Hypoxia-driven pathways in bone development, regeneration and disease

Comparison of remote ischemic preconditioning and intermittent hypoxia training in fracture healing

Fracture healing in elderly patients is an emerging public health concern. As non‑drug treatments, intermittent hypoxia training (IHT) and remote ischemic preconditioning (RIPC) are considered to have substantial advantages and to aid fracture healing in elderly patients. The purpose of the present study was to evaluate and compare the effects of IHT and RIPC on fracture healing. Micro‑computed tomography (micro‑CT) and biomechanical testing were used to assess the morphology and structural properties of bone callus dissected from aged rats with tibial fractures. In addition, hypoxia‑inducible factor‑1α (HIF‑1α) and its target gene, associated with the healing process, were investigated by reverse transcription‑quantitative polymerase chain reaction and western blot analyses. The micro‑CT‑based parameters, including bone mineral density and trabecular number, were measured, and significant differences were identified between the experimental and control groups. The IHT group exhibited superior bone formation and mineralization rates compared with the RIPC group. The biomechanical testing revealed that the ultimate loading and stiffness values were significantly higher in the IHT group compared with those in the RIPC group. In accordance with previous studies, RIPC exerted a similar effect in increasing the expression of HIF‑1α, and its downstream genes, throughout the course of healing. In addition, the IHT group exhibited increased expression levels of HIF‑1α compared with the RIPC group. Taken together, the results suggested that IHT and RIPC significantly enhanced fracture healing; however, IHT exhibited superior bone formation and healing effects compared with RIPC.

Crosstalk Between Sensory Nerves and Cancer in Bone

PURPOSE OF REVIEW

Sensory nerves (SNs) richly innervate bone and are a component of bone microenvironment. Cancer metastasis in bone, which is under the control of the crosstalk with bone microenvironment, induces bone pain via excitation of SNs innervating bone. However, little is known whether excited SNs in turn affect bone metastasis.

RECENT FINDINGS

Cancer cells colonizing bone promote neo-neurogenesis of SNs and excite SNs via activation of the acid-sensing nociceptors by creating pathological acidosis in bone, evoking bone pain. Denervation of SNs or inhibition of SN excitation decreases bone pain and cancer progression and increases survival in preclinical models. Importantly, patients with cancers with increased SN innervation complain of cancer pain and show poor outcome. SNs establish the crosstalk with cancer cells to contribute to bone pain and cancer progression in bone. Blockade of SN excitation may have not only analgesic effects on bone pain but also anti-cancer actions on bone metastases.

Hypoxia Delays Oligodendrocyte Progenitor Cell Migration and Myelin Formation by Suppressing Bmp2b Signaling in Larval Zebrafish

Hypoxia in newborns tends to result in developmental deficiencies in the white matter of the brain. As previous studies of the effects of hypoxia on neuronal development in rodents and human infants have been unable to use in vivo imaging, insight into the dynamic development of oligodendrocytes (OLs) in the central nervous system under hypoxia is limited. Here, we developed a visual model to study OL development using sublethal postnatal hypoxia in zebrafish larvae. We observed that hypoxia significantly suppressed OL progenitor cell migration toward the dorsum using in vivo imaging. Further, we found that hypoxia affected myelination, as indicated by thinner myelin sheaths and by a downregulation of myelin basic protein expression. Bmp2b protein expression was also significantly downregulated following hypoxia onset. Using gain of function and loss of function experiments, we demonstrated that the Bmp2b protein was associated with the regulation of OL development. Thus, our work provides a visual hypoxia model within which to observe OL development in vivo, and reveals the underlying mechanisms involved in these processes.

Hypoxia promotes maintenance of the chondrogenic phenotype in rat growth plate chondrocytes through the HIF-1α/YAP signaling pathway

The Hippo‑yes‑associated protein (YAP) signaling pathway was previously identified to serve an important role in controlling chondrocyte differentiation and post‑natal growth. Growth plate cartilage tissue is avascular, and hypoxia‑inducible factor (HIF)‑1α is essential for chondrocytes to maintain their chondrogenic phenotype in a hypoxic environment. In the present study, the role of hypoxia and HIF‑1α in the regulation of YAP in chondrocytes was investigated. The data demonstrated that hypoxia promoted the maintenance of the chondrogenic phenotype, HIF‑1α expression and YAP activation in chondrocytes in a time‑dependent manner. Hypoxia promoted YAP activation in a Hippo‑independent manner. Inhibiting the expression of HIF‑1α decreased the activation of YAP and downregulated the expression of sex‑determining region‑box 9 protein (SOX9) under hypoxic conditions, while the upregulation of HIF‑1α by cobalt chloride promoted the expression and nuclear translocation of YAP and upregulated the expression of SOX9 and collagen II chain under normoxic conditions. In addition, inhibition of YAP expression under hypoxia did not affect the expression of the HIF‑1α signaling pathway, but inhibited the up‑regulation of SOX9 expression caused by hypoxia. In addition, reoxygenation following hypoxia inhibited the activation of YAP caused by hypoxia in chondrocytes, whereas the upregulation of SOX9 and collagen II chain also appeared to be inhibited. In conclusion, the results of the present study demonstrated that hypoxia promoted YAP activation via HIF‑1α. Therefore, the HIF‑1α/YAP signaling axis may serve an important role in controlling growth plate chondrocyte differentiation and the maintenance of the chondrogenic phenotype in growth plate chondrocytes.

Brain to bone: What is the contribution of the brain to skeletal homeostasis?

The brain, which governs most, if not all, physiological functions in the body, from the complexities of cognition, learning and memory, to the regulation of basal body temperature, heart rate and breathing, has long been known to affect skeletal health. In particular, the hypothalamus - located at the base of the brain in close proximity to the medial eminence, where the blood-brain-barrier is not as tight as in other regions of the brain but rather "leaky", due to fenestrated capillaries - is exposed to a variety of circulating body cues, such as nutrients (glucose, fatty acids, amino acids), and hormones (insulin, glucagon, leptin, adiponectin) [1-3].Information collected from the body via these peripheral cues is integrated by hypothalamic sensing neurons and glial cells [4-7], which express receptors for these nutrients and hormones, transforming these cues into physiological outputs. Interestingly, many of the same molecules, including leptin, adiponectin and insulin, regulate both energy and skeletal homeostasis. Moreover, they act on a common set of hypothalamic nuclei and their residing neurons, activating endocrine and neuronal systems, which ultimately fine-tune the body to new physiological states. This review will focus exclusively on the brain-to-bone pathway, highlighting the most important anatomical sites within the brain, which are known to affect bone, but not covering the input pathways and molecules informing the brain of the energy and bone metabolic status, covered elsewhere [8-10]. The discussion in each section will present side by side the metabolic and bone-related functions of hypothalamic nuclei, in an attempt to answer some of the long-standing questions of whether energy is affected by bone remodeling and homeostasis and vice versa.

