Effect of monocytes/macrophages on the osteogenic differentiation of adipose-derived mesenchymal stromal cells in 3D co-culture spheroids

Osteogenic differentiation of periodontal ligament fibroblasts inhibits osteoclast formation

One of the deficits of knowledge on bone remodelling, is to what extent cells that are driven towards osteogenic differentiation can contribute to osteoclast formation. The periodontal ligament fibroblast (PdLFs) is an ideal model to study this, since they play a role in osteogenesis, and can also orchestrate osteoclastogenesis.when co-cultured with a source of osteoclast-precursor such as peripheral blood mononuclear cells (PBMCs). Here, the osteogenic differentiation of PdLFs and the effects of this process on the formation of osteoclasts were investigated. PdLFs were obtained from extracted teeth and exposed to osteogenic medium for 0, 7, 14, or 21 out of 21 days. After this 21-day culturing period, the cells were co-cultured with peripheral blood mononuclear cells (PBMCs) for an additional 21 days to study osteoclast formation. Alkaline phosphatase (ALP) activity, calcium concentration, and gene expression of osteogenic markers were assessed at day 21 to evaluate the different stages of osteogenic differentiation. Alizarin red staining and scanning electron microscopy were used to visualise mineralisation. Tartrate-resistant acid phosphatase (TRAcP) activity, TRAcP staining, multinuclearity, the expression of osteoclastogenesis-related genes, and TNF-α and IL-1β protein levels were assessed to evaluate osteoclastogenesis. The osteogenesis assays revealed that PdLFs became more differentiated as they were exposed to osteogenic medium for a longer period of time. Mineralisation by these osteogenic cells increased with the progression of differentiation. Culturing PdLFs in osteogenic medium before co-culturing them with PMBCs led to a significant decrease in osteoclast formation. qPCR revealed significantly lower DCSTAMP expression in cultures that had been supplemented with osteogenic medium. Protein levels of osteoclastogenesis stimulator TNF-α were also lower in these cultures. The present study shows that the osteogenic differentiation of PdLFs reduces the osteoclastogenic potential of these cells. Immature cells of the osteoblastic lineage may facilitate osteoclastogenesis, whereas mature mineralising cells may suppress the formation of osteoclasts. Therefore, mature and immature osteogenic cells may have different roles in maintaining bone homeostasis.

Bone-on-a-chip platforms and integrated biosensors: Towards advanced in vitro bone models with real-time biosensing

Bone diseases, such as osteoporosis and bone defects, often lead to structural and functional deformities of the patient's body. Understanding the complicated pathophysiology and finding new drugs for bone diseases are in dire need but challenging with the conventional cell and animal models. Bone-on-a-chip (BoC) models recapitulate key features of bone at an unprecedented level and can potentially shift the paradigm of future bone research and therapeutic development. Nevertheless, current BoC models predominantly rely on off-chip analysis which provides only endpoint measurements. To this end, integrating biosensors within the BoC can provide non-invasive, continuous monitoring of the experiment progression, significantly facilitating bone research. This review aims to summarize research progress in BoC and biosensor integrations and share perspectives on this exciting but rudimentary research area. We first introduce the research progress of BoC models in the study of bone remodeling and bone diseases, respectively. We then summarize the need for BoC characterization and reported works on biosensor integration in organ chips. Finally, we discuss the limitations and future directions of BoC models and biosensor integrations as next-generation technologies for bone research.

The Crosstalk between Mesenchymal Stem Cells and Macrophages in Bone Regeneration: A Systematic Review

Bone regeneration is a complex and well-coordinated process that involves crosstalk between immune cells and resident cells in the injury site. Transplantation of mesenchymal stem cells (MSCs) is a promising strategy to enhance bone regeneration. Growing evidence suggests that macrophages have a significant impact on osteogenesis during bone regeneration. However, the precise mechanisms by which macrophage subtypes influence bone regeneration and how MSCs communicate with macrophages have not yet been fully elucidated. In this systematic literature review, we gathered evidence regarding the crosstalk between MSCs and macrophages during bone regeneration. According to the PRISMA protocol, we extracted literature from PubMed and Embase databases by using "mesenchymal stem cells" and "macrophages" and "bone regeneration" as keywords. Thirty-three studies were selected for this review. MSCs isolated from both bone marrow and adipose tissue and both primary macrophages and macrophage cell lines were used in the selected studies. In conclusion, anti-inflammatory macrophages (M2) have significantly more potential to strengthen bone regeneration compared with naïve (M0) and classically activated macrophages (M1). Transplantation of MSCs induced M1-to-M2 transition and transformed the skeletal microenvironment to facilitate bone regeneration in bone fracture and bone defect models. This review highlights the complexity between MSCs and macrophages, providing more insight into the polarized macrophage behavior in this evolving field of osteoimmunology. The results may serve as a useful reference for definite success in MSC-based therapy based on the critical interaction with macrophages.

