Targeted Reduction of Senescent Cell Burden Alleviates Focal Radiotherapy-Related Bone Loss

Senescence in the bone marrow microenvironment: A driver in development of therapy-related myeloid neoplasms

Therapy-related myeloid neoplasms (t-MN) are a growing concern due to the continued use of cytotoxic therapies to treat malignancies. Cytotoxic therapies have been shown to drive therapy-induced senescence in normal tissues, including in the bone marrow microenvironment (BMME), which plays a crucial role in supporting normal hematopoiesis. This review examines recent work that focuses on the contribution of BMME senescence to t-MN pathogenesis, as well as offers a perspective on potential opportunities for therapeutic intervention.

Activatable Fluorescent Probe for Studying Drug-Induced Senescence In Vitro and In Vivo

Aging represents a significant risk factor for compromised tissue function and the development of chronic diseases in the human body. This process is intricately linked to oxidative stress, with HClO serving as a vital reactive oxygen species (ROS) within biological systems due to its strong oxidative properties. Hence, conducting a thorough examination of HClO in the context of aging is crucial for advancing the field of aging biology. In this work, we successfully developed a fluorescent probe, OPD, tailored specifically for detecting HClO in senescent cells and in vivo. Impressively, OPD exhibited a robust reaction with HClO, showcasing outstanding selectivity, sensitivity, and photostability. Notably, OPD effectively identified HClO in senescent cells for the first time, confirming that DOX- and ROS-induced senescent cells exhibited higher HClO levels compared to uninduced normal cells. Additionally, in vivo imaging of zebrafish demonstrated that d-galactose- and ROS-stimulated senescent zebrafish displayed elevated HClO levels compared to normal zebrafish. Furthermore, when applied to mouse tissues and organs, OPD revealed increased fluorescence in the organs of senescent mice compared to their nonsenescent counterparts. Our findings also illustrated the probe's potential for detecting changes in HClO content pre- and post-aging in living mice. Overall, this probe holds immense promise as a valuable tool for in vivo detection of HClO and for studying aging biology in live organisms.

PCLAF induces bone marrow adipocyte senescence and contributes to skeletal aging

Bone marrow adipocytes (BMAds) affect bone homeostasis, but the mechanism remains unclear. Here, we showed that exercise inhibited PCNA clamp-associated factor (PCLAF) secretion from the bone marrow macrophages to inhibit BMAds senescence and thus alleviated skeletal aging. The genetic deletion of PCLAF in macrophages inhibited BMAds senescence and delayed skeletal aging. In contrast, the transplantation of PCLAF-mediated senescent BMAds into the bone marrow of healthy mice suppressed bone turnover. Mechanistically, PCLAF bound to the ADGRL2 receptor to inhibit AKT/mTOR signaling that triggered BMAds senescence and subsequently spread senescence among osteogenic and osteoclastic cells. Of note, we developed a PCLAF-neutralizing antibody and showed its therapeutic effects on skeletal health in old mice. Together, these findings identify PCLAF as an inducer of BMAds senescence and provide a promising way to treat age-related osteoporosis.

Advances in Molecular Research on Hip Joint Impingement-A Vascular Perspective

With the rise in longevity within the population, medicine continues to encounter fresh hurdles necessitating prompt actions, among which are those associated with hip joint aging. Age-related arthropathies encompass damage to bones' articulating extremities and their supporting structures, such as articular cartilage, and alterations in the quantity and quality of synovial fluid. This study aims to summarize the biomolecular methods of hip joint evaluation focused on its vascularization, using data correlated with biomolecular research on other joints and tissues, in order to reach an objective opinion of the study prospects in this field. Following a retrospective study on most modern biomolecular research methods on the synovium, the capsule, and the articular cartilage of the hip joint, we have hereby concretized certain future research directions in this field that will improve the qualitative and morphofunctional management of the hip joint at an advanced age, even within population categories at risk of developing various degenerative joint pathologies.

SenNet recommendations for detecting senescent cells in different tissues

Once considered a tissue culture-specific phenomenon, cellular senescence has now been linked to various biological processes with both beneficial and detrimental roles in humans, rodents and other species. Much of our understanding of senescent cell biology still originates from tissue culture studies, where each cell in the culture is driven to an irreversible cell cycle arrest. By contrast, in tissues, these cells are relatively rare and difficult to characterize, and it is now established that fully differentiated, postmitotic cells can also acquire a senescence phenotype. The SenNet Biomarkers Working Group was formed to provide recommendations for the use of cellular senescence markers to identify and characterize senescent cells in tissues. Here, we provide recommendations for detecting senescent cells in different tissues based on a comprehensive analysis of existing literature reporting senescence markers in 14 tissues in mice and humans. We discuss some of the recent advances in detecting and characterizing cellular senescence, including molecular senescence signatures and morphological features, and the use of circulating markers. We aim for this work to be a valuable resource for both seasoned investigators in senescence-related studies and newcomers to the field.

Marrow Adipocyte Senescence in the Pathogenesis of Bone Loss

PURPOSE OF REVIEW

Beyond aging, senescent cells accumulate during multiple pathological conditions, including chemotherapy, radiation, glucocorticoids, obesity, and diabetes, even earlier in life. Therefore, cellular senescence represents a unifying pathogenic mechanism driving skeletal and metabolic disorders. However, whether senescent bone marrow adipocytes (BMAds) are causal in mediating skeletal dysfunction has only recently been evaluated.

RECENT FINDINGS

Despite evidence of BMAd senescence following glucocorticoid therapy, additional evidence for BMAd senescence in other conditions has thus far been limited. Because the study of BMAds presents unique challenges making these cells difficult to isolate and image, here we review issues and approaches to overcome such challenges, and present advancements in isolation and histological techniques that may help with the future study of senescent BMAds. Further insights into the roles of BMAd senescence in the pathogenesis of skeletal dysfunction may have important basic science and clinical implications for human physiology and disease.

Interplay Between Skeletal and Hematopoietic Cells in the Bone Marrow Microenvironment in Homeostasis and Aging

PURPOSE OF THE REVIEW

In this review, we discuss the most recent scientific advances on the reciprocal regulatory interactions between the skeletal and hematopoietic stem cell niche, focusing on immunomodulation and its interplay with the cell's mitochondrial function, and how this impacts osteoimmune health during aging and disease.

