Transcriptomic changes during the replicative senescence of human articular chondrocytes

Osteoarthritis (OA) is a degenerative joint disease and a leading cause of disability worldwide. Aging is a major risk factor for OA, but the specific mechanisms underlying this connection remain unclear. Although chondrocytes rarely divide in adult articular cartilage, they undergo replicative senescence in vitro which provides an opportunity to study changes related to aging under controlled laboratory conditions. In this pilot study, we performed bulk RNA sequencing on early- and late-passage human articular chondrocytes to identify transcriptomic changes associated with cellular aging. Chondrocytes were isolated from the articular cartilage of three donors, two with OA (age 70-80 years) and one with healthy cartilage (age 26 years). Chondrocytes were serially passaged until replicative senescence and RNA extracted from early- and late-passage cells. Principal component analysis of all genes showed clear separation between early- and late-passage chondrocytes, indicating substantial age-related differences in gene expression. Differentially expressed genes (DEGs) analysis confirmed distinct transcriptomic profiles between early- and late-passage chondrocytes. Hierarchical clustering revealed contrasting expression patterns between the two isolates from osteoarthritic samples and the healthy sample. Focused analysis of DEGs on transcripts associated with turnover of the extra-cellular matrix and the senescence-associated secretory phenotype (SASP) showed consistent downregulation of Col2A1 and ACAN, and upregulation of MMP19, ADAMTS4, and ADAMTS8 in late passage chondrocytes across all samples. SASP components including IL-1α, IL-1β, IL-6, IL-7, p16INK4A (CDKN2A) and CCL2 demonstrated significant upregulation in late passage chondrocytes originally isolated from OA samples. Pathway analysis between sexes with OA revealed shared pathways such as extracellular matrix (ECM) organization, collagen formation, skeletal and muscle development, and nervous system development. Sex-specific differences were observed, with males showing distinctions in ECM organization, regulation of the cell cycle process as well as neuron differentiation. In contrast, females exhibited unique variations in the regulation of the cell cycle process, DNA metabolic process, and the PID-PLK1 pathway.

Segmental defect healing in the presence or absence of recombinant human BMP2: Novel insights from a rat model

This study defined and compared the course of native, impaired and growth factor-stimulated bone regeneration in a rat femoral defect model. A mid-diaphyseal defect with rigid internal fixation was surgically created in the right femur of male Fischer rats and serially analyzed over 36 weeks. Native bone regeneration was modeled using a sub-critical, 1 mm size defect, which healed uneventfully. Critical size defects of 5 mm were used to analyze impaired bone regeneration. In a third group, the 5 mm defects were filled with 11 µg of recombinant human bone morphogenetic protein 2 (rhBMP2) impregnated onto an absorbable collagen sponge, modeling its clinical use. Native bone regeneration was characterized by endochondral ossification with progressive remodeling to ultimately resemble intact femora. An endochondral response was also observed under conditions of impaired bone regeneration, but by week 8 medullary capping occurred with fibrofatty consolidation of the tissue within the defect, resembling an atrophic non-union. rhBMP2 treatment was associated with prolonged inflammatory cytokine expression and rapid intramembranous bone formation occurring with reduced expression of cartilage-associated collagens. Between weeks 4 and 36, rhBMP2-treated bones demonstrated decreased trabecular number and increased trabecular separation, which resulted in inferior mechanical properties compared with bones that healed naturally. Clinical Significance: Recombinant human bone morphogenetic protein 2 (rhBMP2) is used clinically to promote healing of long bones. Our data suggest that it drives intramembraneous ossification producing an inferior regenerate that deteriorates with time. Clinical outcomes would be improved by technologies favoring endochondral regenerative ossification.

IL-1Ra gene transfer potentiates BMP2-mediated bone healing by redirecting osteogenesis toward endochondral ossification

An estimated 100,000 patients each year in the United States suffer severe disability from bone defects that fail to heal, a condition where bone-regenerative therapies could provide substantial clinical benefits. Although recombinant human bone morphogenetic protein-2 (rhBMP2) is an osteogenic growth factor that is clinically approved for this purpose, it is only effective when used at exceedingly high doses that incur substantial costs, induce severe inflammation, produce adverse side effects, and form morphologically abnormal bone. Using a validated rat femoral segmental defect model, we show that bone formed in response to clinically relevant doses of rhBMP2 is accompanied by elevated expression of interleukin-1 (IL-1). Local delivery of cDNA encoding the IL-1 receptor antagonist (IL-1Ra) achieved bridging of segmental, critical size defects in bone with a 90% lower dose of rhBMP2. Unlike use of high-dose rhBMP2, bone formation in the presence of IL-1Ra occurred via the native process of endochondral ossification, resulting in improved quality without sacrificing the mechanical properties of the regenerated bone. Our results demonstrate that local immunomodulation may permit effective use of growth factors at lower doses to recapitulate more precisely the native biology of healing, leading to higher-quality tissue regeneration.

Gene therapy for bone healing: lessons learned and new approaches

Although gene therapy has its conceptual origins in the treatment of Mendelian disorders, it has potential applications in regenerative medicine, including bone healing. Research into the use of gene therapy for bone healing began in the 1990s. Prior to this period, the highly osteogenic proteins bone morphogenetic protein (BMP)-2 and -7 were cloned, produced in their recombinant forms and approved for clinical use. Despite their promising osteogenic properties, the clinical usefulness of recombinant BMPs is hindered by delivery problems that necessitate their application in vastly supraphysiological amounts. This generates adverse side effects, some of them severe, and raises costs; moreover, the clinical efficacy of the recombinant proteins is modest. Gene delivery offers a potential strategy for overcoming these limitations. Our research has focused on delivering a cDNA encoding human BMP-2, because the recombinant protein is Food and Drug Administration approved and there is a large body of data on its effects in people with broken bones. However, there is also a sizeable literature describing experimental results obtained with other transgenes that may directly or indirectly promote bone formation. Data from experiments in small animal models confirm that intralesional delivery of BMP-2 cDNA is able to heal defects efficiently and safely while generating transient, local BMP-2 concentrations 2-3 log orders less than those needed by recombinant BMP-2. The next challenge is to translate this information into a clinically expedient technology for bone healing. Our present research focuses on the use of genetically modified, allografted cells and chemically modified messenger RNA.

Paracrine effects of living human bone particles on the osteogenic differentiation of mesenchymal stem cells

Bone autografting remains the clinical model of choice for resolving problematic fractures. The precise mechanisms through which the autograft promotes bone healing are unknown. The present study examined the hypothesis that cells within the autograft secrete osteogenic factors promoting the differentiation of mesenchymal stem cells (MSCs) into osteoblasts. Particles of human bone ("chips") were recovered at the time of joint replacement surgery and placed in culture. Then, conditioned media were added to cultures of human, adipose-derived MSCs under both basal and osteogenic conditions. Contrary to expectation, medium conditioned by bone chips reduced the expression of alkaline phosphatase and strongly inhibited mineral deposition by MSCs cultured in osteogenic medium. Real time PCR revealed the inhibition of collagen type I alpha 1 chain (Col1A1) and osteopontin (OPN) expression. These data indicated that the factors secreted by bone chips inhibited the osteogenic differentiation of MSCs. However, in late cultures, bone morphogenetic protein-2 (BMP-2) expression was stimulated, suggesting the possibility of a delayed, secondary osteogenic effect.