Atm-deficient mice: an osteoporosis model with defective osteoblast differentiation and increased osteoclastogenesis

Cell Transitions Contribute to Glucocorticoid-Induced Bone Loss

Glucocorticoid-induced bone loss is a toxic effect of long-term therapy with glucocorticoids resulting in a significant increase in the risk of fracture. Here, we find that glucocorticoids reciprocally convert osteoblast-lineage cells into endothelial-like cells. This is confirmed by lineage tracing showing the induction of endothelial markers in osteoblast-lineage cells following glucocorticoid treatment. Functional studies show that osteoblast-lineage cells isolated from glucocorticoid-treated mice lose their capacity for bone formation but simultaneously improve vascular repair. We find that the glucocorticoid receptor directly targets Foxc2 and Osterix, and the modulations of Foxc2 and Osterix drive the transition of osteoblast-lineage cells to endothelial-like cells. Together, the results suggest that glucocorticoids suppress osteogenic capacity and cause bone loss at least in part through previously unrecognized osteoblast-endothelial transitions.

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.

Intersection of the Orphan G Protein-Coupled Receptor, GPR19, with the Aging Process

G protein-coupled receptors (GPCRs) represent one of the most functionally diverse classes of transmembrane proteins. GPCRs and their associated signaling systems have been linked to nearly every physiological process. They also constitute nearly 40% of the current pharmacopeia as direct targets of remedial therapies. Hence, their place as a functional nexus in the interface between physiological and pathophysiological processes suggests that GPCRs may play a central role in the generation of nearly all types of human disease. Perhaps one mechanism through which GPCRs can mediate this pivotal function is through the control of the molecular aging process. It is now appreciated that, indeed, many human disorders/diseases are induced by GPCR signaling processes linked to pathological aging. Here we discuss one such novel member of the GPCR family, GPR19, that may represent an important new target for novel remedial strategies for the aging process. The molecular signaling pathways (metabolic control, circadian rhythm regulation and stress responsiveness) associated with this recently characterized receptor suggest an important role in aging-related disease etiology.

Inhibition of Poly (ADP-Ribose) Glycohydrolase Accelerates Osteoblast Differentiation in Preosteoblastic MC3T3-E1 Cells

Poly ADP-ribosylation (PARylation) is a post-translational modification catalyzed by poly (ADP-ribose) polymerase (PARP) family proteins such as PARP1. Although PARylation regulates important biological phenomena such as DNA repair, chromatin regulation, and cell death, little is known about the relationship between osteoblast differentiation and the PARylation cycle involving PARP1 and the poly (ADP-ribose)-degrading enzyme poly (ADP-ribose) glycohydrolase (PARG). Here, we examined the effects of PARP inhibitor olaparib, an approved anti-cancer agent, and PARG inhibitor PDD00017273 on osteoblast differentiation. Olaparib decreased alkaline phosphatase (ALP) activity and suppressed mineralized nodule formation evaluated by Alizarin Red S staining in preosteoblastic MC3T3-E1 cells, while PDD00017273 promoted ALP activity and mineralization. Furthermore, PDD00017273 up-regulated the mRNA expression levels of osteocalcin and bone sialoprotein, as osteoblast differentiation markers, and osterix as transcription inducers for osteoblast differentiation, whereas olaparib down-regulated the expression of these genes. These findings suggest that PARG inhibition by PDD00017273 accelerates osteoblast differentiation in MC3T3-E1 cells. Thus, PARG inhibitor administration could provide therapeutic benefits for metabolic bone diseases such as osteoporosis.

iTRAQ-based proteomic analysis reveals potential osteogenesis-promoted role of ATM in strontium-incorporated titanium implant

The present study aims to reveal the osteogenic roles played by DNA damage response biomarkers through implementing isobaric tags for relative and absolute quantitation (iTRAQ) technique. First, sandblasted large-grit double acid-etched (SLA) titanium implant and strontium-incorporated (SLA-Sr) titanium implant were used for inserting in the tibiae of rats. iTRAQ technique was used to detect protein expression changes and identify differentially expressed proteins (DEPs). In total, 19,343 peptides and 4280 proteins were screened out. Among them, 91 and 138 DEPs were identified in the SLA-Sr group after implantation for 3 and 7 days, respectively. Ataxia-telangiectasia mutated (ATM) protein up-regulated on the 3rd day showed a trend of further up-regulation on the 7th day. Moreover, functional enrichment analyses were also conducted to explore the biological function of DEPs during the initial stage of osseointegration in vivo, which revealed that the biological functions of the DEPs on the 7th day were mainly related to "mismatch repair" and "mitotic G1 DNA damage checkpoint." Analysis of the Reactome signaling pathway showed that ATM was associated with TP53's regulation and activation. Finally, DNA damage repair related genes were selected for validation at mRNA and protein expression levels. Real-time reverse transcription-polymerase chain reaction and immunohistochemistry validation results demonstrated that mRNA expression level of ATM was higher in SLA-Sr group. In conclusion, SLA-Sr titanium implant could initiate DNA damage repair by activating expression levels of ATM. This study was striving to reveal new faces of better osseointegration and shedding light on the biological function and underlying mechanisms of this important procedure.

Runx2 (Runt-Related Transcription Factor 2) Links the DNA Damage Response to Osteogenic Reprogramming and Apoptosis of Vascular Smooth Muscle Cells

[Figure: see text].

Skeletal Aging and Osteoporosis: Mechanisms and Therapeutics

Bone is a dynamic organ maintained by tightly regulated mechanisms. With old age, bone homeostasis, which is maintained by an intricate balance between bone formation and bone resorption, undergoes deregulation. Oxidative stress-induced DNA damage, cellular apoptosis, and cellular senescence are all responsible for this tissue dysfunction and the imbalance in the bone homeostasis. These cellular mechanisms have become a target for therapeutics to treat age-related osteoporosis. Genetic mouse models have shown the importance of senescent cell clearance in alleviating age-related osteoporosis. Furthermore, we and others have shown that targeting cellular senescence pharmacologically was an effective tool to alleviate age- and radiation-induced osteoporosis. Senescent cells also have an altered secretome known as the senescence associated secretory phenotype (SASP), which may have autocrine, paracrine, or endocrine function. The current review discusses the current and potential pathways which lead to a senescence profile in an aged skeleton and how bone homeostasis is affected during age-related osteoporosis. The review has also discussed existing therapeutics for the treatment of osteoporosis and rationalizes for novel therapeutic options based on cellular senescence and the SASP as an underlying pathogenesis of an aging bone.