RECENT FINDINGS

Osteoimmunology investigates interactions between cells that make up the skeletal stem cell niche and immune system. Much work has investigated the complexity of the bone marrow microenvironment with respect to the skeletal and hematopoietic stem cells that regulate skeletal formation and immune health respectively. It has now become clear that these cellular components cooperate to maintain homeostasis and that dysfunction in their interaction can lead to aging and disease. Having a deeper, mechanistic appreciation for osteoimmune regulation will lead to better research perspective and therapeutics with the potential to improve the aging process, skeletal and hematologic regeneration, and disease targeting.

Insights into targeting cellular senescence with senolytic therapy: The journey from preclinical trials to clinical practice

Interconnected, fundamental aging processes are central to many illnesses and diseases. Cellular senescence is a mechanism that halts the cell cycle in response to harmful stimuli. Senescent cells (SnCs) can emerge at any point in life, and their persistence, along with the numerous proteins they secrete, can negatively affect tissue function. Interventions aimed at combating persistent SnCs, which can destroy tissues, have been used in preclinical models to delay, halt, or even reverse various diseases. Consequently, the development of small-molecule senolytic medicines designed to specifically eliminate SnCs has opened potential avenues for the prevention or treatment of multiple diseases and age-related issues in humans. In this review, we explore the most promising approaches for translating small-molecule senolytics and other interventions targeting senescence in clinical practice. This discussion highlights the rationale for considering SnCs as therapeutic targets for diseases affecting individuals of all ages.

Telomeres, cellular senescence, and aging: past and future

Over half a century has passed since Alexey Olovnikov's groundbreaking proposal of the end-replication problem in 1971, laying the foundation for our understanding of telomeres and their pivotal role in cellular senescence. This review paper delves into the intricate and multifaceted relationship between cellular senescence, the influence of telomeres in this process, and the far-reaching consequences of telomeres in the context of aging and age-related diseases. Additionally, the paper investigates the various factors that can influence telomere shortening beyond the confines of the end-replication problem and how telomeres can exert their impact on aging, even in the absence of significant shortening. Ultimately, this paper stands as a tribute to the pioneering work of Olovnikov, whose seminal contributions established the solid foundation upon which our ongoing explorations of telomeres and the aging process are based.

Single-cell Transcriptome Analysis Identifies Senescent Osteocytes as Contributors to Bone Destruction in Breast Cancer Metastasis

Breast cancer bone metastases increase fracture risk and are a major cause of morbidity and mortality among women. Upon colonization by tumor cells, the bone microenvironment undergoes profound reprogramming to support cancer progression that disrupts the balance between osteoclasts and osteoblasts, leading to bone lesions. Whether such reprogramming affects matrix-embedded osteocytes remains poorly understood. Here, we demonstrate that osteocytes in breast cancer bone metastasis develop premature senescence and a distinctive senescence-associated secretory phenotype (SASP) that favors bone destruction. Single-cell RNA sequencing identified osteocytes from mice with breast cancer bone metastasis enriched in senescence and SASP markers and pro-osteoclastogenic genes. Using multiplex in situ hybridization and AI-assisted analysis, we detected osteocytes with senescence-associated distension of satellites, telomere dysfunction, and p16Ink4a expression in mice and patients with breast cancer bone metastasis. In vitro and ex vivo organ cultures showed that breast cancer cells promote osteocyte senescence and enhance their osteoclastogenic potential. Clearance of senescent cells with senolytics suppressed bone resorption and preserved bone mass in mice with breast cancer bone metastasis. These results demonstrate that osteocytes undergo pathological reprogramming by breast cancer cells and identify osteocyte senescence as an initiating event triggering bone destruction in breast cancer metastases.

Accelerated Aging in Cancer Survivors: Cellular Senescence, Frailty, and Possible Opportunities for Interventions

The population of cancer survivors has markedly increased due to the rapid improvements in cancer treatment. However, cancer survivors experience accelerated aging, which leads to chronic diseases and other age-related conditions, such as frailty. Those conditions may persist years after cancer diagnosis and treatment. Cellular senescence, a hallmark of aging, is one of the mechanisms that contribute to accelerated aging in cancer survivors. Several aging measures, including measures based on clinical markers and biomarkers, have been proposed to estimate the aging process, and some of them have shown associations with mortality and frailty in cancer survivors. Several anti-aging interventions, including lifestyle changes and anti-aging drugs, have been proposed. Future research, particularly in large-scale studies, is needed to determine the efficiency of these aging measures and anti-aging interventions before considering their application in clinics. This review focuses on the mechanisms of cellular senescence and accelerated aging in cancer survivors, assessment of the aging process using clinical markers and biomarkers, and the high prevalence of frailty in that population, as well as possible opportunities for anti-aging interventions. A deeper understanding of aging measures and anti-aging interventions in cancer survivors will contribute to the development of effective strategies to mitigate accelerated aging in cancer survivors and improve their quality of life.

Recent advances in senescence-associated secretory phenotype and osteoporosis

The worldwide elderly population is on the rise, and aging is a major osteoporosis risk factor. Senescent cells accumulation can have a detrimental effect the body as we age. The senescence-associated secretory phenotype (SASP), an essential cellular senescence hallmark, is an important mechanism connecting cellular senescence to osteoporosis. This review describes in detail the characteristics of SASPs and their regulatory agencies, and shed fresh light on how SASPs from different senescent cells contribute to osteoporosis development. Furthermore, we summarized various innovative therapy techniques that target SASPs to lower the burden of osteoporosis in the elderly and discussed the potential challenges of SASPs-based therapy for osteoporosis as a new clinical trial.

Cellular senescence is associated with osteonecrosis of the femoral head while mesenchymal stem cell conditioned medium inhibits bone collapse

Osteonecrosis of the femoral head (ONFH) is a type of ischemic osteonecrosis that causes pain, loss of function, and femoral head collapse. Here, we analyzed samples of femoral heads excised from patients with ONFH to clarify the relationship between ischemic osteonecrosis and cellular senescence. X-gal staining was strong and p16INK4a-positive cells were abundant in the transitional region of ONFH. The β-galactosidase-positive cells in the transitional region were also positive for nestin, periostin, or DMP-1. In contrast, no β-galactosidase-positive cells were detected in the healthy region. The senescence-associated p16INK4a, p21, and p53 were upregulated in ONFH tissue. We also examined and analyzed a mouse ischemic femoral osteonecrosis model in vivo to verify the association between ONFH and cellular senescence. Human mesenchymal stem cell-conditioned medium (MSC-CM) was administered to determine its therapeutic efficacy against cellular senescence and bone collapse. MSC-CM reduced the number of senescent cells and downregulated the aforementioned senescence-related genes. It also decreased the number of empty lacunae 4 weeks after ischemia induction and promoted bone formation. At 6 weeks post-surgery, MSC-CM increased the trabecular bone volume, thereby suppressing bone collapse. We conclude that cellular senescence is associated with ONFH and that MSC-CM suppresses bone collapse in this disorder.

