Patterns of osteocytic endothelial nitric oxide synthase expression in the femoral neck cortex: differences between cases of intracapsular hip fracture and controls

COX inhibitors and bone: A safer impact on osteoblasts by NO-releasing NSAIDs

Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly prescribed for the treatment of pain and inflammation. Although it is well known that NSAIDs can suppress bone growth, remodelling and repair, they are largely used post-operatively and post-traumatically to achieve analgesia and reduce inflammation in bone tissue.

AIMS

The impact of two NO-releasing, non-selective NSAIDs, NCX-4016 and HCT-3012 (NO-derivatives of Aspirin and Naproxen, respectively) on osteoblasts were evaluated and compared to the non-selective, parent chemicals and to the COX-2-selective inhibitor Celecoxib.

MAIN METHODS

Using MG-63 osteoblast-like cells, we considered proliferation, the early and late stage of differentiation, and the activity of proteinases thought to be involved in osteoid degradation, a preliminary fundamental event of bone remodelling.

KEY FINDINGS

Unlike Aspirin, Naproxen and Celecoxib, the two NO-NSAIDs did not alter proliferation and differentiation of osteoblasts. They also reduced the activity of plasminogen activator, metalloproteinases, and cathepsin B. Similar inhibitory effects against these proteinases were recapitulated by the NO-donor sodium nitroprusside, thereby suggesting a NO-mediated mechanism.

SIGNIFICANCE

Due to a differential effect on cell proliferation and differentiation, the two NO-NSAIDs exhibit a safer impact on osteoblast metabolism compared to Celecoxib and their parent compounds. This suggests an advantageous option for these drugs in individuals with a need of COX-inhibiting treatment, in general. In addition, their capability of modulating the proteinases involved in osteoid degradation may specifically suggest an additional safer use in comorbidity conditions of inflammation or pain with bone disorders characterized by high rate of remodelling, such as high-turnover osteoporosis in post-menopausal women.

Role of cortical bone in hip fracture

In this review, I consider the varied mechanisms in cortical bone that help preserve its integrity and how they deteriorate with aging. Aging affects cortical bone in two ways: extrinsically through its effects on the individual that modify its mechanical loading experience and 'milieu interieur'; and intrinsically through the prolonged cycle of remodelling and renewal extending to an estimated 20 years in the proximal femur. Healthy femoral cortex incorporates multiple mechanisms that help prevent fracture. These have been described at multiple length scales from the individual bone mineral crystal to the scale of the femur itself and appear to operate hierarchically. Each cortical bone fracture begins as a sub-microscopic crack that enlarges under mechanical load, for example, that imposed by a fall. In these conditions, a crack will enlarge explosively unless the cortical bone is intrinsically tough (the opposite of brittle). Toughness leads to microscopic crack deflection and bridging and may be increased by adequate regulation of both mineral crystal size and the heterogeneity of mineral and matrix phases. The role of osteocytes in optimising toughness is beginning to be worked out; but many osteocytes die in situ without triggering bone renewal over a 20-year cycle, with potential for increasing brittleness. Furthermore, the superolateral cortex of the proximal femur thins progressively during life, so increasing the risk of buckling during a fall. Besides preserving or increasing hip BMD, pharmaceutical treatments have class-specific effects on the toughness of cortical bone, although dietary and exercise-based interventions show early promise.

Bone remodeling in the context of cellular and systemic regulation: the role of osteocytes and the nervous system

Bone is a dynamic tissue that undergoes constant remodeling. The appropriate course of this process determines development and regeneration of the skeleton. Tight molecular control of bone remodeling is vital for the maintenance of appropriate physiology and microarchitecture of the bone, providing homeostasis, also at the systemic level. The process of remodeling is regulated by a rich innervation of the skeleton, being the source of various growth factors, neurotransmitters, and hormones regulating function of the bone. Although the course of bone remodeling at the cellular level is mainly associated with the activity of osteoclasts and osteoblasts, recently also osteocytes have gained a growing interest as the principal regulators of bone turnover. Osteocytes play a significant role in the regulation of osteogenesis, releasing sclerostin (SOST), an inhibitor of bone formation. The process of bone turnover, especially osteogenesis, is also modulated by extra-skeletal molecules. Proliferation and differentiation of osteoblasts are promoted by the brain-derived serotonin and hypothetically inhibited by its intestinal equivalent. The activity of SOST and serotonin is either directly or indirectly associated with the canonical Wnt/β-catenin signaling pathway, the main regulatory pathway of osteoblasts function. The impairment of bone remodeling may lead to many skeletal diseases, such as high bone mass syndrome or osteoporosis. In this paper, we review the most recent data on the cellular and molecular mechanisms of bone remodeling control, with particular emphasis on the role of osteocytes and the nervous system in this process.

