Date Published: April 10, 2019
Publisher: Public Library of Science
Author(s): Aaron Dolor, Sara L. Sampson, Ann A. Lazar, Jeffrey C. Lotz, Francis C. Szoka, Aaron J. Fields, Lachlan J. Smith.
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.
Low back pain is the most common and most costly musculoskeletal condition , and is significantly associated with intervertebral disc degeneration . Current medical interventions for disc degeneration are surgical in nature and are often unsuccessful, which motivates development of noninvasive alternatives. Noninvasive treatments to regenerate the disc and alleviate pain are largely experimental and focus on implanting new cells to produce matrix lost during degeneration [3–5], or injecting growth factors , genes , or other small molecules [8, 9] to stimulate matrix synthesis or reduce catabolism and inflammation. Importantly, all of these biologic therapies require a rich nutrient supply to sustain higher cell numbers or metabolic rates. However, the avascular and degenerated disc has a poor nutrient supply , which may limit the utility of biologic therapies [11–13]. Development of treatment strategies to improve disc nutrition may therefore expand the application and utility of biologic therapy as well as inform alternative approaches for slowing or reversing degeneration.
These results show that MMP-8 treatment improves the uptake of a small solute into cadaveric human CEP tissues (p = 0.0004 to 0.06). This increase in uptake was mainly driven by a greater amount of pore space available to the solutes. For example, MMP-8 treatment led to dose-dependent reductions in sGAG in the CEP by up to 20%, which coincided with increases in the uptake of fluorescein by 16–24%. MMP-8 treatment also caused localized reductions to the amount of collagen and alterations to collagen structural order. Importantly, the effects of MMP-8 treatment depended significantly on the extent of non-enzymatic cross-linking of the CEP matrix: overall, treated CEP tissues with the lowest AGE concentrations showed 20% greater fluorescein uptake following treatment compared to treated tissues with the highest AGE concentrations. Moreover, AGE concentrations in the CEP were donor-specific, and the donor with CEP tissues having the highest AGE concentrations showed the lowest solute uptake, despite having a similar amount of sGAG initially and the lowest amount of collagen compared to the other donors. Finally, increasing solute uptake improved cell viability inside diffusion chambers, which supports the biologic relevance of enhancing the transport properties of the CEP and indicates increased nutrient diffusion. Poor nutrient diffusion through the CEP impairs disc nutrition [12, 17] and may limit the efficacy of regenerative therapy [13, 17]. Yet, we are aware of no treatment approaches for improving diffusion. Taken together, our results provide new insights and in vitro proof-of-concept into a treatment approach that could potentially improve 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.