Old age causes de novo intracortical bone remodeling and porosity in mice

Elimination of senescent osteoclast progenitors has no effect on the age-associated loss of bone mass in mice

Both an increase in osteoclast and a decrease in osteoblast numbers contribute to skeletal aging. Markers of cellular senescence, including expression of the cyclin inhibitor p16, increase with aging in several bone cell populations. The elimination of p16-expressing cells in old mice, using the INK-ATTAC transgene, increases bone mass indicating that senescent cells contribute to skeletal aging. However, the identity of the senescent cells and the extent to which ablation of p16-expressing cells may prevent skeletal aging remain unknown. Using mice expressing the p16-3MR transgene, we examined whether elimination of p16-expressing cells between 12 and 24 months of age could preserve bone mass; and whether elimination of these cells from 20 to 26 months of age could restore bone mass. The activation of the p16-3MR transgene by ganciclovir (GCV) greatly diminished p16 levels in the brain, liver, and osteoclast progenitors from the bone marrow. The age-related increase in osteoclastogenic potential of myeloid cells was also abrogated by GCV. However, GCV did not alter p16 levels in osteocytes-the most abundant cell type in bone-and had no effect on the skeletal aging of p16-3MR mice. These findings indicate that the p16-3MR transgene does not eliminate senescent osteocytes but it does eliminate senescent osteoclast progenitors and senescent cells in other tissues, as described previously. Elimination of senescent osteoclast progenitors, in and of itself, has no effect on the age-related loss of bone mass. Hence, other senescent cell types, such as osteocytes, must be the seminal culprits.

Changes in the osteocyte lacunocanalicular network with aging

Osteoporosis is an aging-related disease of reduced bone mass that is particularly prevalent in post-menopausal women, but also affects the aged male population and is associated with increased fracture risk. Osteoporosis is the result of an imbalance whereby bone formation by osteoblasts no longer keeps pace with resorption of bone by osteoclasts. Osteocytes are the most abundant cells in bone and, although previously thought to be quiescent, they are now known to be active, multifunctional cells that play a key role in the maintenance of bone mass by regulating both osteoblast and osteoclast activity. They are also thought to regulate bone mass through their role as mechanoresponsive cells in bone that coordinate adaptive responses to mechanical loading. Osteocytes form an extensive interconnected network throughout the mineralized bone matrix and receive their nutrients as well as hormones and signaling factors through the lacunocanalicular system. Several studies have shown that the extent and connectivity of the lacunocanalicular system and osteocyte networks degenerates in aged humans as well as in animal models of aging. It is also known that the bone anabolic response to loading is decreased with aging. This review summarizes recent research on the degenerative changes that occur in osteocytes and their lacunocanalicular system as a result of aging and discusses the implications for skeletal health and homeostasis as well as potential mechanisms that may underlie these degenerative changes. Since osteocytes are such key regulators of skeletal homeostasis, maintaining the health of the osteocyte network would seem critical for maintenance of bone health. Therefore, a more complete understanding of the structure and function of the osteocyte network, its lacunocanalicular system, and the degenerative changes that occur with aging should lead to advances in our understanding of age related bone loss and potentially lead to improved therapies.

A tale of the good and bad: Cell senescence in bone homeostasis and disease

Historically, cellular senescence has been viewed as an irreversible cell-cycle arrest process with distinctive phenotypic alterations that were implicated primarily in aging and tumor suppression. Recent discoveries suggest that cellular senescence represents a series of diverse, dynamic, and heterogeneous cellular states with the senescence-associated secretory phenotype (SASP). Although senescent cells typically contribute to aging and age-related diseases, accumulating evidence has shown that they also have important physiological functions during embryonic development, late pubertal bone growth cessation, and adulthood tissue remodeling. Here, we review the recent research on cellular senescence and SASP, highlighting the key pathways that mediate senescence cell-cycle arrest and initiate SASP. We also summarize recent literature on the role of cellular senescence in maintaining bone homeostasis and mediating age-associated osteoporosis, discussing both the beneficial and adverse roles of cellular senescence in bone during different physiological stages, including bone development, childhood bone growth, adulthood bone remodeling, and bone aging.