3D Spheroids Derived from Human Lipedema ASCs Demonstrated Similar Adipogenic Differentiation Potential and ECM Remodeling to Non-Lipedema ASCs In Vitro

The growth and differentiation of adipose tissue-derived stem cells (ASCs) is stimulated and regulated by the adipose tissue (AT) microenvironment. In lipedema, both inflammation and hypoxia influence the expansion and differentiation of ASCs, resulting in hypertrophic adipocytes and deposition of collagen, a primary component of the extracellular matrix (ECM). The goal of this study was to characterize the adipogenic differentiation potential and assess the levels of expression of ECM-remodeling markers in 3D spheroids derived from ASCs isolated from both lipedema and healthy individuals. The data showed an increase in the expression of the adipogenic genes (ADIPOQ, LPL, PPAR-γ and Glut4), a decrease in matrix metalloproteinases (MMP2, 9 and 11), with no significant changes in the expression of ECM markers (collagen and fibronectin), or integrin A5 in 3D differentiated lipedema spheroids as compared to healthy spheroids. In addition, no statistically significant changes in the levels of expression of inflammatory genes were detected in any of the samples. However, immunofluorescence staining showed a decrease in fibronectin and increase in laminin and Collagen VI expression in the 3D differentiated spheroids in both groups. The use of 3D ASC spheroids provide a functional model to study the cellular and molecular characteristics of lipedema AT.

Macrophage Effects on Mesenchymal Stem Cell Osteogenesis in a Three-Dimensional In Vitro Bone Model

As musculoskeletal (MSK) disorders continue to increase globally, there is an increased need for novel, in vitro models to efficiently study human bone physiology in the context of both healthy and diseased conditions. For these models, the inclusion of innate immune cells is critical. Specifically, signaling factors generated from macrophages play key roles in the pathogenesis of many MSK processes and diseases, including fracture, osteoarthritis, infection etc. In this study, we aim to engineer three-dimensional (3D) and macrophage-encapsulated bone tissues in vitro, to model cell behavior, signaling, and other biological activities in vivo, in comparison to current two-dimensional models. We first investigated and optimized 3D culture conditions for macrophages, and then co-cultured macrophages with mesenchymal stem cells (MSCs), which were induced to undergo osteogenic differentiation to examine the effect of macrophage on new bone formation. Seeded within a 3D hydrogel scaffold fabricated from photocrosslinked methacrylated gelatin, macrophages maintained high viability and were polarized toward an M1 or M2 phenotype. In co-cultures of macrophages and human MSCs, MSCs displayed immunomodulatory activities by suppressing M1 and enhancing M2 macrophage phenotypes. Lastly, addition of macrophages, regardless of polarization state, increased MSC osteogenic differentiation, compared with MSCs alone, with proinflammatory M1 macrophages enhancing new bone formation most effectively. In summary, this study illustrates the important roles that macrophage signaling and inflammation play in bone tissue formation.

Co-culture systems of osteoblasts and osteoclasts: Simulating in vitro bone remodeling in regenerative approaches

