Molecular Mechanisms of ANKH Function and Regulation in Skeletal Mineralization

Abstract Progressive ankylosis homolog (ANKH) is a transmembrane protein essential for regulating bone mineralization through the export of nucleoside triphosphates, primarily ATP, and citrate into the extracellular matrix. Exported ATP is hydrolyzed by ectonucleotide pyrophosphatase/phosphodiesterase 1 into AMP and inorganic pyrophosphate (PPi). ANKH is widely expressed across tissues, where it limits ectopic mineralization of tissues and other roles. Its functional role is particularly prominent in mineralizing cells such as osteoblasts and chondrocytes that maintain the balance between mineral formation and inhibition required for healthy skeletal function. Mutations in ANKH cause two main mineralization disorders: craniometaphyseal dysplasia, characterized by progressive craniofacial bone thickening, and calcium pyrophosphate deposition disease, marked by crystal deposits causing arthritis-like joint symptoms. Functionally, ANKH operates within a coordinated regulatory axis with ectonucleotide pyrophosphatase/phosphodiesterase 1 and tissue-nonspecific alkaline phosphatase that governs extracellular PPi homeostasis. Although tissue-nonspecific alkaline phosphatase-mediated PPi hydrolysis contributes negligibly to extracellular inorganic phosphate (Pi) levels, this process remains essential for clearing PPi to prevent excessive inhibition of mineral deposition, thereby preserving the Pi/PPi ratio required for physiological mineralization. Emerging evidence also implicates ANKH in citrate export, influencing bone matrix composition and mechanical integrity. Further, ANKH expression and function are regulated by multiple signaling pathways including Wnt, tumor necrosis factor-alpha, and fibroblast growth factor, as well as by post-transcriptional modifications, although these regulatory mechanisms remain poorly characterized. Current mouse models, primarily knock-in lines carrying craniometaphyseal dysplasia-associated mutations, only partially recapitulate human phenotypes while comparable models for calcium pyrophosphate deposition disease associated mutations remain unavailable. Addressing these gaps by elucidating mutation-specific mechanisms, signaling networks, and developing improved animal models will be critical to advance targeted therapies for ANKH-related mineralization disorders and improve patient outcomes.