Study of serum factors potentially involved in the pathogenesis of heterotopic bone formation after severe brain injury

The impact and mechanism of nerve injury on bone metabolism

With an increasing understanding of the mechanisms of fracture healing, it has been found that nerve injury plays a crucial role in the process, but the specific mechanism is yet to be completely revealed. To address this issue and provide novel insights for fracture treatment, we compiled this review. This review aims to study the impact of nerve injury on fracture healing, exploring the role of neurotrophic factors in the healing process. We first revisited the effects of the central nervous system (CNS) and the peripheral nervous system (PNS) on the skeletal system, and further explained the phenomenon of significantly accelerated fracture healing under nerve injury conditions. Then, from the perspective of neurotrophic factors, we delved into the physiological functions and mechanisms of neurotrophic factors, such as nerve growth factor (NGF), Neuropeptides (NPs), and Brain-derived neurotrophic factor (BDNF), in bone metabolism. These effects include direct actions on bone cells, improvement of local blood supply, regulation of bone growth factors, control of cellular signaling pathways, promotion of callus formation and bone regeneration, and synergistic or antagonistic effects with other endocrine factors, such as Sema3A and Transforming Growth Factor β (TGF-β). Finally, we discussed the treatments of fractures with nerve injuries and the future research directions in this review, suggesting that the relationship between nerve injury and fracture healing, as well as the role of nerve injury in other skeletal diseases.

Verteporfin attenuates trauma-induced heterotopic ossification of Achilles tendon by inhibiting osteogenesis and angiogenesis involving YAP/β-catenin signaling

Heterotopic ossification occurs as a pathological ossification condition characterized by ectopic bone formation within soft tissues following trauma. Vascularization has long been established to fuel skeletal ossification during tissue development and regeneration. However, the feasibility of vascularization as a target of heterotopic ossification prevention remained to be further clarified. Here, we aimed to identify whether verteporfin as a widely used FDA-approved anti-vascularization drug could effectively inhibit trauma-induced heterotopic ossification formation. In the current study, we found that verteporfin not only dose dependently inhibited the angiogenic activity of human umbilical vein endothelial cells (HUVECs) but also the osteogenic differentiation of tendon stem cells (TDSCs). Moreover, YAP/β-catenin signaling axis was downregulated by the verteporfin. Application of lithium chloride, an agonist of β-catenin, recovered TDSCs osteogenesis and HUVECs angiogenesis that was inhibited by verteporfin. In vivo, verteporfin attenuated heterotopic ossification formation by decelerating osteogenesis and the vessels densely associated with osteoprogenitors formation, which could also be readily reversed by lithium chloride, as revealed by histological analysis and Micro-CT scan in a murine burn/tenotomy model. Collectively, this study confirmed the therapeutic effect of verteporfin on angiogenesis and osteogenesis in trauma-induced heterotopic ossification. Our study sheds light on the anti-vascularization strategy with verteporfin as a candidate treatment for heterotopic ossification prevention.

A self-amplifying loop of YAP and SHH drives formation and expansion of heterotopic ossification

Heterotopic ossification (HO) occurs as a common complication after injury or in genetic disorders. The mechanisms underlying HO remain incompletely understood, and there are no approved prophylactic or secondary treatments available. Here, we identify a self-amplifying, self-propagating loop of Yes-associated protein (YAP)-Sonic hedgehog (SHH) as a core molecular mechanism underlying diverse forms of HO. In mouse models of progressive osseous heteroplasia (POH), a disease caused by null mutations in GNAS, we found that Gnas^-/- mesenchymal cells secreted SHH, which induced osteoblast differentiation of the surrounding wild-type cells. We further showed that loss of Gnas led to activation of YAP transcription activity, which directly drove Shh expression. Secreted SHH further induced YAP activation, Shh expression, and osteoblast differentiation in surrounding wild-type cells. This self-propagating positive feedback loop was both necessary and sufficient for HO expansion and could act independently of Gnas in fibrodysplasia ossificans progressiva (FOP), another genetic HO, and nonhereditary HO mouse models. Genetic or pharmacological inhibition of YAP or SHH abolished HO in POH and FOP and acquired HO mouse models without affecting normal bone homeostasis, providing a previously unrecognized therapeutic rationale to prevent, reduce, and shrink HO.

