The loss of OPA1 accelerates intervertebral disc degeneration and osteoarthritis in aged mice

NP cells of the intervertebral disc and articular chondrocytes reside in avascular and hypoxic tissue niches. As a consequence of these environmental constraints the cells are primarily glycolytic in nature and were long thought to have a minimal reliance on mitochondrial function. Recent studies have challenged this long-held view and highlighted the increasingly important role of mitochondria in the physiology of these tissues. However, the foundational understanding of mechanisms governing mitochondrial dynamics and function in these tissues is lacking. We investigated the role of mitochondrial fusion protein OPA1 in maintaining the spine and knee joint health in mice. OPA1 knockdown in NP cells altered mitochondrial size and cristae shape and increased the oxygen consumption rate without affecting ATP synthesis. OPA1 governed the morphology of multiple organelles, including peroxisomes, early endosomes and cis-Golgi and its loss resulted in the dysregulation of NP cell autophagy. Metabolic profiling and 13C-flux analyses revealed TCA cycle anaplerosis and altered metabolism in OPA1-deficient NP cells. Noteworthy, Opa1AcanCreERT2 mice with Opa1 deletion in disc and cartilage showed age-dependent disc degeneration, osteoarthritis, and vertebral osteopenia. Our findings underscore that OPA1 regulation of mitochondrial dynamics and multi-organelle interactions is critical in preserving metabolic homeostasis of disc and cartilage.

OPA1 protects intervertebral disc and knee joint health in aged mice by maintaining the structure and metabolic functions of mitochondria

Due to their glycolytic nature and limited vascularity, nucleus pulposus (NP) cells of the intervertebral disc and articular chondrocytes were long thought to have minimal reliance on mitochondrial function. Recent studies have challenged this long-held view and highlighted the increasingly important role of mitochondria in the physiology of these tissues. We investigated the role of mitochondrial fusion protein OPA1 in maintaining the spine and knee joint health in aging mice. OPA1 knockdown in NP cells altered mitochondrial size and cristae shape and increased the oxygen consumption rate without affecting ATP synthesis. OPA1 governed the morphology of multiple organelles, and its loss resulted in the dysregulation of NP cell autophagy. Metabolic profiling and 13 C-flux analyses revealed TCA cycle anaplerosis and altered metabolism in OPA1-deficient NP cells. Noteworthy, Opa1 AcanCreERT2 mice showed age- dependent disc, and cartilage degeneration and vertebral osteopenia. Our findings suggest that OPA1 regulation of mitochondrial dynamics and multi-organelle interactions is critical in preserving metabolic homeostasis of disc and cartilage.

Teaser

OPA1 is necessary for the maintenance of intervertebral disc and knee joint health in aging mice.

GLUT1 is redundant in hypoxic and glycolytic nucleus pulposus cells of the intervertebral disc

Glycolysis is central to homeostasis of nucleus pulposus (NP) cells in the avascular intervertebral disc. Since the glucose importer, GLUT1, is a highly enriched phenotypic marker of NP cells, we hypothesized that it is vital for the development and post-natal maintenance of the disc. Surprisingly, primary NP cells treated with two well-characterized GLUT1 inhibitors maintained normal rates of glycolysis and ATP production, indicating intrinsic compensatory mechanisms. We show in vitro that NP cells mitigate GLUT1 loss by rewiring glucose import through GLUT3. Noteworthy, we demonstrate that substrates, such as glutamine and palmitate, do not compensate for glucose restriction resulting from dual inhibition of GLUT1/3 and inhibition compromises long-term cell viability. To investigate the redundancy of GLUT1 function in NP, we generated two NP-specific knockout mice: Krt19CreERT; Glut1f/f and Foxa2Cre; Glut1f/f. Noteworthy, there were no apparent defects in post-natal disc health or development and maturation in mutant mice. Microarray analysis confirmed that GLUT1 loss did not cause transcriptomic alterations in the NP, supporting that cells are refractory to GLUT1 loss. These observations provide the first evidence of functional redundancy in GLUT transporters in the physiologically hypoxic intervertebral disc and underscore the importance of glucose as the indispensable substrate for NP cells.

