At the symphysis, ANOVAs indicate that between-group differences are related to significantly elevated bone-density levels along the articular surface and external cortical bone in the knockout mice. Both patterns, especially those for the symphysis, appear because of the postweaning effects of increased masticatory stresses in the knockout mice versus normal mice. The greater number of symphyseal differences suggest that bone along this jaw joint may be characterized by elevated plasticity. Significant differences in bone-density levels between normal and myostatin-deficient mice, coupled with the multivariate differences in patterns of plasticity between the corpus and symphysis, underscore the need for a comprehensive analysis of the plasticity of masticatory tissues vis-à-vis altered mechanical loads.

Increased muscle mass with myostatin deficiency improves gains in bone strength with exercise

We tested the hypothesis that increased muscle mass augments increases in bone strength normally observed with exercise. Myostatin-deficient mice, which show increased muscle mass, were exercised along with wildtype mice. Results indicate that increases in bone strength with exercise are greater in myostatin-deficient mice than in wildtype mice, suggesting that the combination of increased muscle mass and physical activity has a greater effect on bone strength than either increased muscle mass or intense exercise alone.

INTRODUCTION

Muscle (lean) mass is known to be a significant predictor of peak BMD in young people, and exercise is also found to increase bone mass in growing humans and laboratory animals. We sought to determine if increased muscle mass resulting from myostatin deficiency would enhance gains in bone strength that usually accompany exercise.

MATERIALS AND METHODS

Male mice lacking myostatin (GDF-8) were used as an animal model showing increased muscle mass. Wildtype and myostatin-deficient mice (n = 10-12 per genotype) were exercised on a treadmill for 30 minutes/day, 5 days/week, for 4 weeks starting at 12 weeks of age. Caged wildtype and myostatin-deficient mice (n = 10-12 per genotype) were included as sedentary controls. Structural and biomechanical parameters were measured from the radius.

RESULTS

Ultimate force (F(u)), displacement (D(u)), toughness (energy-to-fracture; U), and ultimate strain (epsilon(u)) increased significantly with exercise in myostatin-deficient mice but not in normal mice. When F(u) is normalized by body mass, exercised myostatin-deficient mice show an increase in relative bone strength of 30% compared with caged controls, whereas exercised wildtype mice do not show a significant increase in ultimate force relative to caged controls. Relative to body weight, the radii of exercised myostatin-deficient mice are >25% stronger than those of exercised normal mice.

CONCLUSIONS

Increased muscle mass resulting from inhibition of myostatin function improves the positive effects of exercise on bone strength.

Biomineralization and adaptive plasticity of the temporomandibular joint in myostatin knockout mice

Mice lacking myostatin (GDF-8), a negative regulator of skeletal muscle growth, show a significant increase in muscle mass versus normal mice. We compared wild-type and myostatin deficient mice to assess the postnatal effect of elevated masticatory loads due to increased jaw-adductor muscle activity and greater bite forces on mandibular condyle morphology. Microcomputed tomography (microCT) was used to provide details of internal condylar morphology and quantify bone density in three condylar regions. Biomineralization levels, as well as external mandibular dimensions, were used to characterize within-slice, within-joint, within-group and between-group variation. Dimensions of the mandible and mandibular condyle were similar between the myostatin knockout and normal mice. Knockout mice exhibited significantly more biomineralization on the outer surface of the condylar subchondral bone and along the condylar neck, most notably on the buccal side of the condylar neck. The buccal side of the inner aspect of the condyle was significantly less biomineralized in knockout mice, both for the pooled data and for the posterior and anterior condylar slices. Whilst normal mice had symmetric subchondral bone surfaces, those of knockout mice were asymmetric, with a lower, less convex surface on the buccal side versus the lingual side. This appears related to the ontogenetic effects of increased masticatory stress in the mandibles of knockout mice as compared to normal mice. Significant differences in biomineralization between normal and myostatin knockout mice, coupled with the lack of significant differences in certain external dimensions, underscores a need for information on the external and internal morphology of mineralized tissues vis-à-vis altered or excessive mechanical loads.

