Before publication of the original version of this report in 1987, practitioners of bone histomorphometry communicated with each other in a variety of arcane languages, which in general were ...unintelligible to those outside the field. The need for standardization of nomenclature had been recognized for many years, super((1)) during which there had been much talk but no action. To satisfy this need, B Lawrence Riggs (ASBMR President, 1985 to 1986) asked A Michael Parfitt to convene an ASBMR committee to develop a new and unified system of terminology, suitable for adoption by the Journal of Bone and Mineral Research (JBMR) as part of its Instructions to Authors. The resulting recommendations were published in 1987 super((2)) and were quickly adopted not only by JBMR but also by all respected journals in the bone field. The recommendations improved markedly the ability of histomorphometrists to communicate with each other and with nonhistomorphometrists, leading to a broader understanding and appreciation of histomorphometric data.
What old means to bone Manolagas, Stavros C; Parfitt, A. Michael
Trends in endocrinology and metabolism,
06/2010, Letnik:
21, Številka:
6
Journal Article
Recenzirano
Odprti dostop
The adverse effects of aging of other organs (ovaries at menopause) on the skeleton are well known, but ironically little is known of skeletal aging itself. Evidence indicates that age-related ...changes, such as oxidative stress, are fundamental mechanisms of the decline of bone mass and strength. Unlike the short-lived osteoclasts and osteoblasts, osteocytes – former osteoblasts entombed in the mineralized matrix – live as long as 50 years, and their death is dependent on skeletal age. Osteocyte death is a major contributor to the decline of bone strength with age, and the likely mechanisms are oxidative stress, autophagy failure and nuclear pore “leakiness”. Unraveling these mechanisms should improve understanding of the age-related increase in fractures and suggest novel targets for its prevention.
Both aging and loss of sex steroids have adverse effects on skeletal homeostasis, but whether and how they may influence each others negative impact on bone remains unknown. We report herein that ...both female and male C57BL/6 mice progressively lost strength (as determined by load-to-failure measurements) and bone mineral density in the spine and femur between the ages of 4 and 31 months. These changes were temporally associated with decreased rate of remodeling as evidenced by decreased osteoblast and osteoclast numbers and decreased bone formation rate; as well as increased osteoblast and osteocyte apoptosis, increased reactive oxygen species levels, and decreased glutathione reductase activity and a corresponding increase in the phosphorylation of p53 and p66shc, two key components of a signaling cascade that are activated by reactive oxygen species and influences apoptosis and lifespan. Exactly the same changes in oxidative stress were acutely reproduced by gonadectomy in 5-month-old females or males and reversed by estrogens or androgens in vivo as well as in vitro.We conclude that the oxidative stress that underlies physiologic organismal aging in mice may be a pivotal pathogenetic mechanism of the age-related bone loss and strength. Loss of estrogens or androgens accelerates the effects of aging on bone by decreasing defense against oxidative stress.
Mechanical stimulation of cultured osteocytic cells attenuates their apoptosis. We report here that, conversely, reduced mechanical forces in the murine model of unloading by tail suspension ...increases the prevalence of osteocyte apoptosis, followed by bone resorption and loss of mineral and strength.
Introduction: Mechanical loading is critical for the maintenance of bone mass; weightlessness, as with reduced physical activity in old age, bed rest, or space flight, invariably leads to bone loss. However, the cellular and molecular mechanisms responsible for these phenomena are poorly understood. Based on our earlier findings that physiologic levels of mechanical strain prevent apoptosis of osteocytic cells in vitro, we examined here whether, conversely, reduced mechanical forces increase the prevalence of osteocyte apoptosis in vivo and whether this event is linked to bone loss.
Materials and Methods: Swiss Webster mice or OG2‐11β‐hydroxysteroid dehydrogenase type 2 (OG2‐11β‐HSD2) transgenic mice and wildtype littermates were tail‐suspended or kept under ambulatory conditions. Static and dynamic histomorphometry and osteocyte and osteoblast apoptosis by in situ end‐labeling (ISEL) were assessed in lumbar vertebra; spinal BMD was measured by DXA; and bone strength was measured by vertebral compression.
