Human osteoblast-derived factors induce early osteogenic markers in human mesenchymal stem cells

High Bone Mass Disorders: New Insights From Connecting the Clinic and the Bench

Monogenic high bone mass (HBM) disorders are characterized by an increased amount of bone in general, or at specific sites in the skeleton. Here, we describe 59 HBM disorders with 50 known disease-causing genes from the literature, and we provide an overview of the signaling pathways and mechanisms involved in the pathogenesis of these disorders. Based on this, we classify the known HBM genes into HBM (sub)groups according to uniform Gene Ontology (GO) terminology. This classification system may aid in hypothesis generation, for both wet lab experimental design and clinical genetic screening strategies. We discuss how functional genomics can shape discovery of novel HBM genes and/or mechanisms in the future, through implementation of omics assessments in existing and future model systems. Finally, we address strategies to improve gene identification in unsolved HBM cases and highlight the importance for cross-laboratory collaborations encompassing multidisciplinary efforts to transfer knowledge generated at the bench to the clinic. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Strontium Carbonate and Strontium-Substituted Calcium Carbonate Nanoparticles Form Protective Deposits on Dentin Surface and Enhance Human Dental Pulp Stem Cells Mineralization

Strontium acetate is applied for dental hypersensitivity treatment; however, the use of strontium carbonates for this purpose has not been described. The use of Sr-carbonate nanoparticles takes advantage of both the benefits of strontium on dentin mineralization and the abrasive properties of carbonates. Here in, we aimed to synthesize strontium carbonate and strontium-substituted calcium carbonate nanoparticles and test them as potential compounds in active dentifrices for treating dental hypersensitivity. For this, SrCO₃, Sr_0.5Ca_0.5CO₃, and CaCO₃ nanoparticles were precipitated using Na₂CO₃, SrCl₂, and/or CaCl₂ as precursors. Their morphology and crystallinity were evaluated by electron microscopy (SEM) and X-ray diffraction, respectively. The nanoparticles were added to a poly (vinyl alcohol) gel and used to brush dentin surfaces isolated from human third molars. Dentin chemical composition before and after brushing was investigated by infrared spectroscopy (FTIR) and X-ray dispersive energy spectroscopy. Dentin tubule morphology, obliteration, and resistance of the coatings to acid attack were investigated by SEM and EDS. The cytotoxicity and ability of the particles to trigger the mineralization of hDPSCs in vitro were studied. Dentin brushed with the nanoparticles was coated by a mineral layer that was also able to penetrate the tubules, while CaCO₃ remained as individual particles on the surface. FTIR bands related to carbonate groups were intensified after brushing with either SrCO₃ or Sr_0.5Ca_0.5CO₃. The shift of the phosphate-related FTIR band to a lower wavenumber indicated that strontium replaced calcium on the dentin structure after treatment. The coating promoted by SrCO₃ or Sr_0.5Ca_0.5CO₃ resisted the acid attack, while calcium and phosphorus were removed from the top of the dentin surface. The nanoparticles were not toxic to hDPSCs and elicited mineralization of the cells, as revealed by increased mineral nodule formation and enhanced expression of COL1, ALP, and RUNX2. Adding Sr_0.5Ca_0.5CO₃ as an active ingredient in dentifrices formulations may be commercially advantageous since this compound combines the well-known abrasive properties of calcium carbonate with the mineralization ability of strontium, while the final cost remains between the cost of CaCO₃ and SrCO₃. The novel Sr_0.5Ca_0.5CO₃ nanoparticles might emerge as an alternative for the treatment of dental hypersensitivity.

Peek dental implants coated with boron-doped nano-hydroxyapatites: Investigation of in-vitro osteogenic activity

BACKGROUND

PEEK is a high-performance thermoplastic that has many potential uses in orthopaedics and dentistry, and it has been shown to be a substitute for titanium (Ti) implants. However, PEEK is an inherently inert material, and that characteristic limits its cellular adhesion and bone integration. The aim of this study is to determine a suitable surface modification method for increasing the osteogenic potential of polyetheretherketone (PEEK) implants used in periodontal applications.

METHODS

In this work, a nanostructured porous surface is created on PEEK material by sulfonation, in sulfuric acid at room temperature for 20 min, and thus SPEEK samples were obtained. Then, PEEK and SPEEK samples were coated with boron (B) doped hydroxyapatite (HAp) nanoparticles (B-nHAp) precipitated from concentrated synthetic body fluid (10xSBF) by a microwave-assisted method conducted at 600 W. In vitro cell culture studies were carried out with periodontal ligament cells (PDL) on the samples. Osteogenic differentiation of PDL cells on PEEK, SPEEK and SPEEK-B-nHAp was evaluated using ALP activity assay, hydroxyproline assay, and RT-qPCR.