Muscle-bone interactions in dystrophin-deficient and myostatin-deficient mice

We have investigated muscle-bone interactions using two mouse mutants that are known to differ from normal mice in skeletal muscle growth and development: mice lacking myostatin (GDF8) and mice lacking dystrophin (mdx). Myostatin-deficient mice show increased muscle size and strength compared to normal mice, whereas the mdx mouse is a well-established animal model for Duchenne muscular dystrophy. The mdx mice have significantly larger hindlimb muscles than controls, and histological sections of the quadriceps muscles show dystrophic changes with extensive fibrosis. Femoral bone mineral density (BMD) and fracture strength (Fu) are significantly greater in mdx mice than controls, and these variables are more strongly correlated with quadriceps muscle mass than with body mass. In contrast, mdx mice do not shower high bone mineral density in the spine relative to controls, whereas myostatin-deficient mice have significantly increased BMD in the lumbar spine compared to normal mice. Both mdx mice and myostatin-deficient mice have expanded femoral trochanters for attachment of large hindlimb muscles, and both mutant strains show increased cross-sectional area moments of inertia mediolaterally (Iyy) but not anteroposteriorly (Ixx) compared to normal mice. These data suggest that lean (muscle) mass is a significant determinant of bone mineral density and strength in the limb skeleton, even when accompanied by a dystrophic phenotype. Likewise, increased muscle mass produces a marked increase in the external dimensions of muscle attachment sites, even when increased muscle size is accompanied by extensive fibrosis and muscle weakness.

Zebrafishacvr2a andacvr2b exhibit distinct roles in craniofacial development

To examine the roles of activin type II receptor signaling in craniofacial development, full-length zebrafish acvr2a and acvr2b clones were isolated. Although ubiquitously expressed as maternal mRNAs and in early embryogenesis, by 24 hr postfertilization (hpf), acvr2a and acvr2b exhibit restricted expression in neural, hindbrain, and neural crest cells (NCCs). A morpholino-based targeted protein depletion approach was used to reveal discrete functions for each acvr2 gene product. The acvr2a morphants exhibited defects in the development of most cranial NCC-derived cartilage, bone, and pharyngeal tooth structures, whereas acvr2b morphant defects were largely restricted to posterior arch structures and included the absence and/or aberrant migration of posterior NCC streams, defects in NCC-derived posterior arch cartilages, and dysmorphic pharyngeal tooth development. These studies revealed previously uncharacterized roles for acvr2a and acvr2b in hindbrain and NCC patterning, in NCC derived pharyngeal arch cartilage and joint formation, and in tooth development.

Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium

It has been 70 years since the name inhibin was used to describe a gonadal factor that negatively regulated pituitary hormone secretion. The majority of this period was required to achieve purification and definitive characterization of inhibin, an event closely followed by identification and characterization of activin and follistatin (FS). In contrast, the last 15-20 years saw a virtual explosion of information regarding the biochemistry, physiology, and biosynthesis of these proteins, as well as identification of activin receptors, and a unique mechanism for FS action-the nearly irreversible binding and neutralization of activin. Many of these discoveries have been previously summarized; therefore, this review will cover the period from the mid 1990s to present, with particular emphasis on emerging themes and recent advances. As the field has matured, recent efforts have focused more on human studies, so the endocrinology of inhibin, activin, and FS in the human is summarized first. Another area receiving significant recent attention is local actions of activin and its regulation by both FS and inhibin. Because activin and FS are produced in many tissues, we chose to focus on a few particular examples with the most extensive experimental support, the pituitary and the developing follicle, although nonreproductive actions of activin and FS are also discussed. At the cellular level, it now seems that activin acts largely as an autocrine and/or paracrine growth factor, similar to other members of the transforming growh factor beta superfamily. As we discuss in the next section, its actions are regulated extracellularly by both inhibin and FS. In the final section, intracellular mediators and modulators of activin signaling are reviewed in detail. Many of these are shared with other transforming growh factor beta superfamily members as well as unrelated molecules, and in a number of cases, their physiological relevance to activin signal propagation remains to be elucidated. Nevertheless, taken together, recent findings suggest that it may be more appropriate to consider a new paradigm for inhibin, activin, and FS in which activin signaling is regulated extracellularly by both inhibin and FS whereas a number of intracellular proteins act to modulate cellular responses to these activin signals. It is therefore the balance between activin and all of its modulators, rather than the actions of any one component, that determines the final biological outcome. As technology and model systems become more sophisticated in the next few years, it should become possible to test this concept directly to more clearly define the role of activin, inhibin, and FS in reproductive physiology.