Aging-related modifications to G protein-coupled receptor signaling diversity

Aging is a highly complex molecular process, affecting nearly all tissue systems in humans and is the highest risk factor in developing neurodegenerative disorders such as Alzheimer's and Parkinson's disease, cardiovascular disease and Type 2 diabetes mellitus. The intense complexity of the aging process creates an incentive to develop more specific drugs that attenuate or even reverse some of the features of premature aging. As our current pharmacopeia is dominated by therapeutics that target members of the G protein-coupled receptor (GPCR) superfamily it may be prudent to search for effective anti-aging therapeutics in this fertile domain. Since the first demonstration of GPCR-based β-arrestin signaling, it has become clear that an enhanced appreciation of GPCR signaling diversity may facilitate the creation of therapeutics with selective signaling activities. Such 'biased' ligand signaling profiles can be effectively investigated using both standard molecular biological techniques as well as high-dimensionality data analyses. Through a more nuanced appreciation of the quantitative nature across the multiple dimensions of signaling bias that drugs possess, researchers may be able to further refine the efficacy of GPCR modulators to impact the complex aberrations that constitute the aging process. Identifying novel effector profiles could expand the effective pharmacopeia and assist in the design of precision medicines. This review discusses potential non-G protein effectors, and specifically their potential therapeutic suitability in aging and age-related disorders.

Impaired ATM activation in B cells is associated with bone resorption in rheumatoid arthritis

Patients with rheumatoid arthritis (RA) may display atypical CD21^-/lo B cells in their blood, but the implication of this observation remains unclear. We report here that the group of patients with RA and elevated frequencies of CD21^-/lo B cells shows decreased ataxia telangiectasia-mutated (ATM) expression and activation in B cells compared with other patients with RA and healthy donor controls. In agreement with ATM involvement in the regulation of V(D)J recombination, patients with RA who show defective ATM function displayed a skewed B cell receptor (BCR) Igκ repertoire, which resembled that of patients with ataxia telangiectasia (AT). This repertoire was characterized by increased Jκ1 and decreased upstream Vκ gene segment usage, suggesting improper secondary recombination processes and selection. In addition, altered ATM function in B cells was associated with decreased osteoprotegerin and increased receptor activator of nuclear factor κB ligand (RANKL) production. These changes favor bone loss and correlated with a higher prevalence of erosive disease in patients with RA who show impaired ATM function. Using a humanized mouse model, we also show that ATM inhibition in vivo induces an altered Igκ repertoire and RANKL production by immature B cells in the bone marrow, leading to decreased bone density. We conclude that dysregulated ATM function in B cells promotes bone erosion and the emergence of circulating CD21^-/lo B cells, thereby contributing to RA pathophysiology.

Molecular Basis of Bone Aging

A decline in bone mass leading to an increased fracture risk is a common feature of age-related bone changes. The mechanisms underlying bone senescence are very complex and implicate systemic and local factors and are the result of the combination of several changes occurring at the cellular, tissue and structural levels; they include alterations of bone cell differentiation and activity, oxidative stress, genetic damage and the altered responses of bone cells to various biological signals and to mechanical loading. The molecular mechanisms responsible for these changes remain greatly unclear and many data derived from in vitro or animal studies appear to be conflicting and heterogeneous, probably due to the different experimental approaches; nevertheless, understanding the main physio-pathological processes that cause bone senescence is essential for the development of new potential therapeutic options for treating age-related bone loss. This article reviews the current knowledge concerning the molecular mechanisms underlying the pathogenesis of age-related bone changes.

DNA Damage Response: A Molecular Lynchpin in the Pathobiology of Arteriosclerotic Calcification

Vascular calcification is a ubiquitous pathology of aging. Oxidative stress, persistent DNA damage, and senescence are major pathways driving both cellular and tissue aging, and emerging evidence suggests that these pathways are activated, and even accelerated, in patients with vascular calcification. The DNA damage response-a complex signaling platform that maintains genomic integrity-is induced by oxidative stress and is intimately involved in regulating cell death and osteogenic differentiation in both bone and the vasculature. Unexpectedly, a posttranslational modification, PAR (poly[ADP-ribose]), which is a byproduct of the DNA damage response, initiates biomineralization by acting to concentrate calcium into spheroidal structures that can nucleate apatitic mineral on the ECM (extracellular matrix). As we start to dissect the molecular mechanisms driving aging-associated vascular calcification, novel treatment strategies to promote healthy aging and delay pathological change are being unmasked. Drugs targeting the DNA damage response and senolytics may provide new avenues to tackle this detrimental and intractable pathology.

Implication of the p53-Related miR-34c, -125b, and -203 in the Osteoblastic Differentiation and the Malignant Transformation of Bone Sarcomas

The formation of the skeleton occurs throughout the lives of vertebrates and is achieved through the balanced activities of two kinds of specialized bone cells: the bone-forming osteoblasts and the bone-resorbing osteoclasts. Impairment in the remodeling processes dramatically hampers the proper healing of fractures and can also result in malignant bone diseases such as osteosarcoma. MicroRNAs (miRNAs) are a class of small non-coding single-strand RNAs implicated in the control of various cellular activities such as proliferation, differentiation, and apoptosis. Their post-transcriptional regulatory role confers on them inhibitory functions toward specific target mRNAs. As miRNAs are involved in the differentiation program of precursor cells, it is now well established that this class of molecules also influences bone formation by affecting osteoblastic differentiation and the fate of osteoblasts. In response to various cell signals, the tumor-suppressor protein p53 activates a huge range of genes, whose miRNAs promote genomic-integrity maintenance, cell-cycle arrest, cell senescence, and apoptosis. Here, we review the role of three p53-related miRNAs, miR-34c, -125b, and -203, in the bone-remodeling context and, in particular, in osteoblastic differentiation. The second aim of this study is to deal with the potential implication of these miRNAs in osteosarcoma development and progression.

