Cell and tissue interaction in intervertebral disc maintenance

Abstract

The human spine consists of 23 intervertebral discs (IVDs) adjoining the vertebral bodies. These discs enable motion and function to distribute loads from daily activities. Tissues of intervertebral discs are similar to those of diarthordial joints; a thin layer of cartilage lines the interface between the joint and bony elements, and a center rich in extracellular matrix molecules promotes lubrication and sustains osmotic pressure. Like the pathophysiology of other cartilaginous joints, intervertebral discs undergo biomechanical and structural changes as a result of aging and mechanical insults. This affects quality of life and presents a significant burden to healthcare systems globally. The etiology of intervertebral disc degeneration is not fully understood, but is a consequence of the changing structure and environment of its three interconnecting components, the nucleus pulposus, annulus fibrosus, and cartilage endplate, which function cooperatively to transmit load and regulate cellular, biochemical, and nutritional properties of the IVD. Abnormal changes in one or more of these disc tissues compromises integrity and culminates in a degenerated disc state. Mechanistic insights into disc pathology can be gained through an understanding of the development and homeostasis of intervertebral disc tissues. In this thesis, I will highlight the importance of the cooperative action of disc tissues for normal disc function. Previously, we showed a novel lineage extension of Col10a1-expressing hypertrophic chondrocytes in the cartilage endplate to flattened cells in the inner annulus fibrosis. Here, I confirmed that these endplate-derived cells express markers typical of inner annulus fibrosis cells, suggesting they harbor a functional role in the IVD niche. Furthermore, using tail looping and unlooping to apply differential mechanical loads on IVD, I discovered that distention of the annulus fibrous favors the transition of endplate hypertrophic chondrocytes to the inner annulus fibrosis. This indicates that cells in the endplate can replenish lost cells in annulus fibrosus for tissue homeostasis or repair. This finding has potential implications in regenerative strategies that use endogenous progenitor-like cells to regain structural and functional competencies of the IVD. The role of hypertrophic chondrocytes in IVD function was further assessed using a mouse model for metaphyseal chondrodysplasia type Schmid (MCDS). MDCS is a rare disease with protein folding mutations in the COL10A1 gene that cause ER stress activation in hypertrophic chondrocytes, impairing progression of chondrocyte differentiation in growth plates of long bones. Similar activation of ER stress occurs in hypertrophic chondrocytes of the vertebral bodies and cartilage endplate of the spine. Surprisingly, changes are also observed in the nucleus pulposus of MCDS mice. These include an atypical cell phenotype and a reduced matrix aggrecan, which are hallmarks of disc degeneration in humans. How cells in the endplate function to modulate the nucleus pulposus is unclear. It is possible that endplate cells control cell functionality in the nucleus pulposus via a paracrine signaling mechanism or by regulating oxygen and nutrient availability in the disc center. Overall, these findings provide a strong support for cooperation between cells and tissues of the IVD to sustain disc integrity, the failure of which causes malfunction.