The Spectrum of Fundamental Basic Science Discoveries Contributing to Organismal Aging

Mitochondrial oxidative stress or decreased autophagy in osteoblast lineage cells is not sufficient to mimic the deleterious effects of aging on bone mechanoresponsiveness

Exercise-induced mechanical load stimulates bone cells, including osteocytes, to promote bone formation. The bone response to loading is less effective with aging, but the cellular and molecular mechanisms responsible for the impaired mechanoresponsiveness remain unclear. Excessive mitochondrial reactive oxygen species (mtROS) and deficient autophagy are common aging mechanisms implicated in decreased bone formation in old mice. Here, we confirmed that the osteogenic effects of tibia compressive loading are lower in old versus young female mice. We also examined whether an increase in mtROS or decreased autophagy in osteoblast-lineage cells of adult female mice could mimic the deleterious effects of aging. To this end, we loaded mice lacking the antioxidant enzyme superoxide dismutase 2 (Sod2) or autophagy-related 7 (Atg7) in cells targeted by Osterix1 (Osx1)-Cre. Osteocytes in Atg7ΔOsx1 exhibited altered morphology and decreased osteocyte dendrite projections. Two weeks of loading increased cortical bone mass and bone formation rate at both periosteal and endosteal surfaces of Osx1-Cre control mice. Nonetheless, in both Atg7ΔOsx1 and Sod2ΔOsx1 mice the response to loading was identical to that observed in control mice, indicating that compromised Atg7-dependent autophagy or excessive mtROS are not sufficient to impair the bone response to tibial compressive loading. Thus, alternative mechanisms of aging might be responsible for the decreased response of the aged skeleton to mechanical stimuli. These findings also suggest that an intact osteocyte dendrite network is not required for the osteogenic response in this model of bone loading.

The Forgotten Flap: The Pedicled Trapezius Flap's Utility in Pediatric Head and Neck Reconstruction-A Systematic Review

BACKGROUND

 When free tissue transfer is precluded or undesired, the pedicled trapezius flap is a viable alternative for adults requiring complex head and neck (H&N) defect reconstruction. However, the application of this flap in pediatric reconstruction is underexplored. This systematic review aimed to describe the use of the pedicled trapezius flap and investigate its efficacy in pediatric H&N reconstruction.

METHODS

 A systematic review was performed using Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Articles describing the trapezius flap for H&N reconstruction in pediatric patients were included. Patient demographics, surgical indications, wound characteristics, flap characteristics, complications, and functional outcomes were abstracted.

RESULTS

 A systematic review identified 22 articles for inclusion. Studies mainly consisted of case reports (n = 11) and case series (n = 8). In total, 67 pedicled trapezius flaps were successfully performed for H&N reconstruction in 63 patients. The most common surgical indications included burn scar contractures (n = 46, 73.0%) and chronic wounds secondary to H&N masses (n = 9, 14.3%). Defects were most commonly located in the neck (n = 28, 41.8%). The mean flap area and arc of rotation were 326.4 ± 241.7 cm2 and 157.6 ± 33.2 degrees, respectively. Most flaps were myocutaneous (n = 48, 71.6%) and based on the dorsal scapular artery (n = 32, 47.8%). Complications occurred in 10 (14.9%) flaps. The flap's survival rate was 100% (n = 67). No instances of functional donor site morbidity were reported. The mean follow-up was 2.2 ± 1.8 years.

CONCLUSION

 This systematic review demonstrated the reliability of the pedicled trapezius flap in pediatric H&N reconstruction, with a low complication rate, no reports of functional donor site morbidity, and a 100% flap survival rate. The flap's substantial surface area, bulk, and arc of rotation contribute to its efficacy in covering soft tissue defects ranging from the proximal neck to the vertex of the scalp. The pedicled trapezius flap is a viable option for pediatric H&N reconstruction.

Energy metabolism in osteoprogenitors and osteoblasts: Role of the pentose phosphate pathway

Bioenergetic preferences of osteolineage cells, including osteoprogenitors and osteoblasts (OBs), are a matter of intense debate. Early studies pointed to OB reliance on glucose and aerobic glycolysis while more recent works indicated the importance of glutamine as a mitochondrial fuel. Aiming to clarify this issue, we performed metabolic tracing of 13C-labeled glucose and glutamine in human osteolineage cells: bone marrow stromal (a.k.a. mesenchymal stem) cells and bone marrow stromal cell-derived OBs. Glucose tracing showed noncanonical direction of glucose metabolism with high labeling of early glycolytic steps and the pentose phosphate pathway (PPP) but very low labeling of late glycolytic steps and the Krebs cycle. Labeling of Krebs cycle and late steps of glycolysis was primarily from glutamine. These data suggest that in osteolineage cells, glucose is metabolized primarily via the PPP while glutamine is metabolized in the mitochondria, also feeding into the late steps of glycolysis likely via the malate-aspartate shuttle. This metabolic setup did not change after induction of differentiation. To evaluate the importance of this setup for osteolineage cells, we used the inhibitors of either PPP or malate-aspartate shuttle and observed a significant reduction in both cell growth and ability to differentiate. In sum, we observed a distinct metabolic wiring in osteolineage cells with high flux of glucose through the PPP and glutamine flux fueling both mitochondria and late steps of glycolysis. This wiring likely reflects their unique capacity to rapidly proliferate and produce extracellular matrix, e.g., after bone fracture.

A proteomics approach to study mouse long bones: examining baseline differences and mechanical loading-induced bone formation in young-adult and old mice

With aging, bone mass declines and the anabolic effects of skeletal loading diminish. While much research has focused on gene transcription, how bone ages and loses its mechanoresponsiveness at the protein level remains unclear. We developed a novel proteomics approach and performed a paired mass spectrometry and RNA-seq analysis on tibias from young-adult (5-month) and old (22-month) mice. We report the first correlation estimate between the bone proteome and transcriptome (Spearman ρ = 0.40), which is in line with other tissues but indicates that a relatively low amount of variation in protein levels is explained by the variation in transcript levels. Of 71 shared targets that differed with age, eight were associated with bone mineral density in previous GWAS, including understudied targets Asrgl1 and Timp2. We used complementary RNA in situ hybridization to confirm that Asrgl1 and Timp2 had reduced expression in osteoblasts/osteocytes in old bones. We also found evidence for reduced TGF-beta signaling with aging, in particular Tgfb2. Next, we defined proteomic changes following mechanical loading. At the protein level, bone differed more with age than with loading, and aged bone had fewer loading-induced changes. Overall, our findings underscore the need for complementary protein-level assays in skeletal biology research.