Assessment of vitamin D status in common variable immunodeficiency or ataxia-telangiectasia patients

INTRODUCTION AND OBJECTIVES

Vitamin D plays a role in the immune system, however studies regarding this are scarce. This study aimed to evaluate the nutritional status of vitamin D in patients with Common Variable Immunodeficiency (CVID) or Ataxia-Telangiectasia (A-T) and to relate it to body composition, inflammatory and bone metabolism markers.

PATIENTS AND METHODS

This is a cross-sectional and controlled study involving 24 patients of both sexes (59.3% male), aged 8-56 years, with CVID (n=15) or A-T (n=9). The following variables were evaluated: body mass index (BMI), 25-hydroxyvitamin D (25 (OH) D), hepatic profile, parathormone, calcium, phosphorus, alkaline phosphatase, interleukin 6 and high-sensitivity C-reactive protein.

RESULTS

The median age was 26.0 years. A deficiency of 25 (OH) D was found in four A-T patients (44%) and two CVID patients (13%). Nine patients with CVI (60%) and six with A-T (66.7%) were overweight and underweight, respectively. There was a negative correlation between vitamin D and fat mass in the CVID group, and vitamin D and BMI in the A-T group. Vitamin D was negatively associated with the percentage of total fat among the patients (β - 0.842, 95% CI: -1.5-0.17, p=0.015), R²=0.21, after adjusting for sex and age.

CONCLUSION

Vitamin D deficiency occurred in a quarter of the patients although there was no difference between the patient and the control group; without association with bone and inflammation biomarkers. The percentage of fat and BMI were negatively associated with the concentrations of 25 (OH) D.

Murine models of accelerated aging and musculoskeletal disease

The primary risk factor for most musculoskeletal diseases, including osteoarthritis, osteoporosis and sarcopenia, is aging. To treat the diverse types of musculoskeletal diseases and pathologies, targeting their root cause, the aging process itself, has the potential to slow or prevent multiple age-related musculoskeletal conditions simultaneously. However, the development of approaches to delay onset of age related diseases, including musculoskeletal pathologies, has been slowed by the relatively long lifespan of rodent models of aging. Thus, to expedite the development of therapeutic approaches for age-related musculoskeletal disease, the implementation of mouse models of accelerated musculoskeletal aging are of great utility. Currently there are multiple genetically diverse mouse models that mirror certain aspects of normal human and mouse aging. Here, we provide a review of some of the most relevant murine models of accelerated aging that mimic many aspects of natural musculoskeletal aging, highlighting their relative strengths and weaknesses. Importantly, these murine models of accelerated aging recapitulate phenotypes of musculoskeletal age-related decline observed in humans.

DNA damage in aging, the stem cell perspective

DNA damage is one of the most consistent cellular process proposed to contribute to aging. The maintenance of genomic and epigenomic integrity is critical for proper function of cells and tissues throughout life, and this homeostasis is under constant strain from both extrinsic and intrinsic insults. Considering the relationship between lifespan and genotoxic burden, it is plausible that the longest-lived cellular populations would face an accumulation of DNA damage over time. Tissue-specific stem cells are multipotent populations residing in localized niches and are responsible for maintaining all lineages of their resident tissue/system throughout life. However, many of these stem cells are impacted by genotoxic stress. Several factors may dictate the specific stem cell population response to DNA damage, including the niche location, life history, and fate decisions after damage accrual. This leads to differential handling of DNA damage in different stem cell compartments. Given the importance of adult stem cells in preserving normal tissue function during an individual's lifetime, DNA damage sensitivity and accumulation in these compartments could have crucial implications for aging. Despite this, more support for direct functional effects driven by accumulated DNA damage in adult stem cell compartments is needed. This review will present current evidence for the accumulation and potential influence of DNA damage in adult tissue-specific stem cells and propose inquiry directions that could benefit individual healthspan.

Osteogenic differentiation of skeletal muscle progenitor cells is activated by the DNA damage response

Heterotopic ossification (HO) is a pathological condition characterized by the deposition of mineralized tissue in ectopic locations such as the skeletal muscle. The precise cellular origin and molecular mechanisms underlying HO are still debated. In our study we focus on the differentiation of mesoangioblasts (MABs), a population of multipotent skeletal muscle precursors. High-content screening for small molecules that perturb MAB differentiation decisions identified Idoxuridine (IdU), an antiviral and radiotherapy adjuvant, as a molecule that promotes MAB osteogenic differentiation while inhibiting myogenesis. IdU-dependent osteogenesis does not rely on the canonical BMP-2/SMADs osteogenic pathway. At pro-osteogenic conditions IdU induces a mild DNA Damage Response (DDR) that activates ATM and p38 eventually promoting the phosphorylation of the osteogenesis master regulator RUNX2. By interfering with this pathway IdU-induced osteogenesis is severely impaired. Overall, our study suggests that induction of the DDR promotes osteogenesis in muscle resident MABs thereby offering a new mechanism that may be involved in the ectopic deposition of mineralized tissue in the muscle.

ANP32A regulates ATM expression and prevents oxidative stress in cartilage, brain, and bone

Osteoarthritis is the most common joint disorder with increasing global prevalence due to aging of the population. Current therapy is limited to symptom relief, yet there is no cure. Its multifactorial etiology includes oxidative stress and overproduction of reactive oxygen species, but the regulation of these processes in the joint is insufficiently understood. We report that ANP32A protects the cartilage against oxidative stress, preventing osteoarthritis development and disease progression. ANP32A is down-regulated in human and mouse osteoarthritic cartilage. Microarray profiling revealed that ANP32A protects the joint by promoting the expression of ATM, a key regulator of the cellular oxidative defense. Antioxidant treatment reduced the severity of osteoarthritis, osteopenia, and cerebellar ataxia features in Anp32a-deficient mice, revealing that the ANP32A/ATM axis discovered in cartilage is also present in brain and bone. Our findings indicate that modulating ANP32A signaling could help manage oxidative stress in cartilage, brain, and bone with therapeutic implications for osteoarthritis, neurological disease, and osteoporosis.