Neurological heterotopic ossification: novel mechanisms, prognostic biomarkers and prophylactic therapies

Neurological heterotopic ossification (NHO) is a debilitating condition where bone forms in soft tissue, such as muscle surrounding the hip and knee, following an injury to the brain or spinal cord. This abnormal formation of bone can result in nerve impingement, pain, contractures and impaired movement. Patients are often diagnosed with NHO after the bone tissue has completely mineralised, leaving invasive surgical resection the only remaining treatment option. Surgical resection of NHO creates potential for added complications, particularly in patients with concomitant injury to the central nervous system (CNS). Although recent work has begun to shed light on the physiological mechanisms involved in NHO, there remains a significant knowledge gap related to the prognostic biomarkers and prophylactic treatments which are necessary to prevent NHO and optimise patient outcomes. This article reviews the current understanding pertaining to NHO epidemiology, pathobiology, biomarkers and treatment options. In particular, we focus on how concomitant CNS injury may drive ectopic bone formation and discuss considerations for treating polytrauma patients with NHO. We conclude that understanding of the pathogenesis of NHO is rapidly advancing, and as such, there is the strong potential for future research to unearth methods capable of identifying patients likely to develop NHO, and targeted treatments to prevent its manifestation.

Leptin, Anxiety Symptoms, and Hypothalamic-Pituitary-Adrenal Axis Activity among Drug-Free, Female Suicide Attempters

BACKGROUND

Dysregulation of leptin secretion and functioning of the hypothalamic-pituitary-adrenal (HPA) axis may be involved in the pathophysiology of suicide. Preclinical and clinical studies have shown interactions between the HPA axis and leptin. There is also evidence for a negative relationship between leptin and anxiety in humans. However, these possible associations have not been studied in individuals with attempted suicide.

OBJECTIVES

To examine the relationship between leptin, HPA axis activity, and anxiety in individuals with a recent suicide attempt.

METHOD

Sixty-nine individuals with a recent suicide attempt (n = 37 females; n = 32 males) were recruited and subjected to the Dexamethasone Suppression Test (DST), lumbar puncture, and evaluation with the Comprehensive Psychopathological Rating Scale from which the Brief Scale for Anxiety (BSA) was derived. Leptin was analyzed in cerebrospinal fluid (CSF) and cortisol in serum. Leptin was corrected for body mass index (BMI) by dividing CSF-leptin by BMI (CSF-leptin/BMI). Due to gender-related differences in leptin secretion and HPA axis activity, calculations were made for males and females separately.

RESULTS

Significant differences were only found among females; CSF-leptin/BMI levels correlated significantly and negatively with BSA (p < 0.05), pre-DST cortisol, and post-DST serum cortisol at 8 a.m. and 3 p.m. (all p < 0.05). Furthermore, CSF-leptin/BMI was significantly lower in nonsuppressors of dexamethasone as compared to suppressors (p < 0.05).

CONCLUSIONS

These findings suggest that in females with a recent suicide attempt, low CSF leptin may be related to symptoms of anxiety and a hyperactive HPA axis.