Nitric oxide signaling in mechanical adaptation of bone

One of the most serious healthcare problems in the world is bone loss and fractures due to a lack of physical activity in elderly people as well as in bedridden patients or otherwise inactive youth. Crucial here are the osteocytes. Buried within our bones, these cells are believed to be the mechanosensors that stimulate bone formation in the presence of mechanical stimuli and bone resorption in the absence of such stimuli. Intercellular signaling is an important physiological phenomenon involved in maintaining homeostasis in all tissues. In bone, intercellular communication via chemical signals like NO plays a critical role in the dynamic process of bone remodeling. If bones are mechanically loaded, fluid flows through minute channels in the bone matrix, resulting in shear stress on the cell membrane that activates the osteocyte. Activated osteocytes produce signaling molecules like NO, which modulate the activity of the bone-forming osteoblasts and the bone-resorbing osteoclasts, thereby orchestrating bone adaptation to mechanical loading. In this review, we highlight current insights in the role of NO in the mechanical adaptation of bone mass and structure, with emphasis on its role in local bone gain and loss as well as in remodeling supervised by osteocytes. Since mechanical stimuli and NO production enhance bone strength and fracture resistance, these new insights may facilitate the development of novel osteoporosis treatments.

The fragile elderly hip: mechanisms associated with age-related loss of strength and toughness

Every hip fracture begins with a microscopic crack, which enlarges explosively over microseconds. Most hip fractures in the elderly occur on falling from standing height, usually sideways or backwards. The typically moderate level of trauma very rarely causes fracture in younger people. Here, this paradox is traced to the decline of multiple protective mechanisms at many length scales from nanometres to that of the whole femur. With normal ageing, the femoral neck asymmetrically and progressively loses bone tissue precisely where the cortex is already thinnest and is also compressed in a sideways fall. At the microscopic scale of the basic remodelling unit (BMU) that renews bone tissue, increased numbers of actively remodelling BMUs associated with the reduced mechanical loading in a typically inactive old age augments the numbers of mechanical flaws in the structure potentially capable of initiating cracking. Menopause and over-deep osteoclastic resorption are associated with incomplete BMU refilling leading to excessive porosity, cortical thinning and disconnection of trabeculae. In the femoral cortex, replacement of damaged bone or bone containing dead osteocytes is inefficient, impeding the homeostatic mechanisms that match strength to habitual mechanical usage. In consequence the participation of healthy osteocytes in crack-impeding mechanisms is impaired. Observational studies demonstrate that protective crack deflection in the elderly is reduced. At the most microscopic levels attention now centres on the role of tissue ageing, which may alter the relationship between mineral and matrix that optimises the inhibition of crack progression and on the role of osteocyte ageing and death that impedes tissue maintenance and repair. This review examines recent developments in the understanding of why the elderly hip becomes fragile. This growing understanding is suggesting novel testable approaches for reducing risk of hip fracture that might translate into control of the growing worldwide impact of hip fractures on our ageing populations.

The osteocyte: an endocrine cell ... and more

Few investigators think of bone as an endocrine gland, even after the discovery that osteocytes produce circulating fibroblast growth factor 23 that targets the kidney and potentially other organs. In fact, until the last few years, osteocytes were perceived by many as passive, metabolically inactive cells. However, exciting recent discoveries have shown that osteocytes encased within mineralized bone matrix are actually multifunctional cells with many key regulatory roles in bone and mineral homeostasis. In addition to serving as endocrine cells and regulators of phosphate homeostasis, these cells control bone remodeling through regulation of both osteoclasts and osteoblasts, are mechanosensory cells that coordinate adaptive responses of the skeleton to mechanical loading, and also serve as a manager of the bone's reservoir of calcium. Osteocytes must survive for decades within the bone matrix, making them one of the longest lived cells in the body. Viability and survival are therefore extremely important to ensure optimal function of the osteocyte network. As we continue to search for new therapeutics, in addition to the osteoclast and the osteoblast, the osteocyte should be considered in new strategies to prevent and treat bone disease.

