Lack of oxygen in articular cartilage: consequences for chondrocyte biology

Angiogenesis unveiled: Insights into its role and mechanisms in cartilage injury

Osteoarthritis (OA) commonly results in compromised mobility and disability, thereby imposing a significant burden on healthcare systems. Cartilage injury is a prevalent pathological manifestation in OA and constitutes a central focus for the development of treatment strategies. Despite the considerable number of studies aimed at delaying this degenerative process, their outcomes remain unvalidated in preclinical settings. Recently, therapeutic strategies focused on angiogenesis have attracted the growing interest from researchers. Thus, we conducted a comprehensive literature review to elucidate the current progress in research and pinpoint research gaps in this domain. Additionally, it provides theoretical guidance for future research endeavors and the development of treatment strategies.

Preliminary Investigation into the Association between Scoliosis and Hypoxia: A Retrospective Cohort Study on the Impact of Eliminating Hypoxic Factors on Scoliosis Outcomes

OBJECTIVE

This study delves into the implications of adenoidectomy for scoliosis progression, investigating the intricate nexus of hypoxia, spinal curvature, and surgical intervention. With adenoidectomy being a common procedure for addressing pediatric sleep-disordered breathing, this research study explores its potential impact on spinal health.

PATIENTS AND METHODS

Employing a retrospective cohort design, this study gathered data from patients who underwent adenoidectomy, including those with scoliosis, between January 2017 and March 2023. Initial and follow-up evaluations involved clinical and radiological assessments, notably measuring the Cobb angle to quantify spinal curvature.

RESULTS

This study enrolled 218 patients under 10 years old. Among them, 18 exhibited Cobb angles of 10° or more, with a mean Cobb angle of 12.8°. In the follow-up evaluation, 83% of patients with initial Cobb angles of 10° or more were reached out to, along with 84.6% of those with Cobb angles below 10°. The postoperative follow-up revealed a notable decrease in Cobb angles for most patients, particularly those with an initial Cobb angle exceeding 10°.

CONCLUSIONS

This study underscores the potential connection between adenoidectomy, hypoxia, and scoliosis regression, highlighting the importance of early intervention for scoliosis management. Despite certain limitations, this investigation lays the foundation for future research involving larger patient cohorts and multifaceted analyses. The observed interactions between airway function, hypoxia, and spinal health open avenues for refining clinical strategies in scoliosis treatment.

Prosthetic joint infection caused by Mediterraneibacter gnavus following total knee arthroplasty, challenges in anaerobic bacteria identification

BACKGROUND

Mediterraneibacter gnavus is a Gram positive, non-sporulated, obligate anaerobe diplococci. It was first described in 1974 by Moore et al. (under the name Ruminococcus gnavus) from faeces and contents of the gastrointestinal tract of humans. It is a relatively common member of the human gut microbiota, nevertheless its role as a pathogenic bacterium has not been completely elucidated yet and it seems to depend on numerous factors, including those of the host. Here we present a case of prosthetic joint infection following total knee arthroplasty by M. gnavus.

CASE PRESENTATION

A 74 years old patient was admitted to the emergency department presenting with acute onset of left knee pain and swelling 20 days after total left knee arthroplasty. Follow-up revealed erythema and oedema without signs of fluctuation or purulent discharge from the surgical wound and elevated inflammatory reactants. Synovial fluid was taken for bacterial culture and antibiotic treatment with ceftazidime and daptomycin was established. Examination of the synovial fluid revealed abundant polymorphonuclear leucocytes, without visualizing bacteria. After four days of incubation, anaerobic culture exhibit growth of small, grey, umbilicated colonies in pure culture on Schaedler agar. The microorganism was identified as R. gnavus by MALDI-TOF (Bruker Daltonics) and M. gnavus by 16S ribosomal bacterial sequencing. The isolated showed susceptibility to the most commonly used anaerobicidal antibiotics except for clindamycin. Surgical treatment and infection source control included DAIR (debridement, antibiotics, and implant retention) and vacuum assisted therapy. The patient was discharged after six weeks with a 3-month course of oral amoxicillin as consolidation therapy. Subsequent follow-up revealed adequate wound healing with no signs of infection.

CONCLUSIONS

Mediterraneibacter gnavus have been reported as the causal microorganism in a range of human infections, nevertheless its identification remains challenging. Infection of prosthetic joints by anaerobic microorganisms is uncommon and is not considered in its empirical antibiotic treatment, thus, correct and swift identification of anaerobic bacteria in these cases is paramount.

Predisposal of Interferon Regulatory Factor 1 Deficiency to Accumulate DNA Damage and Promote Osteoarthritis Development in Cartilage

OBJECTIVE

Interferon regulatory factor 1 (IRF1) is a transcriptional regulator conventionally associated with immunomodulation. Recent molecular analyses mapping DNA binding sites of IRF1 have suggested its potential function in DNA repair. However, the physiologic significance of this noncanonical function remains unexplored. Here, we investigated the role of IRF1 in osteoarthritis (OA), a condition marked by senescence and chronic joint inflammation.

METHODS

OA progression was examined in wild-type and Irf1-/- mice using histologic assessments and microcomputed tomography analysis of whole-joint OA manifestations and behavioral assessments of joint pain. An integrated analysis of assay for transposase-accessible chromatin with sequencing and whole transcriptome data was conducted for the functional assessment of IRF1 in chondrocytes. The role of IRF1 in DNA repair and senescence was investigated by assaying γ-H2AX foci and senescence-associated beta-galactosidase activity.

RESULTS

Our genome-wide investigation of IRF1 footprinting in chondrocytes revealed its primary occupancies in the promoters of DNA repair genes without noticeable footprint patterns in those of interferon-responsive genes. Chondrocytes lacking IRF1 accumulated irreversible DNA damage under oxidative stress, facilitating their entry into cellular senescence. IRF1 was down-regulated in the cartilage of human and mouse OA. Although IRF1 overexpression did not elicit an inflammatory response in joints or affect OA development, genetic deletion of Irf1 caused enhanced chondrocyte senescence and exacerbated post-traumatic OA in mice.

CONCLUSION

IRF1 offers DNA damage surveillance in chondrocytes, protecting them from oxidative stress associated with OA risk factors. Our study provides a crucial and cautionary perspective that compromising IRF1 activity renders chondrocytes vulnerable to cellular senescence and promotes OA development.

Aberrant methylation and expression of TNXB promote chondrocyte apoptosis and extracullar matrix degradation in hemophilic arthropathy via AKT signaling

Recurrent joint bleeding in hemophilia patients frequently causes hemophilic arthropathy (HA). Drastic degradation of cartilage is a major characteristic of HA, but its pathological mechanisms has not yet been clarified. In HA cartilages, we found server matrix degradation and increased expression of DNA methyltransferase proteins. We thus performed genome-wide DNA methylation analysis on human HA (N=5) and osteoarthritis (OA) (N=5) articular cartilages, and identified 1228 differentially methylated regions (DMRs) associated with HA. Functional enrichment analyses revealed the association between DMR genes (DMGs) and extracellular matrix (ECM) organization. Among these DMGs, Tenascin XB (TNXB) expression was down-regulated in human and mouse HA cartilages. The loss of Tnxb in F8-/- mouse cartilage provided a disease-promoting role in HA by augmenting cartilage degeneration and subchondral bone loss. Tnxb knockdown also promoted chondrocyte apoptosis and inhibited phosphorylation of AKT. Importantly, AKT agonist showed chondroprotective effects following Tnxb knockdown. Together, our findings indicate that exposure of cartilage to blood leads to alterations in DNA methylation, which is functionally related to ECM homeostasis, and further demonstrate a critical role of TNXB in HA cartilage degeneration by activating AKT signaling. These mechanistic insights allow development of potentially new strategies for HA cartilage protection.

Is Obstructive Sleep Apnea-Associated Adenoid Hypertrophy Linked to Scoliotic Attitudes in Children?

