Controversy of physiological vs. pharmacological effects of BMP signaling: Constitutive activation of BMP type IA receptor-dependent signaling in osteoblast lineage enhances bone formation and resorption, not affecting net bone mass

Hyperthyroidism-driven bone loss depends on BMP receptor Bmpr1a expression in osteoblasts

Hyperthyroidism is a well-known trigger of high bone turnover that can lead to the development of secondary osteoporosis. Previously, we have shown that blocking bone morphogenetic protein (BMP) signaling systemically with BMPR1A-Fc can prevent bone loss in hyperthyroid mice. To distinguish between bone cell type-specific effects, conditional knockout mice lacking Bmpr1a in either osteoclast precursors (LysM-Cre) or osteoprogenitors (Osx-Cre) were rendered hyperthyroid and their bone microarchitecture, strength and turnover were analyzed. While hyperthyroidism in osteoclast precursor-specific Bmpr1a knockout mice accelerated bone resorption leading to bone loss just as in wildtype mice, osteoprogenitor-specific Bmpr1a deletion prevented an increase of bone resorption and thus osteoporosis with hyperthyroidism. In vitro, wildtype but not Bmpr1a-deficient osteoblasts responded to thyroid hormone (TH) treatment with increased differentiation and activity. Furthermore, we found an elevated Rankl/Opg ratio with TH excess in osteoblasts and bone tissue from wildtype mice, but not in Bmpr1a knockouts. In line, expression of osteoclast marker genes increased when osteoclasts were treated with supernatants from TH-stimulated wildtype osteoblasts, in contrast to Bmpr1a-deficient cells. In conclusion, we identified the osteoblastic BMP receptor BMPR1A as a main driver of osteoporosis in hyperthyroid mice promoting TH-induced osteoblast activity and potentially its coupling to high osteoclastic resorption.

Impact of PEG sensitization on the efficacy of PEG hydrogel-mediated tissue engineering

While poly(ethylene glycol) (PEG) hydrogels are generally regarded as biologically inert blank slates, concerns over PEG immunogenicity are growing, and the implications for tissue engineering are unknown. Here, we investigate these implications by immunizing mice against PEG to stimulate anti-PEG antibody production and evaluating bone defect regeneration after treatment with bone morphogenetic protein-2-loaded PEG hydrogels. Quantitative analysis reveals that PEG sensitization increases bone formation compared to naive controls, whereas histological analysis shows that PEG sensitization induces an abnormally porous bone morphology at the defect site, particularly in males. Furthermore, immune cell recruitment is higher in PEG-sensitized mice administered the PEG-based treatment than their naive counterparts. Interestingly, naive controls that were administered a PEG-based treatment also develop anti-PEG antibodies. Sex differences in bone formation and immune cell recruitment are also apparent. Overall, these findings indicate that anti-PEG immune responses can impact tissue engineering efficacy and highlight the need for further investigation.

Multiple roles of ALK3 in osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by progressive cartilage degradation, synovial membrane inflammation, osteophyte formation, and subchondral bone sclerosis. Pathological changes in cartilage and subchondral bone are the main processes in OA. In recent decades, many studies have demonstrated that activin-like kinase 3 (ALK3), a bone morphogenetic protein receptor, is essential for cartilage formation, osteogenesis, and postnatal skeletal development. Although the role of bone morphogenetic protein (BMP) signalling in articular cartilage and bone has been extensively studied, many new discoveries have been made in recent years around ALK3 targets in articular cartilage, subchondral bone, and the interaction between the two, broadening the original knowledge of the relationship between ALK3 and OA. In this review, we focus on the roles of ALK3 in OA, including cartilage and subchondral bone and related cells. It may be helpful to seek more efficient drugs or treatments for OA based on ALK3 signalling in future.