Steering the osteoclast through the demineralization-collagenolysis balance

There is a lot of interest for how and how much osteoclasts resorb bone. However, little is known about the mechanism which controls the orientation and the duration of a resorptive event, thereby determining the specific geometry of a cavitation. Here we show that the relative rate of collagenolysis vs. demineralization plays a critical role in this process. First we observed that when culturing osteoclasts on bone slices, excavations appeared either as round pits containing demineralized collagen, or as elongated trenches without demineralized collagen. This suggests that round pits are generated when collagen degradation is slower than demineralization, and trenches when collagen degradation is as fast as demineralization. Next we treated the osteoclasts with a low dose of a carbonic anhydrase inhibitor to slightly decrease the rate of demineralization, thereby allowing collagen degradation to proceed as fast as demineralization. This resulted in about a two-fold increase of the proportion of trenches, thus supporting our hypothesis. The same result was obtained if facilitating collagen degradation by pre-treating the bone slices with NaOCl. In contrast, when decreasing the rate of collagenolysis vs. demineralization by the addition of a cathepsin K specific inhibitor, the proportion of trenches fell close to 0%, and furthermore the round pits became almost half as deep. These observations lead to a model where the osteoclast resorption route starts perpendicularly to the bone surface, forming a pit, and continues parallel to the bone surface, forming a trench. Importantly, we show that the progress of the osteoclast along this route depends on the balance between the rate of collagenolysis and demineralization. We propose that the osteocytes and bone lining cells surrounding the osteoclast may act on this balance to steer the osteoclast resorptive activity in order to give the excavations a specific shape.

Endothelial nitric oxide synthase gene intron 4 polymorphism in non-traumatic osteonecrosis of the femoral head

PURPOSE

Nitric oxide (NO) synthesised by endothelial NO synthase (eNOS) is a potent regulator of internal haemodynamics. A polymorphism in intron 4 of the eNOS is associated with different vascular disorders. We investigated the potential involvement of this polymorphism in idiopathic and secondary osteonecrosis of the femoral head (ONFH) in Polish patients.

METHODS

We performed a study involving 68 patients with ONFH (45 idiopathic and 23 secondary) and 100 healthy controls. All subjects were genotyped for the eNOS4 polymorphism by the polymerase chain reaction followed by agarose gel electrophoresis.

RESULTS

The analysis revealed that the frequencies of eNOS4 genotypes were significantly different in ONFH patients (both idiopathic and secondary) than in controls. The frequencies of the 4a allele were significantly higher in the total group of patients versus controls [22.79 vs 9%, p = 0.00039, odds ratio (OR) 2.98]. In subgroup analysis the 4a allele increased significantly in both idiopathic (20 vs 9%, p = 0.0074, OR = 2.52) and secondary (28.26 vs 9%, p = 0.00047, OR = 3.98) ONFH patients compared to control subjects. The frequency of the 4a/b genotype in the total group of patients (36.76 vs 16%, p = 0.0011, OR = 3.24) as well as patients with idiopathic (35.56 vs 16%, p = 0.0069, OR = 2.96) and secondary (39.13 vs 16 %, p = 0.0073, OR = 3.89) ONFH was higher than in the control group.

CONCLUSIONS

There was a significantly higher frequency of eNOS 4a allele carriers among the total group of patients as well as in idiopathic and secondary ONFH. This suggests that the eNOS gene polymorphism may be associated with increased risk of ONFH.

Endothelial nitric oxide synthase is not essential for nitric oxide production by osteoblasts subjected to fluid shear stress in vitro

Endothelial nitric oxide synthase (eNOS) has long been held responsible for NO production by mechanically stimulated osteoblasts, but this has recently been disputed. We investigated whether one of the three known NOS isoforms is essential for NO production by mechanically stimulated osteoblasts in vitro and revisited the bone phenotype of the eNOS-/- mouse. Osteoblasts, obtained as outgrowths from mouse calvaria or long bones of wild-type (WT), eNOS-/-, inducible NOS-/- (iNOS-/-), or neuronal NOS-/- (nNOS-/-) mice, were subjected to mechanical stimulation by means of pulsating fluid flow (PFF); and NO production was determined. Tibiae and femora from 8-week-old mice were subjected to μCT and three-point bending tests. Deletion of single NOS isoforms did not lead to significant upregulation of alternate isoforms in cultured osteoblasts from WT, eNOS-/-, iNOS-/-, or nNOS-/- mice. Expression of eNOS mRNA in osteoblasts was below our detection limit, and no differences in growth between WT and eNOS-/- osteoblasts were found. PFF increased NO production by approximately fourfold in WT and eNOS-/- osteoblasts and significantly stimulated NO production in iNOS-/- and nNOS-/- osteoblasts. Tibiae and femora from WT and eNOS-/- mice showed no difference in bone volume and architecture or in mechanical parameters. Our data suggest that mechanical stimuli can enhance NO production by cultured osteoblasts singly deficient for each known NOS isoform and that lack of eNOS does not significantly affect bone mass and strength at 8 weeks of age. Our data challenge the notion that eNOS is a key effector of mechanically induced bone maintenance.

Expression of endothelial nitric oxide synthase protein is not necessary for mechanical strain-induced nitric oxide production by cultured osteoblasts

Regulation of nitric oxide (NO) production is considered essential in mechanical load-related osteogenesis. We examined whether osteoblast endothelial NO synthase (eNOS)-derived NO production was regulated by HSP90. We found that HSP90 is essential for strain-related NO release but appears to be independent of eNOS in cultured osteoblasts.