The oral organ: A new vision of the mouth as a whole for a gerophysiological approach to healthy aging

This article brings a new perspective on oral physiology by presenting the oral organ as an integrated entity within the entire organism and its surrounding environment. Rather than considering the mouth solely as a collection of discrete functions, this novel approach emphasizes its role as a dynamic interphase, supporting interactions between the body and external factors. As a resilient ecosystem, the equilibrium of mouth ecological niches is the result of a large number of interconnected factors including the heterogeneity of different oral structures, diversity of resources, external and internal pressures and biological actors. The manuscript seeks to deepen the understanding of age-related changes within the oral cavity and throughout the organism, aligning with the evolving field of gerophysiology. The strategic position and fundamental function of the mouth make it an invaluable target for early prevention, diagnosis, treatment, and even reversal of aging effects throughout the entire organism. Recognizing the oral cavity capacity for sensory perception, element capture and information processing underscores its vital role in continuous health monitoring. Overall, this integrated understanding of the oral physiology aims at advancing comprehensive approaches to the oral healthcare and promoting broader awareness of its implications on the overall well-being.

Regulatory cellular and molecular networks in the bone microenvironment during aging

Age-induced abnormalities in bone metabolism disrupt the equilibrium between bone resorption and formation. This largely stems from disturbances in bone homeostasis, in which signaling pathways exert a significant regulatory influence. Aging compromises the functionality of the bone marrow mesenchymal stem cells (BMSCs), ultimately resulting in tissue dysfunction and pathological aging. Age-related bone degradation primarily manifests as reduced bone formation and the increased accumulation of bone marrow fat. Cellular senescence diminishes bone cell vitality, thereby disrupting the balance of bone remodeling. Intensive osteoclast differentiation leads to the generation of more osteoclasts and increased bone resorption. This review provides insight into the impact of aging on bone, encompassing bone cell states during the aging process and bone signaling pathway transformations. It primarily delves into aging-related signaling pathways, such as the bone morphogenetic protein/Smad, Wnt/β-catenin, osteoprotegerin/receptor activator of NF-κB ligand/receptor activator of NF-κB, connexin43/miR21, and nuclear factor erythroid 2-related factor 2/antioxidant response element pathways, seeking to enhance our comprehension of crucial bone cells and their secretory phenotypes during aging. Furthermore, the precise molecular regulatory mechanisms underlying the interactions between bone signaling pathways and aging are investigated.

The role of reactive oxygen species in bone cell physiology and pathophysiology

Hydrogen peroxide (H2O2), superoxide anion radical (O2-•), and other forms of reactive oxygen species (ROS) are produced by the vast majority of mammalian cells and can contribute both to cellular homeostasis and dysfunction. The NADPH oxidases (NOX) enzymes and the mitochondria electron transport chain (ETC) produce most of the cellular ROS. Multiple antioxidant systems prevent the accumulation of excessive amounts of ROS which cause damage to all cellular macromolecules. Many studies have examined the contribution of ROS to different bone cell types and to skeletal physiology and pathophysiology. Here, we discuss the role of H2O2 and O2-• and their major enzymatic sources in osteoclasts and osteoblasts, the fundamentally different ways via which these cell types utilize mitochondrial derived H2O2 for differentiation and function, and the molecular mechanisms that impact and are altered by ROS in these cells. Particular emphasis is placed on evidence obtained from mouse models describing the contribution of different sources of ROS or antioxidant enzymes to bone resorption and formation. Findings from studies using pharmacological or genetically modified mouse models indicate that an increase in H2O2 and perhaps other ROS contribute to the loss of bone mass with aging and estrogen deficiency, the two most important causes of osteoporosis and increased fracture risk in humans.

Involvement of Inheritance in Determining Telomere Length beyond Environmental and Lifestyle Factors

All linear chromosomal ends have specific DNA-protein complexes called telomeres. Telomeres serve as a "molecular clock" to estimate the potential length of cell replication. Shortening of telomere length (TL) is associated with cellular senescence, aging, and various age-related diseases in humans. Here we reviewed the structure, function, and regulation of telomeres and the age-related diseases associated with telomere attrition. Among the various determinants of TL, we highlight the connection between TL and heredity to provide a new overview of genetic determinants for TL. Studies across multiple species have shown that maternal and paternal TL influence the TL of their offspring, and this may affect life span and their susceptibility to age-related diseases. Hence, we reviewed the linkage between TL and parental influences and the proposed mechanisms involved. More in-depth studies on the genetic mechanism for TL attrition are needed due to the potential application of this knowledge in human medicine to prevent premature frailty at its earliest stage, as well as promote health and longevity.

Cmpk2 regulates mitochondrial function in glucocorticoid-induced osteoblast senescence and affects glucocorticoid-inhibited osteoblast differentiation

Mitochondrial dysfunction plays a crucial role in the development of glucocorticoid-induced osteoporosis (GIO). Cytidine monophosphate kinase 2 (Cmpk2), an essential mitochondria-associated gene, promotes the production of free mitochondrial DNA, which leads to the formation of inflammasome-mediated inflammatory factors. However, the specific role of Cmpk2 in GIO remains unclear. In this study, we report that glucocorticoids induce cellular senescence within the bone, particularly in bone marrow mesenchymal stem cells and preosteoblasts. We discovered that glucocorticoids cause mitochondrial dysfunction in preosteoblasts, increasing cellular senescence. Moreover, we observed elevated expression of Cmpk2 in preosteoblasts following glucocorticoid exposure. Inhibiting Cmpk2 expression alleviates glucocorticoid-induced cellular senescence and promotes osteogenic differentiation by improving mitochondrial function. Our study uncovers new mechanisms underlying glucocorticoid-induced senescence in stem cells and preosteoblasts, highlighting the potential of inhibiting the mitochondrial gene Cmpk2 to reduce senescence and enhance osteogenic differentiation. This finding offers a potential therapeutic approach for the treatment of GIO.

Targeting proteostasis network in osteoporosis: Pathological mechanisms and therapeutic implications

As the most common bone disease, osteoporosis (OP) increases bone fragility and makes patients more vulnerable to the threat of osteoporotic fractures. With the ageing population in today's society, OP has become a huge and growing public health problem. Unfortunately, the clear pathogenesis of OP is still under exploration, and effective interventions are still scarce. Therefore, exploring new targets for pharmacological interventions to develop promising therapeutic drugs for OP is of great clinical value. Previous studies have shown that normal bone remodeling depends on proteostasis, whereas loss of proteostasis during ageing leads to the dysfunctional proteostasis network (PN) that fails to maintain bone homeostasis. Nevertheless, only a few studies have revealed the pathophysiological relationship between bone metabolism and a single component of PN, yet the role of PN as a whole in the pathogenesis of OP is still under investigation. This review comprehensively summarized the role of PN in the pathogenesis of OP and further discussed the potential of PN as innovative drug targets for the therapy of OP.

Sex Specific Global Burden of Osteoporosis in 204 Countries and Territories, from 1990 to 2030: An Age-Period-Cohort Modeling Study

BACKGROUND

Osteoporosis is a highly prevalent disease with distinct sex pattern. We aimed to estimate the sex specific incidence, prevalence, and disability-adjusted life (DALYs) years of osteoporosis between 1990 and 2019, with additional predictions from 2020 to 2034.

