Novel Wnt Regulator NEL-Like Molecule-1 Antagonizes Adipogenesis and Augments Osteogenesis Induced by Bone Morphogenetic Protein 2

Physiologic dentin regeneration: its past, present, and future perspectives

Regenerative dentistry has rapidly progressed since the advancement of stem cell biology and material science. However, more emphasis has been placed on the success of tissue formation than on how well the newly generated tissue retains the original structure and function. Once dentin is lost, tertiary dentinogenesis can be induced by new odontoblastic differentiation or re-activation of existing odontoblasts. The characteristic morphology of odontoblasts generates the tubular nature of dentin, which is a reservoir of fluid, ions, and a number of growth factors, and protects the inner pulp tissue. Therefore, understanding the dynamic but delicate process of new dentin formation by odontoblasts, or odontoblast-like cells, following dentinal defects is crucial. In this regard, various efforts have been conducted to identify novel molecules and materials that can promote the regeneration of dentin with strength and longevity. In this review, we focus on recent progress in dentin regeneration research with biological molecules identified, and discuss its potential in future clinical applications.

Bisphosphonate conjugation enhances the bone-specificity of NELL-1-based systemic therapy for spaceflight-induced bone loss in mice

Microgravity-induced bone loss results in a 1% bone mineral density loss monthly and can be a mission critical factor in long-duration spaceflight. Biomolecular therapies with dual osteogenic and anti-resorptive functions are promising for treating extreme osteoporosis. We previously confirmed that NELL-like molecule-1 (NELL-1) is crucial for bone density maintenance. We further PEGylated NELL-1 (NELL-polyethylene glycol, or NELL-PEG) to increase systemic delivery half-life from 5.5 to 15.5 h. In this study, we used a bio-inert bisphosphonate (BP) moiety to chemically engineer NELL-PEG into BP-NELL-PEG and specifically target bone tissues. We found conjugation with BP improved hydroxyapatite (HA) binding and protein stability of NELL-PEG while preserving NELL-1's osteogenicity in vitro. Furthermore, BP-NELL-PEG showed superior in vivo bone specificity without observable pathology in liver, spleen, lungs, brain, heart, muscles, or ovaries of mice. Finally, we tested BP-NELL-PEG through spaceflight exposure onboard the International Space Station (ISS) at maximal animal capacity (n = 40) in a long-term (9 week) osteoporosis therapeutic study and found that BP-NELL-PEG significantly increased bone formation in flight and ground control mice without obvious adverse health effects. Our results highlight BP-NELL-PEG as a promising therapeutic to mitigate extreme bone loss from long-duration microgravity exposure and musculoskeletal degeneration on Earth, especially when resistance training is not possible due to incapacity (e.g., bone fracture, stroke).

NELL1 Regulates the Matrisome to Promote Osteosarcoma Progression

Sarcomas produce an abnormal extracellular matrix (ECM), which in turn provides instructive cues for cell growth and invasion. Neural EGF like-like molecule 1 (NELL1) is a secreted glycoprotein characterized by its nonneoplastic osteoinductive effects, yet it is highly expressed in skeletal sarcomas. Here, we show that genetic deletion of NELL1 markedly reduces invasive behavior across human osteosarcoma (OS) cell lines. NELL1 deletion resulted in reduced OS disease progression, inhibiting metastasis and improving survival in a xenograft mouse model. These observations were recapitulated with Nell1 conditional knockout in mouse models of p53/Rb-driven sarcomagenesis, which reduced tumor frequency and extended tumor-free survival. Transcriptomic and phosphoproteomic analyses demonstrated that NELL1 loss skews the expression of matricellular proteins associated with reduced FAK signaling. Culturing NELL1 knockout sarcoma cells on wild-type OS-enriched matricellular proteins reversed the phenotypic and signaling changes induced by NELL1 deficiency. In sarcoma patients, high expression of NELL1 correlated with decreased overall survival. These findings in mouse and human models suggest that NELL1 expression alters the sarcoma ECM, thereby modulating cellular invasive potential and prognosis. Disruption of NELL1 signaling may represent a novel therapeutic approach to short-circuit sarcoma disease progression.

SIGNIFICANCE

NELL1 modulates the sarcoma matrisome to promote tumor growth, invasion, and metastasis, identifying the matrix-associated protein as an orchestrator of cell-ECM interactions in sarcomagenesis and disease progression.

Genetic and pharmacologic suppression of PPARγ enhances NELL-1-stimulated bone regeneration

Recent investigations into mechanisms behind the development of osteoporosis suggest that suppressing PPARγ-mediated adipogenesis can improve bone formation and bone mineral density. In this study, we investigated a co-treatment strategy to enhance bone formation by combining NELL-1, an osteogenic molecule that has been extensively studied for its potential use as a therapeutic for osteoporosis, with two methods of PPARγ suppression. First, we suppressed PPARγ genetically using lentiviral PPARγ-shRNA in immunocompromised mice for a proof of concept. Second, we used a PPARγ antagonist to suppress PPARγ pharmacologically in immunocompetent senile osteopenic mice for clinical transability. We found that the co-treatment strategy significantly increased bone formation, increased the proliferation stage cell population, decreased late apoptosis of primary mouse BMSCs, and increased osteogenic marker mRNA levels in comparison to the single agent treatment groups. The addition of PPARγ suppression to NELL-1 therapy enhanced NELL-1's effects on bone formation by upregulating anabolic processes without altering NELL-1's inhibitory effects on osteoclastic and adipogenic activities. Our findings suggest that combining PPARγ suppression with therapeutic NELL-1 may be a viable method that can be further developed as a novel strategy to reverse bone loss and decrease marrow adiposity in age-related osteoporosis.

Isolation of NELL 1 Aptamers for Rhabdomyosarcoma Targeting

NELL1 (Neural epidermal growth factor-like (EGFL)-like protein) is an important biomarker associated with tissue and bone development and regeneration. NELL1 upregulation has been linked with metastasis and negative prognosis in rhabdomyosarcoma (RMS). Furthermore, multiple recent studies have also shown the importance of NELL1 in inflammatory bowel disease and membranous nephropathy, amongst other diseases. In this study, several anti-NELL1 DNA aptamers were selected from a randomized ssDNA pool using a fluorescence-guided method and evaluated for their binding affinity and selectivity. Several other methods such as a metabolic assay and confocal microscopy were also applied for the evaluation of the selected aptamers. The top three candidates were evaluated further, and AptNCan3 was shown to have a binding affinity up to 959.2 nM. Selectivity was examined in the RH30 RMS cells that overexpressed NELL1. Both AptNCan2 and AptNCan3 could significantly suppress metabolic activity in RMS cells. AptNCan3 was found to locate on the cell membrane and also on intracellular vesicles, which matched the location of NELL1 shown by antibodies in previous research. These results indicate that the selected anti-NELL1 aptamer showed strong and highly specific binding to NELL1 and therefore has potential to be used for in vitro or in vivo studies and treatments.

Signaling network regulating osteogenesis in mesenchymal stem cells

Osteogenesis is an important developmental event that results in bone formation. Bone forming cells or osteoblasts develop from mesenchymal stem cells (MSCs) through a highly controlled process regulated by several signaling pathways. The osteogenic lineage commitment of MSCs is controlled by cell-cell interactions, paracrine factors, mechanical signals, hormones, and cytokines present in their niche, which activate a plethora of signaling molecules belonging to bone morphogenetic proteins, Wnt, Hedgehog, and Notch signaling. These signaling pathways individually as well as in coordination with other signaling molecules, regulate the osteogenic lineage commitment of MSCs by activating several osteo-lineage specific transcription factors. Here, we discuss the key signaling pathways that regulate osteogenic differentiation of MSCs and the cross-talk between them during osteogenic differentiation. We also discuss how these signaling pathways can be modified for therapy for bone repair and regeneration.

NELL-1 in Genome-Wide Association Studies across Human Diseases

Neural epidermal growth factor-like (EGFL)-like protein (NELL)-1 is a potent and key osteogenic factor in the development and regeneration of skeletal tissues. Intriguingly, accumulative data from genome-wide association studies (GWASs) have started unveiling potential broader roles of NELL-1 beyond its functions in bone and cartilage. With exploration of the genetic variants of the entire genome in large-scale disease cohorts, GWASs have been used for establishing the connection between specific single-nucleotide polymorphisms of NELL1, in addition to osteoporosis, metabolic diseases, inflammatory conditions, neuropsychiatric diseases, neurodegenerative disorders, and malignant tumors. This review summarizes the findings from GWASs on the manifestation, significance level, implications on function, and correlation of specific NELL1 single-nucleotide polymorphisms in various disorders in humans. By offering a unique and comprehensive correlation between genetic variants and plausible functions of NELL1 in GWASs, this review illustrates the wide range of potential effects of a single gene on the pathogenesis of multiple disorders in humans.

