Simulation of biological therapies for degenerated intervertebral discs

Modeling of glycosaminoglycan biosynthesis in intervertebral disc cells

Loss of proteoglycan (PG) is a potential factor responsible for degeneration of the intervertebral disc (IVD). PG consists of a core protein with covalently attached glycosaminoglycan (GAG) chains. The objective of this study was to develop a mathematical model of GAG biosynthesis to investigate the effects of glycolytic enzymes on GAG biosynthesis of IVD cells. A new mathematical model of GAG biosynthesis was developed for IVD cells by incorporating biosynthesis of uridine diphosphate-sugars into the glycolytic pathway. This new model showed good agreement between the model predictions of intracellular ATP content and GAG biosynthesis and experimental data measured at different external glucose levels. The quantitative analyses demonstrated that GAG biosynthesis may be sensitive to the activities of hexokinase (HK) and phosphofructokinase (PFK), especially at low glucose supply, with GAG biosynthesis being significantly enhanced by a slight increase in activities of HK and PFK. This suggests that metabolic reprogramming could be a potential strategy for promoting PG biosynthesis in IVD cells. Furthermore, it was shown that GAG biosynthesis may be promoted by increasing intracellular glutamine concentration or activity of glutamine:fructose-6-phosphate amidotransferase in the hexamine pathway. This study provides a better understanding of the relationship between glycolysis and PG biosynthesis in IVD cells. The theoretical framework developed in this study is useful for studying the role of glycolysis in disc degeneration and developing new preventive and treatment strategies for degeneration of the IVD.

Intradiscal treatment of the cartilage endplate for improving solute transport and disc nutrition

Poor nutrient transport through the cartilage endplate (CEP) is a key factor in the etiology of intervertebral disc degeneration and may hinder the efficacy of biologic strategies for disc regeneration. Yet, there are currently no treatments for improving nutrient transport through the CEP. In this study we tested whether intradiscal delivery of a matrix-modifying enzyme to the CEP improves solute transport into whole human and bovine discs. Ten human lumbar motion segments harvested from five fresh cadaveric spines (38-66 years old) and nine bovine coccygeal motion segments harvested from three adult steers were treated intradiscally either with collagenase enzyme or control buffer that was loaded in alginate carrier. Motion segments were then incubated for 18 h at 37 °C, the bony endplates removed, and the isolated discs were compressed under static (0.2 MPa) and cyclic (0.4-0.8 MPa, 0.2 Hz) loads while submerged in fluorescein tracer solution (376 Da; 0.1 mg/ml). Fluorescein concentrations from site-matched nucleus pulposus (NP) samples were compared between discs. CEP samples from each disc were digested and assayed for sulfated glycosaminoglycan (sGAG) and collagen contents. Results showed that enzymatic treatment of the CEP dramatically enhanced small solute transport into the disc. Discs with enzyme-treated CEPs had up to 10.8-fold (human) and 14.0-fold (bovine) higher fluorescein concentration in the NP compared to site-matched locations in discs with buffer-treated CEPs (p < 0.0001). Increases in solute transport were consistent with the effects of enzymatic treatment on CEP composition, which included reductions in sGAG content of 33.5% (human) and 40% (bovine). Whole disc biomechanical behavior-namely, creep strain and disc modulus-was similar between discs with enzyme- and buffer-treated CEPs. Taken together, these findings demonstrate the potential for matrix modification of the CEP to improve the transport of small solutes into whole intact discs.

Evaluation of human cartilage endplate composition using MRI: Spatial variation, association with adjacent disc degeneration, and in vivo repeatability

Cartilage endplate (CEP) biochemical composition may influence disc degeneration and regeneration. However, evaluating CEP composition in patients remains a challenge. We used T2* mapping from ultrashort echo-time (UTE) magnetic resonance imaging (MRI), which is sensitive to CEP hydration, to investigate spatial variations in CEP T2* values and to determine how CEP T2* values correlate with adjacent disc degeneration. Thirteen human cadavers (56.4 ± 12.7 years) and seven volunteers (36.9 ± 10.9 years) underwent 3T MRI, including UTE and T1ρ mapping sequences. Spatial mappings of T2* values in L4-S1 CEPs were generated from UTE images and compared between subregions. In the abutting discs, mean T1ρ values in the nucleus pulposus were compared between CEPs with high vs low T2* values. To assess in vivo repeatability, precision errors in mean T2* values, and intraclass correlation coefficients (ICC) were measured from repeat scans. Results showed that CEP T2* values were highest centrally and lowest posteriorly. In the youngest individuals (<50 years), who had mild-to-moderately degenerated Pfirrmann grade II-III discs, low CEP T2* values associated with severer disc degeneration: T1ρ values were 26.7% lower in subjects with low CEP T2* values (P = .025). In older individuals, CEP T2* values did not associate with disc degeneration (P = .39-.62). Precision errors in T2* ranged from 1.7 to 2.6 ms, and reliability was good-to-excellent (ICC = 0.89-0.94). These findings suggest that deficits in CEP composition, as indicated by low T2* values, associate with severer disc degeneration during the mild-to-moderate stages. Measuring CEP T2* values with UTE MRI may clarify the role of CEP composition in patients with mild-to-moderate disc degeneration.

