The Role of Aryl Hydrocarbon Receptor in Bone Biology

Aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, is crucial in maintaining the skeletal system. Our study focuses on encapsulating the role of AhR in bone biology and identifying novel signaling pathways in musculoskeletal pathologies using the GEO dataset. The GEO2R analysis identified 8 genes (CYP1C1, SULT6B1, CYB5A, EDN1, CXCR4B, CTGFA, TIPARP, and CXXC5A) involved in the AhR pathway, which play a pivotal role in bone remodeling. The AhR knockout in hematopoietic stem cells showed alteration in several novel bone-related transcriptomes (eg, Defb14, ZNF 51, and Chrm5). Gene Ontology Enrichment Analysis demonstrated 54 different biological processes associated with bone homeostasis. Mainly, these processes include bone morphogenesis, bone development, bone trabeculae formation, bone resorption, bone maturation, bone mineralization, and bone marrow development. Employing Functional Annotation and Clustering through DAVID, we further uncovered the involvement of the xenobiotic metabolic process, p450 pathway, oxidation-reduction, and nitric oxide biosynthesis process in the AhR signaling pathway. The conflicting evidence of current research of AhR signaling on bone (positive and negative effects) homeostasis may be due to variations in ligand binding affinity, binding sites, half-life, chemical structure, and other unknown factors. In summary, our study provides a comprehensive understanding of the underlying mechanisms of the AhR pathway in bone biology.

Inhibiting MicroRNA-141-3p Improves Musculoskeletal Health in Aged Mice

Emerging evidence shows that the microRNA-141-3p is involved in various age-related pathologies. Previously, our group and others reported elevated levels of miR-141-3p in several tissues and organs with age. Here, we inhibited the expression of miR-141-3p using antagomir (Anti-miR-141-3p) in aged mice and explored its role in healthy aging. We analyzed serum (cytokine profiling), spleen (immune profiling), and overall musculoskeletal phenotype. We found decreased levels of pro-inflammatory cytokines (such as TNF-α, IL-1β, IFN-γ) in serum with Anti-miR-141-3p treatment. The flow-cytometry analysis on splenocytes revealed decreased M1 (pro-inflammatory) and increased M2 (anti-inflammatory) populations. We also found improved bone microstructure and muscle fiber size with Anti-miR-141-3p treatment. Molecular analysis revealed that miR-141-3p regulates the expression of AU-rich RNA-binding factor 1 (AUF1) and promotes senescence (p21, p16) and pro-inflammatory (TNF-α, IL-1β, IFN-γ) environment whereas inhibiting miR-141-3p prevents these effects. Furthermore, we demonstrated that the expression of FOXO-1 transcription factor was reduced with Anti-miR-141-3p and elevated with silencing of AUF1 (siRNA-AUF1), suggesting crosstalk between miR-141-3p and FOXO-1. Overall, our proof-of-concept study demonstrates that inhibiting miR-141-3p could be a potential strategy to improve immune, bone, and muscle health with age.

Circulating Osteoprogenitor Cells Have a Mixed Immune and Mesenchymal Progenitor Function in Humans

BACKGROUND

Circulating osteoprogenitors (COP) are a population of cells in the peripheral circulation that possess functional and phenotypical characteristics of multipotent stromal cells (MSCs). This population has a solid potential to become an abundant, accessible, and replenishable source of MSCs with multiple potential clinical applications. However, a comprehensive functional characterization of COP cells is still required to test and fully develop their use in clinical settings.

METHODS

This study characterized COP cells by comparing them to bone marrow-derived MSCs (BM-MSCs) and adipose-derived MSCs (ASCs) through detailed transcriptomic and proteomic analyses.

RESULTS

We demonstrate that COP cells have a distinct gene and protein expression pattern with a significantly stronger immune footprint, likely owing to their hematopoietic lineage. In addition, regarding progenitor cell differentiation and proliferation pathways, COP cells have a similar expression pattern to BM-MSCs and ASCs.

CONCLUSION

COP cells are a unique but functionally similar population to BM-MSCs and ASCs, sharing their proliferation and differentiation capacity, thus presenting an accessible source of MSCs with strong potential for translational regenerative medicine strategies.

Orchiectomy sensitizes cortical bone in male mice to the harmful effects of kynurenine

Kynurenine (Kyn) is a tryptophan metabolite that increases with age and promotes musculoskeletal dysfunction. We previously found a sexually dimorphic pattern in how Kyn affects bone, with harmful effects more prevalent in females than males. This raises the possibility that male sex steroids might exert a protective effect that blunts the effects of Kyn in males. To test this, orchiectomy (ORX) or sham surgeries were performed on 6-month-old C57BL/6 mice, after which mice received Kyn (10 mg/kg) or vehicle via intraperitoneal injection, once daily, 5×/week, for four weeks. Bone histomorphometry, DXA, microCT, and serum marker analyses were performed after sacrifice. In vitro studies were performed to specifically test the effect of testosterone on activation of aryl hydrocarbon receptor (AhR)-mediated signaling by Kyn in mesenchymal-lineage cells. Kyn treatment reduced cortical bone mass in ORX- but not sham-operated mice. Trabecular bone was unaffected. Kyn's effects on cortical bone in ORX mice were attributed primarily to enhanced endosteal bone resorption activity. Bone marrow adipose tissue was increased in Kyn-treated ORX animals but was unchanged by Kyn in sham-operated mice. ORX surgery increased mRNA expression of the aryl hydrocarbon receptor (AhR) and its target gene Cyp1a1 in the bone, suggesting a priming and/or amplification of AhR signaling pathways. Mechanistic in vitro studies revealed that testosterone blunted Kyn-stimulated AhR transcriptional activity and Cyp1a1 expression in mesenchymal-linage cells. These data suggest a protective role for male sex steroids in blunting the harmful effects of Kyn in cortical bone. Therefore, testosterone may play an important role in regulating Kyn/AhR signaling in musculoskeletal tissues, suggesting crosstalk between male sex steroids and Kyn signaling may influence age-associated musculoskeletal frailty.

DPP4-Truncated CXCL12 Alters CXCR4/ACKR3 Signaling, Osteogenic Cell Differentiation, Migration, and Senescence

Bone marrow skeletal stem cells (SSCs) secrete many cytokines including stromal derived factor-1 or CXCL12, which influences cell proliferation, migration, and differentiation. All CXCL12 splice variants are rapidly truncated on their N-terminus by dipeptidyl peptidase 4 (DPP4). This includes the common variant CXCL12 alpha (1-68) releasing a much less studied metabolite CXCL12(3-68). Here, we found that CXCL12(3-68) significantly inhibited SSC osteogenic differentiation and RAW-264.7 cell osteoclastogenic differentiation and induced a senescent phenotype in SSCs. Importantly, pre-incubation of SSCs with CXCL12(3-68) significantly diminished their ability to migrate toward CXCL12(1-68) in transwell migration assays. Using a high-throughput G-protein-coupled receptor (GPCR) screen (GPCRome) and bioluminescent resonance energy transfer molecular interaction assays, we revealed that CXCL12(3-68) acts via the atypical cytokine receptor 3-mediated β-arrestin recruitment and as a competitive antagonist to CXCR4-mediated signaling. Finally, a reverse phase protein array assay revealed that DPP4-cleaved CXCL12 possesses a different downstream signaling profile from that of intact CXCL12 or controls. The data presented herein provides insights into regulation of CXCL12 signaling. Importantly, it demonstrates that DPP4 proteolysis of CXCL12 generates a metabolite with significantly different and previously overlooked bioactivity that helps explain discrepancies in the literature. This also contributes to an understanding of the molecular mechanisms of osteoporosis and bone fracture repair and could potentially significantly affect the interpretation of experimental outcomes with clinical consequences in other fields where CXCL12 is vital, including cancer biology, immunology, cardiovascular biology, neurobiology, and associated pathologies.

Minocycline-induced disruption of the intestinal FXR/FGF15 axis impairs osteogenesis in mice

Antibiotic-induced shifts in the indigenous gut microbiota influence normal skeletal maturation. Current theory implies that gut microbiota actions on bone occur through a direct gut/bone signaling axis. However, our prior work supports that a gut/liver signaling axis contributes to gut microbiota effects on bone. Our purpose was to investigate the effects of minocycline, a systemic antibiotic treatment for adolescent acne, on pubertal/postpubertal skeletal maturation. Sex-matched specific pathogen-free (SPF) and germ-free (GF) C57BL/6T mice were administered a clinically relevant minocycline dose from age 6-12 weeks. Minocycline caused dysbiotic shifts in the gut bacteriome and impaired skeletal maturation in SPF mice but did not alter the skeletal phenotype in GF mice. Minocycline administration in SPF mice disrupted the intestinal farnesoid X receptor/fibroblast growth factor 15 axis, a gut/liver endocrine axis supporting systemic bile acid homeostasis. Minocycline-treated SPF mice had increased serum conjugated bile acids that were farnesoid X receptor (FXR) antagonists, suppressed osteoblast function, decreased bone mass, and impaired bone microarchitecture and fracture resistance. Stimulating osteoblasts with the serum bile acid profile from minocycline-treated SPF mice recapitulated the suppressed osteogenic phenotype found in vivo, which was mediated through attenuated FXR signaling. This work introduces bile acids as a potentially novel mediator of gut/liver signaling actions contributing to gut microbiota effects on bone.