Bone marrow mesenchymal stem cells with low dose bone morphogenetic protein 2 enhances scaffold-based spinal fusion in a porcine model

High doses bone morphogenetic protein 2 (BMP-2) have resulted in a series of complications in spinal fusion. We previously established a polyelectrolyte complex (PEC) carrier system that reduces the therapeutic dose of BMP-2 in both rodent and porcine spinal fusion models. This study aimed to evaluate the safety and efficacy of the combination of bone marrow mesenchymal stem cells (BMSCs) and low dose BMP-2 delivered by PEC for bone regeneration in a porcine model of anterior lumbar interbody spinal fusion (ALIF) application. Six Yorkshire pigs underwent a tri-segmental (L2/L3; L3/L4; L4/L5) ALIF in four groups, namely: (a) BMSCs + 25 μg BMP-2/PEC (n = 9), (b) 25 μg BMP-2/PEC (n = 3), (c) BMSCs (n = 3), and (d) 50 μg BMP-2/absorbable collagen sponge (n = 3). Fusion outcomes were evaluated by radiography, biomechanical testing, and histological analysis after 12 weeks. Mean radiographic scores at 12 weeks were 2.7, 2.0, 1.0, and 1.0 for Groups 1 to 4, respectively. μ-CT scanning, biomechanical evaluation, and histological analysis demonstrated solid fusion and successful bone regeneration in Group 1. In contrast, Group 2 showed inferior quality and slow rate of fusion, and Groups 3 and 4 failed to fuse any of the interbody spaces. There was no obvious evidence of seroma formation, implant rejection, or any other complications in all groups. The results suggest that the combination of BMSCs and low dose BMP-2/PEC could further lower down the effective dose of the BMP-2 and be used as a bone graft substitute in the large animal ALIF model.

Recent Progresses in the Treatment of Osteoporosis

Osteoporosis (OP) is a chronic bone disease characterized by aberrant microstructure and macrostructure of bone, leading to reduced bone mass and increased risk of fragile fractures. Anti-resorptive drugs, especially, bisphosphonates, are currently the treatment of choice in most developing countries. However, they do have limitations and adverse effects, which, to some extent, helped the development of anabolic drugs such as teriparatide and romosozumab. In patients with high or very high risk for fracture, sequential or combined therapies may be considered with the initial drugs being anabolic agents. Great endeavors have been made to find next generation drugs with maximal efficacy and minimal toxicity, and improved understanding of the role of different signaling pathways and their crosstalk in the pathogenesis of OP may help achieve this goal. Our review focused on recent progress with regards to the drug development by modification of Wnt pathway, while other pathways/molecules were also discussed briefly. In addition, new observations made in recent years in bone biology were summarized and discussed for the treatment of OP.

Novel cell sources for bone regeneration

A plethora of both acute and chronic conditions, including traumatic, degenerative, malignant, or congenital disorders, commonly induce bone disorders often associated with severe persisting pain and limited mobility. Over 1 million surgical procedures involving bone excision, bone grafting, and fracture repair are performed each year in the U.S. alone, resulting in immense levels of public health challenges and corresponding financial burdens. Unfortunately, the innate self-healing capacity of bone is often inadequate for larger defects over a critical size. Moreover, as direct transplantation of committed osteoblasts is hindered by deficient cell availability, limited cell spreading, and poor survivability, an urgent need for novel cell sources for bone regeneration is concurrent. Thanks to the development in stem cell biology and cell reprogramming technology, many multipotent and pluripotent cells that manifest promising osteogenic potential are considered the regenerative remedy for bone defects. Considering these cells' investigation is still in its relative infancy, each of them offers their own particular challenges that must be conquered before the large-scale clinical application.

Crosstalk between adipocytes and M2 macrophages compensates for osteopenic phenotype in the Lrp5-deficient mice

A loss-of-function mutation in the Lrp5 gene in mice leads to a low bone mass disorder due to the inhibition of the canonical Wnt signaling pathway; however, the role of bone marrow microenvironment in mice with this mutation remains unclear. In this study, we evaluated proliferation and osteogenic potential of mouse osteoblasts using the MTT assay and Alizarin red staining. The levels of alkaline phosphatase, tartrate-resistant acid phosphatase, and adiponectin in culture supernatants were measured using the enzyme-linked immunosorbent assay. Osteoclast bone resorbing activity was evaluated by toluidine staining and the number and area of bone resorption pits were determined. We observed increased osteogenesis in osteoblasts co-cultured with the BM-derived myeloid cells compared to the osteoblasts cultured alone. Mice with global Lrp5 deletion had a relatively higher bone density compared to the mice carrying osteoblast/osteocyte-specific Lrp5 deletion. An increased frequency of M2 macrophages and reduced expression of inflammatory cytokines were detected in the myeloid cells derived from the bone marrow of mice with global Lrp5 deletion. Higher adipogenic potential and elevated levels of adiponectin in the global Lrp5 deletion mice contributed to the preferential M2 macrophage polarization. Here, we identified a novel systemic regulatory mechanism of bone formation and degradation in mice with global Lrp5 deletion. This mechanism depends on a crosstalk between the adipocytes and M2 macrophages in the bone marrow and is responsible for partly rescuing osteopenia developed as a result of decreased Wnt signaling.

Trb3 controls mesenchymal stem cell lineage fate and enhances bone regeneration by scaffold-mediated local gene delivery

Aberrant lineage commitment of mesenchymal stem cells (MSCs) in marrow contributes to abnormal bone formation due to reduced osteogenic and increased adipogenic potency. While several major transcriptional factors associated with lineage differentiation have been found during the last few decades, the molecular switch for MSC fate determination and its role in skeletal regeneration remains largely unknown, limiting creation of effective therapeutic approaches. Tribbles homolog 3 (Trb3), a member of tribbles family pseudokinases, is known to exert diverse roles in cellular differentiation. Here, we investigated the reciprocal role of Trb3 in the regulation of osteogenic and adipogenic differentiation of MSCs in the context of bone formation, and examined the mechanisms by which Trb3 controls the adipo-osteogenic balance. Trb3 promoted osteoblastic commitment of MSCs at the expense of adipocyte differentiation. Mechanistically, Trb3 regulated cell-fate choice of MSCs through BMP/Smad and Wnt/β-catenin signals. Importantly, in vivo local delivery of Trb3 using a novel gelatin-conjugated caffeic acid-coated apatite/PLGA (GelCA-PLGA) scaffold stimulated robust bone regeneration and inhibited fat-filled cyst formation in rodent non-healing mandibular defect models. These findings demonstrate Trb3-based therapeutic strategies that favor osteoblastogenesis over adipogenesis for improved skeletal regeneration and future treatment of bone-loss disease. The distinctive approach implementing a scaffold-mediated local gene transfer may further broaden the translational use of targeting specific therapeutic gene related to lineage commitment for clinical bone treatment.

Complex genetic dependencies among growth and neurological phenotypes in healthy children: Towards deciphering developmental mechanisms

The genetic mechanisms of childhood development in its many facets remain largely undeciphered. In the population of healthy infants studied in the Growing Up in Singapore Towards Healthy Outcomes (GUSTO) program, we have identified a range of dependencies among the observed phenotypes of fetal and early childhood growth, neurological development, and a number of genetic variants. We have quantified these dependencies using our information theory-based methods. The genetic variants show dependencies with single phenotypes as well as pleiotropic effects on more than one phenotype and thereby point to a large number of brain-specific and brain-expressed gene candidates. These dependencies provide a basis for connecting a range of variants with a spectrum of phenotypes (pleiotropy) as well as with each other. A broad survey of known regulatory expression characteristics, and other function-related information from the literature for these sets of candidate genes allowed us to assemble an integrated body of evidence, including a partial regulatory network, that points towards the biological basis of these general dependencies. Notable among the implicated loci are RAB11FIP4 (next to NF1), MTMR7 and PLD5, all highly expressed in the brain; DNMT1 (DNA methyl transferase), highly expressed in the placenta; and PPP1R12B and DMD (dystrophin), known to be important growth and development genes. While we cannot specify and decipher the mechanisms responsible for the phenotypes in this study, a number of connections for further investigation of fetal and early childhood growth and neurological development are indicated. These results and this approach open the door to new explorations of early human development.