Targeting Cell Senescence for the Treatment of Age-Related Bone Loss

PURPOSE OF REVIEW

We review cell senescence in the context of age-related bone loss by broadly discussing aging mechanisms in bone, currently known inducers and markers of senescence, the senescence-associated secretory phenotype (SASP), and the emerging roles of senescence in bone homeostasis and pathology.

RECENT FINDINGS

Cellular senescence is a state of irreversible cell cycle arrest induced by insults or stressors including telomere attrition, oxidative stress, DNA damage, oncogene activation, and other intrinsic or extrinsic triggers and there is mounting evidence for the role of senescence in aging bone. Cellular aging also instigates a SASP that exerts detrimental paracrine and likely systemic effects. With aging, multiple cell types in the bone microenvironment become senescent, with osteocytes and myeloid cells as primary contributors to the SASP. Targeting undesired senescent cells may be a favorable strategy to promote bone anabolic and anti-resorptive functions in aging bone, with the possibility of improving bone quality and function with normal aging and/or disease.

Senescent and apoptotic osteocytes and aging: Exercise to the rescue?

Osteocytes are the most prevalent cell in the skeleton and are the master regulator of bone remodeling. Despite the understanding that osteocytes have a multiyear lifespan, and some factors induce apoptosis in osteocytes, much less is understood about the induction and consequences of osteocyte senescence. Filling these gaps in knowledge will provide novel approaches to slowing age-related bone loss and preventing fragility fractures. The purpose of this review is to examine the roles of senescence and apoptosis in osteocytes in age-related bone loss. Based on evidence that exercise can prevent senescence in skeletal muscle, we provide a novel hypothesis by which exercise can prolong skeletal health.

Cellular senescence in bone

Cellular senescence refers to a process induced by various types of stress that causes irreversible cell cycle arrest and distinct cellular alterations, including profound changes in gene expression, metabolism, and chromatin organization as well as activation/reinforcement of anti-apoptotic pathways and development of a pro-inflammatory secretome or senescence-associated secretory phenotype (SASP). However, because of challenges and technical limitations in identifying and characterizing senescent cells in living organisms, only recently have some of the diverse in vivo roles of these unique cells been discovered. New findings indicate that senescent cells and their SASP can have acute beneficial functions, such as in tissue regeneration and wound healing. However, in contrast, when senescent cells accumulate in excess chronically at sites of pathology or in old tissues they drive multiple age-associated chronic diseases. Senotherapeutics that selectively eliminate senescent cells ("senolytics") or inhibit their detrimental SASP ("senomorphics") have been developed and tested in aged preclinical models. These studies have established that targeting senescence is a powerful anti-aging strategy to improve "healthspan" - i.e., the healthy period of life free of chronic disease. The roles of senescence in mediating age-related bone loss have been a recent focus of rigorous investigation. Studies in mice and humans demonstrate that with aging, at least a subset of most cell types in the bone microenvironment become senescent and develop a heterogeneous SASP. Furthermore, age-related bone loss can be alleviated in old mice, with apparent advantages over anti-resorptive therapy, by reducing the senescent cell burden genetically or pharmacologically with the first class of senolytics or a senomorphic. Collectively, these findings point to targeting senescence as a transformational strategy to extend healthspan, therefore providing strong rationale for identifying and optimizing senotherapeutics to alleviate multiple chronic diseases of aging, including osteoporosis, and set the stage for translating senotherapeutics to humans, with clinical trials currently ongoing.

Innervation is higher above Bone Remodeling Surfaces and in Cortical Pores in Human Bone: Lessons from patients with primary hyperparathyroidism

Mounting evidence from animal studies suggests a role of the nervous system in bone physiology. However, little is known about the nerve fiber localization to human bone compartments and bone surface events. This study reveals the density and distribution of nerves in human bone and the association of nerve profiles to bone remodeling events and vascular structures in iliac crest biopsies isolated from patients diagnosed with primary hyperparathyroidism (PHPT). Bone sections were sequentially double-immunostained for tyrosine hydroxylase (TH), a marker for sympathetic nerves, followed by protein gene product 9.5 (PGP9.5), a pan-neuronal marker, or double-immunostained for either PGP9.5 or TH in combination with CD34, an endothelial marker. In the bone marrow, the nerve profile density was significantly higher above remodeling surfaces as compared to quiescent bone surfaces. Ninety-five percentages of all nerve profiles were associated with vascular structures with the highest association to capillaries and arterioles. Moreover, vasculature with innervation was denser above bone remodeling surfaces. Finally, the nerve profiles density was 5-fold higher in the intracortical pores compared to bone marrow and periosteum. In conclusion, the study shows an anatomical link between innervation and bone remodeling in human bone.