The skeletal vascular system - Breathing life into bone tissue

During bone development, homeostasis and repair, a dense vascular system provides oxygen and nutrients to highly anabolic skeletal cells. Characteristic for the vascular system in bone is the serial organization of two capillary systems, each typified by specific morphological and physiological features. Especially the arterial capillaries mediate the growth of the bone vascular system, serve as a niche for skeletal and hematopoietic progenitors and couple angiogenesis to osteogenesis. Endothelial cells and osteoprogenitor cells interact not only physically, but also communicate to each other by secretion of growth factors. A vital angiogenic growth factor is vascular endothelial growth factor and its expression in skeletal cells is controlled by osteogenic transcription factors and hypoxia signaling, whereas the secretion of angiocrine factors by endothelial cells is regulated by Notch signaling, blood flow and possibly hypoxia. Bone loss and impaired fracture repair are often associated with reduced and disorganized blood vessel network and therapeutic targeting of the angiogenic response may contribute to enhanced bone regeneration.

Single-cell RNA-seq analysis reveals the progression of human osteoarthritis

Objectives

Understanding the molecular mechanisms underlying human cartilage degeneration and regeneration is helpful for improving therapeutic strategies for treating osteoarthritis (OA). Here, we report the molecular programmes and lineage progression patterns controlling human OA pathogenesis using single-cell RNA sequencing (scRNA-seq).

Methods

We performed unbiased transcriptome-wide scRNA-seq analysis, computational analysis and histological assays on 1464 chondrocytes from 10 patients with OA undergoing knee arthroplasty surgery. We investigated the relationship between transcriptional programmes of the OA landscape and clinical outcome using severity index and correspondence analysis.

Results

We identified seven molecularly defined populations of chondrocytes in the human OA cartilage, including three novel phenotypes with distinct functions. We presented gene expression profiles at different OA stages at single-cell resolution. We found a potential transition among proliferative chondrocytes, prehypertrophic chondrocytes and hypertrophic chondrocytes (HTCs) and defined a new subdivision within HTCs. We revealed novel markers for cartilage progenitor cells (CPCs) and demonstrated a relationship between CPCs and fibrocartilage chondrocytes using computational analysis. Notably, we derived predictive targets with respect to clinical outcomes and clarified the role of different cell types for the early diagnosis and treatment of OA.

Conclusions

Our results provide new insights into chondrocyte taxonomy and present potential clues for effective and functional manipulation of human OA cartilage regeneration that could lead to improved health.

Osteocytic oxygen sensing controls bone mass through epigenetic regulation of sclerostin

Preservation of bone mass is crucial for healthy ageing and largely depends on adequate responses of matrix-embedded osteocytes. These cells control bone formation and resorption concurrently by secreting the WNT/β-catenin antagonist sclerostin (SOST). Osteocytes reside within a low oxygen microenvironment, but whether and how oxygen sensing regulates their function remains elusive. Here, we show that conditional deletion of the oxygen sensor prolyl hydroxylase (PHD) 2 in osteocytes results in a high bone mass phenotype, which is caused by increased bone formation and decreased resorption. Mechanistically, enhanced HIF-1α signalling increases Sirtuin 1-dependent deacetylation of the Sost promoter, resulting in decreased sclerostin expression and enhanced WNT/β-catenin signalling. Additionally, genetic ablation of PHD2 in osteocytes blunts osteoporotic bone loss induced by oestrogen deficiency or mechanical unloading. Thus, oxygen sensing by PHD2 in osteocytes negatively regulates bone mass through epigenetic regulation of sclerostin and targeting PHD2 elicits an osteo-anabolic response in osteoporotic models.

Osteocytes reside in a low oxygen environment, but it is not clear if oxygen sensing regulates their function. Here, the authors show that deletion of the oxygen sensor prolyl hydroxylase 2 in osteocytes leads to increased bone mass via regulation of sclerostin, and reduces bone loss in mouse models of osteoporosis.

CoCl2 , a mimic of hypoxia, enhances bone marrow mesenchymal stem cells migration and osteogenic differentiation via STAT3 signaling pathway

Mesenchymal stem cells homing and migration is a crucial step during bone fracture healing. Hypoxic environment in fracture site induces bone marrow mesenchymal stem cells (BMSCs) migration, but its mechanism remains unclear. Our previous study and studies by other groups have reported the involvement of signal transducer and activator of transcription 3 (STAT3) pathway in cell migration. However, the role of STAT3 pathway in hypoxia-induced cell migration is still unknown. In this study, we investigated the role of STAT3 signaling in hypoxia-induced BMSCs migration and osteogenic differentiation. BMSCs isolated from C57BL/6 male mice were cultured in the presence of cobalt chloride (CoCl₂ ) to simulate intracellular hypoxia. Hypoxia enhanced BMSCs migration, and upregulated cell migration related gene expression, that is, metalloproteinase (MMP) 7, MMP9, and C-X-C motif chemokine receptor 4. Hypoxia enhanced the phosphorylation of STAT3, and cell migration related proteins: c-jun n-terminal kinase (JNK), focal of adhesion kinase (FAK), extracellular regulated protein kinases, and protein kinase B 1/2 (ERK1/2). Moreover, hypoxia enhanced expression of osteogenic differentiation marker. Inhibition of STAT3 suppressed the hypoxia-induced BMSCs migration, cell migration related signaling molecules phosphorylation, and osteogenic differentiation related gene expression. In conclusion, our result indicates that hypoxia-induced BMSCs migration and osteogenic differentiation is via STAT3 phosphorylation and involves the cooperative activity of the JNK, FAK, and MMP9 signaling pathways.

Exposure to tobacco smoke increases bone loss in spontaneously hypertensive rats

PURPOSE

To define if exposure to tobacco smoke (TS) could induce reduction of bone mass and impairment of bone architecture, features observed in osteoporosis in normotensive rats and the influence of TS exposure on the osteoporotic features exhibited in the spontaneously hypertensive (SH) rats.

METHODS

Normotensive Wistar Kyoto (WKY) and SH rats were exposed to filtered air or TS for 8 weeks, then their proximal femurs were extracted for micro-computed tomography (micro-CT) assessment, histological and immune-histological examinations to quantify the adverse influence of TS exposure on the bone mass and density, as well as bone architecture.

