Defining the phenotype of young healthy nucleus pulposus cells: recommendations of the Spine Research Interest Group at the 2014 annual ORS meeting

Abstract

Low back pain is a major physical and socioeconomic problem. Degeneration of the intervertebral disc and especially that of nucleus pulposus (NP) has been linked to low back pain. In spite of much research focusing on the NP, consensus among the research community is lacking in defining the NP cell phenotype. A consensus agreement will allow easier distinguishing of NP cells from annulus fibrosus (AF) cells and endplate chondrocytes, a better gauge of therapeutic success, and a better guidance of tissue-engineering-based regenerative strategies that attempt to replace lost NP tissue. Most importantly, a clear definition will further the understanding of physiology and function of NP cells, ultimately driving development of novel cell-based therapeutic modalities. The Spine Research Interest Group at the 2014 Annual ORS Meeting in New Orleans convened with the task of compiling a working definition of the NP cell phenotype with hope that a consensus statement will propel disc research forward into the future. Based on evaluation of recent studies describing characteristic NP markers and their physiologic relevance, we make the recommendation of the following healthy NP phenotypic markers: stabilized expression of HIF-1α, GLUT-1, aggrecan/collagen II ratio >20, Shh, Brachyury, KRT18/19, CA12, and CD24.

Keywords: cell phenotype; intervertebral disc; nucleus pulposus.

Figure 1

Morphology of large notochordal cells (NCs) versus small nucleus pulposus cells (SNPCs) from organ cultured porcine discs (A). Control group and (B) Pressurization group. Black arrows, NCs; blue arrows, SNPCs. A large NC was defined as nuclei in direct contact and surrounded by large vacuoles, and an SNPC was defined as nuclei surrounded by matrix with no contact with vacuoles. V, vacuoles (green arrows). Scale bar = 50 μm. Image licensed under Creative Commons Attribution License 4.0 and reproduced with permission from Purmessur D, et al. 2013. Dynamic pressurization induces transition of notochordal cells to a mature phenotype while retaining production of important patterning ligands from development. Arthritis Res. Ther. 15:R122.

Figure 2

Lineage study in control and mutant IVDs. A, B. Detection of fluorescence in frozen sections of NP isolated from E15.5 (A) and 1 month (B) HIF-1α^f/f (a,e,i), HIF-1α^f/f;mTmG (b,f,j), Foxa2^iCre;HIF-1α^f/+;mTmG (c,g,k) and Foxa2^iCre;HIF-1α^f/f;mTmG (d,h,l) mice, respectively. Red fluorescence (a–d), green fluorescence (e–h) and merged filters (i–l) are shown. Red signal is membrane-bound tdtomato fluorescent protein indicating cells in which recombination has not occurred. Green signal is membrane-bound EGFP indicating cells in which Cre mediated recombination has occurred. Bar = 100 μm. Image reproduced with permission from Merceron C, et al. 2014. Loss of HIF-1α in the Notochord Results in Cell Death and Complete Disappearance of the Nucleus Pulposus. PLoS One. 9:e110768.