RESULTS

In vitro cell culture studies disclosed improved adhesion and proliferation of PDL cells on the SPEEK and B-nHAp coated SPEEK surfaces (SPEEK-B-nHAp). Results of these assays confirmed that treated PEEK surfaces, especially SPEEK-B-nHAp, significantly enhanced osteogenic differentiation of PDL cells in vitro compared with untreated PEEK surfaces.

CONCLUSION

Here a simple, easy to-use, and efficient modification method based on the properties of boron is proposed for increasing osteogenic potential of PEEK implants.

Concentrated Growth Factors Promote hBMSCs Osteogenic Differentiation in a Co-Culture System With HUVECs

Osteogenesis is a complex physiologic process that occurs during bone regeneration. This process requires several growth factors that act on bone marrow-derived mesenchymal stem cells (BMSCs). Concentrated growth factor (CGF) is a new-generation platelet-rich derivative that is an appealing autologous material for application in tissue repair and bone regenerative medicine because it contains a variety of fibrin and growth factors. In this study, the effects of CGF on the proliferation and osteogenic differentiation of hBMSCs and human umbilical vein endothelial cells (HUVECs) were explored with in vitro cell co-culture experiments. HBMSCs and HUVECs were directly co-cultured at the ratio of 1:2 under different concentrations (0, 2, 5, 10, 20%) of CGF for 7 days. Alkaline phosphatase (ALP) staining and quantitative reverse transcription polymerase chain reaction were used to detect the effects of CGF on the expression of osteogenic genes (ALP, osteocalcin [OCN], type I collagen [COL-1], Runt-related transcription factor 2 [RUNX2]) and connexin 43 (CX43). RNA sequencing was used to explore potential regulated genes and signaling pathways that affect the osteogenesis of co-cultured hBMSCs exposed to CGF. The results showed higher expressions of ALP, COL-1, RUNX2, OCN, and CX43 in the direct co-culture group containing 10% CGF compared to the direct co-culture group without CGF and the indirect co-culture group. In summary, 10% CGF can significantly promote osteogenesis in hBMSCs directly co-cultured with HUVECs. Intercellular communication between the direct co-culture of hBMSCs and HUVECs through CX43 may be one of the main regulatory mechanisms.

Dual-functional antibacterial and osteogenic nisin-based layer-by-layer coatings

Implanted biomaterials can be regarded in a cornerstone in the domain of bone surgery. Their surfaces are expected to fulfil two particular requirements: preventing the settlement and the development of bacteria, and stimulating bone cells in view to foster osseointegration. Therefore, a modern approach consists in the design of dual functional coatings with both antibacterial and osteogenic features. To this end, we developed ultrathin Layer-by-Layer (LbL) coatings composed of biocompatible polyelectrolytes, namely chondroitin sulfate A (CSA) and poly-l-lysine (PLL). The coatings were crosslinked with genipin (GnP), a natural and biocompatible crosslinking agent, to increase their resistance against environmental changes, and to confer them adequate mechanical properties with regards to bone cell behaviors. Antibacterial activity was obtained with nisin Z, an antimicrobial peptide (AMP), which is active against gram-positive bacteria. The coatings had a significant bactericidal impact upon Staphylococcus aureus, with fully maintained bone cell adhesion, proliferation and osteogenic differentiation.

Three-dimensional cell-laden collagen scaffolds: From biochemistry to bone bioengineering

The bones can be viewed as both an organ and a material. As an organ, the bones give structure to the body, facilitate skeletal movement, and provide protection to internal organs. As a material, the bones consist of a hybrid organic/inorganic three-dimensional (3D) matrix, composed mainly of collagen, noncollagenous proteins, and a calcium phosphate mineral phase, which is formed and regulated by the orchestrated action of a complex array of cells including chondrocytes, osteoblasts, osteocytes, and osteoclasts. The interactions between cells, proteins, and minerals are essential for the bone functions under physiological loading conditions, trauma, and fractures. The organization of the bone's organic and inorganic phases stands out for its mechanical and biological properties and has inspired materials research. The objective of this review is to fill the gaps between the physical and biological characteristics that must be achieved to fabricate scaffolds for bone tissue engineering with enhanced performance. We describe the organization of bone tissue highlighting the characteristics that have inspired the development of 3D cell-laden collagenous scaffolds aimed at replicating the mechanical and biological properties of bone after implantation. The role of noncollagenous macromolecules in the organization of the collagenous matrix and mineralization ability of entrapped cells has also been reviewed. Understanding the modulation of cell activity by the extracellular matrix will ultimately help to improve the biological performance of 3D cell-laden collagenous scaffolds used for bone regeneration and repair as well as for in vitro studies aimed at unravelling physiological and pathological processes occurring in the bone.