Activation of Nell-1 in BMSC Sheet Promotes Implant Osseointegration Through Regulating Runx2/Osterix Axis

Neural epidermal growth factor-like 1 protein (Nell-1) is first studied because of its association with human craniosynostosis. Nell-1 has been used to accelerate the process of fracture healing because of the osteoinductive ability in recent years. However, the role of Nell-1 during the process of osteointegration is unknown. Here we show that activation of Nell-1 in the BMSC sheet promotes osseointegration in vivo and in vitro. We found that overexpression of Nell-1 improved osteogenic differentiation and enhanced matrix mineralization of BMSCs through increasing expression of Runx2 and Osterix. Activation of Nell-1 up-regulated the expression ratio of OPG/RANKL, which might have a negative influence on osteoclast differentiation. Furthermore, we obtained BMSC sheet-implant complexes transfected with lentivirus overexpressing and interfering Nell-1 in in vivo study, and confirmed that overexpression of Nell-1 promoted new bone formation around the implant and increased the bone-implant contacting area percentage. Our results demonstrate that activation of Nell-1 improves implant osteointegration by regulating Runx2/Osterix axis and shows the potential of BMSC sheet-implant complexes in gene therapy.

Contrary Roles of Wnt/β-Catenin Signaling in BMP9-Induced Osteogenic and Adipogenic Differentiation of 3T3-L1 Preadipocytes

Our previous study revealed that 3T3-L1 preadipocytes can differentiate to either osteoblasts or adipocytes in response to bone morphogenic protein 9 (BMP9). In the present study, we try to further investigate whether the Wnt/β-catenin signaling plays a crucial role in this process. It was found that BMP9 effectively activated the Wnt/β-catenin signaling, and induced the expression levels of certain canonical Wnt ligands and their receptors in preadipocytes. Exogenous expression of β-catenin, Wnt1, Wnt3a, and Wnt10b potentiated BMP9-induced alkaline phosphatase (ALP) activity, while β-catenin knockdown or Dickkopf 1 (Dkk1) diminished BMP9-induced ALP activity. Moreover, it was demonstrated that β-catenin overexpression promoted BMP9-induced mineralization, and increased the expression levels of late osteogenic markers osteopontin and osteocalcin. Furthermore, β-catenin inhibited BMP9-induced lipid accumulation and the adipogenic marker adipocyte fatty acid binding protein (aP2). The cell-implantation assay results identified that β-catenin not only augmented BMP9-induced ectopic bone formation, but also blocked adipogenesis in vivo. Mechanistically, it was found that β-catenin and BMP9 synergistically stimulated the osteogenic transcription factors runt-related transcription factor 2 (Runx2) and Osterix (OSX). However, BMP9-induced adipogenic transcription factors, peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT enhancer-binding protein α (C/EBPα), were inhibited by β-catenin. Therefore, these findings suggested that the Wnt/β-catenin signaling, potentially via the modulation of osteogenic and adipogenic transcriptional factors, exerts an opposite effect on BMP9-induced osteogenic and adipogenic differentiation in preadipocytes.

The synergistic effect of NELL1 and adipose-derived stem cells on promoting bone formation in osteogenesis imperfecta treatment

BACKGROUND

Osteogenesis imperfecta (OI) is a rare genetic disorder characterized by bone fragility and deformity. Mesenchymal stem cells (MSCs) infusion can improve bone performance mainly due to their differentiation into osteoblasts in OI therapy. The osteoinductive activity of NELL1 have benefited various bone defect and osteoporotic models by promoting bone formation. The present study investigated the efficacy of combined use of NELL1 and adipose-derived mesenchymal stem cells (ADSCs) in OI treatment.

METHODS

Lentiviral vector carrying mouse Nell1 gene was constructed and lentivirus were used to infect ADSCs. The osteogenic capacity of MC3T3-E1 and ADSCs stimulated by recombinant mouse NELL1 protein (rmNELL1) and Nell1 gene genetically modified ADSCs (lenti-Nell1-ADSCs) were estimated by real-time quantitative PCR. Thirty adult male OI type I mice with single Col1a1 gene knockout were randomly divided into five groups and received intravenously injected PBS, rmNELL1 (1.25 mg/Kg), ADSCs (2 × 10⁵ cells per mice), rmNELL1 (1.25 mg/Kg) combined with ADSCs (2 × 10⁵ cells per mice), or lenti-Nell1-ADSCs (2 × 10⁵ cells per mice) respectively. Six wildtype (WT) mice served as positive control. Bone formation was examined after 4 weeks using micro-CT, histological and immunohistochemical methods.

RESULTS

Three osteoblast related genes of MC3T3-E1 and ADSCs were significantly up-regulated by rmNELL1 in vitro. Lenti-Nell1-ADSCs showed greatly enhanced osteogenic differentiation capacity. The infused lenti-Nell1-ADSCs could migrate to femur and differentiate into ALPL-positive cells. Systemic administration of rmNELL1 combined with ADSCs or lenti-Nell1-ADSCs markedly improved the femoral microstructure and promoted bone formation through increasing the ALPL and osteocalcin (OCN) expression, much better than mice that received single rmNELL1 or ADSCs. And Nell1 gene engineered ADSCs achieved slightly better outcomes than that of combinative use of rmNELL1 and ADSCs.

CONCLUSIONS

NELL1 and ADSCs exhibited synergistic effect on stimulating bone formation of OI mice, which might provide an alternative strategy in OI treatment. Compared with dose escalation or multiple administration of rmNELL1, lentivirus-mediated long term expression of NELL1 might be more feasible and convenient. However, further studies are needed to confirm the safety and optimize the therapeutic regime.

microRNA expression profiles and the potential competing endogenous RNA networks in NELL-1-induced human adipose-derived stem cell osteogenic differentiation

Studies have indicated that Nel-like molecule-1 (NELL-1) was an osteoblast-specific cytokine and some specific microRNAs (miRNAs) could serve as competing endogenous RNA (ceRNA) to partake in osteogenic differentiation of human adipose-derived stem cells (hASCs). The aim of this study was to explore the potential functional mechanisms of recombinant human NELL-1 protein (rhNELL-1) during hASCs osteogenic differentiation. rhNELL-1 was added to osteogenic medium to activate osteogenic differentiation of hASCs. High-throughput RNA sequencing (RNA-Seq) was performed and validated by real-time quantitative polymerase chain reaction. Gene ontology functional annotation and Kyoto Encyclopedia of Genes and Genomes pathway analysis were performed to detect the functions of differentially expressed miRNAs and genes. Coding-noncoding gene co-expression network and ceRNA networks were constructed to predict the potential regulatory role of miRNAs. A total of 1010 differentially expressed miRNAs and 1762 differentially expressed messenger RNAs (mRNAs) were detected. miRNA-370-3p, bone morphogenetic protein 2 (BMP2), and parathyroid hormone like hormone (PTHLH) were differentially expressed during NELL-1-induced osteogenesis. Bioinformatic analyses demonstrated that these differentially expressed miRNAs and mRNAs enriched in Rap1 signaling pathway, PI3K-Akt signaling pathway, p53 signaling pathway, Glucagon signaling pathway, and hypoxia-inducible factor-1 signaling pathway, which were important pathways related to osteogenic differentiation. In addition, miRNA-370-3p and has-miR-485-5p were predicted to interact with circ0001543, circ0002405, and ENST00000570267 in ceRNA networks. Based on the gain or loss of functional experiments by transfection, the results showed that miR-370-3p was a key regulator in osteogenic differentiation by targeting BMP2 and disturbing the expression of PTHLH, and participated in NELL-1-stimulated osteogenesis. The present study provided the primary data and evidence for further exploration on the roles of miRNAs and ceRNAs during NELL-1-induced ossification of hASCs.

Improving the handling properties and long-term stability of polyelectrolyte complex by freeze-drying technique for low-dose bone morphogenetic protein 2 delivery

A variety of controlled release carriers for bone morphogenetic protein 2 (BMP-2) delivery have been developed and tested in animal models. An alginate-based polyelectrolyte complex (PEC) for controlled release of low-dose BMP-2 has shown promising results in preclinical research. However, the poor handling properties and long-term stability of PEC need to be improved for translational applications. This study aimed to address these limitations of alginate-based PEC by employing a freeze-drying technique. The size and structure of freeze-dried PEC (FD-PEC) were maintained with the addition of a cryoprotectant, trehalose. The release profile of BMP-2 from FD-PEC was similar to that of freshly prepared PEC. In vitro bioactivity analysis of the released BMP-2 showed that the carrier performance of PEC was not compromised by freeze-drying up to three-month storage at room temperature. BMP-2-bound FD-PEC induced comparable bone formation to that using freshly prepared regular PEC in a rat posterolateral spinal fusion model. These results suggest that FD-PEC is capable of delivering low-dose BMP-2 and could be developed as an off-the-shelf product for translational applications. The simplicity of this preservation method provides promise for the translational application of PEC.