The mitophagy receptor BNIP3 is critical for the regulation of metabolic homeostasis and mitochondrial function in the nucleus pulposus cells of the intervertebral disc

The contribution of mitochondria to the metabolic function of hypoxic NP cells has been overlooked. We have shown that NP cells contain networked mitochondria and that mitochondrial translocation of BNIP3 mediates hypoxia-induced mitophagy. However, whether BNIP3 also plays a role in governing mitochondrial function and metabolism in hypoxic NP cells is not known. BNIP3 knockdown altered mitochondrial morphology, and number, and increased mitophagy. Interestingly, BNIP3 deficiency in NP cells reduced glycolytic capacity reflected by lower production of lactate/H⁺ and lower ATP production rate. Widely targeted metabolic profiling and flux analysis using 1-2-¹³C-glucose showed that the BNIP3 loss resulted in redirection of glycolytic flux into pentose phosphate and hexosamine biosynthesis as well as pyruvate resulting in increased TCA flux. An overall reduction in one-carbon metabolism was noted suggesting reduced biosynthesis. U¹³C-glutamine flux analysis showed preservation of glutamine utilization to maintain TCA intermediates. The transcriptomic analysis of the BNIP3-deficient cells showed dysregulation of cellular functions including membrane and cytoskeletal integrity, ECM-growth factor signaling, and protein quality control with an overall increase in themes related to angiogenesis and innate immune response. Importantly, we observed strong thematic similarities with the transcriptome of a subset of human degenerative samples. Last, we noted increased autophagic flux, decreased disc height index and aberrant COL10A1/collagen X expression, signs of early disc degeneration in young adult bnip3 knockout mice. These results suggested that in addition to mitophagy regulation, BNIP3 plays a role in maintaining mitochondrial function and metabolism, and dysregulation of mitochondrial homeostasis could promote disc degeneration.Abbreviations: ECAR extracellular acidification rate; HIF hypoxia inducible factor; MFA metabolic flux analysis; NP nucleus pulposus; OCR oxygen consumption rate; ShBnip3 short-hairpin Bnip3.

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

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

Abcc6 Null Mice—a Model for Mineralization Disorder PXE Shows Vertebral Osteopenia Without Enhanced Intervertebral Disc Calcification With Aging

Chronic low back pain is a highly prevalent health condition intricately linked to intervertebral disc degeneration. One of the prominent features of disc degeneration that is commonly observed with aging is dystrophic calcification. ATP-binding cassette sub-family C member 6 (ABCC6), a presumed ATP efflux transporter, is a key regulator of systemic levels of the mineralization inhibitor pyrophosphate (PPi). Mutations in ABCC6 result in pseudoxanthoma elasticum (PXE), a progressive human metabolic disorder characterized by mineralization of the skin and elastic tissues. The implications of ABCC6 loss-of-function on pathological mineralization of structures in the spine, however, are unknown. Using the Abcc6^−/− mouse model of PXE, we investigated age-dependent changes in the vertebral bone and intervertebral disc. Abcc6^−/− mice exhibited diminished trabecular bone quality parameters at 7 months, which remained significantly lower than the wild-type mice at 18 months of age. Abcc6^−/− vertebrae showed increased TRAP staining along with decreased TNAP staining, suggesting an enhanced bone resorption as well as decreased bone formation. Surprisingly, however, loss of ABCC6 resulted only in a mild, aging disc phenotype without evidence of dystrophic mineralization. Finally, we tested the utility of oral K3Citrate to treat the vertebral phenotype since it is shown to regulate hydroxyapatite mechanical behavior. The treatment resulted in inhibition of the osteoclastic response and an early improvement in mechanical properties of the bone underscoring the promise of potassium citrate as a therapeutic agent. Our data suggest that although ectopic mineralization is tightly regulated in the disc, loss of ABCC6 compromises vertebral bone quality and dysregulates osteoblast-osteoclast coupling.

