Nramp2 was first cloned as a gene having a sequence related to Nramp1 (natural resistance associated macrophage protein 1). Expression cloning subsequently identified this protein as an intestinal iron transporter. A G185R mutation in the Nramp2 gene was identified as probably responsible for defective gastrointestinal mucosal iron uptake in microcytic (mk) mice. Remarkably, an identical mutation in this gene was causal for a similar defect in iron exit from reticulocyte endosomes in the Belgrade (b) rat. The names DCT1 (divalent cation transporter 1) and DMT1 (divalent metal transporter 1) reflect the possibility that seven other divalent metals pass through this transporter. We have been examining the role of DMT1 in metal transport by transient expression of DMT1 in HEK293T cells. Preliminary evidence indicates that the G185 DMT1 construct (-IRE form) stimulates uptake of four metals – Fe, Mn, Ni and Co – but R185 cDNA does not. Uptake of two metals, divalent metals Cd and Zn, failed to respond whereas two others, Cu and Pb, lack the commercial radiolabelled isotopes for testing. In the same assay, Cd, Cu and Pb compete with Mn transport via DMT1 but Zn did not. The same methods, combined with specific antibodies and comparison of iron binding proteins under reducing vs. non-reducing conditions, provide strong evidence that Fe (III) uptake depends on a different pathway in multiple cells despite the ferrireductase activity that probably converts it to a substrate for DMT1 in the enterocytes and in endosomes. Tissue levels of these metals in mk/mk mice and b/b rats should reveal where DMT1 plays a substantial role in metal homeostasis. Preliminary data suggest such a role in certain tissues for Fe and Mn but not for Cu and Zn. Alternative splicing generates two isoforms of DMT1 that differ in the C-terminal portion and in the 3Ì UTR: the +IRE and -IRE forms. Very recently N-terminal isoforms have been identified but details of their relationship to the C-terminal isoforms or functionality are currently uncertain. Metal homeostasis includes transporting micronutrients, their subsequent trafficking and management of toxic challenges, so one must consider how the isoforms are used in all of these functions and whether their roles vary for certain metals. Current evidence suggests that the +IRE form is the apical iron transporter in enterocytes. In the lung by contrast, DMT1 -IRE mRNA and -IRE protein levels rise after exposure to oil fly ash. A similar rise occurs in a cell culture model after exposure to exogenous iron; the +IRE form is unresponsive to the metal challenge. Thus the -IRE form might, on such occasions, be part of a detoxification system in which the +IRE form is not a participant. Specific antibodies to the two isoforms show that they have distinct localizations in a number of cell types, also consistent with functional specialization. Most remarkably, the -IRE form is found in the nucleus of cells with neuronal properties. The nuclear role is currently unknown, but tantalizing to consider.

This work was supported by grants from the NIH and EPA.