Abstract
Purpose: :
Retina expresses a complete set of iron-regulatory proteins essential for iron homeostasis. Important among these are HFE, hemojuvelin (HJV), hepcidin, ferroportin, and transferrin receptors, which are expressed in retina in a cell type-specific manner. Matriptase2 (TMPRSS6) is a protease that cleaves hemojuvelin on the cell surface and consequently regulates relative levels of soluble HJV and membrane-bound HJV. Since cell signaling by bone morphogenic proteins is controlled differentially by soluble HJV and membrane-bound HJV, this process is also regulated by matriptase2. Deletion of matriptase2 leads to systemic iron deficiency. Here the retinal expression of this proteolytic regulator of iron homeostasis in wild type mice, and alterations in retinal expression pattern of iron-regulatory genes in matriptase2-null mice were investigated.
Methods: :
Expression of matriptase2 in wild type mouse retina was monitored by RT-PCR and immunofluorescence, and its colocalization with HJV was studied by double labeling. Iron levels in wild type and matriptase2-null retina were measured indirectly by quantifying tissue levels of ferritin by ELISA. Retinal morphology was compared between wild type and matriptase2-null mice by H & E staining. Expression of various iron-regulatory genes in wild type and matriptase2-null mouse retinas was monitored by RT-PCR and immunofluorescence.
Results: :
RT-PCR and immunofluorescence studies showed that matriptase2 is expressed widely in the retina and its expression pattern is similar to that of HJV. In RPE, which is a polarized cell, matriptase2 and HJV colocalize at the apical membrane. The levels of ferritin are much lower in matriptase2-null mouse retina than in wild type mouse retina, indicating reduced levels of iron in the knockout mouse retina. RT-PCR showed that the levels of HJV mRNA and hepcidin mRNA were markedly elevated in matriptase2-null mouse retina compared to wild type mouse retina. However, there was no evidence of any morphological change in the null mouse retina.
Conclusions: :
Retina expresses matriptase2, the proteolytic regulator of iron homeostasis. The expression pattern of matriptase2 is similar to that of HJV, its proteolytic substrate. Iron deficiency is evident in matriptase2-null mouse retina, which is a consequence of increased levels of HJV and hepcidin within the retina.
Keywords: retina • enzymes/enzyme inhibitors • retinal pigment epithelium