Abcc6 null mice a model for mineralization disorder PXE show vertebral osteopenia without enhanced intervertebral disc calcification with aging

Abstract

Chronic low back pain is a highly prevalent health condition intricately linked to intervertebral disc degeneration. One of the prominent features of disc degeneration that is commonly observed with aging is dystrophic calcification. ATP-binding cassette sub-family C member 6 (ABCC6), a presumed ATP efflux transporter, is a key regulator of systemic levels of the mineralization inhibitor pyrophosphate (PPi). Mutations in ABCC6 result in pseudoxanthoma elasticum (PXE), a progressive human metabolic disorder characterized by mineralization of the skin and elastic tissues. The implications of ABCC6 loss-of-function on pathological mineralization of structures in the spine, however, are unknown. Using the ABCC6-/- mouse model of PXE, we investigated age-dependent changes in the vertebral bone and intervertebral disc. ABCC6-/- mice exhibited diminished trabecular bone quality parameters at 7-months which remained significantly lower than the wild-type mice at 18 months-of-age. ABCC6-/- vertebrae showed increased TRAP staining along with decreased TNAP staining, suggesting an enhanced bone resorption as well as decreased bone formation. Surprisingly, however, loss of ABCC6 resulted only in a mild, aging disc phenotype without evidence of dystrophic mineralization. Finally, we tested the utility of oral K3Citrate to treat the vertebral phenotype since it is shown to regulate hydroxyapatite mechanical behavior. The treatment resulted in inhibition of osteoclastic response and an early improvement in mechanical properties of the bone underscoring the promise of potassium citrate as a therapeutic agent. Our data suggest that although ectopic mineralization is tightly regulated in the disc, loss of ABCC6 compromises vertebral bone quality and dysregulates osteoblast-osteoclast coupling. Author SummaryInherited mutations in the ABCC6 transporter gene results in mineralization, often in the form of hydroxyapatite, of connective tissues throughout the body, predominantly affecting the skin, eyes, and blood vessels. Functional loss of ABCC6 causes reduced levels of the potent mineralization inhibitor pyrophosphate (PPi) in blood resulting in these pathologies. Pathological mineralization is also a prominent feature of intervertebral disc degeneration, but the role of ABCC6 and systemic PPi levels and its correlation to disc mineralization and vertebral bone health has remained unexplored. In this study, we show for the first time that loss of ABCC6 in mice results in significant decline in vertebral bone quality and mild age-related disc degeneration without increased incidence of abnormal mineralization. Importantly, treatment of ABCC6 deficient mice with K3Citrate resulted in restoration of early cellular changes which drive bone loss and mechanical function of the vertebrae. In summary, our data reveal that ABCC6 is dispensable for mineralization prevention in the intervertebral disc. Unexpectedly, we found that vertebral bone quality and bone cell activities are linked to ABCC6 function.