INTRODUCTION

NO is a key regulator of bone mass, and its production by bone cells is regarded as essential in mechanical strain-related osteogenesis. We sought to identify whether bone cell NO production relied upon eNOS, considered to be the predominant NOS isoform in bone, and whether this was regulated by an HSP90-dependent mechanism.

METHODS

Using primary rat long bone-derived osteoblasts, the ROS 17/2.8 cell line and primary mouse osteoblasts, derived from wild-type and eNOS-deficient (eNOS(-/-)) mice, we examined by immunoblotting the expression of eNOS using a range of well-characterised antibodies and extraction methods, measured NOS activity by monitoring the conversion of radiolabelled L-arginine to citrulline and examined the production of NO by bone cells subjected to mechanical strain application under various conditions.

RESULTS

Our studies have revealed that eNOS protein and activity were both undetectable in osteoblast-like cells, that mechanical strain-induced NO production was retained in bone cells from eNOS-deficient mice, but that this strain-related induction of NO production was, however, dependent upon HSP90.

CONCLUSIONS

Together, our studies indicate that HSP90 activity is essential for strain-related NO release by cultured osteoblasts and that this is highly likely to be achieved by an eNOS-independent mechanism.

Osteocyte recruitment declines as the osteon fills in: interacting effects of osteocytic sclerostin and previous hip fracture on the size of cortical canals in the femoral neck

There is little information on the distribution of osteocytes within the individual cortical osteon, but using direct 3-D imaging in a single subject, Hannah et al. found a gradient with a two-fold higher density of cells adjacent to the cement line compared to near the canal. Since a limiting factor for bone formation might be the availability of osteoblasts due to their recruitment as osteocytes, we studied distributions of osteonal osteocytes in frozen sections of the femoral neck cortex. Osteocytes were stained with an anti-sclerostin antibody and counter-stained with toluidine blue. Adjacent sections were stained for alkaline phosphatase (ALP). Each osteonal osteocyte was categorised as being sclerostin-positive (scl+) or negative (scl-). ImageJ was used to measure the perimeter and area of each osteon and canal, while special purpose routines were used to measure the minimum distances of each osteocyte from the cement line and the canal. Canal area was strongly correlated with osteon area. Osteocytes were most dense close to the cement line; and their areal density within the matrix declined up to three-fold between the cement line and the canal, depending on osteon diameter. Large and small osteons had similar densities of osteocytes close to the cement line, but fractured neck of femur cases had significantly lower densities of osteocytes close to the canal. Higher osteocyte density close to the canal was associated with ALP expression. It is concluded that entombment of osteocytes newly drawn from the osteoblast pool into the mineralising matrix is independent of preceding bone resorption depth. As osteonal infilling proceeds, osteocyte formation declines more rapidly than matrix formation, leading to a progressive reduction in osteocyte density. A shrinking supply of precursor osteoblasts due to previous osteocyte recruitment, apoptosis, or both could produce this effect. In a statistically significant contrast, sclerostin negative osteocytes adjacent to the canal had the expected effect of reducing canal size in controls but this was not seen in hip fracture. This demonstrated the failure of osteonal osteoblasts to sustain bone formation through a complete remodelling cycle in osteoporosis, perhaps due to insufficient osteoblasts remaining capable of mineralized matrix formation. The failure of osteocytic sclerostin suppression to associate with bone formation in these osteons might alternatively be explained by downstream interference with sclerostin's effect on wnt signalling.

Evidence for pleiotropic factors in genetics of the musculoskeletal system

There are both theoretical and empirical underpinnings that provide evidence that the musculoskeletal system develops, functions, and ages as a whole. Thus, the risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Both bone loss (osteoporosis) and muscle wasting (sarcopenia) are the two sides of the same coin, an involution of the musculoskeletal system. Skeletal loads are dominated by muscle action; both bone and muscle share environmental, endocrine and paracrine influences. Muscle also has an endocrine function by producing bioactive molecules, which can contribute to homeostatic regulation of both bone and muscle. It also becomes clear that bone and muscle share genetic determinants; therefore the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. The aim of this review is to provide an additional evidence for existence of the tight genetic co-regulation of muscles and bones, starting early in development and still evident in aging. Recently, important papers were published, including those dealing with the cellular mechanisms and anatomic substrate of bone mechanosensitivity. Further evidence has emerged suggesting that the relationship between skeletal muscle and bone parameters extends beyond the general paradigm of bone responses to mechanical loading. We provide insights into several pathways and single genes, which apparently have a biologically plausible pleiotropic effect on both bones and muscles; the list is continuing to grow. Understanding the crosstalk between muscles and bones will translate into a conceptual framework aimed at studying the pleiotropic genetic relationships in the etiology of complex musculoskeletal disease. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both musculoskeletal conditions. This may translate into new approaches to reduce the burden of both conditions, which are prevalent in the elderly population.