RESULTS

We found that TS exposure not only induced significant decreases in bone mineral density (BMD), bone volume (BV), cortical and trabecular thickness (Ct.Th and Tb.Th), trabecular surface area (Tb.Ar), expression of hypoxia-inducible factor-1α (HIF-1α) in the trabecular marrow, delayed ossification of cartilage, as well as statistical increases in trabecular separation (Tb.SP) and the number of trabecular marrow adipocytes in both WKY and SH rats, but also exacerbated multiple features of osteoporosis exhibited in SH rats, including decreased BMD, Ct.Th, Tb.Ar, HIF-1α expression, delayed cartilage ossification, and increased Tb.SP.

CONCLUSIONS

Our results show that TS exposure can reduce bone mass and impair bone architecture and exacerbate multiple features of osteoporosis exhibited in SH rats.

Folic acid supplementation repressed hypoxia-induced inflammatory response via ROS and JAK2/STAT3 pathway in human promyelomonocytic cells

Hypoxia is associated with inflammation and various chronic diseases. Folic acid is known to ameliorate inflammatory reactions, but the metabolism of folic acid protecting against hypoxia-induced injury is still unclear. In our study, we examined the inflammatory signal transduction pathway in human promyelomonocytic cells (THP-1 cells) with or without treatment with folic acid under hypoxic culture conditions. Our results indicated that supplementation with folic acid significantly reduced the levels of interleukin-1β and tumor necrosis factor-α in hypoxic conditions. Treating THP-1 cells with folic acid suppressed oxidative stress and hypoxia-inducible factor-1α in a dose-dependent manner. Folic acid targeted the activation of Janus kinase 2, downregulated the phosphorylation of signal transducer and activator of transcription 3, and decreased the expression of nuclear factor-κB p65 protein in cells. However, the absence of folic acid did not make cells more vulnerable under hypoxic conditions. In conclusion, folic acid efficiently inhibited the inflammatory response of THP-1 cells under hypoxic conditions by inhibiting reactive oxygen species production and the Janus kinase 2/signal transducer and activator of transcription 3 signaling pathway. Our study supports a basis for treatment with folic acid for chronic inflammation, which correlated with hypoxia.

Mesoporous silicate nanoparticles/3D nanofibrous scaffold-mediated dual-drug delivery for bone tissue engineering

Controlled delivery systems play a critical role in the success of bone morphogenetic proteins (i.e., BMP2 and BMP7) for challenged bone repair. Instead of single-drug release that is currently and commonly prevalent, dual-drug delivery strategies are highly desired to achieve effective bone regeneration because natural bone repair process is driven by multiple factors. Particularly, angiogenesis is essential for osteogenesis and requires more than just one factor (e.g., Vascular Endothelial Growth Factor, VEGF). Therefore, we developed a novel mesoporous silicate nanoparticles (MSNs) incorporated-3D nanofibrous gelatin (GF) scaffold for dual-delivery of BMP2 and deferoxamine (DFO). DFO is a hypoxia-mimetic drug that can activate hypoxia-inducible factor-1 alpha (HIF-1α), and trigger subsequent angiogenesis. Sustained BMP2 release system was achieved through encapsulation into large-pored MSNs, while the relative short-term release of DFO was engineered through covalent conjugation with chitosan to reduce its cytotoxicity and elongate its half-life. Both MSNs and DFO were incorporated onto a porous 3D GF scaffold to serve as a biomimetic osteogenic microenvironment. Our data indicated that DFO and BMP2 were released from a scaffold at different release rates (10 vs 28 days) yet maintained their angiogenic and osteogenic ability, respectively. Importantly, our data indicated that the released DFO significantly improved BMP2-induced osteogenic differentiation where the dose/duration was important for its effects in both mouse and human stem cell models. Thus, we developed a novel and tunable MSNs/GF 3D scaffold-mediated dual-drug delivery system and studied the potential application of the both FDA-approved DFO and BMP2 for bone tissue engineering.

Association of EGLN1 genetic polymorphisms with SpO2 responses to acute hypobaric hypoxia in a Japanese cohort

Background

Recent studies have explored various genetic and physiological factors related to high-altitude adaptation in highlander populations. However, the effects of single nucleotide polymorphisms (SNPs), influencing such adaptation, on physiological responses to hypobaric hypoxia have not been examined in lowlanders with lowlander ancestry. Thus, we aimed to investigate the association between SNPs around the EGLN1 genomic region, possibly involved in high-altitude adaptation, and physiological changes to hypobaric hypoxia exposure in a cohort of Japanese lowlanders.

Methods

Physiological data were obtained from 46 healthy Japanese male students under different atmospheric pressure conditions (equivalent to sea level and altitudes of 2500 and 4000 m). Genotypes of seven SNPs around EGLN1 were determined in all subjects by PCR-direct sequencing or TaqMan SNP genotyping assay.

Results

Results of the association study suggest that percutaneous arterial oxygen saturation (SpO₂) responses of individuals with rs12097901 and rs2790859 alleles, whose frequencies are high in highlander populations (HL alleles), may be susceptible to acute hypobaric hypoxia. SpO₂ levels of individuals with HL alleles were lower than those of individuals with non-HL alleles. At the same time, the subjects with HL alleles did not appear to have any remarkable hematological or pulmonary features that may counteract the low levels of SpO₂. One may hypothesize that the low SpO₂ levels in HL allele carriers could be a risk factor for acute mountain sickness in Japanese population.

Conclusions

Our findings suggest that rs12097901 and rs2790859 genotypes affect SpO₂ responses and may be associated with the susceptibility to acute hypobaric hypoxia in Japanese population.

Electronic supplementary material

The online version of this article (10.1186/s40101-018-0169-7) contains supplementary material, which is available to authorized users.

Recent Insights into the Contribution of the Changing Hypertrophic Chondrocyte Phenotype in the Development and Progression of Osteoarthritis

Osteoarthritis (OA) is an extremely prevalent age-related condition. The economic and societal burden due to the cost of symptomatic treatment, inability to work, joint replacement, and rehabilitation is huge and increasing. Currently, there are no effective medical therapies that delay or reverse the pathological manifestations of OA. Current treatment options are, without exception, focused on slowing down progression of the disease to postpone total joint replacement surgery for as long as possible and keeping the associated pain and joint immobility manageable. Alterations in the articular cartilage chondrocyte phenotype might be fundamental in the pathological mechanisms of OA development. In many ways, the changing chondrocyte phenotype in osteoarthritic cartilage resembles the process of endochondral ossification as seen, for instance, in developing growth plates. However, the relative contribution of endochondral ossification to the changing chondrocyte phenotype in the development and progression of OA remains poorly described. In this review, we will discuss the current knowledge regarding the cartilage endochondral phenotypic changes occurring during OA development and progression, as well as the molecular and environmental effectors driving these changes. Understanding how these molecular mechanisms determine the chondrocyte cell fate in OA will be essential in enabling cartilage regenerative approaches in future treatments of OA.