Current evidence on potential of adipose derived stem cells to enhance bone regeneration and future projection

Injuries to the postnatal skeleton are naturally repaired through successive steps involving specific cell types in a process collectively termed “bone regeneration”. Although complex, bone regeneration occurs through a series of well-orchestrated stages wherein endogenous bone stem cells play a central role. In most situations, bone regeneration is successful; however, there are instances when it fails and creates non-healing injuries or fracture nonunion requiring surgical or therapeutic interventions. Transplantation of adult or mesenchymal stem cells (MSCs) defined by the International Society for Cell and Gene Therapy (ISCT) as CD105+CD90+CD73+CD45-CD34-CD14orCD11b-CD79αorCD19-HLA-DR- is being investigated as an attractive therapy for bone regeneration throughout the world. MSCs isolated from adipose tissue, adipose-derived stem cells (ADSCs), are gaining increasing attention since this is the most abundant source of adult stem cells and the isolation process for ADSCs is straightforward. Currently, there is not a single Food and Drug Administration (FDA) approved ADSCs product for bone regeneration. Although the safety of ADSCs is established from their usage in numerous clinical trials, the bone-forming potential of ADSCs and MSCs, in general, is highly controversial. Growing evidence suggests that the ISCT defined phenotype may not represent bona fide osteoprogenitors. Transplantation of both ADSCs and the CD105^- sub-population of ADSCs has been reported to induce bone regeneration. Most notably, cells expressing other markers such as CD146, AlphaV, CD200, PDPN, CD164, CXCR4, and PDGFRα have been shown to represent osteogenic sub-population within ADSCs. Amongst other strategies to improve the bone-forming ability of ADSCs, modulation of VEGF, TGF-β1 and BMP signaling pathways of ADSCs has shown promising results. The U.S. FDA reveals that 73% of Investigational New Drug applications for stem cell-based products rely on CD105 expression as the “positive” marker for adult stem cells. A concerted effort involving the scientific community, clinicians, industries, and regulatory bodies to redefine ADSCs using powerful selection markers and strategies to modulate signaling pathways of ADSCs will speed up the therapeutic use of ADSCs for bone regeneration.

Hypoxia and Hypoxia-Inducible Factor-1α Regulate Endoplasmic Reticulum Stress in Nucleus Pulposus Cells: Implications of Endoplasmic Reticulum Stress for Extracellular Matrix Secretion

Endoplasmic reticulum (ER) stress is shown to promote nucleus pulposus (NP) cell apoptosis and intervertebral disc degeneration. However, little is known about ER stress regulation by the hypoxic disc microenvironment and its contribution to extracellular matrix homeostasis. NP cells were cultured under hypoxia (1% partial pressure of oxygen) to assess ER stress status, and gain-of-function and loss-of-function approaches were used to assess the role of hypoxia-inducible factor (HIF)-1α in this pathway. In addition, the contribution of ER stress induction on the NP cell secretome was assessed by a nontargeted quantitative proteomic analysis by sequential windowed data independent acquisition of the total high-resolution mass spectra-mass spectrometry. NP cells exhibited a lower ER stress burden under hypoxia. Knockdown of HIF-1α increased C/EBP homologous protein, protein kinase RNA-like endoplasmic reticulum kinase (PERK), and activating transcription factor 6 (ATF6) levels, whereas HIF-1α stabilization decreased the expression of ER stress markers Ddit3, Hsp5a, Atf6, and Eif2a. Interestingly, ER stress inducers tunicamycin and thapsigargin induced HIF-1α activity under hypoxia while promoting the unfolded protein response. NP cell secretome analysis demonstrated an impact of ER stress induction on extracellular matrix secretion, with decreases in collagens and cell adhesion-related proteins. Moreover, analysis of transcriptomic data of NP tissues from aged mice and degenerated human discs showed higher levels of unfolded protein response markers and decreased levels of matrix components. Our study shows, for the first time, that hypoxia and HIF-1α attenuate ER stress responses in NP cells, and ER stress promotes inefficient extracellular matrix secretion under hypoxia.