Bone is an extremely dynamic tissue, undergoing continuous remodeling for its whole lifetime, but its regeneration or augmentation due to bone loss or defects are not always easy to obtain. Bone tissue engineering (BTE) is a promising approach, and its success often relies on a "smart" scaffold, as a support to host and guide bone formation through bone cell precursors. Bone homeostasis is maintained by osteoblasts (OBs) and osteoclasts (OCs) within the basic multicellular unit, in a consecutive cycle of resorption and formation. Therefore, a functional scaffold should allow the best possible OB/OC cooperation for bone remodeling, as happens within the bone extracellular matrix in the body. In the present work OB/OC co-culture models, with and without scaffolds, are reviewed. These experimental systems are intended for different targets, including bone remodeling simulation, drug testing and the assessment of biomaterials and 3D scaffolds for BTE. As a consequence, several parameters, such as cell type, cell ratio, culture medium and inducers, culture times and setpoints, assay methods, etc. vary greatly. This review identifies and systematically reports the in vitro methods explored up to now, which, as they allow cellular communication, more closely resemble bone remodeling and/or the regeneration process in the framework of BTE. STATEMENT OF SIGNIFICANCE: Bone is a dynamic tissue under continuous remodeling, but spontaneous healing may fail in the case of excessive bone loss which often requires valid alternatives to conventional treatments to restore bone integrity, like bone tissue engineering (BTE). Pre-clinical evaluation of scaffolds for BTE requires in vitro testing where co-cultures combining innovative materials with osteoblasts (OBs) and osteoclasts (OCs) closely mimic the in vivo repair process. This review considers the direct and indirect OB/OC co-cultures relevant to BTE, from the early mouse-cell models to the recent bone regenerative systems. The co-culture modeling of bone microenvironment provides reliable information on bone cell cross-talk. Starting from improved knowledge on bone remodeling, bone disease mechanisms may be understood and new BTE solutions are designed.

Albumin-impregnated bone granules modulate the interactions between mesenchymal stem cells and monocytes under in vitro inflammatory conditions

Bone regeneration around newly implanted biomaterials is a complex process, which in its early phases involves the interactions between Mesenchymal Stem Cells (MSCs) and immune cells. The response of these cells to the biomaterial depends both on the local microenvironment and on the characteristics of the inserted bone substitute. In this work, bone allografts impregnated with albumin are loaded with a co-culture of human MSCs and monocytes; bone granules without albumin are used for comparison. Co-cultures are contextually treated with pro-inflammatory cytokines to simulate the inflammatory milieu naturally present during the bone regeneration process. As revealed by microscopic images, albumin-impregnated bone granules promote adhesion and interactions between cells populations. Compared to control granules, albumin coating diminishes reactive species production by cells. This reduced oxidative stress may be attributable to antioxidant properties of albumin, and it is also reflected in the mitigated gene expression of mitochondrial electron transport chain complexes, where most intracellular reactive molecules are generated. MSCs-monocytes co-cultured onto albumin-impregnated bone granules additionally release higher amounts of immunomodulatory cytokines and growth factors. In summary, this work demonstrates that impregnation of bone granules with albumin positively modulates the interactions between MSCs and immune cells, consequently influencing their mutual activities and immunomodulatory functions.

iTRAQ-Based Quantitative Proteomic Comparison of 2D and 3D Adipocyte Cell Models Co-cultured with Macrophages Using Online 2D-nanoLC-ESI-MS/MS

The demand for novel three-dimensional (3D) cell culture models of adipose tissue has been increasing, and proteomic investigations are important for determining the underlying causes of obesity, type II diabetes, and metabolic disorders. In this study, we performed global quantitative proteomic profiling of three 3D-cultured 3T3-L1 cells (preadipocytes, adipocytes and co-cultured adipocytes with macrophages) and their 2D-cultured counterparts using 2D-nanoLC-ESI-MS/MS with iTRAQ labelling. A total of 2,885 shared proteins from six types of adipose cells were identified and quantified in four replicates. Among them, 48 proteins involved in carbohydrate metabolism (e.g., PDHα, MDH1/2, FH) and the mitochondrial fatty acid beta oxidation pathway (e.g., VLCAD, ACADM, ECHDC1, ALDH6A1) were relatively up-regulated in the 3D co-culture model compared to those in 2D and 3D mono-cultured cells. Conversely, 12 proteins implicated in cellular component organisation (e.g., ANXA1, ANXA2) and the cell cycle (e.g., MCM family proteins) were down-regulated. These quantitative assessments showed that the 3D co-culture system of adipocytes and macrophages led to the development of insulin resistance, thereby providing a promising in vitro obesity model that is more equivalent to the in vivo conditions with respect to the mechanisms underpinning metabolic syndromes and the effect of new medical treatments for metabolic disorders.