METHODS

We collected osteoporosis disease burden data from the Global Burden of Disease study covering the years 1990 through 2019 in 204 countries and territories. The data included information on the number of incident cases of osteoporosis, DALYs, age-standardized incidence rates (ASIR), age-standardized prevalence rates (ASPR) and age-standardized DALYs rates. Additionally, we performed an age-period-cohort analysis to forecast the burden of osteoporosis.

RESULTS

The global number of incidence cases of osteoporosis, in 2019, reached 41.5 million cases. From 1990 to 2019, the low-middle socio-demographic index (SDI) region had the highest estimated annual percentage change in the world. Compared to males, female's ASIR and ASPR were all about 1.5 times higher than males for the same years in the same SDI regions. The projected global total number of incidence cases for osteoporosis between 2030 and 2034 is estimated to reach 263.2 million (154.4 million for females and 108.8 for males). Additionally, the burden in terms of DALYs is predicted to be 128.7 million (with 78.4 million for females and 50.3 million for males).

CONCLUSION

The global burden of osteoporosis is still increasing, mainly observed in high SDI countries. Females bear a burden 1.5 times higher than males in terms of incidence and DALYs. Steps should be taken to reduce the osteoporosis burden, especially in high SDI countries.

Major depression and the biological hallmarks of aging

Major depressive disorder (MDD) is characterized by psychological and physiological manifestations contributing to the disease severity and outcome. In recent years, several lines of evidence have suggested that individuals with MDD have an elevated risk of age-related adverse outcomes across the lifespan. This review provided evidence of a significant overlap between the biological abnormalities in MDD and biological changes commonly observed during the aging process (i.e., hallmarks of biological aging). Based on such evidence, we formulate a mechanistic model showing how abnormalities in the hallmarks of biological aging can be a common denominator and mediate the elevated risk of age-related health outcomes commonly observed in MDD. Finally, we proposed a roadmap for novel studies to investigate the intersection between the biology of aging and MDD, including the use of geroscience-guided interventions, such as senolytics, to delay or improve major depression by targeting biological aging.

Antioxidant Intervention to Improve Cognition in the Aging Brain: The Example of Hydroxytyrosol and Resveratrol

Both physiological and pathological aging processes induce brain alterations especially affecting the speed of processing, working memory, conceptual reasoning and executive functions. Many therapeutic approaches to reduce the impact of brain aging on cognitive functioning have been tested; unfortunately, there are no satisfactory results as a single therapy. As aging is partly contributed by free radical reactions, it has been proposed that exogenous antioxidants could have a positive impact on both aging and its associated manifestations. The aim of this report is to provide a summary and a subsequent review of the literature evidence on the role of antioxidants in preventing and improving cognition in the aging brain. Manipulation of endogenous cellular defense mechanisms through nutritional antioxidants or pharmacological compounds represents an innovative approach to therapeutic intervention in diseases causing brain tissue damage, such as neurodegeneration. Coherently with this notion, antioxidants, especially those derived from the Mediterranean diet such as hydroxytyrosol and resveratrol, seem to be able to delay and modulate the cognitive brain aging processes and decrease the occurrence of its effects on the brain. The potential preventive activity of antioxidants should be evaluated in long-term exposure clinical trials, using preparations with high bioavailability, able to bypass the blood-brain barrier limitation, and that are well standardized.

Respiratory Muscle Strength in Patients with Coronary Heart Disease and Different Musculoskeletal Disorders

Aim. To measure respiratory muscle strength (RMS) in patients with coronary heart disease (CHD) and different musculoskeletal disorders (MSD).

Material and methods. Patients were divided in four groups according to the MSD. Group I included 52 (13.4%) patients with sarcopenia, group II included 28 (7.2%) patients with osteopenia, group III included 25 (6.5%) patients with osteosarcopenia, group IV included 282 (72.9%) patients without MSD. All patients underwent the assessment of maximal expiratory (МЕР) and maximal inspiratory mouth pressures (MIP).

Results. The mean RMS values were lower than the normative values, and the strength of the expiratory muscles was 1.25 times lower compared to the inspiratory muscles. Both of these parameters were within the normal range in 191 (49.3%) patients, and lower values were noted in 196 (50.7%). An isolated decrease in MIP was observed in 24.8% of patients, an isolated decrease in МЕР in 6.5%, a combined decrease in MIP and МЕР in 19.4% of patients. Comparative analysis of МЕР and MIP (depending on the MSD) did not demonstrate statistically significant differences. Lower МЕР (76.9%) and MIP (75%) values were noted mainly in the group of patients with sarcopenia. A similar pattern was notes in patients with osteosarcopenia and in patients without MSD. Normative values of RMS were observed in patients with osteopenia. Correlation analysis revealed a unidirectional relationship between RMS and the parameters of muscle function (hand grip strength, muscle area and musculoskeletal index) and a multidirectional relationship between МЕР and BMI (r -0.743, p=0.013), MIP and patient age (r -0.624, p=0.021).

Conclusion. Respiratory muscle weakness was diagnosed in half of the patients with coronary heart disease. There were no statistically significant differences in RMS between patients with MSD and isolated CHD, despite lower values in the group with MSD. Correlation analysis revealed an association between RMS and muscle function.

Hypoxia and Alpha-Synuclein: Inextricable Link Underlying the Pathologic Progression of Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease, with typical motor symptoms as the main clinical manifestations. At present, there are about 10 million patients with PD in the world, and its comorbidities and complications are numerous and incurable. Therefore, it is particularly important to explore the pathogenesis of PD and find possible therapeutic targets. Because the etiology of PD is complex, involving genes, environment, and aging, finding common factors is the key to identifying intervention targets. Hypoxia is ubiquitous in the natural environment and disease states, and it is considered to be closely related to the etiology of PD. Despite research showing that hypoxia increases the expression and aggregation of alpha-synuclein (α-syn), the most important pathogenic protein, there is still a lack of systematic studies on the role of hypoxia in α-syn pathology and PD pathogenesis. Considering that hypoxia is inextricably linked with various causes of PD, hypoxia may be a co-participant in many aspects of the PD pathologic process. In this review, we describe the risk factors for PD, and we discuss the possible role of hypoxia in inducing PD pathology by these risk factors. Furthermore, we attribute the pathological changes caused by PD etiology to oxygen uptake disorder and oxygen utilization disorder, thus emphasizing the possibility of hypoxia as a critical link in initiating or promoting α-syn pathology and PD pathogenesis. Our study provides novel insight for exploring the pathogenesis and therapeutic targets of PD.