Role of autophagy in intervertebral disc and cartilage function: implications in health and disease

The intervertebral disc and cartilage are specialized, extracellular matrix-rich tissues critical for absorbing mechanical loads, providing flexibility to the joints, and longitudinal growth in the case of growth plate cartilage. Specialized niche conditions in these tissues, such as hypoxia, are critical in regulating cellular activities including autophagy, a lysosomal degradation pathway that promotes cell survival. Mounting evidence suggests that dysregulation of autophagic pathways underscores many skeletal pathologies affecting the spinal column, articular and growth plate cartilages. Many lysosomal storage disorders characterized by the accumulation of partially degraded glycosaminoglycans (GAGs) due to the lysosomal dysfunction thus affect skeletal tissues and result in altered ECM structure. Likewise, pathologies that arise from mutations in genes encoding ECM proteins and ECM processing, folding, and post-translational modifications, result in accumulation of misfolded proteins in the ER, ER stress and autophagy dysregulation. These conditions evidence reduced secretion of ECM proteins and/or increased secretion of mutant proteins, thereby impairing matrix quality and the integrity of affected skeletal tissues and causing a lack of growth and degeneration. In this review, we discuss the role of autophagy and mechanisms of its regulation in the intervertebral disc and cartilages, as well as how dysregulation of autophagic pathways affects these skeletal tissues.

Hypoxic Regulation of Mitochondrial Metabolism and Mitophagy in Nucleus Pulposus Cells Is Dependent on HIF-1α-BNIP3 Axis

Nucleus pulposus (NP) cells reside in an avascular and hypoxic microenvironment of the intervertebral disc and are predominantly glycolytic due to robust HIF-1 activity. It is generally thought that NP cells contain few functional mitochondria compared with cells that rely on oxidative metabolism. Consequently, the contribution of mitochondria to NP cell metabolism and the role of hypoxia and HIF-1 in mitochondrial homeostasis is poorly understood. Using mitoQC reporter mice, we show for the first time to our knowledge that NP cell mitochondria undergo age-dependent mitophagy in vivo. Mechanistically, in vitro studies suggest that, under hypoxic conditions, mitochondria in primary NP cells undergo HIF-1α-dependent fragmentation, controlled by modulating the levels of key proteins DRP1 and OPA1 that are involved in mitochondrial fission and fusion, respectively. Seahorse assays and steady state metabolic profiling coupled with [1-2-¹³ C]-glucose flux analysis revealed that in hypoxia, HIF-1α regulated metabolic flux through coordinating glycolysis and the mitochondrial TCA cycle interactions, thereby controlling the overall biosynthetic capacity of NP cells. We further show that hypoxia and HIF-1α trigger mitophagy in NP cells through the mitochondrial translocation of BNIP3, an inducer of receptor-mediated mitophagy. Surprisingly, however, loss of HIF-1α in vitro and analysis of NP-specific HIF-1α null mice do not show a decrease in mitophagic flux in NP cells but a compensatory increase in NIX and PINK1-Parkin pathways with higher mitochondrial number. Taken together, our studies provide novel mechanistic insights into the complex interplay between hypoxia and HIF-1α signaling on the mitochondrial metabolism and quality control in NP cells. © 2020 American Society for Bone and Mineral Research.

Assessment of Hedgehog Signaling Pathway Activation for Craniofacial Bone Regeneration in a Critical-Sized Rat Mandibular Defect

Importance

Osseous craniofacial defects are currently reconstructed with bone grafting, rigid fixation, free tissue transfer, and/or recombinant human bone morphogenetic protein 2. Although these treatment options often have good outcomes, they are associated with substantial morbidity, and many patients are not candidates for free tissue transfer.

