Comparison of Oxygen Consumption Rates of Nondegenerate and Degenerate Human Intervertebral Disc Cells

Trends and considerations in annulus fibrosus in vitro model design

Annulus Fibrosus (AF) tissue integrity maintains intervertebral disc (IVD) structure, essential to spine mobility and shock absorption. However, this tissue, which confines nucleus pulposus (NP), has been poorly investigated, partially due to the lack of appropriate study models. This review provides a comprehensive analysis of AF in vitro models. By critically assessing the current AF in vitro models, this works thoroughly identifies key gaps in replicating the tissue's complex microenvironment. Finally, we outline the essential criteria for developing more accurate and reliable AF models, emphasizing the importance of biomaterial composition, architecture, and microenvironmental cues. By advancing in vitro models, we aim to deepen the understanding of AF failure mechanisms and support the development of novel therapeutic strategies for IVD herniation. Insights gained from this review may also have broader applications in regenerative medicine, particularly in the study and treatment of other connective tissue disorders. STATEMENT OF SIGNIFICANCE: This review evaluates the current in vitro models of the annulus fibrosus (AF), a key component of the intervertebral disc (IVD). By identifying gaps in these models, particularly in replicating tissue's complex microenvironment, we propose essential criteria for the development of more accurate AF models, to better understand the pathomechanisms and potentially aid the development of therapeutic approaches for spinal disorders. The findings also extend to broader studies of musculoskeletal tissue disorders in the context of regenerative medicine, appealing to a diverse biomedical research readership.

Transcriptional profiling of human cartilage endplate cells identifies novel genes and cell clusters underlying degenerated and non-degenerated phenotypes

BACKGROUND

Low back pain is a leading cause of disability worldwide and is frequently attributed to intervertebral disc (IVD) degeneration. Though the contributions of the adjacent cartilage endplates (CEP) to IVD degeneration are well documented, the phenotype and functions of the resident CEP cells are critically understudied. To better characterize CEP cell phenotype and possible mechanisms of CEP degeneration, bulk and single-cell RNA sequencing of non-degenerated and degenerated CEP cells were performed.

METHODS

Human lumbar CEP cells from degenerated (Thompson grade ≥ 4) and non-degenerated (Thompson grade ≤ 2) discs were expanded for bulk (N=4 non-degenerated, N=4 degenerated) and single-cell (N=1 non-degenerated, N=1 degenerated) RNA sequencing. Genes identified from bulk RNA sequencing were categorized by function and their expression in non-degenerated and degenerated CEP cells were compared. A PubMed literature review was also performed to determine which genes were previously identified and studied in the CEP, IVD, and other cartilaginous tissues. For single-cell RNA sequencing, different cell clusters were resolved using unsupervised clustering and functional annotation. Differential gene expression analysis and Gene Ontology, respectively, were used to compare gene expression and functional enrichment between cell clusters, as well as between non-degenerated and degenerated CEP samples.

RESULTS

Bulk RNA sequencing revealed 38 genes were significantly upregulated and 15 genes were significantly downregulated in degenerated CEP cells relative to non-degenerated cells (|fold change| ≥ 1.5). Of these, only 2 genes were previously studied in CEP cells, and 31 were previously studied in the IVD and other cartilaginous tissues. Single-cell RNA sequencing revealed 11 unique cell clusters, including multiple chondrocyte and progenitor subpopulations with distinct gene expression and functional profiles. Analysis of genes in the bulk RNA sequencing dataset showed that progenitor cell clusters from both samples were enriched in "non-degenerated" genes but not "degenerated" genes. For both bulk- and single-cell analyses, gene expression and pathway enrichment analyses highlighted several pathways that may regulate CEP degeneration, including transcriptional regulation, translational regulation, intracellular transport, and mitochondrial dysfunction.

CONCLUSIONS

This thorough analysis using RNA sequencing methods highlighted numerous differences between non-degenerated and degenerated CEP cells, the phenotypic heterogeneity of CEP cells, and several pathways of interest that may be relevant in CEP degeneration.

Preclinical to clinical translation for intervertebral disc repair: Effects of species-specific scale, metabolism, and matrix synthesis rates on cell-based regeneration

Background

A significant hurdle for potential cell-based therapies is the subsequent survival and regenerative capacity of implanted cells. While many exciting developments have demonstrated promise preclinically, cell-based therapies for intervertebral disc (IVD) degeneration fail to translate equivalent clinical efficacy.

