Bone cell senescence: mechanisms and perspectives

Supplemental magnesium gluconate recovers osteoblastic Wntless ablation-induced degenerative bone complications

INTRODUCTION

Although numerous studies have highlighted the involvement of Wnt-mediated signaling in Mg ion-enhanced bone healing, whether Wnt-stimulated signaling is essential for the Mg ion-triggered bone repair and mass accrual is not yet completely understood.

MATERIALS AND METHODS

We generated Wlsfl/fl wild-type (WT) control and their corresponding mutant (MT), Col2.3-Cre;Wlsfl/fl mice of osteoblastic Wntless (Wls) ablation and explored how supplemental magnesium gluconate (MgG) affects bone mass accrual and defected bone healing in relation to the Wls ablation.

RESULTS

Osteoblastic Wls ablation impaired bone mass accrual and bone healing along with age-related degenerative complications in bone marrow (BM) and BM cells. Oral supplementation of WT mice with MgG did not change natural bone mass accrual, but enhanced regenerative bone healing in femoral defects and the functionalities of BM cells. Supplemental MgG suppressed the Wls ablation-related bone loss and also stimulated new bone formation in the defects of MT mice. The MgG-induced beneficial effects in the MT mice were orchestrated with its potencies to ameliorate senescence, oxidative damage, and functional loss of BM and BM adherent cells, as well as to stimulate osteogenic activity.

CONCLUSION

This study demonstrates that supplemental MgG is able to improve bone homeostatic maintenance by recovering age-related degenerative complications even at the lack of osteoblastic Wnt-stimulated signaling.

n-acetyl-l-cysteine stimulates bone healing by recovering the age-associated degenerative complications relative to osteoblastic Wntless ablation

Dysregulated Wnt signaling causes age-related characteristics such as oxidative stress, stem cell senescence, and abnormal bone homeostasis. Here we explored whether supplemental n-acetyl-l-cysteine (NAC) recovers the age-associated complications relative to osteoblastic Wntless (Wls) ablation and examined the possible mechanisms therein. For this work, we administered Col2.3-Cre;Wlsfl/fl mutant and littermate control (Wlsfl/fl) mice (14 weeks of age) with NAC (40 mM)-supplemented or NAC-free water for four weeks. A proportion of these mice received non-critical-sized femoral defects at 16 weeks of age. Blood, bone, and bone marrow (BM) samples were collected and adjusted for in vivo, ex vivo, and in vitro analyses. Osteoblastic Wls deletion delayed bone mass accrual and the healing of bone defects, stimulated osteoclastic activation and inflammatory factor expression, and decreased antioxidant enzyme activity in the BM. Osteoblastic Wls deletion also promoted oxidative stress, apoptosis, and senescence in BM stromal cells (BMSCs) and decreased BMSC' multipotencies. Supplementation of Wlsfl/fl mice with NAC enhanced bone mass accrual and regenerative bone healing via a Wnt signal-associated osteogenic activation. However, supplemental NAC induced new bone formation in the mutant mice by inhibiting the age-related complications of BM/BMSCs, as well as by restoring endogenous antioxidant system without any alterations in Wnt ligand secretion, hematopoiesis, and expression of osteogenic and growth factors. This study indicates that supplemental NAC protects mice against Wnt deficiency-mediated and age-associated degenerative complications. Overall, this study highlights the therapeutic potency of NAC for restoring the antioxidant system, stem cell function, and regenerative bone homeostasis in osteoblastic Wls-dispensable manner.

Senolytic cocktail dasatinib and quercetin attenuates chronic high altitude hypoxia associated bone loss in mice

Chronic high-altitude hypoxia (CHH) induces irreversible abnormalities in various organisms. Emerging evidence indicates that CHH markedly suppresses bone mass and bone strength. Targeting senescent cells and the consequent senescence-associated secretory phenotype (SASP) with senolytics is a recently developed novel therapy for multiple age-related diseases. The combination of dasatinib and quercetin (DQ) has been proven to selectively target senescent cells and attenuate SASP in multiple tissues. In this study, experimental mice were subjected to an environment simulating 5,000 m above sea level for 8 weeks to induce CHH conditions. Our results indicated that DQ supplementation was well-tolerated with negligible toxicity. In vivo, DQ prevented reductions in BMD and BMC and improved bone microarchitecture against CHH-induced changes. Biomechanical testing demonstrated that DQ significantly improved the mechanical properties of femoral bones in CHH-exposed mice. Furthermore, DQ mitigated senescence in LepR + BMSCs and decreased the population of senescent cells, as evidenced by reduced senescence markers and SA-β-Gal staining. An analysis of serum and bone marrow aspirates showed that DQ treatment preserved angiogenic and osteogenic coupling in the bone marrow microenvironment by maintaining type H vessels and angiogenic growth factors. The results suggest that DQ has significant anti-senescence effects on BMSCs and a positive impact on the bone marrow microenvironment, supporting its clinical investigation as a therapeutic agent for CHH-related osteoporosis.

Targeting MCH Neuroendocrine Circuit in Lateral Hypothalamus to Protect Against Skeletal Senescence

Neuroendocrine regulation is essential for maintaining metabolic homeostasis. However, whether neuroendocrine pathway influence bone metabolism and skeletal senescence is unelucidated. Here, a central neuroendocrine circuit is identified that directly controls osteogenesis. Using virus based tracing, this study is identified that melanin concentrating hormone (MCH) expressing neurons in the lateral hypothalamus (LH) are connected to the bone. Chemogenetic activation of MCH neurons in the LH induces osteogenesis, whereas inhibiting these neurons reduces osteogenesis. Meanwhile, MCH is released into the circulation upon chemogenetic activation of these neurons. Single cell sequencing reveals that blocking MCH neurons in the LH diminishes osteogenic differentiation of bone marrow stromal cells (BMSCs) and induces senescence. Mechanistically, MCH promotes BMSC differentiation by activating MCHR1 via PKA signaling, and activating MCHR1 by MCH agonists attenuate skeletal senescence in mice. By elucidating a brain-bone connection that autonomously enhances osteogenesis, these findings uncover the neuroendocrinological mechanisms governing bone mass regulation and protect against skeletal senescence.

Bazi Bushen attenuates osteoporosis in SAMP6 mice by regulating PI3K-AKT and apoptosis pathways

Osteoporosis (OP), a systemic skeletal disease, is characterized by low bone mass, bone tissue degradation and bone microarchitecture disturbance. Bazi Bushen, a Chinese patented medicine, has been demonstrated to be effective in attenuating OP, but the pharmacological mechanism remains predominantly unclear. In this study, the senescence-accelerated mouse prone 6 (SAMP6) model was used to explore bone homeostasis and treated intragastrically for 9 weeks with Bazi Bushen. In vivo experiments showed that Bazi Bushen treatment not only upregulated the levels of bone mineral density and bone mineral content but also increased the content of RUNX2 and OSX. Furthermore, the primary culture of bone mesenchymal stem cells (BMSCs) in SAMP6 mice was used to verify the effects of Bazi Bushen on the balance of differentiation between osteoblasts and adipocytes, as well as ROS and aging levels. Finally, the pharmacological mechanism of Bazi Bushen in attenuating OP was investigated through network pharmacology and experimental verification, and we found that Bazi Bushen could significantly orchestrate bone homeostasis and attenuate the progression of OP by stimulating PI3K-Akt and inhibiting apoptosis. In summary, our work sheds light on the first evidence that Bazi Bushen attenuates OP by regulating PI3K-AKT and apoptosis pathways to orchestrate bone homeostasis.