Upregulation of the EGFR/MEK1/MAPK1/2 signaling axis as a mechanism of resistance to antiestrogen‑induced BimEL dependent apoptosis in ER+ breast cancer cells

The epidermal growth factor receptor (EGFR) is commonly upregulated in multiple cancer types, including breast cancer. In the present study, evidence is provided in support of the premise that upregulation of the EGFR/MEK1/MAPK1/2 signaling axis during antiestrogen treatment facilitates the escape of breast cancer cells from BimEL‑dependent apoptosis, conferring resistance to therapy. This conclusion is based on the findings that ectopic BimEL cDNA overexpression and confocal imaging studies confirm the pro‑apoptotic role of BimEL in ERα expressing breast cancer cells and that upregulated EGFR/MEK1/MAPK1/2 signaling blocks BimEL pro‑apoptotic action in an antiestrogen‑resistant breast cancer cell model. In addition, the present study identified a pro‑survival role for autophagy in antiestrogen resistance while EGFR inhibitor studies demonstrated that a significant percentage of antiestrogen‑resistant breast cancer cells survive EGFR targeting by pro‑survival autophagy. These pre‑clinical studies establish the possibility that targeting both the MEK1/MAPK1/2 signaling axis and pro‑survival autophagy may be required to eradicate breast cancer cell survival and prevent the development of antiestrogen resistance following hormone treatments. The present study uniquely identified EGFR upregulation as one of the mechanisms breast cancer cells utilize to evade the cytotoxic effects of antiestrogens mediated through BimEL‑dependent apoptosis.

Aryl hydrocarbon receptor (AhR)-mediated signaling as a critical regulator of skeletal cell biology

The aryl hydrocarbon receptor (AhR) has been implicated in regulating skeletal progenitor cells and the activity of bone-forming osteoblasts and bone-resorbing osteoclasts, thereby impacting bone mass and the risk of skeletal fractures. The AhR also plays an important role in the immune system within the skeletal niche and in the differentiation of mesenchymal stem cells into other cell lineages including chondrocytes and adipocytes. This transcription factor responds to environmental pollutants which can act as AhR ligands, initiating or interfering with various signaling cascades to mediate downstream effects, and also responds to endogenous ligands including tryptophan metabolites. This review comprehensively describes the reported roles of the AhR in skeletal cell biology, focusing on mesenchymal stem cells, osteoblasts, and osteoclasts, and discusses how AhR exhibits sexually dimorphic effects in bone. The molecular mechanisms mediating AhR's downstream effects are highlighted to emphasize the potential importance of targeting this signaling cascade in skeletal disorders.

Alterations in bone metabolites with age in C57BL/6 mice model

Understanding the pathophysiology behind age-related diseases is an urgent need as the elderly population continues to grow. With age, there is a high risk of musculoskeletal deterioration and associated morbidity and mortality. Although the exact mechanism behind age-related degeneration is unknown, it is well established that alteration in cellular metabolism is one of the important contributing factors. Alteration in signaling pathways with age leads to the accumulation or depletion of several metabolites that play a vital role in musculoskeletal pathophysiology. This study aimed to identify age-related changes in bone tissue metabolites in C57BL/6 mice. We then correlated the differentially expressed metabolites with their functions in bone biology. In both aged males and females, hydroxyproline, glutamine, and alpha-linolenic acid levels were decreased. In aged females, Ornithine (p value = 0.001), L-Proline (p value = 0.008), Uridine (p value = 0.001), Aspartic Acid (p value = 0.004) levels were significantly decreased, and glutamate (p value = 0.002) was elevated. In aged males, N-acetyl-D-glucosamine (pvalue = 0.010), Adrenic acid (pvalue = 0.0099), Arachidonic acid (p value = 0.029) and Allantoin (p value = 0.004) levels were decreased. Metabolic pathway analysis revealed that purine and D-glutamine and D-glutamate metabolism were significantly altered in both sexes, while arginine biosynthesis in females and lipid metabolism in males were highly affected. These differences in metabolic signaling might be one of the reasons for the discrepancy in musculoskeletal disease manifestation between the two sexes. Understanding the role of these metabolites play in the aging bone will allow for new sex-specific targeted therapies against the progression of musculoskeletal diseases.

Long Non-coding RNA MALAT1 Is Depleted With Age in Skeletal Muscle in vivo and MALAT1 Silencing Increases Expression of TGF-β1 in vitro

Long non-coding RNAs (lncRNAs) are thought to function as “sponges” for microRNAs, but a role for such competing endogenous RNAs (ceRNAs) in muscle aging is not well understood. We therefore examined in skeletal muscles of young (4–6 months) and aged (22–24) male and female mice the expression of lncRNA MALAT1, which is predicted in silico to bind the senescence-associated microRNA miR-34a-5p. Results indicate a significant decrease in lncRNA MALAT1 expression in mouse skeletal muscle with age that coincides with an age-related increase in miR-34a-5p expression. In vitro studies using mouse C2C12 myoblasts demonstrate that MALAT1 silencing using siRNA increases miR-34a expression, consistent with a role for MALAT1 as an inhibitor of miR-34a-5p activity. Levels of reactive oxygen species (ROS) are known to increase in muscle with age, and so we treated C2C12 cells with hydrogen peroxide (10 and 100 μM) to examine changes in MALAT1 expression. MALAT1 expression decreased significantly with H2O2 treatment, but this effect was attenuated with p53 siRNA. Finally, miR-34a-5p is implicated in tissue fibrosis, and so we assessed the expression of TGF-β1 after MALAT1 silencing. MALAT1 siRNA significantly increased the expression of TGF-β1 in C2C12 cells. These findings suggest that age-related fibrosis and muscle atrophy mediated by ROS may result at least in part from an increase in miR-34a bioavailability resulting from a decline in miR-34a “sponging” due to ceRNA MALAT1 depletion. Crosstalk between MALAT1 and miR-34a may therefore represent a therapeutic target for improving muscle function with aging.

Age-associated changes in microRNAs affect the differentiation potential of human mesenchymal stem cells: Novel role of miR-29b-1-5p expression

Age-associated osteoporosis is widely accepted as involving the disruption of osteogenic stem cell populations and their functioning. Maintenance of the local bone marrow (BM) microenvironment is critical for regulating proliferation and differentiation of the multipotent BM mesenchymal stromal/stem cell (BMSC) population with age. The potential role of microRNAs (miRNAs) in modulating BMSCs and the BM microenvironment has recently gained attention. However, miRNAs expressed in rapidly isolated BMSCs that are naïve to the non-physiologic standard tissue culture conditions and reflect a more accurate in vivo profile have not yet been reported. Here we directly isolated CD271 positive (+) BMSCs within hours from human surgical BM aspirates without culturing and performed microarray analysis to identify the age-associated changes in BMSC miRNA expression. One hundred and two miRNAs showed differential expression with aging. Target prediction and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed that the up-regulated miRNAs targeting genes in bone development pathways were considerably enriched. Among the differentially up-regulated miRNAs the novel passenger strand miR-29b-1-5p was abundantly expressed as a mature functional miRNA with aging. This suggests a critical arm-switching mechanism regulates the expression of the miR-29b-1-5p/3p pair shifting the normally degraded arm, miR-29b-1-5p, to be the dominantly expressed miRNA of the pair in aging. The normal guide strand miR-29b-1-3p is known to act as a pro-osteogenic miRNA. On the other hand, overexpression of the passenger strand miR-29b-1-5p in culture-expanded CD271+ BMSCs significantly down-regulated the expression of stromal cell-derived factor 1 (CXCL12)/ C-X-C chemokine receptor type 4 (SDF-1(CXCL12)/CXCR4) axis and other osteogenic genes including bone morphogenetic protein-2 (BMP-2) and runt-related transcription factor 2 (RUNX2). In contrast, blocking of miR-29b-1-5p function using an antagomir inhibitor up-regulated expression of BMP-2 and RUNX2 genes. Functional assays confirmed that miR-29b-1-5p negatively regulates BMSC osteogenesis in vitro. These novel findings provide evidence of a pathogenic anti-osteogenic role for miR-29b-1-5p and other miRNAs in age-related defects in osteogenesis and bone regeneration.

Characterization of Differentially Expressed miRNAs by CXCL12/SDF-1 in Human Bone Marrow Stromal Cells

Stromal cell-derived factor 1 (SDF-1) is known to influence bone marrow stromal cell (BMSC) migration, osteogenic differentiation, and fracture healing. We hypothesize that SDF-1 mediates some of its effects on BMSCs through epigenetic regulation, specifically via microRNAs (miRNAs). MiRNAs are small non-coding RNAs that target specific mRNA and prevent their translation. We performed global miRNA analysis and determined several miRNAs were differentially expressed in response to SDF-1 treatment. Gene Expression Omnibus (GEO) dataset analysis showed that these miRNAs play an important role in osteogenic differentiation and fracture healing. KEGG and GO analysis indicated that SDF-1 dependent miRNAs changes affect multiple cellular pathways, including fatty acid biosynthesis, thyroid hormone signaling, and mucin-type O-glycan biosynthesis pathways. Furthermore, bioinformatics analysis showed several miRNAs target genes related to stem cell migration and differentiation. This study's findings indicated that SDF-1 induces some of its effects on BMSCs function through miRNA regulation.