Introduction Scoliosis, a multifaceted spinal deformity commonly affecting pediatric and adolescent populations, has spurred extensive scientific inquiry to understand its origins and impacts. Early-onset scoliosis (EOS), characterized by spinal curvature exceeding 10° before the age of 10, presents a unique challenge necessitating a comprehensive understanding of its etiological factors. Within this context, the potential role of hypoxia-induced by adenoid hypertrophy in contributing to the pathogenesis of EOS has emerged as an intriguing avenue of investigation. Materials and methods This retrospective study was conducted focusing on radiological and clinical data pertaining to children below 10 years of age who underwent isolated adenoidectomy for adenoid hypertrophy. Preoperative posteroanterior standing chest radiographs were utilized for scoliosis assessment, with Cobb angles serving as the primary measurement metric. To ensure accuracy and reliability, Cobb angle measurements were independently performed by two experienced observers. Statistical analyses encompassed the Mann-Whitney U test, Spearman correlation analysis, and intraclass correlation coefficient calculations to evaluate interobserver agreement. Results Among the cohort of 218 pediatric adenoidectomy patients, 177 individuals had radiographs suitable for EOS evaluation. The mean age of the participants was 5.72±2 years, with a nearly equal distribution of 52.5% male and 47.5% female patients. Strikingly, the study identified a 10.2% prevalence of coronal plane curvatures exceeding the critical threshold of 10°, indicative of EOS. The robust interobserver reliability was demonstrated by a commendable mean interclass correlation coefficient (ICC) value of 0.926, affirming consistent and accurate Cobb angle measurements between the observers. Conclusion In light of the heightened prevalence of EOS observed in children undergoing adenoidectomy, this study provides a compelling impetus for exploring the potential interrelationship between adenoid hypertrophy, hypoxia, and the emergence of early-onset scoliosis. The study underscores the importance of prospective research to elucidate the complex mechanisms connecting these factors, offering insights into potential risk factors and underlying pathogenic pathways associated with the development of early-onset scoliosis.

Pediatric Sleep Questionnaire for Sleep Apnea in Newly Diagnosed Adolescent Idiopathic Scoliosis Patients

Close association has been established between obstructive sleep apnea (OSA) and adolescent idiopathic scoliosis (AIS), with PSQ being employed as a screening method for OSA. A cross-sectional study was conducted among patients aged from 10 to 16 years who presented to a scoliosis outpatient clinic. Patient demographics, radiological assessments, and PSQ scores were gathered. A total of 299 patients were included in the study, with 28.7% males and 71.2% females. The average Cobb angle was 6.20°. PSQ scores revealed a prevalence of 33.4% for significant obstructive sleep apnea. Patients diagnosed with AIS exhibited a prevalence of 32.9% with positive PSQ results. Among those undergoing adenoid and/or tonsil surgery, 27% had positive PSQ scores. Factors such as genetics, abnormal biomechanical forces, environmental factors including melatonin, and intermittent hypoxia were explored for their potential contribution to AIS etiology. The aim of the study is to underscore the importance of early detection and intervention in OSA cases and highlights the effectiveness of the PSQ, as a screening tool in identifying sleep disorders. The findings underscore the complex relationship between OSA and AIS, and moreover any spinal curvature is in relation with OSA.

Dedifferentiated fat cells: current applications and future directions in regenerative medicine

Stem cell therapy is the most promising treatment option for regenerative medicine. Therapeutic effect of different stem cells has been verified in various disease model. Dedifferentiated fat (DFAT) cells, derived from mature adipocytes, are induced pluripotent stem cells. Compared with ASCs and other stem cells, the DFAT cells have unique advantageous characteristics in their abundant sources, high homogeneity, easily harvest and low immunogenicity. The DFAT cells have shown great potential in tissue engineering and regenerative medicine for the treatment of clinical problems such as cardiac and kidney diseases, autoimmune disease, soft and hard tissue defect. In this review, we summarize the current understanding of DFAT cell properties and focus on the relevant practical applications of DFAT cells in cell therapy in recent years.

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.

Maintaining hypoxia environment of subchondral bone alleviates osteoarthritis progression

Abnormal subchondral bone remodeling featured by overactivated osteoclastogenesis leads to articular cartilage degeneration and osteoarthritis (OA) progression, but the mechanism is unclear. We used lymphocyte cytosolic protein 1 (Lcp1) knockout mice to suppress subchondral osteoclasts in a mice OA model with anterior cruciate ligament transection (ACLT), and Lcp1^-/- mice showed decreased bone remodeling in subchondral bone and retarded cartilage degeneration. For mechanisms, the activated osteoclasts in subchondral bone induced type-H vessels and elevated oxygen concentration, which ubiquitylated hypoxia-inducible factor 1 alpha subunit (HIF-1α) in chondrocytes and led to cartilage degeneration. Lcp1 knockout impeded angiogenesis, which maintained hypoxia environment in joints and delayed the OA progression. Stabilization of HIF-1α delayed cartilage degeneration, and knockdown of Hif1a abolished the protective effects of Lcp1 knockout. Last, we showed that Oroxylin A, an Lcp1-encoded protein l-plastin (LPL) inhibitor, could alleviate OA progression. In conclusion, maintaining hypoxic environment is an attractive strategy for OA treatment.

Physioxic Culture of Chondrogenic Cells

Cartilage resides under a low oxygen tension within articulating joints. The oxygen tension within cartilage of the knee joint has been measured to be between 2% and 5% oxygen. Although the literature has historically termed this level of oxygen as hypoxia, particularly when doing experiments in vitro in this range, this is actually the physiological oxygen tension experienced in vivo and is more accurately termed physioxia. In general, culture of chondrogenic cells under physioxia has demonstrated a donor-dependent beneficial effect on chondrogenesis, with an upregulation in cartilage genes (SOX9, COL2A1, ACAN) and matrix deposition (sulfated glycosaminoglycans (sGAGs), collagen II). Physioxia also reduces the expression of hypertrophic markers (COL10A1, MMP13). This chapter will outline the methods for the expansion and differentiation of chondrogenic cells under physioxia using oxygen-controlled incubators and glove box environments, with the typical assays used for qualitative and quantitative assessment of chondrogenesis.

Evaluating the Effect of Hypoxia on Human Adult Mesenchymal Stromal Cell Chondrogenesis In Vitro: A Systematic Review

Human adult mesenchymal stromal cells (MSCs) from a variety of sources may be used to repair defects in articular cartilage by inducing them into chondrogenic differentiation. The conditions in which optimal chondrogenic differentiation takes place are an area of interest in the field of tissue engineering. Chondrocytes exist in vivo in a normally hypoxic environment and thus it has been suggested that exposing MSCs to hypoxia may also contribute to a beneficial effect on their differentiation. There are two main stages in which MSCs can be exposed to hypoxia, the expansion phase when cells are cultured, and the differentiation phase when cells are induced with a chondrogenic medium. This systematic review sought to explore the effect of hypoxia at these two stages on human adult MSC chondrogenesis in vitro. A literature search was performed on PubMed, EMBASE, Medline via Ovid, and Cochrane, and 24 studies were ultimately included. The majority of these studies showed that hypoxia during the expansion phase or the differentiation phase enhances at least some markers of chondrogenic differentiation in adult MSCs. These results were not always demonstrated at the protein level and there were also conflicting reports. Studies evaluating continuous exposure to hypoxia during the expansion and differentiation phases also had mixed results. These inconsistent results can be explained by the heterogeneity of studies, including factors such as different sources of MSCs used, donor variability, level of hypoxia used in each study, time exposed to hypoxia, and differences in culture methodology.

