Hypoxia induces chondrocyte-specific gene expression in mesenchymal cells in association with transcriptional activation of Sox9

Stem-Cell-Driven Chondrogenesis: Perspectives on Amnion-Derived Cells

Regenerative medicine harnesses stem cells' capacity to restore damaged tissues and organs. In vitro methods employing specific bioactive molecules, such as growth factors, bio-inductive scaffolds, 3D cultures, co-cultures, and mechanical stimuli, steer stem cells toward the desired differentiation pathways, mimicking their natural development. Chondrogenesis presents a challenge for regenerative medicine. This intricate process involves precise modulation of chondro-related transcription factors and pathways, critical for generating cartilage. Cartilage damage disrupts this process, impeding proper tissue healing due to its unique mechanical and anatomical characteristics. Consequently, the resultant tissue often forms fibrocartilage, which lacks adequate mechanical properties, posing a significant hurdle for effective regeneration. This review comprehensively explores studies showcasing the potential of amniotic mesenchymal stem cells (AMSCs) and amniotic epithelial cells (AECs) in chondrogenic differentiation. These cells exhibit innate characteristics that position them as promising candidates for regenerative medicine. Their capacity to differentiate toward chondrocytes offers a pathway for developing effective regenerative protocols. Understanding and leveraging the innate properties of AMSCs and AECs hold promise in addressing the challenges associated with cartilage repair, potentially offering superior outcomes in tissue regeneration.

3D printed hybrid scaffolds do not induce adverse inflammation in mice and direct human BM-MSC chondrogenesis in vitro

Biomaterials that can improve the healing of articular cartilage lesions are needed. To address this unmet need, we developed novel 3D printed silica/poly(tetrahydrofuran)/poly(ε-caprolactone) (SiO2/PTHF/PCL-diCOOH) hybrid scaffolds. Our aim was to carry out essential studies to advance this medical device towards functional validation in pre-clinical trials. First, we show that the chemical composition, microarchitecture and mechanical properties of these scaffolds were not affected by sterilisation with gamma irradiation. To evaluate the systemic and local immunogenic reactivity of the sterilised 3D printed hybrid scaffolds, they were implanted subcutaneously into Balb/c mice. The scaffolds did not trigger a systemic inflammatory response over one week of implantation. The interaction between the host immune system and the implanted scaffold elicited a local physiological reaction with infiltration of mononuclear cells without any signs of a chronic inflammatory response. Then, we investigated how these 3D printed hybrid scaffolds direct chondrogenesis in vitro. Human bone marrow-derived mesenchymal stem/stromal cells (hBM-MSCs) seeded within the 3D printed hybrid scaffolds were cultured under normoxic or hypoxic conditions, with or without chondrogenic supplements. Chondrogenic differentiation assessed by both gene expression and protein production analyses showed that 3D printed hybrid scaffolds support hBM-MSC chondrogenesis. Articular cartilage-specific extracellular matrix deposition within these scaffolds was enhanced under hypoxic conditions (1.7 or 3.7 fold increase in the median of aggrecan production in basal or chondrogenic differentiation media). Our findings show that 3D printed SiO2/PTHF/PCL-diCOOH hybrid scaffolds have the potential to support the regeneration of cartilage tissue.

SOX9 switch links regeneration to fibrosis at the single-cell level in mammalian kidneys

The steps governing healing with or without fibrosis within the same microenvironment are unclear. After acute kidney injury (AKI), injured proximal tubular epithelial cells activate SOX9 for self-restoration. Using a multimodal approach for a head-to-head comparison of injury-induced SOX9 lineages, we identified a dynamic SOX9 switch in repairing epithelia. Lineages that regenerated epithelia silenced SOX9 and healed without fibrosis (SOX9on-off). By contrast, lineages with unrestored apicobasal polarity maintained SOX9 activity in sustained efforts to regenerate, which were identified as a SOX9on-on Cadherin6pos cell state. These reprogrammed cells generated substantial single-cell WNT activity to provoke a fibroproliferative response in adjacent fibroblasts, driving AKI to chronic kidney disease. Transplanted human kidneys displayed similar SOX9/CDH6/WNT2B responses. Thus, we have uncovered a sensor of epithelial repair status, the activity of which determines regeneration with or without fibrosis.

Hypoxia and Hypoxia Mimetic Agents As Potential Priming Approaches to Empower Mesenchymal Stem Cells

Mesenchymal stem cells (MSC) exhibit self-renewal capacity and multilineage differentiation potential, making them attractive for research and clinical application. The properties of MSC can vary depending on specific micro-environmental factors. MSC resides in specific niches with low oxygen concentrations, where oxygen functions as a metabolic substrate and a signaling molecule. Conventional physical incubators or chemically hypoxia mimetic agents are applied in cultures to mimic the original low oxygen tension settings where MSC originated. This review aims to focus on the current knowledge of the effects of various physical hypoxic conditions and widely used hypoxia-mimetic agents-PHD inhibitors on mesenchymal stem cells at a cellular and molecular level, including proliferation, stemness, differentiation, viability, apoptosis, senescence, migration, immunomodulation behaviors, as well as epigenetic changes.

Poly(ε-caprolactone)/bioactive glass composite electrospun fibers for tissue engineering applications

In this work, composite electrospun fibers containing innovative bioactive glass nanoparticles were produced and characterized. Poly(ε-caprolactone), benign solvents, and sol-gel B- and Cu-doped bioactive glass powders were used to fabricate fibrous scaffolds. The retention of bioactive glass nanoparticles in the polymer matrix, the electrospinnability of this novel solution and the obtained electrospun composites were extensively characterized. As a result, composite electrospun fibers characterized by biocompatibility, bioactivity, and exhibiting overall properties adequate for both hard and soft tissue engineering applications, have been produced. The addition of these bioactive glass nanoparticles was, indeed, able to impart bioactive properties to the fibers. Cell culture studies show promising results, demonstrating proliferation and growth of cells on the composite fibers. Wettability, degradation rate, and mechanical performance were also tested and are in line with previous results.

Hypoxia Differentially Affects Chondrogenic Differentiation of Progenitor Cells from Different Origins

Background and Objectives

Ear cartilage malformations are commonly encountered problems in reconstructive surgery, since cartilage has low self-regenerating capacity. Malformations that impose psychological and social burden on one's life are currently treated using ear prosthesis, synthetic implants or autologous flaps from rib cartilage. These approaches are challenging because not only they request high surgical expertise, but also they lack flexibility and induce severe donor-site morbidity. Through the last decade, tissue engineering gained attention where it aims at regenerating human tissues or organs in order to restore normal functions. This technique consists of three main elements, cells, growth factors, and above all, a scaffold that supports cells and guides their behavior. Several studies have investigated different scaffolds prepared from both synthetic or natural materials and their effects on cellular differentiation and behavior.

Methods and Results

In this study, we investigated a natural scaffold (alginate) as tridimensional hydrogel seeded with progenitors from different origins such as bone marrow, perichondrium and dental pulp. In contact with the scaffold, these cells remained viable and were able to differentiate into chondrocytes when cultured in vitro. Quantitative and qualitative results show the presence of different chondrogenic markers as well as elastic ones for the purpose of ear cartilage, upon different culture conditions.