Nutrient supply and nucleus pulposus cell function: effects of the transport properties of the cartilage endplate and potential implications for intradiscal biologic therapy

OBJECTIVE

Intradiscal biologic therapy is a promising strategy for managing intervertebral disc degeneration. However, these therapies require a rich nutrient supply, which may be limited by the transport properties of the cartilage endplate (CEP). This study investigated how fluctuations in CEP transport properties impact nutrient diffusion and disc cell survival and function.

DESIGN

Human CEP tissues harvested from six fresh cadaveric lumbar spines (38-66 years old) were placed at the open sides of diffusion chambers. Bovine nucleus pulposus (NP) cells cultured inside the chambers were nourished exclusively by nutrients diffusing through the CEP tissues. After 72 h in culture, depth-dependent NP cell viability and gene expression were measured, and related to CEP transport properties and biochemical composition determined using fluorescence recovery after photobleaching and Fourier transform infrared (FTIR) spectroscopy.

RESULTS

Solute diffusivity varied nearly 4-fold amongst the CEPs studied, and chambers with the least permeable CEPs appeared to have lower aggrecan, collagen-2, and matrix metalloproteinase-2 gene expression, as well as a significantly shorter viable distance from the CEP/nutrient interface. Increasing chamber cell density shortened the viable distance; however, this effect was lost for low-diffusivity CEPs, which suggests that these CEPs may not provide enough nutrient diffusion to satisfy cell demands. Solute diffusivity in the CEP was associated with biochemical composition: low-diffusivity CEPs had greater amounts of collagen and aggrecan, more mineral, and lower cross-link maturity.

CONCLUSIONS

CEP transport properties dramatically affect NP cell survival/function. Degeneration-related CEP matrix changes could hinder the success of biologic therapies that require increased nutrient supply.

Matrix modification for enhancing the transport properties of the human cartilage endplate to improve disc nutrition

Poor solute transport through the cartilage endplate (CEP) impairs disc nutrition and could be a key factor that limits the success of intradiscal biologic therapies. Here we demonstrate that treating the CEP with matrix metalloproteinase-8 (MMP-8) reduces the matrix constituents that impede solute uptake and thereby improves nutrient diffusion. Human CEP tissues harvested from four fresh cadaveric lumbar spines (age range: 38-66 years old) were treated with MMP-8. Treatment caused a dose-dependent reduction in sGAG, localized reductions to the amount of collagen, and alterations to collagen structure. These matrix modifications corresponded with 16-24% increases in the uptake of a small solute (376 Da). Interestingly, the effects of MMP-8 treatment depended on the extent of non-enzymatic glycation: treated CEPs with high concentrations of advanced glycation end products (AGEs) exhibited the lowest uptake compared to treated CEPs with low concentrations of AGEs. Moreover, AGE concentrations were donor-specific, and the donor tissues with the highest AGE concentrations appeared to have lower uptake than would be expected based on the initial amounts of collagen and sGAG. Finally, increasing solute uptake in the CEP improved cell viability inside diffusion chambers, which supports the nutritional relevance of enhancing the transport properties of the CEP. Taken together, our results provide new insights and in vitro proof-of-concept for a treatment approach that could improve disc nutrition for biologic therapy: specifically, matrix reduction by MMP-8 can enhance solute uptake and nutrient diffusion through the CEP, and AGE concentration appears to be an important, patient-specific factor that influences the efficacy of this approach.

Application of Finite Element Analysis for Investigation of Intervertebral Disc Degeneration: from Laboratory to Clinic

Due to the ethical concern and inability to detect inner stress distributions of intervertebral disc (IVD), traditional methods for investigation of intervertebral disc degeneration (IVDD) have significant limitations. Many researchers have demonstrated that finite element analysis (FEA) is an effective tool for the research of IVDD. However, the specific application of FEA for investigation of IVDD has not been systematically elucidated before. In the present review, we summarize the current finite element models (FEM) used for the investigation of IVDD, including the poroelastic nonlinear FEM, diffusive-reactive theory model and cell-activity coupled mechano-electrochemical theory model. We further elaborate the use of FEA for the research of IVDD pathogenesis especially for nutrition and biomechanics associated etiology, and the biological, biomechanical and clinical influences of IVDD. In addition, the application of FEA for evaluation and exploration of various treatments for IVDD is also elucidated. We conclude that FEA is an excellent technique for research of IVDD, which could be used to explore the etiology, biology and biomechanics of IVDD. In the future, FEA may help us to achieve the goal of individualized precision therapy.