Conditional Deletion of HIF-2α in Mouse Nucleus Pulposus Reduces Fibrosis and Provides Mild and Transient Protection From Age-Dependent Structural Changes in Intervertebral Disc

Hypoxia-inducible factors (HIFs) are critical to the development and homeostasis of hypoxic tissues. Although HIF-2α, one of the main HIF-α isoforms, is expressed in nucleus pulposus (NP) cells, its functions remain unknown. We deleted HIF-2α in the NP tissue using a notochord-specific FoxA2Cre allele to study HIF-2α function in the adult intervertebral disc. Unlike observations in HIF-1αcKO mice, fate mapping studies using Rosa26-mTmG reporter showed that HIF-2α loss in NP did not negatively impact cell survival or affect compartment development. Rather, loss of HIF-2α resulted in slightly better attributes of NP morphology in 14-month-old HIF-2αcKO mice as evident from lower scores of degeneration. These 14-month-old HIF-2αcKO mice also exhibited significant reduction in NP tissue fibrosis and lower collagen turnover in the annulus fibrosis (AF) compartment. Imaging-Fourier transform-infrared (FTIR) analyses showed decreased collagen and protein content in the NP and maintained chondroitin sulfate levels in 14-month-old HIF-2αcKO . Mechanistically, global transcriptomic analysis showed enrichment of differentially expressed genes with Gene Ontology (GO) terms related to metabolic processes and cell development, molecular functions concerned with histone and protein binding, and associated pathways, including oxidative stress. Noteworthy, these morphological differences were not apparent in 24-month-old HIF-2αcKO , indicating that aging is the dominant factor in governing disc health. Together these data suggest that loss of HIF-2α in the NP compartment is not detrimental to the intervertebral disc development but rather mitigates NP tissue fibrosis and offers mild but transient protection from age-dependent early degenerative changes. © 2022 American Society for Bone and Mineral Research (ASBMR).

ION MODULATORY TREATMENTS TOWARD FUNCTIONAL SELF-ASSEMBLED NEOCARTILAGE

Signals that recapitulate in vitro the conditions found in vivo, such as hypoxia or mechanical forces, contribute to the generation of tissue-engineered hyaline-like tissues. The cell regulatory processes behind hypoxic and mechanical stimuli rely on ion concentration; iron is required to degrade the hypoxia inducible factor 1a (HIF1α) under normoxia, whereas the initiation of mechanotransduction requires the cytoplasmic increase of calcium concentration. In this work, we propose that ion modulation can be used to improve the biomechanical properties of self-assembled neocartilage constructs derived from rejuvenated expanded minipig rib chondrocytes. The objectives of this work were 1) to determine the effects of iron sequestration on self-assembled neocartilage constructs using two doses of the iron chelator deferoxamine (DFO), and 2) to evaluate the performance of the combined treatment of DFO and ionomycin, a calcium ionophore that triggers cytoplasmic calcium accumulation. This study employed a two-phase approach. In Phase I, constructs treated with a high dose of DFO (100 µM) exhibited an 87% increase in pyridinoline crosslinks, a 57% increase in the Young's modulus, and a 112% increase in the ultimate tensile strength (UTS) of the neotissue. In Phase II, the combined use of both ion modulators resulted in 150% and 176% significant increases in the Young's modulus and UTS of neocartilage constructs, respectively; for the first time, neocartilage constructs achieved a Young's modulus of 11.76±3.29 MPa and UTS of 4.20±1.24 MPa. The results of this work provide evidence that ion modulation can be employed to improve the biomechanical properties in engineered neotissues. STATEMENT OF SIGNIFICANCE: : The translation of tissue-engineered products requires the development of strategies capable of producing biomimetic neotissues in a replicable, controllable, and cost-effective manner. Among other functions, Fe²⁺ and Ca²⁺ are involved in the control of the hypoxic response and mechanotransduction, respectively. Both stimuli, hypoxia and mechanical forces, are known to favor chondrogenesis. This study utilized ion modulators to improve the mechanical properties self-assembled neocartilage constructs derived from expanded and rejuvenated costal chondrocytes via Fe²⁺ sequestration and Ca²⁺ influx, alone or in combination. The results indicate that ion modulation induced tissue maturation and a significant improvement of the mechanical properties, and holds potential as a tool to mitigate the need for bioreactors and engineer hyaline-like tissues.

HIF-1α in Osteoarthritis: From Pathogenesis to Therapeutic Implications

Osteoarthritis is a common age-related joint degenerative disease. Pain, swelling, brief morning stiffness, and functional limitations are its main characteristics. There are still no well-established strategies to cure osteoarthritis. Therefore, better clarification of mechanisms associated with the onset and progression of osteoarthritis is critical to provide a theoretical basis for the establishment of novel preventive and therapeutic strategies. Chondrocytes exist in a hypoxic environment, and HIF-1α plays a vital role in regulating hypoxic response. HIF-1α responds to cellular oxygenation decreases in tissue regulating survival and growth arrest of chondrocytes. The activation of HIF-1α could regulate autophagy and apoptosis of chondrocytes, decrease inflammatory cytokine synthesis, and regulate the chondrocyte extracellular matrix environment. Moreover, it could maintain the chondrogenic phenotype that regulates glycolysis and the mitochondrial function of osteoarthritis, resulting in a denser collagen matrix that delays cartilage degradation. Thus, HIF-1α is likely to be a crucial therapeutic target for osteoarthritis via regulating chondrocyte inflammation and metabolism. In this review, we summarize the mechanism of hypoxia in the pathogenic mechanisms of osteoarthritis, and focus on a series of therapeutic treatments targeting HIF-1α for osteoarthritis. Further clarification of the regulatory mechanisms of HIF-1α in osteoarthritis may provide more useful clues to developing novel osteoarthritis treatment strategies.

Emulating the chondrocyte microenvironment using multi-directional mechanical stimulation in a cartilage-on-chip

The multi-directional mechanical stimulation experienced by articular cartilage during motion is transferred to the chondrocytes through a thin layer of pericellular matrix around each cell; chondrocytes in turn respond by releasing matrix proteins and/or matrix-degrading enzymes. In the present study we investigated how different types of mechanical stimulation can affect a chondrocyte's phenotype and extracellular matrix (ECM) production. To this end, we employed a cartilage-on-chip system which allows exerting well-defined compressive and multi-directional mechanical stimulation on a 3D chondrocyte-laden agarose hydrogel using a thin deformable membrane and three individually addressed actuation chambers. First, the 3D chondrocyte culture in agarose responded to exposure to mechanical stimulation by an initial increase in IL-6 production and little-to-no change in IL-1β and TNF-α secretion after one day of on-chip culture. Exposure to mechanical stimulation enhanced COL2A1 (hyaline cartilage marker) and decreased COL1A1 (fibrotic cartilage) expression, this being more marked for the multi-directional stimulation. Remarkably, the production of glycosaminoglycans (GAGs), one of the main components of native cartilage ECM, was significantly increased after 15 days of on-chip culture and 14 days of mechanical stimulation. Specifically, a thin pericellular matrix shell (1-5 μm) surrounding the chondrocytes as well as an interstitial matrix, both reminiscent of the in vivo situation, were deposited. Matrix deposition was highest in chips exposed to multi-directional mechanical stimulation. Finally, exposure to mechanical cues enhanced the production of essential cartilage ECM markers, such as aggrecan, collagen II and collagen VI, a marker for the pericellular matrix. Altogether our results highlight the importance of mechanical cues, and using the right type of stimulation, to emulate in vitro, the chondrocyte microenvironment.

Oxygen control: the often overlooked but essential piece to create better in vitro systems

Variations in oxygen levels play key roles in numerous physiological and pathological processes, but are often not properly controlled in in vitro models, introducing a significant bias in experimental outcomes. Recent developments in microfluidic technology have introduced a paradigm shift by providing new opportunities to better mimic physiological and pathological conditions, which is achieved by both regulating and monitoring oxygen levels at the micrometre scale in miniaturized devices. In this review, we first introduce the nature and relevance of oxygen-dependent pathways in both physiological and pathological contexts. Subsequently, we discuss strategies to control oxygen in microfluidic devices, distinguishing between engineering approaches that operate at the device level during its fabrication and chemical approaches that involve the active perfusion of fluids oxygenated at a precise level or supplemented with oxygen-producing or oxygen-scavenging materials. In addition, we discuss readout approaches for monitoring oxygen levels at the cellular and tissue levels, focusing on electrochemical and optical detection schemes for high-resolution measurements directly on-chip. An overview of different applications in which microfluidic devices have been utilized to answer biological research questions is then provided. In the final section, we provide our vision for further technological refinements of oxygen-controlling devices and discuss how these devices can be employed to generate new fundamental insights regarding key scientific problems that call for emulating oxygen levels as encountered in vivo. We conclude by making the case that ultimately emulating physiological or pathological oxygen levels should become a standard feature in all in vitro cell, tissue, and organ models.