Tryptophan-Kynurenine Pathway in COVID-19-Dependent Musculoskeletal Pathology: A Minireview

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19), affecting multiple organ systems, including the respiratory tract and lungs. Several studies have reported that the tryptophan-kynurenine pathway is altered in COVID-19 patients. The tryptophan-kynurenine pathway plays a vital role in regulating inflammation, metabolism, immune responses, and musculoskeletal system biology. In this minireview, we surmise the effects of the kynurenine pathway in COVID-19 patients and how this pathway might impact muscle and bone biology.

A Tryptophan-Deficient Diet Induces Gut Microbiota Dysbiosis and Increases Systemic Inflammation in Aged Mice

The gut microflora is a vital component of the gastrointestinal (GI) system that regulates local and systemic immunity, inflammatory response, the digestive system, and overall health. Older people commonly suffer from inadequate nutrition or poor diets, which could potentially alter the gut microbiota. The essential amino acid (AA) tryptophan (TRP) is a vital diet component that plays a critical role in physiological stress responses, neuropsychiatric health, oxidative systems, inflammatory responses, and GI health. The present study investigates the relationship between varied TRP diets, the gut microbiome, and inflammatory responses in an aged mouse model. We fed aged mice either a TRP-deficient (0.1%), TRP-recommended (0.2%), or high-TRP (1.25%) diet for eight weeks and observed changes in the gut bacterial environment and the inflammatory responses via cytokine analysis (IL-1a, IL-6, IL-17A, and IL-27). The mice on the TRP-deficient diets showed changes in their bacterial abundance of Coriobacteriia class, Acetatifactor genus, Lachnospiraceae family, Enterococcus faecalis species, Clostridium sp genus, and Oscillibacter genus. Further, these mice showed significant increases in IL-6, IL-17A, and IL-1a and decreased IL-27 levels. These data suggest a direct association between dietary TRP content, the gut microbiota microenvironment, and inflammatory responses in aged mice models.

Modulation of p75NTR on Mesenchymal Stem Cells Increases Their Vascular Protection in Retinal Ischemia-Reperfusion Mouse Model

Mesenchymal stem cells (MSCs) are a promising therapy to improve vascular repair, yet their role in ischemic retinopathy is not fully understood. The aim of this study is to investigate the impact of modulating the neurotrophin receptor; p75NTR on the vascular protection of MSCs in an acute model of retinal ischemia/reperfusion (I/R). Wild type (WT) and p75NTR-/- mice were subjected to I/R injury by increasing intra-ocular pressure to 120 mmHg for 45 min, followed by perfusion. Murine GFP-labeled MSCs (100,000 cells/eye) were injected intravitreally 2 days post-I/R and vascular homing was assessed 1 week later. Acellular capillaries were counted using trypsin digest 10-days post-I/R. In vitro, MSC-p75NTR was modulated either genetically using siRNA or pharmacologically using the p75NTR modulator; LM11A-31, and conditioned media were co-cultured with human retinal endothelial cells (HREs) to examine the angiogenic response. Finally, visual function in mice undergoing retinal I/R and receiving LM11A-31 was assessed by visual-clue water-maze test. I/R significantly increased the number of acellular capillaries (3.2-Fold) in WT retinas, which was partially ameliorated in p75NTR-/- retinas. GFP-MSCs were successfully incorporated and engrafted into retinal vasculature 1 week post injection and normalized the number of acellular capillaries in p75NTR-/- retinas, yet ischemic WT retinas maintained a 2-Fold increase. Silencing p75NTR on GFP-MSCs coincided with a higher number of cells homing to the ischemic WT retinal vasculature and normalized the number of acellular capillaries when compared to ischemic WT retinas receiving scrambled-GFP-MSCs. In vitro, silencing p75NTR-MSCs enhanced their secretome, as evidenced by significant increases in SDF-1, VEGF and NGF release in MSCs conditioned medium; improved paracrine angiogenic response in HREs, where HREs showed enhanced migration (1.4-Fold) and tube formation (2-Fold) compared to controls. In parallel, modulating MSCs-p75NTR using LM11A-31 resulted in a similar improvement in MSCs secretome and the enhanced paracrine angiogenic potential of HREs. Further, intervention with LM11A-31 significantly mitigated the decline in visual acuity post retinal I/R injury. In conclusion, p75NTR modulation can potentiate the therapeutic potential of MSCs to harness vascular repair in ischemic retinopathy diseases.

MicroRNAs are critical regulators of senescence and aging in mesenchymal stem cells

MicroRNAs (miRNAs) have recently come under scrutiny for their role in various age-related diseases. Similarly, cellular senescence has been linked to disease and aging. MicroRNAs and senescence likely play an intertwined role in driving these pathologic states. In this review, we present the connection between these two drivers of age-related disease concerning mesenchymal stem cells (MSCs). First, we summarize key miRNAs that are differentially expressed in MSCs and other musculoskeletal lineage cells during senescence and aging. Additionally, we also reviewed miRNAs that are regulated via traditional senescence-associated secretory phenotype (SASP) cytokines in MSC. Lastly, we summarize miRNAs that have been found to target components of the cell cycle arrest pathways inherently activated in senescence. This review attempts to highlight potential miRNA targets for regenerative medicine applications in age-related musculoskeletal disease.

Kynurenine Promotes RANKL-Induced Osteoclastogenesis In Vitro by Activating the Aryl Hydrocarbon Receptor Pathway

There is increasing evidence of the involvement of the tryptophan metabolite kynurenine (KYN) in disrupting osteogenesis and contributing to aging-related bone loss. Here, we show that KYN has an effect on bone resorption by increasing osteoclastogenesis. We have previously reported that in vivo treatment with KYN significantly increased osteoclast number lining bone surfaces. Here, we report the direct effect of KYN on receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis in Raw 264.7 macrophage cells, and we propose a potential mechanism for these KYN-mediated effects. We show that KYN/RANKL treatment results in enhancement of RANKL-induced osteoclast differentiation. KYN drives upregulation and activation of the key osteoclast transcription factors, c-fos and NFATc1 resulting in an increase in the number of multinucleated TRAP+ osteoclasts, and in hydroxyapatite bone resorptive activity. Mechanistically, the KYN receptor, aryl hydrocarbon receptor (AhR), plays an important role in the induction of osteoclastogenesis. We show that blocking AhR signaling using an AhR antagonist, or AhR siRNA, downregulates the KYN/RANKL-mediated increase in c-fos and NFATc1 and inhibits the formation of multinucleated TRAP + osteoclasts. Altogether, this work highlights that the novelty of the KYN and AhR pathways might have a potential role in helping to regulate osteoclast function with age and supports pursuing additional research to determine if they are potential therapeutic targets for the prevention or treatment of osteoporosis.

MicroRNA-141-3p Negatively Modulates SDF-1 Expression in Age-Dependent Pathophysiology of Human and Murine Bone Marrow Stromal Cells

Stromal cell-derived factor-1 (SDF-1 or CXCL12) is a cytokine secreted by cells including bone marrow stromal cells (BMSCs). SDF-1 plays a vital role in BMSC migration, survival, and differentiation. Our group previously reported the role of SDF-1 in osteogenic differentiation in vitro and bone formation in vivo; however, our understanding of the post-transcriptional regulatory mechanism of SDF-1 remains poor. MicroRNAs are small noncoding RNAs that post-transcriptionally regulate the messenger RNAs (mRNAs) of protein-coding genes. In this study, we aimed to investigate the impact of miR-141-3p on SDF-1 expression in BMSCs and its importance in the aging bone marrow (BM) microenvironment. Our data demonstrated that murine and human BMSCs expressed miR-141-3p that repressed SDF-1 gene expression at the functional level (luciferase reporter assay) by targeting the 3'-untranslated region of mRNA. We also found that transfection of miR-141-3p decreased osteogenic markers in human BMSCs. Our results demonstrate that miR-141-3p expression increases with age, while SDF-1 decreases in both the human and mouse BM niche. Taken together, these results support that miR-141-3p is a novel regulator of SDF-1 in bone cells and plays an important role in the age-dependent pathophysiology of murine and human BM niche.

Age-related increase of kynurenine enhances miR29b-1-5p to decrease both CXCL12 signaling and the epigenetic enzyme Hdac3 in bone marrow stromal cells

Mechanisms leading to age-related reductions in bone formation and subsequent osteoporosis are still incompletely understood. We recently demonstrated that kynurenine (KYN), a tryptophan metabolite, accumulates in serum of aged mice and induces bone loss. Here, we report on novel mechanisms underlying KYN's detrimental effect on bone aging. We show that KYN is increased with aging in murine bone marrow mesenchymal stem cells (BMSCs). KYN reduces bone formation via modulating levels of CXCL12 and its receptors as well as histone deacetylase 3 (Hdac3). BMSCs responded to KYN by significantly decreasing mRNA expression levels of CXCL12 and its cognate receptors, CXCR4 and ACKR3, as well as downregulating osteogenic gene RUNX2 expression, resulting in a significant inhibition in BMSCs osteogenic differentiation. KYN's effects on these targets occur by increasing regulatory miRNAs that target osteogenesis, specifically miR29b-1-5p. Thus, KYN significantly upregulated the anti-osteogenic miRNA miR29b-1-5p in BMSCs, mimicking the up-regulation of miR-29b-1-5p in human and murine BMSCs with age. Direct inhibition of miR29b-1-5p by antagomirs rescued CXCL12 protein levels downregulated by KYN, while a miR29b-1-5p mimic further decreased CXCL12 levels. KYN also significantly downregulated mRNA levels of Hdac3, a target of miR-29b-1-5p, as well as its cofactor NCoR1. KYN is a ligand for the aryl hydrocarbon receptor (AhR). We hypothesized that AhR mediates KYN's effects in BMSCs. Indeed, AhR inhibitors (CH-223191 and 3',4'-dimethoxyflavone [DMF]) partially rescued secreted CXCL12 protein levels in BMSCs treated with KYN. Importantly, we found that treatment with CXCL12, or transfection with an miR29b-1-5p antagomir, downregulated the AhR mRNA level, while transfection with miR29b-1-5p mimic significantly upregulated its level. Further, CXCL12 treatment downregulated IDO, an enzyme responsible for generating KYN. Our findings reveal novel molecular pathways involved in KYN's age-associated effects in the bone microenvironment that may be useful translational targets for treating osteoporosis.

