Figure 6. Potential role of Zip8 and 14 in retinal iron import in Wt, Cp/Heph DKO and Hepc KO retinas.

Wt retina allows ferrous iron import across cell membranes through Zip8 and Zip14 while simultaneously exporting labile intracellular iron from the intracellular to the extracellular environment via ferroportin (Fpn). On the extracellular side, ferrous iron is oxidized by Cp and/or Heph and then bound by transferrin, which sequesters extracellular iron and prevents catalysis of the Fenton reaction (A). The hypothesized goal of this system is to sequester extracellular ferrous iron intracellularly within ferritin or extracellularly within transferrin. Sequestration prevents the formation of dangerous reactive oxygen species and subsequent deleterious effects while meeting the metabolic iron needs of the cells (Song and Dunaief, 2013). In the Cp/Heph DKO retina, impaired iron export, via mutations in ferroxidases Cp and Heph, results in the build-up of intracellular labile iron levels leading to previously documented increased ferritin levels (Hadziahmetovic et al., 2008). This increase in the amount of labile intracellular iron delays extraction and proteasomal degradation of Zip14 as shown previously (Zhao et al., 2014). We propose that Zip8 levels may increase in Cp/Heph DKO retinas by the same mechanism. This could initiate a positive feedback loop that leads to increased intracellular iron accumulation, eventually exceeding the ability of ferritin to sequester the iron (B). Hepc KO mice also exhibit intracellular retinal iron loading. Based on the data presented herein on the differential regulation of Zip14 in Cp/Heph DKO and Hepc KO retinas, we propose that high Holo-Tf levels in Hepc KO retinas lead to decreased levels of Zip14 protein without affecting Zip8 protein levels. Zip8 levels increase due to inhibition of membrane extraction by iron as proposed in our model of Cp/Heph DKO retinas. We suggest that in the retina, the mechanism for Zip14 degradation may be modulated by HFE. HFE has previously been shown to decrease Zip14 half-life in human hepatoma cells (Gao et al., 2008) and is expressed in the retina (Gnana-Prakasam et al., 2010) (C). Zip8 and Zip14 could play a significant role in retinal iron accumulation via a positive feedback loop whereby intracellular iron delays Zip14, and possibly Zip8, membrane extraction and degradation.