ISSLS PRIZE IN BASIC SCIENCE 2018: Growth differentiation factor-6 attenuated pro-inflammatory molecular changes in the rabbit anular-puncture model and degenerated disc-induced pain generation in the rat xenograft radiculopathy model

Regenerative therapies for lumbar degenerative disc diseases: a literature review

This review aimed to summarize the recent advances and challenges in the field of regenerative therapies for lumbar disc degeneration. The current first-line treatment options for symptomatic lumbar disc degeneration cannot modify the disease process or restore the normal structure, composition, and biomechanical function of the degenerated discs. Cell-based therapies tailored to facilitate intervertebral disc (IVD) regeneration have been developed to restore the IVD extracellular matrix or mitigate inflammatory conditions. Human clinical trials on Mesenchymal Stem Cells (MSCs) have reported promising outcomes exhibited by MSCs in reducing pain and improving function. Nucleus pulposus (NP) cells possess unique regenerative capacities. Biomaterials aimed at NP replacement in IVD regeneration, comprising synthetic and biological materials, aim to restore disc height and segmental stability without compromising the annulus fibrosus. Similarly, composite IVD replacements that combine various biomaterial strategies to mimic the native disc structure, including organized annulus fibrosus and NP components, have shown promise. Furthermore, preclinical studies on regenerative medicine therapies that utilize cells, biomaterials, growth factors, platelet-rich plasma (PRP), and biological agents have demonstrated their promise in repairing degenerated lumbar discs. However, these therapies are associated with significant limitations and challenges that hinder their clinical translation. Thus, further studies must be conducted to address these challenges.

Engineered extracellular vesicle-based gene therapy for the treatment of discogenic back pain

Painful musculoskeletal disorders such as intervertebral disc (IVD) degeneration associated with chronic low back pain (termed "Discogenic back pain", DBP), are a significant socio-economic burden worldwide and contribute to the growing opioid crisis. Yet there are very few if any successful interventions that can restore the tissue's structure and function while also addressing the symptomatic pain. Here we have developed a novel non-viral gene therapy, using engineered extracellular vesicles (eEVs) to deliver the developmental transcription factor FOXF1 to the degenerated IVD in an in vivo model. Injured IVDs treated with eEVs loaded with FOXF1 demonstrated robust sex-specific reductions in pain behaviors compared to control groups. Furthermore, significant restoration of IVD structure and function in animals treated with FOXF1 eEVs were observed, with significant increases in disc height, tissue hydration, proteoglycan content, and mechanical properties. This is the first study to successfully restore tissue function while modulating pain behaviors in an animal model of DBP using eEV-based non-viral delivery of transcription factor genes. Such a strategy can be readily translated to other painful musculoskeletal disorders.

Transcriptomic profiling reveals key early response genes during GDF6-mediated differentiation of human adipose-derived stem cells to nucleus pulposus cells

Background

Stem cell-based therapies show promise as a means of repairing the degenerate intervertebral disc, with growth factors often used alongside cells to help direct differentiation toward a nucleus pulposus (NP)-like phenotype. We previously demonstrated adipose-derived stem cell (ASC) differentiation with GDF6 as optimal for generating NP-like cells through evaluating end-stage differentiation parameters. Here we conducted a time-resolved transcriptomic characterization of ASCs response to GDF6 stimulation to understand the early drivers of differentiation to NP-like cells.

Methods

Human ASCs were treated with recombinant human GDF6 for 2, 6, and 12 h. RNA sequencing and detailed bioinformatic analysis were used to assess differential gene expression, gene ontology (GO), and transcription factor involvement during early differentiation. Quantitative polymerase chain reaction (qPCR) was used to validate RNA sequencing findings and inhibitors used to interrogate Smad and Erk signaling pathways, as well as identify primary and secondary response genes.

Results

The transcriptomic response of ASCs to GDF6 stimulation was time-resolved and highly structured, with "cell differentiation" "developmental processes," and "response to stimulus" identified as key biological process GO terms. The transcription factor ERG1 was identified as a key early response gene. Temporal cluster analysis of differentiation genes identified positive regulation NP cell differentiation, as well as inhibition of osteogenesis and adipogenesis. A role for Smad and Erk signaling in the regulation of GDF6-induced early gene expression response was observed and both primary and secondary response genes were identified.