Cellular Senescence and Inflammaging in the Bone: Pathways, Genetics, Anti-Aging Strategies and Interventions

Advancing age is associated with several age-related diseases (ARDs), with musculoskeletal conditions impacting millions of elderly people worldwide. With orthopedic conditions contributing towards considerable number of patients, a deeper understanding of bone aging is the need of the hour. One of the underlying factors of bone aging is cellular senescence and its associated senescence associated secretory phenotype (SASP). SASP comprises of pro-inflammatory markers, cytokines and chemokines that arrest cell growth and development. The accumulation of SASP over several years leads to chronic low-grade inflammation with advancing age, also known as inflammaging. The pathways and molecular mechanisms focused on bone senescence and inflammaging are currently limited but are increasingly being explored. Most of the genes, pathways and mechanisms involved in senescence and inflammaging coincide with those associated with cancer and other ARDs like osteoarthritis (OA). Thus, exploring these pathways using techniques like sequencing, identifying these factors and combatting them with the most suitable approach are crucial for healthy aging and the early detection of ARDs. Several approaches can be used to aid regeneration and reduce senescence in the bone. These may be pharmacological, non-pharmacological and lifestyle interventions. With increasing evidence towards the intricate relationship between aging, senescence, inflammation and ARDs, these approaches may also be used as anti-aging strategies for the aging bone marrow (BM).

Breaking Osteoclast-Acid Vicious Cycle to Rescue Osteoporosis via an Acid Responsive Organic Framework-Based Neutralizing and Gene Editing Platform

In the osteoporotic microenvironment, the acidic microenvironment generated by excessive osteoclasts not only causes irreversible bone mineral dissolution, but also promotes reactive oxygen species (ROS) production to induce osteoblast senescence and excessive receptor activator of nuclear factor kappa-B ligand (RANKL) production, which help to generate more osteoclasts. Hence, targeting the acidic microenvironment and RANKL production may break this vicious cycle to rescue osteoporosis. To achieve this, an acid-responsive and neutralizing system with high in vivo gene editing capacity is developed by loading sodium bicarbonate (NaHCO3) and RANKL-CRISPR/Cas9 (RC) plasmid in a metal-organic framework. This results showed ZIF8-NaHCO3@Cas9 (ZNC) effective neutralized acidic microenvironment and inhibited ROS production . Surprisingly, nanoparticles loaded with NaHCO3 and plasmids show higher transfection efficiency in the acidic environments as compared to the ones loaded with plasmid only. Finally, micro-CT proves complete reversal of bone volume in ovariectomized mice after ZNC injection into the bone remodeling site. Overall, the newly developed nanoparticles show strong effect in neutralizing the acidic microenvironment to achieve bone protection through promoting osteogenesis and inhibiting osteolysis in a bidirectional manner. This study provides new insights into the treatment of osteoporosis for biomedical and clinical therapies.

Monocyte alteration in elderly hip fracture healing: monocyte promising role in bone regeneration

Individual aged with various change in cell and cellular microenvironments and the skeletal system undergoes physiological changes that affect the process of bone fracture healing. These changes are accompanied by alterations in regulating critical genes involved in this healing process. Unfortunately, the elderly are particularly susceptible to hip bone fractures, which pose a significant burden associated with higher morbidity and mortality rates. A notable change in older adults is the increased expression of activation, adhesion, and migration markers in circulating monocytes. However, there is a decrease in the expression of co-inhibitory molecules. Recently, research evidence has shown that the migration of specific monocyte subsets to the site of hip fracture plays a crucial role in bone resorption and remodeling, especially concerning age-related factors. In this review, we summarize the current knowledge about uniqueness characteristics of monocytes, and their potential regulation and moderation to enhance the healing process of hip fractures. This breakthrough could significantly contribute to the comprehension of aging process at a fundamental aging mechanism through this initiative would represent a crucial stride for diagnosing and treating age related hip fracture.

Alterations in the microenvironment and the effects produced of TRPV5 in osteoporosis

The pathogenesis of osteoporosis involves multiple factors, among which alterations in the bone microenvironment play a crucial role in disrupting normal bone metabolic balance. Transient receptor potential vanilloid 5 (TRPV5), a member of the TRPV family, is an essential determinant of the bone microenvironment, acting at multiple levels to influence its properties. TRPV5 exerts a pivotal influence on bone through the regulation of calcium reabsorption and transportation while also responding to steroid hormones and agonists. Although the metabolic consequences of osteoporosis, such as loss of bone calcium, reduced mineralization capacity, and active osteoclasts, have received significant attention, this review focuses on the changes in the osteoporotic microenvironment and the specific effects of TRPV5 at various levels.

Impact of Environmental and Epigenetic Changes on Mesenchymal Stem Cells during Aging

Many crucial epigenetic changes occur during early skeletal development and throughout life due to aging, disease and are heavily influenced by an individual’s lifestyle. Epigenetics is the study of heritable changes in gene expression as the result of changes in the environment without any mutation in the underlying DNA sequence. The epigenetic profiles of cells are dynamic and mediated by different mechanisms, including histone modifications, non-coding RNA-associated gene silencing and DNA methylation. Given the underlining role of dysfunctional mesenchymal tissues in common age-related skeletal diseases such as osteoporosis and osteoarthritis, investigations into skeletal stem cells or mesenchymal stem cells (MSC) and their functional deregulation during aging has been of great interest and how this is mediated by an evolving epigenetic landscape. The present review describes the recent findings in epigenetic changes of MSCs that effect growth and cell fate determination in the context of aging, diet, exercise and bone-related diseases.

Recapitulation of growth factor-enriched microenvironment via BMP receptor activating hydrogel

Exposure to a growth factor abundant milieu has remarkable regenerative and rejuvenating effects on organ diseases, tissue damage, and regeneration, including skeletal system defects and bone regeneration. Although the introduction of candidate growth factors into relevant fields has been reported, their regenerative effects remain unsatisfactory, mainly because of the experimental challenges with limited types of growth factors, elusive dosage adjustment, and asynchronous stem cell activation with cytokine secretion. Here, an innovative hydrogel recapitulating a growth factor-enriched microenvironment (GEM) for regenerative advantage, is reported. This sulfated hydrogel includes bone morphogenetic protein-2 (BMP-2), an essential growth factor in osteogenesis, to direct mesenchymal stem cell (MSC) differentiation, stimulate cell proliferation, and improve bone formation. The semi-synthetic hydrogel, sulfonated gelatin (S-Gelatin), can amplify BMP-2 signaling in mouse MSCs by enhancing the binding between BMP-2 and BMP-2 type II receptors (BMPR2), which are located on MSC nuclei and activated by the hydrogel. Importantly, the dramatically improved cytokine secretion of MSCs throughout regeneration confirms the growth factor-acquiring potential of S-Gelatin/rhBMP-2 hydrogel, leading to the vascularization enhancement. These findings provide a new strategy to achieve an in situ GEM and accelerated bone regeneration by amplifying the regenerative capacity of rhBMP-2 and capturing endogenous growth factors.

Prion infection modulates hematopoietic stem/progenitor cell fate through cell-autonomous and non-autonomous mechanisms

Studies of PrP^C-derived prion disease generally focus on neurodegeneration. However, little is known regarding the modulation of hematopoietic stem progenitor cells (HSPCs) that express PrP^C in prion infection. Among bone marrow (BM) hematopoietic cells, hematopoietic stem cells (HSCs) strongly express PrP^C. A bioassay revealed the presence of misfolded prion protein (PrP^Sc) in BM cells derived from prion-infected mice; these BM cells demonstrated reproducible prion infectivity. At 5 months after infection with ME7, mice exhibited a significant decrease in the number of HSPCs. This decrease was mainly driven by increased apoptotic cell death, rather than cell cycle progression and senescence, in PrP^C-positive but not PrP^C-negative HSPC populations through a cell-autonomous mechanism. Notably, both PrP^C-positive and PrP^C-negative HSCs underwent cellular senescence, as indicated by high levels of senescence-associated factors and deficits in repopulation and self-renewal capacities at 7 months after infection. Senescence of HSCs occurred in the ME7-impaired BM microenvironment with aging phenotypes through non-cell autonomous mechanisms. These data provide novel evidence that prion infection differentially modulates HSC fate through both cell-autonomous and non-autonomous mechanisms.