Ratiometric Fluorescent Probe for Real-Time Detection of β-Galactosidase Activity in Lysosomes and Its Application in Drug-Induced Senescence Imaging

Senescence is an important biological process, which leads to the gradual degradation of its physiological function and increases morbidity and mortality. Herein, a novel ratiometric fluorescent probe (P1) was constructed by using benzothiazolyl acetonitrile dye as fluorophore, exhibiting significantly enhanced blue-shifted emission to indicate the activity of β-galactosidase (β-gal), a commonly used biomarker for the detection of senescent cells. After incubation with β-gal, the excimer emission of P1 at 620 nm was weakened, while the emission at 533 nm was significantly enhanced, forming an obvious ratiometric probe with high sensitivity and low detection limit (2.7 mU·mL-1). More importantly, probe P1 can locate lysosomes accurately, allowing us to monitor the emergence of living cell senescence in real time. P1 was successfully used to detect β-gal activity in PC-12 cells, Hep G2 cells, and RAW 264.7 cells. It showed strong green fluorescence signal in senescent cells and red fluorescence signal in normal cells, indicating that it can detect endogenous senescence-related β-gal content in living cells. For in vivo drug-induced senescence imaging, after 5 weeks of injection of D-galactose or hydroxyurea, the mice showed significant fluorescence enhancement in specific channels to indicate the activity of β-gal in vivo. At the same time, the senescence of cell-specific organs and skin tissues at the organ level were also detected, which proved that the drug-induced senescence of brain, skin, and muscle tissues was the most serious. These results supported the important application value of P1 in senescence biomedical research.

Inflammatory Processes Affecting Bone Health and Repair

PURPOSE OF REVIEW

The purpose of this article is to review the current understanding of inflammatory processes on bone, including direct impacts of inflammatory factors on bone cells, the effect of senescence on inflamed bone, and the critical role of inflammation in bone pain and healing.

RECENT FINDINGS

Advances in osteoimmunology have provided new perspectives on inflammatory bone loss in recent years. Characterization of so-called inflammatory osteoclasts has revealed insights into physiological and pathological bone loss. The identification of inflammation-associated senescent markers in bone cells indicates that therapies that reduce senescent cell burden may reverse bone loss caused by inflammatory processes. Finally, novel studies have refined the role of inflammation in bone healing, including cross talk between nerves and bone cells. Except for the initial stages of fracture healing, inflammation has predominately negative effects on bone and increases fracture risk. Eliminating senescent cells, priming the osteo-immune axis in bone cells, and alleviating pro-inflammatory cytokine burden may ameliorate the negative effects of inflammation on bone.

Telomere length as a predictive biomarker in osteoporosis (Review)

Telomeres are the ends of chromosomes that protect them from DNA damage. There is evidence to suggest that telomere shortening appears with advanced age. Since aging is a significant risk factor for developing age-related complications, it is plausible that telomere shortening may be involved in the development of osteoporosis. The present review summarizes the potential of telomere shortening as a biomarker for detecting the onset of osteoporosis. For the purposes of the present review, the following scientific databases were searched for relevant articles: PubMed/NCBI, Cochrane Library of Systematic Reviews, Scopus, Embase and Google Scholar. The present review includes randomized and non-randomized controlled studies and case series involving humans, irrespective of the time of their publication. In six out of the 11 included studies providing data on humans, there was at least a weak association between telomere length and osteoporosis, with the remaining studies exhibiting no such association. As a result, telomere shortening may be used as a biomarker or as part of a panel of biomarkers for tracking the onset and progression of osteoporosis.

Cellular senescence in skeletal disease: mechanisms and treatment

The musculoskeletal system supports the movement of the entire body and provides blood production while acting as an endocrine organ. With aging, the balance of bone homeostasis is disrupted, leading to bone loss and degenerative diseases, such as osteoporosis, osteoarthritis, and intervertebral disc degeneration. Skeletal diseases have a profound impact on the motor and cognitive abilities of the elderly, thus creating a major challenge for both global health and the economy. Cellular senescence is caused by various genotoxic stressors and results in permanent cell cycle arrest, which is considered to be the underlying mechanism of aging. During aging, senescent cells (SnCs) tend to aggregate in the bone and trigger chronic inflammation by releasing senescence-associated secretory phenotypic factors. Multiple signalling pathways are involved in regulating cellular senescence in bone and bone marrow microenvironments. Targeted SnCs alleviate age-related degenerative diseases. However, the association between senescence and age-related diseases remains unclear. This review summarises the fundamental role of senescence in age-related skeletal diseases, highlights the signalling pathways that mediate senescence, and discusses potential therapeutic strategies for targeting SnCs.

Clinical Significance of Biological Age in Patients Undergoing Percutaneous Coronary Intervention

OBJECTIVE

To test whether biological age calculated using deficits, functional impairments, or their combination will provide improved estimation of long-term mortality among older adults undergoing percutaneous coronary intervention.

PATIENTS AND METHODS

Cardiovascular deficits, noncardiovascular deficits, and functional impairments were prospectively studied in 535 patients aged 55 years or older from August 1, 2014, to March 31, 2018. Models for biological age included deficits (acquired, increase with age, associated with worse prognosis, did not saturate early), functional impairments (subjective-help with daily activities, difficulty with sensory input, continence, weight, balance, mobility; or objective-timed up and go, functional reach), or their combination.

RESULTS

The mean ± SD age of the study patients was 72.1±9.5 years. For every 5-year increase in chronological age, the mean number of cardiovascular deficits increased from 2.36 among patients younger than 70 years to 3.44 in nonagenarians. The mean number of functional impairments increased from 2.15 for those younger than 70 years to 6.74 for nonagenarians. During a median follow-up of 2.05 years, 99 patients died. Significant improvement in the Harrell concordance index (C index) for prediction of long-term all-cause mortality was noted with biological age calculated from deficits and impairments compared with chronological age (0.77 vs 0.65; P<.001) and when estimating biological age via functional impairments alone vs chronological age (0.75 vs 0.65; P<.001) but not via deficits alone (0.71 vs 0.65; P=.08). Biological age estimates from subjective functional impairments captured most of the prognostic information related to all-cause and noncardiac mortality, whereas deficit-based estimation favored cardiovascular mortality.

CONCLUSION

The derivation of biological age from deficits and functional impairments provides a major improvement in the estimation of survival as estimated by chronological age.