Conclusions

This study identifies a multifactorial early gene response that contributes to lineage commitment, with the identification of a number of potentially useful early markers of differentiation of ASCs to NP cells. This detailed insight into the molecular processes in response to GDF6 stimulation of ASCs is important for the development of an efficient and efficacious cell-based therapy for intervertebral disc degeneration-associated back pain.

Esophageal Symptoms and Lumbosacral Back Pain

BACKGROUND AND AIMS

Esophageal symptoms, that is, heartburn, regurgitation, dysphagia, and chest pain are common in the general population. Also common are symptoms of back pain related to pathology in the lumbosacral spine. The right crus of the diaphragm that forms the esophageal hiatus, originates from lumbar spine, may be affected by lumbar spine pathology resulting in esophageal symptoms. We studied whether there was an association between esophageal symptoms and spine symptoms.

METHODS

Two patient groups of 150 each were investigated: group 1 (ES); patients referred to the esophageal manometry study for assessment of esophageal symptoms, group 2 (SC); patients undergoing screening colonoscopy (control group). Both groups completed standardized questionnaires assessing esophageal and spine symptoms.

RESULTS

Back pain was reported by 74% of patients in the ES group as compared to 55% of patients in the SC group. Thirty percent of patients in the SC group reported one or more esophageal symptoms and these patients were regrouped with the ES group, resulting in 2 groups, ES1 and SC1, with and without esophageal symptoms, respectively. The ES1 group was 3.3 times more likely to experience back pain compared to the SC1 group (95% confidence interval: 1.95-5.46). Thoracolumbar was the most common site of pain in both groups. Pain score was greater for the group with esophageal symptoms compared to controls. Narcotic intake for most patients in the ES1 group was for back pain.

CONCLUSION

A strong association between esophageal symptoms and thoracolumbar back pain raises the possibility that structural and functional changes in the esophageal hiatus muscles related to thoracolumbar spine pathology lead to esophageal dysmotility and symptoms.

Development of a Novel Rat Knee Osteoarthritis Model Induced by Medial Meniscus Extrusion

OBJECTIVE

The medial meniscus extrusion (MME) is associated with increased stress on the knee joint, which leads to cartilage degeneration. To evaluate the etiology of knee osteoarthritis, it is extremely important to create animal models of the disease that more closely resemble actual clinical conditions in terms of symptomatology, molecular biology, and histology. This study aimed to create a clinically relevant model of MME in rats.

DESIGN

Behavioral, molecular biological, and histological changes in the newly developed rat MME model were compared with those in sham and medial meniscus transection and medial collateral ligament transection (MMT) models to examine the characteristics of this model.

RESULTS

In the MME rat model, behavioral evaluation shows abnormalities in gait compared with the other 2 groups, and molecular biological evaluation of the infrapatellar synovia of rats shows that gene expression of inflammatory cytokines, matrix-degrading enzymes, and pain-related nerve growth factor was increased compared with the sham group. Furthermore, histological evaluation reveals that cartilage degeneration was the most severe in the MME group.

CONCLUSIONS

The newly developed MME model reproduced the characteristic pathology of MME in clinical practice, such as severe pain, inflammation, and rapid progression of osteoarthritis. The MME model, which might more closely mimic human knee osteoarthritis (OA), could be a useful model for elucidating the pathophysiology and considering therapeutic management for knee OA.

Intervertebral disc degeneration and how it leads to low back pain

The purpose of this review was to evaluate data generated by animal models of intervertebral disc (IVD) degeneration published in the last decade and show how this has made invaluable contributions to the identification of molecular events occurring in and contributing to pain generation. IVD degeneration and associated spinal pain is a complex multifactorial process, its complexity poses difficulties in the selection of the most appropriate therapeutic target to focus on of many potential candidates in the formulation of strategies to alleviate pain perception and to effect disc repair and regeneration and the prevention of associated neuropathic and nociceptive pain. Nerve ingrowth and increased numbers of nociceptors and mechanoreceptors in the degenerate IVD are mechanically stimulated in the biomechanically incompetent abnormally loaded degenerate IVD leading to increased generation of low back pain. Maintenance of a healthy IVD is, thus, an important preventative measure that warrants further investigation to preclude the generation of low back pain. Recent studies with growth and differentiation factor 6 in IVD puncture and multi-level IVD degeneration models and a rat xenograft radiculopathy pain model have shown it has considerable potential in the prevention of further deterioration in degenerate IVDs, has regenerative properties that promote recovery of normal IVD architectural functional organization and inhibits the generation of inflammatory mediators that lead to disc degeneration and the generation of low back pain. Human clinical trials are warranted and eagerly anticipated with this compound to assess its efficacy in the treatment of IVD degeneration and the prevention of the generation of low back pain.