Accumulation of kynurenine elevates oxidative stress and alters microRNA profile in human bone marrow stromal cells

Kynurenine, a metabolite of tryptophan breakdown, has been shown to increase with age, and plays a vital role in a number of age-related pathophysiological changes, including bone loss. Accumulation of kynurenine in bone marrow stromal cells (BMSCs) has been associated with a decrease in cell proliferation and differentiation, though the exact mechanism by which kynurenine mediates these changes is poorly understood. MiRNAs have been shown to regulate BMSC function, and accumulation of kynurenine may alter the miRNA expression profile of BMSCs. The aim of this study was to identify differentially expressed miRNAs in human BMSCs in response to treatment with kynurenine, and correlate miRNAs function in BMSCs biology through bioinformatics analysis. Human BMSCs were cultured and treated with and without kynurenine, and subsequent miRNA isolation was performed. MiRNA array was performed to identify differentially expressed miRNA. Microarray analysis identified 50 up-regulated, and 36 down-regulated miRNAs in kynurenine-treated BMSC cultures. Differentially expressed miRNA included miR-1281, miR-330-3p, let-7f-5p, and miR-493-5p, which are important for BMSC proliferation and differentiation. KEGG analysis found up-regulated miRNA targeting glutathione metabolism, a pathway critical for removing oxidative species. Our data support that the kynurenine dependent degenerative effect is partially due to changes in the miRNA profile of BMSCs.

Kynurenine suppresses osteoblastic cell energetics in vitro and osteoblast numbers in vivo

Aging is a progressive process associated with declining tissue function over time. Kynurenine, an oxidized metabolite of the essential amino acid tryptophan that increases in abundance with age, drives cellular processes of aging and dysfunction in many tissues, and recent work has focused on understanding the pathways involved in the harmful effects of kynurenine on bone. In this study, we sought to investigate the effects of controlled kynurenine administration on osteoblast bioenergetics, in vivo osteoblast abundance, and marrow fat accumulation. Additionally, as an extension of earlier studies with dietary administration of kynurenine, we investigated the effects of kynurenine on Hdac3 and NCoR1 expression and enzymatic deacetylase activity as potential mechanistic contributors to the effects of kynurenine on osteoblasts. Kynurenine administration suppressed cellular metabolism in osteoblasts at least in part through impaired mitochondrial respiration, and suppressed osteoblastic numbers in vivo with no concurrent effects on marrow adiposity. Deleterious effects of kynurenine treatment on osteoblasts were more pronounced in female models as compared to males. However, kynurenine treatment did not inhibit Hdac3's enzymatic deacetylase activity nor its repression of downstream glucocorticoid signaling. As such, future work will be necessary to determine the mechanisms by which increased kynurenine contributes to aging bone bioenergetics. The current study provides novel further support for the idea that kynurenine contributes to impaired osteoblastic function, and suggests that impaired matrix production by kynurenine-affected osteoblasts is attributed in part to impaired osteoblastic bioenergetics. As circulating kynurenine levels in increase with age, and human bone density inversely correlates with the serum kynurenine to tryptophan ratio, these mechanisms may have important relevance in the etiology and pathogenesis of osteoporosis in humans.

KYNURENINE, AN ENDOGENOUS AHR AGONIST, UPREGULATES CXCL12- AND HDAC3-TARGETING MIRNAS INHIBITING OSTEOGENESIS

Osteoporosis is among the most debilitating disorders for aging men and women. Mechanisms underlying loss of bone mass and impaired fracture healing in aged population are not completely understood. Our lab and others reported increased levels of the oxidized tryptophan metabolite, kynurenine (Kyn), in bone marrow mesenchymal stem cells (BMSCs) and interstitial fluid. Here, we report that Kyn significantly inhibits osteogenic differentiation of BMSCs likely via activating aryl hydrocarbon receptor (AhR). KYN significantly downregulated mRNA levels of pro-osteogenic CXCL12 axis, including CXCL12, and its two main receptors CXCR4 and ACKR3. Secreted protein levels of CXCL12 were significantly reduced with KYN, and were rescued upon the use of AhR antagonist 3’,4’-dimethoxyflavone (DMF). Moreover, KYN upregulated levels of pro-aging and CXCL12-targeting miRNAs miR-29b-1-5p and miR-141-3p. Additionally, KYN significantly inhibited mRNA and protein levels of the epigenetic enzyme Hdac3 which is responsible for activating osteogenesis and inhibiting adipogenesis in BMSCs. Using mutagenesis and luciferase assays, we show evidence that miR-141-3p and miR-29b-1-5p directly target CXCL12 3’-UTR. We also show that miR-29b-1-5p directly targets Hdac3 explaining the significantly increased acetylation levels of H4 with KYN treatment. Finally, we show that mRNA levels of both KYN receptor AhR, and KYN-producing enzyme IDO-1 are regulated by CXCL12. We believe this data explains a novel mechanism of the bone aging phenotype and provides multiple potential clinical intervention targets for controlling osteoporosis.

PICOLINIC ACID, A TRYPTOPHAN METABOLITE, DOESN’T AFFECT BONE MINERAL DENSITY BUT UPREGULATES LIPID STORAGE GENES

Tryptophan is an essential amino-acid broken down initially to kynurenine (kyn), an immunomodulatory metabolite that we have previously shown to promote bone loss. Kyn levels increase with aging and have also been associated with neurodegenerative disorders. Additional tryptophan metabolites include picolinic acid (PA); however, in contrast to kyn, PA is neuroprotective. Thus, we hypothesized that PA might be osteoprotective. In an IACUC-approved protocol, we fed PA to aged (23-month-old) C57BL/6 mice for eight weeks. In an effort to determine potential interactions of PA with dietary protein, we added PA to both a standard (18%) and a low protein diet (8%). The mice were divided into four groups: control (18% protein), +PA (700 ppm); low protein (8%), +PA (700 ppm). There was no difference in weight among the groups [36.1±4.1, 34.6±3.8, 32.8±3.2, 32.6±3.0 gm, (Means±SD, control vs +PA vs 8% vs +PA, p=ns; n=8-10/group). Mice were sacrificed and bones and stromal cells collected for analysis. We found that addition of PA to the diet had no impact on femoral BMD or BMC (BMD: 0.069±0.008 vs 0.075±0.007 vs 0.069±0.005 vs 0.070±007, p=ns). Addition of PA to the diet had no impact of % body fat as measured by DXA; however, stromal cells isolated from the PA-fed mice showed a significant increase in the expression of the lipid storage genes, Plin1 and Cidec. Thus, although PA is downstream of kyn, the kyn-induced detrimental effects on bone mass are no longer observed with PA but instead this kyn metabolite appears to impact energy balance.

AGE-ASSOCIATED INCREASE IN KYNURENINE SUPPRESSES AUTOPHAGY AND PROMOTES APOPTOSIS IN MESENCHYMAL STEM CELLS

The age-related increase of the tryptophan metabolite, kynurenine (KYN), has been associated with osteoporosis progression. Increased activity of by Indoleamine-(2,3)-dioxygenase(IDO), are responsible for the elevation of KYN levels in bone tissue. IDO activity is elevated with age and could be a promising therapeutic target forosteopenia and osteoporosis. Previously, our group has shown that the serum level of KYN is elevated with age and correlates with bone loss in vivo. Kynurenine suppress the expression and activity of chemokine CXCL12 essential for osteogenesis, bone marrow stem cells homing. Bone Marrow Stem Cells (BMSC) cultured in 1% FBS were treated with CXCL12 (100ng/ml) in the presence of saline control or the autophagic flux-inhibition agent chloroquine (CQ). CXCL12 treatment increased autophagy by upregulating the degree of LC3B-II by 20%. CXCL12 treatment also significantly increased co-localization of LC3B and LAMP-2 in serum starved cells. In the present study, we tested the theory that kynurenine plays an opposite role to CXCL12 by suppressing the autophagy cell survival pathway and by inducing apoptosis. Treatment of nutrient-deprived murine BMSCs with 10 or 100 µM of KYN suppresses autophagy in a dose dependent fashion while increasing cellular apoptosis. Treatment of BMSCs with KYN downregulated autophagic flux in BMSC preventing CQ-induced CL3B/LAMP-2 colocalization. KYN treatment prevented conversion of LC3B-I to LC3B-II in CQ-treated cells by 30 percent. At the same time, KYN treatment induces apoptosis, by increasing TUNEL-positive cells number by more than 50 percent. Additionally, KYN treatment significantly increased the levels of cleaved isoforms of PARP and caspase-3.