Toxic Effects of Indoxyl Sulfate on Osteoclastogenesis and Osteoblastogenesis

Uremic toxins, such as indoxyl sulfate (IS) and kynurenine, accumulate in the blood in the event of kidney failure and contribute to further bone damage. To maintain the homeostasis of the skeletal system, bone remodeling is a persistent process of bone formation and bone resorption that depends on a dynamic balance of osteoblasts and osteoclasts. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates the toxic effects of uremic toxins. IS is an endogenous AhR ligand and is metabolized from tryptophan. In osteoclastogenesis, IS affects the expression of the osteoclast precursor nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) through AhR signaling. It is possible to increase osteoclast differentiation with short-term and low-dose IS exposure and to decrease differentiation with long-term and/or high-dose IS exposure. Coincidentally, during osteoblastogenesis, through the AhR signaling pathway, IS inhibits the phosphorylation of ERK, and p38 reduces the expression of the transcription factor 2 (Runx2), disturbing osteoblastogenesis. The AhR antagonist resveratrol has a protective effect on the IS/AhR pathway. Therefore, it is necessary to understand the multifaceted role of AhR in CKD, as knowledge of these transcription signals could provide a safe and effective method to prevent and treat CKD mineral bone disease.

Bone Cells Differentiation: How CFTR Mutations May Rule the Game of Stem Cells Commitment?

Cystic fibrosis (CF)-related bone disease has emerged as a significant comorbidity of CF and is characterized by decreased bone formation and increased bone resorption. Both osteoblast and osteoclast differentiations are impacted by cystic fibrosis transmembrane conductance regulator (CFTR) mutations. The defect of CFTR chloride channel or the loss of CFTRs ability to interact with other proteins affect several signaling pathways involved in stem cell differentiation and the commitment of these cells toward bone lineages. Specifically, TGF-, nuclear factor-kappa B (NF-B), PI3K/AKT, and MAPK/ERK signaling are disturbed by CFTR mutations, thus perturbing stem cell differentiation. High inflammation in patients changes myeloid lineage secretion, affecting both myeloid and mesenchymal differentiation. In osteoblast, Wnt signaling is impacted, resulting in consequences for both bone formation and resorption. Finally, CFTR could also have a direct role in osteoclasts resorptive function. In this review, we summarize the existing literature on the role of CFTR mutations on the commitment of induced pluripotent stem cells to bone cells.

Comprehensive In Vitro Testing of Calcium Phosphate-Based Bioceramics with Orthopedic and Dentistry Applications

Recently, a large spectrum of biomaterials emerged, with emphasis on various pure, blended, or doped calcium phosphates (CaPs). Although basic cytocompatibility testing protocols are referred by International Organization for Standardization (ISO) 10993 (parts 1-22), rigorous in vitro testing using cutting-edge technologies should be carried out in order to fully understand the behavior of various biomaterials (whether in bulk or low-dimensional object form) and to better gauge their outcome when implanted. In this review, current molecular techniques are assessed for the in-depth characterization of angiogenic potential, osteogenic capability, and the modulation of oxidative stress and inflammation properties of CaPs and their cation- and/or anion-substituted derivatives. Using such techniques, mechanisms of action of these compounds can be deciphered, highlighting the signaling pathway activation, cross-talk, and modulation by microRNA expression, which in turn can safely pave the road toward a better filtering of the truly functional, application-ready innovative therapeutic bioceramic-based solutions.

Secretome analysis of in vitro aged human mesenchymal stem cells reveals IGFBP7 as a putative factor for promoting osteogenesis

Aging is a complex biological process, which involves multiple mechanisms with different levels of regulation. Senescent cells are known to secrete senescence-associated proteins, which exert negative influences on surrounding cells. Mesenchymal stem cells (MSCs), the common progenitors for bone, cartilage and adipose tissue (which are especially affected tissues in aging), are known to secrete a broad spectrum of biologically active proteins with both paracrine and autocrine functions in many biological processes. In this report, we have studied the secreted factors (secretome) from human MSCs (hMSCs) and hMSCs-derived adipocytes which were induced to accumulate prelamin A, the immature form of the nuclear lamina protein called Lamin A, known to induce premature aging syndromes in humans and in murine models. Proteomic analysis from two different techniques, antibody arrays and LS-MS, showed that prelamin A accumulation in hMSCs promotes the differential secretion of factors previously identified as secreted by hMSCs undergoing osteogenesis. Moreover, this secretome was able to modulate osteogenesis of normal hMSCs in vitro. Finally, we found that one of the overexpressed secreted factors of this human aging in vitro stem cell model, IGFBP-7, is an osteogenic factor, essential for the viability of hMSCs during osteogenesis.