Long noncoding RNA expression profiles during the NEL-like 1 protein-induced osteogenic differentiation

Long noncoding RNAs (lncRNAs) are important modulators of mesenchymal stem cells (MSCs) in cellular differentiation. However, the regulatory mechanisms of lncRNAs in NEL-like 1 (NELL-1)-induced osteogenic differentiation of human adipose-derived stem cells remain elusive. Expression profiles of lncRNAs and messenger RNAs during NELL-1-induced osteogenesis were obtained using high-throughput sequencing. Gene Ontology, Kyoto Encyclopedia of Genes and Genomes pathway analysis, and gene coexpression networks were performed. We identified 323 statistically differentially expressed lncRNAs during osteogenesis and NELL-1-induced osteogenesis, and three lncRNAs (ENST00000602964, ENST00000326734, and TCONS_00006792) were identified as core regulators. Hedgehog pathway markers, including IHH and GLI1, were downregulated, while the antagonists of this pathway (GLI3 and HHIP) were upregulated during NELL-1-induced osteogenesis. In this process, the antagonist of Wnt, SFRP1, was downregulated. According to the analysis, we speculated that lncRNAs played important roles in NELL-1-induced osteogenesis via the crosstalk between Hedgehog and Wnt pathways.

Neural EGFL like 1 as a potential pro-chondrogenic, anti-inflammatory dual-functional disease-modifying osteoarthritis drug

Arthritis, an inflammatory condition that causes pain and cartilage destruction in joints, affects over 54.4 million people in the US alone. Here, for the first time, we demonstrated the emerging role of neural EGFL like 1 (NELL-1) in arthritis pathogenesis by showing that Nell-1-haploinsufficient (Nell-1^+/6R) mice had accelerated and aggravated osteoarthritis (OA) progression with elevated inflammatory markers in both spontaneous primary OA and chemical-induced secondary OA models. In the chemical-induced OA model, intra-articular injection of interleukin (IL)1β induced more severe inflammation and cartilage degradation in the knee joints of Nell-1^+/6R mice than in wildtype animals. Mechanistically, in addition to its pro-chondrogenic potency, NELL-1 also effectively suppressed the expression of inflammatory cytokines and their downstream cartilage catabolic enzymes by upregulating runt-related transcription factor (RUNX)1 in mouse and human articular cartilage chondrocytes. Notably, NELL-1 significantly reduced IL1β-stimulated inflammation and damage to articular cartilage in vivo. In particular, NELL-1 administration markedly reduced the symptoms of antalgic gait observed in IL1β-challenged Nell-1^+/6R mice. Therefore, NELL-1 is a promising pro-chondrogenic, anti-inflammatory dual-functional disease-modifying osteoarthritis drug (DMOAD) candidate for preventing and suppressing arthritis-related cartilage damage.

Cumulative inactivation of Nell-1 in Wnt1 expressing cell lineages results in craniofacial skeletal hypoplasia and postnatal hydrocephalus

Upregulation of Nell-1 has been associated with craniosynostosis (CS) in humans, and validated in a mouse transgenic Nell-1 overexpression model. Global Nell-1 inactivation in mice by N-ethyl-N-nitrosourea (ENU) mutagenesis results in neonatal lethality with skeletal abnormalities including cleidocranial dysplasia (CCD)-like calvarial bone defects. This study further defines the role of Nell-1 in craniofacial skeletogenesis by investigating specific inactivation of Nell-1 in Wnt1 expressing cell lineages due to the importance of cranial neural crest cells (CNCCs) in craniofacial tissue development. Nell-1^flox/flox; Wnt1-Cre (Nell-1^Wnt1 KO) mice were generated for comprehensive analysis, while the relevant reporter mice were created for CNCC lineage tracing. Nell-1^Wnt1 KO mice were born alive, but revealed significant frontonasal and mandibular bone defects with complete penetrance. Immunostaining demonstrated that the affected craniofacial bones exhibited decreased osteogenic and Wnt/β-catenin markers (Osteocalcin and active-β-catenin). Nell-1-deficient CNCCs demonstrated a significant reduction in cell proliferation and osteogenic differentiation. Active-β-catenin levels were significantly low in Nell-1-deficient CNCCs, but were rescued along with osteogenic capacity to a level close to that of wild-type (WT) cells via exogenous Nell-1 protein. Surprisingly, 5.4% of young adult Nell-1^Wnt1 KO mice developed hydrocephalus with premature ossification of the intrasphenoidal synchondrosis and widened frontal, sagittal, and coronal sutures. Furthermore, the epithelial cells of the choroid plexus and ependymal cells exhibited degenerative changes with misplaced expression of their respective markers, transthyretin and vimentin, as well as dysregulated Pit-2 expression in hydrocephalic Nell-1^Wnt1 KO mice. Nell-1^Wnt1 KO embryos at E9.5, 14.5, 17.5, and newborn mice did not exhibit hydrocephalic phenotypes grossly and/or histologically. Collectively, Nell-1 is a pivotal modulator of CNCCs that is essential for normal development and growth of the cranial vault and base, and mandibles partially via activating the Wnt/β-catenin pathway. Nell-1 may also be critically involved in regulating cerebrospinal fluid homeostasis and in the pathogenesis of postnatal hydrocephalus.

Synergistic Effect of NELL-1 and an Ultra-Low Dose of BMP-2 on Spinal Fusion

Bone morphogenetic protein 2 (BMP-2) is widely used in spinal fusion but it can cause adverse effects such as ectopic bone and adipose tissue in vivo. Neural epidermal growth factor like-like molecule-1 (NELL-1) has been shown to suppress BMP-2-induced adverse effects. However, no optimum carriers that control both NELL-1 and BMP-2 releases to elicit long-term bioactivity have been developed. In this study, we employed polyelectrolyte complex (PEC) as a control release carrier for NELL-1 and BMP-2. An ultra-low dose of BMP-2 synergistically functioned with NELL-1 on bone marrow mesenchymal stem cells osteogenic differentiation with greater mineralization in vitro. The osteoinductive ability of NELL-1 and an ultra-low dose of BMP-2 in PEC was investigated in rat posterolateral spinal fusion. Our results showed increased fusion rate, bone architecture, and improved bone stiffness at 8 weeks after surgery in the combination groups compared with NELL-1 or BMP-2 alone. Moreover, the formation of ectopic bone and adipose tissue was negligible in all the PEC groups. In summary, dual delivery of NELL-1 and an ultra-low dose of BMP-2 in the PEC control release carrier has greater fusion efficiency compared with BMP-2 alone and could potentially be a better alternative to the currently used BMP-2 treatments for spinal fusion. Impact Statement In this study, polyelectrolyte complex was used to absorb neural epidermal growth factor like-like molecule-1 (NELL-1) and bone morphogenetic protein 2 (BMP-2) to achieve controlled dual release. The addition of NELL-1 significantly reduced the effective dose of BMP-2 to 2.5% of its conventional dose in absorbable collagen sponge, to produce solid spinal fusion without significant adverse effects. This study was the first to identify the efficacy of combination NELL-1 and BMP-2 in a control release carrier in spinal fusion, which could be potentially used clinically to increase fusion rate and avoid the adverse effects commonly associated with conventional BMP-2.

Wnt signaling pathway in osteoporosis: Epigenetic regulation, interaction with other signaling pathways, and therapeutic promises

Wnt is a major signaling pathway involved in multifaceted roles of various biological processes. Bones are dynamic tissues which are able to remodel and maintain the tissue homeostasis. Wnt signaling cascade leads to the promotion of bone formation and suppression of bone resorption, leading to a balance in bone remodeling. Recent evidence has reinforced the inevitable role of Wnt signaling in osteoporosis. The complex genetic and epigenetic regulations of Wnt signaling factors and their interaction with other master signaling pathways such as TGF-β, BMP, PI3K/AKT, and Hedgehog outline their importance in diagnosis and treatment of osteoporosis. In this review, we highlighted the recent advances in function of Wnt signaling-related epigenetic regulation, different signaling pathways interacting with Wnt, and their roles in osteoporosis. Finally, we discussed novel promises in molecular targeted therapy of osteoporosis.