Multimorbidity

Multimorbidity (two or more coexisting conditions in an individual) is a growing global challenge with substantial effects on individuals, carers and society. Multimorbidity occurs a decade earlier in socioeconomically deprived communities and is associated with premature death, poorer function and quality of life and increased health-care utilization. Mechanisms underlying the development of multimorbidity are complex, interrelated and multilevel, but are related to ageing and underlying biological mechanisms and broader determinants of health such as socioeconomic deprivation. Little is known about prevention of multimorbidity, but focusing on psychosocial and behavioural factors, particularly population level interventions and structural changes, is likely to be beneficial. Most clinical practice guidelines and health-care training and delivery focus on single diseases, leading to care that is sometimes inadequate and potentially harmful. Multimorbidity requires person-centred care, prioritizing what matters most to the individual and the individual's carers, ensuring care that is effectively coordinated and minimally disruptive, and aligns with the patient's values. Interventions are likely to be complex and multifaceted. Although an increasing number of studies have examined multimorbidity interventions, there is still limited evidence to support any approach. Greater investment in multimorbidity research and training along with reconfiguration of health care supporting the management of multimorbidity is urgently needed.

Cell energy metabolism and bone formation

Energy metabolism plays an important role in cell and tissue ability to effectively function, maintain homeostasis, and perform repair. Yet, the role of energy metabolism in skeletal tissues in general and in bone, in particular, remains understudied. We, here, review the aspects of cell energy metabolism relevant to bone tissue, such as: i) availability of substrates and oxygen; ii) metabolism regulatory mechanisms most active in bone tissue, e.g. HIF and BMP; iii) crosstalk of cell bioenergetics with other cell functions, e.g. proliferation and differentiation; iv) role of glycolysis and mitochondrial oxidative phosphorylation in osteogenic lineage; and v) most significant changes in bone energy metabolism observed in aging and other pathologies. In addition, we review available methods to study energy metabolism on a subcellular, cellular, tissue, and live animal levels.

Skeletal Aging

Aging represents the single greatest risk factor for chronic diseases, including osteoporosis, a skeletal fragility syndrome that increases fracture risk. Optimizing bone strength throughout life reduces fracture risk. Factors critical for bone strength include nutrition, physical activity, and vitamin D status, whereas unhealthy lifestyles, illnesses, and certain medications (eg, glucocorticoids) are detrimental. Hormonal status is another important determinant of skeletal health, with sex steroid concentrations, particularly estrogen, having major effects on bone remodeling. Aging exacerbates bone loss in both sexes and results in imbalanced bone resorption relative to formation; it is associated with increased marrow adiposity, osteoblast/osteocyte apoptosis, and accumulation of senescent cells. The mechanisms underlying skeletal aging are as diverse as the factors that determine the strength (and thus fragility) of bone. This review updates our current understanding of the epidemiology, pathophysiology, and treatment of osteoporosis and provides an overview of the underlying hallmark mechanisms that drive skeletal aging.

Transcriptional regulation of cyclophilin D by BMP/Smad signaling and its role in osteogenic differentiation

Cyclophilin D (CypD) promotes opening of the mitochondrial permeability transition pore (MPTP) which plays a key role in both cell physiology and pathology. It is, therefore, beneficial for cells to tightly regulate CypD and MPTP but little is known about such regulation. We have reported before that CypD is downregulated and MPTP deactivated during differentiation in various tissues. Herein, we identify BMP/Smad signaling, a major driver of differentiation, as a transcriptional regulator of the CypD gene, Ppif. Using osteogenic induction of mesenchymal lineage cells as a BMP/Smad activation-dependent differentiation model, we show that CypD is in fact transcriptionally repressed during this process. The importance of such CypD downregulation is evidenced by the negative effect of CypD 'rescue' via gain-of-function on osteogenesis both in vitro and in a mouse model. In sum, we characterized BMP/Smad signaling as a regulator of CypD expression and elucidated the role of CypD downregulation during cell differentiation.

Osteoblast lineage Sod2 deficiency leads to an osteoporosis-like phenotype in mice

Osteoporosis is a systemic metabolic skeletal disease characterized by low bone mass and strength associated with fragility fractures. Oxidative stress, which results from elevated intracellular reactive oxygen species (ROS) and arises in the aging organism, is considered one of the critical factors contributing to osteoporosis. Mitochondrial (mt)ROS, as the superoxide anion (O₂⁻) generated during mitochondrial respiration, are eliminated in the young organism by antioxidant defense mechanisms, including superoxide dismutase 2 (SOD2), the expression and activity of which are decreased in aging mesenchymal progenitor cells, accompanied by increased mtROS production. Using a mouse model of osteoblast lineage cells with Sod2 deficiency, we observed significant bone loss in trabecular and cortical bones accompanied by decreased osteoblast activity, increased adipocyte accumulation in the bone marrow and augmented osteoclast activity, suggestive of altered mesenchymal progenitor cell differentiation and osteoclastogenesis. Furthermore, osteoblast senescence was increased. To date, there are only a few studies suggesting a causal association between mtROS and cellular senescence in tissue in vivo. Targeting SOD2 to improve redox homeostasis could represent a potential therapeutic strategy for maintaining bone health during aging.

Summary: Osteoblast-lineage specific Sod2 deficiency in mice leads to increased mtROS, impaired osteoblast function, increased adipogenesis, increased osteoclast activity and increased osteoblast senescence, resulting in bone loss.

Serum markers of biological ageing provide long-term prediction of life expectancy-a longitudinal analysis in middle-aged and older German adults

BACKGROUND

lifestyle behaviours and chronic co-morbidities are leading risk factors for premature mortality and collectively predict wide variability in individual life expectancy (LE). We investigated whether a pre-selected panel of five serum markers of biological ageing could improve predicting the long-term mortality risk and LE in middle-aged and older women and men.

METHODS

we conducted a case-cohort study (n = 5,789 among which there were 2,571 deaths) within the European Prospective Investigation into Cancer-Heidelberg cohort, a population cohort of middle-aged and older individuals, followed over a median duration of 18 years. Gompertz models were used to compute multi-adjusted associations of growth differentiation factor-15, N-terminal pro-brain natriuretic peptide, glycated haemoglobin A1c, C-reactive protein and cystatin-C with mortality risk. Areas under estimated Gompertz survival curves were used to estimate the LE of individuals using a model with lifestyle-related risk factors only (smoking history, body mass index, waist circumference, alcohol, physical inactivity, diabetes and hypertension), or with lifestyle factors plus the ageing-related markers.

RESULTS

a model including only lifestyle-related factors predicted a LE difference of 16.8 [95% confidence interval: 15.9; 19.1] years in men and 9.87 [9.20; 13.1] years in women aged ≥60 years by comparing individuals in the highest versus the lowest quintiles of estimated mortality risk. Including the ageing-related biomarkers in the model increased these differences up to 22.7 [22.3; 26.9] years in men and 14.00 [12.9; 18.2] years in women.

CONCLUSIONS

serum markers of ageing are potentially strong predictors for long-term mortality risk in a general population sample of older and middle-aged individuals and may help to identify individuals at higher risk of premature death, who could benefit from interventions to prevent further ageing-related health declines.