Multiparametric senescent cell phenotyping reveals targets of senolytic therapy in the aged murine skeleton

Senescence drives organismal aging, yet the deep characterization of senescent cells in vivo remains incomplete. Here, we apply mass cytometry by time-of-flight using carefully validated antibodies to analyze senescent cells at single-cell resolution. We use multiple criteria to identify senescent mesenchymal cells that are growth-arrested and resistant to apoptosis. These p16 + Ki67-BCL-2+ cells are highly enriched for senescence-associated secretory phenotype and DNA damage markers, are strongly associated with age, and their percentages are increased in late osteoblasts/osteocytes and CD24high osteolineage cells. Moreover, both late osteoblasts/osteocytes and CD24high osteolineage cells are robustly cleared by genetic and pharmacologic senolytic therapies in aged mice. Following isolation, CD24+ skeletal cells exhibit growth arrest, senescence-associated β-galactosidase positivity, and impaired osteogenesis in vitro. These studies thus provide an approach using multiplexed protein profiling to define senescent mesenchymal cells in vivo and identify specific skeletal cell populations cleared by senolytics.

Senolytic and senomorphic interventions to defy senescence-associated mitochondrial dysfunction

The accumulation of senescent cells in the aging individual is associated with an increase in the occurrence of age-associated pathologies that contribute to poor health, frailty, and mortality. The number and type of senescent cells is viewed as a contributor to the body's senescence burden. Cellular models of senescence are based on induction of senescence in cultured cells in the laboratory. One type of senescence is triggered by mitochondrial dysfunction. There are several indications that mitochondria defects contribute to body aging. Senotherapeutics, targeting senescent cells, have been shown to induce their lysis by means of senolytics, or repress expression of their secretome, by means of senomorphics, senostatics or gerosuppressors. An outline of the mechanism of action of various senotherapeutics targeting mitochondria and senescence-associated mitochondria dysfunction will be here addressed. The combination of geroprotective interventions together with senotherapeutics will help to strengthen mitochondrial energy metabolism, biogenesis and turnover, and lengthen the mitochondria healthspan, minimizing one of several molecular pathways contributing to the aging phenotype.

Osteocytes: New Kids on the Block for Cancer in Bone Therapy

The tumor microenvironment plays a central role in the onset and progression of cancer in the bone. Cancer cells, either from tumors originating in the bone or from metastatic cancer cells from other body systems, are located in specialized niches where they interact with different cells of the bone marrow. These interactions transform the bone into an ideal niche for cancer cell migration, proliferation, and survival and cause an imbalance in bone homeostasis that severely affects the integrity of the skeleton. During the last decade, preclinical studies have identified new cellular mechanisms responsible for the dependency between cancer cells and bone cells. In this review, we focus on osteocytes, long-lived cells residing in the mineral matrix that have recently been identified as key players in the spread of cancer in bone. We highlight the most recent discoveries on how osteocytes support tumor growth and promote bone disease. Additionally, we discuss how the reciprocal crosstalk between osteocytes and cancer cells provides the opportunity to develop new therapeutic strategies to treat cancer in the bone.

In vitro and in vivo effects of zoledronic acid on senescence and senescence-associated secretory phenotype markers

In addition to reducing fracture risk, zoledronic acid has been found in some studies to decrease mortality in humans and extend lifespan and healthspan in animals. Because senescent cells accumulate with aging and contribute to multiple co-morbidities, the non-skeletal actions of zoledronic acid could be due to senolytic (killing of senescent cells) or senomorphic (inhibition of the secretion of the senescence-associated secretory phenotype [SASP]) actions. To test this, we first performed in vitro senescence assays using human lung fibroblasts and DNA repair-deficient mouse embryonic fibroblasts, which demonstrated that zoledronic acid killed senescent cells with minimal effects on non-senescent cells. Next, in aged mice treated with zoledronic acid or vehicle for 8 weeks, zoledronic acid significantly reduced circulating SASP factors, including CCL7, IL-1β, TNFRSF1A, and TGFβ1 and improved grip strength. Analysis of publicly available RNAseq data from CD115+ (CSF1R/c-fms+) pre-osteoclastic cells isolated from mice treated with zoledronic acid demonstrated a significant downregulation of senescence/SASP genes (SenMayo). To establish that these cells are potential senolytic/senomorphic targets of zoledronic acid, we used single cell proteomic analysis (cytometry by time of flight [CyTOF]) and demonstrated that zoledronic acid significantly reduced the number of pre-osteoclastic (CD115+/CD3e-/Ly6G-/CD45R-) cells and decreased protein levels of p16, p21, and SASP markers in these cells without affecting other immune cell populations. Collectively, our findings demonstrate that zoledronic acid has senolytic effects in vitro and modulates senescence/SASP biomarkers in vivo. These data point to the need for additional studies testing zoledronic acid and/or other bisphosphonate derivatives for senotherapeutic efficacy.

Senescent Cells: A Therapeutic Target in Cardiovascular Diseases

Senescent cell accumulation has been observed in age-associated diseases including cardiovascular diseases. Senescent cells lack proliferative capacity and secrete senescence-associated secretory phenotype (SASP) factors that may cause or worsen many cardiovascular diseases. Therapies targeting senescent cells, especially senolytic drugs that selectively induce senescent cell removal, have been shown to delay, prevent, alleviate, or treat multiple age-associated diseases in preclinical models. Some senolytic clinical trials have already been completed or are underway for a number of diseases and geriatric syndromes. Understanding how cellular senescence affects the various cell types in the cardiovascular system, such as endothelial cells, vascular smooth muscle cells, fibroblasts, immune cells, progenitor cells, and cardiomyocytes, is important to facilitate translation of senotherapeutics into clinical interventions. This review highlights: (1) the characteristics of senescent cells and their involvement in cardiovascular diseases, focusing on the aforementioned cardiovascular cell types, (2) evidence about senolytic drugs and other senotherapeutics, and (3) the future path and clinical potential of senotherapeutics for cardiovascular diseases.

Deconstructing cellular senescence in bone and beyond

Osteocytes are specialized bone cells that orchestrate skeletal remodeling. Senescent osteocytes are characterized by an activation of cyclin-dependent kinase inhibitor p16Ink4a and have been implicated in the pathogenesis of several bone loss disorders. In this issue of the JCI, Farr et al. have now shown that systemic removal of senescent cells (termed senolysis) prevented age-related bone loss at the spine and femur and mitigated bone marrow adiposity through a robust effect on osteoblasts and osteoclasts, whereas cell-specific senolysis in osteocytes alone was only partially effective. Surprisingly, transplantation of senescent fibroblasts into the peritoneum of young mice caused host osteocyte senescence associated with bone loss. This refined concept of osteocyte senescence and the effects of remote senolysis may help to develop improved senolytic strategies against multisystem aging in bone and beyond.