Arachidonic acid: a bridge between traumatic brain injury and fracture healing

Traumatic brain injury (TBI) is associated with enhanced osteogenesis. The aim of this study was to investigate the effect of serum from TBI rats on fracture healing. Results from this study showed that the serum from TBI rats enhanced the expression of bone gamma carboxyglutamate protein (BGLAP), and promoted in vitro proliferation of MC3T3-E1 cells, a mouse osteoblastic cell line. Furthermore, gas chromatography/mass spectrometry (GC/MS) coupled with multivariate statistical analysis was used to identify the changes in global serum metabolites after TBI. We found that arachidonic acid (AA) was significantly enhanced in serum metabolites in TBI subjects, while hydroxybutyric acid, leucine, malic acid, 5-oxyproline, isocitric acid, mannose, and uric acid were reduced. Finally, we examined the effects of AA on BGLAP expression and cell proliferation in MC3T3-E1 cells. We found that BGLAP expression and proliferation of osteoblasts were positively regulated in the presence of AA. These findings suggest that the increased AA in serum after TBI may play a key role in enhancing the speed of fracture healing.

Low turnover osteoporosis in sheep induced by hypothalamic-pituitary disconnection

The hypothalamus is of critical importance in regulating bone remodeling. This is underscored by the fact that intracerebroventricular-application of leptin in ewe leads to osteopenia. As a large animal model of osteoporosis, this approach has some limitations, such as high technical expenditure and running costs. Therefore we asked if a surgical ablation of the leptin signaling axis would have the same effects and would thereby be a more useful model. We analyzed the bone phenotype of ewe after surgical hypothalamo-pituitary disconnection (HPD + OVX) as compared to control ewe (OVX) after 3 and 12 months. Analyses included histomorphometric characterization, micro-CT and measurement of bone turnover parameters. Already 3 months after HPD we found osteopenic ewe with a significantly decreased bone formation (69%) and osteoclast activity (49%). After a period of 12 months the HPD group additionally developed an (preclinical) osteoporosis with significant reduction (33%) of femoral cortical thickness, as compared to controls (OVX). Taken together, HPD leads after 12 month to osteoporosis with a reduction in both trabecular and cortical bone caused by a low bone turnover situation, with reduced osteoblast and osteoclast activity, as compared to controls (OVX). The HPD-sheep is a suitable large animal model of osteoporosis. Furthermore our results indicate that an intact hypothalamo-pituitary axis is required for activation of bone turnover.

The effects of an injury to the brain on bone healing and callus formation in young adults with fractures of the femoral shaft

In patients with traumatic brain injury and fractures of long bones, it is often clinically observed that the rate of bone healing and extent of callus formation are increased. However, the evidence has been unconvincing and an association between such an injury and enhanced fracture healing remains unclear. We performed a retrospective cohort study of 74 young adult patients with a mean age of 24.2 years (16 to 40) who sustained a femoral shaft fracture (AO/OTA type 32A or 32B) with or without a brain injury. All the fractures were treated with closed intramedullary nailing. The main outcome measures included the time required for bridging callus formation (BCF) and the mean callus thickness (MCT) at the final follow-up. Comparative analyses were made between the 20 patients with a brain injury and the 54 without brain injury. Subgroup comparisons were performed among the patients with a brain injury in terms of the severity of head injury, the types of intracranial haemorrhage and gender. Patients with a brain injury had an earlier appearance of BCF (p < 0.001) and a greater final MCT value (p < 0.001) than those without. There were no significant differences with respect to the time required for BCF and final MCT values in terms of the severity of head injury (p = 0.521 and p = 0.153, respectively), the types of intracranial haemorrhage (p = 0.308 and p = 0.189, respectively) and gender (p = 0.383 and p = 0.662, respectively).

These results confirm that an injury to the brain may be associated with accelerated fracture healing and enhanced callus formation. However, the severity of the injury to the brain, the type of intracranial haemorrhage and gender were not statistically significant factors in predicting the rate of bone healing and extent of final callus formation.

The genetics of heterotopic ossification: insight into the bone remodeling pathway

OBJECTIVES

Heterotopic Ossification (HO) is a significant complication after trauma occurring in 12% to 25% of fractures. Although clinical predictors have been studied to determine the likelihood of developing HO, the results have been inconsistent. This study examines genetic predictors of the HO phenotype to identify the "at-risk" patient and increase the understanding of the genetic contribution to the formation of HO.