Conclusions

We confirmed that auricular perichondrial cells outperform other cells to produce chondrogenic tissue in normal oxygen levels and we report for the first time the effect of hypoxia on these cells. Our results provide updates for cartilage engineering for future clinical applications.

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.

Modeling human skeletal development using human pluripotent stem cells

Chondrocytes and osteoblasts differentiated from induced pluripotent stem cells (iPSCs) will provide insights into skeletal development and genetic skeletal disorders and will generate cells for regenerative medicine applications. Here, we describe a method that directs iPSC-derived sclerotome to chondroprogenitors in 3D pellet culture then to articular chondrocytes or, alternatively, along the growth plate cartilage pathway to become hypertrophic chondrocytes that can transition to osteoblasts. Osteogenic organoids deposit and mineralize a collagen I extracellular matrix (ECM), mirroring in vivo endochondral bone formation. We have identified gene expression signatures at key developmental stages including chondrocyte maturation, hypertrophy, and transition to osteoblasts and show that this system can be used to model genetic cartilage and bone disorders.

The Role of HIF-1α in Bone Regeneration: A New Direction and Challenge in Bone Tissue Engineering

The process of repairing significant bone defects requires the recruitment of a considerable number of cells for osteogenesis-related activities, which implies the consumption of a substantial amount of oxygen and nutrients. Therefore, the limited supply of nutrients and oxygen at the defect site is a vital constraint that affects the regenerative effect, which is closely related to the degree of a well-established vascular network. Hypoxia-inducible factor (HIF-1α), which is an essential transcription factor activated in hypoxic environments, plays a vital role in vascular network construction. HIF-1α, which plays a central role in regulating cartilage and bone formation, induces vascular invasion and differentiation of osteoprogenitor cells to promote and maintain extracellular matrix production by mediating the adaptive response of cells to changes in oxygen levels. However, the application of HIF-1α in bone tissue engineering is still controversial. As such, clarifying the function of HIF-1α in regulating the bone regeneration process is one of the urgent issues that need to be addressed. This review provides insight into the mechanisms of HIF-1α action in bone regeneration and related recent advances. It also describes current strategies for applying hypoxia induction and hypoxia mimicry in bone tissue engineering, providing theoretical support for the use of HIF-1α in establishing a novel and feasible bone repair strategy in clinical settings.

Differential Effects of Hypoxia versus Hyperoxia or Physoxia on Phenotype and Energy Metabolism in Human Chondrocytes from Osteoarthritic Compared to Macroscopically Normal Cartilage

Chondrocyte phenotype and energy metabolism are altered in osteoarthritis (OA). However, most studies characterising the change in human chondrocyte behaviour in OA have been conducted in supraphysiological oxygen concentrations. The purpose of this study was to compare phenotype and energy metabolism in chondrocytes from macroscopically normal (MN) and OA cartilage maintained in 18.9% (standard tissue culture), 6% (equivalent to superficial zone of cartilage in vivo) or 1% oxygen (equivalent to deep zone of cartilage in vivo). MMP13 production was higher in chondrocytes from OA compared to MN cartilage in hyperoxia and physoxia but not hypoxia. Hypoxia promoted SOX9, COL2A1 and ACAN protein expression in chondrocytes from MN but not OA cartilage. OA chondrocytes used higher levels of glycolysis regardless of oxygen availability. These results show that differences in phenotype and energy metabolism between chondrocytes from OA and MN cartilage differ depending on oxygen availability. OA chondrocytes show elevated synthesis of cartilage-catabolising enzymes and chondrocytes from MN cartilage show reduced cartilage anabolism in oxygenated conditions. This is relevant as a recent study has shown that oxygen levels are elevated in OA cartilage in vivo. Our findings may indicate that this elevated cartilage oxygenation may promote cartilage loss in OA.

Construction of Bone Hypoxic Microenvironment Based on Bone-on-a-Chip Platforms

The normal physiological activities and functions of bone cells cannot be separated from the balance of the oxygenation level, and the physiological activities of bone cells are different under different oxygenation levels. At present, in vitro cell cultures are generally performed in a normoxic environment, and the partial pressure of oxygen of a conventional incubator is generally set at 141 mmHg (18.6%, close to the 20.1% oxygen in ambient air). This value is higher than the mean value of the oxygen partial pressure in human bone tissue. Additionally, the further away from the endosteal sinusoids, the lower the oxygen content. It follows that the construction of a hypoxic microenvironment is the key point of in vitro experimental investigation. However, current methods of cellular research cannot realize precise control of oxygenation levels at the microscale, and the development of microfluidic platforms can overcome the inherent limitations of these methods. In addition to discussing the characteristics of the hypoxic microenvironment in bone tissue, this review will discuss various methods of constructing oxygen gradients in vitro and measuring oxygen tension from the microscale based on microfluidic technology. This integration of advantages and disadvantages to perfect the experimental study will help us to study the physiological responses of cells under more physiological-relevant conditions and provide a new strategy for future research on various in vitro cell biomedicines.

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.

HIF signaling: A new propellant in bone regeneration

Bone tissue destruction leads to severe pain, physical flaws, and loss of motility. Bone repair using biocompatible and osteo-inductive scaffolds is regarded as a viable and potential therapeutic approach. However, for large-scale bone regeneration, oxygen and nutrient supply have become limiting factors. Further, a considerable need exists for recruited cell activities and blood vessel growth. Hypoxia-inducible factor (HIF) signaling pathways induced by hypoxia are involved in angiogenesis and osteogenesis. As an important transcription factor, HIF-1 functions by modulating vital genes, such as VEGF, PDK1, and EPO, and is a crucial regulator that influences the final fate of bone regeneration. Collectively, to achieve better osteogenesis results, the in-depth molecular mechanisms that underpin the links between materials, cells, and HIF signaling pathways must be determined. This review aimed to provide an in-depth insight into recent progress in HIF-regulated bone regeneration. Hypoxia and cellular oxygen-sensing mechanisms and their correlations with osteogenesis were determined, and recent studies on hypoxia-inducing and hypoxia-mimicking strategies were briefly described. Finally, the potential applications of HIF signaling in bone regeneration were highlighted. This review provides theoretical support for establishing a novel and viable bone repair strategy in the clinic by harnessing HIF signaling.

Possible roles of anti-type II collagen antibody and innate immunity in the development and progression of diabetic retinopathy

The pathogenesis of both diabetic retinopathy (DR) and rheumatoid arthritis (RA) has recently been considered to involve autoimmunity. Serum and synovial fluid levels of anti-type II collagen antibodies increase early after the onset of RA, thus inducing immune responses and subsequent hydrarthrosis and angiogenesis, which resemble diabetic macular edema and proliferative DR (PDR), respectively. We previously reported that DR is also associated with increased serum levels of anti-type II collagen antibodies. Retinal hypoxia in DR may induce pericytes to express type II collagen, resulting in autoantibody production against type II collagen. As the result of blood-retinal barrier disruption, anti-type II collagen antibodies in the serum come into contact with type II collagen around the retinal vessels. A continued loss of pericytes and type II collagen around the retinal vessels may result in a shift of the immune reaction site from the retina to the vitreous. It has been reported that anti-inflammatory M2 macrophages increased in the vitreous of PDR patients, accompanied by the activation of the NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome, a key regulator of innate immunity. M2 macrophages promote angiogenesis and fibrosis, which might be exacerbated and prolonged by dysregulated innate immunity.