T-786C polymorphism of the endothelial nitric oxide synthase gene and neuralgia-inducing cavitational osteonecrosis of the jaws

OBJECTIVE

We hypothesized that, similar to idiopathic hip osteonecrosis, the T-786C mutation of the endothelial nitric oxide synthase (eNOS) gene affecting nitric oxide (NO) production was associated with neuralgia-inducing cavitational osteonecrosis of the jaws (NICO).

DESIGN

In 22 NICO patients, not having taken bisphosphonates, mutations affecting NO production (eNOS T-786C, stromelysin 5A6A) were measured by polymerase chain reaction. Two healthy normal control subjects were matched per case by race and gender.

RESULTS

Homozygosity for the mutant eNOS allele (TT) was present in 6 out of 22 patients (27%) with NICO compared with 0 out of 44 (0%) race and gender-matched control subjects; heterozygosity (TC) was present in 8 patients (36%) versus 15 control subjects (34%); and the wild-type normal genotype (CC) was present in 9 patients (36%) versus 29 controls (66%) (P = .0008). The mutant eNOS T-786C allele was more common in cases (20 out of 44 [45%]) than in control subjects (15 out of 88 [17%]) (P = .0005). The distribution of the stromelysin 5A6A genotype in cases did not differ from control subjects (P = .13).

CONCLUSIONS

The eNOS T-786C polymorphism affecting NO production is associated with NICO, may contribute to the pathogenesis of NICO, and may open therapeutic medical approaches to treatment of NICO through provision of L-arginine, the amino-acid precursor of NO.

Evaluation of eNOS gene polymorphisms in relation to BMD in postmenopausal women

OBJECTIVE

The aim of the present study was to evaluate the relations between T(-786)C and Glu298Asp polymorphisms of the endothelial nitric oxide synthase (eNOS) gene and BMD in postmenopausal Turkish women.

METHODS

The T(-786)C and Glu298Asp polymorphisms were genotyped by PCR-RFLP method in 311 postmenopausal osteoporotic women (OP) and in 305 age-matched postmenopausal females (CG) with normal BMD.

RESULTS

None of the SNPs of the eNOS gene was significantly associated with BMD at the lumbar spine, femoral neck, Ward's triangle and femoral trochanter in the combined group. Mean BMD values were therefore found to be similar across the genotypes in postmenopausal Turkish women. However, there was a significant association between the T(-786)C polymorphism and BMD values at the lumbar spine in the normal control group (P=0.005), and at the femoral trochanter in the osteoporotic patients (P=0.046). The mean value of the lumbar spine BMD in the normal controls was significantly higher in women with the TC genotype of the T(-786)C polymorphism than in women with the TT genotype (P=0.0012). Women with the CC genotype of the T(-786)C polymorphism in the osteoporotic patients had significantly higher BMD value at the femoral trochanter than those with the TC (P=0.018) and TT genotypes (P=0.024). Frequencies of the TC heterozygotes for T(-786)C polymorphism were significantly higher among osteoporotic subjects than normal controls. Also, the CC and TT genotype frequencies of control group were significantly higher than those of the osteoporotic group at the femoral neck.

CONCLUSIONS

We conclude that, although the biological role of the nitric oxide synthases is well established, our study does not suggest that eNOS gene polymorphisms, T(-786)C and Glu298Asp, are major contributors to adult bone mineral density in the postmenopausal Turkish women.

Levels of endothelial nitric oxide synthase and calcitonin gene-related peptide in the Charcot foot: a pilot study

The pathogenesis of Charcot neuroarthropathy is unclear. To investigate the possibility that decreased levels of calcitonin gene-related peptide and endothelial nitric oxide synthase are involved in the process, we studied bone samples from healthy subjects (n = 4), subjects with diabetic neuropathy (n = 4), and subjects with Charcot neuroarthropathy (n = 4). A statistically significant difference was found in endothelial nitric oxide synthase expression between bone specimens in patients with diabetic neuropathy, Charcot neuroarthropathy, and normal bone (P = .008). A trend toward calcitonin gene-related peptide intensification was observed in normal bone as compared to diabetic neuropathy and Charcot neuroarthropathy bone specimens, but it did not reached statistical significance (P = .23). This pilot study suggests that abnormal calcitonin gene-related peptide and endothelial nitric oxide synthase activity may play a role in the development of Charcot neuroarthropathy.

LEVEL OF CLINICAL EVIDENCE

4.