Leaping the hurdles in developing regenerative treatments for the intervertebral disc from preclinical to clinical

Chronic back and neck pain is a prevalent disability, often caused by degeneration of the intervertebral disc. Because current treatments for this condition are less than satisfactory, a great deal of effort is being applied to develop new solutions, including regenerative strategies. However, the path from initial promising idea to clinical use is fraught with many hurdles to overcome. Many of the keys to success are not necessarily linked to science or innovation. Successful translation to clinic will also rely on planning and awareness of the hurdles. It will be essential to plan your entire path to clinic from the outset and to do this with a multidisciplinary team. Take advice early on regulatory aspects and focus on generating the proof required to satisfy regulatory approval. Scientific demonstration and societal benefits are important, but translation cannot occur without involving commercial parties, which are instrumental to support expensive clinical trials. This will only be possible when intellectual property can be protected sufficiently to support a business model. In this manner, commercial, societal, medical, and scientific partners can work together to ultimately improve patient health. Based on literature surveys and experiences of the co-authors, this opinion paper presents this pathway, highlights the most prominent issues and hopefully will aid in your own translational endeavors.

The Quantitative Structural and Compositional Analyses of Degenerating Intervertebral Discs Using Magnetic Resonance Imaging and Contrast-Enhanced Micro-Computed Tomography

The intervertebral disc (IVD) is susceptible to degenerative changes that are associated with low back pain. Murine models are often used to investigate the mechanistic changes in the development, aging, and diseased states of the IVD, yet the detection of early degenerative changes in structure is challenging because of the minute size of the murine IVDs. Histology is the gold standard for examining the IVD structure, but it is susceptible to sectioning artifacts, spatial biases, and requires the destructive preparation of the sample. We have previously demonstrated the feasibility of using Ioversol for the contrast-enhanced micro-computed tomography (microCT) to visualize and quantitate the intact healthy murine IVD. In this work, we demonstrate utility of this approach to monitor the longitudinal changes of in vitro nucleolytic- and mechanical injury- degeneration models of the murine discs and introduce novel quantitative metrics to characterize the structure and composition of the IVD. Moreover, we compared the imaging quality and quantitation of these in vitro models to magnetic resonance imaging (MRI) and histology. Stab puncture, trypsin injection, and collagenase injection all induced detectable and significant changes in structure and composition of the discs overtime. Compared to MRI and histology, contrast-enhanced microCT produced superior images that capture the degenerative progression in these models. Contrast-enhanced microCT was also capable of monitoring the structural deteriorations via the changes in disc height and volume, and novel the nucleus pulposus intensity/disc intensity (NI/DI) parameter provides a surrogate measure of proteoglycan composition (R = 0.96). Overall, this approach allows for the nondestructive monitoring of the structure and composition of the IVD at very high resolutions.

Simulation of water content distributions in degenerated human intervertebral discs

The objective of this study was to investigate the spatial and temporal variations of water content in intervertebral discs during degeneration and repair processes. We hypothesized that the patterns of water content distribution in the discs are related to the intensity patterns observed in T2-weighted MRI images. Water content distributions in the mildly (e.g., 80% viable cells in the disc, 2.3% decrease in disc height) and moderately (e.g., 40% viable cells in the disc, 9.3% decrease in disc height) degenerated discs were predicted using a finite element model. The variation of water content in the degenerated discs treated with three biological therapies (i.e., increasing the cell density in the nucleus pulposus [Case I], increasing glycosaminoglycan synthesis rate in the nucleus pulposus [Case II], and decreasing glycosaminoglycan degradation rate in the nucleus pulposus [Case III]) were also predicted. It was found that two patterns of water content distributions, a horizontal region with lower water content at the mid-axial plane of nucleus pulposus and a spot with higher water content at the posterior region, were shown during the degeneration progress for the disc simulated in this study. These two patterns disappeared after treatment in Case I, but in Case II and Case III. The implication of these patterns for the horizontal gray band and high intensity zone in T2-weighted MRI images was discussed. This study provided new guidance to develop a novel method for diagnosing disc degeneration and assessing outcomes of biological therapies with MRI techniques. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:147-153, 2017.

Biological Responses of the Immature Annulus Fibrosus to Dynamic Compression in a Disc Perfusion Culture

Mechanical stimuli participate in disc development and remodelling. However, the effects of mechanical load on the immature annulus fibrosus (AF) are largely unclear. This study aimed to investigate how the immature AF responded to dynamic compressive magnitude and duration. Immature porcine discs were bioreactor-cultured for 7 days and then dynamically compressed at various magnitudes (0.1, 0.2, 0.4, 0.8 and 1.3 MPa at a frequency of 1.0 Hz for 2 h/day) and durations (1, 2, 4 and 8 h/day at a magnitude of 0.4 MPa and a frequency of 1.0 Hz). Non-compressed discs were used as controls. The immature AF tissue was analysed for histology, gene expression (aggrecan, collagen I, ADAMTS-4, MMP-3, TIMP-1 and TIMP-3), biochemical content of glycosaminoglycans (GAG) and hydroxyproline (HYP) and aggrecan immunohistochemical staining. In the lower-compressive-magnitude groups (0.1, 0.2 and 0.4 MPa), the immature AF showed an up-regulation in the expression of matrix genes, GAG and HYP content and aggrecan deposition. In the compression duration groups, the GAG and HYP content and aggrecan deposition declined to a minimum in the 8-hour group, in which a catabolic gene expression profile was found. In conclusion, this study indicated that the effects of dynamic compression on the immature AF are magnitude and duration dependent and that catabolic remodelling within the immature AF can be induced by high compressive magnitudes and long compressive durations.