Role of prolyl hydroxylase domain proteins in bone metabolism

Cellular metabolism requires dissolved oxygen gas. Because evolutionary refinements have constrained mammalian dissolved oxygen levels, intracellular oxygen sensors are vital for optimizing the bioenergetic and biosynthetic use of dissolved oxygen. Prolyl hydroxylase domain (PHD) homologs 1-3 (PHD1/2/3) are molecular oxygen dependent non-heme dioxygenases whose enzymatic activity is regulated by the concentration of dissolved oxygen. PHD oxygen dependency has evolved into an important intracellular oxygen sensor. The most well studied mechanism of PHD oxygen-sensing is its regulation of the hypoxia-inducible factor (HIF) hypoxia signaling pathway. Heterodimeric HIF transcription factor activity is regulated post-translationally by selective PHD proline hydroxylation of its HIF1α subunit, accelerating HIF1α ubiquitination and proteasomal degradation, preventing HIF heterodimer assembly, nuclear accumulation, and activation of its target oxygen homeostasis genes. Phd2 has been shown to be the key isoform responsible for HIF1α subunit regulation in many cell types and accordingly disruption of the Phd2 gene results in embryonic lethality. In bone cells Phd2 is expressed in high abundance and tightly regulated. Conditional disruption of the Phd1, Phd2 and/or Phd3 gene in various bone cell types using different Cre drivers reveals a major role for PHD2 in skeletal growth and development. In this review, we will summarize the state of current knowledge on the role and mechanism of action of PHD2 as oxygen sensor in regulating bone metabolism.

Fibronectin Adherent Cell Populations Derived From Avascular and Vascular Regions of the Meniscus Have Enhanced Clonogenicity and Differentiation Potential Under Physioxia

The meniscus is composed of an avascular inner region and vascular outer region. The vascular region has been shown to contain a progenitor population with multilineage differentiation capacity. Strategies facilitating the isolation and propagation of these progenitors can be used to develop cell-based meniscal therapies. Differential adhesion to fibronectin has been used to isolate progenitor populations from cartilage, while low oxygen or physioxia (2% oxygen) enhances the meniscal phenotype. This study aimed to isolate progenitor populations from the avascular and vascular meniscus using differential fibronectin adherence and examine their clonogenicity and differentiation potential under hyperoxia (20% oxygen) and physioxia (2% oxygen). Human vascular and avascular meniscus cells were seeded onto fibronectin-coated dishes for a short period and monitored for colony formation under either hyperoxia or physioxia. Non-fibronectin adherent meniscus cells were also expanded under both oxygen tension. Individual fibronectin adherent colonies were isolated and further expanded, until approximately ten population doublings (passage 3), whereby they underwent chondrogenic, osteogenic, and adipogenic differentiation. Physioxia enhances clonogenicity of vascular and avascular meniscus cells on plastic or fibronectin-coated plates. Combined differential fibronectin adhesion and physioxia isolated a progenitor population from both meniscus regions with trilineage differentiation potential compared to equivalent hyperoxia progenitors. Physioxia isolated progenitors had a significantly enhanced meniscus matrix content without the presence of collagen X. These results demonstrate that combined physioxia and fibronectin adherence can isolate and propagate a meniscus progenitor population that can potentially be used to treat meniscal tears or defects.

The essential anti-angiogenic strategies in cartilage engineering and osteoarthritic cartilage repair

In the cartilage matrix, complex interactions occur between angiogenic and anti-angiogenic components, growth factors, and environmental stressors to maintain a proper cartilage phenotype that allows for effective load bearing and force distribution. However, as seen in both degenerative disease and tissue engineering, cartilage can lose its vascular resistance. This vascularization then leads to matrix breakdown, chondrocyte apoptosis, and ossification. Research has shown that articular cartilage inflammation leads to compromised joint function and decreased clinical potential for regeneration. Unfortunately, few articles comprehensively summarize what we have learned from previous investigations. In this review, we summarize our current understanding of the factors that stabilize chondrocytes to prevent terminal differentiation and applications of these factors to rescue the cartilage phenotype during cartilage engineering and osteoarthritis treatment. Inhibiting vascularization will allow for enhanced phenotypic stability so that we are able to develop more stable implants for cartilage repair and regeneration.

SETD7 regulates chondrocyte differentiation and glycolysis via the Hippo signaling pathway and HIF‑1α

Chondrocytes are well adapted to hypoxia and produce more functional extracellular matrix in low oxygen environments in vitro. In our previous study, methyltransferase SET domain containing (SETD)7 regulated chondrocyte activity in hypoxic conditions. However, the precise association between SETD7 and chondrocyte differentiation under low oxygen partial pressure remains unclear. The association between SETD7 and chondrocyte differentiation was studied by silencing SETD7 in chondrocytes in vitro. The results showed that the silencing of SETD7 in ATDC5 cells inhibited the Hippo signaling pathway, decreased Yes-associated protein (YAP) phosphorylation and increased the levels of YAP and hypoxia inducible factor-1α (HIF-1α) in the nucleus. YAP combined with HIF-1α to form a complex that promoted the expression of genes involved in chondrogenic differentiation and the glycolytic pathway. Thus, SETD7 inhibited chondrocyte differentiation and glycolysis via the Hippo signaling pathway. The present study demonstrated that SETD7 was a potential molecular target that maintained the chondrocyte phenotype during cartilage tissue engineering and cartilage-associated disease.

Cell-Membrane-Derived Nanoparticles with Notch-1 Suppressor Delivery Promote Hypoxic Cell-Cell Packing and Inhibit Angiogenesis Acting as a Two-Edged Sword

Cell-cell interactions regulate intracellular signaling via reciprocal contacts of cell membranes in tissue regeneration and cancer growth, indicating a critical need of membrane-derived tools in studying these processes. Hence, cell-membrane-derived nanoparticles (CMNPs) are produced using tonsil-derived mesenchymal stem cells (TMSCs) from children owing to their short doubling time. As target cell types, laryngeal cancer cells are compared to bone-marrow-derived MSCs (BMSCs) because of their cartilage damaging and chondrogenic characteristics, respectively. Treating spheroids of these cell types with CMNPs exacerbates interspheroid hypoxia with robust maintenance of the cell-cell interaction signature for 7 days. Both cell types prefer a hypoxic environment, as opposed to blood vessel formation that is absent in cartilage but is required for cancer growth. Hence, angiogenesis is inhibited by displaying the Notch-1 aptamer on CMNPs. Consequently, laryngeal cancer growth is suppressed efficiently in contrast to improved chondroprotection observed in a series of cell and animal experiments using a xenograft mouse model of laryngeal cancer. Altogether, CMNPs execute a two-edged sword function of inducing hypoxic cell-cell packing, followed by suppressing angiogenesis to promote laryngeal cancer death and chondrogenesis simultaneously. This study presents a previously unexplored therapeutic strategy for anti-cancer and chondroprotective treatment using CMNPs.

Adipose-PAS interactions in the context of its localised bio-engineering potential (Review)

Adipocytes are a known source of stem cells. They are easy to harvest, and are a suitable candidate for autogenous grafts. Adipose derived stem cells (ADSCs) have multiple target tissues which they can differentiate into, including bone and cartilage. In adipose tissue, ADSCs are able to differentiate, as well as providing energy and a supply of cytokines/hormones to manage the hypoxic and lipid/hormone saturated adipose environment. The plasminogen activation system (PAS) controls the majority of proteolytic activities in both adipose and wound healing environments, allowing for rapid cellular migration and tissue remodelling. While the primary activation pathway for PAS occurs through the urokinase plasminogen activator (uPA), which is highly expressed by endothelial cells, its function is not limited to enabling revascularisation. Proteolytic activity is dependent on protease activation, localisation, recycling mechanisms and substrate availability. uPA and uPA activated plasminogen allows pluripotent cells to arrive to new local environments and fulfil the niche demands. However, overstimulation, the acquisition of a migratory phenotype and constant protein turnover can be unconducive to the formation of structured hard and soft tissues. To maintain a suitable healing pattern, the proteolytic activity stimulated by uPA is modulated by plasminogen activator inhibitor 1. Depending on the physiological settings, different parts of the remodelling mechanism are activated with varying results. Utilising the differences within each microenvironment to recreate a desired niche is a valid therapeutic bio-engineering approach. By controlling the rate of protein turnover combined with a receptive stem cell lineage, such as ADSC, a novel avenue on the therapeutic opportunities may be identified, which can overcome limitations, such as scarcity of stem cells, low angiogenic potential or poor host tissue adaptation.