Polymorphisms in the endothelial nitric oxide synthase gene and bone density/ultrasound and geometry in humans

Nitric oxide (NO), produced by endothelial cells, is a signaling molecule synthesized from l-arginine by nitric oxide synthases (NOS). NO is known to reduce the ratio of receptor activator of nuclear factor KappaB (RANKL)/osteoprotegerin (OPG), leading to decreased osteoclastogenesis and a reduction in bone resorption. Endothelial nitric oxide synthase (eNOS or NOS3) is the predominant constitutive isoform of nitric NOS within bone. Recently, a NOS3 polymorphism, Glu298Asp, previously implicated in osteoporosis, failed to demonstrate an association with bone mineral density (BMD), although there was some indication of an association with selected geometry indices. Since a single polymorphism does not capture all of the potential variants in a given gene, we investigated a broader coverage of the NOS3 gene with bone density/ultrasound and geometry indices in a sample of unrelated individuals from the Framingham Offspring Study. Our results indicated that the Glu298Asp polymorphism was not associated with BMD but suggested some haplotype-based associations in the linkage disequilibrium (LD) region that included the Glu298Asp polymorphism with several geometry indices. Although our findings exhibited several associations with selected bone density/ultrasound and geometry indices, the nominally significant associations are regarded as primarily hypothesis generating and suggest that replication in other samples is needed. Thus, NOS3 genetic variation does not appear to be a major contributor to adult bone density/ultrasound and geometry in our sample.

The biology of osteocytes

Osteocytes, the most abundant cell type in bone, remain the least characterized. Several theories have been proposed regarding their function, including osteolysis, sensing the strains produced in response to mechanical loading of bones, and producing signals that affect the function of osteoblasts and osteoclasts and hence, bone turnover. This review also discusses the role of osteocyte apoptosis in targeted bone remodeling and proposes that the occurrence of osteocyte apoptosis is consistent with the description of apoptosis as an essential homeostatic mechanism for the healthy maintenance of tissues.

Pressure gradients and transport in the murine femur upon hindlimb suspension

Interstitial fluid flow (IFF) is important in a number of processes, including stimulation of cells and nutrient and waste transport. In bone, it arises from the vascular pressure gradient between the medullary cavity and the lymphatic drainage at the periosteal surface and is enhanced by mechanical loading events. However, little is known about the pressure gradients experienced by bone cells in vivo and the role of the induced IFF in bone adaptation. This study investigated IFF changes in bone, in a disuse model and in ambulatory mice, from pressure gradients measured by telemetry, and by fluorescent tracers. The role of IFF-mediated transport of oxygen was assessed by the levels of hypoxic osteocytes in mouse femur after disuse by hindlimb suspension and with or without femoral vein ligation. Femoral intramedullary pressures in alert mice decreased to 77% upon hindlimb suspension and increased by 25% upon ligation, relative to baseline. To determine relative perfusion of cortical bone by IFF, the localization of intracardiac-injected fluorescent albumin conjugate with osteocytes was monitored. The number of osteocytic lacunae per bone area positive for Texas Red albumin was increased by 31% within 20-40 s, in the ligated femur compared to the contralateral sham femur. This confirmed that interstitial fluid flow was increased by femoral vein ligation and indicated that the increase was proportional to the pressure increase. Unloaded bone osteocytes were not hypoxic when compared to loaded controls and venous ligation did not alter these levels significantly. These results support the hypothesis that disuse by hindlimb suspension leads to decreased pressure gradients, which indicate lower IFF. Similarly, the increased pressure gradients, seen upon venous ligation, increased IFF from marrow to periosteum. While a decrease in intramedullary pressure in disuse suggests a decrease in IFF, this did not lead to hypoxia in osteocytes. We conclude that decreased oxygen convective transport in the mouse hindlimb disuse model does not account for cortical bone loss. This study is important in increasing our understanding of the mechanotransductory pathways involved in bone loading and unloading.

The NOS3 (27-bp repeat, intron 4) polymorphism is associated with susceptibility to osteomyelitis

Cytokines generate nitric oxide (NO) in osteoblasts and neutrophils through the induction of NO synthase isoforms, endothelial (NOS3) and inducible (NOS2), thereby producing bone loss. In osteomyelitis (OM), a chronic infection of the bone, homozygosity for the NOS3 (27-bp repeat, intron 4 polymorphism) 4 allele was significantly more frequent among the 80 patients than in 300 healthy controls (p=0.044). No significant differences were found for other polymorphisms of the NOS genes such as NOS3, the promoter (-786T/C), and the missense change (E298D) in exon 7, and for NOS2, the G/A substitution at position 37498 in exon 22, the (CCTTT)(n), and (TAAA)(n) micro-satellites and the -954G/C in the promoter. Serum NO levels were significantly higher only in the OM patients homozygous for the NOS3 (27-bp repeat, intron 4 polymorphism) 4 allele, compared to controls. In the presence of bacteria or bacterial products, the neutrophils of these patients produced more NO. However, immunolabelling of osteoblasts for NOS3 in biopsy tissues did not correlate with the carriage of a determined NOS polymorphism but with the presence of bone inflammation. This is the first report of an association between a NOS3 polymorphism and the risk of developing OM.