Microglia and macrophages contribute to the development and maintenance of sciatica in lumbar disc herniation

ABSTRACT

Lumbar disc herniation (LDH) is a major cause of sciatica. Emerging evidence indicated that inflammation induced by the herniated nucleus pulposus (NP) tissues plays a major role in the pathogenesis of sciatica. However, the underlying mechanisms are still elusive. Although microglia and macrophages have been implicated in nerve injury-induced neuropathic pain, their roles in LDH-induced sciatica largely remain unknown. This study successfully established and modified a mouse model of LDH. We found that nerve root compression using degenerated NP tissues can initiate remarkable and persistent sciatica, with increased and prolonged macrophage infiltration in dorsal root ganglia (DRG) and significant activation of microglia in the spinal dorsal horn. Instead, compression of the nerve root with nondegenerated NP tissues only led to transient sciatica, with transient infiltration and activation of macrophages and microglia. Moreover, continuous treatment of PLX5622, a specific colony-stimulating factor 1 receptor antagonist, ablated both macrophages and microglia, which effectively alleviated LDH-induced sciatica. However, mechanical allodynia reoccurred along with the repopulation of macrophages and microglia after the withdrawal of PLX5622. Using RNA sequencing analysis, the current study depicted transcriptional profile changes of DRG after LDH and identified several macrophage-related potential target candidates. Our results suggested that microglia and macrophages may play an essential role in the development and maintenance of LDH-induced sciatica. Targeting microglia and macrophages may be a promising treatment for chronic LDH-induced sciatica.

Natural products can modulate inflammation in intervertebral disc degeneration

Intervertebral discs (IVDs) play a crucial role in maintaining normal vertebral anatomy as well as mobile function. Intervertebral disc degeneration (IDD) is a common clinical symptom and is an important cause of low back pain (LBP). IDD is initially considered to be associated with aging and abnormal mechanical loads. However, over recent years, researchers have discovered that IDD is caused by a variety of mechanisms, including persistent inflammation, functional cell loss, accelerated extracellular matrix decomposition, the imbalance of functional components, and genetic metabolic disorders. Of these, inflammation is thought to interact with other mechanisms and is closely associated with the production of pain. Considering the key role of inflammation in IDD, the modulation of inflammation provides us with new options for mitigating the progression of degeneration and may even cause reversal. Many natural substances possess anti-inflammatory functions. Due to the wide availability of such substances, it is important that we screen and identify natural agents that are capable of regulating IVD inflammation. In fact, many studies have demonstrated the potential clinical application of natural substances for the regulation of inflammation in IDD; some of these have been proven to have excellent biosafety. In this review, we summarize the mechanisms and interactions that are responsible for inflammation in IDD and review the application of natural products for the modulation of degenerative disc inflammation.

Functionalized self-assembling peptide RADKPS hydrogels promote regenerative repair of degenerated intervertebral discs

Objective: the aim of this study was to investigate whether the functionalized self-assembling peptide hydrogel RADKPS is safe and effective for regenerative repair of degenerative intervertebral discs. Methods: an in vitro degenerative model of human nucleus pulposus cells was constructed by serum starvation culture, and their proliferation, apoptosis and viability were examined after three-dimensional culture with the RADKPS hydrogel. An in vivo degenerative model of the rabbit intervertebral disc was constructed by annulus fibrosus puncture, and the degeneration of the intervertebral disc was evaluated by imaging, histology, immunohistochemistry, and biomechanics after RADKPS hydrogel intervention. Results: through in vitro cell experiments it is shown that human degenerated nucleus pulposus cells after three-dimensional culture with the RADKPS hydrogel still exhibited better proliferation, viability, and low apoptosis rate. Through in vivo animal experiments we found that rabbit degenerated intervertebral discs intervened with the RADKPS hydrogel had higher water content, better histological morphology, more extracellular matrix synthesis, and better biomechanical properties. It is demonstrated that the RADKPS hydrogel may initiate the endogenous repair process through the sustained recruitment and enrichment of nucleus pulposus progenitor cells. Conclusion: it is verified from both in vitro cellular experiments and in vivo animal experiments that the regenerative repair effect of RADKPS, a functionalized self-assembling peptide hydrogel, on degenerated intervertebral discs is safe and effective. It is shown that it would be a new therapeutic approach for the regenerative repair action of intervertebral discs.