AGE-ASSOCIATED INCREASE IN KYNURENINE INHIBITS AUTOPHAGY AND PROMOTES SENESCENCE IN BONE MARROW STEM CELLS

Aging is characterized by progressive decline of tissue functionality and age-related accumulation of cellular and molecular damage leading to multiple pathological conditions including osteoporosis and increased fracture rates. Bone marrow mesenchymal stem cells (BMSCs) play an essential role in bone development and regeneration with their ability to undergo differentiation into osteogenic, chondrogenic, myogenic, and adipogenic cell lines cell lines. Proliferation rate of MSC is declined with ages leading to misbalance between bone resorption and osteogenesis. A recently identified age-related change in bone and bone marrow is an accumulation of tryptophan metabolite, kynurenine (KYN), catalyzed by indoleamine-2,3-dioxygenase (IDO) or free-radical oxidation. We previously reported that KYN suppresses autophagy in BMSC. We now investigated the effect of KYN on BMSC cellular function. In vitro treatment of murine BMSC isolated from 18 month old mice with kynurenine disrupted autophagy suppressing autophagic flux. KYN treatment also induces senescence in BMSC marked by increase in SA-beta-galactosidase activity as well as, increased expression of senescence marker p21. Inhibition of Aryl Hydrocarbon Receptor (AhR) by AhR inhibitors significantly reduced β-galactosidase activity increase and blocked p21 expression elevation suggesting that KYN induces senescence in BMSC through the AhR pathway. Interestingly, KYN treatment failed to up-regulate beta-gal activity in BMSC isolated from 6 month-old mice suggesting that KYN induction of senescence maybe potentiated with aging. Together those data support the idea that KYN shifts the homeostatic balance of BMSC during prolonged stress or in aging through downregulating survival autophagic pathway in favor of driving BMSCs to senescence.

Stromal cell-derived factor-1 (CXCL12) and its role in bone and muscle biology

Musculoskeletal disorders are the leading cause of disability worldwide; two of the most prevalent of which are osteoporosis and sarcopenia. Each affect millions in the aging population across the world and the associated morbidity and mortality contributes to billions of dollars in annual healthcare cost. Thus, it is important to better understand the underlying pathologic mechanisms of the disease process. Regulatory chemokine, CXCL12, and its receptor, CXCR4, are recognized to be essential in the recruitment, localization, maintenance, development and differentiation of progenitor stem cells of the musculoskeletal system. CXCL12 signaling results in the development and functional ability of osteoblasts, osteoclasts, satellite cells and myoblasts critical to maintaining musculoskeletal homeostasis. Interestingly, one suggested pathologic mechanism of osteoporosis and sarcopenia is a decline in the regenerative capacity of musculoskeletal progenitor stem cells. Thus, because CXCL12 is critical to progenitor function, a disruption in the CXCL12 signaling axis might play a distinct role in these pathological processes. Therefore, in this article, we perform a review of CXCL12, its physiologic and pathologic function in bone and muscle, and potential targets for therapeutic development.

Stromal cell-derived factor-1 as a potential therapeutic target for osteoarthritis and rheumatoid arthritis

With age, joints become subject to chronic inflammatory processes that lead to degeneration of articular cartilage. Although multifactorial, cytokines have been shown to play a role in the pathogenesis of these chronic disease states. Stromal cell-derived factor 1 (SDF-1) is a chemokine that has been shown to be active in homeostatic mechanisms and developmental processes throughout the body, such as endochondral bone formation. SDF-1 plays a role in the transition from cartilage to bone. Although it has been shown to be a factor in normal development, it has also been shown to involve in the pathogenesis of rheumatoid arthritis (RA) and osteoarthritis (OA). In RA, SDF-1 has been shown to stimulate the recruitment of proinflammatory cells, as well as osteoclasts to the synovium, aiding in the facilitation of synovial degradation. Similarly, in OA, SDF-1 has been shown to regulate key proteins involved in the degradation of the cartilage of the joint. Because of its role in degenerative joint disease, SDF-1 has been investigated as a potential therapeutic target. Animal studies have been employing SDF-1 inhibitors, such as AMD3100 and T140, to study their effects on attenuating degenerative joint disease. These studies have shown promising results in slowing the progression of cartilage degradation and could potentially be used as therapeutic target for humans OA and RA.

What doesn't kill you makes you stranger: Dipeptidyl peptidase-4 (CD26) proteolysis differentially modulates the activity of many peptide hormones and cytokines generating novel cryptic bioactive ligands

Dipeptidyl peptidase 4 (DPP4) is an exopeptidase found either on cell surfaces where it is highly regulated in terms of its expression and surface availability (CD26) or in a free/circulating soluble constitutively available and intrinsically active form. It is responsible for proteolytic cleavage of many peptide substrates. In this review we discuss the idea that DPP4-cleaved peptides are not necessarily inactivated, but rather can possess either a modified receptor selectivity, modified bioactivity, new antagonistic activity, or even a novel activity relative to the intact parent ligand. We examine in detail five different major DPP4 substrates: glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), peptide tyrosine-tyrosine (PYY), and neuropeptide Y (NPY), and stromal derived factor 1 (SDF-1 aka CXCL12). We note that discussion of the cleaved forms of these five peptides are underrepresented in the research literature, and are both poorly investigated and poorly understood, representing a serious research literature gap. We believe they are understudied and misinterpreted as inactive due to several factors. This includes lack of accurate and specific quantification methods, sample collection techniques that are inherently inaccurate and inappropriate, and a general perception that DPP4 cleavage inactivates its ligand substrates. Increasing evidence points towards many DPP4-cleaved ligands having their own bioactivity. For example, GLP-1 can work through a different receptor than GLP-1R, DPP4-cleaved GIP can function as a GIP receptor antagonist at high doses, and DPP4-cleaved PYY, NPY, and CXCL12 can have different receptor selectivity, or can bind novel, previously unrecognized receptors to their intact ligands, resulting in altered signaling and functionality. We believe that more rigorous research in this area could lead to a better understanding of DPP4's role and the biological importance of the generation of novel cryptic ligands. This will also significantly impact our understanding of the clinical effects and side effects of DPP4-inhibitors as a class of anti-diabetic drugs that potentially have an expanding clinical relevance. This will be specifically relevant in targeting DPP4 substrate ligands involved in a variety of other major clinical acute and chronic injury/disease areas including inflammation, immunology, cardiology, stroke, musculoskeletal disease and injury, as well as cancer biology and tissue maintenance in aging.

Role of MicroRNA-141 in the Aging Musculoskeletal System: A Current Overview

MicroRNA's are small non-coding RNAs that regulate the expression of genes by targeting the 3' UTR's of mRNA. Studies reveal that miRNAs play a pivotal role in normal musculoskeletal function such as mesenchymal stem cell differentiation, survivability and apoptosis, osteogenesis, and chondrogenesis. Changes in normal miRNA expression have been linked to a number of pathological disease processes. Additionally, with aging, it is noted that there is dysregulation in the normal function of stem cell differentiation, bone formation/degradation, chondrocyte function, and muscle degeneration. Due to the change in expression of miRNA in degenerative musculoskeletal pathology, it is believed that these molecules may be at least partially responsible for cellular dysfunction. A number of miRNAs have already been identified to play a role in osteoarthritis, osteoporosis and sarcopenia. One miRNA that has become of interest recently is miRNA 141. The purpose of this article is to review the current literature available on miRNA 141 and how it could play a role in osteoporosis, osteoarthritis and musculoskeletal pathology overall.

Selective serotonin re-uptake inhibitor sertraline inhibits bone healing in a calvarial defect model

Bone wound healing is a highly dynamic and precisely controlled process through which damaged bone undergoes repair and complete regeneration. External factors can alter this process, leading to delayed or failed bone wound healing. The findings of recent studies suggest that the use of selective serotonin reuptake inhibitors (SSRIs) can reduce bone mass, precipitate osteoporotic fractures and increase the rate of dental implant failure. With 10% of Americans prescribed antidepressants, the potential of SSRIs to impair bone healing may adversely affect millions of patients' ability to heal after sustaining trauma. Here, we investigate the effect of the SSRI sertraline on bone healing through pre-treatment with (10 mg·kg-1 sertraline in drinking water, n = 26) or without (control, n = 30) SSRI followed by the creation of a 5-mm calvarial defect. Animals were randomized into three surgical groups: (a) empty/sham, (b) implanted with a DermaMatrix scaffold soak-loaded with sterile PBS or (c) DermaMatrix soak-loaded with 542.5 ng BMP2. SSRI exposure continued until sacrifice in the exposed groups at 4 weeks after surgery. Sertraline exposure resulted in decreased bone healing with significant decreases in trabecular thickness, trabecular number and osteoclast dysfunction while significantly increasing mature collagen fiber formation. These findings indicate that sertraline exposure can impair bone wound healing through disruption of bone repair and regeneration while promoting or defaulting to scar formation within the defect site.

