Astrocytes express DMT1 and transferrin receptors, which transport iron thus activating Ca2+signalling: possible role in neuroprotection against iron overload?

ABSTRACT Iron is the fundamental element for numerous physiological functions. Reduced ferrous (Fe 2+ ) and oxidized ferric (Fe 3+ ) are the two ionized iron states in the living organisms. In the cell membrane, divalent metal ion transporter 1 (DMT1) is responsible for cellular uptake of Fe 2+ , whereas transferrin receptors (TFR) carry transferrin (TF)-bound Fe 3+ . In this study we performed, for the first time, detailed analysis of the action of Fe ions on cytoplasmic free calcium ion concentration ([Ca 2+ ] i ) in astrocytes. Using qPCR and immunocytochemistry we identified DMT1 and TFR in astrocytes in primary cultures, in acutely isolated astrocytes and in brain tissue preparations; in situ both DMT1 and TFR are concentrated in astroglial perivascular endfeet. Administration of Fe 2+ or Fe 3+ in low μM concentrations evoked Ca 2+ signals in astrocytes in vitro and in vivo . Iron ions triggered increase in [Ca 2+ ] i by acting through two distinct molecular cascades. Uptake of Fe 2+ by DMT1 inhibited astroglial Na + -K + -ATPase (NKA), which led to an elevation in cytoplasmic Na + concentration (as measured by SBFI probe), thus reversing Na + /Ca 2+ exchanger (NCX) thereby generating Ca 2+ influx. Uptake of Fe 3+ by TF-TFR stimulated phospholipase C to produce inositol 1,4,5-trisphosphate (InsP 3 ), thus trigering InsP 3 receptor-mediated Ca 2+ release from the endoplasmic reticulum. Iron-induced Ca 2+ signals promote astroglial release of arachidonic acid and prostaglandin E2 cytokines by activating cytosolic phospholipase A2 (cPLA2) and NF-κB signalling cascade. In summary, these findings reveal new mechanisms of iron-induced astrocytic signalling operational in conditions of iron overload, in response to which astrocytes actively accumulate excessive iron and activate neuroprotective pathways.