Disruption of biomineralization pathways in spinal tissues of a mouse model of diffuse idiopathic skeletal hyperostosis

Equilibrative nucleoside transporter 1 (ENT1) mediates passage of adenosine across the plasma membrane. We reported previously that mice lacking ENT1 (ENT1−/−) exhibit progressive ectopic mineralization of spinal tissues resembling diffuse idiopathic skeletal hyperostosis (DISH) in humans. Here, we investigated mechanisms underlying aberrant mineralization in ENT1−/− mice. Micro-CT revealed ectopic mineralization of spinal tissues in both male and female ENT1−/− mice, involving the annulus fibrosus of the intervertebral discs (IVDs) of older mice. IVDs were isolated from wild-type and ENT1−/− mice at 2 months of age (prior to disc mineralization), 4, and 6 months of age (disc mineralization present) and processed for real-time PCR, cell isolation, or histology. Relative to the expression of ENTs in other tissues, ENT1 was the primary nucleoside transporter expressed in wild-type IVDs and mediated the functional uptake of [3H]2-chloroadenosine by annulus fibrosus cells. No differences in candidate gene expression were detected in IVDs from ENT1−/− and wild-type mice at 2 or 4 months of age. However, at 6 months of age, expression of genes that inhibit biomineralization Mgp, Enpp1, Ank, and Spp1 were reduced in IVDs from ENT1−/− mice. To assess whether changes detected in ENT1−/− mice were cell autonomous, annulus fibrosus cell cultures were established. Compared to wild-type cells, cells isolated from ENT1−/− IVDs at 2 or 6 months of age demonstrated greater activity of alkaline phosphatase, a promoter of biomineralization. Cells from 2-month-old ENT1−/− mice also showed greater mineralization than wild-type. Interestingly, altered localization of alkaline phosphatase activity was detected in the inner annulus fibrosus of ENT1−/− mice in vivo. Alkaline phosphatase activity, together with the marked reduction in mineralization inhibitors, is consistent with the mineralization of IVDs seen in ENT1−/− mice at older ages. These findings establish that both cell-autonomous and systemic mechanisms contribute to ectopic mineralization in ENT1−/− mice.