The Impact of Trace Elements on Osteoarthritis

Osteoarthritis (OA) is a progressive degenerative disease characterized by cartilage degradation, synovial inflammation, subchondral sclerosis and osteophyte formation. It has a multifactorial etiology with potential contributions from heredity, endocrine function, abnormal mechanical load and nutrition. Of particular considerations are trace element status. Several trace elements, such as boron and magnesium are essential for normal development of the bone and joint in human. While cadmium correlates with the severity of OA. The present review focuses on the roles of trace elements (boron, cadmium, copper, iron, magnesium, manganese, selenium, zinc) in OA and explores the mechanisms by which they act.

Strategies for Articular Cartilage Repair and Regeneration

Articular cartilage is an avascular tissue, with limited ability to repair and self-renew. Defects in articular cartilage can induce debilitating degenerative joint diseases such as osteoarthritis. Currently, clinical treatments have limited ability to repair, for they often result in the formation of mechanically inferior cartilage. In this review, we discuss the factors that affect cartilage homeostasis and function, and describe the emerging regenerative approaches that are informing the future treatment options.

Hypoxia-Inducible Non-coding RNAs in Mesenchymal Stem Cell Fate and Regeneration

Mesenchymal stem cells (MSCs) can differentiate into multiple cell lines, which makes them an important source of cells for tissue engineering applications. They are defined by the capability to renew themselves and maintain pluripotency. This ability is modulated by the balance between complex cues from cellular microenvironment. Self-renewal and differentiation abilities are regulated by particular microenvironmental signals. Oxygen is considered to be an important part of cell microenvironment, which not only acts as a metabolic substrate but also a signal molecule. It has been proved that MSCs are hypoxic in the physiological environment. Signals from MSCs' microenvironment or niche which means the anatomical location of the MSCs, maintain the final properties of MSCs. Physiological conditions like oxygen tension are deemed to be a significant part of the mesenchymal stem cell niche, and have been proved to be involved in modulating embryonic and adult MSCs. Non-coding RNAs (ncRNAs), which play a key role in cell signal transduction, transcription and translation of genes, have been widely concerned as epigenetic regulators in a great deal of tissues. With the rapid development of bioinformatics analysis tools and high-throughput RNA sequencing technology, more and more evidences show that ncRNAs play a key role in tissue regeneration. It shows potential as a biomarker of MSC differentiation. In this paper, we reviewed the physiological correlation of hypoxia as a unique environmental parameter which is conducive to MSC expansion and maintenance, discussed the correlation of tissue engineering, and summarized the influence of hypoxia related ncRNAs on MSCs' fate and regeneration. This review will provide reference for future research of MSCs' regeneration.

Iron homeostasis in arthropathies: From pathogenesis to therapeutic potential

Iron is an essential element for proper functioning of cells within mammalian organ systems; in particular, iron homeostasis is critical for joint health. Excess iron can induce oxidative stress damage, associated with the pathogenesis of iron-storage and ageing-related diseases. Therefore, iron levels in body tissues and cells must be tightly regulated. In the past decades, excess iron content within joints has been found in some patients with joint diseases including hemophilic arthropathy, hemochromatosis arthropathy, and osteoarthritis (OA). Currently, increased evidence has shown that iron accumulation is closely associated with multiple pathological changes of these arthropathies. This review summarizes system-level and intracellular regulation of iron homeostasis, and emphasizes the role of iron in synovial alterations, cartilage degeneration, and subchondral bone of several arthropathies. Of note, we discuss the potential link between iron homeostasis and OA pathogenesis. Finally, we discuss the therapeutic potential of maintaining iron homeostasis in these arthropathies.

DMOG Negatively Impacts Tissue Engineered Cartilage Development

OBJECTIVE

Articular cartilage exists in a hypoxic environment, which motivates the use of hypoxia-simulating chemical agents to improve matrix production in cartilage tissue engineering. The aim of this study was to investigate whether dimethyloxalylglycine (DMOG), a HIF-1α stabilizer, would improve matrix production in 3-dimensional (3D) porcine synovial-derived mesenchymal stem cell (SYN-MSC) co-culture with chondrocytes.

DESIGN

Pellet cultures and scaffold-based engineered cartilage were grown in vitro to determine the impact of chemically simulated hypoxia on 2 types of 3D cell culture. DMOG-treated groups were exposed to DMOG from day 14 to day 21 and grown up to 6 weeks with n = 3 per condition and time point.

RESULTS

The addition of DMOG resulted in HIF-1α stabilization in the exterior of the engineered constructs, which resulted in increased regional type II collagen deposition, but the stabilization did not translate to overall increased extracellular matrix deposition. There was no increase in HIF-1α stabilization in the pellet cultures. DMOG treatment also negatively affected the mechanical competency of the engineered cartilage.

CONCLUSIONS

Despite previous studies that demonstrated the efficacy of DMOG, here, short-term treatment with DMOG did not have a uniformly positive impact on the chondrogenic capacity of SYN-MSCs in either pellet culture or in scaffold-based engineered cartilage, as evidenced by reduced matrix production. Such 3D constructs generally have a naturally occurring hypoxic center, which allows for the stabilization of HIF-1α in the interior tissue. Thus, short-term addition of DMOG may not further improve this in cartilage tissue engineered constructs.

Methods of Modification of Mesenchymal Stem Cells and Conditions of Their Culturing for Hyaline Cartilage Tissue Engineering

The use of mesenchymal stromal cells (MSCs) for tissue engineering of hyaline cartilage is a topical area of regenerative medicine that has already entered clinical practice. The key stage of this procedure is to create conditions for chondrogenic differentiation of MSCs, increase the synthesis of hyaline cartilage extracellular matrix proteins by these cells and activate their proliferation. The first such works consisted in the indirect modification of cells, namely, in changing the conditions in which they are located, including microfracturing of the subchondral bone and the use of 3D biodegradable scaffolds. The most effective methods for modifying the cell culture of MSCs are protein and physical, which have already been partially introduced into clinical practice. Genetic methods for modifying MSCs, despite their effectiveness, have significant limitations. Techniques have not yet been developed that allow studying the effectiveness of their application even in limited groups of patients. The use of MSC modification methods allows precise regulation of cell culture proliferation, and in combination with the use of a 3D biodegradable scaffold, it allows obtaining a hyaline-like regenerate in the damaged area. This review is devoted to the consideration and comparison of various methods used to modify the cell culture of MSCs for their use in regenerative medicine of cartilage tissue.