A model for intramembranous ossification during fracture healing

We have developed a method to study the molecular basis of intramembranous fracture healing. Unlike intramedullary rods that permit rotation of the fractured bone segments, our murine model relies on an external fixation device to provide stabilization. In this study we compare stabilized fracture callus tissues with callus tissues from non-stabilized fractures during the inflammatory, soft callus, hard callus, and remodeling stages of healing. Histological analyses indicate that stabilized fractures heal with virtually no evidence of cartilage whereas non-stabilized fractures produce abundant cartilage at the fracture site. Expression patterns of collagen type IIa (colIIa) and osteocalcin (oc) reveal that mesenchymal cells at the fracture site commit to either a chondrogenic or an osteogenic lineage during the earliest stages of healing. The mechanical environment influences this cell fate decision, since mesenchymal cells in a stabilized fracture express oc and fail to express colIIa. Future studies will use this murine model of intramembranous fracture healing to explore, at a molecular level, how the mechanical environment exerts its influence on healing of a fracture.

Regulation of cell proliferation, apoptosis, and carcinogenesis by activin

The aim of this review is to provide insight into the molecular mechanisms by which activin A modulates cell proliferation, apoptosis, and carcinogenesis in vitro and in vivo. Activin A, a member of the TGFbeta superfamily, has various effects on diverse biological systems, including cell growth inhibition in many cell types. However, the mechanism(s) by which activin exerts its inhibitory effects are not yet understood. This review highlights activin's effects on activin receptors and signaling pathway, modulation of activin signaling, and regulation of cell proliferation and apoptosis by activin. Based on the experiences of all the authors, we emphasized cell cycle inhibitors such as p16 and p21 and regulators of apoptosis such as p53 and members of the bcl-2 family. Aside from activin's inhibition of cell proliferation and enhancement of apoptosis, other newly developed methods for molecular studies of apoptosis by activin were briefly presented that support the role of activin as an inhibitor of carcinogenesis and cancer progression. These methods include subtractive hybridization based on covalent bonding, a simple and accurate means to determine molecular profile of as few as 20 cells based on an RNA-PCR approach, and a messenger RNA-antisense DNA interference phenomenon (D-RNAi), resulting in a long-term gene knockout effects.

Involvement of activin in the regulation of bone metabolism

Osteogenic activities of activin, a member of TGF-beta superfamily, have been shown in both in vivo and in vitro studies. Local injection of activin promoted fracture healing in rat fibula fracture models. Since both activin and its receptor are expressed during fracture healing, activin would be involved in the healing process via autocrine and/or paracrine mode of action. Activin was abundantly stored also in normal bone matrix, presumably produced by osteoblasts in the process of normal bone formation. It was observed that activin was released in the culture of neonatal mouse calvaria, and the release was strongly coupled with bone resorption. Thus, activin could be involved in the regulation of bone remodeling as one of coupling factors, as was suggested for TGF-ss. Systemic administration of activin in aged ovariectomized rats, in which bone mass decreases due to uncoupling between bone resorption and formation, increased both bone mass and mechanical strength of vertebral bodies. These findings suggest physiological roles of activin in the regulation of bone formation, and further, its possible usefulness for the therapy of fracture and osteoporosis.

Coordinate expression of novel genes during osteoblast differentiation

To achieve new insights into the coordinate regulation of gene expression during osteoblast differentiation we utilized an approach involving global analysis of gene expression to obtain the identities of messenger RNAs (mRNAs) expressed using an established in vitro model of bone development. MC3T3-E1 osteoblast-like cells were induced to differentiate by the addition of beta-glycerophosphate (beta-GP) and ascorbic acid. RNA samples derived from induced and uninduced control MC3T3-E1 cells were used to prepare complementary DNA (cDNA) for serial analysis of gene expression (SAGE). A preliminary SAGE database was produced and used to prepare a hybridization array to further facilitate the characterization of changes in the expression levels of 92 of the SAGE-mRNA assignments after induction of osteoblast differentiation, specifically after 6 days and 14 days of ascorbate treatment. SAGE-array hybridization analysis revealed coordinate induction of a number of mRNAs including Rab24, calponin, and calcyclin. Levels of MSY-1, SH3P2, fibronectin, alpha-collagen, procollagen, and LAMPI mRNAs, present at day 6 postinduction, were markedly reduced by day 14 postinduction. A number of unanticipated and potentially important developmental genes were identified including the transforming growth factor beta (TGF-beta) superfamily member Lefty-1. Lefty-1 transcript and translation product were found to be induced during the course of MC3T3-E1 cell differentiation. We present evidence, using transient transfection and antibody neutralization approaches, that Lefty-1 modulates the induction of alkaline phosphatase (ALP) after treatment of MC3T3-E1 cells with ascorbate and beta-GP. These data should provide useful new information for future analysis of transcriptional events in osteoblast differentiation and mineralization.

