Cell biologists who study iron pathways have known since the early 1980s that iron has at least one way to get into a cellthrough the front door so to speakby pulling in transferrin, a protein that carries iron. But by the end of the decade, researchers showed that iron, a mineral necessary for cell growth and development, must have another way to get into a cellthrough the back door. Scientists disrupted the known pathway and some cells, such as epithelial cells (which line the internal and external surfaces of the body and organs), still received enough iron to function.
Columbia University Health Sciences researchers now have identified a candidate carrier for the back door pathway, called 24p3/Ngal, which transports iron as it induces the formation of kidney tubules. The discovery could have important implications for kidney development, iron delivery in the fetus, and kidney disease.
Dr. Jonathan Barasch, assistant professor of medicine and anatomy and cell biology at P&S, Dr. Jun Yang, associate research scientist, and colleagues found the new iron carrier as part of their continuing efforts to study how epithelial cells are made during kidney development. Epithelial cells form nephrons, the tubules inside the kidney that filter waste from the bloodstream and maintain the body's water balance. Dr. Barasch and his laboratory team were the first to induce kidney tubules to form using purified proteins, a finding they published in Cell in 1999.
Following up the earlier work, Dr. Barasch and his colleagues identified 24p3/Ngal, a member of the lipocalin superfamily of proteins, when isolating proteins that induce the mesenchyme, a group of unspecialized cells found in a developing embryo, to become epithelia. In collaboration with Dr. Roland Strong and Dr. David Goetz of the Fred Hutchinson Cancer Institute, the authors showed that 24p/Ngal contained iron. Confirmation was found by the co-purification of 24p/Ngal and iron, as well as changes in iron-dependent genes upon addition of 24p3/Ngal to the mesenchyme. The findings were published in the November 2002 issue of Molecular Cell.
The other Columbia authors of the recent paper are Dr. Jau-Yi Li, associate research scientist; Dr. Wenge Wang, postdoctoral research fellow; Dr. Kiyoshi Mori, postdoctoral research fellow; Daria Setlik, medical student; and Dr. Tonggong Du, postdoctoral research scientist. The Columbia researchers collaborated with Dr. Hediye Erdjument-Bromage and Dr. Paul Tempst of Memorial Sloan-Kettering Cancer Center, and Dr. Goetz and Dr. Strong.
To test if 24p3/Ngal used a different pathway than transferrin, the researchers put fluorescent tags on the two proteins and added the proteins to developing kidney cells. They found that the iron transporters go to different cells at different stages of development. Specifically, they observed that early epithelial progenitor cells took up the 24p3/Ngal while late epithelial cells absorbed transferrin. In addition, in cells that take up both proteins, 24p3/Ngal and transferrin occupied different compartments, suggesting different mechanisms of iron unloading.
The existence of two different iron loading systems could be significant because iron is known to regulate more than 25 genes. Dr. Barasch speculates that iron, as a gene regulator, may function as one of the mechanisms controlling kidney development.
He is hopeful the findings also can be applied to understanding why iron-deficiency anemia in a pregnant woman can increase the risk of having a low-birth weight baby.
The research may lead to new insights about kidney pathology, as well, since 24p3/Ngal is a highly over-expressed protein in epithelial damage, and may regulate repair of the cells or capture iron released from damaged cells.
Dr. Barasch and the researchers in his lab are continuing their studies of epithelial cell formation using embryonic tissue from mice. Their work in progenitor cell conversion to epithelia differs from other current studies of epithelial cell polarity, which take mature epithelial cells, damage them, and then allow them to repair. Embryonic tissue allows description of epithelial polarity from its origin, while studies of mature epithelial cells are more likely to reflect a different process of reorganization of the regulators of polarity.
They already have identified a group of growth factors that appear to be important for kidney development and are using microarrays to find more of these proteins that induce epithelial development. "We will continue these systematic approaches to define what happens in the induction of epithelia and regulation of kidney development," Dr. Barasch says.
The National Institutes of Health and the March of Dimes provided support for the research.