Objective

To assess whether polysaccharide-based scaffold (PS) constructs that are cross-linked with smoothened agonist (SAG), vascular endothelial growth factor (VEGF), and bone morphogenetic protein 6 (BMP-6) would substantially increase bone regeneration.

Design, Setting, and Participants

This animal model study was conducted at the University of Virginia School of Medicine Cui Laboratory from March 1, 2017, to June 30, 2017. Thirty-three 10-week-old female Lewis rats were acquired for the study. Bilateral nonsegmental critical-sized defects were created in the angle of rat mandibles. The defects were either left untreated or filled with 1 of the 9 PSs. The rats were killed after 8 weeks, and bone regeneration was evaluated using microcomputed tomographic imaging and mechanical testing. Analysis of variance testing was used to compare the treatment groups.

Main Outcomes and Measures

Blinded analysis and computer analysis of the microcomputed tomographic images were used to assess bone regeneration.

Results

In the 33 female Lewis rats, minimal healing was observed in the untreated mandibles. Addition of SAG was associated with increases in bone regeneration and bone density in all treatment groups, and maximum bone healing was seen in the group with BMP-6, VEGF, and SAG cross-linked to PS. For each of the 5 no scaffold group vs BMP-6, VEGF, and SAG cross-linked to PS group comparisons, mean defect bone regeneration was 4.14% (95% CI, 0.94%-7.33%) vs 66.19% (95% CI, 54.47%-77.90%); mean bone volume, 14.52 mm3 (95% CI, 13.07-15.97 mm3) vs 20.87 mm3 (95% CI, 14.73- 27.01 mm3); mean bone surface, 68.97 mm2 (95% CI, 60.08-77.85 mm2) vs 96.77 mm2 (95% CI, 76.11-117.43 mm2); mean ratio of bone volume to total volume, 0.11 (95% CI, 0.10-0.11) vs 0.15 (95% CI, 0.10-0.19); and mean connectivity density 0.03 (95% CI, 0.02-0.05) vs 0.32 (95% CI, 0.25-0.38). On mechanical testing, mandibles with untreated defects broke with less force than control mandibles in which no defect was made, although this force did not reach statistical significance. No significant difference in force to fracture was observed among the treatment groups.

Conclusions and Relevance

In this rat model study, activation of the hedgehog signaling pathway using smoothened agonist was associated with increased craniofacial bone regeneration compared with growth factors alone, including US Food and Drug Administration-approved recombinant human bone morphogenetic protein 2. Pharmaceuticals that target this pathway may offer a new reconstructive option for bony craniofacial defects as well as nonunion and delayed healing fractures.

Level of Evidence

NA.

NFAT5/TonEBP controls early acquisition of notochord phenotypic markers, collagen composition, and sonic hedgehog signaling during mouse intervertebral disc embryogenesis

High osmolarity, bound water, and hydrostatic pressure contribute to notochord mechanics and its morphogenesis into the nucleus pulposus (NP) compartment of the intervertebral disc. Indeed, the osmoadaptive transcription factor, nuclear factor of activated T-cells 5 (NFAT5 aka TonEBP), is robustly expressed by resident cells of the notochord and NP. Nevertheless, the molecular mechanisms that drive notochord osmoregulation and the functions of NFAT5 in disc embryogenesis remain largely unexplored. In this study, we show that deletion of NFAT5 in mice results in delayed vertebral column development and a reduced NP aspect ratio in the caudal spine. This phenotype is associated with lower levels of the T-box transcription factor, Brachyury, delayed expression of notochord phenotypic markers, and decreased collagen II deposition in the perinotochordal sheath and condensing mesenchyme. In addition, NFAT5 mutants showed a stage-dependent dysregulation of sonic hedgehog (Shh) signaling with non-classical expression of Gli1. Generation of mice with notochord-specific deletion of IFT88 (ShhcreER^T2;Ift88^f/f) supported this mode of Gli1 regulation. Using isolated primary NP cells and bioinformatics approaches, we further show that Ptch1 and Smo expression is controlled by NFAT5 in a cell autonomous manner. Altogether, our results demonstrate that NFAT5 contributes to notochord and disc embryogenesis through its regulation of hallmark notochord phenotypic markers, extracellular matrix, and Shh signaling.