Protective Effects of Growth Differentiation Factor-6 on the Intervertebral Disc: An In Vitro and In Vivo Study

Growth differentiation factors (GDFs) regulate homeostasis by amplifying extracellular matrix anabolism and inhibiting pro-inflammatory cytokine production in the intervertebral disc (IVD). The aim of this study was to elucidate the effects of GDF-6 on human IVD nucleus pulposus (NP) cells using a three-dimensional culturing system in vitro and on rat tail IVD tissues using a puncture model in vivo. In vitro, Western blotting showed decreased GDF-6 expression with age and degeneration severity in surgically collected human IVD tissues (n = 12). Then, in moderately degenerated human IVD NP cells treated with GDF-6 (100 ng/mL), immunofluorescence demonstrated an increased expression of matrix components including aggrecan and type II collagen. Quantitative polymerase chain reaction analysis also presented GDF-6-induced downregulation of pro-inflammatory tumor necrosis factor (TNF)-α (p = 0.014) and interleukin (IL)-6 (p = 0.016) gene expression stimulated by IL-1β (10 ng/mL). Furthermore, in the mitogen-activated protein kinase pathway, Western blotting displayed GDF-6-induced suppression of p38 phosphorylation (p = 0.041) under IL-1β stimulation. In vivo, intradiscal co-administration of GDF-6 and atelocollagen was effective in alleviating rat tail IVD annular puncture-induced radiologic height loss (p = 0.005), histomorphological degeneration (p < 0.001), matrix metabolism (aggrecan, p < 0.001; type II collagen, p = 0.001), and pro-inflammatory cytokine production (TNF-α, p < 0.001; IL-6, p < 0.001). Consequently, GDF-6 could be a therapeutic growth factor for degenerative IVD disease.

Proper animal experimental designs for preclinical research of biomaterials for intervertebral disc regeneration

Low back pain is a vital musculoskeletal disease that impairs life quality, leads to disability and imposes heavy economic burden on the society, while it is greatly attributed to intervertebral disc degeneration (IDD). However, the existing treatments, such as medicines, chiropractic adjustments and surgery, cannot achieve ideal disc regeneration. Therefore, advanced bioactive therapies are implemented, including stem cells delivery, bioreagents administration, and implantation of biomaterials etc. Among these researches, few reported unsatisfying regenerative outcomes. However, these advanced therapies have barely achieved successful clinical translation. The main reason for the inconsistency between satisfying preclinical results and poor clinical translation may largely rely on the animal models that cannot actually simulate the human disc degeneration. The inappropriate animal model also leads to difficulties in comparing the efficacies among biomaterials in different reaches. Therefore, animal models that better simulate the clinical charateristics of human IDD should be acknowledged. In addition, in vivo regenerative outcomes should be carefully evaluated to obtain robust results. Nevertheless, many researches neglect certain critical characteristics, such as adhesive properties for biomaterials blocking annulus fibrosus defects and hyperalgesia that is closely related to the clinical manifestations, e.g., low back pain. Herein, in this review, we summarized the animal models established for IDD, and highlighted the proper models and parameters that may result in acknowledged IDD models. Then, we discussed the existing biomaterials for disc regeneration and the characteristics that should be considered for regenerating different parts of discs. Finally, well-established assays and parameters for in vivo disc regeneration are explored.

Painful intervertebral disc degeneration and inflammation: from laboratory evidence to clinical interventions

Low back pain (LBP), as a leading cause of disability, is a common musculoskeletal disorder that results in major social and economic burdens. Recent research has identified inflammation and related signaling pathways as important factors in the onset and progression of disc degeneration, a significant contributor to LBP. Inflammatory mediators also play an indispensable role in discogenic LBP. The suppression of LBP is a primary goal of clinical practice but has not received enough attention in disc research studies. Here, an overview of the advances in inflammation-related pain in disc degeneration is provided, with a discussion on the role of inflammation in IVD degeneration and pain induction. Puncture models, mechanical models, and spontaneous models as the main animal models to study painful disc degeneration are discussed, and the underlying signaling pathways are summarized. Furthermore, potential drug candidates, either under laboratory investigation or undergoing clinical trials, to suppress discogenic LBP by eliminating inflammation are explored. We hope to attract more research interest to address inflammation and pain in IDD and contribute to promoting more translational research.