GIT2-A keystone in ageing and age-related disease

Since its discovery, G protein-coupled receptor kinase-interacting protein 2, GIT2, and its family member, GIT1, have received considerable interest concerning their potential key roles in regulating multiple inter-connected physiological and pathophysiological processes. GIT2 was first identified as a multifunctional protein that is recruited to G protein-coupled receptors (GPCRs) during the process of receptor internalization. Recent findings have demonstrated that perhaps one of the most important effects of GIT2 in physiology concerns its role in controlling multiple aspects of the complex ageing process. Ageing can be considered the most prevalent pathophysiological condition in humans, affecting all tissue systems and acting as a driving force for many common and intractable disorders. The ageing process involves a complex interplay among various deleterious activities that profoundly disrupt the body's ability to cope with damage, thus increasing susceptibility to pathophysiologies such as neurodegeneration, central obesity, osteoporosis, type 2 diabetes mellitus and atherosclerosis. The biological systems that control ageing appear to function as a series of interconnected complex networks. The inter-communication among multiple lower-complexity signaling systems within the global ageing networks is likely coordinated internally by keystones or hubs, which regulate responses to dynamic molecular events through protein-protein interactions with multiple distinct partners. Multiple lines of research have suggested that GIT2 may act as one of these network coordinators in the ageing process. Identifying and targeting keystones, such as GIT2, is thus an important approach in our understanding of, and eventual ability to, medically ameliorate or interdict age-related progressive cellular and tissue damage.

Conditional abrogation of Atm in osteoclasts extends osteoclast lifespan and results in reduced bone mass

Ataxia-telangiectasia mutated (ATM) kinase is a central component involved in the signal transduction of the DNA damage response (DDR) and thus plays a critical role in the maintenance of genomic integrity. Although the primary functions of ATM are associated with the DDR, emerging data suggest that ATM has many additional roles that are not directly related to the DDR, including the regulation of oxidative stress signaling, insulin sensitivity, mitochondrial homeostasis, and lymphocyte development. Patients and mice lacking ATM exhibit growth retardation and lower bone mass; however, the mechanisms underlying the skeletal defects are not fully understood. In the present study, we generated mutant mice in which ATM is specifically inactivated in osteoclasts. The mutant mice did not exhibit apparent developmental defects but showed reduced bone mass due to increased osteoclastic bone resorption. Osteoclasts lacking ATM were more resistant to apoptosis and showed a prolonged lifespan compared to the controls. Notably, the inactivation of ATM in osteoclasts resulted in enhanced NF-κB signaling and an increase in the expression of NF-κB-targeted genes. The present study reveals a novel function for ATM in regulating bone metabolism by suppressing the lifespan of osteoclasts and osteoclast-mediated bone resorption.

Endocrine abnormalities in ataxia telangiectasia: findings from a national cohort

BACKGROUND

Ataxia telangiectasia (AT) is a genetic multisystem disorder, presenting with progressive ataxia, immune deficiency, and propensity toward malignancy. Endocrine abnormalities (growth retardation, reproductive dysfunction, and diabetes) have been described, however detailed information regarding this aspect is lacking. We aimed to characterize endocrine anomalies and growth patterns in a large cohort of AT patients.

METHODS

Retrospective study comprising all 52 patients (aged 2-26.2 y) followed at a national AT Clinic. Anthropometric and laboratory measurements were extracted from the charts.

RESULTS

Median height-SDS was already subnormal during infancy, remaining negative throughout follow up to adulthood. Height-SDS was more impaired than weight-SDS up to age 4 y, thereafter weight-SDS steadily decreased, resulting in progressively lower BMI-SDS. IGF-I-SDS was low (-1.53 ± 1.54), but did not correlate with height-SDS. Gonadal failure was present in all 13 females older than 10 y but only in one male. Two patients had diabetes and 10 had dyslipidemia. Vitamin D deficiency was observed in 52.2% of the evaluated patients.

CONCLUSION

Our results suggest a primary growth abnormality in AT, rather than secondary to nutritional impairment or disease severity. Sex hormone replacement should be considered for female patients. Vitamin D levels should be followed and supplementation given if needed.

Body composition, muscle strength and hormonal status in patients with ataxia telangiectasia: a cohort study

Background

Ataxia-telangiectasia (A-T) is a devastating human autosomal recessive disorder that causes progressive cerebellar ataxia, immunodeficiency, premature aging, chromosomal instability and increased cancer risk. Affected patients show growth failure, poor weight gain, low body mass index (BMI), myopenia and increased fatigue during adolescence.

The prevalence of alterations in body composition, muscle strength and hormonal status has not been well described in classical A-T patients. Additionally, no current guidelines are available for the assessment and management of these changes.

Methods

We analyzed body composition, manual muscle strength and hormonal status in 25 A-T patients and 26 age-matched, healthy controls. Bioelectrical impedance analysis (BIA) was performed to evaluate the body composition, fat-free mass (FFM), body cell mass (BCM), extracellular matrix (ECM), phase angle (PhA), fat mass (FM) and ECM to BCM ratio. Manual muscle strength was measured using a hydraulic hand dynamometer.

Results

The BMI, FFM and PhA were significantly lower in A-T patients than in controls (BMI 16.56 ± 3.52 kg/m² vs. 19.86 ± 3.54 kg/m²; Z-Score: -1.24 ± 1.29 vs. 0.05 ± 0.92, p <0.001; FFM 25.4 ± 10.03 kg vs. 41.77 ± 18.25 kg, p < 0.001; PhA: 4.6 ± 0.58° vs. 6.15 ± 0.88°, p < 0.001). Manual muscle strength was significantly impaired in A-T patients compared with controls (10.65 ± 10.97 kg vs. 26.8 ± 30.39 kg, p < 0.0001). In addition, cortisol and dehydroepiandrosterone sulfate (DHEAS) levels were significantly lower in A-T patients than in controls.