Cytotoxic CD4+ T cells eliminate senescent cells by targeting cytomegalovirus antigen

Senescent cell accumulation has been implicated in the pathogenesis of aging-associated diseases, including cancer. The mechanism that prevents the accumulation of senescent cells in aging human organs is unclear. Here, we demonstrate that a virus-immune axis controls the senescent fibroblast accumulation in the human skin. Senescent fibroblasts increased in old skin compared with young skin. However, they did not increase with advancing age in the elderly. Increased CXCL9 and cytotoxic CD4+ T cells (CD4 CTLs) recruitment were significantly associated with reduced senescent fibroblasts in the old skin. Senescent fibroblasts expressed human leukocyte antigen class II (HLA-II) and human cytomegalovirus glycoprotein B (HCMV-gB), becoming direct CD4 CTL targets. Skin-resident CD4 CTLs eliminated HCMV-gB+ senescent fibroblasts in an HLA-II-dependent manner, and HCMV-gB activated CD4 CTLs from the human skin. Collectively, our findings demonstrate HCMV reactivation in senescent cells, which CD4 CTLs can directly eliminate through the recognition of the HCMV-gB antigen.

Cellular rejuvenation: molecular mechanisms and potential therapeutic interventions for diseases

The ageing process is a systemic decline from cellular dysfunction to organ degeneration, with more predisposition to deteriorated disorders. Rejuvenation refers to giving aged cells or organisms more youthful characteristics through various techniques, such as cellular reprogramming and epigenetic regulation. The great leaps in cellular rejuvenation prove that ageing is not a one-way street, and many rejuvenative interventions have emerged to delay and even reverse the ageing process. Defining the mechanism by which roadblocks and signaling inputs influence complex ageing programs is essential for understanding and developing rejuvenative strategies. Here, we discuss the intrinsic and extrinsic factors that counteract cell rejuvenation, and the targeted cells and core mechanisms involved in this process. Then, we critically summarize the latest advances in state-of-art strategies of cellular rejuvenation. Various rejuvenation methods also provide insights for treating specific ageing-related diseases, including cellular reprogramming, the removal of senescence cells (SCs) and suppression of senescence-associated secretory phenotype (SASP), metabolic manipulation, stem cells-associated therapy, dietary restriction, immune rejuvenation and heterochronic transplantation, etc. The potential applications of rejuvenation therapy also extend to cancer treatment. Finally, we analyze in detail the therapeutic opportunities and challenges of rejuvenation technology. Deciphering rejuvenation interventions will provide further insights into anti-ageing and ageing-related disease treatment in clinical settings.

Context-dependent roles of cellular senescence in normal, aged, and disease states

Cellular senescence is a state of irreversible cell cycle arrest that often emerges after tissue damage and in age-related diseases. Through the production of a multicomponent secretory phenotype (SASP), senescent cells can impact the regeneration and function of tissues. However, the effects of senescent cells and their SASP are very heterogeneous and depend on the tissue environment and type as well as the duration of injury, the degree of persistence of senescent cells and the organism's age. While the transient presence of senescent cells is widely believed to be beneficial, recent data suggest that it is detrimental for tissue regeneration after acute damage. Furthermore, although senescent cell persistence is typically associated with the progression of age-related chronic degenerative diseases, it now appears to be also necessary for correct tissue function in the elderly. Here, we discuss what is currently known about the roles of senescent cells and their SASP in tissue regeneration in ageing and age-related diseases, highlighting their (negative and/or positive) contributions. We provide insight for future research, including the possibility of senolytic-based therapies and cellular reprogramming, with aims ranging from enhancing tissue repair to extending a healthy lifespan.

In vitro and in vivo effects of zoledronate on senescence and senescence-associated secretory phenotype markers

In addition to reducing fracture risk, zoledronate has been found in some studies to decrease mortality in humans and extend lifespan and healthspan in animals. Because senescent cells accumulate with aging and contribute to multiple co-morbidities, the non-skeletal actions of zoledronate could be due to senolytic (killing of senescent cells) or senomorphic (inhibition of the secretion of the senescence-associated secretory phenotype [SASP]) actions. To test this, we first performed in vitro senescence assays using human lung fibroblasts and DNA repair-deficient mouse embryonic fibroblasts, which demonstrated that zoledronate killed senescent cells with minimal effects on non-senescent cells. Next, in aged mice treated with zoledronate or vehicle for 8 weeks, zoledronate significantly reduced circulating SASP factors, including CCL7, IL-1β, TNFRSF1A, and TGFβ1 and improved grip strength. Analysis of publicly available RNAseq data from CD115+ (CSF1R/c-fms+) pre-osteoclastic cells isolated from mice treated with zoledronate demonstrated a significant downregulation of senescence/SASP genes (SenMayo). To establish that these cells are potential senolytic/senomorphic targets of zoledronate, we used single cell proteomic analysis (cytometry by time of flight [CyTOF]) and demonstrated that zoledronate significantly reduced the number of pre-osteoclastic (CD115+/CD3e-/Ly6G-/CD45R-) cells and decreased protein levels of p16, p21, and SASP markers in these cells without affecting other immune cell populations. Collectively, our findings demonstrate that zoledronate has senolytic effects in vitro and modulates senescence/SASP biomarkers in vivo . These data point to the need for additional studies testing zoledronate and/or other bisphosphonate derivatives for senotherapeutic efficacy.

