Activation of nuclear factor-kappa B by TNF promotes nucleus pulposus mineralization through inhibition of ANKH and ENPP1

Extracellular Vesicles Derived Ectonucleoside Triphosphate Diphosphohydrolase 3 Alleviates Mitochondrial Dysfunction of Osteoarthritis Chondrocytes via Ectonucleotide Pyrophosphatase/Phosphodiesterase 1-Induced Suppression of the AKT/Notch2 Pathway

Osteoarthritis (OA) is the most common joint disease that usually starts from joint cartilage injury. Notch2, a versatile signaling in human development and diseases, was recently uncovered to be an important regulator in chondrocyte damage. However, in OA chondrocytes, how Notch2 activation is dysregulated is largely unknown. Here, integrated bioinformatic analysis was performed on GEO datasets (GSE199193 and GSE224255) to search potential extracellular vesicles (EVs) derived regulators of Notch2 in OA chondrocytes. Ectonucleoside triphosphate diphosphohydrolase 3 (Entpd3), a most differentially expressed gene both in LPS-induced macrophage EV and Notch2 mutant chondrocytes, was screened as the candidate regulator of Notch2 in OA chondrocytes. Gain-of-function experiments in cultured human chondrocytes revealed that recombinant Entpd3 protein and macrophage EV both had a protective effect on LPS-induced inflammation, oxidative stress, apoptosis, and collagen loss in chondrocytes. In terms of mechanism, Entpd3 directly interacted with ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and suppressed AKT/Notch2-mediated mitochondrial dysfunction. Finally, we verified that either macrophage EV administration or Entpd3 overexpression was able to alleviate osteoarthritis in mice in vivo. In conclusion, Entpd3 is identified as a new regulator in OA, which alleviates mitochondrial dysfunction induced chondrocyte damage via ENPP1-induced suppression of the AKT/Notch2 pathway.

Inducing Angiogenesis in the Nucleus Pulposus

Bone morphogenetic protein (BMP) gene delivery to Lewis rat lumbar intervertebral discs (IVDs) drives bone formation anterior and external to the IVD, suggesting the IVD is inhospitable to osteogenesis. This study was designed to determine if IVD destruction with a proteoglycanase, and/or generating an IVD blood supply by gene delivery of an angiogenic growth factor, could render the IVD permissive to intra-discal BMP-driven osteogenesis and fusion. Surgical intra-discal delivery of naïve or gene-programmed cells (BMP2/BMP7 co-expressing or VEGF165 expressing) +/- purified chondroitinase-ABC (chABC) in all permutations was performed between lumbar 4/5 and L5/6 vertebrae, and radiographic, histology, and biomechanics endpoints were collected. Follow-up anti-sFlt Western blotting was performed. BMP and VEGF/BMP treatments had the highest stiffness, bone production and fusion. Bone was induced anterior to the IVD, and was not intra-discal from any treatment. chABC impaired BMP-driven osteogenesis, decreased histological staining for IVD proteoglycans, and made the IVD permissive to angiogenesis. A soluble fragment of VEGF Receptor-1 (sFlt) was liberated from the IVD matrix by incubation with chABC, suggesting dysregulation of the sFlt matrix attachment is a possible mechanism for the chABC-mediated IVD angiogenesis we observed. Based on these results, the IVD can be manipulated to foster vascular invasion, and by extension, possibly osteogenesis.

Nucleus Pulposus Cells from Calcified Discs Promote the Degradation of the Extracellular Matrix through Upregulation of the GATA3 Expression

OBJECTIVE

Disc calcification is strongly associated with disc degeneration; however, the underlying mechanisms driving its pathogenesis are poorly understood. This study aimed to provide a gene expression profile of nucleus pulposus cells (NPCs) from calcified discs, and clarify the potential mechanism in disc degeneration.

METHODS

Primary NPCs were isolated from calcified and control discs (CAL-NPC and CON-NPC), respectively. The proliferation and extracellular matrix (ECM) metabolism capacities of the cells were evaluated using MTT and Western blotting, respectively. RNA sequencing was used to identify differentially expressed genes (DEGs) in the CAL-NPCs. The biological functions of the DEGs were analyzed using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. The transcription factor database and Cytoscape software were used to construct the transcription factor-DEGs regulatory network. The role of the verified transcription factor in NPC proliferation and ECM metabolism was also investigated.

