ENPP1 downregulation and FGF23 upregulation in growth‐related calcification of the tibial tuberosity in rats

During tibial tuberosity growth, superficial and deep portions can be observed; however, the deep portion is not observed after the growth period, as it develops into bone tissues. Calcification in vivo is known to be constitutively suppressed by ectonucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) but promoted by tissue-nonspecific alkaline phosphatase (TNAP). FGF23 promotes calcification of enthesis. Gene expression of FGF23 increased rapidly at 13W in this study. Therefore, the tibial tuberosity is speculated to develop via Enpp1 downregulation and Tnap upregulation; however, the understanding of these processes remains unclear. Hence, in the present study, we aimed to explore the age-related structural changes and underlying gene expression changes in the tibial tuberosity of rats. Male Wistar rats were divided into three groups (3-, 7-, and 13-week-old; eight each). The tibial tuberosity superficial and deep portions were clearly observed in 3- and 7-week-old rats, but the presence of the deep portion was not confirmed in 13-week-old rats. The extracellular matrix of hypertrophic chondrocytes was calcified. Furthermore, the Enpp1 expression was the highest in 3-week-old rats and decreased with growth. The TNAP expression did not differ significantly among the groups. The deep portion area was significantly lower in 3-week-old rats than in 7-week-old rats. Generally, the extracellular matrix of the immature chondrocytes is not calcified. Therefore, we speculated that the cartilaginous tibial tuberosity calcifies and ossifies with growth. The Enpp1 expression decreased with growth, whereas the Tnap expression remained unchanged. Thus, we surmise that the tibial tuberosity calcifies with growth and that this process involves Enpp1 downregulation and FGF23 upregulation. As Osgood-Schlatter disease is closely related to the calcification of the tibial tuberosity, these findings may help clarify the pathogenesis of this disease.