Background: Recently, Tie2/TEK receptor tyrosine kinase (Tie2 or syn. angiopoietin‐1 receptor) positive nucleus pulposus progenitor cells were detected in human, cattle, and mouse. These cells show remarkable multilineage differentiation capacity and direct correlation with intervertebral disc (IVD) degeneration and are therefore an interesting target for regenerative strategies. Nevertheless, there remains controversy over the presence and function of these Tie2⁺ nucleus pulposus cells (NPCs), in part due to the difficulty of identification and isolation.Purpose: Here, we present a comprehensive protocol for sorting of Tie2⁺ NPCs from human, canine, bovine, and murine IVD tissue. We describe enhanced conditions for expansion and an optimized fluorescence‐activated cell sorting‐based methodology to sort and analyze Tie2⁺ NPCs.Methods: We present flow cytometry protocols to isolate the Tie2⁺ cell population for the aforementioned species. Moreover, we describe crucial pitfalls to prevent loss of Tie2⁺ NPCs from the IVD cell population during the isolation process. A cross‐species phylogenetic analysis of Tie2 across species is presented.Results: Our protocols are efficient towards labeling and isolation of Tie2⁺ NPCs. The total flow cytometry procedure requires approximately 9 hours, cell isolation 4 to 16 hours, cell expansion can take up to multiple weeks, dependent on the application, age, disease state, and species. Phylogenetic analysis of the TEK gene revealed a strong homology among species.Conclusions: Current identification of Tie2⁺ cells could be confirmed in bovine, canine, mouse, and human specimens. The presented flow cytometry protocol can successfully sort these multipotent cells. The biological function of isolated cells based on Tie2⁺ expression needs to be confirmed by functional assays such as in vitro differentiation. in vitro culture conditions to maintain and their possible proliferation of the Tie2⁺ fraction is the subject of future research.

(1) Background: Intervertebral disc (IVD) repair represents a major challenge. Using functionalised biomaterials such as silk combined with enforced hydrogels might be a promising approach for disc repair. We aimed to test an IVD repair approach by combining a genipin-enhanced fibrin hydrogel with an engineered silk scaffold under complex load, after inducing an injury in a bovine whole organ IVD culture; (2) Methods: Bovine coccygeal IVDs were isolated from ~1-year-old animals within four hours post-mortem. Then, an injury in the annulus fibrosus was induced by a 2 mm biopsy punch. The repair approach consisted of genipin-enhanced fibrin hydrogel that was used to fill up the cavity. To seal the injury, a Good Manufacturing Practise (GMP)-compliant engineered silk fleece-membrane composite was applied and secured by the cross-linked hydrogel. Then, IVDs were exposed to one of three loading conditions: no load, static load and complex load in a two-degree-of-freedom bioreactor for 14 days. Followed by assessing DNA and matrix content, qPCR and histology, the injured discs were compared to an uninjured control IVD that underwent the same loading profiles. In addition, the genipin-enhanced fibrin hydrogel was further investigated with respect to cytotoxicity on human stem cells, annulus fibrosus, and nucleus pulposus cells; (3) Results: The repair was successful as no herniation could be detected for any of the three loading conditions. Disc height was not recovered by the repair DNA and matrix contents were comparable to a healthy, untreated control disc. Genipin resulted being cytotoxic in the in vitro test but did not show adverse effects when used for the organ culture model; (4) Conclusions: The current study indicated that the combination of the two biomaterials, i.e., genipin-enhanced fibrin hydrogel and an engineered silk scaffold, was a promising approach for IVD repair. Furthermore, genipin-enhanced fibrin hydrogel was not suitable for cell cultures; however, it was highly applicable as a filler material.