Bone marrow mesenchymal stem cell's exosomes as key nanoparticles in osteogenesis and bone regeneration: specific capacity based on cell type

Today, communities and their health systems are facing with several challenges associated with the population ageing. Growing number of bone disorders is one of the most serious consequences of aging. According to the reports bone disorders won't just affect the elderly population. Mesenchymal stem cells (MSCs) are multipotent cells that could be derived from a variety of tissues including bone marrow, Wharton's Jelly, adipose tissue, and others. MSCs have been utilized in different researches in the field of regenerative medicine because of their immunosuppression and anti-inflammatory mechanisms (like: inhibiting the activity of antigen presenting cells, and suppressing the activity of T lymphocyte cells, macrophages, and so on.), migration to injured areas, and participation in healing processes. Bone marrow mesenchymal stem cells (BMMSCs) are a type of these cells which can be commonly used in bone research with the promising results. These cells function by releasing a large number of extracellular vesicles (EVs). Exosomes are the most major EVs products produced by BMMSCs. They have the same contents and properties as their parent cells; however, these structures don't have the defects of cell therapy. Proteins (annexins, tetraspannins, etc.), lipids (cholesterol, phosphoglycerides, etc.), nucleic acids (micro-RNAs, and etc.) and other substances are found in exosomes. Exosomes affect target cells, causing them to change their function. The features of BMMSC exosomes' mechanism in osteogenesis and bone regeneration (like: effects on other MSCs, osteoblasts, osteoclasts, and angiogenesis) and also the effects of their micro-RNAs on osteogenesis are the subject of the present review.

P16INK4A-More Than a Senescence Marker

Aging is a biological feature that is characterized by gradual degeneration of function in cells, tissues, organs, or an intact organism due to the accumulation of environmental factors and stresses with time. Several factors have been attributed to aging such as oxidative stress and augmented production or exposure to reactive oxygen species, inflammatory cytokines production, telomere shortening, DNA damage, and, importantly, the deposit of senescent cells. These are irreversibly mitotically inactive, yet metabolically active cells. The reason underlying their senescence lies within the extrinsic and the intrinsic arms. The extrinsic arm is mainly characterized by the expression and the secretory profile known as the senescence-associated secretory phenotype (SASP). The intrinsic arm results from the impact of several genes meant to regulate the cell cycle, such as tumor suppressor genes. P16INK4A is a tumor suppressor and cell cycle regulator that has been linked to aging and senescence. Extensive research has revealed that p16 expression is significantly increased in senescent cells, as well as during natural aging or age-related pathologies. Based on this fact, p16 is considered as a specific biomarker for detecting senescent cells and aging. Other studies have found that p16 is not only a senescence marker, but also a protein with many functions outside of senescence and aging. In this paper, we discuss and shed light on several studies that show the different functions of p16 and provide insights in its role in several biological processes besides senescence and aging.

Effect of Combined Intraosseous and Intraarticular Infiltrations of Autologous Platelet-Rich Plasma on Subchondral Bone Marrow Mesenchymal Stromal Cells from Patients with Hip Osteoarthritis

Osteoarthritis (OA) is a debilitating condition that significantly impacts its patients and is closely associated with advancing age and senescence. Treatment with autologous platelet rich plasma (PRP) is a novel approach that is increasingly being researched for its effects. Subchondral bone mesenchymal stromal cells (MSCs) are key progenitors that form bone and cartilage lineages that are affected in OA. This study investigated the changes in subchondral bone MSCs before and after combined intraosseous (IO) and intraarticular (IA) PRP infiltration. Patient bone marrow aspirates were collected from 12 patients (four male, eight female) aged 40–86 years old (median 59.5). MSCs were expanded in standard media containing human serum to passage 1 and analysed for their colony-forming potential, senescence status, and gene expression. Hip dysfunction and Osteoarthritis Outcome Score (HOOS) at baseline and 6 months post second infiltration were used to assess the clinical outcomes; seven patients were considered responders and five non-responders. The number of colony-forming MSCs did not increase in the post treatment group, however, they demonstrated significantly higher colony areas (14.5% higher compared to Pre) indicative of enhanced proliferative capacity, especially in older donors (28.2% higher). Senescence assays also suggest that older patients and responders had a higher resistance to senescent cell accumulation. Responder and non-responder MSCs tended to differ in the expression of genes associated with bone formation and cartilage turnover including osteoblast markers, matrix metalloproteinases, and their inhibitors. Taken together, our data show that in hip OA patients, combined IO and IA PRP infiltrations enhanced subchondral MSC proliferative and stress-resistance capacities, particularly in older patients. Future investigation of the potential anti-ageing effect of PRP infiltrations and the use of next-generation sequencing would contribute towards better understanding of the molecular mechanisms associated with OA in MSCs.

BMP9 reduces age-related bone loss in mice by inhibiting osteoblast senescence through Smad1-Stat1-P21 axis

Age-related osteoporosis is characterized by the accumulation of senescent osteoblastic cells in bone microenvironment and significantly reduced osteogenic differentiation. Clearing of the senescent cells is helpful to improve bone formation in aged mice. Bone morphogenetic protein 9 (BMP9), a multifunctional protein produced and secreted by liver, was reported to improve osteoporosis caused by estrogen withdrawal. However, the mechanism of BMP9 has not been fully elucidated, and its effect on senile osteoporosis has not been reported. This study reveals that BMP9 significantly increases bone mass and improves bone biomechanical properties in aged mice. Furthermore, BMP9 reduces expression of senescent genes in bone microenvironment, accompanied by decreased senescence-associated secretory phenotypes (SASPs) such as Ccl5, Mmp9, Hmgb1, Nfkb1, and Vcam1. In vitro, Bmp9 treatment inhibits osteoblast senescence through activating Smad1, which suppresses the transcriptional activity of Stat1, thereby inhibits P21 expression and SASPs production. Furthermore, inhibiting the Smad1 signal in vivo can reverse the inhibitory effect of BMP9 on Stat1 and downstream senescent genes, which eliminates the protection of BMP9 on age-related osteoporosis. These findings highlight the critical role of BMP9 on reducing age-related bone loss by inhibiting osteoblast senescence through Smad1-Stat1-P21 axis.

BMP9 inhibits cellular senescence by activation of Smad1, which suppresses the transcription of Stat1, resulting in decreased P21 expression and SASPs production in osteoblast. The anti-aging effect of BMP9 is benefit to improving age-related osteoporosis.