Osseous Remodeling Around Dental Implants

Adequate bone remodeling may be a primary parameter for long-term successful complication-free dental implant treatment. A 1.8-mm osseous thickness around dental implants is thought to be the minimum thickness for adequate vasculature for osteocyte nutrition and function. A dental implant does not provide progenitor cells or angiogenic or osteogenic factors. Thus, the surrounding bone may need to have a 1.8-mm thickness to accommodate the vasculature necessary for nutrients for appropriate remodeling. Additionally, the 1.8-mm dimension may provide for mechanical load resistance. There is no evidence to illustrate the physiologic need for the 1.8-mm dimension. This dimension requirement is based on clinical outcome observations. Basic science research for bone survival around dental implants is needed.

Mandibular Bone Loss after Masticatory Muscles Intervention with Botulinum Toxin: An Approach from Basic Research to Clinical Findings

The injection of botulinum toxin type A (BoNT/A) in the masticatory muscles, to cause its temporary paralysis, is a widely used intervention for clinical disorders such as oromandibular dystonia, sleep bruxism, and aesthetics (i.e., masseteric hypertrophy). Considering that muscle contraction is required for mechano-transduction to maintain bone homeostasis, it is relevant to address the bone adverse effects associated with muscle condition after this intervention. Our aim is to condense the current and relevant literature about mandibular bone loss in fully mature mammals after BoNT/A intervention in the masticatory muscles. Here, we compile evidence from animal models (mice, rats, and rabbits) to clinical studies, demonstrating that BoNT/A-induced masticatory muscle atrophy promotes mandibular bone loss. Mandibular bone-related adverse effects involve cellular and metabolic changes, microstructure degradation, and morphological alterations. While bone loss has been detected at the mandibular condyle or alveolar bone, cellular and molecular mechanisms involved in this process must still be elucidated. Further basic research could provide evidence for designing strategies to control the undesired effects on bone during the therapeutic use of BoNT/A. However, in the meantime, we consider it essential that patients treated with BoNT/A in the masticatory muscles be warned about a putative collateral mandibular bone damage.

Activation of hedgehog signaling by systemic agonist improves fracture healing in aged mice

Fracture healing is a complex process of many coordinated biological pathways. This system can go awry resulting in nonunion, which leads to significant patient morbidity. The Hedgehog (Hh) signaling pathway is upregulated in fracture healing. We hypothesized that the Hh signaling pathway can be pharmacologically modulated to positively affect fracture healing. Diaphyseal femur fractures were created in elderly mice (18 months, C57BL/6 females), which have a blunted and delayed healing response compared to younger mice, and were stabilized with intramedullary pins. To activate the Hh pathway we targeted the receptor Smoothened using an agonist (Hh-Ag1.5 [Hh-Ag]) and compared this to a vehicle control. Expression of Hh target genes were significantly increased in the fracture callus of the agonist group compared to controls, indicating pathway activation. Expression of osteogenic and chondrogenic-related genes was greatly upregulated in fracture callus versus intact femora, although Hh agonist treatment did not consistently enhance this response. Blindly graded, radiographic callus healing scores were significantly higher in the Hh-Ag groups at post operative day (POD) 14, indicating earlier callus bridging. On microCT, Hh-Ag treatment led to greater callus volume (+40%) and bone volume (+25%) at POD21. By day 14, callus vascularity, as assessed by 3D microCT angiography vessel volume, was 85% greater in the Hh-Ag group. Finally, mechanical strength of the calluses in the Hh-Ag groups was significantly greater than in the control groups at POD21. In conclusion, systemic administration of a Hh agonist appears to improve the osseous and vascular healing responses in a mouse fracture healing-impaired model. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