Increased BMP-Smad signaling does not affect net bone mass in long bones

Bone morphogenetic proteins (BMPs) have been used for orthopedic and dental application due to their osteoinductive properties; however, substantial numbers of adverse reactions such as heterotopic bone formation, increased bone resorption and greater cancer risk have been reported. Since bone morphogenetic proteins signaling exerts pleiotropic effects on various tissues, it is crucial to understand tissue-specific and context-dependent functions of bone morphogenetic proteins. We previously reported that loss-of-function of bone morphogenetic proteins receptor type IA (BMPR1A) in osteoblasts leads to more bone mass in mice partly due to inhibition of bone resorption, indicating that bone morphogenetic protein signaling in osteoblasts promotes osteoclast function. On the other hand, hemizygous constitutively active (ca) mutations for BMPR1A (caBmpr1awt/+) in osteoblasts result in higher bone morphogenetic protein signaling activity and no overt skeletal changes in adult mice. Here, we further bred mice for heterozygous null for Bmpr1a (Bmpr1a+/−) and homozygous mutations of caBmpr1a (caBmpr1a+/+) crossed with Osterix-Cre transgenic mice to understand how differences in the levels of bone morphogenetic protein signaling activity specifically in osteoblasts contribute to bone phenotype. We found that Bmpr1a+/−, caBmpr1awt/+ and caBmpr1a+/+ mice at 3 months of age showed no overt bone phenotypes in tibiae compared to controls by micro-CT and histological analysis although BMP-Smad signaling is increased in both caBmpr1awt/+ and caBmpr1a+/+ tibiae and decreased in the Bmpr1a+/− mice compared to controls. Gene expression analysis demonstrated that slightly higher levels of bone formation markers and resorption markers along with levels of bone morphogenetic protein-Smad signaling, however, there was no significant changes in TRAP positive cells in tibiae. These findings suggest that changes in bone morphogenetic protein signaling activity within differentiating osteoblasts does not affect net bone mass in the adult stage, providing insights into the concerns in the clinical setting such as high-dose and unexpected side effects of bone morphogenetic protein application.

Bone morphogenetic protein pathway responses and alterations of osteogenesis in metastatic prostate cancers

BACKGROUND

Prostate cancer is a common cancer in men that annually results in more than 33 000 US deaths. Mortality from prostate cancer is largely from metastatic disease, reflecting on the great strides in the last century of treatments in care for the localized disease. Metastatic castrate resistant prostate cancer (mCRPC) will commonly travel to the bone, creating unique bone pathology that requires nuanced treatments in those sites with surgical, radio and chemotherapeutic interventions. The bone morphogenetic protein (BMP) pathway has been historically studied in the capacity to regulate the osteogenic nature of new bone. New mineralized bone generation is a frequent and common observation in mCRPC and referred to as blastic bone lesions. Less common are bone destructive lesions that are termed lytic.

METHODS

We queried the cancer genome atlas (TCGA) prostate cancer databases for the expression of the BMP pathway and found that distinct gene expression of the ligands, soluble antagonists, receptors, and intracellular mediators were altered in localized versus metastatic disease. Human prostate cancer cell lines have an innate ability to promote blastic- or lytic-like bone lesions and we hypothesized that inhibiting BMP signaling in these cell lines would result in a distinct change in osteogenesis gene expression with BMP inhibition.

RESULTS

We found unique and common changes by comparing these cell lines response and unique BMP pathway alterations. We treated human PCa cell lines with distinct bone pathologic phenotypes with the BMP inhibitor DMH1 and found distinct osteogenesis responses. We analyzed distinct sites of metastatic PCa in the TCGA and found that BMP signaling was selectively altered in commons sites such as lymph node, bone and liver compared to primary tumors.

CONCLUSIONS

Overall we conclude that BMPs in metastatic prostate cancer are important signals and functional mediators of diverse processes that have potential for individualized precision oncology in mCRPC.

Gain-of-Function of FGFR3 Accelerates Bone Repair Following Ischemic Osteonecrosis in Juvenile Mice

Bone collapse, bone deformity, and a long treatment period are major clinical problems associated with juvenile ischemic osteonecrosis (JIO). Accelerating the process of bone repair in JIO is expected to shorten the treatment duration and better maintain morphology. We previously indicated that both bone formation and resorption were accelerated following distraction osteogenesis-mediated limb lengthening in genetically engineered mutant mice with a gain-of-function mutation in fibroblast growth factor receptor 3 (FGFR3) gene (i.e., Fgfr3 mice). The purpose of this study was to investigate the role of FGFR3 in the bone repair process following surgically induced ischemic osteonecrosis in the mutant mice. Epiphyseal deformity was less in the Fgfr3 mice compared to the wild-type mice at 6 weeks following ischemic osteonecrosis in skeletally immature age. Assessment of the morphology by micro-computed tomography (CT) revealed that the trabecular bone volume was increased in the Fgfr3 mice. Dynamic bone histomorphometry revealed increased rates of bone formation and mineral apposition in the Fgfr3 mice at 4 weeks post-surgery. The number of tartrate-resistant acid phosphatase (TRAP)-positive cells rapidly increased, and the numbers of TdT-mediated dUTP nick-end labeling (TUNEL)-positive cells rapidly decreased in the Fgfr3 mice. Vascular endothelial growth factor (VEGF) expression was increased at the earlier phase post-surgery in the Fgfr3 mice. The activation of FGFR3 signaling shortens the time needed for bone repair after ischemic osteonecrosis by accelerating revascularization, bone resorption, and new bone formation. Our findings are clinically relevant as a new potential strategy for the treatment of JIO.