Transient expression of activin betaA mRNA on osteoprogenitor cells in rat bone regeneration after drill-hole injury

We investigated the expression of activin betaA on osteoprogenitor cells in the regenerating bone and bone marrow of the rat femur after drill-hole injury, by immunocytochemistry and in situ hybridization. The periosteum and endosteum adjacent to the wound region showed marked thickening at day 3 and abundant osteoprogenitor cells, which were immunoreactive for proliferating cell nuclear antigen and showed positive reactions for alkaline phosphatase activity, and existed in the inner layer of the periosteum as well as in the endosteum. During the same period, these osteoprogenitor cells began to exhibit activin betaA immunoreactivity and mRNA expression. However, the latter expression gradually reduced the intensity as the cells started to express osteocalcin mRNA during their differentiation to osteoblasts participating in the periosteal and medullary bone formation from day 5. Immunoreactivity for activin type IB and II receptors was also found on activin betaA-immunoreactive cells between days 3 and 7. The above findings suggest that proliferating osteoprogenitor cells, before their transformation to osteoblasts, transiently produce and release activin A, which may play crucial roles in bone and bone marrow regeneration in a receptor-mediated, autocrine and paracrine fashion.

Cytokine-specific induction of the TGF-beta inducible early gene (TIEG): regulation by specific members of the TGF-beta family

Select members of the TGF-beta family of cytokines play key regulatory roles in skeletal development, structure, and turnover. This laboratory has previously reported that TGF-beta treatment of immortalized normal human fetal osteoblast (hFOB) cells results in the rapid induction of the mRNA levels of a TGF-beta inducible early gene (TIEG) followed by changes in cell proliferation and bone matrix protein production. Previous studies have also shown that nonmembers of the TGF-beta superfamily showed little or no induction of TIEG mRNA. This article further addresses the cytokine specificity of this TIEG induction by examining whether activin and select bone morphogenetic proteins, (BMP-2, BMP-4, and BMP-6), which are representative of different subfamilies of this superfamily, also induce the expression of TIEG in hFOB cells. However, TGF-beta remained the most potent of these cytokines, inducing TIEG mRNA steady-state levels at 0.1 ng/ml, with a maximum induction of 24-fold at 2.0 ng/ml. The BMP-2 (16-fold), BMP-4 (4-fold), and activin (1-3-fold) also induced TIEG mRNA levels, but at reduced degrees compared to TGF-beta (24-fold), and only at much higher cytokine concentrations, e.g., 50-100 ng/ml, compared to 2 ng/ml for TGF-beta. BMP-6 showed no effect on TIEG mRNA levels. The TIEG protein levels generally correlated with the mRNA steady-state levels. As with TGF-beta, BMP-2 treatment of hFOB cells was shown by confocal microscopy to induce a rapid translocation of the TIEG protein to the nucleus. In summary, the relative potencies of these TGF-beta family members to induce TIEG expression generally follows the general osteoinductive capacity of these cytokines, with TGF-beta >>> BMP-2 > BMP-4 > activin >> BMP-6.

BMP receptors in limb and tooth formation

Members of the TGF-beta superfamily signal through receptor complexes comprised of type I and type II receptors. These receptors, which are serine/threonine kinases, form two new classes of transmembrane receptor kinases. The activity of both of the kinases is necessary for signal transduction in response to ligand binding. Bone morphogenetic proteins (BMPs), which are members of the TGF-beta superfamily, bind to multiple type I and type II receptors. There is growing evidence to support the hypothesis that the BMP receptors are differentially regulated during development and that they have both unique and overlapping functions. Thus, the nature and distribution of the BMP receptors, which are reviewed here in the context of the development of limbs and teeth, appear to be critical in the control of the diverse activities of BMPs.

Activin release from bone coupled to bone resorption in organ culture of neonatal mouse calvaria

Activin, a member of the transforming growth factor-beta (TGF-beta) superfamily, is present in the bone matrix and assumed to be involved in the regulation of bone formation. In the present study, we investigated whether the release of activin from bone is coupled with bone resorption. Neonatal mouse calvaria were cultured in the presence of various stimulators of bone resorption (parathyroid hormone [PTH], interleukin-1beta, prostaglandin E2) for up to 72 h, and the activin activity in the medium was measured using a specific bioassay for activin. Activin activity was accumulated in proportion to the time- and dose-dependent increase in calcium release from bone into the medium (bone resorption). An inhibition of PTH-dependent bone resorption by a bisphosphonate, disodium dichlormethane-1,1-bisphosphonic acid (Cl2MBP), completely blocked release of activin activity from bone into the medium. In primary culture of calvarial cells, however, neither PTH nor Cl2MBP affected activin production. These findings indicate that release of activin activity from bone tissue is strongly coupled to bone resorption. Because activin possesses osteogenic activities, activin released locally from bone might be involved in the regulation of bone formation in the physiological process of bone remodeling, as has been suggested for TGF-beta.