METHODS

We examined the frequency of 61 single nucleotide polymorphisms (SNPs) in 1095 consecutive trauma patients with fractures. Radiographic studies of these patients were examined for HO in follow-up. Ten percent of the patients in the study demonstrated radiographic evidence of HO. Multivariate logistic regression was used to analyze each SNP independently while adjusting for severity of injury (as measured by the Trauma and Injury Severity Score).

RESULTS

Three SNPs (beta2-adrenergic receptor, toll-like receptor 4, complement factor H) were identified that were associated with an increased or decreased frequency of HO. The less common polymorphism of the beta2-adrenergic receptor gene was associated with increased risk of HO. For toll-like receptor 4 and complement factor H, the less common polymorphism was associated with a decreased risk of HO.

CONCLUSIONS

The SNPs identified as predictors of HO formation are representative of the adrenergic system, immune system, and the alternative complement system. This represents the interplay of multiple pathways that affect bone remodeling, aberrations of which may be found in the genome.

Elevated leptin expression in a rat model of fracture and traumatic brain injury

A few studies have reported a relationship between leptin induced by brain injury and healing of bone tissue. Our objective was to measure serum and callus leptin expression within the setting of fracture and traumatic brain injury (TBI). Sixty-four male Sprague-Dawley rats were randomised equally into four groups: control, TBI group, fracture group and fracture/TBI group. Rats were sacrificed at 2, 4, 8 and 12 weeks after fracture/TBI. Serum leptin was detected using radio-immunoassay, and callus formation was measured radiologically. Callus leptin was analysed with immunohistochemistry. Serum leptin was significantly increased in the fracture, TBI and combined fracture/TBI groups compared with the control group at 2 weeks (P &lt; 0.05). Serum leptin was significantly higher in the combined fracture/TBI group than in the fracture and TBI groups at 4 and 8 weeks (P &lt; 0.05). The percentage of leptin-positive cells in the callus and callus volume were significantly higher in the fracture/TBI group than in the fracture-only group (P &lt; 0.001). Thus, we demonstrated elevated leptin expression within healing bone, particularly in the first 8 weeks of a rat model combining fracture and TBI. A close association exists between leptin levels and the degree of callus formation in fractures.

Leptin receptors and beta2-adrenergic receptor mRNA expression in brain injury-related heterotopic ossification

Heterotopic ossification (HO) frequently occurs after brain injury. Recently, we found that leptin levels were decreased in the serum of patients with HO. Data suggest two mechanisms mediating leptin effects: a central suppressive mechanism acting via the beta(2)-adrenergic system and a direct stimulatory action starting when leptin binds to its receptors in osteoblastic cells. In this study, we analyzed leptin and beta(2)-adrenergic receptors mRNA expression in osteocytes originated from normal or heterotopic bone biopsies to investigate whether direct or indirect pathway signaling might be implicated in this pathological bone formation. We report for the first time the mRNA expression of the leptin receptor isoforms in osteocytes isolated from all biopsies. Moreover, quantitative reverse transcription-polymerase chain reaction allowed us to measure a significant decrease in the level of beta(2)-adrenergic receptor mRNA in cells isolated from heterotopic bone biopsies. These results could suggest an association between hypothalamic leptin signaling and brain injury-related HO.

From brain to bone: evidence for the release of osteogenic humoral factors after traumatic brain injury

PRIMARY OBJECTIVE

The aetiology of the increased osteogenesis associated with severe traumatic brain injury (TBI) remains incompletely understood. The purpose of this article is to review the available evidence regarding the release of osteogenic humoral factors after TBI.

RESEARCH DESIGN

This study is presented in the form of a literature review.

METHODS AND PROCEDURES

To obtain suitable references, Pubmed was searched using keywords 'heterotopic ossification', 'brain', 'trauma', 'injury', 'aetiology'.