Results: We show that within 3 days of tail suspension, mice exhibited an increased incidence of osteocyte apoptosis in both trabecular and cortical bone. This change was followed 2 weeks later by increased osteoclast number and cortical porosity, reduced trabecular and cortical width, and decreased spinal BMD and vertebral strength. Importantly, whereas in ambulatory animals, apoptotic osteocytes were randomly distributed, in unloaded mice, apoptotic osteocytes were preferentially sequestered in endosteal cortical bone—the site that was subsequently resorbed. The effect of unloading on osteocyte apoptosis and bone resorption was reproduced in transgenic mice in which osteocytes are refractory to glucocorticoid action, indicating that stress‐induced hypercortisolemia cannot account for these effects.
Conclusions: We conclude that diminished mechanical forces eliminate signals that maintain osteocyte viability, thereby leading to apoptosis. Dying osteocytes in turn become the beacons for osteoclast recruitment to the vicinity and the resulting increase in bone resorption and bone loss.
There continues to be uncertainty about the classification of available drugs for treating osteoporosis. We find that grouping them into anti‐catabolic and anabolic classes based on the mechanisms of ...their action on bone remodeling and fracture reduction removes ambiguities and provides a relatively straightforward classification.
The recent introduction of teriparatide into clinical practice initiated the era of anabolic therapy for osteoporosis, but it is still unclear how to define an anabolic drug. All drugs that increase bone mass do so by affecting bone remodeling. When their mechanisms of action on bone remodeling and on fracture reduction are considered, we find that anti‐osteoporotic drugs fall naturally into either anti‐catabolic or anabolic classes. Anti‐catabolic drugs increase bone strength and reduce fractures mainly by decreasing the number of bone multicellular units (BMUs). This reduces perforative resorption and preserves skeletal microarchitecture (by preventing further structural damage to trabecular bone and increased porosity in cortical bone induced by high bone remodeling). Reduction in bone remodeling by anti‐catabolic drugs may increase bone mass moderately during the interval in which previously initiated BMUs are completing mineralization. Some anti‐catabolic drugs may also enhance the formation phase of the remodeling cycle, but their major action is to reduce overall bone turnover (i.e., the number of BMUs in bone). In contrast, anabolic drugs increase bone strength and reduce fractures by substantially increasing bone mass as a result of an overall increase in the number of BMUs combined with a positive BMU balance (the magnitude of the formation phase is greater than that of the resorption phase). Some anabolic drugs also induce renewed modeling, increase periosteal apposition and repair of trabecular microstructure. We hope that this classification will serve as a starting point for continued discussion on the important issue of nomenclature.
I devised a method for obtaining information on cancellous bone structure from iliac bone histomorphometry that led to the demonstration that architecture is an important component of bone strength ...and bone fragility. Furthermore, this method contributed to the recognition of the importance of changes in osteoclast and osteocyte apoptosis in response to estrogen deficiency and replacement.
Because osteoblasts and marrow adipocytes are derived from a common mesenchymal progenitor, increased adipogenesis may occur at the expense of osteoblasts, leading to bone loss. Our previous in vitro ...studies indicated that activation of the proadipogenic transcription factor peroxisome proliferator-activated receptor isoform γ 2 with rosiglitazone suppressed osteoblast differentiation. Here, we show that 5-month-old Swiss-Webster mice receiving rosiglitazone for 28 d exhibited bone loss associated with an increase in marrow adipocytes, a decrease in the ratio of osteoblasts to osteoclasts, a reduction in bone formation rate, and a reduction in wall width—an index of the amount of bone formed by each team of osteoblasts. Rosiglitazone had no effect on the number of early osteoblast or osteoclast progenitors, or on osteoblast life span, but decreased the expression of the key osteoblastogenic transcription factors Runx2 and Osterix in cultures of marrow-derived mesenchymal progenitors. These effects were associated with diversion of bipotential progenitors from the osteoblast to the adipocyte lineage, and suppression of the differentiation of monopotential osteoblast progenitors. However, rosiglitazone had no effect on osteoblastic cells at later stages of differentiation. Hence, rosiglitazone attenuates osteoblast differentiation and thereby reduces bone formation rate in vivo, leading to bone loss. These findings provide a mechanistic explanation for the recent evidence that peroxisome proliferator-activated receptor isoform γ activation is a negative regulator of bone mass and suggest that the increased production of oxidized fatty acids with age may indeed be an important mechanism for age-related osteoporosis in humans.