Methodology to Measure the Protective Areal Density of Ceramic Tiles Against Projectile Impact

<p class="p1">The protective areal density of any armour material is the important property required for armour design. In this study, ballistic performance of hot pressed boron carbide tiles, with a tile thickness of 12.2 mm, was evaluated using the protective areal density (PAD) test method, against hard steel 12.7 mm armour piercing (AP) projectiles. The binary response data on complete penetration/ partial penetration obtained from PAD testing was fitted with the standard logistic regression model. A detailed discussion on statistical procedure has been presented. The PAD (pp = 0.5) was estimated to be 82.5 kg/m2 and the lower and upper bounds of 95 per cent confidence interval for (PAD) (pp = 0.5) was found to lie between 79.5 kg/m2 and 85.0 kg/m2.</p>

Future Armour Materials and Technologies for Combat Platforms

<p align="JUSTIFY">The ultimate goal of armour research is to create better armour for battle worthy combat plat forms such as main battle tanks, infantry combat vehicles and light combat vehicles. In each of these applications, the main aim boils down to one of the two things; either reduce the weight without sacrificing protection or enhance the performance at same or even reduced weight. In practice, these ambitions can be fulfilled only if we have with us, appropriate improved armour materials, advanced and innovative technologies and also improved designs, which enable us to use them for creating next generation armour modules. Armour systems have progressed through improvements in metallic, ceramic and lightweight (low areal density) composite materials. Similarly, the advances in development of explosive reactive armour (ERA) and non-explosive reactive armour (NERA) have generated efficient armour system against contemporary high explosive antitank ammunition and missile threats for the armoured vehicles. Yet, to achieve armour performance exceeding that of the current light combat vehicles and main battle tanks, further advancements in armour materials, systems, and survivability technologies are required for new vehicular systems that weigh significantly less than the present combat platforms. Various approaches and advancements in the metallic and composite armour materials, ERA and NERA systems to improve the survivability of armoured vehicles in the futuristic multi-spectral battlefield scenarios are described.</p>

Cross-Talk Between VEGF and BMP-6 Pathways Accelerates Osteogenic Differentiation of Human Adipose-Derived Stem Cells

Deficiency in vascular endothelial growth factor (VEGF) or bone morphogenetic proteins (BMPs) results in fracture non-unions. Therefore, it is indispensable to comprehend the combined effect of VEGF and BMPs on the osteogenic differentiation of osteoprogenitor mesenchymal stem cells (MSCs) that are either naturally occurring at the fracture repair site or exogenously added to enhance the bone repair. We found that the combination of VEGF and BMP-6 enhanced COL1A2 expression, which correlated with upregulated expression of osterix, Dlx5, and Msx2 in human adipose-derived stem cells (hADSCs). Cross-talk between VEGF and BMP-6 pathways upregulated activation of p38 mitogen-activated kinase (p38 MAPK) and inhibited activation of protein kinase B (PKB, also known as Akt), whereas phosphorylation of "mothers against decapentaplegic" homologs 1/5/8 (Smads 1/5/8) and extracellular signal-regulated kinases 1 and 2 (ERK 1/2) was not affected. Consistent with these findings, p38 inhibitor SB203580, or siRNA knockdown of osterix, abrogated crosstalk between the VEGF and BMP-6 pathways and significantly reduced the observed upregulation of COL1A2. Nuclear translocation of the phosphorylated form of osterix was also inhibited by SB203580. Although crosstalk between the VEGF-BMP-6 pathways did not show an effect on the extent of mineralization, inhibition of any one of the three components that were upregulated through the cross-talk, i.e., osterix, Dlx5, and p38 activation, led to a complete inhibition of mineralization. Inhibition of PKB/Akt activation, which is attenuated through the cross-talk, significantly enhanced ALP gene expression. These observations imply that crosstalk between the VEGF and BMP-6 signaling pathways enhances osteogenic differentiation of MSCs.