MAIN OUTCOMES AND RESULTS

Evidence from both clinical and laboratory investigations points to centrally released osteogenic factor(s) that enter the systemic circulation following TBI.

CONCLUSIONS

Further investigation into the identification of these putative osteogenic factor(s), using human tissues and new techniques, is indicated to better understand this phenomenon.

Gene expression in normotopic and heterotopic human bone: increased level of SP7 mRNA in pathological tissue

Head injury-induced heterotopic ossification (HO) develops at vicinity of joints and in severe cases requires surgical intervention. Our previous study demonstrated high mRNA levels of osteocalcin (OC), type 1 collagen (COL1), osteonectin and RUNX2/CBFA1 in osteocytes and lining osteoblasts from non-evolutive HO compared to equivalent healthy cells from the proximal femoral shaft of patients receiving prosthesis. This allowed a first molecular characterisation of this pathological bone. The aims of this study is to extend the analysis to 10 more genes and determine those involved in the high OC mRNA level observed in pathological bone samples. RNAs were prepared from normotopic and heterotopic human bone samples digested by collagenase. After cDNA synthesis, mRNA levels were determined by real-time PCR and normalised using beta actin and glyceraldehyde-3-phosphate dehydrogenase. OSTERIX/SP7 expression was observed for the first time in human adult bone biopsies. In HO samples higher levels of SP7 (four- to sevenfold increase) and 1alpha,25-dihydroxy vitamin D(3) receptor (VDR) (two- to threefold increase) were observed compared to control samples. Moreover, SP7 level was correlated to OC and RUNX2 levels. In control samples, OC and SP7 levels were correlated. Our study further confirms that the involvement of SP7 in bone physiology is not only limited to the developmental step. Moreover, our results support the hypothesis that in HO the high level of OC expression could be due not only to an increase in RUNX2, but also in SP7 or VDR or to an imbalance in their respective activities.

The cannabinoid CB1 receptor regulates bone formation by modulating adrenergic signaling

We have recently reported that in bone the cannabinoid CB1 receptor is present in sympathetic terminals. Here we show that traumatic brain injury (TBI), which in humans enhances peripheral osteogenesis and fracture healing, acutely stimulates bone formation in a distant skeletal site. At this site we demonstrate i) a high level of the main endocannabinoid, 2-arachidonoylglycerol (2-AG), and expression of diacylglycerol lipases, enzymes essential for 2-AG synthesis; ii) that the TBI-induced increase in bone formation is preceded by elevation of the 2-AG and a decrease in norepinephrine (NE) levels. The TBI stimulation of bone formation was absent in CB1-null mice. In wild-type animals it could be mimicked, including the suppression of NE levels, by 2-AG administration. The TBI- and 2-AG-induced stimulation of osteogenesis was restrained by the beta-adrenergic receptor agonist isoproterenol. NE from sympathetic terminals is known to tonically inhibit bone formation by activating osteoblastic beta2-adrenergic receptors. The present findings further demonstrate that the sympathetic control of bone formation is regulated through 2-AG activation of prejunctional CB1. Elevation of bone 2-AG apparently suppresses NE release from bone sympathetic terminals, thus alleviating the inhibition of bone formation. The involvement of osteoblastic CB2 signaling in this process is minimal, if any.

Leptin inhibits bone formation not only in rodents, but also in sheep

This study examines the effect of long-term ICV administration of leptin in ewes. We found that central application significantly decreased osteoblast activity as measured by serum analysis as well as by histomorphometry, resulting in decreased trabecular bone volume. These data provide additional evidence that bone formation and therefore bone remodeling is at least in part centrally controlled.

