Age-related expression of molecular regulators of hypertrophy and maturation in articular cartilage

The purpose of this study was to determine changes in the expression of regulatory molecules in normal equine articular cartilage throughout development up to 18 months of age. The hypothesis was that expression of these regulatory molecules would decrease from birth to postpubescence. Cartilage was harvested from normal femoropatellar or scapulohumeral joints from 34 fresh horse cadavers. Horses were placed in four age groups [prenatal (n = 5); prepubertal, 0-6 months (n = 11); pubertal, 7-14 months (n = 13); and postpubertal, 15-18 months (n = 5)]. Indian hedgehog (Ihh), Gli1, Gli3, Patched1 (Ptc1), Smoothened (Smo), Noggin, bone morphogenetic protein-6 (BMP-6), BMP-2, parathyroid hormone-related peptide (PTHrP), and PTH/PTHrP receptor mRNA expression levels were evaluated by real-time quantitative PCR. Spatial tissue mRNA and protein expression was determined by in situ hybridization and immunohistochemistry. The expression of PTHrP decreased (p = 0.002) in the pubertal group, while PTH/PTHrP receptor expression significantly increased (p = 0.001). No significant difference was found between groups for Ihh (p = 0.6) or Smo (p = 0.3) expression. In contrast, there was significantly increased expression of Ptc1 (p = 0.006), Gli1 (p = 0.04), and Gli3 (p = 0.007) in the pubertal group, and Gli3 (p = 0.007) remained elevated in the postpubertal group. The expression of BMP-6 significantly increased from prenatal to postnatal groups (p = 0.03) while BMP-2 expression increased during puberty and postpuberty (p = 0.03). The changes in expression of hedgehog and BMP signaling molecules in articular cartilage during postnatal development have not been shown previously. The increased expression of hedgehog receptor and transcription factors during puberty may indicate maturation of the deep articular layer during this time period.