Skeletogenic Capacity of Human Perivascular Stem Cells Obtained Via Magnetic-Activated Cell Sorting

Human perivascular stem/stromal cells (PSC) are a multipotent mesenchymal progenitor cell population defined by their perivascular residence. PSC are increasingly studied for their application in skeletal regenerative medicine. PSC from subcutaneous white adipose tissue are most commonly isolated via fluorescence-activated cell sorting (FACS), and defined as a bipartite population of CD146⁺CD34^-CD31^-CD45^- pericytes and CD34⁺CD146^-CD31^-CD45^- adventitial cells. FACS poses several challenges for clinical translation, including requirements for facilities, equipment, and personnel. The purpose of this study is to identify if magnetic-activated cell sorting (MACS) is a feasible method to derive PSC, and to determine if MACS-derived PSC are comparable to our previous experience with FACS-derived PSC. In brief, CD146⁺ pericytes and CD34⁺ adventitial cells were enriched from human lipoaspirate using a multistep column approach. Next, cell identity and purity were analyzed by flow cytometry. In vitro multilineage differentiation studies were performed with MACS-defined PSC subsets. Finally, in vivo application was performed in nonhealing calvarial bone defects in Scid mice. Results showed that human CD146⁺ pericytes and CD34⁺ adventitial cells may be enriched by MACS, with defined purity, anticipated cell surface marker expression, and capacity for multilineage differentiation. In vivo, MACS-derived PSC induce ossification of bone defects. These data document the feasibility of a MACS approach for the enrichment and application of PSC in the field of tissue engineering and regenerative medicine. Impact Statement Our findings suggest that perivascular stem/stromal cells, and in particular adventitial cells, may be isolated by magnetic-activated cell sorting and applied as an uncultured autologous stem cell therapy in a same-day setting for bone defect repair.

Stem cells in Osteoporosis: From Biology to New Therapeutic Approaches

Osteoporosis is a systemic disease that affects the skeleton, causing reduction of bone density and mass, resulting in destruction of bone microstructure and increased risk of bone fractures. Since osteoporosis is a disease affecting the elderly and the aging of the world's population is constantly increasing, it is expected that the incidence of osteoporosis and its financial burden on the insurance systems will increase continuously and there is a need for more understanding this condition in order to prevent and/or treat it. At present, available drug therapy for osteoporosis primarily targets the inhibition of bone resorption and agents that promote bone mineralization, designed to slow disease progression. Safe and predictable pharmaceutical means to increase bone formation have been elusive. Stem cell therapy of osteoporosis, as a therapeutic strategy, offers the promise of an increase in osteoblast differentiation and thus reversing the shift towards bone resorption in osteoporosis. This review is focused on the current views regarding the implication of the stem cells in the cellular and physiologic mechanisms of osteoporosis and discusses data obtained from stem cell-based therapies of osteoporosis in experimental animal models and the possibility of their future application in clinical trials.

Is Tissue Engineering Helping Orthopaedic Care in Trauma?

Tissue engineering in orthopaedic trauma is needed. Progress has been made in all areas including regenerating bone, cartilage, soft tissue, and making up for bone defects with scaffolds. Bone regeneration and managing bone defects with scaffolds continue to be successful in the basic science realm with promising results, but currently, these successes are mostly limited to small animal models. Cartilage defects have more clinically available treatment options, but the benefits of "off-the-shelf" allograft options, and scaffolds, have little clinical evidence in the acute fracture setting. Most of the true chondrocyte replacement therapies such as matrix-induced autologous chondrocyte implantation and osteochondral allografts require delayed treatment while cell growth or graft matching occurs. Soft-tissue defects can be managed with tissue engineering for the skin with success, but muscle and nerve defects are still limited to the basic science arena. Although significant gains have been made in all areas for tissue engineering in basic science, and is very promising, this success currently comes with limited translation into clinical availability for the orthopaedic trauma patient.

Effects of Nel-like molecule-1 and bone morphogenetic protein 2 combination on rat pulp repair

Nel-like molecule-1 (NELL-1) is a novel highly specific growth factor that can induce osteoblast differentiation and bone formation as well as odontoblast differentiation. Recent studies have suggested that NELL-1 can synergistically increase bone formation and regeneration with bone morphogenetic protein 2 (BMP2) and inhibit adverse effects induced by BMP2. This study aimed to evaluate the combined effects of NELL-1 and BMP2 on rat pulp repair. The experiment used healthy non-carious maxillary first molars from 60 Wistar rats. Exposed pulps were capped with NELL-1 plus BMP2, NELL-1 alone, and BMP2 alone, and each was absorbed onto a sterile collagen sponge. In the control samples, the collagen sponge alone and Dycal were used as capping agents. After l, 2 and 4 weeks, the rats were sacrificed. The formation of reparative dentin, as well the situation of pulp repair, was detected by hematoxylin-eosin (HE) staining; moreover, the expression of dentin specific protein-dentin sialophosphoprotein (DSPP) and the pro-inflammatory cytokines interleukin-6 (IL6) and interleukin-8 (IL8) was detected by immunohistochemical staining. Quantitative real-time PCR experiment was used to investigate the mRNA levels of IL6 and IL8. The results showed that pulp capping with NELL-1 plus BMP2 in rats had superior ability in inducing reparative dentin formation with dentin tubules and in reducing the inflammatory cell response compared with the other groups. These findings suggested that combined use of NELL-1 and BMP2 could positively regulate pulp repair.

Age dependent effects of NELL-1 isoforms on bone marrow stromal cells

NELL-1 is an osteogenic protein first discovered to control ossification of the cranium. NELL-1 exists in at least two isoforms. The full-length NELL-1 contains 810 amino acid (aa) (NELL-1810), the N-terminal-truncated NELL-1 isoform contains 570 aa (NELL-1570). The differences in cellular effects between NELL-1 isoforms are not well understood. Methods: Here, BMSC were derived from adult or aged mice, followed by overexpression of NELL-1810 or NELL-1570. Cell morphology, proliferation, and gene expression were examined. Results/Conclusions: Overall, the proliferative effect of NELL-1570 was age dependent, showing prominent induction in adult but not aged mice.

Peroxisome Proliferator-Activated Receptor-γ Knockdown Impairs Bone Morphogenetic Protein-2-Induced Critical-Size Bone Defect Repair

The Food and Drug Administration-approved clinical dose (1.5 mg/mL) of bone morphogenetic protein-2 (BMP2) has been reported to induce significant adverse effects, including cyst-like adipose-infiltrated abnormal bone formation. These undesirable complications occur because of increased adipogenesis, at the expense of osteogenesis, through BMP2-mediated increases in the master regulatory gene for adipogenesis, peroxisome proliferator-activated receptor-γ (PPARγ). Inhibiting PPARγ during osteogenesis has been suggested to drive the differentiation of bone marrow stromal/stem cells toward an osteogenic, rather than an adipogenic, lineage. We demonstrate that knocking down PPARγ while concurrently administering BMP2 can reduce adipogenesis, but we found that it also impairs BMP2-induced osteogenesis and leads to bone nonunion in a mouse femoral segmental defect model. In addition, in vitro studies using the mouse bone marrow stromal cell line M2-10B4 and mouse primary bone marrow stromal cells confirmed that PPARγ knockdown inhibits BMP2-induced adipogenesis; attenuates BMP2-induced cell proliferation, migration, invasion, and osteogenesis; and escalates BMP2-induced cell apoptosis. More important, BMP receptor 2 and 1B expression was also significantly inhibited by the combined BMP2 and PPARγ knockdown treatment. These findings indicate that PPARγ is critical for BMP2-mediated osteogenesis during bone repair. Thus, uncoupling BMP2-mediated osteogenesis and adipogenesis using PPARγ inhibition to combat BMP2's adverse effects may not be feasible.