Roles of oxygen level and hypoxia-inducible factor signaling pathway in cartilage, bone and osteochondral tissue engineering

The repair and treatment of articular cartilage injury is a huge challenge of orthopedics. Currently, most of the clinical methods applied in treating cartilage injuries are mainly to relieve pains rather than to cure them, while the strategy of tissue engineering is highly expected to achieve the successful repair of osteochondral defects. Clear understandings of the physiological structures and mechanical properties of cartilage, bone and osteochondral tissues have been established, but the understanding of their physiological heterogeneity still needs further investigation. Apart from the gradients in the micromorphology and composition of cartilage-to-bone extracellular matrixes, an oxygen gradient also exists in natural osteochondral tissue. The response of hypoxia-inducible factor (HIF)-mediated cells to oxygen would affect the differentiation of stem cells and the maturation of osteochondral tissue. This article reviews the roles of oxygen level and HIF signaling pathway in the development of articular cartilage tissue, and their prospective applications in bone and cartilage tissue engineering. The strategies for regulating HIF signaling pathway and how these strategies finding their potential applications in the regeneration of integrated osteochondral tissue are also discussed.

Synergistic effects of Indian hedgehog and sonic hedgehog on chondrogenesis during cartilage repair

Sonic hedgehog (Shh) and Indian hedgehog (Ihh) have been shown to control the induction of early cartilaginous differentiation. However, it is unclear whether Ihh and Shh exert synergistic effects on chondrogenesis during articular cartilage repair. Herein, we investigate the effects of chondrogenesis of bone-derived mesenchymal stem cells (BMSCs) following co-transfection with Shh and Ihh via adenoviral vectors in vitro and in vivo. A rotary cell culture system (RCCS) and Cytodex 3 microcarriers were used to create a stereoscopic dynamic environment for cell culture. In the RCCS environment, BMSCs co-transfected with Ihh and Shh displayed stronger chondrogenic differentiation and chondrogenesis than BMSCs transfected with Ihh or Shh alone, and exhibited higher expression levels of Sox 9, ACAN and collagen II, stronger toluidine blue and collagen II immunohistochemical staining. After transplanted into the osteochondral defect at 8 weeks, Ihh/Shh co-transfected BMSCs showed a significantly better cartilage repair than BMSCs transfected with Ihh or Shh alone. Ihh and Shh have synergistic effects on the induction of chondrogenic differentiation and chondrogenesis under a microgravity environment, and help to repair damaged cartilage and reverse subchondral defects during the early stages.

Molecular and Cellular Effects of Chemical Chaperone-TUDCA on ER-Stressed NHAC-kn Human Articular Chondrocytes Cultured in Normoxic and Hypoxic Conditions

Osteoarthritis (OA) is considered one of the most common arthritic diseases characterized by progressive degradation and abnormal remodeling of articular cartilage. Potential therapeutics for OA aim at restoring proper chondrocyte functioning and inhibiting apoptosis. Previous studies have demonstrated that tauroursodeoxycholic acid (TUDCA) showed anti-inflammatory and anti-apoptotic activity in many models of various diseases, acting mainly via alleviation of endoplasmic reticulum (ER) stress. However, little is known about cytoprotective effects of TUDCA on chondrocyte cells. The present study was designed to evaluate potential effects of TUDCA on interleukin-1β (IL-1β) and tunicamycin (TNC)-stimulated NHAC-kn chondrocytes cultured in normoxic and hypoxic conditions. Our results showed that TUDCA alleviated ER stress in TNC-treated chondrocytes, as demonstrated by reduced CHOP expression; however, it was not effective enough to prevent apoptosis of NHAC-kn cells in either normoxia nor hypoxia. However, co-treatment with TUDCA alleviated inflammatory response induced by IL-1β, as shown by down regulation of Il-1β, Il-6, Il-8 and Cox2, and increased the expression of antioxidant enzyme Sod2. Additionally, TUDCA enhanced Col IIα expression in IL-1β- and TNC-stimulated cells, but only in normoxic conditions. Altogether, these results suggest that although TUDCA may display chondoprotective potential in ER-stressed cells, further analyses are still necessary to fully confirm its possible recommendation as potential candidate in OA therapy.

Retooling Cancer Nanotherapeutics' Entry into Tumors to Alleviate Tumoral Hypoxia

Anti-hypoxia cancer nanomedicine (AHCN) holds exciting potential in improving oxygen-dependent therapeutic efficiencies of malignant tumors. However, most studies regarding AHCN focus on optimizing structure and function of nanomaterials with presupposed successful entry into tumor cells. From such a traditional perspective, the main barrier that AHCN needs to overcome is mainly the tumor cell membrane. However, such an oversimplified perspective would neglect that real tumors have many biological, physiological, physical, and chemical defenses preventing the current state-of-the-art AHCNs from even reaching the targeted tumor cells. Fortunately, in recent years, some studies are beginning to intentionally focus on overcoming physiological barriers to alleviate hypoxia. In this Review, the limitations behind the traditional AHCN delivery mindset are addressed and the key barriers that need to be surmounted before delivery to cancer cells and some good ways to improve cell membrane attachment, internalization, and intracellular retention are summarized. It is aimed to contribute to Review literature on this emerging topic through refreshing perspectives based on this work and what is also learnt from others. This Review would therefore assist AHCNs researchers to have a quick overview of the essential information and glean thought-provoking ideas to advance this sub-field in cancer nanomedicine.

Physioxia Expanded Bone Marrow Derived Mesenchymal Stem Cells Have Improved Cartilage Repair in an Early Osteoarthritic Focal Defect Model

Focal early osteoarthritis (OA) or degenerative lesions account for 60% of treated cartilage defects each year. The current cell-based regenerative treatments have an increased failure rate for treating degenerative lesions compared to traumatic defects. Mesenchymal stem cells (MSCs) are an alternative cell source for treating early OA defects, due to their greater chondrogenic potential, compared to early OA chondrocytes. Low oxygen tension or physioxia has been shown to enhance MSC chondrogenic matrix content and could improve functional outcomes of regenerative therapies. The present investigation sought to develop a focal early OA animal model to evaluate cartilage regeneration and hypothesized that physioxic MSCs improve in vivo cartilage repair in both, post-trauma and focal early OA defects. Using a rabbit model, a focal defect was created, that developed signs of focal early OA after six weeks. MSCs cultured under physioxia had significantly enhanced in vitro MSC chondrogenic GAG content under hyperoxia with or without the presence of interleukin-1β (IL-1β). In both post-traumatic and focal early OA defect models, physioxic MSC treatment demonstrated a significant improvement in cartilage repair score, compared to hyperoxic MSCs and respective control defects. Future investigations will seek to understand whether these results are replicated in large animal models and the underlying mechanisms involved in in vivo cartilage regeneration.

Stress-activated miR-204 governs senescent phenotypes of chondrocytes to promote osteoarthritis development

A progressive loss of cartilage matrix leads to the development of osteoarthritis (OA). Matrix homeostasis is disturbed in OA cartilage as the result of reduced production of cartilage-specific matrix and increased secretion of catabolic mediators by chondrocytes. Chondrocyte senescence is a crucial cellular event contributing to such imbalance in matrix metabolism during OA development. Here, we identify miR-204 as a markedly up-regulated microRNA in OA cartilage. miR-204 is induced by transcription factors GATA4 and NF-κB in response to senescence signals. Up-regulated miR-204 simultaneously targets multiple components of the sulfated proteoglycan (PG) biosynthesis pathway, effectively shutting down PG anabolism. Ectopic expression of miR-204 in joints triggers spontaneous cartilage loss and OA development, whereas miR-204 inhibition ameliorates experimental OA, with concomitant recovery of PG synthesis and suppression of inflammatory senescence-associated secretory phenotype (SASP) factors in cartilage. Collectively, we unravel a stress-activated senescence pathway that underlies disrupted matrix homeostasis in OA cartilage.

