Changes in Nucleus Pulposus Cell Pools in “Healer” Mice for the Repair of Intervertebral Disc Degeneration
Authors: Zhang Ying , Xiong Chi , Chan Cheuk Wing , Sakai Daisuke , Chan Danny 
Published: May 2015

Presentation Method This abstract was a poster presentation. Introduction Intervertebral disc (IVD) degeneration is a major cause of back pain that can also lead to sciatica, affecting the quality of life. Current treatments are limited to salvage surgical operations. Biological treatments to relieve symptoms or to restore disc are not available, as we know little about the biology of IVD degeneration and its potential to regeneration. Although most people will develop disc degeneration with aging, there are individuals who are protected even at the age (older than 50 years) when over 90% of the population would succumb to the problem, suggesting the presence of protective genes. Furthermore, maintenance of progenitor cells within the nucleus pulposus (NP) is thought to play an important role in disc homeostasis. A hypothesis is that genetic factors, which can confer a protection against disc degeneration via better maintenance of resident progenitor cells. There exist strains of “healer” mice (MRL/MpJ and LG/J) that have better regenerative potentials of cartilage tissues.1,2 Thus, we propose to address the NP progenitor cell pools in these healer mice in relation to the degeneration and potential repair/regeneration potentials of the disc. Materials and Methods Good healer (MRL and LG/J) and poor healer (C57/BL6C, and SM/J) mice were used in this study. Histological comparison of tail disc sections was assessed from 8 to 24 weeks of age. Progenitor cell pools and differentiated NP cells were assessed using immunohistochemistry using specific cell markers, Tie-2 and disialoganglioside (GD2), that were recently identified.3 Tail looping at 8 weeks of age for a fixed period was used as an environmental perturbation that will induce degeneration. Unlooping the tail after the period of looping can assess healing processes with appropriate controls. Results A comparison of MRL and C57 mice showed neither observable histological differences, nor signs of degenerative processes from 8 to 24 weeks of age. Following tail looping for 4, 5, 6, and 8 weeks, there were significant distortion of the annulus fibrosus (AF) and NP at the compressed and distended sides; in terms of loss of NP cells, AF tears and ruptures, and cell death in the AF. After the tails are unlooped for 4 weeks, there are restoration of NP and AF structures such as cell number in both MRL and C57 mice. However, superior healing is seen for mice for MRL at all time-points studied; especially in TL6/TL7, TL7/TL8, and TL8/TL9 disc levels, in which the disc structure restores better via continuous expansion of NP region, cell repopulation, and lamellae orientation recovers in the compressed AF sides with a clear NP AF boundary. In C57 mice, the AF lamellae structure remained disorganized following unlooping. Interestingly, in the absence of tail looping, SM/J tail discs already showed severe degeneration even at the age of 8 weeks, while that of LG/J mice were relatively normal, suggesting an impact on developmental or maturation in SM/J IVDs. Immunohistochemistry analysis of progenitors-related marker Tie2, GD2, and Angiopoietin 1 shows different expression pattern in MRL and C57. Current data show that GD2 positive cells may be the functioning NP cell in disc maintenance, and the extracellular matrix play an importance role in regulating progenitor cell functions. Conclusion By comparing the genetically different healer MRL, LG/J, and nonhealer C57 and SM/J, we have shown that a population of novel-marked NP progenitors may play a key role in the maintaining, healing, or regenerative of IVD structure and function. Abnormal persistent mechanical loading of the spine has negative effects on the healing processes. Furthermore, there appears to be threshold for the disc to self-repair, likely to be related to the onset of fibrotic events. References Clark LD, Clark RK, Heber-Katz E. A new murine model for mammalian wound repair and regeneration. Clin Immunol Immunopathol 1998;88(1):35–45 Rai MF, Hashimoto S, Johnson EE, et al. Heritability of articular cartilage regeneration and its association with ear wound healing in mice. Arthritis Rheum 2012;64(7):2300–2310 Sakai D, Nakamura Y, Nakai T, et al. Exhaustion of nucleus pulposus progenitor cells with ageing and degeneration of the intervertebral disc. Nat Commun 2012;3:1264