Vhl deletion in osteoblasts boosts cellular glycolysis and improves global glucose metabolism

The skeleton has emerged as an important regulator of systemic glucose homeostasis, with osteocalcin and insulin representing prime mediators of the interplay between bone and energy metabolism. However, genetic evidence indicates that osteoblasts can influence global energy metabolism through additional, as yet unknown, mechanisms. Here, we report that constitutive or postnatally induced deletion of the hypoxia signaling pathway component von Hippel-Lindau (VHL) in skeletal osteolineage cells of mice led to high bone mass as well as hypoglycemia and increased glucose tolerance, not accounted for by osteocalcin or insulin. In vitro and in vivo data indicated that Vhl-deficient osteoblasts displayed massively increased glucose uptake and glycolysis associated with upregulated HIF-target gene expression, resembling the Warburg effect that typifies cancer cells. Overall, the glucose consumption by the skeleton was increased in the mutant mice, as revealed by 18F-FDG radioactive tracer experiments. Moreover, the glycemia levels correlated inversely with the level of skeletal glucose uptake, and pharmacological treatment with the glycolysis inhibitor dichloroacetate (DCA), which restored glucose metabolism in Vhl-deficient osteogenic cells in vitro, prevented the development of the systemic metabolic phenotype in the mutant mice. Altogether, these findings reveal a novel link between cellular glucose metabolism in osteoblasts and whole-body glucose homeostasis, controlled by local hypoxia signaling in the skeleton.

Pharmacologic ascorbate (P-AscH-) suppresses hypoxia-inducible Factor-1α (HIF-1α) in pancreatic adenocarcinoma

HIF-1α is a transcriptional regulator that functions in the adaptation of cells to hypoxic conditions; it strongly impacts the prognosis of patients with cancer. High-dose, intravenous, pharmacological ascorbate (P-AscH^-), induces cytotoxicity and oxidative stress selectively in cancer cells by acting as a pro-drug for the delivery of hydrogen peroxide (H₂O₂); early clinical data suggest improved survival and inhibition of metastasis in patients being actively treated with P-AscH^-. Previous studies have demonstrated that activation of HIF-1α is necessary for P-AscH^- sensitivity. We hypothesized that pancreatic cancer (PDAC) progression and metastasis could be be targeted by P-AscH^- via H₂O₂-mediated inhibition of HIF-1α stabilization. Our study demonstrates an oxygen- and prolyl hydroxylase-independent regulation of HIF-1α by P-AscH^-. Additionally, P-AscH^- decreased VEGF secretion in a dose-dependent manner that was reversible with catalase, consistent with an H₂O₂-mediated mechanism. Pharmacological and genetic manipulations of HIF-1α did not alter P-AscH^--induced cytotoxicity. In vivo, P-AscH^- inhibited tumor growth and VEGF expression. We conclude that P-AscH^- suppresses the levels of HIF-1α protein in hypoxic conditions through a post-translational mechanism. These findings suggest potential new therapies specifically designed to inhibit the mechanisms that drive metastases as a part of PDAC treatment.

Mimetic Hierarchical Approaches for Osteochondral Tissue Engineering

In order to engineer biomimetic osteochondral (OC) construct, it is necessary to address both the cartilage and bone phase of the construct, as well as the interface between them, in effect mimicking the developmental processes when generating hierarchical scaffolds that show gradual changes of physical and mechanical properties, ideally complemented with the biochemical gradients. There are several components whose characteristics need to be taken into account in such biomimetic approach, including cells, scaffolds, bioreactors as well as various developmental processes such as mesenchymal condensation and vascularization, that need to be stimulated through the use of growth factors, mechanical stimulation, purinergic signaling, low oxygen conditioning, and immunomodulation. This chapter gives overview of these biomimetic OC system components, including the OC interface, as well as various methods of fabrication utilized in OC biomimetic tissue engineering (TE) of gradient scaffolds. Special attention is given to addressing the issue of achieving clinical size, anatomically shaped constructs. Besides such neotissue engineering for potential clinical use, other applications of biomimetic OC TE including formation of the OC tissues to be used as high-fidelity disease/healing models and as in vitro models for drug toxicity/efficacy evaluation are covered.

HIGHLIGHTS

Biomimetic OC TE uses "smart" scaffolds able to locally regulate cell phenotypes and dual-flow bioreactors for two sets of conditions for cartilage/bone Protocols for hierarchical OC grafts engineering should entail mesenchymal condensation for cartilage and vascular component for bone Immunomodulation, low oxygen tension, purinergic signaling, time dependence of stimuli application are important aspects to consider in biomimetic OC TE.

[Is Oxygen Deficiency Always Harmful?]

Is Oxygen Deficiency Always Harmful? Abstract. The role of the cardiovascular circulation is to supply tissue with oxygen and nutrients. Oxygen deficiency (hypoxia) is considered life-threatening, since cells die, either through apoptotic or necrotic processes. Tissue tries to counteract this by means of evolutionary signalling pathways, such as the nuclear hypoxia-inducible factor, which protects the tissue by promoting cell survival strategies and simultaneously intervening in angiogenesis, haematogenesis and metabolic processes. Recent findings indicate that these conserved signalling pathways can also function as therapeutic approaches in wound healing of bones and skin, as well as in the regeneration of tissues, e.g. in the liver, and in the hematopoietic system.

Promoted Chondrogenesis of Cocultured Chondrocytes and Mesenchymal Stem Cells under Hypoxia Using In-situ Forming Degradable Hydrogel Scaffolds

We investigated the effects of different oxygen tension (21% and 2.5% O₂) on the chondrogenesis of different cell systems cultured in pH-degradable PVA hydrogels, including human articular chondrocytes (hACs), human mesenchymal stem cells (hMSCs), and their cocultures with a hAC/hMSC ratio of 20/80. These hydrogels were prepared with vinyl ether acrylate-functionalized PVA (PVA-VEA) and thiolated PVA-VEA (PVA-VEA-SH) via Michael-type addition reaction. The rheology tests determined the gelation of the hydrogels was controlled within 2-7 min, dependent on the polymer concentrations. The different cell systems were cultured in the hydrogel scaffolds for 5 weeks, and the safranin O and GAG assay showed that hypoxia (2.5% O₂) greatly promoted the cartilage matrix production with an order of hAC > hAC/hMSC > hMSC. The real time quantitative PCR (RT-PCR) revealed that the hMSC group exhibited the highest hypertrophic marker gene expression (COL10A1, ALPL, MMP13) as well as the dedifferentiated marker gene expression (COL1A1) under normoxia conditions (21% O₂), while these expressions were greatly inhibited by coculturing with a 20% amount of hACs and significantly further repressed under hypoxia conditions, which was comparative to the sole hAC group. The enzyme-linked immunosorbent assay (ELISA) also showed that coculture of hMSC/hAC greatly reduced the catabolic gene expression of MMP1 and MMP3 compared with the hMSC group. It is obvious that the hypoxia conditions promoted the chondrogenesis of hMSC by adding a small amount of hACs, and also effectively inhibited their hypotrophy. We are convinced that coculture of hAC/hMSC using in situ forming hydrogel scaffolds is a promising approach to producing cell source for cartilage engineering without the huge needs of primary chondrocyte harvest and expansion.