The Spectrum of Fundamental Basic Science Discoveries Contributing to Organismal Aging

Aging research has undergone unprecedented advances at an accelerating rate in recent years, leading to excitement in the field as well as opportunities for imagination and innovation. Novel insights indicate that, rather than resulting from a preprogrammed series of events, the aging process is predominantly driven by fundamental non-adaptive mechanisms that are interconnected, linked, and overlap. To varying degrees, these mechanisms also manifest with aging in bone where they cause skeletal fragility. Because these mechanisms of aging can be manipulated, it might be possible to slow, delay, or alleviate multiple age-related diseases and their complications by targeting conserved genetic signaling pathways, controlled functional networks, and basic biochemical processes. Indeed, findings in various mammalian species suggest that targeting fundamental aging mechanisms (eg, via either loss-of-function or gain-of-function mutations or administration of pharmacological therapies) can extend healthspan; ie, the healthy period of life free of chronic diseases. In this review, we summarize the evidence supporting the role of the spectrum of fundamental basic science discoveries contributing to organismal aging, with emphasis on mammalian studies and in particular aging mechanisms in bone that drive skeletal fragility. These mechanisms or aging hallmarks include: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Because these mechanisms are linked, interventions that ameliorate one hallmark can in theory ameliorate others. In the field of bone and mineral research, current challenges include defining the relative contributions of each aging hallmark to the natural skeletal aging process, better understanding the complex interconnections among the hallmarks, and identifying the most effective therapeutic strategies to safely target multiple hallmarks. Based on their interconnections, it may be feasible to simultaneously interfere with several fundamental aging mechanisms to alleviate a wide spectrum of age-related chronic diseases, including osteoporosis. © 2018 American Society for Bone and Mineral Research.

Zoledronic acid prevents pagetic-like lesions and accelerated bone loss in the p62P394L mouse model of Paget's disease

Paget's disease of bone (PDB) is an age-related metabolic bone disorder, characterised by focally increased and disorganised bone remodelling initiated by abnormal and hyperactive osteoclasts. The germline P392L mutation of SQSTM1 (encoding p62) is a strong genetic risk factor for PDB in humans, and the equivalent mutation in mice (P394L) causes a PDB-like disorder. However, it is unclear why pagetic lesions become more common with age. Here, we assessed the effect of the p62 P394L mutation on osteoclastogenesis and bone morphometry in relation to ageing, the natural history of lesion progression in p62P394L mice and the effect of zoledronic acid (ZA) on lesion development. p62P394L+/+ osteoclast precursors had increased sensitivity to RANKL (also known as TNFSF11) compared with wild-type (WT) cells, and the sensitivity further increased in both genotypes with ageing. Osteoclastogenesis from 12-month-old p62P394L+/+ mice was twofold greater than that from 3-month-old p62P394L+/+ mice (P<0.001) and three-fold greater than that from age-matched WT littermates. The p62P394L+/+ mice lost 33% more trabecular bone volume in the long bones by 12 months compared with WT mice (P<0.01), and developed pagetic-like lesions in the long bones which progressed with ageing. ZA prevented the development of pagetic-like lesions, and increased trabecular bone volume tenfold compared with vehicle by 12 months of age (P<0.01). This demonstrates that ageing has a pro-osteoclastogenic effect, which is further enhanced by the p62 P394L mutation, providing an explanation for the increased penetrance of bone lesions with age in this model. Lesions are prevented by ZA, providing a rationale for early intervention in humans.