Cellular senescence and the skeleton: pathophysiology and therapeutic implications

Cellular senescence is a fundamental aging mechanism that is currently the focus of considerable interest as a pathway that could be targeted to ameliorate aging across multiple tissues, including the skeleton. There is now substantial evidence that senescent cells accumulate in the bone microenvironment with aging and that targeting these cells prevents age-related bone loss, at least in mice. Cellular senescence also plays important roles in mediating the skeletal fragility associated with diabetes mellitus, radiation, and chemotherapy. As such, there are ongoing efforts to develop "senolytic" drugs that kill senescent cells by targeting key survival mechanisms in these cells without affecting normal cells. Because senescent cells accumulate across tissues with aging, senolytics offer the attractive possibility of treating multiple age-related comorbidities simultaneously.

Ionizing Radiation Activates Mitochondrial Function in Osteoclasts and Causes Bone Loss in Young Adult Male Mice

The damaging effects of ionizing radiation (IR) on bone mass are well-documented in mice and humans and are most likely due to increased osteoclast number and function. However, the mechanisms leading to inappropriate increases in osteoclastic bone resorption are only partially understood. Here, we show that exposure to multiple fractions of low-doses (10 fractions of 0.4 Gy total body irradiation [TBI]/week, i.e., fractionated exposure) and/or a single exposure to the same total dose of 4 Gy TBI causes a decrease in trabecular, but not cortical, bone mass in young adult male mice. This damaging effect was associated with highly activated bone resorption. Both osteoclast differentiation and maturation increased in cultures of bone marrow-derived macrophages from mice exposed to either fractionated or singular TBI. IR also increased the expression and enzymatic activity of mitochondrial deacetylase Sirtuin-3 (Sirt3)-an essential protein for osteoclast mitochondrial activity and bone resorption in the development of osteoporosis. Osteoclast progenitors lacking Sirt3 exposed to IR exhibited impaired resorptive activity. Taken together, targeting impairment of osteoclast mitochondrial activity could be a novel therapeutic strategy for IR-induced bone loss, and Sirt3 is likely a major mediator of this effect.

The Role of SIRT3 in the Osteoporosis

SIRT3 is an NAD⁺-dependent deacetylase in the mitochondria with an extensive ability to regulate mitochondrial morphology and function. It has been reported that SIRT3 participates in the occurrence and development of many aging-related diseases. Osteoporosis is a common aging-related disease characterized by decreased bone mass and fragility fractures, which has caused a huge burden on society. Current research shows that SIRT3 is involved in the physiological processes of senescence of bone marrow mesenchymal stem cells (BMSCs), differentiation of BMSCs and osteoclasts. However, the specific effects and mechanisms of SIRT3 in osteoporosis are not clear. In the current review, we elaborated on the physiological functions of SIRT3, the cell types involved in bone remodeling, and the role of SIRT3 in osteoporosis. Furthermore, it also provided a theoretical basis for SIRT3 as a therapeutic target for osteoporosis.

Ageing-related markers and risks of cancer and cardiovascular disease: a prospective study in the EPIC-Heidelberg cohort

Biological age is an important risk factor for chronic diseases. We examined the associations between five markers of unhealthy ageing; Growth Differentiation Factor-15 (GDF-15), N-terminal pro-brain natriuretic peptide (NT-proBNP), glycated hemoglobin A1c (HbA1C), C-Reactive Protein (CRP) and cystatin-C; with risks of cancer and cardiovascular disease (CVD). We used a case-cohort design embedded in the EPIC-Heidelberg cohort, including a subcohort of 3792 participants along with 4867 incident cases of cancer and CVD. Hazard ratios (HRs) were computed and the strongest associations were used to build weighted multi-marker combinations, and their associations with cancer and CVD risks were tested. After adjusting for common confounders, we observed direct associations of GDF-15 with lung cancer risk, NT-proBNP with breast, prostate and colorectal cancers, HbA1C with lung, colorectal, and breast cancer risks, and CRP with lung and colorectal cancer risks. An inverse association was observed for GDF-15 and prostate cancer risk. We also found direct associations of all 5 markers with myocardial infarction (MI) risk, and of GDF-15, NT-proBNP, CRP and cystatin-C with stroke risk. A combination of the independently-associated markers showed a moderately strong association with the risks of cancer and CVD (HR_Q4-Q1 ranged from 1.78[1.36, 2.34] for breast cancer, when combining NT-proBNP and HbA1C, to 2.87[2.15, 3.83] for MI when combining NT-proBNP, HbA1C, CRP and cystatin-C). This analysis suggests that combinations of biomarkers related to unhealthy ageing show strong associations with cancer risk, and corroborates published evidence on CVD risk. If confirmed in other studies, using these biomarkers could be useful for the identification of individuals at higher risk of age-related diseases.

Modulation of fracture healing by the transient accumulation of senescent cells

Senescent cells have detrimental effects across tissues with aging but may have beneficial effects on tissue repair, specifically on skin wound healing. However, the potential role of senescent cells in fracture healing has not been defined. Here, we performed an in silico analysis of public mRNAseq data and found that senescence and senescence-associated secretory phenotype (SASP) markers increased during fracture healing. We next directly established that the expression of senescence biomarkers increased markedly during murine fracture healing. We also identified cells in the fracture callus that displayed hallmarks of senescence, including distension of satellite heterochromatin and telomeric DNA damage; the specific identity of these cells, however, requires further characterization. Then, using a genetic mouse model (Cdkn2a^LUC) containing a Cdkn2a^Ink4a-driven luciferase reporter, we demonstrated transient in vivo senescent cell accumulation during callus formation. Finally, we intermittently treated young adult mice following fracture with drugs that selectively eliminate senescent cells (‘senolytics’, Dasatinib plus Quercetin), and showed that this regimen both decreased senescence and SASP markers in the fracture callus and significantly accelerated the time course of fracture healing. Our findings thus demonstrate that senescent cells accumulate transiently in the murine fracture callus and, in contrast to the skin, their clearance does not impair but rather improves fracture healing.

Impaired Mitophagy in Neurons and Glial Cells during Aging and Age-Related Disorders

Aging is associated with a decline in cognitive function, which can partly be explained by the accumulation of damage to the brain cells over time. Neurons and glia undergo morphological and ultrastructure changes during aging. Over the past several years, it has become evident that at the cellular level, various hallmarks of an aging brain are closely related to mitophagy. The importance of mitochondria quality and quantity control through mitophagy is highlighted by the contribution that defects in mitochondria-autophagy crosstalk make to aging and age-related diseases. In this review, we analyze some of the more recent findings regarding the study of brain aging and neurodegeneration in the context of mitophagy. We discuss the data on the dynamics of selective autophagy in neurons and glial cells during aging and in the course of neurodegeneration, focusing on three mechanisms of mitophagy: non-receptor-mediated mitophagy, receptor-mediated mitophagy, and transcellular mitophagy. We review the role of mitophagy in neuronal/glial homeostasis and in the molecular pathogenesis of neurodegenerative disorders, such as Parkinson's disease, Alzheimer's disease, and other disorders. Common mechanisms of aging and neurodegeneration that are related to different mitophagy pathways provide a number of promising targets for potential therapeutic agents.