Genetic contributions to self-reported tiredness

Self-reported tiredness and low energy, often called fatigue, are associated with poorer physical and mental health. Twin studies have indicated that this has a heritability between 6 and 50%. In the UK Biobank sample (N=108 976), we carried out a genome-wide association study (GWAS) of responses to the question, 'Over the last two weeks, how often have you felt tired or had little energy?' Univariate GCTA-GREML found that the proportion of variance explained by all common single-nucleotide polymorphisms for this tiredness question was 8.4% (s.e.=0.6%). GWAS identified one genome-wide significant hit (Affymetrix id 1:64178756_C_T; P=1.36 × 10-11). Linkage disequilibrium score regression and polygenic profile score analyses were used to test for shared genetic aetiology between tiredness and up to 29 physical and mental health traits from GWAS consortia. Significant genetic correlations were identified between tiredness and body mass index (BMI), C-reactive protein, high-density lipoprotein (HDL) cholesterol, forced expiratory volume, grip strength, HbA1c, longevity, obesity, self-rated health, smoking status, triglycerides, type 2 diabetes, waist-hip ratio, attention deficit hyperactivity disorder, bipolar disorder, major depressive disorder, neuroticism, schizophrenia and verbal-numerical reasoning (absolute rg effect sizes between 0.02 and 0.78). Significant associations were identified between tiredness phenotypic scores and polygenic profile scores for BMI, HDL cholesterol, low-density lipoprotein cholesterol, coronary artery disease, C-reactive protein, HbA1c, height, obesity, smoking status, triglycerides, type 2 diabetes, waist-hip ratio, childhood cognitive ability, neuroticism, bipolar disorder, major depressive disorder and schizophrenia (standardised β's had absolute values<0.03). These results suggest that tiredness is a partly heritable, heterogeneous and complex phenomenon that is phenotypically and genetically associated with affective, cognitive, personality and physiological processes.

Kynurenine, a Tryptophan Metabolite That Accumulates With Age, Induces Bone Loss

Age-dependent bone loss occurs in humans and in several animal species, including rodents. The underlying causal mechanisms are probably multifactorial, although an age-associated increase in the generation of reactive oxygen species has been frequently implicated. We previously reported that aromatic amino acids function as antioxidants, are anabolic for bone, and that they may potentially play a protective role in an aging environment. We hypothesized that upon oxidation the aromatic amino acids would not only lose their anabolic effects but also potentially become a catabolic byproduct. When measured in vivo in C57BL/6 mice, the tryptophan oxidation product and kynurenine precursor, N-formylkynurenine (NFK), was found to increase with age. We tested the direct effects of feeding kynurenine (kyn) on bone mass and also tested the short-term effects of intraperitoneal kyn injection on bone turnover in CD-1 mice. μCT analyses showed kyn-induced bone loss. Levels of serum markers of osteoclastic activity (pyridinoline [PYD] and RANKL) increased significantly with kyn treatment. In addition, histological and histomorphometric studies showed an increase in osteoclastic activity in the kyn-treated groups in both dietary and injection-based studies. Further, kyn treatment significantly increased bone marrow adiposity, and BMSCs isolated from the kyn-injected mice exhibited decreased mRNA expression of Hdac3 and its cofactor NCoR1 and increased expression of lipid storage genes Cidec and Plin1. A similar pattern of gene expression is observed with aging. In summary, our data show that increasing kyn levels results in accelerated skeletal aging by impairing osteoblastic differentiation and increasing osteoclastic resorption. These data would suggest that kyn could play a role in age-induced bone loss. © 2017 American Society for Bone and Mineral Research.

Association of Plasma SDF-1 with Bone Mineral Density, Body Composition, and Hip Fractures in Older Adults: The Cardiovascular Health Study

Aging is associated with an increase in circulating inflammatory factors. One, the cytokine stromal cell-derived factor 1 (SDF-1 or CXCL12), is critical to stem cell mobilization, migration, and homing as well as to bone marrow stem cell (BMSC), osteoblast, and osteoclast function. SDF-1 has pleiotropic roles in bone formation and BMSC differentiation into osteoblasts/osteocytes, and in osteoprogenitor cell survival. The objective of this study was to examine the association of plasma SDF-1 in participants in the cardiovascular health study (CHS) with bone mineral density (BMD), body composition, and incident hip fractures. In 1536 CHS participants, SDF-1 plasma levels were significantly associated with increasing age (p < 0.01) and male gender (p = 0.04), but not with race (p = 0.63). In multivariable-adjusted models, higher SDF-1 levels were associated with lower total hip BMD (p = 0.02). However, there was no significant association of SDF-1 with hip fractures (p = 0.53). In summary, circulating plasma levels of SDF-1 are associated with increasing age and independently associated with lower total hip BMD in both men and women. These findings suggest that SDF-1 levels are linked to bone homeostasis.

Human Mesenchymal Stem Cells Partially Reverse Infertility in Chemotherapy-Induced Ovarian Failure

INTRODUCTION

Chemotherapy is the most commonly used modality to treat human cancers; however, in many cases it causes irreversible ovarian failure. In this work, we plan to evaluate the restorative function of human bone marrow mesenchymal stem cells (BMSCs) in a chemotherapy-induced ovarian failure mouse model.

METHODS

Acclimatized 4 to 6 week-old female mice (C57BL/6) were assigned randomly to a vehicle-treated control group (group 1), chemotherapy-treated group followed by vehicle alone (group 2), or chemotherapy-treated group followed by stem cell intraovarian injection (group 3). Outcomes were evaluated using immunohistochemistry (IHC), serum hormonal assays, and estrous cycle monitoring and breeding potential.

RESULTS

Post BMSCs administration, group 3 promptly showed detectable vaginal smears with estrogenic changes. Increase in total body weight, ovarian weight, and weight of estrogen-responsive organs (uterus and liver) was observed at 2 weeks and continued to end of the experiment. Hematoxylin and Eosin histological evaluation of the ovaries demonstrated a higher mean follicle count in group 3 than in group 2. Group 3 had lower follicle-stimulating hormone (FSH) levels ( P = .03) and higher anti-Müllerian hormone serum (AMH) levels ( P = .0005) than group 2. The IHC analysis demonstrated higher expression of AMH, FSH receptor, inhibin A, and inhibin B in growing follicles of group 3 versus group 2. Tracking studies demonstrated that human BMSCs evenly repopulated the growing follicles in treated ovaries. Importantly, breeding data showed significant increases in the pregnancies numbers, 2 pregnancies in group 1 and 12 in group 3 ( P = .02).

CONCLUSIONS

Intraovarian administered BMSCs are able to restore ovarian hormone production and reactivate folliculogenesis in chemotherapy-induced ovarian failure mouse model.

Association of DPP-4 activity with BMD, body composition, and incident hip fracture: the Cardiovascular Health Study

There was no association of plasma DPP-4 activity levels with bone mineral density (BMD), body composition, or incident hip fractures in a cohort of elderly community-dwelling adults.

INTRODUCTION

Dipeptidyl peptidase IV (DPP-4) inactivates several key hormones including those that stimulate postprandial insulin secretion, and DPP-4 inhibitors (gliptins) are approved to treat diabetes. While DPP-4 is known to modulate osteogenesis, the relationship between DPP-4 activity and skeletal health is uncertain. The purpose of the present study was to examine possible associations between DPP-4 activity in elderly subjects enrolled in the Cardiovascular Health Study (CHS) and BMD, body composition measurements, and incident hip fractures.

METHODS

All 1536 male and female CHS participants who had evaluable DXA scans and plasma for DPP-4 activity were included in the analyses. The association between (1) BMD of the total hip, femoral neck, lumbar spine, and total body; (2) body composition measurements (% lean, % fat, and total body mass); and (3) incident hip fractures and plasma levels of DPP-4 activity were determined.

RESULTS

Mean plasma levels of DPP-4 activity were significantly higher in blacks (227 ± 78) compared with whites (216 ± 89) (p = 0.04). However, there was no significant association of DPP-4 activity with age or gender (p ≥ 0.14 for both). In multivariable adjusted models, there was no association of plasma DPP-4 activity with BMD overall (p ≥ 0.55 for all) or in gender stratified analyses (p ≥ 0.23). There was also no association of DPP-4 levels and incident hip fractures overall (p ≥ 0.24) or in gender stratified analyses (p ≥ 0.39).

CONCLUSION

Plasma DPP-4 activity, within the endogenous physiological range, was significantly associated with race, but not with BMD, body composition, or incident hip fractures in elderly community-dwelling subjects.

MicroRNA-183-5p Increases with Age in Bone-Derived Extracellular Vesicles, Suppresses Bone Marrow Stromal (Stem) Cell Proliferation, and Induces Stem Cell Senescence

Microvesicle- and exosome-mediated transport of microRNAs (miRNAs) represents a novel cellular and molecular pathway for cell-cell communication. In this study, we tested the hypothesis that these extracellular vesicles (EVs) and their miRNAs might change with age, contributing to age-related stem cell dysfunction. EVs were isolated from the bone marrow interstitial fluid (supernatant) of young (3-4 months) and aged (24-28 months) mice to determine whether the size, concentration, and miRNA profile of EVs were altered with age in vivo. Results show that EVs isolated from bone marrow are CD63 and CD9 positive, and the concentration and size distribution of bone marrow EVs are similar between the young and aged mice. Bioanalyzer data indicate that EVs from both young and aged mice are highly enriched in miRNAs, and the miRNA profile of bone marrow EVs differs significantly between the young and aged mice. Specifically, the miR-183 cluster (miR-96/-182/-183) is highly expressed in aged EVs. In vitro assays demonstrate that aged EVs are endocytosed by primary bone marrow stromal cells (BMSCs), and these aged EVs inhibit the osteogenic differentiation of young BMSCs. Transfection of BMSCs with miR-183-5p mimic reduces cell proliferation and osteogenic differentiation, increases senescence, and decreases protein levels of the miR-183-5p target heme oxygenase-1 (Hmox1). In vitro assays utilizing H₂O₂-induced oxidative stress show that H₂O₂ treatment of BMSCs increases the abundance of miR-183-5p in BMSC-derived EVs, and Amplex Red assays demonstrate that H₂O₂ is elevated in the bone marrow microenvironment with age. Together, these data indicate that aging and oxidative stress can significantly alter the miRNA cargo of EVs in the bone marrow microenvironment, which may in turn play a role in stem cell senescence and osteogenic differentiation by reducing Hmox1 activity.

