Cartilage intermediate layer protein promotes lumbar disc degeneration

Lumbar disc disease (LDD) is one of the most common musculoskeletal disorders, and accompanies intervertebral disc degeneration. CILP encodes cartilage intermediate layer protein, which is highly associated with LDD. Moreover, CILP inhibits transcriptional activation of cartilage matrix genes in nucleus pulposus (NP) cells in vitro by binding to TGF-β1 and inhibiting the phosphorylation of Smads. However, the aetiology and mechanism of pathogenesis of LDD in vivo are unknown. To demonstrate the role of CILP in LDD in vivo, we generated transgenic mice that express CILP specifically in the intervertebral disc tissues and assessed whether CILP exacerbates disc degeneration. Degeneration of the intervertebral discs was assessed using magnetic resonance imaging (MRI) and histology. The level of phosphorylation of Smad2/3 in intervertebral discs was measured to determine whether overexpressed CILP suppressed TGF-beta signalling. Although the macroscopic skeletal phenotype of transgenic mice appeared normal, histological findings revealed significant degeneration of lumbar discs. MRI analysis of the lumbar intervertebral discs indicated a significantly lower signal intensity of the nucleus pulposus where CILP was overexpressed. Intervertebral disc degeneration was also observed. The number of phosphorylation of Smad2/3 immuno-positive cells in the NP significantly was decreased in CILP transgenic mice compared with normal mice. In summary, overexpression of CILP in the NP promotes disc degeneration, indicating that CILP plays a direct role in the pathogenesis of LDD.