The type I BMP receptor BMPRIB is required for chondrogenesis in the mouse limb

Mice carrying a targeted disruption of BmprIB were generated by homologous recombination in embryonic stem cells. BmprIB(−/−) mice are viable and, in spite of the widespread expression of BMPRIB throughout the developing skeleton, exhibit defects that are largely restricted to the appendicular skeleton. Using molecular markers, we show that the initial formation of the digital rays occurs normally in null mutants, but proliferation of prechondrogenic cells and chondrocyte differentiation in the phalangeal region are markedly reduced. Our results suggest that BMPRIB-mediated signaling is required for cell proliferation after commitment to the chondrogenic lineage. Analyses of BmprIB and Gdf5 single mutants, as well as BmprIB; Gdf5 double mutants suggests that GDF5 is a ligand for BMPRIB in vivo. BmprIB; Bmp7 double mutants were constructed in order to examine whether BMPRIB has overlapping functions with other type I BMP receptors. BmprIB; Bmp7 double mutants exhibit severe appendicular skeletal defects, suggesting that BMPRIB and BMP7 act in distinct, but overlapping pathways. These results also demonstrate that in the absence of BMPRIB, BMP7 plays an essential role in appendicular skeletal development. Therefore, rather than having a unique role, BMPRIB has broadly overlapping functions with other BMP receptors during skeletal development.

Bone- and non-bone-derived growth factors and effects on bone healing

In the future, it may be possible to manipulate the fracture site with exogenous growth factors to allow successful union of the bone ends without additional surgery. The complex interaction of growth factors, the timing of their appearance and disappearance at the wound site, and the concentrations necessary to achieve specific effects must be studied more thoroughly. For growth factors to find widespread clinical use, there must be evidence that healing is enhanced. It may be difficult to enhance the healing of fresh fractures in normal animals, and it may also be difficult to demonstrate the healing of nonunion fractures. Because of the great variability in fractures of clinical patients, studies designed to determine the effect of growth factors on bone healing must be carefully designed with appropriate attention given to randomizing patients based on the risk of delayed healing and other patient characteristics.

Local administration of activin promotes fracture healing in the rat fibula fracture model

Osteogenic activities of activin, a member of the transforming growth factor (TGF)-beta superfamily, have been demonstrated in both in vitro and in vivo studies. The present study investigates the effects of topical application of activin on fracture healing using rat fibula fracture models. Activin (0.4-10 microg/day) was injected locally to the fracture once a day for 2 weeks. Activin promoted callus formation in a dose-dependent manner and both callus volume and callus weight were significantly increased with doses of 2-10 microg/day activin. Also, 3 weeks of activin treatment increased the mechanical strength of the healing bone in addition to the callus mass. Histological study 2 weeks after the fracture revealed that activin promoted endochondral bone formation. Immunohistochemical examination of the fractured tibia revealed that activin was localized to osteoblasts and chondrocytes in the region ossified both endochondrally and intramembranously. These findings suggest that activin expressed during fracture healing promotes the healing process through an autocrine/paracrine mode of action.

The potential role of transforming growth factor beta in fracture healing

As shown in previous studies, the transforming growth factor beta superfamily of growth factors is involved in many aspects of skeletal development and regulation, including fracture repair and bone regeneration. Several studies have shown transforming growth factor beta messenger ribonucleicacid and protein expression in cells comprising fracture callus. In healing fractures in a chick model, differential isoform expression of the transforming growth factor betas was observed by in situ hybridization, with more prominent expression of the transforming growth factor beta 2 and transforming growth factor beta 3 isoforms. Small amounts of transforming growth factor beta 1 were present in early callus and increased in expression later during chondrogenesis and endochondral ossification. These findings resemble those reported in rat and human fracture callus. Transforming growth factor beta 4 expression was not significant in the chick fracture model. Transforming growth factor beta can function as a morphogen when injected subperiosteally, inducing cartilage and bone formation that morphologically resembles many of the events occurring in fracture callus. Exogenous transforming growth factor beta has been used in several critical size defect models of bone regeneration and fracture healing, with most of the studies showing increased bone or callus formation and increased mechanical stability. Numerous variables, including markedly different dose ranges and differing isoforms, dosing regimens, delivery methods, animal models, and various times and endpoint measures for analysis, make it difficult to comparatively assess the effects of transforming growth factor beta on bone healing. Additional study is necessary to satisfactorily determine the role of transforming growth factor beta in normal fracture healing and its potential for use in augmenting this process.