Global deletion of tetraspanin CD82 attenuates bone growth and enhances bone marrow adipogenesis

CD82 is a widely expressed member of the tetraspanin family of transmembrane proteins known to control cell signaling, adhesion, and migration. Tetraspanin CD82 is induced over 9-fold during osteoclast differentiation in vitro; however, its role in bone homeostasis is unknown. A globally deleted CD82 mouse model was used to assess the bone phenotype. Based on microCT and 4-point bending tests, CD82-deficient bones are smaller in diameter and weaker, but display no changes in bone density. Histomorphometry shows a decrease in size, erosion perimeter, and number of osteoclasts in situ, with a corresponding increase in trabecular surface area, specifically in male mice. Male-specific alterations are observed in trabecular structure by microCT and in vitro differentiated osteoclasts are morphologically abnormal. Histomorphometry did not reveal a significant reduction in osteoblast number; however, dynamic labeling reveals a significant decrease in bone growth. Consistent with defects in OB function, OB differentiation and mineralization are defective in vitro, whereas adipogenesis is enhanced. There is a corresponding increase in bone marrow adipocytes in situ. Thus, combined defects in both osteoclasts and osteoblasts can account for the observed bone phenotypes, and suggests a role for CD82 in both bone mesenchyme and myeloid cells.

Modeling Rare Bone Diseases in Animals

PURPOSE OF REVIEW

The goal of this review is to highlight some of the considerations involved in creating animal models to study rare bone diseases and then to compare and contrast approaches to creating such models, focusing on the advantages and novel opportunities offered by the CRISPR-Cas system.

RECENT FINDINGS

Gene editing after creation of double-stranded breaks in chromosomal DNA is increasingly being used to modify animal genomes. Multiple tools can be used to create such breaks, with the newest ones being based on the bacterial adaptive immune system known as CRISPR/Cas. Advances in gene editing have increased the ease and speed, while reducing the cost, of creating novel animal models of disease. Gene editing has also expanded the number of animal species in which genetic modification can be performed. These changes have significantly increased the options for investigators seeking to model rare bone diseases in animals.

Oxidation-specific epitopes restrain bone formation

Atherosclerosis and osteoporosis are epidemiologically linked and oxidation specific epitopes (OSEs), such as phosphocholine (PC) of oxidized phospholipids (PC-OxPL) and malondialdehyde (MDA), are pathogenic in both. The proatherogenic effects of OSEs are opposed by innate immune antibodies. Here we show that high-fat diet (HFD)-induced bone loss is attenuated in mice expressing a single chain variable region fragment of the IgM E06 (E06-scFv) that neutralizes PC-OxPL, by increasing osteoblast number and stimulating bone formation. Similarly, HFD-induced bone loss is attenuated in mice expressing IK17-scFv, which neutralizes MDA. Notably, E06-scFv also increases bone mass in mice fed a normal diet. Moreover, the levels of anti-PC IgM decrease in aged mice. We conclude that OSEs, whether produced chronically or increased by HFD, restrain bone formation, and that diminished defense against OSEs may contribute to age-related bone loss. Anti-OSEs, therefore, may represent a novel therapeutic approach against osteoporosis and atherosclerosis simultaneously.

Recent advances in models for screening potential osteoporosis drugs

INTRODUCTION

Osteoporosis is a growing health and health-economic problem due to the increased proportion of elderly people in the population. Basic and clinical advances in research over the past two decades have led to the development of different compounds with antiresorptive or anabolic activity on bone that improved substantially the management of patients with osteoporosis over calcitonin or estrogen replacement. New compounds are in preclinical and clinical development. Areas covered: In this review, the authors review the approaches for the preclinical and clinical development of antiresorptive and anabolic agents for osteoporosis, particularly focusing on the recent advances in technology and in the understanding of skeletal biology, together with their implications on novel osteoporosis drug discovery. Expert opinion: Based on the available evidence from the approved drugs for the treatment osteoporosis as well as from the different compounds under clinical development, it has become clear that long term nonclinical pharmacological studies with either bone quality and off-target effects as the main outcomes should be required for new drugs intended to treat osteoporosis. At the same time, basic and clinical advances in research have underlined the necessity to develop new technologies and new models for a thorough screening of the effects of new drugs on the different components of skeletal aging and bone fragility that cannot be assessed by bone mass measurement.

Inhibiting Cellular Senescence: A New Therapeutic Paradigm for Age-Related Osteoporosis

Context

With the aging of the population and projected increase in osteoporotic fractures coupled with the declining use of osteoporosis medications, there is a compelling need for new approaches to treat osteoporosis. Given that age-related osteoporosis generally coexists with multiple other comorbidities (e.g., atherosclerosis, diabetes, frailty) that share aging as the leading risk factor, there is growing interest in the "Geroscience Hypothesis," which posits that manipulation of fundamental aging mechanisms will delay the appearance or severity of multiple chronic diseases because these diseases share aging as the underlying risk factor. In this context, one fundamental aging mechanism that has received considerable attention recently as contributing to multiple age-related morbidities is cellular senescence. This mini-review provides an overview on cellular senescence with a focus on its role in mediating age-related bone loss.