Inactivation of Nell-1 in Chondrocytes Significantly Impedes Appendicular Skeletogenesis

NELL-1, an osteoinductive protein, has been shown to regulate skeletal ossification. Interestingly, an interstitial 11p14.1-p15.3 deletion involving the Nell-1 gene was recently reported in a patient with short stature and delayed fontanelle closure. Here we sought to define the role of Nell-1 in endochondral ossification by investigating Nell-1-specific inactivation in Col2α1-expressing cell lineages. Nell-1^flox/flox ; Col2α1-Cre⁺ (Nell-1^Col2α1 KO) mice were generated for comprehensive analysis. Nell-1^Col2α1 KO mice were born alive but displayed subtle femoral length shortening. At 1 and 3 months postpartum, Nell-1 inactivation resulted in dwarfism and premature osteoporotic phenotypes. Specifically, Nell-1^Col2α1 KO femurs and tibias exhibited significantly reduced length, bone mineral density (BMD), bone volume per tissue volume (BV/TV), trabecular number/thickness, cortical volume/thickness/density, and increased trabecular separation. The decreased bone formation rate revealed by dynamic histomorphometry was associated with altered numbers and/or function of osteoblasts and osteoclasts. Furthermore, longitudinal observations by in vivo micro-CT showed delayed and reduced mineralization at secondary ossification centers in mutants. Histologically, reduced staining intensities of Safranin O, Col-2, Col-10, and fewer BrdU-positive chondrocytes were observed in thinner Nell-1^Col2α1 KO epiphyseal plates along with altered distribution and weaker expression level of Ihh, Patched-1, PTHrP, and PTHrP receptor. Primary Nell-1^Col2α1 KO chondrocytes also exhibited decreased proliferation and differentiation, and its downregulated expression of the Ihh-PTHrP signaling molecules can be partially rescued by exogenous Nell-1 protein. Moreover, intranuclear Gli-1 protein and gene expression of the Gli-1 downstream target genes, Hip-1 and N-Myc, were also significantly decreased with Nell-1 inactivation. Notably, the rescue effects were diminished/reduced with application of Ihh signaling inhibitors, cyclopamine or GANT61. Taken together, these findings suggest that Nell-1 is a pivotal modulator of epiphyseal homeostasis and endochondral ossification. The cumulative chondrocyte-specific Nell-1 inactivation significantly impedes appendicular skeletogenesis resulting in dwarfism and premature osteoporosis through inhibiting Ihh signaling and predominantly altering the Ihh-PTHrP feedback loop. © 2018 American Society for Bone and Mineral Research.

WISP-1 drives bone formation at the expense of fat formation in human perivascular stem cells

The vascular wall within adipose tissue is a source of mesenchymal progenitors, referred to as perivascular stem/stromal cells (PSC). PSC are isolated via fluorescence activated cell sorting (FACS), and defined as a bipartite population of pericytes and adventitial progenitor cells (APCs). Those factors that promote the differentiation of PSC into bone or fat cell types are not well understood. Here, we observed high expression of WISP-1 among human PSC in vivo, after purification, and upon transplantation in a bone defect. Next, modulation of WISP-1 expression was performed, using WISP-1 overexpression, WISP-1 protein, or WISP-1 siRNA. Results demonstrated that WISP-1 is expressed in the perivascular niche, and high expression is maintained after purification of PSC, and upon transplantation in a bone microenvironment. In vitro studies demonstrate that WISP-1 has pro-osteogenic/anti-adipocytic effects in human PSC, and that regulation of BMP signaling activity may underlie these effects. In summary, our results demonstrate the importance of the matricellular protein WISP-1 in regulation of the differentiation of human stem cell types within the perivascular niche. WISP-1 signaling upregulation may be of future benefit in cell therapy mediated bone tissue engineering, for the healing of bone defects or other orthopaedic applications.

Neurexin Superfamily Cell Membrane Receptor Contactin-Associated Protein Like-4 (Cntnap4) Is Involved in Neural EGFL-Like 1 (Nell-1)-Responsive Osteogenesis

Contactin-associated protein-like 4 (Cntnap4) is a member of the neurexin superfamily of transmembrane molecules that have critical functions in neuronal cell communication. Cntnap4 knockout mice display decreased presynaptic gamma-aminobutyric acid (GABA) and increased dopamine release that is associated with severe, highly penetrant, repetitive, and perseverative movements commonly found in human autism spectrum disorder patients. However, no known function of Cntnap4 has been revealed besides the nervous system. Meanwhile, secretory protein neural EGFL-like 1 (Nell-1) is known to exert potent osteogenic effects in multiple small and large animal models without the off-target effects commonly found with bone morphogenetic protein 2. In this study, while searching for a Nell-1-specific cell surface receptor during osteogenesis, we identified and validated a ligand/receptor-like interaction between Nell-1 and Cntnap4 by demonstrating: 1) Nell-1 and Cntnap4 colocalization on the surface of osteogenic-committed cells; 2) high-affinity interaction between Nell-1 and Cntnap4; 3) abrogation of Nell-1-responsive Wnt and MAPK signaling transduction, as well as osteogenic effects, via Cntnap4 knockdown; and 4) replication of calvarial cleidocranial dysplasias-like defects observed in Nell-1-deficient mice in Wnt1-Cre-mediated Cntnap4-knockout transgenic mice. In aggregate, these findings indicate that Cntnap4 plays a critical role in Nell-1-responsive osteogenesis. Further, this is the first functional annotation for Cntnap4 in the musculoskeletal system. Intriguingly, Nell-1 and Cntnap4 also colocalize on the surface of human hippocampal interneurons, implicating Nell-1 as a potential novel ligand for Cntnap4 in the nervous system. This unexpected characterization of the ligand/receptor-like interaction between Nell-1 and Cntnap4 indicates a novel biological functional axis for Nell-1 and Cntnap4 in osteogenesis and, potentially, in neural development and function. © 2018 American Society for Bone and Mineral Research.

Celastrol inhibits glucocorticoid‑induced osteoporosis in rat via the PI3K/AKT and Wnt signaling pathways

Modern pharmacological studies revealed that Celastrol exhibits anti‑inflammation, anti‑bacteria, anti‑virus, anti‑fertility, insect‑resistance functions and has been used for the treatment of rheumatism, rheumatoid arthritis, blood diseases, skin diseases and agricultural insecticide. The present study aimed to investigate the effects of Celastrol on glucocorticoid‑induced osteoporosis (GIOP) and the underlying molecular mechanisms. The findings of the current study revealed that Celastrol reduced body weight, urine calcium/creatinine, tartrate‑resistant acid phosphatase 5b, C‑terminal telopeptide of type I collagen, and induced osteocalcin in GIOP rats. In addition, alkaline phosphatase, triiodothyronine receptor auxiliary protein and cathepsin K mRNA expression levels were effectively suppressed, and osteocalcin, bone morphogenetic protein 2, type I collagen and runt‑related transcription factor 2 mRNA expression levels were effectively induced in osteoporosis rats treated with Celastrol. Celastrol inhibited prostaglandin E2 and caspase‑3 protein expression levels, and induced phosphoinositol 3‑kinase (PI3K), phosphorylated‑protein kinase B (AKT) and glycogen synthase kinase‑3 phosphorylation, Wnt and β‑catenin protein expression in GIOP rats. The present study demonstrated that Celastrol may inhibit GIOP in rats via the PI3K/AKT and Wnt signaling pathways.

WNT16 induces proliferation and osteogenic differentiation of human perivascular stem cells

Perivascular stem cells (PSC) are a progenitor population defined by their perivascular residence. Recent studies have examined the relative difference in Wnt ligands to induce PSC differentiation, including Wnt16. Here, we examine the role of Wnt16 in the proliferation and osteogenic differentiation of human PSC. Treatment of PSC with WNT16 significantly increased cell proliferation to a greater extent than did WNT3A. In addition, WNT16 showed a significant increase in osteogenic gene expression among PSC. These data demonstrate that WNT16 represents a combined mitogenic/pro-osteogenic stimulus that may play a functional role in human mesenchymal stem cell mediated bone repair.

Tissue engineering using 3D printed nano-bioactive glass loaded with NELL1 gene for repairing alveolar bone defects

The purposes of this study were to construct a novel tissue engineered bone composed of 3D-printed bioactive glass block/chitosan nanoparticles (BD/CSn) composites loaded with Nel-like Type I molecular-1 DNA (pDNA-NELL1) and/or bone marrow mesenchymal stem cells (BMSCs), and study their osteogenic activities by repairing bone defects in rhesus monkeys. CSn with NELL1 gene plasmid and rhesus monkey BMSCs were composited with a BD scaffold to prepare the tissue-engineered bone. Four adult female rhesus monkeys with 10- to 12-years old and 5-7 kg in weight were used in animal experiments. The first and second premolar teeth from four regions of each monkey were removed to form bone defects with size of 10 × 10 × 5 mm, which were then implanted with above-mentioned tissue engineered bone. At 12 weeks after the implantation, gross observations, X-ray and micro-CT observations revealed that the new bone was extremely close to normal bone in mass, density, hardness, and structure. The bony cortex was smooth and closely connected to the surrounding normal bone. Histological observations revealed moderate inflammation in the repair area, and the new bone tissues were similar to normal ones. In conclusion, tissue engineered bone of this study exhibited good osteoconductivity for promoting the formation of new alveolar bone tissue, and NELL1 gene played a promotional role in bone regeneration.

