Mimicking healthy and degenerated intervertebral discs to address back pain

Intervertebral disc degeneration is coordinated by cells and the cause is a complex interaction of both biological and mechanical factors. The intervertebral disc consists of a central nucleus pulposus (NP), which is surrounded by the annulus fibrosus and covered by the cartilage endplates. Degeneration often begins in the central NP, which is therefore a promising target for regenerative treatments. Such treatments must first be tested in cell, tissue, and animal tests before they can be tested in humans. For his thesis, Salzer used NP tissue from cow tails, to study the healthy and degenerated intervertebral disc.

Inducing degeneration in healthy NP tissue

Through a thorough literature review of methods to induce degeneration in healthy intervertebral discs, Salzer discovered that there are two ways to do this. The first is by mimicking human degeneration as closely as possible; the second way mimics only certain processes of degeneration. For his PhD research, Salzer used the second way and mimicked one of the first measurable processes of degeneration, namely a decrease in so-called proteoglycans in the NP. These proteoglycans make the NP tissue swell and are important for the biomechanical function of the intervertebral disc. But when NP tissue is cultured in the laboratory and swells, it will lose its structure.

Therefore, Salzer developed a culture container which controls swelling. By injecting an enzyme that cuts proteoglycans, the potential for swelling could be reduced so that it was comparable to literature values described for mild human degeneration. A biomaterial containing many proteoglycans and bioactive factors was then injected to study if the degeneration can be reversed. Here, no biomechanical improvement was found but it could be confirmed that there was a positive effect on cells and its surrounding extracellular matrix.

Bioreactor to mimic spinal loading

To further mimic the situation in the body, Salzer developed a bioreactor for axial compression, which allowed dynamic loading similar to the human spine. Dynamic loading was previously shown to be an important factor for intervertebral disc cultures in the laboratory, but at the same time could also increase cell activity. Because this could negatively affect cell vitality, he compared dynamic loading with non-dynamic loading under the conditions of healthy or degenerated tissue. In doing so, he found that when he mimicked the conditions of a degenerative intervertebral disc and applied dynamic loading, cell activity increased. This can lead to a decrease in the number of living cells in the center of the tissue and is important to consider for cell therapies.

Progenitor cells of the notochord

Finally, "notochordal progenitor cells", a type of cell found only in young human intervertebral discs, were injected into a degenerated NP using a carrier material that is bioactive and biodegradable. After four weeks of culture, these precursor cells were able to survive, which is a pre-requisite for cell therapies.

The initial results of this bioreactor system developed for this thesis are promising for regeneration using cells and biomaterials. Subsequent studies should then include the unfavorable environment of NP tissue. Salzer's NP model, he says, allows to study the central role of the NP in intervertebral disc degeneration and regeneration and is a relevant model as an intermediate step for cell and animal studies.

With an increase in the complexity and specificity of ex vivo cultures, the possibilities of replacing in vivo animal studies are increasing, hopefully leading to improved clinical translation for patients with back pain in the future.

Title of PhD thesis: “A nucleus pulposus explant model to study intervertebral disc degeneration and regeneration”

Supervisors: Keita Ito and Marianna Tryfonidou (UU)