Differences in the biological properties of mesenchymal stromal cells from traumatic temporomandibular joint fibrous and bony ankylosis: a comparative study

The aim of this study was to compare the functional characteristics of mesenchymal stromal cells (MSCs) from a sheep model of traumatic temporomandibular joint (TMJ) fibrous and bony ankylosis. A sheep model of bilateral TMJ trauma-induced fibrous ankylosis on one side and bony ankylosis on the contralateral side was used. MSCs from fibrous ankylosed callus (FA-MSCs) or bony ankylosed callus (BA-MSCs) at weeks 1, 2, 4, and 8 after surgery were isolated and cultured. MSCs derived from the bone marrow of the mandibular condyle (BM-MSCs) were used as controls. The MSCs from the different sources were characterized morphologically, phenotypically, and functionally. Adherence and trilineage differentiation potential were presented in the ovine MSCs. These cell populations highly positively expressed MSC-associated specific markers, namely CD29, CD44, and CD166, but lacked CD31 and CD45 expressions. The BA-MSCs had higher clonogenic and proliferative potentials than the FA-MSCs. The BA-MSCs also showed higher osteogenic and chondrogenic potentials, but lower adipogenic capacity than the FA-MSCs. In addition, the BA-MSCs demonstrated higher chondrogenic, but lower osteogenic capacity than the BM-MSCs. Our study suggests that inhibition of the osteogenic and chondrogenic differentiations of MSCs might be a promising strategy for preventing bony ankylosis in the future.

Insulin-like growth factor-1 in articular cartilage repair for osteoarthritis treatment

Articular cartilage repair is a critical issue in osteoarthritis (OA) treatment. The insulin-like growth factor (IGF) signaling pathway has been implicated in articular cartilage repair. IGF-1 is a member of a family of growth factors that are structurally closely related to pro-insulin and can promote chondrocyte proliferation, enhance matrix production, and inhibit chondrocyte apoptosis. Here, we reviewed the role of IGF-1 in cartilage anabolism and catabolism. Moreover, we discussed the potential role of IGF-1 in OA treatment. Of note, we summarized the recent progress on IGF delivery systems. Optimization of IGF delivery systems will facilitate treatment application in cartilage repair and improve OA treatment efficacy.

Integrins, cadherins and channels in cartilage mechanotransduction: perspectives for future regeneration strategies

Articular cartilage consists of hyaline cartilage, is a major constituent of the human musculoskeletal system and has critical functions in frictionless joint movement and articular homoeostasis. Osteoarthritis (OA) is an inflammatory disease of articular cartilage, which promotes joint degeneration. Although it affects millions of people, there are no satisfying therapies that address this disease at the molecular level. Therefore, tissue regeneration approaches aim at modifying chondrocyte biology to mitigate the consequences of OA. This requires appropriate biochemical and biophysical stimulation of cells. Regarding the latter, mechanotransduction of chondrocytes and their precursor cells has become increasingly important over the last few decades. Mechanotransduction is the transformation of external biophysical stimuli into intracellular biochemical signals, involving sensor molecules at the cell surface and intracellular signalling molecules, so-called mechano-sensors and -transducers. These signalling events determine cell behaviour. Mechanotransducing ion channels and gap junctions additionally govern chondrocyte physiology. It is of great scientific and medical interest to induce a specific cell behaviour by controlling these mechanotransduction pathways and to translate this knowledge into regenerative clinical therapies. This review therefore focuses on the mechanotransduction properties of integrins, cadherins and ion channels in cartilaginous tissues to provide perspectives for cartilage regeneration.

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.

A scaffold-free approach to cartilage tissue generation using human embryonic stem cells

Articular cartilage functions as a shock absorber and facilitates the free movement of joints. Currently, there are no therapeutic drugs that promote the healing of damaged articular cartilage. Limitations associated with the two clinically relevant cell populations, human articular chondrocytes and mesenchymal stem cells, necessitate finding an alternative cell source for cartilage repair. Human embryonic stem cells (hESCs) provide a readily accessible population of self-renewing, pluripotent cells with perceived immunoprivileged properties for cartilage generation. We have developed a robust method to generate 3D, scaffold-free, hyaline cartilage tissue constructs from hESCs that are composed of numerous chondrocytes in lacunae, embedded in an extracellular matrix containing Type II collagen, sulphated glycosaminoglycans and Aggrecan. The elastic (Young's) modulus of the hESC-derived cartilage tissue constructs (0.91 ± 0.08 MPa) was comparable to full-thickness human articular cartilage (0.87 ± 0.09 MPa). Moreover, we have successfully scaled up the size of the scaffold-free, 3D hESC-derived cartilage tissue constructs to between 4.5 mm and 6 mm, thus enhancing their suitability for clinical application.

Extracellular Vehicles of Oxygen-Depleted Mesenchymal Stromal Cells: Route to Off-Shelf Cellular Therapeutics?

Cellular therapy is a promising tool of human medicine to successfully treat complex and challenging pathologies such as cardiovascular diseases or chronic inflammatory conditions. Bone marrow-derived mesenchymal stromal cells (BMSCs) are in the limelight of these efforts, initially, trying to exploit their natural properties by direct transplantation. Extensive research on the therapeutic use of BMSCs shed light on a number of key aspects of BMSC physiology including the importance of oxygen in the control of BMSC phenotype. These efforts also led to a growing number of evidence indicating that the beneficial therapeutic effects of BMSCs can be mediated by BMSC-secreted agents. Further investigations revealed that BMSC-excreted extracellular vesicles could mediate the potentially therapeutic effects of BMSCs. Here, we review our current understanding of the relationship between low oxygen conditions and the effects of BMSC-secreted extracellular vesicles focusing on the possible medical relevance of this interplay.

Inability of Low Oxygen Tension to Induce Chondrogenesis in Human Infrapatellar Fat Pad Mesenchymal Stem Cells

OBJECTIVE

Articular cartilage of the knee joint is avascular, exists under a low oxygen tension microenvironment, and does not self-heal when injured. Human infrapatellar fat pad-sourced mesenchymal stem cells (IFP-MSC) are an arthroscopically accessible source of mesenchymal stem cells (MSC) for the repair of articular cartilage defects. Human IFP-MSC exists physiologically under a low oxygen tension (i.e., 1-5%) microenvironment. Human bone marrow mesenchymal stem cells (BM-MSC) exist physiologically within a similar range of oxygen tension. A low oxygen tension of 2% spontaneously induced chondrogenesis in micromass pellets of human BM-MSC. However, this is yet to be demonstrated in human IFP-MSC or other adipose tissue-sourced MSC. In this study, we explored the potential of low oxygen tension at 2% to drive the in vitro chondrogenesis of IFP-MSC. We hypothesized that 2% O2 will induce stable chondrogenesis in human IFP-MSC without the risk of undergoing endochondral ossification at ectopic sites of implantation.

METHODS

Micromass pellets of human IFP-MSC were cultured under 2% O2 or 21% O2 (normal atmosphere O2) in the presence or absence of chondrogenic medium with transforming growth factor-β3 (TGFβ3) for 3 weeks. Following in vitro chondrogenesis, the resulting pellets were implanted in immunodeficient athymic nude mice for 3 weeks.

RESULTS

A low oxygen tension of 2% was unable to induce chondrogenesis in human IFP-MSC. In contrast, chondrogenic medium with TGFβ3 induced in vitro chondrogenesis. All pellets were devoid of any evidence of undergoing endochondral ossification after subcutaneous implantation in athymic mice.

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.

ChondroGELesis: Hydrogels to harness the chondrogenic potential of stem cells

The extracellular matrix is a highly complex microenvironment, whose various components converge to regulate cell fate. Hydrogels, as water-swollen polymer networks composed by synthetic or natural materials, are ideal candidates to create biologically active substrates that mimic these matrices and target cell behaviour for a desired tissue engineering application. Indeed, the ability to tune their mechanical, structural, and biochemical properties provides a framework to recapitulate native tissues. This review explores how hydrogels have been engineered to harness the chondrogenic response of stem cells for the repair of damaged cartilage tissue. The signalling processes involved in hydrogel-driven chondrogenesis are also discussed, identifying critical pathways that should be taken into account during hydrogel design.