Aims

This work aims to ascertain the clinical relevance of both a small and large animal model by experimentally investigating and comparing these animal models to human from the perspective of anatomical scale and their cellular metabolic and regenerative potential.

Materials and Methods

First, this work experimentally investigated species-specific geometrical scale, native cell density, nutrient metabolism, and matrix synthesis rates for rat, goat, and human disc cells in a 3D microspheroid configuration. Second, these parameters were employed in silico to elucidate species-specific nutrient microenvironments and predict differences in temporal regeneration between animal models.

Results

This work presents in silico models which correlate favorably to preclinical literature in terms of the capabilities of animal regeneration and predict that compromised nutrition is not a significant challenge in small animal discs. On the contrary, it highlights a very fine clinical balance between an adequate cell dose for sufficient repair, through de novo matrix deposition, without exacerbating the human microenvironmental niche.

Discussion

Overall, this work aims to provide a path towards understanding the effect of cell injection number on the nutrient microenvironment and the "time to regeneration" between preclinical animal models and the large human IVD. While these findings help to explain failed translation of promising preclinical data and the limited results emerging from clinical trials at present, they also enable the research field and clinicians to manage expectations on cell-based regeneration.

Conclusion

Ultimately, this work provides a platform to inform the design of clinical trials, and as computing power and software capabilities increase in the future, it is conceivable that generation of patient-specific models could be used for patient assessment, as well as pre- and intraoperative planning.

Effects of glucose concentration and oxygen partial pressure on the respiratory metabolism of sheep temporomandibular joint disc cells

Temporomandibular joint (TMJ) disc degeneration is a common disease characterized by a decrease in metabolic function. The present study aimed to investigate the pathogenesis of TMJ disc degeneration by analyzing the effects of oxygen and glucose concentrations on metabolism in a simulated TMJ disc cell growth environment. Cell samples were divided into 10 groups and cultured in different nutritional environments, including 21 and 2% O₂ partial pressures and various glucose concentrations (0, 0.5, 3, 5.5 and 22.5 mmol/l). Cell proliferation, extracellular matrix content, mitochondrial function, and cell metabolism were subsequently analyzed. The results demonstrated that hypoxia and a low glucose concentration inhibited cell growth, and low glucose concentration inhibited extracellular matrix synthesis and adenosine 5'-monophosphate-activated protein kinase expression. Hypoxic conditions also induced a compensatory increase in the number of mitochondria, whereas mitochondrial deformation and swelling were observed in the absence of glucose. According to this study, the primary metabolic pathway of TMJ disc cells is glycolysis. It was concluded that hypoxic conditions and normal glucose concentrations are needed for the growth of TMJ disc cells. Glucose is necessary to ensure cell survival, extracellular matrix synthesis and mitochondrial function. Glucose deficiency may be related to disc degeneration, aging and disease mechanisms.

Two- and three-dimensional in vitro nucleus pulposus cultures: An in silico analysis of local nutrient microenvironments

Background

It is well established that the unique biochemical microenvironment of the intervertebral disc plays a predominant role in cell viability and biosynthesis. However, unless the effect of microenvironmental conditions is primary to a study objective, in vitro culture parameters that are critical for reproducibility are both varied and not routinely reported.

Aims

This work aims to investigate the local microenvironments of commonly used culture configurations, highlighting physiological relevance, potential discrepancies, and elucidating possible heterogeneity across the research field.

Materials and Methods

This work uses nutrient-transport in silico models to reflect on the effect of often underappreciated parameters, such as culture geometry and diffusional distance (vessel, media volume, construct size), seeding density, and external boundary conditions on the local microenvironment of two-dimensional (2D) and three-dimensional (3D) in vitro culture systems.

Results

We elucidate important discrepancies between the external boundary conditions such as the incubator level or media concentrations and the actual local cellular concentrations. Oxygen concentration and cell seeding density were found to be highly influential parameters and require utmost consideration when utilizing 3D culture systems.

Discussion

This work highlights that large variations in the local nutrient microenvironment can easily be established without consideration of several key parameters. Without careful deliberation of the microenvironment within each specific and unique system, there is the potential to confound in vitro results leading to heterogeneous results across the research field in terms of biosynthesis and matrix composition.

Conclusion

Overall, this calls for a greater appreciation of key parameters when designing in vitro experiments. Better harmony and standardization of physiologically relevant local microenvironments are needed to push toward reproducibility and successful translation of findings across the research field.