A specific mechanomodulatory role for p38 MAPK in embryonic joint articular surface cell MEK-ERK pathway regulation

Mechanisms regulating cell behavior and extracellular matrix composition in response to mechanical stimuli remain unresolved. Our previous studies have established that the MEK-ERK cascade plays a specific role in the mechano-dependent joint formation process by promoting the assembly of pericellular matrices reliant upon hyaluronan (HA) for their integrity. Here we demonstrate: (i) novel cross-talk between p38 MAPK and MEK-ERK signaling pathways that is specific for mechanical stimuli and (ii) a role for p38 MAPK in facilitating HA production by cells derived from the articular surface of embryonic chick tibiotarsal joints. We find that p38 MAPK blockade restricts pericellular assembly of HA-rich matrices and reduces basal as well as mechanical strain-induced release of HA. p38 MAPK blockers potentiated early strain-induced increases but restricted sustained increases in MEK/ERK phosphorylation at later times; c-Fos hyperphosphorylation at threonine 325 was found to parallel this p38 MAPK-mediated modulation of ERK activation. In contrast, p38 MAPK inhibitors had no detectable effect on the ERK activation induced by fibroblast growth factor 2 or pervanadate, a phosphatase inhibitor, and MEK inhibitors did not influence p38 MAPK phosphorylation, confirming both the specificity and unidirectionality of p38 MAPK-ERK cross-talk. Immunochemical and immunoblotting studies revealed constitutive p38 MAPK activation in cells at, or derived from, developing articular joint surfaces. Unlike the MEK-ERK pathway, however, p38 MAPK was not further stimulated by mechanical stimulation in vitro. Thus, p38 MAPK specifically facilitates ERK activation and downstream signaling in response to mechanical stimuli. These results suggest that constitutively active p38 MAPK serves an essential, permissive role in mechanically induced changes in ERK activation and in the accumulation of HA-rich extracellular matrices that serve a key role in joint development.

Osteoclastic cortical erosion as a determinant of subperiosteal osteoblastic bone formation in the femoral neck's response to BMU imbalance. Effects of stance-related loading and hip fracture

Femoral neck fractures have previously been shown to be associated with increased cortical and endocortical remodeling, reduced wall thickness of endocortical packets and cortical porosity. Femoral neck width is associated positively with history of lifetime physical activity; so we hypothesized that exposure to mechanical loading may influence the subperiosteal osteoblastic response to the weakening effect of intracortical bone resorption. In 21 femoral neck biopsies from female subjects (13 with hip fracture), there was a positive association between osteoblastic periosteal alkaline phosphatase expression shown in frozen sections and the percentage of cortical canals internal to the subperiosteal surface showing evidence of osteoclastic erosion (Goldner's stain; p =0.03). This was stronger in the plane of locomotor loading and particularly strong in the inferior (compression) cortex ( p =0.002). In 35 cases and 23 age/gender-matched postmortem controls, osteoid-bearing cortical canals (%) were significantly elevated in the fracture cases compared with the controls within the anterior region. There was also a significant correlation between cortical and endocortical %OS/BS (percentage osteoid surface to bone surface) (fracture, n =12; control, n =12) over the whole biopsy ( p =0.041). Generally, these associations of intracortical with endocortical remodeling were consistent with both envelopes being regulated by common processes. These results support the concept that the slow growth of femoral neck width by subperiosteal apposition of bone occurs directly or, otherwise, in response to the weakening of the cortex as it is "trabecularized" by imbalance of bone multicellular units (BMU). This process, in turn, depends on cortical thinning and enlargement of canals with the formation of giant, composite osteons, the whole being more marked in cases of future hip fracture.

Sclerostin is a delayed secreted product of osteocytes that inhibits bone formation

Osteocytes are the most abundant cells in bone and are ideally located to influence bone turnover through their syncytial relationship with surface bone cells. Osteocyte-derived signals have remained largely enigmatic, but it was recently reported that human osteocytes secrete sclerostin, an inhibitor of bone formation. Absent sclerostin protein results in the high bone mass clinical disorder sclerosteosis. Here we report that within adult iliac bone, newly embedded osteocytes were negative for sclerostin staining but became positive at or after primary mineralization. The majority of mature osteocytes in mineralized cortical and cancellous bone was positive for sclerostin with diffuse staining along dendrites in the osteocyte canaliculi. These findings provide for the first time in vivo evidence to support the concept that osteocytes secrete sclerostin after they become embedded in a mineralized matrix to limit further bone formation by osteoblasts. Sclerostin did not appear to influence the formation of osteocytes. We propose that sclerostin production by osteocytes may regulate the linear extent of formation and the induction or maintenance of a lining cell phenotype on bone surfaces. In doing so, sclerostin may act as a key inhibitory signal governing skeletal microarchitecture.