Crosstalk Between Senescent Bone Cells and the Bone Tissue Microenvironment Influences Bone Fragility During Chronological Age and in Diabetes

Bone is a complex organ serving roles in skeletal support and movement, and is a source of blood cells including adaptive and innate immune cells. Structural and functional integrity is maintained through a balance between bone synthesis and bone degradation, dependent in part on mechanical loading but also on signaling and influences of the tissue microenvironment. Bone structure and the extracellular bone milieu change with age, predisposing to osteoporosis and increased fracture risk, and this is exacerbated in patients with diabetes. Such changes can include loss of bone mineral density, deterioration in micro-architecture, as well as decreased bone flexibility, through alteration of proteinaceous bone support structures, and accumulation of senescent cells. Senescence is a state of proliferation arrest accompanied by marked morphological and metabolic changes. It is driven by cellular stress and serves an important acute tumor suppressive mechanism when followed by immune-mediated senescent cell clearance. However, aging and pathological conditions including diabetes are associated with accumulation of senescent cells that generate a pro-inflammatory and tissue-destructive secretome (the SASP). The SASP impinges on the tissue microenvironment with detrimental local and systemic consequences; senescent cells are thought to contribute to the multimorbidity associated with advanced chronological age. Here, we assess factors that promote bone fragility, in the context both of chronological aging and accelerated aging in progeroid syndromes and in diabetes, including senescence-dependent alterations in the bone tissue microenvironment, and glycation changes to the tissue microenvironment that stimulate RAGE signaling, a process that is accelerated in diabetic patients. Finally, we discuss therapeutic interventions targeting RAGE signaling and cell senescence that show promise in improving bone health in older people and those living with diabetes.

Melatonin Improves the Resistance of Oxidative Stress-Induced Cellular Senescence in Osteoporotic Bone Marrow Mesenchymal Stem Cells

Accumulation of senescent bone marrow-derived mesenchymal stem cells (BMMSCs) has led to an age-related bone loss. However, the role of stem cell senescence in estrogen deficiency-induced osteoporosis remains elusive. Though melatonin plays a vital role in bone metabolism regulation, the underlying mechanisms of melatonin-mediated antiosteoporosis are partially elucidated. Therefore, this study purposed to explore (1) whether estrogen deficiency causes cellular senescence of BMMSCs, and if so, (2) the potential of melatonin in preventing bone loss via senescence signaling inhibition. BMMSCs derived from ovariectomized (OVX) rats (OVX BMMSCs) showed an impaired osteogenic capacity, albeit having comparable levels of senescence biomarkers than the sham cells. When exposed to low levels of hydrogen peroxide (H₂O₂), OVX BMMSCs rapidly exhibited senescence-associated phenotypes such as the increased activity of senescence-associated β-galactosidase (SA-β-gal) and upregulation of cell cycle inhibitors. Notably, the in vitro treatment with melatonin hindered H₂O₂-induced senescence in OVX BMMSCs and restored their osteogenic capacity. Treatment with either SIRT1 inhibitor (sirtinol) or melatonin receptor antagonists (luzindole and 4-P-PDOT) eliminated melatonin protective effects, thus indicating its potential in preventing stem cell senescence via SIRT1 activation through the melatonin membrane receptors. Following in vivo intravenous administration with melatonin, it successfully protected the bone microstructure and preserved the antisenescence property of BMMSCs in OVX rats. Collectively, our findings demonstrated that melatonin protected against estrogen deficiency-related bone loss by improving the resistance of BMMSCs to cellular senescence. Therefore, melatonin-mediated antisenescence effect on stem cells provides vital information to facilitate the development of a novel and effective strategy for treating postmenopausal OP.

HO-1 in Bone Biology: Potential Therapeutic Strategies for Osteoporosis

Osteoporosis is a prevalent bone disorder characterized by bone mass reduction and deterioration of bone microarchitecture leading to bone fragility and fracture risk. In recent decades, knowledge regarding the etiological mechanisms emphasizes that inflammation, oxidative stress and senescence of bone cells contribute to the development of osteoporosis. Studies have demonstrated that heme oxygenase 1 (HO-1), an inducible enzyme catalyzing heme degradation, exhibits anti-inflammatory, anti-oxidative stress and anti-apoptosis properties. Emerging evidence has revealed that HO-1 is critical in the maintenance of bone homeostasis, making HO-1 a potential target for osteoporosis treatment. In this Review, we aim to provide an introduction to current knowledge of HO-1 biology and its regulation, focusing specifically on its roles in bone homeostasis and osteoporosis. We also examine the potential of HO-1-based pharmacological therapeutics for osteoporosis and issues faced during clinical translation.

Insights into oxidative stress in bone tissue and novel challenges for biomaterials

The presence of Reactive Oxygen Species (ROS) in bone can influence resident cells behaviour as well as the extra-cellular matrix composition and the tissue architecture. Aging, in addition to excessive overloads, unbalanced diet, smoking, predisposing genetic factors, lead to an increase of ROS and, if it is accompanied with an inappropriate production of scavengers, promotes the generation of oxidative stress that encourages bone catabolism. Furthermore, bone injuries can be triggered by numerous events such as road and sports accidents or tumour resection. Although bone tissue possesses a well-known repair and regeneration capacity, these mechanisms are inefficient in repairing large size defects and bone grafts are often necessary. ROS play a fundamental role in response after the implant introduction and can influence its success. This review provides insights on the mechanisms of oxidative stress generated by an implant in vivo and suitable ways for its modulation. The local delivery of active molecules, such as polyphenols, enhanced bone biomaterial integration evidencing that the management of the oxidative stress is a target for the effectiveness of an implant. Polyphenols have been widely used in medicine for cardiovascular, neurodegenerative, bone disorders and cancer, thanks to their antioxidant and anti-inflammatory properties. In addition, the perspective of new smart biomaterials and molecular medicine for the oxidative stress modulation in a programmable way, by the use of ROS responsive materials or by the targeting of selective molecular pathways involved in ROS generation, will be analysed and discussed critically.

Senescent RAW264.7 cells exhibit increased production of nitric oxide and release inducible nitric oxide synthase in exosomes

Aging cells not only cease growing, but also secrete various proteins such as inflammatory cytokines. This secretory phenomenon is known as the senescence‑associated secretory phenotype (SASP). The aim of the present study was to elucidate the effects of senescence on the differentiation of osteoclast precursors (OCPs) and corresponding SASP. RAW264.7 cells were used as OCPs and were cultured to passage (P)5, P10 and P20. Cell proliferation assays, senescence‑associated β‑galactosidase staining and telomere length quantification were subsequently performed, and it was revealed that replicative senescence was induced at P20. In addition, the level of tartrate‑resistant acid phosphatase activity in P20 cells treated with receptor activator of nuclear factor‑κB ligand was significantly lower than that in P5 and P10 cells. The SASP factors interleukin‑6, tumour necrosis factor‑α and nitric oxide were significantly increased in P20 culture supernatants compared with those in P5 and P10 supernatants. Furthermore, the number of exosomes at P20 was increased compared with that at P5 and P10, and inducible nitric oxide synthase (iNOS) was expressed in exosomes at P20, but not in exosomes at P5. In conclusion, the present study revealed that senescent RAW264.7 cells exhibit increased expression of SASP factors and release iNOS in exosomes.

Age-related alterations and senescence of mesenchymal stromal cells: Implications for regenerative treatments of bones and joints

The most common clinical manifestations of age-related musculoskeletal degeneration are osteoarthritis and osteoporosis, and these represent an enormous burden on modern society. Mesenchymal stromal cells (MSCs) have pivotal roles in musculoskeletal tissue development. In adult organisms, MSCs retain their ability to regenerate tissues following bone fractures, articular cartilage injuries, and other traumatic injuries of connective tissue. However, their remarkable regenerative ability appears to be impaired through aging, and in particular in age-related diseases of bones and joints. Here, we review age-related alterations of MSCs in musculoskeletal tissues, and address the underlying mechanisms of aging and senescence of MSCs. Furthermore, we focus on the properties of MSCs in osteoarthritis and osteoporosis, and how their changes contribute to onset and progression of these disorders. Finally, we consider current treatments that exploit the enormous potential of MSCs for tissue regeneration, as well as for innovative cell-free extracellular-vesicle-based and anti-aging treatment approaches.