The challenges of promoting osteogenesis in segmental bone defects and osteoporosis

Conventional clinical management of complex bone healing scenarios continues to result in 5-10% of fractures forming non-unions. Additionally, the aging population and prevalence of osteoporosis-related fractures necessitate the further exploration of novel ways to augment osteogenesis in this special population. This review focuses on the current clinical modalities available, and the ongoing clinical and pre-clinical research to promote osteogenesis in segmental bone defects, delayed unions, and osteoporosis. In summary, animal models of fracture repair are often small animals as historically significant large animal models, like the dog, continue to gain favor as companion animals. Small rodents have well-documented limitations in comparing to fracture repair in humans, and few similarities exist. Study design, number of studies, and availability of funding continue to limit large animal studies. Osteoinduction with rhBMP-2 results in robust bone formation, although long-term quality is scrutinized due to poor bone mineral quality. PTH 1-34 is the only FDA approved osteo-anabolic treatment to prevent osteoporotic fractures. Limited to 2 years of clinical use, PTH 1-34 has further been plagued by dose-related ambiguities and inconsistent results when applied to pathologic fractures in systematic human clinical studies. There is limited animal data of PTH 1-34 applied locally to bone defects. Gene therapy continues to gain popularity among researchers to augment bone healing. Non-integrating viral vectors and targeted apoptosis of genetically modified therapeutic cells is an ongoing area of research. Finally, progenitor cell therapies and the content variation of patient-side treatments (e.g., PRP and BMAC) are being studied. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1559-1572, 2018.

Effect of intracanal medicaments used in endodontic regeneration on the push-out bond strength of a calcium-phosphate-silicate-based cement to dentin

Objective:

To evaluate the effects of various endodontic regeneration agents on the push-out bond strength of Endosequence Root Repair Material (ERRM) to root-canal dentin.

Methods:

Fifty single-rooted human teeth were selected and instrumented to obtain a standard internal diameter of 1.5 mm. Specimens were randomly divided into four experimental groups and treated with an intracanal medicament [calcium hydroxide (CH), double antibiotic paste (DAP), triple antibiotic paste (TAP), TAP with amoxicillin (mTAP)] and a non-treated control group. Medicaments were removed after three weeks, and ERRM was applied to all specimens. The coronal portion of each root was then sliced into 2-mm-thick parallel transverse sections (2 slices per tooth, n=20 slices per group), and a push-out test was used to measure the bond strength of ERRM to dentin. Data were analyzed using Bonferroni-corrected Mann-Whitney tests, with the level of significance set at p<0.05.

Results:

The push-out bond strength of the CH group was significantly higher than that of the TAP, DAP and mTAP groups (p< 0.005). Furthermore, the bond strength of the control group was higher than the bond strength of both the DAP and mTAP groups.

Conclusion:

The use of CH in clinical practice may help improve the adhesion of ERRM to dentin.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

IL-6 potentiates BMP-2-induced osteogenesis and adipogenesis via two different BMPR1A-mediated pathways

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is widely used in the clinic for bone defect reconstruction because of its powerful osteoinductive capacity. However, commercially available rhBMP-2 requires a high concentration in the clinical setting for consistent bone formation. A high dose of rhBMP-2 induces a promising bone formation yield but also leads to inflammation-related events, deteriorated bone quality, and fatty tissue formation. We hypothesize that the seemingly contradictory phenomenon of coformation of new bone and excessive adipose tissue in rhBMP-2-induced bone voids may be associated with interleukin-6 (IL-6), which is significantly elevated after application of rhBMP-2/absorbable collagen sponge (rhBMP-2/ACS). Here, we show that IL-6 injection enhances new bone regeneration and induces excessive adipose tissue formation in an rhBMP-2/ACS-induced ectopic bone formation model in rats. In vitro data further show that IL-6 and its soluble receptor sIL-6R synergistically augment rhBMP-2-induced osteogenic and adipogenic differentiation of human BMSCs (hBMSCs) by promoting cell surface translocation of BMPR1A and then amplifying BMPR1A-mediated BMP/Smad and p38 MAPK pathways, respectively. Our study suggests elevated IL-6 may be responsible for coformation of new bone and excessive adipose tissue in rhBMP-2-induced bone voids.

The Effects of Systemic Therapy of PEGylated NEL-Like Protein 1 (NELL-1) on Fracture Healing in Mice

Fractures are common, with an incidence of 13.7 per 1000 adults annually. Systemic agents have been widely used for enhancing bone regeneration; however, the efficacy of these therapeutics for the management and prevention of fracture remains unclear. NEL-like protein 1 (NELL-1) is a potent pro-osteogenic cytokine that has been modified with polyethylene glycol (PEG)ylation [PEGylated NELL-1 (NELL-PEG)] to enhance its pharmacokinetics for systemic therapy. Our aim was to investigate the effects of systemic administration of NELL-PEG on fracture healing in mice and on overall bone properties in uninjured bones. Ten-week-old CD-1 mice were subjected to an open osteotomy of bilateral radii and treated with weekly injections of NELL-PEG or PEG phosphate-buffered saline as control. Systemic injection of NELL-PEG resulted in improved bone mineral density of the fracture site and accelerated callus union. After 4 weeks of treatment, mice treated with NELL-PEG exhibited substantially enhanced callus volume, callus mineralization, and biomechanical properties. NELL-PEG injection significantly augmented bone regeneration, as confirmed by high expression of bone turnover rate, bone formation rate, and mineral apposition rate. Consistently, the immunohistochemistry results also confirmed a high bone remodeling activity in the NELL-PEG-treated group. Our findings suggest that weekly injection of NELL-PEG may have the clinical potential to accelerate fracture union and enhance overall bone properties, which may help prevent subsequent fractures.

NELL-1, HMGB1, and CCN2 Enhance Migration and Vasculogenesis, But Not Osteogenic Differentiation Compared to BMP2

Currently, autografts still represent the gold standard treatment for the repair of large bone defects. However, these are associated with donor-site morbidity and increased pain, cost, and recovery time. The ideal therapy would use biomaterials combined with bone growth factors to induce and instruct bone defect repair without the need to harvest patient tissue. In this line, bone morphogenetic proteins (BMPs) have been the most extensively used agents for clinical bone repair, but at supraphysiological doses that are not without risk. Because of the need to eliminate the risks of BMP2 use in vivo, we assessed the ability of three putative osteogenic factors, nel-like molecule type 1 (NELL-1), high mobility group box 1 (HMGB1), and CCN2, to enhance the essential processes for bone defect repair in vitro and compared them to BMP2. Although it has been reported that NELL-1, HMGB1, and CCN2 play a role in bone formation, less is known about the contribution of these proteins to the different events involved, such as cell migration, osteogenesis, and vasculogenesis. In this study, we investigated the effects of different doses of NELL-1, HMGB, CCN2, and BMP2 on these three processes as a model for the recruitment and differentiation of resident cells in the in vivo bone defect repair situation, using cells of human origin. Our data demonstrated that NELL-1, HMGB1, and CCN2 significantly induced mesenchymal stem cell migration (from 1.58-fold increase compared to control), but BMP2 did not. Interestingly, only BMP2 increased osteogenesis in marrow stromal cells, whereas it inhibited osteogenesis in preosteoblasts. Moreover, the four proteins studied promoted significantly endothelial cell migration, reaching a maximum of 2.4-fold increase compared to control, and induced formation of tube-like structures. NELL-1, HMGB1, and CCN2 had these effects at relatively low doses compared to BMP2. This work indicates that NELL-1, HMGB1, and CCN2 might enhance bone defect healing via the recruitment of endogenous cells and induction of vascularization and act via different processes than BMP2.

Neural EGFL-Like 1 Regulates Cartilage Maturation through Runt-Related Transcription Factor 3-Mediated Indian Hedgehog Signaling

The pro-chondrogenic function of runt-related transcription factor 2 (Runx2) was previously considered to be dependent on direct binding with the promoter of Indian hedgehog (Ihh)-the major regulator of chondrocyte differentiation, proliferation, and maturation. The authors' previous studies identified neural EGFL like 1 (Nell-1) as a Runx2-responsive growth factor for chondrogenic differentiation and maturation. In this study, it was further revealed that the pro-chondrogenic activities of Nell-1 also rely on Ihh signaling, by showing: i) Nell-1 significantly elevated Ihh signal transduction; ii) Nell-1 deficiency markedly reduced Ihh activation in chondrocytes; and iii) Nell-1-stimulated chondrogenesis was significantly reduced by the specific hedgehog inhibitor cyclopamine. Importantly, the authors demonstrated that Nell-1-responsive Ihh signaling and chondrogenic differentiation extended to Runx2^-/- models in vitro and in vivo. In Runx2^-/- chondrocytes, Nell-1 stimulated the expression and signal transduction of Runx3, another transcription factor required for complete chondrogenic differentiation and maturation. Furthermore, knocking down Runx3 in Runx2^-/- chondrocytes abolished Nell-1's stimulation of Ihh-associated molecule expression, which validates Runx3 as a major mediator of Nell-1-stimulated Ihh activation. For the first time, the Runx2→Nell-1→Runx3→Ihh signaling cascade during chondrogenic differentiation and maturation has been identified as an alternative, but critical, pathway for Runx2 to function as a pro-chondrogenic molecule via Nell-1.