Radiation-Induced Osteocyte Senescence Alters Bone Marrow Mesenchymal Stem Cell Differentiation Potential via Paracrine Signaling

Cellular senescence and its senescence-associated secretory phenotype (SASP) are widely regarded as promising therapeutic targets for aging-related diseases, such as osteoporosis. However, the expression pattern of cellular senescence and multiple SASP secretion remains unclear, thus leaving a large gap in the knowledge for a desirable intervention targeting cellular senescence. Therefore, there is a critical need to understand the molecular mechanism of SASP secretion in the bone microenvironment that can ameliorate aging-related degenerative pathologies including osteoporosis. In this study, osteocyte-like cells (MLO-Y4) were induced to cellular senescence by 2 Gy γ-rays; then, senescence phenotype changes and adverse effects of SASP on bone marrow mesenchymal stem cell (BMSC) differentiation potential were investigated. The results revealed that 2 Gy irradiation could hinder cell viability, shorten cell dendrites, and induce cellular senescence, as evidenced by the higher expression of senescence markers p16 and p21 and the elevated formation of senescence-associated heterochromatin foci (SAHF), which was accompanied by the enhanced secretion of SASP markers such as IL-1α, IL-6, MMP-3, IGFBP-6, resistin, and adiponectin. When 0.8 μM JAK1 inhibitors were added to block SASP secretion, the higher expression of SASP was blunted, but the inhibition in osteogenic and adipogenic differentiation potential of BMSCs co-cultured with irradiated MLO-Y4 cell conditioned medium (CM- 2 Gy) was alleviated. These results suggest that senescent osteocytes can perturb BMSCs’ differential potential via the paracrine signaling of SASP, which was also demonstrated by in vivo experiments. In conclusion, we identified the SASP factor partially responsible for the degenerative differentiation of BMSCs, which allowed us to hypothesize that senescent osteocytes and their SASPs may contribute to radiation-induced bone loss.

MicroRNA-29a Mitigates Osteoblast Senescence and Counteracts Bone Loss through Oxidation Resistance-1 Control of FoxO3 Methylation

Senescent osteoblast overburden accelerates bone mass loss. Little is understood about microRNA control of oxidative stress and osteoblast senescence in osteoporosis. We revealed an association between microRNA-29a (miR-29a) loss, oxidative stress marker 8-hydroxydeoxyguanosine (8-OHdG), DNA hypermethylation marker 5-methylcystosine (5mC), and osteoblast senescence in human osteoporosis. miR-29a knockout mice showed low bone mass, sparse trabecular microstructure, and osteoblast senescence. miR-29a deletion exacerbated bone loss in old mice. Old miR-29a transgenic mice showed fewer osteoporosis signs, less 5mC, and less 8-OHdG formation than age-matched wild-type mice. miR-29a overexpression reversed age-induced senescence and osteogenesis loss in bone-marrow stromal cells. miR-29a promoted transcriptomic landscapes of redox reaction and forkhead box O (FoxO) pathways, preserving oxidation resistance protein-1 (Oxr1) and FoxO3 in old mice. In vitro, miR-29a interrupted DNA methyltransferase 3b (Dnmt3b)-mediated FoxO3 promoter methylation and senescence-associated β-galactosidase activity in aged osteoblasts. Dnmt3b inhibitor 5'-azacytosine, antioxidant N-acetylcysteine, or Oxr1 recombinant protein attenuated loss in miR-29a and FoxO3 to mitigate oxidative stress, senescence, and mineralization matrix underproduction. Taken together, miR-29a promotes Oxr1, compromising oxidative stress and FoxO3 loss to delay osteoblast aging and bone loss. This study sheds light on a new antioxidation mechanism by which miR-29a protects against osteoblast aging and highlights the remedial effects of miR-29a on osteoporosis.

Fitness vs. fatness as determinants of survival in non-institutionalized older adults: The EXERNET multi-center study

BACKGROUND

Physical fitness and body composition are important health indicators, nevertheless their combined pattern inter-relationships and their association with mortality are poorly investigated.

METHODS

This longitudinal study is part of the Spanish EXERNET-Elder project. Person-months follow-up were calculated from the interview date, performed between June 2008 and November 2009, until date of death or censoring on March 2018 (whichever came first). In order to be included, participants had to fulfill the following criteria: 1) be over 65 years old, 2) live independently at home, 3) not suffer dementia and/or cancer and 4) have a BMI above 18.5. Body fat and weight were assessed by a bioelectrical impedance analyzer. Fitness was measured with the Senior Fitness and the one leg static balance tests. The Spanish Death Index was consulted for the death's identification. Cluster analysis was performed to identify Fat-Fit patterns and traditional cut points and percentiles to create the Fat-Fit groups. Cox proportional hazards regression models were used to calculate the hazard ratios of death in clustered Fat-Fit patterns and in traditional Fat-Fit groups.

RESULTS

A total of 2299 older adults (76.8% of women) were included with a baseline mean age of 71.9 ± 5.2 years. A total of 196 deaths (8.7% of the sample) were identified during the 8 years of follow up. Four clustered Fat-Fit patterns (Low fat-Fit, Medium fat-Fit, High fat-Unfit and Low fat-Unfit) and nine traditional Fat-Fit groups emerged. Using the Low fat-Fit pattern as the reference, significantly increased mortality was noted in High fat-Unfit (HR: 1.68, CI: 1.06 - 2.66) and Low fat-Unfit (HR: 2.01, CI: 1.28 - 3.16) groups. All the traditional Fit groups showed lower mortality risk when compared to the reference group (obese-unfit group).

CONCLUSIONS

Physical fitness is a determinant factor in terms of survival in community-dwelling older adults, independently of adiposity levels.

Mitochondrial Sirt3 contributes to the bone loss caused by aging or estrogen deficiency

Altered mitochondria activity in osteoblasts and osteoclasts has been implicated in the loss of bone mass associated with aging and estrogen deficiency — the 2 most common causes of osteoporosis. However, the mechanisms that control mitochondrial metabolism in bone cells during health or disease remain unknown. The mitochondrial deacetylase sirtuin-3 (Sirt3) has been earlier implicated in age-related diseases. Here, we show that deletion of Sirt3 had no effect on the skeleton of young mice but attenuated the age-related loss of bone mass in both sexes. This effect was associated with impaired bone resorption. Osteoclast progenitors from aged Sirt3-null mice were able to differentiate into osteoclasts, though the differentiated cells exhibited impaired polykaryon formation and resorptive activity, as well as decreased oxidative phosphorylation and mitophagy. The Sirt3 inhibitor LC-0296 recapitulated the effects of Sirt3 deletion in osteoclast formation and mitochondrial function, and its administration to aging mice increased bone mass. Deletion of Sirt3 also attenuated the increase in bone resorption and loss of bone mass caused by estrogen deficiency. These findings suggest that Sirt3 inhibition and the resulting impairment of osteoclast mitochondrial function could be a novel therapeutic intervention for the 2 most important causes of osteoporosis.

