Characterization of HOPX expression in notochord/nucleus pulposus development and aging

Abstract

Low back pain is a prominent disability complication that was shown to be correlated to intervertebral disc degeneration. As opposed to a healthy hydrated intervertebral disc, which provides support and withstands mechanical loading in an upright posture, a degenerated disc loses its sponginess and is stiffened due to changes in the extracellular matrix (ECM) composition of a crucial component of the disc, the nucleus pulposus (NP). During development, notochord gives rise to NP, and direct descendant cells of the notochord populate the NP as notochordal-like cells (NCLs). These NCLs express specific sets of genes including Keratins, Type II Collagen, and other matrix components which are thought to maintain the NP in a healthy, hydrated state through natural homeostasis. Yet throughout development and aging, the NCL population diminishes and was observed to be replaced by alternative types of cells, including chondrocyte-like cells (CLCs) and fibroblast-like cells (FBLs). FBLs are considered a hallmark of degeneration as the appearance of the population of cells is associated with a change in the ECM composition. To properly characterize the cells in the NP across the course of development and aging, HOPX was identified as a novel marker for the NCL population via single-cell RNA sequencing (scRNA-seq). The question arose, does HOPX take part in the development of the notochord and NP? More importantly, is HOPX essential to maintain a healthy IVD? To address these research questions, the HOPX expression pattern was characterized in this study by immunohistochemistry in mouse and human samples across various developmental time points. This study showed that HOPX is expressed together with other NCL markers, e.g. T, and the expression diminishes according to age. On top of that, HOPX was found to be expressed in human fetal notochord, and NP tissue, where expression can also be observed in human adult healthy NP as well. HOPX expression was absent only in human degenerated NP. In addition, with an in vitro culture system, this study showed that HOPX expression can be “re-expressed” through altering stiffness of the culture environment. These findings suggested that HOPX might potentially pose as a candidate marker for indicative of the degeneration process. The expression of HOPX might be controlled by mechanical signals, which can be exerted onto the cells via changes in the extracellular matrix composition in the degenerated NP. Regarding known functions of HOPX in alternative systems, HOPX might also pose as a key regulator to develop and maintain nucleus pulposus in a healthy state.