Blood vessel formation and function in bone

In addition to their conventional role as a conduit system for gases, nutrients, waste products or cells, blood vessels in the skeletal system play active roles in controlling multiple aspects of bone formation and provide niches for hematopoietic stem cells that reside within the bone marrow. In addition, recent studies have highlighted roles for blood vessels during bone healing. Here, we provide an overview of the architecture of the bone vasculature and discuss how blood vessels form within bone, how their formation is modulated, and how they function during development and fracture repair.

Hypoxia regulates RhoA and Wnt/β-catenin signaling in a context-dependent way to control re-differentiation of chondrocytes

Cartilage tissue is avascular and hypoxic which regulates chondrocyte phenotype via stabilization of HIFs. Here, we investigated the role of hypoxia and HIFs in regulation of Rho and canonical Wnt signaling in chondrocytes. Our data demonstrates that hypoxia controls the expression of RhoA in chondrocytes in a context-dependent manner on the culturing conditions. Within a 3D microenvironment, hypoxia suppresses RhoA on which hypoxia-driven expression of chondrogenic markers depends. Conversely, hypoxia leads to upregulation of RhoA in chondrocytes on 2D with a failure in re-expression of chondrogenic markers. Similarly to RhoA, hypoxic regulation of Wnt/β-catenin signaling depends on the microenvironment. Hypoxia downregulates β-catenin within 3D hydrogels whereas it causes a potent increase on 2D. Hypoxia-induced suppression of canonical Wnt signaling in 3D contributes to the promotion of chondrogenic phenotype as induction of Wnt signaling abrogates the hypoxic re-differentiation of chondrocytes. Inhibiting Wnt/β-catenin signaling via stabilization of Axin2 leads to a synergistic enhancement of hypoxia-induced expression of chondrogenic markers. The effects of hypoxia on Rho and Wnt/β-catenin signaling are HIF-dependent as stabilizing HIFs under normoxia revealed similar effects on chondrocytes. The study reveals important insights on hypoxic signaling of chondrocytes and how hypoxia regulates cellular mechanisms depending on the cellular microenvironment.

Clinical, radiological, and gene expression analyses in smokers and non-smokers, Part 2: RCT on the late healing phase of osseointegration

BACKGROUND

The mechanisms behind the impact of smoking on osseointegration are not fully understood.

PURPOSE

To investigate the initial clinical and molecular course of osseointegration of different implants in smokers and non-smokers in a randomized controlled trial (RCT).

MATERIALS AND METHODS

Smoking (n = 16) and non-smoking (n = 16) patients received 3 implant types: machined, oxidized, and laser-modified surfaces. Baseline bone biopsies were retrieved from the implant sites. After 60 and 90 days, the pain score, implant stability quotient (ISQ), and peri-implant crevicular fluid (PICF) gene expression were analyzed. Furthermore, radiological and clinical assessments were made at 90 days.

RESULTS

At 90 days, no pain was reported, irrespective of smoking habit. A higher ISQ was found in smokers compared with non-smokers. Marginal bone loss (MBL) was greater in smokers than in non-smokers. The comparison of implant surfaces revealed greater MBL exclusively at the machined implants in smokers. At 90 days in smokers, the PICF around machined implants revealed a higher expression of the proinflammatory cytokine, interleukin-6 (IL-6), and a lower expression of the osteogenic gene, osteocalcin (OC), compared with the PICF around modified implants. Furthermore, OC expression was lower at machined implants in smokers compared with machined implants in non-smokers. After adjustment for age and implant location (maxilla/mandible), multivariate regression revealed the following predictors of MBL: smoking, bleeding on probing at 90 days, hypoxia-inducible factor 1 alpha (HIF-1α) expression at baseline and IL-6 expression in PICF at 90 days.

CONCLUSIONS

During the early phase of osseointegration, non-smokers and smokers present a similar, high implant survival. In contrast, smokers present a greater MBL, particularly at machined implants. HIF-1α baseline expression in the recipient bone and IL-6 expression in PICF cells are important molecular determinants for MBL after 90 days. It is concluded that smoking has an early effect on osseointegration, which is dependent on the implant surface properties and the local host response.

Sex-specific differences in the modulation of Growth Differentiation Factor 15 (GDF15) by hyperoxia in vivo and in vitro: Role of Hif-1α

Male premature neonates are more susceptible than females to the development of bronchopulmonary dysplasia (BPD). The reasons underlying sexually dimorphic outcomes in premature neonates are not known. GDF15 (Growth and differentiation factor 15) is a secreted cytokine and plays a role in cell proliferation, apoptosis, and angiogenesis. In this study, we sought to elucidate the sex-specific expression of Gdf15 in the lung in vivo in neonatal hyperoxic lung injury and its regulation by Hif-1α, and to delineate the differences in GDF15 expression in male and female human umbilical venous endothelial cells in an in vitro model of oxygen toxicity. Following hyperoxia exposure (95% FiO₂, PND (postnatal day 1-5: saccular stage of lung development), neonatal male mice (C57BL/6) show increased GDF15 and decreased HIF-1α expression compared to female mice. For the in vitro experiments, male and female HUVECs were exposed to room air condition (21% O2, 5% CO₂) or in hyperoxia condition (95% O2, 5% CO₂) for up to 72h. Male HUVECs had greater expression of GDF15 mRNA and protein. To study the inter-relationship between GDF15 and HIF-1α, we measured the expression of GDF15 in H441 cells after HIF-1α knockdown using promoter dual luciferase reporter assay, which showed that HIF-1α and GDF15 expression are inversely related under normoxia and hyperoxia. The results indicate that sex differences exist in the expression and modulation of GDF15 by HIF-1α in neonatal hyperoxic injury both in vivo and in vitro. These differences could explain in part the mechanisms behind sex-specific differences in BPD.