Modeling Neuroregeneration and Neurorepair in an Aging Context: The Power of a Teleost Model

Aging increases the risk for neurodegenerative disease and brain trauma, both leading to irreversible and multifaceted deficits that impose a clear societal and economic burden onto the growing world population. Despite tremendous research efforts, there are still no treatments available that can fully restore brain function, which would imply neuroregeneration. In the adult mammalian brain, neuroregeneration is naturally limited, even more so in an aging context. In view of the significant influence of aging on (late-onset) neurological disease, it is a critical factor in future research. This review discusses the use of a non-standard gerontology model, the teleost brain, for studying the impact of aging on neurorepair. Teleost fish share a vertebrate physiology with mammals, including mammalian-like aging, but in contrast to mammals have a high capacity for regeneration. Moreover, access to large mutagenesis screens empowers these teleost species to fill the gap between established invertebrate and rodent models. As such, we here highlight opportunities to decode the factor age in relation to neurorepair, and we propose the use of teleost fish, and in particular killifish, to fuel new research in the neuro-gerontology field.

Age Differences in Motor Recruitment Patterns of the Shoulder in Dynamic and Isometric Contractions. A Cross-Sectional Study

Aging processes in the musculoskeletal system lead to functional impairments that restrict participation. Purpose: To assess differences in the force and motor recruitment patterns of shoulder muscles between age groups to understand functional disorders. A cross-sectional study comparing 30 adults (20–64) and 30 older adults (>65). Surface electromyography (sEMG) of the middle deltoid, upper and lower trapezius, infraspinatus, and serratus anterior muscles was recorded. Maximum isometric voluntary contraction (MIVC) was determined at 45° glenohumeral abduction. For the sEMG signal registration, concentric and eccentric contraction with and without 1 kg and isometric contraction were requested. Participants abducted the arm from 0° up to an abduction angle of 135° for concentric and eccentric contraction, and from 0° to 45°, and remained there at 80% of the MIVC level while isometrically pushing against a handheld dynamometer. Differences in sEMG amplitudes (root mean square, RMS) of all contractions, but also onset latencies during concentric contraction of each muscle between age groups, were analyzed. Statistical differences in strength (Adults > Older adults; 0.05) existed between groups. No significant differences in RMS values of dynamic contractions were detected, except for the serratus anterior, but there were for isometric contractions of all muscles analyzed (Adults > Older adults; 0.05). The recruitment order varied between age groups, showing a general tendency towards delayed onset times in older adults, except for the upper trapezius muscle. Age differences in muscle recruitment patterns were found, which underscores the importance of developing musculoskeletal data to prevent and guide geriatric shoulder pathologies.

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.

Cellular senescence in age-related disorders

Much of the population is now faced with an enormous burden of age-associated chronic diseases. Recent discoveries in geroscience indicate that healthspan in model organisms such as mice can be manipulated by targeting cellular senescence, a hallmark mechanism of aging, defined as an irreversible proliferative arrest that occurs when cells experience oncogenic or other diverse forms of damage. Senescent cells and their proinflammatory secretome have emerged as contributors to age-related tissue dysfunction and morbidity. Cellular senescence has causal roles in mediating osteoporosis, frailty, cardiovascular diseases, osteoarthritis, pulmonary fibrosis, renal diseases, neurodegenerative diseases, hepatic steatosis, and metabolic dysfunction. Therapeutically targeting senescent cells in mice can prevent, delay, or alleviate each of these conditions. Therefore, senotherapeutic approaches, including senolytics and senomorphics, that either selectively eliminate senescent cells or interfere with their ability to promote tissue dysfunction, are gaining momentum as potential realistic strategies to abrogate human senescence to thereby compress morbidity and extend healthspan.

Osteocyte Cellular Senescence

PURPOSE OF REVIEW

Senescent cells are now known to accumulate in multiple tissues with aging and through their inflammation (the senescence-associated secretory phenotype, SASP) contribute to aging and chronic diseases. Here, we review the roles of senescent osteocytes in the context of bone loss.

RECENT FINDINGS

Numerous studies have established that senescent osteocytes accumulate in the bone microenvironment with aging in mice and in humans. Moreover, at least in mice, elimination of senescent cells results in attenuation of age-related bone loss. Osteocyte senescence also occurs in response to other cellular stressors, including radiotherapy, chemotherapy, and metabolic dysfunction, where it appears to mediate skeletal deterioration. Osteocyte senescence is linked to bone loss associated with aging and other conditions. Senescent osteocytes are potential therapeutic targets to alleviate skeletal dysfunction. Additional studies better defining the underlying mechanisms as well as translating these exciting findings from mouse models to humans are needed.

Epigenetic Regulation of Skeletal Tissue Integrity and Osteoporosis Development

Bone turnover is sophisticatedly balanced by a dynamic coupling of bone formation and resorption at various rates. The orchestration of this continuous remodeling of the skeleton further affects other skeletal tissues through organ crosstalk. Chronic excessive bone resorption compromises bone mass and its porous microstructure as well as proper biomechanics. This accelerates the development of osteoporotic disorders, a leading cause of skeletal degeneration-associated disability and premature death. Bone-forming cells play important roles in maintaining bone deposit and osteoclastic resorption. A poor organelle machinery, such as mitochondrial dysfunction, endoplasmic reticulum stress, and defective autophagy, etc., dysregulates growth factor secretion, mineralization matrix production, or osteoclast-regulatory capacity in osteoblastic cells. A plethora of epigenetic pathways regulate bone formation, skeletal integrity, and the development of osteoporosis. MicroRNAs inhibit protein translation by binding the 3'-untranslated region of mRNAs or promote translation through post-transcriptional pathways. DNA methylation and post-translational modification of histones alter the chromatin structure, hindering histone enrichment in promoter regions. MicroRNA-processing enzymes and DNA as well as histone modification enzymes catalyze these modifying reactions. Gain and loss of these epigenetic modifiers in bone-forming cells affect their epigenetic landscapes, influencing bone homeostasis, microarchitectural integrity, and osteoporotic changes. This article conveys productive insights into biological roles of DNA methylation, microRNA, and histone modification and highlights their interactions during skeletal development and bone loss under physiological and pathological conditions.