Expression of CD105 and CD34 receptors controls BMP-induced in vitro mineralization of mouse adipose-derived stem cells but does not predict their in vivo bone-forming potential

Adipose-derived stem cells (ADSCs) can be excellent alternative to bone marrow derived stem cells for enhancing fracture repair since ADSCs can be isolated comparatively in large numbers from discarded lipoaspirates. However, osteogenic potential of ADSCs in vivo is very controversial. We hypothesized that adipose-derived stem cells (ADSCs) that respond maximally to bone morphogenetic proteins (BMPs) in vitro would possess maximum bone-forming potential. Four purified populations of mouse ADSCs: CD105(+) CD34(+), CD105(-) CD34(-), CD105(+) CD34(-) and CD105(-) CD34(+) were obtained using fluorescence-activated cell sorting (FACS) and their BMP-responsiveness was determined in vitro. CD105(+) CD34(-) population showed the strongest response to BMPs in terms of robust increase in mineralization. Expression of CD105 correlated with high BMP-responsive phenotype and larger cell size while expression of CD34 correlated with low BMP-responsive phenotype and smaller cell size. CD105(+) CD34(-) population displayed higher gene expression of Alk1 or Alk6 receptors in comparison with other populations. However, CD105(+) CD34(-) ADSCs failed to induce ectopic bone formation in vivo after they were transplanted into syngeneic mice, indicating that in vitro BMP-responsiveness is not a good indicator to predict in vivo bone forming potential of ADSCs. Therefore greater precautions should be executed during selection of competent ADSCs for bone repair.

BMP-non-responsive Sca1+ CD73+ CD44+ mouse bone marrow derived osteoprogenitor cells respond to combination of VEGF and BMP-6 to display enhanced osteoblastic differentiation and ectopic bone formation

Clinical trials on fracture repair have challenged the effectiveness of bone morphogenetic proteins (BMPs) but suggest that delivery of mesenchymal stem cells (MSCs) might be beneficial. It has also been reported that BMPs could not increase mineralization in several MSCs populations, which adds ambiguity to the use of BMPs. However, an exogenous supply of MSCs combined with vascular endothelial growth factor (VEGF) and BMPs is reported to synergistically enhance fracture repair in animal models. To elucidate the mechanism of this synergy, we investigated the osteoblastic differentiation of cloned mouse bone marrow derived MSCs (D1 cells) in vitro in response to human recombinant proteins of VEGF, BMPs (-2, -4, -6, -9) and the combination of VEGF with BMP-6 (most potent BMP). We further investigated ectopic bone formation induced by MSCs pre-conditioned with VEGF, BMP-6 or both. No significant increase in mineralization, phosphorylation of Smads 1/5/8 and expression of the ALP, COL1A1 and osterix genes was observed upon addition of VEGF or BMPs alone to the cells in culture. The lack of CD105, Alk1 and Alk6 expression in D1 cells correlated with poor response to BMPs indicating that a greater care in the selection of MSCs is necessary. Interestingly, the combination of VEGF and BMP-6 significantly increased the expression of ALP, COL1A1 and osterix genes and D1 cells pre-conditioned with VEGF and BMP-6 induced greater bone formation in vivo than the non-conditioned control cells or the cells pre-conditioned with either VEGF or BMP-6 alone. This enhanced bone formation by MSCs correlated with higher CADM1 expression and OPG/RANKL ratio in the implants. Thus, combined action of VEGF and BMP on MSCs enhances osteoblastic differentiation of MSCs and increases their bone forming ability, which cannot be achieved through use of BMPs alone. This strategy can be effectively used for bone repair.