Turning Nature's own processes into design strategies for living bone implant biomanufacturing: a decade of Developmental Engineering

A decade after the term developmental engineering (DE) was coined to indicate the use of developmental processes as blueprints for the design and development of engineered living implants, a myriad of proof-of-concept studies demonstrate the potential of this approach in small animal models. This review provides an overview of DE work, focusing on applications in bone regeneration. Enabling technologies allow to quantify the distance between in vitro processes and their developmental counterpart, as well as to design strategies to reduce that distance. By embedding Nature's robust mechanisms of action in engineered constructs, predictive large animal data and subsequent positive clinical outcomes can be gradually achieved. To this end, the development of next generation biofabrication technologies should provide the necessary scale and precision for robust living bone implant biomanufacturing.

Characterisation of ovine bone marrow-derived stromal cells (oBMSC) and evaluation of chondrogenically induced micro-pellets for cartilage tissue repair in vivo

Abstract

Bone marrow stromal cells (BMSC) show promise in cartilage repair, and sheep are the most common large animal pre-clinical model.

Objective

The objective of this study was to characterise ovine BMSC (oBMSC) in vitro, and to evaluate the capacity of chondrogenic micro-pellets manufactured from oBMSC or ovine articular chondrocytes (oACh) to repair osteochondral defects in sheep.

Design

oBMSC were characterised for surface marker expression using flow cytometry and evaluated for tri-lineage differentiation capacity. oBMSC micro-pellets were manufactured in a microwell platform, and chondrogenesis was compared at 2%, 5%, and 20% O₂. The capacity of cartilage micro-pellets manufactured from oBMSC or oACh to repair osteochondral defects in adult sheep was evaluated in an 8-week pilot study.

Results

Expanded oBMSC were positive for CD44 and CD146 and negative for CD45. The common adipogenic induction ingredient, 3-Isobutyl-1-methylxanthine (IBMX), was toxic to oBMSC, but adipogenesis could be restored by excluding IBMX from the medium. BMSC chondrogenesis was optimal in a 2% O₂ atmosphere. Micro-pellets formed from oBMSC or oACh appeared morphologically similar, but hypertrophic genes were elevated in oBMSC micro-pellets. While oACh micro-pellets formed cartilage-like repair tissue in sheep, oBMSC micro-pellets did not.

Conclusion

The sensitivity of oBMSC, compared to human BMSC, to IBMX in standard adipogenic assays highlights species-associated differences. Micro-pellets manufactured from oACh were more effective than micro-pellets manufactured from oBMSC in the repair of osteochondral defects in sheep. While oBMSC can be driven to form cartilage-like tissue in vitro, the effective use of these cells in cartilage repair will depend on the successful mitigation of hypertrophy and tissue integration.

Supplementary information

The online version contains supplementary material available at 10.1186/s13287-020-02045-3.

Metabolic regulation of skeletal cell fate and function in physiology and disease

The skeleton is diverse in its functions, which include mechanical support, movement, blood cell production, mineral storage and endocrine regulation. This multifaceted role is achieved through an interplay of osteoblasts, chondrocytes, bone marrow adipocytes and stromal cells, all generated from skeletal stem cells. Emerging evidence shows the importance of cellular metabolism in the molecular control of the skeletal system. The different skeletal cell types not only have distinct metabolic demands relating to their particular functions but also are affected by microenvironmental constraints. Specific metabolites control skeletal stem cell maintenance, direct lineage allocation and mediate cellular communication. Here, we discuss recent findings on the roles of cellular metabolism in determining skeletal stem cell fate, coordinating osteoblast and chondrocyte function, and organizing stromal support of haematopoiesis. We also consider metabolic dysregulation in skeletal ageing and degenerative diseases, and provide an outlook on how the field may evolve in the coming years.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Neurological heterotopic ossification: novel mechanisms, prognostic biomarkers and prophylactic therapies

Neurological heterotopic ossification (NHO) is a debilitating condition where bone forms in soft tissue, such as muscle surrounding the hip and knee, following an injury to the brain or spinal cord. This abnormal formation of bone can result in nerve impingement, pain, contractures and impaired movement. Patients are often diagnosed with NHO after the bone tissue has completely mineralised, leaving invasive surgical resection the only remaining treatment option. Surgical resection of NHO creates potential for added complications, particularly in patients with concomitant injury to the central nervous system (CNS). Although recent work has begun to shed light on the physiological mechanisms involved in NHO, there remains a significant knowledge gap related to the prognostic biomarkers and prophylactic treatments which are necessary to prevent NHO and optimise patient outcomes. This article reviews the current understanding pertaining to NHO epidemiology, pathobiology, biomarkers and treatment options. In particular, we focus on how concomitant CNS injury may drive ectopic bone formation and discuss considerations for treating polytrauma patients with NHO. We conclude that understanding of the pathogenesis of NHO is rapidly advancing, and as such, there is the strong potential for future research to unearth methods capable of identifying patients likely to develop NHO, and targeted treatments to prevent its manifestation.

Key Metabolic Pathways in MSC-Mediated Immunomodulation: Implications for the Prophylaxis and Treatment of Graft Versus Host Disease

Mesenchymal stromal cells (MSCs) are spindle-shaped, plastic-adherent cells in vitro with potent immunosuppressive activity both in vitro and in vivo. MSCs have been employed as a cellular immunotherapy in diverse preclinical models and clinical trials, but most commonly as agents for the prophylaxis or therapy of graft versus host disease after hematopoietic cell transplantation. In addition to the oft studied secreted cytokines, several metabolic pathways intrinsic to MSCs, notably indoleamine 2,3-dioxygenase, prostaglandin E2, hypoxia-inducible factor 1 α, heme oxygenase-1, as well as energy-generating metabolism, have been shown to play roles in the immunomodulatory activity of MSCs. In this review, we discuss these key metabolic pathways in MSCs which have been reported to contribute to MSC therapeutic effects in the setting of hematopoietic cell transplantation and graft versus host disease. Understanding the contribution of MSC metabolism to immunomodulatory activity may substantially inform the development of future clinical applications of MSCs.