Downregulation of Heat Shock Protein 70 Impairs Osteogenic and Chondrogenic Differentiation in Human Mesenchymal Stem Cells

Human mesenchymal stem cells (hMSCs) show promise for bone and cartilage regeneration. Our previous studies demonstrated that hMSCs with periodic mild heating had enhanced osteogenic and chondrogenic differentiation with significantly upregulated heat shock protein 70 (HSP70). However, the role of HSP70 in adult tissue regeneration is not well studied. Here, we revealed an essential regulatory mechanism of HSP70 in osteogenesis and chondrogenesis using adult hMSCs stably transfected with specific shRNAs to knockdown HSP70. Periodic heating at 39 °C was applied to hMSCs for up to 26 days. HSP70 knockdown resulted in significant reductions of alkaline phosphatase activity, calcium deposition, and gene expression of Runx2 and Osterix during osteogenesis. In addition, knockdown of HSP70 led to significant decreases of collagens II and X during chondrogenesis. Thus, downregulation of HSP70 impaired hMSC osteogenic and chondrogenic differentiation as well as the enhancement of these processes by thermal treatment. Taken together, these findings suggest a putative mechanism of thermal-enhanced bone and cartilage formation and underscore the importance of HSP70 in adult bone and cartilage differentiation.

Substrate Microarchitecture Shapes the Paracrine Crosstalk of Stem Cells with Endothelial Cells and Osteoblasts

We examined the hypothesis that substrate microarchitecture regulates the crosstalk between human mesenchymal stem cells (hMSC) and cell types involved in bone regeneration. Compared with polyester flat substrates having uniformly distributed homogenous pores (2D), three-dimensional polystyrene substrates with randomly oriented and interconnected pores of heterogeneous size (3D) stimulated the stromal secretion of IGF-1 while lessened the production of VEGFR-1, MCP-1 and IL-6. The medium conditioned by hMSC cultured in 3D substrates stimulated tube formation by human endothelial cells (hEC) to a higher extent than medium from 2D cultures. 3D co-cultures of hMSC and hEC contained higher secreted levels of IGF-1, EGF and FGF-2 than 2D co-cultures, resulting in increased hEC proliferation and migration. Substrate microarchitecture influenced the secretion of factors related to bone remodeling as the ratio RANKL to OPG, and the levels of M-CSF and IL-6 were higher in 3D co-cultures of hMSC and human osteoblasts (hOB) than in 2D co-cultures. Cytokine microenvironment in 3D co-cultures stimulated osteoblast matrix reorganization while demoted the late steps of osteoblastic maturation. Altogether, data in this study may unveil a new role of scaffold microarchitecture during bone regeneration, as modulator of the paracrine relationships that hMSC establish with hEC and hOB.

Collagen and Hydroxyapatite Scaffolds Activate Distinct Osteogenesis Signaling Pathways in Adult Adipose-Derived Multipotent Stromal Cells

Osteogenic cell signaling pathway disruption varies among bone diseases. This investigation was designed to identify adipose-derived multipotent stromal cell (ASC) and bone graft scaffold combinations for local, targeted restoration of gene expression and extracellular matrix (ECM) deposition. Human ASC osteogenesis on bone graft materials was quantified following culture in stromal (S), osteogenic (O), or osteogenic for 48 h followed by stromal medium (OS) to test the two-part hypothesis: (1) identical ASC isolates on distinct bone graft scaffolds demonstrate unique viability, differentiation, ECM production, and gene expression in the same culture conditions; (2) identical ASC-bone graft scaffold combinations have different cell viability, differentiation, ECM production, and gene expression when cultured in S, O, or OS medium. Three commercially available bone graft scaffold materials, type I bovine collagen (C), hydroxyapatite + β-tricalcium phosphate + type I bovine collagen (HT), and β-tricalcium phosphate + type I bovine collagen (CT) were evaluated. Passage 3 ASCs were loaded onto scaffold blocks with a spinner flask bioreactor, and constructs were cultured up to 28 days. Cell viability, gene expression (alkaline phosphatase [ALPL], osteoprotegerin [TNFRSF11B], osteocalcin [BGLAP], cannabinoid receptors type I [CNR1] and II [CNR2], receptor activator of nuclear factor kappa β ligand [TNFSF11]), as well as ECM DNA, collagen, sulfated glycosaminoglycan, and protein content were quantified. Matrix organization was evaluated with scanning electron microscopy. Effects of scaffold, medium, or culture duration on cell viability were minimal. Significantly higher initial ALPL expression decreased with time, while BGLAP expression increased in HT constructs in O medium, and the constructs had the most abundant ECM components and ultrastructural organization. There was a similar, although delayed, pattern of gene expression and greater ECM collagen with less organization in C constructs in O medium. Higher CNR1 expression in C versus higher TNFRSF11B/TNFSF11 expression in HT constructs throughout the study support stimulation of unique osteogenic signaling pathways by identical cell isolates. These results suggest that bone scaffold composition may be used to selectively target specific osteogenic cell signaling pathways in ASC constructs to stimulate ECM deposition based on therapeutic needs.