The therapeutic effect of bone marrow-derived mesenchymal stem cells on osteoarthritis is improved by the activation of the KDM6A/SOX9 signaling pathway caused by exposure to hypoxia

Abnormal expression of KDM6A and SOX9 is a key factor in the pathogenesis of osteoarthritis (OA). Cellular treatments of OA with articular cartilage chondrocytes (ACCs) and bone marrow mesenchymal stem cells (BMSCs) are promising, but their underlying mechanisms remain to be explored. The pellet size, weight and sulfated glycosaminoglycan/DNA content of ACCs were measured to evaluate the effect of BMSCs on the chondrogenic differentiation of SCCs. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was used to analyze the proliferation of ACCs cultured along or cocultured with BMSCs. Quantitative polymerase chain reaction (qPCR) was performed to evaluate the messenger RNA expression of KDM6A, SOX9, type2 collagen, and Aggrecan in ACCs and OA rats. Western blot and immunohistochemistry were performed to analyze the expression of KDM6A and SOX9 proteins. Bisulfite sequencing PCR was performed to assess the DNA methylation level of the SOX9 promoter. Flow cytometry was used to evaluate the apoptotic status of ACCs. The chondrogenic differentiation of ACCs was significantly enhanced by coculturing with BMSCs, especially under a hypoxic condition. The expression of KDM6A, SOX9, type2 collagen, and Aggrecan was remarkably elevated in ACCs cocultured with BMSCs. Also, the DNA methylation of SOX9 promoter was decreased in ACCs cocultured with BMSCs, along with notably reduced apoptosis. Moreover, ACCs cocultured with BMSCs could repair cartilage lesions and prevent the abnormal expression of KDM6A, SOX9, type2 collagen, and Aggrecan in OA rats. In this study, we cocultured ACCs with BMSCs and used them to treat OA rats. Our findings presented a mechanistic basis for explaining the therapeutic effect of BMSCs on OA treatment.

Fibrin-Hyaluronic Acid Hydrogel (RegenoGel) with Fibroblast Growth Factor-18 for In Vitro 3D Culture of Human and Bovine Nucleus Pulposus Cells

We investigated the effects of a fibrin-hyaluronic acid hydrogel (FBG-HA) and fibroblast growth factor 18 (FGF-18) for nucleus pulposus (NP) regeneration. Healthy bovine (n = 4) and human degenerated NP cells (n = 4) were cultured for 14 days in FBG-HA hydrogel with FGF-18 (∆51-mutant or wild-type) in the culture medium. Gene expression, DNA content, and glycosaminoglycan (GAG) synthesis were evaluated on day 7 and 14. Additionally, histology was performed. Human NP cells cultured in FBG-HA hydrogel showed an increase in collagen type II (COL2) and carbonic anhydrase XII (CA12) gene expression after 14 or 7 days of culture, respectively. GAG release into the conditioned medium increased over 14 days. Healthy bovine NP cells showed increased gene expression of ACAN from day 7 to day 14. Wild type FGF-18 up-regulated CA12 gene expression of human NP cells. Histology revealed an increase of proteoglycan deposition upon FGF-18 stimulation in bovine but not in human NP cells. The FBG-HA hydrogel had a positive modulatory effect on human degenerated NP cells. Under the tested conditions, no significant effect of FGF-18 was observed on cell proliferation or GAG synthesis in human NP cells.

Physioxia Has a Beneficial Effect on Cartilage Matrix Production in Interleukin-1 Beta-Inhibited Mesenchymal Stem Cell Chondrogenesis

Osteoarthritis (OA) is a degenerative condition that involves the production of inflammatory cytokines (e.g., interleukin-1β (IL-1β), tumour necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6)) that stimulate degradative enzymes, matrix metalloproteinases (MMPs) and aggrecanases (ADAMTS) resulting in articular cartilage breakdown. The presence of interleukin-1β (IL-1β) is one reason for poor clinical outcomes in current cell-based tissue engineering strategies for treating focal early osteoarthritic defects. Mesenchymal stem cells (MSCs) are a potential cell source for articular cartilage regeneration, although IL-1β has been shown to inhibit in vitro chondrogenesis. In vivo, articular chondrocytes reside under a low oxygen environment between 2–5% oxygen (physioxia) and have been shown to enhance in vitro MSC chondrogenic matrix content with reduced hypertrophic marker expression under these conditions. The present investigation sought to understand the effect of physioxia on IL-1β inhibited MSC chondrogenesis. MSCs expanded under physioxic (2% oxygen) and hyperoxic (20%) conditions, then chondrogenically differentiated as pellets in the presence of TGF-β1 and either 0.1 or 0.5 ng/mL IL-1β. Results showed that there were donor variations in response to physioxic culture based on intrinsic GAG content under hyperoxia. In physioxia responsive donors, MSC chondrogenesis significantly increased GAG and collagen II content, whilst hypertrophic markers were reduced compared with hyperoxia. In the presence of IL-1β, these donors showed a significant increase in cartilage matrix gene expression and GAG content relative to hyperoxic conditions. In contrast, a set of MSC donors were unresponsive to physioxia and showed no significant increase in matrix production independent of IL-1β presence. Thus, physioxia has a beneficial effect on MSC cartilage matrix production in responsive donors with or without IL-1β application. The mechanisms controlling the MSC chondrogenic response in both physioxia responsive and unresponsive donors are to be elucidated in future investigations.

Advances in Molecular biomarker for early diagnosis of Osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease. The pathogenesis is poorly understood. What is known is that OA is characterized by imbalance in anabolic and catabolic gene expression in articular chondrocytes. This results in bone on bone articulations resulting in impaired mobility and joint pain. Although the cause of OA is unknown, comorbidities include: aging, obesity, and mechanical stress. Currently the only diagnostic modalities are radiology and physical examination, and early detection is rare. Biomarkers are quantifiable substances, and their presence can be suggestive of a certain phenomenon or disease. Biomarkers are popular for early diagnosis for pathological conditions in the fields of oncology, cardiology, and endocrinology. This review has systematically reviewed the literature about biomarkers in the field of OA, specifically protein, miRNA, and metabolic biomarkers found in the blood, urine, and synovial fluid.

Norepinephrine Inhibits Synovial Adipose Stem Cell Chondrogenesis via α2a-Adrenoceptor-Mediated ERK1/2 Activation

In recent years, first evidences emerged that sympathetic neurotransmitters influence osteoarthritis (OA) manifestation. Joint-resident stem cells might contribute to cartilage repair, however, their chondrogenic function is reduced. The neurotransmitter norepinephrine (NE) was detected in the synovial fluid of trauma and OA patients. Therefore, the aim of this study was to analyse how NE influences the chondrogenesis of synovial adipose tissue-derived stem cells (sASCs). sASCs were isolated from knee-OA patients synovia. After adrenoceptor (AR) expression analysis, proliferation and chondrogenic differentiation in presence of NE and/or α- and β-AR antagonist were investigated. Cell count, viability, chondrogenic and hypertophic gene expression, sulfated glycosaminoglycan (sGAG) and type II collagen content were determined. Key AR-dependent signaling (ERK1/2, PKA) was analyzed via western blot. sASC expressed α1A-, α1B-, α2A-, α2B-, α2C-, and β2-AR in monolayer and pellet culture. NE did not affect proliferation and viability, but 10⁻⁷ and 10⁻⁶ M NE significantly reduced sGAG and type II collagen content as well as ERK1/2 phosphorylation. These effects were fully reversed by yohimbine (α2-AR antagonist). Our study confirms the important role of NE in sASC chondrogenic function and provides new insights in OA pathophysiology. Future studies might help to develop novel therapeutic options targeting neuroendocrine pathways for OA treatment.