Conclusion

Altered body composition, characterized by depleted BMI, PhA and BCM; by the need to sit in a wheelchair; by altered hormone levels; and by poor muscle strength, is a major factor underlying disease progression and increased fatigue in A-T patients.

Trial registration

ClinicalTrials.gov NCT02345200

ERS statement on the multidisciplinary respiratory management of ataxia telangiectasia

Ataxia telangiectasia (A-T) is a rare, progressive, multisystem disease that has a large number of complex and diverse manifestations which vary with age. Patients with A-T die prematurely with the leading causes of death being respiratory diseases and cancer. Respiratory manifestations include immune dysfunction leading to recurrent upper and lower respiratory infections; aspiration resulting from dysfunctional swallowing due to neurodegenerative deficits; inefficient cough; and interstitial lung disease/pulmonary fibrosis. Malnutrition is a significant comorbidity. The increased radiosensitivity and increased risk of cancer should be borne in mind when requesting radiological investigations. Aggressive proactive monitoring and treatment of these various aspects of lung disease under multidisciplinary expertise in the experience of national multidisciplinary clinics internationally forms the basis of this statement on the management of lung disease in A-T. Neurological management is outwith the scope of this document.

Mesenchymal stem cell aging: Mechanisms and influences on skeletal and non-skeletal tissues

The aging population and the incidence of aging-related diseases such as osteoporosis are on the rise. Aging at the tissue and organ levels usually involves tissue stem cells. Human and animal model studies indicate that aging affects two aspects of mesenchymal stem cell (MSC): a decrease in the bone marrow MSC pool and biased differentiation into adipocyte at the cost of osteoblast, which underlie the etiology of osteoporosis. Aging of MSC cells is also detrimental to some non-skeletal tissues, in particular the hematopoietic system, where MSCs serve as a niche component. In addition, aging compromises the therapeutic potentials of MSC cells, including cells isolated from aged individuals or cells cultured for many passages. Here we discuss the recent progress on our understanding of MSC aging, with a focus on the effects of MSC aging on bone remodeling and hematopoiesis and the mechanisms of MSC aging.

An unconventional role of BMP-Smad1 signaling in DNA damage response: a mechanism for tumor suppression

The genome is under constant attack by self-produced reactive oxygen species and genotoxic reagents in the environment. Cells have evolved a DNA damage response (DDR) system to sense DNA damage, to halt cell cycle progression and repair the lesions, or to induce apoptosis if encountering irreparable damage. The best studied DDR pathways are the PIKK-p53 and PIKK-Chk1/2. Mutations in these genes encoding DDR molecules usually lead to genome instability and tumorigenesis. It is worth noting that there exist unconventional pathways that facilitate the canonical pathways or take over in the absence of the canonical pathways in DDR. This review will summarize on several unconventional pathways that participate in DDR with an emphasis on the BMP-Smad1 pathway, a known regulator of mouse development and bone remodeling.

Checkpoint kinase Chk2 controls renal Cyp27b1 expression, calcitriol formation, and calcium-phosphate metabolism

Checkpoint kinase 2 (Chk2) is the main effector kinase of ataxia telangiectasia mutated (ATM) and responsible for cell cycle regulation. ATM signaling has been shown to upregulate interferon-regulating factor-1 (IRF-1), a transcription factor also expressed in the kidney. Calcitriol (1,25 (OH)2D3), a major regulator of mineral metabolism, is generated by 25-hydroxyvitamin D 1α-hydroxylase in the kidney. Since 25-hydroxyvitamin D 1α-hydroxylase expression is enhanced by IRF-1, the present study explored the role of Chk2 for calcitriol formation and mineral metabolism. Chk2-deficient mice (chk2 (-/-)) were compared to wild-type mice (chk2 (+/+)). Transcript levels of renal 25-hydroxyvitamin D 1α-hydroxylase, Chk2, and IRF-1 were determined by RT-PCR; Klotho expression by Western blotting; bone density by μCT analysis; serum or plasma 1,25 (OH)2D3, PTH, and C-terminal FGF23 concentrations by immunoassays; and serum, fecal, and urinary calcium and phosphate concentrations by photometry. The renal expression of IRF-1 and 25-hydroxyvitamin D 1α-hydroxylase as well as serum 1,25 (OH)2D3 and FGF23 levels were significantly lower in chk2 (-/-) mice compared to chk2 (+/+) mice. Plasma PTH was not different between the genotypes. Renal calcium and phosphate excretion were significantly higher in chk2 (-/-) mice than in chk2 (+/+) mice despite hypophosphatemia and normocalcemia. Bone density was not different between the genotypes. We conclude that Chk2 regulates renal 25-hydroxyvitamin D 1α-hydroxylase expression thereby impacting on calcium and phosphate metabolism.

Translational initiation regulated by ATM in dendritic cells development

Ataxia telangiectasia mutated (ATM) protein has been implicated in multiple pathways such as DNA repair, cell cycle checkpoint, cell growth, development, and stem cell renewal. In this study, we demonstrate evidence that ATM is involved in granulocyte macrophage colony-stimulating factor (GM-CSF)-induced dendritic cell (DC) development from bone marrow (BM) cells. Inactivation of ATM protein results in decreased BM proliferation, leading to reduced DC development and their activity for T cell activation. Expression of Jak2, STAT5, and mTOR is suppressed in both wild-type and ATM-null BM prior to GM-CSF stimulation. Activation of those proteins is delayed and prolonged hypophosphorylation of 4EBP1 is observed in ATM-null BM when treated with GM-CSF, although Erk and p38 are similarly expressed and activated in both wild-type and ATM-null BM cell types. Akt is also suppressed in wild-type BM, and transduction of constitutively active Akt or STAT5 in ATM-null BM restores DC development. Together, these results illustrate that ATM deficiency causes impaired initiation of protein translation in BM, leading to immature development of DC.

BRAT1 deficiency causes increased glucose metabolism and mitochondrial malfunction

Background

BRAT1 (BRCA1-associated ATM activator 1) interacts with both BRCA1, ATM and DNA-PKcs, and has been implicated in DNA damage responses. However, based on our previous results, it has been shown that BRAT1 may be involved in cell growth and apoptosis, besides DNA damage responses, implying that there are undiscovered functions for BRAT1.