Abstract Osteocytes are long-lived and far more numerous than the short-lived osteoblasts and osteoclasts. Immured within the lacunar–canalicular system and mineralized matrix, osteocytes are ideally ...located throughout the bone to detect the need for, and accordingly choreograph, the bone regeneration process by independently controlling rate limiting steps of bone resorption and formation. Consistent with this role, emerging evidence indicates that signals arising from apoptotic and old/or dysfunctional osteocytes are seminal culprits in the pathogenesis of involutional, post-menopausal, steroid-, and immobilization-induced osteoporosis. Osteocyte-originated signals may also contribute to the increased bone fragility associated with bone matrix disorders like osteogenesis imperfecta, and perhaps the rapid reversal of bone turnover above baseline following discontinuation of anti-resorptive treatments, like denosumab. This article is part of a Special Issue entitled "The Osteocyte".
Glucocorticoids depress bone formation by inhibiting osteoblastogenesis and increasing osteoblast apoptosis. However, the role of bone resorption in the initial rapid phase of bone loss ...characteristic of glucocorticoid-induced osteoporosis is unexplained, and the reason for the efficacy of bisphosphonates in this condition remains unknown. We report that in murine osteoclast cultures, glucocorticoids prolonged the baseline survival of osteoclasts and antagonized bisphosphonate-induced caspase activation and apoptosis by a glucocorticoid receptor-mediated action. Consistent with the in vitro evidence, in a murine model of glucocorticoid-induced osteoporosis, the number of cancellous osteoclasts increased, even though osteoclast progenitor number was reduced. Moreover, in mice receiving both glucocorticoids and bisphosphonates, the expected proapoptotic effect of bisphosphonates on osteoclasts was abrogated, as evidenced by maintenance of osteoclast numbers and, additionally, loss of bone density. In contrast, bisphosphonate administration prevented glucocorticoid-induced osteoblast apoptosis. These results indicate that the early loss of bone with glucocorticoid excess is caused by extension of the life span of pre-existing osteoclasts, an effect not preventable by bisphosphonates. Therefore, the early beneficial effects of these agents must be due, in part, to prolonging the life span of osteoblasts.
Pulmonary hypertension (PH) associated with fibrotic idiopathic interstitial pneumonia (IIP; idiopathic pulmonary fibrosis and nonspecific interstitial pneumonia) confers important additional ...morbidity and mortality.
To evaluate the safety and clinical efficacy of the dual endothelin-1 receptor antagonist bosentan in this patient group.
In a randomized, double-blind, placebo-controlled study, 60 patients with fibrotic IIP and right heart catheter confirmed PH were randomized 2:1 to bosentan (n = 40) or placebo (n = 20). The primary study endpoint was a fall from baseline pulmonary vascular resistance index (PVRi) of 20% or more over 16 weeks.
Sixty patients (42 men; mean age, 66.6 ± 9.2 yr), with a mean pulmonary artery pressure of 36.0 (± 8.9) mm Hg, PVRi 13.0 (± 6.7) Wood Units/m(2) and reduced cardiac index of 2.21 (± 0.5) L/min/m(2) were recruited to the study. Accounting for deaths and withdrawals, paired right heart catheter data were available for analysis in 39 patients (bosentan = 25, placebo = 14). No difference in the primary outcome was detected, with seven (28.0%) patients receiving bosentan, and four (28.6%) receiving placebo achieving a reduction in PVRi of greater than or equal to 20% (P = 0.97) at 16 weeks. There was no change in functional capacity or symptoms between the two groups at 16 weeks, nor any difference in rates of serious adverse events or deaths (three deaths in each group).
This study shows no difference in invasive pulmonary hemodynamics, functional capacity, or symptoms between the bosentan and placebo groups over 16 weeks. Our data do not support the use of the dual endothelin-1 receptor antagonist, bosentan, in patients with PH and fibrotic IIP. Clinical trial registered with www.clinicaltrials.gov (NCT 00637065).
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