Consolidating and re-evaluating the human disc nutrient microenvironment

Background

Despite exciting advances in regenerative medicine, cell-based strategies for treating degenerative disc disease remain in their infancy. To maximize the potential for successful clinical translation, a more thorough understanding of the in vivo microenvironment is needed to better determine and predict how cell therapies will respond when administered in vivo.

Aims

This work aims to reflect on the in vivo nutrient microenvironment of the degenerating IVD through consolidating what has already been measured together with investigative in silico models.

Materials and Methods

This work uses in silico modeling, underpinned by more recent experimentally determined parameters of degeneration and nutrient transport from the literature, to re-evaluate the current knowledge in terms of grade-specific stages of degeneration.

Results

Through modeling only the metabolically active cell population, this work predicts slightly higher glucose concentrations compared to previous in silico models, while the predicted results show good agreement with previous intradiscal pH and oxygen measurements. Increasing calcification with degeneration limits nutrient transport into the IVD and initiates a build-up of acidity; however, its effect is compensated somewhat by a reduction in diffusional distance due to decreasing disc height.

Discussion

This work advances in silico modeling through a strong foundation of experimentally determined grade-specific input parameters. Taken together, pre-existing measurements and predicted results suggest that metabolite concentrations may not be as critically low as commonly believed, with calcification not appearing to have a detrimental effect at stages of degeneration when cell therapies are an appropriate intervention.

Conclusion

Overall, our initiative is to provoke greater deliberation and consideration of the nutrient microenvironment when performing in vitro cell culture and cell therapy development. This work highlights urgency for robust experimental glucose measurements in healthy and degenerating IVDs, not only to validate in silico models but to significantly advance the field in fully elucidating the nutrient microenvironment and refining in vitro techniques to accelerate clinical translation.

Comprehensive analysis of N6-methyladenosine (m6A) modification during the degeneration of lumbar intervertebral disc in mice

Objective

To study the N6-methyladenosine (m⁶A) modification pattern of nucleus pulposus (NP) tissue during intervertebral disc degeneration (IDD).

Methods

A standing mouse model was generated, and staining and imaging methods were used to evaluate the IDD model. Methylated RNA immunoprecipitation with next-generation sequencing (MeRIP-seq) was used to analyze m⁶A methylation-associated transcripts in the NP, and real-time quantitative polymerase chain reaction (qRT-PCR) was used to detect the expression of methylation-related enzymes and conduct bio-informatics analysis.

Results

The standing mouse model caused IDD. Continuous axial pressure changed the expression of related methylases in degenerated NP tissue. Relative to the control group, the expression levels of KIAA1429, METTL14, METTL3, METTL4, WTAP, DGCR8, EIF3A and YTHDC1 in the experimental group were higher, while those of FTO, ELAVL1, HNRNPC1 and SRSF2 were lower. We identified 985 differentially expressed genes through MeRIP-Seq, among which 363 genes were significantly up-regulated, and 622 genes were significantly down-regulated. In addition, among the 9648 genes counted, 1319 m⁶A peaks with significant differences in methylation were identified, among which 933 were significantly up-regulated, and 386 were significantly down-regulated. Genes and pathways that were enriched in IDD have been identified.

Conclusion

The results of this study elucidated the m⁶A methylation pattern of NP tissue in degenerated lumbar intervertebral disc of mice and provided new perspectives and clues for research on and the treatment of lumbar disc degeneration.

The Translational potential of this article

As one of the important causes of low back and leg pain, intervertebral disc degeneration brings a huge economic burden to the society, family and medical system. Therefore, understanding the molecular and cellular mechanisms of intervertebral disc degeneration is of great significance for guiding clinical treatment. In this study, methylated RNA immunoprecipitation with next-generation sequencing on mice lumbar nucleus pulposus tissues found that differentially expressed genes and changes in the expression of related methylases, confirming that RNA methylation is involved in intervertebral disc degeneration. The process provides new vision and clues for future research on intervertebral disc degeneration.

Investigating the physiological relevance of ex vivo disc organ culture nutrient microenvironments using in silico modeling and experimental validation

BACKGROUND

Ex vivo disc organ culture systems have become a valuable tool for the development and pre-clinical testing of potential intervertebral disc (IVD) regeneration strategies. Bovine caudal discs have been widely selected due to their large availability and comparability to human IVDs in terms of size and biochemical composition. However, despite their extensive use, it remains to be elucidated whether their nutrient microenvironment is comparable to human degeneration.

AIMS

This work aims to create the first experimentally validated in silico model which can be used to predict and characterize the metabolite concentrations within ex vivo culture systems.