Cloning of a novel cDNA expressed during the early stages of fracture healing

Using differential mRNA display (DD-PCR), a novel cDNA, FxC1 (Fracture Callus 1) was isolated from the early stages of a healing fractured femur. Utilizing 5' RACE PCR, a 598-bp full-length cDNA was obtained for FxC1 that contains an open reading frame (ORF) of 243 bp, encoding for an 80 amino acid protein. Within this ORF, a leucine zipper motif was present. In vitro transcription/translation of the full-length cDNA generated the expected 9-kDa protein. Northern analysis reveals that this gene is expressed in calluses harvested from post-fracture day 5, 7 and 10, as well as in several other tissues and bone-derived cell lines. During the differentiation of MC3T3 cells along the osteoblast lineage, FxC1 expression increases 3- to 4-fold during the production and deposition of matrix proteins, suggesting a possible role for this protein in cell differentiation.

Immunohistochemical detection of activin A, follistatin, and activin receptors during fracture healing in the rat

Activins are multifunctional proteins that belong to the transforming growth factor-beta superfamily and are thought to play an important role in modulating the formation of bone. Activins exert their cellular effects by way of activin type-I and type-II serine/threonine kinase receptors. Follistatin is an activin-binding protein that can suppress the biological effects of activins. In this study, the immunohistochemical expression of activin A, follistatin, and activin receptors was studied during fracture healing in the rat. Activin A was weakly detected in the periosteum near the fracture ends at an early stage but was absent in the chondrocytes around the fracture gap, where endochondral ossification took place. An antibody to follistatin stained osteogenic cells in the periosteum near the fracture ends; moderate and strong staining were observed in proliferating, mature, and hypertrophied chondrocytes at the sites of endochondral ossification. Levels of activin A and follistatin were high near the osteoblasts on the surface of the newly formed trabecular bone. In addition, an intense localization of activin A was noted where multinucleated osteoclast-like cells were present. This study suggests that the activin-follistatin system may contribute to cellular events related to the formation and remodeling of bone during fracture healing. Activin type-I and type-II receptors were co-expressed in intramembranous and endochondral ossification sites. The expression of activin type-I, type-II, and type-IIB receptors in the absence of activin A in the endochondral ossification suggests that other isoforms of activins may signal by way of these receptors.

Serum 1alpha,25-dihydroxyvitamin D3 accumulates into the fracture callus during rat femoral fracture healing

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is thought to be an important systemic factor in the fracture repair process, but the mechanism of action of 1,25(OH)2D3 has not been clearly defined. In this study, the role of 1,25(OH)2D3 in the fracture repair process was analyzed in a rat closed femoral fracture model. The plasma concentration of 1,25(OH)2D3 rapidly decreased on day 3 and continued to decrease to 10 days after fracture. We assessed whether this decrease was based on the accelerated degradation or retardation of the synthesis rate of 1,25(OH)2D3, from 25(OH)D3. After radiolabeled 3H-1,25(OH)2D3 or 3H-25(OH)D3 was injected i.v. into fractured or control (unfractured) rats, the concentrations of 25(OH)D3 and 1,25(OH)2D3 metabolites were measured by HPLC. The plasma concentrations of these radiolabeled metabolites in fractured group were similar to those in control rats early after operation. However, radioactivity in the femurs of fractured rats was higher than that of the control group. Furthermore, the radioactivity was concentrated in the callus of the fractured group analyzed by autoradiography. 1,25(OH)2D3 receptor gene expression was detected early after fracture and, additionally, both in the soft and hard callus on days 7 and 13 after fracture. These results showed that the rapid disappearance of 1,25(OH)2D3 in the early stages after fracture was not due to either increased degradation or decreased synthesis of 1,25(OH)2D3, but rather to increased consumption. Further, these results suggest the possibility that plasma 1,25(OH)2D3 becomes localized in the callus and may regulate cellular events in the process of fracture healing.