Methods

Using RNA interference against human BRAT1, we generated stable BRAT1 knockdown cancer cell lines of U2OS, Hela, and MDA-MA-231. We tested cell growth properties and in vitro/in vivo tumorigenic potentials of BRAT1 knockdown cells compared to control cells. To test if loss of BRAT1 induces metabolic abnormalities, we examined the rate of glycolysis, ATP production, and PDH activity in both BRAT1 knockdown and control cells. The role of BRAT1 in growth signaling was determined by the activation of Akt/Erk, and SC79, Akt activator was used for validation.

Results

By taking advantage of BRAT1 knockdown cancer cell lines, we found that loss of BRAT1 expression significantly decreases cell proliferation and tumorigenecity both in vitro and in vivo. Cell migration was also remarkably lowered when BRAT1 was depleted. Interestingly, glucose uptake and production of mitochondrial ROS (reactive oxygen species) are highly increased in BRAT1 knockdown HeLa cells. Furthermore, both basal and induced activity of Akt and Erk kinases were suppressed in these cells, implicating abnormality in signaling cascades for cellular growth. Consequently, treatment of BRAT1 knockdown cells with Akt activator can improve their proliferation and reduces mitochondrial ROS concentration.

Conclusions

These findings suggest novel roles of BRAT1 in cell proliferation and mitochondrial functions.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-548) contains supplementary material, which is available to authorized users.

Mouse models of telomere dysfunction phenocopy skeletal changes found in human age-related osteoporosis

A major medical challenge in the elderly is osteoporosis and the high risk of fracture. Telomere dysfunction is a cause of cellular senescence and telomere shortening, which occurs with age in cells from most human tissues, including bone. Telomere defects contribute to the pathogenesis of two progeroid disorders characterized by premature osteoporosis, Werner syndrome and dyskeratosis congenital. It is hypothesized that telomere shortening contributes to bone aging. We evaluated the skeletal phenotypes of mice with disrupted telomere maintenance mechanisms as models for human bone aging, including mutants in Werner helicase (Wrn^−/−), telomerase (Terc^−/−) and Wrn^−/−Terc^−/− double mutants. Compared with young wild-type (WT) mice, micro-computerized tomography analysis revealed that young Terc^−/− and Wrn^−/−Terc^−/− mice have decreased trabecular bone volume, trabecular number and trabecular thickness, as well as increased trabecular spacing. In cortical bone, young Terc^−/− and Wrn^−/−Terc^−/− mice have increased cortical thinning, and increased porosity relative to age-matched WT mice. These trabecular and cortical changes were accelerated with age in Terc^−/− and Wrn^−/−Terc^−/− mice compared with older WT mice. Histological quantification of osteoblasts in aged mice showed a similar number of osteoblasts in all genotypes; however, significant decreases in osteoid, mineralization surface, mineral apposition rate and bone formation rate in older Terc^−/− and Wrn^−/−Terc^−/− bone suggest that osteoblast dysfunction is a prominent feature of precocious aging in these mice. Except in the Wrn^−/− single mutant, osteoclast number did not increase in any genotype. Significant alterations in mechanical parameters (structure model index, degree of anistrophy and moment of inertia) of the Terc^−/− and Wrn^−/−Terc^−/− femurs compared with WT mice were also observed. Young Wrn^−/−Terc^−/− mice had a statistically significant increase in bone-marrow fat content compared with young WT mice, which remained elevated in aged double mutants. Taken together, our results suggest that Terc^−/− and Wrn^−/−Terc^−/− mutants recapitulate the human bone aging phenotype and are useful models for studying age-related osteoporosis.

Mechanisms of vascular calcification in CKD-evidence for premature ageing?

Ageing is a potent, independent risk factor for cardiovascular disease. Calcification of the vascular smooth muscle cell (VSMC) layer of the vessel media is a hallmark of vascular ageing. Young patients with chronic kidney disease (CKD) exhibit an extremely high cardiovascular mortality, equivalent to that seen in octogenarians in the general population. Even children on dialysis develop accelerated medial vascular calcification and arterial stiffening, leading to the suggestion that patients with CKD exhibit a 'premature ageing' phenotype. It is now well documented that uraemic toxins, particularly those associated with dysregulated mineral metabolism, can drive VSMC damage and phenotypic changes that promote vascular calcification; epidemiological data suggest that some of these same risk factors associate with cardiovascular mortality in the aged general population. Importantly, emerging evidence suggests that uraemic toxins may promote DNA damage, a key factor driving cellular ageing, and moreover, that these ageing mechanisms may reiterate some of those seen in patients with genetically induced progeric syndromes caused by nuclear lamina disruption. This new knowledge should pave the way for the development of novel therapies that target tissue-specific ageing mechanisms to treat vascular decline in CKD.

Prelamin A accelerates vascular calcification via activation of the DNA damage response and senescence-associated secretory phenotype in vascular smooth muscle cells

RATIONALE

Vascular calcification is prevalent in the aging population, yet little is known of the mechanisms driving age-associated vascular smooth muscle cell (VSMC) phenotypic change.

OBJECTIVE

To investigate the role of nuclear lamina disruption, a specific hallmark of VSMC aging, in driving VSMC osteogenic differentiation.

METHODS AND RESULTS

Prelamin A, the unprocessed form of the nuclear lamina protein lamin A, accumulated in calcifying human VSMCs in vitro and in vivo, and its overexpression promoted VSMC osteogenic differentiation and mineralization. During VSMC aging in vitro, prelamin A accumulation occurred concomitantly with increased p16 expression and osteogenic differentiation and was associated with increased levels of DNA damage. Microarray analysis showed that DNA damage repair pathways were significantly impaired in VSMCs expressing prelamin A and that chemical inhibition and siRNA depletion of the DNA damage response kinases ataxia-telangiectasia mutated/ataxia-telangiectasia- and Rad3-related effectively blocked VSMC osteogenic differentiation and mineralization. In coculture experiments, prelamin A-expressing VSMCs induced alkaline phosphatase activity in mesenchymal progenitor cells, and this was abrogated by inhibition of ataxia-telangiectasia-mutated signaling, suggesting that DNA damage induces the secretion of pro-osteogenic factors by VSMCs. Cytokine array analysis identified several ataxia-telangiectasia mutated-dependent senescence-associated secretory phenotype factors/cytokines released by prelamin A-positive VSMCs, including the calcification regulators bone morphogenetic protein 2, osteoprotegerin, and interleukin 6.

