ZIP8, Member of the Solute-Carrier-39 (SLC39) Metal-Transporter Family: Characterization of Transporter Properties

Abstract

Cadmium is a dangerous metal distributed widely in the environment. Members of our laboratory recently identified the ZIP8 transporter protein, encoded by the mouse Slc39a8 gene, to be responsible for genetic differences in response to cadmium damage of the testis. Stable retroviral infection of the ZIP8 cDNA in mouse fetal fibroblast cultures (rvZIP8 cells) leads to as much as a 10-fold increase in the rate of intracellular cadmium influx and accumulation. In the present study, we showed that cadmium uptake operated maximally at pH 7.5 and a temperature of 37 degrees C and was inhibited by cyanide. Of more than a dozen cations tested, manganese(II) was the best competitive cation for cadmium uptake. The Km for Cd2+ uptake was 0.62 microM, and the Km for Mn2+ uptake was 2.2 microM; thus, manganese is probably the physiological substrate for ZIP8. Cadmium uptake was independent of sodium, potassium or chloride ions, but strongly dependent on the presence of bicarbonate. By Western blot analysis of rvZIP8 cells, we showed that ZIP8 protein was glycosylated. Using Z-stack confocal microscopy in Madin-Darby canine kidney polarized epithelial cells, we found that ZIP8 was localized on the apical side-implying an important role for manganese or cadmium uptake and disposition. It is likely that ZIP8 is a Mn2+/HCO3- symporter, that a HCO3- gradient across the plasma membrane acts as the driving force for manganese uptake, and that cadmium is a rogue hitchhiker displacing manganese to cause cadmium-associated disease.

Manganese transport in eukaryotes: The role of DMT1

“…More recently, it has been established that Mn can also be transported via high affinity metal transporters such as calcium (Ca) and iron (Fe) transporters. Some of these transporters include the divalent metal transporter (DMT1), which belongs to the family of natural resistance-associated macrophage protein (NRAMP) (Gunshin et al, 1997;Garrick et al, 2003); ZIP-8, a member of the solute carrier-39 (He et al, 2006); transferrin (Tf) receptor (TfR) (Davidsson et al, 1989;Aschner et al, 1994), which is known to be responsible for Fe 3+ uptake; voltage regulated (Lucaciu et al, 1997) and store-operated Ca 2+ channels (Riccio et al, 2002); and the ionotropic glutatmate receptor Ca 2+ channels (Kannurpatti et al, 2000) ( Figure 1). While the relative contribution of each of these transporters remains unknown, it is likely that optimal tissue Mn concentrations are maintained by the involvement of all of these transporters.…”

Manganese neurotoxicity: A focus on the neonate

“…In plants, several ZIP proteins have been implicated in divalent metal transport, including zinc (Zn), Fe, and Mn. A recent in vitro study (He et al, 2006) in mouse fetal fibroblast cultures established that ZIP8 is a high affinity for Mn. The Km of 2.2 μM for Mn2+ is close to physiological concentrations and within the same range determined in many cell lines or tissues.…”

Manganese and its Role in Parkinson’s Disease: From Transport to Neuropathology

“…This family of transporter proteins is members of the solute-carrier-39 (SLC39) metaltransporter family; the family contains 14 members, which are all highly conserved between mouse and human (Eide 2004). It was recently suggested (He et al 2006) ZIP8 possesses high affinity for Mn (K m close to physiological concentrations of Mn in various tissues). Notably, while to date the role of the transporter in Mn has been ascribed only in the testis (He et al 2006), ZIP8 is expressed in brain capillaries (Girijashanker et al 2008).…”

“…It was recently suggested (He et al 2006) ZIP8 possesses high affinity for Mn (K m close to physiological concentrations of Mn in various tissues). Notably, while to date the role of the transporter in Mn has been ascribed only in the testis (He et al 2006), ZIP8 is expressed in brain capillaries (Girijashanker et al 2008). Additional studies have also established that bolus injections of Mn directly into the blood may lead to Mn diffusion across the BBB (Bock et al 2008).…”