Circulating Alpha-Tocopherol Levels, Bone Mineral Density, and Fracture: Mendelian Randomization Study

Recent cohort studies indicate a potential role of the antioxidant α-tocopherol in reducing bone loss and risk of fractures, especially hip fractures. We performed a Mendelian randomization investigation of the associations of circulating α-tocopherol with estimated bone mineral density (eBMD) using heel ultrasound and fractures, identified from hospital records or by self-reports and excluding minor fractures. Circulating α-tocopherol was instrumented by three genetic variants associated with α-tocopherol levels at p < 5 × 10^-8 in a genome-wide association meta-analysis of 7781 participants of European ancestry. Summary-level data for the genetic associations with eBMD in 426,824 individuals and with fracture (53,184 cases and 373,611 non-cases) were acquired from the UK Biobank. Two of the three genetic variants were strongly associated with eBMD. In inverse-variance weighted analysis, a genetically predicted one-standard-deviation increase of circulating α-tocopherol was associated with 0.07 (95% confidence interval, 0.05 to 0.09) g/cm² increase in BMD, which corresponds to a >10% higher BMD. Genetically predicted circulating α-tocopherol was not associated with odds of any fracture (odds ratio 0.97, 95% confidence interval, 0.91 to 1.05). In conclusion, our results strongly strengthen a causal link between increased circulating α-tocopherol and greater BMD. Both an intervention study in those with a low dietary intake of α-tocopherol is warranted and a Mendelian randomization study with fragility fractures as an outcome.

Low magnitude vibration alleviates age-related bone loss by inhibiting cell senescence of osteogenic cells in naturally senescent rats

Dysfunction of bone marrow mesenchymal stem cells (BMSCs), osteoblasts and osteocytes may be one of the main causes of bone loss in the elderly. In the present study, we found osteogenic cells from aged rats all exhibited senescence changes, with the most pronounced senescence changes in osteocytes. Meanwhile, the proliferative capacity and functional activity of osteogenic cells from aged rats were suppressed. Osteogenic differentiation capacity of BMSCs from aged rats decreased while adipogenic capacity increased. The mineralization capacity, ALP activity and osteogenic proteins expression of osteoblasts from aged rats decreased. Additionally, osteocytes from aged rats up-expressed sclerosteosis protein, a negative regulator of bone formation. To inhibit osteogenic cell senescence, we use low magnitude vibration (LMV) to eliminate the senescent osteogenic cells. After LMV treatment, the number of osteogenic cells staining positively for senescence-associated-β-galactosidase (SA-β-Gal) decreased significantly. Besides, the expression of anti-aging protein SIRT1 was upregulated significantly, while p53 and p21 were downregulated significantly after LMV treatment. Thus, the LMV can inhibit the senescence of osteogenic cells partly through the Sirt1/p53/p21 axis. Furthermore, LMV was found to promote bone formation of aged rats. These results suggest that the inhibition of osteogenic cell senescence by LMV is a valuable treatment to prevent or delay osteoporosis.

Hypoxia-inducible factor-2α mediates senescence-associated intrinsic mechanisms of age-related bone loss

Aging is associated with cellular senescence followed by bone loss leading to bone fragility in humans. However, the regulators associated with cellular senescence in aged bones need to be identified. Hypoxia-inducible factor (HIF)-2α regulates bone remodeling via the differentiation of osteoblasts and osteoclasts. Here, we report that HIF-2α expression was highly upregulated in aged bones. HIF-2α depletion in male mice reversed age-induced bone loss, as evidenced by an increase in the number of osteoblasts and a decrease in the number of osteoclasts. In an in vitro model of doxorubicin-mediated senescence, the expression of Hif-2α and p21, a senescence marker gene, was enhanced, and osteoblastic differentiation of primary mouse calvarial preosteoblast cells was inhibited. Inhibition of senescence-induced upregulation of HIF-2α expression during matrix maturation, but not during the proliferation stage of osteoblast differentiation, reversed the age-related decrease in Runx2 and Ocn expression. However, HIF-2α knockdown did not affect p21 expression or senescence progression, indicating that HIF-2α expression upregulation in senescent osteoblasts may be a result of aging rather than a cause of cellular senescence. Osteoclasts are known to induce a senescent phenotype during in vitro osteoclastogenesis. Consistent with increased HIF-2α expression, the expression of p16 and p21 was upregulated during osteoclastogenesis of bone marrow macrophages. ChIP following overexpression or knockdown of HIF-2α using adenovirus revealed that p16 and p21 are direct targets of HIF-2α in osteoclasts. Osteoblast-specific (Hif-2α^fl/fl;Col1a1-Cre) or osteoclast-specific (Hif-2α^fl/fl;Ctsk-Cre) conditional knockout of HIF-2α in male mice reversed age-related bone loss. Collectively, our results suggest that HIF-2α acts as a senescence-related intrinsic factor in age-related dysfunction of bone homeostasis.

Irisin Correlates Positively With BMD in a Cohort of Older Adult Patients and Downregulates the Senescent Marker p21 in Osteoblasts

Irisin is a myokine produced by skeletal muscle during exercise in both mice and humans. We previously showed that irisin treatment ameliorates immobility-induced osteoporosis and muscular atrophy in mice. Data in humans showed a positive association between irisin and bone mineral density (BMD) in athletes and a population of healthy children. However, the role of this myokine regarding the state of muscle and bone in the same population remained to be determined. For this purpose, 62 patients (age 68.71 ± 12.31 years) undergoing total hip or knee replacement were recruited. Our results showed that irisin serum levels negatively correlated with age (R = -0.515; p = .000018) and positively correlated with femoral BMD (R = 0.619; p = .001) and vertebral BMD (R = 0.201; p = .0001). Irisin was also positively associated with Fndc5 mRNA in muscle biopsies (R = 0.248; p = .016), as well as with Osteocalcin (Ocn) mRNA in bone biopsies (R = 0.708; p = .006). In skeletal muscle, FNDC5 positive fibers positively correlate with BMD of total femur (R = 0.765; p = .0014) and BMD of femoral neck (R = 0.575; p = .031), Interestingly, by analyzing patients divided by their T-score, we found lower irisin levels (p = .0011) in patients with osteopenia/osteoporosis (OP) compared to healthy controls matched for age and sex. By analyzing the senescence marker p21, we found a significant increase of its mRNA expression in the bone biopsies of OP patients compared to control ones. Therefore, we investigated in vitro whether rec-irisin had a direct effect on this senescence marker, showing that p21 mRNA expression was significantly downregulated in osteoblasts by the treatment with irisin. Overall, these results indicate that higher irisin levels are associated with a lower rate of age-related osteoporosis and that irisin could be effective in delaying the osteoblast aging process, suggesting a potential senolytic action of this myokine. © 2020 American Society for Bone and Mineral Research (ASBMR).

Adipocyte-induced transdifferentiation of osteoblasts and its potential role in age-related bone loss

Our preliminary findings have lead us to propose bone marrow adipocyte secretions as new contributors to bone loss. Indeed, using a coculture model based on human bone marrow stromal cells, we previously showed that soluble factors secreted by adipocytes induced the conversion of osteoblasts towards an adipocyte-like phenotype. In this study, microarray gene expression profiling showed profound transcriptomic changes in osteoblasts following coculture and confirmed the enrichment of the adipocyte gene signature. Double immunofluorescence microscopic analyses demonstrated the coexpression of adipogenic and osteoblastic specific markers in individual cells, providing evidence for a transdifferentiation event. At the molecular level, this conversion was associated with upregulated expression levels of reprogramming genes and a decrease in the DNA methylation level. In line with these in vitro results, preliminary immunohistochemical analysis of bone sections revealed adipogenic marker expression in osteoblasts from elderly subjects. Altogether, these data suggest that osteoblast transdifferentiation could contribute to decreased bone mass upon ageing.