Baicalin promotes extracellular matrix synthesis in chondrocytes via the activation of hypoxia-inducible factor-1α

Chinese herbal extracts are being used increasingly to treat osteoarthritis (OA) in recent years. Baicalin (BA) is an active component of Scutellaria baicalensis Georgi extracts and protects chondrocytes against damage. The aim of the present study was to examine the mechanism of action of BA on chondrocytes from mouse articular cartilage. In total, 44 µM BA and 10 µM hypoxia-inducible-factor-1α (HIF-1α) inhibitor BAY-87-2243 were screened by the [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] method. Alcian blue and Safran O staining were used to investigate the synthesis of extracellular matrix (ECM) in chondrocytes treated with BA. The expression of HIF-1α and chondrogenic marker genes including SOX9, AGG and Col2α was detected by western blotting or reverse-transcription quantitative (RT-qPCR), the expression of PHD1,2,3 and catabolic genes including ADAMTS5, MMP9 and MMP13 were detected by RT-qPCR. To investigate the effect of BA on the ECM synthesis of chondrocytes, 44 µM BA and 10 µM BAY were chosen for further experimentation. It was confirmed that BA at a concentration of 44 µM could significantly promote the secretion of ECM. The expressions of genes including HIF-1α, SOX9, collagen type 2 (Col2α) and aggrecan (AGG) were elevated following BA pretreatment and decreased by subsequent BAY-87-2243 stimulation for 24 h. Compared with untreated chondrocytes, the expressions of genes including ADAMTS5, MMP9, MMP13, PHD1, PHD2 and PHD3 in chondrocytes treated by BA were downregulated, however, BAY-87-2243 reversed the effect of BA on the genes including ADAMTS5, MMP9, MMP13, PHD1, PHD2 and PHD3 in chondrocytes. The findings of the present study suggest that BA may promote ECM synthesis and marker gene expression in chondrocytes by activating HIF-1α. Therefore, BA may represent a novel clinical drug for OA.

Differentiation Potential of Early- and Late-Passage Adipose-Derived Mesenchymal Stem Cells Cultured under Hypoxia and Normoxia

With an increasing focus on the large-scale expansion of mesenchymal stem cells (MSCs) required for clinical applications for the treatment of joint and bone diseases such as osteoarthritis, the optimisation of conditions for in vitro MSC expansion requires careful consideration to maintain native MSC characteristics. Physiological parameters such as oxygen concentration, media constituents, and passage numbers influence the properties of MSCs and may have major impact on their therapeutic potential. Cells grown under hypoxic conditions have been widely documented in clinical use. Culturing MSCs on large scale requires bioreactor culture; however, it is challenging to maintain low oxygen and other physiological parameters over several passages in large bioreactor vessels. The necessity to scale up the production of cells in vitro under normoxia may affect important attributes of MSCs. For these reasons, our study investigated the effects of normoxic and hypoxic culture condition on early- and late-passage adipose-derived MSCs. We examined effect of each condition on the expression of key stem cell marker genes POU5F1, NANOG, and KLF4, as well as differentiation genes RUNX2, COL1A1, SOX9, COL2A1, and PPARG. We found that expression levels of stem cell marker genes and osteogenic and chondrogenic genes were higher in normoxia compared to hypoxia. Furthermore, expression of these genes reduced with passage number, with the exception of PPARG, an adipose differentiation marker, possibly due to the adipose origin of the MSCs. We confirmed by flow cytometry the presence of cell surface markers CD105, CD73, and CD90 and lack of expression of CD45, CD34, CD14, and CD19 across all conditions. Furthermore, in vitro differentiation confirmed that both early- and late-passage adipose-derived MSCs grown in hypoxia or normoxia could differentiate into chondrogenic and osteogenic cell types. Our results demonstrate that the minimal standard criteria to define MSCs as suitable for laboratory-based and preclinical studies can be maintained in early- or late-passage MSCs cultured in hypoxia or normoxia. Therefore, any of these culture conditions could be used when scaling up MSCs in bioreactors for allogeneic clinical applications or tissue engineering for the treatment of joint and bone diseases such as osteoarthritis.

Differential Production of Cartilage ECM in 3D Agarose Constructs by Equine Articular Cartilage Progenitor Cells and Mesenchymal Stromal Cells

Identification of articular cartilage progenitor cells (ACPCs) has opened up new opportunities for cartilage repair. These cells may be used as alternatives for or in combination with mesenchymal stromal cells (MSCs) in cartilage engineering. However, their potential needs to be further investigated, since only a few studies have compared ACPCs and MSCs when cultured in hydrogels. Therefore, in this study, we compared chondrogenic differentiation of equine ACPCs and MSCs in agarose constructs as monocultures and as zonally layered co-cultures under both normoxic and hypoxic conditions. ACPCs and MSCs exhibited distinctly differential production of the cartilaginous extracellular matrix (ECM). For ACPC constructs, markedly higher glycosaminoglycan (GAG) contents were determined by histological and quantitative biochemical evaluation, both in normoxia and hypoxia. Differential GAG production was also reflected in layered co-culture constructs. For both cell types, similar staining for type II collagen was detected. However, distinctly weaker staining for undesired type I collagen was observed in the ACPC constructs. For ACPCs, only very low alkaline phosphatase (ALP) activity, a marker of terminal differentiation, was determined, in stark contrast to what was found for MSCs. This study underscores the potential of ACPCs as a promising cell source for cartilage engineering.

Incorporation of Bioactive Glasses Containing Mg, Sr, and Zn in Electrospun PCL Fibers by Using Benign Solvents

Poly(ε-caprolactone) (PCL) and PCL/bioactive glass composite fiber mats were produced by electrospinning technique. To improve cell adhesion and proliferation (i) 45S5, (ii) a bioactive glass containing strontium and magnesium oxides, and (iii) a bioactive glass containing zinc oxide were separately added to the starting PCL solution before electrospinning. A good incorporation of bioactive glass particles in PCL electrospun mats was confirmed by SEM and FTIR analyses. Bioactivity was evaluated by immersion of PCL mats and PCL/bioactive glass electrospun fiber mats in simulated body fluid (SBF). Bone murine stromal cells (ST-2) were employed in WST-8 assay to assess cell viability, cell morphology, and proliferation. The results showed that the presence of bioactive glass particles in the fibers enhances cell adhesion and proliferation compared to neat PCL mats. Furthermore, PCL/bioactive glass electrospun mats showed higher wound-healing rate (measured as cell migration rate) in vitro compared to neat PCL electrospun mats. Therefore, the characteristics of the PCL matrix combined with biological properties of bioactive glasses make PCL/bioactive glass composite ideal candidate for biomedical application.

Nanofibrous spongy microspheres to deliver rabbit mesenchymal stem cells and anti-miR-199a to regenerate nucleus pulposus and prevent calcification

Lower back pain is mainly caused by intervertebral disc degeneration, in which calcification is frequently involved. Here novel nanofibrous spongy microspheres (NF-SMS) are used to carry rabbit bone marrow mesenchymal stromal cells (MSCs) to regenerate nucleus pulposus tissues. NF-SMS are shown to significantly enhance the MSC seeding, proliferation and differentiation over control microcarriers. Furthermore, a hyperbranched polymer (HP) with negligible cytotoxicity and high microRNA (miRNAs) binding affinity is synthesized. The HP can complex with anti-miR-199a and self-assemble into "double shell" polyplexes which are able to achieve high transfection efficiency into MSCs. A double-emulsion technique is used to encapsulate these polyplexes in biodegradable nanospheres (NS) to enable sustained anti-miR-199 delivery. Our results demonstrate that MSC/HP-anti-miR-199a/NS/NF-SMS constructs can promote the nucleus pulposus (NP) phenotype and resist calcification in vitro and in a subcutaneous environment. Furthermore, injection of MSC/HP-anti-miR-199a/NS/NF-SMS can stay in place, produce functional extracellular matrix, maintain disc height and prevent intervertebral disc (IVD) calcification in a rabbit lumbar degeneration model.