Interferon gamma and T cells inhibit osteogenesis induced by allogeneic mesenchymal stromal cells

The mesenchymal stromal cells (MSCs) are reported to be immunoprivileged and osteogenic. We hypothesized that the use of allogeneic MSCs for bone repair was possible if they displayed an ability to induce similar osteogenesis in syngeneic as well as in allogeneic hosts. To test this hypothesis we used a cloned bone marrow derived cell, termed D1, isolated from Balb/c mice. The D1 cells were subcutaneously injected in syngeneic Balb/c, allogeneic immunocompetent B6, allogeneic T-cell deficient NCr nude, and allogeneic B6 Pfp-/- Rag2-/- mice that lack matured T and B cells as well as NK-cell cytolytic functions. D1 cells formed ectopic bones only in syngeneic or allogeneic immunocompromised hosts but not in allogeneic B6 hosts. The lack of T cells alone in allogeneic NCr mice was sufficient to promote osteogenesis in allogeneic environment. We observed a significantly higher number of T cells, B cells, macrophages and significantly higher expression of interferon gamma (IFN-γ) in B6 allogeneic implants as compared to the syngeneic implants. These factors correlated with severe inhibition of expression of alkaline phosphatase, osteocalcin, and runx2 genes in the implants from B6 mice. Our data suggest that strategies to inhibit T cells and IFN-γ functions will be useful for bone repair mediated by allogeneic MSCs.

Osteogenic response of human adipose-derived stem cells to BMP-6, VEGF, and combined VEGF plus BMP-6 in vitro

Exogenous addition of three factors-mesenchymal stem cells (MSCs), vascular endothelial growth factor (VEGF), and bone morphogenetic proteins (BMPs)-has proven to be more beneficial than delivery of any single factor for fracture repair in animal models. We studied the osteogenic differentiation of human adipose-derived stem cells (hADSCs) in the presence of VEGF, BMP-6, or VEGF plus BMP-6 to better understand their enhancement of osteoblastic differentiation of MSCs. The VEGF plus BMP-6 group demonstrated an additive effect on the enhancement of mineralization and expression of ALP and Msx2 genes. Unlike VEGF or BMP-6 alone, the combination of VEGF and BMP-6 significantly enhanced the expression of COL1A1, osterix, and Dlx5 genes. The data indicate that a cross-talk between VEGF and BMP-6 signaling pathways enhances osteogenic differentiation of hADSCs.

Combined VEGF and LMP-1 delivery enhances osteoprogenitor cell differentiation and ectopic bone formation

A novel strategy to enhance bone repair is to combine angiogenic factors and osteogenic factors. We combined vascular endothelial growth factor (VEGF) and LIM mineralization protein-1 (LMP-1) by using an internal ribosome entry site to link the genes within a single plasmid. We then evaluated the effects on osteoblastic differentiation in vitro and ectopic bone formation in vivo with a subcutaneously placed PLAGA scaffold loaded with a cloned mouse osteoprogenitor cell line, D1, transfected with plasmids containing VEGF and LMP-1 genes. The cells expressing both genes elevated mRNA expression of RunX2 and β-catenin and alkaline phosphatase activity compared to cells from other groups. In vivo, X-ray and micro-CT analysis of the retrieved implants revealed more ectopic bone formation at 2 and 3 weeks but not at 4 weeks compared to other groups. The results indicate that the combination of the therapeutic growth factors potentiates cell differentiation and may promote osteogenesis.

Peptides from phage display library modulate gene expression in mesenchymal cells and potentiate osteogenesis in unicortical bone defects

Two novel synthetic peptides accelerate bone formation and can be delivered using a collagen matrix. The aim of this study was to investigate the effects on bone repair in a unicortical defect model. Treatment of mesenchymal cells produced an increase in alkaline phosphatase activity, showed nodule formation by the cells, and increased the expression of genes for runx2, osterix, bone sialoprotein, and osteocalcin. A collagen sponge soaked with peptide promoted repair of bone defects, whereas the control was less effective. The results from this study demonstrated that mesenchymal cells treated with peptide in vitro differentiate towards osteogenesis, and, that peptides delivered in vivo using a collagen sponge promote the repair of unicortical defects.