Baghdadite Ceramics Prevent Senescence in Human Osteoblasts and Promote Bone Regeneration in Aged Rats

Bone fractures and critical-sized bone defects present significant health threats for the elderly who have limited capacity for regeneration due to the presence of functionally compromised senescent cells. A wide range of synthetic materials has been developed to promote the regeneration of critical-sized bone defects, but it is largely unknown if a synthetic biomaterial (scaffold) can modulate cellular senescence and improve bone regeneration in aged scenarios. The current study investigates the interaction of Baghdadite (Ca₃ZrSi₂O₉) ceramic scaffolds with senescent human primary osteoblast-like cells (HOBs) and its bone regeneration capacity in aged rats. A senescent HOB model was established by repeatedly passaging HOBs till passage 7 (P7). Compared to the clinically used hydroxyapatite/tricalcium phosphate (HA/TCP), Baghdadite prevented senescence induction in P7 HOBs and markedly negated the paracrine effect of P7 HOB secretomes that mediated the up-regulations of cellular senescence-associated gene expression levels in P2 HOBs. We further demonstrated that conditioned media extracted from Baghdadite corrected the dysfunctional mitochondria in P7 HOBs. In vivo, the bone regeneration capacity was enhanced when 3D printed Baghdadite scaffolds were implanted in a calvaria critical-sized bone defect model in both young and aged rats compared to HA/TCP scaffolds, but a better effect was observed in aged rats than in young rats. This study suggests that Baghdadite ceramic represents a novel and promising biomaterial approach to promote bone regeneration capacity in the elderly by providing an anti-senescent microenvironment.

The effect of age on the microarchitecture and profile of gene expression in femoral head and neck bone from patients with osteoarthritis

Ageing of the skeleton is characterised by decreased bone mineral density, reduced strength, and increased risk of fracture. Although it is known that these changes are determined by the activities of bone cells through the processes of bone modelling and remodelling, details of the molecular mechanisms that underlie age-related changes in bone are still missing. Here, we analysed age-related changes in bone microarchitecture along with global gene expression in samples obtained from patients with osteoarthritis (OA). We hypothesised that changes would be evident in both microarchitecture and gene expression and aimed to identify novel molecular mechanisms that underlie ageing processes in bone. Samples of femoral head and neck were obtained from patients undergoing hip arthroplasty for OA, who were either ≤60 years or ≥70 years of age. Bone microarchitecture was analysed in cores of trabecular bone from the femoral head (17 from the younger group and 18 from the older), and cortical bone from the femoral neck (25 younger/22 older), using a Skyscan 1172 microCT scanner (Bruker). Gene expression was compared between the two age groups in 20 trabecular samples from each group, and 10 cortical samples from each group, using Clariom S Human microarrays (ThermoFisher Scientific). We found no significant changes between the two age groups in indices of trabecular or cortical bone microarchitecture. Gene expression analysis identified seven genes that had higher expression in the older group, including the transcription factor EGR1 and the glucose transporter SLC2A3 (GLUT3), and 21 differentially expressed genes in cortical bone samples (P<0.05, fold change>2). However, none of the comparisons of gene expression had false discovery rate-adjusted P<0.1. In contrast to our working hypothesis, we found only minor differences in gene expression and no differences in bone microarchitecture between the two age-groups. It is possible that pathological processes related to OA provide protection against age-related changes in bone. Our study suggests that in patients with OA, the bone properties measured here in femoral head and neck do not deteriorate significantly from the sixth to the eighth decade of life.

Cheung A. Vitamin K and Osteoporosis

Vitamin K acts as a coenzyme of carboxylase, catalyzing the carboxylation of several vitamin K dependent proteins. Beyond its well-known effects on blood coagulation, it also exerts relevant effects on bone and the vascular system. In this review, we point out the relevance of an adequate vitamin K intake to obtain sufficient levels of carboxylated (active form) vitamin K dependent proteins (such as Osteocalcin and matrix Gla protein) to prevent bone health. Another bone-related action of Vitamin K is being a ligand of the nuclear steroid and xenobiotic receptor (SXR). We also discuss the recommended intake, deficiency, and assessment of vitamin K. Furthermore, we review the few available studies that have as pre-specified outcome bone fractures, indicating that we need more clinical studies to confirm that vitamin K is a potential therapeutic agent for bone fractures.

Osteoblastic Wntless deletion differentially regulates the fate and functions of bone marrow-derived stem cells in relation to age

Although functional association between Wnt signaling and bone homeostasis has been well described through genetic ablation of Wntless (Wls), the mechanisms of how osteoblastic Wls regulates the fate of bone marrow stromal cells (BMSCs) and hematopoietic stem cells (HSCs) in relation to age are not yet understood. Here, we generated Col2.3-Cre;Wls^fl/fl mice that were free from premature lethality and investigated age-related impacts of osteoblastic Wls deficiency on hematopoiesis, BM microenvironment, and maintenance of BMSCs (also known as BM-derived mesenchymal stem/stromal cells) and HSCs. Ablation of osteoblastic Wls deteriorated BM microenvironment and bone mass accrual along with age-independent effects on functions of BMSCs. Osteoblastic Wls deletion impaired HSC repopulation and progeny with skewing toward myeloid lineage cells only at old stage. As proven by hallmarks of stem cell senescence, osteoblastic Wls ablation differentially induced senescence of BMSCs and HSCs in relation to age without alteration in their BM frequency. Our findings support that deletion of Wls in Col2.3-expressing cells induces senescence of BMSCs and impairs BM microenvironment in age-independent manner. Overall, long-term deterioration in BM microenvironment contributes to age-related HSC senescence with impaired progeny and hematopoiesis, which also suggests possible roles of osteoblastic Wls on the maintenance of BM HSCs.

Mesenchymal Stromal Cell-Based Bone Regeneration Therapies: From Cell Transplantation and Tissue Engineering to Therapeutic Secretomes and Extracellular Vesicles

Effective regeneration of bone defects often presents significant challenges, particularly in patients with decreased tissue regeneration capacity due to extensive trauma, disease, and/or advanced age. A number of studies have focused on enhancing bone regeneration by applying mesenchymal stromal cells (MSCs) or MSC-based bone tissue engineering strategies. However, translation of these approaches from basic research findings to clinical use has been hampered by the limited understanding of MSC therapeutic actions and complexities, as well as costs related to the manufacturing, regulatory approval, and clinical use of living cells and engineered tissues. More recently, a shift from the view of MSCs directly contributing to tissue regeneration toward appreciating MSCs as "cell factories" that secrete a variety of bioactive molecules and extracellular vesicles with trophic and immunomodulatory activities has steered research into new MSC-based, "cell-free" therapeutic modalities. The current review recapitulates recent developments, challenges, and future perspectives of these various MSC-based bone tissue engineering and regeneration strategies.

Myeloid-Derived Suppressor Cells at the Intersection of Inflammaging and Bone Fragility

Age-related alteration of the immune system with aging, or immunosenescence, plays a major role in several age-associated conditions, including loss of bone integrity. Studies over the past several years have clearly established the immune system is chronically activated with advanced aging, termed inflammaging, and is characterized by elevated levels of proinflammatory cytokines in response to physiological or environmental cues that essentially result in an arrested immune system that maintains a low-level state of activation. This age-associated inflammation impacts several biological systems including the innate immune system, where aging results in a skewing of the hematopoiesis toward the myeloid lineage, including the expansion of myeloid-derived suppressor cells (MDSCs). This heterogeneous population of myeloid cells classically displays immunosuppressive capacity but they also have the ability to directly differentiate into osteoclasts. This review explores the possibility of inflammaging to be involved in reduction of bone microarchitecture and loss of bone mass/strength through the expansion of MDSCs and the osteoclastogenic capacity and activity.