Shaping the bone through iron and iron-related proteins

Well-controlled iron levels are indispensable for health. Iron deficiency is the most common cause of anemia, whereas iron overload, either hereditary or secondary due to disorders of ineffective erythropoiesis, causes widespread organ failure. Bone is particularly sensitive to fluctuations in systemic iron levels as both iron deficiency and overload are associated with low bone mineral density and fragility. Recent studies have shown that not only iron itself, but also iron-regulatory proteins that are mutated in hereditary hemochromatosis can control bone mass. This review will summarize the current knowledge on the effects of iron on bone homeostasis and bone cell activities, and on the role of proteins that regulate iron homeostasis, i.e. hemochromatosis proteins and proteins of the bone morphogenetic protein pathway, on bone remodeling. As disorders of iron homeostasis are closely linked to bone fragility, deeper insights into common regulatory mechanisms may provide new opportunities to concurrently treat disorders affecting iron homeostasis and bone.

Semaphorin 3A promotes the osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells in inflammatory environments by suppressing the Wnt/β-catenin signaling pathway

After periodontal treatment, the local inflammatory environment surrounding periodontal tissues cannot be entirely eliminated. The means by which alveolar bone repair and regeneration are promoted in inflammatory environments have important clinical significance. As a powerful protein that promotes the differentiation of osteocytes, semaphorin 3A (Sema3A) shows potential for bone regeneration therapy. However, the effect of Sema3A on osteogenic differentiation in an inflammatory environment, as well as the underlying mechanism, have not yet been explored. We used lentivirus to transduce rat bone marrow-derived mesenchymal stem cells (rBMSCs) to stably overexpress Sema3A. Lipopolysaccharide from Escherichia coli (E. coli LPS) was used to stimulate rBMSCs to establish an inflammatory environment. ALP staining, Alizarin red staining, ALP activity tests, quantitative RT-PCR (qRT-PCR), and Western blotting were used to elucidate the effect of Sema3A on the osteogenesis of rBMSCs in inflammatory environments. XAV939 and LiCl were used to determine whether the Wnt/β-catenin signaling pathway was involved in attenuating the inhibition of Sema3A-induced osteogenic differentiation by LPS. The qRT-PCR and Western blot results demonstrated that the lentiviral vector (LV-NC) and lentiviral-Sema3A (LV-Sema3A) were successfully transduced into rBMSCs. An inflammatory environment could be established by stimulating rBMSCs with 1 μg/ml E. coli LPS. After Sema3A overexpression, mineral deposition was exacerbated, and the BSP and Runx2 gene and protein expression levels were increased. Furthermore, E. coli LPS activated the Wnt/β-catenin signaling pathway and decreased rBMSC osteogenesis, but these effects were attenuated by Sema3A. In conclusion, Sema3A could protect BMSCs from LPS-mediated inhibition of osteogenic differentiation in inflammatory environments by suppressing the Wnt/β-catenin pathway.

Targeting the BMP Pathway in Prostate Cancer Induced Bone Disease

From the 33,000 men in the U.S. who die from prostate cancer each year, the majority of these patients exhibit metastatic disease with bone being the most common site of metastasis. Prostate cancer bone metastases are commonly blastic, exhibiting new growth of unhealthy sclerotic bone, which can cause painful skeletal related events. Patient's current care entails androgen deprivation therapy, anti-resorptive agents, radiation, and chemotherapy to help control the spread of the cancer but little intervention is available to treat blastic bone disease. The transforming growth factor beta (TGFβ) and bone morphogenetic protein (BMP) pathways are known to regulate bone growth and resorption of destructive lytic bone lesions, yet the role of TGFβ/BMP signaling in prostate cancer blastic vs lytic bone lesions are not fully understood. We hypothesized that to target the BMP/TGFβ pathway, a useful biomarker of bone lytic or blastic pathology would have superior response. We show distinct BMP vs. TGFβ signaling in clinical samples of human prostate cancer bone metastases with either lytic or blastic pathologies. BMPs exhibit distinct effects on bone homeostasis, so to examine the effect of BMP inhibition on healthy bone, we treated mice with the BMP receptor small molecule antagonist DMH1 and saw a modest temporary improvement in bone health, with increased trabecular bone. We next sought to use the BMP inhibitor DMH1 to treat bone metastasis engraftment seeded by a caudal artery injection of the lytic human prostate cell line PC3 in immunodeficient mice. The colonization by PC3 cells to the bone were restricted with DMH1 treatment and bone health was importantly preserved. We next proceeded to test BMP inhibition in an injury model of established bone metastasis via intratibial injection of the MYC-CaP mouse prostate cell line into FVBN syngeneic mice. DMH1 treated mice had a modest decrease in trabecular bone and reduced lymphocytes in circulation without affecting tumor growth. Taken together we show unique responses to BMP inhibition in metastatic prostate cancer in the bone. These studies suggest that profiling bone lesions in metastatic prostate cancer can help identify therapeutic targets that not only treat the metastatic tumor but also address the need to better treat the distinct tumor induced bone disease.