Multiscale Regulation of the Intervertebral Disc: Achievements in Experimental, In Silico, and Regenerative Research

Intervertebral disc (IVD) degeneration is a major risk factor of low back pain. It is defined by a progressive loss of the IVD structure and functionality, leading to severe impairments with restricted treatment options due to the highly demanding mechanical exposure of the IVD. Degenerative changes in the IVD usually increase with age but at an accelerated rate in some individuals. To understand the initiation and progression of this disease, it is crucial to identify key top-down and bottom-up regulations' processes, across the cell, tissue, and organ levels, in health and disease. Owing to unremitting investigation of experimental research, the comprehension of detailed cell signaling pathways and their effect on matrix turnover significantly rose. Likewise, in silico research substantially contributed to a holistic understanding of spatiotemporal effects and complex, multifactorial interactions within the IVD. Together with important achievements in the research of biomaterials, manifold promising approaches for regenerative treatment options were presented over the last years. This review provides an integrative analysis of the current knowledge about (1) the multiscale function and regulation of the IVD in health and disease, (2) the possible regenerative strategies, and (3) the in silico models that shall eventually support the development of advanced therapies.

Regenerative Response of Degenerate Human Nucleus Pulposus Cells to GDF6 Stimulation

Growth differentiation factor (GDF) family members have been implicated in the development and maintenance of healthy nucleus pulposus (NP) tissue, making them promising therapeutic candidates for treatment of intervertebral disc (IVD) degeneration and associated back pain. GDF6 has been shown to promote discogenic differentiation of mesenchymal stem cells, but its effect on NP cells remains largely unknown. Our aim was to investigate GDF6 signalling in adult human NP cells derived from degenerate tissue and determine the signal transduction pathways critical for GDF6-mediated phenotypic changes and tissue homeostatic mechanisms. This study demonstrates maintained expression of GDF6 receptors in human NP and annulus fibrosus (AF) cells across a range of degeneration grades at gene and protein level. We observed an anabolic response in NP cells treated with recombinant GDF6 (increased expression of matrix and NP-phenotypic markers; increased glycosaminoglycan production; no change in catabolic enzyme expression), and identified the signalling pathways involved in these responses (SMAD1/5/8 and ERK1/2 phosphorylation, validated by blocking studies). These findings suggest that GDF6 promotes a healthy disc tissue phenotype in degenerate NP cells through SMAD-dependent and -independent (ERK1/2) mechanisms, which is important for development of GDF6 therapeutic strategies for treatment of degenerate discs.

Elastic fibers: The missing key to improve engineering concepts for reconstruction of the Nucleus Pulposus in the intervertebral disc

The increasing prevalence of low back pain has imposed a heavy economic burden on global healthcare systems. Intense research activities have been performed for the regeneration of the Nucleus Pulposus (NP) of the IVD; however, tissue-engineered scaffolds have failed to capture the multi-scale structural hierarchy of the native tissue. The current study revealed for the first time, that elastic fibers form a network across the NP consisting of straight and thick parallel fibers that were interconnected by wavy fine fibers and strands. Both straight fibers and twisted strands were regularly merged or branched to form a fine elastic network across the NP. As a key structural feature, ultrathin (53 ± 7 nm), thin (215 ± 20 nm), and thick (890 ± 12 nm) elastic fibers were observed in the NP. While our quantitative analysis for measurement of the thickness of elastic fibers revealed no significant differences (p < 0.633), the preferential orientation of fibers was found to be significantly different (p < 0.001) across the NP. The distribution of orientation for the elastic fibers in the NP represented one major organized angle of orientation except for the central NP. We found that the distribution of elastic fibers in the central NP was different from those located in the peripheral regions representing two symmetrically organized major peaks (±45⁰). No significant differences in the maximum fiber count at the major angles of orientation (±45⁰) were observed for both peripheral (p = 0.427) and central NP (p = 0.788). Based on these new findings a structural model for the elastic fibers in the NP was proposed. The geometrical presentation, along with the distribution of elastic fibers orientation, resulting from the present study identifies the ultrastructural organization of elastic fibers in the NP important towards understanding their mechanical role which is still under investigation. Given the results of this new geometrical analysis, more-accurate multiscale finite element models can now be developed, which will provide new insights into the mechanobiology of the IVD. In addition, the results of this study can potentially be used for the fabrication of bio-inspired tissue-engineered scaffolds and IVD models to truly capture the multi-scale structural hierarchy of IVDs. STATEMENT OF SIGNIFICANCE: Visualization of elastic fibers in the nucleus of the intervertebral disk under high magnification was not reported before. The present research utilized extracellular matrix partial digestion to address significant gaps in understanding of nucleus microstructure that can potentially be used for the fabrication of bio-inspired tissue-engineered scaffolds and disk models to truly capture the multi-scale structural hierarchy of discs.