RESULTS

The CAL-NPCs exhibited a lower proliferation rate and higher ECM degradation capacity than the CON-NPCs. In total, 375 DEGs were identified in the CAL-NPCs. The GO and KEGG analyses showed that the DEGs were primarily involved in the regulation of ribonuclease activity and NF-kappa B and p53 signaling pathways. GATA-binding protein 3 (GATA3) with the highest verified levels was selected for further studies. Overexpression of GATA3 in the CON-NPCs significantly inhibited their proliferation and promoted their ECM degradation function, while the knockdown of GATA3 in the CAL-NPCs resulted in the opposite phenotypes.

CONCLUSION

This study provided a comprehensive gene expression profile of the NPCs from the calcified discs and supported that GATA3 could be a potential target for reversing calcification-associated disc degeneration.

Lumbar spine intervertebral disc gene delivery of BMPs induces anterior spine fusion in lewis rats

Minimally invasive techniques and biological autograft alternatives such as the bone morphogenetic proteins (BMPs) can reduce morbidity associated with spinal fusions. This study was a proof-of-concept for gene-therapy-mediated anterior spine fusion that could be adapted to percutaneous technique for clinical use. Isogeneic bone marrow stromal cells genetically programmed to express b-galactosidase (LACZ, a marker gene), BMP2, BMP7, a mixture of BMP2 and BMP7 infected cells (homodimers, HM), or BMP2/7 heterodimers (HT) were implanted into the discs between lumbar vertebrae 4 and 5 (L4/5) and L5/6 of male Lewis rats. Spine stiffening was monitored at 4, 8 and 12 weeks using noninvasive-induced angular displacement (NIAD) testing. At 12 weeks isolated spines were assessed for fusion and bone formation by palpation, biomechanical testing [four-point bending stiffness, moment to failure in extension, and in vitro angular displacement (IVAD)], faxitron x-rays, microCT, and histology. Progressive loss of NIAD occurred in only the HT group (p < 0.001), and biomechanical tests correlated with the NIAD results. Significant fusion occurred only in the HT group (94% of animals with one or both levels) as assessed by palpation (p < 0.001), which predicted HT bone production assessed by faxitron (p ≤ 0.001) or microCT (p < 0.023). Intervertebral bridging bone was consistently observed only in HT-treated specimens. Induced bone was located anterior and lateral to the disc space, with no bone formation noted within the disc. Percutaneous anterior spine fusions may be possible clinically, but induction of bone inside the disc space remains a challenge.

Mechanisms and clinical implications of intervertebral disc calcification

Low back pain is a leading cause of disability worldwide. Intervertebral disc (IVD) degeneration is often associated with low back pain but is sometimes asymptomatic. IVD calcification is an often overlooked disc phenotype that might have considerable clinical impact. IVD calcification is not a rare finding in ageing or in degenerative and scoliotic spinal conditions, but is often ignored and under-reported. IVD calcification may lead to stiffer IVDs and altered segmental biomechanics, more severe IVD degeneration, inflammation and low back pain. Calcification is not restricted to the IVD but is also observed in the degeneration of other cartilaginous tissues, such as joint cartilage, and is involved in the tissue inflammatory process. Furthermore, IVD calcification may also affect the vertebral endplate, leading to Modic changes (non-neoplastic subchondral vertebral bone marrow lesions) and the generation of pain. Such effects in the spine might develop in similar ways to the development of subchondral marrow lesions of the knee, which are associated with osteoarthritis-related pain. We propose that IVD calcification is a phenotypic biomarker of clinically relevant disc degeneration and endplate changes. As IVD calcification has implications for the management and prognosis of degenerative spinal changes and could affect targeted therapeutics and regenerative approaches for the spine, awareness of IVD calcification should be raised in the spine community.

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.

Biomolecules Orchestrating Cardiovascular Calcification

Vascular calcification, once considered a degenerative, end-stage, and inevitable condition, is now recognized as a complex process regulated in a manner similar to skeletal bone at the molecular and cellular levels. Since the initial discovery of bone morphogenetic protein in calcified human atherosclerotic lesions, decades of research have now led to the recognition that the regulatory mechanisms and the biomolecules that control cardiovascular calcification overlap with those controlling skeletal mineralization. In this review, we focus on key biomolecules driving the ectopic calcification in the circulation and their regulation by metabolic, hormonal, and inflammatory stimuli. Although calcium deposits in the vessel wall introduce rupture stress at their edges facing applied tensile stress, they simultaneously reduce rupture stress at the orthogonal edges, leaving the net risk of plaque rupture and consequent cardiac events depending on local material strength. A clinically important consequence of the shared mechanisms between the vascular and bone tissues is that therapeutic agents designed to inhibit vascular calcification may adversely affect skeletal mineralization and vice versa. Thus, it is essential to consider both systems when developing therapeutic strategies.