Nramp1 modulates iron homoeostasis in vivo and in vitro: evidence for a role in cellular iron release involving de-acidification of intracellular vesicles

Nramp1 controls responses to infection and encodes a biallelic (G169D) macr ophage-restricted divalent-cation transporter. Nramp1(D169) is phenotypical ly null. We demonstrate Nramp1 is implicated in iron regulation in vivo. In spleen, expression is exclusive to Nramp1(G169): strains within the red pu lp. By morphometric analysis, the distribution of splenic iron, following s ystemic overload, correlates with Nramp1 genotype. More iron is located wit hin the red: pulp in Nramp1(D169) strains, whereas in Nramp1(G169) strains iron deposits are localized within the marginal-zone metallophilic cells. N ramp1 immunoreactive protein is not present in control brain, but inducible within a hemorrhagic lesion model in Nramp1(G169) strains. Nramp1 protein expression demonstrates an inverse correlation to the presence of iron. Nra mp1(G169) strains show no Perl's stain-reactive iron within the lesion. In contrast, Nramp1(D169) strains display iron-staining cells. The process of cellular iron regulation was investigated in vitro in Nramp1(G169) transfec tant Raw264.7 macrophages. Greater (30-50%) iron efflux from Nramp1(G169) c ompared with Nramp1(D169) cells was determined. The extent of Nramp1-depend ent iron-release was influenced by bafilomycin Al, and endogenous nitric ox ide synthesis,; both inhibitors of vacuolar-ATPase. This study demonstrates that Nramp1 regulates macrophage iron handling, and probably facilitates i ron release from macrophages undergoing erythrophagocytosis in vivo.