Spinal fusion is a common surgical procedure to address a range of spinal pathologies, like damaged or degenerated discs. After the removal of the intervertebral disc (IVD), a structural spacer is positioned followed by internal fixation, and fusion of the degenerated segment by natural bone growth. Due to their osteoinductive properties, bone morphogenetic proteins (BMP) are applied to promote spinal fusion. Although spinal fusion is successful in most patients, the rates of non-unions after lumbar spine fusion range from 5% to 35%. Clinical observations and recent studies indicate, that the incomplete removal of disc tissue might lead to failure of spinal fusion. Yet, it is still unknown if a secretion of BMP antagonists in intervertebral disc (IVD) cells could be the reason of inhibition in bone formation. In this study, we co-cultured human primary osteoblasts (OB) and IVD cells i.e., nucleus pulposus (NPC), annulus fibrosus (AFC) and cartilaginous endplate cells (CEPC), to test the possible inhibitory effect from IVD cells on OB. Although we could see a trend in lower matrix mineralization in OB co-cultured with IVD cells, results of alkaline phosphatase (ALP) activity and gene expression of major bone genes were inconclusive. However, in NPC, AFC and CEPC beads, an up-regulation of several BMP antagonist genes could be detected. Despite being able to show several indicators for an inhibition of osteoinductive effects due to IVD cells, the reasons for pseudarthrosis after spinal fusion remain unclear.

Aims: The anterior cruciate ligament (ACL) is known to have a poor wound healing capacity, whereas other ligaments outside of the knee joint capsule such as the medial collateral ligament (MCL) apparently heal more easily. Plasmin has been identified as a major component in the synovial fluid that varies among patients. The aim of this study was to test whether plasmin, a component of synovial fluid, could be a main factor responsible for the poor wound healing capacity of the ACL. Methods: The effects of increasing concentrations of plasmin (0, 0.1, 1, 10, and 50 µg/ml) onto the wound closing speed (WCS) of primary ACL-derived ligamentocytes (ACL-LCs) were tested using wound scratch assay and time-lapse phase-contrast microscopy. Additionally, relative expression changes (quantitative PCR (qPCR)) of major LC-relevant genes and catabolic genes were investigated. The positive controls were 10% fetal calf serum (FCS) and platelet-derived growth factor (PDGF). Results: WCS did not differ significantly among no plasmin versus each of the tested concentrations (six donors). The positive controls with PDGF and with FCS differed significantly from the negative controls. However, we found a trend demonstrating that higher plasmin concentrations up-regulate the expression of matrix metalloproteinase 13 ( MMP13), 3 ( MMP3), and tenomodulin ( TNMD). Conclusion: The clinical relevance of this study is the possibility that it is not solely the plasmin, but also additional factors in the synovial fluid of the knee, that may be responsible for the poor healing capacity of the ACL. Cite this article: Bone Joint Res 2020;9(9):543–553.

Diffuse idiopathic skeletal hyperostosis (DISH) is characterised by ectopic ossification along the anterior spine and the outer intervertebral discs (IVD). However, the centre of the IVD, i.e., the nucleus pulposus, always remains unaffected, which could be due to the inhibition of the bone morphogenetic protein (BMP) pathway. In this study, we investigated the transcriptome for the BMP pathway of DISH-IVD cells versus disc cells of traumatic or degenerative discs. The disc cells originated from nucleus pulposus (NP), annulus fibrosus (AF) and from cartilaginous endplate (CEP) tissue. Here, ninety genes of the transforming growth factor β-BMP signalling pathway were screened by qPCR. Furthermore, the protein expression of genes of interest was further investigated by immune-staining and semi-quantitative microscopy. IVDs of three DISH patients were tested against three control patients (same disc level and similar age). Early Growth Response 2 (EGR2) and Interleukin 6 (IL6) were both significantly up-regulated in DISH-IVD cells compared to controls (12.8 ± 7.6-fold and 54.0 ± 46.5-fold, respectively, means ± SEM). Furthermore, Insulin-like Growth Factor 1 (IGF1) tended to be up-regulated in DISH-IVD donors, i.e., 174.13 ± 120.6-fold. IGF1 was already known as a serum marker for DISH and other rheumatoid diseases and is confirmed here to play a possible key role in DISH-IVD.