High mobility group box 1 protein attenuates myocardial ischemia reperfusion injury via inhibition of the p38 mitogen-activated protein kinase signaling pathway

The present study aimed to determine the effects of high mobility group box 1 protein (HMGB1) on myocardial ischemia reperfusion (I/R) injury in rats following acute myocardial ischemia and investigate the underlying molecular mechanisms of these effects. Male Wistar rats were randomly divided into the following groups (n=10/group): Sham operation; I/R; HMGB50 (50 ng/kg HMGB1 before I/R); HMGB100 (100 ng/kg HMGB1 before I/R); and HMGB200 (200 ng/kg HMGB1 before I/R). Serum cardiac troponin I (cTnI), interleukin (IL)-6 and tumor necrosis factor (TNF)-α levels were subsequently measured. Myocardial levels of malondialdehyde (MDA) and superoxide dismutase (SOD) were also determined. Myocardial infarction size (IS) was determined by 2,3,5-triphenyltetrazolium chloride staining. Myocardial expression of hypoxia inducible factor (HIF)-1α and phosphorylated p38 mitogen-activated protein kinase (P-p38 MAPK) protein was measured using western blotting. The results demonstrated that HMGB1 significantly decreased serum levels of cTnI, IL-6 and TNF-α and myocardial IS in I/R rats compared with the sham group (all P<0.05). HMGB1 also significantly decreased and increased myocardial levels of MDA and SOD, respectively (both P<0.05). HMGB1 significantly increased myocardial expression of HIF-1α and decreased expression of P-p38 MAPK following I/R (both P<0.05). These effects of HMGB1 occurred in a dose-dependent manner. The results of the current study indicate that the cardioprotective effects of intravenous HMGB1 are associated with increased myocardial expression of HIF-1α via inhibition of P-p38 MAPK expression, leading to inhibition of the P-p38 MAPK signaling pathway.

Pathological Analysis of Ocular Lesions in a Murine Model of Sjögren's Syndrome

Sjögren’s syndrome (SS) is a systemic autoimmune disease characterized by severe inflammation of exocrine glands such as the salivary and lacrimal glands. When it affects the lacrimal glands, many patients experience keratoconjunctivitis due to severely dry eyes. This study investigated the pathological and immunological characteristics of ocular lesions in a mouse model of SS. Corneal epithelial injury and hyperplasia were confirmed pathologically. The number of conjunctival mucin-producing goblet cells was significantly decreased in the SS model mice compared with control mice. Expression levels of transforming growth factor (TGF)-β, interleukin (IL)-6, tumor necrosis factor (TNF)-α, and C-X-C motif chemokine (CXCL) 12 were significantly higher in the corneal epithelium of the SS model mice than in control mice. Inflammatory lesions were observed in the Harderian, intraorbital, and extraorbital lacrimal glands in the SS model mice, suggesting that the ocular glands were targeted by an autoimmune response. The lacrimal glands of the SS model mice were infiltrated by cluster of differentiation (CD)4⁺ T cells. Real-time reverse transcription-polymerase chain reaction (RT-PCR) revealed significantly increased mRNA expression of TNF-α, TGF-β, CXCL9, and lysozyme in the extraorbital lacrimal glands of the SS model mice compared with control mice. These results add to the understanding of the complex pathogenesis of SS and may facilitate development of new therapeutic strategies.

HIF-2α regulates non-canonical glutamine metabolism via activation of PI3K/mTORC2 pathway in human pancreatic ductal adenocarcinoma

Hypoxia‐inducible factor‐2α (HIF‐2α) plays an important role in increasing cancer progression and distant metastasis in a variety of tumour types. We aimed to investigate its biological function and clinical significance in human pancreatic ductal adenocarcinoma (PDAC). A total of 283 paired PDAC tissues and adjacent normal tissues were collected from patients who underwent surgery or biopsy at Sun Yat‐sen Memorial Hospital between February 2004 and October 2016. In this study, we noted that HIF‐2α expression was significantly up‐regulated in PDAC, positively associated with disease stage, lymph‐node metastasis and patient survival, and identified as an independent prognostic factor of PDAC patients. We demonstrated that HIF‐2α silencing could reduce proliferation, migration and invasion of PDAC cells in vitro. The similar effect on growth was demonstrated in vivo. Furthermore, we noted that knock‐down of HIF‐2α significantly decreased the expression of glutamate oxaloacetate transaminase 1 (GOT1). Importantly, we confirmed that the PI3K/mTORC2 pathway promoted GOT1 expression by targeting HIF‐2α. Our study validated HIF‐2α was an important factor in PDAC progression and poor prognosis and may promote non‐canonical glutamine metabolism via activation of PI3K/mTORC2 pathway. Targeting HIF‐2α represents a novel prognostic biomarker and therapeutic target for patients with PDAC.

Salvianolic acid B regulates gene expression and promotes cell viability in chondrocytes

Articular chondrocytes reside in lacunae distributed in cartilage responsible for the remodelling of the tissue with limited ability of damage repairing. The in vitro expanded chondrocytes enhanced by factors/agents to obtain large numbers of cells with strengthened phenotype are essential for successful repair of cartilage lesions by clinical cell implantation therapies. Because the salvianolic acid B (Sal B), a major hydrophilic therapeutic agent isolated from Salvia miltiorrhiza, has been widely used to treat diseases and able to stimulate activity of cells, this study examines the effects of Sal B on passaged chondrocytes. Chondrocytes were treated with various concentrations of Sal B in monolayer culture, their morphological properties and changes, and mitochondrial membrane potential were analysed using microscopic analyses, including cellular biochemical staining and confocal laser scanning microscopy. The proteins were quantified by BCA and Western blotting, and the transcription of genes was detected by qRT‐PCR. The passaged chondrocytes treated with Sal B showed strengthened cellular synthesis and stabilized mitochondrial membrane potential with upregulated expression of the marker genes for chondrocyte phenotype, Col2‐α1, Acan and Sox9, the key Wnt signalling molecule β‐catenin and paracrine cytokine Cytl‐1. The treatments using CYTL‐1 protein significantly increased expression of Col2‐α1 and Acan with no effect on Sox9, indicating the paracrine cytokine acts on chondrocytes independent of SOX9. Sal B has ultimately promoted cell growth and enhanced chondrocyte phenotype. The chondrocytes treated with pharmaceutical agent and cytokine in the formulated medium for generating large number of differentiated chondrocytes would facilitate the cell‐based therapies for cartilage repair.

Surgically‑induced mouse models in the study of bone regeneration: Current models and future directions (Review)

Bone regeneration has been extensively studied over the past several decades. The surgically‑induced mouse model is the key animal model for studying bone regeneration, of the various research strategies used. These mouse models mimic the trauma and recovery processes in vivo and serve as carriers for tissue engineering and gene modification to test various therapies or associated genes in bone regeneration. The present review introduces a classification of surgically induced mouse models in bone regeneration, evaluates the application and value of these models and discusses the potential development of further innovations in this field in the future.