Methods

This summary is based on the authors' knowledge of the field supplemented by a PubMed search using the terms "senescence," "aging," and "bone."

Results

There is compelling evidence from preclinical models and supportive human data demonstrating an increase in senescent cells in the bone microenvironment with aging. These cells produce a proinflammatory secretome that leads to increased bone resorption and decreased bone formation, and approaches that either eliminate senescent cells or impair the production of their proinflammatory secretome have been shown to prevent age-related bone loss in mice.

Conclusions

Targeting cellular senescence represents a novel therapeutic strategy to prevent not only bone loss but potentially multiple age-related diseases simultaneously.

The Quest for Osteoporosis Mechanisms and Rational Therapies: How Far We've Come, How Much Further We Need to Go

During the last 40 years, understanding of bone biology and the pathogenesis of osteoporosis, the most common and impactful bone disease of old age, has improved dramatically thanks to basic and clinical research advances, genetic insights from humans and rodents, and newer imaging technologies. Culprits of osteoporosis are no longer a matter of speculation based on in vitro observations. Instead, they can be identified and dissected at the cellular and molecular level using genetic approaches; and their effect on distinct bone envelopes and anatomic regions can be functionally assessed in vivo. The landscape of pharmacotherapies for osteoporosis has also changed profoundly with the emergence of several potent antiresorptive drugs as well as anabolic agents, displacing estrogen replacement as the treatment of choice. In spite of these major positive developments, the optimal duration of the available therapies and their long-term safety remain matters of conjecture and some concern. Moreover, antiresorptive therapies are used indiscriminately for patients of all ages on the assumption that suppressing remodeling is always beneficial for bone, but rebound remodeling upon their discontinuation suggests otherwise. In this invited perspective, I highlight the latest state of knowledge of bone-intrinsic and extrinsic mechanisms responsible for the development of osteoporosis in both sexes; differences between the mechanisms responsible for the effects of aging and estrogen deficiency; and the role of old osteocytes in the development of cortical porosity. In addition, I highlight advances toward the goal of developing drugs for several degenerative diseases of old age at once, including osteoporosis, by targeting shared mechanisms of aging. © 2018 American Society for Bone and Mineral Research.

Pathologic conditions of hard tissue: role of osteoclasts in osteolytic lesion

Hard tissue homeostasis is regulated by the balance between bone formation by osteoblasts and bone resorption by osteoclasts. This physiologic process allows adaptation to mechanical loading and calcium homeostasis. Under pathologic conditions, however, this process is ill-balanced resulting in either over-resorption or over-formation of hard tissue. Local over-resorption by osteoclasts is typically observed in osteolytic metastases of malignancies, autoimmune arthritis, and giant cell tumor of bone (GCTB). In tumor-related local osteolysis, tumor-derived osteoclast-activating factors induce bone resorption not by directly acting on osteoclasts but by indirectly upregulating receptor activator of NFκB ligand (RANKL) on osteoblastic cells. Similarly, synovial tissue in the autoimmune arthritis model does overexpress RANKL and contains numerous osteoclast precursors, and like a landing craft, when it comes in contact with eroded bone surfaces, osteoclast precursors are immediately polarized to become mature osteoclasts, inducing rapidly progressive bone destruction at a late stage of the disease. GCTB, on the other hand, is a common primary bone tumor, usually arising at the metaphysis of the long bone in young adults. After the discovery of RANKL, the concept of GCTB as a tumor of RANKL-expressing stromal cells was established, and comprehensive exosome studies finally disclosed the causative single-point mutation at histone H3.3 (H3F3A) in stromal cells. Thus, osteolytic lesions under various pathological conditions are ultimately attributable to the overexpression of RANKL, which opens up a common, practical and useful therapeutic target for diverse osteolytic conditions.