Advanced Strategies for the Regeneration of Lumbar Disc Annulus Fibrosus

Damage to the annulus fibrosus (AF), the outer region of the intervertebral disc (IVD), results in an undesirable condition that may accelerate IVD degeneration causing low back pain. Despite intense research interest, attempts to regenerate the IVD have failed so far and no effective strategy has translated into a successful clinical outcome. Of particular significance, the failure of strategies to repair the AF has been a major drawback in the regeneration of IVD and nucleus replacement. It is unlikely to secure regenerative mediators (cells, genes, and biomolecules) and artificial nucleus materials after injection with an unsealed AF, as IVD is exposed to significant load and large deformation during daily activities. The AF defects strongly change the mechanical properties of the IVD and activate catabolic routes that are responsible for accelerating IVD degeneration. Therefore, there is a strong need to develop effective therapeutic strategies to prevent or reconstruct AF damage to support operational IVD regenerative strategies and nucleus replacement. By the way of this review, repair and regenerative strategies for AF reconstruction, their current status, challenges ahead, and future outlooks were discussed.

Reduced nucleotomy-induced intervertebral disc disruption through spontaneous spheroid formation by the Low Adhesive Scaffold Collagen (LASCol)

Back pain is a global health problem with a high morbidity and socioeconomic burden. Intervertebral disc herniation and degeneration are its primary cause, further associated with neurological radiculopathy, myelopathy, and paralysis. The current surgical treatment is principally discectomy, resulting in the loss of spinal movement and shock absorption. Therefore, the development of disc regenerative therapies is essential. Here we show reduced disc damage by a new collagen type I-based scaffold through actinidain hydrolysis-Low Adhesive Scaffold Collagen (LASCol)-with a high 3D spheroid-forming capability, water-solubility, and biodegradability and low antigenicity. In human disc nucleus pulposus and annulus fibrosus cells surgically obtained, time-dependent spheroid formation with increased expression of phenotypic markers and matrix components was observed on LASCol but not atelocollagen (AC). In a rat tail nucleotomy model, LASCol-injected and AC-injected discs presented relatively similar radiographic and MRI damage control; however, LASCol, distinct from AC, decelerated histological disc disruption, showing collagen type I-comprising LASCol degradation, aggrecan-positive and collagen type II-positive endogenous cell migration, and M1-polarized and also M2-polarized macrophage infiltration. Reduced nucleotomy-induced disc disruption through spontaneous spheroid formation by LASCol warrants further investigations of whether it may be an effective treatment without stem cells and/or growth factors for intervertebral disc disease.

Multiscale and multimodal structure-function analysis of intervertebral disc degeneration in a rabbit model

OBJECTIVES

The objective of this study was to perform a quantitative analysis of the structural and functional alterations in the intervertebral disc during in vivo degeneration, using emerging tools that enable rigorous assessment from the microscale to the macroscale, as well as to correlate these outcomes with noninvasive, clinically relevant imaging parameters.

DESIGN

Degeneration was induced in a rabbit model by puncturing the annulus fibrosus (AF) with a 16-gauge needle. 2, 4, 8, and 12 weeks following puncture, degenerative changes in the discs were evaluated via magnetic resonance imaging (MRI), whole motion segment biomechanics, atomic force microscopy, histology and polarized light microscopy, immunohistochemistry, biochemical content, and second harmonic generation imaging.

RESULTS

Following puncture, degeneration was evident through marked changes in whole disc structure and mechanics. Puncture acutely compromised disc macro and microscale mechanics, followed by progressive stiffening and remodeling. Histological analysis showed substantial anterior fibrotic remodeling and osteophyte formation, as well as an overall reduction in disc height, and disorganization and infolding of the AF lamellae into the NP space. Increases in NP collagen content and aggrecan breakdown products were also noted within 4 weeks. On MRI, NP T2 was reduced at all post-puncture time points and correlated significantly with microscale indentation modulus.