Copper-doped mesoporous hydroxyapatite microspheres synthesized by a microwave-hydrothermal method using creatine phosphate as an organic phosphorus source: application in drug delivery and enhanced bone regeneration

The development of multifunctional biomaterials with drug delivery ability, and pro-osteogenic and pro-angiogenic activities has garnered increasing interest in the field of regenerative medicine. In the present study, hypoxia-mimicking copper (Cu)-doped mesoporous hydroxyapatite (HAP) microspheres (Cu-MHMs) were successfully synthesized through a microwave-hydrothermal method by using creatine phosphate as an organic phosphorus source. The Cu-MHMs doped with 0.2, 0.5 and 1 mol% Cu were prepared. The Cu-MHMs consisting of HAP nanorods or nanosheets exhibited a hierarchically mesoporous hollow structure and a high specific surface area. Then the Cu-MHMs were investigated as a drug nanocarrier using doxorubicin hydrochloride (DOX) as a model drug. The Cu-MHMs showed a relatively high drug-loading capacity and a pH-responsive drug release behavior. Furthermore, the Cu-MHMs were incorporated into a chitosan (CS) matrix to construct a biomimetic scaffold optimized for bone regeneration. The Cu-MHM/CS composite scaffolds maintained high degrees of porosity and showed a sustained release of Cu ions. More importantly, the Cu-MHM/CS scaffolds not only enhanced the osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSCs) but also promoted the migration and tube formation of EA.hy926 cells. When implanted in rat critical-sized calvarial defects, the Cu-MHM/CS scaffolds significantly enhanced bone regeneration accompanied by more new blood vessel formation at 8 weeks post-operation compared with the MHM/CS scaffolds. These results suggest that the hypoxia-mimicking Cu-MHM/CS scaffolds could encourage bone regeneration by enhancing osteogenesis and angiogenesis simultaneously, which bodes well for the reconstruction of vascularized tissue-engineered bone.

Biomimetic strategies for fracture repair: Engineering the cell microenvironment for directed tissue formation

Complications resulting from impaired fracture healing have major clinical implications on fracture management strategies. Novel concepts taken from developmental biology have driven research strategies towards the elaboration of regenerative approaches that can truly harness the complex cellular events involved in tissue formation and repair. Advances in polymer technology and a better understanding of naturally derived scaffolds have given rise to novel biomaterials with an increasing ability to recapitulate native tissue environments. This coupled with advances in the understanding of stem cell biology and technology has opened new avenues for regenerative strategies with true clinical translatability. These advances have provided the impetus to develop alternative approaches to enhance the fracture repair process. We provide an update on these advances, with a focus on the development of novel biomimetic approaches for bone regeneration and their translational potential.

Roles of RUNX in Hypoxia-Induced Responses and Angiogenesis

During the past two decades, Runt domain transcription factors (RUNX1, 2, and 3) have been investigated in regard to their function, structural elements, genetic variants, and roles in normal development and pathological conditions. The Runt family proteins are evolutionarily conserved from Drosophila to mammals, emphasizing their physiological importance. A hypoxic microenvironment caused by insufficient blood supply is frequently observed in developing organs, growing tumors, and tissues that become ischemic due to impairment or blockage of blood vessels. During embryonic development and tumor growth, hypoxia triggers a stress response that overcomes low-oxygen conditions by increasing erythropoiesis and angiogenesis and triggering metabolic changes. This review briefly introduces hypoxic conditions and cellular responses, as well as angiogenesis and its related signaling pathways, and then describes our current knowledge on the functions and molecular mechanisms of Runx family proteins in hypoxic responses, especially in angiogenesis.

Osteoarthritis: from pathogenic mechanisms and recent clinical developments to novel prospective therapeutic options

Osteoarthritis (OA) is a degenerative joint disease that, despite recent progress, has no curative treatment. Considerable research has recently been initiated to identify new potential therapeutic targets. In this review, we will set forth some of the major discoveries in the past 5 years, notably those dealing with the identification of pathogenic factors [hypoxia-inducible factors (HIFs), complement, transforming growth factor (TGF)-β and zinc-ZIP8]. New drugs and concepts currently in clinical development [anti-nerve growth factor (NGF), mesenchymal stromal cells and fibroblast growth factor (FGF)-18] will then be addressed. Finally, we will consider prospective avenues that could lead to mid-to-long-term developments of novel therapeutic concepts, notably those dealing with autophagy regulation and induced pluripotent stem cells.

Hypoxia-Mimicking Nanofibrous Scaffolds Promote Endogenous Bone Regeneration

Utilizing biomimetic materials to potentiate endogenous cell growth or signaling is superior to relying on exogenous cells or signals for bone formation. Desferoxamine (DFO), which is a hypoxia-mimetic agent that chelates iron (Fe³⁺), mimics hypoxia to encourage bone healing. However, high cytotoxicity, off-target effects, and the short half-life of DFO have significantly impeded its further applications. We mitigated these side effects by locally immobilizing DFO onto a gelatin nanofibrous (GF) scaffold that retained DFO's ability to chelate Fe³⁺. Moreover, DFO-functionalized GF (GF-DFO) scaffolds, which have similar micro/macrostructures to GF scaffolds, not only demonstrated decreased cytotoxicity on both human umbilical vein endothelial cells and human mesenchymal stem cells but also significantly increased vascular endothelial growth factor (VEGF) expression in vitro. Most importantly, in our in vivo experiments on a critical-sized cranial bone defect mouse model, a significant amount of bone was formed in most of the GF-DFO scaffolds after six weeks, while very little new bone was observed in the GF scaffolds. These data suggest that use of a hypoxia-mimicking nanofibrous scaffold is a promising strategy for promoting endogenous bone formation.

The histone demethylase KDM4B regulates peritoneal seeding of ovarian cancer

Epithelial ovarian cancer (EOC) has poor prognosis and rapid recurrence because of widespread dissemination of peritoneal metastases at diagnosis. Multiple pathways contribute to the aggressiveness of ovarian cancer, including hypoxic signaling mechanisms. In this study, we have determined that the hypoxia-inducible histone demethylase KDM4B is expressed in ∼60% of EOC tumors assayed, including primary and matched metastatic tumors. Expression of KDM4B in tumors is positively correlated with expression of the tumor hypoxia marker CA-IX, and is robustly induced in EOC cell lines exposed to hypoxia. KDM4B regulates expression of metastatic genes and pathways, and loss of KDM4B increases H3K9 trimethylation at the promoters of target genes like LOXL2, LCN2 and PDGFB. Suppressing KDM4B inhibits ovarian cancer cell invasion, migration and spheroid formation in vitro. KDM4B also regulates seeding and growth of peritoneal tumors in vivo, where its expression corresponds to hypoxic regions. This is the first demonstration that a Jumonji-domain histone demethylase regulates cellular processes required for peritoneal dissemination of cancer cells, one of the predominant factors affecting prognosis of EOC. The pathways regulated by KDM4B may present novel opportunities to develop combinatorial therapies to improve existing therapies for EOC patients.