MATERIALS & METHODS

Finite element models of cultured discs governed by previously established coupled reaction-diffusion equations were created using COMSOL Multiphysics. Experimental validation was performed by measuring oxygen, glucose and pH levels within discs cultured for 7 days, in a static compression bioreactor.

RESULTS

The in silico model was successfully validated through good agreement between the predicted and experimentally measured concentrations. For an ex vivo organ cultured in high glucose medium (4.5 g/L or 25 mM) and normoxia, a larger bovine caudal disc (Cd1-2 to Cd3-4) had a central concentration of ~2.6 %O2, ~8 mM of glucose and a pH value of 6.7, while the smallest caudal discs investigated (Cd6-7 and Cd7-8), had a central concentration of ~6.5 %O2, ~12 mM of glucose and a pH value of 6.9.

DISCUSSION

This work advances the knowledge of ex vivo disc culture microenvironments and highlights a critical need for optimization and standardization of culturing conditions.

CONCLUSION

Ultimately, for assessment of cell-based therapies and successful clinical translation based on nutritional demands, it is imperative that the critical metabolite values within organ cultures (minimum glucose, oxygen and pH values) are physiologically relevant and comparable to the stages of human degeneration.

Evaluation of disc degeneration adjacent to AOspine A fractures: pre- and post-operative MRI analysis

INTRODUCTION

The management of type A thoracolumbar fractures varies from conservative treatment to multiple level fusion. Indeed, although Magerl defined the type A fracture as a strictly bone injury, several authors suggested associated disc lesions or degeneration after trauma. However, the preservation of mobility of the adjacent discs should be a major issue. This study was conducted to analyze the presence of immediate post-traumatic disc injuries and to know if discs degenerate after receiving treatment.

METHODS

We retrospectively reviewed the files of 27 patients with an AOspine A fracture, corresponding to 34 fractures (64 discs) with pre and post-operative MRI (mean follow-up: 32.4 months). Based on Pfirrmann's and Oner's classifications of disc injuries, two observers analyzed independently the type of lesion in the discs adjacent to the fractured vertebra in immediate post-trauma and at the last follow-up.

RESULTS

The immediate post-traumatic analysis according to Pfirrmann's classification found 97% of the cranial adjacent discs and 100% of the caudal discs classified Pfirrmann 3 or less. The analysis on the secondary MRI revealed that 78% of cranial adjacent discs and 88% of caudal adjacent discs still were classified Pfirrmann 3 or less.

CONCLUSIONS

Since, the great majority of type A fractures does not cause immediate disc injuries, these fractures are, as described by Magerl, strictly bony injuries. The quality of the body reduction seems to prevent secondary degeneration. These results may encourage surgeons not to perform arthrodesis on type A fractures even for A3 and A4.

Bioinformatics analysis integrating metabolomics of m6A RNA microarray in intervertebral disc degeneration

Aim: To explore the potential functions and mechanism of N⁶.methyladenosine (m⁶A) abnormality of RNAs in nucleus pulposus from the intervertebral disc degeneration (IDD). Materials & methods: We performed rat model, m⁶A epitranscriptomic microarray, bioinformatics analysis and metabolomics. Results: In IDD, most of the differentially methylated RNAs showed a significant demethylation situation. The competing endogenous RNA network LOC102555094/miR-431/GSK-3β combining downstream Wnt pathway were identified in bioinformatics analysis. For metabolomics, activation of Wnt pathway led to reprogramming of glucose metabolism and enzyme activation of PKM2. Finally, quantitative real-time PCR and methylated RNA immunoprecipitation coupled with quantitative real-time PCR revealed the positive correlation between demethylation of LOC102555094 and expression of both FTO and ZFP217. Conclusion: LOC102555094 might be demethylated by ZFP217, activating FTO and LOC102555094/miR-431/GSK-3β/Wnt played a crucial role in IDD.