Multipotent adult progenitor cells on an allograft scaffold facilitate the bone repair process

Multipotent adult progenitor cells are a recently described population of stem cells derived from the bone marrow stroma. Research has demonstrated the potential of multipotent adult progenitor cells for treating ischemic injury and cardiovascular repair; however, understanding of multipotent adult progenitor cells in orthopedic applications remains limited. In this study, we evaluate the osteogenic and angiogenic capacity of multipotent adult progenitor cells, both in vitro and loaded onto demineralized bone matrix in vivo, with comparison to mesenchymal stem cells, as the current standard. When compared to mesenchymal stem cells, multipotent adult progenitor cells exhibited a more robust angiogenic protein release profile in vitro and developed more extensive vasculature within 2 weeks in vivo. The establishment of this vascular network is critical to the ossification process, as it allows nutrient exchange and provides an influx of osteoprogenitor cells to the wound site. In vitro assays confirmed the multipotency of multipotent adult progenitor cells along mesodermal lineages and demonstrated the enhanced expression of alkaline phosphatase and production of calcium-containing mineral deposits by multipotent adult progenitor cells, necessary precursors for osteogenesis. In combination with a demineralized bone matrix scaffold, multipotent adult progenitor cells demonstrated enhanced revascularization and new bone formation in vivo in an orthotopic defect model when compared to mesenchymal stem cells on demineralized bone matrix or demineralized bone matrix–only control groups. The potent combination of angiogenic and osteogenic properties provided by multipotent adult progenitor cells appears to create a synergistic amplification of the bone healing process. Our results indicate that multipotent adult progenitor cells have the potential to better promote tissue regeneration and healing and to be a functional cell source for use in orthopedic applications.

Effect of Short-Term Stimulation with Interleukin-1β and Differentiation Medium on Human Mesenchymal Stromal Cell Paracrine Activity in Coculture with Osteoblasts

Introduction. Human mesenchymal stromal cells (hMSCs) exhibit the potential to accelerate bone healing by enhanced osteogenic differentiation. Interleukin-1β is highly expressed during fracture healing and has been demonstrated to exert a significant impact on the differentiation behaviour of hMSCs. Here, we investigate the effect of 2-hour IL-1β stimulation on the differentiation and paracrine activity of hMSCs in coculture with osteosarcoma cells in vitro. Methods. hMSCs from 3 donors were incubated for 2 hours with 10 ng/mL IL-1β and subsequently cocultured with MG63-GFP cells either in control or in differentiation medium in a transwell system for 28 days. Genetic and functional effects were investigated. Results. hMSCs cultured in control medium exhibited a regulatory effect on cocultured MG63-GFP cells, resulting in upregulation of osteogenic gene expression in combination with increased ALP activity. However, while stimulated hMSCs cultured under differentiation conditions exhibit signs of osteogenic differentiation, osteogenic differentiation also caused an impaired regulatory effect on the cocultured MG63-GFP cells. Conclusion. Short stimulation of hMSCs has the potential to modify their long-term behaviour. In addition, undifferentiated hMSCs are able to regulate osteoblast differentiation; however, this regulatory function is lost upon osteogenic differentiation in vitro. This offers a novel approach for clinical cell therapy protocols.

Differential osteogenic potential of human adipose-derived stem cells co-cultured with human osteoblasts on polymeric microfiber scaffolds

The osteogenic potential of human adipose-derived stem cells (HADSCs) co-cultured with human osteoblasts (HOBs) using selected HADSCs/HOBs ratios of 1:1, 2:1, and 1:2, respectively, is evaluated. The HADSCs/HOBs were seeded on electrospun three-dimensional poly[(R)-3-hydroxybutyric acid] (PHB) blended with bovine-derived hydroxyapatite (BHA). Monocultures of HADSCs and HOBs were used as control groups. The effects of PHB-BHA scaffold on cell proliferation and cell morphology were assessed by AlamarBlue assay and field emission scanning electron microscopy. Cell differentiation, cell mineralization, and osteogenic-related gene expression of co-culture HADSCs/HOBs were examined by alkaline phosphatase (ALP) assay, alizarin Red S assay, and quantitative real time PCR, respectively. The results showed that co-culture of HADSCs/HOBs, 1:1 grown into PHB-BHA promoted better cell adhesion, displayed a significant higher cell proliferation, higher production of ALP, extracellular mineralization and osteogenic-related gene expression of run-related transcription factor, bone sialoprotein, osteopontin, and osteocalcin compared to other co-culture groups. This result also suggests that the use of electrospun PHB-BHA in a co-culture HADSCs/HOBs system may serve as promising approach to facilitate osteogenic differentiation activity of HADSCs through direct cell-to-cell contact with HOBs.