CONCLUSION

This study defined the time dependent changes in disc structure-function relationships during IVD degeneration in a rabbit annular injury model and correlated degeneration severity with clinical imaging parameters. Our findings identified AF infolding and occupancy of the space as a principle mechanism of disc degeneration in response to needle puncture, and provide new insights to direct the development of novel therapeutics.

Combinatorial conditioning of adipose derived-mesenchymal stem cells enhances their neurovascular potential: Implications for intervertebral disc degeneration

BACKGROUND

Mesenchymal stem cells (MSCs) are becoming an increasingly attractive option for regenerative therapies due to their availability, self-renewal capacity, multilineage potential, and anti-inflammatory properties. Clinical trials are underway to test the efficacy of stem cell-based therapies for the repair and regeneration of the degenerate intervertebral disc (IVD), a major cause of back pain. Recently, both bone marrow-derived MSCs and adipose-derived stem cells (ASCs) have been assessed for IVD therapy but there is a lack of knowledge surrounding the optimal cell source and the response of transplanted cells to the low oxygen, pro-inflammatory niche of the degenerate disc. Here, we investigated several neurovascular factors from donor-matched MSCs and ASCs that may potentiate the survival and persistence of sensory nerve fibers and blood vessels present within painful degenerate discs and their regulation by oxygen tensions and inflammatory cytokines.

METHODS

Donor-matched ASCs and MSCs were conditioned with either IL-1β or TNFα under normoxic (21% O2) or hypoxic (5% O2) conditions. Expression and secretion of several potent neurovascular factors were assessed using qRT-PCR and human magnetic Luminex assay.

RESULTS

ASCs and MSCs expressed constitutive levels of key neurotrophic factors; and stimulation of ASCs with hypoxia triggered increased secretion of both angiogenic factors (Ang-2 and VEGF-A) and neurotrophic (NGF and NT-3) compared to MSCs. We also report increased transcriptional regulation of pain-associated neuropeptides in hypoxia stimulated ASCs compared to those in normoxic conditions. We demonstrate transcriptional and translational upregulation of NGF, NT-3, Ang-1, and FGF-2 in response to cytokines in ASCs in 21% and 5% O2.

CONCLUSIONS

This work highlights fundamental differences between the neurovascular secretome of donor-matched ASCs and MSCs, demonstrating the importance of cell-selection for tissue specific regeneration to reduce ectopic sensory nerve and blood vessel survival and improve patient outcomes.

A novel magnetic resonance imaging postprocessing technique for the assessment of intervertebral disc degeneration-Correlation with histological grading in a rabbit disc degeneration model

INTRODUCTION

Estimation of intervertebral disc degeneration on magnetic resonance imaging (MRI) is challenging. Qualitative schemes used in clinical practice correlate poorly with pain and quantitative techniques have not entered widespread clinical use.

METHODS

As part of a prior study, 25 New Zealand white rabbits underwent annular puncture to induce disc degeneration in 50 noncontiguous lumbar discs. At 16 weeks, the animals underwent multi-echo T2 MRI scanning and were euthanized. The discs were stained and examined histologically. Quantitative T2 relaxation maps were prepared using the nonlinear least squares method. Decay Variance maps were created using a novel technique of aggregating the deviation in the intensity of each echo signal from the expected intensity based on the previous rate of decay.

RESULTS

Decay Variance maps showed a clear and well demarcated nucleus pulposus with a consistent rate of decay (low Decay Variance) in healthy discs that showed progressively more variable decay (higher Decay Variance) with increasing degeneration. Decay Variance maps required significantly less time to generate (1.0 ± 0.0 second) compared with traditional T2 relaxometry maps (5 (±0.9) to 1788.9 (±116) seconds). Histology scores correlated strongly with Decay Variance scores (r = 0.82, P < .01) and weakly with T2 signal intensity (r = 0.32, P < .01) and quantitative T2 relaxometry (r = 0.39, P < .01). Decay Variance had superior sensitivity and specificity for the detection of degenerate discs when compared to T2 signal intensity or Quantitative T2 mapping.

CONCLUSION

Our results show that using a multi-echo T2 MRI sequence, Decay Variance can quantitatively assess disc degeneration more accurately and with less image-processing time than quantitative T2 relaxometry in a rabbit disc puncture model. The technique is a viable candidate for quantitative assessment of disc degeneration on MRI scans. Further validation on human subjects is needed.