The effect of hypoxia on chondrogenesis of equine synovial membrane-derived and bone marrow-derived mesenchymal stem cells

BACKGROUND

Joint injury is extremely common in equine athletes and post-traumatic osteoarthritis (PTOA), a progressive and debilitating disease, is estimated to affect 60% of horses in the USA. The limited potential for intrinsic healing of articular cartilage has prompted intense efforts to identify a cell-based repair strategy to prevent progression of PTOA. Mesenchymal stem cells (MSCs) have the potential to become an ideal source for cell-based treatment of cartilage lesions; however, full chondrogenic differentiation remains elusive. Due to the relatively low oxygen tension in articular cartilage, hypoxia has been proposed as a method of increasing MSC chondrogenesis. The objective of this study was to investigate the effect of hypoxic culture conditions on chondrogenesis in equine synovial membrane-derived MSCs (SM-MSCs) and bone marrow-derived MSCs (BM-MSCs). MSCs were isolated from synovial membrane and bone marrow collected from 5 horses. Flow cytometric analysis was used to assess cell surface marker expression including CD29, CD44, CD90, CD105, CD45, CD-79α, MHCI and MHCII. MSC pellets were cultured in normoxic (21% O₂) or in hypoxic (5% O₂) culture conditions for 28 days. Following the culture period, chondrogenesis was assessed by histology, biochemical analyses and gene expression of chondrogenic-related genes including ACAN, COL2b, SOX9, and COL10A1.

RESULTS

Both cell types expressed markers consistent with stemness including CD29, CD44, CD90, CD105, and MHCI and were negative for exclusion markers (CD45, CD79α, and MHCII). Although the majority of outcome variables of chondrogenic differentiation were not significantly different between cell types or culture conditions, COL10A1 expression, a marker of chondrocyte hypertrophy, was lowest in hypoxic SM-MSCs and was significantly lower in hypoxic SM-MSCs compared to hypoxic BM-MSCs.

CONCLUSIONS

Hypoxic culture conditions do not appear to increase chondrogenesis of equine SM-MSCs or BM-MSCs; however, hypoxia may downregulate the hypertrophic marker COL10A1 in SM-MSCs.

Engineering of gradient osteochondral tissue: From nature to lab

The osteochondral tissue is an interface between two distinct tissues: articular cartilage and bone. These two tissues are significantly diverse with regard to their chemical compositions, mechanical properties, structure, electrical properties, and the amount of nutrient and oxygen consumption. Thus, transition from the surface of the articular cartilage to the subchondral bone needs to face several smooth gradients. These gradients are imperative to study to generate a scaffold suitable for the reconstruction of the cartilaginous and osseous layers of a defected osteochondral tissue, simultaneously. The aim of this review is to peruse the alternation of biochemical, biomechanical, structural, electrical, and metabolic properties of the osteochondral tissue moving from the surface of the articular cartilage to the subchondral bone. Moreover, this review also discusses currently developed approaches and ideal techniques with a focus on gradients present in the interface of the cartilage and bone. STATEMENT OF SIGNIFICANCE: The submitted review paper entitled as "Engineering of the gradient osteochondral tissue: from nature to lab" is a complete review with regard to the osteochondral tissue and transition of different properties between the cartilage and bone tissues. Moreover, previous studies on the osteochondral tissue engineering have been reviewed in this paper. This complete information can be a valuable and useful source for current and future researchers and scientists. Considering the scope of the submitted paper, Acta Biomaterialia would be a suitable journal for publishing this article.

The Importance of Physioxia in Mesenchymal Stem Cell Chondrogenesis and the Mechanisms Controlling Its Response

Articular cartilage covers the surface of synovial joints and enables joint movement. However, it is susceptible to progressive degeneration with age that can be accelerated by either previous joint injury or meniscectomy. This degenerative disease is known as osteoarthritis (OA) and it greatly affects the adult population. Cell-based tissue engineering provides a possible solution for treating OA at its earliest stages, particularly focal cartilage lesions. A candidate cell type for treating these focal defects are Mesenchymal Stem Cells (MSCs). However, present methods for differentiating these cells towards the chondrogenic lineage lead to hypertrophic chondrocytes and bone formation in vivo. Environmental stimuli that can stabilise the articular chondrocyte phenotype without compromising tissue formation have been extensively investigated. One factor that has generated intensive investigation in MSC chondrogenesis is low oxygen tension or physioxia (2⁻5% oxygen). In vivo articular cartilage resides at oxygen tensions between 1⁻4%, and in vitro results suggest that these conditions are beneficial for MSC expansion and chondrogenesis, particularly in suppressing the cartilage hypertrophy. This review will summarise the current literature regarding the effects of physioxia on MSC chondrogenesis with an emphasis on the pathways that control tissue formation and cartilage hypertrophy.

Regeneration of Osteochondral Defects Using Developmentally Inspired Cartilaginous Templates

IMPACT STATEMENT

Successfully treating osteochondral defects involves regenerating both the damaged articular cartilage and the underlying subchondral bone, in addition to the complex interface that separates these tissues. In this study, we demonstrate that a cartilage template, engineered using bone marrow-derived mesenchymal stem cells, can enhance the regeneration of such defects and promote the development of a more mechanically functional repair tissue. We also use a computational mechanobiological model to understand how joint-specific environmental factors, specifically oxygen levels and tissue strains, regulate the conversion of the engineered template into cartilage and bone in vivo.

Role of growth factors and oxygen to limit hypertrophy and impact of high magnetic nanoparticles dose during stem cell chondrogenesis

Due to an unmet clinical need of curative treatments for osteoarthritic patients, tissue engineering strategies that propose the development of cartilage tissue replacements from stem cells have emerged. Some of these strategies are based on the internalization of magnetic nanoparticles into stem cells to then initiate the chondrogenesis via magnetic compaction. A major difficulty is to drive the chondrogenic differentiation of the cells such as they produce an extracellular matrix free of hypertrophic collagen. An additional difficulty has to be overcome when nanoparticles are used, knowing that a high dose of nanoparticles can limit the chondrogenesis. We here propose a gene-based analysis of the effects of chemical factors (growth factors, hypoxia) on the chondrogenic differentiation of human mesenchymal stem cells both with and without nanoparticles. We focus on the synthesis of two of the most important constituents present in the cartilaginous extracellular matrix (Collagen II and Aggrecan) and on the expression of collagen X, the signature of hypertrophic cartilage, in order to provide a quantitative index of the type of cartilage produced (i.e. hyaline, hypertrophic). We demonstrate that by applying specific environmental conditions, gene expression can be directed toward the production of hyaline cartilage, with limited hypertrophy. Besides, a combination of the growth factors IGF-1, TGF-β3, with a hypoxic conditioning remarkably reduced the impact of high nanoparticles concentration.

Hypoxic condition enhances chondrogenesis in synovium-derived mesenchymal stem cells

Background

The chondrogenic differentiation of mesenchymal stem cells (MSCs) is regulated by many factors, including oxygen tensions, growth factors, and cytokines. Evidences have suggested that low oxygen tension seems to be an important regulatory factor in the proliferation and chondrogenic differentiation in various MSCs. Recent studies report that synovium-derived mesenchymal stem cells (SDSCs) are a potential source of stem cells for the repair of articular cartilage defects. But, the effect of low oxygen tension on the proliferation and chondrogenic differentiation in SDSCs has not characterized. In this study, we investigated the effects of hypoxia on proliferation and chondrogenesis in SDSCs.

Method

SDSCs were isolated from patients with osteoarthritis at total knee replacement. To determine the effect of oxygen tension on proliferation and colony-forming characteristics of SDSCs, A colony-forming unit (CFU) assay and cell counting-based proliferation assay were performed under normoxic (21% oxygen) or hypoxic (5% oxygen). For in vitro chondrogenic differentiation, SDSCs were concentrated to form pellets and subjected to conditions appropriate for chondrogenic differentiation under normoxia and hypoxia, followed by the analysis for the expression of genes and proteins of chondrogenesis. qRT-PCR, histological assay, and glycosoaminoglycan assays were determined to assess chondrogenesis.

