Secretory ferritins mediate iron delivery during magnetite tooth formation in chitons
Iron is essential to most organisms, yet unbound iron is toxic, necessitating tight regulation of its storage and transport. Chitons concentrate extraordinary amounts of iron in radular tissue to form magnetite-mineralized teeth, but how iron is supplied to developing teeth remains unclear. Here, synchrotron radiation X-ray fluorescence mapping resolved the spatiotemporal redistribution of iron from the radular epithelium toward developing teeth. Integrated transcriptomic and proteomic analyses identified secretory ferritin subunits in radular tissue whose genes were highly expressed in the midgut gland and whose proteins were also present in the hemolymph. In vivo fluorescence tracking showed that circulating ferritin was selectively taken up by the radular epithelium, and following injection of Ag-loaded hemolymph ferritin, Ag was detected in developing teeth. Together, these findings support a model in which secretory ferritins produced in the midgut gland transport iron through the hemolymph to the radula, enabling the pronounced iron accumulation required for tooth mineralization.