Prolyl Hydroxylase Domain-Containing Protein 2 (Phd2) Regulates Chondrocyte Differentiation and Secondary Ossification in Mice

Endochondral ossification plays an important role in the formation of the primary ossification centers (POCs) and secondary ossification centers (SOCs) of mammalian long bones. However, the molecular mechanisms that regulate POC and SOC formation are different. We recently demonstrated that Prolyl Hydroxylase Domain-containing Protein 2 (Phd2) is a key mediator of vitamin C effects on bone. We investigated the role of Phd2 on endochondral ossification of the epiphyses by conditionally deleting the Phd2 gene in osteoblasts and chondrocytes. We found that the deletion of Phd2 in osteoblasts did not cause changes in bone parameters in the proximal tibial epiphyses in 5 week old mice. In contrast, deletion of Phd2 in chondrocytes resulted in increased bone mass and bone formation rate (normalized to tissue volume) in long bone epiphyses, indicating that Phd2 expressed in chondrocytes, but not osteoblasts, negatively regulates secondary ossification of epiphyses. Phd2 deletion in chondrocytes elevated mRNA expression of hypoxia-inducible factor (HIF) signaling molecules including Hif-1α, Hif-2α, Vegfa, Vegfb, and Epo, as well as markers for chondrocyte hypertrophy and mineralization such as Col10, osterix, alkaline phosphatase, and bone sialoprotein. These data suggest that Phd2 expressed in chondrocytes inhibits endochondral ossification at the epiphysis by suppressing HIF signaling pathways.

Transcriptional control of chondrocyte specification and differentiation

A milestone in the evolutionary emergence of vertebrates was the invention of cartilage, a tissue that has key roles in modeling, protecting and complementing the bony skeleton. Cartilage is elaborated and maintained by chondrocytes. These cells derive from multipotent skeletal progenitors and they perform highly specialized functions as they proceed through sequential lineage commitment and differentiation steps. They form cartilage primordia, the primary skeleton of the embryo. They then transform these primordia either into cartilage growth plates, temporary drivers of skeletal elongation and endochondral ossification, or into permanent tissues, namely articular cartilage. Chondrocyte fate decisions and differentiated activities are controlled by numerous extrinsic and intrinsic cues, and they are implemented at the gene expression level by transcription factors. The latter are the focus of this review. Meritorious efforts from many research groups have led over the last two decades to the identification of dozens of key chondrogenic transcription factors. These regulators belong to all types of transcription factor families. Some have master roles at one or several differentiation steps. They include SOX9 and RUNX2/3. Others decisively assist or antagonize the activities of these masters. They include TWIST1, SOX5/6, and MEF2C/D. Many more have tissue-patterning roles and regulate cell survival, proliferation and the pace of cell differentiation. They include, but are not limited to, homeodomain-containing proteins and growth factor signaling mediators. We here review current knowledge of all these factors, one superclass, class, and family at a time. We then compile all knowledge into transcriptional networks. We also identify remaining gaps in knowledge and directions for future research to fill these gaps and thereby provide novel insights into cartilage disease mechanisms and treatment options.

Intermittent Hypoxia Influences Alveolar Bone Proper Microstructure via Hypoxia-Inducible Factor and VEGF Expression in Periodontal Ligaments of Growing Rats

Intermittent hypoxia (IH) recapitulates morphological changes in the maxillofacial bones in children with obstructive sleep apnea (OSA). Recently, we found that IH increased bone mineral density (BMD) in the inter-radicular alveolar bone (reflecting enhanced osteogenesis) in the mandibular first molar (M1) region in the growing rats, but the underlying mechanism remains unknown. In this study, we focused on the hypoxia-inducible factor (HIF) pathway to assess the effect of IH by testing the null hypothesis of no significant differences in the mRNA-expression levels of relevant factors associated with the HIF pathway, between control rats and growing rats with IH. To test the null hypothesis, we investigated how IH enhances mandibular osteogenesis in the alveolar bone proper with respect to HIF-1α and vascular endothelial growth factor (VEGF) in periodontal ligament (PDL) tissues. Seven-week-old male Sprague-Dawley rats were exposed to IH for 3 weeks. The microstructure and BMD in the alveolar bone proper of the distal root of the mandibular M1 were evaluated using micro-computed tomography (micro-CT). Expression of HIF-1α and VEGF mRNA in PDL tissues were measured, whereas osteogenesis was evaluated by measuring mRNA levels for alkaline phosphatase (ALP) and bone morphogenetic protein-2 (BMP-2). The null hypothesis was rejected: we found an increase in the expression of all of these markers after IH exposure. The results provided the first indication that IH enhanced osteogenesis of the mandibular M1 region in association with PDL angiogenesis during growth via HIF-1α in an animal model.

Cellular Hypoxia Promotes Heterotopic Ossification by Amplifying BMP Signaling

Hypoxia and inflammation are implicated in the episodic induction of heterotopic endochondral ossification (HEO); however, the molecular mechanisms are unknown. HIF-1α integrates the cellular response to both hypoxia and inflammation and is a prime candidate for regulating HEO. We investigated the role of hypoxia and HIF-1α in fibrodysplasia ossificans progressiva (FOP), the most catastrophic form of HEO in humans. We found that HIF-1α increases the intensity and duration of canonical bone morphogenetic protein (BMP) signaling through Rabaptin 5 (RABEP1)-mediated retention of Activin A receptor, type I (ACVR1), a BMP receptor, in the endosomal compartment of hypoxic connective tissue progenitor cells from patients with FOP. We further show that early inflammatory FOP lesions in humans and in a mouse model are markedly hypoxic, and inhibition of HIF-1α by genetic or pharmacologic means restores canonical BMP signaling to normoxic levels in human FOP cells and profoundly reduces HEO in a constitutively active Acvr1(Q207D/+) mouse model of FOP. Thus, an inflammation and cellular oxygen-sensing mechanism that modulates intracellular retention of a mutant BMP receptor determines, in part, its pathologic activity in FOP. Our study provides critical insight into a previously unrecognized role of HIF-1α in the hypoxic amplification of BMP signaling and in the episodic induction of HEO in FOP and further identifies HIF-1α as a therapeutic target for FOP and perhaps nongenetic forms of HEO. © 2016 American Society for Bone and Mineral Research.