Bone-targeted delivery of senolytics to eliminate senescent cells increases bone formation in senile osteoporosis

Systemic elimination of senescent cells using senolytic drugs presents therapeutic effects on age-related diseases, including senile osteoporosis. However, low bioavailability and potential side effects of senolytics restrict clinical application. Therefore, we developed a bone-targeted delivery system for senolytics to effective treatment of senile osteoporosis. In this study, quercetin was screened out as the ideal senolytics for eliminating senescent BMSCs. Treatment of quercetin efficiently decreased the senescence markers in senescent BMSCs models. After treatment with quercetin in vitro, cell mitosis and calcification staining assay confirmed that the proliferation and osteogenesis of the senescent BMSCs populations were enhanced. To enhance the effectiveness and minimize the side effect of treatment, liposomes decorated with bone affinity peptide (DSS)₆ were constructed for bone-targeted delivery of quercetin. After administration of liposomes loading quercetin in two aged mice models, histological and cellular analysis confirmed that bone-targeted treatment with quercetin efficiently eliminated senescent cells in bone, restored the function of BMCSs, and promoted bone formation in aged mice models when compared to non-targeted treatment. Taken together, the bone-targeted delivery of senolytics efficiently eliminates senescent cells to recover bone mass and microarchitecture, showing an effective treatment for senile osteoporosis. STATEMENT OF SIGNIFICANCE: Senile osteoporosis, a common and hazardous chronic disease, has been still lacking effective therapy. How to effectively eliminate the hazards of senescent cells in skeleton to bone formation remains challenge. In this study, quercetin was screened out as the ideal senolytic drug for senescent BMSCs and could effectively eliminated senescent BMSCs to restore the cellular functions of senescent BMSCs models in vitro. Then, the bone-targeted liposomes were designed to encapsulate and deliver senolytics efficiently to senile bone tissue. Based on two aged mice models, we confirmed that bone-targeted delivery of quercetin efficiently eliminated senescent cells in skeleton and enhanced bone formation in vivo, suggesting the bone-targeted elimination of senescent cells is an effective treatment for senile osteoporosis.

Aging and Bone Metabolism

Changes in bone architecture and metabolism with aging increase the likelihood of osteoporosis and fracture. Age-onset osteoporosis is multifactorial, with contributory extrinsic and intrinsic factors including certain medical problems, specific prescription drugs, estrogen loss, secondary hyperparathyroidism, microenvironmental and cellular alterations in bone tissue, and mechanical unloading or immobilization. At the histological level, there are changes in trabecular and cortical bone as well as marrow cellularity, lineage switching of mesenchymal stem cells to an adipogenic fate, inadequate transduction of signals during skeletal loading, and predisposition toward senescent cell accumulation with production of a senescence-associated secretory phenotype. Cumulatively, these changes result in bone remodeling abnormalities that over time cause net bone loss typically seen in older adults. Age-related osteoporosis is a geriatric syndrome due to the multiple etiologies that converge upon the skeleton to produce the ultimate phenotypic changes that manifest as bone fragility. Bone tissue is dynamic but with tendencies toward poor osteoblastic bone formation and relative osteoclastic bone resorption with aging. Interactions with other aging physiologic systems, such as muscle, may also confer detrimental effects on the aging skeleton. Conversely, individuals who maintain their BMD experience a lower risk of fractures, disability, and mortality, suggesting that this phenotype may be a marker of successful aging. © 2023 American Physiological Society. Compr Physiol 13:4355-4386, 2023.

Multiparametric senescent cell phenotyping reveals CD24 osteolineage cells as targets of senolytic therapy in the aged murine skeleton

Senescence drives organismal aging, yet the deep characterization of senescent cells in vivo remains incomplete. Here, we applied mass cytometry by time-of-flight (CyTOF) using carefully validated antibodies to analyze senescent cells at single-cell resolution. We used multiple criteria to identify senescent mesenchymal cells that were growth arrested and resistant to apoptosis (p16+/Ki67-/BCL-2+; "p16KB" cells). These cells were highly enriched for senescence-associated secretory phenotype (SASP) and DNA damage markers and were strongly associated with age. p16KB cell percentages were also increased in CD24+ osteolineage cells, which exhibited an inflammatory SASP in aged mice and were robustly cleared by both genetic and pharmacologic senolytic therapies. Following isolation, CD24+ skeletal cells exhibited growth arrest, SA-βgal positivity, and impaired osteogenesis in vitro . These studies thus provide a new approach using multiplexed protein profiling by CyTOF to define senescent mesenchymal cells in vivo and identify a highly inflammatory, senescent CD24+ osteolineage population cleared by senolytics.

Cellular senescence and senolytics: the path to the clinic

Interlinked and fundamental aging processes appear to be a root-cause contributor to many disorders and diseases. One such process is cellular senescence, which entails a state of cell cycle arrest in response to damaging stimuli. Senescent cells can arise throughout the lifespan and, if persistent, can have deleterious effects on tissue function due to the many proteins they secrete. In preclinical models, interventions targeting those senescent cells that are persistent and cause tissue damage have been shown to delay, prevent or alleviate multiple disorders. In line with this, the discovery of small-molecule senolytic drugs that selectively clear senescent cells has led to promising strategies for preventing or treating multiple diseases and age-related conditions in humans. In this Review, we outline the rationale for senescent cells as a therapeutic target for disorders across the lifespan and discuss the most promising strategies-including recent and ongoing clinical trials-for translating small-molecule senolytics and other senescence-targeting interventions into clinical use.

Cellular senescence: a key therapeutic target in aging and diseases

Cellular senescence is a hallmark of aging defined by stable exit from the cell cycle in response to cellular damage and stress. Senescent cells (SnCs) can develop a characteristic pathogenic senescence-associated secretory phenotype (SASP) that drives secondary senescence and disrupts tissue homeostasis, resulting in loss of tissue repair and regeneration. The use of transgenic mouse models in which SnCs can be genetically ablated has established a key role for SnCs in driving aging and age-related disease. Importantly, senotherapeutics have been developed to pharmacologically eliminate SnCs, termed senolytics, or suppress the SASP and other markers of senescence, termed senomorphics. Based on extensive preclinical studies as well as small clinical trials demonstrating the benefits of senotherapeutics, multiple clinical trials are under way. This Review discusses the role of SnCs in aging and age-related diseases, strategies to target SnCs, approaches to discover and develop senotherapeutics, and preclinical and clinical advances of senolytics.

Psoralen alleviates radiation-induced bone injury by rescuing skeletal stem cell stemness through AKT-mediated upregulation of GSK-3β and NRF2

BACKGROUND

Repairing radiation-induced bone injuries remains a significant challenge in the clinic, and few effective medicines are currently available. Psoralen is a principal bioactive component of Cullen corylifolium (L.) Medik and has been reported to have antitumor, anti-inflammatory, and pro-osteogenesis activities. However, less information is available regarding the role of psoralen in the treatment of radiation-induced bone injury. In this study, we explored the modulatory effects of psoralen on skeletal stem cells and their protective effects on radiation-induced bone injuries.

METHODS

The protective effects of psoralen on radiation-induced osteoporosis and irradiated bone defects were evaluated by microCT and pathological analysis. In addition, the cell proliferation, osteogenesis, and self-renewal of SSCs were explored. Further, the underlying mechanisms of the protective of psoralen were investigated by using RNA sequencing and functional gain and loss experiments in vitro and in vivo. Statistical significance was analyzed using Student's t test. The one-way ANOVA was used in multiple group data analysis.