Molecular Insights Into Lysyl Oxidases in Cartilage Regeneration and Rejuvenation

Articular cartilage remains among the most difficult tissues to regenerate due to its poor self-repair capacity. The lysyl oxidase family (LOX; also termed as protein-lysine 6-oxidase), mainly consists of lysyl oxidase (LO) and lysyl oxidase-like 1-4 (LOXL1-LOXL4), has been traditionally defined as cuproenzymes that are essential for stabilization of extracellular matrix, particularly cross-linking of collagen and elastin. LOX is essential in the musculoskeletal system, particularly cartilage. LOXs-mediated collagen cross-links are essential for the functional integrity of articular cartilage. Appropriate modulation of the expression or activity of certain LOX members selectively may become potential promising strategy for cartilage repair. In the current review, we summarized the advances of LOX in cartilage homeostasis and functioning, as well as copper-mediated activation of LOX through hypoxia-responsive signaling axis during recent decades. Also, the molecular signaling network governing LOX expression has been summarized, indicating that appropriate modulation of hypoxia-responsive-signaling-directed LOX expression through manipulation of bioavailability of copper and oxygen is promising for further clinical implications of cartilage regeneration, which has emerged as a potential therapeutic approach for cartilage rejuvenation in tissue engineering and regenerative medicine. Therefore, targeted regulation of copper-mediated hypoxia-responsive signalling axis for selective modulation of LOX expression may become potential effective therapeutics for enhanced cartilage regeneration and rejuvenation in future clinical implications.

HIF1α-dependent metabolic reprogramming governs mesenchymal stem/stromal cell immunoregulatory functions

Hypoxia-inducible factor 1 α (HIF1α), a regulator of metabolic change, is required for the survival and differentiation potential of mesenchymal stem/stromal cells (MSC). Its role in MSC immunoregulatory activity, however, has not been completely elucidated. In the present study, we evaluate the role of HIF1α on MSC immunosuppressive potential. We show that HIF1α silencing in MSC decreases their inhibitory potential on Th1 and Th17 cell generation and limits their capacity to generate regulatory T cells. This reduced immunosuppressive potential of MSC is associated with a metabolic switch from glycolysis to OXPHOS and a reduced capacity to express or produce some immunosuppressive mediators including Intercellular Adhesion Molecule (ICAM), IL-6, and nitric oxide (NO). Moreover, using the Delayed-Type Hypersensitivity murine model (DTH), we confirm, in vivo, the critical role of HIF1α on MSC immunosuppressive effect. Indeed, we show that HIF1α silencing impairs MSC capacity to reduce inflammation and inhibit the generation of pro-inflammatory T cells. This study reveals the pivotal role of HIF1α on MSC immunosuppressive activity through the regulation of their metabolic status and identifies HIF1α as a novel mediator of MSC immunotherapeutic potential.

Hypoxia Inducible Factor-1α in Osteochondral Tissue Engineering

Damage to osteochondral (OC) tissues can lead to pain, loss of motility, and progress to osteoarthritis. Tissue engineering approaches offer the possibility of replacing damaged tissues and restoring joint function; however, replicating the spatial and functional heterogeneity of native OC tissue remains a pressing challenge. Chondrocytes in healthy cartilage exist in relatively low-oxygen conditions, while osteoblasts in the underlying bone experience higher oxygen pressures. Such oxygen gradients also exist in the limb bud, where they influence OC tissue development. The cellular response to these spatial variations in oxygen pressure, which is mediated by the hypoxia inducible factor (HIF) pathway, plays a central role in regulating osteo- and chondrogenesis by directing progenitor cell differentiation and promoting and maintaining appropriate extracellular matrix production. Understanding the role of the HIF pathway in OC tissue development may enable new approaches to engineer OC tissue. In this review, we discuss strategies to spatially and temporarily regulate the HIF pathway in progenitor cells to create functional OC tissue for regenerative therapies. Impact statement Strategies to engineer osteochondral (OC) tissue are limited by the complex and varying microenvironmental conditions in native bone and cartilage. Indeed, native cartilage experiences low-oxygen conditions, while the underlying bone is relatively normoxic. The cellular response to these low-oxygen conditions, which is mediated through the hypoxia inducible factor (HIF) pathway, is known to promote and maintain the chondrocyte phenotype. By using tissue engineering scaffolds to spatially and temporally harness the HIF pathway, it may be possible to improve OC tissue engineering strategies for the regeneration of damaged cartilage and its underlying subchondral bone.

Lipid availability determines fate of skeletal progenitor cells via SOX9

The avascular nature of cartilage makes it a unique tissue^1–4, but whether and how the absence of nutrient supply regulates chondrogenesis remains unknown. Here, we show that obstruction of vascular invasion during bone healing favours chondrogenic over osteogenic differentiation of skeletal progenitor cells. Unexpectedly, this process is driven by a decreased availability of extracellular lipids. When lipids are scarce, skeletal progenitors activate FoxO transcription factors, which bind to the Sox9 promoter and increase its expression. Besides initiating chondrogenesis, SOX9 acts as a regulator of cellular metabolism by suppressing fatty acid oxidation, and thus adapts the cells to an avascular life. Our results define lipid scarcity as an important determinant of chondrogenic commitment, reveal a role for FoxOs during lipid starvation, and identify SOX9 as a critical metabolic mediator. These data highlight the importance of the nutritional microenvironment in the specification of skeletal cell fate.

Pathomechanisms of Posttraumatic Osteoarthritis: Chondrocyte Behavior and Fate in a Precarious Environment

Traumatic injuries of the knee joint result in a wide variety of pathomechanisms, which contribute to the development of so-called posttraumatic osteoarthritis (PTOA). These pathogenetic processes include oxidative stress, excessive expression of catabolic enzymes, release of damage-associated molecular patterns (DAMPs), and synovial inflammation. The present review focuses on the underlying pathomechanisms of PTOA and in particular the behavior and fate of the surviving chondrocytes, comprising chondrocyte metabolism, regulated cell death, and phenotypical changes comprising hypertrophy and senescence. Moreover, possible therapeutic strategies, such as chondroanabolic stimulation, anti-oxidative and anti-inflammatory treatment, as well as novel therapeutic targets are discussed.

The hypoxic microenvironment: a driving force for heterotopic ossification progression

Heterotopic ossification (HO) refers to the formation of bone tissue outside the normal skeletal system. According to its pathogenesis, HO is divided into hereditary HO and acquired HO. There currently lack effective approaches for HO prevention or treatment. A deep understanding of its pathogenesis will provide promising strategies to prevent and treat HO. Studies have shown that the hypoxia-adaptive microenvironment generated after trauma is a potent stimulus of HO. The hypoxic microenvironment enhances the stability of hypoxia-inducible factor-1α (HIF-1α), which regulates a complex network including bone morphogenetic proteins (BMPs), vascular endothelial growth factor (VEGF), and neuropilin-1 (NRP-1), which are implicated in the formation of ectopic bone. In this review, we summarize the current understanding of the triggering role and underlying molecular mechanisms of the hypoxic microenvironment in the initiation and progression of HO, focusing mainly on HIF-1 and it's influenced genes  BMP, VEGF, and NRP-1. A better understanding of the role of hypoxia in HO unveils novel therapeutic targets for HO that reduce the local hypoxic microenvironment and inhibit HIF-1α activity.