PML overexpression inhibits proliferation and promotes the osteogenic differentiation of human mesenchymal stem cells

The promyelocytic leukemia (PML) gene, as an important tumor-suppressor, has been proven to regulate stem cell function in multiple tissues; however its role in human mesenchymal stem cells (hMSCs) remains unclear. In the present study, the effect of PML on regulating the proliferation and osteogenic differentiation of hMSCs was explored. New downstream genes that may be responsible for the regulation of PML were found, and possible mechanisms were analyzed. The lentiviral vector which encodes full-length human PML cDNA or shRNA against PML was transfected into hMSCs. RT-PCR and western blotting were used to detect mRNA and protein expression. Flow cytometry was used to analyze apoptosis and the cell cycle distribution. Osteogenic differentiation of hMSCs was induced by osteo-inductive medium for 7 to 14 days. cDNA microarray was used to scan the gene expression profile and to identify significant changes in gene expression. In the present study, we found that PML was stably expressed in hMSCs, and the expression was increased time-dependently along with cell osteogenic differentiation. Overexpression of PML inhibited hMSC proliferation by inducing apoptosis and arresting the cell cycle. However, PML enhanced the osteoblast differentiation potential of hMSCs. PML-overexpressing hMSCs had a significant increase in mineralized matrix production and ALP activity on day 7 under osteogenic or non-osteogenic differentiation conditions. Upregulation of integrin-binding sialoprotein (IBSP, bone sialoprotein) induced by PML overexpression was found. Our data indicate that PML regulates hMSCs as an inhibitor of cell proliferation but a promoter of osteogenic differentiation.

Osteoclast-specific cathepsin K deletion stimulates S1P-dependent bone formation

Cathepsin K (CTSK) is secreted by osteoclasts to degrade collagen and other matrix proteins during bone resorption. Global deletion of Ctsk in mice decreases bone resorption, leading to osteopetrosis, but also increases the bone formation rate (BFR). To understand how Ctsk deletion increases the BFR, we generated osteoclast- and osteoblast-targeted Ctsk knockout mice using floxed Ctsk alleles. Targeted ablation of Ctsk in hematopoietic cells, or specifically in osteoclasts and cells of the monocyte-osteoclast lineage, resulted in increased bone volume and BFR as well as osteoclast and osteoblast numbers. In contrast, targeted deletion of Ctsk in osteoblasts had no effect on bone resorption or BFR, demonstrating that the increased BFR is osteoclast dependent. Deletion of Ctsk in osteoclasts increased their sphingosine kinase 1 (Sphk1) expression. Conditioned media from Ctsk-deficient osteoclasts, which contained elevated levels of sphingosine-1-phosphate (S1P), increased alkaline phosphatase and mineralized nodules in osteoblast cultures. An S1P1,3 receptor antagonist inhibited these responses. Osteoblasts derived from mice with Ctsk-deficient osteoclasts had an increased RANKL/OPG ratio, providing a positive feedback loop that increased the number of osteoclasts. Our data provide genetic evidence that deletion of CTSK in osteoclasts enhances bone formation in vivo by increasing the generation of osteoclast-derived S1P.

Crosstalk on cell behavior in interactive cocultures of hMSCs with various oral cell types

When prospectively applied for regenerative therapies, human bone-marrow-derived mesenchymal stem cells (hMSCs) interact with the locally residing host cells. With respect to the developmentally particular origin of oral cells, little is known about the putatively discriminative behavioral responses of hMSCs in interaction with various oral cell types, including human alveolar bone osteoblasts (hOAs), periodontal ligament fibroblasts (hPDLs), and gingival fibroblasts (hGFs). To assess the crosstalk between hMSCs and oral cells, interactive cocultures were established by combining well-characterized hMSCs with hOAs, hPDLs, or hGFs, and the behavioral hMSC aspects, that is, proliferation and gene expression, were measured by employing a 5-bromo-2'-deoxyuridine assay and real-time polymerase chain reaction, while apoptosis was quantified by in situ cell death detection kit. hMSCs expressed the typical antigen spectrum lacking CD34, CD45, CD14, CD19, and HLA-DR, while expressing CD73, CD90, and CD105, and could successfully be transformed into adipocytes, osteocytes, and chondrocytes. Monocultured control hMSCs proliferated readily, whereas a general reduction of BrdU-labeled cells was observed in cocultures. Globally, upon extending time periods, interactive coculture combinations of hMSCs with hOAs reduced both osteogenic gene and stem cell marker transcription in hMSCs, a phenomenon appearing less pronounced by combining hMSCs with hPDLs, such that the observed effects in terms of proliferation and gene expression followed the same ranking: hOAs>hGFs>hPDLs. Vice versa, in interactive hMSC cocultures, the cell survival rate was significantly increased, irrespective from the combined coculture cell counterpart. Our results show for the first time that behavior of hMSCs reflected by proliferation and gene expression was governed by interaction with various oral cells in a cell-type-discriminative manner. In addition, hMSC coculture restrains apoptosis, such that influences on cell behavior appear as a crosstalk. In summary, interactive cocultures render the basis for a prospective prediction of mutual cell behavior in hMSC-based oral tissue regeneration disclosing that oral cells shift hMSC behavior from proliferation to differentiation and apoptosis-repressing features.