CONCLUSIONS

Prelamin A promotes VSMC calcification and aging by inducing persistent DNA damage signaling, which acts upstream of VSMC osteogenic differentiation and the senescence-associated secretory phenotype. Agents that target the DNA damage response and prelamin A toxicity may be potential therapies for the treatment of vascular calcification.

Correlation analyses of clinical and molecular findings identify candidate biological pathways in systemic juvenile idiopathic arthritis

BACKGROUND

Clinicians have long appreciated the distinct phenotype of systemic juvenile idiopathic arthritis (SJIA) compared to polyarticular juvenile idiopathic arthritis (POLY). We hypothesized that gene expression profiles of peripheral blood mononuclear cells (PBMC) from children with each disease would reveal distinct biological pathways when analyzed for significant associations with elevations in two markers of JIA activity, erythrocyte sedimentation rate (ESR) and number of affected joints (joint count, JC).

METHODS

PBMC RNA from SJIA and POLY patients was profiled by kinetic PCR to analyze expression of 181 genes, selected for relevance to immune response pathways. Pearson correlation and Student's t-test analyses were performed to identify transcripts significantly associated with clinical parameters (ESR and JC) in SJIA or POLY samples. These transcripts were used to find related biological pathways.

RESULTS

Combining Pearson and t-test analyses, we found 91 ESR-related and 92 JC-related genes in SJIA. For POLY, 20 ESR-related and 0 JC-related genes were found. Using Ingenuity Systems Pathways Analysis, we identified SJIA ESR-related and JC-related pathways. The two sets of pathways are strongly correlated. In contrast, there is a weaker correlation between SJIA and POLY ESR-related pathways. Notably, distinct biological processes were found to correlate with JC in samples from the earlier systemic plus arthritic phase (SAF) of SJIA compared to samples from the later arthritis-predominant phase (AF). Within the SJIA SAF group, IL-10 expression was related to JC, whereas lack of IL-4 appeared to characterize the chronic arthritis (AF) subgroup.

CONCLUSIONS

The strong correlation between pathways implicated in elevations of both ESR and JC in SJIA argues that the systemic and arthritic components of the disease are related mechanistically. Inflammatory pathways in SJIA are distinct from those in POLY course JIA, consistent with differences in clinically appreciated target organs. The limited number of ESR-related SJIA genes that also are associated with elevations of ESR in POLY implies that the SJIA associations are specific for SJIA, at least to some degree. The distinct pathways associated with arthritis in early and late SJIA raise the possibility that different immunobiology underlies arthritis over the course of SJIA.

A crucial role for bone morphogenetic protein-Smad1 signalling in the DNA damage response

DNA damage and the elicited cellular response underlie the etiology of tumorigenesis and ageing. Yet, how this response integrates inputs from cells' environmental cues remains underexplored. Here we report that the BMP-Smad1 pathway, which is essential for embryonic development and tissue homeostasis, has an important role in the DNA damage response and oncogenesis. On genotoxic stress, Atm phosphorylates BMPs-activated Smad1 in the nucleus on S239, which disrupts Smad1 interaction with protein phosphatase PPM1A, leading to enhanced activation and upregulation of Smad1. Smad1 then interacts with p53 and inhibits Mdm2-mediated p53 ubiquitination and degradation to regulate cell proliferation and survival. Enhanced Smad1 S239 phosphorylation, and Smad1 mutations causing S239 substitution were detected in oesophageal and gastric cancer samples, respectively. These findings suggest that BMP-Smad1 signalling participates in the DNA damage response via the Atm-p53 pathway, thus providing a molecular mechanism whereby BMP-Smad1 loss-of-function leads to tumorigenesis, for example, juvenile polyposis and Cowden syndromes.

Senescence-associated intrinsic mechanisms of osteoblast dysfunctions

Human aging is associated with bone loss leading to bone fragility and increased risk of fractures. The cellular and molecular causes of age-related bone loss are current intensive topic of investigation with the aim of identifying new approaches to abolish its negative effects on the skeleton. Age-related osteoblast dysfunction is the main cause of age-related bone loss in both men and women beyond the fifth decade and results from two groups of pathogenic mechanisms: extrinsic mechanisms that are mediated by age-related changes in bone microenvironment including changes in levels of hormones and growth factors, and intrinsic mechanisms caused by the osteoblast cellular senescence. The aim of this review is to provide a summary of the intrinsic senescence mechanisms affecting osteoblastic functions and how they can be targeted to abolish age-related osteoblastic dysfunction and bone loss associated with aging.

A positive role for c-Abl in Atm and Atr activation in DNA damage response

DNA damage triggers Atm- and/or Atr-dependent signaling pathways to control cell cycle progression, apoptosis, and DNA repair. However, how Atm and Atr are activated is not fully understood. One of the downstream targets of Atm is non-receptor tyrosine kinase c-Abl, which is phosphorylated and activated by Atm. The current view is that c-Abl relays pro-apoptotic signals from Atm to p73 and p53. Here we show that c-Abl deficiency resulted in a broad spectrum of defects in cell response to genotoxic stress, including activation of Chk1 and Chk2, activation of p53, nuclear foci formation, apoptosis, and DNA repair, suggesting that c-Abl might also act upstream of the DNA damage-activated signaling cascades in addition to its role in p73 and p53 regulation. Indeed, we found that c-Abl is required for proper activation of both Atm and Atr. c-Abl is bound to the chromatin and shows enhanced interaction with Atm and Atr in response to DNA damage. c-Abl can phosphorylate Atr on Y291 and Y310 and this phosphorylation appears to have a positive role in Atr activation under genotoxic stress. These findings suggest that Atm-mediated c-Abl activation in cell response to double-stranded DNA breaks might facilitate the activation of both Atm and Atr to regulate their downstream cellular events.