Neural EGF-like protein 1 (NELL-1): Signaling crosstalk in mesenchymal stem cells and applications in regenerative medicine

Bone tissue regeneration holds the potential to solve both osteoporosis and large skeletal defects, two problems associated with significant morbidity. The differentiation of mesenchymal stem cells into the osteogenic lineage requires a specific microenvironment and certain osteogenic growth factors. Neural EGF Like-Like molecule 1 (NELL-1) is a secreted glycoprotein that has proven, both in vitro and in vivo, to be a potent osteo-inductive factor. Furthermore, it has been shown to repress adipogenic differentiation and inflammation. NELL-1 can work synergistically with other osteogenic factors such as Bone Morphogenic Protein (BMP) -2 and -9, and has shown promise for use in tissue engineering and as a systemically administered drug for the treatment of osteoporosis. Here we provide a comprehensive up-to-date review on the molecular signaling cascade of NELL-1 in mesenchymal stem cells and potential applications in bone regenerative engineering.

Combining Smoothened Agonist and NEL-Like Protein-1 Enhances Bone Healing

BACKGROUND

Nonhealing bone defects represent an immense biomedical burden. Despite recent advances in protein-based bone regeneration, safety concerns over bone morphogenetic protein-2 have prompted the search for alternative factors. Previously, the authors examined the additive/synergistic effects of hedgehog and Nel-like protein-1 (NELL-1) on the osteogenic differentiation of mesenchymal stem cells in vitro. In this study, the authors sought to leverage their previous findings by applying the combination of Smoothened agonist (SAG), hedgehog signal activator, and NELL-1 to an in vivo critical-size bone defect model.

METHODS

A 4-mm parietal bone defect was created in mixed-gender CD-1 mice. Treatment groups included control (n = 6), SAG (n = 7), NELL-1 (n = 7), and SAG plus NELL-1 (n = 7). A custom fabricated poly(lactic-co-glycolic acid) disk with hydroxyapatite coating was used as an osteoinductive scaffold.

RESULTS

Results at 4 and 8 weeks showed increased bone formation by micro-computed tomographic analyses with either stimulus alone (SAG or NELL-1), but significantly greater bone formation with both components combined (SAG plus NELL-1). This included greater bone healing scores and increased bone volume and bone thickness. Histologic analyses confirmed a significant increase in new bone formation with the combination therapy SAG plus NELL-1, accompanied by increased defect vascularization.

CONCLUSIONS

In summary, the authors' results suggest that combining the hedgehog signaling agonist SAG and NELL-1 has potential as a novel therapeutic strategy for the healing of critical-size bone defects. Future directions will include optimization of dosage and delivery strategy for an SAG and NELL-1 combination product.

NELL-1 induces Sca-1+ mesenchymal progenitor cell expansion in models of bone maintenance and repair

NELL-1 is a secreted, osteogenic protein first discovered to control ossification of the cranial skeleton. Recently, NELL-1 has been implicated in bone maintenance. However, the cellular determinants of NELL-1's bone-forming effects are still unknown. Here, recombinant human NELL-1 (rhNELL-1) implantation was examined in a clinically relevant nonhuman primate lumbar spinal fusion model. Prolonged rhNELL-1 protein release was achieved using an apatite-coated β-tricalcium phosphate carrier, resulting in a local influx of stem cell antigen-1-positive (Sca-1+) mesenchymal progenitor cells (MPCs), and complete osseous fusion across all samples (100% spinal fusion rate). Murine studies revealed that Nell-1 haploinsufficiency results in marked reductions in the numbers of Sca-1+CD45-CD31- bone marrow MPCs associated with low bone mass. Conversely, rhNELL-1 systemic administration in mice showed a marked anabolic effect accompanied by increased numbers of Sca-1+CD45-CD31- bone marrow MPCs. Mechanistically, rhNELL-1 induces Sca-1 transcription among MPCs, in a process requiring intact Wnt/β-catenin signaling. In summary, NELL-1 effectively induces bone formation across small and large animal models either via local implantation or intravenous delivery. NELL-1 induces an expansion of a bone marrow subset of MPCs with Sca-1 expression. These findings provide compelling justification for the clinical translation of a NELL-1-based therapy for local or systemic bone formation.

Effects of WNT3A and WNT16 on the Osteogenic and Adipogenic Differentiation of Perivascular Stem/Stromal Cells

Human perivascular stem/stromal cells (hPSC) are a multipotent mesenchymogenic stromal cell population defined by their perivascular locale. Recent studies have demonstrated the high potential for clinical translation of this fluorescence-activated cell sorting (FACS)-derived cell population for autologous bone tissue engineering. However, the mechanisms underlying the osteogenic differentiation of PSC are incompletely understood. The current study investigates the roles of canonical and noncanonical Wnt signaling in the osteogenic and adipogenic differentiation of PSC. Results showed that both canonical and noncanonical Wnt signaling activity transiently increased during PSC osteogenic differentiation in vitro. Sustained WNT3A treatment significantly decreased PSC osteogenic differentiation. Conversely, sustained treatment with Wnt family member 16 (WNT16), a mixed canonical and noncanonical ligand, increased osteogenic differentiation in a c-Jun N-terminal kinase (JNK) pathway-dependent manner. Conversely, WNT16 knockdown significantly diminished PSC osteogenic differentiation. Finally, WNT16 but not WNT3A increased the adipogenic differentiation of PSC. These results indicate the importance of regulation of canonical and noncanonical Wnt signaling for PSC fate and differentiation. Moreover, these data suggest that WNT16 plays a functional and necessary role in PSC osteogenesis.

Isolation and characterization of canine perivascular stem/stromal cells for bone tissue engineering

For over 15 years, human subcutaneous adipose tissue has been recognized as a rich source of tissue resident mesenchymal stem/stromal cells (MSC). The isolation of perivascular progenitor cells from human adipose tissue by a cell sorting strategy was first published in 2008. Since this time, the interest in using pericytes and related perivascular stem/stromal cell (PSC) populations for tissue engineering has significantly increased. Here, we describe a set of experiments identifying, isolating and characterizing PSC from canine tissue (N = 12 canine adipose tissue samples). Results showed that the same antibodies used for human PSC identification and isolation are cross-reactive with canine tissue (CD45, CD146, CD34). Like their human correlate, canine PSC demonstrate characteristics of MSC including cell surface marker expression, colony forming unit-fibroblast (CFU-F) inclusion, and osteogenic differentiation potential. As well, canine PSC respond to osteoinductive signals in a similar fashion as do human PSC, such as the secreted differentiation factor NEL-Like Molecule-1 (NELL-1). Nevertheless, important differences exist between human and canine PSC, including differences in baseline osteogenic potential. In summary, canine PSC represent a multipotent mesenchymogenic cell source for future translational efforts in tissue engineering.

Transcriptional Regulation of Adipogenesis

Adipocytes are the defining cell type of adipose tissue. Once considered a passive participant in energy storage, adipose tissue is now recognized as a dynamic organ that contributes to several important physiological processes, such as lipid metabolism, systemic energy homeostasis, and whole-body insulin sensitivity. Therefore, understanding the mechanisms involved in its development and function is of great importance. Adipocyte differentiation is a highly orchestrated process which can vary between different fat depots as well as between the sexes. While hormones, miRNAs, cytoskeletal proteins, and many other effectors can modulate adipocyte development, the best understood regulators of adipogenesis are the transcription factors that inhibit or promote this process. Ectopic expression and knockdown approaches in cultured cells have been widely used to understand the contribution of transcription factors to adipocyte development, providing a basis for more sophisticated in vivo strategies to examine adipogenesis. To date, over two dozen transcription factors have been shown to play important roles in adipocyte development. These transcription factors belong to several families with many different DNA-binding domains. While peroxisome proliferator-activated receptor gamma (PPARγ) is undoubtedly the most important transcriptional modulator of adipocyte development in all types of adipose tissue, members of the CCAAT/enhancer-binding protein, Krüppel-like transcription factor, signal transducer and activator of transcription, GATA, early B cell factor, and interferon-regulatory factor families also regulate adipogenesis. The importance of PPARγ activity is underscored by several covalent modifications that modulate its activity and its ability to modulate adipocyte development. This review will primarily focus on the transcriptional control of adipogenesis in white fat cells and on the mechanisms involved in this fine-tuned developmental process. © 2017 American Physiological Society. Compr Physiol 7:635-674, 2017.