BK ablation attenuates osteoblast bone formation via integrin pathway

Impaired bone formation is one of the major causes of low bone mass and skeletal fragility that occurs in osteoporosis. However, the mechanisms underlying the defects in bone formation are not well understood. Here, we report that big conductance calcium-activated potassium channels (BKs) are required for bone formation and osteoblast function both in vivo and in vitro. By 15 weeks of age, BK knockout (BKO) mice exhibited a decline in bone mineral density and trabecular bone volume of the tibiae and lumbar vertebrae, which were associated with impaired bone formation and osteoblast activity. Mechanistically, BK ablation in bone and bone marrow mesenchymal stem cells (BMSCs) of BKO mice inhibited integrin signaling. Furthermore, the binding of α subunit of BK with integrin β1 protein in osteoblasts was confirmed, and FAK-ERK1/2 signaling was proved to be involved by genetic modification of KCNMA1 (which encodes the α subunit of BK) in ROS17/2.8 osteoblast cells. These findings indicated that BK regulates bone formation by promoting osteoblast differentiation via integrin pathway, which provided novel insight into ion transporter crosstalk with the extracellular matrix in osteoblast regulation and revealed a new potential strategy for intervention in correcting bone formation defects.

Increased Cellular Presence After Sciatic Neurectomy Improves the Bone Mechano-adaptive Response in Aged Mice

The mechano-adaptive response of bone to loading in the murine uniaxial tibial loading model is impaired in aged animals. Previous studies have shown that in aged mice, the amount of bone formed in response to loading is augmented when loads are applied following sciatic neurectomy. The synergistic effect of neurectomy and loading remains to be elucidated. We hypothesize that sciatic neurectomy increases cellular presence, thereby augmenting the response to load in aged mice. We examined bone adaptation in four groups of female C57BL/6J mice, 20-22 months old: (1) sham surgery + 9N loading; (2) sciatic neurectomy, sacrificed after 5 days; (3) sciatic neurectomy, sacrificed after 19 days; (4) sciatic neurectomy + 9N loading. We examined changes in bone cross sectional properties with micro-CT images, and static and dynamic histomorphometry with histological sections taken at the midpoint between tibiofibular junctions. The response to loading at 9N was not detectable with quantitative micro-CT data, but surface-specific histomorphometry captured an increase in bone formation in specific regions. 5 days following sciatic neurectomy, the amount of bone in the neurectomized leg was the same as the contralateral leg, but 19 days following sciatic neurectomy, there was significant bone loss in the neurectomized leg, and both osteoclasts and osteoblasts were recruited to the endosteal surfaces. When sciatic neurectomy and loading at 9N were combined, 3 out of 4 bone quadrants had increased bone formation, on the endosteal and periosteal surfaces (increased osteoid surface and mineralizing surface respectively). These data demonstrate that sciatic neurectomy increases cellular presence on the endosteal surface. With long-term sciatic-neurectomy, both osteoclasts and osteoblasts were recruited to the endosteal surface, which resulted in increased bone formation when combined with a sufficient mechanical stimulus. Controlled and localized recruitment of both osteoblasts and osteoclasts combined with appropriate mechanical loading could inform therapies for mechanically-directed bone formation.

Bench-to-bedside strategies for osteoporotic fracture: From osteoimmunology to mechanosensation

Osteoporosis is characterized by a decrease in bone mass and strength, rendering people prone to osteoporotic fractures caused by low-energy forces. The primary treatment strategy for osteoporotic fractures is surgery; however, the compromised and comminuted bones in osteoporotic fracture sites are not conducive to optimum reduction and rigid fixation. In addition, these patients always exhibit accompanying aging-related disorders, including high inflammatory status, decreased mechanical loading and abnormal skeletal metabolism, which are disadvantages for fracture healing around sites that have undergone orthopedic procedures. Since the incidence of osteoporosis is expected to increase worldwide, orthopedic surgeons should pay more attention to comprehensive strategies for improving the poor prognosis of osteoporotic fractures. Herein, we highlight the molecular basis of osteoimmunology and bone mechanosensation in different healing phases of elderly osteoporotic fractures, guiding perioperative management to alleviate the unfavorable effects of insufficient mechanical loading, high inflammatory levels and pathogen infection. The well-informed pharmacologic and surgical intervention, including treatment with anti-inflammatory drugs and sufficient application of antibiotics, as well as bench-to-bedside strategies for bone augmentation and hardware selection, should be made according to a comprehensive understanding of bone biomechanical properties in addition to the remodeling status of osteoporotic bones, which is necessary for creating proper biological and mechanical environments for bone union and remodeling. Multidisciplinary collaboration will facilitate the improvement of overall osteoporotic care and reduction of secondary fracture incidence.

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.

Mitochondria and cellular senescence: Implications for musculoskeletal ageing

Musculoskeletal ageing and its associated diseases are major contributors to the loss of independence and reduced quality of life in older people. Several recent studies indicate that cellular senescence is a contributor to age-related loss of function in various organs including muscle, bones and joints. Importantly, these studies indicate that therapies targeting specifically senescent cells have great therapeutic potential in improving musculoskeletal health during ageing. Senescent cells are characterised by dramatic changes in mitochondrial function, metabolism and homeostasis. Mitochondrial dysfunction has been shown to contribute to senescence and the SASP. Here we review the role of cellular senescence in musculoskeletal ageing as well as the potential mechanisms by which mitochondrial dysfunction may impact on the induction and development of the senescent phenotype.

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.

The Expanding Life and Functions of Osteogenic Cells: From Simple Bone-Making Cells to Multifunctional Cells and Beyond

During the last three decades, important progress in bone cell biology and in human and mouse genetics led to major advances in our understanding of the life and functions of cells of the osteoblast lineage. Previously unrecognized sources of osteogenic cells have been identified. Novel cellular and molecular mechanisms controlling osteoblast differentiation and senescence have been determined. New mechanisms of communications between osteogenic cells, osteocytes, osteoclasts, and chondrocytes, as well as novel links between osteogenic cells and blood vessels have been identified. Additionally, cells of the osteoblast lineage were shown to be important components of the hematopoietic niche and to be implicated in hematologic dysfunctions and malignancy. Lastly, unexpected interactions were found between osteogenic cells and several soft tissues, including the central nervous system, gut, muscle, fat, and testis through the release of paracrine factors, making osteogenic cells multifunctional regulatory cells, in addition to their bone-making function. These discoveries considerably enlarged our vision of the life and functions of osteogenic cells, which may lead to the development of novel therapeutics with immediate applications in bone disorders. © 2017 American Society for Bone and Mineral Research.

The role of cellular senescence in aging through the prism of Koch-like criteria

Since Hayflick's discovery of cellular senescence (CS), a great volume of knowledge in the field has been accumulated and intensively discussed. Here, we attempted to organize the evidence "for" and "against" the hypothesized causal role of CS in aging. For that purpose, we utilized robust Koch-like logical criteria, based on the assumption that some quantitative relationships between the accumulation of senescent cells and aging rate should exist. If so, it could be expected that (i) the "CS load" would be greater in the premature aging phenotype and lesser in longevity phenotype; (ii) CS would promote age-related diseases, and (iii) the interventions that modulate the levels of senescent cells should also modulate health/lifespan. The analysis shows that CS can be considered a causal factor of aging and an important player in various age-related diseases, though its contribution may greatly vary across species. While the relative impact of senescent cells to aging could overall be rather limited and their elimination is hardly expected to be the "fountain of youth", the potential benefits of the senolytic strategy seems a promising option in combating age-related diseases and extending healthspan.

