Ionomycin Ameliorates Hypophosphatasia via Rescuing Alkaline Phosphatase Deficiency-mediated L-type Ca²⁺ Channel Internalization in Mesenchymal Stem Cells

Abstract

<jats:title>Abstract</jats:title><jats:p>Loss-of-function mutations in ALPL result in hypophosphatasia (HPP), an inborn error of metabolism that causes skeletal mineralization defect. In adults, main clinical involvement includes early loss of primary or secondary teeth, osteoporosis, bone pain, chondrocalcinosis, and fractures. However, guidelines for the treatment of adults with HPP are not available. Here, we show that ALPL deficiency caused reduction of intracellular Ca<jats:sup>2+</jats:sup> influx resulting in osteoporotic phenotype due to downregulated osteogenic differentiation and upregulated adipogenic differentiation in both human and mouse BMSCs. To elevate intracellular level of calcium in bone marrow mesenchymal stem cells (BMSCs) by ionomycin treatment rescues the osteoporotic phenotype in <jats:italic>alpl</jats:italic><jats:sup><jats:italic>+/-</jats:italic></jats:sup> mice and BMSC-specific (<jats:italic>Prrx1-alpl</jats:italic><jats:sup><jats:italic>-/-</jats:italic></jats:sup>) conditional alpl knockout mice. Mechanistically, ALPL is required to maintain intracellular Ca<jats:sup>2+</jats:sup> influx by regulating L-type Ca<jats:sup>2+</jats:sup> channel trafficking <jats:italic>via</jats:italic> binding to the α2δ subunits, which regulates the internalization of L-type Ca<jats:sup>2+</jats:sup> channel. Decreased Ca<jats:sup>2+</jats:sup> flux inactivates Akt/GSK3β/β-catenin signaling pathway that regulates lineage differentiation of BMSCs. This study identifies a previous unknown role of ectoenzyme ALPL in maintenance of calcium channel trafficking to keep stem cell lineage differentiation and bone homeostasis. Accelerating Ca<jats:sup>2+</jats:sup> flux through L-type Ca<jats:sup>2+</jats:sup> channel by ionomycin treatment may be a promising therapeutic approach for adult HPP patients.</jats:p><jats:sec><jats:title>One Sentence Summary</jats:title><jats:p>ALP regulates internalization of L-Type Ca<jats:sup>2+</jats:sup> Channel of BMSCs in Hypophosphatasia.</jats:p></jats:sec>