INTRODUCTION

Genetic studies in mice have identified leptin as a potent inhibitor of bone formation acting through the central nervous system and unraveled the central nature of bone mass control and its disorders. Although these studies have radically enhanced our understanding of skeletal physiology because they have established a hypothalamic regulation of bone remodeling through the sympathetic nervous system, controversy remains about the physiological relevance of these observations because leptin's effect on bone after intracerebroventricular (ICV) application has only been shown in mice. To address whether leptin has a role in regulating bone mass beyond rodents, we treated ewes with long-term ICV application of leptin and analyzed the bone phenotype after a treatment period of 3 months.

MATERIALS AND METHODS

Three groups of corriedale sheep were compared: (1) control entire (control), (2) ovariectomy (OVX) and ICV application of cerebrospinal fluid (CSF); and (iii) OVX and ICV application of leptin (leptin). Analysis included histomorphometric characterization of iliac crest, spine and femur by histology and biomechanical testing and measurement of bone turnover parameters in serum and urine.

RESULTS

Central application of leptin decreased bone formation by 70% and mineralizing surface (MS/BS, 39.4 +/- 3.3% versus 16.1 +/- 2.1%) significantly (p < 0.01). Whereas OVX increased osteoclast indices and urinary cross-lap excretion by two and three times, respectively, serum parameters of osteoblast activity were significantly reduced by ICV application of leptin (p < 0.01). Consequently, ewes treated with leptin were osteopenic (iliac crest BV/TV entire, 22.7 +/- 1.3%; CSF, 18.9 +/- 2.4%; leptin, 12.4 +/- 2.6%), whereas bone torsional failure load reflecting the cortex of the tibia was not yet changed after 3 months of treatment (p < 0.01).

CONCLUSIONS

Taken together, these data suggest that leptin controls bone formation after ICV application, leading to reduction of trabecular bone mass in sheep. Most importantly, however, they show that the central regulation of bone formation is not limited to rodents, but is also found in large animals, providing further evidence that bone remodeling in vertebrates is centrally controlled.

[Pathophysiology and pathomorphology of osteoporosis]

Osteoporosis is a disease that leads to fragility fractures due to loss of bone mass and bone microstructure. This review presents an update on the fundamental pathophysiologic and pathomorphologic mechanisms of bone loss situations. Pathomorphologic characteristics such as perforations and microcallus formations are explained. The physiologic relevance of the remodeling process as well as its control by local-paracrine, systemic-endocrine and central-neural signaling pathways is discussed. Furthermore the role of hormones such as estrogen, FSH and leptin, of transcription-factors such as Runx2 and osterix and as well as that of the wnt signaling pathway for bone cell differentiation and function is presented. On the basis of current knowledge osteoporosis can be diagnosed, treated and fractures can be prevented. However, it is likely that new and even more effective diagnostic and therapeutic strategies will emerge as our understanding of the remodeling process that controls osteoblast and osteoclast function increases.

Postmenopausal osteoporosis and atheroma

Epidemiological studies have established a significant association between two major health problems, osteoporosis and atheroma, which is independent from age, gender, and other cardiovascular risk factors. Bone loss and the development of calcific deposits within vessel walls occur concomitantly. Proteins formerly thought to be specific of bone have been identified in atheroma lesions; and associations linking vertebral and proximal femoral fractures to aortic calcifications have been reported. Bone loss may be a marker for the risk of stroke and cardiovascular death. Bone loss may occur as a consequence of ischemia related to vascular disease. Despite numerous published studies, the mechanism underlying the osteoporosis-atheroma association remains controversial. Factors that perpetuate the debate include the often retrospective design of the studies, small sample sizes, and known limitations of absorptiometry as a tool for measuring osteoporosis severity. We reviewed the available clinical, epidemiological, and animal data on the mechanisms underlying the association between osteoporosis and atheroma. The possible effect of osteoporosis medications on the blood vessel wall is discussed. These issues remain a matter of debate, indicating a need for vast prospective studies to look for a causal link between osteoporosis and atheroma. The results of these studies may lead to new treatment strategies.