Shared genetic aetiology between cognitive functions and physical and mental health in UK Biobank (N=112 151) and 24 GWAS consortia

Causes of the well-documented association between low levels of cognitive functioning and many adverse neuropsychiatric outcomes, poorer physical health and earlier death remain unknown. We used linkage disequilibrium regression and polygenic profile scoring to test for shared genetic aetiology between cognitive functions and neuropsychiatric disorders and physical health. Using information provided by many published genome-wide association study consortia, we created polygenic profile scores for 24 vascular-metabolic, neuropsychiatric, physiological-anthropometric and cognitive traits in the participants of UK Biobank, a very large population-based sample (N=112 151). Pleiotropy between cognitive and health traits was quantified by deriving genetic correlations using summary genome-wide association study statistics and to the method of linkage disequilibrium score regression. Substantial and significant genetic correlations were observed between cognitive test scores in the UK Biobank sample and many of the mental and physical health-related traits and disorders assessed here. In addition, highly significant associations were observed between the cognitive test scores in the UK Biobank sample and many polygenic profile scores, including coronary artery disease, stroke, Alzheimer's disease, schizophrenia, autism, major depressive disorder, body mass index, intracranial volume, infant head circumference and childhood cognitive ability. Where disease diagnosis was available for UK Biobank participants, we were able to show that these results were not confounded by those who had the relevant disease. These findings indicate that a substantial level of pleiotropy exists between cognitive abilities and many human mental and physical health disorders and traits and that it can be used to predict phenotypic variance across samples.

Pleiotropy between neuroticism and physical and mental health: findings from 108 038 men and women in UK Biobank

People with higher levels of neuroticism have an increased risk of several types of mental disorder. Higher neuroticism has also been associated, less consistently, with increased risk of various physical health outcomes. We hypothesised that these associations may, in part, be due to shared genetic influences. We tested for pleiotropy between neuroticism and 17 mental and physical diseases or health traits using linkage disequilibrium regression and polygenic profile scoring. Genetic correlations were derived between neuroticism scores in 108 038 people in the UK Biobank and health-related measures from 14 large genome-wide association studies (GWASs). Summary information for the 17 GWASs was used to create polygenic risk scores for the health-related measures in the UK Biobank participants. Associations between the health-related polygenic scores and neuroticism were examined using regression, adjusting for age, sex, genotyping batch, genotyping array, assessment centre and population stratification. Genetic correlations were identified between neuroticism and anorexia nervosa (rg=0.17), major depressive disorder (rg=0.66) and schizophrenia (rg=0.21). Polygenic risk for several health-related measures were associated with neuroticism, in a positive direction in the case of bipolar disorder, borderline personality, major depressive disorder, negative affect, neuroticism (Genetics of Personality Consortium), schizophrenia, coronary artery disease, and smoking (β between 0.009-0.043), and in a negative direction in the case of body mass index (β=-0.0095). A high level of pleiotropy exists between neuroticism and some measures of mental and physical health, particularly major depressive disorder and schizophrenia.

Total body irradiation is permissive for mesenchymal stem cell-mediated new bone formation following local transplantation

Skeletal injury is a major clinical challenge accentuated by the decrease of bone marrow-derived mesenchymal stem/stromal cells (BMSCs) with age or disease. Numerous experimental and clinical studies have revealed that BMSCs hold great promise for regenerative therapies due to their direct osteogenic potential and indirect trophic/paracrine actions. Increasing evidence suggests that stromal cell-derived factor-1 (SDF-1) is involved in modulating the host response to the injury. Common problems with BMSC therapy include poor cell engraftment, which can be addressed by total body irradiation (TBI) prior to transplantation. In this study, we tested the hypothesis that direct tibial transplantation of BMSCs drives endogenous bone formation in a dose-dependent manner, which is enhanced by TBI, and investigated the potential role of SDF-1 in facilitating these events. We found that TBI is permissive for transplanted BMSCs to engraft and contribute to new bone formation. Bone marrow (BM) interstitial fluid analysis revealed no differences of SDF-1 splice variants in irradiated animals compared to controls, despite the increased mRNA and protein levels expressed in whole BM cells. This correlated with increased dipeptidyl peptidase IV activity and the failure to induce chemotaxis of BMSCs in vitro. We found increased mRNA expression levels of the major SDF-1-cleaving proteases in whole BM cells from irradiated animals suggesting distinct spatial differences within the BM in which SDF-1 may play different autocrine and paracrine signaling roles beyond the immediate cell surface microenvironment.

Mesenchymal stem cell expression of SDF-1β synergizes with BMP-2 to augment cell-mediated healing of critical-sized mouse calvarial defects

Bone has the potential for spontaneous healing. This process, however, often fails in patients with comorbidities. Tissue engineering combining functional cells, biomaterials and osteoinductive cues may provide alternative treatment strategies. We have recently demonstrated that stromal cell-derived factor-1β (SDF-1β) works in concert with bone morphogenetic protein-2 (BMP-2) to potentiate osteogenic differentiation of bone marrow-derived mesenchymal stem/stromal cells (BMSCs). Here, we test the hypothesis that SDF-1β overexpressed in Tet-Off-SDF-1β BMSCs, delivered on acellular dermal matrix (ADM), synergistically augments BMP-2-induced healing of critical-sized mouse calvarial defects. BMSC therapies alone showed limited bone healing, which was increased with co-delivery of BMP-2. This was further enhanced in Tet-Off-SDF-1β BMSCs + BMP-2. Only limited BMSC retention on ADM constructs was observed after 4 weeks in vivo, which was increased with BMP-2 co-delivery. In vitro cell proliferation studies showed that supplementing BMP-2 to Tet-Off BMSCs significantly increased the cell number during the first 24 h. Consequently, the increased cell numbers decreased the detectable BMP-2 levels in the medium, but increased cell-associated BMP-2. The data suggest that SDF-1β provides synergistic effects supporting BMP-2-induced, BMSC-mediated bone formation and appears suitable for optimization of bone augmentation in combination therapy protocols. Copyright © 2015 John Wiley & Sons, Ltd.

Caloric restriction and the adipokine leptin alter the SDF-1 signaling axis in bone marrow and in bone marrow derived mesenchymal stem cells

Growing evidence suggests that the chemokine stromal cell-derived factor-1 (SDF-1) is essential in regulating bone marrow (BM) derived mesenchymal stromal/stem cell (BMSC) survival, and differentiation to either a pro-osteogenic or pro-adipogenic fate. This study investigates the effects of caloric restriction (CR) and leptin on the SDF-1/CXCR4 axis in bone and BM tissues in the context of age-associated bone loss. For in vivo studies, we collected bone, BM cells and BM interstitial fluid from 12 and 20 month-old C57Bl6 mice fed ad-libitum (AL), and 20-month-old mice on long-term CR with, or without, intraperitoneal injection of leptin for 10 days (10 mg/kg). To mimic conditions of CR in vitro, 18 month murine BMSCs were treated with (1) control (Ctrl): normal proliferation medium, (2) nutrient restriction (NR): low glucose, low serum medium, or (3) NR + leptin: NR medium + 100 ng/ml leptin for 6-48 h. In BMSCs both protein and mRNA expression of SDF-1 and CXCR4 were increased by CR and CR + leptin. In contrast, the alternate SDF-1 receptor CXCR7 was decreased, suggesting a nutrient signaling mediated change in SDF-1 axis signaling in BMSCs. However, in bone SDF-1, CXCR4 and 7 gene expression increase with age and this is reversed with CR, while addition of leptin returns this to the "aged" level. Histologically bone formation was lower in the calorically restricted mice and BM adipogenesis increased, both effects were reversed with the 10 day leptin treatment. This suggests that in bone CR and leptin alter the nutrient signaling pathways in different ways to affect the local action of the osteogenic cytokine SDF-1. Studies focusing on the molecular interaction between nutrient signaling by CR, leptin and SDF-1 axis may help to address age-related musculoskeletal changes.

MicroRNAs-141 and 200a regulate the SVCT2 transporter in bone marrow stromal cells

Vitamin C is a micro-nutrient which plays an important role in bone marrow stromal cell (BMSCs) differentiation to osteogenesis. This vitamin is transported into the BMSCs through the sodium dependent vitamin C transporter 2 (SVCT2). We previously reported that knockdown of the SVCT2 transporter decreases osteogenic differentiation. However, our understanding of the post-transcriptional regulatory mechanism of the SVCT2 transporter remains poor. MicroRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally regulate the messenger RNAs of protein-coding genes. In this study, we aimed to investigate the impact of miR-141 and miR-200a on SVCT2 expression. We found that mouse BMSCs expressed miR-141 and miR-200a and repressed SVCT2 expression at the functional level by targeting the 3'-untranslated region of mRNA. We also found that miR-141 and miR-200a decreased osteogenic differentiation. Furthermore, miRNA inhibitors increased SVCT2 and osteogenic gene expression in BMSCs. Taken together, these results indicate that both miRNAs are novel regulators of the SVCT2 transporter and play an important role in the osteogenic differentiation of BMSCs.