Protein palmitoylation regulates osteoblast differentiation through BMP-induced osterix expression

Osteoporosis is one of the most common diseases and can be treated by either anti-resorption drugs, anabolic drugs, or both. To search for anabolic drug targets for osteoporosis therapy, it is crucial to understand the biology of bone forming cells, osteoblasts, in terms of their proliferation, differentiation, and function. Here we found that protein palmitoylation participates in signaling pathways that control osterix expression and osteoblast differentiation. Mouse calvarial osteoblasts express most of the 24 palmitoyl transferases, with some being up-regulated during differentiation. Inhibition of protein palmitoylation, with a substrate-analog inhibitor, diminished osteoblast differentiation and mineralization, but not proliferation or survival. The decrease in differentiation capacity is associated with a reduction in osterix, but not Runx2 or Atf4. Inhibition of palmitoyl transferases had little effect in p53(-/-) osteoblasts that show accelerated differentiation due to overexpression of osterix, suggesting that osterix, at least partially, mediated the effect of inhibition of palmitoyl transferases on osteoblast differentiation. BMPs are the major driving force of osteoblast differentiation in the differentiation assays. We found that inhibition of palmitoyl transferases also compromised BMP2-induced osteoblast differentiation through down-regulating osterix induction. However, palmitoyl transferases inhibitor did not inhibit Smad1/5/8 activation. Instead, it compromised the activation of p38 MAPK, which are known positive regulators of osterix expression and differentiation. These results indicate that protein palmitoylation plays an important role in BMP-induced MAPK activation, osterix expression, and osteoblast differentiation.

Antisense inhibition of ATM gene enhances the radiosensitivity of head and neck squamous cell carcinoma in mice

Background

Treatment failure after radiotherapy of head and neck squamous cell carcinoma (HNSCC) could be a significant problem. Our objective is to sensitize SCCVII cells to ionizing radiation in vitro and in vivo through inhibiting ATM expression using antisense oligodeoxynucleotides (AS-ODNs), and investigate the potential mechanism of radiosensitization.

Methods

We designed and synthesized AS-ODNs that target ATM mRNA to reduce the ATM expression. The influence on the expression of ATM mRNA and protein in SCCVII cells were analysed by real-time quantitative PCR and western blotting respectively. Clonogenic survival assay was performed to detect the survival ability of SCCVII cells after irradiation, while flow cytometry used to analyse the cell cycle and apoptosis. The volume of solid tumors generated with SCCVII cells was measured, and cell apoptosis was analysed by TUNEL assay after irradiation.

Results

The relative ATM mRNA and protein expression in SCCVII cells treated with ATM AS-ODNs were decreased to 25.7 ± 3.1% and 24.1 ± 2.8% of that in untreated cells respectively (P < 0.05). After irradiation, the survival fraction (SF) of cells treated with ATM AS-ODNs was lower than that of other groups at the same dose of radiation (P < 0.05), while the percentage of cells in G2/M phase decreased and apoptotic rate of cells increased(P < 0.05). The inhibition rate in SCCVII cells solid tumor exposed to X-ray alone was 23.2 ± 2.7%, while it was 56.1 ± 3.8% in the group which irradiated in combination with the treatment of ATM AS-ODNs (P < 0.05). The apoptotic index for the group irradiated in combination with ATM AS-ODNs injection was 19.6 ± 3.2, which was significantly higher than that of others (P < 0.05)

Conclusion

Inhibition of ATM expression sensitized SCCVII cells to ionizing radiation in vitro and in vivo. The potential mechanism should be the defective G2/M cell cycle checkpoint control and enhanced radiation-induced apoptosis.

Osteolineage niche cells initiate hematopoietic stem cell mobilization

Recent studies have implicated bone-lining osteoblasts as important regulators of hematopoietic stem cell (HSC) self-renewal and differentiation; however, because much of the evidence supporting this notion derives from indirect in vivo experiments, which are unavoidably complicated by the presence of other cell types within the complex bone marrow milieu, the sufficiency of osteoblasts in modulating HSC activity has remained controversial. To address this, we prospectively isolated mouse osteoblasts, using a novel flow cytometry-based approach, and directly tested their activity as HSC niche cells and their role in cyclophosphamide/granulocyte colony-stimulating factor (G-CSF)-induced HSC proliferation and mobilization. We found that osteoblasts expand rapidly after cyclophosphamide/G-CSF treatment and exhibit phenotypic and functional changes that directly influence HSC proliferation and maintenance of reconstituting potential. Effects of mobilization on osteoblast number and function depend on the function of ataxia telangiectasia mutated (ATM), the product of the Atm gene, demonstrating a new role for ATM in stem cell niche activity. These studies demonstrate that signals from osteoblasts can directly initiate and modulate HSC proliferation in the context of mobilization. This work also establishes that direct interaction with osteolineage niche cells, in the absence of additional environmental inputs, is sufficient to modulate stem cell activity.

Rb regulates interactions between hematopoietic stem cells and their bone marrow microenvironment

Hematopoiesis is maintained by stem cells (HSCs) that undergo fate decisions by integrating intrinsic and extrinsic signals, with the latter derived from the bone marrow (BM) microenvironment. Cell-cycle regulation can modulate stem cell fate, but it is unknown whether this represents an intrinsic or extrinsic effector of fate decisions. We have investigated the role of the retinoblastoma protein (RB), a central regulator of the cell cycle, in hematopoiesis. Widespread inactivation of RB in the murine hematopoietic system resulted in profound myeloproliferation. HSCs were lost from the BM due to mobilization to extramedullary sites and differentiation. This phenotype was not intrinsic to HSCs, but, rather, was the consequence of an RB-dependent interaction between myeloid-derived cells and the microenvironment. These findings demonstrate that myeloproliferation may result from perturbed interactions between hematopoietic cells and the niche. Therefore, RB extrinsically regulates HSCs by maintaining the capacity of the BM to support normal hematopoiesis and HSCs.