RESULTS

Here, we demonstrated that psoralen, a natural herbal extract, mitigated radiation-induced bone injury (irradiation-induced osteoporosis and irradiated bone defects) in mice partially by rescuing the stemness of irradiated skeletal stem cells. Mechanistically, psoralen restored the stemness of skeletal stem cells by alleviating the radiation-induced suppression of AKT/GSK-3β and elevating NRF2 expression in skeletal stem cells. Furthermore, the expression of KEAP1 in skeletal stem cells did not significantly change in the presence of psoralen. Moreover, blockade of NRF2 in vivo partially abolished the promising effects of psoralen in a murine model of irradiation-induced osteoporosis and irradiated bone regeneration.

CONCLUSIONS

In summary, our findings identified psoralen as a potential medicine to mitigate bone radiation injury. In addition, skeletal stem cells and AKT-GSK-3β and NRF2 may thus represent therapeutic targets for treating radiation-induced bone injury.

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.

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.

Short senolytic or senostatic interventions rescue progression of radiation-induced frailty and premature ageing in mice

Cancer survivors suffer from progressive frailty, multimorbidity, and premature morbidity. We hypothesise that therapy-induced senescence and senescence progression via bystander effects are significant causes of this premature ageing phenotype. Accordingly, the study addresses the question whether a short anti-senescence intervention is able to block progression of radiation-induced frailty and disability in a pre-clinical setting. Male mice were sublethally irradiated at 5 months of age and treated (or not) with either a senolytic drug (Navitoclax or dasatinib + quercetin) for 10 days or with the senostatic metformin for 10 weeks. Follow-up was for 1 year. Treatments commencing within a month after irradiation effectively reduced frailty progression (p<0.05) and improved muscle (p<0.01) and liver (p<0.05) function as well as short-term memory (p<0.05) until advanced age with no need for repeated interventions. Senolytic interventions that started late, after radiation-induced premature frailty was manifest, still had beneficial effects on frailty (p<0.05) and short-term memory (p<0.05). Metformin was similarly effective as senolytics. At therapeutically achievable concentrations, metformin acted as a senostatic neither via inhibition of mitochondrial complex I, nor via improvement of mitophagy or mitochondrial function, but by reducing non-mitochondrial reactive oxygen species production via NADPH oxidase 4 inhibition in senescent cells. Our study suggests that the progression of adverse long-term health and quality-of-life effects of radiation exposure, as experienced by cancer survivors, might be rescued by short-term adjuvant anti-senescence interventions.

Bone Marrow Adiposity in Models of Radiation- and Aging-Related Bone Loss Is Dependent on Cellular Senescence

Oxidative stress-induced reactive oxygen species, DNA damage, apoptosis, and cellular senescence have been associated with reduced osteoprogenitors in a reciprocal fashion to bone marrow adipocyte tissue (BMAT); however, a direct (causal) link between cellular senescence and BMAT is still elusive. Accumulation of senescent cells occur in naturally aged and in focally radiated bone tissue, but despite amelioration of age- and radiation-associated bone loss after senescent cell clearance, molecular events that precede BMAT accrual are largely unknown. Here we show by RNA-Sequencing data that BMAT-related genes were the most upregulated gene subset in radiated bones of C57BL/6 mice. Using focal radiation as a model to understand age-associated changes in bone, we performed a longitudinal assessment of cellular senescence and BMAT. Using real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR), RNA in situ hybridization of p21 transcripts and histological assessment of telomere dysfunction as a marker of senescence, we observed an increase in senescent cell burden of bone cells from day 1 postradiation, without the presence of BMAT. BMAT was significantly elevated in radiated bones at day 7, confirming the qRT-PCR data in which most BMAT-related genes were elevated by day 7, and the trend continued until day 42 postradiation. Similarly, elevation in BMAT-related genes was observed in bones of aged mice. The senolytic cocktail of Dasatinib (D) plus Quercetin (Q) (ie, D + Q), which clears senescent cells, reduced BMAT in aged and radiated bones. MicroRNAs (miRNAs or miRs) linked with senescence marker p21 were downregulated in radiated and aged bones, whereas miR-27a, a miR that is associated with increased BMAT, was elevated both in radiated and aged bones. D + Q downregulated miR-27a in radiated bones at 42 days postradiation. Overall, our study provides evidence that BMAT occurrence in oxidatively stressed bone environments, such as radiation and aging, is induced following a common pathway and is dependent on the presence of senescent cells. © 2022 American Society for Bone and Mineral Research (ASBMR).

Targeted clearance of p21- but not p16-positive senescent cells prevents radiation-induced osteoporosis and increased marrow adiposity

Cellular senescence, which is a major cause of tissue dysfunction with aging and multiple other conditions, is known to be triggered by p16Ink4a or p21Cip1 , but the relative contributions of each pathway toward inducing senescence are unclear. Here, we directly addressed this issue by first developing and validating a p21-ATTAC mouse with the p21Cip1 promoter driving a "suicide" transgene encoding an inducible caspase-8 which, upon induction, selectively kills p21Cip1 -expressing senescent cells. Next, we used the p21-ATTAC mouse and the established p16-INK-ATTAC mouse to directly compare the contributions of p21Cip1 versus p16Ink4a in driving cellular senescence in a condition where a tissue phenotype (bone loss and increased marrow adiposity) is clearly driven by cellular senescence-specifically, radiation-induced osteoporosis. Using RNA in situ hybridization, we confirmed the reduction in radiation-induced p21Cip1 - or p16Ink4a -driven transcripts following senescent cell clearance in both models. However, only clearance of p21Cip1 +, but not p16Ink4a +, senescent cells prevented both radiation-induced osteoporosis and increased marrow adiposity. Reduction in senescent cells with dysfunctional telomeres following clearance of p21Cip1 +, but not p16Ink4a +, senescent cells also reduced several of the radiation-induced pro-inflammatory senescence-associated secretory phenotype factors. Thus, by directly comparing senescent cell clearance using two parallel genetic models, we demonstrate that radiation-induced osteoporosis is driven predominantly by p21Cip1 - rather than p16Ink4a -mediated cellular senescence. Further, this approach can be used to dissect the contributions of these pathways in other senescence-associated conditions, including aging across tissues.