Oxidation of the aromatic amino acids tryptophan and tyrosine disrupts their anabolic effects on bone marrow mesenchymal stem cells

Age-induced bone loss is associated with greater bone resorption and decreased bone formation resulting in osteoporosis and osteoporosis-related fractures. The etiology of this age-induced bone loss is not clear but has been associated with increased generation of reactive oxygen species (ROS) from leaky mitochondria. ROS are known to oxidize/damage the surrounding proteins/amino acids/enzymes and thus impair their normal function. Among the amino acids, the aromatic amino acids are particularly prone to modification by oxidation. Since impaired osteoblastic differentiation from bone marrow mesenchymal stem cells (BMMSCs) plays a role in age-related bone loss, we wished to examine whether oxidized amino acids (in particular the aromatic amino acids) modulated BMMSC function. Using mouse BMMSCs, we examined the effects of the oxidized amino acids di-tyrosine and kynurenine on proliferation, differentiation and Mitogen-Activated Protein Kinase (MAPK) pathway. Our data demonstrate that amino acid oxides (in particular kynurenine) inhibited BMMSC proliferation, alkaline phosphatase expression and activity and the expression of osteogenic markers (Osteocalcin and Runx2). Taken together, our data are consistent with a potential pathogenic role for oxidized amino acids in age-induced bone loss.

The crucial role of vitamin C and its transporter (SVCT2) in bone marrow stromal cell autophagy and apoptosis

Vitamin C is an antioxidant that plays a vital role in various biological processes including bone formation. Previously, we reported that vitamin C is transported into bone marrow stromal cells (BMSCs) through the sodium dependent Vitamin C Transporter 2 (SVCT2) and this transporter plays an important role in osteogenic differentiation. Furthermore, this transporter is regulated by oxidative stress. To date, however, the exact role of vitamin C and its transporter (SVCT2) in ROS regulated autophagy and apoptosis in BMSCs is poorly understood. In the present study, we observed that oxidative stress decreased survival of BMSCs in a dose-dependent manner and induced growth arrest in the G1 phase of the cell cycle. These effects were accompanied by the induction of autophagy, confirmed by P62 and LC3B protein level and punctate GFP-LC3B distribution. The supplementation of vitamin C significantly rescued the BMSCs from oxidative stress by regulating autophagy. Knockdown of the SVCT2 transporter in BMSCs synergistically decreased cell survival even under low oxidative stress conditions. Also, supplementing vitamin C failed to rescue cells from stress. Our results reveal that the SVCT2 transporter plays a vital role in the mechanism of BMSC survival under stress conditions. Altogether, this study has given new insight into the role of the SVCT2 transporter in oxidative stress related autophagy and apoptosis in BMSCs.

The aromatic amino acid tryptophan stimulates skeletal muscle IGF1/p70s6k/mTor signaling in vivo and the expression of myogenic genes in vitro

OBJECTIVES

Nutrition plays a key role in the maintenance of muscle and bone mass, and dietary protein deficiency has in particular been associated with catabolism of both muscle and bone tissue. One mechanism thought to link protein deficiency with loss of muscle mass is deficiency in specific amino acids that play a role in muscle metabolism. The aim of this study was to test the hypothesis that the essential amino acid tryptophan, and its metabolite kynurenine, might directly affect muscle metabolism in the setting of protein deficiency.

METHODS

Adult mice (12 mo) were fed a normal diet (18% protein), as well as diets with low protein (8%) supplemented with increasing concentrations (50, 100, and 200 uM) of kynurenine (Kyn) or with tryptophan (Trp; 1.5 mM) for 8 weeks. Myoprogenitor cells were also treated with Trp and Kyn in vitro to determine their effects on cell proliferation and expression of myogenic differentiation markers.

RESULTS

All mice on the low-protein diets weighed less than the group fed normal protein (18%). Lean mass measured by dual-energy X-ray absorptiometry was lowest in mice on the high Kyn diet, whereas percent lean mass was highest in mice receiving Trp supplementation and percent body fat was lowest in mice receiving Trp. Enzyme-linked immunosorbent assays showed significant increases in skeletal muscle insulin-like growth factor-1, leptin, and the myostatin antagonist follistatin with Trp supplementation. mRNA microarray and gene pathway analysis performed on muscle samples demonstrate that mTor/eif4/p70s6k pathway molecules are significantly up-regulated in muscles from mice on Kyn and Trp supplementation. In vitro, neither amino acid affected proliferation of myoprogenitors, but Trp increased the expression of the myogenic markers MyoD, myogenin, and myosin heavy chain.

CONCLUSION

These findings suggest that dietary amino acids can directly affect molecular signaling in skeletal muscle, further indicating that dietary manipulation with specific amino acids could potentially attenuate muscle loss with dietary protein deficiency.

Genetic contributions to variation in general cognitive function: a meta-analysis of genome-wide association studies in the CHARGE consortium (N=53949)

General cognitive function is substantially heritable across the human life course from adolescence to old age. We investigated the genetic contribution to variation in this important, health- and well-being-related trait in middle-aged and older adults. We conducted a meta-analysis of genome-wide association studies of 31 cohorts (N=53,949) in which the participants had undertaken multiple, diverse cognitive tests. A general cognitive function phenotype was tested for, and created in each cohort by principal component analysis. We report 13 genome-wide significant single-nucleotide polymorphism (SNP) associations in three genomic regions, 6q16.1, 14q12 and 19q13.32 (best SNP and closest gene, respectively: rs10457441, P=3.93 × 10(-9), MIR2113; rs17522122, P=2.55 × 10(-8), AKAP6; rs10119, P=5.67 × 10(-9), APOE/TOMM40). We report one gene-based significant association with the HMGN1 gene located on chromosome 21 (P=1 × 10(-6)). These genes have previously been associated with neuropsychiatric phenotypes. Meta-analysis results are consistent with a polygenic model of inheritance. To estimate SNP-based heritability, the genome-wide complex trait analysis procedure was applied to two large cohorts, the Atherosclerosis Risk in Communities Study (N=6617) and the Health and Retirement Study (N=5976). The proportion of phenotypic variation accounted for by all genotyped common SNPs was 29% (s.e.=5%) and 28% (s.e.=7%), respectively. Using polygenic prediction analysis, ~1.2% of the variance in general cognitive function was predicted in the Generation Scotland cohort (N=5487; P=1.5 × 10(-17)). In hypothesis-driven tests, there was significant association between general cognitive function and four genes previously associated with Alzheimer's disease: TOMM40, APOE, ABCG1 and MEF2C.

Mesenchymal stem cell expression of stromal cell-derived factor-1β augments bone formation in a model of local regenerative therapy

Bone has the potential for spontaneous healing. However, this process often fails in patients with co-morbidities requiring clinical intervention. Numerous studies have revealed that bone marrow-derived mesenchymal stem/stromal cells (BMSCs) hold great potential for regenerative therapies. Common problems include poor cell engraftment, which can be addressed by irradiation prior to transplantation. Increasing evidence suggests that stromal cell-derived factor-1 (SDF-1) is involved in bone formation. However, osteogenic contributions of the beta splice variant of SDF-1 (SDF-1β), which is highly expressed in bone, remain unclear. Using the tetracycline (Tet)-regulatory system we have shown that SDF-1β enhances BMSC osteogenic differentiation in vitro. Here we test the hypothesis that SDF-1β augments bone formation in vivo in a model of local BMSC transplantation following irradiation. We found that SDF-1β, expressed at high levels in Tet-Off-SDF-1β BMSCs, augments the cell-mediated therapeutic effects resulting in enhanced bone formation, as evidenced by ex vivo μCT and bone histomorphometry. The data demonstrate the specific contribution of SDF-1β to BMSC-mediated bone formation, and validate the feasibility of the Tet-Off technology to regulate SDF-1β expression in vivo. In conclusion, SDF-1β provides potent synergistic effects supporting BMSC-mediated bone formation and appears a suitable candidate for optimization of bone augmentation in translational protocols.

Abstract T P79: Rapamycin Improves Outcome Post Stroke Injury by Enhancing Autophagy in Neuronal Cells

Background and Purpose: Previous literature and studies from our laboratory indicate that pharmacological inhibition and enhancement of autophagy are both effective in reducing infarct volume after stroke injury.

Methods: Mice were treated with rapamycin to enhance, and chloroquine to suppress autophagy in mice immediately after stroke injury and again 24 hours post stroke injury in a murine stroke model. Survival data and neurological score were taken before sacrifice at 48 hours and lesion size was determined by TTC. HT22 neuronal mouse hippocampal cells were treated for one hour with H2O2 to mimic an oxidative stress injury and treated concurrently with rapamycin to enhance autophagy, chloroquine to inhibit autophagy or ZVAD to inhibit apoptosis. The treatment with pharmacological agents continued for 24 hours. Cells were counted with trypan blue on a hemocytometer. Western blot analysis was used to monitor protein changes. shRNA to Atg5 was used to genetically inhibit autophagy.

Results: Both pharmacological agents showed decreased lesion size, though enhancement of autophagy with rapamycin exhibited a greater reduction in lesion size as well as increased survival and an improved neurological score. Rapamycin improved survival of neurons after oxidative injury and genetic inhibition of autophagy abrogated rapamycin's positive effect.

Conclusions: Rapamycin improves stroke outcome by enhancing autophagy in neuronal cells.