Coculture strategies in bone tissue engineering: the impact of culture conditions on pluripotent stem cell populations

The use of pluripotent stem cell populations for bone tissue regeneration provides many opportunities and challenges within the bone tissue engineering field. For example, coculture strategies have been utilized to mimic embryological development of bone tissue, and particularly the critical intercellular signaling pathways. While research in bone biology over the last 20 years has expanded our understanding of these intercellular signaling pathways, we still do not fully understand the impact of the system's physical characteristics (orientation, geometry, and morphology). This review of coculture literature delineates the various forms of coculture systems and their respective outcomes when applied to bone tissue engineering. To understand fully the key differences between the different coculture methods, we must appreciate the underlying paradigms of physiological interactions. Recent advances have enabled us to extrapolate these techniques to larger dimensions and higher geometric resolutions. Finally, the contributions of bioreactors, micropatterned biomaterials, and biomaterial interaction platforms are evaluated to give a sense of the sophistication established by a combination of these concepts with coculture systems.

Differentiation of mesenchymal stem cells in heparin-containing hydrogels via coculture with osteoblasts

The therapeutic potency of delivered mesenchymal stem cells (MSCs) in tissue engineering applications may be improved by priming cells toward a differentiated state via coculture with native, differentiated cells prior to implantation; however, there is a lack of understanding in what may be the most efficacious method. The objective of this study was to investigate the role of negatively-charged heparin in priming hydrogel-encapsulated MSCs toward the osteoblastic lineage during coculture with a monolayer of osteoblasts in the absence of dexamethasone. MSCs encapsulated with higher amounts of heparin and cocultured with osteoblasts exhibited an over 36-fold increase in alkaline phosphatase activity and 13-fold increase in calcium accumulation by day 21, compared to MSCs cocultured with MSCs at the same heparin content. Moreover, hydrogels with higher amounts of heparin and cocultured with osteoblasts exhibited enhanced mineralization on the edges, suggesting that heparin may be important in sequestering osteoblast-secreted soluble factors, particularly on the surfaces of hydrogels. The ability of heparin to selectively interact with soluble positively-charged proteins from the surroundings was confirmed through protein labeling and microscopy. These results suggest that heparin-containing hydrogels as part of a coculture system can be utilized as a versatile platform to study and enhance priming of MSCs toward various cell types for a wide variety of regenerative medicine-based therapies.

Osteogenic differentiation is synergistically influenced by osteoinductive treatment and direct cell-cell contact between murine osteoblasts and mesenchymal stem cells

PURPOSE

The present study was designed to address whether osteoblasts play a synergistic role in promoting mesenchymal stem cell (MSC) osteogenesis in a direct cell-cell contact co-culture model.

METHODS

Murine C3H10T1/2 and MC3T3-E1 cell lines were mixed and plated onto 12-well culture plates and co-cultured at various ratios of initial cell densities. To compare the possible improvement on osteogenic differentiation, co-culture cells were served with or without osteogenic supplements in culture medium.

RESULTS

Weak osteogenesis was induced in MSCs co-cultured in an untreated medium with different ratios of osteoblasts. An osteoblast-dependent increase in osteogenic gene expression of Runx2, type I collagen, and osteocalcin was observed over time. Moreover, both alkaline phosphatase (ALP) activity and calcium deposition were distinctly enhanced at levels that were proportional to the quantity of osteoblasts in the culture. The increases in mRNA expression and ALP activity were greater in co-cultures treated with osteogenic supplements than in untreated cultures. However, the production of ALP activity followed by a distinct matrix mineralization was lower in osteogenic-treated cultures containing greater numbers of osteoblasts. This suggests that a higher density of osteoblasts may lead to weak osteogenesis of MSCs by direct cell-cell contact co-culture in an untreated environment. Furthermore, additional osteogenic supplements may act synergistically with osteoblasts to accelerate matrix mineralization by reducing the process of osteogenic differentiation in osteogenic treated co-cultures.

CONCLUSIONS

The present work may improve the understanding of MSC osteogenesis and may provide benefits for regenerative medicine.

Osteoblasts on rod shaped hydroxyapatite nanoparticles incorporated PCL film provide an optimal osteogenic niche for stem cell differentiation