Differential regulation of blood vessel formation between traumatic temporomandibular joint fibrous ankylosis and bony ankylosis in a sheep model

OBJECTIVES

Clinical and experimental studies show that the etiology of traumatic temporomandibular joint (TMJ) fibrous ankylosis and bony ankylosis are associated with the severity of trauma. However, how the injury severity affects the tissue differentiation is not clear. We tested the hypothesis that angiogenesis affects the outcomes of TMJ trauma, and that enhanced neovascularization after severe TMJ trauma would promote the development of bony ankylosis.

METHODS

Bilateral condylar sagittal fracture and discectomy were performed for each sheep, with the glenoid fossa receiving either severe trauma to induce bony ankylosis or minor trauma to induce fibrous ankylosis. At days 7, 14, 28, and 56 after surgery, total RNA was extracted from the ankylosed callus. Temporal gene expressions of several molecules functionally important for blood vessel formation were studied by real-time PCR.

RESULTS

Histological examination revealed a prolonged hematoma phase and a lack of cartilage formation in fibrous ankylosis. mRNA expression levels of HIF-1α, VEGF, VEGFR2, SDF1, Ang1, Tie2, vWF, CYR61, FGF2, TIMP1, MMP2, and MMP9 were distinctly lower in fibrous ankylosis compared with bony ankylosis at several time points.

CONCLUSIONS

Our study indicates that inhibition of angiogenesis after TMJ trauma might be a promising strategy for preventing bony ankylosis in the future.

Lipid phosphatase SHIP-1 regulates chondrocyte hypertrophy and skeletal development

SH2‐containing inositol‐5′‐phosphatase‐1 (SHIP‐1) controls the phosphatidylinositol‐3′‐kinase (PI3K) initiated signaling pathway by limiting cell membrane recruitment and activation of Akt. Despite the fact that many of the growth factors important to cartilage development and functions are able to activate the PI3K signal transduction pathway, little is known about the role of PI3K signaling in chondrocyte biology and its contribution to mammalian skeletogenesis. Here, we report that the lipid phosphatase SHIP‐1 regulates chondrocyte hypertrophy and skeletal development through its expression in osteochondroprogenitor cells. Global SHIP‐1 knockout led to accelerated chondrocyte hypertrophy and premature formation of the secondary ossification center in the bones of postnatal mice. Drastically higher vascularization and greater number of c‐kit + progenitors associated with sinusoids in the bone marrow also indicated more advanced chondrocyte hypertrophic differentiation in SHIP‐1 knockout mice than in wild‐type mice. In corroboration with the in vivo phenotype, SHIP‐1 deficient PDGFRα + Sca‐1 + osteochondroprogenitor cells exhibited rapid differentiation into hypertrophic chondrocytes under chondrogenic culture conditions in vitro. Furthermore, SHIP‐1 deficiency inhibited hypoxia‐induced cellular activation of Akt and extracellular‐signal‐regulated kinase (Erk) and suppressed hypoxia‐induced cell proliferation. These results suggest that SHIP‐1 is required for hypoxia‐induced growth signaling under physiological hypoxia in the bone marrow. In conclusion, the lipid phosphatase SHIP‐1 regulates skeletal development by modulating chondrogenesis and the hypoxia response of the osteochondroprogenitors during endochondral bone formation.

Deletion of clock gene Bmal1 impaired the chondrocyte function due to disruption of the HIF1α-VEGF signaling pathway

Several studies have demonstrated the core circadian rhythm gene Bmal1 could regulate the clock control genes (CCGs) expression and maintain the integrity in cartilage tissue. In addition, its abnormal expression is connected with the occurrence and development of several diseases including osteoarthritis (OA). However, the relationship between Bmal1 and cartilage development still needs to be fully elucidated. Here, we bred tamoxifen-induced cartilage-specific knockout mice to learn the effects of Bmal1 on the cartilage development and its underlying mechanisms at specific time points. We observed that Bmal1 ablated mice showed growth retardation during puberty, and the length of whole growth plate and the proliferation zone were both shorter than those in the control group. Deletion of Bmal1 significantly inhibited the chondrocytes proliferation and activated cells apoptosis in the growth plate. Meanwhile, knockout of Bmal1 attenuated the expression of VEGF and HIF1α and enhanced the level of MMP13 and Runx2 in the growth plate chondrocytes. Consistent with these findings in vivo, ablation of Bmal1 could also lead to decrease chondrocytes proliferation, the expression of HIF1α and VEGF and elevate apoptosis in cultured chondrocytes. These findings suggest that Bmal1 plays a pivotal role in cartilage development by regulating the HIF1α-VEGF signaling pathway.

Hypoxia impacts human MSC response to substrate stiffness during chondrogenic differentiation

Tissue engineering strategies often aim to direct tissue formation by mimicking conditions progenitor cells experience within native tissues. For example, to create cartilage in vitro, researchers often aim to replicate the biochemical and mechanical milieu cells experience during cartilage formation in the developing limb bud. This includes stimulating progenitors with TGF-β1/3, culturing under hypoxic conditions, and regulating mechanosensory pathways using biomaterials that control substrate stiffness and/or cell shape. However, as progenitors differentiate down the chondrogenic lineage, the pathways that regulate their responses to mechanotransduction, hypoxia and TGF-β may not act independently, but rather also impact one another, influencing overall cell response. Here, to better understand hypoxia's influence on mechanoregulatory-mediated chondrogenesis, we cultured human marrow stromal/mesenchymal stem cells (hMSC) on soft (0.167 kPa) or stiff (49.6 kPa) polyacrylamide hydrogels in chondrogenic medium containing TGF-β3. We then compared cell morphology, phosphorylated myosin light chain 2 staining, and chondrogenic gene expression under normoxic and hypoxic conditions, in the presence and absence of pharmacological inhibition of cytoskeletal tension. We show that on soft compared to stiff substrates, hypoxia prompts hMSC to adopt more spread morphologies, assemble in compact mesenchymal condensation-like colonies, and upregulate NCAM expression, and that inhibition of cytoskeletal tension negates hypoxia-mediated upregulation of molecular markers of chondrogenesis, including COL2A1 and SOX9. Taken together, our findings support a role for hypoxia in regulating hMSC morphology, cytoskeletal tension and chondrogenesis, and that hypoxia's effects are modulated, at least in part, by mechanosensitive pathways. Our insights into how hypoxia impacts mechanoregulation of chondrogenesis in hMSC may improve strategies to develop tissue engineered cartilage. STATEMENT OF SIGNIFICANCE: Cartilage tissue engineering strategies often aim to drive progenitor cell differentiation by replicating the local environment of the native tissue, including by regulating oxygen concentration and mechanical stiffness. However, the pathways that regulate cellular responses to mechanotransduction and hypoxia may not act independently, but rather also impact one another. Here, we show that on soft, but not stiff surfaces, hypoxia impacts human MSC (hMSC) morphology and colony formation, and inhibition of cytoskeletal tension negates the hypoxia-mediated upregulation of molecular markers of chondrogenesis. These observations suggest that hypoxia's effects during hMSC chondrogenesis are modulated, at least in part, by mechanosensitive pathways, and may impact strategies to develop scaffolds for cartilage tissue engineering, as hypoxia's chondrogenic effects may be enhanced on soft materials.