Results

Low oxygen condition significantly increased proliferation and colony-forming characteristics of SDSCs compared to that of SDSCs under normoxic culture. Similar pellet size and weight were found for chondrogensis period under hypoxia and normoxia condition. The mRNA expression of types II collagen, aggrecan, and the transcription factor SOX9 was increased under hypoxia condition. Histological sections stained with Safranin-O demonstrated that hypoxic conditions had increased proteoglycan synthesis. Immunohistochemistry for types II collagen demonstrated that hypoxic culture of SDSCs increased type II collagen expression. In addition, GAG deposition was significantly higher in hypoxia compared with normoxia at 21 days of differentiation.

Conclusion

These findings show that hypoxia condition has an important role in regulating the synthesis ECM matrix by SDSCs as they undergo chondrogenesis. This has important implications for cartilage tissue engineering applications of SDSCs.

Repair of Damaged Articular Cartilage: Current Approaches and Future Directions

Articular hyaline cartilage is extensively hydrated, but it is neither innervated nor vascularized, and its low cell density allows only extremely limited self-renewal. Most clinical and research efforts currently focus on the restoration of cartilage damaged in connection with osteoarthritis or trauma. Here, we discuss current clinical approaches for repairing cartilage, as well as research approaches which are currently developing, and those under translation into clinical practice. We also describe potential future directions in this area, including tissue engineering based on scaffolding and/or stem cells as well as a combination of gene and cell therapy. Particular focus is placed on cell-based approaches and the potential of recently characterized chondro-progenitors; progress with induced pluripotent stem cells is also discussed. In this context, we also consider the ability of different types of stem cell to restore hyaline cartilage and the importance of mimicking the environment in vivo during cell expansion and differentiation into mature chondrocytes.

Comparing atmospheric and hypoxic cultured mesenchymal stem cell transcriptome: implication for stem cell therapies targeting intervertebral discs

Background

Mesenchymal stem cells (MSCs) represent an attractive avenue for cellular therapies targeting degenerative diseases. MSC in vitro expansion is required in order to obtain therapeutic numbers during the manufacturing process. It is known that culture conditions impact cellular properties and behavior after in vivo transplantation. In this study, we aimed at evaluating the benefit of hypoxic culturing of human bone marrow derived mesenchymal stem cells on cell fitness and whole genome expression and discussed its implication on cellular therapies targeting orthopedic diseases such as chronic lower back pain.

Methods

Human bone marrow mesenchymal stem cells (hBMMSCs) were isolated from fresh human anticoagulated whole bone marrow and were cultured side by side in atmospheric (20% O₂) and hypoxic (5% O₂) oxygen partial pressure for up to 3 passages. Stem cell fitness was assessed by clonogenic assay, cell surface marker expression and differentiation potential. Whole genome expression was performed by mRNA sequencing. Data from clonogenic assays, cell surface marker by flow cytometry and gene expression by quantitative PCR were analyzed by two-tailed paired Student’s t-test. Data from mRNA sequencing were aligned to hg19 using Tophat-2.0.13 and analyzed using Cufflinks-2.1.1.

Results

Hypoxic culturing of hBMMSCs had positive effects on cell fitness, as evidenced by an increased clonogenicity and improved differentiation potential towards adipocyte and chondrocyte lineages. No difference in osteoblast differentiation or in cell surface markers were observed. Only a small subset of genes (34) were identified by mRNA sequencing to be significantly dysregulated by hypoxia. When clustered by biological function, these genes were associated with chondrogenesis and cartilage metabolism, inflammation and immunomodulation, cellular survival, migration and proliferation, vasculogenesis and angiogenesis.

Conclusions

Hypoxic culturing positively impacted hBMMSCs fitness and transcriptome, potentially improving inherent properties of these cells that are critical for the development of successful cellular therapies. Hypoxic culturing should be considered for the in vitro expansion of hBMMSCs during manufacturing of cellular therapies targeting orthopedic disorders such as lower back pain.

Fine-tuning pro-angiogenic effects of cobalt for simultaneous enhancement of vascular endothelial growth factor secretion and implant neovascularization

Rapid neovascularization of a tissue-engineered (TE) construct by the host vasculature is quintessential to warrant effective bone regeneration. This process can be promoted through active induction of angiogenic growth factor secretion or by implementation of in vitro pre-vascularization strategies. In this study, we aimed at optimizing the pro-angiogenic effect of Cobalt (Co²⁺) to enhance vascular endothelial growth factor (VEGF) expression by human periosteum-derived mesenchymal stem cells (hPDCs). Simultaneously we set out to promote microvascular network formation by co-culturing with human umbilical vein endothelial cells (HUVECs). The results showed that Co²⁺ treatments (at 50, 100 or 150 µM) significantly upregulated in vitro VEGF expression, but inhibited hPDCs growth and HUVECs network formation in co-cultures. These inhibitory effects were mitigated at lower Co²⁺ concentrations (at 5, 10 or 25 µM) while VEGF expression remained significantly upregulated and further augmented in the presence of Ascorbic Acid and Dexamethasone possibly through Runx2 upregulation. The supplements also facilitated HUVECs network formation, which was dependent on the quantity and spatial distribution of collagen type-1 matrix deposited by the hPDCs. When applied to hPDCs seeded onto calcium phosphate scaffolds, the supplements significantly induced VEGF secretion in vitro, and promoted higher vascularization upon ectopic implantation in nude mice shown by an increase of CD31 positive blood vessels within the scaffolds. Our findings provided novel insights into the pleotropic effects of Co²⁺ on angiogenesis (i.e. promoted VEGF secretion and inhibited endothelial network formation), and showed potential to pre-condition TE constructs under one culture regime for improved implant neovascularization in vivo.

STATEMENT OF SIGNIFICANT

Cobalt (Co²⁺) is known to upregulate vascular endothelial growth factor (VEGF) secretion, however it also inhibits in vitro angiogenesis through unknown Co²⁺-induced events. This limits the potential of Co²⁺ for pro-angiogenesis of tissue engineered (TE) implants. We showed that Co²⁺ upregulated VEGF expression by human periosteum-derived cells (hPDCs) but reduced the cell growth, and endothelial network formation due to reduction of col-1 matrix deposition. Supplementation with Ascorbic acid and Dexamethasone concurrently improved hPDCs growth, endothelial network formation, and upregulated VEGF secretion. In vitro pre-conditioning of hPDC-seeded TE constructs with this fine-tuned medium enhanced VEGF secretion and implant neovascularization. Our study provided novel insights into the pleotropic effects of Co²⁺ on angiogenesis and formed the basis for improving implant neovascularization.

Validation of the Missouri Osteochondral Allograft Preservation System for the Maintenance of Osteochondral Allograft Quality During Prolonged Storage

BACKGROUND

Fresh osteochondral allografts (OCAs) are limited in availability. The Missouri Osteochondral Allograft Preservation System (MOPS) has been reported to effectively preserve OCAs twice as long as current tissue bank protocols in preclinical studies.

HYPOTHESIS

The viable chondrocyte density (VCD) in OCAs preserved for up to 70 days using the MOPS will not be significantly different from day 0, and the VCD in MOPS-preserved OCAs will be significantly higher than for standard tissue bank preservation. Media changes during preservation will significantly improve the VCD.

STUDY DESIGN

Controlled laboratory study.

METHODS

Femoral condyles harvested from qualified donors (n = 12) were quartered (n = 48), assigned to 1 of 4 treatment groups (tissue bank protocol at 4°C or MOPS at 25°C, with or without media changes), and preserved for 0, 28, 56, or 70 days and assessed for the VCD and histopathological characteristics. In addition, osteochondral explants were created from the femoral condyles of 12 donors (n = 36 explants), assigned to the same groups and time points, and tested for biomechanical properties.

RESULTS

MOPS-preserved OCAs maintained the day 0 VCD through 56 days. OCAs stored using current tissue bank protocols had a significantly lower VCD compared with day 0 and the MOPS by day 28. OCA histological and biomechanical properties did not significantly change from day 0 for any group.

CONCLUSION

The MOPS preserved essential OCA viability and quality at significantly higher levels than current tissue bank protocols for at least 56 days after procurement.

CLINICAL RELEVANCE

Improving the viability and duration of OCA preservation provides potential benefits to tissue banks, donor families, surgeons, and patients with respect to tissue use, financial costs, and outcomes.