Chronic conditions associated with aging have proven difficult to prevent or treat. Senescence is a cell fate defined by loss of proliferative capacity and the development of a pro‐inflammatory senescence‐associated secretory phenotype comprised of cytokines/chemokines, proteases, and other factors that promotes age‐related diseases. Specifically, an increase in senescent peripheral blood mononuclear cells (PBMCs), including T cells, is associated with conditions like frailty, rheumatoid arthritis, and bone loss. However, it is unknown if the percentage of senescent PBMCs associated with age‐associated orthopedic decline could be used for potential diagnostic or prognostic use in orthopedics. Here, we report senescent cell detection using the fluorescent compound C₁₂FDG to quantify PBMCs senescence across a large cohort of healthy and osteoarthritic patients. There is an increase in the percent of circulating C₁₂FDG⁺ PBMCs that is commensurate with increases in age and senescence‐related serum biomarkers. Interestingly, C₁₂FDG⁺ PBMCs and T cells also were found to be elevated in patients with mild to moderate osteoarthritis, a progressive joint disease that is strongly associated with inflammation. The percent of C₁₂FDG⁺ PBMCs and age‐related serum biomarkers were decreased in a small subgroup of study participants taking the senolytic drug fisetin. These results demonstrate quantifiable measurements in a large group of participants that could create a composite score of healthy aging sensitive enough to detect changes following senolytic therapy and may predict age‐related orthopedic decline. Detection of peripheral senescence in PBMCs and subsets using C₁₂FDG may be clinically useful for quantifying cellular senescence and determining how and if it plays a pathological role in osteoarthritic progression.

Background: During aging, perturbation of muscle progenitor cell (MPC) constituents leads to progressive loss of muscle mass and accumulation of adipose and fibrotic tissue. Mesenchymal stem cells (MSCs) give rise to adipocytes and fibroblasts that accumulate in injured and pathological skeletal muscle through constitutive activation of platelet-derived growth factor receptors (PDGFRs). Although the role of the PDGFRα has been widely explored, there is a paucity of evidence demonstrating the role of PDGFRβ in aged skeletal muscle.Methods: In this study, we investigated the role of PDGFRβ lineage cells in skeletal muscle during aging by using Cre/loxP lineage tracing technology. The PDGFR-Cre mice were crossed with global double-fluorescent Cre reporter mice (mTmG) that indelibly marks PDGFRβ lineage cells. Those cells were analyzed and compared at different ages in the skeletal muscle of the mice.Results: Our results demonstrated that PDGFRβ lineage cells isolated from the muscles of young mice are MPC-like cells that exhibited satellite cell morphology, expressed Pax7, and undergo myogenic differentiation producing myosin heavy chain expressing myotubes. Conversely, the PDGFRβ lineage cells isolated from muscles of old mice displayed MSC morphology with a reduced myogenic differentiation potential while expressing adipogenic and fibrotic differentiation markers. PDGFRβ lineage cells also gave rise to newly regenerated muscle fibers in young mice after muscle injury, but their muscle regenerative process is reduced in old mice.Conclusions: Our data suggest that PDGFRβ lineage cells function as MPCs in young mice, while the same PDGFRβ lineage cells from old mice undergo a fate switch participating in adipose and fibrotic tissue infiltration in aged muscle. The inhibition of fate-switching in PDGFRβ lineage cells may represent a potential approach to prevent fibrosis and fatty infiltration in skeletal muscle during the aging process.

Background. Fibrin sealant has been used as a scaffold to deliver genetically modified human muscle-derived stem cells (hMDSCs) for bone regeneration. Alternatively, autologous blood clots are safe, economic scaffolds. This study compared autologous blood clot (BC) with fibrin sealant (FS) as a scaffold to deliver lenti-BMP2/GFP-transduced hMDSCs for bone regeneration. Methods. In vitro osteogenic differentiation was performed using 3D pellet culture and evaluated using microCT and Von Kossa staining. The lenti-GFP transduced cells were then mixed with human blood for evaluation of osteogenic differentiation. Furthermore, a murine critical- sized calvarial defect model was utilized to compare BC and FS scaffolds for lenti-BMP2/GFP-transduced hMDSCs mediated bone regeneration and evaluated with micro-CT and histology. Results. Lenti-BMP2/GFP transduced hMDSCs formed significantly larger mineralized pellets than non-transduced hMDSCs. hMDSCs within the human blood clot migrated out and differentiated into ALP⁺ osteoblasts. In vivo, BC resulted in significantly less new bone formation within a critical-sized calvarial bone defect than FS scaffold, despite no difference observed for GFP⁺ donor cells, osteoclasts, and osteoblasts in the newly formed bone. Conclusions. Human lenti-BMP2/GFP-transduced hMDSCs can efficiently undergo osteogenic differentiation in vitro. Unexpectedly, the newly regenerated bone in BC group was significantly less than the FS group. The autologous blood clot scaffold is less efficacious for delivering stem cells for bone regeneration than fibrin sealant.