Accelerated osteocyte senescence and skeletal fragility in mice with type 2 diabetes

The worldwide prevalence of type 2 diabetes (T2D) is increasing. Despite normal to higher bone density, patients with T2D paradoxically have elevated fracture risk resulting, in part, from poor bone quality. Advanced glycation endproducts (AGEs) and inflammation as a consequence of enhanced receptor for AGE (RAGE) signaling are hypothesized culprits, although the exact mechanisms underlying skeletal dysfunction in T2D are unclear. Lack of inducible models that permit environmental (in obesity) and temporal (after skeletal maturity) control of T2D onset has hampered progress. Here, we show in C57BL/6 mice that a onetime pharmacological intervention (streptozotocin, STZ) initiated in adulthood combined with high-fat diet-induced (HFD-induced) obesity caused hallmark features of human adult-onset T2D, including prolonged hyperglycemia, insulin resistance, and pancreatic β cell dysfunction, but not complete destruction. In addition, HFD/STZ (i.e., T2D) resulted in several changes in bone quality that closely mirror those observed in humans, including compromised bone microarchitecture, reduced biomechanical strength, impaired bone material properties, altered bone turnover, and elevated levels of the AGE CML in bone and blood. Furthermore, T2D led to the premature accumulation of senescent osteocytes with a unique proinflammatory signature. These findings highlight the RAGE pathway and senescent cells as potential targets to treat diabetic skeletal fragility.

The role of cellular senescence in ageing and endocrine disease

With the ageing of the global population, interest is growing in the 'geroscience hypothesis', which posits that manipulation of fundamental ageing mechanisms will delay (in parallel) the appearance or severity of multiple chronic, non-communicable diseases, as these diseases share the same underlying risk factor - namely, ageing. In this context, cellular senescence has received considerable attention as a potential target in preventing or treating multiple age-related diseases and increasing healthspan. Here we review mechanisms of cellular senescence and approaches to target this pathway therapeutically using 'senolytic' drugs that kill senescent cells or inhibitors of the senescence-associated secretory phenotype (SASP). Furthermore, we highlight the evidence that cellular senescence has a causative role in multiple diseases associated with ageing. Finally, we focus on the role of cellular senescence in a number of endocrine diseases, including osteoporosis, metabolic syndrome and type 2 diabetes mellitus, as well as other endocrine conditions. Although much remains to be done, considerable preclinical evidence is now leading to the initiation of proof-of-concept clinical trials using senolytics for several endocrine and non-endocrine diseases.

Independent Roles of Estrogen Deficiency and Cellular Senescence in the Pathogenesis of Osteoporosis: Evidence in Young Adult Mice and Older Humans

Estrogen deficiency is a seminal mechanism in the pathogenesis of osteoporosis. Mounting evidence, however, establishes that cellular senescence, a fundamental mechanism that drives multiple age-related diseases, also causes osteoporosis. Recently, we systematically identified an accumulation of senescent cells, characterized by increased p16^Ink4a and p21^Cip1 levels and development of a senescence-associated secretory phenotype (SASP), in mouse bone/marrow and human bone with aging. We then demonstrated that elimination of senescent cells prevented age-related bone loss using multiple approaches, eg, treating old mice expressing a "suicide" transgene, INK-ATTAC, with AP20187 to induce apoptosis of p16^Ink4a -senescent cells or periodically treating old wild-type mice with "senolytics," ie, drugs that eliminate senescent cells. Here, we investigate a possible role for estrogen in the regulation of cellular senescence using multiple approaches. First, sex steroid deficiency 2 months after ovariectomy (OVX, n = 15) or orchidectomy (ORCH, n = 15) versus sham surgery (SHAM, n = 15/sex) in young adult (4-month-old) wild-type mice did not alter senescence biomarkers or induce a SASP in bone. Next, in elderly postmenopausal women, 3 weeks of estrogen therapy (n = 10; 74 ± 5 years) compared with no treatment (n = 10; 78 ± 5 years) did not alter senescence biomarkers or the SASP in human bone biopsies. Finally, young adult (4-month-old) female INK-ATTAC mice were randomized (n = 17/group) to SHAM+Vehicle, OVX+Vehicle, or OVX+AP20187 for 2 months. As anticipated, OVX+Vehicle caused significant trabecular/cortical bone loss compared with SHAM+Vehicle. However, treatment with AP20187, which eliminates senescent cells in INK-ATTAC mice, did not rescue the OVX-induced bone loss or alter senescence biomarkers. Collectively, our data establish independent roles of estrogen deficiency and cellular senescence in the pathogenesis of osteoporosis, which has important implications for testing novel senolytics for skeletal efficacy, as these drugs will need to be evaluated in preclinical models of aging as opposed to the current FDA model of prevention of OVX-induced bone loss. © 2019 American Society for Bone and Mineral Research.

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.

Sirtuins and FoxOs in osteoporosis and osteoarthritis

The sirtuin family of NAD⁺-dependent protein deacetylases promotes longevity and counteracts age-related diseases. One of the major targets of Sirtuins are the FoxO family of transcription factors. FoxOs play a major role in the adaptation of cells to a variety of stressors such as oxidative stress and growth factor deprivation. Studies with murine models of cell-specific loss- or gain-of-function of Sirtuins or FoxOs and with Sirtuin1 stimulators have provided novel insights into the function and signaling of these proteins on the skeleton. These studies have revealed that both Sirtuins and FoxOs acting directly in cartilage and bone cells are critical for normal skeletal development, homeostasis and that their dysregulation might contribute to skeletal disease. Deacetylation of FoxOs by Sirt1 in osteoblasts and osteoclasts stimulates bone formation and inhibits bone resorption, making Sirt1 ligands promising therapeutic agents for diseases of low bone mass. While a similar link has not been established in chondrocytes, Sirt1 and FoxOs both have chondroprotective actions, suggesting that Sirt1 activators may have similar efficacy in preventing cartilage degeneration due to aging or injury. In this review we summarize these advances and discuss their implications for the pathogenesis of age-related osteoporosis and osteoarthritis.

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.

Potential role of senescence in radiation-induced damage of the aged skeleton

Human aging-related changes are exacerbated in cases of disease and cancer, and conversely aging is a catalyst for the occurrence of disease and multimorbidity. For example, old age is the most significant risk factor for cancer and among people who suffer from cancer, >60% are above the age of 65. Oxidative stress and DNA damage, leading to genomic instability and telomere dysfunction, are prevalent in aging and radiation-induced damage and are major cellular events that lead to senescence. Human exposures from nuclear fallout, cosmic radiation and clinical radiotherapy (RT) are some common sources of irradiation that affect bone tissue. RT has been used to treat malignant tumors for over a century, but the effects of radiation damage on tumor-adjacent normal tissue has largely been overlooked. There is an increase in the percent survivorship among patients post-RT, and it is in older survivors where the deleterious synergy between aging and radiation exposure conspires to promote tissue deterioration and dysfunction which then negatively impacts their quality of life. Thus, an aging skeleton is already pre-disposed to architectural deterioration, which is further worsened by radiation-induced bone damage. Effects of senescence and the senescence associated secretory phenotype (SASP) have been implicated in age-associated bone loss, but their roles in radiation-associated bone damage are still elusive. RT is used in treatment for a variety of cancers and in different anatomical locations, the sequelae of which include long-term morbidity and lifelong discomfort. Therefore, consideration of the growing evidence that implicates the role of senescence in radiation-induced bone damage argues in favor of exploiting current senotherapeutic approaches as a possible prevention or treatment.