Hypoxic stress enhances extension and branching of dorsal root ganglion neuronal outgrowth

It has been shown that painful intervertebral discs (IVDs) were associated with a deeper innervation. However, the effect of the disc's degenerative microenvironment on neuronal outgrowth remains largely unknown. The focus of this study was to determine the influence of hypoxia on dorsal root ganglion (DRG) neurite outgrowth. Toward this aim, the DRG-derived cell line ND7/23 was either directly subjected to 2% or 20% oxygen conditions or exposed to conditioned medium (CM) collected from IVDs cultured under 2% or 20% oxygen. Viability and outgrowth analysis were performed following 3 days of exposure. Results obtained with the cell line were further validated on cultures of rabbit spinal DRG explants and dissociated DRG neurons. Results showed that hypoxia significantly increased neurite outgrowth length in ND7/23 cells, which was also validated in DRG explant and primary cell culture, although hypoxia conditioned IVD did not significantly increase ND7/23 neurite outgrowth. While hypoxia dramatically decreased the outgrowth frequency in explant cultures, it significantly increased collateral sprouting of dissociated neurons. Importantly, the hypoxia-induced decrease of outgrowth frequency at the explant level was not due to inhibition of outgrowth branching but rather to neuronal necrosis. In summary, hypoxia in DRG promoted neurite sprouting, while neuronal necrosis may reduce the density of neuronal outgrowth at the tissue level. These findings may help to explain the deeper neo-innervation found in the painful disc tissue.

HIGHLIGHTS

Hypoxia promoted elongation and branching of neurite outgrowth at single cell level, but reduced outgrowth density at tissue level, possibly due to hypoxia-induced neuronal necrosis; these findings may help to explain the deeper neo-innervation found in clinically painful tissues.

Interaction between Mesenchymal Stem Cells and Intervertebral Disc Microenvironment: From Cell Therapy to Tissue Engineering

Low back pain (LBP) in one of the most disabling symptoms affecting nearly 80% of the population worldwide. Its primary cause seems to be intervertebral disc degeneration (IDD): a chronic and progressive process characterized by loss of viable cells and extracellular matrix (ECM) breakdown within the intervertebral disc (IVD) especially in its inner region, the nucleus pulposus (NP). Over the last decades, innovative biological treatments have been investigated in order to restore the original healthy IVD environment and achieve disc regeneration. Mesenchymal stem cells (MSCs) have been widely exploited in regenerative medicine for their capacity to be easily harvested and be able to differentiate along the osteogenic, chondrogenic, and adipogenic lineages and to secrete a wide range of trophic factors that promote tissue homeostasis along with immunomodulation and anti-inflammation. Several in vitro and preclinical studies have demonstrated that MSCs are able to acquire a NP cell-like phenotype and to synthesize structural components of the ECM as well as trophic and anti-inflammatory mediators that may support resident cell activity. However, due to its unique anatomical location and function, the IVD presents distinctive features: avascularity, hypoxia, low glucose concentration, low pH, hyperosmolarity, and mechanical loading. Such conditions establish a hostile microenvironment for both resident and exogenously administered cells, which limited the efficacy of intradiscal cell therapy in diverse investigations. This review is aimed at describing the characteristics of the healthy and degenerated IVD microenvironment and how such features influence both resident cells and MSC viability and biological activity. Furthermore, we focused on how recent research has tried to overcome the obstacles coming from the IVD microenvironment by developing innovative cell therapies and functionalized bioscaffolds.

Biomaterials for intervertebral disc regeneration: Current status and looming challenges

A biomaterial-based strategy is employed to regenerate the degenerated intervertebral disc, which is considered a major generator of neck and back pain. Although encouraging enhancements in the anatomy and kinematics of the degenerative disc have been gained by biomaterials with various formulations in animals, the number of biomaterials tested in humans is rare. At present, most studies that involve the use of newly developed biomaterials focus on regeneration of the degenerative disc, but not pain relief. In this review, we summarise the current state of the art in the field of biomaterial-based regeneration or repair for the nucleus pulposus, annulus fibrosus, and total disc transplantation in animals and humans, and we then provide essential suggestions for the development and clinical translation of biomaterials for disc regeneration. It is important for researchers to consider the commonly neglected issues instead of concentrating solely on biomaterial development and fabrication.

ENERGY SUPPLY AND DEMAND IN THE INTERVERTEBRAL DISC

ABSTRACT The intervertebral disc (IVD) is one of the parts of the body most commonly affected by disease, and it is only recently that we have come closer to understanding the reasons for its degeneration, in which nutrient supply plays a crucial role. In this literature review, we discuss the basic principles and characteristics of energy supply and demand to the IVD. Specifically, we review how different metabolites influence IVD cell activity, the effects of mechanical loading on IVD cell metabolism, and differences in energy metabolism of the annulus fibrous and nucleus pulposus cell phenotypes. Determining the factors that influence nutrient supply and demand in the IVD will enhance our understanding of the IVD pathology, and help to elucidate new therapeutic targets for IVD degeneration treatment.