Duchenne muscular dystrophy (DMD) is a degenerative muscle disorder characterized by a lack of dystrophin expression in the sarcolemma of muscle fibers. DMD patients acquire bone abnormalities including osteopenia, fragility fractures, and scoliosis indicating a deficiency in skeletal homeostasis. The dKO (dystrophin/Utrophin double knockout) is a more severe mouse model of DMD than the mdx mouse (dystrophin deficient), and display numerous clinically-relevant manifestations, including a spectrum of degenerative changes outside skeletal muscle including bone, articular cartilage, and intervertebral discs. To examine the influence of systemic factors on the bone abnormalities and healing in DMD, parabiotic pairing between dKO mice and mdx mice was established. Notably, heterochronic parabiosis with young mdx mice significantly increased bone mass and improved bone micro-structure in old dKO-hetero mice, which showed progressive bone deterioration. Furthermore, heterochronic parabiosis with WT C56/10J mice significantly improved tibia bone defect healing in dKO-homo mice. These results suggest that systemic blood-borne factor(s) and/or progenitors from WT and young mdx mice can influence the bone deficiencies in dKO mice. Understanding these circulating factors or progenitor cells that are responsible to alleviate the bone abnormalities in dKO mice after heterochronic parabiosis might be useful for the management of poor bone health in DMD.

Duchenne muscular dystrophy (DMD), caused by the loss of dystrophin, remains incurable. Reduction in muscle regeneration with DMD is associated with the accumulation of fibroadipogenic progenitors (FAPs) differentiating into myofibroblasts and leading to a buildup of the collagenous tissue aggravating DMD pathogenesis. Mesenchymal stromal cells (MSCs) expressing platelet-derived growth factor receptors (PDGFRs) are activated in muscle during DMD progression and give rise to FAPs promoting DMD progression. Here, we hypothesized that muscle dysfunction in DMD could be delayed via genetic or pharmacologic depletion of MSC-derived FAPs. In this paper, we test this hypothesis in dystrophin-deficient mdx mice. To reduce fibro/adipose infiltration and potentiate muscle progenitor cells (MPCs), we used a model for inducible genetic ablation of proliferating MSCs via a suicide transgene, viral thymidine kinase (TK), expressed under the Pdgfrb promoter. We also tested if MSCs from fat tissue, the adipose stromal cells (ASCs), contribute to FAPs and could be targeted in DMD. Pharmacological ablation was performed with a hunter-killer peptide D-CAN targeting ASCs. MSC depletion with these approaches resulted in increased endurance, measured based on treadmill running, as well as grip strength, without significantly affecting fibrosis. Although more research is needed, our results suggest that depletion of pathogenic MSCs mitigates muscle damage and delays the loss of muscle function in mouse models of DMD.

Cartilage defect repair and osteoarthritis treatments remain clinical challenges. Microfracture is a commonly used surgical procedure for the treatment of cartilage defects but often leads to fibrocartilage repair. The aim of this study is to compare the effects of 5 bone morphogenetic proteins (BMPs) on chondrogenic differentiation of human bone marrow mesenchymal stem cells, as well as to investigate the use of the heparin/poly (ethylene arginine aspartate diglyceride (PEAD) coacervate sustained release system to deliver these BMPs for microfracture-mediated cartilage repair. Our results indicate that all 5 human BMPs significantly enhance the chondrogenic differentiation of human bone marrow mesenchymal stem cells (hBMMSCs) with BMPs 2,4 and 9 being more potent than BMP6 or BMP7, as revealed by Alcian blue, SO staining, and immunohistochemistry of COL2. Coacervate-BMPs are biocompatible for both hBMMSCs and rat muscle-derived stem cells (MDSCs) and promote their proliferation. In vivo, sustained release of human BMPs 2,4,6,7,9 with heparin/PEAD coacervate significantly enhances microfracture-mediated cartilage repair in a rat osteochondral defect model, as demonstrated by ICRS macroscopic score, Seller's histology score, and COL2 staining. These effects are mediated by increasing SOX9 expression in the regenerated cartilage. In conclusion, BMPs 2,4,9 are the most potent BMPS to promote chondrogenic differentiation, while all BMPs enhanced microfracture-mediated cartilage repair when delivered with heparin/PEAD coacervate without a significant difference between the different BMPs.