After the clinical insertion of a bone biomaterial, the surrounding osteoblasts would migrate and attach to the implant surface and foster a microenvironment that largely determines the differentiation fate of the comigrated mesenchymal stem cells. Whether the fostered microenvironment is suitable for osteogenic differentiation of mesenchymal stem cells is critical for the subsequent osseointegration. In this study, we determined (1) how the spherical or rod-shaped hydroxyapatite nanoparticles (nHA) incorporated poly(ɛ-caprolactone) (PCL) films (PCL-spherical nHA, PCL-rod nHA) interact with primary human osteoblasts (HOBs); (2) how the microenvironment rendered by their interaction affects osteogenic differentiation of adipose tissue-derived mesenchymal stem cells (ASCs). HOBs were seeded on PCL, PCL-spherical nHA, and PCL-rod nHA films, respectively. When cultured alone, the HOBs on PCL-rod nHA films showed most efficient osteoblastic differentiation compared with those on PCL or PCL-spherical nHA films. When cocultured with ASCs in an indirect coculture system, only the HOBs on PCL-rod nHA films up-regulated the gene expression of Runx2, bone sialoprotein, and osteocalcin of ASCs. Additionally, the HOBs on PCL-rod nHA films showed significant up-regulation of bone morphogenic protein 2 gene and protein expression and induced highest phosphorylated Smad1/5 protein level in ASCs. Treatment of the coculture medium with bone morphogenic protein 2 inhibitor (Noggin) largely abolished the osteogenic differentiation of the ASCs induced by the HOBs on PCL-rod nHA films. In conclusion, HOBs can not only best display their osteoblastic phenotype by culturing on PCL-rod nHA films but also render an optimal osteogenic niche for the differentiation of stem cells.

Accelerated angiogenic host tissue response to poly(L-lactide-co-glycolide) scaffolds by vitalization with osteoblast-like cells

BACKGROUND

Bone substitutes should ideally promote rapid vascularization, which could be accelerated if these substitutes were vitalized by autologous cells. Although adequate engraftment of porous poly(L-lactide-co-glycolide) (PLGA) scaffolds has been demonstrated in the past, it has not yet been investigated how vascularization is influenced by vitalization or, more precisely, by seeding PLGA scaffolds with osteoblast-like cells (OLCs). For this reason, we conducted an in vivo study to assess host angiogenic and inflammatory responses after the implantation of PLGA scaffolds vitalized with isogeneic OLCs.

MATERIALS AND METHODS

OLCs were seeded on collagen-coated PLGA scaffolds that were implanted into dorsal skinfold chambers in BALB/c mice (n = 8). Two further groups of animals received either collagen-coated (n = 8) or uncoated PLGA scaffolds (n = 8). Animals that received chambers without implants served as controls (n = 8). Angiogenesis, neovascularization, and leukocyte-endothelial cell interaction were analyzed for 14 days using intravital fluorescence microscopy.

RESULTS

PLGA scaffolds with and without OLCs showed a temporary increase in leukocyte recruitment. At day 3 after implantation, a marked angiogenic host tissue response was observed in close vicinity of all scaffolds studied. At days 6 and 10, the angiogenic response was significantly higher (p < 0.05) in PLGA scaffolds vitalized with OLCs than in uncoated or collagen-coated PLGA scaffolds. The majority of OLCs, however, died within 14 days after implantation.

CONCLUSION

Our study demonstrates that PLGA scaffold vitalization with OLCs accelerates the angiogenic response in the surrounding host tissue. Bone substitutes created by tissue engineering may thus be superior to nonvitalized substitutes although the seeded cells do not survive for long periods.

Platelet-derived growth factor receptor signaling is not involved in osteogenic differentiation of human mesenchymal stem cells

Platelet-derived growth factor (PDGF) receptor signaling plays an important role in the regulation of proliferation and migration of skeletal cells such as osteoblasts or mesenchymal stem cells (MSCs). However, involvement of these receptors in the process of osteoblastic differentiation of MSCs is still a matter of debate. The aim of our study was to examine the role of PDGF receptor signaling in osteogenic differentiation of human MSCs. For this purpose, we performed PDGF receptor stimulation as well as inhibition experiments. Inhibition experiments were carried out with Tyrphostin AG1296, a potent and specific inhibitor of PDGF receptor activity. As expected, Tyrphostin AG1296 treatment caused a concentration-dependent decrease in fetal calf serum and PDGF-BB-induced proliferation of MSCs and effectively inhibited PDGF-BB-induced phosphorylation of extracellular-regulated kinase 1/2. However, PDGF receptor inhibition had no significant effect on osteoblastic differentiation of MSCs, as evaluated histochemically by von Kossa, Alizarin-Red, and osteocalcin stainings. Moreover, mineralized matrix production, as assayed by quantitative Ca(2+)-measurements, was also not modulated by Tyrphostin AG1296 treatment. These results were noticeable irrespective of whether MSCs were grown under nonosteogenic or osteogenic differentiation conditions. Similarly, PDGF-BB treatment of MSCs in receptor stimulation experiments also failed to modulate mineralization. However, expression of alkaline phosphatase was suppressed by Tyrphostin AG1296 treatment at later stages of osteogenesis but not in the early stages, as assessed by enzyme activity and mRNA expression assays. Expression of other osteogenic marker genes such as osteocalcin, runt-related transcription factor 2, osteopontin, collagen type I, and bone sialoprotein was almost unaffected in our perturbation studies. From these experiments, we conclude that PDGF receptor signaling sustains proliferation without affecting osteogenic differentiation of MSCs.