To the Editor:

We appreciate Li and colleagues' interest in our study, and we would like to address their comments as follows. Our data are based on studies not only from genetic mouse models but also from human tissue. In our study, we included β-catenin immunohistochemical analysis using disc tissue derived from patients with disc degeneration and from normal subjects. We found that β-catenin levels were highly up-regulated in most specimens from patients with disc degeneration. In contrast, β-catenin levels were undetectable in samples from normal subjects. We are currently performing detailed studies to determine if the severity or the grade of disc degeneration is correlated with β-catenin protein levels in patient samples.

We generated β-catenin conditional activation mice to mimic the pathologic up-regulation of human β-catenin; we have been using this mouse model to investigate the role of β-catenin signaling in disc tissue during the development of disc degeneration. We found that overexpression of β-catenin in disc cells led to severe osteophyte formation within 3 months, which was associated with significant changes in expression of extracellular matrix genes in disc tissue. We further demonstrated that activation of β-catenin enhances runt-related transcription factor 2–dependent transcriptional activation of Mmp13. In addition, genetic ablation of the Mmp13 or Adamts5 gene in β-catenin conditional activation mice ameliorated the phenotype of disc defects observed in these mice. Since overexpression of β-catenin in disc tissue causes such severe pathologic effects, we propose that β-catenin signaling is a key mediator of a signaling pathway that regulates disc tissue function during normal development and in pathologic processes during disc degeneration.

Anatomic structures differ between human and mouse spines, and the ultimate goal of our ongoing studies is to understand the role of β-catenin signaling in human disc tissue. Genetic mouse models are an essential tool for determining the signaling pathways and for testing clinically relevant hypotheses. As noted above, our data were derived from both of these essential components (animal and human studies).

Patients with persistent back pain typically have lumbar spondylosis defined as degenerative changes in the IVDs and/or the facet joints (1–3). We are now investigating, by quantitative behavioral pain analyses in rodent models, whether dysregulation of β-catenin is associated with discogenic back pain in vivo.

Disc degeneration could be initiated from anulus fibrosus, end plate cartilage tissue, or nucleus pulposus tissue (4–8). Our analyses demonstrated that in Col2CreER mice, only inner anulus fibrosus and growth plate cartilage tissues are targeted, whereas nucleus pulposus is not (9). Because β-catenin up-regulation in nucleus pulposus cells is associated with cell senescence (10), we will address this issue by creating β-catenin conditional activation and conditional knockout mice using Shh-Cre (11) and Aggrecan-CreER (12) mice. Combining these mouse models with our Col2CreER studies, we will be able to define the specific roles of β-catenin signaling in different cell types within disc tissue.

In transgenic mice in which β-catenin is conditionally activated, β-catenin signaling was targeted at an early postnatal stage (age 2 weeks), and shortening of the length of spine tissue was associated with the up-regulation of β-catenin signaling. This suggests that β-catenin also plays a role in normal spine tissue development.

Although our current studies demonstrate the importance of β-catenin signaling in disc tissue, information on the upstream regulators and downstream target genes of β-catenin signaling in disc tissue remains elusive. While our preliminary studies demonstrate that Mmp13 and Adamts5 could be potential targets of β-catenin signaling, deletion of either of these genes could not fully protect against disc defects caused by β-catenin up-regulation. Studies aimed at understanding the upstream and downstream regulatory mechanisms of β-catenin signaling in disc cells are warranted.

Based on the evidence found by our group and others, we are confident in our conclusion that the β-catenin signaling pathway is involved in the pathogenesis of IVD degeneration. Our ongoing studies using transgenic animal models are aimed at revealing a novel pathway and molecular mechanisms that cannot be addressed with clinical protocols in humans and will hopefully establish new experimental avenues and novel research directions regarding disc degeneration and the specific pathophysiologic role of β-catenin signaling in spine development and disc homeostasis.

  • 1
    Kim JS, Kroin JS, Li X, An HS, Buvanendran A, Yan D, et al. The rat intervertebral disk degeneration pain model: relationships between biological and structural alterations and pain. Arthritis Res Ther 2011; 13: R165.
  • 2
    Kim JS, Kroin JS, Buvanendran A, Li X, van Wijnen AJ, Tuman KJ, et al. Characterization of a new animal model for evaluation and treatment of back pain due to lumbar facet joint osteoarthritis. Arthritis Rheum 2011; 63: 296673.
  • 3
    Ellman MB, Kim JS, An HS, Kroin JS, Li X, Chen D, et al. The pathophysiologic role of the protein kinase Cδ pathway in the intervertebral discs of rabbits and mice: in vitro, ex vivo, and in vivo studies. Arthritis Rheum 2011; 64: 19509.
  • 4
    Roberts S, Menage J, Urban JPG. Biochemical and structural properties of the cartilage endplate and its relation to the intervertebral disc. Spine (Phila Pa 1976) 1989; 14: 16674.
  • 5
    Adams MA, Freeman BJC, Morrison HP, Nelson IW, Dolan P. Mechanical initiation of intervertebral disc degeneration. Spine (Phila Pa 1976) 2000; 25: 162536.
  • 6
    Rannou F, Lee TS, Zhou RH, Chin J, Lotz JC, Mayoux-Benhamou MA, et al. Intervertebral disc degeneration: the role of the mitochondrial pathway in annulus fibrosus cell apoptosis induced by overload. Am J Pathol 2004; 164: 91524.
  • 7
    Adler JH, Schoenbaum M, Silberberg R. Early onset of disk degeneration and spondylosis in sand rats (Psammomys obesus). Vet Pathol 1983; 20: 1322.
  • 8
    Hirsch C, Schajowicz F. Studies on structural changes in the lumbar annulus fibrosus. Acta Orthop Scand 1953; 22: 184231.
  • 9
    Jin H, Shen J, Wang B, Wang M, Shu B, Chen D. TGF-β signaling plays an essential role in the growth and maintenance of intervertebral disc tissue. FEBS Lett 2011; 585: 120915.
  • 10
    Hiyama A, Sakai D, Risbud MV, Tanaka M, Arai F, Abe K, et al. Enhancement of intervertebral disc cell senescence by WNT/β-catenin signaling–induced matrix metalloproteinase expression. Arthritis Rheum 2010; 62: 303647.
  • 11
    Harfe BD, Scherz PJ, Tian H, McMahon AP, Tabin C. Evidence for an expansion-based temporal Shh gradient in specifying mammalian digit identities. Cell 2004; 118: 51728.
  • 12
    Henry SP, Jang CW, Deng JM, Zhang Z, Behringer RR, de Crombrugghe B. Generation of aggrecan-CreERT2 knockin mice for inducible Cre activity in adult cartilage. Genesis 2009; 47: 80514.

Meina Wang PhD*, Hee-Jeong Im PhD†, Di Chen MD, PhD†, * Yale University, New Haven, CT, † Rush University Medical Center, Chicago, IL.