Age Related Osteoporosis: Targeting Cellular Senescence

Age-related chronic diseases are an enormous burden to modern societies worldwide. Among these, osteoporosis, a condition that predisposes individuals to an increased risk of fractures, substantially contributes to increased mortality and health-care costs in elderly. It is now well accepted that advanced chronical age is one of the main risk factors for chronical diseases. Hence, targeting fundamental aging mechanisms such as senescence has become a promising option in the treatment of these diseases. Moreover, for osteoporosis, the main pathophysiological concepts arise from menopause causing estrogen deficiency, and from aging. Here, we focus on recent advances in the understanding of senescence-related mechanisms contributing to age-related bone loss. Furthermore, treatment options for senile osteoporosis targeting senescent cells are reviewed.

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.

The osteocyte as a signaling cell

Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone and in distant tissues. Osteocytes are a major source of molecules that regulate bone homeostasis by integrating both mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in both rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of several disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte-secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of several bone therapeutics used in the clinic. Here we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematologic and metastatic cancers in the skeleton.

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.

Radiotherapy-induced bone deterioration is exacerbated in diabetic rats treated with streptozotocin

Following radiotherapy, patients have decreased bone mass and increased risk of fragility fractures. Diabetes mellitus (DM) is also reported to have detrimental effects on bone architecture and quality. However, no clinical or experimental study has systematically characterized the bone phenotype of the diabetic patients following radiotherapy. After one month of streptozotocin injection, three-month-old male rats were subjected to focal radiotherapy (8 Gy, twice, at days 1 and 3), and then bone mass, microarchitecture, and turnover as well as bone cell activities were evaluated at 2 months post-irradiation. Micro-computed tomography results demonstrated that DM rats exhibited greater deterioration in trabecular bone mass and microarchitecture following irradiation compared with the damage to bone structure induced by DM or radiotherapy. The serum biochemical, bone histomorphometric, and gene expression assays revealed that DM combined with radiotherapy showed lower bone formation rate, osteoblast number on bone surface, and expression of osteoblast-related markers (ALP, Runx2, Osx, and Col-1) compared with DM or irradiation alone. DM plus irradiation also caused higher bone resorption rate, osteoclast number on bone surface, and expression of osteoclast-specific markers (TRAP, cathepsin K, and calcitonin receptor) than DM or irradiation treatment alone. Moreover, lower osteocyte survival and higher expression of Sost and DKK1 genes (two negative modulators of Wnt signaling) were observed in rats with combined DM and radiotherapy. Together, these findings revealed a higher deterioration of the diabetic skeleton following radiotherapy, and emphasized the clinical importance of health maintenance.

Accelerated aging in older cancer survivors

This editorial comments on the article by Siddique et al.

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.

Senescent immune cells release grancalcin to promote skeletal aging

Skeletal aging is characterized by low bone turnover and marrow fat accumulation. However, the underlying mechanism for this imbalance is unclear. Here, we show that during aging in rats and mice proinflammatory and senescent subtypes of immune cells, including macrophages and neutrophils, accumulate in the bone marrow and secrete abundant grancalcin. The injection of recombinant grancalcin into young mice was sufficient to induce premature skeletal aging. In contrast, genetic deletion of Gca in neutrophils and macrophages delayed skeletal aging. Mechanistically, we found that grancalcin binds to the plexin-b2 receptor and partially inactivates its downstream signaling pathways, thus repressing osteogenesis and promoting adipogenesis of bone marrow mesenchymal stromal cells. Heterozygous genetic deletion of Plexnb2 in skeletal stem cells abrogated the improved bone phenotype of Gca-knockout mice. Finally, we developed a grancalcin-neutralizing antibody and showed that its treatment of older mice improved bone health. Together, our data suggest that grancalcin could be a potential target for the treatment of age-related osteoporosis.

The role of senolytics in osteoporosis and other skeletal pathologies

The skeletal system undergoes irreversible structural deterioration with aging, leading to increased fracture risk and detrimental changes in mobility, posture, and gait. This state of low bone mass and microarchitectural changes, diagnosed as osteoporosis, affects millions of individuals worldwide and has high clinical and economic burdens. Recently, pre-clinical studies have linked the onset of age-related bone loss with an accumulation of senescent cells in the bone microenvironment. These senescent cells appear to be causal to age-related bone loss, as targeted clearance of these cells leads to improved bone mass and microarchitecture in old mice. Additionally, other pathologies leading to bone loss that result from DNA damage, such as cancer treatments, have shown improvements after clearance of senescent cells. The development of new therapies that clear senescent cells, termed "senolytics", is currently underway and may allow for the modulation of bone loss that results from states of high senescent cell burden, such as aging.

Radiation-Induced Senescence in p16+/LUC Mouse Lung Compared to Bone Marrow Multilineage Hematopoietic Progenitor Cells

We defined the time course of ionizing radiation-induced senescence in lung compared to bone marrow of p16+/LUC mice in which the senescence-induced biomarker (p16) is linked to a luciferase reporter gene. Periodic in situ imaging revealed increased luciferase activity in the lungs of 20 Gy thoracic irradiated, but not 8 Gy total-body irradiated (TBI) mice beginning at day 75 and increasing to day 170. In serial sections of explanted lungs, senescent cells appeared in the same areas as did fibrosis in the 20 Gy thoracic irradiated, but not the 8 Gy TBI group. Lungs from 8 Gy TBI mice at one year did show increased RNA levels for p16, p21, p19 and TGF-β. Individual senescent cells in 20 Gy irradiated mouse lung included those with epithelial, endothelial, fibroblast and hematopoietic cell biomarkers. Rare senescent cells in the lungs of 8 Gy TBI mice at one year were of endothelial phenotype. Long-term bone marrow cultures (LTBMCs) were established at either day 60 or one year after 8 Gy TBI. In freshly removed marrow at both times after irradiation, there were increased senescent cells. In LTBMCs, there were increased senescent cells in both weekly harvested single cells and in colonies of multilineage hematopoietic progenitor cells producing CFU-GEMM (colony forming unit-granulocyte, erythrocyte, monocyte/macrophage, mega-karyocyte) that were formed in secondary cultures when these single cells were plated in semisolid media. LTBMCs from TBI mice produced fewer CFU-GEMM; however, the relative percentage of senescent cell-containing colonies was increased as measured by both p16-luciferase and β-galactosidase. Therefore, 20 Gy thoracic radiation, as well as 8 Gy TBI, induces senescent cells in the lungs. With bone marrow, 8 Gy TBI induced senescence in both hematopoietic cells and in colony-forming progenitors. The p16+/LUC mouse strain provides a valuable system in which to compare the kinetics of radiation-induced senescence between organs in vivo, and to evaluate the potential role of senescent cells in irradiation pulmonary fibrosis.