Microparticles for controlled growth differentiation factor 6 delivery to direct adipose stem cell-based nucleus pulposus regeneration

Currently, there is no effective long‐term treatment for intervertebral disc (IVD) degeneration, making it an attractive candidate for regenerative therapies. Hydrogel delivery of adipose stem cells (ASCs) in combination with controlled release of bioactive molecules is a promising approach to halt IVD degeneration and promote regeneration. Growth differentiation factor 6 (GDF6) can induce ASC differentiation into anabolic nucleus pulposus (NP) cells and hence holds promise for IVD regeneration. Here, we optimised design of novel poly(DL‐lactic acid‐co‐glycolic acid) (PLGA)–polyethylene glycol–PLGA microparticles to control GDF6 delivery and investigated effect of released GDF6 on human ASCs differentiation to NP cells. Recombinant human (rh)GDF6 was loaded into microparticles and total protein and rhGDF6 release assessed. The effect of microparticle loading density on distribution and gel formation was investigated through scanning electron microscopy. ASC differentiation to NP cells was examined after 14 days in hydrogel culture by quantitative polymerase chain reaction, histological, and immunohistochemical staining in normoxic and IVD‐like hypoxic conditions. RhGDF6 microparticles were distributed throughout gels without disrupting gelation and controlled rhGDF6 release over 14 days. Released GDF6 significantly induced NP differentiation of ASCs, with expression comparable with or exceeding media supplemented rhGDF6. Microparticle‐delivered rhGDF6 also up‐regulated sulphated glycosaminoglycan and aggrecan secretion in comparison with controls. In hypoxia, microparticle‐delivered rhGDF6 continued to effectively induce NP gene expression and aggrecan production. This study demonstrates the effective encapsulation and controlled delivery of rhGDF6, which maintained its activity and induced ASC differentiation to NP cells and synthesis of an NP‐like matrix suggesting suitability of microparticles for controlled growth factor release in regenerative strategies for treatment of IVD degeneration.

Annulus fibrosus cell sheets limit disc degeneration in a rat annulus fibrosus injury model

In recent years, studies have explored novel approaches for cell transplantation to enable annulus fibrosus (AF) regeneration of the intervertebral disc in particular for lumbar disc herniation. Nevertheless, successful engraftment of cells is structurally challenging, and no definitive method has yet been established. This study investigated the potential of cell sheet technology to facilitate cell engraftment for AF repair. AF injury was induced by a 1 × 1 mm defect in rat tails after which AF cell sheets were transplanted. Its regenerative effects were compared to a nondegenerated and degeneration only conditions. Degenerative changes of the entire intervertebral disc were examined by disc height measurements, histology, and immunohistochemistry for 4-, 8-, and 12-weeks post-transplantation. Cell engraftment was confirmed by tracing PKH26 fluorescent dyed AF cells. In the transplant group, disc degeneration was significantly suppressed after 4, 8, and 12 weeks when compared with the degenerative group, as indicated by histological scoring and DHI observations. At 2 and 4 weeks after transplant, PKH26 positive cells could be detected in defect region and surrounding AF. The results suggest cell engraftment into AF tissue could be established by the cell sheet technology without additional scaffolding or adhesives. In short, AF cell sheets appear to be an effective and accessible tool for AF repair and to support intervertebral disc regeneration.

Therapeutic potential of growth differentiation factors in the treatment of degenerative disc diseases

Intervertebral disc (IVD) degeneration is a major contributing factor to chronic low back pain and disability, leading to imbalance between anabolic and catabolic processes, altered extracellular matrix composition, loss of tissue hydration, inflammation, and impaired mechanical functionality. Current treatments aim to manage symptoms rather than treat underlying pathology. Therefore, IVD degeneration is a target for regenerative medicine strategies. Research has focused on understanding the molecular process of degeneration and the identification of various factors that may have the ability to halt and even reverse the degenerative process. One such family of growth factors, the growth differentiation factor (GDF) family, have shown particular promise for disc regeneration in in vitro and in vivo models of IVD degeneration. This review outlines our current understanding of IVD degeneration, and in this context, aims to discuss recent advancements in the use of GDF family members as anabolic factors for disc regeneration. An increasing body of evidence indicates that GDF family members are central to IVD homeostatic processes and are able to upregulate healthy nucleus pulposus cell marker genes in degenerative cells, induce mesenchymal stem cells to differentiate into nucleus pulposus cells and even act as chemotactic signals mobilizing resident cell populations during disc injury repair. The understanding of GDF signaling and its interplay with inflammatory and catabolic processes may be critical for the future development of effective IVD regeneration therapies.