Osteocytic expression of constitutive NO synthase isoforms in the femoral neck cortex: a case-control study of intracapsular hip fracture

NO is an osteocytic signaling molecule that can inhibit osteoclasts. The NO synthases eNOS and nNOS were expressed by >50% of osteonal osteocytes in controls. Hip fracture cases showed +NOS osteocytes only in deep osteonal bone, and 25-35% reduced expression overall. These data are consistent with increased osteonal vulnerability to deep osteoclastic attack.

INTRODUCTION

Osteocytes may regulate the response to mechanical stimuli in bone through the production of local signaling molecules such as NO derived from the NO synthase eNOS. Because NO is inhibitory to osteoclastic resorption, it has been suggested that osteocytes expressing eNOS act as sentinels, confining resorption within single osteons. Recently, nNOS has been shown to be present in osteocytes of adult human bone.

MATERIALS AND METHODS

Cross-sections of the femoral neck (eight female cases of intracapsular hip fracture and seven postmortem controls; age, 68-91 years) were analyzed by immunohistochemistry. The percentages of osteocytes expressing each of these two isoforms were calculated, and their distances to the nearest canal surface were measured.

RESULTS

The percentage of +nNOS osteocytes was lower in the fracture cases than in the controls (cases: 43.12 +/- 1.49, controls: 56.68 +/- 1.45; p < 0.0001). Compared with nNOS, eNOS expression was further reduced (p = 0.009) in the cases but was not different in the controls (cases: 36.41 +/- 1.53, controls: 56.47 +/- 2.41; p < 0.0001). The minimum distance of +eNOS or +nNOS osteocytes to a canal surface was higher in the cases compared with controls (eNOS: controls; 44.4 +/- 2.2 microm, cases: 61.7 +/- 2.0 microm; p < 0.0001; nNOS: controls: 52.4 +/- 1.7 microm, cases: 60.2 +/- 2.1 microm; p = 0.0039). +eNOS osteocytes were closer to the canal surfaces than +nNOS osteocytes in the controls by 8.00 +/- 4.0 microm (p = 0.0012).

CONCLUSION

The proportions of osteocytes expressing nNOS and eNOS were both reduced in the fracture cases, suggesting that the capacity to generate NO might be reduced. Furthermore, the reduction in NOS expression occurs in those osteocytes closest to the canal surface, suggesting that the ability of NO to minimize resorption depth might be impaired. Further studies are needed on the regulation of the expression and activity of these distinct NOS isoforms.

NOS isoforms in adult human osteocytes: multiple pathways of NO regulation?

Until now, eNOS has been considered to be the predominant osteocytic nitric oxide synthase (NOS) isoform in bone. We previously studied the distribution of eNOS protein expression in the human femoral neck because of its possible involvement in the response to load. Studies in rat and human fracture callus have shown that nNOS mRNA is expressed sometime after fracture, but no study has yet immunolocalized NOS isoforms in mature adult human bone. In this study, we have examined the distribution of NOS isoforms in iliac osteocytes. Frozen sections (10 microm) were cut from transiliac biopsies from 8 female osteoporotic patients (range, 56-80 years) and from 7 female postmortem femoral neck biopsies (range, 65-90 years). Sections were incubated overnight in antiserum for eNOS, nNOS, or iNOS followed by peroxidase/VIP substrate detection. We used eNOS and iNOS antisera directed against the C-terminus. For nNOS, three different antisera were used, two binding to different C-terminal epitopes and one binding to N-terminal epitope. Sections were then incubated in propidium iodide or methyl green to detect all osteocytes. eNOS antibody was able to detect eNOS epitopes in osteocytes. All three nNOS antibodies detected nNOS epitopes in osteocytes, but those directed against the C-terminus had higher detection rates. iNOS was rarely seen. In the iliac crest, the percentage of osteocytes positive for nNOS was higher than that for eNOS (cortical: nNOS 84.04%, eNOS 61.78%, P < 0.05; cancellous: nNOS 82.33%, eNOS 65.21%, P < 0.05). In the femoral neck, the percentage of osteocytes positive for nNOS (60.98%) was also higher than that for eNOS (40.41%), although this difference was not statistically significant. In conclusion, both eNOS and nNOS isoforms are present in osteocytes in the iliac crest and femoral neck.