Aged rats under zoledronic acid therapy and oral surgery

INTRODUCTION

Aging brings a number of health conditions that can compromise the healing process in elderly individuals, significantly when it comes to bone tissue. The aim of the present study was to analyze the effects of zoledronic acid (ZL) therapy on socket healing of aged male rats.

MATERIALS AND METHODS

Twenty-four Wistar male rats, 20 months old, underwent surgical procedures for the extraction of the upper right incisor and were divided into two groups according to the treatment: Control (C) - intravenous (IV) 0.9% saline, ZL - 0.035 mg/kg of IV zoledronic acid, both every 15 days. At the fifth dose of both substances, tooth extractions were performed and the animals were euthanized after 14 and 28 days.

RESULTS

IV administration of ZL caused OPG-RANKL system imbalance, resulting in deficient bone formation and remodeling and alteration of osteoclast morphology, as well as maintaining persistent inflammation during the healing period.

CONCLUSIONS

IV administration of ZL delayed extracted dental socket healing of aged rats, but not enough to cause osteonecrosis, raising a question about different responses to IV BP therapy considering animal age.

Inhibition of BET proteins and epigenetic signaling as a potential treatment for osteoporosis

Histone modifications are important for maintaining the transcription program. BET proteins, an important class of "histone reading proteins", have recently been described as essential in bone biology. This study presents the therapeutic opportunity of BET protein inhibition in osteoporosis. We find that the pharmacological BET protein inhibitor JQ1 rescues pathologic bone loss in a post-ovariectomy osteoporosis model by increasing the trabecular bone volume and restoring mechanical properties. The BET protein inhibition suppresses osteoclast differentiation and activity as well as the osteoblastogenesis in vitro. Moreover, we show that treated non-resorbing osteoclasts could still activate osteoblast differentiation. In addition, specific inhibition of BRD4 using RNA interference inhibits osteoclast differentiation but strongly activates osteoblast mineralization activity. Mechanistically, JQ1 inhibits expression of the master osteoclast transcription factor NFATc1 and the transcription factor of osteoblast Runx2. These findings strongly support that targeting epigenetic chromatin regulators such as BET proteins may offer a promising alternative for the treatment of bone-related disorders such as osteoporosis.

Senescence: novel insight into DLX3 mutations leading to enhanced bone formation in Tricho-Dento-Osseous syndrome

The homeodomain transcription factor distal-less homeobox 3 gene (DLX3) is required for hair, tooth and skeletal development. DLX3 mutations have been found to be responsible for Tricho-Dento-Osseous (TDO) syndrome, characterized by kinky hair, thin-pitted enamel and increased bone density. Here we show that the DLX3 mutation (c.533 A>G; Q178R) attenuates osteogenic potential and senescence of bone mesenchymal stem cells (BMSCs) isolated from a TDO patient, providing a molecular explanation for abnormal increased bone density. Both DLX3 mutations (c.533 A>G and c.571_574delGGGG) delayed cellular senescence when they were introduced into pre-osteoblastic cells MC3T3-E1. Furthermore, the attenuated skeletal aging and bone loss in DLX3 (Q178R) transgenic mice not only reconfirmed that DLX3 mutation (Q178R) delayed cellular senescence, but also prevented aging-mediated bone loss. Taken together, these results indicate that DLX3 mutations act as a loss of function in senescence. The delayed senescence of BMSCs leads to increased bone formation by compensating decreased osteogenic potentials with more generations and extended functional lifespan. Our findings in the rare human genetic disease unravel a novel mechanism of DLX3 involving the senescence regulation of bone formation.

Identification of Senescent Cells in the Bone Microenvironment

Cellular senescence is a fundamental mechanism by which cells remain metabolically active yet cease dividing and undergo distinct phenotypic alterations, including upregulation of p16^Ink4a , profound secretome changes, telomere shortening, and decondensation of pericentromeric satellite DNA. Because senescent cells accumulate in multiple tissues with aging, these cells and the dysfunctional factors they secrete, termed the senescence-associated secretory phenotype (SASP), are increasingly recognized as promising therapeutic targets to prevent age-related degenerative pathologies, including osteoporosis. However, the cell type(s) within the bone microenvironment that undergoes senescence with aging in vivo has remained poorly understood, largely because previous studies have focused on senescence in cultured cells. Thus in young (age 6 months) and old (age 24 months) mice, we measured senescence and SASP markers in vivo in highly enriched cell populations, all rapidly isolated from bone/marrow without in vitro culture. In both females and males, p16^Ink4a expression by real-time quantitative polymerase chain reaction (rt-qPCR) was significantly higher with aging in B cells, T cells, myeloid cells, osteoblast progenitors, osteoblasts, and osteocytes. Further, in vivo quantification of senescence-associated distension of satellites (SADS), ie, large-scale unraveling of pericentromeric satellite DNA, revealed significantly more senescent osteocytes in old compared with young bone cortices (11% versus 2%, p < 0.001). In addition, primary osteocytes from old mice had sixfold more (p < 0.001) telomere dysfunction-induced foci (TIFs) than osteocytes from young mice. Corresponding with the age-associated accumulation of senescent osteocytes was significantly higher expression of multiple SASP markers in osteocytes from old versus young mice, several of which also showed dramatic age-associated upregulation in myeloid cells. These data show that with aging, a subset of cells of various lineages within the bone microenvironment become senescent, although senescent myeloid cells and senescent osteocytes predominantly develop the SASP. Given the critical roles of osteocytes in orchestrating bone remodeling, our findings suggest that senescent osteocytes and their SASP may contribute to age-related bone loss. © 2016 American Society for Bone and Mineral Research.

In vitro lifespan and senescent behaviour of human periosteal derived stem cells

Periosteum derived progenitor cells (PDPCs) represent promising mesenchymal stem cells (MSCs) for skeletal regeneration and to test bone cell based tissue engineering strategies. Most of regenerative medicine approaches based on MSCs require a noteworthy amount of cells that must be expanded in vitro prior to their use. As culture expansion method may impact on cell behaviour, we assessed the replicative and metabolic capacity (nitric oxide production and glucose consumption), senescence hallmarks of PDPCs serially passaged as well as the expression of selected genes specifically related to early osteoblastic differentiation, bone remodelling and stemness during PDPC sequential passaging. We also scouted a Systems Biology approach to examine and elucidate the experimental results through mathematical modelling and in silico simulations. PDPC subculture led to a progressive proliferative decline but, despite this, PDPCs maintained almost constant their metabolic activity. In vitro, senescent PDPCs displayed the typical "replicative senescence" features, involving p16 and not p53 in the regulation of this phenomenon. Gene expression analysis evidenced the tendency of sub-cultured PDPCs to increase the expression of genes involved in bone resorption. The mathematical analysis of the experimental results showed a strict similarity between replicative senescence and age-related changes, enabling the definition of an in silico model mimicking PDPC behaviour in terms of nitric oxide (NO) production. The relationship between NO production and subculture passages could represent a cutting edge "replicative senescence index". Overall, our findings suggest the possibility to use early-passage PDPCs for bone regenerative approaches based on the local recruitment of stem cells, whilst the later cell passages could be a suitable in vitro tool to validate scaffolds intended for bone regeneration in elderly subjects.